REPOSITORY > RESULTS

Abstract

Sodium niobate (NaNbO3) is the end member of several alkaline niobate-based solid
solutions, which represent an important group of environment-friendly lead-free
piezoceramics. Even though alkaline niobates are of considerable technological
importance, problems in obtaining high densities and a fine-grained microstructure are
still reported and not much is known about the basic mechanisms that take place during
sintering. In addition, NaNbO3 is interesting in its pure form, being a prototype
antiferroelectric, exhibiting a complex, displacive phase-transition behaviour. Besides the
temperature- and electric-field-induced transitions, size-induced phase-transition
phenomena have been observed; however, controversy regarding the phase-transition
behaviour still remains in the literature. In order to investigate the above-described
problems, the present work was focused on the following topics: the preparation of
NaNbO3 powders, the sintering behaviour of NaNbO3 ceramics and an investigation of
the phase-transition behaviour of the obtained samples.
The submicron-sized NaNbO3 powder was prepared by solid-state synthesis, while for
the preparation of the nano-sized powder a top-down processing route was introduced,
combining solid-state synthesis and agitator bead milling. The optimized milling
parameters resulted in the production of the nanopowder with particle sizes of 25–30 nm
and a narrow particle size distribution, which was comparable to the results obtained by
other processing techniques based on the bottom-up approach, such as the solution-based
chemical routes or mechanochemical synthesis. The nanopowder exhibited better
compactability at isostatic pressures above 550 MPa than the submicron-sized powder,
which resulted in higher relative green densities. In all the powder compacts, uniform
pore-size distributions were observed with average pore-radius values of 4–10 nm and
20–30 nm for the nano- and the submicron-sized samples, respectively; however, the
pore-size distributions were much broader for the coarser powder.
The sintering behaviour of the submicron- and nano-sized NaNbO3 powder compacts
was studied using optical dilatometry and microstructure analysis. All the samples
exhibited similar dynamic sintering curves with a very narrow shrinkage interval just
before reaching the melting point at approximately 1410 °C. Despite the different
densification-onset temperatures, both samples reached the same relative density at 1280
°C and above this temperature their densification curves almost coincided, which
indicated that the initial differences in particle sizes disappeared before the samples
reached the main densification stage, as confirmed by the microstructure analysis. Similar
developments of the microstructure were observed in all NaNbO3 samples, regardless of
the initial powder particle size or the applied heating rate, which indicated the dominance
of the grain-growth processes during the initial sintering stage, well before the onset of
densification. The cause of this behaviour was found to be the surface diffusion that, due
to its estimated low activation energy of 50–60 kJ/mol, became activated already during
the initial sintering stage. As a non-densifying material-transport mechanism, the surface
diffusion initiated grain coarsening, reduced the sintering driving force and was thus
identified as the primary cause of the difficulties in obtaining high densities and finegrained
microstructures in NaNbO3. In addition, the investigations of the sintering
behaviour during the intermediate sintering stage indicated the dominance of the grainboundary
diffusion, which was related to the use of very fine particles. The results
obtained from the Knudsen effusion mass spectrometry indicated a negligible vapour
pressure of Na over NaNbO3 during the initial sintering stage, while the vapour pressure
increased to 1.6·10-5 bar at 1350 °C, which suggested that the material transport via the
vapour phase becomes increasingly important during the later stages of sintering.
In order to overcome the densification and coarsening problems, we applied pressureassisted
sintering using different temperatures, 1100 °C and 1150 °C, and different
periods of time, 3 h and 6 h. The application of the external pressure resulted in a
successful impediment of the surface diffusion due to the reduction of the total free
surface area during the initial sintering stage and the increased grain-boundary area,
which provided material transport paths for grain-boundary diffusion and thus enhanced
densification. The highest relative density (98 %) and the smallest average grain size
(0.70±0.29 μm) were obtained when sintering for 6 h at 1150 °C.
The phenomenon of abnormal grain growth (AGG) during conventional atmosphere
sintering of NaNbO3 ceramics was addressed. The samples sintered for 0–10 min at 1350
°C consisted of grains with average sizes of 2.5–2.9 μm and a uniform grain-size
distribution; however, once the sintering time was increased to 15 min, a few large grains
of several 10 μm were observed, while the grain size of the matrix remained as 2.8–2.9
μm. The AGG was related to the decreased quantity of pores and their sizes and the
resulting increase in the grain-boundary-migration velocity, which exceeded the poremigration
velocity and initiated the pore/grain-boundary separation. A further increase of
the sintering time resulted in a microstructure with an average grain size of 35–55 μm and
a relative density of 95–96 %.
The temperature-induced phase transitions were first re-examined on a coarse-grained
NaNbO3 ceramic (grain size of approximately 50 μm) using differential scanning
calorimetry (DSC), dielectric spectroscopy and high-temperature X-ray diffraction
(XRD). We confirmed the presence of the antiferroelectric P phase at room temperature,
which upon heating underwent the following transitions: 371.4 °C (P→R), 511 °C
(S→T(1)), 564.7 °C (T(1)→T(2)), and 642.4 °C (T(2)→U). In addition, another anomaly
was observed in the permittivity curve in the vicinity of 150 °C, but was not detected
using DSC and XRD. The detailed dielectric analysis revealed the frequency dependence
of the ε’’(T) peaks, which excluded the presence of a phase transition in this temperature
region.
The comparison of the coarse-grained ceramics and the submicron-NN powder
(particle size 0.07–0.17 μm) revealed a difference of about 50 °C in the temperatures of
the main transition peak, which was related to the stabilization of the ferroelectric Q
phase in the case of the submicron-NN, as confirmed by XRD. During heating, the
ferroelectric Q phase underwent a Q→R transition between 265 °C and 325 °C.
In order to investigate the size-induced phase transitions in NaNbO3 ceramics, we
prepared a series of samples with grain sizes ranging from about 0.15 μm to 50 μm. The
decrease of the grain size from 50 μm to 0.62 μm resulted in a decrease of the P→R
transition temperature. Upon decreasing the grain size to 0.23 μm, we observed the
occurrence of the ferroelectric Q phase, the amount of which increased with a further
decrease of the grain size to 0.19 μm, while the amount of the antiferroelectric P phase
decreased. At a grain size of about 0.15 μm, only the ferroelectric Q phase was detected.
The critical grain size for the inducement of the P→Q transition was found to be roughly
about 0.2 μm.
Finally, we investigated the phase transition induced by the application of an external
electric field. The antiferroelectric P phase was found to transform to the ferroelectric Q
phase upon applying an electric field of 8 kV/mm at 80 °C, as confirmed by the
occurrence of the ferroelectric polarization hysteresis loops, the butterfly-like strain
hysteresis loops and the XRD analysis. The obtained remanent polarization was 32
μC/cm2 and the d33 piezoelectric coefficient was 28 pC/N. The induced ferroelectric state
remained stable upon removal of the electric field and upon decreasing the temperature to
room temperature, which indicated the irreversible nature of this P→Q phase transition.