1. FINAL PUBLISHABLE SUMMARY REPORT
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A summary of progress towards objectives and details for each task .
The automotive, aerospace, and power generating industries require piezoelectric sensors that can operate at high temperatures. The maximum operating temperature of piezoelectrics is limited by their Curie point and high temperature piezoelectric stability. Modified bismuth titanate compositions can be used for sensor applications up to 500°C. If a higher operating temperature is required there are currently no suitable polycrystalline ceramics available. The objective of this project was to systematically investigate the effect of doping on the properties (ferroelectric, piezoelectric, dielectric, Curie point, electrical conductivity, thermal depoling and ageing) of candidate Aurivillius phase ceramics. Spark Plasma Sintering (SPS) was used to produce highly textured microstructures with enhanced piezoelectric coefficients using two step method.
(i). Materials and characterization - Bismuth layer-structured ferroelectrics (BLFS)
Bi
2
WO
6
is the simplest member (n=1) of the BLSFs family, and has a high Curie point (above 900 o
C). At room temperature, Bi
2
WO
6
has the orthorhombic space group P2
1 ab,
7,8
with a= 5.457 Å, b=5.436 Å and c=16.427 Å. Bi
2
WO
6
has been considered as a high temperature piezoelectric materials because of its high Curie point and the high piezoelectric coefficients of single crystals (d
33
～ 40 pC/N)
9,10
Bi
2
WO
6
single crystals do not exhibit good saturated P-E hysteresis loops because of a high leakage current.
11
To our knowledge, no piezoelectric coefficient and saturated
P-E hysteresis loops have been reported for Bi
2
WO
6 ceramics. Spark Plasma Sintering
(SPS) is a new sintering process. Recently, SPS was used to fabricate some BLSF

ceramics, and proved to be a good sintering technique to obtain fully dense and textured BLSF ceramics.
12
Single phase bismuth tungstate (Bi
2
WO
6
) ceramics with high relative density (>99%) were fabricated by spark plasma sintering (SPS). Ferroelectric, dielectric and piezoelectric properties of Bi
2
WO
6 ceramics were investigated. Almost saturated polarization-electric field （ P-E ） hysteresis loops with a remnant polarization (P r
) of ～ 16.1

C/cm
2 and a coercive field (E c
) of ～ 3.7 kV/mm were obtained (Figure1).
Curie point and second phase transition temperatures were observed at 937 o
C and 665 o
C, respectively (Figure 2). The average piezoelectric constant (d
33
) of this high Curie point ceramic is 15.0

0.2 pCN
-1
.
(ii). Materials and characterization - B site doping
A
2
B
2
O
7
-type ferroelectrics belongs to the perovskite-like layered structure (PLS) family.
13, 14
Their structure is characterized by corner-shared BO
6 octahedra and
12-coordinated A cations within the perovskite-like layers, which are linked by A cations at their boundaries. The n in the formula is the number of octahedral layers in the perovskite layers. Compounds with n = 4, such as Nd
2
Ti
2
O
7
and La
2
Ti
2
O
7
, are ferroelectrics with potentially the highest Tc.
13, 14 These materials may therefore be good candidates for high-temperature piezoelectric and microwave dielectric applications because of their super-high Tc and low loss, respectively.
15, 16
PLS ferroelectrics ceramics have high coercive fields, which restrict the ability to pole them and, therefore, achieve their optimum piezoelectric properties. Just as for perovskite structured ferroelectrics, the addition of donor dopants reduces their coercive field. It has recently been reported that Nd donor doping of A-sites results in spontaneous polarization, Ps, along the c-axis, and enhanced ferroelectric properties, however, some of the results are inconsistent.
6
The Nd
2
Ti
1.95
Nb
0.05
O
7
and
Nd
2
Ti
1.9
Nb
0.1
O
7
were fabricated to investigate the effect of Nd-doping on Nd
2
Ti
2
O
7
.
In order to improve piezoelectric properties, the textured Nd-doping on Nd
2
Ti
2
O
7 ceramics was obtained using a two-step sintering method
(iii).High temperature properties – thermal depoling and ageing
Thermal depoling of poled ferroelectrics determines the upper temperature limit of their potential application as piezoelectrics.
17
In ferroelectric materials, domain wall movement makes a significant contribution to their piezoelectric response, 18 and during annealing produces thermal depoling. Domain wall movement can be influenced by internal stresses
12, 19 and point defects.
20, 21
Internal mechanical stress provides a driving force that assists thermal depolarization.
12, 19
It was found that

textured BLSFs ceramics, which have low internal stress, showed greater resistance to thermal depoling. 19 Point defects formed by vacancies or substituted ions tend to interact with the domain structure, which produce aging effects and inhibit the movement of domain walls at low temperature.
20, 21
However, there is very limited information on the effect of point defects on the thermal depoling of ferroelectrics.
The effect of point defects on the thermal depoling behavior of bismuth layer-structured ferroelectric ceramics fabricated by SPS was studied.The compounds include one-layer compound with orthorhombic structure, Bi
2
WO
6
(BW, m=1), five-layer compound with orthorhombic structure, Sr
2
Bi
4
Ti
5
O
18
(SBT, m=5). Defects dipoles in BW ceramics interact with its domain structure and inhibit the movement of domain walls at low temperature. These defect dipoles in BW ceramics produced pinched P-E hysteresis loops (Figure 3), and a reduction in piezoelectric coefficient d
33
after annealing from room temperature to 200 o C (Figure 4). The defect dipoles were completely decoupled and randomized by annealing at 200 o C, and the d
33 becomes stable with a further increase in temperature up to Curie point. SBT ceramics with low defect concentration show weak frequency dependence of dielectric constant and loss, symmetric P-E hysteresis loops and good piezoelectric stability with increasing temperature.
(iv). Spark Plasma Sintering–texturing ceramics
Spark Plasma Sintering (SPS) involves rapid heating (up to 500°C/min) combined with pressure (up to 100 MPa) during sintering. One of the advantages of the equipment is that it allows you to accurately control the temperature-pressure schedule to achieve highly textured ceramics in Aurivillius ceramics by a processing of dynamic forging.
Anisotropic Ca2Nb2O7 seed particles were synthesized by reacting a stoichiometric mixture of CaCO3 and Nb2O5 in NaCl-KCl at 1000 o
C for 4 hours. Textured
Ca
2
Nb
2
O
7 ceramics were prepared by mixing molten salt powder and solid reaction powder using a two step SPS process. The grain orientation factor, f, was 0.79, which is better than that prepared by solid reaction powder only.
Two step method was used to fabricate high textured ceramics by SPS. In the first step, the powders were sintered in a smaller graphite die to obtain high dense ceramic with minimum grain growth.. In second step, these sintered ceramics were placed in a larger graphite die to experience a hot-forge process to obtain high textured ceramics with high density. In order to understand the texture mechanisms during second step, creep behaviour of ferroelectric ceramics was investigated using SPS machine.

Exponent stress of ferroelectric ceramics at 1300 oC 1350 oC and 1375 oC are all above 1.8, which indicated that the creep of ferroelectric ceramics during second step is not only dominated by diffusion.
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