Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Ken Watanabe Last modified date:2021.06.15

Associate Professor / Department of Advanced Materials Science and Engineering / Faculty of Engineering Sciences

1. Ryunosuke Okuda, Kota Nakano, Koichi Suematsu, Ken Watanabe, Anna Ilnicka, Jerzy P. Łukaszewicz, Kengo Shimanoe, Chemical Activation of Nitrogen-doped Carbon Derived from Chitosan with ZnCl2 to Produce a High-performance Gas Diffusion-type Oxygen Electrode, ELECTROCHEMISTRY, 10.5796/electrochemistry.20-00121, 89, 1, 36-42.
2. Koichi Suematsu, Yuki Hiroyama, Wataru Harano, Wataru Mizukami, Ken Watanabe, Kengo Shimanoe, Double-Step Modulation of the Pulse-Driven Mode for a High- Performance SnO2 Micro Gas Sensor: Designing the Particle Surface via a Rapid Preheating Process, ACS SENSORS, 10.1021/acssensors.0c01365, 5, 11, 3449-3456, 2020.11.
3. Koichi Suematsu, Tokiharu Oyama, Wataru Mizukami, Yuki Hiroyama, Ken Watanabe, Kengo Shimanoe, Selective Detection of Toluene Using Pulse-Driven SnO2 Micro Gas Sensors, ACS Applied Electronic Materials, 2, 2913-2920, 2020.08.
4. Noriko Saito, Hajime Haneda, Ken Watanabe, Kengo Shimanoe, Isao Sakaguchi, Highly sensitive isoprene gas sensor using Au-loaded pyramid-shaped ZnO particles, SENSORS AND ACTUATORS B-CHEMICAL, 10.1016/j.snb.2020.128999, 326, 2021.01.
5. Ryunosuke Okuda, Koichi Suematsu, Ken Watanabe, Kengo Shimanoe, Highly Porous Chitosan-derived Nitrogen-Doped Carbon Applicable for High-performance Gas Diffusion Type Oxygen Electrode, Chemistry Letter, 10.1246/cl.200748, 2021.01.
6. Shingo Ide, Ken Watanabe, Koichi Suematsu, Isamu Yashima, Kengo Shimanoe, Crystal Growth Mechanism of Highly c-Axis-Oriented Apatite-Type Lanthanum Borosilicate Using B2O3 Vapor, ACS OMEGA, 10.1021/acsomega.0c04846, 5, 49, 31936-31942, 2020.12.
7. Koichi Suematsu, Wataru Harano, Shigeto Yamasaki, Ken Watanabe, Kengo Shimanoe, One-Trillionth Level Toluene Detection Using a Dual-Designed Semiconductor Gas Sensor: Material and Sensor-Driven Designs, ACS APPLIED ELECTRONIC MATERIALS, 10.1021/acsaelm.0c00902, 2, 12, 4122-4126, 2020.12.
8. Koichi Suematsu, Ken Watanabe, Masayoshi Yuasa, Tetsuya Kida, Kengo Shimanoe, Effect of ambient oxygen partial pressure on the hydrogen response of SnO2 semiconductor gas sensors, Journal of the Electrochemical Society, 10.1149/2.1391906jes, 166, 8, B618-B622, 2019.01, In this study, the influence of the oxygen partial pressures (PO2) on the sensor response to H2 of SnO2 resistive-type gas sensors was evaluated under various humid atmospheres. SnO2 nanoparticles of 8–15 nm in diameter were synthesized using a hydrothermal technique followed by calcination at 600°C. Additionally, a large amount of pores with diameters greater than 10 nm was confirmed in the nanoparticles. The electrical resistance at 350°C was decreased with decreasing the PO2, and the electrical resistance in the presence of 10 ppm H2 was much smaller than that in the absence of H2 in both dry and humid atmospheres regardless of the PO2. Furthermore, the sensor response to 10 ppm H2 at 350°C increased with decreasing PO2 in both dry and humid atmospheres. Thus, decreasing the amount of oxygen adsorption enhanced the effect of rooted hydroxyl formation on the SnO2 surface through a combustion reaction between H2 and adsorbed oxygen and improved the sensor response to H2. These results are important for understanding the fundamental mechanisms of gas detection and for the material surface design of highly sensitive resistive-type semiconductor gas sensors..
9. Koichi Suematsu, Wataru Harano, Tokiharu Oyama, Nan Ma, Ken Watanabe, Kengo Shimanoe, Ultra-High Sensitive Gas Detection Using Pulse-Driven MEMS Sensor Based on Tin Dioxide, 18th International Symposium on Olfaction and Electronic Nose, ISOEN 2019 ISOEN 2019 - 18th International Symposium on Olfaction and Electronic Nose, Proceedings, 10.1109/ISOEN.2019.8823211, 2019.05, Miniaturized semiconductor gas sensors composed of microheater and a sensor electrode allowed a rapid heater switching, pulse-driven operation. SnO2 nanoparticles was fabricated on the gas sensor device, and the sensor was driven during heater on phase at elevated temperature. Additionally, the gas was introducing into the sensing layer during heater off phase, cooling the sensing layer. On the basis of the behavior of the electrical resistance under pulse-driving, we determined the various types of sensor responses to improve the gas sensing characteristics. The gas sensitivity was drastically enhanced by pulse-driving mode, because utility factor of the pulse-driven sensor was higher than that of conventional sensor. Additionally, newly gas sensing definition also enhanced the gas selectivity, because gas accumulates in the sensing layer during heater off phase. Therefore, pulse-driving mode improved the ability of semiconductor gas sensors..
10. Shingo Ide, Hiroki Takahashi, Isamu Yashima, Koichi Suematsu, Ken Watanabe, Kengo Shimanoe, Effect of Boron Substitution on Oxide-Ion Conduction in c-Axis-Oriented Apatite-Type Lanthanum Silicate, Journal of Physical Chemistry C, 10.1021/acs.jpcc.9b11454, 124, 5, 2879-2885, 2020.02, Apatite-type lanthanum silicate (LSO) is a material with high oxide-ion conductivity in the low- and intermediateerature range (573-873 K) and is, therefore, a promising solid electrolyte for lowerature applications such as solid oxide fuel cells and oxygen sensors. Herein, the effect of B substitution at the Si site in a c-axis-oriented apatite-type lanthanum silicate (La9.7Si5.3B0.7O26.2, c-LSBO) polycrystal on oxide-ion conduction is investigated. A highly c-axis-oriented LSBO polycrystal is fabricated by a vapor-solid reaction in which a dense La2SiO5 disk is heated in B2O3 vapor at ≥1673 K. The oxide-ion conductivity of c-LSBO reaches 16 mS cm-1 at 678 K with an activation energy of 0.4 eV. The obtained oxide-ion conductivity of c-LSBO is approximately 190 times higher than that of yttria-stabilized zirconia and 5.8 times higher than that of the polycrystalline c-axis-oriented nondoped lanthanum silicate. Based on 11B nuclear magnetic resonance measurements, B is located at the SiO4 site as BO4, suggesting the formation of an oxygen vacancy at the O4 site located along the c-axis due to charge compensation. In addition, molecular dynamics simulations indicate that the oxide-ion diffusion coefficient of the B-doped LSO is higher than that of the nondoped LSO. The high oxide-ion conductivity of c-LSBO is likely attributable to the formation of an oxygen vacancy at the O4 site by B doping, which has a lower valency than Si. Therefore, c-LSBO is a promising candidate as a solid electrolyte in electrochemical devices operating at low and moderately high temperatures..
11. Nan Ma, Shingo Ide, Koichi Suematsu, Ken Watanabe, Kengo Shimanoe, Novel Solid Electrolyte CO2 Gas Sensors Based on c-Axis-Oriented Y-Doped La9.66Si5.3B0.7O26.14, ACS Applied Materials and Interfaces, 10.1021/acsami.0c00454, 12, 19, 21515-21520, 2020.05, Nowadays, monitoring and recording CO2 gas has become more and more important in various areas, leading to increasing demand for developing high-sensitive CO2 sensors. In this study, a novel potentiometric CO2 gas sensor is designed based on a new solid electrolyte of Y-doped La9.66Si5.3B0.7O26.14 (Y-LSBO), coated with the Li2CeO3-Au-Li2CO3 composite as a sensing electrode and Pt as a reference electrode. With the optimized composition of a sensing electrode, the electromotive force (EMF) varies linearly with the logarithm of the CO2 concentration in the range of 400-4000 ppm, exhibiting an excellent Nernstian response to CO2 gas in both dry and humid atmospheres. The fabricated CO2 sensor can be well operated at 400 °C in a dry atmosphere and 450 °C in a humid atmosphere. Based on the results, we have proposed that the good CO2 sensing performance may be associated with Li2CeO3 playing a role of "ionic bridge" between the O2- conductor (Y-LSBO) and the Li+ conductor (Li2CO3). This study not only shows the promising potential of a Y-LSBO solid electrolyte utilized in the field of gas sensors but also enriches the research of solid electrolyte-based potentiometric CO2 gas sensors..
12. Koichi Suematsu, Hotaka Uchino, Takaharu Mizukami, Ken Watanabe, Kengo Shimanoe, Oxygen adsorption on ZrO2-loaded SnO2 gas sensors in humid atmosphere, Journal of Materials Science, 10.1007/s10853-018-3020-y, 54, 4, 3135-3143, 2019.02, Oxygen adsorption on the surface of particles plays a key role in gas sensing for SnO2-based resistive-type gas sensors in a humid atmosphere. In this study, we added an extremely small amount of ZrO2 on the SnO2 surface to activate oxygen adsorption and enhance the sensor response of a SnO2-based gas sensor in a humid atmosphere. We evaluated the oxygen adsorption properties and sensor response to 200 ppm H2 in a humid atmosphere (96%) based on variations in the measured electrical resistance. The adsorption of O and O2− ions was activated by a small loading (0.033 mol%) of ZrO2, and the resulting ZrO2–SnO2 composite nanoparticles were in the deep electron-depleted state. This led to a high sensor response of ZrO2–SnO2 to H2 in a humid atmosphere. The results demonstrate that surface modification using an extremely small amount of ZrO2 was effective in improving the response of a SnO2-based gas sensor in a humid atmosphere..
13. Ken Watanabe, Shingo Ide, Takashi Kumagai, Takaaki Fujino, Koichi Suematsu, Kengo Shimanoe, Oxygen pumping based on c-axis-oriented lanthanum silicate ceramics
Challenge toward low operating temperature, Journal of the Ceramic Society of Japan, 10.2109/jcersj2.18172, 1-4, 2019.01, A new electrochemical oxygen separation pump was developed by using c-axis-oriented La 9.66 Si 5.3 B 0.7 O 26.14 (c-LSBO), which has high oxide-ionic conductivity (>10 13 S cm 11 ) up to 300°C. Interfacial resistance between the electrode and c-LSBO was investigated to realize the full potential of LSBO as an oxygen separation material. The formation of a Sm-doped CeO 2 (SDC) thin film (thickness: 300 nm) between the electrode and c-LSBO was effective for suppressing the interfacial resistance. Furthermore, a mixed conductive La 0.6 Sr 0.4 Co 0.78 Ni 0.02 Fe 0.2 O 3 1 ¤ (LSCFN) was applied to the electrode for enhancing the oxygen reduction/evolution activity on the electrode. The LSCFN/SDC/c-LSBO symmetric cell showed an oxygen permeation flux of 3.5 mL cm 12 min 11 (1.0 A cm 12 ) at 600°C under an applied DC voltage of 1.5 V; this value was 67 times that of Pt/c-LSBO. This oxygen pump based on the LSCFN/SDC/c-LSBO symmetric cell would find promising application in oxygen separation at intermediate temperatures. Further reduction of the interfacial resistance and polarization resistance of the electrode may decrease the operating temperatures to below 400°C..
14. Koichi Suematsu, Wataru Harano, Tokiharu Oyama, Yuka Shin, Ken Watanabe, Kengo Shimanoe, Pulse-Driven Semiconductor Gas Sensors Toward ppt Level Toluene Detection, Analytical Chemistry, 10.1021/acs.analchem.8b03076, 90, 19, 11219-11223, 2018.10, Improvements in the responses of semiconductor gas sensors and reductions in their detection limits toward volatile organic compounds (VOCs) are required in order to facilitate the simple detection of diseases, such as cancer, through human-breath analysis. In this study, we introduce a heater-switching, pulse-driven, micro gas sensor composed of a microheater and a sensor electrode fabricated with Pd-SnO2-clustered nanoparticles as the sensing material. The sensor was repeatedly heated and allowed to cool by the application of voltage to the microheater; the VOC gases penetrate into the interior of the sensing layer during its unheated state. Consequently, the utility factor of the pulse-driven sensor was greater than that of a conventional, continuously heated sensor. As a result, the response of the sensor to toluene was enhanced; indeed, the sensor responded to toluene at levels of 1 ppb. In addition, according to the relationship between its response and concentration of toluene, the pulse-driven sensor in this report can detect toluene at concentrations of 200 ppt and even lower. Therefore, the combination of a pulse-driven microheater and a suitable material designed to detect toluene resulted in improved sensor response, and facilitated ppt-level toluene detection. This sensor may play a key role in the development of medical diagnoses based on human breath..
15. Shoichi Sugata, Noriko Saito, Akio Watanabe, Ken Watanabe, Je Deok Kim, Kan Kitagawa, Yosuke Suzuki, Itaru Honma, Quasi-solid-state lithium batteries using bulk-size transparent Li7La3Zr2O12 electrolytes, Solid State Ionics, 10.1016/j.ssi.2018.02.029, 319, 285-290, 2018.06, A thick bulk-sized Li7La3Zr2O12 (LLZ) solid electrolyte is successfully prepared using a polymer complex method and a hot isostatic pressing (HIP) treatment. Despite the difficulties of densification, HIP-treated LLZ can be prepared as a transparent ceramic pellet because of its ~200 μm primary grain size, and its conductivity is improved by its increased density (>99%). An all-solid-state lithium battery is fabricated by using a mixed conducting ionic liquid gel to connect the bulk-sized LLZ electrolyte and a LiFePO4 (LFP) cathode. The introduction of the mixed conducting gel layer reduces the electrode/electrolyte interface resistance. The specific discharge capacity of the active cathode material is 169 mAh g−1 (current density of 0.046 mA cm−2), which suggests that almost the full capacity of LFP can be utilized even under the all-solid-state cell design. The coulombic efficiency (ηc) is ≈1, and no side reactions are observed. The LFP/LLZ/Li cell shows a high electrochemical stability against the Li metal anode, and the cell operates for 20 cycles without dendrite growth causing a short circuit under a charge current density of 0.591 mA cm−2 (1/2 C rate)..
16. Noriko Saito, Ken Watanabe, Hajime Haneda, Isao Sakaguchi, Kengo Shimanoe, Highly Sensitive Ethanol Gas Sensor Using Pyramid-Shaped ZnO Particles with (0001) Basal Plane, Journal of Physical Chemistry C, 10.1021/acs.jpcc.8b01936, 122, 13, 7353-7360, 2018.04, For monitoring of air quality and medical diagnosis, metal-oxide-semiconductor particles with high sensitivity to detect small amount of gases are desirable. Herein, we report the fabrication of ZnO pyramid-shaped particles with remarkably high sensitivity to ethanol gas. The ZnO pyramid-shaped particles were synthesized solvothermally under agitation from the solution of zinc acetate anhydride, hexamethylenetetramine, ethylene glycol, and water. Gas sensing response was evaluated as the ratio of electrical resistance of the ZnO particulate layer in air to that in ethanol. The agitation during the solvothermal process resulted in dispersion of the pyramid-shaped particles rather than spherical aggregates. TEM studies revealed that the base of the pyramid-shaped particles is the (0001) plane and that the six side surfaces are the {101 1} plane. The highest gas sensing response value to 50 ppm ethanol gas was about 10※000, which is remarkably higher than that of previously reported ZnO particles. The influence of the crystal facets and the polarity is discussed..
17. Koichi Suematsu, Nan Ma, Ken Watanabe, Masayoshi Yuasa, Tetsuya Kida, Kengo Shimanoe, Effect of humid aging on the oxygen adsorption in SnO2 gas sensors, Sensors, 10.3390/s18010254, 18, 1, 2018.01, To investigate the effect of aging at 580 °C in wet air (humid aging) on the oxygen adsorption on the surface of SnO2 particles, the electric properties and the sensor response to hydrogen in dry and humid atmospheres for SnO2 resistive-type gas sensors were evaluated. The electric resistance in dry and wet atmospheres at 350 °C was strongly increased by humid aging. From the results of oxygen partial pressure dependence of the electric resistance, the oxygen adsorption equilibrium constants (K1; for O- adsorption, K2; for O2- adsorption) were estimated on the basis of the theoretical model of oxygen adsorption. The K1 and K2 in dry and wet atmospheres at 350 °C were increased by humid aging at 580 °C, indicating an increase in the adsorption amount of both O- and O2-. These results suggest that hydroxyl poisoning on the oxygen adsorption is suppressed by humid aging. The sensor response to hydrogen in dry and wet atmosphere at 350 °C was clearly improved by humid aging. Such an improvement of the sensor response seems to be caused by increasing the oxygen adsorption amount. Thus, the humid aging offers an effective way to improve the sensor response of SnO2 resistive-type gas sensors in dry and wet atmospheres..
18. Yongjiao Sun, Koichi Suematsu, Ken Watanabe, Maiko Nishibori, Jie Hu, Wendong Zhang, Kengo Shimanoe, Determination of effective oxygen adsorption species for CO sensing based on electric properties of indium oxide, Journal of the Electrochemical Society, 10.1149/2.0591807jes, 165, 7, B275-B280, 2018.01, To understand the relationship between adsorbed oxygen on the surface of indium oxide (In
) and its CO-sensing property in wet and dry atmospheres, the oxygen adsorption properties and sensor response to CO were investigated. In
particles were prepared by a precipitation method and characterized using X-ray diffraction, scanning electron microscopy, and nitrogen absorption measurements. The O− and O
adsorption equilibrium constants, K
and K
, respectively, were calculated from the relationship between the electrical resistance and the oxygen partial pressure (P
) using a competitive adsorption model. It was found that the values of both K
and K
decreased with increasing humidity, indicating a reduction in the amount of oxygen adsorption on the In
surface in a wet atmosphere. The In
particles can detect CO at ppm level in dry and wet atmospheres, although they were greatly reduced in wet atmospheres. Moreover, we found that the sensor response to CO is closely related to the values of K
and K
19. Shun Fukami, Munetaka Taguchi, Yutaka Adachi, Isao Sakaguchi, Ken Watanabe, Toyohiko Kinoshita, Takayuki Muro, Tomohiro Matsushita, Fumihiko Matsui, Hiroshi Daimon, Taku T. Suzuki, Correlation between High Gas Sensitivity and Dopant Structure in W-doped ZnO, Physical Review Applied, 10.1103/PhysRevApplied.7.064029, 7, 6, 2017.06, W-doped ZnO films are synthesized by pulsed laser deposition on Al2O3(1120) substrate, and their gas-sensing properties are investigated. Significantly improved sensitivity and response time to ethanol vapor are achieved. In order to study the relation between high sensing performance and local structure around the dopant W site, we carry out two-dimensional photoelectron diffraction on W-doped ZnO thin films at two different annealing temperatures (600 and 1000 °C). The photoelectron-intensity angular-distribution (PIAD) patterns of Zn 3p and W 4f core levels show the presence of forward-focusing peaks of very different relative intensities in the bulk and surface. From the PIAD patterns and their circular dichroism, we find clear evidence for the substitution of a segregated W atom into a Zn site in the second atomic layer. The result shows a direct relationship between the drastic increase in gas-sensing performance via W doping and the segregation of W at the surface. It also demonstrates that this photoelectron-diffraction measurement is a powerful tool for the study of the local structure of dopant sites in gas-sensing materials..
20. Shunsuke Gunji, Masaya Jukei, Yasuhiko Shimotsuma, Kiyotaka Miura, Koichi Suematsu, Ken Watanabe, Kengo Shimanoe, Unexpected gas sensing properties of SiO2/SnO2 core-shell nanofibers under dry and humid conditions, Journal of Materials Chemistry C, 10.1039/c7tc01642e, 5, 25, 6369-6376, 2017.01, SiO2/SnO2 core-shell nanofibers were synthesized using TEMPO-oxidized cellulose nanofibers as templates. The gas sensing properties of the synthesized SiO2/SnO2 core-shell nanofibers were evaluated under dry and humid conditions and compared to practical SnO2 nanoparticles produced by a hydrothermal method. The sensor responses were analyzed by monitoring the electric resistance changes upon the introduction of H2 or CO analytes. Surprisingly, the SiO2/SnO2 nanofibers showed a prominent sensor response at a higher temperature at which the adsorbed oxygen on the SnO2 nanoparticles desorbs under dry conditions. Unexpectedly, the SiO2/SnO2 nanofibers are available in humid atmosphere since they are less influenced by H2O poisoning compared to the SnO2 nanoparticles. Such an intriguing phenomenon could be interpreted in terms of the lower susceptibility of the major adsorbed oxygen species toward water vapor..
21. Yoshinori Higashi, Shoichi Itoh, Minako Hashiguchi, Shuhei Sakata, Takafumi Hirata, Ken Watanabe, Isao Sakaguchi, Hydrogen diffusion in the apatite-water system
Fluorapatite parallel to the c-axis, Geochemical Journal, 10.2343/geochemj.2.0460, 51, 1, 115-122, 2017, Many studies in the past decade have sought to explore the origin and evolution of water in planetary bodies based on the hydrogen isotopic compositions of apatite. However, no investigation has studied hydrogen diffusivity in apatite. This work reports hydrogen diffusion experiments using a natural Durango fluorapatite carried out under a saturated 2H2O/O2 vapor flow at temperatures of 500-700 °C. Diffusion depth profiles for 1H and 2H were measured using secondary ion mass spectrometry (SIMS), indicating that 2H diffusion occurred by an exchange reaction between the original 1H and 2H during annealing. Hydrogen diffusion coefficients were obtained by the fitting of diffusion profiles of 2H using Fick's second law; they followed an Arrhenius-type relationship. The temperature dependence of hydrogen diffusion parallel to the c-axis at 500-700°C can be expressed as D=6.71×10-13exp(-80.5 ±3.3/RT)[m/s] Hydrogen diffusion coefficients in apatite are several orders of magnitude greater than those of other elements. Hydrogen diffusion in apatite occurs at relatively low temperatures (below 700°C). This study indicates that the hydrogen isotopic compositions of apatite are readily affected by the presence of water vapor through the 1H-2H exchange reaction without changing the total water content in the crystal..
22. Kazunori Nishio, Tsuyoshi Ohnishi, Kazutaka Mitsuishi, Narumi Ohta, Ken Watanabe, Kazunori Takada, Orientation alignment of epitaxial LiCoO2 thin films on vicinal SrTiO3 (100) substrates, Journal of Power Sources, 10.1016/j.jpowsour.2016.06.015, 325, 306-310, 2016.09, LiCoO2 is epitaxially grown on SrTiO3 (100) substrates with (104) orientation. Because the LiCoO2 film is grown with its c-axis parallel to four equivalent 〈111〉 axes of the SrTiO3, the (104)-oriented film exhibits four-domain structure on the SrTiO3 (100) substrate. Introducing off-cut angle to the substrate surface breaks the equivalency between the four 〈111〉 axes of the SrTiO3 substrate to induce preferential growth of specific orientation with the c-axis in a descending direction of off-cut surface. Increasing off-cut angle and lowering deposition rate promote the preferential growth, because they facilitate step-flow growth mode, and finally align the c-axes in the domains completely into one 〈111〉 direction of the SrTiO3 substrate. The LiCoO2 film delivers a discharge capacity of 90 mAh g−1 at a low discharge rate of 0.01 C, and 25% of capacity is kept even at a high rate of discharge with 100 C..
23. Masayoshi Yuasa, Koichi Suematsu, Kiyomi Yamada, Ken Watanabe, Tetsuya Kida, Noboru Yamazoe, Kengo Shimanoe, Thermally stable SnO2 nanocrystals
Synthesis and application to gas sensors, Crystal Growth and Design, 10.1021/acs.cgd.6b00087, 16, 8, 4203-4208, 2016.08, In this study, we prepared thermally stable SnO2 nanocrystals (ca. 4 nm) in a mixture of SnCl4, tetraethylene glycol (TEG), and tetrabutylammonium hydroxide (TBAH) under reflux and obtained a highly sensitive semiconductor gas sensor. It has been determined both theoretically and experimentally that the synthesis of oxide semiconductor nanoparticles is an important factor in highly sensitive semiconductor gas sensors. However, as-synthesized nanocrystals generally grow large during calcination at high temperature, and this thermal crystal growth reduces the sensor response. Therefore, to refine the response of the semiconductor gas sensor, we synthesized thermally stable SnO2 nanocrystals by heating under reflux a SnCl4-TBAH-TEG mixture. The obtained SnO2 nanocrystals exhibited high thermal stability even when calcined at a temperature up to 600 °C. The gas-sensing films fabricated from the thermally stable SnO2 nanocrystals exhibited a high sensor response to hydrogen due to their small crystal size, and a change in their surface property as compared with conventional SnO2 nanocrystals synthesized via hydrothermal treatment..
24. Y. Suzuki, Ken Watanabe, S. Sakuma, N. Imanishi, Electrochemical performance of an all-solid-state lithium-oxygen battery under humidified oxygen, Solid State Ionics, 10.1016/j.ssi.2016.03.003, 289, 72-76, 2016.06, The electrochemical performance of an all-solid-state lithium-oxygen battery was examined using a platinum patterned electrode with a lithium conducting solid electrolyte under a humidified O2 atmosphere. The cell consisted of a lithium metal anode, a protective layer of polyethylene oxide with Li(CF3SO2)2N, a lithium-conducting solid electrolyte of Li1 + x + yAlx(Ge,Ti)2 - xSiyP3 - yO12, and a sputtered platinum stripe-patterned air electrode. The open circuit voltage (OCV) of the virgin cell was 3.5 V under humidified O2 and decreased to 3.2 V after slight discharge, which is comparable with that under dry O2 (2.4 V). The cell was successfully discharged and charged at 0.1 mA cm- 2 and 60 °C. A reaction product was observed on the platinum electrode; however, it could not be identified by X-ray diffraction analysis or Raman spectroscopy, which indicated it was amorphous. Nano-secondary ion mass spectrometry (SIMS) analysis suggested that the reaction product contains Li, H, and O..
25. Isao Sakaguchi, Noriko Saito, Ken Watanabe, Yutaka Adachi, Taku T. Suzuki, Evaluation of sensor property for hydrogen and ethanol of zinc-doped tin-dioxide thin films fabricated by RF sputtering, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.16015, 124, 6, 714-716, 2016.06, Zinc-doped Tin-dioxide thin (SnO2) films were synthesized on c-alumina substrates by radio-frequency (rf ) sputtering. To fabricate zinc-doped SnO2 thin films with different concentrations of zinc, an insulating zinc-doped SnO2 target was placed on a conductive non-doped target. Thin films were then deposited at different flow rates of argon gas. Zinc concentration in the deposited SnO2 thin films increased with increasing flow rate of argon gas. Moreover, the SnO2 thin film with a zinc concentration of 2.2 × 1019/cm3 showed the highest gas-sensor response. It is concluded that this rf-sputtering growth method is a useful tool for identifying suitable materials compositions for creating gas sensor with response for hydrogen and ethanol..
26. Yutaka Adachi, Ken Watanabe, Noriko Saito, Isao Sakaguchi, Taku T. Suzuki, Gas sensing properties of C -axis-oriented Al-incorporated ZnO films epitaxially grown on (112-0) sapphire substrates using pulsed laser deposition, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.15319, 124, 6, 668-672, 2016.06, Al-incorporated ZnO films with various Al concentrations were prepared using pulsed laser deposition on the (112-0) face of sapphire substrates, and their gas sensing properties were evaluated. The use of c-axis-oriented epitaxial films with the same thickness suppressed the influence of the surface-to-volume ratio and surface atomic arrangements on the sensing properties, clarifying the role of Al doping in the improvement of ZnO gas sensors. The results of ethanol gas sensing measurements indicated that Al doping significantly improved the sensing response and response time of the ZnO gas sensors. The formation of Al-related impurity phases and/or non-equilibrium defects induced by Al doping improved the sensing performance. On the other hand, changes in the grain size did not significantly affect the sensing response..
27. Ken Watanabe, Isao Sakaguchi, Minako Hashiguchi, Noriko Saito, Emily M. Ross, Hajime Haneda, Takeo Ohsawa, Naoki Ohashi, Isotope tracer investigation and ab-initio simulation of anisotropic hydrogen transport and possible multi-hydrogen centers in tin dioxide, Journal of Applied Physics, 10.1063/1.4953387, 119, 22, 2016.06, Hydrogen as an impurity in single crystals of tin dioxide was investigated through diffusivity and vibrational-mode analyses performed using isotope tracers and density functional theory calculations. It was found that hydrogen diffusion along the 001 axis is very fast, even at relatively low temperatures (400 �C), but is considerably slower within the (001) plane. Using transitional state calculations, this diffusion behavior was determined to be the result of anisotropy in the migration barrier for interstitial hydrogen (Hi). In addition, the two distinct vibrational modes observed in the optical spectrum were identified as the O-H stretching modes of Hi and the substitutional hydrogen at the tin sites..
28. Kazunori Nishio, Tsuyoshi Ohnishi, Minoru Osada, Narumi Ohta, Ken Watanabe, Kazunori Takada, Influences of high deposition rate on LiCoO2 epitaxial films prepared by pulsed laser deposition, Solid State Ionics, 10.1016/j.ssi.2015.06.006, 285, 91-95, 2016.02, This study exhibits influence of deposition rate on LiCoO2 epitaxial films prepared by pulsed laser deposition. High deposition rates tend to destabilize trivalent Co, resulting in formation of an oxygen-deficient phase with a rock-salt structure as an impurity in the deposited films. Because the oxygen-deficient phase originates from incomplete oxidation, post-annealing in air eliminates the impurity phase. Moreover, since it also improves the crystallinity of the film, the post-annealing enhances the electrode performance of the LiCoO2 films..
29. Kengo Shimanoe, N. Ma, T. Oyama, Maiko Nishibori, Ken Watanabe, High performance of SnO2-based gas sensor by introducing perovskite-type oxides, Symposium on Chemical Sensors 12: Chemical and Biological Sensors and Analytical Systems - PRiME 2016/230th ECS Meeting Chemical Sensors 12 Chemical and Biological Sensors and Analytical Systems, 10.1149/07516.0031ecst, 75, 31-37, 2016, Sensing layer mixed perovskite oxide, La0.1Sr0.9Co0.4Fe0.6O3-δ (LSCF), and Pd-loaded SnO2 nanoparticles (LSCF@Pd-SnO2) was deposited on the membrane of MEMS device by injection method, and its gas sensing properties to reducing gases and VOCs were investigated by heating sensor in pulse mode with heaters in ON and OFF state for one second repeatedly. The MEMS sensor showed fast response, high sensitivity and stability to 200 ppm C7H8 at 250°C by pulse heating with extremely low power consumption..
30. Y. Suzuki, K. Kami, Ken Watanabe, N. Imanishi, Characteristics of discharge products in all-solid-state Li-air batteries, Solid State Ionics, 10.1016/j.ssi.2015.06.022, 278, 222-227, 2015.07, Abstract The reaction product and reaction site for solid-state lithium-air batteries were examined using a platinum pattern electrode on a lithium conducting solid electrolyte. The cell consisted of a lithium metal anode, a protective layer consisting of polyethylene oxide with the Li(CF3SO2)2N lithium salt, a lithium conducting solid electrolyte of Li1 + x + yAlx(Ge,Ti)2 - xSiyP3 - yO12, and a sputtered platinum striped pattern air electrode. The cell was discharged at 1.0 μA cm- 2 for 300 h and the reaction product was analyzed using Raman spectroscopy and X-ray photoelectron spectroscopy. The reaction product of Li2O2 was observed at the triple phase boundaries of oxygen, platinum and electrolyte, in addition to on the platinum and solid electrolyte surfaces. This result suggests that the oxygen reduction reaction can proceed on the surface of the Li2O2 reaction product..
31. Takatoshi Orui, Johannes Herms, Yuichiro Hanyu, Shigenori Ueda, Ken Watanabe, Isao Sakaguchi, Naoki Ohashi, Hidenori Hiramatsu, Hideya Kumomi, Hideo Hosono, Toshio Kamiya, Charge compensation by excess oxygen in amorphous In-Ga-Zn-O films deposited by pulsed laser deposition, IEEE/OSA Journal of Display Technology, 10.1109/JDT.2014.2358251, 11, 6, 518-522, 2015.06, We investigated effects of base pressure (Pbase) of the deposition chamber on electrical properties and defect states of amorphous In-Ga-Zn-O (a-IGZO) thin films deposited by pulsed laser deposition. The impurity hydrogen concentration was increased by an order of magnitude when Pbase was deteriorated from <10-5 to 10-3Pa. The optimum oxygen partial pressures (PO2) were 2-4 Pa for an optimized deposition condition with the good Pbase; on the other hand, off-optimized and/or poor Pbase require much higher PO2. This result provides an experimental evidence for a charge compensation model by excess oxygen for H-containing a-IGZO. Thermal desorption spectra indicated that the impurity hydrogens originate mainly from water molecules in the residual gas and exist as -OH chemical bonding states in the a-IGZO films. Hard X-ray photoemission spectroscopy revealed that these -OH states form deep defects above the valence band maximum..
32. Noriko Saito, Kenji Matsumoto, Ken Watanabe, Minako Hashiguchi, Isao Sakaguchi, Hajime Haneda, Microscopic and isotope tracer study on the growth of spherical ZnO particles in water-ethylene glycol solvent, Crystal Growth and Design, 10.1021/cg5014297, 15, 6, 2609-2619, 2015.06, ZnO spherical powder with hierarchical structure was synthesized from zinc acetate anhydride and hexamethylenetetramine in a solvent of water and ethylene glycol (EG), and its time evolution was investigated by microscopic and isotope tracer studies. Initially, spherical ZnO particles made of nanocrystallites were precipitated. From the early growth stage, crystallites were radially aligned along the c-axis. With aging, the nanoparticles transformed into hexagonal prism-like particles with pyramidal tops, which showed clear radial alignment. Subsequently, the ZnO spheres became cracked and broke into wedge-shaped pieces of the pyramid-like particles. The microscopic studies suggested that the spherical particles grew through oriented attachment of small particles during the early growth stage, while Ostwald ripening through dissolution-recrystallization changed the component particles to a pyramidal shape in the later stage. In order to investigate the growth process, we attempted a novel isotope tracer study, where H218O was used for the solvent and the distribution of 18O in the spherical particles was analyzed by a secondary ion mass spectrometer. The concentric 18O distribution in the ZnO spherical powder aided the mechanism of continuous concentric-oriented attachment of nanoparticles to form spherical ZnO particles. The difference in the 18O distribution before and after transformation into the pyramid shape was small, which implies that this change was a local reaction. The oxygen isotope tracer study in water-EG solvent was proven effective in determining the history of the particles and the development of their inner structures..
33. Yosuke Suzuki, K. Kami, Ken Watanabe, A. Watanabe, N. Saito, T. Ohnishi, K. Takada, R. Sudo, N. Imanishi, Transparent cubic garnet-type solid electrolyte of Al2O3-doped Li7La3Zr2O12, Solid State Ionics, 10.1016/j.ssi.2015.06.009, 278, 172-176, 2015.06, A transparent garnet-type lithium-ion conducting solid electrolyte of 1.0 wt% Al2O3-doped Li7La3Zr2O12 (A-LLZ) was prepared using hot isostatic pressing (HIP). The A-LLZ pellet sintered at 1180°C for 36 h was followed by HIP treatment at 127 MPa and 1180°C under an Ar atmosphere. The bulk conductivity of the HIP treated A-LLZ was 9.9 × 10- 4 S cm- 1 at 25°C. The Li/HIP treated A-LLZ/Li cell showed no short-circuit due to lithium dendrite formation at 0.5 mA cm- 2..
34. Tetsuya Kida, Subaru Ninomiya, Hiroki Tatsushiro, Ken Watanabe, Masayoshi Yuasa, Kengo Shimanoe, Oxygen separation from air using Ba0.95La0.05FeO3-δ membranes fitted with porous La1-xSrxFeO3-δ layers, Ceramics International, 10.1016/j.ceramint.2015.02.118, 41, 6, 7830-7835, 2015.01, Selective oxygen separation from air was performed using perovskite-type oxide membranes made of Ba0.95La0.05FeO3-δ. We demonstrated that surface modification of Ba0.95La0.05FeO3-δ membranes with La1-xSrxFeO3-δ catalyst layers led to an increase in oxygen permeation fluxes at 700-930 °C. We studied the effects of oxygen vacancy amounts, surface area, particles size, surface treatment of La1-xSrxFeO3-δ on the oxygen permeability of the membranes fitted with La1-xSrxFeO3-δ catalyst layers. Among the catalyst layers tested, the membranes fitted with La0.9Sr0.1FeO3-δ (x=0.1) showed the highest oxygen permeation flux probably because of its higher porosity and uniform morphology without open voids, which would increase the number of surface reaction sites. The obtained results suggest the feasibility of further upgrading the membrane performance by using surface catalyst layers having a homogeneous morphology and a different composition from that of the mother membrane..
35. Takane Kobayashi, Atsushi Kamoshida, Hideo Akiyama, Ken Watanabe, Tsuyoshi Ohnishi, Kazunori Takada, Taku T. Suzuki, Development of microscopy for lithium analysis using medium-energy ion-stimulated desorption, Applied Physics Express, 10.7567/APEX.7.106601, 7, 10, 2014.10, A medium-energy ion-stimulated desorption spectroscope was combined with a helium ion microscope to develop a microscope for lithium (Li) analysis with high spatial resolution below 10nm. The analysis of Li is based on time-of-flight (TOF) spectroscopy. A clear Li peak was observed in the TOF spectra for the samples containing Li. The capability of the microscope was demonstrated using a LiCoO2/Nb doped SrTiO3 sample. Furthermore, the Li distribution on the sample was successfully observed by measuring the Li+ peak intensity as a function of the helium irradiation position..
36. Ken Watanabe, Takeo Ohsawa, Isao Sakaguchi, Oliver Bierwagen, Mark E. White, Min Ying Tsai, Ryosuke Takahashi, Emily M. Ross, Yutaka Adachi, James S. Speck, Hajime Haneda, Naoki Ohashi, Investigation of charge compensation in indium-doped tin dioxide by hydrogen insertion via annealing under humid conditions, Applied Physics Letters, 10.1063/1.4870425, 104, 13, 2014.03, The behavior of hydrogen (H) as an impurity in indium (In)-doped tin dioxide (SnO2) was investigated by mass spectrometry analyses, with the aim of understanding the charge compensation mechanism in SnO2. The H-concentration of the In-doped SnO2 films increased to (1-2)×1019cm-3 by annealing in a humid atmosphere (WET annealing). The electron concentration in the films also increased after WET annealing but was two orders of magnitude less than their H-concentrations. A self-compensation mechanism, based on the assumption that H sits at substitutional sites, is proposed to explain the mismatch between the electron- and H-concentrations..
37. Tsuyoshi Ogino, Jesse R. Williams, Ken Watanabe, Isao Sakaguchi, Shunichi Hishita, Hajime Haneda, Yutaka Adachi, Takeshi Ohgaki, Naoki Ohashi, Effect of crystalline polarity on microstructure and optoelectronic properties of gallium-doped zinc oxide films deposited onto glass substrates, Thin Solid Films, 10.1016/j.tsf.2013.12.017, 552, 56-61, 2014.02, The effect of crystalline polarity on the microstructure of gallium-doped zinc oxide (GZO) deposited using magnetron sputtering onto glass substrates was investigated. X-ray photoelectron spectroscopy was used to determine the crystalline polarity of c-axis textured GZO films. Grains whose radii were more than 1 μm grew abnormally in 0.2 mol% doped GZO when the film was thicker than ~ 1 μm, and the radius of the grains was much smaller than 100 nm in the heavily (i.e., 4 mol%) doped GZO, regardless of the film thickness. Such abnormal growth of the grains in the 0.2 mol% doped GZO films coincided with a change in the crystalline polarity: the surfaces of unusually large GZO grains were terminated with the (0001̄) face, and those of normal GZO grains were terminated with the (0001) face. The results indicated that polarity flipping is a very important event for controlling the texture of doped zinc oxide films..
38. Noriko Saito, Ken Watanabe, Tangi Aubert, Fabien Grasset, Isao Sakaguchi, Hajime Haneda, Annealing effect on microstructure of ZnO nano-particulate films and VOC gas sensing property, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.122.267, 122, 1424, 267-270, 2014.01, We synthesized ZnO nano-particulate films by a sol-gel method and evaluated their gas sensing properties. A ZnO colloidal solution of 5.6 nm sized nanocrystals was prepared from zinc acetate in n-propanol and dip-coated on silica glass substrates. The as-deposited ZnO film consisted of 30nm grains. Annealing the films resulted in a growth of the particles, which was confirmed by scanning probe microscopy and UV-Vis spectrometry. The ZnO film annealed at 400°C worked well as a gas sensor device for volatile organic compounds (VOC) gases. The sensitivity was the best in the case of ethanol gas at 350°C. The sensing performances decreased by annealing at higher temperatures due to the coalescence of the ZnO particles..
39. Jumpei Suzuki, Kenji Matsumoto, Minoru Ryu, Yoshiki Iwazaki, Toshimasa Suzuki, Ken Watanabe, Minako Hashiguchi, Isao Sakaguchi, Naoki Ohashi, Effects of dielectric film surface on oxygen diffusion, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.122.410, 122, 1426, 410-414, 2014.01, The oxygen diffusivity in BaTiO3 thin films heteroepitaxially grown on SrTiO3 substrates was investigated using a gas/solid exchange technique with 18O-isotope-enriched gas. Deformation of the BaTiO 3 lattice and inhibition of the oxygen diffusivity occurred simultaneously when the YSZ layer, which is assumed to be catalytic for the 18O/16O exchange reaction, was deposited on BaTiO3. The mechanism for the reduction in oxygen diffusivity due to the YSZ cover layer is discussed in terms of residual stress in the strained BaTiO3 layer..
40. Ken Watanabe, Minako Hashiguchi, Isao Sakaguchi, Alex Bryant, Yutaka Adachi, Yuhua Zhen, Takeshi Ohgaki, Takeo Ohsawa, Hajime Haneda, Naoki Ohashi, Hydrogen in tin dioxide films and bulk ceramics
An attempt to identify the most hidden impurity, Applied Physics Letters, 10.1063/1.4863668, 104, 4, 2014.01, Hydrogen impurities in SnO2 films and bulk ceramics were investigated in terms of mass transport and electron transport. The hydrogen concentration (n[H]) in these samples was found to be 1019 cm-3 or higher. Further increase in n[H] could be achieved by annealing the samples in a humid atmosphere. The isotope tracer (1H/2H exchange) study revealed that a part of the hydrogen in these samples showed rapid migration even at 300 °C. However, electrical measurements revealed that the electron concentration in the samples was much less than n[H]. These results could be explained by assuming the presence of defect-hydrogen complexes..
41. Takaya Miyase, Ken Watanabe, Isao Sakaguchi, Naoki Ohashi, Kay Domen, Kenji Nomura, Hidenori Hiramatsu, Hideya Kumomi, Hideo Hosono, Toshio Kamiya, Roles of hydrogen in amorphous oxide semiconductor In-Ga-Zn-O
Comparison of conventional and ultra-high-vacuum sputtering, ECS Journal of Solid State Science and Technology, 10.1149/2.015409jss, 3, 9, Q3085-Q3090, 2014.01, We investigated roles of hydrogen on physical properties of a-IGZO films and thin-film transistors (TFTs) by comparing standard and ultra-high vacuum (UHV) sputtering systems. It was confirmed that the impurity hydrogens come mainly from the residual gas in the deposition chamber and the molecules adsorbed to the surface of the sputtering target. It was found impurity hydrogen has unfavorable effects as follows; (i) enhances selective Zn desorption during film deposition, and (ii) weakens chemical bonds of the resulting film, causing temperature instability. On the other hand, the UHV a-IGZO films with less hydrogen had low density and exhibited structural instability, suggesting that some hydrogens would have a favorable effect to enhance structural relaxation rate and to form denser and more stable structures during film deposition at room temperature. The revealed hydrogen effects are discussed in relation to those in amorphous silicon and silicon dioxide..
42. Noriko Saito, Kenji Matsumoto, Ken Watanabe, Tangi Aubert, Fabien Grasset, Isao Sakaguchi, Hajime Haneda, Solvothermal synthesis of ZnO spherical particles and VOC sensor application, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.122.488, 122, 1426, 488-491, 2014.01, We synthesized micrometer-sized ZnO spherical particles solvothermally from the solutions of zinc acetate anhydride, hexamethylenetetramine (HMT), ethylene glycol (EG) and water, and examined the gas sensing properties. The spherical powders were composed of nano particles, which were radially aligned along the c-axis. We obtained particles with smaller crystallites under conditions of higher EG concentrations and shorter reaction period. Spherical powder made of crystallites of 35 nm was precipitated in the 95 vol%-EG solvent by heating at 120°C for 4 h. The ZnO powder annealed at 450°C worked as a gas sensor device for volatile organic compounds (VOC) gases. The sensitivity was the best in the case of ethanol gas at 350°C..
43. Isao Sakaguchi, Sakyo Hirose, Tomohiro Furuta, Ken Watanabe, Keisuke Kageyama, Shunichi Hishita, Hajime Haneda, Naoki Ohashi, Oxygen diffusion in rare-earth doped BaTiO3 ceramics, 32nd Electronics Division Meeting of the Ceramic Society of Japan Electroceramics in Japan XVI, 10.4028/, 582, 189-193, 2014, The effect of the surface preparation in samarium doped semiconducting barium titanate [(Ba1-xSmx)TiO3] ceramics with (Ba, Sm)/Ti ratio of 1.000 was studied by means of isotope tracer technique using a secondary ion mass spectrometer. The surfaces of specimens were prepared with the chemical mechanical polishing (CMP) with colloidal silica slurry or the mechanical polishing (MP) with diamond paste. The oxygen diffusion coefficients obtained in the CMP samples were small compared to those in the mechanical polished samples. This fact suggests that the surface prepared with CMP has less oxygen defect concentration. Moreover, it was also indicated that high temperature treatment over 1000 °C is required for annihilation of defects formed by MP. The oxygen diffusion study used CMP sample brings the useful information on the oxygen defect chemistry in Sm doped BaTiO3..
44. Shinsuke Ishihara, Pathik Sahoo, Kenzo Deguchi, Shinobu Ohki, Masataka Tansho, Tadashi Shimizu, Jan Labuta, Jonathan P. Hill, Katsuhiko Ariga, Ken Watanabe, Yusuke Yamauchi, Shigeru Suehara, Nobuo Iyi, Dynamic breathing of co2 by hydrotalcite, Journal of the American Chemical Society, 10.1021/ja4099752, 135, 48, 18040-18043, 2013.12, The carbon cycle of carbonate solids (e.g., limestone) involves weathering and metamorphic events, which usually occur over millions of years. Here we show that carbonate anion intercalated layered double hydroxide (LDH), a class of hydrotalcite, undergoes an ultrarapid carbon cycle with uptake of atmospheric CO2 under ambient conditions. The use of 13C-labeling enabled monitoring by IR spectroscopy of the dynamic exchange between initially intercalated 13C-labeled carbonate anions and carbonate anions derived from atmospheric CO2. Exchange is promoted by conditions of low humidity with a half-life of exchange of ∼24 h. Since hydrotalcite-like clay minerals exist in Nature, our finding implies that the global carbon cycle involving exchange between lithosphere and atmosphere is much more dynamic than previously thought..
45. Ken Watanabe, Dong Hee Lee, Isao Sakaguchi, Kenji Nomura, Toshio Kamiya, Hajime Haneda, Hideo Hosono, Naoki Ohashi, Surface reactivity and oxygen migration in amorphous indium-gallium-zinc oxide films annealed in humid atmosphere, Applied Physics Letters, 10.1063/1.4829996, 103, 20, 2013.11, An isotope tracer study, i.e., 18O/16O exchange using 18O2 and H218O, was performed to determine how post-deposition annealing (PDA) affected surface reactivity and oxygen diffusivity of amorphous indium-gallium-zinc oxide (a-IGZO) films. The oxygen tracer diffusivity was very high in the bulk even at low temperatures, e.g., 200°C, regardless of PDA and exchange conditions. In contrast, the isotope exchange rate, dominated by surface reactivity, was much lower for 18O2 than for H218O. PDA in a humid atmosphere at 400°C further suppressed the reactivity of O2 at the a-IGZO film surface, which is attributable to -OH-terminated surface formation..
46. Takeshi Ohgaki, Ken Watanabe, Yutaka Adachi, Isao Sakaguchi, Shunichi Hishita, Naoki Ohashi, Hajime Haneda, Electrical properties of scandium nitride epitaxial films grown on (100) magnesium oxide substrates by molecular beam epitaxy, Journal of Applied Physics, 10.1063/1.4820391, 114, 9, 2013.09, Scandium nitride (ScN) films were grown on (100) MgO single crystals by a molecular beam epitaxy method. The effects of growth conditions, including [Sc]/[N] ratio, growth temperature, and nitrogen radical state, on the electrical properties of the ScN films were studied. The ScN films comprised many small columnar grains. Hall coefficient measurements confirmed that the ScN films were highly degenerate n-type semiconductors and that the carrier concentration of the ScN films was sensitive to the growth temperature and the nitrogen radical states during the film growth. The carrier concentrations of the ScN films ranged from 1019-1021 cm-3 while the Hall mobilities ranged from 50-130 cm2·V -1·s-1 for undoped films. The temperature-dependent Hall coefficient measurements showed that the carrier concentration is nearly independent of temperature, indicating that the change in resistivity with temperature is explained by a change in the Hall mobility. The temperature-dependence of the Hall mobility was strongly affected by the growth conditions..
47. Panneerselvam Mohanapriya, Hiroyo Segawa, Ken Watanabe, Kenji Watanabe, Sadaki Samitsu, T. S. Natarajan, N. Victor Jaya, Naoki Ohashi, Enhanced ethanol-gas sensing performance of Ce-doped SnO2 hollow nanofibers prepared by electrospinning, Sensors and Actuators, B: Chemical, 10.1016/j.snb.2013.07.016, 188, 872-878, 2013.09, Tin oxide (SnO2) hollow nanofibers doped with cerium oxide (CeOx) were synthesized to study the impact of CeO2 additives on their chemical sensing response. X-ray diffraction analysis and Raman scattering spectroscopic analysis showed that crystallization of CeO 2 in the fiber composites was observed only when the Ce concentration was greater than 6 mol%. Also, maximum sensing response to ethanol was seen in the fiber composites with 6 mol% addition of Ce. Accordingly, we have discussed the possible explanations for this, including the role of crystalline CeO 2 as a surface catalyst, and the role of hydrogen liberated during the dehydrogenation reaction of ethanol diffusing into the SnO2 lattice. The results obtained in this study indicate that SnO 2-CeO2 nanofiber composites are potential candidates for use as high-sensitivity gas sensors..
48. Ken Watanabe, I. Sakaguchi, S. Hishita, H. Haneda, N. Ohashi, Oxygen tracer diffusion in BaTiO3 ceramics - Effect of Zr impurity from planetary ball milling, 31st Electronics Division Meeting of the Ceramic Society of Japan Electroceramics in Japan XV, 10.4028/, 262-265, 2013.09, We investigated the effect of Zr impurities on the oxygen diffusion pathway in BaTiO3 ceramics by using an ion-imaging technique. Zr impurities were introduced into BaTiO3 ceramics by a planetary ball milling process. The oxygen tracer diffused quickly from the surface up to the grain boundary and then appeared as discontinuous steps at the grain boundary. The results indicate that the grain boundary acted as a blocking layer against oxygen diffusion. The blocking for oxygen diffusion at the grain boundary is thought to originate during the formation and distribution of complex defects between the charged oxygen vacancies and the Ba vacancies near the grain boundary..
49. Isao Sakaguchi, Ken Watanabe, Yutaka Adachi, Takeshi Ohgaki, Shunichi Hishita, Naoki Ohashi, Hajime Haneda, Oxygen tracer diffusion in a-axis oriented ZnO thin films grown on (011 2) sapphire by pulsed laser deposition, 31st Electronics Division Meeting of the Ceramic Society of Japan Electroceramics in Japan XV, 10.4028/, 566, 266-270, 2013, The a-axis oriented ZnO thin films deposited on (011 2) sapphire substrates by pulsed laser deposition were studied to investigate the effects of pre-annealing on oxygen diffusion. The effect was as follows: the oxygen diffusion coefficient decreased, and the oxygen concentration in the tailing regions of the profiles reduced. Ion images of an oxygen tracer revealed the high-diffusivity paths for oxygen tracer diffusion. The temperature dependence of oxygen tracer diffusion coefficients (Db) in as-deposited and pre-annealed thin films were determined to be Db [cm2/s] = 9.2×102 exp (-405 [kJ/mol] / RT) and Db [cm 2/s] = 1.8×103 exp (- 418 [kJ/mol] / RT), respectively. On basis of these results, the crystal orientation on D b and the mechanism for oxygen diffusion were discussed..
50. Ken Watanabe, Kenji Matsumoto, Yutaka Adachi, Takeshi Ohgaki, Tsubasa Nakagawa, Naoki Ohashi, Hajime Haneda, Isao Sakaguchi, Evaluation of zinc self-diffusion at the interface between homoepitaxial ZnO thin films and (0001) ZnO substrates, Solid State Communications, 10.1016/j.ssc.2012.07.002, 152, 20, 1917-1920, 2012.10, Isotopic ZnO thin films were deposited on the c-plane of ZnO single crystals by pulsed laser deposition. The isotopic abundance of Zn in the films was determined with a secondary ion mass spectrometry before and after the films was diffusion annealed. The diffusion profiles across the film/substrate interface behaved smooth features. The zinc diffusion coefficient (D Zn) was obtained by analyzing the slope of the profile in the annealed sample. The temperature dependence of D Zn was determined to be D Zn(cm 2/s)=8.0×10 4exp(-417[kJ/mol])/ RT, where R and T are gas constant and temperature. The zinc ion diffusion coefficients were of the same order as that in a ZnO single crystal. A comparison of the experimental and theoretical values indicated that the zinc ions diffused in the thin film and the single crystal through a vacancy mechanism..
51. Isao Sakaguchi, Ken Watanabe, Shunichi Hishita, Naoki Ohashi, Hajime Haneda, Oxygen diffusion phenomena and hydrogen incorporation in reducing BaTiO 3 ceramics doped with ho below solubility limit, Japanese Journal of Applied Physics, 10.1143/JJAP.51.101801, 51, 10, 2012.10, Oxygen diffusion phenomena in BaTiO 3 ceramics doped with Ho of 5.2×10 18/cm3 were studied in order to reveal the effect of reduction thermal annealing. The samples were reduced at 850°C in a 3% H2-Ar atmosphere before the oxygen diffusion experiments. The oxygen diffusion was carried out in the temperature range of 700-800°C. The measurement of diffusion profiles and ion images revealed the oxygen defect distribution caused by the annealing. The results show that the grain boundary acts as a layer blocking oxygen diffusion when the diffusion temperature is below the reduction temperature of 850°C..
52. K. Mitsuishi, T. Ohnishi, Y. Tanaka, Ken Watanabe, I. Sakaguchi, N. Ishida, M. Takeguchi, T. Ohno, D. Fujita, K. Takada, Nazca Lines by la ordering in La 2/3-xLi 3xTiO 3 ion-conductive perovskite, Applied Physics Letters, 10.1063/1.4744886, 101, 7, 2012.08, We report an unique planar defect that draws the Nazca Lines in epitaxially grown La 2/3-xLi 3xTiO 3 (LLTO) (x ≈ 0.11) ion-conductive perovskite. Transmission electron microscopy and electron energy loss spectroscopy reveal that the lines are produced by changing the regular arrangement of alternate stacks of La-rich and La-poor layers along the c-axis into all La-rich layers near the defect. The first-principle calculation suggests that the La-rich layers should act as a barrier to Li conductivity and are, therefore, important for the application of epitaxially grown LLTO thin films..
53. Isao Sakaguchi, Ken Watanabe, Yutaka Adachi, Takeshi Ohgaki, Shunichi Hishita, Naoki Ohashi, Hajime Haneda, Simultaneous diffusion of oxygen tracer and lithium impurity in aluminum doped zinc oxide, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes, 10.1143/JJAP.50.125501, 50, 12, 2011.12, Aluminum-doped zinc oxide ceramics were used to study oxygen self-diffusion by the vapor-solid exchange method. The diffusion profile and the quantitative analysis of impurities (H, Li, and Al) were performed by secondary ion mass spectrometry. The concentrations of Al and H in ZnO were evaluated to be 3.2 × 1019 and below 1.5 × 1017/cm3, respectively. The simultaneous diffusion of oxygen tracer and Li impurity were detected in the analysis, and the diffusion profiles were analyzed to evaluate the diffusion coefficients. Enhanced oxygen diffusivity and increased Li impurity concentration was obtained in Al-doped ZnO. We discuss the defect equations for enhancing oxygen diffusivity and increasing Li concentration in Al-doped ZnO..
54. Xiaoxiong Xu, Kazunori Takada, Ken Watanabe, Isao Sakaguchi, Kosho Akatsuka, Bui T. Hang, Tsuyoshi Ohnishi, Takayoshi Sasaki, Self-organized core-shell structure for high-power electrode in solid-state lithium batteries, Chemistry of Materials, 10.1021/cm103665w, 23, 17, 3798-3804, 2011.09, Effect of Al-substitution for Co on the electrode properties of LiCoO 2 is investigated in a sulfide solid electrolyte. The substitution decreases the electrode resistance and thus improves the high-rate capability. Investigation of the post-annealing effect revealed that, although the crystal structure or morphology of the LiAlxCo1-xO2 are not significantly changed by the post-annealing, the electrode properties are highly dependent on the post-annealing conditions, which suggests that the improvement in high-rate capability comes from the change in surface structure of the LiAlxCo1-xO2 particles (i.e., self-organized core-shell structure). The Al-substituted LiCoO2 obtained through this study delivers a discharge capacity of 115 mAh g -1 at 1 C and 84 mAh g-1 even at a high discharge rate of 5 C at 25 °C, which is one of the highest rate capabilities observed in solid-state cells..
55. Ken Watanabe, Isao Sakaguchi, Shunichi Hishita, Naoki Ohashi, Hajime Haneda, Visualization of grain boundary as blocking layer for oxygen tracer diffusion and a proposed defect model in non doped BaTiO3 ceramics, Applied Physics Express, 10.1143/APEX.4.055801, 4, 5, 2011.05, We performed visualization of the oxygen diffusion path in oxidized and reduced BaTiO3 ceramics by utilizing an ion-imaging technique. The oxygen tracer diffused quickly from the surface up to the grain boundary, and then behaved as discontinuous steps at the grain boundary. The grain boundary acted as a blocking layer against oxygen diffusion. The reduction process enhanced the blocking effect. The blocking of oxygen diffusion at the grain boundary originated in the formation and distribution of complex defects between the charged oxygen vacancy and the reduced Ti near the grain boundary..
56. Ken Watanabe, Tetsuya Kida, Isao Sakaguchi, Naoki Ohashi, Kengo Shimanoe, Hajime Haneda, Effect of annealing atmosphere on oxygen diffusion through Ba-Fe-based perovskite oxide, 30th Electronics Division Meeting of the Ceramic Society of Japan Electroceramics in Japan XIV, 10.4028/, 485, 141-144, 2011, To determine the effect of the annealing atmosphere on oxygen diffusion through Ba0.95La0.05FeO3-δ pellets, 18O2 tracer diffusion and high-resolution secondary ion mapping were performed. When annealing in air, the 18O concentration around the surface up to a depth of 40 μm was almost constant. On the other hand, when annealing in vacuum, the 18O concentration obviously decreased. High-resolution secondary ion mapping indicated that the 18O concentration around the grain boundary was reduced. These results suggested that the grain boundary of BLF annealed in vacuum prevents oxygen diffusion..
57. Tetsuya Kida, Subaru Ninomiya, Ken Watanabe, Noboru Yamazoe, Kengo Shimanoe, High oxygen permeation in Ba0.95La0.05FeO 3-δ membranes with surface modification, ACS applied materials & interfaces, 10.1021/am100524k, 2, 10, 2849-2853, 2010.10, Selective oxygen separation from air was carried out using ceramic membranes composed of Ba0.95La0.05FeO3-δ. We demonstrated that surface abrasion of Ba0.95La 0.05FeO3-δ membranes led to a significant increase in oxygen permeation fluxes at 700-930 °C because of an increase in surface reaction sites. Abrasion of the oxygen desorption side of the membrane resulted in a higher oxygen permeability than with abrasion on the oxygen sorption side. Moreover, an increase in the area of the oxygen desorption side also increased the oxygen permeability. The results suggest that oxygen permeation through Ba0.95La0.05FeO3-δ membranes is limited by the desorption of oxygen (surface reaction) and bulk diffusion. Attachment of a porous layer made of Ba0.95La0.05FeO 3-δ to the membrane surface was also effective in increasing oxygen permeability. For example, a high permeation flux of 3.2 cm3 min-1 cm-2 was achieved at 930 °C under a He/air gradient for a membrane (0.5 mm thickness) coated with a porous layer (14.4 μm)..
58. Tetsuya Kida, Atsunori Yamasaki, Ken Watanabe, Noboru Yamazoe, Kengo Shimanoe, Oxygen-permeable membranes based on partially B-site substituted BaFe 1-yMyO3-δ (M=Cu or Ni), Journal of Solid State Chemistry, 10.1016/j.jssc.2010.08.002, 183, 10, 2426-2431, 2010.10, We carried out the partial substitution of the B-site in BaFeO 3-δ perovskite with divalent cations to develop novel oxygen-permeable materials. We demonstrated that the partial substitution of Cu or Ni by more than 10% resulted in the stabilization of the cubic perovskite structure even at room temperature in a highly oxygen-permeable phase, as revealed by the X-ray diffraction (XRD) analysis. The Cu substitution was more effective for the stabilization, because the introduction of Cu in the lattice more effectively made the Goldschmidt tolerance factor (t) close to 1.0. Ni- and Cu-substituted BaFeO3-δ membranes showed higher oxygen permeabilities than their parent BaFeO3-δ membranes particularly at lower temperatures around 600700 °C owing to the stabilization of the cubic phase. Among the fabricated membranes, a BaFe 0.85Cu0.15O3-δ membrane (1.0 mm thickness) showed the highest oxygen permeation flux (1.8 cm3 min-1 cm-2 at 930 °C) under an air/He gradient. The results indicated that Cu-substituted BaFeO is promising as a material for Co-free membranes with high oxygen permeabilities..
59. Takeshi Ohgaki, Ryota Matsuoka, Ken Watanabe, Kenji Matsumoto, Yutaka Adachi, Isao Sakaguchi, Shunichi Hishita, Naoki Ohashi, Hajime Haneda, Synthesizing SnO2 thin films and characterizing sensing performances, Sensors and Actuators, B: Chemical, 10.1016/j.snb.2010.07.036, 150, 1, 99-104, 2010.10, SnO2 thin films were grown on silica glass substrates using the pulsed laser deposition (PLD) method, and their structures, electric properties, and sensor performances were examined to investigate the sensing mechanism of thin-film gas sensors. Single-phase SnO2 films with rutile-type structures were obtained at 650 °C. All the SnO2 films had many columnar grains, and the grain size increased with film thickness. Measurements of the Hall coefficient at room temperature revealed that the Hall mobility of the films was independent of the film thickness. In contrast, the sensing performances of the films for NO2 and H2 gases respectively in an air atmosphere drastically improved for film thicknesses under 100 nm. These results for the film properties and sensing performances of SnO2 thin films are discussed in terms of a space-charge layer formed on the columnar grains..
60. Ken Watanabe, Yasutake Teraoka, Oxygen separation by mixed conductive perovskite-type oxides, Nihon Enerugi Gakkaishi/Journal of the Japan Institute of Energy, 89, 9, 871-879, 2010.09, Mixed ionic-electronic conductors are materials which conduct both oxide ions and electronic charge carriers (electron and/or hole) simultaneously. Perovskite-type oxides (ABO3) with valence-variable cations and oxide ion vacancies such as La1-xSrxCo1-yFe yO3-δ (LSCFs) are representative and most intensively studied mixed conductors. LSCFs and their relatives desorb and absorb a large amount of oxygen reversibly from and into the crystal lattice without changing their fundamental crystal structures. By full use of the reversible oxygen sorption-desorption and mixed conductive properties, oxygen separation from, for example, air with 100% selectivity is possible at temperatures above 300 °C. High temperature oxygen separation is very important technology to recover waste heat from high-temperature facilities and can contribute the improvement of energy efficiencies. Oxygen separation is also a key technology for oxy-fuel combustion, because its exhaust consists, in principle, of H2O and CO2 which is ready for CCS (Carbon dioxide Capture and Storage). In this review article, two types of oxygen separation technologies by using mixed ionic-electronic conductors are outlined; membrane separation at high temperature range (usually above 800 °C) and temperature/pressure swing separation at medium to high temperature range (for example, 300-800 7deg;C). After describing basic aspects of mixed conductive perovskite-type oxides, current research status and future challenges will be reviewed..
61. Ken Watanabe, Subaru Ninomiya, Masayoshi Yuasa, Tetsuya Kida, Noboru Yamazoe, Hajime Haneda, Kengo Shimanoe, Microstructure effect on the oxygen permeation through Ba 0.95La0.05FeO3-δ membranes fabricated by different methods, Journal of the American Ceramic Society, 10.1111/j.1551-2916.2010.03700.x, 93, 7, 2012-2017, 2010.07, In order to investigate the effect of the microstructure on the oxygen permeation in Ba0.95La0.05FeO3-δ membranes, three different methods such as solid-state reaction, nitrate and acetate decomposition (NAD), and amorphous malic acid precursor (AMP) methods were used to fabricate membranes with different grain sizes. The grain size of the membranes was successfully controlled from 35 to 829 μm2 via sintering at 1175°-1275°C. The oxygen permeation fluxes through the Ba0.95La0.05FeO3-δ membranes increased with a decrease in the grain size. The AMP method, using malic acid as a complexing agent, produced a membrane having the highest oxygen permeability (3.10 cm3·(min·cm2)-1 at 930°C) and the smallest grain size. The results obtained again confirmed the significant importance of microstructure control in designing high-performance oxygen permeable membranes..
62. Watenabe, Ken; Yuasa, Masayoshi; Kida, Tetsuya; Teraoka, Yasutake; Yamazoe, Noboru; Shimanoe, Kengo, High-Performance Oxygen-Permeable Membranes with an Asymmetric Structure Using Ba0.95La0.05FeO3-delta Perovskite-Type Oxide, ADVANCED MATERIALS, 10.1002/adma.200903953, 22, 21, 2367-2370, 2010.06.
63. Ken Watanabe, Kenji Matsumoto, Takeshi Ohgaki, Isao Sakaguchi, Naoki Ohashi, Shunichi Hishita, Hajime Haneda, Development of ZnO-based surface plasmon resonance gas sensor and analysis of UV irradiation effect on NO2 desorption from ZnO thin films, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.118.193, 118, 1375, 193-196, 2010.01, In order to perform a high throughput exploration of sensor materials using surface plasmon resonance (SPR), the gas sensing property of a ZnO/Au/SiO 2 chip with SPR and the enhancing effect of UV irradiation on the desorption rate of NO2 from the ZnO surface were investigated. When the ZnO/Au/SiO2 chip was exposed to a high concentration of NO 2 (1000 ppm), a large peak shift was observed in the SPR curve. However, this sensing signal for NO2 gas did not recover to the baseline. In the case of low-concentration NO2 (10 ppm), the peak shift of the SPR curve was lower than that in the case of the high-concentration gas, but recovery to the baseline was observed. From the X-ray photoelectron spectra for N 1s of the ZnO thin films exposed to 1000- and 10-ppm NO 2, two chemisorption states-NO2 - (403.7 eV) and NO3 - (407 eV)-were confirmed. After the ZnO film was irradiated by UV rays, exposed to 10-ppm NO2, all peaks related to N 1s disappeared. However, in the case of the ZnO film exposed to 1000-ppm NO 2, adsorbed NO3 - remained on the surface of ZnO. From these results, it was found that UV irradiation effectively assisted NO2 desorption from the surface of the ZnO thin film exposed to 10-ppm NO2..
64. Isao Sakaguchi, Ken Watanabe, Takeshi Ohgaki, Tsubasa Nakagawa, Shunichi Hishita, Yutaka Adachi, Naoki Ohashi, Hajime Haneda, Ion implantation and diffusion behavior of silver in zinc oxide, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.118.217, 118, 1375, 217-219, 2010.01, Silver implantation (75 keV, 2 × 1014 ions/cm2) in ZnO ceramics was carried out at room temperature to characterize the diffusion phenomenon. Annealing caused Ag evaporation from the sample. Results suggest that it is difficult to incorporate Ag into Zn site by annealing. The Ag profile with the monotonous decrease in concentration below 3 × 10 17/cm3 was observed in ZnO annealed at 900°C. Above results become the basic results for producing the ZnO sensor used the reaction at surface and the grain boundary..
65. Isao Sakaguchi, Kenji Matsumoto, Takeshi Ohgaki, Yutaka Adachi, Ken Watanabe, Naoki Ohashi, Hajime Haneda, Oxygen tracer diffusion in magnesium-doped ZnO ceramics, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi/Journal of the Ceramic Society of Japan, 10.2109/jcersj2.118.362, 118, 1377, 362-365, 2010.01, ZnO samples added with MgO of 1.411.7mol% were used for studying oxygen diffusion by the vaporsolid exchange method. The analyses of oxygen diffusion profiles and impurities were performed by secondary-ion mass spectrometry (SIMS). The results indicate that the increase of MgO concentration enhances oxygen diffusion. The difference between the bulk-diffusion coefficients of oxygen for ZnO samples added with MgO of 1.4 and 11.7mol% was about two orders of magnitude. It was found that the grain boundary diffusion coefficients of oxygen in ZnO with 11.7mol% of MgO were larger than those in other samples, by a factor of about 10. Above results indicate that MgO addition into ZnO increases the concentration of effective defects for that promote oxygen diffusion. As the effective defects, the possibilities of vacancy and interstitial of oxygen are discussed..
66. Isao Sakaguchi, Kenji Matsumoto, Takeshi Ohgaki, Shunichi Hishita, Yutaka Adachi, Tsubasa Nakagawa, Ken Watanabe, Naoki Ohashi, Hajime Haneda, Relationship between aluminum and lithium and annealing for reducing lithium contamination in aluminum-implanted zinc oxide, 9th Electronics Division Meeting of the Ceramic Society of Japan Electroceramics in Japan XIII, 10.4028/, 445, 205-208, 2010, The relationship between Al and Li during diffusion was studied using Al-implanted ZnO. The Al donor in ZnO acts to increase the concentration of Li contamination from the atmosphere during the annealing. It is difficult to decompose the relationship formed by diffusion between Al and Li during high-temperature annealing. The most effective method to decompose the relationship is to anneal the as-implanted ZnO at a pressure of 5×10 -3 torr. This annealing increases the Al solubility limit because the ZnO surface evaporates..
67. Ken Watanabe, Daisuke Takauchi, Masayoshi Yuasa, Tetsuya Kida, Kengo Shimanoe, Yasutake Teraoka, Noboru Yamazoe, Oxygen permeation properties of Co-free perovskite-type oxide membranes based on BaFe 1-yZr yO 3-δ, Journal of the Electrochemical Society, 10.1149/1.3086763, 156, 5, 2009.04, Partially Zr-substituted BaFe 1-yZr yO 3-δ membranes were developed as a Co-free oxygen permeable membrane. In order to stabilize the cubic perovskite structure, Fe sites in BaFeO 3-δ were partially substituted with Zr 4+. In the substitution range of y=0.01-0.1, the cubic perovskite structure was stabilized even at room temperature. Among the membranes prepared, a BaFe 0.975Zr 0.025O3 material (y=0.025) showed the highest oxygen permeation flux of 1.30 cm3 (standard temperature pressure) min-1 cm-2 at 930°C under an air/He gradient. The oxygen permeation flux was higher than that of partially Ce-substituted BaFe 1-yCe yO 3-δ membranes reported previously. From the results obtained by chemical and scanning electron microscope analyses, it appears that the oxygen permeability for BaFe 1-yZr yO 3-δ membranes was well correlated with the amount of oxygen defects in the lattice as well as the grain size. In addition, the oxygen permeation flux of the BaFe 0.975Zr 0.025O3 membrane was significantly increased after decreasing the thickness of the membrane from 2.0 to 0.4 mm. For thin membranes (0.4-1.0 mm), the thickness dependence of the oxygen permeability deviated from the Wagner equation, suggesting that the oxygen permeation of BaFe 0.975Zr 0.025O3 is controlled by not only bulk diffusion of oxide ions but also their surface reactions..
68. Ken Watanabe, Masayoshi Yuasa, Tetsuya Kida, Kengo Shimanoe, Yasutake Teraoka, Noboru Yamazoe, Oxygen permeation of a dense/porous asymmetric membrane using La 0.6 Ca 0.4 CoO 3-δ-BaFe 0.975 Zr 0.025 O 3-δ system, Chemistry Letters, 10.1246/cl.2009.94, 38, 1, 94-95, 2009, To achieve a high oxygen permeation rate at medium and high temperature, an asymmetrically structured membrane, in which a thin dense layer (30μm) made of La 0.6Ca 0.4CoO 3-δ (LCC) and BaFe 0.975Zr 0.025O 3-δ (BFZ) was formed on a porous LCC support, was fabricated and tested for its oxygen permeability. The oxygen permeation flux significantly increased by mixing BFZ with LCC and reached a fairly high value of 1.60 cm 3 min -1 cm -2 even at 780°C..
69. Tetsuya Kida, Daisuke Takauchi, Ken Watanabe, Masayoshi Yuasa, Kengo Shimanoe, Yasutake Teraoka, Noboru Yamazoe, Oxygen permeation properties of partially A-site substituted BaFe O3-δ perovskites, Journal of the Electrochemical Society, 10.1149/1.3231690, 156, 12, 2009, To explore oxygen permeable materials, oxygen permeation properties of partially A-site substituted BaFe O3-δ perovskites were investigated. Ba sites in BaFe O3-δ were substituted with cations such as Na, Rb, Ca, Y, and La by 5%. The partial substitution with Ca, Y, and La, whose ionic radii are smaller than that of Ba, succeeded in stabilizing a cubic perovskite structure that is a highly oxygen permeable phase, as revealed by X-ray diffraction analysis. This can be explained in terms of a decrease in the tolerance factor (t). Among the Ba0.95 M 0.05 Fe O3-δ (M = Na, Rb, Ca, Y, and La) membranes tested, Ba0.95 La0.05 Fe O3-δ showed the highest oxygen permeability at 600-930°C, owing to the stabilization of the cubic phase without the formation of impurity phases. From chemical analysis, the oxygen permeability of Ba1-x Lax Fe O 3-δ membranes was correlated with the amount of oxygen defects (δ) in the lattice. The oxygen permeation flux of Ba0.95 La0.05 Fe O3-δ membrane was significantly increased by reducing its thickness. Furthermore, a Ba0.975 La0.025 Fe O3-δ membrane exhibited good phase stability under He flow at elevated temperatures. The obtained results indicate the promising properties of Ba1-x Lax Fe O3-δ membranes as a cobalt-free material that has a high oxygen permeability, good phase stability, and low cost..
70. Ken Watanabe, Masayoshi Yuasa, Tetsuya Kida, Kengo Shimanoe, Yasutake Teraoka, Noboru Yamazoe, Dense/porous asymmetric-structured oxygen permeable Membranes based on la0.6Ca0.4CoO3 perovskite-type oxide, Chemistry of Materials, 10.1021/cm8013144, 20, 22, 6965-6973, 2008.11, To achieve high-efficiency oxygen permeation using mixed (ionic and electronic) conducting perovskite-type oxides, we examined asymmetric-structured membranes of La0.6Ca0.4CoO3 in which a thin dense membrane was deposited on a porous support. The La0.6Ca 0.4CoO3 porous support was fabricated using irregular-shaped precursor particles prepared through an oxalate method. The fabricated support had good gas permeability and thermal stability, showing sufficient properties as a support for dense thin membranes. A dense membrane of 10 μm thickness was successfully formed on the porous support by coating a La0.6Ca0.4CoO3 slurry and subsequent densification by sintering. The deposited thin membrane was gastight and free from clacks, as revealed by gas permeation tests and SEM observations. The asymmetric membrane exhibited a high oxygen permeability of 1.66 cm3 (STP, standard temperature and pressure) min-1 cm-2 at 930 °C, which was four times higher than that of a typical sintered-disk type membrane with 1200 μm thickness, demonstrating its feasibility as a high-performance oxygen separation membrane..
71. Ken Watanabe, Masayoshi Yuasa, Tetsuya Kida, Kengo Shimanoe, Yasutake Teraoka, Noboru Yamazoe, Preparation of oxygen evolution layer/La0.6Ca0.4CoO3 dense membrane/porous support asymmetric structure for high-performance oxygen permeation, Solid State Ionics, 10.1016/j.ssi.2007.12.092, 179, 27-32, 1377-1381, 2008.09, An asymmetric-structured membrane, in which a dense thin La0.6Ca0.4CoO3 layer of 10 μm was deposited on porous La0.6Ca0.4CoO3 support by a slurry dropping method, were tested for its oxygen permeability at 630-930 °C. In order to increase the oxygen permeability, porous La0.6Ca0.4CoO3 and SrCo0.8Fe0.2O3 - δ oxygen evolution layers were attached on the dense layer. It was found that the asymmetric structured membranes with the porous oxygen evolution layers showed remarkably higher oxygen permeability as compared with a conventional sintered disk-type membrane (1200 μm). This suggests that oxygen permeation through membranes of 10 μm in thickness is rate-determined by both bulk O2- diffusion and oxygen evolution reaction. The maximum oxygen permeability reached 4.77 cm3 (STP) min- 1 cm- 2 (3.54 × 10- 6 mol cm- 2 s- 1) at 930 °C for the asymmetric membrane with porous La0.6Ca0.4CoO3 oxygen evolution layer of 10 μm in thickness (STP = Standard Temperature and Pressure). Further improvements in the oxygen permeability would be achieved through control of the micro-structure of the oxygen evolution layers..