Development of a multiple layer vacuum insulation chip

Abstract

Recent advent of micro thermal devices such as lab-on-a-chip and micro heat pump necessitates development of highly effective insulation chips or layers. This thesis reports the development of a vacuum insulation chip (VIC) having very low effective thermal conductivity and very small thickness. The VIC developed in this work is a multi-layer insulation chip with two vacuum gaps between three plates. A staggered array of support legs in the vacuum layers withstands the atmospheric pressure and increases the heat flow path line. Also, an LCD glass plate is selected by considering the gas permeation, low thermal conductivity, high strength and very small thickness. A complete analysis of radiative transfer in multi-layered structure is performed using electromagnetic wave theory. As the result, fifty nanometer thickness aluminum coating on both sides of the middle LCD glass plate and $5\mu m$ vacuum gaps are found to be enough to virtually eliminate the radiation heat transfer. Design of VIC involves trade-offs between the heat conduction through the multi-layer structure and the mechanical strength. An approximate procedure to determine the actual dimensions is proposed. The results are in reasonable agreement with more refined results using commercial numerical codes. Based on these results, a VIC of $32 \times 32 \times 1.88 mm^3$ is fabricated through glass etching and metal coating. Vacuum bell jar system for evacuation of VIC and guarded hot plate (GHP) for measuring the thermal conductivity of VIC are constructed. The latter apparatus is validated with a reference specimen of 10 mm thick polystyrene. The chip shows effective thermal conductivities of 0.0015 and 0.001 W/m·K at vacuum levels of 1.33 and 0.24 Pa, respectively. The total uncertainty in $\it{k}$ is less than 7.2% for all cases at a 95% confidence level. The results prove the practical applicability of VIC and the design methodology proposed in this thesis.