© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION P. C. Sun, Y. L. Wu, J. W. Gao, G. A. Cheng, G. Chen, and R. T. Zheng* Room Temperature Electrical and Thermal Switching CNT/ Hexadecane Composites P. C. Sun, Y. L. Wu, Prof. G. A. Cheng, Prof. R. T. Zheng Key Laboratory of Radiation Beam Technology and Materials Modification of Ministry of Education College of Nuclear Science and Technology Beijing Normal University Beijing 100875, P. R. China E-mail: rtzheng@bnu.edu.cn Prof. J. W. Gao Institute for Adv. Mater.(IAM) South China Academy of Advanced Optoelectronics South China Normal University Guangzhou 510006, China Prof. G. Chen Department of Mechanical Engineering Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge Massachusetts 02139, USA DOI: 10.1002/adma.201302165 Temperature regulation of electrical and thermal conductivi- ties of materials has attracted extensive attention because of the diverse applications, such as sensors, [1] smart switches, smart building, [2] thermal energy storage [3] and so on. Electrical con- ductivity (EC) regulation could be realized by metal-insulator (M-I) transitions which are widely observed in condensed- matter systems, such as: organic films, [4,5] chalcogenides, [6] metal oxides [7–10] and perovskites. [11,12] Most of the M-I mate- rials have relative high transition temperatures, which the lowest transition temperature among M-I materials is 68 °C (VO 2 ). Although the transition temperature could be decreased by doping, [1] the corresponding process will rise the cost and decrease the temperature coefficient. Furthermore, tempera- ture regulation of thermal conductivity (TC) is much more dif- ficult than EC because of the little variation of TC during the solid-state phase transitions. [13] Materials with EC and TC regu- lating properties near room temperature have great potential applications in daily life. Liquid-solid phase transition materials have the properties to regulate thermal conductivities at phase transition temper- ature, but usually cannot cause a metal-insulator transition. Nanoparticles with high EC and/or TC can be added into liq- uids to increase the EC and/or TC contrast between the liquid and solid states. [14] In the process of liquid freezing, particles are squeezed towards grain boundaries. [15,16] The internal stress generated during freezing improves the contact among particu- lates, increasing the EC and/or TC of the composites. When the solid remelts, particles will restore the chaotic distribution, the EC and/or TC of the composites will decrease simultaneously. By combining these effects, novel switch composites with large EC and TC variations in a narrow room temperature range can be achieved. Base on the above principle, graphite/hexadecane composites with 2 orders of magnitude EC and 3 times of TC variations at 18 °C have been developed. [14] However, the EC contrast of the graphite composites just changes two orders, far less than that of practical PTC thermistors (above four orders). Theoretically, EC contrast could be improved by decrease the volume fraction of graphite flakes, [16] which will makes percola- tion network broken more easily during phase change. How- ever, the liquid composites will be unstable when the addi- tive is dilute. Furthermore, the graphite flakes tend to stick together under the repeated squeeze of the hexadecane crystal, which is hardly to be re-dispersed because of strong Van der Waals interaction. The switching properties of composites will degrade as well. CNTs have attracted great attention in com- posites area because of their high electrical and thermal con- ductivity, remarkable mechanical properties, low density and the unique one-dimensional structures. [17] Using CNTs substi- tute for graphite flakes may result in lower concentration and higher EC contrast composites. In this communication, we first report the functionalized CNTs/hexadecane switching compos- ites with 5 orders of EC variations and 3 times of TC variations around the phase change point of the hexadecane (18 °C). Figure 1 is the schematic diagram of the switching process in composites. In order to enhance the EC contrast of the com- posite, we use functionalized CNTs instead of pure CNTs. By the surface modification with functional groups, the strong Van der Waals interaction among CNTs can be reduced, which makes the CNTs or CNT clusters separate from each other and the composites are almost insulator (Figure 1a). During the freezing of hexadecane, the needle-like hexadecane crystals grow anisotropically (Figure 1b). The strong stress generated by the crystals growth compresses the CNTs contacting with each other and forms a conductive percolation network, the composite turns to be conductor. When the frozen hexadecane remelts, the pressure on the CNTs is released and the CNTs are separated from each other and re-disperse in the liquid again because of the steric effects of functional groups [18] and the liquid convection (Figure 1c). The broken of conductive net- work results in low EC in composite again, which will cause the large EC contrast ratio of the composite. TC has the similar switching property. The composites will have good reversibility in the subsequent repeat freezing and melting circulation if the CNTs or CNT clusters have good re-dispersion property. According to the above strategy, we use the short-cut multi-walled carbon nanotubes (MWCNTs) to fabricate the composites, which could avoid serious aggregation during the freezing of hexadecane. To get the functionalized MWCNTs(F-MWCNTs), the common oxidation strategy is fol- lowed to introduce carboxylic acid groups on MWCNTs, [19,20] then the functionalized F-MWCNTs are produced by thermal Adv. Mater. 2013, DOI: 10.1002/adma.201302165