The Research On Doping Modification And Low Temperature Sintering In Li₃Mg₂NbO₆ Based Microwave Dielectric Ceramics

The rapid development of modern communication technology has made more demands of the performance of microwave devices.Because microwave dielectric ceramic was the key materials of microwave devices,searching for new ceramic system,modifying the microwave dielectric properties and reducing the sintering temperature have became a popular research.Li₃Mg₂NbO₆ ceramics have been found to be a novel low permittivity and low loss microwave dielectric material in recent years.But the high sintering temperature,relatively lower Q×f value and large negative?_f value could not meet the requirement for practical application in microwave device.This paper decided to regulate the crystal structure,improve the sintering properties and optimize the microwave dielectric properties through the replacement of Mg²⁺and Nb⁵⁺.And on the basis,low melting compounds,glasses and composite sintering aids were used to decrease the sintering temperature of Li₃(Mg_0.96Ca_0.04)₂NbO₆ ceramics,the influence of sintering aids on the sintering behavior,phase composition,microstructure and microwave dielectric properties of Li₃(Mg_0.96Ca_0.04)₂NbO₆ ceramics were also studied.1.The Mg²⁺was replaced by A²⁺(Ca²⁺,Mn²⁺,Ni²⁺,Co²⁺)to improve the microwave dielectric properties of Li₃Mg₂NbO₆ ceramics.The results revealed that Li₃(Mg_1-xA_x)₂NbO₆ solid solutions were formed and the sintering characterization and microwave dielectric properties of Li₃(Mg_1-x-x A_x)₂NbO₆ ceramics could be optimized by the substitution of A²⁺for Mg²⁺.The actual ionic polarization of Li₃(Mg_1-xA_x)₂NbO₆ ceramics calculated by the Clausius-Mosotti equation could be well matched with the theoretical ionic polarization.At the same time,it was found that the porosity corrected permittivity(?_pc)was a major factor determining the temperature coefficient of resonant frequency??_f?of Li₃(Mg_1-xA_x)₂NbO₆ ceramics.Otherwise,this paper also introduced some related mathematical relationship between the?_f value and the?_pcc value of Li₃Mg₂NbO₆ based ceramics.?a?.When Mg²⁺was replaced by Ca²⁺,Mn²⁺,Co²⁺respectively,the cell volume of the solid solutions was positively proportional to the x value.The actual ionic polarization and theoretical ionic polarization of ceramics both increased with the increase of x value.When the x value was 0.04,the Li₃(Mg_0.96Ca_0.04)₂NbO₆ specimen sintered at 1140^oC had a superior Q×f value of 122,082 GHz;When the x value was 0.02,The Li₃(Mg_0.98Mn_0.02)₂NbO₆ sample sintered at 1125^oC had a small?_f value of-4.57ppm/^oC and a high Q×f value of 110,582 GHz;When the x value was 0.06,The Li₃(Mg_0.94Co_0.06)₂NbO₆ ceramics sintered at 1150^oC possessed the smallest?_f value of-1.04 ppm/^oC.?b?.When Mg²⁺was replaced by Ni²⁺,the cell volume of the solid solutions was negatively proportional to the x value.the theoretical ionic polarizability and actual ionic polarizability both decreased with the increase of x value.When the x value was 0.04,The Li₃(Mg_0.96Ni_0.04)₂NbO₆ sample sintered at1175^oC had the highest Q×f value of 126,784 GHz and a small?_f value of-2.63ppm/^oC.2.The Nb⁵⁺was replaced by B⁵⁺to improve the microwave dielectric properties of Li₃Mg₂NbO₆ ceramics.The results revealed that Li₃Mg₂(Nb_1-xB_x)O₆ solid solutions were formed by the substitution of B⁵⁺for Nb⁵⁺.The cell volume of the solid solutions was negatively proportional to the x value,the theoretical ionic polarizability and actual ionic polarizability of ceramics both increased with the increase of x value.And the?_f value and?_pcc value of Li₃Mg₂(Nb_1-xB_x)O₆ ceramics both exhibited a similar variation tendency with the increase of x value.?a?.When the x value was 0.02,The Li₃Mg₂(Nb_0.98Ta_0.02)O₆ ceramics sintered at 1150^oC had a high Q×f value of 110,443GHz;?b?.When the x value was 0.06,the Li₃Mg₂(Nb_0.94Sb_0.06)O₆ sample sintered at1150^oC had a Q×f value of 95,487 GHz.3.LiF,Li₂O-B₂O₃-Bi₂O₃-SiO₂?LBBS?,Bi₂O₃,BaCu?B₂O₅??BCB?were used as sintering aids to realize the low temperature sintering of Li₃(Mg_0.96Ca_0.04)₂NbO₆ceramics.The sintering characterization,microstructure and microwave dielectric properties of the ceramics were investigated systematically.The results showed that these four kinds of sintering aids could effectively decrease the sintering temperature of Li₃(Mg_0.96Ca_0.04)₂NbO₆ ceramics.However,Only the LiF additives could decrease the densification temperature of ceramics and the obtained specimen remained superior microwave dielectric properties.When 6wt.%LiF was doped into Li₃(Mg_0.96Ca_0.04)₂NbO₆ ceramics,the obtained sample sintered at 900^oC had excellent microwave dielectric properties of?_r?15.85,Q×f?98,012 GHz,?_f?-28.6ppm/^oC.