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Molecular Orbital Study of an Environmentally Enhanced Crack Growth Process in Silica
Author(s) -
Lindsay Curtis G.,
White Grady S.,
Freiman Stephen W.,
WongNg Winnie
Publication year - 1994
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1994.tb07115.x
Subject(s) - antibonding molecular orbital , molecule , formamide , molecular orbital , ammonia , nitrogen , chemistry , chemical physics , population , materials science , computational chemistry , organic chemistry , electron , atomic orbital , physics , quantum mechanics , demography , sociology
We have used molecular orbital calculations to investigate effects of environmental substances and strain on fracture of the Si─O bond in an H 6 Si 2 O 7 molecule. We believe that the tendencies observed simulate crack growth in silica in the vicinity of a crack tip. This study is focused on the initial Stage of environmental enhancement, in which a molecule of a substance from the environment approaches a reaction site in silica. Five environmental substances—ammonia, water, formamide, nitrogen, and argon—were considered. The total energy of each H 6 Si 2 O 7 –environmental molecule system was calculated with H 6 Si 2 O 7 in each of two strain conditions and with the environmental molecule at two different distances from the bridging O in H 6 Si 2 O 7 . This provides estimates of the average force required to move an environmental molecule toward the reaction site in silica. The energy difference due to environmental molecule position is relatively small for ammonia and water, and is relatively large for nitrogen and argon. Experimentally, ammonia and water have shown the greatest tendency to enhance crack growth in silica, whereas nitrogen has shown virtually no such tendency. The tendency for a substance to enhance crack growth therefore appears to be at least partly an inverse function of the energy required to move a molecule of that substance toward the reaction site on the silica surface. Electron population analyses indicate that oxygen atoms adjacent to the reaction site engage in antibonding interactions with certain environmental‐molecule atoms. These interactions are much smaller in magnitude when H 6 Si 2 O 7 is strained than when it is unstrained. It therefore appears possible that in a crack in silica, where an environmental molecule would experience restricted mobility, strain reduces steric hindrances to the approach of environmental molecules to the reaction site. Straining H 6 Si 2 O 7 also increases the magnitudes of the charges on Si and O at the reaction site, probably leading to increased electrostatic attractions of environmental molecules and H 6 Si 2 O 7 .