| Thermistors with a characteristic of negative temperature coefficient (NTC) of resistivity are widely used in various industrial and domestic applications, e.g. temperature measurement and temperature compen-sation, etc.. Traditionally, most of the NTC thermistors are spinel-type ceramics based on the solid solutions of transition metal oxides, such as Mn-Ni-O and Mn-Ni-Cu-0 systems. However, the applications of NTC thermistors based on the spinel oxides are commonly limited to the temperatures below 300℃for the relaxation of the crystal structure at high temperature. To prepare a novel NTC material for high temperature application, the cobalt-doped BaTiO3 ceramics were studied in this thesis. The research work includes the following aspects.(1)BaTi1-xCoxO3-δ(x=0.01,0.05,0.1,0.2,0.3,0.4) powders were prepared by a wet chemical process and the related ceramics were obtained by the conventional sintering technique.The influence of cobalt contents on the crystal structure and electrical properties of BaTi1-xCoxO3-δceramics were studied. The phase transforms from a tetragonal one to a hexagonal one in the BaTii1-xCoxO3-δceramics with the Co concentration rising, and a pure hexagonal phase formed when x= 0.2. When the Co concentration is higher than x= 0.2, a small amount impurity Ba2TiO4 formed. The resistivity-temperature characteristic of the BaTi1-xCoxO3-δceramics depends on the related Debye temperatures (ΘD). The resistivity increases with temperature rising at the temperature lower thanΘD/2,and decreases with temperature rising at the temperature higher thanΘD/2.The material constants B50/120 are 3,187 K,2,968 K and 2,648 K for x= 0.2,0.3 and 0.4, respectively.The room temperature resistivities are 7.33×105Ω·cm,8.31×104Ω·cm and 7.25×104 fΩ·cm, respectively.The conduction mechanism was proposed by using the small polaron theory. (2) Impedance analysis was employed to study conduction mechanism, and the contribution of grain and/or grain boundary to the electrical properties of BaTi1-xCoxO3-δ(x=0.2,0.3)ceramics was disccused. Both hopping conduction between localized states (i.e. small polaron hopping) and long-range conduction (i.e. band conduction) are considered to be the possible conduction mechanisms in the BaTi0.8Co0.1O3-δceramics. It is also found that the conduction in these ceramics is dominated by the gram.(3) Three kinds of sintering additives were selected to study the influence of liquid phase on the sinter-ability and properties of the BaTi0.8Co0.2O3-δ. The ceramics with sintering additives have higher densities, even were sintered at low temperatures 950℃～1050～,than those without additives. Ceramics with Li-P-Ti-Co-O additive show good NTC effects and have the B50/120 constant of 2533 K. The conduction in these ceramics with Li-P-Ti-Co-O additive is proposed to be dominated by both grain and grain boundary.The B-Zn-Si-Li-O and B2O3 additives have deteriorate the electrical properties.(4) The influence of La, Sb and Bi doping on the NTC properties of BaTi0.8Co0.2O3-δceramics was studied. It is found the resistivity and B50/120 constants can be adjusted by dopants.For most of the donor-doped ceramics, the resistivities and B50/120 constants decreased comparing with the undoped ceramics. However, the resistivity and B50/120 constants will increase at a certain doping concentration. The increase of resistivity and B50/120 constants is related to (VBa"La3+) complex-defects and the decrease of Co2+ concentration. |
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