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Thermistors are thermally sensitive resistors. Several semi-conducting oxide ceramics exhibit large changes in resistivity (~100 to 1000 ohm-cm) over a temperature span of ~100°C. In most such materials, the resistance falls as temperature increases. Consequently, they are known as negative temperature coefficient (NTC) thermistors. Positive temperature coefficient (PTC) thermistors also exist but will not be discussed here. NTC thermistors are used as temperature sensors and in a variety of other control circuitry. Estimates of the market for NTC thermistors range upwards of $50 million.
Typical MaterialsStoichiometric metal oxides such as Mn3O4 are usually insulators. However, if divalent ions are substituted on the Mn lattice, charge neutrality requires the creation of Mn4+ ions. Since Mn3+ and Mn4+ ions are both present, electrons can hop from Mn3+ to Mn4+ ions. This electron hopping mechanism is a thermally activated phenomenon, and thus resistivity is a function of temperature. Spinel structure oxides in the Ni-Mn-O, Ni-Cu-Mn-O and Ti-Fe-O systems are typical of commercial NTC thermistors. By controlled doping, a variety of resistivity-temperature behaviors can be engineered. Thermistor ceramics are polycrystalline. They are usually sintered in air, and the reactions with the ambient oxygen can lead to additional charge carriers within the thermistor material.
Typical Performance ParametersSeveral parameters characterize the performance of a thermistor material. One is the temperature coefficient of resistance, a, which specifies the percent change in resistance per degree Kelvin (K). Thermistors generally have nonlinear resistance (R) - temperature (T) behavior. Alpha is defined as 1/R (dR/dT), where (dR/dT) is the derivative of the R-T curve at the temperature under concern. Typical values for alpha near room temperature are -2 to -6.5% K-1, or ~ 10 times more sensitivity than thermocouples or PTC thermistors.
Another important materials characterization parameter is b, which relates resistance to temperature according to the expression: b= [ T1T2/( T2 - T1 )] ln (R1/R2). Units of b are degrees K, and values typically range from ~2500 to 5000K. Because the resistance-temperature behavior of NTC thermistors is non-linear, equations have been developed to determine T at a given value of resistance. The actual resistance values of a thermistor can be tailored for a specific application by controlling both the material and the geometry, and can vary from 1 to 106 ohm. Current flowing through a thermistor will generate self-heating. Many applications require low current levels (so-called zero power, 100 µA), which generally avoid self-heating, and some use the self-heating effect.
ApplicationsThe most frequent use of NTC thermistors is as temperature sensors in the 200 to 600K range. The thermistor is used as a thermocouple in a standard Wheatstone Bridge circuit. In these applications self-heating must be avoided. If one needs to measure small temperature variations around a fixed temperature (a requirement in many processing industries), accuracies of +/-0.05K are possible at temperatures up to 450K.
An important application for NTC devices is for temperature compensation in electrical circuits. If some elements of a circuit exhibit PTC behavior (a copper coil, for example), then a compensated circuit using an NTC thermistor can flatten the resistance-temperature behavior of the device. Such compensated circuits have been used to stabilize amplifier gain. Small surface mount NTC thermistors are commercially available with a “footprint” as small as 1.5 x 0.8 x 0.5 mm for circuit board application.
Liquid Level and Flow SensingThe self-heating that occurs when a significant current flows though a thermistor can be used as the basis for devices such as liquid level sensing and flow sensing. Heat generated in a thermistor dissipates about 10 times faster in a static liquid than in a static gas. If a thermistor and a light bulb (or LED) are connected in series with a battery so that a constant voltage flows though the thermistor, the temperature—and hence the resistance—of the thermistor will depend on the heat dissipation. At a properly chosen voltage in a static gas, the thermistor will heat up, its resistance will drop, current will flow and the bulb will light.
In a static liquid, the heat dissipation will be sufficiently high to maintain high resistance, thereby limiting current flow and preventing the light from turning on. An array of such sensors in a tank of liquid provides information (and a visual indication) of the liquid level in the tank. Similarly, the differences in heat dissipation between static and flowing gas (or liquid) can be used for flow sensing and control.
Bibliography1. Concise Encyclopedia of Advanced Ceramic Materials, ed. Brook, R.J., Pergamon Press, New York, pp. 328-331 (1991).
2. Mosely, P.T, and Crocker, A.J., Sensor Materials, Institute of Physics Publishing, Philadelphia, pp. 56-58 (1996).
3. Kim, Y.K., Process Improvement for the Fabrication of (Mn,Ni,Ti)3O4-Spinel Negative Temperature Coefficient Thermistors, MS Thesis, Worcester Polytechnic Institute, May 2000.