Compaction and recrystallization kinetics of aluminum magnesium spinel doped with lithium fluoride

Abstract

It is well known that the addition of LiF helps to compact oxides by acting as a liquid phase which reacts with the solid substance [i, 2] or as a mineralizing additive which accelerates the synthesis of aluminum magnesium spinel [3].The present article reports some results from a study of the effect of LiF on the degree and the rate of the compaction and recrystallization of aluminum magnesium spinel grains at temperatures of 20-1400~and pressures of 40-120 MPa.The aluminum magnesium spinel was obtained by recrystallizing magnesium and aluminum sulfates followed by solid phase synthesis at 1300~We used aluminum magnesium spinels with a concentration of 97.8%* (AMS I) or 99.1% (AMS II) of the main material. These spinels were doped with 0.15-0.75% LiF using two chemical methods.The AMS powder was previously impregnated with a LiF solution (method i) or with solutions of LiOH plus HF (method 2).The LiF solution was prepared by mixing solutions of a 9% HF and 7% LiOH until the pH was 5.8.The suspensions of the doped spinel were evaporated at 200~ and the powders calcined at 700~ for 2 h. All the AMS specimens were doped with LiF using the first method except for the AMS I-4 specimens which were doped by the second method.The compaction kinetics of the aluminum magnesium spinel were determined using a dynamic method [2] in accordance with three regimes where the density of the briquette was a function of the following pressing parameters: O=~(P) t=const, dt/d-c =const (regime No. 3),where t is the temperature, ~ ~, time, min; dt/dT, heating rate of 10~ p, pressure, equal to 40 MPa for regime 1 and I00 MPa for regime 2; dl/dT, rate of movement of the traverse of the Instron machine, where d~/dT is 0.02 mm/min.The density of the briquette after pressing was determined by hydrostatic weighing in toluene to an accuracy of _+0.001 g/cm 3. A value of 3.578 g/cm 3 was taken as the theoretical density of the 32iS [4].The concentration of Li in the doped powders was determined using flame photometry in an SP-900 spectrophotometer, and the concentration of F by a photocolorimetric method based on the decolorization of a comnound of zirconium and eriochromcyanine.The microstructure of the hot-pressed specimens was studied using an ~M-6 electron microscope using the carbon replica method from fresh chips in a plane parallel to the direction of the pressing force and passing through the center of the specimen.The grain size was determined using a "secant" method [5]. Tables 1-3 show the results of chemical analyses of the powders of the untreated spinels and of the powders after doping with LiF.The tables also give the compaction parameters when the pressing parameters are varied.With an increase in the pressing temperature from 20 to 700~ the density of the specimens of the experimental powders changes within 0.40-0.50, remaining equal to the density *Here and elsewhere mass fractions are given.