Recent developments in MEMS-based miniature fuel cells

Micro fuel cells (μ-FCs) represent promising power sources for portable applications. Today, one of the technological ways to make μ-FCs is to have recourse to standard microfabrication techniques used in the fabrication of micro-electro-mechanical systems (MEMS). This paper shows an overview on the applications of MEMS techniques on miniature FCs by presenting several solutions developed throughout the world. It also describes the latest developments of a new porous silicon-based miniature fuel cell. Using a silane grafted on an inorganic porous media as the proton-exchange membrane instead of a common ionomer such as Nafion^®, the fuel cell achieved a maximum power density of 58 mW cm^?2 at room temperature with hydrogen as fuel.     

New approach for batch microfabrication of silicon-based micro fuel cells

This paper reports a novel and straightforward approach to the development of a compact micro direct methanol fuel cell. The device consists of a hybrid polymer membrane as a feasible microintegrable electrolyte to be used together with silicon current collectors. These current collectors consist in microfabricated silicon chips that incorporate a fine electrode grid. The membrane combines two polymers with different functionalities, Nafion^® as a proton conducting material and PDMS as a flexible mechanical support. The compatibility of this membrane with MEMS fabrication processes lies in the acknowledged bonding capabilities of the PDMS polymer to materials typically used in microsystems technologies—such as silicon, silicon dioxide and glass—as well as its ability to withstand variations of the Nafion^® volume. The compatibility of all the components with microfabrication processes will permit the application of batch fabrication techniques for the whole device, so contributing to a significant lowering of the fabrication costs.     

Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO₂/Si₃N₄/SiO₂ thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N₂) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm² and of 1.1 μW/cm², respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N₂ flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10^?3 μW/cm² K².     

Impact of thermal contact resistances on micro-gap heat losses for microthermionic power generators

Thermionic power generation is a safe and clean energy source that allows for converting heat into electrical energy using thermionic electrons. The miniaturization is an advantage of this technology that led to the recent development of micro-gap thermionic power generators. In this work, thermal contact resistances between the micro-gap insulators and the emitter as well as between the micro-gap insulators and the collector are measured. A thermal resistance of 48.6 K/W is obtained by downsizing the insulators until 60 × 45 μm² of contact area with the emitter, demonstrating a high impact for decreasing the micro-gap conduction heat loss density from the emitter to the collector from 28 W/cm² (theoretical value obtained without considering contact resistances) to 5.6 W/cm². Downsizing the contact area between the insulators and the emitter from 320 × 300 to 60 × 45 μm² leads to an increase of the power conversion efficiency from 9.1 × 10^?5 until 1.5 × 10^?3.     

Modeling the front side plasmonics effect in nanotextured silicon surface for thin film solar cells application

We report the computational modeling of the front side plasmonics effect arising on gold (Au) nanoparticles array in combination with nanotextured silicon surface for thin film silicon solar cells application. The ultimate efficiency of the optimized silicon nanoholes (SiNH) array textured surface using Au plasmonics effect is 38.58 %, which is 24.01 % greater than SiNH array textured surface without Au plasmonics effect. Furthermore, SiNH array textured surface perform better compared to silicon nanopillar (SiNP) array textured surface for all the parameters studied. The maximum possible short circuit current density and power conversion efficiency of the proposed SiNH array textured surface with Au plasmonics effect are 31.57 mA/cm² and 25.45 % respectively, which compares favorably well to the computed values of 26.17 mA/cm² and 21.12 %, respectively for the SiNP array textured surface with Au plasmonics effect.     

Micro-mixer device with deep channels in silicon using modified RIE process: fabrication,packaging and characterization

In the present work, silicon based micromixer microfluidic devices have been fabricated in silicon substrates of 2-inch diameter. These devices are of 2-input and 1-output port configuration bearing channel depth in the range 80–280 µm. Conventional reactive ion etching (RIE) process used in integrated circuit fabrication was modified to get reasonably high silicon etch rate (~1.2 µm/min). It was anticipated that devices with channel depth in excess of 150 µm would become weak and susceptible to breakage. For such devices, a bonded pair of silicon having a 0.5 µm SiO₂ at the bonded interface was used as the starting substrate. The processed silicon wafer bearing channels was anodically bonded to a Corning^® 7740 glass plate of identical size for fluid confinement. Through-holes for input/output ports were made either in Si substrate or in glass plate before carrying out anodic bonding. Micro-channels were characterized using stylus and optical profiler. Surface roughness of the channel was observed to increase with increasing channel depth. The devices were packaged in a polycarbonate housing and pressure drop versus flow rate measurements were carried out. Reynolds number and friction factor were calculated for devices with 82 µm deep channels. It was observed that up to 25 sccm of gas and 10 ml/min of liquid, the flow was laminar in nature. It is envisaged that using bonded silicon wafer pair and combination of RIE and wet etching, it is possible to get an etch stop at the SiO₂ layer of the bonded silicon interface with much smaller value of surface roughness rendering smooth channel surface.     

Leakage current in a Si-based nanopore structure and its influence on noise characteristics

We have measured leakage current in a silicon substrate-based nanopore membrane device immersed in an aqueous environment which typically shows the current level of few nA. This current level is compared with the measured current density (400 nA/cm² at 1 V) from the pristine Si wafer (p-type, 10¹⁶/cm³ boron doping) indicating that the exposed Si surface in a nanopore membrane device acts as an electrochemical reaction site. The leakage current is drastically reduced from >10 nA to <100 pA at 1 V by the deposition of a dielectric layer to the Si-based nanopore membrane device. We also noted that the root-mean-square noise of the ionic current is also reduced from 38 to 28 pA in correlation with the reduction of leakage current, indicating that electrochemical reaction provides one of the major sources of noise.     

Capillary-based water removal cathode for an air-breathing micro direct methanol fuel cell

This paper presents a capillary-based water removal cathode for an air-breathing micro direct methanol fuel cell (μDMFC). The mechanism of water removal from the cathode is studied and an array of capillaries with hydrophilic surface is designed on the ribs of the cathode structure. Microfabrication techniques, including double-side lithography and ICP, were used to fabricate the anode and cathode plates of the μDMFC on the same silicon wafer simultaneously. The surface of capillary structure was treated by low temperature oxygen plasma to improve the hydrophilicity. One μDMFC with capillary-based water removal cathode and another regular one without were both assembled and characterized. Measured results show that the μDMFC with water removal cathode achieves a power density of 2.35 mW/cm², 12 % larger than that of the regular one with the value of 2.10 mW/cm². And the maximum current density of the novel μDMFC is 30 mA/cm², 20 % larger than that of the regular one, 25 mA/cm². It is also clearly observed during the μDMFC operation that the water is drawn out from the capillary-based water removal cathode expectantly.     

Low power highly sensitive platform for gas sensing application

This paper reports a low power miniaturized MEMS based integrated gas sensor with 36.84 % sensitivity (ΔR/R₀) for as low as 4 ppm (NH₃) gas concentration. Micro-heater based gas sensor device presented here consumes very low power (360 °C at 98 mW/mm²) with platinum (Pt) micro-heater. Low powered micro-heater is an essential component of the metal oxide based gas sensors which are portable and battery operated. These micro-heaters usually cover less than 5 % of the gas sensor chip area but they need to be thermally isolated from substrate, to reduce thermal losses. This paper elaborates on design aspects of micro fabricated low power gas sensor which includes ‘membrane design’ below the microheater; the ‘cavity-to-active area ratio’; effect of silicon thickness below the silicon dioxide membrane; etc. using FEM simulations and experimentation. The key issues pertaining to process modules like fragile wafer handling after bulk micro-machining; lift-off of platinum and sensing films for the realization of heater, inter-digitated-electrodes (IDE) and sensing film are dealt with in detail. Low power platinum microheater achieving 700 °C at 267 mW/mm² are fabricated. Temperature calculations are based on experimentally calculated thermal coefficient of resistance (TCR) and IR imaging. Temperature uniformity and localized heating is verified with infrared imaging. Reliability tests of the heater device show their ruggedness and repeatability. Stable heater temperature with standard deviation (σ) of 0.015 obtained during continuous powering for an hour. Cyclic ON–OFF test on the device indicate the ruggedness of the micro-heater. High sensitivity of the device for was observed for ammonia (NH₃), resulting in 40 % response for ~4 ppm gas concentration at 230 °C operating temperature.     

Morphology and optical performance of graphene-doped organic photovoltaic cells

The optical and electrical properties of graphene doping organic photovoltaics devices are investigated. The root-mean-square roughness, photoluminescence and optical absorption increase with increasing graphene content. Graphene also improves the open-circuit voltage and short-circuit current density. The best performance was obtained with 3 wt% graphene concent, with an open-circuit voltage of 0.62 V, a short-circuit current density value of 6.138 mA cm^?2, a fill factor of 61.912 %, and a power conversion efficiency of 2.35 %.     

A large-area,all-plastic,flexible electroosmotic pump

A large-area, fabric-like pump would potentially have applications, for example, in controlling water transport through a garment, such as a rain jacket, regardless of the external temperature and humidity. This paper presents an all-plastic, flexible electroosmotic pump, constructed from commercially available materials: A polycarbonate membrane combined with the electrochemically active polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate that actively transports water using an electric potential that can be supplied by a small battery. By using electrochemically active polymer electrodes instead of metal electrodes, the electrochemical reaction that drives flow avoids the oxygen and hydrogen gas production or pH changes associated with water electrolysis. We observe a water mass flux up to 23 mg min^?1 per cm² polycarbonate membrane (porosity 10–15%), at an applied potential of 5 V, and a limiting operating pressure of 0.3 kPa V^?1, similar to previously reported membrane-based electroosmotic pumps.     

Zone electrophoresis of proteins in poly(dimethylsiloxane) (PDMS) microchip coated with physically adsorbed amphiphilic phospholipid polymer

The zone electrophoresis of protein in poly(dimethylsiloxane) (PDMS) microchip coated with the physically adsorbed amphiphilic phospholipid polymer (PMMSi) was investigated. PMMSi was composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) and 3-(methacryloyloxy) propyltris (trimethylsiloxy) silane (MPTSSi) units in a random fashion. The membrane of PMMSi can be formed on the PDMS surface by a simple and quick dip-coating method. The membrane showed high hydrophilicity and good stability in water, as determined by contact angle measurement, fourier-transformed infrared absorption by attenuated total reflection (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS) analysis. High suppression of protein adsorption to the PDMS surface and reduction in electroosmotic flow (EOF) were achieved by PMMSi coating due to an increase of hydrophilicity, and a decrease of the ζ-potential on the surface of PDMS. For zone electrophoresis, the PMMSi30 containing 30 % hydrophilic MPC was the most suitable molecular design in terms of the stability of the coated membrane on PDMS surface. The average value of EOF mobility of PDMS microchip coated with PMMSi30 was 1.4 × 10^?4 cm² V^?1 s^?1, and the RSD was 4.1 %. Zone electrophoresis of uranine was further demonstrated with high repeatability and reproducibility. Separation of two FITC-labeled proteins (BSA and insulin) was performed with high efficiency and resolution compared with non-treated PDMS microchip.     

MEMS design and fabrication of an electrostatic vibration-to-electricity energy converter

This paper presents a micro electrostatic vibration-to-electricity energy converter based on the micro-electromechanical system. For the 3.3 V supply voltage and 1 cm² chip area constraints, optimal design parameters were found from theoretical calculation and Simulink simulation. In the current design, the output power is 200 μW/cm² for the optimal load of 8 MΩ. The device was fabricated in a silicon-on-insulator wafer. Mechanical and electrical measurements were conducted. Residual particles caused shortage of the variable capacitor and the output power could not be measured. Fabrication processes are being refined to remove the back silicon substrate to eliminate residual particles and parasitic capacitance.     

Fabrication and characterization of PECVD silicon nitride for RF MEMS applications

The structural, optical and electrical properties of plasma enhanced chemical vapor deposited silicon nitride layers are investigated, which have been used as a dielectric layer during RF MEMS fabrication. During growth, the gas ratio (SiH₄/NH₃) is varied between 0.33 and 0.5 and pressure is varied between 400 and 700 mTorr while deposition time is kept constant. The results in the films show differing properties. The thicknesses of the resultant films are between 150 to 220 nm with different gas flow ratios and pressures whereas the deposition time was kept constant. A Bruggeman effective medium approximation is utilized to model the refractive index of the films. Reflectance measurements were carried out in the range of 210–250 nm. The refractive indexes of the films varied between 1.79 and 2.03, with a dielectric constant varying from 6.66 to 7.22. Capacitance voltage measurements yield a fixed dielectric charge value in the low ?10¹² cm^?2 while a breakdown voltage of 915 V μm^?1 is achieved for films grown at the lowest gas ratio and pressure. The quality of Si/Si_xN_y interface is also considered.     

Fabrication and characterization of a passive silicon-based direct methanol fuel cell

This paper reports on the fabrication and characterization of a passive silicon microfabricated direct methanol fuel cell (μDMFC). The main characteristics of the device are its capability to work without complex pumping systems, only by capillary pressure, and the fact that its performance is not affected by the device orientation. A simple fabrication process based in deep reactive ion etching (DRIE), allows obtaining a reliable and low-cost final device. The device consists of two silicon microfabricated plates mounted together with a commercial membrane electrode assembly (MEA). The impact of current collector design on microfuel cell performance is explored and current–voltage (I–V) and current–power (I–P) curves of the device at different methanol concentration and orientation are presented. Optimal performance was obtained for methanol concentrations between 3 and 5 M, achieving a maximum power density of 12 mW/cm². The results obtained in this work demonstrate the feasibility of the device and give a guideline for design and conditions optimization.     

Optimization of specific energy consumption for Bomaplex Red CR-L dye removal from aqueous solution by electrocoagulation using Taguchi-neural method

In this investigation, firstly, Taguchi method was applied to determine the optimum specific energy consumption (SEC) for dye removal from aqueous solution by electrocoagulation using aluminum electrodes. An orthogonal array (OA₁₆) experimental design that allows to investigate the simultaneous variations of five parameters (Initial dye concentration, Initial pH of the solution, Supporting electrolyte concentration, Supporting electrolyte type and Current density) having four levels was employed to evaluate the effects of experimental parameters with two replicates. According to Taguchi-neural method, while the optimum conditions that dye removal efficiency equals to 62.71 % were found to be initial dye concentration 600 mg/L, initial pH of the solution 6, supporting electrolyte concentration 7.0 mM, supporting electrolyte type NaCl, and current density 0.10 mA/cm². Under these optimum conditions, energy consumption is 0.38 kW h/m³. Alternatively, it can be said that optimum conditions can be modified as follows supporting electrolyte concentration of 10.0 mM and supporting electrolyte type CaCl₂, for 600 mg/L, initial dye concentration initial pH of the solution 6, and current density 0.10 mA/cm². Under these optimum conditions, SEC and dye removal efficiency are 0.45 kW h/m³ and 69.18 %, respectively.     

A MEMS sandwich differential capacitive silicon-on-insulator accelerometer

A differential capacitive accelerometer with simple process is designed, simulated, and fabricated. To achieve a precision structure dimension with fewer processing steps, the silicon device layer transfer technology is being used to built a sandwich accelerometer based on a silicon-on-insulator (SOI) wafer, which was assembled by glass-si-glass multilayer anodic bonding. Deep reactive ion etching is being used to define symmetric beams and large mass block of equal thickness together in SOI device layer (up to 100 μm) in a single step to avoid alignment error in double side process. An actual accelerometer which is designed for 50 g measure range is fabricated with six lithography steps. Measurement results show 0.1166 V/g sensitivity and 0.022 % nonlinearity error in ±1 g gravity static response test. The accelerometer also provides a power spectrum less than 10.49 μVrms/Hz^1/2 (89.97 μg/Hz^1/2) in a non-isolated laboratory environment with a capacitive interface circuit.     

Surface modification of a-Si<Subscript>0.60</Subscript>C<Subscript>0.40</Subscript>:H films by Al-assisted photochemical etching: humidity sensing application

The paper investigates the formation of thin porous amorphous silicon carbide (PASiC) by Al-assisted photochemical etching using HF/AgNO₃ solution under UV illumination at λ = 254 nm. Different etching times varying from 2 to 10 min have been used on thin a-Si_0.60C_0.40:H films, which are elaborated by co-sputtering DC magnetron using a single crystal Si target and who deposited onto 86 of hot pressed polycrystalline 6H-SiC stripes of 12.5 mm³. Because of the high electrical resistivity of the thin a-Si_0.60C_0.40:H film higher than 2 MΩ cm, and in order to facilitate the chemical etching, a thin metallic film of high purity aluminum (Al) has been deposited under vacuum, follow-up of a thin palladium deposited under a grid to reduce attacked surface and reinforced solution etching. The etched surface was characterized by scanning electron microscopy, infrared spectroscopy, spectrophotometer UV, and photoluminescence. Results show that the morphology of etched a-Si_0.60C_0.40:H surface evaluates with etching time and presents a spongy and macroporous layers. Where, the diameter of pore size increases with the increasing etching time. A humidity sensors were fabricated through evaporating coplanar interdigital gold electrodes on PASiC and the humidity sensing properties were tested, it show, that the measured resistance Au-PASiC structure, depends highly on the applied bias voltage. Finally, the sensing performances are attributed to the unique surface structure, morphology of the pore and its size, that provide an effective pathway for vapor transportation and enlarged the sensing area of Au-PASiC.     

Chlorophyll content estimation in arid grasslands from Landsat-8 OLI data

In this study, an arid grassland was selected, and the chlorophyll content of the leaf and canopy level was estimated based on Landsat-8 Operational Land Imager (OLI) data using the PROSAIL radiative transfer (RT) model. Two vegetation indices (green chlorophyll index, CI_green, and greenness index, G) were selected to estimate the leaf and canopy chlorophyll content (LCC and CCC). By analysing the effect of soil background on the two indices, the LCC was divided into low and moderate-to-high levels. A different combination of the two indices was adopted at each level to improve the chlorophyll content estimation accuracy. The results suggested that the chlorophyll content estimated using the proposed method yielded a higher accuracy with coefficient of determination, R² = 0.84, root-mean-square error, RMSE = 9.67 μg cm^?2 for LCC and R² = 0.85, RMSE = 0.43 g m^?2 for CCC than that using CI_green alone with R² = 0.62, RMSE = 20.04 μg cm^?2 for LCC and R² = 0.85, RMSE = 0.71 g m^?2 for CCC. The results also confirmed the validity of this approach to estimate the chlorophyll content in arid areas.     

Chemiluminescence detection for Cu (II) based on 1,10-Phenanthroline-hydrogen peroxide reaction on a PDMS microfluidic chip

A simple, rapid and effective method for the determination of copper (II) in water on a PDMS microfluidic chip with chemiluminescence (CL) detection is presented. The CL reaction was based on oxidation of 1,10-phenanthroline by hydrogen peroxide in basic aqueous solution. Polydimethylsiloxane (PDMS) was chosen as material for fabricating the microfluidic chip with two steps lithography method. Optimized reagents conditions were found to be 6.0 × 10^?5 mol/L 1,10-phenanthroline, 1.2 × 10^?3 mol/L hydrogen peroxide, 6.5 × 10^?2 mol/L sodium hydroxide and 2.0 × 10^?3 mol/L Hexadecyl trimethyl ammonium Bromide (CTMAB). In the continuous flow injection mode the system can perform fully automated detection with a reagent consumption of only 3.4 μL each time. The linear range of the Cu (II) ions concentration was 1.0 × 10^?8 mol/L to 1.0 × 10^?4 mol/L, and the detection limit was 9.2 × 10^?9 mol/L with the S/N ratio of 3. The relative standard deviation was 2.8 % for 1.0 × 10^?6 mol/L Cu (II) ions (n = 8). The most notable features of the detection method are simple operation, rapid detection and easy fabrication of the microfluidic chip.