Papers by Keyword: Vacancy

Abstract: MXenes is a new group of two-dimensional materials via etching of the ‘A’ element from MAX phases. Depending on the functional group, MXenes can be semiconductors or metals. In this paper, first-principles calculations have been performed to investigate the effects of single vacancy defects on a semiconducting MXene, Sc₂CF₂ monolayer. The theoretical results show that V-Sc can induce magnetism in the host monolayer, while V-C and V-F result in n-type conductivity. For V-Sc doped Sc₂CF₂, tensile strains enhance the total magnetic moment which remains constant with applied compressive strains. As a result, by manipulating the fabrication parameters, the magnetic and conductive properties of Sc₂CF₂ can be tuned without the need of chemical doping.

Abstract: The effects of [001] uniaxial strain on the stable structures and structural evolution of vacancy clusters in fcc metals, Cu, Ni, Al and Fe, have been studied and compared. Under uniaxial strain, the clusters in all these metals tend to align parallel or perpendicular to the strain axis under tensile or compressive strain. Moreover, both the body cluster and the {001} planar cluster become the dominant types. In addition, the stacking fault tetrahedron cluster becomes another dominant type in Al under compressive strain. The cluster structures in Fe are disordered under strain possibly because the pure fcc Fe is thermodynamically unstable under the current simulation condition.

Abstract: The preferential sites for vacancies on a series of symmetric tilt grain boundaries in copper have been investigated by molecular dynamics simulation. The regularity of preferential sites for vacancies on these boundaries can be described by the structural unit model. This is essential because of the correspondence between the geometries of the structural units and the local stress field. The vacancies are energetically preferred at the sites with relatively large tensile stress, and these sites are the corner sites of the structural units. Moreover, these preferential sites are mainly related to the structural unit types irrespective of which grain boundary that the structure units locate in. Therefore, the preferential sites for vacancies on various grain boundaries formed by combinations of certain structural units can be readily described and predicted by the structural unit model.

Abstract: A computer simulation of atomic displacement cascades in Fe-9at.%Cr binary alloy has been performed by molecular dynamics method for temperature of 300 K and cascade energies from 100 eV to 20 keV. The average number of Frenkel pairs produced in cascade has been calculated. The data on point defect clusterization have been obtained. Obtained evaluations of effective fraction of surviving defects are well approximated by the sum of power and linear functions of cascade energy. Increased chromium fraction in the self-interstitial (SIA) configurations has been observed and has been explained by combination of two factors: positive binding energy of Cr atom with SIAs and mobility of SIA configuration. The diffusion coefficient of single SIA configuration in the matrix of pure bcc Fe has been evaluated for the temperature range of 300 – 1000 K. We have prepared 100 group neutron cross-sections of effective displacement generation in Fe-9at.%Cr binary alloy. It has been shown that effective dpa generation rate can be 2-3 times lower than corresponding rates of conventional dpa generation rate.

Abstract: In this work investigation on wet etching of ion implanted 4H-SiC has been performed. Starting with the search for a suitable etching solution is followed by investigations on how to damage 4H-SiC in an efficient way involving different implantation species in various doses. With the help of Monte Carlo simulations a model for the experimental findings is proposed to derive the limitations for the wet etch capability.

Abstract: In the present research, imperfect graphene sheets were generated and their vibrational property was studied via finite element analysis. The effect of vacant sites in the arrangement of these nano-structures was examined. The fundamental frequency of the defect free and imperfect nano-sheets was acquired based on two different approaches. The first approach was a pure finite element simulation. The second approach for comparison purpose was a recently reported refined finite element simulation at which the vicinity of a defect was first evaluated according to the density functional theory (DFT) and then the refined geometry was implemented into a finite element model. The findings of this research show that the fundamental frequency of graphene sheets decreases by presenting microscopic imperfection to the formation of these nano-materials. In addition, it was pointed out that the geometry based on the more precise DFT simulations gives a higher decrease in the fundamental frequency of the sheets for all considered cases.

Abstract: Positron lifetimes and momentum distributions of annihilating electron-positron pairs have been calculated for vacancies in ZnSb and Zn₄Sb₃. The calculated positron lifetimes for bulk ZnSb and Zn₄Sb₃ are 203 ps and 208 ps, and for V_Zn in ZnSb and Zn₄Sb₃ are 249 ps and 237 ps respectively. The calculated momentum distribution results indicate the V_Zn in both ZnSb and Zn₄Sb₃ has less characterization from elemental Zn. Using coincidence Doppler broadening spectra combined with lifetime measurements can reveal the vacancy structure in ZnSb and Zn₄Sb₃.

Abstract: Solution annealed type 316L austenitic stainless steels were irradiated using 2 MeV Fe ions at room temperature. The implanted fluences were 2×10¹² ions/cm² and 1×10¹³ ions/cm², respectively. Variable mono-energetic positron beam was performed to characterize the evolution of microstructure and irradiation induced defects. Results show that large amount of vacancy defects formed after heavy ion irradiation. In which, some of mono-vacancies might migrate to form small-sized clusters at room temperature. After irradiation, implanted Fe atoms mainly be interstitials atoms, but some Fe atoms might recombine with vacancies due to their high mobility, which could decrease the defect concentration, effectively.

Abstract: Precursor phenomena of melting in pure metals and alloys have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). The examined materials were the pure metals In, Sn, Pb and Bi, and some alloys of the systems In-Sn and Pb-Bi with different compositions.MS tests have been carried out by means of a novel method developed by us that permits to operate in resonance conditions and employs hollow reeds of stainless steel containing the liquid metal. In all the metals a sharp drop of dynamic modulus and a Q^-1 maximum were observed in a temperature range ΔT before melting that depends on the specific metal and its structure. Such anelastic behaviour is consistent with an increase of the interstitialcies concentration as predicted by the Granato’s theory.Moreover, HT-XRD evidenced that sudden grain re-orientation, shift and broadening of diffraction peaks occur just before the formation of the first liquid, therefore self-interstitials and vacancies seem to play a synergic role in melting. The increase of self-interstitials over ΔT has the effect of weakening interatomic bonds that favours the successive vacancy avalanche leading to the collapse of crystal lattice (melting).

Abstract: Hydrogen in aluminum has been known to be the cause of blister and pore. Some aluminum alloy is susceptible to stress corrosion cracking, which is based on intergranular cracking arisen from hydrogen embrittlement. The behavior of hydrogen in aluminum has not been fully understood yet. Hydrogen gas plasma enables to introduce high hydrogen concentrations into specimen without Al (OH)₃ layer on the surface of specimen. In this paper, we have investigated the behavior of hydrogen in a plasma charged aluminum by means of thermal desorption spectroscopy, a method to evaluate the amount and trap states of hydrogen. Cold-rolled pure aluminum were annealed, electro-polished and charged with hydrogen gas plasma. Immediately after hydrogen gas plasma charging, TDS tests were performed under ultra-high vacuum. The hydrogen desorption spectrums obtained by TDS tests had three peaks corresponding to the co-diffusion of hydrogen-vacancy pair, dislocation and pore. Compared to a sample without charging, in a plasma charged sample, the amount of hydrogen trapped in vacancies especially increased.