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Microstructure characterization of a cobalt-oxide-doped cerium-gadolinium-oxide by analytical and high-resolution TEM
Acta Materialia 55 (2007) 2907–2917 www.actamat-journals.com
Microstructure characterization of a cobalt-oxide-doped cerium-gadolinium-oxide by analytical and high-resolution TEM Zaoli Zhang
a,*
, Wilfried Sigle a, Manfred Ru¨hle a, Eva Jud a
b,1
, Ludwig J. Gauckler
b
MPI fu¨r Metallforschung, Heisenbergstraße 3, D-70569 Stuttgart, Germany b Federal Institute of Technology, Zurich, Switzerland
Received 4 July 2006; received in revised form 22 December 2006; accepted 22 December 2006 Available online 7 March 2007
Abstract The microstructure and chemistry of 2 mol.% and 5 mol.% cobalt-oxide-doped Ce0.8Gd0.2O1.9 sintered at different temperatures were examined by a combination of electron energy-loss spectroscopy and energy-filtering and high-resolution transmission electron microscopy. Co grain boundary excess was evaluated. It is found that Co solubility in Ce0.8Gd0.2O1.9 is low at temperatures between 800 and 1150 C, resulting in a large number of Co precipitates at grain boundaries. With increasing sintering temperature, precipitates grow, influencing the Co redistribution and further altering the segregation amount in the grain boundary. The Co grain boundary concentration is shown to increase with the increase of sintering temperature from 890 to 1050 C, which is suggested to be due to grain growth. It is found that Co grain boundary segregation induces a detectable variation in the ELNES of Ce-M4,5 and O-K absorption edges, indicating a reduction of Ce atoms in the grain boundary region. The phase of the precipitates was identified as CoO at temperatures between 890 C and 1150 C. HRTEM reveals that grain boundaries are less disordered after prolonged sintering time at higher temperature. At a dopant level of 5 mol.% Co oxide in Ce0.8Gd0.2O1.9, the grain boundaries become more disordered, and exhibit a high amount of Co segregation. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: EFTEM; HRTEM; Grain boundary segregation; Sintering; Ceramics
1. Introduction As one of most promising alternative electrolyte materials to yttria-stabilized zirconia (YSZ) for solid oxide fuel cells at intermediate temperature, cerium-gadolinium-oxide (CGO, in text CGO stands for Ce0.8Gd0.2O1.9) has been extensively studied [1–9]. Mostly focusing on sintering, grain growth, conductivity, and microstructure of CGO with different additives, Kleinlogel and Gauckler [4] reported that the addition of small amounts of cobalt oxide makes the low-temperature sintering with nanosized grains *
Corresponding author. Present address: University of Ulm, Electron Microscopy Group, Albert-Einstein-Allee 11, D-89069 Ulm, Germany. E-mail address: [email protected] (Z. Zhang). 1 Present address: Massachusetts Institute of Technology, 77 Massachusetts Avenue 13-4038, Cambridge, MA 02139, USA.
possible, thereby allowing tailoring of the electrical properties from electronic to purely ionic. Lewis et al. [6] investigated similar materials and found that Co addition leads to a decreasing lattice parameter as compared to CGO, thus enhancing the lattice conductivity through the formation of oxygen vacancies (due to the substitution of Co3+ for Ce4+). However, these results seem not to be supported by Fagg et al. [7], who demonstrated that the total electrical conductivity in 2 mol.% cobalt-oxide-doped CGO remains unchanged as long as sintering is performed between 1173 and 1273 K. They found an increased p-type conductivity in these materials. These studies concentrated more on the electrical properties; relatively few studies of the microstructure were performed. Recently, a systematic study on grain growth, conductivity, and sintering of cobalt-oxide-doped CGO was conducted [10], which further showed that the effect of Co oxide additives on
1359-6454/$30.00 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.12.039
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sintering depends on the initial particle size and that the conductivity can be influenced by cobalt redistribution occurring during sintering [8,9]. Here, we report on results of systematic studies of the microstructure, in particular, focusing on the precipitate formation and Co Grain-boundary (GB) segregation that strongly influence the electrical properties [10]. The grain boundary segregation is evaluated by scanning transmission electron microscopy (STEM) [11,12]. Several samples with different sintering temperatures and times were examined using a combination of electron energy-loss spectroscopy (EELS), energy-filtered transmission electron microscopy (EFTEM), and high-resolution TEM (HRTEM). 2. Experimental procedures
magnification on the CCD was adjusted to 12,500·. The width of the energy-selecting slit is 30 eV. Energy-filtered Co-L and Ce-M images were recorded by employing exposure times of 20 and 25 s, respectively. This guarantees sufficient intensity in the elemental map. For the other samples, EFTEM experiments were performed at the SESAM2 (200 keV, field-emission gun, Corrected Omega filter, 2k · 2k Gatan CCD camera with a total size of about 60 · 60 mm2). The magnification on the CCD camera was 504,000· in this case. Gain and dark-current corrections were applied. The width of the energy slit was 30 eV for Co and Ce and 50 eV for Gd, respectively. The exposure times were 10 s for Co and 20–30 s for Ce and Gd. To improve image contrast, EFTEM bright-field images were acquired at about 100 eV or 90 eV.
2.1. Sample preparation
2.3. Co segregation study
The detailed procedure of specimen preparation is shown in Ref. [10]. The CGO used in the present study had the composition Ce0.8Gd0.2O1.9. For doping with 1– 5 mol.% of cobalt oxide, approximately 30 g of CGO powder was dispersed in ethanol. After ultrasonic mixing for 10 min, the desired amount of cobalt nitrate hexahydrate (Fluka Chemie GmbH, Buchs, Switzerland) dissolved in ethanol was added and stirred for another 10 min. The suspension was dried while stirring on a plate heated at 60 C until most of the ethanol had evaporated. Afterwards, the suspension was completely dried at 120 C. Before and after calcination, the powder was thoroughly ground in an agate mortar. Calcinations at 400 C for at least 2 h leads to the decomposition of the cobalt nitrate to cobalt oxide accompanied by a blackening of the powder. After isostatic pressing at 300 MPa, the samples were sintered in a regular box furnace with a heating rate of 1 K min1. In the present report, samples with a dopant concentration of 5 mol.% cobalt oxide (sintered at 900 C for 2 h followed by slow cooling of 0.5 K min1 and air quenching) are denoted in the following by 5CoCGO, and 2 mol.% cobalt oxide (sintered at 800 C, 890 C, 1050 C and 1150 C, respectively) by 2CoCGO.
For EELS and EDXS studies, a dedicated STEM (VG HB 501 UX, Vacuum Generators) was used. The microscope was operated at 100 keV. It is equipped with a cold field-emission gun, an energy-dispersive X-ray spectrometer (Thermo Noran), and an electron energy-loss spectrometer (Gatan UHV ENFINA766). High spatially resolved EDXS measurements were performed from GB regions after orienting the GB parallel to the electron beam. During data acquisition an area of 2 nm by 20 nm containing the GB was illuminated. In order to obtain a statistically realistic picture of the segregation of Co at the GB in the samples measured, generally, 10–15 grain boundaries were analyzed usually with at least three measurements on each grain boundary, and GBs were chosen arbitrarily, i.e. most often well remote from CoO particles at triple junctions The quantification for EDXS was done using Co K lines (Ka-6.929 keV), Ce L lines (La-4.843 keV, Lb-5.266 keV) and Gd L lines (La6.05 keV, Lb1-6.714 keV, Lb2-7.098 keV). The cation ratios Co/Ce were calculated from experimental X-ray intensity ratios using experimentally determined k-factors. Due to the lack of standard samples, the kCo/Ce factor can be determined from experimentally determined kCo/Si kFe/Si = 1.27 [14] (the kCo/Si should be very close to kFe/Si since Co and Fe are neighboring elements in the periodic table), and kCe/Si = 1.4, which were obtained under the same voltage and microscopes [14]. Therefore, a more accurate value of kCo/Ce is (kCo/Si)/(kCe/Si) = 0.91. Since Co-K line and Gd-L lines overlap we used EELS instead of EDXS. The energy resolution in EELS was 0.7 eV. The electron probe size was below 1 nm. The probe currents used were in the range of 0.5–1 nA. Spectra were recorded with a dispersion of 0.5 eV/channel, which allows simultaneous acquisition of the Co L2,3, the Ce M4,5, and Gd M4,5 edges. Measurements were done at GBs with edge-on orientation. The energy scale for the EELS spectra was calibrated by setting the low-energy Ce white line to 883 eV. The convergence
2.2. EFTEM characterization TEM samples were prepared by standard procedures, i.e. grinding, dimpling, polishing, and ion milling as the final step [13]. Ion milling at 3.5 keV was done in a PIPS (Precision Ion Polishing System, Gatan, Pleasanton, USA) and an inclination angle of 12 was used. The samples contain Co, Ce, and Gd, which all display very sharp white lines (Co L3/L2 edges at 779/794 eV, Gd M5/M4 at 1185/1217 eV, Ce M5/M4 at 883/901 eV, respectively). This facilitates both EFTEM and EELS studies. A series of energy-filtered images for samples sintered at 800 C for 0 h were acquired with the Zeiss 912 Omega (120 keV, Omega filter, 1k · 1k Gatan CCD camera). The
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and collection semi-angles were both 6.5 mrad. All data shown here were corrected for dark current and detector gain variation. The spectrum acquisition and processing were done with the Digital Micrograph 3.6 Spectrum Imaging software. The background for each spectrum was subtracted by the fit of a power-law function to the pre-edge background [15]. Hartree–Slater cross-sections were used for Co and Ce EELS quantification. For EELS quantification following signal and background windows were used: 776.0–816.5 eV/746.0–771.5 eV (Co); 878.0–941.0 eV/ 823.5–867.5 eV (Ce). The evaluation of Co excess was performed by a box technique [12] using an area of 1 nm by 10 nm or 2 nm by 20 nm. Quantification of segregation to a GB can be made based on the difference of the amount of a specific element (here, Co) found in two regions: on the GB and off the GB within the nearby bulk [11,12,16]. The results are obtained in terms of interfacial excess for each element which is formulated as
Cexc Co
GB rCe I Co I Bulk Co ¼ wn rCo I bulk Ce
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ð1Þ
r is the scattering cross-section, n is the site density (atoms per volume, here, n is equal to 25.26 atoms nm3), w is the width of the box used, and I is the EDXS or EELS peak intensity. The beam broadening was not included because the specimen thickness was always much less than 100 nm. The TEM sample preparation procedure is as described in our former paper [17]. In addition, a final ion polishing with low-energy Ar ions (0.5 keV) was performed using a LINDA ion mill (TECHNOORG). A Philips CM 200 microscope with a point-to-point resolution of 0.27 nm was used for conventional TEM observations (diffraction, bright-field and dark-field imaging, grain size analysis). The HRTEM work presented here was done with the JEOL ARM1250 using the side-entry objective pole-piece (Cs = 2.7 mm, point-to-point resolution 0.12 nm). The ARM is equipped with a drift compensating system
Fig. 1. (a) Bright-field image and (b) the corresponding diffraction pattern of a sample sintered at 800 C for 0 h. The lattice spacings calculated from the diffraction rings is in agreement with cubic CGO, additional spot reflections denoted by arrows are ascribed to Co3O4. (c) Bright-field image and (d) elemental map with Co indicated in red and Ce in green.
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facilitating the acquisition of high-resolution images of grain boundaries and triple junctions. 3. Results 3.1. Microstructure characterization 3.1.1. 2CoCGO at 800 C for 0 h The bright-field image and corresponding diffraction pattern are shown in Fig. 1a and b. The grain size ranges from 30 to 60 nm. The diffraction pattern mainly consists of diffraction rings which are compatible with the fcc structure of CGO with a lattice parameter of 0.569 nm, the indices are labelled by red arrows. In addition to the rings there are diffraction spots corresponding to dspacings which do not fit to CGO. These are marked by arrows in Fig. 1b. They can be indexed as reflections of the Co3O4 phase. This interpretation is corroborated by differential thermal analysis/thermogravimetry (DTA/
TG) which showed that the cobalt oxide phase is Co3O4 [10] at this stage. Fig. 1c and d shows a bright-field image and the corresponding Co and Ce elemental distributions using EFTEM. The Co-oxide particles appear not to be distributed homogeneously; instead they form a larger cluster in the CGO matrix. 3.1.2. 2CoCGO at 1150 C with different holding times 3.1.2.1. 0 h holding time. The CGO sample doped with 2 at.% CoO sintered at the temperature of 1150 C for 0 h was analyzed using EFTEM. It should be noted that the used heating conditions results in a relative density of more than 99% when measured by dilatometry [18]. Fig. 2a–d shows bright-field image, Ce-M4,5(b), Gd-M4,5(c) and Co-L2,3(d) elemental maps. The elemental maps demonstrate that two small Co-oxide particles precipitate on the grain boundary and Ce and Gd are deficient in these particles. Large-scale observations of this sample
Fig. 2. (a) Bright-field image, and (b) Ce-M4,5, (c) Gd-M4,5, (d) Co-L2,3 maps of a sample sintered at 1150 C for 0 h. In this region two small cobalt particles are distinguishable. (e) and (f) show a 100 eV energy loss image and the Co-L2,3 map, respectively, from a large area containing cobalt precipitates at GBs and triple junctions. Also note that some pores at triple junctions are still present after sintering at this temperature. (g) Enlarged view of a particle in (e), and (h) EELS spectra from this particle and in the nearby bulk.
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Fig. 2 (continued)
reveal that a certain amount of nanosized Co-oxide particles is distributed along grain boundaries. Fig. 2e and f shows another example of triangular- and rectangularshaped cobalt oxide particles acquired from a large area. Fig. 2e was taken at 100 eV energy loss. The Co oxide particles are marked by dashed circles. Small black areas are pores. The average size of cobalt oxide particles is about 80 nm. EELS analysis was done in the triangular particle and the nearby bulk (see Fig. 2g). The absorption edges clearly vary from the bulk (spectrum ‘‘1’’) to the cobalt oxide (spectrum ‘‘3’’). Compared with EELS spectra in standard CoO [19], EELS quantification and comparison of the ELNES features with standard spectra indicate that the cobalt precipitates at this temperature are CoO-like. 3.1.2.2. 2 h holding time. After 2 h holding time at 1150 C the CGO grain size has grown. EFTEM (Fig. 3) shows cobalt oxide precipitation in a GB triple junction. The precipitate is larger than the particles found at 0 h holding time. The average precipitate size is approximately 300 nm. In Fig. 3a, recorded with electrons having suffered 90 eV energy loss, the Co oxide exhibits bright contrast. This is probably due to the Co M-edge located at 60 eV. 3.1.2.3. 26 h holding time. Fig. 4a–d shows a bright-field image and three elemental maps of the sample sintered at 1150 C for 26 h. The Co-L elemental maps illustrate that Co particles have grown as compared to the two samples
shown above. The average particle size of cobalt oxide is in the range of 300–400 nm. To determine the microstructure of the precipitates, electron diffraction analysis was performed on the particles shown in the elemental maps. Fig. 4e presents a series of diffraction patterns acquired at [0 1 1], [1 1 2], and [1 1 1] zone axes. The angles between them were measured as (30.3 ± 2), (34.7 ± 2), and (21 ± 2), respectively, which are, regarding the accuracy of the measurement, in agreement with a cubic lattice. From the diffraction pattern and the d-spacings calculated we obtain a lattice parameter of (0.422 ± 0.003) nm which corresponds to cubic CoO (space group Fm-3m, lattice parameter 0.425 nm). 3.1.3. HRTEM observations A typical HRTEM image of a triple point taken from the sample 2CoCGO sintered at 1150 C for 0 h is shown in Fig. 5a. A slight disorder particularly near the triple junction is visible. In contrast to this image, the HRTEM image in Fig. 5b, acquired from the sample for 26 h, shows more order at GBs as well as at the triple junction. However, the typical HRTEM images from the sample 5CoCGO sintered at 900 C for 2 h by air quenching (Fig. 5c) shows more disorder in the GB compared with the other two HRTEM images. This results most likely from a higher Co GB concentration (see following section). A Fourier-filtered image is shown as an inset in Fig. 5c. From this an approximate disordered-layer thickness of 1.1 nm is discerned.
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Fig. 3. (a) Micrograph taken at 90 eV energy loss of the sample sintered at 1150 C for 2 h. Elemental maps of (b) Ce-M4,5, (c) Gd-M4,5, and (d) Co-L2,3. The cobalt oxide particles are larger compared with 0 h sintering (Fig. 2).
3.2. Co GB segregation 3.2.1. 2CoCGO at 890 C for 2 h and 192 h Fig. 6 shows the results of high-resolution EDXS measurements of Co GB segregation, obtained during electron illumination of a 2 nm · 20 nm area comprising the GB. The data are from samples that were sintered at 890 C for 2 h and 192 h. It should be noted that the temperature of 890 C is above the onset temperature of sintering as was reported in dilatometric sintering studies [18]. The error bar is the standard deviation from the average of the different measurements. It can be seen that the Co excess in the GB decreases from about 1.56 atoms nm2 to 0.60 atoms nm2 as the holding time increases from 2 h to 192 h. The large scatter of all segregation data shown in this paper is quite common for segregation studies in general. It is most likely related to the different GB types that have different GB energies. 3.2.2. 5CoCGO at 900 C for 2 h Fig. 7a and b shows Co segregation in 14–20 GBs for two samples with 5 mol.% cobalt oxide, one being slowly air-cooled after sintering, the other being quenched. The average values and standard deviations are (3.01 ± 1.98) and (1.17 ± 1.14) atoms nm2 for slow cooling and air
quenching samples, respectively. A student’s t-test shows that the averages of these two data sets are different with a very high significance (>99%). 3.2.3. 2CoCGO at 1050 C for 2 h For the sample sintered at 1050 C for 2 h, a higher Co GB concentration is found (Fig. 8) relative to the samples sintered at 890 C (Fig. 6a). The mean value and standard deviation of Co GB excess are (2.86 ± 1.52) atoms nm2. A STEM bright-field image is inserted, in which a typical Co particle is shown at a triple pocket. As was to be expected, the average particle size of the cobalt oxide is larger than in the samples sintered at 890 C. 3.3. EELS of Co precipitate in GBs (5CoCGO at 900 C 2 h) It is obvious from the segregation measurements that Co GB concentration is relatively high in the 5 mol.% Co-oxide-doped sample slowly cooled to room temperature. Moreover, it is conceivable that Co on GBs can induce a variation of the O:Ce ratio as well as of the O-K and Ce-M ELNES. EELS spectra were acquired from an individual GB (‘‘1’’) and from the nearby bulk regions (‘‘2’’ and ‘‘3’’) (Fig. 9a). The spectra are shown
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4. Discussion 4.1. Precipitate formation and phase Two different phases of cobalt oxide are frequently found in the literature: rocksalt-type CoO and spinel-type Co3O4. According to a recent study on the phase transformation in cobalt oxides, Co3O4 is only stable in air up to its decomposition temperature at about 900 C while CoO is stable above 900 C in air [20]. Our EFTEM images (see Fig. 1d) reveal that at 800 C the Co-containing particles are forming clusters. From the diffraction pattern analysis (Fig. 1b) as well as from DTA/TG measurements and elemental maps reported in Ref. [9] on the same samples, it is known that the cobalt oxide is mainly Co3O4 after sintering at 800 C. At higher temperatures, presumably above the temperature of phase transformation, no clustering of precipitates was observed. All precipitates are located at triple junctions and show distinct morphologies. Electron diffraction (see Fig. 4) and EELS analysis indicate that these particles are composed of CoO. Taken together, we thus conclude that increasing sintering temperature leads to a disintegration of Co3O4 particles and a formation of new CoO particles at triple junctions. We suppose that Co transport occurs along GBs during sintering. This result is consistent with the fact that at elevated temperatures CoO is more stable than Co3O4. More importantly, this conclusion explains why densification of cobalt oxide mainly takes place within a very narrow temperature range, i.e. 900–1000 C [18]. 4.2. Solubility of Co in CGO and Co GB segregation
Fig. 4. (a) Bright-field image of the sample sintered at 1150 C for 26 h and (b–d) elemental maps of Ce, Gd, and Co. The cobalt oxide particle is larger than those in the samples sintered for 0 h and 2 h. The electron diffraction analysis performed on the same particle is demonstrated in (e). The orientation and angles between three zone axes are in agreement with the fcc structure, and calculated d-spacings fit with fcc CoO phase.
in Fig. 9b–d. They demonstrate a Co concentration gradient near the GB. Enlarged O-K edge and Ce-M4,5 spectra (Fig. 9c and d) acquired from an individual GB and nearby bulk show detailed changes as Co segregates to the GB. The O-K edge slightly shifts to higher energy while the Ce-M4,5 absorption edge shows a variation of the relative intensities of M4 and M5. EELS quantification of the GB spectrum shows that the Co L3/L2 ratio is about 2.7–2.8, which is close to the CoO bulk value of (2.6 ± 0.1). The corresponding value in Co3O4 is (1.9 ± 0.2) [19]. Moreover, a slight decrease by about 1– 2% in the ratio of Ce to O is observed by comparison with the bulk spectrum.
From our EFTEM and EELS measurements, it is found that the Co concentration in the bulk is below the detection limit (which is probably of the order of 0.1 mol.%) for all the samples doped with 2 mol.% and 5 mol.% Co oxide. This indicates that the solubility of cobalt in CGO is very limited in the temperature range 800–1150 C. This low solubility leads to the formation of Co precipitates along with GB segregation. The limited Co solubility in CGO is in agreement with results of Chen et al. [21] and Lewis et al. [6]. Low Co solubilities in CGO in the range 0.5–2 mol.% were also reported by Sirman et al. [22] and Ranløv et al. [23]. Hrovat et al. [24] reported a 0.5 at.% Co solubility in CeO2 at 1200 C by EDXS. For the 2 mol.% Co samples the Co GB excess increases from 890 C to 1050 C as shown in Fig. 10. This finding is in contradiction with normal segregation isotherms, e.g. the Langmuir–McLean isotherm, which is characterized by a fixed number of adsorption sites and predicts a decreasing GB excess with increasing temperature. The following argumentation based on the different average grain size of the samples offers one view on how to explain the finding shown in Fig. 10 to some extent. Owing to the low Co solubility in CGO, almost
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Fig. 5. HRTEM images of triple junctions from the samples sintered at 1150 C for (a) 0 h and (b) 26 h. Note that the triple junction after 26 h is more ordered than at 0 h. (c) HRTEM image of a GB in 5CoCGO sintered at 900 C for 2 h after slow cooling to room temperature. A Fourier-filtered image is inserted indicating that the GB thickness is about 1.1 nm.
4.0
Co excess at GB (at/nm2)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
50
100
150
200
o
Annealing time at 890 C(hour) Fig. 6. Co grain boundary excess for the samples sintered at 890 C for 2 h and 192 h. The Co GB concentration decreases with increasing sintering time. One corresponding STEM bright-field image is inserted in which a Co particle is visible. It also reveals that the average grain size is increased relative to the samples annealed at 800 C for 0 h (Fig. 1a).
all cobalt ions need to be accommodated at GBs or triple junctions. Because grain growth leads to a smaller total GB area the GB excess would increase. However, it is unclear how much cobalt oxide is adsorbed at the GBs and how much is found in the particles at the triple pockets.
A more basic understanding of the present finding and its consequences can be found in the following consideration. Recently, a second adsorption regime based on Kikuchi’s and Cahn’s lattice gas model [25] with opposite temperature dependence to the Langmuir–McLean regime has been proposed [26,27]. In this regime of prewetting, typically below the eutectic temperature of the system, the number of adsorption sites increases with increasing temperature. With increasing temperature, the GBs become more and more disordered, which in turn creates more adsorption sites. Interestingly, the eutectic temperature of the system CGO–CoO is estimated to be at around 1200 C [21], signifying that the present data originates from the sub-eutectic temperature regime. There is further evidence in the herein presented data for a correlation between GB order and GB segregation. The HRTEM results (Fig. 5a and b) show that the GBs and triple junctions become more ordered in the course of sintering. As shown in Fig. 6, extended sintering also leads to a decrease in Co GB excess. We therefore believe that GB disorder and a high Co GB excess are closely linked. It is likely that prolonged sintering time at elevated temperature enhances the growth of precipitates at GBs, as observed in EFTEM images, thus leading to a reduced Co GB concentration and more ordered GBs. Thus, the GB structure is clearly changed by Co segregation leading to more disordered GBs with increasing temperature. As a consequence, diffusion in the disordered GBs is facilitated and thus their mobility is
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Fig. 7. Co areal density on individual GBs in 5 mol.% cobalt-oxide-doped CGO sintered at 900 C for 2 h, (a) after slow cooling (0.5 K min1) and (b) after air quenching.
Fig. 8. Co GB excess at individual GBs in a 2CoCGO sample sintered at 1050 C for 2 h. A STEM bright-field image of this sample is shown in the right side. A Co particle is marked. The grain size is much larger than in the material sintered at lower temperature, e.g. 800 C (see Fig. 1a).
increased, resulting in enhanced grain growth and sintering as shown in the CGO system doped with cobalt oxide [18,28]. Enhanced sub-eutectic densification is often referred to as activated sintering and has been confirmed for several other systems, such as Bi2O3-doped ZnO [29] and W doped with various transition metals [30,31]. By comparison of the Co ELNES (Figs. 2h and 9), it is found that cobalt is in a similar oxidation state within the CoO precipitates and at GBs. This suggests that the oxygen coordination at GBs resembles that of cubic CoO. On the other hand, the separation of Ce-L and O-K edges is slightly higher at the GB compared to the bulk (Fig. 9). In Ti oxides the same effect is known to be due to a reduction in the oxidation state of Ti [32]. We therefore
assume that the presence of Co at the GB leads to a charge transfer to neighboring O atoms and to a simultaneous reduction of nearby Ce atoms. This is corroborated by the fact that the Ce M4:M5 ratio is lower at the GB compared to the bulk (see Fig. 9d). According to Fortner et al. [33] this ratio decreases by reduction. The substitution of Ce by Gd introduces excess oxygen vacancies in CeO2. So far, detailed analysis on Gd role in CGO is not available in the literature. It should be noted that Gd might play a role in Co segregation at GBs. In some samples we also measured an excess of Gd at GBs, which is in consistent with the results reported by Lei et al. [34]. It would be desirable in the future to know the effects of both Co and Gd in CGO on sintering and other electrical properties.
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Fig. 9. Representative ELNES analysis of 5CoCGO sintered at 900 C for 2 h along the line perpendicular to a GB in steps of few nanometers. (a) A STEM image. (b) Spectra from one GB (‘‘1’’) and from the nearby bulk (‘‘2’’ and ‘‘3’’) showing that the Co concentration gradually decreases away from the GB. (c,d) O-K and Ce-M edges from the GB (black) and nearby bulk (red) showing slight variations.
uted as isolated particles in triple junctions, and gradually grow with a prolonged holding time. At temperatures above 890 C, the precipitate phase is cubic CoO. It was found that Co strongly segregates to GBs, indicating a very low solubility of Co in CGO at temperatures ranging from 800 to 1150 C. The Co GB excess increases with increasing sintering temperature, presumably because of grain growth and/or prewetting. Co segregation is found to modify the grain boundary structure, and consequently change the ELNES of Ce-M4,5 and O-K absorption edges, indicating a reduction of Ce atoms. HRTEM studies show that: (a) the GB microstructure is more ordered after prolonged holding time at elevated temperatures, and (b) high Co GB excess and disordered GB structure are linked. Fig. 10. Average grain sizes and Co GB excess in 2CoCGO and 5CoCGO annealed for 2 h as a function of sintering temperature. Both the Co GB excess and average grain sizes increase with increasing temperature.
5. Summary We have shown that cobalt forms precipitates and shows an excess at GBs in sintered CGO. At low sintering temperatures, the precipitates form clusters. At higher sintering temperatures, the precipitates are regularly distrib-
Acknowledgement One of the authors (Z.L. Zhang) is grateful to the support from Max Planck Society. The authors thank Maria Sycha and Ute Salzberger for the excellent TEM specimen preparation. We also would like to thank Dr. Stephan Kra¨mer for his help in operating the SESAM2 microscope. References [1] Steele BCH. Solid State Ionics 2000;129:95.
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Huijsmans JPP. Curr Opin Solid State Mater Sci 2001;5:317. Yamamoto O. Electrochim Acta 2000;45:2423. Kleinlogel C, Gauckler LJ. Solid State Ionics 2000;135:567. Fagg DP, Kharton VV, Frade JR. J Electroceram 2002;9:199. Lewis GS, Atkinson A, Steele BCH, Drennan J. Solid State Ionics 2002;152:567. Fagg DP, Abrantes JCC, Perez-Coll D, Nunez P, Kharton VV, Frade JR. Electrochim Acta 2003;48:1023. Jud E, Gauckler LJ. J Electroceram 2005;14:247. Jud E, Gauckler LJ. J Electroceram 2005;15:159. Jud Sierra E, Sintering, grain growth and electrical conductivity of cobalt oxide doped Ce1xGdxO2x/2. PhD thesis, Swiss Federal Institute of Technology, Zurich, 2005. Bruley J, Tanaka I, Kleebe H-J, Ru¨hle M. Anal Chim Acta 1994;297:97. Gu H, Cannon RM, Ru¨hle M. J Mater Res 1998;13:376. Strecker A, Salzberger U, Mayer J. Prakt Metallogr 1993;30:482. Williams DB. Practical analytical electron microscopy in materials science. Mahwah (NJ): Philips Electron Optics Publishing Group; 1984. p. 71. Egerton RF. Electron energy loss spectroscopy in the electron microscope, 2nd ed. New York: Plenum Press. Ikeda JAS, Chiang Y-M, Garratt-Reed AJ, Vander Sande JB. J Am Ceram Soc 1993;76:2447. Zhang ZL, Sigle W, Ru¨hle M. Phys Rev B 2002;66:094108.
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[18] Jud E, Huwiler CB, Gauckler LJ. J Amer Ceram Soc 2005;88:3013. [19] Zhang ZL. Ultramicroscopy, in press. [20] Mocala K, Navrotsky A, Sherman DM. Phys Chem Miner 1992;19:88. [21] Chen M, Hallstedt B, Grundy AN, Gauckler LJ. J Am Ceram Soc 2003;86:1567. [22] Sirman JD, Waller D, Kilner JA. In: Worrell WL, editor. Ionic and mixed conducting ceramics III, PV 97-24. Pennington: The Electrochemical Society; 1998. p. 1159. [23] Ranløv J, Poulsen FW, Mogensen M. Solid State Ionics 1993;61:277. [24] Hrovat M, Holc J, Bernik S. J Mater Res 1999;14:1692. [25] Kikuchi R, Cahn JW. Phys Rev B 1980;21:1893. [26] Subramaniam A, Koch CT, Cannon RM, Ru¨hle M. Mater Sci Eng A 2006;422:3. [27] Bishop CM, Tang M, Cannon RM, Carter WC. Mater Sci Eng A 2006;422:102. [28] Jud E, Huwiler CB, Gauckler LJ. J Ceram Soc Jpn 2006;114:963. [29] Luo J, Wang HF, Chiang YM. J Amer Ceram Soc 1999;82:916. [30] Hayden HW, Brophy JH. J Electrochem Soc 1963;110:805. [31] Luo J, Gupta VK, Yoon DH, Meyer III HM. Appl Phys Lett 2005;87:231902. [32] Sankararaman M, Perry D. J Mater Sci 1992;27:2731. [33] Fortner JA, Buck EC, Ellison AJG, Bates JK. Ultramicroscopy 1997;67:77. [34] Lei YY, Ito Y, Browning ND. J Am Ceram Soc 2002;85:2359.
Microstructure characterization of a cobalt-oxide-doped cerium-gadolinium-oxide by analytical and high-resolution TEM Zaoli Zhang
a,*
, Wilfried Sigle a, Manfred Ru¨hle a, Eva Jud a
b,1
, Ludwig J. Gauckler
b
MPI fu¨r Metallforschung, Heisenbergstraße 3, D-70569 Stuttgart, Germany b Federal Institute of Technology, Zurich, Switzerland
Received 4 July 2006; received in revised form 22 December 2006; accepted 22 December 2006 Available online 7 March 2007
Abstract The microstructure and chemistry of 2 mol.% and 5 mol.% cobalt-oxide-doped Ce0.8Gd0.2O1.9 sintered at different temperatures were examined by a combination of electron energy-loss spectroscopy and energy-filtering and high-resolution transmission electron microscopy. Co grain boundary excess was evaluated. It is found that Co solubility in Ce0.8Gd0.2O1.9 is low at temperatures between 800 and 1150 C, resulting in a large number of Co precipitates at grain boundaries. With increasing sintering temperature, precipitates grow, influencing the Co redistribution and further altering the segregation amount in the grain boundary. The Co grain boundary concentration is shown to increase with the increase of sintering temperature from 890 to 1050 C, which is suggested to be due to grain growth. It is found that Co grain boundary segregation induces a detectable variation in the ELNES of Ce-M4,5 and O-K absorption edges, indicating a reduction of Ce atoms in the grain boundary region. The phase of the precipitates was identified as CoO at temperatures between 890 C and 1150 C. HRTEM reveals that grain boundaries are less disordered after prolonged sintering time at higher temperature. At a dopant level of 5 mol.% Co oxide in Ce0.8Gd0.2O1.9, the grain boundaries become more disordered, and exhibit a high amount of Co segregation. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: EFTEM; HRTEM; Grain boundary segregation; Sintering; Ceramics
1. Introduction As one of most promising alternative electrolyte materials to yttria-stabilized zirconia (YSZ) for solid oxide fuel cells at intermediate temperature, cerium-gadolinium-oxide (CGO, in text CGO stands for Ce0.8Gd0.2O1.9) has been extensively studied [1–9]. Mostly focusing on sintering, grain growth, conductivity, and microstructure of CGO with different additives, Kleinlogel and Gauckler [4] reported that the addition of small amounts of cobalt oxide makes the low-temperature sintering with nanosized grains *
Corresponding author. Present address: University of Ulm, Electron Microscopy Group, Albert-Einstein-Allee 11, D-89069 Ulm, Germany. E-mail address: [email protected] (Z. Zhang). 1 Present address: Massachusetts Institute of Technology, 77 Massachusetts Avenue 13-4038, Cambridge, MA 02139, USA.
possible, thereby allowing tailoring of the electrical properties from electronic to purely ionic. Lewis et al. [6] investigated similar materials and found that Co addition leads to a decreasing lattice parameter as compared to CGO, thus enhancing the lattice conductivity through the formation of oxygen vacancies (due to the substitution of Co3+ for Ce4+). However, these results seem not to be supported by Fagg et al. [7], who demonstrated that the total electrical conductivity in 2 mol.% cobalt-oxide-doped CGO remains unchanged as long as sintering is performed between 1173 and 1273 K. They found an increased p-type conductivity in these materials. These studies concentrated more on the electrical properties; relatively few studies of the microstructure were performed. Recently, a systematic study on grain growth, conductivity, and sintering of cobalt-oxide-doped CGO was conducted [10], which further showed that the effect of Co oxide additives on
1359-6454/$30.00 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.12.039
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sintering depends on the initial particle size and that the conductivity can be influenced by cobalt redistribution occurring during sintering [8,9]. Here, we report on results of systematic studies of the microstructure, in particular, focusing on the precipitate formation and Co Grain-boundary (GB) segregation that strongly influence the electrical properties [10]. The grain boundary segregation is evaluated by scanning transmission electron microscopy (STEM) [11,12]. Several samples with different sintering temperatures and times were examined using a combination of electron energy-loss spectroscopy (EELS), energy-filtered transmission electron microscopy (EFTEM), and high-resolution TEM (HRTEM). 2. Experimental procedures
magnification on the CCD was adjusted to 12,500·. The width of the energy-selecting slit is 30 eV. Energy-filtered Co-L and Ce-M images were recorded by employing exposure times of 20 and 25 s, respectively. This guarantees sufficient intensity in the elemental map. For the other samples, EFTEM experiments were performed at the SESAM2 (200 keV, field-emission gun, Corrected Omega filter, 2k · 2k Gatan CCD camera with a total size of about 60 · 60 mm2). The magnification on the CCD camera was 504,000· in this case. Gain and dark-current corrections were applied. The width of the energy slit was 30 eV for Co and Ce and 50 eV for Gd, respectively. The exposure times were 10 s for Co and 20–30 s for Ce and Gd. To improve image contrast, EFTEM bright-field images were acquired at about 100 eV or 90 eV.
2.1. Sample preparation
2.3. Co segregation study
The detailed procedure of specimen preparation is shown in Ref. [10]. The CGO used in the present study had the composition Ce0.8Gd0.2O1.9. For doping with 1– 5 mol.% of cobalt oxide, approximately 30 g of CGO powder was dispersed in ethanol. After ultrasonic mixing for 10 min, the desired amount of cobalt nitrate hexahydrate (Fluka Chemie GmbH, Buchs, Switzerland) dissolved in ethanol was added and stirred for another 10 min. The suspension was dried while stirring on a plate heated at 60 C until most of the ethanol had evaporated. Afterwards, the suspension was completely dried at 120 C. Before and after calcination, the powder was thoroughly ground in an agate mortar. Calcinations at 400 C for at least 2 h leads to the decomposition of the cobalt nitrate to cobalt oxide accompanied by a blackening of the powder. After isostatic pressing at 300 MPa, the samples were sintered in a regular box furnace with a heating rate of 1 K min1. In the present report, samples with a dopant concentration of 5 mol.% cobalt oxide (sintered at 900 C for 2 h followed by slow cooling of 0.5 K min1 and air quenching) are denoted in the following by 5CoCGO, and 2 mol.% cobalt oxide (sintered at 800 C, 890 C, 1050 C and 1150 C, respectively) by 2CoCGO.
For EELS and EDXS studies, a dedicated STEM (VG HB 501 UX, Vacuum Generators) was used. The microscope was operated at 100 keV. It is equipped with a cold field-emission gun, an energy-dispersive X-ray spectrometer (Thermo Noran), and an electron energy-loss spectrometer (Gatan UHV ENFINA766). High spatially resolved EDXS measurements were performed from GB regions after orienting the GB parallel to the electron beam. During data acquisition an area of 2 nm by 20 nm containing the GB was illuminated. In order to obtain a statistically realistic picture of the segregation of Co at the GB in the samples measured, generally, 10–15 grain boundaries were analyzed usually with at least three measurements on each grain boundary, and GBs were chosen arbitrarily, i.e. most often well remote from CoO particles at triple junctions The quantification for EDXS was done using Co K lines (Ka-6.929 keV), Ce L lines (La-4.843 keV, Lb-5.266 keV) and Gd L lines (La6.05 keV, Lb1-6.714 keV, Lb2-7.098 keV). The cation ratios Co/Ce were calculated from experimental X-ray intensity ratios using experimentally determined k-factors. Due to the lack of standard samples, the kCo/Ce factor can be determined from experimentally determined kCo/Si kFe/Si = 1.27 [14] (the kCo/Si should be very close to kFe/Si since Co and Fe are neighboring elements in the periodic table), and kCe/Si = 1.4, which were obtained under the same voltage and microscopes [14]. Therefore, a more accurate value of kCo/Ce is (kCo/Si)/(kCe/Si) = 0.91. Since Co-K line and Gd-L lines overlap we used EELS instead of EDXS. The energy resolution in EELS was 0.7 eV. The electron probe size was below 1 nm. The probe currents used were in the range of 0.5–1 nA. Spectra were recorded with a dispersion of 0.5 eV/channel, which allows simultaneous acquisition of the Co L2,3, the Ce M4,5, and Gd M4,5 edges. Measurements were done at GBs with edge-on orientation. The energy scale for the EELS spectra was calibrated by setting the low-energy Ce white line to 883 eV. The convergence
2.2. EFTEM characterization TEM samples were prepared by standard procedures, i.e. grinding, dimpling, polishing, and ion milling as the final step [13]. Ion milling at 3.5 keV was done in a PIPS (Precision Ion Polishing System, Gatan, Pleasanton, USA) and an inclination angle of 12 was used. The samples contain Co, Ce, and Gd, which all display very sharp white lines (Co L3/L2 edges at 779/794 eV, Gd M5/M4 at 1185/1217 eV, Ce M5/M4 at 883/901 eV, respectively). This facilitates both EFTEM and EELS studies. A series of energy-filtered images for samples sintered at 800 C for 0 h were acquired with the Zeiss 912 Omega (120 keV, Omega filter, 1k · 1k Gatan CCD camera). The
Z. Zhang et al. / Acta Materialia 55 (2007) 2907–2917
and collection semi-angles were both 6.5 mrad. All data shown here were corrected for dark current and detector gain variation. The spectrum acquisition and processing were done with the Digital Micrograph 3.6 Spectrum Imaging software. The background for each spectrum was subtracted by the fit of a power-law function to the pre-edge background [15]. Hartree–Slater cross-sections were used for Co and Ce EELS quantification. For EELS quantification following signal and background windows were used: 776.0–816.5 eV/746.0–771.5 eV (Co); 878.0–941.0 eV/ 823.5–867.5 eV (Ce). The evaluation of Co excess was performed by a box technique [12] using an area of 1 nm by 10 nm or 2 nm by 20 nm. Quantification of segregation to a GB can be made based on the difference of the amount of a specific element (here, Co) found in two regions: on the GB and off the GB within the nearby bulk [11,12,16]. The results are obtained in terms of interfacial excess for each element which is formulated as
Cexc Co
GB rCe I Co I Bulk Co ¼ wn rCo I bulk Ce
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ð1Þ
r is the scattering cross-section, n is the site density (atoms per volume, here, n is equal to 25.26 atoms nm3), w is the width of the box used, and I is the EDXS or EELS peak intensity. The beam broadening was not included because the specimen thickness was always much less than 100 nm. The TEM sample preparation procedure is as described in our former paper [17]. In addition, a final ion polishing with low-energy Ar ions (0.5 keV) was performed using a LINDA ion mill (TECHNOORG). A Philips CM 200 microscope with a point-to-point resolution of 0.27 nm was used for conventional TEM observations (diffraction, bright-field and dark-field imaging, grain size analysis). The HRTEM work presented here was done with the JEOL ARM1250 using the side-entry objective pole-piece (Cs = 2.7 mm, point-to-point resolution 0.12 nm). The ARM is equipped with a drift compensating system
Fig. 1. (a) Bright-field image and (b) the corresponding diffraction pattern of a sample sintered at 800 C for 0 h. The lattice spacings calculated from the diffraction rings is in agreement with cubic CGO, additional spot reflections denoted by arrows are ascribed to Co3O4. (c) Bright-field image and (d) elemental map with Co indicated in red and Ce in green.
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facilitating the acquisition of high-resolution images of grain boundaries and triple junctions. 3. Results 3.1. Microstructure characterization 3.1.1. 2CoCGO at 800 C for 0 h The bright-field image and corresponding diffraction pattern are shown in Fig. 1a and b. The grain size ranges from 30 to 60 nm. The diffraction pattern mainly consists of diffraction rings which are compatible with the fcc structure of CGO with a lattice parameter of 0.569 nm, the indices are labelled by red arrows. In addition to the rings there are diffraction spots corresponding to dspacings which do not fit to CGO. These are marked by arrows in Fig. 1b. They can be indexed as reflections of the Co3O4 phase. This interpretation is corroborated by differential thermal analysis/thermogravimetry (DTA/
TG) which showed that the cobalt oxide phase is Co3O4 [10] at this stage. Fig. 1c and d shows a bright-field image and the corresponding Co and Ce elemental distributions using EFTEM. The Co-oxide particles appear not to be distributed homogeneously; instead they form a larger cluster in the CGO matrix. 3.1.2. 2CoCGO at 1150 C with different holding times 3.1.2.1. 0 h holding time. The CGO sample doped with 2 at.% CoO sintered at the temperature of 1150 C for 0 h was analyzed using EFTEM. It should be noted that the used heating conditions results in a relative density of more than 99% when measured by dilatometry [18]. Fig. 2a–d shows bright-field image, Ce-M4,5(b), Gd-M4,5(c) and Co-L2,3(d) elemental maps. The elemental maps demonstrate that two small Co-oxide particles precipitate on the grain boundary and Ce and Gd are deficient in these particles. Large-scale observations of this sample
Fig. 2. (a) Bright-field image, and (b) Ce-M4,5, (c) Gd-M4,5, (d) Co-L2,3 maps of a sample sintered at 1150 C for 0 h. In this region two small cobalt particles are distinguishable. (e) and (f) show a 100 eV energy loss image and the Co-L2,3 map, respectively, from a large area containing cobalt precipitates at GBs and triple junctions. Also note that some pores at triple junctions are still present after sintering at this temperature. (g) Enlarged view of a particle in (e), and (h) EELS spectra from this particle and in the nearby bulk.
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Fig. 2 (continued)
reveal that a certain amount of nanosized Co-oxide particles is distributed along grain boundaries. Fig. 2e and f shows another example of triangular- and rectangularshaped cobalt oxide particles acquired from a large area. Fig. 2e was taken at 100 eV energy loss. The Co oxide particles are marked by dashed circles. Small black areas are pores. The average size of cobalt oxide particles is about 80 nm. EELS analysis was done in the triangular particle and the nearby bulk (see Fig. 2g). The absorption edges clearly vary from the bulk (spectrum ‘‘1’’) to the cobalt oxide (spectrum ‘‘3’’). Compared with EELS spectra in standard CoO [19], EELS quantification and comparison of the ELNES features with standard spectra indicate that the cobalt precipitates at this temperature are CoO-like. 3.1.2.2. 2 h holding time. After 2 h holding time at 1150 C the CGO grain size has grown. EFTEM (Fig. 3) shows cobalt oxide precipitation in a GB triple junction. The precipitate is larger than the particles found at 0 h holding time. The average precipitate size is approximately 300 nm. In Fig. 3a, recorded with electrons having suffered 90 eV energy loss, the Co oxide exhibits bright contrast. This is probably due to the Co M-edge located at 60 eV. 3.1.2.3. 26 h holding time. Fig. 4a–d shows a bright-field image and three elemental maps of the sample sintered at 1150 C for 26 h. The Co-L elemental maps illustrate that Co particles have grown as compared to the two samples
shown above. The average particle size of cobalt oxide is in the range of 300–400 nm. To determine the microstructure of the precipitates, electron diffraction analysis was performed on the particles shown in the elemental maps. Fig. 4e presents a series of diffraction patterns acquired at [0 1 1], [1 1 2], and [1 1 1] zone axes. The angles between them were measured as (30.3 ± 2), (34.7 ± 2), and (21 ± 2), respectively, which are, regarding the accuracy of the measurement, in agreement with a cubic lattice. From the diffraction pattern and the d-spacings calculated we obtain a lattice parameter of (0.422 ± 0.003) nm which corresponds to cubic CoO (space group Fm-3m, lattice parameter 0.425 nm). 3.1.3. HRTEM observations A typical HRTEM image of a triple point taken from the sample 2CoCGO sintered at 1150 C for 0 h is shown in Fig. 5a. A slight disorder particularly near the triple junction is visible. In contrast to this image, the HRTEM image in Fig. 5b, acquired from the sample for 26 h, shows more order at GBs as well as at the triple junction. However, the typical HRTEM images from the sample 5CoCGO sintered at 900 C for 2 h by air quenching (Fig. 5c) shows more disorder in the GB compared with the other two HRTEM images. This results most likely from a higher Co GB concentration (see following section). A Fourier-filtered image is shown as an inset in Fig. 5c. From this an approximate disordered-layer thickness of 1.1 nm is discerned.
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Fig. 3. (a) Micrograph taken at 90 eV energy loss of the sample sintered at 1150 C for 2 h. Elemental maps of (b) Ce-M4,5, (c) Gd-M4,5, and (d) Co-L2,3. The cobalt oxide particles are larger compared with 0 h sintering (Fig. 2).
3.2. Co GB segregation 3.2.1. 2CoCGO at 890 C for 2 h and 192 h Fig. 6 shows the results of high-resolution EDXS measurements of Co GB segregation, obtained during electron illumination of a 2 nm · 20 nm area comprising the GB. The data are from samples that were sintered at 890 C for 2 h and 192 h. It should be noted that the temperature of 890 C is above the onset temperature of sintering as was reported in dilatometric sintering studies [18]. The error bar is the standard deviation from the average of the different measurements. It can be seen that the Co excess in the GB decreases from about 1.56 atoms nm2 to 0.60 atoms nm2 as the holding time increases from 2 h to 192 h. The large scatter of all segregation data shown in this paper is quite common for segregation studies in general. It is most likely related to the different GB types that have different GB energies. 3.2.2. 5CoCGO at 900 C for 2 h Fig. 7a and b shows Co segregation in 14–20 GBs for two samples with 5 mol.% cobalt oxide, one being slowly air-cooled after sintering, the other being quenched. The average values and standard deviations are (3.01 ± 1.98) and (1.17 ± 1.14) atoms nm2 for slow cooling and air
quenching samples, respectively. A student’s t-test shows that the averages of these two data sets are different with a very high significance (>99%). 3.2.3. 2CoCGO at 1050 C for 2 h For the sample sintered at 1050 C for 2 h, a higher Co GB concentration is found (Fig. 8) relative to the samples sintered at 890 C (Fig. 6a). The mean value and standard deviation of Co GB excess are (2.86 ± 1.52) atoms nm2. A STEM bright-field image is inserted, in which a typical Co particle is shown at a triple pocket. As was to be expected, the average particle size of the cobalt oxide is larger than in the samples sintered at 890 C. 3.3. EELS of Co precipitate in GBs (5CoCGO at 900 C 2 h) It is obvious from the segregation measurements that Co GB concentration is relatively high in the 5 mol.% Co-oxide-doped sample slowly cooled to room temperature. Moreover, it is conceivable that Co on GBs can induce a variation of the O:Ce ratio as well as of the O-K and Ce-M ELNES. EELS spectra were acquired from an individual GB (‘‘1’’) and from the nearby bulk regions (‘‘2’’ and ‘‘3’’) (Fig. 9a). The spectra are shown
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4. Discussion 4.1. Precipitate formation and phase Two different phases of cobalt oxide are frequently found in the literature: rocksalt-type CoO and spinel-type Co3O4. According to a recent study on the phase transformation in cobalt oxides, Co3O4 is only stable in air up to its decomposition temperature at about 900 C while CoO is stable above 900 C in air [20]. Our EFTEM images (see Fig. 1d) reveal that at 800 C the Co-containing particles are forming clusters. From the diffraction pattern analysis (Fig. 1b) as well as from DTA/TG measurements and elemental maps reported in Ref. [9] on the same samples, it is known that the cobalt oxide is mainly Co3O4 after sintering at 800 C. At higher temperatures, presumably above the temperature of phase transformation, no clustering of precipitates was observed. All precipitates are located at triple junctions and show distinct morphologies. Electron diffraction (see Fig. 4) and EELS analysis indicate that these particles are composed of CoO. Taken together, we thus conclude that increasing sintering temperature leads to a disintegration of Co3O4 particles and a formation of new CoO particles at triple junctions. We suppose that Co transport occurs along GBs during sintering. This result is consistent with the fact that at elevated temperatures CoO is more stable than Co3O4. More importantly, this conclusion explains why densification of cobalt oxide mainly takes place within a very narrow temperature range, i.e. 900–1000 C [18]. 4.2. Solubility of Co in CGO and Co GB segregation
Fig. 4. (a) Bright-field image of the sample sintered at 1150 C for 26 h and (b–d) elemental maps of Ce, Gd, and Co. The cobalt oxide particle is larger than those in the samples sintered for 0 h and 2 h. The electron diffraction analysis performed on the same particle is demonstrated in (e). The orientation and angles between three zone axes are in agreement with the fcc structure, and calculated d-spacings fit with fcc CoO phase.
in Fig. 9b–d. They demonstrate a Co concentration gradient near the GB. Enlarged O-K edge and Ce-M4,5 spectra (Fig. 9c and d) acquired from an individual GB and nearby bulk show detailed changes as Co segregates to the GB. The O-K edge slightly shifts to higher energy while the Ce-M4,5 absorption edge shows a variation of the relative intensities of M4 and M5. EELS quantification of the GB spectrum shows that the Co L3/L2 ratio is about 2.7–2.8, which is close to the CoO bulk value of (2.6 ± 0.1). The corresponding value in Co3O4 is (1.9 ± 0.2) [19]. Moreover, a slight decrease by about 1– 2% in the ratio of Ce to O is observed by comparison with the bulk spectrum.
From our EFTEM and EELS measurements, it is found that the Co concentration in the bulk is below the detection limit (which is probably of the order of 0.1 mol.%) for all the samples doped with 2 mol.% and 5 mol.% Co oxide. This indicates that the solubility of cobalt in CGO is very limited in the temperature range 800–1150 C. This low solubility leads to the formation of Co precipitates along with GB segregation. The limited Co solubility in CGO is in agreement with results of Chen et al. [21] and Lewis et al. [6]. Low Co solubilities in CGO in the range 0.5–2 mol.% were also reported by Sirman et al. [22] and Ranløv et al. [23]. Hrovat et al. [24] reported a 0.5 at.% Co solubility in CeO2 at 1200 C by EDXS. For the 2 mol.% Co samples the Co GB excess increases from 890 C to 1050 C as shown in Fig. 10. This finding is in contradiction with normal segregation isotherms, e.g. the Langmuir–McLean isotherm, which is characterized by a fixed number of adsorption sites and predicts a decreasing GB excess with increasing temperature. The following argumentation based on the different average grain size of the samples offers one view on how to explain the finding shown in Fig. 10 to some extent. Owing to the low Co solubility in CGO, almost
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Fig. 5. HRTEM images of triple junctions from the samples sintered at 1150 C for (a) 0 h and (b) 26 h. Note that the triple junction after 26 h is more ordered than at 0 h. (c) HRTEM image of a GB in 5CoCGO sintered at 900 C for 2 h after slow cooling to room temperature. A Fourier-filtered image is inserted indicating that the GB thickness is about 1.1 nm.
4.0
Co excess at GB (at/nm2)
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
50
100
150
200
o
Annealing time at 890 C(hour) Fig. 6. Co grain boundary excess for the samples sintered at 890 C for 2 h and 192 h. The Co GB concentration decreases with increasing sintering time. One corresponding STEM bright-field image is inserted in which a Co particle is visible. It also reveals that the average grain size is increased relative to the samples annealed at 800 C for 0 h (Fig. 1a).
all cobalt ions need to be accommodated at GBs or triple junctions. Because grain growth leads to a smaller total GB area the GB excess would increase. However, it is unclear how much cobalt oxide is adsorbed at the GBs and how much is found in the particles at the triple pockets.
A more basic understanding of the present finding and its consequences can be found in the following consideration. Recently, a second adsorption regime based on Kikuchi’s and Cahn’s lattice gas model [25] with opposite temperature dependence to the Langmuir–McLean regime has been proposed [26,27]. In this regime of prewetting, typically below the eutectic temperature of the system, the number of adsorption sites increases with increasing temperature. With increasing temperature, the GBs become more and more disordered, which in turn creates more adsorption sites. Interestingly, the eutectic temperature of the system CGO–CoO is estimated to be at around 1200 C [21], signifying that the present data originates from the sub-eutectic temperature regime. There is further evidence in the herein presented data for a correlation between GB order and GB segregation. The HRTEM results (Fig. 5a and b) show that the GBs and triple junctions become more ordered in the course of sintering. As shown in Fig. 6, extended sintering also leads to a decrease in Co GB excess. We therefore believe that GB disorder and a high Co GB excess are closely linked. It is likely that prolonged sintering time at elevated temperature enhances the growth of precipitates at GBs, as observed in EFTEM images, thus leading to a reduced Co GB concentration and more ordered GBs. Thus, the GB structure is clearly changed by Co segregation leading to more disordered GBs with increasing temperature. As a consequence, diffusion in the disordered GBs is facilitated and thus their mobility is
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Fig. 7. Co areal density on individual GBs in 5 mol.% cobalt-oxide-doped CGO sintered at 900 C for 2 h, (a) after slow cooling (0.5 K min1) and (b) after air quenching.
Fig. 8. Co GB excess at individual GBs in a 2CoCGO sample sintered at 1050 C for 2 h. A STEM bright-field image of this sample is shown in the right side. A Co particle is marked. The grain size is much larger than in the material sintered at lower temperature, e.g. 800 C (see Fig. 1a).
increased, resulting in enhanced grain growth and sintering as shown in the CGO system doped with cobalt oxide [18,28]. Enhanced sub-eutectic densification is often referred to as activated sintering and has been confirmed for several other systems, such as Bi2O3-doped ZnO [29] and W doped with various transition metals [30,31]. By comparison of the Co ELNES (Figs. 2h and 9), it is found that cobalt is in a similar oxidation state within the CoO precipitates and at GBs. This suggests that the oxygen coordination at GBs resembles that of cubic CoO. On the other hand, the separation of Ce-L and O-K edges is slightly higher at the GB compared to the bulk (Fig. 9). In Ti oxides the same effect is known to be due to a reduction in the oxidation state of Ti [32]. We therefore
assume that the presence of Co at the GB leads to a charge transfer to neighboring O atoms and to a simultaneous reduction of nearby Ce atoms. This is corroborated by the fact that the Ce M4:M5 ratio is lower at the GB compared to the bulk (see Fig. 9d). According to Fortner et al. [33] this ratio decreases by reduction. The substitution of Ce by Gd introduces excess oxygen vacancies in CeO2. So far, detailed analysis on Gd role in CGO is not available in the literature. It should be noted that Gd might play a role in Co segregation at GBs. In some samples we also measured an excess of Gd at GBs, which is in consistent with the results reported by Lei et al. [34]. It would be desirable in the future to know the effects of both Co and Gd in CGO on sintering and other electrical properties.
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Fig. 9. Representative ELNES analysis of 5CoCGO sintered at 900 C for 2 h along the line perpendicular to a GB in steps of few nanometers. (a) A STEM image. (b) Spectra from one GB (‘‘1’’) and from the nearby bulk (‘‘2’’ and ‘‘3’’) showing that the Co concentration gradually decreases away from the GB. (c,d) O-K and Ce-M edges from the GB (black) and nearby bulk (red) showing slight variations.
uted as isolated particles in triple junctions, and gradually grow with a prolonged holding time. At temperatures above 890 C, the precipitate phase is cubic CoO. It was found that Co strongly segregates to GBs, indicating a very low solubility of Co in CGO at temperatures ranging from 800 to 1150 C. The Co GB excess increases with increasing sintering temperature, presumably because of grain growth and/or prewetting. Co segregation is found to modify the grain boundary structure, and consequently change the ELNES of Ce-M4,5 and O-K absorption edges, indicating a reduction of Ce atoms. HRTEM studies show that: (a) the GB microstructure is more ordered after prolonged holding time at elevated temperatures, and (b) high Co GB excess and disordered GB structure are linked. Fig. 10. Average grain sizes and Co GB excess in 2CoCGO and 5CoCGO annealed for 2 h as a function of sintering temperature. Both the Co GB excess and average grain sizes increase with increasing temperature.
5. Summary We have shown that cobalt forms precipitates and shows an excess at GBs in sintered CGO. At low sintering temperatures, the precipitates form clusters. At higher sintering temperatures, the precipitates are regularly distrib-
Acknowledgement One of the authors (Z.L. Zhang) is grateful to the support from Max Planck Society. The authors thank Maria Sycha and Ute Salzberger for the excellent TEM specimen preparation. We also would like to thank Dr. Stephan Kra¨mer for his help in operating the SESAM2 microscope. References [1] Steele BCH. Solid State Ionics 2000;129:95.
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