Study On The Optimization Of BaTiO₃ Based Lead-free PTC Thermistor's Performance

For a long time, it's required of high Curie temperature in many areas of practical application while Ba TiO₃'s Curie temperature was relatively low. In order to improve its Curie temperature, the researchers replaced Ba with Pb. And they made its Curie temperature greatly improved. Then this material was widely used. But in the process of ceramic production Pb would be discharged inevitably, which is very harmful to the environment and human body. With the enhancement of people's environmental protection consciousness, lead-free has become the basic requirement of all kinds of products in the future.?K0.5Bi0.5? TiO₃?KBT? is a ABO₃ type perovskite-material with a high degree of tetragonal-cubic crystal structure transition temperature?That's the Curie temperature with 380??, which is the main choice of lead free ferroelectric, piezoelectric and PTC materials.As far as the author is concerned, the research of Ba TiO₃ based lead-free PTC thermal resistance materials is mainly focused on the replacement of the position of A or B, and the effect of boundary stress on the Curie temperature is seldom reported. In addition, with the progress of science and technology, the performance of the PTC thermistor is no longer limited to high Curie temperature. More sophisticated technology requires more sophisticated control, so a low resistance with a high resistance change rate has become a target for the PTC components. This requires the PTC element with high lift drag ratio and large temperature coefficient.In this thesis, the BaTiO₃ based lead-free PTC ceramics doped with KBT was prepared by solid state method. X ray diffraction, scanning electron microscopy, electric resistance and temperature measuring apparatus were used to characterize the microstructure and thermal properties of the materials. The influence of boundary stress on the Curie temperature?TC? was studied.The effect of KBT on the preparation of the Ba TiO₃ based lead-free PTC ceramics is very important. One of the raw materials for the preparation of KBT has a certain volatility of Bi2O₃. So in the conventional process of preparation, the secondary products would be produced, which leads to the decrease of KBT purity. This experiment found that the experimental group with the increase of 1% of the Bi2O₃ can form a more pure KBT.Ions(such as K⁺, Bi³⁺) in the replacement process cannot be all entered the A placement, there will be a small amount of the accumulation storing in the grain boundaries. And this increases the grain boundary stress, which makes the crystal form change. The phase transition of point Curie temperature is not only Ba TiO₃ ferroelectric phase transition to a paraelectric phase, but also a Ba TiO₃ crystal composed of tetragonal phase to cubic phase transition point. When the boundary stress of Ba TiO₃ is high, the Ba TiO₃ crystal is in a state of compression, and it is easier to enter the cubic phase by the square. So even in the condition of the original phase transition temperature it can also make the Ba TiO₃ from the tetragonal into the cubic phase, namely the ferroelectric phase transition to a paraelectric phase. So Curie temperature decreased. Conversely, the accumulation of ions in the grain boundaries can get into the A position by increasing the relative content of the oxygen octahedrons by increasing the amount of TiO2. It can reduce the boundary stress, and can achieve the effect of improving Curie temperature.In addition, the performance of the Ba TiO₃ based PTC material can be improved by staying in 700? for a period of time during the sintering process: It can increase the temperature coefficient of PTC materials; It can improve the lift-to-drag ratio of PTC material, which has a maximum value, when the maximum value is exceeded, the lift drag ratio will decrease.