The measurement of thermal expansion of incoerent materials with the optical dilatometer

The thermal expansion analysis measures the dimensional variations of a material subjected to a heat treatment as a function of the temperature.

The thermal expansion of incoherent materials (like sands), plastic materials (like glasses or polymers above their glass transition temperature), extremely thin samples (having thickness of tens microns) or soft materials (like felt or wax) cannot be studied by means of traditional dilatometry.

The double beam optical dilatometer allow to carry out thermal expansion measurements with no contact and no interference caused by the measuring system; the use of two beams of light allows to achieve an absolute measurement of the dimensional changes of the sample during the heat treatment.

The following graph represents the thermal expansion of an incoherent material, the Fontainbleu quartz sand.

Before firing (black curve):

• 573°C: reversible allotropic quartz transition, from alpha to beta
• 1470°C: irreversible transition from beta-quartz to beta-cristobalite, which will later reversibly transition to alpha-cristobalite while cooling
• 1600°C: 15 minutes permanence, to complete the transition to beta-cristobalite, which keeps causing expansion
• 573°C while cooling: small transition between residual beta-quartz to alpha-quartz
• 228°C while cooling: steep transition betweeen beta-cristobalite to alpha-cristobalite

After firing (red curve):

• 228°C: first slope deviation of the dilatometric curve, due to the reversible allotropic transition from alpha-cristobalite to beta-cristobalite
• ~573°C: a smaller inflection due to a residual alpha quartz which transitions to its beta form.