REPOSITORY > RESULTS

Abstract

A starting powder with the nominal composition 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (0.65PMN–0.35PT),
which corresponds to the morphotropic phase boundary (MPB) composition, was prepared by
mechanochemical activation followed by attritor milling and low-temperature calcination at 700°C. It had a
narrow particle size distribution with the median particle size equal to 0.3 μm. From the synthesized
powder dense bulk ceramics of 98% of theoretical density were prepared by firing at 1200°C for 2 hours.
The influence of the poling electric field on the electromechanical properties and the phase composition of
0.65PMN−0.35PT ceramics was studied. The phase ratio of the monoclinic Pm to the tetragonal P4mm
phases is changed by the application of a poling electric field and the extent of this change is dependent on
the value of the electric field. This phenomenon was not yet reported for 0.65PMN−0.35PT ceramics. The
piezoelectric coefficient and the coupling coefficients are the highest for ceramics poled at electric fields
between 2 kV/mm and 3.5 kV/mm. This corresponds to largest amount of monoclinic Pm phase in
ceramics.
Thick films with the composition of 0.65PMN–0.35PT were made by screen-printing and firing of the
paste prepared from an organic vehicle and pre-reacted powder. The films were fired for 2 hours at 950°C
on alumina (Al2O3), platinum (Pt), 0.65PMN–0.35PT and aluminium nitride (AlN) substrates. Alumina and
platinum substrates have a higher temperature expansion coefficient (TEC) than 0.65PMN−0.35PT films,
aluminium nitride substrates have a lower TEC, and the TEC of 0.65PMN−0.35PT substrates is the same as
the TEC of the films. Under identical processing conditions the microstructures, phase compositions,
dielectric, ferroelectric and piezoelectric properties strongly depend on the choice of the substrate material.
If the thick films are after firing and cooling under compressive stresses, i.e, films on substrates with higher
TEC, films are dense with grain sizes 1.7 μm on alumina substrates and 1.2 μm on platinum substrates. The
X-ray analyses and Rietveld refinements showed the coexistence of monoclinic and tetragonal phases.
Dielectric constants were 3400 and 2000, piezoelectric coefficients d33eff were 180 pC/N and 140 pC/N and
remanent polarizations were 22 μC/cm2 and 27 μC/cm2 for films on Al2O3 and Pt substrates, respectively.
The electrical properties of films on Al2O3 and Pt substrates and also the coexistence of tetragonal and
monoclinic phases are comparable to results obtained on densly sintered bulk ceramics. The exception is
the lower piezoelectric constant of films, which is partly due to the clamping of the films on rigid
substrates. Films fired and cooled on substrates with lower TEC (AlN) or with the same TEC
(0.65PMN−0.35PT) were porous with grain sizes below or equal to 0.5 μm. In this case we presume that
the tensile stresses were relaxed during cooling and the characteristics are therefore similar for films on
AlN as for stress free films on 0.65PMN−0.35PT substrates. For these films the X-ray analyses and
Rietveld refinements showed the monoclinic phase. The dielectric constants were 700 and 300,
piezoelectric coefficients d33eff were 105 pC/N and 30 pC/N and remanent polarizations were 8 μC/cm2 and
3 μC/cm2 for films on 0.65PMN−0.35PT and AlN substrates, respectively, which is a consequence of the
porous structure. These results indicate that substrates strongly influence the densification and
microstructural characteristics of thick films and, consequently, the structural and electrical properties.
The electrostrictive effect in 0.65PMN−0.35PT films on alumina substrates was measured. The
electrostrctive coefficient M33 of 0.65PMN–0.35PT thick films on alumina substrates was high, i.e.,
7.6·10−16 m2/V2. Due to high electrostriction of thick 0.65PMN−0.35PT films, we decided to prepare the
0.65PMN–0.35PT thick-film actuators.
A novel approach to preparing large-displacement “substrate-free” 0.65PMN–0.35PT/Pt actuators was
developed. After screen printing and firing the 0.65PMN–0.35PT/Pt composites were peeled off from the
alumina substrates. The normalized displacements of 55 μm/cm were achieved for actuators with
dimensions of 1.8 cm × 2.5 mm × 65 μm, which is 5–10 times higher in comparison with the normalized
displacements of bulk PMN–PT or thick-film Pb(Zr,Ti)O3 (PZT) actuators.