Positive temperature coefficient

MMC = multicomponent components PTC = positive temperature coefficient NTC = negative temperature coefficient. 

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 ... 

R. D. Roseman, Influence ofYttria and Zirconia on the Positive Temperature Coefficient of Resistance in Barium Titanate Ceramics, M.S. dissertation. University of Illinois, Urbana, Dl. 1991. 

Ferroelectrics. In the preceding section, positive-temperature-coefficient (PTC) ceramics were mentioned and it was remarked that they are made of a ferroelectric material. 

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. 

Temperature This may be more or less of an important factor, depending on the metal considered. For example, while zinc is characterised by a very low positive temperature coefficient of corrosion rate , steel has a high... 

Underwriters Laboratories (UL) requires that consumer batteries pass a number of safety tests [3]. UL requires that a battery withstand a short circuit without fire or explosion. A positive temperature coefficient (PTC) device [4] is used for external short-circuit protection. The resistance of a PTC placed in series with the cell increases by orders of magnitude at high currents and resulting high temperatures. However, in the case of an internal short, e.g., if the positive tab comes lose and contacts the interior of the negative metal can, the separator could act as a fuse. That is, the impedance of the separator increases by two to three orders of magnitude due to an increase in cell temperature. 

We can notice in Fig. 9.5 that Pt and Rh-Fe thermometers have a positive temperature coefficient, whereas other reported materials have a negative coefficient. They usually are either semiconducting or amorphous materials. 

Although it does not relate to polytype identification, we mention a 14N static and MAS-NMR study of WZ AIN, which revealed a fortuitously small Cq < 10 kHz [167]. For comparison, the 27A1 Cq in the same compound was measured to be 1.913 MHz, and had an unusual positive temperature coefficient that has been seen for other metal nuclei in WZ structures. The 14N MAS-NMR spectrum of BN yielded a Cq of 142 kHz corresponding to an axially-symmetric tensor in the hexagonal ring structure [167]. 

For each type of sensor, a classification framework sets precision tolerance ratings. Platinum temperature sensors have a positive temperature coefficient which is defined as ... 

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. 

Finally, we turn from solutions to the bulk state of amorphous polymers, specifically the thermoelastic properties of the rubbery state. The contrasting behavior of rubber, as compared with other solids, such as the temperature decrease upon adiabatic extension, the contraction upon heating under load, and the positive temperature coefficient of stress under constant elongation, had been observed in the nineteenth century by Gough and Joule. The latter was able to interpret these experiments in terms of the second law of thermodynamics, which revealed the connection between the different phenomena observed. One could conclude the primary effect to be a reduction of entropy... 

Platinum is especially suitable for this application because even at high temperatures it has a good stability and a good resistance to contamination. However, different metals, all having a positive temperature coefficient, may be used, such as tungsten (for very high-temperature applications), nickel and nickel alloys and also (but rarely because of their low resistivity) gold and silver. 

This model was fitted to the data of all three temperature levels, 375, 400, and 425°C, simultaneously using nonlinear least squares. The parameters were required to be exponentially dependent upon temperature. Part of the results of this analysis (K6) are reported in Fig. 6. Note the positive temperature coefficient of this nitric oxide adsorption constant, indicating an endothermic adsorption. Such behavior appears physically unrealistic if NO is not dissociated and if the confidence interval on this slope is relatively small. Ayen and Peters rejected this model also. 

CNTs are also valuable as field emitters because they have a small virtual source size [30], a high brightness, and a small positive temperature coefficient of resistance [31]. The latter means that they can run hot under high emission currents, but not go into thermal runaway. Emission from nanotubes can be visualized by electron holography in a TEM [32],... 

Figure 10. Typical short-circuit behavior of a 18650 lithium-ion cell with shutdown separator and without PTC (positive temperature coefficient) and CID (current interrupt device). This test simulates an external short circuit of a cell.

In an extreme case the surface tension of diphenyl is almost double that of benzene at the same temperature and it would be expected that in a mixture of these substances the benzene would be preferentially adsorbed at the surface, and any attempt to find the mean molecular weight of the two would break down. Certain mixtures of aniline and water were found by Worley (J.G.S. ov. 260, 1914) to have positive temperature coefficients of surfiice tension as exemplified in the following data for a 3-3 °/o aniliiie... 

Figure 6.45 Schematic illustration of negative temperature coefficient (NTC) and positive temperature coefficient (PTC) effects. Reprinted, by permission, from D. M. Moffatt, J. Runt, W. Huebner, S. Yoshikawa, and R. Nenham, in Composite Applications, T. L. Vigo and B. J. Kinzig, eds., p. 52. Copyright 1992 by VCH Publishing, Inc.

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