3.9 Triboluminescence

Triboluminescence is the phenomenon that is observed when coloured light is emitted on grinding, crushing or fracturing certain crystalline compounds, e.g. the bluish white light emitted from sugar. This phenomenon has been known for over 300 years but has remained a curiosity without an obvious application, until very recently when Sage postulated that this effect could be used to detect flaws in composite materials.83

This was based on the reported high triboluminescence efficiency exhibited by certain crystalline compounds, especially complexes with lanthanides (Table 3.15), some of which have already been described under electroluminescence (section 3.8.5.1). The light emission from a few of these materials is sufficient for the phenomenon to be observed in a well-lit room.

The mechanism of the origin of the triboluminescence effect is not fully understood but there appears to be three types.83

• Many of the brightest triboluminescent compounds are also piezoelectric and photoluminescent. Thus triboluminescence in these compounds can be explained by a fracturing of the crystal causing electrical charges to be formed along the new surfaces, from which an electrical discharge occurs in the crack line. Nitrogen from the air present in this discharge emits UV light, which is absorbed into the crystal and then re-emitted as visible light.

• For centrosymmetric crystals this cannot happen as the surfaces will be the same...