Properties of Electrical Materials Flashcards
conducting materials
allow the movement of charge with quantum states for free electrons and electrons that can be liberated with very little energy input
dielectric (insulating) medium
- an ideal dielectric medium contains no free charges
- all electrons are bonded to atoms
- the medium will not conduct electricity
- in reality, a dielectric medium can be overcome by a strong enough electric field (called a breakdown)
dielectric polarization
- the electric field polarizes the dielectric medium
- atoms and molecules are aligned and stretched in the direction of the field
- positive nuclei are pushed in one direction, electrons in the opposite direction
- directions depend on direction of the field
- this creates an induced electric dipole
semiconducting materials
materials though which charge easily flow more or less easily
hole
- a hole is positively charged atom that attracts electrons
- when voltage is applied, electrons are drawn to positive charge
- hole attracts an electron from a neighboring atom, most likely from direction of negative charge
- hole atom become neutral, neighboring atom becomes new hole
- hole migrates toward negative charge, though atoms do not move
- in this way, both electrons and holes carry current
p-type material
majority carrier: holes
valance electrons in dope atoms: 3 electrons (Al, B, ln, Ga)
no. of covalent bonds formed by dope atom: 3, resulting in one neighbor atom unbonded
dope atom charge: retains static negative charge, hole is free to move
dope atom name: acceptor
minority carrier: free electrons
n-type
majority carrier: electrons
valence electrons in dope atoms: 5 electrons ( Ph, As, Sb)
no. of covalent bonds formed by dope atom: 4, donates a free electron
dope atom charge: retains positive charge, electron is free to move
dope atom name: donor
minority carrier: holes
permeability, μ
- a property of a material
- measure how readily a magnetic field is created within the material when an external magnetic field is applied
- due to behavior of electrons in presence of external magnetic field
- examples of high permeability: iron and steel
- examples of low permeability: wood and water
nonmagnetic materials
negligibly affected by presence of magnetic field (i.e. have low permeability)
types include:
- diamagnetic
- paramagnetic
- antiferromagnetic
diamagentism
- due to electrons circulation in their orbits
- exhibited by all materials
- cancels due to random orientation of the spins
paramagnetism
- due to circulation of unpaired electrons in their orbitals
- spins align with magnetic field
antiferromagnetism
- leads to tiny increase in permeability
- due to magnetic dipole moments that align
- moment of one atom has the opposite orientation of its neighbor
magnetic materials
- significantly affected by presence of magnetic field
- cause circulating currents in plane perpendicular to magnetic field
types include:
- ferromagnetic
- ferrimagnetic
ferromagnetism
- leads to large increase in permeability
- due to magnetic domains with fully aligned magnetic dipole moments
- results in spinning electrons even without external magnetic field
ferrimagnetism
- leads to increase in permeability, but less so than ferromagnetism
- due to ordered spin structures that neither cancel fully (as in antiferromagnetism) nor add fully (as in ferromagnetism)
ferromagnetism
- fully aligned
- large increase in permeability
ferrimagnetism
- neither fully aligned nor fully canceling
- moderate increase in permeability
antiferromagnetism
- fully canceling
- tiny increase in permeability
coefficient of thermal expansion
- rate at which material expands and contracts as temperature increases and decreases α = E/delta T - engineering strain: E = delta L/ Lo -change in linear dimension: delta L = Lo α delta T
The coefficient of thermal expansion is a property of both electrical and magnetic materials
However, the change in dimensions is not considered in resistance, capacitance, and so on, because the change is so small
electric flux Φ
- measures electric field
- does not flow equally well through all materials
- units are volt-meters (V * m)
permittivity, ε
- determines the flux that passes through the medium
- total electric flux generated by a point charge is proportion to charge
ϕ = q/ E
Eo is permittivity of free space
for air E= Eo= 8.85x 10^-12 F/m
dielectric constant (or relative permittivity), k
- dimensionless comparison to permittivity of free space, Eo
E= k Eo - informal definition: ratio of flux in a medium to flux in a vacuum
- for a vacuum, k =1
- formal definition: ratio of capacitances for a given voltage and separation
k = C with dielectric/ C vacuum - The NCEES Handbook sometimes uses the symbol, Er for relative permittivity
electric flux density, D
- relationship between density and electric flux is
D= εE - E is electric field intensity in (N/C)
Capacitance, C
- measures of capacitor’s ability to store electric charge
- ratio of stored charge to applied voltage
C = q/V
-for a parallel plate capacitor,
C = εA/d
relative electric susceptibility, Xe
- like relative permittivity, a dimensionless comparison to free space εo
ε = K εo = (1 + Xe) εo
conductivity,
- measure of how easily current flows through a medium (how much current is created by an electric field)
- ratio of current density to electric field strength
- units are siemens per meter (S/m)
resistivity, p
inverse of conductivity: p = 1/σ
small conductivity means large resistivity (and vice versa)
units are ohm-meters
conductivity and resistivity
- intrinsic properties of materials
- small conductivity, large resistivity: even a strong field creates little current (for example, rubber)
- large conductivity, small resistivity: strong field creates large current (for example, copper)
conductance, G
- measure of ease of current flow in circuit or circuit element
- units are siemens (S)
Resistance, R
- measure of opposition to current flow in circuit or circuit element
- inverse of conductance R= 1/G
- units are ohms
resistance
- proportional to resistivity of material
- proportional to length of element
-inversely proportional to cross-sectional area of element
R= pL/A
resistivity and resistance
- both depend on temperature
- as temperature increase, both p and R increase for most conductors, decrease for most semiconductors
piezoelectric effect
- certain crystals and ceramics respond to mechanical stress with change in electric charge
- for example: quartz exhibits very stable piezoelectric effects
- practical uses include sensors for sound, pressure, mechanical stress
- also used in reverse: voltage applied between faces of certain crystals and ceramics to produce mechanical distortion
magnetic field strength, H
- measure of strength of magnetic field in free space
- independent of medium
- units are amperes per meter (A/m)
magnetic flux density, B
- includes magnetic response of material that field passes through
- dependent on medium
- units are teslas (T)
- in strongly magnetic material, B-field is greater than in free space
corrosion
degradation of a material ( usually a metal) by chemical reaction with its environment
four main types:
- galvanic action
- fretting corrosion
- stress corrosion
- cavitation
Stress corrosion and cavitation rarely apply to electrical materials
galvanic action
- caused when metallic ions differ in oxidation potential
- if metals are similar in oxidation potential, corrosion can be slow or negligible
- if metals are very dissimilar, corrosion is much faster
anode reaction
- metal with higher potential acts as anode and will corrode
cathode reaction
metal with lower potential acts as cathode and is unchanged
fretting corrosion
- when metals rub and slide
- combination of wear and chemical corrosion
-happens on metals that depend on film of surface oxide for protection (aluminum, stainless steel)
can be reduced by - lubricating rubbing surfaces
- sealing surfaces
- reducing vibration and movement
law of mass action (mass action law)
The number of holes, p, multiplied by the number of free electrons, n, in a semiconductor, doped or not, is equal to the intrinsic carrier concentration, ni, squared
concentration of holes
- in a p-type material, is approximately the same as the concentration of the acceptor (p-dopant) atoms
- in an n-type material, is approximately the same as the concentration of the donor (n-dopant) atoms
