Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.

A field model interpretation of crack initiation and growth behavior in ferroelectric ceramics: change of poling direction and boundary condition

Strain energy density expressions are obtained from a field model that can qualitatively exhibit how the electrical and mechanical disturbances would affect the crack growth behavior in ferroelectric ceramics. Simplification is achieved by considering only three material constants to account for elastic, piezoelectric and dielectric effects. Cross interaction of electric field (or displacement) with mechanical stress (or strain) is identified with the piezoelectric effect; it occurs only when the pole is aligned normal to the crack. Switching of the pole axis by 90degrees and 180degrees is examined for possible connection with domain switching. Opposing crack growth behavior can be obtained when the specification of mechanical stress sigma(infinity) and electric field E-infinity or (sigma(infinity), E-infinity) is replaced by strain e and electric displacement D-infinity or (epsilon(infinity), D-infinity). Mixed conditions (sigma(infinity),D-infinity) and (epsilon(infinity),E-infinity) are also considered. In general, crack growth is found to be larger when compared to that without the application of electric disturbances. This includes both the electric field and displacement. For the eight possible boundary conditions, crack growth retardation is identified only with (E-y(infinity),sigma(y)(infinity)) for negative E-y(infinity) and (D-y(infinity), epsilon(y)(infinity)) for positive D-y(infinity) while the mechanical conditions sigma(y)(infinity) or epsilon(y)infinity are not changed. Suitable combinations of the elastic, piezoelectric and dielectric material constants could also be made to suppress crack growth. (C) 2002 Published by Elsevier Science Ltd.

Crystal-growth in floating zone with phase-change and thermosolutal convections

Floating zone crystal growth in microgravity environment is investigated numerically by a finite element method for semiconductor growth processing, which involves thermocapillary convection, phase change convection, thermal diffusion and solutal diffusion. The configurations of phase change interfaces and distributions of velocity, temperature and concentration fields are analyzed for typical conditions of pulling rates and segregation coefficients. The influence of phase change convection on the distribution of concentration is studied in detail. The results show that the thermocapillary convection plays an important role in mixing up the melt with dopant. The deformations of phase change interfaces by thermal convection-diffusion and pulling rods make larger variation of concentration field in comparison with the case of plane interfaces.

Coupling of melting-solidification phase-change convection with floating-zone convection under microgravity

A finite element algorithm is used to analyze the process of floating zone crystal growth under microgravity. The effect of phase change convection coupled with surface tension convection is considered. The results show that the rate of crystal growth is very important. The single-crystal-melt interface is steeper than the feed-melt interface during the process of crystal growth. When the rate exceeds a critical value, the Marangoni vortex near the feed-melt interface will become so large that a secondary vortex will exist.

The catastrophic change of steady flow in collapsible tubes

Based on a general background we have developed a mechanical model of the catastrophic change of steady flow in collapsible tubes and got the critical conditions for the catastrophic change analyzing the effects of viscosity, gravity and the longitudinal gradient of external pressure. Several simple flows have been discussed. Furthermore, we conducted three types of model experiments, Results agree with the theoretical critical conditions qualitatively.