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“A new class of materials, called the relaxor ferroelectronics, show high strain response and are
actively being investigated for transducer and dielectric applications. The theoretical and
experimental investigations of the relaxor behaviour is an active field of research and a better
understanding would lead to more efficient, smarter materials."
unmodified
Ferroelectric Ceramics - A Unique Class of
Smart Materials
By D. C. Agrawal
Ferroelectrics - Properties and Applications
SBN
KTN
PLT
BST
BST
PT
BT
PMN
PZT
BZT
PLZT
KTN
PZT
SBT
BLT
BST
PST
The ferroelectric materials are technologically
an important class of materials that display a
wide variety of phenomena. Some of these are
illustrated in Figure 1. T he name
“ferroelectric” is derived from
“ferromagnetic”, the iron (ferro) based
materials in which a permanent magnetization
was first noticed. The ferroelectrics do not
usually have Fe but show analogous electric
behaviour i.e. they have a permanent electric
polarization which can be reversed by the
application of an electric field. Both classes of
materials are now included under the banner
of “ferroic” materials.
water. At present there are more than 38
structural families of ferroelectrics, the
perovskites being one of the most
technologically impor tant families.
Figure 2 depicts the structure of barium
titanate, BaTiO 3 . The discovery of
ferroelectricity in barium titanate in the early
1940s led to a burst of research and
applications which is continuing to this day.
4+
The Ti ion in BaTiO3 is slightly displaced
from the centre of the “cube” so that there is
a separation between the centres of the +ve
and the -ve charges in the unit cell, leading to
an electric dipole moment. Ideally, all the unit
Ferroelectricity was first discovered in Rochelle cells in the ceramic should have the same
salt, a double tartarate of sodium and direction of polarization to impart maximum
potassium crystallizing with four molecules of overall polarization to the ceramic body.
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However, to minimize the total energy, the
direction of polarization is same in only small
regions in a grain of the ceramic. These regions
are called domains. The domains form
spontaneously when the ceramic is cooled
from its processing temperature below a
characteristic temperature, called the Curie
temperature, Tc (~ 120o C for barium titanate).
unmodified
Figure 2: Unit cell of BaTiO 3 ; the green spheres
are Ba2+, the red are O 2- and the blue are Ti 4+
The polarization direction in the various
domains is randomly oriented so that the net
polarization in the as prepared ceramic is nearly
zero. The ceramic is subjected to an operation
called “poling” in which a high electric field (~
3 kV mm-1) is applied to it. The polarization in
the domains tends to orient in the direction of
the external field. The favourably oriented
domains grow. A high temperature, below Tc is
used to facilitate this process. The ceramic is
then cooled with the field on and remains in the
poled condition upon removal of the field due
to the randomization process having a
negligible rate at room temperature.
Piezoelectricity - the basis of smart
electromechanical behaviour
These field related electromechanical coupling
constants are very large in ferroelectrics and are
the basis for their application for efficient
transduction between the electrical and
mechanical signals leading to uses in
acceleration and pressure sensing, smart
mirrors, automobile suspension control, AFM
cantilevers, non destructive evaluation (NDE),
sonics and ultrasonics and a host of other
transducing and sensing applications. The
electromechanical coupling coefficients are
found to be particularly high for the
ferroelectric ceramic lead zirconate titanate,
(Pb,Zr)TiO3 or PZT which can be looked at as a
solid solution of PbTiO3 and PbZrO3 . When
these two constituents are in the ratio
0.535:0.465, the piezoelectric constants of
PZT are found to be the largest. This is the so
called morphotropic phase boundary (MPB)
composition of PZT. The PZT compositions
near the MPB are therefore almost exclusively
used for the various electromechanical and
transducer applications. The property of the
PZT ceramics can be further tailored for a
given application by incorporating additional
cations, called dopants, in the PZT lattice.
Special care has to be taken to minimize the loss
of Pb due to volatilization during the
processing of the ceramic.
Ferroelectric Thin Films
Coatings and thin films of PZT are of interest
in transducer, dielectric and nonvolatile
memory applications. Figure 3 shows the
“hysteresis loop” of a PZT film which is a plot
of the externally applied electric field vs.
polarization in the film. The nonzero remnant
polarization at zero field and the ability to
reverse the direction of polarization by going
along the loop by half a cycle, is the basis for
the application of these films in information
storage (e.g. nonvolatile random access
memory), parametric bistable devices,
microwave frequency tripler, etc.
All insulators experience a strain upon
application of an electric field due to the so
called electrostriction effect. However, in the
ferroelectrics large piezoelectric strains are
produced due to the reorientation of the
electric domain states upon application of an The thin film of PZT can be deposited by
electric field; conversely, the application of a various methods. In the sol-gel method, a liquid
stress leads to a change in the polarization. precursor solution is prepared from the
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(b)
(a)
Figure 4 : (Ankur 280nm )RBS spectra from PZT film
deposited on (a) sapphire and (b) quartz ; substantial
diffusion of Pb into the quartz substrate has taken
place in the latter case.
The spectra on platinum, a much used coating
on the substrate to provide the electrical
contact to the bottom of the film, is similar to
that for sapphire. In case of quartz (also
silicon), there is a large depletion of Pb from
the film due to the diffusion into the substrate
and only a slight loss from the top surface due
to volatilization. In case of the sapphire or
platinum substrates, the loss by diffusion is
negligible. The Pb deficiency in the film in the
former case causes the film to assume a
pyrochlore phase which is not ferroelectric.
Figure 3:( Ankur2Hystloopundoped)
Hysteresis loop of PZT thin film
alkoxides (e.g. titanium butoxide, zirconium
isopropoxide) and salts (lead acetate)
containing the required metal ions. The “sol” is
then deposited on the substrate by spin coating
or dip coating. A heat treatment at about 700oC
results in the removal of the organic matter and
the formation of a crystalline ceramic film. The
problem of loss of Pb during the heat
treatment is also significant in the film
preparation. However, unlike the bulk
ceramics, the loss can be more due to diffusion
into the substrate, rather than by volatilization.
This is illustrated in Figures 4 and 5 which show
the RBS (Rutherford Back Scattering) spectra
of films deposited on sapphire and quartz and
their analysis.
X-ray phase
Perovskite
Pyrochlore
In most applications involving polarization
reversal, an important problem is the
degradation of the remnant polarization by the
cycling of the polarization state, as during the
read and write cycles of a ferroelectric memory
cell.
RBS simulation
structure
Layer thickness Layer #
(A)
Pb1.0Zr0.53Ti 0.47O3
3000
3000
Al2O3
20000
3000
Pb0.85Zr0.53Ti0.47O3
3000
1
Pb0.82Zr0.53Ti0.47O3
200
2
Pb0.71Zr0.53Ti0.47O2.5Si0.5
400
3
Pb1.4Si2O6
1000
4
Pb1.2Si2O6
Pb1.0Si2O6
2000
1200
5
6
SiO2
10000
7
Figure 5: Results of the analysis of the spectra of Figure 4; initial PZT film thicknesses are290 nm
and 390 nm respectively on sapphire and quartz.
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(b)
(a)
Figure 6: Hysteresis loops of (a) undoped and (b) 3 at % Ce doped PZT thin films before and
after cycling for a million cycles; doped sample shows significantly higher resistance to degradation
Figure 6 shows the hysteresis loops of (a)
undoped and (b) 3 at(atomic) % Ce doped)
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PZT film at the beginning and after 10 cycles
of switching at a frequency of 1 kHZ. In both
cases, the remnant polarization has degraded;
however, the Ce doped sample has withstood
the “fatigue” process much better in fact, the
undoped sample can hardly be called
ferroelectric after fatigue.
The results imply that the Ce doping decreases
the drift mobility of the oxygen vacancies. The
Ce as a dopant is somewhat unique in that it can
exist as Ce3+ as well as Ce4+. Binding with Ce is
believed to lead to deeper potential wells for
the oxygen vacancies resulting in decreased
degradation kinetics.
Relaxors - A New Class of Smart
Ferroelectrics
In the PZT films, the charged oxygen vacancies
are known to be highly mobile. The movement
of these charged species in the presence of As mentioned above, the PZT ceramics show
field is primarily responsible for their the largest coupling between the electric field
redistribution leading to the formation of a and strain amongst all the ferroelectric
nonswitching layer at the electrode. The net ceramics. The PZT is therefore the material of
drift of these charged species can be shown to choice for such transduction applications.
be proportional to N/f2 where N is the total However, a new class of materials, called the
number of cycles imposed at a frequency f. The relaxor ferroelectrics, show an even higher
experiments show that the model is able to strain response and are actively being
reproduce the observed scaling behaviour as investigated for transducer and dielectric
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N/f (Figure 7).
applications. A distinguishing feature of the
relaxor ferroelectrics is that the peak in the
dielectric constant vs temperature plot shifts to
higher temperatures with an increase in the
frequency of measurement.; also, the peak is
much broader than the peak in the regular
ferroelectric. Some examples of relaxors are :
Lead magnesium niobate Pb(Mg1/3Nb2/3)O3
(PMN), lead zinc niobate Pb(Zn1/3Nb2/3)O3
PZN, etc. The relaxor ferroelectrics are
characterized by very high dielectric constants
(e.g. Figure 8). This leads to very high strains.
Figure 7: Normalized polarization as a function
As mentioned earlier, the total strain produced
of N/f2 where N is the number of cycles and f the
on application of a field E = Ek in (say) Z
frequency for doped and Ce doped samples for
frequencies differing in three orders of magnitude;
direction is stated in the first expression on
the solid line is fitted to the model.
page 21.
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2
The Q constants are not very large for the
relaxor ferroelectrics - in fact they are smaller
Here dijk are the piezoelectric constants and Mykk
than the normal ferroelectrics. However,
are the electrostriction coefficients. To because the dielectric constants are very high,
understand the origin of high strains in the
the strains produced are 10-4 to 10-3, the highest
relaxor, we can write the strain in terms of
that are obtained in these materials (Table 1).
polarization:
xij = dijkEk + MykkEk
xij = gijkEijPk + QijklPkPl
Table 1: Comparison of electrostrains
achievable in different materials
The most accepted mechanism for the relaxor
behaviour is that there are microcompositional
fluctuations which produce a distribution of
Curie temperatures. Thus at any temperature,
the material is a mixture of the polar and the
nonpolar regions, becoming steadily more
polar as the temperature is decreased. It is the
interfaces between the polar and the nonpolar
regions that may hold the key to the extremely
high dielectric constants in these materials. The
theoretical and experimental investigations of
the relaxor behaviour is an active field of
research and a better understanding would lead
to more efficient, smarter materials.
Figure 8: Dielectric constant vs temperature for
Relaxor ferroelectrics as exemplified by the series
lead zinc niobate (PZN)-lead iron niobate (PFN);
dielectric constant values higher than those shown
here are achieved in many other relaxors
Table 1
Material
Non ferroelectrics
Normal ferroelectics
Relaxor ferroelectrics
Q
10
10-1
10-2
Dielectric constant
10
103
104
Strain
10-7
10-5
10-4
References:
·
Sambit K. Saha and D. C. Agrawal,
American Ceramic Society Bulletin, 71 (1992) 1424
·
S.B. Majumder, V.N. Kulkarni, Y.N. Mohapatra
and D.C. Agrawal, Bul. Mat. Sc., 17 (1994).
·
S. B. Majumder, Y. N. Mohapatra and
D. C. Agrawal, App. Phy. Let. 70 (1997)138
About the author: Dr. D. C. Agrawal is a professor in the Materials Science
Programme. His research interests include nanoparticle preparation and
their assembly, ceramic thin films and sol-gel processing.
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