The effect of alumina and Cu addition on the electrical properties and the SOFC performance of Gd-doped CeO2 electrolyte

Alumina and transition metals were added to Gd-doped ceria and the electrical properties of doped ceria were studied. For 10 mol% Gd-doped ceria (GDC), 1 at.% transition metals (Fe, Co, Ni, Cu, or Ga) were added as a sintering aid and 1, 2, 5, 10, and 20 at.% alumina was added as a possible strengthening media. The electrical conductivity was measured as a function of temperature (250–700 °C) in air. The open circuit voltage (OCV) and the impedance spectra of the cell in dual atmosphere (air on one side, hydrogen on another side) were measured as a function of time. Samples added with both 1 at.% Cu and 2 at.% alumina showed good sinterability without much loss of electrical conductivity. The time-dependence of the OCV value and the impedance spectra showed that either the alumina-added or the Cu and alumina co-added sample exhibited the slower degradation of OCV and the less electrode polarization than GDC. The results indicate that Cu and alumina addition improved the stability of the cell.     

Effect of Fe doping on the structural and gas sensing properties of ZnO porous microspheres

Fe-doped ZnO porous microspheres composed of nanosheets were prepared by a simple hydrothermal method combined with post-annealing, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer–Emmett–Teller N₂ adsorption–desorption measurements and photoluminescence (PL) spectra. In this paper we report Fe doping induced modifications in the structural, photoluminescence and gas sensing behavior of ZnO porous microspheres. Our results show that the crystallite size decreases and specific surface area increases with the increase of Fe doping concentration. The PL spectra indicate that the 4 mol% Fe-doped ZnO has higher ratio of donor (V_O and Zn_i) to acceptor (V_Zn) than undoped ZnO. The 4 mol% Fe-doped ZnO sample shows the highest response value to ppb-level n-butanol at 300 °C, and the detected limit of n-butanol is below 10 ppb. In addition, the 4 mol% Fe -doped ZnO sample exhibits good selectivity to n-butanol. The superior sensing properties of the Fe-doped porous ZnO microspheres are contributed to higher donor defects contents combined with larger specific surface area.     

Role of the microstructure on the transport properties of Y-doped zirconia and Gd-doped ceria

Transmission electron microscopy characterizations and XPS analyses have allowed us to show the influence of the microstructure and nanochemistry on the transport properties of Y₂O₃-(9 mol%)-stabilized zirconia (YSZ) and Gd₂O₃ (10 mol%)-doped ceria (GDC). The grain boundary electrical conductivity (σ_gb) and oxygen diffusion coefficient (D_o) of conventional YSZ ceramics increase with the grain size, while an opposite behavior was found for GDC samples. This difference was attributed to glassy precipitates present at YSZ grain boundaries. Furthermore, it was shown that kinetic demixing processes take place during cooling, at the end of sintering. This causes important changes in the cationic species distribution at interfaces and plays an important role on the transport properties of these two materials. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Different conduction behaviors of grain boundaries in SiO2-containing 8YSZ and CGO20 electrolytes

Both doped zirconia and ceria have been widely recognized as promising electrolytes in solid oxide fuel cells (SOFC). Total conductivity is an important parameter to evaluate solid electrolytes. It is well know that the contribution to the total conductivity by grain boundaries is especially pronounced for SiO₂-contaminated electrolytes. In this study, we report on the different conduction behaviors of grain boundaries (GB) found in SiO₂-containing (impure) 8YSZ (8 mol% Y₂O₃-doped ZrO₂) and CGO20 (10 mol% Gd₂O₃-doped CeO₂) ceramics. In the grain size range (∼ 0.5–10 μm) studied, the GB conductivity of impure CGO20 ceramics constantly decreases with increasing grain size, in contrast to that observed in impure 8YSZ electrolytes whose GB conductivity increases almost linearly with grain size. It is also found that the variation in GB conductivity versus grain size is different from case to case, depending on the sintering/annealing conditions used to fabricate the ceramics. Two mechanisms were proposed to explain the GB behaviors of the impure 8YSZ and CGO20 ceramics. For doped ceria, the GB phases are supposed to be inert, which do not react with or dissolve into the matrix. Increasing sintering temperature leads to not only grain growth but also change in viscosity and wetting nature of the GB phases. These two factors promote further propagation of the GB phases along the grain boundaries, leading to an increased GB coverage fraction. For doped zirconia, however, the major factor dominating the GB conduction is the further dissolution of SiO₂ into zirconia lattice as a result of increase in sintering temperature or/and time. In addition, we will also evaluate and discuss the validities of the three models that are widely used to analyze the GB conduction in solid electrolytes.     

Electrical study and dielectric relaxation behavior in?nanocrystalline Ce0.85Gd0.15O2?δ material at intermediate temperatures

The nanocrystalline material of 15 mol% Gd-doped ceria (Ce_0.85Gd_0.15O_2−δ ) was prepared by citrate auto ignition method. The electrical study and dielectric relaxation technique were applied to investigate the ionic transport process in this nanocrystalline material with an average grain size of 13 nm and the dynamic relaxation parameters are deduced in the temperature range of 300–600°C. The ionic transference number in the material is found to be 0.85 at 500°C at ambient conditions. The oxygen ionic conduction in the nanocrystalline Ce_0.85Gd_0.15O_2−δ material follows the hopping mechanism. The grain boundary relaxation is found to be associated with migration of charge carriers. The frequency spectra of modulus M″ exhibited a dielectric relaxation peak corresponding to defect associates (Gd-V_{o^{\blacksquare \blacksquare}})^{_\blacksquare}(\mathrm{Gd}\mbox{-}\mathrm{V}_{\mathrm{o}}^{_{_{{\blacksquare\,\blacksquare}}}})^{_{_{{\blacksquare}}}}. The material exhibits very low values of migration energy and association energy of the oxygen vacancies in the long-range motion, i.e., 0.84 and 0.07 eV, respectively.     

Improved total conductivity of nanometric samaria-doped ceria powders sintered with molten LiNO3 additive

Nanometric 20% molar Sm-doped ceria (SDC20) powders were synthesized by co-precipitation in the presence of N, N, N′, N′ tetramethylethylendiamine (TMEDA). SDC20 powders were sintered using lithium nitrate salt in various concentrations (0.1, 1, 3, and 10 mol% with respect to the SDC20 total moles) as an additive to promote the liquid phase sintering and without additive for comparison. The addition of the Li salt allowed reducing significantly the sintering temperature of SDC. Electrochemical impedance spectroscopy (EIS) measurements were performed to estimate the contribution of grain boundary and bulk to the electrical conductivity in different sintering conditions. Liquid phase sintering allowed to produce dense samples with enhanced ionic conductivity especially at the grain boundary when compared to the samples sintered without additive. The additive liquid phase was evaporated in large part at the high temperatures throughout the sintering process. Residual extra phases were segregated at the grain boundary, generated probably by reaction of the Li salt with impurities, which were removed by a chemical etching.     

The effects of thermal annealing on the structure and the electrical transport properties of ultrathin gadolinia-doped ceria films grown by pulsed laser deposition

Ultrathin crystalline films of 10 mol% gadolinia-doped ceria (CGO10) are grown on MgO (100) substrates by pulsed laser deposition at a moderate temperature of 400°C. As-deposited CGO10 layers of approximately 4 nm, 14 nm, and 22 nm thickness consist of fine grains with dimensions ≤∼11 nm. The films show high density within the thickness probed in the X-ray reflectivity experiments. Thermally activated grain growth, density decrease, and film surface roughening, which may result in the formation of incoherent CGO10 islands by dewetting below a critical film thickness, are observed upon heat treatment at 400°C and 800°C. The effect of the grain coarsening on the electrical characteristics of the layers is investigated and discussed in the context of a variation of the number density of grain boundaries. The results are evaluated with regard to the use of ultrathin CGO10 films as seeding templates for the moderate temperature growth of thick solid electrolyte films with improved oxygen transport properties.     

Studies on grain boundary effects in spray deposited BICOVOX 0.1 films on platinum-coated stainless steel substrate

Thin film of Bi₂Co_0.1V_0.9O_5.35 is deposited by using spray pyrolysis technique on platinum-coated stainless steel substrate. Impedance measurements done in the frequency range 1 to 10 MHz and in the temperature range 502 to 720 K revealed two relaxation processes with distinguishable time constants. The first corresponds to the grain interior charge transfer while the second could be due to grain boundary. The change in polarization seems to be associated with hopping of charge carriers showing Arrhenius behavior with increase in temperature. The relaxation frequency of grain interior transport for the thin film ranges from 96 kHz to 2.59 MHz. The blocking factor was found to be increasing with increase in temperature at low temperature region from 502 to 640 K. At higher temperature above 640 K, diffusive nature of grain boundaries is inferred with the decrease in blocking factor. The same inference is derived by specific grain boundary conductivity calculations since specific grain boundary conductivity decreases in low temperature region while it increases rapidly at higher temperature. These observations prove the grain boundaries to be blocking in lower temperatures while at higher temperatures above 640 K they turn diffusive. These changes are attributed to structural phase transition, or ordering of vacancies in Bi₂Co_0.1V_0.9O_5.35 films.     

Ionic and electronic conduction in nanostructured solids: Concepts and concerns,consensus and controversies

This paper summarizes some advances achieved in the first 10 years of studies on nanosized ionic and mixed conductors. It addresses first concepts: boundary core and space charge effects, impedance and percolation models. Then, some consensual evidence is presented: nanocrystals show no particular structural disorder and their electrical conductivity can be nicely interpreted using space charge theory, as shown for the model case of nanosized ceria. However, there are also controversial cases, such as doped zirconia, for which very different results were reported. Finally, concerns on long-time stability of nanosized solids are addressed. Recent studies of phase stability and grain growth show that these issues might be overcome.     

Effects of Fe doping on crystalline and optical properties of yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) samples with different Fe concentrations were prepared to study the effects of Fe doping on crystalline and optical properties of YSZ. The former properties were determined by X-ray diffraction, while the latter properties were determined by diffuse reflectance (DR) and photoluminescence (PL) spectroscopies. Lattice contraction of YSZ caused by the Fe doping was observed. We revealed that the DR spectra of the 3 and 6 mol% Fe-doped YSZ samples originate from the Fe ions dissolved and undissolved in the YSZ, respectively. Moreover, two PL bands centered around 440 and 530 nm were observed for the YSZ sample, whereas one PL band centered around 440 nm was observed for the Fe-doped YSZ samples. The Fe doping reduced the PL intensity of YSZ and quenched the PL band around 530 nm. This could be explained by considering that the concentration of Fe ions near the surface of YSZ is much larger than that in the bulk of YSZ or by considering that the Fe doping enhances surface band bending of YSZ.     

Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%)

Yttria–zirconia doped ceria, 10% ZrO₂–10% Y₂O₃–CeO₂ (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (> 95%) at relatively low sintering temperature and long soaking time.     

Modelling of diffusion and conductivity relaxation of oxide ceramics

A two-dimensional square grain model has been applied to simulate simultaneously the diffusion process and relaxation of the dc conduction of polycrystalline oxide materials due to a sudden change of the oxygen partial pressure of the surrounding gas phase. The numerical calculations are performed by employing the finite element approach. The grains are squares of equal side length (average grain size) and the grain boundaries may consist of thin slabs of uniform thickness. An additional (space charge) layer adjacent to the grain boundary cores (thin slabs) either blocking (depletion layer) or highly conductive for electronic charge carriers may surround the grains. The electronic transport number of the mixed ionic-electronic conducting oxide ceramics may be close to unity (predominant electronic conduction). If the chemical diffusion coefficient of the neutral mobile component (oxygen) of the grain boundary core regions is assumed to be higher by many orders of magnitude than that in the bulk, the simulated relaxation curves for mass transport (diffusion) and dc conduction can deviate remarkably from each other. Deviations between the relaxation of mass transport and dc conduction are found in the case of considerably different electronic conductivities of grain boundary core regions, space charge layers, and bulk. On the contrary, the relaxation curves of mass transport and electronic conductivity are in perfect coincidence, when either effective medium diffusion occurs or the effective conductivity is unaffected by the individual conductivities of core regions and possible space charge layers, i.e. the grain boundary resistivity is negligible.     

Microstructure and electrical properties of YSZ–NiO composites

The microstructure and electrical properties of YSZ (ionic)–NiO (electronic) composites were investigated. The electrical properties of the composites were examined using impedance spectroscopy between 160 and 630°C. The effects of grains and grain boundaries on the electrical conductivity of the composites vary with composition and temperature. With increasing temperature, the contribution of grain boundary to the total resistivity decreases for YSZ-rich compositions (NiO≤30 mol%). For NiO-rich compositions (NiO>50 mol%), the contribution of grain boundary resistance is variable with temperature, showing maxima near 530°C. Above 630°C, the grain boundary resistance is small for all compositions. With varying composition, grain boundary resistance is greater when the number of YSZ/NiO contacts is greater than the number of YSZ/YSZ or NiO/NiO contacts below 500°C. The temperature- and composition-dependence of grain and grain boundary conductivity is explained by the microstructure and thus by the distribution of the two phases together with variations in activation energy according to the composition.     

Impedance studies on bismuth-ruthenate-based electrodes

Doped bismuth ruthenates and bismuth ruthenate-stabilized bismuth oxide composites were studied as prospective cathode material for solid oxide fuel cells. Symmetric cells were fabricated on gadolinium-doped ceria electrolytes and studied by electrochemical impedance spectroscopy. Ca- and Ag-doped bismuth ruthenate electrodes (5–10 mol%) showed the same characteristic frequency as undoped bismuth ruthenate but with higher activation energy and slightly better performance above ∼550 °C. At 700 °C, area-specific resistance (ASR) of undoped, 5 mol% Ca and 5 mol% Sr-doped bismuth ruthenate electrode was 1.45, 1.24, and 1.41  Ωcm², respectively. The change in ASR as a function of oxygen partial pressure and current bias suggests that the rate-limiting steps for oxygen reduction in bismuth ruthenate systems are charge transfer and surface diffusion of dissociatively adsorbed oxygen to triple phase boundaries. Introduction of the erbia-stabilized bismuth oxide (ESB) phase reduced both the rate-limiting steps resulting in much improved electrode performance. At 700 °C, composite electrodes containing 31.25–43.75 wt% ESB exhibited an ASR of 0.08–0.11 Ωcm².     

铁掺杂TiO2/膨润土光催化剂的制备及表征

以溶胶-凝胶法制备了过渡金属铁掺杂的Fe-TiO2/膨润土复合光催化剂,通过XRD、FTIR技术对该系列催化剂的结构进行了表征,并以亚甲基蓝染料废水为模型,考察了紫外光和日光下铁掺杂量对催化剂性能的影响。结果表明,催化剂中有锐钛矿型TiO2生成,铁掺杂TiO2进入了膨润土的蒙脱石层间,改变了蒙脱石层间的有序性,生成了Ti—O—Si键,实现了TiO2粒子与膨润土的复合;适量Fe3+的掺杂降低了TiO2粒子的粒径,并且部分Fe3+还可能进入了TiO2的晶格,形成了缺陷位,拓宽了TiO2的光谱利用范围,提高了光催化活性。     

Fe3O4 Segregation and Transport Properties of La0.67Ca0.33MnO3

Effect of Fe3O4 segregation at grain boundaries on the electrical transport and magnetic properties of La0.67Ca0.33MnO3 is investigated. The experimental results show that the Fe3O4 segregation not only shifts the paramagnetic-ferromagnetic transition temperature of La0.6TCa0.33MnO3 to a lower temperature region but also induces a new transition in a lower temperature region. Meanwhile, the transition processes observed in both the resistivity and magnetization curves are obviously widened. Compared to pure La0.67Ca0.33MnO3, we assume that the Fe3O4 segregation level at the grain boundaries can modify the electrical transport and magnetic properties of La0.67Ca0.33MnO3.     

A Surface Study of Ultrathin Ceria Nanoparticles Decorated with Transition‐Metal Ions

A wide range of nanoparticle properties can be tuned by changing their surface characteristics, especially when dealing with ultrathin nanomaterials. Surface modification with transition‐metal ions may affect a variety of the nanoparticles' properties including the surface charge, the electronic structure, and the electrical and optical characteristics. In this work, a surface study of ceria nanoparticles modified by attachment of various transition‐metal ions to their surface is conducted. Characterization of the decorated particles as well as of the modifying transition‐metal ion is carried out using zeta potential in organic solution, UV–Vis absorption, and electron paramagnetic resonance measurements, together with isothermal titration calorimetry, X‐ray photoelectron spectroscopy, and energy dispersive X‐ray spectroscopy. All measurements confirm the attachment of the cation to the surface of ceria, both in solid state and in colloidal suspension. It is suggested that the modifying ion‐complex attaches to ceria both via chemical or strong physical interactions and weak physical interactions, demonstrated by a case‐study modification of ceria using a copper‐oleylamine complex. The metalization has a significant effect on the surface charge of the nanoparticles by shifting the zeta potential to more positive values and on the optical properties of the modifying transition‐metal ions by red‐shifting their absorption peak.     

Electron backscatter diffraction study of changes in the grain structure of Ni3Fe ordering alloy upon an A1 → L12 phase transition

The rearrangement of the grain structure of Ni₃Fe alloy upon an A1 → L1₂ phase transition is studied via transmission electron microscopy in combination with the electron backscatter diffraction. It is established that the formation of fresh grain boundaries of general and special types occurs, with the proportion of special-type grain boundaries in the grain boundary ensemble growing. The spectrum of the special grain boundaries changes, due to an increase of the proportion of grain boundaries Σ3 and Σ9. This strengthens the texture of the long-range ordered alloy.     

Nanostructured PLD-grown gadolinia doped ceria: Chemical and structural characterization by transmission electron microscopy techniques

The morphology as well as the spatially resolved elemental and chemical characterization of 10 mol% gadolinia doped ceria (CGO10) structures prepared by pulsed laser deposition (PLD) technique are investigated by scanning transmission electron microscopy accompanied with electron energy loss spectroscopy and energy dispersive X-ray spectroscopy. A dense, columnar and structurally inhomogeneous CGO10 film, i.e. exhibiting grain size refinement across the film thickness, is obtained in the deposition process. The cerium M_4,5 edges, used to monitor the local electronic structure of the grains, indicate apparent variation of the ceria valence state across and along the film. No element segregation to the grain boundaries is detected. These results are discussed in the context of solid oxide fuel cell applications.     

Heterogeneous ceramics formed by grain boundary engineering

Two examples were selected to emphasize the potential of grain boundary engineering in the performance design of heterogeneous ceramics. Gadolinium-doped ceria-based powders were co-fired with additions of silica, and silica and lanthanum oxide, to test the silica scavenging role of lanthanum. The formation of one ionic conducting secondary phase, instead of an insulating phase, was attempted. The structural, microstructural, and electrical characterization of these samples confirmed the formation of one apatite-type lanthanum silicate-based phase and a significant enhancement of the grain boundary conductivity of these materials. One second approach addressed the formation of one mixed conductor, with electronically conductive grain boundaries, surrounding the grains of one lanthanum gallate-based electrolyte (core-shell type microstructure). Fe-doped grain boundaries were formed by selective Fe-diffusion (thermally assisted) from lanthanum ferrite screen printed layers. Combined microstructural and electrical characterization showed that the adopted solution was also effective.