Molybdenum Seed Holder

Sapphire is a very important technical crystal with excellent optical properties, mechanical properties and chemical stability, high strength, high hardness, erosion resistance, and can be used at high temperatures. Therefore, it is widely used in infrared military devices, satellite space technology, window material for high-intensity lasers.

The crystal grown by the melt growth method has the characteristics of fast growth rate, good integrity and high crystal purity, so most sapphire crystals use this method. Melt growth methods mainly include Czochralski, EFG technique, Kyropoulos, TGT crystal growth and Heat Exchange Method (HEM).

When using Czochralski, the feed is placed in a cylindrical crucible and heated to hundreds of degrees until it reaches its melting point. After the raw material is completely melted, use seed chuck assembly to make the seed crystal drop close to the aluminum oxide liquid level, then contact the aluminum oxide liquid surface, and lift the seed crystal; repeat the above operation to complete the crystal washing; after the crystal washing is completed, the seed crystal is lowered to below the liquid level of Al2O3, and the seed crystal is welded. After the seed crystal is welded, the seed chuck assembly continues to be pulled and rotated at the same time to increase the diameter of the crystal until the weight of the crystal is within the required weight range.

schematic diagram of crystal growth

In the process of sapphire crystal growth, high-quality single crystal ingots are extracted by controlling heating power, pulling speed and rotation speed. The seed holder/chuck is a device that connects the seed rod (seed) and plays a very important role in the growth of single crystals.

There are various structures of seed holder devices for crystal growth by Czochralski. The seed holder made of one-piece graphite, on the one hand, affects the centering accuracy of the sag of the flexible shaft due to insufficient weight; on the other hand, due to the low tensile strength of graphite, the contact surface between the holder and the end of the flexible shaft is easily damaged. Currently, seed holders made of molybdenum are the most commonly used.

Molybdenum has a high melting point, high thermal and electrical conductivity, and has a low coefficient of expansion and excellent strength, so it is widely used in the crystal growth industry. As the heat insulation part of the sapphire growth furnace, the molybdenum heat shield can prevent heat loss and reflect the heat back into the furnace; as a seed chuck, the molybdenum seed holder is safe, reliable and stable in structure.

molybdenum seed holder photo

Molybdenum seed holder is made of high-quality molybdenum raw material, with purity not less than 99.95%. Its high temperature resistance can reach 1600°-1700°, density is ≥9.90g/cm3, size tolerance is within ±0.02mm, the thread is required to be smooth, and the surface finish requires high. At the same time, there are strict requirements for concentricity as precise concentricity can improve the force of the tungsten wire rope. Typical diameters are between 10mm and 100mm, and lengths are between 100mm and 1000mm.

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