SiO Evaporation Research Articles

Investigation of the microstructure and oxidation behavior of monolayer and bilayer ZrB2-SiC-Y2O3/SiC coatings on C/C composites

To improve the oxidation resistance of carbon/carbon composites in high-temperature environments, the monolayer SiC and ZrB2-SiC-Y2O3 coatings and bilayer ZrB2-SiC-Y2O3/SiC coatings were prepared on carbon/carbon composites by atmospheric plasma spray technique. The chemical composition, morphology, and microstructure of the coating were characterized by XPS, SEM, XRD, and TEM. The oxidation behavior of each layer was evaluated at 1723 K for 10 h in the air. The result showed that the bilayer coating could effectively prevent the degradation of the C/C substrates. After 10 h oxidation, its mass loss was only 1.39 % compared to 15.27 % for the monolayer ZrB2-SiC-Y2O3-coated sample and 8.77 % for the SiC-coated sample. Based on XRD pattern results and microstructure observations, the failure mechanism of the monolayer SiC coating was mainly attributed to the evaporation of SiO2 at elevated temperatures. The coefficient of thermal expansion (CTE) mismatch between the substrate and coating was the main reason for the spallation of the monolayer ZrB2-SiC-Y2O3 coating. For the bilayer coating, the inner layer of SiC was responsible for the superior because it blocked the inward diffusion of oxygen and was used as a transition layer to reduce thermal stress, which made the coating more durable. Meanwhile, the amorphous-nanocrystalline composite structure is believed to be able to reduce the evaporation rate of SiO2 and prevent the inner diffusion of oxygen.

Formation mechanism of (Ti, Zr, Hf)C medium-entropy carbide derived from polymer precursors and its modification on the ablative mechanism of C/C-SiC composites

Medium-entropy carbide (MEC) of (Ti, Zr, Hf)C was synthesized and its modification on the ablative mechanism of C/C-SiC composites was investigated. After 60 s oxyacetylene flame test, the mass and linear ablation rates were 1.43 mg/s and 11.1 μm/s, respectively. The evolution of ceramics oxide scale dominated the anti-ablation process, which transfers from (Ti, Zr, Hf)xCyOz, (Ti, Zr, Hf)xSiyOz to (Ti, Zr, Hf)O2, (Zr, Hf)TiO4. With further evaporation of SiO2, the generated (Ti, Zr, Hf)O2 skeleton and the residual liquid oxides ((Zr, Hf)TiO4 and (Zr, Hf)SiO4) can block the oxygen channel and further reduce oxidation of the inner matrix.

Microstructural evolution and ablation behaviors of NbSi2/SiO2–Nb2O5/X (X=MoSi2, MoSi2-Yb2O3, MoSi2-Yb2O3–ZrC) multilayer coatings on Nb alloy in different ablation environments

To improve the high-temperature ablation resistance of Nb alloys, multiphase-multilayer ceramic coatings were prepared using a novel strategy combining halide activated pack cementation (HAPC) and liquid plasma-assisted particle deposition and sintering (LPDS) methods. The MoSi2, Yb2O3, and ZrC particles were deposited individually or co-deposited on the NbSi2 bottom layer to obtain NbSi2/SiO2–Nb2O5/MoSi2 (Mo), NbSi2/SiO2–Nb2O5/MoSi2-Yb2O3–SiO2 (MoYb), NbSi2/SiO2–Nb2O5/MoSi2-Yb2O3–ZrO2–ZrC (MoYbZr) multilayer ceramic coatings, respectively. Results showed that all three coatings remained intact after propane ablation (1500 °C), exhibiting excellent ablation resistance. For the harsher oxyacetylene ablation environments with higher temperatures (1800 °C), Mo coating suffered from long cracks, bubbling, and localized spalling and MoYb coating sprouted massive cracks without spalling. The oxide scale integrity of the MoYbZr coating was well maintained without cracks, exhibiting excellent resistance to oxyacetylene ablation attributed to the incorporation of Yb2O3 and ZrC. The mass and linear ablation rates of MoYbZr coating were only 0.137 mg/s and 0.196 μm/s after oxyacetylene ablation, which were 25.14 % and 49.61 % lower than that of MoYb coating, and 88.01 % and 75.37 % lower than that of the Mo coating, respectively. The Yb2O3 strengthened the Si–O bonding in the SiO2 network and increased the viscosity of the SiO2 scale, thus enhancing the self-healing ability of the oxide scale and slowing down the evaporation of SiO2. The oxidation products (ZrO2 with SiO2) from MoSi2 and ZrC further reacted to produce ZrSiO4, which can inhibit microcrack creation and extension. Micropores in the Zr–Si–O oxide scale mitigated thermal stresses, inhibited crack initiation and extension. The Yb2O3, Yb2SiO5, and ZrSiO4 embedded in the SiO2–ZrO2 scale acted as pinning effects, enhancing the high-temperature structural stability, thereby improving the ablation resistance of the MoYbZr coatings.

Construction of C/SiC–Cu3Si–Cu interpenetrating composites for long-duration thermal protection at 2500 °C by cooperative active-passive cooling

Developing materials which can resist long-term ablation at ultra-high temperatures above 2200 °C in oxygen-containing environments is of prime importance for the thermal protection of aerospace vehicles, but remains a great challenge. C/SiC composites are attractive candidates for long-term use, but they are often severely ablated above 1650 °C due to the damage of protective oxide scale. Here, C/SiC–Cu3Si–Cu interpenetrating composites with interconnected ceramic and metal matrixes are developed by the selective ceramization of carbon-fiber reinforced carbon aerogel preforms. The heat radiation of C/SiC–Cu3Si–Cu, transpiration of Cu2O, evaporation of SiO2, as well as inhibition of oxygen diffusion in the glassy scale, jointly lead to the cooperative active-passive cooling during ablation. Resultantly, their surface temperatures are 150 °C, 450 °C and 750 °C lower than those for C/SiC (1900 °C, 2200 °C and 2500 °C) with a duration of 600 s ablation, respectively. They also exhibit remarkably low mass and linear ablation rates with respective 0.054–0.064 mg cm−2 s−1 and -0.157–0.018 μm s−1. Additionally, the composites can endure 1160 s ablation with a surface temperature of 1750 °C under the ablation condition where the surface temperature of C/SiC is 2200 °C. The developed C/SiC–Cu3Si–Cu interpenetrating composites are so far the only known light-weight (<3.0 g cm−3), non-ablation materials which can offer long-duration (≥600 s) thermal protection at 2200–2500 °C in high-enthalpy-flow and oxidizing environments.

Effects of melt depth on oxygen transport in silicon crystal growth by continuous-feeding Czochralski method

The continuous-feeding Czochralski method (CCz) is an effective and promising way for the high-quality and large-weight monocrystalline silicon growth. In the CCz technology, the inner crucible is employed to prevent unmelted silicon granules from being transported to the melt-crystal interface, which introduces a new source of oxygen impurities and makes the temperature and flow distribution in the melt complicated. So it is challenging to regulate the oxygen impurities at the m-c interface in the CCz process. The melt depth stays constant during the CCz process, and it determines the crucible area where oxygen impurities are dissolved. Therefore, in order to manage and minimize the oxygen impurities at the m-c interface, controlling the melt depth may be a good way. In the study, the influence of melt depth on oxygen impurity distribution at the melt-crystal interface is investigated using a series of global simulations coupled with the transport of oxygen and carbon impurities. The findings indicate that as melt depth increases, the temperature of the crucible wall inside the inner crucible drops, and oxygen impurities transported from the melt between the inner and outer crucibles to the interior of the inner crucible likewise decrease in content. However, the rate of SiO evaporation at the free melt surface inside the inner crucible essentially stays the same. Therefore, the oxygen impurity content at the melt-crystal (m-c) interface decreases with the increase of melt depth. The paper also suggests that while constructing the scheme for lowering oxygen content, it is necessary to take into account the position relation between the heater and the crucible, the height of the crucible above the free melt surface, and the rotation of the inner crucible.

Ablation property and oxide scale evolution of Y2O3 modified C/C-SiC composites fabricating via precursor infiltration and pyrolysis method

Employed precursors of Y(NO3)3·6H2O and polycarbosilane, Y2O3 doped C/C-SiC composites were prepared by utilizing precursor infiltration and pyrolysis method. Results show that Y2O3 alters the composition of C/C-SiC composites with the generation of β-Y2Si2O7 and Y2O3 together with slight Y2SiO5, Y4.46(SiO4)3O and Y2C3. During ablation, β-Y2Si2O7 is stable and directly liquefied to rapidly fill in the empties in the substrate; the other three are transformed to δ-Y2Si2O7, evolving the oxide scale from SiO2-β-Y2Si2O7-Y2O3 (semi-fused) layer to SiO2-β-Y2Si2O7-δ-Y2Si2O7 ones. The ablation rates after ablation for 120 s were respectively 0.210 mg/s and 14.58 µm/s, severally decreased by 57.32% and 8.59% compared with that for 60 s. Further evaporation of SiO2 is suppressed and reconcilement of oxide layer is hence confirmed, implying decent anti-ablation resistance of oxide layer.

Long-term and cyclic ablation behavior of La2O3 modified Cf/ZrB2-SiC composites at 2500 ℃

In this work, long-term and cyclic ablation behavior of the La2O3 modified Cf/ZrB2-SiC composites were studied by air plasma torch at 2500 ℃. It is indicated that a micron-sized ZrO2-La2O3-SiO2 liquid phase layer forms on the ablation surface at 2500 ℃, which reduces the evaporation of SiO2 and decreases the mass recession rates of the composites. However, the ZrO2-La2O3-SiO2 liquid phase layer exhibits poor structure stability. In addition, the La2O3 weakens the pinning effect of bulk ZrO2. Consequently, the oxide layer with low viscosity tends to sputter out due to the mechanical scouring by plasma flame. Therefore, the linear recession rates of the composites are increased during cyclic ablation with the increase of the La2O3 content.

Microstructure and ablation behavior of Zr-Ti-Si-C multiphase coating fabricated by solid solution and in-situ reaction

The Zr-Ti-Si-C solid solution coatings were fabricated by solid solution and in-situ reaction methods, and effects of TiC on the microstructure and phase evolution, and ablation behavior of Zr-Ti-Si-C solid solution coatings were investigated. Results show that TiC was reacted with zirconium or silicon compounds to generate even-distributed ternary ZrxTi1−xC and quaternary (TixZr1−x)3SiC2 solid solution. The solid solution structure decreases the active oxidation of Si source, and reduces the depletion and evaporation of SiO2, forming a strengthened ZrO2-ZrTiO4-TiO2-SiO2 oxides layer with low oxygen diffusion coefficient, thereby enhancing the ablative property of the Zr-Ti-Si-C solid solution coatings.

Investigations on Ba diffusion and SiO evaporation during BaSi2 film formation on Si substrates by thermal evaporation

Thermal evaporation is a simple and rapid process to fabricate BaSi2 films for solar cell applications. In this study, we investigated the extent of Ba diffusion into the Si substrate and SiO evaporation, which occur during the BaSi2 film formation, aiming at elucidating BaSi2 film formation processes. Composition depth profiles determined by Auger electron spectroscopy reveal that SiO evaporates at 650 °C and above concurrently with surface oxidation, which is supported by surface roughening observed by secondary electron microscopy. Ba diffusion into the substrate is also detected from composition depth profiles. It is demonstrated that the amount of Ba in-diffusion decreases as the deposition rate increases. The present findings serve for controlling the composition of BaSi2 evaporated films.

Effect of Argon Flow on Oxygen and Carbon Coupled Transport in an Industrial Directional Solidification Furnace for Crystalline Silicon Ingots

Transient global simulations were carried out to investigate the effect of argon flow on oxygen and carbon coupled transport in an industrial directional solidification furnace for quasi-single crystalline silicon ingots. Global calculation of impurity transport in the argon gas and silicon melt was based on a fully coupled calculation of the thermal and flow fields. Numerical results show that the argon flow rate affects the flow intensity along the melt–gas surface, but has no significant effect on the flow patterns of silicon melt and argon gas above the melt–gas surface. It was found that the evaporation flux of SiO along the melt–gas surface decreases with the increasing argon flow rate during the solidification process. However, the net flux of oxygen atoms (SiO evaporation flux minus CO dissolution flux) away from the melt–gas surface increases with the increasing argon flow rate, leading to a decrease in the oxygen concentration in the grown ingot. The carbon concentration in the grown ingot shows an exponential decrease with the increase of the argon flow rate, owing to the fact that the dissolution flux of CO significantly decreases with the increasing argon flow rate. The numerical results agree well with the experimental measurements.

Multi-cycle and long-term ablation behavior of Cf/ZrB2-SiC composites at 2500 °C

Multi-cycle and long-term ablation behavior of Cf/ZrB2-SiC composite was evaluated by plasma ablation flame at 2500 °C. The composites can be protected by the oxide layer with uniform distribution of ZrO2 particles in SiO2 melt. However, the structural stability of the oxide layer could be destroyed due to the phase transition of ZrO2 and violent evaporation of SiO2, resulting from the aggregation and growth of ZrO2 during multi-cycle and long-term ablation. It is of great significance to optimize the composition and microstructure of the oxide layer to retard the aggregation, growth and phase transition of ZrO2.

Evaporation behavior of SiO2 glass doped with various transition metal oxides

AbstractTo investigate the effect of transition metal oxides on the evaporation characteristic of SiO2 glass at 1700°C, nine transition metal oxides were added to SiO2 glass. The results show that V2O5, MoO3, and WO3 have little effect on the high‐temperature stability of SiO2 glass due to their evaporation at 1500‐1700°C. Besides, Nb2O5 and Ta2O5 reduce the evaporation of SiO2 by covering the surface of the sample. Moreover, by improving the content of cristobalite, the evaporation of the SiO2‐Cr2O3 sample decreases. Meanwhile, TiO2, ZrO2, and HfO2 are beneficial to decreasing the evaporation of SiO2 glass because Ti, Zr, and Hf diffuse into c‐SiO2. Meanwhile, the SiO2‐HfO2 sample possesses the least evaporation.

Ablation properties of ZrC-SiC-HfB2 ceramic with different amount of carbon fiber under an oxyacetylene flame

In the present work, a carbon fiber reinforced ZrC-20 vol% SiC-15 vol% HfB2 composite was produced by pressureless sintering method. Carbon fiber with various weight percentages (0, 10, 20, 30) was added to the milled ZrC-SiC-HfB2. The mixed powders were pressed and sintered at 2200 °C for 2 h. The microstructure and mechanical properties of the specimens were investigated by Scanning Electron Microscope (SEM) equipped with EDS spectroscopy, XRD analysis and hardness and toughness tests. The ablation resistance of the composites was performed by oxyacetylene torch. The results showed that as the carbon fiber content was increased, the porosity and toughness were increased as well, while the hardness and density were decreased. During the ablation test, the outer surface of the composite underwent a sintering process and formed a dense layer of Zr-Hf-Si-O on the surface. ZrSiO4 and HfSiO4 were also created after cooling, so these phases played a pinning effect and prevented the crack propagation. The results also showed that linear and mass ablation rates were increased with increasing the weight percentage of the carbon fiber. SEM analysis with EDS further revealed that with increasing the carbon content, the defects in the formed oxide layer on the composite surface were increased due to the evaporation of SiO2 and the decrease in the pinning effect of ZrSiO4 and HfSiO4 phases as well as the phase transformation of the remained ZrO2 and HfO2. As a result, as the carbon fiber was increased, the ablation resistance of the composite was decreased.

Silicon nanowhisker formation during SiO2 evaporation by arc discharge

Nanoscale silicon is interesting in various areas, including electronics, photovoltaics, solar and Li-ion energetics. This work describes processes of electric arc sputtering of SiO2 in a graphite electrode in a helium medium, leading to formation of silicon nanowhiskers. A fan- shaped turbulent jet of vapors of the sprayed electrode flowing from the interelectrode space, resulting in formation of areas with different component compositions and temperatures, is considered. Interaction of SiO and Si vapors leads to the formation of the silicon nanowhisker structure.

Effects of SnO&lt;sub&gt;2&lt;/sub&gt;-SiO&lt;sub&gt;2&lt;/sub&gt;-MgO-Bi&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Additions on Liquid Phase Sintering Silicon Nitride

Si3N4 ceramics were sintered at relatively low temperature of 1400-1650 °C with additives from the SnO2-SiO2-MgO-Bi2O3-Y2O3 system. The degree of densification, α-β transformation, mass loss and microstructure were measured as a function of additives composition, sintering temperature, and nitrogen gas pressure. Specimen prepared from 3 mass% SnO2, 3 mass% SiO2, 3 mass% MgO, 6 mass% Bi2O3, and 5 mass% Y2O3 as the sintering additive could be sintered to almost full density at relatively low temperature as 1550 °C for 2 h under ambient pressure of nitrogen atmosphere. The α-β transformation was not completed at this condition. The mass loss was relatively high due to the evaporation of SiO2, MgO and Bi2O3. However, the resulted dense materials have high bending strength.

Advances in the monitoring of the SiO2 evaporation loss in transparent YAG ceramics by LIBS

Abstract The effect of silica on the densification of transparent YAG ceramics has long been investigated. The present paper makes a step forward on the quantification of the Si amount in YAG. The study shows how to measure with a high level of accuracy the silicon concentration in YAG in different steps of the fabrication process. The Laser-Induced Breakdown Spectroscopy (LIBS) has been used to quantify the amount of silicon in a set of samples with 0–0.275 wt% silica. Nanometric silica soot or its precursor, TEOS, has been used as silica source. The analysis shows that more than 80 wt % of SiO2 evaporates during reactive-sintering under vacuum, most probably because SiO gas forms during sintering under low oxygen partial pressure. In-depth concentration maps were obtained by laser drilling with LIBS pulses. The results show the possibility to modify the amount of residual Si in YAG ceramics by adjusting the sintering and annealing process.

Ablation Behaviors of ZrC-20vol.%MoSi&lt;sub&gt;2&lt;/sub&gt; Composite Coating under Different Heat Fluxes

ZrC-20vol.%MoSi2 (ZM) composite coating was fabricated by vacuum plasma spray and the ablation resistance was assessed using plasma flame under low (1.94 MW/m2) and high (3.01 MW/m2) heat fluxes, respectively. Results showed that the ultimate surface temperatures of ZM coating were about 2100 °C and 2400 °C, respectively. ZM coating exhibited good ablation resistance at low heat flux, which benefited from the low evaporation of SiO2 and the diffusion of Si derived from MoSi2 decomposition. However, bubble-burst event took place under high heat flux. The different ablation behaviors of ZrC-MoSi2 coating were analyzed, which might contribute to the application of ultra-high temperature ceramic coatings.

Thermal ablation behavior of SiC coating for 3D braided carbon fiber reinforced ZrC-SiC composites in different heat fluxes

In this study, CVD-SiC coating was prepared on 3D braided carbon fiber reinforced ZrC-SiC (abbreviated as Cf/ZrC-SiC) composites, and the microstructure and morphology of the composites and coating were characterized by scanning electron microscopy coupled with energy dispersive spectrometry (SEM–EDS), X-ray diffraction (XRD). The effect and mechanism of CVD-SiC coating on the ablation properties of Cf/ZrC-SiC composites under different ablation conditions were studied in details. The results showed that the CVD-SiC coating could reduce the ablation temperature and improve the compactness, which will promote the ablation properties of the Cf/ZrC-SiC composites in the heat flux of 2.38 MW/m2. But with the heat flux increasing to 4.18 MW/m2, ablation properties of Cf/ZrC-SiC composites decreased significantly, it was mainly difficult to form dense and stable ZrO2-SiO2 protective oxides due to accelerated mechanical erosion which makes the CVD-SiC coating lose its protection for Cf/ZrC-SiC composites during ablation and higher ablation temperature which will aggravate active oxidation of SiC and the evaporation of SiO2.

Incorporating Zr to achieve self-protecting and enhancement of silica sol bonded SiC castables at active oxidation condition

SiC castables exhibit degraded properties in static air at 1700°C, due to the formation of gaseous products. The efficiency of different contents of Zr in SiC castables was evaluated by considering sintered properties, mechanical performance, isothermal oxidation behavior, and microstructural analysis of the SiC castables. Specimens with more Zr exhibited enhanced mechanical behavior and anti-oxidation capability. The addition of Zr decreased the evaporation of SiO2 by reducing its equilibrium partial pressure (g), and formed a dense ZrO2-SiO2 protective layer (e.g., the sample with 0.9wt% Zr) to prevent further degradation of the SiC castable. The Zr that was preferentially oxidized to ZrO2 reduced the partial pressure of the oxidizing gases (O2 and CO2) in the matrix, and increased SiO (g) content, which facilitates formation of SiC fibers, which, in turn, improves the anti-oxidation capability and mechanical behavior of SiC castables, preventing their degradation in static air at 1700°C. The addition of Zr created a ZrO2-SiO2 protective layer on the surface and prevented the decrease in SiC content, by forming SiC fibers. This made the silica sol bonded SiC castable a self-protecting refractory.

Shifting Lu2SiO5 crystal to eutectic structure by laser floating zone

Starting from stoichiometric mixtures of lutetium and silicon oxides, Lu2SiO5 (LSO) crystals to eutectic ceramic rods were obtained using the laser floating zone (LFZ) technique in air at growth rates ranging from 200 to 5mm/h. The newly obtained eutectic is formed by Lu2O3 fibrils and/or lamellae into a LSO matrix being its formation induced by a SiO2 evaporation process during the LFZ growth.Scanning electron microscopy (SEM) and Raman spectroscopy studies allowed observing how growth rate affects microstructural evolution, namely the transition from single crystal to eutectic. SEM observations show that the eutectics present alternated monophasic oxyorthosilicate regions with biphasic Lu2SiO5/Lu2O3 ones identified by X-Ray diffraction as C2/c monoclinic Lu2SiO5 and Ia3¯ cubic lutetium oxide.While the luminescence phenomena were found to be only dependent on the excitation energy, a pronounced UV light-induced reversible photochromic effect was observed in the samples grown at slower rates.