Preparation, Structure And Dielectric Properties Of (Ba,La)₅(Ti,Ta)₄O₁₅ Microwave Dielectric Ceramics

With the development of microwave technique, Microwave Dielectric Ceramics (MWDC) is becoming the key fundamental material for mobile communication technique, and people have showed great interests in their investigation and application. Now mobile communication systems are developing toward high frequency, miniaturization, integration, high reliability, which intensively requires exploration and research of MWDC systems with high dielectric constant.The aim of this thesis is to explore new MWDC with cation-deficient perovskite structure in A₅B₄O₁₅ system. According to the characteristic of A₅B₄O₁₅ perovskite structure, series of new titanates / tantalates Ba_5-xLa_xTi_xTa_4-xO₁₅ (x=1³) were prepared, characterized. The dielectric properties were measured. Furthermore, the effects of Nb⁵⁺ substitution Ta⁵⁺ at B-site in Ba₃La₂Ti₂Ta₂O₁₅ ceramic on structure and dielectric properties were studied.Firstly, Ba_5-xLa_xTi_xTa_4-xO₁₅(x=1³)ceramics were prepared by high temperature solid-state reaction route. Sintering properties were studied and established. The phase and structure of the ceramics were characterized by X-ray diffraction (XRD) , Raman spectrum analysis and scanning electron structure microscope (SEM). The results show that these three ceramics are all single-phase, with A₅B₄O₁₅ hexagonal perovskite structure; the indexed XRD data of Ba_5-xLa_xTi_xTa_4-xO₁₅(x=1³) and refined unit cell parameters were obtained.Then, dielectric properties of Ba_5-xLa_xTi_xTa_4-xO₁₅(x=1³ )ceramics at microwave frequency were studies. The results of the test at microwave frequency show that with the value of x the dielectric constants decrease from 47.6 to 44.6,and the temperature coefficients of resonance frequency decrease from 331 to 2.5 ppm/℃, but the quality factor (Q-f) are opposite and increase from 24840 to 33030GHz.Finally, Ba₃La₂Ti₂Nb_xTa_2-x O₁₅ (x=0²) ceramics have been prepared by conventional solid-state ceramic route. Ba₃La₂Ti₂Nb_xTa_2-x O₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 45.1 to 49.8, quality factor Q-f from 31040 to 22006 GHz and temperature variation of resonant frequency from +14.3 to +32.1 ppm/℃ as the value of x increases.