Preparation of the mixtures of ultrafine powders of nickel, cobalt, and copper

Abstract

Ultrafine (ultradispersed) metal powders (UFP) obtained according to the method of low-temperature thermal dissociation of the salts of organic acids belong to the high-energy nonequilibrium systems. Their high sinterability is due to the excess reserve of the surface energy which, in turn, is due to their significantly developed surface and the high density of the lattice defects. The relaxation processes occurring at the defects act as additional sources of energy and lead to an increased diffusional mobility of the atoms of the metal [1, 2]. Because of the unique properties of the particulate materials, they are used in the technology of bonding dissimilar materials as intermediate packing strips (fillers) that facilitate reduction of the main parameters of diffusional welding, viz., temperature and compressive force. The two-component strips obtained from UFP obtained according to the method of thermal dissociation of the mixtures of the formiates of nickel, cobalt, and copper make it possible to improve the mechanical strength of the weldments, impart special properties of the assembly, and reduce the duration of the technological cycle of welding. We studied the effect of the production technology of the original formiates and the regimes of their thermal dissociation on the composition, the degree of dispersion, and the physical and mechanical properties of the particulate nickel-copper, nickel-cobalt, and cobalt-copper composite materials containing different weight fractions of the components. The process of obtaining the heterogeneous composites in the form of UFP mixtures includes the following stages, viz., mixing of the formiates, maintaining the required proportions by weight, their homogenization, thermal dissociation at a constant temperature in an inert furnace atmosphere, cooling, and protecting against oxidation during storage. The production technology of UFP of the solid solutions envisages preparation of the solid solutions of the formiates in the given ratios and their subsequent decomposition during pyrolysis into a metal and a gaseous product without dissociating the original system. The solid solutions of the formiates were obtained from the nitrate salts according to the method involving precipitation of the corresponding cations using sodium carbonate and subsequent reprecipitation of the carbonate using formic acid. Just as the formiates of nickel, cobalt, and copper, the solid solutions of the formiates crystallize in the form of crystallohydrates containing two water molecules. They differ from the original formiates with respect to the color of the preparation and the structure of the salt crystals. We studied the kinetics of thermal dissociation in an inert-gas atmosphere using a derivatograph. The specific surface area of the powders was measured according to the method based on the thermal desorption of argon. The microstructure of the particles was studied using a "Stereoscan-150" electron microscope and phase analysis was carried out using a DRON-2 diffractometer under iron-radiation. The solid solution dissociates thermally as a unicomponent salt (Fig. la). The processes of dehydration and thermal dissociation of the solid solutions occur at lower temperatures (140 and 160~ respectively) than those in the mechanical mixtures. The DTA and TG curves have a third endothermic peak at 270~ where successive decomposition of the constituent formiates of the mixture (Fig. lb) occurs; furthermore, one observes higher temperatures of dehydration and thermal dissociation of the salt. The DTA curve of the mechanical mixture is widened in the temperature range corresponding to the decomposition of the salt and, also, in the range corresponding to dehydration and structural changes. During the pyrolysis of the mixtures containing copper formiate, metallizing (coating) of the particles of the salt and the walls of the crucible takes place due to the volatile phase of copper formed during the decomposition of its formiate [3]. During the pyrolysis of the solid solutions of the formiates with copper, no volatile copper phase forms.