Influence of sintering aids and excess Lithium on the conductivity of perovskite-type Li3/8Sr7/16Zr1/4Nb3/4O3 solid electrolyte

Perovskite-structured Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ solid-state Lithium-conductors were prepared by conventional solid-state reaction method. Influence of sintering aids (Al₂O₃, B₂O₃) and excess Lithium on structure and electrical properties of Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ (LSNZ) has been investigated. Their crystal structure and microstructure were characterized by X-ray diffraction analysis and scanning electron microscope, respectively. The conductivity and electronic conductivity were evaluated by AC-impedance spectra and potentiostatic polarization experiment. All sintered compounds are cubic perovskite structure. Optimal amount of excess Li₂CO₃ was chosen as 20 wt% because of the total conductivity of LSNZ-20% was as high as 1.6×10⁻⁵ S cm⁻¹ at 30 °C and 1.1×10⁻⁴ S cm⁻¹ at 100 °C, respectively. Electronic conductivity of LSNZ-20% is 2.93×10⁻⁸ S cm⁻¹, nearly 3 orders of magnitude lower than ionic conductivity. The density of solid electrolytes appears to be increased by the addition of sintering aids. The addition of B₂O₃ leads to a considerable increase of the total conductivity and the enhancement of conductivity is attributed to the decrease of grain-boundary resistance. Among these compounds, LSNZ-1 wt%B₂O₃ has lower activation energy of 0.34 eV and the highest conductivity of 1.98×10⁻⁵ S cm⁻¹ at 30 °C.     

High Li-ion conductivity of Al-doped Li7La3Zr2O12 synthesized by solid-state reaction

Li₇La₃Zr₂O₁₂ (LLZO) has cubic garnet type structure and is a promising solid electrolyte for next-generation Li-ion batteries. In this work, Al-doped LLZO was prepared via conventional solid-state reaction. The effects of sintering temperature and Al doping content on the structure and Li-ion conductivity of LLZO were investigated. The phase composition of the products was confirmed to be cubic LLZO via XRD. The morphology and chemical composition of calcined powders were investigated with SEM, EDS, and TEM. The Li-ion conductivity was measured by AC impedance. The results indicated the optimum sintering temperature range is 800–950 °C, the appropriate molar ratio of LiOH·H₂O, La(OH)₃, ZrO₂ and Al₂O₃ is 7.7:3:2:(0.2–0.4), and the Li-ion conductivity of LLZO sintered at 900 °C with 0.3 mol of Al-doped was 2.11×10⁻⁴ S cm⁻¹ at 25 °C.     

Preparation of dual-phase composite BaCe0.8Y0.2O3/Ce0.8Y0.2O2 and its application for hydrogen permeation

Dual-phase ceramic membranes composed of BaCe_0.8Y_0.2O₃ (BCY) and Ce_0.8Y_0.2O₂ (CYO) were successfully synthesized by solid state reaction method for hydrogen permeation. The influences of the BCY/CYO volume ratios on phase composition, microstructure, chemical stability and electrical property were investigated. The hydrogen permeation of the dual-phase composite was characterized as a function of temperature and feed side hydrogen partial pressure. The results showed that there was no reaction between the two constituent oxides observed under the preparation conditions. The dual-phase composite with different BCY/CYO volume ratios after sintering at 1550 °C exhibited dense structure, as well as good stability in 4% H₂/Ar, wet Ar and pure CO₂ atmosphere. The conductivity of the dual-phase composite increased with the content of CYO increasing and 30BCY–70CYO exhibited the highest total conductivity of 2.6×10⁻² S cm⁻¹ at 800 °C in 4% H₂/Ar. The hydrogen permeability of 30BCY–70CYO sample was improved as the temperature and the hydrogen partial pressure in feed gas increased. The hydrogen permeation flux of 1.7 μmol cm⁻² s⁻¹ was achieved at 850 °C.     

Semiconducting properties of cold sintered V2O5 ceramics and Co-sintered V2O5-PEDOT:PSS composites

Recently we established a sintering approach, namely Cold Sintering Process (CSP), to densify ceramics and ceramic-polymer composites at extraordinarily low temperatures. In this work, the microstructures and semiconducting properties of V₂O₅ ceramic and (1-x)V₂O₅-xPEDOT:PSS composites cold sintered at 120 °C were investigated. The electrical conductivity (25 °C), activation energy (25 °C), and Seebeck coefficient (50 °C) of V₂O₅ are 4.8 × 10⁻⁴ S/cm, 0.25 eV, and −990 μV/K, respectively. The conduction mechanism was studied using a hopping model. A reversible metal-insulator transition (MIT) was observed with V₂O₅ samples exposed to a N₂ atmosphere, whereas in a vacuum atmosphere, no obvious MIT could be detected. With the addition of 1–2 Vol% PEDOT:PSS, the electrical conductivity (50 °C) dramatically increases from 10⁻⁴ to 10⁻³ ∼ 10⁻² S/cm, and the Seebeck coefficient (50 °C) shifts from −990 to −(600 ∼ 250) μV/K. All the results indicate that CSP may offer a new processing route for the semiconductor electroceramic development without a compromise to the all-important electrical properties.     

Synthesis of BaCeO3 and BaCe0.9Y0.1O3−δ from mixed oxalate precursors

An oxalate precipitation route is proposed for the synthesis of BaCe_1−xY_xO₃ (x = 0 and 0.1) after calcination at 1100 °C. The precipitation temperature (70 °C) was a determinant parameter for producing a pure perovskite phase after calcination at 1100 °C for 1 h. TG/DTA measurements showed that the co-precipitated (Ba, Ce and Y) oxalate had a different thermal behaviour from single oxalates. Despite a simple grinding procedure, sintered BaCe_0.9Y_0.1O_3−δ pellets (1400 °C, 48 h) presented 90.7% of relative density and preliminary impedance measurements showed an overall conductivity of around 2 × 10⁻⁴ S cm⁻¹ at 320 °C.     

Effect of B2O3 on the sintering and microwave dielectric properties of M-phase LiNb0.6Ti0.5O3 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics have been investigated. It is found that low-level doping of B₂O₃ (≤2 wt.%) can significantly improve the densification and dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics. Due to the liquid phase effect of B₂O₃ addition, LiNb_0.6Ti_0.5O₃ ceramics could be sintered to a theoretical density higher than 95% even at 880 °C. No secondary phase was observed for the B₂O₃-doped ceramics. There is no obvious degradation in dielectric properties for the ceramics with B₂O₃ additions. In the case of 1 wt.% B₂O₃ addition, the ceramics sintered at 880 °C show good microwave dielectric properties of ɛ_r = 70, Q × f = 5400 GHz, τ_f = −6.39 ppm/°C. It represents that the ceramics could be promising for multilayer low-temperature co-fired ceramics (LTCC) applications.     

Electrical and thermal properties of SiC-Zr2CN composites sintered with Y2O3-Sc2O3 additives

SiC-Zr₂CN composites were fabricated from β-SiC and ZrN powders with 2 vol% equimolar Y₂O₃-Sc₂O₃ additives via conventional hot pressing at 2000 °C for 3 h in a nitrogen atmosphere. The electrical and thermal properties of the SiC-Zr₂CN composites were investigated as a function of initial ZrN content. Relative densities above 98% were obtained for all samples. The electrical conductivity of Zr₂CN composites increased continuously from 3.8 × 10³ (Ωm)⁻¹ to 2.3 × 10⁵ (Ωm)⁻¹ with increasing ZrN content from 0 to 35 vol%. In contrast, the thermal conductivity of the composites decreased from 200 W/mK to 81 W/mK with increasing ZrN content from 0 to 35 vol%. Typical electrical and thermal conductivity values of the SiC-Zr₂CN composites fabricated from a SiC-10 vol% ZrN mixture were 2.6 × 10⁴ (Ωm)⁻¹ and 168 W/m K, respectively.     

Electrical conductivity and nonstoichiometry in doped Sr3Ti2O7

A series of doped Ruddlesden–Popper phases, of general formula Sr₃Ti_2-xM_xO_7-δ (M=Al, Ga, Co), were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O₂)> 10⁻⁵ atm, followed by a p(O₂)-independent electrolytic regime, and n-type electronic conductivity at very low p(O₂). The electrolytic regime exhibits activation energies in the range 1·7–1·8 eV. Doping with transition metals such as Co results in a very significant increase in total conductivity with a p-type conductivity at high p(O₂). Furthermore, an apparent ionic regime at intermediate p(O₂) is observed, characterized by high conductivity (> 10⁻² S/cm at 700°C) and low activation energy (0·6 eV). This interpretation is consistent with iodometric measurements as interpreted by a defect chemical model. Other measurements are in progress to confirm this conclusion.     

Ion-exchange characteristics of 12CaO·7Al2O3 for halide and hydroxyl ions

Substitution characteristics of the halide ions F⁻ Cl⁻ for the OH⁻ ions in the crystal lattice of 12CaO·7Al₂O₃ solid solution were investigated. Single phases of composition 11CaO·7Al₂O₃·CaF₂ and 11CaO·7Al₂O₃·CaCl₂ were formed at 900 °C or above. The OH⁻ ions in 12CaO·7Al₂O₃ solid solution, i.e. 11CaO·7Al₂O₃·Ca(OH)₂, could be replaced wholly or partially by F⁻ or Cl⁻ ions from the corresponding calcium halide, forming 11CaO·7Al₂O₃·Ca(OH,F)₂ and 11CaO·7Al₂O₃·Ca(OH,Cl)₂ solid solutions above 500 °C and above 700 °C, respectively. Lattice constants of 12CaO·7Al₂O₃ solid solution changed continuously with the proportion of F⁻ ions or Cl⁻ ions. The F⁻ ions in 11CaO·7Al₂O₃·CaF₂ could be wholly or partially substituted by Cl⁻ ions from CaCl₂ at 900 °C or more, forming the solid solution 11CaO·7Al₂O₃·Ca(F,Cl)₂. The Cl⁻ ions in 11CaO·7Al₂O₃·CaCl₂ could be partially replaced F⁻ ions from CaF₂ at 1000 °C or above, apparently due to slow chloride loss by evaporation.     

Synthesis of glass–ceramics in the CaO–MgO–SiO2 system with B2O3, P2O5, Na2O and CaF2 additives

Glass–ceramics based on the CaO–MgO–SiO₂ system with limited amount of additives (B₂O₃, P₂O₅, Na₂O and CaF₂) were prepared. All the investigated compositions were melted at 1400 °C for 1 h and quenched in air or water to obtain transparent bulk or frit glass, respectively. Raman spectroscopy revealed that the main constituents of the glass network are the silicates Q¹ and Q² units. Scanning electron microscopy (SEM) analysis confirmed liquid–liquid phase separation and that the glasses are prone to surface crystallization. Glass–ceramics were produced via sintering and crystallization of glass-powder compacts made of milled glass-frit (mean particle size 11–15 μm). Densification started at 620–625 °C and was almost complete at 700 °C. Crystallization occurred at temperatures >700 °C. Highly dense and crystalline materials, predominantly composed of diopisde and wollastonite together with small amounts of akermanite and residual glassy phase, were obtained after heat treatment at 750 °C and 800 °C. The glass–ceramics prepared at 800 °C exhibited bending strength of 116–141 MPa, Vickers microhardness of 4.53–4.65 GPa and thermal expansion coefficient (100–500 °C) of 9.4–10.8 × 10⁻⁶ K⁻¹.     

Influence of Yb3+ doping on phase stability and thermophysical properties of (Y1-xYbx)3Al5O12 under high temperature

In this work, Yb³⁺ was selected to replace the Y³⁺ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y_1-xYb_x)₃Al₅O₁₂ (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y_1-xYb_x)₃Al₅O₁₂ ceramics were comprised of a single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xYb_x)₃Al₅O₁₂ ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y_0.7Yb_0.3)₃Al₅O₁₂ had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m⁻¹ K⁻¹ at 1000 °C, around 30% lower than that of pure YAG (2.3 W m⁻¹ K⁻¹, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y_1-xYb_x)₃Al₅O₁₂ ceramics and the CTE was approximate 10.7×10⁻⁶ K⁻¹ at 1200 °C. In addition, (Y_0.7Yb_0.3)₃Al₅O₁₂ ceramic remained good phase stability when heating from room temperature to 1450 °C.     

Oxygen ionic transport in SrFe1−yAlyO3−δ and Sr1−xCaxFe0.5Al0.5O3−δ ceramics

The oxygen permeability of mixed-conducting Sr_1−xCa_xFe_1−yAl_yO_3−δ (x=0–1.0; y=0.3–0.5) ceramics at 850–1000 °C, with an apparent activation energy of 120–206 kJ/mol, is mainly limited by the bulk ionic conduction. When the membrane thickness is 1.0 mm, the oxygen permeation fluxes under pO₂ gradient of 0.21/0.021 atm vary from 3.7×10⁻¹⁰ mol s⁻¹ cm⁻² to 1.5×10⁻⁷ mol s⁻¹ cm⁻² at 950 °C. The maximum solubility of Al³⁺ cations in the perovskite lattice of SrFe_1−yAl_yO_3−δ is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr_1−xCa_xFe_0.5Al_0.5O_3−δ system corresponds to x>0.75. The oxygen ionic conductivity of SrFeO₃-based perovskites decreases moderately on Al doping, but is 100–300 times higher than that of brownmillerites derived from CaFe_0.5Al_0.5O_2.5+δ. Temperature-activated character and relatively low values of hole mobility in SrFe_0.7Al_0.3O_3−δ, estimated from the total conductivity and Seebeck coefficient data, suggest a small-polaron mechanism of p-type electronic conduction under oxidising conditions. Reducing oxygen partial pressure results in increasing ionic conductivity and in the transition from dominant p- to n-type electronic transport, followed by decomposition. The low-pO₂ stability limits of Sr_1−xCa_xFe_1−yAl_yO_3−δ seem essentially independent of composition, varying between that of LaFeO_3−δ and the Fe/Fe_1−γO boundary. Thermal expansion coefficients of Sr_1−xCa_xFe_1−yAl_yO_3−δ ceramics in air are 9×10⁻⁶ K⁻¹ to 16×10⁻⁶ K⁻¹ at 100–650 °C and 12×10⁻⁶ K⁻¹ to 24×10⁻⁶ K⁻¹ at 650–950 °C. Doping of SrFe_1−yAl_yO_3−δ with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations.     

Electrical characterization of La9.33Si2Ge4O26 oxyapatite for prospective intermediate-temperature solid oxide fuel cells

La_9.33Si₂Ge₄O₂₆ materials have been fabricated from La₂O₃, SiO₂ and GeO₂ powders by high speed mechanical alloying followed by conventional and microwave hybrid sintering at different temperatures and holding times. XRD data showed that the apatite phase is formed after 1 h of mechanical alloying at 850 rpm. This phase remained stable after conventional sintering in an electric furnace with density increasing as sintering temperatures and holding times were increased. However, the highest density was achieved for samples sintered in the microwave furnace (5.44 g cm⁻³), corresponding to a relative density of 98%. The electrical conductivity of the samples microwave sintered at 700 and 800 ºC are 4.72×10⁻³ and 1.93×10⁻² S.cm⁻¹, respectively, with a correspondent activation energy of 0.952 eV.     

CoAl2O4–mullite composites prepared by sol–gel processes

The formation of CoAl₂O₄–mullite composites from diphasic sol–gel precursors with 3:2 mullite composition doped with 1, 2 and 3 at.% Co²⁺ was studied by differential scanning calorimetry (DSC), X-ray diffraction and Rietveld structure refinement. The course of thermal reactions is dominated by the intermediate formation of two faint crystallized phases having different composition and activation energies. The former phase with smaller activation energy (822 kJ mol⁻¹) is attributed to cobalt-containing spinel structure and the latter with larger activation energy (about 1200 kJ mol⁻¹) to Al–Si spinel. With temperature increase Co-containing spinel transforms progressively in CoAl₂O₄, while Al–Si spinel forms mullite above 1100 °C. Mullite lattice parameters, Rietveld refinement data and the CoAl₂O₄/Co²⁺ ratio in annealed samples points out that the majority of cobalt is incorporated in CoAl₂O₄ and only about 0.6 at.% enters mullite structure or the glassy phase, or both.     

High-energy storage performance in lead-free (1-x)BaTiO3-xBi(Zn0.5Ti0.5)O3 relaxor ceramics for temperature stability applications

In this paper, we prepared lead-free (1-x)BaTiO₃-xBi(Zn_0.5Ti_0.5)O₃ (x=0.04, 0.08, 0.10, and 0.14) ceramics by a conventional solid-state reaction technique. Pure perovskite structures and dense microstructures were demonstrated for all the compositions. Interestingly, it was found that the sintering temperature tended to decrease with increasing the Bi(Zn_0.5Ti_0.5)O₃ content. It should be stressed that a low sintering temperature of 1050 °C was utilized for the composition of x=0.14. Moreover, the dielectric permittivity-temperature curve became more flat and the relaxor degree became stronger with the augment in Bi(Zn_0.5Ti_0.5)O₃ content. We also conducted a detailed study on the energy storage performance for all the compositions from 25 °C to 180 °C.We found that relatively temperature-stable energy storage performance could be obtained in the compositions with x=0.08, 0.10 and 0.14 regardless of the evolution of dielectric constant during the test temperature range. In particular, due to a higher field of 12 MV m⁻¹, the discharge energy storage densities of x=0.14 could reach 0.81 J cm⁻³, 0.80 J cm⁻³, 0.78 J cm⁻³, 0.72 J cm⁻³, and 0.67 J cm⁻³ with high efficiencies of 94%, 92%, 94%, 88% and 77% at 25 °C, 50 °C, 100 °C, 150 °C, and 180 °C, respectively. All these results demonstrate the (1-x)BaTiO₃-xBi(Zn_0.5Ti_0.5)O₃ ceramics are quite promising for temperature-stable energy storage applications.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

Fabrication of reaction-bonded Cr2O3 ceramics

Reaction-bonding to form Cr₂O₃ can be achieved by gaseous oxidation of a Cr phase. The reaction-bonding process is best conducted by complete oxidation before sintering. Below ≈800 °C, the activation energy for oxidation is 220 kJ mol⁻¹, indicating the predominance of Cr³⁺ outward diffusion along high diffusivity paths, e.g., grain boundaries and dislocations. At higher temperatures, the activation energy is reduced to 52 kJ mol⁻¹ as a result of oxygen transport along lower-energy paths, e.g., along microcracks, and the internal and external surfaces. In spite of the decrease in activation energy, the access of oxygen to the inside of the powder compact is hindered by the progressive densification of the oxidizing powder compacts. Maximum densification is achieved for fully oxidized Cr/Cr₂O₃ compacts when the oxygen partial pressure is close to that of the Cr-Cr₂O₃ equilibrium. 0.1 wt% MgO addition increases the density and reduces the grain size of the reaction-bonded Cr₂O₃ samples due to the possible formation of the spinel phase MgCr₂O₄. ZrO₂ and MgO additions improve the fracture strength and toughness of conventionally sintered Cr₂O₃ and change its fracture mode from intergranular to intragranular. For reaction-bonded Cr₂O₃ samples with or without MgO addition, their fracture strength and toughness data are roughly the same as those of sintered Cr₂O₃ doped with ZrO₂ and MgO and their fracture surfaces are predominantly intragranular.     

Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends

Samarium borate (SmBO₃) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm₂O₃) and boron oxide (B₂O₃) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B₂O₃ phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700–1250 °C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV–vis) techniques. Fine-grained and pure SmBO₃ powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO₃ powders were synthesized after mechanical activation for 1 h and annealing at 700 °C for 2 h. The polymorphic transformation temperature of SmBO₃ powders from triclinic to hexagonal is about 1080 °C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO₃ phase and its transformation to hexagonal form were found to take place at ∼50–100 °C lower temperatures than those reported in other methods. Mainly hexagonal SmBO₃ powders were obtained after annealing at 1150 °C in the presence of a very small amount of triclinic SmBO₃. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO₃ phase) and 0.68% (hexagonal SmBO₃ phase) depending on their crystal structures.     

Development of solid oxide cells by co-sintering of GDC diffusion barriers with LSCF air electrode

The effects of a Cu-based additive and nano-Gd-doped ceria (GDC) sol on the sintering temperature for the construction of solid oxide cells (SOCs) were investigated. A GDC buffer layer with 0.25–2 mol% CuO as a sintering aid was prepared by reacting GDC powder and a CuN₂O₆ solution, followed by heating at 600 °C. The sintering of the CuO-added GDC powder was optimized by investigating linear shrinkage, microstructure, grain size, ionic conductivity, and activation energy at temperatures ranging from 1000 to 1400 °C. The sintering temperature of the CuO–GDC buffer layer was decreased from 1400 °C to 1100 °C by adding the CuO sintering aid at levels exceeding 0.25 mol%. The ionic conductivity of the CuO–GDC electrolyte was maximized at 0.5 mol% CuO. However, the addition of CuO did not significantly affect the activation energy of the GDC buffer layer. Buffer layers with CuO-added GDC or nano-GDC sol-infiltrated GDC were fabricated and tested in co-sintering (1050 °C, air) with La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF). In addition, SOC tests were performed using button cells (active area: 1 cm²) and five-cell (active area: 30 cm²/cell) stacks. The button cell exhibited the maximum power density of 0.89 W cm⁻² in solid oxide fuel cell (SOFC) mode. The stack demonstrated more than 1000 h of operation stability in solid oxide electrolysis cell (SOEC) mode (decay rate: 0.004%/kh).     

Ionic conductivities and phase transitions of lanthanide rare-earth substituted La2Mo2O9

The ion conductivities and phase transitions of lanthanum molybdate (La₂Mo₂O₉) substituted with lanthanide rare-earths are investigated using impedance spectroscopy, dilatometry, and X-ray powder diffraction. Among the substituted La₂Mo₂O₉ of 10 mol% Ce, Nd, Sm, Gd, Dy, Er, Yb, the specimens containing Er, and Dy exhibit depressed α–β phase transformation and high conductivities. Their 700 °C conductivities are approximately five to seven times that of La₂Mo₂O₉, around 0.26 S cm⁻¹, comparable with those of (LaSr)(GaMg)O₃ and Gd-substituted CeO₂. Among the three compositions of 10 mol% Gd, Dy, Er showing depressed phase transition, Er- and Dy-substituted La₂Mo₂O₉ possess relatively low thermal expansion coefficient 11×10⁻⁶ K⁻¹, compared with that of the Gd-substituted La₂Mo₂O₉, 18×10⁻⁶ K⁻¹, which is near that of La₂Mo₂O₉. Hence, Dy and Er are valuable dopants in improving the La₂Mo₂O₉ properties. Across the lanthanide series, 10 mol%-substituted La₂Mo₂O₉ demonstrates systematic variations in the conductivity–temperature relation. Hysteresis phenomena in both of conductivity and thermal expansion are also observed in those compositions which display phase transition.