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Thermal Decomposition of Molecules Relevant to Combustion and Chemical Vapor Deposition by Flash Pyrolysis Time-of-Flight Mass Spectrometry


Flash pyrolysis coupled with vacuum-ultraviolet photoionization time-of-flight mass spectrometry was used to study the thermal decomposition mechanisms of molecules relevant to fuel combustion and the chemical vapor deposition (CVD) of SiGe, SiC, and GeC. For combustion research, the thermal decomposition of benzyl radical, n-alkanes CnH2n+2 (n = 5-8 and 10), 1-butyl radical, and 1-pentyl radical was performed. Benzyl was confirmed to decompose primarily by ejection of H atom after significant isomerization with loss of methyl observed as a minor decomposition pathway. Thermal decomposition of n-alkanes by C-C bond homolysis was directly observed with preferential fission of central C-C bonds. Methyl radical formation by homolysis of the α C-C bond was observed to be the least significant C-C bond homolysis pathway. Decomposition of 1-butyl by β-scission to form ethene and ethyl radical was directly observed. Significant isomerization of 1-pentyl radicals by 1,4-H migration followed by β-scission was observed at temperatures below 900 K with direct β-scission of 1-pentyl becoming increasingly prevalent at higher temperatures. Gas phase CVD research included pyrolysis of SiH4/GeH4 mixture, tetramethylsilane, tetramethylgermane, and several methylchlorosilanes. Formation of highly unsaturated SiGeHz, Si2GeHz, Si3GeHz, SiGe2Hz, SiGe3Hz and SiGe5Hz clusters in the pyrolysis of a 1 to 1 SiH4/GeH4 mixture by a silylene/germylene insertion mechanism was observed. The decomposition of tetramethylsilane and tetramethylgermane by Si-C bond homolysis was confirmed and newly observed secondary decomposition mechanisms were demonstrated. The decomposition of methyltrichlorosilane (MTS), an established SiC CVD precursor, was studied. Decomposition occurred primarily via Si-C bond homolysis at temperatures below 1350 K with elimination of CH3Cl to form SiCl2 becoming increasingly important as the pyrolysis temperature was increased. The mechanism of MTS decomposition was also compared to two other potential precursors, dimethyldichlorsilane and methyldichlorosilane.

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