Download 4.5 Solid fast-ion conductors 1

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 4.5 Solid Fast-Ion Conductors
 4.5.1 Cubic stabilized zirconia (CSZ)
 4.5.2 The β-alumina
 4.6 Humidity and Gas Sensors
 4.6.1 humidity sensors
 4.6.2 Gas sensors
4.5 Solid fast-ion conductors 1

Inoic transport comprises the movement of ions into vacant sites over
substantial energy barrier that result in very low mobilities

Fast-ion conductors, optimal ionic conductors, superionic conductors

there clearly must be a supply of sites into which the mobile ion can move.
lattice vacancies ,features of lattice structure such as tunnels,
underpopulated planes within which specific ions can move which a low
activation energy

Important factors of movement :size and charge of mobile ions ,size of the
spaces within a lattice .

Small singly charged cations(Li+,La+) are mobile in a number of environments,
in contrast with more highly charged and smaller cations.
 this is the polarization of the anions by the smaller more highly charged
cations. This interaction constitutes a high-energy barrier to the movement of
cations(chap 2.6.3). However, there are factors other than geometry and
size that have important effects
4.5 Solid fast-ion conductors 2

ex)on a rigid sphere model, the interstices between the anions(radius rA) in a
face-centred cubic lattice would only allow penetration by a cation(0.15rA).
however, in the case of I- ions(radius 220pm), interstices offer an aperture of
only 34pm radius, the transport of Li+ (r3=55pm) is significant and the room
temperature ionic Conductivity of LiI is 5×10-5 Sm-1

This can be attributed to a number of factors
1. ions are not rigid spheres.
2. Simple concepts relating to the interaction of massive bodies are not
applicable In detail to the interactions of ions.
3.Thermal energy will lead to random enlargements of the interstices and to the
creation of anion and cation vacancies facilitating the passage of ions

LiI melts at 450℃ so that at room temperature it is only 430K below its melting
point and the concentration of vacant sites will be high.
4.5 Solid fast-ion conductors 3

RbAg4I5 ,a fast-ion conductor (in polycrystalline form) has an ionic conductivity
due to Ag+ ions of 21Sm-1 at 25 ℃

even though silver has a large radius(r6=115pm), the RbAg4I5 structure is such
as to offer apertures approaching this diameter for the Ag+ ions

Ionically conducting ceramic oxides are not as conductive as iodides and
sulphides with their larger anions.

However, the oxygen conductor-zironia and the sodium-conductor and β’and β”-alumina are sufficiently conductive, particularly at elevated temperature
to have been adopted for a number of applications.

In particular they can form the electrolytes in electrolytic cells which can be
used both to measure the concentration of specific elements and as a means
of storing electrical power.
4.5 Solid fast-ion conductors 4

e.m.f. E of cell which has a source of ions at a high concentration (activity a1)
on one side of an electrolyte and at a low concentration (activity a2) on the
other side is given by
ΔGOT = -zFE
ΔGOT : standard free-energy change of the cell reaction at temperature
F : Faraday constant(9.649×104Cmol-1)

z : charge number of the ion
In terms of the activity differences the change in free energy is
ΔGOT =-RTln(a1/a2) • R: gas constant(8.314JK-1mol-1)
• a1/a2: by the ratio of gas pressures at the active electrodes on each side of
the electrolyte or by the rate of the concentrations of ions in solution.
 E

RT
a1
ln( )
ZF
a2
To obtain efficient conversion between chemical and electrical energy, the
resistance of the electrolyte must be low, which is assisted by operation at
high temperatures.
4.5.1 Cubic stabilized zirconia(CSZ) 1
 Pure zirconia(ZrO2): purified from mineral zircon(ZrSiO4), baddeleyite
 Monoclinic(1170℃)tetragonal(2370℃)cubic(2680 ℃:melting point)
Contraction in volume of 5% which can cause mechanical failure in ceramic piece

This difficulty is overcome in CSZ by stabilizing the cubic form down to room
temperature by substituting lower-valency cations
for the Zironia.

Cubic ZrO2 has the fluorite structure with the O2ions arranged in simple cubic packing and half the
interstices in this lattice occupied by Zr4+ ions as
indicated. (Fig 4.26)

The vacancies that stabilize the structure also lead
to high mobility in the oxygen sublattice and to be
haviour as a fast-ion conductor.
Fig.4.26 The ideal fluorite structure showing
half a unit cell including a dopant cation and
a charge-compensating oxygen vacancy.
4.5.1 Cubic stabilized zirconia(CSZ) 2









conductivity-temperature characteristic(fig4.27)
Elements (stabilize the cubic fluorite structure in zi)
: lanthanides, scandium(Sc),yttrium (Y), magnesium
(Mg), calcium(Ca), manganese(Mn), indium(In)
The main qualification would appear to be an ionic radius
close to that of Zr4+ (r8=84pm)
Ce3+(r8=114pm)
: one of the largest ions fulfilling this function
Fig 4.27 Electrical conductivity of Zr0.85Ca0.15O1.85
Ca2+(r8=112pm)
:the most commonly used substituent, 15mol.%
Yttrium(r8=101pm)
:
stabilizes the cubic phase,13-68mol.% but maximum conductivity (7-8mol.%)
Calcium
:
resulting in a partially stabilized zi ,7-8mol.%
Scandium
:
give a material with a higher conductivity, which is particularly valuable at lower temperature.
Thorium
:
yttria-containing bodies as a grain growth inhibitor
4.5.1 Cubic stabilized zirconia(CSZ) 3





CSZ finds important applications in the control of
the oxygen content of gases and in solution in
molten metal
For this purpose electrodes of platinum gauze are
pressed into the surface of the CSZ compact
before firing.
Gauzes have platinum wires welded to them which
are connected to a high-impedance voltmeter
Platinum catalyses the dissociation and
recombination of oxygen molecules so that O2ions can be formed at one electrode & converted Fig. 4.28 Schematic diagram of a solid
electrolyte oxygen probe suitable for the
into O2 molecules at the other.
measurement of the oxygen content of gases.
z=4 for O2 (4.29) so that, if the oxygen pressures
at the electrodes are p1 and p2, the e.m.f is
E

RT
p1
ln( )
4F
p2
Air is used as a reference material so that p2=0.209 atm or 21.2kPa
4.5.1 Cubic stabilized zirconia(CSZ) 4

CSZ oxygen sensors are used to monitor automobile exhaust gas emissions.

Variations in the cell temperature can be estimated from a measurement of
the cell resistance.
•
•
•
Check the oxygen content of steels
Sintering of high-quality ferrite inductor cores
Manufacture of some thick-film microcircuits
Fig. 4.29 Section through a ZrO2 oxygen sensor for
use in a internal combustion engine
4.5.2 The β-aluminas 1

Na2O·11Al2O3(β-alumina),Na2O·8Al2O3(β’-alumina),Na2O·5Al2O3(β’’-alumina)

conductivity is high within these plane but negaligible in the perpendicular
direction.

Conductivity parallel to planes is 1Sm-1:room temperature,30Sm-1:300℃

Anisotropy is a disadvantage in ceramic because of the random orientation of
the crystallites, but nevertheless useful levels of conduction are obtainable.

nasicon (Na superionic conductor)
:Na3Zr2Si2PO12
:Na+ ion conduction is high and isotropic

higher conductivites can be obtained
by treating β’’-Al2O3 in molten LiCl
:La+ Li+
Fig. 4.30 Structure of β’’-alumina;(a) alternating spinel blocks and conduction
planes; (b) Migration pathway of Na+ ions indicating paths of concerted motion
4.5.2 The β-aluminas 2
•
•
•


Produced by heating the mixture of sodium carbonate and alumina
(additive,1250℃)
Resulting calcine is milled to a fine powder, and the required shape is formed
Volatilization of sodium during sintering is suppressed either by surrounding
the piece with a β-alumina buffer powder or by ‘zone sintering’ which involves
passing the article through a very hot zone(1700℃),10mm/min-1.
Intrinsic conductivity anisotropy of β-alumina demands that careful
consideration is given to controlling the texture of the ceramic
Na-S battery (Na(l)/β-alumina(s)/S(l))
2Na+5S <-> Na2S5
Cell runs at about 300℃ with both sodium
and sulphur in the liquid state
Fig 4.31 Schematic diagram of an Na-S cell
(Courtesy of chloride Silent Power)
4.6.1 Humidity sensors 1

Essentially thin discs of porous material that change in resistivity when
exposed to atmospheres containing certain gases.


Structure of interconnected pores and a high specific surface area( chap 3.9).
fall in resistivity : exposed to humid atmospheres

Water is adsorbed from low-humidity atmospheres by most oxide surfaces
when it dissociates, with OH- (attaching itself to a surface cation) and H30+
(attaching itself to a surface O2- ion).

Movement of the ions to alternative sites under the influence of a field give
initial reduction in resistivity.

With increased humidities liquid water may condense in the pores as a result of
the curvature of the liquid surface induced in the confined space by surface
tension.
4.6.1 Humidity sensors 2


formation of liquid : pore diameter, surface tension ,contact angle between the
solid, liquid and gas phases.
contact angle = zero, the vapour pressure (p) in equilibrium with a concave
liquid meniscus relative to the vapour pressure (p0) in equilibrium with a flat
surface is given by
ln(

p0
2Vm
)
p
RTr
humidity (h) = 100p/p0 ,diameter (d) of a pore in which water will condense is
given by d=2r, the relation between d and h is
d


γ: surface tension Vm: molar volume of the liquid
R: gas constant
T: temperature
r: radius of curvature of the meniscus.
4Vm
RT ln( 100 / h)
To speed of response a humidity element , element must be thin in section
and may contain a system of larger pores to reduce the length of the finer pore
pathways into which gas must penetrate. Electrodes covering major surfaces
must be porous.
The response of the surfaces of the element to moisture must remain constant
 teat the sensor to a temperature of ~500℃ so that the surface is restored to
a reproducible state.
4.6.1 Humidity sensors 3

Sensing element comprises a solid solution of TiO2 in MgCr2O4 with a minor
phase of MgTi2O5.

It is formed by mixing the constituent oxides, pressing them into compacts
0.25 mm thick at the relatively low pressure of about 70MPa(about 104 p.s.i)

Firing for several hours at about 1350oC to yield a sintered body (35%
porosity).

Electrodes are RuO2 with the minimum amount of added glaze for adhesion so
that the layer is porous.

MgCr2O4 is a normal spinel with Mg2+ (tetrahedral site),Cr3+ (octahedral site).
30mol.%TiO2 can be incorporated without losing the spinel structure.

Ti4+ and Mg2+ ions occupy beighbouring octahedral sites and the excess
charge is compensated by the formation of Cr2+ ions which occupy tetrahedral
sites.

It is a p-type semiconductor, in contrast with its behaviour as a gas sensor
discussed below.
4.6.1 Humidity sensors 4

Typical resistance-humidity characteristics. (fig 4.32)

High charge on the Cr3+ ions exposed in the pore
surfaces promotes the adsorption and dissociation
of water molecules according to
2H2O<-> H3O++OHleading to the conduction mechanism outlined above.

In practice, the sensor is a plate 4mm square mounted
within a heater coil, periodically by heating to 450oC for
a few seconds
Fig 4.32 Resistance versus humidity
for MgCr2O4+30mol.%TiO2
4.6.2 Gas sensors 1

A number of semiconducting oxides have been found to show changes in
electrical resistivity in the presence of small concentrations of certain gases.
( n-type SnO2, ZnO, γ-Fe2O3, TiO2 and Ag2O)

Used at temperatures in excess of about 350℃

The sensing behaviour (n-type) :adsorption of oxygen in the neck regions
between grains.
Fig. 4.33 (a) Schematic diagram of a section through a porous SnO2 compact
with a porous electrode; (b) detail of the ‘neck’ region between the grains
showing the effect of adsorbed oxygen
4.6.2 Gas sensors 2




a thin layer of porous SnO2 ceramic
carried on a small alumina tube (1.0
mm outside diameter ×0.5mm inside
diameter ×4mm long).
Two metallized gold electrodes are
deposited in the form of bands so
that the doped SnO2 makes contact
with them.
A coiled heater located in the tube
is capable of raising the temperature
of the SnO2 layer to 300-400℃.
Fig 4.34 Tin oxide gas sensor; (a) schematic diagram; (b) commercial unit
4.6.2 Gas sensors 3

Typical operational characteristics are shown in Figs 4.35 and 4.36

p02 is the equilibrium oxygen pressure in pascals for the metal oxide
system at 1000k
Fig. 4.35 Effect of pO2 on the resistance of an SnO2 gas sensor.
Fig 4.36 Resistance of a typical SnO2 gas sensor at 400℃
in various gas-air mixtures
4.6.2 Gas sensors 4


Surface states that are essential to the functioning of sensors must be
destroyed at the interface with the electrode. high affinity for oxygen
Table 4.2 Work functions and oxygen affinities for electrode metals (after
Kulwichi[10])
Oxygen affinity is taken as –log pO2 ,where p02 is the equilibrium oxygen pressure in pascals for the metal
oxide system at 1000k