Effect Of Tungsten Oxide Powders With Different Characteristics On The Characterization Of Hydrogen Reduced Tungsten Powders

Effect of calcining temperature,exhausting frequency,rotation rate,and furnace atmosphere on the characterization of tungsten oxides powder prepared by calcining APT(ammonium paratungstate)powder have been detailedly studied,and the change mechanisms of particle size and morphology,phase transformation mechanisms of tungsten oxides,and growth mechanisms of W03 and WO2.72 grains during calcining APT are also detailedly investigated.According to the phase structures and compositions and the particle(grain)sizes and morphologies in tungsten oxides detected by XRD and SEM,respectively,the tungsten oxide powders are divided into four groups,and then three particle size grades(0.7m,2.5pm,and 8.O?m)of tungsten powders are prepared by hydrogen reducing the above four groups of tungsten oxide powders.Effect of W03 grain sizes,WO2.72 grain sizes and morphologies,W03 crystal structure,and phase compositions in the starting tungsten oxide powders on the characterization of reducing tungsten powders have been detailedly studied,and the growth mechanisms of tungsten grain(particle)are also detailedly investigated.In 400?600? or at the higher temperature than 600? with a high exhausting frequency(equivalent to decrease hydrogen content in the furnace),a pure W03 powder can be obtained.With increasing the calcining temperature,the W03 grains begin to grow up obviously at 500-600?(10-20nm),and the dominated growth mechanism is the grain boundary diffusion in the particle,and accompanying a vapor deposition growth mechanism at the surface of the particle.The size of W03 grain is about 50-70nm at 700-750?,and 200-300nm at 850?.The needle-like WO2.72 grains begin to generate at the calcining temperature of below 600-700?,with increasing temperature,the content of WO2.72 increases gradually,and the pure component W02.72 can be obtained when the calcining temperature is higher than 750?800?.With increasing temperature,the needle-like WO2,72 grains grow up.The ultra-fine and elongated single crystal whisker WO2.72 grains are formed when the calcining temperature is lower than 800?,and the main growth mechanism is "Root growth of a leading whisker,accompanying "a tip growth via the gas phase" mechanism at the top of the nucleus.When the calcining temperature is higher than 800?,multiple WO2.72 nucleuses nucleate simultaneously,and then the above WO2.72 nucleuses grows simultaneously along axial to form a rod-like grain by the "Root growth" mechanism,but also accompanying "the tip growth via the gas phase" mechanism at the top of the rod-like grain.The initial nucleus number determines the diameter of the rod-like grain.With increasing the exhausting frequency or the rotation rate,the broken degree of the particles increases.With increasing the rotation rate,which is equivalent to decrease the holding time and improve the motion energy of particles,the broken degree of the particles increases.With increasing the calcining temperature,the broken degree of APT particles gradually decreases due to the thermal closure of microcracks in particles.After APT is completely decomposed into WO3,the basic process of the following crystal structural transformation for tungsten oxides is hexagonal-W03?monoclinic-WO3?triclinic-WO3?rhombic-WO3?tetragonal-WO2.9?mornoclirnic-WO2.9?monoclinic-WO2.72.Fine tungsten powders can be obtained by reducing YTO,BTO,or VTO used as the raw materials at a suitable reduction technique,and the ultra-fine and elongated needle-like VTO powder is more suitable for preparing ultrafine tungsten powder.Using high valence tungsten oxide(such as YTO)as the raw material,the dominant growth mechanism of particle is the volatilization-deposition growth mechanism in the hydrogen reducing process,and the morphology of particle for the hydrogen reducing tungsten powder is near-spherical and smooth,and there are obvious growth steps formed by a volatilization-deposition growth mechanism on the surface of particle.Using low valence tungsten oxide(such as VTO)as the raw material,the dominant growth mechanism of particle is the atomic diffusion growth mechanism in the hydrogen reducing process,and the morphology of particle for the hydrogen reducing tungsten powder is polyhedral,and there are not obvious growth steps on the surface of particle.With increasing the reducing temperature,the number of crystallographic plane on the surface of the polyhedral tungsten grain decreases,and eventually the cubic tungsten grains are formed.With increasing the grain size in the tungsten oxides,it is easier for tungsten grains to grow by the atomic diffusion mechanism in the hydrogen reducing process.