Oxygen Ion Conduction in Bulk and Grain Boundaries of Nominally Donor‐Doped Lead Zirconate Titanate (PZT): A Combined Impedance and Tracer Diffusion Study

Oxygen ion conduction in Nd³⁺‐doped Pb(Zr_xTi_1?x)O₃ (PZT) was investigated by impedance spectroscopy and ¹⁸O‐tracer diffusion with subsequent secondary ion mass spectrometry (SIMS) analysis. Ion blocking electrodes lead to a second relaxation feature in impedance spectra at temperatures above 600°C. This allowed analysis of ionic and electronic partial conductivities. Between 600°C and 700°C those are in the same order of magnitude (10^?5–10^?4 S/cm) though very differently activated (2.4 eV vs. 1.2 eV for ions and electron holes, respectively). Oxygen tracer experiments showed that ion transport mainly takes place along grain boundaries with partly very high local ionic conductivities. Numerical analysis of the tracer profiles, including a near‐surface space charge zone, revealed bulk and grain‐boundary diffusion coefficients. Calculation of an effective ionic conductivity from these diffusion coefficients showed good agreement with conductivity values determined from impedance measurements. Based on these data oxygen vacancy concentrations in grain boundary and bulk could be estimated. Annealing at high temperatures caused a decrease in the grain‐boundary ionic conductivity and onset of additional defect chemical processes near the surface, most probably due to cation diffusion.     

Low temperature sintering of LaNbO4 proton conductors from freeze-dried precursors

A synthesis method based on freeze-dried precursors was used to obtain nanocrystalline powders of pure and Ca-doped LaNbO₄ at 800 °C. Dense ceramics were prepared at temperature as low as 1100 °C. The LaNbO₄ ceramics were examined by scanning electron microscopy (SEM) to study the microstructure evolution with the sintering temperature. The densification and grain growth rate are lower in Ca-doped samples. The bulk and grain boundary contributions to the overall conductivity were studied by impedance spectroscopy under different gases. Samples sintered at low temperature and with smaller grain size exhibit higher grain boundary resistance and consequently lower total conductivity.     

Toward nanograined gadolinia doped ceria via two-step sintering at ultralow temperature and its electrical conductivity

Highly dense (>98%) and nanograined (∼60 nm) gadolinia doped ceria are obtained from ultrafine powders by adopting two-step sintering (TSS) procedure at an ultralow temperature of 750 °C with a dwell time of 20 h, which is the lowest sintering temperature for ceria family without sintering aids up to now. Electrochemical impedance spectroscopy investigations suggest that the electrical conductivities of densified electrolytes are closely related to sintering temperature and grain size, and GDC900-750 exhibits the highest total electrical conductivity of 3.640 S m⁻¹ at 700 °C in air. Fitting calculation indicates partial grain-size dependence of oxygen vacancy association enthalpy and grain-size independence of oxygen ion migration enthalpy. Grain boundary maturity influences on grain boundary conductivity to some extent, and younger grain boundary endues the densified electrolytes with higher grain boundary conductivity.     

Effect of divalent cation addition on structure,conductivity and grain boundary properties in La doped ceria oxygen ion conductors

The aim of this work is to investigate the effect of divalent cations on the structure and electrical properties of Ce_0.85La_0.1D_0.05O_2-δ (D = Ca, Sr and Ba) oxygen ion conductors. The X-Ray structural analysis confirms the presence of CeDO₃ minor phase in addition to cubic fluorite phase of ceria in Sr²⁺ and Ba²⁺ added compositions. The lattice parameter of the compositions significantly depends on the ionic radius of dopants and the presence of D²⁺ ions in ceria lattice. The Ca²⁺ added composition shows the highest free oxygen vacancy concentration due to its lowest association energy and complete dissolution of Ca²⁺ ions into ceria lattice. The dopant-vacancy association energy and grain interior conductivity changes with the ionic radii of the divalent dopants. The grain boundary capacitance depends on dielectric constant, grain size and grain boundary thickness. The grain boundary conductivity shows 46% over total conductivity for Sr²⁺ added composition. The presence of CeDO₃ phase and space charge layer promotes the grain boundary resistances and affects the ion dynamics. Schematic models are proposed to understand the ion migration in grain boundaries. The scavenging effect is found to be highest in Sr²⁺ ions added composition. The defect structures, the presence of CeDO3 phase and electrical properties are correlated with each other.     

Ionic conductivity of tetragonal ZrO2 polycrystal doped with TiO2 and GeO2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO₂ and 2 mol% GeO₂ on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO₂ polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti⁴⁺ and Ge⁴⁺ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.     

Design of Electroceramics for Solid Oxides Fuel Cell Applications: Playing with Ceria

Nanostructured samaria- and gadolinia-doped ceria (SDC and GDC) powders were synthesized at low temperature (400°C) using diamine-assisted direct coprecipitation method. Fast-firing (f.f.) processes, where sintering temperatures are reached in a short time to promote lattice diffusion, were compared with conventional sintering, for the formation of dense microstructures from the nanostructured powders. Highly dense SDC and GDC samples (96%) with reduced grain size (150 nm) were obtained by f.f. even at 1300°–1400°C and, unexpectedly, high electrical conductivity and low blocking effect at grain boundary was obtained. Conventionally sintered samples showed that the grain boundary resistivity decreased with increasing the grain size, in agreement with the increase in geometrical bulk volume/grain boundary area ratio. Conversely, f.f. samples showed grain boundary resistivity smaller for small grain size. The above effect was observed only for high dopant (>10% molar) contents. The combined effect of powder grain size, dopant content, and sintering temperature–time profile, can be exploited to tune ceria microstructures for specific ionic device applications.     

Electrical properties of reduced 3YTZP ceramics consolidated by spark plasma sintering

3 mol% Yttria doped zirconia ceramics were consolidated by spark plasma sintering (SPS) at two sintering temperatures with the aim of achieving two different reduction levels. Microstructural characterization of the ceramics was performed by scanning electron microscopy (SEM). Electrical properties were investigated by means of impedance spectroscopy from room temperature up to 500 °C. The two ceramics presented a remarkably different electrical behavior. The effect of the extra electrons introduced by reduction during SPS on both the bulk and the grain boundary conductivity was analyzed and discussed.     

Complex plane analysis of the impedance and admittance of polycrystalline sodium-β-alumina doped with transition metal ions

The bulk and grain boundary conductivities of undoped and doped β-alumina have been analyzed using complex plane techniques. Dopants included iron, chromium, cobalt, manganese and nickel. Except for chromium which is present as Cr(IV) all dopants increased the bulk conductivity. In nearly all cases an increase in sodium content was also observed in the doped samples and was a significant factor in the observed conductivity increase.The grain boundary conductivity did not show any marked change with doping although high doping levels of iron (8%) and manganese (4%) showed a large decrease in grain boundary conductivity.     

Effects of Space Charge, Grain-Boundary Segregation, and Mobility Differences Between Grain Boundary and Bulk on the Conductivity of Polycrystalline Al2O3

The potential difference between grain boundary and bulk and the concentrations of native and foreign point defects in the bulk, the grain boundary ( gb ), and the subgrain boundary space-charge region ( sg ) of polycrystalline Al₂O₃ doped with acceptors are computed for the case that the dopants segregate at the grain boundaries, with either the ionized or the nonionized acceptor as the dominant species. Expressions are derived for the effective dc conductivity of a polycrystalline material on the basis of a model in which the grain has one or two shells with conductivities different from that of the bulk. Combination of the two results yields expressions for the effective ionic and electronic conductivity of doped A1₂O₃ as a function of grain size with distribution coefficients gb/sg , mobility ratios in the various regions, and equilibrium constants as parameters. At impurity concentrations normally found in ceramics, the contribution by subgrain boundaries to conductivity may be neglected. The theoretical results are compared to published data for Al₂O₃:Mg, Fe, and Ti.     

Defect and Mixed Conductivity in Nanocrystalline Doped Cerium Oxide

The results obtained from a study on the microstructure and the electrical properties of Gd-doped CeO₂ thin films were reported. Dense, nanocrystalline films on sapphire substrates are prepared using a polymeric precursor spin coating technique. The electrical conductivity was studied as a function of temperature and oxygen activity and correlated with the grain size. For nanocrystalline Gd-doped CeO₂ thin films, the ionic conductivity increased with decreasing activation energy as the grain size decreased. A conductivity model was developed to analyze P _O2 behavior of the electrical conductivity. Using the conductivity model, the hopping energy of electron conduction and the enthalpy of oxygen vacancy formation were determined for different microstructures.     

Effect of cation doping on lattice and grain boundary diffusion in superplastic yttria-stabilized tetragonal zirconia

Lattice diffusion coefficients D_l and grain boundary diffusion D_gb coefficients of hafnium were studied for 0.5 and 1 mol% cation-doped yttria-stabilized tetragonal zirconia at the temperature range from 1283 to 1510 °C. The diffusion profiles were determined by two experimental techniques: secondary ion mass spectroscopy and electron microprobe analysis. Additionally the first principle calculations of the electronic states of Zr⁴⁺, dopant cations and O^2? anions and elastic properties in 3Y-TZP were performed. Superplastic strain rate versus stress and inverse temperature was also measured. For 1 mol% doped samples the significant increase of the grain boundary diffusion and superplastic strain rate was observed. Correlations between the calculated ionic net charges and D_gb indicate that enhancement of D_gb was caused by the reduction of ionic bonding strength between metal cation and oxygen anion in zirconia. The new constitutive equation for superplastic flow of yttria-stabilized tetragonal zirconia ceramics was obtained.     

Thermal Conductivity in Nanocrystalline Ceria Thin Films

The thermal conductivity of nanocrystalline ceria films grown by unbalanced magnetron sputtering is determined as a function of temperature using laser‐based modulated thermoreflectance. The films exhibit significantly reduced conductivity compared with stoichiometric bulk CeO₂. A variety of microstructure imaging techniques including X‐ray diffraction, scanning and transmission electron microscopy, X‐ray photoelectron analysis, and electron energy loss spectroscopy indicate that the thermal conductivity is influenced by grain boundaries, dislocations, and oxygen vacancies. The temperature dependence of the thermal conductivity is analyzed using an analytical solution of the Boltzmann transport equation. The conclusion of this study is that oxygen vacancies pose a smaller impediment to thermal transport when they segregate along grain boundaries.     

Microstructures and grain boundaries of cubic boron nitrides

Two types of chemically pristine polycrystalline cubic boron nitrides (c-BN) are sintered at different conditions using the starting cubic and hexagonal BN powders and their microstructures and grain boundaries are investigated systematically by transmission electron microscopy. The two c-BN samples are found to exhibit a number of twins inside their grains and have a similar grain size, despite their huge differences in the grain size of the starting powders and sintering conditions. Twin width for the c-BN sintered from hexagonal BN is significantly smaller than that for the c-BN sintered from c-BN. Grain boundaries in the two samples can be atomically abrupt without any amorphous or secondary layers and oxygen is detected merely at the grain boundaries of the c-BN sintered from the c-BN powders. Such microstructural differences have a direct impact on mechanical behaviors of the c-BN, as the Vickers hardness of the c-BN sintered from hexagonal BN powders is found to be higher than that of the c-BN sintered from the c-BN powders.     

Electrical Conduction in β-Bi2O3 Doped with Sb2O3

Electrical conduction in tetragonal β-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, ionic transference number, and Seebeck coefficient. The β-Bi₂O₃ doped with 1 to 10 mol% Sb₂O₃ was stable up to 600°C and showed an oxygen ionic and electronic mixed conduction, where the electron conduction was predominant at low oxygen pressures. The oxygen-ion conductivity showed a maximum at 4 mol% Sb₂O₃, whereas the activation energy for the ionic conduction remained unchanged for 4 to 10 mol% Sb₂O₃-doped specimens. These results were interpreted in terms of the oxygen vacancy concentration and the distortion of the tetragonal structure. The electron conductivity and its oxygen pressure dependence decreased with increasing Sb₂O₃ content. The fact that Sb⁵⁺ is partially reduced by excess electrons in heavily doped β specimens at low oxygen pressures is explained.     

Re-examination of bulk and grain boundary conductivities of Ce1−xGdxO2−δ ceramics

Powders of gadolinium-doped ceria solid solutions, Ce_1−xGd_xO_2−δ (x = 0.05, 0.1, 0.2, 0.3 and 0.4), were prepared by a freeze-drying precursor route. Dense ceramic pellets with average grain sizes in the range of several microns were obtained after sintering at 1600 °C. Cobalt nitrate was added to the powders to obtain dense ceramic samples with grain sizes in the submicrometer range at 1150 °C. The ionic conduction was analysed by impedance spectroscopy in air, to de-convolute the bulk and grain boundary contributions. The bulk conductivity at low temperature clearly decreases with increasing content of Gd whereas the activation energy increases. An alternative method is proposed to analyse the extent of defect interactions on conduction. For samples without addition of Co, the specific grain boundary conductivity increases with increasing Gd content. Addition of cobalt does not alter the bulk properties but produces an important increase in the specific grain boundary conductivity, mainly in samples with lower Gd-concentration (x = 0.05 and 0.1). Segregation of Gd and its strong interaction with charge carriers may explain the blocking effects of grain boundaries.     

Ionic transport properties and their variations with grain size in nanostructured double doped cerias such as Ce0.8Gd0.1Pr0.1O2−δ and Ce0.8Gd0.15Pr0.05O2−δ

This work presents the ionic transport properties in some nanocrystalline double doped cerias, i.e., Ce_0.8Gd_0.1Pr_0.1O_2−δ and Ce_0.8Gd_0.15Pr_0.05O_2−δ with various average grain sizes, in the intermediate temperature region. The correlations between electrical and dielectric properties of these materials have been established and variation of conductivity with respect to temperature has been thoroughly discussed. All the materials are found to be ionic in nature and show high value of ionic conductivity at intermediate temperatures. The nanocrystalline Ce_0.8Gd_0.1Pr_0.1O_2−δ material (irrespective of grain size), shows lowest association energy, i.e., 0.03 eV, which is close to the theoretically predicted lowest value (0.02 eV) in double doped ceria. A repulsive force is expected between the free oxygen vacancies at the grain boundary regions at higher temperatures, which restricts the rise in grain boundary conductivity and results in decrease in total conductivity.     

Reduced grain boundary energies in rare-earth doped MgAl2O4 spinel and consequent grain growth inhibition

Grain growth inhibition in MgAl₂O₄ spinel nanostructure was achieved by grain boundary (GB) segregation of rare-earth dopants. Microcalorimetric measurements showed that dense spinel compacts doped with 3 mol% of R₂O₃ (R = Y, Gd, and La) had decreased GB energies as compared to the undoped spinel, representing reduction in the driving force for grain growth. Segregation energies of the three dopants to the Σ3 (111) GB were calculated by atomistic simulation. The dopants with higher ionic radius tend to segregate more strongly to GBs. The GB energies were calculated from atomistic simulation and, consistent with experiments, a systematic reduction in GB energy with dopant ionic size was found. High temperature grain growth experiments revealed a significant reduction of grain growth in the doped nanostructures as compared to the undoped one, which was attributed to increased metastability or possibly also a GB dragging originated from the dopant segregation.     

Synthesis,sintering and characterization of ceria-based solid electrolytes codoped with samaria and gadolinium using the Pechini method

Studies on fuel cell components have attracted interest due to the growing demand for sustainable energy sources. In the present study, synthesis of nanometric powders of the Ce_0.8Sm_0.2−xGd_xO_1.9 system (x = 0; 0.05; 0.1) system was carried out using the polymeric precursor method (Pechini), followed by calcination at 600 °C for 1 h and pressureless sintering. Characterizations were carried out with differential thermal analysis (DTA), thermogravimetric analysis (TG), infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The grain and grain boundary contributions to the ionic conductivity of the ceramic disks obtained were assessed by complex impedance spectroscopy (CIS). Microstructural characterization was conducted by SEM. Electrical characterization showed greater grain conductivity for samples that were codoped with increasing levels of gadolinium, likely due to less deformation in the crystalline lattice with the addition of an element that contains an ionic radius closer to that of the host matrix of ceria (Ce < Gd < Sm). Grain boundary conductivity was lower than grain conductivity with a gradual rise in the same codoping element. This results from changes in microstructural characteristics due to increased codoping, including a reduction in relative density.     

Multiscale modeling of the ionic conductivity of acceptor doped ceria

The ionic conductivity of acceptor doped ceria is strongly influenced by grain boundaries and interfaces, with most experiments showing a conductivity decrease in these regions. Classical models explain this observation by the formation of space charge layers, that are depleted of mobile ionic charge carriers. However, some experiments demonstrate an increase in ionic conductivity and recent models show that the space charge layers can also be enriched in mobile ionic species. Because of these discrepancies, it is still not certain whether nanocrystalline or thin film ceria can offer superior ionic conductivity or not. Recently, we have demonstrated by means of Monte Carlo simulations that the ionic conductivity in space charge layers can indeed exceed the bulk value. In this work, we combine these Monte Carlo simulations with a continuum model to predict charge carrier concentration profiles. This multiscale approach allows for a realistic prediction of the grain boundary ionic conductivity.     

Al2O3掺杂YSZ的电性能

研究了Al2O3掺杂对Y2O3稳定的ZrO2材料电导性能的影响。因为Al与自由氧离子空穴缔合的能力大于Y，所以YSZ的晶粒电导随Al2O3含量的增大而减小。Al2O3一方面能清除晶界上的SiO2，同时又与晶界中的氧离子空穴缔合，在这两方面影响下，随Al2O3含量的增大，晶界电阻先减小后增大。