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Gerbrand Ceder - One of the best experts on this subject based on the ideXlab platform.
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the thermodynamics of Decohesion
Acta Materialia, 2004Co-Authors: Gerbrand CederAbstract:We present a thermodynamic description of Decohesion that provides a link between first principles studies of Decohesion and cohesive zone models used in continuum simulations of crack growth. The properties of a cohesive zone are described by thermodynamic excess variables extracted from first-principles calculations. Applied to Decohesion of fcc aluminum, we find that the excess energy for Decohesion along adjacent (1 1 1) planes is well described by the universal binding relation of Rose et al. [Phys. Rev. Lett. 47 (1981) 675; Phys. Rev. B 28 (1983) 1835]. We also present a first principles model to investigate the effect of impurity atoms on slow Decohesion when the impurity chemical potential can remain constant by modifying the impurity concentration in the decohering zone. In studying the effect of hydrogen or oxygen impurities on Decohesion of aluminum along a pair of (1 1 1) planes, the model predicts a Van der Waals transition above a critical impurity chemical potential. This transition involves the saturation with impurity atoms of the region between the decohering planes and leads to a dramatically reduced maximum stress for Decohesion.
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impurity induced van der waals transition during Decohesion
Physical Review B, 2003Co-Authors: Gerbrand CederAbstract:Current understanding of the mechanisms leading to me- chanical failure of materials is far from complete. This has its origin in the immense complexity associated with an event such as fracture in which many phenomena operate at varying length scales. While dislocation nucleation and glide often play an important role, it is the propagation of cracks that ultimately leads to fracture. Crack growth results in the separation of atomic planes either along grain boundaries or through grains. Hence efforts have been made to relate the atomic-level description of bond breaking to fracture resis- We present a first-principles thermodynamic model to study the effect of impurities at constant chemical potential on the traction curve of a uniformly decohering solid. We apply it to Decohesion of fcc aluminum along a pair of ~111! planes accounting for the presence of hydrogen or oxygen atoms in the decohering region at constant chemical poten- tial. We find that while the tractions at constant impurity concentration have a form similar to those of previously ex- plored systems, 1 a constant impurity chemical potential can lead to a discontinuity in the relation between Decohesion distance and stress. This first-order transition in the stress- displacement diagram, reminiscent of a van der Waals tran- sition, leads to an abrupt drop in the maximal cohesive stress once a critical impurity chemical potential is exceded. In- stead of a gradual change in the resistance to crack growth with increasing impurity chemical potential, this result indi- cates that an abrupt change in crack growth mechanism should occur at a characteristic impurity chemical potential. The traction curve for Decohesion can be obtained as the derivative of the energy of a solid as two slabs of bulk ma- terial are uniformly separated along a pair of adjacent atomic planes. This stress-separation relation then serves as a first- principles input to describe the resistance to crack growth of the cohesive zone in continuum simulations of fracture. 3,4 Impurity atoms can segregate between the decohering atomic planes. Calculating the energy of Decohesion as a function of slab separation from first principles is straightforward if the impurity concentration and arrangement between the separat- ing atomic planes is kept fixed. 5-7 Obtaining the traction curve at constant impurity chemical potential though requires additional thermodynamic considerations. Thermodynamically, the decohering region can be consid- ered as a subregion of the solid characterized by excess ex- tensive quantities 8 as in the standard thermodynamic descrip- tion of surface properties. The excess internal energy for the decohering region per unit area ~of the separating atomic planes! u is related to the other excess extensive quantities according to
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Impurity-induced van der Waals transition during Decohesion
Physical Review B - Condensed Matter and Materials Physics, 2003Co-Authors: Anton Van Der Ven, Gerbrand CederAbstract:We investigate the thermodynamics of impurity segration between the separating atomic planes of a decohering solid from first principles. We find that the traction curve for Decohesion at constant impurity chemical potential differs qualitatively from that at constant impurity concentration. In fact, for hydrogen and oxygen segregation between separating (111) planes during Decohesion of fcc aluminum, a first-order van der Waals transition is predicted above a critical impurity chemical potential that results in a dramatic drop in the maximum stress of Decohesion.
Liang Tian - One of the best experts on this subject based on the ideXlab platform.
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High temperature damping behavior and dynamic Young's modulus of magnesium matrix composite reinforced by Ti 2 AlC MAX phase particles
Mechanics of Materials, 2019Co-Authors: Wenbo Yu, Maxime Vallet, Xiaobo Li, Liang TianAbstract:The evolutions with the temperature of damping capacities and dynamic Young's moduli of ternary Ti 2 AlC particles reinforced AZ91D composites were investigated. This work has highlighted that Ti 2 AlC significantly contributes to the damping capacity and to dynamic moduli of composites. With increasing temperature, especially above 200 °C, the interfacial damping capacity becomes dominant and dynamic moduli of composites decrease. These trends were revealed from the transformed tensile fracture mechanisms. At room temperature, Ti 2 AlC particles delaminated without any Decohesion between Ti 2 AlC particles and the Mg matrix. However, the interfacial Decohesion occurred above 200 °C. One explanation of this change of Decohesion mechanism could be ascribed to the inherent structure of the Ti 2 AlC MAX phase and to the formation of a robust amorphous magnesium layer at the interface between Ti 2 AlC and magnesium matrix that was evidenced by HRTEM. This amorphous layer further gives rise a high activation energies (H) of damping peaks for composites, which was calculated to 128 kJ/mol. This value is higher than those calculated for SiC or graphite reinforced AZ91D composites, making Ti 2 AlC particles particularly interesting for application in reinforcing magnesium composites.
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High temperature damping behavior and dynamic Young's modulus of magnesium matrix composite reinforced by Ti2AlC MAX phase particles
Mechanics of Materials, 2019Co-Authors: Wenbo Yu, Maxime Vallet, Xiaobo Li, Liang TianAbstract:Abstract The evolutions with the temperature of damping capacities and dynamic Young's moduli of ternary Ti2AlC particles reinforced AZ91D composites were investigated. This work has highlighted that Ti2AlC significantly contributes to the damping capacity and to dynamic moduli of composites. With increasing temperature, especially above 200 °C, the interfacial damping capacity becomes dominant and dynamic moduli of composites decrease. These trends were revealed from the transformed tensile fracture mechanisms. At room temperature, Ti2AlC particles delaminated without any Decohesion between Ti2AlC particles and the Mg matrix. However, the interfacial Decohesion occurred above 200 °C. One explanation of this change of Decohesion mechanism could be ascribed to the inherent structure of the Ti2AlC MAX phase and to the formation of a robust amorphous magnesium layer at the interface between Ti2AlC and magnesium matrix that was evidenced by HRTEM. This amorphous layer further gives rise a high activation energies (H) of damping peaks for composites, which was calculated to 128 kJ/mol. This value is higher than those calculated for SiC or graphite reinforced AZ91D composites, making Ti2AlC particles particularly interesting for application in reinforcing magnesium composites.
Anton Van Der Ven - One of the best experts on this subject based on the ideXlab platform.
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Impurity-induced van der Waals transition during Decohesion
Physical Review B - Condensed Matter and Materials Physics, 2003Co-Authors: Anton Van Der Ven, Gerbrand CederAbstract:We investigate the thermodynamics of impurity segration between the separating atomic planes of a decohering solid from first principles. We find that the traction curve for Decohesion at constant impurity chemical potential differs qualitatively from that at constant impurity concentration. In fact, for hydrogen and oxygen segregation between separating (111) planes during Decohesion of fcc aluminum, a first-order van der Waals transition is predicted above a critical impurity chemical potential that results in a dramatic drop in the maximum stress of Decohesion.
Reinhold H Dauskardt - One of the best experts on this subject based on the ideXlab platform.
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Decohesion kinetics in polymer organic solar cells
ACS Applied Materials & Interfaces, 2014Co-Authors: Christopher Bruner, Fernando Novoa, Stephanie R Dupont, Reinhold H DauskardtAbstract:We investigate the role of molecular weight (MW) of the photoactive polymer poly(3-hexylthiophene) (P3HT) on the temperature-dependent Decohesion kinetics of bulk heterojunction (BHJ) organic solar cells (OSCs). The MW of P3HT has been directly correlated to its carrier field effect mobilities and the ambient temperature also affects OSC in-service performance and P3HT arrangement within the BHJ layer. Under inert conditions, time-dependent Decohesion readily occurs within the BHJ layer at loads well below its fracture resistance. We observe that by increasing the MW of P3HT, greater resistance to Decohesion is achieved. However, failure consistently occurs within the BHJ layer representing the weakest layer within the device stack. Additionally, it was found that at temperatures below the glass transition temperature (∼41–45 °C), Decohesion was characterized by brittle failure via molecular bond rupture. Above the glass transition temperature, Decohesion growth occurred by a viscoelastic process in the B...
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Decohesion kinetics of pedot pss conducting polymer films
Advanced Functional Materials, 2014Co-Authors: Stephanie R Dupont, Fernando Novoa, Eszter Voroshazi, Reinhold H DauskardtAbstract:The highly conductive polymer PEDOT:PSS is a widely used hole transport layer and transparent electrode in organic electronic devices. To date, the mechanical and fracture properties of this conductive polymer layer are not well understood. Notably, the Decohesion rate of the PEDOT:PSS layer and its sensitivity to moist environments has not been reported, which is central in determining the lifetimes of organic electronic devices. Here, it is demonstrated that the Decohesion rate is highly sensitive to the ambient moisture content, temperature, and mechanical stress. The kinetic mechanisms are elucidated using atomistic bond rupture models and the Decohesion process is shown to be facilitated by a chemical reaction between water molecules from the environment and strained hydrogen bonds. Hydrogen bonds are the predominant bonding mechanism between individual PEDOT:PSS grains within the layer and cause a significant loss in cohesion when they are broken. Understanding the Decohesion kinetics and mechanisms in these films is essential for the mechanical integrity of devices containing PEDOT:PSS layers and yields general guidelines for the design of more reliable organic electronic devices.
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The effect of anneal, solar irradiation and humidity on the adhesion/cohesion properties of P3HT:PCBM based inverted polymer solar cells
2012 38th IEEE Photovoltaic Specialists Conference, 2012Co-Authors: Stephanie R Dupont, Eszter Voroshazi, Paul Heremans, Reinhold H DauskardtAbstract:We use a thin-film adhesion technique that enables us to precisely measure the energy required to separate adjacent layers in OPV cells. We demonstrate the presence of weak interfaces in prototypical inverted polymer solar cells, either prepared by spin, spray or slot-die coating, including flexible and non flexible solar cells. In all cases, we observed adhesive failure at P3HT:PCBM/PEDOT:PSS interface, indicating the intrinsic material dependence of this mechanism. The impact of temperature, solar irradiation and humidity on the adhesion and cohesion properties of this particular interface is discussed. First, we have found that post-deposition annealing increases the adhesion significantly. Annealing changes the morphology in the photoactive layer and consequently alters the chemical properties at the interface. Second, solar irradiation on fully encapsulated solar cells has no damaging but in contrast an enhancing effect on the adhesion properties, due to the heat generated from IR radiation. Finally, the synergetic effect of stress and an environmental species like moisture greatly accelerates the Decohesion rate in the weak hygroscopic PEDOT:PSS layer. This results in a loss of mechanical integrity and device performance. The insight into the mechanisms of delamination and Decohesion yields general guidelines for the design of more reliable organic electronic devices.
F P E Dunne - One of the best experts on this subject based on the ideXlab platform.
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competing mechanisms of particle fracture Decohesion and slip driven fatigue crack nucleation in a pm nickel superalloy
International Journal of Fatigue, 2020Co-Authors: Alexander Bergsmo, F P E DunneAbstract:Abstract Fatigue cracks may initiate around non-metallic inclusions via particle fracture, particle Decohesion and slip-driven nucleation. Cohesive zone techniques within microstructurally faithful crystal plasticity modelling validated by micromechanical experiments (HR-DIC and HR-EBSD) are employed to investigate these nucleation phenomena. Particle fracture and Decohesion lead to stress redistribution which influences subsequent energy storage driving slip-driven fatigue crack nucleation. Particle fracture and Decohesion strengths were determined and using a stored energy criterion, the number of cycles to initiation of the fatigue microcrack was predicted. A threshold applied stress below which Decohesion and fracture do not occur was obtained, thus modestly increasing fatigue life.
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crack nucleation using combined crystal plasticity modelling high resolution digital image correlation and high resolution electron backscatter diffraction in a superalloy containing non metallic inclusions under fatigue
Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 2016Co-Authors: Tiantian Zhang, Jun Jiang, Ben Britton, Barbara A Shollock, F P E DunneAbstract:A crystal plasticity finite-element model, which explicitly and directly represents the complex microstructures of a non-metallic agglomerate inclusion within polycrystal nickel alloy, has been developed to study the mechanistic basis of fatigue crack nucleation. The methodology is to use the crystal plasticity model in conjunction with direct measurement at the microscale using high (angular) resolution-electron backscatter diffraction (HR-EBSD) and high (spatial) resolution-digital image correlation (HR-DIC) strain measurement techniques. Experimentally, this sample has been subjected to heat treatment leading to the establishment of residual (elastic) strains local to the agglomerate and subsequently loaded under conditions of low cyclic fatigue. The full thermal and mechanical loading history was reproduced within the model. HR-EBSD and HR-DIC elastic and total strain measurements demonstrate qualitative and quantitative agreement with crystal plasticity results. Crack nucleation by interfacial Decohesion at the nickel matrix/agglomerate inclusion boundaries is observed experimentally, and systematic modelling studies enable the mechanistic basis of the nucleation to be established. A number of fatigue crack nucleation indicators are also assessed against the experimental results. Decohesion was found to be driven by interface tensile normal stress alone, and the interfacial strength was determined to be in the range of 1270–1480 MPa.
