February 01, 2021

Thermocouples: Technical Ceramics in Sensor Tubes

Technical ceramics, equally referred to as engineering or fine ceramics, are a high-performance class of inorganic materials known to withstand the extremes of industrial environments. Reliable over extended service lengths in the most challenging conditions, technical ceramics are routinely deployed in conditions where other materials would surely fail. Although technical ceramics are a broad group of non-metallic alloys with varying thermodynamic and chemical properties, they are unified by comprehensively good resistance to corrosion, wear, and high-temperatures—relative to other engineering materials. 
Saint Gobain Specialty Ceramics' products allow customers to solve tough problems in thermal and chemical processing, industrial metrology, temperature sensing, brazing, mechanical seals, semi-conductor processing, sandblasting, and renewable energy generation applications. Specialty Ceramics' portfolio consists of more than a dozen products, sold as well-known brands: Hexoloy® materials, Norbide® nozzles, Noralide® materials, Silit® mat
This means they can be deployed in demanding environments like foundries where carburising, oxidizing, nitriding, and sulfurizing atmospheres—in conjunction with operating temperatures exceeding 1,200°C (2,190°F)—will cause rapid degradation of metallic components.

Using Technical Ceramics for Process Monitoring

Process monitoring is a challenge in glass and metal foundries. Sampling melt or flue gas temperatures means exposing sensitive probes to high-temperature, corrosive environments.
Thermocouples are used across the full spectrum of engineering and industry to obtain accurate temperature readings in situ. This is calculated as a function of the temperature differential at a junction between two temperature-sensitive wires—usually chromel-alumel, or what is known as a Type K thermocouple.

Although reliable and accurate at temperatures over 1,000°C (1,832°F), thermocouples are prone to corrosion in harsh atmospheres and cannot withstand the severe thermal shock associated with foundry applications. Additionally, the accuracy of temperature readings is relative to the dimensional integrity of thermocouple wires, meaning there is low tolerance for thermally-induced expansion.

An easy solution to this problem is to sheathe thermocouples in a protection tube composed of a technical ceramic like alumina, silicon carbide, or zirconia.

Technical Ceramics for Thermocouple Tubes

Although alumina and zirconia can be used for thermocouple protection tubes, high-purity silicon carbide provides the greatest level of performance owing to its high density, low porosity, and outstanding thermal conductivity.

At Saint-Gobain Performance Ceramics & Refractories, we supply a proprietary alpha silicon carbide under the Hexoloy® product line. Prior to our unique technical ceramic offering, silicon carbide was used sparingly in foundry applications due to problems associated with purity and porosity. Our single-phase silicon carbide resolves those challenges with excellent uniformity and absolute chemical integrity, providing universal corrosion resistance. This is achieved through a pressureless sintering and extrusion process which yields self-bonded fine-grained silicon carbide without the need for binders or fillers.

Our Hexoloy® silicon carbide thermocouple protection tubes are subsequently ideal for temperature sensor tubes in challenging working environments, guaranteeing long-lasting performance for reduced maintenance requirements and increased up-time for critical processes.

Interested in learning about more technical ceramics applications?

This article functions as a brief introduction to the need for precision thermocouple sheaths, and our unique solution to that challenge. If you would like more details, you can find a full breakdown of the specifications of our Hexoloy® SiC sensor tubes here.