Types of Piezo Electric Materials – Properties, and Characteristics

With their amazing characteristic to produce electricity from unused vibrations of the devices, piezoelectric materials are emerging as revolutionary power harvesters. Owing to the research done on these materials, today there is a wide range of piezoelectric materials to choose from. Different specifications characterize these materials. But, how to choose a material for our requirement? What to look for? What are the types of piezoelectric materials? In this article, we look into different types of piezoelectric materials along with their properties. The article describes the five basic merits to look for while choosing a piezoelectric material for the product.

Types of Piezoelectric Materials

The different types of piezoelectric materials include the following.

1).Natural Existing

These crystals are anisotropic dielectrics with non-centrosymmetric crystal lattice. Crystal materials like Quartz, Rochelle salt, Topaz, Tourmaline-group minerals, and some organic substances as silk, wood, enamel, bone, hair, rubber, dentin comes under this category.

2). Manmade Synthetic Materials

Materials with ferroelectric properties are used to prepare piezoelectric materials. Manmade materials are group into five main categories – Quartz analogs, Ceramics, Polymers, Composites, and Thin Films.

• Polymers: Polyvinylidene difluoride, PVDF or PVF2.
• Composites: Piezocomposites are the upgrade of piezopolymers. They can be of two types:
  Piezo polymer in which piezoelectric material is immersed in an electrically passive matrix.
  Piezo composites that are made by using two different ceramics example BaTiO3 fibers reinforcing a PZT matrix.
• Manmade piezoelectric with crystal structure as perovskite: Barium titanate, Lead titanate, Lead zirconate titanate (PZT), Potassium niobate, Lithium niobate, Lithium tantalate, and other lead-free piezoelectric ceramics.

Properties of Different Piezoelectric Materials

The properties of different piezoelectric materials include the following.

Quartz

• Quartz is the most popular single crystal piezoelectric material. Single crystal materials exhibit different material properties depending on the cut and direction of the bulk wave propagation. Quartz oscillator operated in thickness shear mode of the AT-cut are used in computers, TV’s and VCR’s.
• In S.A.W. devices ST-cut quartz with X-propagation is used. Quartz has extremely high mechanical quality factor QM > 105.

Lithium Niobate and Lithium Tantalate

• These materials are composed of oxygen octahedron.
• Curies temperature of these materials is 1210 and 6600c respectively.
• These materials have a high electromechanical coupling coefficient for surface acoustic wave.

Barium Titanate 

• These materials with dopants such as Pb or Ca ions can stabilize the tetragonal phase over a wider temperature range.
• These are initially used for Langevin -type piezoelectric vibrators.

PZT

• Doping PZT with donor ions such as Nb5+ or Tr5+ provides soft PZT’s like PZT-5.
• Doping PZT with acceptor ions such as Fe3+ or Sc3+ provides hard PZT’s like PZT-8.

Lead Titanate Ceramic

• These can produce clear ultrasonic imaging because of there extremely low planar coupling.
• Recently, for ultrasonic transducers and electromechanical actuators single crystal relaxor ferroelectrics with morphotropic phase boundary (MPB) are being developed.

Piezoelectric Polymers

Piezoelectric polymers have certain common characteristics as

• Small piezoelectric d constant which makes them a good choice for the actuator.
• Large g constant which makes them a good choice as sensors.
• These materials have good acoustic impedance matching with water or human body due to there light weight and soft elasticity.
• Broad resonance bandwidth due to low QM.
• These materials are highly-opted for directional microphones and ultrasonic hydrophones.

Piezoelectric Composites

• Piezoelectric composites made up of piezoelectric ceramic and polymer phases form excellent piezoelectric materials
• High coupling factor, low acoustic impedance, mechanical flexibility characterizes these materials.
• These materials are especially used for underwater sonar and medical diagnostic ultrasonic transducer applications.

Thin films

For bulk acoustic and surface acoustic wave devices thin films of ZnO are widely used because of there large piezoelectric coupling.

Which is the Best Piezoelectric Material?

Piezoelectric materials are chosen based on the requirement of our applications. The material that could easily meet our requirement can be considered the best. There are a few factors to be considered while choosing piezoelectric materials.

The five important merits of piezoelectric are

1. The electromechanical coupling factor k

k2 = (Stored mechanical energy / Input electrical energy) or
k2 = (Stored electrical energy / Input mechanical energy)

2. Piezoelectric strain constant d

Describes the relation of magnitude of induced strain x to the electric field E as x = d.E.

3. Piezoelectric voltage constant g

g defines the relation between the external stress X and induced electric field E as E = g.X.
Using the relation P = d.X. we can state g = d/ε0 .ε. where ε = permittivity.

4. Mechanical quality factor QM

This parameter characterizes the sharpness of the electromechanical resonance system.

QM= ω0/2 ω.

5. Acoustic Impedance Z

This parameter evaluates the acoustic energy transfer between two materials. This is defined as

Z2 = (pressure/volume velocity).

In solid materials Z = √ρ.√ϲ where ρ is the density and ϲ is the elastic stiffness of the material.

Piezoelectric Characteristics Table

• Polymers have low piezoelectric constant compared to ceramics.
• Shape change of ceramic-based materials is more than that of polymer-based materials when the same amount of voltage is applied.
• Piezoelectric voltage coefficient of PVDF makes is a better material for sensor applications.
• Due to the larger electromechanical coupling coefficient, PZT is used in an application where mechanical stress has to be converted to electrical energy.
• Three parameters to be considered for selecting piezoelectric materials for applications working under mechanical resonance are the mechanical quality factor, electromechanical coupling factor, and dielectric constant. Higher the magnitude of these parameters best is the material for the application.
• Materials with large piezoelectric strain coefficient, large non-hysteretic strain levels are best for an actuator.
• Materials with high electromechanical coupling factor and high dielectric permittivity are best as transducers.
• Low dielectric loss is important for materials used in off-resonance frequency applications accounting for low heat generation.

Based on these physical, material, electromechanical properties we can easily distinguish between piezoelectric materials. These properties help us to choose the best piezoelectric material for our application. Which material have you used for your application? What modifications are required for the existing materials to overcome their limitations?