Piezoelectric phases

Various types of work in addition to pV work are frequently involved in experimental studies. Research on chemical equilibria for example may involve surfaces or phases at different electric or magnetic potentials [11], We will here look briefly at field-induced transitions, a topic of considerable interest in materials science. Examples are stress-induced formation of piezoelectric phases, electric polarization-induced formation of dielectrica and field-induced order-disorder transitions, such as for environmentally friendly magnetic refrigeration. 

Order parameters may also refer to underlying atomic structure or symmetry. For example, a piezoelectric material cannot have a symmetry that includes an inversion center. To model piezoelectric phase transitions, an order parameter, r], could be associated with the displacement of an atom in a fixed direction away from a crystalline inversion center. Below the transition temperature Tc, the molar Gibbs free energy of a crystal can be modeled as a Landau expansion in even powers of r (because negative and positive displacements, 77, must have the same contribution to molar energy) with coefficients that are functions of fixed temperature and pressure,... 

Figure 19.11 Phase maps f(x, y) of a 6x4/um region of a fatigued FeCap (108 cycles) after negative (left) and positive (right) poling and evolution map of the piezoelectric phase signal f(x, E) (central picture) under varying (triangular shape) electric field E of the horizontal line indicated by the horizontal arrows. PI, P2, LI and L2 are discussed in detail later.

Fig. 7.27 ElFeetive hydrostatic piezoelectric coefficients for 1-3 composite. Piezoelectric phase is PZT5 [Table I. in Smith (1993)] with...

Also 3-3 composites were reported (Bowen et al. 2001). Such stmctures exhibit better poling properties due to the piezoelectric phase cormected in all 3 dimensions however the technology of making it is more comphcated. 

Piezoelectric Composite Sensors. The concept of piezoelectric composites comes from the idea to connect an active piezoelectric phase with a passive matrix phase in a way that the best features from both components can be enhanced and the shortcomings can be minimized correspondingly. Common examples are composites from a stiff ceramic with a soft polymer. 

Thus, in a composite the ME effect is given by the product between the magnetostriction of the magnetic phase and piezoresponse of the piezoelectric phase [50], The two possible ME coefficients in case of composites are... 

The research works on toughening mechanisms and fracture toughness of ceramic composites with piezoelectric phases have been limited, while those for monolithic piezoelectric/ferroelectric materials have been available, as mentioned above. Therefore, details of toughening behavior and mechanisms in composite materials with piezoelectric phases have been still not clear. 

Figure 3.39 shows the strain versus field relationship of various types of electromechanical ceramics. Hard piezoelectric materials. Types I to IV, are typically formulated to operate well below the transition region. The piezoelectric phase is highly stable and typically formulated for high coercive force and minimal domain wall movement (minimal... 

FIgumSI Cakidsted values of piezoekcttk oomtana and d, for piezoelectric phase of PHB... 

Piezoelectric effect may take place not only in the crystalline phase but also in the interface between the crystalline phase and the amorphous phase, where molecules arc well oriented. The oriented rcgioiis where molecules are aligned parallel, but with non-uniform intermolecular spacing may have a symmetry required to piezoelectric effect. However, such oriented amorphous phase may show relaxatkmal behavior, as indicated in the temperature variatioo of and di in Figure 31. We assume that piezoelectric phase... 

The P(VDF-TrFE) copolymer is particularly useful since it exhibits a very stable piezoelectric phase at room temperature. ... 

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