Impact of a Supersonic Dissociated Air Flow on the Surface of HfB<Subscript>2</Subscript>–30 vol % SiC UHTC Produced by the Sol–Gel Method

A new method to produce ultra-high-temperature ceramic composites under rather mild conditions (1700°C, 30 MPa, treatment time 15 min) was applied to synthesize a relatively dense (ρ_rel = 84.5%) HfB₂–30 vol % SiC material containing nanocrystalline silicon carbide (average crystallite size ～37 nm). The elemental and phase compositions, microstructure, and some mechanical properties of this material and also its thermal behavior in an air flow within the temperature range 20–1400°C were investigated. Using a high-frequency induction plasmatron, a study was made of the effect of a supersonic dissociated air flow on the surface of the produced ultra-high-temperature ceramic composite shaped as a flat-end cylindrical sample installed into a copper water-cooled holder. On 40-min exposure of the sample to the supersonic dissociated air flow, the sample did not fail, and the weight loss was 0.04%. Although the heat flux was high, the temperature on the surface did not exceed 1400–1590°C, which could be due to the heat transfer from the sample to the water-cooled model. The thickness of the oxidized layer under these conditions was 10–20 μm; no SiC-depleted region formed. Specific features of the microstructure of the oxidized surface layer of the sample were noted.     

Impact of a Subsonic Dissociated Air Flow on the Surface of HfB<Subscript>2</Subscript>–30 vol % SiC UHTC Produced by the Sol–Gel Method

A new method, which included the sol–gel synthesis of a HfB₂–(SiO₂–C) reactive composite powder and its subsequent consolidation by hot pressing (1700°C, 30 MPa, 15 min) with simultaneous carbothermic synthesis of nanocrystalline silicon carbide, was used to produce HfB₂–SiC ultra-high-temperature ceramic material promising for using in an air atmosphere at temperatures above 2000°C. Its elemental and phase compositions, as well as its microstructure were investigated. The density and calculated porosity were 7.6 g/cm³ and 13.5%, respectively. The behavior of a cylindrical sample of the material was studied on long-term (40 min) exposure to a subsonic dissociated air flow in a high-frequency induction plasmatron. The change in the temperature of the surface of the material was examined in the context of its relationship with the HfB₂ and SiC oxidation and the evaporation of the oxidation products. The phase composition and microstructure were determined in regions of the oxidized surface of a HfB₂–SiC sample containing 30 vol % SiC that were heated on exposure to high-enthalpy flows to 2600–2700°C and in regions the temperature of which was 1850–1950°C. By scanning electron microscopy, the thickness, microstructure, and composition of the oxidized layer were found.     

ZrB<Subscript>2</Subscript>/HfB<Subscript>2</Subscript>–SiC Ultra-High-Temperature Ceramic Materials Modified by Carbon Components: The Review

The review has been made of recent publications on modification of ZrB₂/HfB₂–SiC ultra-hightemperature ceramic composite materials (UHTC) by carbon components: amorphous carbon, graphite, graphene, fibers, and nanotubes. Available data have been presented on some aspects of oxidation of such materials at temperatures ≥1500°C and both at the atmospheric pressure and at the reduced oxygen partial pressure; structural features of the formed multilayer oxidized regions have been noted. It has been considered how the type and content of the carbon component and the conditions (first of all, temperature) of UHTC production affect the density, flexural strength, hardness, fracture toughness, and thermal and oxidation resistance of the modified ceramic composites.     

Study of the Thermal Behavior of Wedge-Shaped Samples of HfB<Subscript>2</Subscript>–45 vol % SiC Ultra-High-Temperature Composite in a High-Enthalpy Air Flow

By spark plasma sintering, HfB₂–45 vol % SiC ultra-high-temperature ceramic was prepared, from which wedge-shaped samples were cut. The behavior of the samples was examined in a flow of dissociated air produced by an induction plasmatron, where the surface temperature of the leading edges of the samples reached ~2700°C. The dependence of the temperature distribution gradient on the distance from the leading edges of the samples was experimentally investigated. For the samples after the experiments, the elemental and phase compositions were determined, and features of the surface microstructure in various regions of the sample and on its polished surface were studied.     

Sol–Gel Derived Rh/TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> Nanocomposite Membranes

A porous borosilicate substrate has been coated with base catalysed SiO₂ sol–gel nanoparticles. Onto these were deposited the vapour of titanium isopropoxide, where it reacted with the surface OH groups to give a TiO₂-overcoat. This nanocomposite sol–gel derived TiO₂/SiO₂ membrane was then doped with 1%Rh giving Rh/TiO₂/SiO₂ membranes. These are shown to be coherent and crack-free, to have good permeability and activity in the isomerisation of butanes. It appears that sol–gel chemistry will allow such membranes to be totally engineered at a nm level.     

Preparation of HfB<Subscript>2</Subscript>/SiC composite powders by sol–gel technology

Sol–gel technology was used to chemically modify the surface of HfB₂ powders with highly dispersed silicon carbide using two carbothermy protocols: (1) under heating to 1500°С in flowing argon (100 mL/min) without exposure and (2) under dynamic vacuum conditions (p ~ 1 × 10^–5–1 × 10^–6 MPa) at 1400°С with 4-h exposure. The phase composition and microstructural features of the thus-prepared HfB₂/xSiC (x = 10–65 vol %) composite powders were studied. The products prepared by the second protocol showed an enhanced oxidation resistance in the range 20–1400°C in flowing air compared to individual HfB₂.     

Single-Step Synthesis of SiO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> Hydrophobic Core–Shell Nanocomposite by Hydrothermal Method

The objective of this work is the synthesis and characterization of an appropriate hydrophobic nanoparticle as a collector for flotation of hematit in Gol-E-Gohar Iron Mine Iran. In this investigation, SiO₂–TiO₂ nanocomposites have been successfully synthesized by hydrothermal process. The morphology, structure, and composition of the as-synthesized nanostructures have been investigated by scanning electron microscopy and transmission electron microscopy. The ability of SiO₂–TiO₂ nanocomposite to facilitate the froth flotation of pyrite was correlated to the hydrophobicity of the nanoparticles. Furthermore, the efficiency of the mineral flotation process was evaluated in terms of final recovery and grade of S in Gol-e Gohar Iron Ore Complex, Sirjan, Iran.     

H<Subscript>2</Subscript>-Sensing Characteristics of Nanocrystalline La<Subscript>0.8</Subscript>Pb<Subscript>0.2</Subscript>FeO<Subscript>3</Subscript> Prepared by Sol–Gel Method

Nanocrystalline La_0.8Pb_0.2FeO₃ has been prepared by the sol-gel method. XRD patterns show that the nanocrystalline La_0.8Pb_0.2FeO₃ is a perovskite phase with the orthorhombic structure and its mean crystallite size is about 19 nm. The influence of Pb ions which replaced the La ions on A-sites can be directly observed from the electrical and sensing properties to H₂ gas. The conductance of La_0.8Pb_0.2FeO₃-based sensor is considerably higher than that of LaFeO₃-based sensor, and Pb-doping can enhance the sensitivity to H₂ gas. An empirical relationship of with α = 0.668 was obtained.     

Synthesis of ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript>–Cu(II) xerogels by joint hydrolysis in an aqueous ammonia atmosphere

ZrO₂–SiO₂ xerogels have been synthesized through hydrolysis of a mixture of tetrabutoxyzirconium and tetraethoxysilane in a desiccator in a vapor of a 15% aqueous NH₃ atmosphere. ZrO₂–SiO₂–Cu(II) xerogels were synthethized analogously through joint hydrolysis of a mixture of the organometallic precursors and copper(II) chloride. The effect of synthesis conditions on the physical and chemical properties of the resulting material has been studied.     

Sensor Performance of Nanostructured TiO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript> Films Prepared by an Aqueous Sol–Gel Process

Nanostructured TiO₂–SnO₂ thin films and powders were prepared by a facile aqueous particulate sol–gel route. The prepared sols showed a narrow particle size distribution with hydrodynamic diameter in the range 17.2–19.3 nm. Moreover, the sols were stable over 5 months, since the constant zeta potential was measured during this period. The effect of Sn:Ti molar ratio was studied on the crystallisation behaviour of the products. X-ray diffraction analysis revealed that the powders were crystallised at the low temperature of 400 °C containing anatase-TiO₂, rutile-TiO₂ and cassiterite-SnO₂ phases, depending on annealing temperature and Sn:Ti molar ratio. Furthermore, it was found that SnO₂ retarded the anatase to rutile transformation up to 800 °C. The activation energy of crystallite growth was calculated in the range 0.96–6.87 kJ/mol. Transmission electron microscope image showed that one of the smallest crystallite sizes was obtained for TiO₂–SnO₂ binary mixed oxide, being 3 nm at 600 °C. Field emission scanning electron microscope analysis revealed that the deposited thin films had nanostructured morphology with the average grain size in the range 20–40 nm at 600 °C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption.     

Preparation and Optical Properties of SiO<Subscript>2</Subscript> Sol–Gel Made Glass Colored with Carminic Acid

SiO₂ based films and bulk samples, containing dispersed carminic acid molecules, were prepared using the traditional sol–gel process. The precursor material was a mixture of water, TEOS, ethanol and carminic acid, the components were added at ratios known to provide material with good structure. The carminic acid in powder form was introduced in the solution. Films and bulk samples with intense red color were obtained. The optical absorption and emission spectra of this material contain a number of visible and UV bands corresponding to transitions between discrete electronic energy levels. It is found that optical and thermal stability of the coloration produced by the carminic acid is enhanced when the acid molecules are embedded into the SiO₂ matrix, especially in bulk samples. The energy spectrum of the carminic molecule was obtained from a quantum mechanical model based on the modified FEMO approach. This model provides a good agreement with the experiment without any adjustable parameters.     

Fourier Transform Infrared Spectroscopy and Atomic Force Microscopy Studies of a SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript>-Zeolite Matrix for a CuO-CoO Catalyst Prepared by a Sol–Gel Method

Combinations of SiO₂–TiO₂ mixed with zeolites may open new opportunities as base supports for acid-base and oxidation reactions. These solids would endow the catalysts with specific acid sites, which may not necessarily be found in those supports with only Si-Al oxides as basic materials. Using a standard sol–gel methodology, SiO₂–TiO₂ mixtures are prepared and mixed with β, Y and ZSM-5 zeolites. By partially hydrolyzing the SiO₂ precursor at the beginning of the sol–gel procedure it is possible to maximize the number of Si–O–Ti bridges formed. FTIR and AFM analyses show that the size of sols formed depend upon the type of zeolite. Metal loading helps surface dehydroxylation of the materials, possibly indicating a good insertion of the metal into the network.     

Preparation and Characterizations of ZnTiO<Subscript>3</Subscript> Powders by Sol–Gel Process

ZnTiO₃ powders were prepared by a sol–gel method. The gelation conditions and the gelation mechanism were investigated. The crystallization behavior and characteristics of the ZnTiO₃ were also investigated in detail. The experimental results show that homogenous and translucent gels can be prepared when the gelation conditions are appropriate and the gelation temperature remains constant. The gel structure can be described as a network of Ti and O, in which zinc ions are well distributed. The ZnTiO₃ phase can be detected by XRD in the powders calcined above 700 °C. The formation of ZnTiO₃ is a slow reaction process, which leads to the development of large ZnTiO₃ particles, with dimensions after calcination at 900 °C for 2 h in the range of 30–50 m.     

Heat-Treatment-Induced Evolution of the Mesostructure of Finely Divided Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> Produced by the Sol–Gel Method

A method was developed for the low-temperature sol–gel synthesis of one of the most popular components of functional and structural materials—nanostructured yttrium aluminum garnet Y₃A_l5O₁₂—using precursors from the class of alkoxoacetylacetonates produced from the corresponding acetylacetonates. It was determined that increasing duration of heat treatment of yttrium-aluminum-containing xerogen in air to 6 h reduces the crystallization temperature of the Y₃A_l5O₁₂ phase from 920–930 to 750–800°C, which was confirmed by IR spectroscopy and X-ray powder diffraction analysis. The microstructure of nanocrystalline yttrium aluminum garnet obtained at 800°С was studied; it was found that the size of crystallites is 30–40 nm, the size of particles is 30–50 nm, and the size of pores is 20–30 nm. Small-angle neutron scattering demonstrated that, in the powders synthesized at 700–800°C, there is structural ordering of the short-range type, whereas in the nanocrystalline samples heat-treated at a higher temperature (850°С), there is no such ordering.     

Effects of Calcination on the Physical and Photocatalytic Properties of TiO<Subscript>2</Subscript> Powders Prepared by Sol–Gel Template Method

TiO₂ powders were prepared by sol–gel template method and calcined under different conditions. XRD, BET and TEM were used to characterize the TiO₂ powders obtained. The photocatalytic activity of TiO₂ was investigated by the degradation of methyl orange. It was found that TiO₂ powder has the highest photocatalytic activity at a calcination temperature of 673 K. The effects of physical properties such as surface area, crystallinity and crystal phase on the photocatalytic activity of TiO₂ were discussed.     

Physicochemical properties of TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> xerogel modified by hydrogen peroxide

How the specific surface area and the amorphous-to-crystalline titania phase ratio in TiO₂–SiO₂ (14 mol % TiO₂) xerogels change during the fivefold repeated cycles comprising the hydrogen peroxide treatment of the xerogel followed by drying and calcining of the binary material, was traced by the BET method, X-ray powder diffraction, and IR spectroscopy.     

Phosphorus modified MoO<Subscript>3</Subscript>–Bi<Subscript>2</Subscript>SiO<Subscript>5</Subscript>/SiO<Subscript>2</Subscript> catalyst for gas-phase epoxidation of propylene by molecular oxygen

The phosphorus (P) modified MoO₃–Bi₂SiO₅/SiO₂ catalyst was prepared by a simple co-impregnation method and investigated in the epoxidation of propylene by molecular oxygen. The catalyst was characterized by X-ray diffraction (XRD), N₂ adsorption–desorption analysis, NH₃-temperature-programmed desorption (NH₃-TPD), transmission electron microscopy, and Raman spectroscopy. It was found that the P-modified MoO₃–Bi₂SiO₅/SiO₂ catalyst with a P/Mo molar ratio of 0.5 exhibits the best catalytic performance for epoxidation of propylene by O₂, the TOFs for propylene oxide (PO) formation was four times higher than that of the unmodified one at 633 K. The modification by P could promote the dispersion of MoO₃ nanoparticles and increase the number of weak and moderate acid sites with respect to the phosphorus-free MoO₃–Bi₂SiO₅/SiO₂ catalyst, which were beneficial to the formation of PO. Moreover, the introduction of P also could protect the mesoporous structure by inhibiting the formation of Bi₂Mo₃O₁₂, which was beneficial to the dispersion of active species. We suppose that the phosphorus, bismuth and molybdenum species of P-modified MoO₃–Bi₂SiO₅/SiO₂ catalyst play important roles for propylene epoxidation by molecular oxygen.     

ZrO<Subscript>2</Subscript> and Cu/ZrO<Subscript>2</Subscript> Sol–Gel Materials Spectroscopic Characterization

Copper-doped zirconia (1% mol) and zirconia powders were prepared by the sol–gel process, using zirconium n-butoxide and copper nitrate as precursors. The resulting xerogels are nanocrystalline and exhibit different properties from the corresponding microcrystalline materials. The copper nitrate salt was dissolved and co-gelled in situ at the initial stage of the reaction. The properties of the resulting materials were studied by XRD, FTIR and UV-Vis. The as-prepared samples were amorphous and crystallized to the tetragonal zirconia phase at 400 °C. At temperatures higher than 600 °C, the monoclinic phase was also obtained. No evidence of discrete crystalline copper compounds was observed, consistent with good dispersion of the dopant. Several bands were observed by FTIR in the 4400–3000 cm^–1 region, which diminishes in intensity and shifted to higher wavenumbers with heating. The bandgap energy (E_g) was strongly modulated by the presence of the dopant and heating temperature, with increasing temperature leading to a corresponding decrease in E_g.     

Preparation of Polyvinyl Acetate (PVAc) and PVAc–Ag–Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Composite Nanofibers by Electro-spinning Method

Magnetite (Fe₃O₄) and silver nanoparticles were synthesized via simple chemical reactions at room temperature. Poly vinyl acetate (PVAc) nano-fibers and their nanocomposites with Ag and Fe₃O₄ were prepared by electro-spinning method. Effect of various electric potentials and distance on the morphology and diameter of fibers were investigated. Photocatalytic properties of silver in degradation five different dyes as organic pollutants in water were investigated. Fe₃O₄ nanoparticles exhibit a super-paramagnetic behavior at room temperature. Nontoxic nanoparticles appropriately enhanced both thermal stability and flame retardant property of the PVAc matrix. In the presence of flame, Fe₃O₄ nanoparticles remain together (show resistance to drip falling because of magnetic interaction) and build a barrier against flame.     

Sol–gel immobilization of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> on hydrophobic clay and its removal of methyl orange from water

In this study, SiO₂/TiO₂–organoclay hybrids with high adsorption capability and high photocatalytic activity were synthesized by immobilizing mixed silica and titanium dioxide nanoparticles on organically modified clay via a hydrothermal sol–gel method. Addition of negatively charged silica particles enhanced the uniform dispersion of titanium dioxide nanoparticles on organoclay layers by decreasing the system tension, which resulted in high photocatalytic activity of SiO₂/TiO₂–organoclay hybrids. The high adsorption capability endowed by organically modified clay enriched the organic compounds around the photoactive sites, and thus greatly improved the photodegradation efficiency. Combining the high adsorption capability of organoclay with the high photocatalytic activity of TiO₂ nanoparticles, SiO₂/TiO₂–organoclay hybrids were promising and cost-effective photocatalysts in removal of pollutants from wastewater.