Lezione 14: Electrical properties Flashcards
What are resistivity and conductivity?
Electrical resistivity is the measure of a material’s resistance to current passing through it. The values for materials have a large range: 1 - 10^24
Definition of capacitance of a capacitor
Measures the ability for a capacitor to store charge under a potential V
Definition of dielectric constant
Measures the ability of a material to polarize
Definition of dielectric loss factor
Measures the energy dissipated when radio-frequency waves pass through a material
Definition of dielectric breakdown potential
Electrical potential gradient at which an insulator breaks down and a damaging surge of current flows through it
What is the atomic meaning of conductivity?
Electrons of an atom occupy discrete energy states or orbits, arranges in shells. Each shell is made up of sub shells, each of which contains 1, 3, 5 or 7 orbits respectively. Electrons fill the shells with the lowest energy with 2 electrons of opposite spin in each orbit. When a large number of atoms are brought together to form a solid, the inner electrons remain with their initial host atom, but the outer ones interact. Fields created by neighboring electrons in a crystal cause energy levels of individual atoms to split into bands of closely spaces levels. Whether a material is a conductor or insulator depends on how full the bands are:
- conductors have a partially filled outer band, allowing electrons to move easily
- insulators have a filled outer band that is separated from the nearest unfilled band by a band gap.
What is electrical resistance?
Conductivity is proportional to the n° of free electrons and to the mobility of the electrons, which is proportional to the free mean path. Impurities, dislocations and thermal vibrations act as scattering centers and shorten the distance electrons scan travel through a material. Strengthening mechanisms of metals increase electrical resistivity by introducing scattering centers: precipitation or work hardening is the best way to strengthen conductors. Electric resistivity increases with T.
What is dielectric behavior
An electric field exerts a force on a charge, pushing positive charges in the direction or the field and negative charge the opposite way. The dipole moment, which determines the dielectric constant, depends on the magnitude and separation of the charges.
What happen with dielectric loss?
Materials with high dielectric loss become asymmetrical and form a dipole moment: these materials respond to oscillating fields by rotating, but because of their shape they interfere with each other, which causes heat to dissipate.
What happens during dielectric breakdown?
If an electron is torn from a filled outer band, it’s accelerated by the force of the electric field until it collides with another atom. IF the field is sufficiently large, the kinetic energy of the removed electron will collide with and remove new electrons from the atom it hits, causing a cascade of similar events. The critical field strength to make this happen is the breakdown potential.
Talk about piezo-electric materials
They contain molecules with a permanent dipole that results from a non symmetrically distributed charge. A 2 sided relationship exists between polarization and deformation: a field induces deformation and deformation induces charge differences between its surfaces, thus creating a field..
Talk about ferro-electric materials
They have a natural dipole moment: they are polarized to start with, and each polarized molecule lines up so their dipole moments are parallel. The direction of polarization can be changed by applying and electric field: this causes a change in shape. These materials have a huge dielectric constant so they’re perfect to make super capacitors.
Analyzing their hysteresis curve, we can see that as the strength of the field is increased, polarization increases too reaching a max at the saturation polarization. If the field is then removed, a large part of the polarization remain, which is only removed by reversing the field.
How is electrical power transmitted?
The objective in electrical power transmission is to minimize power loss: PL = i^2R
R, resistance, is proportional to the material’s resistivity (pe), so minimizing it would achieve the desired result. However, strength and density are important, as power is often transferred through lines supported by widely spaced pylons. Resistivity and strength generally have an inverse relationship, so hybrid materials are used to achieve the desired property profile.
How is electrical insulation achieved?
Polymers are excellent insulators, used widely for cable insulation, connectors, plugs casings. The latter, as example, has to be based on a minimum strength constraint and an objective of maximizing electrical resistivity. Polymers are the best choice. Another constraint requires a thermoset material which will not melt in the event of circuit overload or poor contact.
How is electrical insulation with thermal conduction achieved?
Heat sinks are used to absorb energy dissipated from microchips, which requires materials with high thermal conductivity. Metals have the highest conductivity, but they are also electrically conductive, which could slow data transfer by electrically coupling with the microchip. . For this reason ceramics (alumina, silicon carbide) offer the best combination of thermal conductivity and electrical resistivity.
