Ryota Gemma

Defect studies of ZnO single crystals electrochemically doped with hydrogen

Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3at.% H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to ∼30at.%) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O–H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape ...

Microstructural evolution of Cu-1 at% Ti alloy aged in a hydrogen atmosphere and its relation with the electrical conductivity

Copper alloys with titanium additions between 1 and 6at% Ti emerge currently as attractive conductive materials for electrical and electronic commercial products, since they exhibit superior mechanical and electrical properties. However, their electrical conductivity is reduced owing to the residual amount of Ti solutes in the Cu solid solution (Cu(ss)) phase. Since Cu shows only poor reactivity with hydrogen (H), while Ti exhibits high affinity to it, we were inspired by the idea that hydrogenation of Cu-Ti alloys would influence their microstructure, resulting in a significant change of their properties. In this contribution, the influence of aging under a deuterium (D(2)) atmosphere of Cu-1at% Ti alloys on their microstructure is investigated to explore the effects on the electrical conductivity. The specimens were investigated by means of transmission electron microscopy (TEM), field ion microscopy (FIM), computer-aided field ion image tomography (cFIIT), and atom probe tomography (APT). At an early aging stage at 623K in a D(2) atmosphere of 0.08MPa, ellipsoidal alpha-Cu(4)Ti precipitates are formed in the alloy, and during subsequent aging, delta-TiD(2) is competitively nucleated instead of growth of alpha-Cu(4)Ti particles. The co-precipitation of alpha-Cu(4)Ti and delta-TiD(2) efficiently reduces the Ti concentration of Cu(ss) matrix, particularly in the later aging stages in comparison to the aging in vacuum conditions. The electrical conductivity of the alloy aged in the D(2) atmosphere increases steeply up to 48% International Annealed Copper Standard (IACS) after 1030h, while it saturates to approximately 20% IACS in the alloy aged in vacuum. The outstanding increase of electrical conductivity during aging in D(2) atmosphere can be basically explained by the reduction of Ti solute concentration in Cu(ss) matrix.

APT analyses of deuterium-loaded Fe/V multi-layered films

Interaction of hydrogen with metallic multi-layered thin films remains as a hot topic in recent days. Detailed knowledge on such chemically modulated systems is required if they are desired for application in hydrogen energy system as storage media. In this study, the deuterium concentration profile of Fe/V multi-layer was investigated by atom probe tomography (APT) at 60 and 30K. It is firstly shown that deuterium-loaded sample can easily react with oxygen at the Pd capping layer on Fe/V and therefore, it is highly desired to avoid any oxygen exposure after D(2) loading before APT analysis. The analysis temperature also has an impact on D concentration profile. The result taken at 60K shows clear traces of surface segregation of D atoms towards analysis surface. The observed diffusion profile of D allows us to estimate an apparent diffusion coefficient D. The calculated D at 60K is in the order of 10(-17)cm(2)/s, deviating 6 orders of magnitude from an extrapolated value. This was interpreted with alloying, D-trapping at defects and effects of the large extension to which the extrapolation was done. A D concentration profile taken at 30K shows no segregation anymore and a homogeneous distribution at c(D)=0.05(2)D/Me, which is in good accordance with that measured in the corresponding pressure-composition isotherm.

Phase transformations in non-isothermally annealed as-cast and cold-rolled AlMnScZr alloys

Abstract The effect of Mn addition on the microstructure, thermal and mechanical properties in as-cast and cold-rolled Al–Sc–Zr alloys was studied. Electrical resistometry, differential scanning calorimetry and microhardness measurements were used. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction of specimens quenched from temperatures of pronounced changes in resistivity helped to identify the microstructural processes responsible for resistivity changes. The distinct microhardness increase observed after annealing above ∼320°C is caused by precipitation of the Al3Sc particles. The cold-rolling prior to a heat treatment has no substantial effect on temperature position of the Al3Sc-phase precipitation. The formation of Al6Mn and/or Al6(Mn,Fe) particles is responsible for the intensive resistivity decrease of the cold-rolled materials compared to the as-cast materials. Precipitation of these particles has an insignificant effect on microhardness. The apparent activation energy for the precipitation of the Al3Sc particles was determined.

Hydrogen Interaction with Defects in Nanocrystalline, Polycrystalline and Epitaxial Pd Films

Hydrogen interaction with defects and structural development of Pd films with various microstructures were investigated. Nanocrystalline, polycrystalline and epitaxial Pd films were prepared and electrochemically loaded with hydrogen. Structural changes of Pd films caused by absorbed hydrogen were studied by in-situ X-ray diffraction combined with acoustic emission and measurement of electromotorical force. Development of defects during hydrogen loading was investigated by positron annihilation spectroscopy. It was found that hydrogen firstly fills open volume defects existing already in the films and subsequently it occupies also interstitial sites in Pd lattice. Absorbed hydrogen causes volume expansion, which is strongly anisotropic in thin films. This introduces high stress into the films loaded with hydrogen. Acoustic emission measurements revealed that when hydrogen-induced stress achieves a certain critical level rearrangement of misfit dislocations takes place. The stress which grows with increasing hydrogen concentration can be further released by plastic deformation and also by detachment of the film from the substrate.

Hydrogen Absorption in Ion Beam Sputtered Titanium Thin Films

Titanium films were prepared on sapphire substrates in an UHV chamber, by means of ion beam sputter deposition under Ar-atmosphere at the pressure of 1.5ּ10-4 mbar, with a deposition rate of 2,1 nm/min. The crystal structure was investigated by means of X-Ray diffraction using a Phillips X-Pert diffractometer with a Co-Kα radiation. For electrochemical hydrogen loading, the films were covered by a 30 nm thick layer of Pd in order to prevent oxidation and facilitate hydrogen absorption. The samples were step-by-step loaded with hydrogen by electrochemical charging, which was carried out in a mixed electrolyte of phosphoric acid and glycerine (1:2 in volume). An Ag/AgCl (sat.) and Pt wires were used as the reference and the counter electrode, respectively. XRD measurements were performed before and after hydrogenation in order to investigate the effect of hydrogen loading on the microstructure. The main characteristics of hydrogens absorption behaviour, as well as the thermodynamics and phase boundaries of titanium-hydrogen thin films are discussed in detail with specific emphasis on the comparison to titanium-hydrogen bulk system.

Laser-Induced Reversion of δ′ precipitates in an Al-Li Alloy

Atom probe tomography (APT) technique has been improved significantly, making it a wellestablished nano-analysis tool in the field of material science. It has been extensively applied to the investigation of different types of materials due to its ability to map the distribution of single atoms in a material in real space on a nearly atomic scale [1]. In this paper, we investigate the details of the laser pulse mode APT analyses, in particular the laser induced specimen-heating effect using an interface reaction in an Al-Li alloy as a model system. This alloy is known to have a low-temperature, metastable, miscibility gap [2]. It has been shown that under classical conditions of ageing of this alloy, including solution treatment, fast quench to room temperature and thermal ageing at intermediate temperatures (e.g., 100–200°C), the precipitation behavior was dominated by the presence of the metastable phase with an L12 structure [3]. The influence of the laser power on the morphology, the composition, and the diffusion of the constituents of the precipitates in the aluminum-lithium-based alloy is identified during an APT analysis. Prior to the laser exposure all sample were prepared by ageing an Al2wt.%Li binary specimen at 190 °C for 3h to produce comparable initial state with an average diameter of (14.2±3) nm and a number density of (10±0.1)x10 m as confirmed by APT-voltage pulsing analysis. A simple model is used to explain the observed experimental behavior and to estimate the corresponding tip-apex temperature for various laser energies. APT analyses were performed with both a CAMECA laser assisted wide angle tomographic atom probe (LAWATAP) for the field ion microscopy (FIM) mode and the CAMECA local electrode atom probe (LEAP 4000X HR). Data were acquired utilizing either the voltageor the laser pulse mode. A diode-pumped (Nd: YAG) solid-state laser operating in the frequency tripled ultraviolet region with a wavelength of 355 nm, a pulse duration of approximately 12 ps and a repetition rate of 200 kHz was used. The laser pulse energy was systematically varied through the following values: 10, 30, 40, 50, 60, 80, and 100 pJ. The reconstruction algorithm used was the standard evolution algorithm [4].

Hydrogen-Induced Buckling of Pd Films Deposited on Various Substrates

A Pd-H system is a model system suitable for studying interactions of hydrogen with metals. In the present work, we studied hydrogen-induced buckling of thin Pd films deposited on various substrates with different bonding strengths (sapphire, glimmer) and also the effect of deposition temperature. Lattice expansion and phase transitions were investigated by X-ray diffraction of synchrotron radiation. The influence of the substrate and microstructure of the film on the buckling process and phase transformation to palladium hydride are discussed.

A Study on Hydrogen in Titanium Thin Films

Titanium and its conventional alloys reveal a high affinity for hydrogen, being capable to absorb up to 60 at.% hydrogen at 600°C, and even higher contents can be alloyed with titanium at lower temperatures. Hydrogen exhibits a low solubility in the low-temperature hexagonal closed-packed (hcp) α phase and a very high solubility (up to 50 at.%) in the high temperature body-centered cubic (bcc) β phase. The presence of hydrogen in the amount exceeding 200 ppm leads to formation of hydrides in α and α + β titanium alloys. While the aforementioned hydrogen behavior within bulk titanium has been well-established and reviewed, this is not the case with titanium thin films. The interpretation of results in these nanosized systems is complicated because the exact determination of the hydrogen concentration is difficult. However, using electrochemical hydrogen loading technique under the proper conditions, the hydrogen concentration can be accurately determined via Faraday’s law. In this study the thermodynamics of the titanium films during hydrogen absorption were investigated by electromotive force (EMF) measurements. Titanium films of different thicknesses were prepared on sapphire substrates in an UHV chamber with a base pressure of 10-8 mbar, using ion beam sputter deposition under Ar-atmosphere at the pressure of 1,5ּ10-4 mbar. The crystal structure was investigated by means of X-Ray diffraction using a Co-Kα radiation. For electrochemical hydrogen loading, the films were covered by a 30 nm thick layer of Pd in order to prevent oxidation and facilitate hydrogen absorption. The samples were step-by-step loaded with hydrogen by electrochemical charging, which was carried out in a mixed electrolyte of phosphoric acid and glycerin (1:2 in volume). An Ag/AgCl (sat.) and Pt wires were used as the reference and the counter electrode, respectively. XRD measurements were performed before and after hydrogenation in order to investigate the effect of hydrogen loading on the films microstructure. The role of varying thicknesses on the main characteristics of hydrogens absorption behavior, as well as hydrogen-induced microstructural changes in titanium thin films, are discussed in detail.