There are a wide range of systems available that integrate the elements as well as a suitable insulation package into a single unit. Normally the elements used in these systems are either iron-chrome-aluminum alloys or nickel chrome alloys. Standard shapes are semi-circular, flat panels or cylindrical in a range of power, size and voltage ratings.
Normal construction is either a pressed or a cast ceramic support with a vacuum cast or an optional ceramic package that offers insulation and support. They are rated to achieve maximum chamber temperatures range between 1100 and 1400ºC.
The higher temperature ranges usually for special packaged heaters normally have strict operating restrictions.
Metal sheath heaters are a special type of package heaters that consist of Wire elements shielded in metallic sheath. These types of heaters do not use insulation panels.
Certain characteristics of oxygen-sensitive materials are listed below:
- Oxygen-sensitive materials can be used at considerably high temperatures and watt loadings provided they are not exposed to oxygen. Protection from oxygen is ensured by either maintaining appropriate vacuum levels or by an inert atmosphere.
- The materials show a considerable increase in resistance as temperatures increase. Hence it is essential to protect the furnace, elements and power supply from increased power draw especially in cold startup conditions.
- The materials are sensitive to thermal and mechanical shock. Hence a gradual and slow ramping function is required.
- Many of these materials operate at voltages lower than standard line voltages. It is important to note that since these materials have all the above features there is a requirement for a costly and complicated power supply.
Oxygen sensitive materials include tungsten, tantalum, graphite and molybdenum. Details regarding these materials are detailed below.
The use of molybdenum in industrial furnaces began in 1930 and it has been used extensively in industrial applications since 1940. The material is offered in different forms that include rod, wire, strip and tubes. Some of the characteristics of molybdenum are listed below:
- Molybdenum has a high level of affinity for oxygen at increased temperatures and can only be heated in the presence of a reducing, vacuum or a pure inert atmosphere. Oxidation of the material takes place between 250 and 300ºC forming molybdenum dioxide that offers a restricted amount of protection against more oxidation up to around 600°C.
- Molybdenum dioxide turns to molybdenum trioxide that is extremely volatile and exposes the base metal to more oxidation.
- At around 800°C catastrophic oxidation occurs with molybdenum dioxide clouds being generated.
- It must be ensured not to expose molybdenum to oxygen at temperatures over 400ºC.
- Ductility of the material is good when the material us new but with changes in temperature Molybdenum turns from ductile to brittle.
- Ideally molybdenum when utilized with high purity Al2O3 ceramic panels can be supplied as a part of a previously packaged system.
- The material may be used in dry cracked ammonia, dry hydrogen and ultra-pure inert helium or argon atmospheres up to 1900°C and, also in higher temperatures.
Tungsten exhibits many of the properties shown by molybdenum in its mechanical, electrical, thermal and chemical properties. It is also found in the same forms as molybdenum. Tungsten is less ductile and is more difficult to work with. Certain unique characteristics of tungsten are listed below:
- Tungsten has a high melting point. Hence it can be used at higher temperatures and higher vacuum levels than molybdenum.
- The maximum use temperature is around 2500°C.
- At vacuum levels of below 10-2 Torr it can be used up to 2000°C. At levels of less than 10-4 Torr it can be used up to 2400°C after which tungsten evaporation occurs.
- Oxidation begins at around 500°C with catastrophic oxidation occurring around 1200°C.
Tantalum can be heated in an inert atmosphere or in a vacuum below 10-4 Torr. Certain unique characteristics of tantalum are listed below:
- This substance exhibits a rather strong affinity to bonding with most of the gas molecules present in common industrial heating atmospheres.
- In the presence of oxygen, hydrogen, nitrogen or carbon, chemical reactions take place leading to formation of oxides, hydrides, nitrides or carbides. Due to this erosion occurs
- Hydride formation can be reversed and tantalum oxide can also be removed hence it is used as a “getter” in critical applications.
- Tantalum is highly ductile and needs to be protected from these gases to ensure it maintains its ductility in spite of repeated heating and total recrystallization.
- When used in vacuum levels below 10-4 Torr, in highly pure helium or argon atmospheres, tantalum may be used up to 2400ºC. Under higher vacuum conditions, tantalum is highly susceptible to evaporation.
- Tantalum is very expensive hence is used only for specialized usage.
These include platinum/rhodium alloys, pure platinum and pure rhodium. Some of the characteristics of these are listed below:
- These elements exhibit significantly increases in resistance with increasing temperature. The changes in resistance may cause a high current draw during cold startup conditions unless proper care is taken.
- The metals are not highly sensitive to thermal cycling though their support structures are. There should be a ramping function in the control system that would enable limiting of amperage to design specifications to ensure system protection against damage.
- Pure platinum has a low melting temperature of about 1770°C, and has a considerably high vapor pressure hence is prone to losing material in the form of oxide volatilization at increased temperatures.
- Standard use temperatures for pure exposed platinum is listed around 1450°C. Oxide and metal losses can be reduced by embedding the platinum in approved refractory cement. Hence an operating temperature of up to 1600°C on a normal basis and up to 1700°C in special cases with strict operating parameters is possible.
- Rhodium has a higher melting point of around 1960°C, better vapor pressure and oxide evaporation rates, improved hot strength, and higher temperatures for grain growth inception yet it has poor ductility and an adverse resistance curve making it difficult to work with.
- Rhodium can be alloyed with platinum, it is possible to create new compounds with remarkable improvements in use temperature, vapor pressure, oxidation rates and brittleness with some reduction in ductility when compared to pure platinum.
- Standard alloys typically contain 10, 20, 30 or 40% rhodium with the balance being platinum. These alloys have use temperatures of 1550, 1650, 1720 and 1770°C respectively compared to 1450°C for pure platinum and 1850 to 1900°C for pure rhodium.
- The 10 and 20% rhodium/ platinum alloys are used mostly because of ductility, temperature and cost.
- These are used for specialized applications in the glass industry and in R&D owing to their high costs.
Molybdenum, tungsten, tantalum and precious metals can be used as heating elements and have their own advantages and disadvantages.
This information has been sourced, reviewed and adapted from materials provided by Thermcraft.
For more information on this source, please visit Thermcraft.