Greg H. Evans

A Mathematical Model of the Fluid Mechanics and Gas‐Phase Chemistry in a Rotating Disk Chemical Vapor Deposition Reactor

We describe a mathematical model of the coupled fluid mechanics and gas-phase chemical kinetics in a rotating-disk chemical vapor deposition reactor. The analysis uses a similarity transformation that reduces the problem to a one-dimensional boundary value problem. The deposition of silicon from silane is used as an example system. We present predictions of deposition rate as a function of susceptor temperature, spin rate, and carrier gas. 12 refs.

Design and Verification of Nearly Ideal Flow and Heat Transfer in a Rotating Disk Chemical Vapor Deposition Reactor

A research chemical vapor deposition reactor design is presented for a rotating disk configuration. The reactor can be operated under conditions such that nearly ideal, one-dimensional, infinite-radius disk behavior is achieved over most of the disk surface. Boundary conditions, flow stability under both isothermal and heated-disk conditions, and gas temperatures are addressed with both one- and two-dimensional numerical fluid mechanics models. In this paper experimental verification of the design using flow visualization and laser Raman thermometry are presented.

A study of traveling wave instabilities in a horizontal channel flow with applications to chemical vapor deposition

Abstract The flow and heat transfer of helium in a horizontal channel of height H and length L with a heated bottom surface and a cooled top surface are studied. Numerical solutions of the transient, two-dimensional Navier-Stokes and energy equations reveal that for conditions of interest in chemical vapor deposition (CVD), a thermal instability in the fluid (the Rayleigh-Benard instability), produced by the temperature difference between the horizontal surfaces of the channel, can result in traveling, transverse waves. The results show the effects of these waves on the flow and the heat transfer over a range of Reynolds numbers, Re = u H v o (10 −1 2 ) , Grashof numbers, Gr = gϵH 3 v o 2 (2.5 × 10 3 5 ) , aspect ratios, L H (4 L H , for two temperature ratios, ϵ = (T s − T o ) T o , 0.0333 and 2.333, corresponding to constant and variable property flow, respectively. The existence of transverse, traveling waves is shown to enhance the heat transfer from 50 to more than 300% over the condition without traveling waves. The important effect of the aspect ratio on the results is also emphasized.

EFFECTS OF BOUNDARY CONDITIONS ON THE FLOW AND HEAT TRANSFER IN A ROTATING DISK CHEMICAL VAPOR DEPOSITION REACTOR

Numerical solutions of the Navier-Stokes and energy equations have been obtained to predict the fluid flow, temperature profiles, and heat transfer in a rotating disk reactor. The effects of buoyancy, variable properties, and finite geometry have been included for helium. It is shown that recirculation of the gas can be reduced or eliminated by increasing the uniform velocity at the inlet of the reactor above the asymptotic value for a one-dimensional, variable-properties, infinite rotating disk. This is true for both adiabatic and isothermal reactor walls. Furthermore, a cooled reactor wall as opposed to an adiabatic reactor wall is shown to result in dramatically altered velocity and temperature fields and reduced recirculation of the gas. When recirculation of the gas is reduced or eliminated the disk heat transfer that results is highly uniform and in good agreement with the one-dimensional, variable-properties, infinite rotating disk result. These results are useful for the design and operation of ro...

Numerical Studies of Methane Catalytic Combustion Inside a Monolith Honeycomb Reactor Using Multi-Step Surface Reactions

Abstract The heterogeneous oxidation of methane-air mixture in a honeycomb catalytic reactor is investigated numerically in the present study. An improved multi-step surface reaction mechanism for methane oxidation on platinum is proposed so that surface ignition of lean methane-air mixtures is better modeled. First, this surface mechanism is used to determine the apparent activation energy of methane-air catalytic combustion. The predicted activation energies are found to agree well with the experimental data by Trimm and Lam (1980) and by Griffin and Pfefferle (1990). The chemical model indicates that, depending on the surface temperature, the surface reaction rate is dominated by either the oxygen desorption rate or by the methane adsorption rate. Second, the surface chemistry model is used to model a methane-air catalytic reactor with a two-dimensional flow code. The substrate surface temperatures are solved directly with a thermal boundary condition derived by balancing the energy fluxes at the gas-c...

Unsteady three-dimensional mixed convection in a heated horizontal channel with applications to chemical vapor deposition

Abstract The flow and heat transfer of a gas in a horizontal channel with a heated bottom surface, a cooled top surface, and adiabatic side walls is studied. Numerical solutions of the transient, threedimensional Navier-Stokes and energy equations reveal that the fluid is unstable thermally for conditions of interest in chemical vapor deposition (CVD). The instability appears as a combination of transverse, traveling waves and longitudinal rolls. The unsteady nature of the flow and the heat transfer is shown for two Reynolds numbers, Re = u H/ν 0 (5 and 10), Grashof number, Gr = geH3/ν20 = 5000, Prandtl number, Pr = ν 0 /α 0 = 2 3 , aspect ratios, L/H = 8 and W/H = 2, for the temperature ratio e = (Ts-T0)/T0 = 0.01, corresponding to constant property flow. The instability results in an increase in the average heat transfer from 15% to more than 40% above the fully developed condition in the absence of the instability.

Thermally unstable convection with applications to chemical vapor deposition channel reactorst

Abstract The three dimensional, thermally unstable flow and heat transfer of a gas have been studied in a horizontal channel with applications to chemical vapor deposition (CVD). The cases examined include flows that exhibit a longitudinal roll instability and a combination of both transverse, traveling waves and longitudinal rolls. Detailed results are presented for two values of the temperature ratio e = (T 1 −T 0 ) T o = 0.01 and 2.33 (helium), Grashof number Gr = geH 3 v 2 0 = 125000 , Prandtl number Pr = v 0 α 0 = 2 3 , aspect ratios L H = 10 and W H = 2 . For e = 0.01 and Reynolds number Re = u H v 0 = 250 , longitudinal rolls result whereas for Re = 100 a combination of transverse waves and longitudinal rolls occurs. For e = 2.33 and Re = 100 the longitudinal roll instability is present.

Buoyant instabilities in downward flow in a symmetrically heated vertical channel

This study of the downward flow of nitrogen in a tall, partially heated vertical channel (upstream isothermal at Ts∗, heated region isothermal at Tin∗, downstream adiabatic) shows the strong effects of buoyancy even for small temperature differences. Time-dependent oscillations including periodic flow reversals occur along the channel walls. Although the flow and heat transfer are asymmetric, the temperature and axial component of velocity show symmetric reflections at two times that are half a period apart and the lateral component of velocity shows antisymmetric reflections at the two times. There is strong interaction between the downward flow in the central region of the channel and the upward flow along the heated channel walls. At the top of the heated region, the upward buoyant flow turns toward the center of the channel and is incorporated into the downward flow. Along the channel centerline there are nonmonotonic variations of the axial component of velocity and temperature and a large lateral component of velocity that reverses direction periodically. Results are presented for Re = 219.7 and GrRe2=1.83, 8.0 and 13.7. The heat transfer and the frequency of the oscillations increase and the flow and temperature fields become more complex as GrRe2 increases. The results have applications to fiber drying, food processing, crystal growth, solar energy collection, cooling of electronic circuits, ventilation, etc.

A two-dimensional model of the chemical vapor deposition of silicon nitride in a low-pressure hot-wall reactor including multicomponent diffusion

Abstract A multidimensional model has been developed and applied to simulate the chemical vapor deposition (CVD) of silicon nitride from silicon tetrafluoride and ammonia in a low-pressure hot-wall reactor. The purpose of this work is to evaluate the effects of gas-phase transport and reactant depletion on the uniformity and rate of deposition of silicon nitride by CVD in order to provide a basis for reactor scaling and process control. Two irreversible surface reactions are used to model the deposition chemistry. Diffusion is shown to be important relative to convection in transporting gas-phase reactants to the surface where the chemical reactions occur. Reactant depletion also has a significant impact on the deposition. Multicomponent diffusion of the five reacting species is studied by solving the Stefan-Maxwell equations; the results are compared with those obtained using the simpler mixture-averaged approximation to multicomponent diffusion.

Mixed binary convection in a rotating disk chemical vapor deposition reactor

This study examines the effects of gas mixing on flow behavior in a rotating disk reactor. We restrict the study to an isothermal binary gas system flowing over a high speed rotating disk in a cylindrical reactor. Complex flow fields are produced as a result of the interaction that occur between the solutal buoyant force, the forced flow, and the flow induced by the rotation of the disk. Deviation from the ideal rotating disk flow is quantified with a radial shear stress parameter.