Consolidation Of Ceramics With Composite Structure And Fabrication Of Transparent Ceramics By Gel-casting

α-Al₂O₃,the only Thermodynamic stable phase in three structures（β-Al₂O₃、γ-Al₂O₃）,is very stable under the high temperature.It has advantages such as compact structure,low activity,good mechanical properties.Y₂O₃ is the material with optical function,whose melting point is high and which is heat-resisting,anti-corrosion,and no double refraction.It is mainly applied to infrared missile window,cowling,and infrared lens and so on.Composite structure has great significance.For example,pure YAG and Sapphire,in transparent ceramics with composite structure,serves the function of heat dissipation,which has been effective in decreasing the heat effect during the operation and improving the laser performances.The thesis mainly focus on the ceramics with cylinder composite structure and transparent ceramics that are made through gel-casting and vacuum sintering technology.Firstly,it studies the Al₂O₃ doped Cr³⁺ and determines the optimal technological parameters.It turns out that the optimal addition of dispersing agent is 1.5wt%、2.5wt%and 3.0wt%respectively when solids loading is 20vol%、25vol%、30vol%,which makes the respective stability of suspension liquid in a good condition.Cold isostatic pressing technology plays critical role in interfacial adhesion.Among the solids loading,two groups conform to the requirement of experiment.①For internal 30vol%-external 20vol%,the highest relative density of sample is up to 99.5%.②For internal 30vol%-external 25vol%,the relative density of sintered body of composite structure is up to 98.52%.Systematic study for performance of ceramics with cylindrical composite structure(internal 25vol%Al₂O₃-external 30vol%Al₂O₃:Cr³⁺)suggests that as the temperature of sintered body increase,its relative density and compressive strength increase,the average size of crystal grain enlarge apparently,and,in the border,the dispersal distance of Cr³⁺ enlarges,which is in favor of the physical performance of ceramics with composite structure.According to SEM graph,sample interface under various temperatures is of good quality and microstructure is uniform.The transmittance of transparent ceramics,which is made through traditional water base gel system,dosen’t conform to the theory requrement.The main reason is that when hydrolytic action occurs in Y³⁺,slurries is unable to reach stability state within short time.Thus,to disinhibit hydrolytic action of Y³⁺,coating technology is made on the Y₂O₃ powder surface.It turns out that the pH of powder is in a stable state after calcining at 1000℃.Diffraction peak diverges to left in the graph of XRD phase analysis.The image of transmission electron microscope shows that there is apparent coating layer on the powder surface.When the Y₂O₃ transparent ceramic is made of coated powders calcined at 1000℃,the crystal boundary is more clear,with an average grain size of 3.5μm,and a high optical transmittance of up to 71%at 800nm.