A brief tangent: Piezoelectric dielectrics

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In 1880, two French scientists Jacques and Pierre Curie, were also brothers discovered that pressure applied to certain crystals such as quartz, tourmaline, Rochelle salts and cane sugar creates an electrical charge in that material. They referred to this strange phenomenon as the piezoelectric effect. Soon afterwards, they observed the inverse piezoelectric effect where application of electric voltage across these crystals caused mechanical deformation. They subsequently obtained enough data to prove quantitatively the complete reversibility of this effect. Note that all the crystals listed above are dielectrics (electrical insulators).

In piezoelectric materials, the crystal structure is such that the mechanical properties of the crystal are couple to its electronic properties at microscopic level so that any change in one causes a corresponding change in the other. This may also be referred to as electro-elasto-mechanical coupling. Consequently, a piezoelectric crystal responds to an applied mechanical stress by developing an electric charge, hence a voltage, across it. Conversely, when a voltage is applied across a piezoelectric, it responds by changing its dimensions, say by expanding; and on reversing polarity of the voltage, it contracts. The figure below illustrates this phenomenon.

The field of piezoelectric materials has come far since the discovery of the phenomenon. These materials are employed in some of the most sophisticated devices used in research and industry. For example, in modern microscopes such as Scanning Probe Microscope (SPM), Atomic Force Microscope (AFM), etc. the sample stage needs to be moved with the precision of fraction of a nanometer. Mechanical motors with their bulky size, backlash issue, and irreproducible performance are highly unreliable for such sensitive applications. Honestly, such advanced microscopy wouldn’t have been possible without piezoelectric materials.

Another area piezoelectrics find application is in weight scales. Mechanical strain caused by the weight of an object placed on a piezoelectric induces an electric voltage across it. This voltage is recorded for several known standard weights and then the piezoelectric is calibrated as a sensor to measure weight. The precision and reproducibility in piezo weight scales is remarkable, so much so that they are used to track thickness of thin films on nanometer scale in situ (during growth) in growth techniques such as sputtering, Molecular Beam Epitaxy, Pulsed Laser Deposition, etc.

Application of an AC voltage causes a piezoelectric to contract and expand at the frequency of the applied voltage. If the frequency is in audible range, the induced vibrations produce sound waves. There are buzzers that use this phenomenon to make simple mono-frequency beep sounds. Since piezoelectrics are low power devices (they are insulators, hence, do not draw much current), they can be easily integrated with low power electronic circuits based on ICs (Integrated Chips). Conversely, piezoelectrics are also used to measure pressure variation in a medium.

To list all of interesting applications of dielectrics would be an exhausting, nevertheless enjoyable, task. The framework we are developing here to study dielectrics is quite general and is routinely employed in understanding, tailoring and fine-tuning properties of dielectrics in research as well as in industry. There are other equally interesting families of dielectrics that exhibit different sets of properties and are applied to different ends but they all fit in our current framework.