Increasing Oxidation Temperature Research Articles

Increasing CO2 gasification rates of longan-seed char by a technique of char pre-oxidation

A simple and effective technique was presented to increase the longan-seed char gasification rate during CO2 activation by pre-oxidizing the char with air. The kinetics of char gasification was studied in a thermogravimetric analyzer (TGA) for the temperatures of 850–950 °C for char samples being oxidized in a tube furnace at the temperatures of 250, 280, and 300 °C. It was found that the gasification rate of the oxidized char was about twice higher than that of the non-oxidized char. The increase in the char reactivity for CO2 gasification is due to the increasing number of reaction sites caused by the disruption of functional group bonding created during the oxidation step. Additionally, the increase in the oxidation temperature gave rise to the increase in the char reactivity but the maximum temperature was limited by the ignition temperature of the longan-seed char. Four gasification kinetic models were tested against the experimental rate data and it was discovered that the modified volume reaction model (MVRM) provided the best description of the gasification conversion. The pre-exponential factor and the activation energy of the gasification reaction were determined from the Arrhenius equation. The estimated activation energy was found to be in the range of 156.87–218.98 kJ/mol and decreased with the increase of char oxidation temperature.

Experimental study on the effects of pre-oxidation on the pore structure and fractal characteristics of coal

ABSTRACT To assess the effects of pre-oxidation on pore structures and fractal features of coal samples, pre-oxidized coals at various oxidation temperatures were prepared and compared. Pore structure parameters of coal samples were analyzed qualitatively and quantitatively using electron scanning microscopy (SEM) and N2 adsorption. The pore volume of coal samples increased first and then decreased with the increase of oxidation temperature. Among them, the pre-oxidized coal at 150°C is the highest, which is 27.92% higher than the raw coal. The fractal features of coal samples were calculated by fractal theory. It was found that pre-oxidation makes pore fissures of coal samples develop with obvious pore expansion effect, which is more obvious at the higher pre-oxidation temperature. In addition, fractal dimension D 2 of coal samples was increased by pre-oxidation, indicating that pore structures of pre-oxidized coal are more complex than those of raw coal.

High temperature oxidation and microstructural evolution of SiC layer of TRISO particles in the presence of NaCl

With the different types of advanced nuclear fission reactors, the reliability of the SiC layer in tristructural-isotropic (TRISO) particles exposed to different corrosive environments is of vital important. In the present work, the oxidation behaviors of the SiC layer of TRISO particles exposed in NaCl salts with the concentrations of 0.2–2 wt% at 800–1400 °C were investigated. The effects of NaCl concentration and oxidation temperature on phase composition, microstructure, and chemical composition of the TRISO particle samples were systematically investigated. The results show that, with the increase of oxidation temperature and NaCl concentration, the oxidation rate of SiC layer in TRISO particles was accelerated. However, the effect of NaCl concentration alone on the oxidation rate of SiC layer was not significant. This could be attributed to the volatility of NaCl at high temperature. In addition, just like other halogen salts, NaCl salts also reacted with the oxide layer to form silicate compounds, such as Na2SiO3, which provided a channel for both oxygen and fluoride ions to diffuse into the inner SiC layer. The results obtained in the present work can provide a guide for the application of TRISO particles in molten salt stacks of chlorine salt.

Oxidation Property of a Fourth-Generation Powder Metallurgy FGH4108 Nickel-Based Superalloy

Isothermal oxidation kinetics of a fourth-generation powder metallurgy FGH4108 nickel-based superalloy is investigated at 800 °C to 1100 °C by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). At 800 °C and 900 °C, the oxidation kinetic curves of the FGH4108 superalloy follow parabolic law. At 1000 °C, the oxidation kinetic curve follows cubic law. At 1100 °C, the oxidation kinetic curve has two distinct parts: the first part follows a parabolic law, and the second one obeys a linear law. Cross-sectional morphologies and elemental distributions show that the oxide film consists of two parts at 800 °C: the outer layer is a continuous dense protective Cr2O3 oxide film, and the inner layer is a discontinuous Al2O3 oxide layer. At 900–1100 °C, the oxides consist of three layers: the outermost is the oxides of Cr2O3 and TiO2, the middle is a continuous oxide of Al2O3, and the innermost is dotted oxides of TiO2. The thickness of the inner TiO2 oxide layer increases with the increase of oxidation temperature. On this basis, the oxidation behavior of the FGH4108 superalloy at high temperatures is confirmed to be controlled by the diffusion of Cr, Al, Ti, and O. From the aspect of oxidation resistance, the long-term service temperature of the FGH4108 superalloy should not exceed 1000 °C.

Physicochemical properties, combustion kinetics and thermodynamics of oxidized lignite

Lignite oxidizes easily during storage and transportation. Furthermore, the surface characteristics of lignite change during oxidation, affecting its thermal conversion process. Herein, the surface functional groups and pore structures of lignite oxidized at temperatures below 190 °C were investigated. Moreover, the kinetics and thermodynamics of the combustion of lignite were analyzed using thermogravimetric analysis. The relationship between the surface structure and combustion performance of oxidized lignite was established, and the effect of oxidation on the characteristics of the combustion process was evaluated. The results revealed that the value of CH3/CH2 increased considerably upon increasing oxidation temperature from 70 to 150 °C. The number of –OH groups decreased with increasing in oxidation temperature, whereas the number of CO groups decreased gradually, then increased significantly, and lastly decreased with increasing oxidation temperature. The mesoporous structure of lignite was severely damaged during oxidation, the specific surface area and pore volume of oxidized lignite were lower than those of raw coal, and the average pore diameter of oxidized lignite was significantly higher than that of raw lignite. The poor combustion performance of oxidized lignite was attributed to its low mesopore specific surface area and low pore volume. The activation energy (Ea) of lignite during combustion was calculated to be 35.29 kJ/mol using the Ginstling-Broushtein equation, and the Ea of oxidized lignite ranged between 40.53 and 45.93 kJ/mol. These results indicate that a large amount of energy was required to burn oxidized lignite. Therefore, the oxidation of lignite should be avoided during the storage process to damage the pore structure, thus affecting the combustion performance; the pore structure of oxidized lignite can be adjusted to enhance its combustion performance. This will effectively improve the utilization efficiency and economic value of lignite.

CHARACTERISATION OF MICROMECHANICALAND TRIBOLOGICAL PROPERTIES OF TITANIUM GRADE 2AFTER CYCLIC OXIDATION

This paper presents the characterisation of micromechanical and tribological properties of titanium Grade2 before and after cyclic oxidation. The oxidation process was carried out at temperatures of 600C, 650Cand 700C in 4 and 12 cycles. Microscopic studies showed that oxide particle size increased with increasingoxidation temperature and the number of cycles. Titanium Grade 2 showed up to 3 times higher hardnessafter cyclic oxidation. The highest hardness (8.4 GPa) was obtained after 12 cycles of titanium oxidation at650C. Tribological tests were conducted in pairs with different materials (Al2O3, ZrO2, bearing steel 100Cr6).The presence of oxide layers obtained on the titanium surface resulted in a significant reduction in specificwear rate. Titanium Grade 2 showed the best resistance to sliding wear after cyclic oxidation at 600C duringsliding interaction with ZrO2 and 100Cr6 balls (unmeasurable wear under assumed test conditions). In theother test variants, the reduction in wear ranged from 37 to 96%.

Room Temperature Detection of Chlorpyrifos Using Thermally Oxidized Manganese and Copper Thin Films

The usage of pesticides has increased rapidly over the past three decades to boost crop production and meet the global food demand. According to UN FAO, about 66% of the countries have reported an average increase in pesticide consumption of about 1.85 kg hectare−1 between 1990 and 2019. Chlorpyrifos is a widely used but hazardous pesticide, popular for domestic and agricultural applications. Real-time monitoring of Chlorpyrifos is necessary for its safe application due to its toxicity, even at the residue level. In this work, an attempt has been carried out to detect chlorpyrifos at room temperature using thermally oxidized manganese and copper thin films. Thermal oxidation of thin films has been carried out under air ambient at different temperatures ranging from 400 °C to 550 °C for manganese and 500 °C to 800 °C for copper thin films. Morphological investigation depicts nanobar-like morphology and irregular-sized clusters for various oxidation temperatures of manganese oxide thin films, and granular-like morphology with an increase in grain size has been observed for increasing oxidation temperature. Structural investigation using the XRD technique depicts the evolution of various phases of manganese oxide thin film and a single monoclinic phase for copper oxide thin film. Further, the RMS roughness of the manganese and copper film was found to vary differently with the rise in oxidation temperature. Copper oxide thin films have shown higher chlorpyrifos sensing response with sensitivity than thermally oxidized manganese thin films better sensing response towards chlorpyrifos. The sensitivity at room temperature is found to be around 30 and 250 for manganese oxide and copper oxide thin films, respectively.

Oxidation behavior of Ti2AlC MAX-phase foams in the temperature range of 600–1000 °C

The isothermal oxidation behavior of Ti2AlC gel-cast foams with an almost completely open porosity of 87 vol% was investigated in the temperature range of 600–1000 °C, in static air, and for exposure times of up to 6.5 h. It was found that the oxidation process obeyed the parabolic rate law. The determined parabolic oxidation rate constants were low, indicating that the material’s resistance to oxidation at high temperatures was very good, especially given its highly porous form. The nature of the scale formed on the strut surface of Ti2AlC foams changed significantly with increasing oxidation temperature. After oxidation at 600 °C, gaps/cracks were observed between Ti2AlC layers on the strut surface, and the oxidation products were mixed Ti and Al oxides. At 700–800 °C, the scales were integrated and composed of Ti and Al oxides. At 900–1000 °C, the oxidation products were composed of large elongated plates of TiO2 (rutile) and small, needle-like Al2O3 (corundum) grains.

The oxidation behaviors of TiB1.73, Al0.59Ti0.41N, and TiB1.73/Al0.59Ti0.41N coatings deposited by high-power impulse magnetron sputtering method

Transition metal nitrides and diborides have high melting points, excellent chemical stability, and good thermal conductivity. However, these materials typically suffer from oxidation when served in high-temperature environment. In the present work, the single layer TiB1.73, Al0.59Ti0.41N, and bilayer TiB1.73/Al0.59Ti0.41N coatings with ~4–6 μm thickness have been deposited on cemented carbide inserts by high-power impulse magnetron sputtering (HiPIMS) method. The oxidation behavior and phase transition of these coatings at elevated temperatures (400–900 °C) are investigated by means of a hybrid approach of experiments and phase diagram calculations. TiB1.73 and Al0.59Ti0.41N coatings grow in columnar form. Anatase is the first generated product during TiB1.73 coating oxidation at 400°C for 1 h, then the amount of rutile phase gradually increases with the rise of oxidation temperature. Oxide layers for TiB1.73 coating are porous and show oxidation delamination phenomenon, which are related to the evaporation of B2O3 (g) and the transformation of anatase to rutile. Anatase is also firstly formed during the oxidation process at 700 °C for Al0.59Ti0.41N coating. Subsequently, a protective Al-rich oxide layer is generated and this layer is broken after oxidation at 900 °C. The initial oxidation behavior of bilayer TiB1.73/Al0.59Ti0.41N coating is similar to that of TiB1.73 coating, but aluminum oxides are observed after oxidation at 650 °C. The B element on TiB1.73/Al0.59Ti0.41N coating surface is gradually exhausted with the increase of oxidation temperature.

Oxidation Behaviors of ZrB2–SiC Ceramics with Different Porosity

ZrB2–SiC ceramics have great application potential in thermal protection system of hypersonic aircraft, due to their excellent oxidation resistance and mechanical properties. Porosity is one of the main factors which will obviously affect the oxidation behaviors of ZrB2–SiC ceramics. Herein, ZrB2–SiC ceramics with three different porosity are sintered by spark plasma sintering at different temperatures. Then, a series of oxidation tests of the ceramics with different porosity are carried out at different temperatures and with different time. Morphologies and microstructures of the oxidation surface and cross section, as well as the oxide layer thickness and weight change are discussed in detail. Results show that the porosity of ZrB2–SiC ceramics has great influence on the oxidation behaviors of ceramics. The oxide layer thickness and weight change gradually increase with the increase of oxidation temperature and oxidation time for ZrB2–SiC ceramics with different porosity. As the porosity of ZrB2–SiC ceramics decreases, the oxide layer thickness and weight change generally decrease, which shows that the ceramics with lower porosity have stronger oxidation resistance. This work provides a relative systematic analysis of the effects of porosity on the oxidation behaviors of ZrB2–SiC ceramics.

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.

Oxidative modification of industrial basic oxygen furnace slag for recover iron-containing phase: Study on phase transformation and mineral structure evolution

Basic oxygen furnace (BOF) slag is the main co-product in the steelmaking process, its recycling within the iron and steel enterprises is crucial for the industry to ensure greater sustainability. According to the composition characteristics of BOF slag, a method of separating and recovering iron resources by regulating the composition of BOF slag with silica as an additive at high temperature was proposed. In this study, the growth of iron-containing phase during oxidative modification of BOF slag was in-situ observed by ultra-high-temperature confocal scanning laser microscope, and the phase transformation and mineral structure evolution were analyzed by chemical analysis, X-ray diffraction and field emission scanning electron microscopy. The results demonstrated that the increase of oxidation temperature effectively promoted the transformation of iron oxides to (Mg,Fe2+)O·Fe2O3, but too high temperature could lead to the increase of iron-containing melilite phase and reduce the iron recovery. In the oxidative modification process, the appropriate holding time and cooling mode facilitated the growth and development of (Mg,Fe2+)O·Fe2O3 particles, which was conducive to the separation and recovery of iron resources. After BOF slag was oxidized at 1300 ℃ for 240 min and cooled in the furnace (1 ℃/min), the grade of magnetic slag obtained by magnetic separation had a grade of 40.26%, a recovery of 70.95%, and a P and S content of 0.042 and 0.456, respectively, which may be used as an important supplement to the raw materials for sintering. The regulated secondary slag may be used to diversify into higher value-added production. Therefore, the systematic research and development of oxidative modification-crushing-magnetic separation technology may promote the efficient utilization of BOF slag components and heat energy.

Packaging Materials Based on Styrene-Isoprene-Styrene Triblock Copolymer Modified with Graphene.

This study presents the improved stabilization effects of graphene on a polymer substrate, namely a styrene-isoprene-styrene triblock copolymer (SIS) which creates opportunities for long-term applications and radiation processing. The added graphene has a remarkable activity on the protection of polymer against their oxidation due to the penetration of free macroradical fragments into the free interlayer space. The chemiluminescence procedure used for the evaluation of the progress of oxidation reveals the delaying effect of oxidative degradation by the doubling extension of oxidation induction time, when the material formulation containing graphene is oxidized at 130 °C. The pristine polymer that is thermally aged requires an activation energy of 142 kJ mol-1, while the modified material needs 148, 158 and 169 kJ mol-1, for the oxidative degradation in the presence of 1, 2 and, respectively, 3 wt% of graphene. The contribution of graphene content (1 wt%) on the stability improvement of SIS is demonstrated by the increase of onset oxidation temperature from 190 °C for neat polymer to 196 °C in the presence of graphene and to 205 °C for the polymer stabilized with graphene and rosemary extract. The addition of graphene into the polymer formulations is a successful method for enlarging durability instead of the modification of receipt with synthesis antioxidants. The presumable applications of these studied materials cover the areas of medical wear, food packaging, commodities, sealing gaskets and others that may also be included through the products for nuclear power plants.

Oxidation behavior of Cr-coated Zr-4 alloy prepared by multi-arc ion plating at 1000-1200°C

The oxidation behavior of Cr-coated Zr-4 alloy is studied in air environment at temperatures ranging from 1000 °C to 1200 °C and for oxidation times ranging from 1 h up to 4 h. The oxidation weight gain, sample appearance, surface and cross-sectional microstructure, Cr-Zr diffusion and phase at different oxidation times for the three temperatures were analyzed in detail. The results reveal that the oxidation weight gain is parabola with oxidation time, and the parabolic rate constant increases with the increase of oxidation temperature. At 1000 °C and 1100 °C, the Cr coating can provide oxidation protection for the internal Zr-4 alloy after 4 h of oxidation. While at 1200 °C, Cr coating can only provide effective oxidation protection for the internal Zr-4 alloy for 1 h. At 1000 °C and 1100 °C, the thickness of the Cr-Zr diffusion layer increases linearly with the oxidation time, while at 1200 °C, the thickness of the Cr-Zr diffusion layer increases linearly with the oxidation time after 1–3 h of oxidation, and after 4 h of oxidation, it suddenly increases sharply. And the XRD results imply that the preferred orientation of Cr2O3 grains is related to the oxidation temperature.

Growth and characterization of Cobalt Oxide thin films

Thin films of cobalt were deposited on glass substrates at room temperature using an electron beam evaporation technique. Formation of different cobalt nanostructures has been carried out by controlled thermal oxidation of the as-deposited Co films in controlled ambient condition. Oxidation temperature was varied within a range up to 700 °C. Structure and morphology of the oxide films along with their chemical and electrical properties have been investigated using various surface characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and I-V measurements. All results are in good correlation and suggest that the CoO phase is only stable for a lower oxidation temperature below 400 °C whereas Co3O4 phase usually appears at higher oxidation temperature. Moreover, the average crystallite size of the oxide nano-particles increases with the oxidation temperature and duration. Electrical measurement shows that resistance of cobalt oxide thin films decreases with an increase in oxidation temperature and duration. All these findings can be explained in terms of thermal diffusion related to different surface morphologies and grain size effects.

Study on Characteristics of Coal Spontaneous Combustion in Kerjian Mining Area, Xinjiang, China

The spontaneous combustion of coal is a disaster associated with coal mining. In this study, the authors investigated the characteristics of spontaneous combustion of coal at different temperatures (room temperature, 50–500 °C with 50 °C interval) using Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), etc. The results showed the aromatic structure was mainly naphthalene. The aliphatic hydrocarbons were long chain. Oxygen, nitrogen, and sulphur existed as C-O, pyridine, pyrrole nitrogen, aliphatic sulphur, and sulfone. The molecular structural formula is C142H112N2O22. The stable 3D structural was obtained through optimization. Thermogravimetric analysis results showed the critical and dry-cracking temperatures of coal samples showed downward trends overall, whereas the acceleration and thermal-decomposition temperatures varied greatly with increase in oxidation temperature. The activation energy change pattern of 4 stages is not obvious. The FTIR results showed the contents of self-associated OH changed greatly. The aliphatic hydrocarbons changed greatly at 30–150 °C and 300–500 °C. The C-O showed increasing trends, whereas the C=O decreased consistently. The HRTEM results showed the aromatic fringes in coal samples were dominated by 1 × 1 and 2 × 2, the contents of which accounted for more than 80% of the total fringes.

Influence of heat treatment on H2S gas sensing features of NiO thin films deposited via thermal evaporation technique

In this study, NiO thin films are prepared on Ni foil using thermal evaporation method and the effect of deposition conditions on structural, morphological and H2S gas sensing properties of NiO thin films is investigated. Structural analysis of the NiO films are conducted by means of X-ray diffraction (XRD), and the films are found to have the FCC phase with preferred (111) and (200) Bragg reflections. Based on XRD and scanning electron microscopy results, it is shown that the crystallite size and the size of homogeneous nanoclusters on the surface of the films increase after increasing oxidation temperature. Also, the EDS patterns of NiO films demonstrate a rise in the intensity of O peak upon increasing the oxidation temperature due to the enhancement of oxygen in the NiO films. The Raman spectrum of NiO thin films several bands correspond to Ni–O vibrations. The obtained gas sensing results demonstrate the sensing response to 20 ppm of hydrogen sulfide at 700 °C. Upon increasing the amount of hydrogen sulfide gas to more than 20 ppm, the fabricated sensors show no significant change in their response, indicating that no additional active sites were exist to interplay with hydrogen sulfide molecules for the higher concentration of hydrogen sulfide. The response of the sensors, recorded 10 s after H2S gas exposure, depended on the deposition temperature. The current study shows that NiO thin films-based sensors prepared by a simple thermal evaporation method are potentially useful for the detection of H2S gas.

Effects of oxidation temperature on the microstructure and tribological performance of Cu-based brake pad for high-speed train

The braking process of different speeds causes varying degrees of oxidation of the Cu-based brake pad, which will significantly affect its tribological performance. To simulate the process, Cu-based brake pads for high-speed trains were oxidized in atmospheric conditions at 25, 200, 400, and 600 °C, respectively. The microstructure and composition of the oxide layers were analyzed, and the tribological properties of the oxidized surfaces were investigated. The results show that, as the oxidation temperature rises, the surface oxides change from Cu2O + Fe3O4 to CuO + Fe3O4, along with the increase in the thickness and hardness of the oxide layer. The ablation of graphite particles is aggravated. The formed oxides and ablated graphite result in high friction coefficient and wear rate. Due to the loss of plastic deformation ability, the dominant wear mechanism of Cu-based brake pad transforms from delamination wear to abrasive wear with the increase of oxidation temperature.

Characteristics of the oxides and suboxides on Zr-1.0Nb-0.01Cu isothermally oxidised in steam at 650–1200 ℃

The texture of monoclinic ZrO2, grain morphology and crystal structure of suboxides ZrO and Zr3O, and orientation relationships near the ZrO2/α-Zr(O) interface of Zr-1.0 Nb-0.01Cu isothermally oxidised in steam at 650–1200 ℃ were investigated. The findings show that the morphology and distribution of the suboxide phase ZrO vary significantly with temperature, and both ZrO and Zr3O grains have hexagonal structures. The number density of orientation relationships of different phases near the interface decreases as the oxidation temperature increases. The characteristics of the oxides and suboxides can provide some insights into the understanding of the high-temperature oxidation and cooling processes.

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.