Rectifying and thermocouple junctions based on Portland cement
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Rectifying and thermocouple junctions have been achieved using electrically dissimilar Portland cement pastes. The preferred junction is a pn-junction involving steel fiber cement paste (n-type) and carbon fiber cement paste ( p-type). For this junction, the thermocouple sensitivity is 70 V/°C.
I. INTRODUCTION
Junctions between electrically dissimilar materials are used to provide electric current rectification (as in the case of pn-junctions) and to provide thermocouples. Rectifying junctions are useful as diodes for electric circuits. Various junctions can be used together to provide transistors. They are used on semiconductors, mostly commonly silicon, as the doping of a semiconductor gives n-type and p-type materials for making pn-junctions. Thermocouple junctions are useful for temperature sensing. They require dissimilar materials. One example of dissimilarity is that one is n-type and the other is p-type. A thermocouple junction does not have to be a pn-junction, though a pn-junction is a junction of electrically very dissimilar materials and can serve as a relatively sensitive thermocouple junction. The function of a thermocouple junction is based on the Seebeck effect that occurs in each of the two materials making up the junction. The more dissimilar are the two materials, the greater is the thermocouple sensitivity. Hence, the phenomenon behind a thermocouple is not an interfacial phenomenon but is associated with the difference in bulk properties between the two materials. On the other hand, the function of a rectifying junction involves an interfacial phenomenon, as the junction gives rise to a contact potential, which is key to the rectifying ability of the junction. Therefore, the quality of the interface at the junction is important to a rectifying junction but is relatively unimportant to a thermocouple junction. Thermocouple materials are commonly metals and semiconductors with large values of the absolute thermoelectric power (i.e., large Seebeck effect). Metals are most commonly used because of their availability in the form of wires and the ease of joining metal wires by welding to form a junction. In sharp contrast to the metals and semiconductors widely used for rectifying and thermocouple junctions, this paper uses materials based on Portland cement. J. Mater. Res., Vol. 16, No. 7, Jul 2001
Cement is a low-cost, mechanically rugged, and electrically conducting material which can be rendered n-type or p-type by the use of appropriate admixtures, such as short carbon fibers (which contribute holes) for attaining p-type cement and short steel fibers (which contribute electrons) for attaining n-type cement.1–5 (Cement itself is weakly n-type in relation to electronic/ionic conduction.1) The fibers also improve the structural properties, such as increasing the flexural strength and toughness and decreasing the drying shrinkage.6 –13 Furthermore, cement-based junctions can be easily made by pouring the dissimilar cement mixes side by side. In addition, since cement is a structure material, cem
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