Reaney- Varistors and Thermistors

Question 1

What is a varistor?

Answer 1

A voltage dependent resistor. A resistor which has high resistance at low voltages and low resistance at high voltages.

Question 2

Major application of varistors

Answer 2

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.

Question 3

Typical voltage current graph for vaistor

Answer 3

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.

Question 4

Basic circuit arrangement for a varistor

Answer 4

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.

Question 5

Structure of varistors

Answer 5

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).

Question 6

How do varistors work at low fields?

Answer 6

Electrons can only pass over the barrier by thermionic emission resulting in a high resistance. See slide 8 diagram

Question 7

How do varistors work at high fields?

Answer 7

The electrons can tunnel to the positive space charges of the second barrier. See slide 8 diagram

Question 8

Role of IGL

Answer 8

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 Electrons withdrawn from the region near the layer resulting in a positive charge.

Question 9

How does Bi affect production of ZnO?

Answer 9

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

Question 10

Another application of varistors

Answer 10

Suppression of sparks in switches and relays in highly conductive circuits.

Question 11

Two types of thermistors

Answer 11

Those which have a positive temperature coefficient of resistance (TCR) referred to as PTC
Those which have a negative TCR referred to as NTC

Question 12

Main applications of thermistors

Answer 12

Temperature controls or sensors. Compensating for variation in properties of other components as a function of temperature

Question 13

What does a thermistor generally require?

Answer 13

A ceramic which has a large TCR

Question 14

How do NTCs get a large TCR?

Answer 14

Intrinsic semiconductor properties giving an exponential fall in resistivity over a wide temperature range

Question 15

How do PTCs get a large TCR?

Answer 15

A phase transition which gives rise to a change in the conduction mechanism from semiconducting to conducting

Question 16

Exponential decay in resistance for NTCs formula

Answer 16

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

Question 17

Formula for temperature coefficient of resistance

Answer 17

Differentiate exponential decay of resistivity formula.

Get αr=-B/T^2

Question 18

TCR of Si

Answer 18

B is 116 and T at 300K gives
αr=-1300MK^-1 or -0.13%
Industry demands a value of -3 to -5%

Question 19

What are units MK^-1 equivalent to?

Answer 19

ppm

Question 20

Logσ Vs 1/T graph for typical semiconductor

Answer 20

Straight negative gradient of -B=EA/k

B is sensitivity of material to changes in temperature and related to activation energy for conduction

Question 21

Materials used for NTCs

Answer 21

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

Question 22

Structure of spinel

Answer 22

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+)

Question 23

Polyhedral structure of spinel for doped Mn3O4

Answer 23

Get edge shared (Mn/Ni)O6 octahedra. This is the conduction pathway. Also get MnO4 tetrahedra

Question 24

What are properties of NTCs sensitive to?

Answer 24

The firing schedule in oxygen. Must be controlled carefully to obtain desire resistivity and B values

Question 25

What are NTC thermistors used for?

Answer 25

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

Question 26

What are PTCs associated with?

Answer 26

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

Question 27

Why is PTC effect only associated with ceramics not single crystals?

Answer 27

Is related to the grain boundary structure

Question 28

Typical material used for PTC

Answer 28

La doped BaTiO3 where segregation of La occurs at grain boundary

Question 29

Resistivity Vs temperature graph for La doped BaTiO3

Answer 29

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)

Question 30

What happens at grain boundary for La doped BaTiO3? (in terms of diagram)

Answer 30

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

Question 31

Defect chemistry of BaTiO3 in reducing conditions

Answer 31

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

Question 32

Interface between insulating grain boundary and adjacent grain interiors for PTC

Answer 32

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.

Question 33

What controls the resistivity either side of para to ferro phase transition?

Answer 33

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

Question 34

Applications for PTCs

Answer 34

Current limiters in televisions and most high-tech electronic equipment
Automotive electric chokes
Crankcase heaters for refrigeration
Air conditioning compressors
Hair dryers
Food warmers