Increasing Oxidation Temperature Research Articles

Raising the Oxidation Resistance of Low-Alloyed Mg-Ca Alloys Through a Preheating Treatment in an Argon Atmosphere

With the rise and development of aerospace, communications, electronics, medical, transportation and other fields, magnesium (Mg) and its alloys have attracted much attention for their high specific strength and stiffness, good electromagnetic shielding properties, excellent damping properties and other advantages. However, magnesium has a high affinity for oxygen, producing magnesium oxide (MgO), and MgO’s Pilling–Bedworth ratio (PBR) of 0.81 is not protective. The occurrence of catastrophic oxidation is unavoidable with the increase of oxidation time and temperature. A promising approach is to perform an appropriate pretreatment in conjunction with alloying to obtain a dense and compact composite protective film. In this work, the effect of a preheating treatment on the oxidation resistance (OR) of Mg-xCa (x = 1, 3 and 5 wt. %) was investigated. The preheating was carried out in an Ar atmosphere at 400 °C for 8 h. Upon it, a dense and compact MgO/CaO composite protective film was formed on the surface, which is CaO-rich especially in the vicinity to the surface. The alloys’ oxidation resistance was strongly increased due to the composite protective film formed during the preheating treatment, in particular for Mg-3Ca. Relative to the Mg-hcp phase, the OR of the Mg2Ca phase was significantly raised.

Characterization of hot‐pressed ZrC–TiC composites

AbstractSynthesis and consolidation of (Zr,Ti)C were investigated in the present work. ZrC–TiC powder mixture was synthesized at 1650°C for 90 min by the carbothermal method. The prepared powder mixture was hot‐pressed at 1800 C for 60 min under a pressure of 40Mpa and then heat‐treated at 2100 C for 60 min. There were ZrC‐rich and TiC‐rich phases together with zirconium oxide phases in powder mixture and hot‐pressed samples, while there were no oxide phases in the heat‐treated sample and only ZrC‐rich and TiC‐rich phases were present in the sample. The FESEM image of the fracture surface of the samples shows that the porosity of the samples decreases with increasing temperature. Relative density, microhardness, and flexural strength increased from 93.8% to 98.6%, from 15.26 to 21.41 GPa, and from 185.02 to 220.69 MPa, respectively, with increasing temperatures from 1800 C to 2100 C. In contrast to these properties, the fracture toughness of the samples was reduced from 3.71 to 3.47 MPa·m1/2. The investigation of oxidation behavior also showed that oxidation is influenced by oxygen diffusion and exhibits nonlinear behavior as a function of time. With increasing oxidation temperature of hot‐pressed and heat‐treated samples from 600 to 1000°C, parabolic oxidation rate constants (kp) increased from .267 to 11.793 and from .367 to 10.993, respectively.

Effect of Water Immersion on the Reoxidation Capacity of Coal with Different Secondary Oxidation Degrees

ABSTRACT Water accumulation and secondary oxidation of residual coal are commonly encountered in goaf, which can endanger the safety of coal mines. Both factors can affect the oxidation capacity of coal. Hence, it is essential to investigate the reoxidation capacity of residual coal with secondary oxidation in goaf after water immersion. This helps reveal the influence of different degrees of water immersion on coal samples with varying degrees of secondary oxidation. In this study, a programmed temperature system was used to examine the re-burning capacity of bituminous coal under the combined effects of secondary oxidation and water immersion. An electron spin resonance spectrometer was employed to examine the alteration in free radicals present for analyzing the evolution of microscopic groups of coal. According to the research results, under the same soaking conditions, an increase in secondary oxidation temperature leads to a decrease in the reoxidation capacity of coal. During re-oxidation, the immersion in water accelerates the process in bituminous coal with a secondary oxidation temperature of 120°C, while it has a certain inhibitory effect on coal samples with a secondary oxidation temperature of 180°C. This research lays a theoretical foundation for comprehending the re-oxidation of coal under complex conditions.

Influence of phospholipid structures on volatile organic compounds generation in model systems

To investigate the regularities and differences in oxidation products of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) by gas chromatography-mass spectrometry (GC–MS), and examine the influence of variations in fatty acid compositions and head groups on the kinds and contents of volatile organic compounds (VOCs) generated. A total of 42 VOCs were identified from PE (16:0–18:2), PC (16:0–18:2), and PC (16:0–18:1), with aldehydes and ketones being the main VOCs in three phospholipids (PLs). The content of most VOCs produced by PE (16:0–18:2), PC (16:0–18:2), and PC (16:0–18:1) increases with the increase of oxidation temperature and time. Reached peak at 175 °C for 60 min. The total VOCs contents generated by PE (16:0–18:2) and PC (16:0–18:2) were higher than those produced by PC (16:0–18:1), with PC (16:0–18:2) showing the highest total VOCs contents. PLs exhibited three mass loss processes with increasing temperature, namely stability, reduction, and stabilization. PC (16:0–18:2) experienced the highest mass loss, followed by PE (16:0–18:2), while PC (16:0–18:1) showed the least mass loss. These findings showed that polyunsaturated fatty acids were more susceptible to oxidation and degradation during oxidation, and the presence of choline groups in the form of PE may enhance the oxidative stability of fatty acyl groups compared to PC.

Preparation and properties of Ta fiber reinforced high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2-SiC composite ceramics

High-entropy boride ceramics are expected to be widely used in aerospace, automotive turbines, and armor protection due to their advantages of high melting point, high hardness, adjustable performance, high-temperature stability, and good oxidation resistance. However, it is urgent to solve the problem of low fracture toughness before application. Therefore, in this paper, a single-phase high-purity (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 powder was prepared by boron/carbon thermal reduction method using a vacuum furnace. The effects of synthesis temperature and C content on the powder were studied. Secondly, HEB ((Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2) powder, SiC powder, and chopped Ta fiber were mixed uniformly, and Ta fiber toughened HEB-SiC composite ceramics were prepared by spark plasma sintering (SPS). The effects of Ta fiber content on the phase composition, microstructure, mechanical properties, and oxidation resistance of the composite ceramics were investigated. The results show that with the increase in synthesis temperature, the HEB powder gradually dissolves, and the solid solution is completely formed at 1700 °C. As the C content increases, the oxygen content and particle size of the powder gradually decrease. Single-phase high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 powders with high purity were prepared at 1700 °C for 1 h with 6 wt% C content. The addition of C will promote the boron/carbon thermal reduction method, reduce the oxygen content, and inhibit grain growth. With the increase of Ta fiber content, the density of HEB-SiC-Taf composite ceramics increased first and then decreased. The hardness gradually decreased, and the fracture toughness gradually increased. When the Ta fiber content was 7 vol%, the fracture toughness was the highest, reaching 5.12 ± 0.39MPa⋅m1/2, which was nearly 45 % higher than that of the composite ceramics without Ta fiber. This is because of the synergistic toughening mechanism of metal toughening and fiber toughening, such as crack deflection, crack bridging, fiber debonding, and fiber pullout, which improves the fracture toughness of the composite ceramics. With the increase in oxidation temperature, B2O3, SiO2, Ta2O5, and various metal oxides appear on the surface of HEB-SiC-Taf composite ceramics. The oxidation depth and weight gain per unit area gradually increase. When the Ta fiber content is 5 vol%, the composite ceramics exhibit the best high temperature stability and oxidation resistance. This is due to the Ta2O5 formed by the oxidation of Ta fibers, which dissolves into the B2O3 glass phase, increasing viscosity and improving high temperature stability while reducing the oxygen diffusion rate.

Evolution of high temperature oxidation properties of SiCf/SiCN composites with BN interphase

In this study, SiC/SiCN composites with BN interphase were prepared using chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP) methods. The evolution and related mechanisms of mechanical and microwave absorption properties of the composites after oxidation at 800 °C, 1000 °C, 1200 °C, and 1400 °C were investigated. The results showed that with the elevating of oxidation temperature, the flexural strength of SiC/SiCN composites gradually decreased, with a retention rate of 56.7 % after oxidation at 1400 °C. The main reasons for the deterioration of mechanical properties were the changes in the crystal structure of SiC fibers under high-temperature conditions and the formation of brittle SiO2 phases on the surface and interior of the SiCN matrix. With the increase in oxidation temperature, the microwave absorption performance of the composites showed an enhancing trend. The main reasons were that after oxidation treatment, the reduction of free carbon content and the formation of SiO2 film layers from the oxidation of SiC and SiCN led to a decrease in the complex permittivity of the composites, resulting in better impedance matching and dielectric loss capabilities. The contributions of BN interphase to stress dispersion, interface polarization, and oxidation protection significantly enhance the mechanical and electromagnetic absorption properties of the composites. Therefore, SiC/SiCN composites are high-temperature structural microwave absorbing materials with great application prospects.

Evolution of macromolecular structure during coal oxidation via FTIR, XRD and Raman

The analysis of the macromolecular structure and morphology in coal during oxidation is the basis to explore the mechanism of spontaneous combustion. To explore the evolutionary rules of coal macromolecular structure during oxidation, Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Raman Spectroscopy (Raman) were employed to analyze the coal samples with different oxidation degrees. The results revealed that the oxidation action led to the decrease of the aliphatic structures and aromatic hydroxyl groups in coal, while promoting the formation of oxygen-containing functional groups and aromatic structures. It also led to a relative increase of free hydroxyl groups linked to hydrogen bonds. The aromatic layer spacing (d002) decreased with increasing oxidation degree, while the microcrystal stacking height (Lc), the aromatic layer diameter (La), the average number of crystal stacking layers (n) generally increased. It indicated that small aromatic ring molecules in coal could undergo continuous polymerization during oxidation to form a single aromatic layer structure. The variation of Raman spectrum parameters exhibited a consistent decreasing trend in WD/WG, ID/IG, AD/AG, and A(GR+SL)/AG value, indicating an increase in the vibration of sp2 hybridization carbon atoms within the lattice structure of coal. Conversely, PG-D, AS/AD and A(GR+VL+VR)/AD value increased overall, suggesting that small aromatic rings decreased in content during oxidation while polymerizing into larger aromatic rings. The coal structure underwent a brief stage of disordered evolution during oxidation, followed by removal of impurity structures and condensation of aromatic structures due to increasing oxidation temperatures, ultimately leading to a highly ordered crystalline state. The oxidation process significantly influenced the development of coal's aromatic structure, particularly in less metamorphic coal. The research findings provide a theoretical basis for analyzing the underlying mechanism behind spontaneous combustion induced by coal oxidation.

Effect of nano-graphene lubricating oil on particulate matter of a diesel engine

Nano-graphene lubricating oil with appropriate concentration shows excellent performance in reducing friction and wear under different working conditions of diesel engines, and has been widely concerned. Lubricating oil has a significant impact on particulate matter (PM) emissions. At present, there are few studies on the impact of nano-graphene lubricating oil on the physicochemical properties of PM. In order to comprehensively evaluate the impact of nano-graphene lubricating oil on diesel engines, this paper mainly focused on the effects of lubricating oil nano-graphene additives on the particle size distribution and physicochemical properties of PM. The results show that, compared with pure lubricating oil, the total number of nuclear PM and accumulated PM of nano-graphene lubricating oil is significantly increased. The fractal dimension of PM of nano-graphene lubricating oil increases and its structure becomes more compact. The average fringe separation distance of basic carbon particles decreases, the average fringe length increases. The degree of ordering and graphitization of basic carbon particles are higher. The fringe tortuosity of basic carbon particles decreases, and the fluctuation of carbon layer structure of basic carbon particles decreases. Aliphatic substances in PM are basically unchanged, aromatic components and oxygen functional groups increase. The initial PM oxidation temperature and burnout temperature increase, the maximum oxidation rate temperature and combustion characteristic index decrease, and the activation energy increases, making it more difficult to oxidize. This was mainly caused by the higher graphitization degree of PM of nano-graphene lubricating oil and the increased content of aromatic substances.

Superior oxidation resistance of a Y-Hf co-doped Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy at 1100–1300 °C

We report a study on the oxidation behavior of Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy co-doped with reactive elements (RE) Y and Hf at 1100–1300 °C. The highly stable eutectic microstructure of γ’/β phases within this alloy contributes to a low coefficient of thermal expansion, thus a low residual stress in the Al2O3 scale. The spinel with a low thickness is formed at the top of Al2O3 scale at 1100 °C due to the low Al diffusion coefficient of γ’-phase and low aspect ratio of β-phase in maze-like eutectic region. As the oxidation temperature increases, the oxidation products transition to exclusive Al2O3 at 1200 °C and 1300 °C, facilitated by the increased Al diffusion coefficient. The Al2O3 scale exhibits a dominated columnar grain microstructure at 1100–1300 °C, suggesting that the inward O diffusion plays more critical role than Al diffusion and thus leads to the low oxidation rate. Besides, the segregation of S to scale/metal interface is inhibited by the uniform RE distribution in the alloy resulting from the fine eutectic microstructure in the alloy. Overall, this alloy exhibits low residual stress in the oxide scale, slow oxidation rate, and the lack of interfacial sulfur segregation, thus contributing to its superior oxidation resistance. The strong oxidation resistance of Y-Hf co-doped Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy makes it a potential candidate for advanced oxidation-resistant alloy or coating applications.

Study on the Mechanical Degradation and Destruction Characteristics of Coal Oxidation

ABSTRACT Deep coal bodies are often in high stress and high temperature environments. To study the effects of stress and temperature on the mechanical degradation and failure characteristics of coal samples, nuclear magnetic resonance experiments were carried out before and after coal sample oxidation to analyze the effect of oxidation on the porosity of the coal samples. Through mechanical property experiments, the changes in the mechanical properties of the coal samples at different oxidation temperatures is analyzed. Then coal samples of different particle sizes are screened and statistically analyzed to determine the relationship between the fractal dimension of the broken block size and the oxidation temperature. A stress model for the oxidized coal samples is established to analyze its mechanical degradation and its breaking mechanism. Based on the results, As the oxidation temperature increases, The total porosity of the coal samples show an overall increasing trend, the elastic modulus, secant modulus, Poisson’s ratio, and shear modulus show a downward trend, and the destructive strain energy density decreases from 43.33 kg•m−3 to 15.47 kg•m−3, indicating the oxidized coal sample increases in plasticity and decreases in stiffness, resulting in a decrease in the amount of load the sample can carry. The fractal dimensions of the broken block size from raw coal to 200°C coal samples are 2.008, 2.092, 2.150, 2.159, 2.175, 2.183, 2.314, and 2.328. Oxidation leads to the formation a network of tiny fractures inside the coal samples; thus, the broken block size becomes more complex with worsened integrity. A model for the ability of the oxidized coal samples to resist destruction is established, and the diameters of unoxidized areas in oxidized coal samples are calculated to be 45.053, 44.956, 43.031, 41.931, 37.442, 28.081 and 13.778 mm. An analysis of the breaking mechanism of oxidized coal samples was carried out. Our results are beneficial for the safe operation of deep coal seam mining.

High temperature oxidation behaviors of Al/Cr composite coating on Ti2AlNb alloy

The Al/Cr coatings were deposited on Ti2AlNb alloy by magnetron sputtering and double glow plasma alloying technology to improve the oxidation resistance. The morphology of the protective coating was investigated by scanning electron microcopy and the phase composition of the coating was analyzed using x-ray diffraction. The results suggested that the protective coating with multilayer structure was dense and homogeneous. The formation of Cr inner diffusion layer was beneficial to the bonding strength between the coating and substrate, which was attributable to the double glow technology. The oxidation behaviors of Al/Cr coating were investigated by the isothermal oxidation tests for 100 h at 700 °C, 800 °C and 900 °C, respectively. The results indicated that the protective coating showed a lower oxidation rate. At 700 °C, the Al2O3 oxidation products were formed on the surface of coating at high temperature, which were dense and homogeneous against oxygen diffusion. With the increase of oxidation temperature, the external diffusion of Cr element reacted with oxygen to form Cr2O3, which provided the further protection. As for 900 °C, the volatile CrO3 was formed by the reaction of the external diffusion of Cr and Cr2O3, and the oxidation products Al2O3 and Cr2O3 continued to provide protection for Ti2AlNb alloy.

Molecular simulation study of microstructural evolution during low-temperature oxidation of coal

In this study, industrial analysis, 13C NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), and in-situ infrared spectroscopy were employed to determine the elemental composition and structure of Tingnan coal. The peak area, pore volume, and specific surface area were used as the main parameters to investigate the structural changes of coal molecules and pore evolution during low-temperature coal oxidation, using an automated surface area and pore size analyzer. The analysis revealed that with increasing oxidation degree, the CH groups in the aromatic structure gradually decreased, while the total content of oxygen-containing functional groups such as alcohols, phenols, carboxyls, and carbonyls remained relatively constant. However, the relative content of hydroxyl groups and ether linkages increased with increasing oxidation temperature. Additionally, coals with different oxidation degrees exhibited well-developed micropores, and the specific surface area gradually decreased while the pore size increased with temperature. Molecular models of Tingnan coal in its original state and oxidized at temperatures of 50 °C, 70 °C, and 110 °C were constructed using Materials Studio software. Based on these models, pores with diameters of 2.79 nm, 3.07 nm, 3.07 nm, and 3.11 nm and a length of 4.5 nm were constructed using the coal molecular structure as the pore walls.

Surface characteristics and re-ignition law of water-soaked coal in a coal mine closed fire area

There are different degrees of oxidized coal samples in the gobs of coal mines, and there will be hidden dangers of re-ignition of water-soaked coal after the fire zone is closed. Using an atomic force microscope (AFM) and a simultaneous thermal analyzer, the re-ignition properties of coal samples and the pore dynamic evolution in different water-immersed areas of the goaf were studied. The results show that the IO (oxidation after water immersion) coal sample has a more microporous structure and larger fractal dimension when oxidized at 300 °C. The roughness of OI (water immersion after oxidation) coal samples decreases as the oxidation temperature increases. The re-ignition temperature points of the IO300 and O200I coal samples are lower, and the combustion heat release is greater. Before reaching 311 °C, the oxidation kinetics curve conforms to the cubic polynomial ln[-ln(1-a))/T2] =A(1/T)3-B(1/T)2 + C(1/T)-D. After reaching 311 °C, the activation energy of IO300 and O200I coal samples is smaller and more prone to oxidation and re-ignition, which conforms to ln [- ln (1-a))/T2] =A (1/T) – B. Different control measures can be taken in different areas and points based on the oxidized degree and different immersion methods to prevent coal spontaneous combustion (CSC) in closed fire areas.

Effect of Resveratrol on physicochemical structure evolution of lignite during spontaneous combustion

Studying the evolution of the physicochemical structure of coal during coal spontaneous combustion (CSC) by antioxidants is of great significance for revealing the mechanism of CSC and preventing it. The effects of three concentrations of Resveratrol (RES) on the physicochemical structure evolution of coal during the low-temperature oxidation (L-TO) process of CSC were comprehensively analyzed through low-field nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) experiments. The results showed that the pore cracks in the coal develop more fully with the increase in oxidation temperature. Besides, there was an increase in porosity and permeability, and a full development of pore throat. Based on the T1-T2 2D fluid identification results, it was found that as the oxidation temperature increased, the adsorption capacity and permeability of the coal were stronger. RES can effectively inhibit the CSC process of coal, with a maximum reduction of 23 % in porosity and a maximum reduction of 82 % in permeability. The hydroxyl groups and CH3/CH2 of coal samples treated with RES decreased by a maximum of 42 % and 43 %, respectively. The oxygen-containing functional groups increased by a maximum of 77 %. RES can not only cover on the coal surface to prevent oxygen from seeping into the coal, but also infiltrate into the pores of coal to inhibit the combination of the active functional groups and oxygen molecules.

Optimization of multiscale structure and electrochemical properties of bamboo-based porous activated biochar by coordinated regulation of activation and air oxidation

Activated biochar prepared by a single pyrolysis process often has poor pore structure and insufficient surface oxygen-containing groups, which severely inhibit its application in supercapacitors. Herein, we synthesized bamboo-based porous activated biochar (PAB) via coordinated regulation including carbonization at 450 °C, ZnCl2 activation at 400 ∼ 800 °C, and air oxidation at 200 ∼ 350 °C. The results showed that the coordinated regulation could efficiently improve the physicochemical structure and electrochemical properties of PAB. Air oxidation exhibited an obvious improving effect on the activated carbon prepared with mid-temperature activation (optimal at 600 °C), which was strengthened with increasing oxidation temperature from 200 °C to 350 °C. Air oxidation following 600 °C activation could improve the mesoporous rate by etching the pore, and simultaneously introduce more oxygen-containing groups such as carbonyl (C=O) and carboxyl (–COOH) groups, which could enhance the wettability of PAB. The optimal synergistic temperature of ZnCl2 activation and air oxidation was 600 °C and 350 °C (PAB-600-350), respectively. PAB-600-350 is a wonderful supercapacitor electrode material with a higher surface oxygen content (20.74%) and a superior mesoporous rate (35%). At 1 A/g, PAB-600-350 showed the highest capacitance of 256 F/g, which was 2-fold that of PAB-600 (128 F/g). PAB-600-350 capacitors also supplied an excellent energy density of 12.54 Wh/kg at the power density of 225 W/kg in 1 M Na2SO4 electrolyte. Furthermore, a density functional theory (DFT) analysis was performed to investigate the interaction between PAB and electrolyte ions. The results showed that oxygen-containing functional groups on the surface can increase the dipole moment of the material and enhance the adsorption of PAB for electrolyte ions (K+).

Improved wear and corrosion resistance of biomedical TiZrNbTaMo medium-entropy alloy by thermal oxidation treatment

Thermal oxidation treatment was carried out on novel TiZrNbTaMo biomedical Medium-entropy alloy (bio-MEAs) at 400∼600 °C for 2 h in air. The extracted results demonstrated that the oxide layer was mainly composed of oxides of Zr and Ti, in which the content of ZrO2 and TiO2 with high hardness increased with the increase of oxidation temperature, leading to enhanced wear resistance (wear rate reduced from ∼10−4 to ∼10−5 mm3/N⋅m). Likewise, the oxide layer also improves the corrosion resistance of the alloys. The results showed that thermal oxidation can be used as an effective surface treatment method to broaden the application prospects of bio-MEAs.

High-temperature oxidation behavior of laser-cladded refractory Al0.75NbNi0.1Mo0.1Cr0.5Si0.1Wx high-entropy coatings

Refractory High Entropy Alloys (RHEAs) have great potential for ultra-high temperature environments and are one of the potential candidates for high temperature applications. In this work, a series of Al0.75NbNi0.1Mo0.1Cr0.5Si0.1Wx (x = 1, 1.5, 2) refractory high-entropy alloy coatings is prepared by laser cladding technology. The microstructure and oxidation behavior at 1000 °C and effect of changing oxidation temperature on oxidation behavior of the coatings is investigated. The results show that Al0.75NbNi0.1Mo0.1Cr0.5Si0.1Wx RHEA coatings are composed of BCC, W5Si3 and AlNi3 phases. The microstructure of RHEA coatings exhibits dendritic-cellular grain transition with increasing W content. The specific mass gains of coatings due to high temperature oxidation is 65.5 mg/cm2 for x = 1, 8.5 mg/cm2 for x = 1.5 and 14.5 mg/cm2 for x = 2 at 1000 °C for 70 h. After oxidation in 1000 °C for 70 h under air atmosphere, a protective oxide film (such as Al2O3 and SiO2) adhere around NiWO4 on the alloy coatings, which impedes the inward oxygen transport. Moderate addition of W elements is found to be beneficial in enhancing the oxidation resistance of coatings. However, excessive addition of W elements prevents the formation of protective oxides (Al2O3 and SiO2) and reduces the high-temperature oxidation resistance of the coatings. With the increase of oxidation temperature, simple oxides are significantly reduced while compound oxides are increased on the surface of the coatings. The surface appears hill-shaped oxides with the increase of NiWO4.

Oxidized Cellulose Nanocrystals from Durian Peel Waste by Ammonium Persulfate Oxidation.

Cellulose nanocrystals (CNCs) have gained much attention due to their biodegradable, renewable, nontoxic, and inexpensive nanomaterials with some remarkable properties. In this study, cellulose nanocrystals from durian peel waste were isolated by chemical oxidation. This process involved two stages of a chemical process, namely, bleaching followed by oxidation of ammonium persulfate (APS). The impact of process parameters (APS concentrations and oxidation temperatures) on the oxidized CNC was assessed. The properties of CNC were investigated by attenuated total reflection-infrared (ATR-IR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). ATR-IR results showed that the structure of cellulose did not change during APS oxidation. XRD results indicated that APS oxidation improved the crystallinity index by 103% due to the removal of the amorphous components. The resulting CNC was needlelike in shape and had an average width range of 5.00-7.81 nm, a length range of 114.52-126.83 nm, and an aspect ratio range of 16.76-24.20. From the TGA analysis, the thermal stability was found to increase with increasing oxidation temperature. The optimum conditions for a maximum crystallinity index and the highest thermal stability were obtained at 80°C oxidation with 1 M APS. Therefore, APS oxidation was a remarkable method for increasing the value of durian peel waste into high-value nanocellulose.

Effect of oxidation temperature on the properties of niobium in view of its biomedical applications

Four-hour oxidation processes of niobium in an air atmosphere at temperatures of 400 °C, 425 °C, 450 °C and 500 °C were carried out. In order to characterise the layers produced, the cross-sectional microstructure, chemical and phase composition as well as surface roughness were examined. The mechanical properties of the surface were determined by performing Vickers microhardness tests. In order to verify the properties from a biological point of view, contact angle analysis and corrosion tests in Ringer's solution were carried out. The results revealed the formation of layers composed of a solid solution of oxygen in niobium Nb(O) at oxidation temperatures of 400 °C, a solution of Nb(O) and niobium pentoxide Nb2O5 at 425 °C, and Nb2O5 at 450 °C and 500 °C. Increased oxidation temperature resulted in an increase in hardness and surface roughness, and each process contributed to improved corrosion resistance. Oxidation at too high temperature (≥450 °C) caused degradation of the material's surface due to niobium's low heat resistance. At 450 °C the first cracks in the material were visible, and at 500 °C the layer was inhomogeneous, brittle and underwent significant chipping. The highest hardness, roughness and hydrophobic properties were shown by niobium oxidised at 500 °C, which underwent surface degradation at this temperature. In turn, niobium oxidised at 400 °C and 425 °C showed outstanding properties in the biological aspect, achieving both high hydrophilicity and the highest corrosion resistance.

Low temperature synthesis of VO2 and hysteresis free VOx thin films with high temperature coefficient of resistance for bolometer applications

Vanadium dioxide (VO2) is an insulator to metal phase transition material, which finds many electronic and optical applications due to its reversible switching while VOx-based bolometers are widely used in commercial uncooled infrared detectors due to their high-temperature coefficient of resistance and low resistivity. Atmospheric pressure thermal oxidation (APTO) of Vanadium (V) is a simple process used to synthesize VO2 films without the need for precise control of oxygen partial pressure unlike other synthesis methods. In this work, we report the effect of reducing the V oxidation temperature to 250–450 °C during APTO and synthesize films with different oxidation durations. For an optimized oxidation duration at each oxidation temperature, VO2 films are obtained with more than two orders of resistance switching. This optimum oxidation duration is found to decrease exponentially with increase in oxidation temperature with an activation energy of 1.05 eV. The phase transition temperature of these VO2 films is found to decrease monotonically with decrease in oxidation temperature. We also observe that partially oxidized (oxidation time less than optimized oxidation duration to form VO2) VOx films show broad resistance switching from room temperature up to 70 °C instead of steep switching at the phase transition temperature. The partially oxidized film shows a temperature coefficient of resistance (TCR) as high as ∼10%/K, which is 3–4 times greater than TCR of commercial VOx thin films used in bolometers while showing similar resistivity. The resistivity of these films also exhibits almost zero hysteresis with temperature, an important requirement for bolometer applications. This study will widen the usefulness of the APTO process for fabricating VO2-based devices and high TCR VOx films for bolometer applications. The demonstrated low temperature process will especially be suitable to integrate VO2/VOx on substrates which can sustain only low temperatures such as some of the flexible substrates.