Research On The Preparation,Optical And Microwave Dielectric Properties Of Translucent Polycrystalline Alumina

The development of microwave dielectric materials for applications in communication systems has attracted significant attention for the last decade.As the most widely used ceramic material,alumina ceramic is a well-known microwave dielectric material because of its distinguished performances,including its suitable dielectric constant,low dielectric loss,good thermal conductivity and high mechical properties.In this work,the precursor of?-Al2O3 preparation,microstructure,optical and microwave dielectric properties of translucent polycrystalline alumina(PCA)have been investigated.Firstly,ammonium aluminum carbonate hydroxide(AACH)was synthesized via co-precipitation process by using ammonium aluminum sulfate(AAS)and ammonium hydrogen carbon(AHC)as raw materials.Effects of aging and washing processes on AACH have been studied.AACH could keep stable only under the low temperature(room temperature,?20?)and a suitable pH value(9.0?10.0).AACH wouldtransform to others Al-(oxy)hydroxide phases as the pH value changed,and the transformation process would be speeded up when the temperature elevated.Compared with deionized water,dilute ammonia washing can make the AACH powder better dispersivity and less-aggregated.Based on the above study,AACH washed with dilute ammonia could easier to transform into ?-Al2O3 completely at 1100? for 2h.Then,high-purity ?-Al2O3 powder was used as the raw material to fabricate transcluent PCA,MgO dopant were introduced into alumina via precipitating method.The effect of the amounts of MgO dopant on the microstructure,optical performances and microwave dielectric properties of translucent PCA was investigated systematically.Results show that the amounts of MgO has no significant influences on the densities and dielectric constants of the translucent PCA samples.However,the effect of dopants on the optical transmittance and dielectric loss were obvious.Compared with the undoped PCA,the translucent PCA doped with 500 ppm MgO showed a lower dielectric loss of 5.20×10-5 and a high Q×f value of 199,231 GHz.Besides,the real in-line transmission(RIT)at the wavelength of 600 nm was as high as?21%.Based on the above investigation,ZrO2 was chosed to co-dope with 500 ppm MgO to further improve the performance of translucent PCA.Samples co-doped with 500 ppm ZrO2 and 500 ppm MgO showed the lower dielectric loss of 4.78×10-5,the Q×f value could increase to 220,084 GHz.Besides,effects of the MgO/ZrO2 content ratio on the microstructure and dielectric properties of translucent PCA were discussed.As the content of MgO increased,the abnormal grain growth could be restrained gradually;When 300 ppm ZrO2and 500 ppm MgO were co-doped,the Q×f value maximized at 200,000 GHz;When 800 ppm or 1000 ppmZrO2 were co-doped,more content of MgO addition could help reduce the sensitivity of Q×fvalue on the doping level,Q×f value varied in a relative narrow range from 100,000 GHz to 160,000 GHz.Finally,La2O3 was chosed to co-dope with 500 ppm MgO to improve the performance of translucent PCA.For samples co-doped with 500 ppm La2O3 and 500 ppm MgO,the most optimal RIT and mirowave dielectric properties could be obtained,which were?30%and 4.91×10-5 respectively.After that,effects of the diffent content ratio of MgO/La2O3 on the microstructure,optical and dielectric properties of samples were discussed.When the content ratio of MgO/La2O3?1,the second phase at the grain boundaries disappeared.The RIT of samples could increase to?30%,and Q×fvalue was as high as?300,000 GHz.Besides,with the increasing content of dopants,the fluctuation on the Q×f value was much slighter than before.