The basic structure of sapphire:  
Sapphire is composed of aluminum oxide (Al2O3), which is combined by three oxygen atoms and two aluminum atoms in the form of Covalent bond. Its crystal structure is hexagonal lattice structure. Sapphire crystal belongs to the intermediate crystal family and Trigonal crystal system in the crystal symmetry classification.  
In terms of color, pure aluminum oxide crystals are transparent and colorless, as different colored elemental ions penetrate into the growing sapphire, resulting in the sapphire showing different colors. When titanium ions (Ti3) and iron ions (Fe3) are present in sapphire crystals in nature, the crystals become blue sapphire.  
 
 

Crystal properties
Chemical formula	Al2O3
crystal structure	Hexagonal crystal family
Lattice constant	A=b=0.4758nm, c=1.2991nm α=β= 90 °, γ= 120 °
Space group	R3c
Number of molecules in unit cell	two
optical performance
Through the band（ μ M)	0.14-6 (where T ≈ 80% within the range of 0.3-5)
Dn/dt (/K @ 633nm)	13x10-6
Refractive index	N0=1.768 ne=1.760
absorption coefficient α	three μ M-0.0006 4 μ M-0.055 5 μ M-0.92
Refractive coefficient n	three μ M-1.713 4 μ M-1.677 5 μ M-1.627
Mechanical and thermal properties
Density (g/cm3)	3.95-4.1
Mohs scale hardness	9 (second only to diamonds: 10)
Young's modulus (GPa-1)	three hundred and eighty
Breaking strength (MPa-1)	four hundred
bending strength	895MPa
compressive strength	2GPa
Melting point (℃)	2040-2050
Boiling point (℃)	three thousand
Thermal conductivity (W · m-1 · K-1)	twenty-four
Thermal expansion coefficient (K-1)	eight point eight × 10-6
Specific heat (J/g)	zero point seven eight two
Heat capacity (J/mol · K)	seventy-seven
Poisson's ratio	0.27-0.29
Resistivity (Ω · cm)	one thousand and fourteen
Dielectric constant (F · m-1)	11.5 (Å c), 9.3 (Å c)

 
 Characteristics and Applications of Sapphire  
 1. Sapphire, with its best comprehensive performance, has become the most widely used oxide substrate material（Substrate materials are mainly used as semiconductor thin film substrate materials, large-scale integrated circuit substrates, etc.  
 2. Sapphire crystal is also an excellent window material for infrared military devices, missiles, submarines, satellite space technology, detection, and high-power intense lasers, as well as high-quality optical materials and wear-resistant bearing materials, especially in missile fairing, submarine window, Atomic clock and other military products.  
 3. Optical components, watch mirrors, optical windows, detection windows and their applications  
Sapphire single crystal can be made into experimental optical components such as lenses, prisms, and reflectors due to its ideal optical and mechanical properties; And detection instruments for scientific research, such as gravity wave detectors.  
Sapphire single crystal can be used in various high temperatures Observation and detection windows for equipment working in harsh environments such as high pressure, such as high-temperature resistant thermocouples and boiler water level gauges, and wear-resistant commodity barcode scanner windows.  
It is currently an excellent alternative material for multispectral sulfur based window materials.  
 4. Sapphire Fiber Optic Sensor and Its Applications  
Sapphire single crystal fiber optic sensors have excellent optical transmission characteristics in the visible to near-infrared range and the high-temperature resistance of sapphire single crystals, making them suitable for high-temperature sensing and near-infrared laser transmission in the biomedical field.  
Sapphire single crystal fiber temperature sensor not only has the advantages of large dynamic range, high sensitivity, fast response, and electromagnetic interference resistance of ordinary fiber temperature sensors, but also can achieve large range (room temperature -2000 ℃), high accuracy (0.2%, at 1000 ℃), and high signal-to-noise ratio (1 × 106dB (at 1000 ℃), wide bandwidth (10kHz) temperature measurement, and widely used in fields such as plasma deposition, high-frequency electric heating furnaces, and high-temperature hot gas flow.  
In addition to measuring temperature and near-infrared laser transmission, sapphire fiber optic sensors can also continuously monitor the operation of systems in harsh environments such as high temperature, acidic, and alkaline environments up to 1600 ℃, such as structural performance, material degradation, and measurement of physical and chemical parameters such as pressure, stress, strain, and chemical concentration.  
 5. Doped Sapphire Single Crystal Thermoluminescence Materials and Their Applications  
α- Al2O3: C crystal is used to manufacture Thermoluminescence detectors, which has the following characteristics:  
(1) Thermoluminescence has high sensitivity, 40-60 times of the common Thermoluminescence crystal LiF: (Mg, Ti); The emission peak near 187 ℃ is single, and the effective Atomic number is relatively low (10.2);  
(2) The critical value of low background dose response (10-6Gy), the Radiation exposure response is linear sublinear, and the linear response range is wide (10-6-10Gy);  
(3) α- The emission peak at 420nm of Al2O3: C crystal is just the best peak of Photomultiplier tube response, α- Al2O3: C crystal detector can be reused without annealing treatment.