SnO<Subscript>2</Subscript>-core/SiO<Subscript>x</Subscript>-shell coaxial whiskers with a needle-like morphology

Needle-shaped structures of tin oxide (SnO₂) were coated with a shell layer of SiO_x via a sputtering method. The diameters of the SiO_x-coated structures gradually became thinner, leading to the formation of sharp tips. The whiskers consisted of a crystalline SnO₂ core surrounded by an amorphous SiO_x shell. The photoluminescence (PL) spectrum with a Gaussian fitting exhibited yellow-green and orange light emission bands, and the overall shape and intensity of the PL spectrum were not changed by the SiO_xcoating. The results of this study suggest that sputtering can be employed to achieve the layered growth of shell layers on a variety of nanostructures.     

New environmental barrier coating system on carbon-fiber reinforced silicon carbide composites

Carbon-fiber-reinforced silicon carbide composites (C/SiC) are promising materials for high-temperature, light weight structural components. However, a protective coating and environmental barrier coating (EBC) are necessary to prevent the oxidation of the carbon and the reaction of the formed silica scale with water vapor. Current EBC systems use multiple layers, each serving unique requirements. However, any mismatch in the coefficients of thermal expansion (CTE) creates internal stresses and might lead to crack formation. In this case, oxygen and water vapor penetrate through the EBC, reducing the lifetime of the component. Mullite (Al₆Si₂O₁₃) is used in many known EBC systems on silicon-based ceramics either as an EBC itself or as a bondcoat. Due to its low CTE and its sufficient thermal cycling behavior, mullite was chosen in this investigation as a first layer. As mullite suffers loss of SiO₂ when exposed to water vapor at high temperatures, an additional protective top coat is needed to complete the EBC system. Different oxides were evaluated to serve as top coat, especially high-temperature oxides with low coefficients of thermal expansion (LCTE). An EBC containing mullite as bondcoat and the LCTE oxide La₂Hf₂O₇ as a top coat is proposed. Both layers were applied via atmospheric plasma spraying. In this paper, results of the influence of processing conditions on the microstructure of single mullite and LCTE oxide layers as well as mullite/LCTE oxide systems are presented. Special emphasis was directed toward the crystallinity of the mullite layer and, in the top layer, toward low porosity and reduced crack density.     

Reaction behavior of kaolinite with ferric oxide during reduction roasting

The pre-separation of silica and alumina in aluminosilicates is of great significance for efficiently treating alumina-/silica-bearing minerals for alumina production. In this work, the reaction behavior of kaolinite with ferric oxide during reduction roasting was investigated. The results of thermodynamic analyses and reduction roasting experiments show that ferrous oxide obtained from ferric oxide reduction preferentially reacts with alumina in kaolinite to form hercynite, meanwhile the silica in kaolinite is transformed into quartz solid solution and/or cristobalite solid solution. With increasing roasting temperature, fayalite formed by reaction of surplus ferrous oxide with silica at low temperature is reduced to silica and metallic iron in the presence of sufficient carbon dosage. However, increasing roasting temperature and decreasing Fe₂O₃/Al₂O₃ molar ratio favor mullite formation. The complete conversion of kaolinte into free silica and hercynite can be obtained by roasting raw meal of kaolin, ferric oxide and coal powder with Fe₂O₃/Al₂O₃/C molar ratio of 1.2:2.0:1.2 at 1373 K for 60 min. This work may facilitate the development of a technique for comprehensively utilizing silica and alumina in aluminosilicates.     

Effect of Fe/SiO2 and CaO/SiO2 mass ratios on metal recovery rate and metal content in slag in oxygen-enriched direct smelting of jamesonite concentrate

The oxygen-enriched direct smelting of jamesonite concentrate was carried out at 1250 °C by changing the slag composition. The effects of Fe/SiO₂ and CaO/SiO₂ mass ratios on the metal recovery rate as well as metal content in slag were investigated. Experimental results indicated that the metal (Pb+Sb) recovery rate was up to 88.30%, and metal (Pb+Sb) content in slag was below 1 wt.% under the condition of slag composition of 21−22 wt.% Fe, 19−20 wt.% SiO₂ and 17−18 wt.% CaO with Fe/SiO₂ mass ratio of 1.1:1 and CaO/SiO₂ mass ratio of 0.9:1. The microanalysis of the alloy and slag demonstrated that the main phases in the alloy contained metallic Pb, metallic Sb and a small amount of Cu₂Sb and FeSb₂ intermetallic compounds. The slag was mainly composed of kirschsteinite and fayalite. Zinc in the raw material was mainly oxidized into the slag phase in the form of zinc oxide.     

Cathodoluminescence Analysis for the Nondestructive Evaluation of Silica Scale on an Iron-Based Alloy

Information on the composition, morphology, and thickness of surface oxide scale helps to control the performance of heat-resistant alloys. Currently, there is a lack of adequate analytical methods that allow the rapid and nondestructive evaluation of these properties. In this study, we demonstrate a nondestructive method for identifying silica (SiO₂) scale and evaluating its morphology and thickness within 1 min by acquiring the cathodoluminescence (CL) images and spectra of SiO₂ scale on an Fe–5%Si alloy heated to 900 °C. SiO₂ scale emitted yellow–orange, violet, or red luminescence, whereas the other scale products that form on Fe–Si alloys, such as FeO, Fe₃O₄, Fe₂O₃, and Fe₂SiO₄, did not. Thus, we can identify SiO₂ scale and observe its morphology on the basis of luminescent color in the CL image. The thickness of SiO₂ scale can be correlated to the intensity of the CL peak at 645 nm. Therefore, the acquisition of CL images and spectra is a novel analytical method, which allows one to control the performance of SiO₂-forming Fe–Si alloys.

     

The influence of silicon on the high-temperature oxidation of nickel

An investigation of the oxidation of nickel-silicon alloys has been carried out in order to ascertain the mode of development of partially or fully protective SiO₂ layers. The addition of 1% Si has little effect on the oxidation rate of nickel at 1000°C but is sufficient for partial-healing layers of amorphous SiO₂ to be established. These layers are incorporated into the inner part of the duplex NiO scale but do not react with the oxide to form a double oxide. Increasing the silicon concentration to 4% or 7% facilitates the development of apparently continuous amorphous SiO₂ layers at the base of the NiO scale, resulting in reduced rates of oxidation. However, these layers develop imperfections, possibly microcracks resulting from oxide growth stresses, and are unable to prevent some continued transport of Ni²⁺ ions into the NiO scale and oxygen into the alloy, particularly for Ni-4% Si. Although the formation of SiO₂-healing layers can reduce the rate of oxidation of nickel, they provide planes of weakness that result in considerable damage under the differential thermal contraction stresses during cooling. In particular, severe scale spalling occurs for Ni-4% Si and Ni-7% Si as failure occurs coherently within the SiO₂ layer.     

Corrosion Resistance of a High-Silicon Alloy in Metal-Dusting Environments

A Si-containing, high-temperature alloy (Fe–17Cr–9Ni–8Mn–4Si) was exposed to high-carbon activity and low-oxygen partial pressure environments (CO–H₂) over a temperature range from 650 to 950 °C. No metal dusting corrosion was observed in this alloy. The structure and composition of the surface films formed were characterized in detail at the nanometer level. At a temperature of 650 °C, the surface-oxide films formed are made up of an inner, continuous, amorphous-silica (SiO₂) layer and an outer crystalline manganese chromate (MnCr₂O₄) spinel layer with manganese oxide (MnO) crystals on the surface. By contrast, at a higher temperature of 950 °C, a more-complex layered structure is developed, comprising inner, continuous, amorphous SiO₂ and crystalline manganese-silicate (Mn₂SiO₄) layers and an outer crystalline Cr₂O₃/MnCr₂O₄ duplex layer with MnO crystals having variable textures on the surface.     

Protective Al-rich mullite coatings on Si- based ceramics against hot corrosion at 1200 °C

The hot-corrosion behavior of uncoated SiC, bulk mullite and CVD grown mullite coatings in contact with Na₂SO₄ were investigated at 1200 °C. The range of thermodynamic activity of Na₂O (10^− 4 to 10^− 6) simulated in this study makes the oxide very basic leading to high reaction rates with SiO_2. Uncoated SiC corroded severely, forming various Na₂O·SiO₂ compounds with a significant weight gain. Even though the thermodynamic activity of silica is reduced in mullite, several compounds in Na₂O·SiO₂·Al₂O₃ system were formed in case of bulk mullite. CVD based mullite coatings with high alumina content at the top surface of the coating, and therefore reduced SiO₂ activity, offered protection to the underlying SiC in corrosive environments. As predicted correctly by thermodynamic calculations, the trend in weight gain with the coated SiC samples followed the theoretically calculated SiO₂ activity in Al-rich mullite.     

Effects of Temperature and Straining on the Oxidation Behavior of Electrical Steels

The effect of Si content (in the range of 0.01–1.91 wt%) on scale formation of electrical steels in dry air at temperatures ranging from 850 to 1200 °C was investigated. The effect of applied tensile strain on oxidation behavior was also explored. A thermo-mechanical simulator (Gleeble machine) was employed to conduct the oxidation tests at different load conditions. The experimental results showed that at 1000 °C the oxidation rate decreased with increasing Si content in the steel. The formation of an inner scale, mainly consisting of amorphous silica, was responsible for the improved oxidation resistance. However, a substantial increase in oxidation rate due to the formation of molten eutectic fayalite (Fe₂SiO₄) was observed when the temperature was raised to 1200 °C. Under straining conditions at a very short oxidation time, the inner scale structure was slightly modified though the scale thickness remained almost unchanged for the steel containing 1.91 wt% Si.     

Design and fabrication of compositionally graded inorganic oxide thin films: Mechanical,optical and permeation characteristics

Different types of inorganic oxide films composed of a chemical composition gradient single layer were designed, fabricated and characterized. Compositionally graded thin films were created by power-controlled co-sputtering of alumina (Al₂O₃) and silica (SiO₂) at room temperature, allowing the structural design of the film to be tailored at the nanometer scale. Two distinct graded thin films were fabricated: one with a compositionally asymmetric structure consisting of a SiO₂-rich bottom interface and a Al₂O₃-rich top surface, and the other with a compositionally balanced sandwich structure consisting of both the top surface and bottom interface rich in SiO₂ and a core rich in Al₂O₃ (referred to as SGS for ‘sandwich graded structure’). Smoothly graded thin films without interfacial boundaries were verified by Auger electron spectroscopy profiles. X-ray photoelectron spectroscopy demonstrated that the Al₂O₃/SiO₂ graded structures consisted of Si–O and Al–O bonds, as well as Al–O–Si bonds in the transition layer. Neat Al₂O₃ or SiO₂ and their graded ones were all investigated for their mechanical, optical and permeation properties. A SGS thin film presented the best mechanical stability (i.e., about three times improved film toughness of a neat Al₂O₃ single layer), demonstrating that balanced internal stresses and alternating bonding structures, achieved via a graded structure without interfaces, are crucial for enhancing mechanical stability. Furthermore, neat and graded thin films exhibited the similar level of optical transmittance and the permeation properties for the graded films were well matched with the behaviors of mechanical stability.     

Thermophysical properties of refractory W-50.4%Re and Mo-39.5%Re thin alloy layers deposited on silicon and silica substrates

The examined samples were W-Re and Mo-Re thin alloy layers with high content of Rhenium, applied in industry as protective coatings. The 550 nm of W-50.4%Re and 580 nm of Mo-39.5%Re alloys were deposited on crystalline silicon and amorphous silica substrates by magnetron sputtering method. For thermal characterization of investigated layers the scanning thermal microscopy (SThM) and high frequency photothermal radiometry (HF-PTR) measurements were carried out. The thermal methods were supported by the microstructural studies performed by the atomic force microscopy (AFM) and X-ray diffraction (XRD). The SThM method allowed determining the thermal conductivity (κ) of the alloy layers. From the HF-PTR the effective thermal conductivity (κ_eff) was determined directly. Estimation of the thermal boundary resistance (R_th) between particular alloy layer and its substrate enabled determination of the real κ of the layer with its thermal diffusivity (α) and effusivity (ε) from the HF-PTR fitting. The results showed, that W-Re and Mo-Re layers deposited on Si are characterized by higher κ, α, and ε values, and lower R_th in comparison to those deposited on SiO₂. The κ values for W-Re and Mo-Re layers deposited on Si and SiO₂ substrates were estimated in the range from 7 W·m⁻¹ K⁻¹ to 12 W·m⁻¹ K⁻¹, and 1.6 W·m⁻¹ K⁻¹ to 4 W·m⁻¹ K⁻¹, respectively. The correlation between the alloy layer structure, it's thermophysical properties and substrate structure was observed. The AFM and XRD measurements confirmed the short range order formation in Mo-Re layers deposited on Si.     

Recovery of valuable metals from spent lithium ion batteries by smelting reduction process based on FeO–SiO2–Al2O3 slag system

A novel smelting reduction process based on FeO–SiO₂–Al₂O₃ slag system for spent lithium ion batteries with Al cans was developed, while using copper slag as the only slag former. The feasibility of the process and the mechanism of copper loss in slag were investigated. 98.83% Co, 98.39% Ni and 93.57% Cu were recovered under the optimum conditions of slag former/battery mass ratio of 4.0:1, smelting temperature of 1723 K, and smelting mass ratio of time of 30 min. The FeO–SiO₂–Al₂O₃ slag system for the smelting process is appropriate under the conditions of m(FeO):m(SiO₂)=0.58:1–1.03:1, and 17.19%–21.52% Al₂O₃ content. The obtained alloy was mainly composed of Fe–Co–Cu–Ni solid solution including small amounts of matte. The obtained slag mainly consisted of fayalite and hercynite. Meanwhile, the mechanism of copper loss is the mechanical entrainment from strip-like fayalite particles in the main form of copper sulfide and metallic copper.     

Effects of silicate concentration on anodic films formed on AZ91D magnesium alloy in solution containing silica sol

Anodic films were prepared on the AZ91D magnesium alloy in 1.0 M and 1.5 M Na₂SiO₃ with varied silica sol addition under the constant current density of 20 mA/cm² at 18 °C. The surface and cross-section morphologies of the anodic films were characterized by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). The results showed that both the surface morphologies and the thickness of the anodic film were affected by the concentration of Na₂SiO₃ and the additions of silica sol. The effects of Na₂SiO₃ concentration and silica sol addition on the solution properties were also investigated. The results showed that the addition of silica sol into Na₂SiO₃ solution could decrease the surface energy and the conductivity of the solution. Moreover, the anodic film formed in 1.5 M Na₂SiO₃ with addition of silica sol was more uniform and compact than that formed in 1.0 M Na₂SiO₃ with addition of silica sol. And the electrochemical impedance spectroscopy (EIS) results also indicated that the anodic film formed in 1.5 M Na₂SiO₃ solution with 5 vol.% silica sol addition could provide higher corrosion resistance than that formed in 1.0 M Na₂SiO₃ with the same silica sol addition for the AZ91D Mg alloy substrate.     

Effect of creep on the oxidation characteristics of Fe-Si alloys at 973–1073 K

Studies of the simultaneous creep and oxidation of Fe-1Si and Fe-4Si alloys at a constant tensile stress of 16 N· mm^–2 at 973–1073 K have shown that scales formed at oxygen partial pressures of 20–1013 mbar were thicker by a factor of 2 than those formed on uncrept specimens. Scales on uncrept alloys comprised alternate layers of wustite and fayalite, whereas scales on crept alloys exhibited an additional external layer of magnetite. Only intergranular oxidation (fayalite) was observed in uncrept alloys, but crept alloys showed both intra- and intergranular oxidation (silica). Uniquely nodular scales were formed only on the Fe-4Si alloy on crept and uncrept specimens. Oxidized, uncrept Fe-1Si showed a fine-grained ferrite substrate which was absent in the crept alloy. It is believed that oxide growth stresses stimulated a recrystallization process.     

Thin films of titanium and tin oxides and semiconductor structures on their basis obtained by pyrolytic pulverization: Preparation,characterization, and corrosion properties

Peculiarities of obtaining of tin oxide and titanium oxide layers and semiconductor structures on their basis are described. The X-ray diffraction data show that the SnO₂ and TiO₂ layers possess the tetragonal crystal structure (anatase modification for TiO₂). The results of analysis of the elemental composition and impedance investigations of the fabricated structures in model chloride-sulfate solutions demonstrate that the oxide/SiO₂/Si structures are obtained if Si substrates are used. In the case of InP substrates, the oxide layer at the interface is not detected and the corresponding structure is oxide/InP. The results of investigations of corrosion show that a substantial shift of the corrosion potential to the anode region is observed in the case of deposition of SnO₂ and TiO₂ oxide layers on Si and InP crystals and fabrication of corresponding semiconductor structures. This demonstrates the possibility of the use of these materials in photoelectrochemical applications.     

Amorphous sol-gel SiO2 film for protection of Ti6Al4V alloy against high temperature oxidation

Amorphous SiO₂ thin films were deposited on Ti6Al4V alloy by sol-gel processing. Isothermal and cyclic oxidation tests of the coated and uncoated specimens were performed at 700 and 800 °C. The SiO₂ film exhibited beneficial effects on the oxidation resistance of the alloy. Titania scales formed on the uncoated specimens, and severe spallation and stratification of the scales were observed. The oxidation rates of the silica coated specimens were decreased significantly. The silica film shrunk to about a quarter in thickness, probably by mechanism of crystallization of silica and evaporation of the organic additments. The oxide scales formed on the coated specimens were multilayered. Beneath the silica film, formation of a thick rutile titania layer followed by a thin alumina layer occurred. Above the silica film, alumina plus minor titania layer formed. It is deduced therefore that the growth of the multilayered and mixed oxide scales was dominated by both outward diffusion of metal and inward diffusion of oxygen.     

Characterization and corrosion behavior of ceramic coating on magnesium by micro-arc oxidation

In this study, the commercial pure magnesium was coated in different aqueous solutions of Na₂SiO₃ and Na₃PO₄ by the micro-arc oxidation method (MAO). Coating thickness, phase composition, surface and cross sectional morphology and corrosion resistance of coatings were analyzed by eddy current method, X-ray diffraction (XRD), scanning electron microscope (SEM) and tafel extrapolation method, respectively. The average thickness of the coatings ranged from 52 to 74 μm for sodium silicate solution and from 64 to 88 μm for sodium phosphate solution. The dominant phases on the coatings were detected as spinal Mg₂SiO₄ (Forsterite) and MgO (Periclase) for sodium silicate solution and Mg₃(PO₄)₂ (Farringtonite) and MgO (Periclase) for sodium phosphate solution. SEM images reveal that the coating is composed of two layers as of a porous outer layer and a dense inner layer. The corrosion results show the coating consisting Mg₂SiO₄ is more resistant to corrosion than that containing Mg₃(PO₄)₂.     

Durability of aminosilane-silica hybrid gas-barrier coatings on polymers

Aminosilane solutions with pH equal to 8 and to 11 were applied and cured on SiO₂-coated poly(ethylene terephthalate) (PET) films. The permeability of the silane-silica hybrid coating on PET was reduced twofold compared to that of the SiO₂/PET film, at solution concentrations as low as 1 wt.%, irrespective of the pH. This concentration level led to a dense silane monolayer crosslinked to the silica surface. The oxygen transport mechanisms in the hybrid coatings were determined based on the thermally activated rate theory. Permeation experiments were also performed under tensile loading, and the critical strain for loss of barrier performance was found to be improved by a factor of two, only in case of basic pH. The defect population and morphology of the hybrid coating subjected to hydrothermal aging were analyzed using a reactive ion etching method and atomic force microscopy, respectively. These experiments confirmed the defect healing action of the aminosilane at low concentrations in solution, through the formation of a densely crosslinked polysiloxane layer at the silane-silica interface for both pH8 and pH11. The influence of the silane treatment was emphasized in case of basic pH due to the dissolution of superficial oxide layers.     

Mullite-gadolinium silicate environmental barrier coatings for melt infiltrated SiC/SiC composites

Slurry based mullite/gadolinium silicate (Gd₂SiO₅) environmental barrier coatings (EBCs) were developed for melt infiltrated (MI) SiC/SiC composites. The coating chemically adhered well on the substrates. Thermal cycling of uncoated MI-SiC/SiC composites conducted between 1350 °C and 90 °C (one hour hot and 15 min cold) in a 96.5% H₂O-3.5% O₂ environment caused severe oxidation damage after 100 cycles resulting in the formation of dense silica layer of about 25 μm maximum thickness. Mullite/Gd₂SiO₅ EBCs provided excellent protection to MI-SiC/SiC against moisture damage with significantly less oxidation of the substrate; only about a 2 μm thick oxide layer formed even after 400 similar thermal cycles. The hair-line cracks formed at the coating/substrate interface after 400 cycles causing partial coating de-lamination.     

Oxidation behavior of silicon carbide at 1200 °C in both air and water–vapor-rich environments

Oxidation of SiC in both air and water–vapor–rich environments was carried out at 1200 °C to examine the effects of different oxidation conditions on the early-stage oxidation behavior of SiC. Two different types of SiC oxidation behavior were found, passive or active, depending on the oxidation environment. All the samples possessed amorphous oxide layers, regardless of the oxidation environment. Three Si oxidation states (SiO, Si₂O₃, and SiO₂) were observed in this layer. The amorphous Si₂O₃ state was dominant, and the ratio of the three different states changed with the test conditions.