Thermistors are sensitive components that are developed early, have many types, and are relatively mature. The thermistors are composed of semiconductor ceramic materials, and the principle of use is temperature-induced resistance change. If the concentrations of electrons and holes are n, p, respectively, migration The rates are μn, μp, respectively, and the conductance of the semiconductor is: σ=q(nμn+pμp) Since n, p, μn, and μp are all functions dependent on the temperature T, the conductance is a function of temperature, so the temperature can be derived from the measured conductance and the resistance-temperature characteristic curve can be made. This is the semiconductor thermistor. How it works. Thermistors include positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors, as well as critical temperature thermistors (CTR). Their resistance-temperature characteristics are shown in Figure 1. The main characteristics of the thermistor are :1 high sensitivity, its temperature coefficient of resistance is 10 to 100 times larger than metal, can detect temperature changes of 10-6 ° C; 2 wide operating temperature range, room temperature device is suitable for -55 ° C ~ 315 ° C, high temperature device Applicable temperature is higher than 315 °C (currently up to 2000 °C), low temperature device is suitable for -273 °C ~ 55 °C; 3 small volume, can measure the temperature of voids, cavity and blood vessels in other organisms that can not be measured by other thermometers; Convenient, the resistance value can be arbitrarily selected between 0.1 ~ 100kΩ; 5 easy to process into a complex shape, can be mass-produced; 6 good stability, strong overload capacity. Due to its unique performance, semiconductor thermistors can be used not only as measuring components (such as measuring temperature, flow, liquid level, etc.) but also as control components (such as thermal switches, current limiters) and circuits. Compensation components. Thermistors are widely used in household appliances, power industry, communications, military science, aerospace and other fields, and the development prospects are extremely broad. First, PTC thermistor PTC (Positive Temperature Coeff1Cient) is a thermistor phenomenon or material with a sharp increase in resistance at a certain temperature and a positive temperature coefficient. It can be used as a constant temperature sensor. The material is mainly composed of BaTiO3 or SrTiO3 or PbTiO3. a sintered body in which a small amount of an oxide such as Nb, Ta, Bi, Sb, Y, or La is doped to be semi-conducting, and such a semi-conductive material such as BaTiO3 is often simply referred to as a semiconducting body. Porcelain; also added Mn, Fe, Cu, Cr oxides and other additives to increase the temperature coefficient of positive resistance, using general ceramic process, high temperature sintering to make titanate and its solid solution semi-conductive The thermistor material with positive characteristics is obtained. The temperature coefficient and Curie point temperature vary with the composition and sintering conditions (especially the cooling temperature). Barium titanate crystal belongs to perovskite structure and is a ferroelectric material. Pure barium titanate is an insulating material. Adding trace rare earth elements to barium titanate material, after proper heat treatment, near Curie temperature, The resistivity increases steeply by several orders of magnitude, producing a PTC effect, which is related to the ferroelectricity of BaTiO3 crystals and the phase transition of the material near the Curie temperature. Barium titanate semiconducting ceramics are polycrystalline materials, grains There is a grain-to-grain interface. When the semi-conductive porcelain reaches a certain temperature or voltage, the crystal grain boundary changes, and the resistance changes sharply. The PTC effect of barium titanate semiconducting porcelain is caused by grain boundary (grain boundary). For conductive electrons, the intergranular interface corresponds to a barrier. When the temperature is low, due to the electric field inside the barium titanate, When the electrons easily cross the barrier, the resistance value is small. When the temperature rises to the vicinity of the Curie point temperature (ie, the critical temperature), the internal electric field is destroyed, and it cannot help the conductive electrons to cross the barrier. This is equivalent to the barrier. When it rises, the resistance value suddenly increases, and the PTC effect is generated. The physical model of the PTC effect of the barium titanate semi-conducting porcelain is the sea-eye surface barrier model, the annihilation model of the Daniels et al., and the superimposed barrier model. The PTC effect is explained reasonably from different aspects. Experiments have shown that the resistance-temperature characteristics of PTC thermistors can be approximated by experimental formulas over the operating temperature range: RT=RT0expBp(T-T0) Where RT, RT0 represents the resistance value when the temperature is T, T0, and Bp is the material constant of the material. The PTC effect originates from the nature of the grain boundary between the grain boundary and the grain boundary of the ceramic, and varies significantly with the type, concentration, sintering conditions, etc. of the impurity. Recently, there has been a temperature sensitivity of silicon using a silicon wafer in a practical thermistor. Component, this is a body shape and high precision PTC thermistor, which is composed of n-type silicon. The electron scattering caused by impurities increases with temperature, and the resistance increases. PTC thermistors appeared in 1950, and then in 1954, PTC thermistors with barium titanate as the main material appeared. PTC thermistors can be used as temperature measurement and control in industrial applications, and also used in certain parts of automobiles. Temperature detection and adjustment, also used in a large number of civilian equipment, such as controlling the water temperature of the instant water heater, the temperature of the air conditioner and the cold storage, using the heating itself for gas analysis and the wind speed machine. The following is an example of the heater, motor, transformer, Applications for heating and overheat protection of appliances such as high power transistors. In addition to being used as a heating element, the PTC thermistor can also function as a "switch". It has three functions of sensitive components, heaters and switches. It is called a "thermal switch", as shown in Figures 2 and 3. It shows that the temperature rises after passing through the component, that is, the temperature of the heating element rises. When the temperature exceeds the Curie point, the resistance increases, thereby limiting the current increase, so that the decrease of the current causes the temperature of the component to decrease, and the decrease of the resistance value causes The circuit current increases, the temperature of the component rises, and it repeats. Therefore, it has the function of keeping the temperature within a certain range, and functions as a switch. The temperature is used as a heating source, and the heater is used as a heating element. Soldering iron, drying closet, air conditioner, etc., can also protect the appliance from overheating. Second, NTC thermistor NTC (Negative Temperature Coeff1Cient) is a thermistor phenomenon and material with a negative temperature coefficient that decreases exponentially with increasing temperature. The material is made of manganese, copper, silicon, cobalt, iron, nickel, zinc, etc. Or a semiconductor ceramic obtained by sufficiently mixing, molding, sintering, or the like of two or more metal oxides, can be made into a thermistor having a negative temperature coefficient (NTC). Its resistivity and material constant vary with material composition ratio, sintering The atmosphere, sintering temperature and structural state vary. Now there are non-oxide NTC thermistor materials represented by silicon carbide, tin selenide and tantalum nitride. NTC heat-sensitive semi-conductive porcelain is mostly a spinel structure or other structure of oxide ceramics, with a negative temperature coefficient, the resistance value can be approximated as: In the formula, RT and RT0 are the resistance values ​​at temperatures T and T0, respectively, and Bn is the material constant. The ceramic grains themselves change the resistivity due to temperature changes, which is determined by the semiconductor characteristics. The development of NTC thermistors has gone through a long period. In 1834, scientists discovered for the first time that silver sulfide has a negative temperature coefficient. In 1930, scientists discovered that cuprous oxide-copper oxide also has a negative temperature coefficient performance, and It has been successfully used in the temperature compensation circuit of aviation instruments. Subsequently, due to the continuous development of transistor technology, significant progress has been made in the research of thermistors. In 1960, N1C thermistors were developed. NTC thermistors are widely used. In the aspects of temperature measurement, temperature control, temperature compensation, etc. The following is an application example of temperature measurement. The principle of NTC thermistor temperature measurement is shown in Figure 4. Its measurement range is generally -10 ~ +300 ° C, can also be -200 ~ +10 ° C, and can even be used in the environment of +300 ~ +1200 ° C for temperature measurement. RT is NTC thermistor; R2 and R3 is the bridge balance resistance; R1 is the starting resistance; R4 is the full scale resistance, the check meter, also called the check resistor; R7, R8 and W are the voltage divider resistors, which provide a stable DC power supply for the bridge. R6 is connected in series with the meter head (microampere meter) to correct the head scale and limit the current flowing through the meter head. R5 is connected in parallel with the meter head for protection. The unbalanced bridge arm (ie R1, RT) is connected. Enter a thermal element RT as a temperature sensing probe. Since the resistance of the thermistor changes with temperature, the indication of the head connected between the diagonals of the bridge changes accordingly. This is thermal The working principle of the resistor thermometer. The thermistor thermometer can achieve an accuracy of 0.1 ° C, and the temperature sensing time can be as low as 10 s. It is not only suitable for granary thermometers, but also for food storage, medical and health, scientific farming, ocean, deep well, high altitude, Temperature measurement in glaciers, etc. Third, CTR thermistor The critical temperature thermistor CTR (Crit1Cal Temperature Resistor) has a negative resistance mutation characteristic. At a certain temperature, the resistance value decreases sharply with the increase of temperature, and has a large negative temperature coefficient. The constituent materials are vanadium, niobium and tantalum. A mixed sintered body of elemental oxide such as phosphorus is a semi-glassy semiconductor, also called CTR is a glass-state thermistor. The sudden change temperature varies with the addition of oxides such as lanthanum, tungsten, molybdenum, etc. This is due to different impurities. The incorporation of vanadium oxide has a different lattice spacing. If vanadium pentoxide is changed to vanadium dioxide in a suitable reducing atmosphere, the temperature of the electrical resistance becomes sharper; if it is further reduced to vanadium trioxide, it is rapidly changed. Disappeared. The temperature at which the resistance changes rapidly corresponds to the position where the semi-glass semiconductor material changes rapidly, thus producing a semiconductor-metal phase shift. The CTR can be used as a temperature control alarm. The theoretical research and application development of thermistors have achieved remarkable results. With the application of high, precise and sharp technology, the deeper exploration of the conduction mechanism and application of thermistors, as well as the new performance In-depth research on materials will achieve rapid development. 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