Study On Preparation Of Ultrafine Cobalt Powders By Freeze-Drying

Ultrafine cobalt powders have been widely used in the fields of hard alloy, battery, catalysts, magnetic material, wave-adsorbed, heat resisting powder metallurgy, and show excellent performance because of the particular physical and chemical characters. How to prepare pure spheric cobalt powders with uniform diameter distribution become one of the hot topics in the advancd material fields, and the development of production technology tends to more environment friedndly and power-saved. Spheric cobalt powders with varied diameter distribution have been prepared by freeze-drying technology, CoC₂O₄·2H₂O is raw material and particular parameters are adopted in the process in this particle. The structure and characters of the product have been studied by means of XRD, BET, IR, TEM, SEM and TG-DTA. The forming processes and mechanics of the precursor and cobalt powders with controllable diameter distribution, the course loading ultra-fine cobalt powders on aluminium oxide base and the catalysis also have been discussed in this particle.The impact of freezing rate, concentration and pH of solution, surface active agent have been studied. The pH should be bigger than 12 to avoid the forming of glassy stuff under the over-cooling state and to make sure that there are enough NH3 in solution which is useful to help ice sublimating in the drying process. Adequate freezing rate is necessary to avoid aliquation in the freezing process, so liquid nitrogen is choosed to be freezing medium, and spray-freezing is appropriate. The eutectic point of cobalt ammonia complex compound solution will fall with lifting of the concentration of solution, and lift with falling of the pH of solution, detailed trending curve is gived. The average size of precursors will increase with lifting of solution's concentration, and the trending curve is also gived. The addition of 0.5%Wt PVP will not influence precursors'size and shape. Most of precursors are massive with smooth surface, some of them are in regular shape. Co exist in the precursor in the way of [Co(NH₃)_x]₂(C₂O₄)₃.The influencing mechanics in the thermal decomposition of temperature, atmosphere, precursor and gas flow rate are investigated in this paper, and according the mechanics, detailed controllable programmes are proposed. The cobalt powders are analysed by XRD, IR, TG-DSC, SEM, TEM and Laser Size Analysis. The mixture of cobalt and cobalt oxide powders are prepared under the nitrogen, the percentage of the cobalt powders in the mixture increases with lifting of temperature. Hydrogen is choosed to be the reductive atmosphere. The reductive temperature of precursors is 250℃, which is 200℃lower than temperature of other regular methods. The size of cobalt powders would increase, if the reductive temperature was continued going up. The results of XRD show that some unkown crystalline products appear in process that precursors turn into cobalt powders. Pictures of SEM show that a single precursor breaks up into plenty of cobalt powders, but the size of precursor still contribute to the cobalt powders'average diameter. The reductive course tends to be faster under the larger hydrogen flow rate. Cobalt powders prepared by freezing-dry technology are spheric with narrow size distribution and without impurity.Aim at to solve the disadvantage of supported catalysts, a new immersion method is designed to prepare supported cobalt catalysts. XRD shows that cobalt powders have been loaded on the aluminum oxide base successfully, and the spectrums of electronic probe scan indicate that the catalysts prepared by freeze-drying can load more cobalt powders and the distribution is even more homogeneous. Catalysts'activity is also test in the system of carbon monoxide oxidation, temperature at 50 and 100 percentages both are lower than other catalysts.