Study On The Al₂O₃-TiO₂ Based Composite Microwave Dielectric Ceramics

The innovation of modem communication requests that the microwave electronic components based on microwave dielectric ceramics have better performances. The new type of microwave dielectric ceramics with lowε_r, high quality factor and near zeroÏ„_f, is a good candidate of millimeter-wave devices. The Al₂O₃ based ceramic, which have low dielectric constant, high quality factor and high thermal conductivity, is a kind of excellent materials. Low temperature sintering of alumina based ceramics and dielectric properties of ceramics have been investigated in this work.Al₂O₃-TiO₂ ceramics had been chosen and investigated in this work. Proper amount of rutile (+450 ppm/℃) was added to alumina ceramics (-60 ppm/℃) to adjust theÏ„_f value. Sol-gel method was carried to synthesis MCAS glass powder as sintering additives. Two kinds of raw materials, typical and nano-sized powders of Al₂O₃ and TiO₂, were used and different amount of MCAS glass were added as sintering aid to synthesis 0.88Al₂O₃-0.12TiO₂ ceramics. The phase composition and microstructures of ceramics was investigated by X-ray diffraction (XRD) patterns, scanning electron microscope (SEM). The microwave dielectric properties of the ceramics were investigated by HP Agilent 8722ET Network Analyzer. The results show that the phase composition and the dielectric properties of Al₂O₃-TiO₂ ceramics are determined by the amount of sintering aid and post-annealing treatments.MCAS glass added 0.88Al₂O₃-0.12TiO₂ ceramics, which used typical Al₂O₃ and TiO₂ powder as raw materials, could reach 95% relative density after sintering process (at 1350℃for 4h). The dielectric properties of the ceramics change with the MCAS glass content. The dielectric constants and quality factors of ceramics decrease with the MCAS glass content increasing. TheÏ„_f values of ceramics as-sintered are large negative values, because Al₂TiO₅ phase formed in the sintering process. Ceramics which have nearÏ„_f value by post-annealing treatments can be obtained, for Al₂TiO₅ phase decreases to Al₂O₃ and TiO₂ by annealing treatment. With the increasing of the MCAS glass content, the decomposition of Al₂TiO₅ phase is restrained gradually, but Al₂TiO₅ phase decomposes more completely when long annealing hold time is used, and ceramics with nearÏ„_f values can be obtained.Nano-sized Al₂O₃ and TiO₂ powder were used as raw materials and MCAS glass added 0.88Al₂O₃-0.12TiO₂ ceramics was fabricated. Dielectric properties of ceramics depend on its sintering temperature and MCAS glass content. Al₂TiO₅ phase can't be detected in ceramics sintered at 1250℃, and ceramics with 8wt% MCAS glass content have near zeroÏ„f values. Al₂TiO₅ phase appears in ceramics sintered at 1300℃and increases with MCAS glass increasing, therefore dielectric properties of ceramics change with the phase compositions. Samples with low MCAS glass content, 2wt%, have dielectric properties of near zeroÏ„_f value (+0.7 ppm/℃),ε_r value of 11.6 and Q×f value of 16873GHz. The phase compositions of samples sintered at 1350℃are same as the ceramics fabricated from typical powder and sintered at 1350℃.The sintering temperature of 0.88Al₂O₃-0.12TiO₂ ceramics by adding MCAS glass can be lowered. The phase compositions can be controlled by adjusting the content of sintering additives and annealing hold time. Ceramics which have dielectric constant range of 11.5～12.5, near zeroÏ„_f values and good quality factors can be obtained.