Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications
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Handbook of dielectric, piezoelectric and ferroelectric materials920
CN cationic sites. Two different kinds of behaviour appeared in the thermal
evolution of
′
ε
r
versus frequency. For 0 ≤ x < 0.075, there was one relatively
sharp peak of
′
ε
r
, no dependence on frequency at T
C
and a shoulder in the
slope with a dispersion of
′
ε
r
and an increase of
′
T
m
with frequency. As an
example the result for a ceramic corresponding to x = 0.040 is reported in
Fig. 30.21: the behaviour is of ferroelectric type. For 0.075 ≤ x ≤ 1, only one
peak was observed, showing the typical relaxor
′
ε
r
= f (T, f) dependences. In
addition, there was a deviation from the Curie–Weiss law and a strong dielectric
dispersion was evidenced, leading to a Vogel–Fulcher behaviour [6, 88]. All
these dielectric characteristics are typical of relaxor behaviour [1]. Figure
30.22 shows the variations in T
C,
′
T
m
and
′′
T
m
with the composition at 1kHz.
′′
T
m
decreased from x = 0.075 to x = 0.8. For 0.8 < x ≤ 1, the values of
′′
T
m
are not plotted, due to low-temperature experimental limitations.
Examination of Fig. 30.22 shows that for a relatively low Zr–Ti substitution
rate, the value of T
C
is decreasing while that of
′
T
m
is increasing up to a
composition x ≈ 0.07, beyond what remains a single effect at
′′
T
m
between a
relaxor state and a paraelectric one. This value of
′′
T
m
slowly decreases to
values close to 100K when x further increases. The very peculiar features of
the phase diagram displayed on Fig. 30.22 can be pointed out as follows. For
compositions close to Ba
0.9
Bi
0.067
TiO
3
, i.e x ≤ 0.07, the ceramics undergo
two phase transitions on cooling. The first, at T
C
, is from a paraelectric to the
ferroelectric state similar to BaTiO
3
. The second one, which occurs within
the ferroelectric state, is of relaxor type. This is very unusual in relaxors
where the succession of transitions is always in the thermodynamically
favourable sequence on cooling: paraelectric → relaxor → ferroelectric. The
latter ordering transition can be either electric field induced as in PMN [89,
90] or chemically driven as in PSN and PST [91]. The very peculiar behaviour
of Ba
0.9
Bi
0.067
(Ti
1–x
Zr
x
)O
3
is ascribed to the dual substitution on the A and B
sites. The Bi substitution on the A site leads to the low-temperature relaxor
state while the Zr substitution on the B site keeps the high-temperature
ferroelectric state at low rates. This special chemistry explains the apparent
contradiction of such results with thermodynamics which always calls for
ordering on cooling. Detailed investigations of the possible segregation of
Table 30.6
Values of
T
m
, ∆
T
m
and
∆
′′
εε
rr
/
for ceramics
with composition Ba
1–
x
Bi
2
x
/3
TiO
3
xT
m
(K) at 1 kHz ∆
T
m
(K)
∆
′′
εε
rr
/
0.090 390 2 0.050
0.110 365 5 0.045
0.135 328 10 0.120
0.150 288 26 0.325
WPNL2204
Lead-free relaxors 921
both substituted cations on nanoscale are needed to clarify the microscopic
origin of the transition sequence.
With a view to future applications in electronic components, attention
was focused on the ceramics with composition Ba
0.9
Bi
0.067
(Ti
0.92
Zr
0.08
)O
3
.
Its relaxor characteristics, reported in Table 30.7, show that high permittivities
50 100 150 200 250 300 350 400 450
T
(K)
(a)
Relaxor
Ferroelectric
Paraelectric
0.1 kHz
0.5 kHz
1 kHz
5 kHz
10 kHz
50 kHz
100 kHz
200 kHz
′
ε
r
4000
3500
3000
2500
2000
1500
1000
500
0
50 100 150 200 250 300 350 400 450
T
(K)
(b)
Relaxor
Paraelectric
0.1 kHz
0.5 kHz
1 kHz
5 kHz
10 kHz
50 kHz
100 kHz
200 kHz
1500
1000
500
0
′
ε
r
30.21
Temperature dependence of
′
ε
r
for a ceramic with composition
Ba
0.9
Bi
0.067
(Ti
0.96
Zr
0.04
)O
3
(a) or Ba
0.9
Bi
0.067
(Ti
0.85
Zr
0.15
)O
3
(b).
WPNL2204
Handbook of dielectric, piezoelectric and ferroelectric materials922
are achieved close to room temperature for this composition. In addition,
there is also a strong deviation from the Curie–Weiss law (
′′
T
m
< T
0
)
characteristic of dipole interactions responsible of some type of short range
order. The large curvature of
1/
r
′
ε
around
′′
T
m
is in good agreement with a
strongly diffuse phase transition. The value of the frequency dispersion
∆
′′
εε
rr
/
at a normalised temperature T =
′′
T
m
– 60K is about 0.091. The good fit of
the log f =
fT()
m
′′
data to the Vogel–Fulcher law leads to the value T
VF
= 238
± 10K. It appears that such a composition satisfies the criteria for applications:
• The value of
′′
T
m
is next to room temperature, allowing relaxor properties
to be used in normal conditions.
• The values of
∆
′′
T
m
and that of
∆
′′
εε
rr
/
are high (∆T
m
= 32K,
∆
′′
εε
rr
/
=
30.22
Variations of
T
C
,
′
T
m
and
′′
T
m
, at 1 kHz, for ceramics with
composition Ba
0.9
Bi
0.067
(Ti
1–
x
Zr
x
)O
3
.
′′
T
m
T
(K)
500
400
300
200
100
0
′
T
m
T
C
′′
T
m
0.00 0.05 0.10 0.15 0.20
x
T
C
T
(K)
500
400
300
200
100
0
′
T
m
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
x
Table 30.7
Some relaxor characteristics of a ceramic with
composition Ba
0.9
Bi
0.067
(Ti
0.92
Zr
0.08
)O
3
f
(kHz)
′′
T
m
(K)
′′′
ε
rmax. m
at T
FWHM (K)
0.1 255 3930 227
1 264 3810 224
10 275 3650 221
100 287 3470 219
FWHM: full width at half maximum.
WPNL2204
Lead-free relaxors 923
0.91); they are higher than those of PMN ceramics (∆T
m
= 15K, ∆
′′
εε
rr
/
= 0.19), usually used for applications.
• The value of
′
ε
r
lies between 3400 and 4000; these values are promising.
Current studies will lead to an increase in these values, thanks to an
optimisation of processing parameters.
Other compositions have also been studied in the BaO–Bi
2
O
3
–TiO
2
ternary
system [92].
30.6.1 Ceramics of TTB type
From a crystallographic point of view, it is interesting to note the two
compositions BaA䊐Nb
5
O
15
(A = La, Bi) from the TTB family (Table 30.4),
in which only two over three of the large A and B sites (15 and 12-CN) are
occupied [93]. Such a composition type is original since it has one vacancy
in the (2A + B) sites and the two C sites of 9-CN are also empty compared
with the general formulation A
2
BC
2
M
5
O
15
.
Table 30.8 shows the dielectric properties of some lanthanum and bismuth
relaxor compositions in which the value of T
m
increases systematically from
lanthanum to bismuth. This increase becomes greater as the amount of A
3+
cation increases compared with that of Ba
2+
:
• BaLaNb
5
O
15
(205 K) and BaBiNb
5
O
15
(295K) (there are as many A
3+
as
Ba
2+
cations).
• Ba
2
La(Nb
3
Ti
2
)O
15
(195K) and Ba
2
Bi(Nb
3
Ti
2
)O
15
(185K) (there are fewer
A
3+
than Ba
2+
cations).
The value of T
m
decreases as the ratio A
3+
/Ba
2+
decreases but this also
occurs simultaneously as the relative amount of Ti
4+
increases (it is thus
difficult to attribute this phenomenon to one or the other type of composition
replacement). When K
2
ANb
5
O
15
and BaANb
5
O
15
are compared, the value of
T
m
is found to decrease from the barium to the potassium compounds. This
Table 30.8
Relaxor characteristics of some TTB ceramics
T
m
(K) at 1 kHz (±10 K) ∆
T
m
(K)*( ±10 K)
K
2
LaNb
5
O
15
165 20
K
2
BiNb
5
O
15
225 80
BaLa䊐Nb
5
O
15
205 55
Ba
1.5
La䊐
0.5
(Nb
4
Ti)O
15
190 35
Ba
2
La(Nb
3
Ti
2
)O
15
185 20
BaBi䊐Nb
5
O
15
295 75
Ba
1.5
Bi䊐
0.5
(Nb
4
Ti)O
15
255 65
Ba
2
Bi(Nb
3
Ti
2
)O
15
200 30
∆
T
m
=
T
m
(200 kHz) –
T
m
(0.1 kHz)
WPNL2204
Handbook of dielectric, piezoelectric and ferroelectric materials924
is also accompanied by a decrease in the ratio from 1A
3+
/1Ba
2+
to the ratio
1A
3+
/2K
+
. All these results show the favourable influence of these A
3+
cations
on the highest value of T
m
.
Of all the lead-free TTB compounds studied, the highest value of T
m
was
obtained for BaBiNb
5
O
15
(T
m
= 295K, at 1 kHz). In addition, independently
of the value of T
m
, the bismuth compounds allow the highest values of ∆T
m
:
∆T
m
= 80 and 75 K for the potassium and barium niobates, respectively.
Such a result is in good agreement with that obtained for BaBiNb
5
O
15
[71].
30.7 Conclusions
In each lead-free family investigated, i.e. perovskite or TTB-type, the relaxor
behaviour appears when at least two different cations occupy the same
crystallographic site. This condition is necessary but not sufficient. The lack
of relaxor effect in perovskite BaMg
1/3
Nb
2/3
O
3
is a good example. One of
the M
n+
cations in the octahedral site has to be ferroelectrically active (e.g.
Ti
4+
, Nb
5+
or Ta
5+
). In any case, a local polarisation leads to a relaxor behaviour
while a macroscopic one gives rise to a ferroelectric ordering.
Lead-free perovskites derived from BaTiO
3
are solid solutions. The only
local polarisation comes from dilution of the ferroelectric character. The
relaxor effect is greater if the substitution affects the octahedral site. The
higher the substitution rate, the greater the relaxor effect. For TTB-type
compounds (not only solid solutions), the effect is due either to a cationic
disorder in the A or B site or to a dilution of the ferroelectric character by
cationic or anionic substitution, on any crystallographic sites.
The increase of substitution rate from either BaTiO
3
or a TTB ferroelectric
oxide leads to an increase of the main relaxor dielectric characteristics but
also a decrease of T
C
, then of T
m
. A continuous change from ferroelectric to
relaxor either driven by composition or temperature variation was evidenced.
It reflects the ionic feature of lead-free solid solutions (contrary to lead-
containing compounds). The corresponding studies were helpful to understand
the physics of such relaxors.
Among all the compounds investigated, those containing bismuth possess
simultaneously the highest value of T
m
(close to room temperature) and the
best relaxor characteristics. Such ceramics present a real interest for applications
as a lead-free dielectric for capacitors or actuators.
30.8 References
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WPNL2204
Lead-free relaxors 925
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WPNL2204
Handbook of dielectric, piezoelectric and ferroelectric materials926
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–CaTiO
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–BaZrO
3
–BaLiF
3
system’, Mater. Lett., 2000 42
189–193.
WPNL2204
Lead-free relaxors 927
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TiO
3
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Zr
0.35
)O
3
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Ti
0.75
Zr
0.25
O
3
compositions: influence of electric field’, Solid State
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–
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3
–CaTiO
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54. Aydi A., Khemakhem H., Boudaya C., Von Der Mühll R. and Simon A., ‘New
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’,
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61. Kherfah A., Taibi K., Guehria–Laidoudi A., Simon A. and Ravez J., ‘New oxyfluoride
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composition Ba
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Ca
0.08
Ti
0.75
Zr
0.25
O
3
with both ferroelectric and relaxor properties’,
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Handbook of dielectric, piezoelectric and ferroelectric materials928
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