Ceramic materials have a wide range of electrical properties, and can be divided into insulators, semiconductors, and conductors according to their resistivity. Most structural ceramics (such as Al2O2, Si3N4, AIN) are good insulators with high breakdown field strength, and current cannot flow through them even in a very strong field. Some ceramics have semiconductor properties, that is, they can allow current to pass under certain conditions, such as SiC, ZnO, SnO2, CdS, etc. Conductive ceramics refer to doped ZrO2, β-Al2O3, LaSrCrO3, etc., and high-temperature superconductors are Y-Ba-Cu-O, La-Ba-Cu-O, Bi-Sr-Ca-Cu-O, and other systems. In addition to the conductive properties, the dielectric constant is also an important electrical property of ceramic materials, especially in the application fields of electronics and vacuum devices. The dielectric constant, dielectric loss and breakdown field strength of ceramics are very important indicators.
The conductivity of materials depends mainly on carriers. The passage of current means the directional movement of charged particles. These charged particles carry charges for directional transport to form a current, so they are called “carriers”.
In metal materials, the carriers are electrons. Due to the nature of metal bonds, these electrons can move relatively freely, resulting in high conductivity, especially for pure metals, because the size and stacking arrangement of atoms in pure metals are uniform, and there is no obstacle to the free movement of electrons. For alloy materials, the uniformity of their structure is destroyed to a certain extent, thereby reducing the conductivity. When the temperature rises, the structure will also be destroyed, resulting in a decrease in conductivity.
Most organic materials have poor conductivity because such materials have no carriers, so they are used as electrical insulators or dielectrics. Usually, the resistivity of organic polymers is greater than 10Ω·cm. Adding conductive fillers such as metal powder or graphite to polymers can reduce the resistivity of organic materials. Some special functional polymer organic materials have lower resistivity.
Ceramics are generally good insulators because there are no free electrons in the internal structure of ceramics. For example, the resistivity of ordinary porcelain under room temperature is greater than 107Ω·cm, and that of alumina ceramics is greater than 1014Ω·cm. However, due to the movement of ions inside some ceramics, ion conductivity is generated, or there is an energy band that is not filled with electrons, resulting in electronic conduction, which reduces the resistivity of the ceramic or shows a certain conductivity or semiconductor properties. Therefore, ceramic materials show a wide range of conductivity.
ZrO2 is doped or has typical ion conductivity at high temperatures and is conductive. This property of ZrO2 can be used in oxygen-sensitive detection devices and high-temperature heating elements.
Adding some low-valent oxides such as CaO, MgO, Y2O3, and Yb2O3, whose cation radius is within 12% of the Zr4+ ion radius, to ZrO2, after high-temperature solution treatment, the low-valent cations partially replace the high-valent Zr4+ ions; To maintain the electrical neutrality of the system, an oxygen vacancy type solid solution is formed in the structure. Oxygen ion vacancies and oxygen ions near oxygen vacancies diffuse in the lattice to form conductivity, and the conductivity depends on the ambient temperature and the partial pressure of oxygen. The diffusion of oxygen is measured in the device by voltage or current, depending on the device structure, and the response time is less than 1s. Such devices can be used in car and truck engines to monitor air-gas mixtures and combustion efficiency. To improve the ion migration number and detection accuracy of ZrO2, and fully consider the thermal shock resistance during manufacturing and use, appropriate types and amounts of additives should be selected. The more commonly used are CaO-ZrO2 system and Y2O3-ZrO2 system.
ZrO2 can be transformed from an insulator to a conductor when heated to a certain range in air. At about 1000℃, ionic conductivity accounts for more than 95% of its total conductivity. Therefore, the conductivity form at this time is called ionic conductivity. The main factors affecting the conductivity of ZrO2 are: the type of stabilizer and its addition amount, the size of the ZrO2 grain, the porosity, and the temperature environment. For example, at a temperature of 1000°C, the resistivity of ZrO2 stabilized with 12% CaO is 5.5×10-2Ω·cm; in the temperature range of 1500-2000°C, the resistance of ZrO2 heating elements stabilized with various stabilizers is between 4-19Ω.