| Due to the progress of the internet technology and modern communications, microwave dielectric ceramic materials are obtaining the unprecedented development. With the working frequency of communication equipment expanding to the high frequency range, entering the millimeter wave band, the size of the communication device size requirements are more stringent, hope microwave dielectric ceramics which has low dielectric constant, high quality factor and the near-zero temperature coefficient of resonant frequency. Combined with the current research direction of microwave dielectric ceramics, ZnAl2O4-based ceramics were chosen as research objective in this dissertation. We choose Cu2+ ions which substituting at A-site of ZnAl2O4 to promote the sintering, meanwhile keep the high-quality. Then TiO2 and SrTiO3 which have large temperature-coefficient-frequency and high dielectric constant are chosen to regulate the temperature-coefficient-frequency of ZnAl2O4-based ceramics closed to zero and to increase the dielectric constant. At last, the sintering additives are used to further reduce the sintered temperature.Firstly, Zn1-xCuxAl2O4(x=0-0.015) microwave dielectric ceramics were prepared via the conventional solid-state route, the effect of different amounts of Cu2+ on the crystal phase composition, microstructure and microwave dielectric properties were investigated systematically. It’s fund that x in the range of 0 to 0.015, Zn1-xCuxAl2O4^ system forms a complete solid solution, ZnAl2O4 spinel crystal structure, and the second phase does not appear, the lattice parameters and cell volume increases as the value of x decreases. With the increase of x, dielectric constant and quality factor increases then decreases. The best microwave dielectric properties were obtained in Zn0.99Cu0.01Al2O4 with the dielectric constant εr= 8.85, the quality factor Q×f= 69300 GHz and the frequency temperature coefficient Υf=-59.4×10-6/℃ which was sintered at 1520℃ for 3 h in air.Secondly, (1-x)Zn0.99Cu0.01Al2O4-xSrTiO3 (abbreviated ZCAST, x=0,0.02,0.03, 0.04,0.045) microwave dielectric ceramics were prepared via the conventional solid-state route. The effect of different amounts of SrTiO3 on sintering properties, composition and the microwave dielectric properties were investigated systematically. The results show that ZCAST ceramics is dominated by spinel ZnAl2O4 crystal phase and perovskite SrTiO3 phase, no impurity phase appears. With the increase of the amount of SrTiO3, ZCAST ceramic sintering temperature was reduced by nearly 40℃, but the dielectric constant increases, while the frequency of the temperature coefficient of the material increases. When the addition amount of SrTiO3 was 0.04 mol, the frequency temperature coefficient can be adjusted to near zero (Υf=-1.8 × 10-6/℃). In this case εr= 14.12 and Q×f= 28700 GHz.We studied the (1-x) Zn0.99Cu0.01Al2O4-xTiO2(abbreviated ZCAT, x= 0,0.17,0.20,0.23,0.25) microwave dielectric ceramics with the same method and the result shows that Zn0.99Cu0.01Al2O4 and TiO2 could coexist with each other and form a two-phase system. The sintering temperature could be significant decreased by increasing TiO2 content. The bulk volume density and the quality factor of ZCAT decreased with the content of TiO2, while the dielectric constant of ZCAT increased with the content of TiO2. The frequency temperature coefficient could adjust nearly-zero by adding an appropriate amount of TiO2. The optimal microwave dielectric properties were obtained in 0.77Zn0.99Cu0.01Al2O4-0.23TiO2with εr=11.4, Q×f= 38155 GHz and Υf=-2.4×10-6/℃ which was sintered at 1380℃ for 3 h in air.Finally, in order to effectively reduce the sintered temperature of 0.77 Zno.99Cuo.oiAl204-0.23Ti02(abbreviated 0.77ZCAT) ceramics, we adopted solid-phase method to study the influences to sintering character, structural composition and microwave dielectric properties of 0.77ZCAT ceramics by mixing different amounts of B2O3(0.25,0.5,0.75,1.0,2.0,3.0 wt%). The result shows that the added B2O3 doses not change the phase composition but significantly reduce sintering temperature of 0.77ZCAT ceramic. Excessively added B2O3 causes much more liquid phase, blurs the grain boundary and reduces the bulk density, dielectric constant and quality factor. The 0.75 wt% B2O3 addition in 0.77ZCAT ceramics sintered at 1300℃ exhibits the optimal dielectric properties:εr= 9.96, Q×f= 33280 GHz and Υf= 3.09×10-6/℃。... |
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