Question

For which refractory material the thermal conductivity decreases with increasing temperature?

• A. Zirconia
• B. Silica
• C. Magnesite
• D. Alumina

Hint:

Basic refractories like magnesite can show decreasing thermal conductivity at high temperatures due to internal structural changes and increased phonon scattering.

Answer 1

The Correct Option is C

Refractory materials are used in high-temperature industrial processes and are chosen based on their ability to withstand thermal, mechanical, and chemical stresses.
One of the key thermal properties of refractories is thermal conductivity — the ability of the material to conduct heat. This property is crucial because it determines how heat flows through the refractory lining and affects the efficiency and safety of furnaces and kilns.
Generally, for most materials, thermal conductivity may increase with temperature. However, magnesite (MgCO$_3$ or after calcination, MgO) behaves differently. In magnesite and similar basic refractories, thermal conductivity tends to decrease with rising temperature. This is due to phonon scattering effects and changes in microstructure at elevated temperatures, which reduce the ability of the material to transfer heat.
Let's evaluate the options:
- Zirconia: Known for stable thermal conductivity at high temperatures, often increasing slightly with temperature.
- Silica: Shows a complex trend, but generally not a consistent decrease across a wide temperature range.
- Magnesite: Correct. Exhibits decreasing thermal conductivity with increasing temperature.
- Alumina: Like zirconia, it maintains or slightly increases its conductivity at higher temperatures.
Therefore, magnesite is the correct answer.