Takehisa Saito

Ferroelectric NaNbO3 Ceramics Fabricated by Spark Plasma Sintering

High-density ceramic samples of NaNbO3 (NN) have been successfully fabricated by spark plasma sintering (SPS), which is a process that uses microscopic electrical discharge between particles under pressure. The obtained NN ceramic shows a hysteresis loop characteristic of ferroelectrics. The coercive electric field, Ec, and remanent polarization, Pr, are determined to be 12 kV/cm and 0.42 µC/cm2, respectively, for an unpoled sample and 16 kV/cm and 27 µC/cm2, respectively, for a poled sample. The NN ceramic also shows piezoelectricity with a planar electromechanical coupling factor, Kp, of about 20%. The ferroelectricity and piezoelectricity in the NN ceramic are stable at high temperatures. The piezoelectric characteristic does not change after annealing at 300°C but it disappears after annealing at 350°C.

Pulsed laser deposition of high-quality (K, Na)NbO3 thin films on SrTiO3 substrate using high-density ceramic targets

High-quality (K,Na)NbO3 thin films were successfully deposited on a (100) SrTiO3 (STO) substrate by pulsed laser deposition. High-density KNbO3 (KN), (K0.5Na0.5)NbO3 (KNN) and NaNbO3 (NN) ceramic targets were prepared by spark plasma sintering (SPS). The crystallographic analyses of the film were performed by conventional X-ray diffraction (XRD) analysis and rocking curve measurement. The XRD reciprocal space map was also measured to determine the lattice constants of the film and analyze the crystallographic relationship between the grown film and the STO substrate. The fluctuation change in the orientation of crystals in the grown film decreased and the smoothness of the film surface improved with increasing sodium content of the film. For the NN films, the full width at half maximum (FWHM) of the rocking curve was as small as 0.12°. The XRD reciprocal space map measurements showed that the lattices of the KN and KNN films relaxed on the STO substrate but the NN film was restricted to it.

Pulsed-Laser Deposition of Ferroelectric NaNbO3 Thin Films

High-quality NaNbO3 (NN) thin films were epitaxially grown on a (100)SrTiO3 substrate by pulsed-laser deposition. NN films with a flat surface morphology were obtained, when the films were grown slowly at high substrate temperatures, high oxygen partial pressures, and low laser energy densities. In order to characterize their dielectric properties, the NN films were epitaxially grown on (100)SrRuO3∥(100)SrTiO3 substrates. The relative dielectric constant, er, and dielectric loss, tan δ of the film were 252 and 0.03 at 1 kHz, respectively. When the temperature dependence of er was measured, er was found to suddenly increase at 377°C, which corresponds to the transition temperature of NN between the antiferroelectric and paraelectric phases. The P–E hysteresis loop of the NN films exhibited characteristic ferroelectric behavior.

Fabrication of Lead-Free Piezoelectric KNbO3 Ceramics by Modified Solid State Reaction Method

Potassium niobate (KNbO3) fine powder has been successfully synthesized at a low temperature of 500 °C through a modified solid state reaction, in which urea [CO(NH2)2] plays an important role. High-density KNbO3 ceramics can be obtained by the conventional sintering of the synthesized KNbO3 fine powder with the addition of 1 mol % Co3O4, which is a sintering additive. Their crystal structure, microstructure, and dielectric and piezoelectric properties were characterized. The KNbO3 ceramics show a comparatively saturated P–E hysteresis loop. The remnant polarization and coercive field are 14 µC/cm2 and 16 kV/cm, respectively. The KNbO3 ceramics also show piezoelectricity with a planar electromechanical coupling factor, kp, of 16%.

Pulsed laser deposition of ferroelectric (Na0.5K0.5)NbO3-based thin films

Ferroelectric (Na0.52K0.44Li0.04)(Nb0.84Ta0.10Sb0.06)O3 thin films were epitaxially grown on a (100)SrRuO3∥(100)SrTiO3 substrate by pulsed laser deposition. Crystallographic analysis of the film was performed using conventional X-ray diffraction analysis and rocking curve measurements. High resolution X-ray diffraction reciprocal space map was also measured to analyze the crystallographic relationship between the grown film and the SrTiO3 substrate and to determine the strain state of the film. The full width at half maximum of the rocking curve was as small as 0.19° and the determined pseudo-tetragonal lattice parameters were a=3.947 A and c=3.955 A. The P–E hysteresis loop of the film was characteristic of ferroelectric behavior.

Fabrication of lead-free piezoelectric (Na 0.5 K 0.5 )NbO 3 ceramics by a modified solid-state reaction method

Sodium potasium niobate, (Na_0.5K_0.5)NbO₃ fine powder has been successfully synthesized at a low temperature of 550degC through a modified solid state reaction, in which urea [CO(NH₂)₂] plays an important role. High-density (Na_0.5K_0.5)NbO₃ ceramics can be obtained by a conventional sintering with an addition of 0.03 mol% Co₃O₄. Their crystal structure, microstructure, and dielectric and piezoelectric properties were characterized. The (Na_0.5K_0.5)NbO₃ ceramics show a comparatively saturated P-E hysteresis loop. The (Na_0.5K_0.5)NbO₃ ceramics also show piezoelectricity with a piezoelectric constant, d₃₃, of 126 pC/N and a planar electromechanical coupling factor, k_p, of 33%..

Observation of domain structure in 001 orientated NaNbO3 films deposited on (001)SrTiO3 substrates by laser beam scanning microscopy

A laser beam scanning microscope (LSM) was used to observe the domain structure of 001 orientated NaNbO3 (NN) films on SrRuO3/(001)SrTiO3 substrates. The LSM micrograph clearly showed the domains and the domain walls of NN film. Using a polarization plate revealed that the deposited NN film had antiferroelectric 90° domains separated by {100} domain walls. When 001 orientated NN film was applied by an electric field, the 90° domains and the {100} domain walls disappeared and a new domain structure and {110} domain walls were created. This drastic change is due to the formation of new 60° domains.

Preparation of ferroelectric NaNbO 3/thin films on MgO substrate by pulsed laser deposition

We successfully fabricated good quality NaNbO3 (NN) films on MgO substrate by pulsed laser deposition using a K-Ta-O (KTO) buffer layer. An SrRuO3 (SRO) lower electrode layer was deposited on a KTO buffer layer/(100)MgO substrate and then the NN film was deposited on top. X-ray diffraction showed that the SRO and NN films were epitaxially grown on (100)MgO substrate. Transmission electron microscopy showed a crystal- lographic relationship of [001]NN//[001]MgO between NN and MgO. The relative dielectric constant, epsivr, and dielectric loss, tan delta, of the film were 350 and 0.05 at 1 kHz, respectively. The polarization vs. electric field (P-E) hysteresis loop of the NN film was characteristic of ferroelectric behavior.

Preparation of Ferroelectric NaNbO 3 Thin Films on MgO Substrate by Pulsed Laser Deposition

We have successfully fabricated good quality NaNbO₃ films on MgO substrate by pulsed laser deposition by using K-Ta-O (KTO) buffer layer. SrRuO₃ (SRO) lower electrode layer was deposited on KTO/(100)MgO substrate and then the NaNbO₃ film was deposited. X-ray diffraction showed that the NaNbO₃ film was epitaxially grown on a (100)SRO/KTO/(100)MgO substrate. The <i>P-E</i> hysteresis loop of the NaNbO₃ film was characteristic of ferroelectric behavior. The crystallographic and dielectric properties are discussed with a comparison of those of the NaNbO₃ films epitaxially grown on (100)SrTiO₃ substrate.

Fabrication of Lead-Free Piezoelectric (Na 0.5 K 0.5 )NbO 3 Ceramics by Modified Solid State Reaction Method

Sodium potassium niobate, (Na_0.5K_0.5)NbO₃, fine powder has been successfully synthesized at the low temperature of 550degC through a modified solid-state reaction method, in which urea [CO(NH₂)₂] plays an important role. High-density (Na_0.5K_0.5)NbO₃ ceramics could be obtained by conventional sintering of the synthesized (Na_0.5K_0.5)NbO₃ fine powder with the addition of 0.03 mol% Co₃O₄ as a sintering additive. The crystal structure, microstructure, and dielectric and piezoelectric properties were characterized. The (Na_0.5K_0.5)NbO₃ ceramic showed a comparatively saturated P-E hysteresis loop. The (Na_0.5K_0.5)NbO₃ ceramic also displayed piezoelectricity with a piezoelectric constant d₃₃ of 126 pC/N and a planar electromechanical coupling factor k_p of 33%.