Table of contents

The c-axis oriented polycrystalline thin film of (Na,Bi)TiO₃–BaTiO₃ (NBT–BT) around the morphotropic phase boundary (MPB) was prepared on an LaNiO₃-buffered Si substrate by rf magnetron sputtering. The NBT–BT film showed a large crystal lattice distortion in the out-of-plane direction, and a voltage shift of the hysteresis loop along the negative field axis as large as −200 kV/cm, which indicates that the film is internally biased and strongly self-polarized towards the top electrode. Owing to the large internal bias field, the NBT–BT film exhibited a linear piezoelectric response with the piezoelectric coefficient, $e_{31}^{*}$, reaching −4.8 C/cm² in the unipolar excitation and a low dielectric permittivity of 230. The temperature-dependent dielectric properties revealed that the permittivity maximum temperature, T_m, of the NBT–BT film was significantly enhanced to ∼550 °C from the ∼300 °C of bulk NBT–BT, accompanied by the disappearance of the depolarization temperature, T_d, which is confirmed by the structural data where the crystal lattice remained unchanged up to ∼400 °C. These stable temperature properties would lead to an expansion of the temperature range of use of NBT–BT film.

Sr₂Bi₄Ti₅O₁₈ (SBTO) films with a- and b-axis orientations, and thicknesses of 0.9–1.2 µm were sputter-deposited on conductive Nb:TiO₂(101) substrates containing 0.79 mass % Nb. The deposition temperature was varied from 575 to 700 °C under a fixed gas pressure of 0.4 Pa, and the structural and ferroelectric characteristics of the films were investigated. SBTO films deposited at 625–700 °C had a mostly single-phase orthorhombic structure, with a high degree of a- and b-axis orientations [α_(h00)/(0k0)] of 99.0–99.8%. In addition, the full width at half maximum of the (200) diffraction peak was 0.69–0.86°, which indicated good crystallinity. SBTO films deposited at 625–650 °C had a nanoplate-like microstructure with the plates aligned along the [010] direction. The real room-temperature remanent polarization $(2P_{\text{r}}^{*})$, taking the porosity between the nanoplates into account, exhibited a maximum of 40 µC/cm² at 650 °C. Thus, the optimal deposition temperature for heteroepitaxial growth of SBTO nanoplates with a high α_(h00)/(0k0) of ≥99.0% and excellent ferroelectric properties on conductive Nb:TiO₂ substrates is 650 °C under a gas pressure of 0.4 Pa.

Single-crystalline BaTiO₃ nanocubes were synthesized by a hydrothermal method using a water-soluble titanium complex and surfactants. Ordered assemblies of BaTiO₃ nanocubes were directly fabricated on substrates by the dip-coating method. To optimize the conjugation of the nanocubes, the sintering temperature was changed in the range of 750 to 900 °C to compare with the dielectric properties. The microstructures and dielectric properties of BaTiO₃ nanocube assemblies sintered at various temperatures were characterized. The structure–property relation and the sintering temperature dependence of the dielectric properties are discussed. The assembly sintered at 850 °C showed the most enhanced dielectric properties. The face-to-face conjugation of the nanocubes was completed and there was good adhesion between the electrode and the assembly at 850 °C. The enhanced dielectric properties were considered to be due to the combination of the single-crystalline nanocubes and the interface between them.

The effect of the heat treatment conditions on the constituent phases and electrical properties of (Hf_0.5Zr_0.5)O₂ films deposited by the metalorganic chemical vapor deposition was investigated. By using a low temperature or short duration for post-heat treatment after the deposition, the volume fraction of the tetragonal phase increases, resulting in a high dielectric constant. On the other hand, the volume fraction of the monoclinic phase increased in the films that were heat-treated at higher temperatures and exposed to longer heat treatment duration. The ferroelectric with and dielectric properties of these films were greatly inferior. Superior ferroelectric properties a significant volume fraction of orthorhombic phase were achieved for intermediate heat treatment conditions. These results give useful information to understand the origin of the ferroelectricity and to control the phases and electrical properties in HfO₂-based films.

Pb(Zr,Ti)O₃ (PZT) thin films for ferroelectric probe data storage technology were studied. (001)-oriented PZT thin films were deposited on SrRuO₃/SrTiO₃ substrates by rf magnetron sputtering. Dc voltage was applied on the films using a metal-coated tip and the poling region was observed by scanning nonlinear dielectric microscopy (SNDM). The contrasts in the positive and negative poling regions in the SNDM images obtained were improved by using the PZT films after ion-beam irradiation. This suggests that a surface layer of a few nanometers in thickness was formed on the as-grown PZT film and the polarization was not invertible in the surface layer. The deposition condition was examined with focus on deposition temperature. Nanosized domain dots were successfully formed on a PZT film deposited at 550 °C.

We evaluated the compositional dependence of Pb(Mg_1/3,Nb_2/3)O₃–PbTiO₃ (PMN–PT) polycrystalline thin films by combinatorial sputtering. We prepared compositional gradient (1 − x)PMN–xPT polycrystalline thin films with preferential orientation along the 〈001〉 direction in the composition range of x = 0–0.62. We determined that the morphotropic phase boundary (MPB) composition of PMN–PT polycrystalline thin film existed at around x = 0.35, from the X-ray diffraction (XRD) measurements. The maximum value of relative dielectric constants (ε_r = 1498) was obtained at approximately x = 0.23. On the other hand, the piezoelectric coefficients (|e_31,f| = 14.1 C/m²) peaked at the determined MPB composition of x = 0.35. From the results of the compositional dependence of dielectric and piezoelectric characteristics, the FOM ($e_{31,\text{f}}^{2}/\varepsilon _{0}\varepsilon _{\text{r}}$) of the PMN–PT (x = 0.35) thin film reached 21 GPa, which is much higher than that of the other polycrystalline piezoelectric thin films. These results suggest that PMN–PT (x = 0.35) thin film is a promising material for high-efficiency piezoelectric MEMS energy harvesters.

We report on oxide-channel ferroelectric-gate thin-film transistors (FeTFTs) with the bottom-contact structure for source and drain electrodes. FeTFTs with the bottom-contact structure have faster switching characteristics than those with the top-contact structure, especially for the switching from ON to OFF. First, we confirmed that FeTFTs with the top- or bottom-contact structure exhibit similar I_D–V_G characteristics and retention properties to each other. Next, we measured the C–V characteristics and confirmed that the oxide semiconductor channel under the source and drain electrodes is depleted in the top-contact structure. This depleted channel impedes the switching from ON to OFF. Finally, we demonstrated that FeTFTs with the bottom-contact structure possess fast switching characteristics for the switching from both OFF to ON and ON to OFF.

ZnO films with c-axis (0002) orientation have been successfully grown by RF magnetron sputtering on interdigital transducer/Al₂O₃/glass substrates. Alumina films were deposited on glass substrates by electron beam evaporation. The crystalline structure and surface roughness of the films were investigated by X-ray diffraction and atomic force microscopy, respectively. The phase velocity and coupling coefficient of surface acoustic wave (SAW) device apparently increased when we increased the thickness of the alumina films. In addition, an excellent temperature coefficient of frequency of the SAW device was obtained by increasing the thickness of the alumina films. This experimental result is beneficial for enhancing the performance of ZnO thin-film SAW devices on inexpensive glass substrates.

(Li_xK_0.1Na_0.9−x)NbO₃ (x = 0, 0.05, 0.1, and 0.2) epitaxial films were prepared by pulsed laser deposition on SrRuO₃/(001)SrTiO₃ substrates. Reciprocal space mapping indicated that the crystal systems of the films were antiferroelectric orthorhombic at x = 0, antiferroelectric orthorhombic and/or ferroelectric monoclinic depending on film thickness at x = 0.05, ferroelectric monoclinic at x = 0.1, and ferroelectric rhombohedral at x = 0.2. Polarization–electric field responses were ferroelectric for all the films. At x = 0.05, the remanent polarization was maximized to ∼35 µC/cm², which was more than four times larger than that at x = 0. This was probably due to a stabilized monoclinic phase under a high electric field.

KNbO₃ films were synthesized at 120–240 °C by a hydrothermal method. The deposition amount of KNbO₃ films decreased with decreasing deposition temperature because the deposition rate decreased together with the increase in the starting time of precipitation. The KNbO₃ phase was ascertained at temperature as low as 120 °C in films deposited on (100)_cSrRuO₃//(100)SrTiO₃ substrates. On the other hand, powders prepared at 150 °C included the Nb₂O₅ phase as well as the KNbO₃ phase. In addition, powders prepared at 120 °C consist of the K₃Nb₇O₁₉ phase instead of the KNbO₃ phase. KNbO₃ films were deposited at 150 °C on LaNiO₃/Pt/Ti/polysulfone substrates. Films consisting of the polycrystalline KNbO₃ phase were obtained, and their ferroelectricity and piezoelectricity were observed. The remanent polarization and piezoelectric coefficient were 4.2 µC/cm² and 20–32 pm/V, respectively.

Thin films of the BaTiO₃–Bi(Mg_1/2Ti_1/2)O₃ (BT–BMT) solid-solution system were fabricated with the aim of achieving a stable temperature coefficient of capacitance (TCC) favorable for high-temperature electronics. A single perovskite phase with pseudocubic symmetry was obtained for the films fabricated by chemical solution deposition on (111)Pt/TiO₂/(100)Si substrates in the composition range of x = 0–0.80 for (1 − x)BT–xBMT. BMT added to the BaTiO₃-based films enhanced the crystallinity of the perovskite phase and resulted in saturated P–E hysteresis behavior with remanent polarization of up to 13 µC/cm². BMT addition led to gradual dielectric relaxation, which also resulted in stable TCC behavior with a relative dielectric constant of approximately 400 in the temperature range of RT − 400 °C, especially for the BT–BMT films with x = 0.20–0.40.

Lead- and alkali-metal-free BaTiO₃–Bi(Mg_0.5Ti_0.5)O₃–BiFeO₃ solid-solution thin films were prepared on (111)_cSrRuO₃/(111)Pt/TiO₂/SiO₂/(100)Si substrates by chemical solution deposition (CSD) and their crystal structure and dielectric properties were investigated. The lattice spacing as a function of z/(x + z) in xBaTiO₃–0.1Bi(Mg_0.5Ti_0.5)O₃–zBiFeO₃ indicated the existence of phase boundaries (pseudocubic/rhombohedral) in the range of z/(x + z) = 0.33–0.56, where the relatively high relative dielectric constant, ε_r, of above 800 was obtained. On the other hand, dielectric loss, tan δ, of below 0.2 was confirmed in the range z/(x + z) = 0–0.87, which rapidly increased toward z/(x + z) = 1.0. The relatively high ε_r values of these films deposited on Si substrates by a solution-based process suggest that they can be used as alternative to Pb(Zr,Ti)O₃, KNbO₃, and (Bi_1/2Na_1/2)TiO₃-based films.

We evaluated the thicknesses of domain walls (DWs) in rhombohedral BiFeO₃ thin films epitaxially grown on SrRuO₃-covered SrTiO₃ (100), (110), and (111) single crystals by scanning nonlinear dielectric microscopy (SNDM). The SNDM phase signal revealed an abrupt change in the sign of polarization components normal to the surface at DWs within one or two unit cells. On the other hand, the SNDM amplitude signals gradually changed across DWs, corresponding to the change in the electrostatic potential. The minimum thicknesses estimated from the SNDM amplitude signals are 4, 1, and 2 nm for 71, 109, and 180° DWs, respectively. The relationship between these DW thicknesses and the nature and situation of DWs is discussed taking account of the polarization configuration in adjacent domains.

The piezoelectric properties of (100)-orientated BiFeO₃ thin films grown on (100)LaNiO₃/SiO₂/(100)Si were investigated. 200-nm-thick LaNiO₃ and 250-nm-thick BiFeO₃ was deposited by the rf magnetron sputtering method and sol–gel method, respectively. The (100)-oriented BiFeO₃ films were distorted to the in-plane tensile direction owing to the low thermal expansion coefficient of the Si substrate. While no significant dependence of the in-plane lattice distortion on the dielectric and ferroelectric properties and d_33,AFM piezoelectric coefficient measured by scanning probe microscopy was observed, it was found that the e_31,f piezoelectric coefficient increases with increasing the lattice distortion. The maximum e_31,f and figure of merit (FOM) were −4.0 C/m and 14 GPa, respectively, which are comparable to those of epitaxially grown (100)BiFeO₃ films.

⁵⁷Fe-enriched BiFeO₃ (BFO) thin films are fabricated on Pt/Ti/SiO₂/Si substrates from a stoichiometric precursor solution by chemical solution deposition process. The microstructure of the thin films is controlled by a changing the sintering time at 550 °C. The polycrystalline thin film fabricated at 550 °C for 5 min shows well-saturated polarization–electric field (P–E) hysteresis loops and the remnant polarization P_r and coercive field E_c at room temperature are 52 µC/cm² and 365 kV/cm, respectively, at an applied electric field of 1200 kV/cm. The Mössbauer spectra show that the BFO thin film has the valence state of Fe³⁺ only, consisting of antiferromagnetic and paramagnetic components. The paramagnetic component with an area fraction from 11 to 18%, which is not amorphous or Bi₂Fe₄O₉, seems to distribute in the surface shell of the grains and the grain boundaries. This component must strongly influence the ferroelectric properties at room temperature.

Epitaxial BiFeO₃ (BFO) thin films with striped- and single-domain structures have been grown on SrTiO₃ (STO) (103) and (113) substrates by radio-frequency planar magnetron sputtering. The domain structure of BFO was controlled by the orientation of the STO substrate. Piezoelectric force microscopy revealed that BFO thin films on STO (103) and STO (113) had a striped-domain structure with 71° domain walls running along 〈010〉_STO, and a single-domain structure, respectively. To confirm the photovoltaic property, rectangular Pt electrodes with widths of 150–200 µm were deposited on BFO surfaces with interelectrodes distances of 200–250 µm. I–V characteristics were measured under an illumination of a collimated violet laser (λ = 405 nm) with a power density of 380 W/cm². In the striped-domain-structure BFO film with Pt electrodes fabricated along domain walls, above-band-gap open-circuit voltage (V_OC) of 29 V was observed. In addition, single-domain-structured BFO thin film with Pt electrodes fabricated along $\langle \bar{1}10\rangle$ also showed above-band-gap V_oc of 26 V despite the absence of domain walls. It is considered that these large V_oc values originated from the photovoltaic effect not at the domain walls but in bulk BFO.

Polycrystalline Mn-doped BiFeO₃ thin films with a relatively narrow band gap were prepared on Pt/TiO_x/SiO₂/Si and MgO(100) substrates by chemical solution deposition. Their photoinduced electrical properties under visible light irradiation (400–700 nm) were characterized. The rapid on/off response of the photocurrent to light in unpoled BiFeO₃-based thin films was demonstrated. From the direction of the electric current, the internal bias electric field caused by the space charge distribution in the unpoled film is considered to have an important effect on photocurrent generation and photovoltaic phenomena. Although Mn doping did not greatly affect the band gap and ferroelectric remanent polarization, it had an influence on the photocurrent behavior and photovoltaic properties. The magnitude of the photoinduced current of the film decreased with increasing Mn doping. In this case, the energy level created in BiFeO₃ by Mn doping traps carriers generated by photoexcitation. The interfacial state between the thin film and the top or bottom electrode and the electrical resistivity at low applied voltages were also found to be related to the photoinduced behavior of the Mn-doped BiFeO₃ thin films.

Crystal structures and microstructures in Ba_1−xSr_xAl₂O₄ solid solutions between the end members of BaAl₂O₄ and SrAl₂O₄ have been carefully investigated by powder X-ray diffraction, electron diffraction and transmission electron microscopy (TEM) imaging experiments. With the help of fast Fourier transform (FFT) calculation, high-resolution TEM images suggested that diffuse streaks along three equivalent 〈110〉 directions in the (001) plane, which appear in the P6₃ structure of Ba_1−xSr_xAl₂O₄ for x = 0.4, originate from the large structural fluctuation of the AlO₄ tetrahedral network. On the other hand, the monoclinic P2₁ structure in Ba_1−xSr_xAl₂O₄ with x = 0.7 was found to consist of a modulated structure with $\boldsymbol{{q}} = 0,1/2,0$. The present experimental results reveal that a structural phase boundary exists at approximately x = 0.6 between the P6₃ structure with a large structural fluctuation and a monoclinic P2₁ phase with the single-q modulated structure.

A space-charge-controlled optical beam deflector made of a KTa_1−xNb_xO₃ (KTN) single crystal utilizes electrons that are injected through the cathode by applying voltage. With the deflector made of lithium-doped KTN (K_0.95Li_0.05Ta_0.73Nb_0.27O₃, KLTN/0.05/0.27), we observed large increases in the capacitance of the deflector when we injected electrons. The increases were not caused by changes in the electrode interface but by changes in the permittivity of the bulk crystal. In the paraelectric phase, the KLTN/0.05/0.27 crystal exhibited nonlinearity in the dielectric response with double hysteresis loops in the D–E curves. We ascribed the permittivity change to this nonlinear phenomenon. We also discuss this nonlinearity in terms of the Landau–Devonshire phenomenological theory. The coefficient g₄ of the fourth power term in the expanded free energy was negative in the paraelectric phase near the phase transition temperature as it is for other materials that exhibit a first-order phase transition. However, g₄ depended on the temperature and its sign became positive about 15 °C above the phase transition temperature.

The microwave dielectric properties and crystal structure of spinel-structured (1 − x)MgGa₂O₄–xZnGa₂O₄ ceramics were characterized in this study. From the linear variation in the lattice parameters, the formation of solid solution was obtained over the whole composition range. On the basis of the refined atomic coordinates, the variations in the volume of tetrahedron and octahedron were estimated and the expansion of the MO₄ (M = Mg, Zn, and Ga) tetrahedron was recognized. The covalency of the cation–oxygen bond in the 8a site decreased with increasing composition x, suggesting the preferential Zn substitution for Mg in 8a site. The ε_r values of Zn-substituted ceramics were higher than that of MgGa₂O₄ ceramic. The Q · f values higher than 1.8 × 10⁵ GHz were obtained when the ceramics were sintered at temperatures higher than 1475 °C.

Ba(Zr_xTi_1−x)O₃ (BZT) thin films were deposited on a (100)MgO substrate by RF-magnetron reactive sputtering using metal targets. In this paper, a model of the reactive sputtering process using a metal composite target at different reactive gas flow rates is presented. 500-nm-thick BZT thin (x = 0.26) films for high frequency performance showed a single perovskite phase and a high crystallinity on the MgO substrate with only a (001)/(100) orientation at an optimum reactive gas flow rate in Ar + O₂. The BZT films had a stoichiometric Ba/Ti ratio and epitaxially grew on the MgO substrate. They showed a dense microstructure without cracks or voids. Their low capacitance and loss tangent showed little dispersion at 1–18 GHz. These results indicated that we succeeded in depositing high-quality and potentially tunable ferroelectric BZT films for high-frequency applications by RF-magnetron reactive sputtering using metal targets.

To develop ceramic capacitors with a high effective dielectric constant, we attempted to fabricate BaTiO₃ (BT) complexes with embedded Ag nanoparticles by wet chemical processes. Ag nanoparticle-adsorbed dendritic BT particles, Ag–BT hybrid particles, were synthesized from the sol–gel-derived precursor gel powders containing Ag, Ba, and Ti by hydrothermal treatment. These particles were pressed with BT fillers and TiO₂ precursor nanoparticles into green compacts, and then, the green compacts were chemically converted into the Ag/BT nanocomplex compacts in Ba(OH)₂ aqueous solution under the hydrothermal condition at 160 °C. The effective dielectric constant of the resultant Ag/BT nanocomplexes increases with an increase in Ag content. The maximal effective dielectric constant of approximately 900 was recorded for the nanocomplex with the Ag content of 10.7 vol %.

BaTi₂O₅ (BT2) calcined powder synthesized by a solid-phase reaction decomposed into BaTiO₃ (BT) and Ba₆Ti₁₇O₄₀ (B6T17) as the firing temperature was raised. This decomposition was suppressed by adding 1 wt % SiO₂ as an additive. SiO₂ did not dissolve in BT2 crystal grains and segregated as a secondary phase including Ba and Ti atoms. The incorporation of Ba and Ti into SiO₂ probably suppresses the decomposition of BT2. By using another TiO₂ powder with small particle sizes as one of the raw material powders, perfect BT2 single-phase calcined powder was obtained. The ceramic fired from this calcined BT2 powder maintained its BT2 single phase. This is probably attributed to the non-existence of any chemical compounds other than BT2 that can be the origin of BT2 decomposition during firing. On the one hand, in the case of using 1 wt % Fe₂O₃ as the additive, Fe₂O₃ dissolved in BT2 crystal grains, and the sintered samples exhibited relaxor behavior.

The effects of CuO or NiO addition on the sintering temperature and electrical properties of 0.92(K_0.47Na_0.47Li_0.06)NbO₃–0.07BaZrO₃–0.01(Bi_0.5Na_0.5)TiO₃ ceramics with a temperature-stable morphotropic phase boundary were investigated. The ceramics without any additives and those with NiO exhibited a tetragonal phase. The ceramics with CuO exhibited the coexisting orthorhombic and tetragonal phases. The sintering temperature was reduced by the addition of NiO or CuO. NiO addition was also effective for broadening the range of the sintering temperature window. The d₃₃, radial-mode electromechanical coupling factor, and remanent polarization of the ceramics with NiO were 238 pC/N, 0.39, and 14 µC/cm², respectively; moreover, the measured values of the ceramics with CuO were all lower than those of the ceramics with NiO. The results suggest that NiO acted as a sintering aid among grains, and the sintering temperature window was consequently broadened. In the case of CuO addition, Cu ions replaced the A- and B-sites of the ceramics, and such replacement reduced the tetragonality. As a result, the electrical properties of the ceramics with CuO were lower than those of the ceramics with NiO.

Platelet NaNbO₃ grains were grown at 1150–1225 °C by single-step molten salt synthesis. The structural and compositional transformation from the precursor Aurivillius phase to perovskite NaNbO₃ by the topochemical conversion reaction was studied. No compositional distribution was confirmed for the platelet grains grown at 1150 °C, whereas it was observed that the expulsion of bismuth and incorporation of sodium were simultaneously initiated in spots for the grains grown at 1170 °C. With increasing the growth temperature the topochemical conversion reaction was promoted, and single-phase NaNbO₃ grains were eventually grown with heat treatment at 1225 °C for 6 h. In order to trace the structural transformation due to the topochemical conversion reaction, preconversion and postconversion platelet grains were chosen for characterizing the microstructure. It was found that the precursor Aurivillius phase is a mixed phase described as Bi_2.5Na_m−1.5Nb_mO_3m+3 (m = 5, 6, and 8). In the interior of the platelet grains, migration paths vertically elongated to the principal surface are created, and bismuth is expelled via the vertical path as well as the horizontal path along the (Bi₂O₂)²⁺ layer. It was concluded that the distinctive migration network contributed to the structural transformation while maintaining the epitaxy.

Load-bearing applications, such as actuators, require sufficient mechanical properties to guarantee long lifetime and reliability. Lead zirconate titanate (PZT) ceramics show relatively low mechanical strength which decreases after applying an electric field. Thus far, evaluations of the mechanical properties have not been the focus in the case of alkali niobate-based (NKN) ceramics. For this purpose, differently poled Li_x(Na_0.5K_0.5)_1−xNbO₃ ceramics have been observed by means of 3-point bending tests. Best results were achieved with Li_0.02Na_0.49K_0.49NbO₃, with a flexural strength of 115 MPa in unpoled state. This value was maximized at a 90° domain switching fraction η of about 20% to 134 MPa. Other compositions showed similar behavior, which led to the idea that domain switching can be used to enhance the mechanical properties of NKN ceramics. Internal stresses induced via domain reorientation might be the cause of this phenomenon and will be examined in this study.

It has been reported that poly(β-phenethyl l-aspartate) (PPLA) exhibits irreversible inversion from right-handed to left-handed helixes in a solid state at 130–140 °C from the results of circular dichroism measurement and X-ray diffraction. On the other hand, it is well known that semicrystalline chiral polymers show shear piezoelectricity due to their asymmetric crystal structure. The results of macroscopic shear piezoelectric measurement include information on macroscopic and microscopic properties due to the helix structure of the chain molecules. However, in general, no direct correspondence has been reported between the macroscopic piezoelectric properties and crystal characteristics because of the existence of a unique complex high-order structure in polymer films. We measured the temperature dependence of the piezoelectricity of a PPLA film in order to obtain more information concerning the helix inversion process. As a result, the sign inversion of the piezoelectric constant was observed at the temperature at which the helix inversion took place. It was also shown that the macroscopic orientation of the crystalline structure was maintained in the PPLA film after the sign inversion, even though molecular motion such as the segmental motion and side-chain motion of PPLA molecules was activated.

Recently, the application of uniaxially stretched poly(l-lactic acid) (PLLA) films to speakers, actuators, and pressure sensors has been attempted, taking advantage of their piezoelectric performance. However, the shear piezoelectric constant d₁₄ of uniaxially stretched PLLA film is conventionally 6–10 pC N⁻¹. To realize a high sensitivity of pressure sensors, compact speakers, and actuators, and a low driving voltage, further improvement of the piezoelectric performance is desired. In this study, we carried out solid-state extrusion (SSE) to stretch and orient poly(d-lactic acid) (PDLA) and verified its effects on piezoelectric performance. By SSE, we were able to improve the mechanical strength and elastic modulus of PDLA samples. Furthermore, the d₁₄ of the samples was significantly increased to approximately 20 pC N⁻¹.

In this study, we focus on vertical alignment ferroelectric liquid crystals (VA-FLCs), because an FLC display with a high contrast ratio is expected. In the VA, although the normal of smectic layers is vertical to the substrate surface, the director of FLC molecules must tilt to the substrate surface at the tilt angle of FLC molecules. Thus, it is usually difficult to obtain a uniform VA in FLCs. We determine the optimum conditions for the fabrication of VA-FLC cells in terms of the tilt angle of FLC molecules, the anchoring strength of the alignment film, and the cell gap. Results indicate that the FLC with a small tilt angle tends to form good VA, which can be obtained by utilizing alignment films with high surface anchoring strengths. Good VA strongly depends on the cell gap.

We developed a piezoelectric polymer film that was an electret using a porous poly(tetrafluoroethylene) (p-PTFE) film with high piezoelectricity and high heat resistance. First, we found that the p-PTFE electret had a piezoelectric constant d₃₃ of 100 pC/N after the optimization of its pore size. This value was about five times as large as that of poly(vinylidene fluoride) (PVDF) and was retained up to a temperature of as high as 120 °C. Then a new device using the laminated film with perfluoroalkoxy (PFA) laminated on one side of the p-PTFE electret was developed for the demonstration of pressure sensing. A new flexible device with a large area was realized. Then, for the demonstration of pressure sensing, a plastic touch pen for a touch panel was traced on the surface of the device at a constant speed of 80 mm/s under a compressive load of 0.05 N. The results confirmed that the device exhibited superior sensing responsiveness.

We have investigated the structure and ferroelectric properties of vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymer thin films fabricated on amorphous alloy electrodes. Thin films of amorphous Pd–Cu–Si alloys with various compositions were successfully prepared by sputtering deposition. An atomically flat surface resulting from a fully amorphous structure of Pd–Cu–Si was obtained upon achieving a uniform surface of a spin-coated VDF–TrFE copolymer thin film. X-ray diffraction (XRD) measurements revealed that the crystalline structure of the VDF–TrFE copolymer thin films was of ferroelectric β-phase, being independent of the composition and crystalline state of the Pd–Cu–Si alloy. The clearly observed D–E hysteresis loops showed a remanent polarization of 0.075 C/m² and a coercive field of 90 MV/m at a measurement frequency of 10 Hz for a 50-nm-thick film, which is almost consistent with the results obtained with Pt electrode samples. We also observed the thinning-induced reduction of remanent polarization, which was explained by the depolarization field induced by the surface dead layer in VDF–TrFE copolymer rather than the oxidized layer in Pd–Cu–Si alloy electrode.

The atomic-scale structure of a solid solution of BaTiO₃ (BT) and KNbO₃ (KN) has been studied using high-energy X-ray diffraction, X-ray absorption fine structure, and atomic pair-distribution function analysis techniques. We prepared BT–KN solid solution using KNbTiO₅, in which Ti and Nb atoms are arranged randomly. The average structure of the BT–KN solid solution was cubic and the local structure is also reproduced by the cubic structure. It is rare that a solid solution synthesized from ferroelectric materials has the local structure of a paraelectric material. Since the original correlation of BT or KN was lost, ferroelectricity disappeared in the BT–KN solid solution.

The paraelectric cubic structure of (Bi_1/2Na_1/2)TiO₃ was analyzed precisely by high-energy synchrotron radiation X-ray powder diffraction measurement and the maximum entropy method (MEM)/Rietveld method. The 〈100〉-favored rotator-like thermal behavior of the Bi ion was observed. The 〈100〉 off-centering of Bi led to the Bi–O distance of ∼2.5 Å that was shorter than the average Bi–O distance in the structure of the Bi on-centered model. The off-centering of Bi can be attributed to the orbital hybridization between the Bi and O ions.

The complex dielectric permittivity of single-domain BaTiO₃ in the region of 30–700 cm⁻¹ (0.90–21 THz) was directly measured using a far-infrared spectroscopic ellipsometer. The dielectric responses of the Slater, Last, Axe, and O₄ torsional modes, which are all infrared-active phonon modes in BaTiO₃ were clearly observed. The resonance frequencies of the phonon modes estimated in this study were in agreement with the previous data estimated by analysis of reflection spectra using a harmonic oscillator model. These results indicate that the far-infrared ellipsometric technique is effective for analyzing the phonon modes of BaTiO₃ single crystals. Additionally, we examine the dielectric response and the damping of the soft mode with respect to the order–disorder model.

The electric field–temperature (E–T) phase diagram of Ba_1−xK_xTiO_3−xF_x (x = 0.027) was established to investigate the relationship between critical phenomena and enhanced piezoelectric and dielectric properties after KF substitution. By measuring the dielectric constant under an electric field, the critical end point (CEP) was determined to be E = 4 kV/cm and T = 380 K. E at the CEP is markedly less than that of BaTiO₃, indicating that the electric fields necessary for large polarization fluctuations decrease owing to KF substitution. We suggest that the CEP line ends at a tricritical point located at x = 0.10, T ∼ 308 K, and E = 0 kV/cm in the x–E–T phase diagram.

The dielectric permittivity along the [001]_c direction of the pseudocubic coordinate in Pb(Zn_1/3Nb_2/3)O₃–8%PbTiO₃ (PZN–8%PT) was investigated in a wide temperature range of 30–640 °C. The determination of the Curie constant in PZN–8%PT was attempted, using the dielectric permittivity above the Burns temperature, to evaluate the bulk property in the ferroelectric phase as an average structure with reduced heterogeneity. It was confirmed that the reported temperature dependence of the spontaneous polarization can be quantitatively well reproduced on the basis of the Landau-type free energy.

We have deposited 1.0-µm-thick BaTiO₃ (BTO), SrTiO₃ (STO), Ba_0.5Sr_0.5TiO₃ (BST) thin films on MgO substrates by pulsed laser deposition technique and measured their dielectric properties by terahertz-time domain spectroscopy (THz-TDS) and an impedance analyzer with interdigital surface electrodes in the temperature range from 20 to 290 K. By comparing the real part of the dielectric constant at 1 MHz and 1 THz, we have estimated the temperature dependence of the orientational polarization. The results suggest that the orientational polarization emerges in the ferroelectric phase of (Ba,Sr)TiO₃, and the concentration of orientational polarization components of BST exceeds 90% of the total below 150 K, while it is about 30% at room temperature.

Terahertz radiation by optical rectification has been observed at room temperature in a hydrogen-bonded organic molecular ferroelectric crystal, 2-phenyl malondialdehyde (PhMDA). The radiated electromagnetic wave consisted of a single-cycle terahertz pulse with a temporal width of ∼0.5 ps. The terahertz radiation amplitude divided by the sample thickness in PhMDA was nearly equivalent to that in a typical terahertz wave emitter ZnTe. This is attributable to a long coherence length in the range of 130–800 µm for the terahertz radiation from PhMDA. We also discussed the possibility of PhMDA as a terahertz wave emitter in terms of the phase-matching condition.

We report on the emission of terahertz radiation by irradiation of femtosecond laser pulses in non-centrosymmetric paraelectric and ferroelectric phases of Co₃B₇O₁₃I boracite. The generation of the terahertz waves in both phases is caused by optical rectification via a second-order nonlinear optical effect. In the ferroelectric phase, we successfully visualized ferroelectric domains by analyzing the polarization state of the terahertz wave radiated from the crystal. In a large area of the crystal (∼500 × 500 µm²), the observed polarization vector of the radiated terahertz wave was tilted from directions of spontaneous polarization, i.e., $[100]_{\text{cub}}$, $[010]_{\text{cub}}$, and $[001]_{\text{cub}}$ directions in cubic setting, which can be explained by the presence of a ferroelectric 90° domain wall of the $(101)_{\text{cub}}$ plane.