Communication Enhancing the Seebeck effect in carbon fiber-reinforced cement by using intercalated carbon fibers Sihai Wen, D.D.L. Chung* Composite Materials Research Laboratory, State University of New York at Buffalo, Box 604400, Furnas Hall, Buffalo, NY 14260-4400, USA Received 24 January 2000; accepted 13 June 2000 Abstract The absolute thermoelectric power of carbon fiber - reinforced cement paste was rendered as negative as  17 mV/ °C by using bromine - intercalated carbon fibers, which had a high concentration of holes. The corresponding paste with pristine carbon fibers exhibited absolute thermoelectric power as negative as  0.8 mV/ °C only. D 2000 Elsevier Science Ltd. All rights reserved. Keywords: Fiber reinforcement; Cement paste; Electrical properties; Silica fume; Thermoelectric 1. Introduction The Seebeck effect is a thermoelectric effect that is the basis for thermocouples. Its occurrence in cement - based materials is potentially valuable for the use of concrete as a temperature sensor, which is needed for thermal control. By using a structural material as a sensor, the need for attached or embedded sensors is eliminated, thereby reducing cost, enhancing durability, increasing the sensing volume and avoiding mechanical property degradation, which occurs in the case of embedded sensors. The Seebeck effect has been reported to occur in carbon fiber-reinforced cement [1±3]. The absolute thermoelectric power is positive (2.0 mV/ °C) in the absence of fibers, but becomes negative (as negative as  0.76 mV/ °C) in the presence of a sufficient amount of carbon fibers [3]. The higher the carbon fiber content, the more negative is the absolute thermoelectric power. This is because the cement matrix contributes to electron conduction, whereas the carbon fibers contribute to hole conduction, such that the two contributions are equal at the percolation threshold [3]. For practical use of the Seebeck effect, a much larger magnitude of the absolute thermoelectric power is desirable. Attaining this by increasing the fiber content is not practical because the workability and compressive strength decrease with increasing fiber content. In this paper, this has been attained by using bromine-intercalated carbon fibers. Intercalation is a chemical reaction involving the inser- tion of a foreign species called the intercalate (bromine in this case) between the graphite layers, thereby forming an intercalation compound. The charge transfer between the intercalate and the carbon host results in a large increase in the carrier concentration and the material becomes metallic. In the case of bromine as the intercalate, a hole metal results, since bromine is an electron acceptor (accepting electrons from the carbon host) [4±14]. Thus, by using bromine- intercalated carbon fibers in cement, the hole conduction is enhanced and the absolute thermoelectric power becomes as negative as  17 mV/ °C, as reported here. 2. Experimental methods 2.1. Materials The carbon fibers (Thornel P-100) were mesophase pitch-based, unsized and of length  5 mm, as obtained from Amoco Performance Products (Ridgefield, CT). This type of carbon fiber is quite graphitic, thereby allowing intercalation to occur. In contrast, the isotropic pitch-based carbon fiber used in previous work [3] is amorphous and thus cannot be intercalated. * Corresponding author. Tel.: +1 - 716 - 645 - 2593 ext. 2243; fax: +1 - 716 - 645 - 3875. E-mail address: ddlchung@acsu.buffalo.edu (D.D.L. Chung). 0008-8846/00/$ ± see front matter D 2000 Elsevier Science Ltd. All rights reserved. PII:S0008-8846(00)00341-0 Cement and Concrete Research 30 (2000) 1295 ± 1298