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

Abstract

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 photoacoustic absorption spectroscopy measurements.

Rapid mechanochemical synthesis of amorphous alloys

“…3 Typically SPEX milling is conducted in a mode that maximizes impact energy, as it is widely believed that low impact energy cannot induce alloying. 3 Therefore milling conditions that employ a few large (5 -8 g, 1.3 -1.5 cm diameter [6][7][8][9][10] ) balls are widely used. 3 Very nice results can sometimes be achieved under these conditions 10,11 but the resulting materials can also be microcrystalline as opposed to amorphous or nano-structured.…”

“…3 Therefore milling conditions that employ a few large (5 -8 g, 1.3 -1.5 cm diameter [6][7][8][9][10] ) balls are widely used. 3 Very nice results can sometimes be achieved under these conditions 10,11 but the resulting materials can also be microcrystalline as opposed to amorphous or nano-structured. 6,8 Recently it has been shown that low energy milling can result in the formation of alloys that are more amorphous than SPEX milled alloys.…”

Structural and Photoacoustic Study of Cr11Ge19 Prepared by Mechanical Alloying

“…Probably the nucleation of GeO 2 occurred during manipulation of blended powders to carry out the XRD measurements at ambient conditions, and is probably restricted to the region close to the particle surface. In a previous article 21 , this oxide phase was also observed, but not discussed, in the XRD pattern of blended powders before milling. For a milling time of 5 h, the intensity of the diffraction peaks of pure Ge was smaller than in the initial mixture and the peaks of GeO 2 are absent.…”

“…As shown in Ref. 21, an ideal solution with Cr 11 Ge 19 nominal composition has ΔV/V 0 = 0.415. For 15 h of milling, the interfacial and amorphous volume fractions of the nanostructured Cr 11 Ge 19 alloy occupy an excess volume ΔV/V 0 = 0.248.…”

“…For longer milling times, both Cr 11 Ge 19 and amorphous phases decomposed and the Cr 3 Ge phase emerged, remaining stable up to the highest milling time (32 h). Although the Cr-Ge phase diagram 5 shows that the Cr 11 Ge 19 phase is stable up to ≈ 928 o C, decomposition under milling was observed 21 . To the best of our knowledge, this fact has not been reported in the literature.…”