Recently, carbon materials were reported to work as catalysts for the production of H2 from hydrocarbons. However, the hydrogen production rate is extremely low. To broaden the scope of carbon materials as catalysts for the decomposition of hydrocarbons, a higher H2 production rate is required. Thus, the hydrogen production rate under conditions employing a low flow rate of hydrocarbon per unit area and a large quantity of carbon materials as the catalyst should be evaluated, and the influence of these conditions on the rate of H2 production by carbon materials should be discussed. Herein, the H2 production rate was measured under conditions employing a low flow rate of CH4 and a large quantity of carbon black (CB) as a catalyst. H2 production started at 600°C and the production rate was less than 0.1%. The transmission electron microscope image of CB heated in CH4 atmosphere demonstrated that a low flow rate of CH4 does not affect the reaction site (i.e., graphite edges) of CB. However, the H2 production rate was extremely low compared to that achieved with a high flow rate. It is proposed that there is an optimal flow rate for maintaining the catalytic activity at the graphite edge of CB for decomposition of CH4.
Feature: Functional Ceramics for Energy Conversion and Storage Devices: Express letter
Inorganic-bio nanocomposites combining photoproteins (aequorin, AEQ) with a Pt-doped α-Fe2O3 (Pt-α-Fe2O3) thin film could induce a photoelectrochemical reaction without an external light source. Blue emission originated from Ca2+ binding to AEQ excited the n-type semiconducting Pt-α-Fe2O3 under an anodic bias, then an anodic photocurrent was clearly observed in a basic solution.
Feature: Functional Ceramics for Energy Conversion and Storage Devices: Special Article-Invited Review
Electrochemical impedance spectroscopy is a well suited method for studying the properties of electrochemical systems. In recent decades electrochemical systems were investigated at different scales, from small model electrodes to high power devices, such as fuel cells and batteries. In the latter case, the measurement of reliable spectra and their evaluation is challenging because (i) the impedance is usually very low (<<1 Ω), (ii) there is more than one rate limiting, electrochemical process per electrode, (iii) their charge transfer and transport processes are coupled and (iv) cathodic and anodic contributions overlap in the frequency domain. The Distribution of Relaxation Times (DRT) is supportive when deconvoluting complex impedance spectra, and has therefore gained increased attention. In this paper we introduce selected results of advanced impedance analysis. We discuss the impact of impedance data quality, statistically distributed noise and single errors in the spectra. Furthermore, the applicability of DRT for establishing adequate equivalent circuit models for ceramic electrochemical devices, such as batteries and fuel cells will be discussed.
Feature: Functional Ceramics for Energy Conversion and Storage Devices: Full papers
Various compositions of mixture of silver (Ag) and nickel oxide (NiO) powders are studied for use as a cathode contact material in solid oxide fuel cell stacks. The addition of NiO is intended to mitigate Ag evaporation and maintain low resistivity of the contact material during stack operation. It is found that the mixture in the form of pellets consisting of 20 mol % Ag and 80 mol % NiO can yield acceptable area specific resistance (ASR) 2.24 mΩ cm2 at 800°C with reduced weight loss. However, in the steel/Ag–NiO paste/steel assemblies prepared with 30–50 mol % Ag in mixed powders, each paste containing 64 wt % mixed powder, high ASR values of ∼100–200 mΩ cm2 after 1552 h thermal exposure at 800°C are observed. The low resistivity of the pellet with 20 mol % Ag is attributed to the formation of effective conducting pathways by complete sintering of the densely packed Ag particles through the pressing process in producing the pellets. For the steel/Ag–NiO paste/steel assemblies, Ag particles are scattered within the Ag–NiO layer and cannot readily get sintered. It is found that the resistance of Ag–NiO mixture critically depends on the mass concentration of Ag powder.
Metal-supported solid oxide fuel cells (SOFCs) are expected to lower operating temperature and improve reliability. The residual crushing strength and electrical conductivity of porous metallic nickel microtubes were higher than those of nickel-gadolinia-doped ceria (Ni-GDC) cermets. The maximum power density of the metal-supported microtubular SOFC with GDC electrolyte was approximately three times higher than that of a conventional anode-supported microtubular SOFC with yttria-stabilized zirconia electrolyte at a relatively low temperature of 550°C. The metal-supported microtubular SOFCs have a potential to be applied not only in stationary devices, such as residential combined heat/power systems, but also in portable power sources and vehicles.
A feasible study for an in-situ stress evaluation method for solid oxide fuel cells based on Raman scattering spectroscopy was performed using two anode-supported cells with a cathode interlayer made from Samarium (Sm) doped ceria. Model cells with a dense cathode interlayer demonstrated a compressive stress of −100s MPa at room temperature, and the results were comparable to those from X-ray diffraction. Model cells with a porous interlayer showed complex stress conditions that deviated from in-plane stress due to their porosity, and the model cell was confirmed not to be suitable for this method. At high temperatures, compressive stress was found to decrease as temperature increased and was almost neutral at the operating temperatures of the fuel cell, with good replication. Hence, the feasibility of this method for model cells with a dense cathode interlayer was confirmed.
We have examined the long-term durability of a La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)–samaria-doped ceria (SDC) composite oxygen electrode with SDC interlayer for reversible solid oxide cells (R-SOCs). A symmetrical cell with the configuration: LSCF–SDC|SDC interlayer|yttria-stabilized zirconia (YSZ) electrolyte|SDC interlayer|LSCF–SDC, was operated at 900°C and a constant current density of 0.5 A cm−2 with the top electrode as the anode (O2 evolution). The IR-free overpotentials at both the anode and cathode were virtually constant during 5500 h of operation. The value of ohmic resistance at the anode side (RA) increased slightly, whereas that at the cathode side (RC) increased markedly. The I–E performance of the bottom electrode (operated as the cathode), that was measured from −1.0 to 1.0 A cm−2 every 1000 h, degraded specifically at high current densities. It was found that the thickness, pore size, and porosity in both electrodes were unchanged, but the distribution of the Sr component changed markedly at both the LSCF–SDC/SDC interlayer and SDC interlayer/YSZ interfaces. While the diffusion of the Sr component from the anode was limited within the SDC interlayer, the Sr component from the cathode reached the SDC interlayer/YSZ interface, which could increase the RC, likely due to the formation of SrZrO3. However, the diffusion rates of Sr were found to be noticeably slowed down at dense portions of the SDC interlayer. Hence, it is essential to prepare a dense, uniform SDC interlayer to improve both the durability and performance of R-SOCs.
Nb-substituted or Ta-substituted strontium titanates were synthesized by conventional solid-state reactions. For perovskite-type titanates, Sr1−yTi1−xMxO3, the lattice constant a was found to increase with the amount of dopant (x) added, to maximum values of 3.927 and 3.930 Å for M = Nb and Ta respectively, regardless of Sr deficiency (y). X-ray diffraction studies revealed that the Ta-substituted strontium titanate possesses a wider stable region in its perovskite-type structure against the amount of added dopant (x) and strontium deficiency (y), compared with the Nb-substituted structure. Even in the single phase regions of SrTi1−xMxO3 (M = Nb and Ta) perovskites, scanning electron microscopy images confirmed that after firing at 1500°C for 10 h some SrTi0.9Ta0.1O3 grains showed strontium and tantalum enrichment and titanium dilution, whereas no inhomogeneity was found in SrTi0.9Nb0.1O3. This cationic compositional fluctuation is consistent with the lower electrical conductivity of SrTi0.9Ta0.1O3 compared with SrTi0.9Nb0.1O3 at low temperatures.
In order to evaluate potential of PrNi1−xFexO3−δ as new cathode material for solid oxide fuel cells, single phase preparation, crystal structure at various temperatures, thermal expansion property and electrical conduction behavior at high temperature were investigated. Single phase of PrNi1−xFexO3−δ (0.3 ≤ x ≤ 1.0) with orthorhombic distorted perovskite structure was successfully prepared by Pechini method. No structural phase transition was observed between room temperature and 1000°C in air. Linear thermal expansion coefficient was evaluated to be around 1.16 × 10−5 K−1, showing fair agreement with that of yttria-stabilized zirconia and gadolinia-doped ceria which were frequently employed as electrolyte material. Electrical conductivity of PrNi1−xFexO3−δ increased with increasing Ni content; however, the conductivity did not exceed that of LaNi1−xFexO3−δ. From the comparison of activation energy of variable range hopping conduction and Mössbauer spectroscopy between PrNi1−xFexO3−δ and LaNi1−xFexO3−δ, it was suggested that hole carrier concentration of PrNi1−xFexO3−δ was lower than that of LaNi1−xFexO3−δ owing to variation of valence of Pr ion.
In this study, La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM)-supported micro tubular solid oxide fuel cells (T-SOFCs) with two different configurations, one containing an LSGM–Ce0.6La0.4O2−δ (LDC) bi-layer electrolyte (Cell A) and one containing an LDC–LSGM–LDC tri-layer electrolyte (Cell B), were fabricated using extrusion and dip-coating. After optimizing the paste formulation for extrusion, the flexural strength of the dense and uniform LSGM micro-tubes sintered at 1500°C was determined to be approximately 144 MPa. Owing to the insertion of an LDC layer between LSGM electrolyte and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)–LSGM cathode, the ohmic resistances of Cell B were slightly larger than those of Cell A at the operating temperatures investigated, mainly because of interfacial resistance, but Cell B exhibited slightly lower polarization resistance than Cell A. The maximum power densities (MPDs) of Cell A were 0.25, 0.35, 0.43, and 0.47 W cm−2 at 650, 700, 750, and 800°C, respectively, which are slightly larger than those of Cell B, i.e., 0.23, 0.33, 0.42, and 0.41 W cm−2, respectively, owing to the facts that Cell A exhibited a slightly higher open-circuit voltage and a smaller Rt value. Cell A containing the LSGM (288 µm)–LDC (8 µm) bi-layer electrolyte can be operated at approximately 650°C with an MPD value of approximately 0.25 W cm−2; however, a similarly structured single cell containing a Zr0.8Sc0.2O2−δ (ScSZ) (210 µm) electrolyte need to be operated at 900°C, and one containing an Ce0.8Gd0.2O2−δ (GDC; 285 µm)–ScSZ (8 µm) bi-layer electrolyte has to be operated at 700°C. Thus, the advantage of using LSGM as an electrolyte for micro T-SOFC single cells is apparent.
To investigate the influence of orthorhombic-based perovskite structures on oxygen permeation, this study focused on (Sm, Ca)FeO3. The (Sm1−xCax)FeO3 (x = 0.20, 025, 0.30) samples were prepared using a conventional solid-state reaction method. The relative density of the prepared samples was larger than 95%. All the prepared samples were orthorhombic perovskites with space groups of Pbnm. The lattice volume decreased with increases in the Ca content. The tilt angles of the FeO6 octahedral decreased with increases in the Ca content. The oxygen permeation flux JO2 values increased and the activation energy EJ for the JO2 decreased with increases in the Ca content. The total conductivity σt value of the x = 0.25 sample was highest among the three different Ca content samples, suggesting that the oxygen vacancy formation energy decreases with increases in the Ca content from x = 0.20 to 0.30. Based on the investigation results, we assume that lower oxygen vacancy formation energy with doping with Ca ions is an important factor in improving oxygen permeation as well as ion conductive properties in the (Sm1−xCax)FeO3.
The perovskite-type proton conductor with the composition of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) has been reported to exhibit the highest proton conductivity among proton conductors. However, cerate-based perovskite materials such as BZCYYb are also known to react with carbon dioxide which causes phase decomposition through the formation of barium carbonate. This is a significant issue because chemical stability is an important property to enable these materials to be utilized for fuel cell applications. In this study, the chemical stability of BZCYYb was investigated in CO2 or CO2 + H2 atmosphere, with or without nickel addition as sintering aid. Some nickel addition is assumed to occur from nickel diffusion in anode-support-type fuel cells. The enhancement of reactivity with carbon dioxide species by adding nickel into BZCYYb was attributed to barium enrichment at grain boundary regions and the formation of an impurity phase of Ba(Y(1−x)Ybx)2NiO5. Moreover, different decomposition reactions depending on the atmosphere have been inferred. In a pure CO2 atmosphere, barium carbonate formation occurred without appearance of the CeO2-based phase, in other words, without decomposition of the perovskite phase. On the other hand, in hydrogen-containing CO2 atmosphere, both the barium carbonate and CeO2-based phase were observed.
Electrochemical synthesis of ammonia (NH3) using a proton conducting solid electrolyte and nickel (Ni) cermet electrodes has been studied. As an electrolyte, BaCe0.9Y0.1O3−δ (BCY) perovskite-type oxide was prepared by a co-precipitation method and its electrolyte pellet was synthesized by the sintering method with small amount of nickel oxide (NiO) as a sintering aid. The Ni–BCY cermet was employed as electrodes of anode and cathode. To evaluate the performance of this cell, electrochemical synthesis of NH3 from dry nitrogen (N2) and wet hydrogen (H2) was conducted at 500°C. As a result, the maximum formation rate of NH3 was 3.36 × 10−10 mol s−1 cm−2 at the applied voltage of −0.2 V against open circuit voltage using the Ni–BCY|BCY(NiO)|Ni–BCY cell, and the Faradic efficiency was 0.63%. Durability test of NH3 electrochemical synthesis from dry N2 and wet H2 revealed that current density of cell was relatively stable for 15 h but NH3 formation rate was decreased slightly. In addition, using Ni–BCY based cell, NH3 was successfully synthesized from dry N2 and steam diluted argon with the applied voltage of −1.8 V at a rate of 2.79 × 10−10 mol s−1 cm−2 and the Faradic efficiency was 0.15%. And this fabricated cell kept the high NH3 formation rate during the long-term test at −2.0 V for 15 h.
Perovskite-type mixed protonic and oxide ionic conductors for electrolyte material of solid oxide fuel cells were investigated, focusing on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) due to its high ionic conductivity and chemical stability. BZCYYb and NiO-added BZCYYb were evaluated using electrolyte-supported cell (ESC) and anode-supported cell (ASC) samples. 2 wt.% NiO was solid solute into the BZCYYb, resulting in improvement in the sinterability and thermal-expansion behavior. The addition of NiO, however, lead to the deterioration of cell performance. Compared with the ESC, power density of the ASC was much higher due to thin electrolyte, whereas its open-circuit voltage (OCV) was lower. This is due to Ni diffusion from the NiO–BZCYYb anode into the BZCYYb electrolyte during high-temperature co-sintering process at 1350°C. From the results of OCV measurements, 0–2 wt.% NiO was considered to be dissolved in the BZCYYb electrolyte of the ASC, suggesting that controlling Ni diffusion during co-sintering process is essential to achieve higher-performance ASCs using BZCYYb.
A lithium-rich layered cathode material [0.4Li2MnO3·0.6Li(Mn0.43Ni0.36Co0.21)O2)] containing nanosized grains (50–100 nm) was prepared from an aqueous precursor solution via a sequential two-step process composed of ultrasonic spray pyrolysis and post-calcination. The microsized lithium-rich layered composites show a high initial discharge capacity of 251 mA h g−1 at 0.1 C. The reversible capacities of 206 mA h g−1 at 0.5 C and 189 mA h g−1 at 1 C are obtained between 4.6 and 2.0 V. These are comparable to the values reported previously for these materials, without the need for doping or surface modification. The improved electrochemical performance may have resulted from the presence of nanosized grains, which can lead to an improvement in electronic and ionic transport, and the homogeneously dispersed Li2MnO3 phase in the LiMO2 (M = Mn, Ni, Co) phase. These results suggest that spray pyrolysis is an effective technique for the preparation of multi-component composite materials and can be used to control the microstructure of the materials, ultimately improving the electrical performance.
Solid solution of Li2TiS3 (Li8/3Ti4/3S4) and Li3NbS4 were prepared using mechanochemical syntheses. The materials with cubic rock-salt structure were obtained over the full range of compositions (Li(8+x)/3Ti(4−4x)/3NbxS4; 0 ≦ x ≦ 1). Lithium, titanium, and niobium randomly share the cationic site. Each material showed electrode activity and had a large reversible capacity of more than 350 mAh g−1 with more than 3 electron process per composition formulas of Li(8+x)/3Ti(4−4x)/3NbxS4.
We investigated the Li-ion conductivity and crystal structure of the Ta-doped Li7La3M2O12 (M = Hf, Sn) samples. All of the Ta-doped samples exhibited a relatively high conductivity of ∼10−4 S cm−1 at room temperature, and the activation energies of Li6.5La3Hf1.5Ta0.5O12 and Li6.5La3Sn1.5Ta0.5O12, which were determined from the Arrhenius plots in measured temperature range, are Ea = 0.400(6) and 0.451(1) eV, respectively. The crystal structure was analyzed by Rietveld method using powder X-ray diffraction data. From a view point of the Li–O polyhedral volume in unit cell, Li6.5La3Hf1.5Ta0.5O12 has a most suitable Li-ion environment among the Li7La3M2O12 (M = Zr, Hf, Sn) compounds.
Lithium-ion conductivity of Li2.2C0.8B0.2O3 pellet is 2.1 × 10−6 S cm−1 at 30°C after sintering at 450°C by spark-plasma sintering (SPS) process, which is about three times higher than the conductivity of the pellet sintered at 660°C in conventional furnace. The ionic resistance at grain boundary was especially minimalized by SPS process, which would be related to the grain-growth suppression under densification as confirmed by surficial SEM image. Improvement of charge–discharge capacity could be measured at all-solid-state lithium battery assembled by the SPS process (LiCoO2–Li2.2C0.8B0.2O3 composite electrolyte/Li2.2C0.8B0.2O3/dry polymer/Li foil) compared with the same compositions of the battery assembled by conventional furnace sintering, which would be due to the enhancement of the ionic transfer at LiCoO2/Li2.2C0.8B0.2O3 hetero interface as well as Li2.2C0.8B0.2O3/Li2.2C0.8B0.2O3 grain-boundary resistance.
Lithium iron metasilicate, LiFeSi2O6, was synthesized via a hydrothermal route and was identified as the pyroxene structure with space group C2/c using the synchrotron X-ray diffraction. Galvanostatic charge and discharge tests showed that the large charge and discharge capacities were obtained for the carbon-LiFeSi2O6 composite prepared by planetary ball-milling. The 10 wt % carbon-LiFeSi2O6 composite delivered a specific discharge capacity of 174 mAh·g−1 at 25°C in the voltage range from 1.5 to 4.8 V because of the enhancement of the electric conductivity. The valence state of iron was estimated by the X-ray Absorption Near Edge Structure spectra, where the oxidation states changed from charge to discharge state. The Galvanostatic Intermittent Titration measurement was applied for LiFeSi2O6 and carbon-LiFeSi2O6 composite, and then the improved Li+ diffusion was derived from the fully electron supply by the enhanced electric conductivity.
Li2S–P2S5 (LPS) film was prepared using colloidal process by employing electrophoretic deposition (EPD) technique followed by warm pressing for advanced processing of all-solid-state lithium ion battery. LPS precursor film with positive surface charge was obtained from its suspension that was synthesized by liquid-phase shaking method. The homogeneous film with thickness of 10–100 µm was controllably prepared. Resulted LPS film exhibited high conductivity (1.98 × 10−4 S cm−1) at ambient temperature and significantly low activation energy (16.6 kJ mol−1) compared with conventional LPS materials. Thus, good solid–solid interfacial contact can be obtained in the sulfide-based ionic conductor by employing EPD process followed by warm pressing.
As potential cathode materials for thin-film energy storage devices, Mn–Ni oxide nanosheets with the chemical composition H0.46Mn0.81Ni0.19O2 (M81N19) were prepared. Upon restacking in HNO3 aqueous solution and re-exfoliation, the Mn–Ni oxide nanosheets produced the novel H0.58Mn0.81Ni0.13O2 (M81N13) nanosheets with vacancy defects. The chemical composition of the nanosheets was characterized using X-ray absorption spectroscopy, inductively coupled plasma atomic emission spectroscopy, and thermogravimetry. The sizes of the M81N19 and M81N13 nanosheets were compared, and no structural change was observed after Ni dissolution from the nanosheets. The optical properties, the local structure and the electrochemical properties when used as cathodes in Li-ion batteries of the M81N19 and M81N13 nanosheets, were compared.
In this work, we directly observed the reaction distribution formed in a composite cathode of the all-solid-state lithium-ion battery (ASSLIB) LiCoO2/Li2.2C0.8B0.2O3|Li2.2C0.8B0.2O3|PEO|Li during charging by using the two-dimensional X-ray absorption spectroscopy (2D-XAS) at Co K-edge. The two-dimensional mapping of the state of charge in a composite cathode indicated that the reaction distribution was formed during charging process in the in-plane direction due to the disconnection in electron pathway at cracks. It was experimentally confirmed that the 2D-XAS technique was a useful tool for evaluating the reaction distribution in a composite cathode of an ASSLIB and investigating the influence factors for the formation of the reaction distribution.
Effects of NH4Cl addition to perovskite CH3NH3PbI3 precursor solutions on photovoltaic properties were investigated. TiO2/CH3NH3PbI3(Cl)-based photovoltaic devices were fabricated by a spin-coating technique, and the microstructures of the devices were investigated by X-ray diffraction and scanning electron microscopy. Current density–voltage characteristics were improved by a small amount of Cl-doping, which resulted in improvement of the efficiencies of the devices. The structure analysis indicated formation of a homogeneous microstructure by NH4Cl addition to the perovskite phase, and formation of PbI2 was suppressed by the NH4Cl addition.
The thermoelectric properties of Ca1−xBixMnO3−δ sintered bodies prepared by the electrostatic spray deposition method and sintering technology were evaluated. Ca0.95Bi0.05MnO3−δ showed the maximum power factor value of 230 µW·m−1·K−2 among Ca1−xBixMnO3−δ compounds at room temperature. From Seebeck coefficient, Hall coefficient and power generation efficiency measurements on CaMnO3−δ and Ca0.95Bi0.05MnO3−δ under high temperature, it was found that the power factor value of Ca0.95Bi0.05MnO3−δ increased with temperature in the range of 300–873 K, and was 2 to 5 times higher than that of CaMnO3−δ. The carrier concentration (n = 5.9 × 1020 cm−3) of Ca0.95Bi0.05MnO3−δ is two orders of magnitude higher than CaMnO3−δ (n = 7.1 × 1018 cm−3) at 300 K. The increase in carrier concentration contributed to higher conductivity, power factor and power generation density in the Bi-substituted compound. In thermoelectric performance evaluations, the power density reached 625 mW·cm−2 for Ca0.95Bi0.05MnO3−δ with a temperature difference of 444 K, an 11-fold increase compared to the parent compound CaMnO3−δ (57 mW·cm−2).
Feature: Functional Ceramics for Energy Conversion and Storage Devices: Technical reports
The performances of innovative planar copper-based anode supported solid oxide fuel cells were investigated. Li-containing Gadolinia-doped Ceria (GDC) and Lanthanum Strontium Cobalt Ferrite/Li-GDC were used for the electrolyte and the cathode, respectively; anodes consisting of 35 or 45 vol % CuO/GDC were produced and the corresponding green cells were sintered at 950 and 900°C, respectively. Polarization and power density measurements revealed an important dependence of the electrochemical parameters on sintering temperature and anodic composition. Cu redistribution within the anodic cermet during cell operation was investigated and energy dispersive spectroscopy analysis of anode structure before and after operation revealed Cu migration towards the anode/electrolyte interface.
The automatic multilayer coating system based on electrophoretic deposition process, including coating, drying, and washing, is proposed for stable preparation of multilayered films. As demonstration of the system, multilayered films consisting of layers of LiNi1/3Mn1/3Co1/3O2 (NMC) and Al2O3 particles were fabricated under constant applied voltage and current modes. The smoothly coated films were formed without any cracks or defects in both modes. The films with uniform thickness of each layer (average thickness of NMC and Al2O3 are 3.2 and 6.5 µm respectively) were obtained at low constant current density. Results indicate that ordered film structure such as thickness of inside layers and layers number are controllable by simply varying the process flow conditions. It is expected that the system is able to be employed to make not only multilayered films but also particle-reinforced composite materials.
Feature: Functional Ceramics for Energy Conversion and Storage Devices: Note
We here prepared polystyrene-derived carbon-coated Na3V2(PO4)3 composite (NVP/C) cathode materials via a solid-phase reaction using a mixture of NVP precursor and polystyrene (as the carbon source), followed by calcination at high temperatures. Using this method, we were able to obtain active NVP materials with relatively high purity while allowing a uniform surface coating with carbon layers about 3–4 nm thick. This structure significantly prevented NVP particle growth even after calcination at high temperatures. The NVP/C material calcined at 700°C exhibited the best charge–discharge performance among the samples studied.
High-temperature corrosion behavior of volcanic ash and artificial calcium-magnesium-aluminosilicate (CMAS) on sintered ytterbium monosilicate (Yb2SiO5), and the corrosion resistance of Yb2SiO5 as an environmental barrier coating, are investigated. Dense sintered Yb2SiO5 specimens were prepared using the spark plasma sintering method at 1400°C for 10 min. These specimens were subjected to a hot corrosive environment of molten Iceland volcanic ash and CMAS at 1400°C for 2, 12, and 48 h. Different corrosion phenomena, i.e., continuous reaction with CMAS and weak reaction with the volcanic ash, were observed. From the results of in-situ high temperature X-ray diffraction measurement and scanning electron microscope-energy dispersive X-ray spectrometry studies, Yb2SiO5 exhibits excellent resistance to volcanic ash but lacks resistance to CMAS attack.
Bandgap-tunable (2.5 ≤ Eg ≤ 4.3 eV) amorphous Cd–Ga–O films with mobilities of ∼10 cm2 V−1 s−1 were annealed in vacuum (∼10−4 Pa), Ar, and O2 atmospheres. The Cd concentrations ([Cd]/([Cd] + [Ga])) of 70, 50, and 20% films, with respective bandgap energies of ∼2.5, ∼3.0, and ∼4.0 eV, were employed for representative narrow, middle, and wide bandgap samples, respectively. The carrier concentrations of all of the annealed films, except those with a Cd concentration of 70% annealed in a vacuum and Ar, showed minimum values at annealing temperatures between 200 and 300°C. This was probably due to structural relaxation, which leads to low in-gap states. The threshold carrier concentrations for demonstrating high mobility (∼10 cm2 V−1 s−1) were 1018 cm−3 for films with a Cd concentration of 70% and ≤1017 cm−3 for films with a Cd concentration of 50% films. O2 annealing of films with a Cd concentration of 20% effectively decreased the carrier concentration to ∼ 1015 cm−3, with a Hall mobility of ∼3 cm2 V−1 s−1, which is a distinctively high mobility for amorphous oxide with bandgap energy of ∼4 eV. The marked decreases in the carrier concentration and mobility of the Cd concentration of 20% film at 600°C with O2 annealing were assumed to be due to the formation of microcrystalline Ga2O3.
Hydrogen permeation properties of Gd0.1Ce0.9Ox/BaCe0.80Y0.20O3−δ (GDC/BCY) dual-phase membranes were evaluated. Nongalvanic hydrogen permeation was observed for 46 vol % GDC-containing BCY membranes: the permeation rate was 0.30 ml·min−1·cm−2 at 800°C. This is almost comparable to the reported value of Ni–BCY cermet (0.50 ml·min−1·cm−2). This nongalvanic hydrogen permeation unambiguously indicates that the BCY–GDC composite functions as a mixed proton–electron conductor in an H2 reducing atmosphere. From a conductivity analysis, it was deduced that 46 vol % GDC/BCY possesses proton and electron conductivities comparable to the bulk states of BCY and GDC. The composite membrane was stable in an atmosphere switching between H2 and air. TGA analysis indicated that addition of GDC enhanced the stability of the BCY phase in CO2.
We prepared a borosilicate glass, in which the ZrO2 phase is singly crystallizable, i.e., 15Na2O–15ZrO2–30B2O3–40SiO2 glass, and investigated the texture and morphology of the resulting glass-ceramics. ZrO2 dendrites with a tetragonal system (high-temperature phase) were developed as initial phase, and the tetragonal phase was transformed to the monoclinic phase (low-temperature phase) by elongation of the heat-treatment time, and finally the needle-/rod-like crystals on a scale of few hundred nanometers were obtained. The glass-ceramics with monoclinic ZrO2 showed photoluminescence around 480 nm by excitation in the ultraviolet region. The effect of TiO2-addition on the photoluminescent property was also considered.
Nearly single-phase and almost fully-crystallized glass-ceramics of lithium chloroboracite Li4+xB7O12+x/2Cl (x = 0–1) were prepared by crystallization of precursor glasses derived from the Li2O–B2O3–LiCl ternary system. The crystal structure of Li4B7O12Cl (x = 0) was refined by the Rietveld method in the space group F43c (no. 219). The precursor glass partially crystallized into Li2B4O7 at ∼500°C and was subsequently converted to Li4B7O12Cl at ∼600°C. The decomposition of Li4B7O12Cl started at ∼700°C. The conductivity of Li4B7O12Cl was much higher than that of Li5B7O12.5Cl (x = 1). The glass-ceramic sample with the highest weight fraction of Li4B7O12Cl (∼0.98 of crystalline part with a degree of crystallinity of ∼0.96) exhibited a conductivity of ∼4.6 × 10−4 S cm−1 at 200°C and an activation energy of conductivity of ∼0.52 eV.
Clay-based nanosheets were produced from montmorillonite modified with organic cations. Exfoliation of the clay was achieved by ion exchange with imidazolium and ammonium cations having long-chain alkyl groups. Dispersible nanosheets as a monolayer clay was obtained through the exfoliation by adjusting of the alkyl-chain length. Homogeneous dispersion of the nanosheets was demonstrated in a non-protonic polar liquid and polymeric solid media.
The densification behavior during the isothermal sintering of 7.8 mol % Y2O3-stabilized zirconia was examined in the intermediate and final stages of sintering. In the intermediate stage, it is shown that the relationship between the grain size and the relative density is not invariant, but is temperature dependent. The relationship is combined with the measured densification rate to evaluate the grain size exponent, the activation energy and an unspecified function of density. The evaluation of the characteristic parameters indicates that the densification and the grain growth are related to a mechanism of grain-boundary diffusion and sliding. The densification rate predicted with the evaluated parameters shows a good consistence to the measured rate. In the final stage, the densification rate is inversely proportional to the grain size, and the kinetics is well explained by using the diffusive model modified with a gas pressure in closed pores.
We investigated the crystal structures of high- and low-temperature phases in Sr4[Al6O12]SO4, and their thermal behavior by high-temperature X-ray powder diffraction (Cu Kα1), differential thermal analysis, and temperature-dependent Raman spectroscopy. The crystal structure at 298 K was isomorphous with that of Ca4[Al6O12]SO4 (space group Pcc2 and Z = 4). The structural model at 573 K (space group I23 and Z = 2) was characterized by the positional disordering of oxygen atoms that form [SO4] tetrahedra. The maximum-entropy method-based pattern fitting method was used to confirm the validity of these structural models, in which conventional structure bias caused by assuming intensity partitioning was minimized. The starting temperature of the cubic-to-orthorhombic transformation during cooling (= 524 K) was slightly higher than that of the reverse transformation during heating (= 519 K). The negative thermal hysteresis (= −5 K) strongly suggested the transformation being thermoelastic. At around the transformation temperature during heating, the vibrational spectra, corresponding to the Raman-active [SO4] internal stretching mode, showed the continuous and gradual change in the slope of full width at half maximum versus temperature curve. This strongly suggests that the phase transformation would be principally accompanied by the statistical disordering of oxygen-atom positions, without distinct dynamical reorientation of the [SO4] tetrahedra.
Gallium nitride (GaN) crystals were synthesized through the reaction of Ga with LiNH2 at temperature ranging from 450 to 800°C under NH3 atmosphere. Hexagonal GaN crystals with a diameter of approximately 100 µm were obtained. A mechanism that can increase the crystal size during the synthesis of GaN crystals has been discovered. Ga reacted with LiNH2 to form GaN particles at the first step. Afterward, Li3GaN2 was formed through the reaction of GaN particles and LiNH2. At the second step, the growth of GaN crystals continuously occurred through the reaction of Li3GaN2.
Porous alumina with the surface layer was fabricated from an alumina platelet/novolac-HexaMethyleneTetramine (HMT) composite. HMT was used as a blowing agent as well as a curing one for the novolac binder. When the alumina platelet/novolac composite was heated with covering its surface with filter papers, a porous structure formed inside the composite due to blowing from HMT. On the other hand, the blowing gas near the surface was evolved outward through the filter paper, which resulted in the formation of the surface layer composed of the aligned alumina platelets. Effect of the surface layer on the mechanical strength was examined by three-point bending measurement. The bending strength of the obtained porous alumina increased by 94% compared to the porous alumina without the surface layer.
Functional nanocrystals have received considerable attention because of their various applications. Although glass-ceramic (GC) processing is recognized as promising for preparation of nanostructured material, it is difficult to use the materials as nanocrystals as in. This study provides a possible way to obtain nanocrystals by irradiation of fundamental wave of Q-switched Nd:YAG laser to nanocrystallized GCs in liquid. In this study, we employed Mn4+:Li2Ge4O9 nanocrystallized GCs as the target, and succeeded in fabricating deep-red emissive nanocrystals with a size of ∼100 nm.
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Edited and published by : The Ceramic Society of Japan Produced and listed by : Komiyama Printing Co., Ltd.(Vol.115 No.1344-Vol.116 No.1351, Vol.118 No.1376-) Letterpress Co., Ltd.(Vol.116 No.1352-Vol.118 No.1375)