Polymers, plastics, and electric wire's polyvinyl chloride are called "insulators" that do not conduct electricity.
"Dielectric" is a form of insulator which is sandwiched between the + and - electrodes of the capacitor element and has the property of storing electricity.
The dielectric has similar properties to insulators, like not conducting DC electricity, even though it is between + and - electrodes.
Insulators and dielectrics are same in that they do not conduct electricity but have different functions with various properties.
Dielectrics can store electricity as they cause electric polarization.
The ability to store electrical energy is expressed by dielectric constant.
Dielectrics are used extensively for capacitors and are extremely important as next-generation materials.
Learn more about dielectrics and insulators.
Comparison of Dielectrics and Insulators
While dielectrics and insulators both have in common in that they obstruct DC, the dielectrics allow AC to pass since a capacitor element using the dielectric will have resistance (impedance) corresponding to the frequency. It means that the dielectrics are conductors of AC. On the other hand, the insulators do not conduct electricity in either DC or AC circuits. What difference does this make?
Dielectric Polarization and Constant
Dielectric materials are also used in capacitors, but electricity does not directly pass through the dielectric.
Therefore, the dielectrics are categorized into insulators.
However, the dielectrics have a property of passing high-frequency AC currents with an impedance (resistance).
The dielectric material will get polarized when sandwiched between positive and negative electrodes.
Dielectric polarization occurs when an electric field is applied. An alignment of positive and negative charges takes place at the molecular level, resulting in polarization corresponding to positive and negative electricity.
The polarized dielectrics have the property of attracting electrons onto the electrodes. The polarization prevents some of the electricity (electrons) from leaving the electrodes of the capacitor, which causes it to store electricity. In AC circuit, as the frequency increases, the positive and negative charges on both poles of the capacitor are frequently switched, and some of the electricity that cannot leave the capacitor electrodes resists the switching, which becomes the impedance.
While dielectrics are categorized into insulators, they can store electricity and provide electrical resistance in AC circuits.
The dielectric constant is the ratio of the applied electric field strength. It is also an indicator of polarizability. The dielectric constant often shows the ratio in vacuum, and it varies depending on the type of dielectrics. Dielectric substances have an impact on the capacitor performance.
| Material | Dielectric constant |
|---|---|
| Barium titanate | approx. 5000 |
| Water | 80.4 (20°C *Varies greatly depending on temperature) |
| Alumina | 8.5 |
| Mica | 7.0 |
| Quartz | 3.8 |
| Glass | 5.4 to 9.9 |
| Rubber | 2.0 to 3.5 |
| Paper | 2.0 to 2.6 |
| Paraffin | 2.1 to 2.5 |
| Air | 1.00059 |
*from Wikipedia (CC BY-SA 3.0)
Electrical Difference Between Conductor, Semiconductor, and Insulators
What is the difference in atomic structure between conductors and insulators? Conductors such as metals that can easily conduct electricity are used in electric wires. When metals are considered at the atomic level, they possess an incomparably greater number of "free electrons" that can move around freely than other materials. These free electrons move when voltage is applied, resulting in the flow of electricity.
What about semiconductors? They do not have as many free electrons as metals, but they do have the property of ejecting free electrons when energy is applied to them from the outside. Depending on materials and manufacturing methods, it is also possible to create semiconductors with many free electrons (n-type semiconductors) and semiconductors with many holes (p-type semiconductors), which are the counterpart of electrons (p-type semiconductors).
By combining these two types of semiconductors, it is possible to make a diode that allows current to flow in only one direction, or a transistor that controls the flow of current only when it is desired. When a diode allows current to flow in one direction but not the other that acts as an insulator and does not permit current to flow. In other words, a "semiconductor" can be a conductor like a metal or an insulator.
Insulators have no conduction and no free electrons in their atoms. They have very low conductivity and emit free electrons when energy is applied, the same as semiconductors. Still, the energy required to reach this point is so great that electricity cannot easily pass through it. Vinyl, plastics, rubber, etc. are insulator materials. Dielectrics are insulators in a sense which do not easily conduct electricity.
Insulator Breakdown Condition
Insulators secure our lives daily, however they have limitations. If excessive voltages are applied to the insulators, insulator breakdown will occur. Semiconductors emit free electrons as energy is applied. Due to their atomic-level structure, insulators emit free electrons when energy above a specific voltage is applied.
For example, air is also generally an insulator that does not conduct electricity. But when static electricity in clouds causes a high-voltage condition, electricity flows due to the voltage difference with the earth (lightning strike). This natural phenomenon is also an insulator breakdown caused by the energy of a certain voltage being applied to the air. It is also applied to wire sheathing and dielectrics such as vinyl wires when a voltage over the allowable voltage is supplied. So, we must take care of them not to occur the insulator breakdown.
Electrostatic Charge and Discharge
Static electricity is an imbalance of electrical charges within or on the surface of an insulating material such as plastic or vinyl. It is generated by friction and separation of the materials like plastics with a fleece blanket, etc. The rubbing releases negative charges, called electrons, which can transfer and build upon one object to produce static electricity (a state in which the number of electrons and holes are different and have different voltages.
The materials will become "charged " with static electricity. After they are charged, the free electrons give them the ability to "discharge" when they come close to another conductor with a different potential. Some substances are negatively charged (easily receive electrons) and others are positively charged (easily emit electrons). When a finger or metal touches a charged substance, it emits electrons or inflows electrons and returns to a stable state.
The electrostatic discharge or ESD also may be responsible for damaging electronic components such as ICs and capacitors when static electricity can reach voltages as high as several kV, unlike standard electronic circuits. Therefore, the prevention of ESD is necessary to reduce the generation of static electricity or add circuits to protect electronic components from static discharge.
Dielectric in Capacitor
A capacitor consists of dielectric sandwiched between electrodes. Varied capacitors are used depending on the dielectric materials as follows.
Film capacitor
As film capacitors use vinyls such as polyethylene between electrodes, they are completely isolated. While it has not only a long life but is inexpensive, the film capacitor has a dielectric constant of approximately 3 with lower capacitance than others. We can see the film capacitor used for noise reduction in audio equipment.
Aluminum Electrolytic Capacitors
As aluminum electrolytic capacitors use electrolytic paper between electrodes which are made of aluminum foil.
The aluminum oxide formed on the surface of the aluminum foil is used as the dielectric.
The aluminum electrolytic capacitor has a dielectric constant of approximately 8 with high capacity.
However, the electrolytic paper is filled with electrolyte, and current may leak between the electrodes from there, so the electrodes cannot be completely isolated.
The electrolyte is a consumable material that evaporates gradually and needs to be replaced periodically.
The capacitors featuring high capacitance are often used in power electronics applications such as power inverters and solar inverters.
Advancing Ceramic Capacitors
In recent years, some capacitors use new ceramic materials as dielectrics. There have been ceramic capacitors using ceramic as the dielectrics. The dielectric constant of typical ceramic capacitors with limited applications was almost as good as that of film capacitors.
A newly developed barium titanate ceramic has a high capacity and dielectric constant of 4500, which is nothing compared to typical dielectric materials. There is currently underway on the use of barium titanate for batteries taking advantage of the capacitor's electricity-hoarding properties.
Electric Insulation materials for Ultra-High Voltage Transmission
As for ultra-high voltage operation, it is essential to ensure the high voltage isolators. Insulators are crucial in maintaining safety and stability as mechanical strength is required for high-voltage isolation. For many years, the insulators have been used to fix and insulate transmission towers and power lines standing on the ground because of their exceptionally high mechanical tensile strength and isolation capability.
Rubber materials are also used for high-voltage isolation.
Such rubber materials, referred to as insulating materials, must have high insulation resistance and dielectric strength without isolation breakdown.
Dielectric breakdown tests are conducted to measure the dielectric strength as the voltage is used to ensure reliability. In the dielectric breakdown testing, a high voltage is applied to the insulating material, and the voltage at which it is destroyed is measured. If the material can withstand high voltage for a specified period, it is judged to have isolation strength to the test voltage.
High dielectric strength for high voltage applications makes the product safe and high quality, leading to longer service life.
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