Mechanical characterization and thermal modeling of a MEMS thermal switch

Kevin Richard Crain

This thesis summarizes the research involved in mechanically characterizing a liquid metal micro droplet array thermal switch for use with the P3 micro heat engine under development at Washington State University. The mechanical characterization of the thermal switch was completed with the development of a thermal model that predicts the thermal resistance of the droplet array when in contact with the P3 engine at various loadings. The thermal switch was mechanically characterized statically, dynamically, as well as statistically. Static deformation experiments of single large droplets of 200µm diameters provided a means for validating the mechanical and geometrical aspects of the thermal model, such as force, deflection, and contact diameter. Deflection and contact diameter were measured simultaneously under static conditions with applied forces ranging from 0 to 0.1N. Dynamic droplet array behavior was experimentally observed by visualizing cyclic actuation of the droplet arrays against a rigid, glass surface. Actuation frequencies of 20Hz and 400 Hz were applied to the droplet arrays using a piezoelectric actuator stack, over varying lengths of actuation time, for total cycle spans of one, ten, and one hundred thousand cycles, as well as one million cycles. Statistical characterization was obtained using computer-assisted droplet diameter measurement of digital photographs of the droplet arrays. Total array population was also statistically inferred from sample population data analysis. Analytical models were derived for geometry, mechanical deformation, and thermal behavior. Geometrical relationships between contact radius and deformed droplet height were derived, assuming constant volume and contact angle of the liquid/solid/vapor interface. This was converted into contact diameter and deflection for comparison and validation with experimental measurements. Experimental mechanical deformation data was compared and validated against a model based on capillary pressure of the liquid droplet. This model was used to obtain a corresponding relationship between force and deflection of a deformed droplet. These models were then incorporated into a thermal model which calculates an effective thermal resistance for a network of resistances of the droplets and gas gaps in the array. The geometric and force models were used to calculate the theoretical thermal resistance of every droplet in an array under a specific loading. Forces between 0 and 1N were used to calculate the thermal resistance of a droplet array. Different interstitial gas environments of air, vacuum, and xenon, were applied to the thermal model to investigate possible increases in performance of the droplet arrays.

Mechanical characterization and thermal modeling of a MEMS thermal switch

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