The rare earth polysulfides and polyselenides of Tm, Yb, and Lu are unknown. The ionic radius of the rare earth has become too small to allow the structure common to the known members of the series, and with the polysulfides the stable temperature zone is too low to give the high sulfur pressure necessary for synthesis of RS_2. It was felt that high pressure, high temperature techniques could be used to overcome both problems and to allow synthesis of compounds not possible by ordinary methods. Synthesis studies were carried out on mixtures of the rare earth element plus sulfur in the 1:2 mole ratio for Nd, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y. The known polysulfide series was extended three members, TmS_2, YbS_2, and LuS_2. A high pressure pseudo-cubic polymorph was found for nine members, GdS_2, TbS_2, DyS_2, HoS_2, ErS_2, TmS_2, YbS_2, LuS_2, and YS_2. The minimum pressure of formation for the cubic polymorphs was found to be a smooth function of the ionic radius of the rare earth elements except for Y. YS_2 required a higher pressure to form the cubic polymorph than expected from the usual value of the ionic radius of Y, but the compressibility of Y is somewhat higher than Dy, which Y otherwise closely resembles. The reaction product diagrams of the rare earth polysulfides were all very similar. Regions were found where Ho_2S_3 and Yb_2S_3 were found on their respective diagrams. These compounds had the recently reported cubic Th_3P_4 type structure. Synthesis studies were carried out on 1:2 molar mixtures of rare earth element plus selenium for Er, Tm, Yb, and Lu. The known polyselenide series was extended three members to include TmSe_2, YbSe_2, and LuSe_2. Cubic Er_2Se_3 with the Th_3P_4 type structure was found at the highest pressure and temperature tried for the Er + 2 Se system. Prior to this work Er_2Se_3 was known only with the orthorhombic Sc_2S_3 type structure. The results of this investigation suggest that high pressure, high temperature techniques can be used to extend other series of rare earth compounds and several possibilities are suggested.

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Thermodynamic properties are calculated for mixtures of ideal gases in equilibrium for temperatures up to 15,000° K. Results for air and a model Mars atmosphere are given. The equilibrium composition of a gas mixture at a given temperature and pressure is determined by minimizing the Gibbs free energy. Species of the following types are included in the mixture: atoms and atomic ions, diatomic and linear triatomic molecules and ions, and electrons. Quantum statistical mechanics is used to determine thermodynamic properties of each species. For the diatomic species, accurate evaluations of vibrational anharmonicity, vibration-rotation interaction, and rotational stretching corrections are carried out. Comparisons are made between results with and without the above corrections. / Master of Science

Ultrasonic velocity and attenuation were both measured on samples containing various degrees of damage due to high temperature creep. These results were compared with parameters associated with creep damage such as strain and cavity formation, in order to better understand the mechanisms of creep in Si/SiC and to determine if ultrasonics can be used in evaluating the severity of damage. The data indicated that both ultrasonic velocity and attenuation are directly related to creep strain and can be used in evaluating creep damage. Ultrasonic velocity was found to be exponentially related to creep strain. Cavity formation was found not to significantly affect either of the measured ultrasonic properties. The results indicated that Si/SiC behaves as a two phase material in that high frequency ultrasound propagates primarily through the silicon carbide phase and not by the silicon phase. / Master of Science

Two framework oxide materials of the MO��� network type have been synthesized and structurally characterized by synchrotron and X-ray powder diffraction and the Rietveld method in the temperature range 25~500 K. The results show one of them to be a low thermal expansion material. Theoretical studies of negative thermal expansion (NTE) in framework oxides were conducted with two methods, geometrical modeling by Rigid Unit Mode (RUM) method and lattice dynamic calculations by free energy minimization (FEM) method, the results are compared with each other as well as with experimental observations. RUM analysis of all five types of framework oxide structures negates any simple and direct correlation between presence or absence of RUMs in a structure and the sign of its thermal expansion. Instead, results suggest that NTE of a crystalline solid can not be explained by pure geometrical considerations over its structure alone, and for a better understanding of structure-relationship in negative thermal expansion structures, specific interatomic interactions present in each one must be brought in explicitly. FEM calculation of two negative thermal expansion structures indicates on a structure by structure basis NTE could be predicted and understood within the Gruneisen model, which attributes NTE of a structure to special vibration modes in a structure that softens when the lattice shrinks. The soft NTE modes are, however, not necessarily RUM or RUM like vibration motions. / Graduation date: 2003

Thesis (Ph. D.)--University of Alabama at Birmingham, 2006. / Additional advisors: Heng Ban, Renato P. Camata, Krishan K. Chawla, Joseph G. Harrison. Description based on contents viewed Feb. 13, 2009; title from PDF t.p. Includes bibliographical references (p. 124-126).

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2016. / Metal dusting (MD) is a severe type of corrosion that occurs mainly in petrochemical industries. The occurrence of MD is mainly due to syngas attacking Fe-, Ni- and Co-based alloys at elevated temperatures. More recently, literature has shown that apart from syngas, liquid hydrocarbon sources have been causing problems on platformer units in refineries. In the first part of this study a highly corrosion resistant Ni-based alloy (Hastelloy C276), in its polished form, was subjected to MD conditions at 800 °C using a liquid hydrocarbon (toluene) and helium (carrier gas) for 1 h. Exposure to these conditions revealed the formation of carbon nanofilaments and graphite layers which were confirmed by laser Raman spectroscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Burning off the carbon nanofilaments and the graphite layers in laboratory air for 1 h at 800 °C revealed that pits were formed on the Hastelloy C276. These same pits were not evident when Hastelloy C276 was exposed to either the carrier gas (helium) or laboratory air alone. Besides MD being a continuous problem in industry, this mechanism has been shown to be beneficial in the synthesis of carbon nanofilaments viz., carbon nanofibers (CNTs) and nanotubes (CNFs). In the second part of this study, unpolished Hastelloy C276 blocks (as opposed to polished blocks) were used to synthesize carbon nanofilaments. This was done as prior studies had shown that carbon nanofilaments were produced with better quality and greater yields this way. Here the flow rate (80, 160 and 240 mL/min) and reaction duration (10, 15, 30, 45, 60, 120 and 240 min) were studied using toluene (a liquid hydrocarbon). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the quality and quantity of the carbon nanofilaments synthesized. Besides the formation of carbon nanofilaments, a less important material known as graphite particle structures (GPSs) were also synthesized. These studies collectively showed that MD had taken place on the surface of Hastelloy C276 when exposed to toluene at 800 °C. / TG2016