Synthesis And Modification On The Dielectric Properties Of Ba(Mg_1/3Nb_2/3)O₃ Microwave Dielectric Ceramics

Ba(Mg_1/3Nb_2/3)O₃ microwave dielectric ceramic is one of theimportant microwave dielectric material.But the pretty high temperature coefficient of resonance frequency(（32 ppm/ ℃）and low quality factor restrict the application and development of Ba(Mg_1/3Nb_2/3)O₃ ceramics.Based on the result of the research work on the microwave dielectric ceramics,A-site substitution and second phase addition with negative temperature coefficient of resonance frequency are used to modify the structure and properties of Ba(Mg_1/3Nb_2/3)O₃ ceramics.The synthesis techniques of Ba(Mg_1/3Nb_2/3)O₃ ceramics was researched,the appropriate calcination and sintering process were confirmed.Firstly,the Ba(Mg_1/3Nb_2/3)O₃ phase was synthesiseed by calcining at 1300°C for 2 h.And then,the samples sintered at 1500 ℃ for 4 h get the pretty excellent properties: εr=32.3,Q×f=41614GHz,τf=31.13ppm/ ℃。The Ca²⁺ and rare earth ion(Ln³⁺: Eu³⁺;Sm³⁺;La³⁺;Nd³⁺)were chosen to substitute the A-site of the Ba(Mg_1/3Nb_2/3)O₃ ceramics,which does influence the phase,microstructure and dielectric properties.The results show that:（1）The Ca-substitution does improve the 1:2 ordering structure of B-site,which can reduce the dielectric loss of ceramics.And with the 0.5 mol% Ca-substitution,the 1:2 ordering parameter（S）and the dielectric properties reach best value: S=53.5;εr=31.46,Q×f= 82802 GHz,τ_f＝ 14.59 ppm/ ℃.（2）the few second phase BaLn₂O₄ will appear when the Ln-substitution ceramics sintered at lower temperature,and disappear with the rise of sintering temperature.For the charge imbalance between Ln³⁺ and Ba²⁺,the best dielectric properties of Ln-substitution Ba(Mg_1/3Nb_2/3)O₃ ceramics deteriorate:（a）Eu-substitution samples sintered at 1400 ℃ get the best properties: εr=32.5;Q×f= 39180GHz;tf= 25.32 ppm/ ℃.（b）Sm-substitution samples sintered at 1500 ℃ get the best properties: εr=32.5;Q×f= 25578GHz;tf= 29.43 ppm/ ℃.（c）La-substitution samples sintered at 1550 ℃ get the best properties: εr=31.82;Q×f = 50592GHz;τ_f＝30 ppm/ ℃.（d）Nd-substitution samples sintered at 1450 ℃ get the best properties: εr=32.81;Q×f = 30610GHz;τ_f＝ 34 ppm/ ℃.（3）The variations in Raman spectra of the Ba(Mg_1/3Nb_2/3)O₃ ceramics with A-site substation have been revealed.The results show that the variation modes related to the Ba ions and O ions are affected by the A-site substitution obviously.For the Ca-substitution,the A1g（O）mode shift increases and the FWHM changes with the S variation;For the Ln-substitution,both of the F2g（Ba）mode and A1g（O）mode shift increase.According to the stoichiometric ratio of Ba(Mg_1/3Nb_2/3)O₃: LaAlO3 =4:1,the LaAlO3 phase was added to the Ba(Mg_1/3Nb_2/3)O₃ ceramics.After sintering procession,there is no Ba(Mg_1/3Nb_2/3)O₃ and LaAlO3 coexistence,but the new phase Ba_0.8La_0.2[(Mg_1/3Nb_2/3)_0.8Al_0.2]O₃ appears.And the 1:2 ordering structure disappeared,which contains the development of quality factor.The best microwave dielectric properties were obtained with the samples sintered at 1500 ℃ for 4 h: εr=23.2;Q×f = 44164GHz;τ_f＝ 7.5 ppm/ ℃.