Thermal Coefficient of Electrical Resistance

Borides have metallic characteristics such as high electrical conductivity and positive coefficients of electrical resistivity. Many of them, particularly the borides of metals of Groups 4 (IVB), 5 (VB), and 6 (VIB), the MB compounds of Groups 2(11) and 13(111), and the borides of aluminum and siUcon, have high melting points, great hardness, low coefficients of thermal expansion, and good chemical stabiUty. 

Thermistors are usually made from ceramic metal oxide semiconductors, which have a large negative temperature coefficient of electrical resistance. Thermistor is a contraction of thermal-sensitive-resistor. The recommended temperature range of operation is from -55 to 300°C. The popularity of this device has grown rapidly in recent years. Special thermistors for cryogenic applications are also available [12]. 

However, in most practical cases, the two coefficients of thermal expansion are generally much smaller than the temperature coefficient of electrical resistivity. Therefore, if dimensional variations are negligible, values of the coefficient of electrical resistivity and that of electrical resistance can be assumed to be identical. 

The electronic configuration for an element s ground state (Table 4.1) is a shorthand representation giving the number of electrons (superscript) found in each of the allowed sublevels (s, p, d, f) above a noble gas core (indicated by brackets). In addition, values for the thermal conductivity, the electrical resistance, and the coefficient of linear thermal expansion are included. 

From the interatomic distances the conclusion is to be drawn that the bonds in the hexagonal layers of atoms in these metals are stronger than those between layers. This conclusion is substantiated by the properties of the crystals, which show basal cleavage and have larger values of the compressibility, coefficient of thermal expansion, and electrical resistance in the direction perpendicular to the basal plane than in this plane. Moreover, measurements of the intensities of... 

SiC-BN composites show good thermal conductivity. The electrical resistivity depends on the BN content and can be adjusted from a few Q/cm to more than 1010 Q/cm. When the SiC content of the composites is high, the composites can be used as cutting tools [135]. BN additions reduce the friction coefficient. Thus these composites are used for sliding parts [136],... 

The Seebeck coefficient a and figure of merit Z for B4C-B ceramics as a function of C content are given in Fig. 7 and Fig. 8, respectively. The a was always positive, and its absolute value increases with increasing carbon content except for B4C+5B. a (0.30 0.38 mV/K), whose maxima was observed at 20 at.% C (B4O sintered at 2250 °C, showed opposite tendency of electric resistivity (7X 10 6 X10" Qm), whose minimum was at B4C+8B sample fired at 2250°C. Though the figure of merit Z is evaluated from electrical resistivity, thermal conductivity and Seebeck coefficient, the Z values showed maximum of 2.4 X 10 K at B4C+8B composite fired at 2250 °C. Therefore, the electric resistivity affects more than the Seebeck coefficient. 

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 ... 

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

To determine the efficiency of such a device, let us first consider a simpler model, with only one leg (see Figure 4.2), of electrical resistivity p, thermal conductivity, k and Seebeck coefficient, S. 

Tensile modulus Electrical resistivity(a) Magnetic peimeability Specific heat capacity(a) Thermal conductivity(a) Thermal coefficient of linear expansion(b)... 

Physical characteristics, such as electrical conductivity, thermal coefficient of expansion, and thermal conductivity, are important considerations for selection of materials. For instance, cupronickel (90-10 Cu-Ni) are excellent materials for heat exchanger tubes in thermal desalination plants employing raw seawater, because of their excellent conductivity and corrosion resistance. The choice of material is narrowed down by eliminating low conductivity and low corrosion resistant materials. 

The thermal expansion coefficient, the electric resistivity, and the specific heat of metallic nanosolids or alloys increase inversely with solid size whereas the temperature coefiicient of resistivity and the critical temperature for magnetic transition drop inversely with solid size [46]. The emerging freedom of size... 

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... 

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