Duanming Zhang

Fractal geometry model for effective thermal conductivity of three-phase porous media

An approximate model, a fractal geometry model, for the effective thermal conductivity of three-phase/unsaturated porous media is proposed based on the thermal-electrical analogy technique and on statistical self-similarity of porous media. The proposed thermal conductivity model is expressed as a function of porosity (related to stage n of Sierpinski carpet), ratio of areas, ratio of component thermal conductivities, and saturation. The recursive algorithm for the thermal conductivity by the proposed model is presented and found to be quite simple. The model predictions are compared with the existing measurements. Good agreement is found between the present model predictions and the existing experimental data. This verifies the validity of the proposed model

Improved two-temperature model and its application in femtosecond laser ablation of metal target

An improved two-temperature model to describe femtosecond laser ablation of metal target was presented. The temperature-dependent heat capacity and thermal conductivity of the electron, as well as electron temperature-dependent absorption coefficient and absorptivity are all considered in this two-temperature model. The tailored two-temperature model is solved using a finite difference method for copper target. The time-dependence of lattice and electron temperature of the surface for different laser fluence are performed, respectively. The temperature distribution of the electron and lattice along with space and time for a certain laser fluence is also presented. Moreover, the variation of ablation rate per pulse with laser fluence is obtained. The satisfactory agreement between our numerical results and experimental data indicates that the temperature dependence of heat capacity, thermal conductivity, absorption coefficient and absorptivity in femtosecond laser ablation of metal target must not be neglected. The present model will be helpful for the further experimental investigation of application of the femtosecond laser.

A new comprehensive model for the pyroelectric property of 0-3 ferroelectric composites

A new comprehensive model has been developed for the pyroelectric activity of 0-3 ferroelectric composites which includes additional contributions from the electrical conductivity of the constituents and the frequency of measurement when the secondary pyroelectric effect resulting from the coupled thermoelectroelastic behaviour is considered. The boundary conditions are obtained in terms of the Laplace equation and Maxwells equations. The analytical expressions have been derived for primary and secondary pyroelectric coefficients. The effects of the electrical conductivity of the matrix σm and the measuring frequency f on the pyroelectric property have been investigated in detail. Possible methods for improving the pyroelectric effect are also discussed. Our model can reflect comprehensively the pyroelectric property of the material and shows reasonably good agreement with experimental data.

Effect of precursor solution concentration on the grain orientation of Bi3.15Nd0.85Ti3O12 thin films

Polycrystalline Bi3.15Nd0.85Ti3O12 (BNT) thin films were fabricated on Pt/Ti/SiO2/Si by the sol–gel method. The dependence of precursor solution concentration (PSC) on the grain orientation of the films was observed, and a model was proposed to explain it by taking into account the effect of bulk nucleation. With increasing PSC, the BNT thin film turned from c-axis-preferential-oriented to random-oriented. It is suggested that a competition exists between the nucleation and grain growth on the grain orientation. When the PSC is low, the former plays a predominant role, while in the opposite case, the latter is dominant. The bulk nucleation inhibits the homoepitaxial grain growth, and it gradually replaces the heterogeneous nucleation as the main nucleation mechanism with the increase in PSC.

Pyroelectric properties of ferroelectric ceramic/ferroelectric polymer 0-3 composites

A general analytic expression is derived for pyroelectric coefficient p of a 0-3 composite composed of ferroelectric ceramic particles dispersed in a ferroelectric polymer by employing the Lapalace equation in the static case where three factors are considered including the interaction between ceramic particles, the contributions of the polymer, and the poling ratios of the two constituents. Good agreement is obtained between theoretical and experimental results when the volume fraction v(2) of the ceramic particles is below 0.5. The limitation for the particle volume fraction v(2)>0.5 is also discussed according to a simple model

Effect of electrical conductivity on the polarization behaviour and pyroelectric, piezoelectric property prediction of 0–3 ferroelectric composites

We have investigated the effect of electrical conductivity of the constituents on the poling behaviour of the ceramic inclusions in 0–3 ferroelectric composites which comprise a dilute suspension of spherical particles uniformly distributed in the matrix material. A new model for the pyroelectric and piezoelectric properties in terms of the poling conditions (poling field and poling time) has been developed to include electrical conductivity. Simulated results show that conductivity plays an important role in the poling process. Properly increasing the conductivity of the matrix σm can enhance the polarization in the ceramic inclusion of the composite Pi, thereby making the poling of the composite more efficient. In contrast, higher conductivity of the ceramic inclusion σi results in lower polarization Pi, which is unfavourable to the poling of the composite. These results provide insights into the observed behaviour of 0–3 composites. The model predicts the pyroelectric and piezoelectric properties under different poling conditions, which agree well with the corresponding experimental data.

Hysteresis loops of first-order ferroelectric bilayers or superlattices and their size effect

Based on Landau–Ginzburg phenomenological theory, a first-order ferroelectric bilayer or superlattice with an antiferroelectric interface coupling has been studied by taking into account the spatial variation of polarization within each constituent film and the surface effect. The hysteresis loops are obtained and the size effect of a bilayer is discussed. The loop patterns vary between typically antiferroelectric and typically ferroelectric depending on the thickness ratio, the coupling constant, the thickness and the extrapolation length. The antiferroelectric coupling has a great effect on the phase transition of a bilayer. It is shown that the size-driven phase transition cannot be observed in a ferroelectric bilayer in the case of strong antiferroelectric coupling.

A new model for studying the plasma plume expansion property during nanosecond pulsed laser deposition

A physical model is proposed to study the laser-induced plasma dynamics expansion during irradiation of material by a high-intensity nanosecond pulsed laser beam. Based on a consideration of the plasma ionization effect and local conservations of mass, momentum and energy, combined with the assumption that plasma can be viewed as a compressible ideal fluid and a high temperature–high pressure ideal gas, we developed a new dynamics expansion mechanism for plasma produced by pulsed laser radiation. A set of new plasma expansion dynamics equations based on our model are first deduced. Then, taking the example of the carbon target, using the finite difference method, the plasma flow dynamics into a vacuum, such as plasma spacial number density distribution, plasma number density and velocity evolvement in radial and axial directions, are studied in detail. The results show that there are some main factors affecting the velocity distribution of the plasma, including the temperature of the plasma, the ionization fraction of the plasma and the dynamic source provided by the vaporized material. The velocity uz in the longitudinal direction is different from ur in the transverse direction because of huge initial eject velocity and evaporated dynamic source. The velocities of the plasma calculated by this model are found to be in better agreement with the experimental results derived from the work of Sanz et al (1985 Plasma Phys. Control. Fusion 27 329) compared with the conventional model.

Model for the magnetoresistance of the silver-rich Ag2+δSe and Ag2+δTe thin films

This work proposes a model for linear magnetoresistance (MR) by considering Ag2+δSe and Ag2+δ Te thin films as two-dimensional two-component (silver metal component and semiconductor component) compositions. The model addresses the dependence of the MR on the magnetic field, temperature and the conductivities of the two components without the magnetic field. The model predictions agree well with the available experimental data. It is found that there is a critical volume fraction of the silver component at which the MR reaches a maximum value. We interpret the occurrence of the critical volume fraction as a consequence of the percolation between silver particles and the variation of the disorder of the microstructure in the material.

The topology of an accelerated growth network

We present and investigate a general nonlinear growth network model which incorporates accelerated growth of nodes and edges, where the growth rates of edges and nodes are all time-dependent power-law functions. The acceleration of edges determines the proportion of the internal edges to the external edges, which play a key role influencing the structure of the network. On the other hand, the effects of the acceleration of nodes on the topology of the network are discussed in the present work. This model predicts an observable two-regime scale-free degree distribution, where the scaling exponents are γ1 < 2 and γ2 ≈ 3, respectively. The crossover point kcross of the degree distribution is adjusted by the growth rates of nodes and edges. The nontrivial clustering coefficient and degree assortativity coefficient are relevant to the acceleration of nodes and edges.