Thin films of three iron oxide polymorphs, hematite, maghemite, and magnetite, were produced on KBr substrates using a conventional electron beam deposition technique coupled with thermal annealing. This method allowed for iron oxide thin films free from chemical precursor contaminants. The films were characterized using Fourier-transform infrared spectroscopy (FTIR), Raman microspectroscopy, and ellipsometry. These spectroscopic techniques allowed for a clear assignment of the phase of the iron oxide polymorph films produced along with an examination of the degree of crystallinity possessed by the films. The films produced were uniform in phase and exhibited decreasing crystallinity as the thickness increased from 40 to 250 nm.

Vibrational Spectroscopic Characterization of Hematite, Maghemite, and Magnetite Thin Films Produced by Vapor Deposition

Electrochemical methods for the codeposition of Cr or Mo with α-Fe2O3 (hematite) have been developed to produce doped iron oxide films with varying compositions of Cr and Mo which are active photoanodes for photoelectrochemical (PEC) water decomposition (“water splitting”). The films were characterized by scanning electron microscopy, X-ray diffraction, UV−vis optical spectroscopy, and X-ray photoelectron spectroscopy to determine the effect of the dopants on the hematite structure and PEC performance. Upon doping, the microstructures of the films varied; however, no preferred crystallographic orientation or dopant phase segregation was observed. The best performing samples were 5% Cr and 15% Mo doped which had Incident Photon Conversion Efficiencies (IPCE’s) at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4V vs Ag/AgCl. These IPCE values were 2.2× and 4× higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. The increase in performance is attributed to an imp...

Electrodeposition of α-Fe2O3 Doped with Mo or Cr as Photoanodes for Photocatalytic Water Splitting

With the reduction in solid oxide fuel cell (SOFC) operating temperatures into the range 650–800°C, cost effective metallic materials, in particular high temperature oxidation resistant alloys, have become promising candidates, in place of ceramic counterparts, for construction of interconnects in SOFC stacks. For a lifetime of thousands of hours, the alloys must demonstrate excellent surface, structural and electrical stability, and compatibility with other components in SOFC stacks in a very challenging environment. The present paper provides a critical review of metallic interconnects for SOFCs with a focus on recent progress in materials development, as well as advances in understanding materials degradation and interfacial phenomena under the SOFC operating conditions. It also attempts to provide guiding principles for the development of optimised, cost effective metallic interconnect materials that can demonstrate satisfactory life time stability and performance.

Recent advances in metallic interconnects for solid oxide fuel cells

How the vacancies in excess of equilibrium concentration are created and destroyed in solids is crucial for understanding many of their physical characteristics and processes. Grain boundaries are known as sources and sinks for bulk vacancies, but what exchange will occur between the grain boundary and the bulk under a stress is still obscure. In the present paper, we show that grain boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted. This is the sixth method of introducing and annihilating vacancies in solids in addition to the five methods mentioned in literature. At the same time, this physical process will produce solute nonequilibrium grain-boundary segregation/dilution. A set of kinetic equations is established to describe such a physical process. Additionally an attempt has been made to simulate Misra’s experimental data with the kinetic equations to justify the physical process.

Creating and destroying vacancies in solids and nonequilibrium grain boundary segregation

Initial oxidation and chromium diffusion. I. Effects of surface working on 9–20% Cr steels

The metal dusting resistance of various 2 1/4Cr to 20%Cr steels has been tested in flowing CO-H 2 -H 2 O atmospheres at 525, 560, 600 and 650 °C. For the 2 1/4Cr-steel immediately a constant rate of metal wastage is reached, for the high alloy steels the approach to this rate where all surface area is attacked, is more or less retarded by an oxide layer. The formation of a protective Cr-rich oxide layer is favoured by a high Cr-content of the steels, by the ferritic structure, by a fine-grained microstructure, by surface working and by high temperatures -these factors hinder or suppress metal dusting, whereas low Cr-concentration, austenitic structure, coarse-grained microstructure, removal of surface deformation by etching and a critical temperature of ≤ 600 °C accelerate and enhance metal dusting.

Resistance of 9–20%Cr-steels against metal dusting

Layers of the oxides FeO, Fe3O4, Fe2O3, Cr2O3 and Al2O3 were prepared by evaporation of the corresponding metal layers onto a preoxidized Si substrate and oxidation at elevated temperature The oxidation was conducted under thermodynamically well-defined conditions so that the pure oxide phases noted were obtained The oxide layer thickness was calculated from the thickness of the initial metal film, which was determined by chemical analysis and from the frequency change of a quartz crystal upon evaporaton Auger electron spectroscopy depth profiles of the oxide layers were measured during Ar+ sputtering, and the sputter time necessary to reach the interface oxide/substrate was determined The determination of this sputter time was not very exact, because the transition from oxide to substrate in the profile was rather gradual, owing to the roughness and crystalline character of the oxide layers that causes inhomogeneous sputtering However, from the sputter time, oxide thickness and ion beam current, the sputter rates were obtained for the oxides noted (in nm μA−1 cm−2 min−1) and were compared to the sputter rate of amorphous Ta2O5, which is generally used as the sputter reference standard Considerable differences were found The results will lead to more reliable determinations of oxide scale thicknesses by sputtering

Ion sputter rates and yields for iron-, chromium- and aluminium oxide layers

The phosphorus grain boundary segregation kinetics during tempering at 680 °C and aging at 500 °C of 2.7Cr-0.7Mo-0.3V steels with phosphorus mass contents of 0.004, 0.014, and 0.027 % was investigated. To determine the grain boundary concentrations of phosphorus the Auger electron spectroscopy was used. Chemical compositions of carbide particles were determined by means of EDX/STEM. Xu Tingdong's and McLean's models of non-equilibrium and equilibrium segregations, respectively, were used to analyze experimental data. It was shown that a phosphorus grain boundary enrichment during tempering was mainly caused by non-equilibrium segregation. During aging the mechanism of the equilibrium grain boundary segregation was prevalent. Slow phosphorus segregation kinetics was observed in the experimental steels during aging.

Kinetics of phosphorus segregation in 2.7Cr-0.7Mo-0.3V steels with different phosphorus contents

Metal dusting is a disintegration of metals and alloys into graphite and metal particles, caused by strongly carburizing gas mixtures mainly in the temperature range 400–700°C. Protection of steels against metal dusting is possible through the formation of dense chromium rich oxide scales but it is not guaranteed that such scales are formed at low temperatures, even on high Cr-steels.Surface analytical studies have been conducted on the formation and composition of the oxide scales on 9–20%Cr steels. The growth of oxide films was followed by AES for 3 hours at 10–7 mbar O2 great differences were observed in dependence on surface finish. On ground samples, Mn and Si appeared early and Cr-rich oxide was formed, whereas on chemically etched samples Fe-rich oxides grew.After long term exposures (240 h) under metal dusting conditions, i.e. in CO–H2–H2O mixtures at 600°C, thin Cr-rich scales were observed on ground steels which were impermeable to carbon whereas on chemically etched steels thick Fe-rich...

Formation of chromium rich oxide scales for protection against metal dusting

The surface segregation of antimony was studied using AES on a 1.8% Si steel alloyed with different mass contents of Sb (0.052 and 0.1%). The dependence of surface segregation on grain orientation and on the presence of other solute atoms (S, P, C, Al, Si) were investigated under UHV conditions in the temperature range from 450 to 900° C. The results lead to an explanation for the formation of a texture rich in (100)-grains in Si steel caused by the presence of Sb. Also grain boundaries were analysed by AES after annealing at 850, 700 and 550° C for different times. Since the grain boundary segregation was negligible one can conclude that the texture formation results from orientation dependent surface segregation of Sb causing differences in surface energies and stabilities of different grains

Manfred Lucas

Papers

Resistance of 9–20%Cr-steels against metal dusting

Ion sputter rates and yields for iron-, chromium- and aluminium oxide layers

Kinetics of phosphorus segregation in 2.7Cr-0.7Mo-0.3V steels with different phosphorus contents

Formation of chromium rich oxide scales for protection against metal dusting

Orientation dependent surface segregation of antimony on non‐oriented electrical steel sheet