Study of the oxidation kinetics of finely-divided magnetites IV. Influence of zinc substitution

The oxidation kinetics of zinc substituted magnetites (Fe²⁺_1?xZn²⁺_xFe³⁺₂)O^2?₄ (O < x < 1) into the γ defect phase of type x(Zn²⁺Fe³⁺₂O^2?₄), $\text{3}\text{2}\text{(1?}\text{x}\text{)}\text{Fe}{}^{\mathrm{3+}}{}_2\text{O}{}^{\mathrm{2?}}{}_3$ is found to be governed by the diffusion with a chemical diffusion coefficient depending on vacancy concentration. For compounds rich in zinc the diffusion coefficient is constant and the rate law written as v = k Pⁿ_(O2) with $\text{n}\text{=}\text{1}\text{4}$ is in agreement with a partial association of defects. For low substitution extents the diffusion coefficient varies with the transformation extent α and exponent n from the pressure law is equal to $\text{1}\text{2}$, which agrees with a total association of defects.     

Study of the oxidation kinetics of finely-divided magnetites III — Influence of chromium and aluminum surstitution

The oxidation kinetics of aluminum and chromium-substituted magnetites (Fe²⁺Cr³⁺_xAl³⁺_2?x)O^2?₄ (0< x < 2) into the γ defect phase of type $\text{γ(}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{Al}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext><mtext>?</mtext><mtext>y</mtext>}}\text{Cr}{}^{\mathrm{3+}}{}_{\mathrm{<mtext>y</mtext>}}\text{)}{}_2\text{O}{}^{\mathrm{2?}}{}_3$ is found to be governed by the diffusion under variable working conditions for samples prepared at 700°C and whose size is less than 400 Å. When particle size is caused to increase by annealing the kinetic curves are sigmoidal but only for specimens with high chromium substitution. For compounds rich in chromium observations of morphology show facetting during oxidation but no fracturing was observed.     

Etude de la cinetique d'oxydation de magnetites finement divisees. I - Influence de la substitution par l'aluminium

The kinetics of finely divided, (Fe²⁺Fe³⁺_2?xAl³⁺_x)O₄ type (0 ? x ? 2), substitution magnetites oxidation into metastable phase γ(Fe³⁺_1?yAl³⁺_y)₂O₃ (x = 3y) has been accounted for by means of the law of diffusion, under variable working conditions, of the cationic vacancies generated at solid-gas interface with a diffusion coefficient that decreases when the stoichiometric difference is high. The crystallites size and the trivalent substituant percentage increases, in both cases, slow down the reaction rate.     

Influence of chromium diffusion and different surface finishes on the oxidation behaviour of chromium steels

Abstract

The present contribution is focused on the systematic investigation of the effects of different surface finishes (ground, polished, electropolished) on the oxidation behaviour of chromium steels. The specimens were oxidized in a H2–2.5%H₂O atmosphere at 872 K for 1 h to 100 h. Depth profiles were recorded by secondary neutral mass spectrometry (SNMS) to determine the elemental composition of the oxide scale and the diffusion profiles below the scale. The surface finish was found to influence both the thickness of the oxide scale and the depletion of the selectively oxidized elements.     

Electrical conductivity study of oxidation process in manganese-substituted magnetites

During oxidation in air of finely-grained manganese-substituted magnetites (Mn _0.8x ²⁺ Fe _1–0.8x ³⁺ )_A– (Fe _1+0.6x ³⁺ Fe _1–0.8x ²⁺ Mn _0.2x ³⁺ )_BO ₄ ^2– (A=tetrahedral, B=octahedral) the temperature dependence of the electrical conductivity over a temperature range of 100 to 700° C was investigated. Below 500° C the evolution of electrical conductivity might be closely associated with the position and nature of cations in the spinel lattice. The profile of the =f(t) curves show that the mechanism of electrical conduction in the temperature range 150 to 300° C can be explained in terms of the oxidation of Fe²⁺ to Fe³⁺ ions at octahedral sites. For the temperature range 300 to 400° C the conductivity involves the hopping of electrons from tetrahedral-site Mn²⁺ ions to tetrahedral-site Mn³⁺ ions. Above 500° C the oxidation of Mn²⁺ ions leads to an increase in conductivity with the generation of new phases of -Fe₂O₃, Mn₂O₃ and -(MnFe)₂O₃.     

Influence of microstructure and chromium content on oxidation behaviour of spin cast high speed steels

Abstract

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.     

Sintering kinetics of pure and doped chromium oxide

The mechanism of sintering in chromium oxide in the presence of varying amounts of magnesium oxide in a firing condition that simulates a controlled reducing atmosphere has been investigated. The investigation is based on isothermal shrinkage measurements at different temperatures. The data suggest grain boundary diffusion to be the main process with evidence of vapour transport.     

Ellipsometric examination of the thermal oxidation process for plated chromium films

The process of thermal oxidation of ion-plated chromium films having the A15 structure deposited on the glass BK-7 and on quartz glass was investigated in air and in oxygen at 676 K. It has been stated that the optical constants of plated chromium films do not depend on the nature of the substrate; the substrate, however, does influence the rate and character of the process. The oxidation of chromium on quartz substrates proceeds according to the inverse logarithmic rule and on glass according to the parabolic rule. A layer of chromium oxide is always formed on chromium films irrespective of the atmospheric composition (oxygen or air); in air the layer of chromium oxide is thicker than in an atmosphere of pure oxygen. Ellipsometric measurements allow the determination of both the refractive index and the thickness of the chromium oxide formed on chromium films.     

SiC颗粒的高温氧化动力学

研究了SiC颗粒在927℃、1027℃和1127℃空气中的高温氧化动力学.结果表明,温度越高SiC颗粒的氧化速率常数越大,氧化反应越容易进行;SiC颗粒的高温氧化分为氧化前期和氧化后期两个阶段.氧化前期的反应速率受界面化学反应的控制;氧化后期受扩散控制,其表观活化能远比氧化前期的大.SiC颗粒的高温氧化过程符合两个阶段式模型:氧化前期的氧化速率常数k_c=143.37exp(-70994/RT)(mg·m~(-2)·min~(-1)),氧化后期的氧化速率常数k_D=3.61×10~8exp(-192758/RT)(mg·m~(-2)·min~(-1)).     

Hydrolysis of iron and chromium fluorides: mechanism and kinetics

Fluoride complexes of metallic ions are one of the main problems when processing industrial effluents with high content of fluoride anion. The most important case is derived from pickling treatment of stainless steel, which is performed with HNO3/HF mixtures to remove oxides scale formed over the metal surface. Waste from this process, spent pickling liquor, must be treated for recovering metallic and acid content. Conventional treatments produce a final effluent with high quantity of fluoride complexes of iron and chromium. This work proposes a hydrolysis treatment of these solid metal fluorides by reacting them with a basic agent. Metal oxides are obtained, while fluoride is released to solution as a solved salt, which can be easily recovered as hydrofluoric acid. Solid iron and chromium fluorides, mainly K2FeF5(s) and CrF3(s), obtained in the UCM treatment process, were employed in this work. Optimal hydrolysis operating conditions were obtained by means of a factorial design: media must be basic but pH cannot be higher than 9.5, temperature from 40 to 70 degrees C and alkali concentration (potassium hydroxide) below 1.1 mol L(-1). Secondary reactions have been detected, which are probably due to fluoride adsorption onto obtained oxides surface. Mechanism of reaction consists of several stages, involving solid fluoride dissolution and complexes decomposition. Hydrolysis kinetics has been modeled with classical crystal dissolution kinetics, based on mass transfer phenomena.     

Influence of chromium on the formation of the bismuth-system superconductors

The influence of chromium on the bismuth-system superconductors (Bi-SC) was found as a by-product in a crystal synthesis experiment using a high-temperature-pressure-vessel (HTPV). The main product in the HTPV was found to be a 2 2 0 1 phase of the Bi-SC, while the product in air from the same raw materials was a 2 2 1 2 phase. This difference is attributed to an impurity from the HTPV. It was found from other experiments that chromium stabilizes the 2 2 0 1 phase of the Bi-SC and chromium decreases the 2 2 1 2 phase.     

Effect of tungsten on the oxidation kinetics of diamond nanopowder

The oxidation kinetics of ACM5 grade statically synthesized nanopowder of 0.1/0 grit size has been studied without addition and with addition of 1, 3, and 5 wt % tungsten nanopowder. It has been shown that tungsten is an inhibitor of the diamond reaction with oxygen. The kinetic scheme of the reaction that explains the mechanism of the tungsten inhibitory action has been substantiated.     

Atmospheric oxidation of chromium between 270 and 490°C

Auger electron spectroscopy was used to study the oxidation behavior of pure chromium in the temperature range 270–490°C for exposures ranging from 1 h to more than 100 h in air. Regression analysis indicated that the oxide thickness on mechanically polished bulk chromium samples increased logarithmically with time, although some spallation was observed at the highest temperature. Oxidation of mechanically polished samples was slower than that for electropolished bulk chromium or that for chromium films on glass substrates. Possible factors responsible for the slower logarithmic growth rate of the oxide on the mechanically polished samples are discussed.     

Titanium-to-rutile oxidation kinetics in the direct-oxidation fabrication of thin-wall ceramics

The oxidation kinetics of titanium preforms for the direct-oxidation fabrication of thin-wall ceramics have been studied in the temperature range 750–1100°C using titanium plates up to 5 mm in thickness. Empirical relations for the kinetics of these processes have been obtained. The preform weight has been shown for the first time to influence the oxidation rate of titanium and the amount of forming rutile, which is inconsistent with the known patterns of metal oxide formation. One possible mechanism of rutile formation during the direct-oxidation fabrication of thin-wall ceramics is considered.     

Influence of the steady background turbulence level on second sound dynamics in He II - II

We report complementary results to our previous publication [Dalban-Canassy M, Hilton DK, Van Sciver SW. Influence of the steady background turbulence level on second sound dynamics in He II. Adv Cryo Eng 2006;51:371-8], both of which are aimed at determining the influence of background turbulence on the breakpoint energy of second sound pulses in He II. The apparatus consists of a channel 175 mm long and 242 mm² in cross section immersed in a saturated bath of He II at 1.7 K. A heater at the bottom end generates both background turbulence, through a low level steady heat flux (up to q_s = 2.6 kW/m²), and high intensity square second sound pulses (q_p = 100 or 200 kW/m²) of variable duration Δt₀ (up to 1 ms). Two superconducting filament sensors, located 25.4 mm and 127 mm above the heater, measure the temperature profiles of the traveling pulses. We present here an analysis of the measurements gathered on the top sensor, and compare them to similar results for the bottom sensor [1]. The strong dependence of the breakpoint energy on the background heat flux previously illustrated is also observed on the top sensor. The present work shows that the ratio of energy received at the top sensor to that at the bottom sensor diminishes with increasing background heat flux.     

The oxidation kinetics of spherically shaped palladium powder

The oxidation kinetics of spherically shaped palladium (Pd) powders was investigated over the temperature range between 350 and 800°C. Two types of Pd powders differing from each other in particle sizes were used in this study, 0.30 and 0.85 m in diameter. The fraction of total oxide formed was measured in situ by means of a thermobalance method. In this system, the volume of product palladium oxide (PdO) increases significantly vs. the volume of reactant Pd, and consequently the experimental data were explained well by Valensi-Carter equation. Apparent activation energies of oxidation for the two types of Pd powders were both estimated to be 140 kJ/mol.