Microscopic Evaluation of Electrical and Thermal Conduction in Random Metal Wire Networks

Event Date: March 22, 2017
Authors: R. Gupta, A. Kumar, S. Sadasivam, S. Wali, G.U. Kulkarni, T.S. Fisher, and A.M. Marconnet
Journal: ACS Applied Materials & Interfaces
Paper URL: Link to Full Text
ACS Applied Materials & Interfaces, vol. 9, no. 15, pp. 13703-13712, 2017. doi: 10.1021/acsami.7b00342

Ideally, transparent heaters exhibit uniform temperature, fast response time, high achievable temperatures, low operating voltage, stability across a range of temperatures, and high optical transmittance. For metal network heaters, unlike for uniform thin film heaters, all of these parameters are directly or indirectly related to the network geometry.In the past, at equilibrium, the temperature distributions within metal networks have primarily been studied using either a physical temperature probe or direct infrared thermography, but there are limits to the spatial resolution of these cameras and probes and thus only average regional temperatures have typically been measured. But knowledge of local temperatures within the network with very high spatial resolution is required for ensuring safe and stable operation. Here, we examine the thermal properties of random metal network thin film heaters fabricated from crack templates using high resolution infrared (IR) microscopy. Importantly, the heaters achieve predominantly uniform temperatures throughout the substrate despite of the random crack network structure (e.g., unequal sized polygons created by metal wires), but the temperatures of wires in the network are observed to be significantly higher than the substrate due to significant thermal contact resistance at the interface between the metal and the substrate. Lastly, the electrical breakdown mechanisms within the network are examined through transient IR imaging. In addition to experimental measurements of temperatures, an analytical model of the thermal properties of the network is developed in terms of geometrical parameters and material properties providing insight into key design rules for such transparent heaters. Beyond this work, the methods and understanding developed here extends to other network-based heaters and conducting films including those that are not transparent.