With the development of science and technology, in the region of industrial production and scientific research, the objects (information) requiring inspection and control have increased rapidly. The acquisition of information depends on sensors, that is, various sensitive components, of which ceramic sensitive components occupy a very important position.
1. Classification and application of sensitive ceramics
Sensitive ceramics are one of the key materials in some sensors and are used to make sensitive components. Sensitive ceramics are mostly semiconductive ceramics (Semiconductive Ceramics), which is another new type of polycrystalline semiconductor electronic ceramics after single crystal semiconductor materials. Sensitive ceramic material refers to a certain external condition used for components made of these materials, such as temperature, pressure, humidity, atmosphere, electric field, light and radiation, etc., which can cause changes in certain physical properties of the material. Therefore, a certain useful signal can be obtained accurately and quickly from this component. According to their corresponding characteristics, these materials can be called heat-sensitive, pressure-sensitive, humidity-sensitive, photosensitive, gas-sensitive and ion-sensitive ceramics. Most of these materials are semiconductor ceramics, such as ZnO, SiC, SnO2, TiO2, FezO3, BaTiO3 and SrTiO3.
In addition, there are pressure, position, speed, acoustic wave sensitive ceramics with piezoelectric effect, magneto-sensitive ceramics with ferrite properties, and multi-functional sensitive ceramics with various sensitive characteristics. These sensitive ceramics have been widely used in industrial detection, control instruments, transportation systems, automobiles, robots, pollution prevention, disaster prevention, public security, and household appliances.
2. The structure and performance of sensitive ceramics
Modern electronic technology requires ceramic sensors to output the detected information (such as temperature, humidity, etc.) in the form of electrical signals. Therefore, sensor ceramics are often semiconductor materials. Through the doping of trace impurities, controlling the sintering atmosphere (deviation of the stoichiometric ratio) and the microstructure of the ceramics, the traditional insulating ceramics can be semiconductorized and have certain properties.
Ceramics is a multi-phase system composed of crystal grains, grain boundaries, and pores. Through artificial doping, the composition of the crystal grain surface is deviated, and solid solution, segregation and lattice defects are generated on the surface of the crystal grain. The precipitation of heterogeneous phases, the accumulation of impurities, lattice defects and lattice anisotropy, etc. occur at the particle boundary, heteroplasmic boundary and intergranular phase. Changes in the composition and structure of these grain boundary layers have significantly changed the electrical properties of the grain boundaries, leading to significant changes in the electrical properties of the entire ceramic.
The grain boundary effect of semiconductor ceramics shows properties that many single crystals do not have. Recently, it has begun to study how to use water vapor and certain gases to diffuse into the ceramic body through pores and adsorb on the surface of the grain boundary to change the electrical conductivity of the ceramic, which provides a new way for the development of humidity and gas-sensitive ceramics. In short, one can adjust the chemical composition and porosity (from dense to porous) on the macro level; control the composition of the micro domains (mainly grain boundary components) and microstructure (grains, grain boundaries, etc.) from the microcosm. Through the combination of the above various factors, a series of special functional materials are produced. Although the application characteristics of these functional materials are related to the nature of the crystal grains, they mainly use the characteristics of grain boundaries and ceramic surfaces, which are beyond the reach of single crystals.
① Mainly use the nature of the crystal itself: NTC thermistor, high temperature thermistor, oxygen sensor.
② Mainly use the properties of grain boundaries: PTC thermistor, ZnO varistor.
③ Mainly use surface properties: various gas sensors and humidity sensors.