Thermal expansion behavior of polycrystalline ceramics

Thermal energy input causes atoms to vibrate with greater amplitude and, as indicated earlier, this usually causes expansion. As the atomic spacing increases, the spring constants of the atomic bonds decrease and the atoms vibrate more slowly. The tendency for expansion is opposed by the bulk modulus. The quantitative development of this theory leads to 

The data in Table 3.5 show the trends one expects for an inverse dependence on bulk modulus, i.e., thermal expansion coefficients increase in the order carbides, silicides, oxides and halides. The main exception appears to be the silicates, which often have quite low thermal expansion coeffiecients. Silicate structures are often open and coupled transverse modes of vibration can lead to a decrease 

F ore 3.25 A simple model for the expansion of three rods with different thermal expansion coefficients. 

Tabk 3.5 Thermal expansion coefficients of selected ceramic polycrystals 

The presence of porosity in a material is not expected to influence the thermal expansion behavior. This follows from Eq. (3.18) but, in addition, it can be envisioned that a hole in a body will not constrain the thermal expansion of the sur- 

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