Jiecai Han

Cracks problem for non-homogeneous composite material subjected to dynamic loading

Abstract In this paper, we present a method to analyse the dynamic and steady response of non-homogeneous composite materials. Differing from the existing works reported in literature, the present method can be used for arbitrarily varying material properties through thickness direction and the crack number can be larger than one. It is assumed that the composite material is orthotropic and all the material properties depend only on the coordinates y (along the thickness direction). The material non-homogeneity is simulated by dividing the plate into a number of layers, each layer is assigned slightly different material properties. The method is based upon the Fourier and Laplace transforms to reduce the boundary value problem to a system of generalized singularity integral equations in the Laplace transform domain. The singular integral equations for the problem are derived and numerically solved by weighted residual value methods. By utilized numerical Laplace inversion the time-dependent full field solutions are obtained. As the numerical illustrates, three different cracked specimens, a functionally graded material, a metal-ceramic joint with functionally graded interlayer, and a metal substrate/functionally graded film structure are presented for various material non-homogeneity parameters and/or functionally graded layer thickness. The results obtained demonstrate that the present model is an efficient tool in the fracture analysis of composite materials with properties varying in the thickness direction.

Strong and stiff aramid nanofiber/carbon nanotube nanocomposites.

Small but strong carbon nanotubes (CNTs) are fillers of choice for composite reinforcement owing to their extraordinary modulus and strength. However, the mechanical properties of the nanocomposites are still much below those for mechanical parameters of individual nanotubes. The gap between the expectation and experimental results arises not only from imperfect dispersion and poor load transfer but also from the unavailability of strong polymers that can be effectively utilized within the composites of nanotubes. Aramid nanofibers (ANFs) with analogous morphological features to nanotubes represent a potential choice to complement nanotubes given their intrinsic high mechanical performance and the dispersible nature, which enables solvent-based processing methods. In this work, we showed that composite films made from ANFs and multiwalled CNTs (MWCNTs) by vacuum-assisted flocculation and vacuum-assisted layer-by-layer assembly exhibited high ultimate strength of up to 383 MPa and Youngs modulus (stiffness) of up to 35 GPa, which represent the highest values among all the reported random CNT nanocomposites. Detailed studies using different imaging and spectroscopic characterizations suggested that the multiple interfacial interactions between nanotubes and ANFs including hydrogen bonding and π-π stacking are likely the key parameters responsible for the observed mechanical improvement. Importantly, our studies further revealed the attractive thermomechanical characteristics of these nanocomposites with high thermal stability (up to 520 °C) and ultralow coefficients of thermal expansion (2-6 ppm·K(-1)). Our results indicated that ANFs are promising nanoscale building blocks for functional ultrastrong and stiff materials potentially extendable to nanocomposites based on other nanoscale fillers.

S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient Oxygen Reduction Reaction Electrocatalysts

Replacement of rare and precious metal catalysts with low-cost and earth-abundant ones is currently among the major goals of sustainable chemistry. Herein, we report the synthesis of S, N dual-doped graphene-like carbon nanosheets via a simple pyrolysis of a mixture of melamine and dibenzyl sulfide as efficient metal-free electrocatalysts for oxygen reduction reaction (ORR). The S, N dual-doped graphene-like carbon nanosheets show enhanced activity toward ORR as compared with mono-doped counterparts, and excellent durability in contrast to the conventional Pt/C electrocatalyst in both alkaline and acidic media. A high content of graphitic-N and pyridinic-N is necessary for ORR electrocatalysis in the graphene-like carbon nanosheets, but an appropriate amount of S atoms further contributes to the improvement of ORR activity. Superior ORR performance from the as-prepared S, N dual-doped graphene-like carbon nanosheets implies great promises in practical applications in energy devices.

Rapid prototyping and combustion synthesis of TiC/Ni functionally gradient materials

Abstract TiC–Ni functionally gradient materials (FGM) parts were fabricated by laminated object manufacturing (LOM), one of rapid prototyping (RP) techniques and combustion synthesis technique. The microstructure and phases of TiC–Ni FGM were analyzed with SEM and XRD. TiC–Ni FGM had anisotropic mechanical properties. It was stronger in the direction parallel to the thickness than in the direction perpendicular to the thickness. The maximum strength was 950 MPa in the TiC–20wt.%Ni region. The hardness of TiC–Ni FGM was larger than HRA 84 and the density was larger than 5.2 g cm −3 . When the content of Ni raised the density increased, then there was the largest relative density in the TiC–20wt.%Ni region.

Combustion synthesis and densification of titanium diboride–copper matrix composite

Abstract The quasi-static consolidation in uniaxial compression of combustion synthesized TiB 2 –Cu composite with 40 wt.% Cu from Ti, B and Cu powders was investigated. Thermodynamics of the system was calculated theoretically. It was found that TiB 2 was a stable phase in the composite and TiCu interphase compound can convert into stable phase. The phases of the synthesized product were identified using X-ray diffraction and the results showed only TiB 2 and Cu phases, no other phases existed in the product. It is consistent with the calculated result of thermodynamics. The consolidated product was dense and no apparent pores existed in the microstructure. Fine TiB 2 reinforcement grows in near equivalent axis-like and block-like shapes. As a binder phase, copper metal was found between TiB 2 and TiB 2 particles. It is shown that copper improves the densification behavior during combustion synthesis, compared with synthesized pure TiB 2 ceramic. Due to the addition of copper, the relative density, bend strength and fracture toughness of TiB 2 –Cu composite are much higher than those of TiB 2 ceramic. Crack-tip plastic blunting by a ductile metallic phase and crack deflection by the stress due to the difference of thermal expansion coefficient and elastic modulus between TiB 2 and Cu are the principal mechanisms of toughness improvement of TiB 2 –Cu composite.

Electroelastic fracture dynamics for multilayered piezoelectric materials under dynamic anti-plane shearing

This paper describes a method to analyze the dynamic response of a multilayered piezoelectric material plate containing some non-collinear cracks. It is assumed that the multilayers is composed of numerous laminae with cracks located at the interface of composite layers. Based upon Fourier transforms and Laplace transforms, the boundary value problem is reduced to a system of generalized singularity integral equations in the Laplace transform domain. By utilized numerical Laplace inversion, the time-dependent full field solutions are obtained in the time domain. Numerical results are plotted to illustrate how the loading state and material non-homogeneity influence the stress fields and the electric displacement fields ahead of the crack tip.

Magnetic properties of Mn-doped 6H-SiC

We report the synthesis and characterizations of low Mn-doped (<10−3 molar fraction) 6H-SiC. Raman scattering studies show an unusual shift in Raman peak with altering Mn contents. The magnetic properties measurement shows the typical ferromagnetic order was established at as low Mn-doped concentration as 10−4 molar fraction at around 250 K. It is speculated that the defects-related effects other than the Mn content play a more important role to determine the magnetic ordering.

Combustion synthesis and densification of large-scale TiC-xNi cermets

Abstract Large-scale TiC– x Ni cermets with 240-mm diameter were fabricated by self-propagating high-temperature synthesis and combined with pseudo heat isostatic pressing. Combustion-synthesized products consisted of TiC phase and Ni binder phase. Spheroidal TiC particles were enveloped by nearly continuous Ni binder phases. Size of TiC particles decrease with Ni content increase. Synthesized products have excellent mechanical properties and the bending strength of TiC–20Ni and TiC–30Ni is close to K151A and K152B, respectively, produced by traditional powder metallurgy technology.

Graphene nanosheet reinforced ZrB2–SiC ceramic composite by thermal reduction of graphene oxide

A graphene nanosheet reinforced ZrB2–SiC ceramic composite (GNs/ZrB2–SiC) using graphene oxide (GO) was hot pressed at 1950 °C and 30 MPa for 1 h. Raman and XPS analysis showed multilayer GNs structures were successfully introduced into the composite by in situ thermal reduction of GO during the hot pressing process. The homogeneous dispersion of GO guaranteed the uniform distribution of GNs structures in the composite. Mechanical properties such as bending strength, fracture toughness and hardness were studied for the ZrB2–SiC composite with different volumes fraction of GO. The addition of approximately 5 vol% graphene nanosheets improved the fracture toughness of ZrB2–SiC up to 7.32 MPa m0.5, and the strength was also raised to 1055 MPa. The toughening mechanisms were graphene crack bridging and pulling out induced crack deflection in the GNs reinforced composite.

Photovoltaic characteristics of amorphous silicon solar cells using boron doped tetrahedral amorphous carbon films as p-type window materials

Boron doped tetrahedral amorphous carbon (ta-C:B) was prepared by filtered cathodic vacuum arc deposition. A band gap of 2.0eV and a conductivity of 1.42×10−7S∕cm were obtained at the doping ratio of 2.13at.%. A device structure was deduced from the conventional amorphous silicon (a-Si:H) solar cell using the ta-C:B window layer. Photovoltaic parameters of the cells were studied by varying the boron content in the ta-C:B films. A roughly 10% relative improvement of conversion efficiency was observed compared to the normal a-Si:H solar cell. The improved cell performance results from the enhancement of short wavelength response.