Metalloid Elements (graphite, Te) Structural Changes In The Mechanical Alloying Process

Mechanical alloying (MA) method has been considered as a useful process and as a widely used technique to fabricate novel materials with unique properties, in particularly, for those materials which are difficult to be obtained by the traditional methods. A large number of materials including carbides have been synthesized through mechanochemical reaction at a lower temperature. However, most researchers' interest was focused on the formation of new and high-powered materials, and correspondingly the analysis of the MA process had been neglected.In this work, the transformation of metalloid elements (graphite, Te) with aluminum during MA has been studied. The raw materials, including graphite, diamond, aluminum and tellurium, are elemental powders. The ball milling is carried out with a planetary ball mill. The milling experiments are interrupted at the predetermined times and the milled powders are taken out for analysis. Some samples are annealed at high temperatures in Ar atmosphere. The characterization of the ball milled powders is carried out by means of X-ray diffractometry (XRD), Raman spectroscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and others. Some new phenomena and encouraging results have been found.(1). Computer simulation on the XRD pattern of graphite indicates that the moving of diffraction lines to smaller diffraction angles and the increase of the full width at half maximum (FWHM) increase with decreasing crystallite size, although the lattice parameters is kept as constants. The size effects are more obvious when the crystallite is smaller than about 6 nm. Comparing with simulated XRD patterns of turbostratic graphite, turbostratic graphite is obtained when the graphite-diamond mixture is milled for longer than 20 hours. The turbostratic graphite is so stable that it will not transform into graphite even it is annealed at 1700℃ for 3 hours.(2) The XRD peaks of diamond are almost not broadened when the mixture of graphite and diamond is milled for a long time, and the grain size of diamond is always in the range of 57-50 nm. But the intensity of diffraction peaks of diamonddecreases with increasing milling time, which reveals that a critical size effect may exist for diamond. If the grain size is smaller than about 50 nm, the diamond would transform into amorphous phase easily during ball milling. Diamond powder serves as a powerful milling medium due to its super-high hardness to accelerate the milling of graphite in the ball milling.(3) Diffraction peaks of graphite in the XRD patterns disappear for the ball milled graphite-diamond mixture. Experiments on ball milled graphite by means of Raman spectroscopy and annealing experiments indicate that some of the nanocrystalline graphite still exists even though the graphite has been milled with powerful abrasive of diamond for 500hr.(4) AI4C3 produced in the ball milled graphite-Al mixture. The grain sizes of graphite and Al decrease at the initial stage of ball milling, and the C atoms dissolve gradually into the lattice of Al to form a (A1,C) solid solution with increasing milling time. When the amount of C in Al reached a certain value, it began to transform into AI4C3 phase. The reaction is a diffusion controlled process. The (A1,C) solid solution transforms into AI4C3 after annealing at 500°C for 2 hours, and the (A1,C) solid solution can also transform into AI4C3 by ball milling for a prolonged time.(5) A new structural Al2Te3 produced in the ball milled Te-Al mixture. Just as the Al-graphite, an Al-Te solid solution is formed firstly by milling the mixture of Te and Al for just 2 hours, and the solid solution transforms into AbTe3 with increasing milling time. The reaction is also a diffusion controlled process. The conventional A\{Te3 phase has a monoclinic structure with a=7.181 A, b= 12.84 A, c= 14.167 A, p=90.040° and is stable below 895 °C. But the Al2Te3 obtained by ball milling Al-Te mixture has an fee structure with a lattice parameter of a—5.957 A. It is a new phase and is stable below 120°C, which has never been reported to our knowledge.