Edward Mark Russick

Alkylene-bridged polysilsesquioxane aerogels: highly porous hybrid organic-inorganic materials

Abstract Alkylene-bridged polysilsesquioxane gels were prepared by sol-gel polymerizations of α, ω-bis(triethoxysilyl)alkanes 1–5. The gels were extracted with supercritical carbon dioxide to afford a novel class of hybrid organic-inorganic aerogels. The effect of the length of the alkylene bridging group and catalyst (HCl and NaOH) on the structure was examined. The molecular structure was characterized by solid-state 13C and 29Si cross polarization magic angle spinning nuclear magnetic resonance spectroscopy. The alkylene bridging groups survived sol-gel polymerization to give materials with average degrees of condensation of 79 and 90% for the acid- and base-catalyzed aerogels, respectively. Scanning electron microscopy was used to examine the macroscopic structure of the gels and nitrogen sorption porosimetry was used to measure their surface areas and pore structures. Most of the alkylene-bridged aerogels were mesoporous, high-surface-area materials. As with alkylene-bridged polysilsesquioxane xerogels, the surface area decreased with increasing alkylene bridging group length. Only the base-catalyzed tetradecylene-bridged aerogel was found to be non-porous.

Synthesis of titanium dioxide particles in supercritical CO2

Abstract Spherical particles of titanium dioxide, anatase, were prepared in a supercritical carbon dioxide medium from titanium alkoxides and water. The dissolution of the alkoxide and stabilization of water dispersions in supercritical CO2 were found to be required for the formation of spherical particles. An anionic fluorinated surfactant was used to stabilize water dispersions in supercritical CO2. This could not be realized with hydrocarbon-based surfactants. The solubility of titanium alkoxides in CO2 appears to parallel their vapor pressure which is dependent on the oligomerization of the unhydrolyzed alkoxides. The polydispersity in particle sizes is due to nucleation occurring simultaneously with particle growth, owing to changes in the degree of supersaturation during the transition from the liquid to the supercritical state.

Corrosive effects of supercritical carbon dioxide and cosolvents on metals

Abstract With the eventual phase-out of chlorofluorocarbons and hydrochlorofluorocarbons, and restrictive regulations concerning the use of other volatile organic compounds as cleaning solvents, it is essential to seek new, environmentally acceptable cleaning processes. We are investigating supercritical carbon dioxide (C02) as an alternative solvent for precision cleaning of machined metal parts in governmental and industrial cleaning processes. The compatibility of metals in supercritical-fluid cleaning media with respect to corrosion must be addressed. In this work, a screening study of the corrosive effects of supercritical CO 2 and several supercritical cosolvents on selected metals was conducted. Sample coupons of stainless steel (grades 304LSS, 316SS), aluminum (grades 2024, 6061, 7075), carbon steel (1018), and copper (CDA 101) were statically exposed to pure supercritical C0 2 , water-saturated supercritical C02, 10 wt % methanol/CO 2 cosolvent, and 4 wt % tetrahydrofurfuryl alcohol (THFA)/C0 2 at 24,138 kPa (3500 psig) and 323 K (50 °C) for 24 h. Gravimetric analysis and magnified visual inspection of the coupons were performed before and after the exposure tests. Surface analyses including electron microprobe analysis (EMPA), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) were done where visual and gravimetric changes were indicative of corrosive attack. The metal alloys were found to be compatible with the supercritical test media barring a few exceptions. Corrosive attack was observed on 1018 carbon steel in the water-saturated CO 2 environment, and also on 2024 aluminum and CDA 101 copper, both in the 10 wt % methanol-CO 2 cosolvent. The results of all compatibility testing are reported, and hypotheses are formed in an attempt to explain possible corrosion mechanisms.

Supercritical carbon dioxide solvent extraction from surface-micromachined micromechanical structures

Results are presented supporting the use of supercritical carbon dioxide (SCCO2) drying to enhance the yield of surface-micromachined micromechanical devices following the final release etch. The equipment and extraction process of the SCCO2 system are described, and results of successfully released cantilevered beams and microengines are presented. A new system capable of drying 6 inch wafers is also described.

Preparation of aryl-bridged polysilsesquioxane aerogels

We report the preparation of a new class of organic/inorganic hybrid aerogels from aryl-bridged polysilsesquioxanes. 1,4- Bis(triethoxysilyl)benzene and 4,4{prime}-bis(triethoxysilyl)- biphenyl were sol-gel processed to form phenyl- and biphenyl-bridged polysilsesquioxane gels. The gels were then dried using supercritical carbon dioxide extraction. It was discovered that aryl-bridged polysilsesquioxane aerogels are indeed formed, but with a pronounced influence on surface area from the reaction conditions used in preparing the initial gels. Specifically, high surface area aerogels (up to 1750 m{sup 2}/g) are obtained from gels prepared with either acid or base catalysts. With reduced concentrations of base catalyst, however, supercritical processing afforded phenyl-bridge xerogel-like materials. The materials were characterized by nitrogen sorption surface analysis and by transmission electron microscopy. 8 refs., 5 figs., 1 tab.

Experiments for foam model development and validation.

A series of experiments has been performed to allow observation of the foaming process and the collection of temperature, rise rate, and microstructural data. Microfocus video is used in conjunction with particle image velocimetry (PIV) to elucidate the boundary condition at the wall. Rheology, reaction kinetics and density measurements complement the flow visualization. X-ray computed tomography (CT) is used to examine the cured foams to determine density gradients. These data provide input to a continuum level finite element model of the blowing process.

Electrooptic Inspection of Vector Leakage in Radiofrequency Multichip Modules

This paper demonstrates a new approach for the measurement of vector electromagnetic leakage within tightly integrated electronic subsystems. Specifically, the effect of shielding and encapsulation techniques commonly used in radiofrequency (RF) multichip modules is investigated using an electrooptic (EO) near-field measurement system. The test vehicle used for this study is a single-chip 50-W microwave power amplifier integrated into a multilayer low-temperature cofired ceramic module. The measured near-field data show power intensity at 2.5 GHz with up to 0.2-mm resolution for each of the three x-, y-, and z-directed electric field vectors. The resulting images illustrate 1) the polarization-dependent shortcomings of the ubiquitous “via-fence” shielding technique and 2) the formation of surface waves in microstrip substrates due to impedance mismatch. These results are evidence of the applicability of the EO technique to the diagnosis of electromagnetic compatibility issues within compact RF multichip modules.