Electrical resistivity varies with temperature

The thermal conductivity, coefficient of thermal expansion, and electrical resistivity vary with temperature as shown in Fig. 6.6.l l... 

Electrical Properties. The electrical properties of pyrolytic graphite also reflect the anisotropy of the material and there is a considerable difference between the resistivity in the ab and the c directions. Pyrolytic graphite is considered to be a good electrical conductor in the ab directions, and an insulator in the c direction. Its electrical resistivity varies with temperature as shown in Fig. 7.11. 

We can model the performance of a bolometer (and microbolometer) using basic thermal physics and a knowledge of how the resistance varies with temperature. The dependence of resistance on temperature is captured in the thermal coefficient of resistance (TCR). The electrical resistor is connected to a heat sink through thermal... 

Electrical resistance monitors use the fact that the resistance of a conductor varies inversely as its cross-sectional area. In principle, then, a wire or strip of the metal of interest is exposed to the corrodent and its resistance is measured at regular intervals. In practice, since the resistance also varies with temperature, the resistance of the exposed element is compared in a Wheatstone bridge circuit to that of a similar element which is protected from the corrodent but which experiences the same temperature. 

The numerator of the above ratio can vary with changes in either resistivity or thermal expansion. The resistivity changes with temperature are small for most of the resistive alloys thus, the effect of dimensional changes with temperature cannot be ignored when determining the resistivity ratio. In the case of Chromel-P thermocouple wire, the thermal expansion data are not known below room temperature. Therefore, we have elected to determine the resistance ratio instead of the resistivity ratio the electrical resistivity ratio can be calculated using (1), the data presented and thermal expansion data when available. 

If two or more types of different materials are mixed up and treated in defined conditions (varying with temperature, pressure, and other chemical and physical processes), a composite material with a clear interfacial boundary will be obtained. If a major part of the produced composite consists of polymer, then it is called a polymeric composite. A polymeric composite material is one of the most developed areas of modern science and technology. In addition to composite materials, modern science and technology use nano-sized materials. Such composites are called nanocomposites, whose main attraction is related to very high operation properties, such as flexibility, elasticity, recycling, hardness, resistance to abrasion, and optical and electrical transmission [9]. 

With respect to the electrical resistivity, the Kondo temperature roughly separates a high temperature region (T S> Tg) where the resistivity varies linearly with the logarithm of the temperature, and a low temperature region (T < Tn) where the resistivity saturates to the so-called unitary limit as T O. For temperatures well above Tg, the magnetic susceptibility resembles a Curie-Weiss law with a Curie-Weiss temperature which is of the order of several times Tg, whereas for temperatures well below Tg, the susceptibility exhibits at most a weak temperature dependence and approaches a finite value as T O. The specific heat and thermoelectric power exhibit broad maxima as a function of temperature which peak in the vicinity of Tg. 

Actually, the electric resistance, as defined in the previous paragraph, also involves the length and the cross-sectional area of the conductor, so the dimensional change of the conductor due to the temperature change must also be taken into account. We know that both dimensional quantities vary with temperature according to their respective coefficient of linear thermal expansion (o ) and coefficient of surface thermal expansion ( ) in addition to that of electrical resistivity. Hence the exact equation giving the variation of the resistance versus temperature is given by ... 

It has been shown that all the properties of T, Hy, 0 and electrical resistivity at room temperature (Prt) for the Al-R amorphous alloys were essentially independent of the atomic number of the R metals. The atomic size of the R metals varies systematically with the atomic number and hence the atomic size factor also seems to have little effect on the above-mentioned properties. On the other hand, it is generally known that the inherent chemical nature of the lanthanide metals results from 4f-electrons which lie at the inner side in their atoms. Although the number of 4f-electrons varies systematically with the atomic number, the electrons are screened by 5s - and 5p -electrons which lie at the outer side of the atoms, resulting in a similarity in chemical properties of the lanthanide metals. Accordingly, it may reasonably be assumed that the independence of the properties of the Al-R amorphous alloys as a function of atomic number is due to the unique electronic structure in which the 4f-electrons are screened by 5s- and 5p-electrons. 

The insulation resistance between two conductors or plated holes is the ratio of the voltage to the total current between the conductors. Two measures of electrical resistance are volume and surface resistivities. Since these properties can vary with temperature and humidity, testing is normally performed at two standardized environmental conditions, one involving humidity conditioning, the other involving elevated temperature. Humidity conditioning subjects the sample to 90 percent relative humidity and 35°C for 96 hours (96/35/90).The elevated temperature conditioning normally subjects the sample to 125°C for 24 hours (24/125). 

Although the ceramic phase of the composite makes the material electroactive, many of the important properties of the material are derived primarily from the properties of the polymer. Also, the choice of polymer can determine whether the best ceramic sensitivity can be realized. The electrical properties to be considered are the resistivity p, the relative permittivity (dielectric constant) the dielectric loss, the dissipation factor D, the power factor F and the dielectric strength. The variation of these properties with changes in the likely environment should also be considered, since many of them vary with temperature, frequency and humidity. 

The situation is a little different in the stoichiometric compound. Tsuda and his collaborators argued that the temperature dependence of the electrical resistivity is well explained by a semiconductor model where the energy gap, E, varies with temperature through the change of the degree of ordering,... 

Sihcone fluids have good dielectric properties, loss factor, specific resistance, and dielectric strength at normal operating conditions, and the properties vary only slightly with temperature (59,328,350). The properties in combination with relatively low flammabiUty have led to the use of siUcones in transformers and other large electrical appHcations (351). The dielectric constant of a 1000-cSt oil is 2.8 at 30°C and 2.6 at 100°C. The loss factor is low,... 

Fig. 3.6 Electrical resistivity p of Gd3 3CvxS4 for varying compositions x as a function of temperature V. (a) with no external magnetic field applied (b) in the presence of an applied field of 32kOe. Number of carriers (in cm" 3) (1) 5.6 x 10t9 (2) 8.7 x 1019 (3) 1.6 x 1020 (4) 2.5 x 10. From von Molnar and Holtzberg (1973).

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