# Passive Thermal Management of Tablet PCs Using Phase Change Materials: Intermittent Operation
## 1 Introduction  
Tablet computers (tablet PCs) have grown in use and capabilities in recent years and have several useful features that require high-density power sources to operate. However, the thermal management of these tablet PCs still relies on very basic passive cooling systems (internal heat spreaders, external natural convection). These passive cooling techniques can fall short in maintaining the required internal temperature of the tablet PC. Therefore, thermal management in a compact package has become a major concern for portable electronics manufacturers, mostly because active cooling is not an option in portable handheld devices. Localized high temperatures in handheld devices can lead to many potential threats towards the user or the device itself. Several studies [1; 2; 3] showed that handheld devices should maintain external operating temperatures below 42 \({}^{\circ}\)C-45 \({}^{\circ}\)C for usercomfort. In addition to user comfort, the longevity of electronic devices is also directly related to operating temperature. One study [4] revealed that 1 \({}^{\circ}\)C increase past operating temperature limit results in a 4% reduction of the reliability of electronic devices. More recently, users suffered serious injuries due to the explosion of smartphone batteries (which can happen to tablet PCs as well) caused by excessive device temperature. A passive thermal management system is optimal for portable electronics cooling, as no extra power is required to operate the system with the additional advantage of silent operation. Phase change materials (PCMs) have gained significant attention for passive cooling of electronics due to their low cost, the wide range of physical properties they offer which facilitate flexibility in design, and their reliable service. PCMs have been studied for the temperature management of numerous types of larger scale electronics, more often than not combined for various heat sink designs. Those studies are not relevant to the work presented here since their sizes were at least one order of magnitude greater than what can be employed in thin portable electronics, and the power dissipation studied was also typically one to two orders of magnitude greater. The reader can find references to this research in the open literature. Of greater importance to this work, a few studies have successfully implemented PCM-based thermal energy storage (TES) units for temperature stabilization in portable electronics having sizes approaching today's tablet PCs. [PERSON] et al. [5,6] showed both experimentally and numerically that paraffin wax is useful for the thermal management of electronic devices. In their earlier study [6], the authors considered both continuous and intermittent heat addition to the experimental system (a rectangular aluminum enclosure with 128 mm \(\times\) 10 mm internal cavity). Paraffin wax (PCM) provided a good transient response of the system for both continuous and intermittent usage. Since lower temperatures of the heater and external casing were encountered during intermittent operation, the authors indicated that for practical design, consideration of intermittent operation can reduce the TES unit weight and size greatly. In their later work [5], they used PCM-filled heatsinks (minimum height 10 mm) on top of a dummy electronic system. They reported improvement of thermal performance over the baseline condition by using PCM filled heatsinks. No significant influence of the inclination of the device was reported in their studies. [PERSON] and [PERSON] [7] experimentally investigated a heat storage unit having cavities (depth not reported) filled with \(n\)-eicosane. They reported that the PCM can stabilize the temperature of the system and extend the operation time. In another investigation, [PERSON] et al. [8] used three heatsinks (21 mm height, with and without fins) filled with \(n\)-eicosane and tested those under different usage loads.