## 3 Results and Discussion
### Transient Performance under High Heat Flux Millisecond Pulses
#### 3.2.3 The Effects of the Heat Pluses' Peak Heat Flux and Duty Cycle  
Combined with the variations in \(T_{\mathrm{max}}\) and the phase change process during a heat pulse cycle shown in Figures 10 and 11, it can be found that after reaching the melting point, the temperature increase rate is small when the underlying liquid fraction \(\theta\) is small, while the temperature increases rapidly when the underlying PCM is almost complete phase change. Therefore, if the high heat flux lasts for a longer time, the buffer effect of the thin-layer PCM will not be obvious. With the same average heat flux and pulse period, the millisecond pulses with a peak heat flux of 300 W/cm\({}^{2}\) and a duty cycle of 0.5 were employed to study the buffer performance of the thin-layer PCM (\(\delta\) = 0.05 mm) under low peak heat flux and large duty cycle pulse heating. As shown in Figure 13, after reaching the periodic state, the temperature fluctuates in the same period as the heat pulses and rises for half of the cycle. \(T_{\mathrm{max}}\) without PCM reaches 64.4 \({}^{\circ}\)C, 2.2 \({}^{\circ}\)C higher than the steady state value of 62.2 \({}^{\circ}\)C and 2.5 \({}^{\circ}\)C lower than the \(T_{\mathrm{max}}\) (66.9 \({}^{\circ}\)C) of high peak heat flux and a short heating period. The use of a thin-layer metallic PCM makes the temperature decrease overall, and the highest \(T_{\mathrm{max}}\) is 59.1 \({}^{\circ}\)C, which is 5.3 \({}^{\circ}\)C lower than that without PCM. At the initial moment of the pulse, the temperature increase rate with PCM is greater than that without PCM due to the lower thermal conductivity of the metallic PCM. When the temperature increases to the melting point, the temperature with PCM increases slower than that without PCM as a result of the latent heat. Then, with the expansion of the phase change region and the increase in the underlying liquid fraction \(\theta\), the temperature growth rate increases. As shown in Figure 14, compared with the high peak heat flux and short-period heating, the average phase change percentage \(\gamma\) decreases, but the phase change extends to more than 1/2 of the thickness in the perpendicular direction, and the region of the PCM bottom with high phase change degree is smaller, showing a better buffering performance. Therefore, low peak heat flux and large duty cycle pulse heating can take full advantage of metallic PCMs. Figure 12: The change processes of heat flux on the upper side of the metallic PCM during a heat pulse cycle: (**a**) 43–45 ms, the duty period with heat flux of 600 W/cm\({}^{2}\) on the chip; (**b**) 45–47 ms, the idle period with zero heat flux on the chip. Figure 14: The phase change processes of metallic PCM during a heat pulse cycle under low peak heat flux and large duty cycle pulses heating. Figure 13: The variations in \(T_{\max}\) and \(\gamma\) with time under low peak heat flux and large duty cycle pulses heating. 2.4 The Comparison of Thermal Contact Performance between the Metallic PCM and Thermal Silicon Grease  
Tight contact between the heating surface and heat-dissipating devices is required for effective heat management. In the studies above, the interface thermal resistance between the heat sink and the heating surface was not taken into consideration. Generally, thermal interface materials (TIMs) are used to fill the roughness of the coupled surfaces while eliminating the air with low thermal conductivity [42]. Thermal silicon grease is one of the traditional TIMs and has been widely used [43], while low melting point metal materials show several advantages owing to their unique properties [44]. The thin-layer metallic PCM embedded between the contact surface of the chips and the heat sink reacts as a TIM exactly when dealing with power surges, improving the thermal contact performance compared to the thermal silicon grease.