Zirconia (ZrO2) refractories: identify the application where they are generally not recommended, considering thermal cycling and service conditions.

Introduction / Context:
Zirconia refractories are valued for very high refractoriness and corrosion resistance. However, their use demands attention to stability during heating and cooling. This question asks where zirconia is generally not preferred, highlighting thermal-cycling concerns.

Given Data / Assumptions:

• Zirconia can undergo polymorphic transformations with volume changes unless stabilized.
• Thermal cycling can trigger stresses and cracking in poorly stabilized products.
• Many zirconia products are used successfully in steady high-temperature duty.

Concept / Approach:
Frequent rapid temperature fluctuations are risky for zirconia refractories if stabilization and microstructure are not optimized. In contrast, applications such as thermocouple sheaths, kiln linings at steady conditions, or specialized induction components may suit zirconia when temperatures are relatively stable and design accounts for its characteristics.

Step-by-Step Solution:
Assess which environment imposes repeated thermal shocks.Recognize zirconia’s susceptibility without perfect stabilization.Conclude that fluctuating-temperature furnaces are generally unsuitable.

Verification / Alternative check:
Refractory handbooks caution that zirconia’s phase changes can degrade cycling performance unless carefully stabilized and engineered, making fluctuating service a concern.

Why Other Options Are Wrong:
Thermocouple sheaths/kiln linings/induction components: feasible with proper grade and design.

Common Pitfalls:
Assuming “highest refractoriness” automatically implies the best thermal-shock resistance under severe cycling.

Final Answer:
Furnaces subjected to fluctuating temperature