Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The most common refractory metals include tungsten, molybdenum, tantalum, niobium, and rhenium. These metals are characterized by their extremely high melting points, which are significantly higher than those of most other metals
Refractory metal thermocouples are designed for use in extremely high temperature environments where other types of thermocouples would fail. The most common types of refractory metal thermocouples are made from combinations of tungsten and rhenium. These thermocouples can be used in extreme temperatures, making them ideal for applications in high-temperature furnaces, nuclear reactors, and the aerospace industry.
On this page we first explain more about refractory metals and then about refractory metals thermocouples.
What are refractory metals?
The most commonly recognized refractory metals include:
- Tungsten (W): has the highest melting point of any metal at 3.422°C (6,192°F), making it ideal for high-temperature applications such as lighting, electrical, and heating elements.
- Molybdenum (Mo): known for its strength and stability at high temperatures, as well as its high melting point of 2.623°C (4,753°F), molybdenum is used in aerospace, electronics, and industrial applications.
- Tantalum (Ta): with a melting point of 3.017°C (5,463°F), tantalum is highly resistant to corrosion and is used in electronics, chemical processing, and high-temperature environments.
- Niobium (Nb): has a melting point of 2.468°C (4,474°F) and is used in superalloys for jet engines and rockets, as well as in the manufacture of superconducting materials.
- Rhenium (Re): one of the rarest elements in the Earth’s crust with a melting point of 3,180°C (5,756°F), rhenium is used in high-temperature turbine engines and as an additive to improve the properties of other refractory metals.
Why use refractory metals in thermocouples?
- High temperature resistance: the primary characteristic of refractory metals is their ability to retain strength and stability at temperatures where most other metals would melt or significantly degrade.
- Corrosion resistance: many refractory metals exhibit excellent resistance to corrosion, making them suitable for chemical processing environments.
- High density: most refractory metals have a high density, which contributes to their durability and wear resistance.
- High melting points: the defining feature of refractory metals is their exceptionally high melting points, which make them indispensable in industries such as aerospace, defense, and nuclear energy.
In summary, refractory metals are indispensable in applications that demand materials capable of withstanding extreme conditions. Their unique properties enable advancements in technology and industry, particularly in areas subjected to the highest temperatures and most challenging environments.
Types of refractory metal thermocouples
- Type C: consists of a tungsten 5% rhenium (W-5% Re) versus tungsten 26% rhenium (W-26% Re) combination. This type is known for its ability to perform well in high temperatures up to 2.300°C (4,172°F) in inert or vacuum environments.
- Type D: another combination of tungsten and rhenium, with different proportions compared to Type C, offering slightly different temperature ranges and stability characteristics.
Characteristics of refractory metal thermocouples
- High temperature capability: refractory metal thermocouples can measure temperatures much higher than those possible with noble metal (e.g., platinum-based Type S) thermocouples.
- Inert or vacuum use: they are best used in inert atmospheres or vacuum conditions because refractory metals can oxidize or react with certain atmospheres at high temperatures, leading to degradation.
- Durability: these thermocouples maintain structural integrity and measurement accuracy at high temperatures, making them suitable for harsh conditions.
- Stability: they provide stable temperature measurements in extreme conditions, making them reliable for critical applications in harsh environments.
Applications
They are commonly used in industries that operate at high temperatures, such as:
- Aerospace: for monitoring the temperature of rocket engines and other propulsion components during testing and operation.
- Nuclear industry: in reactor control and safety systems where high temperatures occur.
- Materials science: for the study of materials at high temperatures, including sintering processes, melting, and heat treatment.
- Industrial processes: in furnaces, kilns, and other high-temperature industrial processes where accurate temperature control is essential.
Limitations refractory metal thermocouples
Despite their high temperature capabilities, refractory metal thermocouples have limitations, including:
- Sensitivity to oxidation: without proper protective atmospheres, they can rapidly degrade in oxidizing environments.
- Rather difficult manufacturing process: the precise care needed in junctioning to prevent damage, the requirement for special cutting tools for accuracy and strict cleanliness standards makes the manufacturing process for Type C thermocouple wires challenging. More about this topic on this page.
- Brittleness: at lower temperatures, refractory metals can be brittle and difficult to handle, requiring careful installation and maintenance.
In summary, refractory metal thermocouples are critical tools for temperature measurement in extreme conditions, chosen for their unparalleled ability to withstand high temperatures while maintaining accuracy and stability. Their use in specific industries highlights the importance of these devices in advancing technological capabilities and understanding materials under extreme conditions.
What about platinum?
Although platinum is a metal with a high melting point (1.768°C or 3,214°F) and excellent resistance to corrosion, it is typically classified separately from the traditional group of refractory metals. Platinum is often categorized as a “noble” or “precious” metal due to its rarity, chemical inertness, and extensive use in jewelry, catalysis, and electrical applications.
The distinction comes from the specific criteria used to define refractory metals, primarily their melting points which are generally above 2.000°C. While platinum has remarkable properties that make it suitable for high-temperature applications, its melting point is below the threshold commonly used to classify metals as refractory.
Nonetheless, platinum and its alloys are crucial in many industrial applications where high temperature resistance, along with chemical stability, is required. Making it a good metal and alloy to use in high-temperature thermocouples.
Like to know more about other high temperature metals? Discover titanium and Haynes®.
At Kamet, our years of experience in the extreme heat applications makes us well-positioned to advise you which combination of our refractory metal thermocouples and sheath materials will best suit your situation. You can call or email to get in touch with one of our advisors.
