Graphene-enhanced microwave absorption properties of Fe3O4/SiO2 nanorods

Fe₃O₄/SiO₂/graphene composite composed of Fe₃O₄/SiO₂ core–shell nanorods and graphene nanosheets were synthesized by a facile wet chemical method. Structure and morphology studies reveal that the Fe₃O₄/SiO₂ nanorods with porous structure and large aspect ratio are densely wrapped by the graphene nanosheets. By changing the graphene content, the electromagnetic properties of the Fe₃O₄/SiO₂/graphene composite can be well tuned. When the weight ratio of Fe₃O₄/SiO₂ to graphene reaches an appropriate value, excellent microwave absorption performance is achieved due to the large electromagnetic losses and good impedance matching. The Fe₃O₄/SiO₂/graphene composite with graphene content of 5 wt.% shows the minimum reflection loss of −27.1 dB at 12.2 GHz when the coating layer thickness is only 1.5 mm.     

Mechanical reinforcement while remaining electrical insulation of glass fibre/polymer composites using core–shell CNT@SiO2 hybrids as fillers

Carbon nanotubes (CNTs) have been widely used as mechanical reinforcement agents of composites. However, their aggregations, weak interfacial interaction with polymer, as well as high electrical conductivity limit their use in some especial applications. In this paper, the silicon oxide (SiO₂)-coated (CNT@SiO₂) core–shell hybrids with different SiO₂ thickness were prepared and employed to reinforce glass fibre-reinforced bismaleimide–triazine (BT) resin (GFRBT) composites. The results indicated the mechanical properties, including tensile strength and Young’s modulus increased with the increase of SiO₂ thickness and CNT@SiO₂ loading. Such enhanced mechanical properties were mainly attributed to the intrinsically nature of CNTs, homogeneous dispersion of the hybrids, as well as improved interfacial interaction. Meanwhile, the composites remained high electrical insulation (9.63 × 10¹² Ω cm) due to the existence of SiO₂ layer on CNT surface. This study will guide the design of functionalized CNTs and the construction of high-performance composites.     

Effect of silica coating thickness on the thermal conductivity of polyurethane/SiO2 coated multiwalled carbon nanotube composites

Silica coated multiwalled carbon nanotubes (SiO₂@MWCNTs) with different coating thicknesses of ∼4 nm, 30–50 nm, and 70–90 nm were synthesized by a sol–gel method and compounded with polyurethane (PU). The effects of SiO₂@MWCNTs on the electrical properties and thermal conductivity of the resulting PU/SiO₂@MWCNT composites were investigated. The SiO₂ coating maintained the high electrical resistivity of pure PU. Meanwhile, incorporating 0.5, 0.75 and 1.0 wt% SiO₂@MWCNT (70–90 nm) into PU, produced thermal conductivity values of 0.287, 0.289 and 0.310 W/mK, respectively, representing increases of 62.1%, 63.3% and 75.1%. The thermal conductivity of PU/SiO₂@MWCNT composites was also increased by increasing the thickness of the SiO₂ coating.     

Enhanced photocatalytic property of nanoporous TiO2/SiO2 micro-particles prepared by aerosol assisted co-assembly of nanoparticles

Nanoporous TiO₂/SiO₂ composite micro-particles were prepared by an aerosol assisted co-assembly (AACA) and their characteristics were investigated for photocatalytic application. The average diameter of resulting co-assembled TiO₂/SiO₂ particles was ranged 4–10 μm, and increased as the precursor concentration increased. The TiO₂/SiO₂ particles were spherical in shape and pores ranged 1–100 nm in diameter. Photocatalytic activity of the as-prepared nanoporous TiO₂/SiO₂ particles was evaluated by measuring the photodegradation of methylene blue (MB) and NO_x. Furthermore, the photocatalytic activity of nanoporous TiO₂/SiO₂ particles was compared with those of commercial TiO₂ nanoparticles and nanoporous TiO₂ particles. The nanoporous TiO₂/SiO₂ particles exhibited the highest photodegradation of MB and NO_x among three samples, which was 80% after 3 h and 55% at 10 min, respectively.     

Effect of SiO2 nanoparticles on the phase transformation of TiO2 in micron-sized porous TiO2–SiO2 mixed particles

Spherical and nanoporous TiO₂ and TiO₂–SiO₂ mixed micro-particles with four different compositions (20/80, 50/50, 80/20, 90/10 in weight ratio of TiO₂/SiO₂) were prepared by spray drying method from colloidal mixtures of amorphous silica and anatase titania nanoparticles. The as-prepared particles were heat-treated at 900 °C for 0.5–5 h. The TiO₂ and TiO₂–SiO₂ particles were spherical in shape and the average particle diameter was about 1 μm. The anatase mass fraction and the specific surface area of TiO₂–SiO₂ (50 wt.% SiO₂) mixed particles were kept to 61.5% and 30.6%, respectively, of their initial values after 5 h heat-treatment whereas these values of TiO₂ particles were rapidly decreased to 13.0% and 1.2% of their initial values, respectively, within 30 min after heat-treatment. And the anatase mass fraction and specific surface area increased as SiO₂ content in the TiO₂–SiO₂ mixed particles increased.     

Tunable BT@SiO2 core@shell filler reinforced polymer composite with high breakdown strength and release energy density

Barium titanate@silicon dioxide (BT@SiO₂) core@shell fillers with an average diameter of 100 nm were prepared by a facile sol–gel synthesis. The thickness of SiO₂ shell can be easily tuned by varying different mass ratio of BT to tetraethyl orthosilicate (TEOS). Polyvinylidene fluoride (PVDF) based composite films reinforced by BT and BT@SiO₂ were fabricated via a solution casting method. The effects of SiO₂ shell on morphology structure, wettability, interfacial adhesion, dielectric, electrical and energy performances of composites were investigated. Compared with BT/PVDF, BT@SiO₂/PVDF composites show significantly increased breakdown strength due to enhanced interfacial adhesion and suppressed charge carrier conduction. Benefiting from enhanced breakdown strength and reduced remnant polarization induced by SiO₂ shell, BT@SiO₂/PVDF shows increased release energy density (energy density which can be fully discharged and applicable). Especially, BT@SiO₂/PVDF with SiO₂ thickness of 4 nm exhibits the highest release energy density of 1.08 J/cm³ under applied electric field of 145 kV/mm.     

Electrospinning of PAN/DMF/H2O containing TiO2 and photocatalytic activity of their webs

Polyacrylonitrile (PAN) nanofiber webs containing titanium dioxide (TiO₂) were prepared via electrospinning. Either dimethyl formamide (DMF) or its mixture with small amount of water (3 and 5%w/w.) was employed to prepare 5%w/w.PAN/DMF or PAN/DMF/H₂O solution, respectively. Introducing non-solvent water in PAN/DMF/H₂O solution was attempted to induce phase separation, which may lead to formation of porous structure on nanofibers surface. Different amounts of TiO₂ (1 to 3 wt.%) were added into PAN/DMF/(H₂O) solutions and then electrospun into nanofiber webs. From SEM, nanofibers possessed rough surfaces and had averaged diameters in ranges of 170–430 nm., showing tendency to increase with amount of TiO₂ and water. Porous structure on fiber surfaces was not clearly observed, which was suspected to be due to insufficient amount of water employed. Less homogeneity in polymer solution due to presence of TiO₂ disfavored increasing water content higher than 5%w/w. EDS data confirmed presence of TiO₂ in electrospun webs. From photocatalysis evaluation, webs containing 2 and 3 wt.% TiO₂ showed good photocatalytic activity such that 80 percent of 10 ppm. methylene blue degraded in 24 hours. Slight increase in photocatalytic activity was observed in webs obtained from PAN/DMF/H₂O solutions.     

Brazing of SiO2f/SiO2 composite modified with few-layer graphene and Invar using AgCuTi alloy

Due to the poor wettability of the AgCuTi alloy on the SiO_2f/SiO₂ composite, direct brazing of the composite with an Invar alloy could hardly achieve a reliable joint. To overcome that, the SiO_2f/SiO₂ composite was decorated with few-layer graphene (FLG) by a plasma enhanced chemical vapor deposition (PECVD) method. Sessile drop experiments indicate that the contact angle dropped from 123.8° to 50.7° after FLG was grown on the surface of the SiO_2f/SiO₂ composite. Afterwards, the effects of brazing temperature and Ti contents on the microstructure evolution and mechanical properties of joints (Invar/SiO_2f–SiO₂ modified with FLG) were investigated. The typical interface structure of the joint is SiO_2f–SiO₂/Ti₅Si₃ + TiO₂ + Cu_xTi_6 − xO(x = 2,3)/Ag(s,s) + Cu(s,s) + Cu–Ti blocks/wave-like Fe₂Ti + Ni₃Ti/Ag(s,s) + Cu(s,s) + Fe₂Ti + Ni₃Ti blocks/Invar. As the brazing temperature and Ti contents increase, the reaction layer on the SiO_2f/SiO₂ side becomes thicker and cracks gradually propagate. Meanwhile, a few dispersive Fe₂Ti + Ni₃Ti phases change into large-area wave-like compounds and more Cu–Ti compounds form with the increase of the Ti content. The microstructure evolution significantly affects the shear strength of the brazed joints. The highest shear strength is 26 MPa brazed at 860 °C for 10 min with 4.5 wt.% Ti content.     

Synthesis of polyaniline-modified Fe3O4/SiO2/TiO2 composite microspheres and their photocatalytic application

Polyaniline-modified Fe₃O₄/SiO₂/TiO₂ composite microspheres have been successfully synthesized by sol–gel reactions on Fe₃O₄ microspheres followed by the chemical oxidative polymerization of aniline. The synthesized multilayer-structured composites were characterized by TEM, XRD, TGA, UV–vis diffuse reflectance spectra and magnetometer. The photocatalytic activity was evaluated by the photodegradation of methylene blue under visible light. The effect of polyaniline (PANI) amounts on the photocatalytic activity was investigated. The photocatalytic activity results show that the Fe₃O₄/SiO₂/TiO₂ composites with about 2.4 wt.%–4.1 wt.% PANI could show higher photocatalytic efficiency than that of Fe₃O₄/SiO₂/TiO₂. Furthermore, the PANI-Fe₃O₄/SiO₂/TiO₂ photocatalyst could be easily recovered using a magnet.     

Preparation and properties of sliver and nitrogen co-doped TiO2 photocatalyst

TiO₂ photocatalysts co-doped with different content of Ag and N were prepared by sol–gel method combined with microwave chemical method. The samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscope (TEM), ultraviolet–visible diffuse reflectance spectrum (UV–vis) and photo-luminescence emission spectrum (PL). The photocatalytic activity was investigated by photocatalytic degradation of methylene blue (MB) under irradiation of fluorescent lamp. The results indicate that Ag and N co-doping can restrain the increase of grain size, broaden the absorption spectrum to visible light region, and inhibit the recombination of the photo-generated electron–hole pairs. Moreover, the photocatalytic activity of Ag–N–TiO₂ in MB degradation is remarkable improved. The degradation rate of the sample with Ag:TiO₂ = 0.05 at%, N:TiO₂ = 18.50 wt% in 5 h is 93.44%, which is much higher than that of Degussa P25 (39.40%).     

The photoluminescence and optical constant of ZnSe/SiO2 thin films prepared by sol–gel process

In this paper, ZnSe/SiO₂ thin films were prepared by sol–gel process. X-ray diffraction results indicate that the phase structure of ZnSe particles embedded in SiO₂ thin films is sphalerite (cubic ZnS). The dependence of ellipsometric angle ψ on wavelength λ of ZnSe/SiO₂ thin films was investigated by spectroscopic ellipsometers. The optical constant, thickness, porosity and the concentration of ZnSe/SiO₂ composite thin films were fitted according to Maxwell–Garnett effective medium theory. The thickness of ZnSe/SiO₂ thin films was also measured by surface profile. The photoluminescence properties of ZnSe/SiO₂ thin films were investigated by fluorescence spectrometer. The photoluminescence results reveal that the emission peak at 487 nm (2.5 eV) excited by 395 nm corresponds to the band-to-band emission of sphalerite ZnSe crystal (2.58 eV). Strong free exciton emission and other emission peaks corresponding to ZnSe lattice defects are also observed.     

Effect of Na2SiO3 on synthesis of TiO2 nanopowders by thermal processing of the precursor

TiO₂ nanopowders could be prepared by thermal processing of the precursor of titanium hydroxide, urea and Na₂SiO₃. The powders were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The results show that Na₂SiO₃ has an important effect on the average crystallite size and dispersity of TiO₂ nanopowders, and other phases (Na₂SO₄ and SiO₂) will be introduced. However, Na₂SO₄ has distinctive intercalation ability and catalytic activity; SiO₂ coating layers can effectively inhibit the agglomeration of TiO₂ nanopowders. TiO₂ (anatase) powders with well dispersity can be prepared at ～600 °C for 2 h with addition of 2–6 wt.% Na₂SiO₃, and the average crystallite size is 15.5–22.1 nm. The surface of the sample mainly consists of Ti, O, C, Si, Na and S six species elements.     

Core/shell structured sSiO2/mSiO2 composite particles: The effect of the core size on oxide chemical mechanical polishing

In a typical chemical mechanical polishing (CMP) process, the type, morphology, structure, mechanical, and surface characteristics of abrasive particles play an important role in influencing the material removal process. The novel abrasive particles with special mechanical and/or tribochemical properties have been introduced into CMP processes for the improvement of surface quality and finishing efficiency. In this work, the composite particles containing solid silica (sSiO₂) cores and mesoporous silica (mSiO₂) shells were prepared via a developed Stöber method using cetyltrimethylammonium bromide as a structure-templating surfactant. The as-synthesized core/shell structured sSiO₂/mSiO₂ composite particles were characterized by powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and nitrogen sorption–desorption measurements. The effect of the sSiO₂ core size of the composite particles on oxide CMP performance was evaluated in terms of surface roughness and material removal rate (MRR). The root-mean-square surface roughness (0.15–0.31 nm) of the polished substrates slightly increased with increasing of the sSiO₂ core size (168–353 nm) of the composites with a comparable mSiO₂ shell thickness (16–18 nm). The sSiO₂/mSiO₂ composite particles with a relatively smaller or larger core presented a relatively high MRR for silicon oxide films. These oxide CMP results could be rationalized according to the contact area mechanism and indentation-based mechanism, incorporating the total contact area and chemical reactivity between particles and wafers, and the indentation depth of an abrasive particle onto the substrate surface.     

Fabrication of TiO2 hollow fibers with surface nanostructure

Polyvinyl alcohol-TiO₂ (PVA-TiO₂) core sheath nanofibers were fabricated by electrospinning an aqueous solution of PVA and introducing the thread-like droplets directly into a titanium tetraisopropoxide (TTIP)/hexane solution. Rod-like and sheet-like structures of lepidocrocite-type layered titanate formed on the surface of the TiO₂ sheath of the nanofibers by alkaline treatment in 1 mol L⁻¹ aqueous NaOH solution at 363 K. The nanofibers were converted to hollow TiO₂ nanofibers with surface nanostructure and anatase crystallinity by acid treatment to remove sodium ions and heat treatment at 773 K. The surface nanostructures enhanced the crystallinity and external surface area of the nanofiber and contributed to the improvement of photocatalytic oxidation activity.     

Sol–gel auto-combustion synthesis of PVP/CoFe2O4 nanocomposite and its magnetic characterization

Poly(vinyl pyrrolidone)/CoFe₂O₄ nanocomposite has been fabricated by a sol–gel auto-combustion method. Poly(vinyl pyrrolidone) was used as a reducing agent as well as a surface capping agent to prevent particle aggregation and stabilize the particles. The average crystallite size estimated from X-ray line profile fitting was found to be 20 ± 7 nm. The high field irreversibility and unsaturated magnetization behaviours indicate the presence of the core–shell structure in the sample. The exchange bias effect observed at 10 K suggests the existence of the magnetically aligned core surrounded by spin-disordered surface layer. The reduced remanent magnetization value of 0.6 at 10 K (higher than the theoretical value of 0.5) shows the PVP/CoFe₂O₄ nanocomposite to have cubic magnetocrystalline anisotropy according to the Stoner–Wohlfarth model.     

Preparation and characterization of V/TiO2 nanocatalyst with magnetic nucleus of iron

A magnetic composite containing V/TiO₂ was prepared by combination of sol–gel and wetness impregnation methods. The effects of synthesis temperature, different weight percents of Fe supported on TiO₂, vanadium loading and the heating rate of calcination on the structure and morphology of nanocatalyst were investigated. The optimum conditions for synthesized catalyst were 40 wt.% of Fe, 15 wt.% of V and synthesis temperature equal to 30 °C. Characterization of catalyst is carried out using XRD, TGA, DSC, SEM, FTIR and N₂ physisorption measurements. The magnetic character of nanocatalyst was measured using VSM, which showed the typical paramagnetic behavior of sample at room temperature with a saturation magnetization value equal to 8.283 emu/g. The nanocatalyst has a particle size about 56 nm and can easily be separated from medium by a magnet.     

Structure and microwave dielectric characteristics of lithium-excess Ca0.6Nd0.8/3TiO3/(Li0.5Nd0.5)TiO3 ceramics

Compositions based on (1−x)Ca_0.6Nd_8/3TiO₃−x(Li_1/2Nd_1/2)TiO₃ + yLi (CNLNTx + yLi, x = 0.30–0.60, y = 0–0.05), suitable for microwave applications have been developed by systematically adding excess lithium in order to tune the microwave dielectric properties and lower sintering temperature. Addition of 0.03 excess-Li simultaneously reduced the sintering temperature and improved the relative density of sintered CNLNTx ceramics. The excess Li addition can compensate the evaporation of Li during sintering process and decrease the secondary phase content. The CNLNTx (x = 0.45) ceramics with 0.03 Li excess sintered at 1190 °C have single phase orthorhombic perovskite structure, together with the optimum combination of microwave dielectric properties of ɛ_r = 129, Q × f = 3600 GHz, τ_f = 38 ppm/°C. Obviously, excess-Li addition can efficiently decrease the sintering temperature and improve the microwave dielectric properties. The high permittivity and relatively low sintering temperatures of lithium-excess Ca_0.6Nd_0.8/3TiO₃/(Li_0.5Nd_0.5)TiO₃ ceramics are ideal for the development of low cost ultra-small dielectric loaded antenna.     

Band gap tuning and nonlinear optical characterization of TiO2–SiO2 nanocomposites with respect to composition and phase

We report on the linear and nonlinear optical studies on TiO₂–SiO₂ nanocomposites with varying percentage ratio. It is found that optical band gap of the material varies with respect to the amount of the SiO₂ in the composite. Nonlinear optical characterization of these samples was studied by using open as well as closed aperture Z-scan technique using an Nd:YAG laser (532 nm, 7 ns, 10 Hz). The nanocomposites showed enhanced nonlinear optical properties than pure TiO₂ and this can be attributed to the surface states and weak dielectric confinement of TiO₂ nanoparticles by SiO₂ matrix. The nanocomposites were thermally treated and similar studies were performed. The anatase form of TiO₂ in the nanocomposites showed superior properties relative to the amorphous and rutile phase of the composite. The involved mechanism is explained by taking into account the dominant role played by the excitons in the TiO₂ nanoparticles.     

Grafting of nano-TiO2 onto flax fibers and the enhancement of the mechanical properties of the flax fiber and flax fiber/epoxy composite

In this article, a flax fiber yarn was grafted with nanometer sized TiO₂, and the effects on the tensile and bonding properties of the single fibers and unidirectional fiber reinforced epoxy plates were studied. The flax fiber yarn was grafted with nanometer sized TiO₂ through immersion in nano-TiO₂/KH560 suspensions under sonification. The measured grafting content of the nano-TiO₂ ranged from 0.89 wt.% to 7.14 wt.%, dependent on the suspension concentration. With the optimized nano-TiO₂ grafting content (∼2.34 wt.%), the tensile strength of the flax fibers and the interfacial shear strength to an epoxy resin were enhanced by 23.1% and 40.5%, respectively. The formation of Si–O–Ti and C–O–Si bonds and the presence of the nano-TiO₂ particles on the fiber surfaces contributed to the property enhancements. Unidirectional flax fiber reinforced epoxy composite (V_f = 35.4%) plates prepared manually showed significantly enhanced flexural properties with the grafting of nano-TiO₂.     

Photocatalytic degradation of gaseous toluene over TiO2–SiO2 composite nanotubes synthesized by sol–gel with template technique

TiO₂–SiO₂ composite nanotubes were successfully synthesized by a facile sol–gel technique utilizing ZnO nanowires as template. The nanotubes were well characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, N₂ adsorption–desorption analysis and UV–vis diffuse reflectance spectroscopy. The nanotubular TiO₂–SiO₂ composite photocatalysts showed diameter of 300–325 nm, fine mesoporous structure and high specific surface area. The results indicated that the degradation efficiency of gaseous toluene could get 65% after 4 h reaction using the TiO₂–SiO₂ composite as the photocatalyst under UV light illumination, which was higher than that of P25.