Study Of The Pb-based Perovskite High Dielectric Constant Microwave Ceramics

The present dissertation firstly reviewed the development of research on microwave dielectric ceramics. In succession the relationship between the microwave dielectric behavior and microstructure as well as the crystal structure was discussed. Based on the relationship among the composition, processing, microstructure and dielectric property, high microwave dielectric constant ceramic (Pb1-xCax)(Fe0.5Nb0.5)O3(PCFN)and its improved system were produced and their processing technique and dielectric characteristics were discussed. The main contents are as following:The origin of high dielectric constant, relationship between the crystal structure, microstructure and dielectric properties were stated. The mechanisms of dielectric loss and temperature coefficient of resonant frequency was studied based on which and with the utilization of XRD,SEM,EDS and microwave network analysis(ADVANTEST R3767C), the author investigated the improvement of PCFN microwave dielectric ceramics and the relationships between the microstructure and microwave dialectic constant. Experimental results show that the substitution of Zr4+and Sn4+ at B site can improve the quality factor of PCFN system at a certain degree while keeping the high dielectric constant and low temperature coefficient(τf<10ppm/℃). These was resulted from the decreased tolerance factor and in (Pb0.45Ca0.55)[(Fe0.5Nb0.5)0.9Zr0.1]O3 system dielectric constant εr=84.3,quality factor Qf=5330GHz and temperature coefficient τf≈+9.3ppm/℃ can be obtained and the Qf value was improved by 12% more that (Pb0.45Ca0.55)(Fe0.5Nb0.5)O3 system. Additionally in (Pb1-xCax){(Fe0.5Nb0.5)1-ySny}O3(PCFNS)system single phase with dense microstructure could be obtained through B site precursor method. In three proforma substitution of La3+ at A site, the charge-unbalanced formation has the best effect on microwave properties. Whilst it decreased dielectric constant slightly, it could suppressτf value and improve Qf value significantly. The charge-unbalanced substitution of Nd3+ can increase the B-site bond valence of PCFN perovskite system,which could control τf value and improve εr. In [(Pb0.48Ca0.52)0.95La0.05](Fe0.5Nb0.5)O3 system microwave dielectric properties of εr=79.9, Qf=5810GHz and τf≈+4.1ppm/℃ could be obtained. The effect of co-substitution at both A and B site of PCFN using CeO2 was investigated. The results show that the addition of CeO2 can effectively improve the microwave dielectric constant,including improving the Qf value remarkably while keeping high dielectric constant and suppressing the τf value. A near zeroτf value could be obtained when CeO2 doping content was 2.2mol%(τf=+3.5ppm/℃),and the εr and Qf value were 96.9 and 6770GHz respectively. The study of the relationship between temperature coefficient of dielectric constant and the microstructure parameters A,B and C indicates that the change of crystal structure of volume has an significant effect on the temperature coefficient of dielectric constant. Additionally we utilized PCFNS system with high εr, low τf value to combine with PCLFN system possessing high Qf value to produce excellent comprehensive properties ceramic. However the experiment results seemed not match with the additive rule of theory because of the complex chemical reaction and the inharmonic lattice vibration which is left for further study.To satisfy the development of microwave circuit, a multiplayer microwave filter is needed which require the ceramic to be sintered under 1050℃. Bi2O3,MnO2 and their combination Bi2O3/MnO2 with different ratio were added to PCLFN system. The results showed that Bi2O3 and MnO2 can both effectively lower the sintering temperature and improve the dielectric properties. The ratio of Bi2O3/MnO2(k)has strong influence on the microwave properties. Through microstructure and EDS study the sintering mechanism was revealed and the co-doping can form lower molten solution and decrease the sintering temperature more effectively. When sintered at 1050℃ for 4h, in PCLFN system with k=1,w=1, micr...