Publications

2010
Aksay, I. A. ; Gurdag, S. ; Javaherian, N. ; Lettow, J. S. ; Pino, G. ; Redmond, K. ; Vatansever, A. ; Yildirim, I. O. Tire cords comprising spun monofilaments of polymer matrix-graphene sheets and an adhesive, 2010.Abstract
Tire cords comprising fibers including at least one polymer and graphene sheets. Graphene sheets are added to poly(ethylene terephthalate) (PET) by melt compounding in an extruder to yield a PET compn. comprising about 0.25 wt.% graphene sheets. The PET compn. is then solid phase polymd. at 215° to an IV of about 1 dL/g. The compn. is spun into monofilaments that are then post drawn to a draw ratio of about 4 to 5. After drawing, the filaments have a diam. of about 120 μ. The storage modulus of the monofilaments is then measured as a function of temp. using a dynamic mech. analyzer (DMA). [on SciFinder(R)]
Fang, Z. H. ; Punckt, C. ; Leung, E. Y. ; Schniepp, H. C. ; Aksay, I. A. Tuning of structural color using a dielectric actuator and multifunctional compliant electrodes. Applied Optics 2010, 49, 6689-6696.Abstract
We have developed electrically conducting silicone elastomer nanocomposites that serve both as compliant electrodes in an electrostatic actuator and, at the same time, as optically active elements creating structural color. We demonstrate the capabilities of our setup by actuating an elastomeric diffraction grating and colloidal-crystal-based photonic structures. (C) 2010 Optical Society of America
2009
Herrera-Alonso, M. ; McAllister, M. J. ; Aksay, I. A. ; Prud'homme, R. K. Bridged graphite oxide materials for hydrogen storage, 2009.Abstract
The present invention is a bridged graphite oxide material, comprising at least two graphite oxide sheets in which a plurality of graphite oxide sheets are bridged to at least one other graphite oxide sheet by at least one diamine bridging group. The bridged graphite oxide is formed by the covalent reaction of one amino group of a diamine with a reactive group on the surface or edge of one graphite oxide sheet and the covalent reaction of another amino group of the same diamine with a reactive group on the surface or edge of another graphite oxide sheet. The bridged graphite oxide material may be incorporated in polymer composites or used in hydrogen adsorption media. [on SciFinder(R)]
Punckt, C. ; Aksay, I. A. Dissolution dynamics of thin films measured by optical reflectance. Journal of Chemical Physics 2009, 131.Abstract
Measuring the dissolution dynamics of thin films in situ both with spatial and temporal resolution can be a challenging task. Available methods such as scanning electrochemical microscopy rely on scanning the specimen and are intrinsically slow. We developed a characterization technique employing only an optical microscope, a digital charge coupled device camera, and a computer for image processing. It is capable of detecting dissolution rates of the order of nm/min and has a spatial and temporal resolution which is limited by the imaging and recording setup. We demonstrate the capabilities of our method by analyzing the electrochemical dissolution of copper thin films on gold substrates in a mild hydrochloric acid solution. Due to its simplicity, our technique can be implemented in any laboratory and can be applied to a variety of systems such as thin film sensors or passive coatings.
Castrillon, S. R. V. ; Giovambattista, N. ; Aksay, I. A. ; Debenedetti, P. G. Effect of Surface Polarity on the Structure and Dynamics of Water in Nanoscale Confinement. Journal of Physical Chemistry B 2009, 113, 1438-1446.Abstract
We present a molecular dynamics simulation study of the structure and dynamics of water confined between silica surfaces using beta-cristobalite as a model template. We scale the surface Coulombic charges by means of a dimensionless number, k, ranging from 0 to 1, and thereby we can model systems ranging frorn hydrophobic apolar to hydrophilic, respectively. Both rotational and translational dynamics exhibit a nonmonotonic dependence on k characterized by a maximum in the in-plane diffusion coefficient, D-parallel to, at values between 0.6 and 0.8, and a minimum in the rotational relaxation time, tau(R), at k = 0.6. The slow dynamics observed in the proximity of the hydrophobic apolar surface are a consequence of beta-cristobalite templating an ice-like water layer. The fully hydrophilic surfaces (k = 1.0), on the other hand, result in slow interfacial dynamics due to the presence of dense but disordered water that forms strong hydrogen bonds with surface silanol groups. Confinement also induces decoupling between translational and rotational dynamics, as evidenced by the fact that TR attains values similar to that of the bulk, while D-parallel to is always lower than in the bulk. The decoupling is characterized by a more drastic reduction in the translational dynamics of water compared to rotational relaxation.
Kou, R. ; Shao, Y. Y. ; Wang, D. H. ; Engelhard, M. H. ; Kwak, J. H. ; Wang, J. ; Viswanathan, V. V. ; Wang, C. M. ; Lin, Y. H. ; Wang, Y. ; et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochemistry Communications 2009, 11, 954-957.Abstract
Electrocatalysis of oxygen reduction using Pt nanoparticles supported on functionalized graphene sheets (FGSs) was studied. FGSs were prepared by thermal expansion of graphite oxide. Pt nanoparticles with average diameter of 2 nm were uniformly loaded on FGSs by impregnation methods. Pt-FGS showed a higher electrochemical surface area and oxygen reduction activity with improved stability as compared with the commercial catalyst. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical characterization suggest that the improved performance of Pt-FGS can be attributed to smaller particle size and less aggregation of Pt nanoparticles on the functionalized graphene sheets.
Castrillon, S. R. V. ; Giovambattista, N. ; Aksay, I. A. ; Debenedetti, P. G. Evolution from Surface-Influenced to Bulk-Like Dynamics in Nanoscopically Confined Water. Journal of Physical Chemistry B 2009, 113, 7973-7976.Abstract
We use molecular dynamics simulations to study the influence of confinement on the dynamics of a nanoscopic water film at T = 300 K and rho = 1.0 g cm(-3). We consider two infinite hydrophilic (beta-cristobalite) silica surfaces separated by distances between 0.6 and 5.0 nm. The width of the region characterized by surface-dominated slowing down of water rotational dynamics is similar to 0.5 nm, while the corresponding width for translational dynamics is similar to 1.0 nm. The different extent of perturbation undergone by the in-plane dynamic properties is evidence of rotational-translational decoupling. The local in-plane rotational relaxation time and translational diffusion coefficient collapse onto confinement-independent "master" profiles as long as the separation d >= 1.0 nm. Long-tithe tails in the perpendicular component of the dipole moment autocorrelation function are indicative of anisotropic behavior in the rotational relaxation.
Sabourin, J. L. ; Dabbs, D. M. ; Yetter, R. A. ; Dryer, F. L. ; Aksay, I. A. Functionalized Graphene Sheet Colloids for Enhanced Fuel/Propellant Combustion. ACS Nano 2009, 3 3945-3954.Abstract
We have compared the combustion of the monopropellant nitromethane with that of nitromethane containing colloidal particles of functionalized graphene sheets or metal hydroxides. The linear steady-state burning rates of the monopropellant and colloidal suspensions were determined at room temperature, under a range of pressures (3.35-14.4 MPa) using argon as a pressurizing fluid. The ignition temperatures were lowered and burning rates increased for the colloidal suspensions compared to those of the liquid monopropellant alone, with the graphene sheet suspension having significantly greater burning rates (i.e., greater than 175%). The relative change in burning rate from neat nitromethane increased with increasing concentrations of fuel additives and decreased with increasing pressure until at high pressures no enhancement was found.
Aksay, I. A. ; Milius, D. L. ; Korkut, S. ; Prud'homme, R. K. Functionalized graphene sheets having high carbon to oxygen ratios for polymer composite, 2009.Abstract
Functionalized graphene sheets having a carbon to oxygen molar ratio of at least about 23:1. A polymer composite comprises the functionalized graphene sheet and ≥1 polymer. [on SciFinder(R)]
Wu, H. ; Wang, J. ; Kang, X. H. ; Wang, C. M. ; Wang, D. H. ; Liu, J. ; Aksay, I. A. ; Lin, Y. H. Glucose biosensor based on immobilization of glucose oxidase in platinum nanoparticles/graphene/chitosan nanocomposite film. Talanta 2009, 80, 403-406.Abstract
The bionanocomposite film consisting of glucose oxidase/Pt/functional graphene sheets/chitosan (GOD/Pt/FGS/chitosan) for glucose sensing is described. With the electrocatalytic synergy of FGS and Pt nanoparticles to hydrogen peroxide, a sensitive biosensor with a detection limit of 0.6 mu M glucose was achieved. The biosensor also has good reproducibility, long-term stability and negligible interfering signals from ascorbic acid and uric acid comparing with the response to glucose. The large surface area and good electrical conductivity of graphene suggests that graphene is a potential candidate as a sensor material. The hybrid nanocomposite glucose sensor provides new opportunity for clinical diagnosis and point-of-care applications.
Kang, X. H. ; Wang, J. ; Wu, H. ; Aksay, I. A. ; Liu, J. ; Lin, Y. H. Glucose Oxidase-graphene-chitosan modified electrode for direct electrochemistry and glucose sensing. Biosensors & Bioelectronics 2009, 25, 901-905.Abstract
Direct electrochemistry of a glucose oxidase (GOD)-graphene-chitosan nanocomposite was studied. The immobilized enzyme retains its bioactivity, exhibits a surface confined, reversible two-proton and two-electron transfer reaction, and has good stability, activity and a fast heterogeneous electron transfer rate with the rate constant (k(s)) of 2.83 s(-1). A much higher enzyme loading (1.12 x 10(-9) mol/cm(2)) is obtained as compared to the bare glass carbon surface. This GOD-graphene-chitosan nanocomposite film can be used for sensitive detection of glucose. The biosensor exhibits a wider linearity range from 0.08 mM to 12 mM glucose with a detection limit of 0.02 mM and much higher sensitivity (37.93 mu A mM(-1) cm(-2)) as compared with other nanostructured supports. The excellent performance of the biosensor is attributed to large surface-to-volume ratio and high conductivity of graphene, and good biocompatibility of chitosan, which enhances the enzyme absorption and promotes direct electron transfer between redox enzymes and the surface of electrodes.
Wang, D. H. ; Choi, D. W. ; Li, J. ; Yang, Z. G. ; Nie, Z. M. ; Kou, R. ; Hu, D. H. ; Wang, C. M. ; Saraf, L. V. ; Zhang, J. G. ; et al. Self-Assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion. ACS Nano 2009, 3 907-914.Abstract
We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in the presence of a percolated graphene network embedded into the metal oxide electrodes.
Hirata, Y. ; Hara, A. ; Aksay, I. A. Thermodynamics of densification of powder compact. Ceramics International 2009, 35, 2667-2674.Abstract
This paper established a necessary condition for the sintering of powder compacts by examining the total free energy balance in terms of the particle size, neck size and contact number. The thermodynamic analysis of the proposed model clarifies the relation of shrinkage (q) of powder compact-contact angle (phi)-relative density at a given dihedral angle (phi(e)) of a grain boundary. Faster densification proceeds in the region with a larger coordination number (n) of particles at a small q value. A large shrinkage is needed to eliminate the large pores formed in the structure of small n value. Full density can be achieved in the range of 117 degrees
2008
Schniepp, H. C. ; Kudin, K. N. ; Li, J. L. ; Prud'homme, R. K. ; Car, R. ; Saville, D. A. ; Aksay, I. A. Bending Properties of Single Functionalized Graphene Sheets Probed by Atomic Force Microscopy. ACS Nano 2008, 2 2577-2584.Abstract
We probe the bending characteristics of functionalized graphene sheets with the tip of an atomic force microscope. Individual sheets are transformed from a flat into a folded configuration. Sheets can be reversibly folded and unfolded multiple times, and the folding always occurs at the same location. This observation suggests that the folding and bending behavior of the sheets is dominated by pre-existing kink (or even fault) lines consisting of defects and/or functional groups.
Korkut, S. ; Saville, D. A. ; Aksay, I. A. Collodial Cluster Arrays by Electrohydrodynamic Printing. Langmuir 2008, 24, 12196-12201.Abstract
A "stable" electrohydrodynamic jet is used to print arrays of colloidal suspensions on hydrophobic surfaces. Printed lines break up into sessile drops, and capillary forces guide the self-assembly of colloidal particles during the evaporation of the liquid, resulting in arrays of colloidal single particles or particle clusters depending on the concentration of the suspensions. The clusters differ from those formed in the absence of a substrate when the number of particles is larger than three. Multiple structures are found for the same number of particles.
Korkut, S. ; Aksay, I. A. Comment on "Enhanced Stability of Electrohydrodynamic Jets through Gas Ionization" - Korkut and Aksay reply. Physical Review Letters 2008, 101.Abstract
A Reply to the Comment by Manuel Gamero-Castano.
Prud'homme, R. K. ; Ozbas, B. ; Aksay, I. A. ; Register, R. A. ; Adamson, D. H. Electrically conductive polymer nanocomposites containing functional graphene, 2008.Abstract
A polymer compn. comprises a polymer matrix comprising an elastomer, and a functional graphene which displays no signature of graphite and/or graphite oxide, as detd. by X-ray diffraction. The functional graphene-contg. polymer compns. have excellent strength, toughness, thermal stability, elec. cond., and can be used for prodn. of gas-barrier materials. Thus, films (200-400 μm) made of a silicone rubber RTV 615 compn. contg. 5% of functional graphene nanoparticles had a Young's modulus 12.18 and a tensile strength 6.52 times higher than the resp. values for neat rubber. Similar films made of a silicone rubber RTV 615 compn. contg. 5% of clay nanoparticles had a Young's modulus 1.28 and a tensile strength 2.38 times higher than the resp. values for neat rubber. [on SciFinder(R)]
Ristenpart, W. D. ; Jiang, P. ; Slowik, M. A. ; Punckt, C. ; Saville, D. A. ; Aksay, I. A. Electrohydrodynamic Flow and Colloidal Patterning near Inhomogeneities on Electrodes. Langmuir 2008, 24, 12172-12180.Abstract
Current density inhomogeneities on electrodes (of physical, chemical, or optical origin) induce long-range electrohydrodynamic fluid motion directed toward the regions of higher current density. Here, we analyze the flow and its implications for the orderly arrangement of colloidal particles as effected by this flow on patterned electrodes. A scaling analysis indicates that the flow velocity is proportional to the product of the applied voltage and the difference in current density between adjacent regions on the electrode. Exact analytical solutions for the streamlines are derived for the case of a spatially periodic perturbation in current density along the electrode. Particularly simple asymptotic expressions are obtained in the limits of thin double layers and either large or small perturbation wavelengths. Calculations of the streamlines are in good agreement with particle velocimetry experiments near a mechanically generated inhomogeneity (a "scratch") that generates a current density larger than that of the unmodified electrode. We demonstrate that proper placement of scratches on an electrode yields desired patterns of colloidal particles.
Korkut, S. ; Saville, D. A. ; Aksay, I. A. Enhanced stability of electrohydrodynamic jets through gas ionization. Physical Review Letters 2008, 100.Abstract
Theoretical predictions of the nonaxisymmetric instability growth rate of an electrohydrodynamic jet based on the measured total current overestimate experimental values. We show that this apparent discrepancy is the result of gas ionization in the surrounding gas and its effect on the surface charge density of the jet. As a result of gas ionization, a sudden drop in the instability growth rate occurs below a critical electrode separation, yielding highly stable jets that can be used for nano- to microscale printing.
Prudhomme, R. K. ; O'Neil, C. D. ; Ozbas, B. ; Aksay, I. A. ; Register, R. A. ; Adamson, D. H. Functional graphene-polymer nanocomposites for gas barrier applications, 2008.Abstract
A gas diffusion barrier contains a polymer (e.g., natural rubber) matrix and a functional graphene which displays no signature of graphite and/or graphite oxide, as detd. by X-ray diffraction. [on SciFinder(R)]
Ramanathan, T. ; Abdala, A. A. ; Stankovich, S. ; Dikin, D. A. ; Herrera-Alonso, M. ; Piner, R. D. ; Adamson, D. H. ; Schniepp, H. C. ; Chen, X. ; Ruoff, R. S. ; et al. Functionalized graphene sheets for polymer nanocomposites. Nature Nanotechnology 2008, 3 327-331.Abstract
Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications(1). Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content(2-11). However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for polyacrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet - poly methyl methacrylate) rivaling those for single-walled carbon nanotube-poly methyl methacrylate) composites.
Murira, C. M. ; Punckt, C. ; Schniepp, H. C. ; Khusid, B. ; Aksay, I. A. Inhibition and Promotion of Copper Corrosion by CTAB in a Microreactor System. Langmuir 2008, 24, 14269-14275.Abstract
We report on an optical microscopy technique for the analysis of corrosion kinetics of metal thin films in microreactor systems and use it to study. the role of cetyltrimethylammonium bromide surfactant as a corrosion inhibitor in a copper-gold galvanic coplanar microsystem. A minimum in the dissolution rate of copper is observed when the surfactant concentration is similar to 0.8 mM. To explain why the inhibitory role of the surfactant does not extend to higher concentrations, we use zero resistance ammetry with separated half cells and show that while the surfactant inhibits cathodic reactions on gold, it also promotes the corrosion of copper because of the catalytic action of bromide counterions. These two competing processes lead to the observed minimum in the dissolution rate.
Poon, H. F. ; Saville, D. A. ; Aksay, I. A. Linear colloidal crystal arrays by electrohydrodynamic printing. Applied Physics Letters 2008, 93.Abstract
We use electrohydrodynamic jets of colloidal suspensions to produce arrays of colloidal crystalline stripes on surfaces. A critical factor in maintaining a stable jet is the distance of separation between the nozzle and the surface. Colloidal crystalline stripes are produced as two wetting lines of the deployed suspension merge during drying. To ensure that the two wetting lines merge, the "deployed-line-width" to "particle size" ratio is kept below a critical value so that the capillary forces overcome the frictional forces between the particles and the substrate. (C) 2008 American Institute of Physics.
Schniepp, H. C. ; Shum, H. C. ; Saville, D. A. ; Aksay, I. A. Orientational order of molecular assemblies on rough surfaces. Journal of Physical Chemistry C 2008, 112, 14902-14906.Abstract
Using atomic force microscopy, we show that previous observations on the orientational order of micelles on atomically smooth crystals with directions dictated by the crystal symmetry is only valid for the case of perfectly smooth crystals. On rough surfaces, orientations are independent of the lattice symmetry and the observed directions can be explained by considering the guiding influence of topographic surface features.
Kudin, K. N. ; Ozbas, B. ; Schniepp, H. C. ; Prud'homme, R. K. ; Aksay, I. A. ; Car, R. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Letters 2008, 8 36-41.Abstract
We investigate Raman spectra of graphite oxide and functionalized graphene sheets with epoxy and hydroxyl groups and Stone-Wales and 5-8-5 defects by first-principles calculations to interpret our experimental results. Only the alternating pattern of single-double carbon bonds within the sp(2) carbon ribbons provides a satisfactory explanation for the experimentally observed blue shift of the G band of the Raman spectra relative to graphite. To obtain these single-double bonds, it is necessary to have sp(3) carbons on the edges of a zigzag carbon ribbon.
Schniepp, H. C. ; Saville, D. A. ; Aksay, I. A. Tip-induced orientational order of surfactant micelles on gold. Langmuir 2008, 24, 626-631.Abstract
Using liquid-cell atomic force microscopy, we investigate aqueous solutions of alkyltrimethylammonium halide surfactants at the Au(III) surface. The long, micellar surfactant surface aggregates cover the gold surface completely and exhibit two types of orientational order for chloride and bromide counterions, respectively. We observe lateral forces perpendicular to the scanning direction, which we explain by anisotropic friction between the probe and the oriented micelles. Conversely, we show that these friction forces can be employed to modify the spatial conformation of the micellar adlayer. Where previous methods have failed to provide control over the orientation down to the level of individual micelles, we use this technique to achieve a very high degree of order over more than 100 micelle diameters.
2007
Ristenpart, W. D. ; Aksay, I. A. ; Saville, D. A. Electrically driven flow near a colloidal particle close to an electrode with a faradaic current. Langmuir 2007, 23, 4071-4080.Abstract
To elucidate the nature of processes involved in electrically driven particle aggregation in steady fields, flows near a charged spherical colloidal particle next to an electrode were studied. Electrical body forces in diffuse layers near the electrode and the particle surface drive an axisymmetric flow with two components. One is electroosmotic flow (EOF) driven by the action of the applied field on the equilibrium diffuse charge layer near the particle. The other is electrohydrodynamic (EHD) flow arising from the action of the applied field on charge induced in the electrode polarization layer. The EOF component is proportional to the current density and the particle surface (zeta) potential, whereas our scaling analysis shows that the EHD component scales as the product of the current density and applied potential. Under certain conditions, both flows are directed toward the particle, and a superposition of flows from two nearby particles provides a mechanism for aggregation. Analytical calculations of the two flow fields in the limits of infinitesimal double layers and slowly varying current indicate that the EOF and EHD flow are of comparable magnitude near the particle whereas in the far field the EHD flow along the electrode is predominant. Moreover, the dependence of EHD flow on the applied potential provides a possible explanation for the increased variability in aggregation velocities observed at higher field strengths.
Ku, A. Y. ; Saville, D. A. ; Aksay, I. A. Electric-field-induced orientation of surfactant-templated nanoscopic silica. Langmuir 2007, 23, 8156-8162.Abstract
While exhibiting a well-defined nanometer-level structure, surfactant-templated nanoscopic silicas produced via self-assembly do not always possess long-range order. We demonstrate that long-range order can be controlled by guiding the self-assembly of nanostructured silica-surfactant hybrids with low-strength electric fields (E similar to 200 V/m) to produce nanoscopic silica with both the micrometer- and nanometer-level structures oriented parallel to the applied field. Under the influence of the electric field, nanoscopic silica particles migrate, elongate, and merge into fibers with a rate of migration proportional to the applied field strength. The linear dependence with the field strength indicates that the process is governed by electroosmotic flow but not by polarization effects. Realignment of the short-range ordered surfactant nanochannels along the fiber axis accompanies the migration.
Ristenpart, W. D. ; Aksay, I. A. ; Saville, D. A. Electrohydrodynamic flow around a colloidal particle near an electrode with an oscillating potential. Journal of Fluid Mechanics 2007, 575, 83-109.Abstract
Electrohydrodynamic (EHD) flow around a charged spherical colloid near an electrode was studied theoretically and experimentally to understand the nature of long-range particle-particle attraction near electrodes. Numerical computations for finite double-layer thicknesses confirmed the validity of an asymptotic methodology for thin layers. Then the electric potential around the particle was computed analytically in the limit of zero Peclet number and thin double layers for oscillatory electric fields at frequencies where Faradaic reactions are negligible. Streamfunctions for the steady component of the EHD flow were determined with an electro-osmotic slip boundary condition on the electrode surface. Accordingly, it was established how the axisymmetric flow along the electrode is related to the dipole coefficient of the colloidal particle. Under certain conditions, the flow is directed toward the particle and decays as r(-4), in accord with observations of long-range particle aggregation. To test the theory, particle-tracking experiments were performed with fluorescent 300 nm particles around 50 mu m particles over a wide range of electric field strengths and frequencies. Treating the particle surface conductivity as a fitting parameter yields velocities in excellent agreement with the theoretical predictions. The observed frequency dependence, however, differs from the model predictions, suggesting that the effect of convection on the charge distribution is not negligible as assumed in the zero Peclet number limit.
Herrera-Alonso, M. ; Abdala, A. A. ; McAllister, M. J. ; Aksay, I. A. ; Prud'homme, R. K. Intercalation and stitching of graphite oxide with diaminoalkanes. Langmuir 2007, 23, 10644-10649.Abstract
The intercalation reaction of graphite oxide with diaminoalkanes, with the general formula H2N(CH2)(n)NH2 (n = 4-10), was studied as a method for synthesizing pillared graphite with tailored interlayer spacing. Interlayer spacings from 0.8 to 1.0 nm were tailored by varying the size of the intercalant from (CH2)(4) to (CH2)(10). X-ray diffraction and infrared spectroscopy were used to confirm intercalation, and the frequency of the CH2 stretch confirmed that the intercalants are in a disordered state, with an important contribution from the gauche conformer. Sequential intercalation of diaminoalkanes followed by dodecylamine demonstrated the inability of these "stitched" systems to undergo expansion along the c-direction, indicative of cross-linking. Finally, the reaction of graphite oxide with diaminoalkanes under reflux and for extended periods (> 72 h) resulted in the chemical reduction of the graphite oxide to a disordered graphitic structure.
Adamson, D. H. ; Dabbs, D. M. ; Pacheco, C. R. ; Giotto, M. V. ; Morse, D. E. ; Aksay, I. A. Non-peptide polymeric silicatein alpha mimic for neutral pH catalysis in the formation of silica. Macromolecules 2007, 40, 5710-5717.Abstract
We have synthesized a catalytically active polymer inspired by the naturally occurring protein silicatein alpha and have shown it to catalyze the formation of silica from tetraethoxysilane under near-neutral pH and ambient temperatures. We based the composition of the polymer on the functionalities found in silicatein alpha, specifically those essential components of the catalytically active site for the hydrolysis of silicon alkoxides. Our bioinspired polymer is a block copolymer of poly(2-vinylpyridine-b-1,2-butadiene), functionalized by the addition of hydroxyl groups via hydroboration chemistry. The catalytic action of our polymer on tetraethoxysilane at neutral pH and ambient temperature conditions has been confirmed using a modified molybdic acid assay method, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The structure of the resulting gel is investigated by scanning electron microscopy and solid-state nuclear magnetic resonance. The microscopic features of the material formed resemble that of gels formed by the acid-catalyzed hydrolysis of tetraethoxysilane.
McAllister, M. J. ; Li, J. L. ; Adamson, D. H. ; Schniepp, H. C. ; Abdala, A. A. ; Liu, J. ; Herrera-Alonso, M. ; Milius, D. L. ; Car, R. ; Prud'homme, R. K. ; et al. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chemistry of Materials 2007, 19, 4396-4404.Abstract
A detailed analysis of the thermal expansion mechanism of graphite oxide to produce functionalized graphene sheets is provided. Exfoliation takes place when the decomposition rate of the epoxy and hydroxyl sites of graphite oxide exceeds the diffusion rate of the evolved gases, thus yielding pressures that exceed the van der Waals forces holding the graphene sheets together. A comparison of the Arrhenius dependence of the reaction rate against the calculated diffusion coefficient based on Knudsen diffusion suggests a critical temperature of 550 degrees C which must be exceeded for exfoliation to occur. As a result of their wrinkled nature, the functionalized and defective graphene sheets do not collapse back to graphite oxide but are highly agglomerated. After dispersion by ultrasonication in appropriate solvents, statistical analysis by atomic force microscopy shows that 80% of the observed flakes are single sheets.
Schniepp, H. C. ; Shum, H. C. ; Saville, D. A. ; Aksay, I. A. Surfactant aggregates at rough solid-liquid interfaces. Journal of Physical Chemistry B 2007, 111, 8708-8712.Abstract
We demonstrate improved atomic force microscopic imaging of surfactant surface aggregates, featuring an increase in the topography contrast by several hundred percent with respect to all previous studies. Surfactant aggregates on rough gold surfaces, which could not be imaged previously because of low resolution, display substantially different morphologies when compared with atomically smooth materials.
Prud'homme, R. K. ; Aksay, I. A. ; Adamson, D. ; Abdala, A. Thermally exfoliated graphite oxide and its use in nanocomposites, 2007.Abstract
A modified graphite oxide material contains a thermally exfoliated graphite oxide with a surface area of from ∼300 m2/g to 2600 m2/g, wherein the thermally exfoliated graphite oxide displays no signature of the original graphite and/or graphite oxide, as detd. by x-ray diffraction. The modified graphite oxide is useful in nanocomposites with, e.g., PMMA. [on SciFinder(R)]
2006
Chun, J. ; Li, J. L. ; Car, R. ; Aksay, I. A. ; Saville, D. A. Anisotropic adsorption of molecular assemblies on crystalline surfaces. Journal of Physical Chemistry B 2006, 110, 16624-16632.Abstract
Orientational order of surfactant micelles and proteins on crystalline templates has been observed but, given that the template unit cell is significantly smaller than the characteristic size of the adsorbate, this order cannot be attributed to lattice epitaxy. We interpret the template-directed orientation of rodlike molecular assemblies as arising from anisotropic van der Waals interactions between the assembly and crystalline surfaces where the anisotropic van der Waals interaction is calculated using the Lifshitz methodology. Provided the assembly is sufficiently large, substrate anisotropy provides a torque that overcomes rotational Brownian motion near the surface. The probability of a particular orientation is computed by solving a Smoluchowski equation that describes the balance between van der Waals and Brownian torques. Torque aligns both micelles and protein fibrils; the interaction energy is minimized when the assembly lies perpendicular to a symmetry axis of a crystalline substrate. Theoretical predictions agree with experiments for both hemi-cylindrical micelles and protein fibrils adsorbed on graphite.
Chen, C. H. ; Saville, D. A. ; Aksay, I. A. Electrohydrodynamic "drop-and-place" particle deployment. Applied Physics Letters 2006, 88.Abstract
The "drop-and-place" paradigm aims at delivering and positioning liquid drops using a pulsed electrohydrodynamic jet. On-demand drops much smaller than the diameter of the delivery nozzle may also contain particles. We report proof-of-concept experiments on the delivery of single 2 mu m diameter particles using a 50 mu m nozzle and identify the control parameters for dosing and positioning accuracies. A positioning accuracy at the micrometer level is achieved by eliminating contact line pinning on a hydrophobic surface and minimizing impingement-induced motion. The dosing statistics follow the random Poisson distribution, indicating that single-particle accuracy can be achieved using a gating mechanism. (c) 2006 American Institute of Physics.
Bhansali, S. H. ; Jarvis, J. M. ; Aksay, I. A. ; Carbeck, J. D. Enhanced resonator sensitivity with nanostructured porous silica coatings. Langmuir 2006, 22, 6676-6682.Abstract
We present a strategy to increase the sensitivity of resonators to the presence of specific molecules in the gas phase, measured by the change in resonant frequency as the partial pressure of the molecule changes. We used quartz crystals as the resonators and coated them with three different thin films (< 1 mu m thick) of porous silica: silica xerogel, silica templated by an ordered hexagonal phase of surfactant micelles, and silica templated by an isotropic L-3 phase surfactant micellar system. We compared the sensitivity of coated resonators to the presence of water vapor. The crystals coated with hexagonal phase-templated silica displayed a sensitivity enhancement up to 100-fold compared to an uncoated quartz crystal in the low-pressure regime where adsorption played a dominant role. L-3 phase-templated silica displayed the highest sensitivity (up to a 4000-fold increase) in the high partial pressure regimes where capillary condensation was the main accumulation mechanism. Three parameters differentiate the contributions of these coatings to the sensitivity of the underlying resonator: (i) specific surface area per unit mass of the coating, (ii) accessibility of the surfaces to a target molecule, and (iii) distribution in the characteristic radii of curvature of internal surfaces, as measured by capillary condensation.
Schniepp, H. C. ; Li, J. L. ; McAllister, M. J. ; Sai, H. ; Herrera-Alonso, M. ; Adamson, D. H. ; Prud'homme, R. K. ; Car, R. ; Saville, D. A. ; Aksay, I. A. Functionalized single graphene sheets derived from splitting graphite oxide. Journal of Physical Chemistry B 2006, 110, 8535-8539.Abstract
A process is described to produce single sheets of functionalized graphene through thermal exfoliation of graphite oxide. The process yields a wrinkled sheet structure resulting from reaction sites involved in oxidation and reduction processes. The topological features of single sheets, as measured by atomic force microscopy, closely match predictions of first-principles atomistic modeling. Although graphite oxide is an insulator, functionalized graphene produced by this method is electrically conducting.
Saville, D. A. ; Chun, J. ; Li, J. L. ; Schniepp, H. C. ; Car, R. ; Aksay, I. A. Orientational order of molecular assemblies on inorganic crystals. Physical Review Letters 2006, 96.Abstract
Surfactant micelles form oriented arrays on crystalline substrates although registration is unexpected since the template unit cell is small compared to the size of a rodlike micelle. Interaction energy calculations based on molecular simulations reveal that orientational energy differences on a molecular scale are too small to explain matters. With atomic force microscopy, we show that orientational ordering is a dynamic, multimolecule process. Treating the cooperative processes as a balance between van der Waals torque on a large, rodlike micellar assembly and Brownian motion shows that orientation is favored.
Li, J. L. ; Kudin, K. N. ; McAllister, M. J. ; Prud'homme, R. K. ; Aksay, I. A. ; Car, R. Oxygen-driven unzipping of graphitic materials. Physical Review Letters 2006, 96.Abstract
Optical microscope images of graphite oxide (GO) reveal the occurrence of fault lines resulting from the oxidative processes. The fault lines and cracks of GO are also responsible for their much smaller size compared with the starting graphite materials. We propose an unzipping mechanism to explain the formation of cracks on GO and cutting of carbon nanotubes in an oxidizing acid. GO unzipping is initiated by the strain generated by the cooperative alignment of epoxy groups on a carbon lattice. We employ two small GO platelets to show that through the binding of a new epoxy group or the hopping of a nearby existing epoxy group, the unzipping process can be continued during the oxidative process of graphite. The same epoxy group binding pattern is also likely to be present in an oxidized carbon nanotube and cause its breakup.
Chen, C. H. ; Saville, D. A. ; Aksay, I. A. Scaling laws for pulsed electrohydrodynamic drop formation. Applied Physics Letters 2006, 89.Abstract
A pulsed electrohydrodynamic jet can produce on-demand drops much smaller than the delivery nozzle. This letter describes an experimentally validated model for electrically pulsed jets. Viscous drag in a thin nozzle limits the flow rate and leads to intrinsic pulsations of the cone jet. A scale analysis for intrinsic cone-jet pulsations is derived to establish the operating regime for drop deployment. The scaling laws are applicable to similar electrohydrodynamic processes in miniaturized electrospraying systems. (c) 2006 American Institute of Physics.
Schniepp, H. C. ; Saville, D. A. ; Aksay, I. A. Self-healing of surfactant surface micelles on millisecond time scales. Journal of the American Chemical Society 2006, 128, 12378-12379.
Malik, A. S. ; Dabbs, D. M. ; Katz, H. E. ; Aksay, I. A. Silica monoliths templated on L-3 liquid crystal. Langmuir 2006, 22, 325-331.Abstract
Dimensionally stable, optically clear, highly porous (similar to 65% of the apparent volume), and high surface area (up to 1400 m(2)/g) silica monoliths were fabricated as thick disks (0.5 cm) by templating the isotropic liquid crystalline L-3 phase with silica through the hydrolysis and condensation of a silicon alkoxide and then removing the organic constituents by supercritical ethanol extraction. The L3 liquid crystal is a stable phase formed by the cosurfactants cetylpyridinium chloride monohydrate and hexanol in HCl(aq) solvent. Extracted 0.5 cm thick disks exhibited a low ratio of scattered to transmitted visible light (1.5 x 10(-6) at 22 from the surface normal). The degree of silica condensation in the monoliths was high, as determined by Si-29 NMR measurements of Q(3) and Q(4) peak intensities (0.53 and 0.47, respectively). As a result, the extracted and dried monoliths were mechanically robust and did not fracture when infiltrated by organic solvents. Photoactive liquid monomers were infiltrated into extracted silica monoliths and polymerized in situ, demonstrating the possible application of templated silica to optical storage technology.
Dabbs, D. M. ; Mulders, N. ; Aksay, I. A. Solvothermal removal of the organic template from L-3 ("sponge") templated silica monoliths. Journal of Nanoparticle Research 2006, 8 603-614.Abstract
We compare the methods of continuous solvent (Soxhlet) and supercritical solvent extractions for the removal of the organic template from nanostructured silica monoliths. Our monoliths are formed by templating the L-3 liquid crystal phase of cetylpyridinium chloride in aqueous solutions with tetramethoxy silane. The monoliths that result from both Soxhlet and supercritical extraction methods are mechanically robust, optically clear, and free of cracks. The Soxhlet method compares favorably with supercritical solvent extraction in that equivalent L-3-templated silica can be synthesized without the use of specialized reactor hardware or higher temperatures and high pressures, while avoiding noxious byproducts. The comparative effectiveness of various solvents in the Soxhlet process is related to the Hildebrand solubility parameter, determined by the effective surface area of the extracted silica.
Bhansali, S. H. ; Malik, A. S. ; Jarvis, J. M. ; Akartuna, I. ; Dabbs, D. M. ; Carbeck, J. D. ; Aksay, I. A. The stability of L-3 sponge phase in acidic solutions. Langmuir 2006, 22, 4060-4064.Abstract
In the synthesis of the disordered lyotropic liquid crystalline L-3 sponge phase prepared with the cosurfactants cetylpyridinium chloride and hexanol, aqueous NaCl solution is used as the solvent. When this sponge phase is used as the template for L3 silica-phase processing, we replace NaCl with HCl to facilitate the acid catalysis of tetramethoxysilane in forming a templated silica gel, assuming that changing the solvent from NaCl(aq) to HCl(aq) of equivalent ionic strength does not affect the stability range of the L3 phase. In this work, we confirm that changing the pH of the solvent from neutral to acidic (with HQ has negligible effect on the L3 phase region. Equivalent ionic strength is provided by either NaCl(aq) or HCl(aq) solvent; therefore, a similar phase behavior is observed regardless of which aqueous solvent is used.
2005
Dabbs, D. M. ; Ramachandran, U. ; Lu, S. ; Liu, J. ; Wang, L. Q. ; Aksay, I. A. Inhibition of aluminum oxyhydroxide precipitation with citric acid. Langmuir 2005, 21, 11690-11695.Abstract
Citric acid has been shown to act as an agent for increasing the solubility of aluminum oxyhydroxides in aqueous solutions of high (>2.47 mol/mol) hydroxide-to-aluminum ratios. Conversely, citric acid also colloidally stabilizes particles in aqueous suspensions of aluminum-containing particles. Solutions of aluminum chloride, with and without citric acid added, were titrated with NaOH(aq), The presence and size of particles were determined using quasi-elastic light scattering. In solutions that contained no citric acid, particles formed instantaneously when NaOH(aq) was added but these were observed to rapidly diminish in size, disappearing at OH/Al ratios below 2.5 mol/mol. When the OH/Al ratio was raised beyond 2.5 by adding more NaOH(aq), suspensions of colloidally stable particles formed. Large polycations containing 13 aluminum atoms were detected by Al-27 solution NMR in citric-acid-free solutions with OH/Al ratios slightly lower than 2.5. In comparison, adding citric acid to solutions of aluminum chloride inhibited the formation of large aluminum-containing polycations. The absence of the polycations prevents or retards the subsequent formation of particles, indicating that the polycations, when present, act as seeds to the formation of new particles. Particles did not form in solutions with a citric acid/aluminum ratio of 0.8 until sufficient NaOH(aq) was added to raise the OH/Al ratio to 3.29. By comparison, lower amounts of citric acid did not prevent particles from forming but did retard the rate of growth.
Aksay, I. A. ; Wahl, C. M. ; Dabbs, D. M. ; Yilgor, I. L3-silica/polyurethane thermally insulating nanocomposite, 2005.Abstract
The present invention provides thermal insulator composites based upon nanostructured L3-silica microparticles and polyurethane foam chem. that are both easy to process and have superior insulating properties for use in household and com. refrigeration, construction, and shipping applications. The composite material retains many of the attractive processing characteristics of polyurethane foams such as vol. expansion and shape-filling during polymn. and demonstrates a total thermal cond. between 32% and 44% that of com. available polyurethane foams. [on SciFinder(R)]
Martin, C. R. ; Aksay, I. A. Low-cost Patterning of Ceramic Thin Films. In Electroceramic-based MEMS, Fabrication Technology and Applications; Setter, N., Ed. Springer Science+Business Media, Inc. New York, 2005; pp. 387-410.Abstract
The patterning of ceramic thin films is of great interest for use in MEMS and other applications as discussed in the chapters by Maeda et al. and by Muralt and Baborowski. However, the complex chemistries of certain materials make the use of traditional photolithography techniques prohibitive. In this chapter, a number of low-cost, high throughput techniques for the patterrning of ceramic thin films derived from chemical solution precursors, such as sol-gels and ceramic slurries, are presented. Most of these methods are derived from soft lithographic methods using elastomer molds, a method that is categorized as the next generation lithography in the chapter by Alexe et al. Two categories of techniques are discussed: first, the focus is on methods that rely on the principles of confinement within the physical features of the mold to define the pattern on the substracte surface. Then, subtractive patterning techniques that rely on transferring a pattern to a spin-cast large-area continuous thin film are described. Most techniques have been demonstrated with fidelities on the order of 100 nm; however, their inability to precisely register and align the patterns as part of a hierarchical fabrication scheme has hindered their commercial implementation thus far. This chaper has been updated from the original manuscript to reflect the most recent available literature and complements the chapter by Baborowshi on pattern formation by micromachining techniques.
Martin, C. R. ; Aksay, I. A. Microchannel molding: A soft lithography-inspired approach to micrometer-scale patterning. Journal of Materials Research 2005, 20, 1995-2003.Abstract
A new patterning technique for the deposition of sol-gels and chemical solution precursors was developed to address some of the limitations of soft lithography approaches. When using micromolding in capillaries to pattern precursors that exhibit large amounts of shrinkage during drying, topographical distortions develop. In place of patterning the elastomeric mold, the network of capillary channels was patterned directly into the substrate surface and an elastomer membrane is used to complete the channels. When the wetting properties of the substrate surfaces were carefully controlled using self-assembled monolayers (SAMs), lead zirconate titanate thin films with nearly rectangular cross-sections were successfully patterned. This technique, called microchannel molding (mu CM), also provided a method for aligning multiple layers such as bottom electrodes for device fabrication.
Li, J. L. ; Chun, J. ; Wingreen, N. S. ; Car, R. ; Aksay, I. A. ; Saville, D. A. Use of dielectric functions in the theory of dispersion forces. Physical Review B 2005, 71.Abstract
The modern theory of dispersion forces uses macroscopic dielectric functions epsilon(omega) as a central ingredient. We reexamined the formalism and found that at separation distance < 2 nm the full dielectric function epsilon(omega,k) is needed. The use of epsilon(omega,k) results in as much as 30% reduction of the calculated Hamaker constants reported in the current literature. At larger distances, the theory reduces to the traditional method, which uses dielectric functions in the long-wavelength limit. We illustrate the formalism using the example of interaction between two graphite slabs. This example is of importance for intercalation and exfoliation of graphite and for the use of exfoliated graphite in composite materials. The formalism can also be extended to study anisotropic van der Waals interactions.
2004
Ristenpart, W. D. ; Aksay, I. A. ; Saville, D. A. Assembly of colloidal aggregates by electrohydrodynamic flow: Kinetic experiments and scaling analysis. Physical Review E 2004, 69.Abstract
Electric fields generate transverse flows near electrodes that sweep colloidal particles into densely packed assemblies. We interpret this behavior in terms of electrohydrodynamic motion stemming from distortions of the field by the particles that alter the body force distribution in the electrode charge polarization layer. A scaling analysis shows how the action of the applied electric field generates fluid motion that carries particles toward one another. The resulting fluid velocity is proportional to the square of the applied field and decreases inversely with frequency. Experimental measurements of the particle aggregation rate accord with the electrohydrodynamic theory over a wide range of voltages and frequencies.
Abdala, A. A. ; Milius, D. L. ; Adamson, D. H. ; Aksay, I. A. ; Prudhomme, R. K. Inspired by abalone shell: Strengthening of porous ceramics with polymers. Polymeric Materials: Science & Engineering 2004, 90, 384-385.
Adamson, D. H. ; Dabbs, D. M. ; Aksay, I. A. ; Morse, D. E. Non-peptide, silicatein alpha inspired silica condensation catalyst. Polymeric Materials: Science & Engineering 2004, 90, 239-240.
Martin, C. R. ; Aksay, I. A. Submicrometer-scale patterning of ceramic thin films. Journal of Electroceramics 2004, 12, 53-68.Abstract
The patterning of ceramic thin films is of great interest for use in MEMS and other applications. However, the complex chemistries of certain materials make the use of traditional photolithography techniques prohibitive. In this paper, a number of low-cost, high throughput techniques for the patterning of ceramic thin films derived from chemical solution precursors, such as sol-gels and ceramic slurries, are presented. A particular emphasis is placed on methods that are derived from soft lithographic methods using elastomer molds. Two categories of techniques are discussed: first, the focus is on methods that rely on the principles of confinement within the physical features of the mold to define the pattern on the substrate surface. Then, subtractive patterning techniques that rely on transferring a pattern to a spin-cast, large-area continuous thin film are described. While most techniques have been demonstrated with fidelities on the order of 100 nm, their inability to precisely register and align the patterns as part of a hierarchical fabrication scheme have thus far hindered their commercial implementation.
2003
Ristenpart, W. D. ; Aksay, I. A. ; Saville, D. A. Electrically guided assembly of planar superlattices in binary colloidal suspensions. Physical Review Letters 2003, 90.Abstract
Binary colloidal suspensions are assembled into planar superlattices using ac electric fields. Either
triangular or square-packed arrays form, depending on the frequency and relative particle concentrations.
The frequency dependence is striking since superlattices develop at low and high frequencies
but not at intermediate frequencies. We explain the low frequency behavior <3 kHz in terms of
induced-dipole repulsion balanced by attraction resulting from electrohydrodynamic (EHD) flow. At
high frequencies (20–200 kHz), EHD flow is negligible but aggregation occurs since dipole-dipole
interactions become attractive.
Materials Research to Meet 21st Century Defense Needs; National Academies Press: Washington, DC, 2003; pp. 330. Publisher's VersionAbstract
In order to achieve the revolutionary new defense capabilities offered by materials science and engineering, innovative management to reduce the risks associated with translating research results will be needed along with the R&D. While payoff is expected to be high from the promising areas of materials research, many of the benefits are likely to be evolutionary. Nevertheless, failure to invest in more speculative areas of research could lead to undesired technological surprises. Basic research in physics, chemistry, biology, and materials science will provide the seeds for potentially revolutionary technologies later in the 21st century.
Vartuli, J. S. ; Ozenbas, M. ; Chun, C. M. ; Trau, M. ; Aksay, I. A. Micropatterned lead zirconium titanate thin films. Journal of Materials Research 2003, 18, 1259-1265.Abstract
Micropatterning of Pb(Zr0.52Ti0.48)O3 (PZT) thin films with line features as small as
350 nm was demonstrated through capillary molding of organometallic solutions within
the continuous channels of an elastomeric mold. Despite the large stresses that develop
during the evaporation of the solvent, pyrolysis of the organics, and the densification
and crystallization of the inorganic gel, the patterned crystalline PZT films were
crack-free and mechanically robust. Flawless regions as large as 1 cm2 were obtained.
The cross-sectional shape of the patterned PZT lines was trapezoidlike. Single
perovskite PZT grains that formed during annealing at 600–700 °C completely filled
the cross-sectional area of the patterned lines. Lead acetate, zirconium propoxide, and
titanium isopropoxide were used as the starting materials. Substrates used included
silver tape, stainless steel plate, silicon wafer, and platinum-coated silicon wafer.
Gleason, N. J. ; Nodes, C. J. ; Higham, E. M. ; Guckert, N. ; Aksay, I. A. ; Schwarzbauer, J. E. ; Carbeck, J. D. Patterning proteins and cells using two-dimensional arrays of colloids. Langmuir 2003, 19, 513-518.Abstract
Asimple method is described for controlling the organization of proteins on surfaces using two-dimensional arrays of micron-sized colloidal particles. Suspensions of colloids functionalized with proteins are deposited onto coverslips coated with gold using a combination of gravitational settling and applied electrical fields. Varying settling time and particle concentration controls the density of particles on the substrate. Surface coverage ranged from an essentially continuous coating of protein on close-packed arrays to domains of protein separated by distances as large as 16 ím. Colloidal particle arrays were also patterned into 500 µm islands on substrates using elastomeric lift-off membranes. The applicability of this approach to the promotion of fibroblast cell adhesion and spreading was demonstrated using particles coated with the cell adhesion protein fibronectin. Behavior of adherent cells varied with particle density. This method provides a general strategy for controlling the organization of functional proteins at surfaces on three length scales: the size of individual colloidal particles, the spacing between particles, and the organization of particles in patterned arrays.
Martin, C. R. ; Aksay, I. A. Topographical evolution of lead zirconate titanate (PZT) thin films patterned by micromolding in capillaries. Journal of Physical Chemistry B 2003, 107, 4261-4268.Abstract
The patterning of sol-gel-derived thin films by micromolding in capillaries can produce unintended topographical deviations from the shape of the original mold that may limit the utility of the technique in potential applications. During drying and heat treatment, nonuniform shrinkage across the film due to the densification of the gel matrix results in “double-peak” film topographies whereby the film thickness is greater at the lateral edges than in the middle. Using the same framework used to understand the imbibition and wetting of the sol-gel in the capillary channels, we developed a mechanism to explain the formation of the double-peak profile. As a model system, patterned Pb(Zr0.52Ti0.48)O3 thin films were studied. Atomic force microscopic characterization was used to quantify the effect of the rate of gelation on the topography of the patterned thin films. Modifications to the channel mold design eliminate the peak formation, producing more homogeneous patterns that better replicate the features of the mold.
2002
Li, X. P. ; Shih, W. Y. ; Vartuli, J. ; Milius, D. L. ; Prud'homme, R. ; Aksay, I. A. ; Shih, W. H. Detection of water-ice transition using a lead zirconate titanate/brass transducer. Journal of Applied Physics 2002, 92, 106-111.Abstract
We have examined experimentally and theoretically the resonance frequency of a lead zirconate titanate ~PZT!/brass unimorph disk transducer with a water ~ice! layer on the brass surface. We showed that the flexural resonance frequency decreased with the presence of a water layer and the decrease in resonance frequency increased with an increasing water amount. Upon lowering the temperature, the freezing transition of the deposited water layer was detected when the resonance frequency of the transducer increased abruptly at the freezing temperature. In contrast to water, an ice layer increased the resonance frequency and the increase in the resonance frequency increased with the ice layer thickness. Theoretically, an analytic expression for the flexural resonance frequency of a unimorph transducer in the presence of an ice ~water! layer on the brass surface was obtained in terms of the Young’s moduli, densities, and thickness of the PZT, brass, and ice ~water layers. The theoretical predictions were shown to agree with the experimental results.
Vican, J. ; Gajdeczko, B. F. ; Dryer, F. L. ; Milius, D. L. ; Aksay, I. A. ; Yetter, R. A. Development of a microreactor as a thermal source for microelectromechanical systems power generation. Proceedings of the Combustion Institute 2002, 29, 909-916.Abstract
An alumina ceramic 12.5x12.5x5.0 mm microreactor was constructed using a modified stereolithography process. The design was based on a ‘‘Swiss roll’’ concept of double spiral-shaped channels to facilitate a high level of heat transfer between the reactants and combustion products and wall surface contact of the flow through the microreactor body. Self-sustained combustion of hydrogen and air mixtures was demonstrated over a wide range of fuel/air mixtures and flow rates for equivalence ratios from 0.2 to 1.0 and chemical energy inputs from 2 to 16 W. Depositing platinum on gamma alumina on the internal walls enabled catalytic ignition at or near room temperature and self-sustained operation at temperatures to 300 C. Catalyst degradation was observed at higher operating temperatures and reignition capabilities were lost. However, sustained operation could be obtained at wall temperatures in excess of 300 C, apparently stabilized by a combination of surface and gas-phase reaction phenomena. A global energy balance model was developed to analyze overall reactor performance characteristics. The reactor design and operating temperature range have potential applications as a heat source for thermoelectric and pyroelectric power generation at small scales compatible with microelectromechanical systems applications.
Li, X. P. ; Shih, W. Y. ; Vartuli, J. S. ; Milius, D. L. ; Aksay, I. A. ; Shih, W. H. Effect of a transverse tensile stress on the electric-field-induced domain reorientation in soft PZT: In situ XRD study. Journal of the American Ceramic Society 2002, 85, 844-850.Abstract
The effect of a transverse tensile stress on the electric-fieldinduced 90°-domain reorientation in tetragonal lead zirconate titanate (PZT) near the morphotropic phase boundary was investigated in situ using X-ray diffraction (XRD). The XRD intensity ratio, I(002)/I(200), which represents the ratio of the volume of the c-domains to that of the a-domains on the PZT surface, was examined as a function of the electric field at various stress levels. It was found that a transverse tensile stress changes the electric-field dependence of I(002)/I(200), especially at higher electric fields. Without a transverse tensile stress, I(002)/I(200) began to saturate at E 800 kV/m. With a transverse tensile stress of 75 MPa, I(002)/I(200) increased with an upward curvature with the electric field, indicating that the transverse tensile stress enhanced the field-induced 90°- domain reorientation, and increased the effective piezoelectric coefficients at larger electric fields. At E 900 kV/m, the estimated d31,domain changed from 200 1012 V/m at zero stress, to 350 1012 V/m at 75 MPa.
Brown, C. L. ; Aksay, I. A. ; Saville, D. A. ; Hecht, M. H. Template-directed assembly of a de novo designed protein. Journal of the American Chemical Society 2002, 124, 6846-6848.Abstract
A number of biological materials owe their unusual structural characteristics and mechanical properties to long-range order induced by the lamination of â-sheet proteins between layers of inorganic mineral.1 In such composites, both the protein layer and the mineral layer adopt structures different from those they assume in isolation. Interactions between such layers and the ordered structures that result from these interactions enable nature to produce biomaterials that are simultaneously hard, strong, and tough.
2001
Lee, T. ; Yao, N. ; Imai, H. ; Aksay, I. A. Barium titanate nanoparticles in block copolymer. Langmuir 2001, 17, 7656-7663.Abstract
CubicBaTiO3 particles of10nmwere crystallized predominantly within the hydroxylated polybutadiene matrix of a phase separated triblock copolymeric thin film of polystyrene-polybutadiene-polystyrene (Kraton D1102). The barium titanated Kraton thin film had remnants of the cylindrical morphology of a plain Kraton thin film with an interdomain spacing of 23 nm. The procedure of barium titanation consisted of three steps: (1) in situ hydroxylation of the polybutadiene matrix of an annealed Kraton thin film, (2) regioselective deposition of barium titanium methoxypropanoxide (BaTi(OCH2CH(CH2)OCH3)6) on the hydroxylated polybutadiene matrix, and (3) hydrothermal reduction of the organometallic complexes in anNH3/H2Oatmosphere at 80 °C for 24 h. Isolated water clusters in step 3, condensed from theNH3/H2O atmosphere of 1 M NH4OH(aq) at 80 °C in the alkoxide-Kraton films, were believed to have a typical diameter of no more than 23 nm at a pH of 14. They gave a high pH environment to weaken the chelating effectamongalkoxides and the organic matrix and provided a spatial confinement for the localized nucleation and growth of cubic BaTiO3 nanoparticles.
Xu, G. F. ; Aksay, I. A. ; Groves, J. T. Continuous crystalline carbonate apatite thin films. A biomimetic approach. Journal of the American Chemical Society 2001, 123, 2196-2203.Abstract
In contrast to extensive studies on hydroxyapatite thin films, very little has been reported on the thin films of carbonated apatite (dahllite). In this report, we describe the synthesis and characterization of a highly crystalline dahllite thin film assembled via a biomimetic pathway. A free-standing continuous precursor film of carbonated calcium phosphate in an amorphous phase was first prepared by a solution-inhibited templating method (template-inhibition) at an air-water interface. A stearic acid surface monolayer acted as the template, while a carbonate-phosphate solution composed a binary inhibition system. The precursor film formed at the air/water interface was heated at 900 °C and transformed into a dense crystalline film that retained the overall shape of the precursor. The crystalline phase was characterized by XRD and IR to be a single-phase carbonate apatite, with carbonate substitutions in both type A (OH-) and type B (PO4 3-) lattice positions.
Lee, J. H. ; Prud'homme, R. K. ; Aksay, I. A. Cure depth in photopolymerization: Experiments and theory. Journal of Materials Research 2001, 16, 3536-3544.Abstract
The depth of photocuring for a model resin system was investigated as a function of photoinitiator concentration. Direct measurements of gel thickness were made from thin films of cross-linked multifunctional methacrylate monomer. The monomer, 2,2-bis{4-[2-hydroxy-3-(methacryloxy)propoxy]phenyl}propane, was polymerized in a solution of trichloroethylene with an ultraviolet laser light source at 325 nm. The monomer solutions were photocured using varying levels of both photonic energy and photoinitiator concentration. An optimal photoinitiator concentration that maximized the gel cure depth was observed. Additionally, two regimes were shown to exist in which the shrinkage (upon solvent removal) was minimized or maximized. A model was developed to probe the physics of the system. Good agreement with experiment was obtained, and the model may be employed to predict both the existence and location of the optimal photoinitiator concentration and the corresponding cure depth. The study showed that photoinitiator plays a significant role in controlling the quality and performance of the formed gel network, with special regard to thickness of cured layers. This has potential application to fields as diverse as industrially cured coatings and dental fillings, and more generally, 3-dimensional rapid prototyping techniques.
Wang, R. Z. ; Suo, Z. ; Evans, A. G. ; Yao, N. ; Aksay, I. A. Deformation mechanisms in nacre. Journal of Materials Research 2001, 16, 2485-2493.Abstract
Nacre (mother-of-pearl) from mollusc shells is a biologically formed lamellar ceramic. The inelastic deformation of this material has been experimentally examined, with a focus on understanding the underlying mechanisms. Slip along the lamellae tablet interface has been ascertained by testing in compression with the boundaries oriented at 45° to the loading axis. The steady-state shear resistance tss has been determined and inelastic strain shown to be as high as 8%. The inelastic deformation was realized by massive interlamellae shearing. Testing in tension parallel to the tablets indicates inelastic strain of about 1%, occurring at a steady-state stress, sss » 110 MPa. The strain was associated with the formation of multiple dilatation bands at the intertablet boundaries accompanied by interlamellae sliding. Nano-asperities on the aragonite tablets and their interposing topology provide the resistance to interfacial sliding and establish the level of the stress needed to attain the inelastic strain. Detailed mechanisms and their significance for the design of robust ceramics are discussed.
Malik, A. - S. ; Dabbs, D. M. ; Aksay, I. A. ; Katz, H. E. Disordered Mesoporous Silicates formed by Templation of a Liquid Crystal (L3). Mater. Res. Soc. Symp. Proc. 2001, 658, 7.5.1-5.Abstract
For a wide range of technological applications the need for optically transparent, monolithic, mesoporous silicates is readily apparent. Potential areas of utility include filtration,catalysis, and optoelectronics among many others. This laboratory has previously reported on the synthesis of such materials that are formed through the addition of tetramethoxysilane to a liquid crystal solution of hexanol, cetylpyridinium chloride, and 0.2 M hydrochloric acid, and our investigation into the properties of these materials is a continuing process. We have achieved defect and fracture free material of suitable size (0.5 cm x 3 cm diameter disks) via supercritical drying of the silicate under ethanol or CO2. The dried materials are remarkably similar to ordinary glass in strength, texture, and clarity. They possess pore volumes of ca. 1.0 cm3/g, with BET surface areas >1000 m2/g. We can re-infiltrate the dried monolith with hydroxyethylacrylate, a photo-polymerizable monomer, to create an inorganic/organic nanocomposite. There is fracturing upon re-infiltration, but preliminary tests show that the polymerization proceeds despite the mechanical failure. These findings suggest many possible applications for these unique nanocomposites.
Hayward, R. C. ; Poon, H. F. ; Xiao, Y. ; Saville, D. ; Aksay, I. Electrohydrodynamically patterned colloidal crystals, 2001.Abstract
A method for assembling patterned cryst. arrays of colloidal particles using UV illumination of an optically-sensitive semiconducting anode while using the anode to apply an electronic field to the colloidal particles. The UV illumination increases c.d., and consequently, the flow of the colloidal particles. As a result, colloidal particles can be caused to migrate from nonilluminated areas of the anode to illuminated areas of the anode. Selective illumination of the anode can also be used to permanently affix colloidal crystals to illuminated areas of the anode while not affixing them to nonilluminated areas of the anode. [on SciFinder(R)]
Li, X. P. ; Vartuli, J. S. ; Milius, D. L. ; Aksay, I. A. ; Shih, W. Y. ; Shih, W. H. Electromechanical properties of a ceramic d(31)-gradient flextensional actuator. Journal of the American Ceramic Society 2001, 84, 996-1003.Abstract
We examined the static axial displacement of a ceramic d31-gradient flextensional transducer both experimentally and theoretically. Two lead zirconate titanate systems, (PZT)/PZT and PZT/ZnO, were studied. The PZT/PZT transducers consisted of two PZT layers of different d31 coefficients. The PZT/ZnO transducers consisted of a PZT and a ZnO layer. The PZT/PZT transducers were of an inner-type dome structure. The PZT/ZnO transducers were either flat, or had an inner- or outer-type dome structure by varying the thickness ratio between the two layers or the Sb2O3 content in the ZnO layer. An inner (outer)-type transducer has the large-d31 layer on the inside (outside) of the dome structure. For the PZT/PZT transducers, the axial displacement varied with the thickness ratio and reached a maximum when the two layers had similar thickness, in agreement with the calculations. With a conductive nonpiezoelectric layer, the PZT/ZnO transducers had higher axial displacements, which varied with the thickness ratio and the Sb2O3 content, than the PZT/PZT transducers. With 6 wt% Sb2O3, the transducers were flat and the measured displacements at various thickness ratios were similar to the calculated values. With 4 wt% Sb2O3, the transducers were of an outer type. The measured axial displacements were about twice the calculated values, suggesting an enhanced d31 value because of the tensile bending stress in the PZT layer. The scaled axial displacements of the PZT/ZnO transducers with 4 wt% Sb2O3 were comparable to that of the Rainbow transducers. With 8 wt% Sb2O3, the displacements of transducers with thin PZT layers (<0.3 mm) were lower than the calculated values because of increased conductivity in the PZT layer.
Lee, T. ; Aksay, I. A. Hierarchical structure-ferroelectricity relationships of barium titanate particles. Crystal Growth & Design 2001, 1 401-419.Abstract
The aim of this paper is to dissect the hierarchical structures of BaTiO3 particles into primary, secondary, tertiary, and quaternary structural levels and to thoroughly review the corresponding structureferroelectricity relationships at all levels. This analytical framework is of growing importance for the fundamental understanding of the ferroelectric properties of BaTiO3 in microelectronic devices, especially as their structural levels are all approaching the same scale in the process of miniaturization. The identification of the most influential structure among others and the deduction of a meaningful structure-ferroelectricity relationship depend on a thorough understanding of the relationships.
Evans, A. G. ; Suo, Z. ; Wang, R. Z. ; Aksay, I. A. ; He, M. Y. ; Hutchinson, J. W. Model for the robust mechanical behavior of nacre. Journal of Materials Research 2001, 16, 2475-2484.Abstract
The inelastic deformation of nacre that leads to its structural robustness has been characterized in a recent experimental study. This article develops a model for the inelastic behavior, measured in tension, along the axis of the aragonite plates. The model is based on observations for abalone nacre that the inelasticity is associated with periodic dilatation bands. These bands contain coordinated separations at the periphery of the plates. The separations open as the material strains. The response is attributed to nanoscale asperities on the surfaces of the plates. The model calculates the stresses needed to displace the plates, resisted by elastic contacts at the asperities. The results are compared with the measured stress/strain curves.
Vartuli, J. S. ; Milius, D. L. ; Li, X. ; Shih, W. H. ; Shih, W. Y. ; Prud'homme, R. K. ; Aksay, I. A. Multilayer ceramic piezoelectric laminates with zinc oxide conductors, 2001.Abstract
A modification of the traditional unimorph flextensional actuator is provided by replacing the metal shim with an elec. conducting oxide. Comprised of Pb zirconate titanate (PZT) and ZnO that is co-sintered, the laminate composite obtains large axial displacements while maintaining moderate axial loads. The varistor properties of ZnO dictate that the conductance increases several orders of magnitude when a crit. elec. field is applied. The versatility of the processing over other actuator system facilitates miniaturization, while maintaining comparable performance characteristics. Functional gradients in the material properties are created in the green body by layering thin tape cast sheets. The unique PZT-ZnO composite not only controls the piezoelec. gradient, but permits control of the sintering kinetics leading to the processing of either flat or highly domed structures. [on SciFinder(R)]
Aksay, I. A. ; Ker, H. L. Plastically deformable aqueous ceramic slurries and methods for their manufacture, 2001.Abstract
The ceramic particles from plastically deformable aq. ceramic slurrys have on their surface a closely-packed anionic surfactant bilayer or an anionic/nonionic surfactant bilayer. Optionally, ceramic particles have on their surface a closely-packed cationic surfactant bilayer or a cationic/nonionic surfactant bilayer. Slurries prepg. includes (a) dispersing ceramic particles in an amt. of water to form an aq. ceramic slurry, (b) adding an anionic surfactant or a mixt. of an anionic and nonionic surfactant, and (c) adjusting the pH value to adsorb on the ceramic particle surface a closely-packed anionic surfactant bilayer or anionic/nonionic surfactant bilayer. [on SciFinder(R)]
Wang, R. Z. ; Suo, Z. ; Evans, A. G. ; Yao, N. ; Aksay, I. A. Scanning Electron Microscopic Analysis on the Deformation Mechanisms in Nacre. Proc. Microscopy and Microanalysis, 2001, 1118-19.
Shih, W. Y. ; Li, X. P. ; Gu, H. M. ; Shih, W. H. ; Aksay, I. A. Simultaneous liquid viscosity and density determination with piezoelectric unimorph cantilevers. Journal of Applied Physics 2001, 89, 1497-1505.Abstract
We have examined both experimentally and theoretically a piezoelectric unimorph cantilever as a liquid viscosity-and-density sensor. The fabricated piezoelectric unimorph consisted of a PbO•ZrO2•TiO2 ~PZT! layer on a thin stainless-steel plate. In addition to a driving electrode, a sensing electrode was placed on top of the PZT layer, permitting the direct measurement of the resonance frequency. The cantilever was tested using water–glycerol solutions of different compositions. In all three of the tested modes, the resonance frequency decreased while the width of the resonance peak increased with increasing glycerol content. To account for the liquid effect, we consider the cantilever as a sphere of radius R oscillating in a liquid. By including the high and low frequency terms in the induced mass and the damping coefficient of the liquid, we show that for a given liquid density and viscosity the oscillating-sphere model predicts a resonance frequency and peak width that closely agree with experiment. Furthermore, the viscosity and the density of a liquid have been determined simultaneously using the experimentally measured resonance frequency and peak width as inputs to the oscillating-sphere model. The calculated liquid viscosity and density closely agreed with the known values, indicating that our cantilever-based sensor is effective in determining viscosity and density, simultaneously. We also show that scaling analysis predicts an increase in the width of the resonance peak with decreasing cantilever size, an observation in agreement with the large peak widths observed for microcantilevers.
Yao, N. ; Ku, A. Y. ; Saville, D. A. ; Aksay, I. A. ; Musselman, I. H. TEM Studies of Disorder-Order Transition in the Acid-Catalyzed Growth of Mesoscopic Silica Thin Films. Proc. Microscopy and Microanalysis, 2001, 1226-27.
2000
Yao, N. ; Ku, A. Y. ; Nakagawa, N. ; Lee, T. ; Saville, D. A. ; Aksay, I. A. Disorder-order transition in mesoscopic silica thin films. Chemistry of Materials 2000, 12, 1536-1548.
Hayward, R. C. ; Saville, D. A. ; Aksay, I. A. Electrophoretic assembly of colloidal crystals with optically tunable micropatterns. Nature 2000, 404, 56-59.
Blohowiak, K. Y. ; Garrigus, D. F. ; Luhman, T. S. ; McCrary, K. E. ; Strasik, M. ; Aksay, I. ; Dogan, F. ; Hicks, W. C. ; Martin, C. B. Making large, single crystal, 123 YBCO superconductors, 2000.Abstract
Large (in excess of 2 cm in diam.), single crystal YBa2Cu3O7-x [123 YBCO] crystals, where x ≤ 0.6, can be grown in a seventeen step process or some variant thereof from finely ground and well mixed 123 YBCO and 211 YBCO powders with a small amt. of Pt by controlling the rate of cooling from within a compact of the powders using a temp. gradient in the radial and axial planes (independently) of ∼1-20°/in. diam. of compact to nucleate the crystal growth. Crystal growth is also promoted as well using a Sm oxide seed crystal, preferably SmBa2Cu3O7-y, where y ≤ 1.6. After nucleation the compact is cooled slowly at a rate from ∼0.1-1°/h to promote the single crystal development. [on SciFinder(R)]
Lee, J. H. ; Prud'homme, R. K. ; Aksay, I. A. Processing of organic/inorganic composites by stereolithography. In Solid Freeform and Additive Fabrication-2000; Danforth, S. C. ; Dimos, D. ; Prinz, F. B., Ed. 2000; Vol. 625, pp. 165-172.
Sidis, Y. ; Bourges, P. ; Fong, H. F. ; Keimer, B. ; Regnault, L. P. ; Bossy, J. ; Ivanov, A. ; Hennion, B. ; Gautier-Picard, P. ; Collin, G. ; et al. Quantum impurities and the neutron resonance peak in YBa2Cu3O7: Ni versus Zn. Physical Review Letters 2000, 84, 5900-5903.
Dabbs, D. M. ; Aksay, I. A. Self-assembled ceramics produced by complex-fluid templation. Annual Review of Physical Chemistry 2000, 51, 601-+.
McGrath, K. M. ; Dabbs, D. M. ; Yao, N. ; Edler, K. J. ; Aksay, I. A. ; Gruner, S. M. Silica gels with tunable nanopores through templating of the L-3 phase. Langmuir 2000, 16, 398-406.
Hsu, M. F. ; Cates, G. D. ; Kominis, I. ; Aksay, I. A. ; Dabbs, D. M. Sol-gel coated glass cells for spin-exchange polarized He-3. Applied Physics Letters 2000, 77, 2069-2071.
Fong, H. F. ; Bourges, P. ; Sidis, Y. ; Regnault, L. P. ; Bossy, J. ; Ivanov, A. ; Milius, D. L. ; Aksay, I. A. ; Keimer, B. Spin susceptibility in underdoped YBa2Cu3O6+x. Physical Review B 2000, 61, 14773-14786.
Liu, X. H. ; Chun, C. M. ; Aksay, I. A. ; Shih, W. H. Synthesis of mesostructured nickel oxide with silica. Industrial & Engineering Chemistry Research 2000, 39, 684-692.
1999
Aksay, I. A. ; Trau, M. ; Manne, S. ; Honma, I. Biomimetic pathways for assembling inorganic thin films and oriented mesoscopic silicate patterns through guided growth, 1999.Abstract
A process directed to prepg. surfactant-polycryst. inorg. nanostructured materials having designed microscopic patterns. The process includes forming a polycryst. inorg. substrate having a flat surface and placing in contact with the flat surface of the substrate a surface having a predetd. microscopic pattern. An acidified aq. reacting soln. is then placed in contact with an edge of the surface having the predetd. microscopic pattern. The soln. wicks into the microscopic pattern by capillary action. The reacting soln. has an effective amt. of a silica source and an effective amt. of a surfactant to produce a mesoscopic silica film upon contact of the reacting soln. with the flat surface of the polycryst. inorg. substrate and absorption of the surfactant into the surface. Subsequently an elec. field is applied tangentially directed to the surface within the microscopic pattern. The elec. field is sufficient to cause electro-osmotic fluid motion and enhanced rates of fossilization by localized Joule heating. [on SciFinder(R)]
Chang, H. L. ; Chun, C. M. ; Aksay, I. A. ; Shih, W. H. Conversion of fly ash into mesoporous aluminosilicate. Industrial & Engineering Chemistry Research 1999, 38, 973-977.Abstract
Mesoporous aluminosilicate in the hexagonal phase (MCM-41) has been synthesized from fused fly ash solutions and cationic cetyltrimethylammonium bromide (CTAB) surfactants. We provide direct evidence that an MCM-41 aluminosilicate with a homogeneous chemical composition of Si/Al = 13.4 can be prepared with cationic surfactant. Our results indicate that coal combustion byproducts can be utilized for producing mesoporous molecular sieves even though they contain significant amounts of impurities.
Fong, H. F. ; Bourges, P. ; Sidis, Y. ; Regnault, L. P. ; Bossy, J. ; Ivanov, A. ; Milius, D. L. ; Aksay, I. A. ; Keimer, B. Effect of nonmagnetic impurities on the magnetic resonance peak in YBa2Cu3O7. Physical Review Letters 1999, 82, 1939-1942.Abstract
The magnetic excitation spectrum of a YBa2Cu3O7 crystal containing 0.5% of nonmagnetic (Zn) impurities has been determined by inelastic neutron scattering. Whereas in the pure system a sharp resonance peak at E similar or equal to 40 meV is observed exclusively below the superconducting transition temperature T-c, the magnetic response in the Zn-substituted system is broadened significantly and vanishes at a temperature much higher than T-c. The energy-integrated spectral weight observed near q = (pi, pi) increases with Zn substitution, and only about half of the spectral weight is removed at T-c.
Shih, W. Y. ; Shih, W. H. ; Aksay, I. A. Elastic and yield behavior of strongly flocculated colloids. Journal of the American Ceramic Society 1999, 82, 616-624.Abstract
We have investigated theoretically the elastic and yield behaviors of strongly flocculated colloids by first examining the yield forces between two particles within the framework of Derjaguin-Landau-Verwey-Overbeck (DLVO) interactions. Under highly attractive conditions, i,e,, in the absence of the secondary minimum in the DLVO potential, the radial (tensile) motion between particles is nonelastic because of the lack of an inflection point in the DLVO potential. However, the lateral (shear) motion is shown to be elastic up to a distance y(max), providing a mechanism for the observed elasticity in colloidal gels. If r(0) and s(0) are, respectively, the closest center-to-center and surface-to-surface distances between two particles, y(max) proportional to (1- 0.5 alpha zeta(2))(s(0)r(0))(1/2) where zeta is the zeta potential of the particles and a a defined constant. Moreover, the yield force between two particles is much smaller in the lateral direction than in the radial direction. These results suggest that yielding of a particulate network is likely to occur through the lateral movements between particles. The yield strain can be approximated as that at which all the bonds in a certain direction have a perpendicular displacement >y(max), resulting in epsilon(yield) = y(max)/r(0) proportional to (1 - 0.5 alpha zeta(2))(s(0)/r(0))(1/2) The shear modulus of the network, G', can be deduced by combining the elastic constant of the lateral movement with the existing elastic theory of a particulate network. The yield stress can be approximated as sigma(yield) approximate to G'epsilon(yield) proportional to (1 - 1.5 alpha zeta(2))A/24s(0)(3/2) 1/Rd-3/2 where A is the Hamaker constant and R the particle radius. These predictions are shown to compare favorably with existing experiments.
Torquato, S. ; Yeong, C. L. Y. ; Rintoul, M. D. ; Milius, D. L. ; Aksay, I. A. Elastic properties and structure of interpenetrating boron carbide aluminum multiphase composites. Journal of the American Ceramic Society 1999, 82, 1263-1268.Abstract
We study the elastic moduli and structure of boron carbide/aluminum (B4C/Al) multiphase composites using rigorous bounding and experimental characterization techniques. We demonstrate that rigorous bounds on the effective moduli are useful in that they can accurately predict (i) the effective elastic moduli, given the phase moduli and volume fractions, or (ii) the phase moduli (volume fractions), given the effective moduli and phase volume fractions (moduli). Using the best available rigorous bounds on the effective elastic moduli of multiphase composites involving volume-fraction information, we are able to predict the bulk and shear moduli of the AI,BC phase, a reaction product that forms during heat treatment. These theoretical predictions are in very good agreement with recent experimental measurements of the moduli of the AI,BC phase. Moreover, we evaluate more-refined bounds involving three-point structural correlation functions by extracting such information from an image of a sample of the B4C/Al composite. Although experimental data for the effective moduli are unavailable for this sample, our predictions of the effective moduli based on three-point bounds should be quite accurate.
Li, X. P. ; Shih, W. Y. ; Aksay, I. A. ; Shih, W. H. Electromechanical behavior of PZT-brass unimorphs. Journal of the American Ceramic Society 1999, 82, 1733-1740.Abstract
We have examined the static and dynamic electromechanical responses of PZT-brass (piezoelectric-nonpiezoelectric) plate unimorphs of various brass/PZT thickness ratios. The study was performed both experimentally and theoretically. The static measurements showed that, given a PZT layer thickness, there exists a brass/PZT thickness ratio that gives the unimorph the highest static displacement under an applied field. The effects of geometric shape and external loading on the displacement of the unimorphs were also examined. The dynamic measurements showed that, given a PZT layer thickness, the bending-mode resonance frequencies increase with an increasing brass/PZT thickness ratio. These results were in good agreement with the theoretical predictions that were obtained with the plate geometry.
Dabbs, D. M. ; Yao, N. ; Aksay, I. A. Nanocomposite mullite/mullite powders by spray pyrolysis. Journal of Nanoparticle Research 1999, 1 127-130.Abstract
A mullite/mullite nanocomposite powder has been synthesized, composed of nanometer-size 3Al(2)O(3)center dot 2SiO(2) ('3 : 2') mullite precipitates within a matrix of the high alumina 2Al(2)O(3)center dot SiO2 ('2 : 1') mullite. Historically, the transition from the metastable high-alumina phase to the thermodynamically stable '3 : 2' phase of mullite has been thought to be a continuous process, involving a continuous solid solution between the two forms of mullite. In contradiction to this widely held view, our high resolution transmission electron microscopic characterization confirms that a first order phase transition between two distinct mullites occurs. The high degree of interface coherence between the precipitates and the matrix allows us to speculate that the mechanical properties of the matrix could be enhanced by a process similar to the precipitation hardening of metals.
Aksay, I. A. ; Vicenzi, E. P. ; Milius, D. L. ; Lettow, J. S. Producing ceramic superconductor single crystals, 1999.Abstract
A bulk high-temp. superconductor single crystal MBa2Cu3O7-x, where M = Y, Sm, Eu, Gd, Dy, Ho, Er, or Yb; and x = ∼0.1 to ∼1.0, are produced by a novel process incorporating: (i) starting powders produced by combustion spray pyrolysis; (ii) a novel setter powder; and/or (iii) a monitored isothermal growth process. [on SciFinder(R)]
Keimer, B. ; Bourges, P. ; Fong, H. F. ; Sidis, Y. ; Regnault, L. P. ; Ivanov, A. ; Milius, D. L. ; Aksay, I. A. ; Gu, G. D. ; Koshizuka, N. Resonant spin excitations in YBa2Cu3O6+x and Bi2Sr2CaCu2O8+delta. Journal of Physics and Chemistry of Solids 1999, 60, 1007-1011.Abstract
A summary of some recent results of neutron scattering studies of high temperature superconductors is given, with a focus on resonant spin excitations in the superconducting states of YBa2CU3O6+x and Bi2Sr2CaCu2O8+delta. The opportunities offered by advances in neutron scattering instrumentation, such as focusing and polarization techniques, for these experiments are discussed, (C) 1999 Published by Elsevier Science Ltd. All rights reserved.
Cates, Gordon D., J. ; Aksay, I. A. ; Happer, W. ; Hsu, M. F. ; Dabbs, D. M. Sol-gel coated polarization vessels, 1999.Abstract
The invention relates to a polarization cell which is coated with glass deposited from a sol-gel used for hyperpolarizing noble gases. The invention also includes a method for hyperpolarizing noble gases utilizing the polarization cell coated with glass deposited from a sol-gel. These polarization cells can also be incorporated into containers used for storage and transport of the hyperpolarized noble gases. [on SciFinder(R)]
Patrick, H. N. ; Warr, G. G. ; Manne, S. ; Aksay, I. A. Surface micellization patterns of quaternary ammonium surfactants on mica. Langmuir 1999, 15, 1685-1692.Abstract
The microscopic equilibrium structures of adsorbed films of quaternary ammonium surfactants on mica have been investigated by noncontact atomic force microscopy imaging as a function of alkyl chain length and headgroup structure. Spherical and cylindrical surface micelles were observed; these were found to be related to bulk. solution self-assembly and the surfactant packing parameter, v/a(0)l(c). Shape transitions in the surface aggregates were observed on changing the counterion between chloride, bromide, and salicylate.
1998
Garg, R. ; Prud'homme, R. K. ; Aksay, I. A. ; Liu, F. ; Alfano, R. R. Absorption length for photon propagation in highly dense colloidal dispersions. Journal of Materials Research 1998, 13, 3463-3467.Abstract
The absorption length for photon propagation in highly concentrated colloidal dispersions calculated from temporal intensity profiles of 100 femto-second pulses is much longer than the absorption length obtained from the measurements of static light transmission in the pure continuous phase fluid. The difference between these two values is explained on the basis of small interparticle spacing at high particle concentration and hence shorter paths traveled by photons through the absorbing medium relative to the total diffusive path in the dispersion. The two values are in good agreement when the absorption length is rescaled with the interparticle separation.
Keimer, B. ; Aksay, I. A. ; Bossy, J. ; Bourges, P. ; Fong, H. F. ; Milius, D. L. ; Regnault, L. P. ; Vettier, C. Bilayer spin dynamics in underdoped YBa2Cu3O6+x. Journal of Physics and Chemistry of Solids 1998, 59, 2135-2139.Abstract
The bilayer structure of YBa2Cu3O6+x supports spin excitations that are either even or odd under a symmetry operation that exchanges two directly adjacent CuO2 layers. Both types of excitation have been observed in underdoped YBa2Cu3O6+x by inelastic neutron scattering. Even and odd response functions are split in energy. The temperature evolution in the normal state is parallel in both channels, but different behavior is observed in the superconducting state where a resonant enhancement is observed only in the odd channel. The measurements shed new light on the nature of interlayer interactions in the cuprates. (C) 1998 Elsevier Science Ltd. All rights reserved.

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