SiO2 Protective Layer Research Articles

Preparation and anti-oxidation performance of TaC-SiC nanocomposites by precursor-derived ceramic method

As a kind of ultra-high temperature ceramic, TaC is widely used in aerospace field. In order to improve its high temperature oxidation resistance, SiC is usually added as a second phase. TaC-SiC nanocomposites prepared by precursor-derived ceramic method have the advantages of low pyrolysis temperature, small particle size and uniform phase composition. Some studies have used solid polycarbosilane (PCS) as the precursor of SiC. However, it has problems such as high cost and need to add solvent. In this work, TaC-SiC nanocomposites were prepared by precursor-derived ceramic method with polytantaloxane (PTO) and vinyl-containing liquid polycarbosilane (LPVCS) instead of PCS as raw materials after 2 h pyrolysis at 1600 °C. The obtained TaC-SiC nanocomposites have a particle size of about 200 nm and a grain size of 15–40 nm, wrapped by 1–2 nm amorphous carbon shells, which is beneficial to inhibit grain growth. By analyzing the oxidation mechanism of TaC-SiC nanocomposites, it was found that higher SiC content was conducive to antioxidant, because the SiO2 protective layer formed by oxidation insulated O2 diffusion.

Etching-free pixel definition in InGaN green micro-LEDs

The traditional plasma etching process for defining micro-LED pixels could lead to significant sidewall damage. Defects near sidewall regions act as non-radiative recombination centers and paths for current leakage, significantly deteriorating device performance. In this study, we demonstrated a novel selective thermal oxidation (STO) method that allowed pixel definition without undergoing plasma damage and subsequent dielectric passivation. Thermal annealing in ambient air oxidized and reshaped the LED structure, such as p-layers and InGaN/GaN multiple quantum wells. Simultaneously, the pixel areas beneath the pre-deposited SiO2 layer were selectively and effectively protected. It was demonstrated that prolonged thermal annealing time enhanced the insulating properties of the oxide, significantly reducing LED leakage current. Furthermore, applying a thicker SiO2 protective layer minimized device resistance and boosted device efficiency effectively. Utilizing the STO method, InGaN green micro-LED arrays with 50-, 30-, and 10-µm pixel sizes were manufactured and characterized. The results indicated that after 4 h of air annealing and with a 3.5-μm SiO2 protective layer, the 10-µm pixel array exhibited leakage currents density 1.2 × 10−6 A/cm2 at −10 V voltage and a peak on-wafer external quantum efficiency of ~6.48%. This work suggests that the STO method could become an effective approach for future micro-LED manufacturing to mitigate adverse LED efficiency size effects due to the plasma etching and improve device efficiency. Micro-LEDs fabricated through the STO method can be applied to micro-displays, visible light communication, and optical interconnect-based memories. Almost planar pixel geometry will provide more possibilities for the monolithic integration of driving circuits with micro-LEDs. Moreover, the STO method is not limited to micro-LED fabrication and can be extended to design other III-nitride devices, such as photodetectors, laser diodes, high-electron-mobility transistors, and Schottky barrier diodes.

Designed fabrication of lightweight SiC(Hf, rGO) bulk composites derived from Hafnium-modified precursors for hypersonic vehicle components

Demand for SiC-based polymer-derived ceramics (PDCs) employed as hypersonic vehicle components with light weight, high strength, excellent high-temperature/ablation resistance is increasingly urgent. Herein, bulk SiC(Hf, rGO) nanocomposite PDCs were fabricated via re-pyrolyzing ball-milling blends of unique SiC(Hf, rGO)p/polyhafniumcarbosilane-vinyltriethoxysilane-graphene oxide (PHVG). Highly cross-linked PHVG can effectively optimize compact formability and improve ceramic yield. In-situ formed HfO2 embedded in ceramic network toughens microstructure, while interfacial HfOSi increases structure disorder to improve high-temperature properties. As demonstrated, SiC(Hf, rGO) modified by 3 wt.% Hf and re-pyrolyzed at 1300 °C possess good mechanical performances including hardness (5.83 GPa), fracture toughness (4.29 MPa·m1/2), compressive strength (181.83 MPa) and flexural strength (46.55 MPa). More interestingly, self-healing HfO2(HfSiO4)/SiO2 protective layer emerging during 20 min surface ablation test at 1300 °C, timely covers defects and avoids destructive structure, thus retains stable structure and mechanical properties of specimens. Such nanocomposites can promote uses in thermal protection systems (TPS) for hypersonic vehicles against extreme high-temperature environments.

High temperature oxidation of additively manufactured silicon carbide/carbon fiber nanocomposites

An additive manufacturing process and subsequent pyrolysis cycle were used to fabricate SiC matrix/carbon fiber hybrid composites. The matrix was fabricated using a mixture of preceramic polymer and acrylate monomers, while polyacrylonitrile (PAN) precursor was used to fabricate fibers via electrospinning. The precursor matrix and reinforcing fibers at 0, 2, 5, or 10 wt% were printed using digital light processing, and both were simultaneously pyrolyzed to yield the final ceramic matrix composite structure. After pyrolysis, XRD and SEAD analysis proved the existence of SiC nanocrystals and turbostratic carbon structure in the matrix, while the reinforcement phase was shown to have a turbostratic carbon structure similar to commercial carbon fibers. Thermogravimetric analysis (TGA) in air up to 1400 °C was used to evaluate the oxidation resistance of this material. TGA results showed some weight loss due to oxidation of SiC and/or carbon up to about 900 °C, followed by weight gain to about 1200 °C due to the formation of a protective SiO2 layer. Although increasing carbon fiber content negatively impacted the total mass loss for the first heating cycle, exposure of the composite to second-run air revealed negligible weight loss. This is explained by SiO2 layer formation which acts as a protective film that prevents oxygen diffusion. Oxidation of SiC and the formation of a glassy layer has been proven to protect the sample from further oxidation, as well as provide healing of surface cracks and defects as revealed by SEM analysis.

Insight into (Electro)magnetic Interactions within Facet-Engineered BaFe12O19/TiO2 Magnetic Photocatalysts.

A series of facet-engineered TiO2/BaFe12O19 composites were synthesized through hydrothermal growth of both phases and subsequent deposition of the different, faceted TiO2 nanoparticles onto BaFe12O19 microplates. The well-defined geometry of the composite and uniaxial magnetic anisotropy of the ferrite allowed alternate interfaces between both phases and fixed the orientation between the TiO2 crystal structure and the remanent magnetic field within BaFe12O19. The morphology and crystal structure of the composites were confirmed by a combination of scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses together with the detailed study of BaFe12O19 electronic and magnetic properties. The photocatalytic activity and magnetic field effect were studied in the reaction of phenol degradation for TiO2/BaFe12O19 and composites of BaFe12O19 covered with a SiO2 protective layer and TiO2. The observed differences in phenol degradation are associated with electron transfer and the contribution of the magnetic field. All obtained magnetic composite materials can be easily separated in an external magnetic field, with efficiencies exceeding 95%, and recycled without significant loss of photocatalytic activity. The highest activity was observed for the composite of BaFe12O19 with TiO2 exposing {1 0 1} facets. However, to prevent electron transfer within the composite structure, this photocatalyst material was additionally coated with a protective SiO2 layer. Furthermore, TiO2 exposing {1 0 0} facets exhibited significant synergy with the BaFe12O19 magnetic field, leading to 2 times higher photocatalytic activity when ferrite was magnetized before the process. The photoluminescence emission study suggests that for this particular combination, the built-in magnetic field of the ferrite suppressed the recombination of the photogenerated charge carriers. Ultimately, possible effects of complex electro/magnetic interactions within the magnetic photocatalyst are shown and discussed for the first time, including the anisotropic properties of both phases.

Preparation of carbon-based monolith with macroporous-mesoporous structure derived from PVDF hollow fiber membranes for supercapacitors

In this work, a carbon-based monolith with macroporous-mesoporous structure derived from polyvinylidene fluoride (PVDF) hollow fiber membranes is reported. The Co2+ soaked PVDF hollow fiber membranes are treated with vinyltrimethoxysilane (VTMS) and calcined at relatively low temperatures (600 °C) to obtain a carbon-based material with a distinctive honeycomb porous structure. The VTMS treatment is the committed step, which provides a hard SiO2 protective layer for the PVDF hollow fiber membrane during the calcination process to support the skeleton structure and prevent the macrostructure collapse and also plays a role in expanding mesopores. The obtained hierarchically porous structure can improve the electrochemical activity of the carbon-based material. In a three-electrode system, the specific capacitance of 701.0 F g−1 can be achieved at a current density of 1 A g−1, and the retention rate is 112.1 % after 10,000 cycles at 10 A g−1. The assembled quasi-solid-state symmetric supercapacitor can be operated in a 0–1.4 V voltage window, and it demonstrates an energy density of 26.3 Wh kg−1 at the power density of 349.9 W kg−1.

Ablation properties of SiC-reinforced PBI resin matrix composites under high-energy continuous laser ablation

In this work, poly-p-phenylene benzobisoxazole (PBO) fibers reinforced polybenzimidazole (PBI) resin-based laser ablation composites (thickness = 5 mm) with different SiC contents were prepared. SiC/PBI/PBO composites exhibited good mechanical properties, excellent thermal stability and laser ablation properties. The laser ablation properties and ablation mechanism of the composites were investigated using a high-energy continuous-wave laser (30 MW·m−2). Increasing the SiC content led to increase in composites density, while decreasing the bending performance of the composites. However, it improves the mass and linear ablation rates, resulting in an overall improvement in ablation properties. At a SiC content of 70 %, the composite exhibited a mass loss of 8.2 %, a mass ablation rate of 9.2 mg·s−1, and a linear ablation rate of 0.11 mm·s−1. The ablation process of SiC/PBI/PBO composites was delineated into pre-ablation and post-ablation stages, with the ablated composites divided into the ablation center, transition region, and outer region based on morphological characteristics. After ablation, numerous white hollow SiO2 spheres and SiO2 protective layers were formed in the ablation centers, the transition regions were deformed and cracked, while the outer region remained unscathed.

Effect of SiO2 protective layer on LiNbO3 structured SAW resonators and temperature characteristics study

In this paper, using LiNbO3 as the piezoelectric substrate, Pt as the electrode and SiO2 as the protective layer, a multi-physics field coupled finite element model was used to investigate the effect of the protective layer thickness on the Rayleigh wave propagation characteristics, and the relationship between the protective layer thickness and the frequency–temperature coefficient TCF and electromechanical coupling coefficient K 2 is established. The frequency characteristics of the device and the microscopic changes of the electrode and substrate structures under high temperature were analyzed experimentally. The results show that no acoustic mode shift occurs when the normalized thickness is less than 31.25%. As the thickness of the protective layer increases, the vibration displacement in the direction of L wave, SH wave and SV wave decreases, and the thickness of the protective layer can be increased appropriately to reduce the interference of SH wave to Rayleigh wave; |TCF| decreases with increasing thickness of the protective layer. Changes in the thickness of the protective layer at different temperatures lead to fluctuations in K 2; the protective layer structure leads to a slight decrease in Q value. As the temperature increases, the fluctuation of the resonant frequency of the SAW resonator increases. The SiO2 protective layer can effectively protect LiNbO3 materials while improving the high-temperature working stability of Pt electrodes.

Influence of sintering aids on properties of dense Si2N2O ceramic developed by spark plasma sintering

Through reactive liquid phase sintering, dense and homogenous pellets with a diameter of 40 mm were successfully achieved after 50 min. These samples contain at least 95 vol% of Si2N2O. The microstructure, fracture toughness, coefficient of thermal expansion (673–1673 K), dielectric properties and high-temperature oxidation resistance were investigated to determine the influence of sintering aids such as lanthanide oxides (Sm2O3 and Y2O3) and metal oxide (Al2O3). The nano-sized grains generated have a uniform distribution. The composition of the sintering aid has no influence on the coefficient of thermal expansion or the dielectric properties. The crack propagation mode shifts from transgranular to intergranular in Al2O3 samples, which increases the fracture toughness. The lanthanide oxide samples, on the other hand, outperform the Al2O3 samples in terms of oxidation resistance, as a protective SiO2 and Ln2Si2O7 layer forms on the surface of the oxidized samples.

Reactive molecular dynamics simulations on the mechanism of the different protection methods of KAPTON during atomic oxygen impact

KAPTON-type polyimide (PI) is a crucial polymer that is often used in the aerospace industry, but the high levels of atomic oxygen (AO) in low-earth orbit (LEO) can cause the polymer to deteriorate and peel, so it is especially important to protect PI from AO. Incorporating nano-reinforcements into PI and applying a protective coating to the surface of PI are the two basic strategies for shielding PI from AO impact. Herein, to investigate the effect and mechanism of different protection methods, molecular models of PI reinforced with graphene (Gr) and carbon nanotubes (CNTs) as well as covered by silica (SiO2) and Gr, respectively, are constructed. Reactive molecular dynamics simulations are then performed to explore the degradation mechanisms of PI under AO impact. Results indicated that the addition of CNT and Gr can improve the resistance of PI to AO erosion, and their protective effect is mainly by burying in the PI substrate to prevent the penetration of AO downward. Additionally, the protective effect of covering the PI surface with a protective layer is significantly better than that of mixing nanomaterials into the PI substrate. Meanwhile, the protective effect of the Gr protective layer is weaker than that of SiO2 due to its smooth surface, which tends to slip when AO is incident, and it plays a protective effect mainly by bouncing the incident AO. By comparison, the SiO2 protective layer can be embedded into the substrate due to its serrated structure, which better protects the substrate, and it protects the substrate mainly by absorbing the incident AO. This study provides useful guidance for the identification of promising candidate materials for LEO applications, as well as for the development of molecular design for AO-resistant polymeric materials.

Effect of Reaction Temperature on the Molten Salt Electrodeposition of Silicide Coating on Molybdenum Substrate and its Oxidation Behavior

In this study, silicide coatings were successfully prepared on Mo substrates using NaCl-KCl-NaF molten salt as the reaction medium and K2SiF6 as the silicon source. The electrochemical reduction mechanism of Si(IV) was studied by cyclic voltammetry and chronoamperometry technology. The effect of reaction temperature on the phase composition, microstructure, and growth kinetics of the electrodeposited coatings were investigated in detail. The long-term oxidation behaviors of Mo and MoSi2-coated Mo samples at 873 K were also compared. The results revealed that the electrodeposition of silicide coatings was closely related to the reaction temperature. Dense and coherent single-layer MoSi2 coatings could be obtained at 1023 K and 1073 K, while double-layer coatings with inner MoSi2 layer and outer Si layer could be obtained at 1123 K. Under the same electrodeposition time, the overall thickness of the silicide coatings increased with the reaction temperature. Moreover, long-term oxidation tests at 873 K proved that MoSi2 coatings could effectively prevent the Mo substrate from oxidation. The mass gain of the MoSi2-coated Mo sample was only 0.46 wt% after 500 h of oxidation, which could be attributed to the formation of the continuous SiO2 protective layer.

Thermal chemical erosion behavior of the burning-free Al2O3–C–Si bottom nozzle

Burning-free Al2O3–C–Si bottom nozzle, a key refractory component connecting the intermediate ladle slide to the immersion spout, is always prone to failure under the extreme environment of repeated exposure to chemical attack and thermal shock of the steel. Here, an approach of adding Si powder and the change in Al2O3 aggregate aims to enhance the product's quality and performance of the burning-free Al2O3–C–Si bottom nozzle. And the different composition ratios effects on the chemical erosion behavior of the burning-free Al2O3–C–Si bottom nozzle were investigated based on the measured instantaneous thermal-expansion coefficient and young's modulus of the bottom nozzle materials in the high-temperature range of 300–1473 K. Moreover, the high-temperature folding resistance, thermal-shock resistance test and slag resistance experiments have also been probed. Obtained results reveal that the absence of 6 wt.% Si can promote the formation of SiC phases in the form of flakes and whiskers inside the bottom nozzles, which significantly improves the density, strength, thermal shock resistance and slag erosion resistance of refractory materials. Compared with the bauxite-Al sample, the slab corundum-Si-Al sample reduced the porosity by 7.8% and the erosion index decreased by 4.7% due to the formation of SiC and a thin protective layer of SiO2, as well as the mullite crystals structure. Here, an optimized bottom nozzle with a composite structure has also been proposed. This work can make significant contribution to improve the performance of burning-free refractory materials, bottom nozzles and reducing costs.

Microstructures and high temperature oxidation behaviors of AlMo0.5NbTa0.5TiZr high entropy alloys coated by silicon pack cementation

In this study, we fabricated silicon coatings on AlMo0.5NbTa0.5TiZr refractory high entropy alloy using pack cementation for the first time. The growth kinetics of the coating layer were analyzed by considering coating temperature, time, amount of Si, and NaF. The growth coefficient constant and activation energy were determined to be 96,823 µm/h1/2 and 117 kJ/mol, respectively. Microstructural analysis revealed the formation of a protective layer of SiO2 on the specimen surface, which increased its oxidation resistance at 1300 °C. Comparing the uncoated and coated specimens provided insight into the potential use of AlMo0.5NbTa0.5TiZr for high-temperature applications.

Fabrication, Characterization, and Corrosion Protection of Siloxane Coating on an Oxygen Plasma Pre-treated Silver-Copper Alloy

In this study, siloxane has been used for the protection of metal artifacts from corrosion in the form of transparent barrier coating films because of their good adhesion to the metal substrate. The effect of oxygen plasma pre-treatment on the adhesion properties of the siloxane thin film on the silver-copper alloy substrate was investigated. Radiofrequency plasma-enhanced chemical vapor deposition (RF-PECVD) was used for the deposition process. Surface identification and characterization of the deposited films were carried out using Scanning Electron Microscopy coupled with energy-dispersive x-ray spectroscopy (SEM–EDX) and Fourier-transform infrared spectroscopy (FT-IR). Surface topography and roughness were investigated by atomic force microscopy (AFM). The hydrophobic characteristic was measured by water contact angle measurement (WCA). The film thickness was evaluated using a spectroscopic ellipsometer (SE). Colorimetric measurement (CM) was used to evaluate changes in the appearance of the surface following the PECVD deposition of the SiO2 protective layer. The corrosion protection ability of siloxane films for metal substrates as a function of RF power and gas feed composition was examined by the electrochemical impedance spectroscopy (EIS) technique. It was found that the deposited film improved the protective efficiency for samples from 55.29 to 92.93%. Besides, after the oxygen plasma pretreatment step, the film showed better corrosion resistance of the tested samples.

Deposition of Thick SiO2 Coatings to Carbonyl Iron Microparticles for Thermal Stability and Microwave Performance.

Thick dielectric SiO2 shells on the surface of iron particles enhance the thermal and electrodynamic parameters of the iron. A technique to deposit thick, 500-nm, SiO2 shell to the surface of carbonyl iron (CI) particles was developed. The method consists of repeated deposition of SiO2 particles with air drying between iterations. This method allows to obtain thick dielectric shells up to 475 nm on individual CI particles. The paper shows that a thick SiO2 protective layer reduces the permittivity of the 'Fe-SiO2-paraffin' composite in accordance with the Maxwell Garnett medium theory. The protective shell increases the thermal stability of iron, when heated in air, by shifting the transition temperature to the higher oxide. The particle size, the thickness of the SiO2 shells, and the elemental analysis of the samples were studied using a scanning electron microscope. A coaxial waveguide and the Nicholson-Ross technique were used to measure microwave permeability and permittivity of the samples. A vibrating-sample magnetometer (VSM) was used to measure the magnetostatic data. A synchronous thermal analysis was applied to measure the thermal stability of the coated iron particles. The developed samples can be applied for electromagnetic compatibility problems, as well as the active material for various types of sensors.

The Effects of Alloying Elements Cr, Al, and Si on Oxidation Behaviors of Ni-Based Superalloys.

Oxidation behaviors of three Ni-based model alloys and pure Ni in the temperature range of 700–1200 °C are investigated to reveal effects of Cr, Al, and Si on the oxidation resistance of Ni-based superalloys. The formation and integrity of consecutive chromia or alumina layers are important for excellent oxidation resistance. The addition of 20 at.% Cr can effectively improve the oxidation resistance of Ni-based alloys by forming a thin chromia film below 1000 °C, while adding 15 at.% Al has a beneficial effect on the oxidation resistance of Ni-based alloys at temperatures above 900 °C. The addition of 2 at.% Si to Ni-Al alloy is insufficient to form a protective SiO2 layer but can accelerate the formation of alumina, which enables Ni-Al alloy to form a consecutive inner alumina layer at a relatively low temperature of 800 °C and further improve the oxidation resistance above 800 °C.

Nanostructured Magnetically Sensitive Catalysts for the Fenton System: Obtaining, Research, Application

Nanostructured “shell-shell” type catalysts, which consist of a magnetically sensitive core of cobalt ferrite and a protective layer of porous SiO2, have been synthesized. On the surface of porous SiO2 clusters of copper oxide are situated playing the role of catalytic centers. The structure of CoFe2O4 / SiO2 / CuO catalyst was confirmed by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Studies of the catalytic activity of the obtained catalysts were performed in the Fenton system on a model solution of methylene blue (MB). The catalytic activity of the composite in MB destruction reaches 99%. The high magnetic sensitivity of the obtained catalysts ensures their easy removal from the reaction medium. The catalysts demonstrated the possibility of reusability without loss of activity.

Enhanced atomic-oxygen resistance of surface-siliconized polyimide film via an in-situ precursor network at the interface

In order to allow polyimide (PI) to resist the erosion of atomic oxygen (AO) in Low Earth Orbit to extend the life of the aircraft, an in-situ precursor network approach was proposed to manufacture the inorganic gradient microstructure across the organic/inorganic transition interface and simplify the process, which create a three-tier microstructure film and lead the dense SiO2 protective layer firmly grasps the PI film. Benefiting from the integrated structure, SiO2-coated PI film shows remarkable surface and interface state after 3000 times mechanical bending, which able to delay the initial AO corrosion caused by the defect of the protective layer, resistance to AO reported as AO erosion yield of 2.39 × 10−26 cm3 O atom−1 under 7 × 1021O atoms cm−2 AO fluence. This work proposes a novel approach for the fabrication of high-performance and AO resistant PI to enable the aircraft to complete more efficient and highly integrated flight missions and shows great potential in the general preparation of organic-inorganic interface.

Study of KCl-induced hot corrosion behavior of high velocity oxy-fuel sprayed NiCrAlY and NiCrBSi coatings deposited on 12CrMoV boiler steel at 700 ℃

The hot corrosion behavior of two kind of HVOF-sprayed NiCr-based coatings with different oxide-forming elements (such as Al and Si) was investigated and compared in the presence of KCl salt at 700 ℃. The results showed that the hot corrosion resistance of NiCrBSi coating was superior to that of NiCrAlY coating. NiCrAlY coating was seriously corroded and severe internal oxidation was produced along the splat boundaries, while the corrosion of NiCrBSi coating was relatively mild and the inner coating remained intact, which was ascribed to the denser structure and the generation of a SiO2 protective layer.

Improving oxidation resistance of TZM alloy by deposited Si–MoSi2 composite coating with high silicon concentration

The oxidation behavior of Si–MoSi2 composite coating deposited on titanium-zirconium-molybdenum alloy (TZM) at 1200 °C in air was discussed. The oxidation tests show that the mass loss of coated samples is only 0.021 mg/cm2 after oxidation at 1200 °C for 9 h, whereas the bare TZM suffers serious volatilization and decomposition, and the mass loss is only 2.488 mg/cm2 after oxidation for 20 min. The coated samples presents good oxidation resistance compared to the bare TZM alloy at medium temperature, which is due to the formation of self-healing SiO2 protective layer and the dense structure of coating. Moreover, the high surface silicon concentration also plays a positive role in the rapid formation of SiO2 protective layer.