# Passive Thermal Management of Tablet PCs Using Phase Change Materials: Intermittent Operation
## 2 Experimental Setup
### Apparatus  
Figure 1a shows the test stand which held the experimental tablet PC using four 3D printed clamps allowing it to rotate at any angle. As the performance of the tablet PC might be affected by the natural convection occurring within the melted PCM and on the tablet PC's surface, it was necessary to include device inclination as a test parameter. A more complete description of the overall apparatus can be found in reference [15]. A thermal camera (ICI 7320 USB, accuracy level: \(\pm\)2 \({}^{\circ}\)C) was also attached to the stand with a holder housed in a rack and pinion system used to obtain an appropriate focus on the test tablet PC. (The experimental tablet PC used for the study is shown in Figure 1b). It consisted of the original Windows tablet PC (Dell Venue 8 Pro) parts as a substrate (battery, screen, and casing) and had the following dimensions: 216 mm \(\times\) 116 mm \(\times\) 9 mm. The original printed circuit board (PCB) including the CPU of the tablet PC was replaced by a mock PCB made from an FR4 copper clad board, which has similar thermal conductivity to the original PCB (through the plane and in-plane thermal conductivities of 0.343 and 1.059 W/m\(\cdot\)K, respectively [23]). The heat dissipation from the tablet's electronics (mostly the CPU) was applied using a 1\({}^{\prime\prime}\) square (25.4 \(\times\) 25.4 mm\({}^{2}\)) Kapton heater (Omega, Biel Biene, Switzerland, KHLV-101/10) with a rated power density of 10 W/in\({}^{2}\) at 28 VAC. This specific heater is the best one available to replicate the dimensional and power attributes of an actual CPU in the tablet PC. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Properties** & \(n\)**-Eicosane** & **PT-37** \\ \hline \(\Delta_{flu}H\) & 239 \(\pm\) 24 J/g & 206 \(\pm\) 21 J/g \\ \(T_{m}\) & 35.6 \(\pm\) 1.5 \({}^{\circ}\)C & 36.4 \(\pm\) 1.5 \({}^{\circ}\)C \\ \(C_{p,s}\) & 1.8 \(\pm\) 0.2 J/g K for \(T\) = 20 \({}^{\circ}\)C & 1.8 \(\pm\) 0.2 J/g K for \(T\) = 20 \({}^{\circ}\)C \\ \(C_{p,l}\) & 2.3 \(\pm\) 0.2 J/g K for \(T\) = 50 \({}^{\circ}\)C & 2.1 \(\pm\) 0.2 J/g K for \(T\) = 50 \({}^{\circ}\)C \\ \(k_{s}\) & 0.46 \(\pm\) 0.05 W/m K for \(T\) = \(-\)21 to 29 \({}^{\circ}\)C & 0.28 \(\pm\) 0.03 W/m K for \(T\) = \(-\)21 to 29 \({}^{\circ}\)C \\ \(\rho\) & 0.79 kg/m\({}^{3}\) & 0.84 kg/m\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermophysical properties of \(n\)-eicosane and PT-37 [21]. Nine Type-T thermocouples (Omega SA1-T-SC) were used in the mock tablet PC to measure temperatures on the front surface, back cover, and internal components. The locations of these thermocouples in the mock tablet PC are shown in Figure 2a. The front thermocouples are denoted using the letter F, and the letter B for the thermocouples on the back surface. Three encapsulations with different areas and masses were used: Size 1: 152.4 \(\times\) 66 mm\({}^{2}\), 16 g; Size 2: 177.8 \(\times\) 38 mm\({}^{2}\), 11 g; and Size 3: 177.8 \(\times\) 86 mm\({}^{2}\), 25 g; the average thickness was 2 mm for all encapsulations. Different sizes of encapsulation with similar thickness were chosen to determine the effect of size and PCM amount on the thermal behavior of the system. Moreover, while the smallest (Size 2 in Figure 2b) encapsulation is mostly useful to control the temperature of the electronics, Size 1 and Size 3 can help reduce temperature of the battery as well (since parts of these two encapsulations sit on the top of the battery) [15]. However, in the current study the battery has not been considered as a heat source. Figure 1: Photograph of the experimental setup showing (**a**) the inclined test stand and (**b**) the mock tablet PC with Size 2 PCM (phase change material) encapsulation on display. IR: Infrared. Figure 2: _Cont._  
Aluminized laminate film was used as the encapsulation material, and due to its flexible nature, the thickness of the encapsulations varied from a minimum of 1.7 mm to a maximum of 2.2 mm; additional information on the laminate film used can be found in references [22,24].