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The rapid development of advanced manufacturing technologies in micro-electro-mechanical system (MEMS) fields has promoted the advance in wearable electronics. Among them, wearable biomechanical micro-energy harvesters have attracted much attention in recent years to meet the unique power supply demands of wearable electronics serving as sustainable power sources. In the meanwhile, they also have been successfully demonstrated to be either self-powered sensors or other functional devices for wearable electronic applications. Herein, we overview the state-of-the-art of biomechanical micro-energy harvesters for wearable electronic applications, including the summary of working principles and structural configurations of biomechanical micro-energy harvesters. In detail, three promising technologies for biomechanical micro-energy harvesting are studied, i.e. electromagnetic effect, piezoelectric effect, and electrostatic effect. As for the structural configurations, two essential factors to affect the properties of biomechanical micro-energy harvesters are emphasized, i.e. new materials, and advanced manufacturing technologies. For wearable application scenarios, fiber/fabric-based materials and biodegradable/ecofriendly materials are investigated. For advanced manufacturing technologies, micro/nano fabrication technologies for precise fabrication and large-scale fabrication technologies for mass fabrication are summarized. Moreover, we review the wearable electronic applications of biomechanical micro-energy harvesters for powering, sensing, and actuating, respectively, which reveals the feasibility of constructing the smart wearable microsystems based on the abundant function of biomechanical micro-energy harvesters. Finally, we conclude the review and discussed the future development trends of biomechanical micro-energy harvesters for wearable electronic applications. [ABSTRACT FROM AUTHOR]

This paper reports a novel piezoelectric energy harvester incorporated on a waistbelt for power generation from waist movements. The device is composed of multiple piezoelectric disks in 3 D-printed encapsulations and rectifying circuity, which are connected to a battery for energy storage. The harvester was simulated and tested through different body activities. Measured open circuit voltages up to 19 V, μW-level AC power generation, and energy storage test results clearly demonstrate that the energy harvester presented here can effectively convert waist movements into electrical power, store energy in a battery, and support the batteries of wearable electronic devices for extended operations. [ABSTRACT FROM AUTHOR]

We report the design and microfabrication of a MEMS turbopump with thermal insulation, as the core component of a steam power plant-on-a-chip for waste heat recovery based on the Rankine thermodynamic cycle. Based on a first-generation microturbopump for ambient temperature operation, this second-generation device introduces materials for thermal insulation and design improvements to the rotordynamic components for operation at high temperatures. Thermal management is required to prevent water boiling in the pump and vapor from condensing in the turbine, bearings and seals. The device is a five-wafer stack that encloses a 4-mm diameter rotor disk with microturbine blades on one side and a viscous micropump on the other. This second-generation microturbopump implements unique thermal insulation strategies to isolate the pump from the surrounding hot flows. An out-off-plane thick molded glass embedded into the rotor prevents heat conduction from the turbine to the pump. In addition, an in-plane thick array of oxidized trenches prevents heat conduction from the thrust bearing region to the pump. The assembled microturbopump is fabricated and tested, resulting in the first MEMS turbopump demonstrated at high temperature. Design and fabrication are covered herein, whereas experimental demonstration and characterization are presented in Part II of this two-part paper. [2020-0050] [ABSTRACT FROM AUTHOR]

Due to the enormous energy densities of liquid hydrocarbon fuels for future utilization on micro scale, there is a concern about the feasibility of scaling down reciprocating internal combustion engines from small scale to meso scale. By building a specialized test bench, the performance and combustion characteristics of a miniature internal combustion engine with a displacement of 0.99 cc were tested, and the thermodynamic simulation was carried out to achieve a more complete understanding of in-cylinder mass and energy change of the miniature internal combustion engine. The miniature internal combustion engine had higher brake-specific fuel consumption, lower thermal efficiency, lower brake mean effective pressure, and serious cyclic variation; however, friction mean effective pressure seems to be less sensitive to engine speed. Simulation results showed that the miniature internal combustion engine had a poor volumetric efficiency, which was not more than 50%. The step-by-step processes of scaling down the miniature internal combustion engine were also simulated; it was found that the maximum indicated mean effective pressure loss was due to the imperfection of gas exchange processes, and the next was the imperfection of combustion. It is considered that for the scaled-down miniature internal combustion engines, more attention should be pay on improving the processes of gas exchange and combustion, and achieving meso-scale internal combustion engines with cylinder bore less than 1 mm is thermodynamically possible in future if these imperfections, especially that of the gas exchange process, can be effectively perfected. [ABSTRACT FROM AUTHOR]

Because of the difficulty in fabricating iron-core solenoid coils by micro electro-mechanical system (MEMS) processes, existing micromotors typically have an axial-flux structure with planar spiral coils or air-core solenoid coils. However, solenoid iron-core microcoils have a higher inductance and a lower magnetic resistivity than spiral coils and air-core solenoid coils. In this study, a radial-flux permanent magnet (PM) micromotor with 3D iron-core MEMS in-chip coils of high aspect ratio was designed and fabricated. The three-phase PM brushless direct current (BLDC) micromotor has four Nd-Fe-B magnet poles on a steel rotor and six solenoid Cu coils of high aspect ratio (coil height/line width = 14) in the silicon stator fabricated using MEMS processes. Iron cores were inserted into the coils, and a low-resistivity magnetic circuit was established. At 100 Hz, the inductance of a 15-turn coil was $13.2\mu \text{H}$. The coils were embedded tightly in the silicon substrate, resulting in a good heat dissipation performance and thus a high current-carrying capacity. The hexagonal sheet-type micromotor weighed 2.25 g with a volume of 586.5 mm3 before bearing packaging. The modeling results showed that at 10000 rpm, the torque was $676\mu \text{N}\cdot \text{m}$ and the power density was 314 W/kg. This micromotor incorporates a conventional large-scale BLDC motor structure and the proposed design will help improve the power density and efficiency of micromotors, thereby broadening their application prospects. [ABSTRACT FROM AUTHOR]

In this letter, we report the design and measurement of a 3D solenoid inductor that is embedded in a Si substrate and can integrate an iron core. Various inductor designs were fabricated with good structural integrity and repeatability via a CMOS-compatible MEMS fabrication process. The average inductance and quality factor peak-to-peak variation of the inductors was below 10%, which indicates that the fabrication process is repeatable. Among the inductors without iron cores, the highest quality factor (37.6 at 21 MHz) was found in a 5-turn inductor, and the highest inductance and inductance density (respectively, 86.6 nH and 21.7 nH/mm2) were found in a 20-turn inductor. Among the iron-core inductors, the 15-turn inductor had an inductance of 1063 nH and an inductance density of 354.3 nH/mm2, nearly 18 times higher than the same design without an iron core, which is the highest inductance density for a MEMS microinductor to the best of our knowledge. This type of inductor is an important component in RF MEMS and electromagnetic power MEMS devices and can improve their performance and efficiency. [ABSTRACT FROM AUTHOR]

An electret electrostatic MEMS vibration energy harvester for tire sensors mounted inside of the tire tread is reported. The device was designed so as to linearly change an electrostatic capacitance between the corrugated electret and output electrode according to the displacement of the proof mass. The electromechanical linearity was effective at reducing the power loss. The output power reached 495 μW under sinusoidal vibration despite the footprint size being as small as 1 cm2. Under impact vibration inside of the tire tread, the output power reached 60 μW at a traveling speed of 60 km/h. It was revealed that a higher mechanical resonance frequency of the harvester adjusted within the frequency band of a low-power spectral density of impact vibration acceleration was effective for high efficiency harvest impact vibration energy.

The performance of more than 60 different electromagnetic energy harvesters described in more than 100 publications is benchmarked. The benchmarking is based on earlier published parameters from literature as well as on two novel parameters introduced in this paper. The former allow to compare different harvester conversion principles as well as harvesters of different electrodynamic design principles. The latter consider the impact of ambient and boundary conditions for the most important sub-group, namely the resonant electrodynamic harvesters. The special consideration of how the mechanical damping and the energy conversion effectiveness depend on these conditions enables a fairer benchmarking of this common harvester type. High performing prototypes are identified, and the key parameters are provided for explanation. Finally, beneficial design approaches and the main challenges to maximize the output power are pointed out.

Beginning with a short historical sketch, electrodynamic energy harvesters with focus on vibration generators and volumes below 1dm3 are reviewed. The current challenges to generate up to several milliwatts of power from practically relevant flows and vibrations are addressed, and the variety of available solutions is sketched. Sixty-seven different harvester concepts from more than 130 publications are classified with respect to excitation, additional boundary conditions, design and fabrication. A chronological list of the harvester concepts with corresponding references provides an impression about the developments. Besides resonant harvester concepts, the review includes broadband approaches and mechanisms to harvest from flow. Finally, a short overview of harvesters in applications and first market ready concepts is given.

The focus of this study is on the manufacturing of micro air bearings (MABs) using silica film assisted processing. Structure of the three-layer micro air bearing is described in detail and the salient process flow of etching and bonding is illustrated. The main manufacturing challenges and the methods adopted to overcome them are also presented. The uniformity of wet etching for nozzles with 20 μm in diameter to silica film is improved by adopting an ultrasound assisted method. Particular attention is given to the novel fabrication procedures for the second layer of MAB (with three depths on aft side). This paper develops new applications of silica film in Micro Electro Mechanical System (MEMS) processing for MABs to realize the etching of multi-depth on the same side and efficient three-layer bonding with increased bonding areas. A silica etch mask is proven to achieve a higher accuracy in surface topography when compared to a photoresist mask for multi-depth etching, resulting in precise depth and vertical control. The bonding rate of three-layer direct bonding for MAB is increased by 50% from 0.05 to 0.1 with the novel silica film protection method.

Modeling of a near-field concentrated solar thermophotovoltaic (STPV) microsystem is carried out to investigate the use of STPV-based solid-state energy conversion as high power density MEMS power generator. Near-field radiation can be realized between two closely separated surfaces (order of radiation characteristic wavelength), resulting in the enhancement of the heat radiation flux orders of magnitudes higher than the blackbody limit, consequently increasing cell output power density. The near-field STPV model incorporates a photonic crystal absorber which transfers absorbed concentrated solar radiation to a tungsten emitter. Thermal radiation from the emitter illuminates an In 0.18 Ga 0.82 Sb photovoltaic (PV) cell generating electrical power; waste heat is rejected from the backside of the PV cell via a microcooler. Based on the model, the near-field STPV performance is estimated for different emitter-to-PV cell separation distances d c , emitter temperatures T e , and emitter/absorber area ratios A R . Results from the numerical study showed significant enhancement of the heat fluxes due to tunneling of the near-field radiation, resulting in power densities as high as 60 W / cm 2 which is 30 times higher than the equivalent far-field power density for d c = 20 nm, T e = 2000 K and solar concentration of × 4350 . For a emitter/absorber area ratio of A R = 1 , the emitter/absorber thermal efficiency and the overall solar to electrical conversion efficiency were 73% and 15.5%, respectively. Higher power densities are achievable (up to 50 times that of far-field values) however cooling requirements and solar concentration could be a concern. [ABSTRACT FROM AUTHOR]

The catalytic combustion characteristics of methane-air mixture inside the combustion chamber of a micro-engine were investigated experimentally and numerically. The catalytic combustion experiments of the micro-engine coated with the supported noble metal catalysts Pt-La/γ-Al2O3were investigated. Numerical simulations of methane-air combustion with detailed gas phase and surface catalytic reaction kinetics, heat transfer, and leakage were investigated to interpret the experimental results of micro-engine catalytic combustion. The experimental results indicate catalytic micro-combustion of methane-air mixture is possible in the spaces approximately the length of 0.34 mm and diameter of 3 mm. Catalytic micro-combustion of methane-air mixture is demonstrated at the equivalence ratios of 0.8 and 0.2. The free-piston position traces are identical during the compression stroke, but different during the expansion stroke. The mass loss causes product temperatures to decrease faster during the expansion stroke, and significantly decreases the cycle work of the micro-engine. Some theoretical evidences are provided for the application of catalytic combustion to a power micro-electromechanical system and the extension of the micro-combustion limits. [ABSTRACT FROM AUTHOR]

This work reports the design and electromagnetic optimization of a MEMS-scale turbo generator for harvesting fluidic energy in human exhalation. The device is composed of a turbine with dual-layer integrated permanent magnets, ball bearings, and stators with micro coils. For an efficient energy conversion, number of magnetic poles should be optimized to obtain maximum flux density in a given device geometry, where magnetic reluctance and leakage act as two competing effects. This optimization has been performed for different commercially available magnet thicknesses with fixed inner and outer radii of 2 mm and 8 mm, respectively. It has been shown that the optimum number of poles for 0.5 mm-thick magnets is 28, while pole numbers as high as 32 lead to higher flux densities for thinner magnets. The results presented here shed light on the efficient design of magnetic micromachines in similar scales. [ABSTRACT FROM AUTHOR]

The performance of more than 60 different electromagnetic energy harvesters described in more than 100 publications is benchmarked. The benchmarking is based on earlier published parameters from literature as well as on two novel parameters introduced in this paper. The former allow to compare different harvester conversion principles as well as harvesters of different electrodynamic design principles. The latter consider the impact of ambient and boundary conditions for the most important sub-group, namely the resonant electrodynamic harvesters. The special consideration of how the mechanical damping and the energy conversion effectiveness depend on these conditions enables a fairer benchmarking of this common harvester type. High performing prototypes are identified, and the key parameters are provided for explanation. Finally, beneficial design approaches and the main challenges to maximize the output power are pointed out. [ABSTRACT FROM AUTHOR]

Beginning with a short historical sketch, electrodynamic energy harvesters with focus on vibration generators and volumes below 1 dm³ are reviewed. The current challenges to generate up to several milliwatts of power from practically relevant flows and vibrations are addressed, and the variety of available solutions is sketched. Sixty-seven different harvester concepts from more than 130 publications are classified with respect to excitation, additional boundary conditions, design and fabrication. A chronological list of the harvester concepts with corresponding references provides an impression about the developments. Besides resonant harvester concepts, the review includes broadband approaches and mechanisms to harvest from flow. Finally, a short overview of harvesters in applications and first market ready concepts is given. [ABSTRACT FROM AUTHOR]

Abstract: The miniaturization of electro-mechanical devices, and the resulting need for micro-power generation (milliwatts to watts) with low weight, long life devices, has lead to the recent development of the field of micro-scale combustion and power generation. The primary objective of this new field is to leverage the high energy density of fuels, specifically liquid hydrocarbon fuels relative to batteries and all other energy storage devices other than nuclear fission, fusion or decay. As such, a miniaturized device even with a moderately efficient conversion of hydrocarbon fuels to power would result in increased lifetime and/or reduced weight of an electronic or mechanical system that are currently most often powered by electrochemical cells. Furthermore, improvements in this field may make possible novel applications and/or capability. In addition to the interest in miniaturization, the field is also driven by the potential fabrication of the devices using Micro-Electro-Mechanical Systems (MEMS) or rapid prototyping techniques, with their favorable characteristics for mass production and/or low unit cost. The micro-power generation field is very young, and still is in most cases in the feasibility stage. However, considering that it is a new frontier of technological development, and that only a few projects have been funded, it can be said that significant progress has been made to date. Currently there is consensus, at least among those working in the field, that combustion at the micro-scale is possible with proper thermal and chemical management. Several meso-scale and micro-scale combustors have been developed that appear to operate with good combustion efficiency. Some of these combustors have been applied to energize thermoelectric systems to produce power, although with low overall efficiency. Several turbines/engines have also been, or are being, developed, some of them currently producing positive power, albeit with low efficiency. Micro-rockets using solid or liquid fuels have been built and shown to produce thrust. More detailed scaling/modeling efforts are required to improve existing designs. Improvements in diagnostic, control and computational tools are expected to have a significant impact on the development of the field. Some brief scaling arguments are given in this work, and more detailed efforts are referred. A brief introduction to several of the fabrication techniques is presented in this work. Hydrogen-based and some preliminary specialty fuel micro-fuel cells have been successfully developed, and there is a need to develop reliable reformers (or direct conversion fuel cells) for liquid hydrocarbons so that the fuel cells become competitive with the batteries. In this work, the technological issues related to micro-scale combustion and the development of thermochemical devices for power generation will be discussed. Some of the systems currently being developed will be presented, ongoing critical research issues under investigation, and other potential areas of development discussed. Comments regarding the opportunities and limitations of each of the techniques are also presented where applicable. [Copyright &y& Elsevier]

The development of a microfabricated turbopump that is capable of delivering liquid fuel with the flow rates and pressures required for portable power generation is reported. The device is composed of a spiral-groove viscous pump driven by a radial in-flow microturbine and supported using a newly developed encapsulated microball bearing. A planar-contact bearing raceway is employed using 285-μm-diameter 440C stainless steel microballs. A modification to the raceway geometry, as compared to previous designs, has enabled repeatable operation of themicroturbine at speeds up to 87 000 r/min, showing negligible variations in performance for over 6 h and 3.8 million revolutions. Pumping has been demonstrated with water as the working fluid for flow rates of 10-80 mL/h and pressures rises of up to 8.2 kPa. This is the first application to incorporate an encapsulated microball bearing support mechanism, to achieve rotational speeds in excess of 50 000 r/min using a contact bearing, and to demonstrate reliable operation of more than 1 million revolutions. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides (i) a comparison of the energy harvesting capabilities of three different electrostatic mechanisms, and (ii) discussion on the relations among the contributing parameters involved in maximizing the energy output that can be harvested from an electrostatic microelectromechanical system (MEMS) device. The three mechanisms considered in this paper are namely the in-plane overlap, in-plane gap closing and out-of-plane gap closing converters. In this analytical modeling, the mass of the movable loads as well as the cross-sectional areas of the devices’ active regions were set to be the same for all the mechanisms, while we assumed these electrostatic mechanisms are operated in ideal vacuum environment. A maximum output energy density of 0.547μJmm−3 has been obtained for the case in which the movable load has a volume of 5mm3. [Copyright &y& Elsevier]

Palm-sized microturbomachinery have broad potential applications in micropower generation areas, such as air/fuel management systems for various fuel cells, propulsion engine for unmanned micro-air vehicles, power generation turbines for robots, small satellites, etc. This paper introduces design and manufacturing processes of mesoscale foil gas bearings applicable to the microturbomachinery and also presents its performances predicted from nonlinear orbit simulations. X-ray and ultraviolet lithography were explored as promising manufacturing tools of elastic foundations for the mesoscale foil gas bearings. Designed and manufactured mesoscale foil gas bearings have unique design features that precision-machined foil bearings cannot provide, such as easy control of mechanical properties of elastic foundations, a simple assembly process, and easy control of bearing preload through lithographic pattern. The manufactured bearing performance was predicted using a time-domain orbit simulation, and results are presented. [ABSTRACT FROM AUTHOR]

We demonstrate and discuss the performance of fully integrated and flexible micro thermoelectric generators (μTEGs). The devices are fabricated with a low-cost microfabrication process based on electrochemical deposition of a thermoelectric material into a polymer mold. Overall system optimization is demonstrated by means of NiCu and p- and n-type Bi2Te3-based μTEGs. Influences of design, material, fabrication, and performance parameters on device performance are explained by means of measurements and model calculations. The fabricated devices generate up to 2.6 * 10-3 μW · cm-2 K-2 for devices with NiCu thermocouples and up to 0.29 μW · cm-2 K-2 for Bi2Te3-based generators in planar state. Mechanical testing on NiCu μTEGs demonstrated functionality of the generator when bent to curvatures down to 7.5 mm. This allows for enhanced thermal contact to nonplanar surfaces. [ABSTRACT FROM AUTHOR]

This paper presents a microfabricated electro-quasi-static (EQS) induction turbine generator that has generated net electric power. A maximum power output of 192 μW was achieved under driven excitation. We believe that this is the first report of net-electric-power generation by an EQS induction machine of any scale found in the open literature. This paper also presents self-excited operation in which the generator resonates with an inductor and generates power without the use of external active-drive electronics. The generator comprises five silicon layers, fusion-bonded together at 700 °C. The stator is a platinum-electrode structure formed on a thick (approximately 20 μm) recessed oxide island. The rotor is a thin film of lightly doped polysilicon also residing on a 10-μm-thick oxide island. Carrier depletion in the rotor conductor film limited the performance of the generator. This paper also presents a generalized state-space model for an EQS induction machine that takes into account the machine and its external electronics and parasitics. This model correlates well with measured performance and was used to find the optimal drive conditions for all driven experiments. [ABSTRACT FROM AUTHOR]

The purpose of this work is to establish the maximum theoretical efficiency that a viscous flow turbine (such as a Tesla turbine) can achieve. This is very much in the spirit of the Betz limit for wind turbines. The scaling down of viscous flow turbines is thought not to alter this result, whereas the scaling down of conventional turbines, whether axial or radial flow, results in an ever lowering of their efficiencies. A semiempirical scaling law is developed for conventional gas turbines using published machine performance data, which is fitted to a simple boundary layer model of turbine efficiency. An analytical model is developed for a viscous flow turbine. This is compared to experimental measurements of the efficiency of a Tesla turbine using compressed air The semiempirical scaling law predicts that below a rotor diameter of between about 11 mm and 4 mm, a practical Brayton cycle is not possible. Despite that, however and for rotor diameters less than between about 7 mm and 2 mm, a viscous flow turbine, compressor or pump will be more efficient than a conventional design. This may have a significant impact on the design of microelectromechanical system devices. [ABSTRACT FROM AUTHOR]

Abstract: A variety of micro medical devices have been developed to provide more advanced and less invasive medical treatment. An effective power supply is crucial to the operation of these devices. Currently, two types of power supply are used: small batteries or radio-power transmission. However, the former limits the operating time of the devices, while radio-power transmission affects other medical devices due to the electromagnetic waves. In this paper, we report on a gastric-fluid-utilizing micro battery (GMB) that utilizes the gastric fluid in the stomach as an electrolyte. The GMB is designed to be used in the stomach and to generate electricity based on the principle of the voltaic cell. It consists only of biocompatible materials, including the electrodes, porous ceramic filter, and polydimethylsiloxane casing. Platinum deposited on a thin glass plate and a zinc plate work as positive and negative electrodes, respectively. Though zinc dissolves during the generation of electricity, it is an essential trace metal for humans and, given the small amount released, is not toxic. The porous ceramic filter placed between the electrodes filters out any foreign materials in the stomach fluid and holds the gastric fluid by capillary attraction. In experiments, GMB successfully generated 1.0mW (0.42V, 2.41mA) with a 200-Ω external load. It generated a stable output voltage of 0.6V for more than 39min with a 5-kΩ external load. We demonstrated the feasibility of the GMB for medical applications by successfully supplying power to a telemetric system and a tiny DC motor that is commercially available. [Copyright &y& Elsevier]

Abstract: This paper presents the design and experiment with a micro-energy storage system using an axial-flux permanent magnet and a high-temperature superconductor (HTS). This system consists of a three-phase planar stator, flywheel, and HTS. The three-phase planar stator is fabricated with five lines, width of 100μm, and height of 50μm for low eddy current loss and high current density. The flywheel with a diameter of 50mm is integrated by eight motor/generator magnets on the upper side and a single bearing magnet on the bottom side. The HTS is made of melt-textured yttrium-barium-copper-oxide (YBCO) and is used as the non-contact bearing by a levitation force. The maximum rotational speed of the flywheel is 51,000rpm at 12V and 0.68A. At this speed, the flywheel stores a rotational kinetic energy of 337J and the no-load three-phase electric output is 802mVrms. By using a three-phase full-wave rectifier, an electric output power of 50mW is measured. [Copyright &y& Elsevier]

Abstract: A new approach of a MEMS power generator based on a piezoelectric diaphragm is presented. A MEMS design is introduced taking into account an optimum diaphragm thickness and a beneficial electrode structure. Moreover, experiments for small and large deflections demonstrate the functionality of the energy converter. Electrical measurements point out dependencies regarding electrical load, residual stress within the diaphragm and electrode design. The frequency response for small deflections and the time response for large deflections of the device are also determined. [Copyright &y& Elsevier]

Abstract: This paper reports on the design and analysis of a capacitive vibration-to-electrical energy converter. A theoretical design model of a parallel-plate electrostatic spring-mass-system is presented, based on state space equations. The charging of the parallel-plate capacitor takes place by utilizing materials with different work functions for the electrodes. Numerical simulations are performed in order to optimize design parameters targeting a maximum output power. The shown Micro-Electro-Mechanical System (MEMS) based capacitive energy converter is able to provide an output power of 4.28μW at an external vibration with a frequency of 1kHz and an amplitude of 1.96m/s2 (0.2g). A basic electronic circuit for storing the electrical energy is also introduced. Furthermore, a MEMS design and a process flow are presented. [Copyright &y& Elsevier]

Abstract: A micro-electro-mechanical-system internal combustion engine (MEMS IC engine) is fabricated to demonstrate the energy conversion from heat to mechanical power. The dimensions and surface-to-volume ratio of combustion chamber are 5mm×3mm×1mm (0.015cc) and 3.07mm−1, respectively. The reciprocation motion of a piston can be sustained by the force balance between combustion pressure and the repulsion force of an elastic spring. A premixture gas composed of hydrogen and oxygen is used for fuel under stoichiometric combustion (equivalent ratio is 1). The combustion is confirmed by pressure measurement and the direct observation of flame propagation. A high-speed CMOS camera is adopted for the flame observation. A maximum combustion pressure of 142.6kPa, a displacement of 9.5mm3, and a mechanical power of 29.1mW were obtained for the MEMS IC engine. This experimental evidence provides a positive outlook for the materialization of the MEMS heat engine concept. [Copyright &y& Elsevier]

This paper discusses the history, current state of the art, and ongoing challenges for compact (less than a few cubic centimeters) magnetic power generation systems in the microwatts to tens of watts power range. These systems are of great interest for powering sensor networks, robotics, wireless communication systems, and other portable electronics. The paper considers the following topics. 1) The theoretical and practical implications of miniaturizing magnetic power generators. 2) The design and performance of previously demonstrated devices, which are summarized and compared. 3) Ongoing challenges for implementation, including integrated high-performance hard magnetic materials, microscale core laminations, low-friction bearings, high-speed rotor dynamics, and compact, high-efficiency power converters. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the design, fabrication and electrical characterization of MEMS tunable volume capacitors to be used as voltage controlled devices in tuning telecommunication circuits. A novel fabrication process to obtain parallel plate metallic capacitors based on deep reactive ion etching (DRIE) bulk micromachining techniques have been developed. Devices of three different electrode areas have been fabricated and designed and the influence of the area in the main characteristics of the MEMS capacitors established. The interaction of the DC tuning voltage and AC signal and their influence on the tuning behavior has been analyzed and the top operation limits evaluated. The obtained results make this kind of MEMS tunable metallic capacitors appropriate candidates for its use in the new power MEMS scenario. [Copyright &y& Elsevier]

A two-dimensional simulation on the unit structure of a novel thermoelectric module consisting of rectangular-fin elements was conducted to obtain the detailed temperature and electric potential fields of the element, and to compare the results with those of the one-dimensional theory. Numerically predicted power density and thermoelectric conversion efficiency agreed well with those predicted by one-dimensional theory at a small ratio of the element height to thickness, H/t < 3.5, and surprisingly showed better performance at H/t > 4.0. This was caused by a decrease of the thermal conductance of the rectangular-fin element at H/t > 4.3, which was not predicted by the one-dimensional theory. The electrical power to pumping power ratio of more than one was found to be possible by reducing the mass flow rate even when the module is scaled down. [ABSTRACT FROM AUTHOR]

Abstract: Different types of axial-flux permanent magnet electromagnetic μ-generators have been developed in LEG in collaboration with Laboratoire d’Electronique de Technologie de l’Information (LETI), CEA Grenoble. A Ø8mm planar generator consists of a permanent magnet disc rotor cut out of bulk SmCo5 or NdFeB, and a silicon stator with electroplated three-phase planar coils. High speed electrical tests exhibit an output power of 5W per stator at 380000rpm. A first prototype of integrated μ-turbo-generator driven by compressed air delivering an electric output power of 14.6mW at 58000rpm is also presented. Possible applications of the μ-generators range from high speed μ-gas engines for the generation of electrical power at the Watt level, to autonomous low-power μ-systems for wireless flow or air-speed sensing. [Copyright &y& Elsevier]

Most microscale electric and magnetic machines studied in the last decade lack the power density to support many practical applications. This paper introduces a design for a magnetic induction machine that offers power densities in excess of 200 MW/m³ and efficiencies of up to 50%, while providing more than 10 W of mechanical power. This is a substantial performance increase in MEMS electromagnetic machines studied to date. [ABSTRACT FROM AUTHOR]

This paper reports on the design, fabrication, and testing of an axial-flux permanent magnet electromagnetic generator. The generator comprises a polymer rotor with embedded permanent magnets sandwiched between two silicon stators with electroplated planar coils. Finite element simulations have been carried out using ANSYS to determine the effects on performance of design parameters such as the number of layers in the stator coils, and the rotor-stator gap. The effect of including soft magnetic pole pieces on the stators has also been studied. A prototype device with a diameter of 7.5 mm has been tested, and shown to deliver an output power of 1.1 mW per stator at a rotation speed of 30000 rpm. The generator has been integrated with a microfabricated axial-flow microturbine to produce a compact power conversion device for power generation and flow sensing applications. [1265] [ABSTRACT FROM AUTHOR]

A single-stage axial microturbine has been developed with a rotor diameter of 10 mm. This turbine is a first step in the development of a microgenerator that produces electrical energy from fuel. The turbine is made of stainless steel using die-sinking electro-discharge machining. It has been tested to speeds up to 160,000 rpm and generates a maximum mechanical power of 28 W with an efficiency of 18%. When coupled to a small generator, it generates 16 W of electrical power, which corresponds to an efficiency for the total system of 10.5%. [Copyright &y& Elsevier]

Hydrocarbon fuels have much higher energy to weight ratios than batteries. A research project is being carried out at the University of Birmingham to develop microengines to replace batteries. The project will be carried out into two stages. In the first stage a cryogenic CO2 engine is to be produced, followed in the second stage by the development of a micro combustion engine. This paper presents the first stage work to develop a micro Wankel engine from the design to the fabrication of a CO2 microengine. The design of the micro Wankel engine is based on its macro counterpart, but in the CO2 engine the housing curve has been modified to eliminate the compression stage of the Otto cycle. Finite-element analysis has been carried out during the design for both cryogenic and combustion engines to ensure that the engines can withstand the pressure released from their respective energy sources, and that deformation will not cause leakage. An advanced UV-lithography process has been developed, which can produce ultra thickness and high aspect ratio engine components. The precision and geometry of the engine components satisfy very strict design requirements. At present, a CO2 engine has been successfully fabricated and is under test. [Copyright &y& Elsevier]

The fluidic packaging of Power MEMS devices such as the MIT microengine and microrocket requires the fabrication of hermetic seals capable of withstanding temperature in the range 20–600 °C and pressures in the range 100–300 atm. We describe an approach to such packaging by attaching Kovar metal tubes to a silicon device using glass seal technology. Failure due to fracture of the seals is a significant reliability concern in the baseline process: microscopy revealed a large number of voids in the glass, pre-cracks in the glass and silicon, and poor wetting of the glass to silicon. The effects of various processing and materials parameters on these phenomena were examined. A robust procedure, based on the use of metal-coated silicon substrates, was developed to ensure good wetting. The bending strength of single-tube specimens was determined at several temperatures. The dominant failure mode changed from fracture at room temperature to yielding of the glass and Kovar at 600 °C. The strength in tension at room temperature was analyzed using Weibull statistics; these results indicate a probability of survival of 0.99 at an operational pressure of 125 atm at room temperature for single tubes and a corresponding probability of 0.9 for a packaged device with 11 joints. The residual stresses were analyzed using the method of finite elements and recommendations for the improvement of packaging reliability are suggested. [ABSTRACT FROM AUTHOR]

The present work is to show the development of micropower systems or power MEMS characterized by thermal, electrical and mechanical power density of 1∼20 Watts in sub-centimeter-sized packages. Three kinds of typical micro power systems, namely, the micro-gas turbine engine, the MEMS rotary micro engine, and a micro thermoelectric system, are described in this paper. A novel microthermophotovoltaic (micro-TPV) system, which is being developed in NUS, is also introduced in this work. The system uses hydrogen as fuel and is able to deliver electrical power in the order of watts in a package less than 1 cubic centimeter in volume. The main components of the system are a heat source, a selective emitter, and a photovoltaic (PV) array. The performance of the system is largely affected by the materials of the emitter and the PV cells. When the emitter is made of Co/Ni-doped MgO, and the PV cells are made of GaSb, a 4.4 W electrical power output can be expected. [ABSTRACT FROM AUTHOR]

High-precision fabrication is indispensable for high-speed silicon micro-rotors for power MEMS applications so as to minimize the rotor imbalance that deteriorates the rotor performance. Etch variation of deep reactive ion etch (DRIE) process results in differences in rotor blade heights and thus rotor imbalance. A Fourier transform of the etch non-uniformity along the rotor circumference revealed the global etch variation across the wafer and local variations in etch rates depending on the concentration or proximity of the patterned geometry. Rotor imbalance arising from the global etch variation of DRIE process was estimated, which compared favorably to results obtained from spinning experiments. The global etch non-uniformity which culminates in rotor imbalance could be alleviated to 0.25% across a rotor of 4.2 mm diameter by optimizing the plasma chamber pressure. The developed DRIE recipe successfully reduced the rotor imbalance and thus enhanced the rotordynamic performance. The manufacturing processes presented herein are readily applicable to the constructions of other microstructures containing intricate geometries and large etched areas. [Copyright &y& Elsevier]

This paper reports development of a hydrocarbon-fueled micro-combustion system for a micro-scale gas turbine engine for power generation and micro-propulsion applications. A three-wafer catalytic combustor was fabricated and tested. Efficiencies in excess of 40% were achieved for ethylene–air and propane–air combustion. A fabrication process for a six-wafer catalytic combustor was developed and this device was successfully constructed. [Copyright &y& Elsevier]

This paper presents the development of micro-fabricated “on-chip” polysilicon igniters and temperature sensors for the combustion system of a micro-gas turbine engine. We have reported the design and fabrication results of a novel through-wafer interconnect scheme that could greatly facilitate electrical contacts in multi-level MEMS devices by allowing direct electrical access to the backside of a wafer. This paper presents the results of a further effort that uses these interconnects to make electrical contacts to a thin-film polysilicon resistor so as to evaluate its ignition capability and its use as a wall temperature sensor for the micro-gas turbine engine. An application of the through-wafer interconnects to a concept demonstration of thin-film polysilicon resistive igniters for the micro-engine showed that it was possible to initiate combustion and locally raise the temperature of the igniter to 900 °C so long as the chip is thermally isolated. The results were found to be in good agreement with the predictions of an FEM thermal model. The possibility of using the resistors as temperature sensors is also examined. The non-linear variation of polysilicon resistivity with annealing temperatures due to complex effects resulting from dopant atom segregation, secondary grain growth and crystallographic relaxation reduced the operating range of the sensors to 450 °C. [Copyright &y& Elsevier]

This paper reports the investigation of low-temperature silicon wafer fusion bonding for MEMS applications. A bonding process utilizing annealing temperatures between 400 and 1100 °C was characterized. The silicon–silicon bonded interface was analyzed by infrared transmission (IT) and transmission electron microscopy (TEM) and the bond toughness was quantified by a four-point bending–delamination technique. [Copyright &y& Elsevier]