查看更多>>摘要：The various high aspect ratio micro/nano structures that are duplicated by the widely applicable micro/nanomolding process would require excellent wettability of the structured mold for full filling of the liquid-like materials into microcavities. Herein, by employing molecular dynamics (MD) simulations, the influence of a monolayer graphene (MLG) on the wetting behaviors of grooved copper is thoroughly investigated, indicating that MLG is an effective approach in modulating the wettability of the structured copper surfaces. Moreover, the extracted outcomes signify that the coating with MLG reduces remarkably the required activation energy for the infiltration of water molecules into the copper groove, thereby triggering the wetting transition from the Cassie-Baxter (CB) to the Wenzel (WZ) state. In addition, by varying the coating locations and dimension of the MLG, it is unveiled that the sidewall coating of MLG plays a crucial role in reducing the free energy barrier needed to surmount the transition state. Interestingly, as the coating height of sidewall MLG increases, the energy barrier is reduced, and the grooved surface becomes more attractive to water molecular, attaining a stable WZ state by employing a height beyond the critical value of MLG coating of 12.4 A in our model. Our work provides useful insights into the tunable wettability of the grooved surfaces by nanocoating engineering, suggesting that MLG can be an effective coating solution in the fabrication of novel micro/nanostructures with high aspect ratios.
查看更多>>摘要：In the MAX phase, the nucleation and motion of basal dislocations dominate the plastic deformation at room temperature. Therefore, information on the dislocation core properties and energy barrier for dislocation glide is essential to understand their plasticity. Dislocations in MAX phases are mainly < a >-type basal edge dislocations (b = a/3[11 (2) over bar0]) and can glide in the basal plane (typical slip) or form kink bands depending on the load direction and crystal orientation owing to the strong plastic anisotropy. In this study, we determined the core structure, Peierls barrier, and corresponding stress of a/3(0001)[11 (2) over bar0] edge dislocations in Ti3AlC2 MAX phase by multiple computational approaches using the semidiscrete variational Peierls-Nabarro method, density functional theory, and classical bond-order-potential-based molecular statics calculations. The a/3[11 (2) over bar0] basal edge dislocation was dissociated into two Shockley partial dislocations with a 2-2.5b-wide stacking fault in between. These partial dislocations glide between the Ti(4f) and Al atomic layer. For straight dislocation motion, the Peierls barrier was single-humped and the corresponding Peierls stress was estimated to be approximately 182 MPa.
查看更多>>摘要：Recent works have paved the way to theoretical predictions of the conditions governing the transition from internal to external oxidation of metals and alloys: such conditions directly result from Wagner (1959)'s classical analytical model, provided that it is made to incorporate a heuristic decrease of diffusion coefficients upon the fraction of oxides, aimed at representing their "barrier effect"upon diffusion. The aim of this paper is to extend these works by removing some of the very restrictive hypotheses introduced by Wagner (1959). First, the formulation initially limited to small fractions of oxides is extended to arbitrarily large fractions. Even in their modified form, the equations are still solvable entirely analytically, albeit with a change of the predicted value of the "critical"fraction of oxides, above which internal oxidation must give way to external oxidation. The new value is in better agreement than previous ones with the scarce available experimental estimates. Second, the formulation is extended to finite - instead of infinitesimal - values of the solubility product governing local chemical equilibrium between the oxide and the chemical elements dissolved in the metallic matrix. The nonlinear equations of the diffusion/precipitation problem then become much more complex and amenable only to some hybrid analytical/numerical solution. The results, although interesting, raise a number of issues essentially tied to the basic hypothesis made of instantaneous local thermodynamic equilibrium. It is finally shown, using a simplistic, prototype kinetic model of oxide precipitation, that relaxation of this hypothesis should permit to solve at least some of these issues.
查看更多>>摘要：This work presents a methodology for predicting a hybrid microstructure and defect processing map for laser powder bed fusion (LPBF) additive manufacturing (AM). A finite element (FE) analysis based thermal model is coupled with a phase field model (PFM) in order to predict both defect formation and microstructure across the LPBF process parameter space. The parameter space is first categorized into defect regions such as keyholing, balling, and lack of fusion. These regions are predicted using the FE thermal model, allowing a defect map to be generated based on the geometries of the melt pools for a defined powder layer thickness. A region of the parameter space that leads to successful AM of defect-free parts is then identified. Segregation across the parameter space is predicted by an integrated computational model, which couples a finite interface dissipation PFM and the aforementioned FE thermal model. The parameter space is then separated into regions that result in cellular or planar microstructures based on the area fraction of cellular-dendritic segregation predicted within the melt pools. By merging the microstructure and defect maps, a hybrid microstructure-defect printability map is generated that distinguishes a process parameter region for fabricating defect-free parts with homogeneous microstructures. This hybrid map is validated with experimental observations for a Ni-5wt.%Nb alloy. The hybrid printability map can be used in the rapid selection and optimization of process parameters for additively manufacturing any alloy (provided models and their parameterization are available, and the alloy is printable), and can potentially help design alloys best suited for AM.
查看更多>>摘要：The microstructure evolution of single crystal Al under shock loading is investigated using both numerical simulations and shock wave theory. Using momentum mirror method with loading velocity in the range of 0.8 similar to 1.2 km/s, the results show the generation of a split two-wave structure, in which the (fcc -> bcc) transition wave and preceding elastic precursor are connected by the plastic zone. The Hugoniot state of the body-centered phase does not deviate from the theoretical Hugoniot curve considering both the lattice vibration and thermal activation of electrons. The metastable bcc phase would change into fcc phase again once the uniaxial stress is released during the unloading stage. For shock loading tests with piston velocity larger than 1.5 km/s, the shock Hugoniot would enter into melting state. While the impact velocity is larger than 4.0 km/s, solitary wave can propagate stably at the shock front. Further analysis of wave velocity show that the competition between the transition wave propagation and dislocation mobility determines the full development of plastic zone. This new finding suggests that the dynamic response of solids might be tuned by engineering the distribution of microstructures to control the dislocation multiplication.
查看更多>>摘要：Molecular dynamics simulation, which is considered an important tool for studying physical and chemical processes at the atomic scale, requires accurate calculations of energies and forces. Although reliable energies and forces can be obtained by electronic structure calculations such as those based on density functional theory this approach is computationally expensive. In this study, we propose a full-stack model using a deep neural network (NN) to enhance the calculation of force and energy. The NN is designed to extract the embedding feature of pairwise interactions of an atom and its neighbors. These are aggregated to obtain its feature vector for predicting atomic force and potential energy. By designing the features of the pairwise interactions, we can control the performance of models. We also consider the many-body effects and other physics of the atomic interactions. Moreover, using the Coulomb matrix of the local structures in complement to the pairwise information, we can improve the prediction of force and energy for silicon systems. Furthermore, the transferability of our models to larger systems is confirmed with high accuracy.
Krasnov, Pavel O.Shkaberina, Guzel S.Polyutov, Sergey P.
8页
查看更多>>摘要：Schwarzites, due to their high porosity, are among prospective materials for the sorption of different gases, including hydrogen. Their surface possesses negative Gaussian curvature that intimately determines how many carbon atoms each hydrogen molecule will interact with, which, in turn, defines the fraction of hydrogen that would be sorbed in the schwarzite. The critical question about contributions to the sorption of the surface topology and electronic effects is solved here. Within the framework of the QTAIM theory, the topological parameters of the electron density distribution function at the bond critical points characterizing the dispersion interaction of the H2 molecule with the carbon surface are estimated. On the example of molecules [6]circulene and [7]circulene, it was shown that, despite the electronic effects arising from the bending of this surface, on average, the energy of physical sorption of hydrogen obtained using MP2 calculations changes insignificantly - by about 0.1 kJ/mol in the case of, for example, vertical orientation of the molecule. By calculating the thermochemical properties by the PM6-D3 method, the dependence of the weight fraction of hydrogen sorbed in P216-schwarzite on the external gas pressure and the temperature has been established. In particular, it was shown that at 300 K and 10 MPa, this value is 4.6%, slightly higher than other carbon nanostructures with similar density, porosity, and accessible surface area values.
查看更多>>摘要：Reactive molecular dynamics was used to simulate the decomposition processes of the HMX nanoparticle (NP) and core-shell structured HMX@Al NP. The results indicate that the decomposition of HMX@Al NP is earlier than that of the HMX NP. As the reaction proceeded, the Al shell underwent a process of melting-reactionaggregation from a surface shell structure to a bulk clusters. The amount of NO, NO2, N2, H2O, and CO2 produced by the HMX@Al NP is lower than that by the HMX NP since the high active Al atoms are easy to react with these products to form a series of aluminized clusters such as AlmNn, AlmCn, and AlmOn. Besides, the C clusters in the HMX@Al system are larger, indicating that Al is helpful for the growth of C clusters. The solid decomposition products of HMX@Al NP are mainly AlmOn and C clusters, consistent with the experimental report that the solid combustion matter of the HMX-based aluminum propellant is mainly composed of Al, O, and a small amount of C clusters. This work may provide a theoretical basis for the reaction mechanism of energetic aluminized composites and has a guiding significance for the design of high energy aluminized explosives.
查看更多>>摘要：The chloride and sulphate attacks, either individually or parallelly, are the main reasons for the corrosion of the rebar. In this study, the interactions between chloride or/and sulphate and the Fe (100) surface were simulated by using the density functional theory to reveal the mechanism of rebar corrosion led by the chloride-sulphate adsorption. The adsorption of lone chloride and sulphate on the Fe (100) surface has been modelled separately. The results show that the interaction of the iron surface with the sulphate is stronger than that with the chloride. The adsorption of sulphate causes more electron loss of the iron surface. In the case of combined chloride sulphate attack, these two ions mutually compete and thereby, weaken the further interactions with the iron surface. This work may provide guidance for studying the corrosion performance of rebar when subjected to the combined chloride-sulphate attack.
查看更多>>摘要：Polymorphism is a very common phenomenon for optoelectronic functional materials. The in-depth study of polymorph is an important basis for understanding the structure-activity relationship, polymorph effect and multifunctional application of optoelectronic functional materials. In this work, the crystal microstructure, electronic structure and optical properties of 9 polytypes of CuAlO2 are compared and studied in detail by using density functional theory calculation. We found that the structural motif and its connection mode in the polytype determine the stability of crystal phase and the main characteristics of electronic structure. The coordination environment of Cu is the key factor to determine the stability of crystal phase: the delafossite polytype containing two-fold-coordinated Cu is the most stable; followed by chalcopyrite and wurtzite polytype containing four-fold-coordinated Cu; the polytypes containing high-fold-coordinated Cu or more complex connection mode of motif are the least stable. Moreover, the binding energy and stability of these nine crystalline polytypes are mainly determined by the coordination number of Cu atom, the degree of deformity of structural motif and the connection mode of structural motif. When CuAlO2 contains linear two-coordinated Cu and is connected by sharing vertices, the absolute value of binding energy is the largest and most stable. At the same time, the type of Cu containing structural motif and the connection mode between structural motifs are the key factors to determine the fundamental band gap of CuAlO2. These 9 polytypes of CuAlO2 have well-defined indirect bandgap semiconductor characteristics. Finally, according to the main characteristics of electronic structures and optical properties, we also predict the potential applications of these polytypes of CuAlO2 in different opto-electronic fields.
NSTL
Elsevier