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Protonic ceramic fuel or electrolysis cells (PCFC/PCEC) have shown promising performance at intermediate temperatures. However, these technologies have not yet been demonstrated in a stack, hence the oxidation behavior of the metallic interconnect under relevant operating environments is unknown. In this work, ferritic stainless steels 430 SS, 441 SS, and Crofer 22 APU were investigated for their use as interconnect materials in the PCFC/PCEC stack. The bare metal sheets were exposed to a humidified air environment in the temperature range from 450 °C to 650 °C, to simulate their application in a PCFC cathode or PCEC anode. Breakaway oxidation with rapid weight gain and Fe outward diffusion/oxidation was observed on all the selected stainless steel materials. A protective coating is deemed necessary to prevent the metallic interconnect from oxidizing. To mitigate the observed breakaway oxidation, state-of-the-art protective coatings, Y2O3, Ce0.02Mn1.49Co1.49O4, CuMn1.8O4 and Ce/Co, were applied to the stainless steel sheets and their oxidation resistance was investigated. Dual atmosphere testing further validated the effectiveness of the protective coatings in realistic PCFC/PCEC environments, with a hydrogen gradient across the interconnect. Several combinations of metal and coating material were found to be viable for use as the interconnect for PCFC/PCEC stacks.

Increasing the density of percolated triple phase boundaries (TPBs) by infiltrating nanoscale electrocatalysts can improve the performance of solid oxide fuel cell (SOFC) anodes. However, the complex microstructure of these infiltrated nanocatalysts creates challenges in quantifying their role in anode performance improvements. In this research, scanning electron microscopy of fractured cross-sections of a Ni-nanocatalyst infiltrated anodic symmetric cell along with three-dimensional (3-D) reconstruction of the same anode have been used to quantify the changes in percolated TPB densities due to infiltration. This change in percolated TPB density has been compared to the improvement in anode activation polarization resistance measured by electrochemical impedance spectroscopy (EIS). It was found that increased TPB densities only partially accounted for the measured performance improvement. Distribution of relaxation times (DRT) analyses showed that a reduction in the time constants of the catalytic processes in the anode also play a role, suggesting that the added nanoscale percolated TPB boundaries are more electrochemically active as compared to the cermet TPB boundaries.

To prevent Cr-poisoning in solid oxide fuel cells (SOFCs) and to decrease area specific resistance by limiting the rate of chromia growth on Crofer 22 APU interconnects, a protective coating layer on the interconnects is necessary. In this study, the glycine nitrate process (GNP) was used to synthesize Cu1.3Mn1.7O4 submicron powders, which then were deposited on Crofer 22 APU by electrophoretic deposition (EPD), followed by post-deposition thermo-mechanical treatments. The phase purity, uniformity, thickness and porosity of the coating layers were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effectiveness of the coating layer to prevent oxygen and chromium diffusion was evaluated by thermogravimetric analysis and energy dispersive X-ray spectroscopy (EDX), respectively. The area specific resistance (ASR) of the coated samples was measured as a function of temperature.

The search for low-loss and robust optical fibers in the infrared spectrum has always been an important research topic for many investigators. Over the years, fibers of various materials groups have been proposed to obtain ‘the ultimate infrared fiber’. Recently, a new competitive alternative has emerged: the semiconductor-core glass-cladded optical fibers. The excellent bulk materials properties and integrated circuit applications reveals the potential of semiconductors as fiber materials. One of the important photonic materials that has been proposed as a fiber-core material is germanium. In this paper, the development of Ge-based fibers and their optical properties in the mid-infrared spectrum are discussed. The performance of Ge-based fibers has been compared with other semiconductor-core fibers. Recent developments in the area of semiconductor fibers and the future prospects of semiconductors as infrared fiber materials are also discussed.

We present a systematic synchrotron radiation photoemission spectroscopy study of the interaction of O2 with defective La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surfaces at low temperatures. First, the surface chemical evolution during low energy Ar+ ion sputtering is investigated and is found to create oxygen vacancies. Subsequently, the interaction of the O2 molecules with La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surface is observed to create an insulating phase. We performed a detailed core-level peak-fitting analysis to identify the chemical nature of this phase and to probe the role of vacancies in the formation of this phase.

The reduction kinetics of tungsten trioxide (WO 3 ) powder using hydrogen gas was studied with titanium dioxide (TiO 2 ) or 8 mol% yttria-stabilized zirconia (8YSZ) mixed into the powder bed. TiO 2 was selected as an inert volume filler, and 8YSZ was selected to increase the ionic conductivity of the powder bed. The reduction sequence of tungsten trioxide in hydrogen is known to take place in the following steps: WO 3 → WO 2.9 → WO 2.72 → WO 2 → W. When the electronic and ionic conductivities of the mixed powder bed were both sufficiently high, reduction rates were found to increase. Specifically, reaction kinetics improved with the addition of 8YSZ to the WO 3 when compared to experiments in which TiO 2 was added to the WO 3 powder bed. Chemical reaction rates were determined by first recording changes in mass as a function of time during reduction using thermogravimetry, and then reaction rates were calculated using relevant kinetic theory.

Mitigating activation polarization in the anode is one of the major challenges in intermediate-temperature operation of solid oxide fuel cells (SOFCs). Liquid phase infiltration of nanoscale electrocatalysts has been shown to result in significant reductions in activation polarization in SOFC anodes. In this study, we explore liquid-phase infiltration of nickel, gadolinium doped ceria (GDC), and Ni/GDC electrocatalysts into two different types of cermet anodes: one with a conventional Ni-YSZ composition, and the other with a Ni-MIEC cermet anode where the YSZ has been doped with 3 mol% TiO2 to impart electronic conductivity. The principal goal of this study is to explore the role of electronic transport in the MIEC phase in effective utilization of the infiltrated nanoscale electrocatalysts. The role of temperature, infiltration cycles and the type of electrocatalysts have been experimentally studied in symmetric cells using electrochemical impedance spectroscopy (EIS). Distribution of relaxation times (DRT) modeling has been used to elucidate the contributions of various charge transfer processes.

The infiltration of nanoparticle electrocatalysts into solid oxide fuel cell (SOFC) electrodes has been proven to produce a high density of electrochemically active sites, and reduce charge transfer polarization losses for SOFC electrodes. This is crucial for intermediate temperature operation, as these losses increase greatly at lower temperatures. Nickel-yttria stabilized zirconia (Ni-YSZ) cermets are low-cost, and exhibit excellent stability, but their main disadvantage stems from nickel coarsening and performance loss over their operational lifetimes. Infiltration of electrocatalyst nanoparticles has been shown to mitigate nickel coarsening and the consequent anode degradation. In this study, the effects of these infiltrants are observed in a standard Ni-YSZ electrode. In addition to nickel, mixed ionic and electronic conducting (MIEC) phases were infiltrated into Ni-YSZ scaffolds and their performances were characterized using electrochemical impedance spectroscopy (EIS). Cross-sectional microscopy of fractured cells was used to compare electrode microstructure and particle statistics. A model is proposed for how the nanoparticle electrocatalysts improve the anode performance.

a b s t r a c t Formation of oxygen vacancies by introducing various mixed-valent cation dopants is a common practice to improve the cathode performance in solid oxide fuel cells. A computational procedure is developed in this work to predict the equilibrium oxygen vacancy non-stoichiometric concentrations at experimen- tally relevant temperatures and oxygen partial pressures for both bulk and surface oxide phases. The calculations are based on the first-principles density functional theory and a constrained free-energy functional. Quantitative agreements are found by direct comparisons to the thermogravimetry and solid electrolyte coulometry measurements for the strontium-doped lanthanum cobalt iron oxides at different compositions. Our results indicate that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and all oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures.

Thermal barrier coatings (TBCs) and environmental barrier coatings (EBCs) improve the efficiency and durability of turbine blades. Plasma-sprayed yttria-stabilized zirconia (YSZ) coupled with dense, chemical vapor deposited functionally graded mullite is a promising TBC–EBC system. A computational model is constructed to consider effects of functional grading of the EBC, coating geometry, as well as density and depth of vertical cracks in the TBC on the stress distribution in the coating system. Functional grading of the EBC coating is found to reduce interfacial σxx mismatch at the interfaces of the coating system. The ratio of EBC to TBC thicknesses has a much smaller effect compared with the effect of grading. Vertical cracks increase TBC compliance but introduce substantial σxx fluctuations in the EBC, especially when cracks are through-thickness. Increased spacing between cracks is accompanied by greater σxx fluctuation. While TBC stresses are minimized with through-thickness and more distanced cracks, this geometry is not ideal for the EBC. The absence of cracks produces the best results for the EBC layer. If cracks are present, sub through-thickness, closely spaced cracks achieve the lowest magnitude and most uniform stresses in the EBC.

Energy storage and recovery using the redox reaction of tungsten/tungsten-oxide is proposed. The system will store energy as tungsten metal by reducing the tungsten oxide with hydrogen. Thereafter, steam will be used to reoxidize the metal and recover the hydrogen. The volumetric energy density of W for storing hydrogen by this process is 21 kWh/L based on the lower heating value (LHV) of hydrogen. The main objective of this investigation was to study the kinetics of the reduction process of tungsten oxide (WO3) and determine the optimum parameters for rapid and complete reduction. Theoretical treatment of isothermal kinetics has been extended in the current work to the reduction of tungsten oxide in powder beds. Experiments were carried out using a thermogravimetric technique under isothermal conditions at different temperatures. The reaction at 1073 K (800 °C) was found to take place in the following sequence: WO3 → WO2.9 → WO2.72 → WO2 → W. Expressions for the last three reaction rate constants and activation energies have been calculated based on the fact that the intermediate reactions proceed as a front moving at a certain velocity while the first reaction occurs in the entire bulk of the oxide. The gas–solid reaction kinetics were modeled mathematically in terms of the process parameters. This model of the reduction has been found to be accurate for bed heights above 1.5 mm and hydrogen partial pressures greater than 3 pct, which is ideal for implementing the energy storage concept.

The hot-corrosion behavior of uncoated SiC, bulk mullite and CVD grown mullite coatings in contact with Na 2 SO 4 were investigated at 1200 °C. The range of thermodynamic activity of Na 2 O (10 − 4 to 10 − 6 ) simulated in this study makes the oxide very basic leading to high reaction rates with SiO 2. Uncoated SiC corroded severely, forming various Na 2 O·SiO 2 compounds with a significant weight gain. Even though the thermodynamic activity of silica is reduced in mullite, several compounds in Na 2 O·SiO 2 ·Al 2 O 3 system were formed in case of bulk mullite. CVD based mullite coatings with high alumina content at the top surface of the coating, and therefore reduced SiO 2 activity, offered protection to the underlying SiC in corrosive environments. As predicted correctly by thermodynamic calculations, the trend in weight gain with the coated SiC samples followed the theoretically calculated SiO 2 activity in Al-rich mullite.

This paper discusses the requirements of sensors that could be used both for advanced real-time control as well as manual torch operator control of plasma spray processes, and reviews the critical issues associated with design and implementation of such sensors. The focus is on yttria-stabilized zirconia for thermal barrier coatings. The overarching requirement is that the sensor must capture the subset of particles that contributes most to coating properties and that the resulting measurements of these critical subdistributions must be aggregated in a manner that correlates best to the coating buildup. For deposition rate control, the focus of this work, experiments show that the mass flux of molten particles correlates better with coating thickness than bulk-average temperature or light intensity. However, measurement of molten mass flux for control requires sensors that are capable of sensing particle states at high rates from across a large portion of the full plume.

In this paper, we discuss optical emission, energy transfer and electroluminescence from a superlattice structure containing small (∼2 nm diameter) amorphous silicon (Si) clusters coupled to erbium (Er) ions. The superlattice structure is fabricated by direct co-sputtering of thin (∼3–5 nm) Er-doped silicon-rich nitride/Si (Er:SRN/Si) layers subsequently annealed at different temperatures in order to induce the nucleation of Si clusters and to activate Er emission. In this paper, we discuss efficient Er emission and sensitization with nanosecond-fast non-radiative transfer time and we report on low turn-on voltage (∼7 V) electroluminescence from simple electrical device structures. Our results demonstrate that small Si clusters embedded in silicon nitride-based superlattice structures provide a viable approach for the fabrication of Si-compatible optical devices.

FeCrAl/Al2O3 and FeCrAlY/Al2O3 interfaces were formed by sputter deposition of alloy thin films on A-plane sapphire substrates. The microstructure of both Y-containing and Y-free films consisted of single-phase BCC columnar grains of fairly uniform composition across the film thickness. Both films exhibited a high degree of 〈110〉 texture along the growth direction normal to the interface with no preferred in-plane orientation. In both cases, a thin amorphous layer was observed at the interface in the as-deposited state. Annealing the films for 16 h at 850 °C improved the crystalline quality of the film/substrate interface and led to grain growth within the films. The annealing did not lead to any significant changes in the film composition in the Y-free film. In the Y-containing film, yttrium oxide precipitates formed in the near interface region, depleting the adjacent alloy matrix of Y completely; accompanied by a significant enhancement of Al in the film, very close to the interface.

The interfacial strengths of the FeCrAl/Al 2 O 3 and FeCrAlY/Al 2 O 3 interfaces in both as-deposited and annealed state were measured quantitatively by the laser spallation technique. The results show that in the as-deposited state, the presence of Y significantly improved the interfacial strength from 330±31 to 686±36 MPa. However, after annealing the films at 850 °C for 16 h, the interfacial strength of the Y-free samples increased to 545±68 MPa, while the interfacial strength of Y-containing sample decreased to 599±22 MPa. The increase in the interfacial strength of the Y-free films was attributed to an improvement in the crystalline quality of the interface. The decrease in interfacial strength of the annealed Y-containing film in spite of the improvement in the crystalline quality of the interface was attributed to the depletion of Y at the interface due to formation of yttrium oxide precipitates. This is further proved that the presence of Y improves the strength of FeCrAl/Al 2 O 3 interfaces significantly.

Observation of long range atomic ordering in InGaN films grown by molecular beam epitaxy on A-plane sapphire is reported, based on x-ray diffraction and transmission electron microscopy studies. The InGaN films have a domain structure, with alternating domains of ordered and disordered phases, close to the film/substrate interface. Closer to the film surface, disordered domains are no longer observed. The degree of ordering was found to increase with growth rate (at the low growth rates used in these materials), which is consistent with ordering being a surface phenomenon.

In this study, we investigated phase separation and long-range atomic ordering phenomena in InGaN alloys produced by molecular beam epitaxy. Films grown at substrate temperatures of 700–750 °C with indium concentration higher than 35% showed phase separation, in good agreement with thermodynamic predictions for spinodal decomposition. Films grown at lower substrate temperatures (650–675 °C) revealed compositional inhomogeneity when the indium content was larger than 25%. These films, upon annealing to 725 °C, underwent phase separation, similar to those grown at the same temperature. The InGaN films also exhibited long-range atomic ordering. The ordering parameter was found to increase with the growth rate of the films, consistent with the notion that ordering is induced at the growth surface. The ordered phase was found to be stable up to annealing temperatures of 725 °C. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice...

The high temperature oxidation behavior of WC-Co samples has been investigated. The oxidation rate of WC-Co was found to increase rapidly with temperature above 600 °C and with the oxygen content of the atmosphere. Increasing the Co content led to an improvement in the oxidation behavior. In all cases, only WO3 and CoWO4 phases were present in the oxide. Although the morphology of the two oxide phases depended on the oxidation conditions, their volume fractions were determined solely by the Co content of the sample. The WO3 formed during the initial stages of oxidation, changed from a strong {{001}} texture at 600 °C to a weak {{200}} texture at 800 °C. As the flow rate of the oxidizing gas was increased, the oxidation rate of WC-Co increased at lower flow rates due to an increase in the oxygen supply to the oxide/sample interface. However, at higher flow rates, the oxidation rates decreased with increasing flow rate.

The microstructures of GaN films, grown on (001) and (111) Si substrates by a two-step method using Electron Cyclotron Resonance assisted-Molecular Beam Epitaxy (ECR-MBE), were studied by electron microscopy techniques. Films grown on (001) Si had a predominantly zinc-blende structure. The GaN buffer layer, grown in the first deposition step, accommodated the 17% lattice mismatch between the film and substrate by a combination of misoriented domains and misfit dislocations. Beyond the buffer layer, the film consisted of highly oriented domains separated by inversion domain boundaries, with a substantial decrease in the defect density away from the interface. The majority of defects in the film were stacking faults, microtwins, and localized regions having the wurtzite structure. The structure of the GaN films grown on (111) Si was found to be primarily wurtzite, with a substantial fraction of twinned zinc-blende phase. Occasional wurtzite grains, misoriented by a 30°twist along the [0001] axis, were also observed. A substantial diffusion of Si was seen in films grown on both substrates.

An extensive characterization study of sputtered YBa 2 Cu 3 O 7- x thin films deposited on single-crystal (001) LaAlO 3 substrates was carried out using electron-microscopy techniques to investigate the effect of the deposition parameters on the microstructure of the films. The research focused mainly on the study of the phases present in the film and the orientation and morphology of the grains. The film microstructure was found to be strongly dependent on the atomic mobility during deposition. Lower deposition temperatures and higher oxygen partial pressures led to a lower mobility of atoms favoring smaller grain sizes, larger a -axis coverage and a higher density of stacking faults. A computer model was developed to simulate the microstructural devlopment in the film. With the use of this model, the nucleation density of a -axis grains was found to range from 10 7 to 8×10 11 nuclei/cm 2 .

The relationship between the deposition conditions and the structural and electrical properties of in situ superconducting YBa2Cu3O7−x thin films deposited by off‐axis magnetron sputtering has been investigated. High‐quality films have been produced with a transition temperature TC (R=0) of 92 K, a critical current density JC (zero field) of 3.3×107 A/cm2 at 4.2 K and 4.8×106 A/cm2 at 77 K, and a microwave surface resistance RS of 2.6×10−6 Ω at 1.5 GHz and 4.2 K which rises to 8.3×10−6 Ω at 77 K. Among the deposition conditions explored, substrate temperature was identified as the most influential in producing these high‐quality films. A quantitative relationship was established between substrate temperature and TC, normal‐state resistivity ρ, JC, orientation distribution, x‐ray‐diffraction peak broadening, lattice expansion, RS, and penetration depth λ. Increasing substrate temperature results in an increase in TC, a decrease in ρ, an increase in JC, an increase in grain size, an increase in the ratio of...

The surface morphology and microstructure of in situ and ex situ derived YBa2Cu3O7−x (YBCO) thin films have been investigated. In situ films were deposited by single-target off-axis sputtering and three-target co-sputtering. Ex situ films were derived by metalorganic deposition (MOD) of trifluoroacetate precursors. Surface defects resulting from mixed a-axis and c-axis orientation as well as secondary phases have been identified in these films. Despite these defects, films with excellent electrical properties have been formed. However, defects interfere with film patterning and the fabrication of multi-layered structures. Several secondary phase precipitates have been identified, including CuO, Y2O3, Cu-Ba-O, and Y2Cu2O5. Secondary phases resulting from a lack of stoichiometry can be eliminated by direct composition control in the MOD and three-target sputtering techniques, and by composition control through the application of an externally applied magnetic field in single-target off-axis sputtering. Secondary phases caused by contamination were also identified: Cr-Ba-O in off-axis sputtering, resulting from contamination by the oxidized heater block; and BaSO4 in MOD, resulting from gas phase impurities. These results suggest that cation composition control is not sufficient to prevent the formation of secondary phases and that small levels of contamination may prevent phasepure material from being formed.

The microstructures of YBa2Cu3O7−x thin films deposited by laser ablation on single crystal (001) LaAlO3 substrates have been investigated. The orientation of the YBa2Cu3O7−x layer next to the interface is found to be completely c-perpendicular, with a high degree of epitaxy between the film and the substrate. Misfit dislocations, with a periodic spacing of around 13 nm, are present at the interface. Two distinct interfacial structures are seen in these films. At a film thickness of around 400 nm, nucleation of c-parallel grains occurs, leading to a switchover from a c, and, and-perpendicular to a c-parallel microstructure. Amorphous particulates, ejected from the target during processing, lead to the formation of misoriented grains, giving rise to high-angle grain boundaries in the film.

The microstructure of YBa2Cu3O7−δ thin films deposited on (001)LaAlO3 substrates by a laser ablation process has been investigated by scanning electron microscopy, x‐ray diffraction, and cross‐sectional transmission electron microscopy. Adjacent to the substrate, the film is entirely oriented with the c‐axis perpendicular to the surface. At a thickness of about 0.4 μm, the occurrence of 90° boundaries brings about a transition to grains with their c‐axes parallel to the surface (aligned along the [100] and [010] directions of the pseudocubic LaAlO3 substrate). This transition is discussed in terms of the crystal growth anisotropy and the retained strain that may precipitate the transition.

We investigated phase separation and long-range atomic ordering in InGaN alloys grown on (11–20) sapphire substrates by molecular beam epitaxy (MBE). Compositional inhomogeneities were found in films grown at substrate temperatures ranging from 650 – 675°C when the indium content was larger than 20%. Upon annealing to 725°C, these films underwent phase separation, similar to films grown at 725° C.These InGaN films also showed the evidence of long-range atomic ordering, with the order parameter being a function of film composition and the growth parameters. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice strain in the alloy.

The evolution of the Mn charge state, chemical composition, and electronic structure of La{sub 0.8}Sr{sub 0.2}MnO{sub 3} (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O K-edge and Mn L{sub 3,2}-edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn L{sub 3} resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment (800 C ambient pressure of O{sub 2}) alone results in La deficiency (severest near the surface with Sr doping >0.55) and a stronger Mn{sup 4+} contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O K-edge X-ray absorption measurements support Sr/La enrichment nearer the surface, along with the formation of mixed Sr{sub x}Mn{sub y}O{sub z} and/or passive MnO{sub x} and SrO species.

Dense and well-adhered Cumn 1.8 O 4 spinel-oxide coatings were successfully deposited on Crofer 22 APU substrates by a cost-effective electrophoretic deposition technique. The coatings were effective in reducing the oxidation rate of the stainless steel substrates. A diffusion model has been developed that indicates that the coated stainless steel substrates exhibit paralinear oxidation kinetics. The effective diffusivities in the coating and in the thermally grown oxide scale were calculated to be 2 X 10 -15 and 2.5 X 10 -16 cm 2 /s, respectively, at 800°C and 5 X 10 -16 and 1.6 X 10 -17 cm 2 /s, respectively, at 750°C. The as-processed coated sample showed only 4.6 mΩ cm 2 of area specific resistance at 800°C.