Modelling failure of composite specimens with defects under compression loading

Composite structures exhibit many different failure mechanisms, but attempts to model composite failure frequently make a priori assumptions about the mechanism by which failure will occur. Wang et al. [1] conducted compressive tests on four configurations of composite specimen manufactured with out-of-plane waviness created by ply-drop defects. There were significantly different failures for each case. Detailed finite element models of these experiments were developed which include competing failure mechanisms. The model predictions correlate well with experimental results-both qualitatively (location of failure and shape of failed specimen) and quantitatively (failure load). The models are used to identify the progression of failure during the compressive tests, determine the critical failure mechanism for each configuration, and investigate the effect of cohesive parameters upon specimen strength. This modelling approach which includes multiple competing failure mechanisms can be applied to predict failure in situations where the failure mechanism is not known in advance. © 2013 Elsevier Ltd. All rights reserved.

α-Synuclein senses lipid packing defects and induces lateral expansion of lipids leading to membrane remodeling.

There is increasing evidence for the involvement of lipid membranes in both the functional and pathological properties of α-synuclein (α-Syn). Despite many investigations to characterize the binding of α-Syn to membranes, there is still a lack of understanding of the binding mode linking the properties of lipid membranes to α-Syn insertion into these dynamic structures. Using a combination of an optical biosensing technique and in situ atomic force microscopy, we show that the binding strength of α-Syn is related to the specificity of the lipid environment (the lipid chemistry and steric properties within a bilayer structure) and to the ability of the membranes to accommodate and remodel upon the interaction of α-Syn with lipid membranes. We show that this interaction results in the insertion of α-Syn into the region of the headgroups, inducing a lateral expansion of lipid molecules that can progress to further bilayer remodeling, such as membrane thinning and expansion of lipids out of the membrane plane. We provide new insights into the affinity of α-Syn for lipid packing defects found in vesicles of high curvature and in planar membranes with cone-shaped lipids and suggest a comprehensive model of the interaction between α-Syn and lipid bilayers. The ability of α-Syn to sense lipid packing defects and to remodel membrane structure supports its proposed role in vesicle trafficking.

High purity GaAs nanowires free of planar defects: Growth and characterization

We investigate how to tailor the structural, crystallographic and optical properties of GaAs nanowires. Nanowires were grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. A high arsine flow rate, that is, a high ratio of group V to group III precursors, imparts significant advantages. It dramatically reduces planar crystallographic defects and reduces intrinsic carbon dopant incorporation. Increasing V/III ratio further, however, instigates nanowire kinking and increases nanowire tapering. By choosing an intermediate V/III ratio we achieve uniform, vertically aligned GaAs nanowires, free of planar crystallographic defects, with excellent optical properties and high purity. These findings will greatly assist the development of future GaAs nanowire-based electronic and optoelectronic devices, and are expected to be more broadly relevant to the rational synthesis of other III-V nanowires. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

Controllable nanodomain defects in ferroelectric/ferroelastic biferroic thin films

Ferroelectric thin films have been intensively studied at the nanometre scale due to the application in many fields, such as non-volatile memories. Enhanced piezo-response force microscopy (E-PFM) was used to investigate the evolution of ferroelectric and ferroelastic nanodomains in a polycrystalline thin film of the simple multi-ferroic PbZr0.3Ti0.7O 3 (PZT). By applying a d.c. voltage between the atomic force microscopy (AFM) tip and the bottom substrate of the sample, we created an electric field to switch the domain orientation. Reversible switching of both ferroelectric and ferroelastic domains towards particular directions with predominantly (111) domain orientations are observed. We also showed that along with the ferroelectric/ferroelastic domain switch, there are defects that also switch. Finally, we proposed the possible explanation of this controllable defect in terms of flexoelectricity and defect pinning. © 2013 IEEE.

Oxygen vacancy defects in Ta2O5 showing long-range atomic re-arrangements

The structure, formation energy, and energy levels of the various oxygen vacancies in Ta2O5 have been calculated using the λ phase model. The intra-layer vacancies give rise to unusual, long-range bonding rearrangements, which are different for each defect charge state. The 2-fold coordinated intra-layer vacancy is the lowest cost vacancy and forms a deep level 1.5 eV below the conduction band edge. The 3-fold intra-layer vacancy and the 2-fold inter-layer vacancy are higher cost defects, and form shallower levels. The unusual bonding rearrangements lead to low oxygen migration barriers, which are useful for resistive random access memory applications. © 2014 AIP Publishing LLC.

Defects in gallium nitride nanowires: First principles calculations

Atomic configurations and formation energies of native defects in an unsaturated GaN nanowire grown along the [001] direction and with (100) lateral facets are studied using large-scale ab initio calculation. Cation and anion vacancies, antisites, and interstitials in the neutral charge state are all considered. The configurations of these defects in the core region and outermost surface region of the nanowire are different. The atomic configurations of the defects in the core region are same as those in the bulk GaN, and the formation energy is large. The defects at the surface show different atomic configurations with low formation energy. Starting from a Ga vacancy at the edge of the side plane of the nanowire, a N-N split interstitial is formed after relaxation. As a N site is replaced by a Ga atom in the suboutermost layer, the Ga atom will be expelled out of the outermost layers and leaves a vacancy at the original N site. The Ga interstitial at the outmost surface will diffuse out by interstitialcy mechanism. For all the tested cases N-N split interstitials are easily formed with low formation energy in the nanowires, indicating N-2 molecular will appear in the GaN nanowire, which agrees well with experimental findings.

First-principle study of native defects in CuScO2 and CuYO2

This paper studies the electronic structure and native defects intransparent conducting oxides CuScO2 and CuYO2 using the first-principle calculations. Some typical native copper-related and oxygen-related defects, such as vacancy, interstitials, and antisites in their relevant charge state are considered. The results of calculation show that, CuMO2 (M = Sc, Y) is impossible to shown-type conductivity ability. It finds that copper vacancy and oxygen interstitial have relatively low formation energy and they are the relevant defects in CuScO2 and CuYO2. Copper vacancy is the most efficient acceptor, and under O-rich condition oxygen antisite also becomes important acceptor and plays an important role in p-type conductivity.

First-principles study of defects in CuGaO2

Using the first-principles methods, we study the electronic structure, intrinsic and extrinsic defects doping in transparent conducting oxides CuGaO2. Intrinsic defects, acceptor-type and donor-type extrinsic defects in their relevant charge state are considered. The calculation result show that copper vacancy and oxygen interstitial are the relevant defects in CuGaO2. In addition, copper vacancy is the most efficient acceptor. Substituting Be for Ga is the prominent acceptor, and substituting Ca for Cu is the prominent donors in CuGaO2. Our calculation results are expected to be a guide for preparing n-type and p-type materials in CuGaO2.

First-principle study of extrinsic defects in CuScO2 and CuYO2

Using first-principles methods, we studied the extrinsic defects doping in transparent conducting oxides CuMO2 (M=Sc, Y). We chose Be, Mg, Ca, Si, Ge, Sn as extrinsic defects to substitute for M and Cu atoms. By systematically calculating the impurity formation energy and transition energy level, we find that Be-Cu is the most prominent extrinsic donor and Ca-M is the prominent extrinsic acceptor. In addition, we find that Mg atom substituting for Sc is the most prominent extrinsic acceptor in CuSCO2. Our calculation results are expected to be a guide for preparing n-type and p-type materials through extrinsic doping in CuMO2 (M=SC, y). (C) 2008 Elsevier B.V. All rights reserved.

Evolution of structural defects in SiOx films fabricated by electron cyclotron resonance plasma chemical vapour deposition upon annealing treatment

We study the structural defects in the SiOx film prepared by electron cyclotron resonance plasma chemical vapour deposition and annealing recovery evolution. The photoluminescence property is observed in the as-deposited and annealed samples. [-SiO3](2-) defects are the luminescence centres of the ultraviolet photoluminescence (PL) from the Fourier transform infrared spectroscopy and PL measurements. [-SiO3](2-) is observed by positron annihilation spectroscopy, and this defect can make the S parameters increase. After 1000 degrees C annealing, [-SiO3](2-) defects still exist in the films.