Dense SiO2 Research Articles

Oxidation of TiC–SiC nanocomposite

AbstractThis work aims at understanding the oxidation mechanism of a TiC–SiC nanocomposite ceramic material. Samples were subjected to heat treatments up to 1400°C under air, varying the relative density of the composite, the heating rate, and the dwell time. The weight variations were followed by thermogravimetric analyses coupled with mass spectrometry. The oxidized samples were then characterized by scanning electron microscopy on cross‐sections (SEM‐EDX) and their microstructure and composition were studied by transmission electron microscopy (TEM‐EDX). The oxidation process was also followed by in situ high‐temperature X‐ray diffraction and high‐temperature environmental scanning electron microscopy. Until 1000°C, the formation of a multilayer alteration scale was observed, with a dense and protective SiO2 layer. Above 1200°C, this layer showed cracks, and the oxidation was increased. Based on the results, a three‐step mechanism was proposed for the temperature‐dependent conversion of TiC and SiC to the subsequent oxides.

Preparation and the Photoelectric Properties of ZnO-SiO2 Films with a Sol-Gel Method Combined with Spin-Coating.

This study explores the fabrication of ZnO-SiO2 composite films on silicon substrates via a sol-gel method combined with spin-coating, followed by annealing at various temperatures. The research aims to enhance the UV emission and photoelectric properties of the films. XRD showed that the prepared ZnO sample has a hexagonal structure. SEM images revealed the formation of ZnO nanorods within a dense SiO2 substrate, ranging from 10 μm to 30 μm in length. Photoluminescent analysis showed that the film exhibited strong UV emission centered at 360 nm. The response time measurements indicated that the optimal photoresponse time was approximately 2.9 s. These results suggest the potential of ZnO-SiO2 films as efficient UV-emitting materials with high photoconductivity and a stably reproducible response under visible light, thereby laying the foundation for their application in advanced optoelectronic devices.

Exploring the Relationship Between Electrical Characteristics and Changes in Chemical Composition and Structure of OSG Low-K Films Under Thermal Annealing

The influence of annealing temperature on the chemical, structural, and electrophysical properties of porous OSG low-k films containing terminal methyl groups was investigated. The films were deposited via spin coating, followed by drying at 200 °C and annealing at temperatures ranging from 350 °C to 900 °C. In the temperature range of 350–450 °C, thermal degradation of surfactants occurs along with the formation of a silicon-oxygen framework, which is accompanied by an increase in pore radius from 1.2 nm to 1.5 nm. At 600–700 °C, complete destruction of methyl groups occurs, leading to the development of micropores. FTIR spectroscopy reveals that after annealing at 700 °C, the concentration of silanol groups and water reaches its maximum. By 900 °C, open porosity is no longer observed, and the film resembles dense SiO2. JV measurements show that the film annealed at 450 °C exhibits minimal leakage currents, approximately 5 × 10−11 A/cm2 at 700 kV/cm. This can be attributed to the near-complete removal of surfactant residues and non-condensed silanols, along with non-critical thermal degradation of methyl groups. Leakage current models obtained at various annealing temperatures suggest that the predominant charge carrier transfer mechanism is Poole–Frenkel emission.

Multi-confinement structured carbon dots with long room temperature phosphorescence lifetime and efficiency for sensing thiram residues assisted by copper ions.

Room temperature phosphorescent carbon dots(NCCDs@SiO2) were obtained by encapsulating hydrothermally synthesized CDs in a dense Si-O network structure after high-temperature calcination using silica as the matrix. Thiscan avoid the quenching effect of dissolved oxygen in water and has a phosphorescence lifetime of up to 2.41 s. Using the phosphorescence property of NCCDs@SiO2, a phosphorescence quenching sensor was developed for the sensitive and selective detection of thiram with the assistance of Cu2+. Cu2+-thiram complexesled to a rapid phosphorescence quenching of NCCDs@SiO2 within 30 s through the inner filter effect. The linear range of phosphorescence for thiram was 0.5-100 µM with a detection limit of 0.121 µM. Theproposed method was able to detect thiram in real samples and was validated by high-performance liquid chromatography (HPLC) confirmingthe potential of this phosphorescence sensing method for thiram detection. This work opens up a new avenue for the detection of thiram residues in fruits and vegetables and also provides a new idea for the design of a rapid detection platform using other room temperature phosphorescent materials.

Laser ablation behavior and mechanisms of 3D carbon fiber reinforced ZrB2-SiC composite

The high-performance 3D Cf-PyC/ZrB2-SiC composite was fabricated by vibration-assisted slurry impregnation and low-temperature hot pressing. The composite exhibited superior laser ablation resistance, leading to a low linear loss rate of 1.56×10−2 mm/s at laser power density of 4878 W/cm2 (Power 300 W, Laser beam diameter 2.8 mm) for 90 s. The coupling effect of a dense SiO2 layer in fringe region followed by dense ZrO2 layer in transitional region and next porous ZrO2 layer in center region achieved superior laser ablation resistance.

Ablation resistant C/C-HfC-ZrC-TaC-SiC composites prepared by reactive melt infiltration

Ultra-high temperature ceramics (UHTCs) modified C/C composites for extreme high-temperature environments are fascinating, the rapid introduction of UHTCs and structural temperature resistance are essential for the long-term service. Herein, C/C-HfC-ZrC-TaC-SiC composites with continuous ceramic-rich layers were designed and prepared to improve the ablation resistance. HZT441 (HfC:ZrC:TaC = 4:4:1) sample exhibited pseudo-plastic fracture with a flexural strength of 195.27 ± 11.11 MPa. After 80 s of oxyacetylene ablation (4.2 MW/m2), the mass and linear growth rates were 0.10 mg/s and 1.02 μm/s. This is attributed to the dense oxide layer, including (Hf,Zr)6Ta2O17, (Hf,Zr)O2 and SiO2, which effectively inhibits oxygen intrusion and carbon fiber damage. In addition, the thermal response behavior and stress distribution of ceramic-rich layers to C/C composites were investigated, which is contributed to provide a theoretical basis for the material design of thermal protection systems in extreme high-temperature environments.

Tribological characteristics and degradation mechanism of typical synthetic lubricants from room temperature to 300 ℃

In this study, the tribological characteristics of several synthetic lubricants—including poly alpha olefin (PAO), pentaerythritol ester (PET), perfluoropolyether (PFPE), ethyl siloxane fluid, chlorophenyl silicone oil (CPSO), and polydimethylsiloxane—were assessed from room temperature to 300 ℃. The results show that PAO 6 and PET exhibit exceptional lubrication properties at elevated temperatures. PAO 6, PET and ethyl siloxane fluid exhibits slight wear at high temperatures. Silicon oils, notably, tend to form Si aggregates at the Si3N4 and M50 (8Cr4Mo4V) wear interface. Among the silicone-based lubricants, ethyl siloxane fluids demonstrated the lowest wear rate, attributed to the generation of dense Si-O films at the wear interface that offer protective benefits. This work provides essential data support for the application of high-temperature lubricants.

Growth of CsPbX3 nanocrystals using sol–gel SiO2 solid powders as reactors without capping agents towards anomalous stable emission membranes

To overcome the poor stability of metal halide perovskite nanocrystals (NCs) prepared in organic solvents, a solid reaction method at high temperature was developed using sol–gel porous SiO2 as a reactor. Sol-gel SiO2 powders were used to encapsulate CsPbX3 NCs at high temperatures (600–800 °C) in air to get SiO2/CsPbX3 glass by dispersing growth cites in advance and no ligands added. After NaOH etching, a thin dense SiO2 shell was remained on the surface of CsPbX3 NCs (named as CsPbX3@SiO2).The photoluminescence (PL) quantum yield of the CsPbX3@SiO2 was 86.7 %, in which their PL peak wavelengths were adjusted from 410-707 nm by changing halogen components. The dense SiO2 shell on CsPbX3 NCs blocked the connection of CsPbX3 cores with water and oxygen in the external environment, resulting in CsPbX3@SiO2 revealed extraordinary high stability. After immersing in water for half year, the PL intensity of CsPbX3@SiO2 composites was remained unchanged and the composites still revealed bright PL after heat-treating in boiling water for 14 h. These highly stable composites were used to fabricate a white light-emitting diode, in which the color coordinates are located at (0.36, 0.33), and the color temperature reaches 5169 K which belongs to the warm white light range.

Mesoporous collapsing encapsulated carbon dots: Direct evidence for the effect of multiple-confined interactions of silica on efficient long-lived phosphorescence

Coupling with a matrix is the common strategy to improve the phosphorescence performance of carbon dots (CDs). However, there is a lack of clear and direct evidence about the formation mechanism of the inner relationship between CDs and matrix, and the specific effect of matrix on the phosphorescence performance. Mesoporous silica nanospheres (MSNs) were used as the research model to provide the direct evidence of the dense structure formation process and to construct a quantum chemical theoretical computational model of the effect of dense matrix on phosphorescence performance. Due to the multiple domain-limiting effects of rigid Si-C covalent bonds and SiO2 dense structure, the calcined-carbonized polymer dots-mesoporous silica nanospheres (C-CPDs-MSNs) can exhibit thermally activated delayed fluorescence (TADF) by tuning the HOMO-LUMO gap and enriching the electrons in the surface region. With a long lifetime of 3.07 s, the composite can also maintain stable phosphorescence performance under harsh conditions and has been successfully applied in multilevel information encryption, fingerprint identification, and cellular imaging. This work provides the direct evidence for the process of dense SiO2 structure enhancing the phosphorescence performance of CDs and offers a general strategy and method for the design and preparation of efficient and practical CDs-based phosphorescent materials.

High temperature oxidation behaviors of C/C composites with SiOC coatings

The oxidation behaviors of C/C composites with various silicon oxycarbide (SiOC) coatings prepared at various deposition temperatures (1300 - 1513 K) were studied. The SiOC coating improved the oxidation resistance of C/C composites by forming dense SiO2 glass, but such an improvement was sensitive to the deposition temperature. The SiOC coatings prepared at low temperatures exhibited limited oxidation resistance improvement given the appearance of penetrating cracks, fundamentally caused by their thin, amorphous (high elastic modulus), dense, and brittle characteristics. The SiOC coatings prepared at high temperatures had thick, polycrystalline (low elastic modulus), loose, and resilient structures, thus exhibiting higher oxidation resistance.

The dominant factor of C/SiC composites in ablation environment with heat and particle flow

As thermal structure components in scramjet engines, carbon-reinforced silicon carbide (C/SiC) composites are often subject to an ablation coupled with heat and particle flows. This study simulates the working environment of the scramjet engine's thermal structure component by coupling an Al2O3 particle flow with a plasma flow. The ablation behaviour of C/SiC composites in the presence and absence of particle impact have been comprehensively analysed. The results show that the factor dominating the ablation behaviour varies with temperature. At relatively low temperatures, the particles have difficulty breaking down the dense SiO2 layer formed from the oxidation of SiC in the presence of heat flow which is the dominant factor. As the temperature rises, the SiC begins to shift towards active oxidation. A few defects appear in the oxide layer, leading to a significant particle mechanical flaking effect. Particle flow is the dominant factor. However, a higher temperature led to the substantial active oxidation of SiC, and the oxide layer dissipated, exposing more defects. The heat flow causes a strong oxidation reaction, and the scouring effect of the particles is significant as well, both of which play a dominant role in the ablation process.

Oxidation behaviors of electrodeposited MoSi2 coating in a wide temperature range from 773 K to 1673 K

In this study, a MoSi2 coating was prepared on the surface of a molybdenum (Mo) substrate using the molten salt electrodeposition method, and the oxidation behavior of the coating over a wide temperature range from 773 K to 1673 K was studied in detail. The phase composition, surface and cross-sectional morphologies of the MoSi2 coatings after oxidation at various temperatures were characterized, and the evolution of the phase and microstructure were analyzed to elucidate the oxidation mechanism. The experimental results showed that the MoSi2 coating exhibited excellent oxidation resistance at lower temperatures (773 K) and did not experience the pesting phenomenon. When exposed to medium and high temperatures, a continuous and dense SiO2 layer was formed on the surface of the MoSi2 coating, which effectively prevented further oxidation inside the coating, thereby protecting the Mo substrate. Meanwhile, at higher temperatures, especially at 1473 K and 1673 K, the solid-phase diffusion of Si towards the substrate gradually transformed the MoSi2 into Mo5Si3 with a lower Si content. Notably, the thickness of the Mo5Si3 layer that was distributed between the original MoSi2 coating and the Mo substrate increased with the oxidation time and temperature. At 1673 K, a Mo3Si layer with a low Si content was formed beneath the Mo5Si3 layer. Thus, the diffusion of Si was an important factor restricting the service life of MoSi2 coating.

Reactively grown Al/Si-based top coatings protecting TM-diborides (TM = W, Ti, Hf) against high-temperature oxidation

Architectural designs involving Al- and/or Si-containing top coatings present an effective approach to protect oxidation-sensitive materials such as binary transition metal (TM) diborides against high-temperature oxidation. Using reactive arc evaporation, Al-Si-N, Al-Si-O, and Si-O top coatings were applied on sputter-deposited WB1.9, TiB2.7, and HfB2.4 thin films, respectively. A pure Si adhesion layer was introduced prior to the deposition of both oxide-based top coatings (Al-Si-O & Si-O) to prevent in-situ oxidation of the diboride base layers. During thermal annealing up to 1200 °C, Al-Si-O and Si-O provided outstanding oxidation resistance on top of TiB2.7 and HfB2.4, while limited adhesion was observed on WB1.9 at elevated temperatures. Contrary, Al-Si-N protected architectures suffered from accelerated oxidation beyond 800 to 1000 °C. Isothermal annealing in ambient air at 1200 °C for 30 h confirmed the long-term stability of the oxide-based top coatings, revealing no decohesion or oxygen inward diffusion beyond the Si interlayer. Complementary TEM analysis showed that the Si-O top coating forms a dense SiO2 scale above the columnar HfB2.4, while the formation of a mullite scale (3Al2O3·2SiO2) and pronounced recrystallization is recorded for the Al-Si-O protected HfB2.4. In summary, the Al-Si-O- and Si-O-based top coatings demonstrated exceptional high-temperature oxidation resistance, fully protecting the TM-diborides from oxidative attack.

Multilayer synergistic design of NbSi2/Nb2O5-SiO2/MoSi2 ceramic coating on niobium alloys for multiple thermal protection properties

A novel NbSi2/Nb2O5-SiO2/MoSi2 multilayer thermal protection ceramic coating is designed and fabricated via halide-activated pack cementation and liquid-plasma-assisted particle deposition and sintering technology, endowing the coating with multiple high-temperature protective properties. The incorporation of MoSi2 nanoparticles not only gives superior high-temperature oxidation resistance/hot corrosion resistance of coating by forming a dense SiO2 barrier layer, but also promotes passive radiative heat dissipation. When NbSi2/Nb2O5-SiO2/MoSi2 coating was prolonged exposure to air at 1250 °C for 50 h of oxidation and at 900 °C for 100 h of corrosion, the mass gains are only 0.37 mg/cm2 and − 0.52 mg/cm2, respectively. The multilayer coating exhibits enhanced emissivity (>0.88) over the thermal infrared range of 3–20 µm, due to the lattice vibrational absorption-matching synergy of Nb2O5, SiO2, and MoSi2, as well as the mutual doping of SiO2/Nb2O5, mismatch of atoms at the interface, and a high degree of lattice distortion within the coating.

Effect of Si content on the microstructure and high temperature oxidation resistance of TiAlCrNiSi high-entropy alloy films synthesized by multi-target magnetron co-sputtering

In this study, we prepared TiAlCrNiSi high-entropy alloy films (HEFs) with different contents of four elements by multi-target magnetron co-sputtering with regulated sputtering power, and the elemental contents of the films all met the high-entropy range (5 at%−35 at%). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the surface of the films was relatively smooth, the increase of sputtering power and Si element in the films reduced the roughness of the films. This demonstrates the effect of Si in refining the grains and reducing the roughness of the film. X-ray diffraction (XRD) showed that the average grain size of films decreases with the increase of Si content. This phenomenon proves that Si has the effect of refining grains and reducing the roughness of the film. The TC4 substrate and the films were treated at 700 ℃ for 50 h. It was found that TiAlCrNiSi HEFs could improve the high temperature oxidation resistance of TC4. The mechanism of oxidation resistance of the films was investigated by XRD, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), and it was found that the generation of dense oxides Al2O3, Cr2O3 and SiO2, the grain refining effect of Si and the appearance of amorphous structure were the main reasons for the increased oxidation resistance.

CsPbBr3@Glass@SiO2 Composite Materials with Ultrahigh Water Resistance and Thermal Reversibility of Photoluminescence

AbstractPerovskite quantum dots usually possess excellent optical properties, such as high color purity, high photoluminescence quantum yield, and tunable emission wavelength. However, their relatively low stability against heat, water, oxygen, and light hinders their practical applications in the field of optoelectronic devices. Herein, melt‐quenching and heat treatment crystallization methods are used to fabricate the CsPbBr3@glass. Atomic layer deposition and the sol–gel method are applied to encapsulate the CsPbBr3@glass with a dense SiO2 layer and hydrophobic SiO2 layer, respectively, to further improve the water resistance, thermal reversibility of photoluminescence, and photostability. The CsPbBr3@glass@ASG can be synthesized with the triple layer encapsulation of the glass matrix, dense SiO2 layer, and hydrophobic SiO2 layer. During the water resistance test for seven weeks, CsPbBr3@glass@ASG can preserve ≈100% of initial PL intensity. Similarly, it can preserve ≈100% of PL intensity after five continuous heating–cooling cycles between 30 and 100 °C. In addition, the internal and external quantum efficiencies of CsPbBr3@glass@ASG can still be maintained at 42.0% and 33.7%, respectively. Results indicate that CsPbBr3@glass@ASG can provide a balance between optical properties and extrinsic stability successfully, thereby becoming a potential candidate material for practical applications related to optoelectronic devices in the future.

The high temperature oxidation and thermal shock behavior of a dense WSi2–TaSi2 coating on Ta substrate prepared by a novel two-step process

In order to improve the oxidation resistance of the Ta substrate, a novel two-step process including molten salt electrodeposition (Na2WO4-WO3 system) and halide activated pack cementation was adopted to prepare a WSi2–TaSi2 coating on tantalum substrate. During the electrodeposition process, dense tungsten coatings were fabricated at current densities of 30 mA/cm2, 40 mA/cm2 and 50 mA/cm2. It was observed that the grain size exhibited a log-normal distribution. When the current density was 40 mA/cm2, the grain size and flattest surface of the tungsten coating reached 9.50 ± 0.23 μm and 6.792 μm, respectively. When performing the static oxidation test, the WSi2–TaSi2 coating could effectively protect the Ta substrate oxidized at 1600 °C for 30 h. This is attributed to the presence of dense SiO2 and Ta2O5, which acted as a protective layer and suppressed the further penetration of oxygen. Furthermore, due to the matching thermal expansion coefficient between each layer and the sealing ability of semi-molten SiO2, the four-layer SiO2–W5Si3–WSi2–Ta5Si3 coating could successfully pass 721 thermal shock tests from 1600 °C to room temperature.

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

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

Design of lightweight and antioxidant SiCnws/SiC(Mox, rGO) nanocomposite from molybdenum-modified precursors for aerospace vehicle components

Making lightweight SiC composite ceramics with good load-bearing and remarkable surficial antioxidant capacity for use in thermal protection system is challenging. In this contribution, an ingenious strategy is reported to produce SiC(Mox, rGO) nanocomposite polymer-derived ceramics (PDCs) for the first time by reactively re-pyrolyzing flexible polymolybdenumcarbosilane-vinyltriethoxysilane-graphene oxide (PMVG)/rigid SiC(Mox, rGO)p fillers blends. Highly cross-linked PMVG precursors connect up to five polycarbosilane (PCS) to optimize their formability. In-situ formed MoSi2 and Mo4.8Si3C0.6, compatible with β-SiC/SiOxCy/Cfree(rGO) matrix, can effectively strengthen interface bonding, promote nailing cracks, and even hinder β-SiC grain growth. More interestingly, using in-situ SiC nanowires (SiCnws) by Mo-containing catalyst to decorate samples is an efficient route to reinforce the ceramic network, and SiCnws/SiC(Mo0.1, rGO) nanocomposite PDCs display optimal Vickers hardness (5.19 GPa), enhanced fracture toughness (4.26 MPa·m1/2), high compressive strength (151.73 MPa), and low linear shrinkage (4.75%). Such products remain stability/integrity under flame scouring, their surface is covered by a dense SiO2 passivation layer with good self-healing capacity owing to unique non-destructive oxides of embedded MoSi2/Mo4.8Si3C0.6. This work can further provide a feasible way to overcome the critical fabrication process in lightweight porous PDCs with well-balanced high load-bearing and good thermal insulation integration for aerospace vehicle components.

Microstructure and oxidation resistant of Si–NbSi2 coating on Nb substrate at 800°C and 1000°C

The Si–NbSi2 composite coating with a smooth surface was successfully prepared on Nb substrate by hot dip silicon-plating (HDS) technology. The composite coating is composed of Si outer layer, NbSi2 interlayer and Nb5Si3 interfacial layer. And the average surface roughness (RSa) and specific surface area growth rate (Sdr) are only 0.275 μm and 2.85%, respectively. The cyclic oxidation test shows that the Si–NbSi2 composite coating has a very excellent oxidative resistance after oxidation at 800 °C for different times. After oxidation for 40 h, the Δm/S and oxide layer thickness of the coating are only 3.72 mg/cm2 and 8 μm, respectively. After oxidation at 1000 °C for 20 h, the coating surface is almost completely covered by a dense SiO2 layer, the Δm/S and oxide layer thickness of the coating are 7.28 mg/cm2 and 15 μm, respectively. The Si–NbSi2 composite coating presents good self-healing ability and excellent oxidation resistance, which can significantly prolong the service life of bare Nb in oxidation environment.