What is a Piezoelectric Material? Working, Advantages and Limitations

Piezoelectric materials have been around since the late 80’s and paved way for many game-changing inventions. Serving in the form of SONAR in world war these materials have now caught eyes of the inventors for their mystic characteristics. Wireless Sensor Networks, Internet of Things rules the technical era of the 21st century. To keep these novelties up and running, power requirement has become the greatest challenge. Hunt for a sustainable, reliable, renewable energy source caused researchers to stumble upon trailblazing power harvesters- the piezoelectric materials. Let’s set on a voyage to explore these new age power harvesters.

What is Piezoelectric Material?

To know what a piezoelectric material is one has to know what does the term piezoelectric stand for?. In PIEZOELECTRICITY the term” piezo” stands for pressure or stress. Thus piezoelectricity is defined as “Electricity generated by application of mechanical stress or tension” and the materials that exhibit this property comes under the category of piezoelectric materials. The credit for the discovery of these materials goes to Sir Jacques Curie (1856–1941) and Pierre Curie (1859–1906). While experimenting with certain crystalline minerals like quartz, cane sugar, etc… they found that application of force or tension on these materials generated voltages of opposite polarities with magnitudes propositional to the applied load. This phenomenon was named as Direct Piezoeffect.

In the following year, Lippman discovered the Converse effect stating that one of these voltage-generating crystals, when exposed to an electric field, lengthened or shortened according to the polarity of the applied field. Piezoelectric materials came into recognition with their role in WW1 when Quartz was used as resonators in SONAR. During the period of WW2, synthetic piezoelectric material was discovered, which later led to the intense development of piezoelectric devices. Before using a piezoelectric material one must know what characteristics make these materials piezoelectric.

Properties of Piezoelectric Material and How does it Work?

The secret of piezoelectric materials lies in their unique atomic structure. Piezoelectric materials are ionically bonded and contain positive and negative ions in the form of pairs called unit cells. These materials are available in nature as an anisotropic dielectric with non-Centrosymmetric crystal lattice i.e. they don’t have any free electrical charges and the ions lack a center of symmetry.

Direct Piezoelectric Effect

When mechanical stress or friction is applied on these materials the geometry of the atomic structure of the crystal changes due to net movement of positive and negative ions with respect to each other, resulting in electric dipole or Polarization. Thus the crystal changes from a dielectric to a charged material. The amount of voltage generated is directly proportional to the amount of stress or tension applied to the crystal.

Converse Piezoelectric Effect

When electricity is applied to these crystals electric dipoles appear, forming the dipole movement which causes deformation of the crystal, thus giving rise to converse piezoelectric effect as shown in the figure.

Synthetic Piezoelectric Materials

Manmade piezoelectric materials like piezoelectric ceramics exhibit spontaneous polarization (ferroelectric property) i.e. dipole exists in their structure even when no electric field is applied. Here the amount of piezoelectric effect produced strongly depends on their atomic structure. The dipoles present in the structure forms domains-regions where the neighboring dipoles have the same alignment. Initially, these domains are randomly oriented thus causing no net polarization.

By applying a strong DC electric field to these ceramics when they pass through their Curie point the domains get aligned in the direction of applied electric field. This process is called poling. After cooling to room temperature and removing the applied electric field, all domains maintain their orientation. After completion of this process, the ceramic exhibits the piezoelectric effect. Natural existing piezoelectric materials like quartz do not display ferroelectric behavior.

Piezoelectric Equation

Piezoelectric Effect can be described with the following Piezoelectric Coupling Equations

Direct piezoelectric effect: S = sE .T+ d. E
Converse piezoelectric effect: D = d.T+εT.E

Where,

D = electric displacement vector

T = the stress vector

sE = matrix of elastic coefficients at constant electric field strength,

S = strain vector

εT = dielectric matrix at constant mechanical strain

E = electric field vector

d = direct or converse piezoelectric effect

The electric field applied in different directions generates different amounts of stress in piezoelectric materials. So sign conventions are used along with coefficients to know the direction of applied field. To determine the direction, axes 1, 2, 3 are used analogously to X, Y, Z. Poling is always applied in the direction of 3. The coefficient with double subscripts relates electrical and mechanical characteristics with the first subscript describing the direction of the electric field in accordance with the voltage applied or charge produced. The second subscript gives the direction of mechanical stress.

Electromechanical coupling coefficient occurs in two forms. The first is the actuation term d, and the second is the sensor term g. The piezoelectric coefficients along with their notations can be explained with d33

Where,

d specifies applied stress is in 3rd direction.

3 specifies electrodes are perpendicular to the 3rd axis.

3 specifies piezoelectric constant.

How Piezoelectric Material Works?

As explained above piezoelectric materials can work in two modes:

• • The direct piezoelectric effect
  • Converse piezoelectric effect

Let take an example for each to understand the application of these modes.

Heal-Strike Generator using Direct Piezoelectric Effect:

DARPA has developed this device to equip soldiers with a portable power generator. The piezoelectric material implanted in the shoes experiences mechanical stress when the soldier walks. Due to direct piezoelectric property, the material produces electric charge due to this mechanical stress. This charge is stored in the capacitor or batteries which thereby can be used to charge their electronic devices on the go.

Quartz Crystal Oscillator in Watches using Converse Piezoelectric Effect

Watches contain a quartz crystal. When electricity from the battery is applied to this crystal through a circuit converse piezoelectric effect occurs. Due to this effect upon application of electric charge the crystal starts to oscillate with a frequency of 32768 times per second. The microchip present in the circuit counts these oscillations and generates a regular pulse per second that spins the second hands of the watch.

Uses of Piezoelectric Materials

Due to its unique characteristics, piezoelectric materials have acquired an important role in various technological inventions.

Use of Direct Piezo Effect

• • In Japan’s train stations the concept of “crowd farm” was tested where the footsteps of the pedestrians’ on the piezoelectric tiles embedded on the road can generate electricity.

• • In 2008 a nightclub in London builds first eco-friendly floor made up of piezoelectric material that can generate electricity to power up light bulbs when people dance on it.

• • Piezoelectric effect finds useful application as mechanical frequency filters, surface acoustic wave devices, bulk acoustic wave devices, etc…

• • Sound and ultrasound microphones and speakers, ultrasonic imaging, hydrophones.

• • Piezoelectric pickups for guitars, biosensors to power up pacemaker.
  • Piezoelectric elements are also used in the detection and generation of sonar waves, single-axis and dual-axis tilt sensing.

Uses of Converse Piezoelectric Effect

• Actuators and motors
• Micro-precision placement and micro-precision adjustments in lenses for microscopes.
• Needle driver in printers, miniaturized motors, bimorph actuators.
• Multilayered actuators for fine positioning in optics
•  Injection systems in automotive fuel valves etc…

  

By Coupling Electrical and Mechanical Fields:

• • For investigation of the atomistic structure of materials.
  • To monitor structural integrity and detect faults at early stages in civil, industrial and aerospace structures.

Advantages and Limitations of Piezoelectric Materials

The advantages and limitations of piezoelectric materials include the following.

Advantages

• • Piezoelectric materials can operate at any temperature conditions.

• • They have low carbon footprint making them the best alternative for fossil fuel.

• • Characteristics of these materials make them the best energy harvesters.

• • Unused energy lost in the form of vibrations can be tapped to generate green energy.
  • These materials can be reused.

Limitations

• • While working with vibrations these devices are prone to pick up unwanted vibrations also.

• • Resistance and Durability apply limits to devices when used to tap energy from pavements and roads.

• • The mismatch between stiffness of piezoelectric material and pavement material.
  • Less known details of these devices and the amount of research done till date is not sufficient to exploit full usage of these devices.

As its being said “Necessity is the mother of invention” our necessity for a hustle free, Low carbon footprint energy harvesting device has brought piezoelectric materials into the limelight again. How can these materials overcome their limitations? Are we moving towards a future where instead of worrying about the amount of fuel used up to travel, we would only wonder about the amount of power our car generated? What do you think? Here is a question for you, what is the best piezoelectric material?