| 1. |
- Gao, Chan, et al.
(författare)
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Defect evolution behaviors from single sulfur point vacancies to line vacancies in monolayer molybdenum disulfide
- 2021
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 23:35, s. 19525-19536
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional monolayer transition metal dichalcogenides (TMDs) are promising candidates for many novel nanoelectronic and optoelectronic applications due to their exceptional electronic, optical, chemical and mechanical properties. Experimentally, single chalcogen point vacancies caused by electron beam irradiation are found to agglomerate into line vacancy defects in monolayer TMDs. Herein, the corresponding defect evolution behaviors from single sulfur point vacancies to line vacancies in the monolayer molybdenum disulfide (MoS2) have been systematically studied using molecular dynamics and first principles calculations. The experimental observations of the defect evolution from single sulfur point vacancies to line vacancies are reproduced at the atomic level. The results indicate that the di-vacancy line defect and a point vacancy separated by a sulfur atom in a line evolve into tri-vacancy line defects, and the di-vacancy line defects can rotate 60 degrees clockwise or counterclockwise. Moreover, two adjacent di-vacancy line defects with an angle of 120 degrees can evolve into tri-vacancy line defects. High temperature and large vacancy concentrations promote the defect evolution from point vacancies to line vacancies. Intriguingly, compared with the randomly distributed point vacancy defects, the line vacancy defects formed after the defect evolution significantly decrease the mechanical properties, such as the ultimate strength, ultimate strain and Young's modulus of monolayer MoS2. In addition, the mechanical properties decrease with increasing vacancy concentration and temperature for the final configurations after defect evolution in monolayer MoS2 with different vacancy concentrations at different temperatures. The band gaps of monolayer MoS2 with line vacancy defects are smaller than those with randomly distributed point vacancy defects. Therefore, our study clarifies the defect evolution behaviors from single sulfur point vacancies to line vacancies in monolayer MoS2 and opens an opportunity for the novel nanoelectronic and optoelectronic applications of monolayer TMDs.
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| 2. |
- Gao, Chan, et al.
(författare)
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Synergistic vacancy defects and mechanical strain for the modulation of the mechanical, electronic and optical properties of monolayer tungsten disulfide
- 2021
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 23:10, s. 6298-6308
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Tidskriftsartikel (refereegranskat)abstract
- Monolayer transition metal dichalcogenides (TMDs) are the potential candidate materials in nanoelectronic and optoelectronic applications due to their unique physical and chemical properties. Although both defect and strain greatly alter the structural, physical and chemical properties of monolayer TMDs, the defective monolayer TMDs under applied strain have not been adequately studied. In this paper, the synergistic effects of sulfur vacancy defects and mechanical strain on the mechanical, electronic and optical properties of monolayer tungsten disulfide (WS2) have been systematically studied using first principles density functional theory. The results indicate that the sulfur vacancy formation energy increases linearly with increasing sulfur vacancy concentration under different strains. The strain energy and stress of monolayer WS2 with different sulfur vacancy concentrations increase with increasing applied strain in the strain range of -10% to 10%. The band gap of monolayer WS2 decreases with increasing sulfur vacancy concentration under different strains. Moreover, compared with unstrained conditions, 5% compressive strain increases the band gap at a larger vacancy concentration and the case is just opposite at a smaller vacancy concentration, while 5% tensile strain decreases the band gap. The band gap of monolayer WS2 with different sulfur vacancy concentrations firstly increases and then shrinks with increasing applied strain under compressive strain, whereas it decreases monotonically under tensile strain in the strain range of -10% to 10%. In the visible-light wavelength region, the out-of-plane absorption coefficient under different strains increases with increasing sulfur vacancy concentration. Furthermore, 5% compressive strain enhances the absorption coefficient and 5% tensile strain decreases the absorption coefficient. Hence, the synergistic effects of sulfur vacancy defects and mechanical strain in monolayer TMDs can open new avenues for their applications in nanoelectronic and optoelectronic devices.
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| 3. |
- Li, Shujun, et al.
(författare)
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A representative volume element based on translational symmetries for FE analysis of cracked laminates with two arrays of cracks
- 2009
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 46:7-8, s. 1793-1804
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Tidskriftsartikel (refereegranskat)abstract
- A methodology is proposed for the construction of a representative volume element (RVE) for analysis of laminated composites containing two arrays of ply cracks running in different directions. The only requirement is that the cracks in any ply are uniformly spaced, and if more than one ply of a given orientation is cracked, then the crack spacing of individual plies must only be in exact multiples of each other. The spacing of cracks in the two directions can be fully independent. The RVE is constructed through a systematic consideration of translational symmetries present in the cracked laminate. As a result, the boundary conditions on the RVE can be imposed without compromising accuracy. Examples of the application of the RVE methodology are given to illustrate its broad capability and a finite element (FE) stress analysis is performed for these cases to illustrate results such as the crack surface displacements, local stress fields and RVE-averaged elastic properties. For one case, the average properties are compared with experimental results, showing good agreement.
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| 4. |
- Pakharenko, Viktoriya, et al.
(författare)
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Chemical and molecular structure transformations in atomistic conformation of cellulose nanofibers under thermal environment
- 2022
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Ingår i: npj Materials Degradation. - : Springer Nature. - 2397-2106. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- The structural changes of the glucopyranose chain and the chemical compositional response of cellulose nanofibers (CNFs) under thermal exposure (at 190 °C for 5 h) have remained a significant gap in the understanding of the long-term performance of nanocellulose. Herein, CNF films with different chemical compositions were investigated to confirm the structural transformation of glucopyranose (coupling constant of OH groups changed up to 50%) by nuclear magnetic resonance (NMR) analysis. Remarkably, the glucopyranose rings underwent partial dehydration during the thermal exposure resulting in enol formation. This study confirms the chain mobility that could lead to the conformational and dimensional changes of the CNFs during thermal exposure. The broad range of conformations was defined by the dihedral angles that varied from ±27° to ±139° after thermal exposure. Investigation into the mechanism involving chemical transformation of the substrates during heating is important for the fabrication of the next generation of flexible electrical materials.
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| 5. |
- Pakharenko, Viktoriya, et al.
(författare)
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Thermoconformational Behavior of Cellulose Nanofiber Films as a Device Substrate and Their Superior Flexibility and Durability to Glass
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:34, s. 40853-40862
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Tidskriftsartikel (refereegranskat)abstract
- The design and high-throughput manufacturing of thin renewable energy devices with high structural and atomic configurational stability are crucial for the fabrication of green electronics. Yet, this concept is still in its infancy. In this work, we report the extraordinary durability of thin molecular interlayered organic flexible energy devices based on chemically tuned cellulose nanofiber transparent films that outperform glass by decreasing the substrate weight by 50%. The nanofabricated flexible thin film has an exceptionally low thermal coefficient of expansion of 1.8 ppm/K and a stable atomic configuration under a harsh fabrication condition (over 190 °C for an extended period of 5 h). A flexible optoelectronic device using the same renewable cellulose nanofiber film substrate was found to be functionally operational over a life span of 5 years under an intermittent operating condition. The success of this device’s stability opens up an entirely new frontier of applications of flexible electronics.
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| 6. |
- Singh, Chandra Veer, et al.
(författare)
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A multiscale approach to modeling of composite damage
- 2016
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Ingår i: Modeling damage, fatigue and failure of composite materials. - Cambridge : Woodhead Publishing Limited. - 9781782422983 ; , s. 329-345
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Bokkapitel (refereegranskat)abstract
- Damage in composite materials initiates at the length scale of one or a few fiber diameters, governed by the local stress fields. Further progression of the failure events is governed by conditions existing in a material volume representative of geometrical aspects such as fiber orientation and thickness of the plies, as well as the extent of stress field perturbations caused by damage entities. Failure of a composite structure occurs at attainment of a critical state in its response related to the designed functionality. Assessment of failure must therefore involve analyzing failure events from initiation until the relevant criticality state, with proper account of the length scales at which the respective failure mechanisms occur. Approaches for this purpose are necessarily of a multiscale nature. This chapter discusses a particular approach that incorporates the micro-, meso-, and macroscales in one single framework and is aimed at describing the deformational response of multidirectional composite laminates with multiple cracking in different orientations.
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| 7. |
- Singh, Chandra Veer, et al.
(författare)
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A synergistic damage mechanics approach for composite laminates with matrix cracks in multiple orientations
- 2009
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Ingår i: Mechanics of materials. - : Elsevier BV. - 0167-6636 .- 1872-7743. ; 41:8, s. 954-968
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Tidskriftsartikel (refereegranskat)abstract
- This paper treats the problem of elastic response of composite laminates containing matrix cracks in plies of multiple orientations. The approach taken has been described as synergistic damage mechanics (SDM) and has been previously illustrated for [0 m / ± θ n / 0 m / 2] s laminates with cracks of equal density in + θ and - θ plies [Singh, C.V., Talreja, R., 2008. Int. J. Solids Struct. 45(16), 4574-4589]. The current work extends the approach to [0 m / ± θ n / 90 r] s and [0 m / 90 r / ± θ n] s laminates with cracks additionally in the 90°-plies. The interaction between the ± θ-cracks and the 90°-cracks is analyzed in terms of the crack surface displacements using a three-dimensional finite element (FE) model and found to be significant only for crack orientations close to 90°. The stiffness degradation of the laminate with all cracking modes simultaneously present is formulated by continuum damage mechanics using a second order tensor characterization of damage. The elastic moduli changes predicted by the SDM procedure are validated by independent three-dimensional FE calculations. For a particular case of quasi-isotropic [0 / 90 / ∓ 45] s laminate, the elastic moduli predictions are evaluated against experimental data. Finally, a parametric study is performed to examine the effects of ply thickness changes on stiffness properties.
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| 8. |
- Singh, Chandra Veer, et al.
(författare)
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A synergistic damage mechanics approach to mechanical response of composite laminates with ply cracks
- 2013
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Ingår i: Journal of composite materials. - : SAGE Publications. - 0021-9983 .- 1530-793X. ; 47:20-21, s. 2475-2501
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Tidskriftsartikel (refereegranskat)abstract
- We treat selected test cases in the third world wide failure exercise by the approach described as synergistic damage mechanics. This approach utilizes micromechanics and continuum damage mechanics to predict the overall mechanical response of composite laminates with ply cracking in multiple orientations. The material constants needed in the continuum damage mechanic formulation are calculated from stiffness property changes incurred in a reference laminate. For other laminate configurations, the stiffness changes are derived using a relative constraint parameter which is calculated from the constraint on the opening displacement of ply cracks within the given cracked laminate evaluated numerically by a finite element analysis of appropriately constructed representative unit cell. The number density of ply cracks (cracks per unit length normal to the crack planes) under quasi-static loading is calculated by an energy-based approach. Finally, the stress–strain response of a laminate is determined by combining stiffness property changes and evolution of crack number density.
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| 9. |
- Singh, Chandra Veer, et al.
(författare)
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Analysis of multiple off-axis ply cracks in composite laminates
- 2008
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 45:16, s. 4574-4589
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a synergistic methodology to analyze damage behavior in composite laminates with transverse matrix cracks in plies of multiple orientations. The approach combines the strengths of micro-damage mechanics (MDM) and continuum damage mechanics (CDM) in predicting the stiffness degradation due to presence of transverse cracks. The micromechanics is performed on a representative unit cell using a three-dimensional finite element analysis to calculate the crack opening displacement (COD) accounting for the influence of the surrounding plies, the so-called constraint effect. This information is then incorporated in the CDM formulation dealing with laminates containing cracks in different ply orientations through a 'constraint parameter'. In CDM, a separate damage mode is defined for each type of crack and the expressions for engineering moduli of the damaged laminate are derived in terms of crack density and the constraint parameter. The COD and stiffness degradation predictions agree well with published experimental data for [0 / ± θ 4 / 0 1 / 2] s laminate configuration. To enable damage analysis of other configurations of [0 m / ± θ n / 0 m / 2] s laminate, a parametric study of the CODs is performed and using the computations a master equation is developed
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| 10. |
- Singh, Chandra Veer, et al.
(författare)
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Damage mechanics of composite laminates with transverse matrix cracks in multiple orientations
- 2007
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Ingår i: A collection of technical papers. - Reston, Va. : American Institute of Aeronautics and Astronautics, AIAA. - 9781563478925 ; , s. 7047-7056
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Konferensbidrag (refereegranskat)abstract
- This paper presents a synergistic methodology to analyze damage behavior in composite laminates with transverse matrix cracks in plies of multiple orientations. The approach combines the strengths of micromechanics and continuum damage mechanics (CDM) to evaluate constraint of undamaged plies over cracked plies and this information is suitably carried through the constraint parameter in the formulation to predict stiffness reduction. The damage state in laminate with cracks in off-axis plies is characterized through CDM formulation by defining a separate damage mode for each type of crack. The methodology is validated for [0/±θ 4/0 1/2] s laminate with experimental results obtained in a previous work by Varna et al. The results show remarkable agreement with experiments. Parametric studies are performed to evaluate the constraint effects of neighboring 0-deg plies over cracked ±θ plies. Stiff outer plies are found to reduce the opening of cracks in inner off-axis plies resulting in less severe degradation in stiffness properties of the composite laminate.
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