Zhu Zhen-Gang

Relative Energy Dissipation: Sensitive to Structural Changes of Liquids

Energy dissipation techniques, widely used in solid physics previously, are proven to be sensitive also to changes in liquid structure. It has been suggested from relative energy dissipation that changes in liquid structure can occur as a function of temperature in some ordinary binary systems such as Pb-Sn, In-Sn and In-Bi. This finding may be helpful to understand liquid structure changing patterns, therefore enriching the phenomenology of liquid state physics. This is significant for engineering practices.

Mechanical properties and damping capacity of (SiCw+B4Cp)/ZK60A mg alloy matrix composite

Mg alloys possess the advantages of low density and high damping capacity and they are conventionally used in the aerospace and aviation industries. The mechanical properties of Mg alloy matrix composites reinforced with continuous fibers, short fibers or whiskers and particles are improved over those of unreinforced Mg alloys. The hybrid composite, that means its reinforcements are produced by an appropriate combination of short fibers (or whiskers) and particles, can satisfy the demands for reducing cost and improving properties. So the interest in Mg alloy matrix hybrid composites is increasing recently. In order to gain more data of this new kind of composites and to search for a better combination of different reinforcements, a Mg alloy matrix composite reinforced by hybrid SiC whiskers and B{sub 4}C particles was fabricated and its mechanical properties and damping capacity were studied in this paper.

Measurement of Electrical Conductivity of Porous Titanium and Ti6Al4V Prepared by the Powder Metallurgy Method

Porous titanium and Ti6Al4V are produced by the powder metallurgy method. Dependence of the electrical conductivity on the porosity and pore size is investigated and the experimental results are correlative and compared with several earlier models. A newly modified Mori–Tanaka relationship based on the effective field method is proposed, which is successfully applied to describe the dependence of the electrical conductivity of porous titanium and Ti6Al4V on the porosity. The pore size has a minor effect on the electrical conductivity of both samples.

Turing Patterns in a Reaction-Diffusion System

We have further investigated Turing patterns in a reaction-diffusion system by theoretical analysis and numerical simulations. Simple Turing patterns and complex superlattice structures are observed. We find that the shape and type of Turing patterns depend on dynamical parameters and external periodic forcing, and is independent of effective diffusivity rate σ in the Lengyel–Epstein model. Our numerical results provide additional insight into understanding the mechanism of development of Turing patterns and predicting new pattern formations.

Low-Frequency Internal Friction Study on the Structural Changes in Polymer Melts

With the help of the low-frequency internal friction method, we investigate the structural properties of polymer melts, such as amorphous polystyrene (PS), poly(methyl methacrylate) (PMMA), and semi-crystalline poly(ethylene oxide) (PEO). An obvious peak of relaxation type is found in each of the internal friction curves. The peak temperature Tp follows the relation Tp ≈ (1.15 – 1.18) Tg for PS and PMMA melts, while it follows Tp ≈ 1.22Tm for PEO melt, with Tg being the glass transition temperature and Tm the melting temperature. Based on the analysis of the features of this peak, it is found that this peak is related to the liquid-liquid transition temperature Tu of polymer melts. Mechanism of the liquid-liquid transition is suggested to be thermally-activated collective relaxation through cooperation. This finding may be helpful to understand the structural changes in polymer melts. In addition, the internal friction technique proves to be effective in studying dynamics in polymer melts.

Apparent Electrical Conductivity of Porous Titanium Prepared by the Powder Metallurgy Method

Porous titanium is produced by the powder metallurgy method. Dependence of the electrical conductivity on the porosity and pore size is investigated and the experimental results are compared with a number of models. It is found that the minimum solid area model could be successfully applied to describe the relationship between the electrical conductivity and the porosity of porous titanium. This kind of conductivity increases with increasing pore sizes.

UNIVERSAL SCALING LAW FOR ATOMIC DIFFUSION AND VISCOSITY IN LIQUID METALS

The recently proposed scaling law relating the diffusion coefficient and the excess entropy of liquid [Samanta A et al. 2004 Phys. Rev. Lett. 92 145901; Dzugutov M 1996 Nature 381 137], and a quasi-universal relationship between the transport coefficients and excess entropy of dense fluids [Rosenfeld Y 1977 Phys. Rev. A 15 2545], are tested for diverse liquid metals using molecular dynamics simulations. Interatomic potentials derived from the glue potential and second-moment approximation of tight-binding scheme are used to study liquid metals. Our simulation results give sound support to the above-mentioned universal scaling laws. Following Dzugutov, we have also reached a new universal scaling relationship between the viscosity coefficient and excess entropy. The simulation results suggest that the reduced transport coefficients can be expressed approximately in terms of the corresponding packing density.

Molecular Dynamics Simulations of Liquid Phosphorus at High Temperature and Pressure

By performing ab initio molecular dynamics simulations, we have investigated the microstructure, dynamical and electronic properties of liquid phosphorus (P) under high temperature and pressure. In our simulations, the calculated coordination number (CN) changes discontinuously with density, and seems to increase rapidly after liquid P is compressed to 2.5 g/cm3. Under compression, liquid P shows the first-order liquid-liquid phase transition from the molecular liquid composed of the tetrahedral P4 molecules to complex polymeric form with three-dimensional network structure, accompanied by the nonmetal to metal transition of the electronic structure. The order parameters Q6 and Q4 are sensitive to the microstructural change of liquid P. By calculating diffusion coefficients, we show the dynamical anomaly of liquid P by compression. At lower temperatures, a maximum exists at the diffusion coefficients as a function of density; at higher temperatures, the anomalous behavior is weakened. The excess entropy shows the same phenomena as the diffusion coefficients. By analysis of the angle distribution functions and angular limited triplet correlation functions, we can clearly find that the Peierls distortion in polymeric form of liquid P is reduced by further compression.

Final Structures of Crystallization of Liquid Copper Studied by Molecular Dynamics Simulation

Molecular dynamics simulation is performed to study the cooling rate effects on the resulting crystallization microstructure of bulk liquid copper (homogeneous nucleation). Our studies reveal that mixtures of the metastable hcp phase and most-stable fcc phase, in all sorts of proportion and in various forms such as layering and phase separation, can be obtained by controlling the cooling rate. We have also found that, below the glass forming critical cooling rate, the ratio of the number of fcc-type polyhedra to the number of hcp-type polyhedra at 0 K decreases with increase of the cooling rate.

Structural Characteristics of Liquid Sn

We investigate the structural properties of liquid Sn. With the help of the internal friction (tan Φ) method, it is found that a peak appears in the tan Φ−T curve, suggesting that an anomalous discontinuous temperature-induced structure change may take place in liquid Sn. From the experimental data of pair distribution functions, we calculate the viscosity η and the excess entropy S and it is found that there are a peak of viscosity in the η−T curve and a bend of excess entropy in the S−T curve, which give a positive support to the appearance of the internal-friction peak in the tan Φ−T curve.