Sapphire: An Extreme Performer
Synthetic sapphire’s versatility and ability to withstand extreme conditions make it a viable candidate for a number of applications.
Sapphire, the single crystal form of alumina (Al2O3), was first synthesized more than a century ago, but the most exciting advances associated with this versatile material are taking place today. Shaped by the dual forces of application demands and technological advances in the fabrication and finishing process, synthetic sapphire is often the material of choice for design engineers dealing with extreme conditions of high temperature, high pressure, and harsh chemical environments.
Sapphire is second only to diamond in hardness and scratch resistance, having a rating of 9 on the Mohs scale. Its melting point of 2,050°C, physical strength, resistance to impact and corrosion, inertness, durability under extreme pressure, and the unusual combination of excellent thermal conductivity and high heat resistance further set sapphire apart from other advanced ceramics and glasses (see Table 1). In addition, sapphire’s optical properties are extraordinary: it is transparent to wavelengths of light between 150 nm (UV) and 5,500 nm (IR).
Applications are many and varied, from miniature bearings to large missile nose domes. Table 2 lists a sampling of typical applications. Optimizing performance for a particular application requires knowledge of not only the various sapphire crystal growth technologies, but also the effect of specific finishing techniques.
Sapphire, the single crystal form of alumina (Al2O3), was first synthesized more than a century ago, but the most exciting advances associated with this versatile material are taking place today.
Six main methods are used to produce sapphire; they all involve two basic steps: melting the raw material (alumina), then cooling and solidifying it in such a way that the crystals are aligned. Each crystal growth method has advantages and limitations.
Verneuil Method (late 19th century)
This flame-fusion (melt and drip) technique uses a Verneuil furnace. Alumina powder is first fused in a hydrogen-oxygen flame, then falls on the molten surface of an oriented seed crystal. As more fused powder falls on the resulting molten ball, the crystal enlarges.
While this is still the least expensive way to make synthetic sapphire for some applications (e.g., synthetic sapphire gemstones, watch jewels, watch windows), only limited sizes and shapes are possible. In addition, curved striations appear throughout the crystal, making it less suitable for optical applications.
Czochralski Method (1916)
Seed material for this “pull from the melt” method is placed in a crucible and melted in a growth chamber. A thin seed of sapphire with precise orientation is dipped into the melt and withdrawn at a controlled rate while the crystal and crucible are rotated in opposite directions. The process is repeated continuously, with crystal layers being added during each cycle.
Sapphire made in this way has good optical properties suitable for lasers, IR and UV windows, transparent electronic substrates, high-
temperature process windows, and other optical applications. However, the growth process can last up to eight weeks, requiring continuous power and monitoring.
Kyropoulos Method (1926)
This method involves direct crystallization of the melt. In a crucible, a seed crystal is mounted on a holder and the raw material is melted. Crystallization starts when the seed contacts the melt, with the crystal growing into the melt to form a hemisphere. When the crystal gets bigger and approaches the walls of the crucible, the seed crystal holder with the grown crystal rises, and the growth continues until the crystal reaches the walls of the crucible again.
The size of the grown crystal is limited only by the size of the crucible, and the crystals are free of the cracks and damage that can result from restricted containment. Also, resulting sapphire crystals are of very high optical quality, suitable for manufacturing ingots and substrates for LED and RF applications, windows, lenses and precision optics. As-grown Kyropoulos boules do have circular traces around their side surface, however.
Edge-Defined Film-Fed Growth, or EFG (Stepanov) (1965)
Alumina melted in a crucible wets the surface of a die and moves up by capillary attraction. A sapphire seed of specific crystallinity is dipped into the melt on top of the die and drawn out, solidifying into sapphire in the shape of the die.
Sapphire crystals of any shape, including tubes, rods, sheets, ribbons and fibers can be produced with this method, reducing machining and finishing costs. The crystals can have different crystallographic orientations, including A, C and random.
Extra time and cost are associated with producing the dies to create the shaped crystals, though, and low to medium optical quality limits this method’s use to mechanical, industrial, and lesser grade optical applications.
Heat Exchanger Method, or HEM (1967)
In this inverted or modified Kyropoulos method, a sapphire seed crystal is placed at the bottom of a crucible, which is then loaded with alumina crackle and placed in a furnace. Precise manipulation of heating and cooling causes the crystal to grow in three dimensions.
Large boules can be created with this method, and the extended cooling process yields exceptional crystal quality. It can be expensive, though, due to the cost of replacing the molybdenum and tungsten crucibles that are often used in the process.
Horizontal Directional Solidification, or HDS (1975)
This method involves a horizontal slab growth technique in which crystals are grown from the melt in a horizontal container at a growth rate of 8-10 mm per hour. Large slabs with nearly perfect edges of any crystal orientation can be produced. HDS also enables the fabrication of very thick windows and components with excellent optical quality. It is often used in blue LEDs.
This process usually produces only thick material. As with HEM, cost may also be an issue because of the use of molybdenum or tungsten boats.
Given the variety of growth technologies, selecting a fabrication method is a challenge best met in collaboration with someone skilled in matching the fabrication technology and finishing process (e.g., shaping, slicing, grinding, polishing) to the intended application of the sapphire material or component. While some applications may demand sapphire’s superb optical properties and scratch resistance, others may require the material’s high operating temperature and mechanical durability. It is important to consider every physical and functional aspect of the proposed sapphire component in order to write specifications that result in a cost-effective (and feasible) part.
In terms of finishing, how the sapphire crystal is cut can have a profound effect on performance. A sapphire crystal is a rhombohedral structure with three axes—R, A and C—and planes A, C, R, M and N. As an anisotropic crystal of non-uniform dimensions, it displays properties that are specific to crystal orientation. These properties can be thermal, physical, optical or electrical. Although a specific crystal orientation may be unimportant for many applications, it is wise to consider its possible significance whenever specifying sapphire.
Recent advances in the ability to increase the size of sapphire parts—tubes up to 40 in. long, wafers approaching 12 in. in diameter, domes more than 8 in. in diameter—have opened the door to exciting design innovations. The move toward more near-net-shape crystal growth is also resulting in increased design flexibility. After more than 100 years, the story of synthetic sapphire is most definitely still being written.
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