ionic conductors
TRANSCRIPT
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Ionic conductors
Ionic solids contain defects that allow the migration ofions in an electric field
Some solid materials have very high ionicconductivities at reasonable temperatures
– useful in solid state devices
mobile interstitialmobile vacancy
Applications of solid ionic conductors
Membranes in separation processes
Electrolytes in sensors
Electrolytes in fuel cells and batteries
– should be a poor electronic conductor
Electrode materials in solid state batteries
– should be a good electronic and ionicconductor
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Factors effecting the conductivity
σ = n Z e µ Conductivity is influenced by 1)the carrier concentration n,
2) the carrier mobility µ Usually, defects act as the charge carriers
– not many defects in most ionic solids
– mobility is usually low at room temperature
< 10-10Insulators
10-3-104Semiconductors
103-107MetalsElectronic conductors
10-1-103Liquid electrolytes
10-1-103Solid Electrolytes
< 10-16 – 10-2Ionic crystalsIonic conductors
Conductivity (S m-1)Material
Ionic conductivity in NaCl
NaCl is a poor ionicconductor
Conduction involvesmigration of cationvacancies
Cation vacancies are present due to
– doping - extrinsic defects
– Schottky defects - intrinsicdefects
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Conduction is an activated process
µ = µ0 exp (-Ea/kT) - Arrhenius equation
Temperature dependence of conductivity
σ = (σ0/T) exp(-Ea/kT) – Contribution from mobility and defect formation
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Idealized conductivity for NaCl
At low T conductivity is
dominated by mobility of
extrinsic defects
At High T, conductivity is
due to thermally formed
(intrinsic) defects
Intrinsic versus extrinsic conductivity
Extrinsic conductivity
– σ = (σ0/T) exp(-Ea/kT)
– carrier concentration is fixed by doping
Intrinsic conductivity
– carrier concentration varies with temperature – σ = (σ’0/T) exp(-Ea/kT) exp(-∆HS/2kT)
– slope of plot gives Ea + ∆HS/2
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Cation vacancy migration mechanism
Cations can not hop from site to site via a
direct route
– not enough space
Cations migrate via an interstitial site
– this is a tight squeeze and requires energy
Experimental conductivity of NaCl
Broadly as expected – Get deviation at low T due
to vacancy pairing
– Get deviation at high T due
to screening of mobile
defects by defects of
opposite charge» Debye-Huckle type model
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Energetics of ionic conduction in NaCl
0.27-0.50Dissociation of vacancy –
Mn2+ pair
~1.3Dissociation of vacancy pair
2.18-2.38Formation of Schottky pair
0.90-1.10Migration of Cl-
0.65-0.85Migrationof Na+, Em
Activation energy (eV)Process
AgCl
The predominant defect in AgCl is cationFrenkel
Cation interstitials are more mobile than cationvacancies
Cation interstitials can migrate by one of twomechanisms
– direct movement
– indirect movement
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Migration mechanism in AgCl
Two possible pathways for interstitial migration:
1) move directly from interstitial to interstitial
2) interstitial displaces regular cation onto
interstitial position
Migration actually occurs by second pathway
Evidence for the indirect mechanism
Both charge and mass transport through a crystal
can be measures
– conductivity gives charge mobility
– diffusion measurements using radiolabelled Ag+ gives
mobility of Ag+
Charge is transported twice as fast as Ag+ ions
suggesting the indirect mechanism is correct
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Doping in AgCl
Doping AgCl with a divalent impurity like Cd2+
reduces the ionic conductivity of the specimen
There is an equilibrium between cation vacancies
and Ag+ interstitials
– doping increases vacancy concentration
– doping decreases interstitial concentration
Cd2+ doped AgCl
Schematic showing effect of Cd2+ impurity
on conductivity – Presence of Cd2+ reduces
number of Ag+ interstitials and hence
lowers conductivity
Get minimum in conductivity
curve when doped – at high
impurity concentrationsconductivity is dominated by
cation vacancy migration, at
low concentrations interstitial
migration dominates
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Solid electrolytes
There is a technological need for solids that havevery high ionic conductivities
Such materials are referred to as FAST IONCONDUCTORS
They include:
– α AgI
– Na β alumina
– NASICON, Na1+xZr 2[(PO4)3-x(SiO4)x]
– Stabilized zirconias
Ionic conductivity of some good solid electrolytes
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β - alumina
Na1+xAl11O17+x/2 (β) and Na1+xMgxAl11-xO17 (β”) aregood sodium ion conductors at moderate temperatures
Na ions have high mobility and can be ion exchangedwith a wide variety of other cations
M2O.x Al2O3 x = 5 - 11
– M = Alkali+, Cu+, Ag+, Ga+, In+, Tl+, NH4+
– x = 5-7 usually produces β” material
– x = 8 - 11 gives β material
– β” material usually stabilized by addition of Li+
or Mg2+
The structures of β and β” alumina
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The structure of β - alumina
Conduction plane of β alumina
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The sodium sulfur cell
Sodium sulfur cells have ahigh energy density
– useful for electric vehicles
There are safety concerns
– molten sodium
2Na(l) --> 2Na+ + 2e-
2Na+ + 5S(l) + 2e- ---->
Na2S5(l)
Sodium sulfur phase diagram Need to operate at high temperatures
Can not fully discharge cell (solidifies)
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Silver iodide
At low temperatures AgI adopts either a Wurtzite
or zinc blende structure
– Ag+ fills half of the tetrahedral holes in a close packed
I- array
Above 146o C it transforms to a BCC structure
with the Ag+ filling a small fraction of the
available tetrahedral sites
– the cation sublattice “melts”
σ ~ 130 Sm-1
The structure of α - AgI
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Cation sites in α - AgI
Ionic conduction in α - AgI
There are many possible sites for Ag+
– 12 tetrahedral
– 24 trigonal
– 6 octahedral
There are only 2 Ag+ ions per unit cell!
– these ions are found disordered on the tetrahedral sites
Motion between sites is facile – ~0.05 eV activation barrier
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RbAg4I5
AgI is polymorphic. The high
temperature α phase has a
high ionic conductivity
associated with a melted Ag+
sublattice
At low T ionic conductivity
drops
RbAg4I5 discovered while
trying to find materials that stillhad α – AgI structure at low T
RbAg4I5
Highest room temperature ionic conductivity ofany crystalline solid, 0.25 S cm-1
– Not stable < ~25 °C
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Cu2HgI4
Material shows an order disorder phasetransition similar to AgI
– color change at phase transition
– marked increase in ionic conductivity at phasetransition
Structure has FCC array of I- with cationsfilling tetrahedral holes
– at low T cations are ordered – at high T they are disordered over all sites
The structure of Cu2HgI4 at low T
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Stabilized zirconias
Y2O3 and CaO can be
dissolved in ZrO2
– creates a lot of oxygen
vacancies
At high temperatures
the defects are mobile
– oxide ion conductor
Applications of stabilized zirconia
Oxide conductors are of use for
– oxygen sensors
» based on concentration cell, can be used to measure O2 inexhaust gases, molten metals …
– fuel cell membranes
ZrO2 is only usable at high temperatures
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An oxygen sensor
An O2 concentration cell can be built
E = [2.303RT/4F] log(p’/pref )
Fuel cells
Fuel cells are
devices for the
direct conversion
of fuels such as
CH3OH, H2, CO to
electrical energy
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Solid oxide fuel cells
Fuel cells offer an
efficient and clean
way of using fossil
fuels, but
– high cost
– thermal cycling
problems
Solid oxide fuel cell performance
from a paper by S.C. Singhal in Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells, 1995
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Electrochromic devices
Color changes such
as those needed in
smart windows can
be achieved by
moving ions into a
suitable solid
Lithium batteries
Batteries based on
lithium are attractive
as they can be light a
have a very high
voltage output
– Considerable current
research on cathodes
and electrolytes for
these devices