Synthesis of SiC–TiO2 hybrid aerogel via supercritical drying combined PDCs route

The SiC-TiO₂ hybrid aerogels were obtained from polycarbosilane (PCS) and tetrabutyltitanate (TBT) as precursors using supercritical drying and polymer derived ceramics route. The polymer to ceramic conversion and the crystallization behavior were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM), suggesting the preceramic aerogels converted to the SiC-TiO₂ ceramic aerogels through pyrolysis process at different temperatures. At 1200 °C, the ceramic aerogels were homogeneous with well-distributed element components, composed of rutile TiO₂ and the β-SiC crystalline phases. The results show that the SiC-TiO₂ ceramic aerogels with netwoks structure have 23.36 nm average pore size, high surface area (58 m²/g) and pore volume (0.22 cm³/g).     

Synthesis of well-defined norbornene–lactone-functionalized polymers via ATRP

Well-defined norbornene–lactone-functionalized polymers were synthesized by atom transfer radical polymerization (ATRP) of 5-methacryloxy-6-hydroxynorbornene-2-carboxylic-6-lactone (MNL) and 5-acryloxy-6-hydroxynorbornene-2-carboxylic-6-lactone (ANL) monomers. The ATRP of MNL initiated by ethyl 2-bromopropionate (EBrP), in both N,N-dimethylformamide (DMF) and o-dichlorobenzene (ODCB) solvents was successfully carried out in the presence of CuCl/CuBr and N,N,N′′,N′′,N′′-pentamethyltriethylenetetramine (PMDETA) at 70 °C. The CuCl/ODCB catalyst system gave rise to a lower M _w/M _n (≦1.20) than CuBr/DMF catalyst system. The ATRP of ANL was feasible in the presence of CuBr and PMDETA at 70 °C but showed lower reactivity than MNL. The resulting polymers were characterized by means of gel permeation chromatography (GPC) and ¹H NMR spectroscopy.     

Densification of MCMB–SiC composites via two-step hot pressing

Mesocarbon microbead–SiC (MCMB–SiC) composites with 30 wt% MCMBs were densified using a two-step hot pressing method. Based on the pyrolysis of the initial MCMB powders, the effects of the pressing schedule on densification were investigated and the optimal first-step pressing temperature was determined. To reveal the influence of temperature on their microstructures, the raw MCMB powders were heat-treated at different temperatures in the range 400–1400 °C. The morphologies and degrees of carbonisation at different temperatures were additionally studied. The results showed that densification was mainly affected by the micro-gaps in the lamellar structure formed during the pyrolysis of the MCMBs. When the samples were first hot-pressed at a lower temperature and then at a higher temperature, the densification pressure required was effectively decreased. Furthermore, when the samples were first pressed at an appropriate temperature, the relative density of the composites was improved to a rather high value of 98.6%. The two-step hot pressing method was effective in fabricating dense C–SiC composites with high C content.     

Creep of SiC–SiC microcomposites

The creep behavior of SiC/C/SiC microcomposites at 1200–1400 °C and 140–450 MPa was investigated in the presence and absence of matrix cracking. The microcomposites consisted of single Hi Nicalon or Carborundum fibers coated with a CVD carbon interlayer and a CVD SiC matrix. Since the fibers and matrix had been examined by the identical experimental technique, direct comparisons of the creep of the composite and of the constituents were performed. The creep of uncracked microcomposites was successfully modeled using a simple rule of mixtures algorithm. When matrix cracks were present, the microcomposites were modeled using a series composite consisting of intact microcomposite, exposed fiber at the matrix crack, and the debonded region in between. Trends for behavior with respect to the various mechanical and structural parameters that control creep are presented.     

Reaction Synthesis and Mechanical Properties of TiB2–AlN–SiC Composites

TiB₂–AlN–SiC (TAS) ternary composites were prepared by reactive hot pressing at 2000°C for 60 min in an Ar atmosphere using TiH₂, Si, Al, B₄C, BN and C as raw powders. The phase composition was determined to be TiB₂, AlN and β-SiC by XRD. The distribution of elements Al and Si were not homogeneous, which shows that to obtain a homogeneous solid solution of AlN and SiC in the composites by the proposed reaction temperatures higher than 2000°C or time duration longer than 60 min are needed. The higher fracture toughness (6·35±0·74 MPa·m^1/2 and 6·49±0·73 MPa·m^1/2) was obtained in samples with equal molar contents of AlN and SiC (TAS-2 and TAS-5) in the TAS composites. The highest fracture strength (470±16 MPa) was obtained in TAS-3 sample, in which the volume ratio of TiB₂/(AlN+SiC) was the nearest to 1 and there was finer co-continuous microstructure. ©     

Growth of In2O3 Nanowires Catalyzed by Cu via a Solid–Liquid–Solid Mechanism

In₂O₃ nanowires that are 10–50 nm in diameter and several hundred nanometers to micrometers in length have been synthesized by simply annealing Cu–In compound at a relatively low temperature of 550°C. The catalysis of Cu on the growth of In₂O₃ nanowires is investigated. It is believed that the growth of In₂O₃ nanowires is via a solid–liquid–solid (SLS) mechanism. Moreover, photoluminescence (PL) peaks of In₂O₃ nanowires at 412 and 523 nm were observed at room temperature, and their mechanism is also discussed.     

Enhanced Activity via Surface Modification of Fe-Based Fischer–Tropsch Catalyst Precursor with Titanium Butoxide

The surface of a model iron catalyst precursor was modified with titanium butoxide to introduce Fe–O–Ti interactions in a controlled manner and to investigate the role of these interactions in the catalyst. The reduction of the model catalyst precursors in hydrogen at 350 °C for 16 h leads to the formation of α-Fe and an iron–titanium mixed oxide, due to the incorporation of Ti into the iron oxide structure. The α-Fe phase is transformed into χ-Fe₅C₂ during the Fischer–Tropsch synthesis at 250 °C, whilst the Fe–Ti mixed oxide phase is preserved. A higher reaction temperature of 300 °C is required to transform some the oxide phase into a carbide phase under Fischer–Tropsch conditions. The intrinsic activity of the iron carbide phase in samples also containing the Fe–Ti mixed oxide phase is at a reaction temperature of 250 °C ca. 20 % more active than in the sample, which does not contain the mixed oxide.     

Synthesis of the Precursor of Aluminum–Calcium Oxide Systems by Anodic Oxidation of Aluminum in Aqueous Solutions

An aluminum oxide system is obtained by electrolysis with a soluble anode and by the subsequent thermal treatment of the precipitate. It is shown that this system can be modified with calcium oxide in the process of synthesis. The influence of the synthesis conditions (the anodic current density and the electrolyte composition) on the formation of finely dispersed particles of the aluminum–calcium systems is studied. The structure and properties of the powder particles obtained by the electrochemical method are studied using the methods of laser diffraction, X-ray diffraction analysis, and electron microscopy.     

Synthesis and characterization of novel Ti3SiC2–cBN composites

In this paper, synthesis of novel super hard and high performance composites of titanium silicon carbide–cubic boron nitride (Ti₃SiC₂–cBN) was evaluated at three different conditions: (a) high pressure synthesis at ~ 4.5 GPa, (b) hot pressing at ~ 35 MPa, and (c) sintering under ambient pressure (0.1 MPa) in a tube furnace. From the analysis of experimental results, the authors report that the novel Ti₃SiC₂–cBN composites can be successfully fabricated at 1050 °C under a pressure of ~ 4.5 GPa from the mixture of Ti₃SiC₂ powders and cBN powders. The subsequent analysis of the microstructure and hardness studies indicates that these composites are promising candidates for super hard materials.     

Synthesis of SiC precursors by a two-step sol–gel process and their conversion to SiC powders

A two-step sol–gel processing was developed to synthesize phenolic resin–SiO₂ hybrid gels as SiC precursors, with tetraethoxysilane (TEOS) and novolac phenolic resin being the starting materials, and oxalic acid (OA) and hexamethylenetetramine (HMTA) being the catalysts. At the first step TEOS was prehydrolyzed under the catalysis of OA. At the second step HMTA was added to facilitate gelation. The influences of the molar ratio of OA/TEOS and prehydrolysis time on the sol–gel reaction were investigated. There existed an optimum OA/TEOS ratio where prehydrolysis time needed to form transparent gels was the shortest. The increase of temperature could accelerate sol–gel reaction. The dried hybrid gels were yellowish transparent glassy solids, with uniform microstructure composed of nanometer-sized particles. The conversion of the gels to silicon carbide powders was complete when heated at 1650°C for 30 min in vacuum. The oxygen and free carbon were 0.43 and 0.50 wt%, respectively, in the powder produced from the gel prepared with starting resin/TEOS being 0.143 g/ml.     

Reactive infiltration processing of SiC/Fe–Si composites using preforms made of coked rice husks and SiC powder

A reactive infiltration processing of SiC/Fe–Si composites using preforms made of coked rice husks (RHs) and SiC powder in different ratios is reported, in which FeSi₂ alloy was used as infiltrant. The preforms were heat-treated at 1550 °C for 6 h prior to the infiltration. The coked RHs, which are composed of SiO₂ and C, were converted to SiC and poorly crystallized C by carbothermal reduction during the heat treatment. The study of the microstructure and mechanical properties of the composites shows that molten Fe–Si alloy had good wetting of the heat-treated preforms and adequate infiltration properties. Free carbon in the preform reacted with Si in the molten FeSi₂ during infiltration forming new SiC, the composition of the intermetallic liquid being moved towards that of FeSi. As a result, the infiltrated composites are composed of SiC, FeSi₂ and FeSi phases. Vickers hardness, elastic modulus, three-point flexural strength and indentation fracture toughness of the composites are found to increase with SiC additions up to 30% w/w in the preforms, reaching the values of 18.2 GPa, 290 GPa, 213 MPa and 4.9 MPa m^1/2, respectively. With the SiC addition further raised to 45% w/w, the elastic modulus, flexural strength and fracture toughness of the composite turned down probably due to high residual stress and hence the more intense induction of microcracks in the composite. De-bonding of SiC particles pulled out of the Fe–Si matrix, transgranular fracture of part of the SiC particles and in the Fe–Si matrix, and crack bridging all exist in the fracture process of the composites.     

Synthesis of Fe3 Al–TiC-SiC Nanocomposite by Mechanical Alloying of FeSiTiO3–Al–C Powders

Silicon - The purpose of this study was to produce Fe3Al/TiC-SiC nanocomposite by mechanical alloying of the FeSiTiO3, Al and C powder mixture. The phases made of the samples were characterized by...     

Synthesis and characterization of polybenzimidazole–nanodiamond hybrids via in situ polymerization method

Poly[2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole] (OPBI) was polymerized in poly(phosphoric acid) (PPA) with the presence of the pristine nanodiamonds (NDs) (0.2–5 wt %) to fabricate NDs-g-OPBI/OPBI nanocomposites via Friedel–Crafts (F-C) reaction. The OPBI chains were successfully attached to the NDs through F-C reaction between carboxylic acid from OPBI and NDs, which was proved by nuclear magnetic resonance, X-ray photoelectron, and X-ray diffraction. NDs-g-OPBI/OPBI nanocomposites show more homogeneous dispersion than the physical blending containing pristine NDs and OPBI matrix, as showed through scanning electronic microscopy images. The mechanical properties, including Young's modulus, yield strength, and tensile strength are all improved by the introduction of NDs (<1 wt %) without loss of ductility, which overcomes the brittleness brought by the addition of inorganic reinforced agent in traditional composites. Dynamic mechanical analysis results showed that the modulus of the ND-g-OPBI/OPBI nanocomposites was significantly higher than OPBI matrix, and the NDs-g-OPBI/OPBI nanocomposites displayed more pronounced improvement than the physical blending, which could be ascribed to the homogeneous dispersion of NDs particles and the covalent bonding between NDs and OPBI via F-C reaction. Thermogravimetric analysis indicated that all the OPBI nanocomposites containing NDs displayed the improved thermal stability. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis and properties of acrylate functionalized alkyds via a Diels–Alder reaction

The α-eleosterate pendent fatty acid of a tung oil based alkyd was functionalized via a Diels–Alder reaction with three different acrylate groups: (1) 2,2,2-trifluoroethyl methacrylate, (2) 3-methacryloxypropyl trimethoxysilane, and (3) triallyl ether acrylate. The modified alkyds were characterized by using ¹H NMR, ¹³C NMR, and gel permeation chromatography (GPC). The drying time was measured at ambient temperature. The viscoelastic properties of the alkyd-modified films were measured using dynamic mechanical thermal analysis. The viscoelastic and drying time results show that the alkyd modified with siloxane and triallyl group affords a faster drying time, higher crosslink density, and higher glass transition temperature compared to the unmodified alkyd, whereas the fluorinated alkyd possesses surface active properties, but suffers in terms of drying and crosslinking density.     

Phenomena and mechanism of local oxidation microlithography of 4H–SiC via electrochemical jet anodisation

Local oxidation microlithography (LOM) of 4H–SiC wafers based on the spatial confinement of electrochemical oxidation inside a microelectrolyte jet, namely, electrochemical jet anodisation (EJA), is presented. EJA enables selective growth of the oxide layer on a micro-scale local area with no masks because the anodic reaction occurs exclusively in the jet-substrate interaction area. The shape and height profile of the oxide layer were highly dependent on the anodising conditions. The change in the current density resulted in two distinct oxidation regimes, leading to the formation of either Gaussian-type or doughnut-shaped oxide spots. The oxide growth mechanism was revealed by SEM, AFM and XTEM characterisation of the oxide at the micro/nanoscale. A flat oxide layer with uniform thickness was obtained by applying parametric control. Following a chemical etching process, microstructures were readily created at the patterned oxide locations, demonstrating EJA as a potential lithography technique for microfabrication.     

Microstructure and texture of polycrystalline 3C–SiC thick films characterized via EBSD

Cubic silicon carbide (3C–SiC) is an excellent protective film on graphite and has been fabricated via laser chemical vapor deposition (LCVD) with an extremely high deposition rate by our research group. To understand comprehensively its growth behavior, the polycrystalline 3C–SiC thick films with the preferred orientation of <111> and <110> were characterized by diverse measurements, especially electron back-scatter diffraction (EBSD). Along the growth direction of the <110>-oriented 3C–SiC, the microstructure changed from equiaxed grains to elongated grains with <111> orientation, and eventually the <110>-oriented columnar grains. The stacking faults in the <110>-oriented 3C–SiC could be marked as <11–20>-oriented 6H–SiC. On the other hand, in the <111>-oriented 3C–SiC films, the microstructure changed from mainly equiaxed grains to large columnar grains. The high-density stacking faults in <111>-oriented 3C–SiC films may lead to the identification of the nominal 2H, 4H and 6H polytypes by Raman spectra and EBSD. The (0001) planes of 2H-, 4H–SiC are perpendicular to the growth direction, while that of 6H–SiC are parallel.     

Microstructure,interfacial characteristics,and wear performances of Cu–Fe–SiC cermet composites

The Cu–Fe metal-based ceramic grinding wheel material with SiC as abrasive was prepared by the powder metallurgy process of ball milling and hot pressing sintering. Cu–Fe–SiC cermets with Cu:Fe mass ratios of 4:1, 1:1, and 1:4 were designed by changing the composition of metal binder. The phase composition, microstructure, mechanical properties, and grinding properties of Cu–Fe–SiC cermets were systematically studied. The effect of Cu–Fe binder ratio on the microstructure and properties of cermets was analyzed. The results show that with the increase of Fe content, the density and hardness of cermets increase gradually, indicating that the mechanical properties are improved. Because the Fe in the adhesive can react with the abrasive SiC to form the reaction bonding interface, the Cu–80Fe–SiC cermets with higher Fe content have better adherence. The grinding test results of Cu–80Fe–SiC cermet show that the friction coefficient is .341, the surface roughness is 6.64 μm, the residual stresses parallel to the grinding direction are 353.3 MPa, and the residual stresses perpendicular to the grinding direction are 140.9 MPa. With the increase of Fe content, the wear mechanism changes from ploughing and cutting to friction.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m⁻¹ K⁻¹. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB₂–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

Replication Route Synthesis of Mesoporous Titanium–Cobalt Oxides and Their Photocatalytic Activity in the Degradation of Methyl Orange

Mesoporous Ti–Co oxides were synthesized via a replication route, using a 3-D wormlike mesoporous silica as template and tetra-tert-butyl orthotitanate (TBOT) and Co(NO₃)₂ as source materials. The prepared materials were characterized by X-ray diffraction (XRD), N₂-physisorption, TEM, EDS, and UV/Vis-DRS and found to possess a spherical morphology and a 3-D wormhole-like mesoporous structure, with the average pore size between 4.5 and 16.0 nm. The pore walls consisted mainly of a cobalt-incorporated anatase phase. The Co³⁺ ions were generated in the replicated mesoporous Co–Ti oxides, via the transfer of electrons from Co²⁺ to Ti⁴⁺ ions. The formation of cobalt-incorporated anatase phase and Co³⁺ ions were both favored by larger Co/Ti atomic ratios and by relatively low calcination temperatures. The specific surface area decreased and the mesopore sizes increased, with increasing Co/Ti atomic ratio or calcination temperature. The average crystal size of the anatase phase decreased with increasing Co/Ti atomic ratio but increased with increasing calcination temperature. The photocatalytic activity of the replicated mesoporous Co–Ti oxides in the degradation of methyl orange dye was investigated. It was observed that the photocatalytic activity increased with increasing Co/Ti atomic ratio and exhibited a maximum with increasing calcination temperature. With the exception of those prepared at too high calcination temperatures, the replicated mesoporous Co–Ti oxides were much more active than the pure titania. It is concluded that, in addition to a higher diffusion, the cobalt-containing anatase, as the active phase, and the Co³⁺ ions, as the active sites, are responsible for the high photocatalytic activity of the replicated mesoporous Co–Ti oxide.