Why in News?

• A pair of chemists at Michigan State University has observed the piezoelectric effect in liquids for the first time.
• In their paper published in The Journal of Physical Chemistry Letters, Iqbal Hossain and G. J. Blanchard, describe accidently observing the property while studying ionic liquids.

What’s in today’s article?

• About Piezoelectric Effect (Concept, History, Materials, Applications, etc.)
• Discovery of Piezoelectric Effect in Liquids (About the Research, Significance)

What is the Piezoelectric Effect?

• The piezoelectric effect is a phenomenon where specific types of material (e.g., quartz, topaz, etc.) produce an electric charge proportional to the mechanical stress applied to them.
• It occurs when there is a conversion of kinetic or mechanical energy due to crystal deformation, into electrical energy.
• Piezoelectric materials are materials that can produce electricity due to mechanical stress.
• When a piezoelectric material is placed under mechanical stress, there is a shift of the positive and negative charge centres in the material, which then results in an external electric field.

What is the History behind the Piezoelectric Effect?

• In 1880, Jacques and Pierre Curie discovered that pressure generates electrical charges in certain types of crystals such as quartz and tourmaline.
• They called this phenomenon the "piezoelectric effect". The word "piezo" is derived from the Greek word piezein, which means to squeeze or press.
• Later they noticed that electric fields can deform piezoelectric materials. This effect is called the "inverse piezoelectric effect".
• After these first demonstrations, more work was undertaken to explore the great potential of the piezoelectric effect.

About Piezoelectric Materials:

• A crystal is any solid with atoms or molecules that are arranged in a very orderly way based on repetitions of the same basic atomic building block (the unit cell).
• In most crystals (such as in metals), the unit cell is symmetrical; in piezoelectric crystals, it is not.
• Normally, piezoelectric crystals are electrically neutral.
• The atom arrangement may not be symmetrical, but the electrical charges are perfectly balanced: a positive charge in one place cancels out a negative charge nearby.
• However, stretching or squeezing a piezoelectric crystal deforms the structure, pushing some of the atoms closer together or further apart.
• This upsets the balance of positive and negative, and causes net electrical charges to appear.

Examples of Piezoelectric Materials:

• There are a variety of piezoelectric materials, both man-made and natural, that can produce electricity.
• Natural –
  • The most well-known, and the first commercially available piezoelectric material used in electronic devices is the quartz crystal.
  • Other naturally-occurring piezoelectric materials include topaz, tourmaline, and even bone.
• Man-made –
  • A breakthrough came when scientists developed a new class of man-made materials, called ferroelectrics, which exhibited piezoelectric characteristics many times more powerful than piezoelectric crystals.
  • Scientists kept searching for higher performance materials and this resulted in the development of barium titanate and lead zirconate titanate.

Applications of Piezoelectric Effect:

• Developed in the 1900s to detect icebergs, the sonar device is the first practical application for piezoelectric devices.
• Sensors, high voltage generators, electronic frequency generators, microbalances, inkjet printers, and ultra-fine focusing and alignment of optical assemblies among others all exploit piezoelectric technology.
• It is also the basis of a number of scientific instrumental techniques with atomic resolution, such as scanning tunnelling microscopes.

 Discovery of Piezoelectric Effect in Liquids:

• The researchers at Michigan State University were studying properties of ionic liquids.
  • Ionic liquids are made from salts with unsymmetrical, flexible organic cations and symmetrical weakly coordinating anions.
• The liquid piezoelectric material was discovered as the researchers applied pressure with a piston to a sample of an ionic liquid in a cylinder.
• To their surprise, they found that this led to the release of electricity.
• They also found that the amount of electricity released was proportional to the amount of pressure applied.

What is the Significance of this Discovery?

• The reason the piezoelectric effect has only been expected in solids thus far is that the body being squeezed needs to have an organised structure, like the pyramids of quartz.
• Liquids do not have such structure; instead, they take the shape of their container.
• The researchers suggest that liquid piezoelectric materials could prove to be useful, especially ones made using ionic liquids, because they would be more environmentally friendly than solid materials.
• They also note that liquid piezoelectric materials could allow more variety in device shape, opening up wider design opportunities.