Preparation And Investigation Of Al₂O₃ Based Microwave Dielectric Ceramics

In the recent 20 years,the wide application of communication equipments have brought unprecedented applications for microwave dielectric ceramics,and the functional materials of microwave ceramics have also developed rapidly.However,the updated communication technology in recent years also needs the support of microwave ceramic with better performance.Modern communication equipment has high requirement for low signal delay,which limits the dielectric constant of ceramic materials as low as possible.Al₂O₃ is a kind of material with low dielectric constant and high performance.At the same time,its raw materials are relatively cheap,and its potential application value is very high,but it has some shortcomings that have not been completely overcome,such as high sintering temperature and low temperature stability.In this thesis,systematic experiments were carried out in the system of alumina materials.According to the data obtained,the effects of doping and two-phase modification on the microstructure and microwave dielectric properties of the material were analyzed.Firstly,the influence of MgO doping on the properties of Al₂O₃ ceramics was studied,and the main performance was quality factor.In this section of the experiment,the purity of Al₂O₃ is 99.9%,the mole fraction of MgO is 0.05%～0.225%,and the sintering temperature is 1600℃.The results show that the Q×（？）of the ceramics can be improved with appropriate MgO contents,The performances of the ceramics with MgO doping mole fraction of 0.075%is the best:Q×（？）=94268GHz,ε_r=9.95.From the chemical analysis of raw materials,we know that the impurity content of the raw material is equal to the doping amount,in order to reduce the impact of impurities,the purity of 99.99%of the raw material is used for the experiment.In this section of the experiment,the mole fraction of MgO doping is 0.1%～0.25%,and the sintering temperature is 1450℃～1600℃.The experimental results showed that appropriate amount of MgO is beneficial to the growth of grains in the sintering process.When the sintering temperature is 1600℃and the doping amount is 0.15%,its microwave dielectric properties were Q×（？）=163264GHz,ε_r=10.72.Compared with the raw materials with lower purity,its Q×（？）is significantly improved.Secondly,the dielectric properties of Al₂O₃-TiO₂ multiphase ceramics were studied.The main research performance is the resonance frequency temperature coefficient.In order to study the influence of temperature on the properties of Al₂O₃-TiO₂ multiphase ceramics,the formula used in the experiment is 0.15%MgO+0.9Al₂O₃+0.1TiO₂,and the sintering temperature is 1300℃～1600℃.According to the experimental data,when the sintering temperature is higher than 1300℃,the third phase Al₂TiO₅ will be generated.The dielectric properties of ceramics will be greatly affected.At 1300℃,ceramics with the resonance frequency temperature coefficient of-6ppm/℃can be obtained,but Q×（？）was only 57683 GHz.In order to make the temperature coefficient of resonance frequency near zero,based on the original formula,the doping amount of MgO was adjusted to reduce the sintering temperature of Al₂O₃-TiO₂ multiphase ceramics.In this section of the experiment,the formula was x MgO+0.9Al₂O₃+0.1TiO₂（x=0.15%,0.175%,0.2%,0.225%,x=mole fraction）,and the sintering temperature was 1275℃～1350℃.When the mole fraction of MgO is 0.175%and the sintering temperature is 1300℃,the microwave dielectric properties of ceramics were Q×（？）=85188GHz,ε_r=12.81,τ_f=-1.9ppm/℃.In order to obtain high performance microwave dielectric ceramics with adjustable frequency temperature coefficient,the ratio of Al₂O₃ and TiO₂ was adjusted,the formula used in the experiment is 0.175%MgO+（1-x）Al₂O₃+x TiO₂（x=0.06,0.08,0.1,0.12,0.14）,and the sintering temperature is 1275℃～1350℃.High performance ceramics have been obtained with following properties:Q×（？）=68000GHz～85000GHz,ε_r=13.54～11.18,τ_f=+10ppm/℃～-28ppm/℃.At last,the preparation processes of Al₂O₃-TiO₂ multiphase ceramics were studied by controlling variables,and the effects of annealing temperature,annealing time,holding time and heating rate on microwave properties of the ceramics were studied.The results show that the Al₂TiO₅ can be decomposed by annealing process,and the properties of the ceramics are not affected in the range of annealing temperature 900℃～1200℃and annealing time 5h～20h.Reducing the heating rate during sintering can improve the quality of ceramics Q×（？）Up to 104194GHz.The experiment results show that the holding time（2h～5h）has no significant effect on the properties of the ceramics,this phenomenon proves that the material has good process adaptability.