History and Production Methods
Synthetic Sapphire is one of the hardest man-made materials – second only to Synthetic Diamond. It is a 99.99% pure single crystal Aluminium Oxide and is normally completely colourless as it does not contain the impurities that lend naturally occurring sapphire gemstones their blue colour. However, sometimes a few parts per million of a particular chromate are added to the raw powder producing variously coloured crystals, such as dark red synthetic Ruby.
Since the late 19th century the majority of Synthetic Sapphire and Ruby is still grown by the Verneuil flame fusion process named after its inventor, however it is only economical for items up to Ø40mm. This material is mostly used for mechanical applications as the resulting crystals can suffer from crystalline inhomogeneities including lineage, strain, striae and macroscopic bubbles. As a result, most of the optical grade sapphire supplied by Agate today is grown by the Kyropoulos method – also named after its inventor. This method, for direct crystallization of the melt, yields large-diameter homogenous boules of exceptionally high optical quality.
A similar process is used to grow sapphire rod, tube and even flat ribbon in long lengths and of various configurations. This Edge-defined, Film-fed Growth (EFG) method involves pulling the crystal from above a molybdenum die of the desired shape floating on the surface of the melt. The end product has a transparent ribbed finish, which can be machined to a smooth optical surface with minimal stock removal, however the body of the material is not as homogenous as Kyropoulos sapphire.
Sapphire is a single crystal aluminium oxide normally indexed on hexagonal axes, though it is categorised as a Rhombohedral crystal. It is uni-axial, negative to visible light and anisotropic in its physical, thermal and dielectric properties. As an amphoteric semiconductor, sapphires energy band gap is one of the greatest of any oxide crystal at approximately 10eV, permitting useful transmission reaching from 1450A to 5.5nm.
The unique properties of sapphire make it ideal as an optical material for hostile environments. These properties include high mechanical strength and temperature stability, high UV, visible and infrared transmission, as well as high wear resistance, abrasion resistance, volume resistivity, hardness and thermal conductivity. Other characteristics of sapphire are zero porosity, chemical inertness, low optical dispersion and dielectric loss.
Sapphire is a birefringent crystal with the C-axis usually growing at approximately 60 ͦ to the axis of the boule. Such crystals are normally cut at right angles to the mechanical axis of the boule in order to give the greatest yield and economy. Such parts are designated as being of `random` orientation. However, with some loss of material, the boule can be cut to a specific orientation, normally to within +/- 2 degrees. Sapphire that is cut with the C-axis perpendicular to the faces is generally referred to as being of `zero degree orientation` and the faces are considered to be in the c-plane (0001). For specific applications, the material can be supplied with a `90 degree orientation`, this being known as the r-plane (1102). Alternatively, the crystal can be cut in the a-plane (1120), especially when it is required to have a uniform dielectric constant.
Sapphire is used in the chemical, medical and optical industries, electronic engineering, high pressure and low vacuum technologies, fibre optics, avionics and space technologies. Also for the measuring, metering, pumping, and textile industries. It is also used for furnace and missile windows, transparent armour, and maser tubes. By far the greatest use of clear synthetic sapphire today is as scratch proof watch `glasses` and this scratch resistance has also been made use of for the protective windows of laser barcode readers at supermarket check-out tills.
At Agate Products we specialise in supplying the numerous markets that exist for low volume, high accuracy requirements such as for sapphire substrates, IR detectors, high pressure windows, lenses, prisms, probes, bearings, crucibles, high pressure nozzles, laser rods, tape guides and wire guides, insulators and many more diverse and varied applications. We also stock large quantities of high precision sapphire and ruby spheres ranging in size from Ø0.15mm to Ø12.00mm.
The greatest demand from our customers is for sapphire windows of various orientations and sizes. These can be as small as Ø0.8mm x 0.02mm thick or as large as Ø200mm x 12mm or more in thickness. They are normally supplied to a tolerance of +0.00/-0.05mm on the diameter and +/-0.10mm on the thickness with an 80:50 Scratch:Dig surface finish, unless requested otherwise. Total diameter tolerances of 0.01mm and 0.05 on thickness with chemically polished surface finishes as fine as 20:10 can be readily achieved. Flatness is usually in the order of 3 to 5 fringes but flatnesses of 1λ down to λ/4 can be supplied on windows whose thickness is at least equal to a tenth of the diameter.
These windows can comfortably be used at temperatures of up to 1,800 ͦ C and pressures as high as 20,000 psi (1,500 bar). Many of our windows are specially annealed as a final operation to prevent failure at high pressures. This process has also been found to improve the surface quality especially with respect to any slight sub-surface damage.
We are also able to supply windows with metalising on the edges or around the polished face as well as windows that have been brazed into metal housings of your design. In fact, nearly all our sapphire parts are made to customers drawings and specifications, often following discussions and advice from ourselves.
We have also formed special working relationships with some of the leading coating companies in the UK and beyond to provide a one stop service to our customers and can arrange any type of coating to sapphire windows to suit the individual application.
Typical Properties of Sapphire
|Hardness||Knoop||1900 parallel to c-axis|
|2200 perpendicular to c-axis|
|Compressive strength||MPa @ Room Temp.||2000|
|Tensile Strength||MPa @ Room Temp.||250 – 400|
|Young’s modulus||GPa||435 parallel to c-axis @ 25°C|
|386 parallel to c-axis @ 1000°C|
|Flexural Strength||MPa @ Room Temp.||760 – 1035|
|Poisson’s ratio||orientation dependent||0.27 – 0.30|
|Coefficient of thermal expansion 25-1000°C||1x10E-6||8.8 parallel to c-axis|
|7.9 perpendicular to c-axis|
|Thermal conductivity||W/(cmK)||0.4 @ 25°C|
|Volume resistivity||ohm-cm||10E16 @ 25°C|
|10E6 @ 1000°C|
|Dielectric constant||10E3 to 10E9 Hz||11.5 parallel to C axis|
|@ 25°C||9.3 perpendicular to c-axis|
|Loss tangent||10E10 @ 25°C||8.6E-5 parallel to c-axis|
|3.0 perpendicular to c-axis|