Papers by Author: Qing Jiang

Abstract: Nanocrystalline (NC) metals and alloys always exhibit extremely high strength but quite limited ductility. This disappointing ductility might be caused by the preparation artifacts and the weak strain hardening ability of NC materials. In order to optimize the mechanical properties of NC metals and alloys , especially to enhance their ductility, and investigate the underlying deformation mechanism, nanocrystalline Ni, dual phase Ni-Co alloy and Cu were synthesized via electrodeposition and electro-brush deposition respectively, and then a series of mechanical tests were carried out. The results show that all the materials exhibit a combination of high strength and remarkable ductility. The high strength can be attributed to the “true” nanocrystalline grain sizes ranges from 15 to 30 nm. In addition, three factors are revealed to contribute to the enhanced ductility of these materials, respectively: (a) modified deformation mode (the tensile-relaxation cycle test) for nanocrystalline Ni, (b) high strain hardening and cooperative deformation for dual-phase Ni-Co alloy, and (c) high strain rate sensitivity for nanocrystalline Cu.

Abstract: An extension of the classical thermodynamics to nanometer scale has been conducted to elucidate information regarding size dependence of phase transition functions and binary phase diagrams. The theoretical basis of the extension is Lindemann′s criterion for solid melting, Mott′s expression for vibrational melting entropy, and Shi′s model for size dependent melting temperature. These models are combined into a unified one without adjustable parameters for melting temperatures of nanocrystals. It is shown that the melting temperature of nanocrystals may drop or rise depending on interface conditions and dimensions. The model has been extended and applied to size dependences of melting enthalpy, melting entropy, atomic cohesive energy. Moreover, the above modeling has been utilized to determine the size-dependent continuous binary solution phase diagrams. These thermodynamic approachs have extended the capability of the classical thermodynamics to the thermodynamic phenomena in the nanometer regime.

Abstract: The nanostructured metals and alloys are under intensive research worldwide and being developed into bulk forms for application. While these new materials offer record-high strength, their ductility is often inadequate and sometime rendering them unusable. Besides tailoring the nanostructure to achieve coexisting high strength and high ductility, to uncover the coherent property of this material is also important. This article reviews the recent researches finished in our lab. A set of nanostructured metals and alloys were synthesized by a direct current electrodeposition technique, and the effect of grain size and strain rate on the mechanical properties stressing on tensile ductility was systemically studied by tensile test at room temperature.

Abstract: The size-dependent valence-conduction bandgap of semiconductor nanocrystals are predicted based on a model for size-dependent cohesive energy, without any adjustable parameter. The model predicts an increase of the bandgap of semiconductors with decreasing crystalline sizes. It is found that the model predictions are in good agreement with the available experimental results for Si, ZnS, ZnSe, CdS, and CdSe nanocrystals.

Abstract: A unified model is developed for the finite size effect on the glass transition temperature of polymers Tg(D) where D denotes diameter of particles or thickness of films. In terms of this model, Tg(D) depends on both the size and interface conditions. The predicated results are consistent with the experimental evidences of polystyrene (PS) and poly (methyl methacrylate) (PMMA) films and nanoparticles with different interface situations.

Abstract: The size dependent miscibility of binary polymer blend films of polystyrene (PS) and tetramethylbisphenol-A polycarbonate (TMPC) is studied by the molecular dynamics (MD) simulation in the way of computing Flory-Huggins interaction parameter, χ, of the blend films, which determines the blend films compatibility. It is found that the miscibility of the two polymers decreases as the film thickness D decreases. After that, the size dependent glass transition temperature Tg(w,D) of the two polymers blend films in miscible ranges are determined by computer simulation and the Fox equation where w is the weight fraction of the second component.

Abstract: The pressure-dependent melting temperature of bulk Si, bulk Ge and nanocrystalline (nc) Si are predicted by the Clapeyron equation where the pressure-dependent volume difference is modeled by introducing the effect of surface stress induced pressure. The predictions are found to be consistent with the present experimental and other theoretical results.

Abstract: Through considering the packing density and the lattice contraction, volume changes of elements with different structures DV on melting are modeled. An agreement between the model predictions and the experimental as well as theoretical data is found. Moreover, the alloy design principle for GFA of bulk metallic glasses (BMG) based on the size of DV is also suggested that amorphous metallic alloys will have the best GFA when its packing density approaches that of A2 crystal.