Microwave Synthesis And Low-temperature Sintering Of BaO-Ln₂ O₃-TiO₂ (Ln=La,Nd,Sm) Ceramics

Multilayer chip microwave design based on the low temperature co-fired ceramic (LTCC) technology can effectively reduce the size of the devices and it plays an important role to meet the high demands of miniature, lightweight, multifunctional and high-performance for communication devices. However, the exploitation of LTCC technique in China is still in its infancy, and the LTCC materials are highly dependent upon import. Therefore, the research and development of high-performance LTCC materials is essential to us.BaO-Ln₂O₃-TiO₂ (BLnT, Ln=La, Nd or Sm. It is BNT, BLT or BST when Ln is La, Nd or Sm respectively, and the general pormula is Ba6-3xLn8+2xTi18O54) ceramics were studied in this paper, and the microwave synthesis and low-temperature sintering of BLnT ceramics were investigated. The microwave synthesis process of BLnT ceramics were studied, and the properties were analyzed by X-ray diffraction analysis, scanning electron microscopy, differential scanning calorimetry and other testing methods. The BLnT ceramics can be sintered at low temperature by adding low melting point glass. The properties of the glass/ceramics composites were studied.The influence of the parameters of the microwave synthesis process on the BNT ceramics were studied. The results indicate that the BNT ceramics, with fine and uniform crystallites, can be rapidly synthesized at low temperature by microwave heating. The effects of the heating temperature, time and rate on the dielectric constant of BNT ceramics were significant, while they were negligible on the phases of the ceramics. The single BaNd2Ti4O12 ceramics was synthesized by heating to 1100℃at 15℃/min and kept for 30min. It has the highest dielectric constant,εr=77.3, among the obtained ceramics.The effects of microwave heating on the properties of BLnT ceramics with different Ln elements and contents were investigated. The single BaNd2Ti4O12 ceramics was synthesized by microwave heating with x in the range 0.3<x≤0.75 and the highest dielectric constantεr was 77.3 at x=0.67. The single BaLa2Ti4O12 ceramics was synthesized at x in the range 0.5<x≤0.75, with the highestεr of 96.6 at x=0.67. In the case of BST ceramics, single BaSm2Ti4O12 ceramics was difficult to be synthesized by microwave heating.The influences of the glass content, the synthesizing temperature and the Nd content in the BNT ceramics on the properties of the glass/BNT composites were studied. The glass/BNT composites, sintered at 900℃for 2h with the BNT (x=0.67) ceramics synthesized at 1100℃and the glass content being 45wt%, demonstrate the best properties, with the dielectric constant (εr) of 38.3, the dielectric loss (tanδ) of 19×10-4, the flexural strength of 106.1MPa, and thermal conductivity of 2.328W/(m·K). As the glass content was increased, the density of the sintered ceramics increased and the interfacial reaction between glass and ceramics was weakened. With the increase of the synthesizing temperature of BNT ceramics, the grain size increased, the homogeneity decreased, and the dielectric constant increased initially, then decreased, then increased again. The glass-added BNT ceramics synthesized at 1100℃had the highest dielectric constant. The crystallization of the glass and the interfacial reaction was weakened with the increase of the Nd content. The BLT and BST ceramics were also densified with the addition of 45wt% glass at 900℃, but the properties are worse compared with that of the glass/BNT composites.