Crystallization and oxidation resistance properties of boron modified silicon oxycarbides derived from polymeric precursors by sol–gel method

Boron modified silicon oxycarbides (SiBOCs) were prepared from sol–gel derived pre-ceramic polymeric gels followed by pyrolysis at 950 °C under nitrogen. As-prepared SiBOC was found to be amorphous in nature and partially crystallized to SiO₂ at 1500 °C. The effect of boron incorporation on the crystallization of SiBOC was studied and the result revealed that the tendency to crystallization decreased with increasing boron content. This is due to the formation of Si–O–B bridges at higher temperatures, which retards the crystallization of SiO₂, evidenced from FTIR studies. SiBOC also exhibited excellent oxidation resistance ability at high temperature.     

Analyses of nano-crystalline LiCoVO4 prepared by solvothermal reaction

LiOH·H₂O, Co(NO₃)₂·6H₂O and NH₄VO₃ were used to prepare nano-crystalline LiCoVO₄ by 150 °C solvothermal reaction in isopropanol for 10–360 h and subsequent calcination at 300–500 °C for 6 h. XRD, TEM and selected area electron diffraction (SAED) revealed the presence of nano-crystalline LiCoVO₄ with inverse spinel structure. The V–O stretching vibration modes of VO₄ tetrahedrons were detected by FTIR over the range 617–835 cm^− 1 and by Raman spectrometer at 805.7 and 783.1 cm^− 1. Co, V and O were detected by EDX. TGA of solvothermal products shows weight loss due to the evaporation and decomposition processes at 40–648 °C.     

High-temperature stability of the mechanical and optical properties of Si–B–C–N films prepared by magnetron sputtering

Quaternary Si–B–C–N materials are becoming increasingly attractive due to their possible high-temperature and harsh-environment applications. In this work, amorphous Si–B–C–N films with two compositions (Si₃₄B₉C₄N₄₉ and Si₃₆B₁₃C₇N₄₀) and low contamination level (H + O + Ar < 4 at.%) were deposited on silicon substrates by reactive dc magnetron co-sputtering using two different targets and gas mixtures. Thermal stability of these films was investigated in terms of composition, bonding structure, as well as mechanical and optical properties after annealing in helium up to a 1300°C substrate limit. Films with a high nitrogen content (Si₃₄B₉C₄N₄₉, i.e. N/[Si + B + C]~ 1.0) were found to be stable up to 1300°C. After annealing, the hardness and elastic recovery of those films slightly increased up to 27 GPa and 84%, respectively, and the reduced Young's modulus remained practically constant (~ 170 GPa). The refractive index and the extinction coefficient at 550 nm were evaluated at 2.0 and 5 × 10^− 4, respectively, and the optical band gap was approximately 3.0 eV. In contrast, films with a lower nitrogen content (Si₃₆B₁₃C₇N₄₀, i.e. N/[Si + B + C]~ 0.7) were stable only up to 1200°C. Both Si–B–C–N materials studied here exhibited extremely high oxidation resistance in air up to the 1300°C substrate limit.     

Preparation of SrAl2O4: Eu, Dy nanometer phosphors by detonation method

SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer phosphors were synthesized by detonation method. The particle morphology and optical properties of detonation soot that was heated at different temperatures (600–1100 °C) had been studied systematically by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results indicated SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer powders in monoclinic system (a = 8.442, b = 8.822, c = 5.160, β = 93.415) can be synthesized by detonation method, when detonation soot was heated at 600–800 °C. The particle size of SrAl₂O₄: Eu²⁺, Dy³⁺ is 35 ± 15 nm. Compared with the solid-state reaction and sol-gel method, synthesis temperature of the detonation method is lower about 500 and 200 °C respectively. After being excited under UN lights, detonation soot and that heated at 600–1100 °C can emit a green light.     

Structural, electrochemical and optical comparisons of tungsten oxide coatings derived from tungsten powder-based sols

Tungsten trioxide (WO₃) electrochromic coatings have been formed on indium tin oxide-coated glass substrates by aqueous routes. Coating sols are obtained by dissolving tungsten powder in acetylated (APTA) or plain peroxotungstic acid (PTA) solutions. The structural evolution and electrochromic performance of the coatings as a function of calcination temperature (250 °C and 400 °C) have been reported. Differential scanning calorimetry and X-ray diffraction have shown that amorphous WO₃ films are formed after calcination at 250 °C for both processing routes; however, the coatings that calcined at 400 °C were crystalline in both cases. The calcination temperature-dependent crystallinity of the coatings results in differences in optical properties of the coatings. Higher coloration efficiencies can be achieved with amorphous coatings than could be seen in the crystalline coatings. The transmittance values (at 800 nm) in the colored state are 35% and 56% for 250 °C and 400 °C-calcined coatings, respectively. The electrochemical properties are more significantly influenced by the method of sol preparation. The ion storage capacities designating the electrochemical properties are found in the range of 1.62–2.74 × 10^− 3 (mC cm^− 2) for APTA coatings; and 0.35–1.62 × 10^− 3 (mC cm^− 2) for PTA coatings. As a result, a correlation between the microstructure and the electrochromic performance has been established.     

Chemical deposition of Al2O3 thin films on Si substrates

Uniform Al₂O₃ films were deposited on silicon substrates by the sol–gel process from stable coating solutions. The technological procedure includes spin coating deposition and investigating the influence of the annealing temperature on the dielectric properties. The layers were studied by Fourier transform infrared spectroscopy and Scanning Electron Spectroscopy. The electrical measurements have been carried out on metal–insulator–semiconductor (MIS) structures. The C–V curves show a negative fixed charge at the interface and density of the interface state, Dit, 3.7 × 10¹¹ eV^− 1cm^− 2 for annealing temperature at 750 °C.     

Si diffusion in magnetron sputtered silicon carbide films deposited on silicon and carbon substrates

Self-diffusion of silicon in magnetron sputtered silicon carbide films deposited on different substrates (crystalline silicon and glassy carbon) is investigated. Since crystallization of amorphous silicon carbide films strongly depends on the substrate, the diffusivity of silicon is expected to depend on the substrate as well. Isotope hetero-structures and secondary ion mass spectrometry were used for analysis. For amorphous samples an upper limit of the diffusivity of 1 × 10^− 21 m²/s is derived at 1100 C°. For crystallized films diffusivities between 1350 °C and 1600 °C are found to be not significantly different for the two types of substrates. For samples deposited on glassy carbon substrates an activation enthalpy ΔH_D = (8.7 ± 0.9) eV was found for the self-diffusion of Si. The consequences of our findings for crystallization are discussed.     

Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

Synthesis of Ti2SnC from Ti/Sn/TiC powder mixtures by pressureless sintering technique

Ti/Sn/TiC powder mixtures were first employed to synthesize Ti₂SnC powder by pressureless sintering in the temperature range of 950–1250 °C at vacuum atmosphere. Ti₂SnC began to form at 950 °C, its content increased with increasing temperature. High purity of Ti₂SnC was obtained by sintering the mixtures with deficient Sn and TiC at 1200 °C for 15 min. A reaction mechanism was proposed to explain the formation of Ti₂SnC. The Ti₂SnC powder was characterized by scan electron microscopy (SEM) and X-ray diffraction (XRD). Using the above mixtures and process, the Ti₂SnC ceramic powder can be obtained on a larger scale.     

Template-free synthesis of NiO hollow microspheres covered with nanoflakes

α-Ni(OH)₂ hollow microspheres precursors were synthesized without any template through a solvothermal method. Hydrolyzing NiCl₂ in alkaline solution, the Ni(OH)₂ hollow microspheres covered with a disordered covering of perpendicular nanoflakes were prepared. Subsequently, the similar microstructured NiO hollow microspheres with BET area around 222.8 m²/g and specific pore volume around 0.5 cm³/g were obtained by calcining the above precursor at 250–300 °C with a slow heating rate (about 1 K/min). The effects of the surfactant and the calcining temperature on the morphology and the mesostructure of the NiO hollow microspheres were also discussed.     

Characterization of precipitates in a 7.9Cr–1.65Mo–1.25Si–1.2V steel during tempering

In this paper, the precipitates formed during the tempering after quenching from temperature 1150 °C for 7.90Cr–1.65Mo–1.25Si–1.2V steels are investigated using an analytical transmission electron microscope (A-TEM).The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hot hardness.Tempering is performed in the range of 300–700 °C. Coarse primary carbides retained after heat treatment are V-rich MC and Cr–Mo-rich M₇C₃ types. In turn, it gives a significant influence on the precipitation of fine secondary carbides, that is, secondary hardening during tempering. The major secondary carbides are Cr–Mo–V-rich M′C (and/or) Cr–Mo-rich M₂C type. The peak hardness is observed in the tempering range of 450–500 °C. In the end, we observe between 600 and 700 °C, that this impoverished changes the phase. At these high temperatures of tempering, we observe that there is a carbide formation of the types M₆C developing at the expense of the fine M₇C₃ carbides previously formed.     

Bulk preparation and characterization of mesoporous carbon nanotubes by catalytic decomposition of cyclohexane on sol–gel prepared Ni–Mo–Mg oxide catalyst

Multiwalled carbon nanotubes were synthesized using Ni–Mo–Mg oxide catalyst prepared by sol–gel technique. Carbon nanotubes were formed in situ by the reduction of nickel oxide (NiO) and molybdenum oxide (MoO₃) to Ni and Mo by a gas mixture of nitrogen, hydrogen and cyclohexane at 750 °C. Scanning Electron Microscopy (SEM) was used to confirm the formation of carbon nanotubes (CNTs). The pore size distribution of carbon nanotubes (CNTs) was investigated by N₂ adsorption and desorption. It was found that the pore size fell into the mesopore range: 2 < d < 50 nm. Interpretation was also made using Raman spectroscopy, Diffuse reflectance spectroscopy, X-ray diffraction and ESR spectra. This method is found to produce a very high yield weighing over 20 times of the catalyst. Based on the experimental conditions and results obtained a possible growth mechanism of the carbon nanotubes is proposed.     

Conversion of polycarbosilane (PCS) to SiC-based ceramic Part II Pyrolysis and characterisation

The conversion to ceramic of a commercial polycarbosilane (PCS) under various pyrolysis conditions has been investigated. The products of pyrolysis have been characterised by solid state ²⁹Si and ¹³C NMR spectroscopy and X-ray diffraction (XRD). Some of the phases identified in the present study were found to differ from those reported previously, particularly in the earlier literature. Oxidation-cured PCS, when pyrolyzed up to 1400 °C in argon, generally produced silicon oxycarbide (SiO _x C _y ) as the second major phase with -SiC as the major phase, and smaller amounts of free carbon. With increasing temperature above 1200 °C, the silicon oxycarbide phase decomposed to give -SiC. Silica (SiO₂) was also found to evolve from this silicon oxycarbide phase. Loss of some of the silica, probably by reaction with carbon, was found at 1400 °C, possibly yielding SiO, CO and SiC. At 1500 °C, crystalline -cristobalite was found as a minor phase with -SiC as the major phase and a lower amount of free carbon. Pyrolysis in vacuum leads to production and crystallization of -SiC at a lower temperature than required if pyrolyzed in argon flow. After pyrolysis at 1600 ° in vacuum, the cured PCS converted to almost stoichiometric -SiC.     

Low-temperature fabrication of 0.65 PMN–0.35 PT by a mixed oxide method

Single-phase perovskite 0.65 PMN–0.35 PT was achieved at low temperature by a conventional mixed oxide method. It was prepared by ball-milling a mixture of PbO(orthorhombic), TiO₂, Nb₂O₅ and (MgCO₃)₄Mg(OH)₂·5H₂O instead of MgO and heat treatment at 800 °C for 2 h. The formation was studied by means of DSC, FT-IR, Coupled TG-Mass, XRD, and SEM. It proceeded via formation of PbO(tetragonal) and Pb₂Nb₂O₇(P₂N) intermediates to form perovskite phase. The pure perovskite PMN-PT powder was obtained in particle size of 0.5–0.8 μm, agglomerate-free, and pseudo-cube. The powder calcined at 600 °C was sintered to 97% T.D. at 900–1000 °C for 2 h and showed room temperature dielectric constant of 3200, loss of 1–2%, and specific resistance of 5 × 10¹¹ Ω cm.     

Enhancement of fatigue endurance and retention characteristic in Bi3.25Eu0.75Ti3O12 thin films

The effects of moderate annealing temperature (600–800 °C) on the microstructure, fatigue endurance, retention characteristic, and remnant polarization (2P_r) of Bi_3.25Eu_0.75Ti₃O₁₂ (BET) thin films prepared by metal-organic decomposition (MOD) were studied in detail. 2P_r (66 µC/cm² under 300 kV/cm), fatigue endurance (3% loss of 2P_r after 1.2 × 10¹⁰ switching cycles), and retention characteristic (no significant polarization loss after 1.8 × 10⁵s) for BET thin film annealed at 700 °C are better than those for thin films annealed at other temperature. The mechanisms concerning the dependence of microstructure and ferroelectric properties on the annealing temperature were discussed.     

Photoactivity of anatase–rutile TiO2 nanotubes formed by anodization method

Titania (TiO₂) nanotubes were prepared by anodizing titanium (Ti) foils in an electrochemical bath consisting of 1 M glycerol with 0.5 wt.% NH₄F.The pH of the bath was kept constant at 6 and the anodization voltage was varied from 5 V, 20 V to 30 V. It is found that the morphology of the anodized titanium is a function of anodization voltage with pits-like oxide formed for the sample made at 5 V and samples made at 20 V and 30 V consisted of well-aligned nanotubes growing perpendicularly on the titanium foil. However, the nanotubes formed on the samples made at 30 V were not uniform in terms of the nanotubes' diameter and wall thickness. Regardless of the anodization voltage, as anodised samples were amorphous. The crystal structure evolution was studied as a function of annealing temperatures and was characterised by X-ray diffraction and Raman spectroscopy analyses. Crystallization of the nanotubes to anatase phase occurred at 400 °C while rutile formation occurred at 700 °C. Disintegration of the nanotube arrays was observed at 600 °C and the structure completely vanished at 700 °C. TiO₂ nanotube annealed at 400 °C and containing 100% anatase revealed the highest photocatalytic activity for the degradation of methyl orange. Consequently, these results indicate that diameter, wall thickness, crystal structure and degree of crystallinity of the TiO₂ nanotube arrays are the important factors influencing the efficiency of the photocatalytic activity.     

The effect of Sb2O5 additions on the dielectric properties of Ag(Nb0.8Ta0.2)O3 ceramics

The sintering behavior and dielectric properties for perovskite Ag(Nb_0.8Ta_0.2)O₃ ceramic with Sb₂O₅ doping was explored. A small amount of Sb₂O₅ (2.5 wt.%) led to high densification at temperatures < 1060 °C. The dielectric constant increased and the temperature coefficient decreased with increasing concentration of Sb₂O₅, and the dielectric constant reached 673, combined with a low temperature coefficient of 147 ppm/°C, and dielectric loss of 0.0044 (at 1 MHz) for the sample with 3.5 wt.% Sb₂O₅ sintered at 1080 °C.     

Preparation of high surface area LaTiO2N photocatalyst

Nanocrystalline LaTiO₂N with a surface area of 27.5 m²/g was synthesized by nitridation of amorphous La₂O₃/TiO₂ composite powder at 900 °C for 8 h using NH₃ as the reactant gas. X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) results revealed that the as-prepared LaTiO₂N nanocrystals had a mean diameter of about 30 nm. It was found that the absorption edge of the oxynitride is significantly red-shifted compared with that of La₂Ti₂O₇ as increasing the nitridation temperature. The UV–vis absorption spectra indicated that the synthesized oxynitrides displayed good light absorption properties not only in the ultraviolet light but also in the visible-light region.     

Release of BSA from porous matrices constituted of alginate–Ca and PNIPAAm-interpenetrated networks

The synthesis of thermosensitive Interpenetrating Polymer Network (IPN) hydrogels and the release of Bovine Serum Albumin (BSA) from the hydrogels were reported. The hydrogels, constituted of poly(N-isopropyl acrylamide) PNIPAAm network interpenetrated in alginate–Ca²⁺ network, were synthesized in a two-stepped process. In the first step, PNIPAAm network was synthesized from an aqueous solution containing N-isopropyl acrylamide (NIPAAm) monomers and N,N′-methylene-bis-acrylamide (MBAAm) co-monomers, and sodium alginate (SA) (1 or 2% w/v). The concentration of NIPAAm monomers in the hydrogel-forming solution was always 2.5, 5.0 or 10.0% (w/v). In the second step, alginate–Ca²⁺ networks were formed by immersion of the membrane, obtained on the first step, in a 1.0% (w/v) aqueous calcium chloride. The IPN hydrogels were characterized as a function of temperature (from 25 to 45 °C) through the following measurements: drop water contact angle (DWCA), compression elastic modulus (E) and cross-linking density (ν_e). The morphology was investigated using scanning electronic microscopy (SEM). In vitro release of BSA from the hydrogels was monitored by UV–Vis spectroscopy at 22 °C and 37 °C. DWCA results showed a decrease in the hydrogel hydrophilicity when the temperature and/or the PNIPAAm amount on hydrogels were increased. PNIPAAm-loader hydrogels are more compacted and presented elevated rigidity, mainly above 35 °C. This trend was attributed to the collapsing of PNIPAAm chains as the hydrogels were warmed above its Lower Critical Solution Temperature (LCST), which in aqueous solution is ca. 32–33 °C. The amount of BSA released from the alginate–Ca²⁺/PNIPAAm hydrogels changes inversely to both amount of PNIPAAm and temperature. The transport of BSA from the hydrogels was evaluated through a conventional model. In the lesser-compacted hydrogels the release occurs mostly by diffusion. In the more compacted ones the chain relaxation contributes to the BSA release. Thus, the alginate–Ca²⁺/PNIPAAm IPN-typed matrixes may be considered as smart hydrogels for the release of BSA, because the amount and rate of BSA released may be tailored by both the NIPAAm concentration in the hydrogel-forming solution and the control of temperature of hydrogel.     

New powellite type oxides in Ca–R–Nb–Mo–O system (R = Y, La, Nd, Sm or Bi)—Their synthesis, structure and dielectric properties

New complex oxides having powellite (CaMoO₄) type structure in the Ca–R–Nb–Mo–O system (R = Y, La, Nd, Sm or Bi) were prepared employing the method of solid state reaction between the component oxides at high temperature (1000–1100 °C). The new compounds, CaRNbMoO₈ (R = Y, La, Nd, Sm, Bi) are colorless and electrical insulators. The dielectric constants (K at 1 MHz) of these compounds are in the range 14–33 and K shows very little variation in the temperature range 30–100 °C. Their temperature coefficient of dielectric constant (TCK) is negative, which varies from − 21 to − 220 ppm/°C.