Estudo de propriedades mecanicas em ceramicas supercondutoras com base em Bi-Sr-Ca-Cu-O dopadas com chumbo

Orientador: Vicente Roberto DumkeDissertação (mestrado) - Universidade Federal do ParanaResumo: Estudos realizados em cerâmicas supercondutoras com base em Bi-Sr-Ca-Cu-O, dopadas com chumbo, de acordo com a formula estequiometrica Bi2_xPbxSr2Ca2Cu30," mostraram que a introdução do chumbo altera as propriedades mecânicas do material, em particular a plasticidade. Usando um microdurometro Vickers, observou-se que a amostra com concentração de chumbo x = 0,4 possui a maior microdureza Vickers. Percebeu-se, ainda, com a utilização de um porosímetro de mercúrio, que nesta concentração houve uma redução drástica do volume de cada poro, mostrando que o chumbo melhorou o processo de sinterização, já que essa amostra ( x = 0,4 ) possui o menor intervalo em que 80 % dos poros se encontram ( 0,19 a 0,79 fim ). Através da microscopia ótica e eletrônica, na observação das fases, pode-se perceber a presença de duas fases principais: fase com propriedades isolantes e outra com propriedades supercondutoras. Uma fase menor, com propriedades birrefringentes, foi também observada tendo a sua maior concentração na amostra x = 0,2. O aumento da microdureza, atingindo o seu máximo na amostra com concentração x = 0,4, deve-se a redução do volume da fase com propriedades isolantes ( com uma dureza menor, conforme observado do polimento ), pois esta atinge o seu mínimo na amostra com a mesma concentração. Também, através da microscopia eletrônica, percebeu-se que a maior microdureza na amostra com concentração x = 0,4 não foi influenciada pela variação do diâmetro ou da forma dos cristais, já que todos possuem a forma de placas lamelares. Da literatura cientifica, sabe-se que a amostra com concentração x = 0,4 possui a menor resistividade, para uma certa temperatura, em relação as demais amostras. Deste trabalho, percebeu-se que isto deve-se ao processo de sinterização ( redução drástica do volume de cada poro ), aumentando, portanto, a adesão entre os grãos e da diminuição da fase com propriedades isolantes fazendo com que a resistência intergranular da amostra diminua. Foram realizados também ataque químico, com acido acético e agua, na verificação da diferença de solubilidade das fases presentes. Observou-se que demais. Foi observado também sobre as superfícies das amostras o aparecimento de uma película, produto da reação do acido com a amostra. O diâmetro destas diminuiu ate que na concentração de chumbo x = 1,0 não houvesse mais reação. Novos estudos estão sendo realizados com o objetivo de verificar a natureza destas.Abstract: Investigations performed in superconducting ceramics based on Bi-Sr-Ca- Cu-O, doped with lead, according to the stoichiometric formulae Bi2-xPbI Sr2 Ca2Cu3Oy, demonstrated the insertion of lead alters the mechanical properties of the material, particularly the plasticity. When employing an Hardness Testing, it was observed that the sample with a lead concentration of x = 0.4 holds the highest Vickers microhardness. It was also noticed, when using a mercury porosimeter, that on this concentration there was a drastic reduction of the volume of each pore, evincing the lead enhanced the sinterization process, considering that the sample with a lead concentration of x = 0.4 holds the minimum interval in which 80 % of the pores meet ( 0.19 a 0.79 /zm ). Through optical and electronic microscopy, one can notice the presence of two main phases: a phase presenting isolating properties and another with superconducting ones. A minor phase, with birefringent properties, was also observed showing its highest concentration in the sample x = 0.2. The increase in microhardness, reaching its top in sample x = 0.4, is due to the volume reduction of the phase with isolating properties ( with a lower hardness, as it was observed in the polishing ), for it reaches its minimum in the sample presenting the same concentration. Also, through electronical microscopy, it was noted the highest microhardness in the sample x = 0.4 was not influenced by the diameter variance or by the crystal shape, since they all present the shape of lamelars plates. From the scientific literature its known that the sample with concentration x = 0.4 holds the least resistivity to a certain temperature in relation to the other samples. From this task, it is remarkable to perceive this is due to the process of sintierization ( drastical reduction of the volume of each pore ), increasing, therefore, the adhesion among the grains and, also, to the diminution of the phase with isolating properties causing the reduction of the intergranular resistance of this sample. Chemical etching, based on acetic acid and water, have been performed in order to verify the solubility of the present phases. It was observed that the phase showing biref. ingent properties were much more soluble than the others. It was also observed the features of sheet, product of the reaction between the acid and the sample, on the surfaces of the samples. The diameter of these decreased until the moment when in lead concentration x = 0.1 there was no more reaction. New investigations are now in progress to verify the nature of these sheet

The upper critical field in superconducting MgB_2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB_2 superconductor was investigated in magnetic fields up to 16 T. The upper critical field of the major fraction of the investigated sample was determined from ac susceptibility and resistance data and was found to increase up to Hc2(0) = 13 T at T = 0 corresponding to a coherence length of 5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc and at intermediate temperatures. This positive curvature of Hc2(T) is similar to that found for the borocarbides YNi_2B_2C and LuNi_2B_2C indicating that MgB_2 is in the clean limit.Comment: 8 pages with 4 figure

A microscopic approach to heating rate of ferrofluid droplets by a magnetic field

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORIn this work, we study the heating process of colloidal ferrofluids by a magnetic field. The heating of the fluid occurs by the magnetic relaxation of the nanoparticles which provide thermal energy for the host liquid. In the limit of small volumes, the relaxation process occurs through the Neel mechanism since the magnetic nanoparticles present superparamagnetic behavior. Within this limit, we have used a microscopic model for the coupling to phonons and external magnetic field in order to model the relaxation mechanism and to obtain an expression for the heating rate of the fluid as a function of microscopic parameters. The analysis allows determining appropriate conditions for an optimal heating rate for ferrofluids based on superparamagnetic nanoparticles.1254111CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIOR307466/2016-7PNPD 028/2010E.C.S. acknowledges support from the CNPq agency under Grant No. 307466/2016-7. P.A.O. acknowledges financial support from FONDECYT under Grant No. 1140571. F.F.F. and E.C.S. acknowledge CAPES agency financial support (No. CAPES PNPD 028/2010)

Depth-Sensing Indentation on REBa2Cu3O(7-\delta) Single Crystals obtained from Xenotime Mineral

A natural mixture of heavy rare earths oxides extracted from xenotime mineral have been used to prepare large single crystals of high-temperature REBa2Cu3O(7-\delta) superconductor grown using the CuO-BaO self-flux method. Its mechanical properties along the ab-plane were characterized using instrumented indentation. Hardness and elastic modulus were obtained by the Oliver and Pharr method and corresponds to 7.4 \pm 0.2 GPa and in range 135-175 GPa at small depths, respectively. Increasing the load promotes the nucleation of lateral cracks that causes a decrease in hardness and the measured elastic modulus by instrumented indentation at higher loads. The indentation fracture toughness was estimated by measuring the radial crack length from cube-corner indentations at various loads and was 0.8 \pm 0.2 MPa.m1/2. The observed slip systems of REBa2Cu3O(7-\delta) single crystals were [100](001) and [010](001), the same as for YBa2Cu3O(7-\delta) single crystals. The initial stages of deformation and fracture in the indentation process were investigated. The hardness and elastic modulus were not strongly modified by the crystallographic orientation in the ab-plane. This was interpreted in terms of the resolved shear stresses in the active slip systems. Evidence of cracking along the {100} and {110} planes on the ab-plane was observed. As a conclusion, the mechanical properties of REBa2Cu3O(7-\delta) single crystals prepared from xenotime are equivalent to those of YBa2Cu3O(7-\delta) single crystals produced by conventional rare earths oxides.Comment: The paper will appear in Volume 42 (2012) of the Brazilian Journal of Physic

Magneto-transport and magnetic susceptibility of SmFeAsO1-xFx (x = 0.0 and 0.20)

Bulk polycrystalline samples, SmFeAsO and the iso-structural superconducting SmFeAsO0.80F0.20 are explored through resistivity with temperature under magnetic field {\rho}(T, H), AC and DC magnetization (M-T), and Specific heat (Cp) measurements. The Resistivity measurement shows superconductivity for x = 0.20 sample with Tc(onset) ~ 51.7K. The upper critical field, [Hc2(0)] is estimated ~3770kOe by Ginzburg-Landau (GL) theory. Broadening of superconducting transition in magneto transport is studied through thermally activated flux flow in applied field up to 130 kOe. The flux flow activation energy (U/kB) is estimated ~1215K for 1kOe field. Magnetic measurements exhibited bulk superconductivity with lower critical field (Hc1) of ~1.2kOe at 2K. In normal state, the paramagnetic nature of compound confirms no trace of magnetic impurity which orders ferromagnetically. AC susceptibility measurements have been carried out for SmFeAsO0.80F0.20 sample at various amplitude and frequencies of applied AC drive field. The inter-granular critical current density (Jc) is estimated. Specific heat [Cp(T)] measurement showed an anomaly at around 140K due to the SDW ordering of Fe, followed by another peak at 5K corresponding to the antiferromagnetic (AFM) ordering of Sm+3 ions in SmFeAsO compound. Interestingly the change in entropy (marked by the Cp transition height) at 5K for Sm+3 AFM ordering is heavily reduced in case of superconducting SmFeAsO0.80F0.20 sample.Comment: 18 pages text + Figs: comments/suggestions welcome ([email protected]

Fluctuation Conductivity of Polycrystalline Y

We studied the effect of superconducting fluctuations on the electrical conductivity of granular samples of Y 1−x Pr x Ba 2 Cu 3 O 7−δ superconductors, with x =0.01, 0.03, 0.05, 0.07 and 0.10. Samples were prepared by the standard solid-state reaction technique, with two different types of calcination process, in air at 900˚C ( x ≤ 0.07 ) and in vacuum at 850˚C ( 0.05 ≤ x ≤ 0.10 ). For the samples prepared in air, our results revealed a splitting of the bulk transition, denoted by T C1 and T C2 , besides the coherence transition. It was observed fluctuation regimes above the highest transition ( T C1 ) and the lowest transition ( T C2 ). For the samples calcinated in vacuum and high concentrations of Pr, changes were observed in the critical region with chemical substitution of the Pr ion for the Y ion. In the regime of approach to the zero resistance state it was observed an occurrence of a coherence transition for all concentrations of praseodymium

Estudo de propriedades mecanicas em ceramicas supercondutoras com base em Bi-Sr-Ca-Cu-O dopadas com chumbo

Orientador: Vicente Roberto DumkeDissertação (mestrado) - Universidade Federal do ParanaResumo: Estudos realizados em cerâmicas supercondutoras com base em Bi-Sr-Ca-Cu-O, dopadas com chumbo, de acordo com a formula estequiometrica Bi2_xPbxSr2Ca2Cu30," mostraram que a introdução do chumbo altera as propriedades mecânicas do material, em particular a plasticidade. Usando um microdurometro Vickers, observou-se que a amostra com concentração de chumbo x = 0,4 possui a maior microdureza Vickers. Percebeu-se, ainda, com a utilização de um porosímetro de mercúrio, que nesta concentração houve uma redução drástica do volume de cada poro, mostrando que o chumbo melhorou o processo de sinterização, já que essa amostra ( x = 0,4 ) possui o menor intervalo em que 80 % dos poros se encontram ( 0,19 a 0,79 fim ). Através da microscopia ótica e eletrônica, na observação das fases, pode-se perceber a presença de duas fases principais: fase com propriedades isolantes e outra com propriedades supercondutoras. Uma fase menor, com propriedades birrefringentes, foi também observada tendo a sua maior concentração na amostra x = 0,2. O aumento da microdureza, atingindo o seu máximo na amostra com concentração x = 0,4, deve-se a redução do volume da fase com propriedades isolantes ( com uma dureza menor, conforme observado do polimento ), pois esta atinge o seu mínimo na amostra com a mesma concentração. Também, através da microscopia eletrônica, percebeu-se que a maior microdureza na amostra com concentração x = 0,4 não foi influenciada pela variação do diâmetro ou da forma dos cristais, já que todos possuem a forma de placas lamelares. Da literatura cientifica, sabe-se que a amostra com concentração x = 0,4 possui a menor resistividade, para uma certa temperatura, em relação as demais amostras. Deste trabalho, percebeu-se que isto deve-se ao processo de sinterização ( redução drástica do volume de cada poro ), aumentando, portanto, a adesão entre os grãos e da diminuição da fase com propriedades isolantes fazendo com que a resistência intergranular da amostra diminua. Foram realizados também ataque químico, com acido acético e agua, na verificação da diferença de solubilidade das fases presentes. Observou-se que demais. Foi observado também sobre as superfícies das amostras o aparecimento de uma película, produto da reação do acido com a amostra. O diâmetro destas diminuiu ate que na concentração de chumbo x = 1,0 não houvesse mais reação. Novos estudos estão sendo realizados com o objetivo de verificar a natureza destas.Abstract: Investigations performed in superconducting ceramics based on Bi-Sr-Ca- Cu-O, doped with lead, according to the stoichiometric formulae Bi2-xPbI Sr2 Ca2Cu3Oy, demonstrated the insertion of lead alters the mechanical properties of the material, particularly the plasticity. When employing an Hardness Testing, it was observed that the sample with a lead concentration of x = 0.4 holds the highest Vickers microhardness. It was also noticed, when using a mercury porosimeter, that on this concentration there was a drastic reduction of the volume of each pore, evincing the lead enhanced the sinterization process, considering that the sample with a lead concentration of x = 0.4 holds the minimum interval in which 80 % of the pores meet ( 0.19 a 0.79 /zm ). Through optical and electronic microscopy, one can notice the presence of two main phases: a phase presenting isolating properties and another with superconducting ones. A minor phase, with birefringent properties, was also observed showing its highest concentration in the sample x = 0.2. The increase in microhardness, reaching its top in sample x = 0.4, is due to the volume reduction of the phase with isolating properties ( with a lower hardness, as it was observed in the polishing ), for it reaches its minimum in the sample presenting the same concentration. Also, through electronical microscopy, it was noted the highest microhardness in the sample x = 0.4 was not influenced by the diameter variance or by the crystal shape, since they all present the shape of lamelars plates. From the scientific literature its known that the sample with concentration x = 0.4 holds the least resistivity to a certain temperature in relation to the other samples. From this task, it is remarkable to perceive this is due to the process of sintierization ( drastical reduction of the volume of each pore ), increasing, therefore, the adhesion among the grains and, also, to the diminution of the phase with isolating properties causing the reduction of the intergranular resistance of this sample. Chemical etching, based on acetic acid and water, have been performed in order to verify the solubility of the present phases. It was observed that the phase showing biref. ingent properties were much more soluble than the others. It was also observed the features of sheet, product of the reaction between the acid and the sample, on the surfaces of the samples. The diameter of these decreased until the moment when in lead concentration x = 0.1 there was no more reaction. New investigations are now in progress to verify the nature of these sheet