Influence Of Non-stoichimetry On The Structure Of CaTiO₃-NdAlO₃Based Microwave Dielectric Ceramics

(Ca,Nd)（Ti,Al）O₃-based microwave ceramics with a medium permittivity,low dielectric loss and near zero temperature resonant frequency have receivedmore and more attention recently, especially,0.7CaTiO₃-0.3NdAlO₃（CTNA30）which exhibits excellent dielectric properties. However, calcium loss duringelevated temperature sintering results in the deviation of stoichiometry, whichcan induce the defects and change the structure and microwave properties. Thus,the influence of A-site and B-site non-stoichiometry, including calcium,aluminum and titanium, on the structure of CTNA is investigated in the presentdissertation so as to establish the foundation for studying the relationship amongchemical composition, structure and dielectric property.In a combination with high energy ball milling technique and conventionalsolid state reaction, CTNA-based microwave ceramics with an accurate controlof stoichiometry were prepared. Various materials characterization techniques,including X-ray Diffraction （XRD）, Scanning Electron Microscopy （SEM）equipped with Energy Dispersive X-ray Spectroscopy （EDS）, Ramanspectroscopy （Raman）, and Fourier transform infrared spectroscopy （FTIR）, wereused to characterize the structure and microstructure, such as grain morphology,second phase structure morphology and distribution.It is found that the structure and microstructure of CTNA are closely relatedto calcium or titanium non-stoichiometry that is dependent on the way by whichthey enter the CTNA matrix. When Ca excess is slight, Ca2+ions tends to occupyA sites, creating corresponding B site and oxygen vacancies. When Ti excess isslight, Ti4+ions tend to occupy B sites, creating corresponding A site and oxygenvacancies. With increasing Ca and Ti excess, these elements occupysimultaneously both A sites and B sites. The different in the ways by whichexcessive Ca and Ti ions enter the CTNA matrix result in substitution fordifferent types and quantities of elements, changing the ratio of calcium toaluminum and affecting the formation of different secondary phases. When theratio of calcium to aluminum is appropriate, the low melting-point Ca₃Al₂O₆secondary phase may be formed, which can promote the densification of CTNAby liquid sintering. Raman and FTIR spectra results show that the ordering ofboth A sites and B sites in the CTNA decreases with increasing Ca or Tideviation from stoichiometry. When the Al deficiency is slight, Nd3+ions tendsto occupy A sites, creating corresponding B site and oxygen vacancies. When the Al deficiency is large, Nd3+ions tend to occupy simultaneously both A sites andB sites. Nd3+ions substitute for different elements to form different secondaryphases. Furthermore, secondary phases won’t be disapear with alumiumdecreasing. The addition of2wt.%or3wt.%Li₂ZnTi₃O₈in CTNA matrix canimprove the sintering properties. At the temperature of1450°C, liquid sinteringmay be realized to promote the desinification of the material.