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Jun 21st, 2012
 
Brilliant performance in sapphire growth
 
Growing sapphire in a crucible has become child's play thanks to tungsten crucibles from PLANSEE.
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Their particularly smooth surface gives them a longer service life and reduces costs. Expansion plans have already begun to be implemented within YESCO Electronics® and will significantly grow the Logan, Utah facility.

Ultra-pure sapphire is used in the production of LED lights. The Kyropoulos 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. Not only that: The crucible itself is also damaged in the process and has to be reworked or replaced earlier than planned.

A fully automated manufacturing process coupled with more than 90 years of experience in the field of powder metallurgy allow PLANSEE to exercise precise control over the material properties of refractory metals such as tungsten and molybdenum. In order to reduce the costs of sapphire growth, the materials experts developed a special cost-saving pressed-sintered tungsten crucible with an ultra-smooth surface. With its surface roughness of less than 0.8 µm, sapphire growth is now a smooth operation: The sapphire can be extracted from the crucible without difficulty and without damaging the surface of the crucible. For the sapphire producers, this results in less complex and expensive reworking of the surface of the crucible. The cycles run smoothly and deliver high-quality ingots. And there's another advantage: The smooth surface is less susceptible to corrosion caused by the aggressive melted sapphire. This increases the service life of the reusable tungsten crucibles.

And the tungsten crucible from PLANSEE also scores in terms of density. Why is that important? 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 seriously deformed or even break. PLANSEE, the experts in powder metallurgy, finely adjusts 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. Even under high temperatures and rapid changes of temperature, the crucible is guaranteed to retain its shape.

So what about purity? Contaminants in the plant components can be transferred to the sapphire in a way that is detrimental to its quality. Iron, titanium and chromium are particularly critical in this respect. 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 %.

Nowadays, crucibles large enough for 35 kg up to 100 kg of sapphire are commonplace. And yet the industry is already considering a new generation for up to 200 kg of sapphire. PLANSEE's extremely large sintering plants and processing machines mean that they are already geared up to take this step.

 

 
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