C. D. Richards

Design, fabrication and testing of the P3 micro heat engine

The development and testing of a micro heat engine is presented. For the first time the production of electrical power by a dynamic micro heat engine is demonstrated. The prototype micro heat engine is an external combustion engine that converts thermal power to mechanical power through the use of a novel thermodynamic cycle. Mechanical power is converted into electrical power through the use of a thin-film piezoelectric membrane generator. This design is well suited to photolithography-based batch fabrication methods, and is unlike any conventionally manufactured macro-scale engine.

Evaluation of contacts for a MEMS thermal switch

In this paper we present the evaluation of contact materials for a MEMS thermal switch. The contact materials considered were liquid-metal micro-droplets and aligned carbon nanotube arrays. Five switch configurations were tested. The thermal interfacial resistance of the switches was measured. The liquid metal switch had a minimum thermal interfacial resistance of 1.1 mm2 K W−1. The minimum thermal interfacial resistance of the best carbon nanotube thermal switch was 20.4 mm2 K W−1. The ratio of the switch off to on thermal resistance was measured for each configuration. Based on these measurements, a switch incorporating liquid-metal droplets is found to be superior to switches incorporating aligned carbon nanotubes.

Efficiency of energy conversion by piezoelectrics

The efficiency of energy conversion by piezoelectric devices depends upon the quality factor Q and electromechanical coupling coefficient k2. In this study the efficiency Q and k2 are measured for a piezoelectric cantilever, a piezoelectric stack, and a micromachined piezoelectric membrane. The experimental values are compared to a model which predicts efficiency at resonance in terms of k2 and Q. Results of the experiment and model are within 2% of each other.

Power production by a dynamic micro heat engine with an integrated thermal switch

Mechanical power production by a dynamic micro heat engine with integrated thermal switch is demonstrated. A microengine operated from a constant heat source of 60 °C is shown to produce 350 µW of mechanical power. Employing an active thermal switch to control heat rejection from the microengine enables power to be increased to 2500 µW. Power consumption by the thermal switch is shown to be minimized by operating the cantilever switch at its resonant frequency. Thermal switch power requirements can be reduced to less than 20 µW for operational speeds up to 100 Hz.

Evaluation of a thermal interface material fabricated using thermocompression bonding of carbon nanotube turf

In this work a thermal interface material fabricated by thermocompression bonding of vertically aligned carbon nanotube turf (VACNT) to metallized substrates was characterized. The VACNT structure was fabricated onto silicon substrates using chemical vapor deposition. The structures were then transferred to metallized substrates using thermocompression bonding. The resulting structure consisted of VACNT turf sandwiched between two layers of Au. Two configurations of VACNT, full coverage and patterned, were fabricated and tested. In addition, the thermal interface resistance of structures at intermediate steps in the thermocompression bonding process were measured. For the full coverage turf a thermal interface resistance of 1.082 cm(2) degrees C W(-1) at an applied load of 1 N was measured, while a thermal interface resistance of 0.044 cm(2) degrees C W(-1) at a load of 1 N was measured for the patterned turf configuration.

Characterization of a dynamic micro heat engine with integrated thermal switch

Progress toward the realization of an external combustion dynamic micro heat engine is documented. First, the development of a thermal switch suitable to control heat transfer to and from the micro heat engine is described. Second, the integration of a thermal switch with an engine is detailed. The thermal switch is shown to be an effective means to control heat transfer into the engine from a continuous heat source and out of the engine to a continuous heat sink. The use of the thermal switch is shown to enable engine cycle speeds up to 100 Hz, engine efficiencies up to 0.095% and power output up to 1.0 mW. The internal irreversibility of the engine is measured to be 23%.

A facility for characterizing the dynamic mechanical behavior of thin membranes for microelectromechanical systems

A bulge testing system capable of applying static and dynamic loads to thin film membranes is described. The bulge tester consists of a sealed cavity, filled with a fluid, bounded on the bottom by a circular stainless steel diaphragm and on the top by the thin film membrane of interest. An actuator is used to apply either a static or a periodic force to the stainless steel diaphragm. The force is transmitted through the water to the thin film membrane. This facility provides for both accelerated lifetime testing and simulated service environment testing. The thin film membranes tested are composite stacks consisting of thin films of silicon, glass, metallic electrodes, and lead-zirconate-titanate. Pressure and deflection of a membrane are acquired simultaneously during loading. An image capture system coupled with an interferometer provides the means to capture interferograms of deflected membranes during both static and dynamic testing conditions. Images are then postprocessed to construct deflection ver...

A dielectric liquid contact thermal switch with electrowetting actuation

We present the design, fabrication and characterization of a new kind of MEMS thermal switch based on electrowetting actuation of a dielectric liquid contact. The thermal switch consists of a thin layer (30–120 µm thick) of a dielectric liquid, such as glycerin or water, squeezed between two silicon dies. The gas pressure in the gap can be varied from ambient down to 0.6 T. The switch operates by changing the conductive path between the two silicon dies by moving the thin layer of dielectric liquid using electrowetting. The result is a bi-stable thermal switch that can change between a low thermal resistance state and a high thermal resistance state. Electrowetting measurements indicate switching time on the order of 2–40 s with switching time increasing as gap width decreases. The power required to switch states is less than 2 mW. Heat transfer measurements indicate thermal resistance ratios of up to ROFF/RON = 14, with the highest thermal resistance ratios found for the smallest gap (30 µm) and the lowest pressure (0.6 T).

Influence of structure and chemistry on piezoelectric properties of lead zirconate titanate in a microelectromechanical systems power generation application

Lead zirconate titanate (PZT) films between 1 and 3 μm thick were grown using solution deposition techniques to study the effects of crystal structure and orientation on the direct piezoelectric output of these films on platinized Si membranes. By varying the chemistry of the film from Zr-rich to Ti-rich, the {100}/(111) relative intensity increased for films grown on randomly oriented Pt films. The 40:60 PZT had a tetragonal crystal structure and produced greater electrical output at a given strain than the rhombohedral film (Zr:Ti concentrations less than 50:50), while having a similar e 3 1 constant, between 4.8 and 6.3 C/m 2 . Orientation and voltage output at a given strain were not strongly influenced by thickness in the ranges investigated. Defects in internal PZT/PZT crystallization interfaces were identified and include porosity on the order of tens of nm, with a corresponding depletion in Pb and accumulation of O at these interfaces. The {100} texture of rhombohedral films deposited upon (111) textured Pt films is significantly greater than the {100} texture of tetragonal films, which show both a {100} and {111} orientation on the same Pt film.

Power Output Force Generation by a MEMS Phase Change Actuator

A microelectromechanical-systems-based phase change actuator has been developed and tested for high-speed mechanical power output and force generation. This actuator is well suited for a variety of advanced devices like tactile displays or micro fluidic systems. The device features two thin membranes that bound a cavity filled with working fluid. The working fluid boils at low temperature. Two sizes of actuator are tested, an actuator with membrane sidelengths of 5 mm and an actuator with top membrane sidelength of 10 mm. Two top membrane materials are explored consisting of 2 μm thick silicon and 300 nm thick silicon nitride. Heat addition is through the lower membrane which is fabricated with novel capillary structures designed to increase the efficiency of actuator operation. The actuator is shown to produce up to 2.6 mW of mechanical power output and generate an applied force of 43 mN. Operating speeds up to 100 Hz are demonstrated.