When mechanical strain is applied to piezoelectric material, the dipoles present in its crystalline structure get distorted, leading to the generation of electric charges.
Based on its structure, piezoelectric materials are classified into four types: ceramics, single crystals, polymers, and composites.
Piezoelectric characteristics that need to be evaluated before selecting piezoelectric energy harvester material include the piezoelectric charge constant, piezoelectric voltage constant, electromechanical coupling factor, mechanical quality factor, permittivity constant or dielectric constant, and Young's Modulus.
Heel-strike system used for LED lighting in shoes
As I was searching for a heel-strike system, I came across piezoelectric energy harvesters, which generate electrical power from heel strikes. Piezoelectric materials are used in heel strike circuits, which convert kinetic energy into electrical energy. The devices that utilize piezoelectric materials to harvest electrical energy are called piezoelectric harvesters. Piezoelectric materials are capable of converting oscillating or vibrational mechanical energy into electrical energy. The piezoelectric energy harvester is able to meet electrical energy needs from motion. Before selecting piezoelectric energy harvester material, it is important to understand its charge and voltage constant, electromagnetic coupling factor, mechanical quality factor, permittivity constant, and Young’s Modulus.
Principle of Piezoelectric Energy Harvesters
Piezoelectric materials are employed in electronic devices that convert human motion, low-frequency seismic vibrations, and acoustic noises into electrical energy. Piezoelectric energy harvesters have become popular since they eliminate the need for a battery replacement process in electronic devices. Compared to other mechanical or electrical energy conversion principles, such as electromagnetic induction and electrostatic induction, piezoelectric energy harvesting possesses high power density and high integration capability.
Piezoelectric material can be considered the best source to generate power in the mW or µW range in electronic devices. Piezoelectric materials are crystalline structures with non-overlapping positive and negative charges. As the centers of the charges are not overlapping, they form dipole moments in the materials. When mechanical strain is applied to the piezoelectric material, the dipoles are distorted, leading to the generation of electric charges. If the electric charges are allowed to flow, they form the electric current and power the circuit. Otherwise, the charge can be stored in capacitors or batteries.
Classification of Piezoelectric Materials
Based on the structure, piezoelectric materials are classified into the following types:
Ceramics-Ceramics are polycrystalline materials with grains that have the same chemical composition. Lead zirconate titanate (PZT), barium titanate (BT), and strontium titanate (ST) are examples of piezoelectric ceramics. They are widely used in motion sensors, ultrasonic power transducers, high-frequency loudspeakers, and watches.
Single crystals-In single crystal materials, positive and negative ions follow a periodic arrangement. They find applications in sensors, transducers, and actuators. The solid solution of lead magnesium niobate and lead titanate, known as PMN-PT and Lithium Niobate ( LiNbO3), are examples of piezoelectric crystals.
Polymers-Carbon-based materials forming chains with repeated structures called monomers are good piezoelectric polymers. The flexibility of polymers makes them suitable to withstand high strain compared to ceramics and single crystals. The piezoelectric polymers are suitable for applications where the host device undergoes frequency bends. Polyvinylidene fluoride (PVDF) is an example of a piezoelectric polymer.
Composite materials-By combining ceramics, polymers, and single crystals, improved piezoelectric characteristics can be achieved. This idea has led to the formation of piezoelectric composite materials. PZT-polymer composites are one of the most commonly used piezoelectric composites, formed by combining PZT ceramics with polymers.
Selection Criteria of Piezoelectric Energy Harvester Materials
The piezoelectric property of materials differs between ceramics, single crystals, polymers, and composites. The selection of piezoelectric materials used in piezoelectric energy harvesters is not only based on the piezoelectric property, but also based on the application and design of the host device or harvesting system. The input frequency, occupiable volume, and type of mechanical input are some factors of the application system that need to be considered in piezoelectric material selection. Some of the piezoelectric characteristics that need to be evaluated for the selection of the piezoelectric energy harvester are below:
Piezoelectric Charge Constant, d -The piezoelectric charge constant indicates the suitability of the material for actuator applications. It is a constant that represents the polarization induced per stress applied.
Piezoelectric voltage constant, g-The piezoelectric charge constant assesses whether the piezoelectric material is suitable for sensor applications. It is the constant indicating the electric field generated per unit of mechanical stress applied.
Electromechanical coupling factor, k-The electromechanical coupling factor, k, is an index that is used to show the effectiveness of piezoelectric material in converting mechanical strain to electrical energy. It can be calculated by finding the square root of mechanical-electrical energy conversion efficiency.
Mechanical quality factor, Q-The mechanical quality factor, Q, is the representation of the sharpness of the resonance frequency in piezoelectric materials. It is the ratio of the reactance to the resistance, which is obtained from the series equivalent circuit of a piezoelectric material. This constant indicates the degree of damping in the piezoelectric material.
Permittivity constant or Dielectric constant, 𝜀-The permittivity, or dielectric constant, ε, is the dielectric displacement per unit electric field in a piezoelectric material. It represents the ability of the piezoelectric material to store charge.
Young's Modulus, Y-The elasticity or stiffness of the piezoelectric material can be evaluated from its Young’s Modulus value. It is the ratio of mechanical stress applied on the piezoelectric material to the strain in the same direction.
Typical Piezoelectric Energy Harvesting System
So far, we have seen the principles of piezoelectric energy harvesters, the types of materials involved, and its selection criteria—now we need to understand how they operate. The figure above shows the schematic of a typical piezoelectric energy harvester. From the figure, we can see that the piezoelectric patch is placed on a cantilever beam in the host system. When the beam is subjected to mechanical vibrations, a large strain is put on the piezoelectric material and results in the generation of alternating voltage. The interface circuits shown in the figure are used for converting the generated power according to the load specifications. The interface unit also handles the delivery of the harvested electrical energy to storage elements such as batteries and capacitors.
Piezoelectric energy harvesters are one means to achieve self-sufficiency. If we can utilize our motions to charge electronic gadgets, it would be one of the best strategies for energy self-sufficiency. As portable electronics become more important and heavily used, piezoelectric harvesters bonded to the human body might be a good idea in order to convert our motion to electrical energy.