Before understanding the dielectric properties of solids, you need to have a clear understanding of what dielectric materials are and their characteristics. So, first we are discussing here dielectric materials in brief before explaining dielectric properties of solids.
An electrical insulator that can be polarized by an applied electric field is called dielectric or dielectric material. When materials which are conductors of electricity are placed under an electric field then electric charge or electrons flow through them. While when a dielectric is placed under an electric field, no electric charges flow through them. This is the reason they are called insulators. But these dielectric insulators under applied an electric field show polarization. It means when we apply an electric field on dielectric materials then electrons and positively charged nucleus of their atoms slightly shift from their average equilibrium positions. Due to this polarization in dielectric materials under electric field, positive charges are displaced in the direction of electric field while negative charges are displaced in direction opposite to the electric field. It means the nucleus (positively charged) gets polarized towards the direction of the electric field while electrons get polarized in the opposite direction of the applied electric field.
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For example, if the applied electric field is moving in the positive x – axis, then negative charges or electrons will shift in the negative x – axis. This polarization in dielectric material due to the electric field creates an internal electric field that reduces the overall field within the dielectric itself. When dielectric material is made up of weakly bonded molecules then these molecules not only get polarized on applying electric field but also reorient themselves so that their symmetry axes align to the field. This polarization of charges in dielectric materials results in the formation of dipoles. These dipoles in atoms or molecules of dielectric materials can arrange themselves in a systematic manner so that they will possess a net dipole moment, or they can arrange themselves in a way so that the net dipole moment will be zero. Although there are conditions in which dielectric materials possess no dipole in the crystal and only ions are present. Dielectric materials are important for explaining electronics, optics, solid – state physics and cell biophysics.
Anti – ferroelectricity
Piezoelectricity – The solids in which individual dipoles are formed and align itself in an ordered manner in such a way so that a net dipole moment of the solid (crystal) shows piezoelectricity. When pressure is applied in such solids, their atoms or ions are displaced and produce electricity. Piezoelectricity is electric charge which accumulates in some crystals due to mechanical stress. It means piezoelectricity is electricity resulting from pressure and latent heat. The word piezoelectricity is derived from the Greek word piezein which means ‘to squeeze or press’ and elecktron, which means ‘amber’(an ancient source of electric charge). Piezoelectricity was discovered by French Physicists Jacques and Pierre Curie in 1880.
This dielectric property of solids is used in the medical field, automotive industry, information technology and telecommunications.
Pyroelectricity – The word ‘Pyroelectricity’ is derived from the two Greek words pyr which means ‘fire’ and elecktron which means ‘amber’(an ancient source of electric charge) or ‘electricity’. Pyroelectricity is the ability of certain crystals to produce a temporary voltage when they are heated or cooled. Some piezoelectric crystals produce electricity on heating, thus produced electricity is called pyroelectricity and this phenomenon is called pyroelectric effect. Pyroelectric crystals are generally naturally electrically polarized and as a result contain large electric fields. Due to change in temperature positions of the atoms change within a crystal structure. Now due to change in crystal structure, polarization of the crystal changes which causes rise to a voltage across the crystal. Now if the temperature remains constant at its new value, the pyroelectric voltage disappears due to leakage current.
They are used in heat sensors, power generation and nuclear fusion. They can be used in PIR (passive infrared) sensors, infrared non – contact thermometers and motion detector thermal sensors. Motion detectors thermal sensors are used to detect the movement of human beings, animals, and objects etc.
Ferroelectricity – In some crystals the dipoles are permanently aligned even in absence of electric field. They possess spontaneous electric polarization. On application of an external electric field on such crystals their electrical polarization gets reversed. It was discovered by Valasek in Rochelle salt in 1920. The word ferroelectricity is made up of two words ferro which means iron and electricity. All ferroelectric materials are pyroelectric as well.
It is used in ferroelectric capacitors, ferroelectric RAM, high quality infrared cameras, fire sensors, sonar, vibration sensors and fuel injectors on diesel engines. It is also used in ferroelectric tunnel junctions (FTJ). Ferroelectrics show catalytic properties. So, they can be used for catalysis as well. They can also act as energy harvesters. Materials which possess both ferroelectric and ferromagnetic properties are called multiferroics. Many researches are going on in multiferroics.
Anti – Ferroelectricity – As the name suggests it is opposite to ferroelectricity. The relation between anti - ferroelectricity and ferroelectricity is analogous to the relation of ferromagnetism and anti – ferromagnetism. Crystals which possess anti – ferromagnetism property consist of an ordered array of electric dipoles but with adjacent dipoles oriented in opposite (antiparallel) directions. This results in a net zero dipole moment. They possess zero spontaneous electric polarization since the adjacent dipoles cancel each other. This property of crystal can appear or disappear depending on temperature, pressure, growth method and external electric field etc. The temperature at which anti – ferroelectricity disappears is called Neel point or Curie point.
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It is used in supercapacitors, integration with ferromagnetic materials, high energy storage devices etc.
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