Publications by Year: 1995

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.
Vicenzi, E. P. ; Lettow, J. S. ; Milius, D. L. ; Aksay, I. A. Platinum distribution in large single crystals of YBa2Cu3O7-x; Etz, E. S., Ed. V C H Publishers: New York, 1995; pp. 421-422.
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.