Electrically conductive silicon carbide with the addition of TiNbC

The work deals with the preparation of dense SiC based ceramics with high electrical conductivity. SiC samples with different content of conductive TiNbSiCO based phase were hot pressed at 1820 °C for 1 h in Ar atmosphere under mechanical pressure of 30 MPa. The conductive phase is a mixture of 50 wt% TiNbC (molar ratio of Ti/NbC is 1:1.8) and 50 wt% eutectic composition of Y₂O₃SiO₂. Composite with 30% of conductive TiNbSiCO phase showed the highest electrical conductivity 28.4 S mm^?1, while the good mechanical properties of SiC matrix were preserved (fracture toughness K_IC = 5.4 MPa m^1/2 and Vickers hardness 17.8 GPa).The obtained results show that the developed additive system is suitable for the preparation of SiC-based composite with sufficient electrical conductivity for electric discharge machining.     

Degradation of mechanical and electrical properties after long-term oxidation and corrosion of non-oxide structural ceramic composites

This work summarizes the results related to the influence of the starting composition and of microstructure on properties degradation, due to oxidation and corrosion, relatively to the following structural ceramics: Si₃N₄TiN, Si₃N₄MoSi₂, AlNSiCMoSi₂, AlNSiC.The effects of: (i) long-term oxidation in air (100 h), in the temperature range 600–1500 °C and (ii) of long-term corrosion (400 h) in acid or basic aqueous solution at RT, 40 and 70 °C, on the electrical resistivity and mechanical strength of the composites are analysed and compared. The degradation of the properties are related to the characteristics of the surface and sub-surface damage after oxidation and corrosion treatments.     

A novel approach for preparation of dense TiC–SiC nanocomposites by sol–gel infiltration and spark plasma sintering

Dense TiC–SiC nanocomposite ceramics were prepared by infiltration of porous TiC scaffolds with a SiOC sol, followed by spark plasma sintering (SPS). The porous nano TiC scaffold was first synthesized by direct carbothermal reduction of a monolithic TiOC precursor obtained from a controlled sol–gel process. The TiC scaffold was infiltrated with a SiOC sol and then the sample was aged in a container for 48 h at 80 °C to convert the sol into gel. After this, the sample was heated at 550 °C to remove the organic components and then 1350 °C to convert the SiOC gel to SiC by carbothermal reduction reaction. The cycle of the infiltration and carbothermal reduction was repeated several times to obtain relatively dense TiC–SiC composite samples. Dense TiC–SiC composite with a uniform nano-sized grain microstructure was obtained by spark plasma sintering at 1800 °C for 5 min under 40 MPa uniaxial pressure. Compared with conventional powder mixing methods, the sol–gel infiltration approach has shown distinct advantages of achieving dense TiC–SiC composites with uniform nano-sized grain structures.     

Thermally stable pseudo-third-generation Grubbs ruthenium catalysts with pyridine–phosphinimine ligand

The ligand precursors 2-(R₃PN)CH₂Py (R = Ph(1a), Cy(2a)) were prepared from reaction of pyridine azide with various phosphine ligands. Reaction of 1a or 2a with RuCl₂(CHPh)(Py)₂(H₂IMes) (Py = pyridine) afforded the ruthenium alkylidene complex RuCl₂(CHPh)(PyCH₂(NPR₃))(H₂IMes) (R = Ph(1), Cy(2)). Both catalysts showed good thermal stability and latent behavior toward RCM and ROMP reactions.     

Low temperature sintering and microwave dielectric properties of (Mg3 − xZnx)(VO4)2 ceramics

This paper describes the effects of Zn substitution for Mg on the microwave dielectric properties of (Mg_3 − xZn_x)(VO₄)₂ ceramics. As for the XRPD patterns of (Mg_3 − xZn_x)(VO₄)₂ ceramics sintered at the sintering temperature of 750 °C, no secondary phase was detected over the whole composition range. However, in the case of the sample sintered at 850 °C, the Zn₄V₂O₉ and Zn₂V₂O₇ compounds were identified by using XRPD; this result was attributed to the decomposition of Zn₃(VO₄)₂ phase. From crystal structure analysis, it was found that the atomic distances of M(1)O (M = Mg and Zn) and M(2)O in MO₆ octahedra increased, though that of VO in VO₄ tetrahedron decreased. Moreover, the slight tilting of M(2)O₆ octahedron was observed by the Zn substitution. As for the covalency of cation–oxygen bond, the covalency of MO bond in M(1)O₆ and M(2)O₆ octahedra decreased because of the increase in the atomic distance of MO, whereas that of VO increased with increasing the Zn addition. However, as a result, the slight decrease in the covalency of cation–oxygen bond was recognized because the variation in the covalency of MO bond is predominant in this crystal structure. The dielectric constants of the samples range from 4.4 to 11.1. The decrease in the covalency may be related to the difference in the dielectric constant of each composition. The maximum Q · f value of bulk densities is effected by varying the chemical composition of (Mg_3 − xZn_x)(VO₄)₂ ceramics and it shifts toward lower sintering temperature with an increase in x within the temperature region of 800–1050 °C. The temperature coefficient of resonant frequency (τ_f) of the samples decreased with increasing of Zn, and then a variation in τ_f value was attributed to the tilting of M(2)O₆ octahedron caused by Zn substitution for Mg.     

Measurement of the hydroxyl radical formation from H2O2, NO3−, and Fe(III) using a continuous flow injection analysis

Production of hydroxyl radical (OH) is of significant concern in engineered and natural environment. A simple in situ method was developed to measure OH formation in UV/H₂O₂, UV/Fe(III), and UV/NO₃^? systems using trapping of OH by benzoic acid (BA) and measuring fluorescence signals from hydroxylated products of BA. Method development included characterization of OH trapping mechanism and measurement of quantum yields (Φ_OH) for OH. The distribution of OHBA isomers was in the order of o-OHBA > p-OHBA > m-OHBA, although it changed with the H₂O₂ concentration and light intensity. This supports that OH attacks dominantly on the benzene rings. The quantum yields for OH formation in the UV/H₂O₂ process were 1.02 and 0.59 at 254 and 313 nm, which were in good agreement with the literature values, confirming that the method is suitable for the measurement of OH production from UV/H₂O₂ processes. Using the continuous flow method developed, quantum yields for OH in UV/H₂O₂, UV/Fe(III), and UV/NO₃^? systems were measured varying the initial concentration of OH precursors. The Φ_OH values increased with increasing concentrations of H₂O₂, Fe(III), and NO₃^? and approached constant values as the concentration increased. The Φ_OH values were 0.009 for H₂O₂ at 365 nm, showing that OH production is not negligible at such high wavelength. The Φ_OH values during the photolysis of Fe(OH)²⁺ (pH 3.0) and Fe(OH)₂⁺ (pH 6.0) at 254 nm were 0.34 and 0.037, respectively. The Φ_OH values for NO₃^? approached a constant value of 0.045 at 254 nm at the initial concentration of 10 mM.     

Carbothermal synthesis of β-sialon from mechanochemically activated precursors

Reaction mixtures of halloysite clay and fine carbon for carbothermal reduction and nitridation (CRN) synthesis of β-sialon were ground in a planetary ball mill under flowing nitrogen for varying periods before being converted to sialon by heating in nitrogen at 1200–1400 °C. After 4 h grinding the XRD reflections of the halloysite were destroyed and some of the octahedrally-coordinated Al was converted to four- and five-fold coordination. ²⁷Al and ²⁹Si MAS NMR gave no evidence of the formation of AlN or SiN bonds upon grinding. Upon subsequent heating in nitrogen, the ground samples show significant differences from the unground control, the intermediate compound mullite being replaced by β-sialon (z ≈ 2) a temperature at least 100 °C lower, but the formation of corundum (α-Al₂O₃) also occurs at a lower temperature and is more persistent than in the unground control. MAS NMR spectroscopy shows that the products from the ground mixtures contain relatively less AlON units and that the formation of SiC (a transient reaction intermediate) is also facilitated by grinding. The optimum grinding time for this system was found to be 12 h.     

Towards osseointegration of bioinert ceramics: Can biological agents be immobilized on alumina substrates using self-assembled monolayer technique?

Immobilization of biological agents on inert alumina surfaces could promote bone growth and improve osseointegration. We hypothesize that functional groups on alumina surfaces can be used to link biological agents as a supporting factor e.g. for cell attachment. CH₂, OH, COOH, and NH₂ groups were linked to alumina surfaces using self-assembled monolayer technique (SAM). Subsequently, bovine serum albumin (BSA) was immobilized on each functionalized surface. Contact angle, bicinchoninic acid assay and immunofluorescence were used to detect immobilized BSA. The amount of BSA linked to functionalized surfaces increased in the following order CH₂ < OH < COOH = NH₂. The greatest amount, 26.1 μg/cm² of BSA was found on both, NH₂- and COOH-terminated surfaces. Cell tests confirmed cytocompatibility of all surfaces. The highest proliferation was detected on NH₂-terminated samples. Using the model protein, the results confirmed feasibility for immobilization of biological agents to inert alumina ceramic surfaces using SAM technique.     

Fabrication of polymer derived ceramic parts by selective laser curing

Polymer derived ceramic parts of complex shape were fabricated by selective laser curing (SLC). The ceramic parts were built similar to the selective laser sintering process by sequentially irradiating layers consisting of SiC loaded polysiloxane powder with a CO₂-laser beam (λ = 10.6 μm), which locally induces curing reaction of the polymer phase at moderate temperatures around 400 °C. The laser-cured bodies were converted to SiOC/SiC ceramic parts in a subsequent pyrolysis treatment at 1200 °C in argon atmosphere. The scanning speed and the power of the CO₂-laser beam were varied, leading to pronounced differences in material properties. Laser irradiating a powder mixture containing 50 vol.% polymer/50 vol.% SiC resulted in relative green densities between 38 and 60%. Relative densities decreased to 32–50% after pyrolysis due to the polymer shrinkage. A post-infiltration with liquid silicon was carried out in order to produce dense parts. While before infiltration the bending strength only attained 17 MPa as a result of both microcracks in the SiOC matrix and a pronounced porosity, an average value of 220 MPa was achieved after post-infiltrating with Si. In the case of 50 vol.% SiC content the linear shrinkage after pyrolysis was only 3%. Thus, the SLC approach can be considered as a near-net-shape forming process of ceramic components with complex geometries.     

Accelerating the crosslinking process of hyperbranched polycarbosilane by UV irradiation

A liquid hyperbranched polycarbosilane (LHBPCS) with stoichiometric C/Si ratio but without unsaturated groups was synthesized. Different from traditional thermal crosslinking, ultraviolet (UV) irradiation crosslinking was taken. The molecular weight, the consumption of SiH group and ceramic yield of LHBPCS showed an increase trend with increasing the UV irradiation time. After 30 min of UV irradiation, 71.8 wt% ceramic yield was obtained. In addition, extra divinyldimethylsilane was added into LHBPCS. Under UV irradiation, both the SiH group and vinyl group of divinyldimethylsilane were consumed. But the reaction extend of vinyl group was much faster than that of SiH group. Compared with pure LHBPCS, the mixture of LHBPCS and 5 wt% divinyldimethylsilane gave a higher ceramic yield of 79 wt% after 30 min of UV irradiation. By heating the crosslinked LHBPCS to 1000 °C, a near stoichiometric SiC ceramic was got. It exhibited excellent thermal stability at 1400 °C in air.     

Liquid-phase hydrogenation of o-chloronitrobenzene over supported nickel catalysts

The effect of supports (SiO₂, ZrO₂, TiO₂ and Al₂O₃) on the performance of nickel catalysts for the hydrogenation of o-chloronitrobenzene was investigated. It was found that the supports strongly influenced the catalytic performance of the catalysts, and Ni/TiO₂ showed the best catalytic performance. Over Ni/TiO₂, 99.9% conversion of o-chloronitrobenzene and 99.5% selectivity of o-chloroaniline were obtained at 1.5 MPa and 363 K. The good performance of Ni/TiO₂ was perhaps attributed to the strong polarization of NO band induced by oxygen vacancies of TiO_x which was produced by a high temperature reduction, and the NO band polarized was attacked easily by hydrogen dissociatively adsorbed on the nickel particles.     

Characterization of carbon nanofibers synthesized by microwave plasma-enhanced CVD at low-temperature in a CO/Ar/O2 system

The low-temperature synthesis of carbon nanofibers by microwave plasma-enhanced chemical vapor deposition using a CO/Ar/O₂ system and their characterizations were performed. At the optimum oxygen concentration of O₂/CO = 7/1000, vertically aligned CNFs can be synthesized at temperatures as low as 180 °C with growth rates of 4–6 nm/s. The diameter of bulk CNFs is about 50–100 nm and the surface of CNFs is covered by branching fibers and their nuclei with a diameter of about 5–20 nm. Not only the peaks originating from carbon chains, but also oxygen containing groups, such as CO and COC, are observed in the FTIR spectra. The CNFs growth rate is almost independent from the substrate temperature and it is concluded that an elementary process not on the substrates, but in the gas phase, is the rate-determining step in the present CO/Ar/O₂ microwave-plasma-enhanced CVD system.     

Crystalline MoVO based complex oxides as selective oxidation catalysts of propane

Three single crystalline MoVO based oxides, MoVO, MoVTeO and MoVTeNbO, all of which have the same orthorhombic layer-type structure with the particular arrangement of MO₆ (M = Mo, V, Nb) octahedra forming slabs with pentagonal, hexagonal, and heptagonal rings in (1 0 0) plane, were synthesized by hydrothermal method and their catalytic performance in the selective oxidation of propane to acrylic acid were compared in order to elucidate the roles of constituent elements and crystal structure in the course of the propane oxidation. It was observed that the rate of propane oxidation was almost the same over all three catalysts, revealing that Mo and V, which were indispensable elements for the structure formation, were responsible for the catalytic activity for propane oxidation. The Te-containing catalysts showed much higher selectivity to acrylic acid than the MoVO catalyst. Since propene was formed as a main product at low conversion levels over every catalyst, it can be concluded that Te located in the central position of the hexagonal ring promoted the conversion of intermediate propene effectively to acrylic acid. The catalyst with Nb occupying the same structural position of V clearly showed the improved selectively to acrylic acid particularly at high conversion region, because the further oxidation of acrylic acid to CO_x was greatly suppressed. These conclusions were further supported by the additional studies of the determination of activation energy and catalytic oxidations of intermediate products of the propane oxidation.     

Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part II: Effect of Al2O3

The hydration and microstructural evolution of three alkali activated slags (AAS) with Al₂O₃ contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O. The formation of C(A)–S–H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al₂O₃ content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(A)-S-H and led to the formation of strätlingite. Increasing Al₂O₃ content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al₂O₃ content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.     

Catalytic reactions of dioxygen with ethane and methane on platinum clusters: Mechanistic connections,site requirements,and consequences of chemisorbed oxygen

C₂H₆ reactions with O₂ only form CO₂ and H₂O on dispersed Pt clusters at 0.2–28 O₂/C₂H₆ reactant ratios and 723–913 K without detectable formation of partial oxidation products. Kinetic and isotopic data, measured under conditions of strict kinetic control, show that CH₄ and C₂H₆ reactions involve similar elementary steps and kinetic regimes. These kinetic regimes exhibit different rate equations, kinetic isotope effects and structure sensitivity, and transitions among regimes are dictated by the prevalent coverages of chemisorbed oxygen (O^*). At O₂/C₂H₆ ratios that lead to O^*-saturated surfaces, kinetically-relevant CH bond activation steps involve O^*O^* pairs and transition states with radical-like alkyls. As oxygen vacancies (¹) emerge with decreasing O₂/alkane ratios, alkyl groups at transition states are effectively stabilized by vacancy sites and CH bond activation occurs preferentially at O^** site pairs. Measured kinetic isotope effects and the catalytic consequences of Pt cluster size are consistent with a monotonic transition in the kinetically-relevant step from CH bond activation on O^*O^* site pairs, to CH bond activation on O^** site pairs, to O₂ dissociation on ^** site pairs as O^* coverage decrease for both C₂H₆ and CH₄ reactants. When CH bond activation limits rates, turnover rates increase with increasing Pt cluster size for both alkanes because coordinatively unsaturated corner and edge atoms prevalent in small clusters lead to more strongly-bound and less-reactive O^* species and lower densities of vacancy sites at nearly saturated cluster surfaces. In contrast, the highly exothermic and barrierless nature of O₂ activation steps on uncovered clusters leads to similar turnover rates on Pt clusters with 1.8–8.5 nm diameter when this step becomes kinetically-relevant at low O₂/alkane ratios. Turnover rates and the O₂/alkane ratios required for transitions among kinetic regimes differ significantly between CH₄ and C₂H₆ reactants, because of the different CH bond energies, strength of alkylO^* interactions, and O₂ consumption stoichiometries for these two molecules. Vacancies emerge at higher O₂/alkane ratios for C₂H₆ than for CH₄ reactants, because their weaker CH bonds lead to faster scavenging of O^* and to lower O^* coverages, which are set by the kinetic coupling between CH and OO activation steps. The elementary steps, kinetic regimes, and mechanistic analogies reported here for C₂H₆ and CH₄ reactions with O₂ are consistent with all rate and isotopic data, with their differences in CH bond energies and in alkyl binding, and with the catalytic consequences of surface coordination and cluster size. The rigorous mechanistic interpretation of these seemingly complex kinetic data and cluster size effects provides useful kinetic guidance for larger alkanes and other catalytic surfaces based on the thermodynamic properties of these molecules and on the effects of metal identity and surface coordination on oxygen binding and reactivity.     

The effect of Mg(OH)2 on furfural oxidation with H2O2

The oxidation of furfural in H₂O₂ and H₂O₂–Mg(OH)₂ system were systematically investigated and a rational explanation for the reaction mechanism was proposed. 2-formyloxyfuran, from selective oxidation of HCO group in furfural, was a crucial intermediate. The addition of Mg(OH)₂ suppressed the oxidation of furan ring of furfural and enhanced selectivities of 2(5H)-furanone (44.8%) and succinic acid (38.0%). FT-IR, Gaussian calculation and experimental results indicated that the process of furfural oxidation with H₂O₂ is homogeneous, and the synergy between dissolved Mg^2 + cations and OH⁻ ions facilitates the HOO⁻ attacking the carbon atom of HCO other than the CC bound of furan ring.     

High efficient adsorption of gold ions onto the novel functional composite silica microspheres encapsulated by organophosphonated polystyrene

The novel functional composite silica microspheres encapsulated by organophosphonated polystyrene (SGPSNP) has been successfully synthesized. SGPSNP was employed to adsorb Au(III) from simulated wastewater, and it exhibited excellent performance, and the maximum adsorption capacity was 980.39 mg/g at 35 °C. The adsorption process optimization was performed using response surface methodology (RSM), and the analysis of variance (ANOVA) of the quadratic model demonstrated that the model was highly significant. Moreover, the regeneration capacities of SGPSNP were investigated, and it has been found that the adsorption capability remains high after several cycles of adsorption–desorption.     

Crystal structural refinement of corundum-structured A4M2O9 (A = Co and Mg,M = Nb and Ta) microwave dielectric ceramics by high-temperature X-ray powder diffraction

The microwave dielectric properties and crystal structure of corundum-structured Mg₄Nb₂O₉ (MN), Co₄Nb₂O₉ (CN) and Mg₄Ta₂O₉ (MT) compounds were investigated in this study. The crystalline phases of the ceramics were characterized by using the high-temperature X-ray powder diffraction and the crystal structures of the compounds were refined in terms of Rietveld analysis. The temperature coefficient of resonant frequency (τ_f) varied from −70 to −10 ppm/°C with increasing the composition x when Mg was substituted by Co. From the calculation of covalency of cation–oxygen bonds, it is found that the covalency of CoO bond is lower than that of MgO bond and the difference in the covalency of these cation–oxygen bonds influences on the temperature dependence of dielectric constant on the compounds. As for the dielectric constant (ɛ_r) and the quality factor (Q·f) of the ceramics, these values ranged from 10 to 16 and from 210,000 to 5000 GHz, depending on the composition x.     

Synthesis,pyrolysis of a novel liquid SiBCN ceramic precursor and its application in ceramic matrix composites

SiBCN ceramic precursor, polyborosilazane, was synthesized through a novel method which used sodium borohydride as boron source. Vinyl silazane with SiCl was converted to vinyl silazane with SiH structure, followed by hydroboration reaction and subsequent high-temperature reaction to form soluble polyborosilazane liquid. The process of precursor-to-ceramic conversion was almost completed before 800 °C and the cross-linked polyborosilazane precursor exhibited higher ceramic yield 75.6% at 1200 °C. The SiBCN ceramic annealed at 1400 °C contained BN, SiN and SiC bonds with smooth and dense surface and still retained principally amorphous structure up to 1600 °C. In addition, the viscosity of the polyborosilazane was 65 mPa.s, which can efficiently prepare ceramic matrix composite by means of precursor infiltration and pyrolysis (PIP). The density of as-obtained ceramic matrix composite (CMC) was 1.82 g/cm³, and the average bending strength, bending modulus and tensile strength were 265.2 MPa, 37.5 GPa and 158.6 MPa, respectively.