An Introduction
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 dielectric materials here in brief before explaining dielectric properties of solids.
About Dielectric Materials and Their Characteristics
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 the electric field, positive charges are displaced in the direction of the electric field while negative charges are displaced in the opposite direction 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.
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.
Dielectric Properties of Solids
The study of dielectric properties concerns storage and dissipation of electric and magnetic energy in materials. Some dielectric properties of solids are as follows –
Piezoelectricity
Pyroelectricity
Ferroelectricity
Anti – ferroelectricity
Piezoelectricity
The solids in which individual dipoles are formed and align themselves 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 an 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 and 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.
It is used in supercapacitors, integration with ferromagnetic materials, high energy storage devices etc.
Overview of Dielectric Properties of Solids
‘Dielectric properties of Solids’ is a vast topic. This was a brief on Dielectric properties of solids, if you are looking for detailed study notes on this topic or solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.
FAQs on Dielectric Properties of Solids
1. What are dielectric materials and how do they differ from electrical conductors?
Dielectric materials are electrical insulators that can be polarised by an applied electric field. Unlike conductors, which allow electric charges (electrons) to flow freely through them, dielectrics do not conduct electricity. Instead, when a dielectric is placed in an electric field, its positive and negative charges shift slightly from their equilibrium positions, creating an internal electric field that opposes the external field. This phenomenon is known as polarisation.
2. What are the four main types of dielectric properties observed in solids as per the NCERT syllabus?
Solids can exhibit several distinct dielectric properties based on the arrangement of dipoles within their crystal structure. The four main types are:
- Piezoelectricity: The ability of certain crystals to generate an electric voltage in response to applied mechanical stress.
- Pyroelectricity: The property of certain crystals to produce a temporary voltage when they are heated or cooled.
- Ferroelectricity: The characteristic of materials that have a spontaneous electric polarisation that can be reversed by applying an external electric field.
- Anti-ferroelectricity: A property where adjacent dipoles in a crystal align themselves in opposite (antiparallel) directions, resulting in a net zero dipole moment.
3. How does applying an external electric field cause polarisation in a dielectric solid?
When a dielectric solid is subjected to an external electric field, the forces exerted by the field cause a slight displacement of its charged particles. The positively charged nuclei are pushed in the direction of the field, while the negatively charged electron clouds are pushed in the opposite direction. This separation of charge centres creates or reorients tiny electric dipoles throughout the material. The alignment of these dipoles results in a net dipole moment for the material, a state known as polarisation.
4. What is piezoelectricity and where is it applied in real-world technology?
Piezoelectricity is the generation of electricity in certain crystalline solids when they are subjected to mechanical pressure or stress. This 'pressure electricity' is a result of the displacement of ions in the crystal, creating a net dipole moment. This effect is reversible, meaning the material will deform if an electric field is applied. Common applications include gas lighters, pressure sensors in automotive electronics, microphones, and sonar transducers.
5. Explain the key difference between ferroelectric and anti-ferroelectric solids.
The key difference lies in the alignment of electric dipoles and the resulting net polarisation. In ferroelectric solids, the dipoles are permanently aligned in the same direction, even without an external electric field, resulting in a strong spontaneous polarisation. In anti-ferroelectric solids, the dipoles are ordered but align in an alternating, antiparallel pattern. This arrangement causes the individual dipole moments to cancel each other out, leading to a zero net dipole moment.
6. Why are all ferroelectric materials also pyroelectric, but the reverse is not always true?
This relationship is based on the underlying crystal structure and polarisation. Ferroelectric materials have a spontaneous polarisation that can be reversed by an electric field. This inherent polarisation is sensitive to temperature changes, causing them to exhibit pyroelectricity (generating a voltage upon heating or cooling). However, some pyroelectric materials have a spontaneous polarisation that is fixed and cannot be reversed by an electric field. Therefore, they are pyroelectric but not ferroelectric.
7. What is the dielectric constant of a solid, and what does it signify?
The dielectric constant (κ) is a dimensionless quantity that measures a material's ability to store electrical energy when placed in an electric field. It signifies how much more charge a capacitor can store with the dielectric material compared to a vacuum. A higher dielectric constant means the material is more effective at reducing the internal electric field, allowing for greater energy storage. It is a crucial parameter for designing capacitors and other electronic components.
