Study on properties and application of sapphire material

Sapphire, commonly known as corundum, mainly composed of Al2O3, is a common simple coordination type oxide crystal.Sapphires in nature exhibit different colors due to impurities such as titanium ions (Ti3+) and iron ions (Fe3+), chromium ions (Cr3+), and nickel ions (Ni3+), which give the crystals their yellow color.Pure alumina is a colorless transparent crystal, because it has the unique crystal structure, excellent mechanical properties, optical properties and chemical stability, and can be applied to the high temperature of 2000 ℃ environment, so they are widely used in the infrared military equipment, satellite space technology, high intensity laser window materials and semiconductor, large scale integrated circuit substrate material.

There are many growing methods of artificial sapphire crystal, and the main methods at present are bubble generation, heat exchange and lift.The crystals grown by heat exchange method can reach 430 mm in diameter and weigh more than 150 kg, which is the leading level among all sapphire growth methods. Sapphire crystal with a diameter of 500 mm is also under research.In recent years, with the national attention to the application of LED lighting technology and the deepening of people's awareness of this advanced environmental protection material, the sapphire industry has developed rapidly. The research and production of large size and high-quality sapphire crystal has gradually become one of the most dynamic new material industries at present.

The structure, properties, main preparation methods and main application fields of sapphire materials are described in this paper.

1. Structure and properties of sapphire

Sapphire is composed of Al2O3 and belongs to colorless and transparent crystal material.Sapphire belongs to hexagonal crystal system in crystallography, crystal space group is R3C, crystal cell structure is shown in Figure 1, oxygen atom and aluminum atom are combined by covalent bond, lattice parameter a=b=0.4758 nm, C =1.2991 nm, = =90°, =120°[1].In the cell structure, O2- is the most densely packed hexagonal layer. The accumulation layer is perpendicular to the cubic axis, while Al3+ fills in 2/3 octahedral void, and is connected into layers perpendicular to the C axis [AlO6] octahedral.The other two solid [AlO6] octahedrons are formed by connecting parallel to the C axis [AlO6] octahedrons in the direction of their plane or vertex Angle. The two solid [AlO6] octahedrons are helically symmetric along the C axis, and an O2- enclosed octahedrons are arranged in alternate lines.Therefore, the main crystal types of sapphire include hexagonal column, hexagonal bicone, rhombohedron and parallel double crystal, etc., as shown in FIG. 2 [1].

FIG. 1 Crystal structure of sapphire FIG. 2 Crystal morphology of sapphire

Sapphire crystal has many excellent properties, as shown in Table 1, the Mohs hardness of sapphire is 9, second only to diamond, high strength, high thermal conductivity, thermal shock resistance and chemical stability, high surface smoothness, good dielectric properties.These properties are closely related to their unique crystal structure.In addition, sapphire in 3 ~ 5?The M-band has a high optical transmittance, and its single crystal has a wide range of light transmittance, covering vacuum ultraviolet, visible, near-infrared to mid-infrared and other bands [2].These superior properties make sapphire play an increasingly important role in military and civil fields.

Table 1 Main properties of sapphire material

Application of sapphire material

1) Military field.

Sapphire single crystal has a place in the military field, the most important is in the application of mid-infrared optical port materials and missile fairing.

Nowadays, in order to realize the high precision guidance of missile, the material requirements of each part are very high.As a key component of no-load photoelectric equipment, large-size infrared optical window has more stringent requirements on its material properties. In addition to high infrared transmittance, it also needs to have mechanical properties required to meet adverse combat conditions, including high surface hardness, thermal shock resistance, corrosion resistance and rain corrosion.Sapphire single crystal, on the one hand, has high transmittance in the infrared band range, on the other hand, it also has excellent mechanical and physical properties, such as high tensile strength, corrosion resistance, high thermal conductivity, and stable mechanical properties.Therefore, this superior comprehensive performance makes it especially suitable for making infrared optical Windows, and the most outstanding performance is the fairing applied to high Mach missiles.As the missile flies fast, the fairing has to bear high aerodynamic heating temperature and aerodynamic pressure, so the fairing must have good thermal shock resistance, strength, stiffness and high temperature stability performance.In addition, the fairing is also subject to wind, rain and hail erosion, so its hardness and corrosion resistance is also very high.So far, sapphire is an ideal choice for high Mach missile fairing in infrared transmissible materials.

At present, the infrared optical window and fairing made of sapphire single crystal in the world have been applied in the field of military aviation in many countries, but the high production cost is still an important problem that needs to be solved urgently.

2) Civil field.

In civil use, sapphire, as an important technical crystal, is mainly used in substrate materials such as semiconductor lighting, large-scale integrated circuits and superconducting nanostructured films [3].Because gallium nitride is difficult to grow single crystal, it must be grown on the similar structure of sunken bottom material.Sapphire, Si, AIN, ZnO can be used as gallium nitride substrate candidate materials.Although there is a large lattice mismatch between sapphire substrate and gallium nitride, compared with other substrate materials, sapphire substrate has the advantages of high-temperature chemical stability, easy to obtain large size, and low price, so it is used as a substrate material for GaN epitaxial growth.With the continuous improvement of technology and the rapid growth of the market of Gan-based luminescent materials and devices, sapphire has become an important gallium nitride substrate material, and the demand for sapphire also increases rapidly.China lags far behind the developed countries in the field of semiconductor materials and semiconductor lighting, which puts forward urgent requirements for the development of sapphire research and industrialization in China. At the same time, it is also a great opportunity and challenge for the emerging sapphire enterprises.

In addition, due to its high hardness and good wear resistance, sapphire is widely used in jewelry decoration, medical equipment, environmental protection industry, etc.Because of its good optical and mechanical properties, sapphire single crystal can be used in a variety of optical elements, such as lenses, prisms, reflectors, detection instruments for scientific research, and equipment and instrument observation Windows and detection Windows in various high temperature and high pressure harsh environments.Another important civilian application is used as a substitute for traditional glass on a variety of table mirrors and panels.Apple's announcement in November 2013 that I Phone6 might use sapphire glass as a phone panel drew widespread attention at home and abroad.In fact, Apple has long used sapphire material on the camera lens covering the glass and function of the primary key, to improve the hardness of the phone's panel and anti-wear ability, so as to avoid surface scratches affecting the shooting and fingerprint identification effect.Sapphire is a corundum crystal with a hardness of up to 9 Mowell, while ordinary glass is only 6 to 7 hard. Sapphire is twice as durable as Gorilla glass, which is famous for its brightness, chemical stability and thermal stability.But the biggest problem with sapphire materials is the complexity and high cost of sapphire production, especially for large sapphire phone screens, which cost $30 to $40 per piece, nearly 10 times higher than glass panels.However, with the mass production of sapphire, the cost will be greater to reduce the room.

3) Application of doped sapphire crystal.

The thermoluminescence and photoluminescence characteristics of doped sapphire single crystals were first discovered in the 1950s. After a series of studies, the properties of sapphire single crystals have been improved, which makes them more in-depth and extensive research and application in the fields of dosimeter and detector.

At present, Ti: Al2O3 laser has made great achievements in physics, biology, chemistry and other basic disciplines. It has also been widely used in military and engineering fields.This is all due to the discovery and research of single crystal doped with titanium and sapphire (Ti3+ : Al2O3). With the continuous deepening of the research, various lasers with Ti: Al2O3 as the working substance have also been developed rapidly [4].At present, such lasers are mainly used in the military fields such as laser ranging and photoelectric jamming, as well as in the fields such as laser communication, ocean detection, atmospheric environment monitoring and micro-machining.

In general, the cost of single crystal preparation and processing of sapphire is still high, and it is mainly used in military, optical and LED applications.In addition, the appearance of sapphire mobile phone panel makes the application of sapphire in the mobile phone industry increasingly prominent.In the future, the research and application of sapphire single crystal will continue to grow, but the premise must be optimized to improve the growth technology and processing technology of sapphire single crystal, so as to further reduce the production cost.