Thermistors include positive temperature coefficient and negative temperature coefficient thermistors

2019-09-27  4583

The main characteristics of thermistors are as follows: 1. The sensitivity of thermistors is higher than that of non-metals, and their resistance-temperature coefficient is 10-100 times larger than that of non-metals; 2. The scope of task-temperature sensing is wide, and the practical temperature of room temperature components is - 55 ~315 C, and that of cryogenic components is higher than 315 (- 2000 C at present) and that of high temperature components is - 273 ~55 C; 3. Volume. Small, it can measure the sensation temperature of empty space, cavity and blood vein in organism that other thermometers can't help measuring; 4. Easy to use, resistance value can be arbitrarily chosen between 0.1 and 100 k; 5. Easy to process into a simple shape, can be consumed in a small amount; 6. Good stability, strong overload power.

Semi-superconductor thermistors have common functions, because they can be used as measurement components (such as temperature, flow, liquid level, etc.), as well as control components (such as temperature gates, current limiters) and circuit compensation components. Thermistors are widely used in household appliances, wind light industry, telecommunications, military superstitions, aerospace and other fields, and their development prospects are extremely broad.

I. PTC Thermistor

PTC (Positive Temperature Coeff1Cient) refers to the phenomenon or material of thermistor with positive temperature coefficient, which increases rapidly at a certain temperature and can be used as a temperature sensor. The material is a sintered body consisting mainly of BaTi3 or SrTi3 or PbTi3, in which a small amount of Nb, Ta, Bi, Sb, Y, La oxides are doped to control the atomic valence and make it semi-conductive. The semiconducting materials such as BaTi3 are often referred to as semi-conducting (bulk) ceramics, and Mn, Fe, which increase the temperature coefficient of positive resistance, are also added. The oxides of copper, chromium and other additives are semi-conductive to platinum titanate and its solid solution by common ceramic forming and high temperature sintering, thus obtaining positive temperature thermistor materials. Their temperature coefficients and Curie point temperatures vary with composition and sintering conditions (especially cooling temperature).

Barium titanate crystal belongs to perovskite structure, it is a ferroelectric material, pure barium titanate is an insulating material. After adding trace rare earth elements into barium titanate material and proper heat treatment, the resistivity increases by several orders of magnitude near Curie temperature, resulting in PTC effect, which is related to the ferroelectricity of BaTiO3 crystal and the phase transformation of the material near Curie temperature. Barium titanate semi-conducting ceramics are polycrystalline materials with grain-to-grain interfaces. When the semi-conducting ceramics reach a certain temperature or voltage, the grain boundary of the crystal will change, and the resistance will change sharply.

The PTC effect of barium titanate semiconducting ceramics results from grain boundaries (grain boundaries). For conductive electrons, the interface between grains is equivalent to a barrier. When the temperature is low, because of the electric field in barium titanate, it is easy for electrons to cross the barrier, and the resistance value is small. When the temperature rises to the vicinity of Curie point temperature (i.e. critical temperature), the internal electric field is destroyed, which can not indicate that the conducting electrons cross the barrier. This corresponds to the increase of the barrier and the sudden increase of the resistance, resulting in the PTC effect. The physical models of PTC effect of barium titanate semi-conducting ceramics include the potential barrier model of the sea surface, the barium vacancy model of Daniels et al. and the superposition barrier model. They explain the PTC effect reasonably from different aspects.

PTC thermistors appeared in 1950, and then in 1954 PTC thermistors with barium titanate as the main material appeared. PTC thermistor can be used for temperature measurement and control in industry, temperature detection and regulation in a certain part of automobile, and also widely used in civil equipment, such as controlling the water temperature of instantaneous water heater, the temperature of air conditioner and cold storage, heating the working gas analysis and wind turbine by itself, etc.

PTC thermistor can not only be used as heating component, but also play the role of "switch". It has three functions: sensitive component, heater and switch. It is called "thermistor switch". When the current passes through the module, the temperature of the heater rises. When the temperature exceeds Curie point, the resistance increases and the current increases. Then the temperature of the module decreases with the decrease of the current, and the circuit current increases with the decrease of the resistance value. The temperature of the module rises again and again. Specific range of functions, but also play a switching role. Using this resistance to temperature as a heating source, heating components can be used as heaters, soldering iron, drying wardrobes, air conditioning, etc. It can also play an overheating protection role for electrical appliances.

NTC Thermistor

NTC (Negative Temperature Coeff1Cient) refers to the phenomena and materials of thermistor with negative temperature coefficient, which decreases exponentially with the rise of temperature. The material is a semiconductor ceramics made by fully mixing, forming and sintering two or more metal oxides, such as manganese, copper, silicon, cobalt, iron, nickel, zinc, etc., which can be used to produce a thermistor with negative temperature coefficient (NTC). Its resistivity and material constant vary with the proportion of material composition, sintering atmosphere, sintering temperature and structure state. At present, non-oxide NTC thermistors such as silicon carbide, tin selenide and tantalum nitride have also appeared.

The development of NTC thermistors has gone through a long period. In 1834, scientists first discovered that silver sulfide has a negative temperature coefficient. In 1930, scientists found that cuprous oxide - cuprous oxide also has negative temperature coefficient, and successfully applied it in the temperature compensation circuit of Aeronautical instruments. Subsequently, due to the continuous development of transistor technology