Various Heating Elements for High Temperature Uses

Heating elements form an integral part of high-temperature equipment and are widely employed in materials processing, semiconductor device fabrication, metallurgy, research labs, and various high-tech industries. Contemporary high-temperature reactors employ a number of diverse heating materials, each designed to operate under specific thermal, chemical, or mechanical conditions.

Metallic Heating Elements

Tungsten Heating Elements - Unmatched at Extremely High Temperature Ranges

Tungsten (W) as heating elements works well at the highest possible operating temperature. Its melting point stands at 3,422°C. These heating elements are useful in vacuum furnaces, sintering machines, sapphire crystal growth machines, and high-temperature evaporation plants. They work best under vacuum or an inert atmosphere. It exhibits low vapor pressure, high thermal conductivity, and stability at higher temperatures.

However, it readily oxidizes at temperatures above 500°C in air. As a consequence, it is normally used at low pressure and in an inert atmosphere. The heating elements have normally been made as rods, wires, and mesh heaters.

Molybdenum Heating Elements - Superior Performance in Vacuum And Reducing Environments

Molybdenum (Mo) is another widely used metal heater due to its good high-temperature properties and strong resilience at elevated temperatures. It melts at 2,623°C. Although not as noble as tungsten, molybdenum can be more easily machined and relatively less expensive. Molybdenum heating elements are common in vacuum heating furnaces. Crystal pulling and vacuum metalizing were initially done using molybdenum.

Like tungsten, molybdenum is also an air-reactive metal that requires inert atmosphere protection. Molybdenum-La alloys and molybdenum-Zr alloys have better ductility and increase the life of heaters.

Tantalum Heating Elements - Outstanding Corrosion Resistance

Tantalum heating elements have several desirable properties. It should be noted that it has an extremely high melting temperature of 3,017°C. Also, it presents high corrosion resistance and acts as an inert metal, primarily against acids. Additionally, it will oxidize and form a non-protective oxide layer when exposed to air at high temperatures. It is useful for high purity processing, which includes semiconductor crystal growth.

It is costlier compared with either tungsten or molybdenum and thus requires consideration only if purity or corrosion resistance issues are present.

Ceramic-based Heating Elements

Silicon Carbide (SiC) Heating Elements - Resistant to Air up to Approximately 1,600°C

Silicon carbide components rank among the most flexible ceramic heaters. These heating elements function effectively in air without requiring either a vacuum or an inert atmosphere. Silicon carbide components possess good oxidation resistance, thermal shock resistance, and a stable service life. These components have various applications, ranging from laboratory heating furnaces and heating treatment ovens to glass processing and metal annealing.

Their electrical resistances increase with usage, and they undergo deterioration as a result of oxidation. Nonetheless, they are still among the most cost-effective materials for operation at mid- and high-temperatures.

Molybdenum Disilicide (MoSi₂) Resistance Heating Elements

MoSi₂ heating elements extend the maximum operating temperature beyond that of SiC heating elements because they can be operated successfully in air up to 1,800°C. They develop a safeguarding layer of silica, SiO₂, on the surface, thus preventing any more oxidation.

MoSi₂ components are relatively more brittle compared with metal heaters but offer high thermal stability and life even in air.

Carbon-Based Heating Elements

Graphite Heating Elements –Conductivity and Stability At Very High Temperatures

Graphite components are capable of operating at temperatures above 2,000°C in an inert ambient and reaching 3,000°C in a vacuum. The high thermal conductivity of graphite ensures efficient heating. It finds a large number of applications as an efficient heating source at very high temperatures in various metallurgy processes and epitaxy.

Graphite cannot be heated in air at high temperatures because it will combine with oxygen. As a result, vacuum or inert atmospheres are necessary in graphite furnace designs. Graphite heating elements are sold as rods, plates, tubes, and more.

Comparison Table and How to Choose

All data presented above are for reference only and may vary depending on materials, processing conditions, and specific application requirements. Related reading: Heating Elements: Molybdenum Disilicide vs Silicon Carbide

• Metallic materials with high melting points, including metals such as tungsten, molybdenum, and tantalum, offer superior performance under vacuum and inert atmospheres and enable high-temperature processing with extremely pure semiconductor materials.
• Silicon carbide and molybdenum disilicides have been successfully employed as ceramic heaters that can operate effectively with air as a medium.
• Carbon heaters, especially graphite, have no equal in terms of uniformity and stability for some of the highest-temperature processes done in controlled atmospheres.

Every type of heating element, metal heating elements, ceramic heating elements, and carbon-based heating elements, has its own set of advantages suited for different heating conditions. Please visit Stanford Advanced Materials (SAM) for different types of heating elements. U-shape, W-shape, and H-shape, and customization are available.