Here’s a fun one — I’m reviewing literature papers about various topics on how piezoelectricity can be used to harvest energy. This is a multi-part series, where all the parts form a single review paper. Here’s the first 2 chapters!
The way the world is currently heading, energy usage is killing us. Majority of greenhouse gas emissions worldwide are caused by energy usage and only a small portion of total energy usage is sustainable! Clearly this is a problem. Piezoelectricity brings a fresh, new, unique and potentially world changing solution to a problem tormenting humanity for possibly the rest of its existence! New energy harvesting technologies aren’t common, especially ones with as big of a potential as piezoelectricity. In fact, piezoelectricity is already in much of modern technology, from lighters, phones, speakers, earpieces and so much more. Innovating on this established technology is a surefire way to lead the way in sustainable energy usage.
1. What is Piezoelectricity?
Piezoelectricity is the creation of electrical polarization in an object when mechanical energy is applied to it. Popular piezoelectric materials include PZT (lead zirconate titanate), quartz, bone, barium titanate and more.
1.1. The Piezoelectric Effect
Hemihedral crystals are crystals that are semi-symmetrical. They have an unequal number of faces on opposite sides, but are symmetrical in all other sides. When they face pressure on the hemihedral axis, they have polarization. The crystal gets an imbalance of positively and negatively charged atoms on both unequal sides.
The piezoelectric effect was discovered in 1880 by Pierre and Jacques Curie. They wrote “every time that a nonconducting hemihedral crystal with inclined faces contracts, there is a formation of the electric poles in a certain direction; every time that the crystal expands, the release of electricity takes place in opposite direction”. This basically means that when a crystal contracts by any means, it gains an electric charge and when it expands, this flips. Tapping and any type of mechanical force on a piezoelectric crystal makes it contract, and this is the piezoelectric effect. The piezoelectric effect applies in reverse too, where applying electricity to a crystal makes it change shape.
1.2. Current Uses
While piezoelectricity isn’t the most known to the average person, it actually is everywhere. Many lighters use it, where pressing a button hits a piezoelectric material which creates a spark, starting a fire. Many earphones and speaker devices use piezoelectricity as well. Using a reverse piezoelectric effect, an earpiece has electricity that is transferred to a crystal that changes shape, generating vibrations as sound waves. Many parts of vehicles and industrial machines actually use piezoelectricity as well, especially with safety features. Any sort of intense vibrations or mechanical energy will trigger the electricity because of the polarization.
2. Popular Piezoelectric Materials
Different materials have different uses in technologies that use piezoelectricity. Some factors determining this are the intensity of the effect, the hardness and malleability of the material, conductivity, etc.
2.1. Material Categories
There are 6 categories of piezoelectric materials. Depending on the structure of their lattice, this can increase or decrease the intensity of the piezoelectric effect displayed in the material.
Single crystalline materials
Materials like quartz (natural) or PMN-PT (synthetic) have a lattice structure that is monocrystalline. This means that any sample of the material will have the same continuous, unbroken atom structure.
Unlike single crystalline materials, there’s no crystalline pattern in the lattice structure of piezoceramics except for the fact that the piezoelectric effect is shown. Lead Zirconate Titanate (PZT) is the most widely used piezoelectric material and falls into this category along with other synthetic materials. Since the effect isn’t uniform and is averaged, they can’t be used in very precise instruments.
A semiconductor has the conductivity between that of an insulator and most metals. Piezoelectric semiconductors include many piezoceramics such as PZT.
These are materials that are organic and lead-free. Some of these include polyvinylidene fluoride and other liquid crystal polymers. They are used for extremely precise instruments.
These are composites between piezoceramics and polymers. This way, you can get the high effects of the ceramics but the targeted capabilities of the polymers.
Piezoelectric glass ceramics are highly theorized. They are materials that are crystalline on the surface, where they display this effect.
There is wide debate over what type of piezoelectric material is best for different applications.
2.2. The Best Materials
Quartz is one of the more commonly used materials. It’s easy and cheap to find and refine, and it’s also a stable material since it’s single-crystalline.
PZT is used mostly for things like earpieces and loudspeakers. This is because it is one of the materials where the piezoelectric effect is most common.
Now this is just the start of your journey learning about piezoelectricity with me. Stay tuned for more!