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Electrical properties and oxygen diffusion in yttria-stabilised zirconia (YSZ)–La0.8Sr0.2MnO3±δ (LSM) composites
Solid State Ionics 176 (2005) 937 – 943 www.elsevier.com/locate/ssi
Electrical properties and oxygen diffusion in yttria-stabilised zirconia (YSZ)–La0.8Sr0.2MnO3Fd (LSM) composites Y. Jia, J.A. Kilnera,*, M.F. Carolanb a Department of Materials, Imperial College London, SW7 2AZ, UK Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501, USA
b
Received 24 November 2003; received in revised form 26 November 2004; accepted 27 November 2004
Abstract Dense YSZ–LSM composites were fabricated for application as an oxygen separation membrane. A density about 95% was achieved for a sintering temperature of 1350 8C; however, an insulating La2Zr2O7 phase was formed during this high-temperature sintering. The percolation threshold was identified at about 30 wt.% (or 28 vol.%) of LSM. Isotopic exchange depth profiling (IEPD)/secondary ion mass spectrometry (SIMS) method was used to investigate the oxygen diffusion and surface exchange properties of the composites. The two composites, YSZ– 30 wt.%LSM and YSZ–40 wt.%LSM (or 38 vol.% LSM), with a 3-D interconnected network, had higher oxygen diffusivity than the pure LSM, but slightly lower than the parent YSZ material. The oxygen surface exchange coefficient of the YSZ was significantly enhanced by the introduction of the electronic conductor LSM, which confirmed the supposition that the availability of electrons limits the oxygen surface exchange reaction on the pure YSZ materials. D 2004 Elsevier B.V. All rights reserved. PACS: 66.30 (diffusion in solids); 68.35F (diffusion at solid surfaces and solid–solid interfaces); 82.65F (ion exchange surface processes); 82.80K (mass spectroscopy, chemical analysis); 72.60 (electrical conductivity, mixed conductivity) Keywords: Mixed conductor; Electrical conductivity; Oxygen diffusion; Oxygen surface exchange
1. Introduction Mixed conductors, which have both high ionic and electronic conductivity, have attracted a lot of attention in applications such as fuel cell electrodes, oxygen separation membranes, and membrane reactors for the partial oxidation of methane to syngas [1–3]. Both single-phase ceramic and dual-phase composites can be used to produce such a mixed conductor. The mixed conducting properties can easily be tailored for a composite having an oxide ionic conductor for one phase and a second phase with a high electronic conductivity. There have been some previous investigations on such dual phase mixed conducting materials, such as
* Corresponding author. Tel.: +44 20 75946733; fax: +44 20 75946736. E-mail address: [email protected] (J.A. Kilner). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.11.019
ZrO2–Mn2O3 [4], ZrO2–In2O3 [5], ZrO2–TiO2 [6], and CeO2–LSM composites [7]. For application in oxygen separators, it is desirable to maximise the flux of oxygen flowing through the ceramic. The most important factor for optimising this oxygen flux is the rate at which oxygen is incorporated into the oxide from the gas phase and subsequently diffuses through the oxide; however, the investigation of such properties on dual phase materials is rare. Conventional methods to study the oxygen diffusion and exchange are prone to errors, particularly for mixed conducting oxides with a high electronic conductivity [8]. A technique involving studies of oxygen isotope exchanged samples is being developed in our laboratories at Imperial College aimed at developing suitable materials and extending our fundamental understanding of the oxygen exchange process in single-phase and dual-phase materials. YSZ (yttria-stabilised zirconia) is a commonly used electrolyte material in solid electrochemical devices because
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of its high oxygen ionic conductivity and stability. For solid oxide fuel cells (SOFCs) based on YSZ, strontium-doped lanthanum magnate (LSM) is the most commonly used as a cathode material due to its high electronic conductivity, fast transfer of oxygen at gas/solid interfaces, combined with its thermal expansion and chemical compatibility with the yttria-stabilised zirconia solid electrolyte. In this study, the electronic conductor LSM was added to YSZ to produce a dense mixed conducting composite material for potential use as an oxygen separator. The oxygen diffusion and exchange properties of these composites were investigated by an isotopic exchange depth profiling (IEPD)/secondary ion mass spectrometry (SIMS) method.
2. Experimental 8 mol% Y2O3-doped ZrO2 (YSZ) (TOSOH, Japan, purity 99.9%) and La0.8Sr0.2MnO3Fd (LSM) (PRAXAIR Speciality Ceramics, Seattle, purity 99.9%) powders were used as starting materials. The surface area of the YSZ powder is 14.9 m2/g and crystallite size is 22 nm. The average particle size of the LSM powder is 2.2 Am and the surface area is 4.5 m2/g. YSZ and LSM powder with the desired amount of LSM in the final product were mixed in ethanol for 24 hours with ZrO2 milling media. The powder mixtures were then oven-dried at 70 8C and subsequently reground. After sieving through a 53-Am sieve, the powder blends were uniaxially pressed into pellets at a pressure of 75 MPa followed by isostatic pressing at 300 MPa. The green bodies were then sintered in air at different temperatures. The density of the composite was determined by the Archimedes’ method. Phase identification was performed using X-ray diffractometry (XRD) on a Philips PW1710 Xray diffractometer. The total conductivity of dense ceramic bar samples was measured as a function of temperature using a 4-probe D.C. technique. Each bar was ground to produce flat, parallel faces with the approximate dimensions 3220 mm. Four platinum wires were then attached along the length of the bar by making small notches in the bar. Sliver paste was baked on the wires to ensure good electrical contract. A Solatron 1186 Electrochemical interface was used to supply the two outmost wires with a continuous current. The specimens were maintained for at least 0.5 h at each temperature before taking measurement. The diffusion of oxygen ions was investigated by introducing 18O to the materials at high temperatures. SIMS line scanning was used for the YSZ and YSZ–LSM composites, whereas SIMS depth profiling was used for the LSM samples. Samples for 18O/16O exchange experiments were all ground flat and polished with successive grades of diamond down to 1/4 Am finish. Prior to isotopic exchange anneals, the samples were equilibrated in research grade oxygen (99.996%) at the required temperature and pressure for duration of 4–10 times of the exchange time to
ensure chemical equilibrium with the exchanging gas. After quenching the samples to room temperature, an 18O isotope enriched gas was then introduced and the samples annealed for the required time. For line scanning analysis, the 18O exchanged samples were cut perpendicular to the diffusion surface and the cross-sections were mechanically polished to 1/4 Am finish. The isotope concentration was measured by a secondary ion mass spectrometer (ATOMIKA 6500) using an 8 keV 132Xe+ beam. In all cases, the primary ion beam was at normal incidence to the sample. To prevent electrical charging of the pure YSZ samples, they were cobombarded with a 2 keV electron beam. For the LSM samples, the polished surfaces were used for depth profile analysis after 18O annealing. The intensities of 16O and 18O as a function of sputtering time from the central 300300 Am of the crater were recorded. The crater depth was then calibrated by interference microscopy (Zygo New View 200) after SIMS analysis. The solution of the diffusion equation for isotope tracer diffusion in a semi-infinite medium has been given by Crank [9]: C ð xÞ Cbg x CVð xÞ ¼ erf c pffiffiffiffiffiffiffiffi exp hx þ h2 DT t Cg Cbg 2 DT t pffiffiffiffiffiffiffiffi x erf c pffiffiffiffiffiffiffiffi þ h DT t ð1Þ 2 DT t where CV(x) is the normalised fraction of 18O, C(x) is the isotopic fraction as a function of depth x which is obtained from SIMS measurements, C bg is the natural background level of 18O in the sample at some distance below the sample surface (0.20%), C g is the fraction of 18O in the gas phase, and D T is the bulk tracer diffusion coefficient. The parameter h is given by h=k/D T, where k is the oxygen surface exchange coefficient. The values of D T (cm2/s) and k (cm/s) were fitting using a nonlinear least squares regression based upon Eq. (1). In tracer diffusion measurements, LSM exhibits short circuit diffusion and is manifested by a btailQ to the diffusion profile which extends to considerable distances. Mathematical treatment of the grain boundary diffusion problem represent the boundary by an isolated, planar, isotropic slab of width y, within which rapid diffusion occurs at a rate D*gb. The grain boundary diffusion product, D*gb y, is determined by [10] " # BlnC ð xÞ B 3=2 1=2 A T ¼ DT t 10 ð2Þ dDgb Bg6=5 where x g ¼ pffiffiffiffiffiffiffiffi DT t
ð3Þ
and A and B are parameters whose values depend on the experimental value of the tail slope, BlnC(x)/Bg 6/5, between 6VgV10. Details of A and B values can be obtained from Ref. [10].
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3. Results 3.1. Densification behaviour Sufficient density is a pre-requisite for ceramic membranes to be used as oxygen separators as the diffusion of molecular species across the membrane is undesirable. Therefore, there should be no interconnected porosity in the material. The densification behaviour of YSZ–LSM composites was studied by sintering at different temperatures and times in air. The theoretical density of YSZ and LSM was calculated as 6.04 g/cm3 and 6.57 g/cm3, respectively, using the lattice parameters obtained from by XRD. As shown in Fig. 1, at 1350 8C all the composite ceramics reached their highest densities and were about 95% theoretical density. Thus all the composites and the LSM samples for conductivity and oxygen diffusion measurement were consolidated at this temperature. The YSZ sample was sintered at 1500 8C in order to achieve full density. As expected, the sintered YSZ was a cubic phase material and the pure LSM was a single-phase perovskite with rhombohedral perovskite structure as determined by XRD. Apart from c-ZrO2 and LSM, La2Zr2O7 (LZO) was identified in all the composites made in this study and the amount increases with increasing LSM content in the composites. It is also noted that all the peaks from c-ZrO2 were moved to lower angles. This probably indicates that La dissolved into the ZrO2 lattice as La3+ is a larger cation compared to Zr4+ which causes a lattice expansion in YSZ. The formation of more insulating LZO phase may be undesirable in terms of the electrical conductivity and oxygen transport properties. 3.2. Electrical conductivity The electrical conductivity of the YSZ, LSM, and YSZ– LSM composites was measured by a 4-probe DC method
Fig. 2. Arrhenius plot of the dc electrical conductivity in air of the YSZ– LSM composites.
and the results are shown in Fig. 2. The conductivity of all samples studied here follows Arrhenius behaviour, i.e. the data obey the equation Ea rT ¼ Aexp ð4Þ kT Therefore, the activation energy E a was calculated using the slopes of the lines. The fit of data to Eq. (4) is generally good and the activation energies are plotted in Fig. 3. It is known that YSZ is a pure ionic conductor in the normal temperature and oxygen partial pressure range. The activation energy of 86 kJ/mol is in agreement with literature values [4,11]. LSM is an electronic conductor and the activation energy is expected to be much lower. Adding up to 20 wt.% LSM to YSZ the composites followed the electrical behaviour of the more insulating phase, i.e. YSZ.
98 120
Activation energy (kJ/mol)
Density (%th)
96
94
92 LSM YSZ-10wt%LSM YSZ-20wt%LSM
90
88 1200
100 80 60 40 20 0 0
1300
1400
1500
10
20
30
40 50 60 70 LSM content (wt%)
80
90
100
Temperature (oC) Fig. 1. Densification behaviour of the YSZ–LSM composites.
Fig. 3. Activation energy of the electrical conductivity of the YSZ–LSM composites.
Y. Ji et al. / Solid State Ionics 176 (2005) 937–943
(a)
0.0035
1.0E-07
log(DT/cm-2s-1)
Normalised18O concentration
940
0.0025 raw data fitted data 0.0015
0.0005
1.0E-09 YSZ YSZ-30%LSM YSZ-40%LSM LSM
1.0E-11
1.0E-13
-0.0005 0
0.02
0.04
0.08
0.06
1.0E-15 7.5
0.1
8
Depth (cm) 1.0
Normalised18O concentration
8.5
9
9.5
104/T (K-1) Fig. 5. Arrhenius plot of D T for the YSZ, LSM, YSZ–30 wt.%LSM, and YSZ–40 wt.%LSM composite.
(b) 0.8 raw data fitted data
0.6
the effect of the insulating La2Zr2O7 phase. When the LSM content is higher than 30 wt.% in the final product, the composites closely followed the behaviour of the pure LSM; in fact, the data for the activation energy of these composites are comparable to the LSM. This suggests that, at 30% LSM, the composite shifted from an ionic conductor to an electronic conductor and a 3-dimensional interconnecting LSM network is formed.
0.4
0.2
0.0 0.0
0.2
0.4
0.6
0.8
Depth (µm) 0.2
Typical 18O diffusion profiles obtained from the YSZ, LSM and YSZ-LSM composites exchanged at 1000 8C are shown in Fig. 4(a), (b) and (c), respectively. The nonlinear regression fits are also presented in the same figures. The results of D T and k as a function of inverse
(c) 0.15 raw data fitted data 0.1
1.0E-05 YSZ YSZ-30%LSM YSZ-40%LSM LSM
0.05 1.0E-06
0 0
0.01
0.02
0.03
Depth (cm)
log(k/cms-1)
Normalised18O concentration
3.3. Oxygen diffusion and surface exchange of YSZ, LSM, and the YSZ–LSM composites
1.0E-07
1.0E-08 Fig. 4. 18O diffusion profile at 1000 8C for the (a) YSZ, (b) LSM, and (c) YSZ–30 wt.%LSM composites, together with the fitted curves using Eq. (1).
The activation energy is 109 and 113 kJ/mol for YSZ with 10 wt.% and 20 wt.% LSM, respectively. These values are much higher than that of the pure YSZ and may represent
1.0E-09 7.5
8
8.5
9
9.5
104/T (K-1) Fig. 6. Arrhenius plot of k for the YSZ, LSM, YSZ–30 wt.%LSM, and YSZ–40 wt.%LSM composite.
Y. Ji et al. / Solid State Ionics 176 (2005) 937–943
log(DT/cm-2s-1)
1.E-07
1.E-09
1.E-11
800C 900C 1000C
1.E-13
1.E-15 0
20
40
60
80
100
LSM content (%) Fig. 7. Measured effective oxygen diffusion coefficient of the YSZ–LSM composites as a function of the LSM content.
1.E-06
log(k/cms-1)
temperature are shown in Figs. 5 and 6. For the YSZ, the fitted curve based on Eq. (1) agrees with the experimental data and the results are in general agreement with literature [12]. It is obvious that the 18O surface concentration is low compared to that of the annealing atmosphere and the diffusion length is long (Fig. 4(a)). This indicates a relatively low surface exchange rate between the gas phase and the bulk material. The 18O diffusion profile of the pure LSM is obtained by SIMS depth profiling (Fig. 4(b)). The 18O concentration near the sample surface approaches the isotopic concentration in the gas phase. This suggests that the isotope exchange on the surface is relatively fast compared to the bulk diffusion, and the sample surface achieves isotopic equilibrium with the gas phase. This high surface concentration presents a problem for the accurate determination of k value for this sample. Eq. (1) does not completely describe these conditions, thus the fit of the data points to the curve is insufficient. Bulk diffusion is thought to be a rate-limiting step in the oxygen transport in LSM. It is clear that grain boundary diffusion is active as characterised by a dtailT to the diffusion profile. It should note that the D T values presented in Fig. 5 refer to the LSM bulk transport and not the grain boundary diffusivity. A good fit to the experimental data was achieved at the depths where the bulk diffusion was dominant. These values are in good agreement with data on La0.8Sr0.2MnO3 by De Souza [13]. The oxygen exchange measurements were conducted only on the YSZ–30 wt.%LSM and YSZ–40 wt.%LSM composites where percolation has occurred. There was no grain boundary diffusion found in these two composites as shown in Fig. 4(c). The 18O surface concentrations and the diffusion depth of the composite are intermediate between those for YSZ and LSM. The effective tracer diffusion coefficients of the two composites were much higher than the values of the LSM material, whereas they were slightly lower than that of the YSZ at the same temperature (see Fig.
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1.E-07
1.E-08 800C 900C 1000C 1.E-09 0
20
40
60
80
100
LSM content (%) Fig. 8. Measured effective surface oxygen exchange coefficient of the YSZ–LSM composites as a function of the LSM content.
7). Interestingly both the two composites showed an enhanced effective surface exchange coefficient i.e. greater than either of the parent materials (see Fig. 8).
4. Discussion 4.1. Electrical conductivity The formation of La2Zr2O7 (LZO) was identified by XRD in the YSZ–LSM composites sintered at 1350 8C for 10 h. There have been some previous reports on the reaction between YSZ and LSM [14–17]. The amount of LZO was found to increase with increasing reaction time and sintering temperature [14]. Taimatu et al. [16] attributed the formation of LZO to the faster diffusion of Mn ions than La ions into YSZ. Mori et al. [17] suggested that it was caused by the different solubility of La and Mn in YSZ. Although there is no unanimity on the formation of such a phase, the much lower ion conductivity of LZO (104 S/cm at 1000 8C) than that of 8YSZ (101 S/cm at 1000 8C) is undesirable [18]. It is also catalytically inactive as well. The detrimental effect of LZO formation on the electrical conductivity was clearly shown by the YSZ-LSM composites with low content of LSM. For the composites with 10 wt.% and 20 wt.% LSM, LSM particles surrounded by a LZO layer act as discrete phases. They would block ionic transport and thus behave as an insulator for the conduction of oxygen ions. Consequently the conductivity of the composites decreased because of this insulating phase and the activation energy is increased. With increasing LSM content to 30 wt.% in the composite, an interpenetrating network starts to form. The detrimental effect of LZO on the conductivity is overcome by the percolating 3-dimensional network, as ionic and electronic pathways are provided, respectively. The conductivity of YSZ–LSM with 30 wt.% and 40 wt.% LSM follows a similar behaviour to pure LSM. Therefore, in this region, the conductivity is mostly determined by the highly conducting LSM phase, as it is also shown by the activation
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energy data with nearly constant values. A recent study on the electrical conductivity of La0.65Sr0.3MnO3–8 mol% YSZ showed a similar behaviour [19]. It was found that the percolation threshold is above 20 vol.% LSM in the composites. 4.2. Ionic transport The driving force for oxygen transport in a mixed conductor is a gradient in oxygen activity applied across the ceramic membrane. At the high oxygen pressure side, the gaseous oxygen molecules interact with the electron at dactiveT sites on the surface, are ionised and incorporated into the oxide. The oxide ions then diffuse through the material to the oxygen lean side where the reverse process occurs. It is well known that the diffusion of oxygen ions through YSZ and the perovskite structure is via a vacancy mechanism. Therefore the concentration of vacancies plays an important role in the diffusivity of oxygen. YSZ is a good oxygen ionic conductor. The oxygen vacancies are introduced to the zirconia lattice by doping with yttrium, as shown below using Krfger–Vink notation ð5Þ The addition of substantial amounts of dopant (typical 8– 10 mol%) also has the effect of stabilising the cubic phase over the entire temperature range. According to the defect model of Nowotny and Rekas [20], the concentration of oxygen vacancies in the doped lanthanum manganites is determined by the following reaction x ˙ Y V˙˙ Oox þ 2MnMn O þ 2MnMn þ 1=2O2
ð6Þ
x where Mn˙Mn and Mn Mn are Mn 4+ and Mn 3+ ions, respectively. Berenov [21] calculated the vacancy concentration in La0.8Sr0.2MnO3 using isotopic diffusion coefficient data. It was found that oxygen ion transport through this material is negligible and LSM is thus considered as virtually a pure electronic conductor. As both the ionic conductivity of LSM and the electronic conductivity of YSZ for the temperature range and compositions considered here are low, the connectivity of the two phases is crucial to produce a mixed conducting composite. This is determined by particle size ratio and volume fraction [18]. In this study, the interconnection started to form when 30 wt.% LSM is added to YSZ. Below that point, LSM particles would block the conduction of oxygen ions. Thus the oxygen diffusivity of the composite is expected to initially decrease as LSM is added to YSZ. For practical application, the oxygen transport properties were only investigated on the YSZ–30 wt.%LSM and YSZ–40 wt.%LSM. It is seen from Fig. 6 that the pure YSZ showed the highest diffusivities, whereas the LSM exhibited the lowest values in the temperature range studied. Adding LSM to the YSZ produced slightly
lower diffusivities than that of the YSZ as discussed above; however, these values are improved greatly compared with LSM. It thus seems clear that the effective oxygen diffusion properties of the composite materials are determined simply by the volume fraction and connectivity of the YSZ phase. The oxygen ion conductivities, r ion, can be calculated from diffusion data with the Nernst–Einstein equation, which relates to tracer diffusivity, D T, by rion ¼
DT N q2ion f kT
ð7Þ
where N is the concentration of anion sites, q ion is the charge of anion and f, the correlation factor, is assumed to be equal to 0.69. A comparison of the calculated ionic conductivity and the measured total conductivity at 900 8C is given in Table 1 together with the calculated transport number. It can be seen that for LSM r ion is very much lower than the r total. In contrast, the YSZ–LSM composites exhibit much higher ionic conductivities and consequently the transport numbers of the composites are improved compared to single-phased LSM. 4.3. Surface exchange In contrast to the behaviour of the diffusion coefficient the measured effective surface exchange coefficients showed an unexpected behaviour. Adding LSM to the YSZ produced a significant increase in the surface exchange coefficient and the two composites, YSZ–30 wt.%LSM and YSZ–40 wt.%LSM, showed values higher than the two individual end-members as shown in Fig. 8. The implications of these findings are discussed below. The surface exchange reaction is commonly described by an equation of the type 1=2O2 þ V˙˙O þ 2eVYOOx
ð8Þ
Manning et al. [22] investigated the oxygen diffusion and surface exchange in YSZ and suggested that surface reaction is limited by the availability of free electronic species. De Souza [23] further proposed different surface exchange processes for two extreme cases, i.e. pervoskite type LaTMO3 (TM=Co, Fe, Mn, Ni ect) compounds with high electronic concentration and fluorite oxides with high ionic conductivity. It was suggested that, for LaTMO3, the vacancy concentration limits the rate of surface exchange. In the fluorite oxides with low electron concentration, only the few surface vacancies that have trapped electrons are able to Table 1 Comparison of calculated ionic conductivity of YSZ–LSM composites with measured total conductivity at 900 8C YSZ–30wt.%LSM YSZ–40wt.%LSM LSM
r ion (S/cm)
r total (S/cm)
t ion
5e-3 3e-3 4e-8
0.3 26 200
2e-2 1e-4 2e-10
Y. Ji et al. / Solid State Ionics 176 (2005) 937–943
participate in surface exchange. Therefore, the concentration of electrons limits the kinetics of the exchange process. Both vacancies and electrons need to be taken into account in the surface exchange process. The data on the pure YSZ and LSM in this study indicated that these two materials have markedly different diffusivity and surface exchange kinetics. As shown in Fig. 4(a), a very low concentration of 18O on the YSZ surface is observed with a long diffusion length. For the LSM, the 18O surface concentration is close to the concentration in the gas phase (Fig. 4(b)) with a very shallow diffusion length (b1 micrometer). The diffusion length and 18O surface concentration of the two composites fall in between YSZ and LSM (Fig. 4(c)). The issues raised by this work are very interesting, however; unfortunately yet there is no other published data with which to make a direct comparison. What can be done is to compare this data with the electrochemical measurements of composite electrodes. There have been a considerable number of studies on YSZ/LSM composite electrodes applied to the SOFCs. Composites have been fabricated in order to improve the electrochemical performance by introducing both electrons and surface vacancies [9,24,25]. The composites are more electrochemically active than the pure LSM and would support the finding that the extended three phase boundaries are responsible for the enhanced effective exchange coefficient found in this work. If this analysis is correct, then this would imply that the three-phase boundaries on the surface of the composite materials should be highly active for the exchange of oxygen. Hence adding LSM to YSZ should increase the effective surface exchange coefficient, measured by these experiments, as is observed. However, one possible explanation of this finding must be included as a note of caution. The k values for LSM could be in error due to subtle experimental problems in the accurate determination of the surface 18O concentration when the surface concentration approaches that in the gas phase implying the k value for LSM is much higher than previously thought. Work is currently underway to check this possibility including high spatial resolution SIMS measurements of composite materials to deduce the distribution of 18O in the component oxides of the composite. This work will be reported in a subsequent publication.
5. Conclusions YSZ–LSM composites can be sintered to high density (above 95%) at a temperature of 1350 8C. The formation of the insulating La2Zr2O7-phase during high temperature consolidation is detrimental in terms of electrical properties. A 3-dimensional interpenetrating microstructure was formed by adding about 30 wt.% (or 28 vol.%) LSM to the YSZ. The IEPD/SIMS method was used to characterise the
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effective oxygen transport in the YSZ, LSM, YSZ–30 wt.%LSM, and YSZ–40 wt.%LSM composites. The two composites showed higher oxygen diffusivity than the pure LSM, but slightly lower than the parent YSZ. In contrast, the effective oxygen surface exchange coefficients of the two composites are apparently enhanced over that of the two parent materials. It is suggested that the availability of electronic species limits the kinetics of the exchange process on the YSZ surface and the oxygen exchange properties can be improved by adding the electronic conductor LSM to YSZ to form an YSZ–LSM composite.
Acknowledgement The authors would like to thank Air Product and Chemicals for the funding for this work.
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Electrical properties and oxygen diffusion in yttria-stabilised zirconia (YSZ)–La0.8Sr0.2MnO3Fd (LSM) composites Y. Jia, J.A. Kilnera,*, M.F. Carolanb a Department of Materials, Imperial College London, SW7 2AZ, UK Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501, USA
b
Received 24 November 2003; received in revised form 26 November 2004; accepted 27 November 2004
Abstract Dense YSZ–LSM composites were fabricated for application as an oxygen separation membrane. A density about 95% was achieved for a sintering temperature of 1350 8C; however, an insulating La2Zr2O7 phase was formed during this high-temperature sintering. The percolation threshold was identified at about 30 wt.% (or 28 vol.%) of LSM. Isotopic exchange depth profiling (IEPD)/secondary ion mass spectrometry (SIMS) method was used to investigate the oxygen diffusion and surface exchange properties of the composites. The two composites, YSZ– 30 wt.%LSM and YSZ–40 wt.%LSM (or 38 vol.% LSM), with a 3-D interconnected network, had higher oxygen diffusivity than the pure LSM, but slightly lower than the parent YSZ material. The oxygen surface exchange coefficient of the YSZ was significantly enhanced by the introduction of the electronic conductor LSM, which confirmed the supposition that the availability of electrons limits the oxygen surface exchange reaction on the pure YSZ materials. D 2004 Elsevier B.V. All rights reserved. PACS: 66.30 (diffusion in solids); 68.35F (diffusion at solid surfaces and solid–solid interfaces); 82.65F (ion exchange surface processes); 82.80K (mass spectroscopy, chemical analysis); 72.60 (electrical conductivity, mixed conductivity) Keywords: Mixed conductor; Electrical conductivity; Oxygen diffusion; Oxygen surface exchange
1. Introduction Mixed conductors, which have both high ionic and electronic conductivity, have attracted a lot of attention in applications such as fuel cell electrodes, oxygen separation membranes, and membrane reactors for the partial oxidation of methane to syngas [1–3]. Both single-phase ceramic and dual-phase composites can be used to produce such a mixed conductor. The mixed conducting properties can easily be tailored for a composite having an oxide ionic conductor for one phase and a second phase with a high electronic conductivity. There have been some previous investigations on such dual phase mixed conducting materials, such as
* Corresponding author. Tel.: +44 20 75946733; fax: +44 20 75946736. E-mail address: [email protected] (J.A. Kilner). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.11.019
ZrO2–Mn2O3 [4], ZrO2–In2O3 [5], ZrO2–TiO2 [6], and CeO2–LSM composites [7]. For application in oxygen separators, it is desirable to maximise the flux of oxygen flowing through the ceramic. The most important factor for optimising this oxygen flux is the rate at which oxygen is incorporated into the oxide from the gas phase and subsequently diffuses through the oxide; however, the investigation of such properties on dual phase materials is rare. Conventional methods to study the oxygen diffusion and exchange are prone to errors, particularly for mixed conducting oxides with a high electronic conductivity [8]. A technique involving studies of oxygen isotope exchanged samples is being developed in our laboratories at Imperial College aimed at developing suitable materials and extending our fundamental understanding of the oxygen exchange process in single-phase and dual-phase materials. YSZ (yttria-stabilised zirconia) is a commonly used electrolyte material in solid electrochemical devices because
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of its high oxygen ionic conductivity and stability. For solid oxide fuel cells (SOFCs) based on YSZ, strontium-doped lanthanum magnate (LSM) is the most commonly used as a cathode material due to its high electronic conductivity, fast transfer of oxygen at gas/solid interfaces, combined with its thermal expansion and chemical compatibility with the yttria-stabilised zirconia solid electrolyte. In this study, the electronic conductor LSM was added to YSZ to produce a dense mixed conducting composite material for potential use as an oxygen separator. The oxygen diffusion and exchange properties of these composites were investigated by an isotopic exchange depth profiling (IEPD)/secondary ion mass spectrometry (SIMS) method.
2. Experimental 8 mol% Y2O3-doped ZrO2 (YSZ) (TOSOH, Japan, purity 99.9%) and La0.8Sr0.2MnO3Fd (LSM) (PRAXAIR Speciality Ceramics, Seattle, purity 99.9%) powders were used as starting materials. The surface area of the YSZ powder is 14.9 m2/g and crystallite size is 22 nm. The average particle size of the LSM powder is 2.2 Am and the surface area is 4.5 m2/g. YSZ and LSM powder with the desired amount of LSM in the final product were mixed in ethanol for 24 hours with ZrO2 milling media. The powder mixtures were then oven-dried at 70 8C and subsequently reground. After sieving through a 53-Am sieve, the powder blends were uniaxially pressed into pellets at a pressure of 75 MPa followed by isostatic pressing at 300 MPa. The green bodies were then sintered in air at different temperatures. The density of the composite was determined by the Archimedes’ method. Phase identification was performed using X-ray diffractometry (XRD) on a Philips PW1710 Xray diffractometer. The total conductivity of dense ceramic bar samples was measured as a function of temperature using a 4-probe D.C. technique. Each bar was ground to produce flat, parallel faces with the approximate dimensions 3220 mm. Four platinum wires were then attached along the length of the bar by making small notches in the bar. Sliver paste was baked on the wires to ensure good electrical contract. A Solatron 1186 Electrochemical interface was used to supply the two outmost wires with a continuous current. The specimens were maintained for at least 0.5 h at each temperature before taking measurement. The diffusion of oxygen ions was investigated by introducing 18O to the materials at high temperatures. SIMS line scanning was used for the YSZ and YSZ–LSM composites, whereas SIMS depth profiling was used for the LSM samples. Samples for 18O/16O exchange experiments were all ground flat and polished with successive grades of diamond down to 1/4 Am finish. Prior to isotopic exchange anneals, the samples were equilibrated in research grade oxygen (99.996%) at the required temperature and pressure for duration of 4–10 times of the exchange time to
ensure chemical equilibrium with the exchanging gas. After quenching the samples to room temperature, an 18O isotope enriched gas was then introduced and the samples annealed for the required time. For line scanning analysis, the 18O exchanged samples were cut perpendicular to the diffusion surface and the cross-sections were mechanically polished to 1/4 Am finish. The isotope concentration was measured by a secondary ion mass spectrometer (ATOMIKA 6500) using an 8 keV 132Xe+ beam. In all cases, the primary ion beam was at normal incidence to the sample. To prevent electrical charging of the pure YSZ samples, they were cobombarded with a 2 keV electron beam. For the LSM samples, the polished surfaces were used for depth profile analysis after 18O annealing. The intensities of 16O and 18O as a function of sputtering time from the central 300300 Am of the crater were recorded. The crater depth was then calibrated by interference microscopy (Zygo New View 200) after SIMS analysis. The solution of the diffusion equation for isotope tracer diffusion in a semi-infinite medium has been given by Crank [9]: C ð xÞ Cbg x CVð xÞ ¼ erf c pffiffiffiffiffiffiffiffi exp hx þ h2 DT t Cg Cbg 2 DT t pffiffiffiffiffiffiffiffi x erf c pffiffiffiffiffiffiffiffi þ h DT t ð1Þ 2 DT t where CV(x) is the normalised fraction of 18O, C(x) is the isotopic fraction as a function of depth x which is obtained from SIMS measurements, C bg is the natural background level of 18O in the sample at some distance below the sample surface (0.20%), C g is the fraction of 18O in the gas phase, and D T is the bulk tracer diffusion coefficient. The parameter h is given by h=k/D T, where k is the oxygen surface exchange coefficient. The values of D T (cm2/s) and k (cm/s) were fitting using a nonlinear least squares regression based upon Eq. (1). In tracer diffusion measurements, LSM exhibits short circuit diffusion and is manifested by a btailQ to the diffusion profile which extends to considerable distances. Mathematical treatment of the grain boundary diffusion problem represent the boundary by an isolated, planar, isotropic slab of width y, within which rapid diffusion occurs at a rate D*gb. The grain boundary diffusion product, D*gb y, is determined by [10] " # BlnC ð xÞ B 3=2 1=2 A T ¼ DT t 10 ð2Þ dDgb Bg6=5 where x g ¼ pffiffiffiffiffiffiffiffi DT t
ð3Þ
and A and B are parameters whose values depend on the experimental value of the tail slope, BlnC(x)/Bg 6/5, between 6VgV10. Details of A and B values can be obtained from Ref. [10].
Y. Ji et al. / Solid State Ionics 176 (2005) 937–943
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3. Results 3.1. Densification behaviour Sufficient density is a pre-requisite for ceramic membranes to be used as oxygen separators as the diffusion of molecular species across the membrane is undesirable. Therefore, there should be no interconnected porosity in the material. The densification behaviour of YSZ–LSM composites was studied by sintering at different temperatures and times in air. The theoretical density of YSZ and LSM was calculated as 6.04 g/cm3 and 6.57 g/cm3, respectively, using the lattice parameters obtained from by XRD. As shown in Fig. 1, at 1350 8C all the composite ceramics reached their highest densities and were about 95% theoretical density. Thus all the composites and the LSM samples for conductivity and oxygen diffusion measurement were consolidated at this temperature. The YSZ sample was sintered at 1500 8C in order to achieve full density. As expected, the sintered YSZ was a cubic phase material and the pure LSM was a single-phase perovskite with rhombohedral perovskite structure as determined by XRD. Apart from c-ZrO2 and LSM, La2Zr2O7 (LZO) was identified in all the composites made in this study and the amount increases with increasing LSM content in the composites. It is also noted that all the peaks from c-ZrO2 were moved to lower angles. This probably indicates that La dissolved into the ZrO2 lattice as La3+ is a larger cation compared to Zr4+ which causes a lattice expansion in YSZ. The formation of more insulating LZO phase may be undesirable in terms of the electrical conductivity and oxygen transport properties. 3.2. Electrical conductivity The electrical conductivity of the YSZ, LSM, and YSZ– LSM composites was measured by a 4-probe DC method
Fig. 2. Arrhenius plot of the dc electrical conductivity in air of the YSZ– LSM composites.
and the results are shown in Fig. 2. The conductivity of all samples studied here follows Arrhenius behaviour, i.e. the data obey the equation Ea rT ¼ Aexp ð4Þ kT Therefore, the activation energy E a was calculated using the slopes of the lines. The fit of data to Eq. (4) is generally good and the activation energies are plotted in Fig. 3. It is known that YSZ is a pure ionic conductor in the normal temperature and oxygen partial pressure range. The activation energy of 86 kJ/mol is in agreement with literature values [4,11]. LSM is an electronic conductor and the activation energy is expected to be much lower. Adding up to 20 wt.% LSM to YSZ the composites followed the electrical behaviour of the more insulating phase, i.e. YSZ.
98 120
Activation energy (kJ/mol)
Density (%th)
96
94
92 LSM YSZ-10wt%LSM YSZ-20wt%LSM
90
88 1200
100 80 60 40 20 0 0
1300
1400
1500
10
20
30
40 50 60 70 LSM content (wt%)
80
90
100
Temperature (oC) Fig. 1. Densification behaviour of the YSZ–LSM composites.
Fig. 3. Activation energy of the electrical conductivity of the YSZ–LSM composites.
Y. Ji et al. / Solid State Ionics 176 (2005) 937–943
(a)
0.0035
1.0E-07
log(DT/cm-2s-1)
Normalised18O concentration
940
0.0025 raw data fitted data 0.0015
0.0005
1.0E-09 YSZ YSZ-30%LSM YSZ-40%LSM LSM
1.0E-11
1.0E-13
-0.0005 0
0.02
0.04
0.08
0.06
1.0E-15 7.5
0.1
8
Depth (cm) 1.0
Normalised18O concentration
8.5
9
9.5
104/T (K-1) Fig. 5. Arrhenius plot of D T for the YSZ, LSM, YSZ–30 wt.%LSM, and YSZ–40 wt.%LSM composite.
(b) 0.8 raw data fitted data
0.6
the effect of the insulating La2Zr2O7 phase. When the LSM content is higher than 30 wt.% in the final product, the composites closely followed the behaviour of the pure LSM; in fact, the data for the activation energy of these composites are comparable to the LSM. This suggests that, at 30% LSM, the composite shifted from an ionic conductor to an electronic conductor and a 3-dimensional interconnecting LSM network is formed.
0.4
0.2
0.0 0.0
0.2
0.4
0.6
0.8
Depth (µm) 0.2
Typical 18O diffusion profiles obtained from the YSZ, LSM and YSZ-LSM composites exchanged at 1000 8C are shown in Fig. 4(a), (b) and (c), respectively. The nonlinear regression fits are also presented in the same figures. The results of D T and k as a function of inverse
(c) 0.15 raw data fitted data 0.1
1.0E-05 YSZ YSZ-30%LSM YSZ-40%LSM LSM
0.05 1.0E-06
0 0
0.01
0.02
0.03
Depth (cm)
log(k/cms-1)
Normalised18O concentration
3.3. Oxygen diffusion and surface exchange of YSZ, LSM, and the YSZ–LSM composites
1.0E-07
1.0E-08 Fig. 4. 18O diffusion profile at 1000 8C for the (a) YSZ, (b) LSM, and (c) YSZ–30 wt.%LSM composites, together with the fitted curves using Eq. (1).
The activation energy is 109 and 113 kJ/mol for YSZ with 10 wt.% and 20 wt.% LSM, respectively. These values are much higher than that of the pure YSZ and may represent
1.0E-09 7.5
8
8.5
9
9.5
104/T (K-1) Fig. 6. Arrhenius plot of k for the YSZ, LSM, YSZ–30 wt.%LSM, and YSZ–40 wt.%LSM composite.
Y. Ji et al. / Solid State Ionics 176 (2005) 937–943
log(DT/cm-2s-1)
1.E-07
1.E-09
1.E-11
800C 900C 1000C
1.E-13
1.E-15 0
20
40
60
80
100
LSM content (%) Fig. 7. Measured effective oxygen diffusion coefficient of the YSZ–LSM composites as a function of the LSM content.
1.E-06
log(k/cms-1)
temperature are shown in Figs. 5 and 6. For the YSZ, the fitted curve based on Eq. (1) agrees with the experimental data and the results are in general agreement with literature [12]. It is obvious that the 18O surface concentration is low compared to that of the annealing atmosphere and the diffusion length is long (Fig. 4(a)). This indicates a relatively low surface exchange rate between the gas phase and the bulk material. The 18O diffusion profile of the pure LSM is obtained by SIMS depth profiling (Fig. 4(b)). The 18O concentration near the sample surface approaches the isotopic concentration in the gas phase. This suggests that the isotope exchange on the surface is relatively fast compared to the bulk diffusion, and the sample surface achieves isotopic equilibrium with the gas phase. This high surface concentration presents a problem for the accurate determination of k value for this sample. Eq. (1) does not completely describe these conditions, thus the fit of the data points to the curve is insufficient. Bulk diffusion is thought to be a rate-limiting step in the oxygen transport in LSM. It is clear that grain boundary diffusion is active as characterised by a dtailT to the diffusion profile. It should note that the D T values presented in Fig. 5 refer to the LSM bulk transport and not the grain boundary diffusivity. A good fit to the experimental data was achieved at the depths where the bulk diffusion was dominant. These values are in good agreement with data on La0.8Sr0.2MnO3 by De Souza [13]. The oxygen exchange measurements were conducted only on the YSZ–30 wt.%LSM and YSZ–40 wt.%LSM composites where percolation has occurred. There was no grain boundary diffusion found in these two composites as shown in Fig. 4(c). The 18O surface concentrations and the diffusion depth of the composite are intermediate between those for YSZ and LSM. The effective tracer diffusion coefficients of the two composites were much higher than the values of the LSM material, whereas they were slightly lower than that of the YSZ at the same temperature (see Fig.
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1.E-07
1.E-08 800C 900C 1000C 1.E-09 0
20
40
60
80
100
LSM content (%) Fig. 8. Measured effective surface oxygen exchange coefficient of the YSZ–LSM composites as a function of the LSM content.
7). Interestingly both the two composites showed an enhanced effective surface exchange coefficient i.e. greater than either of the parent materials (see Fig. 8).
4. Discussion 4.1. Electrical conductivity The formation of La2Zr2O7 (LZO) was identified by XRD in the YSZ–LSM composites sintered at 1350 8C for 10 h. There have been some previous reports on the reaction between YSZ and LSM [14–17]. The amount of LZO was found to increase with increasing reaction time and sintering temperature [14]. Taimatu et al. [16] attributed the formation of LZO to the faster diffusion of Mn ions than La ions into YSZ. Mori et al. [17] suggested that it was caused by the different solubility of La and Mn in YSZ. Although there is no unanimity on the formation of such a phase, the much lower ion conductivity of LZO (104 S/cm at 1000 8C) than that of 8YSZ (101 S/cm at 1000 8C) is undesirable [18]. It is also catalytically inactive as well. The detrimental effect of LZO formation on the electrical conductivity was clearly shown by the YSZ-LSM composites with low content of LSM. For the composites with 10 wt.% and 20 wt.% LSM, LSM particles surrounded by a LZO layer act as discrete phases. They would block ionic transport and thus behave as an insulator for the conduction of oxygen ions. Consequently the conductivity of the composites decreased because of this insulating phase and the activation energy is increased. With increasing LSM content to 30 wt.% in the composite, an interpenetrating network starts to form. The detrimental effect of LZO on the conductivity is overcome by the percolating 3-dimensional network, as ionic and electronic pathways are provided, respectively. The conductivity of YSZ–LSM with 30 wt.% and 40 wt.% LSM follows a similar behaviour to pure LSM. Therefore, in this region, the conductivity is mostly determined by the highly conducting LSM phase, as it is also shown by the activation
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energy data with nearly constant values. A recent study on the electrical conductivity of La0.65Sr0.3MnO3–8 mol% YSZ showed a similar behaviour [19]. It was found that the percolation threshold is above 20 vol.% LSM in the composites. 4.2. Ionic transport The driving force for oxygen transport in a mixed conductor is a gradient in oxygen activity applied across the ceramic membrane. At the high oxygen pressure side, the gaseous oxygen molecules interact with the electron at dactiveT sites on the surface, are ionised and incorporated into the oxide. The oxide ions then diffuse through the material to the oxygen lean side where the reverse process occurs. It is well known that the diffusion of oxygen ions through YSZ and the perovskite structure is via a vacancy mechanism. Therefore the concentration of vacancies plays an important role in the diffusivity of oxygen. YSZ is a good oxygen ionic conductor. The oxygen vacancies are introduced to the zirconia lattice by doping with yttrium, as shown below using Krfger–Vink notation ð5Þ The addition of substantial amounts of dopant (typical 8– 10 mol%) also has the effect of stabilising the cubic phase over the entire temperature range. According to the defect model of Nowotny and Rekas [20], the concentration of oxygen vacancies in the doped lanthanum manganites is determined by the following reaction x ˙ Y V˙˙ Oox þ 2MnMn O þ 2MnMn þ 1=2O2
ð6Þ
x where Mn˙Mn and Mn Mn are Mn 4+ and Mn 3+ ions, respectively. Berenov [21] calculated the vacancy concentration in La0.8Sr0.2MnO3 using isotopic diffusion coefficient data. It was found that oxygen ion transport through this material is negligible and LSM is thus considered as virtually a pure electronic conductor. As both the ionic conductivity of LSM and the electronic conductivity of YSZ for the temperature range and compositions considered here are low, the connectivity of the two phases is crucial to produce a mixed conducting composite. This is determined by particle size ratio and volume fraction [18]. In this study, the interconnection started to form when 30 wt.% LSM is added to YSZ. Below that point, LSM particles would block the conduction of oxygen ions. Thus the oxygen diffusivity of the composite is expected to initially decrease as LSM is added to YSZ. For practical application, the oxygen transport properties were only investigated on the YSZ–30 wt.%LSM and YSZ–40 wt.%LSM. It is seen from Fig. 6 that the pure YSZ showed the highest diffusivities, whereas the LSM exhibited the lowest values in the temperature range studied. Adding LSM to the YSZ produced slightly
lower diffusivities than that of the YSZ as discussed above; however, these values are improved greatly compared with LSM. It thus seems clear that the effective oxygen diffusion properties of the composite materials are determined simply by the volume fraction and connectivity of the YSZ phase. The oxygen ion conductivities, r ion, can be calculated from diffusion data with the Nernst–Einstein equation, which relates to tracer diffusivity, D T, by rion ¼
DT N q2ion f kT
ð7Þ
where N is the concentration of anion sites, q ion is the charge of anion and f, the correlation factor, is assumed to be equal to 0.69. A comparison of the calculated ionic conductivity and the measured total conductivity at 900 8C is given in Table 1 together with the calculated transport number. It can be seen that for LSM r ion is very much lower than the r total. In contrast, the YSZ–LSM composites exhibit much higher ionic conductivities and consequently the transport numbers of the composites are improved compared to single-phased LSM. 4.3. Surface exchange In contrast to the behaviour of the diffusion coefficient the measured effective surface exchange coefficients showed an unexpected behaviour. Adding LSM to the YSZ produced a significant increase in the surface exchange coefficient and the two composites, YSZ–30 wt.%LSM and YSZ–40 wt.%LSM, showed values higher than the two individual end-members as shown in Fig. 8. The implications of these findings are discussed below. The surface exchange reaction is commonly described by an equation of the type 1=2O2 þ V˙˙O þ 2eVYOOx
ð8Þ
Manning et al. [22] investigated the oxygen diffusion and surface exchange in YSZ and suggested that surface reaction is limited by the availability of free electronic species. De Souza [23] further proposed different surface exchange processes for two extreme cases, i.e. pervoskite type LaTMO3 (TM=Co, Fe, Mn, Ni ect) compounds with high electronic concentration and fluorite oxides with high ionic conductivity. It was suggested that, for LaTMO3, the vacancy concentration limits the rate of surface exchange. In the fluorite oxides with low electron concentration, only the few surface vacancies that have trapped electrons are able to Table 1 Comparison of calculated ionic conductivity of YSZ–LSM composites with measured total conductivity at 900 8C YSZ–30wt.%LSM YSZ–40wt.%LSM LSM
r ion (S/cm)
r total (S/cm)
t ion
5e-3 3e-3 4e-8
0.3 26 200
2e-2 1e-4 2e-10
Y. Ji et al. / Solid State Ionics 176 (2005) 937–943
participate in surface exchange. Therefore, the concentration of electrons limits the kinetics of the exchange process. Both vacancies and electrons need to be taken into account in the surface exchange process. The data on the pure YSZ and LSM in this study indicated that these two materials have markedly different diffusivity and surface exchange kinetics. As shown in Fig. 4(a), a very low concentration of 18O on the YSZ surface is observed with a long diffusion length. For the LSM, the 18O surface concentration is close to the concentration in the gas phase (Fig. 4(b)) with a very shallow diffusion length (b1 micrometer). The diffusion length and 18O surface concentration of the two composites fall in between YSZ and LSM (Fig. 4(c)). The issues raised by this work are very interesting, however; unfortunately yet there is no other published data with which to make a direct comparison. What can be done is to compare this data with the electrochemical measurements of composite electrodes. There have been a considerable number of studies on YSZ/LSM composite electrodes applied to the SOFCs. Composites have been fabricated in order to improve the electrochemical performance by introducing both electrons and surface vacancies [9,24,25]. The composites are more electrochemically active than the pure LSM and would support the finding that the extended three phase boundaries are responsible for the enhanced effective exchange coefficient found in this work. If this analysis is correct, then this would imply that the three-phase boundaries on the surface of the composite materials should be highly active for the exchange of oxygen. Hence adding LSM to YSZ should increase the effective surface exchange coefficient, measured by these experiments, as is observed. However, one possible explanation of this finding must be included as a note of caution. The k values for LSM could be in error due to subtle experimental problems in the accurate determination of the surface 18O concentration when the surface concentration approaches that in the gas phase implying the k value for LSM is much higher than previously thought. Work is currently underway to check this possibility including high spatial resolution SIMS measurements of composite materials to deduce the distribution of 18O in the component oxides of the composite. This work will be reported in a subsequent publication.
5. Conclusions YSZ–LSM composites can be sintered to high density (above 95%) at a temperature of 1350 8C. The formation of the insulating La2Zr2O7-phase during high temperature consolidation is detrimental in terms of electrical properties. A 3-dimensional interpenetrating microstructure was formed by adding about 30 wt.% (or 28 vol.%) LSM to the YSZ. The IEPD/SIMS method was used to characterise the
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effective oxygen transport in the YSZ, LSM, YSZ–30 wt.%LSM, and YSZ–40 wt.%LSM composites. The two composites showed higher oxygen diffusivity than the pure LSM, but slightly lower than the parent YSZ. In contrast, the effective oxygen surface exchange coefficients of the two composites are apparently enhanced over that of the two parent materials. It is suggested that the availability of electronic species limits the kinetics of the exchange process on the YSZ surface and the oxygen exchange properties can be improved by adding the electronic conductor LSM to YSZ to form an YSZ–LSM composite.
Acknowledgement The authors would like to thank Air Product and Chemicals for the funding for this work.
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