Structure, microstructure and magnetic investigation of the hexagonal δ-FeSe nanophase produced by mechanochemical synthesis.

We present a systematic structural, microstructural and magnetic characterization of the hexagonal δ-FeSe nanophase produced by a simple one-step mechanochemical synthesis route, by using conventional X-ray powder diffraction (XRPD), Rietveld refinement, transmission electron microscopy (TEM) and magnetometry techniques. We observed the simultaneous formation of tetragonal ß-FeSe and δ-FeSe after 3 h of milling (with minor amounts of unreacted iron), followed by complete ß-FeSe → δ-FeSe phase transition as milling time increases to 6 h (no unreacted iron). The average crystallite size of the δ-FeSe phase of about 16 nm after 3 h milling time decreases by about 31% up to the final milling time (24 h). TEM images and electron diffraction patterns confirm the nanometric size of the crystalline domains in the irregularly-shaped agglomerated particles. Two ferromagnetic phases with distinct coercivity spectra were assumed here by considering an assembly of randomly-oriented weakly-anisotropic ferromagnetic particles, mixed at a 4 to 6 volume ratio with other randomly-oriented ferromagnetic grains. Four years after synthesis, the aged samples milled for less than 9 h revealed a certain amount of the ß-FeSe phase that slightly affects the δ-FeSe (micro)structure but causes some variations (decreasing) in magnetic parameters. Milling times as long as 12 h were shown to be necessary to guarantee the δ-FeSe nanophase stability and to retain its magnetic properties over time.

Nanosized tetragonal ß-FeSe phase obtained by mechanical alloying: structural, microstructural, magnetic and electrical characterization.

Nanocrystalline tetragonal ß-FeSe phase was prepared mechanochemically using ball milling procedures in an inert atmosphere, starting from Fe x Se powder mixtures with x = 1.00, 1.25 and 1.50, with x = 1.25 and 1.50 leading to more than 93% of pure phase after annealing at 400 °C for 1 hour under vacuum. X-ray powder diffraction provides information on phase formation and phase transitions with milling time and temperature. The Rietveld method was used to refine the crystal structure, including the z coordinate of Se and occupancies, to determine the microstructure and to assess the amount of contaminant phases observed. Lattice contraction is found in the ab-plane more than along the c-axis, the small average size of crystalline domains (<22 nm) and the high microstrain (>1%) indicate the formation of highly strained nanoparticles. Magnetic and electrical characterization showed a poor superconductivity at 4 K and semiconducting properties only for thermally treated samples. These observations are explained by the presence of ferromagnetic impurity phases (residual Fe, hexagonal δ-FeSe phase and monoclinic Fe3Se4), but other effects caused by the mechanochemical synthesis must be considered, such as small average size, large/non-uniform size distribution and high microstrain of the nanosized tetragonal ß-FeSe phase. The increase of the ß-FeSe phase content with increasing storage time (ageing) above a few days to months in air, at RT and in the dark was observed for all as-milled samples. Preliminary data on the ageing effect are shown while a systematic study on this is in progress and will be presented elsewhere.

Dissolution enhancement of Deflazacort using hollow crystals prepared by antisolvent crystallization process.

Deflazacort (DFZ), a derivate of prednisolone, is a poorly soluble drug which has been proposed to have major advantages over other corticosteroids. Poorly soluble drugs present limited bioavailability due to their low solubility and dissolution rate and several strategies have been developed in order to find ways to improve them. In general, pharmaceutical laboratories use a micronized process to reduce the particle size in order to increase the dissolution of the drugs. However, this process causes changes such as polymorphic transitions, particle agglomeration and a reduction in fluidity and wettability. These solid-state properties affect the dissolution behavior and stability performance of drugs. Crystallization techniques are widely used in the pharmaceutical industry and antisolvent crystallization has been used to obtain ultrafine particles. In this study, DFZ was investigated in terms of its antisolvent crystallization in different solvents and under various preparation conditions (methanol/water ratio, stirring and evaporation rate, etc.), in order to compare the physicochemical properties between crystallized samples and raw materials available on the Brazilian market with and without micronization. Crystalline structure, morphology, and particle size, and their correlation with the Intrinsic Dissolution Rate (IDR) and dissolution profile as relevant biopharmaceutical properties were studied. Crystallization conditions were achieved which provided crystalline samples of hollow-shaped crystals with internal channels, which increased the dissolution rate of DFZ. The antisolvent crystallization process allowed the formation of hollow crystals, which demonstrated a better dissolution profile than the raw material (crystalline and micronized), making this a promising technique as a crystallization strategy for improving the dissolution and thus the bioavailability of poorly soluble drugs.

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.