Reaney- Varistors and Thermistors Flashcards
What is a varistor?
A voltage dependent resistor. A resistor which has high resistance at low voltages and low resistance at high voltages.
Major application of varistors
To protect a circuit from a high power transience by providing a path across the power supply which takes a low current normally but a large current if the voltage rises abnormally. Such transience is very common in most power supplies like in televisions and computers.
Typical voltage current graph for vaistor
Voltage vs log(current). Curve up for a false origin with decreasing gradient. Then shallow straight line with slope 1/α. Then curve up with increasing gradient.
Basic circuit arrangement for a varistor
Connected in parallel to the circuit to be protected. At large voltages it will become the path of least resistance so most of current will flow through it.
Structure of varistors
Grains of a semiconducting material separated by an intergranular layer (IGL). Can use ZnO doped with Bi to have Bi2O3 rich IGL. Or use SiC mixed with clay or other siliceous materials (fired at high temperature). You get back to back Schottky barriers in neighbouring grains (they are the interfaces between the grains and IGL).
How do varistors work at low fields?
Electrons can only pass over the barrier by thermionic emission resulting in a high resistance. See slide 8 diagram
How do varistors work at high fields?
The electrons can tunnel to the positive space charges of the second barrier. See slide 8 diagram
Role of IGL
Not completely understood. Is believed that it allows electron states to form at the interface with the semiconducting matrix. The surface states may act as acceptors for electrons from the n-type semiconducting matrix. Electrons withdrawn from the region near the layer resulting in a positive charge.
How does Bi affect production of ZnO?
ZnO normally sintered at 1200C. Bi2O3 has low melting point so forms a liquid phase which wets the grain boundaries. This results in dissolution of ZnO and reduces temperature of firing
Another application of varistors
Suppression of sparks in switches and relays in highly conductive circuits.
Two types of thermistors
Those which have a positive temperature coefficient of resistance (TCR) referred to as PTC
Those which have a negative TCR referred to as NTC
Main applications of thermistors
Temperature controls or sensors. Compensating for variation in properties of other components as a function of temperature
What does a thermistor generally require?
A ceramic which has a large TCR
How do NTCs get a large TCR?
Intrinsic semiconductor properties giving an exponential fall in resistivity over a wide temperature range
How do PTCs get a large TCR?
A phase transition which gives rise to a change in the conduction mechanism from semiconducting to conducting
Exponential decay in resistance for NTCs formula
ρ(T)=ρsubμ exp(B/T)
Where ρ is resistivity
ρsubμ is constant independent of temperature
B is constant related to energy required to activate electrons to conduct
T is temperature
Formula for temperature coefficient of resistance
Differentiate exponential decay of resistivity formula.
Get αr=-B/T^2
TCR of Si
B is 116 and T at 300K gives
αr=-1300MK^-1 or -0.13%
Industry demands a value of -3 to -5%
What are units MK^-1 equivalent to?
ppm
Logσ Vs 1/T graph for typical semiconductor
Straight negative gradient of -B=EA/k
B is sensitivity of material to changes in temperature and related to activation energy for conduction
Materials used for NTCs
Most based on spinel structure as value of B intrinsically high. Most common based on solid solutions of Fe3O4 with ZnCr2O4 or MgCr2O4. If particularly low resistivity required as well as large TCR use Mn3O4 doped with Ni, Co and Cu. This is normal spinel with Mn2+ in tet and Mn3+ in oct. Dopants place ions of different valence state on same site so conductivity increases
Structure of spinel
Based on 2x2x2 FCC array of O2- ions. Means 64 tetrahedral A sites and 32 octahedral B sites available. Normal spinel has 8 tet (2+) and 16 Oct (3+) occupied. Inverse spinel has 8 tet (3+), 8 oct (3+) and 8 oct (2+)
Polyhedral structure of spinel for doped Mn3O4
Get edge shared (Mn/Ni)O6 octahedra. This is the conduction pathway. Also get MnO4 tetrahedra
What are properties of NTCs sensitive to?
The firing schedule in oxygen. Must be controlled carefully to obtain desire resistivity and B values
What are NTC thermistors used for?
Sense temperature of cooling water in car engines, fuel gauges. Maintain picture stability in televisions by compensating for heat increase in the beam focussing coils
What are PTCs associated with?
Paraelectric to ferroelectric phase transitions. At the phase transition there is typically a jump in the value of resistivity by several orders of magnitude. Below this temperature the device exhibits normal NTC behaviour associated with semiconductors
Why is PTC effect only associated with ceramics not single crystals?
Is related to the grain boundary structure
Typical material used for PTC
La doped BaTiO3 where segregation of La occurs at grain boundary
Resistivity Vs temperature graph for La doped BaTiO3
Orthorhombic straight negative. Tetragonal fairly flat but curves down a bit. Then curves up to almost vertical big increase in height for cubic (dipoles lost)
What happens at grain boundary for La doped BaTiO3? (in terms of diagram)
Close to it the CB curves up from being flat to sharp peak at the boundary. Comes back down as mirror image other side. Acceptor states exist at the boundary. Barrier height is height difference between normal flat level and peak
Defect chemistry of BaTiO3 in reducing conditions
Oo=1/2O2 + Vo°° + 2e’ into CB
Oxygen diffusion in a post sinter anneal fills the Vo and causes grain boundaries to become insulating. Overall effect is creation of grain boundary region whose defect chemistry differs from that of the semiconducting grain
Interface between insulating grain boundary and adjacent grain interiors for PTC
Forms back to back double Schottky barrier. At paraelectric to ferroelectric phase transition on heating the acceptors are no longer compensated by polarisation. Barrier height increases and so does resistivity.
What controls the resistivity either side of para to ferro phase transition?
Low temperature (ferro) bulk resistivity controlled by matrix phase. High temperature (para) bulk resistivity controlled by grain boundary structure or phase. Characteristics can change with grain size
Applications for PTCs
Current limiters in televisions and most high-tech electronic equipment Automotive electric chokes Crankcase heaters for refrigeration Air conditioning compressors Hair dryers Food warmers