Decrease In Hydrogen Content Research Articles

Grain Enlargement of Polycrystalline Silicon by Multipulse Excimer Laser Annealing: Role of Hydrogen

The role of hydrogen introduced into melt-Si during excimer laser annealing (ELA) is examined from the viewpoint of grain enlargement. An amorphous silicon (a-Si)/SiN/quartz glass structure is successfully prepared by a catalytic chemical vapor deposition (Cat-CVD) method for a SiN film, in which the hydrogen concentration of the SiN film is controlled. The grain size increases as the hydrogen concentration decreases, and it partially exceeds 2 µm when the hydrogen concentration of the SiN film is fixed at 2.3 at. %. The relationship between defects at grain boundary and hydrogen is also considered.

Preparation of hydrogenated microcrystalline silicon films with hot-wire-assisted MWECR-CVD system

Intrinsic hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by hot-wire-assisted microwave electron-cyclotron-resonance chemical vapour deposition (HW-MWECR-CVD) under different deposition conditions. Fourier-transform infrared spectra and Raman spectra were measured. Optical band gap was determined by Tauc plots and experiments of photo-induced degradation were performed. It was observed that hydrogen dilution plays a more essential role than substrate temperature in microcrystalline transformation at low temperatures. Crystalline volume fraction and mean grain size in the films increase with the dilution ratio (R=H2/(H2+SiH4)). With the rise of crystallinity in the films, the optical band gap tends to become narrower while the hydrogen content and photo-induced degradation decrease dramatically. The samples, were identified as μc-Si:H films, by calculating the optical band gap. It is considered that hydrogen dilution has an effect on reducing the crystallization activation energy of the material, which promotes the heterogeneous solid-state phase transition characterized by the Johnson–Mehl–Avrami (JMA) equation. The films with the needed structure can be prepared by balancing deposition and crystallization through controlling process parameters.

Thin-Film Silicon in Photovoltaics: The Role of Structure and Microstructure

Structure and microstructure of hydrogenated silicon thin films and the influence of the hydrogen dilution of silane plasma at the PEVCD deposition on the film properties were investigated. The results show that at dilution between 20 and 30 the transition between amorphous and crystalline phase occurs. The sample becomes a mixture of amorphous silicon, polycrystalline silicon with nano-sized grains and voids with decreasing hydrogen concentration at grain boundaries and post-deposition oxygen contamination. The dominance of Si-H bonds and presence of interstitial oxygen and oxygen bonded in surface Si-OH groups was observed.

Improved magnetic properties and fracture strength of NdFeB by dehydrogenation

Effects of the dehydrogenation of the hydrogen decrepitated (HD) powders on the magnetic properties and the fracture strength of sintered NdFeB magnets were studied. It was found that the lattice parameters and the crystal phase of NdFeB changed significantly with the various hydrogen contents of the resultant HD powders due to the different degrees of dehydrogenation. The magnetic properties and fracture strength increased with decreasing hydrogen content, reaching the maximum increases of 200% for both intrinsic coercivity and bending strength, which can be ascribed to the improved microstructure of the sintered NdFeB magnets. The hydrogen remaining in the HD powders diffused out and affected drastically the grain and grain boundaries by the hydrogen out-take channel during the subsequent sintering process.

Experimental evidence for extended hydrogen diffusion in silicon thin films during light-soaking

We have used mass spectrometry to detect hydrogen effusing from silicon thin films exposed to light. Our results indicate a long range diffusion of hydrogen through the whole film, which ends with its release into the vacuum system. The changes in the film properties are characterized by dark and photoconductivity and hydrogen exodiffusion measurements. From the evolution of dark conductivity measurements after turning off the light, we show that this long range motion of hydrogen is not due to the heating of the sample. A comparison of hydrogen exodiffusion spectra of as-deposited and light-soaked samples shows that the weakly-bonded hydrogen content decreases by 30% for a-Si:H films and that the tightly-bonded hydrogen migrates to grain boundaries of crystalline regions in the case of pm-Si:H films. These results clearly demonstrate the long range motion of hydrogen during light soaking.

Mathematical models for kinetics of batchwise hydrogenation of shale oil

Hydrogenation of shale oil in a batch autoclave is described by a second order kinetic equation depending on the hydrogen current concentration ( H) and on the current value of a total characteristic of the oil ( x i ), both approaching to their equilibrium levels ( H ∞, x i∞ ). Algorithms are proposed to estimate the values of hydrogenation rate coefficient ( k) and H ∞ from the plot of decrease in hydrogen concentration in time versus H at isothermal conditions. A new concept “specific change of the characteristic” ( β i = d x i / d H) is introduced to handle various x i (oil yield, viscosity, density, iodine number, and yield of the fraction 200–275 °C). The equilibrium constants ( K i ) and β i for the characteristics are estimated on the basis of experimental results. The temperature-dependencies for k, K i and β i are determined. A scheme is proposed to predict current values of H and x i for different H 0, T max and oil mass under isothermal and non-isothermal conditions. The model deduced and the coefficients and constants found can be applied for quantitative evaluation of catalysts and initial oils for hydrogenation.

Optical characterization of polysilicon thin films for solar applications

We report on the results of the investigation of optical properties and structure of PECVD deposited thin films of hydrogenated polysilicon determined by UV–Vis and IR spectroscopy. The influence of the hydrogen dilution of silane plasma at the PEVCD deposition on the film properties was investigated. The refractive index, the optical band gap energy and the microstructure of hydrogen and oxygen were analysed. The changes are discussed and correlated with the structure, the changes of the surface morphology and the hydrogen to silicon bonding. The optical band gap becomes larger than that of the undiluted sample. The results show that at dilution between 20 and 30 the transition between amorphous and crystalline phase occurs and the sample becomes a mixture of amorphous, polycrystalline phase with nano-sized grains and voids with decreasing hydrogen concentration. The presence of interstitial oxygen and oxygen bonded in surface Si–OH groups was detected.

Hydrogen diffusion and ortho– para conversion in absorption and Raman spectra of germanosilicate optical fibers hydrogen-loaded at 150–170 MPa

IR absorption and spontaneous Raman scattering spectra of germanosilicate optical fibers loaded with molecular hydrogen at pressures of 150–170 MPa, as well as the change of these spectra during hydrogen out-diffusion from the fibers are investigated. Purely rotational transitions of molecular hydrogen in the Raman spectra of optical fibers are observed for the first time. Changes in spectral characteristics of the fiber Bragg gratings subjected to hydrogen processing are analyzed. It is found that after loading at so high pressures a decrease in hydrogen concentration in the fiber core exhibits two different stages, the initial stage being faster as compared with that for the fibers loaded at 10–15 MPa. To explain this phenomenon, the influence of gradient of internal stress caused by hydrogen dissolved in the glass network as well as the ortho– para hydrogen conversion in germanosilicate glass are considered. An increase in solubility of molecular hydrogen in the glass network subjected to UV irradiation is revealed.

Improvement of cyclic durability of BCC structured Ti–Cr–V alloys

Durabilities of Ti–Cr–V alloys were investigated using the cyclic hydrogen absorption–desorption test. The XRD pattern obtained for Ti 24Cr 36V 40 alloy after 100 cycles suggested the production of a highly distorted secondary phase. A temperature swing cyclic test was conducted and it became clear that the degradation of hydrogen transfer was caused by the decrease of total hydrogen content in the alloy. Vanadium content is an important factor in improving durability. High vanadium content prevents rapid degradation in the initial cycles. Floating zone melting is effective also for improving durability and the effect is remarkable for alloys with high vanadium content. Highly homogenized Ti 8Cr 12V 80 alloy made by floating zone melting shows only 1.4% degradation from the initial hydrogen capacity after 500 cycles. As a behavior different from other Ti–Cr–V alloys, the floating zone melted Ti 8Cr 12V 80 alloy shows an increase of the hydrogen absorption plateau pressure with the progression of cycles.

Characterization of SiC Thin Film Obtained by Magnetron Reactive Sputtering: IBA, IR and Raman Studies

Co-sputtering of silicon and carbon in a hydrogenated plasma (20%Ar-80%H2) at temperatures, Ts, varying from 200°C to 600°C has been used to grow SiC thin films. We report on the influence of Ts on the crystallization, the ratio Si/C and the hydrogen content of the grown films. Film composition is determined by ion beam analysis via Rutherford backscattering spectrometry, nuclear reaction analysis via the 12C(d,p0)13C nuclear reaction and elastic recoil detection analysi(ERDA) for hydrogen content. Infrared absorption (IR) has been used to determine the crystalline fraction of the films and the concentration of the hydrogen bonded to Si or to C. Complementary to IR, bonding configuration has been also characterized by Raman spectroscopy. As Ts is increased, the crystalline fraction increases and the hydrogen content decreases, as observed by both ERDA and IR. It also appears that some films contain a few Si excess, probably located at the nanograin boundaries.

Role of Hydrogen in Polycrystalline Si by Excimer Laser Annealing

The role of hydrogen in the Si film during excimer laser annealing (ELA) has been successfully studied by using a novel sample structure, which is stacked by a-Si film and SiN film. Hydrogen contents in the Si films during ELA are changed by preparing samples with hydrogen content of 2.3-8.2 at.% in the SiN films with a use of catalytic (Cat)-CVD method. For the low concentration of hydrogens in the Si film, the grain size increases by decreasing hydrogen concentration in the Si film, and the internal stress of the film decreases as increasing the shot number. For the high concentration of hydrogens in the Si film, hydrogen burst was observed at 500 mJ/cm 2 and the dependence of the internal stress on the shot number becomes weak even at 318 mJ/cm 2 . These phenomena can be understood basically using the secondary grain growth mechanism, which we have proposed.

Performances of a-Si:H films using hot wire assisted microwave ECR CVD technique

We report on the growth and properties of a-Si:H films using a combination of the hot wire and microwave ECR CVD growth techniques. The films were prepared using W filaments. Films were grown at powers from 175 to 500 W, and the corresponding optical and electronic properties of the films were measured. The temperature of the substrate was varied between ∼200 and 230 °C. It was discovered in the current research that the hot wire could significantly increase the growth rate, about 30 Å/s. The H bonding and electronic properties were found to be dependent critically upon the growth rate and the substrate temperature. In such a system, the hot wire plays an important role in perfecting the microstructure as well as improving the stability and the optoelectronic properties of a-Si:H films. The experimental results indicate that the decrease in hydrogen content in the microstructure of a-Si:H films was the primary reason for the improvement of the stability and the optoelectronic properties of a-Si:H films.

A Study on the Tribological Properties of DLC Films Deposited with Different Reaction Gases

AbstractDiamond like carbon (DLC) coatings have attracted great attention for use in various applications in automobile industry and machinery because they have excellent properties such as low friction coefficient and ultra high strength. The low friction coefficient of DLC comes from anti-adhesion with other materials, smooth surface, lubrication of graphite structure at the contacting point of films, and the hydrogen content of the films. Many researches have been focused on the microstructure effects on the tribological properties of DLC films but few have been reported on the effect of hydrogen content. In this study, the effect of hydrogen content on the friction coefficient of DLC films has been investigated.DLC films have been deposited on D2 steel by plasma enhanced CVD (Pulsed DC PECVD) method with different precursor gas of C2H2and CH4and different gas pressure. The effects of gas composition on the hydrogen content in DLC films and the resulting tribological properties have been reviewed. Si interlayer was deposited on D2 to improve adhesion of DLC on steel substrates. The characteristics of microstructure were evaluated by Raman spectroscope and composition was measured by RBS and EDS. The tribological behaviors of DLC films were investigated using ball on disk tribometer. The hardness of films was examined by nano-indenter. The failure mechanism of DLC deposited on steel substrates was examined using optical microscope and SEM/EDS. The results showed that the friction coefficient of DLC films deposited with C2H2was 0.06 and that of the film with CH4was 0.15. The friction coefficient improved with decreasing hydrogen content in DLC films.

Experimental investigation of the structure of a sooting premixed benzene/oxygen/argon flame burning at low pressure

The structure of a one-dimensional premixed benzene–oxygen–argon flame burning at 50 mbar with a fuel equivalence ratio ( ϕ) of 2.0 has been determined experimentally. Mole fraction profiles of species up to C 22H 12 have been measured by using molecular beam mass spectrometry and the temperature profile by a Pt/PtRh10% thermocouple. Large maximum mole fractions (>10 −3) have been measured for acetylene, diacetylene, cyclopentadiene, and vinylacetylene. Linear acetylenic intermediates remain in the burned gases significantly longer than olefinic and aromatic compounds. Also, at a given carbon content in a hydrocarbon intermediate, its maximum concentration shifts downstream when the hydrogen content decreases. The comparison with a leaner flame ( ϕ = 1.8) studied by Bittner and Howard has shown a substantial increase in the maximum concentration of aromatics (PAH) and therefore the detection of larger intermediates as a consequence of the equivalence ratio effect. The comparison with an ethylene flame ( ϕ = 2.5) investigated by Renard et al. has illustrated the strong impact of benzene as the initial fuel which enhances the formation of cyclopentadiene and heavier hydrocarbons. Light species (<C 5) occur in similar maximum quantities in all discussed flames. Some discrepancies with the results issuing from a model validated in rich ethylene flames have illustrated the importance of the C 4 route compared to the C 3 path in the oxidation and degradation mechanism of benzene in flames.

Stress Relaxation of Poly-Si Film Formed by Excimer Laser Annealing

We investigated the low-temperature processed polycrystalline-silicon (poly-Si) film formed by carrying out low-energy-density and multishot excimer laser annealing (ELA) of amorphous Si (a-Si) film on a glass or quartz substrate. The influence of secondary grain growth which grain size becomes larger than the film thickness on the tensile stress relaxation of the poly-Si film was clarified. Relationship between hydrogens which are supplied to the melt-Si from the bottom catalytic-chemical vapor deposition (Cat-CVD) SiN film and the stress relaxation was also examined. The tensile stress in the poly-Si film relaxes upon the appearance of the secondary grains, and the stress decreases with increasing diameter of the secondary grains. However, the hydrogen supply to melt-Si suppresses the secondary grain growth and the stress relaxation becomes marked as the hydrogen concentration decreases. Furthermore, the relationship between the dangling bond density corresponding to the crystal defect density and grain size was investigated, and the defect site was clarified. Lastly, the recrystallization model by ELA at low energy densities was discussed. The model was constructed based on the experimental data that the secondary grain growth occurs suddenly at the critical energy density and the critical shot number, and it was shown that the present model is consistent with the results related to the hydrogen concentration and the secondary grain growth.

Lignin-based carbon fibers: Oxidative thermostabilization of kraft lignin

The thermostabilization of lignin fibers used as precursors for carbon fibers was studied at temperatures up to 340 °C at various heating rates in the presence of air. The glass transition temperature ( T g) of the thermally treated lignin varied inversely with hydrogen content and was found to be independent of heating rate or oxidation temperature. A continuous heating transformation (CHT) diagram was constructed from kinetic data and used to predict the optimum heating rate for thermostabilization; a heating rate of 0.06 °C/min or lower was required in order to maintain T g > T during thermostabilization. Elemental and mass analyses show that carbon and hydrogen content decrease during air oxidation at constant heating rates. The hydrogen loss is sigmoidal, which is consistent with autocatalytic processes. A net increase in oxygen occurs up to 200–250 °C; at higher temperatures, oxygen is lost. Spectroscopic analyses revealed the oxidation of susceptible groups within the lignin macromolecule to ketones, phenols and possibly carboxylic acids in the early stage of the reaction; the later stage involving the loss of CO 2 and water and the formation of anhydrides and possibly esters. Slower heating rates favored oxygen gain and, consequently, higher glass transition temperatures ( T g) as opposed to faster heating rates.

The effect of the substrate bias voltage and the deposition pressure on the properties of diamond-like carbon produced by inductively coupled plasma assisted chemical vapor deposition

Diamond-like carbon (DLC) films were deposited by inductively coupled plasma (ICP) assisted CVD using a gas mixture of Ar and C 2H 2. The film showed one (37 GPa) of the highest hardness values for a DLC film produced by CVD at the optimum process condition. The film hardness increased rapidly with decreasing hydrogen content in the film. By applying ICP, the hydrogen content could be reduced to approximately 20%, which is the lowest value among those reported in the literature. It is believed that the film hardness is affected by the hydrogen content in the film up to a certain (∼25%) hydrogen concentration.

Hydrogenation and dehydrogenation properties for DyNi 5–H system

We have investigated the hydrogenation and dehydrogenation properties of DyNi 5 binary intermetallic compound in the pressure range from 0.1 to 35 MPa. After initial activation, DyNi 5 compound absorbs hydrogen to reach H/DyNi 5 = 3.8 at 248 K. By evaluating the pressure–composition (P–C) isotherm part by part in detail, we found the P–C isotherm for DyNi 5–H system comprises three steps of phase transitions during hydrogenation and dehydrogenation processes due to the presence of one solid solution ( α) and three kinds of hydride ( β, γ and δ). Below H/DyNi 5 = 1.0, the first α– β phase transition appears with large hysteresis, whereas the second β– γ and third γ– δ phase transitions are superposed upon each other around H/DyNi 5 = 2.3. Consequently, the P–C isotherm apparently shows two pressure plateaux during both hydrogenation and dehydrogenation. Between these plateaux during dehydrogenation, the hydrogen pressure decreases logarithmically with decreasing hydrogen content or log P ∝ H/DyNi 5. This complex behavior of P–C isotherm is reversible.

Crystal structure and hydrogen occupation of LaNi4.9Al0.1Dx (5.0 ≤ x ≤ 6.1) on the desorption isotherm studied by in situ neutron powder diffraction

We performed in situ neutron powder diffraction measurements of LaNi4.9Al0.1Dx along the desorption P–C isotherm using 90° banks of a TOF diffractometer. Structure of the hydride phase for various hydrogen contents was determined by Rietveld refinement. Variation of hydrogen site occupation in the hydride phase was clearly observed. The full hydride LaNi4.9Al0.1D6.1 has a P6mm structure with four hydrogen sites; 3c sites (O[La2Ni4]) with occupancy of 0.95(2) and three T sites, 6e1 (T[La2Ni2]), 6e2 (T[La1Ni3]) and 2b (T[Ni4]), with occupancies of 0.324(6), 0.195(6) and 0.22(1), respectively. In the two-phase region, the hydride phase has the limiting composition LaNi4.9Al0.1D5.0. Decreased hydrogen content from x = 6.1 to 5.0 in the hydride phase involved decrease of occupation in all sites, but behavior of the decrease depended on the sites. The 3c and 6e1 sites are preferably occupied in the limiting composition, whereas extra occupation caused by hydrogen solution in the hydride single-phase region is most preferable in the 6e2 sites.

Influence of alloying on hydrogen-assisted cracking and diffusible hydrogen content in Cr-Mo steel welds

Study of hydrogen-assisted cracking and measurement of diffusible hydrogen content in different Cr-Mo steel welds shows that under identical conditions, susceptibility to cracking increased and diffusible hydrogen content decrease with increase in alloy content. Hydrogen permeation studies show that hydrogen diffusivity decreases and solubility increases with increase in alloy content. Thus decrease in diffusible hydrogen content with increase in alloying is attributed to increase in apparent solubility and decrease in apparent diffusivity of hydrogen. Analysis of the results indicates that variation of diffusible hydrogen content and apparent diffusivity of hydrogen with alloy content can be represented as a function of carbon equivalent CE1 originally proposed to predict the hardness in the heat-affected zone of alloy steel welds.