No abstract

A numerical model was developed to predict the behavior of a particle in a radio-frequency (rf ) plasma. The analysis obtained the heat, mass, and momentum transfer of a single particle injected into an rf plasma. The governing equations for vaporization of a liquid particle were taken from the model which was used to simulate the vaporization process of liquid droplets in a rocket combuster. All the thermodynamical and transport properties were calculated as functions of temperature in order to simulate the actual behavior more precisely. The boundary conditions are a finite temperature at the particle center and a heat balance at the particle surface. Thermodynamical equilibrium at the particle surface was always assumed. ’’Moving-boundary problems’’ in the case of melting or vaporizing processes were solved by the use of a ’’moving grid system’’. Four sizes of iron particles injected into the rf argon plasma have been computed: r0=20, 40, 60, and 80 μ. The iron particles with radii less than 70 μ could be melted under the assumed plasma conditions. It was found that the potentiality as concerns powder processing of an rf plasma was overestimated by previous models.

A numerical model has been developed for predicting the two-dimensional flow and temperature fields in a hybrid plasma which is characterized by the superposition of an rf plasma and an arc jet. Calculations have been made for the confirmation of a prominent feature of the hybrid plasma. As might be expected, the derived results suggested that the hybrid plasma has a possibility to offer higher efficiencies for practical processing than other conventional plasmas. Particularly noteworthy is the disappearance of a recirculation eddy found in a usual rf plasma, by the presence of the high velocity and high-temperature channel caused by an arc jet flow. Moreover, the relative insensitive character of the exit gas enthalpy to the rf input power was found. Based on the theoretical investigations, a reactor was designed to confirm the effectiveness of the hybrid plasma for chemical syntheses. Using the reactor, ultrafine amorphous Si3N4 with the nitrogen content of 37±0.5 wt. % and the size of 10–30 nm, which can meet the purity and size requirements of an ideal Si3N4 powder, could be prepared successfully by a reaction of SiCl4 and NH3. These theoretical and experimental investigations confirmed that the hybrid plasma torch is an effective reactor for the preparation of ultrafine refractory compounds by plasma chemical vapor deposition (CVD) processes.

No abstract

No abstract