PHYSICS

G. G. DEVYATYKH, G. K. BORISOV

SEPARATION OF SILICON ISOTOPES IN MONOSILANE BY THE THERMODIFFUSION METHOD

(Presented by Academician N. M. Zhavoronkov, October 4, 1962)

Natural silicon contains three stable isotopes, \(\mathrm{Si}^{28}\), \(\mathrm{Si}^{29}\), \(\mathrm{Si}^{30}\). In small quantities, separation of silicon isotopes has been carried out on a calutron \((^{1,2})\). This method is still very expensive. A small separation was obtained in the rectification of monosilane \((^3)\). The isotope effect in the vapor pressure of monosilane is very small; therefore separation of silicon isotopes by rectification of monosilane is a technically difficult problem.

In the present work the possibility of separating silicon isotopes by the thermodiffusion method was investigated. Monosilane was used as the working gas, because the relative difference in the masses of its isotopic molecules is greater than for any other silicon compound. At temperatures above \(300^\circ\), the rate of thermal decomposition of monosilane becomes appreciable \((^4)\). Therefore the temperature of the hot wall cannot be maintained above \(300^\circ\).

Experiments were carried out in a metal column of the coaxial-cylinder type. Such columns are more effective than wire-type columns when operating with low values of the hot-wall temperature. The length of the working part of the column was \(102\) cm, the diameter of the inner cylinder \(46\) mm, and the width of the gap between the cylinders \(2\) mm. The inner cylinder was maintained at a temperature of \(250^\circ\), and the outer one was cooled with tap water to a temperature of \(\sim 15^\circ\). Samples of monosilane taken during the separation process were converted into silicon tetrafluoride and analyzed on an MI-1305 mass spectrometer. From the results of the analysis the separation factor was determined.

Fig. 1. Dependence of the separation factor on pressure. \(a\) — \(\mathrm{Si}^{30}\mathrm{H}_4\), \(b\) — \(\mathrm{Si}^{29}\mathrm{H}_4\)

\[ F=\frac{(N_1/N_2)_{\text{bottom of column}}}{(N_1/N_2)_{\text{top of column}}}; \]

\(N_1\) is the atomic fraction of the isotope \(\mathrm{Si}^{30}\) or \(\mathrm{Si}^{29}\); \(N_2\) is the atomic fraction of the isotope \(\mathrm{Si}^{28}\).

The experimental results are shown in Fig. 1. They are described by the equations

\[ \ln F_{30}=\frac{0.134P^2}{1+0.0847P^4}, \qquad \ln F_{29}=\frac{0.0645P^2}{1+0.0759P^4}, \]

in which \(P\) is the pressure of monosilane in atmospheres (the subscripts 30 and 29 refer to the isotopes \(\mathrm{Si}^{30}\) and \(\mathrm{Si}^{29}\), respectively).

By comparing the experimental results with the theory of the thermodiffusion method \((^5)\), the value of the ratio \(K_p/K_c = 1.81\) was determined, where \(K_c\) is the coefficient of convective mixing and \(K_p\) is the coefficient of para-

mixing. This ratio has an admissible value. It is smaller than, for example, the average value of \(K_p/K_c\) in the thermodiffusion separation of carbon isotopes in carbon monoxide \({}^{(6)}\). In calculating the value of \(F\) from \({}^{(5)}\), the value of the viscosity of monosilane was taken from \({}^{(7)}\).

The theoretical ratio \(\ln F_{30}/\ln F_{29} = 1.97\). In our experiments the mean value of this ratio was 1.96.

The walls of the column (steel 3) catalyze the decomposition of monosilane. In the first (preliminary) experiments, partial decomposition of monosilane occurred. After the walls of the column became coated with a thin film of silicon, decomposition of monosilane was not observed*.

We express our gratitude to E. I. Ovcharenko for assistance in carrying out the experiment.

Scientific Research Institute of Chemistry
at Gorky State University
named after N. I. Lobachevsky

Received
28 VI 1962

CITED LITERATURE

1. W. Groth, Zs. Elektrochem. u. angew. phys. Chem., 54, 5 (1950).
2. C. P. Keim, J. Appl. Phys., 24, 1255 (1953).
3. G. G. Devyatykh, G. K. Borisov, A. M. Pavlov, DAN, 138, No. 2, 402 (1961).
4. T. R. Hogness, T. L. Wilson, W. C. Johnson, J. Am. Chem. Soc., 58, 108 (1936).
5. K. Jones, V. Ferri, Separation of Isotopes by the Thermodiffusion Method, Moscow, 1947.
6. N. G. Tunginy, G. G. Devyatykh et al., ZhTF, 28, No. 4, 881 (1958).
7. A. O. Rankine, C. J. Smith, Proc. Phys. Soc., 34, 181 (1922).

* Duration of one experiment: 6 h.