Study On BaO-Al₂O₃-SiO₂/Al₂O₃ LTCC Substrate Materials With Low Loss At High Frequency

For low temperature co-fired ceramic?LTCC?substrate materials,the glass-ceramic/ceramics system is expected to be widely used in microwave/millimeter wave circuit components due to its good process adaptability and excellent high-frequency dielectric properties.How to effectively control the thermodynamic properties?including softening characteristics and crystallization characteristics?of glass-ceramic is the key to achieve sintering densification and to obtain excellent comprehensive performance of LTCC materials.In this paper,BaO-Al₂O₃-SiO₂?BAS?glass-ceramic/Al₂O₃ system was studied.By adjusting the composition of BAS glass-ceramic,the effects of glass former?B₂O₃?,network modifier?BaO?,network intermediate?Al₂O₃?,as well as alkaline earth metal oxide doping?SrO,CaO,MgO?on the thermodynamic properties of the glass were studied.On this basis,the low temperature sintering behaviors and comprehensive performance of BAS/Al₂O₃ composites were investigated.The results show that the increase of both B₂O₃ and BaO can lower the softening temperature and devitrification temperature of the glass,but the increase of BaO content leads to a decrease of glass sintering shrinkage rate as well.The increase of Al₂O₃ can reduce the devitrification temperature of glass,but the softening temperature of glass will go down at first and then up.By comparing the effects of various components on the softening and devitrification properties of glass,the optimum composition of BAS glass ceramics was determined to be B₂O₃ 13 mol%,BaO 24 mol%,Al₂O₃12 mol%,SiO₂ 50mol%,ZrO₂ 1 mol%.For this BAS glass,the softening temperature is 650?,the main crystalline phase is BaAl₂Si₂O₈,and the crystallization activation energy is 292.25 kJ/mol.The doping of BAS glass-ceramic was carried out by using SrO,CaO and MgO.It was found that the doping of alkaline earth metal oxides leads to an increase in glass softening and devitrification temperature,and a decrease in crystallization activation energy.The doping of Sr O,CaO and MgO can effectively improve bending strength and reduce thermal expansion coefficient without degrading the dielectric properties of BAS/Al₂O₃ composites.Compared with the other two alkaline earth metal oxides,CaO has better effects on the comprehensive performance of BAS/Al₂O₃ composite.Furthermore,the increase of CaO content is beneficial to improve bending strength and reduce thermal expansion coefficient of BAS/Al₂O₃ composite,but excessive CaO will deteriorate the dielectric properties at high frequencies.The 55BCAS/45Al₂O₃ composite has the optimum performance when the molar ratio of BaO/CaO is 2:1.For55BCAS/50Al₂O₃ composite,the dielectric constant is 5.818@10GHz,the loss is 2.043×10^-3,the bending strength is 155.26MPa.The effects of glass content on the structure and comprehensive properties of BCAS/Al₂O₃ were studied.The results show that the main crystalline phases of BCAS/Al₂O₃ are BaAl₂Si₂O₈ and Al₂O₃.With the increase of BCAS glass content,the dielectric constant of the composite increases,the bending strength and dielectric loss increases first and then decreases,which is consistent with the compactness of the composites.Considering the dielectric properties and mechanical properties,50 wt.%can be determined to be the optimum glass content of BCAS/Al₂O₃ composite.For the50BCAS/50Al₂O₃ composite,the dielectric constant is 5.675@10GHz,the loss is 1.721×10^-3,the bending strength is 164.58MPa.