Positive temperature coefficient resistivity

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. 

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. 

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. 

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. 

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. 

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],... 

The detection of sharp plasmon absorption signifies the onset of metallic character. This phenomenon occurs in the presence of a conduction band intersected by the Fermi level, which enables electron-hole pairs of all energies, no matter how small, to be excited. A metal, of course, conducts current electrically and its resistivity has a positive temperature coefficient. On the basis of these definitions, aqueous 5-10 nm colloidal silver particles, in the millimolar concentration range, can be considered to be metallic. Smaller particles in the 100-A > D > 20-A size domain, which exhibit absorption spectra blue-shifted from the plasmon band (Fig. 80), have been suggested to be quasi-metallic [513] these particles are size-quantized [8-11]. Still smaller particles, having distinct absorption bands in the ultraviolet region, are non-metallic silver clusters. 

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 crystal structure of cadmium rhenium(V) oxide, as determined by single-crystal technique,1 is of the face-centered cubic pyrochlore type (a = 10.219 A.). The only positional parameter for the 48 (/) oxygens is x = 0.309 0.007 when rhenium is at the origin. The density, determined pycnometrically, is 8.82 0.03 g./cc., compared with the theoretical value of 8.83 g./cc. for Z = 8. The resistivity between 4.2 K and room temperature is very low (10-3-10-4 J2-cm.) and has a positive temperature coefficient. Over the same temperature range the magnetic susceptibility is low and temperature-independent. These properties indicate that cadmium rhenium(V) oxide exhibits metallic conductivity. 

The variability in the resistance-temperature characteristics at temperatures below about 800 °C can be attributed to impurities, the materials behaving as very complex, doped semiconductors (see Section 2.6.2 and Problem 2.10). The positive temperature coefficient of resistance observed at the higher temperatures suggests the effect of decreasing charge-carrier mobility with increasing temperature (see Section 2.6.2), but because of the complex nature of the materials both from the chemical and microstructural standpoints, this has to be regarded as speculation. 

In many applications, temperatures of stator windings and bearings of motors are continuously monitored during operation by Pt-100 resistance thermometers or PTC (positive temperature coefficient) temperature sensors. 

The trend toward greater metallic character of the elements is complete at polonium, which has two allotropes, both with typically metallic structures a cubic, converting at 36°C to /3 rhombohedral (mp 254°C). Each of these has resistivity typical of a true metal with a positive temperature coefficient. 

It was pointed out in the introduction to this chapter that an experimental criterion for metallicity is the observation of a positive temperature coefficient to the electrical resistivity. The so-called bad or marginal metals are those that meet this criterion, but in which the value for the resistivity is relatively high (p > 10 flcm). Many transition metal oxides behave in this manner, while others (e.g. ReOs and RuOa) have very low electrical resistivities, similar in scale to those of conventional metals (p < 10 " O cm). Consider the Ruddlesden-Popper mthenates. Both strong Ru 4d-0 2p hybridization and weaker intrasite correlation effects compared to the 3d transition metals are... 

The problem of reducing sintering temperature is also crucial for Bao sSrg sTiOj, which possesses ferroelectric properties but is used more often as a material with a positive temperature coefficient of resistance (PTCR). The citrate synthesis method is often considered as an alternative to freeze-drying synthesis, but in the following case the solution of barium, strontium, and titanium citrates was dried by various methods (i.e., oven drying and freeze-drying). Careful... 

Figure I shows the temperature dependence of the electrical resistivity between 4 and 300 K. At room temperature the resistivity has a positive temperature coefficient of 0.8x10 K . However, the temperature coefficient changes sign at -2S0 K, and the resistivity increases by 30% before reaching the superconducting state at 12.8 K. The inset of fig. 1 shows the superconducting transition in detail. The temperature dependence of the...

The trend towards greater metallic character in the elements is complete at polonium. Whereas sulfur is a true insulator (specific resistivity in fiQ-cm = 2 x 1023), selenium (2X1011) and tellurium (2 x10s) are intermediate in their electrical conductivities, and the temperature coefficient of resistivity in all three cases is negative, which is usually considered characteristic of non-metals. Polonium in each of its two allotropes has a resistivity typical of true metals ( 43 juQ-cm) and a positive temperature coefficient. The low-temperature allotrope, which is stable up to about 100°, has a cubic structure, and the high temperature form is rhombohedral. In both forms the coordination number is six. 

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. 

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