Study On The Preparation And The Mechanism Of Ultrafine Ni Powder By Freeze-Drying

Ultrafine nickel powders have been used as catalysts, magnetic materials, sintered activators, electroconductive sizing materials, hard metal adhesives, etc. The research on ultrafine nickel powder is a topic of general interest in the new materials fields at present. Recently, as an advanced and eco-friendly method of powder preparation, Freeze-drying has been developing quickly.The ultrafine Ni powder with the average size of 50nm was prepared by freeze-drying form the NiSO4 and NaOH and was studied by means of XRD, IR, FESEM, TEM, and DSC. The catalysis of nickel powders to NH4ClO4 and how the precursor forms and the processes of the amorphous precursor to the final nickel powders are discussed.The precursor of freeze-drying is amorphous with residual water and ammonia, having a smooth surface. The morphology is flaky, club-shaped and a little hexagonal. At the beginning of the reduction, the spherical Ni powders precipitated at the tip of the precursor where specific surface energy is higher, then more and more Ni powders appeared at the edges and the surface of the precursor; finally the spherical ultrafine Ni powders take the place of the whole precursor. The lattice parameter of obtained ultrafine Ni powders is 0.35288nm which is larger than that of the perfect Ni single crystal.The ultrafine nickel powders have a prominent influence on the heat decomposition of NH4ClO4. The high temperature exothermic peak of NH4ClO4 added 5wt% and 10wt% is respective 63K and 130K lower than the pure NH4ClO4.The"segregation of solute model"is established and discussed. The process of rapid freezing contains two stages, low-temperature-freezing and temperature-raising. In the low-temperature-freezing stage, the water is frozen as ice phase, while the solute precipitate as the salt phase. The velocity of ice phase growth (vf) in the solution and the velocity of solute diffusion (vd) decide that whether the heavy segregation of solute appears. In the temperature-raising stage, the amorphous ice would crystallize. The microcrystal would segregate at the process of ice...