The intrinsic origin of the grain-boundary resistance in Sr-doped LaGaO3

Abstract  

In this paper I summarize our recent investigations (Park and Kim, Phys Chem C 111:14903, 2007; Solid State Ionics 179:1329, 2008) on the origin of the grain-boundary resistance in a doped LaGaO₃, a perovskite-structured solid electrolyte. The partial electronic and ionic resistances of the bulk and the grain boundaries, as well as the total resistance, in 1 mol% Sr-doped LaGaO₃ were measured separately by means of a dc-polarization method and ac-impedance spectroscopy. Both of the partial resistances at the grain boundaries were greater than the bulk counterparts, indicating that the grain boundaries impede the ionic as well as the electronic transport in this material. The transference number of the partial electronic conductivity at the grain boundary was however greater than that in the bulk. This fact strongly suggests that both electronic and ionic charge carriers deplete at the grain boundaries to form the space-charge zones and that the grain-boundary cores in this material are positively charged. In light of the fact that the effective charge of the oxygen vacancy (+2) is greater than that of the electron hole (+1), the oxygen vacancies deplete more sharply in the space-charge zones compared to the electron holes such that the grain boundaries become more mixed conducting relative to the bulk. These observations verify that the electrical conduction across the grain-boundaries in 1 mol% Sr-doped LaGaO₃ is governed by the space charge.     

Preparation and properties of mechanically alloyed and electrochemically etched porous Ti–6Al–4V

Formation of porous Ti–6Al–4V nanostructure biomaterial was described. The alloy was prepared by mechanical alloying followed by pressing, sintering and subsequent anodic electrochemical etching in 1 M H₃PO₄ + 2% HF electrolyte at 10 V for 30 min. Mechanically alloyed Ti–6Al–4V has nanostructure with grain size of about 35 nm and large grain boundaries volume fraction, which essentially improve etching process. The electrolyte penetrates sintered compacts through the grain boundaries, resulting in effective material removing and pores formation. The pore diameter reaches up to 60 μm, which is very attractive for strong bonding with bone. The anodization of the microcrystalline alloy ingot results in selective etching, revealing of the two-phase structure with relatively flat surface. The corrosion properties were investigated in Ringer’s solution. Mechanically alloyed samples shows worse corrosion resistance than the bulk microcrystalline alloy ingot, but electrochemical etching results in improving corrosion resistance.     

The intrinsic origin of the grain-boundary resistance in Sr-doped LaGaO<Subscript>3</Subscript>

Abstract  In this paper I summarize our recent investigations (Park and Kim, Phys Chem C 111:14903, 2007; Solid State Ionics 179:1329, 2008) on the origin of the grain-boundary resistance in a doped LaGaO₃, a perovskite-structured solid electrolyte. The partial electronic and ionic resistances of the bulk and the grain boundaries, as well as the total resistance, in 1 mol% Sr-doped LaGaO₃ were measured separately by means of a dc-polarization method and ac-impedance spectroscopy. Both of the partial resistances at the grain boundaries were greater than the bulk counterparts, indicating that the grain boundaries impede the ionic as well as the electronic transport in this material. The transference number of the partial electronic conductivity at the grain boundary was however greater than that in the bulk. This fact strongly suggests that both electronic and ionic charge carriers deplete at the grain boundaries to form the space-charge zones and that the grain-boundary cores in this material are positively charged. In light of the fact that the effective charge of the oxygen vacancy (+2) is greater than that of the electron hole (+1), the oxygen vacancies deplete more sharply in the space-charge zones compared to the electron holes such that the grain boundaries become more mixed conducting relative to the bulk. These observations verify that the electrical conduction across the grain-boundaries in 1 mol% Sr-doped LaGaO₃ is governed by the space charge. Graphical Abstract        

Influence of microstructure and crystalline phases on impedance spectra of sodium conducting glass ceramics produced from glass powder

Crystallization of highly ionic conductive N5 (Na₅YSi₄O₁₂) phase from melted Na_3+3x-1Y_1-xP_ySi_3-yO₉ parent glass provides an attractive pathway for cost-effective manufacturing of Na-ion conducting thin electrolyte substrates. The temperature-dependent crystallization of parent glass results in several crystalline phases in the microstructure (N3 (Na₃YSi₂O₇), N5 and N8 (Na8.1Y Si6O18) phases) as well as in rest glass phase with temperature dependent viscosity. The electrical properties of dense parent glass and of compositions densified and crystallized at 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C are investigated by impedance spectroscopy and linked to their microstructure and crystalline phase content determined by Rietveld refinement. The parent glass has high isolation resistance and predominantly electrons as charge carriers. For sintering at ≥ 900 °C, sufficient N5 phase content is formed to exceed the percolation limit and form ion-conducting pathways. At the same time, the highest content of crystalline phase and the lowest grain boundary resistance are observed. Further increase of the sintering temperature leads to a decrease of the grain resistance and an increase of grain boundary resistance. The grain boundary resistance increases remarkably for samples sintered at 1100 °C due to softening of the residual glass phase and wetting of the grain boundaries. The conductivity of fully crystallized N5 phase (grain conductivity) is calculated from thorough impedance spectra analysis using its volume content estimated from Rietveld analysis, density measurements and assuming reasonable tortuosity to 2.8 10⁻³ S cm⁻¹ at room temperature. The excellent conductivity and easy processing demonstrate the great potential for the use of this phase in the preparation of solid-state sodium electrolytes.

     

A Perovskite Electrolyte That Is Stable in Moist Air for Lithium‐Ion Batteries

Solid‐oxide Li⁺ electrolytes of a rechargeable cell are generally sensitive to moisture in the air as H⁺ exchanges for the mobile Li⁺ of the electrolyte and forms insulating surface phases at the electrolyte interfaces and in the grain boundaries of a polycrystalline membrane. These surface phases dominate the total interfacial resistance of a conventional rechargeable cell with a solid–electrolyte separator. We report a new perovskite Li⁺ solid electrolyte, Li_0.38Sr_0.44Ta_0.7Hf_0.3O_2.95F_0.05, with a lithium‐ion conductivity of σ_Li=4.8×10^?4 S cm^?1 at 25 °C that does not react with water having 3≤pH≤14. The solid electrolyte with a thin Li⁺‐conducting polymer on its surface to prevent reduction of Ta⁵⁺ is wet by metallic lithium and provides low‐impedance dendrite‐free plating/stripping of a lithium anode. It is also stable upon contact with a composite polymer cathode. With this solid electrolyte, we demonstrate excellent cycling performance of an all‐solid‐state Li/LiFePO₄ cell, a Li‐S cell with a polymer‐gel cathode, and a supercapacitor.     

Conductivity and thermal expansion of the Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> solid electrolyte in the oxidizing and reducing atmospheres

Highly compact (99%) solid electrolyte Ce_0.8Gd_0.2O_1.9 with submicron (0.3 μm) grains is synthesized. The dilatometric (20–850°C) and conductivity (180–350°C) measurements are performed on the electrolyte in air and as a function of the partial oxygen pressure \(p_{O_2 } \) (0.21?1×10^?25 atm) at 600, 700, and 800°C. An inflection is found in the temperature dependences of the thermal coefficient of linear expansion and conductivity (impedance measurements) at ～230°C, which is the evidence for a phase transition. The activation energies for conduction in the grain bulk and boundaries differ only slightly, indicating that the grain boundaries’ resistance is caused not by the precipitation of the second phase at the boundaries, but most probably by the presence of intergranular nanopores. The dilatometric measurements confirm a significant increase in the linear dimensions of Ce_0.8Gd_0.2O_1.9 in the reducing atmospheres with a parallel increase in its electron conductivity. The electron conductivity and specific elongation increase proportionally to \(p_{O_2 }^{ - 1/4} \) at all temperatures. The \(p_{O_2 } \) values, at which the transport numbers of ions t _i = 0.5, are determined. They are 10^?22.5, 10^?20, and 10^?18 atm at 600, 700, and 800°C, respectively.     

Two types of diffusions at the cathode/electrolyte interface in IT-SOFCs

Analytical transmission electron microscopy, in particular with the combination of energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS), has been performed to investigate the microstructure and microchemistry of the interfacial region between the cathode (La_0.6Sr_0.4Co_0.8Fe_0.2O₃, LSCF) and the electrolyte (Gd-doped ceria, GDC). Two types of diffusions, mutual diffusion between cathode and electrolyte as well as the diffusion along grain boundaries, have been clarified. These diffusions suggest that the chemical stability of LSCF and GDC are not as good as previously reported. The results are more noteworthy if we take into consideration the fact that such interdiffusions occur even during the sintering process of cell preparation.     

Mechanoelectrochemical synthesis of porous Ti-based nanocomposite biomaterials

In this work we have synthesized a new class of nanocomposites based on Ti with the addition of hydroxyapatite (HA) and glass 45S5. The nanocomposites were prepared by mechanical alloying of the pure microcrystalline Ti powders with different amount of ceramics. The powder mixture was milled up to 48 h, pressed and sintered, which resulted in nanocomposite structure with the grain size of about 20–36 nm. The ultra low grain size structure improves mechanical properties of the implants in comparison to commonly used microcrystalline Ti-based implants. For example, the hardness of the Ti-HA nanocomposites reaches a value of 1500 HV and is five times greater than the microcrystalline Ti.To improve bonding of the implants with human tissue, the implants were electrochemically etched in 1 M H₃PO₄ + 2–10% HF electrolyte at 10 V vs. OCP for times up to 60 min. The treatment results in highly porous surface covered with Ti-oxide. The nanocrystalline structure is very useful during etching, due to the easy access of the electrolyte to the large volume of the grain boundaries. The nanocomposites with modified surface show very good corrosion resistance in Ringer’s solution.     

Central role of TiO2 anatase grain boundaries on resistivity of CaCu3Ti4O12-based materials probed by Raman spectroscopy

This study focuses on characterization and control of grain boundaries to enhance the properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics capacitors for industrial applications. A novel factor deals with TiO₂ anatase revealed by Raman scattering in grain boundaries, found as a dominant parameter of largest sample resistivity, consistent with higher grain boundary resistivity and higher breakdown voltage. Four selected samples of CCTO-based compositions showing very different properties in terms of permittivity ranging from 1000 to 684 000 measured at 1 kHz, capacitance of grain boundaries ranging from 8 10⁻¹⁰ to 4.5 10⁻⁷ F cm⁻¹, grain boundary resistivity ranging from 193 to 30,000,000 Ω cm and sample resistivity extending from 450 to 10¹¹ Ω cm. The relationship between permittivity weighted by grain size and capacitance of grain boundaries confirms the internal barrier layer capacitance model over 5 orders of magnitude.     

The Effect of Chromium Oxide on the Conductivity of Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2</Subscript>, a Solid-Oxide Fuel Cell Electrolyte

To study the effect of chromium oxide on the electric properties of Ce_0.9Gd_0.1O₂, a solid-oxide fuel cell electrolyte, two approaches were used: (a) the studying of electrochemical properties of the Ce_0.9Gd_0.1O₂- electrolyte after the spontaneous adsorption of chromium-containing molecules from a gas phase and (b) the analyzing of transport properties of the Ce_0.9Gd_0.1O₂-based chromium-containing compositions obtained by the mixing of solid-oxide electrolyte with chromium(III) oxide. It was found that the chromium reduction at the electrolyte surface dominates when chromium is adsorbed from gas phase. Both approaches allow concluding that the chromium presence in Ce_0.9Gd_0.1O₂ deteriorates the electrolyte transport properties at temperatures above 735°С. This is caused by the chromium incorporation into the electrolyte’s fluorite structure, as well as surface microheterogeneity induced by the chromium presence at the Ce_0.9Gd_0.1O₂ surface and the cerium and gadolinium cation redistribution between the grains’ bulk and surface. At intermediate temperatures (below 735°С) the electric conductivity of the Ce_0.9Gd_0.1O₂-based chromium-containing composition exceeds that of the initial solid-oxide electrolyte, which can be due to changes in transport properties of the chromium-containing phases formed at the Ce_0.9Gd_0.1O₂ surface and grain boundaries.     

Features of behavior of solid-state CO<Subscript>2</Subscript> sensors at low temperatures

The dependence of the impedance of low-temperature sensors for carbon dioxide based on the solid-state electrochemical cells Na_0.5WO₃/Na₅GdSi₄O₁₂/SnO₂(Sb₂O₄) on the concentration of carbon dioxide in the air was studied. The reversible change in the sensor resistance was shown to be due to adsorption processes at intergrain boundaries of the solid electrolyte. The composition of the products of the electrochemical processes occurring in the sensors was established. Electronic Publication     

Cathodes based on (La,Sr)MnO<Subscript>3</Subscript> modified with PrO<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>

The electrochemical characteristics of composite cathodes made of (La, Sr) MnO₃-(Zr, Sc)O₂ (LSM-SSZ), modified with PrO_{2 − x} additive, and designed for application in solid oxide fuel cells at moderately high temperatures were studied. The relationship between activity of catalytically modified composite LSM-SSZ cathodes and dispersity of electrocatalyst was revealed. The boundaries of the temperature range with the maximum dispersity of electrocatalyst and electrochemical activity of cathodes were found. The composite LSM-SSZ cathodes modified with PrO_{2 − x} were shown inert with respect to oxidation reactions of hydrocarbon fuel (methane) and highly active electrochemically with respect to oxygen reaction in non-equilibrium gas mixture of CH₄ and O₂. In cells with (Ce, Sm)O₂ (SDC) and (Zr, Y)O₂ (YSZ) electrolytes, their overvoltage is below 80 mV at the current density about 0.5 A/cm² and temperature of 600°C. These electrodes can be used as cathodes in single-chamber fuel cells. Long-term experiments were carried out to study time stability of characteristics of the said composite cathodes. The studied electrodes show parabolic or damped exponential time curves of polarization resistance if contacting with YSZ or SDC electrolyte, respectively. According to the forecast based on the experimental regularities, the polarization resistance of LSM-SSZ cathodes in 10,000 h will not exceed 0.4 or 0.13 Ohm cm², respectively, if YSZ or SDC electrolyte is used.     

Temperature effects in carbon monoxide and methanol electrooxidation on platinum–ruthenium: influence of grain boundaries

The temperature dependence of methanol and CO monolayer oxidation is studied on carbon-supported PtRu (1:1 atomic ratio) electrodes with different metal percentages (5, 30, and 60 wt.%) in an aqueous H₂SO₄ electrolyte. High-resolution transmission microscopy confirms that at high (30 or 60 wt.%) metal percentage PtRu nanostructures with a high concentration of intercrystalline boundaries are formed. These nanostructures comprise multiple-twinned particles, particles with intersecting randomly oriented intergrain boundaries, or particles with parallel intergrain boundaries. Formation of such nanostructures leads to a decrease of the apparent activation energy of the methanol and CO monolayer oxidation, while the Tafel slope and the reaction order in methanol show minor dependence on the type of nanostructure. Materials with a high concentration of grain boundary regions may be of interest for practical applications in direct methanol or proton exchange fuel cells fed with reformate.     

Effects of Charge Compensation on Colossal Permittivity and Electrical Properties of Grain Boundary of CaCu3Ti4O12 Ceramics Substituted by Al3+ and Ta5+/Nb5+

The effects of charge compensation on dielectric and electrical properties of CaCu₃Ti_4-x(Al_1/2Ta_1/4Nb_1/4)_xO₁₂ ceramics (x = 0−0.05) prepared by a solid-state reaction method were studied based on the configuration of defect dipoles. A single phase of CaCu₃Ti₄O₁₂ was observed in all ceramics with a slight change in lattice parameters. The mean grain size of CaCu₃Ti_4-x(Al_1/2Ta_1/4Nb_1/4)_xO₁₂ ceramics was slightly smaller than that of the undoped ceramic. The dielectric loss tangent can be reduced by a factor of 13 (tanδ ~0.017), while the dielectric permittivity was higher than 10⁴ over a wide frequency range. Impedance spectroscopy showed that the significant decrease in tanδ was attributed to the highly increased resistance of the grain boundary by two orders of magnitude. The DFT calculation showed that the preferential sites of Al and Nb/Ta were closed together in the Ti sites, forming self-charge compensation, and resulting in the enhanced potential barrier height at the grain boundary. Therefore, the improved dielectric properties of CaCu₃Ti_4-x(Al_1/2Ta_1/4Nb_1/4)_xO₁₂ ceramics associated with the enhanced electrical properties of grain boundaries. In addition, the non-Ohmic properties were also improved. Characterization of the grain boundaries under a DC bias showed the reduction of potential barrier height at the grain boundary. The overall results indicated that the origin of the colossal dielectric properties was caused by the internal barrier layer capacitor structure, in which the Schottky barriers at the grain boundaries were formed.     

Electrochemical impedance spectroscopy of mesoporous Al-stabilized TiO<Subscript>2</Subscript> (anatase) in aprotic medium

High-frequency electrochemical impedance spectroscopy was used to investigate the mesoporous film of Al-stabilized TiO₂ on F-doped SnO₂ support in 1 M Li(CF₃SO₂)₂N in ethylene carbonate/dimethoxyethane (1:1 v/v). Kinetic parameters, viz. charge transfer resistance and chemical diffusion coefficient, were determined. Charge transfer resistance increased with time of contact of electrode in the above aprotic electrolyte solution. The increase followed exponential dependence, whereas the double layer capacitance, simultaneously, decreased exponentially with time. These effects were discussed in terms of the solid–electrolyte interface, which undergoes chemical changes upon contact with the electrolyte solution. Adel Attia is currently on leave from the Department of Physical Chemistry, National Research Center, El-Tahrir St., Dokki 12622, Cairo, Egypt.     

Influence of grain size on the oxygen permeation performance of perovskite-type (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3−δ membranes

The effect of grain size distribution in perovskite-type (Ba_0.5Sr_0.5)(Fe_0.8Zn_0.2)O_3−δ (BSFZ) ceramics on their oxygen permeation behaviour has been investigated by variation of calcination temperature in powder production and sintering time for the ceramics. The membranes were examined via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. We found that the dwell time during sintering has an important influence on the microstructure of the ceramic. The longer the dwell time, the further proceeds the grain coarsening, which affects the oxygen permeation in a positive way and leads to an enhanced permeation. Supplementary, decreasing calcination temperature in perovskite powder synthesis delivers fine powders with grain sizes less than one micrometer and thus smaller grains in the ceramic. Unfortunately, the grain size distribution in sintered membranes is not constant through membrane cross-sections since grains in the bulk are smaller compared to those at the surface which is not favorable for the oxygen permeation of the ceramics. The activation energy was determined to be in the range of 51–53 kJ/mol and its variation does not exhibit a dependence of grain size changes. High-resolution transmission electron microscopy proved that grain boundaries are atomically thin without any interfacial phases. We come to the conclusion that the transport rate of the oxygen permeation is limited predominantly by bulk diffusion and due to the fact that grain boundaries in BSFZ act as barriers for bulk diffusion, this material is a high mobility material.     

Behavior of manganite electrodes in contact with LSGM electrolyte: the nature of low electrochemical activity

The electrochemical cells with electrodes based on La_0.8Sr_0.2MnO₃ (LSM) and supporting solid electrolytes La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 (LSGM) and Ce_0.80Sm_0.20O_1.90 (SDC) were studied comparatively. Characteristics of LSM electrodes and composite electrodes comprising a mixture of LSM and electrolytes of different origins [LSGM, SDC, and Zr_0.82Sc_0.18O_1.91 (SSZ) in the mass ratio of 1:1] were analyzed. It was shown that: 1) the electrode polarization conductivity and the ohmic resistance of the cells with the LSM–LSGM composite electrodes on the LSGM and SDC electrolytes had very similar values, while they were largely different from all the other electrodes, 2) the electrochemical activity of the electrodes on the SDC electrolyte was much higher than on the LSGM electrolyte, and 3) the ohmic resistance of the cells with the SDC electrolyte corresponded to the electrolyte resistance, whereas, the ohmic resistance of the cells with the LSGM electrolyte was much larger than the electrolyte resistance. The obtained results are due to the interaction between the LSM and LSM-containing electrodes with the LSGM electrolyte during sintering, leading to the formation of a product with a very low conductivity.     

Li2S-P2S5体系电解质及其在全固态锂离子电池中的应用研究进展

全固态锂离子电池具有安全性能好、能量密度高、工作温区广等优点,被广泛应用于便携式电子设备。固态电解质是全固态锂离子电池的关键材料之一,其中的硫化物电解质具有离子电导率高、电化学窗口宽、晶界电阻低和易成膜等特点,被认为最有希望应用于全固态锂离子电池。本文综述了Li_2S-P_2S_5体系电解质的发展状况,包括固态电解质的制备、改性、表征以及电极/固态电解质之间的固-固界面的稳定兼容问题。本文还涉及了以Li_2S-P_2S_5为电解质的全固态锂离子电池性能的研究进展。     

Electrocatalytic properties of La0.9Sr0.1MnO3-based electrodes for oxygen reduction

Electrochemical reduction of oxygen at the interface between a La_0.9Sr_0.1MnO₃ (LSM)-based electrode and an electrolyte, either yttria-stabilized-zirconia (YSZ) or La_0.8Sr_0.2Ga_0.9Mg_0.1O₃ (LSGM), has been investigated using DC polarization, impedance spectroscopy, and potential step methods at temperatures from 1053 to 1173 K. Results show that the mechanism of oxygen reduction at an LSM/electrolyte interface changes with the type of electrolyte. At an LSM/YSZ interface, the apparent cathodic charge transfer coefficient is about 1 at high temperatures, implying that the rate-determining step (r.d.s.) is the diffusion of partially reduced oxygen species, while at an LSM/LSGM interface the cathodic charge transfer coefficient is about 0.5, implying that the r.d.s. is the donation of electrons to atomic oxygen. The relaxation behavior of the LSM/electrolyte interfaces displays an even more dramatic dependence on the type of electrolyte. Under cathodic polarization, the current passing through an LSM/YSZ interface increases with time whereas that through an LSM/LSGM interface decreases with time, further confirming that it is the triple phase boundaries (TPBs), rather than the surface of the LSM or the LSM/gas interface, that dominate the electrode kinetics when LSM is used as an electrode. Electronic Publication     

Surface characteristics of AZ91D alloy anodized with various conditions

Anodic oxidation of an AZ91D magnesium alloy was carried out in an attempt to increase the corrosion resistance. The alloy was placed in an electrolyte containing 0.1 M sodium silicate (Na₂SiO₃), 2.0 M sodium hydroxide (NaOH) and 0.1 M sodium phosphate (Na₃PO₄), and treated with a current density of 100–400 mA/cm² for 1 to 4 min. After the anodic oxidation treatment, the surface characteristics were analyzed by SEM, X‐ray diffraction (XRD) and a surface roughness tester. The corrosion resistance was determined by measuring the corrosion potential and corrosion current density using potentiodynamic polarization in a 3.5 wt% NaCl electrolyte solution. Although the anodic oxidation treatment with the base electrolyte resulted in an arrival voltage ranging from 60 to 70 V, the addition of silicate tended to reduce this arrival voltage by approximately 10–20 V and decrease the critical voltage required for the formation of a porous oxide film. The pore size and film thickness increased with increasing applied current and treatment time. The addition of silicate to the electrolyte resulted in films with a homogeneous pore size and a film thickness increasing with the increasing applied current and treatment time. XRD showed the formation of a new MgO and Mg₂SiO₄ phase. The formation of Mg₂SiO₄ was attributed to the presence of SiO₄^4? in the film. After the addition of silicate, the corrosion potential increased and corrosion current decreased, resulting in improved corrosion resistance. Copyright © 2008 John Wiley & Sons, Ltd.