Study On Preparation Of Yttrium Oxide Powders And Properties Of Yttrium Materials

As a kind of rare earth oxide with excellent performance and low price,yttrium oxide(Y2O3)has excellent optical,electrical and magnetic properties,as well as high corrosion resistance and chemical stability at high temperature.Due to these properties above,Y2O3 is widely used in the fields of glass ceramics,petrochemicals,metallurgical machinery,electronic information and high-tech materials,et al.However,Y2O3 materials are difficult to be sintered and densified,and the thermal shock resistance is poor,which extremely limited its application.Therefore,great attention has been paid to investigate the further improve of sintering densification and thermal shock resistance of Y2O3 materials.In this paper,a series of research works were carried out on the preparation of Y2O3 powders with special morphology and Y2O3 ceramic materials with excellent comprehensive performance.The test methods and characterization methods were developed,such as TG-DSC,XRD,optical microscope,SEM,bulk density,apparent porosity,thermal shock resistance and fracture to analyze the phase,microstructure,macrostructure and mechanical properties of the prepared Y2O3 powders and Y2O3 ceramic materials.The effects of initial concentration of mother salt solution,precipitant species,reaction temperature,magnitude and frequency of pulse current on the preparation of powders were studied.The effects of preparation technologies,such as pulse current sintering,wet chemical method and infiltration technique of liquid phase precursor on the Y2O3 properties and the corresponding mechanism were discussed.The main research results are as follows:(1)The precipitant species,initial concentration of mother salt solution and reaction temperature have great influence on the particle size and morphology of Y2O3 powders prepared by liquid precipitation method,which must be strictly controlled to obtain the desired powders.Y2(CO3)3·2H2O precursor was obtained when by using NH4HCO3 as a precipitant,which have three morphologies,i.e.,two-dimensional diamond,long strip and whisker-like structure.Y;2(OH)x(NO3)(6-x)·1.5H2O precursor was obtained when by using NH3·H2O as a precipitant,and the received precursor was a flaky with adhesion,self-assembled three-dimensional spherical particle structure.When the reaction temperature was 35°C and the Y3+concentration was 0.1mol·L-1,the Y2O3 powders with regular rhombohedral structure were obtained by using the NH4HCO3 as precipitant.However,when the reaction temperature was raised to 55℃ and 75℃,a long strip shape with few granules was obtained.When the Y3+concentration was raised to 0.5mol·L-1,Y2O3 with bundle whisker was obtained when the reaction temperature at 35℃.(2)The internal energy of the reaction system was increased when the current pulse applied to the solution during the precipitation process.The outer electron layers of the cluster in the system undergo repeated distortion and relaxation under the action of the current pulse,which promoted the combination between the cluster and its surrounding ion pair.The combination further affected the microstructure of the prepared Y2O3 powders.The magnitude and frequency of the current pulse have a great influence on the degree of crystallization and morphology of the Y2O3 powders.When a current pulse with 10A and 10kHz was applied to the precipitation process,uniform Y2O3 powders with large length-diameter ratio were obtained.The other conditions at the precipitation process were that,the concentrations of the mother salt solution Y(NO3)3 and NH4HCO3 solution respectively were 0.5mol·L-1 and 1.5mol·L-1,and the reaction temperature was 55℃.(3)Y2O3 powders with special morphologies(rhombus-like and whisker-like)were used as raw materials to prepared high density Y2O3 sinter,which was obtained at lower sintering temperature and shorter holding time by using the spark plasma sintering(SPS)method.Among them,the whisker-like Y2O3 powders were easier to sinter than the rhombus-like Y2O3 powders.Due to the Y2O3 sinter prepared by SPS inherited the layered structure of the raw material,the Y2O3 sinter prepared by rhombus-like Y2O3 powders showed higher thermal shock resistance than the Y2O3 sinter prepared by whisker-like Y2O3 powders.(4)Containing a small amount of CeO2 or CaO played a role in the promoting sintering,and a higher density densification of Y2O3一based composite sintered body obtained at a lower temperature.CeO2 effectively inhibited the grain growth of CeO2/Y2O3 composites and made the grains fine and uniform.The CeO2/Y2O3 composites with 23.1%mole fraction of CeO2 have a maximum hardness and fracture toughness after sintering at 1600℃ for 4h.CaO promoted the growth of CaO/Y2O3 composites,while the abnormal growth of grain reduced the hardness of the CaO/Y2O3 materials.(5)Liquid phase precursor infiltration technology was used to compose single or composite impregnation solution by Ca(NO3)2·4H2O,Ce(NO3)3·6H2O,Nd(NO3)3·6H2O,La(NO3)3·6H2O and Cr(NO3)3*9 solution.The concentration gradient was formed due to the impregnation solution infiltration into the material from top to bottom,which induced the different microstructure of Y2O3 at different parts were obtained in the subsequent sintering process.The well-defined microstructures could divide into fully impregnated layer,infiltrated transition layer and native layer.After impregnated with Ca(NO3)2·4H2O solution,the sintered body has abnormally grown crystal grains.After impregnated with Ce(NO3)3·6H2O solution,the sintered body has fine crystal grains and uniform size.After impregnated with the mixed solution of Ca(NO3)2·4H2O and Cr(NO3)3·9H2O,the sintered body was poor densification with many pores.After impregnated with Nd(NO3)3·6H2O solution and impregnated with the mixed solution of Ca(NO3)2·4H2O,Ce(NO3)3·6H2O and La(NO3)3·6H2O,sintered body was compact with uniform grain size.The results show that the corresponding gradient materials could be prepared by liquid phase precursor infiltration technology.