1. Ingredients
At present, most domestic and foreign manufacturers use solid-phase synthesis technology. The raw materials used are BaCO3, SrCO3, Pb3O4, TiO2, Nb2O5, MnCO3, SiO2 and other powders, and the purity should generally be above 99.5%. The purity of the raw materials, the variety and content of impurities, the chemical activity of the raw materials, etc. all have a very large impact on the function of the very end product of PTC. The accuracy of the ingredients also has a great influence on the performance of the product.
2. Ball mill destroys and removes water
Put several weighed raw material powder, grinding balls, and pure water into the ball mill and destroy them, and mix them evenly. The ball-milled raw materials can be simply put into an oven for drying, or they can be dried after dewatering by filtering, centrifugal separation, vacuum filtration and other techniques.
Three, solid phase synthesis
Solid phase synthesis is also called calcination. It is to put the mixed raw materials into a high-temperature furnace for reaction to form a uniform solid solution. The general formula of the solid solution can be written as (Ba x, Sr y, Pb z)TiO3, where x+y+ z=1, the temperature of solid-phase synthesis is selected between 1000~1250℃ according to the material and the ratio, and the holding time is 2~4 hours.
Fourth, the second ball mill
After solid-phase synthesis, the materials agglomerate and certain crystal grains grow up, which need to be destroyed by ball milling to facilitate firing. The technique of secondary ball milling is similar to that of ball milling before synthesis.
Five, forming
PTC materials can be made into various patterns, such as: round, square, honeycomb, etc. The sheet-like components are formed by dry pressing, and a viscous PVA solution is added to the material. Then the sheet is granulated by the sieving method or spray drying method, and then press-formed on the punching machine. The honeycomb elements are formed by extrusion techniques.
Six, firing
Put the formed green sheet into a high-temperature furnace, and according to the necessary firing premise, fire the semiconductor ceramic with the required PTC characteristics. The firing curve and firing air have a great influence on the function of the product, so it must be strictly controlled in production to improve the product function and product rate.
PTC materials are therefore BaTiO3-based semiconductor ceramic materials. The resistivity of this material rises sharply with the increase in temperature in a certain area, and the temperature at which the resistivity rises suddenly is called the Curie temperature. The Curie temperature of BaTiO3 is 120°C. When Ba2+ is replaced with a piece of Pb2+, it becomes Ba(1-X)PbX TiO3 material, and its Curie temperature rises with the increase of Pb2+ content. The currently applicable PTC fever material has a very high temperature of 300°C.
The PTC effect of BaTiO3 semiconductor ceramics originates from the abnormal change of the material's dielectric constant. After replacing Ba2+ with Pb2+, the abnormal change of the dielectric constant is reduced, so the PTC effect is not as great as the material without Pb. The so-called PTC effect is the ratio of the very large resistance to the very small resistance in the material resistance-temperature curve. The PTC effect of high-temperature PTC materials containing Pb is shown in Fig. 1 with the change of Curie temperature TC. In addition, with the advancement of Pb content, it is not easy to control the volatilization of PbO gas during firing. The volatilization of PbO gas during firing deviates the composition of the PTC material, making it impossible to fire a ceramic body with a uniform layout. For these reasons, high-temperature PTC materials are not as long as low-temperature PTC materials with long life and high reliability.