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Thermal Expansion of Technical Ceramics: Understanding the Relationship Between Thermal Expansion and Ceramic Structure

A fascinating phenomenon that occurs in ceramics is thermal expansion. Objects tend to increase in volume or length as their temperature increases. This expansion is caused by an increase in the amplitude of atomic vibrations within the object’s structure when heated. To quantify this expansion, we use the coefficient of thermal expansion, denoted α.

The relationship between thermal expansion and ceramic structure is very interesting. Ceramics with covalent bonds generally exhibit lower coefficients of thermal expansion. For example, SiC, Si3N4, diamond, B4C and other materials have thermal expansion coefficients ranging from 1.0×10-6/°C to 5.0×10-°/°C, which makes them highly resistant to expansion in high temperature environments. Temperature range 20°. C to 1000°C.

The directionality of the covalent bonds in these ceramics leads to the formation of voids that can absorb atomic vibration amplitudes and bond angles when heated. Therefore, the overall scaling of the component is minimal. On the other hand, ceramic or metallic materials with ionic bonds tend to have a close-packed structure. The amplitude of each atom increases when heated, resulting in a relatively large expansion of the entire material. For example, the coefficient of thermal expansion of ceramics such as Al2O3 and ZrO2 in the temperature range of 200°C to 1000°C is greater than 10×10-6/°C.

The specific crystal structure also plays a crucial role in determining the coefficient of thermal expansion. For monocrystalline and polycrystalline ceramics of the cubic crystal system, the coefficient of thermal expansion remains the same in all directions. However, for non-equiaxed crystals, each crystallographic axis direction can have a different thermal expansion coefficient, resulting in thermal expansion anisotropy.

An example of anisotropic thermal expansion is graphite, where there are strong covalent bonds within the layers and weak van der Waals bonds between the layers. Therefore, the coefficient of thermal expansion of each layer perpendicular to the c-axis is significantly smaller, only 1×10-6/°C. However, the thermal expansion coefficient parallel to the c-axis direction is very large, reaching 27×10-6/℃.

Similarly, ceramics such as aluminum titanate and calcite, although not having a layered structure like graphite, exhibit anisotropic thermal expansion coefficients due to different bonding and atomic packing in different crystallographic orientations. These factors lead to the different thermal expansion behavior observed in ceramics.

In summary, understanding the relationship between thermal expansion and ceramic structure is critical for various applications. Covalently bonded ceramics tend to have a lower coefficient of thermal expansion due to the presence of voids that absorb vibration amplitudes. On the other hand, ceramic or metallic materials with ionic bonds exhibit a large coefficient of thermal expansion. Furthermore, specific crystal structures induce anisotropic thermal expansion, resulting in different expansion behaviors in different directions. By understanding these concepts, researchers and engineers can design ceramics with desired thermal expansion properties for specific applications.