FORMATION OF HYDROGEN-ATOMS IN SILANE PYROLYSIS BEHIND SHOCK-WAVES - KINETICS AND THERMOCHEMISTRY OF SIH4, SIH3, AND SIH2 DECOMPOSITION

The kinetic of the formation of hydrogen and oxygen atoms during pyrol ysis of SiH4-Ar and SiH4-A-Ar (A = H-2, O2, and CO2) mixtures behind r eflected shock waves is studied over the temperature range of 1200 - 2 300 K, using the ARAS detection technique. For the first time, a homog eneous production of H atoms in the pyrolysis of silane is observed. A n analysis shows that the H atoms are generated in secondary reactions between fragments of the SiH4 decomposition. To describe the experime ntally measured H-atom profiles, a tentative 15-step mechanism is prop osed. The influence of several additives (H-2, O2, and CO2) on the H-a tom buildup is tested. The O-atom monitoring in the runs with a SiH4-O 2-Ar mixture is used to determine the rate constant for SiH4 dissociat ion. The RRKM analysis is applied to treat our data and that in the ex perimental literature. The low- (k0) and high-pressure (k(infinity)) r ate constants for SiH4, SiH3, and SiH2 dissociation are described by t he following expressions (E, in kJ/mol): k0(SiH4) = 5.2 x 10(18)(T/100 0)-3.95 x exp(-243/RT) cm3 mol-1 s-1, k(infinity)(SiH4) = 1.4 x 10(14) (T/1000)0.68 x exp(-243/RT) s-1, k0(SiH3) = 1.4 x 10(17)(T/1000)-3.05 x exp(-187/RT) cm3 mol-1 s-1, k(infinity)(SiH3) = 1.0 x 10(13)(T/1000) -0.55 x exp(-187/RT) s-1, k0(SiH2) = 5.2 x 10(15)(T/1000)-1.76 x exp(- 160/RT) cm3 mol-1 s-1. The possibility that a chain process occurs dur ing SiH4 pyrolysis is discussed.