Effects of photoacoustic measurements on a nanostructured ZnSe mechanically alloyed.

Zinc-blende ZnSe powder, with particles containing nanometric grains, was prepared using the mechanical alloying technique, from an equiatomic mixture of elemental Zn and Se powders. An important interfacial component was observed in the as-milled powder, which affects its thermal and optical properties. In order to obtain a high-quality zinc-blende ZnSe powder an annealing process was applied. The structural properties of both as-milled and annealed powders were characterized by an x-ray diffraction technique. The effects of defect centers on the optical band gap of both as-milled and annealed zinc-blende ZnSe powders were studied through optical absorbance measurements. The results showed a substantial broadening and a shift of the band gap energy to lower values in the as-milled powder, while for the annealed sample no effects were observed. For the annealed sample, the measured band gap energy value is in good agreement with those reported in the literature. A sequence of photoacoustic absorption measurements of the as-milled ZnSe sample showed that the absorbed energy promotes a structural relaxation. This relaxation causes a change in the heat transfer mechanism from thermal diffusion to thermoelastic bending and a reduction in the thermal diffusivity values. Similar measurements of the annealed nanostructured ZnSe sample showed the thermal diffusivity value to have a high degree of stability, and to be in good agreement with values reported in the literature.

The pressure-induced phase transition of mechanically alloyed nanocrystalline GaSb.

Ga K-edge energy dispersive x-ray absorption spectroscopy and Raman spectroscopy measurements were employed to follow the pressure-induced semiconductor-metal phase transition of nanocrystalline GaSb produced by mechanical alloying up to 26 GPa. The results showed a slight increase of the phase transition pressures for both as-milled (8 GPa) and annealed (10 GPa) GaSb samples, as compared to that for the bulk one. The extended x-ray absorption fine structure analysis of the zinc blende (ZB) pressure domain (<10 GPa) showed that the microscopic compressibility of the bonds in the as-milled/annealed samples is higher/lower than the crystalline bulk modulus (56 GPa). The comparison between x-ray absorption near edge structure regions of the spectra and multiple scattering calculations suggests that the ZB structure evolves to a short-range chemically ordered ß-Sn structure for pressures as high as 8 GPa. Raman measurements confirm the semiconductor-metal phase transitions of ZB-GaSb between 8 and 11 GPa for both as-milled and annealed samples, showing that the semiconductor character was not recovered on releasing the pressure down to 3.9 and 1.8 GPa, indicating a very strong hysteresis effect (or even irreversible transitions). The well-known transverse effective charge reduction with pressure was also observed. Furthermore, resonance behaviour is clearly seen for transverse optical phonons and the resonance maxima peak occurs at about 1.2 GPa, corresponding to 2.11 eV in the E(1) scale, smaller by 0.3 eV than the incident photon energy.

Structural and photoacoustic studies of mechanically alloyed Ga(40)Sb(38)Se(22) powder.

GaSb, GaSe and Ga(2)Se(3) alloys were produced by the mechanical alloying technique. Their structural, thermal and optical properties were studied. Some of the results obtained have been reported in some papers referenced here. As an extension to those studies, some mixtures of elemental Ga, Sb and Se powders are now being investigated. Starting from a mixture with nominal Ga(62)Sb(27)Se(11) composition, 9 h of milling resulted in a final product with Ga(40)Sb(38)Se(22) composition and containing nanometric cubic GaSb and an amorphous GaSe phase. Part of this as-milled sample was annealed in order to study the amorphous-crystalline phase transformation. The crystalline cubic GaSb, hexagonal and rhombohedral GaSe phases form the measured x-ray diffraction (XRD) pattern for the annealed sample. The structural and thermal properties of both as-milled and annealed samples were studied by x-ray diffraction, differential scanning calorimetry and photoacoustic spectroscopy (PAS) techniques. We observe that the thermoelastic bending contribution is dominant in the PAS signal for both as-milled and annealed samples. The thermal diffusivity value was calculated for both samples by fitting the PAS signal phase.

Mechanical alloying: a pressure induced reaction for obtaining zinc blende GaSb and multiphase states.

The cubic zinc blende GaSb phase was produced by a mechanical alloying technique, which is a solid state route based on the action of non-hydrostatic pressures. The thermal stability of this phase was tested using the differential scanning calorimetry technique and, in order to clarify the results, an annealing process was performed. Comparing x-ray diffraction patterns for as-prepared and annealed samples, the improvement in crystallinity of the cubic phase and Sb segregation and/or crystallization can be easily seen. Optical phonons frequencies were measured for both as-milled and annealed samples by means of the Raman spectroscopy technique. Raman profiles of as-milled samples showed typical zinc blende GaSb optical modes and revealed new features that can be associated with multiphase states.

Reverse Monte Carlo simulations, Raman scattering, and thermal studies of an amorphous Ge30Se70 alloy produced by mechanical alloying.

The short- and intermediate-range orders of an amorphous Ge30Se70 alloy produced by mechanical alloying were studied by reverse Monte Carlo simulations of its x-ray total structure factor, Raman scattering, and differential scanning calorimetry. The simulations were used to compute the G(Ge-Ge) (RMC)(r), G(Ge-Se) (RMC)(r), and G(Se-Se) (RMC)(r) partial distribution functions and the S(Ge-Ge) (RMC)(K), S(Ge-Se) (RMC)(K), and S(Se-Se) (RMC)(K) partial structure factors. We calculated the coordination numbers and interatomic distances for the first and second neighbors and the bond-angle distribution functions Theta(ijl)(cos theta). The data obtained indicate that the structure of the alloy has important differences when compared to alloys prepared by other techniques. There are a high number of Se-Se pairs in the first shell, and some of the tetrahedral units formed seemed to be connected by Se-Se bridges.

Structural study of Cu(2-x)Se alloys produced by mechanical alloying.

The crystalline structures of the superionic high-temperature copper selenides Cu(2-x)Se (0 < x < 0.25) produced using mechanical alloying were investigated using X-ray diffraction (XRD). The measured XRD patterns showed the presence of peaks corresponding to the crystalline superionic high-temperature alpha-Cu(2)Se phase in the as-milled sample, and its structural data were determined by means of a Rietveld refinement procedure. After heat treatment in argon at 473 K for 90 h, this phase transforms to the superionic high-temperature alpha-Cu(1.8)Se phase, whose structural data were also determined by Rietveld refinement. In this phase, a very low occupation of the trigonal 32(f) sites ( approximately 3%) by Cu ions is found. In order to explain the evolution of the phases in the samples, two possible mechanisms are suggested: (i). the high mobility of Cu ions in superionic phases and (ii). the important diffusive processes in the interfacial component of samples produced by mechanical alloying.