Characteristics and applications of thermal sensitive ceramics

2020-03-14 00:00

Thermistor is a kind of sensitive component which has been developed early, has many kinds and is mature. The thermistor is composed of semiconductor ceramic materials. The principle is that the temperature causes the change of resistance. If the concentration of electron and hole is n and P, and the mobility is μ N and μ P respectively, then the conductivity of semiconductor is:


σ=q(nμn+pμp)


Because n, P, μ N and μ P are all functions dependent on temperature T, the conductivity is a function of temperature. Therefore, the temperature can be calculated by measuring the conductance, and the resistance temperature characteristic curve can be drawn. This is the working principle of semiconductor thermistor


Thermistors include positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors, and critical temperature thermistors (CTR). Their resistance temperature characteristics are shown in Fig. 1. The main characteristics of thermistors are: ① high sensitivity, the temperature coefficient of resistance is 10-100 times larger than that of metal, and can detect the temperature change of 10-6 ℃; ② the working temperature range is wide and constant The temperature apparatus is suitable for - 55 ℃ ~ 315 ℃, the high temperature device is suitable for temperature higher than 315 ℃ (up to 2000 ℃), and the low temperature device is suitable for - 273 ℃ - 55 ℃; ③ it is small in size and can measure the temperature of voids, cavities and blood vessels in the organism that can not be measured by other thermometers; ④ it is easy to use, and the resistance value can be selected arbitrarily from 0.1 to 100 K Ω; ⑤ it is easy to be processed into complex shapes, It can be produced in large quantities; 6. Good stability and strong overload capacity


Because of its unique performance, semiconductor thermistors can be used not only as measuring elements (such as measuring temperature, flow, liquid level, etc.), but also as control elements (such as thermal switches, current limiters) and circuit compensation components. Thermistors are widely used in various fields such as household appliances, power industry, communication, military science, aerospace and so on Extremely broad


1、 PTC thermistor


PTC(Positive Temperature Coeff1cient) refers to the phenomenon or material of thermistor with positive temperature coefficient and rapid increase of resistance at a certain temperature. It can be used as a constant temperature sensor. The material is a sintered body with BaTiO3 or SrTiO3 or PbTiO3 as the main component, which is semiconducted by doping a small amount of Nb, Ta, Bi, Sb, y, La and other oxides to make it semiconducting BaTiO3 and other materials are referred to as semiconducting (bulk) ceramics for short. At the same time, oxides of Mn, Fe, Cu, Cr that increase the temperature coefficient of their positive resistance and other additives are added. Platinum titanate and its solid solution are semiconducted by general ceramic process and sintering at high temperature. Thus, the thermistor materials with positive characteristics are obtained (especially the cooling temperature) varies


Barium titanate crystal belongs to perovskite structure and is a kind of ferroelectric material. Pure barium titanate is a kind of insulating material. After adding trace rare earth elements into barium titanate material and conducting appropriate heat treatment, the resistivity increases several orders of magnitude near Curie temperature, resulting in PTC effect. This effect is related to the ferroelectric property of BaTiO3 crystal and the phase transition of material near Curie temperature Semiconducting porcelain is a kind of polycrystalline material. There is an interface between grains. When the semiconducting ceramic reaches a certain temperature or voltage, the crystal grain boundary changes and the resistance changes sharply


The PTC effect of BaTiO3 semiconducting ceramics results from the grain boundary (grain boundary). For conducting electrons, the interface between grains is equivalent to a potential barrier. When the temperature is low, due to the effect of internal electric field of barium titanate, the electrons easily cross the barrier, and the resistance value is small. When the temperature rises to the Curie point temperature (critical temperature), the internal electric field is destroyed, which can not help The physical models of PTC effect of BaTiO3 semiconducting ceramics include the sea view surface barrier model, Daniels' barium vacancy model and superposition barrier model. They explain the PTC effect reasonably from different aspects


The experimental results show that the resistance temperature characteristic of PTC thermistor can be approximately expressed by the experimental formula in the range of working temperature


RT=RT0expBp(T-T0)


Where RT and RT0 are the resistance values at t and T 0, and BP is the material constant of the material


The PTC effect originates from the properties of ceramic grain boundaries and precipitates, and changes significantly with the type, concentration and sintering conditions of impurities. Recently, silicon temperature sensitive elements using silicon wafers are used in practical thermistors. This is a high-precision PTC thermistor, which is composed of n-type silicon, As a result, the resistance increases