| Microwave dielectric ceramics have attracted the attention of researchers due to their wide applications in communication applications,radar systems and detection area.In this paper,low temperature sintered La2Zr3?MoO4?9-based microwave dielectric ceramics were prepared by traditional solid state method.In order to improve the dielectric properties of the ceramics,La2(Zr1-xTix)3?MoO4?9?x=0.00-0.10?microwave dielectric ceramics were prepared by quantitative doping of Ti4+in the Zr4+site,and the sintering characteristic,apparent morphology,phase composition and microwave dielectric properties of the matrix were systematically investigated.In addition,the mechanism of the improvement of dielectric properties was investigated by calculating the polarizability of ceramic matrix,the packing fraction,the bond valence and the chemical bond parameters.Besides,the contribution of lattice vibration to dielectric constant and dielectric loss could be obtained by far infrared spectra.The main findings of this paper are as follows:Firstly,La2Zr3?MoO4?9 microwave dielectric ceramics were prepared by traditional solid state method and sintered at 550?-800?.The results of the apparent density and SEM showed that the dense La2Zr3?MoO4?9 ceramics could be obtained at 650?and exhibited the best microwave dielectric properties of?r=10.8,Q·f=61,790 GHz and?f=-29.1 ppm/?.Besides,X-ray diffraction analysis and Rietveld refinement indicated that La2Zr3?MoO4?9belonged to a trigonal system with space group R-3c and the lattice parameters were a=b=9.8481?,c=59.1385?,?=?=90°,?=120°and V=4967.0830?3.Based on the chemical bond theory,the?r value depended on the bond ionicity of La-O bond.Mo-O bond possessed higher lattice energy,bond energy and near-zero thermal expansion coefficient which indicated that the Mo-O bond played a leading role in the stability of the crystal structure.Secondly,in order to optimize the microwave dielectric properties of ceramics,La2(Zr1-x-x Tix)3?MoO4?9?x=0.00-0.10?ceramics were prepared.Ceramics with different doping amounts sintered at 650?-750?possessed dense structure.With the increase of Ti4+amount,the dielectric constant of ceramics decreases slightly,and the quality factor and temperature coefficient of resonance frequency were improved.The La2(Zr0.92Ti0.08)3?MoO4?9ceramics sintered at 750?exhibited the best dielectric properties of?r=10.3,Q·f=80,700GHz and?f=3.5 ppm/?.The analysis of the intrinsic factors affecting the dielectric properties of ceramics showed that the decrease of the theoretical polarizability and the ionicity of La-O bonds were the main reasons for the decrease of dielectric constant of ceramics.In addition,the increase of packing fraction of ceramic and lattice energy of Mo-O bond were the main influencing factor of quality factor;Besides,the increase of bond energy of the Mo-O bond and the decrease of the bond valence made the temperature coefficient of the resonant frequency greatly improved.What's more,the dielectric constant and dielectric loss obtained by far-infrared fitting extrapolation were consistent with the measured results,indicating that the main contribution to polarization for La2(Zr0.92Ti0.08)3?MoO4?9 ceramics was the absorption of structural phonon oscillation. |
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