Al2O3 Sol Research Articles

Anti-CMAS corrosion mechanism of Al2O3 pore sealing and laser remelting on thermal barrier coatings

The sol-dip-coating method and surface laser remelting technology are applied to form an Al2O3 layer on a YSZ coating surface to effectively block the environmental sediment CMAS. The behaviour and mechanism for CMAS corrosion of the coating are investigated, and the interfacial reliability of the coating and matrix is calculated by using the Monte Carlo method. The bonding force between the Al2O3 sol and YSZ coating can be effectively improved by laser surface treatment. Samples subjected to a laser pretreatment and posttreatment (YL-AL) of the YSZ coating are found to show the best interfacial bonding strength between Al2O3 and YSZ. Furthermore, the YL-AL sample shows a higher CMAS resistance than the laser posttreatment (Y-AL) samples, which effectively combines the chemical resistance of Al2O3 to CMAS and the physical resistance of the laser re-melted densification layer against CMAS penetration.

Preparation of high-performance silica-based ceramic cores with B4C addition using selective laser sintering and SiO2–Al2O3 sol infiltration

Silica-based ceramic cores are commonly employed to fabricate hollow aero-engine blades during investment casting. However, achieving excellent performance and high precision remains an extremely challenging problem. In this study, silica-based ceramic cores with boron carbide (B4C) addition were prepared by selective laser sintering (SLS) and SiO2–Al2O3 sol infiltration. The effects of B4C content and mass ratio of alumina to silica in the SiO2–Al2O3 sol on the mechanical properties of the obtained silica-based ceramic cores were explored. The results suggested that the room temperature flexural strength was gradually improved from 3.39 MPa to 27.35 MPa at B4C content from 0 to 2 wt% due to the oxidation of B4C, and the linear shrinkage gradually increased to 4.61%. Furthermore, the high temperature flexural strength was enhanced from 4.86 MPa (non-infiltrated) to 11.84 MPa at mass ratio from 0 to 1 because of the generated mullite phase and the linear shrinkage decreased to 1.38%, which can meet the application requirements for the preparation of ceramic cores. Consequently, it shows the capability to prepare high-performance silica-based ceramic cores with B4C addition by combining SLS technology and SiO2–Al2O3 sol infiltration process.

Sol–Gel Synthesis of Oxonitridoaluminosilicates (SiAlON)

Specific features of the synthesis of sialons (SiAlON) in the Si–Al–O–N system by firing at 1600–1700°C in a nitrogen atmosphere of mixtures of different initial compositions corresponding to the same composition of the final product Si3Al3O3N5 have been studied. β-Sialon was synthesized from reaction mixtures based on crystalline silicon and aluminum nitrides and amorphous highly dispersed hydrated oxides—(1) AlN + SiO2(sol), (2) Si3N4 + Al2O3(sol), and (3) AlN + Si3N4 + Al2O3(sol)—obtained by applying sols based on silicon or aluminum alkoxides (based on the β-sialon stoichiometry) to the surface of nitride powders by the sol-gel method. It has been demonstrated that β-Si3Al3O3N5 can form in the reaction at 1600°C of highly dispersed mixed xerogels made of silicon and aluminum alkoxides—(4) (SiO2 + Al2O3) (mixed sol)—with molecular nitrogen without the participation of the initial nitrides. The influence of the starting composition of xerogel mixtures and synthesis conditions on the phase composition and morphological features of sialons obtained in one stage of calcination in a nitrogen atmosphere has been studied by thermogravimetry, X-ray powder diffraction analysis, and electron microscopy. It has been shown that the proposed reactions of the formation of sialons involving crystalline nitrides, xerogels, and molecular nitrogen are related to reduction–nitridation reactions, in which the reducing agents are organic products of hydrolysis of silicon and aluminum alkoxides and their pyrolytic decomposition in a nitrogen atmosphere.

Preparation of High Hardness Transparent Coating with Controllable Refractive Index by Sol-Gel Technology

SiO2/ZrO2 and SiO2/ZrO2/Al2O3 composite organic–inorganic coatings were prepared by the sol-gel technology. The structure of the coating was characterized by IR, particle size analyzer, SEM, and AFM, respectively. The results showed that ZrO2, SiO2 (inorganic component), and siloxane had undergone a hydrolytic condensation reaction, and the composite organic–inorganic coatings were formed with -O-M-O- (M is Si, Zr) as molecular skeleton network structure. Adding an appropriate ratio of ZrO2 sol, no agglomeration and phase separation occurred, which could significantly improve the refractive index, hardness, and light transmittance of the coatings. Al2O3 sol could greatly improve the friction resistance of the composite organic–inorganic coatings, and the Bayer ratio of the composite coatings could reach 7.86. By adjusting the proportioning of composite sol solution, the refractive index of the composite transparent coatings could be controlled from 1.52 to 1.65.

Preparation of Sol-Enhanced Ni–P–Al2O3 Nanocomposite Coating by Electrodeposition

A Ni–P–(sol)Al2O3 coating was prepared on the surface of Q235 steel by direct-current electrodeposition. This method was combined with sol-gel and electrodeposition technique, instead of the traditional nanopowder dispersion, to prepare highly dispersible oxide nanoparticle-reinforced composites. The effects of temperature, pH value, and current density and Al2O3 sol on the hardness of composite coating were investigated. The coating surface morphology and structure were characterized by scanning electron microscopy and energy dispersive spectroscopy, respectively. The corrosion resistance of coatings in the presence of intermediate layers was evaluated by electrochemical measurement in 3.5% NaCl solution by open-circuit potential measurement at room temperature. The hardness and wear resistance of the coating were measured by a microindentation instrument and friction wear machine, respectively. The results showed that Al2O3 sol can effectively improve Ni–P alloy coating structure and refine grain. When the bath temperature was 55°C, the pH value was 4.5, the amount of sol was 80 mL/L, the current density was 1 A/dm2, and the hardness of the nanosol coating was 569 HV. Compared with Ni–P, the friction coefficient increases slightly, but the wear rate was only 1.768×10−6 g·m−1. The corrosion resistance was also better than that of Ni–P coating.

Effect of Aluminum Doping on the Performance and Microstructure of Methyl-Modified SiO2 Sol

With methyltriethoxysilane as hydrophobic precursors and aluminum isopropoxide as the aluminum source, the Al2O3/SiO2 sols and their gel materials were prepared. The effects of aluminum content (nAl) on the viscosity (η), density (ρ), reaction rate constant (k), Gibbs energies of activation for viscous flow (ΔG*), particle size of Al2O3/SiO2 sols were studied. And the high temperature calcined materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). The results show that, with the increase of nAl, the η, n and ΔG* values of the Al2O3/SiO2 sols decrease while the k and ρ values and average particle sizes increase. There is strong intermolecular interaction between the Al2O3 and SiO2 sol molecules. The Al-O-Si bond exists in the Al2O3/SiO2 materials before and after calcination at 350 °C. Calcination at 350 °C in N2 atmosphere can change the phase structure of Al2O3/SiO2 sample greatly, which can make the γ-AlOOH in the Al2O3/SiO2 gel material convert to γ-Al2O3 through dehydration.

Self-standing MWCNTs based gas sensor for detection of environmental limit of CO2

In this work, we have demonstrated multi walled carbon nanotubes (MWCNTs) based CO2 gas sensor using a reliable and low cost gel-cast method with MWCNT concentration of 0.6, 1.0, 1.5, 2.0 and 3.0 by wt% in alumina Al2O3 sol which acts as a host matrix for the composite. Sensing response of the as prepared highly uniform self-standing MWCNTs film was measured at room temperature by exposure to different CO2 concentrations, ranging from 50 ppm to 450 ppm. The adsorption effect of CO2 on the varying concentration of CNTs was investigated by measuring a significant change in the resistance. Upon exposure to CO2 molecules, the device shows a drastic increase in resistance attributed to the charge transfer between CNTs and CO2 molecules. High sensor response of 7.3% with a response time of 53.7 s exhibited by sensor S3 (1.5 wt% of MWCNTs) is reported when the concentration of CO2 is 450 ppm in ambient conditions. In addition, different recovery modes were examined and the results reveal that there is a significant decrease in the recovery time of sensor upon UV light exposure and heat application. Besides being fast recoverable with heat energy, the fabricated gas sensor shows favorable stability in response over multiple exposure of analyte gas.

Microstructure and mechanical properties of sol-enhanced nanostructured Ni–Al2O3 composite coatings and the applications in WC-Co/steel joints under ultrasound

Reliable brazing of WC-Co to steel is crucial for the applications in shield tunneling and temping machines. However, the interface could not be effectively connected with high strength due to the large difference in coefficients of thermal expansion. We developed a sol-enhanced method to electroplate Ni–Al2O3 nanocomposite coatings on steel in order to improve the bonding and strength of the joints. Al2O3 sol was fabricated with mean size of 59 nm, which was identified through TEM analysis. The change of Al2O3 concentration of the coatings with different Al2O3 sol addition was investigated through LA-ICP-MS test. The phase, microstructure, mechanical properties and corrosion performance of the coating were strongly affected by the sol concentration through the XRD and TEM analysis. At very high sol concentration, cracks appeared on the coating surface, which deteriorated the properties. It was found that the Ni–Al2O3 coating improved the micro-hardness and corrosion resistance by polarization Tafel tests and EIS analysis. Specifically, 35CrMo with a Ni–Al2O3 coating was joined with tungsten carbide by ultrasound-assisted brazing in air. The shear strength of the joints was increased by 31% (approximately 427 MPa) than conventional method.

Acid-catalysed Al2O3 sol and its gel material: influence of HNO3 content and stability

A series of Al2O3 sols were prepared by HNO3 catalysis of aluminum isopropoxide (AIP), using different molar ratios of HNO3/AIP (nH). The changes in sol densities, viscosities, refractive indices, stability and particle sizes were reported. Derived properties such as Gibbs energies (ΔG*) and the reaction rate constant (k) have been calculated from the experimental density and viscosity data. The phase-chemical structures of Al2O3 materials were characterized using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) measurement. The results show that, when the molar ratio of HNO3/AIP is 0.25, the prepared Al2O3 sol was transparent, uniform and stable, and the sol average particle size gets the minimum. The chemical structure of the non-calcined Al2O3 gel material is dominated by Al–O–H and Al–O bonds. After calcination at 350 °C, the Al–O–H bond will break and is transformed into Al–O bonds, and the phase structure is changed from γ-AlOOH to γ-Al2O3.

Improvement of high-temperature resistance on carbon fiber felt/portland cement composite friction material by Al2O3 sol–gel coating

The extrusion impregnation method is used to prepare carbon fiber felt/portland cement (CFF/PC) composite materials. The samples are placed into muffle furnace for ablation, but carbon fiber is oxidized at around 450 °C. To improve the oxidation resistance of carbon fiber, Al2O3 sol–gel (AS) is prepared to coat the surface of carbon fiber. The extrusion impregnation method is also used to prepare AS–CFF/PC composite materials. The samples are also ablated into muffle furnace at different temperatures. The results indicate that carbon fiber is uniformly dispersed and closely arranged in the matrix, presenting a three-dimensional network distribution. The bending and compressive strengths of CFF/PC composite materials are increased by 200% and 12% when compared with that of PC material at room temperature. The AS coating layer can effectively improve the antioxidant capacity of carbon fiber at high temperatures. The comparison reveals that AS–CFF/PC composite materials exhibit improved high-temperature resistance performance.

Preparation, characterization, and properties of Pt/Al2O3/cordierite monolith catalyst for hydrogen generation from hydrolysis of sodium borohydride in a flow reactor

High-purity hydrogen can be generated by hydrolysis of sodium borohydride and used for operating portable proton exchange membrane fuel cells. The monolith supported catalyst is suitable for practical NaBH4-based hydrogen generation system due to its simple reactor structure miniaturizing for small size applications and easy separation from the spent solution. In the present study, a structured catalyst was prepared by wash-coating the Al2O3 sol over the wall of cordierite monolith followed by depositing Pt using incipient wet impregnation method; then the monolithic catalysts were characterized by XRF, XRD, SEM, HRTEM and XPS. The catalytic activity of the Pt-based monolithic catalyst towards hydrolysis of NaBH4 was tested using a flow reactor under ambient conditions in an autothermal manner. The characterization results show that Pt nanoparticles are highly dispersed on the surface of the Al2O3-coated layer. A continuous and stable hydrogen generation can be obtained by feeding the reactant (10 wt% NaBH4–5 wt % NaOH) into the tube reactor loaded with the monolithic catalyst at feed rates of 0.5–2.0 mL min−1.

Plasma Electrolytic Oxidation of Magnesium Alloy AZ31B in Electrolyte Containing Al₂O₃ Sol as Additives.

Plasma electrolytic oxidation (PEO) coatings were produced on AZ31B magnesium alloys in alkaline electrolytes with the addition of various concentrations of Al2O3 sols. Effects of Al2O3 sol concentrations on the microstructure, phase composition, corrosion resistance and hardness of PEO coatings were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness testing and potentiodynamic polarization measurements, respectively. It was revealed that the Al2O3 sol mostly participated in the formation of the ceramic coating and transferred into the MgAl2O4 phase. With the increase of the Al2O3 sol concentration in the range of 0–6 vol%, the coating performance in terms of the microstructure, diffraction peak intensity of the MgAl2O4 phase, corrosion resistance and microhardness was improved. Further increase of Al2O3 sol addition did not generate better results. This indicated that 6 vol% might be the proper Al2O3 sol concentration for the formation of PEO coatings.

Superhydrophobic properties induced by sol-gel routes on copper surfaces

Superhydrophobic surfaces are attracting increasing attention in different fields such as energy, transportation, building industry and electronics, as they exhibit many interesting properties such as high water repellence, anti-fogging, anti-corrosion, anti-fouling and self-cleaning abilities. Here, superhydrophobic nanostructured hybrid materials obtained by depositing alumina nanoparticles on copper surfaces via dip coating in Al2O3 sol are presented. Two different preparation routes were explored, based on either an alcoholic or an aqueous Al2O3 sol, and the resulting wetting properties were compared. Wettability measurements showed that when the alcoholic sol is used superhydrophobicity is attained, with values of water contact angle very close to the upper limit of 180°, while highly hydrophobic coatings are obtained with the aqueous sol. These findings were further supported by electron microscopy and X-ray photoelectron spectroscopy, which revealed that the surface layer deposited on Cu is more homogenous and richer in alumina nanoparticles when the alcoholic sol was used. Durability of the superhydrophobic coating was assessed by performing ageing tests in chemically aggressive environments. A remarkable resistance is displayed by the superhydrophobic coating in acid environment, while alkaline conditions severely affect its properties. Such behaviors were investigated by XPS and FE-SEM measurements, which disclosed the nature of the surface reactions under the different conditions tested. The present results underline that a thorough investigation of surface morphology, chemical composition and wetting properties reveals their strongly connection and helps optimizing the combination of substrate nanostructuring and suitable chemical coating for an improved durability in different aggressive environments.

Heat resistance investigation and mechanical properties of fabric coated with Al2O3 sol–gel

The Inflatable Reentry and Descent Vehicle is a new kind of ballistic atmospheric entry vehicle which generates a piece of large aerodynamic drag area for deceleration when the resistance cone is deployed by gas-filling expansion. However, the aerothermal heating during planetary entry poses a significant challenge to a flexible fabric. In this paper, the para-aramid fabrics with three different sol–gel coatings (i.e. Al2O3 sol coating, supercritical drying Al2O3 aerogel coating, and atmospheric pressure drying Al2O3 xerogel coating) were proposed for heat resistance analysis and strength analysis. The thermal and mechanical properties of flexible fabric materials were conducted by infrared spectrum analysis, thermogravimetric analysis, scanning electron microscopy analysis, 50s ablation and uniaxial tension testing. All the results indicate that fabric coated with the supercritical drying Al2O3 aerogel possesses high heat resistance and nearly 100% residues strengthen.

Electrodeposition of Cu-Sn-Graphite-Al<sub>2</sub>O<sub>3</sub> Composite Coatings and their Tribological Properties

In order to improve the wear resistance, ensure the self-lubricating property, Cu-Sn coatings were prepared on the basis of non-cyanide pyrophosphate-stannate and strengthened by combining sol-gel method and composite electroplating technology. In the paper, graphite Sol and Al2O3 Sol were added to the Cu-Sn electroplating solution to obtain nanoparticles enhanced Cu-Sn-graphite and Cu-Sn-graphite-Al2O3. The effect of graphite Sol and Al2O3 Sol addition on composite coatings was investigated, such as surface morphology, micro-hardness and tribological properties. The result shows that the incorporation of the graphite Sol and the Al2O3 Sol lead to change the structure of the composite coatings. When the additions of graphite Sol and Al2O3 Sol were 110mL/L and 40mL/L respectively, the graphite and Al2O3 particles are uniformly distributed in the Cu-Sn matrix and contribute greatly to improve the hardness and tribological properties of Cu-Sn alloy coating.

Dispersion Property of Al2O3 Nanosol Prepared by Various Dispersion Factors and Silane Modification under Non-Aqueous Solvent

Al2O3 nanosol dispersed under ethanol or N-Methyl-2-pyrrolidone(NMP) was studied and optimized with various dispersion factors and by utilizing the silane modification method. The two kinds of Al2O3 powders used were prepared by thermal decomposition method from aluminum ammonium sulfate(AlNH4(SO4)2) while controlling the calcination temperature. Al2O3 sol was prepared under ethanol solvent by using a batch-type bead mill. The dispersion properties of the Al2O3 sol have a close relationship to the dispersion factors such as the pH, the amount of acid additive(nitric acid, acetic acid), the milling time, and the size and combination of zirconia beads. Especially, Al2O3 sol added 4 wt% acetic acid was found to maintain the dispersion stability while its solid concentration increased to 15 wt%, this stability maintenance was the result of the electrostatic and steric repulsion of acetic acid molecules adsorbed on the surface of the Al2O3 particles. In order to observe the dispersion property of Al2O3 sol under NMP solvent, Al2O3 sol dispersed under ethanol solvent was modified and solventexchanged with N-Phenyl-(3-aminopropyl)trimethoxy silane(APTMS) through a binary solvent system. Characterization of the Al2O3 powder and the nanosol was observed by XRD, SEM, ICP, FT-IR, TGA, Particles size analysis, etc.

Preparation of a Novel Al2O3/SiO2/Nano-SiO2 ARC on GaAs Wafer with Broadband Spectrum

In the paper, a novel Al2O3/SiO2/Nano-SiO2 triple-layer antireflection coating (ARC) on GaAs wafer with lower reflectance range on broadband spectrum was developed by sol-gel method. The experiments showed that: (1) to obtain stable and transparent liquid Al2O3 sol, the three crucial factors the concentration of aluminum isopropoxide, hydrolysis temperature and reflux time should be controlled well, which could be 0.68~1.04mol/L, 90~95℃, and longer than 10h respectively. (2) The Al2O3 film and SiO2 film would be thicker with the acceleration of withdrawal speed, be thinner with the rising of annealing temperature; and the refractive index would be lower with the rising of annealing temperature also. (3) The Al2O3 film would put forward Boehmite (γ-AlOOH) structure after 200℃ annealing; while when the annealing temperature was higher than 200℃, the film showed γ- Al2O3 phase, with refractive index 1.71, and film thickness could be adjustable from 74.1 nm to 161 nm.(4) The amorphous SiO2 thin-film could be adjustable also, which refractive index ranged from 1.21 to1.45, thickness varied from 132.8nm to 219nm.

Control of the Spring Back in Rice Husk by Al<sub>2</sub>O<sub>3</sub> Sol

The effects of Al2O3 sol on the mechanical, morphological, and water sorption properties of rice husk were studied by means of compression-strain tester, and scanning electron microscopy (SEM) associated with the energy spectrum analysis. The results showed that the spring back of materials is found to decrease with increasing coating concentration. With the increase in Al2O3 sol concentration, the gaining of mass fraction of the materials increased proportionally, while the water absorption, which measured at ambient temperature, was gradually decreased with increasing Al2O3 sol concentrations. The gaining of mass fraction, lowest water absorption and lowest of compressive recovery ratio of 10.15%, 34.5% and 16.58%, respectively, were obtained when Al2O3 sol solid content was 8%, and resulted mainly from an increase in the rigidity increase. The Al2O3 sol had a positive effect on the morphology stabilization of rice husk.

Surface modification of WC-based cemented carbide by one-pot non-vapor deposition method derived Al2O3 coatings

Al2O3 films are usually coated on WC-based cemented carbide by chemical/physical vapor deposition techniques. In the paper, a novel one-pot non-vapor deposition method is introduced, which is achieved by dip-coating of Al2O3 sol on the cemented carbide green compact and following liquid phase sintering. The coated surface is mainly composed of ~0.5μm Al2O3 fine grains embedded with ~2μm rounded-edged WC particles. The Co capping phenomenon on the surface of cemented carbide disappears after coating, which is attributed to the carbon-deficit condition created by the dip-coating process. Simultaneously, the introduction of Al2O3 results in a W-rich Co binder, which facilitates the transformation of WC shape to round-edged shape on the surface compared with the faceted truncated triangular prism shape in inner area. There is hardness increase and transverse rupture strength decrease after Al2O3 coating, which is attributed to the existence of W3Co3C on the surface.

Nanostructured Er3+-doped SiO2–TiO2 and SiO2–TiO2–Al2O3 sol–gel thin films for integrated optics

The nanostructured multilayer silica–titania or silica–titania–alumina films doped with Er3+ were prepared by sol–gel method. The sol–gel method is a flexible and convenient way to prepare oxide films on several types of substrates, and for this reason it was extensively investigated for optical waveguides fabrication. The selected molar composition was 90%SiO2–10%TiO2 or 85%SiO2–10%TiO2–5% Al2O3 and 0.5% Er2O3.The films were characterized by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Spectroellipsometry (SE), as well as by Atomic Force Microscopy (AFM) and photoluminescence (PL). The films deposited on Si/SiO2 substrate by dip-coating or spin-coating, followed by annealing at 900°C, presented homogenous and continuous surface and good adherence to the substrate. Differences were noticed in the structure and properties of the prepared films, depending on the composition and the number of deposited layers.Channel optical waveguides were obtained by patterning Er3+-doped SiO2–TiO2 and SiO2–TiO2–Al2O3 sol–gel layers deposited on oxidized silicon wafers.