Publications by Year: 1999

Aksay, I. A. ; Trau, M. ; Manne, S. ; Honma, I. Biomimetic pathways for assembling inorganic thin films and oriented mesoscopic silicate patterns through guided growth, 1999.Abstract
A process directed to prepg. surfactant-polycryst. inorg. nanostructured materials having designed microscopic patterns. The process includes forming a polycryst. inorg. substrate having a flat surface and placing in contact with the flat surface of the substrate a surface having a predetd. microscopic pattern. An acidified aq. reacting soln. is then placed in contact with an edge of the surface having the predetd. microscopic pattern. The soln. wicks into the microscopic pattern by capillary action. The reacting soln. has an effective amt. of a silica source and an effective amt. of a surfactant to produce a mesoscopic silica film upon contact of the reacting soln. with the flat surface of the polycryst. inorg. substrate and absorption of the surfactant into the surface. Subsequently an elec. field is applied tangentially directed to the surface within the microscopic pattern. The elec. field is sufficient to cause electro-osmotic fluid motion and enhanced rates of fossilization by localized Joule heating. [on SciFinder(R)]
Chang, H. L. ; Chun, C. M. ; Aksay, I. A. ; Shih, W. H. Conversion of fly ash into mesoporous aluminosilicate. Industrial & Engineering Chemistry Research 1999, 38, 973-977.Abstract
Mesoporous aluminosilicate in the hexagonal phase (MCM-41) has been synthesized from fused fly ash solutions and cationic cetyltrimethylammonium bromide (CTAB) surfactants. We provide direct evidence that an MCM-41 aluminosilicate with a homogeneous chemical composition of Si/Al = 13.4 can be prepared with cationic surfactant. Our results indicate that coal combustion byproducts can be utilized for producing mesoporous molecular sieves even though they contain significant amounts of impurities.
Fong, H. F. ; Bourges, P. ; Sidis, Y. ; Regnault, L. P. ; Bossy, J. ; Ivanov, A. ; Milius, D. L. ; Aksay, I. A. ; Keimer, B. Effect of nonmagnetic impurities on the magnetic resonance peak in YBa2Cu3O7. Physical Review Letters 1999, 82, 1939-1942.Abstract
The magnetic excitation spectrum of a YBa2Cu3O7 crystal containing 0.5% of nonmagnetic (Zn) impurities has been determined by inelastic neutron scattering. Whereas in the pure system a sharp resonance peak at E similar or equal to 40 meV is observed exclusively below the superconducting transition temperature T-c, the magnetic response in the Zn-substituted system is broadened significantly and vanishes at a temperature much higher than T-c. The energy-integrated spectral weight observed near q = (pi, pi) increases with Zn substitution, and only about half of the spectral weight is removed at T-c.
Shih, W. Y. ; Shih, W. H. ; Aksay, I. A. Elastic and yield behavior of strongly flocculated colloids. Journal of the American Ceramic Society 1999, 82, 616-624.Abstract
We have investigated theoretically the elastic and yield behaviors of strongly flocculated colloids by first examining the yield forces between two particles within the framework of Derjaguin-Landau-Verwey-Overbeck (DLVO) interactions. Under highly attractive conditions, i,e,, in the absence of the secondary minimum in the DLVO potential, the radial (tensile) motion between particles is nonelastic because of the lack of an inflection point in the DLVO potential. However, the lateral (shear) motion is shown to be elastic up to a distance y(max), providing a mechanism for the observed elasticity in colloidal gels. If r(0) and s(0) are, respectively, the closest center-to-center and surface-to-surface distances between two particles, y(max) proportional to (1- 0.5 alpha zeta(2))(s(0)r(0))(1/2) where zeta is the zeta potential of the particles and a a defined constant. Moreover, the yield force between two particles is much smaller in the lateral direction than in the radial direction. These results suggest that yielding of a particulate network is likely to occur through the lateral movements between particles. The yield strain can be approximated as that at which all the bonds in a certain direction have a perpendicular displacement >y(max), resulting in epsilon(yield) = y(max)/r(0) proportional to (1 - 0.5 alpha zeta(2))(s(0)/r(0))(1/2) The shear modulus of the network, G', can be deduced by combining the elastic constant of the lateral movement with the existing elastic theory of a particulate network. The yield stress can be approximated as sigma(yield) approximate to G'epsilon(yield) proportional to (1 - 1.5 alpha zeta(2))A/24s(0)(3/2) 1/Rd-3/2 where A is the Hamaker constant and R the particle radius. These predictions are shown to compare favorably with existing experiments.
Torquato, S. ; Yeong, C. L. Y. ; Rintoul, M. D. ; Milius, D. L. ; Aksay, I. A. Elastic properties and structure of interpenetrating boron carbide aluminum multiphase composites. Journal of the American Ceramic Society 1999, 82, 1263-1268.Abstract
We study the elastic moduli and structure of boron carbide/aluminum (B4C/Al) multiphase composites using rigorous bounding and experimental characterization techniques. We demonstrate that rigorous bounds on the effective moduli are useful in that they can accurately predict (i) the effective elastic moduli, given the phase moduli and volume fractions, or (ii) the phase moduli (volume fractions), given the effective moduli and phase volume fractions (moduli). Using the best available rigorous bounds on the effective elastic moduli of multiphase composites involving volume-fraction information, we are able to predict the bulk and shear moduli of the AI,BC phase, a reaction product that forms during heat treatment. These theoretical predictions are in very good agreement with recent experimental measurements of the moduli of the AI,BC phase. Moreover, we evaluate more-refined bounds involving three-point structural correlation functions by extracting such information from an image of a sample of the B4C/Al composite. Although experimental data for the effective moduli are unavailable for this sample, our predictions of the effective moduli based on three-point bounds should be quite accurate.
Li, X. P. ; Shih, W. Y. ; Aksay, I. A. ; Shih, W. H. Electromechanical behavior of PZT-brass unimorphs. Journal of the American Ceramic Society 1999, 82, 1733-1740.Abstract
We have examined the static and dynamic electromechanical responses of PZT-brass (piezoelectric-nonpiezoelectric) plate unimorphs of various brass/PZT thickness ratios. The study was performed both experimentally and theoretically. The static measurements showed that, given a PZT layer thickness, there exists a brass/PZT thickness ratio that gives the unimorph the highest static displacement under an applied field. The effects of geometric shape and external loading on the displacement of the unimorphs were also examined. The dynamic measurements showed that, given a PZT layer thickness, the bending-mode resonance frequencies increase with an increasing brass/PZT thickness ratio. These results were in good agreement with the theoretical predictions that were obtained with the plate geometry.
Dabbs, D. M. ; Yao, N. ; Aksay, I. A. Nanocomposite mullite/mullite powders by spray pyrolysis. Journal of Nanoparticle Research 1999, 1 127-130.Abstract
A mullite/mullite nanocomposite powder has been synthesized, composed of nanometer-size 3Al(2)O(3)center dot 2SiO(2) ('3 : 2') mullite precipitates within a matrix of the high alumina 2Al(2)O(3)center dot SiO2 ('2 : 1') mullite. Historically, the transition from the metastable high-alumina phase to the thermodynamically stable '3 : 2' phase of mullite has been thought to be a continuous process, involving a continuous solid solution between the two forms of mullite. In contradiction to this widely held view, our high resolution transmission electron microscopic characterization confirms that a first order phase transition between two distinct mullites occurs. The high degree of interface coherence between the precipitates and the matrix allows us to speculate that the mechanical properties of the matrix could be enhanced by a process similar to the precipitation hardening of metals.
Aksay, I. A. ; Vicenzi, E. P. ; Milius, D. L. ; Lettow, J. S. Producing ceramic superconductor single crystals, 1999.Abstract
A bulk high-temp. superconductor single crystal MBa2Cu3O7-x, where M = Y, Sm, Eu, Gd, Dy, Ho, Er, or Yb; and x = ∼0.1 to ∼1.0, are produced by a novel process incorporating: (i) starting powders produced by combustion spray pyrolysis; (ii) a novel setter powder; and/or (iii) a monitored isothermal growth process. [on SciFinder(R)]
Keimer, B. ; Bourges, P. ; Fong, H. F. ; Sidis, Y. ; Regnault, L. P. ; Ivanov, A. ; Milius, D. L. ; Aksay, I. A. ; Gu, G. D. ; Koshizuka, N. Resonant spin excitations in YBa2Cu3O6+x and Bi2Sr2CaCu2O8+delta. Journal of Physics and Chemistry of Solids 1999, 60, 1007-1011.Abstract
A summary of some recent results of neutron scattering studies of high temperature superconductors is given, with a focus on resonant spin excitations in the superconducting states of YBa2CU3O6+x and Bi2Sr2CaCu2O8+delta. The opportunities offered by advances in neutron scattering instrumentation, such as focusing and polarization techniques, for these experiments are discussed, (C) 1999 Published by Elsevier Science Ltd. All rights reserved.
Cates, Gordon D., J. ; Aksay, I. A. ; Happer, W. ; Hsu, M. F. ; Dabbs, D. M. Sol-gel coated polarization vessels, 1999.Abstract
The invention relates to a polarization cell which is coated with glass deposited from a sol-gel used for hyperpolarizing noble gases. The invention also includes a method for hyperpolarizing noble gases utilizing the polarization cell coated with glass deposited from a sol-gel. These polarization cells can also be incorporated into containers used for storage and transport of the hyperpolarized noble gases. [on SciFinder(R)]
Patrick, H. N. ; Warr, G. G. ; Manne, S. ; Aksay, I. A. Surface micellization patterns of quaternary ammonium surfactants on mica. Langmuir 1999, 15, 1685-1692.Abstract
The microscopic equilibrium structures of adsorbed films of quaternary ammonium surfactants on mica have been investigated by noncontact atomic force microscopy imaging as a function of alkyl chain length and headgroup structure. Spherical and cylindrical surface micelles were observed; these were found to be related to bulk. solution self-assembly and the surfactant packing parameter, v/a(0)l(c). Shape transitions in the surface aggregates were observed on changing the counterion between chloride, bromide, and salicylate.