R. F. 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.

A MEMS fabricated flexible electrode array for recording surface field potentials

We developed a method to microfabricate flexible electrode arrays on a thin Kapton substrate, which was engineered to minimize trauma when inserted between the dura and skull to obtain surface EEG recordings. The array consisted of 64 gold electrodes, each 150microm in diameter on a 750microm spaced 8x8 grid. Using photolithographic procedures, any arrangement of electrodes can be implemented. We used the electrode array to record evoked response signals to create topographical maps of the whisker barrels on the cortical surface with excellent signal stability over a period of 8h. The materials used for this fabrication are potentially biologically inert and, with some additional modifications to the design, can be chronically implanted with minimal side effects. Retinal prosthesis, human neurosurgery, and neurological research are all limited to some degree by the resolution and biological compatibility of the implants used. This type of array could greatly enhance the spatial resolution, signal quality, and stability of implantable surface electrode arrays.

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%.

Thermocompression bonding of vertically aligned carbon nanotube turfs to metalized substrates

Vertically aligned carbon nanotube turfs (VACNTs), consisting of entwined, nominally vertical carbon nanotubes, are being proposed for use as electrical and thermal contact materials. Issues in their implementation include high contact resistance, the van der Waals interactions of carbon nanotubes, and a low temperature limit during processing. One route for circumventing the 750 degrees C temperatures required for VACNT growth using chemical vapor deposition is for the VACNTs to be grown separately, and then transferred to the device. A method of mechanical transfer, using thermocompression bonding, has been developed, allowing dry mechanical transfer of the VACNTs at 150 degrees C. This method can be used for the construction of both a thermal switch or a permanent conducting channel. The conductivity of the bonded structure is shown to be independent of the imposed strain, up to strains in excess of 100%.

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.