Yanping Wei

Molecular dynamics simulation of plastic deformation of nanotwinned copper

The plastic deformation of polycrystalline Cu with ultrathin lamella twins has been studied using molecular dynamics simulations. The results of uniaxial tensile deformation simulation show that the abundance of twin boundaries provides obstacles to dislocation motion, which in consequence leads to a high strain hardening rate in the nanotwinned Cu. We also show that the twin lamellar spacing plays a vital role in controlling the strengthening effects, i.e., the thinner the thickness of the twin lamella, the harder the material. Additionally, twin boundaries can act as dislocation nucleation sites as they gradually lose coherency at large strain. These results indicate that controlled introduction of nanosized twins into metals can be an effective way of improving strength without suppression tensile ductility.

Thermal conductivity of composites with nanoscale inclusions and size-dependent percolation

An analytical model for thermal conductivity of composites with nanoparticles in a matrix is developed based on the effective medium theory by introducing the intrinsic size effect of thermal conductivity of nanoparticles and the interface thermal resistance effect between two phases. The model predicts the percolation of thermal conductivity with the volume fraction change of the second phase, and the percolation threshold depends on the size and the shape of the nanoparticles. The theoretical predictions are in agreement with the experimental results.

Size-dependent interface phonon transmission and thermal conductivity of nanolaminates

An analytical model for size-dependent interface phonon transmission and thermal conductivity of nanolaminates is derived based on the improved acoustic mismatch theory and the Lindemann melting theory by considering the size effect of phonon velocity and the interface lattice mismatch effect. The model suggests that the interface phonon transmission is dominant for the cross-plane thermal conductivity of nanolaminates and superlattices, and the intrinsic variety of size effect of thermal conductivity for different systems is proposed based on the competition mechanism of size effect of phonon transport between two materials constituting the interfaces. The models prediction for thermal conductivity of nanolaminates agrees with the experimental results

Spectroscopic properties of Pr3+ ions in NaGd(WO4)2 crystal

A Pr3+-doped NaGd(WO4)2 single crystal has been grown by the Czochralski technique along the (0 0 1) orientation. The refractive indices were measured precisely as a function of wavelength. A Raman spectrum, polarized absorption spectra, fluorescence spectra, and fluorescence decay curve have been recorded at room temperature. The standard and modified Judd-Ofelt theories have been performed to analyze the absorption spectra to determine the spectroscopic parameters, including the Judd-Ofelt intensity parameters Ωt (t=2,4,6), radiative transition probabilities, radiative lifetimes, and branching ratios. The stimulated emission cross sections and fluorescence lifetime of the promising laser level were obtained.

Interface energy and its influence on interface fracture between metal and ceramic thin films in nanoscale

A theoretical model about the size-dependent interface energy between two thin films with different materials is developed by considering the chemical bonding contribution based on the thermodynamic expressions and the structure strain contribution based on the mechanical characteristics. The interface energy decreases with reducing thickness of thin films, and is determined by such available thermodynamic and mechanical parameters as the melting entropy, the melting enthalpy, the shear modulus of two materials, etc. The predicted interface energies of some metal/MgO and metal/Al2O3 interfaces based on the model are consistent with the results based on the molecular mechanics calculation. Furthermore, the interface fracture properties of Ag/MgO and Ni/Al2O3 based on the atomistic simulation are further compared with each other. The fracture strength and the toughness of the interface with the smaller structure interface energy are both found to be lower. The intrinsic relations among the interface energy, the interface strength, and the fracture toughness are discussed by introducing the related interface potential and the interface stress. The microscopic interface fracture toughness is found to equal the structure interface energy in nanoscale, and the microscopic fracture strength is proportional to the fracture toughness

An experimental study on the dependence of the strength of adhesively bonded joints with thickness and mechanical properties of the adhesives

Adhesive bonding joints are widely applied in many engineering fields. Their overall strength is much dependent on the thickness of adhesive layers. Many previous experimental studies have found that the ultimate failure strength of the bonding structure increases with the decrease of the adhesive thickness. However, few of them consider the effect of adhesive intrinsic material parameters on the relation between the overall strength and adhesive thickness. In the present investigation, the effect of the adhesive thickness on the overall strength of the lightweight metallic adhesive bonding joints was experimentally studied, considering the effect of the adhesive toughness. The results show that the variations of overall strength resulting from the adhesive thicknesses have remarkable discrepancy due to the toughness of the adhesive, which is in agreement with the previous model prediction.

Optical properties of NaLa(WO4)2 : Ho3+single crystal

Crystals of NaLa(WO4) 2 : Ho3+ were grown by the Czochralski method. The spectroscopic properties have been investigated at room temperature. Intensity parameters, radiative lifetimes and branching ratios have been calculated in the framework of Judd-Ofelt theory. The luminescence spectra measured under 450 nm pumping are dominated by a strong green emission in the 550 nm region connected with the S-5(2) + F-5(4). I-5(8) transition. The stimulated emission cross-section for this transition was estimated. The luminescence decay curve of the 5S2 + 5 F-4 level was also measured at room temperature.

Optical and thermal properties of Co2+:ZnWO4 crystal

The crystal of ZnWO4 doped with Co2+ has been grown by the Czochralski technique. The orientation procedure was presented. Along the crystallographic axes, the principal coefficients of thermal expansion and absorption spectra were measured and discussed. Based on the data, the crystal field parameter 10Dq, Racah parameter B, as well as the absorption cross sections were calculated. The nonlinear dependence of optical absorption on concentration of cobalt ions was found and discussed in single crystal Co2+:ZnWO4. The results indicate that inner defects such as natural cleavage or slip faces in crystal are important in the study of optical and thermal properties.

Optical properties of Pr3+: NaLa(WO4)2 single crystal

The polarized absorption spectra, polarized fluorescence spectra and fluorescence decay curve of Pr3+-doped NaLa(WO4)2 single crystals were measured at room temperature. The emission cross section has been presented and Judd–Ofelt theory has been applied to the analysis of the absorption spectra. The calculated radiative lifetime is compared with the experimental data for the 3P0 emitting level. The result indicates that the relaxation of the excited 3P0 level of praseodymium is governed by radiative transitions, which makes the 3P0 manifold of the Pr3+-doped NaLa(WO4)2 crystal promising as the upper laser level.

Multiple-scale interface fracture analysis for thin film/substrate system under temperature mismatch

Based on the Wei-Xu model (Int J Plast, 21:2123, 2005), a multiple-scale interface fracture analysis for the thin film/substrate system under temperature mismatch is carried out. The thin film delamination process at the macroscopic scale is analyzed by using the strain gradient plasticity theory. The plastic shielding effects on the interface crack propagation are investigated for both the elastic film/plastic substrate and the plastic film/elastic substrate systems. On the other hand, by presenting a bridging model (equivalent interface K’-field) and by using the Rice-Thomson discrete dislocation model, the microscopic fracture process of the thin film/substrate system is analyzed, and the dislocation shielding effects on interface cracking at the microscale are investigated. The results show that the variation of interface fracture energy has an inverse size effect with respect to the dislocation-free zone size.