Non-isothermal Oxidation Research Articles

The influence mechanism of thermal invasion on spontaneous combustion of different coals using continuous adiabatic heating and non-isothermal oxidation method

Within deep coal mines, the elevated ground temperature has the potential to induce thermal invasion, thereby exacerbating coal spontaneous combustion (CSC). To investigate the influence mechanism of thermal invasion on CSC, experiments were conducted using coals of different metamorphic degrees (lignite, bitumite, and anthracite) with a continuous adiabatic heating and non-isothermal oxidation method in an oxidation furnace and an in-situ ESR spectrometer. Thermal invasion markedly enhanced the inherent susceptibility of spontaneous combustion of coals by increasing the oxygen consumption rate and lowering the apparent activation energy. This enhancement results from active free radicals generated during thermal invasion which accelerated coal-oxygen reactions during non-isothermal oxidation. These free radicals were found to be mainly carbon-centred radicals adjacent to an oxygen atom (alkyl radicals). An increase in both the g-factor value and free radical concentration was observed with thermal invasion temperature and invasion duration, especially in lignite, leading to a surge of free radical concentration during non-isothermal oxidation. The active free radicals generated during thermal invasion can easily react with oxygen, providing heat for coal-oxygen reactions and enabling rapid free radical generation. These findings offer insights for developing CSC evaluation and prevention strategies in deep mines where high ground temperatures are encountered.

Heterogeneous oxidation involving different atomic clusters in sintering-free amorphous SiBCN ceramic with MA@PDC-SiBCN structure

Amorphous SiBCN monoliths featuring a structure of three-dimensional PDC-SiBCN network encapsulating MA-SiBCN nanoparticles (MA@PDC-SiBCN), were prepared without the need for sintering densification (>1800 °C), enabling preparation of dense ceramics at a much lower temperature (1100 °C). The continuous PDC-SiBCN network effectively inhibits oxygen diffusion, reducing the mass loss from B–C–N cluster oxidation and mass gain from silicon-containing clusters by 51.5 % and 86.9 %, respectively. Besides, the phase-separation coupled heterogeneous oxidation behaviors and kinetics of different atomic clusters in MA@PDC-SiBCN ceramic during non-isothermal oxidation up to 1500 °C were investigated. Finally, the evolution of the different atomic clusters within the oxide layer during the heterogeneous oxidation process was analyzed to elucidate the micro-mechanisms behind the enhanced oxidation resistance.

Preparation of β-SiAlON/SiC composite ceramic foam filters and their oxidation resistance

In this study, ceramic foam filters with high porosity, strength, and oxidation resistance are produced at low temperatures by preparing porous β-SiAlON/SiC composite ceramic foams using the foam impregnation method with Si, Si, α-Al2O3, AlN, and SiC as the raw materials. The presence of SiC inhibits the growth of β-SiAlON grains, which reduces the amount of β-SiAlON produced. As the proportion of β-SiAlON increases, the porosity of the β-SiAlON/SiC composite ceramic foam decreases, whereas the bulk density and compressive strength increase. Non-isothermal oxidation experiments show that β-SiAlON/SiC composite ceramic foams exhibit high oxidation resistance and their macroscopic morphology does not change significantly after oxidation. As the proportion of β-SiAlON increases, the smooth SiO2 oxide layer on the surface of the ceramic foam gradually changes to acicular and columnar mullite crystals, and the oxidation resistance increases. Therefore, β-SiAlON acts as a high-temperature binder when it is added to SiC. Hence, the prepared β-SiAlON/SiC composite ceramic foams can be used as ceramic filters in the field of molten metal filtration.

Size-dependent thermal oxidation kinetics in Sm2(Co, Fe, Cu, Zr)17 alloy powder

This study investigates the interrelationship between the oxide growth kinetics and the microstructural changes induced by dry, thermal oxidation in Sm2(Co, Fe, Cu, Zr)17 alloy powder (of varying particle sizes and compositions). Whereas, model-free iso-conversional methods are used to determine the kinetic-triplets during the non-isothermal oxidation stage, power-law dependent kinetic model is considered during the entire oxidation process to calculate the reaction order, rate constant and the apparent-activation energy. A distinctly different oxide growth kinetics, with strong dependency on the initial particle size(s) and the alloy composition-induced oxide microstructure is seen here.

Heterogeneous oxidation of different atomic clusters in amorphous SiBCN ceramic associated with phase separation

In this study, the microstructure of amorphous SiBCN ceramic derived from polyborosilazane and the crystalline SiBCN ceramic heat treated at 1600 ℃ was investigated and their oxidation behavior and kinetics involving different atomic clusters or phases during the non-isothermal oxidation process up to 1500 ℃ were analyzed to obtain their different micro mechanisms. Due to higher oxidation activity, the atomic clusters in amorphous structure can be oxidized at lower temperatures and exhibit lower activation energy than the crystal phases in crystalline structure, but have much higher oxidation resistance due to the phase separation-coupled heterogeneous oxidation of different atomic clusters.

Improved oxidation resistance and infrared emissivity of tantalum disilicide particles with sol-gel derived SiO2 coatings

As an important high-temperature structural material, tantalum disilicide (TaSi2) suffers from performance degradation associated with high temperature oxidation due to the poor oxygen diffusion barring effect of the mixed Ta2O5/SiO2 oxidation scale. In this work, silica coating was prepared on the surface of TaSi2 powder by a sol-gel method. The oxidation resistance was studied by thermogravimetric tests in non-isothermal oxidation and isothermal oxidation, as well as by oxidation treatment in a tube furnace at temperatures ranging from 800 °C to 1400 °C. The spectral emissivity of raw TaSi2, coated TaSi2 and the oxidized ones was compared and analyzed. The amorphous SiO2 coating is composed of numerous nanoparticles and covers most of the surface of TaSi2 particles. The oxidation weight gain of coated TaSi2 during heating from RT to 1200 °C and 1400 °C decreases from 29.8% and 30.9–3.0% and 8.6%, respectively. And the oxidation is controlled by diffusion in the overall process. The coated TaSi2 combines the advantageous radiation property of TaSi2 core and SiO2 coating. Compared with that of raw TaSi2, the emissivity of coated TaSi2 at the wavelength range of 200–2500 nm and 2.5–25 µm has increased by 7.5% and 13.2%, respectively, and the property did not deteriorate after oxidation at 1200 °C, suggesting that the coated TaSi2 is an excellent emitting agent during the entire service life.

Enhancing oxidation resistance of MoAlB based on its anisotropic oxidation mechanism: A theoretical calculation guided experimental investigation

MoAlB is considered the most promising material for oxidation resistance among the MAB phase family. However, the anisotropic oxidation mechanism at the atomic scale has not been fully investigated, which may limit its practical applications. In this work the anisotropic oxidation mechanism of MoAlB was first explored through systematic theoretical calculations, based on which an effective strategy to enhance its oxidation resistance was identified and successfully implemented in the experimental work. The theoretical calculation revealed that the anisotropic oxidation of MoAlB was primarily related to the difference in free energy of surface oxidation reactions due to the degrees of electron interaction localization of O-Al bonding. A series of candidates of various transition metal-doped MoAlB materials were then designed to improve the weaker surface of MoAlB for oxidation resistance enhancement. Among these candidates, Zr-doped MoAlB was selected to further evaluate the effects of doping on the anisotropic oxidation through the electronic structure and Ab initio molecular dynamics (AIMD). Finally, Zr-doped MoAlB was experimentally synthesized to validate the theoretical design. Non-isothermal oxidation kinetics analysis indicated that Zr doping can enhance the oxidation resistance of MoAlB by promoting the formation of a protective oxide scale and increasing the formation activation energy of volatile products, which finally reduced the thickness of the oxidation scale on the surface after isothermal oxidation of the bulk material. This work not only contributes to an in-depth understanding of the oxidation mechanisms in MoAlB but also presents a new approach to enhance its oxidation resistance and other Al containing ceramic materials beyond MAB phases.

Preparing β-SiAlON ceramic foam filters with high oxidation resistance

Herein, to develop ceramic foam filters (CFFs) with high porosity and strength, porous β-SiAlON ceramic foam was prepared using Si, α-Al2O3, and AlN via foam impregnation. Experiments were performed to investigate the effects of the slurry solid content, impregnation times, and sintering temperature on the sample properties. The best green bodies were obtained at a slurry solid content of ∼70 wt%, and the sample subjected to two impregnation times effectively filled the defects. The sintering temperature was found to affect the phase composition, microstructure, sintering behavior, mechanical properties, and oxidation resistance of β-SiAlON ceramic foams. Granular β-SiAlON started to develop at 1550 °C and was almost completely developed at 1600 °C. After sintering at 1650 °C, the porosity remained as high as 85.93% and the compressive strength reached 1.57 MPa. In a nonisothermal oxidation experiment, the maximum oxidation weight gain rate was 14.1% and the sample strength still reached 1.54 MPa at 1600 °C. A high sintering temperature promoted the development of material phases and liquefaction of grains, while the oxidized surface generated an amorphous layer with a high melting point, effectively preventing oxygen diffusion. Therefore, β-SiAlON can be employed to develop CFFs with high strength and excellent oxidation resistance.

Green synthesis, formation mechanism and oxidation of Ti3SiC2 powder from bamboo charcoal, Ti and Si

Using low-cost and highly reactive bamboo charcoal, Ti and Si elemental powders as starting materials, Ti3SiC2 powder was synthesized via a simple and cost-efficient pressureless sintering technique in argon atmosphere. The influences of synthesis temperature, holding time and Si content on the Ti3SiC2 content of the synthesized products were investigated, and the analysis indicated that the relative content of Ti3SiC2 reached 98.9 wt% with a molar ratio of 3Ti/1.2Si/2.2C at 1400 °C for 1.5 h. The Ti3SiC2 with good crystallinity and homogeneous nanolayered structure was synthesized at lower temperatures due to the high reactivity and high specific surface area of bamboo charcoal. The non-isothermal oxidation behavior showed that Ti3SiC2 powder was stable in air below 540 °C. With the temperature increasing up to 1300 °C, continuous and dense TiO2 and SiO2 oxidation layers were formed on the surface of Ti3SiC2 particles, which conferred good oxidation resistance to Ti3SiC2 powder.

Investigating non-isothermal oxidation kinetics of a non-stoichiometrically synthesized Ti3AlC2 MAX phase

Synthesis protocol plays a crucial role in the large-scale production of Ti3AlC2 MAX phases and its derivative MXenes. A non-stoichiometric pressure-less sintering approach is proposed to obtain a highly pure Ti3AlC2 MAX phase at 1500 ºC. The variation in aluminum content plays a vital role during the synthesis of Ti3AlC2. The impurity phase (TiC) was minimal when the non-stoichiometric ratio was Ti:TiC:Al = 1:0.75:1.3. Microstructure analysis confirmed the typical nano-laminated structure of the MAX phase. Non-isothermal oxidation stability of Ti3AlC2 was also examined through TGA/DTA technique. Deconvolution kinetic analysis was performed to distinguish three peaks corresponding to the reactions responsible for the oxidation of the MAX phase. The kinetic triplets were estimated by following iso-conversional kinetic methods. Kissinger-Akahira-Sunose (KAS) and Freidman (FR) iso-conversional methods were used to calculate activation energy for oxidation peak I (KAS: 107.25 kJ/mol, FR: 104.80 kJ/mol), II (KAS: 140.61 kJ/mol, FR: 178.24 kJ/mol) and III (KAS: 173.72 kJ/mol, FR: 217.63 kJ/mol). The integral master plot method revealed that F1 reaction mechanism dominated oxidation peaks I and III; for oxidation peal II, A2 reaction mechanism was responsible.

Non-isothermal oxidation induration mechanism of vanadium titanomagnetite pellets

To understand the non-isothermal oxidation induration mechanism of vanadium titanomagnetite (VTM) pellets, change of compressive strength, porosity, phase composition, and morphology were investigated in this study. It was found that the compressive strength of VTM pellets gradually increased under non-isothermal oxidation induration process; however, the porosity showed an opposite trend. The phase transformations of valuable elements could be described as follows: Fe3O4 → Fe2O3; FeTiO3 → Fe2TiO4 → Fe2Ti3O9 → Fe2TiO5; and Fe2VO4 → V2O3. The number of pores decreased and the size of hematite grains increased with the increase of induration temperature. The non-isothermal oxidation induration mechanism of VTM pellets was discussed by combining the experimental and characterization results. These findings can provide theoretical support for the utilization of VTM.

A Theoretical Description of the Ignition Conditions for TC17 Alloy in Oxygen-Enriched Atmospheres

ABSTRACT Titanium alloys can be ignited under extreme conditions including high pressure, high temperature, and high-speed friction. A theoretical description of the ignition conditions for titanium alloys is critical for their safe use in aerospace industries. In this study, a chemical-absorption-controlled ignition model for bulk TC17 alloy in oxygen-enriched atmospheres was developed based on Semenov’s thermal ignition theory. The adsorption coefficient, reaction order, and activation energy for the ignition of TC17 alloy were obtained by fitting the model with the experimental critical oxygen pressure and critical ignition temperatures. The fitting results showed that the chemical-absorption-controlled ignition model fitted better with the experimental data compared with the diffusion-controlled ignition model, implying that the ignition process was controlled by chemical absorption rather than oxygen diffusion through the oxidation layer. Moreover, the activation energy for the ignition of TC17 alloy bars was obtained to be 50.01 kJ/mol, which was only less than one-fourth of that of non-isothermal oxidation (231.98 kJ/mol), indicating that the ignition requires much lower activation energy than oxidation. The chemical-absorption-controlled ignition model successfully predicted the ignition temperature of TC17 alloy bars with different diameters with the relative error within 4.41%, demonstrating the validity of the model.

Thermo-oxidative behavior and kinetics analysis of light and heavy oils based on TG, DSC, and FTIR

In-situ combustion (ISC), as a time-honored replacement technology for heavy reservoir development, needs to improve its application effect urgently. The oxidation exothermic behavior and reaction activity of heavy oil directly determine the EOR effect of ISC technology during air injection. In this work, the oxidation heat effect and combustion activity of light and heavy crude oils, and the physical properties of their oxidized oils were studied and compared through thermogravimetry (TG), differential scanning calorimetry (DSC), static non-isothermal oxidation and Fourier transform infrared spectroscopy (FTIR). The results indicate that the main reaction pathway of MH light oil is distillation and volatilization under 200 °C, the mass loss is obvious and the heat release is more than that at the high temperature oxidation (HTO) stage. After the full reaction of FC heavy oil at 350 °C, there is still about 30% low temperature oxidation (LTO) left. After the LTO and fuel deposition (FD) stages, heavy oil can produce more coke. MH light oil and FC heavy oil show an obvious mass loss at the stage of LTO and HTO, which is 68.8% and 27.3% (10 °C/min) respectively, the corresponding enthalpies are 3.02 kJ/g and 15.668 kJ/g as well. Between 470 and 500 °C, the endothermic reaction of heavy oil becomes weak, which may relate to the condensation of heavy components such as resins that release heat. The LTO activation energy of light oil decreases with the conversion, and temperature increase can promote the oxidation reaction of active components. The LTO activation energy of heavy oil increases with the conversion, while that in FD and HTO stage increases obviously, which indicates that coke combustion activity is low. The content of C1–C6 in heavy oil decreases from 5.81% to 3.57% with heating rates. The oxidation reaction in the full temperature range can improve the quality of heavy crude oil. The vibration peak at 1707 cm-1 of the MH oxidized oil gets weak, the vibration peak at 1539 cm-1 disappears, and the number of carboxyls, ether groups, aromatic rings, and miscellaneous molecules in the oxidized oil declines. The vibration peak at 3460 cm-1 of the oxidized oil enhances significantly, and the number of carboxyls and hydroxyl molecules increased.

Crystalline structure development of coal during low-temperature oxidation under lean-oxygen environments caused by methane emission in gob

The residual coal in gobs is usually oxidized under a lean-oxygen condition due to methane emissions, thus affecting the crystalline structure of coal that is closely associated with heat transfer during low-temperature oxidation. This study set out to investigate the crystalline structure development of coal during low-temperature oxidation under this practical condition. For this purpose, non-isothermal oxidation and X-ray diffraction (XRD) experiments were employed to conduct lean-oxygen oxidation of coal and crystalline structure analysis, respectively. The results indicate that crystalline structure was closely associated with methane at different temperatures. Methane in gobs reduced the oxidation degree of coal, making the crystalline structure became less orderly; it restrained the condensation of crystalline structure, resulting in a less compact aromatic structure compared with that under aerial conditions. The condensation occurs between aromatic layers and along the plane of the aromatic layer (methane < 20.0 %), while it mainly happens between aromatic layers when methane was higher(>20.0 %). During self-heating, the rising methane increased the interlayer spacing of aromatic layers and reduces the crystallite height, the graphitization degree, and the number of effectively stacked aromatic layers. Methane’s influence on crystallite diameter was found to be sensitive to oxidation temperature. Crystallite diameter consistently grew with oxidation temperature regardless of methane, but the growth rate became apparent only when the oxidation temperature exceeded 150 ℃ and methane concentration was lower than 20.0 %. The findings of this research will promote our understanding of crystalline structure development under oxygen-lean conditions caused by methane emission in gobs.

High temperature oxidation behavior and mechanism of SiC-TaB2 composites

In order to improve the oxidation resistance of SiC in its ceramic matrix composites (CMCs), monolithic SiC ceramics doped with various proportions of TaB2 were prepared, isothermal and non-isothermal oxidation was investigated in detail as well. It was found that even a small proportion addition of TaB2 can effectively alleviate the intense oxidation of SiC between 1200 ºC to 1600 ºC, i.e., an oxidation degree of SiC can be reduced by 37% in the case with 10 wt% TaB2 under non-isothermal oxidation from room temperature to 1600 ºC. The oxidation resistance mechanism was also studied via analyzing the composition and microstructure of the formed oxide layer, it was found that the transformation of Ta2O5 in the multiphase oxide layer from grainy to flaky shape at about 1500 ºC plays the most important role in the excellent high temperature oxidation resistance of composites.

Experimental study on non-isothermal oxidative characteristics and kinetics of methane in accelerating rate calorimetry tests

The non-isothermal oxidative characteristics and kinetics of natural gas have important effects on performance of gas reservoir combustion, which directly determines temperature distribution, playing a significant role in hydrogen generation boundary. In this paper, the non-isothermal oxidation of methane, as main component of natural gas, is in depth analysis, which is rarely studied. At first, the non-isothermal oxidative characteristics at different pressure, oxygen/methane ratio, and debris were investigated by accelerating rate calorimetry (ARC). Afterwards, the non-isothermal oxidation kinetic parameters were analyzed and calculated. The difference of onset self-exothermic, ignition temperature, and kinetics parameters under different conditions were extraordinarily emphasized, aiming achieving successful and stable gas reservoir combustion for high efficiency hydrogen production. The results indicated that there is no even self-exothermic phenomenon at depleted reservoir condition (7.1–9.8 MPa and 100–150 °C), and the minimum starting self-exothermic temperature ignition temperature of methane oxidation are 220.6 °C and 312.7 °C. The pressure did not have effect on oxidation temperature in methane oxidation while did in catalytic oxidation. It was observed that rock debris has catalytic effect on methane oxidation suggested by obviously lower onset self-exothermic temperature detected and significantly smaller activation energy calculated in oxidation process of methane with debris. Moreover, the oxidation reaction rate of methane with debris is lower than that of methane, assisting more methane undergoes a more thorough HTO reaction conducive to combustion and decreasing the possibility of explosion in the injection air ignition process. According to the results obtained by ARC tests, we suggested that the near-wellhole places of gas reservoirs should be better heated over 313 °C for successfully achieving stable combustion. Some new knowledge got from this study is conducive to achieve rapid and stable combustion, thus achieving high-efficiency hydrogen generation.

Fabrication and oxidation behavior of β-SiAlON powders in presence of trace Y2O3

β-SiAlON powders were fabricated using Al, Si, α-Al2O3 and trace Y2O3 powders as starting materials in flowing nitrogen atmosphere. The effect of Y2O3 on oxidation behavior of the prepared β-SiAlON powders has been investigated. The results show that the Y2O3 contributed to the generation of β-SiAlON during the fabricated process. The weight gain ratio of β-SiAlON in presence of Y2O3 is higher than that of without Y2O3 during non-isothermal oxidation up to 1723 K, which may be due to the Y2O3 dissolving into the lattice of β-SiAlON resulting in lattice deformation. At the initial oxidation period, the incorporation of Y2O3 increases the oxidation rate and decreases the oxidation activation energy (Ea) of β-SiAlON due to the lattice deformation increasing the reactivity of β-SiAlON. However, at the final oxidation period, denser glass film formed on the surface of β-SiAlON because of the incorporation of Y2O3, which reduces the opportunity for O2 contact with β-SiAlON, thus decreasing the oxidation rate and increasing the Ea of β-SiAlON.

New insights into the non-isothermal oxidation of tight oil: Experimental study and theoretical prediction

A better understanding of the influence factor investigation and oxidation behavior prediction for the crude oil could supply valuable insights into the reactivity of crude oil and feasibility of air injection process. In this study, non-isothermal oxidation experiments have been conducted via thermal analyses (thermogravimetry/differential scanning calorimetry/pressurized differential scanning calorimetry, TG/DSC/PDSC) with different heating rates (5, 10, and 15 °C/min) and experimental pressures (1, 3, and 5 MPa) in order to investigate the influences on oxidation behaviors, exothermic characteristics, and kinetic parameters of typical tight oil. In addition, the distributed activation energy method (DAEM) was employed to determine accurate kinetic parameters for whole oxidation process and to further predict the oxidation behavior with the reconstruction of TG and conversion curves. The results showed that, heating-rate increment would lead to inadequate oxidation process with more reaction occurrence in a shorter time. Inversely, more rapid heat release, higher exothermic peaks with lower temperature ranges were detected in PDSC curves, indicating high pressure in actual oil reservoir would strengthen the oxidation progress with more heat generation to enhance oil recovery. Then, based on specific activation energy distribution and DAEM, the calculated TG and conversion curves presented a satisfactory fitting relation with experimental data, demonstrating a reasonable prediction of oxidation behavior and kinetic characteristic for the crude oil under specific heating-rate. The understandings in this study not only provided new insights on the verification of obtained kinetic parameters, but also the accurate prediction of crude oil oxidation behavior.

Control of ZM5 alloy ignition via PEO/aluminum phosphate composite coating

A composite coating consisting of the plasma electrolytic oxidation (PEO) and aluminum phosphate layer was fabricated to improve the ignition resistance of ZM5 alloy. The influences of PEO coating and PEO/aluminum phosphate composite coating on the ignition resistance of ZM5 alloy were investigated by torch ignition and non-isothermal oxidation test. The ZM5 alloy with PEO/aluminum phosphate composite coating possesses the best ignition resistance and does not ignite when exposes to flame for 20 min, about 16 min longer than that of ZM5 alloy with PEO coating, and about 18 min longer than that of bare ZM5 alloy. The ignition protection was achieved through synergetic combination increased oxidation resistance, high strengthening and thermal expansion coefficient match of the composite coating. The incombustible ceramic component and metallurgical bond to the substrate endow the PEO coating with a self-extinguishing property. In addition, three characteristic stages (protective oxidation, non-protective oxidation and burning) of ZM5 alloy and its coating were identified based on the non-linear mass gain behaviors at different temperatures. The ZM5 alloy with PEO/aluminum phosphate composite coating had higher non-protective oxidation initial temperature (605 °C) than 437 °C of the bare ZM5 alloy. Meanwhile, the non-protective oxidation energy of the ZM5 alloy coated with PEO/aluminum phosphate composite coating was remarkably improved from 61 kJ/mol to 136 kJ/mol.

New Insights into the Non-Isothermal Oxidation of Tight Oil: Experimental Study and Theoretical Prediction

New Insights into the Non-Isothermal Oxidation of Tight Oil: Experimental Study and Theoretical Prediction