Grain Boundary Resistance Research Articles

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.

Synthesis and lithium-ion conductivity for perovskite-type Li3/8Sr7/16Ta3/4Zr1/4O3 solid electrolyte by powder-bed sintering

Perovskite-type compounds ABO 3 in which A = La, Li and B = Ti (Li 2/3 − 3x La x TiO 3 , LLT) exhibit very high bulk ionic conductivity of 1.2 × 10 − 3 S cm − 1 at 30 °C, but LLT has very large grain-boundary resistance and is only stable above 1.6–1.8 V vs. Li/Li + because Ti 4 + can be easily reduced to Ti 3 + below this potential. Li 3/8 Sr 7/16 Ta 3/4 Zr 1/4 O 3 (LSTZ), in which A and B cations of SrZrO 3 are partially substituted by Li and Ta, has been reported to be more stable against the lithiated negative electrode than LLT. In this paper, we synthesized LSTZ by using “powder-bed sintering” method, which is commonly used for preparation of dense garnet-structured lithium-ion conductor Li 7 La 3 Zr 2 O 12 (LLZ). At sintering process, the LSTZ pellet was covered with the same mother powder to suppress the excess Li loss during high temperature treatment and the formation of secondary phases. For LSTZ sintered with covering LSTZ mother powder, secondary phases such as SrTa 2 O 6 and Sr 2 Ta 2 O 7 were significantly reduced and LSTZ grains in the pellet were in good contact with each other. According to these improvement, the bulk and total (bulk + grain-boundary) ionic conductivity at 27 °C of LSTZ attained to 3.5 × 10 − 4 S cm − 1 and 2.7 × 10 − 4 S cm − 1 , respectively. The total conductivity at room temperature is approximately 3 times higher than that as previously reported and comparable with LLT and LLZ. • Perovskite Li–Sr–Ta–Zr–O solid electrolyte was prepared by powder-bed sintering. • Powder-bed sintering is effective to densify the pellet and reduce impurity phase. • Total ionic conductivity of Li–Sr–Ta–Zr–O pellet is above 2 × 10 − 4 S cm − 1 at 27 °C.

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.

Atomic-scale origin of the large grain-boundary resistance in perovskite Li-ion-conducting solid electrolytes

Li-ion-conducting solid electrolytes are the potential solution to the severe safety issues that occur with conventional batteries based on solvent-based electrolytes. The ionic conductivity of solid electrolytes is in general too low, however, due to a high grain-boundary (GB) resistance. A thorough understanding of the ionic transport mechanism at GBs in these materials is critical for a revolutionary development of next-generation Li batteries. Herein we present the first atomic-scale study to reveal the origin of the large GB resistance; (Li3xLa2/3−x)TiO3 was chosen as a prototype material to demonstrate the concept. A strikingly severe structural and chemical deviation of about 2–3 unit cells thick was revealed at the grain boundaries. Instead of preserving the ABO3 perovskite framework, such GBs were shown to consist of a binary Ti–O compound, which prohibits the abundance and transport of the charge carrier Li+. This observation has led to a potential strategy for tailoring the grain boundary structures. This study points out, for the first time, the importance of the atomic-scale grain-boundary modification to the macroscopic Li+ conductivity. Such a discovery paves the way for the search and design of solid electrolytes with superior performance.

Effects of the fabrication process on the grain-boundary resistance in BaZr0.9Y0.1O3−δ

Download options Please wait... Associated articles Correction: Effects of the fabrication process on the grain-boundary resistance … Article information DOI https://doi.org/10.1039/C4TA02848A Article type Paper Submitted 05 Jun 2014 Accepted 10 Aug 2014 First published 14 Aug 2014 Download Citation J. Mater. Chem. A, 2014,2, 16107-16115 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Effects of the fabrication process on the grain-boundary resistance in BaZr0.9Y0.1O3−δ S. Ricote, N. Bonanos, A. Manerbino, N. P. Sullivan and W. G. Coors, J. Mater. Chem. A, 2014, 2, 16107 DOI: 10.1039/C4TA02848A To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author S. Ricote N. Bonanos A. Manerbino N. P. Sullivan W. G. Coors

Infiltrated Lanthanum Nickelate Cathodes for Use with BaCe0.2Zr0.7Y0.1O3 − δ Proton Conducting Electrolytes

Symmetric cells (electrode/electrolyte/electrode) were prepared using BaCe0.2Zr0.7Y0.1O3 − δ (BCZY27) as a proton conducting electrolyte. Porous BCZY27 backbones were sintered on each side of the electrolyte. Lanthanum nickelate, La2NiO4 + δ (LN), was infiltrated into the porous BCZY27 backbone to act as the electrode. Pure phase LN was formed after annealing in air at 850°C for 4 h. Electron microscopy was used to observe LN nano-particles ( ∼ 50 nm) infiltrated throughout the backbone. Impedance spectra were recorded at 500, 600, and 700°C with four oxygen partial pressures and four steam partial pressures. Four arcs were observed in the spectra corresponding to: BCZY27 backbone grain-boundary resistance, charge-transfer, oxygen dissociation/adsorption, and gas transport. Total area specific resistances (ASRs) were found to decrease with increasing oxygen partial pressure. Total cathode ASRs of 161.8, 29.3, and 3.7 Ω · cm2 were measured in 3% humidified air at 500, 600, and 700°C respectively.

Electrical properties of nanometric CGO-thin films prepared by electron-beam physical vapour deposition

Thin-film oxide-ion-conducting electrolytes are of considerable interest for the development of micro-solid oxide fuel cells (μ-SOFCs). In this study, nanocrystalline CGO thin films (with thickness ranging from 0.3 to 1.35 μm) were deposited from a Ce0.9Gd0.1O2−δ target by electron-beam physical vapour deposition onto a selection of substrates (Si, glass, fused silica and Al2O3) which were either maintained at room temperature or at 700 °C during deposition. Films have a columnar microstructure with (111) texture in the surface plane. The crystallinity of the films deposited at a substrate temperature of 700 °C was found to be much superior to those deposited at RT and subsequently heated. Impedance spectroscopy revealed that the in-plane resistivity of the films in air at 900 °C is greater than that of bulk CGO by around two orders of magnitude, with this difference increasing with decreasing temperature. The poor electrical properties of the nanocrystalline films may be considerably affected by the expected higher concentration of strains in comparison with the micrometric sample. The composition of the thin films was also affected by a de-doping process. The in-plane charge transport in films is limited by grain boundaries, mainly due to both a greater grain-boundary volume and higher specific grain-boundary resistivity. Conductivity measurements as a function of oxygen partial pressure reveal a considerably narrower electrolytic domain for the thin films than that of bulk CGO.

Microstructure and dielectric properties of Ca1–3/2xBixCu3Ti4O12 (x = 0, 0.05, 0.10, 0.15 and 0.20) ceramics

Influence of bismuth substitution on calcium site in CaCu3Ti4O12 has been investigated. Compositions of Ca1-3/2xBixCu3Ti4O12 (x = 0, 0.05, 0.10, 0.15 and 0.20) were fabricated by solid-state sintering method. Crystal structure is remained cubic. X-ray diffraction indicates the presence of secondary phase of CuO in CCTO ceramics. Bismuth doping restrains the formation of CuO phase apparently. The grain size of CaCu3Ti4O12 ceramics was greatly decreased by Bi3+ doping, resulting from the ability of bismuth to inhibit the grain growth. The dielectric and electric properties of CCTO ceramics were found to be influenced by bismuth doping. The fitting results of the complex impedance spectra showed an increase of the resistance of grain and grain boundary by bismuth substitution. Ca0.70Bi0.20Cu3Ti4O12 showed the highest dielectric constant in the low frequency range. A modest composition such as Ca0.85Bi0.10Cu3Ti4O12 expressed the optimized dielectric properties of higher dielectric constant (1.3 × 104) and lower dielectric loss (0.06) than pure CCTO. The low and high temperature dielectric loss spectra demonstrate the interfacial polarization of the initial and secondary oxygen ionization, relating with the grain and grain boundary (the electrode contact for Ca0.70Bi0.20Cu3Ti4O12) respectively.

Influence of the sintering process on the PTCR effect of chip-type Ba1.022–x Sm x TiO3 ceramics prepared by the reduction sintering-reoxidation method

We investigated the effects of sintering temperature and reoxidation annealing on the positive temperature coefficient of resistance (PTCR) effect of Ba1.022–xSmxTiO3 ceramics that were sintered at 1,180–1,260 °C for 30 min in a reducing atmosphere and reoxidized at 800 °C for 1 h. Results indicated that the room-temperature (RT) resistivity and resistance jump of the ceramics decreased with increasing sintering temperature; moreover, the samples exhibited a remarkable PTCR effect with a resistance jump of 3.3 orders of magnitude and achieved a low RT resistivity of 374.4 Ω cm at a lower sintering temperature. Furthermore, the higher grain-boundary resistivity of the ceramics obtained at a high reoxidation temperature after sintering at low temperature was estimated using an impedance analyzer. In addition, the voltage versus current behavior was investigated in present study.

Effect of composition and microstructure on electrical properties and CO2 stability of donor-doped, proton conducting BaCe1−(x+y)ZrxNbyO3

Acceptor-doped BaCeO3 perovskites have frequently been studied as high temperature proton conductors (HTPCs) where conduction occurs due to hydroxyl protons that occupy the oxide ion vacancies in the perovskite-type structure. Recently, donor-doped perovskites of the nominal composition BaCe0.7Zr0.2Nb0.1O3 have demonstrated mixed proton–electron conduction in humidified H2 and N2. The proton conduction mechanism is assumed to be similar to that of acceptor-doped perovskites, where the reduction of B-site cations is suggested. In this study, BaCe1−(x+y)ZrxNbyO3 (x = 0.05–0.2, y = 0.05–0.1) are characterized using X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and AC impedance spectroscopy. The role of composition and sintering temperature on microstructural, chemical, stoichiometric, and electrical properties is thoroughly investigated. PXRD shows the formation of orthorhombic perovskites in all samples and SEM images reveal that a decrease in porosity and increase in grain size are significantly affected by relative Zr and Nb dopant levels. The improved densification caused by the Nb doping leads to enhanced total conductivity by lowering the grain-boundary resistance. Surface analysis by XPS indicates that Ba vacancies in as-prepared BaCe1−(x+y)ZrxNbyO3 can be controlled by composition and temperature control. The co-doping of Nb and Zr lead to enhanced chemical stability in CO2 at elevated temperatures compared to the parent compound. The bulk electrical conductivity decreases with increasing Zr content in BaCe1−(x+y)ZrxNbyO3 in air. Among the samples investigated in this work, BaCe0.9Zr0.05Nb0.05O3 exhibits the highest bulk conductivity of 10−3 S cm−1 at 400 °C with an activation energy of 0.43 eV (400–700 °C).

Metallic magnetism and change of conductivity in the nano to bulk transition of cobalt ferrite

Variations in conductivity with particle size have been observed in cobalt ferrite, when synthesized by solgel auto-combustion method. Impedance analysis reveals metallic and semiconducting behavior at room temperature for a particle size of 6 nm and 52 nm, respectively. Upon thermal activation, metallic to semiconducting phase transition has been observed as a function of particle size and vice-versa. Grainboundary Resistance (Rgb), increased drastically with particle size (19 MΩ for 6 nm and 259 MΩ for 52 nm) at room temperature. AC conductivity and dielectric constants exhibit similar metallic to semiconducting phase transition at 6 nm and semiconducting behavior at 52 nm with temperature in the selected frequencies. Enhanced magnetic moment with an increase in the grain size along with decreased coercivity (1444 G to 1146 G) reveals transition from single domain to multi-domain. Increased inter-particle interaction is responsible for metallicity at the nano level and on the contrary semiconductivity is attributed to bulk.

Comparative Study on Preparation and Properties of BaZr<sub>0.8</sub>Y<sub>0.2</sub>O<sub>3-</sub><sub>δ</sub> Solid Electrolyte

The preparation of the BaZr0.8Y0.2O3-δ solid electrolyte obtained by a nitrate-citrate combustion is reported. This synthesis method is compared with solid state reaction. The phase formation process was studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The phase evolution during the gel-combustion process and solid state process as a function of heat-treatment temperature was also studied by X-ray powder diffraction (XRD). When using nitrate-citrate combustion method a pure phase was formed at 800°C, which is much lower than that of conventional solid state reaction (at 1200°C). The sinterability of electrolyte powder was also investigated. At 1700°C very dense pellets can be obtained for both samples. The conductivity of BaZr0.8Y0.2O3-δ (BZY20)electrolyte prepared using two different processes was measured by AC impedance spectroscopy. Impedance spectra analyses showed that the grain boundary conductivity is much lower than that of the bulk for BZY20. Furthermore, the grain-boundary resistance of the BZY20 prepared via nitrate-citrate combustion process was reduced markedly compared to that of the BZY20 prepared by conventional solid state reaction.

Influence of grain boundaries on the EPIR effect in Nd0.7Sr0.3MnO3 ceramics

• Use the circuit analogy grain boundary connection. • Simulate the resistances of grain boundaries by using non-ohmic contact resistances. • An excessive number of grain boundaries result in a loss of the EPIR effect. • Provide an insight into the conditions of the EPIR effect in ceramics. Nd 0.7 Sr 0.3 MnO 3 polycrystalline ceramics were synthesized by solid-state reaction under high pressure. The experimental results show that the electrical pulse induced resistance switching effect cannot be induced from the grain boundaries even though they harbor a space charge region with nonlinear I – V behavior. In order to investigate the reason why there is non-detectable EPIR effect originating from double Schottky barrier type at grain boundaries, we simulate the electrical behavior of grain boundaries by connecting them in series, parallel and mixing mode and by taking the interfacial resistance between the electrode and the ceramic for the grain boundary resistance. The simulating results show that the EPIR effect decreases with increasing number of components for both series and parallel modes, even disappears completely if more components are added. That indicates that the reduction of the voltage of the stimulating electrical pulse is responsible for the disappearance of the EPIR effect in the grain boundaries even though there is a nonlinear I – V characteristic and a space charge region. The results also prove that the EPIR effect in Nd 0.7 Sr 0.3 MnO 3 ceramics has to be ascribed to the Schottky barrier which develops between the Ag electrode and Nd 0.7 Sr 0.3 MnO 3 ceramic.

High temperature strain hardening behavior in double forged and potassium doped tungsten

The strain-hardening behavior of two recently developed double forged and K-doped tungsten grades in the 300–2000°C range was analyzed applying a phenomenological model describing the evolution of the flow stress as a function of the dislocation density. The applied model allowed establishing a correlation between the strain hardening curvature and the size of microstructural features controlling the dislocation multiplication. The obtained results demonstrated that plastic deformation was controlled by the resistance of the low angle grain boundaries below 1000°C and the high angle grain boundaries at 1500°C and above. The experimental results obtained at different loading rates showed that thermal activation was essential for the passage of dislocations through grain boundary interfaces at 1000°C and above. The limitations of the applied model and need for further development of the physical model accounting for stress- and temperature-induced grain growth are discussed.

Annealing effect on structural, optical and electrical properties of pure and Mg doped tin oxide thin films

This study describes the effect of annealing at different temperatures (400–600 °C) on structural, optical and electrical behaviors of pure and Mg doped tin oxide thin films grown on the glass substrate by electron beam evaporation technique. The transformation of tetragonal to orthorhombic form due to annealing, introduced a change in the optical and electrical properties of pure and Mg doped tin oxide thin films. X-ray diffraction studies or analysis revealed the phase transformation and change in the crystalline size with increase in the annealing temperature. The morphology and roughness of the thin films were studied by Atomic force microscopy. Optical band gap increased with annealing temperature confirms the improvements of crystallinity. The quality of thin films transparency was investigated by UV/Vis-spectroscopy. Photoluminescence of pure and Mg doped tin oxide thin films shows two extra peaks one at 486 nm and other at 538 nm is due to the crystal defect created as a result of annealing temperature. These peaks became stronger and shifted to longer wavelength with increasing the annealing temperature. The complex plot (Nyquist plot) showed the data point laying on two semicircles and the resistance of grains and grain boundaries increases with the increase in annealing temperature for both pure and Mg doped tin oxide thin films.

Effect of Al- or Ga-additive on ionic conductivity of thin-film Gd-doped ceria

Abstract For acceptor-doped ceria, thin films have usually shown lower ionic conductivity compared to the bulk. In this communication, the effect of additives (1 mol% of Ga2O3 or Al2O3) has been studied in terms of the film quality and the magnitude of the ionic conductivity of Gd-doped ceria (GDC). With additives, the films show strong (111)-orientation and the increased magnitude of ionic conductivity. The GDC film with the addition of 1 mol% Ga2O3 shows the highest conductivity among films and the conductivity is comparable to that of bulk samples. This proves that Ga and Al additives are useful to densify the film and that they reduce the grain-boundary resistivity of the thin films of acceptor-doped ceria.

Slip transmission in bcc FeCr polycrystal

Grain boundaries induce heterogeneities in the deformation response of polycrystals. Studying these local variations in response, measured through high resolution strain measurement techniques, is important and can improve our understanding of fatigue damage initiation in the vicinity of grain boundaries and material hardening. In this work, strain fields across grain boundaries were measured using advanced digital image correlation techniques. In conjunction with strain measurements, grain orientations from electron back-scattered diffraction were used to establish the dislocation reactions at each boundary, providing the corresponding residual Burgers vectors due to slip transmission across the interfaces. A close correlation was found between the magnitude of the residual Burgers vector and the local strain change across the boundary. When the residual Burgers vector magnitude (with respect to the lattice spacing) exceeds 1.0, the high strains on one side of the boundary are paired with low strains across the boundary, indicating the difficulties for slip dislocations to penetrate the grain interfaces. When the residual Burgers vector approaches zero, the strain fields vary smoothly across the boundary due to limited resistance to slip transmission. The results suggest that the residual Burgers vector magnitude, which relates to the GB (Grain Boundary) resistance to slip transmission, enables a quantitative analysis of the accumulation of strain at the microstructural level and the development of strain heterogeneities across grain boundaries. The results are presented for FeCr bcc alloy which exhibits single slip per grain making the measurements and dislocation reactions rather straightforward. The work points to the need to incorporate details of slip dislocation–grain boundary interaction (slip transmission) in modeling research.

Dielectric relaxation of NdMnO3 nanoparticles

The neodymium manganate (NdMnO3) nanoparticles are synthesized by the sol–gel process. The phase formation and particle size of the sample are determined by X-ray diffraction analysis and transmission electron microscopy. The band gap of the material is obtained by UV–visible absorption spectroscopy using Tauc relation. Dielectric properties of the sample have been investigated in the frequency range from 42Hz to 1MHz and in the temperature range from 303K to 573K. The dielectric relaxation peaks are observed in the frequency dependent dielectric loss spectra. The Cole–Cole model is used to explain the dielectric relaxation mechanism of the material. The complex impedance plane plot confirms the existence of both the grain and grain-boundary contribution to the relaxation. The temperature dependence of both grain and grain-boundary resistances follow the Arrhenius law with the activation energy of 0.427 and 0.431eV respectively. The frequency-dependent conductivity spectra follow the power law.

A gradient plasticity grain boundary yield theory

A strain gradient crystal plasticity theory is presented that accounts for the resistance of grain boundaries against plastic flow based on an interface yield condition. This theory incorporates the previously presented numerically efficient visco-plastic treatment by the gradient of an equivalent plastic strain ∇γeq in Wulfinghoff and Böhlke (2012). The finite element implementation is discussed and the three-dimensional numerical model is fitted to experimental data of polycrystalline copper micro-tensile tests. The size dependent yield strength is reproduced notably well.

Influence of Grain and Grain-Boundary Resistances on Dielectric Properties of KNbO3 Under Small DC Bias Field

Lead-free piezoelectric potassium niobate (KNbO3) system was synthesized by conventional solid-state ceramic route. Rietveld analysis of X-ray diffraction data of this system revealed that the sample crystallized in pure orthorhombic perovskite phase at room temperature. SEM micrograph of this system depicted presence of grains having diffuse brick structure with an average grain size of 500 nm. Dielectric properties of KNbO3 ceramic were investigated under different DC bias voltage in a broad frequency (from 20 Hz to 1 MHz) and temperature (from 200°C to 500°C) ranges in its three crystalline phases. The dielectric constant was found to increase with increasing bias field in all three phases. The loss tangent of this system was found to increase first, and then it becomes constant with increasing bias field. These properties have been explained in terms of variation of grain and grain-boundary resistances with bias field.