Tham khảo tài liệu 'crystalline silicon properties and uses part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Defect Related Luminescence in Silicon Dioxide Network A Review 139 I u Ọ JO 110 120 130 140 150 160 Si-O-Si bond angle a degree 170 180 9 w Fig. Total energy as a function of Si-O bond length and Si-O-Si bond angle. Contour lines in units of kJ mol van Santen et al. 1991 . Due to the high Si-O bond energy eV the crystalline quartz is resistant to chemical weathering it is only soluble in hydrofluoric acid and in hot alkalis and to corrosion Lamkin et al. 1992 Lide 2004 . It is hard Mohs hardness 7 brittle and has a very high melting point at around 1710 C Lide 2004 . Due to its wide energy band gap of about 9 eV it is optically transparent and shows low electrical conductivity Fan et al. 1998 . Generally a homobond is electrostatically neutral although both Si-Si and O-O bonds may become positively charged by trapping holes. Irrespective of their electrical charges defects can be divided into two classes diamagnetic and paramagnetic. As a general role all stable paramagnetic defects have optical absorption bands associated with them since they represent half-occupied energy transitions to the valence band and electron transitions to the conduction band are both possible. Diamagnetic defects may have absorption bands associated with electron transitions to the conduction band. The confirmed examples of diamagnetic defects in ữ-SiO2 have electron absorption bands in the ultraviolet or vacuum ultraviolet spectral regions implying that the uppermost filled levels of these states lie below the middle of the 9 eV band gap Griscom 1977 . A variety of defect structures are known to exist in silica materials and were one of the major subjects of extensive experimental and theoretical studies Stevens-Kalceff 2000 Song et al. 2001 Lu et al. 2002 . Many aspects regarding the nature of the defects and their correlated properties are still controversial and not yet completely understood. Quite a lot of defect types have been discussed in the .
