Embedded phase change material microinclusions for thermal control of surfaces

Abstract

The performance and lifetime of sensors, microelectronic devices, mini air vehicle components and other systems can benefit from maintenance of a constant temperature profile or a local temperature that does not exceed a pre-selected, critical value during short-term transient loading events. In this work, the utility of surfaces featuring phase change materials (PCMs) encapsulated within micro-reservoirs was evaluated as a passive thermal management system for mitigation of transient temperature spikes. Patterned silicon substrates with 40 μm diameter, 10 μm deep trenches were prepared by reactive ion etching in an Ar/CF4 plasma. The features were filled with an organic phase change material possessing a high latent heat and known to undergo melting at a target operating temperature of approximately 60°C. An infrared microscope was used to produce temporally and spatially resolved temperature maps of the surface during heating. When a constant heat flux of approximately 200Wm−2 was used for uniform heating of the sample area from 50 to 75°C the PCM encapsulated materials demonstrated an isothermal plateau period lasting 5–8s near the PCM melting point. A similar plateau was also observed during cooling below the PCM melting point. From the isothermal time during heating the areal thermal energy storage density was estimated to be approximately 800Jm−2, close to that predicted by the calculated volume of PCM contained within the sample. The effects of PCM inclusions on surface temperature gradients during pulsed laser heating were also investigated. An infrared (980nm) laser at a fixed power and repetition rate was focused into a 1mm diameter beam on surfaces with and without PCM encapsulation for rapid heating (analogous to heating from resistive contacts in microelectronic circuits) of different duration from 0.5 to 5.0s. The surface temperature stayed below the desired temperature limit for 100mW laser pulses lasting up to 2s in duration. Transient temperature profiles of Si/PCM surfaces showed that micro-scale volumes of embedded phase change material stored sufficient quantities of heat to stabilize and otherwise control surface temperatures for potentially useful periods of time in selected applications.