Which characteristic is associated with piezoelectric materials?

Piezoelectric materials are unique because they have the ability to generate an electric charge in response to mechanical stress. This characteristic, known as the piezoelectric effect, allows these materials to convert mechanical energy into electrical energy. When a piezoelectric material is deformed—whether compressed, stretched, or subjected to shear stresses—it creates a voltage across its surface. This property makes piezoelectric materials invaluable in various applications such as sensors, actuators, and energy harvesting devices.

The other characteristics mentioned in the options do not accurately define the piezoelectric effect. For instance, the expansion of materials when heated is more closely related to thermal expansion properties rather than piezoelectric behavior. Color changes with temperature are often associated with thermochromic materials, which have their unique properties unrelated to piezoelectricity. Lastly, hardening under pressure is a characteristic of certain materials, but it does not pertain to the generation of electrical energy from mechanical deformation, which is the defining feature of piezoelectric materials.