Standard Seebeck coefficient

We will now discuss the Seebeck coefficient in the system Fej xMxO (M — Ti). The experimental result is shown in Figure 18.10. In the case of X = 0 the standard Seebeck coefficient is very high because of a large energy gap according to Eq. (2). The Ti-substitution decreases an absolute value of the Seebeck... 

Several zirconias doped with yttria in the concentration range 4-10 mol Vo were investigated [25]. At temperatures and oxygen partial pressures at which unit oxide transport number prevails [21], the standard Seebeck coefficient of the materials was found to be independent of temperature. This allows the application of Equation (26-24), a linear form of Equation (26-21) ... 

The quantity (d Th,m/dr) in the integral of Equation (26-36) is defined standard Seebeck coefficient and is a characteristic property of a glass-forming melt. It is determined by means of zirconia microelectrodes (Figure 26-14), which eliminate temperature-dependent redox potentials inherently included in the emf if platinum electrodes were applied to these measurements [12]. The cell scheme for measuring standard Seebeck coefficients according to Figure 26-14 is... 

Figure 26-15. Relative potentials of a Zr02 electrode with 1 bar oxygen partial pressure in glass-forming melts with temperature-independent (A, B) and temperature-dependent (C, D) standard Seebeck coefficients. Reference potential 9, = 0. (a) Fiolax klar (b) (Na20)oo7 (K20)o 78(CaO)o, (Si02)o.737 (c) (Na20)o,s6(CaO)ojB7(Si02) .737 (d) BK7 (e) phos-phate-ba optical glass.

Standard Seebeck coefficients of oxidic glass-forming melts were found to be between —0.3 and —1.0 mV/K in the temperature range 800-1600 °C and to be dependent on or, in some cases, independent of temperature. Examples are given in Figure 26-15. Melts with temperature-independent standard Seebeck coefficients allow the application of linear equations such as Equation (26-24) for calculating standard thermoelectric emfs of the melt for an application in Equation (26-35). For melts with temperature-dependent standard Seebeck coefficients, however, the thermoelectric emf of the melt, Tb.m ( c m) according to Equa-... 

The final example is concerned with standard Seebeck coefficients of glass melts, which were accessible after zirconia electrodes had been developed [12]. Continuously working glass melting units are characterized by nonisothermal operation, and metals, eg, platinum-type metals, contacting the melt and often short-circuited are subject to electrode reactions, eg, generation and consumption of oxygen, which can indirectly impair the production of the melters. Thermoelectric emfs of such nonisothermal cells. 

Figure 26-20. Practical signincance of thermoelectric potentials. The relative potentials, tRpt< of a platinum electrode in melts satisfying Equation (26-33) were obtained from standiud thermoelectric potentials, of a zirconia electrode and temperature-dependent emfs, E, of cell (VI) according to Equation (26-37). The temperature dependence of Pp, is determined by the standard Seebeck coefficients of the melts and is positive (a), negative (b), and, depending on the temperature, both negative and positive (c).

In agreement with the literature, standard Seebeck coefficients of yttria-doped zirconias are independent of temperature and proportional to the molar yttria concentration of the ceramics [10, 12]. For the material most frequently applied during this investigation they can be represented by the linear relationship [11] ... 

First, zirconia tube electrodes employ a reference gas (and an outside determined source of oxygen). An advantage is thus a temperature-independent reference oxygen partial pressure and a strictly isothermal functioning. A disadvantage, on the other hand, is their temperature shock sensitivity due to the fragility of the ceramic tube. Zirconia tube electrodes are thus the choice in research and development and even the only choice if standard Seebeck coefficients of glass melts are to be determined [17]. 

To measure the Seebeck coefficient a, heat was applied to the sample which was placed between the two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature difference (JT <2 E) between the both ends of the sample. The Seebeck coefficient a of the compound was determined from the E/JT. The electrical resistivity p of the compound was measured by the four-probe technique. The repeat measurement was made rapidly with a duration smaller than one second to prevent errors due to the Peltier effect [3]. The thermal conductivity k was measured by the static comparative method [3] using a transparent Si02 ( k =1.36 W/Km at room temperature) as a standard sample in 5x10 torr. 

Electrical resistivity measurement adopted conventional four probes method. Seebeck coefficient was measured by the standard DC method. Thermal conductivity k was calculated from density, specific heat, and thermal diffiisivity. Specific heat measurement was carried out by use of a differential scanning calorimeter (DSC model 8230, Rigaku, Japan) compared with a standard material of a -AI2O3. The values of thermal diffiisivity obtained from a differential phase analysis of photo-pyroelectric signal (AL- A 0 analysis) [9]. All measiu ements were done at room temperature. 

Thermocouple Materials. Thermoelectric properties of different materials can be represented by their respective Seebeck coefficients. Usually this is done with reference to Eq. 16.20, assuming a standard reference material (material B) and a standard reference temperature T0 (usually 0°C see Fig. 16.17). 

Some environmental limitations of the standard thermocouple materials compiled by ASTM [36] are reproduced in Table 16.11. The thermal EMF of standard thermoelements relative to platinum is shown in Fig. 16.20 [36]. Seebeck coefficients (first derivative of thermal EMF with respect to temperature) for each of the standard thermocouples as a function of temperature are tabulated in Table 16.12. 

The voltages generated by thermocouples used for temperature measurement are generally quite small being on the order of tens of microvolts per °C. Thus, for most biomedical measurements where there is only a small difference in temperature between the sensing and reference junction, very sensitive voltmeters or amplifiers must be used to measure these potentials. Thermocouples have been used in industry for temperature measurement for many years. Several standard alloys to provide optimal sensitivity and stability of these sensors have evolved. Table 2.5 lists these common alloys, the Seebeck coefficient for thermocouples of these materials at room temperature, and the full range of temperatures over which these thermocouples can be used. 

A summary of the common standardized thermocouples is given in Table 1. Besides the thermoelectric properties of the Seebeck coefficient, the thermal and electrical conductivities are important for the sensitivity and time response of the thermoelectric flow sensor. 

Type E Thermocouples. The ASTM designation type E indicates a thermocouple pair consisting of a Ni-Cr alloy and a Cu-Ni alloy. This type of thermocouple has the highest Seebeck coefficient, 5, of the three ASTM standard thermocouple types commonly used at low temperatures, types E, K, and T. Also, both elements of this thermocouple have low thermal conductivity, reasonable homogeneity, and corrosion resistance in moist atmospheres. This type is the best thermocouple to use for temperatures down to about 40 K. 

Rectangular shape samples with typical sizes of 10 mm x 4 mm x 1.5 mm were employed to simultaneously measure electrical conductivity o and Seebeck coefficient S by the standard four-probe methods in a He atmosphere (ULVAC-RIKO ZEM-3). Thermal conductivity k was calculated using the equation k = apC from the thermal diflfusivity a obtained by a flash diflusivity method (LFA 457, Netzsch) on a round disk sample with diameter of about 13 mm and thickness of 2 mm, and specific heat Cp was determined by a differential scanning calorimeter method (DSC Q2000, Netzsch). 

Akin et al. (1998) An integrated thermopile structure with high responsivity using any standard CMOS process by T. Akin, Z. Olgun, O. Akar, and H. Kulah. Sensors Actual A66, 218-224. This describes a relatively modern application using a CMOS process they report a net Seebeck coefficient of 150 35 pW/K, responsivity of 49.8 V/W, andD = 5.15 x 10 cm Hz / W for their two-arm bridge structure. Their article references other CMOS processes. 

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