Preparation, Structure And Dielectric Properties Of (Ba,Sr)₆ RNb₄O₁₈ Microwave Dielectric Ceramics

With the development of microwave technology, dielectric resonators and filters with excellent properties which made up of microwave dielectric ceramics (MWDC) are widely used in many fields, such as mobile phone, satellite communications, radar, computer, modern medicine, and so on. Now the study of MWDC materials abstracts many attentions. The aim of this thesis is to explore new MWDC with cation-deficient perovskite structure. According to the characteristic of A6B5O18 perovskite-like structure, series of new niobates Ba6-xSrxTiNb4O18 (x=0-6) were prepared, characterized. The dielectric properties were measured. Furthermore, the effects of Sn4+ substitution Ti4+ at B-site on structure and dielectric properties were studied.Firstly, Ba6-xSrxTiNb4O18 (x=0-6) 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) and scanning electron structure microscope (SEM). The results show that these seven ceramics are all single-phase, with A6B5O18 hexagonal perovskite structure; the cell parameters decrease with increasing content of Sr substitution; the indexed XRD data of Ba6TiNb4O18 and refined unit cell parameters were a =5.7730 (2) A, c=42.420 (1) A, V=1224.33 A3, and theoretical density is 6.245 g·cm"3.Then, dielectric properties of Ba6-xSrxTiNb4O18 ceramics at low frequency (1MHz) and microwave frequency were studies. The results show that with increasing content of Sr element, dielectric constants increase from 43.31 (x=0) to 52.35 (x=2), then decrease to 31.69 (x=6), the quality factor (Qxf) changes between 3463-11529GHz, and the temperature coefficients of resonance frequency are opposite and decrease from 64.84 ppm℃-1 (x=0) to 7.02 ppm℃-1 (x=4), then increase to 42.63 ppm℃-1(x=6).Ba6-xSrxSnNb4O18 (x=0,3,6) ceramics were prepared through B-site Ti4+ substitution by Sn4+ and influence of B-site substitution on materials structure and properties were studied. The results show that three compounds are pure phase and belong to hexagonal perovskite structure, the cell parameters increase; for Ba6-xSrxSnNb4O18 (x=0,3,6), the dielectric constants decrease, and are 34.54,25.42, 21.25 respectively, the quality factor (Qxf) changes between 4801-17600 GHz,and the temperature coefficients of resonance frequency decrease obviously, change from positive to negative value and they are 0 ppm℃-1, -10.33 ppm℃-1, -23.77 ppm℃-1, respectively.Finally, the influence of A-site or B-site cation substitution on microwave dielectric properties was discussed.