Cecilia D. Richards

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.

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%.

Concentration measurements in a self-excited jet

The near field of helium-air jets exhausting into an air environment has been experimentally investigated using an aspiration probe and flow visualization. Jets with varying density ratios and Reynolds numbers were studied. Pure helium jets with density ratios of 0.14 were found to display a self-excited behavior characterized by intense mixing. The centerline concentration decay was found to be substantially increased for the self-excited jet. Flow visualization revealed the expulsion of side jets from the potential core region of low density jets. Radial profiles of concentration provide additional evidence that side-jets produce vigorous mixing.

On the Coupling Between Standing-Wave Thermoacoustic Engine and Piezoelectric Transducer

Small thermoacoustic engines integrated with piezoelectric elements can be effective small-scale power sources to convert heat to electricity. A simplified mathematical model is developed to illustrate the effect of transducer parameters on the frequency and onset temperature difference in a standing-wave engine and to estimate efficiencies of energy conversion. Results of sample calculations show that efficiencies for the acoustic-electric energy conversion on the order of 10% are feasible.Copyright

The Effect of Design and Process Parameters on Electromechanical Coupling for a Thin-Film PZT Membrane

In this paper, a finite-element model is used to analyze the performance of a thin-film piezoelectric laminate for power generation applications. The focus is on the effects of residual stress, the ratio of PZT to substrate thickness, the substrate material, electrode coverage, boundary conditions, and side length on the electromechanical coupling coefficient. The results show that the residual stress has the most substantial effect on the electromechanical coupling coefficient and should be minimized to increase the electromechanical coupling. Attention to other design parameters can be used to further optimize electromechanical coupling. Electrode coverage should be kept close to 50%. For Si substrates, a Si/PZT ratio of 4 maximizes the electromechanical coupling coefficient. Substrates with higher stiffness lead to superior electromechanical coupling. The results show that design changes, which lead to increased electromechanical coupling, also lead to greater efficiency

Modeling of a resonant heat engine

A resonant heat engine in which the piston assembly is replaced by a sealed elastic cavity is modeled and analyzed. A nondimensional lumped-parameter model is derived and used to investigate the factors that control the performance of the engine. The thermal efficiency predicted by the model agrees with that predicted from the relation for the Otto cycle based on compression ratio. The predictions show that for a fixed mechanical load, increasing the heat input results in increased efficiency. The output power and power density are shown to depend on the loading for a given heat input. The loading condition for maximum output power is different from that required for maximum power density.

Droplet and vapor transport in a turbulent jet

An experimental study of the role of large-scale structures in droplet and vapor transport in dropletladen jets has been conducted. Flow visualization was used to document the effect of droplet injection location on dispersion and to characterize the convective transport of droplet vapor. Experiments were conducted in an acoustically forced turbulent jet with point-source injection of a monodisperse-droplet stream. A reactive Mie scattering technique was employed to visualize vortex structures, droplet distribution. and vapor transport simulataneously in the near-field region of the jet. Two droplet sizes, 40 and 100 μm, wre investigated, resulting in Stokes numbers of 0.9 and 5.6, respectively. The injection location was varied from the jet centerline to the jet edge. The results show that injection location has a substantial impact on both droplet and vapor convection. In general, for a given injection location the 40 μm droplets experienced greater dispersion than the 100 μm droplets. For a given injection location the 40 μm droplets experienced greater dispersion than the 100 μm droplets. For a given droplet size (Stokes number), dispersion increased as the injection location moved radially ontward from the centerline. For both Stokes mumbers investigated, droplets injected into the core region of the jet (R0

Low Frequency Operation of the P3 Micro Heat Engine

The development and low frequency testing of a micro heat engine is presented. Production of electrical power by a dynamic micro heat engine is demonstrated. The prototype micro heat engine is an external combustion engine in which thermal power is converted to mechanical power through 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 peak-to-peak voltage of .84 volts, and power output of 1.5 microwatts have been realized at operating speeds of 10 Hz. Measurements are also presented for the engine operating at resonant conditions. Cycle speeds up to 240 Hz have been obtained, with peak-to-peak voltages of 70 millivolts.Copyright

CHARACTERIZATION OF PIEZOELECTRIC MATERIALS FOR THERMOACOUSTIC POWER TRANSDUCTION

In this work an experimental study of the performance of piezoelectric transducers for power production from a small-scale thermoacoustic engine is presented. Four piezoelectric samples are identified and characterized. These samples are tested on a variable acoustic driver and electrical power produced is measured. Finally, the samples are tested on four experimental thermoacoustic engines to verify the results from the acoustic setup. The maximum power produced is 177 μW from a closed thermoacoustic engine coupled to a 15mm PZT disk.Copyright

Electro-Mechanical Coupling and Power Generation in a Pzt Micro-Engine

A piezoelectric thin film MEMS device for generating power from a novel heat engine which approaches a Carnot cycle has been developed. The structure of the underlying electrode and PZT thin film generator has been optimized for increased adhesion. Atomic force microscopy was used to track electrode grain size and roughness; generating grain sizes of approximately 100 and 200 nm in diameter and a roughness of about 14-20 nm provide substantial improvements in film adhesion over systems with smaller grains and smoother surfaces. This has led to the ability to operate the engine at frequencies between 10 and 1500 Hz. The system of interest (a fluid filled cavity sealed by a micromachined silicon membrane and the PZT film) shows increased deflections for a given pressure applied to the membrane at frequencies where the system resonates. By operating the system dynamically, it is possible to generate more than 2 V from a single generator structure.