Microalloying Mediated Segregation and Interfacial Oxidation of FeCr Alloys for Solid-Oxide Fuel Cell Applications

Ferritic stainless steels (FeCr-based alloys with low Ni content) have gained recent interest due to their excellent corrosion resistance, mechanical strength, and competitive price, which make them an attractive choice for many energy applications, for example, energy conversion and exhaust systems. The corrosion resistance results from the Cr-rich protective oxide layer that forms spontaneously under oxidizing conditions. At elevated temperatures, oxidation resistance can be enhanced by controlled surface treatments or by microalloying with elements that affect the oxide layer formation through segregation and interfacial oxidation. Today, further alloy development is required concerning the high-temperature oxidation resistance due to the increased operation temperatures. Furthermore, new alloy materials that form an electrically conductive non-volatile oxide layer under high-temperature conditions are required for the solid-oxide fuel cell interconnect applications. In this thesis, the segregation and oxidation phenomena on non-stabilized and Ti–Nb stabilized ferritic stainless steel alloys were investigated at 50–800 °C by photoemission spectroscopy, inelastic electron energy-loss background analysis, and electrochemical impedance spectroscopy. Firstly, the influence of controlled surface treatments on the initial stages of oxidation was investigated. The surface enrichment of Cr was induced by H2O preadsorption at low temperatures and by thermally induced cosegregation with N at high temperatures. The Cr-enriched surface was found beneficial against further oxidation by O2, but the effect was the most pronounced at low temperatures where the thermal diffusion of ions is not fast enough to support the oxidation. Secondly, microalloying with Nb was shown to improve the electrical properties of ferritic stainless steel alloys at 650 °C. The role of excess Nb was attributed to its high segregation rate and formation of conductive oxides at the oxide–metal interface. Furthermore, the Nb alloying induced the formation of (FeNbSi)-type Laves intermetallic phase in the alloy, which resulted in the non-uniform distribution of electrically resistive SiO2 at the interface. Therefore, the results presented in this thesis can be applied to design ferritic stainless steel alloys and surface treatments that facilitate the formation of the protective oxide layer with the optimum composition under various demanding application conditions, particularly, in the solid-oxide fuel cells

Low residual dissolved phosphate in spent medium bioleaching enables rapid and enhanced solubilization of rare earth elements from end-of-life NiMH batteries

Successful heterotrophic bioleaching with high metal yields requires an efficient leaching agent production and minimization of secondary reactions such as precipitation of leached metals with growth medium components. In this study, the role of the secondary reactions on bioleaching of spent nickel-metal-hydride batteries was investigated. Substitution of K2HPO4 by yeast extract (YE) reduced precipitation of both base metals and rare earth elements (REEs). REEs were proportionally more affected by precipitation than base metals. Optimizing the ratio of YE to glucose in the growth medium resulted in glucose to gluconic acid conversion yield of 90% by Gluconobacter oxydans. In one-day leaching with YE medium, 28.8% Mn, 52.8% Fe, 22.9% Co, 12.0% Ni, and 19.5% of total REEs were extracted. The leach liquor obtained with the YE medium resulted in leaching of 1.5 and 11.0 times more of total base metals and REEs, respectively, than with phosphate medium. Experimental results were consistent with geochemical modeling results corroborating the benefit of low phosphate concentrations in leaching systems at neutral to moderately acidic pH. In summary, substitution of K2HPO4 with YE in gluconic acid production phase with G. oxydans reduced subsequent base metal and especially REE precipitation during leaching and, thus, enhanced the overall metal extraction.publishedVersionPeer reviewe

Grain orientation dependent Nb-Ti microalloying mediated surface segregation on ferritic stainless steel

Surface segregation and oxide formation anisotropy on Ti-Nb stabilized ferritic stainless steel (EN 1.4521) were studied by XPS and Electron Backscatter Diffraction. Competitive surface segregation of Si, Nb and Ti was initiated at ∼550. °C, and segregation was favored to the open surface sites of 〈111〉 oriented grains. Furthermore, the surface segregation of Cr was strongly limited at the locations of stable Ti(CN)- and (NbTi)C-type precipitates. Consequently, the oxidation resistance of stainless steels can be enhanced cost-efficiently, without alloy additions, by optimizing the microstructure to facilitate the fast and uniform growth of protective oxide scale.acceptedVersionPeer reviewe

Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO2Thin Films Grown by Atomic Layer Deposition

The physicochemical properties of titanium dioxide (TiO2) depend strongly on the crystal structure. Compared to anatase, rutile TiO2 has a smaller bandgap, a higher dielectric constant, and a higher refractive index, which are desired properties for TiO2 thin films in many photonic applications. Unfortunately, the fabrication of rutile thin films usually requires temperatures that are too high (>400 °C, often even 600-800 °C) for applications involving, e.g., temperature-sensitive substrate materials. Here, we demonstrate atomic layer deposition (ALD)-based fabrication of anatase and rutile TiO2 thin films mediated by precursor traces and oxide defects, which are controlled by the ALD growth temperature when using tetrakis(dimethylamido)titanium(IV) (TDMAT) and water as precursors. Nitrogen traces within amorphous titania grown at 100 °C inhibit the crystal nucleation until 375 °C and stabilize the anatase phase. In contrast, a higher growth temperature (200 °C) leads to a low nitrogen concentration, a high degree of oxide defects, and high mass density facilitating direct amorphous to rutile crystal nucleation at an exceptionally low post deposition annealing (PDA) temperature of 250 °C. The mixed-phase (rutile-brookite) TiO2 thin film with rutile as the primary phase forms upon the PDA at 250-500 °C that allows utilization in broad range of TiO2 thin film applications.publishedVersionPeer reviewe

Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes

We employ atomic layer deposition to prepare 50 nm thick hematite photoanodes followed by passivating them with a 0.5 nm thick Ta2O5-overlayer and compare them with samples uniformly doped with the same amount of tantalum. We observe a three-fold improvement in photocurrent with the same onset voltage using Ta-overlayer hematite photoanodes, while electrochemical impedance spectroscopy under visible light irradiation shows a decreased amount of surface states under water splitting conditions. The Tadoped samples have an even higher increase in photocurrent along with a 0.15 V cathodic shift in the onset voltage and decreased resistivity. However, the surface state capacitance for the Ta-doped sample is twice that of the reference photoanode, which implies a larger amount of surface hole accumulation. We further utilize transient absorption spectroscopy in the sub-millisecond to second timescale under operating conditions to show that electron trapping in both Ta2O5-passivated and Ta-doped samples is markedly reduced. Ultrafast transient absorption spectroscopy in the sub-picosecond to nanosecond timescale shows faster charge carrier dynamics and reduced recombination in the Ta-doped hematite photoanode resulting in the increased photoelectrochemical performance when compared with the Ta2O5-overlayer sample. Our results show that passivation does not affect the poor charge carrier dynamics intrinsic to hematite based photoanodes. The Ta-doping strategy results in more efficient electron extraction, solving the electron trapping issue and leading to increased performance over the surface passivation strategy.Peer reviewe

Pinhole-resistant nanocrystalline rutile TiO2 photoelectrode coatings

Atomic layer deposited (ALD) TiO2 thin films have a wide range of applications in photonics which are, however, limited by the chemical instability of the amorphous as-deposited TiO2. Post-deposition annealing is required for improving the performance by inducing phase transitions and oxide defects. ALD precursor traces remaining in the TiO2 film affect the thermally-induced processes but the understanding of the effect of growth temperature on precursor traces in the film as well as on the thermally-induced processes is weak. In this study 30 nm ALD TiO2 was grown on Si wafer from tetrakis(dimethylamido)titanium and water at 100–200 °C. TiO2 was subsequently annealed in vacuum at 200–500 °C. Increasing the growth temperature decreased the amount of N bearing precursor traces and thus makes the TiO2 more easily reducible. The reduction takes place simultaneously with the crystallization and formation of O1− defects. Vacuum annealing of TiO2 with less than 0.3 at% of N results in nanocrystalline rutile whereas samples with more N containing traces crystallized as microcrystalline anatase. Nanocrystalline rutile TiO2 was chemically stable and resistant to the dissolution at the grain boundaries under alkaline conditions making it a suitable material for protective photoelectrode coatings used in artificial photosynthesis.publishedVersionPeer reviewe

Design aspects of all atomic layer deposited TiO2–Fe2O3 scaffold-absorber photoanodes for water splitting

Iron and titanium oxides have attracted substantial attention in photoelectrochemical water splitting applications. However, both materials suffer from intrinsic limitations that constrain the final device performance. In order to overcome the limitations of the two materials alone, their combination has been proposed as a solution to the problems. Here we report on the fabrication of an atomic layer deposited (ALD) Fe2O3 coating on porous ALD-TiO2. Our results show that successful implementation requires complete mixing of the TiO2 and Fe2O3 layers via annealing resulting in the formation of a photoactive iron titanium oxide on the surface. Moreover, we found that incomplete mixing leads to crystallization of Fe2O3 to hematite that is detrimental to the photoelectrochemical performance. IPCE and transient photocurrent measurements performed using UV and visible light excitation confirmed that the iron titanium oxide extends the photocurrent generation to the visible range. These measurements were complemented by transient absorption spectroscopy (TAS), which revealed a new band absent in pristine hematite or anatase TiO2 that we assign to charge transfer within the structure. Taken together, these results provide design guidelines to be considered when aiming to combine TiO2 and Fe2O3 for photoelectrochemical applications.Peer reviewe

Is carrier mobility a limiting factor for charge transfer in TiO2/Si devices? A study by transient reflectance spectroscopy

TiO2 coatings are often deposited over silicon-based devices for surface passivation and corrosion protection. However, the charge transfer (CT) across the TiO2/Si interface is critical as it may instigate potential losses and recombination of charge carriers in optoelectronic devices. Therefore, to investigate the CT across the TiO2/Si interface, transient reflectance (TR) spectroscopy was employed as a contact-free method to evaluate the impact of interfacial SiOx, heat-treatments, and other phenomena on the CT. Thin-film interference model was adapted to separate signals for Si and TiO2 and to estimate the number of transferred carriers. Charge transfer velocity was found to be 5.2 × 104 cm s−1 for TiO2 heat-treated at 300 °C, and even faster for amorphous TiO2 if the interfacial SiOx layer was removed using HF before TiO2 deposition. However, the interface is easily oversaturated because of slow carrier diffusion in TiO2 away from the TiO2/Si interface. This inhibits CT, which could become an issue for heavily concentrated solar devices. Also, increasing the heat-treatment temperature from 300 °C to 550 °C has only little impact on the CT time but leads to reduced carrier lifetime of ¡3 ns in TiO2 due to back recombination via the interfacial SiOx, which is detrimental to TiO2/Si device performance.publishedVersionPeer reviewe

Visible to near-infrared broadband fluorescence from Ce-doped silica fiber

We investigate the fluorescence characteristics of a purely Ce-doped silica fiber and demonstrate broad-bandwidth fluorescence across the visible and near-infrared. The Ce-doped fiber is fabricated using standard modified chemical vapor deposition technology. Trace metal analysis by inductively coupled plasma mass spectrometry confirmed the purity of Ce-doping. The Ce valence state of 3+ was revealed by X-ray photoelectron spectroscopy. The optimum pump wavelength for the broadest luminescence from a fiber is scanned between 405 nm to 440 nm wavelength of diode lasers operating under continuous-wave regime. The strongest pump absorption is observed at the wavelength of 405 nm. Variation of pump power and fiber length results in the demonstration of broad-bandwidth fluorescence with spectral widths up to 301 nm (at -10 dB). The measured fluorescence spectra cover the wavelength range from ∼458 nm to ∼819 nm with spectral power densities of up to 2.4 nW/nm

Enhancing the Microstructure of Perovskite-Inspired Cu-Ag-Bi-I Absorber for Efficient Indoor Photovoltaics

Lead-free perovskite-inspired materials (PIMs) are gaining attention in optoelectronics due to their low toxicity and inherent air stability. Their wide bandgaps (≈2 eV) make them ideal for indoor light harvesting. However, the investigation of PIMs for indoor photovoltaics (IPVs) is still in its infancy. Herein, the IPV potential of a quaternary PIM, Cu2AgBiI6 (CABI), is demonstrated upon controlling the film crystallization dynamics via additive engineering. The addition of 1.5 vol% hydroiodic acid (HI) leads to films with improved surface coverage and large crystalline domains. The morphologically-enhanced CABI+HI absorber leads to photovoltaic cells with a power conversion efficiency of 1.3% under 1 sun illumination-the highest efficiency ever reported for CABI cells and of 4.7% under indoor white light-emitting diode lighting-that is, within the same range of commercial IPVs. This work highlights the great potential of CABI for IPVs and paves the way for future performance improvements through effective passivation strategies.</p