John B. Wachtman

Electronic structure and bonding in silicon oxynitride films: An XPS study

Abstract Silicon oxynitride films on sapphire as well as silicon made by RF sputtering of silicon in a reactive atmosphere of O2 and N2 have been examined by X-ray photoelectron spectroscopy. Bremsstrahlung X-rays from the Mg anode have sufficient intensity to initiate the Si(KL2.3L2.3) Auger line, whose chemical line shift is about three times that of the Si(2p) level. There is a continuous shift in the Si(KLL) line as the oxygen in SiO2 is replaced by nitrogen. From the peak shape and shifts of Si(KLL), N(KLL), O(KLL), Si(2p) and the valence band, we show that oxygen and nitrogen are mixed locally on an atomic scale. The thin films do not consist of regions of SiO2 and Si3N4. This mixed condition persists even up to 1000°C.

A comparative study of processing methods and property relations of Y-Ba-Cu-O superconductors

Superconducting YBa/sub 2/Cu/sub 3/O/sub 6+x/ samples were made from powders prepared by three different routes: solid state reaction, coprecipitation of all components from nitrates and coprecipitation of Y and Ba oxalates on CuO particles. The powders produced by solid-state reaction were given different degrees of milling, calcined and either sintered or hot-pressed. The powders synthesized from solutions were precipitated either by mixing or spraying and were subsequently sintered and then annealed in oxygen at 600 C. In all cases, sintering at 920 C was found to give the lowest normal state resistivity. Samples hot pressed at 875 C required a subsequent annealing at higher temperatures to regain the superconducting phase. The solid-state reaction and coated-CuO methods gave single-phase material after sintering. Samples made from the conventional coprecipitation method always showed some nonsuperconducting phases. The latter specimens did exhibit superconductivity with T/sub c/ values of about 70 K; subsequent annealing did not significantly improve T/sub c/. Optimized attrition milled samples and those made by the coated-CuO method gave the lowest values of resistivity in the normal state. The extent of corrosion by interaction with water was studied. It was found that within one hour in water at 60 C, the samples lost most of their superconductivity. Ba was preferentially leached out compared to yttrium and copper.<<ETX>>

Ceramics and Ceramic Composites: Opportunities and Cooperative Research Modes

Ceramics and ceramic composites are a rapidly evolving category of materials capable of being tailored to have unique combinations of electrical, optical, mechanical, and chemical properties that make them essential and irreplacable in many engineering applications. Conventional ceramics have both enabled and enhanced many important aspects of our modern technological society; the next generation of ceramics will play a further enabling role for still higher performance devices. A briefing panel of the National Academies of Science and Engineering recently identified four areas into which promising research areas for advanced ceramics can be grouped: 1. New thin films and layer structures with improved properties. 2. Exploration of completely new, multicomponent ceramic crystal structures and composites. 3. The mechanical behavior of ceramics and tough composites. 4. Ceramic processing of large parts and assemblies. Three mechanisms for achieving critical mass in ceramics research were recommended: 1. University/industry programs. 2. A summer institute. 3. A large interdisciplinary center.

Thermal Properties of Mullite-Cordierite Composites

The thermal conductivity and specific heat of mullite-cordierite composites have been measured in the temperature range 90 K to 420 K. The sound velocity, as determined from a fit to the specific heat data, agrees well with velocities determined from mechanical measurements. The temperature and composition dependence of the phonon mean free path is established. The thermal conductivity as a function of composition passes through a minimum for the mullite-rich compositions due to a minimum in the upper limit of the phonon mean free path. Solid solution formation is a possible cause.

An XPS investigation of the surface layer formed on ‘123’ high Tc superconducting films annealed in O2 and CO2 atmospheres

YBa2Cu3O7 thin films produced by laser deposition typically possess a layer consisting of several oxide forms. We attempt to identify this oxide layer using X‐ray Photoelectron Spectroscopy (XPS). We alter the composition of the oxide layer by in‐situ annealing in O2 and CO2 atmospheres. O2 annealing predominantly produces a BaxCuyO layer, whereas CO2 predominantly produces a BaCO3 layer.