Effects of growth rate on the microcrystalline characteristics of plasma-deposited μc-Si:H

Hydrogenated microcrystalline silicon (μc-Si:H) has been prepared by the rf glow discharge of a SiH₄ + H₂ gas mixture. The average grain size and the volume fraction of the crystalline are simultaneously increased by decreasing the growth rate of μc-Si:H. Furthermore, the average grain size of the crystallites is increased by increasing the substrate temperature while the volume fraction of the crystallites is less sensitive to the substrate temperature. The orientation of crystallites in μc-Si:H is randomly distributed and independent of the silane concentration and substrate temperature as well. The dependence of the average grain size on growth rate is qualitatively explained by a phenomenological theory.     

Development of high efficiency p-i-n amorphous silicon solar cells with the p-μc-Si:H/p-a-SiC:H double window layer

A structure is developed to help improve the TCO/p contact and efficiency of the solar cell. A p-i-n amorphous silicon (a-Si:H) solar cell with high-conversion efficiency is presented via use of a double p-type window layer composed of microcrystalline silicon and amorphous silicon carbide. The best efficiency is obtained for a glass/textured TCO/p-μc-Si:H/p-a-SiC:H/buffer/i-a-Si:H/n-μc-Si:H/GZO/Ag structure. Using a SnO₂/GZO bi-layer and a p-type hydrogenated microcrystalline silicon (p-μc-Si:H) layer between the TCO/p-a-SiC:H interface improves the photovoltaic performance due to reduction of the surface potential barrier. Layer thickness, B₂H₆/SiH₄ ratio and hydrogen dilution ratio of the p-μc-Si:H layer are studied experimentally. It is clearly shown that the double window layer can improve solar cell efficiency. An initial conversion efficiency of 10.63% is achieved for the a-Si:H solar cell.     

Thermochemical hydrogen preparation—Part V. A feasibility study of the sulfur iodine cycle

A sulfur-iodine cycle consists of the following three reactions:

$\text{2H}{}_2\text{O + SO}{}_2\text{+ I}{}_2\text{→ H}{}_2\text{SO}{}_4\text{+ 2HI,}$

$\text{H}{}_2\text{SO}{}_4\text{→ H}{}_2\text{O + SO}{}_2\text{+}\text{1}\text{2O}{}_2\text{,}$

$\text{2 HI → H}{}_2\text{+ I}{}_2\text{.}$

It was found that the first reaction can be performed as a cell reaction without the addition of external energy. The sulfuric acid and the hydriodic acid are produced separately in the anode and cathode compartments, respectively. The second and third reactions can be carried out as catalytic thermal decompositions. A process flow sheet of this cycle and its mass balance was based on experimental results, and the heat balance for this cycle was made. It was found that internal heat exchange for this cycle was very large (about 2600 kcal/mol H₂), due mainly to the low yield of the decomposition reaction of hydrogen iodide. Theoretical and experimental studies were made to improve the yield of this reaction. The following three methods seem to be promising for this purpose: (1) continuous removal of the hydrogen produced in the reaction zone; (2) performance of the reaction at low temperature (185–250°C) and high pressure (100 atml; and (3) substitution of the benzene-cyclohexane cycle (6HI ? C₆H₆ → C₆H₁₂ + 3I₂; C₆H₁₂ → C₆H₆ ? 3H₂) for the hydrogen iodide decomposition step.     

Influence of hydrogen in magnetic properties of Ce(Fe1 − xAlx)2

Results on the effect in magnetic and structural properties of hydrogen in Ce(Fe_{1 − x}Al_x)₂ samples, dissolved up to the terminal solid solubility, are presented. A comparison of our data with previously reported studies on the magnetic properties of the system under pressure allow us to determine that hydrogen affects magnetic properties of this system further than structural effects. In addition, magnetic and structural preliminary results on the hydride phase of this system are presented.     

Optimization of flame stabilization methods in the premixed microcombustion of hydrogen–air mixture

The premixed combustion of a lean hydrogen–air mixture is analyzed in this study to examine various properties and flame stabilization. A two-dimensional (2D) analysis of a microscale combustor is performed with various shapes of bluff bodies (e.g., circular and triangular). Nine bluff bodies are placed at the entrance of the microscale combustor and solved with 2D governing equations. The analysis is performed with the three velocities of 10, 20, and 30 m/s, but the equivalence ratio is fixed in all cases. The various characteristics of the microscale combustor are studied such as the temperature of the wall, difference in peak temperature, the mean velocity at the outlet, and temperature of the exhaust gases. Flame stabilization depends on various factors such as bluff body shape and size, and the velocity of the fuel–air mixture at the inlet and recirculation zone. In comparison to all bluff body cases, we observe that the wall blade bluff body is the most efficient (low exhaust gas temperature, large recirculation zone, low mean velocity at the outlet of the microcombustor, and high wall temperature) compared with all eight other bluff body cases. Combustion efficiency is directly proportional to the wall temperature, meaning that the microcombustor with wall blade bluff bodies is more efficient with a stabilized flame. The simulation results are compared with published data on an L/D ratio of 15.     

The temperature dependence of the characteristics of sputtered a-SiH solar cells

It is now well established that the properties of hydrogenated amorphous silicon are highly dependent on the preparation conditions. In this paper we describe the Schottky barrier characteristics of cells incorporating a-SiH grown at different substrate temperatures and in various hydrogen partial pressures. The characteristics of the cells in the dark and under illumination are highly dependent on the type of the dominant conduction process. The illuminated cell characteristics are described for cells with efficiencies of 2%. The open-circuit voltage V_oc and the short-circuit current I_sc are shown to be temperature dependent and the dependence is more pronounced for non-optimum cells than for optimum devices. The spectral response for the cells is also described.     

Prospects of “solar” hydrogen for desert development in the Arab world

In this paper, the Arab world is surveyed initially for the availability of new and renewable energy sources (NRES) including solar energy. A classification is made based on the level of development and on the energy balance of each Arab country. The target is to utilize these NRES for hydrogen production and hence for desert development claiming more arable land. The emphasis is on using solar energy.Hydrogen will be harnessed along the following avenues: (a) to provide energy for land development, (b) to provide energy for pumping and irrigation, (c) to produce fresh water, (d) to produce fertilizers based on ammonia as a starting raw material. Case studies are presented for Egypt and Saudi Arabia.     

Evaluation of thermodynamic parameters for hydrogen in the storage device ST-90®

The thermodynamic parameters such as partial molar enthalpy (ΔH_H), partial molar entropy (ΔS_H), and partial molar excess entropy (ΔS^E_H) of the dissolved hydrogen in the hydrogen storage unit ST-90^® containing 18.6 kg of misch metal based AB₅ alloy are evaluated based on the calculation of Sieverts constant and from Clausius–Clapeyron equation. The partial molar enthalpy at infinite dilution (ΔH⁰_H) and the partial molar excess entropy at infinite dilution (ΔS^E,0_H) have been obtained from the dependence of and on the hydrogen concentration. The nature of metal-hydrogen and hydrogen interactions are also evaluated for the three single phases namely α, β and γ. The hydrogen–hydrogen interaction is attractive in the α phase and repulsive in the β and γ phases. For the mixed phases α+β and β+γ, it is very difficult to predict the nature of the interactions either as attractive or repulsive.     

Hydrogen microscopy – Distribution of hydrogen in buckled niobium hydrogen thin films

Hydrogen absorption in thin metal films clamped to rigid substrates results in mechanical stress that changes the hydrogen's chemical potential by Δμ_H(σ) = −1.124σ kJ/mol_H for σ measured in [GPa]. In this paper we show that local stress relaxation by the detachment of niobium hydrogen thin films from the substrate affects the chemical potential on the local scale: using coincident proton–proton scattering at a proton microprobe, the hydrogen concentration is determined with μm resolution, revealing that hydrogen is not homogenously distributed in the film. The local hydrogen solubility of the film changes with its local stress state, mapping the buckled film fraction. In niobium hydrogen thin films loaded up to nominal concentrations in the two-phase coexistence region, the clamped film fraction remains in the solid solution phase, while the buckles represent the hydride phase. These results are compared to a simple model taking the stress impact on the chemical potential into account.     

Wind hydrogen energy system and the gradual replacement of natural gas in the State of Ceará – Brazil

The original model for the solar hydrogen energy system created by Veziroglu and Basar in the 70’s was adapted to the State of Ceará – Brazil. The State of Ceará has one of the greatest wind potentials in Brazil and it is estimated to be around 35 GW. At the present year, there are 494 MW wind farms in operation. The aforementioned State also has a natural gas grid of pipelines serving a great number of consumers. There are studies in literature considering the injection of hydrogen into the natural gas pipeline up to 20% in volume without substantial modifications in the natural gas infrastructure. The main objective of this article is to use that model in order to evaluate long term scenarios in which the off peak wind generated hydrogen gradually replaces natural gas in such important State of Brazil. The system is supposed to start in the year 2015 and the economical revenue when it is fully implemented can reach respectively US$ 730 million or US$ 1 billion in the slow or fast scenario of hydrogen introduction into the energy matrix of that important State of Brazil.     

NICKEL–COBALT oxide-coated electrodes: influence of the preparation technique on oxygen evolution reaction (OER) in an alkaline solution

The oxygen evolution reaction (OER) was investigated at 60°C in 15 wt% NaOH solution on NiCo₂O₄ electrodes. Nickel–cobalt oxides were obtained by different preparation techniques and they were applied to chemically pickled nickel plates. Fresh and artificially aged electrodes were subjected to electrochemical tests. The ageing was performed for 21 days. The correlation between the coating preparations and their electrochemical performances was investigated by analysis of the electrochemical data. The efficiency of the NiCo₂O₄ electrodes changed significantly with the changing of the preparation method. Two Tafel slopes for the OER were observed in most cases. This behaviour was interpreted on the basis of a change in the valence state of the oxide. The kinetic parameters obtained on NiCo₂O₄ electrodes were compared and discussed according to the preparation methods.     

Hydrogen environment embrittlement of an ODS RAF steel – Role of irreversible hydrogen trap sites

The microstructure and the effects of 10 MPa hydrogen atmosphere on the tensile properties of a oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel were investigated. The microstructure consists of a fine grained ferritic matrix with Me₃O₄ (Me = Cr, Fe or Mn), VN and Cr₂₃C₆ grain boundary precipitates as well as dispersed yttrium oxide nano precipitates in the ferritic matrix. The yield and ultimate tensile strength were unaffected by the H₂ atmosphere whereas elongation at fracture and reduction in area were markedly reduced. In H₂ atmosphere, the fracture morphology was found to be a mixture of intergranular H-assisted fracture and a smaller amount of transgranular hydrogen enhanced localized plasticity (HELP) fracture. The sensitivity of the ODS RAF steel to hydrogen embrittlement is attributed to the large number grain boundary precipitates which enhance the tendency for intergranular fracture.     

Experimental study of hydrogen explosion in repeated pipe congestion – Part 2: Effects of increase in hydrogen concentration in hydrogen-methane-air mixture

Hydrogen is seen as an important energy carrier for the future which offers carbon free emissions. At present it is mainly used in refueling hydrogen fuel cell cars. However, it can also be used together with natural gas in existing gas fired equipment with the benefit of lower carbon emissions. This can be achieved by introducing hydrogen into existing natural gas pipelines. These pipelines are designed, constructed and operated to safely transport natural gas, which is mostly methane. Because hydrogen has significantly different physical and chemical properties than natural gas, any addition of hydrogen my adversely affect the integrity of the pipeline network, increasing the likelihood and consequences of an accidental leak. Since it increases the likelihood and consequences of an accidental leak, it increases the risk of explosion. In order to address various safety issues related to addition of hydrogen in to a natural gas pipeline a EU project NATURALHY was introduced. A major objective of the NATURALHY project was to identify how much hydrogen could be introduced into the natural gas pipeline network. Such that it does not adversely impact the safety of the pipeline network and significantly increase the risk to the public. This paper reports experimental work conducted to measure the explosion overpressure generated by ignition of hydrogen-methane-air mixture in a highly congested region consisting of interconnected pipes. The composition of the methane/hydrogen mixture used was varied from 0% hydrogen (100% methane) to 100% hydrogen (0% methane) to understand its effect on generated explosion overpressure. It was observed that the maximum overpressures generated by methane-hydrogen mixtures with 25% (by volume) or less hydrogen content are not likely to be significantly greater than those generated by methane alone. Therefore, it can be concluded that the addition of less than 25% by volume of hydrogen into pipeline networks would not significantly increase the risk of explosion.     

A study on the hydrogen storage performance of graphene–Pd(T)–graphene structure

The 1–6 H₂ molecule adsorption energy and electronic properties of sandwich graphene–Pd(T)–Graphene (G–Pd(T)–G) structure were studied by the first-principle analysis. The binding energies, adsorption energies, and adsorption distances of Pd atoms-modified single-layer graphene and bilayer graphene with H₂ molecules at B, H, T adsorption sites were calculated. In bilayer graphene, the adsorption properties at T sites were found to be more stable than those at B and H sites. The binding energy of Pd atoms (4.16 eV) on bilayer graphene was higher than the experimental cohesion energy of Pd atoms (3.89 eV), and this phenomenon eliminated the impact of metal clusters on adsorption properties. It was found that three H₂ molecules were stably adsorbed on the G–Pd(T)–G structure with an average adsorption energy of 0.22 eV. Therefore, it can be speculated that G–Pd(T)–G is an excellent hydrogen storage material.     

Experimental study on the purification of HIx phase in the iodine–sulfur thermochemical hydrogen production process

Purification of hydriodic phase (HIx) plays an important role in avoiding the undesirable side reactions between HI and H₂SO₄ in the sulfur–iodine thermochemical cycle. In this paper, a series of experiments on HIx phase purification were conducted by means of a stirred reactor using N₂ as stripping gas. The effects of the iodine concentration, reaction temperature, and the striping gas flow rate on HIx phase purification were investigated systematically in terms of the conversion of H₂SO₄ and the reaction types during purification. It was observed that the iodine concentration played a significant role in dictating the reactions during purification. The quantitative analysis of the compositions of the initial and purified HIx phases showed that not only the conversion of H₂SO₄ was enhanced but also the side reactions were effectively impeded by increasing the iodine concentration, temperature and the stripping gas flow rate. Based on the experimental data, the suitable operating conditions for HIx phase purification were proposed.     

Membrane electrolysis of Bunsen reaction in the iodine–sulphur process for hydrogen production

In traditional IS process for production of hydrogen by water decomposition, the Bunsen reaction (SO₂ + I₂ + 2H₂O → H₂SO₄ + 2HI) was carried out by direct contact of SO₂ with aqueous solution of I₂ where a large excess of I₂ (8 mol) and H₂O (16 mol) were required. Excess amounts of these chemicals severely affected the overall thermal efficiency of the process and new ways including membrane electrolysis was reported in literature for carrying out Bunsen reaction where the amount of excess chemicals can be greatly reduced. We have carried out Bunsen reaction in a two-compartment membrane electrolysis cell containing graphite electrodes and Nafion 117 membrane as a separator between the two-compartments. Electrolysis was carried out at room temperature with continuous recirculation of anolyte and catholyte. Electrolysis was done in constant-current mode with current density in the range of 1.6 A/dm² to 4.8 A/dm². Initial concentrations of H₂SO₄ and HI were about 10 and 5 N, respectively and I₂/HI molar ratio in the catholyte was varied in the range of 0.25–1.5. Current efficiency was found to be close to 100% indicating absence of any side reaction at the electrodes. Cell voltage was found to vary linearly with current densities up to 80 A/dm² and for I₂/HI molar ratio in the range of 0.25–1.5 the cell voltage was found to be lowest for the value of 0.5.