| As one kind of key information functional materials, Microwave dielectric ceramics (MWDC) are widely applied in microwave communication technology. Recently microwave dielectric ceramics with high dielectric constant have attracted more and more special interests due to their potential advantage of miniaturization and integration of microwave resonant components. Usually the application of single MWDC material is limited by its large temperature coefficient of resonant frequency. The modification of microwave dielectric properties of MWDC with perovskite structure was investigated in this thesis. Binary compounds were chosen to compensate the temperature coefficient, with the desire to search for perovskite structure solid solutions with high-s, high Qf and near-zero Tf.Perovskite structure CaTiO3 exhibits good dielectric properties of high e and high Qf except for large positive f. Hence, (Li1/2Ln1/2)TiO3 and Ca(Mg1/3Nb2/3)O3 with negative f were combined with CaTiO3 based materials to conduct A-site and B-site substitution respectively. The compounds were prepared by means of solid state reaction. The effect of composition change and sintering condition upon the phase constitution, microstructure and microwave dielectric properties was investigated as well as the relationship between the structure and the property. XRD analysis, SEM observation and the measurement of dielectric properties at low frequency and microwave frequency are employed in the present study.The best sintering condition and solid solution limits of (l-x)CaTiO3-x(Li1/2Ln1/2)TiO3 MWDC system were discussed. Infinite perovskite structure solid solution could be found for Ln=Nd, Sm system and the solid solution limits were confirmed to be x 0.5 for Ln=La. Dielectric constant and quality factor values decreased with the increase of x value in the solid solution compounds. Tunable microwave dielectric properties of from 80 to 140, Qf values from 2500GHz to 6000GHz and f values from +400ppm/ C to -200ppm/C wereobtained. Near-zero Tf value can be achieved by adjusting the x value: Ln=La, x=0.5, f=-12.3ppm/ C, Ln=Nd, x=0.6, f=2.2ppm/ C and Ln=Sm, x=0.7, Tf=3.8ppm/ C, respectively. The results of substitution of Ba2+ to Ca2+ in 0.3CaTiO3-0.7(Li1/2Sm1/2)TiO3 ceramic showed that the modification can improve the dielectric constant.Powders of (l-x)CaTi03-xCa(Mgi/3Nb2/3)O3 system were synthesized by the precursor method. A single orthorhombic perovskite structure formed and microwave dielectric properties continuously changed with the increasing x: decreased, Qf increased and Tf moved to negative. The better microwave dielectric properties of e= 49.54, Qf=28900GHz, f= -22ppm/C were obtained at x=0.6. La3+ was chosen to dope into Ca [(Mgi/3Nb2/3)o.6Tio.4]O3 and improved Qf values from 28900GHz to 31400 GHz and f values from -22ppm/ C to -0.02ppm/ C but decreased e were found when the dopant amount is 0.2.The results showed that two or more compounds with negative and positive temperature coefficient values employed to form a solid solution are the most promising method to obtain near-zero temperature coefficient of resonant frequency. Microwave dielectric properties strongly depend on the distortion of oxygen octahedron and inner electric fields in perovskite structure.
|
PDF Full Text Request