Ferroelectricity Explained: Concepts, Properties & Applications

Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field. All ferroelectrics are pyroelectric, with the additional property that their natural electrical polarization is reversible. The term is employed in analogy to ferromagnetism, during which a cloth exhibits a permanent moment of a magnet. Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle salt by Valasek. Thus, the prefix Ferro, meaning iron, was used to describe the property despite the fact that most ferroelectric materials do not contain iron. Materials that are both ferroelectric and ferromagnetic are known as multiferroics. 

Examples of Ferroelectric Materials Include: 

• Lead titanate, (PbTiO₃)

• Lead zirconate titanate (PZT) 

• Lead lanthanum zirconate titanate (PLZT)

Ferroelectric Compound Polarization

When most of the materials are polarized, the polarization induced, which is denoted as P. It is almost exactly proportional to the applied external electric field E. Thus the polarization is a linear function. This type of polarization is called linear dielectric polarization. 

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Some materials are also known as paraelectric materials, These materials sometimes show a more enhanced nonlinear polarization. Whereas the electric permittivity, such as the slope of the polarization curve, is not constant as compared to that of the linear dielectrics but it may be a function of the external field.

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In addition to being nonlinear, the ferroelectric materials demonstrate spontaneous nonzero polarization even in the application of the electric field E. The different feature of ferroelectrics is that the spontaneous polarization can be reversed by applying a suitable strong electric field within the opposite direction. Therefore the polarization is dependent not only on the present field but also on its history, which yields to a hysteresis loop. They are called ferroelectrics by analogy to ferromagnetic materials, which have spontaneous magnetization and exhibit similar hysteresis loops.

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Typically, these materials demonstrate the ferroelectricity only below a particular phase of change of temperature, this change is called the Curie temperature and denoted as TC. These materials are paraelectric above this temperature, thus the spontaneous polarization vanishes, and therefore the ferroelectric crystal transforms into the paraelectric state. Many ferroelectric materials lose their piezoelectric properties completely above the Tc. Because the paraelectric phase of these materials has a centrosymmetric crystal structure.

Ferroelectric Properties

There are two types of properties that are exhibited by the ferroelectric compounds: 

1. Pyroelectric Properties and Spontaneous Polarisation: 

All ferroelectric materials are pyroelectric in nature, however pyroelectric materials are not ferroelectric. Below a transition temperature called the Curie temperature, the ferroelectric and the pyroelectric materials are polar and possess an electric dipole moment that is also known as spontaneous polarization. But this polarity can be re-oriented or reversed partially or fully by the application of an electric field with ferroelectric materials. Complete reversal of the spontaneous polarization is known as “switching”. The non-polar phase that can be encountered above the Curie Temperature is known as the paraelectric phase.

The direction of this spontaneous polarization conforms to the crystal symmetry of the material. While the re-orientation of the spontaneous polarization occurs as a result of atomic displacements. The magnitude of the electric dipole moment is greatest at a temperature below the Curie temperature and approaches zero as the temperature is nearer to the Curie temperature.

2. Piezoelectric Properties: 

Due to the reason that all the pyroelectric materials are piezoelectric, this means that the ferroelectric materials are inherently piezoelectric. This means that in response to an applied mechanical load, the material is capable of producing an electric charge that is proportional to the load. Similarly, in response to the applied voltage, the material will produce a mechanical deformation.

Properties including the piezoelectric, dielectric, and electro-optic coefficients may vary by several orders of magnitude in the narrow temperature band around the Curie temperature. Especially the changes to these coefficients are much more gradual when these are compared to other temperature ranges. The piezoelectric coefficient is much greater in the region of CT. Other properties such as dielectric strength and electro-optic properties also change more markedly in the region of the Curie temperature when compared to other temperature ranges.

Ferroelectricity Applications

• The nonlinear nature of the ferroelectric materials is often required for the purpose to make capacitors with adjustable capacitance. Typically, a ferroelectric capacitor simply consists of a pair of electrodes that are sandwiching a layer of ferroelectric material. The permittivity of these ferroelectrics is not only adjustable but is commonly also very high. This happens especially when the phase of the temperature changes. Because of this reason, the ferroelectric capacitors are small in physical size when compared to dielectric capacitors of comparable capacitance.

• The spontaneous polarization of ferroelectric materials, when plotted on the graph, leads to a hysteresis effect which may be used as a memory function. Ferroelectric capacitors are sometimes required to make ferroelectric RAM that is used for computers and RFID cards. In these applications, a thin film of ferroelectric materials is typically used. Due to the reason that this allows the required field to switch in between the polarization that has to be achieved with a moderate voltage. However, while using thin films an excellent deal of attention must be paid to the interfaces, electrodes, and sample quality for devices to figure out the reliability.

• Ferroelectric materials are piezoelectric and pyroelectric. These combined properties of piezoelectricity, memory, and pyroelectricity make the ferroelectric capacitors very useful for example in sensor applications. Ferroelectric capacitors are utilized in medical ultrasound machines, top-quality infrared cameras, in different sensors such as vibration sensors, fire sensors, sonar, and even in the fuel injectors on diesel engines.

• The recent interest is the ferroelectric tunnel junction (FTJ) during which a contract is formed by a nanometer-thick ferroelectric film that is placed in between the metal electrodes. The thickness of the ferroelectric layer is enough to permit the tunnelling of electrons. The piezoelectric and the interface effects are also because the depolarization field may cause an enormous electroresistance (GER) switching effect.

• Another burgeoning application is multiferroics. In this, the researchers are looking for ways to couple the magnetic and ferroelectric ordering within a material or heterostructure.

• The catalytic properties found in ferroelectrics have been studied since 1952. It was observed by Parravano, in the anomalies of oxidation rates of CO over the ferroelectric sodium and potassium niobates near the Curie temperature of those materials. The surface-perpendicular component of the ferroelectric polarization can dope polarization-dependent charges on the surface of the ferroelectric materials. This opens the possibility of performing catalysis beyond the limits of the Sabatier principle. On the other hand, ferroelectric polarization-dependent chemistry can offer the possibility of switching the surface of adsorbate interaction from strong adsorption to strong desorption. Hence a compromise between desorption and adsorption is not needed. Ferroelectric polarization can also act as an energy harvester. 

Conclusion

Energy harvesters convert various energy sources to electrical energy. Ferroelectric polarization can have an important role to increase the output power of energy harvesters by enhancing internal potential. Strong ferroelectric polarization produces high piezoelectric potential and surface potential.