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Thermistor is a sensitive element, and its resistance value will change with the change of temperature. So it is variable resistance and is widely used in various electronic components. Thermistors usually achieve high accuracy in a limited temperature range (-90℃ to 130℃).
The thermistor becomes a thermosensitive element due to the features of thermistor that the resistance value of semiconductor varies with change of temperature and so on. Within a certain temperature range, the temperature change of the measured medium can be known according to the change of the thermistor value. At the same temperature change, the resistance change of thermistor is about 10 times that of lead thermal resistor.
The temperature characteristics of semiconductor comes from that the conduction mode of semiconductor is carrier conduction. Since the number of carriers in semiconductor is far less than that of free electrons in metal, its resistivity becomes larger.With the increase of temperature, the number of carriers participating in conduction increases, the conductivity increases and the resistivity decreases. Thermistor resistor includes positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors, and critical temperature thermistors (CTR).
NTC refers to thermistor phenomena and materials with negative temperature coefficient, whose resistance decreases exponentially with temperature rise. The material is a semiconductor ceramic made of two or more metal oxides such as manganese, copper, silicon, cobalt, iron, nickel and zinc, which are fully mixed, molded and sintered, and can be made into a thermistor with negative temperature coefficient (NTC).The resistivity and material constant vary with the material composition ratio, sintering atmosphere, sintering temperature and structural state. At present, non-oxide NTC thermistor materials, such as silicon carbide, tin selenide and tantalum nitride, have also appeared.
NTC thermosensitive semi-conductive ceramics are mostly oxide ceramics with spinel structure or other structures, and have negative temperature coefficient. The development of NTC thermistor has gone through a long period. In 1834, scientists first discovered that silver sulfide has negative temperature coefficient. In 1930, scientists discovered that cuprous oxide-copper oxide also has negative temperature coefficient, and it is successfully applied to the temperature circuit of aviation instruments. Subsequently, due to the continuous development of transistor technology, great progress has been made in the research of thermistor.