Preparation And Spectral Properties Of A₅B₄O₁₅ Microwave Dielectric Ceramics

With the development of microwave technology, its application has been foundin every relative part of science and technology.Due to its high performance in themicrowave region, they are playing more and more important role in microwavetechnology.Vibrational spectra was considered as an ideal tool for detecting the molecularstructure.For example,narrow and intense peaks always include useful information.Itis easy to get the structure-property relatiomship of ion-replaced ceramics andmodification theoretical guidance by a detailed study of spectral analysis combinedwith the conventional methods,such as: XRD,SEM.In the dissertation,the preparation, characterization and microwave dielectricproperties of Ba_5-xLa_xTi_xTa_4-xO₁₅ and Ba_5-xLa_xTi_xNb_4-xO₁₅ ceramics were investigated.The relationship between dielectric performance and structures were obtained inA₅B₄O₁₅ system on a basis of spectra analysis.1.Complex perovskites Ba_5-xLa_xTi_xMn_4-xO₁₅(M=Nb,Ta)(x=1,2,3)ceramics wereprepared by conventional solid-state ceramic route.The phase and structure of theceramics were characterized by X-ray diffraction (XRD) and scanning electronstructure microscope (SEM). The results show that all single-phase, with A₅B₄O₁₅hexagonal perovskite structure in these ceramics, the dielectric properties of ceramicsat microwave frequency were examined, the similar trend were observed are asfollows: the dielectric constants decrease with the value of x,however, the Q-factors(Q.f) exhibit the increasing trend.2.According to the Raman spectra,paying particular attention to A_1g (O)modeband., the extent of splitting is becoming larger and the degree of B-site orderincrease with the substituents of La and Ti in the tantalates, Also, analogic changewere observed in the niobates.3.Trace amount of Dy²⁺+and Dy³⁺ were detected in tantalates and niobatesrespectively by photoluminescence spectra analysis. There are several narrow bandsin the emission spectra of Ba_5-xLa_xTi_xNb_4-xO₁₅ system,which peaks at 906nm,795nm and 775nm,they are corresponded to the ⁶F_7/2→⁶H_15/2,⁶F_5/2→⁶H_15/2 and ⁶F_3/2→⁶H_15/2 transition of Dy³⁺,However, An array of broader bands are observed in the emissionspectra of Ba_5-xLa_xTi_xTa_4-xO₁₅ system, these bands are related to the behavior ofelectronic transition of Dy²⁺.4.The value of the Q·f in tantalates and niobates can be correlated to thevariation of symmetric stretching vibrationas of [TaO₆]^n- and [NbO₆]^n-,respectively.Narrow and intense peaks are usually observed for samples which include welllong-range ordering of B-site cations,accordingly, the Q·f are higher in theseceramics.