| Li4Mg3Ti2O9 based microwave dielectric ceramics have been widely concerned by researchers in recent years because of their excellent microwave dielectric properties.In order to further optimize the performance,(Mg1/3Ta2/3)4+and(Mg1/3Nb2/3)4+were used to investigate the modification.On this basis,the optimum ceramics were selected to lower sintering temperature,and the specific research contents are as follows:A series of Li4Mg3[Ti1-x(Mg1/3A2/3)x]2O9?A=Ta,Nb?ceramics were prepared via the traditional solid-state method.The effects of the substitution of(Mg1/3A2/3)4+on the sintering characteristic,microstructure,phase composition and dielectric properties of Li4Mg3Ti2O9were investigated for the first time.The analysis of the X-ray diffraction pattern indicated that the pure cubic Li4Mg3Ti2O9 phase in Fm-3m space group could be formed in the whole compositions range.Dense and homogeneous microstructures of ceramics sintered at their optimum sintering temperatures could be revealed from the scanning electric microscope.The results indicated that appropriate(Mg1/3Ta2/3),(Mg1/3Nb2/3)-substitution could improve dielectric properties of the matrix.The optimum microwave dielectric properties,with a permittivity 15.77,a quality factor 160,575 GHz and a?f value 0 ppm/°C,were achieved in the Li4Mg3[Ti0.8(Mg1/3Ta2/3)0.2]2O9 ceramic sintered at 1550°C;the best properties of?r=16.83,Q·f=144,350 GHz as well as ?f=-3.05 ppm/°C were obtained in Li4Mg3[Ti0.6(Mg1/3Nb2/3)0.4]2O9 ceramics sintered at 1450°C.TheintrinsicpropertiesofLi4Mg3[Ti0.8(Mg1/3Ta2/3)0.2]2O9and Li4Mg3[Ti0.6(Mg1/3Nb2/3)0.4]2O9 were explored by the far-infrared spectrum,and the calculated microwave dielectric properties were in good agreement with the measured values.It could be concluded that the microwave dielectric property was mainly ascribed to the ion polarization rather electronic.The crystal structure of Li4Mg3[Ti1-x(Mg1/3Ta2/3)x]2O9 was further investigated through the Rietveld refinement results,and the cell parameters,Rp and Rwpp values were obtained.The chemical bonds theory of complex crystals was employed to investigate the structure-property relations of samples in detail,and the variations of?r,Q·f andtf values could be mainly caused by the Li/Ti-O bond parameters.Espeacially,the variation in the?Ti/?Mg1/3Ta2/3?-O versus the x presented similar behavior to that oftf.The effects of Li F additives on the phase composition,sintering characteristic,microstructure and microwave dielectric properties of Li4Mg3[Ti0.8(Mg1/3Ta2/3)0.2]2O9 and Li4Mg3[Ti0.6(Mg1/3Nb2/3)0.4]2O9 ceramics were investigated in detail for the first time.The sintering temperatures of ceramics could be effectively lowered to 950°C by appropriate addition of LiF.A single rock salt crystalline phase structure belonging to a space group of Fm-3m?No.225?was obtained through X-ray diffraction patterns.Dense uniform morphology wasobservedat950°CforLi4Mg3[Ti0.8(Mg1/3Ta2/3)0.2]2O9and Li4Mg3[Ti0.6(Mg1/3Nb2/3)0.4]2O9 ceramics with 3-5 wt%Li F in terms of scanning electron microscopy photographs.Particularly,the Li4Mg3[Ti0.8(Mg1/3Ta2/3)0.2]2O9-4 wt%LiF ceramics sintered at 950?°C for 4h possessed optimum microwave dielectric properties of?r=16.10, Q·f=114,313 GHz andtf=-7.72 ppm/°C;the optimum dielectric properties of?r=16.11,Q·f=107,828 GHz and?f=-6.3 ppm/°C were obtained in Li4Mg3[Ti0.6(Mg1/3Nb2/3)0.4]2O9-4 wt%LiF ceramics sintered at 950?°C.In addition,the excellent chemical compatibility with silver metalelectrodesindicatedthattheLi4Mg3[Ti0.8(Mg1/3Ta2/3)0.2]2O9,Li4Mg3[Ti0.6(Mg1/3Nb2/3)0.4]2O9-4 wt%LiF ceramics might be promising candidates for the low temperature co-fired ceramics?LTCC?applications in microwave devices. |
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