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Dielectric properties of solids are generally observed in insulators. In a strong electric field, the nucleus and electrons of the insulators are pulled in opposite directions. This results in the formation of dipoles in the atoms or molecules of the insulators. There are three possible ways in which the dipoles align. The dipole moments can cancel each other out, they can align themselves in such a way that there is a net dipole moment or the crystal may contain no dipoles but ions are present.
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Key terms: Dielectric, Insulators, Piezoelectricity, Pyroelectricity, Ferroelectricity, Anti-Ferroelectricity, Dipoles, Electric Fields
Piezoelectricity
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In the simplest terms, piezoelectricity is the process of using crystals to convert mechanical energy into electrical energy, or vice versa. Regular crystals have an organized and repeating structure of atoms that are held together by bonds, This is termed as a unit cell. Most crystals have a symmetrical unit cell. This makes them useless for pyroelectric purposes. But some crystals aren't symmetrical but they still exist in an electrically neutral balance.
Exertion of pressure to a piezoelectric crystal leads to deformation and the result is a crystal that can conduct electricity. Applying electrical energy to a piezoelectric crystal results in the crystal expanding and contracting, converting electrical energy into mechanical energy.
Piezoelectricity in a Lighter
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| Related Articles | ||
|---|---|---|
| Electrical properties of solids | Conductors | Corrosion |
| Classification Of Crystalline Solids | Density of Unit Cell | Metallic Bonds |
Pyroelectricity
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Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The positions of atoms are modified slightly within the crystal structure by the change in temperature, such that the polarization of the material changes.
The change in polarization creates a voltage within the crystal. If the new temperature remains constant, the pyroelectric voltage gradually disappears due to leakage current. The cause of the leakage can be electrons moving through the crystal, ions moving through the air, or current leaking through a voltmeter attached across the crystal.
Pyroelectric Sensor
Ferroelectricity
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Certain materials have a spontaneous electric polarization that can be reversed by the application of an external electric field. This property is called ferroelectricity. All ferroelectrics are pyroelectric but they also have an additional property. Their natural electrical polarization is reversible. The structural units of crystals of ferroelectric materials are tiny electric dipoles. In each unit, the centers of positive charge and negative charge are slightly separated.
In certain crystals, these electric dipoles spontaneously line up in clusters called domains. In the case of ferroelectric crystals, the domains can be oriented predominantly in one direction by a strong external electric field. The predominant orientation of the ferroelectric domains is reversed by reversing the external field. But the switching to a new direction lags somewhat behind the change in the external electric field. This lag is referred to as ferroelectric hysteresis.
Hysteresis(P-E) Curve in Ferroelectric Materials
Anti-Ferroelectricity
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The structure of an anti-ferroelectric material is an ordered crystalline array of electric dipoles but with adjacent dipoles oriented in opposite directions. In an antiferroelectric material, the total, macroscopic spontaneous polarization is zero, since the adjacent dipoles cancel each other out.
Anti-ferroelectricity can strengthen or weaken depending on temperature, pressure, external electric field, growth method, and other parameters. At a certain high temperature, anti-ferroelectricity disappears. This temperature is called the Néel point or Curie point.
Typical anti-ferroelectric double hysteresis loop
Things to Remember
- The electrons and nucleus of the atoms of insulators are pulled in different directions in the presence of a strong electric field.
- When electrical energy is applied to a piezoelectric crystal, it results in the crystal expanding and contracting, converting electrical energy into mechanical energy.
- The property of certain materials to have spontaneous electric polarization that can be reversed by the application of an external electric field is called ferroelectricity.
- The total macroscopic spontaneous polarization of anti-ferroelectric materials is zero since the adjacent dipoles cancel each other out.
- Antiferroelectricity can vary depending on factors such as temperature, pressure, external electric field, growth method, and other parameters.
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Sample Questions
Ques. What is meant by dielectric constant? (2 Marks)
Ans. The dielectric constant of a substance can be defined as the ratio of the permittivity of a substance to the permittivity of free space. It is the electrical equivalent of relative magnetic permeability and is the extent to which a material can hold in electric flux.
Ques. Which material has the highest dielectric constant?
(1)Water b. Calcium Copper Titanate c. Plastic d. Porcelain (2 Marks)
Ans. The material that has the highest dielectric constant is Calcium Copper Titanate. Its dielectric constant is over 10,000 and goes deeper at higher frequencies. Its main use is in developing supercapacitors. There are some limitations in its application, however.
Ques. What are the properties of a perfect dielectric? (2 Marks)
Ans. A perfect dielectric material has zero electrical conductivity. Thus, it only exhibits displacement current. Therefore, a perfect dielectric functions like an ideal capacitor – it stores and returns electrical energy. The electrically insulating material between the metallic plates of a capacitor is a common dielectric.
Ques. What are the applications of dielectrics? (3 Marks)
Ans. Dielectric components are used in electronic components such as capacitors (responsible for energy storage properties of the device) and high-K / low-K materials widely used in semiconductors to enhance performance and reduce device size(K being the permittivity of dielectric constant).
Ques. Explain the difference between dielectric and insulator? (2 Marks)
Ans. Insulators do not conduct electricity in an electric field because they do not have any free electrons. While dielectrics are also insulators, they are insulators that can be polarized. In a strong electric field, the nucleus and electrons of dielectrics are pulled in opposite directions.
Ques. Is water dielectric? (2 Marks)
Ans. By definition, a dielectric material is an electrical insulator. An insulator is a material that does not allow the conduction of electrical charges as electrons or ionic chemical species. Water is not an insulator. Hence, following this definition, it is not a dielectric.
Ques. How does a pyroelectric generator work? What are its advantages? Construct a diagram of a pyroelectric nanogenerator based on a composite structure of pyroelectric nanowires. (5 Marks)
Ans. In general, pyroelectric generators are thermal generators. Temperature changes bring about a charge on the surface of pyroelectric crystals, which produces a corresponding electrical signal. The absorption of light creates a temperature gradient. Pyroelectric generators have potentially lower operating temperatures, less bulky equipment, and fewer moving parts. Although several models have attempted to get patented, pyroelectric generators are nowhere near commercialization.
(a), heated (b) and cooled (c) conditions. The angles marked in the diagrams represent the degrees to which the dipole would oscillate as driven by statistical thermal fluctuations.
Ques. What is ferroelectricity used for? (2 Marks)
Ans. A hysteresis effect is implied by the spontaneous polarization of ferroelectric materials which can be used as a memory function. Hence, ferroelectric capacitors are used to make ferroelectric RAM for computers and RFID cards.
Ques. What is a ferroelectric domain? (2 Marks)
Ans. A ferroelectric domain can be defined as an area of oriented spontaneous polarization. Ferroelectrics are very important materials for applications such as data storage devices or optical frequency converters because of the controlled formation of domains.
Ques. Define ferroelectric hysteresis? (1 Marks)
Ans. Like ferromagnetic materials, ferroelectric materials exhibit a hysteresis loop. They also retain their polarization state after being exposed to an electric field, making them useful as memories.
Ques. What factors does the value of dielectric constant depend on? How does it vary with temperature? (3 Marks)
Ans. The value of dielectric constant depends on a variety of factors like temperature, moisture content and frequency. When the value of dielectric constant is measured all these values should be kept constant. When temperature increases, the mobility of polar molecules increases. This causes the value of dielectric constant to increase. This effect of temperature on dielectric constant is similar to that of frequency.
Ques. Why are dielectrics used in capacitors? How do they increase capacitance? (5 Marks)
Ans. Dielectrics are mainly used in capacitors to keep the conducting plates from coming in contact. The dielectric placed between the plates of a capacitor reduces the strength of the electric field between them. This results in a small voltage between the plates for the same charge. Thus, the capacitor stores a more charge for a smaller value of voltage. Hence, the capacitance increases. They also reduce the possibility of shorting out by sparking during operation at high voltage.
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