The study of silicates and refractory materials at high pressures and temperatures

To better understand high pressure behavior of solids, both silicates and oxides have been investigated to clarify the high pressure melting, phase transformations and thermal parameters as well as their size dependences, both theoretically and experimentally. ^ To judge the precision of data determined experimentally, the reliabilities of different high pressure techniques have been discussed. A thermodynamic model has been developed and demonstrated to be able to closely reproduce the melting of solids by comparison between results calculated and data obtained experimentally, including metals (Al, Ni and Pt), Silicates (Mg3Al 2Si3O12 and CaMgSi2O6), Halides (NaCl, CsCl and LiF) and Oxides (MgO, FeO and Al2O3). The melting data obtained have been discussed to address the dynamics of the Earth's interior. ^ Results obtained with Raman spectroscopy and x-ray diffraction show that solids including silicates (andradite and pyrope) and oxides (CeO2 and TiO2) undergo a series of pressure-induced phase transformations. The effects of particle size under high pressures have been investigated. The results obtained indicate that the reduction of particle size leads to the enhancement of the bulk modulus and a significant decrease of transition pressure in TiO2 (rutile) and CeO2. The pressure-induced amorphization in anatase also results from the size effects. ^ Combining the data obtained with global seismic tomography, the physics and chemistry of the Earth's mantle and the dynamics of the core-mantle interaction have been discussed. The high pressure phases of Al3+- and Fe3+-bearing minerals play important roles in the dynamics of the lower mantle. ^

Industrial Discharges of Metals in Kigali, Rwanda, and the Impact on Drinking Water Quality

Rwanda is a landlocked country located in Africa's Central-East Great Lakes region. It has a population of 7.5 million which occupies 26,338 km'. Its population density (285/km') is one of the highest in the world and has prompted fear of a rapid degradation of the ecosystem. There are no central sewer systems in Rwanda. The use of pit latrines and septic tanks is common in urban and rural areas. People still defecate in the fields (World Bank, 1989). Less than half of the urban population is served by a central water supply. The majority of people get their water untreated from rivers that have been polluted by chemicals and human excreta. In and around the capital city of Kigali, there is a concentration of people, farms, and industries which discharge wastewater into the Nyabarongo River and its tributaries. The Nyabarongo River, a tributary of the Nile, empties into the Akagera River which flows into Lake Victoria. Nyabarongo River water is used for drinking water, cooking, bathing, and agriculture in the Kigali area. There has been very little monitoring of the water quality of the Nyabarongo River and of industrial outfalls located on tributaries of the Nyabarongo River. As a first step in understanding the water quality of the Nyabarongo River, wastewater samples were collected in 1993 from industrial outfalls located on tributaries of the Nyabarongo River. Most of the facilities sampled had no wastewater treatment. The impact of these discharges on the water quality of the Nyabarongo River was evaluated.

Study of the physical properties of metals and oxides at extreme pressure and temperature conditions

The high-pressure and temperature investigations on transition metals, metal doped-oxide system, nanocrystalline materials are presented in this dissertation. The metal-doped oxide systems are technologically important because of their applications, e.g. LSC, opto electronic applications, luminescence from lasers, etc., and from the earth sciences point of view, e.g. the study of trace elements in the MgO-SiO2 system, which accounts for 50% of the Earth's chondritic model. We have carried out thorough investigations on Cr2O3 and on chromium bearing oxides at high PT-conditions using in situ X-ray diffractometry and florescence spectroscopy techniques. Having obtained exciting results, an attempt to focus on the mechanism of the coordination of transition metals in oxides has been made. Additionally, the florescence from the metals in host oxides was found to be helpful to obtain information on structural variations like changes in the coordination of the doped element, formation of new phases, the diffusion processes. The possible reactions taking place at extreme conditions in the MgO-SiO2 system has been observed using florescence as markers. A new heating assemblage has been designed and fabricated for a precise determination of temperature at high pressures. An equation combining pressure shifts of ruby wavelength and temperature has been proposed. We observed that the compressibility of nanocrystalline material (MgO and Ni) is independent of crystallite size. A reduction in the transition pressure of nanocrystalline ceria at high-pressure has been observed as compare to the corresponding bulk material. ^

Sources of metals and sulfur in the mineral deposits of central Idaho, USA

A variety of mineral deposits occur in the Paleozoic sedimentary rocks and Late Cretaceous granitic rocks of central Idaho. The main objective of this project is to identify the sources of metals and sulfur in central Idaho ores. Lead isotope compositions of various crustal rocks were determined and compared with the ore lead composition in order to trace sources of lead, and by inference other metals. Sulfur isotope compositions of various sulfide minerals were also determined to trace the sources of sulfur and to explore the coupling or decoupling of metal and sulfur sources. ^ On the basis of lead and sulfur isotope compositions, two groups of ores are recognized: a sedimentary group and an igneous group. The sedimentary group ores are characterized by radiogenic lead and heavy sulfur typical of upper crustal rocks. The sedimentary group ores were formed by meteoric water-dominated hydrothermal systems that leached metals and sulfur from host Paleozoic sedimentary rocks and the underlying Precambrian crystalline basement rocks. The igneous group ores can be divided into two types, the Carrietown-type, and the non Carrietown-type. The Carrietown-type ores are isotopically different from their host granites and are characterized by low uranogenic lead isotope ratios (206Pb/204Pb and 207Pb/ 204Pb) and variable thorogenic lead isotope ratios (208Pb/ 204Pb) typical of lower crustal rocks. The non Carrietown-type ores are similar to host granites and are more radiogenic in their uranogenic lead isotope ratios when compared to the Carrietown-type ores. The differences in the lead isotope compositions of the igneous group ores are attributed to two different phases of magmatic activity. The magmatic phase exposed on the surface involved melting of shallow crustal Precambrian crystalline rocks as well as mid/lower crustal rocks while the underlying phase was derived by melting of mid/lower crustal rocks only. Igneous group ores have both light and heavy sulfur associated with them and it is a function of interaction of hydrothermal fluids with Paleozoic sedimentary rocks. ^ Paleozoic sedimentary rocks and Precambrian basement rocks are the sources of radiogenic lead, and the granites are the sources of light sulfur. Heavy sulfur comes almost entirely from Paleozoic sedimentary rocks. ^

Synthesis, characterization and study of physical properties of novel M(n+1)AX(n) compounds

Mn+1AXn compounds, the ternary layered nanolaminates have gathered momentum in the last decade since its advent because of their unusual but exciting properties. These technologically important compounds combine some of the best properties of metals and ceramics. Like ceramics they are refractory, oxidation resistant, elastically stiff and relatively light. They also exhibit metallic properties like excellent machinability, thermal and electrical conductivity. This dissertation concentrates on the synthesis of germanium-based 211 Mn+1AXn compounds. The main objective of the research was to synthesize predominantly single phase samples of Cr2GeC, V2GeC and Ti2GeC. Another goal was to study the effect of solid substitutions on the M-site of Mn+1AXn compounds with Ge as an A-element. This study is in itself the first to demonstrate the synthesis of (Cr0.5V0.5)2GeC a novel Mn+1AXn compound. Scanning electron microscopy coupled with energy dispersive spectroscopy, x-ray diffraction and electron probe microanalysis were employed to confirm the presence of predominantly single phase samples of M2GeC compounds where M = Ti, V, Cr and (Cr 0.5V0.5). A large part of the dissertation also focuses on the effect of the compressibility on the Ge-based 211 Mn+1AXn compounds with the aid of diamond anvil cell and high energy synchrotron radiation. This study also concentrates on the stability of these compounds at high temperature and thereby determines its suitability as high temperature structural materials. In order to better understand the effect of substitutions on A-site of 211 Mn+1 AXn compounds under high pressure and high temperature, a comparison is made with previously reported 211 Mn+1AXn compounds with Al, Ga and S as A-site elements.

Isotopic studies of the Guerrero composite terrane, west-central Mexico: Implications for provenance of crustal rocks and genesis of ore metals

A variety of world-class mineral deposits occur in Mesozoic and Tertiary rocks of the Guerrero terrane. New Pb isotope analyses of various crustal units and ores from distinct subterranes of the Guerrero terrane are presented to trace metal sources in these deposits and infer source reservoirs. New Sr and Nd isotope results are provided to gain insight into the provenance of the crustal rocks from the Guerrero terrane. Triassic schist samples from the Arteaga Complex and Triassic-Jurassic phyllite and slate samples from the Tejupilco metamorphic suite contain radiogenic Pb (206Pb/204Pb = 18.701–19.256) relative to bulk earth models. Cretaceous sedimentary rocks of the Zihuatanejo Sequence are more radiogenic (206Pb/204Pb = 18.763–19.437) than samples from the Huetamo Sequence (206Pb/204Pb = 18.630–18.998). Tertiary intrusive rocks from La Verde, Inguaran, La Esmeralda, and El Malacate plot to the right of the average Pb crust evolution curve of Stacey and Kramers (206Pb/204Pb = 18.705–19.033). Ores from the La Verde and La Esmeralda porphyry copper deposits yield isotopic ratios (206Pb/204Pb = 18.678–18.723) that are generally less radiogenic than the host igneous rocks, but plot within the field defined by the sedimentary rocks from the Huetamo Sequence. Tertiary intrusive rocks from the Zimapan and La Negra districts in the Sierra Madre terrane plot above and to the right of the Stacey-Kramers reference line (206Pb/204Pb = 18.804–18.972). Lead isotope ratios of ore minerals from the Zimapan and La Negra skarn mines ( 206Pb/204Pb = 18.775–18.975) resemble those of the associated igneous rocks, implying a magmatic Pb input in the skarn deposits. New Sr and Nd isotope data on metamorphic rocks (87Sr/ 86Sr = 0.707757–0.726494 and 143Nd/144 Nd = 0.512109–0.512653) suggest that the basement of the Guerrero terrane originated from sources that had been derived from an old cratonic area. The narrow ranges and generally low 87Sr/86Sr ratios (0.704860–0.705755) and 143Nd/144Nd values (0.512765–0.512772) above that of bulk earth for igneous rocks from Inguaran, El Malacate, and La Esmeralda suggest a relatively low degree of crustal contamination. However, the isotopic values for the La Verde site (87Sr/86Sr = 0.708784 and 143Nd/144Nd = 0.512640) may indicate the involvement of a more evolved crustal component.

Vacuum brazing of alumina ceramic to titanium for biomedical implants using pure gold as the filler metal

One of the many promising applications of metal/ceramic joining is in biomedical implantable devices. This work is focused on vacuum brazing of C.P titanium to 96% alumina ceramic using pure gold as the filler metal. A novel method of brazing is developed where resistance heating of C.P titanium is done inside a thermal evaporator using a Ta heating electrode. The design of electrode is optimized using Ansys resistive heating simulations. The materials chosen in this study are biocompatible and have prior history in implantable devices approved by FDA. This research is part of Boston Retinal implant project to make a biocompatible implantable device (www.bostonretina.org). ^ Pure gold braze has been used in the construction of single terminal feedthrough in low density hermetic packages utilizing a single platinum pin brazed to an alumina or sapphire ceramic donut (brazed to a titanium case or ferrule for many years in implantable pacemakers. Pure gold (99.99%) brazing of 96% alumina ceramic with CP titanium has been performed and evaluated in this dissertation. Brazing has been done by using electrical resistance heating. The 96% alumina ceramic disk was manufactured by high temperature cofired ceramic (HTCC) processing while the Ti ferrule and gold performs were purchased from outside. Hermetic joints having leak rate of the order of 1.6 × 10-8 atm-cc/ sec on a helium leak detector were measured. ^ Alumina ceramics made by HTCC processing were centreless grounded utilizing 800 grit diamond wheel to provide a smooth surface for sputtering of a thin film of Nb. Since pure alumina demonstrates no adhesion or wetting to gold, an adhesion layer must be used on the alumina surface. Niobium (Nb), Tantalum (Ta) and Tungsten (W) were chosen for evaluation since all are refractory (less dissolution into molten gold), all form stable oxides (necessary for adhesion to alumina) and all are readily thin film deposited as metals. Wetting studies are also performed to determine the wetting angle of pure gold to Ti, Ta, Nb and W substrates. Nano tribological scratch testing of thin film of Nb (which demonstrated the best wetting properties towards gold) on polished 96% alumina ceramic is performed to determine the adhesion strength of thin film to the substrate. The wetting studies also determined the thickness of the intermetallic compounds layers formed between Ti and gold, reaction microstructure and the dissolution of the metal into the molten gold.^