Y2O2S:Eu Red Phosphor Powders Coated with Silica

Y₂O₂S:Eu red phosphor powders were coated with silica (SiO₂), using sol–gel and heterocoagulation techniques. Phosphor powders were dispersed in ethanol with tetraethyl orthosilicate and water. Hydrochloric acid was used to catalyze the sol–gel reaction, and an amorphous film 10–20 nm thick was observed via transmission electron microscopy (TEM). Colloidal SiO₂ powders 10–70 nm in size were used, and the SiO₂ powder coating was made by controlling pH values in the range of 4.5–8, in which a negatively charged surface of SiO₂ powder and a positively charged surface of red phosphor powder were formed. Then, SiO₂ powders were adsorbed electrically onto the phosphor powder surface, as evidenced by TEM, dissolution, and zeta potential measurements. Chemical bonding in the coating was studied using electron spectroscopy for chemical analysis and Fourier transform infrared spectroscopy.     

Synthesis of Blue-Emitting CaMgSi2O6:Eu2+ Phosphor Using an Electrostatic Self-Assembly Deposition Method

Eu²⁺-doped CaMgSi₂O₆ phosphor was prepared by depositing mixed hydroxides of Ca, Mg, and Eu over spherical SiO₂ particles (300 nm) pre-coated with polycations (polyethyleneimine), followed by calcination at 1200°C in a reducing atmosphere. The prepared phosphor showed intense blue emission, ascribable to the 4f⁷-4f⁶5d transition of Eu²⁺. In contrast, the luminescence intensity of the phosphor was considerably decreased when prepared without polycations. It was suggested that negatively charged hydroxides are deposited on positively charged SiO₂ surfaces pre-coated with polycations through electrostatic self-assembly interaction. On calcination, the hydroxide shells react with the SiO₂ cores to produce Eu²⁺:CaMgSi₂O₆.     

AI2O3 Coatings as Diffusion Barriers Deposited from Particulate-Containing Sol-Gel Solutions

A procedure for the formation of A1₂O₃ coatings as diffusion barriers between ductile reinforcements (e.g., Nb and Ta) and intermetallic matrices (e.g., MoSi₂ and NiAl) is described. The coating technique involved sol-gel processing of alumina -forming sols with the addition of submicrometer-sized A1₂O₃ particles. Cracking in the coatings, a typical shortcoming of alumina sol-gel coating, was overcome by the addition of the fine particles into the sols. The surface charge of the A1₂O₃ particles was adjusted to be the same as the AIO(OH) colloids in the sols and electrophoresis was used to codeposit A1₂O₃ and AIO(OH) onto the surfaces of the reinforcements. The alumina gel derived from the sols acted as binder for the alumina particles, while the particles reduced the shrinkage of the sol-gel coatings and promoted the formation of dense coatings. The thickness of the coatings could be easily controlled without cracking and the effectiveness of the coatings as diffusion barriers was improved substantially.     

Dy:YAG Phosphor Coating Using the Solution Precursor Plasma Spray Process

Dy:YAG phosphor coatings were deposited using the solution precursor plasma spray process. The phase composition, microstructure, and photoluminescent properties of the as-deposited coatings were investigated. X-ray diffraction analysis confirmed that the coating is mainly composed of the YAG phase with a small amount of an intermediate YAP phase. Scanning electron microscopy micrograph revealed that the as-sprayed coating has columnar structures with a thickness of ∼60 μm. The measurement of photoluminescent properties indicated that the phosphor coating exhibits two predominant emission regions: a blue region (470–500 nm) and a yellow region (560–600 nm), which are assigned to ⁴F_9/2–⁶H_15/2 and ⁴F_9/2–⁶H_13/2 transitions, respectively.     

High-Temperature Stability of Lanthanum Orthophosphate (Monazite) on Silicon Carbide at Low Oxygen Partial Pressures

The stability of lanthanum orthophosphate (LaPO₄) on SiC was investigated using a LaPO₄-coated SiC fiber at 1200°–1400°C at low oxygen partial pressures. A critical oxygen partial pressure exists below which LaPO₄ is reduced in the presence of SiC and reacts to form La₂O₃ or La₂Si₂O₇ and SiO₂ as the solid reaction products. The critical oxygen partial pressure increases from ∼0.5 Pa at 1200°C to ∼50 Pa at 1400°C. Above the critical oxygen partial pressure, a thin SiO₂ film, which acts as a reaction barrier, exists between the SiC fiber and the LaPO₄ coating. Continuous LaPO₄ coatings and high strengths were obtained for coated fibers that were heated at or below 1300°C and just above the critical oxygen partial pressure for each temperature. At temperatures above 1300°C, the thin LaPO₄ coating becomes morphologically unstable due to free-energy minimization as the grain size reaches the coating thickness, which allows the SiO₂ oxidation product to penetrate the coating.     

Preparation of TiO2 Nanoparticles on Different SiO2 Supports by the Adsorption Phase Technique

The influence of supports on the preparation of TiO₂ nanoparticles by the adsorption phase technique is studied in detailed. Series temperature experiments of two types of supports (named as SiO₂ A and B) were used. Energy-dispersive analysis by X-ray indicates that the concentration of TiO₂ on both supports decreases with temperature increasing. TiO₂ quantity on SiO₂ A decreases sharply between 40° and 60°C, whereas the temperature range for SiO₂ B is between 30° and 50°C. X-ray diffraction (XRD) shows that grain size of TiO₂ particles on two SiO₂ surfaces is all below 7 nm. It is also shown by XRD that particles on SiO₂ A decrease sharply as in the quantity curve of TiO₂, but particles on SiO₂ B all change gradually and TiO₂ particles on SiO₂ B are more uniform in transmission electron spectroscopy. The similarly of both supports is considered to be the reason for the similar changes in Ti concentration, and the different characteristics of the internal/external surface lead to variant quantity and grain size, as well as characteristics of TiO₂.     

Optimization of Chemical Vapor Deposition Parameters for Fabrication of Oxidation-Resistant Mullite Coatings on Silicon Nitride

Fabrication of mullite (3Al₂O₃·2SiO₂) coatings by chemical vapor deposition (CVD) using AlCl₃–SiCl₄–H₂–CO₂ gas mixtures was studied. The resultant CVD mullite coating microstructures were sensitive to gas-phase composition and deposition temperature. Chemical thermodynamic calculations performed on the AlCl₃–SiCl₄–H₂–CO₂ system were used to predict an equilibrium CVD phase diagram. Results from the thermodynamic analysis, process optimization, and effects of various process parameters on coating morphology are discussed. Dense, adherent crystalline CVD mullite coatings ∼2 μm thick were successfully grown on Si₃N₄ substrates at 1000°C and 10.7 kPa total pressure. The resultant coatings were 001 textured and contained well-faceted grains ∼0.3–0.5 μm in size.     

Mullite Coatings on SiC and C/C–Si–SiC Substrates Characterized by Infrared Spectroscopy

Mullite (3Al₂O₃·2SiO₂) coatings on SiC substrates and SiC precoated carbon/carbon composite (C/C-Si-SiC) substrates were produced by pulsed laser deposition (PLD) using pressed mullite powder targets. The layers can be characterized efficiently by IR reflection spectroscopy in the spectral range between 650 and 5000 cm⁻¹. The deposited coatings turn into mullite upon oxidation in air at temperatures between 1400° and 1600°C. Fabry-Perot interferences indicate a high quality and homogeneity of the mullite coating/SiC substrate interface. Amorphous SiO₂ gradually forms during prolonged heating or at higher temperatures.     

Evolution of the Morphology of Zinc Oxide/Silica Particles Made by Spray Combustion

Pure and mixed ZnO–SiO₂ particles were made by flame-spray pyrolysis of zinc acetate and hexamethyldisiloxane or SiO₂ sol dispersed in methanol or water-in-oil emulsion, respectively. The product particles were characterized by nitrogen adsorption, infrared absorption, and X-ray diffraction. The evolution of solid or hollow particle formation along the flame axis was unraveled by transmission electron microscopy after collection by thermophoretic sampling. The effects of silicon precursor and solvent on product particle characteristics were evaluated. The characteristics of the product particles were controlled by the Si precursor and solvent.     

Hydration of Tricalcium Silicate

The kinetics of paste, bottle, and ball-mill hydration of 3CaO SiO₂ and the effects of additions of electrolytes and alcohols were studied. Paste and bottle hydrations proceed through periods of induction, acceleration, and decay. If 3CaO SiO₂ is hydrated in an excess of H₂O, as in bottle hydration, the reaction rate is lower than that for paste hydration. The ball-mill hydration rate is much the highest and is controlled by the removal of the hydrate layer coating the 3CaO SiO₂ particles. Electrolytes always accelerate and alcohols retard the reaction rate. Experimental results are discussed with reference to modern theories of the 3CaO SiO₂ hydration mechanism.     

Reactive Coating of Zircon on Mullite and Mullite—Alumina Substrates

A stable zirconia coating of 20–30 pm thickness on a mullite substrate with a different alumina/silica ratio was obtained by reactive coating of zircon. It is shown that the Al₂O₃/ SiO₂ ratio of the mullite substrate drastically affects the morphology of the zirconia coating. The results are explained on the basis of the Al₂O₃–SiO₂–ZrO₂ phase equilibria.     

Structural Control of Phase Formation in Low-Tem perature AIPO4—Sio2 Reactions

The phase relation along the binary join of AIPO₄-SiO₂ were investigated up to 400°C using starting materials made by a solution route. Precursor structures used were boehmite (AIOOH), H₃PO₄, noncrystalline silica, and quartz. The silica precursors acted as structural seeds for the epitaxial growth of AIPO₄. Studies showed that SiO₂ and AIPO₄ were the only crystalline and noncrystalline phases present along the binary join, and no substantial crystalline solution or any ternary phase was observed. Three polymorphic forms of AIPO₄, i.e., berlinite, tridymite, and cristobalite, coexisted as low as 200°C. The nature of the silica precursor greatly influenced the development of the polymorphic phases of AIPO₄. The low-quartz precursor suppressed the formation of the cristobalite form of AIPO₄ and favored berlinite (AIPO₄ quartz) production. On the other hand, noncrystallin silica with a cristobalite-like broad XRD peak suppressed the formation of berlinite and enhaned that of the cristobalite form of AIPO₄. These precursor effects indicate that heteroepitaxy is very significant during the nucleation and growth of AIPO₄ phases on the surface of SiO₂ particles even in these low-temperature reactions.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Control of the Interfacial Reactions in Nb-Toughened MoSi2

Toughening of MoSi₂ for high-temperature applications can be achieved by incorporating ductile refractory-metal reinforcements, provided that a coating is applied to prevent interdiffusion and reaction between the matrix and the reinforcements. In the present study, three different coating techniques for applying a thin Al₂O₃ film on Nb reinforcements as a diffusion barrier have been studied. The techniques consisted of (i) sol-gel coating; (ii) physical vapor deposition (PVD); (iii) hot dipping in molten Al, followed by anodizing Al to form Al₂O₃. The processing parameters for the techniques were evaluated and the effectiveness of each coating as a diffusion barrier was assessed. For the present MoSi₂ matrix which contains SiO₂, PVD coatings provided the most effective diffusion barrier for processing MoSi₂/Nb composites.     

Vacuum Plasma Spray Deposition of Titanium Particle/Glass-Ceramic Matrix Biocomposites

The vacuum plasma spray technique (VPS) has been successfully employed to coat Ti-6A1-4V substrates with bioactive glasses and Ti-particle/glass-ceramic matrix biocomposites. The composites were prepared by sintering, under an Ar flow, green bars of bioactive glass powders and 30% volume Ti particles. The bioactive glasses have the two following compositions: SCB (48.8SiO₂−48.8CaO−2.4B₂O₃) and TSCB (46.6SiO₂−48.7CaO−2.5B₂O₃−2.2TiO₂) (mol%). The VPS bioactive coatings were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and mechanical tests (Vickers indentations and tensile and shear tests). Their bioactivity was tested by soaking the samples in a simulated body fluid (SBF) and by analyzing the growth of hydroxylapatite (HA) by SEM, EDS, and XRD. Leaching tests of Ca, Si, and P in SBF were made by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Perkin-Elmer 5000) to study the in vitro bioactivity of the samples versus time. Each coating was found to be bioactive and well bonded to the substrate; the composites showed better mechanical properties than the pure glass matrices and the hydroxylapatite coatings deposited by the same VPS technique.     

Novel Deposition of Columnar Y3Al5O12 Coatings by Electrostatic Spray-Assisted Vapor Deposition

A novel and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) was used to deposit Y₃Al₅O₁₂ (YAG) coatings. Polycrystalline single-phase Y₃Al₅O₁₂ coatings were synthesized using the ESAVD method in an open atmosphere at 650°C, and then annealed at 700°–900°C for 1 h. The ESAVD process involves the decomposition and chemical reactions of charged aerosol in vapor phase. The low-temperature coating deposition characteristics of the ESAVD process using a suitable sol precursor decreases the reaction and crystallization temperatures for forming Y₃Al₅O₁₂ coatings. The microstructure of the Y₃Al₅O₁₂ coating prepared using the ESAVD method is columnar and such strain-resistance microstructure could be useful for thermal barrier coating applications.     

Fabrication of Bifunctional Titania/Silica-Coated Magnetic Spheres and their Photocatalytic Activities

A magnetic photocatalyst is prepared by coating a magnetic core with a dual layer of inert silica (SiO₂) and photoactive titania (TiO₂). The complete photoactive TiO₂ shell is directly formed on the SiO₂-coated magnetic spheres. Subsequent hydrothermal treatment improves the degree of crystallinity of TiO₂. TiO₂/SiO₂-coated magnetic spheres show good photocatalytic activities in the degradation of methylene blue in the aqueous solution. The photocatalytic TiO₂ surface shell is electrically insulated by the intermediate SiO₂ layer from the magnetic core to maintain good photocatalytic activity of the TiO₂ shell. The magnetic photocatalyst can be easily recovered by applying external magnetic field.     

Stabilization and Characterization of Nanosized Niobium and Tantalum Oxide Sols — Optical Applications for High-Power Lasers

Stable nanosized particles of niobium and tantalum oxides were obtained by hydrolysis of the metal ethoxides in ethanol with triethylamine. These sols were amorphous hydrated oxides M₂O₅· n H₂O. They were used to prepare thin porous films on SiO₂ disks via spin-coating techniques. These coatings displayed thicknesses ranging from 100 to 300 nm and a refractive index of about 1.7. Laser damage tests at 1053-nm wavelength with a pulse duration of 3 ns were conducted on the single-layer systems. Ta₂O₅ coatings exhibited much higher threshold values (averaging 14.5 ± 2.1 J/cm²) than those of Nb₂O₅ (averaging 8.3 ± 1.6 J/cm²). Buildup of multilayer stacks of high- and low-index materials was prevented because of internal stress.     

Effect of Carbon Precursors on the Microstructure and Bonding State of a Boron–Carbon Compound Grown by LPCVD

Methane (CH₄) and propylene (C₃H₆) were used to fabricate a boron–carbon coating by a low-pressure chemical vapor deposition (LPCVD) technique. The effects of carbon precursors on the phase, microstructure, and bonding state of the deposits have been investigated. X-ray diffraction results show that the 2θ value of the deposit from the C₃H₆ precursor shifts to 25.78° when the coating is deposited at 1223 K, and shifts to 26.1° when deposited at 1273 K, compared with the 2θ value of the pyrocarbon (PyC) peak deposited by LPCVD, which is 25.42°, and no boron–carbon (B–C) compound peak exists. However, the phases of coating deposited from CH₄ include B₂₅C, B₁₃C₂, elemental carbon, and boron. X-ray photoelectron spectroscopy (XPS) results show that the percent contents of boron atom in the coatings from the CH₄ precursor are 61.18% and 67.78% when deposited at 1223 and 1273 K, respectively, much higher than that from the C₃H₆ precursor, 10.85% and 15.30%, respectively. Scanning electron microscopy (SEM) results show that the coatings deposited from CH₄ have a coarse crystal-like morphology; however, the coatings deposited from the C₃H₆ precursor are smooth. The formation of PyC from C₃H₆ is more facile than that from CH₄, which leads to differences in the phase, atom content, and microstructure of coatings from CH₄ and C₃H₆ precursors.