The composition of the synthesised powders was checked by ICP-MS analysis. Table 1
shows that the concentrations of Sm and Sr are within the nominal values; however, that of Ce is lower than expected. The decreased Ce concentration is approximately equal to the concentration of the Gd impurity. This suggests that Gd was introduced as an impurity in the cerium nitrate. The concentration of impurities found in the powders are within the specified purities of the metal nitrates used in the synthesis: 99.5% for the cerium nitrate hexahydrate and 99.9% for the samarium nitrate hexahydrate. Impurities like the minor lanthanide impurities are likely to be present in most doped ceria electrolytes, as reagents of 99.9% purity are among the most commonly used. The concentration of the Gd impurity, however, is high enough that it may have a small, but significant, effect on the material properties. A commonly used reagent is 99.5% cerium nitrate hexahydrate [5
]; however, it is not known if the Gd impurity is present in such materials, as elemental analysis is not usually carried out. There is no appreciable systematic difference between materials synthesised with 99.5% or 99.9% pure reagents in the literature, though differences would likely be obscured by other larger differences between the materials.
Examination of previous studies on strontium doping indicated that strontium should be fully soluble in Ce0.8
at the synthesised levels of doping. The linear variation in XRD lattice parameter and retention of a single phase Fm-3m crystal structure confirmed this. The values for the crystallite size were in the range expected from previous work using the same synthesis method [25
]. The decrease in the crystallite size of the powder with increasing strontium content is, however, in contrast to the increases in density and grain size in the final sintered materials. The crystallite size of the powder is determined by the conditions of the sol-gel synthesis and subsequent calcination. There are two other reports of the relationship between strontium doping and crystallite size of co-doped ceria. These studies looked at the Ce0.8+x
] (x = 0–0.06) and Ce0.9
] (x = 0–0.06). Both showed slight variation in crystallite size, but no overall trend, suggesting that there was no direct relationship to the strontium content. These two systems vary from the one in this study, however, in that their oxygen vacancy concentrations were kept constant as strontium content was changed. In addition, it has also been observed previously that crystallite size decreased from 11 nm to 8 nm when the samarium content of Ce1−x
was increased from x = 0.1 to x = 0.3 in materials made under the same conditions as used in the present study, and that crystallite size was strongly affected by the calcination conditions [25
]. In this system oxygen vacancy concentration did increase. There is a possible mechanism by which an increase in oxygen vacancy concentration could cause a drop in cation diffusivity, inhibiting crystallite growth during calcination. Chen et al. [28
] proposed that the rate-limiting diffusion step in aliovalently doped ceria is interstitial cation diffusion through oxygen vacancies and that this process can be hindered by the association of vacancies with other species. It is also known that as the concentration of dopants and vacancies increases, there is a greater tendency for these associates to form [29
]. It follows that the parallel increase in vacancy and Sr concentration in the Ce0.8−x
materials studied here could reduce cation diffusion, and therefore slow crystallite growth at the intermediate temperatures used for their synthesis and calcination. It should be noted that this effect does not contradict the observed improvements in sintering and grain growth with increasing Sr content, since associated defects are thought to be mostly absent by 1000 °C [31
The increase in the final density of the sample with increasing strontium content is a clear sign of an improved rate of sintering. As was mentioned in the Introduction, almost all studies on strontium co-doping showed that sinterability was improved with strontium addition. The exception is a study on Ce0.8+x
by Jaiswal et al. [22
], which does not show a clear trend, but does suggest a decrease. The observation of a plateau in density at strontium doping levels of more than 1 mol% is in agreement with Zheng et al., who also found improvements in density were negligible above this level of strontium in Ce0.8
]. Only two previous studies give a mechanism for the improvement in sintering. Zheng et al. [15
] suggest that viscous flow could be occurring during sintering, though they do not provide any details, whilst Lane et al. [10
] point out the possibility of liquid phase sintering by the formation of a strontia-silica phase which is liquid at sintering temperatures. Neither mechanism is proven, however. Sintering is ultimately governed by diffusion, so improvements must result from an enhancement in diffusion. Possible mechanisms for diffusion enhancement are discussed in relation to grain growth in the next section.
The samples for conductivity measurements were sintered at 1450 °C for 4 h. This sintering regime was used as it is close to the regime found to give maximum conductivity for Ce0.8
powders synthesised by the same method [25
]. Therefore, the undoped sample is treated as the standard material against which the new materials should be compared. None of the strontium doped compositions showed a significant improvement in conductivity compared to the undoped sample at any measurement temperature, though the relative differences in total conductivity did change with temperature.
Whilst the trends and relative values of the results are clear, in order to establish if the results are significant, it is necessary to look at the absolute values of conductivity and compare them to the literature. First, Ce0.8
will be considered in order to establish the validity of the reference sample. The total conductivity of the Ce0.8
sample at 600 °C was 0.020 ± 0.001 S cm−1
, whilst that of a sample synthesised by Kosinski et al. [25
] using the same method, though sintered at 1450 °C for 6 h, had a similar total conductivity of 0.018 S cm−1
at 600 °C. The similarity of the values confirms that the sample is a reliable standard. For a broader comparison, the total conductivity at 600 °C of the Ce0.8
sample in other studies on strontium doping varies from 0.0031 S cm−1
(Gao et al.) [19
] to 0.013 S cm−1
(Jaiswal et al.) [22
], respectively, 85% and 35% less conductive than the same sample in this study. This wide range of conductivities for materials which are nominally the same is mostly due to variation in the grain boundary conductivity. While this comparison of Ce0.8
samples is useful to compare the baseline, the most useful comparison, in terms of absolute values of conductivity, is between the conductivity of the most conductive sample in this and other studies on strontium doping. For total conductivity at 600 °C, the conductivity of Ce0.8
in this study (0.020 S cm−1
) compares favourably to the highest total conductivities in other studies which include, 0.008 S cm−1
] 0.007 S cm−1
] 0.013 S cm−1
] and 0.027 S cm−1
]. The latter sample, produced by Jaiswal et al. [22
] is, in fact, the only sample in any strontium doping study to show higher conductivity than the pure Ce0.8
in this study. It should be noted that the value reported by these authors is significantly higher than any published previously for any aliovalently doped ceria. Therefore, further work to confirm such a significant effect in the case of strontium-doping with constant oxygen vacancy concentration would be desirable. The implication of this comparison of conductivity values of the most conductive samples is that strontium co-doping may not be an effective strategy to increase the conductivity of aliovalently doped ceria materials past that of the best existing materials.
In order to gain an understanding of the causes of the changes in total conductivity the bulk and grain boundary conductivity components must be considered. Generally, the bulk conductivity depends mostly on the intrinsic conductivity of the material, while the grain boundary conductivity depends on the microstructure and grain boundary composition as well as the intrinsic conductivity [33
]. As can be seen in Figure 11
, changes in total conductivity are the result of corresponding changes in both the bulk and grain boundary components of conductivity. By examining each component in turn, the changes in total conductivity can be explained.
The grain boundary conductivity will be considered first to explain microstructural and processing effects, then the remaining intrinsic effects will be considered. It is generally found that the intrinsic conductivity of the grain boundary is directly linked to that of the bulk, if microstructural and impurity effects are discounted. Therefore, it is useful to consider the blocking factor, as this quantifies the magnitude of the grain boundary resistance, Rgb
, relative to the bulk resistance, Rb
(Equation (2)). Figure 12
a shows that the blocking factor decreases with increasing strontium content. This means that the effects of strontium doping on impurities at the grain boundary or on the microstructure must be beneficial. Figure 12
a also shows, however, that the decrease is not linear and that the trend changes with temperature. There is a step change in blocking factor between the doped and the undoped samples, the magnitude of which decreases with increasing temperature. The fact that the size of this step changes with temperature implies that it is due to at least one factor other than grain size, which is invariant with temperature, and these factors will be considered after the effect of grain size is quantified. It is clear, from the results of the grain size study, that doping with strontium does lead to increased grain size and therefore will have the effect of decreasing the blocking factor. Figure 4
c shows the variation of grain size with strontium content for samples sintered under the same conditions as the impedance samples. There is a notable symmetry between the shape of this curve and that of the blocking factor in Figure 11
a; a jump between Sr00 and Sr02 followed by a linear section as strontium content is increased further. This suggests that the blocking factor may be strongly determined by grain size. To examine this possibility, the normalised blocking factor, plotted in Figure 12
b, takes into account the expected effect of grain size on blocking factor. It appears that the normalised blocking factor also displays a step followed by a linear region. If the changes in blocking factor were due only to the changes in grain size then one would expect the normalised blocking factor to be constant for a given temperature. From Figure 12
b, it appears that this may essentially be the case from 0.20 cation% upwards, although there is still some variation. Between 0 and 0.20 cation%, however, there is still a change in normalised blocking factor. This analysis of the blocking factor and the normalised blocking factor together indicate that increased grain size is responsible for the decrease in blocking factor from 0.20 to 3 cation% strontium doping. As the decrease from 0 to 0.20% is not fully accounted for by grain size, however, it must be due to a change in the structure or local composition of the grain boundary. It is likely that this is related to silicon impurities. As discussed in the Introduction, it has been reported that only a few cation% of strontium doping is sufficient to mitigate the effects of hundreds of ppm of silicon impurity [10
]. XRF analysis shows the level of silicon in the samples of the current study to be less than 10ppm. It is therefore probable that 0.20 cation% addition of strontium is sufficient to fully mitigate the effects of silicon impurities in these samples and that further additions will not have any further effect in this regard. This would result in the variation of the normalised blocking factor with strontium content that we observe. In summary, there are improvements in the blocking factor with increasing strontium content due to silicon scavenging and increased grain size. Now, considering this analysis of the blocking factor the trends in grain boundary conductivity can be explained. The observed grain boundary conductivity at 300 °C, shown in Figure 11
c, shows a slight increase up to 0.5 cation% strontium followed by a decrease. This trend results from the combination of positive local effects on the grain boundary, described by the blocking factor, and the underlying decrease in intrinsic bulk conductivity, which can be seen in Figure 11
b. In addition, the trend in the activation energy associated with grain boundary processes in Figure 9
shows a minimum at 0.20 cation% strontium. It is probable that this too is linked to the scavenging of silicon by strontium, so reducing the overall energy barrier to conduction of the grain boundaries.
The bulk component of the conductivity will now be examined. As previously explained, once microstructural factors are accounted for, the intrinsic conductivity of the bulk is the main factor in determining the overall trend in total conductivity. The changes in bulk conductivity with strontium content will be discussed with reference to the oxygen vacancy concentration, average dopant ionic radius and the valency of the dopants. Oxygen vacancies play a crucial role by enabling oxygen mobility. However, if the vacancy concentration is too high the vacancies tend to cluster and become trapped leading to a decrease in ionic conductivity. As a result, in aliovalently doped ceria, a vacancy concentration of 0.05–0.1 is found to be optimal, depending on the identity of the dopants and the measurement temperature. Dopant ionic radius is also considered to have an optimal value and this is often thought of in terms of minimisation of the elastic strain in the lattice which occurs due to the mismatch between the radius of the dopant cation and that of the host lattice. Finally, dopant valency is observed to strongly affect conductivity with trivalent dopants, resulting in consistently higher conductivities than for divalent dopants. It can be seen from Figure 8
a that increasing strontium doping decreased the bulk conductivity at all temperatures for which the components of conductivity could be resolved. It is well established that the oxygen vacancy concentration and average dopant radius are the two main variables which must be optimised to maximise conductivity in aliovalently doped ceria [34
]. As Ce0.8
is known to be near optimal in these respects at intermediate temperatures, changes in these two variables may have a negative impact on conductivity. Assuming ideal behaviour, as the strontium content in the samples increased up to the maximum of 3 cation% the molar oxygen vacancy concentration would increase from 0.10 to 0.13 and the average dopant radius would increase from 1.079 to 1.100 Å. Generally, variations in conductivity of doped ceria with vacancy concentration and dopant ionic radius follow distinct trends which allow a comparison to be made with the samples in this study. These trends are studied by varying the dopant concentration or dopant species, respectively. A comparison between the magnitude of the changes in conductivity in this study with selected others should reveal the likely reasons for the decrease in bulk conductivity with increasing strontium content.
Considering first the effect of vacancy concentration, a comparison can be made with purely samarium doped ceria. The effect of an increase from 20 mole% to 25 and 30 mole% samarium—i.e., a 25 or 50% increase in oxygen vacancy concentration-on total conductivity—has been reported [25
]. All of these studies found a decrease in total conductivity at intermediate temperatures when molar oxygen vacancy concentration was increased beyond 0.1. Kosinski et al. [25
] provide directly comparable conductivity values for the bulk component of conductivity for oxygen vacancy concentrations of 0.1 (Ce0.8
) and 0.15 (Ce0.7
) at 300 °C. Interpolating linearly between the bulk conductivity of the two samples at 300 °C gives an approximate value of 50% for the expected decrease in bulk conductivity for an increase in vacancy concentration from 0.10 to 0.13. Therefore, by comparison with the 76% decrease in bulk conductivity observed on going from Sr00 to Sr30 at 300 °C, it seems likely that the majority of the decrease in bulk conductivity observed in the present study is a result of the increase in oxygen vacancy concentration.
Now the effect of the variation in average dopant radius will be examined. It is increasingly considered to be the case that the change in bulk conductivity with varying average dopant radius for co-doped materials follows a similar trend to singly doped materials [40
]. The average dopant radius of 1.10 Å, for Sr30, is intermediate between those of Sm, 1.079 Å, and Nd, 1.109 Å. Salih et al. found the bulk conductivity of Ce0.8
at 300 °C to be 7.2 × 10−5
. Interpolating between that value and the bulk conductivity of Ce0.8
at 300 °C from the present study, 9.5 × 10−5
, implies a decrease in conductivity of 17% due to decreased average ionic radius on going from Sr00 to Sr30.
A final change which occurs with strontium doping, which has not yet been addressed, is the divalency of the Sr2+
cations compared to the trivalency of Sm3+
. It has been noted that the attraction of a strontium cation, which has an effective charge of 2−, for an oxygen ion vacancy, which has an effective charge of 2+, is likely to be stronger than that of a samarium cation, which has an effective charge of only −1 [15
]. In addition to this, a computational study showed that the binding energy of divalent dopant clusters is higher than for trivalent clusters [29
]. The implication of this increased attraction is that divalent doping would lead to fewer free vacancies and therefore lower ionic conductivity. It is the case that divalently doped ceria generally shows lower ionic conductivity than trivalently doped [26
]. Therefore, it would be expected that this may be responsible for a small part of the decreased bulk conductivity observed in these samples.
In summary, the observed decreases in conductivity are most likely to be due to a combination of increased oxygen vacancy concentration, increased average dopant ionic radius and the presence of more strongly charged strontium cations. The oxygen vacancy concentration is responsible for the majority of the decrease, with a large minority due to the ionic radius and a minor amount due to the more strongly charged cations.