Pengwan Chen

Edge-to-Edge Assembled Graphene Oxide Aerogels with Outstanding Mechanical Performance and Superhigh Chemical Activity

Aerogels, an extremely important aggregation state of various self-assembled nanoscale building blocks, have great potential in fields ranging from energy storage to thermal insulation. However, the porosity of aerogels makes them mechanically weak in most cases, and the chemical activity of the resulting aerogel needs consideration. Herein, chemically crosslinked graphene oxide (GO) 3D aerogels with large specific surface areas (up to 850 m(2) g(-1) ), outstanding mechanical performance (up to 20 MPa Youngs modulus, 1 MPa yield strength and 45 J g(-1) specific energy adsorption), and superhigh chemical activity (toward some reducing gases such as H2 S, HI, and SO2 ), are fabricated by assembling 2D GO sheets edge-to-edge into uniform, 3D hydrogel networks with subsequent supercritical fluid drying. These aerogels are superior to other 3D frameworks (e.g. graphene aerogels) assembled via partial overlapping of the basal planes of the 2D building blocks.

Large-Scale Spinning Assembly of Neat, Morphology-Defined, Graphene-Based Hollow Fibers

Large-scale assembly of graphenes in a well-controlled macroscopic fashion is important for practical applications. We have developed a facile and straightforward approach for continuous fabrication of neat, morphology-defined, graphene-based hollow fibers (HFs) via a coaxial two-capillary spinning strategy. With a high throughput, HFs and necklace-like HFs of graphene oxide have been well-controlled produced with the ease of functionalization and conversion to graphene HFs via simply thermal or chemical reduction. This work paves the way toward the mass production of graphene-based HFs with desirable functionalities and morphologies for many of important applications in fluidics, catalysis, purification, separation, and sensing.

Characterization of the condensed carbon in detonation soot

A number of pure and composite explosives with a negative oxygen balance were detonated in a hermetic steel chamber under different environmental conditions. After detonation, solid carbonaceous products (detonation soot) and ultrafine diamond separated from the soot were collected for examination. Elemental analysis, high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier-IR, X-ray photoelectron spectroscopy and small angle X-ray scattering were used to characterize the structure, composition and surface properties of the condensed carbon in the soot. Crystallite size and microstrain of ultrafine diamond and the graphitization index of the graphite phase were calculated according to XRD patterns. The yields of the soot and ultrafine diamond from the explosives were obtained. The influences of charge conditions and environmental conditions on the formation mechanisms and properties of condensed carbon were analyzed. Detonation soot contains ultrafine diamond, graphite and amorphous carbon. Two types of graphite structures were present in the detonation soot.

Spherical nanometer-sized diamond obtained from detonation

Abstract Ultrafine diamond (UFD) was synthesized under high pressure and high temperatures generated by explosive detonation. The structure, composition, surface and thermal stability of UFD were studied by use of XRD, TEM, Raman Spectroscopy, FTIR, etc. The influences of the synthesis conditions and purification conditions on the properties of UFD were analyzed. The UFD had an average size of 4–6 nm, commonly exhibiting a spherical shape. The highest yield was of up to 10 mass% of the explosive. Attempts were made to use UFD as an additive to metal–diamond sintering and as crystallite seeds of CVD diamond films. The results show that UFD can decrease the coefficient of friction of the composite by 30%, and raise the nucleation density in CVD diamond films by 2–3 times.

Cross-Sectional Residual Stresses in Thermal Spray Coatings Measured by Moiré Interferometry and Nanoindentation Technique

A plasma-sprayed thermal barrier coating (TBC) was deposited on a stainless steel substrate. The residual stresses were firstly measured by moiré interferometry combined with a cutting relaxation method. The fringe patterns in the cross-section of the specimen clearly demonstrate the deformation caused by the residual stress in thermal spray coatings. However, restricted by the sensitivity of moiré interferometry, there are few fringes in the top coat, and large errors may exist in evaluating the residual stress in the top coat. Then, the nanoindentation technique was used to estimate the residual stresses across the coating thickness. The stress/depth profile shows that the process-induced stresses after thermal spray are compressive in the top coat and a tendency to a more compressive state toward the interface. In addition, the stress gradient in the substrate is nonlinear, and tensile and compressive stresses appear simultaneously for self-equilibrium in the cross-section.

Buckling modes of polymer membranes restricted by metal wires

Using field emission SEM, the thermal deformation of metal-wire/polymer-membrane/glass structures was observed. The thermal mismatch between the metal wire and the polymer membrane can trigger new instability modes of the polymer membrane. In the direction parallel to the axis of the wire, the orthogonal wave, oblique wave, or crumpled wave buckling mode with micro wavelength arises, while in the perpendicular direction, harmonic nano wrinkles appear on the polymer membrane. These two kinds of mutually orthogonal multi-scale instability modes can form a novel micro/nano buckling pattern. This paper discusses the formation mechanisms of these buckling patterns. The research results can provide insights into the optimization design and manufacturing process control of metal-wire/polymer-membrane structures.

Theoretical analysis of moiré fringe multiplication under a scanning electron microscope

In this study, theoretical analysis and experimental verification of fringe multiplication under a scanning electron microscope (SEM) are presented. Fringe multiplication can be realized by enhancing the magnification or the number of scanning lines under the SEM. A universal expression of the pitch of moire fringes is deduced. To apply this method to deformation measurement, the calculation formulas of strain and displacement are derived. Compared to natural moire, the displacement sensitivity is increased by fringe multiplication while the strain sensitivity may be retained or enhanced depending on the number of scanning lines used. The moire patterns are formed by the interference of a 2000 lines mm−1 grating with the scanning lines of SEM, and the measured parameters of moire fringes from experimental results agree well with theoretical analysis.

Quasi-static tensile deformation and fracture behavior of a highly particle-filled composite using digital image correlation method

Polymer bonded explosives (PBXs) are highly particle-filled composite materials. This paper experimentally studies the tensile deformation and fracture behavior of a PBX simulation by using the semi-circular bending (SCB) test. The deformation and fracture process of a pre-notched SCB sample with a random speckle pattern is recorded by a charge coupled device camera. The displacement and strain fields on the observed surface during the loading process are obtained by using the digital image correlation method. The crack opening displacement is calculated from the displacement fields, the initiation and propagation of the crack are analyzed. In addition, the damage evolution and fracture mechanisms of the SCB sample are analyzed according to the strain fields and the correlation coefficient fields at different loading steps.

Hydrothermal growth of VO2 nanoplate thermochromic films on glass with high visible transmittance

The preparation of thermochromic vanadium dioxide (VO2) films in an economical way is of interest to realizing the application of smart windows. Here, we reported a successful preparation of self-assembly VO2 nanoplate films on TiO2-buffered glass by a facile hydrothermal process. The VO2 films composed of triangle-shaped plates standing on substrates exhibit a self-generated porous structure, which favors the transmission of solar light. The porosity of films is easily controlled by changing the concentration of precursor solutions. Excellent thermochromic properties are observed with visible light transmittance as high as 70.3% and solar modulating efficiency up to 9.3% in a VO2 film with porosity of ~35.9%. This work demonstrates a promising technique to promote the commercial utilization of VO2 in smart windows.

A 2D mark shearing technique for characterizing the mechanical properties of material

The digital image correlation (DIC) method has been widely used in mechanical experiments due to their non-contact, full-field feature and high accuracy in displacement measurement. Yet the precision in strain determination is not so desirable. In this paper, a 2D mark shearing technique was proposed to improve the accuracy of the strain measurement in both vertical direction and horizontal direction. A four-prism system was designed and used to introduce a shearing distance of the marks, which were made on the specimen to calculate the two strain components. The measurement principle of the mark shearing technique is described in detail. A tensile experiment using an aluminum sample was conducted, and the Youngs modulus and Poisson ratio were measured. The successful experimental results demonstrated the feasibility of this method.