b The present work shows that the observed HR-XRD 002 RSMs of PZN-xPT single crystals, 0.045 ≤ x ≤ 0.09, can be understood from the micro- and nanotwin structures of both R and T phase
Trang 1Chapter 10 Conclusions
Based on the results presented in the previous chapters, the following conclusions
can be drawn concerning the phase and domain structures in unpoled (annealed)
PZN-(4.5-9)%PT single crystals
(a) The surface layer of carefully-polished PZN-PT single crystals is covered with a
deformed layer and is not suitable for typical XRD study A fracture technique
has been devised which exposes the relatively strain-free crystal bulk for the
XRD study
(b) The present work shows that the observed HR-XRD (002) RSMs of PZN-xPT
single crystals, 0.045 ≤ x ≤ 0.09, can be understood from the micro- and
nanotwin structures of both R and T phases in the material
(c) For PZN-xPT of lower PT contents (i.e x ≤ 0.07) at room temperature, the
{100}R diffractions manifest as an extremely broad peak at 2θ = 44.50-44.65°
Bragg’s position, being the convoluted peak of the four degenerated R
microtwins In addition to the extremely broad convoluted R peak, nanotwin
diffractions arising from {100}-type and {110}-type R nanotwins have also been
Trang 2detected in selected samples, suggesting that the R phase exist in a mixture of
micro and nanotwins in PZN-(4.5-8)%PT single crystals at room condition
(d) In addition to the R diffractions, T micro- and nanotwin domains could also be
detected in PZN-(6-8)%PT single crystals at room condition Our HR-XRD
RSM results indicate that the T phase in these crystal compositions is likely to be
a metastable phase stabilized by the cooling-cum-transformation stresses in the
crystal This is manifested by the disappearing of the (100)T diffractions on the
fracture surface, a result attributed to the stress relaxation effect in the surface
layer of the fractured crystal Such stress relaxation effects are expected to be
more pronounced for the R+T domains with their {110} R//{110}T interface lying
at about 45° to the (100)pc diffraction planes
(e) For PZN-9%PT, the dominant phase at room condition is the T phase of both
micro- and nanotwin domains, although a small amount of the R phase may also
be present
(f) Strong evidence of T nanotwins, manifested by their streaked diffractions, has
been detected in PZN-(4.5-8)%PT in the vicinity of T R-T phase transformation,
suggesting that it presence being promoted by the transformation stress possibly
as a means to relax the transformation stresses in the crystal
Trang 3(g) A revised phase diagram for the PZN-PT system has been constructed Two new
evident features of this revised phase diagram are: (a) the expanded (R+T)
two-phase MPB region, and (b) a (T+C) two-two-phase region at high temperature before
the crystal transforms completely into the single C phase
(h) The expanded (R+T) MPB region can be further divided into two regions In the
lower PT region, 0.06 ≤ x ≤ 0.08, the T phase is metastable stabilized by the
residual stress in the crystal In the high PT region, 0.09 ≤ x ≤ 0.10, both the
(R+T) phase are thermodynamically stable phases at room temperature
(i) The ease with which perovskite crystals may form micro- and nanotwins may
play an important role in the reported superior piezoelectricity of PZN-PT and
PMN-PT single crystals, especially near their MPBs
(j) This present work does not support the existence of M phases in PZN-PT single
crystals considering the transformations from M to C, R to M C , and M B to T are
not allowed in perovskite structure as according to Landau theory In addition,
experimental analysis involving polarization and structural characteristics
suggests that the out-of-plane diffractions pertained to coherent diffraction
phenomenon associated with micro/nanotwin of R and T
Trang 4Chapter 11 Recommendations for Future Work
The present work has shown that micro- and nanotwin domains in both the R
and T phases serve to relax the transformation stresses leading to an expanded MPB
across 0.06 ≤ x ≤ 0.10 in PZN-xPT single crystals This expanded (R+T) MPB region
can be further divided into two different regions In the low PT region (i.e., 0.06 ≤ x ≤
≈ 0.08), the room temperature T phase is metastable stabilized by the residual stresses
in the material In the high PT region (i.e., 0.09 ≤ x ≤ 0.10), both the R and T are
thermodynamically stable phases The presence of (R+T) micro- and nanotwin
domains is believed to play a role in the superior piezoelectricity of relaxor
ferroelectric single crystals In order to provide a better understanding of the
structural-property relationship in relaxor ferroelectric single crystals, the following topics are
recommended for future studies:
(a) The fine (002) XRD spectra obtained from SSLS provide circumstantial evidence
for the presence of R micro- and nanotwin domains in the PZN-PT single crystals
Despite so, the ultra fine nanotwin structure in R, are difficult to resolve with
high-synchrotron x-ray as available in SSLS Higher energy x-ray of improved
Trang 5resolution is required to resolve these fine nanotwin diffractions and to decipher
the detailed configurations of the R* domains, which are expected to shed light
on how the “engineered domain state” would give rise to enhanced properties of
the material In addition, higher energy x-ray with larger penetration depth
provides the advantage to detect the (001)T domains in PZN(6-8)%PT crystals,
which are shadowed by the R domains upon fracturing
(b) To investigate the effects of temperature and E-field on the R and T micro- and
nanotwin domains in relaxor single crystal and how this may affect the
structure-property-relation of the crystal
(c) To investigate how the R and T micro- and nanotwin domains are affected by
different crystal cuts, e.g (011) cut, etc., and this may affect the
structure-property-relation of different crystal cuts
(d) Combined state-of-the-art experimental analysis and computational modeling
(e.g., first-principle calculations and three-dimensional numerical models for the
nano-domain structure of relaxor materials) are of extreme value to provide
fundamental insight into the hierarchy of domain structures and its role in the
superior piezoelectricity of relaxor ferroelectric single crystals It is
recommended that such an approach be pursued in future work
Trang 6References
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