Chapter 6.3 - Heat Capacity Study of Functional Ceramics

Abstract

The role of heat capacity study of functional materials was discussed with the examples of dielectric materials. The stable state of materials is determined by the Gibbs energy, which includes both enthalpy and the entropy terms. This means that the knowledge of entropy , which is a measure of the disorder in the material, is essential to understand the thermodynamic properties at finite temperature. The heat capacity measurement is the only way to determine the entropy, and the importance of the measurements was demonstrated by the studies of relaxor behavior in Pb(Mg 1/3 Nb 2/3 )O 3 (PMN), Pb(Mg 1/3 Ta 2/3 )O 3 (PMT), and Pb(Sc 1/2 Ta 1/2 )O 3 (PST). In the relaxors with perovskite structure, a broad heat capacity anomaly was observed around room temperature. The excess heat capacity was caused by the formation and growth of ferroelectric nanoregion in relaxors. The entropy change due to the formation of the nanoregion is limited to only one fifth of the perfect ferroelectric ordering in the materials. It indicates that the disorder remains and freezes at the lower temperature. Such information can be obtained only by the heat capacity measurements, and the usefulness of the thermodynamic study was clearly shown. In the second example, the giant particle size effect on the phase transition in dielectric materials was discussed from the thermodynamic point of view. In general, the particle-size effects on ceramics can be analyzed in terms of the influence of the surface. The relative population of the atom near surface increases gradually with decreasing the particle size, and the surface effect becomes considerable only in nanometer scale. However, the peculiar particle-size effect on the phase transition was found in some dielectric substances BaZnGeO 4 (BZG) and CsZnPO 4 (CZP), in which the critical size for the phase transition locates around 1 mm. The sample with large size above 1 mm showed a heat capacity anomaly due to the phase transition, but no heat capacity anomaly was observed in the small samples. In addition to the appearance and disappearance of the phase transition, the phase transition observed in large crystals showed a kinetic behavior. The heat capacity anomaly grew with increasing the annealing time just below the phase transition temperature. The kinetics of the growth of the heat capacity anomaly was analyzed using Avrami model, and the results indicated a heterogeneous nucleation and growth mechanism. The mechanism of this curious giant particle size effect of the phase transition have not been understand clearly yet. However, the high-temperature phase is ferroelastic in both materials, and the ferroelastic domain may influence the phase transition property. It became clear that all the possible types of thermal agitation contribute to the heat capacity of materials, and thus, the heat capacity measurements have an advantage to find novel phenomena occurring with the temperature change. The thermodynamic study of materials is classic but still effective in the investigations of functional materials.