The protective properties of a scale forming on a metal are intimately relat-ed to the way in which matter is transported through the oxide. This report re-views the oxidation behaviour the bi-nary iron-chromium alloys which form the basis of many technologically important materials and attempts to relate it to the defect structure and transport properties of (Fe, Cr)2O3 oxides. Existing information on this system is mainly restricted to the end members, Cr2O3 and Fe2O3. The former is a cation deficient, p-type oxide and cation transport dominates. Fe2O3 is n-type, probably containing iron interstitials and oxygen vacancies, with comparable transport rates for both species, but the defect structure and properties of both oxides have been only partly characterized. New measurements by thermogravimetric, electrical and high temperature deformation techniques of these properties are briefly described.



Electrical measurements support the hypothesis that certain solid solutions of Cr2O3-Fe2O3have lower defect concentrations and ionic transport rates than the pure oxides. There is evidence that scales of about the required composition form on the binary alloy containing about 20% Cr and are responsible for its low oxidation rate. Possible extensions of the theory to the design of highly stoichiometric scales for the protection of tornary and more complex alloys are briefly mentioned.



Transporteigenschaften und Defektstruktur des auf FeCr-Legierungen entstehenden Oxids (Fe, Cr2)O3



Die Schutzeigenschaften einer auf einem Metall entstehenden Zunderschicht sind eng verbunden mit der Art des Stofftransports durch das Oxid. In der vorliegenden Arbeit wird das Oxydations- verhalten der binaren FeCr-Legierungen behandelt, welche die Grundlage zahlreicher technisch wichtiger Werkstoffe sind; gleichzeitig wird versucht, eine Beziehung zwischen Oxydationsverhalten einerseits und Defektstruktur und Transporteigenschaften der Oxide vom Typ (Fe, Cr)2O3, andererseits herzustellen. Die uber dieses System vorhandenen Informationen be- schranken sich im wesentlichen auf die Endprodukte, namlich Cr2O3 und Fe2O3. Das Chromoxid ist ein Oxid vom p-Typ rnit Kationenunterschus, so das der Kationentransport uberwiegt. Das Eisenoxid hingegen ist ein Oxid vom n-Typ und enthalt wahrscheinlich Eisen auf Zwischengitterplatzen sowie Sauerstoffleerstellen; die Transportgeschwindigkeiten fur diese beiden Spezies sind vergleichbar, doch die Defektstruktur und die Eigenschaften der beiden Oxide sind bisher nur teilweise bekannt. Neue Messungen unter Verwendung von thermogravimetrischen, elektrischen und Hochtemperatur-Verformungstechniken zur Ermittlung der genannten Eigenschaften werden kurz beschrieben.



Elektrische Messungen stutzen die Hypothese, wonach bestimmte feste Losungen von Cr2O3-Fe2O3 durch niedrigere Defektkonzentrationen und niedrigere Ionentransportgeschwindigkeiten als die reinen Oxide gekennzeichnet sind. Es gibt Anzeichen dafur, das die auf einer binaren Legierung mit etwa 20% Chrom entstehenden Zunderschichten etwa die Zusammensetzung haben, welche fur niedrige Oxydationsgeschwindigkeiten erforderlich ist. Kurz eingegangen wird auf die mogliche Erweiterung der Theorie fur die Bildung won stochiometrisch zusammengesetzten Zunderschichten zum Schutz won ternaren und noch komplexeren Legierungen.

The Transport Properties and Defect Structure of the Oxide (Fe, Cr)2O3 formed on Fe?Cr alloys

This review is focused on the rapidly expanding field of porous materials for the sensor, actuator, energy storage and energy generation application. The synthesis methods to produce excellent poro...

A review on porous polymer composite materials for multifunctional electronic applications

The main objective of this work is the catalyst optimization of Fe2O3-, Co3O4-, NiO- and/or PdO- (transition element oxides—TEO) functionalized CeO2 nanoparticles to maximize the conversion of asphaltenes under isothermal conditions at low temperatures ( CePd2 > CeNi0.66Pd0.66 > CeNi2 > CePd1 > CeNi1 > CeO2. It is worth mentioning that bi-elemental nanoparticles reduced the gasification temperature of asphaltenes in a larger degree than mono-elemental nanoparticles at a fixed amount of adsorbed asphaltenes of 0.02 mg·m−2, confirming the synergistic effects between Pd and Fe, Co, and Ni. Further, optimized nanoparticles (CeNi0.89Pd1.1) have the best performance by obtaining 100% asphaltenes conversion in less than 90 min at 220 °C while reducing 80% the activation energy.

https://www.mdpi.com/2079-4991/9/3/401/pdf

Optimization of the Load of Transition Metal Oxides (Fe₂O₃, Co₃O₄, NiO and/or PdO) onto CeO₂ Nanoparticles in Catalytic Steam Decomposition of n -C₇ Asphaltenes at Low Temperatures.

Alkaline (lithium, potassium, rubidium) and alkaline-earth (magnesium, barium) doped hematite materials were studied for NO 2 sensing application. The synthesized materials were characterized by laser granulometry, X-ray diffraction and scanning electronic microscopy. A temperature of 1300 °C was chosen as the optimal heat treatment in order to obtain the densest material. Humidity dependence of the electrical properties revealed a strong influence in the case of rubidium doped hematite material while the other doped materials were less sensitive. The AC impedance analyses underlined the n-type intrinsic semi-conduction of pure hematite. Alkaline-earth doped hematite materials showed two semi-conducting regions, below and above 500 °C, corresponding to extrinsic and intrinsic n-type semi-conduction, respectively. These electrical analyses associated with SEM observations suggested instability of the ferrites formed in rubidium and potassium doped materials. AC electrical measurements were performed in the 0–500 ppm NO 2 partial pressure range. The alkaline-earth doped hematite materials exhibited the most promising behavior.

New NOx sensors based on hematite doped with alkaline and alkaline-earth elements

Silicon carbide amorphous thin films have been bombarded with 100 keV N ions. Infrared-absorption spectroscopy has been used to study the effect of increasing ion doses, up to 5×1017 N+ cm−2, on the evolution of chemical bonding between Si, C, and N. The changes induced by thermal annealing at different temperatures, up to 973 K, on the stability of the bombardment induced SiCxNy phase are investigated, together with the effect of surface oxidation during the postannealing cooling of the films. The new phase is thermally stable within the temperature range investigated. At the highest implantation dose a threshold is reached above which N atoms are located as interstitial, or are weakly bonded in the host film. Annealing at high enough temperature induces migration toward the surface and subsequent loss of interstitial N.

Phase formation and stability of N+ implanted SiC thin films

Two raw materials were chiefly used, namely laboratory ferric oxide and a high grade hematite. All experiments were carried out in an oxygen atmosphere, in the range 600 to 1300° C.

Electrical conductivity and sintering in iron oxides at high temperatures

The mechanism of the high shrinkage that occurs in chromic oxide in reducing conditions has been investigated by micrographic, analytical, electrical resistance, and other means. The effect of the reducing conditions is to modify the surface of the oxide particles, the core remaining normal oxide (i.e. oxygen excess). The modification is predominantly a change to a lower oxide or oxides and/or metal, although transitional formation of oxygen-deficient oxide is implied. In oxidising conditions (which cause fritting, but not shrinkage), modification to the extent that surfaces become of approximately stoichiometric proportions also occurs, but this change is apparently caused by loss of trioxide and results in unreactive oxide.

The morphology of oxide reduction: Chromic oxide

Compacts have been pressed from (i) four, finely ground, iron ores and (ii) mixtures of subsieve ferric oxide with up to 40 wt % of either lime, alumina, silica, or water, and have been sintered in oxygen at temperatures in the range 1000 to 1400‡ C. Shrinkage, macro- and micro-appearance were recorded at each sintering stage. The iron ores were classified into two groups: one was characterised by the red ferric oxide appearance and by significant shrinkage at 1000‡ C; the other was predominantly black and required a temperature of 1200‡ C to initiate shrinkage in 1 h. The shrinkages of the ores are discussed in terms of the presence of black ferric oxide, particle size, lime content, and dilution of the iron-bearing grains with inert oxides. The mixtures were used to study the effect of inert oxides on the sintering of ferric oxide. The reduction in shrinkage in the case of alumina and silica is shown to be largely a consequence of dilution; the abnormally high shrinkage at 1000‡ C induced by 5 wt % of lime is probably due to the presence of low-melting-point ferrites.

H. E. N. Stone

Papers

Electrical conductivity and sintering in iron oxides at high temperatures

The morphology of oxide reduction: Chromic oxide

Shrinkage in Compacts of Iron Oxides and Ores