Hydrogen permeation behavior and mechanism of Cr2O3/Al2O3 bipolar oxide film under plasma discharging environment

To improve the thermal stability between aluminide and stainless steel substrate and obtain thermodynamically stable phase of alpha-Al₂O₃, a new Cr₂O₃/Al₂O₃ bipolar oxide barrier was proposed, in which metallic Al was sputtered on the preoxidation coating of electroplated chromium and then oxidizing by oxygen plasma. It was found that Cr₂O₃ film exhibits P-type semiconducting properties while Al₂O₃ acts as N-type. Hydrogen discharging plasma was used to simulate the in-pile hydrogen permeation. Raman spectra and atomic force microscopy (AFM) were employed to analyze phase structure and surface morphology. Electrochemical impedance spectroscopy and Mott–Schottky were utilized to qualitatively evaluate effective thickness and the integrity for the oxide film. The depth profile and surface chemical states of involving elements were analyzed by auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), respectively. The result shows that Cr₂O₃/Al₂O₃ bipolar oxides have improved hydrogen permeation resistance and would be a potential candidate for barrier application.     

Hydrogen interaction characteristics of a Cr2O3Y2O3 coating formed on stainless steel in an ultra-low oxygen environment

Ceramics are the most promising candidates for tritium permeation barriers for fusion reactors due to their high thermal and chemical stabilities and low hydrogen isotope permeation reduction factors. However, hydrogen embrittlement and a large number of defects in ceramic coatings are new challenges for first wall materials in nuclear reactors. To address this issue, a new Cr₂O₃Y₂O₃ coating with a thickness of about 100 nm was synthesized and placed in an ultra-low oxygen partial pressure (8 × 10⁻²⁰ Pa) environment, in which a compact CrY alloy coating was successfully deposited on the stainless-steel substrate by pulsed electrochemical deposition. The interactions between the coating and hydrogen plasma were comprehensively analyzed and compared via surface analysis techniques, including TEM, XPS and electrochemical impedance spectroscopy (EIS). The mechanical properties of the coating before and after hydrogen permeation were studied by tensile testing. It was found that this ceramic coating effectively reduced the defect concentration and retained a high protective performance upon hydrogen exposure. Therefore, this new Cr₂O₃Y₂O₃ coating has potential as a promising hydrogen permeation barrier.     

Hydrogen permeation properties of CrxCy@Cr2O3/Al2O3 composite coating derived from selective oxidation of a CrC alloy and atomic layer deposition

Metal oxides and carbides are promising tritium permeation barrier coatings for fusion reactors. However, the thermomechanical mismatch between the coating and substrate poses a threat to their interface's integrity during fabrication and operation. To address this issue, a metallic interlayer coating was introduced followed by selective oxidation in which a compact and uniform CrC amorphous alloy coating was successfully deposited on the stainless steel substrate by pulsed electrochemical deposition. A new composite coating of Cr_xC_y@Cr₂O₃/Al₂O₃ was formed by subsequent controlled oxidation conversion and atomic layer deposition. The phase, morphology, chemical state and defects of the films were analyzed and compared both before and after hydrogen exposure at 300 °C. The results show that this new kind of composite coating, based on the principles of grain boundary pinning of chromic oxide with carbide and defect healing of alumina, can remarkably improve the hydrogen permeation barrier performance of these materials.     

Effect of helium implantation on the hydrogen retention of Cr2O3 films formed in an ultra-low oxygen environment

To simulate and investigate the irradiation damage of neutron and transmutation effect (He production) on the hydrogen isotope trapping behavior, in the present study, a new monoclinic hydrogen permeation barrier composed of Cr₂O₃ was fabricated and helium implantation with different fluences was tentatively employed. First, pure chromium samples were oxidized in an ultra-low oxygen partial pressure (1.7 × 10⁻²³ Pa) environment to obtain a single Cr₂O₃ layer. Then a dense layer of helium bubbles was formed in a substrate using a helium ion implantation method. Finally, the samples were treated in a hydrogen plasma environment at 500 °C. The damage, vacancy, and helium distribution in these samples were then simulated by SRIM. The morphology, phase, surface characteristics, thermal desorption, and electrochemical properties were subsequently characterized and evaluated. Thermal desorption spectrum analysis (TDS) was used to study the thermal desorption of hydrogen and helium at different temperatures. Our results showed that the inhibitory effect of the composite hydrogen barrier layer on the hydrogen diffusion in the substrate first increased and then decreased with the increase of the helium ion implantation fluence.     

Effect of annealing conditions on in situ formation of Cr2O3 diffusion barrier

The previous investigation suggested the approach for an in situ formation of Cr₂O₃ diffusion barrier by annealing the cold-sprayed Ni coatings on 310SS. In this paper, the influences of annealing conditions on the growth kinetics of Cr₂O₃ and substrate microstructure were investigated. Results show that Cr₂O₃ formed at the selected annealing temperatures of 850, 900 and 950°C for different durations of 4, 8 and 20?h. Increasing temperature enhanced the growth kinetics of Cr₂O₃ and the Mn content in the oxide layer. The annealing process for the growth of Cr₂O₃ improves the coating adhesion compared to the as-deposited coating. However, annealing at 950°C resulted in the precipitation of chromium carbides and enhanced the element inter-diffusion across the substrate/coating interface.     

Improved hydrogen permeation through thin Pd/Al2O3 composite membranes with graphene oxide as intermediate layer

Here we proposed the decreasing in the roughness of asymmetric alumina (Al₂O₃) hollow fibers by the deposition of a thin graphene oxide (GO) layer. GO coated substrates were then used for palladium (Pd) depositions and the composite membranes were evaluated for hydrogen permeation and hydrogen/nitrogen selectivity. Dip coating of alumina substrates for 45, 75 and 120 s under vacuum reduced the surface mean roughness from 112.6 to 94.0, 87.1 and 62.9 nm, respectively. However, the thicker GO layer (deposited for 120 s) caused membrane peel off from the substrate after Pd deposition. A single Pd layer was properly deposited on the GO coated substrates for 45 s with superior hydrogen permeance of 24 × 10⁻⁷ mol s⁻¹m⁻² Pa⁻¹ at 450 °C and infinite hydrogen/nitrogen selectivity. Activation energy for hydrogen permeation through the Al₂O₃/GO/Pd composite membrane was of 43 kJ mol⁻¹, evidencing predominance of surface rate-limiting mechanisms in hydrogen transport through the submicron-thick Pd membrane.     

Superhydrophobic Pd@CeO2/Al2O3 catalyst coating for hydrogen mitigation systems in nuclear power plants

Passive autocatalytic recombination (PAR) was employed as an effective hydrogen mitigation system for nuclear power plants. Despite the great progress achieved in catalysts for PAR, few reports could be found on PAR catalysts, which exhibit strong resistance to poisoning by iodine compounds and the detrimental effect of water, as well as good anti-sintering properties. In this study, a superhydrophobic Pd@CeO₂/Al₂O₃ catalyst coating was prepared through a self-assembly method and the grafting of 1H,1H,2H,2H-perfluorooctyltriethoxysilane. The superhydrophobicity of the catalyst coating effectively alleviated water poisoning. The ceria shell of Pd@CeO₂/Al₂O₃ served as a protective layer against poisoning by iodine compounds because it was difficult for iodine vapor to access the catalyst core of the Pd nanoparticles when the shell pore diameter was below 1.2 nm. Given the interface effect between palladium and ceria, the CeO₂ shells also served as an effective barrier to prevent the Pd nanoparticles from aggregating at high temperatures. The superhydrophobic Pd@CeO₂/Al₂O₃ coating showed excellent potential for the mitigation of hydrogen containing various poisons during a nuclear accident.     

Coating the porous Al2O3 substrate with a natural mineral of Nontronite-15A for fabrication of hydrogen-permeable palladium membranes

Substrate surface modification is a key pretreatment during fabrication of composite palladium membranes for hydrogen purification in hydrogen energy applications. The suspension of a natural porous material, Nontronite-15A mineral, without any organic additives was employed in dip-coating of the porous Al₂O₃ substrate. The Nontronite-15A mineral was characterized by SEM, XRD, TG−DSC and granulometry analysis. The surface and cross-section of the coated porous Al₂O₃ tubes were observed by SEM, and their pore size distribution and nitrogen flux were also measured. Palladium membranes were fabricated over the coated Al₂O₃ tubes by a suction-assisted electroless plating. The optimal loading amount of the Nontronite-15A mineral is just to fill in and level up the surface cavities of the Al₂O₃ substrate rather than to form an extra continuous layer. A thin and selective palladium membrane was successfully obtained, and its permeation performances were tested. The kinetic analyses on the hydrogen flux indicate that the hydrogen permeation behavior exhibits typical characteristics for most of the palladium membranes. During the stability test at 450 °C for 192 h, no membrane damage was detected, and the hydrogen flux increased slightly.     

Preparation and hydrogen penetration performance of TiO2/TiCx composite coatings

Titanium carbide is a good candidate for tritium permeation barrier in a fusion reactor. However, its oxidation susceptibility and the mismatch between the ceramic coating and substrate are still a challenge. In this study, a promising candidate as a hydrogen permeation barrier, comprising a titanium-based ceramic TiO₂/TiC_x composite coating, was proposed. The preparation process of this TiO₂/TiC_x composite coating involves two steps of carbon ion implantation and oxidation under ultra-low oxygen partial pressure. According to the results, the optimal oxidation temperature for TiO₂ coating is 550 °C, with the increase of the oxidation temperature, the particles on the surface of the oxide layer become coarse and loosely arranged, and the protective performance of the oxide layer is greatly reduced. The hydrogen barrier permeation behavior of the composite coating in a fusion reactor was simulated via hydrogen plasma discharge environment, the results show that the hydrogen barrier permeation performance of the composite is significantly better than that of a single TiO₂ coating. In addition, the coatings treated with hydrogen plasma showed a certain self-repairing performance through the diffusion growth of the TiC_x layer. These findings illustrate a novel method for preparing composite coatings to restrain hydrogen permeation, providing insight into the development of hydrogen permeation barrier materials.     

Synthesis of Cr2O3–Al2O3 powders with various Cr2O3/Al2O3 molar ratios and their applications as support for the preparation of nickel catalysts in CO2 methanation reaction

The Methanation of CO₂ to CH₄ is a significant route to save energy and reduce CO₂ emission. In this work, a series of Cr₂O₃–Al₂O₃ powders were synthesized by a novel and simple solid-state method and considered as the carrier for the nickel catalysts in CO₂ methanation. The BET area and pore volume of the supports decreased with the decrease in Al₂O₃/Cr₂O₃ molar ratio. The results indicated that the increase in Cr₂O₃ content improved the catalytic performance and 15 wt%Ni/Cr₂O₃ catalyst exhibited the highest CO₂ conversion of 80.51%, and 100% CH₄ selectivity at 350 °C. The results indicated that the CO₂ conversion improved with the increment in H₂/CO₂ molar ratio from 2 to 5. The improvement in CO₂ conversion was also observed with decreasing GHSV due to the longer residence time of the reactants on the catalyst surface. Also, the results showed that increasing calcination temperature led to a decrease in CO₂ conversion. The 15 wt%Ni/Cr₂O₃ catalyst exhibited high stability in carbon dioxide methanation reaction.     

Investigation of Al2O3 diffusion barrier layer fabricated by atomic layer deposition for flexible Cu(In,Ga)Se2 solar cells

The use of Al₂O₃ fabricated by atomic layer deposition (ALD) as a metal diffusion barrier between the stainless steel substrate and the back contact layer in flexible Cu(In,Ga)Se₂ (CIGS) photovoltaic (PV) devices was found to reduce metal ion diffusion from the substrate and reduce the number of defects at the CIGS absorber layer, as determined from the secondary ion mass spectrometry (SIMS) depth profile and quantitative defect analysis using C–V measurements. Cells with Al₂O₃ barrier layers were found to show higher efficiency and uniformity compared to cells with ZnO barrier layers. XRD pattern analysis showed the Al₂O₃ barrier layer's amorphous characteristic which can form a complex diffusion path. In addition, quantum efficiency (QE) analysis of the cells showed that the main advantage of using an Al₂O₃ barrier layer is derived from the increase in the current density due to the decrease in the number of recombination sites resulting from the decrease in the number of defects due to the amorphous nature of the layer. Therefore, cells with an Al₂O₃ barrier layer fabricated by ALD showed better average conversion efficiency and uniformity (11.23 ± 1.86%) compared to cells with a ZnO barrier layer fabricated by sputtering. Ongoing advancements in ALD processes make the use of Al₂O₃ barrier layers promising for obtaining large-scale flexible solar cells.     

Plasma-enhanced catalytic ammonia decomposition over ruthenium (Ru/Al2O3) and soda glass (SiO2) materials

Catalytic ammonia (NH₃) decomposition has been identified as a COx-free, sustainable hydrogen production method for fuel cell applications. In this study, the performance of plasma–catalyst-based NH₃ decomposition over ruthenium (Ru/Al₂O₃) and soda glass (SiO₂) catalytic materials at atmospheric pressure and ambient temperature was investigated. NH₃ decomposition reactions were conducted in a dielectric barrier discharge plasma plate-type reactor. NH₃ was fed into the plate catalytic microreactor at flow rates of 0.1–1 L/min and plasma voltages of 12–18 kV. Compared to plasma NH₃ decomposition without a catalyst, plasma–catalyst-based NH₃ decomposition showed a significant enhancement of the hydrogen production rate and energy efficiency. Furthermore, the hydrogen concentration results obtained over the Ru/Al₂O₃ catalyst were higher than those over the SiO₂ catalyst because Ru/Al₂O₃ possesses good electronic properties and exhibits high sensitivity to NH₃ decomposition. In addition, the resulting plasma heat enhanced the activation of the catalytic material, subsequently leading to an increase in the hydrogen production rate from NH₃. The maximum conversion rates were 85.65% and 84.39% for Ru/Al₂O₃ and SiO₂, respectively. Moreover, the energy efficiency of NH₃ decomposition over the Ru-based catalyst material was higher than that over the SiO₂ material. The presence of the catalyst active sites and plasma enhanced the mean electron energy, which could enhance the dissociation of NH₃. It can be concluded that the SiO₂ material can be utilised as a catalyst and that its combination with plasma accelerates the decomposition process of NH₃ and incurs a lower cost compared to Ru materials.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells

Cr₂O₃ evaporation from Cr₂O₃-forming metallic interconnects during operation of the solid oxide fuel cells (SOFC) can poison other cell components and cause degradation. Protective NiFe₂O₄ spinel coatings on interconnect alloys were developed by electroplating and screen printing, respectively. Results indicate that NiFe₂O₄ coatings can significantly improve the oxidation resistance of the alloy while providing effective conducting path with inherent low resistance, and also are expected to serve as a diffusion barrier to effectively reduce the Cr₂O₃ evaporation. Two coating techniques were evaluated in terms of the performances of the coatings. A very interesting and smart coating structure was reported.     

Hydrogen interaction characteristic of nanoscale oxide films grown on iron–nickel based stainless steel by selective thermal oxidation

Hydrogen isotopes, the reaction ingredients in the nuclear fusion plant, can easily permeate through the stainless steel (SS) substrate, leading to the so-called hydrogen degradation. Generally, a widely accepted way to reduce the hydrogen permeation is to prepare a barrier coating on the substrate. Nevertheless, the coated layer has the inherent problem of incompatibility with the heterogeneous base materials. In this work, in-situ selective oxidation was used to explore the optimal oxides with the improved hydrogen resistance. Two types of layers thermally formed at 450 °C and 750 °C, respectively, were selected to investigate their hydrogen interaction characteristics. Comprehensive analyses, including Raman spectra, XPS, EIS and AES, indicate that the oxide formed at 450 °C is a better candidate of hydrogen permeation barriers, probably due to the formation of protective layers of chromia and FeCr₂O₄, while the oxides obtained at 750 °C, though exhibiting a much more stable phase, can rarely reduce hydrogen diffusion through the shortcuts of defects. This finding provides a potential new way to prepare a hydrogen permeation barrier.     

ITO free MoO3/Au/MoO3 structures using Al2O3 as protective barrier between MoO3 and PEDOT:PSS in organic solar cells

A MoO₃/Au/MoO₃ structure with a protective barrier Al₂O₃ was developed to suppress the reactions between MoO₃ and the PEDOT:PSS film in organic solar cells (OSCs). Though the maximum optical transmittance of this structure was 66% at 550 nm wavelength, the power conversion efficiency of a MoO₃/Au/MoO₃/Al₂O₃/PEDOT:PSS based OSCs was 2.77%, comparable to the 2.89% of an ITO-based OSCs. The introduction of a very thin Al₂O₃ layer between the MoO₃ and the acidic PEDOT:PSS film effectively protected the MoO₃ from the acidic and water dispersed PEDOT:PSS film, increasing the J_sc, V_oc and FF of the structure above those of the MoO₃/Au/MoO₃/PEDOT:PSS structure. The Al₂O₃ (1 nm) introduced to the MoO₃/Au/MoO₃ structure improved J_sc because it suppressed the reactions between MoO₃ and PEDOT:PSS and lowered the work function of the PEDOT:PSS film. The MoO₃/Au/MoO₃/Al₂O₃ electrode was shown to be a promising replacement of ITO for use in flexible optoelectronic devices.     

The structural,electrical and optical properties of ZnO/Al2O3 multilayer deposited on PET substrates by RF sputtering

In this work ZnO/Al₂O₃ multilayer is deposited on PET(polyethylene terephthalate) substrate by RF sputtering in argon atmosphere and the effect of Al₂O₃ layer on optical, electrical and structural properties of ZnO is investigated. It is observed that the presence of Al₂O₃ layer between ZnO and PET reduce the transmission of light from ZnO/Al₂O₃/PET structure 11% but if a thin layer of SiO₂ ∼100 nm is applied between Al₂O₃ and PET it can be compensated. The XRD results show that the ZnO film had hexagonal wurtzite structure with (002) preferred orientation and the application of Al₂O₃ and SiO₂ layers do not have any effect on its structure. The effect of Al₂O₃ and SiO₂ layers on optical band gap of ZnO has been investigated, also the surface morphology of ZnO film is studied by scanning electron microscopy.     

The strain and transition metal doping effects on monolayer Cr2O3 for hydrogen evolution reaction: The first principle calculations

Chromic oxide (Cr₂O₃) monolayer is a promising alternative hydrogen evolution reaction (HER) catalyst compared with expensive platinum (Pt) due to its advantages such as low cost, large specific surface area, high reserves, and designability. In this study, the two practical strategies, strain engineering and transition metal (TM) doping (Mn, Fe, Zn, etc.), are proposed to activate the catalytic sites of Cr₂O₃ monolayer for the HER. The density functional theory (DFT) calculations demonstrate that the strained Cr₂O₃ monolayer can stimulate the HER activity with the Gibbs free energy of hydrogen adsorption (ΔG_H1) close to 0.09eV, which can be considered as a performable strategy to tune the HER catalytic behavior of Cr₂O₃ monolayer. For the TM doping, it also plays a role in the performance adjustment. These results provide a guideline to optimize the HER performance of Cr₂O₃ monolayer.     

Spontaneous dissociation of H2 and high energy barrier of H invasion on W doped Al2O3(0001) surface

In view of the wide use of tungsten in fusion experimental devices and the importance of hydrogen isotopes permeation, here we studied the adsorption, dissociation, diffusion and invasion behavior of hydrogen on W doped α-Al₂O₃ (0001) surface. Based on the first-principle approaches, we found the W substitution for a top surface Al atom is the most energetically favorable. H₂ molecule prefers to be adsorbed on the surface W and spontaneously dissociates into two H anions. Near the W defects, H atoms favor to be adsorbed at the W and Al sites rather than O sites on the surface, and within the subsurface layer H can only bond to W stably. As a result, H migration to subsurface should occur around W with an energy barrier as large as 4.22 eV which is much larger than the 1.91 eV around the O atom on undoped α-Al₂O₃ (0001) surface. These findings suggest that W surface doping is beneficial to α-Al₂O₃ as tritium permeation barrier.     

Influence of the grain structure of yttria thin films on the hydrogen isotope permeation

Steel components are required in the infrastructure and the facilities of the hydrogen economy. The high hydrogen pressures in the hydrogen economy lead to embrittlement and surface corrosion of the steels. For the functionality of the facilities it is necessary to suppress the embrittlement and the surface corrosion of the steels by protective layers, e.g. ceramic thin films. With regard to fusion power plants ceramic thin films on the structural steel materials are also required. These thin films work as a tritium permeation barrier that is necessary to prevent the loss of the radioactive fuel inventory. Oxide thin films, e.g. Al₂O₃, Er₂O₃, and Y₂O₃, are promising candidates as tritium permeation barrier layers. In terms of the application in the first wall, this is especially true for yttrium due to its favorably short decay time after neutron activation compared to the other candidates. The Y₂O₃ layers with thicknesses of 0.5 μm–1 μm are deposited on both substrate sides by RF magnetron sputter deposition. Since the microstructure of the barrier layer plays an important role for the permeation reduction, layers with three different magnetron process modes and thus three different microstructures are prepared. After annealing the cubic crystal structure of all thin films is verified by X-ray diffraction and the different microstructures are investigated by scanning electron microscopy and transmission electron microscopy. The Y₂O₃ stoichiometry of all thin films and a chromium oxide material segregation at the interface are verified by analysis methods such as X-ray photoelectron spectroscopy. The permeation reduction factors of all thin films are determined in gas-driven deuterium permeation experiments. Corresponding to the three different microstructures, reduction factors of 25, 45, and 1100 are identified. Thus, the permeation reduction is strongly dependent on the Y₂O₃ microstructure. The measurement results suggest that a high density of grain boundaries leads to a high hydrogen permeation.