Local stress in an atomic system, which provides an average stress measurement within a spatial volume containing a collection of atoms, is essential for determining the mechanical properties of a nanoscale structure as well as developing a proper multiscale modeling technique. Theoretically, the smaller averaging volume where a local stress can converge, the closer this atomistic stress definition can approach the ideal continuum stress. As a result, the more accurate stress concentration can be evaluated for the inhomogeneous case. With reference to the previous studies focusing on the spherical averaging volume, dependent on the type of crystals, the convergent radius of the virial stress or Hardy stress usually spans the size of several lattice constants. In this paper, we find that, once the averaging volume is periodic, the convergence of the virial stress and Hardy stress can be accomplished within one single lattice, which is much smaller than what is required by other non-periodic volumes such as a sphere. In the final section, a cracked sodium chloride crystal is considered to demonstrate that the crack opening stress described by the periodic lattices captures the stress concentration near the crack tip. 相似文献
Magneto-electro-elastic (MEE) materials usually consist of piezoelectric (PE) and piezomagnetic (PM) phases. Between different constituent phases, there exist lots of interfaces with discontinuous MEE properties. Complex interface distribution brings a great difficulty to the fracture analysis of MEE materials since the present fracture mechanics methods can hardly solve the fracture parameters efficiently of a crack surrounded by complex interfaces. This paper develops a new domain formulation of the interaction integral for the computation of the fracture parameters including stress intensity factors (SIFs), electric displacement intensity factor (EDIF) and magnetic induction intensity factor (MIIF) for linear MEE materials. The formulation derived here does not involve any derivatives of material properties and moreover, it can be proved that an arbitrary interface in the integral domain does not affect the validity and the value of the interaction integral. Namely, the interaction integral is domain-independent for material interfaces and thus, its application does not require material parameters to be continuous. Due to this advantage, the interaction integral becomes an effective approach for extracting the fracture parameters of MEE materials with complex interfaces. Combined with the extended finite element method (XFEM), the interaction integral is employed to solve several representative problems to verify its accuracy and domain-independence. Good results show the effectiveness of the present method in the fracture analysis of MEE materials with continuous and discontinuous properties. Finally, the particulate MEE composites composed of PE and PM phases are considered and four schemes of different property-homogenization level are proposed for comparing their effectiveness. 相似文献
We report on the fabrication and characterization of InAs/GaAs chirped multilayer quantum-dot superluminescent diodes (CMQD-SLDs) with and without direct Si doping in QDs. It was found that both the output power and the spectral width of the CMQD-SLDs were significantly enhanced by direct Si doping in the QDs. The output power and spectral width have been increased by approximately 18.3% and 40%, respectively. Moreover, we shortened the cavity length of the doped CMQD-SLD and obtained a spectral width of 106 nm. In addition, the maximum output power and spectral width of the CMQD-SLD doped directly with Si can be further increased to 16.6 mW and 114 nm, respectively, through anti-reflection coating and device packaging. The device exhibited the smallest spectral dip of 0.2 dB when the spectrum was widest. The improved performances of the doped CMQD-SLD can be attributed to the direct doping of Si in the QDs, optimization of device structure and device packaging. 相似文献
As concepts closely related to microwave absorption properties, impedance matching and phase matching were rarely combined with material parameters to regulate properties and explore related mechanisms. In this work, reduction-diffusion method was innovatively applied to synthesize rare earth alloy Y$_{2}$Fe$_{17}$. In order to regulate the electromagnetic parameters of absorbers, the Y$_{2}$Fe$_{17}$N$_{3-\delta }$ particles were coated with silica (Y$_{2}$Fe$_{17}$N$_{3-\delta }$@SiO$_{2}$) and absorbers with different volume fractions were prepared. The relationship between impedance matching, matching thickness, and the strongest reflection loss peak (${\rm RL}_{\rm min}$) was presented obviously. Compared to the microwave absorption properties of Y$_{2}$Fe$_{17}$N$_{3-\delta }$/PU absorber, Y$_{2}$Fe$_{17}$N$_{3-\delta }$@SiO$_{2}$/PU absorbers are more conducive to the realization of microwave absorption material standards which are thin thickness, light weight, strong absorbing intensity, and broad bandwidth. Based on microwave frequency bands, the microwave absorption properties of the absorbers were analyzed and the related parameters were listed. As an important parameter related to perfect matching, reflection factor ($\sqrt {ărepsilon_{\rm r}/\mu_{\rm r}}$) was discussed combined with microwave amplitude attenuation. According to the origin and mathematical model of bandwidth, the formula of EAB (${\rm RL}<-10$ dB) was derived and simplified. The calculated bandwidths agreed well with experimental results. 相似文献
The \begin{document}$ \alpha $\end{document}-particle preformation factors of nuclei above doubly magic nuclei \begin{document}$ ^{100} $\end{document}Sn and \begin{document}$ ^{208} $\end{document}Pb are investigated within the generalized liquid drop model. The results show that the \begin{document}$ \alpha $\end{document}-particle preformation factors of nuclei near self-conjugate doubly magic \begin{document}$ ^{100} $\end{document}Sn are significantly larger than those of analogous nuclei just above \begin{document}$ ^{208} $\end{document}Pb, and they will be enhanced as the nuclei move towards the \begin{document}$ N = Z $\end{document} line. The proton–neutron correlation energy \begin{document}$ E_{p-n} $\end{document} and two protons–two neutrons correlation energy \begin{document}$ E_{2p-2n} $\end{document} of nuclei near \begin{document}$ ^{100} $\end{document}Sn also exhibit a similar situation, indicating that the interactions between protons and neutrons occupying similar single-particle orbitals could enhance the \begin{document}$ \alpha $\end{document}-particle preformation factors and result in superallowed \begin{document}$ \alpha $\end{document} decay. This also provides evidence of the significant role of the proton–neutron interaction on \begin{document}$ \alpha $\end{document}-particle preformation. Also, the linear relationship between \begin{document}$ \alpha $\end{document}-particle preformation factors and the product of valence protons and valence neutrons for nuclei around \begin{document}$ ^{208} $\end{document}Pb is broken in the \begin{document}$ ^{100} $\end{document}Sn region because the \begin{document}$ \alpha $\end{document}-particle preformation factor is enhanced when a nucleus near \begin{document}$ ^{100} $\end{document}Sn moves towards the \begin{document}$ N = Z $\end{document} line. Furthermore, the calculated \begin{document}$ \alpha $\end{document} decay half-lives fit well with the experimental data, including the recent observed self-conjugate nuclei \begin{document}$ ^{104} $\end{document}Te and \begin{document}$ ^{108} $\end{document}Xe [Phys. Rev. Lett. 121, 182501 (2018)]. 相似文献
A new methodology is developed to conjugate hyaluronic acid (HA) hydrogel with novel nano‐fibrous architectures via non‐covalent assembly that specifically allows for targeted adipose‐derived stem cells (ASCs) differentiation and soft tissue engineering. The assembly of non‐covalently associated hydrogel network produced via the interaction of a low molecular weight heparin (LMWH) modified HA derivative and heparin interacting protein (HIP). The multifunctional star poly(ethylene glycol) (PEG) and HIP copolymer has the capability to mediate the non‐covalent assembly of nano‐fibrous HA hydrogel networks via affinity interactions with LMWH. The effect of the HIP mediation on in vitro gelation, rheological characteristics, degradation, equilibrium swelling, adipose‐derived stem cells (ASCs) proliferation and differentiation of nano‐fibrous hydrogel is examined. The results suggest the potential utility of this unique design of the bioactive nano‐fibrous HA hydrogel in directing the differentiation of ASCs and adipogenesis in ECM‐mimetic scaffolds in vitro. These studies demonstrate that this nano‐fibrous HA hydrogel can render the formulation of a therapeutically effective platform for in vitro adipogenesis applications.
Although it is known that low signal-to-noise ratio (SNR) can affect tensor metrics, few studies reporting disease or treatment effects on fractional anisotropy (FA) report SNR; the implicit assumption is that SNR is adequate. However, the level at which low SNR causes bias in FA may vary with tissue FA, field strength and analytical methodology. We determined the SNR thresholds at 1.5 T vs. 3 T in regions of white matter (WM) with different FA and compared FA derived using manual region-of-interest (ROI) analysis to tract-based spatial statistics (TBSS), an operator-independent whole-brain analysis tool. Using ROI analysis, SNR thresholds on our hardware-software magnetic resonance platforms were 25 at 1.5 T and 20 at 3 T in the callosal genu (CG), 40 at 1.5 and 3 T in the anterior corona radiata (ACR), and 50 at 1.5 T and 70 at 3 T in the putamen (PUT). Using TBSS, SNR thresholds were 20 at 1.5 T and 3 T in the CG, and 35 at 1.5 T and 40 at 3 T in the ACR. Below these thresholds, the mean FA increased logarithmically, and the standard deviations widened. Achieving bias-free SNR in the PUT required at least nine acquisitions at 1.5 T and six acquisitions at 3 T. In the CG and ACR, bias-free SNR was achieved with at least three acquisitions at 1.5 T and one acquisition at 3 T. Using diffusion tensor imaging (DTI) to study regions of low FA, e.g., basal ganglia, cerebral cortex, and WM in the abnormal brain, SNR should be documented. SNR thresholds below which FA is biased varied with the analytical technique, inherent tissue FA and field strength. Studies using DTI to study WM injury should document that bias-free SNR has been achieved in the region of the brain being studied as part of quality control. 相似文献
Structural, magnetic and magnetocaloric properties of manganites series with the AMn1−xGaxO3 (A=La0.75Ca0.08Sr0.17 and x=0, 0.05, 0.1 and 0.2) composition have been investigated to shed light on Ga-doping influence. Solid-state reaction method was used for preparation. From XRD study, all samples are found single phase and crystallize in the orthorhombic structure with the Pnma space group. The variation of the magnetization M vs. temperature T, under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The experimental results indicate that TC decreases from 336 to 135 K with increasing Ga substitution. Magnetocaloric effect (MCE) was estimated, in terms of isothermal magnetic entropy change (−ΔSM), using the M(T, μ0H) data and employing the thermodynamic Maxwell equation. The maximum entropy change and Relative Cooling Power (RCP) show non-monotonic behaviors with increasing the concentration of Gallium. In fact, the maximum value of ΔSMmaxof AMn1−xGaxO3 for x=0.00 and 0.2 samples is found to be, respectively, 2.87 and 1.17 J/kg/K under an applied magnetic field change of 2 T. For the same applied magnetic field (μ0H=2 T), the RCP values are found to vary between 97.58 and 89 J/kg. 相似文献