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Fast oxygen transport in bismuth oxide containing nanocomposites
SOSI-12875; No of Pages 6 Solid State Ionics xxx (2013) xxx–xxx
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Fast oxygen transport in bismuth oxide containing nanocomposites V. Sadykov a,b,⁎, N. Mezentseva a,b, M. Arapova a, T. Krieger a, E. Gerasimov a, G. Alikina a, V. Pelipenko a, A. Bobin a, V. Muzykantov a, Y. Fedorova a, E. Sadovskaya a, N. Eremeev a, V. Belyaev a, Y. Okhlupin c, N. Uvarov c a b c
Boreskov Institute of Catalysis, 630090 Novosibirsk, Russian Federation Novosibirsk State University, 630090 Novosibirsk, Russian Federation Institute of Solid State Chemistry and Mechanochemistry, 630128 Novosibirsk, Russian Federation
a r t i c l e
i n f o
Article history: Received 15 September 2012 Received in revised form 18 February 2013 Accepted 3 March 2013 Available online xxxx Keywords: Mixed ionic–electronic conducting composites Bi-doped lanthanum manganite Y–Sm-doped bismuth oxide Structure Oxygen mobility Isotope exchange
a b s t r a c t Bi-doped lanthanum manganite La1 − xBixMnO3 + y (x = 0–1) (LBM) and fluorite-like Bi1.5Y0.3Sm0.2O2 oxide (BYS) were synthesized via Pechini route and characterized by a complex of physical–chemical methods. Oxides with reasonable high lattice oxygen mobility and reactivity were selected for LBM + BYS nanocomposites preparation. LBM + BYS composites were prepared via ultrasonic dispersion of the mixture of perovskite and fluorite powders in isopropanol with addition of polyvinyl butyral. Studies of the real structure evolution of composites at sintering under air up to 850 °C revealed some redistribution of elements between the phases without new phase formation. The oxygen mobility and reactivity of powdered LBM, BYS and composites were estimated by the oxygen isotope exchange (using both static and flow (SSITKA) modes) and O2 TPD, while weight relaxation technique was applied for studies of oxygen mobility in dense composite pellets. The oxygen mobility of composites was shown to increase with sintering temperature exceeding additive level, which suggests a positive role of interfaces as fast oxygen migration paths. For best LBM + BYS composites, the value of the oxygen chemical diffusion coefficient exceeds that of well known LSFC–GDC composite. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Design of mixed ionic–electronic conducting (MIEC) composite materials composed of phases with predominantly ionic or electronic conductivity is one of the most important tasks in the development of IT SOFC cathodes and oxygen separation membranes [1–4]. Among many oxide-ion electrolytes, Bi-based oxides (δ-Bi2O3, BiMeVOx, etc.) provide the highest ionic conductivity in the intermediate temperature range [5,6]. However, combination of Bi-based electrolytes with perovskites possessing the highest electronic conductivity (LSC, etc.) results in the incorporation of Bi in their structure and/or segregation of low-conducting phases leading to transport properties deterioration [7]. Among electronic conductors, systems containing precious metals (Bi ruthenate or Pd/Ag alloys [4,8,9]) as well as less expensive La0.87MnO3 and Ca–La–Bi–MnO3 [3,10–12] were shown to be compatible with Bi-based electrolytes. Moreover, doping of lanthanum manganite by Bi was even shown to increase its oxygen mobility [13]. However, in earlier studies [10–13], Bi content in lanthanum manganite was not optimized as related to its effect on oxygen mobility in nanocomposites. For synthesis of nanocomposites with mixed ionic–electronic conductivity,
⁎ Corresponding author at: Boreskov Institute of catalysis SB RAS, prosp. Acad. Lavrentieva, 5, 630090, Novosibirsk, Russian Federation. Tel.: +7 383 3308763; fax: +7 383 3308056. E-mail address: [email protected] (V. Sadykov).
Bi1.5Y0.3Sm0.2O2 (BYS) oxide was shown to be attractive due to its high oxide-ion conductivity and moderate interaction with complex perovskites without formation of new phases [10]. This paper presents results of studies on synthesis and characterization of structural and transport properties of La1 − xBixMnO3 + y (x = 0–1) and fluorite-like Bi1.5Y0.3Sm0.2O2 oxides as well as their mixed ionic–electronic conducting P + F nanocomposites. 2. Experimental Nanocrystalline complex oxides La1 − xBixMnO3 + y (LBM, x = 0–1) and Bi1.5Y0.3Sm0.2O2 (BYS) were synthesized via polymerized complex precursor (Pechini) route [11,13–17]. For nanocomposite preparation LBM oxides with reasonably high oxygen mobility and reactivity were selected. Nanocomposites LBM + BYS (BYS content in the range of 30–70%) were synthesized via powerful ultrasonic dispersion of the mixture of oxide powders in isopropanol (a T25 ULTRA-TURRAX IKA, Germany) homogenizer with addition of 1 wt.% polyvinyl butyral (PVB). The suspension was dried at room temperature, pressed into pellets under ~20 MPa and calcined at 700–900 °C (Tsint) in air for 2 h. For subsequent characterization, samples were finely ground using an agate pestle and mortar. X-ray diffraction patterns were obtained with an ARLX'TRA diffractometer (Thermo, Switzerland) using CuKα radiation in 2θ scanning range of 10–90°. The lattice parameters and the X-ray particle sizes were determined from the positions and half-widths of the non-overlapping
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peaks of the perovskite (P) and fluorite (F) phases. The X-ray particle (domain) sizes were estimated from the Scherrer equation with uncertainty ~ 1 nm. Transmission Electron Microscopy (TEM) micrographs were obtained with a JEM-2010 instrument (lattice resolution 1.4 Å, acceleration voltage 200 kV). Analysis of the local elemental composition was carried out by using an energy-dispersive EDX spectrometer equipped with Si(Li) detector (energy resolution 130 eV). For powdered samples, the reactivity of surface oxygen forms and bulk oxygen mobility were characterized by using the oxygen isotope heteroexchange following known approaches [17,18]. In this work, results of experiments carried out in a static installation (V = 680 cm3) with on-line control of the gas phase isotope composition by QMS-200 (Stanford Research System, USA) mass-spectrometer are presented. These experiments were carried out in two modes: 1. Isothermal isotope exchange (IIE) at pO2 1.5–4.5 Torr at 360–700 °C. 2. Temperature-programmed isotope exchange (TPIE) with the temperature ramp 5 K/min from 100 to 750 °C. The initial 18O content in the gas phase was equal to 96%. Before experiments, samples were pretreated for 2 h under air at 650 °C [17,18]. Experiments on oxygen isotopic exchange in SSITKA mode were carried out in a flow reactor in the temperature range of 500–850 °C. A flow of 1% 16O2 in He (the flow rate 300 ml/min) was isothermally fed through the catalyst bed (the sample weight 0.05 g) for an hour and then was replaced in a step-like manner by the flow of 18O2 (C 18O2) of the same concentration. Simple data analysis revealed that dynamic adsorption–desorption equilibrium in all experiments was established fast (~ 10 s) due to a small total surface of loaded samples. Variation of the concentration of isotopic 16O2/C16O2, 16O18O/ C16O18O and 18O2/C18O2 molecules in time at the reactor outlet was registered by SRS UGA 200 mass-spectrometer. Analysis of oxygen isotope exchange data was carried out using approaches earlier described in details [18–21]. The temperature-programmed desorption of oxygen (O2 TPD) in He stream was used to characterize the bonding strength of oxygen with the surface of nanocomposites and the amount of easily desorbed/mobile oxygen [16,18]. It was studied in a flow reactor system after pretreatment of samples in O2 at 500 °C for 2 h using the temperature ramp 5 °C/min from 25 to 880 °C followed by the isotherm for 60 min at 880 °C. For dense LBM–BYS nanocomposite pellets, the oxygen chemical exchange and diffusion coefficients were estimated by analysis of their weight relaxation after step-wise change of O2 content in the N2 stream from 14 to 1.4% using a STA 409 PC “LUXX” NETZSCH machine as described previously [19–22]. 3. Results and discussion 3.1. Structural characteristics Specific surface area of both LBM and BYS samples calcined at 700 °C is in the range of 5–15 m 2/g decreasing with Bi content in LBM. According to TEM data (not shown for brevity), platelet particles of samples with typical sizes in the range of 1000 Å are composed of stacked domains with typical sizes in the range of 50–100 Å. Diffraction patterns of La1 − xBixMnO3 + y samples calcined at 700 °C contain reflections corresponding to several phases (Fig. 1). Thus, for samples with Bi molar fractions 0.1, 0.2, and 0.3, rhombohedral (R3-2/c) LaMnO3 perovskite-like phase [89-048] dominates. The intensity of this phase reflections decreases with the Bi content in samples, while the lattice parameters increase. The latter trend can be explained by a partial substitution of La by Bi in the A sublattice of perovskite, since ionic radius of Bi3+ cation (0.116 nm) is bigger than that of La3+ cation (0.104 nm). In addition, already for samples with x (Bi) = 0.1 and 0.2, a weak halo in the range of the most intense reflections of orthorhombic Bi2Mn4O10
Fig. 1. Diffraction patterns of La1 − xBixMnO3 oxides calcined at 700 °C.
phase [74-1092] appears. For sample with x (Bi) = 0.3, reflections of this phase are quite clearly observed. Samples with x (Bi) = 0.4, 0.5 are composed of two phases — perovskite La(Bi)MnO3 and Bi2Mn4O10 with their ratio ~1. At x (Bi) = 0.6, the third phase appears being identified as a cubic Fm3m phase of either δ-Bi2O3 [77-374] or Bi3.69Mn0.31O6.15 [43-185]. At x (Bi) = 0.6, the admixture of α-Bi2O3 with the monoclinic structure appears. BYS prepared via Pechini route and calcined at 700 °C is characterized by diffraction pattern corresponding to rhombohedral Bi0.775Sm0.225O1.5 phase [89-4391, 44-0043], which agrees with our previous results [10]. After calcinations at 850 °C, it is transformed into cubic fluorite phase of Bi1.5Y0.5O3 type [33-0223] (not shown for brevity). Hence, rhombohedral distortion of BYS structure can be caused by some clustering of doping cations, while sintering at higher temperatures provides their uniform distribution leading to more symmetric structure. In diffraction patterns of LBM + BYS composites with different ratios of phases calcined at 700 °C, only reflections corresponding to both phases are observed without any detectable variation of their positions. Some broadening of perovskite phase reflections is observed, suggesting disordering of its structure. For composite 50%Bi1.5Y0.3Sm0.2O3 + 50%MnLa0.3Bi0.7O3 calcined at 700 °C, reflections with high intensity corresponding to Bi0.775Sm0.225O1.5 [89-4391] and/or Bi0.775La0.225O1.5 [89-4388] rhombohedral phase (R-3m) are observed. LaMnO3 perovskite-like phase [89-048], Bi-containing cubic phase Fm-3m phase (Bi2O3 or/and Bi1.5Y0.5O3) and traces of Bi2Mn4O10 have been observed as well. With the increase of the temperature of composite calcinations up to 850 °C, the intensity of LaMnO3 and Bi-containing cubic phase reflections increases while reflections correspond to rhombohedral phase decreases. TEM images (not shown for brevity) revealed progressive sintering of LBM and BYS particles in composites with the calcination temperature increase. Substantial redistribution of elements between perovskite and BYS phases was found by EDX analysis even in regions when only single phase is present as confirmed by digital diffraction pattern analysis. As a typical example, Fig. 2 shows a part of BYS domain with the rhombohedral structure in 50%Bi1.5Y0.3Sm0.2O3 + 50%MnLa0.3Bi0.7O3 composite sintered at 800 °C. According to EDX data, the atomic composition of this region corresponds to 6%La + 10%Sm + 5%Mn + 70%Bi + 9%Y. Hence, Mn and La are transferred from LBM phase into BYS, while Y migrates into the perovskite phase. In different BYS domains, Mn concentration was found to vary from 1 to 5%, La — from 3 to 10%. Similarly, in LBM domains, up to 8% Sm and up to 5% Y were revealed. Specific surface area of composites calcined at 700 °C corresponds to the mixture of initial phases, while after calcination at 850 °C it decreases to ~0.5–1 m2/g reflecting sintering. Pellets of composites sintered at 900 °C possess density ~95% of the theoretical one.
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Fig. 2. Typical high-resolution TEM image of BYS domain in 50%Bi1.5Y0.3Sm0.2O3 + 50%MnLa0.3Bi0.7O3 composite sintered at 800 °C with respective digital diffraction pattern corresponding to rhombohedral phase and EDX spectra.
3.2. Transport properties 3.2.1. Powders 3.2.1.1. O2 TPD. For LBM samples oxygen desorption observed in the range from 400 to 700 °C (Fig. 3) corresponds to removal of terminal M–O oxygen forms (heat of desorption ~ 60 kJ/mol), while the main O2 TPD peak situated in the range of 800–900 °C (several oxygen monolayers desorbed, Table 1) is due to removal of the lattice oxygen [13,18,20,21]. For samples with comparable content of La and Bi (x = 0.4–0.6), the maximum rate of O2 desorption (~1 ∗ 1016 molec. O2/m2 s) varies rather moderately being close to that for LaMnO3 and La0.8Bi0.2MnO3 calcined at 500–700 °C [13]. At the same time, this rate exceeds by an order of magnitude the value for Sr-doped lanthanum manganite (~0.5 ∗ 1015 molec. O2/m2 s) known for rather low bulk oxygen mobility [18]. For samples with a higher Bi content (x (Bi) = 0.7–0.8), the rate of O2 desorption increases further. For BiMnO3 this value reaches a maximum (~1.6 ∗ 10 15 molec. O2/m2 s), which can be assigned to enhanced oxygen mobility in the multiphase oxide system (vide supra). For BYS, the oxygen desorption is much slower (Fig. 3). However, up to 40% of the oxygen monolayer is removed from BYS in TPD run (Table 1), desorption occurring mainly in the range from 400 to 600 °C (removal of the surface oxygen species) and at temperatures >800 °C (more strongly bound surface and/or subsurface oxygen species). Hence, mainly surface oxygen forms are removed from BYS. For LBM–BYS composites sintered at 800 °C both the maximum rate of desorption and the amount of oxygen desorbed in TPD run (up to >10 monolayers) are strongly increased as compared with LBM samples (Table 1). The maximum rate of oxygen desorption from LBM–BYS composites (~80 ∗ 1015 molec. O2/m2 s) substantially exceeds that for LSFN– GDC nanocomposites (~15 ∗ 1015 molec. O2/m2 s) known for very high bulk oxygen mobility [18–21].
3.2.1.2. Isotope exchange. For LBM samples, specific rates of oxygen heteroexchange characterizing ability of surface sites to activate O2 molecules (~1017 molec. O2/m2 s at 600 °C, Fig. 4) are very close to those of LaMnO3 and La0.8Ca(Sr)0.2MnO3 samples prepared via Pechini
route [13,20,21]. This suggests that at temperatures b650 °C, active sites on the surface of perovskite-like manganites are mainly associated with Mn cations. Indeed, at these temperatures, specific rates of oxygen exchange on BYS are much lower than those on LBM. However, at 700 °C, specific rates of exchange on LBM and BYS are rather close. This apparently requires existence of sites on the BYS surface with a moderate Bi\O bond strength, which agrees with O2 TPD data (vide supra). Since Bi cations in oxides can exist in different oxidation states (2+, 3+, 5+), defect sites on the surface of BYS composed of anion vacancies and coordinatively unsaturated Bi cations are apparently able to provide transfer of electron density required for activation of O2 molecule. Analysis of kinetics of oxygen isothermal exchange in the static reactor revealed that for BYS so called 3rd type of exchange (with participation of two oxygen atoms of oxide [18]) dominates with a share of 95%. According to the mechanistic scheme of the oxygen isotope exchange by Muzykantov [23] considered earlier in details for analysis of the oxygen exchange on ionic/mixed ionic–electronic conductors [17,18,21], this means that for BYS the stage of O2 molecule
Fig. 3. O2 TPD spectra of La1 − xBixMnO3 samples with x (Bi) = 0.8 (1), 0.7 (2), 0.6 (3), 0.5 (4), 0.4 (5) and BiYSmO (6).
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Table 1 Specific surface area (S), the maximum rate of oxygen desorption (W) and amount of oxygen monolayers (θ) desorbed in TPD run for selected samples. Samples
T calc., °C S, m2/g θ, WO2, 1015 molec. monolayers O2/m2 s
Bi1.5Y0.3Sm0.2O2 La0.2Bi0.8MnO3 La0.3Bi0.7MnO3 50%La0.2Bi0.8MnO3 50%La0.2Bi0.8MnO3 70%La0.3Bi0.7MnO3 70%La0.3Bi0.7MnO3
700 700 700 700 800 700 800
+ + + +
50%BYS 50%BYS 30%BYS 30%BYS
8 4.8 5.5 4.1 1 5.1 0.5
0.4 11.8 6.9 5.3 16.4 5.3 40.9
0.3 50 22 16 52 10 85
dissociation with formation of weakly bound ZOads and strongly bound (O)s oxygen forms O2 + Zads + ( )s ⇔ ZOads + (O)s is the ratelimiting, while the stages of oxygen redistribution between different surface sites ZOads + ( )s ⇔ Zads + (O)s and incorporation into the subsurface vacancy (O)s + [ ]v ⇔ ( )s + [O]v proceed much faster. For Sr- or Ca-doped lanthanum manganite the share of the 3rd type of exchange is 0.4 [18], which agrees with well-known low bulk oxygen mobility in lanthanum manganite. For LBM perovskites with much higher bulk oxygen mobility, the share of the 3rd type of exchange increases up to 0.7. For LBM–BYS composites calcined at 700 °C, even in the lowtemperature range, specific rates of exchange coincide with those for LBM despite dilution by less active electrolyte (Fig. 4). After sintering at 800 °C, specific rate of exchange strongly increases exceeding by up to an order of magnitude the values for LSFN–GDC and LSM–ScCeSZ nanocomposites sintered at 900 °C [18–21]. This suggests that new active sites appear due to interaction between LBM and BYS domains leading to generation of defects, migration of Mn cations onto the surface of BYS domains (vide supra EDX data) and even variation of the phase composition. Since Mn was detected in the bulk of BYS domains, at least comparable concentration of Mn on their surface is expected. Since Mn4+ cation has a small radius, its preferential segregation in the surface layer is expected. Earlier, for LSM–ScCeSZ nanocomposite, enrichment of the surface by Mn with increasing the temperature of sintering was demonstrated [18]. For BYS, the dynamic extent of exchange characterizing the oxygen mobility in the temperature-programmed mode [17] is rather close to values earlier found for Gd-doped ceria — up to 15 monolayers at 700 °C [20,21] (Fig. 5). For these electrolytes, dynamic extent of exchange is controlled both by the surface reaction and bulk oxygen mobility [21]. For Bi-doped manganite, dynamic extent of exchange at 650 °C (up to 20 monolayers) greatly exceeds values for Ca, Sr-doped lanthanum manganites (1–2 monolayers [20,21]). With a due regard
for very close specific rates of exchange for lanthanum manganites either pure or doped by Ca, Sr or Bi (vide supra), this clearly demonstrates much higher bulk oxygen mobility in Bi-containing manganites, which can be in general assigned to a lower Bi\O bonding strength as compared with that for La–O, though effect of microstructure (presence of layered Bi–Mn oxides, etc., vide supra) can be important as well. For LBM–BYS composites even calcined at 700 °C, non-additive increase of dynamic degree of exchange is apparent, the effect being much stronger for sample sintered at 800 °C (Fig. 5). The dynamic extent of exchange achieved for the latter sample at 750 °C (350 monolayers) is close to that for dense LSFN–GDC nanocomposite sintered at 1200 °C possessing very high oxygen mobility [19–21]. SSITKA data for exchange of all studied samples with both C 18O2 (Fig. 6) and 18O2 (not shown for brevity) were well fitted by a model with the exchange controlled by the uniform bulk diffusion. As follows from Fig. 7a, the average Deff. values estimated by using both types of labeled molecules agree with each other. For LBM–BYS nanocomposite Deff. is by an order of magnitude higher than that for LBM, which agrees with results of O2 TPD and temperature-programmed isotope exchange studies (vide supra). As is seen from Fig. 7b, for LBM–BYS nanocomposite specific rates of O2 exchange R are as high as those for LBM, while specific rates of CO2 exchange are apparently higher for LBM–BYS nanocomposite agreeing with results obtained for O2 exchange in a static system. Though a detailed discussion of all these features related to the problem of estimation of both the rate of the surface reaction and oxygen diffusion coefficients from one set of the oxygen exchange data is beyond the scope of this paper and will be
Fig. 4. The temperature dependence of specific rates of oxygen heteroexchange for LBM and BiYSm oxides and their composites sintered at 700 °C or 800 °C. PO2 = 2.5 Torr. BiYSm (1), La0.3Bi0.7MnO3 (2), 30%La0.3Bi0.7MnO3 + 70%BYS (700 °C) (3), 50%La0.3Bi0.7MnO3 + 50%BYS (700 °C) (4), 30%La0.3Bi0.7MnO3 + 70%BYS (800 °C) (5).
Fig. 6. Variation of 18O (α) and C16O18O (X1) fractions during C18O2 exchange at 600 °C with 50%Bi1.5Y0.3Sm0.2O3 + 50%La0.8Bi0.2MnO3 calcined at 850 °C. Points — experiment, lines — fitting.
Fig. 5. Temperature dependence of the dynamic extent of exchange for BiYSm (1), La0.3Bi0.7MnO3 (2), 30%La0.3Bi0.7MnO3 + 70%BYS (700 °C) (3), 50%La0.3Bi0.7MnO3 + 50%BYS (700 °C) (4), 30%La0.3Bi0.7MnO3 + 70%BYS (800 °C) (5).
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of the structure of interfaces determined both by redistribution of cations between coexisting phases and heteroepitaxy between perovskite–fluorite surface faces. While depletion of the surface layers of perovskite domains by A-site cations (La, Sr, etc.) due to their preferential transfer into the fluorites generates disordered oxygen layers (such as A1 − xO3 densely-packed plane in cubic perovskites) with a plenty of vacancies, their enrichment by transition metal cations with disordered coordination spheres helps to decrease the average metal–oxygen strength as well as to enhance the electronic conductivity due to clustering of transition metal cations. In frames of known models of oxygen diffusion in nanostructured perovskites [21,24,25], this rearrangement decreases the barrier for the oxygen ion/atom migration, thus explaining fast oxygen self-diffusion/chemical diffusion along these interfaces demonstrated in our research. 4. Conclusions Procedures based upon polymerized polyester precursor (Pechini) route and ultrasonic treatment of binary oxide mixture in isopropanol were successfully applied for synthesis of Bi-doped lanthanum manganites (LBM), Y–Sm-doped Bi oxide (BYS) and their composites with mixed ionic–electronic conductivity. Basic features of their structure and microstructure were elucidated by XRD and TEM revealing coexistence of several phases in manganites with a high Bi content as well as their chemical compatibility with BYS. In LBM, the lattice oxygen mobility was found to increase with Bi content. In LBM + BYS composites, the oxygen mobility increases with sintering temperature and strongly exceeds that of separate phases due to positive role of perovskite–fluorite interfaces. Acknowledgments Fig. 7. Linear plot of Deff (a) and R (b) temperature dependence derived from SSITKA 18O2 and C18O2 experiments.
The authors gratefully acknowledge support from OCMOL FP7 EC Project, Project 57 of RAS Presidium Program No. 27 and Federal Program “Scientific and Educational Cadres of Russia”.
given elsewhere, well-known more facile activation of CO2 molecules as compared with that of O2 apparently provides a higher sensitivity to the surface composition and defect structure of mixed oxides/nanocomposites.
References
3.2.2. Dense pellets For dense nanocomposite LBM–BYS pellets, weight relaxation studies also revealed a high lattice oxygen mobility and reactivity. Thus, for 70%La0.3Bi0.7MnOx + 30%Bi1.5Y0.3Sm0.2O3 nanocomposite, at 800 °C the value of the oxygen chemical diffusion coefficient Dchem. (~ 2 ∗ 10 − 5 cm 2 s − 1) exceeds those for LSFC + 30%GDC (10−5 cm2 s−1) and LSFN + GDC nanocomposites (4 ∗ 10−6 cm2 s−1) [19–21]. The chemical coefficient of oxygen exchange kchem. (~6 ∗ 10 −4 cm s −1) is close to that of LSFC or LSFN composites with GDC varying in the range of 10 −3–10−4 cm s−1 as dependent upon the GDC content [21,22]. Hence, dense ceramics composed of Bicontaining nanocomposites indeed provide high oxygen mobility required for oxygen separation membrane application. In general, results of this research demonstrated the positive effect of perovskite–fluorite interfaces on the bulk oxygen mobility earlier observed for other systems such as LSFN–GDC [14,19,21], ScCeSZ– LSM [18], LSFC–GDC [21] etc. provided preparation procedures are properly optimized to ensure developed interfaces and prevent formation of isolating pyrochlore-type phases. The most straightforward demonstration of the positive role of these interfaces was given for LSFN–GDC system where detailed analysis of SSITKA results allowed one to estimate oxygen self-diffusion coefficient along the perovskite– fluorite interface shown to be up to 6 orders of magnitude higher than that in GDC [14,19]. Apparently, this effect is explained by specificity
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Fast oxygen transport in bismuth oxide containing nanocomposites V. Sadykov a,b,⁎, N. Mezentseva a,b, M. Arapova a, T. Krieger a, E. Gerasimov a, G. Alikina a, V. Pelipenko a, A. Bobin a, V. Muzykantov a, Y. Fedorova a, E. Sadovskaya a, N. Eremeev a, V. Belyaev a, Y. Okhlupin c, N. Uvarov c a b c
Boreskov Institute of Catalysis, 630090 Novosibirsk, Russian Federation Novosibirsk State University, 630090 Novosibirsk, Russian Federation Institute of Solid State Chemistry and Mechanochemistry, 630128 Novosibirsk, Russian Federation
a r t i c l e
i n f o
Article history: Received 15 September 2012 Received in revised form 18 February 2013 Accepted 3 March 2013 Available online xxxx Keywords: Mixed ionic–electronic conducting composites Bi-doped lanthanum manganite Y–Sm-doped bismuth oxide Structure Oxygen mobility Isotope exchange
a b s t r a c t Bi-doped lanthanum manganite La1 − xBixMnO3 + y (x = 0–1) (LBM) and fluorite-like Bi1.5Y0.3Sm0.2O2 oxide (BYS) were synthesized via Pechini route and characterized by a complex of physical–chemical methods. Oxides with reasonable high lattice oxygen mobility and reactivity were selected for LBM + BYS nanocomposites preparation. LBM + BYS composites were prepared via ultrasonic dispersion of the mixture of perovskite and fluorite powders in isopropanol with addition of polyvinyl butyral. Studies of the real structure evolution of composites at sintering under air up to 850 °C revealed some redistribution of elements between the phases without new phase formation. The oxygen mobility and reactivity of powdered LBM, BYS and composites were estimated by the oxygen isotope exchange (using both static and flow (SSITKA) modes) and O2 TPD, while weight relaxation technique was applied for studies of oxygen mobility in dense composite pellets. The oxygen mobility of composites was shown to increase with sintering temperature exceeding additive level, which suggests a positive role of interfaces as fast oxygen migration paths. For best LBM + BYS composites, the value of the oxygen chemical diffusion coefficient exceeds that of well known LSFC–GDC composite. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Design of mixed ionic–electronic conducting (MIEC) composite materials composed of phases with predominantly ionic or electronic conductivity is one of the most important tasks in the development of IT SOFC cathodes and oxygen separation membranes [1–4]. Among many oxide-ion electrolytes, Bi-based oxides (δ-Bi2O3, BiMeVOx, etc.) provide the highest ionic conductivity in the intermediate temperature range [5,6]. However, combination of Bi-based electrolytes with perovskites possessing the highest electronic conductivity (LSC, etc.) results in the incorporation of Bi in their structure and/or segregation of low-conducting phases leading to transport properties deterioration [7]. Among electronic conductors, systems containing precious metals (Bi ruthenate or Pd/Ag alloys [4,8,9]) as well as less expensive La0.87MnO3 and Ca–La–Bi–MnO3 [3,10–12] were shown to be compatible with Bi-based electrolytes. Moreover, doping of lanthanum manganite by Bi was even shown to increase its oxygen mobility [13]. However, in earlier studies [10–13], Bi content in lanthanum manganite was not optimized as related to its effect on oxygen mobility in nanocomposites. For synthesis of nanocomposites with mixed ionic–electronic conductivity,
⁎ Corresponding author at: Boreskov Institute of catalysis SB RAS, prosp. Acad. Lavrentieva, 5, 630090, Novosibirsk, Russian Federation. Tel.: +7 383 3308763; fax: +7 383 3308056. E-mail address: [email protected] (V. Sadykov).
Bi1.5Y0.3Sm0.2O2 (BYS) oxide was shown to be attractive due to its high oxide-ion conductivity and moderate interaction with complex perovskites without formation of new phases [10]. This paper presents results of studies on synthesis and characterization of structural and transport properties of La1 − xBixMnO3 + y (x = 0–1) and fluorite-like Bi1.5Y0.3Sm0.2O2 oxides as well as their mixed ionic–electronic conducting P + F nanocomposites. 2. Experimental Nanocrystalline complex oxides La1 − xBixMnO3 + y (LBM, x = 0–1) and Bi1.5Y0.3Sm0.2O2 (BYS) were synthesized via polymerized complex precursor (Pechini) route [11,13–17]. For nanocomposite preparation LBM oxides with reasonably high oxygen mobility and reactivity were selected. Nanocomposites LBM + BYS (BYS content in the range of 30–70%) were synthesized via powerful ultrasonic dispersion of the mixture of oxide powders in isopropanol (a T25 ULTRA-TURRAX IKA, Germany) homogenizer with addition of 1 wt.% polyvinyl butyral (PVB). The suspension was dried at room temperature, pressed into pellets under ~20 MPa and calcined at 700–900 °C (Tsint) in air for 2 h. For subsequent characterization, samples were finely ground using an agate pestle and mortar. X-ray diffraction patterns were obtained with an ARLX'TRA diffractometer (Thermo, Switzerland) using CuKα radiation in 2θ scanning range of 10–90°. The lattice parameters and the X-ray particle sizes were determined from the positions and half-widths of the non-overlapping
0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.03.016
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peaks of the perovskite (P) and fluorite (F) phases. The X-ray particle (domain) sizes were estimated from the Scherrer equation with uncertainty ~ 1 nm. Transmission Electron Microscopy (TEM) micrographs were obtained with a JEM-2010 instrument (lattice resolution 1.4 Å, acceleration voltage 200 kV). Analysis of the local elemental composition was carried out by using an energy-dispersive EDX spectrometer equipped with Si(Li) detector (energy resolution 130 eV). For powdered samples, the reactivity of surface oxygen forms and bulk oxygen mobility were characterized by using the oxygen isotope heteroexchange following known approaches [17,18]. In this work, results of experiments carried out in a static installation (V = 680 cm3) with on-line control of the gas phase isotope composition by QMS-200 (Stanford Research System, USA) mass-spectrometer are presented. These experiments were carried out in two modes: 1. Isothermal isotope exchange (IIE) at pO2 1.5–4.5 Torr at 360–700 °C. 2. Temperature-programmed isotope exchange (TPIE) with the temperature ramp 5 K/min from 100 to 750 °C. The initial 18O content in the gas phase was equal to 96%. Before experiments, samples were pretreated for 2 h under air at 650 °C [17,18]. Experiments on oxygen isotopic exchange in SSITKA mode were carried out in a flow reactor in the temperature range of 500–850 °C. A flow of 1% 16O2 in He (the flow rate 300 ml/min) was isothermally fed through the catalyst bed (the sample weight 0.05 g) for an hour and then was replaced in a step-like manner by the flow of 18O2 (C 18O2) of the same concentration. Simple data analysis revealed that dynamic adsorption–desorption equilibrium in all experiments was established fast (~ 10 s) due to a small total surface of loaded samples. Variation of the concentration of isotopic 16O2/C16O2, 16O18O/ C16O18O and 18O2/C18O2 molecules in time at the reactor outlet was registered by SRS UGA 200 mass-spectrometer. Analysis of oxygen isotope exchange data was carried out using approaches earlier described in details [18–21]. The temperature-programmed desorption of oxygen (O2 TPD) in He stream was used to characterize the bonding strength of oxygen with the surface of nanocomposites and the amount of easily desorbed/mobile oxygen [16,18]. It was studied in a flow reactor system after pretreatment of samples in O2 at 500 °C for 2 h using the temperature ramp 5 °C/min from 25 to 880 °C followed by the isotherm for 60 min at 880 °C. For dense LBM–BYS nanocomposite pellets, the oxygen chemical exchange and diffusion coefficients were estimated by analysis of their weight relaxation after step-wise change of O2 content in the N2 stream from 14 to 1.4% using a STA 409 PC “LUXX” NETZSCH machine as described previously [19–22]. 3. Results and discussion 3.1. Structural characteristics Specific surface area of both LBM and BYS samples calcined at 700 °C is in the range of 5–15 m 2/g decreasing with Bi content in LBM. According to TEM data (not shown for brevity), platelet particles of samples with typical sizes in the range of 1000 Å are composed of stacked domains with typical sizes in the range of 50–100 Å. Diffraction patterns of La1 − xBixMnO3 + y samples calcined at 700 °C contain reflections corresponding to several phases (Fig. 1). Thus, for samples with Bi molar fractions 0.1, 0.2, and 0.3, rhombohedral (R3-2/c) LaMnO3 perovskite-like phase [89-048] dominates. The intensity of this phase reflections decreases with the Bi content in samples, while the lattice parameters increase. The latter trend can be explained by a partial substitution of La by Bi in the A sublattice of perovskite, since ionic radius of Bi3+ cation (0.116 nm) is bigger than that of La3+ cation (0.104 nm). In addition, already for samples with x (Bi) = 0.1 and 0.2, a weak halo in the range of the most intense reflections of orthorhombic Bi2Mn4O10
Fig. 1. Diffraction patterns of La1 − xBixMnO3 oxides calcined at 700 °C.
phase [74-1092] appears. For sample with x (Bi) = 0.3, reflections of this phase are quite clearly observed. Samples with x (Bi) = 0.4, 0.5 are composed of two phases — perovskite La(Bi)MnO3 and Bi2Mn4O10 with their ratio ~1. At x (Bi) = 0.6, the third phase appears being identified as a cubic Fm3m phase of either δ-Bi2O3 [77-374] or Bi3.69Mn0.31O6.15 [43-185]. At x (Bi) = 0.6, the admixture of α-Bi2O3 with the monoclinic structure appears. BYS prepared via Pechini route and calcined at 700 °C is characterized by diffraction pattern corresponding to rhombohedral Bi0.775Sm0.225O1.5 phase [89-4391, 44-0043], which agrees with our previous results [10]. After calcinations at 850 °C, it is transformed into cubic fluorite phase of Bi1.5Y0.5O3 type [33-0223] (not shown for brevity). Hence, rhombohedral distortion of BYS structure can be caused by some clustering of doping cations, while sintering at higher temperatures provides their uniform distribution leading to more symmetric structure. In diffraction patterns of LBM + BYS composites with different ratios of phases calcined at 700 °C, only reflections corresponding to both phases are observed without any detectable variation of their positions. Some broadening of perovskite phase reflections is observed, suggesting disordering of its structure. For composite 50%Bi1.5Y0.3Sm0.2O3 + 50%MnLa0.3Bi0.7O3 calcined at 700 °C, reflections with high intensity corresponding to Bi0.775Sm0.225O1.5 [89-4391] and/or Bi0.775La0.225O1.5 [89-4388] rhombohedral phase (R-3m) are observed. LaMnO3 perovskite-like phase [89-048], Bi-containing cubic phase Fm-3m phase (Bi2O3 or/and Bi1.5Y0.5O3) and traces of Bi2Mn4O10 have been observed as well. With the increase of the temperature of composite calcinations up to 850 °C, the intensity of LaMnO3 and Bi-containing cubic phase reflections increases while reflections correspond to rhombohedral phase decreases. TEM images (not shown for brevity) revealed progressive sintering of LBM and BYS particles in composites with the calcination temperature increase. Substantial redistribution of elements between perovskite and BYS phases was found by EDX analysis even in regions when only single phase is present as confirmed by digital diffraction pattern analysis. As a typical example, Fig. 2 shows a part of BYS domain with the rhombohedral structure in 50%Bi1.5Y0.3Sm0.2O3 + 50%MnLa0.3Bi0.7O3 composite sintered at 800 °C. According to EDX data, the atomic composition of this region corresponds to 6%La + 10%Sm + 5%Mn + 70%Bi + 9%Y. Hence, Mn and La are transferred from LBM phase into BYS, while Y migrates into the perovskite phase. In different BYS domains, Mn concentration was found to vary from 1 to 5%, La — from 3 to 10%. Similarly, in LBM domains, up to 8% Sm and up to 5% Y were revealed. Specific surface area of composites calcined at 700 °C corresponds to the mixture of initial phases, while after calcination at 850 °C it decreases to ~0.5–1 m2/g reflecting sintering. Pellets of composites sintered at 900 °C possess density ~95% of the theoretical one.
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Fig. 2. Typical high-resolution TEM image of BYS domain in 50%Bi1.5Y0.3Sm0.2O3 + 50%MnLa0.3Bi0.7O3 composite sintered at 800 °C with respective digital diffraction pattern corresponding to rhombohedral phase and EDX spectra.
3.2. Transport properties 3.2.1. Powders 3.2.1.1. O2 TPD. For LBM samples oxygen desorption observed in the range from 400 to 700 °C (Fig. 3) corresponds to removal of terminal M–O oxygen forms (heat of desorption ~ 60 kJ/mol), while the main O2 TPD peak situated in the range of 800–900 °C (several oxygen monolayers desorbed, Table 1) is due to removal of the lattice oxygen [13,18,20,21]. For samples with comparable content of La and Bi (x = 0.4–0.6), the maximum rate of O2 desorption (~1 ∗ 1016 molec. O2/m2 s) varies rather moderately being close to that for LaMnO3 and La0.8Bi0.2MnO3 calcined at 500–700 °C [13]. At the same time, this rate exceeds by an order of magnitude the value for Sr-doped lanthanum manganite (~0.5 ∗ 1015 molec. O2/m2 s) known for rather low bulk oxygen mobility [18]. For samples with a higher Bi content (x (Bi) = 0.7–0.8), the rate of O2 desorption increases further. For BiMnO3 this value reaches a maximum (~1.6 ∗ 10 15 molec. O2/m2 s), which can be assigned to enhanced oxygen mobility in the multiphase oxide system (vide supra). For BYS, the oxygen desorption is much slower (Fig. 3). However, up to 40% of the oxygen monolayer is removed from BYS in TPD run (Table 1), desorption occurring mainly in the range from 400 to 600 °C (removal of the surface oxygen species) and at temperatures >800 °C (more strongly bound surface and/or subsurface oxygen species). Hence, mainly surface oxygen forms are removed from BYS. For LBM–BYS composites sintered at 800 °C both the maximum rate of desorption and the amount of oxygen desorbed in TPD run (up to >10 monolayers) are strongly increased as compared with LBM samples (Table 1). The maximum rate of oxygen desorption from LBM–BYS composites (~80 ∗ 1015 molec. O2/m2 s) substantially exceeds that for LSFN– GDC nanocomposites (~15 ∗ 1015 molec. O2/m2 s) known for very high bulk oxygen mobility [18–21].
3.2.1.2. Isotope exchange. For LBM samples, specific rates of oxygen heteroexchange characterizing ability of surface sites to activate O2 molecules (~1017 molec. O2/m2 s at 600 °C, Fig. 4) are very close to those of LaMnO3 and La0.8Ca(Sr)0.2MnO3 samples prepared via Pechini
route [13,20,21]. This suggests that at temperatures b650 °C, active sites on the surface of perovskite-like manganites are mainly associated with Mn cations. Indeed, at these temperatures, specific rates of oxygen exchange on BYS are much lower than those on LBM. However, at 700 °C, specific rates of exchange on LBM and BYS are rather close. This apparently requires existence of sites on the BYS surface with a moderate Bi\O bond strength, which agrees with O2 TPD data (vide supra). Since Bi cations in oxides can exist in different oxidation states (2+, 3+, 5+), defect sites on the surface of BYS composed of anion vacancies and coordinatively unsaturated Bi cations are apparently able to provide transfer of electron density required for activation of O2 molecule. Analysis of kinetics of oxygen isothermal exchange in the static reactor revealed that for BYS so called 3rd type of exchange (with participation of two oxygen atoms of oxide [18]) dominates with a share of 95%. According to the mechanistic scheme of the oxygen isotope exchange by Muzykantov [23] considered earlier in details for analysis of the oxygen exchange on ionic/mixed ionic–electronic conductors [17,18,21], this means that for BYS the stage of O2 molecule
Fig. 3. O2 TPD spectra of La1 − xBixMnO3 samples with x (Bi) = 0.8 (1), 0.7 (2), 0.6 (3), 0.5 (4), 0.4 (5) and BiYSmO (6).
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Table 1 Specific surface area (S), the maximum rate of oxygen desorption (W) and amount of oxygen monolayers (θ) desorbed in TPD run for selected samples. Samples
T calc., °C S, m2/g θ, WO2, 1015 molec. monolayers O2/m2 s
Bi1.5Y0.3Sm0.2O2 La0.2Bi0.8MnO3 La0.3Bi0.7MnO3 50%La0.2Bi0.8MnO3 50%La0.2Bi0.8MnO3 70%La0.3Bi0.7MnO3 70%La0.3Bi0.7MnO3
700 700 700 700 800 700 800
+ + + +
50%BYS 50%BYS 30%BYS 30%BYS
8 4.8 5.5 4.1 1 5.1 0.5
0.4 11.8 6.9 5.3 16.4 5.3 40.9
0.3 50 22 16 52 10 85
dissociation with formation of weakly bound ZOads and strongly bound (O)s oxygen forms O2 + Zads + ( )s ⇔ ZOads + (O)s is the ratelimiting, while the stages of oxygen redistribution between different surface sites ZOads + ( )s ⇔ Zads + (O)s and incorporation into the subsurface vacancy (O)s + [ ]v ⇔ ( )s + [O]v proceed much faster. For Sr- or Ca-doped lanthanum manganite the share of the 3rd type of exchange is 0.4 [18], which agrees with well-known low bulk oxygen mobility in lanthanum manganite. For LBM perovskites with much higher bulk oxygen mobility, the share of the 3rd type of exchange increases up to 0.7. For LBM–BYS composites calcined at 700 °C, even in the lowtemperature range, specific rates of exchange coincide with those for LBM despite dilution by less active electrolyte (Fig. 4). After sintering at 800 °C, specific rate of exchange strongly increases exceeding by up to an order of magnitude the values for LSFN–GDC and LSM–ScCeSZ nanocomposites sintered at 900 °C [18–21]. This suggests that new active sites appear due to interaction between LBM and BYS domains leading to generation of defects, migration of Mn cations onto the surface of BYS domains (vide supra EDX data) and even variation of the phase composition. Since Mn was detected in the bulk of BYS domains, at least comparable concentration of Mn on their surface is expected. Since Mn4+ cation has a small radius, its preferential segregation in the surface layer is expected. Earlier, for LSM–ScCeSZ nanocomposite, enrichment of the surface by Mn with increasing the temperature of sintering was demonstrated [18]. For BYS, the dynamic extent of exchange characterizing the oxygen mobility in the temperature-programmed mode [17] is rather close to values earlier found for Gd-doped ceria — up to 15 monolayers at 700 °C [20,21] (Fig. 5). For these electrolytes, dynamic extent of exchange is controlled both by the surface reaction and bulk oxygen mobility [21]. For Bi-doped manganite, dynamic extent of exchange at 650 °C (up to 20 monolayers) greatly exceeds values for Ca, Sr-doped lanthanum manganites (1–2 monolayers [20,21]). With a due regard
for very close specific rates of exchange for lanthanum manganites either pure or doped by Ca, Sr or Bi (vide supra), this clearly demonstrates much higher bulk oxygen mobility in Bi-containing manganites, which can be in general assigned to a lower Bi\O bonding strength as compared with that for La–O, though effect of microstructure (presence of layered Bi–Mn oxides, etc., vide supra) can be important as well. For LBM–BYS composites even calcined at 700 °C, non-additive increase of dynamic degree of exchange is apparent, the effect being much stronger for sample sintered at 800 °C (Fig. 5). The dynamic extent of exchange achieved for the latter sample at 750 °C (350 monolayers) is close to that for dense LSFN–GDC nanocomposite sintered at 1200 °C possessing very high oxygen mobility [19–21]. SSITKA data for exchange of all studied samples with both C 18O2 (Fig. 6) and 18O2 (not shown for brevity) were well fitted by a model with the exchange controlled by the uniform bulk diffusion. As follows from Fig. 7a, the average Deff. values estimated by using both types of labeled molecules agree with each other. For LBM–BYS nanocomposite Deff. is by an order of magnitude higher than that for LBM, which agrees with results of O2 TPD and temperature-programmed isotope exchange studies (vide supra). As is seen from Fig. 7b, for LBM–BYS nanocomposite specific rates of O2 exchange R are as high as those for LBM, while specific rates of CO2 exchange are apparently higher for LBM–BYS nanocomposite agreeing with results obtained for O2 exchange in a static system. Though a detailed discussion of all these features related to the problem of estimation of both the rate of the surface reaction and oxygen diffusion coefficients from one set of the oxygen exchange data is beyond the scope of this paper and will be
Fig. 4. The temperature dependence of specific rates of oxygen heteroexchange for LBM and BiYSm oxides and their composites sintered at 700 °C or 800 °C. PO2 = 2.5 Torr. BiYSm (1), La0.3Bi0.7MnO3 (2), 30%La0.3Bi0.7MnO3 + 70%BYS (700 °C) (3), 50%La0.3Bi0.7MnO3 + 50%BYS (700 °C) (4), 30%La0.3Bi0.7MnO3 + 70%BYS (800 °C) (5).
Fig. 6. Variation of 18O (α) and C16O18O (X1) fractions during C18O2 exchange at 600 °C with 50%Bi1.5Y0.3Sm0.2O3 + 50%La0.8Bi0.2MnO3 calcined at 850 °C. Points — experiment, lines — fitting.
Fig. 5. Temperature dependence of the dynamic extent of exchange for BiYSm (1), La0.3Bi0.7MnO3 (2), 30%La0.3Bi0.7MnO3 + 70%BYS (700 °C) (3), 50%La0.3Bi0.7MnO3 + 50%BYS (700 °C) (4), 30%La0.3Bi0.7MnO3 + 70%BYS (800 °C) (5).
Please cite this article as: V. Sadykov, et al., Solid State Ion. (2013), http://dx.doi.org/10.1016/j.ssi.2013.03.016
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of the structure of interfaces determined both by redistribution of cations between coexisting phases and heteroepitaxy between perovskite–fluorite surface faces. While depletion of the surface layers of perovskite domains by A-site cations (La, Sr, etc.) due to their preferential transfer into the fluorites generates disordered oxygen layers (such as A1 − xO3 densely-packed plane in cubic perovskites) with a plenty of vacancies, their enrichment by transition metal cations with disordered coordination spheres helps to decrease the average metal–oxygen strength as well as to enhance the electronic conductivity due to clustering of transition metal cations. In frames of known models of oxygen diffusion in nanostructured perovskites [21,24,25], this rearrangement decreases the barrier for the oxygen ion/atom migration, thus explaining fast oxygen self-diffusion/chemical diffusion along these interfaces demonstrated in our research. 4. Conclusions Procedures based upon polymerized polyester precursor (Pechini) route and ultrasonic treatment of binary oxide mixture in isopropanol were successfully applied for synthesis of Bi-doped lanthanum manganites (LBM), Y–Sm-doped Bi oxide (BYS) and their composites with mixed ionic–electronic conductivity. Basic features of their structure and microstructure were elucidated by XRD and TEM revealing coexistence of several phases in manganites with a high Bi content as well as their chemical compatibility with BYS. In LBM, the lattice oxygen mobility was found to increase with Bi content. In LBM + BYS composites, the oxygen mobility increases with sintering temperature and strongly exceeds that of separate phases due to positive role of perovskite–fluorite interfaces. Acknowledgments Fig. 7. Linear plot of Deff (a) and R (b) temperature dependence derived from SSITKA 18O2 and C18O2 experiments.
The authors gratefully acknowledge support from OCMOL FP7 EC Project, Project 57 of RAS Presidium Program No. 27 and Federal Program “Scientific and Educational Cadres of Russia”.
given elsewhere, well-known more facile activation of CO2 molecules as compared with that of O2 apparently provides a higher sensitivity to the surface composition and defect structure of mixed oxides/nanocomposites.
References
3.2.2. Dense pellets For dense nanocomposite LBM–BYS pellets, weight relaxation studies also revealed a high lattice oxygen mobility and reactivity. Thus, for 70%La0.3Bi0.7MnOx + 30%Bi1.5Y0.3Sm0.2O3 nanocomposite, at 800 °C the value of the oxygen chemical diffusion coefficient Dchem. (~ 2 ∗ 10 − 5 cm 2 s − 1) exceeds those for LSFC + 30%GDC (10−5 cm2 s−1) and LSFN + GDC nanocomposites (4 ∗ 10−6 cm2 s−1) [19–21]. The chemical coefficient of oxygen exchange kchem. (~6 ∗ 10 −4 cm s −1) is close to that of LSFC or LSFN composites with GDC varying in the range of 10 −3–10−4 cm s−1 as dependent upon the GDC content [21,22]. Hence, dense ceramics composed of Bicontaining nanocomposites indeed provide high oxygen mobility required for oxygen separation membrane application. In general, results of this research demonstrated the positive effect of perovskite–fluorite interfaces on the bulk oxygen mobility earlier observed for other systems such as LSFN–GDC [14,19,21], ScCeSZ– LSM [18], LSFC–GDC [21] etc. provided preparation procedures are properly optimized to ensure developed interfaces and prevent formation of isolating pyrochlore-type phases. The most straightforward demonstration of the positive role of these interfaces was given for LSFN–GDC system where detailed analysis of SSITKA results allowed one to estimate oxygen self-diffusion coefficient along the perovskite– fluorite interface shown to be up to 6 orders of magnitude higher than that in GDC [14,19]. Apparently, this effect is explained by specificity
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