Molybdenum Crucibles vs Tungsten Crucibles: How to Choose for High-Tem Applications

High-temperature crucibles are essential in industries dealing with the melting of metals, crystal growth, and chemical processes at very high temperatures. Applications requiring high-performance crucibles mainly employ molybdenum and tungsten. The following will provide a detailed comparison of molybdenum and tungsten crucibles, including practical guidance on selecting the appropriate material for specific applications.

1. Molybdenum Crucibles

Molybdenum crucibles are widely used in metallurgy, electronics, and crystal growth owing to their very high thermal conductivity and excellent compatibility with molten metals. They find common applications in melting non-ferrous metals, pulling silicon or CVD crystals, and the processing of special alloys either under vacuum or inert atmospheres. Due to their non-polluting properties, for instance, Mo crucibles are commonly employed in the melting of semiconductor-grade silicon to provide an inert, stable environment.

The important features and benefits of molybdenum crucibles include:

• Melting Point: 2,623 °C permits application in high-temperature metal processing below the maximum of tungsten.
• Density: 10.2 g/cm³ is less than that of tungsten, hence of great advantage when handling large crucibles.
• Thermal conductivity: It is about 138 W/m·K; this ensures good heat distribution and minimizes thermal gradients that may be caused by warping of the crucible.
• Chemical Compatibility - Resistant to many molten metals, like copper, silver, and silicon, under vacuum or inert gases.
• Mechanical Strength: The structural integrity is maintained up to ~2,000 °C in inert atmospheres.
• Molybdenum crucibles are a little less expensive than tungsten ones. Their relatively low densities and easy machinability facilitate their production in custom shapes and sizes, such as cylindrical, conical, or hemispherical designs, for laboratory or industrial furnaces.

2. Tungsten Crucibles

Tungsten crucibles are preferred in extremely high-temperature applications where molybdenum may reach its thermal limits. Tungsten's melting point of 3,422 °C makes it suitable for processing refractory metals, high-purity metals, and certain ceramics that require ultra-high temperatures. Common applications range from the melting of tantalum, rhenium, or molybdenum alloys to acting as containers for high-temperature CVD diamond synthesis or specialty crystal growth.

Important characteristics and advantages of tungsten crucibles are as follows:

• Melting Point: 3,422 °C - provides an increased safety margin for high-temperature operations.
• Density: 19.3 g/cm3 (Higher than molybdenum), strong, yet large crucibles heavier.
• Thermal Conductivity: Closer to 173 W/m·K, as compared to molybdenum, thus providing uniformity in temperature for ultra-high-temperature applications.
• Chemical Compatibility: Excellent resistance to most molten metals under vacuum and inert atmospheres. Oxidation in air should be avoided above 400 °C.
• Mechanical Strength: Has structural integrity up to 2,500 °C in vacuum; its applications involve heavy-duty and high-temperature operations.

• Tungsten crucibles are extremely dependable, particularly at high temperatures, in operations that may take long periods of time. They are heavier and more costly compared to molybdenum; as a result, they are more adaptable to priority high-temperature work than to routine services.

3. When Choosing between Mo and W Crucibles

Selection of the proper crucible depends on several temperature requirements, chemical compatibility, size, and cost. These factors listed below will guide selection:

Molybdenum is the appropriate choice for processes operating below approximately 2,000–2,200 °C, while tungsten should be selected for ultra-high-temperature applications above 2,500 °C.

Both metals perform well in vacuum and inert atmospheres, but each must be protected from oxygen at high temperatures. Tungsten, in particular, oxidizes much more rapidly in air than molybdenum does.

Molybdenum offers the advantage of a much lower density, which translates to easier handling and lighter crucibles, especially in larger sizes. Tungsten, however, has a higher thermal conductivity and significantly greater strength at extreme temperatures, providing superior thermal uniformity under high stress.

From a cost and fabrication standpoint, molybdenum is generally less expensive and easier to machine. Tungsten is more costly due to its higher density, greater difficulty in fabrication, and the higher price of the raw material itself.

For application-specific guidance, molybdenum crucibles are commonly used for melting semiconductor silicon. Tungsten crucibles are preferred for melting other refractory metals or for high-temperature chemical vapor deposition (CVD) processes. In vacuum furnace crystal growth, both may be suitable, but tungsten is favored where higher temperatures or longer-duration melts are required.

How to Choose

Molybdenum and tungsten crucibles both have very vital uses in high-temperature industrial and laboratory applications.

• Mo Crucibles: for applications requiring moderately high temperatures; excellent in thermal conductivity, chemical resistance, and cost efficiency for silicon melting of non-ferrous metals and general crystal growth.
• The tungsten crucible offers extreme temperature operations, providing far superior melting point, mechanical strength, and thermal uniformity for refractory metals, ultra-high-temperature CVD, and specialty crystal growth.
• For more refractory metal products, please check Advanced Refractory Metals (ARM).