Study On Structure And Property And Application Of Nanometer V₈C₇ Powder

V₈C₇ is the most effective grain growth inhibitor in the ultrafine cemented carbide,the nanometer V₈C₇ powder taking the place of micrometer V₈C₇ powder is useful to prepare the ultrafine cemented carbide which the grain size is smaller and the structure is more uniform.So the ultrafine cemented carbide has more high mechnical properties. The nanometer V₈C₇ powder has prepared and its use in the ultrafine cemented carbide and the V₈C₇→δ-VC_1-X phase transformation in the process of preparation of V₈C₇ powder and the propoties of the powders were studied;The conclusions are as follows:(1) Changing the carbon-dopping methods in the conventional carbon-thermal reaction.The V₈C₇ powders was first prepared by "V₂O₅ dissolved in oranic acid solutions→spray drying→vacuum carburization at 1100℃".Because of organic acid solution but dissolved V₂O₅ and reduction to V₂O₃ also can provide the carbon. V₂O₃ and carbon can be homogenously mixed at molecule's level in the vacuum carburization,the diffusion distance is shortened and the carburizing temperature decreased to inhibit grain growth. So the weak points of the conventional carbothermal synthesis of V₈C₇ powder such as high reaction temperature and large particle size can be overcomed.(2) The phase transformation and morgphy change during the process of preparation of nanometer V₈C₇ powder were analysied by X-ray Diffraction, SEM, TEM, Differential Thermal Analysis, Chemical Analysis.During the preparation of vanadium carbide by vacuum carburization,when the temperature rises to 1300℃from the room temperature,the process of phase transformation is as follows:V₂O₃→VC_1-X→V₈C₇→δ-VC_1-X(3) By calculation, The law of X diffraction lines is obtained. X diffraction super structure lines are absencet during the V₈C₇→VC_0.875 transformation are explain Because of vacancy ordered distribution in the V₈C₇ lattice.by calculation internal energy and entropy, It is concluded that the ordered phase free energy is a function of order parameter and temperature,with the temperature,the free energy remains min,so the lattice transforms from ordering to disordering,The temperature of V₈C₇→δ-VC_1-X transformation was determined,By calculation.the order parameter decreases from 0.23 to 0 suddenly.(4) Through the determination of the lattice parameters of ordered and disordered vanadium carbide with different carbon contents,and the relationship of ordering parameter vs lattice parameter by octrodon model.the max order parameter and the lattice parameter of VG_0.802,VC_0.833,VC_0.865 was obtained.(5) The electrical resistivity is determined by the method "double-armed electric bridge".When the carbon contents rise in the disordered vanadium carbide, because of the vancancy decreased,so the diffraction to the electron decreased,the electrical resistivity is decreased and Given the same carbon content, the electrical resistivity of the ordered phase is lower than that of the disordered phase.(6) In the preparation of 12%Co ultrafine cemented carbide, Compared with the micrometer-level vanadium carbide powder.Nanometer vanadium carbide powder can effectively inhibit grain growth,the grain size is smaller,the structure is uniform and the transverse rupture strength increased by 7-8%.