Relaxor-PbTiO3 single crystals and polycrystals: processing, growth and characterisation

Acoustic transducers operate using polycrystalline lead zirconate titanate (PZT) since
the 1950s’. Recently, relaxor-lead titanate (relaxor-PT) single crystals have been developed
and exhibit up to 650% higher piezoelectric charge coefficient and up to
50% higher electromechanical coupling factor, compared with PZT. Transducers built
with relaxor-PT crystals show increased bandwidth, lower power consumption and
increased sensitivity. Despite the significant advantages over PZT, a growth method
for relaxor-PT single crystals that is both economical and able to produce homogeneous,
highly dense crystals is yet to be found. Furthermore, one of the most important
relaxor-PT solid solution is Mn modified Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-
PbTiO3 due to its thermal stability and low electrical and mechanical losses compared
with other relaxor-PT solid solutions. However, the behaviour of Mn in this compound
is not fully understood and control of properties, such as the mechanical quality factor,
is difficult. The aim of this study was to compare Bridgman and Solid State techniques
for growth of single crystalline Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3.
Subsequent to this, several Mn modified compositions were designed and investigated
to determine the mechanisms by which mechanical and electrical losses are lowered
upon Mn incorporation. Bridgman experiments have shown that a 20 mm ø x 60 mm
length and highly dense single crystal can be obtained, with (011) the natural growth
direction. Seeded Bridgman was also investigated as a method of controlling the orientation
of the grown crystal, but was proven challenging due to nucleation of several
crystallites. Solid State experiments, which involve attaching a single crystal seed to
a polycrystalline matrix and promoting boundary migration of the seed into the matrix,
showed that crystal growth is encouraged when a Pb-based interlayer exists in
between the seed and matrix. An epitaxially deposited, uniform thin film as interlayer
was found particularly beneficial. More research is needed to determine conditions for
growth of a single crystal of useful size by Solid State. Analysis of several relaxorPT
compositions modified with the same amount of Mn revealed that alterations of
the relaxor-PT formulation affects behaviour of the multi-valent Mn which in turn in-
fluences electromechanical properties. These findings are of use to the industrial and
scientific communities. Crystal growth results indicate Bridgman as suitable method
if growth of crystals is desired in a short time frame, whilst Solid State results provide
the basis of a new approach for growing relaxor-PT crystals. The study of Mn modified
compositions provides new insights into the role of Mn substitution for manipulating
the electrical and mechanical properties of complex, relaxor-PT solid solutions.