C.M. Poffo

Structural, thermal, optical, and photoacoustic study of nanocrystalline Bi2Te3 produced by mechanical alloying

Nanocrystalline Bi2Te3 was produced by mechanical alloying and its properties were investigated by differential scanning calorimetry (DSC) x-ray diffraction (XRD), Raman spectroscopy (RS), and photoacoustic spectroscopy (PAS). Combining the XRD and RS results, the volume fraction of the interfacial component in as-milled and annealed samples was estimated. The PAS results suggest that the contribution of the interfacial component to the thermal diffusivity of nanostructured Bi2Te3 is very significant.

Mechanical alloying of Co and Sb: Structural, thermal, optical, and photoacoustic studies

High-purity elemental Co and Sb powders were blended with nominal composition CoSb3 and submitted to mechanical alloying. After 30 h of milling, the x-ray diffraction (XRD) pattern was indexed to skutterudite CoSb3 and monoclinic CoSb2. The differential scanning calorimetry (DSC) thermogram of the as-milled powder showed an exothermic peak at 615 K, which seems to be related to monoclinic-orthorhombic transformation of CoSb2. To confirm this transformation, annealing at 823 K of as-milled powder was performed and the XRD pattern of the product was indexed to skutterudite CoSb3 and orthorhombic CoSb2. The Raman spectra of as-milled and annealed samples showed only the active Raman modes of CoSb3. From the photoacoustic measurements of as-milled and annealed samples, the thermal diffusivity and other transport properties were computed.

Determination of thermal and photothermal properties of an amorphous GaSe9 alloy

We investigated the thermal and photothermal properties of an amorphous GaSe9 alloy produced by mechanical alloying considering the photoacoustic spectroscopy and differential scanning calorimetry (DSC) techniques. The room temperature thermal diffusivity of GaSe9 was determined using the open photoacoustic cell configuration considering the thermal diffusion and thermoelastic bending effects. The glass transition and crystallization processes were investigated through DSC measurements obtained at five heating rates, and glass transition, crystallization temperatures, and activation energies were determined. The crystallization mechanism was also determined.

Structural and optical studies of FeSb2 under high pressure

Nanostructured orthorhombic FeSb2 was formed along with an amorphous phase, by mechanical alloying from a mixture of Fe and Sb powders. The influence of pressure on the structural and optical properties of the material was investigated by X-ray diffraction (XRD) and Raman spectroscopy (RS) up to 28.2 and 45.2 GPa, respectively. The volume fraction of the amorphous phase increased with increasing pressure. For pressures above 14.3 GPa, a tetragonal FeSb2 phase was also observed. For the orthorhombic FeSb2 phase, the pressure dependence of the volume fitted to a Birch–Murnaghan equation of state gave a bulk modulus B0=75.5±3.2 GPa, and its derivative B0′=7.2±0.7. For the orthorhombic FeSb2 phase, the Raman active Ag2 mode was observed up to 28.3 GPa, while the B1g2 mode was not for pressures larger than 14 GPa. For pressures above 21 GPa, the Raman active A1g mode of a tetragonal FeSb2 phase was observed.

Structural, thermal, and photoacoustic study of nanocrystalline Cr3Ge produced by mechanical alloying

A thermodynamic analysis of the Cr-Ge system suggested that it was possible to produce a nanostructured Cr3Ge phase by mechanical alloying. The same analysis showed that, due to low activation energies, Cr-poor crystalline and/or amorphous alloy could also be formed. In fact, when the experiment was performed, Cr11Ge19 and amorphous phases were present for small milling times. For milling times larger than 15 h these additional phases decomposed and only the nanostructured Cr3Ge phase remained up to the highest milling time used (32 h). From the differential scanning calorimetry measurements, the Avrami exponent n was obtained, indicating that the nucleation and growth of the nanostructured Cr3Ge phase may be restricted to one or two dimensions, where the Cr and Ge atoms diffuse along the surface and grain boundaries. In addition, contributions from three-dimensional diffusion with a constant nucleation rate may be present. The thermal diffusivity of the nanostructured Cr3Ge phase was determined by photoa...

Synthesis and properties of nanostructured GeSb 4 Te 7 prepared by mechanical alloying

A nanostructured GeSb4Te7 (GST147) phase was produced by mechanical alloying (MA), starting from a mixture with nominal GeSb2Te4 composition of high-purity elemental Ge, Sb and Te powders. The effect of milling on z-coordinates of Wyckoff sites 2c and 2d occupied by the Sb and Te atoms was studied using an approach combining the Rietveld method and pair distribution function analysis. The thermal stability of GST147 and the Raman active modes of GST147 were investigated. The results of photoacoustic absorption spectroscopy show that the thermal diffusivity of nanostructured GST147 produced by MA is similar that of its bulk counterpart.

Structural, thermal and optical studies of nanocomposite powder NiSb + Sb produced by mechanical alloying

Abstract A nanostructured composite powder formed from 75 wt.% NiSb and 25 wt.% Sb was obtained by mechanical alloying binary mixtures of elemental Ni and Sb powders in the composition NiSb2 for 39 hours. The structural evolution was followed by X-ray diffraction measurements and Rietveld structural refinement. At the end of milling, the crystallites of Sb and NiSb reached a mean size of 17 and 15 nm, respectively. Differential scanning calorimetry measurement showed two exothermic processes at 263 °C and 309 °C. Variable temperature powder X-ray diffraction measurements at 25, 100, 200 and 300 °C showed that the sample is structurally stable until 100 °C. The beginning of the transition NiSb + Sb → NiSb2 was observed at 200 °C and the nucleation of Sb2O3 was observed at 300 °C in accordance with differential scanning calorimetry measurement. Raman spectroscopy measurements showed neither NiSb nor NiSb2 compounds produce significant Raman signals when compared with Sb and Sb2O3.