In this work, the high-temperature thermal stability of nanocrystalline Cr2O3 films on Si wafers deposited at various bias voltages was systematically investigated by means of a symmetrical ...high-resolution thermogravimetric system. In the meantime, the effects of substrate bias voltage on the morphology, microstructure, crack area percentage, phase constituents, and grain size of the heat-treated Cr2O3 films were also studied in detail. The results showed that the Cr2O3 films presented the higher thermal stability in pure nitrogen than in air up to 1200°C because the brittle oxidation product was more prone to cracking and chipping. As the bias voltage was −100V, the Cr2O3 film showed the highest thermal stability which was attributed to its most compact structure and the lowest defect density. After the heat treatments, all the films cracked due to the big difference in thermal expansion coefficient between the Cr2O3 film and Si wafer, which caused large thermal stresses. And some obvious micro-cavities were left in the film cross section after oxidation owing to the vaporization of Cr2O3 in oxygen containing atmosphere. In addition, the heat treatment also had a strong influence on the grain size of the Cr2O3 films.
•High-temperature thermal stability of AIP Cr2O3 films was studied by a high-resolution thermogravimetric system.•The Cr2O3 films presented the higher thermal stability in pure nitrogen than in air.•The Cr2O3 film possessed the highest thermal stability as the substrate bias voltage is −100V.•Heat treatment had a strong influence on the grain size of the Cr2O3 films.
An experimental test facility was developed to perform thermo-mechanical fatigue crack growth experiments. The thermal cycles were generated using hot and cold air flows distributed by a nozzle onto ...the test specimen. With the equipment it was possible to obtain TMF cycles with cycle times of 120
s with less than 10
°C temperature variation over the measurement length of the sample, cycling between 200
°C and 550
°C. The equipment allows TMF crack propagation tests up to, at least, 600
°C. The crack length and the crack closure level were determined using the potential drop technique. Thermo-mechanical fatigue crack propagation experiments were performed in-phase and out-of-phase with various
R-values on samples of Inconel 718. Cylindrical specimens with a small starter crack were cycled in nominal total strain control. The crack propagation rate was determined and the correlation to the effective
J-integral range, Δ
J
eff
, corrected for crack closure presented. The fracture surfaces showed a dominance of trans-granular crack propagation with striations, indicating a low degree of time dependency in the procedure.
Nanocrystalline Cr2O3 thin films were deposited on silicon wafers with (100) orientation by arc ion plating (AIP) technique at various negative bias voltages. By virtue of X-ray diffraction analysis, ...scanning electron microscope, and high-resolution transmission electron microscope, the influence of substrate bias voltage on the film growth process, microstructure, and characteristics was investigated systematically, including the phase constituents, grain size, lattice constant, chemical compositions, as well as surface and cross-section morphologies. With increasing the bias voltage, the grain size and lattice constant of AIP Cr2O3 films first decreased slightly, and then increased gradually again. Both reached the minimum (35nm and 13.57Å) when the bias voltage was −100V. However, the bias voltage had little effect on the phase constituents and chemical compositions of AIP Cr2O3 films. During the film growth process, the surfaces of Cr2O3 films were getting smoother with the negative bias voltage increase, in the meantime, their microstructures evolved from coarse columnar grains to fine columnar grains, short columnar recrystallized grains, and fine columnar grains again.
► Nanocrystalline Cr2O3 thin films were prepared by arc ion plating technique. ► Influence of substrate bias voltage on film growth process, microstructure, and characteristics was studied. ► Grain size and lattice constant of AIP Cr2O3 films were slightly changed by applying different bias voltages. ► Microstructure and surface morphology of AIP Cr2O3 films were greatly altered by substrate bias voltage and ion current density.
•We present numerically evidence that the Poisson’s ratio is size dependent.•We show that the Poisson’s ratio varies with crystallographic orientation.•Materials display negative ratios at nanoscale ...despite positive at macroscale.•For small beams Poisson’s ratios are initially negative, but turn to positive.•Negative ratios exist at the nanoscale and values below −0.5 are possible.
Elastic simulations of single-crystal copper nanobeams, of different cross section sizes and with crystallographic orientations 100 and 110 along their length directions, have been performed applying tensile mechanical loading. The molecular dynamics code LAMMPS was employed for the simulations. The Poisson’s ratio, which is one of the fundamental measures of the elastic deformation behavior of materials, has been determined. In this paper we present numerical evidence that the Poisson’s ratio of nanobeams loaded by finite strains varies with both size and crystallographic orientation. In particular, we provide numerical evidence for that, of the two Poisson’s ratio that naturally can be defined for nanobeams loaded in the 110-direction, one is negative whereas the other one remains almost constant, irrespective of applied strain. We also show that for nanobeams loaded in the 100-direction the values of Poisson’s ratio initially decrease, reaches a minimum and thereafter increase with applied strain. For the smallest 100 cross sections the Poisson’s ratios are initially negative, but turn positive at larger strains.
Molecular dynamic simulations inevitably demand large computational resources for structures of liner measures even as small as a few tens or hundreds of nanometers. Thus, a computationally efficient ...method to simulate larger structures and, at the same time, retain the properties and the mechanical response at the atomic scale is in demand. One such approach is peridynamics, which is a nonlocal extension of continuum mechanics. In this study, we investigate the possibility to efficiently reproduce results from molecular dynamic (MD) simulations by calibration of two parameters inherent in peridynamics: the length scale parameter and the interparticle bond strength. The free-ware LAMMPS supports both numerical approaches, and thus LAMMPS has been used as the common framework. Beams of single-crystal fcc copper of various sizes and under tension along the crystallographic 100- and 110-directions act as the modeling example. The force-displacement curves and the elastic-plastic transitions have been compared between the approaches. The conclusion is that proper calibration of the peridynamic two parameters to MD simulations results in proper reproduction of the molecular dynamic results. This in turn allows for geometrical upscaling or simulation of geometrically more complicated structures, without loss of features derived from the atomic scale but to a much lower computational cost.
We investigate the impact of impurity configuration on electronic and optical properties of YVO4 through application of plane-wave DFT. Since this crystal is a common host material for optical ...devices and it has been widely used as a phosphor and a laser host material, it is important to identify a correlation between impurity location and the intrinsic properties of the material. To further improve the accuracy of the results several semi-local and a hybrid functional are tested. To find the most stable structure among possible configurations of doping, the electronic structures and formation energies of the available configurations are calculated and compared. We report that while the valence is formed by O-2p orbitals and conduction is made of V-3d orbitals in all configurations, the band width varies with the impurity configuration. Additionally, the formation energy is changing depending on where the dopant is located. Then the absorption coefficient and the refractive index are obtained using a modified HSE functional.
•The location of impurity in the host affects the intrinsic properties of the host.•Notably, the band width varies with dopant configuration.•Concentration of an impurity plays a significant role on formation energy.•The effect of dopant configuration on formation energy is small.
We investigate the impact of impurity configuration on electronic and optical properties of YVO4 through application of plane-wave DFT. Since this crystal is a common host material for optical ...devices and it has been widely used as a phosphor and a laser host material, it is important to identify a correlation between impurity location and the intrinsic properties of the material. To further improve the accuracy of the results several semi-local and a hybrid functional are tested. To find the most stable structure among possible configurations of doping, the electronic structures and formation energies of the available configurations are calculated and compared. We report that while the valence is formed by O-2p orbitals and conduction is made of V-3d orbitals in all configurations, the band width varies with the impurity configuration. Additionally, the formation energy is changing depending on where the dopant is located. Then the absorption coefficient and the refractive index are obtained using a modified HSE functional.
We investigated the role of phosphorus (P) impurities on the fracture toughness and underlying failure mechanisms by means of classical atomistic modeling for a set of ⟨110⟩ symmetric tilt tungsten ...grain boundaries (GBs). This entailed the utilization of a quasi-static mode I displacement-controlled setup with cohesive zone volume elements (CZVEs) to study failure mechanisms and evaluate the fracture toughness of the GB cracks. The fracture toughness was estimated using three approaches: computing (i) the individual and (ii) the average energy release rate of CZVEs along the fractured surfaces and using them as inputs for the Griffith model, and(iii) relating the fracture toughness to crack propagation initiation. The cracks in all the pristine GBs evolved in a brittle fashion, occasionally forming facetted cleavage planes. Upon introduction of impurities, other mechanisms such as void formation and crack-tip transformation were also observed. Depending on the GBproximity of the occupied segregation sites, local strengthening was seen occasionally for individual CZVEs and at the crack-tip, which was triggered by local impurity-induced crack deflection onto planes with higher cohesion. But when the fracture toughness from the averaged energy release rate was considered, an overall reduction with increasing impurity segregation was found, although to a varying degree for different GBs. This indicates an overall increased degree of embrittlement with increasing P-segregation at the GBs, which concurs with most experimental results reported in the literature.
Tungsten is a prime candidate material for use in plasma facing components in nuclear fusion reactors. This would entail weathering extreme conditions, such as high thermal loads and particle ...bombardment. It is of vital importance to understand how tungsten mechanically responds to these conditions, and how it is impacted by the defects that form. The current communication considers how the mechanical properties of tungsten are affected by the presence of several nanosized lattice defects: interstitial helium (both scattered throughout the sample and centrally clustered), isolated vacancies and vacancy clusters. All defects were found to lower the yield stress of the crystal, with vacancies and vacancy clusters having negligible influence during the elastic phase. Interstitial helium formed clusters, leading to the displacement of tungsten atoms, and a lowered stiffness at high strains. These negative effects of interstitial helium — along with the decrease in yield stress — were found to be partially negated by the presence of vacancies.
We investigate the variation of elastic stiffness moduli and the thermodynamic properties of yttrium orthosilicate (Y2SiO5, YSO) under various doping concentrations of Eu3+ ions. The model is based ...on a low temperature approximation (T<<θD) , and the plane-wave density functional theory (DFT) is used to carry out the calculations. The results show that the Eu3+ ions primarily occupy the Y1 site of the basic molecule for all applied concentrations. The overall shear, bulk, and Young’s moduli exhibit a decreasing trend with increasing Eu3+ concentration. The overall anisotropy shows a very small increase with increasing concentration. The Debye temperature as well as the Grünesien parameter for each concentration are predicted. Lastly, the predicted heat capacity at constant volume is calculated and compared to experimental values. Our study reveals that there is almost linear relationship between concentration and mechanical properties of YSO. The decrease of the Grünesien parameter with concentration increase might decrease the anharmonic effects in YSO, although this effect is small. In addition, the change in heat capacity with concentration rise is negligible.
•The overall shear, bulk, and Young’s moduli exhibit a decreasing trend with increasing Eu3+ concentration.•There is almost a linear relationship between dopant concentration and crystal parameters.•The overall anisotropy shows a small increase with increasing concentration.•The calculated heat capacity in constant volume does not change with increasing concentration.
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