Research On The Structure Regulation And Properties Of A Novel Medium-high Permittivity With A General Formula A₃B₈CO₂₁ Microwave Dielectric Ceramics

Microwave dielectric ceramic is a kind of novel functional ceramic materials with the development of several decades,which can achieve one or more function in the microwave regions.Application of the dielectric ceramic materials were determined by the microwave dielectric properties,and the relative permittivity?_r,quality factor Q×f and the temperature coefficient of resonance frequency?_f are as the three key parameters.Excellent microwave dielectric properties were reported in the tungsten bronze-like structured ceramics with a general formula A₃B₈CO₂₁.According to the crystal structure of A₃B₈CO₂₁1 compounds,the cationic composition and placeholder design of B and C sites were carried out and prepared a series of A₃B₈CO₂₁1 materials.And the relationship between crystal structure,spectral characteristics and dielectric properties were investigated systematically.1.A series of Ba₃Nb_4-4xTi_4+5xO₂₁?x=0.1,0.2,and 0.3?ceramics with hexagonal structure were prepared via a solid-state reaction method.The results show that when the x value is 0.1and 0.2,a single Ba₃Nb₄Ti₄O₂₁1 phase can be obtained,and when x=0.3,the second phase Ba₆Nb₂Ti₁₄O₃₉9 appears,indicating that it can be completely formed solid solution when x?0.2 in the ceramic system.The Ti element reacts better than the Nb element,so that the increase of Ti content can lower the sintering temperature,as the x values increases from 0.1to 0.3,the optimum sintering temperature of the ceramic sample reduces from 1230°C to1170°C.The lattice structure became more stable after the cations entered into C sites formed solid solution,thus reducing the dielectric loss and ionic polarizability of the ceramics.Therefore,when the value of x increases from 0.1 to 0.3,the?_r is reduced from 63.2 to 54.4,and the?_f is from 123 reduced to 96.3 ppm/°C,and the Q×f increased from 8,100 to 9,400 and then reduced to 7,300 GHz.The excellent microwave dielectric properties were obtained in Ba₃Nb_3.2Ti₅O₂₁1 ceramic sintered at 1210°C for 6 h,with?_{r^</sub>54.9,Q×f=9,400 GHz,?f¹09.7ppm/°C.And the intrinsic dielectric properties of Ba3Nb3.2Ti5O211 with?r^</sub>55,Q×f=19,480GH were obtained by infrared reflectivity spectra.2.Consindering that Ta and Nb possess the same ionic radius and different extranuclear electronic arrangements,which may change the crystal structure and affects its microwave dielectric properties.In the Ba3Nb4-4xTi4+5xO211 ceramics system,Ba3Ta4-4xTi4+5xO21?x=0.1,0.2,and 0.3?ceramics were synthesized with Ta instead of Nb.As x=0.2,all the ceramic samples appear green after sintered,and the XPS results show that the part of the valence state of Ti⁴⁺changed to Ti³⁺,leading to the microwave dielectric properties of Ba3Ta4-4xTi4+5xO211 and Ba3Nb4-4xTi4+5xO211 exhibited a different variation.Thus,the ceramic sample at x=0.2 revealed a low value of Q×f and?f.In addition,the introduction of Ta element can improve the dielectric properties in the Ba3Ta4-4xTi4+5xO211 system.The microwave dielectric properties of Ba3Ta3.6Ti4.5O211 ceramic was obtained at 1240°C for 6 h,with?r^</sub>46.2,Q×f=12,400 GHz,?f⁺50 ppm/°C.And the intrinsic dielectric properties of Ba3Ta3.6Ti4.5O211 with?r^</sub>44,Q×f=34,850 GHz were obtained by infrared reflectivity spectra.3.In Ba3Nb4-4xTi4+5xO211 ceramic systems,it can be completely formed solid solution and lower the sintering temperature as the value of x?0.2,but the sintering temperature is still too high to co-fired with metal electrodes,therefore the Li element contented componds with low melting point is introduced in the ceramics system,a series of ceramic samples of LixBa3Nb4-x-x Ti4+xO21?x=0.2,0.4,0.6,0.8 and 1.0?were prepared.The results show that all the ceramic samples are single phase,and with x increases from 0.2 to 1.0,the optimum sintering temperature from 1100°C gradually reduces to 1020°C.The introduction of Li reduces the driving force required for crystal growth,so that the sintering temperature is lowered,the growth of crystal grains is suppressed,and the grain size is gradually reduced from 5-6?m to 2-3?m.The introduction of Li can also lead to a change in the oxygen polyhedron,so that the ionic polarizability is improved and increased the relative dielectric constant from 67.8 to 74.8.On the one hand,the introduce of Li strengthened the crystal structure and lower the dielectric loss,on the other hand,some crystal defects enhance the dielectric loss simultaneously,the quality factor only determined by one factor under certain conditions.The optimum quality factor Q×f values was obtained at x=0.4 with Q×f¹4,600GHz and the intrinsic dielectric constant?r⁴9 and Q×f¹6,470 GHz of Li0.4Ba3Nb3.6Ti4.4O21ceramic were obtained by infrared reflectivity spectra.4.Similarly,the Li element is introduced in the Ba3Ta4-4xTi4+5xO211 system,and prepared a series of ceramic samples of LixBa3Ta4-xTi4+xO21?x=0.2,0.4,0.6,0.8 and 1.0?with different filling degrees of C sites.The results show that the small secondary phase of Ba6Ti2Ta8O300 was detected at x=1.0.As the x value from 0.2 to 1.0,the optimum sintering temperature of the ceramic decreases from 1140°C to 1060°C,the?rincreases from 45.8 to56.8,the Q×f decreases from 23,400 GHz to 2,320 GHz,and the?f from+58 ppm/°C increases to+142 ppm/°C.The microwave dielectric properties of Li0.2Ba3Ta3.8Ti4.2O21ceramic sintered at 1140°C for 6 h was obtained with?r^</sub>45.8,Q×f=23,400 GHz,?f⁺58ppm/°C and the intrinsic dielectric properties with?r^</sub>42.2,Q×f=26,470 GHz were obtained by infrared reflectivity spectra.}