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<p>Tension and compression experiments have been carried out on a series of Mg-Y alloys with Y content up to 1.3 at.%, in a range of temperatures between 4.2K and 298K, to study the effect of Yttrium on mechanical properties and strain hardening. The alloys show strong difference in the hardening behavior under tension and compression attributed to the effect of texture. The yield strength scales with concentration of the solute as cⁿ, where c is the concentration of the solute in atomic percent and n~2/3. The results suggest that in addition to the atomic size and modulus misfit effects, the valence may be responsible for the enhanced strengthening of Y in Mg. Strain rate sensitivity measurements carried out under tension and compression reveal that Mg-Y alloys show decreasing SRS with increasing Y content at 298K and exhibit a negative SRS in highly concentrated alloys. At low temperatures the alloys show positive SRS increased with Y content. Texture measurements suggest that increasing Y content in alloys decreases the amount of basal component and enhances non-basal orientations. The reduced yield asymmetry between tension and compression observed in higher Y content alloys is being attributed to the weakening of the basal texture.</p> / Master of Applied Science (MASc)

京都大学 / 新制・課程博士 / 博士(理学) / 甲第25482号 / 理博第5063号 / 新制||理||1722(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 北川 宏, 教授 有賀 哲也, 教授 堀毛 悟史 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Dans le domaine des cristaux piézoélectriques, le quartz est l'un des plus employés dans l'industrie électroniques pour des applications comme oscillateurs ou dans le domaine temps-fréquence. Le quartz-alpha SiO2 montre une décroissance de ses propriétés au-delà de 250°C, une transition de phase alpha-beta à 573°C et un faible coefficient de couplage électromécanique k autour de 8%. Bien que ses propriétés d'optique non-linéaire soient bien connues, son faible coefficient chi2 ne lui permet pas d'être utilisé dans des dispositifs doubleurs de fréquence. L'objectif de cette étude est d'augmenter la distorsion structurale et la polarisabilité de ce matériau en substituant une partie des atomes de silicium par des atomes plus volumineux tels que le germanium ou d'autres éléments. Afin de faire croitre des cristaux de taille centimétrique, la technique hydrothermale a été employée dans des autoclaves hautes pressions. Des cristaux de quartz-alpha de type Si(1-x)GexO2 ont été réalisés sur des germes de quartz-alpha SiO2 (001). Des cristaux volumineux avec différentes teneurs en germanium ont été obtenus puis analysés par spectroscopie infrarouge et par EPMA. Les propriétés piézoélectriques et d'optique non-linéaire ont été mesurées sur ces cristaux montrant une augmentation des propriétés physiques. Puis des croissances cristallines avec des atomes plus volumineux que le germanium ont été réalisées afin d'en augmenter davantage les propriétés physiques. Des substitutions par les éléments suivants ont été entreprises : Mn, V, Ti, et Sn / In the field of piezoelectric crystals, quartz is one of the widely used materials in industry for electronic device application as oscillators for the time-frequency domain. alpha-Quartz SiO2 shows a decrease of its piezoelectric properties above 250°C, an alpha-quartz to beta-quartz phase transition at 573°C and a low electromechanical coupling factor of about 8%. Although its nonlinear optics properties are well known, its low chi2 coefficient prevent it to be used in frequency doubling devices. The goal of this study is to increase the structural distortion and the polarizability of this material by substituting part of the silicon atoms with larger atoms such as germanium or other elements. In order to grow centimeter-size single crystals we use hydrothermal methods in high-pressure autoclaves. Crystal growth of mixed alpha-quartz Si(1-x)GexO2 crystals was successfully performed on pure alpha-quartz SiO2 (001) seeds. Large crystals with different germanium content were obtained and analyzed by infrared spectroscopy and EPMA. Piezoelectric and nonlinear optical properties were measured on these crystals, which exhibit a improved physical properties. Then crystal growths with larger elements than germanium were performed in order to further improve their physical properties. Substitution by the following elements: Mn, V, Ti and Sn were investigated.

Made available in DSpace on 2014-12-17T15:42:13Z (GMT). No. of bitstreams: 1 ValdeliceRS_TESE.pdf: 2586359 bytes, checksum: e3d9d5d8e2c97ad0990f43fd755545a2 (MD5) Previous issue date: 2010-11-26 / One of the main applications of methane is in the production of syngas, a mixture of hydrogen and carbon monoxide. Procedures used in this process are steam reforming, CO2 reforming, partial oxidation and autothermal reforming. The present study evaluated and compared the behavior of nickel catalysts supported on mixed oxides of cerium and manganese in the partial oxidation of methane with that of nickel catalysts supported on mixed oxides of cerium and zirconium. Mixed oxides of cerium and zirconium or cerium and manganese were synthesized using two different preparation methods, the polymeric precursor based on Pechini method and combustion reaction using a microwave. This was followed by impregnation with nickel content of 15 %. Samples were calcined at 300, 800 and 900 ?C and characterized by specific surface area (SSA), X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed reduction (TPR) and the reaction of partial oxidation of methane. The specific areas of samples decrease with the rise in calcination temperature and after nickel impregnation. Metal-cerium solid solution was formed and the presence of other manganese species outside the solid solution structure was confirmed in the compound with the highest amounts of manganese oxides showed. With regard to scanning electron microscopy, supports based on cerium and zirconium prepared by Pechini method exhibited agglomerated particles without uniform geometry or visible pores on the surface. However, compounds containing manganese presented empty spaces in its structure. Through synthesis by combustion reaction, morphology acquired independently of the proposed composition demonstrated greater porosity in relation to Pechini synthesis. Although catalysts were prepared using different synthesis methods, the insertion of nickel showed very similar reduction profiles (TPR). In relation to nickel catalysts supported on mixed oxide of cerium and zirconium, there is an initial reduction of NiO species that present certain interaction with the support. This is followed by the reduction of Ce4+ in Ce3+ surface, with subsequent bulk reduction. For catalysts containing manganese, a reduction of nickel oxide species occurs, followed by two stages of reduction for species Mn2O3 in Mn3O4 and Mn3O4 in MnO, with subsequent reduction of bulk. With respect to partial oxidation reactions, the nickel catalyst supported on mixed oxide of cerium and zirconium, prepared using the Pechini method, exhibited CH4 conversion of approximately 80 %, with conversion of 81 % when prepared by combustion. This behavior continued for 10 hours of reaction. Manganese content was also found to directly influence catalytic activity of materials; the greater the manganese oxide content, the faster deactivation and destabilization occurred in the catalyst. In both synthesis methods, the nickel catalyst supported on mixed oxide of cerium and zirconium maintained an H2/CO ratio very close to 2 during the 10 hours of partial oxidation reaction. Samples containing manganese displayed smaller H2/CO ratios and lower performance in partial oxidation. / Uma das principais aplica??es do metano ? a produ??o de g?s de s?ntese, mistura de hidrog?nio e mon?xido de carbono. Os processos utilizados na produ??o de g?s de s?ntese a partir do metano s?o: reforma a vapor, reforma com CO2, oxida??o parcial e reforma autot?rmica. Neste trabalho, o comportamento de catalisadores de n?quel suportados em ?xidos mistos de c?rio e mangan?s na rea??o de oxida??o parcial do metano foi avaliado e comparado com o catalisador de n?quel suportados no ?xido misto de c?rio e zirc?nio. Os ?xidos mistos de c?rio e zirc?nio ou c?rio e mangan?s foram sintetizadas usando dois diferentes m?todos de prepara??o; o de precursores polim?ricos baseado no processo Pechini e por rea??o de combust?o usando um micro-ondas, seguido da impregna??o de n?quel com teor de 15 %. As amostras foram calcinadas a 300, 800 e 900 ?C e caracterizados por ?rea espec?fica (ASE), fluoresc?ncia de raios X (FRX), difra??o de raios X (DRX), microscopia eletr?nica de varredura (MEV), redu??o ? temperatura programada (RTP) e a rea??o de oxida??o parcial do metano. As ?reas espec?ficas das amostras diminuem com o aumento da temperatura de calcina??o e ap?s a impregna??o com n?quel. A solu??o s?lida c?rio-metal foi formada e nos composto com as maiores quantidades de ?xidos de mangan?s verificou-se a presen?a de outras esp?cies de mangan?s fora da estrutura da solu??o s?lida. Quanto ? microscopia eletr?nica de varredura os suportes a base de c?rio e zirc?nio preparados via Pechini exibem part?culas aglomeradas, sem geometria uniforme e sem a visualiza??o de poros na superf?cie, enquanto os compostos contendo mangan?s apresentaram alguns vazios na sua estrutura. Atrav?s da s?ntese por rea??o de combust?o a morfologia adquirida independente da composi??o proposta apresentou uma maior porosidade em rela??o ? s?ntese Pechini. Mesmo sendo os catalisadores preparados por diferentes m?todos de s?ntese, a inser??o de n?quel deixou seus perfis de redu??o (RTP) muito semelhantes. Para os catalisadores de n?quel suportados no ?xido misto de c?rio e zirc?nio, h? em primeiro lugar redu??o de esp?cies NiO que apresentam certa intera??o com o suporte, seguido da redu??o de Ce4+ em Ce3+ superficiais, com posterior redu??o do bulk. Para os catalisadores contendo mangan?s h? a redu??o das esp?cies de ?xido de n?quel, seguido de duas etapas de redu??o para as esp?cies Mn2O3 em Mn3O4 e Mn3O4 em MnO, com posterior redu??o do bulk. Quanto ?s rea??es de oxida??o parcial, o catalisador de n?quel suportados no ?xido misto de c?rio e zirc?nio preparado via m?todo Pechini, apresentou uma convers?o de CH4 de cerca de 80 %, sendo 81 % a convers?o quando preparado via combust?o. Esse comportamento manteve-se durante 10 horas de rea??o. Observou-se tamb?m que o teor de mangan?s influencia diretamente na atividade catal?tica dos materiais, quanto maior o teor de ?xido de mangan?s mais r?pido o catalisador apresentava desativa??o e desestabiliza??o. Para ambos os m?todos de s?ntese o catalisador de n?quel suportados no ?xido misto de c?rio e zirc?nio manteve a raz?o H2/CO bem pr?xima de 2 durante as 10 horas em que ocorre a rea??o de oxida??o parcial. As amostras contendo mangan?s apresentaram menores raz?es de H2/CO e menor desempenho na oxida??o parcial.

Understanding of the interrelationship between structure, thermodynamic properties and phase diagrams is very useful for rationalizing the behavior of materials and development of predictive models, which can be used to optimize the composition of materials and their fabrication processes. The properties of materials are governed by its electronic and crystallographic structure. Chemical bonding determines the electronic structure of materials. Furthermore, the electronic structure plays a predominant role in determining the physical, electrical, magnetic, thermal and optical properties of materials. Crystal structure also influences most properties of materials. Since changes in thermodynamic variables such as temperature, pressure, and composition dramatically alter the physical properties of materials and its structure, it is desirable to study the thermodynamic stability of materials in conjunction with phase relations. Phase diagrams can indicate the ranges of pressure, temperature and chemical composition where specific phases and mixtures of phases are stable. If the Gibbs energies of all the phases involved are known, phase diagram can be computed using Gibbs energy minimization algorithms. In recent times, one of the important uses of thermodynamics in materials science has been in the computation of phase diagrams. To materials scientists phase diagrams are like maps to travelers. They guide the path through the composition space to find phases, fulfilling specific materials performance requirements. As phase diagrams are the graphic representations of minimizations of Gibbs energy under given constraints, computational thermodynamics significantly expands our capability to walk in the multi-component space of engineering materials. High-temperature phase-equilibrium studies, thermodynamics and materials processing have had a close relationship over a number of decades. Successful utilization of ceramic materials under different environmental conditions at high temperatures requires accurate thermodynamic data. Focus of the present investigation is to obtain correct phase relations and accurate thermodynamic data in selected technologically important ceramic oxide systems in which the data are either not available or are inconsistent. Based on the experimental data, different types of phase diagrams are computed for the systems of contemporary relevance. After a brief introduction, Chapter 1 discusses the brief overview of the experimental techniques available for determining the phase relations and thermodynamic properties at high temperatures and the methods used in this study. The chapter reviews the possible sources of errors in experimental techniques and tests for correct functioning. In Chapter 2, systematic studies on high-temperature phase equilibria and thermodynamic properties of compounds in the ternary systems Ln-Pd-O (Ln = La, Pr, Eu, Gd, Tb, Dy, Ho and Er) are presented. Some of the ternary oxides on the Ln-Pd-O systems have potential application in catalysis and electrochemistry. To optimize the parameters for the synthesis and to understand the behavior of the catalysts, it is useful to have information on the thermodynamic stability domain of each compound. Quantitative information on the stability of the ternary oxides is also useful for assessing the interaction of metal Pd with ceramic compounds containing rare-earth elements under different environments. Furthermore, the thermodynamic data are beneficial for the design of processes for the recovery of rare earth and precious metals from scrap. There is very little thermodynamic and phase diagram information on the Ln-Pd-O systems. Isothermal sections of phase diagram for the ternary system La-Pd-O at 1200 K and for the systems Ln-Pd-O (Ln = Pr, Eu, Gd, Tb, Dy, Ho and Er) at 1223 K, were established by the isothermal equilibration technique at high temperatures. Phases were identified after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). Based on the phase relations, the thermodynamic properties of ternary interoxide compounds were determined by the solid-state galvanic cell technique over a range of temperature between 925 - 1400 K. An advanced version of the solid-state cell incorporating a buffer electrode was used for high temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. These novel features enhanced the accuracy of thermodynamic data. From electrochemical measurements, the standard enthalpies of formation of these oxides from elements and their standard entropies at 298.15 K were also evaluated. The variation of the lattice parameters and unit cell volume as a function of rare earth atomic number for the three ternary compounds Ln4PdO7, Ln2PdO4 (Ln = La, Pr, Nd, Sm, Eu, Gd) and Ln2Pd2O5 (Ln = La to Er) are discussed. The systematic variations of thermodynamic properties of all the ternary compounds as a function of rare earth atomic number are presented and correlated with structural features. Thermodynamic and structural parameters of uninvestigated Ln-Pd-O systems (Ln = Ce, Pm) can be obtained by interpolation. Based on the thermodynamic information obtained in this study and auxiliary data on binary compounds available in the literature, different types of phase diagrams, isothermal oxygen potential diagrams, isobaric phase diagrams, isothermal two dimensional and three-dimensional chemical potential diagrams for the systems Ln-Pd-O (Ln = La, Pr, Eu, Gd, Tb, Dy, Ho and Er) are constructed. Chapter 3 contains the studies on partial phase diagrams of the systems M-Ru-O (M = Ca and Sr) at 1300 K and determination of Gibbs energies of formation of calcium and stronsium ruthenates in the temperature range from 925 to 1350 K using solid-state cells with yttria-stabilized zirconia as the electrolyte and Ru + RuO2 as the reference electrode. Gibbs energies, enthalpies and entropies of formation of calcium and strontium ruthenates from their component binary oxides were deduced. The standard enthalpies of formation of these oxides from elements and their standard entropies at 298.15 K were also evaluated. Based on the thermodynamic data obtained in this study and auxiliary information from the literature, the three dimensional representation of oxygen potential diagram for the M-Ru-O systems (M = Ca and Sr) as a function of composition and temperature are computed. The purpose of this chapter is to determine the thermodynamic stability of alkaline earth metal ruthenates in the perovskite related layered system Mn+1RunO3n+1 (n = 1, 2, and ¥ for Ca-Ru-O system and n = 1, 2, 3 and µ for Sr-Ru-O system) since these calcium and stronsium ruthenates have interesting magnetic and electronic device applications. Moreover, there is no literature available for thermodynamic properties on first and second members of the Ruddelsdon-Popper (R-P) series in Ca-Ru-O system, Ca2RuO4, Ca3Ru2O7 and third member of R-P series in Sr-Ru-O system, Sr4Ru3O10. Some of the available literature information on thermodynamic properties for other compounds of R-P series in Mn+1RunO3n+1 (M = Ca, Sr) are found to be based on incorrect assumptions and erroneous calculation. Thus, this chapter provides the complete thermodynamic information for all the electronically and magnetically applicable alkaline earth metal ruthenates for optimizing the deposition condition in device fabrications. Chapter 4 gives the structure-properties correlations of 2-3 spinel compounds and spinel-corundum equilibria for the system NiO-Al2O3-Cr2O3 at 1373 K. Nickel, aluminum and chromium are important base-constituent elements of high-temperature oxidation-resistant alloys. A spinel phase is usually found in the protective scale formed on the surface of the alloys. There is no thermodynamic data on spinel solid solution NiAl2O4-NiCr2O4. Thus, the phase relations and mixing properties of the spinel solid solution have been determined in this chapter. The inter-crystalline ion-exchange equilibrium between NiAl2+2xO4+3x-NiCr2O4 spinel solid solution and Al2O3-Cr2O3 solid solution with corundum structure in pseudo-ternary system NiO-Al2O3-Cr2O3 have been determined by the conventional tie-line rotation method at 1373 K. The nonstoichiometry of NiAl2+2xO4+3x has been taken into consideration. Lattice parameters were used to obtain the compositions of the corundum and spinel solid solutions at equilibrium. Formation of homogeneous solid solutions and attainment of equilibrium were confirmed by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). From the experimental tie-line information and thermodynamic data on Al2O3-Cr2O3 solid solution available in the literature, the activities in the spinel solid solution were derived by using a modified Gibbs-Duhem integration technique. Gibbs energy of mixing of the spinel solid solution has been calculated from the derived activity data. Since high temperature data generation is expensive and time consuming, it is useful to develop models, which relate thermodynamic properties to electronic and crystallographic structure, leading to predictive modeling of mixing properties. By comparing the results from models with experimental information, one can evolve methodologies for the prediction of the properties of uninvestigated system. A model can be used to discriminate among conflicting experimental data and extrapolate the data into regions where direct measurements are lacking or difficult to perform. In this chapter, a model approach has also been considered to analyze the activity-composition relationship in the NiAl2O4-NiCr2O4 spinel solid solution in terms of the intra-crystalline exchange of cations between the tetrahedral and octahedral sites of the spinel structure governed by site preference energies of the cations. Since Ni2+ and Cr3+ ion in tetrahedral coordination exhibits Jahn-Teller distortion, an entropy corresponding to randomization of the distortion in the cubic phase has been incorporated in the cation distribution model. The thermodynamic mixing properties of stoichiometric spinel solid solution NiAl2O4-NiCr2O4 in terms of one mole of mixing species were computed at 1373 K. The strain energy caused by size mismatch was added as a separate term to the Gibbs energy of mixing using empirical relationship between enthalpy of mixing for a pair of ions and the difference in their ionic volumes. Madelung constant and electrostatic contribution of energy of mixing of the spinel solid solution have also been computed. Comparison of Gibbs energy of mixing calculated using the cation mixing model for the stoichiometric spinel solid solution NiAl2O4-NiCr2O4 with that of the experimental tie-line data for nonstoichiometric spinel solid solution NiAl2+2xO4+3x-NiCr2O4 were included in this chapter. The thermodynamic mixing properties obtained in this study would be helpful in understanding the formation of complex spinel protective layers on alloys containing nickel, aluminium and chromium in high-temperature applications. The summary of the important finding and the conclusions arrived at on the basis of results obtained from the present investigations are presented in Chapter 5.

In this thesis research on synthesis, microstructure and properties of carbide-based coatings is reported. These coatings are electrically conducting, and can be tailored for high hardness, low friction and wear, along with load-adaptive behaviour. Tailoring these properties is achieved by controlling the relative phase content of the material. Coatings have been synthesised by dc magnetron sputtering, and their structures have been characterised, mainly by X-ray photoelectron spectroscopy and X-ray diffraction. It has been shown that nanocomposites comprising of a nanocrystalline transition metal carbide (nc-MeCx, Me = Ti, Nb or V) and an amorphous carbon (a-C) matrix can result in low contact resistance in electrical contacts. Such materials also exhibit low friction and high resistance to wear, making them especially suitable for application in sliding contacts. The lowest contact resistance is attained for small amounts of the amorphous carbon phase. It has been shown that specific bonding structures are present in the interface between nc-TiCx and the a-C phases in the nanocomposite.  It was found in particular that Ti3d and C2p states are involved, and that considerable charge transfer occurs across the interface, thereby influencing the structure of the carbide. Further design possibilities were demonstrated for TiCx-based nanocomposites by alloying them with weakly carbide-forming metals, i.e., Me = Ni, Cu or Pt.  Metastable supersaturated solid solution carbides, (T1-xMex)Cy, were identified to result from this alloying process. The destabilisation of the TiCx-phase leads to changes in the phase distribution in the deposited nanocomposites, thus providing further control over the amount of carbon phase formed. Additional design possibilities became available through the decomposition of the metastable (Ti1-xMex)Cy phase through an appropriate choice of annealing conditions, yielding either more carbon phase or a new metallic phase involving Me. This alloying concept was also studied theoretically for all 3d transition metals using DFT techniques. It has also been demonstrated that Ar-ion etching (commonly used in the analysis of carbide based nanocomposites) can seriously influence the result of the analysis, especially for materials containing metastable phases. This implies that more sophisticated methods, or considerable care are needed in making these analyses, and that many of the earlier published results could well be in error.

Made available in DSpace on 2014-10-09T12:54:28Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:42Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:54:28Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:42Z (GMT). No. of bitstreams: 0 / Este trabalho tem como objetivo principal investigar o comportamento frente à corrosão de aços inoxidáveis avançados em meios fisiológicos. Foram selecionados para o estudo quatro aços inoxidáveis visando avaliar o potencial destes para aplicações em implantes cirúrgicos: um aço superferrítico (DIN W. Nr. 1.4575), a Incoloy MA 956, contendo alumínio e óxido de ítrio, um aço austenítico DIN W. Nr. 1.4970 e um aço superaustenítico obtido por meio da adição de 0,87% de nitrogênio ao aço dúplex DIN W. Nr. 1.4460. Os três primeiros aços contêm baixo teor de níquel e suas películas protetoras são ricas em cromo, enquanto a Incoloy MA 956 é isenta de níquel, e rica em alumínio, o que influencia o seu filme passivo. Os materiais foram analisados usando técnicas de espectroscopia de impedância eletroquímica (EIE), polarização potenciodinâmica, técnica do eletrodo vibrante, microscopia eletroquímica de varredura, microscopia eletrônica de varredura de emissão de campo, microscopia ótica e microscopia eletrônica de varredura. Os meios escolhidos para avaliação da resistência à corrosão foram a solução de Hanks, um meio de cultura e uma solução tamponada com fosfato. Os resultados de EIE foram interpretados usando circuitos elétricos equivalentes que simularam uma camada passiva dúplex em todos os materiais analisados. Todos os materiais analisados apresentaram resistência à corrosão superior à do aço inoxidável AISI 316L, correspondente ao ASTM F-138, que é o mais utilizado na fabricação de implantes metálicos. Também foi destacada a importância do tratamento de solubilização nos aços com alto teor de nitrogênio. O DIN W. Nr. 1.4970 foi considerado citotóxico e sua potencialidade para uso como biomaterial, rejeitada. O DIN W. Nr. 1.4575 e Incoloy MA 956 podem ser usados como biomateriais, mas somente em próteses odontológicas ou de fácil remoção, devido ao seu comportamento ferromagnético. O DIN W. Nr. 1.4460 com 0,87% de nitrogênio foi o que apresentou as condições mais apropriadas para uso como biomaterial, inclusive para próteses ortopédicas. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP