Grafite ou diamante?

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Hardness and elasticity in cubic ruthenium dioxide

The Knoop hardness of the highly incompressible cubic phase of ruthenium dioxide was found to be 19–20 GPa from indentation tests. This value scales well with the shear modulus approximated by the elastic constant C44 of 144 GPa obtained from Brillouin scattering measurements. This work provides evidence that the shear modulus is a better indicator of hardness than the bulk modulus for ionic and covalent materials

Efeito de altas pressões sobre o gradiente de campo elétrico em metais sp

Neste trabalho foi estudado o efeito de altas pressões sobre o gradiente de campo elétrico (GCE) presente em núcleos de 111 Cd imersos em matrizes dos seguintes metais s-p: Zn, In, Sn e Tl. As medidas foram realizadas através da técnica de correlação angular diferencial perturbada. A alta pressão, aproximadamente hidrostática, foi produzida por um aparelho do tipo pistão-cilindro e cobriu a região de zero até 40 kbar para o Tl e até 30 kbar para as outras matrizes. O GCE decresceu com a pressão nas matrizes de Zn e Ti e cresceu para o Sn e In. Em to dos os casos o valor estimado para a taxa de variação do GCE com o volume, (MnVzz/âlnV) c/a,T , esta de acordo com os resultados sistemáticos, que mostram uma grande dependência volumétrica do GCE em metais. Com os resultados obtidos deste estudo foi possível estimar também a contribuição nau-harmônica responsável pela variação do GCE com a temperatura, que mostrou-se de magnitude comporável à contribuição harmônica. Os resultados foram com parados com as previsões baseadas no modelo de Nishiyama e Riegel, produzindo boa concordância apenas em Zn e In. O modelo de cargas pontuais também não conseguiu explicar todos os resultados, apresentando, no caso do Sn, um comportamento muito diferente do observado experimentalmente. Este fato foi interpretado como uma evidência de que a chamada "correlação de Raghavan" e originaria de uma similar dependência estrutural das contribuições ao GCE provenientes da rede e dos elétrons de condução.The effect of high pressure on the electric field gra dient (EFG) present at 111 Cd impurity nuclei in s-p host metals of Zn, In, Sn and Ti has been measured by the time-differential perturbed angular correlation technique. The nearly hydrostatic pressure was produced by a special piston-in-cylinder chamber, and covered a range up to 40 kbar for Ti and up to 30 kbar for the other hosts. The EFG decreases with pressure for Zn and Ti hosts and increases for Sn and In. In all hosts the estimated value of the rate of change of the EFG with volume, (ãlnVzz/B1nV) cia,1" is in agreement with systematics, showing a strong volume depen dence for the EFG in metals. With the obtained results it was possible to separate the anharmonic part of the EFG temperature dependence, which is shown to be of the same arder as the harmonic one. An attempt to interpret the experimental results using the Nishiyama and Riegel model gives good agreement only in the case of Zn and In. With the point charge model the agreement was good only for In, giving a very different trend in the case of Sn. This fact was interpreted as evidence that the so called "Raghavan correlation" results from a similar structural dependence of the electronic and ionic part of the EFG

Efeito de altas pressões sobre o gradiente de campo elétrico em metais sp

Neste trabalho foi estudado o efeito de altas pressões sobre o gradiente de campo elétrico (GCE) presente em núcleos de 111 Cd imersos em matrizes dos seguintes metais s-p: Zn, In, Sn e Tl. As medidas foram realizadas através da técnica de correlação angular diferencial perturbada. A alta pressão, aproximadamente hidrostática, foi produzida por um aparelho do tipo pistão-cilindro e cobriu a região de zero até 40 kbar para o Tl e até 30 kbar para as outras matrizes. O GCE decresceu com a pressão nas matrizes de Zn e Ti e cresceu para o Sn e In. Em to dos os casos o valor estimado para a taxa de variação do GCE com o volume, (MnVzz/âlnV) c/a,T , esta de acordo com os resultados sistemáticos, que mostram uma grande dependência volumétrica do GCE em metais. Com os resultados obtidos deste estudo foi possível estimar também a contribuição nau-harmônica responsável pela variação do GCE com a temperatura, que mostrou-se de magnitude comporável à contribuição harmônica. Os resultados foram com parados com as previsões baseadas no modelo de Nishiyama e Riegel, produzindo boa concordância apenas em Zn e In. O modelo de cargas pontuais também não conseguiu explicar todos os resultados, apresentando, no caso do Sn, um comportamento muito diferente do observado experimentalmente. Este fato foi interpretado como uma evidência de que a chamada "correlação de Raghavan" e originaria de uma similar dependência estrutural das contribuições ao GCE provenientes da rede e dos elétrons de condução.The effect of high pressure on the electric field gra dient (EFG) present at 111 Cd impurity nuclei in s-p host metals of Zn, In, Sn and Ti has been measured by the time-differential perturbed angular correlation technique. The nearly hydrostatic pressure was produced by a special piston-in-cylinder chamber, and covered a range up to 40 kbar for Ti and up to 30 kbar for the other hosts. The EFG decreases with pressure for Zn and Ti hosts and increases for Sn and In. In all hosts the estimated value of the rate of change of the EFG with volume, (ãlnVzz/B1nV) cia,1" is in agreement with systematics, showing a strong volume depen dence for the EFG in metals. With the obtained results it was possible to separate the anharmonic part of the EFG temperature dependence, which is shown to be of the same arder as the harmonic one. An attempt to interpret the experimental results using the Nishiyama and Riegel model gives good agreement only in the case of Zn and In. With the point charge model the agreement was good only for In, giving a very different trend in the case of Sn. This fact was interpreted as evidence that the so called "Raghavan correlation" results from a similar structural dependence of the electronic and ionic part of the EFG

Efeito de altas pressões sobre a interação quadripolar elétrica no háfnio metálico

O presente trabalho descreve medidas da interação quadripolar elétrica em háfnio metálico policritalino, como função da pressão. As medidas foram realizadas através da correlação angular diferencial perturbada da cascata gama-gama de 133-482 kev do181Ta dentro da rede do Hf. As frequências de interação quadripolar (wQ), medidas em pressões de 0, 20, 40 e 50 Kilobares foram, respectivamante, 48, 55, 62 e 63 Mrd/s, mostrando um claro aumento da frequência com a pressão. Os resultados mostram ainda que o parâmetrode assimetria e a largura da distribuição de frequência são praticamente insensíveis à pressão, dentro da gama de pressões investigadas.This present work describes measurements of the eletric quadrupole interaction in policristaline metalic hafnium, as a function of pressure. The measurements were performed by means of the differential perturbed angular correlation technique, through the 133-482 kev gamma-gamma cascade of 181Ta in a metalic Hf lattice. The quadrupolar interaction frequency (wQ) measured at pressures of 0, 20, 40 and 50 Kilobars,were respectivelly 48, 55, 62 and 63 Mrd/s, showing a clear increase of frequency with pressure. Our results also demonstrated that the assimetry parameter and the frequency distribution with, are pratically insensitive to the pressure, within the investigated range of pressure

Piston-cylinder apparatus for high-pressure impedance spectroscopy

A simple piston–cylinder apparatus was developed for impedance spectroscopy at pressures up to 3 GPa and temperatures up to 500 °C. The apparatus is of the compound cylinder type, with the innermost cylinder made of a commercially available low-cost alumina-based ceramic, with pistons of hard metal that allow electric connections. Cell electrical resistance was higher than 1 MV at T>300 °C and higher than 100 kV at 400 °C. The inductivity in the wires and pistons was about 2 mH. The sample was enclosed in a h-BN capsule, which insured a good hydrostaticity without disturbing the measurement. The equipment proved to be very useful in determining the activation volume of the conduction process in several ionic conductors

Piston-cylinder apparatus for high-pressure impedance spectroscopy

A simple piston–cylinder apparatus was developed for impedance spectroscopy at pressures up to 3 GPa and temperatures up to 500 °C. The apparatus is of the compound cylinder type, with the innermost cylinder made of a commercially available low-cost alumina-based ceramic, with pistons of hard metal that allow electric connections. Cell electrical resistance was higher than 1 MV at T>300 °C and higher than 100 kV at 400 °C. The inductivity in the wires and pistons was about 2 mH. The sample was enclosed in a h-BN capsule, which insured a good hydrostaticity without disturbing the measurement. The equipment proved to be very useful in determining the activation volume of the conduction process in several ionic conductors

Effects of high pressure on the electric field gradient in sp metals

The effect of high pressure on the electric field gradient (EFG) present at 111Cd impurity nuclei in sp host metals of Zn, In, β-Sn, and TI has been measured by the time-differential perturbed angular correlation technique. The pressure range was up to 40 kbar for TI and up to 30 kbar for other hosts. The EFG decreases with pressure for Zn and TI hosts and increases for Sn and In. In all hosts the rate of change of the EFG with volume (∂ lnq /∂ In V) c/a,T is in agreement with systematics. The results are compared with calculations based on existing theoretical approaches

Instrumento para medida de condutividade térmica de materiais sólidos

15

Effects of high pressure on the electric field gradient in sp metals

The effect of high pressure on the electric field gradient (EFG) present at 111Cd impurity nuclei in sp host metals of Zn, In, β-Sn, and TI has been measured by the time-differential perturbed angular correlation technique. The pressure range was up to 40 kbar for TI and up to 30 kbar for other hosts. The EFG decreases with pressure for Zn and TI hosts and increases for Sn and In. In all hosts the rate of change of the EFG with volume (∂ lnq /∂ In V) c/a,T is in agreement with systematics. The results are compared with calculations based on existing theoretical approaches