Oxidation In Dry Air Research Articles

Synthesis of Mn(III)X3 (X = Cl, Br, I) Compounds with Phosphine (R3P) Ligands.

This work details the synthesis and characterization of complexes of the form [MnIIIX3(PR3)2] (X = Cl, Br, I), which is a rare type of coordination compound. Prior to this work, the only mode of synthesis was oxidizing mixtures of MnIIX2 and PR3 with dry air, but these procedures give low yields and variable outcomes. By taking advantage of the starting material [MnIIICl3(OPPh3)2] (1) and other new strategies, we present robust synthetic protocols for both new and known Mn(III) halido phosphine complexes. In addition to [MnIIIX3(PR3)2], these include [MnIIICl2(dmpe)2]+ salts and [LMnIIX3]- manganates (L = phosphine oxide). Furthermore, the full characterization of these [MnIIIX3(PR3)2] species enabled a definitive revisitation of some controversial chemistry surrounding the compounds and products from the dry air oxidation of certain MnIIX2 and phosphine ligand mixtures.

Preparation of Cr2AlC powder and its isothermal oxidation behavior in dry air and pure steam

As a promising candidate ceramic for using in harsh environments, Cr2AlC ternary carbide possesses numerous unique properties at elevated temperatures. Herein, high-purity Cr2AlC powders had been synthesized by using a facile method with a mixture of Cr7C3, Cr3C2 and Al powders at 1200 °C for 3 h. Subsequently, the oxidation behaviors of Cr2AlC powders were investigated in dry air and pure steam at temperatures ranging from 900 to 1100 °C, respectively. In addition, the oxidation products were characterized and the mass gain curves were analyzed via the real physical picture (RPP) kinetic model. As a result, thicker Al2O3-rich oxide scale and porous Cr7C3 subscale were generated during dry air oxidation compared to pure steam oxidation. Notably, the oxidative active energy of Cr2AlC was 446.5 kJ/mol in pure steam, while in dry air it was decreased by 14.4%. Theoretical calculations further verified that Cr2AlC oxidizes more sluggishly in steam than in dry air.

Zinc Ferrite Nanoparticle Coatings on Austenitic Alloy Steel.

The phase transition of austenitic stainless steel of commercial label CL20ES and zinc ferrite nanoparticles was studied in an oxidative atmosphere of dry air to develop a low-cost, effective technique for covering-layer fabrication. CL20ES powder and zinc ferrite powder were mechanically mixed. This mixture was studied in an atmosphere of dry air at different annealing temperatures from room temperature to 900 °C. The employed characterization techniques are X-ray powder diffraction, Mössbauer spectroscopy in the transmission geometry, and scanning electron microscopy with elemental mapping. The fabricated layers were also characterized by surface-specific techniques such as conversion electron Mössbauer spectroscopy and grazing incidence X-ray powder diffraction. The analyzed powder mixture shows resistance against oxidation in dry air and high temperatures. These results were employed to produce zinc ferrite covering layers on 3D-printed cylinders of CL20ES. The results show a predisposition of zinc ferrite to be recrystallized at temperatures above 350 °C without the production of corrosive substances on steel. The zinc ferrite layers were analyzed by an ultrasonic hardness tester as well, which proved the hardness enhancement.

High‐Temperature Oxidation of 60Si2Mn Spring Steel in Dry Air and Wet Air: Insights from an Experimental and First‐Principles Study

During the hot‐rolling process, the 60Si2Mn spring steel is exposed to both dry air and water vapor at high temperatures. However, no precise mechanism or theory is developed to explain the effect of water vapor on the oxide layer from a microscopic atomic perspective. The high‐temperature oxidation of 60Si2Mn spring steel with dry and wet air is investigated herein using a combination of experiments and first‐principles calculations. After high‐temperature oxidation in both dry and wet air, the 60Si2Mn spring steel generates a typical three‐layer structure consisting of Fe2O3, Fe3O4, and FeO + SiO2/Fe2SiO4; however, the oxide layer produced in wet air is significantly thicker. Furthermore, the phase transformation from Fe3O4 to Fe2O3 occurs in the middle Fe3O4 layer, which is associated with H protons derived from H2O molecules penetrating into oxide layers and promoting the development of Fe vacancies in the center of the tetrahedral interstices. The high porosity of the Fe3O4 layer essentially encourages Fe and O diffusion, hence enhancing the growth of the oxide layer and facilitating the transformation from Fe3O4 to Fe2O3. These findings provide a more detailed mechanistic explanation of how water vapor affects the high‐temperature oxidation of 60Si2Mn spring steel at the atomic level.

Cerium modified birnessite-like MnO2 for low temperature oxidation of formaldehyde: Effect of calcination temperature

Cerium modified birnessite-like MnO2 were synthesized (CexMn; x = 0.01; 0.1) from a simple and inexpensive redox method involving the reduction of potassium permanganate by sodium lactate in the presence of Ce(NO3)3 at ambient temperature. The as-synthesized samples were calcined at different temperatures (Tc: 200, 300 and 400 °C) to be tested in HCHO total oxidation in dry air at low temperature. The Ce0.01Mn catalyst calcined at 400 °C showed the best catalytic activity in HCHO oxidation with a T50 (temperature at which 50 % of HCHO was converted into CO2) of 50 °C and was found to be air and moisture stable (up to 75 %RH) upon time. The beneficial effects of a calcination step at 400 °C allowed to get enhanced asymmetric Ce-O-Mn surface interactions along with highly dispersed Ce4+/3+ related species and enhanced TCMn3+ (TC: triple corner) amounts which contributed to a great extent for its high activity in HCHO oxidation.

Degradation mechanism of near stoichiometric SiC fibers after air and Ar–H2O–O2corrosion at 1000–1500°C

AbstractThe near stoichiometric SiC fiber has been reported to play significant roles in the application of aeroengine field. An in‐depth understanding on the degradation mechanism of the fiber during its corrosion in air and under a simulated aeroengine environment (PH2O:PO2:PAr = 14:8:78 kPa) will shine a light on the performance evaluations of the near stoichiometric SiC fiber‐based materials as well as the development of their potential applications. In this study, X‐ray diffraction, scanning electron microscope, and FIB‐TEM were utilized to analyze the mechanical properties and microstructure of the fiber. After oxidation in dry air and Ar–H2O–O2for 1 h, respectively, the fiber strength retention rate has been found to decrease with the increased oxidation temperature. The raise in oxidation temperature also led to the increase of the thickness and the crystallization rate of the oxide scale. The most different oxidation behaviors of SiC being treated under the simulated environment than in air are the lower oxidation activation energy and the higher crystallization activation energy for cristobalite. Water vapor can promote the oxidation reaction and inhibit the crystallization of cristobalite in the oxide scale. Few significant differences have been observed otherwise in the oxidation process and oxidation chromatography crystallization mechanism of fibers being treated under different conditions. The increase of oxide layer thickness and the formation of cristobalite impair the structural integrity and compactness of the oxide scale and thus lead to the deterioration of the mechanical properties of SiC fibers. Therefore, it is proposed that oxidation resistance of SiC fiber can be improved by insulating the reaction between the oxidizing agents and the SiC fiber or by increasing the crystallization temperature of cristobalite in the oxidation process and reducing the crystallization rate.

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

AbstractA primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si‐bond coating, where the formation of SiO2 at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si‐bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1‐h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.

Enhancing the Oxidation Behavior of Zr-Modified Aluminide Coatings on Inconel 600 at Different Temperatures

In the recent search nickel superalloy Inconel 600 was coated with Zr-modified aluminide diffusion coating using pack cementation technique. Diffusion coating was done in a single step utilizing a conversion reaction of 10% Al, 2% ZrO2, 4% NaCl, and 84 percent Al2O3 (wt. percent ) and a simultaneous aluminizing-zirconizing process. The diffusion coating operations were performed in an argon environment at 1050 °C for 10 hours. The test of the isothermal oxidation in dry air was performed on the Inconel Alloy 600 (IA600) without and with Zr-modified aluminide coating for 800-1000 °C. The oxidation kinetic of IA600 and its coated system was found to follow the parabolic law. The activation energy is 243 kJ/mol. for the coated system and 457 kJ/mol. for the uncoated system. XRD analysis show that oxide phases are formed on an uncoated IA600 surface during most of the oxidation exposure conditions are NiO, Cr2O3, Fe2O3, NiCr2O4 and NiFe2O4, , whereas alumina scale is the major oxide that is obtained on the surface of coated samples.

A Comparison of the Oxidation and Nitridation Properties of Selected Chromia- and Alumina-Forming Alloys at 800 °C

Three FeCrAl alloys and two chromia-formers (a stainless steel, and a Ni-base alloy) have been exposed in four environments (dry air, air + 20% H2O, 20% H2 + 20% H2O + Ar and 95% N2 + 5% H2) for 168 h at 800 °C. The corroded samples were investigated by SEM/EDS, XRD and gravimetry, and the formation of CrO2(OH)2(g) was measured as a function of time using a denuder technique. The Fe-base alloy formed a Cr-rich protective oxide scale in dry air and wet air but suffered break-away oxidation in 20% H2 + 20% H2O + Ar. In contrast, the Ni-base alloy suffered extensive NiO formation and internal oxidation in dry air and wet air but formed a protective chromia scale in 20% H2 + 20% H2O. All three FeCrAl alloys formed protective alumina scales in dry air, wet air and 20% H2 + 20% H2O + Ar. The FeCrAl alloy Kanthal APMT was severely nitrided in the 95% N2 + 5% H2 environment due to defects in the oxide scale associated with RE-rich inclusions which allowed nitrogen to enter the alloy. In contrast, the two Cr-lean FeCrAl alloys Kanthal EF101 and Kanthal EF100 did not suffer nitridation at all.

Chemical coupling between oxidation and hydrolysis in polyamide 6 - A key aspect in the understanding of microplastic formation

The formation mechanism of secondary microplastics (issued from larger plastics) in the oceans is still an open question. However, it is commonly accepted that the chemical degradation undergone by polymers leads to their embrittlement and finally fragmentation. In the marine environment, polymers are subjected to different types of chemical and physical degradations. This study focuses on the coupling between two types of chemical degradation: hydrolysis and oxidation in the case of polyamide 6. To do so, polymer films (250 µm thick) were aged for durations up to 2 years and then characterized at the molecular scale using FTIR and at the macromolecular scale using mainly DSC and GPC. Results clearly show the existence of a chemical coupling between oxidation and hydrolysis in polyamide 6 that induces a large increase in the chain scission rate. The degradation rate is indeed 80 times faster in water with oxygen than in water without oxygen. Then, a two-stage ageing condition (first thermal oxidation in dry air and then hydrolysis in water without oxygen) was used to further study the nature of the coupling. These results have shown for the first time that a strong interaction occurs between thermal oxidation and hydrolysis of polyamide materials at the macromolecular scale.

Air oxidation of yttrium hydride as a high temperature moderator for thermal neutron spectrum fission reactors

Yttrium hydride (YH x ) is an attractive moderator material for thermal neutron spectrum fission reactors requiring a small reactor core volume and has been selected as the neutron moderator for the Transformational Challenge Reactor (TCR), an advanced gas-cooled microreactor . Before YH x can be used in this application, it is important to understand the material response to off-normal conditions. In the present study, 550–650 °C isothermal dry air oxidation was performed to simulate a depressurized loss of force circulation (DLOFC) event. The oxidation was performed using thermogravimetric analysis (TGA) on bulk crack-free YH x coupons. Oxidation studies were also performed on Y coupons to elucidate the impact of H on oxidation. Both the chemistry and distribution of processing impurities were found to strongly affect oxidization behavior on a batch-to-batch basis. Regardless of batch, YH x oxidized at a significantly lower rate than Y at all temperatures, and the lower rate was directly correlated with increased hydride content. Metallic Y exhibited complex exponential kinetics, whereas YH x also exhibited complex kinetics but gained considerably less mass. According to literature reports on protonic and native-ion conductivities of Y 2 O 3 and mass spectrometry analysis of gaseous reaction products formed during the oxidation of YH x , a mechanism for the reduced oxidation rate of yttrium hydride is suggested.

Antioxidation enhancement and siliconization mitigation for SiC coated C/C composites via a nano-structural SiC porous layer

A reactive nano-structural SiC porous layer on C/C surface was pre-prepared via a slurry-dipping and reaction-sintering method, using Si, C and Al2O3 powders as raw materials. Pre-introduction of nano-structural SiC porous layer helped to form a dense sintered SiC coating by the pack cementation (PC), which provided excellent oxidation protection for C/C substrate under different conditions at 1773 K and mitigated the siliconization reaction of free silicon during the PC process. Results showed that the modified SiC coating had a mass loss of only ~0.03% after 50 thermal cycles between room temperature and 1773 K in air. And the mass loss reduced by ~4.31 and ~2.25 times after isothermal oxidation in dry air and wet oxygen atmosphere at 1773 K, respectively. Furthermore, because of the delaying effect of SiC porous layer on free silicon infiltrating into C/C substrate, the bending strength of modified SiC-coated C/C composites has increased by 93.0%.

Surface‐Bound MoO3 on Single MoS2 Flakes: Direct Identification of Surface Bound MoO3 on Single MoS2 Flakes Heated in Dry and Humid Air (Adv. Mater. Interfaces 13/2021)

In article number 2100328, Maciej Rogala, Stanislaw Sokolowski, Robert Szoszkiewicz, and co-workers investigate chemical origins of the MoOx species appearing on single MoS2 flakes due to their oxidation in dry and humid air. MoOx species on the oxidatively etched MoS2 surface are shown to originate from loosely surface-bound (MoO3)n (n > 0) single clusters with an average height of 0.32 ± 0.05 nm. Their presence has been confirmed topographically and chemically via combined AFM, XPS, and XAS studies.

Oxidation of 3D‐printed SiC in air and steam environments

AbstractThe high‐temperature oxidation of additively manufactured and chemically vapor infiltrated (3D‐printed SiC) has been compared to chemical vapor deposited (CVD) SiC. 100‐h isothermal exposures were conducted at 1425° and 1300°C at 1 atm under both dry air and steam environments. A SiC reaction tube was utilized to reduce silica volatility. After steam oxidation at 1425° and 1300°C, on the 3D‐printed SiC surface, which was intrinsically rougher than the CVD surface, scales were 70%–90% thicker at the convex regions compared to concave/flat regions. In the convex regions, large cracks perpendicular to the oxidizing interface were observed. After dry air oxidation, scale thicknesses were comparable between 3D‐printed SiC and CVD SiC, regardless of geometry. Finite element modeling, conducted to elucidate the relationship between SiC geometry and ß‐ to α‐cristobalite transformation stress, determined cristobalite transformation tensile stresses to be on the order of 103 MPa during cool down, assuming a 6 vol% reduction. Compared to flat SiC substrates, tensile transformation stresses were elevated at concave regions and relaxed at convex regions. Combined with specimen mass gain (accounting for the rougher surface) of 3D‐printed SiC being 15%–32% higher for 3D‐printed SiC after 1300°C and 1425°C steam oxidation, the work presented concludes that the increased oxidation of 3D‐printed SiC is primarily caused by tensile hoop stresses driven by oxidation volume expansion. Lastly, the efficacy of the 3D‐printing method is demonstrated through the production of tristructural isotropic imbedded 3D‐printed SiC fuel forms.

Effect of coalbed methane airflow with optimum action concentration on coal oxidation under low temperatures

AbstractTo explore the effect of coalbed methane (CBM) airflow on coal oxidation under low temperatures, an experimental apparatus was developed and applied to simulate coal oxidation under different oxidizing atmospheres. The formation of carbon monoxide CO, the index gas, in CBM airflow was analyzed to obtain the optimum concentration of CH4 that exerted the strongest influence on coal oxidation. Then, a comparison experiment was performed on coal oxidation in dry air and in CH4 at the optimum concentration by using electron spin resonance (ESR) and Fourier transform infrared (FTIR) technology. The results suggest that 25% of CH4 has the strongest effect on coal oxidation at low temperatures. The free radical concentration (Ng) in coal increases with temperature in both dry air and 25% CH4. At this concentration, CH4 can inhibit the reactivity of coal substantially while raising the excitation temperature. The oxidation of aliphatic hydrocarbons is severely inhibited by 25% CH4, which reduces its consumption rate and abates its decrement in the initial phase. Although 25% CH4 restricts the consumption of –OH and –COOH in oxidation, it promotes the formation of –C–O.

The Oxidation Behaviors of Indefinite Chill Roll and High Speed Steel Materials

Indefinite chill (IC) roll and high speed steel (HSS) materials have been widely employed to manufacture work rolls as latter and former stands in hot rolling mills. The oxidation of work rolls is of importance for the surface quality of the rolled workpieces. The isothermal oxidation of the IC and HSS materials was conducted at 650 °C and 700 °C in both dry air and humid air. The isothermal oxidation curves indicate that HSS shows faster kinetics than the IC materials in dry air, whereas the opposite occurred in humid air. The oxide scales of the IC materials after the oxidation in both dry air and humid air are made up of two oxide phases. Two oxide phases were found when the HSS oxidized in the dry air and three oxide phases were found when oxidized in the humid air.

Size Distribution, Mechanical and Electrical Properties of CuO Nanowires Grown by Modified Thermal Oxidation Methods.

Size distribution, Young’s moduli and electrical resistivity are investigated for CuO nanowires synthesized by different thermal oxidation methods. Oxidation in dry and wet air were applied for synthesis both with and without an external electrical field. An increased yield of high aspect ratio nanowires with diameters below 100 nm is achieved by combining applied electric field and growth conditions with additional water vapour at the first stage of synthesis. Young’s moduli determined from resonance and bending experiments show similar diameter dependencies and increase above 200 GPa for nanowires with diameters narrower than 50 nm. The nanowires synthesized by simple thermal oxidation possess electrical resistivities about one order of magnitude lower than the nanowires synthesized by electric field assisted approach in wet air. The high aspect ratio, mechanical strength and robust electrical properties suggest CuO nanowires as promising candidates for NEMS actuators.

Микроструктура и коррозионная стойкость сплавов Mo – 15 ат. % Si – РЗЭ (Sc, Nd) на воздухе и в паровоздушной среде

The research results of the REEs effect, in particular, Sc and Nd, on the structural-phase state and behavior of a high-temperature Mo – Si composite with a hypereutectic composition, which crystallizes from the melt with the formation of a two-phase natural (in situ) composite during oxidation in dry and moist air were presented. Samples of model alloys with different REE concentrations were obtained by vacuum-arc melting of a mixture consisting of the Mo – 15 Si (at. %) binary alloy and additives of alloying elements. Phase composition, microstructure features and distribution pattern of alloying components in Mo – Mo3Si metal-silicide composites containing from 0,5 to 3,0 at. % Sc or Nd were identified by X-ray diffraction and electron microscopy. Corrosion resistance of Mo – 15 at. % Si – REE (Sc, Nd) alloys during continuous heating to 700 and 850 °C in air and in a vapor-air medium was evaluated by thermal analysis methods (DSC and DTA). It was shown that microalloying of the Mo – 15 at. % Si with scandium or neodymium within 0,5 at. % leads to decrease in the oxidation rate when heated in air to 850 °C. Increasing the concentration of alloying elements to 3,0 at. % REE during oxidation in air adversely affects the corrosion resistance of samples, moreover the oxidation of alloys with neodymium proceeds at a higher rate than with scandium. During non-isothermal heating in a vapor-air medium, the alloy doped with scandium oxidizes more slowly than the binary one, and the sample with neodymium, by way of contrast, oxidizes at a much higher rate compared to dry air.

Paralinear Oxidation of Cr-Si-C-Coated C/SiC at 1300°C in Wet and Dry Air Environments

The oxidation kinetics of a Cr3Si-Cr7C3/SiC/SiC-coated C/SiC were comparatively investigated in dry and wet air at 1300°C under 1 atm. After oxidation for 10 h, Cr2O3 and SiO2 were produced in both oxidations. However, a larger amount of SiO2 formed during the wet air oxidation. After the dry air oxidation, the surface of Cr3Si-Cr7C3/SiC/SiC coating became relatively loose with many pores. After the wet air oxidation, the surface of Cr3Si-Cr7C3/SiC/SiC coating became undulating and loose with notable damage at the position of bulges. The Cr3Si-Cr7C3/SiC/SiC-coated C/SiC showed paralinear weight gains in both oxidation environments. Although the initial weight gains within the first 1 h in the wet air oxidation condition were higher, the gradual weight gains thereafter were lower. The parabolic rate constant for oxide formation and the linear rate constant for oxide volatilization in the wet air oxidation condition were higher than those in the dry air oxidation condition.

Effects of H2O on HCHO and CO oxidation at room-temperature catalyzed by MCo2O4 (M=Mn, Ce and Cu) materials

The abatement of HCHO and CO at room temperature by catalytic oxidation over metal oxides can be achieved. However, the catalytic performance of metal oxides is influenced by H2O vapor in the reaction air and the mechanism of H2O role is still debated. In this paper, the effect of H2O on the catalytic performance of an MCo2O4 (M = Mn, Ce and Cu) catalyst for HCHO and CO oxidation at room temperature was investigated. For both HCHO oxidation and CO oxidation, all the MCo2O4 catalysts in humid air are less active than in dry air except for HCHO oxidation catalyzed by CuCo2O4 catalyst. The CuCo2O4 is inactive for HCHO oxidation in dry air, but active for HCHO oxidation in humid air. XRD and XPS results indicated that there are no obvious changes of crystal structure and valance states of element between fresh and used MCo2O4 catalysts. The active sites of MCo2O4 catalysts and reaction/deactivation mechanism of HCHO and CO oxidation were studied by H2O-TPD and DRIFT. The associatively adsorbed H2O on MCo2O4 contributed to the active HOH sites for HCHO oxidation. Formate species are intermediates of HCHO oxidation. However, if formate cannot be further transferred into CO2, the formate adsorbed on active sites will result in catalyst deactivation. In humid air, hydrogen-bonded OH and free OH generated and covered active sites accelerated the catalyst deactivation in HCHO oxidation. The CO was oxidized by the active oxygen species but not included OH species. Thus, the H2O adsorbed on oxygen vacancies blocked the generation of active oxygen species, which resulted in MCo2O4 deactivation for CO oxidation in humid air. This work provide a fundamental understanding of the key role of H2O in the HCHO and CO oxidation at room temperature, which is helpful for the design of high activity and long lifetime catalyst used in dry or humid conditions.