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Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Hiroshi YAMAGUCHI, Junichi TATAMI and Motoyuki IIJIMA
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The measurement of the mechanical properties of electroceramics is important for improving the reliability of ceramic components and devices. Although information of the mechanical properties at the microscale level, especially under tensile or bending stress, is desired, this has not yet been reported. In this study, a bending test using a microcantilever beam specimen was applied to measure the mechanical properties of the BaTiO3 layer in a multi-layered ceramic capacitor. As a result, a nonlinearity relationship between stress and strain was observed during the bending test. The bending strength and failure strain, measured using the microcantilever beam specimen, were much higher than those measured using bulk specimens and almost the same or higher than those of conventional Si3N4 ceramics, as the fracture origin was nano-size.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Issei SUZUKI, Yuki IGUCHI, Chiyuki SATO, Hiroshi YANAGI, Naoki OHASHI and Takahisa OMATA
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The electronic structures of delafossite-type α-AgGaO2 and β-NaFeO2-type β-AgGaO2 were calculated based on density-functional theory using the local density approximation functional including the Hubbard correction. We compared the electronic structures of α- and β-AgGaO2 with previously reported electronic structures of α- and β-CuGaO2. We found that the Ag–Ag distances in α- and β-AgGaO2 are almost the same as the Cu–Cu distances in α- and β-CuGaO2, respectively, despite the ionic radius of Ag+ being larger than that of Cu+, because the frameworks of their crystal structures are determined by the linkages of GaO6 octahedra in the α-phase and GaO4 tetrahedra in the β-phase. It is indicated that transfer of electrons in Ag 4d states of AgGaO2 can occur between Ag atoms more easily than in Cu 3d states of CuGaO2 because high-electron-density regions surrounding Ag atoms are closer to each other than those of Cu atoms. This resulted in larger dispersion of the valence band of AgGaO2 than CuGaO2 for both α- and β-phases. We propose that the ratio of ionic radii of the monovalent and trivalent cations, rAI/rBIII, can provide an indication of the dispersion of the valence band of delafossite-type and β-NaFeO2-type ternary oxide semiconductors.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Shinya KONDO, Tomoaki YAMADA, Alexander TAGANTSEV, Nava SETTER, Masahito YOSHINO and Takanori NAGASAKI
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The influence of compressive strain on the linear electro-optic (EO) effect in c-domain (BaxSr1−x)TiO3 (BST) thin films was theoretically investigated using a phenomenological thermodynamic model. We considered that the c-domain BST thin films were grown on cubic substrates and an electric field was applied parallel to the direction of spontaneous polarization. To estimate the EO coefficients, we considered not only the intrinsic EO effect but also the elasto-optic effect by converse piezoelectric response. We found that the EO response is mostly determined by the difference in temperature from the ferroelectric-to-paraelectric phase transition boundary modified by the strain, and the elasto-optic effect appreciably affects the overall EO coefficient when the intrinsic EO effect is weak.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Sangwook KIM, Gopal Prasad KHANAL, Hyunwook NAM, Ichiro FUJII, Shintaro UENO and Satoshi WADA
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The structural and electrical properties of 0.75BiFeO3–0.25BaTiO3 (BF25BT) ceramics, with and without heat treatment, were investigated. The polished and cut BF25BT ceramics were thermally annealed at 800°C for 20 h and then quenched in water at room temperature (heat-treated ceramics). The PE hysteresis loop and SE curve were significantly improved after the heat treatment process. Meanwhile, the leakage current density at an electric field of 50 kV/cm was reduced from 4.847 × 10−7 (as-sintered ceramics) to 3.213 × 10−8 A/cm2 (heat-treated ceramics). The heat-treated ceramics was poled at 120°C with applying a DC-bias field of 50 kV/cm, and the maximum phase angle was −39°, which was evaluated from the analysis of frequency-dependence of impedance and phase angle. Because of the small maximum phase angle obtained by poling under DC-bias field alone, the poling process was modified; that is, an AC-bias field was applied prior to the DC-bias field poling. The ceramics poled with the modified poling procedure exhibited the maximum phase angle of 63°. The appreciable increase of the maximum phase angle suggested the higher degree of domain alignment than that of its counterpart, and the effect was explained in terms of improved domain switching. The piezoelectric constant d33 was increased twice with adopting the modified poling process, which signifies that poling with both AC- and DC-bias field is highly effective in the lead-free Bi-based systems.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Junji NISHIYAMA, Kazuki KANEHARA, Hiroaki TAKEDA, Takaaki TSURUMI and Takuya HOSHINA
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THz dielectric spectra of Nb-doped SrTiO3 single crystals were measured using the far-infrared spectroscopic ellipsometer to understand the doping effect of Nb on ionic polarization. The frequency of the Slater transverse optical (TO) mode increased with the doping amount of Nb, meaning the decrease in the ionic polarization by Nb doping. From the change in the Slater TO mode frequency, it was estimated that the ionic polarizability of 1% Nb-doped SrTiO3 was 10.4% lower than that of undoped SrTiO3. This result suggested that the intrinsic permittivity of Nb-doped SrTiO3 is lower than that of that of undoped SrTiO3. First principles calculations revealed that the Nb 4d-orbitals contributes to the covalency between B-site cations and O ions, however, the covalency is suppressed due to the electron carrier doping. The doped electrons causes the decrease in the long-range Coulomb force and the increase in short-range elastic force, and therefore the ionic polarizability is decreased by Nb doping.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Gopal Prasad KHANAL, Ichiro FUJII, Sangwook KIM, Shintaro UENO, Tohru S. SUZUKI and Satoshi WADA
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Layered titanate H1.08Ti1.73O4·nH2O (HTO) particles with plate-like morphology were used as a template for the fabrication of grain-oriented bismuth potassium titanate [(Bi0.5K0.5)TiO3, or BKT] ceramics by a reactive-templated grain growth method. Single perovskite-phase textured BKT ceramics were successfully fabricated with optimizing the preparation conditions such as mixing procedure of matrix and template particles with a binder solution, temperature programs for binder removal and sintering, and sintering conditions. Microstructure evaluation suggested that the sintering with or without weight-pressing and a temperature program used could significantly affect the degree of texture development. The debinding and sintering with weight-pressing helped on the proper alignment of oriented particles and/or grains along with promoting the growth of the oriented plate-like BKT particles, formed by an in situ topotactic transformation reaction of plate-like HTO templates, resulting in a high degree of orientation. Using optimum preparation conditions, single perovskite-phase BKT ceramics with grain-orientation over 85% were successfully fabricated for the first time via a conventional sintering method.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Hyunwook NAM, Ichiro FUJII, Sangwook KIM, Tomoya AIZAWA, Shintaro UENO and Satoshi WADA
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Mn-added 0.3BaTiO3–0.1Bix(Mg1/2Ti1/2)O3–0.6BiFeO3 (x = 0.93, 0.96, 1.00, 1.04, 1.07, and 1.10) ceramics were fabricated by conventional solid-state reaction. The crystal systems of all the ceramics were pseudo-cubic, and no secondary phase was observed. It was found that the ceramics with x = 1.00 exhibited an optimum piezoelectric property of d33 = 106.16 pC/N, while the Bi-deficient and excess ceramics lowered the piezoelectric properties to 81.09 and 102.29 pC/N, respectively. The reason of the decreases could be associated with domain wall pinning effects by defect complexes and the presence of the residual small amount of Bi2O3.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Naoki HAMAO and Junji AKIMOTO
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We successfully synthesized a garnet-type Li6.5La3Zr1.5Ta0.5O12 polycrystalline sample at a relatively low temperature of 700°C using pyrochlore-type La2Zr2O7 and weberite-type La3TaO7 as precursor oxides. The cubic garnet-type structure with the lattice parameter of a = 1.29577(1) nm was confirmed using powder X-ray diffraction data. The primary particle size of the present Li6.5La3Zr1.5Ta0.5O12 sample was about 2 µm, which is considerably smaller than that prepared using conventional solid-state reaction at higher temperatures. Thermogravimetry differential thermal analysis data confirmed that the reaction between LiOH·H2O and precursor oxides occurred above 500°C after the dehydration reaction of LiOH·H2O.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Tomoya KANEKO, Kohei NOSUE, Tomoki UCHIYAMA, Masaru NAGANO, Naoki ANKEI, Kazutaka KAMITANI and Maiko NISHIBORI
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The oxygen non-stoichiometry (δ) and structural properties of a brownmillerite-type oxide of La0.1Sr0.9Co0.9Fe0.1O3–δ (LSCF1991) during oxygen sorption/desorption were investigated by simultaneously applying a temperature-programmed technique and high-temperature X-ray diffraction. The structural changes of perovskite-type (P-type) to brownmillerite-type (B-type) and B-type to P-type occurred within limited p(O2) ranges, and a B-type phase-stable region was observed in the presence of oxygen at high temperature. Moreover, P-type LSCF1991 changed into B-type LSCF1991 through tetragonal P-type LSCF1991 as an intermediate phase. B-type LSCF1991 showed repeating structural changes with a certain limited p(O2), which is 0.5% < p(O2) < 1.0%, whereas such changes could not be observed at p(O2) > 1.0%. These behaviors correspond with the temperature programmed desorption of oxygen profiles, as well as the amount of lattice oxygen (3–δ). The p(O2) dependence of the structural change will provide a basis for a further investigation into the durability of an oxygen-permeated membrane by predicting the strain across the membrane under practical conditions.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Tomoya AIZAWA, Ichiro FUJII, Shintaro UENO, Tohru S. SUZUKI and Satoshi WADA
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The effects of oxygen partial pressure (pO2) during sintering and post-annealing treatment on the microstructure and electrical properties of Mn-added 0.3BaTiO3–0.1Bi(Mg1/2Ti1/2)O3–0.6BiFeO3 ceramics were investigated. The ceramics sintered at pO2 of 0.1–1 atm showed pinched polarization–electric field (PE) loops, while the ceramics sintered in nitrogen gas could not be measured due to dielectric breakdown. By annealed at 800°C for 5 h and cooled slowly, the ceramics showed increased remanent polarizations although their P–E loops were still pinched. When the as-sintered ceramics were annealed at 800°C for 5 h and then quenched into water, well-developed ferroelectric hysteresis loops were observed. These results were discussed with grain size and defect dipoles consisted of oxygen vacancies and bismuth vacancies or iron ions with a different valence.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Takahisa SHIRAISHI, Yuta MUTO, Yoshiharu ITO, Akinori TATEYAMA, Hiroshi UCHIDA, Takanori KIGUCHI, Minoru K. KUROSAWA, Hiroshi FUNAKUBO and Toyohiko. J. KONNO
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Li substituted (K,Na)NbO3 films with K-rich composition against polymorphic phase boundary were deposited at 240°C on (100)La:SrTiO3 substrates by hydrothermal method. The amount of Li content in films was controlled by changing the nominal composition of raw materials, A = [LiOH]/([KOH] + [NaOH] + [LiOH]). X-ray diffraction measurement showed that {001}c-oriented epitaxial (K,Na,Li)NbO3 films were obtained for all nominal compositions (A = 0–0.1). However, the diffraction peaks originated from a secondary phase were also detected in the range of A = 0.05–0.1. Plan-view scanning electron microscopy observation revealed that (K,Na,Li)NbO3 films possessed square-shape feature in accordance with the epitaxial relationship between film and substrate, while the secondary phase exhibited triangular- and pyramid-shaped features. The distribution of the elements across the thickness was investigated by time-of-flight secondary ion mass spectrometry. Transmission electron microscopy revealed the columnar structure of the film. In addition, nanoscale-pores have been found to exist in the boundary of each column by scanning transmission electron microscopy. (K,Na,Li)NbO3 films, A up to 0.03, showed ferroelectricity, and the maximum remanent polarization was observed at A = 0.01.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Kodai AOYAMA, Takao SHIMIZU, Hideto KURAMOCHI, Masami MESUDA, Ryo AKIIKE, Yoshisato KIMURA and Hiroshi FUNAKUBO
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We firstly prepared SrSi2 thin films on insulating substrates and measured their thermoelectric properties. Thin films of Sr–Si system were deposited on (0001) Al2O3 substrates by radio frequency magnetron sputtering method at various deposition temperatures and under various total deposition pressure. Constituent phases primary depend on the deposition temperature. The films deposited below 600°C consisted of amorphous or the metastable CaSi2 structure phase. CaSi2 structure phase was obtained at 600°C irrespective of the pressure and finally stable α-SrSi2 (α-phase) above 700°C. The films with CaSi2 structure phase had low power factor below 10 µW m−1 K−2 for the temperature range between 100 and 400°C. On the other hand, the film with α-phase showed p-type conduction and good thermoelectric power factor beyond 700 µW m−1 K−2 at room temperature. This value is larger than the reported value of (111) one-axis-oriented Mg2Si films prepared by the same deposition process, maximum 130 µW m−1 K−2 at 300°C. The present result shows that α-phase is one of the promising candidates as thermoelectric materials.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Hideaki NAGAI, Kunimitsu KATAOKA and Junji AKIMOTO
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H2Ti12O25 (HTO) is a negative-electrode oxide material for Li-ion batteries with high capacity and repetition stability. To improve the conductive performance of HTO, porous titanium hydroxide powder containing fine carbon particles was synthesized by hydrolyzing TiOSO4 solution with fine carbon particles, and a HTO containing fine carbon particles was synthesized by impregnation method using its porous titanium hydroxide. The charge–discharge properties of this sample were characterized, and its Li insertion and extraction performance was generally higher than that of HTO without C prepared by solid-state and impregnation methods.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Terumasa TADANO and Shinji TSUNEYUKI
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The cubic-to-tetragonal phase transition temperature Tc of SrTiO3 is predicted from first-principles anharmonic lattice dynamics simulation. By using the self-consistent phonon approach, we compute finite-temperature phonon dispersion of cubic and tetragonal SrTiO3. By comparing the Helmholtz free energies of the two phases calculated with anharmonic effects included, we obtain Tc ∼ 80 K, which is comparable with the experimental value of 105 K. In addition, we show that the nuclear quantum effect is significant for predicting Tc. Our approach is efficient and provides a convenient way of predicting the phase stability of solids including ceramics, where the conventional approach based on the harmonic approximation often breaks down.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Ryo NAGAI, Yue Jin SHAN and Keitaro TEZUKA
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The luminescent center Er3+ doped upconversion (UC) phosphors generally exhibit strong green and weak red emission, however, some UC phosphors with Bi-containing host materials exhibit an intense red emission, or the emission color can change from green to red as the Er3+, Yb3+ concentration increases. This paper examined the relationship between the red UC emission and Bi-containing host materials by comparing the UC emission colors of host materials containing or not containing Bi with different or identical crystal structures. Furthermore, novel UC phosphors Bi2Ti4O11:xEr, yYb and Bi2Sn2O7:xEr, yYb were also synthesized by solid-state reaction method to certify those facts. These UC phosphors exhibit intense red emission with increasing Er, Yb concentrations in the same way as the reported UC phosphors with Bi-containing host materials. Based on the experimental results, it can be thought that energy transfer exists between Bi3+ and luminescence center Er3+, and inferred that non-radiative relaxation (energy-loss) is more easily to occur. Therefore, Er3+ doped UC phosphors with Bi-containing host materials tend to exhibit lower energy red emission than green emission.
Feature: Cutting edge research on electroceramics 2018
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published : vol. 127, no.6, June 2019
Ryoko MURAKAMI, Shintaro UENO, Ichiro FUJII and Satoshi WADA
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We investigated an influence of post-annealing treatment on dielectric and ferroelectric properties of BaTiO3 (BT) ceramics with a high relative density of around 90% prepared by a solvothermal solidification method at 200°C. In the solvothermal solidification method, green compacts consisting of TiO2 nanoparticles were converted into the BT ceramics in Ba(OH)2 ethanol/water reaction solutions, and the BT formation reaction accompanied by a volume increase was utilized for densification of the BT ceramics. By reducing mixing ratios of organic binder in formation of TiO2 green compacts, the relative density of the BT ceramics reached to around 90% after the solvothermal treatment at 200°C for 36 h. The as-prepared BT ceramics with pseudo-cubic symmetry show the lower dielectric constant of around 500 in spite of the high relative density due possibly to the size effect and/or incorporation of hydroxyl groups in crystal lattices. Therefore, we tried to enhance the dielectric constant by the post-annealing treatment at 500–1200°C for 2 h in air. Although the BT ceramics showed the pseudo-cubic symmetry even after the post-annealing at 500–1100°C, the dielectric constant increases with the post-annealing temperature and the polarization–electric field (P–E) loops and strain–electric field (S–E) curves show the improved ferroelectric behavior. The post-annealing treatment was effective to enhance the electrical properties of the BT ceramics prepared by the solvothermal solidification method owing mainly to facilitating grain growth. The dielectric constant of over 1000 could be obtained by annealing at 800°C.
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published : vol. 127, no.6, June 2019
Junyan WU, Xueqian ZHANG, Zhihao WANG, Hao WU, Zhi WANG, Qinggang LI, Qian LIU and Fangfang QI
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Nb and Nb2O5 were used, respectively, as sintering additives for pressureless sintering of Ti/Al2O3 composites, and the effect of elemental Nb on the microstructures and mechanical properties of the composites were investigated. The X-ray diffraction results indicated that the addition of both Nb and Nb2O5 resulted in the solution of Nb in Ti. Compared to metal Nb, the addition of Nb2O5 made the composites easier to sinter, and the density of the Ti/Al2O3 composite rose with increases in the Nb2O5 addition. When metal Nb was added, the changes in flexural strength and micro-hardness were basically consistent with the variations in density. The strength of the Ti/Al2O3 composites was found to increase at first and then to decrease, however when Nb2O5 was added. The phase formation and strengthening mechanisms are illustrated. The best performance occurred when 5 vol.% Nb2O5 was added, with a relative density, micro-hardness and flexural strength of 89.3%, 5.6 GPa and 115.78 MPa, respectively.
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published : vol. 127, no.6, June 2019
Surangkana WANNAPOP, Asanee SOMDEE, Titipun THONGTEM and Somchai THONGTEM
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ZnO nanoplates and ZnO nanostructure flowers were successfully synthesized in alkaline solutions containing different contents of sodium hydroxide by a cyclic-microwave method. The as-synthesized products were characterized by X-ray Diffraction, Field Emission Scanning Electron Microscopy and ultraviolet (UV)–visible spectroscopy. The crystal structure of ZnO nanoplates and nanostructure flowers are wurtzite structure with energy band gap of 3.201–3.217 eV. In this research, these products were applied as energy harvesting materials in dye sensitized solar cells (DSSCs) and also used photocatalytic degradation of methylene blue (MB) under UV irradiation. In the case of photo conversion materials, the DSSCs based ZnO nanoplates showed the best performance of photoconversion to electricity at 1.5 AM illumination. Moreover, it also well performed at dye degradation of MB under UV irradiation.
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published : vol. 127, no.6, June 2019
Takuma TAKAHASHI, Junichi TATAMI, Hiromi NAKANO and Satoshi TANAKA
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High-thermal-conductivity Si3N4 ceramics are desired for substrates in SiC power devices. We successfully fabricated highly c-axis-oriented Si3N4 ceramic by casting in a rotating-magnetic field and then sintered using a conventional gas pressure. As a result, the c-axis orientation of Si3N4 grains effectively improves the thermal conductivity. The sintering behavior of c-axis-oriented Si3N4 ceramics was investigated. The linear shrinkage ratio in the c-axis-oriented direction was smaller than that in the direction normal to the c-axis-oriented direction. Fine c-axis-oriented structures resulted from the orientation of β-Si3N4 seeds during the casting because they played as templates controlling the grain growth of β-Si3N4 nuclei and inducing the anisotropic sintering shrinkage.
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published : vol. 127, no.6, June 2019
Chunfu LIN, Takashi TANAKA, Akio NISHIYAMA, Tadashi SHIOTA, Osamu SAKURAI, Naoki WAKIYA, Kazuo SHINOZAKI and Kouichi YASUDA
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Al2O3 thin films were deposited on (001)Si substrate through Cr2O3/yttria-stabilized-zirconia (YSZ) buffer layer by cold-wall type chemical vapor deposition method with tri-methyl aluminum as a raw material. By changing the deposition temperature, different polytypes of Al2O3 thin films were formed. At lower temperatures (1123–1173 K), η-Al2O3 and amorphous Al2O3 were found in mixture. With increasing the deposition temperature, the series of Al2O3 polytypes (γ, κ and α) appeared in the order of decreasing the unit cell volume per Al atom. At 1323 K, single-phase α-Al2O3 thin film was obtained in success. On (00l)Cr2O3/YSZ/Si substrate, epitaxial (00l)α-Al2O3 thin-film was grown, however, on (00l)YSZ/Si substrate, epitaxial (00l)κ-Al2O3 thin film was formed. It shows that buffer layer also has much influence on polytype of Al2O3 thin film. On the other hand, there exist the polycrystalline α-Al2O3 and Cr2O3 have the same Miller index (h k l), therefore, polycrystalline α-Al2O3 thin film was deposited on the polycrystalline Cr2O3 buffer layer in which each Al2O3 and Cr2O3 grain has an epitaxial relation. This epitaxial growth can be explained by both the similarity in crystal structure between Al2O3 film and Cr2O3 buffer layer, and also their moderate lattice mismatch.
Technical Report
published : vol. 127, no.6, June 2019
Teruhiko SETSU and Hideki YAGI
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Sodium bismuth titanate [(0.15BaTiO3–0.85(Bi0.5Na0.5)TiO3], hereafter 0.15BT–0.85BNT, powder was prepared using hydrothermal synthesis. The particle size was approximately 0.1 µm. In addition, 0.15BT–0.85BNT ceramics were fabricated using the spark plasma sintering technique. Sintering was performed at 1273 K for 10 min. The relative density, grain size, and piezoelectric constant of the obtained ceramics were approximately 97%, 0.25 µm, and 12 pC/N, respectively. To our knowledge, this would be the first attempt to synthesize 0.15BT–0.85BNT piezoceramics featuring submicron-sized grains.