Papers by Keyword: Vacancy Defects

Abstract: This study investigates the mechanical behavior of single and multi-wall carbon nanotubes (SWCNT/MWCNT) during torsional loading using the molecular dynamics (MD) simulation technique. The open-source software Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) is utilized to conduct MD simulations to gain valuable insights into the response of pristine and defective carbon nanotubes. The torsional behavior of armchair SWCNTs with chiralities (5,5); (7,7); (10,10); (12,12) and (15,15) and zigzag SWCNTs (12,0); (17,0) and (22,0) is explored to understand the effect of chirality on the torsional properties. Furthermore, the impact of the aspect ratio is examined by varying the diameter of SWCNTs while keeping the length constant. The findings reveal a notable decrease in shear modulus with increasing tube diameter, providing a crucial understanding of the torsional behavior concerning SWCNT geometry. To assess the effect of vacancy defects, 1%, 2%, and 4% vacancy defects are introduced on (10,10) armchair SWCNTs, and their torsional response is analyzed. The predictions highlight a significant reduction in shear modulus by 25% for SWCNTs with the rising concentration of vacancy defects from 1% to 4%. Overall, this study contributes to a deeper comprehension of the mechanical properties of carbon nanotubes under torsional loading, paving the way for potential applications in nanotechnology and nanocomposite design.

Abstract: The oscillatory behaviors of an oscillator made from double-walled carbon nanotubes (DWCNTs) with vacancy defects were systematically investigated via molecular dynamics simulation method. We found that the vacancy defects change the off-axial rocking motion and the van der Waals potential, resulting in more energy dissipation. Unlike the case in the C60–nanotube oscillators (Song, et al., Phys. Lett. A. 373 2009, 1058-1061) that one vacancy can make the oscillators more stable, our study showed that the vacancies cannot improve the performance of DWCNT-based oscillators no matter where vacancy defects are located.

Abstract: Molecular structural mechanics based finite element method has been applied to study the effects of two types of Stone-Wales (SW) defects and vacancy defect on elastic buckling properties of single-layered graphene sheets (SLGSs). The defect effect factors of critical buckling stresses are calculated for the defective SLGSs with different chirality and geometrical dimensions. It is proved that defect effect factors are size-dependent and chirality-dependent. The results show that the vacancy defects will always weaken the SLGSs’ stability, and two types of SW defects have different effects on zigzag and armchair SLGSs. What’s more, the positions of defects also have remarkable influence on the critical buckling stress of SLGSs.

Abstract: In the present study interfacial characteristics of carbon nanotubes (CNTs) and their composite with different types of defects are calculated using molecular dynamics (MD) simulation. Although single-walled CNTs (SWCNTs) are found to have ultra-high stiffness and strength, an enormous scatter is also observed in available experimental results due to the presence of nanoscale defects such as vacancy and Stone-Wales defects. The current paper investigates the influences of Stone-Wales (S-W) and vacancy defects on the mechanical properties of composites containing defective CNTs.

Abstract: Positron Annihilation Lifetime Spectroscopy (PALS) is used to study the nanoporosity and fractional free volume in Ultra High Molecular Weight Polyethylene (UHMWPE) and composites with the addition of Martian Regolith (UHMWPE-MR) as-made and irradiated with ⁵⁶Fe heavy ions at an energy of 600 MeV/u to three different doses (10, 32, 64 Gy). The positron lifetime spectra were obtained using ²²Na positron source and the spectra were analyzed to two lifetime components using POSFIT program. First short lifetime component around 0.28 ns is related to positron annihilation in material including vacancy defects and the second long lived component around 1.7 ns is due to Positronium formation in free volume pores. UHMWPE-MR composites were shown to be less porous with much lower nanopores concentration compared to the UHMWPE polymer. The average size of the nanopores is around 0.5 nm (obtained from a simple model). Larger variations in positron lifetime parameters are observed with increasing irradiation dose for UHMWPE polymer compared to UHMWPE+MR composites. The 3-point bend test results also showed larger variations with increasing irradiation dose for the UHMWPE polymer. The variations in PALS parameters may indicate an increasing competition between two processes at higher irradiation doses: 1) vacancy defects aggregation and 2) escape of vacancy defects as the local temperature increases at higher doses resulting in increased vacancy defects mobility. Present results clearly indicate a qualitative inverse relationship between nanoscale porosity measured by positron life time and mechanical properties of UHMWPE and its composite with MR.

Abstract: Ultra-high-temperature ceramics (UHTC) such as ZrB₂ and HfB₂ with SiC nanofiller are useful for propulsion and thermal protection systems. ZrB₂ and HfB₂ with 10-20 wt% SiC were prepared using ultra-sonication, rotary evaporation, and spark plasma heat treatment to high temperatures (~2,000°C) and pressures (50-60 MPa). We used positron annihilation lifetime spectroscopy (PALS) to study the nanoporosity, SEM for particle size distribution, and microhardness tester for Vickers hardness. The PALS studies were performed using a ²²Na positron source and the positron lifetime spectra were analyzed to three components using POSFIT program. The first and second components are related to positrons annihilating in bulk and in vacancy clusters, respectively; and the third component to positronium annihilation in nanopores within the granules. The PALS results indicate that HfB₂ has larger vacancy clusters and nanopores with lesser concentrations compared to ZrB₂ and SiC. The SEM observations showed that HfB₂ has larger particles compared to ZrB₂ and SiC showed wide range of size distribution. The Vickers-Hardness Number (VHN) is measured for spark plasma heat treated composites using a microhardness tester and the results indicate that 10wt%SiC composite has higher hardness compared to 20wt%SiC in both ZrB₂-SiC and HfB₂-SiC composites. HfB₂-SiC composites seem to be more brittle compared to ZrB₂-SiC composites. This may be due to larger size and smoother surface of HfB₂ particles (600 nm) compared to ZrB₂ particles (240 nm).

Abstract: Atomic defects play an important role in the brittle deformation of nanowires at low temperatures. With molecular dynamics simulations, we study the influence of vacancy defects on the deformation and breaking behaviors of [10 oriented single-crystal gold nanowires at 50 and 150 K. The size of the nanowire is 10a × 10a × 30a (a stands for lattice constant, 0.408 nm for gold). It is shown that good crystalline structure appears in the whole deformation process, and it is in a brittle way at low temperature. The nanowire breaking behavior is sensitive to atomic vacancies when the atomic vacancy ratio is 1% in single-layer crystalline plane. Within the limitation of vacancy-induced breaking of the nanowire, the mechanical strengths increase under atomic vacancies. However, it decreases with the defect ratio increasing.

Abstract: The application of variable energy positron annihilation spectroscopy (VE-PAS) methods to the study of perovskite oxide, ABO₃, material thin films and near-surface regions is reviewed. The primary focus is on ferroic perovskite titanate oxide materials SrTiO₃ and Pb (Zr_xTi_1-x)O₃, but studies of BaTiO₃, La_xSr_1-xCoO₃, La_0.67Sr_0.33MnO₃ and YBa₂CuO_7-δ are also included. Characterization of single layer and multilayer structures is discussed. The methods, in particular positron annihilation lifetime spectroscopy, allow the identification of cation vacancy defects at both the A-and B-sites with parts per million sensitivity. Varying oxygen deficiency is often observed to result in marked changes in PAS spectra; these effects are reviewed and discussed.

Abstract: Depth resolved positron beam results of Si and metal silicides are highlighted in terms of identifying the nature of vacancy defects in amorphous and disordered Si and phase transformation and defect evolution as a consequence of silicide formation, respectively.

Abstract: Molecular dynamics simulations are used to investigate the junction formation of two crossed single-walled carbon nanotubes (SWCNTs) with or without preexisting structural defects by heat welding. The junction formation of the chiral SWCNTs by heat welding is discussed. Furthermore, both the single vacancy defects and double vacancy defects are introduced in SWCNTs to explore the effect on junction formation by heat welding. We found the single vacancy defects and double vacancy defects pairs distributed on both crossed SWCNTs will accelerate the heat welding process and make the junction easier, and the required temperature for junction formation will be significantly reduced.