High Temperature Stability Of The New Mlcc Dielectric Material

Multilayer ceramic capacitor (MLCC) is one of the fastest growing SMD electronic components in recent years, which is dominantly applied in the oscillation,coupling,filtering and by-pass circuits of various types of military and civil electronic machines. Its applied field has extended to automatic control instrument,computer,mobile phones,home digital appliances and automobile electric appliances etc. The ferroelectric Barium titanate (BaTiO₃, BT) with the pervoskite strucutere has been widely used because of its high dielectric constant and excellent insulating properties. However, the dielectric constant of BT sharply decreases above its lower Curie temperature (Tc=120oC). For the modified BT ceramics, the capacitance changes was still large at high temperatures (over 200 oC) so that it can not satisfy the requirements of high-temperature applications, such as aerospace, petroleum exploration and so on.(Bi_0.5Na_0.5)TiO₃ (BNT) is perovskite type ferroelectric, with high Curie temperature (Tc=320oC) and remanent polarization (Pr=38μC/cm2). In recent years, BNT has been paid much attention as one of the most important lead-free piezoelectric materials. However, the dielectric properties of BNT are seldom investigated due to its low dielectric constant at room temperature. Moreover, BNT has not been reported as MLCC material so far. In this thesis, the high-temperature stable MLCC materials were investigated on the basis of BNT. The main content and results are as follows:1. The microstructure and dielectric properties of (Bi_0.5Na_0.5)_1-xCa_xTiO₃(BNCT) ceramics were studied. XRD indicated there are second phase of TiO2; The temperature characteristic displayed that the dielectric peak shifted to low temperature and was depressed and broadened with increasing the content of Ca, The room-temperature dielectric constant was increased and the dielectric loss was reduced when Ca was added. It was considered that this system was hopeful to be used as high-temperature stable MLCC materials.2. The influences of MnCO₃,CuBi₂O₄,ZnO,CeO₂,Co₂O₃ dopants on the dielectric properties of BNCT ceramics were studied. The results suggested that the temperature characteristic of capacitance was markedly improved in the case of MnCO₃ and CeO₂ additions; CuBi₂O₄ can significantly boost the dielectric constant of high temperature; The addition of ZnO can promote sintering and reduce the sintering temperature, resulting in relatively high density; Co₂O₃ can improve the insulation of ceramics.3. The orthogonal experiment of Zn, Mn, Ca, Ce four variables on the dielectric properties of BNCT ceramics was carried out. The best doping amount of four variables was achieved: MnCO₃ is 0.1～0.2wt%, ZnO is about 2wt%, CeO₂ is about 0.6wt%, Ca is about 14mol% respectively. Eventually the MLCC materials that satisfied the requirement of -55～250℃,(?)C/C25℃≤±15% was obtained.4. The effect of rare-earth elements on the dielectric properties of BNCT ceramics was studied. The results revealed that the capacitance changes at lower temperatures became smaller and the dielectric loss decreased with the addition of Dy and Y. However, the dielectric properties deteriorated when excessively doped.5. The influences of the calcining temperature and secondary milling time on the dielectric properties of BNCT system materials were investigated. It was observed that the calcining temperature and secondary milling time caused the dielectric constant to change, but had little effect on the temperature characteristic of capacitance.