This study introduced differential photocalorimetry (DPC) as a method for real-time monitoring of the photo-oxidation kinetics of vegetable oils. DPC measures the heat flow generated during the ...oxidation of oils upon light exposure. Experiments conducted with stripped linseed oil (SLSO), an oil depleted from its natural antioxidants, showed no induction time (τ). Conversely, spiking SLSO with increasing concentrations of trans-ferulic acid resulted in an induction time (τ) proportional to the antioxidant concentration (R2 = 0.99). A comparative study among different vegetable oils revealed that rice bran oil exhibited the highest resistant to photo-oxidation, followed by corn, soybean, and sunflower oils. The results are discussed in terms of sample oxidizability and antioxidant efficiency (A.E.), and validated through high-performance liquid chromatography with diode array detection (HPLC-DAD). Furthermore, the measured heat flow enabled the determination of the rates of inhibited (Rinh) and uninhibited (Runi) periods, as well as the rate constant of propagation (kp) and inhibition (kinh) reactions.
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•DPC allowed real-time monitoring of oil photo-oxidation kinetics.•An increase of the light intensity showed a consistent decrease in induction time.•DPC enabled determination of antioxidant efficiency, capacity and oil oxidizability.
Mine fires caused by spontaneous coal combustion are major disasters in coal mines. The staged oxidation kinetic parameters of various coal samples at oxygen concentrations of 21 %, 15 %, 10 %, 5 %, ...and 3 % were analyzed using a programmed temperature testing system. Herein, the temperature increase rate of coal, the temperature difference between the furnace and coal, and the oxygen consumption characteristics were obtained. Based on the amount of CO produced and the temperature sensitivity coefficient, three characteristic temperatures and four stages of low-temperature oxidation (LTO) were identified. The results showed that at a critical temperature (TC), the amount of CO gas released from the coal samples increased with increasing oxygen concentration, and the difference in the oxygen consumption rate increased. After the limit temperature (Tu), the amount of CO gas increased steadily, and the increase in the oxygen consumption rate stagnated. CO production, the maximum heating rate, and the maximum heat release rate were positively correlated with the oxygen concentration. As the oxygen concentration increased, the activation energy during the oxygen absorption stage gradually decreased. The average reaction enthalpy (ΔH) of pre-oxidized water-immersed coal was 19.37 kJ/kg greater than that of raw coal. The equation for the conservation of energy of the coal oxidation warming process was normalized. The theoretical values of the awakening stage and the stable stage were τν and τν (1-B), respectively. When B was >1, pre-oxidized water-immersed coal at a low oxygen concentration was prone to crossover points during the oxygen absorption stage, which increased the risk of coal spontaneous combustion (CSC). The research results could provide a theoretical basis for the staged control of the spontaneous combustion of water-immersed coal in goaf areas.
The isothermal steam oxidation experiments of Cr-coated Zr alloy cladding at 1350 ℃ were conducted, and the influence of Cr-Zr eutectic reaction on the oxidation behavior was investigated. The ...oxidation duration, ranging from 50 s to 3600 s, covers the cases from the initial formation of eutectic liquid to the complete oxidation of the cladding. The microstructures of the coating and the substrate were characterized. The eutectic-oxidation degradation mechanism of the coating is summarized. The migration of the eutectic liquid and the evolution of Cr-rich phases in the post-oxidation eutectic structure are studied and discussed, confirming the process of ZrCr2 reacting with α-Zr(O) to form Zr3Cr3O. The degraded Cr coating and the Cr retained in ZrO2 during the migration of the eutectic liquid can hardly affect the oxidation of cladding substrate. However, the Cr-rich phases, which eventually aggregated in the radial center of the cladding, can reduce the oxidation rate.
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•Systematic isothermal steam oxidation experiments of Cr-coated Zr alloy cladding at 1350 ℃ were conducted.•Microstructural evolution mechanism of Cr coating and Zr substrate induced by eutectic and oxidation reactions is summarized.•The formation of Zr3Cr3O during the oxidation of Cr-Zr eutectic liquid is confirmed.•The oxidation kinetics are summarized and the influence of Cr-rich phases is analyzed.
The isothermal steam oxidation behavior of Cr-coated Zr-1Nb alloy at 1350 ℃ and 1400 ℃ was investigated. The temperatures are above the Cr-Zr eutectic temperature. The reliability of the experimental ...setup and methodology was confirmed by conducting isothermal steam oxidation experiments on bare Zr-1Nb specimens at 1100∼1400 ℃, and the corresponding oxidation kinetics correlation was obtained. After the eutectic-oxidation reaction, the surface of the coated specimens exhibited a “crocodile skin” morphology composed of “ridges” and “hollows”, which was attributed to the non-uniform eutectic reaction. Due to the much higher O affinity of Zr compared to Cr, and the higher solubility of Cr in β-Zr compared to α-Zr(O) and ZrO2, the ZrO2 and α-Zr(O) layers initially grew on the Cr-Zr eutectic during oxidation. Cr continuously diffused inward and eventually precipitated as Cr-rich phases within the metallic matrix. Three types of Cr-rich phases were observed, with varying oxygen content and reflecting the oxidation process of the Cr-rich phases. The Cr-rich phases were ultimately oxidized into a mixture of Cr2O3 and ZrO2. The parabolic weight gain rates for the oxidation of Cr-Zr eutectic are compared with the oxidation kinetics correlation derived from the bare Zr-1Nb specimens, and the model applicability of the oxidation kinetics correlations of Zr alloys in the simulations of Cr-Zr eutectic-oxidation reaction is discussed.
In this study, the isothermal oxidation kinetics of magnetite (OM), high-Mg magnetite (MM), titanomagnetite (TM), and chromite (CM) were investigated by applying thermogravimetry (TG) analysis at ...temperatures ranging from 1073 K to 1223 K. The results show that different high-FeO spinels possess distinct oxidizability. The oxidation process of OM in the temperature range from 1073 K to 1223 K is faster than others, followed by MM and TM. While CM exhibits the poorest oxidizability, and generally undergoes complex phase transitions. In the initial stage of oxidation, high FeO spinels have a higher oxidation rate due to the surface oxidation of spinel particles. However, the oxidation rate gradually declines in the later stages of oxidation due to increased internal diffusion resistance. The results of oxidation kinetics indicate that the initial oxidation stage of four spinels can be described as random nucleation and subsequent growth mechanism. The average apparent activation energies of the initial oxidation stage of OM, MM, TM, and CM are 25.09 kJ/mol, 32.39 kJ/mol, 58.10 kJ/mol, and 82.42 kJ/mol, respectively.
This study aimed to analyze how the addition of titanium (Ti) to TiAlTa alloys affects their mechanical properties (Young’s modulus and hardness) and oxidation behavior. Alloys with three different ...Ti additions and equi-atomic Al and Ta were fabricated with nominal compositions of 33Ti-33Al-33Ta at%, 50Ti-25Al-25Ta at%, and 70Ti-15Al-15Ta at%. Phase identification of as-processed and compositionally homogenized alloys was conducted by coupling various electron microscopy, diffraction, and chemical analysis techniques. Composition and structures were modeled using CALPHAD-based phase predictions and compared with experimental results. Each of the alloys exhibited a unique set of microstructures with distinctly different ordered precipitates, dependent on cooling rate and chemical composition. Subsequently, nanoindentation tests were performed at temperatures of 25°C, 250°C, 500°C, and 750°C. Oxidation studies were conducted under static air at 750°C for up to 200 h. 33Ti-33Al-33Ta at% and 70Ti-15Al-15Ta at% alloys showed higher hardness and modulus than 50Ti-25Al-25Ta at% at room temperature. The 50Ti-25Al-25Ta at% alloy exhibited higher hardness and modulus values but lower oxidation resistance at 750°C when compared with the other specimens. The X-ray photoelectron spectroscopy experiments were conducted to further analyze the oxides present in the samples. Despite each sample displaying combinations of mixed oxide layers, TiO2 was the preferred oxide forming in the 50Ti-25Al-25Ta composition, while the other samples preferred to form Al2O3. In conclusion, the findings from this systematic exploration of phase evolution in the TiAlTa alloy space provide insights for optimized material chemistries, processing, and properties.
The isothermal oxidation behavior of magnetron-sputtered Cr-coated Zr-1Nb alloy cladding in 1000–1300 ℃ steam environment is studied. The phenomena at each temperature are summarized and the ...oxidation kinetics are discussed. Cr coating can maintain the oxidation resistance for more than 6 h at 1000 ℃ and 1100 ℃ under double-sided oxidation condition, while suffered degradation during long-duration oxidation at 1200 ℃ and 1300 ℃. After coating degradation, Zr substrate was oxidized prior to residual metallic Cr and the metallic Cr was not completely oxidized until the ZrO2 layer beneath the coating had grown to a certain thickness. Mainly due to the redox reaction between Zr and Cr2O3 and the preferential oxidation of Zr substrate, the thickness of Cr2O3 film decreased after a certain oxidation duration. The Cr coating after degradation at 1200 ℃ can still hinder the oxidation of Zr substrate to a certain extent, while that at 1300 ℃ almost lost the protective effect.
•Isothermal steam oxidation of Cr-coated Zr alloy was performed in temperature range of 1000–1300 ℃.•Oxidation kinetics of Cr-coated Zr alloy cladding was summarized.•Oxidation behavior of Zr substrate after coating degradation were discussed.
Karst environments are an important but often overlooked global sink of atmospheric CH4. Little is known about methanotrophic bacterial communities in karst soils; consequently, the effects of land ...use change (LUC) and soil management practices on them and how they function as part of the soil CH4 sink are unclear. In this study, we compared net soil CH4 fluxes, kinetic parameters (Km and Vmax) of CH4 oxidation, and the pmoA gene of CH4 oxidizers at five test soil plots in karst areas that have different soil types and land use types. The maximum atmospheric CH4 uptake rate (−5.62 ± 3.14 kg ha-1 y-1) occurred in reverting scrublands just 10 years after they had been utilized for crop cultivation; followed by natural forest (−4.30 ± 4.02 kg ha-1 y-1) in limestone soils. The cultivated soils in the cropland emitted CH4 into the atmosphere. The difference in net CH4 fluxes (−8.21 kg ha–1 y–1) between soils in cropland and soils in reverting scrubland from cropland was 4–8 times higher than the global average values, suggesting that reforestation on limestone soils in karst cropland areas has much greater potential to enhance soil CH4 oxidation than in non-karst areas. Soil moisture is the most likely environmental factor to trigger the enhancement effect of reforestation. The remarkable corresponding relationships between Km, net soil CH4 fluxes, and methanotrophic compositions in these plots suggest that bacteria play an essential role in regulating the effects of LUC on soil CH4 oxidation rates. It is type Ⅰ methanotroph USCγ and JR3 (USCγ sensu lato), not type II methanotrophs USCα as in previous studies, that dominate in enhancing soil oxidation of atmospheric CH4 since reforestation on limestone soils. The specific unstable habitats in karst settings favor USCγ who, as r-strategists, can change rapidly to adapt to new conditions formed from LUC and then grow presumably at an exponential rate. This coupling of environmental and microbial factors can well explain the great and fast response of CH4 oxidation in limestone soils. Our study provides new insights that reforestation of cropland in karst areas may be a noticeably more efficient way to potentially mitigate atmospheric CH4 build-up and thus should be strongly encouraged.
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High temperature isothermal oxidation of equimolar AlFeCoNi was conducted at 1000 °C by confocal scanning laser microscopy (CSLM) and thermogravimetric analysis (TGA), respectively, to investigate ...the properties of oxide scales. The formation of a heterogeneous oxide scale was observed in-situ by CSLM. Combined with SEM images and EDS analysis, it was found that the bulk oxide scale was Al2O3, with some dispersed Fe-Co-Ni-rich oxides after 3 h of oxidation. After 24 h of oxidation, a Fe-Co-Ni-rich oxide scale with dendritic morphology was observed, and Al2O3 scale spalling tended to occur in adjacent areas. According to TGA during the initial isothermal oxidation period (ca. 0.2 h), the mass gain and oxidation time followed a parabolic relationship well with an estimated parabolic rate constant of 1.46 × 10−4 mg2·cm−4·s−1. As oxidation continued, the mass change experienced a short decrease due to the spalling of the Al2O3 scale. After around 6.7 h of oxidation, the mass gain then had an approximately linear relationship with oxidation time, whose linear rate constant was estimated to be 1.06 × 10−6 mg·cm−2·s−1. The results indicated that the Al2O3 scale re-formation occurred on the exposed metal surface, which showed a self-healing property.
•The surface morphology changes during oxidation were observed in-situ.•The spalling of Al2O3 scale was confirmed.•The entire oxidation process could be divided into three stages based on the mass change per unit area.•The parabolic rate constant was estimated as 1.46 × 10−4 mg2·cm−4·s−1 during the initial period of oxidation.•In the wake of mass loss, mass gain occurred again with an estimated linear rate constant of 1.06 × 10−6 mg·cm−2·s−1.
The oxidation behavior of a Mo51Co34B15 metallic glass was investigated at 873–973 K in dry air. The oxidation kinetics generally followed a parabolic-rate law, which indicated the rate-controlling ...step in the oxidation process is diffusion, and the parabolic-rate constants increased with increasing temperature, showing the oxidation rate is closely related to temperature. The thinner oxide layer was mainly composed of CoMoO4, MoO3, CoO, B2O3, and MoB, while the CoMoO4 and B2O3 oxides resulted in a lower oxidation rate, and the presence of MoB also suggested that small amounts of crystallization were taking place beneath the inner layer.
•The oxidation behavior of refractory MoCoB metallic glass is studied initially.•This system shows a superior oxidation resistance at higher temperatures.•Spinel-structured oxide provides a beneficial effect on reduced oxidation rates.
