Thermal Reaction of Silane with Acetylene and the Thermal Decomposition of Ethynylsilane
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THERMAL REACTION OF SILANE WITH ACETYLENE AND THE THERMAL DECOMPOSITION OF ETHYNYLSILANE H. E. O'Neal, J. W. Erwin and D. S. Rogers, Department of M. A. Ring, Chemistry, San Diego State University, San Diego, CA 92182
Decomposition of SiH4 in Acetylene The volatile products from the thermal reaction (414*C) of silane in excess acetylene are hydrogen, ethylene, vinylsilane, ethynylsilane, vinylethynylsilane (possibly divinylsilane) and ethynyl-divinylsilane (1,2). We have reexamined this reaction using a 3 C2 H2 /1 SiH4 reaction mixture and have obtained product yield curves for these products versus percent silane loss. We have also found that product curves are unaffected when propylene at pressures equal to that of acetylene is also present. Since only trace quantities of propylsilane are produced in the presence of propylene, we can rule out reactions involving silyl radicals. Thus the SiH4 -C 2 H2 reaction involves silylene and silene intermediates. The products can be explained by a mechanism similar to one proposed by Barton and Burns (3). SiH4 + (M) ---
SiH 2 + HCECH
SiH2 + H2 + (M)
HC=CH
->
->
~H
HC-CH2
L
I! ~ --
II
H2 C=CH 2
1/)
HCC
jHSiC H 2 3
LIV IV
->
j
H2 C=CHSiH >
H2 C=CHSiH + IIC=CH
7
HCECSiH3 (ES)
-
III
C-CH2
III
III
I
2
Si
Sil
SiH 2
+
Si
E-
SiC H] 2
V]
HC=aCSiH 2 C2 H3 (EVS)
Mat. Res. Soc. Symp. Proc. Vol. 32 (1984) Published by Elsevier Science Publishing Co., Inc.
384
V
>
H2 C-CHSiC 2 H3 + HCECH
H2 C=CHSiC 2 H3
LSi K j 1C=CH
->
7
(C 2H3 )
VI
VI
>
-
HCECSiH(C 2 H3 ) 2 (EDVS)
In addition to the volatile products, significant yields of solid polymer are also formed. The yield of volatile silicon products was 54% at 10% silane loss and dropped to only 18% at 50% silane loss. Polymerization of the highly unsaturated products is therfore inferred.
The Thermal Decomposition of Ethynylsilane The product yield curve for ethynylsilane reached a maximum value early This suggests that in the silane - acetylene reaction (T - 415%C). We have thus ethynylsilane is relatively unstable at this temperature. examined the decomposition of ethynylsilane (1% in argon) in a stirred-flow reactor at reaction pressures ranging from 5.8 to 8.5 torr between 589.6 and 783.7 0 K. No prior quantitative data on the thermal stability of ethynylsilane exists in the literature. An Arrhenius plot of the kinetic data (ln k vs l/T) showed considerable curvature with slopes which increased by as much as a factor of three between the lower and upper temperatures. The very low activation energy corresponding to the low temperature slopes strongly suggests wall induced decomposition and we have treated the data accordingly
(i'e:w h c = kwall + kgas phase),
data: whi,
fits
On this basis, a computer fit of the
the experimental observations quite well,
gives the
following rate constants: kwall = 1.5 x 103 x e-17,6000/RT sec- 1 kgas = 3.0 x 101 3 e-53,000/RT sec- 1 The volatile products of the ethynylsilane decomposition are predominantly acetylene and ethylene,
with very small
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