San-Qiang Shi

Direct measurement of giant electrocaloric effect in BaTiO3 multilayer thick film structure beyond theoretical prediction

The electrocaloric (EC) effect of BaTiO3 multilayer thick film structure was investigated by direct measurement and theoretical calculation. The samples were prepared by the tape-casting method, which had 180 dielectric layers with an average thickness of 1.4 μm. The thermodynamic calculation based on the polarization-temperature curves predicted a peak heat adsorption of 0.32 J/g at 80 °C under 176 kV/cm electric field. The direct measurement via differential scanning calorimeter showed a much higher EC effect of 0.91 J/g at 80 °C under same electric field. The difference could result from the different trends of changes of electric polarization and lattice elastic energy under ultrahigh electric field.

Criteria for fracture initiation at hydrides in zirconium alloys. I: Sharp crack tip

Abstract A theoretical framework for the initiation of delayed hydride cracking (DHC) in zirconium is proposed for two different types of initiating sites, i.e., a sharp crack tip (considered in this part) and a shallow notch (considered in part II). In the present part I, an expression for KIH is derived which shows that KIH depends on the size and shape of the hydride precipitated at the crack tip, the yield stress and elastic moduli of the material and the fracture stress of the hydride. If the hydride at the crack tip extends in length at constant thickness, then KIH increases as the square root of the hydride thickness. Thus a microstructure favouring the formation of thicker hydrides at the crack tip would result in an increased KIH. KIH increases slightly with temperature up to a temperature at which there is a more rapid increase. The temperature at which there is a more rapid increase in KIH will increase as the yield stress increases. The model also predicts that an increase in yield stress due to irradiation will cause an overall slight decrease in KIH compared to unirradiated material. There is good agreement between the overall predictions of the theory and experimental results. It is suggested that more careful evaluations of some key parameters are required to improve on the theoretical estimates.

Formation energy of Stone-Wales defects in carbon nanotubes

A Stone–Wales (SW) defect is a dipole of 5–7 ring pair in a hexagonal network, which is one of the most important defective structures in carbon nanotubes (CNTs) that will affect mechanical, chemical, and electronic properties of CNTs. Using the extended Huckel method, we calculated the formation energy of SW defects in carbon nanotubes. The formation energy of SW defects was then fitted to a simple formula as a function of the tube radius and the orientation of a SW defect in the tube. This result provides a convenient tool for the study of thermodynamics and kinetics of SW defects, as well as the interaction of SW defects with other types of defects in CNTs.

Current-induced magnetization dynamics in Co/Cu/Co nanopillars

We studied current-induced magnetization dynamics in Co∕Cu∕Co nanopillars using the Landau-Lifshitz-Gilbert equation incorporating the spin transfer torque effect. We show that the magnetization dynamics can be grouped into four types according to its characteristics and the current density value under zero external field. It is found that an external field can significantly affect the magnetization dynamics, either favoring or impeding the magnetization switching depending on its direction.

Molecular dynamic simulations on tensile mechanical properties of single-walled carbon nanotubes with and without hydrogen storage

Abstract Using a bond order potential, molecular dynamics (MD) simulations have been performed to study the mechanical properties of single-walled carbon nanotubes (SWNTs) under tensile loading with and without hydrogen storage. (10,10) armchair and (17,0) zigzag carbon nanotubes have been studied. Up to the necking point of the armchair carbon nanotube, two deformation stages were identified. In the first stage, the elongation of the nanotube was primarily due to the altering of angles between two neighbor carbon bonds. Youngs Modulus observed in this stage was comparable with experiments. In the second stage, the lengths of carbon bonds are extended up to the point of fracture. The tensile strength in this stage was higher than that observed in the first stage. Similar results were also found for the zigzag carbon nanotube with a lower tensile strength. Hydrogen molecules stored in the nanotubes reduced the maximum tensile strength of the carbon nanotubes, especially for the armchair type. The effect may be attributed to the competitive formation between the hydrogen–carbon and the carbon–carbon bonds.

Micro-mechanical properties and morphological observation on fracture surfaces of carbon nanotube composites pre-treated at different temperatures

In an early stage of the decade, most of works on carbon nanotubes were centred on chemistry and physics. Due to extraordinary mechanical and thermal properties, these nanotubes have been used as nano-fibres for the improvement of the global mechanical properties of advanced composite structures. This paper reports the micro-hardness and flexural properties of nanotube/epoxy composites with different amounts of nanotubes content. Experimental measurements and microscopic observations of the composites pre-treated at different temperatures are discussed in detail. The results show that the hardness of the nanotube composites varied with different nanotube weight fractions. The flexural strength decreased by 10% for a composite beam with 2 wt.% of nanotubes. The fracture surfaces of nanotube composite were perpendicular to the nanotube pre-treated at 70 degreesC, and parallel to nanotubes pre-treated at -180 degreesC. The SEM images also revealed that all nanotubes were completely pulled out after the flexural strength test due to a weak-bonding strength between the nanotube and matrix

Hydrogen concentration limit and critical temperatures for delayed hydride cracking in zirconium alloys

Abstract An experimental study was carried out to determine the hydrogen concentration limit as a function of temperature at which delayed hydride cracking (DHC) commences in Zr-2.5 Nb pressure tube material. For a given hydrogen content of the specimen, two critical temperatures were observed in this work — a DHC initiation temperature, T c at which DHC would initiate when approaching the test temperature from above the solvus (or terminal solid solubility) for hydride dissolution (TSSD) and a DHC arrest temperature, T h , obtained by heating the same specimen from T c after DHC had started. Both of T c and T h are close to, but below, temperatures defined by TSSD for the specific hydrogen content of the specimen. A theoretical analysis was carried out to quantitatively derive the hydrogen concentration limit and these critical temperatures. The theoretical por T c depends sensitivity on the particular solvus or terminal solid solubility curve for hydride precipitation (TSSP) used, since there is a wide range of values for TSSP depending on the thermal-mechanical history of the material. It is also suggested that T h is governed by the TSSP for hydride growth, in contrast to T c , which is governed by the TSSP for hydride nucleation. A model for a previously observed critical temperature ( T A ) is also proposed. T A is a DHC arrest temperature, obtained by approaching the test temperature from a lower temperature. The model suggests that T A is controlled by the energy difference between TSSD, TSSP and the hydrostatic stress at the crack tip.

Elastoplastic phase field model for microstructure evolution

Success has been obtained in predicting the dynamic evolution of microstructures during phase transformation or cracking propagation by using the time-dependent phase field methodology (PFM). However, most efforts of PFM were made in the elastic regime. In this letter, stress distributions around defects such as a hole and a crack in an externally loaded two-dimensional representative volume element were investigated by a proposed phase field model that took both the elastic and plastic deformations into consideration. Good agreement was found for static cases compared to the use of finite element analysis. Therefore, the proposed phase field model provides an opportunity to study the dynamic evolution of microstructures under plastic deformation.

Debond induced by strain recovery of an embedded niti wire at a NiTi/epoxy interface: micro-scale observation

Abstract Rapid development in smart structures has enhanced the great efforts in understanding the mechanical and thermo-mechanical behaviour of shape-memory alloy (SMA) materials. SMAs, in the form of wires and strips, have been embedded into advanced composite structures to control the shape and residual stress of the structures. It is well recognised that the mechanical properties of embedded SMA composites are highly dependent on the integrity of the interface, particularly by the existence of high thermal-induced shear stress at the SMA wire/epoxy interface. This paper discusses the debonding failure mechanism of embedded pre-strained SMA wires in an epoxy matrix environment using the scanning electron microscopy (SEM) technique. It was found that debonds occurred at the SMA wire/matrix interface for a wire with a pre-strained level of 8% at a temperature above Af. Several cracks in the matrix were found around the wire-end region and arrayed along the circumferential direction. A sharp crack, which was propagated towards the radial direction of the wire, was induced in a matrix with high air-bubble content during the wire/matrix debonding process. The effect of high-temperature treatment of SMAs was also studied. It was found that the surface layer of the wire peeled off when the wire was heated at 773 K for 10 min.

Micro-hardness and Flexural Properties of Randomly-oriented Carbon Nanotube Composites:

The carbon nanotubes possess many unique mechanical and electrical properties, and have been appreciated as new advanced materials for nanocomposite structures, particularly for the development of nanocomposite films. Nanotubes may also be used as nano-reinforcements for matrix system for fibre-reinforced plastic structures in order to improve out-of-plane properties, thus increasing the delamination resistance. However, those properties are highly relied on the structural integrity and homogeneity of the nanotube composites. Unfortunately, only a little works have paid much attention on these issues recently. It has been obviously proved that the atomic architecture on the nanotube’s surface may be affected after the nanotubes were chemically reacted with polymer matrix. The weak bonding force among the different layers (bonded by a weak Van Der Waals attractive force) of multiwalled nanotubes may also cause a discontinuous stress transfer from the outer-shell to the inner of the composites. This paper reports the micro-hardness and flexural properties of nanotube composites with different amounts of nanotubes content. Experimental measurements and microscopic observations of the nanotube-epoxy composites before and after the tests are discussed in detail. The results show that the hardness of the nanotube composites varied with different nanotube weight fractions. The flexural strength decreased by 10% for a nanotube composite beam with 2 wt.% of nanotubes. The SEM images also revealed that all nanotubes were completely pulled out after the flexural strength test due to a weak-bonding strength between the nanotube and matrix.