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 (Si2 H6 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 we re performed. In the first group of experiments mixtures of 0.1 to 0.2 5 ppm Si2H6 diluted in Ar were shock heated to temperatures 2000 K les s than or equal to T less than or equal to 3135 K and pressures betwee n 0.8 and 1.4 bar. From the Si-measurements silane and silylene were f ound to be the main primary products during disilane decomposition. Th e SiH2 absorption measurements were carried out at temperatures 1070 K less than or equal to T less than or equal to 1381 K and pressures be tween 0.35 and 1.3 bar. In these experiments the SiH2 formation was fo und to be mainly caused by the dissociation reaction Si2H6 reversible arrow SiH4 + SiH2 (R1). The rare coefficient k(1) was determined by fi tting SiH2 profiles calculated based on a simplified reaction mechanis m to measured profiles with k(1) to be a variable parameter. The value obtained was interpreted based on RRKM calculations. For bath gas con centrations of 3.1 x 10(-6) less than or equal to [M] less than or equ al to 4.6 x 10(-6) mol cm(-3) the rate coefficient k(1) can be summari zed by the Arrhenius expression k(1) = 5.2 x 10(10) exp (-16850 K/T) s (-1). For longer reaction times the SiH2 radicals were consumed due to the reaction SiH2+SiH2 reversible arrow, Si2H2 + H-2 (R4). The rate c oefficient k(4) was determined to be k(4) = 6.5 x 10(14) cm(3) mol(-1) s(-1) (+/- 70%). The second group of experiments was carried out in mi xtures with molecular hydrogen as an additive. SiH2 measurements were performed in the temperature range 1082 K less than or equal to T less than or equal to 1417 K at pressures 0.24 bar less than or equal to p less than or equal to 1.23 bar by using initial mixtures of 15 to 30 ppm Si2H6 and 5 to 50% H-2 diluted in Ar. The aim of this part was to increase the influence of the recombination reaction SiH2+H-2-->SiH4 ( R-2) and to measure the dissociation barrier for the silane decomposit ion via the equilibrium constant. The data evaluation results in a mea n value of E(0.2) = 244.1 kJ/mol which is in good agreement to previou s investigations. In the last part of this study the influence of sila ne used as an additive during Si2H6 decomposition was investigated. In itial mixtures of 30 ppm Si2H6 and 300 to 1000 ppm SiH4 diluted in Ar were used under conditions 1060 K less than or equal to T less than or equal to 1198 K at pressures between 0.36 and 1.3 bar. From SiH2 conc entration measurements an estimation for the rate coefficient of the d isproportionation reaction SiH2+SiH4-->H3SiSiH+H-2 (R5),k(5) = 1.3x10( 13) cm(3) mol(-1) s(-1), was found.