Sapphire is a precious and versatile mineral with unique properties that make it useful for a wide range of industrial and scientific applications. Its chemical, thermal, optical, mechanical, and electrical properties are explored and detailed in this article.

Sapphire Hardness, the most unique property of Sapphires

Sapphire is a unique mineral known for its durability and strength, as well as its remarkable optical properties. It is a type of corundum mineral and is one of the hardest natural materials known to man, measuring a 9 on the Mohs hardness scale. It is second only to diamond in terms of hardness, which makes it an ideal material for a wide range of applications.

Chemical Properties of Sapphires

One of the most striking features of sapphire is its chemical properties. Unlike alumina ceramics, sapphire is non-porous and unaffected by weathering or hydration. It is also virtually unaffected by any solvents or acids at room temperature, although some etching can occur at temperatures exceeding 600°C – 800°C when exposed to hot phosphoric acid or strong caustics. This makes sapphire a highly desirable material for use in harsh environments where other materials would deteriorate quickly.

Thermal Properties of Sapphires

Sapphire's thermal properties are equally impressive. It has a melting point of approximately 2053°C (3727°F) and most of its properties remain useful up to a maximum temperature of around 1800°C (3272°F). It has a thermal conductivity of 40 W/M°K at 298°K and an expansion rate of 4.5×10-6/K-1 at 25°C, increasing to 9.0×10-6/K-1 at 1000°C. Its specific heat capacity is 750 J/K at 300°K and its viscosity at 2053°C is 0.0584 Pa•s.

Optical Properties of Sapphires

Sapphire's optical properties are equally impressive. Its refractive indices are 1.768 for the ordinary ray (C-axis direction) and 1.760 for the extraordinary ray (perpendicular to C-axis direction). It has a birefringence (Ne – No) of 0.008 and a temperature coefficient of the refractive index of 13×10-6/°C (@ 0.57 µm, 20°C). It has a high transmittance of over 85% between 0.3-4.0 µm (@ 0.1 mm thick) uncorrected, as well as an emissivity of 16%, 25%, and 70% at 3,4,5 µm (@500°C) respectively. Its absorption rate at 0.66 mm @ 1600°C is between 0.1 – 0.2 cm-1.

Mechanical Properties of sapphires

In terms of mechanical properties, sapphire is one of the strongest and most durable materials available. It has a hardness of 2000 kg/mm2 on the Knoop scale, and a coefficient of friction of 0.14 on steel. It's Young’s modulus is 400 GPa at 20°C, and its Poisson's ratio is 0.29. It has a compressive strength of 2.0 GPa, and its creep rate at 100 Mpa and 1600°C is 1.5×10-4/hr. Its fracture toughness is 2.0 MPa (m½), and its flexural strength is 900 MPa. It has a bulk modulus of 240 GPa and a shear modulus of 175 GPa (Rigidity Modulus). Its tensile strength ranges from 300 to 400 MPa, and its rupture modulus is between 65 – 100,000 psi.

Electrical Properties of sapphires

Finally, sapphire possesses impressive electrical properties, including a bulk resistivity of 1016 ohm-cm at 25°C and 1011 ohm-cm at 500°C, a dielectric strength of <0.48 kv/mm (1.2kv/mil), a dielectric constant of 9.4 perpendiculars to the c-axis and 11.6 parallel to the c-axis between 10Hz and 3×109 Hz, a loss tangent of 3.0 – 8.6×10-5 between 10 Hz and 3×109 Hz, and a magnetic susceptibility of -0.21×10-6 to -0.25×10-6.

Conclusion

In conclusion, sapphire's unique properties make it highly desirable in various applications, including electronics, optics, and aerospace industries. Sapphire's properties make it highly resistant to heat, pressure, and chemical reactions, making it a go-to mineral for extreme environmental conditions.