|
Sheath Materials | |||
|
Material |
Upper Service Temperature |
Melting Point |
Remarks |
|
Molybdenum (Mo) |
3400° F (1871° C) |
4730° F (2610° C) |
Molybdenum offers excellent mechanical properties at elevated temperatures and is exceptional for use in reducing or neutral atmospheres. Molybdenum offers superb thermal shock resistance and has potentially unlimited life at high temperatures in a vacuum. Pure hydrogen, argon and helium atmospheres are completely inert to molybdenum. Performance is poor in oxidizing atmospheres. Above 800° F (427° C), molybdenum will react with oxygen. |
|
Tungsten (W) |
4200° F (2315° C) |
6100° F (3380° C) |
Tungsten has the best strength at high temperatures of all the common refractory metals. Tungsten provides good abrasion resistance. It can be used in hydrogen at all temperatures, and in dry argon and helium. Tungsten is not recommended for use in oxygen bearing environments, as oxygen is known to cause physical degradation in only a few hours depending on temperature and oxygen content. |
|
Tantalum (Ta) |
4500° F (2482° C) |
5425° F (2996° C) |
Tantalum is best used in reducing or neutral atmospheres and in a vacuum environment. Its performance is poor in oxidizing atmospheres, and it must be protected from gases such as oxygen and nitrogen at temperatures above 570° F (300° C). Tantalum is outranked by tungsten for high temperature strength. Tantalum is excellent for thermal cycling applications and is the most corrosion resistant of the refractory materials. It is almost completely immune to attack by acids except hydrofluoric acid. |
|
Alumina (Al2O3) |
3450° F (1900° C) |
3686° F (2030° C) |
High-purity aluminum oxide offers high thermal conductivity, high electrical resistivity and good resistance to thermal shock. It also offers excellent resistance to chemical attack. Aluminum oxide functions well in oxidizing, reducing or high vacuum applications, but is not recommended for high vacuum environments in the presence of graphite at temperatures exceeding 2372° F (1300° C). |
|
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