Dielectric Properties Of Low Temperature Sintering BSCT-MgO Composite Ferroelectrical Materials

In order to achieve the application of LTCC on barium strontium calcium titanate(BSCT)-based tuning materials,this article aims to prepare moderate dielectric constant, low dielectric loss, high tunability and good dielectric temperature stability of BSCT based ceramics at low sintering temperature. In this thesis, we have had more focus on role mechanism of MgO and Mn-doping and B₂O₃-ZnO glass doping on low-temperature sinterability and dielectric properties of Ba_0.55Sr_0.4Ca_0.05TiO₃–MgO composite ferroelectric ceramics. Nano-MgO powder and Ba_0.55Sr_0.4Ca_0.05TiO₃ powder prepared by co-precipitation both had superfine structure. Using B₂O₃-Li₂CO₃ as sintering additives, we found that a single phase material (pure BSCT or MgO) sintered at 1000℃contain more pores and non-dense structure. After doping MgO low-dielectric, non-ferroelectric phase, BSCT-MgO composite material become less pores and more densely. When increasing the content of MgO in composite material, the size of BSCT grains and dielectric constant gradually reduced, curie temperature also shifted to lower temperature. When w(MgO)=60wt%, the size of BSCT grains reduced to 0.7-1.5μm, whose dielectric constant, dielectric loss and curie temperature were about 90, 1×10^-3 and -27℃, respectively. BSCT-1wt%MgO composite material sintered at 950℃had more relatively flat dielectric-temperature curve and higher tunability 18% (room temperature, 1kV/mm), these excellent performance are compatible with the tunability 20% of pure BSCT sintered at 1200℃, this technology can have realized low-temperature sintering of BSCT based ferroelectric materials with high tunability, stable dielectric-temperature properties. MgO doping did not only improve the viscosity of B₂O₃-Li₂O liquid to increase the flow of liquid and mass diffusion, but also increased wettability of B₂O₃-Li₂O liquid at high temperature for BSCT grains, thus "Small particles—MgO-B₂O₃-Li₂O liquid—BSCT grains "pattern was carried out more smoothly by good wetting environment, Grains growth were accelerated by mass transfer in liquid phase, therefore densification of BSCTM material also increased.With the increasing of the amount of Mn ions, the curie temperature of BSCTM composite material shifted to lower temperature and tunability first increased and then decreased. When sintering temperature lower to 950℃and Mn= 0.50mol%, the tunability of BSCT-45wt%MgO composite material under a DC field of 0.71 kV/mm achieves 4.8%. It had realized preparation technology of high tunability of ferroelectric material at lower sintering temperatures. After doping 10wt% B₂O₃-ZnO glass, BSCT-45wt% MgO material sintered at 1000℃had very small tunability because a large number of glass phase destroyed the continuity of the non-harmonic movement of Ti ions in BSCT. In order to improve this poor performance, we can increase the ferroelectric phase and reduce the amount of B₂O₃-ZnO glass. Adding 2.5wt% and 5.0wt% B₂O₃-ZnO glass to BSCT-22wt%MgO composite material can be sintered at 1000℃with excellent dielectric properties, when measure at 1MHz, theirε, tanδand tunability (under a DC field) were 235.7, 1.52×10^-3, 5.29% (0.66kV/mm) and 299.8, 1.43×10^-3, 4.17% (0.75kV/mm), respectively, the temperature stability of dielectric constant is improved, dielectric constant remains stable between 20Hz and 2MHz.