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

Abstract

AbstractIn the present investigation the thermal dissociation of disilane (Si2H6) 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 SiH2 radicals. Three different series of experiments were performed.In the first group of experiments mixtures of 0.1 to 0.25 ppm Si2H6 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 SiH2 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 SiH2 formation was found to be mainly caused by the dissociation reaction Si2H6⇌ SiH4+SiH2 (R1). The rate coefficient k1 was determined by fitting SiH2 profiles calculated based on a simplified reaction mechanism to measured profiles with k1 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 k1 can be summarized by the Arrhenius expression k1 = 5.2×1010 exp(‐16850 K/T) s−1. For longer reaction times the SiH2 radicals were consumed due to the reaction SiH2+SiH2 ⇌Si2H2 (R4). The rate coefficient k4 was determined to be k4 = 6.5×1014 cm3 mol−1 s−1 (70%).The second group of experiments was carried out in mixtures with molecular hydrogen as an additive. SiH2measurements 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 Si2H6 and 5 to 50% H2 diluted in Ar. The aim of this part was to increase the influence of the recombination reaction SiH2 + H2→ SiH4 (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 E0,2 = 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 Si2H6 decomposition was investigated. Initial mixtures of 30 ppm Si2H6 and 300 to 1000 ppm SiH4 diluted in Ar were used under conditions 1060 K ≤ T ≤ 1198 K at pressures between 0.36 and 1.3 bar. From SiH2 concentration measurements an estimation for the rate coefficient of the disproportionation reaction SiH2+SiH4 →H3SiSiH+H2 (R5), k5 = 1.3 × 1013 cm3 mol−1 s−1, was found.