Reason for poor thermal-shock resistance of silica bricks below about 600°C: which property dominates their tendency to spall in this lower-temperature range?

Introduction / Context:
Silica bricks are staple refractories for coke ovens, glass tank crowns, and hot-blast stove checkers. However, they are notorious for poor thermal-shock resistance at lower temperatures. Understanding the governing property helps operators avoid damaging heat-up or cooldown profiles.

Given Data / Assumptions:

• Temperature range of interest: ambient to roughly 600°C.
• Silica undergoes polymorphic transitions with significant volume/expansion changes, especially near 573°C (alpha–beta quartz transition).
• Thermal shock relates to rapid temperature changes coupled with material expansion behavior.

Concept / Approach:
Below ~600°C, silica’s effective thermal expansion is high due to phase transitions and structural changes. This high coefficient of thermal expansion (CTE) causes large strain for a given temperature change, generating tensile stresses that lead to cracking and spalling. Thus, in this temperature band, the CTE dominates the thermal-shock response, more than conductivity or nominal refractoriness.

Step-by-Step Solution:
Identify key transition: alpha–beta quartz near 573°C.Relate high CTE to stress under rapid thermal cycling.Conclude that elevated CTE up to ~600°C drives poor shock resistance.

Verification / Alternative check:
Heat-up schedules for silica refractories specifically slow the ramp through ~573°C to limit damage, confirming the central role of high expansion.

Why Other Options Are Wrong:
High thermal conductivity / diffusivity: these can reduce gradients; not the main cause of spalling.Low refractoriness: silica actually has very high refractoriness; not the issue here.Low density: not the principal driver of shock failure.

Common Pitfalls:
Ramping too fast through the 500–600°C range.Confusing high-temperature creep with low-temperature shock sensitivity.

Final Answer:
High coefficient of thermal expansion up to this temperature