C. L. Carvalho

Evidence of distinct phase transformations in lithium phosphate glass Li2O–P2O5

The purpose of this work is to study the Li2O–P2O5 glass using the differential scanning calorimetry (DSC) and x-ray diffraction (XRD) techniques to understand the crystallization process in this glass matrix. To study the glass by DSC, screened samples with different particle sizes to resolve the crystallization peaks were used. Both crystallization peaks were attributed to Li6P6O18 and LiPO3 phases. This evidence was corroborated by XRD analysis on glasses annealed at different temperatures in order to crystallize these phases.

Effect of PbS impurity on crystallization mechanism of phosphate glasses studied by differential scanning calorimetry

Abstract In this work, differential scanning calorimetry (DSC) was used to study effect of PbS impurity on crystallization mechanism of phosphate glasses. Bulk glasses presented one crystallization peak while powdered glasses presented two distinct crystallization peaks. For both undoped and doped glasses were determined the activation energies for the crystallization and the Avrami n parameters. The activation energies for undoped phosphate glass were 336±6 and 213±3 kJ mol−1, respectively, associated with first and second crystallization peaks. For doped glass, the obtained energies were 373±9 and 286±7 kJ mol−1. The calculated Avrami parameters, based on first crystallization peaks, for undoped and doped glasses were 2.25±0.01 and 1.75±0.02, respectively. These values suggest that the first DSC peak, in both glasses, may be associated with surface crystallization.

Construção de um sistema de caracterização das propriedades de transporte de filmes finos pelo efeito Hall

In this work was built a system to determine transport properties such as electrical resistivity, charge carrier density and mobility of thin films using Hall effect in special conditions of undergraduate laboratory using no special materials either instruments. SnO2 thin films doped with fluor obtained by chemical process were characterized using the developed methodology and presented consistent results those reported in literature.

Preparation and characterization of a homemade Josephson junction prepared from a thin film sintered in a domestic microwave oven

A homemade Josephson junction was successfully obtained using a superconductor thin film of the BSCCO system. The film was deposited on a lanthanum aluminate, produced from a commercial powder with a nominal composition Bi1.8Pb0.4Sr2CaCu2Ox, was thermally treated by a domestic microwave oven. The XRD analysis of the film indicated the coexistence of Bi-2212 and Bi-2223 phases and SEM images revealed that a typical superconductor plate-like morphology was formed. From the electrical characterization, performed using DC four probes technique, it was observed an onset superconducting transition temperature measured around 81K. At the current-voltage characteristics curve, a step of electric current at zero-voltage could be observed, an indicative that the tunneling Josephson occurred.

Experimento demonstrativo de levitação supercondutora: Ferramenta para problematização de conceitos físicos

In the middle of 1980’s, the ceramic superconductors were novel materials that exhibit superconductivity at temperatures larger than that of liquid nitrogen (T = 77 K). This cryogenic liquid is easier to be obtained and cheaper than liquid helium (T = 4.2 K). Thus, the possibility of applications of such materials increased and they are suited to the efforts to search clean energy sources and the growth of a sustainability society. This is possible due to the properties of the superconductors as zero resistivity and perfect diamagnetism. In this work we will present the arrangement of a superconducting levitation experiment that can be used as a motivation for high school students and also to encourage them to choose a scientific career.

Synthesis of Bi-based superconductor by microwave-assisted hydrothermal method

In this work we studied the synthesis of BSCCO-2212 superconducting phase associating a quite similar method developed by Pechini (PM) with the microwave-assisted hydrothermal method, MAH. To study the influence of MAH on the properties of BSCCO system, we synthesized two samples by such method. For one sample we used carbonates (CMAH) and for the other one we used nitrates (NMAH) as chemical reagents. We also produced a reference sample (REF) just using carbonates by PM to compare their morphological and superconducting properties. The structural properties of the samples were analyzed by scanning electron microscopy and X-ray diffraction. It can be noted that the Bi-2212 phase is predominant in all samples; MAH samples present larger grains than the REF one. However, we can observe that the magnetic behaviour of NMAH is closer to that presented by REF.

Planejamento e produção de um criostato de baixo custo para caracterização elétrica de materiais

Superconductor systems based on BSCCO (BiSrCaCuO) show three main structures, Bi2Sr2CuO, Bi2Sr2Ca Cu2O, and Bi2Sr2CaCu3O, with critical temperatures about 20 K, 80 K, and 110 K, respectively. Therefore, it is fundamental to characterize these systems at low temperatures. In this work it was projected a cryogenic system with capacity to reach temperatures below the liquid nitrogen temperature (77 K). Controlling the pressure on the liquid nitrogen system, according to usual thermodynamic principles, it has been possible to reach a range of temperatures above 64 K in the sample holder. With the availability of a larger temperature range, it became possible to identify at least two superconducting phases in the BSCCO system. There is a favorable cost/benefit ratio with respect to using liquid helium as the refrigerator system. Also, this cryostat provides an application of the standard p-T diagrams (Gay-Lussac law), which are studied in undergraduate courses, and can be used as a pedagogical experimental demonstration.

Electrical devices obtained from polymeric precursors process

Physics and Chemistry Universidade Estadual Paulista, Av. Brasil, 56, Ilha Solteira, Sao Paulo, 15385-000

Study of electrical, chemical and structural characteristics of superconductor thin film obtained by polymeric precursors method

Superconductor films of the BSCCO system have been grown by dip coating technique with good success. The chemical method allows us to grow high temperature superconductor thin films to get better control of stoichiometry, large areas and is cheaper than other methods. There is a great technological interest in growth oriented superconductor films due anisotropic characteristics of superconductor materials of high critical temperature, specifically the cuprates, as we know that the orientation may increase the electrical transport properties. Based on this, the polymeric precursor method has been used to obtain thin films of the BSCCO system. In this work we have applied that method together with the deposition technique known as dip coating to obtain Bi-based superconductor thin films, specifically, Bi 1.6 Pb 0.4 Sr 2.0 Ca 2.0 Cu 3.0 O x+δ , also known as 2223 phase with a critical temperature around 110 K. The films with multilayers have been grown on crystalline substrates of LaAlO 3 and orientated (100) after being heat treated around 790°C - 820°C in lapse time of 1 hour in a controlled atmosphere. XRD measurements have shown the presence of a crystalline phase 2212 with a critical temperature around 85 K with (001) orientation, as well as a small fraction of 2223 phase. SEM has shown a low uniformity and some cracks that maybe related to the applied heat treatment. WDS has also been used to study the films composition. Different heat treatments have been used with the aim to increase the percentage of 2223 phase. Measurements of resistivity confirmed the presence of at least two crystalline phases, 2212 and 2223, with T c around 85 K and 110 K, respectively.