Fabrication of carbon-core/TiO2-sheath nanofibers by carbonization of poly(vinyl alcohol)/TiO2 composite nanofibers prepared via electrospinning and an interfacial sol–gel reaction

Carbon-core/TiO₂-sheath nanofibers have been fabricated from poly(vinyl alcohol) (PVA)/TiO₂ composite nanofibers that were prepared by electrospinning an aqueous solution of PVA and introducing the thread-like droplets directly into a titanium tetraisopropoxide (TTIP)/heptane solution. The PVA/TiO₂ composite nanofibers were transformed into carbon/TiO₂ nanofibers by iodine vapor treatment at 353 K, which induced dehydration of PVA, followed by carbonization in N₂ at 873 K. The carbon/TiO₂ nanofibers had diameters of a few hundred nanometers and sheathes of tens of nanometers. The nanofibers were composed of carbon cores with a low degree of graphitization and TiO₂ sheaths with an anatase crystal structure. The electrical conductivity of the carbon/TiO₂ composite nanofibers was enhanced by 4 orders of magnitude compared with TiO₂ hollow nanofibers.     

Isolation of nanofibers from soybean source and their reinforcing capability on synthetic polymers

The focus of this work is to study nanofibers in three different polymers: polyvinyl alcohol (PVA), polypropylene (PP) and polyethylene (PE). The nanofibers were isolated from a soybean source by combining chemical and mechanical treatments. Isolated nanofibers were shown to have diameter between 50 and 100 nm and the length in micrometer scale which results in very high aspect ratio. The mechanical properties demonstrated an increase in tensile strength from 21 MPa of PVA/UNF5 (untreated-fiber (5 wt%) reinforced PVA) and 65 MPa of pure PVA to 103 MPa of PVA/SBN5 (nanofiber (5 wt%) reinforced PVA). The increased stiffness of PVA/SBN5 nanocomposites was also very promising; it was 6.2 GPa compared to 2.3 GPa of pure PVA and 1.5 GPa of PVA/UNF5. In solid phase melt-mixing, nanofiber was directly incorporated into the polymer matrix using a Brabender. The nanofiber addition significantly changed the stress–strain behavior of the composites: modulus and stress were increased with coated nanofibers by ethylene–acrylic oligomer emulsion as a dispersant; however, elongation was reduced. The dynamic mechanical analysis showed the addition of the soybean nanofiber (SBN) improved the thermal properties for PVA and how the addition of different contents of SBN influenced the tan δ peak and storage modulus of PVA.     

Synthesis of mesoporous tungsten oxide nanofibers using the electrospinning method

Mesoporous tungsten oxide nanofibers were synthesized via a 500 °C thermal treatment of composite nanofibers prepared by electrospinning an ethanol solution consisting of tungsten ethoxide, P123 triblock copolymer, and polyvinylpyrrolidone. The as-electrospun composites exhibited unwoven nanofibers with an average diameter of 233 nm and a smooth surface morphology. During the calcination process, the composite nanofibers were shrunk to 85 nm in diameter and converted into rough, wormhole-like nanofibers. These were formed by agglomerating polycrystalline WO₃ particles of 10–30 nm along the axial direction. Furthermore, a measured pore-size distribution indicated that this nanofiber mat had different types of meso-sized porosities, which may have resulted from their wormhole-like structures and inter-fiber voids. In addition, it was observed to have the intra-grain porosity with the diameter of about 1.0 nm.     

Upconversion luminescence of Ba(MoO4)h(WO4)1−h:Yb3+/Er3+ nanocrystals synthesized through hydrothermal method

A series of Yb³⁺/Er³⁺ co-doped Ba(MoO₄)_h(WO₄)_1−h upconversion nanocrystals (UCNCs) were prepared via hydrothermal method. The effects of different concentration ratios of Yb³⁺/Er³⁺ and Mo₄O²⁻/WO₄²⁻ on the upconversion luminescence were investigated, and the optimum doping concentrations of Yb³⁺ and Er³⁺ in the Ba(MoO₄)_0.5(WO₄)_0.5 host were found to be 3 mol% and 1 mol%, respectively. Structure of Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺ was identified as the tetragonal in the X-ray diffraction (XRD) results and the particle size observed in the scanning electron microscope (SEM) was about 40 nm. Under excitation of 980 nm semiconductor laser, three emission bands centered at 528, 550 and 660 nm, originating from ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively, were observed for Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺. The pump power dependence research suggested that these bands arise due to two-photon absorption. The variation of CIE coordinate at different excitation powers was observed.     

Xylan polysaccharides fabricated into nanofibrous substrate for myocardial infarction

Myocardial infarction, a main cause of heart failure, leads to loss of cardiac tissue impairment of left ventricular function. Repair of diseased myocardium with in vitro engineered cardiac muscle patch/injectable biopolymers with cells may become a viable option for myocardial infarction. We attempted to solve these problems by in vitro study by selecting a plant based polysaccharides beech wood Xylan for the normal functioning of infarcted myocardium. The present study fabricated Xylan based nanofibrous scaffolds cross-linked with glutaraldehyde (Glu) vapors for 24 h, 48 h and 1% Glu blended fibers for the culture of neonatal rat cardiac cells for myocardial infarction. These nanofibers were characterized by SEM, FT-IR, tensile testing and cell culture studies for the normal expression of cardiac proteins. The observed results showed that the Xylan/polyvinyl alcohol (PVA) 24 h Glu vapor cross-linked nanofibers (427 nm) having mechanical strength of 2.43 MPa and Young modulus of 3.74 MPa are suitable for the culture of cardiac cells. Cardiac cells proliferation increased only by 11% in Xylan/PVA 24 h Glu cross-linked nanofibers compared to control tissue culture plate (TCP). The normal cardiac cell morphology was observed in 24 h cross-linked Xylan/PVA nanofibers but 48 h cross-linked fibers cell morphology was changed to flattened and elongated on the fibrous surfaces. Confocal analysis for cardiac expression proteins actinin, connexin 43 was observed normally in 24 h Glu cross-linked nanofibers compared to all other nanofibrous scaffolds. The fabricated Xylan/PVA nanofibrous scaffold may have good potential for the normal functioning of infarcted myocardium.     

Structural and microstructural behaviour of SnO2 dense ceramics doped with ZnO and WO3

SnO₂-based varistors doped with ZnO and WO₃ were prepared by mixed oxide method. Experimental evidence shows that the increase in ZnO amount increases the volume and microstrain of unit cell while the WO₃ promotes a decrease. The effect of ZnO and WO₃ additives could be explained by the substitution of Sn^4 +  by Zn^2 +  and W^6 + . The addition of WO₃ inhibits the grain growth due to the segregation in the grain boundary without influence in the densification of the samples. Besides that, an increase in the electrical resistance of the SnO₂–ZnO–WO₃ system was observed independent of the WO₃ concentration.     

Effects of WO3 addition on the structure and electrical properties of Pb3O4 modified PZT–PFW–PMN piezoelectric ceramics

The ceramics were prepared successfully by Pb₃O₄ and WO₃ additions to 0.90Pb(Zr,Ti)O₃–0.03Pb(Fe_2/3W_1/3)O₃–0.07Pb(Mn_1/3Nb_2/3)O₃ (0.90PZT–0.03PFW–0.07PMN). Effects of the additions on the structure, bulk density and electrical properties of ceramics were investigated. The results revealed that the proper additions of WO₃ with 2.0 wt.% Pb₃O₄ excess could form liquid phase that promoted the densification of the ceramics. The fracture mode changed from transgranular to intergranular as increasing WO₃ with 2.0 wt.% Pb₃O₄ excess. The piezoelectric and dielectric properties were also promoted by excess of Pb₃O₄ and WO₃ additions. The optimized electrical properties were obtained at excess of 2.0 wt.% Pb₃O₄ and 0.15 wt.% WO₃. The parameters were as follows: d₃₃ = 351 pC/N, K_p = 0.64, Q_m = 1882, ɛ_r = 1798, tan δ = 0.0052, P_r = 19.94 μC/cm² and E_c = 11.98 kV/cm, which shows high K_p, Q_m, d₃₃ and low tan δ can be obtained simultaneously by adding WO₃ addition to Pb₃O₄ modified PZT–PFW–PMN system.     

Crystal structure and electrical properties of CaxWO3 (0.01 ≤ x ≤ 0.15) prepared by hybrid microwave synthesis

Calcium tungsten bronzes Ca_xWO₃ (0.01 ≤ x ≤ 0.15) were synthesized by hybrid microwave method from mixtures of CaO, WO₃ and tungsten powder. Single-phased samples can be obtained by microwave heating within 40 min. With the increase of calcium content, the crystal structure of Ca_xWO₃ transforms from orthorhombic (0.01 ≤ x ≤ 0.02) to tetragonal (0.03 ≤ x ≤ 0.11) and then to cubic (0.12 ≤ x ≤ 0.15). The average size of crystallites is in the range 1–5 μm. All samples show semiconductor behaviour in their temperature dependence of resistivity. The electrical conduction mechanism changes from variable-range hopping to the thermally activated mechanism when x > 0.12.     

Characterisation of micro-sized and nano-sized tungsten oxide-epoxy composites for radiation shielding of diagnostic X-rays

Characteristics of X-ray transmissions were investigated for epoxy composites filled with 2–10 vol% WO₃ loadings using synchrotron X-ray absorption spectroscopy (XAS) at 10–40 keV. The results obtained were used to determine the equivalent X-ray energies for the operating X-ray tube voltages of mammography and radiology machines. The results confirmed the superior attenuation ability of nano-sized WO₃-epoxy composites in the energy range of 10–25 keV when compared to their micro-sized counterparts. However, at higher synchrotron radiation energies (i.e., 30–40 keV), the X-ray transmission characteristics were similar with no apparent size effect for both nano-sized and micro-sized WO₃-epoxy composites. The equivalent X-ray energies for the operating X-ray tube voltages of the mammography unit (25–49 kV) were in the range of 15–25 keV. Similarly, for a radiology unit operating at 40–60 kV, the equivalent energy range was 25–40 keV, and for operating voltages greater than 60 kV (i.e., 70–100 kV), the equivalent energy was in excess of 40 keV. The mechanical properties of epoxy composites increased initially with an increase in the filler loading but a further increase in the WO₃ loading resulted in deterioration of flexural strength, modulus and hardness.     

Effect of hydrothermal temperature on structure and photochromic properties of WO3 powder

Tungsten trioxide (WO₃) powders were prepared via a simple hydrothermal method. The morphology, structure and photochromic activity of the synthesized WO₃ powders were studied by X-ray diffraction, scanning electron microscopy and UV–vis spectrophotometer combined with color difference meter. The results showed the synthesized WO₃ powders with hexagonal phase got much better photochromic properties than the WO₃ powders with cubic phase, the ones not appear until about 160 °C. Besides, the WO₃ powder synthesized at 120 °C exhibited the best photochromic properties of the samples prepared below 160 °C, the particles of which formed a shape of clusters of cactus with uniform size and good dispersion.     

Investigation of the localized oxide ion interstitials of PbWO4-based oxide ion conductors by means of impedance spectroscopy

Impedance spectra were measured for two types of lanthanum-substituted PbWO₄ systems (Pb_1 − xLa_2x/3WO₄ and Pb_1 − xLa_xWO_4 + x/2), from which the relaxation phenomena associated with oxide ion conduction were investigated. Profiles of the imaginary part of the electric modulus normalized at the peak position were not essentially varied with temperature or composition for Pb_1 − xLa_2x/3WO₄ system, whereas broadening of the modulus peak was observed for Pb_1 − xLa_xWO_4 + x/2 system. We attributed this broadening to the localization of oxide ion interstitials since it appeared only for the latter system and the peak broadening grew larger with decreasing temperature.     

Preparation and thermomechanical properties of Ag-PVA nanocomposite films

Metal–polymer hybrid nanocomposites have been prepared from an aqueous solution of polyvinyl alcohol (PVA) and silver nitrate (AgNO₃). The silver nanoparticles were generated in PVA matrix by the reduction of silver ions with PVA molecule at 60–70 °C over magnetic stirrer. UV–vis analysis, X-ray diffraction studies, transmission electron microscopy, scanning electron microscopy and current–voltage analysis were used to characterize the nanocomposite films prepared. The X-ray diffraction analysis reveals that silver metal is present in face centered cubic (fcc) crystal structure. Average crystallite size of silver nanocrystal is 19 nm, which increases to 22 nm on annealing the film at 150 °C in air. This result is in good agreement with the result obtained from TEM. The UV–vis spectrum shows a single peak at 433 nm, arising from the surface plasmon absorption of silver nanocolloids. This result clearly indicates that silver nanoparticles are embedded in PVA. An improvement of mechanical properties (storage modulus) was also noticed due to a modification of PVA up to 0.5 wt% of silver content. The current–voltage (I–V) characteristic of nanocomposite films shows increase in current drawn with increasing Ag-content in the films.     

Biomimetic mineralization of calcium phosphate crystals in polyacrylamide hydrogel: Effect of concentrations of calcium and phosphate ions on crystalline phases and morphology

Calcium phosphate crystals were synthesized in polyacrylamide (PAAm) hydrogel, and the effects of the concentrations of calcium and phosphate ions on the crystalline phases and morphology were investigated. PAAm hydrogels containing diammonium hydrogen phosphate ((NH₄)₂HPO₄) were transformed into calcified materials by diffusion of calcium ions from calcium nitrate (Ca(NO₃)₂) aqueous solution into the gels. Several kinds of calcium phosphate crystals were precipitated at various Ca(NO₃)₂ concentrations (0.5–4.0 mol·dm^? 3), or (NH₄)₂HPO₄ contents (3.6–21.6 mmol) in the gels. The crystalline phases were mainly determined by the (NH₄)₂HPO₄ content in the gels. When the (NH₄)₂HPO₄ content was ≥ 10.8 mmol, hydroxyapatite (HAp) formed near the interfaces between Ca(NO₃)₂ solution and the gels, whereas octacalcium phosphate (OCP) formed in gels with ≤ 10.8 mmol (NH₄)₂HPO₄. HAp crystals were granular in form and about 200 nm in diameter, and OCP crystals were spherulitic with diameter 10–70 μm.     

Strong and optically transparent biocomposites reinforced with cellulose nanofibers isolated from peanut shell

Cellulose nanofibers–reinforced PVA biocomposites were prepared from peanut shell by chemical–mechanical treatments and impregnation method. The composite films were optically transparent and flexible, showed high mechanical and thermal properties. FE-SEM images showed that the isolated fibrous fragments had highly uniform diameters in the range of 15–50 nm and formed fine network structure, which is a guarantee of the transparency of biocomposites. Compared to that of pure PVA resin, the modulus and tensile strength of prepared nanocomposites increased from 0.6 GPa to 6.0 GPa and from 31 MPa to 125 MPa respectively with the fiber content as high as 80 wt%, while the light transmission of the composite only decreased 7% at a 600 nm wavelength. Furthermore, the composites exhibited excellent thermal properties with CTE as low as 19.1 ppm/K. These favorable properties indicated the high reinforcing efficiency of the cellulose nanofibers isolated from peanut shell in PVA composites.     

Spectroscopic and laser characterization of Yb,Tm:KLu(WO4)2 crystal

We report on a comprehensive spectroscopic and laser characterization of monoclinic Yb,Tm:KLu(WO₄)₂ crystals. Stimulated-emission cross-section spectra corresponding to the ³F₄ → ³H₆ transition of Tm³⁺ ions are determined. The radiative lifetime of the ³F₄ state of Tm³⁺ ions is 0.82 ms. The maximum Yb³⁺ → Tm³⁺ energy transfer efficiency is 83.9% for 5 at.% Yb – 8 at.% Tm doping. The fractional heat loading for Yb,Tm:KLu(WO₄)₂ is 0.45 ± 0.05. Using a hemispherical cavity and 5 at.% Yb – 6 at.% Tm doped crystal, a maximum CW power of 227 mW is achieved at 1.983–2.011 μm with a maximum slope efficiency η = 14%. In the microchip laser set-up, the highest slope efficiency is 20% for a 5 at.% Yb– 8 at.% Tm doped crystal with a maximum output power of 201 mW at 1.99–2.007 μm. Operation of Yb,Tm:KLu(WO₄)₂ as a vibronic laser emitting at 2.081–2.093 μm is also demonstrated.     

Sol–gel synthesis,characterization and microwave absorbing properties of nano sized spherical particles of La0.8Sr0.2Mn0.8Fe0.2O3

LSMFO (La_0.8Sr_0.2Mn_0.8Fe_0.2O₃) nano sized powders were synthesized by modified EDTA assisted sol–gel synthesis using EDTA:metal ion = 0.5. Another new synthesis method, sol–gel self combustion using PVA, was also devised for the synthesis of LSMFO and the effects of the PVA mole ratio was investigated. The characterization techniques, XRD, FE-SEM, TG/DTA and ICP, confirmed the formation of the pure phase LSMFO by both the methods. The material attained spherical morphology instead of previously reported rod like structure. Sizes of synthesized powders vary from 14.5 nm to 23 nm. TG/DTA results suggest that LSMFO can be synthesized by the self combustion at temperatures as low as 200 °C. Synthesized LSMFO has excellent microwave absorbing properties in the range of 0.05–18 GHz and appreciable effective bandwidth, 2 GHz, with microwave absorption in excess of 10 dB and peak reflection loss of 25 dB which suggests that the materials can be used as effective microwave absorbers.     

Structure and luminescence properties of Ba3WO6:Eu3+ nanowire phosphors obtained by conventional solid-state reaction method

For the first time, novel Ba_3−xWO₆:xEu³⁺ (x = 0.01, 0.03, 0.05, 0.08, 0.1) nanowire phosphors were synthesized by the conventional solid state method. The X-ray pattern indicates that Ba₃WO₆ belongs to the cubic system with space group Fm-3m. The photoluminescence (PL) spectra demonstrate that the phosphors emit strong red light centered at 595 nm corresponding to ⁵D₀ → ⁷F₁ transition of Eu³⁺ ion under CT band excitation. The position of charge transfer (CT) band of Ba_2.95WO₆:0.05Eu³⁺ shifts to a lower energy region (red shift) with the increase of annealing temperature. The co-doped effect of alkali-metal ions (Li⁺, Na⁺, and K⁺) on the luminescence behavior of Ba₃WO₆:Eu³⁺ has been discussed in this paper. The luminescence properties suggest that the Ba₃WO₆:Eu³⁺ phosphor may be a promising candidate in solid-state lighting applications.     

Crystal structure,oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.1M0.1O3−δ (M = Fe,Cr, Mn,Zr) oxygen-permeable membranes

Oxygen-permeable ceramic membrane materials of the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) and partially Fe-substituted Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr, Mn, Zr) were synthesized by solid-state reaction method. These materials possess purely cubic perovskite structure with the exception of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn, Zr), in which minor impurities exist. Oxygen permeability across these dense membrane disks were measured under an air/He oxygen partial pressure gradient in the temperature range of 973–1123 K. The results demonstrated that the oxygen permeation fluxes of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Mn, Zr) membranes increased in the following order: Fe (BSCFO) > Cr > Zr > Mn. The corresponding activation energies for oxygen permeation of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Fe, Cr, Zr) membranes were calculated to be similar (53 ± 4 kJ/mol), which was remarkably lower than that (99 ± 3 kJ/mol) of Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Mn) membrane. In addition, good oxygen permeation stability of the Ba_0.5Sr_0.5Co_0.8Fe_0.1M_0.1O_3−δ (M = Cr) membrane was achieved at the temperature lower than 1123 K. The X-ray diffraction (XRD) and differential thermal analysis (DTA) experiments showed that the structural stability of BSCFO could be significantly improved when Fe ions in the BSCFO material were partially substituted by Cr, Mn or Zr ions.     

Fabrication of indium sulfide nanofibers via a hydrothermal method assisted by AAO template

β-In₂S₃ nanofibers were successfully synthesized via a hydrothermal method with AAO membrane as a template at 150 °C for 15 h. XRD patterns indicated the perfect crystallization of β-In₂S₃. SEM images showed that the β-In₂S₃ nanofibers grew up from the channel ends of the AAO template. TEM images confirmed that the nanofibers had a high aspect ratio of ca. 40–50 and diameters of about 10 nm. The room temperature photoluminescence (PL) spectrum of the β-In₂S₃ nanofibers indicated its potential applications in light-emission devices.     

Single-crystal oxoborate (Pb3O)2(BO3)2WO4: Growth and characterization

An oxoborate, (Pb₃O)₂(BO₃)₂WO₄, has been prepared by solid-state reaction methods below 620 °C. Single-crystal XRD analysis shows that it crystallizes in the orthorhombic group Cmcm with a = 18.480(4) Å, b = 6.3567(13) Å, c = 11.672(2) Å, Z = 4. The crystal structure is composed of one-dimensional 1/∞ [Pb₃O]⁴⁺ chains formed by corner-sharing OPb₄ tetrahedra. BO₃ and WO₄ groups are located around the chains to hold them together via PbO bonds. The IR spectra further confirmed the presence of BO₃ groups. Furthermore we have performed theoretical calculations by employing the all-electron full potential linearized augmented plane wave (FP-LAPW) method to solve the Kohn Sham equations. Starting from our XRD data we have optimized the atomic positions by minimizing the forces. These are used to calculate the electronic band structure, the atomic site-decomposed density of states, electron charge density and the chemical bonding features. The calculated electronic band structure and densities of states suggest that this oxoborate possesses a wide energy band gap. The valence band maxima and the conduction band minima are located at Y point in the Brillouin zone resulting in a direct energy band gap of 2.3 eV using the local density approximation and 2.6 eV for the Engel–Vosko generalized gradient approximation. This compares well with our experimentally measured energy band gap of 2.9 eV. From our calculated electron charge density distribution, we obtain an image of the electron clouds that surround the molecules in the unit cell of the crystal. The chemical bonding features were analyzed and the substantial covalent interactions are observed between Pb and O, B and O and W and O atoms.