7. Mineral and ceramic lenses

In general, glasses show a rapid increase in internal friction as a function of temperature, linked to the decrease in viscosity at the glass transition temperature and associated with a decrease in Young's modulus [12] .

The addition of alkali ions reveals two relaxation peaks at low temperatures. The first, located between – 40 C and – 25 C, is attributed to stress diffusion of alkali ions. The interpretation of the second peak, located between 100 C and 250 C, is more controversial; several authors have shown that the presence of non-bridging oxygen atoms and OH groups is necessary for its existence. The replacement of some of the alkaline ions by ions of another species leads to a more intense internal friction peak located at an intermediate temperature between the two preceding peaks. The origin...