Xiang-Fa Wu

Elasticity of planar fiber networks

A micromechanics model is proposed for the elasticity of planar fiber networks (FNs). The FN is created by random deposition of linearly elastic straight rods within a region. The rods are bonded rigidly at contacts. Under external in-plane loading, the FN deformation consists of fiber bending, elongation, and contraction. An effective constitutive relation for fiber network is developed by averaging the strain energy dissipated by all possible fiber deformations in all directions. Numerical calculations are performed to analyze the effects of fiber aspect ratio and fiber concentration on the effective stiffness of the planar random FN. Finite element analysis (FEA) is performed and compared with the theoretical predictions of the effective FN moduli at several fiber concentrations. FEA results are in good agreement with theoretical predictions. The present model can be used for the prediction of mechanical properties, scaling analysis, and optimization of fiber assemblies.

A moving mode-III crack at the interface between two dissimilar piezoelectric materials

Abstract A moving mode-III crack at the interface between two dissimilar piezoelectric materials is considered. The integral representation of a general solution is given in terms of Fourier consine integrals. Solving the system of dual integral equations derived from the mixed boundary value problem considered, the elastic and electric fields are obtained for a moving impermeable crack and for a moving crack where the electric displacement is continuous across the crack surfaces, respectively. The results obtained show that the singularity of all the field variables depends on the remote mechanical loading as well as electric loading for the former, while it depends only on the remote mechanical loading for the latter. Unlike a stationary impermeable crack, the remote electric loading or electric field can disturb the stress field for a moving impermeable crack.

Preparation and properties of electrospun soy protein isolate/polyethylene oxide nanofiber membranes.

Soy protein isolate (SPI) and polyethylene oxide (PEO) were dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and nonwoven nanofiber membranes were prepared from the solution by electrospinning. PEO functioned as a cospinning polymer in the process to improve the spinnability of SPI. The ratio of SPI to PEO was varied and the rest spinning conditions remained unchanged. The morphology of the nanofiber membranes, SPI and PEO distribution and phase structure in the fiber, crystallization and interaction between SPI and PEO, thermal properties and wettability of the membranes were studied. The results showed that the diameter of most of the nanofibers was in the range of 200-300 nm. SPI and PEO showed high compatibility in the fiber and SPI was homogeneously dispersed at nanoscale. Crystallization of SPI and PEO in the fiber was significantly different from that of their pure forms. All the nanofiber membranes showed superhydrophilicity. These nanofiber membranes can find importance in filtration and biomedical applications.

Nickel incorporated carbon nanotube/nanofiber composites as counter electrodes for dye-sensitized solar cells

A nickel incorporated carbon nanotube/nanofiber composite (Ni-CNT-CNF) was used as a low cost alternative to Pt as counter electrode (CE) for dye-sensitized solar cells (DSCs). Measurements based on energy dispersive X-rays spectroscopy (EDX) showed that the majority of the composite CE was carbon at 88.49 wt%, while the amount of Ni nanoparticles was about 11.51 wt%. Measurements based on electrochemical impedance spectroscopy (EIS) showed that the charge transfer resistance (R(ct)) of the Ni-CNT-CNF composite electrode was 0.71 Ω cm(2), much lower than that of the Pt electrode (1.81 Ω cm(2)). Such a low value of R(ct) indicated that the Ni-CNT-CNF composite carried a higher catalytic activity than the traditional Pt CE. By mixing with CNTs and Ni nanoparticles, series resistance (R(s)) of the Ni-CNT-CNF electrode was measured as 5.96 Ω cm(2), which was close to the R(s) of 5.77 Ω cm(2) of the Pt electrode, despite the significant difference in their thicknesses: ∼22 μm for Ni-CNT-CNF composite, while ∼40 nm for Pt film. This indicated that use of a thick layer (tens of microns) of Ni-CNT-CNF counter electrode does not add a significant amount of resistance to the total series resistance (R(s-tot)) in DSCs. The DSCs based on the Ni-CNT-CNF composite CEs yielded an efficiency of 7.96% with a short circuit current density (J(sc)) of 15.83 mA cm(-2), open circuit voltage (V(oc)) of 0.80 V, and fill factor (FF) of 0.63, which was comparable to the device based on Pt, that exhibited an efficiency of 8.32% with J(sc) of 15.01 mA cm(-2), V(oc) of 0.83, and FF of 0.67.

Mechanical characterization of single high-strength electrospun polyimide nanofibres

Ultimate tensile strength and axial tensile modulus of single high-strength electrospun polyimide [poly(p-phenylene biphenyltetracarboximide), BPDA/PPA] nanofibres have been characterized by introducing a novel micro tensile testing method. The polyimide nanofibres with diameters of around 300 nm were produced by annealing their precursor (polyamic acid) nanofibres that were fabricated by the electrospinning technique. Experimental results of the micro tension tests show that polyimide nanofibres had an average ultimate tensile strength of 1.7 ± 0.12 GPa, axial tensile modulus of 76 ± 12 GPa and ultimate strain of ∼3%. The ultimate tensile strength and axial tensile modulus of the electrospun polyimide nanofibres in this study are among the highest ones reported in the literature to date. The precursor nanofibres with similar diameters and molecular weights had an average ultimate tensile strength of 766 ± 41 MPa, axial tensile modulus of 13 ± 0.4 GPa and ultimate strain of ∼43%. The experimental stress–strain curves obtained in this study indicate that under axial tension, the precursor (polyamic acid) nanofibres behave as linearly strain-hardening ductile material without obvious softening at final failure, while the polyimide nanofibres behave simply as brittle material with very high tensile strength and axial tensile modulus. Furthermore, by using a transmission electron microscope, detailed fractographical analysis was performed to examine the tensile failure mechanisms of the polyimide nanofibres, which include chain scission, pull-out, chain bundle breakage, etc. X-ray diffraction analysis of the highly aligned polyimide nanofibres shows the high chain alignment along the nanofibre axis that was formed in the electrospinning process and responsible for the high tensile strength and axial tensile stiffness. (Some figures in this article are in colour only in the electronic version)

Growth of carbon nanostructures on carbonized electrospun nanofibers with palladium nanoparticles.

This paper studies the mechanism of the formation of carbon nanostructures on carbon nanofibers with Pd nanoparticles by using different carbon sources. The carbon nanofibers with Pd nanoparticles were produced by carbonizing electrospun polyacrylonitrile (PAN) nanofibers including Pd(Ac)(2). Such PAN-based carbon nanofibers were then used as substrates to grow hierarchical carbon nanostructures. Toluene, pyridine and chlorobenzine were employed as carbon sources for the carbon nanostructures. With the Pd nanoparticles embedded in the carbonized PAN nanofibers acting as catalysts, molecules of toluene, pyridine or chlorobenzine were decomposed into carbon species which were dissolved into the Pd nanoparticles and consequently grew into straight carbon nanotubes, Y-shaped carbon nanotubes or carbon nano-ribbons on the carbon nanofiber substrates. X-ray diffraction analysis and transmission electron microscopy (TEM) were utilized to capture the mechanism of formation of Pd nanoparticles, regular carbon nanotubes, Y-shaped carbon nanotubes and carbon nano-ribbons. It was observed that the Y-shaped carbon nanotubes and carbon nano-ribbons were formed on carbonized PAN nanofibers containing Pd-nanoparticle catalyst, and the carbon sources played a crucial role in the formation of different hierarchical carbon nanostructures.

Modeling of solvent evaporation from polymer jets in electrospinning

Solvent evaporation plays a critical role in nanofiber formation in electrospinning. Here, we present a nonlinear mass diffusion-transfer model describing the drying process in dilute polymer solution jets. The model is used to predict transient solvent concentration profiles in polyacrylonitrile/N,N-dimethylformamide (PAN/DMF) jets with the initial radii ranging from 50 μm down to 100 nm. Numerical simulations demonstrate high transient inhomogeneity of solvent concentration over the jet cross-section in microscopic jets. The degree of inhomogeneity decreases for finer, submicron jets. The simulated jet drying time decreases rapidly with the decreasing initial jet radius, from seconds for microjets to single milliseconds for nanojets. The results demonstrate the need for further improved coupled multiphysics models of electrospinning jets.

Size effect in polymer nanofibers under tension

Due to their high porosity, strength, and surface area to volume ratio, fiber networks and fibrous materials have found extensive applications in thermal and sound insulators, gas and fluid filters, chemical carriers, tissue templates, and various paper products. In these materials and products, individual fibers are the key constituents and their mechanical properties play a crucial role in the global response of fibrous materials subjected to external loadings. As a matter of fact, the mechanical properties of fibrous materials can be remarkably enhanced through improving the properties of the individual fibers, the interface properties of fiber-fiber contacts, and the fiber lay-ups within the fibrous materials. Research has indicated that the mechanical properties of individual fibers e.g., ultimate tensile strength, fatigue and damage tolerance, etc. can be improved with decreasing fiber diameters under proper spinning conditions. Furthermore, recent ultrathin fibers produced by the electrospinning technique 1‐4 further extend the applications of classic fibrous materials. So far, continuous ultrathin fibers with diameters ranging from a few microns down to a few nanometers were fabricated efficiently by means of the electrospinning technique. With their ultrahigh surface area to volume ratio and tensile strength, continuous nanofibers and their derivatives are expected to form fiber networks with high porosity and tensile strength for the use of electromagnetic shielding, chemical catalyst carriers, tissue templates, artificial skins, nanofiber composites, etc. 5‐8

Electrospun carbon nanofibers surface-grown with carbon nanotubes and polyaniline for use as high-performance electrode materials of supercapacitors

This paper reports the synthesis and electrochemical performance of carbon nanofibers (CNFs) surface-grown with carbon nanotubes (CNTs) and nanostructured polyaniline (PANI) films, i.e., PANI/CNT/CNF, for use as a high-performance electrode material of pseudosupercapacitors. The PANI/CNT/CNF films were synthesized via in situ polymerization of aniline onto the surface of CNT-coated CNFs. The CNT-coated CNFs were prepared via electrospinning continuous polyacrylonitrile (PAN) nanofibers, followed by controlled carbonization and CNT growth. The morphology and microstructure of the PANI/CNT/CNF were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. The electrochemical properties of the novel nanofiber films were characterized by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge/discharge (GCD) in a 1 M aqueous H2SO4 solution as electrolyte. This unique porous nanofibrous structure exhibited low equivalent series resistance (ESR) and interfacial charge-transfer resistance (Rct) of 1.46 Ω and 0.55 Ω, respectively. Supercapacitors based on the present PANI/CNT/CNF electrodes behaved as with high specific capacitance of ∼503 F g−1 at a current density of 0.3 A g−1 and ∼471 F g−1 (only 6% decrease) at 3 A g−1. The maximum energy and power densities of ∼70 W h kg−1 and ∼15 kW kg−1 were achieved. In addition, over 92% of the initial capacitance was retained after 1000 charge/discharge cycles at a current density of 15 A g−1. The results of the present experimental study suggested that such a unique multifunctional nanofibrous material can be utilized for developing high-performance electrochemical energy storage devices such as pseudosupercapacitors, battery–supercapacitor hybrids, etc.

Electrospun carbon nanofibers surface-grafted with vapor-grown carbon nanotubes as hierarchical electrodes for supercapacitors

This letter reports the fabrication and electrochemical properties of electrospun carbon nanofibers surface-grafted with vapor-grown carbon nanotubes (CNTs) as hierarchical electrodes for supercapacitors. The specific capacitance of the fabricated electrodes was measured up to 185 F/g at the low discharge current density of 625 mA/g; a decrease of 38% was detected at the high discharge current density of 2.5 A/g. The morphology and microstructure of the electrodes were examined by electron microscopy, and the unique connectivity of the hybrid nanomaterials was responsible for the high specific capacitance and low intrinsic contact electric resistance of the hierarchical electrodes.