Characterization of alkali niobate single crystals prepared by the seeded polycrystalline conversion

1
Characterization of alkali niobate single
crystals prepared by the seeded polycrystalline
conversion methodAndreja Bencan, John Fisher,
Janez Holc, Marija KosecJozef Stefan Institute,
Ljubljana, Slovenia A. Sophie Vernay, Daniel
RytzForschungsinstitut fur mineralische und
metallische werkstoffe edelsteine/edelmetalle
GMBH, Idar-Oberstein, Germany
The most widely used piezoelectric ceramics today
are lead oxide based ferroelectrics, such as
(Pb,Zr)TiO3 (PZT) solid solution. Although PZT
exhibits high piezoelectric properties, the
toxicity of lead oxide has led to a demand for
alternative lead-free piezoelectric materials.
Among them, (K,Na)NbO3 has been considered as a
good candidate for lead-free piezoelectric
ceramics. The solid solution with the composition
K/Na 50/50, close to the morphotropic phase
boundary, exhibits a moderate dielectric constant
?300-800 and a piezoelectric response d3380-100
pC/N. In this study we report results on
microstructural investigations of Ka0.5Na0.5NbO3
(KNN) single crystals using analytical electron
microscopy. Crystal growth experiments have been
carried out on KNN samples using the seeded
polycrystalline conversion method (SPC).
Experimental procedure
Ceramics The conventional mixed oxide technique
was used to prepare the powders of
(K0.5Na0.5)NbO3 followed by calcination at 700?C
for 4 hours and hot pressed at 1040oC for 2
hours. Crystal growth experiments A KTaO3 seed
crystal 2mm?2mm?0.5mm was buried in KNN powder
and cold isostatically pressed, followed by
annealing at 1100oC for 10 hours. Some samples
were hot-pressed at 1050oC for 2 hours prior to
annealing in order to increase the density of the
grown crystal. Analytical electron microscopy a
JEOL 5800 scanning electron microscope (SEM) and
an analytical electron microscope (JEM 2010F)
were used. Both microscope are equipped with a
LINK ISIS 300 energy-dispersive X-ray
spectrometer (EDS). SEM Samples were thermally
etched and gold coated prior to analysis. XRD
analysis Philips PW 1710 X-ray diffractometer
with Cu K? radiation.
Results
Digital picture of the TEM sample sample was
prepared by mechanical thinning, dimpling and ion
milling using 3.8 keV argon ions.
KNN ceramics
Comments Since in the SPC method a dense matrix
is necessary to produce a dense single crystal
we tried first to hot press KNN. The calcined
powder was examined by powder X-ray diffraction
(XRD) and found to be single phase (Fig B).
Samples of (Na0.5K0.5)NbO3 with an Archimedes
density of 4.48 g.cm-3 (99.4 TD) have been
produced by hot pressing at 1040?C and 50 MPa for
2 hours (Fig A).
Fig B) X-ray powder diffraction pattern of
calcined KNN powder.
Fig A) SEM micrograph of KNN ceramics after hot
pressing at 1040oC for 2 hours.
Crystal growth experiments
Fig F) Line - EDS analysis across the seed and
single crystal for Ta and Nb.
Fig C) SEM Micrograph of the cross-section of the
sample after annealing at 1100oC for 10 hours.
The sample consists of the lt001 gt oriented KTaO3
seed crystal, a single porous crystal layer and
matrix grains which surround the single crystal
layer.
Fig D) SEM Micrograph of the cross-section of a
sample that was hot pressed at 1050oC for 2 hours
before annealing. A dense single crystal layer is
surround by matrix grains.
Comments Crystal growth experiments have been
carried out for the first time on KNN samples
using the seeded polycrystalline conversion
method (SPC). After annealing KNN powder with a
KTaO3 seed crystal embedded inside, a single
crystal approx. 100µm thick grew onto the seed
crystal (Fig C). If the sample was hot pressed
prior to annealing, a more dense single crystal
was obtained (Fig D). Transmission electron
microscope analysis has shown that the
orientation of the crystal is identical with the
orientation of the embedded seed (Fig E) and that
Ta has not migrated from the seed into the grown
crystal (Fig F) .
Fig E) high resolution TEM image of the KTaO3 /
single crystal boundary in Z110.
Acknowledgment
The study was done in the frame of the EU
co-operative research project IMMEDIATE.