Alfredo G. Cunha

Formation and stability of HgCaO2, a competing phase in the synthesis of Hg1−xRexBa2Ca2Cu3O8+δ superconductor

Abstract Thanks to a novel technique (thermobaric analyser or TBA) for measuring the in-situ pressure in quartz tubes, we have investigated the formation and stability of HgCaO2 and its competition with the Hg,Re-1223 in the superconductor synthesis. The pressure determination has enabled us to obtain thermodynamic values such as ΔHf, ΔSf, from which formation temperatures and stability of the binary oxide were determined. This fact has made possible to minimise the presence of this impurity applying suitable heating and cooling ramps in the thermal treatment leading to the formation of Hg,Re-1223. It has been observed that the faster the heating ramp, the less HgCaO2 phase was formed. However, the cooling ramp only influenced the Hg,Re-1223 stability, being more advantageous to perform slow cooling rates. In summary, the in-situ pressure measurement has permitted to ascertain the way to minimise the HgCaO2 impurity content and hence to improve the quality of the ceramic Hg,Re-1223 superconductor.

Influence of precursor oxygen stoichiometry on the formation of Hg, Re-1223 superconductors

Thanks to a novel technique (thermobaric analyser or TBA) for measuring the in situ pressure in quartz tubes, we have investigated the precursor quality for the synthesis of the superconductor Hg-1223 which is an essential parameter to control. We have made this study on the samples and we can conclude that this superconductor phase requires a ceramic precursor sintered in a low mixture gas flux. An excessively oxygenated precursor leads to overdoped superconducting phases, presence of other members with lower n, and some unreacted precursor. The precursor oxygenation degree also modifies the kinetics of formation of and the partial melting of the superconductor material may be affected.

Improvement of superconducting (Hg,Re)-1223 ceramics synthesised by the sealed quartz tube technique

We analyse the effect of several synthesis conditions and parameters on the superconducting properties of (Hg,Re)-1223 ceramics obtained by the sealed quartz tube technique. The incorporation of liquid mercury results in an increase of the superconducting volume fraction, probably as a consequence of the displacement of the reaction towards the superconducting phase formation. On the other hand, the oxygen content of the precursor is shown to have a significant effect on the sample compactness, and therefore on the intergrain connectivity of the superconductor. Finally, wrapping the unreacted superconductor pellet in a gold foil has also beneficial effects on the quality of the synthesised samples.

Effects of re-doping on superconducting properties and formation of Hg-1223 superconductors

Abstract The influence of the rhenium content on the formation of the Hg 1− x Re x Ba 2 Ca 2 Cu 3 O 8+ δ superconductors is reported. For the same synthesis conditions, different samples are obtained, as shown by XRD and magnetic measurements. This proves that the synthesis conditions have to be adapted precisely to the rhenium content. Nevertheless, the Re concentration in the superconductor sample has the tendency to form a stable ratio with respect to Hg; i.e., 1/4 Re and 3/4 Hg, as was found by EDX analysis. Moreover, a clear influence of the Re content on the oxygen δ of the HgO δ plane was observed by micro-Raman analysis.

Preparação e caracterização de carvão ativado quimicamente a partir da casca de arroz

This work consists in a study about the chemical activation of charred rice hulls using NaOH as the activation agent. The influence of the naturally-occurring silica was particularly evidenced. X-ray diffraction patterns showed the formation of sodium carbonate and silicates in the activated samples, whereas thermogravimetric curves revealed a strong reduction in the ash content of these samples after washing with water. Nitrogen adsorption data indicated a microporosity development only in the washed samples, with BET surface area values of 450 and 1380 m2/g achieved for the samples activated at 800 °C starting from the precursor with or without silica, respectively.

Comparison of practical techniques to develop latent fingermarks on fired and unfired cartridge cases

We have tested some widely used and practical fingermark enhancement techniques such as powdering (regular powder dusting and magnetic powder application), cyanoacrylate fuming, fluorescent dying (basic yellow 40), gun blueing solutions and acidified hydrogen peroxide solutions. The results were evaluated and compared in order to establish best procedures on processing cartridge cases. The tests were performed on brass discs subjected to three different temperatures (room temperature, 63 and 200°C), and on fired and unfired cartridge cases. All the samples were processed after three different periods of time (24h, 7 days and 14 days) after deposition. The best results for both fired and unfired cartridge cases were obtained by the sequential application of cyanoacrylate, gun blueing solution and basic yellow 40. Some stages of the firing process were isolated in order to identify their effects over the final amount and quality of the remaining latent fingermarks on cartridge cases. Good state fingermarks were developed on unfired cartridge cases cycled through the gun, showing that friction inside the gun without firing does not cause significant damage to the fingermarks. On the other hand, fired cartridge cases are significantly affected by the firing effects, exhibiting low quality ridge details which are mainly located next to base. An unexpected phenomenon was observed on most of the brass discs heated to 200°C and developed with gun blueing solutions; they presented a reverse development compared to the expected one, with darkening of the ridges instead of the background.

Physical and chemical properties of a Brazilian peat char as a function of HTT

Physical and chemical properties of a Brazilian peat char as a function of heat treatment temperature (HTT) from 300 to 1000°C are reported. Powdered and compacted cylindrical samples of peat char were prepared at each HTT. The samples were characterized by ultimate analysis, proximate analysis, thermogravimetric analysis and measurements of dimensional changes, density, calorific value and electrical resistivity. The char has advantages over natural dried peat, such as higher calorific value and lower volatile matter. An advantage of the peat char studied in comparison with most wood charcoals is the higher apparent density (0.9 g cm−3) obtained for the compacted samples without any binding agent.

Hg0.95Re0.05Ba2Ca2Cu3O8+delta superconductor: sample preparation and transport properties under hydrostatic pressure

Samples of the Hg1-x Rex Ba2 Ca2 Cu3 O8+ superconductor (Hg, Re-1223), with low rhenium (Re) content (x = 0.05), have been produced with the help of a novel thermobaric analysis technique which could monitor the total pressure inside the quartz tube during synthesis at high temperature. The sample quality was verified by means of Rietveld analysis of the x-ray diffraction data, and by ac susceptibility measurements. The formation and stability of the Hg, Re-1223 phase has also been investigated by varying the oxygen content of the Re0.05 Ba2 Ca2 Cu3 O8+ precursor, and the mercury filling factor (ffHg = 0.00 to 0.014 g cm-3). It was observed that the reduction of oxygen content in the precursor (Re0.05 Ba2 Ca2 Cu3 O8+) and the increase of the mercury partial pressure inside the quartz tube enhanced the yield of the superconductor phase. Resistance measurements as a function of temperature under hydrostatic pressure (0-1.0 GPa) show an increase in the superconducting transition temperature which can be fitted with a parabolic curve. The reduction of the lattice volume induced by external hydrostatic pressure is similar to the one induced by chemical pressure due to rhenium (Re) doping (0.00 < x < 0.10); however TC does not depend on the chemical pressure. This behaviour can be understood on the basis of the pressure-induced charge transfer model (PICTM) modified by Almasan et al.

Rhenium effect in the formation and stability of HgCaO2 and Hg1-xRexBa2Ca2Cu3O8+δ superconductor

Abstract Using a novel technique (thermobaric analysis or TBA) to measure in situ the pressure in quartz tubes, we have investigated the influence of Hg and Re x content in the range 0.00⩽ x ⩽0.25 on the formation and stability of HgCaO 2 . Moreover the competition between the synthesis of HgCaO 2 and of the superconductor Hg 1− x Re x Ba 2 Ca 2 Cu 3 O 8+ δ (Hg,Re)-1223 was investigated. The pressure determination has enabled us to obtain all thermodynamic parameters such as the enthalpy Δ d H 0 , entropy Δ d S 0 and the Gibbs free energy Δ d G 0 and follow the decomposition reactions by monitoring the pressure behavior. This possibility of pressure measurements has also allowed us to acquire a better knowledge on the effects of Re substitution in the formation of (Hg,Re)-1223. The TBA technique has also permitted us to minimize the HgCaO 2 impurity content and hence to improve the quality of the ceramic (Hg,Re)-1223 superconductor.

A novel thermobaric analyser: in situ measurement of gas pressure during synthesis in sealed quartz tube at high temperatures

We have developed a novel technique (thermobaric analysis or TBA) to measure, in situ up to 900 °C, the pressure of gases such as Hg and O2 in sealed quartz tubes. The pressure determination in closed systems enables us to obtain information on the synthesis of compounds which involve solid-gas reactions. The concept of the TBA set-up is described, including the calibration method and the verification with HgO decomposition. The technique is applied to the optimized synthesis of the ceramic Hg, Re-1223 superconductor.