Investigation of mechanical properties and deformation behavior of single-crystal Al-Cu core-shell nanowire generated using non-equilibrium molecular dynamics simulation

Molecular dynamics (MD) simulation studies were carried out to generate a cylindrical single-crystal Al-Cu core-shell nanowire and its mechanical properties like yield strength and Young’s modulus were evaluated in comparison to a solid aluminum nanowire and hollow copper nanowire which combines to constitute the core-shell structure respectively. The deformation behavior due to changes in the number of Wigner-Seitz defects and dislocations during the entire tensile deformation process was thoroughly studied for the Al-Cu core-shell nanowire. The single-crystal Al-Cu core-shell nanowire shows much higher yield strength and Young’s modulus in comparison to the solid aluminum core and hollow copper shell nanowire due to tangling of dislocations caused by lattice mismatch between aluminum and copper. Thus, the Al-Cu core-shell nanowire can be reinforced in different bulk matrix to develop new type of light-weight nanocomposite materials with greatly enhanced material properties.     

Atomistic simulations to characterize the influence of applied strain and PKA energy on radiation damage evolution in pure aluminum

Knowledge of defects generation, their mobility, growth rate, and spatial distribution is the cornerstone for understanding the surface and structural evolution of a material used under irradiation conditions. In this study, molecular dynamics simulations were used to investigate the coupled effect of primary knock-on atom (PKA) energy and applied strain (uniaxial and hydrostatic) fields on primary radiation damage evolution in pure aluminum. Cascade damage simulations were carried out for PKA energy ranging between 1 and 20 keV and for applied strain values ranging between ?2% and 2% at the fixed temperature of 300 K. Simulation results showed that as the atomic displacement cascade proceeds under uniaxial and hydrostatic strains, the peak and surviving number of Frenkel point defects increases with increasing tension; however, these increments were more prominent under larger volume changing deformations (hydrostatic strain). The percentage fraction of point defects that aggregate into clusters increases under tension conditions; compared to the reference conditions with no strain, these increases are around 13% and 7% for interstitials and vacancies, respectively (under 2% uniaxial strain), and 19% and 11% for interstitials and vacancies, respectively (under 2% hydrostatic strain). Clusters formed of vacancies and interstitials were both larger under tensile strain conditions, with increases in both the average and maximum cluster sizes. The rate of increase/decrease in the number of Frenkel pairs, their clustering, and their size distributions under expansion/compression strain conditions were higher for higher PKA energies. Overall, the present results suggest that strain effects should be considered carefully in radiation damage environments, specifically for conditions of low temperature and high radiation energy. Compressive strain conditions could be beneficial for materials used in nuclear reactor power systems.     

Anisotropic Migration of Defects under Strain Effect in BCC Iron

The basic properties of defects(self-interstitial and vacancy) in BCC iron under uniaxial tensile strain are investigated with atomic simulation methods. The formation and migration energies of them show different dependences on the directions of uniaxial tensile strain in two different computation boxes. In box-1, the uniaxial tensile strain along the100direction influences the formation and migration energies of the110dumbbell but slightly affects the migration energy of a single vacancy. In box-2, the uniaxial tensile strain along the 111 direction influences the formation and migration energies of both vacancy and interstitials. Especially, a 110 dumbbell has a lower migration energy when its migration direction is the same or close to the strain direction, while along these directions, a vacancy has a higher migration energy. All these results indicate that the uniaxial tensile strain can result in the anisotropic formation and migration energies of simple defects in materials.     

Nontrivial tensile behavior of rutile TiO<Subscript>2</Subscript> nanowires: a molecular dynamics study

In this paper, we study the tensile behavior of cylindrical rutile TiO₂ nanowires, employing molecular dynamics (MD) simulation technique. The third-generation charge optimized many-body (COMB3) has been used for interatomic potential modeling. The influence of temperature and nanowire diameter on Young’s modulus is investigated. Our simulations exhibit the anisotropic behavior of Young’s modulus as a function of diameter for different crystallographic orientations. Although our results are in good accord with the existing results in [1 0 0] direction, Young’s modulus adds up monotonically with increasing the cross-sectional diameter of nanowire in [0 0 1] direction. It is found that Young’s modulus of the nanowires are lower (higher) than the bulk value for [0 0 1] ([1 0 0]) direction. Furthermore, simulation results also indicate that Young’s modulus of rutile TiO₂ nanowire increases as a function of temperature for a given diameter, unexpectedly. The obtained results may be useful in the field of nanotechnology for optimizing mechanical performance to gain specific applications.     

金纳米管力学性能的分子动力学模拟

采用分子动力学模拟方法, 研究了金纳米管沿不同晶向拉伸与压缩载荷下的力学性能, 并分析了金纳米管的半径对其力学行为的影响. 在模拟计算中, 采用镶嵌原子势描述金原子之间的相互作用. 模拟结果表明, 在拉伸及压缩过程中, 不同晶向的金纳米管力学性能相差较大, 在拉伸和压缩载荷下金纳米管<110>向的屈服强度最大; 在三个晶向<100>, <110>, <111>的金纳米管中, <100>晶向的金纳米管其屈服强度和杨氏模量都远远小于其他晶向. 研究结果还发现, 当纳米管的半径小于3.0 nm时, 金纳米管的屈服强度没有大的变化, 而当半径大于3.0 nm后, 随着半径的增大, 其屈服强度明显降低. 关键词： 分子动力学模拟 金纳米管 力学性能     

Investigation of mechanical properties of twin gold crystal nanowires under uniaxial load by molecular dynamics method

Twin gold crystal nanowires, whose loading direction is parallel to the twin boundary orientation, are simulated.We calculate the nanowires under tensile or compressive loads, different length nanowires, and different twin boundary nanowires respectively. The Young modulus of nanowires under compressive load is about twice that under tensile load.The compressive properties of twin gold nanowires are superior to their tensile properties. For different length nanowires,there is a critical value of length with respect to the mechanical properties. When the length of nanowire is greater than the critical value, its mechanical properties are sensitive to length. The twin boundary spacing hardly affects the mechanical properties.     

Impact of the displacement damage in channel and source/drain regions on the DC characteristics degradation in deep-submicron MOSFETs after heavy ion irradiation

This paper mainly reports the permanent impact of displacement damage induced by heavy-ion strikes on the deep-submicron MOSFETs.Upon the heavy ion track through the device,it can lead to displacement damage,including the vacancies and the interstitials.As the featured size of device scales down,the damage can change the dopant distribution in the channel and source/drain regions through the generation of radiation-induced defects and thus have significant impacts on their electrical characteristics.The measured results show that the radiation-induced damage can cause DC characteristics degradations including the threshold voltage,subthreshold swing,saturation drain current,transconductance,etc.The radiation-induced displacement damage may become the dominant issue while it was the secondary concern for the traditional devices after the heavy ion irradiation.The samples are also irradiated by Co-60 gamma ray for comparison with the heavy ion irradiation results.Corresponding explanations and analysis are discussed.     

Radiation enhanced diffusion in face centered cubic cu-zn alloys

The enhancement of diffusion by neutron irradiation has been investigated on a Cu-36 percent Zn alloy for various neutron fluxes and irradiation temperatures by means of in-pile measurements of electrical resistivity. For fresh samples the diffusion rate depends on temperature with an activation energy of 0.35 eV. During repeated irradiations the diffusion rate decreases and becomes nearly temperature independent. The variation of the concentration of interstitials and vacancies with irradiation time has been numerically calculated for various neutron fluxes, irradiation temperatures and sink concentrations. A comparison of the experimental and theoretical results shows that the point defects annihilate in fresh samples mainly by pair recombination and in samples which had been repeatedly cycled by pair recombination and at fixed sinks. Point defect clusters acting as sinks are created during the course of the irradiation as shown by electron microscope investigations. The radiation enhanced diffusion rate was found to depend on interstitials only, the activation energy of which was determined to 0.70 eV.     

Examination of optical properties and estimation of mechanical energies of irradiated polypropylene and teflon

The optical absorption and mechanical yielding energy has been studied under exposure of γ-radiation. Two crystalline polymers, polypropylene (PP) and Teflon, were irradiated with a ⁶⁰Co source, with doses ranging up to 6 kGy. The observed optical energy gap (E _opt) and energy gap tail (Δ E) for irradiated thin sheets of PP were determined from the measured absorption spectra. The average values of (E _opt) and (Δ E) were 5.85 and 0.5 eV, respectively. There is no detectable change in the optical energy gap under the applied γ-ray doses. On the other hand, the effect of γ-radiation on mechanical properties of irradiated Teflon was much pronounced. It was found that Young’s modulus and yield stress increase with radiation dose, whereas the yield strain decreases. The calculated yield energy increases with radiation dose from 0.27 to 0.35 Mpa per unit volume. The enhancement in the mechanical properties of irradiated Teflon was attributed mainly to crosslinking process and other structural changes occuring during irradiation with γ-rays.     

Consideration of mechanical properties of single-walled carbon nanotubes under various loading conditions

In this article, mechanical properties of single-walled carbon nanotubes (SWCNTs) with various radiuses under tensile, compressive and lateral loads are considered. Stress–strain curve, elastic modulus, tensile, compressive and rotational stiffness, buckling behaviour, and critical axial compressive load and pressure of eight different zigzag and armchair SWCNTs are investigated to figure out the effect of radius and chirality on mechanical properties of nanotubes. Using molecular dynamic simulation (MDS) method, it can be explained that SWCNTs have higher Young’s modulus and tensile stiffness than compressive elastic modulus and compressive stiffness. Critical axial force of zigzag SWCNT is independent from the radius, but that of armchair type rises by increasing of radius, also these two types show different buckling modes.     

Single and paired point defects in a 2D Wigner crystal

Using the path-integral Monte Carlo method, we calculate the energy to form single and pair vacancies and interstitials in a two-dimensional Wigner crystal of electrons. We confirm that the lowest energy point defects of a 2D electron Wigner crystal are interstitials, with a creation energy roughly 2/3 that of a vacancy. The formation energy of the defects goes to zero at melting, suggesting that point defects may be the melting mechanism and that the melting could be a continuous transition. In addition, we find that the interaction between defects is strongly attractive, so that most defects will exist as bound pairs.     

Effect of vacancy defect clusters on the optical property of the aluminium filter used for the space solar telescope

We investigate the microstructures of the pure aluminium foil and filter used on the space solar telescope, irradiated by photons with different doses. The vacancy defect clusters induced by proton irradiation in both samples are characterized by transmission electron microscopy, and the density and the size distribution of vacancy defect clusters are determined. Their transmittances are measured before and after irradiating the samples by protons with energy E=100~keV and dose φ =6× 10¹¹/mm². Our experimental results show that the density and the size of vacancy defect clusters increase with the increase of irradiation doses in the irradiated pure aluminium foils. As irradiation dose increases, vacancies incline to form larger defect clusters. In the irradiated filter, a large number of banded void defects are observed at the agglomerate boundary, which results in the degradation of the optical and mechanical performances of the filter after proton irradiation.     

Molecular dynamic investigation of mechanical properties of armchair and zigzag double-walled carbon nanotubes under various loading conditions

Using molecular dynamic simulation (MDS), effects of chirality and Van der Waals interaction on Young's modulus, elastic compressive modulus, bending, tensile, and compressive stiffness, and critical axial force of double-walled carbon nanotube (DWCNT) and its inner and outer tubes are considered. Achieving the highest safety factor, mechanical properties have been investigated under applied load on both inner and outer tubes simultaneously and on each one of them separately. Results indicate that as a compressive element, DWCNT is more beneficial than single-walled carbon nanotube (SWCNT) since it carries two times higher compression before buckling. Except critical axial pressure and tensile stiffness, in other parameters zigzag DWCNT shows higher amounts than armchair type. Outer tube has lower strength than inner tube; therefore, most reliable design of nanostructures can be attained if the mechanical properties of outer tube taken as the properties of DWCNT.     

Positron annihilation in the quenched and electron-irradiated NiSb intermetallic compound

Samples of the near equiatomic NiSb compound were irradiated by 3 MeV electrons at 20 K or quenched from 1103 K and 1333 K and subsequently annealed isochronally. The behaviour of defects created by quench or irradiation were studied by the positron annihilation technique. Only one recovery stage was found around 425 K for quenched specimens, but two distinct stages (100 K and 425 K) were observed after irradiation. The 425 K stage is ascribed to the migration of Ni vacancies giving dislocation loops. The recombination of mobile interstitials with vacancies after irradiation is assumed to occur between 100 K and 250 K. Doppler broadening and lifetime variations of positrons as a function of the measuring temperature in these irradiation samples are discussed.     

A study of lattice defects in alkali fluorides by thermoluminescence experiments

Summary Lattice defects in KF, NaF and LiF are investigated by recording the thermoluminescent emission of X-irradiated samplesvs. temperature and wavelength. Experimental results show the presence of cation vacancies and interstitials in KF and NaF and of F⁽⁻⁾ vacancies and interstitials in NaF and LiF. In NaF, the ionization energy of V_F centres is found to be 6.4 eV, that of H centres 5.6 eV.     

First-principles study of native defects in rutile TiO2

Native point defects in the rutile TiO₂ are studied via first-principles pseudopotential calculations. Except for the two antisite defects, all the native point defects have low formation energies. Under the Ti-rich growth condition, high concentrations of titanium interstitials and oxygen vacancies would form spontaneously in p-type samples; whereas high concentrations of titanium vacancies would form spontaneously in n-type samples regardless of the oxygen partial pressure.     

First-principles investigation of the intrinsic defects in Ti3SiC2

First-principles calculations have been carried out to investigate intrinsic defects including vacancies, interstitials, antisite defects, Frenkel and Schottky defects in the 312 MAX phase Ti₃SiC₂. The formation energies of defects are obtained according to the elemental chemical potentials which are determined by the phase stability conditions. The most stable self-interstitials are all found in the hexahedral position surrounded by two Ti(2) and three Si atoms. For the entire elemental chemical potential range considered, our results demonstrated that Si and C related defects, including vacancies, interstitials and Frenkel defects are the most dominant defects. Besides, the present calculations also reveal that the formation energies of C and Si Frenkel defects are much lower than those of all Schottky defects considered. In addition, the calculated profiles of densities of states for the defective Ti₃SiC₂ indicate that these defects should have great influence on its thermal and electrical properties.     

Effect of various components of slightly diluted tungsten-based alloys on the formation of defects in atomic displacement cascade regions: Field-ion microscopic analysis

New complex field-ion microscopic data for the formation of radiation-induced defects in VChV ultrapure tungsten, VA-3 commercial-grade tungsten, and four slightly diluted tungsten-based alloys (W-Hf-C, P39A; W-1.5% ThO₂, VT-15; W-5% Re, VR-5; and W-2% Fe, VZh-2) are reported. Samples were irradiated in an external unit by Ar⁺ and Ni⁺ ions of energy 35 keV. In the experiments, the ion current is kept at j=2.0 μA and the irradiation fluence equals Φt=5×10¹⁴ ions/cm². The clustering of single vacancies in samples irradiated is studied in relation to the impurity concentration and type. The distribution of vacancy clusters over the number of aggregated single vacancies is studied. These distributions are found to differ noticeably inside and outside the depletion zones. The mean lengths of focused substitutional atomic collision chains in samples with different impurity concentrations and types are measured indirectly. From these data, the efficiency of trapping intrinsic interstitials by various impurities in tungsten is estimated.     

Defects in electron irradiated amorphous SiO2 probed by positron annihilation

Defects in 3-MeV electron irradiated amorphous (a-) SiO₂ were studied by the positron annihilation technique. A high formation probability of positronium (Ps) was found for un-irradiated a-SiO₂ specimens. These Ps atoms were considered to be trapped by vacancy-type defects. The formation probability of Ps was drastically decreased by the electron irradiation. This fact can be attributed to an introduction of vacancy-type defects such as oxygen vacancies by the electron irradiation and a resultant trapping of positrons by such defects.     

Diffusion coefficients of vacancies and interstitials along tilt grain boundaries in molybdenum

The diffusion coefficients of vacancies and interstitials along symmetrical tilt grain boundaries in molybdenum have been calculated using the molecular dynamics method. The migration energies of defects have been obtained. The activation energy and coefficients of grain boundary self-diffusion have been deter-mined. A comparison of the obtained results with the studies of other authors indicates that boundaries formed between particles in the powder in sintering experiments have a higher diffusion activity as compared to stable grain boundaries in polycrystals.