Fast Oxide-ion Conductor Research Articles

Crystal Structures and Conduction Paths of Sc-Doped CaTiO3 Fast Oxide-Ion Conductors with Orthorhombic Symmetry

The crystal structures and electron density distributions (EDDs) of Ca-deficient Sc-doped CaTiO3 fast oxide-ion conductors, Ca0.97(Ti0.97Sc0.03)O3-δ (CTS3) and Ca0.96(Ti0.9Sc0.1)O3-δ (CTS10), were investigated in the temperature range of 298-1173 K in N2 to analyze the effect of composition on the perovskite structure and oxide-ion transport mechanism. CTS3 and CTS10 exhibited orthorhombic Pnma symmetry in temperature ranges of 298-1173 K and 973-1173 K, respectively, with CTS10 exhibiting lower symmetry and reduction in oxide-ion conductivity below 973 K. The EDDs of CTS3 and CTS10 at 1173 K indicated unique chemical bonds and conduction paths. CTS3 and CTS10 showed covalent bonds between (Ti,Sc) and O1 (or O2) sites. CTS3, with a lower oxide-ion conductivity than that of CTS10, exhibited pseudo-one-dimensional (1D) zig-zag curved conduction paths for oxide-ions along the a-axis, unlike previously reported curved migration paths along the b-axis in CaTiO3, and chemical bonds between Ca and O1 sites, indicating oxide-ion conduction suppression. In CTS10, additional conduction paths were observed along the a-axis, forming three-dimensional (3D) zig-zag curved conduction paths in the ac-plane and along the b-axis, with the weakening of the chemical bonds between the Ca and O1 sites. The oxide-ion conductivity and mobile oxide-ion concentration of CTS10 were 3.6 and 2.0 times those of CTS3, respectively, at 1173 K; the higher oxide-ion conductivity of CTS10 could be attributed to an increase in the mobile oxide-ion concentration and mobility with a 1D to 3D change in the conduction paths and a weakening of chemical bonds between the Ca and O1 sites.

How to Make Ceramic Gas Sensor Films at Room Temperature - the Powder Aerosol Deposition Method

Introduction Typically, gas sensors are ceramic devices. They are manufactured in ceramic techniques like tape technology and/or conventional thick-film techniques (typically screen-printing with subsequent firing) [1]. During firing, interdiffusion processes occur between substrate and gas sensitive film. At least partly, they may deteriorate the gas sensing properties of the functional oxides. Some materials can even hardly be processed without decomposition. Therefore, room temperature deposition techniques are advantageous. The Powder Aerosol Deposition Method The Powder Aerosol Deposition Method (PAD) bases on the Room Temperature Impact Consolidation (RTIC) as the film densification mechanism. This allows producing dense ceramic films without any high-temperature processes directly from an initial ceramic powder on almost any substrate material. This contribution gives examples for PAD-applications in the field of gas sensing.Although the process is rather simple, the results are very promising. Driven by a pressure difference to a vacuum deposition chamber (evacuated only to rough vacuum), the aerosol is accelerated by a slit nozzle to several hundred m/s. This aerosol jet ejects particles into the deposition chamber. Here, the particles impact on a movable substrate and get fractured into nanometer-sized fragments that are compacted by subsequently impacting particles. Fig. 1 depicts the basic principle. Further process details can be found in the reviews [2], [3], or [4]. Results and Discussion Some examples for PAD-based sensors are surveyed here. Besides conventional conductometric gas sensors based on SnO2 and other metal oxides to detect limited components, applications for temperature independent oxygen sensors are reported, e.g. of SrTi1-x Fe x O3 [5], [6] or Alumina-doped BaFe1-x Ta x O3 (BFATx) [7]. Simultaneous powder aerosol co-deposition of inert and functional oxides to fine-tune the sensing properties may be promising [6].Figure 2 is a measurement protocol at 900 °C of the logarithm of the film conductivity of BFAT30 fabricated by PAD. The material is rapidly and reproducibly responding to stepwise changes of the oxygen partial pressure, pO2. In Figure 3, the log-log plot of the conductivity (log s vs. log pO2) of the sensor between 600 °C and 900 °C in the pO2 range from 0.01 to 1 bar is shown. Neither the sensitivity to oxygen (slope in the log-log-plot) nor the base line resistance changes with temperature. Log s depends linearly on log pO2 between 700 °C and 900 °C, showing a slope of 0.24. An improved formulation contains 1 % alumina (BFATx). Amongst the BFATx samples examined, particularly good properties were found with regard to temperature independency for BFAT25 (BaFe0.74Al0.01Ta0.25O3).Combined resistive and thermoelectric oxygen sensors with almost temperature-independent characteristics of both conductivity and Seebeck coefficient were manufactured using BFAT30 [8].Classical tin-oxide conductometric metal oxide gas sensors were deposited by PAD at room temperature on interdigital electrodes. As expected, they show a pronounced sensitivity to nitrogen oxides at around 300 °C and to hydrogen at around 450 °C [9].With PAD, solid electrolyte electrochemical gas sensors have also been successfully manufactured [10]. A nitrogen oxide sensor using the novel pulsed-polarization method, which attracted much attention recently with YSZ as solid electrolyte, can now be operated at lower temperatures due to the use of a Bismuth-based fast oxide ion conductors (Fig. 4).The powder aerosol deposition method is also suitable to produce thermistors with a negative temperature coefficient of resistance (NTCR) [11]. They may serve as sensitive detectors for pellistors. Conclusions It is possible to manufacture ceramic gas sensor films completely without any high-temperature process and directly from an initial ceramic powder on almost any substrate material. Even temperature sensors can be realized. Future research directions may focus on sensors on polymers or textiles by applying the powder aerosol deposition method.

The Powder Aerosol Deposition Method - Making Ceramic Gas Sensor Films at Room Temperature

Introduction Most gas sensors are ceramic devices. They are manufactured in ceramic techniques like tape technology and/or conventional thick-film techniques (typically screen-printing with subsequent firing) [1]. During firing, between substrate and gas sensitive film interdiffusion processes occur. They may (at least partly) deteriorate the gas sensing properties of the functional oxides. Some materials can even hardly be processed without decomposition. Therefore, room temperature deposition techniques may be advantageous. The Powder Aerosol Deposition Method The Powder Aerosol Deposition Method (PAD) bases on a film densification mechanism called Room Temperature Impact Consolidation (RTIC). This allows producing dense ceramic films without any high-temperature processes directly from an initial ceramic powder on almost any substrate material. This contribution gives examples for PAD-applications in the field of gas sensing.The results are very promising, despite the process is rather simple. Driven by a pressure difference to a vacuum deposition chamber (evacuated only to a rough vacuum), the aerosol is accelerated by a slit nozzle to several hundred m/s. This aerosol jet ejects particles into the deposition chamber. Here, the particles impact on a movable substrate and get fractured into nanometer-sized fragments that are compacted by subsequently impacting particles. Fig. 1 depicts the basic principle. Further process details can be found in the reviews [2], [3], or [4]. Results and Discussion Some examples for PAD-based sensors are surveyed here. Besides conventional conductometric gas sensors based on SnO2 and other metal oxides to detect limited gaseous components, applications for temperature independent oxygen sensors are reported, e.g. of SrTi1-x Fe x O3 [5], [6] or BaFe1-x Ta x O3 (BFTx) [7]. Simultaneous aerosol co-deposition of inert and functional oxides to fine-tune the sensing properties may be promising [6].Figure 2 is a measurement protocol at 900 °C of the logarithm of the film conductivity of BFAT30 fabricated by PAD. The material is rapidly and reproducibly responding to stepwise changes of the oxygen partial pressure, pO2. In Figure 3, the log-log plot of the conductivity (log s vs. log pO2) of the sensor between 600 °C and 900 °C in the pO2 range from 0.01 to 1 bar is shown. Neither the sensitivity to oxygen (slope in the log-log-plot) nor the base line resistance changes with temperature. Log s depends linearly on log pO2 between 700 °C and 900 °C, showing a slope of 0.24. An improved formulation contains 1 % alumina (BFATx). Amongst the BFATx samples examined, particularly good properties were found with regard to temperature independency for BFAT25 (BaFe0.74Al0.01Ta0.25O3).Combined resistive and thermoelectric oxygen sensors with almost temperature-independent characteristics of both conductivity and Seebeck coefficient were manufactured using BFAT30 [8].Classical tin-oxide conductometric metal oxide gas sensors were deposited by PAD at room temperature on interdigital electrodes. As expected, they show a pronounced sensitivity to nitrogen oxides at around 300 °C and to hydrogen at around 450 °C [9].With PAD, solid electrolyte electrochemical gas sensors have also been successfully manufactured [10]. A nitrogen oxide sensor using the novel pulsed-polarization method, which attracted much attention recently with YSZ as solid electrolyte, can now be operated at lower temperatures due to the use of a Bismuth-based fast oxide ion conductors (Fig. 4).The powder aerosol deposition method is also suitable to produce thermistors with a negative temperature coefficient of resistance (NTCR) [11]. They may serve as sensitive detectors for pellistors. Conclusions It is possible to manufacture ceramic gas sensor films completely without any high-temperature process and directly from an initial ceramic powder on almost any substrate material. Even temperature sensors can be realized. Future research directions may focus on sensors on polymers or textiles by applying the powder aerosol deposition method.

High oxide ion and proton conductivity in a disordered hexagonal perovskite.

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties, and the discovery of new materials is a challenging research focus. Here, we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm-1), and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell that will combine the advantages of both oxide ion and proton-conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.

Ion dynamics of non-stoichiometric Na0.5+xBi0.5-xTiO3-δ: A degradation study

Sodium bismuth titanate, a well-known piezoelectric material is reported as the new family of fast oxide ion conductors. In the current study, we investigated the influence of A-site non-stoichiometry on the electrical conductivity and ion dynamics of sodium bismuth titanate ceramics. For this, we synthesized the system Na0.5+xBi0.5-xTiO3–δ with x = −0.02, −0.01, 0.00, 0.01 and 0.02. The samples synthesized were almost single phase with rhombohedral (R3c) symmetry. The scanning electron micrographs showed the formation of dense grains with bulk density > 96%. The microstructure appeared to be very sensitive to non-stoichiometry. The conductivity data isotherms with frequency showed non-linear variation and were well fitted with Jonscher power law. We have observed super linear frequency dependent behaviour in Bi rich composition and sub-linear behaviour with the increase in Na content. To study the degradation mechanism, Na0.5+xBi0.5-xTiO3-δ (x = −0.01, 0.00, 0.01) samples were kept in reducing environment for 48 h. Further, for the analysis of conduction mechanism and the qualitative/quantitative estimation of charge carriers before and after reducing conditions, various models were used.

Tailoring Mixed Ionic/Electronic Conductivity with Grain Boundaries: (La,Sr)(Ga,Mg)O3-X Case Study

Modified point defect chemistry at grain boundaries can be leveraged to turn a predominantly ionic conductor or a predominantly electronic conductor into a mixed conductor, by changing the grain boundary density [1,2]. There are reports indicating that introducing some electronic conductivity into the ionically conducting electrolyte of a solid oxide electrolysis cell could help to alleviate the problem of delamination at or near the electrolyte-oxygen electrode interface [3]. For this reason, we revisited the possible role of grain boundaries in tailoring mixed conduction in the fast oxide-ion conductor La0.9Sr0.1Ga0.9Mg0.1O3-x (LSGM). LSGM is a candidate electrolyte as it exhibits good thermo-chemo-mechanical compatibility with adjacent perovskite-structured electrodes. Previous work has indicated enhanced p-type electronic transference numbers at the grain boundaries of LSGM compared to the bulk, attributed to space charge effects on the basis of the observed non-linear current-voltage behavior of the grain boundaries at sufficiently high voltages [4]. However, the interplay of processing route, grain size, and dopant concentration in tailoring the extent of this effect had not been investigated. In this work [5] we fabricated dense LSGM pellets with non-dilute doping levels typical of electrolytes, with a range of average grain sizes from ~100 nm to ~6 μm, using field- and pressure- assisted sintering with various post-annealing steps as well as conventional sintering. The electrical transport behavior of grain cores and grain boundaries (GBs) was subsequently studied by impedance spectroscopy as a function of temperature, oxygen partial pressure, and dc bias. The brick layer model and the nano-grain composite model were applied to determine specific GB conductivities and electrical widths from the impedance data. Structure and composition were evaluated by X-ray diffraction, thermogravimetric analysis, transmission electron microscopy, scanning transmission microscopy, and energy dispersive X-ray spectroscopy. With increasing sintering temperature (and grain size), the following trends were observed: increasing pO2-dependence of the GB conductivity indicating higher local electronic transference numbers, higher apparent GB potentials, lower specific GB conductivity, lower pre-exponential factor for specific GB conductivity, increasing electrical GB widths, and greater dc bias dependence of GB conductivity. On the other hand, as expected, grain core electrical behavior was independent of processing route, grain size, and pO2. The results are suggestive of an increasing grain boundary space charge effect with increasing sintering temperature (and grain size), as has been observed for other ionic conductors where grain size and sintering temperature/time were coupled. Additionally, in all cases, the GB electronic transference numbers in these highly doped samples appeared to be smaller than those reported for lightly doped LSGM. Microscopy and EDS revealed that while grain boundaries appeared to be well-crystalline and “clean” in all samples, there was increasing microstructural and compositional homogeneity in samples that were sintered at progressively higher temperatures. The significance of the nanoscale compositional and microstructural fluctuations in the small-grained samples is not yet clear, as the apparent phase purity and density of all the samples was very similar macroscopically. Nonetheless, it seems possible that such inhomogeneity (pores and Sr- rich and Mg-rich areas not limited to GB regions) might limit the space charge contribution at GBs, and that some of the pores could have arisen from residual volatile impurities trapped during the pressure-assisted sintering. Overall, the results confirm that 1) grain boundaries can be sites of mixed conduction in an otherwise predominantly ionically conducting electrolyte and 2) processing route (thermal, pressure, and electrical history) is an important variable for tailoring the magnitude of the local ionic and electronic conductivities and transference numbers. [1] Y.M. Chiang et al., Applied Physics Letters, 69(2), 185-187 (1996).[2] A.M. Saranya et al., Advanced Energy Materials, 5(11), 1500377 (2015). [3] A.V. Virkar et al., International Journal of Hydrogen Energy, 40, 5561-5577 (2015). [4] C.T. Chen et al., Physical chemistry chemical physics, 14, 9047-9049 (2012). [5] T. Chen et al., “Modifying grain boundary ionic/electronic transport in nano-Sr- and Mg- doped LaGaO3-δ by sintering variations,” (under review)

Dual Oxygen Defects in Layered La1.2Sr0.8-xBaxInO4+δ (x = 0.2, 0.3) Oxide-Ion Conductors: A Neutron Diffraction Study.

The title compounds exhibit a K2NiF4-type layered perovskite structure; they are based on the La1.2Sr0.8InO4+δ oxide, which was found to exhibit excellent features as fast oxide-ion conductor via an interstitial oxygen mechanism. These new Ba-containing materials were designed to present a more open framework to enhance oxygen conduction. The citrate-nitrate soft-chemistry technique was used to synthesize such structural perovskite-type materials, followed by annealing in air at moderate temperatures (1150 °C). The subtleties of their crystal structures were investigated from neutron powder diffraction (NPD) data. They crystallize in the orthorhombic Pbca space group. Interstitial O3 oxygen atoms were identified by difference Fourier maps in the NaCl layer of the K2NiF4 structure. At variance with the parent compound, conspicuous oxygen vacancies were found at the O2-type oxygen atoms for x = 0.2, corresponding to the axial positions of the InO6 octahedra. The short O2–O3 distances and the absence of steric impediments suggest a dual oxygen-interstitial mechanism for oxide-ion conduction in these materials. Conductivity measurements show that the activation energy values are comparable to those typical of ionic conductors working by simple vacancy mechanisms (~1 eV). The increment of the total conductivity for x = 0.2 can be due to the mixed mechanism driving both oxygen vacancies and interstitials, which is original for these potential electrolytes for solid-oxide fuel cells.

Extension of the A nB nO3 n+2 Slicing/Oxidizing Process from 2D Layered to 1D Columnar Perovskites: The La7W7-xM xO30 Structural Family with Di-, Tri-, and Tetravalent Transition Metal M Substitutes to Tungsten.

The A7B7O30 structural type is a columnar-perovskite-type arrangement originally evidenced in La7Mo7O30 [Goutenoire et al. J. Solid State Chem. 1999, 142, 228], an intermediate reduction product of fast oxide-ion conductor La2Mo2O9. Subsequently, this new structural family has been extended to La7W6+4M5+3O30 (M = Nb, Ta) [Goutenoire et al. J. Solid State Chem. 2005, 178, 2811]. Here we show that it is possible to further extend the solid solution through partial substitution of hexavalent tungsten by di-, tri-, or tetravalent transition metals. The general formula is La3+7W6+7- xM m+ xO30 with x = 3/(6 - m), or m = 6 - 3/ x. Single phase samples with M m+ = Zn2+, Fe3+, and Ti4+ have been prepared, and their crystal structure refined using X-ray powder diffraction. In the structure, the transition metal with lower valence is preferentially located in the octahedral site shared by consecutive trans-connected perovskite cages, whose diagonal runs along the threefold axis of the structure. Close similarities are noticed with the cationic repartition and distortion of coordination polyhedra in compounds belonging to the A nB nO3 n+2 series of layered perovskites. A rationale is proposed, which includes the A7B7O30 structural type as a subsequent 1D step to the 3D → 2D A nB nO3 n+2 scheme of perovskite framework slicing by progressive oxygen insertion through decreasing n.

Structure Evolution and Reactivity of the Sc(2- x)V xO3+δ (0 ≤ x ≤ 2.0) System.

Solid oxide fuel cells (SOFCs) are solid-state electrochemical devices that directly convert chemical energy of fuels into electricity with high efficiency. Because of their fuel flexibility, low emissions, high conversion efficiency, no moving parts, and quiet operation, they are considered as a promising energy conversion technology for low carbon future needs. Solid-state oxide and proton conducting electrolytes play a crucial role in improving the performance and market acceptability of SOFCs. Defect fluorite phases are some of the most promising fast oxide ion conductors for use as electrolytes in SOFCs. We report the synthesis, structure, phase diagram, and high-temperature reactivity of the Sc(2- x)V xO3+δ (0 ≤ x ≤ 2.00) oxide defect model system. For all Sc(2- x)V xO3.0 phases with x ≤ 1.08 phase-pure bixbyite-type structures are found, whereas for x ≥ 1.68 phase-pure corundum structures are reported, with a miscibility gap found for 1.08 < x < 1.68. Structural details obtained from the simultaneous Rietveld refinements using powder neutron and X-ray diffraction data are reported for the bixbyite phases, demonstrating a slight V3+ preference toward the 8b site. In situ X-ray diffraction experiments were used to explore the oxidation of the Sc(2- x)V xO3.0 phases. In all cases ScVO4 was found as a final product, accompanied by Sc2O3 for x < 1.0 and V2O5 when x > 1.0; however, the oxidative pathway varied greatly throughout the series. Comments are made on different synthesis strategies, including the effect on crystallinity, reaction times, rate-limiting steps, and reaction pathways. This work provides insight into the mechanisms of solid-state reactions and strategic guidelines for targeted materials synthesis.

A protocol to detect the phase transition in La2Mo2O9 oxide ion conductor

The fast oxide ion conductor, La2Mo2O9 undergoes a structural phase transition around 580 °C from room temperature monoclinic (α) to high temperature cubic (β). The β phase is the one of interest as a fast oxide ion conductor, and thus efforts are made to stabilize it to lower temperatures. The α and β phases have very similar X-ray diffraction patterns, posing characterization problem in this quest. A new reliable protocol is proposed to clearly identify the phase in a short time. This is done without referring to peak splitting, which can be problematic with fine powders. One can use scan speed of 12 s/step in the 2θ = 14°–16.5° range to detect three peaks that vanish after phase transition to the β form. This examination is extended further to detect the cubic β phase of La2Mo1.0W1.0O9 in the same scan range.

Mechanical and impedance spectroscopy studies on the fast oxide ion conductor Na0.54Bi0.5Ti0.94Mg0.06O2.94

The perovskite structure Na0.5Bi0.5Ti0.94Mg0.06O2.94 sample was prepared by the conventional solid state reaction. The electrical properties and the mechanism of oxide ion diffusion in the Na0.5Bi0.5Ti0.94Mg0.06O2.94 compounds were investigated with the method of AC impedance spectroscopy and internal friction spectroscopy. The bulk oxide ionic conductivity of the Na0.5Bi0.5Ti0.94Mg0.06O2.94 compound at 673 K is about 3.3 × 10−4 S/cm, 1.2 times higher than that of the Na0.5Bi0.5TiO3 sample at the same temperature. A prominent relaxation internal friction peak was observed and the relaxation parameters is E = 0.92 eV, \({\tau _0}=\) 1.6 × 10−13 s. Compared with the Na0.5Bi0.5TiO3 compound, the factors of the higher oxygen vacancy content and better oxygen vacancy mobility are responsible for the oxide ionic conductivity improvement of the Na0.5Bi0.5Ti0.94Mg0.06O2.94 sample. By the investigation of dielectric frequency spectroscopy in the temperature range from 473 to about 720 K, the Arrhenius plot of the Na0.5Bi0.5Ti0.94Mg0.06O2.94 sample was divided into two parts by an inflection point (540 K), which corresponds to the transition between the rhombohedral anti-ferroelectric phases to the tetragonal para-electric phase in the Na0.5Bi0.5Ti0.94Mg0.06O2.94 sample. The activation energies of oxide ionic diffusion in the rhombohedral and tetragonal structure Na0.5Bi0.5Ti0.94Mg0.06O2.94 sample are about 0.94 and 0.62 eV, respectively. The oxygen vacancy mobility of the tetragonal structure Na0.5Bi0.5Ti0.94Mg0.06O2.94 sample is superior to that of the rhombohedral structure Na0.5Bi0.5Ti0.94Mg0.06O2.94 sample, which is meaningful to improve the oxide ionic conductivity of the Na0.5Bi0.5TiO3-based materials.

Genesis, structural, and transport properties of La2Mo2-xWxO9 prepared via mechanochemical activation

Fast oxide-ion conductors La2Mo2-xWxO9 (x = 0–1) have been prepared using mechanochemical activation (MA) of starting oxides in a high-power planetary ball mill. Studies of La2Mo2-xWxO9 genesis and structural properties using thermal analysis, XRD, SEM, IR, and Raman spectroscopy have revealed that MA results in the formation of an amorphous precursor, while the cubic β-phase is formed after calcination at 700–900 °C. Due to a high dispersion of powders, high-density pellets of W-LAMOX ceramics have been obtained already after sintering at 950 °C. Their electrical conductivity measured by the impedance spectroscopy depends on the W concentration being sufficiently high (up to 5.6∙10−3 S/cm at 630 °C) at temperatures below 650 °C.

Mechanical and dielectric relaxation studies on the fast oxide ion conductor Na0.54Bi0.46TiO2.96

The electrical properties and the mechanism of oxide ion diffusion in the Na0.54Bi0.46TiO2.96 compounds were investigated. The bulk ionic conductivity of the Na0.54Bi0.46TiO2.96 sample at 673K is about 1.6×10−3S/cm, nearly five times higher than that of the Na0.5Bi0.5TiO3 sample at the same temperature. In the internal friction measurement, a prominent relaxation peak was observed around 340K at the measuring frequency 1Hz. As for the dielectric measurement, the relaxation peak was observed above 450K when the measurement frequency is greater than 500Hz. The activation energy and the preexponential factor of the relaxation time measured were determined as (0.76eV, 0.72×10−12s) and (0.77eV, 1.8×10−12s) for the internal friction measurement and the dielectric measurement, respectively. The relaxation parameters are in the same range as that for oxide ion diffusion in the perovskite oxide ion conductors, indicating that the relaxation peak may be caused by the oxide ion diffusion by the Na–Bi–Ti pathways in the Na0.54Bi0.46TiO2.96 sample. In additional, compared with the Na0.5Bi0.5TiO3 sample, the phase transition temperature of the ferroelectric phase to the anti-ferroelectric phase in the Na0.54Bi0.46TiO2.96 sample increases from about 430K to 495K, widening the application temperature range of the Na0.5Bi0.5TiO3-based materials. These results demonstrated that the Na0.5Bi0.5TiO3-based material system is promising as fast oxygen ionic conductors and the application temperature range of the Na0.5Bi0.5TiO3-based materials can be widened with the bismuth ion partly substituted by the sodium ion, which is meaningful to improve the ferroelectric and piezoelectric performance of Na0.5Bi0.5TiO3-based materials.

Sinterability, reducibility, and electrical conductivity of fast oxide-ion conductors La1.8R0.2MoWO9 (R=Pr, Nd, Gd and Y)

To examine the effect of La-site substitution on the performance of the fast oxide-ion conductor β-La2MoWO9, doped La1.8R0.2MoWO9 (R=Pr, Nd, Gd and Y) electrolytes are synthesized via solid-state reaction. The structures and properties of the electrolytes are studied by X-ray diffraction, dilatometric analysis, scanning electron microscopy, and impedance spectroscopy. Pr, Nd, and Y substitutions decrease the sintering temperature and increase the average grain size. The as-sintered specimens are annealed at 873K in humidified 25% H2–N2 to test their structural stabilities. Compared with Nd-doped and Gd-doped samples, Pr-doped and Y-doped specimens do not exhibit obvious surface degradation after reduction. Although all substitutions relatively enhance the low-temperature (<500°C) conductivity of La2MoWO9, only Pr substitution obviously increases the high-temperature conductivity of the corresponding electrolyte. The temperature dependence of La1.8R0.2MoWO9 conductivity were well fitted to an Arrhenius model below 500°C and a Vogel–Tamman–Fulcher model above 500°C.

Multinuclear Solid-State NMR Studies of Ionic Conduction Mechanisms in Low-Cost and Rare-Earth-Free Superior Fast Oxide-Ion Conductor Sr3-3xNa3xSi3O9-1.5x

One of the most promising next-generation energy conversion technologies is the solid oxide fuel cell (SOFC), which is more efficient and less polluting than conventional internal combustion engines (ICEs). However, in order to commercially compete with ICEs, SOFCs must be cost-competitive and operation-reliable. To achieve this goal, lowering the operating temperature of a SOFC from the current 700-1000oC to ≤ 600oC is vitally important. The realization of such intermediate-temperature SOFCs (IT-SOFCs) depends critically upon the availability of high-conductivity electrolytes and high-activity electrode catalysts. In recent years, significant progress has been made in the area of cathode catalysts through new material discovery and microstructural optimization, which has demonstrated a polarization resistance (area-specific resistance) as low as 0.1 W×cm2 even at 500oC. 1 2 In contrast, the development of high-conductivity oxide-ion conductors significantly lags behind that of IT-SOFC cathodes. Therefore, identification of new IT solid-state fast oxide-ion conductors is in high demand. A practical criterion for an electrolyte to be suitable for an IT-SOFC is its oxide-ion conductivity so ≥0.01 S/cm at an operating temperature < 600oC. None of the existing chemically stable oxide-ion conductors can satisfy this requirement at a temperature ≤500oC. Recently, the Goodenough group at the University of Texas at Austin discovered a new class of oxide-ion conductors that can meet this conductivity-temperature requirement. These new oxide-ion conductors have a generic formula Sr3-3xNa3xSi3O9-1.5x (or SNS hereinafter) and layered structure as shown in Fig.1 (a). 3 4 The oxide-ion conductivity of SNS is so»0.01 S/cm at 500oC, Fig.1 (b), the highest among all known chemically stable oxide-ion conductors. More importantly, it is stable over a broad pO2 range of approximately from 10-30 to 1 atm and an extended lifetime, making it an ideal electrolyte for IT-SOFCs. 5 Despite the exciting discovery of this superior ion conductor, so far very little is known about the transport mechanisms governing the high oxide-ion conductivity in the SNS. This study investigates the origin of high oxide-ion conductivity by probing the local structure and dynamics of ionic motion with high-temperature multinuclear (17O, 23Na, and 29Si) solid-state NMR techniques. It reveals the inhomegeneity of chemical phases in this electrolyte. The increased level of Na-doping has led to increased degree of amorphorization in structure, which may be correlated to the enhancement in ionic conductivity. The foundational knowledge gained from this research may have significant impact on understanding the critical structure-conductivity relationship and designing new solid-state fast ion conductors for advanced electrochemical energy conversion and storage systems. References (1) Liang, F. L.; Zhou, W.; Li, J.; Zhu, Z. H.: Microwave-plasma induced reconstruction of silver catalysts for highly efficient oxygen reduction. J Mater Chem A 2013, 1, 13746-13749. (2) Zhu, Y. L.; Chen, Z. G.; Zhou, W.; Jiang, S. S.; Zou, J.; Shao, Z. P.: An A-Site-Deficient Perovskite offers High Activity and Stability for Low-Temperature Solid-Oxide Fuel Cells. Chemsuschem 2013, 6, 2249-2254. (3) Singh, P.; Goodenough, J. B.: Monoclinic Sr1-xNaxSiO3-0.5x: New Superior Oxide Ion Electrolytes. J Am Chem Soc 2013, 135, 10149-10154. (4) Singh, P.; Goodenough, J. B.: Sr1-xKxSi1-yGeyO3-0.5x: a new family of superior oxide-ion conductors. Energ Environ Sci 2012, 5, 9626-9631. (5) Wei, T.; Singh, P.; Gong, Y. H.; Goodenough, J. B.; Huang, Y. H.; Huang, K.: Sr3-3xNa3xSi3O9-1.5x (x=0.45) as a superior solid oxide-ion electrolyte for intermediate temperature-solid oxide fuel cells. Energ Environ Sci 2014, 7, 1680-1684.

Temperature-programmed C18O2 SSITKA for powders of fast oxide-ion conductors: Estimation of oxygen self-diffusion coefficients

For powders of oxide-ion conductors based on Al/Fe- doped lanthanum silicates as well as Sc+Ce-doped zirconia, the temperature –programmed exchange with C18O2 in the SSITKA mode was applied for estimation of the oxygen self-diffusion coefficients Do. Comparison with results obtained for dense ceramics of these electrolytes using SIMS isotope profiling and conductivity measurements demonstrated a reasonable agreement for powders sintered at high temperatures, thus providing required verification of C18O2 SSITKA approach. For powders calcined at moderate temperatures much lower values of DO were obtained thus suggesting a strong negative effect of inhomogeneity of dopants spatial distribution on the oxygen mobility.

“Free” Volume Expansion and Formation Enthalpy of Defects as Key Parameters Tuning the Oxide Ionic Conductivity in Derivatives of β-La2Mo2O9

The crystal structure of fast oxide-ion conductor β-La1.85Ba0.15Mo2O8.925 and its thermal evolution have been studied using neutron powder diffraction, and compared to those already published [G. Corbel et al., Chem. Mater. 2011, 23, 1288] of a bismuth counterpart in the LAMOX family, β-La1.7Bi0.3Mo2O9. Comparable evolutions have a tendency to suggest that the observed behavior, a specific combination of rotation and distortion of cationic building units, is a common feature of La-substituted β-LAMOX compounds. For the first time in crystallized solids, a quantitative link is made for both compounds between the high-temperature (high-T) conductivity increase relative to Arrhenius behavior and the volume expansion of the voids, through consecutive fits to crystallographic and conductivity data of the Dienes–Macedo–Litovitz (DML) equation. Both the expansion of the “free” volume (VF) and the formation enthalpy of the Frenkel defects (ΔHf) are the key parameters tuning the conductivity of LAMOX compounds above T0 = 400–450 °C. A substitution strategy of La in La2Mo2O9 is proposed in order to optimize anionic conductivity, based on crystallographic grounds.

Sintering and densification behavior of pure and alkaline earth (Ba2+, Sr2+ and Ca2+) substituted La2Mo2O9

Abstract The present work studies densification and grain growth of fast oxide ion conductor, lanthanum molybdenum oxide (La 2 Mo 2 O 9 ). Sintering variables are experimentally analyzed using sintering laws. This paper investigates sintering of pure as well as Ca 2+ , Sr 2+ and Ba 2+ doped compositions by linear shrinkage measurement using dilatometer, relative theoretical density calculation and microstructural analysis using scanning electron microscope. In this work fine powder prepared by citrate–nitrate auto-combustion synthesis is used as source of powders for different compositions. The investigations indicate that sintering is enhanced in compare to the powders prepared by the other synthesis routes reported earlier. The evaluation of activation enthalpy of pure La 2 Mo 2 O 9 and its dependence with dopant addition are also studied with the help of sintering models and equations. The sintering mechanism is found to be dominated by grain boundary diffusion. The sintering inhibition is observed for the substituted compositions.

Partial substitution of Mo6+ by S6+ in the fast oxide ion conductor La2Mo2O9: Synthesis, structure and sulfur depletion

Powder-solid state reaction route using La2(SO4)3 as sulfur source was used to prepare compositions of the solid solution La2Mo2−ySyO9. Single phases were only obtained in the substitution range extending up to y=0.8 (40mol% S) at the annealing temperature of 850°C with regard to the limit of stability of the lanthanum sulphate reactant. Within the synthesis conditions, a stabilization of the high temperature β-form is observed from and above y=0.1 (5mol% S). Temperature-controlled X-ray diffraction and thermogravimetric analyses have shown that La2Mo2−ySyO9 raw powders undergo thermal decompositions in two steps. Heating above 900°C, a sulfur depletion to the benefit of molybdenum in La2Mo2−ySyO9 raw powders leads to the formation of La2SO6. At higher temperature, the exsolved La2SO6 phase then decomposes into La2O3, which in turn reacts with the sulfur-depleted La2Mo2O9 phase to form La2MoO6. The present study also reveals that depending on the substitution rate y, the sulfur depletion can be induced by ball-milling of raw powders. Along the La2Mo2−ySyO9 series, the isovalent substitution of molybdenum by sulfur tends to restrict in magnitude, or even to suppress above 400°C, the distortive thermal expansion of the cubic β-type structure, thus strongly decreasing the conductance at high temperature.

Internal Friction Study of the Rare Earth Ion Substituted Fast Oxide-Ion Conductors (La&lt;sub&gt;1-&lt;i&gt;Χ&lt;/i&gt;&lt;/sub&gt;re&lt;sub&gt;&lt;i&gt;Χ&lt;/i&gt;&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;Mo&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;9&lt;/sub&gt; (Re=Nd, Gd)

The relaxation and phase transition behaviors of rare-earth ion substituted fast oxide-ion conductors (La1-xRex)2Mo2O9 (Re=Nd, Gd) were investigated by internal friction (IF) measurement in the temperature range 300 K - 950 K. Three different IF peaks (labeled as PL, PH, and PG, respectively) were observed in the rare-earth ion doped La2Mo2O9 samples. Peak PL corresponds to short diffusion processes of oxygen ions among different oxygen vacancy sites. Peak PH is associated with the static/dynamic disorder transition in oxygen ion distribution. Peak PG is a newly discovered peak embodying phase transition-like characteristics and is suggested to be related to order-disorder transition associated with the rearrangement of La/ Re sub-lattice.