Facile Synthesis and Optical Performance of Y2.9Ce0.1Al5O12 Phosphor Thick Films from Sol–Gel Method

Transparent luminescent Y_2.9Ce_0.1Al₅O₁₂ (YAG:Ce³⁺) phosphor films (PFs) with thickness of 4.30 μm and homogeneous grain size about 80 nm were successfully synthesized by a facile sol–gel method via 40 layers deposition process on quartz substrate. The crystallization, morphology, and optical properties of the as‐prepared samples were investigated by X‐ray diffraction, scanning electron microscopy, ultraviolet‐visible spectrophotometer, spectroffluorophotometry, and absolute photoluminescence quantum yield (QY) measurement system. The as‐synthesized YAG:Ce³⁺ PFs exhibited high transparency, broad absorb peaks at blue‐light range and excellent photoluminescence performance with a QY of 47% at 457 nm excitation, which could be used in solid‐state lighting and display system applications.     

Thin‐Film Catalytic Coating of a Microreactor for Preferential CO Oxidation over Pt Catalysts

Different Pt‐based catalyst layers have been prepared and tested in a stacked foil microreactor for CO oxidation and preferential oxidation of CO in presence of hydrogen. The reactions were performed on Pt without support by impregnation of a pre‐oxidized microstructured metal plate, Pt/Al₂O₃ and Pt/CeO₂ based on sol methods as well as Pt/nano‐Al₂O₃, a combined method of sol‐gel and nanoparticle slurry coating. The ceria based sol‐gel catalyst was much more active for CO oxidation than alumina based sol‐gel catalysts at low temperature. However, total oxidation was only obtained at higher temperature on the alumina based catalysts. The combined method seems to have advantages in terms of less internal mass transfer limitation when trying to increase the catalyst coating thickness based on sol‐gel approaches due to no reduction of CO selectivity up to 300 °C reaction temperature. Experiments on CO oxidation with the Pt/CeO₂ catalyst have been conducted in an oxygen supply microreactor to evaluate the catalyst performance under sequential oxygen supply to reaction zone (CO excess).     

Development of ZrO2‐MgO nanocomposite powders by the modified sol‐gel method

The ZrO₂‐MgO nanocomposites were synthesized using a new sol‐gel method with sucrose and tartaric acid as a gel agent. The samples were characterized by thermal analysis (TG/DTA), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy‐dispersive X‐ray mapping (EDX mapping), and Ultraviolet‐visible spectroscopy (UV‐vis). The results showed that the cubic phase of ZrO₂‐MgO was formed in the presence of both gel agents. The average particle size of the samples synthesized with sucrose was lower (30 nm) than that of tartaric acid (50 nm). Finally, the formation mechanism and the optical properties of zirconia‐magnesia have been discussed.     

Effect of rheological properties of AlOOH sol on the preparation of Al2O3 nanofiltration membrane by sol-gel method

In order to directly prepare an High Flux Al₂O₃ nanofiltration membrane on an Al₂O₃ support with an average pore size of 4 μm, AlOOH sol was prepared with aluminum isopropoxide as the precursor, The effect of rheology on the dip-coating of AlOOH sol and the effect of viscoelasticity on the heat treatment of AlOOH gel film to prepare defect-free Al₂O₃ nanofiltration membrane were studied. The results indicate that AlOOH sol will increase its viscosity with the increase of the standing time. When the viscosity increases to a certain extent, the colloidal particles will gradually transform into gels, and change from Bingham fluid to Herschel-Bulkley pseudoplastic fluid. The thickness of the AlOOH gel film is related to the viscosity of the AlOOH sol. The flow viscosity of AlOOH sol should be about 0.0025～0.005 Pa·s, while the thickness of the AlOOH gel film after dip-coating is about 6.5～12 μm. The storage modulus and loss modulus of AlOOH gel film increase with the increase of aging time. Only when the storage modulus of the AlOOH gel is greater than the saturated vapor pressure of the solvent under normal pressure (0.1 MPa), it will not crack due to the evaporation of the sol during the heat treatment process. After the storage modulus exceeds 0.1 MPa, the surface of the heat-treated Al₂O₃ ceramic membrane is smooth and crack-free, the rejection rate for crystal violet dye is 99.8%, and its average pore size is 2.75 nm, that has the capability of nanofiltration. Due to the lack of intermediate layer, the pure water flux of the Al₂O₃ nanofiltration membrane is as high as 201.7 l.m^-2bar^-1h^-1, and the steady-state filtration flux is 48.7 l.m^-2bar^-1h^-1 when filtering 20 mg/l crystal violet solution. By controlling the rheological properties of AlOOH sol, a high flux Al₂O₃ nanofiltration membrane was prepared.     

Sol–gel synthesis and investigation of un-doped and Ce-doped strontium aluminates

The preparation of various strontium aluminates including SrAl₂O₄ (SrO–Al₂O₃), Sr₃Al₂O₆ (3SrO–Al₂O₃), and Sr₄Al₄O₁₀ (4SrO–2Al₂O₃) was performed by using a simple aqueous sol–gel processing. The formation of strontium aluminate phases was investigated as a function of temperature in the range of 700–1200 °C. The obtained compounds were doped with cerium in the concentration range of 0.25–3.00 mol%. The sol–gel derived samples were characterized by X-ray powder diffraction (XRD) analysis, infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and UV–Vis diffuse reflectance spectroscopy. It was demonstrated that optical properties of obtained phosphor materials slightly depend on the nature, crystal structure and phase purity of the strontium aluminate matrix.     

Direct Formation and Luminescence Properties of Yttrium Niobate YNbO4 Nanocrystals via Hydrothermal Method

Yttrium niobate YNbO₄ nanocrystals with ellipsoidal morphology were directly formed from the precursor solution mixtures of YCl₃ and NbCl₅ under weakly basic conditions in the presence of aqueous ammonia by hydrothermal method. The hydrothermal treatment at 180°C for 5 h was necessary to obtain nanocrystals (18 nm) with sufficient crystallinity. The optical band gap of the as‐prepared samples was 3.6 eV. The as‐prepared YNbO₄ nanocrystals showed UV‐blue and broadband emission centered at 405 nm under excitation at 235 nm, which was due to the blue recombination luminescence, associated with charge‐transfer transitions involving the tetrahedral NbO₄ group. The emission intensity increased with increased hydrothermal treatment temperature. The photoluminescence intensity of the YNbO₄ was extremely improved via heating above 1000°C in air, which was accompanied by the increase in the optical band gap from 3.6 to 4.0 eV. By heat treatment at 1300°C, the intensity of the UV‐blue and broadband emission (with maximum at 400 nm) for the YNbO₄ became more than 22.5 times as strong as that before heat treatment.     

Effect of glass-ceramics network intermediate Al2O3 content on up-conversion luminescence in Er3+/Yb3+ co-doped NaYF4 oxy-fluoride glass-ceramics

Effect of alumina as a glass network intermediate on the up-conversion luminescence (UCL) in NaYF₄:Er³⁺/Yb³⁺ co-doped oxy-fluoride glass-ceramics (GCs) was investigated. Combinations of smaller NaYF₄ nanocrystals (10 and 13 nm) and lower Al₂O₃ contents (5% and 10%), as well as larger NaYF₄ nanocrystals (26 and 40 nm) and higher lower Al₂O₃ contents (15% and 20%) were prepared after heat-treatment, respectively. The glass network of intermediate partial replacement of SiO₂ with Al₂O₃ was investigated, and the consequence on the response to the up-conversion of the lanthanide ions was also studied. The UCL properties of Er³⁺ ions were changed in accordance with the addition of Al₂O₃, the red UCL intensity decreased with an increased Al₂O₃ concentration, while the green emission intensity showed opposite tendency. Our results showed that adding Al₂O₃ to 20 mol% is an effective strategy to simultaneous control of the magnitude and luminescence properties of lanthanide ion doped GCs.     

Surface charges and optical characteristic of colloidal cubic SiC nanocrystals

Colloidal cubic silicon carbide (SiC) nanocrystals with an average diameter of 4.4 nm have been fabricated by anisotropic wet chemical etching of microsized cubic SiC powder. Fourier transform infrared spectra show that these cubic SiC nanocrystals contain carboxylic acid, SiH, CH, and CH_x groups. UV/Vis absorption and photoluminescence (PL) spectroscopy clearly indicate that water and ethanol colloidal suspensions of the as-fabricated colloidal samples exhibit strong and above band gap blue and blue-green emissions. The cubic SiC nanocrystals show different surface charges in water and ethanol solutions due to the interaction of water molecules with polar Si-terminated surfaces of cubic SiC nanocrystals. The results explain the distinctive optical characteristics of colloidal cubic SiC nanocrystals in water and ethanol, and reveal that quantum confinement and surface charges play a great role in determining the optical characteristics of colloidal cubic SiC nanocrystals.     

Preparation of alumina ultrafine powders through acrylamide,starch and glutaric dialdehyde mediated sol–gel method

Ultrafine alumina powders were synthesized through acrylamide (AM), starch and glutaric dialdehyde mediated aqueous sol–gel process, respectively. Sol and gel formed gradually during drying of the solution due to polymerization reaction between functional groups. The used AM, starch and glutaric dialdehyde could be applied to form perfect matrix for the entrapment of metal ions, giving rise to ultrafine crystalline alumina particles during heating treatment. Al₂O₃ nanoparticles with γ crystalline phase were obtained via heating treatment of the dried precursor in air. Then the γ phase transforms to α phase and pure α-Al₂O₃ powders could be obtained when the heating temperature was 1473 K. Our results provide a new way of aqueous sol–gel process.     

Silver nanoparticles supported on α-, η- and δ-alumina

Silver monodispersed nanopaticles supported on α- and η-Al₂O₃ have been obtained by a colloidal processing route. Precursor (silver acetate) was transformed into Ag metallic nanoparticles deposited on Al₂O₃ powder after a drying and reduction process. Silver particle size varies between 100 and 1 nm depending on the alumina phase. On the basis of HRTEM studies, and according to the nature of alumina surface, a mechanism of nucleation and growth of silver nanoparticles has been proposed. Finally, optical spectra of these samples revealed the presence of silver surface plasmon, which supplies information about the dispersion of nanoparticles into alumina matrices.     

Facile synthesis of monolithic carbon/alumina composite aerogels with high compressive strength using different inorganic aluminium salts

Resorcinol–formaldehyde/alumina composite (RF/Al₂O₃) gels were initially prepared using sol–gel techniques, and then dried to aerogels with supercritical fluid CO₂. RF/Al₂O₃ aerogels were successfully converted to monolithic carbon/alumina composite (C/Al₂O₃) aerogels after carbonization under flowing Ar at 800 °C. The samples were characterized by Brunauer–Emmett–Teller, scanning electron microscopy, transmission electron microscope and X-ray diffraction, and the compressive strengths were also measured. The results indicated that the resulting C/Al₂O₃ aerogels prepared from hydrated AlCl₃ possessed microstructures containing highly reticulated networks of fibers, 2–5 nm in diameter and of varying lengths, whereas the samples prepared from hydrated Al(NO₃)₃ were amorphous with microstructures comprised of interconnected spherical particles with diameters in the 5–15 nm range and the alumina were surrounded by amorphous carbon. The difference in microstructure resulted in each type of aerogels displaying distinct physical and mechanical properties. In particular, the as-prepared C/Al₂O₃ aerogels with the weblike microstructure were far more mechanically robust than those with the colloidal network. Correspondingly, the compressive strengths are 5.6 and 2.8 MPa, respectively.     

Effects of Al2O3 addition on microstructure and luminescence of transparent germanosilicate glass-ceramics with incorporated spinel Ga-oxide nanocrystals

Ga-oxide spinel nanocrystals are wide band gap systems, which can be incorporated in a glass matrix by phase separation mechanisms. In suitable conditions, this kind of processes can give rise to transparent nanostructured glass-ceramics with UV excitation and luminescence properties potentially interesting in several technological areas. Nanophase size dispersion and volume fraction have been demonstrated to be controllable, at some extent, by suitable thermal treatments for nucleation and nano-crystallization in low-alkali gallium germanosilicate system. Here we report the results on the role of Al₂O₃ additions on the microstructure and optical response of the glass-ceramics fabricated in this system. Data of differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, absorption and fluorescence spectroscopy show that Al₂O₃ addition, up to 4.5 mol%, turns out to have a considerable impact on the size and number density of precipitated nanocrystals, which are solid solutions of γ-Ga_2-xAl_xO₃ resulting from the partial incorporation of Al³⁺ ions into the crystalline phase. We show that the use of Al₂O₃ as an additive in the composition of gallium germanosilicates facilitates glass melting and leads to glass-ceramics with significantly modified photoluminescence characteristics such as decay lifetime and integrated intensity of light emission. The possible reasons are discussed.     

Thermal,Structural, and Enhanced Photoluminescence Properties of Eu3+‐doped Transparent Willemite Glass–Ceramic Nanocomposites

The precursor glass in the ZnO–Al₂O₃–B₂O₃–SiO₂ (ZABS) system doped with Eu₂O₃ was prepared by the melt‐quench technique. The transparent willemite, Zn₂SiO₄ (ZS) glass–ceramic nanocomposites were derived from this precursor glass by a controlled crystallization process. The formation of willemite crystal phase, size, and morphology with increase in heat‐treatment time was examined by X‐ray diffraction (XRD) and field‐emission scanning electron microscopy (FESEM) techniques. The average calculated crystallite size obtained from XRD is found to be in the range 18–70 nm whereas the grain size observed in FESEM is 50–250 nm. The refractive index value is decreased with increase in heat‐treatment time which is caused by the partial replacement of ZnO₄ units of ZS nanocrystals by AlO₄ units due to generation of vacancies. Fourier transform infrared (FTIR) reflection spectroscopy was used to evaluate its structural evolution. Vickers hardness study indicates marked improvement of hardness in the resultant glass‐ceramics compared with its precursor glass. The photoluminescence spectra of Eu³⁺ ions exhibit emission transitions of ⁵D₀→⁷F_j (j = 0, 1, 2, 3, and 4) and its excitation spectra show an intense absorption band at 395 nm. These spectra reveal that the luminescence performance of the glass–ceramic nanocomposites is enhanced up to 17‐fold with the process of heat treatment. This enhancement is caused by partitioning of Eu³⁺ ions into glassy phase instead of into the willemite crystals with progress of heat treatment. Such luminescent glass–ceramic nanocomposites are expected to find potential applications in solid‐state red lasers, phosphors, and optical display systems.     

Effect of peptiser species on crystallisation of alumina gel produced by sol–gel process

The present work reports a study on the effect that a peptiser species has on the crystallisation of alumina gel produced by a sol–gel process to help develop a method for producing α-Al₂O₃ at low temperature. The white precipitate of aluminium hydroxide, which was prepared with a homogeneous precipitation method using aluminium nitrate and urea in an aqueous solution, was peptised using various peptisers at room temperature to form a transparent alumina sol. The alumina gel obtained from the alumina sol, which was produced using formic acid as the peptiser, was most dominantly crystallised into α-Al₂O₃ by annealing at 900°C. The optimal [peptiser]/[Al³⁺] (P/A) molar ratio for the crystallisation into α-Al₂O₃ was 0.2. The alumina gel began to crystallise into α-Al₂O₃ with annealing at as low as 500°C when formic acid and a P/A ratio of 0.2 were used.     

Effect of metal‐support interface on hydrogen permeation through palladium membranes

Thin palladium membranes of different thicknesses were prepared on sol‐gel derived mesoporous γ‐alumina/α‐alumina and yttria‐stabilized zirconia/α‐alumina supports by a method combining sputter deposition and electroless plating. The effect of metal‐support interface on hydrogen transport permeation properties was investigated by comparing hydrogen permeation data for these membranes measured under different conditions. Hydrogen permeation fluxes for the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes are significantly smaller than those for the Pd/YSZ/α‐Al₂O₃ membranes under similar conditions. As the palladium membrane thickness increases, the difference in permeation fluxes between these two groups of membranes decreases and the pressure exponent for permeation flux approaches 0.5 from 1. Analysis of the permeation data with a permeation model shows that both groups of membranes have similar hydrogen permeability for bulk diffusion, but the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes exhibit a much lower surface reaction rate constant with higher activation energy, due possibly to the formation of Pd‐Al alloy, than the Pd/YSZ/α‐Al₂O₃ membranes. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Structural and luminescence properties of silica powders and transparent glass‐ceramics containing LaF3:Eu3+ nanocrystals

In this work, silica powders and transparent glass‐ceramic materials containing LaF₃:Eu³⁺ nanocrystals were synthesized using the low‐temperature sol‐gel technique. Prepared samples were characterized by TG/DSC analysis as well as X‐ray diffraction and IR spectroscopy. The transformation from liquid sols toward bulk powders and xerogels was also examined and analyzed. The optical behavior of prepared Eu³⁺‐doped sol‐gel samples were evaluated based on photoluminescence excitation (PLE: λ_em = 611 nm) and emission (PL: λ_exc = 393 nm, λ_exc = 397 nm) spectra as well as luminescence decay analysis. The series of luminescence lines located within reddish‐orange spectral scope were registered and identified as the intra‐configurational 4f⁶‐4f⁶ transitions originated from Eu³⁺ optically active ions (⁵D₀ → ⁷F_J, J = 0‐4). Moreover, the R/O‐ratio was also calculated to estimate the symmetry in local framework around Eu³⁺ ions. The luminescence spectra and double‐exponential character of decay curves recorded for fabricated nanocrystalline sol‐gel samples (τ₁(⁵D₀) = 2.07 ms, τ₂(⁵D₀) = 8.07 ms and τ₁(⁵D₀) = 0.79 ms, τ₂(⁵D₀) = 9.76 ms for powders and glass‐ceramics, respectively) indicated the successful migration of optically active Eu³⁺ ions from amorphous silica framework to low phonon energy LaF₃ nanocrystal phase.     

Direct foaming of macroporous ceramics containing colloidal alumina

Liquid foams containing Al₂O₃ nanoparticles were obtained after direct foaming of a colloidal alumina suspension with ammonium stearate. These systems were stable for at least 24 h and were comprised by small cells (<35 μm). Up to 10 wt% of these foams were added to an ultrastable Al₂O₃-stabilised one and resulted in macroporous samples with high total porosity (>70%). Their green mechanical strength was proportional to the amount of colloidal alumina added, but lower than a composition with calcium aluminate cement. When compared with compositions prepared with colloidal alumina suspension, the colloidal foams resulted in samples with a higher number of small pores (<30 μm) and lower linear shrinkage after firing at 1600 °C for 5 h (~9%). Thus, colloidal alumina foams can be used for processing macroporous refractory ceramics with smaller pores, lower dimensional changes after firing and higher porosity.     

Synthesis mechanisms,optical and structural properties of η-Al2O3 based nanoparticles prepared by DC arc discharge in environmentally friendly liquids

The staple topic of this work is synthesis of η-Al₂O₃ nanoparticles via direct current (DC) arc discharge in different carrier media. The effects of surrounding liquid environment on composition, crystal structure and optical properties of η-Al₂O₃ nanoparticles were studied. DC arc discharge was made between two pure aluminum electrodes in water, ethanol and methanol as environmentally friendly liquid carrier media. For all media a 40 A electrical current was used for arc discharge. Scanning electron microscopy observations, X-ray diffraction and optical transmission spectroscopy, were employed for characterization of particles size, shape, crystal structure and optical properties respectively. X-ray diffraction results reveal the synthesis of pure η-Al₂O₃, Al/η-Al₂O₃ and a mixture of η-Al₂O₃ and γ-Al₂O₃ in water, ethanol and methanol respectively. The mean particle size for the nanoparticles in ethanol is 50 nm. In deionised water and also in methanol, Al₂O₃ nanoparticles with mean size of 37 nm were observed. Nanoparticles in ethanol and methanol, were completely precipitated within 48 h. In all the liquid media, optical transmissions are the optical characteristics of Al₂O₃ and aluminum nanoparticles in accordance with the data obtained from X-ray diffraction. The lowering of the band gaps with respect to the bulk value of alumina due to some oxygen deficiency reveals gradual oxidation of nanoparticles in water. The composition and formation mechanisms of the nanoparticles are discussed based on the chemical nature of the liquids and the behaviour of carrier medium under DC arc discharge condition. This results demonstrate an environmentally friendly pathway for rapid mass production of η-Al₂O₃ and mixed-Al₂O₃ and γ-Al₂O₃ nanoparticles that is essential for catalytic application of Al₂O₃ nanostructures.     

Enhanced Luminescence in the SrMgAl10O17: Eu2+ Blue Phosphor Prepared by a Hybrid Urea‐Sol Combustion Route

A phase‐pure and high‐brightness blue phosphor, Eu²⁺ doped SrMgAl₁₀O₁₇ (SAM), was synthesized through a hybrid urea‐sol combustion route. The structure, photoluminescence spectra, and thermal stability of the SAM were investigated in this work. The phosphor had a homogeneous and rod‐like microstructure, showing a broad emission band centered at 465 nm under the 330 nm excitation. The concentration for luminescence quenching of SAM: Eu²⁺ occurred at 10 mol%, which doubled that of the phosphor prepared by the conventional combustion method. Compared with the traditional combustion method, the hybrid route led to improvements in luminescence by 52.2%, external quantum efficiency by 16.2%, and thermal stability by 8.8% at 435 K. The blue phosphor prepared by the new method could thus be potentially used with near ultraviolet light‐emitting diodes.     

Development of highly reinforced maleated natural rubber nanocomposites based on sol–gel‐derived nano alumina

In the present study, sol–gel synthesized alumina (Al₂O₃) nanoparticles were characterized by Fourier transform infrared spectra, X‐ray diffraction, field‐emission scanning electron microscopy. Then, Al₂O₃ nanoparticles were employed to improve cure, mechanical, and thermal properties of maleated natural rubber (MNR) nanocomposites. The MNR nanocomposite with 2 phr nano Al₂O₃ exhibited excellent value of cure rate index and exceptionally high value of mechanical properties like modulus and tensile strength in comparison to unfilled MNR compound. Thermogravimetric analysis indicated that nano Al₂O₃ was able to improve the thermal stability of MNR composites to some extent. Additionally, the present study revealed that the interfacial interaction between MNR and nano Al₂O₃ was far better than that between NR and nano Al₂O₃ as confirmed from crosslinking degree measurement and morphological analysis. The present article offers a fresh approach to prepare high performance nano Al₂O₃‐based MNR compounds for future industrial application. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46248.