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thermal resistor[′thər·məl ri′zis·tər]
a semiconductor resistor that has the property of significantly changing its electrical resistance upon a change in temperature. It is one of the simplest semiconductor devices. The main parameters of a thermal resistor are the operating temperature range and the temperature coefficient of resistance, which is defined as the relative percentage increase in the resistance upon a temperature change of 1°K.
A distinction is made between thermistors, which have a negative temperature coefficient of resistance (the resistance decreases with increasing temperature), and posistors, which have a positive coefficient (the resistance increases with increasing temperature). Graphs of the coefficients of both types are shown in Figure 1. Thermal resistors are made of mixtures of oxides of transition metals (for example, Mn, Co, Ni, and Cu), Ge and Si alloyed with various additives, silicon carbide (SiC), semiconductors of the AIIIBV type, synthetic diamond, or organic semiconductors. The operating temperature range of most thermistors is in the interval from 170°–210°K to 370°–570°K, with the temperature coefficient of resistance at room temperature ranging from – 2.4 to – 8.4%/°K. Other types of thermistors include high-temperature thermistors, operating in the range from 900° to 1300°K, and low-temperature thermistors, which operate in the range from 4.2° to 77°K and have temperature coefficients of resistance of at least –15 to –20%/°K.
The most important of the posistors are those made of solid solutions based on barium titanate, BaTiO3, alloyed with lanthanum, cerium, bismuth, or other metals. In the temperature region close to the ferroelectric phase transition (seeFERROELECTRIC), the resistance of such thermal resistors increases by several orders of magnitude upon an increase in temperature, and in a short temperature interval (about 5°K), their temperature coefficient of resistance may reach 50%/°K or more. A change in the composition of the solid solution may shift the region of the phase transition in a temperature interval from ~200°K up to ~500°K. Posistors are also made of Si alloyed with B.
Thermal resistors are produced in the form of rods, tubes, disks, plates, and beads. Their size ranges from several micrometers to several centimeters. They are used in systems and devices for remote and centralized temperature measurement and control, for fire alarms and heat control, for balancing the temperatures of various components of electrical circuits, and for the measurement of vacuum and the speed of liquid and gaseous flows. They are also used in power meters.
REFERENCESShashkov, A. G. Termorezistory i ikh primenenie. Moscow, 1967.
Sheftel’, I. T. Termorezistory. Moscow, 1973.
I. T. SHEFTEL’