Lu Wenqiang

Preliminary study of non-isothermal phase change phenomena in vertical Bridgman crystal growth

Axisymmetric dual reciprocity boundary element method (DRBEM) with augmented items is extended to simulate the heat and mass transfer problems in the vertical Bridgman method (VBM) crystal growth of HgCdTe and CdZnTe. Axial solute concentration redistribution of three regions numerically reappears, and the influence of the pulling rate of the ampoule on it is further studied. Secondly, one dimensional transient phase change phenomena is studied, and non-isothermal phase change phenomena is obtained from the initial transient region through the steady growth region to the final transient region. Thirdly, the two-dimensional axisymmetric phase change interface position, interfacial shape and the temperature field of the melt and the crystal are numerically captured under the condition to arrive at the steady state with zero pulling rate of the ampoule. Finally, the study of transient axisymmetric non-isothermal phase change phenomena is stressed and the results are compared with those in isothermal phase change. The influence of the pulling rate on non-isothermal phase change phenomena is revealed.

The stratification phenomena in the thermal and solutal fields of double-diffusive convection in a circular cylinder

The structure of double diffusive convection in a circular cylinder cavity has been numerically studied. The numerical results exhibit some new characters of non-horizontal stratifications of thermal and solutal fields: in the stratification state, the isothermal lines near the sidewall are higher than that near the symmetry axis, while the isoconcentration lines near the symmetry axis are relatively high. The mechanism of these non-horizontal stratifications is illustrated by comparing double-diffusive convection with natural convection driven by thermal buoyancy or solutal buoyancy alone. The effects of Lewis number Le and buoyancy ratio N on the non-horizontal stratifications of thermal and solutal fields are also investigated. The results show that: at a given time (t = 0.2), with an increase in Le (Le = 0–15), the area influenced by solute diffusion decreases; for isothermal line, the gradients of it initially increase, but it tends to be horizontal at the top of the cavity when the Lewis number is higher than 10. When N varies from 0 to 2, the isoconcentration lines tend to be horizontal while the gradients of isothermal line increase.

Study on constant-pressure specific heat of non-equilibrium phase change process in gas-liquid two-phase flow system

In this paper, the air-water vapor-water system is taken as an example, and the formula of constant-pressure specific heat during non-equilibrium phase change process in the two-phase flow system is deduced using the theory of two-phase flow and thermophysics. The constant-pressure specific heat of non-equilibrium phase change process is calculated with the corresponding numerical model, and the numerical results are compared to those of the equilibrium phase change process. It is shown that in evaporation process, the variational rate of the non-equilibrium specific heat increases with increasing initial fluid temperature and particle mass fraction. The smaller particle radius is, the faster the variational rate is. Meanwhile, the constant-pressure specific heat of equilibrium process is higher than that of the non-equilibrium process all the time.

Controllable synthesis of large scale, catalyst-free, lateral ZnO nanowires network

We report a novel method for large scale catalyst-free zinc oxide nanowires (ZnO NWs) network growth on Si substrate using chemical vapor deposition (CVD). The SiO2 layer of the Si substrate is photoetched to SiO2 square pillar array which works as the growth core of urchin-like ZnO NW lump, and the NWs cross over each other forming an integrated network. This method achieves controlled growth of lateral ZnO NW network and allows the in situ nano device fabrication. Meanwhile, the vapor deposition mechanism is discussed and confirmed by contrast experiments along with different growth conditions, which realizes growth on large size substrate. According to the result of Field Emission Scanning Electron Microscope (FE-SEM), the ZnO NWs are wurtzite hexagonal single-crystalline and the network is intact with massive intersection points.