Correlation between microstructure and electrical conductivity in composite electrolytes containing Gd-doped ceria and Gd-doped barium cerate

Composite electrolytes with nominal compositions, Ce_0.8Gd_0.2O_1.9 + xBaO (x = 0.2 and 0.3), have been synthesized through the citrate route. Formation of two phases, namely Gd-doped ceria and Gd-doped barium cerate, has been confirmed through XRD and SEM studies. The impedance spectra show three distinct semi-circles, all originating from the composite electrolytes. In the temperature range 175-350 °C, the activation energies for the conductivity values extracted from the high frequency and intermediate frequency parts of the impedance spectra remains the same, irrespective of compositional and micro-structural variation. On the other hand, the activation energies for the conductivity values associated with the low frequency impedance spectra show a significant change with micro-structural variation. Solid oxide fuel cells constructed using these composite electrolytes exhibit a higher open circuit voltage compared to those based on single phase 20 mol% Gd-doped ceria.     

Lowering the sintering temperature of Gd-doped ceria by mechanochemical activation

Two kinds of Ce_0.8Gd_0.2O_2?δ pellets were synthesized by a solid-state reaction using two types of commercial CeO₂ and Gd₂O₃. In contrast to previous reports, pellets with a sintered density of 99% at 1300 °C were obtained regardless the powder used. Mechanochemical activation of the starting materials by 7 h of high energy milling, which resulted in particles several tens of nanometer in size, was effective in reducing the sintering temperature. Ce_0.8Gd_0.2O_2?δ pellets could be synthesized by direct sintering without calcination due to the homogeneous distribution of fine starting materials. The final phase was confirmed by the ionic conductivity, X-ray diffraction patterns and lattice parameters.     

Effect of preparation route on the microstructure and electrical conductivity of co-doped ceria

Dense Ce_0.8Sm_0.1Gd_0.1O_2?δ electrolytes were fabricated by sintering of CeO₂ solid solutions which were prepared from metal nitrates and NaOH using self propagating room temperature synthesis (SPRT). Three different routes were employed to obtain CeO₂ solid solution powders: (I) hand mixing of reactants, (II) ball milling of reactants and (III) ball milling of Ce_0.8Sm_0.2O_2?δ and Ce_0.8Gd_0.2O_2?δ solid solutions previously prepared by ball milling of corresponding nitrates and NaOH. Density measurements showed that ball milling, which is more convenient than hand mixing, is an effective way to obtain almost full dense samples after presureless sintering at 1550 °C for 1 h. These samples had larger grain size and consequently higher conductivity than the samples obtained by hand mixing. The highest conductivity of 2.704×10^?2 (Ω cm)^?1was measured at 700 °C in a sample prepared by route II. It was found that reduced grain size in samples obtained by hand mixing leads to a decrease in grain boundary conductivity and therefore decrease in the total conductivity. The results showed that mixing of single doped ceria solid solutions improved densification and inhibited grain growth.     

Effect of precursor characteristics on zirconia and ceria particle morphology in spray pyrolysis

Ultrasonic spray pyrolysis of acetate-based precursors with precisely measured precursor drop size was employed to produce ZrO₂ and CeO₂ particles. A bimodal size distribution of the product particles indicates a significant influence of the gas-to-particle conversion mechanism in addition to the conventionally accepted one-particle-per-drop mechanism. Due to the differences in solubility of the precursors, ZrO₂ particles are spherical in shape and smooth on their surfaces while the CeO₂ particles are bowl-like in shape with uneven surfaces. Spherical and monodispersed particles with a peak diameter <100 nm can be obtained by reducing the precursor concentrations to 0.01 wt.% in both the different precursor system.     

Enhanced ionic conductivity of Sm,Gd-doped ceria induced by modification of powder synthesis procedure

Nanostructured Ce_0.85Gd_0.05Sm_0.10O_2?δ powders have been obtained by a classical and a modified Pechini method. The textural parameters of the synthesized powders were evaluated using N₂ adsorption–desorption. X-ray diffraction (XRD) showed that the powders were single phase with fluorite-type structure. The dilatometry measurements evidenced a strong influence of the synthesis procedure of Gd, Sm-co-doped ceria on its sintering behavior. A decrease in powder sintering temperature down to 1200 °C was obtained when Triton X-100 was added into the synthesis reaction mixture. The AC impedance spectroscopy of the sintered pellets was also performed in the 200–800 °C temperature range, in air. The sample prepared using the non-ionic surfactant exhibited higher ionic conductivities and lower activation energies than the sample synthesized by classical Pechini method over the entire investigated temperature range.     

Field-assisted heating of Gd-doped ceria thin film

Flash sintering has recently been used to sinter various bulk ceramics under reduced sintering temperatures and sintering time by applying an electric field across the sample. In this work, we have demonstrated field-assisted heating of 10 mol% Gd-doped CeO₂ thin films deposited by pulsed laser deposition. Microstructure analysis revealed the elongated grains aligned in the out-of-plane direction which is perpendicular to the direction of electric field. The overall microstructure of the flash-heated thin film also contained a matrix of porous and clustered regions, which are distributed throughout the thin film from the anode to cathode electrode regions. The flash-heated thin film showed significantly different conductivity and optical permittivity compared to the as-grown thin films. This demonstration suggests a feasible approach for post-deposition synthesis of thin films using field-assisted heating toward novel morphologies and properties.     

Effects of microstructure on electrochemical reactivity and conductivity in nanostructured ceria thin films

The proton conductivity in functional oxides is crucial in determining electrochemistry and transport phenomena in a number of applications such as catalytic devices and fuel cells. However, single characterization techniques are usually limited in detecting the ionic dynamics at the full range of environmental conditions. In this report, we probe and uncover the links between the microstructure of nanostructured ceria (NC) and parameters that govern its electrochemical reaction and proton transport, by coupling experimental data obtained with time‐resolved Kelvin probe force microscopy (tr‐KPFM), electrochemical impedance spectroscopy (EIS), and finite element analysis. It is found that surface morphology determines the water splitting rate and proton conductivity at 25°C and wet conditions, when protons are mainly generated and transported within surface physisorbed water layers. However, at higher temperature (i.e., ≥125°C) and dry conditions, when physisorbed water evaporates, grain size, and crystallographic orientation become significant factors. Specifically, the proton generation rate is negatively correlated with the grain size, whereas proton diffusivity is facilitated by surface {111} planes and additional conduction pathways offered by cracks and open pores connected to the surface.     

Conventional and field-assisted sintering of nanosized Gd-doped ceria synthesized by co-precipitation

Gadolinium-doped ceria is an attractive electrolyte for potential application in SOFCs operating at intermediate temperature; for such use, the fundamental compositions typically contain 10–20 mol% Gd₂O₃. In this work, we produced nanosized 10 mol% gadolinium-doped ceria powder by co-precipitation, starting from Ce and Gd nitrate solutions and using ammonia solution as precipitating agent. The co-precipitate was characterized by DTA-TG, TEM, XRD and nitrogen adsorption analyses. We studied the behavior of the nanopowder under both conventional and Flash sintering. Very different behavior was seen: the conventional sintering cycle produced a poorly densified material, while Flash sintering allowed production of a perfectly densified material, with uniform sub-micrometric grain size.     

Effect of pyrolysis temperature on the electric conductivity of polymer-derived silicoboron carbonitride

The electric conductivity of polymer-derived SiBCNs pyrolyzed at different temperatures was studied. We showed that the boron impeded the graphitization of the free-carbon phase in the SiBCN, leading to a higher characteristic temperature and activation energy as compared to the SiCN. Such an impeding effect is due to the interaction between h-BN and graphite phase. We also provided a credible evidence to show that the increase in the electric conductivity of the SiBCN with pyrolysis temperature is likely due to the increase in the conductivity of the free-carbon phase.     

热解温度对无烟煤焦微观结构和脱硝特性的影响

在氮气气氛和600～1000℃条件下, 使用水平管式炉对龙坪无烟煤进行了热解制焦实验, 利用拉曼光谱分析了热解煤焦的微观结构变化, 使用热重分析仪(TGA)进行了无烟煤焦的程序升温脱硝实验, 研究了热解温度对无烟煤焦微观结构和脱硝特性的影响。研究结果表明:热解温度对无烟煤焦的微观结构有显著的影响, 随着热解温度升高, 煤焦的拉曼光谱G峰与D1峰的位置差逐渐减小, 谱峰积分面积逐渐缩小, 积分面积比值I_D1/I_G和I_D3/I_G先增大后减小, I_G/I_All先减小后增大。热解温度还对无烟煤焦的脱硝特性有明显的影响, 无烟煤随着热解温度升高由炭化向初步石墨化转变, 由于活性结构数量的减少和碳结构变得更加有序, 无烟煤焦的脱硝反应性不断降低, 脱硝反应活化能增大。     

Processing and performance of solid oxide fuel cell with Gd-doped ceria electrolyte

Fuel Cell performance was measured at 792-1095 K for Ni-GDC (Gd-doped ceria) anode-supported GDC film (60 μm thickness) with a (La_0.8Sr_0.2)(Co_0.8Fe_0.2)O₃ cathode using H₂ fuel containing 3 vol% H₂O. A maximum power density, 436 mW/cm², was obtained at 1095 K. The electrical conductivity of GDC electrolyte in N₂ atmosphere of 10⁻¹⁵-10⁰ Pa oxygen partial pressures (Po₂) at 773-1073 K was independent of Po₂, which indicated the diffusion of oxide ions. The conductivity of GDC in H₂O/H₂ atmosphere increased because of the further formation of electrons due to the dissociation of hydrogen in GDC (H₂ → 2H⁺ + 2e⁻). The hole conductivity was observed at 873 K in Po₂ = 10⁰-10⁴ Pa. The key factors in increasing power density are the increase of open circuit voltage and the suppression of H₂ fuel dissolution in GDC electrolyte. These are controlled by the cathode material and Gd-dopant composition.     

Effects of precursor concentration on the microstructure and properties of ZrC modified C/C composites prepared by precursor infiltration and pyrolysis

To improve the ablation resistance of C/C composites, ZrC modified composites were fabricated by precursor infiltration and pyrolysis combined with gradient chemical vapor infiltration process. The effects of ZrC precursor concentration on the microstructure, mechanical and ablation properties of the composites were studied. Results showed that with the increase of ZrC precursor concentration, the ZrC content and macroscopic uniformity of the composites increased but with obvious ZrC particle aggregation and the flexural strength decreased gradually. As the concentration of ZrC precursor improved to 60%, the fracture mode of the composites transformed from toughness to brittleness which was mainly attributed to the improved graphitization degree and reaction damage of carbon fiber in the precursor pyrolysis process. However, the ablation resistance was enhanced with the increasing precursor concentration which was resulted from the formation of ZrO₂ in center ablation region and continuous ZrO₂ coating in brim region serving as a barrier to heat and oxygen transfer.     

Influence of temperature and alumina catalyst on pyrolysis of corncob

Corncob has been investigated as an alternative feedstock to obtain fuels and chemicals via pyrolysis in fixed-bed reactor. The influence of pyrolysis temperature in the range 300-800 °C as well as the catalyst effects on the products was investigated in detail and the obtained results were compared. The results indicated that a maximum oil yield of 22.2% was obtained at a moderate temperature of 600 °C. The oil yield was reduced when the temperature was increased from 600 to 800 °C, whereas the gas yield increased.Pyrolysis oils were examined by using instrumental analysis, ¹H NMR spectroscopy and GC/MS. This analysis revealed that the pyrolysis oils were chemically very heterogeneous at all temperatures. It was determined that the most abundant compounds composing the bio-oil were phenolics.It was observed that the catalyst decreased the reaction temperature. Most of the components obtained using a catalyst at moderate temperatures was close to those obtained at high temperatures without using a catalyst. Moreover, the use of a catalyst and the high temperatures of the reactions also decreased the amount of oxygenated compounds produced.According to these results, corncob bio-oils can be used as fuel and constitute a valuable source of chemical raw materials.     

The ionic conductivity of Sm-doped ceria

The oxygen ion conductivity of polycrystalline samples of Sm-doped ceria and of Gd-doped ceria is studied as a function of doping fraction and temperature using impedance spectroscopy allowing the separation of bulk and grain boundary conductivity. The introduction of a fine spacing for the Sm dopant fraction allows the clear identification of the dopant fraction leading to the largest bulk conductivity. At 267°C, the largest bulk conductivity is shown for Ce_0.93Sm_0.07O_1.965. With increasing temperature, indications of an increase in the dopant fraction, which leads to the maximum in conductivity, are found. For the grain boundary conductivity, the maximum appears at larger dopant fractions compared to the bulk conductivity. The largest total conductivity for both dopants is again found for Sm-doped ceria. In literature, different syntheses and sample preparation methods led to larger total conductivities for Gd-doped ceria. In this work, we demonstrate that the variation of sintering conditions leads to scattering in the conductivity over one order of magnitude. Finally, we demonstrate that, in nominally pure cerium oxide, impurities dominate the ionic conductivity.     

Electrical conductivity of praseodymia doped ceria

The electrical conductivity () and oxide ion transference number (t ₀) of praseodymia doped ceria systems were measured. The former increased rapidly with the praseodymia content, while the latter decreased. At 600° C, for instance, CeO₂ and Ce_0.6Pr_0.4O₂ under 0.21 atm of oxygen were 2.0×10^–5 and 3.6×10^–2 S cm^–1; andt ₀ in them were 0.59 and 0.11, respectively. This mixed conductor having high electrical conductivity might be useful as a fuel cell electrode if it could be combined with a suitable solid electrolyte.     

Synthesis,sintering and ionic conductivity of scandia-doped ceria ceramic materials obtained by different procedures

Crystalline scandia-doped ceria powders, with Ce_0.92Sc_0.08O_2?δ and Ce_0.82Sc_0.18O_2?δ compositions have been prepared by three different procedures: (a) mechanochemical route from the starting oxides; (b) co-precipitation of hydroxides from an aqueous solution; (c) solid state reaction between oxides. Nanopowders were only obtained from the two first routes. Isothermal sintering was carried out between 1200 and 1500 °C. Apparent density as high as 99% D_th was achieved by sintering at 1300 °C for 2 h from co-precipitated powders. Temperature was further reduced, and 99% D_th was also obtained by SPS at 950 °C of Ce_0.82Sc_0.18O_2?δ mechanically activated powders. Sc₂O₃ was observed as a secondary phase for all the samples containing 18 at% Sc, whereas only traces were pointed out in the 8 at% Sc samples. Total conductivity attains lower values than those measured in other ceria solid solutions, at difference of that observed in cubic Sc-stabilized zirconia.     

Effect of the temperature of precursor pyrolysis on luminescence characteristics of luminophores based on REE compounds

The effect of the pyrolysis temperature on luminescence characteristics of luminophores based on REE (rare-earth elements) compounds in the extraction-pyrolysis synthesis method has been investigated. The optimal synthesis conditions have been found, and the techniques for producing efficient nanosized luminophores based on REE oxides, oxysulfides, and phosphates have been developed.     

Low temperature densification by lithium co-doping and its effect on ionic conductivity of samarium doped ceria electrolyte

Low temperature densification and improving the ionic conductivity of doped ceria electrolyte is important for the realization of efficient intermediate temperature solid oxide fuel cell system. Herein, we report the effect of lithium co-doping (1, 3, 5 and 7?mol%) in 20?mol% samarium doped ceria on the low temperature sinterability and conductivity. The synthesized nanoparticles by citrate-nitrate combustion method showed a decrease in lattice parameter and increase in oxygen vacancy with lithium content after calcination due to the substitution of Li⁺ into CeO₂ lattice. Upon sintering at 900?°C, the density improved and reached a maximum value of 98.6% for 5% Li which exhibited a dense microstructure than at 7% Li. 5%Li co-doping exhibited the best conductivity of 3.65?×?10^?04–1.81?×?10^?3 S?cm^?1 in the operative temperature range of IT-SOFC (550–700?°C).Our results demonstrate the significance of lithium as co-dopant for efficient low temperature sintering as well as improving the electrolyte conductivity.