The Bauschinger and size effects in the thinfilm plasticity theory arising from the defect-energy of geometrically necessary dislocations （GNDs） are analytically investigated in this paper. Firstly, this defect-ener...The Bauschinger and size effects in the thinfilm plasticity theory arising from the defect-energy of geometrically necessary dislocations （GNDs） are analytically investigated in this paper. Firstly, this defect-energy is deduced based on the elastic interactions of coupling dislocations （or pile-ups） moving on the closed neighboring slip plane. This energy is a quadratic function of the GNDs density, and includes an elastic interaction coefficient and an energetic length scale L. By incorporating it into the work- conjugate strain gradient plasticity theory of Gurtin, an energetic stress associated with this defect energy is obtained, which just plays the role of back stress in the kinematic hardening model. Then this back-stress hardening model is used to investigate the Bauschinger and size effects in the tension problem of single crystal Al films with passivation layers. The tension stress in the film shows a reverse dependence on the film thickness h. By comparing it with discrete-dislocation simulation results, the length scale L is determined, which is just several slip plane spacing, and accords well with our physical interpretation for the defect- energy. The Bauschinger effect after unloading is analyzed by combining this back-stress hardening model with a friction model. The effects of film thickness and pre-strain on the reversed plastic strain after unloading are quantified and qualitatively compared with experiment results.展开更多
The discovery of ideal spin-1/2 kagome antiferromagnets Herbertsmithite and Zn-doped Barlowite represents a breakthrough in the quest for quantum spin liquids(QSLs),and nuclear magnetic resonance(NMR)spectroscopy play...The discovery of ideal spin-1/2 kagome antiferromagnets Herbertsmithite and Zn-doped Barlowite represents a breakthrough in the quest for quantum spin liquids(QSLs),and nuclear magnetic resonance(NMR)spectroscopy plays a prominent role in revealing the quantum paramagnetism in these compounds.However,interpretation of NMR data that is often masked by defects can be controversial.Here,we show that the most significant interaction strength for NMR,i.e.the hyperfine coupling(HFC)strength,can be reasonably reproduced by first-principles calculations for these proposed QSLs.Applying this method to a supercell containing Cu-Zn defects enables us to map out the variation and distribution of HFC at different nuclear sites.This predictive power is expected to bridge the missing link in the analysis of the low-temperature NMR data.展开更多
It is difficult to establish structure-property relationships in a defective solid because of its inhomogeneous-geometry microstructure caused by defects. In the present research, the effects of pores and cracks on th...It is difficult to establish structure-property relationships in a defective solid because of its inhomogeneous-geometry microstructure caused by defects. In the present research, the effects of pores and cracks on the Young’s modulus of a defective solid are studied. Based on the law of the conservation of energy, mathematical formulations are proposed to indicate how the shape, size, and distribution of defects affect the effective Young’s modulus. In this approach, detailed equations are illustrated to represent the shape and size of defects on the effective Young’s modulus. Different from the results obtained from the traditional empirical analyses, mixture law or statistical method, for the first time, our results from the finite element method （FEM） and strict analytical calculation show that the influence of pore radius and crack length on the effective Young’s modulus can be quantified. It is found that the longest crack in a typical microstructure of ceramic coating dominates the contribution of the effective Young’s modulus in the vertical direction of the crack.展开更多
Based on the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by Kumar et al., we gave the theoretical analysis using the Frank elastic theory. When a direct-curren...Based on the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by Kumar et al., we gave the theoretical analysis using the Frank elastic theory. When a direct-current electric field normal to the plane of the substrate is applied to the parallel aligned nematic liquid crystal cell with weak anchoring, the rotation of 4-1 defects in the narrow inversion walls can be exhibited. The free energy of liquid crystal molecules around the ＋1 and - 1 defect sites in the nematic inversion walls under the electric field was formulated and the electric-field-driven structural changes at the defect site characterized by polar and azimuthal angles of the local director were simulated. The results reveal that the deviation of azimuthal angle induced by flexoelectric effect are consistent with the switching of extinction brushes at the - 1 and -1 defects obtained in the experiment conducted by Kumar et al.展开更多
基金supported by the National Natural Science Foundation of China (10772096)
文摘The Bauschinger and size effects in the thinfilm plasticity theory arising from the defect-energy of geometrically necessary dislocations （GNDs） are analytically investigated in this paper. Firstly, this defect-energy is deduced based on the elastic interactions of coupling dislocations （or pile-ups） moving on the closed neighboring slip plane. This energy is a quadratic function of the GNDs density, and includes an elastic interaction coefficient and an energetic length scale L. By incorporating it into the work- conjugate strain gradient plasticity theory of Gurtin, an energetic stress associated with this defect energy is obtained, which just plays the role of back stress in the kinematic hardening model. Then this back-stress hardening model is used to investigate the Bauschinger and size effects in the tension problem of single crystal Al films with passivation layers. The tension stress in the film shows a reverse dependence on the film thickness h. By comparing it with discrete-dislocation simulation results, the length scale L is determined, which is just several slip plane spacing, and accords well with our physical interpretation for the defect- energy. The Bauschinger effect after unloading is analyzed by combining this back-stress hardening model with a friction model. The effects of film thickness and pre-strain on the reversed plastic strain after unloading are quantified and qualitatively compared with experiment results.
基金supported by the National Natural Science Foundation of China(11774196)Tsinghua University Initiative Scientific Research Program+5 种基金supported by the National Postdoctoral Program for Innovative Talents of China(BX201600091)the Funding from China Postdoctoral Science Foundation(2017M610858)the support from US-DOE(DEFG02-04ER46148)supported by National Key Research and Development Program of China(2016YFA0300202)National Natural Science Foundation of China(11774306)the Strategic Priority Research Program of Chinese Academy of Sciences(XDB28000000)
文摘The discovery of ideal spin-1/2 kagome antiferromagnets Herbertsmithite and Zn-doped Barlowite represents a breakthrough in the quest for quantum spin liquids(QSLs),and nuclear magnetic resonance(NMR)spectroscopy plays a prominent role in revealing the quantum paramagnetism in these compounds.However,interpretation of NMR data that is often masked by defects can be controversial.Here,we show that the most significant interaction strength for NMR,i.e.the hyperfine coupling(HFC)strength,can be reasonably reproduced by first-principles calculations for these proposed QSLs.Applying this method to a supercell containing Cu-Zn defects enables us to map out the variation and distribution of HFC at different nuclear sites.This predictive power is expected to bridge the missing link in the analysis of the low-temperature NMR data.
基金Project supported by the National Natural Science Foundation of China (Grant No. 50801005)
文摘It is difficult to establish structure-property relationships in a defective solid because of its inhomogeneous-geometry microstructure caused by defects. In the present research, the effects of pores and cracks on the Young’s modulus of a defective solid are studied. Based on the law of the conservation of energy, mathematical formulations are proposed to indicate how the shape, size, and distribution of defects affect the effective Young’s modulus. In this approach, detailed equations are illustrated to represent the shape and size of defects on the effective Young’s modulus. Different from the results obtained from the traditional empirical analyses, mixture law or statistical method, for the first time, our results from the finite element method （FEM） and strict analytical calculation show that the influence of pore radius and crack length on the effective Young’s modulus can be quantified. It is found that the longest crack in a typical microstructure of ceramic coating dominates the contribution of the effective Young’s modulus in the vertical direction of the crack.
基金National Natural Science Foundation of China !under Grant No. 59671039.
文摘The effects of distribution of induced defects on the positron diffusion was studied by using the point-source diffusion model and 6 function method.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11374087,11274088,and 11304074)the Natural Science Foundation of Hebei Province,China(Grant Nos.A2014202123 and A2016202282)+1 种基金the Research Project of Hebei Education Department,China(Grant Nos.QN2014130and QN2015260)the Key Subject Construction Project of Hebei Province University,China
文摘Based on the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by Kumar et al., we gave the theoretical analysis using the Frank elastic theory. When a direct-current electric field normal to the plane of the substrate is applied to the parallel aligned nematic liquid crystal cell with weak anchoring, the rotation of 4-1 defects in the narrow inversion walls can be exhibited. The free energy of liquid crystal molecules around the ＋1 and - 1 defect sites in the nematic inversion walls under the electric field was formulated and the electric-field-driven structural changes at the defect site characterized by polar and azimuthal angles of the local director were simulated. The results reveal that the deviation of azimuthal angle induced by flexoelectric effect are consistent with the switching of extinction brushes at the - 1 and -1 defects obtained in the experiment conducted by Kumar et al.
