News Article

Super Smooth Sapphire Growth

New tungten crucibles made by Plansee make it simpler and cheaper to grow sapphire ingots used in the production of LEDs


Plansee says its new .tungsten crucibles make growing sapphire in a crucible much easier. The firm says their very smooth surface gives the crucibles a longer service life and reduces costs.



Plansee tungsten crucibles

Ultra-pure sapphire is used in the production of LEDs. The Kyropoulos method, which involves melting a sapphire crystal in a heat-resistant crucible made from molybdenum or tungsten, is in widespread use. One particularly critical stage during this production process is when the sapphire crystal is extracted from the crucible.

The easier it is to release the sapphire 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. This 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 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 sapphire manufacturers, this results in less complex and expensive reworking of the surface of the crucible. Plansee says the cycles run smoothly and deliver high-quality ingots. What's more, the smooth surface is less susceptible to corrosion caused by the aggressive melted sapphire. This increases the service life of the reusable tungsten crucibles.

Plansee's tungsten crucibles also score well in terms of density. You may wonder why is this 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.

The company has fine tuned its 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 have a purity of over 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 says its extremely large sintering plants and processing machines mean that it is already geared up to take this step.

On 19th June, Plansee will  be showcasing its products for sapphire growth at the Display Taiwan trade show at stand I112.

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