Ceramic items cannot be rapidly cooled from a heated state, primarily due to the thermophysical properties and internal structural characteristics of ceramic materials. The specific reasons are as follows:
1. Stress Concentration Caused by Thermal Expansion and Contraction
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Expansion During Heating
When ceramics are heated, the vibration of internal atoms (or molecules) intensifies, causing the material to expand overall. If the temperature of the ceramic is uniform across all parts during this process, the expansion is relatively consistent, and internal stress remains low. -
Non-Uniform Contraction During Rapid Cooling
When a high-temperature ceramic suddenly comes into contact with a low-temperature environment (such as cold water or cold air), the surface cools rapidly and contracts, while the interior cools more slowly and contracts with a delay. This difference in contraction rates between the surface and the interior generates enormous thermal stress within the ceramic (with tensile stress on the surface and compressive stress in the interior). -
Stress Exceeding the Strength Limit
Ceramic materials have low tensile strength (typically only 1/10 to 1/20 of their compressive strength). When thermal stress exceeds their tensile strength, it triggers the initiation and propagation of cracks, ultimately leading to the fracture of the ceramic.
2. The Brittle Nature of Ceramics
- Lack of Plastic Deformation Capacity: Ceramics are brittle materials and do not have the good ductility of metals. When thermal stress arises, ceramics cannot release stress through plastic deformation and can only respond through fracture.
- High Sensitivity to Cracks: Ceramics may contain tiny pores, grain boundary defects, or other initial cracks internally. The thermal stress generated by rapid cooling can cause these cracks to expand rapidly and even penetrate the entire material, resulting in fragmentation.
3. Differences in Sensitivity Among Ceramic Types
Not all ceramics have the same resistance to rapid cooling and heating. This depends on their coefficient of thermal expansion and thermal conductivity:
| Property | Ceramics with Poor Thermal Shock Resistance (e.g., Ordinary Daily-Use Ceramics) | Ceramics with Good Thermal Shock Resistance (e.g., Heat-Resistant Ceramics) |
|---|---|---|
| Coefficient of Thermal Expansion | High (e.g., traditional clay ceramics, α≈6–8×10⁻⁶/℃) | Low (e.g., cordierite ceramics, α≈1–2×10⁻⁶/℃; glass ceramics, α≈0) |
| Thermal Conductivity | Low (slow heat conduction, prone to temperature difference accumulation) | High (e.g., some silicon carbide ceramics, thermal conductivity λ≈100–400 W/m·K) |
| Typical Applications | Ordinary bowls and plates (should avoid rapid temperature changes) | Oven tableware, laboratory crucibles (can withstand sudden temperature changes) |
4. Practical Usage Precautions
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Avoid Sudden Temperature Changes
- Do not directly place heated ceramics (such as dishes just taken out of a microwave oven or oven) into cold water or a refrigerator.
- When using ceramic tableware in winter, it is recommended to preheat them with a small amount of warm water before adding hot food.
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Choose Heat-Resistant Ceramics
- For applications requiring frequent temperature changes, opt for high-alumina ceramics (e.g., Al₂O₃ content >75%), cordierite ceramics, or glass ceramics (e.g., Pyrex). These materials have low coefficients of thermal expansion and significantly better thermal shock resistance than ordinary ceramics.
Conclusion
The characteristics of thermal expansion and contraction, brittle nature, and internal structural defects of ceramics make them prone to cracking due to thermal stress concentration during rapid cooling or heating. This phenomenon essentially reflects the material’s insufficient thermal shock stability (resistance to thermal shock). When using ceramic products, it is necessary to avoid sudden temperature changes or select specially designed heat-resistant ceramic varieties.