Ultra-pure sapphire is used in the production of LED lights and the Kyropoulos production method is in widespread use. This involves melting a sapphire crystal in a heat-resistant crucible made from molybdenum or tungsten.
One particularly critical stage during this production process is when the sapphire crystal is extracted from the crucible. The easier the sapphire can be released from the crucible, the greater the yield and quality of the sapphire. If the surface of the crucible is too rough, the sapphire cannot be released or can only be partially released from the wall of the crucible, which results in cracks and faults in the crystal. Additionally, the crucible itself can be damaged in the process and has to be reworked or replaced earlier than planned.
In order to reduce the costs of sapphire growth, Plansee SE, Austria, has developed a special cost-saving pressed-sintered tungsten crucible with a surface roughness of less than 0.8 µm. For sapphire producers, this results in less complex and expensive reworking of the surface of the crucible.
The tungsten crucible from Plansee also scores in terms of density. Crucibles with a low density and a large number of porous areas can shrink under the high process temperatures. This means that they become more dense in use and can thus become deformed or even break.
Plansee states that it is able to finely adjust the sintering process for the crucibles. Sintering is performed at very high temperatures and a uniform temperature distribution. This gives the tungsten crucibles a very high material density of 93 %, coupled with an extremely homogeneous density distribution.
To ensure that only flawless sapphire comes out of the crucible, Plansee has adopted an ultra-clean production process. Everything from the metal powder to the finished crucibles is manufactured in-house. This allows every step to be carefully monitored, and the resulting crucibles are of an exceptional purity in excess of 99.97%.
Today, crucibles large enough for 35 kg to 100 kg of sapphire are commonplace, however the industry is already considering a new generation of crucibles for up to 200 kg of sapphire.
Plansee states that its extremely large sintering plants and processing machines mean that they are already geared up to take this step.