Increasing Oxidation Temperature Research Articles

Oxidation behaviour and its effect on fracture toughness of Niobium metal

Niobium metal was studied for its oxidation behaviour at different temperatures (800, 1000 & 1200 °C) and time (1, 3 & 5 h). The study revealed that Nb underwent oxidation forming pro-active Nb oxides with initial formation of meta-stable Nb12O29 which forms as an oxidisation resistant layer. However, formation of Nb2O5 from a meta-sable Nb12O29 did not act as protective layer as it exposed underneath to further oxidation leading to cracks. The oxidation kinetics of Nb shown a linear behaviour and the devised mathematical model using Deal Grove determined the thickness of oxide layers. The un-protectiveness of Nb2O5 was verified by the Vickers hardness (8.06 ± 0.4 GPa) and indentation fracture toughness (28.14 ± 0.4 MPa.m1/2) of material which decreased with increase in oxidation temperature.

Evolution and mechanism for the terahertz dielectric spectrum of coal during oxidation

In this paper, the dielectric spectrums of lignite during oxidation and its corresponding dielectric response mechanisms were investigated by means of THz-TDS and synchronous thermal analyzer, combined with the Clausius-Mossotti equation and chemical reaction pathway. Results indicate that dipole polarization is the dominant polarization mechanism during coal oxidation in the THz field. There are four dielectric response mechanisms, which gradually participate in the competition of polarity changes with the increase of oxidation temperature. The dielectric response of lignite shows significant temperature dependence. The dielectric constant first decreases until 150 °C due to the dewatering. From 150 °C to 275 °C, the dielectric constant tends to increase. Before 225 °C, the increase is due to the formation of more oxygen-containing bonds. Beyond 225 °C, the coupling effect of the formation of oxygen-containing bonds and the twisting deformation of the molecular chain skeleton become the main reason. Then, due to the fracture of polar bonds, the dielectric constant drops significantly until 400 °C. Beyond 400 °C, the dielectric constant increases, possibly due to enhanced electron migration. The outcomings of the current study help to identify the oxidation stage of coal and enrich the application basis of THz technology in the fields of coal fire identification and mineral processing.

Discussion on the presence condition of suboxide ZrO beneath the oxide in zirconium alloys

Characteristics related to the oxide-metal interface of oxidized (corroded) zirconium alloys in an autoclave have been extensively explored using both experimental and theoretical methods during the last ten years. A thin intermediate or suboxide layer with an approximate 1:1 ZrO stoichiometry was observed at the oxide-metal interface in several zirconium alloys. Through investigating its chemical composition, morphology, grain size and orientation relationship with the substrate α-Zr, some researchers assumed that the suboxide layer is related to the oxidation (corrosion) rate, whereas others did not. The majority of earlier studies used aqueous corrosion samples that were exposed to high pressure and low temperature, simulating in-service conditions. However, the interfacial characteristics at higher temperature have been scarcely reported. In this study, we focused on the microstructure and micro-chemistry of the oxide and suboxide ZrO in Zircaloy-4 and Zr-1.0Nb-0.01Cu oxidized at 650–1000 °C by combining Backscattered Electron (BSE) mode in Scanning Electron Microscope (SEM), Transmission Electron Backscatter Diffraction (t-EBSD) and (Scanning) Transmission Electron Microscope fitted with an Energy Dispersive X-ray Spectroscope ((S)TEM-EDS). The results show that the suboxide ZrO in oxidized Zircaloy-4 and Zr-1.0Nb-0.01Cu alloys has different morphologies. With increasing oxidation temperature, the suboxide ZrO distribution varies significantly, from a continuous layer to a few occasional blocky grains, until any ZrO grains are undetectable at the ZrO2/α-Zr(O) interface. The hexagonal crystallographic structure of ZrO has also been confirmed. According to discussions combining our investigations and earlier reports, the presence of suboxide ZrO and their morphology are probably related to the oxidation temperature, chemical composition of alloys and complex stress accumulation adjacent to the interfacial region.

Effects of Thermal Oxidation on Sensing Properties of Porous Silicon

We report the effects of thermal oxidation on the sensing properties of porous silicon. Porous silicon substrates were prepared by electrochemical etching and thermally oxidized at different temperatures. A comparative EDS analysis shows that porous surfaces oxidized at higher temperatures have higher oxygen-to-silicon ratios. Our results indicate that the chemoresistive response due to the presence of isopropanol vapors at room temperature also increases with an increasing oxidation temperature. The presence of oxygen atoms in the PS layer could both protect the sensor from further atmospheric oxidation and increase its sensitivity.

Operando infrared imaging of ozone-assisted catalysis for high-throughput screening of catalytic activity

The interest in applying plasma catalysis for various applications is significantly growing; therefore, a rapid discovery and optimization of catalysts in the plasma system is highly demanded. In this study, a new operando infrared (IR) thermography system was developed for high-throughput screening of the catalytic reactivity in ozone-assisted oxidation under realistic operating conditions. In the developed system, the gas flow was maintained under laminar conditions in a flow-through catalyst bed using a porous disc, and the enthalpy of the system was directly correlated with the catalyst activity. The performance of the developed system was confirmed for ozone decomposition and CO oxidation over different metal loaded-TiO2 catalysts at wide operating parameters. Among four metals tested, Ag exhibited the highest activity for both ozone decomposition and CO oxidation, and its loading amount clearly corresponded with the CO oxidation rate and temperature increase measured by the operando IR thermography. Meanwhile, Mn had poor activity for CO oxidation, despite its high activity for ozone decomposition. Therefore, the operando IR thermography provided a rapid screening and deep understanding of catalysts for ozone-assisted oxidation. Unlike conventional point temperature measurements, IR thermography provides a two-dimensional temperature distribution in the catalyst, which can be sensitive in detecting undesirable “hot-spot” formation. Hence, the developed system can be used as a tool for reaction engineering and catalyst design and development.

Mechanical properties and microstructural evolution of Cansas-III SiC fibers after thermal exposure in different atmospheres

Cansas-III SiC fibers were exposed in argon, air and wet oxygen (12%H2O+8%O2+80%Ar) atmospheres for 1 h at 1000–1500 °C. The pristine fiber consisted of β-SiC, free carbon and SiCxOy phases. After exposure in air and wet oxygen, an amorphous SiO2 layer with embedding α-cristobalite crystals formed, while stacking faults were generated in the SiC core to release the residual stress. With the increasing oxidation temperature, lots of pores formed in the oxide layer, accompanied with the thickening, cracking and spallation of oxide layer. The average tensile strength decreased with the exposure temperature increasing and the exposure atmosphere deteriorating (argon→air→wet oxygen). After exposure at 1400 °C in argon and air, the fiber strength retention rates were 84% and 70%, respectively. However, after exposure at 1300 °C in wet oxygen, the strength retention rate was only 51%, indicating the accelerating oxidation and severe strength degradation of fibers.

Enhanced photocatalytic activity of ZnO nanostructures deposited on mesh through electrochemical deposition and thermal oxidation

This article reported that the properties of zinc oxide (ZnO) nanostructures were modified by the thermal oxidation process at different temperatures in the range of 200℃ to 600℃ for 1 h. The Zn films were deposited on a stainless steel mesh by using the electrochemical deposition technique. The elemental composition of Zn and O was exhibited via energy-dispersed X-ray spectroscopy in which the atomic ratio of O/Zn increased with the increase of oxidation temperature. The results showed that oxidation temperature has a significant effect on the morphological and crystal structure. The nanosheet structure of the as-deposited film transferred to the intermixing of porous nanosheet and urchin-like structure at the oxidation temperature of 600℃ while the crystallinities of mixing Zn and ZnO were improved to only ZnO when increasing the oxidation temperature. ZnO films were tested for photocatalytic activity in methylene blue under various ultraviolet irradiation times. The best condition of ZnO for photocatalytic activity was an oxidation temperature of 600℃ with the highest crystallinity and surface area that showed the highest decomposition rate and percentage degradation of 8.150 ´ 10-3 min-1 and 74.06%, respectively.

Analysis of Sequential Adsorption–Oxidation of VOCs on Atomic Layer-Deposited PtNi/ZrO2–SiO2 Dual-Function Materials

To develop efficient dual-function materials (DFMs) for capture and destruction of volatile organic compounds (VOCs), we prepared a series of composite materials comprising PtNi nanoparticles supported on ZrO2-promoted SiO2 (PtNi/ZrO2–SiO2) via atomic layer deposition (ALD) method and investigated them for sequential adsorption and desorption/catalytic oxidation of benzene, as a model VOC compound. ZrO2 was first deposited on a unimodal SiO2 support, followed by deposition of Pt and Ni nanoparticles with varried amount. Dynamic adsorption experiments were conducted using a 500 ppmv-laden feed at 25 °C and 1 atm, followed by in situ catalytic oxidation at 200 °C. Among the DFMs investigated, 1Pt2Ni/ZrSi emerged as the material with overall in situ conversion of ∼96% due to its retaively high PtNi acid sites density compared with other DFMs. Kinetic analysis was conducted on 1Pt2Ni/ZrSi by investigating the effects of feed concentration, regenerative feed flow rate, and oxidation temperature on the benzene conversion in the sequential adsorption–oxidation process. The results of the kinetic analysis revealed that a complete oxidation can be attained over 1Pt2Ni/ZrSi and its performance can be improved upon decreasing adsorptive feed benzene concentration (maximum value of 99.7% at 125 ppmv) and regenerative feed flow rate (100% at 5 mL/min), and increasing oxidation temperature (100% at 200 °C). Overall, the obtained results highlighted the optimal operation conditions for sequential adsorption and oxidation of VOCs over ALD-based DFMs.

High Temperature Oxidation Resistance Performance of TiC/Mo Composite by Spark Plasma Sintering

In this paper, the Titanium carbide/Molybdenum (TiC/Mo) alloy was prepared by spark plasma sintering (SPS). The oxidation process of the TiC/Mo alloy at different oxidation temperatures was studied, and the oxidation mechanism was discussed in depth. The focus is on the influence of the introduction of TiC particles on the high-temperature oxidation properties of Mo alloys. The phase composition and morphology of the oxide film were analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results showed that, after oxidation, the surface oxide film is mainly composed of Titanium dioxide (TiO2), Molybdenum dioxide (MoO2), Molybdenum trioxide (MoO3) and Molybdenum oxide hydrate (MoO3(H2O)2) phases. As the oxidation temperature increases, the surface of the oxide film will warp, and thereby increase porosity. The dense MoO2 will form a protective inner oxide layer and inhibit further progress of the TiC/Mo alloy. Oxygen will undergo violent oxidation through the pores and the inside of the matrix, and the protective MoO2 internal oxide film will disappear. TiC particles dispersed in the matrix will be oxidized to form TiO2, which will be gradually deposited on the surface of the oxide film, hindering the diffusion of oxygen to the matrix. The quantity loss during the entire oxidation process is significantly reduced. Therefore, the introduction of TiC can greatly improve the oxidation resistance of Mo alloys.

Investigation of Dry Sliding Friction, Wear and Mechanical Behavior of the Ti-6Al-7Nb Alloy after Thermal Oxidation.

The mechanical and tribological characteristics of the Ti-6Al-7Nb alloy were investigated within a wide range of temperature and time parameters of thermal oxidation. The hardness, HIT, and indentation modulus, EIT, of the alloy in question, with and without an anti-wear oxide layer, were determined. The tribological properties of sliding couples were studied under technically dry friction conditions, using a ball-on-disc tribometer. The test pieces were non-oxidized and oxidized Ti-6Al-7Nb alloy discs, and Al2O3, ZrO2, and 100Cr6 balls were used as counter specimens. After thermal oxidation, the surface of the titanium alloy was characterized by a significantly higher hardness, HIT (8–10 GPa), compared to the surface not covered with oxide layers (3.6 GPa). The study showed that the curvature of the loading segments increased with an increasing oxidation temperature, indicating a strong positive dependence of hardness on the thermal oxidation temperature. The value of the indentation modulus, EIT, was also found to increase with the increasing oxidation temperature. The intensity of the tribological processes was strictly dependent on the oxidation parameters and the couple’s material (Al2O3, ZrO2, 100Cr6). It has been shown that the thermal oxidation process makes it possible to control, within a wide range, the friction-wear characteristics of the Ti-6Al-7Nb alloy.

Porous Silicon on Insulator Technology

In this research, the structure, optical, and electrical properties of porous silicon prepared by electrochemical etching at current density 50 mA /cm2 with etching time 10 min, before and after rapid thermal oxidation process (RTO) at different oxidation temperature from (500-700) Ċ and constant oxidation time 60 s. Under optimum conditions, the pore size decreased after RTO leads to decrease porosity, the refractive index decreased for porous silicon (PS) and increased after oxidation. Electrical properties were showed an increase in rectification ratio after oxidation, where it varied from (3.93 to 158.8), the ideality factor which describes an approach the device from ideality characteristics founded decrease with increasing oxidation temperature where it varied from (9 to 5.4), the barrier height increased from (0.561 to 0.698 eV). The oxidation process improves electrical conductivity for electronic device by decreasing saturation current density that initiated from minority carriers, where (OPS/PS/Si) work as anti-reflected coating that benefit to eliminate swings effect and standing waves in photoresist.

Optimization of the microwave absorption properties of FeSiCr@Fe2O3 core‐shell nanoparticles by controlling the thickness and crystallinity of Fe2O3 shell

AbstractFe2O3‐coated FeSiCr(FeSiCr@Fe2O3) nanoparticles were prepared by plasma arc discharge method and oxidation heat treatment process, and the crystallinity of Fe2O3 was improved by vacuum heat treatment. By adjusting the oxidation temperature, Fe2O3 shell layer is formed on the surface of FeSiCr nanoparticles. The thickness of the Fe2O3 shell layer increases with the increase of oxidation temperature. The complex permittivity () and complex permeability () show a trend of first increasing and then decreasing. When the oxidation temperature is 125°C, the minimum reflection loss (RLmin) of FeSiCr@Fe2O3 nanoparticles is calculated to be −23.7 dB at 11.2 Ghz (d = 2.2 mm). With the increase of the temperature of vacuum heat treatment, the crystallinity of Fe2O3 shell keeps increasing. The real and imaginary values ( and ) of the complex permittivity of FeSiCr@Fe2O3 nanoparticle increase with the increase of the crystallinity of Fe2O3 shell, and the basically remains unchanged. When the vacuum heat treatment temperature is 150°C, the FeSiCr@Fe2O3 nanoparticle obtains RLmin of −49.9 dB at 15.9 Ghz (d = 1.7 mm). And the corresponding effective absorption bandwidth within 1‐18 GHz (RL ≤ −10 dB) is 5.9 GHz. The improved microwave performance is due to the interface polarization of the core‐shell structure, and the synergistic effect between FeSiCr and Fe2O3 also helps to balance the electromagnetic parameters.

Investigation on gas and particle emission characterization of carbon black oxidation process promoted by catalyst/ash

The study of the gas and particle emission characteristics during the oxidation of carbon black particles is the basis for exploring the particle emission characteristics during diesel particulate filter (DPF/CDPF) regeneration process. The gas and particle emission of Printex U (PU) particles oxidation under different oxidation temperatures and sample masses are investigated based on fixed bed and thermogravimetric analysis, and the oxidation processes are conducted under the cooperative effect of catalyst (CeO2)/ash (ZnO). The experimental results show that, with the increase of oxidation temperature, the CO/CO2 concentration, the extent of oxidation (η) and the total emitted solid particulate number (PN) increase, but the average particulate diameter (Da) range reduces from 60 to 100 nm to 20–40 nm. On the other hand, with the increase of PU mass, the CO/CO2 concentration and the total PN increase together, and the Da rises from about 30 nm to 40–160 nm, while the η decreases markedly. Both the CeO2 and ZnO nanoparticles can promote the PU oxidation process, and then reduce CO emissions, which lead to the rise the CO2 emissions. Beneficially, the η and the Da increase under the effect of CeO2 and ZnO, and the total PN decreases sharply. Adding catalyst (CeO2) and ash (ZnO) can effectively reduce the emission of small particle size (<23 nm) during oxidation process, which provides a research basis for controlling the emission of particles below 23 nm during DPF regeneration process.

The effect of mullite coating and microwave sintering on high temperature oxidation resistance of MWCNTs

Multi-walled carbon nanotubes (MWCNTs) are considered as a promising material for nanocomposites fabrication due to their magnificent characteristics. However, challenges including low oxidation resistance (oxidized above 530 °C in the air) and weak interfacial adhesion, etc, restrict their use at high temperatures. Coating the surface of MWCNTs seems a promising way to control its oxidation and to increase interfacial adhesion. To the best of our knowledge, the coating of MWCNTs by mullite has not been reported yet, so that in this paper, mullite-coated MWCNTs (20 wt %) was fabricated by sol-gel method to improved oxidation resistance of MWCNTs. The uniform mullite shell was sintered through two methods of microwave and conventional heating. The effect of sintering temperature and type of heating on mullitization, crystallization and oxidation resistance of MWCNTs were investigated. Microstructural investigations revealed that the surface of MWCNTs is uniformly covered by mullite and TGA results showed the increase in oxidation temperature (≈300 °C). In addition, the obtained results confirm that the microwave method could be a better technique for obtaining a dense, smooth and crystallized coating layer than the conventional one for improving oxidation resistance of MWCNTs.

Low-temperature plasma atomic layer etching of molybdenum via sequential oxidation and chlorination

In this study, an atomic layer etching (ALE) process for molybdenum was developed in two steps: plasma oxidation and plasma chlorination. In the plasma oxidation step, molybdenum was oxidized with oxygen plasma to form molybdenum oxide. As the plasma oxidation time increased, the atomic ratio of O-to-Mo, determined by x-ray photoelectron spectroscopy, increased, and then saturated to a value of 2.3. The oxidation depth of molybdenum was found to increase with increasing oxidation temperature—from 3.0 nm at 40 °C to 22.0 nm at 300 °C. It also increased with increasing RF (radio frequency) power—from 2.0 nm at 5 W to 5.5 nm at 25 W. In the plasma chlorination step, it is believed that molybdenum oxide was removed from the surface by forming molybdenum oxychloride (MoOCl2, MoOCl4, and MoO2Cl2) in chlorine plasma in the temperature range of 40–300 °C. The etch per cycle (EPC) continuously increased at temperatures above 100 °C; however, at temperatures below 40 °C, it was saturated. The RF power increased the EPC from 2.2 to 5.8 nm/cycle in the range of 5–25 W. It was found that the removal depth matched the oxidation depth at each RF power in ALE at 40 °C. The atomic composition of molybdenum after ALE was almost identical to that before ALE. This study demonstrates that the ALE of molybdenum at 40 °C can be realized by sequential plasma oxidation and chlorination.

Effects of wet oxidation on residual stress variation of SiCf/SiC composites

Continuous silicon carbide fiber-reinforced silicon carbide matrix (SiCf/SiC) composites, used as thermal structures for gas turbines, are prone to degrade under the gas stream (mainly composed of O2 and H2O). Here, the degradation of the composites in wet oxygen environment was investigated by Raman spectroscopy. Results show that the composites exhibit significant residual stress as the oxidation proceeds. SiC matrix in the as-received condition exhibits average hydrostatic tensile stresses of approximately 1350 MPa, while SiC fiber exhibits compression stress of approximately 1450 MPa. Both the residual stresses in matrix and fiber decrease with the increasing oxidation temperature. Moreover, the residual stress evolution is found to be related to the formation of cristobalite and the volatilization of Si(OH)4, CO, H2. This work can open new alternatives for understanding the degradation mechanism of SiCf/SiC composites in gas turbine environment, and for developing mathematical model to predict their service life.

Enhanced Mechanical Properties and Oxidation Resistance of Zirconium Diboride Ceramics via Grain-Refining and Dislocation Regulation.

Zirconium diboride (ZrB2) is considered as one of the most promising ultra‐high temperature materials for the applications in extreme environments. However, the difficulty in fabrication of ZrB2 limits its industrial applications. In this study, fully dense and grain‐refined ZrB2 is prepared under ultra‐high pressure of 15 GPa at low temperature of 1450 °C. The as‐prepared ZrB2 exhibits excellent mechanical and oxidation‐resistant properties. Compared with raw powder, the grain size decreases 56%. Compared with high‐temperature sintered control specimen beyond 2000 °C, the hardness and fracture toughness increase about 46% and 69%, respectively, the dislocation density increase 3 orders of magnitude, while the grain size considerably decrease 96%. According to work hardening, Hall–Petch and Taylor dislocation hardening effects, the refined grains, substructures, and high dislocation density caused by plastic deformation during sintering can enhance the mechanical properties. The unique structure contributes to a threshold oxidation temperature increase of ≈250 °C relative to the high‐temperature sintered ZrB2, achieving one of the highest values (1100 °C) among the reported monolithic ultra‐high temperature ceramics. A developed densification mechanism of dislocation multiplication with grain refining is proposed and proved to dominate the sintering, which is responsible for simultaneous improvements in mechanical and oxidation‐resistant properties.

Oxidation resistance performance of SiC-AlON ceramic composites at high temperature

The microstructure and oxidation behaviour of the pure AlON ceramics and 8 wt.% SiC-AlON composite were investigated at 700-1500?C. With increasing oxidation temperature, the surface morphology of two ceramics showed a change from acicular to flaky-like and then to granular features. With the addition of nanosized SiC particles, the oxidation resistance of the ceramics was remarkably enhanced above 1100?C. This is attributed to the formation of a dense oxide layer composed of Al2O3, SiO2 and mullite, which could cover the whole matrix and suppress further penetration of oxygen. Due to the dense oxide layer, the oxidation kinetics of the 8 wt.% SiC-AlON composites conformed to a parabolic law, while that of the pure AlON conformed to a linear law. After oxidation at 1500 ?C for 40 h, the weight gain of the 8 wt.% SiC-AlON composites was 3.07mg/cm2, which was only 22.5% of that of the pure AlON.

High temperature oxidation behavior and mechanism of Al0.3CuCrFeNi2 high-entropy alloy with a coherent γ/γ' microstructure

In this study, the oxidation behavior of the Al0.3CuCrFeNi2 HEA with a typical coherent γ/γ' microstructure was systematically investigated by the isothermal oxidation tests at 600, 750 and 900 ℃ in air. The results show that the oxidation kinetics of the Al0.3CuCrFeNi2 HEA follow the parabolic law at all three temperatures. As the oxidation temperature increases, the surface segregation of Cu near the grain boundary occurs, leading to different oxidation behavior in the intragranular regions and near the grain boundary regions. The associated formation mechanisms of the oxide scales are proposed and discussed in detail.

Preparation of Copper Oxide/TiO2 Composite Films by Mechanical Ball Milling and Investigated Photocatalytic Activity

The Cu/Ti composite coatings were prepared by the mechanical ball milling, the CuO/TiO2 and Cu2O/TiO2 composite photocatalytic films were obtained by the subsequent oxidation process. The microstructure of the composite films was analyzed by X-ray Diffraction (XRD）and scanning election microscope (SEM). The photocatalytic activity was evaluated, the effects of ball milling time on the formation of the Cu/Ti coatings were investigated, and the effects of the oxidation temperature and oxidation atmosphere on microstructure and photocatalytic activity of the films were studied. The results illustrate that the ball milling time has significant effects on the formation of the coatings and the coatings are continuous and compact by ball milling for 15 h. The photocatalytic activity of the CuO/TiO2 composite films is increased first and then decreased with the oxidation temperature increases, and the photocatalytic activity is the best at 500 °C. The CuO/TiO2 composite films are obtained by the oxidation of Cu/Ti coatings at 500 °C for 15 h in the air, while the Cu2O/TiO2 composite films are oxidized in carbon atmosphere. Photocatalysis efficiency of the films is obviously enhanced with the help of the p-n junction heterostructure in the Cu2O/TiO2 composite films.