Preparation And Properties Of Substituted Lanthanide Hexaaluminate Powders Based On Reverse Microemulsion Method

In recent years,technologies in the fields of aerospace and energy have been developing rapidly.Therefore,it is necessary to research new high temperature materials.The substituted hexaaluminate is a new kind of promising high temperature material,with special layered crystal structure and excellent high temperature stability.The performance of hexaaluminate depends largely on the preparation method.Hexaaluminate prepared by reverse microemulsion method has better properities than hexaaluminate prepared by traditional methods,such as higher surface area and purer phase.There are few reports on the preparation of substituted hexaaluminate by reverse microemulsion,and there is a lack of systematic research on LaMgAl₁₁O₁₉?LMA?and LaZnAl₁₁O₁₉?LZA?preparation by reverse phase microemulsion method.In this paper,LMA and LZA powders were prepared by reverse microemulsion method,the preparation process was systematically studied and the properties of the material were tested.Reverse microemulsion system was prepared using TritionX-100 as surfactant?S?,n-hexanol/n-octylalchhol as cosurfactant?AS?,n-heptane/cyclohexane as oil phase?O?.The effects of reagent type,temperature and ion concentration on microemulsion system were studied by drawing pseudo ternary phase diagram.The maximum water solubility of the reverse microemulsion system is 30%,when TritionX-100 is selected as the surfactant,hexanol as the cosurfactant and heptanes as the oil phase,and the mass ratio of S,AS and O is controlled as 1:1:2.The effects of various factors on the preparation of LMA powders by reverse microemulsion were studied.LMA powder with pure phase and high specific surface area was finally obtained,investigating the effects of pH value,water content and concentration of reactant on the reaction.The specific surface area of LMA powder prepared by reverse microemulsion is 55.08m²/g,when the temperature of system is30?,the ion concentration of solution in microemulsion system A is 4.7mol/L,the concentration of ammonia in microemulsion system B is 3mol/L,the water content of system is 30%,the pH value of system is 10.The high temperature evolution of LMA from precursor to product was studied.The inorganic process of precursor was completed before 400?,LMA was generated at 1148?.LMA powders prepared by reverse microemulsion method were sintered into ceramics,whose relative density was 98.04%.Some properities of LMA ceramics were characterized.The thermal conductivity of LMA is 2.14W·m^-1·K^-1?1473K?.The average coefficient of thermal expansion is 19.56×10^-6·K^-1?from room temperature to 1673K?.The LMA ceramics were obviously sintered after heated at 1300?,90%H2O?g?condition.The contents of Al³⁺,Mg²⁺,La³⁺in the soak liquid were 2.893mg/L,0.146mg/L and 0.402 mg/L after the LMA was immersed in deionized water for 168h.The effects of various factors on the preparation of LZA powders by reverse microemulsion were studied referring to the research of LMA.LZA powders were synthesized,through the research on the effects of pH value,water content and concentration of reactant on the reaction.The specific surface area of LZA powder prepared by reverse microemulsion is 48.40m²/g,when the temperature of system is30?,the ion concentration of solution in microemulsion system A is 4.7mol/L,the concentration of ammonia in microemulsion system B is 3mol/L,the water content of system is 30%,the pH value of system is 8.The high temperature evolution of LZA from precursor to product was studied too.The inorganic process of precursor was completed before 400?,LZA was generated at 1120?.LZA powders were sintered into ceramics with the relative density of 97.62%.Some properities of LZA ceramics were characterized,such as thermal physical properties,high temperature stability and so on.The thermal conductivity of LZA is3.70W·m^-1·K^-1?1473K?.The average coefficient of thermal expansion is 32.63×10^-6·K^-1?from room temperature to 1673K?.After heated at 1300?,90%H2O?g?condition,nothing had changes with LZA ceramics,which indicates a better water-vapor resistance than LMA ceramics.