Electrochemical properties of silicon deposited on patterned wafer

An amorphous silicon thin-film deposited on a patterned wafer is prepared by radio-frequency (rf) magnetron sputtering and is characterized by X-ray diffraction, galvanostatic cycle testing and field emission scanning electron microscopy. The specimen is assembled in cell of configuration: silicon working electrode/1 M LiPF₆ in EC/DMC, electrolyte/lithium metal, counter electrode (EC = ethylenecarbonate; DMC = dimethyl carbonate). A patterned silicon (1 0 0) wafer prepared by photolithography and KOH etching is used as the electrode substrate. The size of the patterns, which are composed of arrays of the negative square pyramids, is 5 μm/side.The patterned specimen (silicon film on patterned substrate) is compared with a normal specimen (silicon deposited on a flat substrate). The rate of capacity fade on cycling is monitored as a function of the voltage window and current density. The patterned specimen displays better cycle behaviour at a high current density (high C-rate).During the cycle tests at 200 μA cm⁻², the silicon electrodes yield an initial capacity of 327 μAh (cm² μm)⁻¹. After 100 cycles, the capacity is 285 μAh (cm² μm)⁻¹ and the capacity retention is 86%. Capacity retention is 76 and 61% at cycles 200 and 300, respectively.     

Enhanced nanocrystalline silicon solar cell with a photonic crystal back-reflector

Nanocrystalline silicon solar cells were enhanced with a photonic crystal back-reflector. Rigorous scattering matrix simulations were used to optimize a photonic crystal back-reflector consisting of a triangular lattice of nano-holes, with a pitch near 800 nm. The photonic crystal back-reflector with a pitch of 800 nm was fabricated on the crystalline silicon substrate by photolithography and reactive-ion etching, and coated with silver and zinc oxide. Nanocrystalline silicon solar cells were grown on the patterned substrates. We observed ～7% enhancement of the absorption and photo-generated current relative to a Ag/ZnO substrate, with an enhancement ratio of 1.5 near the band edge. Significant enhancement occurred in photon absorption at near infrared wavelengths greater than 700 nm, due to diffraction resonances of the incoming light.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Biodiesel from palm oil via loading KF/Ca–Al hydrotalcite catalyst

The solid base catalyst KF/Ca–Al hydrotalcite was obtained from Ca–Al layered double hydroxides and successfully used in the transesterification of methanol with palm oil to produce biodiesel. With the load of KF, the activity of Ca–Al mixed-oxides had been improved much. For the mass ratio 80 wt.%(KF·6H₂O to Ca–Al mixed-oxides) catalyst, under the optimal condition: 338 K, catalyst amount 5%(wt./wt. oil) and methanol/oil molar ratio 12:1, after 5 h reaction, the fatty acid methyl esters yield could reach 97.98%; for the mass ratio 100 wt.%(KF·6H₂O to Ca–Al mixed-oxides) ones, under the same reaction condition, only needed 3 h to get the FAME yield of 99.74%, and even only reacted 1 h, the FAME yield could obtain 97.14%.     

Improving thin-film crystalline silicon solar cell efficiency with back surface field layer and blaze diffractive grating

Both 2D electromagnetic and electrical semiconductor simulations are performed sequentially in this study in order to better understand the structural principles of thin-film crystalline solar cells with back surface field and blaze diffractive grating. In the absence of adequate approximations for blazed gratings, we simulate silicon solar cells electromagnetically and electrically in order to deal with the geometrical complexity produced by the blazed grating with a BSF on top of it. Thin-film crystalline silicon solar cells (TF-c-Si SCs) typically exhibit poor quantum efficiency both at shorter wavelengths and longer wavelengths with sharp drops in spectral response. Longer wavelength spectral response (from 0.6 μm to 1.2 μm) is addressed here first by considering the influence of blaze gratings on the enhancement of effective optical absorption in thin-film crystalline silicon (TF-c-Si) solar cells. The effect of the back surface field layer (BSF) in terms of improving minority carrier collection is also taken into account. In the 2D electromagnetic simulation, polarization dependent two-dimensional (2D) numerical simulations based on rigorous coupled wave analysis (RCWA) and finite element method (FEM) are implemented for the optimization of optical absorption of the solar cell structure. A rather large tolerance in design parameters of the optimized blaze grating structure was found. The optimized blaze grating structures help in improving the cell efficiency, especially for weak absorption thin cell structures. The enhancement of equivalent optical path length reveals the efficient light trapping effect caused by the diffractions of the blaze grating structures, especially in the longer wavelength range. In the electrical semiconductor simulation, the BSF, which arises from the heavy acceptor doping that creates the concentration gradient, is set atop the blaze grating in order to provide an extra small drift field for the collection of minority electrons. Incorporating the optimized antireflection coating along with a BSF layer and a blaze-grating in the 2 μm cell doubles cell efficiency. The use of blazed gratings in thin-film solar cells, which can be performed upon silicon by means of lithography and ion-beam etching, is promising for low cost and high-efficient solar cell applications.     

Redirection of sunlight by microstructured components – Simulation,fabrication and experimental results

This paper presents a non-tracking microstructured light redirecting device, which can be integrated into architectural glass. When fixed in the upper area of a window above eye level it redirects the light from solar altitudes between 15° and 65° and illuminates a room without causing glare.Ray-tracing calculations are employed as a tool for identifying suitable configurations and geometries. The results of the simulations show the advantage of combinations of lens-like with prism-like geometries in comparison to conventional microprism arrays regarding the overall light redirection efficiency as well as the producibility. The redirecting device is more lightweight, gives better integration options and is producible in a more economic manufacturing process as systems with similar performance. Measurements of cast silicone prototypes (100 mm × 100 mm × 4 mm) confirmed the simulation results. By now the performance has also been shown by large scale industrially produced acrylic panels with dimensions of 1500 mm × 400 mm × 4 mm.     

Combustion of micro aluminum–water mixtures

An experimental investigation on the combustion behavior of micro-sized aluminum (μAl)–water mixtures was conducted. It was easily ignited and self-deflagrated on μAl and liquid water when using a paper shell tube. Linear burning rates of quasi-homogeneous mixtures of μAl and liquid water as a function of pressure, mixture composition, density and environment gas medium were measured. Steady-state burning rates were obtained at room temperature using a windowed vessel for a pressure range of 1–80 bar in a nitrogen atmosphere, particle size of 0.5 × 30 × 30 μm and overall mixture equivalence ratios from 0.67 to 2.0. The pressure exponent was obtained as 0.47 at room temperature and compared to the case of nano-sized aluminum (nAl) and liquid water. When a wire was inserted into the sample, for increasing local heat transfer, burning rates were found to be faster.     

Ag–Al based air braze for high temperature electrochemical devices

Silver–aluminum based air brazing was attempted using an in situ alloying and brazing process. In this process, layers of foils of aluminum and silver were laid up between alumina plates in alternating fashion to achieve three target compositions representing Ag, Ag₃Al, and Ag₂Al phases. Each alloy composition revealed different microstructure, mechanical properties and fracture mechanisms. Joints brazed with foils containing 9.8 at% Al formed a long continuous layer parallel to the direction of the original aluminum foil. The fracture occurred at low bend strength (6–12 MPa) mainly through the interface between this newly formed long alumina layer and the braze filler. Joints containing 26.5 at% Al in the braze filler metal experienced the series of phase transformations, leading to cracks in as-brazed specimens. The fracture initiated through these pre-existing cracks, thus the joint strength observed in these specimens was extremely low. The joints prepared using foils with 35.1 at% Al exhibited a good interface even though interfacial alumina particles formed during air brazing. Crack propagation occurred along the interface between the alumina substrate and in situ formed interfacial alumina particles or directly through these particles. Due to the good interface, the best bend strength (46–52 MPa) was achieved for the braze filler containing 35 at% Al.     

Enhancement of pool-boiling heat transfer using nanostructured surfaces on aluminum and copper

Enhanced pool-boiling critical heat fluxes (CHF) at reduced wall superheat on nanostructured substrates are reported. Nanostructured surfaces were realized using a low temperature process, microreactor-assisted-nanomaterial-deposition. Using this technique we deposited ZnO nanostructures on Al and Cu substrates. We observed pool-boiling CHF of 82.5 W/cm² with water as fluid for ZnO on Al versus a CHF of 23.2 W/cm² on bare Al surface with a wall superheat reduction of 25–38 °C. These CHF values on ZnO surfaces correspond to a heat transfer coefficient of ～23,000 W/m² K. We discuss our data and compare the behavior with conventional boiling theory.     

Effect of oxide shell growth on nano-aluminum thermite propagation rates

Flame propagation rates for nanometric particle composites of aluminum (Al) and molybdenum trioxide (MoO₃) were examined. The Al particles were prepared by thermally treating the particles at 480 °C for time increments up to 180 min in oxygen and 90 min in argon. This treatment caused the aluminum passivation shell to grow and there is also evidence of shell damage due to treatment. Results reveal several interesting behaviors: flame speeds initially on the order of several hundred meters per second were reduced with damage to the oxide shell, and there is a weak dependence of the flame speed on the ratio of particle radius to shell thickness (M) in the range 6.1 < M < 13.4. The sharp drop in flame rate at further reduction in M down to 5.0 is consistent with a similar drop observed for adding alumina to the reactive mixture. All observations are consistent with the melt dispersion mechanism associated with Al nanoparticle oxidation.     

Interfacial heat transfer during microdroplet evaporation on a laser heated surface

A comprehensive experimental and numerical investigation on water microdroplet impingement and evaporation is presented from the standpoint of phase-change cooling technologies. The study investigates microdroplet impact and evaporation on a laser heated surface, outlining the experimental and numerical conditions necessary to quantify the interfacial thermal conductance (G) of liquid-metal interfaces during two-phase flow. To do this, continuum-level numerical simulations are conducted in parallel with experimental measurements facilitating high-speed photography and in-situ time-domain thermoreflectance (TDTR). During microdroplet evaporation on laser heated Al thin-films at room temperature, an effective interfacial thermal conductance of G_eff = 6.4 ± 0.4 MW/m² is measured with TDTR. This effective interfacial thermal conductance (G_eff) is interpreted as the high-frequency (ac) interfacial heat transfer coefficient measured at the microdroplet/Al interface. Also on a laser heated surface, fractal-like condensation patterns form on the Al surface surrounding the evaporating microdroplet. This is due to the temperature gradient in the Al surface layer and cyclic vapor/air convection patterns outside the contact line. Laser heating, however, does not significantly increase the evaporation rate beyond that expected for microdroplet evaporation on isothermal Al thin-film surfaces.     

Electrochemical behaviour of aluminium in non-aqueous electrolytes over a wide potential range

The electrochemical behaviour of aluminium in LiClO₄–propylene carbonate electrolyte is studied by cyclic voltammetry, steady-state polarisation, and ac impedance spectroscopy in the potential range −0.4–4.2 V versus Li/Li⁺. The open-circuit potential of Al is 1.57 V versus Li/Li⁺, which is about 0.2 V above the thermodynamic value of Al due to the presence of a surface passive film. In the positive potential region, Al is fairly stable between 1.57 and 3.5 V versus Li/Li⁺ owing to the presence of the surface film. Nevertheless, the oxidation of Al occurs at potentials >3.5 V versus Li/Li⁺. The ac impedance data are analysed by using a non-linear least-squares fitting procedure, and the surface film resistance is found to be between 498 and 1032 kΩ cm⁻². In the potential range 3.6–4.2 V versus Li/Li⁺, there is a breakdown of the passive film as demonstrated by a decrease in its resistance to 1.2–4.8 kΩ cm⁻². This breakdown accompanies anodic oxidation of Al. Thus, there is a possibility of anodic degradation of the Al substrate that is usually used as the current-collector of positive electrodes of Li-ion batteries, if Al is exposed to the electrolyte. In the negative potential region, the deposition of uniform and non-dendritic Li occurs, which can be anodically stripped in a quasi-reversible process with high coulombic efficiency. Diffusion of Li into Al results in the formation of a surface layer of Li–Al alloy, as suggested by X-ray diffraction patterns. The quasi-reversible cathodic deposition and anodic stripping of Li with an exchange current density of 0.16 mA cm⁻² indicates that Al is useful as a negative electrode in Li-batteries.     

Degradation behaviour of Al–Fe coatings in wet-seal area of molten carbonate fuel cells

The corrosion resistance of Al–Fe coatings increases as a protective LiAlO₂ layer forms. If, however, the Al–Fe coatings lack sufficient aluminium for maintaining this protective layer, the corrosion resistance of the coating is degraded by the growth of non-protective scales, such as LiFeO₂. In this study, the degradation behaviour of Al–Fe coatings is investigated in the wet-seal environment of molten carbonate fuel cells (MCFC). Al–Fe coated specimens with various amounts of aluminium in the range 8–70 at.% and bulk specimens of Fe–23.9 Al (at.%) are prepared. A corrosion test is performed in Li/K carbonate systems at 650 °C with a single-cell and an immersion test. Test results reveal that aluminium contents in the coatings should be higher than 25 at.% in order to form and maintain a protective LiAlO₂ layer. In addition to aluminium content, the influence of microstructural features on the degradation behaviour of Al–Fe coatings is discussed.     

Absorbing photonic crystals for silicon thin-film solar cells: Design,fabrication and experimental investigation

In this paper, we present the integration of an absorbing photonic crystal within a thin-film photovoltaic solar cell. Optical simulations performed on a complete solar cell revealed that patterning the hydrogenated amorphous silicon active layer as a 2D photonic crystal membrane enabled to increase its integrated absorption by 28 % between 300 and 720 nm, comparing to a similar but unpatterned stack. In order to fabricate such promising cells, we developed a high throughput process based on holographic lithography and reactive ion etching. The influences of the parameters taking part in those processes on the obtained patterns are discussed. Optical measurements performed on the resulting “photonized” solar cell structures underline the regularity of the 2D pattern and a significant absorption increase above 550 nm, similarly to what is observed on the simulated absorption spectra. Moreover, our patterned cells are found to be robust with regards to the angle of incidence of the light.     

Ebeam fabrication of silicon nanodome photovoltaic devices without metal catalyst contamination

In this paper, the fabrication of silicon nanodome solar cells on crystalline wafers is reported. Crystalline silicon was patterned by ebeam lithography to define the silicon nano pillars with diameter of 100 nm, 1 μm and 5 μm. Unlike conventional bottom up growth of silicon nanowire from gold (Au), our method is free from contaminant. Consequently, it is a valuable method to fully evaluate the effect of nanostructures on solar cell performances. The fabricated devices were characterized through scanning electron microscopy, absorption measurements, illuminated solar cell I–V characteristics and monochromatic incident photon-to-electron conversion efficiency.     

Evaporation characteristics of kerosene droplets with dilute concentrations of ligand-protected aluminum nanoparticles at elevated temperatures

The evaporation characteristics of kerosene droplets containing dilute concentrations (0.1%, 0.5%, and 1.0% by weight) of ligand-protected aluminum (Al) nanoparticles (NPs) suspended on silicon carbide fiber were studied experimentally at different ambient temperatures (400–800 °C) under normal gravity. The evaporation behavior of pure and stabilized kerosene droplets was also examined for comparison. The results show that at relatively low temperatures (400–600 °C), the evaporation behavior of suspended kerosene droplets containing dilute concentrations of Al NPs was similar to that of pure kerosene droplets and exhibited two-stage evaporation following the classical d²-law. However, at relatively high temperatures (700–800 °C), bubble formation and micro-explosions were observed, which were not detected in pure or stabilized kerosene droplets. For all Al NP suspensions, regardless of the concentration, the evaporation rate remained higher than that of pure and stabilized kerosene droplets in the range 400–800 °C. At relatively low temperatures, the evaporation rate increased slightly. However, at relatively high temperatures (700–800 °C), the melting of Al NPs led to substantial enhancement of evaporation. The maximum increase in the evaporation rate (56.7%) was observed for the 0.5% Al NP suspension at 800 °C.     

Creep behavior of Ni-12 wt.% Al anodes for molten carbonate fuel cells

Ni-12 wt.% Al anodes are fabricated for use in molten carbon fuel cells by tape casting and sintering. Sintering is performed in three steps, first at 1200 °C for 10 min in argon, then at 700 °C for 2.5 h in a partial oxidation atmosphere (P_H2/P_H2O=10⁻²), and finally at 950 °C for 5 min, 30 min or 1.5 h in hydrogen. Three anodes with different phases or microstructures are produced at different reduction times. One anode contains three phases, namely Ni–Al solid solution, Ni₃Al, and Al₂O₃. The amount of Al₂O₃ is extremely small at 5 min. A second anode also contains the three phases with the amount of Al₂O₃ comparable with that of Ni₃Al at 30 min. Third anode contains two phases, i.e. Ni–Al solid solution and Al₂O₃ formed at 1.5 h. The creep strains measured for the three anodes after a 100-h creep test are practically the same with an average value of 0.85%.     

Synthesis and characterization of lithium aluminum-doped spinel (LiAlxMn2−xO4) for lithium secondary battery

LiAl_xMn_2−xO₄ has been synthesized using various aluminum starting materials, such as Al(NO₃)₃, Al(OH)₃, AlF₃ and Al₂O₃ at 600–800°C for 20 h in air or oxygen atmosphere. A melt-impregnation method was used to synthesize Al-doped spinel with good battery performance in this research. The Al-doped content and the intensity ratio of (3 1 1)/(4 0 0) peaks can be important parameters in synthesizing Al-doped spinel which satisfies the requirements of high discharge capacity and good cycleability at the same time. The decrease in Mn³⁺ ion by Al substitution induces a high average oxidation state of Mn ion in the LiAl_xMn_2−xO₄ material. The electrochemical behavior of all samples was studied in Li/LiPF₆-EC/DMC (1:2 by volume)/LiAl_xMn_2−xO₄ cells. Especially, the initial and last discharge capacity of LiAl_0.09Mn_1.97O₄ using LiOH, Mn₃O₄ and Al(OH)₃ complex were 128.7 and 115.5 mAh/g after 100 cycles. The Al substitution in LiMn₂O₄ was an excellent method of enhancing the cycleability of stoichiometric spinel during electrochemical cycling.     

Micro thermoelectric cooler: Planar multistage

A suspended, planar multistage micro thermoelectric (TE) cooler is designed using thermal network model to cool MEMS devices. Though the planar (two-dimensional) design is compatible with MEMS fabrication, its cooling performance is reduced compared to that of a pyramid (three-dimensional) design, due to a mechanically indispensable thin dielectric substrate (SiO₂) and technical limit on TE film thickness. We optimize the planar, six-stage TE cooler for maximum cooling, and predict ΔT_max = 51 K with power consumption of 68 mW using undoped, patterned 4–10 μm thick co-evaporated Bi₂Te₃ and Sb₂Te₃ films. Improvement steps of the planar design for achieving cooling performance of the ideal pyramid design are discussed. The predicted performance of a fabricated prototype is compared with experimental results with good agreements.     

Electrooxidation of ascorbic acid on polyaniline and its implications to fuel cells

l-Ascorbic acid (AA) has been shown to undergo oxidation on polyaniline (PANI) without a platinum-group catalyst. A direct ascorbic acid fuel cell (DAAFC) has been assembled by employing an anode coated with PANI catalyst. From the experimental studies using cyclic voltammetry, amperometry and IR spectroscopy, it has been concluded that PANI facilitates the oxidation of AA. It has been possible to achieve a maximum power density of 4.3 mW cm⁻² at a load current density of 15 mA cm⁻² at 70 °C. As both AA and PANI are inexpensive and environmental-friendly, the present findings are expected to be useful for the development of cost-effective DAAFCs for several low power applications.