Mechanical Alloying Preparation Of Carbides

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 reseachers' 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 synthesis of metal carbides by MA and the effect of ball-milling with different technical parameters have been researched. The process and mechanism of the MA have been discussed. The ball milling was carried out with a planetary ball and the starting reactant materials were elemental powders, metal oxide powders and graphite powders. They were mixed with some ratio and sealed in cylindrical vials and milled with the planetary ball mill. The milling experiments were interrupted at predetermined times and small amounts of the milled powders were taken out of the vial for analysis. Some samples were annealed at high temperatures in Ar atmosphere. The characterization of the powders was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and others.Crystalline vanadium carbide was synthesized by ball milling V2O5, Mg and C powders at room temperature with a planetary ball mill. In spit of the similarity between the V2O5-Mg-C and WO3-Mg-C systems, the formation mechanisms for WC and VC are quite different. WC was synthesized through gradual reaction between WO3, Mg and graphite powders. But the MA mechanism for V2O5-Mg-C system is a self-sustained reaction, in which the phase transformation from reactants to resultants is completed in a short time. The time of starting reaction is 24 to 24.5 hours of milling. When the as-milled VCX is annealed at a temperature higher than about 750℃, it transformed into V8C7. Both the VCX and V8C7 have NaCl-like structure, C vacancies in the as-milled VCX being randomly distributed in the lattice. When VCX was annealed it transformed into V8C7 with ordered C vacancies, the phasetransformation from VCX to V8C7 is a vacancy-ordering transformation.Tungsten and graphite powders were mixed with an atomic ratio of 1:1 and milled, the carbon atoms would entered into the interstices and defects of the metal lattice and formed interstitial solid solution. In the process of the formation of solid solution, the size of W gradually decreased while the lattice parameter of W was firstly decreased and then increased because C gradually diffused from dislocations, grain boundaries and free surfaces to the lattice of W.Wet milling with dry milling were carried out for the same powders for comparison, the effect of the ball-milling in the two situations is almost the same at the early stage of the milling. But with ball-milling time extended, the wet milling is nothing better than the dry milling on the effect of ball-milling. The liquid lubricated the reactant materials and ball-milling media, which lowered the friction, moreover, the friction is the main means that diminished the grain size when the particles were quite fine.For the first time, Carbon nanotube (CNTs) and W was milled. No other new phase formed but an amorphous phase. The amorphous phase gradually increased with increasing the milling time. This means the energy of ball-milling is not high enough to place a premium on the formation of WC or W2C. The mechanochemical reaction was controlled not only the thermodynamic factors but also the kinetic factors.Several reaction systems including WO3-Mg, WO3-V2O5-Mg, WO3-Mg-C and CrO3-Mg-C were also carried out and discussed. The MA mechanism for the reaction systems are all involved in Self-propagating High-temperature Synthesis(SHS) reactions. But the time of starting reaction is quite different. The principal condition of SHS reaction occurrence is that the exothermic reaction can release a...