Hydrogen-doped Samples Research Articles

Structural and electronic properties of hydrogen doped Wurtzite ZnO

We have carried out comprehensive studies on the various types of hydrogen doping including, interstitial and substitutional (for oxygen) position (Hi, HO), HO + Hi, and oxygen vacancy plus interstitial hydrogen (VO + Hi) complexes using DFT + U calculations. The effect of Hi, HO, HO + Hi, and VO + Hi dopants on structural and electronic properties of Wurtzite ZnO at 0.020, 0.0625, and 0.125 nH/nZn concentrations, were studied. We found that cell parameters and volume of the supercells decrease in Hi, HO, and HO + Hi defects, whereas these values increase in the VO + Hi case. Bader analysis shows that H− ion locates at VO+2 in HO doping and it also places beside VO0 in VO + Hi. Furthermore, the HO + Hi complex is a H2 molecule that is almost neutral, electrically inactive and is trapped in VO+2. So, it seems that this complex is an appropriate model for formation of H2 molecule at high temperature. ZnO band gap is slightly influenced by Hi in all concentrations while, HO has prominent effect on increment of it which is consistent with experimental results too. In addition, it is shown that growth of H-doped sample under O-poor condition leads to improved optical properties which is applicable in transparent conductive oxide devices. It is predicted that band gap of hydrogen doped samples will decrease by annealing due to trapping of HO by Hi and formation of H2 molecule. Finally, hydrogen beside oxygen vacancy can create two deep levels, decreasing the optical band gap and shifting absorption edge to lower energy, which leads to a red shift and possible interest in photocatalytic applications.

In situ x-ray diffraction study of crystal structure of Pd during hydrogen isotope loading by solid-state electrolysis at moderate temperatures 250−300 °C

Hydrogen isotopes and metal interaction with respect to Pd under high hydrogen isotope potential at moderate temperature region around 300 °C was studied. A dry electrolysis technique using BaZr1−x YxO3 solid state electrolyte was developed to generate high hydrogen isotope potential. Hydrogen or deuterium was loaded into a 200 nm thick Pd cathode. The cathode is deposited on SiO2 substrate and covered with the solid state electrolyte and a Pd anode layer. Time resolved in situ monochromatic x-ray diffraction measurement was performed during the electrolysis. Two phase states of the Pd cathodes with large and small lattice parameters were observed during the electrolysis. Numerous sub-micron scale voids in the Pd cathode and dendrite-like Pd precipitates in the solid state electrolyte were found from the recovered samples. Hydrogen induced super-abundant-vacancy may take role in those phenomena. The observed two phase states may be attributed to phase separation into vacancy-rich and vacancy-poor states. The voids formed in the Pd cathodes seem to be products of vacancy coalescence. Isotope effects were also observed. The deuterium loaded samples showed more rapid phase changes and more formation of voids than the hydrogen doped samples.

Electrical conductivity of enstatite as a function of water content: Implications for the electrical structure in the upper mantle

The electrical conductivity of Ca-free aluminous enstatite with various water contents has been determined at a pressure of 3GPa in a Kawai-type multi-anvil apparatus. Impedance spectroscopy was performed for both hydrogen-doped and -undoped samples in a frequency range from 0.1Hz to 1MHz to examine the effect of water on conductivity. Two conduction mechanisms were identified for hydrogen-undoped samples at temperature of 1000–1723K and for hydrogen-doped samples at relatively lower temperature range of 500–900K to minimize dehydration of samples. For the hydrogen-undoped samples, the activation enthalpy is around 1.9eV at the higher temperatures range (>1300K) suggesting that the dominant charge transfer mechanism is Fe2+−Fe3+ hopping (small polaron) conduction. For the hydrogen-doped samples measured below 900K, the activation enthalpy decreases from 1.11 to 0.70eV, and the conductivity values systematically increase with increasing water content, suggesting that proton conduction is the dominant conduction mechanism. Taking hopping conduction and water content dependence of activation enthalpy for proton conduction into account, all electrical conductivity data were fitted to the formula σ=σ0hexp(−Hh/kT)+σ0pCwexp[−(Hp0−αCw1/3)/kT], where σ0 is pre-exponential factor, Cw is the water content in weight percent, H is the activation enthalpy, Hp0 is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolution temperature and subscripts h and p represent hopping and proton conductions, respectively. Using the present results, a laboratory-based conductivity-depth profile in the Earth's upper mantle has been constructed as a function of water content. Comparison of our model with the currently available geophysical observations beneath the Eastern Pacific Rise indicates that hydrous aluminous enstatite cannot account for the high conductivity anomaly at the top of the asthenosphere as well as hydrous olivine.

The effect of water on the electrical conductivity of olivine aggregates and its implications for the electrical structure of the upper mantle

The electrical conductivity of San Carlos olivine aggregate of various water content was measured at a pressure of 10 GPa in a Kawai-type multi-anvil apparatus. Conductivity measurements were performed on two sets of samples to determine the effect on conductivity of water in olivine: 1) a hydrogen-doped sample and 2) a hydrogen-undoped sample. To minimize water escape from the hydrogen-doped samples, the conductivity measurement was carried out below 1000 K. Three conduction mechanisms were identified from the Arrhenian behavior of the undoped samples, which include a small amount of water. A change in the activation enthalpy indicated that the dominant conduction mechanism changed from proton conduction to small polaron conduction with increasing temperature. At temperatures above 1700 K, the activation enthalpy exceeds 2 eV suggesting that the dominant mechanism of charge transport would be ionic conduction. The conductivity increased with increasing water content. The activation enthalpy for proton conduction tends to decrease slightly with increasing water content. The activation enthalpy determined for each run had similar values (~ 0.9 eV). Taking the water concentration dependence of activation enthalpy into account for proton conduction, all data were fitted to the electrical conductivity formula σ= σ 0Iexp[− E I/ kT]+ σ 0Hexp[− E H/ kT]+ σ 0P C Wexp[−( E 0− αC W 1/3)/ kT], where σ 0 represents a pre-exponential term, C W is the water content in weight percent, E is the activation enthalpy, E 0 is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolute temperature and subscripts I, H and P denote ionic, hopping (small polaron) and proton conductions, respectively. The conductivity jump at the 410 km discontinuity (olivine–wadsleyite transition) is much smaller than that previously predicted. Since the contribution of proton conduction to the bulk electrical conductivity decreases with increasing temperature the high conductivity anomaly at the top of the asthenosphere cannot be explained by olivine hydration.

Effect of hydrogen on internal friction and elastic modulus in titanium alloys

The effect of hydrogen on the variation with temperature of internal friction ( Q − I ) and elastic modulus ( E) of a number of Ti-based alloys has been studied in the Hz and kHz frequency ranges. A relaxation peak of internal friction with a high degree of relaxation ( Q −I max ∼ 10 −1) and with a Δ E effect is observed in all hydrogen-doped samples at T ∼ 600 K at ∼1 kHz, and at T ∼ 500 K at ∼1 Hz. Such a peak is not present in samples without hydrogen. The activation energy W and the frequency factor v 0 of the observed relaxation are determined to be W ∼ 1.55 eV, v 0 ∼ 10 17 s −1. It is shown that the observed effects are connected with the mechanism of grain boundary relaxation, as the introduction of hydrogen into titanium alloys leads to the formation of fine-grained structures.

Formation and Properties of Iron-Phosphorus and Iron-Phosphorus-Hydrogen Complexes in Silicon

Hitherto unreported ESR signal, labeled TU10, was detected after annealing of electronirradiated silicon samples doped with phosphorus, iron and hydrogen. The ESR spectrum corresponds to a complex having monoclinic-I symmetry and S = 3/2 spin-state. Hyperfine structure of the TU10 spectrum suggests participation of two nucleus with spin I = 1/2 and 100% abundance in the core of the related defect. Doping of samples with hydrogen-deuterium mixture revealed presence of one hydrogen atom in the complex. The second nucleus with I = 1/2 is apparently a phosphorus atom. Presence of single iron atom was verified by doping with iron heaving modified isotope content. An intensity of the previously reported TU6 signal, related to iron-phosphorus complex, was significantly suppressed in hydrogen-doped samples.

Influence of hydrogen dopant on the structure and crystallization of the partially crystalline Zr 76Ni 24 metallic glass

The influence of hydrogen dopant on the structure and crystallization of partially crystalline Zr 76Ni 24 metallic glass during heat treatment at different heating rates has been investigated by means of differential scanning calorimetry (DSC) in the temperature interval from 300 to 823 K and by X-ray powder diffraction (XRD) at room temperature. According to the XRD results, the as quenched Zr 76Ni 24 samples were mostly glassy (about 90%) with about 10% orthorhombic metastable Zr 3Ni crystalline phase. A systematic DSC analysis shows that adding hydrogen to these Zr 76Ni 24 samples raises the temperature of crystallization, T x1 for the hydrogen content up to x=0.054 ( x indicate at.%). Also, the hydrogen absorption causes an increase of the lattice parameter a and a decrease of the lattice parameter b of the orthorhombic metastable Zr 3Ni crystalline phase. The as quenched Zr 76Ni 24 samples showed to be oxidation resistant, whereas the oxidation rate of the hydrogen doped sample increases with increasing hydrogen content during heat treatment up to 823 K. The scales formed on the surface of the annealed hydrogen doped samples during the oxidation have a nanocrystalline microstructure consisting mainly of monoclinic ZrO 2.

NMR of 1H and 2D in hydrogen-doped and deuterium-doped YBa2Cu3O7.

The hydrogen and deuterium nuclear-magnetic-resonance absorption lines were obtained for ${\mathrm{H}}_{\mathit{x}}$(${\mathrm{D}}_{\mathit{x}}$)${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ where x=0.21, 0.60, and 1.00 for the hydrogen-doped samples and x=0.10, 0.20, 0.50, and 1.00 for the deuterium-doped samples. The $^{1}\mathrm{H}$ line shape was both temperature and hydrogen concentration dependent. While a relatively narrow line was seen for low hydrogen concentrations, a line of the order of 650 kHz emerged at higher concentrations. The quadrupole powder pattern of the deuterated sample was independent of the deuterium concentration and yielded only a single discernible line, having a quadrupole coupling constant ${\mathit{e}}^{2}$qQ/h of the order of 230 kHz and an asymmetry parameter of \ensuremath{\eta}\ensuremath{\approxeq}0.05. While the pattern can be explained by a single $^{2}\mathrm{D}$ site, the nuclei represented by the wide portion of the $^{1}\mathrm{H}$ traces give a washed-out $^{2}\mathrm{D}$ quadrupolar pattern and a second site for these nuclei cannot be ruled out. A point-charge model using accepted values for the metal and oxygen charges gives results consistent with the deuterium (or hydrogen) being located between the O(1) chain oxygens and the O(4) bridging oxygens. All other sites proposed by various investigators yield quadrupole-coupling-constant values much smaller than that obtained in our measurements.

Structure and copper valence of hydrogen-doped Nd2CuO x H y

There has been an explosion of activity in the field of high-temperature superconductivity since the discovery of superconductivity in Laz_xBaxCuO 4 [1]. Subsequently, superconductivity near 100 K has been found in the layered-perovskite family [1-4]. There are controversies as to whether holes in oxygen play an important role in superconductivity or not. Recently, an n-type superconductor of Ndz_xCexCuO 4 (x = 0.15) showing a critical temperature (To) of 30 K has been reported by Tokura et al. [5]. Superconductivity was also found in fluorine-doped NdzCuO 4 [6]. As well as the doping with fluorine, the introduction of hydrogen into transition-metal oxides has been employed to produce n-type semiconductors [7]. In fact, hydrogendoped YBazCu3O7_~Hy was prepared by the direct reaction of these superconducting oxides with hydrogen gas at a relatively low temperature [8-10]. In this study we attempted to prepare hydrogen-doped NdzCuO4, and the effects of introduced hydrogen on the crystal structure, the formal valence of copper, and superconductivity are examined. Stoichiometric mixtures of Nd203 and CuO were heated to 950 °C in air for 10 h. After the calcination was repeated twice, the mixtures were fired at 1100°C in air for 10h, and slowly cooled in a furnace. The resulting samples were confirmed to be a single phase of NdzCuO 4 by X-ray diffraction (XRD). Hydrogen-doped samples were prepared as follows: tlhe single-phase powders of NdzCuO 4 were heated at 250 °C in 101.3 kPa H 2 for 20 h, and quenched to room temperature. The presence of hydrogen contained in the hydrogen-doped sample was detected by a pulsed hydrogen nuclear magnetic resonance (H-NMR; Bruker PC20C) instrument. In order to obtain the relationship between the hydrogen concentration and the initial intensity of a free induced decay (FID) curve, FID curves were taken for mixtures of ( 1 x) CuO and xCuSO4"5H20, where x ranged from 1 to 0.05. It was confirmed that a linear relationship existed between x and the initial intensity of the FID curves. From this calibration curve, the hydrogen content of the H-doped sample, Nd2CuOxHy, was estimated. The oxygen content of the hydrogen-doped sample was determined by hydrogen reduction using a thermogravimetric analysis system (Shimazu TGA-30); the hydrogen-doped sample was heated in flowing hydrogen (50 cm 3 min -1) to 700 °C at 5 °Cmin -1. The formal oxidation state of copper in the hydrogendoped sample was determined by the iodiometry method. These hydrogen-doped samples were pressed into pellets, and then the superconducting

Electronic properties and localization effects in some hydrogen-doped 4d-3d metallic glasses

Magnetoresistance and magnetic susceptibility data obtained on hydrogen-doped samples of ZrNi and ZrCu systems are reported. A mobile-stage sample container in conjunction with a 6 T superconducting coil was used to measure magnetoresistance at temperatures down to 1.7 K with a relative precision of 10 −6. The temperature profile of the magnetic susceptibility was also measured down to 2 K by Faraday's method using a Cahn electrobalance combined with a conventional magnet. A precision range of 10 −7 J T −2 mol −1 was maintained in this measurement. The samples were produced by the common melt-spinning method and were used in the as-obtained condition. Hydrogen strongly influences the quantum-mechanical interference at defects, considerably enhances the quasi-elastic electron scattering and depresses the relative contribution of the spin-orbit interaction. The Maki-Thompson interaction is likewise depressed with increasing hydrogen concentration. The results can be interpreted on the basis of current theoretical concepts based on weak localization in three-dimensional systems in the presence of strong spin-orbit interaction and superconducting fluctuations. The magnetic susceptibility data are interpreted in terms of the hydrogen influence on the electronic density of states at the Fermi level.

Phonon density of states of amorphous silicon

Inelastic neutron scattering, Raman spectroscopy and infrared absorption measurements have been used to obtain the vibrational spectra for two large (5 gm) sputtered samples of pure and 12% hydrogen doped samples of amorphous silicon. The SiH wagging mode at 640 cm −1 is observed in both neutron scattering and infrared data with similar peak shapes. The mode at 480 cm −1 corresponding to the TO (520 cm −1) mode in crystalline silicon is observed in all three measurements independent of hydrogen concentration. A significant spectral density is observed in the neutron scattering data in the Si mode region from 480 cm −1 down. Of particular interest is a feature near 200 cm −1 which appears in the hydrogen doped samples in both neutron scattering and infrared data. For the first time, results from all three types of measurements are reported on the same samples.

Depth dependent void swelling rates in self-ion irradiated nickel

Specimens of high purity nickel were irradiated with high energy heavy ions and the resultant microstructure examined along the ion path using a cross sectioning sample preparation technique. Void nucleation was found to be very sensitive to hydrogen introduced into the samples by electropolishing prior to irradiation. Samples containing hydrogen had void densities of about two orders of magnitude greater than outgassed samples. A series of hydrogen doped samples were irradiated with 14 MeV nickel ions to fluences from 2 × 10 15 ions/ cm 2 to 1.4 × 10 17 ions/ cm 2 (peak damage from 2 to 150 dpa). The increase in the collision cross section as the incident ion slows down causes an increase in displacement rate with depth. The variation in the observed void density, void size and void swelling rates with depth indicate the importance of displacement rate on void simulation studies.

An anelastic study of the diffusion coefficient of hydrogen in α-zirconium

Internal friction measurements have been carried out as a function of temperature in annealed and electrolytically-hydrogen-charged thin sheets of high-purity zirconium. The vibration frequency ranged from 4.4 to 8.5 Hz, the thickness was about 6.5 μm. A Gorsky peak was observed near 700 K in the hydrogen-doped samples, and the diffusion coefficient was deduced both from the shape of the relaxation curves and from the shift of the peak with the vibration frequency.