Triboluminescence and the dynamics of crystal fracture

Triboluminescence appears concurrently with the sudden force decreases in the fracture region of the force versus compression curves of tartaric-acid, sugar, ammonium-tartrate, lithium-sulfate-monohydrate, and citric-acid-monohydrate crystals. This relationship suggests that the motion of cracks and thereby the creation of new surfaces is responsible for the excitation of triboluminescence in these crystals. All of these crystals exhibit triboluminescence from the second positive group of molecular nitrogen. The intensity and time dependence of triboluminescence is measured as a function of the velocity of impact of a piston on single crystals. A quantitative expression for time and impact velocity dependence of the triboluminescence intensity. $I=\left(\frac{\eta \mathrm{bV}{u}^3{v}^3}{{\alpha }^2{h}^3}\right)e^{-\alpha t}{(1-e^{-\alpha t})}^2\exp [-(\frac{\beta u^2{v}^2}{{\alpha }^2{h}^2}\left){(-e^{-\alpha t})}^2\right],$ is derived from the movement and the interaction of mobile cracks. This equation explains the observed dependence of the triboluminescence intensity on the impact velocity, the applied stress, the temperature, and the crystal dimensions. The values of $\alpha$ and $\beta$ are determined from triboluminescence measurements. $\alpha$ is related to the viscosity of the crystal and $\beta$ is related to the attrition coefficient of mobile cracks in the crystal. The triboluminescence activity decreases over three orders of magnitude from sugar to citric-acid crystals.