This paper reviews the state-of-the art in vibration energy harvesting for wireless, self-powered microsystems. Vibration-powered generators are typically, although not exclusively, inertial spring and mass systems. The characteristic equations for inertial-based generators are presented, along with the specific damping equations that relate to the three main transduction mechanisms employed to extract energy from the system. These transduction mechanisms are: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge when mechanically stressed. A comprehensive review of existing piezoelectric generators is presented, including impact coupled, resonant and human-based devices. Electromagnetic generators employ electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electromagnetic generators presented in the literature are reviewed including large scale discrete devices and wafer-scale integrated versions. Electrostatic generators utilize the relative movement between electrically isolated charged capacitor plates to generate energy. The work done against the electrostatic force between the plates provides the harvested energy. Electrostatic-based generators are reviewed under the classifications of in-plane overlap varying, in-plane gap closing and out-of-plane gap closing; the Coulomb force parametric generator and electret-based generators are also covered. The coupling factor of each transduction mechanism is discussed and all the devices presented in the literature are summarized in tables classified by transduction type; conclusions are drawn as to the suitability of the various techniques.

/pdf/energy-harvesting-vibration-sources-for-microsystems-46t885pwdp.pdf

Energy harvesting vibration sources for microsystems applications

https://cmapspublic3.ihmc.us/rid=1N3976C22-12M3P49-9V/Holladay%20et%20al.%20-%202009%20-%20An%20overview%20of%20hydrogen%20production%20technologies.pdf

An overview of hydrogen production technologies

A three-dimensional serpentine microchannel design with a "C shaped" repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a "square-wave" channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring.

/pdf/passive-mixing-in-a-three-dimensional-serpentine-2csmc8h1zg.pdf

Passive mixing in a three-dimensional serpentine microchannel

Microreaction engineering * is small better?

Micro total analysis systems. Recent developments.

We present a suspended-tube chemical reactor/heat exchanger for high-temperature fuel processing in micro energy conversion systems, primarily for hydrogen production in portable fuel cell systems. This reactor, designed to thermally isolate a high-temperature reaction zone, consists of four free-standing silicon nitride tubes comprising two independent U-shaped fluidic channels. Portions of the tubes are encased in silicon to enable heat exchange between the fluids in these channels. A thin-film platinum resistor is embedded for localized heating and temperature sensing. This paper describes the design and fabrication process for the MEMS fuel processor. Fluidic testing, thermal characterization up to 825/spl deg/C, and preparation of catalyst washcoats in the reactor microchannels are discussed. In addition, results from catalytic autothermal butane combustion and ammonia cracking in the reactor are presented.

A microfabricated suspended-tube chemical reactor for thermally efficient fuel processing

The present invention relates to gas separation membranes including a metal-based layer (17) having sub-micron scale thicknesses. The metal-based layer (17) can be a palladium alloy supported by ceramic layers such as a silicon oxide layer and a silicon nitride layer. By using MEMS, a series of perforations (holes) (11) can be patterned to allow chemical components to access both sides of the metal-based layer. Heaters and temperature sensing devices can also be patterned on the membrane (16). The present invention also relates to a portable power generation system at a chemical microreactor comprising the gas separation membrane. The invention is also directed to a method for fabricating a gas separation membrane. Due to the ability to make chemical microreactors of very small sizes, a series of reactors can be used in combination on a silicon surface to produce an integrated gas membrane device.

Integrated palladium-based micromembranes for hydrogen separation and hydrogenation/dehydrogenation reactions

A thermoelectric generator with integrated catalytic combustion has been microfabricated and successfully tested. The device consists of a high-temperature silicon-germanium thermopile supported on a thermally insulating silicon nitride membrane. Heat is supplied by catalytic combustion of fuels on the underside of the membrane. Power output has been generated from on-chip autothermal combustion of hydrogen, ammonia and butane, with external power used only to drive mass-flow controllers. The device was stable at temperatures up to 500°C, with thermopile voltages up to 7V and device thermal efficiencies up to 0.02%.

A Combustion-Based MEMS Thermoelectric Power Generator

A micromachined device for efficient thermal processing at least one fluid stream includes at least one fluid conducting tube having at least a region with wall thickness of less than 50 μm. The device optionally includes one or more thermally conductive structures in thermal communication with first and second thermally insulating portions of the fluid conducting tube. The device also may include a thermally conductive region, and at least a portion of the fluid conducting tube is disposed within the region. A plurality of structures may be provided projecting from a wall of the fluid conducting tube into an inner volume of the tube. The structures enhance thermal conduction between a fluid within the tube and a wall of the tube. A method for fabricating, from a substrate, a micromachined device for processing a fluid stream allows the selective removal of portions of the substrate to provide desired structures integrated within the device. As an example, the micromachined device may be adapted to efficiently react fluid reactants to produce fuel for a fuel cell associated with the device, resulting in a system capable of conversion of chemical to electrical energy.

Thermally efficient micromachined device

Etching of silicon with molecular fluorine for the micromachining of micro electro mechanical systems (MEMS) has been evaluated. The etching process is carried out in a continuous flow etching system that uses a 25 vol% mixture of F2 in N2 and operates at room temperature and atmospheric pressure. Fluorine etches silicon isotropically at a rate of 0.2 µm min−1 and is a viable etchant for bulk silicon micromachining. The F2 etch results in the formation of pits about 10–50 µm in size within the silicon features and roughness in the micrometer and sub-micrometer length scales. SiO2, Pt, Ni, Al and Ta do not etch or roughen after several hours of F2 exposure. In addition, F2 does not etch low-stress silicon nitride; a solid layer forms on the silicon nitride surface upon exposure to F2 that protects the underlying silicon nitride from further attack. In contrast, a commercial XeF2 etching system etches low-stress silicon nitride with a silicon nitride:silicon selectivity of approximately 1:200. The F2 etching system is used to release tubes of a MEMS fuel processor that have 2 µm thick low-stress silicon nitride walls.

https://iopscience.iop.org/article/10.1088/0960-1317/17/2/026/pdf

Aleksander Franz

Papers

A microfabricated suspended-tube chemical reactor for thermally efficient fuel processing

Integrated palladium-based micromembranes for hydrogen separation and hydrogenation/dehydrogenation reactions

A Combustion-Based MEMS Thermoelectric Power Generator

Thermally efficient micromachined device

Isotropic etching of silicon in fluorine gas for MEMS micromachining