As mentioned previously, since PZT materials are not single crystal, relaxation occurs in the material which also lowers the piezoelectric coefficients. This relaxation is logarithmic in nature and is measured in percent decrease per decade of time. Determining the actual values of the piezoelectric coefficients can be done using a variety of methods. Other methods for calibrating piezoelectric constants include using an interferometer, a capacitive displacement sensor, or the double piezoelectric response 16 of the piezoelectric tube. Even after determining the d 31 constant for a given piezoelectric tube, determining the actual displacement for a given voltage may still be difficult.
Many piezoelectric materials suffer from hysteresis and creep effects.
Furthermore, at high voltages the piezoelectric tubes do not respond linearly. Thus, at large deflection the actual displacement is difficult to determine. The condition of the tunneling probe is critical for obtaining atomic resolution. The first STM probe used a tungsten rod 1mm in diameter which was mechanically ground at an angle to produce a sharp probe. Since then, many superior techniques have been developed for manufacturing probes for scanning tunneling microscopes.