Effect Of Hydrogen Dilution Research Articles

Pulsating one-dimensional detonation in ammonia-hydrogen–air mixtures

In this study, one-dimensional detonations in ammonia/hydrogen-air mixtures are numerically investigated by solving the fully compressible Navier-Stokes equations with detailed chemistry. Pulsating instabilities with single-mode are observed during the detonation wave propagation, accompanied by periodic coupling and decoupling of the lead shock wave and the reactive front. The ratio between driver pressure and initial pressure determines the overdrive degree and thus the oscillatory mode of detonation for a premixture with certain composition. The effects of hydrogen dilution and mixture equivalence ratio on pulsating detonations are also examined under a constant driver pressure. The growing hydrogen fraction in fuel blends significantly increases the oscillation frequency. In addition, the pulsating detonation frequency rises with increasing equivalence ratio under fuel-lean conditions, peaks under stoichiometric conditions, and falls under fuel-rich conditions as the equivalence ratio increases further. Evolutions of reactants, main intermediate radicals, and products are analysed in both fuel-lean and fuel-rich conditions. A chemical explosive mode analysis further confirms the highly-autoignitive nature of the mixture in the induction zone between reaction front and shock front where thermal diffusion plays a negligible role.

A sequential CFD-chemistry investigation of the chemical and dilution effects of hydrogen addition on the combustion of PRF-85 in an HCCI engine

The effects of hydrogen addition on the combustion characteristics of a primary reference fuel PRF-85 in a Homogeneous Charge Compression Ignition engine have been investigated analytically. The numerical modeling was focused on both chemical and dilution effects of hydrogen enrichment. The analysis used a sequential procedure combining a fluid mechanics code (IC Engine coupled with ANSYS FLUENT) to determine temperature and mass distributions as a function of crank angle, assuming motored conditions, and a mutli-zone model (ANSYS Chemkin multi-zone HCCI model) to calculate combustion and engine efficiency using a detailed chemical kinetic mechanism containing 1550 species evolved in 6000 reactions. Analysis was performed using an HCCI engine with a constant speed (1200 rpm) and an intake manifold pressure of 1 bar. The obtained results indicate that hydrogen dilution effect retarded combustion phasing, decreased both combustion duration and specific fuel consumption, and boosted the engine thermal efficiency. Opposite trends were observed for the chemical effect. In all cases, it was found that, whatever the hydrogen percentage in the fuel mixture, the dilution effect was very important as compared to the chemical one, suggesting that combustion characteristics and engine power output variations with hydrogen enrichment were essentially due to the dilution effect.

Effect of hydrogen dilution on the silicon cluster volume fraction of a hydrogenated amorphous silicon film prepared using plasma-enhanced chemical vapor deposition

We investigated the effect of hydrogen dilution on the Si cluster volume fraction of hydrogenated amorphous films by varying the hydrogen dilution ratio at 0.5 Torr and compared it to that obtained at pure silane discharge at 0.3, 0.4, and 0.5 Torr. The correlation between the plasma emission characteristic, deposition rate, and cluster volume fraction in the hydrogen dilution plasma was described. The cluster volume fractions of films under hydrogen dilution conditions were similar to those of the pure silane but showed a higher deposition rate. The results suggest that under hydrogen dilution conditions, it is possible to maintain a higher deposition rate with a lower cluster incorporation rate.

Intrinsic thin film properties study of hydrogenated silicon using the method of RF-PECVD

Effect of hydrogen dilution on deposition rate, energy band gap, localized state defect - through observation of energy absorption Urbach, and surface roughness of intrinsic thin film was investigated using RF-PECVD. Films were grown on ITO glass substrate. Analysis of energy band gap was conducted to determine changes in the structure of a thin film of a-Si:H. Energy band gap is important to determine the portion of the spectrum of sunlight that is absorbed solar cells. The sun’s rays with energy greater than the energy band gap will be absorbed by the solar cell, but the rest will be the thermal energy which results in low efficiency. Characterization using UV-Vis spectrometer and Tauc’s plot methods showed that wide energy band gap was greater for larger hydrogen dilution. Moreover, the increase of the hydrogen dilution will decrease the rate of deposition and the Urbach energy. It is estimated that with an increased dilution of hydrogen will be obtained μc-Si:H film structure. This structure is more conductive due to the reduction of residual bandtail defect or dangling bond defects.

The effects of hydrogen dilution, carbon monoxide poisoning for a Pt–Ru anode in a proton exchange membrane fuel cell

The effects of carbon monoxide (CO) poisoning and hydrogen dilution are investigated for a series of simulated reformate gases. A simple mathematical model with an internal air bleed term is proposed to understand the interaction between CO poisoning, hydrogen dilution, and the internal air bleed (IAB). According to the steady-state modeling results, the presence of nitrogen can be considered as a diluting agent for a relatively dilute H2–N2 mixture. For a CO–H2–N2 mixture, hydrogen dilution and CO poisoning “magnify” each other in their combined effect on the surface coverage of hydrogen. The beneficial effect of IAB was found to be affected by the competitive adsorption between CO and hydrogen. The steady-state surface coverage of CO is influenced by the adsorption and desorption of CO, the interaction between CO and the electro-oxidation of hydrogen, the interaction between IAB/hydrogen dilution/CO, the interaction between IAB/CO. However, the steady-state surface coverage of CO is largely determined by its own adsorption and desorption properties. Analytical solutions are also derived from the proposed mode for the transient surface coverage of hydrogen and CO. The mathematical treatment has been proved extremely effective for the understanding of the steady-state and transient behavior of the CO–H2 and CO–H2–N2 mixtures.

Modeling of Single and Dual Frequency Capacitive Discharge in Argon Hydrogen Mixture − Dynamic Effects and Ion Energy Distribution Functions

Single (SF) and dual frequency capacitively coupled plasma (DF CCP) in (0–50%) H2/Ar mixtures was studied numerically with 1D hybrid (Particle‐in‐cell with Monte Carlo Collision + fluid) model. In SF (81 MHz) discharge the effect of hydrogen dilution is studied for the fixed discharge input power. Mechanisms of the plasma density decrease and variation of ions composition with H2 percentage are discussed. The effect of low‐frequency voltage on plasma density, ions composition, ions fluxes, and ions energy was studied in DF CCP (1.76–81 MHz) discharge in 5% H2/Ar gas mixture at low (20 mTorr) and high (100 mTorr) pressures.

The Effect of Hydrogen Dilution on the Morphology of Carbon Nanowalls Decorated Carbon Nanotubes Deposited by Hot-Wire r.f. Plasma Enhanced Chemical Vapour Deposition

An investigation on the effects of hydrogen (H2) gas dilution on the morphology and growth of carbon nanowalls (CNWs) decorated carbon nanotubes (CNTs) by hot-wire r.f. plasma enhanced chemical vapour deposition technique is presented. With the assistance of nickel nanoparticle catalyst, CNWs decorated CNTs formed only under the presence of 25% of H2 gas, relative to the methane (CH4) gas precursor. By varying the amount of H2 incorporated with CH4, the role of H2 dilution in the development of CNWs decorated CNTs was studied. Based on the FESEM and HRTEM results, it is hypothesized that H2 density and relative carbon radical concentration are the important parameters for the deposition of CNWs decorated CNTs. The effect of H2 dilution on the formation of CNWs decorated CNTs is presented.

Structural, hydrogen bonding and in situ studies of the effect of hydrogen dilution on the passivation by amorphous silicon of n-type crystalline (1 0 0) silicon surfaces

Hydrogenated amorphous silicon (a-Si : H) layers deposited by chemical vapour deposition provide an attractive route to achieve high-performance crystalline silicon (c-Si) solar cells due to their deposition at low temperatures and their superior passivation quality. Hydrogen certainly plays an additional crucial role by passivating the dangling bonds, and thus improving the electrical and optical properties.In this work, we present the variation of the effective lifetime with the hydrogen dilution ratio R = (H2/SiH4). We find that at lower hydrogen dilution rates (R < 2), the polymerization reaction of silane molecules in the plasma bulk as well as the excess of dihydride (Si–H2) incorporation in as-deposited layers result in higher microvoid density and worse passivation quality. In contrast, the deposition at higher hydrogen dilution rates (R > 5) leads to a degradation in the film quality with very low hydrogen content and crystalline epitaxial growth at the a-Si/c-Si interface. Thus, the best material quality is obtained just below the onset of amorphous-to crystalline transition (R = 3), which favours further improvement of the passivation during a post-deposition annealing.

Deoxygenation and cracking of free fatty acids over acidic catalysts by single step conversion for the production of diesel fuel and fuel blends

Gas phase deoxygenation of oleic acid was accomplished over palladium containing acidic catalysts under solvent-free conditions. Aluminum oxide catalysts were prepared out of three different boehmite precursors by calcination at different temperatures between 500°C and 1100°C forming different aluminum oxide phases. All catalysts were impregnated with 1wt.% palladium and subsequently tested for their deoxygenation performance either batch wise in autoclaves or continuously in a fixed bed plug flow reactor or in a trickle bed reactor. Conversion of oleic acid and selectivity to the desired diesel-like C17 hydrocarbons heptadecane and heptadecenes were studied with the best aluminum oxide found in the catalyst screening at different reaction temperatures, catalyst amounts and palladium amounts on the catalyst. Using boehmite found as best alumina precursor, different mixtures of such aluminum oxide with zeolites were prepared in order to increase the catalyst acidity and thus the deoxygenation performance. The optimum reaction conditions found for aluminum oxide catalysts were applied for the screening of these composite catalysts. Using the best composite catalyst, the effect of hydrogen dilution with nitrogen, the addition of water and catalyst regeneration by thermal treatment was tested. In addition catalyst containing silica aluminates doped with 1wt.-% Pd were prepared and tested for their catalytic activity in deoxygenation and cracking of free fatty acids. Some reactions were conducted under elevated hydrogen pressure. The catalysts were characterized by nitrogen adsorption isotherms (BET), temperature programmed desorption of ammonia (TPD), X-ray powder diffraction (XRD), thermo gravimetric analysis (TGA) and field emission scanning electron microscopy (FESEM).

A novel method to make boron-doped microcrystalline silicon thin films with optimal crystalline volume fraction for thin films solar cell applications.

Highly conducting boron-doped microcrystalline silicon (p-type μc-Si:H) thin films have been prepared by radio frequency plasma-enhanced chemical-vapor deposition (RF-PECVD). In this work, the effects of hydrogen dilution, doping ratio, plasma power, deposition pressure and substrate temperature on the growth and the properties of boron-doped microcrystalline silicon (p-type μc-Si:H) thin films are investigated. The electrical, chemical and structural properties are improved with increasing crystallite, which depends on the plasma conditions. For various plasma parameters, the crystalline volume fraction (X(c)), dark conductivity (σ(d)), activation energy (E(a)), hydrogen content (C(H)), surface roughness (S(r)), and micro void fraction (R*) were measured, and they were 0-72%, 4.17-10(-4) S/cm-1.1 S/cm, 0.041-0.113 eV, 3.8-11.5 at.%, 3.2 nm-12.2 nm, and 0.47-0.80, respectively. The film with R* of 0.47 and C(H) of about 5 at.% belonged to a region of low disorder, and acted as a good passivation layer.

Raman spectroscopic investigation of boron doped hydrogenated amorphous silicon thin films

We report on the effects of hydrogen dilution and boron doping on the structural properties of amorphous silicon thin films prepared by plasma enhanced chemical vapor deposition. Raman studies were performed on samples differing in both boron concentration and hydrogen dilution of the silane precursor prepared at two growth temperatures. Changes in the transverse optical and acoustic phonon modes were analyzed to determine the effect of boron and hydrogen on short- and mid-range order of the amorphous crystal structure. The results show that, with either an increase of hydrogen dilution or growth temperature, the short- and mid-range order improves. However, the effect of growth temperature on the ability of hydrogen to improve the short- and mid-range order decreases as the growth temperature is increased. This is attributed to dissociation of weakly bonded species at higher growth temperature, leading to less hydrogen being absorbed. In addition, for fixed hydrogen dilution, an increase in boron doping results in a decrease in the short-range order due to a rearrangement of the chemical bonding in the thin film. Finally, a direct correlation is seen between the electrical resistivity and the short-range order for samples of differing hydrogen dilution grown at the higher growth temperatures.

Silicon–hydrogen bond effects on aluminum-induced crystallization of hydrogenated amorphous silicon films

The effects of hydrogen dilution on aluminum-induced crystallization (AIC) of hydrogenated amorphous silicon (a-Si:H) films have been studied. The Raman spectra showed that the short-range order (SRO) and the intermedium-range order (IRO) of the as-deposited a-Si films increased with the increase of the H2 dilution from 0% to 20%. The optical microscope (OM) and X-ray diffraction (XRD) observation revealed that, compared to the a-Si:H film deposited in pure Ar, the a-Si:H films deposited with H2 dilution in the range of 3–8% possessed a lower crystallization rate while the a-Si:H films deposited with high H2 dilution in the range of 15–20% possessed a faster crystallization rate. It was found that majority of the hydrogen existed in the form of monohydride (SiH) bond in the a-Si:H films with H2 dilution ratio of 3–8%, the bonding energy of which was higher than that of Si–Si bond, leading to a lower crystallization rate of a-Si:H films. While the dihydride (SiH2) bond became dominant in the a-Si:H films with high H2 dilution of 15–20%, the bonding energy of which was lower than that of Si–Si bond, thus accelerating the crystallization rate. Therefore, it was illustrated that not the hydrogen concentration but the form of silicon–hydrogen bond determined the AIC process of a-Si:H films.

Effects of hydrogen dilution on CNx film properties deposited using rf PECVD from a mixture of ethane, nitrogen and hydrogen

Carbon nitride (CNx) thin films were deposited using radio frequency plasma enhanced chemical vapor deposition (rf PECVD) from a mixture of ethane (C2H6), nitrogen (N2) and hydrogen (H2) gases. The C2H6 and N2 flow rates were kept constant, while the H2 flow rate was varied. The effects of hydrogen dilution on the growth rate and structural properties of the films were studied. It was found that a significant increase in the films growth rate was observed with the introduction of H2 at as low as 25 standard cubic centimeters per minute (sccm). A set of CNx films deposited from C2H6:N2 mixture without any inclusions of H2 were also presented in this work as a reference to compare the differences between those two sets and to understand the roles of H2 to the films properties. At highest H2 flow rate, the structure of the films changed from polymeric to graphitic and the quenching of PL was observed. Furthermore, higher N incorporation with lower Eg was obtained for these films compared to those of C2H6:N2 films. The change in the structure of the films corresponds to changes in their chemical bonding. As N incorporation increased, the porosity of the films increases and thus affects the disorder in the film structures.

Chemical and structural properties of polymorphous silicon thin films grown from dichlorosilane

We have examined the effects of hydrogen dilution (RH) and deposition pressure on the morphological, structural and chemical properties of polymorphous silicon thin films (pm-Si:H), using dichlorosilane as silicon precursor in the plasma enhanced chemical vapor deposition (PECVD) process. The use of silicon chlorinated precursors enhances the crystallization process in as grown pm-Si:H samples, obtaining crystalline fractions from Raman spectra in the range of 65–95%. Atomic Force Microscopy results show the morphological differences obtained when the chlorine chemistry dominates the growth process and when the plasma–surface interactions become more prominent. Augmenting RH causes a considerable reduction in both roughness and topography, demonstrating an enhancement of ion bombardment and attack of the growing surface. X-ray Photoelectron Spectroscopy results show that, after ambient exposure, there is low concentration of oxygen inside the films grown at low RH, present in the form of SiO, which can be considered as structural defects. Instead, oxidation increases with deposition pressure and dilution, along with film porosity, generating a secondary SiOx phase. For higher pressure and dilution, the amount of chlorine incorporated to the film decreases congruently with HCl chlorine extraction processes involving atomic hydrogen interactions with the surface. In all cases, weak silicon hydride (SiH) bonds were not detected by infrared spectroscopy, while bonding configurations associated to the silicon nanocrystal surface were clearly observed. Since these films are generally used in photovoltaic devices, analyzing their chemical and structural properties such as oxygen incorporation to the films, along with chlorine and hydrogen, is fundamental in order to understand and optimize their electrical and optical properties.

Effect of Hydrogen Dilution on Properties of Microcrystalline Silicon Thin Films Prepared by VHF-PECVD

Microcrystalline silicon thin films were deposited on glass substrates by VHF-PECVD varying the ratio of hydrogen dilution from 88% to 98%. The structural characteristics, deposition rate and photosensitivity of the films were investigated. With the improvement of the hydrogen dilution ratio, crystallization rate of the films had been improved which was much more stable than amorphous silicon that the films transmit from amorphous silicon to microcrystalline silicon. However the deposition rate had been reduced with the increase of the hydrogen dilution and the highest deposition rate was 0.43nm/s. The samples showed a downward trend of photosensitivity with optical and dark conductivity both decreasing first then increasing. Thus suitable hydrogen dilution ratio should be chosen according to the different needs in preparation of microcrystalline silicon film.

Effects of Hydrogen Dilution on ZnO Thin Films Fabricated via Nitrogen-Mediated Crystallization

Hydrogenated ZnO thin films have been successfully deposited on glass substrates via a nitrogen mediated crystallization (NMC) method utilizing RF sputtering. Here we aim to study the crystallinity and electrical properties of hydrogenated NMC-ZnO films in correlation with substrate temperature and H2 flow rate. XRD measurements reveal that all the deposited films exhibit strongly preferred (001) orientation. The integral breadth of the (002) peak from the hydrogenated NMC-ZnO films is smaller than that of the conventional hydrogenated ZnO films fabricated without nitrogen. Furthermore, the crystallinity and surface morphology of the hydrogenated NMC-ZnO films are improved by increasing substrate temperature to 400 °C, where the smallest integral breadth of (002) 2θ–ω scans of 0.83° has been obtained. By utilizing the hydrogenated NMC-ZnO films as buffer layers, the crystallinity of ZnO:Al (AZO) films is also improved. The resistivity of AZO films on NMC-ZnO buffer layers decreases with increasing H2 flow rate during the sputter deposition of buffer layers from 0 to 5 sccm. At a H2 flow rate of 5 sccm, 20-nm-thick AZO films with low resistivity of 1.5×10-3 Ω cm have been obtained.

Effect of Hydrogen Dilution on the Nanostructural and Electrooptical Characteristics of Hydrogenated Nanocrystalline Silicon Thin Films Prepared by Plasma Enhanced Chemical Vapor Deposition

Nanocrystalline hydrogenated amorphous silicon (nc-Si:H) thin films were deposited on silicon wafers and glass by plasma-enhanced chemical vapor deposition. The hydrogen dilution in the precursor gases, [SiH4/H2], were varied from 1 to 0.01 with the other deposition factors kept constant. The nanocrystallite size and volume fraction increased steadily with increasing hydrogen dilution ratio in the gas from 1 to 0.01. The mean size of the nanocrystallites ranged from ∼1 to ∼7 nm. The band gap of the films varied according to the hydrogen dilution, indicating the nanostructural features of the films. Film resistivity was dependent on the crystallite size and volume fraction in the films. In particular, the resistivity of a simple P–I–N type device decreased with increasing nanocrystallite size. The increased crystallinity can be explained by the predominance of Si–H bonds in the films.

Improvement of a-Si1−xGex:H single-junction thin-film solar cell performance by bandgap profiling techniques

In this work, the effect of hydrogen dilution on the Ge content of the film and the effect of bandgap grading in the a-Si1−xGex:H absorber near the p/i and the i/n interfaces on cell performance were discussed. The a-Si1−xGex:H single-junction solar cells were improved by employing both p/i grading and i/n grading. The i/n grading increased the VOC and the FF while it also reduced the JSC as compared to the cell without grading. Presumably the potential gradient established by the i/n grading facilitates the hole transport hereby improved by the FF. On the contrary, the potential barrier established by the p/i grading seemed to limit the cell performance and constrain the p/i grading width below 20nm due to the drop in FF and JSC. Combining the effects of bandgap grading on the VOC, JSC and FF, the suitable thicknesses of the p/i and the i/n grading were 20nm and 45nm, respectively. Finally, the grading structures accompanied with further optimization in doped layers were integrated to achieve a cell efficiency of 8.59%.

Low-Temperature Epitaxy of Compressively Strained Silicon Directly on Silicon Substrates

We report epitaxial growth of compressively strained silicon directly on (100) silicon substrates by plasma-enhanced chemical vapor deposition. The silicon epitaxy was performed in a silane and hydrogen gas mixture at temperatures as low as 150°C. We investigate the effect of hydrogen dilution during the silicon epitaxy on the strain level by high-resolution x-ray diffraction. Additionally, triple-axis x-ray reciprocal-space mapping of the samples indicates that (i) the epitaxial layers are fully strained and (ii) the strain is graded. Secondary-ion mass spectrometry depth profiling reveals the correlation between the strain gradient and the hydrogen concentration profile within the epitaxial layers. Furthermore, heavily phosphorus-doped layers with an electrically active doping concentration of ~2 × 1020 cm−3 were obtained at such low growth temperatures.

Effect of Hydrogen Dilution on the Metastability of Hydrogenated Amorphous Silicon Oxide Solar Cells

Effect of Hydrogen Dilution on the Metastability of Hydrogenated Amorphous Silicon Oxide Solar Cells