The New Dielectric Ceramics With Tungsten-Bronze Structure In The BaO-La₂O₃-TiO₂-Ta₂O₅ And SrO-Sm₂O₃-TiO₂-Nb₂O₅ Systems

In order to search new high- dielectric ceramics for high frequency applications, the BaO-La203-TiO2-Ta2O5 quaternary system with filled tungsten-bronze structure was investigated with respect to the relationship of composition, crystal structure and dielectric properties. The modification was performed by A- and B-sites substitution in BaO-La2O3-TiO2-Ta2O5 ceramics. The ceramics with filled tungsten-bronze structure in SrO-Sm2O3-TiO2-Nb2O5 system were also investigated.In BaO-La2O3-TiO2-Ta2O5 quaternary system, BaxLa6-xTi8-xTa2+xO30 (3 x 5) formed the solid solutions with single tetragonal tungsten-bronze structure, and the ceramics based on these compositions had high dielectric constant( =123~143) and low dielectric loss tan5 in the order of 10-4~10-3, and relatively lower temperature coefficient of dielectric constant( =-1347 ppm/0C -728 ppm/0C). The dielectric constant and loss reached the minimum for x=4, and the temperature coefficient of dielectric constant increased with increasing x. With increasing x, the axial ratio( 10 c/a) increased first then decreased while the tolerance factor and the electronegativity difference showed reverse trend.In (Ba1-xSrx)3La3Ti5Ta5O30 ceramics, the dielectric constant, dielectric loss and temperature coefficient of dielectric constant reached the maximum at x=0.5. With increasing x, the axial ratio increased first then decreased.With increasing x, (Ba1-xCax)5LaTi3Ta7O30 and Ba5La(Ti1-xZrx)3Ta7O30 ceramics showed the decreased dielectric constant, increased dielectric loss and the temperature coefficient of dielectric constant approaching to zero. The temperature coefficient of dielectric constant was improved significantly when Ca ions were substituted for Ba ions completely in (Ba1-xCax)5LaTi3Ta7O30 ceramics, where =65, tanS=0.0017 and = -135 ppm/0C. The paraelectric nature was found in BaxLa6-xTi8-xTa2+xO30 and the modified BaxLa6-xTi8-xTa2+xO30 ceramics. It should be emphasized that the ion polarization and the electronic polarization were the primary polarization mechanisms and showed the dominating effects upon the dielectric characteristics. The above relationship could be analyzed by using the parameters of the axial ratio, tolerance factor and electronegativity difference in the present solid solutions.In the SrO-Sm2O3-TiO2-Nb2O5 quaternary system, Sr5SmTi3Nb7O30 formed single tetragonal tungsten-bronze phase, but the unknown second phase were found inSr4Sm2Ti4Nb6O30 and Sr3Sm3Ti5Nb5O3o, and even the tetragonal tungsten-bronze phase was not the major phase in the latter. Sr4Sm2Ti4Nb6O3o and Sr5SmTi3Nb7O30 were weak ferroelectrics, where Pr is 0.09 C/cm2 for the former and 0.15uC/cm2 for the latter. The displacement type ferroelectric phase transition was observed in Sr4Sm2Ti4Nb6O30 ceramics near 300 , and SrsSniTi3Nb7O30 showed the diffused ferroelectrics phase transition.Generally, the effects of composition upon the crystal structures and dielectric properties were clarified in BaO-La2O3-TiO2-Ta2O5 solid solutions. Furthermore, the ferroelectrics phase transitions were investigate in the quaternary system.