A Shock Tube Study of the Thermal Decomposition of Si₂H₆ Based on Si and SiH₂ Measurements

In the present investigation the thermal dissociation of disilane (Si 2 H 6 ) mixed with different additives was measured behind reflected shock waves. Atomic Resonance Absorption Spectroscopy (ARAS) was applied for detecting Si atoms and Ring Dye Laser Absorption Spectroscopy (RDLAS) for detecting SiH 2 radicals. Three different series of experiments were performed. In the first group of experiments mixtures of 0.1 to 0.25 ppm Si 2 H 6 diluted in Ar were shock heated to temperatures 2000 K≤ T ≤3135 K and pressures between 0.8 and 1.4 bar. From the Si‐measurements silane and silylene were found to be the main primary products during disilane decomposition. The SiH 2 absorption measurements were carried out at temperatures 1070 K ≤ T ≤ 1381 K and pressures between 0.35 and 1.3 bar. In these experiments the SiH 2 formation was found to be mainly caused by the dissociation reaction Si 2 H 6 ⇌ SiH 4 +SiH 2 (R1). The rate coefficient k 1 was determined by fitting SiH 2 profiles calculated based on a simplified reaction mechanism to measured profiles with k 1 to be a variable parameter. The value obtained was interpreted based on RRKM calculations. For bath gas concentrations of 3.1 × 10 −6 ≤[M]≤ 4.6 × 10 −6 mol cm −3 the rate coefficient k 1 can be summarized by the Arrhenius expression k 1 = 5.2×10 10 exp(‐16850 K/ T ) s −1 . For longer reaction times the SiH 2 radicals were consumed due to the reaction SiH 2 +SiH 2 ⇌Si 2 H 2 (R4). The rate coefficient k 4 was determined to be k 4 = 6.5×10 14 cm 3 mol −1 s −1 (70%). The second group of experiments was carried out in mixtures with molecular hydrogen as an additive. SiH 2 measurements were performed in the temperature range 1082 K≤ T ≤1417 K at pressures 0.24 bar ≤ p ≤ 1.23 bar by using initial mixtures of 15 to 30 ppm Si 2 H 6 and 5 to 50% H 2 diluted in Ar. The aim of this part was to increase the influence of the recombination reaction SiH 2 + H 2 → SiH 4 (R‐2) and to measure the dissociation barrier for the silane decomposition via the equilibrium constant. The data evaluation results in a mean value of E 0,2 = 244.1 kJ/mol which is in good agreement to previous investigations. In the last part of this study the influence of silane used as an additive during Si 2 H 6 decomposition was investigated. Initial mixtures of 30 ppm Si 2 H 6 and 300 to 1000 ppm SiH 4 diluted in Ar were used under conditions 1060 K ≤ T ≤ 1198 K at pressures between 0.36 and 1.3 bar. From SiH 2 concentration measurements an estimation for the rate coefficient of the disproportionation reaction SiH 2 +SiH 4 →H 3 SiSiH+H 2 (R5), k 5 = 1.3 × 10 13 cm 3 mol −1 s −1 , was found.