fndian Journal of Pure & Applied Physics
Vol. 37. April 1999. pp. 294-301
dc Polarisation: An experimental tool in
the study of ionic conductors
Rakesh Chandra Agrawal
So lid State lonics Research Laboratory. School of Studies in Physics,
Pt Ravishankar Shukla University, Raipur 492 0 I Q
Received 3 February 1999
dc Pol arization technique has long been in use as an important tool to look macroscopically at the transporting ions and to
understand ion transport mechanism in number of sol id electrolyte systems. Wagner's dc polarization method is one of the most
widely used technique to evaluate the extent of ionic/electronic contribution to the total conductivity in the ionic/mi xed
conductors. Variety of new experiments based on dc polarization techniques have.been developed in the last - 2 decades. The
present article deals with some simple but powerfu l dc polarization technique as well as a novel polari zation/self- depolarization
method. recentl y developed in the present laboratory. to study persistent-polari zation/memory- type/electret-type phenomenon
in some Ag+ ion conducting systems.
1 Introduction
Fast ion cond ucting so lids, also termed as ' superionic
so lids' or 'solid electrolytes ', are a specia l c lass of so lid
state io nic materials w hich exh ibit exceptiona lly hi g h
1
I
io nic conductivity (-10- _ 10-4 S.cm- ) comparable to
the co nductivity of liquid/aq ueous electrolytes. These
so lids show g reat promises in recent times to develop
re li able and effi cient so lid state e lectroc hemical devices
such as batteries, fuel ce ll s, se nsors, memory and elecl9
trochromic di splay devices, supercapacitors etc . . The
mecha ni sm o f hi gh ionic conduction in these so lids is
governed by number of ioni c transport parameters viz.
ionic cond uctivity (a), mob il ity (~), mobile ion concentrati on (n), ionic tran sference number (lion) , ioni c drift
ve loc ity (Vd) as we ll as the energ ies invo lved in varioLi s
thermally activated processes. Hence, to understand fast
ion conduction vis-a-vis to characterize th e ion transport
ph eno menon in these solids, it is imperative to have
quantitative information of these bas ic transport parameters. A wide variety of experimental techniques are
3
employed to determine these parameters . Some of the
widely used techniques are:
• Imp edance spectroscopy (IS) for a - m eas urements
10
;
• T rans ient ionic current (TIC) techn ique for ~ - meas. .lon 0 f n
urements II . 12 . S ub seq uent Iy, determlnat
from a a nd ~ data using the well- known general
equation for conductivity : aCT) = neT) q~!(T) where
a , ~ and n are temperature dependent parameters;
•
l3
Wagner' s method for li on - meas urement . Subseq uently, Vd can be determined from the data obtained in the above experiments .
IS is basically an ac technique whi le TIC and Wagner's method s are essentially dc polarization method s.
Since the pure ioni c/superionic solids obey the Ohm's
law pretty well i.e. th e in stant initial tota l c urrent (fT)
varies directl y as a function of the dc potential (II)
applied across th e s pec imen when V is ke pt be low the
deco mpos ition potentia l of the sample material. Hence,
de po larization method can be considered to be o ne of
the app ro priate technique to determine some bas ic io ni c
parameters which in turn would he lp us to ex plain th e
ion tra ns port be haviour in these so lids . As the present
paper is aimed at to deal with dc polarizat io n studies, we
would , therefore, co nfine ourse lves to this technique
only. In the subsequent section various experimental
methods, based on dc polarization, have been dealt with
including Wagner 's and TIC techniques along with the
results obtained earlier from these studies on some Ag +
ion conductin g systems. A novel polarizatio n/self-depol4
larizati o n technique , recently developed in o Lir laboratory to study persistent polarization/ e lectret-type
phenom enon in so me Ag+ io n conducting sol ids, has
also been incorporated in this sect io n. Apal1 from TIC
technique, other method suggested for the estimat ion of
295
AGRA W AL: de POLARISATION
I5
•
nand)l are: homovalent doping method for glasses - ' 7,
20 21
field assisted diffusion method '8 ,'9, Hall effect . (for
details, please refer to the original papers).
2 dc Polarization Technique
The use of dc potential as an experimental tool can be
thought to have initiated just after f araday proposed his
laws of electrolysis. It was employed mainly for the
purpose of electrolysis of electrolyte solutions. However, in the present times, many modem industries extensively employ dc potential for electroplating various
kinds of metal s.
Blocking electro
'-
Non-block In9
eltetrod.
r-----=(+..::):tI
lottlry
(_ )
som~le
R
Key
(0)
-----------------------1-1~
~on= "':T'" IT
~
\
(b)
lion
...
:::J
2.1 Review of earlier experimental results
Ionic transference number and drift velocity measurements - The tran sference number g ives a quantitative information of the extent of ionic and electronic
(electrons and holes) contribution to the total conducti vity (crT)' Since, crT = crion + cre.h, the ionicl electronic
transference number can be defin ed as :
tion = crion I crT = lion IIr
le.h = cre.h IcrT = le.h Ih
where crion Icre.h and l ion Il e.h are the conductiv ity and
current contributions du e to ions/(e lectrons/ho les) respective ly. The tota l current is expressed by the usua l
equati on: Ir = n qVd A , w here q is the charge on the ion
and A is the area of c ross-secti on. T he ion ic tran sference
number can be determin ed acc urate ly eith er by Tubandt
method or by Wagner' s meth od . The ioni c drift ve loc ity
can be evaluated usi ng Ir and n data obta ined from
Wagner's ' current versus ti me ' pl ot a nd T IC technique
respective ly.
Tubandt 's melhod - Proba bl y, T ubandt 22 for the first
time used dc po lari zati on method as a too l to dete rmine
io ni c tra ns fe re nce numb e r (tion) in io ni c so lid s .
T ubandt ' s method was prin c ipa ll y based on Faraday ' s
laws of e lectro lys is. When a dc pote ntia l appears across
an ioni c so lid sandw iched between two e lectrodes, th e
mobile pos itive and negati ve ions move towards the
electrodes of oppos ite po lariti es. If the e lectrodes a re
such that the io ns woul d d isso lve into the m, then the
mass o f the electrodes w ill increase. T he measurements
of change in mass of the e lectrodes as we ll as tota l
amount of ch arge passed through the e lectro lyte of a
coulometer, connected in the externa l c ircuit, he lp us to
determ in e ionic transfere nce numbe r. An exce ll ent descri ption of the Tuban dt ' s techniq ue appeared in the
literature 3.23 . Improving the des ign of hi s original ce ll,
Tubandt carried out tion-measuremellts on a-Agl . A dc
potential was applied across three cylindrical pellets of
U
Tlml'
Fig. I - (a) Schemati c experimental circuit fo r lion - measurement by Wagner' s meth od ; (b) typi cal current versus time plot
AgI packed together in between Ag and Pt-metal electrodes. A g + ions, the only mobile ions in Agl, move fro m
Ag-anode toward s Pt- cath ode . He fo und that the weight
lost by th e Ag-anode was equi va lent to th e weight
ga ined by Pt-e lectrode and Ag l cy linder attached to it.
Thi s was in tum equivalent to the tota l am ount of charge
passed through the coulom eter. Hence, IAg+ = '1 which
was indi cati ve of the fact th at AgT ions are the sole
24
charge carri e rs in a-Agl. Ta kahashi el al. modified the
T ubandt ' s geo metry a littl e and ca rri ed out tion - measurements in num ber o f fa st Ag + ion conducting syste ms.
Wagner 's method - This is a most co nvenient and
w id ely used meth od suggested by W agner and Wagner
in 1957' 3 to measure ionic/electro ni c transference num be r in number of so lid electrol yte syste ms. The tec hnique g ives very re liable results particularl y in case of
Ag + ion conducting so lids. Hence, th e present d isc uss ion
is limited to study Ag+ ion conductin g systems on ly.
Howeve r, it should be menti oned here that thi s tec hnique is exten sive ly e mpl oyed, in general , to study oth er
so li d e lectro lyte systems a lso . T he experimental arrangement for th e determ in ation of tion in a Ag + ion
cond uctin g system is sc hem aticall y shown in Fi g. I (a).
A cy lin drical pe ll et of the sam p le is sandw iched between
blocki ng (graph ite) and )1 on- bl oc ki ng (si lver) electrodes . A constant dc potenti al (V - 0.5 V) is appl ied
across the samp le w ith the po lari ty shown and the current in the c ircuit is monitored as a function oftime with
the help of an x-y-t recorder. A typical ' current versus
296
INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1999
-1
~----.-.---------------~
« -phase
I
.t--- Transition renion
...
I
: ,
~-i-
-2
,I,
0(-
Rl9ion
"
I'
Il -ptBse
- --r-- --I-
I
I
~'
20 - lr {2I00C)
2
2.5
3
3.5
1oo0lT [K')
6
It
.... IT CC)9·C)
10
1,.<12S-c)
IT - TOTAL CURRENT
Ir (152·C)
Ir(187·C)
Fig. 3 -
2
ITC60·C)
0.6
0
IT (27"C)
Fig. 2 -
2
4
6
8
10
(h)
Current versu s time plot for Agl
time ' plot is shown in Fig. I (b). Silver ion conducting
systems are generally pure ionic systems with Ag+ ions
as sole charge carriers. For such a system the total
current h approaches zero as a result of complete desoJution of Ag + ions in Ag-anode. Since, h ~ lion ; li on = I.
However, if the so lid is a mixed Ag+ ionic/electronic
system, the total current h levels off at some non-zero
value, as shown. T he final residual current (Ie.h) is due
to the moving electrons/holes in the system . Hence, lion
and te.h can be known separately with the help of the
ratios: lion / hand le.l,lh respectively.
The current versus time plot of Wagner's dc polarization method can also be employed to estimate the drift
ve locity (Vd) of mobile ions in a pure ionic solid, as
discussed above. This novel approach was used for the
first time by the present group to determine Vd in number
of Ag+ ion conducting systems 25 viz . AgJ 26 .27 ; a
quenched [0.7SAgl:0 .2SAgCI] mixed-system/solid- so. 2829
.
d at our Ia boratory as an alternate
' ·Il1vestlgate
IutlOn
host in place of AgJ; glass systems: 0.7
[0.75AgI:0.25AgCI]: 0.3[Ag 20 :B 20 3] and 0 . 75
rO.75AgJ 0.25AgCI]: 0.25[Ag20:Cr03]30J I and com posite systems: 0 .7[0 .75 Ag1:0.25AgC I] :0 .3 A b03,
O. 8[0 . 75Agl: 0.25AgCI]: 0 .2Sn02 and 0 . 9
[0.75Agl:0 .25AgCI]: 0 . 1Si0 2 32-34. A representative
temperature dependent current versus time plots for (3
and a phases of AgJ (Region-I and Region-II respectively) is shown in Fig. 2 . The temperature dependence
of current versus time plots for all other systems were
Log Vd versus l i T Arrheniu s plot fo r Ag l
identical and exhibited following significant features in
common:
I. The initial current (h) approached to zero with time
at all temperatures of our measurements . This was indicative of the fact that all the system s remain purely
ionic with Ag + ions as sole charge carriers and hence,
l ion ~ I in the entire range of temperature.
2. The polarizing time (i.e. time in which h ~ 0)
increased as the sample temperature increased . This is
expected, as at higher temperatures the mobile ions are
thermally more agitated , hence , wo uld require lo nger
time to get polarized at a fixed va lu e of applied dc
potential as compared to the time required at lo wer
temperatures .
3 . The magnitude of initial total current (h,) al so
increased with increasing temperature s.
The increase in I h I may either due to the increase in
nor Vd. If n can be measured independ e ntly, Vd can be
eva lu ated at various temperatures with the help of h
-data obtai ned from the above stud ies (in fact, n has been
determ in ed independently with the hel p of (J and /1 data,
as discussed below). A representative log Vd versus I/T
plot for Ag l is shown in Fig. 3 . Sim i lar pl ots were drawn
for other systems and the energy (Ed), involved in the
thermally activated process, was computed from the
s lope of the straight Iine for all the syste ms2 5 -3 ~ . It ha s
been verified that Ohm ' s law obeyed we ll during these
measurements. Since, Vd is directly proportional to /1 at
a fixed value of applied dc fie ld E (i.e. Vd = /1E) . hence,
log Vd versus liT and log ~l versus I /T va riations must
be analogous and the energies Ed and E111 involved in
these two thermally activated processes respecti vely
wou ld be identical. The validity of Vd measurements on
all the above system s were cross-checked by direct
determination o f ~l using TIC technique , as discu ssed
below.
297
AGRA WAL: de POLARISATION
Ionic mobility and mobile ion concentration measurements - Transient Ionic Current (TIC) technique:
TIC technique, originally suggested by Watanabe et a/. II
12
and Chandra et a/. , was used for direct detennination
of 'ionic mobility (11). This is also a dc polarization
method, like Wagner's method, except for both the
electrodes being blocking. The sample is first polarized
by applying a constant dc potential across the thickness
of sample pellet for sufficient long time to ensure that a
state of complete polarization has been attained. At this
state, the mobile ions are polarized and remain blocked
at the respective bulk/electrode interfaces. The polarity
of the applied potential is then reversed, simultaneously,
the current in the circuit is monitored with time. The
instant the polarity is reversed, the polarized ion clouds
start travelling in the bulk towards electrodes of opposite
polarity. This results in a flow of current through an
external circuit. The moment the ion cloud arrives the
other end of the pellet, a peak occurs in the current versus
time plot, and then the current drops sharply. Fig. 4
shows TIC plot for a typical system in which only one
type of ionic species are mobile. The inset shows the
basic experimental arrangement. I f more than one ion ic
species are mobile in the system, number of peaks would
appear in TIGplot when suitable blocking electrodes are
used. Each individual peak would correspond to one
type ofmobile ionic species. The position of the peak on
the time axis directly measures the time offlight"t of the
mobile ion species to cross the thickness d of the sample
pellet. Hence, the ionic mobility 11 can be determined
with the help of equation : 11 = if / V "t, where V is the
applied fixed dc potential. Using the Il-data obtained
above and a-values from conductivity measurements,
mobile ion concentration n can be evaluated conveniently for the systems with one type of mobile ionic
species. The temperature dependent measurements of 11
and n can also be carried out by placing the specimen in
a furnace. The energies (Em and Ef) involved in the
thermally activated processes can be computed from the
slopes oflog 11 versus I/ Tand log n versus I /T Arrheniu s
plots respectively. These measurements were carried out
25
on the above mentioned Ag+ ion conducting system s 34 . On th e basis of the experimental resu Its, the phenomenon contro ll ing the basic ion transport mechani sm in
these sol id electro lyte systems can be 'easi Iy und erstood.
Detailed discussion on the mechanism of ion transport
in these Ag t- ion conductin g solids appeared elsewhere
in the literature 26.34 .
River,
kty
-
2
Furno!' ,
c
Sample
~
.a...
...c
~
lOO.n.
.......
I
I
I
:)
U
\
\
,,
<
0
0
10
20
30
60
90
120
150
Tlmt' (,)
Fig. 4 - Typical TIC plot for !-I-measurement. Inset:
the basic experimental circuit
Miscellaneous dc polarization experiments - Various miscellaneous experiments, based on dc polarization, have recently been suggested by a number of
35
workers. Yoo and coworkers designed an experiment
based on the polarization in an ion-blocking electrode
condition and determined ionic-charge-of-transport
(a*) and chemical diffusivity (D) in mixed conductors.
36
Preis and Sitte have recently given an excellent theoretical treatment for the polarization process occurring
in mixed ionic/electronic conductors. Assuming the polarization as a chemical diffusion-induced phenomenon ,
they developed experimental model based on Weppner
37
and Huggins asymmetric electrochemical cell configuration and determined chemical diffusion coefficient in mixed conductors with comparable ionic and
electronic conductivities by means of galvanostatic po38
larization experiments . Mizusaki has recentl y suggested a novel and improved experimental technique to
study the bulk and interfacial properties of sol id electrolyte systems. His technique was based on Hebb- Wag39
ner ' s ion blocking method by dc polarization field
using the cell configuration : (- )Agi AgX (X = C I, Br, I)
IC or Pt(+). It was shown that the complete ion- bloc kin g
can be realized when the chemical equilibrium is attained not onl y at the Agl Ag X interface but at AgX IC
interface al so.
2.2 Polarization/self-depolarization and persistentpolarization/electret··type effects in some Ag + ion
conductors
Another novel id ea, based o n dc po lari zat ion method,
has recentl y bee n deve loped in our Laboratory to study
po larizati on/self-depo larization phenomenon in some
A g~ ion conducting systems . The polarization procedure
298
INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1999
was exactly similar to TIC technique discussed above.
The polarization/accumulation of mobile Ag + ions at
negative polarity end of the bulk specimen results in a
potential difference across the sample pellet which can
be measured experimentally 'on the removal of the external dc potential. The magnitude of the potential difference, obtained at the instant the external dc potential
is removed, corresponds to a peak value and can be
referred to as instant peak potential (Vp). Vp decays
rapidly due to redistribution/self-diffusion (chemical
diffusion) of accumulated ions throughoutthe bulk. This
process has been termed as a self-depolarization phenomenon. In order to explore the time exactly required
by the specimen to attain the state of complete polarization, the external polarizing dc potential was applied for
different durations and Vp-values were measured. Fig. 5
shows instant peak potential Vp-values measured at
room temperature on pellets of different thicknesses of
AgI, [0.75AgJ: 0.25AgCI] and a superionic borate glass
system : 0.7[0.75AgJ:0.25AgCI]: 0.3 [Ag20:B 20 3]. The
abscissa corresponds to the time (I) for which the samples were initially polarized. One can note from tlie
figure that Vp increases initially as the polarizing time
increases then attains a saturation value afterwards. This
corresponds to the state of complete polarization and
gives an information about the minimum time needed
for the above Ag + ion conducting systems to attain the
state of complete polarization . One can also note that the
magnitude of Vp increases with the thickness of the
samples. This may be due to the reason that in thicker
specimen, number of mobile Ag+ ions are expected to
be more which in turn get polarized and give rise to
larger Vp-va lues as compared to that for the thinner
sample. This is a qualitative and not a quantitative
statement. The experimental results in Fig. 5 clearl y
indicate the fact that the magn itude of Vp at the state of
complete polarization gives a qualitative information
regarding the number of mobile Ag~ ions available in
the system at a particular temperature. We carried out
temperature dependent Vp- measurements . The assertion
drawn from the above study regarding Vp-values giving
infonnation about the mobile ion concentration n is
further supported when we compared Vp versus temperature plots with the plots of temperature variatio n of
n for these systems obtained earlier in the independent
studies26.28.10. Fig. 6 shows log Vp versus I/ T plot for:
Agi (thickness -0.205 cmt, [0.75Agl : O.25AgCI]
(t hi c k n e s s -0 .2 c m) and sup e rio n i c g Iass :
0.7[0. 75AgI:0.25AgCI] :0.3 [Ag 20:B 20 1] (t hi c k n e s s
&Xl
... - _. ..- . _ ....... - --..--.---.--.- -...... - Thickness
I ~O.2C6cm -O.2fficm O.~ ~
=--=-=.
>
£.:m
Co
>
I
/
~
I
(i-ti
200
!:
::;;,
=
=
!!
]
"'
~
(a)
Temperature -2'fC
i
100 L
- -- ----- - - - -. - -_.--- _._-- - -
- - - - - -- - 1!
0.2 em I i
Thickness 1~ 0.135 cm - 0.167 cm
i
.~
VSOi(
>
£..m
wi
no
...
i
r== .J
"If
~. ~
Ig.like ohase at
.....:
=/
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-2f:ftc l
~I
..
_
7
_
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_ __ _ . .. . . . - .
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ThiCk'ness l ~ 0.155cm
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(b)
~
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1
. ~
____ ~ .~..... r ... .,.......",...~
=0. 215cml~
".
~~~-----------------------~
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>200
Temperature 2'fC
Fig. 5 -
I 'p
(c)
versus polarizing lime plols for : (a) Ag J:
(b) [0.75A gJ:0.2 SAgC I) : (e) 0.7[0 .75A g J:0.25AgC I)
0.31Ag20B201J
-0 .15 5 cm). Log n versus l i T Arrheni us plots for these
systems are reproduced in Fig. 6 for direct comparison.
One can obviously visua lize that Vp and n vary almost
analogously with temperature for all the systems . We
note an abrupt increase in Vp-values for AgI and the new
host [0.75AgI :0.25AgCI] , after f3 --+ a transition temperature. The abrupt increase in Vp after the phase tran sition further justifies our other assertion we made
earlier26 .28 , regarding the superionic conduction of a AgI or a -like phase of new host, as due to an abrupt
increase in the mobile ion concentration (n).
299
AGRA W AL: de POLARISATION
3
>'
.5.
'-\l-~--:-'-.'---Tra-ns-it- i-o-n-reg-io-n-(-a)----,j :
2.8 ~....
13 iNse
1
_...........
~
~2.6
300
2ID
18:;,
16]'
! 2.4 ~
I_
1~
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14.Jo
•
L
c
.4aJ
.i
'
iAt instant I
CooUng cycle
. .::. Healing cycle
iI
~
=r···
:1 ~~~"----,,---,-_.~~.~_;~,~~~~a--,)
-~
2.9 ... ----",.--,: - - - -- '
~ Transition region
.
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2.8
~
2.6
!r
2.4
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2.2
-
- - - --
•• -
•• •.•.. • ,. - .... -" .• - , ~ . ~ - ... ~ - . - - I
I I
After 8 h
13 -like phase
3 .---- --
(a)
~
-
L..
---, 24
r
400
12
3.5
L.....__'_~~_'__'~~~...........~__'_~
2,0
2.5
3.0
1000lT [K"'l
Fig. 6 - Log Vp versus IIT and log n versus IIT plots for :
(a) AgI; (b) [O.75AgI:O .25AgCI];(e) O.7[O.7SAgI :O.2SAgCI] :
O.3[Ag20:B203]
r------.-.. -.-..-.'-...
~---.-. P=
'A=fte=r'''''
i =h'''1
'''
u
:;,:m
!.200
(b)
i
Q.
> 100
o
~~~~~~~~~~~~~~~
3lJ
200
100
In addition to the above novel information, a phenomenon of persistent-polarization/electret-type effects
I4
were also observed in AgI and the new host
[0.75AgI:0.25AgCI] during self-depolarization cycle.
This is probably another remarkable feature exhibited
by mobile Ag + ions of the system . These effects correspond to electret-type behaviour commonly observed in
several dielectric materials such as polymers, divalent
40
impurity doped ionic salts viz. KCI , Kl, AgCI etc.
Kumar and Chandra 41 have reported electret-type effects
in solid electrolyte mixture : RbAg4I5 + KEr, which they
referred to as ionic polarates. Electret-type effect, in
fact, refers to a phenomenon in which the polarization
state persists for a long time after the dc polarizing
potential across the sample is removed. For details,
~
.. I I'Iterature 42 .43 .
relerences
may be rna de to the onglOa
The electret-type effects become more predominant or
the polarizati~n states persist for longer duration in
'thermally stimulated polarized ' samples i.e. samples
polarized at higher temperature. We studied this effect
in thermally stimulate polarized Agl (thickness -0 .205
o
~~~~~~~~~~~~~~~
2
2.5
3
3.5
1000/T (1<"1)
Fig. 7 -
Vp ve rsus l i T plots showing persistent-polari zalion in:
(a) Ag\; (b & c) [O.7S AgI:O.2S AgC I]
300
INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1999
cm) and [O.75Agl:0.25AgCI] (thickness -0.2 cm). The
samples were poll;lrized by an external dc potential (-0.5
V) for 10 min at 200°C (i.e. well above (3 ~ a transition
temperatures, -147°C for Agl and -135°C for [O.75Agl
:0.25AgCI]), then the external dc potential was removed. The potential difference developed across the
sample was measured during different thermal heating/
coo ling cycles in the time span ranging from I to many
hours. Fig. 7 shows the 'peak potential versus temperature' plots fo r these systems. The upper plot in both Figs
7a & 7b gives the variation of potential immediately
after the removal of the field at 200°C and cooling the
sample to room temperature (1st cooling cycle), then
heating the sample back to 200°C (1 st heating cycle). A
hysteresis type behaviour was observed in both the
cases. Hysteresis generally corresponds to some kind of
energy loss in the systems . The other plots correspond
to the variation of potential difference in subsequent
heaJingicooling cycles of the same samples which were
left open at room temperature for several hours. It can
be obviously noted that, although, the magnitude of
potential decreased with time, but, the polarization
states persisted for very long time . The potential difference measured at higher temperatures may be thought
to be due to the existence of temperature gradient between upper and lower electrodes i.e. a typical thermoemf measurement in the usual thermoelectric power
studies on ionic/superionic systems . However;this was
overruled after performing the same measurement on an
unpola rized Agf sample . This is obvious from the bottom-m ost plot in Fig. 7(a) which shows that the potential
difference remains close to zero at all temperatures. The
persistence or retention of polarization field substantial ly improved in the samples cooled to room temperature from 200°C with polarizatic n field on . This can be
obviously seen in Fig. 7(c) showing the similar plots
during various heating / cooling cycles for
[0. 75Agl:0.25AgCI] sample cooled from 200°C to room
temperature with polarizing field on and then the field
was removed. The polarization state in this sample
persisted for more than 350 hrs. In the dielectric electret
material s this kind of polarization-state-retention have
been reported due to homo-charge formati on and deca/ 2,4 3 . The similar reasons may probably be assigned
to above ionic systems also . However, an extensive
investigation is needed to explain such phenomenon in
ionic solid:;. Nevertheless, to give an approximate explanation, one can think of a memory-type-effect for
mobile Ag + ions of the above systems polarized in the
high conducting phase and make a vague statement as :
it seems as if 'the mobile Ag + ions have retained the
memory of their polarization state at a particular temperature and the memory died out slowly with time '.
3 Conclusion
On the basis of various experimental results discussed, it can be concluded that the dc polarization
technique can certainly be employed as an important
tool to study ion transport (macroscopic properties) in
several ionic/superionic and mixed ionic/electronic systems. What is actuall y required is ; an ingenious designing and development of new experiments. The dc
technique is widely used in many Solid State fonics
Research Laboratories, including the present LaboratorY" to measure ionic/electronic trans ference number,
ionic mobility and ionic drift velocity etc . Based on this
technique, another novel method has recentl y been developed, to study the polarization/se lf- depolarization
phenomenon and persistent-polarization effect in some
Ag+ ion conducting solids. The results have been di scussed with reference to electret-type effects, commonly observed in dielectric material s.
Acknowledgment
The author gratefully acknowledges the financial
support provided by the MPCOST, Bhopal, through
project No . P-86/92 cit. 16/12/94.
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