Showing papers in "Chemistry of Materials in 1991"

Structural chemistry and Raman spectra of niobium oxides

Abstract: The pyrolysis of TVSb has been investigated in a flow tube reactor using Dz and He carrier gases. For TVSb alone, the most likely pyrolysis reaction involves an Sbcentered reductive elimination pathway. A less likely possibility is pyrolysis via homolysis of the Sb-C bonds, yielding vinyl radicals. Unfortunately, examination of the organic byproducts in both He and D, yields insufficient information to form a definitive hypothesis. However, in He the pyrolysis rate for TVSb is more rapid than for TMSb. Since vinyl radicals form stronger bonds than methyl radicals, this datum contradicts the Sb-C bond homolysis mechanism. Again, the activation energy for pyrolysis is less than the expected Sb-vinyl bond strength. Finally, the addition of C7D, produces no CH,=CHD, indicative of the absence of vinyl radicals. To elucidate our understanding of GaSb growth by using TMGa and TVSb, the pyrolysis rates for this combination of reactants were also studied. CH, radicals from (CH3N), pyrolysis were found to enhance TVSb pyrolysis in He. TMGa also increases the TVSb pyrolysis rate, mainly due to the methyl radicals produced. A heterogeneous pyrolysis reaction appears a t high surface area. At V/III ratios normally used for OMVPE growth, carbonaceous deposits were formed. Thus, TVSb may be a useful precursor for OMVPE only a t V/III ratios less than unity.

α'-Sialon ceramics : a review

Abstract: a' -Sialon ceramics promise the possibility of a reduction of the amount of glassy grain boundary phases by incorporating the oxides, present in the starting mixture as either sintering additives or impurities, into its rmallattice. It has been shown that techniques such as gas pressure sintering can be used successfully to obtain fully reacted and dense a'- or (a' + s')-sialons. By combination of suitable modifier cations and heat treatment the amount of grain boundary phases can be kept very low. This may lead to improved mechanical properties, especially at elevated temperatures. The equiaxed microstructure of a' -sialons, compared with the needlelike structure of s'sialons, leads to specific differences in mechanical behavior. However, at this moment the complicated interrelationships between the formation sequence, fabrication conditions, properties, and microstructures are still insufficiently understood. Yet, even with the resent-day materials, wear resistance, high-temperature mechanical strength, thermal shock resistance, and oxidation resistance are such that further studies are worth-while.

Molecular and Supermolecular Origins of Enhanced Electronic Conductivity in Template-Synthesized Polyheterocyclic Fibrils. Part 1. Supermolecular Effects

Abstract: : The pores in a nanoporous membrane can be used as templates for the synthesis of nanostructures. We have recently shown that conductive polymer fibrils, obtained via this template synthetic method, can show dramatically higher electronic conductivities than conventional versions of the analogous polymers. In this and a succeeding paper we explore the molecular and supermolecular origins of this enhanced electronic conductivity. This paper focuses on supermolecular effects. We have used DC and optical measurements of conductivity, X-ray diffraction, and polarized infrared absorption spectroscopy to show that the polymer chains in the narrowest template-synthesized fibrils are preferentially oriented parallel to the axes of these fibrils. This preferential polymer chain orientation is partially responsible for the observed conductivity enhancements. We also show that template-synthesis can yield poly(3-methylthiophene) fibrils with conductivities as high as 6600 S cm-1. This is the highest conductivity ever reported for a heterocyclic polymer.

Volatile Barium Beta-Diketonate Polyether Adducts. Synthesis, Characterization and Metalorganic Chemical Vapor Deposition

Abstract: : The 2,5,8,11,14-pentaoxaheptadecane (tetraglyme) adducts of barium complexes of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (H(hfacac(), 1,1,1,- trifluoro-2,4-pentanedione (Htfacac) and 2,2,6,6-tetramethyl-3,5-heptanedione (H(thd)) have been prepared and characterized by X-ray crystallography, H and C nmr spectroscopies, melting and sublimation points, and elemental analysis. The thermal stability of the complexes is related to their solid state structure in which the coordination number around the barium atom is nine with a monocapped, twisted square prism structure. Metalorganic chemical vapor deposition experiments were performed using the 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate and 1,1,1-trifluoro-2,4-pentanedionato complex; the 2,2,6,6-tetramethyl-3,5- heptanedionate complex dissociated prior to sublimation. The as-deposited barium fluoride films were characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy.