SHRINKAGE OVER A WIDE T EMPERATURE R ANGE

On the basis of experimental data for the sintering of nickel powder under a regime of alternate isothermal and non-isothermal periods, it is shown that a mathematical model of pore volume decrease is capable of describing the process over the temperature range 550-900°C. It appears that in all cases both the time and temperature dependence of pore volume shrinkage are determined by parameter-constants which depend on the method of production of the initial powder. A mathematical model was created to describe the process of pore volume shrinkage (PVS) under various sintering conditions. It includes, in addition, the initial prerequisites for analyzing the kinetic features of the elementary processes which together determine the time and temperature dependence of PVS. In earlier publications it was shown that the function W(r) (W = -(9/v) is the relative rate of PVS and r = time) calculated with the aid of this model for various sintering conditions, including alternating isothermal and nonisothermal periods, is in good agreement with experimental data [1]. Since these experiments were carried out over a relatively narrow temperature range, it seemed necessary to test the model over a wider range of temperatures in order to confu:m its utility. Experimental data on this function for the sintering of powder bodies under a regime of equal time intervals of heating between two isothermal periods have been obtained over a relatively broad temperature range (550-900°C). Use of these data made it possible to determine the kinetic parameters of the elementary processes much more accurately, and to confirm the utility of the model under the selected conditions. It was determined that a substantially more accurate choice of kinetic parameters for the elementary processes, particularly of the activation energies, is needed in order to describe the investigated relationships over a wide temperature range. The basic parameters of the model are the activation energies for annihilation of initial imperfections E x, generation of active imperfections E x + E a , disappearance of active imperfections Ey x, creep ("flow") of the crystalline substance under the influence of active imperfections Ey w, (U/g-atom), and also the kinetic, properties of the concentration of initial x and active y imperfections (h'l), more thoroughly described in [1]. The mathematical model of PVS includes the following equations: