Chemical Vapor Deposition and Isotope Separation

Abstract

Because chemical vapor deposition (CVD) is based on diffusion, isotope separation should take place if the process is in the diffusion‐limited range. The enrichment in a horizontal reactor as used for silicon epitaxy is calculated including thermal diffusion effects. It is found that whereas the deposition rate decreases exponentially along the reactor with characteristic length Λ, the isotope enrichment decreases linearly over the length from maximum to zero at x=normalΛ . The maximum value depends in a complex way on the masses and concentrations of the gas to be separated and the second gas, present as a reaction component or carrier gas. The requirements for efficient CVD isotope enrichment, which include fast mixing, a high deposition rate, and high enrichment ratio can be satisfied better by a liquid‐source CVD system. An experimental arrangement is described and discussed in terms of heat transfer by film boiling. The absence of an unstable range in the plot of the heat flux against the temperature is ascribed to the effects of the decomposition process which dominates the evaporation. The importance of the increased rate of heat transfer due to the reversible accumulation of energy in chemical reaction products is stressed. The effect of an applied electric field is mentioned. Experiments with carbon deposition from C12H26 have shown a 12C/13C enrichment ratio up to 0.8%, whereas normalΔM/2M=0.3% . This effect is attributable to mass and thermal diffusion, selective evaporation, spray fractionation by drops acting as microultracentrifuges, and geometry effects. The possibility of achieving uranium isotope separation with a CVD process has been considered theoretically. Thermodynamically possible reactions are specified by which liquid UF6 is transformed into solid UF3 , UF4 , and UF5 . In this way uranium should be transported quickly from one condensed phase to another with an increasing lighter isotope concentration in the deposited reaction product. The reactor products can easily be retransformed into UF6 so that the enrichment cycle can be repeated until the desired isotope ratio is achieved. In principle CVD can also be used as a collector mechanism with laser‐induced isotope enrichment.

Chemical Vapor Deposition of Erosion‐Resistant TiB2 Coatings

“…The TiB~ coatings were deposited according to the reaction TIC14 + 2BCI:~ + 5H~ --* TiB._, + 10 HC1 [1] This reaction has been used by various researchers (2)(3)(4)6) to deposit TiB2. The thermodynamics and kinetics of the reaction have been studied extensively (7,8). Lower depo-'Battelle Columbus Laboratories.…”

Thermodynamics of ZrC — Equilibrium Condition Calculated for Deposition from a CH 4 ‐ ZrCl4 ‐ H 2 Gaseous Mixture

“…CVD Diagram of ZrxCl-x from a CH4-ZrCI4-H2 Gaseous Mixture Thermodynamic calculations based on the minimization of the total free energy of a complex chemical system have been used by others to determine conditions for chemical vapor deposition (10)(11)(12). In the present case, the conditions for deposition of zirconium carbide have been calculated as a function of the input gas pressures (P~ P~ P~ at 1400 and 1900 K, under a total pressure of 1 atm.…”

Isotopes Separation Method using Physical Vapor Deposition Technique

“…235 UF 6 and 238 UF 6 . Severin and Lydtin [17] also reported the uranium isotope separation through chemical vapor deposition (CVD) method.…”