Observation And Research Of The Eutectic Process Of Na₃AlF₆-Al₂O₃ System

Na3Al F6-Al2O3 system melt are the important to material system, it is widely used in the field of production, processing and research of alumina. Especially, it is a significance to manufacturing and processing of component of alumina high-temperature ceramics. In recent years, it is widely used in the field of chemical industry and aviation on account of its stronger corrosion as melts at high temperature. The current research of Na3 Al F6-Al2O3 system is mainly based on aluminum electrolytic industry as electrolyte, and the report of component of alumina high-temperature ceramics is really little. The basic research which contain eutectic phenomenon, thermodynamics and kinetic is scarce.In this work, the eutectic process of Na3 Al F6-Al2O3 system which the raw materia is cryolite(Na3Al F6) and alumina(Al2O3) under heating process was in situ observed by Linkam TS1500 hot-stage combined with polarizing microscope, the effect of particle size and crystal form of sample to eutectic reaction process was discussed. Investigating and summarizing the effect of heating rate to eutectic reaction process and the component proportion to crystallographic morphology under cooling process; Kinetic parameters of Na3 Al F6-Al2O3 system was calculated utilized the method of thermal analysis kinetics, replenishing kinetic datas of this system. The specific work are as follows:First, Combined with solid solution theory, the eutectic process was conjectured in situ observed. We consider that Eutectic reaction process occurs only in the first catch of heterogeneous crystal particles. And before the temperature do not reach the eutectic point of two components of Na3 Al F6-Al2O3 system the reaction occurred. Two components start generating limited type solid solution during 560℃~570℃. With the increase of temperature, solubility of solute solvent reaches the limit in solid solution, and the system generate the third phase, that is liquid phase. At this point the temperature reaches the eutectic point, two components start melting.Second, discussing the effect of particle size of aluminium oxide to eutectic reaction process. Compared the reaction process of different sizes of alumina particles and cryolite, we found that reaction rate of nanoparticles in preference to reaction rate of ordinary particles. Shall be investigated for the reasons, it is nanometer size effect of aluminium oxide. The increase of specific surface area provides more reflex point of alumina and cryolite, thus the efficiency of the reaction greatly improve.Third, discussing the effect of crystal form of aluminium oxide to eutectic reaction process. Compared the reaction process of different crystal form of alumina and cryolite, we found that dissolution rate of different crystal form of alumina is different. The activity of gamma alumina is best. As from gamma alumina to alpha alumina, their activity gradually reduce,and the melting rate of alpha alumina is smallest.Fourth, discussing the effect of heating rate to eutectic reaction process. Investigating and summarizing the effect of component proportion to crystallographic morphology. Found that with the increase of heating rate, the exothermic peak of eutectic reaction corresponding delay. This is because the quick heating makes reaction in higher temperature and causes lag; By comparing the melting enthalpy of reaction process, it is concluded that the melting enthalpy of eutectic reaction of this system is not the simple sum of melting enthalpy of two separate component. there is an interaction between alumina and cryolite, that is the eutectic reaction; The component proportion of alumina and cryolite is different, the crystallographic morphology of eutectic mixture during cooling is different.Fifth, calculating the kinetic parameters of eutectic reaction of Na3 Al F6-Al2O3 system. We used Starink method, in combination with Kissinger and Ozawa method,to calculate the apparent activation energy and pre-exponential factor of eutectic reaction of Na3 Al F6-Al2O3 system,Ea=239.197 k J/mol, ln A=15.9278s-1, thus, the Arrhenius equation of eutectic reaction of Na3 Al F6-Al2O3 system, lnk=15.9278-238.598×103/RT; Combined with the Arrhenius equation and related parameters,we calculate reaction order of eutectic reaction by Kissinger method,it is about 2.2; Combined with Kissinger method,we calculate the most probable mechanism function of this system, obtain the dynamics equation of Na3 Al F6-Al2O3 system by Coats- Redfern method.