Preparation of samaria-doped ceria for solid-oxide fuel cell electrolyte by a modified sol-gel method

Samaria-doped ceria powders were prepared by the sol-gel method with different processes. The characteristics of the sample were investigated by particle size distribution, X-ray diffraction, crystallite size, and density analyses. A modified process of the sol-gel method was proposed in this work. It involves treating the gel with high-carbon (long chain, high boiling point) alcohol. It yields near-completely soft-agglomerated nanocrystalline powders which are easily sintered in air to yield near-fully relative density at 1300°C, which is significantly lower than temperatures of 1400–1500°C required by the doped-ceria powder prepared via the previous processes of the sol-gel method.     

Effect of sintering temperature on electrical properties of gadolinium-doped ceria ceramics

Journal of Materials Science - Electrical properties of 10 mol% Gd2O3–90 mol% CeO2 (GDC10) and 20 mol% Gd2O3–80 mol% CeO2 (GDC20) ceramics,...     

Study of gadolinia-doped ceria solid electrolyte surface by XPS

Gadolinia-doped ceria (CGO) is an important material to be used as electrolyte for solid oxide fuel cell for intermediate temperature operation. Ceria doped with 10 mol% gadolinia (Ce_0.9Gd_0.1O_1.95) was prepared by conventional solid state synthesis and found to be single phase by room temperature X-ray diffraction (XRD). The chemical states of the surface of the prepared sample were analyzed by X-ray photoelectron spectroscopy (XPS). Though Gd was present in its characteristic chemical state, Ce was found in both Ce⁴⁺ and Ce³⁺ states. Presence of Ce³⁺ state was ascribed to the differential yield of oxygen atoms in the sputtering process.     

Influence of the diamond grain size on the electrical properties of nano-crystalline diamond film detectors

In this work, we developed X-ray radiation detectors with sandwich structure fabricated from nano-crystalline diamond (NCD) films. These NCD films with different grain size ranging from 15 nm to 160 nm were grown on silicon substrates using a hot-filament chemical vapor deposition technique. I-V measurement results indicate that with reducing of the grain size, the resistivity of diamond films decreases from 9.5 × 10⁸ to 6.20 × 10⁷ Ω cm and the ratio of the photocurrent to the dark-current (I_ph/I_d) of the detectors decreases rapidly from 0.45 to 0.09 at an electric field of 50 kV/cm. Typical spectral response to 5.9 keV ⁵⁵Fe X-rays shows that counting efficiency and energy resolution of NCD detectors with large grains are better than those of detectors with small grains, due to the less defects and grain-boundaries contained in the film.     

Effect of powder thermal treatment on the microstructure and electrical properties of doped ceria ceramics

Ce_0.8Sm_0.2O_2 − δ electrolytes were prepared by sol-gel methods with different thermal treatment conditions. It was found that the particle size of the powders thermally treated in N₂ could be controlled less than 10 nm, due to the fact that carbons that enwrapped the precursors could inhibit their crystallization. At the same time, the nano-sized precursors could form larger nuclei at the initial stage of sintering, resulting in the formation of electrolyte with higher density and larger grain size. At the same consent of oxygen vacancies, the enhancement of electrical properties for the electrolyte was correlated to its microstructure obtained by the N₂ thermal treatment.     

过渡和碱土金属掺杂对LaGaO3固体电解质电性能影响

采用固相反应法合成碱土金属Sr、Mg与过渡金属Fe或Co复合掺杂的LaGaO3基氧化物LSGMC和LSGMF;通过TG/DTA研究LSGMC和LSGMF的形成过程;采用直流四端子法测量样品在400℃-850℃温度区间的电导率;利用EDS和XRD等技术分析样品的微区成分和晶体结构等.结果表明,约在1000℃开始形成钙钛矿相,经1400℃烧结,获得单一正交钙钛矿结构;在测试温度范围,电导率与温度的关系符合Arrhenius方程,合成的LSGMC和LSGMF为氧离子导体;在相同温度下,Sr、Mg和Fe或Co复合掺杂氧化物LSGMC和LSGMF与仅Sr、Mg双掺杂氧化物LSGM相比具有较高的离子电导率,说明少量过渡金属Fe或Co的掺杂,有利于提高离子电导率.     

Effect of grain morphology and grain size on the mechanical properties of Al2O3 ceramics

Bending strength, fracture toughness, fracture energy and crack extension resistance were evaluated for Al₂O₃ ceramics with equi-axed and platelet grains. Bending strength was proportional to grain size^–1/2, but flaws with a size of 10 m controlled the strength when the microstructure was finer than 10 m. Fracture toughness, measured by the single etched precracked beam (SERB) method, was proportional to fracture energy^1/2, and increased with the grain size of Al₂O₃ ceramics with equi-axed and platelet grains. However, the toughness of platelet grain ceramics was higher than the ceramics with equi-axed grains, and increased up to 6.6 MPam^1/2 with grain size. Therefore, it is thought that fracture toughness not only depends on grain size, but also on grain morphology; equations were derived to account for this phenomenon.     

Inkjet printing of gadolinium-doped ceria electrolyte on NiO-YSZ substrates for solid oxide fuel cell applications

A sol–gel-based precursor solution of gadolinium-doped ceria (CGO) was developed for deposition by inkjet printing. A stable precursor was synthesised by dissolving stoichiometric amounts of cerium (III) acetate hydrate and gadolinium (III) acetate hydrate in propionic acid, and diluted to 0.75 M with 1-propanol. The sintering behaviour of the printed precursor was investigated. Since the commonly used method of dilatometry is only applicable to bulk samples, an alternative approach using Differential scanning calorimetry (DSC) has been explored. The sintering temperature of the printed precursor was estimated by subtracting contributions from energy stored due to heat capacity and activation energy of ionic mobility from the DSC heat flow signal. Based on this modelling it was found that the optimum sintering temperature of the acetate-based CGO precursor was 1100 ± 55 °C, a result independently confirmed by SEM imaging of printed precursor coating on NiO-YSZ cermet. A gadolinium-doped ceria (CGO) thin film was then directly deposited on a porous NiO-YSZ cermet anode composite by inkjet printing. After co-sintering, it was shown that crack-free and continuous coating thinner than 10 μm of CGO can be readily produced. These results suggest that the inkjet printing technique can be successfully implemented to fabricate a thin film of dense electrolyte (>98%) for solid oxide fuel cell (SOFC) applications.     

Effect of nanoprecipitates and grain size on the mechanical properties of advanced structural steels

The microstructure and nanometric precipitates present in advanced structured steel have been studied by high resolution transmission electron microscopy equipped with energy dispersion X-ray microanalysis, in order to relate the nanometric precipitates and grain size with the improvement of the yield strength value of the API steel. The microstructure and nanometric precipitates of the advanced steel were obtained by a combination of thermo-mechanical controlled hot rolling and accelerated cooling procedures. The API steel composition consisted of hot rolled Nb-Ti microalloyed with: 0.07C, 1.40Mn, 0.24Si, 0.020Al, 0.009P, 0.001S, 0.05Mo, 0.5Cr, 0.05Nb, 0.25Ni, 0.10Cu, 0.012Ti, 0.05N in wt%. As a result, this hot rolled steel tested at a strain rate of 5 × 10⁻³ s⁻¹ showed an improved yield strength from 798 MPa to 878 MPa due to the micrometric grain size of 2.2 μm and to the nanometric precipitates with a size of around 5 nm in the microstructure of the steel studied.     

Effect of initial grain size on the joint properties of friction stir welded aluminum

In this study, 2 mm-thick commercial 1050-Al plates with an ultrafine grained (UFG) structure were obtained by the accumulative roll bonding (ARB) technique after a 5 cycle process and were subsequently joined by friction stir welding (FSW) at various revolution pitches (welding speed/rotation speed) of 1 mm/r, 1.67 mm/r and 2.5 mm/r. To understand the effect of the initial grain size on the welding properties, ARB processed samples followed by annealing under H24 conditions as well as the as-received samples in the fully annealed state were also applied to the FSW process. The microstructure evolution and Vickers hardness in the stir zone of all the samples were investigated. It was revealed that the annealed samples with an intermediate grain size finally obtained the most refined grain size and highest value of Vickers hardness in the stir zone. However, for the UFG samples, significant grain growth and corresponding decrease in hardness can be found in the stir zone.     

Synthesis, sintering and electrical properties of gadolinia- calcia-doped ceria

Crystalline gadolinia and calcia co-doped ceria powders, with Ce_0.85Gd_0.15-xCa_xO_2-δ composition, with x = 0.01, 0.015, 0.02 have been prepared by a co-precipitation procedure from the corresponding nitrates of component cations. Nanopowders were obtained after thermal treatment of the co-precipitated mixtures at 700 °C for 2 h. Isostatically pressed pellets were prepared from the powder. Isothermal sintering was carried out between 1300º and 1400 °C. Apparent densities as high as 98% of the theoretical one, D_th, were attained after sintering at 1300 °C 4h. Microstructure of the ceramics was observed by Scanning Electron Microscopy (SEM). Bulk and grain boundary conductivities, determined by Complex Impedance Spectroscopy, have similar values, and the total conductivity attains good values compatible with the use as electrolyte in SOFC.     

Effect of grain size on the electrical and magnetic properties of MgB2 thick films deposited on the Al2O3 single crystal substrates

MgB₂ films having thicknesses of approximately 10 μm have been prepared on the Al₂O₃ single crystal substrates by using ultrasonic spray pyrolysis technique. The films fabricated were then analyzed by means of microstructural, structural, transport and magnetic properties. The influence of grain size, depending on the ex-situ and in situ heating, were investigated. The results obtained showed that the size of MgB₂ grains has an effect on the transport and magnetic properties of the films. Films prepared with in situ heating have small particle size (<50 nm) and were found to be much better properties than the films prepared ex-situ heating, which have larger grains (~1 μm). The best T _c value and critical current density, J _c ^mag , were obtained to be ~36 K and 9.88 × 10⁵ A cm^?2 respectively for the films prepared with small grains. The results were showed that the smaller grain size and in situ heating cycles are essential points for quality thick film MgB₂ fabrication.     

Effect of rare-earth oxides on fracture properties of ceria ceramics

The influences of the sintering additive content of rare-earth oxide (Y₂O₃, Gd₂O₃, Sm₂O₃) on microstructure and mechanical properties of ceria ceramics were investigated by scanning electron microscopy and small specimen technique. A small punch testing method was employed to determine the elastic modulus and biaxial fracture stress of the ceria-based ceramics, and the fracture toughness was estimated by Vickers indentation method. Grain growth in the rare-earth oxides doped ceria ceramics was significantly suppressed, compared to the pure ceria ceramics. However, the elastic modulus, fracture stress and fracture toughness were decreased significantly with increasing additive content of the rare-earth oxides, possibly due to the oxygen vacancies induced by the rare earth oxides doping. The experimental results suggest that the change in the mechanical properties should be taken into account in the use of ceria-based ceramics for solid oxide fuel cells, in addition to the improvement of oxygen ion conductivity.     

Study of grain boundaries influence on electrical properties of nitrides

The electronic conduction mechanism through grain boundaries in heteroepitaxial gallium nitride layers was explained by applying the model, which included three effects: thermionic emission over potential barrier, thermionic field emission through potential barrier and thermionic field emission through scattering barrier. Space charge potential barriers height at the grain boundary layer was estimated to be 80 meV from the measurement of the temperature dependence of layer resistivity. Influence of the deep traps location in the different regions of active layers of the MSM detector on the device performance was evaluated by 2D numerical simulation.