Publications by Type: Journal Article

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.
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.
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.
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.
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. ; 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.
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.
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.
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.
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.
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.
Aksay, I. A. ; Weiner, S. Biomaterials - Is this really a field of research?. Current Opinion in Solid State & Materials Science 1998, 3 219-220.
Xu, G. F. ; Yao, N. ; Aksay, I. A. ; Groves, J. T. Biomimetic synthesis of macroscopic-scale calcium carbonate thin films. Evidence for a multistep assembly process. Journal of the American Chemical Society 1998, 120, 11977-11985.Abstract
Biologically controlled mineralization features an orchestrated balance among various controlling factors such as spatial delineation, template promotion, crystal growth modification and cessation, and so on. Highly ordered calcium carbonate lamellae formed in the nacreous layers of mollusk (aragonite), the foliated calcitic layers of mollusk (calcite), or the semi-nacre of brachiopods (calcite) are excellent examples of the outcome of such synergistic control. Mimicking the concerted interplay of template promotion and growth inhibition as often utilized in biomineralization, we have synthesized macroscopic and continuous calcium carbonate thin films with thickness ranging from 0.4 to 0.6 mu m. The thin films were prepared at air/subphase interfaces by promoting mineral deposition with amphiphilic porphyrin templates, coupled with growth inhibition by the use of poly(acrylic acid) as a soluble inhibitor. Films formed at 22 degrees C were found to have a biphasic structure containing both amorphous and crystalline calcium carbonate. The crystalline regions were identified to be calcite oriented with the (00.1) face parallel to the porphyrin monolayer at the air/subphase interface. Films obtained in the early stage of formation at lower temperature (4 degrees C) displayed characteristics of a single amorphous phase. These observations suggest that films formed through a multistage assembly process, during which an initial amorphous deposition was followed by a phase transformation into the ultimate crystalline phase and the orientation of the crystalline phase was controlled by the porphyrin template during the phase transformation. The results provide new insights into the template-inhibitor-biomineral interaction and a new mechanism for synthesizing ceramic thin film under mild conditions.
Sigmund, O. ; Torquato, S. ; Aksay, I. A. On the design of 1-3 piezocomposites using topology optimization. Journal of Materials Research 1998, 13, 1038-1048.Abstract
We use a topology optimization method to design 1-3 piezocomposites with optimal performance characteristics for hydrophone applications. The performance characteristics we focus on are the hydrostatic charge coefficient d(h)((*)), the hydrophone figure of merit d(h)((*))g(h)((*)), and the electromechanical coupling factor k(h)((*)). The piezocomposite consists of piezoelectric rods embedded in an optimal polymer matrix. We use the topology optimization method to design the optimal (porous) matrix microstructure. When we design for maximum d(h)((*)) and d(h)((*))g(h)((*)) the optimal transversally isotopic matrix material has negative Poisson's ratio in certain directions. When we design for maximum k(h)((*)), the optimal matrix microstructure is layered and simple to build.
Patrick, H. N. ; Warr, G. G. ; Manne, S. ; Aksay, I. A. Interfacial self-assembly of cationic surfactants on mica: Variation with headgroup size. Abstracts of Papers of the American Chemical Society 1998, 215, U491-U491.
Garg, R. ; Prud'homme, R. K. ; Aksay, I. A. ; Liu, F. ; Alfano, R. R. Optical transmission in highly concentrated dispersions. Journal of the Optical Society of America A - Optics Image Science and Vision 1998, 15, 932-935.Abstract
The intensity temporal profiles of diffusive light propagation in highly concentrated (up to volume fraction phi similar to 0.55) dispersions measured by 100-fs laser pulses showed an increase in transport scattering mean free path above a critical concentration. This observation confirms the previous theoretical predictions of enhanced transmission at high particle concentrations due to correlated scattering. The correlation effects are accounted for by incorporating a hard sphere Percus-Yevick static structure factor into the prediction of transport mean free path. (C) 1998 Optical Society of America.
Trau, M. ; Saville, D. A. ; Aksay, I. A. Assembly of colloidal crystals at electrode interfaces. Langmuir 1997, 13, 6375-6381.Abstract
We describe a theory for a new type of colloid behavior whereby particles deposited on a surface by electrophoresis are manipulated to form two-dimensional crystals. Since the particles are equally charged, the clustering is opposite that expected from electrostatic considerations. Such behavior is consistent with migration due to electrohydrodynamic flows associated with polarization layers and ion currents. Provided colloid stability is maintained, the assembly processes take place with both dc and ac fields and may be modulated by adjusting the field strength or frequency. No migration is present at frequencies above 1 MHz. Two-dimensional fluid and crystalline states can be formed on the electrode surface. Experiments with patterned electrodes demonstrate the presence of the electrohydrodynamic now. A mathematical model of the electrohydrodynamics provides insight into the assembly process.
Keimer, B. ; Fong, H. F. ; Lee, S. H. ; Milius, D. L. ; Aksay, I. A. Doping dependence of the magnetic resonance peak in YBa2Cu3O6+x. Physica C 1997, 282, 232-235.Abstract
We report inelastic neutron scattering experiments on the doping dependence of the energy and spectral weight of the sharp magnetic resonance peak in YBa2Cu3O6+x. These measurements also shed light on the relationship between the magnetic excitations in the normal and superconducting states.
McGrath, K. M. ; Dabbs, D. M. ; Yao, N. ; Aksay, I. A. ; Gruner, S. M. Formation of a silicate L-3 phase with continuously adjustable pore sizes. Science 1997, 277, 552-556.Abstract
The lyotropic L-3 phase was used as a template to form nanoporous monolithic silicates with continuously adjustable pore sizes. The monolith was optically isotropic and transparent with a nonperiodic network. The pore size was adjusted by a change in the solvent volume fraction rather than by a change of the surfactant. Unlike other silicates, the bicontinuous pores were water-filled; removal of surfactant was not necessary to access the pores. Measured characteristic dimensions were from six to more than 35 nanometers. For a given solvent fraction, x-ray scattering indicated little variation of pore widths, in marked contrast to the polydisperse pores of aerogels.
Manne, S. ; Schaffer, T. E. ; Huo, Q. ; Hansma, P. K. ; Morse, D. E. ; Stucky, G. D. ; Aksay, I. A. Gemini surfactants at solid-liquid interfaces: Control of interfacial aggregate geometry. Langmuir 1997, 13, 6382-6387.Abstract
Recent work has shown that conventional surfactants form ordered aggregates of well-defined shape and size at solid-liquid interfaces.(1, 2) Here we report interfacial aggregate structures as a function of surfactant geometry by using gemini surfactants with varying tail and spacer lengths. On the anionic cleavage plane of mica, aggregates tend to favor a lower curvature than in solution but follow the same general variation with surfactant geometry (i.e., with larger headgroup areas resulting in greater curvature). These morphologies on mica correlate well with those observed in surfactant-silicate mesophases, where electrostatic binding of headgroups also plays a dominant role. In addition, interfacial sphere-to-rod transitions are induced on mica (as in free solution) by binding with a headgroup-specific counterion. In contrast to mica, the hydrophobic cleavage plane of graphite interacts with surfactant tailgroups, giving rise to interfacial aggregates that are surface-controlled and relatively independent of surfactant geometry. This interaction is used to heterogeneously nucleate a surfactant-silicate mesophase which is interfacially controlled and differs from the bulk phase.
Bourges, P. ; Fong, H. F. ; Regnault, L. P. ; Bossy, J. ; Vettier, C. ; Milius, D. L. ; Aksay, I. A. ; Keimer, B. High-energy spin excitations in YBa2Cu3O6.5. Physical Review B 1997, 56, 11439-11442.Abstract
Inelastic neutron scattering has been used to obtain a comprehensive description of the absolute dynamical spin susceptibility chi ''(q, omega) of the underdoped superconducting cuprate YBa2Cu3O6.5 (T-c = 52 K) over a wide range of energies and temperatures (2 meV less than or equal to (h) over bar omega less than or equal to 120 meV and 5 K less than or equal to T less than or equal to 200 K). Spin excitations of two distinct symmetries (even and odd under the exchange of two adjacent CuO2 layers) are observed which exhibit two different gaplike features (rather than a single ''spin pseudogap''). The excitations show dispersive behavior at high energies. [S0163-1829(97)51142-8].
McHale, J. M. ; Yurekli, K. ; Dabbs, D. M. ; Navrotsky, A. ; Sundaresan, S. ; Aksay, I. A. Metastability of spinel-type solid solutions in the SiO2-Al2O3 system. Chemistry of Materials 1997, 9 3096-3100.Abstract
The addition of small amounts (2-10 wt %) of SiO2 to gamma-Al2O3 increases the temperature of heat treatment necessary for transformation to alpha-Al2O3 by similar to 100 K. We have studied this system using high-temperature solution calorimetry in molten 2PbO . B2O3 at 1043 K, Our results indicate that the spinel-type Al2O3-SiO2 solid solutions with 2-10 wt % SiO2 are always energetically metastable by 30-35 kJ.mol(-1) (on a 4 O2- per mole basis) with respect to alpha-Al2O3 and quartz. Calculation of the maximum configurational entropy of the solid solutions allowed determination of the likely most negative value of the Gibbs free energy of the materials, The solid solutions are somewhat entropy stabilized, but still thermodynamically metastable by > 10 kJ.mol(-1) at 1400 K, Therefore, SiO2 addition appears to provide mainly a kinetic hindrance to alpha-Al2O3 formation.
Trau, M. ; Yao, N. ; Kim, E. ; Xia, Y. N. ; Whitesides, G. M. ; Aksay, I. A. Microscopic patterning of orientated mesoscopic silica through guided growth. Nature 1997, 390, 674-676.Abstract
The supramolecular assembly of surfactant molecules at a solid-liquid interface can produce tubular structures with diameters of around 10 nm (refs 1-4), which can be used for the templated polymerization of mesoporous silica thin films(3-5). The orientation of the tubules depends primarily on the nature of the substrate-surfactant interaction. These nanostructured films hold much promise for applications such as their use as orientated nanowires(6), sensor/actuator arrays(7-9) and optoelectronic devices(10), But a method of patterning the tubules and orientating them into designed arrangements is required for many of these possibilities to be realized. Here we describe a method that allows the direction of growth of these tubules to be guided by infiltrating a reaction fluid into the microcapillaries of a mould in contact with a substrate(11). An electric field applied tangentially to the surface within the capillaries induces electro-osmotic flow, and also enhances the rates of silica polymerization around the tubules by localized Joule heating. After removal of the mould, patterned bundles of orientated nanotubules remain on the surface. This method permits the formation of orientated mesoporous channels on a non-conducting substrate with an arbitrary microscopic pattern.
Lee, T. ; Yao, N. ; Aksay, I. A. Nanoscale patterning of barium titanate on block copolymers. Langmuir 1997, 13, 3866-3870.Abstract
A triblock copolymer of polystyrene-polybutadiene-polystyrene (Kraton D1102) has been used to pattern barium titanate precursor with nanoscale modulations. The copolymer self-assembles to yield cylindrical polystyrene nanodomains in a polybutadiene matrix. The preorganized thin films of polymer are then selectively OH-functionalized in situ on the unsaturated carbon bonds in the polybutadiene matrix with antistereochemistry. Anchoring the barium titanate precursor onto the hydroxylated polymer thin films is possible only in the trans-1,2 polybutadiol matrix through the condensation between the barium titanium double alkoxides and the hydroxyl groups. The regioselective deposition of the barium titanium double alkoxides on the original polybutadiene matrix of the Kraton thin films was verified by transmission electron microscopy and electron energy loss spectroscopy. The spacing of the coordinated barium titanium double alkoxide pattern was similar to 23 nm, equivalent to the interdomain spacing of the original polybutadiene matrix.
Lahiri, J. ; Xu, G. F. ; Dabbs, D. M. ; Yao, N. ; Aksay, I. A. ; Groves, J. T. Porphyrin amphiphiles as templates for the nucleation of calcium carbonate. Journal of the American Chemical Society 1997, 119, 5449-5450.
Shih, W. Y. ; Shih, W. H. ; Aksay, I. A. Scaling analysis for the axial displacement and pressure of flextensional transducers. Journal of the American Ceramic Society 1997, 80, 1073-1078.Abstract
We have examined the axial displacement, Delta h, and maximum axial pressure, P-max, of flextensional transducers such as the moonies and the rainbows with both scaling and mechanical analyses. For a constant electric field E across the transducer, Delta h/t alpha E/t(2) where t is the thickness of the rainbow or the thickness of the metal end cap of the moonie and Delta h/t, the relative axial displacement. Thus, for a constant voltage V across the transducer, Delta h/t alpha V/t(3). As for the maximum pressure, P-max alpha t(2) for the rainbows and P-max alpha wt for the moonies where t is the thickness of the rainbow or the thickness of the metal end cap of the moonie and w the thickness of the piezoelectric disk of the moonie. These predictions agree well with the experimental results found in the rainbows and the moonies, Our analysis showed that although the rainbows and the moonies differ in design and processing, the underlying physics for the enhancement in the axial displacement are essentially the same: The nonuniform distribution of d(31) through the thickness of the transducer causes the transducer to arch or flatten with an applied electrical field, which leads to the enhancement in the axial displacement, The only difference is that, for the transducer to arch, the applied field is in the opposite direction to the polarization in the rainbows but in the same direction as the polarization in the moonies.
Patrick, H. N. ; Warr, G. G. ; Manne, S. ; Aksay, I. A. Self-assembly structures of nonionic surfactants at graphite/solution interfaces. Langmuir 1997, 13, 4349-4356.Abstract
The interfacial self-assembly structures of a series of poly(oxyethylene) n-dodecyl ether (C12En) nonionic surfactants on graphite has been imaged by atomic farce microscopy using only the steric stabilization force as the contrast mechanism. Aggregates are arranged in parallel stripes perpendicular to the underlying graphite symmetry axes for C12E5-C12E10. These are interpreted as hemicylindrical micelles, consistent with previous studies of ionic surfactants adsorbed on graphite. C12E23 shows a featureless layer and C12E3 forms an anchored lamellar phase growing normal to the graphite surface. We relate the interfacial structures to those formed in bulk solution and show that the initially adsorbed molecules template the interfacial aggregates, modifying their self-assembly behavior.
Keimer, B. ; Aksay, I. A. ; Bossy, J. ; Bourges, P. ; Fong, H. F. ; Milius, D. L. ; Regnault, L. P. ; Reznik, D. ; Vettier, C. Spin excitations and phonons in YBa2Cu3O6+x: A status report. Physica B 1997, 234, 821-829.Abstract
A review is made of recent developments in inelastic neutron scattering experiments on spin excitations and phonons in the high-temperature superconductor YBa2Cu3O6+x and its antiferromagnetic precursor YBa2Cu3O6.2. These experiments include the detection of high-energy ''optical'' spin waves and the determination of the full spin Hamiltonian in YBa2Cu3O6.2, detailed investigations of the 40 meV magnetic resonance peak in the superconducting state of YBa2Cu3O7 and its precursors in underdoped YBa2Cu3O6+x, and experiments on the effect of superconductivity on phonon lifetimes in YBa2Cu3O7.
Fong, H. F. ; Keimer, B. ; Milius, D. L. ; Aksay, I. A. Superconductivity-induced anomalies in the spin excitation spectra of underdoped YBa2Cu3O6+x. Physical Review Letters 1997, 78, 713-716.Abstract
Polarized and unpolarized neutron scattering have been used to determine the effect of superconductivity on the magnetic excitation spectra of YBa2Cu3O6.5 (T-c = 52 K) and YBa2Cu3O6.7 (T-c = 67 K); Pronounced enhancements of the spectral weight centered around 25 and 33 meV, respectively; are observed below T-c in both crystals, compensated predominantly by a loss of spectral weight at higher energies. The data provide important clues to the origin of the 40 meV magnetic resonance peak in YBa2Cu3O7.
Manne, S. ; Aksay, I. A. Thin films and nanolaminates incorporating organic/inorganic interfaces. Current Opinion in Solid State & Materials Science 1997, 2 358-364.Abstract
Recent research on the solution-based fabrication of inorganic thin films and organic/inorganic nanolaminates has ranged from fundamental studies of biomineralization to the synthesis of never materials and devices. Highlights include the elucidation of how biogenics and model organic nucleants affect the form of the biomineral; synthesis of mesoscale nanocomposite films by surfactant templating at interlaces; and fabrication of heterostructures with enhanced electronic and mechanical properties.
Putnam, R. L. ; Nakagawa, N. ; McGrath, K. M. ; Yao, N. ; Aksay, I. A. ; Gruner, S. M. ; Navrotsky, A. Titanium dioxide-surfactant mesophases and Ti-TMS1. Chemistry of Materials 1997, 9 2690-+.Abstract
Several attempted syntheses of Ti-TMS1, a hexagonal mesoporous TiO2 reported by Antonelli and Ying, have resulted in a lamellar structure as determined by two-dimensional powder X-ray diffraction and transmission electron microscopy (TEM). Regions of partially calcined lamellar materials, when observed by TEM can be mistaken for hexagonal material. In no cases are specimens produced that were unambiguously hexagonal. It is concluded that the hexagonal material exists, if at all, only as a minor component of a larger lamellar structure when phosphate surfactants are used. Ti-TMS1 therefore remains elusive.
Aksay, I. A. ; Trau, M. ; Manne, S. ; Honma, I. ; Yao, N. ; Zhou, L. ; Fenter, P. ; Eisenberger, P. M. ; Gruner, S. M. Biomimetic pathways for assembling inorganic thin films. Science 1996, 273, 892-898.Abstract
Living organisms construct various forms of laminated nanocomposites through directed nucleation and growth of inorganics at self-assembled organic templates at temperatures below 100 degrees C and in aqueous solutions. Recent research has focused on the use of functionalized organic surfaces to form continuous thin films of single-phase ceramics. Continuous thin films of mesostructured silicates have also been formed on hydrophobic and hydrophilic surfaces through a two-step mechanism. First, under acidic conditions, surfactant micellar structures are self-assembled at the solid/liquid interface, and second, inorganic precursors condense to form an inorganic-organic nanocomposite. Epitaxial coordination of adsorbed surfactant tubules is observed on mica and graphite substrates, whereas a random arrangement is observed on amorphous silica. The ability to process ceramic-organic nanocomposite films by these methods provides new technological opportunities.
Reznik, D. ; Bourges, P. ; Fong, H. F. ; Regnault, L. P. ; Bossy, J. ; Vettier, C. ; Milius, D. L. ; Aksay, I. A. ; Keimer, B. Direct observation of optical magnons in YBa2Cu3O6.2. Physical Review B 1996, 53, 14741-14744.Abstract
We have used high-energy inelastic neutron scattering to detect optical magnons directly in antiferromagnetic YBa2Cu3O6.2. The optical magnon gap is 67+/-5 meV. This implies an intrabilayer superexchange constant perpendicular to the CuO2 layers of J(perpendicular to) = 0.08 J(parallel to) where J(parallel to) is the in-plane nearest-neighbor superexchange constant.
Trau, M. ; Saville, D. A. ; Aksay, I. A. Field-induced layering of colloidal crystals. Science 1996, 272, 706-709.Abstract
An electrohydrodynamic methodology has been developed that makes possible the precise assembly of two- and three-dimensional colloidal crystals on electrode surfaces. Electrophoretically deposited colloidal particles were observed to move toward one another over very large distances (greater than five particle diameters) to form two-dimensional colloidal crystals for both micrometer- and nanometer-size particles, This coalescence of particles with the same charge is opposite to what is expected from electrostatic considerations and appears to result from electrohydrodynamic fluid flow arising from an ionic current flowing through the solution. The ability to modulate this ''lateral attraction'' between particles, by adjusting field strength or frequency, facilitates the reversible formation of two-dimensional fluid and crystalline colloidal states on the electrode surface. Further manipulation allows controlled structures to be assembled.
Trau, M. ; Saville, D. A. ; Aksay, I. A. Field-induced two- and three-dimensional colloidal crystals. Abstracts of Papers of the American Chemical Society 1996, 212, 153-INOR.
Shih, W. Y. ; Shih, W. H. ; Aksay, I. A. Heteroflocculation in binary colloidal suspensions: Monte Carlo simulations. Journal of the American Ceramic Society 1996, 79, 2587-2591.Abstract
We have examined heteroflocculation in binary colloidal suspensions by Monte Carlo simulations based on a diffusion-limited-cluster-aggregation (DLCA) model modified with finite attraction energies, The simulations were performed in two dimensions, Under heteroflocculation conditions, i.e., attraction between unlike particles and repulsion between like particles, cluster size undergoes a maximum as the concentration of the second species of particles is increased, similar to the experimental results in both binary suspensions and suspensions with adsorbing polymers. The initial increase in cluster size at low concentrations of the second species of particles is due to the mutual attraction between unlike particles, The decrease in cluster size at higher concentrations of the sea,nd species of particles is due to the repulsion between the second species of particles, The distinction between heteroflocculation and particulate depletion flocculation is discussed.
Aksay, I. A. Mesoscale patterning of thin films through template-assisted self-assembly. Abstracts of Papers of the American Chemical Society 1996, 212, 213-INOR.
Treadwell, D. R. ; Dabbs, D. M. ; Aksay, I. A. Mullite (3Al(2)O(3)-2SiO(2)) synthesis with aluminosiloxanes. Chemistry of Materials 1996, 8 2056-2060.Abstract
High-yield mullite (3A1(2)O(3)-2SiO(2)) precursors consist of aluminosiloxanes :synthesized from mixtures of aluminum and silicon alkoxides. Atomic level mixing of the aluminum and silicon oxides is demonstrated by the low-temperature conversion (<1000 degrees C) of the aluminosiloxanes to phase-pure mullite. The proper selection of monomeric side groups serves several functions: (i) controlling reactivity of the silicon and aluminum monomers, thereby favoring atomic-level mixing; (ii) maintaining the tractability of the resulting aluminosiloxane; (iii) improving the yield during mullitization of the aluminosiloxane through easy thermolytic removal. The tractability of the aluminosiloxane compounds permits these materials to be used, in fiber spinning, the casting of thin films and monoliths, and as impregnants to powder compacts.
Fong, H. F. ; Keimer, B. ; Reznik, D. ; Milius, D. L. ; Aksay, I. A. Polarized and unpolarized neutron-scattering study of the dynamical spin susceptibility of YBa2Cu3O7. Physical Review B 1996, 54, 6708-6720.Abstract
We report an extensive study of magnetic excitations in fully oxygenated YBa2Cu3O7, using neutron scattering with and without spin polarization analysis. By calibrating the measured magnetic intensity against calculated structure factors of optical phonons and against antiferromagnetic spin waves measured in the same crystal after deoxygenation to YBa2Cu3O6.2, we establish an absolute intensity scale for the dynamical spin susceptibility, chi ''(q,omega). The integrated spectral weight of the sharp magnetic resonance at h omega=40 meV and q(parallel to)=(pi/a,pi/a) in the superconducting state is integral d(h omega)chi(res)''(q,w)=(0.52+/-0.1) at low temperatures. The energy and spectral weight of the resonance are measured up to T=0.8T(c). The resonance disappears in the normal state, and a conservative upper limit of 30 states/eV is established for the normal state dynamical susceptibility at q(parallel to)=(pi/a,pi/a) and 10 meV less than or equal to h omega less than or equal to 40 meV. Our results are compared to previous neutron-scattering data on YBa2Cu3O7, theoretical interpretations of NMR data and current models of the 40 meV resonance.
Rintoul, M. D. ; Torquato, S. ; Yeong, C. ; Keane, D. T. ; Erramilli, S. ; Jun, Y. N. ; Dabbs, D. M. ; Aksay, I. A. Structure and transport properties of a porous magnetic gel via x-ray microtomography. Physical Review E 1996, 54, 2663-2669.Abstract
A three-dimensional digitized image of a porous magnetic gel is determined by x-ray microtomographic techniques. The complex connected pore-space topology is quantitatively characterized by measuring a variety of statistical correlation functions, including the chord-length distribution function, the pore-size distribution function, and the Lineal-path function. This structural information is then employed to estimate transport properties, such as the fluid permeability and trapping rate, of the gel.
Slamovich, E. B. ; Aksay, I. A. Structure evolution in hydrothermally processed ( <100°C) BaTiO3 Films. Journal of the American Ceramic Society 1996, 79, 239-247.Abstract
Thin films of cubic BaTiO3 were processed hydrothermally at 40 degrees-80 degrees C by reacting thin layers of titanium organometallic liquid precursors in aqueous solutions of either Ba(OH)(2) or a mixture of NaOH and BaCl2. All films (thickness similar to 1 mu m) were polycrystalline with grain sizes ranging from nano- to micrometer dimensions, BaTiO2 formation was facilitated by increasing [OH-], [Ba2+], and the temperature, The film structure was related to the nucleation and growth behavior of the BaTiO3 particles, Films processed at relatively low [OH-], [Ba2+], and temperatures were coarse grain and opaque, but increasing [OH-], [Ba2+], and temperature caused the grain size to decrease, resulting in transparent films.
Trau, M. ; Sankaran, S. ; Saville, D. A. ; Aksay, I. A. Electric-Field-Induced Pattern-Formation in Colloidal Dispersions . Nature 1995, 374, 437-439.Abstract
THE formation of patterned colloidal structures from dispersions of particles has many potential uses in materials processing(1-3). Structures such as chains of particles that form in the presence of electric or magnetic fields are also central to the behaviour of electrorheological fluids(4-6) and ferrofluids(7). Electrohydrodynamic effects in aqueous suspensions have been described by Rhodes et al.(8). Here we show that such effects can be used to create structures within a non-aqueous colloidal dispersion of dielectric particles, When the conductivity of a particle-rich spherical region (bolus) is higher than that of the surrounding fluid, an electric field deforms the bolus into a prolate ellipsoid. If the conductivities are reversed (by adding salt to the surrounding fluid, for example), a disk-like shape results. In this way, we form colloidal columns, disks and more complex structures. Once formed, these could be frozen in place by solidifying the fluid matrix by gelation or polymerization(9).
Shih, W. Y. ; Shih, W. H. ; Aksay, I. A. Elimination of an Isolated Pore - Effect of Grain Size. Journal of Materials Research 1995, 10, 1000-1015.Abstract
The effect of grain size on the elimination of an isolated pore was investigated both by the Monte Carlo simulations and by a scaling analysis. The Monte Carlo statistical mechanics model for sintering was constructed by mapping microstructures onto domains of vectors of different orientations as grains and domains of vacancies as pores. The most distinctive feature of the simulations is that we allow the vacancies to move. By incorporating the outer surfaces of the sample in the simulations, sintering takes place via vacancy diffusion from the pores to the outer sample surfaces. The simulations were performed in two dimensions. The results showed that the model is capable of displaying various sintering phenomena such as evaporation and condensation, rounding of a sharp corner, pore coalescence, thermal etching, neck formation, grain growth, and growth of large pores. For the elimination of an isolated pore, the most salient result is that the scaling law of the pore elimination time t(p) with respect to the pore diameter d(p) changes as pore size changes from larger than the grains to smaller than the grains. For example, in sample-size-fixed simulations, t(p) similar to d(p)(3) for d(p) < G and t(p) similar to d(p)(2) for d(p) > G with the crossover pore diameter d(c) increasing linearly with G where G is the average grain diameter. For sample-size-scaled simulations, t(p) similar to d(p)(4) for d(p) < G and t(p) similar to d(p)(3) for d(p) > G. That t(p) has different scaling laws in different grain-size regimes is a result of grain boundaries serving as diffusion channels in a fine-grain microstructure such as those considered in the simulations. A scaling analysis is provided to explain the scaling relationships among t(p), d(p), and G obtained in the simulations. The scaling analysis also shows that these scaling relationships are independent of the dimensionality. Thus, the results of the two-dimensional simulations should also apply in three dimensions.
Trau, M. ; Sankaran, S. ; Saville, D. A. ; Aksay, I. A. Pattern formation in nonaqueous colloidal dispersions via electrohydrodynamic flow. Langmuir 1995, 11, 4665-4672.Abstract
We describe a new electrohydrodynamic phenomenon observed in inhomogeneous, nonaqueous colloidal dispersions with a spatially varying particle number concentration. In the presence of an external electric field, the dielectric constant and conductivity gradients in these systems engender fluid motion which results in the formation of patterned colloidal structures: columns, disks, and other more complicated structures. Other workers found similar effects in high conductivity systems, where the particles are dispersed in water with dissolved electrolyte. Our experimental results with barium titanate dispersed in low conductivity, apolar liquids indicate that electrical forces due to free charge and dielectric constant variations each play a role in inducing now. This pattern forming phenomenon differs from previously observed field-induced pattern formation in colloidal dispersions (e.g., colloidal string formation in electrorheological and ferrofluids) largely as a result of the induced fluid flow. A mathematical model has been developed which predicts, qualitatively, the initial now patterns encountered in our system. The theory may also help explain the formation of more complicated field-induced particle morphologies which have been reported in aqueous and nonaqueous media as well as the observation of dispersion band broadening during electrophoresis.
Fong, H. F. ; Keimer, B. ; Anderson, P. W. ; Reznik, D. ; Dogan, F. ; Aksay, I. A. Phonon and Magnetic Neutron-Scattering at 41 MEV in YBa2Cu3O7. Physical Review Letters 1995, 75, 316-319.
Lee, Y. S. ; Yang, J. Z. ; Sisson, T. M. ; Frankel, D. A. ; Gleeson, J. T. ; Aksay, E. ; Keller, S. L. ; Gruner, S. M. ; Obrien, D. F. Polymerization of Nonlamellar Lipid Assemblies. Journal of the American Chemical Society 1995, 117, 5573-5578.Abstract
The formation of nonlamellar lipid structures in model lipid membranes has been extensively studied in recent years. These hydrated lipid phases include the inverted hexagonal phase and various bicontinuous cubic phases, which occur at selected lipid concentrations, temperatures, and pressures. Cubic phases that are bicontinuous with respect to the polar and nonpolar regions are especially interesting as organic analogs of zeolites. The recently developed methods used to polymerize and stabilize lamellar assemblies offer certain strategies that are applicable to nonlamellar phases. Here we report the successful stabilization of a nonlamellar phase via the polymerization of reactive amphiphiles. A 3:1 molar mixture of polymerizable mono-dienoyl-substituted phosphoethanolamine and bis-dienoyl-substituted phosphocholine were hydrated to yield bilayers. X-ray diffraction of the unpolymerized mixture at 60 degrees C showed the formation of an inverted hexagonal phase which on prolonged incubation changed to a bicontinuous cubic phase of Pn ($) over bar 3m symmetry. Polymerization of the hexagonal phase produced a stabilized hexagonal structure over the range of 20 to 60 degrees C. The same lipids at lower concentration were characterized by P-31-NMR and transmission electron microscopy (TEM) before and after polymerization. The NMR shows the formation of a sample with isotropic symmetry as expected for a cubic phase. The polymerized sample retained a nonlamellar structure after cooling and extended storage at room temperature or near 0 degrees C. The TEMs show a polydomain square lattice with 6 +/- 1 nm diameter aqueous channels. This stabilized nonlamellar phase is the first representative of a new family of materials with interpenetrating water channels with high surface area and potentially bicompatible lipid-water interfaces.
Sakka, Y. ; Bidinger, D. D. ; Aksay, I. A. Processing of Silicon Carbide-Mullite-Alumina Nanocomposites. Journal of the American Ceramic Society 1995, 78, 479-486.Abstract
Nanocomposite materials in the form of nanometer-sized second-phase particles dispersed in a ceramic matrix have been shown to display enhanced mechanical properties. In spite of this potential, processing methodologies to produce these nanocomposites are not well established. In this paper, we describe a new method for processing SiC-mullite-Al2O3 nanocomposites by the reaction sintering of green compacts prepared by colloidal consolidation of a mixture of SIC and Al2O3 powders, In this method, the surface of the SIC particles was first oxidized to produce silicon oxide and to reduce the core of the SiC particles to nanometer size. Next, the surface silicon oxide was reacted with alumina to produce mullite. This process results in particles with two kinds of morphologies: nanometer-sized SiC particles that are distributed in the mullite phase and mullite whiskers in the SiC phase. Both particle types are immersed in an Al2O3 matrix.
Reznik, D. ; Keimer, B. ; Dogan, F. ; Aksay, I. A. Q-Dependence of Self-Energy Effects of the Plane Oxygen Vibration in YBa2Cu3O7. Physical Review Letters 1995, 75, 2396-2399.Abstract
We have measured the temperature dependence of the peak position and linewidth of the 42.5 meV phonon branch in a twinned single crystal of YBa2Cu3O7 as a function of wave vector q. In the (100)/(010) direction in the Brillouin zone, considerable softening and broadening occur below the superconducting transition temperature T-c at some values of q. We observe an order of magnitude smaller softening and no linewidth broadening for q in the (110)/(1(1) over bar0$) direction. Possible implications of these findings for the symmetry of the superconducting order parameter are discussed.
Keimer, B. ; Fong, H. F. ; Reznik, D. ; Dogan, F. ; Aksay, I. A. Resonant neutron scattering from YBa2Cu3O7. Journal of Physics and Chemistry of Solids 1995, 56, 1927-1929.Abstract
We have developed a new scattering geometry for magnetic neutron scattering experiments on YBa2Cu3O7 in which the phonon background around q similar to (pi/alpha,pi/alpha), h omega similar to 40meV is significantly reduced. We use this new approach to study the previously detected, sharp magnetic excitation at similar to 40meV in the superconducting state in detail. The excitation does not shift substantially in energy up to at least 75K (similar to 0.8T(c)). Polarized neutron scattering experiments (horizontal minus vertical field) confirm the magnetic origin of the 40meV excitation and put stringent limits on the magnetic scattering intensity in the normal stale.
Keimer, B. ; Shih, W. Y. ; Aksay, I. A. ; Lynn, J. W. ; Erwin, R. W. Vortex Lattice Symmetry and Electronic-Structure in YBa2Cu3O7 - Reply. Physical Review Letters 1995, 75, 1423-1423.
Son, Y. B. ; Kim, C. H. ; Jang, S. D. ; Liu, J. ; Sarikaya, M. ; Aksay, I. A. Crystallization behavior of cordierite-based glass with excess SiO2 and Al2O3 at initial-stage. Japanese Journal of Applied Physics Part 1-Regular Papers Short Notes & Review Papers 1994, 33, 1101-1108.Abstract
Crystallization behavior of cordierite-based glass with excess SiO2 and Al2O3 (CM glass) was studied at the initial crystallization stage. Two kinds of glass-in-glass phase separation occurred during the heat treatment of glass powder. The secondary glass-in-glass phase separation, which consists of an SiO2-rich phase and an Al2O3-rich phase significantly affects the crystallization of the glass and the resultant microstructures of the crystallized glass. Mullite nucleated first in the Al2O3-rich amorphous phase which is no longer a stable glass-forming region. Metastable mu-cordierite crystallized in the SiO2-rich amorphous phase and grew rapidly. A fine-grained cordierite-mullite composite with mullite as the grain boundary phase was produced.
Shih, W. H. ; Shih, W. Y. ; Kim, S. I. ; Aksay, I. A. Equilibrium-state density profiles of centrifuged cakes. Journal of the American Ceramic Society 1994, 77, 540-546.Abstract
We have examined the equilibrium-state density profiles of centrifuged cakes both theoretically and experimentally. The theoretical density profiles were obtained by implementing the experimental pressure-density relationship into the general differential equation for centrifugation with appropriate boundary conditions. With a power-law pressure-density relationship, P = beta phi(n), we show that phi(Z)/phi(max) = (1 - Z/Z(m))(1/(n - 1)) where phi(max) is the density at the bottom of the cake, Z the distance measured from the bottom of the cake, and Z(m) the distance at which the cake density vanishes. Experimentally, the density profiles were examined with gamma-ray densitometry. The predicted density profiles are in good agreement with the experimental ones. We also show that form phi(Z)/phi(max) = (1 - Z/Z(m))(1/(n - 1)) applies to sedimentation cakes as well, provided the pressure-density relationship of sedimentation cakes is also a power-law one.
Reznik, D. ; Keimer, B. ; Dogan, F. ; Aksay, I. A. High-resolution inelastic neutron-scattering study of phonon self-energy effects in YBCO. Physica C 1994, 235, 1733-1734.Abstract
We report preliminary results of the high resolution inelastic neutron scattering measurements of the 42.5 meV optical phonon branch in YBCO, whose behavior at momentum transfer q=0 has been extensively studied by Raman scattering. The experiment was done on a large (75g) single crystal of YBa2Cu3O7-x (T-c approximate to 90K) with the resolution of 2 meV (full width at half maximum). In cooling from 100K to 50K we observe a small softening of the phonon energy at q=0.25,0.25,0, but no significant linewidth change.
Aksay, I. A. Processing of complex ceramics with the aid of biopolymers and biomonomers. Abstracts of Papers of the American Chemical Society 1994, 207, 207-IEC.
Aksay, I. A. Processing of complex ceramics with the aid of biopolymers and biomonomers. Abstracts of Papers of the American Chemical Society 1994, 207, 11-ANYL.
Jun, Y. N. ; Dabbs, D. M. ; Aksay, I. A. ; Erramilli, S. Processing of monolithic magnetic gels for magnetophoresis. Langmuir 1994, 10, 3377-3379.Abstract
Magnetite particles and monosized polystyrene beads were trapped in a silica-gel, which was then dried by using supercritical fluid extraction. When the monolithic dried gel is sintered, the polystyrene beads are pyrolyzed, leaving a porous magnetized piece of ceramic with controlled pore sizes. These ''magnetic gel'' ceramics provide a novel class of materials for use in gel magnetophoresis and other biophysical applications.
Abel, J. S. ; Stangle, G. C. ; Schilling, C. H. ; Aksay, I. A. Sedimentation in flocculating colloidal suspensions. Journal of Materials Research 1994, 9 451-461.Abstract
A combined experimental and theoretical investigation of the sedimentation of unstable colloidal ceramic suspensions has been performed. Suspensions containing submicron-sized alpha - Al2O3 particles were prepared at various pH values in order to modify suspension stability. Particle volume fraction during sedimentation was determined as a function of position and time by gamma-ray densitometry. A population balance model was developed to account for various coagulation and decoagulation mechanisms that affect sedimentation behavior in flocculating suspensions. Model predictions were then compared with experimental measurements, in order to establish the validity of the theoretical model.
Shih, W. Y. ; Shih, W. H. ; Aksay, I. A. Size dependence of the ferroelectric transition of small BaTiO3 particles - effect of depolarization. Physical Review B 1994, 50, 15575-15585.
Yogo, T. ; Aksay, I. A. Synthesis of Mullite Fiber from an Aluminosiloxane Precursor. Journal of Materials Chemistry 1994, 4 353-359.Abstract
Mullite fibre was successfully synthesized by pyrolysis of aluminosiloxane formed from ethyl 3-oxobutanoatodiisopropoxyaluminium and di-(sec-butoxy)aluminoxytriethoxysilane. Aluminosiloxane increased in viscosity with increasing coordination number of aluminium, which was analysed by Al-27 NMR spectroscopy. The viscosity of aluminosiloxane was controlled by the amount of added glacial acetic acid as well as the working temperature for the spinning of precursor fibre. Aluminosiloxane gave amorphoUS SiO2-Al2O3 at 500-degrees-C, which began to crystallize to mullite at 930-degrees-C. Single-phase mullite was produced on heating to 1000-degrees-C for 1 h. The polymer fibre spun from aluminosiloxane was pyrolysed yielding crack-free mullite fibre at 1000-degrees-C for 1 h.
Keimer, B. ; Shih, W. Y. ; Erwin, R. W. ; Lynn, J. W. ; Dogan, F. ; Aksay, I. A. Vortex Lattice Symmetry and Electronic-Structure in YBa2Cu3O7. Physical Review Letters 1994, 73, 3459-3462.
Keimer, B. ; Lynn, J. W. ; Erwin, R. W. ; Dogan, F. ; Shih, W. Y. ; Aksay, I. A. Vortex Structures in YBa2Cu3O7. Journal of Applied Physics 1994, 76, 6778-6783.
Blohowiak, K. Y. ; Garrigus, D. F. ; Luhman, T. S. ; McCrary, K. E. ; Strasik, M. ; Aksay, I. A. ; Dogan, F. ; Hicks, W. B. ; Liu, J. ; Sarikaya, M. Evaluation of YBa2Cu3O7-x bulk superconductors for high field magnet applications. IEEE Transactions on Applied Superconductivity 1993, 3 1049-1052.Abstract
Processing of YBCO single crystals was carried out by solidification of semi-liquid YBCO composition using a seeding technique. Microstructural characterization of the pinning centers was investigated by TEM. Characterization of single crystals was carried out, relating grain size and shape to the corresponding flux profiles. Current densities were calculated based on measured trapped fields. Once circulating currents were established, flux pumping and quenching experiments were conducted. These large single crystals will be incorporated into electromagnetic forming devices for use in the military and commercial aircraft manufacturing and service industries.
Keimer, B. ; Dogan, F. ; Aksay, I. A. ; Erwin, R. W. ; Lynn, J. W. ; Sarikaya, M. Inclined-field structure, morphology, and pinning of the vortex lattice in microtwinned YBa2Cu3O7. Science 1993, 262, 83-86.Abstract
A detailed small-angle neutron scattering study of the vortex lattice in a single crystal of YBa2Cu3O7 was made for a field of 0.5 tesla inclined at angles between 0 and 80 degrees to the crystalline c axis. The vortex lattice is triangular for all angles, and for angles less than or equal to 70 degrees its orientation adjusts itself to maximize the pinning energy to densely and highly regularly spaced twin planes. These observations have important implications for the microscopic flux-pinning mechanism, and hence for the critical current achievable in YBaCu3O7. For large angles (about 80 degrees) the vortex lattice consists of independent chains in the orientation predicted by anisotropic London theory.
Bergstrom, L. ; Schilling, C. H. ; Aksay, I. A. Consolidation behavior of flocculated alumina suspensions. Journal of the American Ceramic Society 1992, 75, 3305-3314.Abstract
The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time-dependent and equilibrium density profiles were measured by a gamma-ray absorption technique. The interparticle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, phi(m), obtained, with phi(m) almost-equal-to 0.54 for the most strongly flocculated suspension to phi(m) almost-equal-to 0.63 for the most weakly flocculated suspension. The compressive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated suspensions.
Ren, T. ; Pellerin, N. B. ; Graff, G. L. ; Aksay, I. A. ; Staley, J. T. Dispersion of small ceramic particles (Al2O3) with azotobacter-vinelandii. Applied and Environmental Microbiology 1992, 58, 3130-3135.Abstract
The high surface charge of small ceramic particles such as alumina particles prevents them from dispersing evenly in aqueous suspensions and forming high-density compacts. However, suspensions of 400-nm-diameter alumina particles treated with alginate from the bacterium Azotobacter vinelandii were well dispersed. The alginate bound firmly to the particle surface and could not be removed by repeated washing with distilled water (2.82 mg of the bacterial alginate adsorbed to 1 g of the alumina particles). Furthermore, A. vinelandii grew and produced alginate in the presence of up to 15% (vol/vol) alumina particles. These results suggest that an in situ process using this bacterium to coat ceramic particles with alginate might be developed. In in situ processing experiments, the particle-packing densities were significantly increased and the viscosities of 5 and 10% (vol/vol) suspensions were reduced 4- and 60-fold, respectively, over those of controls. The bacteria were readily removed from the alumina particles by washing.
Shih, W. Y. ; Liu, J. ; Shih, W. H. ; Aksay, I. A. Aggregation of colloidal particles with a finite interparticle attraction energy. Journal of Statistical Physics 1991, 62, 961-984.Abstract
Aggregation of colloidal particles with a finite attraction energy was investigated with computer simulations and with gold particles coated with a surfactant. Computer simulations were carried out with the Shih-Aksay-Kikuchi (SAK) model, which incorporates a finite nearest-neighbor attraction energy - E into the diffusion-limited-cluster-aggregation (DLCA) model. Both the computer simulations and the experiments showed that (i) with a finite interparticle attraction energy, aggregates can still remain fractal, and (ii) the fractal dimension remains unchanged at large interparticle attraction energies and increases when the interparticle attraction energy is smaller than 4k(B)T, where T is the temperature and k(B) is the Boltzmann constant. The agreement between the simulations and the experimental results suggests that the reversible aggregation process in a colloidal system can be represented by the SAK model.
Yasrebi, M. ; Shih, W. Y. ; Aksay, I. A. Clustering of binary colloidal suspensions - experiment. Journal of Colloid and Interface Science 1991, 142, 357-368.
Liu, J. ; Shih, W. Y. ; Kikuchi, R. ; Aksay, I. A. Clustering of binary colloidal suspensions - theory. Journal of Colloid and Interface Science 1991, 142, 369-377.
Aksay, I. A. Molecular and colloidal engineering of ceramics. Ceramics International 1991, 17, 267-274.Abstract
During the last decade, significant advances have been made in the processing of ceramics by a combination of techniques utilizing molecular precursors and colloids for powder consolidation. Powder consolidation methods have mainly dealt with the formation of unagglomerated powders in the size range of 0.1-1-mu-m, the preparation of colloidal suspensions that are suitable for the formation of high density compacts by filtration and/or plastic forming techniques, the removal of the processing aids, and the role of consolidation methods on microstructural evolution. In contrast, the molecular and/or sol-gel techniques dealt with processing at a finer dimensional scale of 10-1000 angstrom with either molecularly homogeneous precursors or nanometer-sized particulates that are used in the preparation of gels that display linear viscoelastic behavior. Similar to green compacts of micron-sized powders, these gels are then converted to dense ceramics by heat treatment. This review summarizes the concepts that are common to both of these regimes and points to the synergistic benefits of coupling molecular precursors with colloids in a process path. The emphasis is on the control of the structure of a final product at scale lengths ranging from molecular to micro- and macroscopic dimensions.
Aksay, I. A. ; Dabbs, D. M. ; Sarikaya, M. Mullite for structural, electronic, and optical applications. Journal of the American Ceramic Society 1991, 74, 2343-2358.Abstract
Mullite (3Al2O3.2SiO2) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mol% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for low-temperature (< 1000-degrees-C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for low-temperature (1250-degrees to 1500-degrees-C) densification through viscous deformation. Flexural strength and creep resistance at elevated temperatures are significantly affected by the presence of glassy boundary inclusions; in the absence of glassy inclusions, polycrystalline mullite retains > 90% of its room-temperature strength to 1500-degrees-C and displays very high creep resistance. Because of its low dielectric constant, mullite has now emerged as a substrate material in high-performance packaging applications. Interest in optical applications mainly centers on its applicability as a window material within the mid-infrared range.
Sundaresan, S. ; Aksay, I. A. Mullitization of diphasic aluminosilicate gels. Journal of the American Ceramic Society 1991, 74, 2388-2392.Abstract
Recent studies have shown that the mullitization of diphasic aluminosilicate matrices comprising transitional alumina and amorphous silica occurs via a nucleation and growth process. Nucleation is preceded by a temperature-dependent incubation period. Following this incubation period, rapid nucleation of mullite occurs, producing about 1.8 x 10(11) nuclei/cm3, which remains constant throughout the rest of the transformation. Both incubation and mullite growth are thermally activated processes with apparent activation energies of 987 +/- 63 and 1070 +/- 200 kJ/mol, respectively. The growth rate of mullite grains under isothermal conditions is time dependent. An interpretation of these results is proposed on the basis of the nucleation and growth concepts of LaMer and Dinegar which supports the concept that the growth rate of mullite grains is controlled by the dissolution of transitional alumina into the amorphous matrix.
Aksay, I. A. ; Wiederhorn, S. M. Symposium for mullite processing, structure, and properties. Journal of the American Ceramic Society 1991, 74, 2341-2341.
Staley, J. T. ; Pellerin, N. ; Ren, T. ; Graff, G. L. ; Treadwell, D. R. ; Aksay, I. A. Use of acidic biopolymers as dispersants for processing of ceramics. Abstracts of Papers of the American Chemical Society 1991, 201, 146-IEC.
Liu, J. ; Shih, W. Y. ; Sarikaya, M. ; Aksay, I. A. Fractal colloidal aggregates with finite interparticle interactions - energy-dependence of the fractal dimension. Physical Review A 1990, 41, 3206-3213.
Shih, W. H. ; Shih, W. Y. ; Kim, S. I. ; Liu, J. ; Aksay, I. A. Scaling behavior of the elastic properties of colloidal gels. Physical Review A 1990, 42, 4772-4779.
Shih, W. Y. ; Shih, W. H. ; Aksay, I. A. Semidilute athermal polymer-solutions near a hard-wall. Macromolecules 1990, 23, 3291-3296.
Stangle, G. C. ; Aksay, I. A. Simultaneous momentum, heat and mass-transfer with chemical-reaction in a disordered porous-medium - application to binder removal from a ceramic green body. Chemical Engineering Science 1990, 45, 1719-1731.
Sundaresan, S. ; Aksay, I. A. Sintering with rigid inclusions - pair interactions. Journal of the American Ceramic Society 1990, 73, 54-60.
Choi, J. S. ; Sarikaya, M. ; Aksay, I. A. ; Kikuchi, R. Theory of oxygen diffusion in the YBa2Cu3O7-x superconducting compound. Physical Review B 1990, 42, 4244-4254.
Qian, M. ; Stern, E. A. ; Ma, Y. ; Ingalls, R. ; Sarikaya, M. ; Thiel, D. ; Kurosky, R. ; Han, C. ; Hutter, L. ; Aksay, I. NICKEL IMPURITIES IN THE 90K SUPERCONDUCTOR. Physica B-Condensed MatterPhysica B-Condensed Matter 1989, 158, 477-479.