SiSiC Thermocouple Protective Tube

Reaction-bonded silicon carbide (RBSC/SiSiC) is a fully dense, zero-porosity material with excellent gas-tightness, making it ideally suited for sealed temperature-measurement applications in atmosphere furnaces, flue-gas monitoring, and similar environments. However, because it contains residual free silicon, it is susceptible to oxidation and surface degradation at elevated temperatures. The maximum continuous service temperature is 1350 °C, which limits its use to medium-temperature conditions.

SiSiC protection tubes are produced from a blend of silicon carbide powder and carbon black, which is formed and then subjected to high-temperature debinding to create a porous green body. This preform is infiltrated with molten silicon in a high-temperature argon atmosphere; capillary action drives the liquid silicon into the pores, where it reacts in situ with the carbon black to form secondary silicon carbide. Any remaining free silicon fills the residual porosity, achieving full densification in a single sintering step. After sintering at a relatively moderate temperature (1500–1600 °C), the tubes are finished by simple grinding and shaping.

This route offers high production yield, low cost, excellent density, superior gas tightness, simple processing, and good dimensional accuracy. Its main limitation is the presence of residual free silicon, which caps the maximum service temperature (generally not exceeding 1400 °C) and can act as a weak point in certain aggressive corrosive media.

Thermocouple protection tubes are widely used in high‑temperature industrial sectors including iron and steel metallurgy (continuous temperature measurement of molten steel and iron), petrochemicals (high‑temperature cracking furnaces), coal chemical industry (gasifiers operating at 1400 °C and above), power generation (circulating fluidised‑bed boilers), and glass/ceramic firing. Different process conditions demand specifically tailored tube materials: atmospheres such as carburising, nitriding, sulphidising, or oxidising each require protection tubes with compatible resistance characteristics.

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