Type & Properties of Electroceramics

EBB 443 – Technical Ceramics

Dr. Sabar D. HutagalungSchool of Materials and Mineral Resources Engineering, Universiti Sains Malaysia

Ceramic insulators High-k ceramic dielectrics Piezoelectric ceramics Ferroelectric ceramics Magnetic ceramics Superconductors Photonic ceramics

Ceramic insulators

The primary function of insulation in electrical circuits is physical separation of conductors and regulation or prevention of current flow between them.

Other functions are to provide mechanical support, heat dissipation, and environmental protection for conductors.

Ceramic materials which in use these functions are classified as ceramic insulators.

They include most glasses, porcelains, and oxide and nitride materials.

The advantage of ceramics as insulators is their capability for high-temperature operation.

Insulation Resistance

Conductivity

= d/(R A) and 1/ = = (R A)/d Or in terms of the material parameters

= nqwhere is the electrical resistivity (m), R () the sample resistance, A is area (m2), and d thickness (m).

If more than one type of charge carrier being present, the resultant conductivity can be defined as the sum of component conductivities (I) as follow

= i ni (ez)i i = i i

Insulation Resistance

Depending on which charge carriers predominate, the solid may be classified as primarily an electronic (n or p type) or ionic conductor.

However, mixed conduction is

= electronic + ionic

Insulation Resistance

For an ionic solid, mobility is related to the diffusion coefficient D (cm2/sec) by the Einstein relationship

= ezD/kT Diffusion and conductivity are related by the

Nernst-Einstein equation: = n(ez)2D/kT

Since both diffusion and N (number of defects generated) are activated processes, where

N = n exp(-w/2kT)

Insulation Resistance

D = Do exp(- /kT)Then

= o exp(-E/kT) and

E = w/2 +

Where w and are activation energies for defect generation and migration.

For extrinsic conduction w=0 and E=; that is, the ionic mobility becomes the controlling factor in the conduction.

Ionic Conduction

What is an ion? An ion is a positive or negative loaded atom

caused by electron deficiency or electron excess.

This electron deficiency/excess arises at the reaction of two atoms (ionic connection).

Positive loaded ions are called cations and negative loaded ions are called anions.

In ionic crystal, the individual lattice atoms transfer electron between each other to form positivily charged cations and negatively charged anions.

Ionic Conduction

The binding forces between ions are electrostatic in nature and thus very strong.

The RT conductivity of ionic crystals is much lower than the conductivity of typical metallic conductors.

The large difference in conductivity can be understood by realizing that the wide bandgap in insulators allows only extremely few electrons to become excited from the valence band into the conduction band.

Ionic Conduction

Ionic conduction is caused by the movement of some negatively (or positively) charged ions which “hop” from lattice site to lattice site under the influence of an electric field.

This ionic conductivity:

(1)

Nion is the number of ions per unit volume that can change their position under the influence of an electric field

ion is the mobility of these ions.

ionionion eN

Ionic Conduction

In order for ions to move through a crystalline solid, they must have sufficient energy to pass over an “energy barrier” (see schematic).

Thus, Nion in eq.(1) depends on the vacancy concentration in the crystal (i.e., on the number of Schottky defects).

Ionic Conduction

d

Q

E

distance

(a)

E

distance

d

(b)

Figure: Schematic representation of a potential barrier, which an ion ( ) has to overcome to exchange its site with a vacancy ( ).

(a) Without an external electric field, (b) with an external electric field. D = distance between two adjacent, equivalent lattice sites, Q = activation energy.

Ionic Conduction

The D varies with temperature; this dependence is commonly expressed by an Arrhenius equation:

(2)

Where Q is the activation energy , Do is a pre-exponential factor that depends on the vibrational frequency of the atoms and some structural parameters.

Tk

QDD

Bo exp

Ionic Conduction

Combining (1) through (2) yields,

(3)

Equation (3) is shortened by combining the pre-exponential constant:

(4)

Taking the natural logarithm yields: (5)

Tk

Q

Tk

DeN

BB

oionion exp

2

Tk

Q

Boion exp

Tk

Q

Boion

1lnln

Ionic Conduction

If plotted ln ion vs. 1/T, a straight line with a negative slope would result. (See figure).

The slopes in Arrhenius plots are utilized to calculate the activation energy, Q or Ea.

Ionic Conduction

Slope = - Q/kFor SiO2 from graph:Slope = (ln 10-7 – ln 10-14)/(1.5x10-3 – 2.55x10-3) = [ln (10-7/10-14)]/(-1x10-3) = ln 107/(-1x10-3) = - 1.612x104

From Slope = - Q/k

Q = (1.612x104)x(1.3806x10-23) = 2.225x10-19 JQ = 1.39 eV (1 eV = 1.6x10-19 J)

Ionic Conduction800 600 400

ln

1/T

T(oC)

Sometimes, ln vs. 1/T plot will give us two (2) line regions representating of two different Q values.

Figure: Schematic representation of ln versus 1/T for Na+ ions in NaCl. (Arrhenius plot).

Ionic Conduction

At low T, the Q is small, the thermal energy is just sufficient to allow the hopping of ions into already existing vacancy sites. This T range is commonly called the “extrinsic region”.

At high T, the thermal energy is large enough to create additional vacancies.

The related Q is thus the sum of the Q for vacancy creation and ion movement. This T range is called the “intrinsic region”.

High Conducting Ceramics

Ceramics are generally classified as electronic conductors, ionic conductors, mixed (electronic/ionic) conductors, and insulators.

The electronic conductors include superconductors, and semiconductors.

Ionic conductors generally exhibit conductivities in the range 10-1 to 100 S m-1 that increase exponentially with temperature.

Insulators such as high-purity alumina are at the lower extreme of the conductivity of 10 -13 S m-1.

Temperature Sensitive Resistor

Some ceramic resistors exhibit high value of the temperature coefficient of resistance (TCR) and they may be negative (NTC) or positive (PTC).

Temperature Sensitive Resistor

In a ceramic a large temperature coefficient of resistivity can arise from 3 causes: The intrinsic characteristic. A structure transition which accomponied by a change in

the conduction mechanism from semiconducting to metallic.

A rapid change in dielectric properties in certain ceramics which affects the electronic properties in the intergranular region to give rise to a large increase in resistivity with temperature over small temperature range.

The 3rd Mechanism has led to important TCR devices.

Typical resistance-temperature response for various sensor materials

NTC Thermistor

The TCR of a semiconductor is expected to be negative.

In each case the resistivity depends on temperature according to

where is approximately independent of T and B is a constant related to the energy required to active the electron to conduct.

Differentiating this equation leads to TCR value R:

T

BT exp)(

2

1

T

B

dT

dR

NTC Thermistor

The most NTC materials are based on solid solutions of oxides with spinel structure, e.g. Fe3O4-ZnCr2O4 and Fe3O4-MgCr2O4.

A series that gives favourable combinations of low resistivity and high coefficients is based on Mn3O4 with a partial replacement of Mn by Ni, Co and Cu.

PTC Thermistor

PTC thermistors exhibit an increase in resistance at a specified temperature.

PTC resistor could be classified as critical temperature resistors because, in the case of the most widely used type,

The positive coefficient is associated with the ferroelectric Curie point.

PTC Thermistor

Most PTC has the negative resistivity-temperature characteristic up to about 100 oC and above about 200 oC.

While between these temperatures there is an increase of several orders of magnitude in resistivity.

The PTC effect is exhibited by specially doped and processed (eg. BaTiO3).

Application of PTC Thermistor

The are two main groups: Applications such as temperature measurement,

temperature control, temperature compensation and over-temperature protection.

The second group includes applications such as over-current protection, liquid level detection and time delay.

Voltage-dependent Resistors (Varistors) There are a number of situations in which it is valuable to have a

resistor which offers a high resistance at low voltages and a low resistance at high voltages.

Such a devices can be used to protect a circuit from high-voltage transients by providing a path across the power suply that takes only a small current under normal conditions but takes

large current if the voltage rises abnormally, thus preventing high-voltage pulses from reaching the circuit.

Schematic use of a VDR to protect a circuit against transients,

VDRCircuit to be protectedSource

Varistors-VDR

Ceramics based on SiC and ZnO are two materials in everyday use for VDR.

The VDR behaviour in ZnO varistors for example is governed by electron states that are formed on the surfaces of crystals as a consequence of the discontinuity.

These surface states act as acceptors for electrons from the n-type semiconductor.

Electrons will be withdrawn from region near the surface and replaced by a positive space charge.

Oppositely oriented Schottky barrier will be created at surface of neihbouring crystals so that a high resistance will be offered to electron flow in either direction.

Illustrations of actual microstructure of a varistor

Basic principles of Varistors-VDR

At low applied fields small thermally activated currents pass over the reverse biased junction.

At high fields tunneling through the junction will occur, accounting for the low resistance.

The behaviour is similar in some respects to Zener diodes. From varistor I-V characteristic, the linear part can be represented by the

relation,

Where kI is a constant and falls off at low voltages.

If I1 and I2 are currents at voltages that differ by factor of 10,

UkI I

212

110 ,log III

I

Basic principles of Varistors-VDR Alternatively, where

The resistance at a given voltage is

Power dissipated is

with = 25, a 10 % increase in voltage would increase the power dissipation by a factor about 2.5.

IkU V /1/1 IV kkand

)1(1 1 Uk

IkRI

V

1 UkIUP I