Function Of Hydrogen Concentration Research Articles

Measurement of thermodynamic properties of some hydrogen absorbing alloys

In this paper, the effect of hydrogen concentration on the reaction enthalpies of some metal hydride alloys during hydriding and dehydring is presented. Pressure–concentration–temperature characteristics of the metal hydride alloys are measured under nearly isothermal condition during both absorption and desorption. Reaction enthalpies and entropies of LaNi 5, LaNi 4.7Al 0.3, LmNi 4.91Sn 0.15, Ti 0.99Zr 0.01V 0.43Fe 0.09Cr 0.05Mn 1.5 and MmCo 0.72Al 0.87Fe 0.04Ni 3.91 are estimated by constructing van't Hoff plots at different hydrogen concentrations. It is observed that the effect of hydrogen concentration on reaction enthalpies is more significant for the alloys having larger plateau slopes. At the initial stage of hydrogenation, metal hydrides are found to have larger reaction enthalpies which decrease gradually by about 5–15% at the end of the hydrogen absorption. At any given temperature, desorption enthalpies of LaNi 5, LmNi 4.91Sn 0.15, MmCo 0.72Al 0.87Fe 0.04Ni 3.91, LaNi 4.7Al 0.3 and Ti 0.99Zr 0.01V 0.43Fe 0.09Cr 0.05Mn 1.5 are found to be higher by about 5, 8, 10, 28 and 32% than their respective absorption enthalpies. Reaction enthalpies of the selected metal hydride alloys are expressed as a function of hydrogen concentration by a fourth order polynomial equation obtained from fitting with the experimental data.

Optimized Ring Tensile Test Method and Hydrogen Effect on Mechanical Properties of Zircaloy Cladding in Hoop Direction

Data pertaining to the mechanical properties of the fuel cladding in the hoop direction are required for the analysis of high burn-up fuel behavior under reactivity-initiated accident (RIA) conditions. In the present study, by minimizing undesirable effects of friction and bending, the ring tensile test method was optimized to obtain precise data pertaining to the mechanical properties of the fuel cladding in the hoop direction. The optimized specimen has a single gauge section and is stretched using the tooling consisting of two half-mandrels. The gauge section is set on top of a half-mandrel; this arrangement is unique in comparison with other methods. Using the optimized test method, the mechanical properties of the prehydrided Zircaloy-4 cladding in the hoop direction were evaluated as functions of hydrogen concentration and test temperature. When the hydrogen concentration is below 500 ppm, the decrease in ductility due to hydriding is relatively small at all test temperatures. When the hydrogen concentration is above 600 ppm, the ductility in the tests at 300K remarkably decreases due to hydriding, while the hydrogen effect decreases in magnitude in the tests above 473 K.

Properties of hydrogen absorption by nano-structured FeTi alloys

Abstract In this study, two different nano-structured samples of the FeTi compound were prepared by mechanical alloying and mechanical grinding. For these samples, kinetics of the initial rate of hydrogen absorption, and the equilibrium hydrogen pressure as a function of hydrogen concentration were measured. Mechanical alloying of Fe and Ti atoms produced the FeTi compound powder samples with microstructures of a mixture of nano-structured FeTi grains and amorphous phases. This sample exhibited a high initial rate of hydrogen absorption even at 298 K, however, a strongly reduced hydrogen storage capacity. Mechanical grinding of the FeTi produced samples of particles with a particular microstructure: surface layers with a mixture of nano-structured FeTi grains and amorphous phases, and a single crystalline phase of FeTi below the surface layers for each particle. This sample exhibited a high initial rate of hydrogen absorption without a significant reduction of the hydrogen storage capacity compared with that of the standard FeTi sample. This mechanical grinding treatment was found to be an effective method of surface modification to improve the initial activation of the FeTi hydrogen storage alloy.

On the composition analysis of nc-TiC/a-C : H nanocomposite coatings

Using a set of ion beam analysis (IBA) techniques the compositions of hydrogenated diamond-like carbon (DLC) nanocomposite coatings are scrutinized, including the hydrogen content. The coatings are composed of two constituents: amorphous hydrocarbon matrix (a-C : H) and nanocrystalline titanium carbide (nc-TiCa) which makes it challenging to quantify the sp3/sp2 ratio of the a-C : H matrix. Using IBA and SEM data, the partial mass densities of nc-TiCa and a-C : H matrix are obtained. The latter is analysed as a function of hydrogen content and compared with the density of pure sp3 bonding DLC and of sp2 graphite containing different contents of hydrogen. In addition, the fraction of sp3 bonding in the matrix is estimated assuming a linear dependence on the mass densities of the matrix.

Phononic engineered materials and devices

Disorder in hydrogenated amorphous silicon leads to short electronic diffusion lengths and poor electronic lifetimes relative to crystalline silicon. Interestingly this same disorder can lead to longer phonon diffusion lengths and lifetimes. These properties suggest disorder-induced truncation of the Brillouin zone results in fewer phonon decay possibilities thereby reducing Umklapp scattering. Phonon propagation speed is a function of the mechanical properties of the matrix and therefore a function of hydrogen content. Therefore, it is possible to engineer structures to guide phonons using methods previously developed to pattern the refractive index. Taken together these properties offer a means of explaining observed phononic behavior of amorphous silicon and offer a platform for engineering devices.

Hydrogen on graphene: Electronic structure, total energy, structural distortions and magnetism from first-principles calculations

Density-functional calculations of electronic structure, total energy, structural distortions, and magnetism for hydrogenated single-layer, bilayer, and multilayer graphenes are performed. It is found that hydrogen-induced magnetism can survive only at very low concentrations of hydrogen (single-atom regime) whereas hydrogen pairs with optimized structure are usually nonmagnetic. Chemisorption energy as a function of hydrogen concentration is calculated, as well as energy barriers for hydrogen binding and release. The results confirm that graphene can be perspective material for hydrogen storage. Difference between hydrogenation of graphene, nanotubes, and bulk graphite is discussed.

In situ electrochemical XRD study of (de)hydrogenation of MgyTi100−y thin films

X-Ray diffraction and electrochemical (de)hydrogenation were performed in situ to monitor the symmetry of the unit cells of MgyTi100−y thin film alloys (70 ≤ y ≤ 90 at.% Mg) along the pressure–composition isotherms at room temperature. The diffraction patterns show that the crystal structures of all as-deposited alloys have a hexagonal closed packed symmetry. Inserting hydrogen transforms the structure to a body centered tetragonal structure for Mg90Ti10, whereas the unit cells of Mg70Ti30 and Mg80Ti20 transform into a face centered cubic symmetry. The structural change of the hydrides along with the ability to rapidly (de)hydrogenate the films emphasize the influence of the symmetry of the host lattice on the hydrogen transport properties. The lattice spacings of the unit cell of Mg90Ti10Hx as a function of hydrogen content do not change much in the phase transformation region, indicating that only the fractions of the phases change. Remarkably, the lattice spacings found for Mg70Ti30Hx in the two phase coexistence region reveal that not only the fractions but also the Mg-to-Ti ratio of both phases continuously change. Evidently, there is a large spread in the thermodynamic stability of the available sites for hydrogen. Since the X-ray diffraction patterns rule out large scale segregation, the results imply a nanostructured alloy with Ti-poor and Ti-rich regions and illustrate that the Mg and Ti atoms in Mg70Ti30 are not randomly distributed.

Unit cell expansion in Laves phase‐type compounds caused by interstitial hydrogen

AbstractThe Laves phase‐type compounds of yttrium and rare earths with manganese (RMn2) easily absorb hydrogen which locates at the interstitial sites. It is well known that the lattice expands with increasing hydrogen concentration and the volume increases up to about 30%. The increase of the unit volume is nonlinear as a function of hydrogen content. To explain this behaviour a model in which the volume changes were correlated with the changes of the bulk modulus with hydrogen concentration was proposed by Hirata and co‐workers. This model describes properly the initial volume increase, but does not reflect the changes at higher concentrations. For the hydrides with R = Y, Gd, Er, Dy, Tb, Ho at x ≈ 3 the volume of AB3 interstitials becomes large enough for hydrogen to enter. According to this observation we extended this model. We show that the calculated volume expansion fits very well to the experimental data for RMn2 (R = Y, Gd, Dy, Tb, Ho). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical and electrical properties of holmium thin films as a function of hydrogen concentration

We measure the evolution of the optical transmission (200–760 nm) and the electrical resistivity of Ho films, 50 nm thick, as a function of H concentration up to 2.9 [H]/[Ho]. The Ho films are covered with a 15 nm thick Pd overcoat for hydrogenation and ex situ measurements. Concentration is measured using a quartz crystal microbalance; the films are deposited in a high vacuum chamber and all measurements are made at room temperature. From the measured transmission spectra, using an inversion method, the complex refraction index of the Ho film is determined; from it the dielectric function, ɛ = ɛ 1 + i ɛ 2, is calculated. This procedure is applied to the hydrogenated Pd coated Ho films finding the evolution of ɛ as a function of hydrogen concentration. Furthermore, using the inversion method in the trihydrided films the plasma frequency and the band gap are calculated.

Hydrogen absorption and desorption properties of [formula omitted] alloys

The hydrogen absorption–desorption pressure-composition (PC) isotherms and kinetics of Ho 1 - x Mm x Co 2 ( x = 0 , 0.1 , 0.2 , 0.3 and 0.4, Mm = mischmetal ) have been carried out in the temperature and pressure ranges 50– 200 ∘ C and 0.001–1 bar using Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydrides stability has been discussed. The kinetics of hydrogen absorption at near equilibrium plateau region have been analyzed by a rate function. The reaction rate constants in the two-phase coexistence region and diffusion coefficient in the hydride phase region were calculated and its pressure and temperature dependence have been discussed. The hydrogen desorption from interstitial sites of Ho 1 - x Mm x Co 2 H has been performed using differential scanning calorimetry (DSC) and thermogravimetry (TG) analysis. The results are explored to identify the preferential occupation of different interstitial sites of hydrogen atoms and complete desorption temperature. The observed phase transformations as a function of hydrogen concentration and temperature have been discussed.

Characterization of Photoconductive Amorphous Si:H Films for Photoconducting Sensor Applications

Photoconductive properties of a-Si:H films deposited on Si(100) substrates by reactive sputtering at room temperature were characterized as a function of hydrogen concentration in the films and the film thickness. In 230 nm thick a-Si:H films, the difference between dark- and photoresistance increases with increasing hydrogen concentration within the films. The films deposited with an H 2 flow rate of 2 sccm show a dark-resistance of approximately 1 × 10 6 Ω/□ and a photoresistance of 2 × 10 4 Ω/□. A decrease in the difference between dark- and photoresistance values with increasing film thickness above 230 nm was attributed to the increase of polymeric bonding such as SiH 2 within a-Si:H films. a-Si:H films deposited on Si(100) substrates are considered to be suitable for photoconductivity materials in photoconducting sensor applications.

Acoustic characteristics of Ni–Nb–Zr–H glassy alloys by ultrasonic shear waves

AbstractThe propagation characteristics of shear horizontally polarized (SH) waves passing through (Ni42Nb28Zr30)100–x Hx (x = 0–15.2) glassy alloys were investigated as a function of hydrogen content. With an increase in hydrogen content, the propagation time and main frequency of the receiving waves show increase and decrease, respectively, indicating expan‐ sion in average atomic distance which comes from solution of hydrogen. In sharp contrast to crystalline alloys, the decrease in damping ratio and the delay in phase with increasing hydrogen suggest a strong settlement of hydrogen into four‐coordination sites surrounded tetrahedrally by four Zr atoms and the resulting increase in dynamic elasticity, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gas-expanded liquids: Determination of the expansion coefficient by an acoustic technique

This paper presents an experimental method for liquid level detection in gas-expanded liquids by the sound speed and amplitude. Volumetric expansion of acetonitrile and 2-propanol as a function of CO 2 pressure was determined between 298–333 K and 0–6 MPa. Propane expansion of 2-propanol was done up to 393 K and up to V/ V 0 = 8. It was clearly demonstrated that this technique is useful also for multiphase detection with high accuracy and for obtaining accurate speed of sound values without calibration. The paper also presents an experimental method for gas composition sensing. The speed of sound and sound attenuation as a function of hydrogen concentration in CO 2 and in propane atmosphere at ambient conditions were measured to obtain information whether an acoustic technique could be an alternative method for the determination of H 2 solubility in liquefied gases and in gas-expanded solvents.

Hydrogen-induced localization of Ce-4f electrons in mixed-valent CeNiIn: Observation of heavy-fermion and magnetically ordered states

Hydriding studies have been carried out on the mixed-valent compound CeNiIn having a Kondo temperature T K ∼ 100 K. Two distinct hydride phases, CeNiInH y, with y = 0.3–0.6 and 1.6 are obtained which retain the hexagonal structure of the parent compound. The higher hydride phase with a composition of y = 1.6 is relatively unstable and it transforms into the lower hydride phase over a short period of time. Magnetization measurements indicate valence transformation of cerium from mixed-valent for the parent compound to Ce 3+ for the hydrides. While the higher hydride composition CeNiInH 1.6 shows ferromagnetic ordering below ≅7 K, the lower hydride phase with a composition CeNiInH 0.6 shows heavy fermion behaviour. Thus, Ce valence changes from mixed-valent to normal trivalence via a heavy-fermion like state as a function of hydrogen concentration.

Assessment of Hydrogen-Induced Precipitation in a Nickel-Copper Alloy Using Thermoelectric Power

Abstract Hydrogen sensing using thermoelectric power (TEP) was performed on a nickel-copper alloy to assess the hydrogen content resulting from cathodic protection. The measured TEP as a function of hydrogen content developed an anomaly at a specific hydrogen content, indicating a hydrogen-induced phase transformation in the alloy. The formation of Ni2H phase was found in a hydrogen-charged alloy (UNS N05500) using x-ray diffraction (XRD). The transmission electron microscopy (TEM) investigation of the hydrogen-charged alloy specimen revealed nano-sized Ni2H participates dispersed in the nickel-copper matrix. The measured TEP profile correlated with an alloy hydride phase formation on a hydrogen-metal pressure-composition-temperature (PCT) activity diagram. These results indicate that the TEP surface contact probe measurements can assist in the nondestructive evaluation (NDE) identification of a microstructural change in hydrogen-charged nickel-copper alloys. With this TEP surface probe measurement techni...

Laser crystallization of compensated hydrogenated amorphous silicon thin films

Raman backscattering and hydrogen effusion measurements were performed on compensated, highly P- and B-doped laser crystallized polycrystalline silicon. From hydrogen effusion spectra the hydrogen chemical potential, μH, is determined as a function of hydrogen concentration, which can be related to the hydrogen density-of-states distribution. Interestingly, hydrogen bonding is affected by doping of the amorphous starting material. Below the hydrogen transport states, four peaks are observed in the hydrogen density-of-states at 2.0, 2.2, 2.5 and 2.8eV. The latest peak is not observed in B-doped samples. The hydrogen effusion results will be correlated with the results obtained from Raman backscattering measurements.

Boron Doped Polycrystalline Silicon Produced By Step-by-Step XeCl Excimer Laser Crystallization

AbstractA series of boron doped polycrystalline silicon were produced using step-by-step laser crystallization process from amorphous silicon. The influence of doping concentrations on laser- induced dehydrogenation and crystallization of amorphous silicon and on hydrogen bonding have been investigated employing Raman spectroscopy and hydrogen effusion measurements. From hydrogen effusion spectra the hydrogen chemical potential is determined as a function of hydrogen concentration, which can be related to the hydrogen density-of-states distribution. The results from hydrogen effusion are consistent with the results obtained from Raman spectroscopy.

Spectroscopic optimization of abnormal glow conditions for plasma ion nitriding

Optical emission spectroscopy is used to characterize the production of active species as a function of hydrogen concentration in the mixture under different operating conditions. A major concern is to enhance the concentration of the active species by hydrogen addition that carry several electron volts energy above their ground states, and thus affect the surface chemistry. The emission intensity of the selected optical transitions of molecular and atomic species is measured to determine the functional dependence of their radiative states. The relative ground state molecular ion density [N 2 + ] is measured from the emission intensity of the first negative band head ( λ = 391.4 nm, 0–0) by considering the fact that in low temperature plasma, ion with single charge is produced by the electron impact, and the ion density is proportional to the electron density. It is found that the concentration of the active species may be enhanced significantly by selecting an appropriate gas composition and operating parameters. The SS-304 samples are nitrided under the optimum conditions for 4, 8, 12 and 16 hours and hardness values are found to increase five times for 16 hours treatment time. The optimized discharge conditions are found favorable for plasma ion nitriding.

Optical and electrical properties of terbium films as a function of hydrogen concentration

AbstractThe absorption of hydrogen in the rare earth metals produces interesting changes in their optical and electrical properties. In this work we study these properties on Tb films, 50 nm thick, as a function of hydrogen concentration. The Tb films are covered with a 15 nm thick Pd overcoat which allows hydrogenation while protecting the highly reactive Tb film from contamination. Electrical resistivity is measured using the van der Pauw method, the transmission spectra are measured in the visible region and the concentration using a quartz crystal microbalance. The maximum concentration reached was 2.95 H/Tb at a hydrogen pressure of 450 Torr. From the measured transmission spectra, using a Spectral Projected Gradient Method, we obtain the Tb film's complex refractive index. From this we calculate the real and imaginary parts of the dielectric function, ε1 and ε2, and follow their evolution as a function of hydrogen concentration. ε1 changes from negative values, characteristic of metals, to positive values characteristic of semiconductors and dielectrics. ε2 decreases indicating a significant decrease in light absorption. These results are similar to those observed on Dy, Gd, La and Y films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Studies on the hydrogen absorption properties of LaNi5−xSnx and La0.95Ni4.6Sn0.3 alloys using magnetic and thermoelectric power measurement

Magnetic and thermoelectric power (TEP) measurements are used to quantify hydrogen availability, absorption, and desorption in materials for nickel-hydride batteries and hydrogen storage. The effect of the concentration of conduction electrons is introduced and used to investigate hydride properties of these alloys. To investigate hydrogenation properties the calculated average number ofd-band electrons in hydrogen storage material is related to the measured thermoelectric power. Also, a thermodynamic exprssion of equilibrium is applied to demonstrate the relationship between the TEP and the activity of hydrogen for hydrogen adsorption. Magnetization decreases with increasing absorbed hydrogen in the stoichiometric LaNi4.78Sn0.22 alloy. However, magnetization increases with increasing hydrogen content in the nonstoichiometric La0.95Ni4.6Sn0.3 alloy. The TEP for the stoichiometric LaNi4.78Sn0.22 alloy monotonically increases as a function of hydrogen content. However, the TEP of the nonstoichiometric La0.95Ni4.6Sn0.3 alloy decreases as hydrogen content increases.