Orientador: Carlos K. Suzuki / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-07-31T20:22:15Z (GMT). No. of bitstreams: 1 Argonz_Raquel_D.pdf: 6011460 bytes, checksum: 288a8671c5c8ed96310d3c57226ce9c8 (MD5) Previous issue date: 2001 / Resumo: O Brasil é na atualidade um dos principais produtores de silício para o mundo, sendo que a quantidade de quartzo extraído para a sua produção incluindo o ferro-silício, é da ordem de 2 milhões de toneladas/ano. Para a obtenção do quartzo destinado à redução carbotérmica em silício, nos diversos estágios de extração, britagem, seleção, transporte, e lavagem, cerca de 300.000 toneladas/ano de lascas de quartzo tomam-se rejeitos. Neste trabalho foi desenvolvida uma metodologia ambientalmente correta, denominada "quench-Ieaching" e "crush-leaching", que se utiliza da lixiviação aquosa para a purificação deste material. Os resultados mostram que ocorre uma remoção efetiva de impurezas majoritárias nas lascas de quartzo, tais como, AI, Fe, Na, K, Ca, Mn, ..., dando-lhe uma pureza de 99,9% de SI 'O IND 2'. Uma comparação com diversos insumos de quartzo produzidos no exterior para uso em tecnologia avançada, como para produção de sílica vítrea translucente e "fillers" de "micro-chips", revela que este material purificado com esta tecnologia toma-se de qualidade equivalente ao pó de quartzo internacional / Abstract: Nowadays, Brazil is one of the main silicon metal and iron-silicon producer in the world. But on the other hand, the amount of natural quartz that has been extracted for this purpose is up to 2 milliontons/year. The key-point is the large quantity of rejected quartz lascas, approximately 300,000 tons/year, generated during the various stages of extraction, crushing, selection, transportation, and washing. A new environrnentally mendly purification methodology denominated "quench-Ieaching" and "crush-leaching, that only uses aqueous leaching, has been developed. The result shows an effective elimination of major quartz impurities, such as Al, Fe, Na, K, Ca, Mn, ... , that transforms this rejected material into a 99.9% purity SI 'O IND 2'. The quality of this material is as high as the quartz powder commercially available in the intemational market for use as "fillers" and translucent silica glass raw material for semiconductor industries / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

The knowledge of diffusion parameters provides important understanding of many physical and mechanical properties of materials. In most of the applications silicides are grown by a diffusion controlled process mainly in thin film condition. Because of this reason, most of the studies till date are available in thin film condition. Although more than one phase is present in all these systems, mainly disilicides were found at the interface. In this thesis bulk interdiffusion studies are conducted by coupling pure refractory metals (group IVB, VB and VIB elements) with single crystal Si. Several phase layers grow between binary refractory metal and Si systems. The layer thicknesses of the phases are measured from the microstructures. Composition profiles were measured in electron probe microanalyzer. Different diffusion parameters are estimated such as parabolic growth constants, integrated diffusion coefficients, activation energy for diffusion and ratio of tracer diffusivities of the components are estimated. Growth mechanisms of the phases are discussed with the help of diffusion parameters. The atomic mechanism of the diffusion is discussed considering crystal structure of the phases along with possible defects present. Solid diffusion couple experiments are conducted to analyse the growth mechanism of the phases and the diffusion mechanism of the components in the Ti-Si system. The calculation of the parabolic growth constant and of the integrated diffusion coefficients substantiates that the analysis is intrinsically prone to erroneous conclusions if it is based just on the parabolic growth constants determined for a multiphase interdiffusion zone. The location of the marker plane is detected based on the uniform grain morphology in the TiSi2 phase, which indicates that this phase grows mainly because of Si diffusion. The growth mechanism of the phases and morphological evolution in the interdiffusion zone are explained with the help of imaginary diffusion couples. The activation enthalpies for the integrated diffusion coefficient of TiSi2 and the Si tracer diffusion are calculated as 190±9 and 170±12 kJ/mol, respectively. The crystal structure, details on the nearest neighbours of the elements and the relative mobilities of the components indicate that the vacancies are mainly present on the Si sublattice. Diffusion controlled growth of the phases in the Hf-Si and Zr-Si are studied by bulk diffusion couple technique. Only two phases grow in the interdiffusion zone, although several phases are present in both the systems. The location of the Kirkendall marker plane detected based on the grain morphology indicates that the disilicides grow by the diffusion of Si. Diffusion of the metal species in these phases is negligible. This indicates that vacancies are present mainly on the Si sublattice. The activation energies for integrated diffusion coefficients in the HfSi2 and ZrSi2 are estimated as 394 ± 37 and 346 ± 34 kJ/mol, respectively. The same is calculated for the HfSi phase as 485±42 kJ/mol. The activation energies for Si tracer diffusion in the HfSi2 and ZrSi2 phases are estimated as 430 ± 36 and 348 ± 34 kJ/mol, respectively. We conducted interdiffusion studies to understand the atomic mechanism of the diffusing species and the growth mechanism of the phases. Integrated diffusion coefficients and the ratio of tracer diffusion coefficients were estimated for these analyses. The activation energies for the integrated diffusion coefficients were calculated as 550 ± 70 and 410 ± 39 kJ/mol in the TaSi2 and the Ta5Si3 phases, respectively. In the TaSi2 phase, Ta has a slightly lower but comparable diffusion rate with respect to Si, although no TaTa bonds are present in the crystal. In the Ta5Si3 phase, Si has higher diffusion rate, which is rather unusual, if we consider the atoms in the nearest-neighbor positions for both the elements. The ratio of Si to Ta tracer diffusion coefficients is found to be lower in the Si-rich phase, TaSi2, compared to the Si-lean phase, Ta5Si3, which is also unusual. This indicates the type of structural defects present. An analysis on the growth mechanism of the phases indicates that duplex morphology and the Kirkendall marker plane should only be present in the TaSi2 phase. This is not present in the Ta5Si3 phase because of the very high growth rate of the TaSi2 phase, which consumes most of the Ta5Si3 phase layer. The problems in the calculation method used previously by others in this system are also explained. Experiments are conducted in the W-Si system to understand the diffusion mechanism of the species. The activation energies for integrated diffusion are found to be 152±7 and 301±40 kJ/mol in the WSi2 and W5Si3 phases, respectively. In both the phases, Si has a much higher diffusion rate compared to W. The result found in the WSi2 phase is not surprising, if we consider the nearest neighbors in the crystal. However, it is rather unusual to find that Si has higher diffusion rate in the W5Si3 phase, indicating the presence of high concentration of Si antisites in this phase. In the group IVB, VB and VIB M-Si systems are considered to show an interesting pattern in diffusion of components with the change in atomic number in a particular group. MSi2 and M5Si3 are considered for this discussion. Except in the Ta-Si system, activation energy for integrated diffusion of MSi2 is always lower than M5Si3. Interestingly, in both the phases, the relative mobilities measured by the ratio of tracer D* diffusion coefficients, S i decreases with the increase in atomic number in both the DM* groups. Both the phases have similar crystal structures in a particular group in which these parameters are calculated. In both the phases Si has higher diffusion rate compared to M. Absence of any M-M bonds in MSi2 and increase in the diffusivities of M with the increase in atomic number substantiates the increasing concentration of M anti-sites and higher interactions of M with vacancies. Only one or two Si-Si bonds are present in M5Si3, however, the higher diffusion rate of Si indicates the presence of vacancies mainly D* on its sublattice. On the other hand, increase in S i with increasing atomic number in DM* Both the groups substantiates increasing interactions of M and vacancies.

Metal silicon systems have a wide range of applications, ranging from the use in electronic industry, as superconductors, protective coatings and as high temperature structural materials. Mo- and Nb-based silicides have emerged as suitable high temperature materials and extensive studies are being conducted make it suitable for various applications. Because of very good strength to density ratio, Nb-based silicides have attracted maximum attention. This is basically a mixture of Nb solid solution and Nb5Si3 intermetallic compound. A very small amount of NbCr2 Laves phase could also be present because of Cr addition. Incorporation of other alloying elements, which are mainly partitioned to these phases, helps to achieve a property balance like, high temperature strength, high fracture toughness, high creep and oxidation resistance. The knowledge on diffusion parameters is useful to understand many physical and mechanical properties. In this thesis, diffusion couple technique is used in different temperature ranges to study the growth kinetics and diffusion of the phases in an interdiffusion zone in binary silicides, Nb/Si, Mo/Si and V/Si, binary solid solutions, Nb/Mo, Nb/Ti, Nb/Zr and ternary silicides, Nb-Mo/Si, Nb-Ti/Si, Nb-Zr/Si. The parabolic growth constant, the integrated diffusion coefficients and the tracer diffusion coefficients are calculated from the experimental results obtained in this study and also from the results already available in the literature on the binary silicides. The activation energy for growth kinetics and the diffusion coefficients are also calculated to gain knowledge on the diffusion mechanism. The atomic mechanism of the diffusing species in all the phases of Nb and Mo silicide are discussed with the help of crystal structure and possible defects present. Also, a detailed analysis is done on the growth mechanism of the phases in Nb/Si and Mo/Si systems. In the Nb/Si system, Si is found to have higher diffusion rate in both the NbSi2 and Nb5Si3 phases. The number of nearest neighbour Si bonds is higher than nearest neighbour Nb bonds and hence one may predict high concentration of Nb antisites to be present in the NbSi2 phase. The growth mechanism analysis following the physico chemical approach explains the absence of the Kirkendall plane in the Nb5Si3 phase and duplex morphology in the NbSi2 phase in the Nb/Si couple. In the Mo/Si system, Si diffusion is faster than Mo in all the three phases. In the MoSi2 phase, Mo is practically immobile due to the absence of vacancies on the Mo sublattice. Similar defect structure is expected in the Mo5Si3 and Mo3Si phases also with additional Si antisite defects to assist Si diffusion. The growth mechanism analysis explains the absence of the Kirkendall plane in the Mo5Si3 and Mo3Si phases and continuous columnar grains in the MoSi2 phase in the Mo/Si couple. In the V/Si system, the activation energy for integrated diffusion coefficient of the VSi2 phase is found to be reasonably lower than the other phases which could happen because of very high concentration of defects, and/or because of contribution from the grain boundary diffusion as it shows the presence of columnar grains. Problems associated with the analysis done in literature are also discussed. A diffusion study is performed in different temperature ranges for the three binary metallic solid solution systems to determine the interdiffusion coefficients over the entire composition range using the relation developed by Wagner. The change in activation energy for interdiffusion with composition is also determined. It is found that activation energy for interdiffusion in Nb/Mo system is much higher than that for Nb/Ti and Nb/Zr system. Further the impurity diffusion coefficients of the species are determined and compared with the available data in literature. It is found that the activation energy for the impurity diffusion of Nb in Ti, Zr and Mo is higher than that of Ti, Zr and Mo in Nb. Interdiffusion study is done in the ternary silicides with the aim to examine the role of alloying additions, such as, Ti, Mo and Zr on the growth kinetics and diffusion behaviour of the phases in the Nb/Si system. The average interdiffusion (or integrated) coefficients are calculated when possible. The reaction and dissociation of the species at the interfaces are considered to understand the growth mechanism of the phases. An attempt is made to understand the change in diffusion mechanism because of the presence of third element. It is found that none of the alloying elements participate in the diffusion process although they do alter the growth kinetics and diffusion rate in both the phases, NbSi2 and Nb5Si3. It is also found that Nb becomes immobile in the NbSi2 phase in the presence of the alloying elements. Mo reduces the growth of both the phases while Ti addition does not cause any change in the growth but affects the diffusivity. Zr addition also reduces growth of the Nb5Si3 phase. It however complicates the interdiffusion zone in the Nb(Zr)/Si couple, which limits to qualitative study only. The Growth and consumption rate of the end members become very significant in many practical applications. Hence, relations for the growth and consumption rate in systems with finite end member thickness is developed considering single and double phase layer in the interdiffusion zone. Two different methodologies are used, the diffusion based and the physico-chemical approach to develop the same relations. We have shown that the diffusion based approach is rather straightforward; however, the physico-chemical approach is much more versatile than the other method. It is found that the position of the marker plane becomes vague in the second stage of the interdiffusion process in such a system, where two phases grow simultaneously.

No description available.