Grain Boundary Conductivity Research Articles

Theoretical Modeling of Defect Segregation and Space-Charge Formation in Proton-Conducting Barium Zirconate

Introduction Acceptor-doped barium zirconate (BaZrO3) shows considerable potential as an electrolyte material for intermediate temperature solid oxide fuel cells due to its high bulk proton conductivity and chemical stability. However, in polycrystalline materials the grain boundaries (GBs) display high proton resistivity, severely limiting the overall proton transport in the material. The low GB conductivity has been thought to be due to an intrinsic mechanism, and recently the so called space charge model has been used to interpret and model the GB blocking behavior in BaZrO3 by several research groups [1-5]. Modeling According to the space charge model, charged defects accumulate in the structurally distorted region at the GB plane, the GB core. The resulting core charge gives rise to an electrostatic potential barrier and causes depletion of mobile charge carriers of the same polarity in the regions adjacent to the core, the space charge layers. The existence of a positive core charge is crucial to the application of the space charge model to BaZrO3. In a previous study, we used density functional theory (DFT) to investigate oxygen vacancy segregation to a symmetric tilt GB and we found that oxygen vacancies segregate to the GB and create a positive core charge [4]. Here, we extend our previous studies [4,6] to several other tilt GBs. Segregation energies of both oxygen vacancies and protons are determined and used in a one-dimensional thermodynamic space charge model. We also investigate the effect of segregation of dopants as well as interaction between oxygen vacancies in the GB core. Our first-principles electronic-structure calculations are performed within the framework of DFT using the generalized gradient PBE functional to approximate exchange and a layer-by-layer space charge model [6] is employed to determine the potential barrier and core charge resulting from defect segregation. Results and Discussion Using DFT proton and oxygen vacancy segregation have been investigated for an extensive set of tilt GBs. Both defect species are found to segregate to the GB core, with typical segregation energies of -1.5 eV and -1.0 eV for oxygen vacancies and protons, respectively. Below 900 K and at wet conditions potential barrier heights around 0.6V are obtained, which is consistent with experimental results. For many GBs we find the protons to be responsible for the major part of the excess core charge and thereby the electrostatic potential. We have also investigated dopant segregation and its effect on the potential barrier. Experimental evidence suggests that an increased dopant content should decrease the potential barrier and that this effect should be especially pronounced if the dopants are able to segregate to the GB. Indeed, we find that if low-energy sites in the core are saturated with positive defects, an increase in the dopant concentration does lower the potential barrier. However, if the low-energy sites are not saturated an increased dopant concentration in the core could also lead to an increase in the concentration of positive defects and the core concentration is instead limited by the electrostatic potential. An increase in the number of negative defects is therefore followed by an increase in the number of positive defects, keeping the total core charge constant. Furthermore, our results suggest that interactions among oxygen vacancies may effectively reduce the number of available sites in the GB core. This has consequences for the effects of dopant segregation since it implies that the core can become saturated with positive defects at an earlier stage and segregation of dopant ions may then actually decrease the core charge, and reduce the potential barrier height. Conclusions To conclude, segregation of oxygen vacancies and protons plays an important role in the formation of space charge potentials at the grain boundary interface. Additionally, our results show that protons may be the main source to the space-charge potentials under conditions relevant for fuel cell applications.

Enhanced ionic conductivity of Sm0.2Ce0.8O2−δ electrolyte for solid oxide fuel cells through doping transition metals

Ceria-based electrolytes are considered as candidate electrolyte materials for intermediate temperature solid oxide fuel cells due to the high oxygen-ion conductivity. Lowing sintering temperature for ceria-based electrolyte is favorable for co-firing process as well as the reduction of fabrication cost. Transition metals (Fe, Co, Ni) were added to Sm-doped ceria (SDC) and the crystal structure, sintering performance and ionic conductivity were investigated by X-ray diffraction, SEM and AC impedance spectroscopy. The transition metal elements added to SDC as sintering aids reduced the sintering temperature of SDC electrolyte by 100–150 °C and the grain size decreased with increased doping amount of transition metals. The ionic conductivity, especially the grain-boundary conductivity was significantly improved by a small addition of transition metals. Among Fe, Co, Ni doped SDC electrolytes, the 0.01 mol Fe3+ doped SDC sintered at 1350 °C exhibited the highest ionic conductivity about 0.063 S cm−1 at 800 °C and the lowest activation energy of 0.73 eV.

Grain-Boundary Structures Associated with Ionic Transport in Gd-Doped Ceria Nanostructured Electrolyte

High resistive ionic transport at the grain boundary is one of the most decisive factors for designing sophisticated nanostructured electrolytes as oxide ceramics. Here, the grain boundary structures are investigated for nanostructured electrolytes, ceria (CeO2), Gd-doped ceria (GDC), and Li-doped GDC (Li-GDC), by means of element-specific positron annihilation spectroscopy. The angstrom-scale spaces with negative charge are identified in addition to the grain-to-grain contacts essentially identical to the bulk and void-like open spaces created by lattice mismatching. The grain-boundary structures associated with oxygen ionic transport are demonstrated on the basis of space charge theory for the grain-boundary conductivity.

First-principles calculations of oxygen vacancy formation and metallic behavior at a β-MnO2 grain boundary.

Nanostructured MnO2 is renowned for its excellent energy storage capability and high catalytic activity. While the electronic and structural properties of MnO2 surfaces have received significant attention, the properties of the grain boundaries (GBs) and their contribution to the electrochemical performance of the material remains unknown. Through density functional theory (DFT) calculations, the structure and electronic properties of the β-MnO2 Σ 5(210)/[001] GB are studied. Our calculations show this low energy GB has a significantly reduced band gap compared to the pristine material and that the formation of oxygen vacancies produces spin-polarized states that further reduce the band gap. Calculated formation energies of oxygen vacancy defects and Mn reduction at the GB core are all lower than the equivalent bulk value and in some cases lower than values recently calculated for β-MnO2 surfaces. Oxygen vacancy formation is also shown to produce a metallic behavior at the GB with defect charge distributed over a number of oxygen and manganese sites. The low energies of oxygen defect formation and the potential creation of conductive GB pathways are likely to be important to the electrochemical performance of β-MnO2.

Electrical conductivity and grain boundary composition of Gd-doped and Gd/Pr co-doped ceria

We characterize electrical conductivity, microstructure, nano-scale grain boundary structure and chemistry of ceria electrolytes with nominal compositions of Gd0.2Ce0.8O2-δ (GDC) and Gd0.11Pr0.04Ce0.85O2-δ (GPDC). The electrolytes are fabricated using mixed oxide nanopowders synthesized by spray drying. AC impedance spectroscopy was performed from 150°C to 700°C in air to determine grain-interior electrical conductivity. Grain-boundary conductivity was determined below 300°C. The grain-interior conductivity of the GPDC was higher than that of GDC by as much as 10 times, depending on the temperature. The GPDC specific grain-boundary conductivity was measured to be approximately 100 times higher than that of GDC. Energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) confirmed the grain-to-grain compositional uniformity of both materials following heat treatments. Grain boundaries were free of glassy intergranular phases; dopant concentration and Ce oxidation state were found to vary significantly near grain boundaries. Boundary core composition was estimated from STEM EELS to be Gd0.62Ce0.38O2-δ, and Gd0.29Pr0.16Ce0.55O2-δ in GDC and GPDC, respectively. Pr segregation to grain boundaries in the GPDC is hypothesized to enhance conductivity by both decreasing oxygen vacancy migration energy, and inducing mixed ionic–electronic conductivity in the near-boundary region.

Connectivity between electrical conduction and electrode structure in Mn–Co–Ni–O thick-film thermistors

In this paper, the electrical conduction of grain and grain boundary (GB) in Mn1.85Co0.3Ni0.85O4 negative-temperature-coefficient (NTC) thick-film thermistors with different finger electrode structures was studied carefully through the temperature-dependent ac impedance spectroscopy analyzing. It was found that the increase in finger length will drop the resistance of grain and GB sharply. The conduction mechanism of the grain was independent on the electrode structure and followed the small-polaron hopping model. However, the conduction mechanism of GB was dependent on the electrode structure. With the finger length increasing, the resistance–temperature (R–T) relation of GB was transformed from nonlinear to linear relation. These studies on the mechanisms concerning the dependence of conduction behaviors of grain and GB on electrode structure will provide a comprehensive understanding on the transformation behaviors of R–T relation of thick-film NTC thermistors.

Influence of Lithium Content on the Structure and Ionic Conductivity of Perovskite \(\mbox{La}_{(2/3) - x}\mbox{Li}_{3x}\mbox{TiO}_{3}\) Made by Double Mechanical Alloying Method

Perovskite La\(_{(2/3) - x}\)Li\(_{3x}\)TiO\(_{3}\) samples with 0.06 \( \leq x \leq 0.15\) were prepared by a double mechanical alloying method. Structure and Li$^{ + }$-ion conductive properties of the La\(_{(2 / 3) - x}\)Li\(_{3x}\)TiO\(_{3}\) samples were investigated. Most of the analyzed perovskite samples exhibit a double unit cell. In these samples, a change of symmetry from tetragonal to orthorhombic is observed for sample with lithium content x = 0.06. Structural modifications were obtained mainly due to the cation vacancies ordering along the c-axis, which disappeared gradually when the lithium content increased. At room temperature, the maximum values of grains and grain boundaries conductivities of the La\(_{(2 / 3) - x}\)Li\(_{3x}\)TiO\(_{3}\) samples were found to be of \(1.5\times 10$^{ - 3}\) S/cm and \(5.8 \times 10^{ - 5}\) S/cm, respectively. The temperature dependence of ionic conductivity obeyed a non-Arrhenius behaviour. At temperature from 30 to 125\(^{\circ}\)C, the activation energy for grain and grain-boundary conductivity was found to be of $\sim $ 0.23 eV and $\sim $ 0.32 eV, respectively.

Microstructure and dielectric properties of (Nb + In) co-doped rutile TiO2 ceramics

The (Nb + In) co-doped TiO2 ceramics recently attracted considerable attention due to their colossal dielectric permittivity (CP) (∼100,000) and low dielectric loss (∼0.05). In this research, the 0.5 mol. % In-only, 0.5 mol. % Nb-only, and 0.5–7 mol. % (Nb + In) co-doped TiO2 ceramics were synthesized by standard conventional solid-state reaction method. Microstructure studies showed that all samples were in pure rutile phase. The Nb and In ions were homogeneously distributed in the grain and grain boundary. Impedance spectroscopy and I-V behavior analysis demonstrated that the ceramics may compose of semiconducting grains and insulating grain boundaries. The high conductivity of grain was associated with the reduction of Ti4+ ions to Ti3+ ions, while the migration of oxygen vacancy may account for the conductivity of grain boundary. The effects of annealing treatment and bias filed on electrical properties were investigated for co-doped TiO2 ceramics, where the electric behaviors of samples were found to be susceptible to the annealing treatment and bias field. The internal-barrier-layer-capacitance mechanism was used to explain the CP phenomenon, the effect of annealing treatment and nonlinear I-V behavior for co-doped rutile TiO2 ceramics. Compared with CaCu3Ti4O12 ceramics, the high activation energy of co-doped rutile TiO2 (3.05 eV for grain boundary) was thought to be responsible for the low dielectric loss.

Low dielectric loss, dielectric response, and conduction behavior in Na-doped Y2/3Cu3Ti4O12 ceramics

The Na-doped Y2/3Cu3Ti4O12 system has been prepared and investigated. Na doping facilitates the formation of oxygen vacancies, which is of great benefit to the growth of the grain size. Proper amount of Na substitution in NaxY(2−x)/3Cu3Ti4O12 ceramics makes the dielectric loss significantly decreased. As x = 0.050, Na0.050Y0.650Cu3Ti4O12 ceramics exhibit the lowest dielectric loss (about 0.022 at 1 kHz) and a relatively high dielectric constant (about 7500 at 1 kHz). The lowered dielectric loss is closely associated with the enhanced resistance of grain boundary. The conduction and dielectric processes of grain boundary become much more difficult after Na doping. Impedance analysis suggests that the same charge defects are responsible for the conduction and dielectric relaxation behaviors of grain boundary. Scaling behaviors indicate that the physical nature of their dielectric relaxation and conduction behavior are independent of the measurement temperature and the Na concentration.

Varistor and dielectric properties of Cr2O3 doped SnO2–Zn2SnO4 composite ceramics

Cr2O3 doped SnO2–Zn2SnO4 composite ceramics were prepared by traditional ceramic processing and the varistor, dielectric properties were investigated. With increasing Cr2O3 content, the breakdown electrical field EB increases from 11 to 92 V/mm and the relative dielectric constant εr measured at 1 kHz, 50 °C decreases from 11,028 to 3412, respectively. The barrier height ϕB about 0.8–0.84 eV and the decreasing of SnO2 grain size suggest that the varistor behavior with high εr is originated from SnO2–SnO2 or SnO2–Zn2SnO4 grain boundary. In the dielectric spectra lower than 1 kHz, a dielectric peak is presented and depressed with increasing bias voltage. Similarly, at high temperature, the dielectric constant also presents a dielectric peak in the temperature spectra and the peak becomes faint with increasing frequency. The exhibition of the dielectric peak is thought to be attributed to the conduction of grain boundary since it is accompanied by the sharp increase of dielectric loss. In addition, a dielectric relaxation with the activation energy about 0.4–0.5 eV was observed in the temperature range of 20–100 °C. Based on the results, the formation mechanism of Schottky barriers at grain boundaries and the varistor behavior with high dielectric constant are well understood.

Microscopic Origin of the Ionic Transport Behaviors in Solid Electrolytes for Li Batteries

Recently Li-ion-conducting solid electrolytes have received intensive research interest, as they provide solutions for issues associated with the organic liquid electrolytes in conventional Li-ion batteries. However, despite the relatively high bulk conductivity achieved in many solid electrolytes, the grain-boundary conductivity is frequently low and limits the application. Detailed atomic structure and elemental distribution on the grain boundaries in these materials must be revealed in order to fundamentally understand the origin of large grain-boundary resistance. Such studies, however, are currently very rare. Here we present a detailed atomic scale study on the conduction behaviors of an extensively studied solid electrolyte (Li3x La2/3-x )TiO3 (LLTO). Using aberration-corrected scanning transmission electron microscopy (STEM) and the associated electron energy loss spectroscopy (EELS) analysis, we found that the large grain-boundary resistance of LLTO arose from the intrinsic local structural reconstruction, which is energetically not favorable for Li accommodation. Therefore, large flux of charge carriers was unlikely to occur both along and across the grain boundary, leading to the poor grain-boundary conductivity. The relationship among the processing condition, microstructure, and ionic conductivity was established. And the ionic transport behavior in LLTO is further compared to that in garnet-type Li7La3Zr2O12. These results paved the way for further improvement of the Li-ion-conducting solid electrolytes.

Investigations on structural and electrical properties of calcium substituted LSGM electrolyte materials for IT-SOFC

Strontium and magnesium doped lanthanum gallate, La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) and a few compositions of calcia (CaO) incorporated LSGM, La(0.9−x)CaxSr0.1Ga0.8Mg0.2O2.85; x≤0.10, were prepared by conventional solid state ceramic route. The X-ray diffraction patterns showed orthorhombic LSGM phase along with a few impure secondary phases. SEM micrograph depicted fairly dense grain morphology of the system. The textural properties were also found to be quite suitable for solid oxide fuel cell electrolyte applications. The electrical conductivity of the systems was measured in the frequency range 20Hz to 1MHz and temperature range 300–800°C. The total conductivity of the system was found to first increase up to x=0.05 and thereafter it decreased for higher concentrations of the dopant. A small amount of calcia (upto x=0.05) scavenges the intergranular secondary impure phases resulting in a significant increase in grainboundary conduction. The presence of extra secondary phases in the higher compositions has been attributed to decrease in electrical conductivity.

Influence of small DC bias field on the electrical behaviour of Sr- and Mg-doped lanthanum gallate

One of the promising electrolyte materials for solid oxide fuel cells application, Sr- and Mg-doped lanthanum gallate La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM), is synthesized by conventional solid state ceramic route. X-ray Rietveld analysis confirms the formation of main orthorhombic phase at room temperature along with a few minor secondary phases. SEM micrograph reveals the grain and grainboundary morphology of the system. Electrical conductivity of the LSGM sample is measured in the temperature range 573–873 K and in the frequency range 20 Hz–1 MHz at a few small DC bias fields (at 0.0, 0.5, 1.0, 1.5 and 2.0 V). The conductivity spectra show power-law behaviour. Electrical conductivity of the sample is found to be weakly dependent on DC bias field. This is attributed to field-dependent bulk and grainboundary conduction processes. In the present system, under investigated bias field range, the possibility of formation of Schottky barrier is ruled out. The concept of grainboundary channel (pathway) modulation on the application of bias field is proposed.

Relationship between lattice mismatch and ionic conduction of grain boundary in YSZ

The grain boundary plays an important role in the electrical behaviors of solid oxide electrolytes for solid state fuel cells. To reveal the relationship between the structure and the ionic conductivity of grain boundary, the conductive properties of {111} and {110} twist grain boundaries in 8mol% yttria-stabilized zirconia have been examined. These boundaries have a series of Σ values defined by the coincident site lattice model. It has been found that the activation energy of {111} twist grain boundary increases and then decreases with the Σ value, while that of the {110} boundary shows an opposite trend. It is suggested that the properties can reflect the balance of the effects of lattice mismatch on the diffusion ability of oxygen vacancies and the segregation of oxygen vacancies and Y3+ ions. Therefore, the properties in polycrystalline electrolyte can be adjusted by controlling the grain boundary structures.

Cu and Gd co-doped BaCeO3 proton conductors: Experimental vs SEM image algorithmic-segmentation results

Segmentation algorithms for the quantitative description of the surface microstructure and the evaluation of the grain-boundary conductivity of ceramic materials surface using SEM microphotographs analysis was developed and applied in the present work for first time in barium cerate based solid solutions. To this purpose novel polycrystalline ceramic materials based on Cu and Gd co-doped BaCeO3 proton conductors exhibiting high proton conductivity have been prepared and characterized. The influence of copper oxide as a dopant on the microstructure and the electrical properties of BaCe0.9–xGd0.1CuxO3–δ (0≤x≤0.1) have been examined in detail. It is shown that the dependence of the electrical conductivity is correlated with the average oxide grain diameter variation in air at various temperatures. Moreover, it was found that the algorithmic segmentation evaluation results are in good agreement with the experimental ones, verifying the considerable contribution of the grain-boundary conductivity to the electrical transport of Cu and Gd co-doped BaCeO3 proton conductors.

The interplay between strain and size effects on the thermal conductance of grain boundaries in graphene

The effect of strain on the thermal transport across grain boundaries in graphene is investigated using molecular dynamics simulations. The thermal boundary conductance is found to decrease significantly under biaxial tension as expected. In contrast, under biaxial compression, the thermal boundary conductance is strongly affected by the dimensions of the graphene monolayer, increasing with strain for specimen with length-to-width ratio of less than 20 and being insensitive to strain for length-to-width ratio above 20. This rather unexpected size-dependence under biaxial compression is found to be a result of geometric instabilities.

The Lowered Dielectric Loss and Grain‐Boundary Effects in La‐doped Y2/3Cu3Ti4O12 Ceramics

The effects of La concentration on the electrical conductivity and electric modulus of Y2/3−xLaxCu3Ti4O12 ceramics (0.00 ≦ x ≦ 0.20) were investigated in detail. Proper amount of La substitution in Y2/3−xLaxCu3Ti4O12 ceramics made the dielectric loss decreased. When x = 0.10, Y2/3−0.10La0.10Cu3Ti4O12 ceramics exhibited the highest grain‐boundary resistance (0.893 MΩ) and the lowest dielectric loss (about 0.025 at 1 kHz), meanwhile the samples exhibited a relatively high dielectric constant above 6000 over a wide frequency range from 40 Hz to 1 MHz. The decreased dielectric loss was attributed to the enhanced grain‐boundary resistance. With the increase in La concentration, the dielectric relaxation behaviors correlated with the grain‐boundary effects were significantly enhanced. By La doping, the activation energies for the conduction in grain boundaries were slightly depressed, and the activation energies for the relaxation process in grain boundaries were slightly changed. Based on the activation values, it can be concluded that the doubly ionized oxygen vacancies had substantial contribution to the conduction and relaxation behaviors in grain boundaries.

Oxygen diffusion in niobia-doped zirconia as surrogate for oxide film on Zr–Nb alloy: AC impedance analysis

The oxygen conductivities and crystallographic properties of niobia-doped yttria-stabilized tetragonal zirconia with 0.0–2.6wt% Nb2O5 were evaluated by the AC impedance analysis and the X-ray diffraction measurement, respectively. The tetragonality of zirconia increased with niobia content and approached ∼1.017 while the tetragonal-to-monoclinic phase transition occurred above ca. 1wt% Nb2O5. On the other hand, oxygen conductivities of bulk and grain-boundary (GB) decreased with increasing niobia content. The bulk conductivity controlled the total ionic conductivity at high temperatures, and its activation energy had smaller dependence on temperature than that of GB. In addition to the effect of [VO] depletion by niobia addition, the behaviors of bulk and GB conductivities might be explained by the decrease of mobility of oxygen ion due to Coulomb repulsion between Nb5+ and VO and by no segregation of Nb ions in the space-charge layers, respectively.

Electron interaction, charging, and screening at grain boundaries in graphene

Electronic, transport, and spin properties of grain boundaries (GBs) are investigated in electrostatically doped graphene at finite electron densities within the Hartree and Hubbard approximations. We demonstrate that depending on the character of the GBs, the states residing on them can have a metallic character with a zero group velocity or can be fully populated losing the ability to carry a current. These states show qualitatively different features in charge accumulation and spin polarization. We also demonstrate that the semiclassical Thomas-Fermi approach provides a satisfactory approximation to the calculated self-consistent potential. The conductance of GBs is reduced due to enhanced backscattering from this potential.

Synthesis, characterization, and dielectric properties of low loss LaBO3 ceramics

Abstract The preparation, sintering behaviour, and dielectric properties of low loss LaBO 3 ceramics have been investigated. Single-phase LaBO 3 powder was synthesized by the conventional solid-state ceramics route and dense ceramics (relative density >96%) with uniform microstructure (grain size ∼30 μm) were obtained by sintering at 1300 °C in air. The electrical conductivity of LaBO 3 follows the Arrhenius law and the related activation energies for electrical conduction of bulk and grain boundary are 0.62 eV and 0.90 eV, respectively. The LaBO 3 ceramics sintered at 1300 °C exhibit excellent microwave dielectric properties with a relative permittivity, ɛ r ∼ 11.8, a quality factor, Q × f 0 value ∼76,869 GHz (at ∼15 GHz), and a negative temperature coefficient of resonant frequency τ f ∼ −52 ppm/°C.