Research Of CaO-B₂O₃-La₂O₃/Al₂O₃ System LTCC Tape Casting Slurry

The CaO-B₂O₃-La₂O₃（CBL）glass system was an LTCC material with superior high frequency performance and great potential,and this material was of high research value.Due to the small particle size and high surface energy of the CBL glass powder,it could easily lead to defects such as dark lines during the tape casting.In addition,the binder system currently in use could gel with the CBL glass powder and the binder could not be completely pyrolysed during sintering,resulting in the reduction of the densification of the sintered substrate.In response to these problems,this paper selected polymethyl methacrylate（PMMA）as the slurry binder.The stable slurry was prepared and the green tapes with excellent performance were obtained by studying both the surface modification of CBL glass powder and the optimisation of the organic components of slurry.This study provided technical support for the application of LTCC materials.With the aim of changing the surface properties of glass powder,the CBL glass powder was modified by silane coupling agent KH-570 without adding dispersant.The optimum modification conditions were established by studying the effect of powder modification at different pre-hydrolysis ratios.The effects of modified powders on the kinetic stability and rheological properties of the slurry were clarified,and the surface modification mechanism of glass powders by silane coupling agents was elaborated.The results showed that when 2 wt.%KH-570 was added under optimum modification conditions,the relative settling coefficient of the powder in the solvent increased significantly to 0.57 and the viscosity of the cast slurry decreased to 1500 m Pa·s.In contrast to the unmodified or excessive additions of coupling agents,the dispersion of slurry was stable and the uniform green tapes that well-matching with silver paste were casted.With the aim of optimising the organic component of the slurry,the effects of castor oil,BYK-22552,polyvinylpyrrolidone and TEGO-700 on the dispersion of casting slurry were investigated.The rheological properties,thixotropy,solid content and sedimentation properties of the slurry were studied.The anchoring groups of the four different dispersants were characterised and the respective dispersion mechanisms were analysed.The results showed that when 2 wt.%TEGO-700 dispersant was used,the casting slurry had the minimum viscosity and excellent sedimentation properties.When the optimum viscosity of tape casting was 2000 m Pa·s,the slurry had the maximum solid volume content of 37.1 vol.%.The thickness of the green tapes formed by this slurry was uniform.The surface of the sintered substrate was free from obvious defects such as pores and cracks.Aiming at the rheological properties of casting slurry,the effect of the ratio of plasticizer dioctyl phthalate（DOP）to binder PMMA（P/B）on the slurry was further studied on the basis of adding 2 wt.%TEGO-700 dispersant.The results showed that when the binder content reached an optimum value of 10 wt.%,the thixotropic properties of the slurry first increased and then decreased as the plasticiser content increased,gradually changing to viscous fluid properties in the linear viscoelastic zone.The structural recovery ratio increased and then decreased,with the maximum recovery ratio of 47.679%.When the plasticiser content was increased to P/B=0.6,the best combination of tensile strength（//）3.34 MPa and elongation at break（//）22.4155%was achieved for the green tapes.The sintered substrate was highly densified and had the maximum flexural resistance of 222.784 MPa.The co-firing of the green tapes with the conductor silver paste was well matched and no significant silver diffusion occured after sintering.