Judy Odinek

Monolithic periodic mesoporous silica gels

We have synthesized monolithic, surfactant-templated particulate gels that have pore volumes and surface areas comparable to silica xerogels and aerogels. The gels have a complex microstructure with micro-, meso- and macroscopic features that emerge over five orders of magnitude in length (1) amorphous silica walls (characterized by a broad distribution of Si-Si spacings of ≈0.4 nm), (2) periodic hexagonal arrays of 1-d channels within each particle (≈3 nm channel diameter), (3) a feature in the gas adsorption measurements that indicates a second class of ≈10–50 nm diameter mesopores, (4) particles that are ≈ 150 to ≈500 nm in diameter, (5) interparticle pores that are on the order of the particle size, and (6) fractal domains larger than the particle size (>10 000nm). The microstructure can be controlled by the varying the initial silica content, the template size, the drying conditions, or the calcination conditions. The wet gel monoliths exhibit calculated densities as low as ≈0.02 g/cm3; the dried and calcined gels have bulk densities that range from 0.3 to 0.5 g/cm3. The materials possess large interparticle (1.1–2.2 cm3/g) and intraparticle (0.3–0.6 cm3/g) porosities.

A light‐scattering study of the nonlinear dynamics of electrorheological fluids in oscillatory shear

We report a real time, two‐dimensional light‐scattering study of the nonlinear dynamics of field‐induced structures in an electrorheological (ER) fluid subjected to oscillatory shear. We have developed a simple kinetic chain model of the observed dynamics by considering the response of a fragmenting/aggregating particle chain to the prevailing hydrodynamic and electrostatic forces. This theory of the dynamics is then used to describe the nonlinear rheology of ER fluids, and it is concluded that light scattering allows one to indirectly probe the stress in an ER fluid.

Characterization of Fe/KClO4 Heat Powders and Pellets

Pellets of Fe/KClO4 mixtures are used as a heat source for thermally activated (“thermal”) batteries. They provide the energy necessary for melting the electrolyte and bringing the battery stack to operating temperature. The effects of morphology of the Fe and the heat-pellet density and composition on both the physical properties (flowability, pelletization, and pellet strength) and the pyrotechnic performance (burn rate and ignition sensitivity) were examined using several commercial sources of Fe.

Synthesis of Periodic Mesoporous Silica Thin Films

We have synthesized periodic mesoporous silica thin films from homogeneous solutions. To synthesize the films, a thin layer of a pH 7 micellar coating solution that contains TMOS (tetramethoxysilane) is dip or spin-coated onto Si wafers, borosilicate glass, or quartz substrates. NH3 gas is diffused into the solution and causes rapid hydrolysis and condensation of the TMOS and the formation of periodic mesoporous thin films within 10 seconds. Combination of homogenous solutions and rapid product formation maximizes the concentration of the desired product and provides a controlled, predictable microstructure. The films have been made continuous and crack-free by optimizing initial silica concentration and film thickness. The films are being evaluated as high surface area, size-selective coatings for surface acoustic wave (SAW) sensors.

On the turbidity of silica gels: Aggregation in the presence of coulomb interactions

Abstract Silica gels are readily formed by the hydrolysis and condensation of a dilute solution of a silicon alcoxide under acidic or basic conditions. Sols formed from basic solutions of tetramethoxysilicon, where the hydrolysis products (silicic acids) are negatively charged, condense very slowly and yield rigid, transparent gels. However, we have found that the addition of small amounts of salt catalyzes the reaction by several orders of magnitude, and leads to opaque gels. We present a quasielastic and elastic light scattering study of the growth and structure of silica clusters that indicates that the increase in the turbidity is due to an increase in the fractal dimension of the dilute sol, due in turn to the ionic screening of Coulomb interactions.

Synthesis of surfactant-templated mesoporous materials from homogeneous solutions

Mesoporous is defined as 20{le}d{le}500{angstrom}. Mesoporous materials with narrow pore size distributions may be useful as hosts, supports, catalysts, or separation media for small molecules. An ensemble of organic molecules to create a larger template has been used to synthesize ordered mesoporous materials. The silicon alkoxide precursors TEOS and TMOS were examined. Cosolvents were used to control pore size and the structure of the mesophase. Effects of anions (salts) on mesophase formation were examined. Properties of mesophases made from homogeneous solutions are discussed.

LIGHT SCATTERING STUDIES OF THE ELECTRORHEOLOGICAL TRANSITION

Abstract Real-time, two-dimensional light scattering studies of the evolution of structure in an electrorheological fluid in the quiescent state and under shear are reported. It is found that when an electric field is applied to the quiescent fluid, particles chain along the electric field lines and cause strong light scattering lobes to appear at a finite scattering wavevector, q , orthogonal to the field lines. These lobes then brighten as they move to q = 0, indicating the existence of an unstable concentration fluctuation that signifies the segregation of chains into columns. In fact, the observed power law growth kinetics of the characteristic length, as well as the form of the structure factor, are qualitatively similar to two-dimensional spinodal decomposition in a system with a conserved order parameter. When the sample is subjected to shear, we find that the scattering pattern approaches a steady state, with lobes that are rotated in the direction of fluid vorticity. The angle of rotation is found to increase as the cube root of the shear rate, in agreement with a theoretical prediction of the steady state structure of fragmenting particle chains.

LIGHT SCATTERING STUDIES OF AN ELECTRORHEOLOGICAL FLUID IN OSCILLATORY SHEAR

We have conducted a real time, two-dimensional light scattering study of the nonlinear dynamics of field-induced structures in an electrorheological fluid subjected to oscillatory shear. We have developed a kinetic chain model of the observed dynamics by considering the response of a fragmenting/aggregating particle chain to the prevailing hydrodynamic and electrostatic forces. This structural theory is then used to describe the nonlinear rheology of ER fluids.

Periodic mesoporous silica gels

We have synthesized monolithic particulate gels of periodic mesoporous silica by adding tetramethoxysilane to a homogeneous alkaline micellar precursor solution. The gels exhibit 5 characteristic length scales over 4 orders of magnitude: fractal domains larger than the particle size (>500 nm), particles that are {approximately}150 to 500 nm in diameter, interparticle pores that are on the order of the particle size, a feature in the gas adsorption measurements that indicates pores {approximately}10-50 nm, and periodic hexagonal arrays of {approximately}3 nm channels within each particle. The wet gel monoliths exhibit calculated densities as low as {approximately}0.02 g/cc; the dried and calcined gels have bulk densities that range from {approximately}0.3-0.5 g/cc. The materials possess large interparticle ({approximately}1.0-2.3 cc/g) and intraparticle ({approximately}0.6 cc/g) porosities.

Light Scattering Studies of an Electrorheological Fluid in Oscillatory Shear

We have conducted a real time, two-dimensional light scattering study of the nonlinear dynamics of field-induced structures in an electrorheological fluid subjected to oscillatory shear. We have developed a kinetic chain model of the observed dynamics by considering the response of a fragmenting/aggregating particle chain to the prevailing hydrodynamic and electrostatic forces. This structural theory is then used to describe the nonlinear rheology of ER fluids.