Thermal ceramics are ceramic materials that are sensitive to temperature changes. It can be divided into ceramic materials such as thermistor, thermistor, thermoelectric and pyroelectric.
Among the various types of sensitive components, thermistors are the most widely used. Thermistors have been introduced in the 1930s and used as circuit temperature compensation components. Later, semiconductor ceramic thermistors made of metal oxides were discovered, which have a large negative temperature coefficient (NTC), glass thermistors with vanadium oxide as the main body, silicon and germanium single crystal thermistors, using V2O3 , P2O5, SiO2, BaO, SrO, CaO and other oxides synthesized critical thermistor (CTR) and barium titanate as the main body positive temperature coefficient (PTC) thermistor. In recent years, with the maturity of the planar technology of silicon components and the development of integration technology, silicon carbide single crystals and silicon carbide thin film thermistors have emerged.
Thermistor is a resistance element (or resistor) whose resistance value changes with temperature. The resistance value increases as the temperature rises and is called a positive temperature coefficient (PTC) thermistor, on the contrary, it is called a negative temperature coefficient (NTC) thermistor. The thermistor whose resistance changes linearly with temperature is called linear thermistor, and the resistance whose resistance changes exponentially (or logarithm) with temperature is called nonlinear thermistor. Among the non-linear thermistors, there is a type of thermistor whose resistance can be changed (rising or falling) several orders of magnitude within a very narrow temperature range, which is called a switching thermistor.
1. Positive temperature coefficient thermistor
Positive temperature coefficient thermal elements are often called PTC (Positive Temperature Coefficient) elements, which means that the resistivity of this element increases with temperature.
2. NTC resistance material
Most NTC thermistor materials are spinel-type semiconductors, including binary and multi-component oxides. Binary metal oxides mainly include CuO-MnO-O2, CoO-MnO-O2, NiO-MnO-O2 and other systems. Ternary systems include Mn systems such as MnO-CoO-NiO and MnO-CuO-CoO, and non-Mn systems such as CuO-FeO-NiO and CuO-FeO-CoO. this
In addition, there are thick film materials that are constantly being developed and rapidly developed.
3. CTR material
The resistance value of some transition metal oxides changes sharply at a certain temperature. This change is reproducible and reversible, so it can be used as an electrical switch or temperature detector. This specific temperature is called the critical temperature. The abrupt change of the resistance value is usually with the increase of temperature, and the resistance value decreases sharply near the critical temperature. This material is called a critical (temperature) thermistor material.
4. High temperature thermistor materials
The thermistor (NTC) with an operating temperature above 300°C is often called a high temperature thermistor. Such materials must have stable physical and chemical properties at high temperatures, that is, their R~T curve does not change with time, has no phase change, and has a stable structure at high temperatures. The use of ceramic materials as high temperature thermistors has outstanding advantages. Therefore, it has a wide range of application prospects, especially in automotive air/fuel ratio sensors.
5. Low temperature thermistor materials
The thermistor materials whose operating temperature is below -60°C are called low-temperature thermistor materials. This material is based on transition metal oxides, adding La, Nd. Pd, Nb and other oxides. Common temperature zones are 4-20K, 20-80K, 77-300K. Its main advantages are stability, mechanical strength, and good performance against magnetic field interference. The main materials are Mn-Ni-Fe-Cu, Mn-Cu-Co, Mn-Ni-Cu, etc.
Thermistor is the most widely used in temperature sensors. Although it is not suitable for high-precision measurement, its price is low, so it is mostly used in household appliances, automobiles, etc. PTC thermistors have two purposes: one is for thermostatic electric heaters. The PTC thermistor works by self-heating. After reaching the set temperature, it will automatically maintain a constant temperature. Therefore, no additional control circuit is required, such as electric heating. Mosquito device, constant temperature electric iron, heater, etc. The second is used as current limiting components, such as color TV degaussers, electronic ballasts for energy-saving lamps, refrigerator motor starters, etc.