​Thermal properties of sapphire crystals

Thermal properties of sapphire crystals

Melting temperature is 2,323 K.

Boiling temperature is 3,253 K.

The linear expansion coefficient (α) of sapphire depends on temperature and orientation (Fig. 2.67) [78]. Similar to other thermal parameters listed in catalogues established by different authors, the value of (α) varies widely .

The most commonly used linear expansion coefficient values:

⊥C 293-323K 5.0•10-6K-1

‖C 6.6•10-6 K-1

60º direction 293-323 K 5.8 10-6 K-1

⊥C 1,273 K (7.9-8.3) 10-6 K-1

‖C (8.8-9.0) 10-6 K-1

60º direction 1.273 K 7.7 10-6 K-1

The linear expansion coefficient of the lattice constant at room temperature (295.65 K) is: αa=5.22•10-6 K-1; αc=5.92•10-6K-1.

The study of thermal elongation of sapphire in the low temperature range, which is the topic of this material for superconducting resonators, shows that below 50Kα and below 10-7K-1[79], and this in most cases It can be ignored. But impurities may produce low temperature anomalies. For example, ruby containing magnetic impurity Cr2+ has a temperature expansion anomaly caused by the Yan-Teller effect. Anomalies are also found in sapphire deviating from stoichiometric composition [80]. With 1018 cm The results of the thermal expansion study of the sample with -3 (10-3%) anion vacancies are shown in Figure 2.68. These measurements were made with a volumetric dilatometer on a sample with a diameter of 16 mm and a width of 100 mm.

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