Q. Jiang

Melting thermodynamics of organic nanocrystals

The size-dependent melting temperature and the size-dependent melting entropy of organic nanocrystals are predicted by use of our simple model being free of any adjustable parameter. The model predictions for the size-dependent melting temperature and the size-dependent melting entropy are supported by the experimental results on benzene, chlorobenzene, heptane, and naphthalene nanocrystals.

Fabrication of TiC particulate reinforced magnesium matrix composites

Abstract A TiC particulate reinforced magnesium matrix composite (PRMMC) was fabricated by adding a TiC–Al master alloy processed via self-propagating high-temperature synthesis reaction into molten magnesium and using the semi-solid slurry stirring technique. The properties of the PRMMC are higher than those of the unreinforced magnesium alloy.

Size-dependent melting point of noble metals

A simple model, without any free parameter, is introduced to predict the size-dependent melting temperature of noble metals in this contribution. It is found that the model predictions for the melting point depression of both Au and Ag nanoparticles correspond to the experimental and computer simulation results well.

In situ synthesis of TiC/Mg composites in molten magnesium

Abstract Magnesium reinforced by in situ particulates was synthesized utilizing the self-propagating high-temperature synthesis (SHS) reaction of Al–Ti–C preforms in molten magnesium. The result showed that the preform preheat temperature has a great effect on the SHS reaction. For a preheat temperature of 450 °C, in situ TiC/Mg composite was fabricated successfully.

Field emission properties of N-doped capped single-walled carbon nanotubes: a first-principles density-functional study.

The geometrical structures and field emission properties of pristine and N-doped capped (5,5) single-walled carbon nanotubes have been investigated using first-principles density-functional theory. The structures of N-doped carbon nanotubes are stable under field emission conditions. The calculated work function of N-doped carbon nanotube decreases drastically when compared with pristine carbon nanotube, which means the enhancement of field emission properties. The ionization potentials of N-doped carbon nanotubes are also reduced significantly. The authors analyze the field emission mechanism in terms of energy gap between the lowest unoccupied molecular orbital and the highest occupied molecular orbital, Mulliken charge population, and local density of states. Due to the doping of nitrogen atom, the local density of states at the Fermi level increases dramatically and donor states can be observed above the Fermi level. The authors results suggest that the field emission properties of carbon nanotubes can be enhanced by the doping of nitrogen atom, which are consistent with the experimental results.

Size Effect on the Phase Stability of Nanostructures

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 Lindemannufffb s criterion for solid melting, Mottufffb s expression for vibrational melting entropy, and Shiufffb 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 tem- perature of nanocrystals may drop or rise depending on interface conditions and dimensions. The model has been applied to size de- pendences of melting enthalpy and atomic cohesive energy, critical temperatures for glass transition, ferromagnetic transition, ferroelec- tric transition, superconductor transition and ferromagnetic-antiferromagnetic transition. Moreover, the above modeling has been utilized to determine the size-dependent continuous binary solution phase diagrams, bi-layer transition diagrams of metallic multilayers, and solid transition phase diagrams after modeling the transition entropy and atomic interaction energy functions of nanocrystals. Moreover, the model has been used to predict size dependence of diffusion activation energy and diffusion coefficient. These thermodynamic approachs have extended the capability of the classical thermodynamics to the thermodynamic phenomena in the nanometer regime.

FREE ENERGY OF CRYSTAL-LIQUID INTERFACE

It is demonstrated that a simple model, free of any adjustable parameter, can be developed to predict the free energy of a crystal-liquid interface ssl based on the Gibbs-Thomson equation and a model for the size-dependent melting temperature. This model has improved Turnbulls empirical equation. In the model, ssl depends on not only the melting enthalpy of crystals, but also the vibrational component of the overall melting entropy of the crystals. The predicted values of ssl for diAerent types of crystals, such as true metals, meta metals, semiconductors, ionic crystals and organic crystals, are confirmed by available ex- perimental results in the experimental error range. # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Stacking fault energy of iron-base shape memory alloys

Abstract The stacking fault energy of six iron-base shape memory alloys can be calculated by the extended dislocation node method. The results show that Ni and Mn increase the stacking fault energy of the alloys, while Cr and Si decrease the stacking fault energy. An expression relating the alloy elements Ni, Cr, Mn, and Si to the stacking fault energy of the iron-base shape memory alloys is provided. The effective ratio of the alloying elements on the Stacking fault energy of iron-base alloys is given: %Cr: %Ni: %Mn: %Si=−1.1: 1.64: 0.21: −4.45.

Entropy for solid-liquid transition in nanocrystals

It is demonstrated that a simple model, free of any adjustable parameter, can be developed for the melting entropy of nanocrystals based on Motts equation for the melting entropy and a model for the size-dependent melting. It is shown that the entropy of melting for a nanocrystal decreases as size decreases, suggesting that the difference in the degree of order between the liquid and solid phase is size-dependent. Supported by both the available experimental results on Sn and Al nanocrystals and the first principle calculations on Al nanocrystals, the model suggests that the size dependence of the entropy of melting for non-semiconductor nanocrystals is determined by the size dependence of vibrational component of the melting entropy.

Finite size effect on glass transition temperatures

Abstract A simple and unified model, free of any adjustable parameters, is developed for the finite size effect on glass transition temperatures of polymers and organic particles. As the thickness of polymer thin films and the radius of organic particles decrease, their glass transition temperatures decrease. For polymers, this decrease is independent of their molecular weight, but dependent on the correlation length for intermolecular cooperative rearrangement and the presence of substrates. The model predictions are consistent with available experimental results on size dependence of the glass transition temperatures for free-standing polystyrene thin films, polystyrene films supported on passivated substrates and o-terphenyl and benzyl alcohol nanoparticles.