Bauschinger effect in thin metallic films by fem simulations

Unpassivated free-standing gold and aluminum thin films (thickness ~ 200-400 nm, mean grain size dm,Au≈ 70-80nm, dm,Al≈ 120-200nm), subjected to tensile tests show Bauschinger effect (BE) during unloading [1, 2]. The focus of this work is to investigate the effect of microstructural heterogeneity such as grain sizes on the BE and the macroscopic deformation behavior in thin metallic films. The finite element code LAGAMINE is used to model the response of films involving sets of grains with different strengths. The numerical results are compared with experimental results from tensile tests on aluminum thin films from the work of Rajagopalan, et al. [2]

Subcritical crack growth in freestanding silicon nitride and silicon dioxide thin films using residual stress-induced crack on-chip testing technique

Thin film materials are ubiquitous in a large number of applications like flexible electronics, microelectromechanical / nanoelectromechanical systems (MEMS/NEMS) and functional coatings. In the present work, a new mechanical testing method on a chip is developed to characterize the fracture behavior of freestanding thin films. This on-chip technique is based on the residual stress inside what is called here actuator material. Two beams are fabricated with the actuator film and attached to a specimen, incorporating a notch induced by lithography. The residual stress upon release by chemical etching leads to the actuator contraction, hence pulling on the central notched specimen. A crack is initiated at the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value. This crack arrest measurement avoids the problem of introducing a sufficiently sharp precrack. Besides, using a freestanding film leads to extract the real intrinsic fracture resistance of the film without any substrate effect. By tracking the crack length growth over different time intervals as well as environments using this crack on-chip testing method, the subcritical crack growth mechanisms can be investigated without monopolizing any test equipment. Thin film materials that are showing time-dependent failure are used in numerous devices that its reliability is determined by the understanding of the mechanisms causing the subcritical crack growth. Low-pressure chemical vapor deposition (LPCVD) silicon nitride (SiN) and silicon dioxide (SiO2) films deposited by electron beam-evaporation technique are studied with a variety of thicknesses. The specimens are tested in laboratory air and dry nitrogen environments under various temperature conditions. The stress intensity factor (K) and the crack velocity (v); K-v curve in different environments is determined based on both experimental data and finite element simulation results (FE), following classical exponential law

Evaluation of the properties of a new circular building composite material to upcycle building wastes

peer reviewedA new circular composite material for building applications is developed, made of two secondary raw materials coming from waste recycling: fibers and sand. Hydraulic lime is added as a binder. This new composite targets a low environmental impact thanks to benefits of upcycling buildings waste, low energy production process, lifetime up to 60 years and of a high potential of reversibility, reuse and upcycling. The research focused on mechanical and physical properties, as well as analysis of the microstructure by X-ray 3D microtomography and in-situ compression testing. The mechanical and physical test results show good and unexpected properties: a density of 390 to 1300 kg/m³; a compressive strength between 0.2 and 2.2 MPa, a bending strength of 0.1 to 1.9 MPa and a thermal conductivity of 0.06 to 0.14 W/mK. Further research will focus on circular construction and environmental aspects. Three applications are envisioned, according the standards of the building sector9. Industry, innovation and infrastructure11. Sustainable cities and communities12. Responsible consumption and production13. Climate actio

Band gap reduction in highly-strained silicon beams predicted by first-principles theory and validated using photoluminescence spectroscopy

A theoretical study of the band gap reduction under tensile stress is performed and validated through experimental measurements. First-principles calculations based on density functional theory (DFT) are performed for uniaxial stress applied in the [001], [110] and [111] directions. The calculated band gap reductions are equal to 126, 240 and 100 meV at 2$\%$ strain, respectively. Photoluminescence spectroscopy experiments are performed by deformation applied in the [110] direction. Microfabricated specimens have been deformed using an on-chip tensile technique up to ~1$\%$ as confirmed by back-scattering Raman spectroscopy. A fitting correction based on the band gap fluctuation model has been used to eliminate the specimen interference signal and retrieve reliable values. Very good agreement is observed between first-principles theory and experimental results with a band gap reduction of, respectively, 93 and 91 meV when the silicon beam is deformed by 0.95$\%$ along the [110] direction

Effect of hydriding on nanoscale plasticity mechanisms in nanocrystalline palladium thin films

Thin palladium (Pd) membranes constitute an enabling material in hydrogen permeation and sensing applications. During hydriding of Pd, as long as the H/Pd (atomic ratio) stays below αSSmax≈0.02, the α-Pd with face centered cubic (fcc) lattice will expand from 3.889 Å to 3.895 Å. When the ratio reaches 0.02 a β-phase, again fcc based, having a lattice constant near 4.025 Å appears which induces a 10% volume change. In the present work, nanoscale plasticity mechanisms activated in sputtered nanocrystalline (nc) Pd thin films subjected to hydriding at different hydrogen pressures have been investigated for the first time using advanced TEM. The in-situ measurement of the evolution of the internal stress during hydriding shows that the internal stress increases rapidly and reaches a constant value of 120 MPa tensile stress for α phase and 920 MPa compressive stress for β phase transformation. The automated crystallographic orientation mapping in TEM (ACOM-TEM) before and after hydriding to α and β phase did not reveal significant changes of the grain size and the crystallographic texture, excluding grain boundary mediated processes as dominant hydrogen induced plasticity mechanisms. High resolution TEM (HRTEM) investigation of ∑3 {111} coherent twin boundaries (TBs) in Pd films shows clear loss of the coherency of these boundaries after hydriding to β phase. However, significant changes of microstructure have not been observed in Pd films hydrated to α phase. These results confirm that hydrogen induced plasticity is mainly controlled by dislocation activity at higher hydrogen pressures. Surprisingly, an fcc→9R phase transformation at Σ3 {112} incoherent TBs as well as a high density of stacking faults (SFs) (Fig. 1a) have been observed after hydriding to β phase indicating a clear effect of hydrogen on the stacking fault energy of Pd. Shear type faulted loops rarely reported in nc materials were also observed within the Pd grains after hydriding to β-phase (Fig. 1b). In order to investigate the stability of this shear type loops, different internal stress fields originating from the neighboring dislocation (dislocation d3 ) and surface effects (image forces) have been computed using a Finite Element method (Fig. 1c). Such calculations confirm that high attractive forces exist between the dislocation “d2” and “d3” forming the dipole. On the other hand, although the Peach Koehler force on the dislocation “d1” tends to extend the SF, the force magnitude is much smaller than the force induced by the fault on the partial segments. Therefore, an extra shear stress of +385MPa (τdis.) acting on the glide plane of the dislocation “d1” is required in order to counter balance the attractive force of the SF which thus explains the stability of this dislocation in the TEM thin foil after dehydriding. This shear stress can not be compensated by the negligible image force in such thin foil. Moreover, no residual hydrides were detected using high resolution electron energy loss spectroscopy. Therefore, the stability of glissile intrinsic SF loops in nc Pd films after dehydriding can thus be attributed to the presence of large internal stress heterogeneities typical of nc materials

Graphene effect on mechanical response of copper film

This research is investigated the effect of the presence of a single layer graphene on the development of the contact plasticity inside a copper underlying substrate. As a matter of fact, a film of copper (deposited on a Si wafer) is the substrate used in the CVD process for graphene production, there is no need for transferring graphene which avoids any possible artifacts. Moreover, the adhesion between CVD-grown graphene and the underlying Cu film is larger than transferred graphene, since during transfer, wrinkles and ripples may form, thus weakening the interaction between graphene and the substrate. The bare Cu-film in the same condition as to produce graphene except that no methane was introduced into the chamber (the last step in graphene production). Nanoindentation was performed on the Cu-film with and without graphene. Nanoindentation was performed on the bare Cu-film also Cu-film with graphene. The same process, as the growth of graphene on Cu-film, was performed on bare Cu just without introducing the methane flow at the last step. The analysis of the force-displacement curves indicates that the presence of graphene modifies the onset of plasticity which appears in the form of a burst which is called pop-in. The first pop-in occurs at lower loads and the pop-in lengths are smaller with graphene in comparison to the bare Cu-film. The magnitude of the effect of the presence of a graphene cap layer varies also with respect to the orientation of the indented Cu grain. In order to understand the root causes of these effects of the presence of graphene on the plastic flow, transmission electron microscopy is used to compare samples after nanoindentation in terms of dislocation structures. 3D discrete dislocation dynamics simulations are performed to analyze the long-range back stress that are generated by the dislocation arrangements with and without graphene. To further extend this research and investigate the known effect of hardening by graphene insertion into metals, another system has been addressed which involves the deposition of a Cu film on top of the graphene layer, lying itself on top of the annealed Cu substrate. The presence of graphene caused marked effect on the indentation response in this case, even larger than in the first configuration

Multifunctional sandwich structure for electromagnetic absorption and mechanical performances

A sandwich panel based on a multiscale architectured material is developed for structural and EM absorption performances. At the nanoscale level, carbon nanotubes are dispersed in a polymer to obtain a conductive material. This composite is then foamed into a micro porous solid to improve EM absorption and to decrease the density. The foam is inserted in a millimeter scale hexagonal metallic honeycomb lattice. The combination of the metallic honeycomb and the polymeric foam provides high bending, impact and crushing performances and a moderate thermal conductivity. This hybrid is used as core for sandwich panels, produced by the addition of two EM transparent face-sheets made of glass fiber reinforced polymers. EM absorption around 90% is achieved in the 10-40 GHz frequency band with a 8.8 mm thick sandwich panel

A maximum stress at a distance criterion for the prediction of crack propagation in adhesively-bonded joints

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Strain gradient plasticity analysis of the strength and ductility of thin metallic films using an enriched interface model

The mechanical response of thin metallic films is simulated using a two-dimensional strain gradient plasticity finite-element model involving grain boundaries in order to investigate the effect of the thickness, grain shape and surface constraint on the strength, ductility and back-stress. The grain boundaries and surface layers are modeled as initially impenetrable to dislocations while allowing for relaxation at a critical stress level. The model captures the variation of the strength with grain size, film thickness, and with the presence or not of constraining surface layers, in agreement with experimental results on Al and Cu films. A decrease in the uniform elongation is predicted with decreasing film thickness due to a loss of strain-hardening capacity and the possible presence of imperfections. These two effects dominate over the stabilizing contribution of the plastic strain gradients. Accounting for the relaxation of the interface constraint affects the magnitude of the back-stress as well as the drop in ductility.Institute of Mechanics, Materials and Civil Engineering, Universite´ catholique de Louvain, 1348 Louvain-la-Neuve, Belgium b Universite´ Libre de Bruxelles, Building, Architecture & Town Planning Dept. (BATir) CP 194/02, Avenue F.D. Roosevelt 50, 1050 Bruxelles, Belgiu

Kinked silicon nanowires-enabled interweaving electrode configuration for lithium-ion batteries

A tri-dimensional interweaving kinked silicon nanowires (k-SiNWs) assembly, with a Ni current collector co-integrated, is evaluated as electrode configuration for lithium ion batteries. The large-scale fabrication of k-SiNWs is based on a procedure for continuous metal assisted chemical etching of Si, supported by a chemical peeling step that enables the reuse of the Si substrate. The kinks are triggered by a simple, repetitive etch-quench sequence in a HF and H2O2-based etchant. We find that the inter-locking frameworks of k-SiNWs and multi-walled carbon nanotubes exhibit beneficial mechanical properties with a foam-like behavior amplified by the kinks and a suitable porosity for a minimal electrode deformation upon Li insertion. In addition, ionic liquid electrolyte systems associated with the integrated Ni current collector repress the detrimental effects related to the Si-Li alloying reaction, enabling high cycling stability with 80% capacity retention (1695 mAh/gSi) after 100 cycles. Areal capacities of 2.42 mAh/cm2 (1276 mAh/gelectrode) can be achieved at the maximum evaluated thickness (corresponding to 1.3 mgSi/cm2). This work emphasizes the versatility of the metal assisted chemical etching for the synthesis of advanced Si nanostructures for high performance lithium ion battery electrodes