GTBL042-12 GTBL042-Callister-v3 October 2, 2007 13:41
2nd Revised Pages508 • Chapter 12 / Electrical Properties(a) (b)0.398 nmTi4+ Ba2+ O2–0.403 nm0.398 nm0.006 nm
0.006 nm0.009 nmFigure 12.35 A barium titanate (BaTiO 3 ) unit cell (a) in an isometric projection, and
(b) looking at one face, which shows the displacements of Ti^4 +and O^2 −ions from the center
of the face.potassium niobate (KNbO 3 ), and lead zirconate–titanate (Pb[ZrO 3 ,TiO 3 ]). Ferro-
electrics have extremely high dielectric constants at relatively low applied field fre-
quencies; for example, at room temperature,rfor barium titanate may be as high
as 5000. Consequently, capacitors made from these materials can be significantly
smaller than capacitors made from other dielectric materials.12.25 PIEZOELECTRICITY
An unusual property exhibited by a few ceramic materials is piezoelectricity, or, lit-
erally, pressure electricity: polarization is induced and an electric field is established
across a specimen by the application of external forces. Reversing the sign of an ex-
ternal force (i.e., from tension to compression) reverses the direction of the field. The
piezoelectric effect is demonstrated in Figure 12.36. This phenomenon and examples
of its application were discussed in the Materials of Importance piece that follows
Section 13.10.
piezoelectric Piezoelectricmaterials are utilized in transducers, which are devices that con-
vert electrical energy into mechanical strains, or vice versa. Some other familiar+––––––
+
++
+
++––––––++++++ ++++++
+
++
+
+(a) (b)VFigure 12.36 (a) Dipoles within a
piezoelectric material. (b) A voltage is
generated when the material is subjected to a
compressive stress. (From Van Vlack, L.,
ELEMENTS OF MATERIALS SCIENCE
AND ENGINEERING, 6/E,©c1989, p. 482.
Adapted by permission of Pearson Education,
Inc., Upper Saddle River, New Jersey.)