Synthesis of Glass-like Cordierite from Metal Alkoxides and Characterization by 27Al and 29Si MASNMR

A sol-gel process for the preparation of glass-like cordierite and ceramic oxide powder is described. Metal alkoxide precursors were dissolved in 2-methoxyethanol and the metal cations were complexed using 2,4-pentanedione to overcome different hydrolysis rates of metal alkoxides which lead to microscopic inhomogeneities during gelation. Heating the gel in an aerated furnace resulted in ultrafine cordierite powder of stoichiometric composition at a relatively low temperature. Cordierite gel and glass were also prepared by other methods and compared with the above gel. The environments of aluminum and silicon in the glass and gels heated at various temperatures were studied using ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Most of the heated gels and the glass showed resonances due to pentacoordinated aluminum along with 4- and 6-coordinated aluminum. The homogeneities of the resulting gels and glass are compared using the MASNMR data.     

New metal-rich germanide and carbide non-crystalline solids

Non-crystalline metal-rich (Au, Pt, W, Ta, Fe, Ni, Sn) germanide and (Au, Pt, Ir, Fe, Sn, Cd) carbide films were prepared by rf-sputtering. Compositions as high as 60 at.% metal could be made without any special techniques for substrate cooling. A number of these films crystallized above 400°C. No phase separation was detected by either electron microscopy or heat treatment studies.     

Preparation and properties of sputtered “glassy” carbon films

Some remarkable “glassy” carbon films with densities from 0.5 to 1.79 have been prepared by sputtering from carbon targets. Conditions for the preparation of various carbon films by rf sputtering and data on the density, mechanical properties, microhardness, microstructure and crystal structure of the films formed are presented. Optimum sputtering conditions for obtaining mechanically stable films up to 40μ thick are described. It is shown that density and microhardness of the carbon can be controlled within wide limits, and both increase with decreasing deposition rate. The higher density carbon films adhere poorly to glass, while the low density carbon films on glass are stable. All sputtered carbon films are non-crystalline and have a grainy microstructure. Potential applications exclude corrosion protection but include novel catalyst supports.     

Thermal Expansion Anisotropy and Acoustic Emission of NaZr2P3O12 Family Ceramics

Most members of the NaZr₂P₃O₁₂ (NZP) family possess low, near zero, overall thermal expansion coefficients. However, they also exhibit anisotropy of axial thermal expansion. Some compounds have opposite anisotropy; for example, the a parameter of CaZr₄P₆O₂₄ contracts on heating and that of SrZr₄P₆O₂₄ expands, while the c parameter expands for the Ca compound and contracts for the Sr compound. The anisotropy of the axial thermal expansion of these materials is believed to induce microcracking. The acoustic emission method was employed here to detect microcracking in ceramics due to the axial thermal expansion anisotropy. Acoustic signals were observed during cooling of the Ca and Sr compounds from 500°C, and Na and K compounds from 600°C. On the other hand, no acoustic emission signal is detected in Ca_0.5Sr_0.5Zr₄P₆O₂₄ ceramics, in which the lattice parameters a and c remain nearly unchanged in the temperature range of room temperature to 500°C. Thus, a direct correlation between microcracking of ceramics and their anisotropic axial thermal expansion coefficients was established by employing acoustic emission monitoring techniques.     

Enhanced Densification by Seeding of Sol-Gel-Derived Aluminum Titanate

Aluminum titanate was prepared by two different methods. A monophasic gel was obtained by cohydrolysis and polycondensation reactions of chelated aluminum sec -butoxide and titanium isopropoxide. A diphasic gel was obtained from boehmite and anatase. Effects of homogeneous dispersion of magnesium oxide and iron oxide incorporated as alkoxides in the monophasic gel and also the effects of isostructural seed on the crystallization and densification of aluminum titanate obtained from monophasic and diphasic gels were investigated. Seed particles of MgTi₂O₅ facilitated lowering of crystallization temperature and enhanced densification of aluminum titanate. Microstructure studies indicated that almost-microcrack-free sintered bodies with smaller grains were formed from the seeded monophasic and diphasic gels.     

Alternative Radwaste Solidification Route for Three Mile Island Wastes

Present plans being considered for Three Mile Island (TMI) wastes involve radionuclide adsorption on zeolites, transportation, and possible melting with pit into glass. Several zeolites were treated with a simulated TMI waste and directly sintered at 1000°C to fix the adsorbed ions. The Cs leach rates of the sintered products are similar to those of borosilicate glasses. Thus, simple sintering (on-site) of zeolites may be a cost-effective and less cumbersome alternative for such waste solidification.     

Effect of lattice disorder and strain on TC in sputtered “Nb3Ge” thin films

Disorder and strain introduced into sputtered “Nb₃Ge” films by mechanical polishing slightly lowers the onset temperature and/or very markedly broadens the width of the superconducting transition. The structural damage is reversible and annealing restores the superconducting behavior of the film very nearly to its initial state.