Ferroelectric materials Properties, processing and perspectives

Anticipated for decades at the end of the 19th century, ferroelectricity was formally discovered in 1920. By analogy with ferromagnetics, and although they rarely contain iron, compounds with a spontaneous polarization that can be reversed by an electric field are referred to as ferroelectrics. The dielectric permittivity of ferroelectrics is of the order of several thousand, i.e. at least a hundred times greater than all other insulators (polymers, oxides, nitrides, etc.). These compounds are therefore used to make high-density capacitors, an application for which they are indispensable. However, this very high permittivity does not mean that ferroelectrics are perfect in every respect. Indeed, their very high polarizability is accompanied by dielectric losses that can reach values of several percent, incompatible with certain applications, particularly at high frequencies. The first challenge is to counteract these losses, either through a physicochemical approach, or by developing materials science strategies (ceramics, composites, interfaces, etc.).

All other ferroelectric functionalities result from the fact that their polarization is highly dependent on external stresses. Depending on the nature of these constraints, exceptional effects are obtained, and different applications are possible:

• pyroelectric under temperature change; this allows the realization of thermal sensors, in particular infra-red detectors;

• This coupling is widely used in ultrasonic sensors (medical imaging), underwater communications (sonar), actuators (controlled displacement devices) and energy recovery (transformation of vibrations into electrical energy);

• This effect has applications in a wide range of frequencies, notably at gigahertz frequencies for tunable telecommunications devices (phase shifters, resonators, antennas), in optics for electro-optical modulators or harmonic generators in lasers. In the latter case, it is the electric field of the light wave that induces the stress required to change the optical index.

For all so-called non-linear applications (stress-dependent response), the magnitude of the applied stress imposes increased control of defects present in the material. Since the vast majority of ferroelectrics used in applications are ternary oxides (e.g. ABO ₃ ), this is an ongoing problem for researchers and engineers in the field. Ferroelectrics have not yet reached the degree of purity and reliability of semiconductors and metals, materials with which they coexist in all the devices where they are implemented. Irreplaceable in many applications, ferroelectrics require in-depth research to meet...