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Research Papers: Micro/Nanoscale Heat Transfer

# A Micro-Insulation Concept for MEMS Applications

[+] Author and Article Information
Rui Yao

Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706

James Blanchard

Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706blanchard@engr.wisc.edu

J. Heat Transfer 131(5), 052401 (Mar 16, 2009) (7 pages) doi:10.1115/1.3084121 History: Received August 24, 2007; Revised November 18, 2008; Published March 16, 2009

## Abstract

Small scale, thermally driven power sources will require appropriate insulation to achieve sufficiently high thermal conversion efficiencies. This paper presents a micro-insulation design, which was developed for a thermionic microbattery, which converts the decay heat from radioactive isotopes directly to electricity using a vacuum thermionic diode. The insulation concept, which is suitable for any small scale application, separates two planar surfaces with thin, semicircular posts, thus reducing conduction heat transfer and increasing the relative radiation heat transfer. In this case, the surfaces are silicon wafers and the columns are SU-8, a photoresist material. The experimental results indicate that this design is adequate for a practical power source concept, and they are supported by a numerical model for the effective thermal conductivity of the structure. The results show that a typical design of $20 columns/cm2$ with a $200 μm$ diameter and a $10 μm$ wall thickness has an apparent thermal conductivity on the order of $10−4 W/m K$ at a pressure of 1 Pa. System models of a thermionic power source indicate that this is sufficiently low to provide practical efficiency.

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## Figures

Figure 11

The apparent thermal conductivity and heat flow contributions as a function of the number of columns at a pressure of 1 Pa

Figure 12

The apparent thermal conductivity and heat flow contributions as a function of the top layer emissivity at a pressure of 1 Pa

Figure 1

Schematic of micro-insulation design (not to scale)

Figure 2

(a) An SU-8 photoresist layer has been deposited on top of the bottom silicon substrate and soft-baked to evaporate the solvent and densify the film before exposure. (b) The SU-8 layer is exposed under UV lights through a photomask and baked to selectively cross-link the exposed portions of the film. (c) The exposed SU-8 is developed using MicroChem’s SU-8 developer to form the designed structure. (d) The top silicon layer with gold deposited on the downward surface is placed on top of the patterned SU-8 structure.

Figure 3

Heat transfer modeling through micro-insulation. Note that Qs represents the total heat conduction through all columns.

Figure 4

Thermal conductivity of air at 400 K as a function of Knudsen number

Figure 5

The apparatus for thermal conductivity measurement

Figure 6

Apparent thermal conductivity of micro-insulation at atmospheric pressure as a function of hot-side temperature

Figure 7

Apparent thermal conductivity of micro-insulation at 6.5 Pa as a function of hot-side temperature

Figure 8

Apparent thermal conductivity as a function of pressure. The cold-side temperature began at room temperature and reached a steady state temperature of approximately 50°C before the apparent thermal conductivity was determined.

Figure 9

The apparent thermal conductivity and hot-side temperature as a function of pressure

Figure 10

The fraction of heat flow contribution from radiation, gas conduction, or solid conduction as a function of pressure

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