Wiedemann

To a good approximation, thermal conductivity at room temperature is linearly related to electrical conductivity through the Wiedemann-Eran2 rule. This relationship is dependent on temperature, however, because the temperature variations of the thermal and the electrical conductivities are not the same. At temperatures above room temperature, thermal conductivity of pure copper decreases more slowly than does electrical conductivity. Eor many copper alloys the thermal conductivity increases, whereas electrical conductivity decreases with temperature above ambient. The relationship at room temperature between thermal and electrical conductivity for moderate to high conductivity alloys is illustrated in Eigure 5. 

Eig. 5. The Wiedemann-Eran2 relationship at 20°C between electrical and thermal conductivities of copper alloys having moderate to high conductivities. 

The thermal conductivity of copper having an electrical conductivity of 100% lACS is 391 W/ (m-K) at 20°C. The Wiedemann-Eranz ratio of thermal conductivity and the product of electrical conductivity times absolute temperature are approximately constant. Many copper alloys have increasing thermal conductivity with increase in temperature, whereas electrical conductivity decreases. 

As described above, quantum restrictions limit tire contribution of tire free electrons in metals to the heat capacity to a vety small effect. These same electrons dominate the thermal conduction of metals acting as efficient energy transfer media in metallic materials. The contribution of free electrons to thermal transport is very closely related to their role in the transport of electric current tlrrough a metal, and this major effect is described through the Wiedemann-Franz ratio which, in the Lorenz modification, states that... 

GEPI9924668 R. Fisher, O. Kretschik, T. Schenke, R. Schenkel, J. Wiedemann,... 

Irith Wiegand, Bernd Wiedemann Institut fur Mikrobiologie Biotechnologie, Rheinische Friedrich-Wilhelms-Universitat, Bonn, Germany... 

E. Wiedemann (1876) replaced the heating coil and metal boxes by-metal tubes filled with metal turnings, thus exposing a larger surface to the gas. 

Regnault and Wiedemann, cf. Haber s Thermodynamics of Technical Gas Reactions, Eng. trans., lect. 6. 

The results of Regnault and of E. Wiedemann (which are not in good agreement) indicate that ... 

Measurements in this held have been made by Berthelot and Ogier with nitrogen tetroxide Ann. de Chim. et Phys., [v.], 30, 382 (1883)), and with acetic acid ibid., 400), and some calculations with reference to steam have been made by Nernst Verhandl. Deutsch. Phys. Ges., 15, 313) and Levy ibid., 330), who utilised the vapour-pressure measurements of Holborn and Henning Ann. der Physik, (1906), 21 (1907), 22, 23). Wiedemann had previously observed that the specific heats of ethylbromide, ethyl-acetate, and benzene increase with temperature at about the same rate as that of nitrogen tetroxide at 200°. In the case of steam it was assumed that (i.) the polymerisation is to double molecules... 

The gas chromatograph (GC) resembles the MS in providing both qualitative and quantitative EGA but is significantly slower in operation. The interval between analyses is normally controlled by the retention time of the last component to be eluted from the column such delay may permit the occurrence of secondary reactions between primary products [162]. Several systems and their applications have been described [144,163— 167] sample withdrawal can be achieved [164] without the necessity for performing the reaction in an atmosphere of carrier gas. By suitable choice of separation column or combination of columns [162], it is possible to resolve species which are difficult to measure in a small low-resolution MS, e.g. H20, NH3, CH4, N2 and CO. Wiedemann [168] has made a critical comparison of results obtained by MS and GC techniques and adjudged the quality of data as being about equal. 

Direct kinetic measurements from the changes in diffracted beam intensities with time during heating of the reactant are illustrated in the work of Haber et al. [255]. Gam [126] has reviewed the apparatus used to obtain X-ray diffraction measurements in thermal analysis. Wiedemann [256] has designed equipment capable of giving simultaneous thermo-gravimetric and X-ray data under high vacuum. X-Ray diffraction studies enable the presence, or absence, of topotactic relationships between reactant and product to be detected [92,102,257—260], Results are sometimes considered with reference to the pseudomorphic shape of residual crystallites. 

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