Behavior of HfB<Subscript>2</Subscript>-SiC (10, 15, and 20 vol %) ceramic materials in high-enthalpy air flows

HfB₂–SiC ceramic samples containing 10, 15, and 20 vol % silicon carbide were prepared by spark plasma sintering. The samples were characterized by X-ray powder diffraction, SEM, and other methods. Their densities and calculated porosities were determined. The behavior of the materials under heating by a subsonic dissociated air flow was studied on a VGU-4 high-frequency inductive plasmatron. The average surface temperatures of the 10 and 15 vol % SiC samples were shown to increase up to 2550–2675°C during heating, due to the generation of surface localities having temperatures of 2600–2700°C (the initial surface temperature was ~1700–1900°C) and the progressive growth of these regions in area. The overall time during which the average surface temperatures of these samples were higher than 2000°C, was about 31–32 min. For the 20 vol % SiC sample, heat removal (when the sample touched a water-cooled holder) was shown to influence the surface temperature and surface temperature distribution. The variation in gas-phase composition over the central area of the sample surface during an experiment was studied using emission spectroscopy. Explanations are proposed to the variation of boron and silicon concentrations in the course of exposure to high-enthalpy flows. The elemental and phase compositions were determined and the microstructures were studied on the surface and sections of samples after long-term (~40-min) exposure to high-enthalpy air flows.     

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.     

Production of ultrahigh temperature composite materials HfB2-SiC and the study of their behavior under the action of a dissociated air flow

Ultrahigh temperature composite materials HfB₂-SiC containing 25, 35, and 45 vol % SiC were produced by spark plasma sintering. Modeling of heating under the action of a dissociated air flow for selected samples using a VGU-4 induction plasma generator showed that these materials do not degrade even while keeping at a surface temperature of more than 2000°C (up to 2600°C) for 11 min. A combination of optical microscopy, scanning electron microscopy (with EDX analysis), and X-ray computed microtomography were used for investigating the microstructure and composition of the oxidized layer before and after heating.     

Temperature effect on the chemical composition of a polytetrafluoroethylene surface under the irradiation of synchrotron radiation by means of the X-ray photoelectron spectroscopy

The chemical composition and components of a polytetrafluoroethylene (PTFE) surface was investigated as a function of the temperature under the irradiation of synchrotron radiation (SR) by the X-ray photoelectron spectroscopy (XPS). When the temperature of PTFE under the SR irradiation was less than 100 °C, the C-rich surface appeared. With increasing the temperature more than 150 °C, the relative intensity of the F 1s peak to the C 1s peak increased markedly. At the temperatures of 150–180 °C, the C–C component became small and the CF₂ component was dominant. With further increasing the temperature more than 200 °C, CF₃, CF and C–CF components grew in addition to CF₂ component. Based on these XPS results, the temperature effect on the chemical composition and components is discussed.     

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.     

Growth and thermal kinetics of pure and cadmium doped barium phosphate single crystal

Non-isothermal kinetic parameter of pure and cadmium-doped barium phosphate single crystal grown by room temperature solution technique have been investigated. Single crystal X-ray diffraction establishes grown crystal to be orthorhombic in nature. Scanning electron microscopy supplemented with energy dispersive X-ray analysis was used to study the surface features and to find the exact stoichiometric composition of the grown crystal. Fourier transform infrared spectroscopy studies confirm the presence of various functional groups. The effect of cadmium doping on pure barium phosphate single crystal was studied using thermogravimetry analysis. Thermogravimetry studies shows that the pure crystal was stable up to a temperature of 330 °C whereas doped crystal was stable up to a temperature of 240 °C, i.e., pure crystals were more stable than doped ones. Various solid-state reaction kinetics, i.e., activation energy (E _a), frequency factor (Z), and entropy (ΔS*) was calculated out to find the mechanism of thermal decomposition at different stages for pure and cadmium doped barium phosphate.     

Low-temperature formation and characterization of Cu2O nanoparticles in the binary gold alloys

Thermal analyses, using differential scanning calorimetry (DSC) and dilatometry, reveal an important anomaly at low temperature for Au-25 wt.% Cu composition after homogenization at 700°C during 2 hours under vacuum followed by heating up to 160°C before water quenching. This anomaly has been already observed and not explained. Surface characterization, using scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM), exhibits a specific topography, consisting of a nanostructured surface. The precipitates of nanostructured particles are homogeneously scattered all over the surface for this 18-carat gold alloy. Moreover, X-ray photoelectron spectroscopy (XPS) shows that the composition of the observed particles corresponds to cuprous oxide phase (Cu₂O). The formation of such material can be explained by the diffusion of copper atoms from the lattice to the surface at 160°C. Pulsed radio-frequency glow discharge optical emission spectroscopy (RF GD-OES) further proves the proposed Cu₂O formation through a diffusion process. The appearance of such cuprous oxide nanoparticles on the Au-Cu alloy surface explains the low-temperature DSC and dilatometry anomaly and affects directly the surface electrical resistance at low temperature. These results might open a large gate for new ideas to investigate in catalytic, electronic, and antimicrobial activities.     

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.     

Synthesis and characterization of perovskite-type La1-yCayMn1-xB″xO3±δ nanomaterials (B″ = Ni,Fe; x = 0.2, 0.5; y = 0.4, 0.25)

Perovskite-type nanomaterials of the compositions La_1-yCa_yMn_1-xB″_xO_3±δ with B’’ = Ni, Fe; x = 0.2, 0.5 and y = 0.4, 0.25 were prepared using two different preparation routes (synthesis by precipitation and the PVA/sucrose method) at 500 °C–700 °C. The calcined products of the syntheses were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and physisorption measurements. The materials from the PVA/sucrose method contain particles with diameters from 33 nm to 48 nm, generate specific surface areas up to 33 m²/g and form pure compared to 45 nm–93 nm and up to 18 m²/g from precipitation method which contain a significant amount of sodium ions. The agglomeration process was analyzed for one nanomaterial (B’’ = Fe, x = 0.2, y = 0.4) from the PVA/sucrose method using temperature dependent XRD showing only a slight growth (4.3%) of nanoparticles at 600 °C. The materials from the PVA/sucrose method turned out to be more suitable as electrode materials in electrochemical applications (SOFC, sensors) because of smaller particle sizes, higher specific surface areas and purity.     

Electrical properties of the YSZ/STO/YSZ-STO superlattice electrolyte film at low temperatures

This study is focused on characterization of the low temperature properties of the YSZ/STO/YSZ superlattice film deposited onto unilateral polished SrTiO₃ (STO) monocrystalline substrates using pulsed laser deposition (PLD). The phase composition, structure, surface morphology and electrical properties of the oxygen ion conducting electrolyte YSZ/STO/YSZ multilayers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The minimum conductivity activation energy of YSZ/STO/YSZ is 0.76 eV at 300–500°C. The YSZ/STO/YSZ superlattice film shows an enhancement in conductivity by three orders of magnitude compared to bulk YSZ at a temperature of 300°C.     

Carbon deposits on a resistive FeCrAl catalyst for the suboxidative pyrolysis of methane

The carbon deposits forming upon the suboxidative pyrolysis of methane on resistive FeCrAl catalysts heated with electric current were studied. The suboxidative pyrolysis of methane was carried out in a flow reactor at the ratio CH₄: O₂ = 15: 1 in a catalyst-coil temperature range of 600–1200°C; a cold reaction mixture (～20°C) was supplied. The morphology and structure of the carbon deposits and changes in the composition and structure of the catalyst were characterized by scanning electron microscopy, transmission electron microscopy with EDX analysis, Raman spectroscopy, and X-ray diffraction analysis. Various forms of carbon deposits, including branched nanotubes, and metal carbides formed by catalyst constituents were detected. It was found that the carbon deposits on the catalyst surface were morphologically different from the deposits on quartz reactor walls. The reasons for these differences were considered.     

Effects of vacuum annealing on the particle size and phase composition of nanocrystalline tungsten carbide powders

The effects of the vacuum annealing temperature (400–1400°C) on the phase and chemical composition, particle size, and microstresses of the nanocrystalline powders of tungsten carbide WC with 20–60 nm particles were studied by X-ray diffraction and electron microscopy. Vacuum annealing of WC nano-powders at T _ann ≤ 1400°C was accompanied by decarbonization, resulting from the interaction of carbon with the oxygen impurity. Changes in the chemical composition of the nanocrystalline powder of tungsten carbide led to changes in its phase composition. The annealing was accompanied by growth of powder particles due to the aggregation of nanoparticles and by a decrease of microstresses.     

Ceramic and Transport Characteristics of Electrolytes Based on Mg-Doped LaYO<Subscript>3</Subscript>

Effect of magnesium on the sinterability, phase composition, microstructure, and transport properties of proton-conducting materials of composition LaY_1–xMg_xO_3–δ (х = 0, 0.05, 0.1) was studied. Ceramic samples were obtained by using the citrate-nitrate synthesis method at various sintering temperatures (1250–1400°C). It was shown that, for the samples with x = 0.05 and 0.1, the relative density was no less than 95% at a sintering temperature of 1350°C, whereas undoped lanthanum nitrate has this density at 1450°C. An X-ray diffraction analysis and scanning electron microscopy demonstrated that introduction of a small amount of magnesium (x = 0.05) is sufficient for forming the single-phase and high-dense ceramics. Electrical conductivity data show that the LaY_0.95Mg_0.05O_3–δ sample has high overall and ionic conductivities.     

Temperature Hysteresis in the Reaction of Methane Oxidation on a Palladium-Doped Manganese Hexaaluminate Catalyst

A study of the Pd-containing catalyst based on manganese hexaaluminate by X-ray diffraction analysis, transmission electron microscopy, thermo-programmed reduction with hydrogen, and thermoprogrammed oxidation showed the hysteresis manifested itself in a difference between the temperature dependences of its catalytic activity in the oxidation of methane under the conditions of sample heating or cooling is related to the decomposition of PdO to metallic palladium at temperatures higher than 700–800°C and the subsequent formation of PdO nanoparticles, which are localized on the surface of metallic Pd (30–50 nm) in the form of polycrystalline films with a thickness of 2–5 nm, at temperatures lower than 600–700°C. A near-surface oxide film is formed under the conditions of cooling in oxygen-containing atmospheres, including in the presence of methane. The value of the hysteresis depends on the heat treatment temperature of the catalyst, and it reaches a maximum on the catalysts calcined at 900 and 1000°C.     

Insertion of iron oxide in calciumsilicate gel-derived materials

Samples of the composition of 10Fe₂O₃·10CaO·80SiO₂ were prepared by the sol-gel method and heat-treated in different atmospheres. They were investigated by X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. In the heat-treated samples in air iron is present up to 1000 °C in form of hematite and as Fe³⁺ in the tetrahedral sites. A wide range of hematite particle sizes was observed, the average size increased with heating temperature. At 1000 °C wollastonite was observed, at 1200 °C tridymite was formed and all the iron was incorporated in hematite. A heat-treatment at 500 °C under reducing conditions led to poorly crystallized maghemite and at 700 °C to metallic iron and fayalite formation.     

Effect of synthesis parameters on characteristics of expanded graphite

Effect of synthesis parameters on the characteristics of expanded graphite were studied. The starting sample, intercalated graphite, was treated by several methods: thermal shock (400, 1000°C) and programmed heating (400–700°C). The samples were examined by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction analysis, and low-temperature nitrogen adsorption. The programmed-heating method yields better texture characteristics as compared with the thermal shock. The programmed-heating method was used to obtain high-quality expanded graphite with high specific surface area (299 m² g^–1) at a comparatively moderate temperature of 400°C.     

Low CO content hydrogen production from oxidative steam reforming of ethanol over CuO-CeO2 catalysts at low-temperature

CuO-CeO₂ catalysts were prepared by a urea precipitation method for the oxidative steam reforming of ethanol at low-temperature. The catalytic performance was evaluated and the catalysts were characterized by inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, temperature-programmed reduction, field emission scanning electron microscopy and thermo-gravimetric analysis. Over CuO-CeO₂ catalysts, H₂ with low CO content was produced in the whole tested temperature range of 250–450 °C. The non-noble metal catalyst 20CuCe showed higher H₂ production rate than 1%/oRh/CeO₂ catalyst at 300–400 °C and the advantage was more obvious after 20 h testing at 400 °C. These results further confirmed that CuO-CeO₂ catalysts may be suitable candidates for low temperature hydrogen production from ethanol.     

Carbonization and graphitization of pitch applied for anode materials of high power lithium ion batteries

The artificial graphite materials were prepared by carbonizing coal tar pitch using two methods, namely, one- and two-step processes, and all sintered samples were graphitized at 2800 °C. Effects of different heat treatments on the performance of the samples were characterized by scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction, Brunauer–Emmett–Teller, electrochemical impedance spectroscopy (EIS), particle size analysis, polarized light microscopy, and charge–discharge measurements. All samples show a typical graphite crystalline structure; moreover, the degree of graphitization (g factor) and crystallite size along the c-axis (L _c ) were calculated from (002) peak. The polarized light microscopy indicates that the coke with carbonization at 700 °C has an obvious wide domain (D) optical structure, while that with two-step sintering at 400 and 700 °C has a mixed optical structures of wide D, flow domains, and mosaics. TEM analysis revealed a number of irregular graphene layer images which are caused by the defects of graphite. EIS shows that the sample carbonized by two-step has a larger diffusion coefficient than the sample carbonized at 700 °C by one step. Higher carbonization temperature leads to better cycle performance as the temperature increasing from 500 to 700 °C in the one-step route. Specifically, the charge (Li⁺ extraction) capacity at the 50th cycle increases from 318 mA?h?g^?1 to 357 mA?h?g^?1. The results show that the rate performance of the artificial graphite is improved with the addition of the presintering at 400 °C.     

Preparation of manganese dioxide loaded activated carbon adsorbents and their desulfurization performance

Manganese dioxide loaded activated carbon adsorbents (MnO₂/AC) were prepared and characterized by N₂ adsorption-desorption, BET method, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FT-IR) and scanning electron microscopy (SEM). Effects of preparation conditions and adsorption conditions on desulfurization performance of the adsorbents were studied in a fixed-bed adsorption apparatus. Experimental results show that the surface area and pore volume of MnO₂/AC decreased compared with the unmodified activated carbon, but the adsorption capacity to H₂S was improved greatly. A suitable H₂S removal activity was obtained with manganese dioxide to activated carbon ratio of 1.1: 1 and the calcination temperature of 250°C. At the adsorption temperature of 40°C and gas flow rate of 20 mL/min, the H₂S saturation capacity and H₂S removal rate reached up to 713.25 mg/g and 89.9%, respectively.     

Characterization of Fine Hydroxyapatite Powders Synthesized by Wet Process

Abstract

We prepared fine hydroxyapatite powders by dropping ammonium bi-phosphate into a calcium acetate solution and by vigorously stirring at 3O°C. We measured the powders' specific surface area to be 290 m²/g by the BET technique. The powders were mono-dispersed ultrafine particles by transmission electron microscopy investigation. No phase other than hydroxyapatite (JCPDS: 9–432) was revealed by X-ray diffractometry. A quantitative chemical analysis gave a Ca/P ratio very close to the exact hydroxyapatite stoichiometry (Ca/P: 1.67). Shrinkage started up to 800°C according to a dilatometric measurement and the dense products were obtained when heated at 1000°C for 2 h in air. We used a quadrupole mass spectrometer to monitor the gases desorbed from the hydroxyapatite powders at a constant heating rate of 5°C/min in a high vacuum. It is noted here that there were two peaks of H₂O and CO₂, respectively, and that we observed an increase in desorption of H₂. The two peaks were explained clearly with an infrared spectrometry analysis and a thermal analysis we made separately.