Alumina oxide sheath material for thermocouple sensors

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Alumina oxide (Al₂O₃) is a widely used ceramic material that can be used to protect high temperature thermocouples. Its unique combination of characteristics makes it an relatively low cost choice for applications requiring high performance and reliability.

Properties of alumina

  • Chemical Formula: Al₂O₃
  • Appearance: White crystalline solid
  • Recommended max temperature: 1550°C
  • Melting Point: 2040°C
  • Thermal Conductivity: High
  • Electrical Insulation: Excellent
  • Mechanical Strength: High hardness and wear resistanc
  • Allowable Environment: Oxidizing
  • Corrosion Resistance: Resistant to most chemicals
  • Minimum Bend Radius:  Non-bendable

Advantages of alumina sheath

  1. High temperature resistance: Alumina sheaths can withstand extremely high temperatures, making them suitable for use in environments where other materials might fail. This property is crucial for applications in high-temperature furnaces and reactors.
  2. Electrical insulation: With excellent insulating properties, alumina prevents electrical interference and ensures the accuracy and safety of sensor readings. This is particularly important in sensitive electronic applications.
  3. Mechanical strength: The high hardness and mechanical strength of alumina protect the internal components of sensors from physical damage, extending their operational life.
  4. Chemical inertness: Alumina’s resistance to chemical attack allows it to perform well in harsh environments, including those with corrosive gases and liquids. This property makes alumina sheaths ideal for use in chemical processing plants and other corrosive environments.
  5. Thermal conductivity: Alumina’s ability to conduct heat effectively helps in the rapid response of temperature sensors, ensuring precise and timely measurements.

Alumina sheaths are not bendable. They are a economical alternative for some refractory metals, but in comparison to other sheaths not bendable.

Applications of alumina sheathed sensors

  • Industrial furnaces: Used to monitor and control temperatures in high-temperature environments.
  • Aerospace: Employed in aircraft and spacecraft where high temperature and corrosive resistance are required.
  • Nuclear industry: Used in reactors for temperature measurement and control due to its radiation resistance.
  • Chemical processing: Ideal for applications in corrosive environments where chemical resistance is essential.
  • Power generation: Used in power plants for monitoring temperatures in boilers and turbines.

Property comparison of alumina versus other refractory metals

PropertyAlumina (Al₂O₃)Refractory metals (e.g., Tungsten,
Molybdenum, Tantalum, Niobium)
Melting Point 2040°CExtremely high (e.g., Tungsten 3422°C, Molybdenum 2623°C)
Thermal ConductivityHighVery high (e.g., Tungsten ~173 W/m·K, Molybdenum ~138 W/m·K)
Electrical ConductivityExcellent insulatorExcellent conductors
Mechanical StrengthHigh hardness and wear resistanceVery high strength, particularly at elevated temperatures
Chemical ResistanceResistant to most chemicalsVariable, but generally good, especially against acids and bases
CostRelatively lowHigh, particularly for tungsten and tantalum
DensityLightweightVery dense (e.g., Tungsten ~19.3 g/cm³, Molybdenum ~10.28 g/cm³)
Oxidation ResistanceExcellent at room temperature; degrades at high temperaturesOften requires protective coatings in high-temperature oxidizing environments
Bend radius Non-bendableNiobium 10x diameter, others non-bendable

Conclusion

Choosing between alumina and refractory metal sheaths depends on the specific requirements of your application. Alumina offers excellent electrical insulation, high-temperature resistance, and cost-effectiveness, making it suitable for a wide range of industrial applications. Refractory metals, with their superior high-temperature tolerance, thermal conductivity, and mechanical strength, are ideal for extreme conditions, particularly in aerospace, defense, and high-performance electronics.

Understanding the distinct properties and advantages of each material will help you select the most appropriate sheath for your sensors, ensuring optimal performance and reliability in your specific application.

Contact Kamet to help you choosing the right sheath material for your installation.