The evolution and wonders of piezoelectricity

“Small steps make big changes” is something most of us have heard at one point in our lives. But what if these small steps could power the world?

Scientists and Researchers around the world have been probing into different sources of energy that are efficient, environmentally friendly, and cost-efficient.

Piezo-Electricity is one such domain where a simple small change in the structure of lattice results in the release of electrical energy. It was discovered in 1880 by Jacques Curie and Pierre Curie. A Piezo-electric material has an electrical symmetry in its lattice structure however when tensile or compressive stress is applied to it the electrical stability of the crystal is disturbed and unequal charges appear. These charges can be accumulated to generate current.

The piezoelectric effect cannot occur in centrosymmetric crystals, since any mechanical action (which has the centre of symmetry) on a centrosymmetric crystal leads to a symmetry group with a centre of symmetry (stretching, compression, or shear actions are centrosymmetric actions). The presence of a centre of symmetry in a deformed crystal means that there is no reason for mechanically induced electrical polarization arising because there are no polar directions in such a crystal.

There are three kinds of Piezo-electric effects:

· Longitudinal Piezoelectric Effect: A one-dimensional crystal is supplied by electrodes and placed under longitudinal mechanical stress which lengthens the crystal giving rise to an electrical moment within it.

· Transverse Piezoelectric effect: A two-dimensional crystal (hexagonal shaped) when applied with directional stress produces a transverse effect. Due to this stretching electric polarization is induced not in the same direction but perpendicular to it. This gives rise to a transverse Piezo-electric effect.

· Shear Piezoelectric Effect: In a three-dimensional crystal (Cubic) any stress applied will result in both longitudinal and transverse stress on it. This gives rise to both types of Piezo-electric effects and is known as the shear effect (3-L and 6-T effects are observed for a cubic lattice).

The functioning of several electronic devices is not possible without the use of Piezo-electric parts like sonar radiators, antennas, frequency stabilizers in computers, sensors for acceleration, vibration, acoustic emission, medical instruments, etc. Some of these parts have even been used to generate electricity just by walking by using the stress applied on the ground using Piezo-electric transducers to convert sound waves to electrical signals.

Researches over the years have led to discoveries of several materials that are piezo-electric in nature like Rochelle Salt, Quartz Crystal, topaz, etc. with Rochelle salt being the first and highly experimented salt.

The history of Rochelle Salt dates back to around 1660 when Elie Seignette and Jehan Seignette first synthesized the double salt of tartaric acid i.e., Potassium Sodium Tartrate tetrahydrate or Rochelle Salt (KNaC4H4O6·4H2O). Amongst several minerals and organic naturally occurring crystals, Rochelle Salt had the maximum piezo-electric constant approximating to 10 esu per kg at the time. Raw and untreated crystals possess a large number of electrical dipoles which are desiccated and their exposed parts are covered with electrodes (Tinfoil or Aluminum). Upon application of pressure, unequal charges appear on the electrodes which are connected to wires and load.

The piezo characteristic of Rochelle Salt makes it a highly useful crystal in sensitive acoustics and vibrational devices. They are used in electromechanical transducers such as ultrasonic generators, microphones, and electromechanical resonators. During World War 1 physicists and electrical engineers like Paul Langevin in France and A.M. Nicholson in the USA independently perfected an ultrasonic submarine detector using crystal salt transducers.

The application of Rochelle salt for Piezo-electricity is beneficial due to its inherent cheapness and simplicity, long life, the flexibility of design, and freedom from the necessity of exciting currents, permanent magnets, or polarizing bias since it carries its field of excitation.

As non-renewable energy sources get depleted future prospects of sustainable energy sources like Piezo-electricity are expanding. Small scale projects of Piezo-electric speed breakers, footpaths, and stairs are being applied around the world to generate power. Its potential to revolutionize wearable technology is also being developed where simple movements could power devices like watches, clothing, and accessories. Research on piezo effects of Rochelle salt has also brightened up the prospects of using simple salts for power generation owing to their low costs, better efficiency, and heightened durability.

-Ganpati Nayak