# Passive Thermal Management of Tablet PCs Using Phase Change Materials: Intermittent Operation
## 1 Introduction
They found that it is reasonable to use PCM to reduce the rate of temperature increase in a mobile device during the transient heating phase. However, they concluded that using a PCM cooling system is effective only for intermittent-use mobile phones and not for continuous usage. In an experimental study, [PERSON] et al. [9] showed that a PCM-filled TES (101 \(\times\) 68 \(\times\) 12 mm\({}^{3}\)) is useful in controlling the temperature of electronic devices. In a more recent study, [PERSON] et al. [10] investigated the cooling performance of paraffin as PCM in both continuous and intermittent operational regime. They showed that intermittent operation with increasing PCM melting-solidification cycle can substantially increase the critical time (defined as time to reach 77 \({}^{\circ}\)C). Breaking down a single 200 min of continuous heat addition cycle into eight consecutive cycles of melting-solidification resulted in a 320% increase of the critical time. [PERSON] et al. [11] evaluated a heat pipe charged with both tricosane and water. Their study showed that while mimicking an actual application with cyclic heat addition, the PCM can provide significantly lower heater temperature (almost 17 \({}^{\circ}\)C reduction). The common factor of the thicker PCM TES units used in all the studies mentioned above challenges the applicability of existing experimental work in practice, as typical tablet PCs are less than 10 mm thick, restricting space for PCMs within the device to only a few millimeters. Only a few papers in the literature deal with such thin systems. [PERSON] et al. [12] approached this issue by trapping PCM (PMCD-32 SP paraffin) in polyethylene in a composite sheet form, referred to as MPCM/polyethylene composite. In one case, an MPCM/polyethylene sheet of 1.60 mm in thickness, which is ideal for installation in a tablet PC in a practical manner, was used. Although the MPCM/polyethylene sheet was successful in reducing the rate of temperature increase during transient heating, they showed that PCM alone is the best option for providing the highest delay before reaching the saturation temperature of approximately 50 \({}^{\circ}\)C for this study. The only other published experimental studies of thermal management of thin tablet PCs using PCM are from the authors' laboratory. [PERSON] et al. [13], using a simplified experimental setup focusing on the thin (<2 mm) PCM TES units, found that they can delay the temperature increase of both the electronics and the cover by up to 34% compared to a case without PCM. Through this study, both continuous and intermittent operations were assessed. Additionally, [PERSON] et al. [14] studied the effect of different PCM melting temperatures on the thermal behavior of tablet PCs. The authors concluded that PCMs with melting temperatures below 39 \({}^{\circ}\)C are most suitable for thermal management of tablet PCs. In a previous study presenting the first approach to the current work, i.e., testing under continuous operation [15], PCM TES unit implementation issues in a real tablet PC were worked out and presented, and experimental results showed that under continuous operation (60 min of continuous heating at 2 W, 4 W, 6 W or 8 W), the temperatures of both the system on a chip (SOC) and the cover were reduced compared to the case without PCM. Consideration of an intermittent regime is of great importance since, during normal usage of tablet PCs, computational loads on the chip vary randomly from time to time based on the user utilization and the type of software, or apps, being used. It is expected that the typical tablet PC user will perform a high-power operation (e.g., online video call, HD recording and editing or power-intensive gaming) for a relatively short duration followed by a period of low usage or even idling of the device, releasing the chip from the heavy computational load and the associated heat generation.