Application of Tungsten and Molybdenum Products in Sapphire Growth Furnaces

I. Introduction to Sapphire Growth Furnaces

Sapphire single crystals are typically grown using the Kyropoulos (KY), Czochralski (CZ), or Heat Exchange Method (HEM). These processes require extremely high temperatures between 1800–2100°C to melt high-purity alumina and allow it to crystallize slowly. Therefore, all high-temperature structural materials used in the furnace must exhibit excellent heat resistance, chemical stability, and thermal conductivity.

II. Main Applications of Tungsten and Molybdenum Materials

1. Heating Elements

Molybdenum wires, tungsten wires, molybdenum rods, and tungsten rods are commonly used as resistance heating elements. They provide excellent high-temperature electrical stability and can operate at 1800–2200°C without significant evaporation, suitable for long-term use in vacuum or inert atmospheres.

Typical materials: Molybdenum (99.95%), Tungsten (99.95%), and MoLa (molybdenum-lanthanum alloy).

2. Heat Shields and Reflectors

Made from molybdenum or tungsten sheets, these multilayer shields reduce heat radiation loss and improve temperature uniformity within the furnace. A 3–7 layer structure is typically used, with molybdenum as the inner layer and tungsten as the outer layer. Mo + MoLa combinations are often used to enhance mechanical strength.

3. Crucibles

Crucibles are used to hold molten alumina. Molybdenum crucibles are preferred due to their low reactivity with Al₂O₃ melts. For higher temperatures or more reducing atmospheres, tungsten crucibles can also be used. They must have smooth surfaces and high purity (>99.95%) to avoid contaminating the sapphire crystal.

4. Support and Structural Components

Molybdenum rods, bolts, rings, and tungsten bases serve as structural supports under high temperatures. They have excellent thermal expansion compatibility, ensuring furnace stability. TZM (titanium-zirconium-carbon molybdenum alloy) is often used to enhance strength and creep resistance.

5. Seed Holders and Fixtures

Made from molybdenum or tungsten alloys, these parts must maintain excellent mechanical stability at high temperatures and must not react with or contaminate the sapphire crystal.

III. Comparison of Tungsten and Molybdenum Properties

IV. Typical Furnace Structure

From the inside out, the typical sapphire growth furnace structure includes:
Sapphire crystal → Seed rod (Mo/W) → Crucible (Mo) → Heating element (Mo/W wire) → Heat shield (Mo/W sheet) → Support frame (Mo/TZM) → Outer shell (stainless steel).

V. Summary