Interdiffusion Barrier Research Articles

Hydrogen permeation through porous stainless steel for palladium-based composite porous membranes

Surface topography and hydrogen permeation properties of Porous Stainless Steel (PSS) substrates for thin films deposition of Pd-based hydrogen separation membrane were investigated. Hydrogen permeance through the as received PSS substrates demonstrated a wide range, despite a similar average surface pore size of ~15µm determined by SEM and confocal laser microscopy analyses. The surface pores of the PSS substrates were modified by impregnation of varying amounts of tungsten (W) powder. Maximum hydrogen flux reduction of 28% suggested that W has a limited effect on the hydrogen permeation through the PSS substrate. Therefore, it appears that hydrogen transport through PSS substrates is mainly controlled by the substrate geometrical factor (ετ), that is the ratio of the porosity to tortuosity. In addition, tungsten was shown to inhibit the iron inter-diffusion between the PSS substrate and the deposited Pd60Cu40 film at temperature as high as 800°C. Thus, tungsten layer also serves as an effective inter-diffusion barrier. The variation in the permeance between the nominally similar PSS substrates indicates the importance to independently assess the hydrogen transport characteristics of each of the components in a composite membrane.

High performance solid-oxide fuel cell: Opening windows to low temperature application

A key hindrance of operating solid oxide fuel cells (SOFCs) at low temperature is the relatively high cell resistance resulting in low power output density. In this work, we report an SOFC design based on an anode-supported cell (ASC) with thin film Yttria stabilized zirconia electrolyte (YSZ), capable of high power output densities of 1050 mW cm−2 using H2 as fuel, at an operating temperature of 873 K. Such high cell performances have been realized by applying three optimization steps: (1) using La0.6Sr0.4CoO3−δ-perovskite (LSC) as high performing cathode material at low temperature; (2) integration of an optimized Ce0.8Gd0.2O1.9 (CGO) inter-diffusion barrier layer and (3) optimization of the microstructure of the anode substrate by means of increasing the substrate porosity.

Preparation and characterization of metallic supported thin Pd–Ag membranes for hydrogen separation

This paper reports the preparation and characterization of thin-film (4–5μm thick) Pd–Ag metallic supported membranes for high temperature applications. Various thin film membranes have been prepared by depositing a ceramic interdiffusion barrier layer prior to the simultaneous Pd–Ag electroless plating deposition. Two deposition techniques for ceramic layers (made of zirconia and alumina) have been evaluated: Atmospheric Plasma Spraying and dip coating of a powder suspension. Initially, the prepared ceramic layers have been characterized for nitrogen permeation at room temperature and surface roughness for the selection of the appropriate type of ceramic layer. The most promising membranes have been tested at 400–600°C for single gas permeation (H2 and N2), and have shown extremely high H2/N2 permselectivities (>200,000).

Notice of Removal: A Novel Metal-Ferroelectric-Insulator-Silicon FET With Selectively Nucleated Lateral Crystallized Pb(Zr,Ti)O3 and ZrTiO4 Buffer for Long Retention and Good Fatigue

Removed.

Restricted-Access Al-Mediated Material Transport in Al Contacting of PureGaB Ge-on-Si p + n Diodes

The effectiveness of using nanometer-thin boron (PureB) layers as interdiffusion barrier to aluminum (Al) is studied for a contacting scheme specifically developed for fabricating germanium-on-silicon (Ge-on-Si) p + n photodiodes with an oxide-covered light entrance window. Contacting is achieved at the perimeter of the Ge-island anode directly to an Al interconnect metallization. The Ge is grown in oxide windows to the Si wafer and covered by a B and gallium (Ga) layer stack (PureGaB) composed of about a nanometer of Ga for forming the p + Ge region and 10 nm of B as an interdiffusion barrier to the Al. To form contact windows, the side-wall oxide is etched away, exposing a small tip of the Ge perimeter to Al that from this point travels about 5 μm into the bulk Ge crystal. In this process, Ge and Si materials are displaced, forming Ge-filled V-grooves at the Si surface. The Al coalesces in grains. This process is studied here by high-resolution cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy that confirm the purities of the Ge and Al grains. Diodes are fabricated with different geometries and statistical current–voltage characterization reveals a spread that can be related to across-the-wafer variations in the contact processing. The I–V behavior is characterized by low dark current, low contact resistance, and breakdown voltages that are suitable for operation in avalanching modes. The restricted access to the Ge of the Al inducing the Ge and Si material transport does not destroy the very good electrical characteristics typical of PureGaB Ge-on-Si diodes.

Sigma-phase formation in high chromium ferritic steels at 650 °C

A binary Fe–30wt.%Cr alloy and corresponding ternary alloys containing manganese, molybdenum or tungsten were studied with respect to σ-phase formation at 650°C. Although even after 3000h exposure complete equilibration was not attained, the presence of tungsten and especially molybdenum was found to promote σ-phase formation. More extensive σ-phase formation was observed in the tungsten and especially in the molybdenum-containing alloys than in the binary and manganese-containing alloy. Apparently the bulk free energy decrease driving the nucleation of σ-phase is substantially larger when tungsten or molybdenum are present in the alloy.The presence of a nickel layer, to simulate the contact between ferritic steel interconnects and nickel mesh in a Solid Oxide Fuel Cell (SOFC) results in the formation of an austenitic zone and in accelerated formation of a σ-phase rich layer at the ferrite/austenite interface, due to interdiffusion processes. This interface acts as a highly efficient heterogeneity for the nucleation of σ-phase. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface. Several possible modes for the growth of the σ layer were identified but the available experimental data were not sufficient to distinguish among these. The σ-rich layer, which appears to act as an interdiffusion barrier, is thicker in the case of the binary Fe–Cr and the Fe–Cr–Mn alloy than for the molybdenum- or tungsten-rich alloys.The results show that the stability range of σ-phase is larger than indicated by the presently used thermodynamic data bases.

Ferroelectric/ferrimagnetic composite ceramics with depressed interfacial reaction and low dielectric loss

(1−x)BaTiO3/xNi0.5Zn0.5Fe2O4 (NZFO) ferroelectric/ferrimagnetic composite ceramics with restricted interfacial reaction were prepared by adopting fine NZFO precursors synthesized by combustion method. The dielectric dispersion, loss, and conductivity are significantly reduced at most compositions, particularly at concentrations below the percolation threshold. At x = 0.3, a frequency-stable permittivity of 2300 and a low loss of 0.04 at 1 kHz is realized. The recovery of the dielectric/electric properties is attributed to the interfacial amorphous phase introduced by the fine NZFO precursors, which can act as barrier for ionic inter-diffusion between the two phases and hopping conduction among ferrites.

Plasma spray deposition of tri-layer environmental barrier coatings

Abstract An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb 2 SiO 5 top coat on SiC substrates. Solidified droplets in as-deposited Yb 2 SiO 5 and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb 2 SiO 5 + Yb 2 O 3 two-phase top coat and 2:1 mullite (2Al 2 O 3 *SiO 2 ) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb 2 O 3 in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al 2 O 3 . Mud-cracks were found in the Yb 2 SiO 5 layer and in precipitated Al 2 O 3 due to the thermal expansion mismatch between these coating phases and the substrate.

Durability of high performance Ni–yttria stabilized zirconia supported solid oxide electrolysis cells at high current density

We report the durability of a solid oxide electrolysis cell (SOEC) with a record low initial area specific resistance (ASR) and a record low degradation rate. The cell consists of a Ni–yttria stabilized zirconia (YSZ) cermet as support and active fuel electrode, a YSZ electrolyte, a gadolinia doped ceria (CGO) inter-diffusion barrier, and a strontium doped lanthanum cobaltite (LSC)–CGO composite oxygen electrode. The cell was tested at 800 °C and −1 A cm−2 converting 31% of a 0.1:0.45:0.45 mixture of H2:H2O:CO2 for approximately 2700 h, demonstrating an initial ASR of 200 mΩ cm2 and a steady degradation rate of ≤12 mV (or 0.9%) per 1000 h. Electrochemical impedance spectroscopy (EIS) was used to study in situ changes in the electrochemical response of the cell and the retrieved data was treated to deconvolute resistive contributions from the physiochemical processes occurring within the cell. The results showed rapid initial fuel electrode degradation during the first 350 h followed by partial reactivation. The serial resistance was found to increase with time but in an exponentially decaying behavior. A discussion is made based on the detailed electrochemical results together with post-mortem microstructural analysis.

Role of electroless nickel diffusion barrier on the combinatorial plating characteristics of dense Pd/Ni/PSS composite membranes

Abstract This work addresses the combinatorial plating characteristics of dense Pd/Ni/porous stainless steel (PSS) composite membranes in comparison with Pd/PSS membranes. While Pd/PSS membranes were fabricated using 0.1 μm nominal pore size PSS supports, Pd/Ni/PSS membranes were fabricated using 0.5 and 0.1 μm nominal pore size PSS supports. Both Ni and Pd films were deposited using an identified novel electroless plating process that characterizes the optimal utilization of surfactant, sonication and reducing agent contacting pattern in Pd electroless plating baths. It was observed that the combinatorial plating characteristics for Pd/Ni/PSS membranes were significantly different and poorer in comparison with those obtained for the Pd/PSS membranes. In summary, it has been inferred that the introduction of nickel interdiffusion barrier was not fruitful to reduce the critical thickness of dense Pd film without jeopardizing upon the pore densification.

Fabrication of thin-film gadolinia-doped ceria (GDC) interdiffusion barrier layers for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition (CSD)

A dense gadolinia-doped ceria (GDC) interdiffusion barrier layer as thin as 300nm was successfully fabricated on a rigid anode/electrolyte bilayer substrate using the chemical solution deposition (CSD) process for intermediate temperature solid oxide fuel cells (SOFCs). Drying-related macro-defects were removed by employing drying control chemical additives (DCCA), which effectively relieved drying stresses. The major process flaws caused by the constraining effects of the rigid substrate were completely eliminated by the addition of GDC nanoparticles into the chemical solution, which suppressed the generation of microstructural anisotropy by mitigating the predominant bi-axial substrate constraints. As a consequence, a thin film GDC interlayer was successfully deposited with a high volumetric density, effectively preventing the chemical interaction between the electrolyte and cathode during the fabrication process and subsequent operation. The cell test and microstructural analysis confirmed excellent electrochemical performance and structural and chemical stability. The CSD process presented in this paper is considered to be a promising technology for the practical preparation of GDC thin film barrier layers for intermediate temperature SOFCs based on the film quality, processing costs and potential for large-scale production.

Performance and Stability of High Temperature Solid Oxide Electrolysis Cells (SOECs) for Hydrogen Production

Solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), which are composed of Ni-yttria-stabilized zirconia (YSZ) fuel electrode, YSZ electrolyte, gadolinia-doped ceria (GDC) interdiffusion barrier layer, and (La0.8Sr0.2)0.95Co0.2Fe0.8O3 (LSCF) air electrode, were fabricated and electrochemically tested. The performance of SOEC was dominated by the concentration polarization of the fuel electrode and activation polarization of the air electrode. The concentration polarization of the fuel electrode was reduced by optimization of the pore structure, and the eletrocatalytic activity of the air electrode was enhanced by infiltration of nanocatalysts into the porous electrodes. The cells showed excellent long-term stability up to 300 hour operation.

Paramagnetic FexTa1-x alloys for engineering of perpendicularly magnetized tunnel junctions

Exchange coupling between two magnetic layers through an interlayer is of broad interest for numerous recent applications of nano-magnetic systems. In this letter, we study ferromagnetic exchange coupling through amorphous paramagnetic Fe-Ta alloys. We show that the exchange coupling depends exponentially on spacer thickness and scales with the Fe-Ta susceptibility, which can be tuned via the alloy composition and/or temperature. Such materials are of high interest for the engineering of perpendicularly magnetized CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of magnetic layers with differing crystalline lattices, suppresses dead layers, and can act as an inter-diffusion barrier during annealing.

Fabrication and characterization of all-ceramic solid oxide fuel cells based on composite oxide anode

All-ceramic solid oxide fuel cells (SOFCs), which offer advantages in carbon tolerance, sulfur resistance and redox stability, are fabricated and evaluated. The electrolyte-supported cells are composed of a La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM)–Ce0.9Gd0.1O1.95−δ (GDC) anode, an Y2O3-stabilized ZrO2 (YSZ) electrolyte, a GDC interdiffusion barrier layer, and a La0.8Sr0.2Co0.2Fe0.8O3−δ (LSCF)–GDC cathode. A particle-dispersed glycine-nitrate process is developed to synthesize extremely fine and homogeneous LSCM–GDC ceramic composite powders. The electrochemical performance of the LSCM–GDC anode is comparable to that of conventional Ni-based anodes. The impedance spectra of the all-ceramic SOFCs are successfully interpreted by the independent characterization of the individual electrodes via half-cell measurements. The impedance of the LSCM–GDC anode is dominated by a low-frequency arc originating from the “chemical capacitance”, which is associated with the variation of the oxygen nonstoichiometry in the mixed conducting ceramic electrode. In addition, the impedance arc associated with the electrode–gas interaction is observed in the LSCM–GDC anode. The rate-limiting processes for the LSCF–GDC cathode are observed to be solid-state oxygen diffusion and surface chemical exchange. Herein, the reaction mechanisms and rate-limiting processes of the all-ceramic SOFCs are discussed in detail and compared with those of conventional Ni-based SOFCs.

Influence of the oxygen electrode and inter-diffusion barrier on the degradation of solid oxide electrolysis cells

Two Solid Oxide Electrolysis Cells (SOECs) with different oxygen electrodes have been tested in galvanostatic tests carried out at −1.5 Acm−2 and 800 °C converting 60% of a 50:50% mixture of H2O and CO2 (co-electrolysis). One of the cells had an LSM:YSZ oxygen electrode. The other had an CGO inter-diffusion barrier sandwiched between the YSZ electrolyte and an LSCF:CGO oxygen electrode. Impedance Spectroscopy was used during the tests to diagnose the change in electrochemical response of the different components of the SOECs. The results showed a significantly lower degradation rate for the cell with an LSCF:CGO oxygen electrode. This was attributed to three different effects: (1) The LSCF electrode was found to be more stable than its LSM:YSZ based counterpart. (2) The crack propagation in the YSZ electrolyte was less severe in the cell with an LSCF electrode. (3) The Ni:YSZ degradation was less severe in the cell with an LSCF electrode. We hypothesize further that (2) is an effect of the CGO inter-diffusion layer and that (3) is an indirect effect of (2).

TiWOx interfacial layer for current reduction and cyclability enhancement of phase change memory

TiWOx interfacial layer was proposed and implemented to act as both heater and inter-diffusion barrier for phase change memory through a complementary metal-oxide semiconductor compatible oxidization process. Significant reduction of RESET current was obtained due to more efficient Joule heating and better thermal confinement. About one order of magnitude endurance increase was achieved for the device with TiWOx due to suppression of inter-diffusion between Ge2Sb2Te5 and TiW. The change of the minimum RESET voltage against cycling was reduced by TiWOx layer with shorter RESET pulse, which would benefit device cyclability.

ZnO films grown using a novel procedure based on the reactive evaporation method

A novel procedure based on the reactive evaporation method was developed to deposit highly transparent thin films of i-ZnO and n+-ZnO in-situ. The opto-electrical properties of the ZnO films were optimized for using them as TCO layer in solar cells. The optimization of the preparation parameters was achieved through a figure of merit defined in terms of both, the transmittance and the resistivity. n+-ZnO films with resistivities around 8×10−4Ωcm and i-ZnO films with resistivities around 105Ωcm and transmittances greater than 80% (in the visible region) were obtained with this method.The applicability of the i-ZnO and n+-ZnO thin films in photovoltaic devices has been demonstrated by using them as interdiffusion barrier and TCO layer in CuInS2 based solar cells. Conversion efficiencies of 9.1% were achieved with CIS based solar cells using ZnO thin films deposited by reactive evaporation.

Improvement in Coating Quality and Corrosion Characteristics of Galvanized Steels by Pre-Electrodeposition of Cu

A dual-phase steel was electrodeposited with Cu prior to reduction annealing in order to improve coating quality and resultant corrosion characteristics of the galvanized material. This pre-eletrodeposition process allowed a tenfold reduction in coating defects (typically found in uncoated areas and empty pores), thereby yielding a 8.4% coating weight gain and a noticeable decrease (24%) in the average spangle diameter. However, the Cu pre-electrodeposition layer acted as an inter-diffusion barrier, thereby limiting Fe-Al interactions during galvanization, and resulted in the formation of imperfect Fe-Al compounds. Nevertheless, this Cu layer could extend the incubation time of the Fe-Zn phases. Consequently, the presence of this Cu electrodeposition layer could effectively prevent concentration polarization, thereby decreasing corrosion current density even after 4 days of immersion in 3.5% NaCl solution.

Preparation of dense Pd composite membranes on porous Ti–Al alloy supports by electroless plating

Dense Pd/TiO 2/Ti–Al alloy composite membranes on porous Ti–Al alloy supports were prepared by electroless plating. The TiO 2 membrane was applied as an inter-diffusion barrier between the Pd layer and the Ti–Al alloy support to prevent intermetallic diffusion. Electron probe microanalysis (EPMA) and EDX cross-sectional line scans showed that the intermediate layer was effective as the diffusion barrier for Pd membranes on Ti–Al alloy supports after 40 h of treatment at 973 K in pure hydrogen. The thickness of the resulting Pd composite membrane was approximately 14 μm. The hydrogen permeance through the Pd composite membrane was 1.07 × 10 −3 mol m −2 s −1 Pa −0.5 at 773 K. The permeance and selectivity of the Pd composite membrane were measured at different temperatures and pressures. The permeance tests showed that the Pd/TiO 2/Ti–Al alloy composite membranes have high H 2/N 2 selectivity. In addition, heat cycles indicated that the structure of the Pd composite membranes was stable.

SOFC LSM:YSZ cathode degradation induced by moisture: An impedance spectroscopy study

The cause of the degradation effect of moisture during operation of LSM cathode based SOFCs has been investigated by means of a detailed impedance characterization on LSM:YSZ composite cathode based SOFCs. Further the role of YSZ as cathode composite material was studied by measurements on SOFCs with a LSM:CGO composite cathode on a CGO interdiffusion barrier layer. It was found that both types of cathodes showed similar electrochemical characteristics towards the presence of moisture during operation. Upon addition and removal of moisture in the fed air the impedance study showed a change in the high frequency cathode arc, which is associated with the charge transport/transfer at the LSM/YSZ interface. On prolonged operation with the presence of moisture an ongoing increase in the high frequency cathode arc resulted in a permanent loss of cathode/electrolyte contact and thus increase in the serial resistance and the high and low frequency cathode arcs. On the basis of these results and reports within literature a mechanism for the effect of moisture was proposed, which attribute to moisture the role of participating in an enhanced removal of manganese from the LSM/YSZ interface and thus eventually a decomposition of LSM.