Jon K. West

Processing materials with microwave energy

Abstract Microwave energy (microwave frequency, in this case, includes radio frequencies and ranges from 0.3 MHz to 300 GHz) is being developed as a new tool for high-temperature processing of materials. Examples of the advantages associated with microwave processing include: rapid and uniform heating; decreased sintering temperatures; improved physical and mechanical properties; and, unique properties which are not observed in conventional processes. These advantages observed in materials processed using microwave energy are being attributed to ‘microwave effects’ which are particular to this technology. Researchers at the University of Florida are working to identify and to qualitatively and quantitatively define the mechanisms of microwave–material interactions. A new model has been developed based on the molecular orbital model which predicts the behavior of specific pure materials in a microwave field. Experimental work as well as dielectric property measurements confirm the accuracy of this model in specific cases.

Chemical processing of advanced materials

Sol-Gel Science A Sol-Gel Route to Very High Porosity Silica Aerogels Kinetics and Equilibrium of Acid-Catalyzed Tetraethoxysilane Hydrolysis NMR Studies of the Sol-Gel Process Various Oxide and Multicomponent Systems The Preparation and Characterization of Sn-Si-O Gels Raman Spectroscopy of Phosphorous-Doped Silica Gels Sol-Gel Applications New Group 2 Organometallic Precursors to Metal Oxides Optical and Thermal Properties of Silica Aerogels Thin Films and Coatings: Ultrastructure Processing of Thin Crystalline Films Size Effects in Ferroelectric Thin Films Micromorphology Science Ultrastructural Polymers Chemically Processed Fibres and Composites Microwave Syntheses of Materials and Their Precursors Advanced Optical Materials Nanomodulated Ceramic Superlattices by Electrodeposition Future Directions.

Computational model for protein-mediated biomineralization of the diatom frustule

The mechanism of silicification from which the intricate cell walls of diatoms emerge, as well as the stereochemical relationship between the walls organic casing and the siliceous materials within, continue to elude current technology. The present study further develops Hecky et al.s standing model of the organicinorganic interface with semi-empirical computer simulations of a biosilicification pathway. Polycondensation reactions between silicic acid molecules and a hydroxyl-rich β-sheet protein template results in a stereochemically-compatible chemisorbed tetrasiloxane ring. The 24-stage reaction pathway has an activation barrier of +15.4 kcal mol-1 and results in a net stabilization of-28.0 kcal mol-1. Spatial matching and favorable thermodynamics support the theory of protein-mediated biomineralization of the diatom.

Interactions of water with trisiloxane rings. I. Experimental analysis

Abstract The structure of the pore surface of stabilized silica gels is dominated by D2 trisiloxane ring. The surface structure is investigated by curve-fitting Raman spectra measured during the adsorption of water. This allowed the concentration of the trisiloxane rings, [D2], and surface silanols, [SisOH], to be calculated. For a gel sintered at 650°C with a BET surface area of 640m2/g and a pore radius ≈1.2 nm, [D2] ≈ 1 D2/nm2 and [SisOH] ≈ 1.8 SisOH/ nm2 is found. After rehydration, the water content = 0.31 g/g, [D2] ≈ 0.38 D2/nm2 and [SisOH] ≈ 3.1 SisOH/nm2. During D2 ring hydrolysis, the [H2O molecules]/[D2 rings] ratio was found to be ≈ 24 at 25°C.

Molecular modeling study of adsorption of poly-L-lysine onto silica glass

A research program was initiated with both experimental and computational chemistry based molecular modeling components to investigate specific amino acid-surface interactions. The experimental portion of this study, with details reported elsewhere, investigated the adsorption of selected molecular weights of poly(L-lysine) onto silica glass microspheres with the adsorption enthalpy per adsorbed mer determined to be -0.23 +/- 0.13 kcal/mol (mean +/- 95% confidence interval). Molecular modeling of this system was then conducted using two approaches: an AM1 semiempirical molecular orbital method to predict L-lysine/glass interaction energy and an MM2 molecular mechanics method to investigate the structural configuration for poly(L-lysine). The modeling predicted a minimum energy configuration of a rotational backbone structure for poly(L-lysine) with approximately one full rotation occurring about every 8 mers, and that the amine side chains of the L-lysine will hydrogen bond with the silica surface with an average adsorption energy of approximately -0.34 kcal/mol/mer. The molecular modeling results are in good agreement with the experimentally measured value and provide insights into possible molecular-level behavior which would be very difficult to determine by experimental analyses alone. This work demonstrates the use of molecular modeling in conjunction with experimental studies to investigate complex molecular interactions.

A PM3 molecular orbital model of silica rings and their vibrational spectra

Abstract The PM3 molecular orbital model was applied to various silica rings from twofold to sixfold on size. From these optimized structures, the bridging oxygen (BO) and non-bridging oxygen (NBO) bond lenths, the SiOSi (φ) angles, the OSiO (θ) angles and final heats of formation were tabulated. These data were compared with earlier MNDO and ab initio molecular orbital calculations. The theoretical vibrational spectra were then calculated and compared with earlier work as well as the experimental IR response. The PM3 method was found to be a viable calculational tool for modeling silica structures.

Quantum chemistry of sol-gel silica clusters

Abstract Quantum calculations of rings and chains containing up to six hydrolized silica tetrahedra have been made using intermediate neglect of differential overlap (INDO) molecular orbital theory. Differences in molecular energies between rings and chains appears to be responsible for limiting the growth of particles in the sol prior to gelation. Changes in the structure and molecular energies of a 4-membered silicon ring with adsorption and desorption of H2O indicates a possible mechanism for the observed dilotometric behavior of porous Type VI sol-gel silica.

Interactions of water with trisiloxane rings. II. Theoretical analysis

Abstract The Austin model (AM1) semi-empirical molecular orbital (MO) modeling was used to calculate the reaction path for the hydrolysis of D2 trisiloxane rings. The lowest energy path was found using a dissociative proton-transfer model involving a transition state containing a pentavalent Si atom bonded to a H2O molecule containing a tricoordinate O atom. The H2O molecule acts a nucleophile, and adsorbs on an acidic Si atom in the D2 ring to form the pentavalent Si transition state. The activation energy, δE, for this exothermic reaction was calculated to be † E = 2.8 kcal/mol . This is very small, in agreement with the experimental analysis of the hydrolysis of trisiloxane rings on the internal pore surface of Type VI a-silica.

Effect of long-term in vitro testing on the properties of bioactive glass-polysulfone composites.

The combination of bioactive ceramics and polymers can allow the preparation of composites with tailorable mechanical properties and bioactive behavior. In these composites, bioactive ceramics can act as a source of both reinforcement and bioactivity, while the polymer matrix can add toughness and processability to the material. On the other hand, the effect of using a highly dimensional unstable phase as a reinforcing agent on the long-term properties of the composite is a major concern regarding the lifetime of possible applications. In this work, a bioactive glass-polysulfone particulate composite was prepared by hot-pressing at 215 degrees C a mixture of polysulfone and different concentrations of bioactive glass particles (Bioglass 45S5, particle size range: 125-106 microm) to yield composites having 20 and 40 vol % of bioactive glass particles. The obtained composites were exposed to a simulated body fluid at 37 degrees C for different periods of time ranging from 1 h to 60 days. After the test, the mechanical properties of the composites were investigated by a four-point bending test, while DMS (dynamic mechanical spectroscopy) was used to identify the effect of water on the structure and behavior of the composite. The interface between glass particles and the polymer was also investigated by SEM/EDX and diffuse reflection infrared spectroscopy. The results showed that a decay in the mechanical properties of the composites within the first 20 h of test can occur. Otherwise, after this initial decay, no more pronounced reduction in properties could be noted. The analyses of the fracture surface of composites tested in vitro indicated the hydration of the surface of the particles. Therefore, it was concluded that water migration through the interface of the composite causes surface dissolution of glass particles and formation of voids, which were responsible for the observed decay in mechanical properties. Composites with modified interfaces revealed less damaged fracture surfaces than composites with untreated interfaces.

The UV-visible spectrum in porous type VI silica : application and theory

The ultraviolet (UV) through visible (VIS) absorptions are determined and analyzed for porous type VI sol-gel-derived silica optical matrices. Changes in the UV-VIS spectral response are related to changes in the texture and molecular structure of the gel silica. There is a significant increase in UV cutoff as the pore size increases from 12 A to 90 A radius pores. This result is analyzed using classical scattering theory in combination with semi-empirical molecular orbital (MO) calculations. The concentration of small (three- and four-member) silica rings is a function of the processing temperature and history. As a result the UV absorption is observed to move to larger wavelengths as the number of three-member silica rings increases.