Oxygen Diffusion Channels Research Articles

High-temperature oxidation behavior and mechanical properties of PS-PVD sprayed Yb2Si2O7 environmental barrier coatings

The high-temperature oxidation behavior and mechanical properties of the Yb2Si2O7/Si environmental barrier coatings (EBCs) prepared by plasma spray-physical vapor deposition at 1250 °C and 1350 °C were investigated, and obvious delamination was observed after oxidation for 4000 h. The results showed that element diffusion occurred during oxidation, and the diffusion of oxygen led to the formation of thermal growth oxides (TGO), which induced pores and microcracks. The TGO played a role in hindering element diffusion, but the mismatching coefficients of thermal expansion and phase transition caused vertical and transverse cracks in the TGO layer. These cracks became channels for oxygen diffusion and ultimately resulted in delamination between TGO and the bonding coating (BC) layer. Furthermore, temperature significantly affects the oxidation kinetics of EBCs, and the oxidation within the BC layer alters its elastic modulus. The elastic modulus of the BC layer first increased and then decreased. The oxidation damage was more pronounced at 1350 °C.

Atomic-level insight into the carburization process of iron-based catalysts: A ReaxFF molecular dynamics study

Transition metal carbide catalysts have garnered widespread attention due to their outstanding catalytic performance. The carbide catalysts, such as FexC and MoxC are typically obtained from their oxide carburization. The reduction and carburization processes are key steps during the activation of oxide precursors, which determine the activity and stability. An understanding of the carburization process mechanism is very important for the optimization of carbide catalysts. Iron-based catalysts are widely used in the large-scale coal-to-liquid industry because of their low price and high activity. In this paper, the carburization process of iron oxide nanoparticles was simulated by the Reactive force field (ReaxFF) molecular dynamics method. The results illustrate that the competition between CO dissociation and reduction of oxides occurs at the 100 ps time scale, which can be tuned by particle size and oxygen vacancy. We have explained the counter-intuitive finding that small oxide particles are more difficult to achieve fully carburization than larger ones; this is because rapid carbon accumulation on the surface blocks the oxygen diffusion channels from bulk to surface. The atomic distribution heat map was carried out to demonstrate the carbon and oxygen penetration processes. Meanwhile, the volumetric strain analysis reveals the driving force comes from the strong tendency to form Fe-C bonds. The dependence on surface orientation was further systematically investigated. This work provides microscopic insights into the carburization process of iron-based materials and the fundamental knowledge for the optimization of catalyst synthesis procedures.

In-vitro fretting tribocorrosion and biocompatibility aspects of laser shock peened Ti-6Al-4V surfaces

Laser shock peening without coating (LSPwC), a prospective surface modification technique for improving the mechanical aspects of Ti-6Al-4V alloy for automotive/aerospace sector, is also expected to dictate the efficiency of this material class for implant application. Here we unravel the impact of LSPwC on Ti-6Al-4V material surface characteristics, in-vitro tribocorrosion and biocompatibility. Micrography shows the presence of nano and sub-micron sized pores after LSPwC process. The role of nano and sub-micron sized pores along with topography modification induced by LSPwC to serve as cues for controlling gene expressions, cell adhesion and activities offer novel insights in this research direction. A detailed X-ray photoelectron spectroscopy analysis detected local chemical non-stoichiometry with reduced number of oxygen diffusion channels. A crucial outcome of this oxide layer modification is the negative skewness (−0.55 ± 0.11) and reduced kurtosis (3.49 ± 0.14) of the surface, along with localized plastic deformation. These factors are correlated with the shift in potential during fretting tribo-corrosion from −800 to −250 mV after LSPwC, accompanied by a lower coefficient of friction of 0.4. Furthermore, the presence of well-spread cells and the up-regulation of beneficial genetic markers (Ki67) on LSPwC surfaces have the potential to form a better bone-material interface. The findings open new frontiers of the LSPwC-treated Ti-6Al-4V surface to synergistically modulate the tribocorrosion and biocompatibility aspects, with exciting possibilities for biomedical implants.

Influence of precursor content on the microstructure and ablative resistance of CLVD-C/C-SiC-ZrC composites

Precursor content is a key to control ablation resistance of CLVD-C/C-SiC-ZrC composites. The ceramic content of the composites prepared with 20 wt% precursor was only 8.38 vol%, so an oxide protective coating was not formed. High-content precursors promoted the formation of oxide coating, especially the composites prepared with 50 wt% precursor had a complete oxide coating. Thus, mass and line ablation rates were only 0.64 mg/s and 0.45 µm/s. When precursor content was 65 wt%, the composites had high-content ceramics, but porosity was great and many pores and cracks acted as channels for oxygen diffusion, causing a poor ablation resistance.

A peridynamic model for coupled thermo-mechanical-oxygenic analysis of C/C composites with SiC coating

The oxidation induced by coating cracking is a general failure mode of high-temperature composites such as C/C and C/SiC. However, this failure mode has not been fully investigated due to the lack of a method suitable for both crack simulation and oxidation interface capture. In this paper, a coupled thermo-mechanical-oxygenic peridynamic model is proposed for SiC-coated C/C composites in an oxidizing environment. The ordinary state-based peridynamics is extended to plane strain thermoelastic problems for orthotropic media to calculate the crack distribution. Cracks in the coating are modeled as oxygen diffusion channels and the peridynamic oxidation equation is developed to simulate the oxidation of the C/C substrate. Furthermore, the peridynamic non-Fourier heat transfer equation is employed to consider the finite thermal propagation speed and the model is validated by experimental results. Numerical examples are presented to investigate the oxidation failure after thermal shock, non-Fourier thermal shock fracture, and stress oxidation failure of SiC-coated C/C composites.

Effects of alloying ZrB<sub>2</sub>(HfB<sub>2</sub>)–SiC with tantalum on the structure and resistance to high-temperature oxidation and ablation: A review

This review presents a comprehensive analysis of the impact of tantalum alloying on the structure, heat resistance, and ablation resistance of ZrB2(HfB2)–SiC ultra-high-temperature composites. The influence of the primary phase content on the effects on the structural and morphological features of the oxide layers and their protective efficiency is analyzed. It is shown that alloying positively affects the composite's behavior by enhancing the viscosity and thermal stability of the glass phase, decreasing anionic conductivity, partially stabilizing the ZrO2(HfO2) lattice, and forming temperature-resistant complex oxides, such as Zr11Ta4O32 or Hf6Ta2O17 on the surface. It has been established that the alloying can have negative effects, including an increase in the liquid phase content, oxide film discontinuity, ZrO2(HfO2) grain damage due to TaB2 oxidation, or a significant amount of gas release due to TaC oxidation, as well as the formation of oxygen diffusion channels during the verticalization of Zr11Ta4O32 or Hf6Ta2O17 platelets. It is essential to note that the oxidation and ablation resistance, as well as the mechanisms driving composite behavior, differ depending on the alloying compounds and test conditions. Overall, this study sheds light on the role of tantalum alloying in enhancing the performance of ZrB2(HfB2)–SiC UHTC and highlights the importance of understanding the underlying mechanisms that govern their behavior.

Insight into combustion characteristics of micro- and nano-sized boron carbide

The combustion characteristics of micro- and nano-sized boron carbide (B4C) powders were investigated to exploit the role of carbon atoms on combustion. Various combustion parameters had been evaluated for B4C powders. Results demonstrated a direct correlation between the gas oxide of carbon atoms with the morphology of condensed combustion products. The thrust generated when the gas diffused from the B4C matrix to the environment could not only disperse the burning B4C powder, but also break through the liquid oxide layer and leave holes on the surface of micro-sized B4C. The formed holes on the surface were considered oxygen diffusion channels to the interior of B4C particle, which accelerated the mass transfer rate and improved the oxidation efficiency. The evolution images of confined combustion and the characterizations of condensed combustion products suggested that the B4C powder released most of energy potential after stable combustion and transformed into boron oxide. Given the combustion process, the B4C powders exhibited the higher combustion heat and the larger peak area of combustion pressure than boron powders with similar diameter distribution, and the B4C powders in nano-sized generated strongest energy output among all samples.

Anchoring NiO Nanosheet on the Surface of CNT to Enhance the Performance of a Li-O2 Battery.

Li2O2, as the cathodic discharge product of aprotic Li-O2 batteries, is difficult to electrochemically decompose. Transition-metal oxides (TMOs) have been proven to play a critical role in promoting the formation and decomposition of Li2O2. Herein, a NiO/CNT catalyst was prepared by anchoring a NiO nanosheet on the surface of CNT. When using the NiO/CNT as a cathode catalyst, the Li-O2 battery had a lower overpotential of 1.2 V and could operate 81 cycles with a limited specific capacity of 1000 mA h g−1 at a current density of 100 mA g−1. In comparison, with CNT as a cathodic catalyst, the battery could achieve an overpotential of 1.64 V and a cycling stability of 66 cycles. The introduction of NiO effectively accelerated the generation and decomposition rate of Li2O2, further improving the battery performance. SEM and XRD characterizations confirmed that a Li2O2 film formed during the discharge process and could be fully electrochemical decomposed in the charge process. The internal network and nanoporous structure of the NiO/CNT catalyst could provide more oxygen diffusion channels and accelerate the decomposition rate of Li2O2. These merits led to the Li-O2 battery’s better performance.

Effect of pre-strain on oxidation behaviour of nickel-based superalloys in air at 700 °C

The effect of pre-strain on oxidation behaviour of nickel-based superalloys was analysed using alloys 263 and 740H. Both alloys were subjected to tensile tests followed by oxidation tests at 700 °C in air. At higher pre-strain, both alloys exhibited higher oxidation rates and more oxide spallation. Microstructures were examined using various techniques (XRD, SEM, EBSD, and TEM). It was found that the pre-strain accelerated the formation of internal oxides. The defects that formed during the tensile tests apparently acted as oxygen diffusion channels from the oxide/metal interface to the inside of the metal substrate.

Oxidation behaviors of carbon fiber reinforced multilayer SiC-Si3N4 matrix composites

Oxidation behaviors of carbon fiber reinforced SiC matrix composites (C/SiC) are one of the most noteworthy properties. For C/SiC, the oxidation behavior was controlled by matrix microcracks caused by the mismatch of coefficients of thermal expansion (CTEs) and elastic modulus between carbon fiber and SiC matrix. In order to improve the oxidation resistance, multilayer SiC-Si3N4 matrices were fabricated by chemical vapor infiltration (CVI) to alleviate the above two kinds of mismatch and change the local stress distribution. For the oxidation of C/SiC with multilayer matrices, matrix microcracks would be deflected at the transition layer between different layers of multilayer SiC-Si3N4 matrix to lengthen the oxygen diffusion channels, thereby improving the oxidation resistance of C/SiC, especially at 800 and 1000 °C. The strength retention ratio was increased from 61.9% (C/SiC-SiC/SiC) to 75.7% (C/SiC-Si3N4/SiC/SiC) and 67.8% (C/SiC-SiC/Si3N4/SiC) after oxidation at 800 °C for 10 h.

Phase stability, thermo-physical property and thermal cycling durability of Yb2O3 doped Gd2Zr2O7 novel thermal barrier coatings

Novel double-ceramic-layer (DCL) thermal barrier coatings (TBCs) composed of (Yb 0.1 Gd 0.9 ) 2 Zr 2 O 7 (YbGZO) top layer and YSZ bottom layer were deposited via EB-PVD technology. The phase stabilities and thermo-physical properties of the YbGZO bulk ceramics as well as the diffraction peak constituent, elemental content, microstructure and thermal exposure behavior of DCL thermal barrier coatings were evaluated. After calcination from room temperature up to 1573 K, the YbGZO ceramic powders do not form new phases and still maintain excellent thermal stability while the thermal diffusivity of YbGZO bulk reaches the minimum value of 0.207 ± 0.004 mm 2 /s at 1273 K. Meanwhile, the thermal conductivity of YbGZO ceramics is in the range of 0.88–1.04 W/(m⋅K), which is correspondingly lower than that of YSZ bulk material ((1.20–1.46) W/(m⋅K)) due to the fact that the ceramic top layer of YbGZO has more irregular columns distribution and a larger intercolumnar gap. After spallation failure, DCL coating is mainly composed of YbGZO with fluorite phase and Y 0.08 Zr 0.92 O 1.06 of tetragonal phase which the pyramidal morphologies on top have disappeared with an obvious densification of ceramic coatings. In comparison, the sintering behavior of YSZ coating is more serious than that of YbGZO coating. Moreover, the penetration of vertical microcracks through the whole ceramic coatings could promote the formation of oxygen diffusion channels and further accelerate the internal oxidation behavior of grain boundaries and voids within the bond coat. In addition, the irregular distribution of transverse microcracks has led to the transgranular fracture of columnar grains.

High temperature oxidation resistance of Ti-5553 alloy with electro-deposited SiO2 coating

Ti–5Al–5V–5Mo–3Cr (Ti-5553) metastable β titanium alloy possesses excellent high temperature mechanical properties, however, the oxidation resistance is fairly poor. In this work, a SiO2 coating was prepared on Ti-5553 alloy by cathodic electro-deposition. The high temperature oxidation resistance of the deposited SiO2 coating was evaluated at 800 °C. The results show that the optimal current density of deposition is −2.0 mA/cm2 for 300 s, and the thickness of the compact SiO2 film reaches 5.5 μm. The SiO2 film can react with Ti-5553 substrate and form protective Ti5Si3 layer, which inhibits the inward diffusion of oxygen from air, leading to the remarkable improvement of oxidation resistance. The mass gain is only 1.13 mg/cm2 after oxidation at 800 °C for 80 h. The oxide scale consists of TiO2, Al2O3, V2O5, Cr2O3 and SiO2, but Mo2O5 is absent. The occurrence of Kirkendall voids as well as the cracks induced by internal stress should be responsible for the deterioration of oxidation resistance, which may provide extra channels for oxygen diffusion inwards the substrate.

Ablation resistance and mechanism of SiC–LaB6 and SiC–LaB6–ZrB2 ceramics under plasma flame

In this study, silicon carbide-lanthanum hexaboride (SiC–LaB6) and silicon carbide–lanthanum hexaboride–zirconium boride (SiC–LaB6–ZrB2) ceramics were fabricated by spark plasma sintering at 1900 °C, and their ablation resistance was tested under plasma flames over 2300 °C. The results indicate that the SiC–LaB6–ZrB2 ceramic exhibits better ablation resistance than the SiC–LaB6 ceramic. After ablation under the plasma flame for 60 s, the mass and linear ablation rates of the SiC–LaB6 ceramic were 15.83 μg/s and 1.08 μm/s, respectively, while those of SiC–LaB6–ZrB2 were -8.42 μg/s and -0.27 μm/s. With the addition of ZrB2, SiC–LaB6–ZrB2 ceramic attained a high density and fewer inner oxygen diffusion channels. Moreover, the ZrO2–La2O3–SiO2 oxide scale with good self-healing ability and excellent stability was formed in the ablation centre, which can retard the further oxidation during ablation.

Nitrogen and iodine dual-doped 3D porous graphene as a bi-functional cathode catalyst for Li-O2 batteries

A novel nitrogen and iodine co-doped porous graphene with well-defined hierarchical microstructure is synthesized via a hydrothermal self-assembly strategy followed by heat treatment at elevated temperature. Its electrochemical properties as the cathode catalyst for Li–O2 batteries are investigated. Compared to un-doped, nitrogen or iodine single doped porous graphene, nitrogen and iodine co-doped porous graphene shows much higher activity for oxygen reduction reaction and oxygen evolution reaction. As oxygen cathode for Li–O2 batteries, nitrogen and iodine co-doped porous graphene exhibits an outstanding specific reversible capacity up to 14000 mAh g−1 at 200 mA g−1, a superior high-rate capability (5000 mAh g−1 at 1000 mA g−1), and an excellent cycling stability enduring over 225 cycles with a limited capacity of 1000 mA h g−1 at a current density of 500 mA g−1. The fantastic cycling performance can be attributed to a combination of porous structure and dual-doping, which not only provides a large number of channels for oxygen diffusion and electrolyte infiltration, but also supplies abundant active catalytic sites to affect the formation and decomposition of the discharge products with various morphologies. This study provides an effective approach to develop highly-efficient carbon-based catalysts with optimized pore structure and tunable surface chemistry.

Hierarchical NiCo2S4@NiO Core–Shell Heterostructures as Catalytic Cathode for Long‐Life Li‐O2 Batteries

AbstractThe critical challenges of Li‐O2 batteries lie in sluggish oxygen redox kinetics and undesirable parasitic reactions during the oxygen reduction reaction and oxygen evolution reaction processes, inducing large overpotential and inferior cycle stability. Herein, an elaborately designed 3D hierarchical heterostructure comprising NiCo2S4@NiO core–shell arrays on conductive carbon paper is first reported as a freestanding cathode for Li‐O2 batteries. The unique hierarchical array structures can build up multidimensional channels for oxygen diffusion and electrolyte impregnation. A built‐in interfacial potential between NiCo2S4 and NiO can drastically enhance interfacial charge transfer kinetics. According to density functional theory calculations, intrinsic LiO2‐affinity characteristics of NiCo2S4 and NiO play an importantly synergistic role in promoting the formation of large peasecod‐like Li2O2, conducive to construct a low‐impedance Li2O2/cathode contact interface. As expected, Li‐O2 cells based on NiCo2S4@NiO electrode exhibit an improved overpotential of 0.88 V, a high discharge capacity of 10 050 mAh g−1 at 200 mA g−1, an excellent rate capability of 6150 mAh g−1 at 1.0 A g−1, and a long‐term cycle stability under a restricted capacity of 1000 mAh g−1 at 200 mA g−1. Notably, the reported strategy about heterostructure accouplement may pave a new avenue for the effective electrocatalyst design for Li‐O2 batteries.

High-temperature oxidation resistance of VCps-reinforced Fe-matrix composites using an in-situ reaction

The oxidation kinetics and behavior of vanadium carbide particle-reinforced Fe-based composites (VCps-Fe-MC) were investigated at 600, 800, and 950 °C. The microstructure of the surface and cross-section of the oxidation layer was characterized by scanning electron microscope (SEM). The composition of the surface of the oxide scales was examined by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The micro hardness of the specimens was measured by a micro-hardness tester after a high-temperature treatment and subsequent air cooling. The results showed that the growth of the oxide scales on the VCps-Fe-MC followed a parabolic curve with an increase in the oxidation time. In addition, the weight gain increased with the oxidation temperature during the same period. Different oxide scales and oxidation rates occurred in some regions due to different textures. The minimum oxidation rate was in the austenite region because of a high silicon and chromium contents. At the same time, severe cracking and large shedding of the oxide scales were discovered due to growth stress and thermal stress. These defects can provide channels for oxygen diffusion. Based on the micro-hardness values, temperatures greater than 900 °C can be regarded as austenitizing temperatures during heat treatment to improve the mechanical properties and the wear resistance.

Oxidation behaviour of plasma-sprayed ZrB2-SiC coatings

Current study aimed at investigating oxidation behaviour of plasma-sprayed ZrB2-SiC coatings, which were deposited using different spray powers. Moreover, oxidation tests were carried out within temperature range of 400–1200 °C. Results from X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) suggested that ZrB2-SiC coatings had been markedly oxidized at 600 °C. Besides, oxidation behaviour of ZrB2-SiC block ceramics, which had achieved different densities using spark plasma sintering (SPS) and pressureless sintering (PS), was also studied to explain oxidation behaviour of plasma-sprayed ZrB2-SiC coatings. Results revealed that high-density ZrB2-SiC block ceramics displayed better oxidation resistance than low-density counterparts, indicating that unfavorable oxidation resistance of ZrB2-SiC coatings could be ascribed to low density, since there were more pores in ZrB2-SiC coatings, which provided channels for oxygen diffusion.

Exposure of sufficient edge sites on well-crystallized MoSe2 induced by nitrogen doping (Mo−Nx) for Pt: Enhanced co-catalytic activity and methanol tolerance for oxygen reduction

To endow catalyst supports with excellent co-catalytic activity is an effective way to strengthen methanol-tolerance of Pt-based catalysts towards oxygen reduction reaction (ORR). In this study, nitrogen-doped molybdenum selenide/biomass-derived carbon (N-MoSe2/BC) composite as a Pt-support/co-catalyst is prepared via a synchronous synthesis method to enhance methanol tolerance. The porous structure of N-MoSe2/BC with N-doping can improve the exposure of coordinated Mo–Sex sites along MoSe2 edges and provide the oxygen diffusion channels to promote ORR activity. Pt-N-MoSe2/BC (Pt, 5 wt.%) shows high activity (14.83 mA cm−2) and selectivity (4e− pathway) towards ORR, promising durability (11.9% decline) and excellent tolerance against methanol-crossover effects, which are superior to those of commercial Pt/C (10 wt.%). With the introduction of pyridinic N, graphitic N and Mo−Nx in MoSe2/BC, more active sites on Pt (111) facets are activated to enhance charge transfer efficiency and ORR activity. Both N-species in BC and exposed edge sites in N-MoSe2 contribute to high methanol-tolerance and co-catalytic activity towards ORR. Therefore, the remarkable ORR activity is originated from the synergistic effects among well-distributed Pt, N-species, and active edge sites of MoSe2. Design of porous (N)-MoSe2/BC provides a promising direction for preparation of co-catalyst/support with strong methanol tolerance and ORR activity.

Durable anti-oxidation mechanism and failure analysis of the ZrSiO4 compound glass coating for carbon/carbon composites

In order to improve the anti-oxidation property of glass coating for aviation thermal structural components such as C/C airplane brake disc under extreme conditions up to 1473 K, ZrSiO4 is doped to improve the stability of the glass coating and to prolong its service life. The microstructures of the ZrSiO4 compound glass (ZSO-glass) coating after different oxidation time at 1473 K were studied. The anti-oxidation mechanism and failure of the coating was comprehensively analyzed. The results show that the novel ZSO-glass coating prepared by slurry method displayed excellent oxidation resistance and protected the carbon/carbon matrix at 1473 K for 573 h with only 0.33 wt% weight-loss, and it also displayed good thermal shock resistance. The excellent oxidation resistance of the coating system is attributed to self-healing effect of glass phase and toughening effect of ZrSiO4 in the coating. The fine ZrSiO4 particles can inhibit crystallization of glass coating while the agglomerated ZrSiO4 particles with a little larger size can pin microcracks in the glass phase during oxidation. The reason for the failure of ZSO-glass coating is that bubbles gradually grow up during oxidation and finally break through the ZSO-glass coating forming main oxygen diffusion channels. The service life of the ZSO-glass/SiC coating can be predicted by the bubble growth model.

Ignition-proof performance and mechanism of AZ91D-3Nd-xDy magnesium alloys at high temperatures

This study focused on the synergistic effect of alloying elements neodymium (Nd) and dysprosium (Dy) on the ignition-proof performance of AZ91D alloy. The ignition-proof mechanism of AZ91D-3Nd-xDy (x = 0.5, 1.0, 1.5, 2.0 and 2.5wt.%) alloy was discussed in depth through ignition-proof testing and microstructure observation. The results showed that the AZ91 D-3Nd-2Dy alloy exhibited the highest ignition-point of 893 K, increased by 69 K as compared to the AZ91D alloy. The ignition-proof mechanism of Nd and Dy additions lay in three aspects: (1) the formation of denser oxide film consisting of Dy2o3 and MgO improves the oxidation resistance of the alloy, (2) the great reduction of the low melting-point phase β-Mg17Al12, which leads to the decrease in the oxygen diffusion channels, and (3) the newly formed high melting-point phases (Al2Nd and Al2Dy), which block the oxygen diffusion channels and prevent the chemical reaction of Mg and oxygen.