Research On Doping Modification Of BT/BST Series Capacitor Ceramics

BT(BaTiO₃)/BST((BaSr)TiO₃) ceramics have been becoming one of the highlights in the research on functional materials and devices for their potential applications in many fields. Most important of all is the combination of BT/BST ceramics with semiconductor integrated technique, and according to their effects of ferroelectric, piezoelectric, thermoelectric, dielectric, electro-optic and non-linear optical to realize their functions in signal memory, transition, modulation, switch, sensor and et al. For the limitation of sintering temperature and temperature stability have affected the properties of BT/BST ceramics and the practical applies in industries, in this dissertation, doped impurity to modify the properties and strcture of BT/BST have been presented. The main work is as the follows:1. The influence of the dopant BaSiO₃ on the dielectric properties for BT ceramic capacitors has been discussed. It is found that doping BaSiO₃ can improve the sintering and microstructure of the capacitor ceramics. The results showed that the dielectric properties of doped BaSiO₃ with 3mol% are as follows:ε_r=1792, tanδ=0.01,ρ= 6.1×10¹¹Î©Â·cm,E_b=3.5 kV·mm^-1,â–³C/C=18.0%(20～150℃),â–³C/C=-23.1%(-55～5℃), and the sintering temperature had decreased 40℃. SEM studies showed that doping BaSiO₃ can suppress the grain to grow up for exception, and the structure of ceramic is compact with little pore. And XRD studies showed that there are a little SiO₂ phases.2. The influence of doping rare-earth metal on the dielectric properties of BT series ceramic has been investigated by means of orthogonal design experiments. The major and secondary influencing factors and the influencing tendency of various factors' levels for the dielectric properties of this ceramics have been obtained. The optimally compounding is BT+1%CeO₂+1%Dy₂O₃+0.9%Y₂O₃+0.9%Yb₂O₃ (mole fraction) for dielectric constant and BT+1.5%CeO₂+1.5%Dy₂O₃+0.6%Y₂O₃+0.6%Yb₂O₃ (mole fraction) for dielectric loss. The performance of the former ceramic which was sintering at 1280℃for 60 minutes isε_r=6911, tanδ=0.0124; and the latter isε_r=3800, tanδ= 0.0012. The optimiallyρ=7.8×10¹¹Î©Â·cm of 3 sample and the optimially E_b=8.2 kV·mm^-1 of 8 sample,â–³C/C= 61.8% (20～80℃)of 5 sample. Studied by SEM found that the grain size small and uniform when the samples had high dielectric properties, and the crystal grains ranged compaction with little pore when the samples had low dielectric loss. SEM studies showed that the samples all formed perovskite structure (ABO₃), but there were some SiO₂ phases at about 2θ=29°.3. The influence of the doping Bi₄Ti₃O₁₂ on the dielectric properties and sintering temperature for (Ba_0.71Sr_0.29TiO₃ (BST) ceramic capacitors has been discussed. It is found that when w(Bi₄Ti₃O₁₂)=10%, the dielectric properties are as follows:ε_r=2558, tanδ=0.0050,â–³C/C=-39.2%(20～150℃),â–³C/C=12.21%(-55～5℃), and the sintering temperature decreased to 1180℃, when w(Bi₄Ti₃O₁₂)=8%,ρ=6.2×10¹¹Î©Â·cm, E_b=7.4 kV·mm^-1 which are the maximal value; when w(Bi₄Ti₃O₁₂)=50%, the dielectric properties are as follows:ε_r=1045, tanδ=0.0040,â–³C/C =-13.2%(20～150℃),â–³C/C= 1.22%(-55～5℃),ρ=1.24×10¹¹Î©Â·cm, E_b=4.7kV·mm^-1, and the sintering temperature decreased to 1090℃.The change of C and tanδof BST ceramic capacitors followed frequency became small when doping Bi₄Ti₃O₁₂ in BST ceramic. But when vv(Bi₄Ti₃O₁₂)≥10wt%, the effect was small for C. Doping high content of Bi₄Ti₃O₁₂ can make the curve of tanδ-f flater. The influence of various Bi₄Ti₃O₁₂ additive amounts on microstructure and phases compositions of BST ceramics were studied by SEM and XRD. The result shows that Bi₄Ti₃O₁₂ additives influence the properties and microstructure of BST ceramics by means of forming defect solid solution, strong segregation in crystal boundary, suppress the grain's growth and fine crystal grains. But when w(Bi₄Ti₃O₁₂)≥46wt %, there appeared banding Bi in the microstructure, and the diffracting intensity of main phases became larger, the crystal grains began bigger, the doping Bi₄Ti₃O₁₂ can't fine crystal grains go on.