## 1 Introduction  
Portable electronic devices, such as tablet computers and smartphones, are now immensely popular. It is estimated that the number of tablet users around the world was 591 million in 2015 while the forecast for 2018 was 1.43 billion, nearly a threefold increase [1]. Tablet computers and smartphones are devices with 5- to 14-inch displays and touch-screen interfaces that run a lightweight operating system, such as Apple iOS, Android OS, or Windows [2]. Nowadays, these devices can execute sophisticated applications, including video editing, 3D gaming, and on-demand access to the internet. These applications require high-performance components capable of processing more data, more rapidly. Particularly, the requirements for the central processing unit (CPU) and the graphic processing unit (GPU), which are integrated into the system on a chip (SoC), have increased substantially [3]. While the performance capability is increasing, the thickness of the tablet computersand smartphones is decreasing. The newest tablet generation of Apple (iPad Air 2) and Samsung (Galaxy Tab S2) have an overall thickness of only 6 and 5.7 mm, respectively [4]. Due to compact construction, these devices do not have any active cooling parts (heat sinks and fans), which are found in more spacious desktop personal computer (PC) or notebooks. Therefore, thermal management and, in particular, the cooling capability have become a serious concern for current tablet computer and smartphone manufacturers [5]. A modern SoC contains over a billion transistors. Failure mechanisms, such as dielectric breakdown, are exponentially dependent on temperature [6]. Moreover, the leakage power consumption of these devices, which is the result of an unwanted subthreshold current in transistors when turned off, also exponentially increases with temperature [7]. The probability of thermal runaway processes in Li-ion batteries is increased under abuse conditions, such as overheating [8]. This all points to the need for improved thermal management in handheld electronic devices to avoid overheating of internal electronic components. The temperature increase of the internal electronic components is a serious concern, but the resulting temperature increase of the outer surface is also problematic. Portable electronic devices are in direct contact with the consumer's hands. According to the investigations performed by [PERSON] [9], cover temperatures above 43 to 45 \({}^{\circ}\)C lead to users' discomfort when touching the device. Accordingly, the thermal management system of tablet computers and smartphones needs to address both a comfortable surface touch temperature and maximum temperature limitations of critical power-consuming electronic components. An interesting approach that could lead to a reduction of peak temperatures and therefore increased time of operation of electronic components is based on phase change materials (PCMs) integration into the design of tablets [10; 11; 12]. Typically, portable electronic devices operate in transient cycles, meaning that heat is produced during short periods of demanding use, followed by longer periods of idling or less demanding use. PCMs will absorb heat dissipated from the electronics through a solid-liquid phase change. From the isothermal phase transition, this heat will be stored by the PCM over a small temperature range. It would be exchanged to the environment later when the device is idle. A delay of the overheating and reduction of the maximum temperature of the electronics and cover could be achieved. With PCMs possessing large thermal energy storage densities, small amounts of PCMs would be needed to reach the requirement level of thermal management in these compact handheld electronic devices, with no integration of any moving parts. Studies have been published where the use of PCM as a heat sink of electronic components has been investigated.