Solid Solution Modification On The Microwave Dielectric Properties Of Tungsten Bronze Ceramics With Perovskite Additives

The present dissertation firstly reviewed the development of research on microwave dielectric ceramics. And then, the modification foundation of dielectric properties for microwave dielectric ceramics was discussed theoretically along with the feasibility of perovskite additives on the modification of Ba_6-3xLn_8+2xTi₁₈O₅₄ (BLnT, in short) with tungsten bronze structure. Based on the relationship among the types of additives, microstructure and dielectric property, BLnT ceramic and its improved systems were produced. The Raman Spectrum of BLnT ceramics with perovskite additives was also analyzed elementarily.The relationships between polarization mechanism and dielectric loss of microwave dielectric ceramics were analyzed. The investigation result of tolerance factor reveals that the tilting of O-ocahedra is an important factor that associate with the temperature coefficient of resonate frequency. The influences of ions substitution and dopants on the microstructure and microwave dielectric were summarized, whereafter, the modification mechanisms of perovskite addtitives on microwave dielectric ceramics with tungsten bronze structure were investigated, the perovskite additives can form solid solutions with tungsten bronze ceramics in a certain extent. So, different perovskite additives were used to modified the dielectric properties of BLnT ceramics from the aspect of three important parameters which are Qf values, temperature coefficient of resonate frequency and permittivities.It was found that as the Al₂O₃ doping amounts increased, the c-axis lattice parameter and unit cell volume of Ba_4.2(Nd_0.5Sm_0.5)_9.2Ti₁₈O₅₄ (BNST, in short) decreased, the density and dielectric constant decreased, while Qf increased. After 0.5 wt% Al₂O₃ was added to BNST, the dielectric constant and Qf initially increased and then decreased as the SiO₂ content increased. The properties ofε_r =74.3, Qf =8700 GHz and TCf =8.8 ppm/℃had been attained with the addition of 0.5 wt.% Al₂O₃ and 0.2 wt.% SiO₂ to BNST sintered at 1320℃for 2 h.The fired densities of Ba_4.2(Sm_0.8Nd_0.17Bi_0.03)_9.2Ti₁₈O₅₄ (BSNBT, in short) ceramics were enhanced by the small amounts of TiO₂ additions at 1340℃. With the addition amounts of TiO₂ increased, the BaTi₄O₉ phase decreased while the Ba₂Ti₉O₂₀ phase increased, TiO₂ second phase appeared when 5.0 wt.% TiO₂ was added. The additions of TiO₂ deteriorated the dielectric constant and the maxima Qf values could be attained at approximately 0.5～1.0 wt.% addition of TiO₂, the TCf values showed a decrease with increasing content of the additives up to 3.0 wt.% when the sintering temperature below 1340℃because of the development of second phases. BSNBT doped with 0.5 wt.% TiO₂ exhibited dielectric ceramics withε_r = 80.5, Qf= 9009 GHz and TCf = 6.5 ppm/℃when sintered at 1340℃for 2 hours.The influences of NdAlO₃ additives with high Qf values on the microstructure and dielectric properties of Ba_4.2Nd_9.2Ti₁₈O₅₄ (BNT, in short) ceramics were investigated systemically. The results showed that small additions of NdAlO₃ could be accommodated in BNT, but between 15～20 wt.% NdAlO₃ there was the development of second phase. The relative permittivity and the TCf values showed a linear decrease with increasing additions of NdAlO₃. It was observed that near zero TCf values could be achieved with addition of NdAlO₃ additive to BNT ceramics. On the other hand, the Q values exhibited a non-linear behaviour with NdAlO₃ additions with maxima attained at approximately 10 wt.% NdAlO₃. With the addition of 10 wt.% NdAlO₃ to BNT and sintered at 1340℃for 2h, excellent microwave dielectric properties ofε_r=66.3, Qf=11400 GHz and TCf=-0.7 ppm/℃could be achieved. Using LaAlO₃ and SmAlO3 as additives showed the same tendency as NdAlO₃. The Raman Spectrum of BLnT ceramics with NdAlO₃ additives was also analyzed. The peaks that near 179 and 139 cm^-1 are associated with the movements of the rare earth element and Ba ions, while the peaks that near 276 and 227 cm^-1 can be attributed to the tilt of oxygen octahedral when the large cation sites are occupied by the rare earth element and Ba ions, respectively. The correspongding lattice vibration modes of each peak in Raman Spectrum of BLnT ceramics need further investigation. The Qf values of the samples showed a linear relationship with the reciprocal of FWHM of A1g(O) mode.The influences of SrTiO₃ additives with large TCf values on the microstructure and dielectric properties of Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST, in short) ceramics were investigated. The relative permittivities and the TCf values increased with the increasing additions of SrTiO₃. It was observed that near zero TCf values could be achieved with addition of SrTiO₃ Badditives to BST ceramics. The modest addition contents of SrTiO₃ (less than 4 wt.%) had no obviously effect on the Qf values of BST based ceramics, but further addition of SrTiO₃ led to a rapid decrease in the Qf values. With 4 wt.% SrTiO₃ addition and sintered at 1360℃ for 2 hours, BST ceramics exhibited excellent dielectric properties:ε_r=81.2, Qf =8470 GHz and TCf =-1.5 ppm/℃.The microstructure and dielectric properties of Ba_4.2(Sm_0.9,Bi_0.1)_9.2Ti₁₈O₅₄ (BSBT, in short) ceramics doped with ferroelectric Na_0.5Bi_0.5TiO₃ (NBT, in short) have been discussed. Additions of NBT improved the densifications process. The addition of small amounts of ferroelectric NBT increased the relative permittivities remarkably while the Qf values decreased. The TCf values increased slightly with the increasing additions of NBT and the sintering temperatures. It was observed that near zero TCf values could be achieved with addition of NBT dopants to BSBT ceramics. BSBT doped with 5 wt.% NBT additives exhibited excellent dielectric properties withε_r=90.1, Qf=5450 GHz and TCf=0.2 ppm/℃when sintered at 1320℃for 2 hours.