ULTRAFINE REFRACTORY-METAL PARTICLES PRODUCED BY HYBRID PLASMA PROCESS

Ultrafine particles of W, Mo, Ta and Nb have been formed by the plasma evaporation method in a "Hybrid Plasma", characterized by the super-imposition of a DC arc jet on a radio-frequency plasma. The raw refractory metal powders were injected into the DC arc jet. All particles obtained were black in color and fluffy. Mean particle size was between 15 approximately 50 nm. The maximum formation rate of W ultrafine particle was 41.7 g.K.s-1. The ultrafine particle of W was heat-treated between 673 approximately 1673 K in vacuum. Heat-treatment above 873 K showed an increase of the mean particle size. and the oxygen content was decreased above 1073 K through the transformation of W3O to WO2 which is followed sublimation. The ultrafine W particle and conventional W powder having a size of 1.8-mu-m and the mixtures of both were sintered in H-2 atmosphere. In the cases of the mixture containing more than 20% of the ultrafine particle, the sintering temperature was decreased by 400 K compared to the conventional powder. Vicker's hardness of sintered compacts was increased with the addition of the ultrafine particle. The decrease of sintering temperature was also observed in the case of the conventional WC powder and the ultrafine particle mixture. The mean ultrafine particle size of Mo was larger than that of W which is explained by the lower melting and boiling temperature of Mo. The heat-treatment of the Mo ultrafine particle in vacuum shows the increase in particle size over 873 K and the decrease in oxygen content over 1273 K. The ultrafine Ta and Nb particles formed hydride TaH0.9, NbH0.89 respectively, by using H2 as a sheath gas. The heat-treatment of Ta ultrafine particle in vacuum showed an increase in particle size above 873 K and a decrease in hydrogen content above 673 K, whereas the oxygen content was not decreased in this heat-treatment.