Thermistors positive temperature coefficient

Thermistors are resistors with a temperature-dependent value of their resistance. There are three types of thermistors CTT (critical temperature thermistors), NTC (negative temperature coefficient), and PTC (positive temperature coefficient) thermistors. Their thermal behavior is shown in Figure 9.7. 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

Typical positive temperature coefficient (PTC) device behavior for a doped polycrystaHine BaTiO thermistor is presented in Figure 4. At temperatures below - 100° C and above - 200° C the material shows the expected negative resistivity vs temperature associated with semiconductors as expressed by ... 

Thermistors are temperature-dependent resistances, normally constructed from metal oxides. The resistance change with temperature is high compared with the metallic resistances, and is usually negative the resistance decreases with temperature increase. The temperature characteristics are highly nonlinear. Such thermistors, having a negative temperature coefficient, are called NTC thermistors. Some thermistors have a positive temperature coefficient (PTC), but they are not in common use for temperature measurement. 

Positive temperature coefficient (PCT) thermistors are solids, usually consisting of barium titanate, BaTiOi, in which the electrical resistivity increases dramatically with temperature over a narrow range of temperatures (Fig. 3.38). These devices are used for protection against power, current, and thermal overloads. When turned on, the thermistor has a low resitivity that allows a high current to flow. This in turn heats the thermistor, and if the temperature rise is sufficiently high, the device switches abruptly to the high resisitvity state, which effectively switches off the current flow. 

Whereas the RTD exhibits a small positive temperature coefficient, the thermistor has a large negative temperature coefficient and the resistance/temperature relationship is highly non-linear. The latter is typically ... 

The class of ferroelectric materials have a lot of useful properties. High dielectric coefficients over a wide temperature and frequency range are used as dielectrics in integrated or in smd (surface mounted device) capacitors. The large piezoelectric effect is applied in a variety of electromechanical sensors, actuators and transducers. Infrared sensors need a high pyroelectric coefficient which is available with this class of materials. Tunable thermistor properties in semiconducting ferroelectrics are used in ptcr (positive temperature coefficient... 

Small amounts of Y or La are used to dope BaTiC>3 which is the main component of all PTCs (positive temperature coefficient). Demand for PTC thermistors is high. Yttrium iron garnets are used in soft ferrites at very high frequencies (microwave region) for radar equipment. 

Crytal chemitry. The effect of solid solution on the transition behavior of perovskite (ABX3) structures has been intensively scrutinized for more than 50 years. These materials have merited continuous attention because of their enormous technological versatility. As multilayer capacitors, piezoelectric transducers, and positive temperature coefficient (PTC) thermistors they generate a market of over 3 billion every year (Newnham 1989, 1997). In addition to ease of fabrication, these compounds exhibit a number of attributes required of ideal actuators (1) They display very large field-induced strains (2) They offer quick response times and (3) Their strain-field hysteresis can be chemically controlled to be very large or negligibly small, depending on the application. Details of their technical applications can be found in Jaffe et al. (1971) and Cross (1993). 

Resistive materials used in thermometry include platinum, copper, nickel, rhodium-iron, and certain semiconductors known as thermistors. Sensors made from platinum wires are called platinum resistance thermometers (PRTs) and, though expensive, are widely used. They have excellent stability and the potential for high-precision measurement. The temperature range of operation is from -260 to 1000°C. Other resistance thermometers are less expensive than PRTs and are useful in certain situations. Copper has a fairly linear resistance-temperature relationship, but its upper temperature limit is only about 150°C, and because of its low resistance, special measurements may be required. Nickel has an upper temperature limit of about 300°C, but it oxidizes easily at high temperature and is quite nonlinear. Rhodium-iron resistors are used in cryogenic temperature measurements below the range of platinum resistors [11]. Generally, these materials (except thermistors) have a positive temperature coefficient of resistance—the resistance increases with temperature. 

Common thermistors are based on semiconductors whose resistance decreases significantly as the temperature increases. Currently, thermistors are manufactured from many different materials including polymers and other ceramics and their resistance may either increase with temperature (positive temperature coefficient—PTC) or decrease with temperature (negative temperature coefficient— NTC). The conductivity of pure metals increases with increasing temperatures and instruments based on metals are referred to as RTDs. 

One another major application in this field is the use of Y and La as doping agents for BaTiOs - the main component, which preserves the positive temperature coefficient of thermistors. 

Metal Bolometer. In contrast to thermistor materials, metals have a positive temperature coefficient of resistance, i.e., the resistance increases as the temperature increases. Usually the absolute value of the temperature coefficient... 

Conductive blacks are used in high-voltage cables, electronic packaging, antistatic flooring, EM shielding, fuel injectors and tanks, and computer assembly stations. Carbon black can be used in polyethylene and its copolymers to produce positive temperature coefficient (PTC) materials for use in thermistors and other electrical devices. 

The resistance of this semiconductor called the positive temperature coefficient (PTC) thermistor drastically increases above the Curie temperature (Tc), up to the temperature (Tn) where the resistance reaches its maximum value. The characterized temperature is divided into three regions (1, 11, and 111 in Figure 2.1.1) according to the resistance behavior. 

Thermistors are specially prepared metal oxide semiconductors that exhibit a strong negative temperature coefficient, in sharp contrast to the weak positive temperature coefficient of RTDs. Nominal thermistor resistance, usually specified for 25°C, ranges from less than 1000 S2 to more than 1 Mf2, with... 

Sauer, H. A. and S. S. Flaschen Positive Temperature Coefficient of Resistance Thermistor Materials for Electronic Applications, Proc. 1956 Elec. Comput. Conf., pp. 41 6, 1956. 

Thermistors cover a temperature range from -80 °C to +350 °C, and have either a negative (NTC) or positive (PTC) temperature coefficient. Typical resistance ranges from around 100Q up to the megaohm (MQ) region. Their response is related... 

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