Thermal properties of materials
Red-hot quartz glass (fused silica) rarely cracks when dropped into water, whereas soda-lime glass cracks easily. What is the main reason for the high thermal shock resistance of quartz glass?
- AIts linear expansion coefficient is very small, about 0.5×10⁻⁶ K⁻¹, so the thermal stress from a temperature difference is small
- BIts thermal conductivity is as high as a metal's, so the surface–interior temperature difference vanishes quickly
- CIts fracture toughness is as high as a metal's, so even if thermal stress starts surface cracks they hardly grow
Show answer and explanation
Answer: A. Its linear expansion coefficient is very small, about 0.5×10⁻⁶ K⁻¹, so the thermal stress from a temperature difference is small
On quenching, only the surface tries to contract first and is held back by the interior, creating a tensile thermal stress σ ≈ EαΔT/(1 − ν) at the surface. The linear expansion coefficient of quartz glass is about 0.5×10⁻⁶ K⁻¹, only about one twentieth of the roughly 9×10⁻⁶ K⁻¹ of soda-lime glass, so the thermal stress is far smaller for the same temperature difference and it does not crack. The thermal conductivity of quartz glass is low, about 1.4 W/(m·K), and its fracture toughness of about 0.8 MPa·√m is on par with soda-lime glass, both far below metals. Thermal conductivity acts through how the temperature difference between surface and interior develops; under mild heat transfer, materials with higher thermal conductivity are more resistant to thermal shock.