Change In Oxidation Kinetics Research Articles

Water vapor oxidation behavior and mechanism of high-entropy (TiZrNbTaCr)C at 1200°C

High-entropy (TiZrNbTaCr)C ceramics with different Cr content were oxidized at 1200°C in water vapor. Zr6(Nb, Ta)2O17, (Nb, Ta)2O5 and (Ti, Nb, Ta, Cr)O2 were initial oxidation products. After a 4-h oxidation, TiO2-(Nb, Ta)2O5 was produced and changed from Ti(Nb, Ta)2O7 to Ti2(Nb, Ta)10O29 with increasing Cr content, resulting in better oxidation resistance. TiO2-(Nb, Ta)2O5 with large size and relatively dense microstructure impeded the inward diffusion of water vapor, slowing down the oxidation rate and causing the change of oxidation kinetics from linear to parabolic. The formation process of the oxide scale was discussed and the corrosion mechanism was proposed.

Investigation of the pH-dependence of the oxidation of FAD in VcCRY-1, a member of the cryptochrome-DASH family.

Vibrio cholerae cryptochrome-1 (VcCRY-1) is a member of the cryptochrome DASH family. The flavoprotein appears to use blue light both for repair of cyclobutane pyrimidine dimers (CPDs) on DNA and signal transduction. Earlier, we found that it was almost impossible to oxidize the FADH· state upon binding to a CPD, and, in the absence of substrate, the rate of FADH· oxidation was much larger at high pH (Gindt et al. in Biochemistry 54:2802-2805, 2015). Here, we present the pH-dependence of the oxidation of FADH· by ferricyanide, which revealed a switch between slow and fast oxidation with a pKa ≈ 7.0. Stopped-flow mixing was used to measure the oxidation of FADH- to FADH· at pH 6.7 and 7.5. Substrate binding was required to slow down this oxidation such that it could be measured with stopped flow, but there was only a small effect of pH. In addition, resonance Raman measurements of FADH· in VcCRY-1 at pH 6.5 and 7.5 were performed to probe for structural changes near the FAD cofactor related to the observed changes in rate of FADH· oxidation. Only substrate binding seemed to induce a change near the FAD cofactor that may relate to the change in oxidation kinetics. The pH-effect on the FADH· oxidation rate, which is rate-limited by the proton acceptor, does not seem to be due to a protein structural change near the FAD cofactor. Instead, a conserved glutamate in CRY-DASH may control the deprotonation of FADH· and give rise to the pH-effect.

A Systematic Testing Procedure to Investigate the Influence of Oxide Morphology, Composition and Thickness on Changes in the High Temperature Oxidation Kinetics of AISI 316L Stainless Steel

A systematic testing procedure has been employed to investigate the high temperature oxidation kinetics of AISI 316L. Thermo-gravimetric (TG) analysis was carried out at 950°C, 1050°C, 1150°C and 1250°C for 8h. Alongside this, isothermal furnace treatments were carried out on samples of the same material at the same temperatures for time periods of 0.5h, 1h, 2h, 4h and 8h. Changes in oxidation kinetics were observed on mass gain curves plotted from data derived from the TG analysis. When a change in oxidation kinetics was identified, the structure, thickness and composition of the oxides formed on the isothermal treatment samples at time periods before and after the change occurred could be studied. It was found that this systematic testing procedure provided a great deal of useful information allowing more meaningful conclusions to be made on the influence of oxide layer thickness, structure and composition on high temperature oxidation kinetics.

Technical aspects of 〈$ \bf 11\bar 20 $〉 4H–SiC MOSFET processing

AbstractIn order to process n‐type channel MOSFETs on 〈$ 11\bar 20 $〉‐oriented 4H–SiC wafers one has to optimise, independently, many different steps. First is the growth of the active epitaxial layer, including the effect of p‐type doping. Next is the oxidation and, finally, the n‐type implantation for source and drain formation. In this work, we investigate the growth and doping of the p‐type doped epitaxial layer on the 〈$ 11\bar 20 $〉‐oriented substrate using, both, SIMS, low temperature photoluminescence measurements and Hall effect measurement to control the final doping level. Next we investigate the change in oxidation kinetics with respect to the conventional 〈0001〉 surfaces. Finally, we evaluate the efficiency of implantation and annealing processes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Oxidation Instability of SiC and Si3 N 4 Following Thermal Excursions

The effect of thermal excursion and thermal cycling on the oxidation stability of chemical vapor‐deposited (CVD) and was studied at 1350°C. Thermal cycling alone produced no noticeable change in oxidation kinetics. However, transmission electron microscopy showed that oxide scales grown in cycles consist of alternating layers of and . When the oxidation of CVD or at 1350°C was interrupted with a 1.5‐h annealing in Ar at 1500°C, the kinetics of reoxidation at 1350°C were found to be drastically increased. The and then oxidized essentially at the same rate, which is over 50 times the preannealing rate, and comparable to the expected oxidation rate of these materials at 1500°C. This loss of passivity induced by excursion to higher temperature has also been reported for Si, and is considered an intrinsic instability in materials that oxidize to silica.

Change of oxidation kinetics in the Zr-Al system

The oxidation of ZrAl 2 in dry oxygen in the temperature range 883–1038 K obeys a logarithmic rate law, while the oxidation of Zr 3Al, Zr 2Al, ZrAl and Zr 2Al 3 follows a parabolic rate law. An activation energy of 115 kJ mol −1 is estimated for the oxidation of ZrAl 2. The reaction starts by chemisorption of oxygen and rapid formation of a ZrO 2 film. Subsequently, the reaction slows down and proceeds further at a low rate. Aluminium escapes oxidation and diffuses to the ZrAl 2 bulk, forming the ZrAl 3 phase at the oxide-alloy interface. The remaining zirconium is therefore selectively oxidized. Due to the higher content of aluminium in ZrAl 2 than in the Zr-Al phases mentioned, aluminium accumulates very quickly at the oxide-alloy interface, releasing a correspondingly small amount of zirconium. Thus, the reaction is rapid as long as there is enough zirconium available for oxidation; subsequently, the reaction rate decreases abruptly.

Ignition Behavior of Plutonium Metal and Certain Binary Alloys

The ignition behavior of plutonium and binary alloys containing nominally 2 a/o additives was studied by two methods in air and oxygen. Ignition temperatures in both air and oxygen lie in two regimes. Cubes, 5 mm on an edge, ignite near 500°C while thin foils, less than 0.2 mm thick, ignite near 300°C. The transition from one regime to the other occurs sharply at a specific area that depends on whether ignition takes place in air or in oxygen. The existence of two regimes is consistent with a change of oxidation kinetics between 300° and 400°C which is characterized by a minimum in oxidation rate near 400°C. The effects of additives are sometimes significant and must be considered in each regime and on the position of the transition between them. In the high temperature regime: (i) Increased resistance to ignition and oxidation is found on addition of aluminum, copper, gallium, or silicon; (ii) The same ignition and oxidation behavior as plutonium occurs with addition of nickel, chromium, uranium, cerium, or iron; and (iii) Decreased resistance to ignition and oxidation results from addition of carbon, cobalt or manganese. In the low temperature regime: (i) Increased resistance to ignition and oxidation is produced by additions of aluminum, cerium, carbon, cobalt, gallium, manganese or possibly chromium; (ii) The same ignition and oxidation behavior as plutonium is obtained with addition of nickel or silicon; and (iii) Decreased resistance to ignition and oxidation follows from addition of iron or uranium.The transition from the high temperature regime to the low temperature regime does not occur in air for alloys of aluminum, cerium, carbon, or cobalt for specimens with specific areas of up to about 1 cm2/g. Addition of silicon, nickel, manganese, or uranium have little influence on the transition in air which occurs at a specific area of 1.5 cm2/g for pure plutonium. The presence of moisture had a negligible effect on the ignition temperatures of pure plutonium and the aluminum alloy.

An X‐Ray Study of Oxidized Uranium Surfaces

not Available.