Francisco G. Emmerich

Evolution with heat treatment of crystallinity in carbons

Abstract The variation with heat treatment of the dimensions La and Lc of the graphite-like crystallites of graphitizable and non-graphitizable carbons is studied. The increases of La and Lc with heat treatment temperature (HTT) owing to three processes (crystallite growth in-plane, coalescence of crystallites along the c-axis and coalescence of crystallites along the a-axis) are functionally separated. The evolution with HTT of the number of crystallites (Ncr), the mean volume of the crystallites (vcr) and the total volume occupied by the crystallites (Vcr) are determined in terms of the changes of La, Lc and d002. Since among other characteristics the crystallites form the electrical and thermal conducting phase of the carbon, Ncr, vcr and Vcr are important parameters in many physical properties of these materials. The developed expressions were applied to a non-graphitizable and to a graphitizable carbon.

Investigation of biomass- and polymer-based carbon materials using 13C high-resolution solid-state NMR

Carbon materials produced through the thermal decomposition of biomass (rice hulls) and a polymer (polyvinyl chloride) were investigated by 13C high-resolution solid-state NMR under two different magnetic fields (2.0 and 9.4 T). The details revealed by the high-field NMR spectra provide important information about the chemical changes in the initial stages of pyrolysis: These are shown to be directly related to the original structure of the precursors and the results complement well some conclusions existent in the literature. From a heat treatment temperature of about 600°C upwards, the general shape of the 13C NMR spectra, attained with low applied magnetic field, is very similar for both chars, with a strong resonance line near 125 ppm from TMS (carbon nuclei in aromatic planes). The analysis of the evolution of the main parameters associated with this resonance line shows a behavior typical of heat-treated carbon materials, which is interpreted on the basis of the structural evolution of both chars. We show that the results are well understood when a comparison is made with the features of the 13C NMR spectrum of polycrystalline graphite.

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.

ESR in heat treated carbons from the endocarp of babassu coconut

The results of an electron spin resonance (ESR) study for the endocarp of babassu coconut with heat treatment temperature up to 2,200°C are presented. Correlations between the transition range (700 ≤ HTT < 1,300°C) with the low and high HTT ranges are emphasized. Similar to other heat treated carbons, free radicals were observed in the low HTT range, and free charge carriers in the high HTT range. In the transition range, the usual broadening of the linewidth was observed for the macroscopic samples (φ −1 mm, where φ is the particle size). On the other hand, the ground samples (φ < 37 μm and φ < 74 μm) showed two well-defined resonances (g ≤ 2.004; ΔH ≤ 2 Gandg ≥2.000; ΔH ≥ 10 G). The temperature dependence of the ESR spectra show that the narrow line is associated to remainder free radicals and the broad one to free charge carriers that dominate in the high HTT range. Although the electrical conductivity becomes appreciable about 700°C HTT, when the material experiences a nonmetal-metal percolation transition, the resonance of the charge carriers for the macroscopic samples is clearly observed only above 1,300°C HTT. This fact is explained by the silicon effect, which plays an important role in this material.

Tensile strength and fracture toughness of brittle materials

The fractureproperties of brittlematerials under tension have been explained by many authors; however, questions such as the dependence of the tensile strength on the crack tip radius of curvature and the scatter of fracture toughness are still not well explained from fundamental principles. This work aims to address this question by using a force-atomistic approach: we analyze the forces that act in the solid down to the smallest dimensions in an atomistic context, verifying the satisfaction of the static equilibrium condition given by Newton’s second law up to the beginning of the rupture. We take into account the forces due to the applied stress, which may be very large at crack tips, and the material cohesion forces, particularly at the point of largest local strain and stress concentration, where the local hyperelasticity of the material plays a governing role. By considering and connecting microstructure and atomicity, and using an experimentally proved maximum tensile-stress criterion for fracture, here we obtain an expression for the tensile strength of the brittlematerials, where an effective local cohesive stress is defined. Thus, we explain in a unified framework from fundamental principles a set of established experimental results of brittlefracture of materials under tension, including the dependence of the tensile strength on the crack tip radius of curvature and some scatter in reported values of fracture toughness and cleavage surface energy. This work can be useful to make more realistic predictions of fractureproperties of brittlematerials taking into account microstructure and atomicity.

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.

Young's modulus of heat-treated carbons: A theory for nongraphitizing carbons

Abstract An expression to calculate the Youngs modulus of nongraphitizing carbons as a function of heat treatment temperature (HTT) is developed. A microscopic model involving parameters of the carbon granular structure is used, where the cross-linkings between microcrystallites play a fundamental role. The theoretical expression is applied to the endocarp of babassu coconut heat treated from 400°C up to 2200°C; it explains satisfactorily the behaviour of the modulus with HTT, including the maximum that generally occurs around 1100°C HTT.

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.