Anisotropic heat transfer prediction of multiscale wires using pulse laser thermal relaxation technique Jin W. Tan, Yue Cheng, Denis S.G. Yap, Feng Gong, Son T. Nguyen, Hai M. Duong ⇑ Department of Mechanical Engineering, National University of Singapore, Singapore article info Article history: Received 27 June 2012 In final form 7 October 2012 Available online 24 October 2012 abstract A theoretical model is developed that predicts the thermal characterization of multiscale wires using the pulse laser thermal relaxation technique. This into account anisotropic heat transfer and radiation heat lost to the surroundings. The simulation results have better agreement with the experimental results than previous models. Using the validated model, the heat transfer characteristics of multiscale wires with various morphologies (radius of 15–25 lm, length of 500–1000 lm, radial thermal conductivities of 0.05–5.00 W/mK, axial thermal conductivities of 10–2000 W/mK) and experimental conditions (laser power outputs of 20–50 kW and laser pulse width of 5–9 ns) are studied. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Technology has evolved significantly in recent years and more technological products are focusing on being smaller, lighter and better. In the electronic industry, heat dissipation is gaining in- creased importance due to the increased levels of dissipated power [1]. Hence, it is important to source for materials which are able to conduct heat well. This justifies the recent significant research ef- forts on materials with excellent thermal conductivities such as carbon nanotubes (CNTs) to replace current materials. CNTs have been extensively studied since they were first discovered by Iijima in 1991 [2]. CNTs have great strength, light weight, high stability and excellent electrical and heat conductivities [1,3–5]. Due to their size, CNTs are ideal materials for nano-scale devices. In this Letter, specific focus is made on the excellent thermal conductivity of CNTs. The thermal conductivity of CNTs and CNT bundles has been reported to range from 8 to 3000 W/mK [1,3–15]. The wide range of thermal conductivity is due to the morphology of the CNTs (multi-walled or single-walled, length, diameter of bundle). There are various methods for measuring the thermal conduc- tivity of the CNT. One of the earliest experimental methods of mea- suring their thermal conductivity is the 3x technique [6–8,10].A CNT sample is connected between two metal bases forming a bridge structure. A constant-amplitude AC current is then passed through the CNT which will create a temperature fluctuation at 2x, where x is the frequency of the AC current. Subsequently, a third-harmonic voltage signal would be induced by the tempera- ture fluctuation and this voltage signal would be recorded. For this method, the thermal conductivity of the CNT can be determined by selecting an optimal x whereby the voltage signal can be clearly distinguished. However, there are several limitations of the 3x technique. The 3x method requires the CNT to have a linear cur- rent to voltage relationship within the applied AC voltage range. It is also difficult to carry out the 3x method for wire samples with low thermal conductivity as the heat transfer between the two bases will become less accurate to measure [12,15]. To overcome the limitations of the 3x method, the transient electrothermal technique (TET) has been developed [9,12,15]. For the TET technique, a CNT sample is suspended between two copper electrodes which are excellent heat sinks. A step DC current is then applied through the CNT which results in a temperature rise due to electrical heating. This temperature change would result in a resis- tance change and hence, a change in the measured voltage. The change in voltage is directly proportional to the temperature change. In this way, the thermal conductivity of the CNT can be determined from the voltage evolution curve which has the same profile as the temperature evolution curve. However, the TET tech- nique is not accurate as the slow rising time would make it difficult for experimentalists to measure short wires with relatively high thermal conductivity like CNTs [13]. This is due to the short char- acteristic time of heat transfer which is comparable to the rising time of the electric current. Thus, the pulse laser thermal relaxation (PLTR) technique is developed to complement the TET technique [13]. The experiment setup of the PLTR method is similar to that of the TET method. The main difference will be the usage of a laser, operating in a pulsed mode, to heat the sample instead of the electrical heating in the TET technique. Nonetheless, a small DC current would still be fed through the CNT so that the temperature change would lead to a change in the resistance. The thermal conductivity of the CNT is 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.021 ⇑ Corresponding author. E-mail address: mpedhm@nus.edu.sg (H.M. Duong). Chemical Physics Letters 555 (2013) 239–246 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett