Publications by Year: 1997

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.
Vartuli, J. S. ; Milius, D. L. ; Li, X. P. ; Shih, W. Y. ; Shih, W. H. ; Prudhomme, R. K. ; Aksay, I. A. Multilayer electromechanical composites with controlled piezoelectric coefficient distribution; Simmons, W. C. ; Aksay, I. A. ; Huston, D. R., Ed. Spie - Int Soc Optical Engineering: Bellingham, 1997; Vol. 3040, pp. 93-98.Abstract
We have fabricated multilayer electromechanical composites with controlled piezoelectric coefficient distributions using tape casting. Tapes of doped lead zirconate titanate were cut and stacked in accordance with their characteristic electromechanical coupling values and modulus of elasticity. This technique is an extremely versatile method to fabricate displacement actuators to fabricate monolithic ceramic parts with controlled material property gradients. To obtain a quantifiable method to optimize this type of transducer, we have devised a processing model. Given the functional distribution of the electromechanical coupling coefficient, d(31), and the functional distribution of elastic modulus through the thickness of the transducer, the analysis predicts the displacement as a function of loading. The tape casting method coupled with the model provides an actuator that maximizes displacement and generated force for the given material properties.
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.
Aksay, I. A. ; Milius, D. L. ; Vicenzi, E. P. ; Lettow, J. S. Preparation of ceramic superconductor single crystals, 1997.Abstract
A single crystal of a high-temp. superconductor of the formula MBa2Cu3O7-x, where M = Y, Sm, Eu, Gd, Dy, Ho, Er, or Yb and x = 0.1-1.0, is prepd. by forming a starting powder by combustion spray pyrolysis, then growth of a crystal on a settling powder of the compn. Ba4Cu2PtOx and/or controlled isothermal growth. [on SciFinder(R)]
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.