In addition to being a structural ceramic with excellent performance, zirconia ceramics are also a special functional ceramic material. For example, zirconia has unique electrical properties. Simply put, zirconia has the characteristics of low temperature insulation and high temperature conductivity. One characteristic makes zirconium oxide an important application in sensors, solid-state batteries, inorganic heating elements, etc.
● Electrical properties of ZrO2
Whether it is pure ZrO2 or doped ZrO2, they are insulators at room temperature, with a resistivity greater than 1010Ω·cm, but their high temperature conductivity is good, with a negative temperature coefficient of resistance, the resistivity is 104Ω·cm at 1000°C, 1700°C When it is only 6～7Ω·cm.
According to first-principle calculations, in the electronic structure of ZrO2, the electron orbital energy levels of the valence band and conduction band are different in the number of electron orbitals depending on the crystal structure. The valence band of ZrO2 is full band, and the conduction band is also filled with a certain amount of electrons. The room temperature insulation performance of ZrO2 is mainly due to the excessive bandwidth between the valence band and the conduction band, and it is impossible to conduct electricity at room temperature. . After doping, a new energy level (donor energy level or acceptor energy level) can be formed in the forbidden band, so that the forbidden band width is reduced, but there is still no conductivity at room temperature, mainly due to the ZrO2 at room temperature The electron mobility is too low. Therefore, whether it is high-purity ZrO2 or doped ZrO2, both exhibit high insulation properties at room temperature.
The main conductive mechanism of ZrO2 comes from the directional migration of oxygen vacancies, and the conductivity increases with the increase of temperature and oxygen partial pressure difference. In an environment where the temperature is higher than about 800°C, the conductivity of ZrO2 is greatly improved, and the conductivity of Zr02 changes linearly with temperature, that is, the higher the temperature, the stronger the conductivity of ZrO2.
● Will conductivity be infinitely improved?
will not! A German scholar GuoX pointed out in a review that the conductive properties of ZrO2 are different from those of electronic conductivity. The ion-conducting ZrO2 material can achieve the maximum ionic conductivity under the condition of a suitable vacancy concentration, which is higher than the optimal vacancy. Adding more vacancies on the basis of the concentration will result in a decrease in ion conductivity. Therefore, the conductivity of ZrO2 at high temperatures cannot be increased indefinitely. For example, the decrease in ion conductivity of nanostructured ZrO2 is mainly caused by the excessive influence of the internal interface, which leads to a decrease in ion migration. Since the space charge layer in the stabilized ZrO2 is about 2.5nm, only when the size of the nanoparticle is less than 5nm will direct electron migration caused by the quantum size effect occur. Obviously, this situation is difficult to achieve on a large scale.
