Molybdenum Spun Crucibles for LED Sapphire Crystal Growing
Molybdenum Spun Crucible
H.C. Starck Perfects Molybdenum Crucibles
The spinning process for molybdenum is an adaptation of metal spinning for materials like steel. Working with a conventional spinning machine manufacturer, they developed a machine with capabilities for spinning material while being heated. At the same time, H.C. Starck's engineers were developing the input material to be used for spinning crucibles. As the process development progressed, the machine and input material were refined until H.C. Starck was able to repeatedly make defect-free crucibles with little down time in the crucible production process.
The sapphire crystal growing method used does tend to affect the crucible selection. During crystal growth via HEM, the temperature required for melting the alumina crackle is held constant. Because the thermal stress affecting the crucible is low, thin-wall spun molybdenum crucibles can be used. In other methods such as Kyropoulous, the thermal gradient is quite severe and requires thicker-walled crucibles made from molybdenum, molybdenum alloys, or tungsten.
Innovative Crucible Spinning Process
The core component of H.C. Starck's crucible is our pure molybdenum plate, which is input into the spinning process. We have developed and follow a very strict rolling and post processing procedure to ensure our input material delivers a perfect crucible. Beyond the input material, the spinning procedure is performed the same way it would be on steel, but the material is heated during the spinning process to improve formability. After spinning, the crucibles are trimmed as required and cleaned to remove any surface oxidation created during hot forming.
There are other ways to make crucibles, but not large diameter, tall, thin-walled crucibles. Spinning is ideal, because the material yield is high, wall thickness can be precision-controlled, and post processing beyond trimming and cleaning is generally not required. The cycle time to spin a thin-walled crucible is very short and well-suited for high volume production. For shallow crucibles, deep drawing is sometimes used, but the height of a spun crucible cannot be matched with deep drawing. Pressing, sintering and machining can also be used to make crucibles, but it's not practical to get such a thin-wall using this method. Handling prior to sintering is very difficult and sintering time increases cycle time. Moreover, slight deformations in sintering can require machining afterwards to finish the part. Although, there are other methods to make crucibles, spin forming remains the most economical.
Choice Materials for Spin Forming
Molybdenum is well-suited for spun crucibles, because its thermal properties match the process requirements of growing sapphire crystals extremely well. The high melting temperature and creep resistance of molybdenum are required to survive the harsh environment found in sapphire crystal growing. Molybdenum has a low thermal expansion coefficient (CTE), which is excellent for the design of the components in the sapphire growing furnace. Molybdenum has excellent thermal conductivity, which combined with low CTE, makes the entire crystal growing system very stable. In addition, molybdenum and other refractory metals do not contaminate the alumina melt, resulting in a higher quality sapphire.
H.C. Starck has a vertically integrated supply chain with the capability to refine molybdenum powder, press and sinter ingots, roll input material for spinning, spinning to form, trim, and clean molybdenum and molybdenum alloy crucibles. H.C. Starck can manufacture a wide-range of diameter, height and thickness crucible sizes to suit our customers' needs.Molybdenum and Molybdenum/Tungsten Crucibles – Powder Metallurgy
- Pure Molybdenum (99.95% minimum)
- MoW Blend (percentages vary depending on customer requirements)
|Input Thickness ||2||8||Input thickness requirement is driven by overall crucible height and the amount of stretching done. In general, consider 2 mm for shorter narrower crucibles and 8 mm for longer wide crucibles. For non-consumable parts, thicker material can also add to the expected lifetime.|
|Outer Diameter||100||600||<100 mm request a quote|
|Length of Finished Part||No limit||1000|
Molybdenum alloys and tungsten can be utilized in spin forming crucibles. Just like molybdenum, molybdenum alloys and tungsten needs to be heated to become more ductile and improve formability. In the case of tungsten, the temperature required for spin forming is higher than that of molybdenum, so special capabilities are used to handle the higher temperature. Regardless of the material used, the quality of the input plate is critical to the spinning success.
Molybdenum and molybdenum alloy crucibles have been used in other sapphire growing methods such as Edge Defined Film Fed Growth (EFG) and are commonly produced by machining from a rolled plate or a pressed and sintered near-net shape input material. In all of these processes, the crucibles, normally cylindrical in shape, contain or hold the molten alumina during the crystal growing process.
H.C. Starck also manufactures molybdenum and tungsten hot zones used in sapphire crystal growing furnaces.