Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 9
2. Literature review
In this chapter, the main points relating to the glasses studied here (tellurite and
flurotellurite) will be outlined. It will begin with a brief outline of the glass transition,
then the general theories, structural and kinetic, of glass formation will be summarised.
Important results from early studies on tellurite glasses will subsequently be reviewed,
followed by the structure of tellurite glasses. Finally, the impact of fibre optics will be
presented, concluding with research in that area on tellurite and fluorotellurite glasses.
2.1. The nature of glass
2.1.1. Behaviour of ‘typical’ glass-forming materials on melt-cooling
Many inorganic elements and compounds (e.g. Zn, LiCl, Fe, CdBr2 and Na) melt to form
liquids with a viscosity of the order of magnitude as water (≈ 10 -4 to 10 -3 Pa.s) [1]. When
these types of liquids are cooled, rapid crystallisation occurs at the melting point. Even
when the cooling is fast, in general, crystallisation cannot be avoided. In some cases,
small droplets of these types of materials may be supercooled far below the freezing
point, but cooling cannot be carried out indefinitely without crystallisation occurring,
although theoretically any liquid can be cooled to form a glass if quenching is rapid
enough. However, a number of materials exist which melt to form very viscous liquids,
of the order of ≈ 10 4 to 10 6 Pa.s (e.g. SiO2, BeF2, B2O3, As2O3, P2O5, and GeO2) [1]. If