Positive temperature coefficient of resistance

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. 

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. 

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 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. 

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... 

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. 

The resistivity of all the Er-based alloys is shown in fig. 90. It can be seen that the alloys containing Al, Ga, and Au all have positive temperature coefficients of resistivity at high temperatures whereas the alloys containing transition metals have negative temperature coefficients. Hadjipanayis et al. (1980) interpret this behaviour in terms of the Ziman theory of liquid metals. 

CPs have rarely been produced in a form ordered enough to exhibit a small positive temperature coefficient of resistivity. This so-called metallic behavior is normally not seen in the transport properties of as-grown CPs, like PPys, PThs, or PANIs, where the negative temperature coefficient of the resistivity is generally attributed to hopping. 

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... 

Mallette J G, Quej L M, Marquez A and Manero O (2001) Carbon black-filled PET/HDPE blends Effect of the CB structure on rheological and electric properties, J Appl Polym Sci 81 562-569. Xu X B, Li Z M, Dai K and Yang M B (2006) Anomalous attenuation of the positive temperature coefficient of resistivity in a carbon-black-filled poljoner composite with electrically conductive in situ microfibrils, Appl Phys Lett 89 032105. 

Ferroelectric materials, especially polyciystalhne ceramics, are utihzed in various devices such as high-permittivity dielectrics, ferroelectric memories, pyroelectric sensors, piezoelectric transducers, electrooptic devices, and PTC (positive temperature coefficient of resistivity) components. 

Miraceram Barium titanate ceramics with positive temperature coefficient of resistance, used to make heating elements for domestic appliances, which then need no separate overheating cutout. The heaters are made as honeycomb with high specific surface. 

PTCR. Positive temperature coefficient of resistance. Ceramics with PTCR s include barium titanates. 

By the early 1990s, several groups have started making high-quality materials of PPV, PPy, PANI, and polyalkylthiophene (PAT) [3]. In doped oriented PPV samples, room-temperature conductivity values on the order of 10 S cm were observed [1117]. In high-quality PF6-doped PPy and PT samples, prepared by low-temperature electrochemical polymerization, the conductivity was nearly 500 S cm [1118]. In these samples, for the first time in doped conducting polymers, a positive temperature coefficient of resistivity (TCR) was observed at temperatures below 20 K, demonstrating the real metallic qualities. 

The dynamic movement of polymer chains has been used to achieve materials with a positive temperature coefficient of resistivity PTC materials. The characteristic of these materials is that the resistance does not decrease with increase in temperature, as is observed with most semiconductors, but increases at some characteristic temperature (Figure 14.6). 

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. 

CSA by PMMA since the localization length decreases for concentrations that approach the percolation threshold. However, the persistence of the positive temperature coefficient of resistivity near room temperature, even for samples near the percolation threshold, indicates that increased disorder does not drastically affect the metallic properties of PANI-CSA upon dilution. 

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