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Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%)
Solid State Ionics 180 (2009) 1587–1592
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%) G.S. Godoi a,⁎, D.P.F. De Souza b a b
Department of Chemistry, Federal University of Sergipe, São Cristovão, Brazil Department of Materials Engineering, Federal University of São Carlos, São Carlos, Brazil
a r t i c l e
i n f o
Article history: Received 21 July 2008 Received in revised form 18 June 2009 Accepted 8 October 2009 Keywords: Doped ceria Microstructure Electrical conductivity SOFC
a b s t r a c t Yttria–zirconia doped ceria, 10% ZrO2–10% Y2O3–CeO2 (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (N 95%) at relatively low sintering temperature and long soaking time. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ceria-based ceramics are an alternative to yttria-stabilized zirconia as electrolyte materials in solid oxide fuel cells (SOFCs) [1]. However, doped ceria such as yttria-doped ceria has the disadvantage of increasing the electronic conductivity in a reductive atmosphere, i.e., the atmosphere prevailing on the anode side of SOFCs. The parameter used to establish the degree of electrolyte reducibility is the electrolytic domain boundary (EDB), which is defined as the partial pressure of oxygen (PO2) below which the electronic conductivity exceeds the ionic conductivity. When the ionic conductivity is constant, if the electronic conductivity increases the SOFC performance will decrease. To devise an electrolyte with a wide EDB, attempts have been made to use ternary electrolytes such as ceria–zirconia–yttria solid solutions as substitutes for ceria doped with only one type of dopant. The EDB can extend to pO2 = 10− 20 in these ternary ceramics [2]. Zirconia-rich systems were studied initially but were later replaced by ceria-rich systems because of the latter's better properties. Lee et al. [3], who studied the nY2O3–10 mol% ZrO2–CeO2 system (n = 0, 2, 4, 6, 8 and 10%) prepared by coprecipitation, obtained an almost threefold increase in the ionic conductivity, i.e., 4 × 10− 2 S cm− 1 at 800 °C, compared with the zirconia-rich system. For a material to be used as an electrolyte it must display not only high ionic conductivity but also high sinterability. Powders prepared by chemical routes normally meet these requirements, but at a high fabrication cost. On the other hand, electrolytic material produced
⁎ Corresponding author. E-mail address: [email protected] (G.S. Godoi). 0167-2738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2009.10.006
from powders prepared by the traditional oxide mixture process is a good option due to its low fabrication cost. However, oxide raw materials usually contain impurities that may be deleterious to electrical conductivity [4–7]. To avoid this problem in some systems, for example yttria-stabilized zirconia (YSZ), an additive such as alumina has already been used, which optimizes the electrical properties by acting as a silica scavenger [8,9]. In this work, alumina was used as an additive in ceria-based ceramics and an evaluation was made of its effect on the electrical and microstructural properties of these ceramics. A two-step sintering procedure was also tested to minimize some deleterious effects resulting from high temperature sintering, which are common in powders prepared by the traditional oxide mixture process. This procedure was originally designed to obtain materials with very small grain sizes [10], but our aim here was to control the distribution of second phase at the grain boundaries by sintering at low temperatures. 2. Experimental 2.1. Preparation of specimens Two ceria-based ceramic compositions, 10 mol%Y2O3–10 mol% ZrO2–CeO2 (CZY) and 0.5 mol%Al2O3–10 mol%Y2O3–10 mol%ZrO2– CeO2 (CZYA), were prepared by the oxide mixture process. The following raw materials were used: ZrO2 TZ-0 (99.9%) and Y2O3 (99.99%) from TOSOH and Alfa-Aesar, respectively, and Al(NO3)3–9H2O (N98% purity) and CeO2 (99.9% purity with 140 ppm SiO2) from Aldrich. The appropriate amount of each raw material was mixed in a vibratory mill in isopropanol alcohol containing 2 wt.% of polyvinyl butyral (Butvar B98 Solutia) for 12 h. The slurry was dried at room temperature and
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the powder obtained was calcined at 800 °C for 2 h. After two milling and calcination steps, the slurry was ball-milled again and dried at room temperature following the same procedure described. The dried powder was deagglomerated and sieved to 80# mesh. Pellets of CZY and CZYA powders were prepared by uniaxial pressing (die diameter = 10 mm) followed by isostatic pressing at 200 MPa. The pellets were sintered using isothermal sintering (IS) and two-step sintering (TSS) as described in Table 1. The heating rate of 15 °C/min was used for all the sintering procedures. The characteristics of the sintered samples were examined to ascertain the effect of sintering temperature and soaking time. 2.2. Characterization The true densities of CZY and CZYA solid solutions were determined by the gas pycnometer technique (Pycnometer Micromeritics Accu Pyc 1330). The samples used in this measurement were prepared by crushing the sintered bodies and sieved them through a 600mesh nylon sieve. The bulk densities of the sintered samples were measured by the Archimedes method, using a sensitive analytical balance (Metler Toledo AX 204). The electrical conductivity of the sintered samples was measured by electrochemical impedance spectroscopy (EIS) (HP 4192A impedance analyzer). Prior to deposition of Pt electrodes (Demetron 308A Platinum paste) on both sides of the pellets, a 600-mesh paper-grit was used to polish pellets' surface to improve the adhesion of the paste and the quality of the contacts. After the electrode deposition the samples were fired at 1100 °C for 0.5 h. The geometric areas of the electrodes were 0.60–0.65 cm2. Measurements were made in twoelectrode configuration at a temperature range of 250 to 700 °C in air, over a frequency range of 5 Hz to 13 MHz, with an applied peak-topeak voltage of 50 mV. The temperature dependence of the grain (bulk) and grain boundary resistances, Rg and Rgb respectively, were obtained by fitting the complex impedance data using equivalent circuits (fitting performed by specific software). The resistance values were converted to conductivity values (σ) by the equation σ = L / (RA), where L is the sample thickness and A the geometric area of the electrode [5]. The microstructures of polished and thermally etched surfaces were analyzed by scanning electron microscopy (Philips XL30 SEMFEG) equipped with an X-ray energy dispersive spectrometer (EDS) in order to evaluate the modifications caused by the additive and sintering procedure. The average grain size of sintered samples was
Table 1 Sintering schedules used for CZY and CZYA samples. Sintering condition denomination Isothermal sintering
Two-step sintering
1 2 3 4 5 6 7 8 9
10 11
Sintering temperature (°C)
Soaking time (h)
1400 1450 1450 1480 1480 1500 1600 1st step sintering (1500 °C–0.1 h) + 2nd step sintering at reduced temperature of 1450 °C 1st step sintering (1500 °C–0.1 h) + 2nd step sintering at reduced temperature of 1420 °C
2 2 10 2 10 2 2 1 2
calculated based on an examination of about 500 grains using an imaging software. The measured grain diameters were multiplied by 4/π to get a better approximation of the real grain diameter (under the spherical grain approximation) [11]. Selected samples were also analyzed by transmission electron microscopy (Philips CM 120). 3. Results and discussion 3.1. Densities of sintered samples According to the procedure described in Section 2.2, the true density obtained for CZY and CZYA was practically the same, i.e., 6.87 g cm− 3. Table 2 shows the percentage of true densities of sintered samples. The highest values were obtained for isothermal sintering at 1500 °C–2 h. 3.2. Electrical properties of CZY and CZYA The electrical conductivity of CZY and CZYA samples sintered under isothermal and two-step conditions was measured by two-electrode configuration ac impedance spectroscopy. One of the main advantages of this technique is that the contributions of the grain boundary and the bulk to the sample's total electrical conductivity can be evaluated separately. To highlight the effect of the sintering conditions on the electrical properties of the samples, this section is separated into two parts: the results obtained with the isothermal sintering mode (2 h of soaking time at all the sintering temperatures), and those obtained by two-step sintering. 3.2.1. Isothermal sintering (IS) mode Fig. 1 shows the impedance spectra of CZY and CZYA samples sintered at different temperatures (soaking time = 2 h), measured at 400 °C in air. This measurement temperature was chosen to make comparison among all the samples' impedance spectra easier. The ac impedance of an ionic conductor measured by two-electrode configuration contains the contributions from bulk (high frequency), grain boundaries (intermediate frequency), and electrode–electrolyte interface (low frequency), that can be represented in a complex plane by three consecutive semicircles [12]. For impedance analysis, a serial equivalent circuit, with a series of two RC (resistance/capacitance) parallel circuits was assumed, each one representing bulk and grain boundary contributions. For a more detailed conductivity analysis, Fig. 2 shows the bulk, grain boundary and total conductivity obtained at 500 °C for CZY and CZYA samples sintered at 1400 °C ≤ T ≤ 1600 °C. For all samples the measured conductivities were corrected for the porosity by dividing with the true density fraction (% true density/100) [13]. For samples sintered in high temperature region (T ≥ 1500 °C), the bulk electrical conductivity of both compositions was insensitive to the sintering temperature. However, the grain boundary conductivity was sensitive to both sintering temperature and additive, as follows: i) the increase
Table 2 Bulk densities of CZY and CZYA sintered samples.
Isothermal sintering
10 20 Two-step sintering
Sintering condition denomination (see Table 1)
% True density CZY
CZYA
1 2 3 4 5 6 7 8 9 10 11
81.6 90.9 98.2 98.1 97.6 98.5 97.9 90 94 97.7 98.4
85.6 91.4 96.6 95.6 97.8 98.1 97.8 93.4 94.4 95.3 96.3
G.S. Godoi, D.P.F. De Souza / Solid State Ionics 180 (2009) 1587–1592
Fig. 1. Impedance spectra of CZY and CZYA samples sintered at different temperatures (soaking time = 2 h), measured at 400 °C in air: (■) 1400 °C, (●) 1450 °C, (▲) 1480 °C, (□) 1500 °C, and (○) 1600 °C. The numbers (2, 3, 5, and 7) give the log10(signal frequency) for the corresponding point.
of sintering temperature decreased the grain boundary conductivity; ii) the additive decreased the grain boundary conductivity. This effect is more pronounced for samples sintered at 1500 °C; samples sintered at 1600 °C were insensitive to the additive. The total conductivity fol-
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lows the same behavior of the grain boundary because low grain boundary conductivity, even in a conductor with high bulk conduction, will result in poor overall conductivity hence the grain and grain boundary impedance contributions occur in series. For samples sintered in low temperature region (T b 1500 °C), Fig. 2 indicates the following relationship for bulk conductivity: σCZY, CZYA 1480 °C N σCZY, CZYA 1450 °C N σCZY, CZYA 1400 °C. Two possible reasons for this behavior are the high density of samples sintered at high temperatures and the better dissolution of the dopant in the ceria matrix. Concerning the lower bulk conductivity for CZYA samples sintered at 1450 and 1480 °C, a possible reason is the partial dissolution of alumina in the bulk producing large lattice distortion. This effect can decrease the oxygen vacancy mobility, and consequently the bulk conductivity. Similar result was obtained for alumina-doped YSZ as reported by Feighery and Irvine [13]. With regard to the grain boundary conductivity, an interesting fact was observed. Unlike what occurred with the samples sintered in the high temperature range, the additive had a positive effect here, i.e., increased the grain boundary conductivity. Similar results were reported by Butler and Drennan for yttria-stabilized zirconia [8], who suggested that alumina acts as a scavenger of silica impurities located at the grain boundaries. The total conductivity was strongly affected by the grain boundary conductivity, as was discussed for samples sintered in the high temperature region. An important point to keep in mind is that even when calculating the grain boundary conductivity considering the microstructural features, i.e., the conductance per square centimeter of grain boundary surface (Cgb, S cm− 2) [14], Cgb = σgb / d, where σgb is the grain boundary conductivity obtained directly from the impedance data and d is the average grain size, the results of all samples maintained the same relationship, as indicated in Fig. 3, because the average grain size was similar in both samples sintered at the same temperature. Dessemond [15] demonstrated that the blocking effect on the grain boundaries can be evaluated consistently by the blocking factor αR, which is defined from either admittance diagram parameters or impedance diagram parameters. Using the impedance approach, αR can be calculated by: αR = Rgb = ðRg + RbgÞ; where Rg and Rgb are the electrical resistance of the grain (bulk) and grain boundary, respectively. Based on this factor, different samples can be compared with regard to the fraction of electrical charge carriers being blocked at the impermeable internal surfaces, under the measuring conditions, with respect to the total number of electric carriers in the sample [15]. Fig. 4 shows the effect of the sintering temperature on the CZY and CZYA blocking factor measured at 500 °C. As can be seen, alumina promotes the increase of grain boundary conductivity only for samples sintered below 1500 °C. A similar result was obtained for calcia-stabilized zirconia (CSZ) samples [16,17], in
Fig. 2. Effect of sintering temperature on bulk, grain boundary and total conductivity of (■) CZY and (□) CZYA samples, measured at 500 °C in air.
Fig. 3. Effect of sintering temperature on conductance per square centimeter of grain boundary surface of: (■) CZY and (□) CZYA samples, measured at 500 °C in air.
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Fig. 4. Effect of sintering temperature on the blocking factor of (■) CZY and (□) CZYA samples sintered at different temperatures, measured at 500 °C in air.
which alumina was also used as an additive and promoted an increase of grain boundary conductivity only for samples sintered below 1600 °C. The deterioration effect in the grain boundary conduction was attributed to the dissolution of alumina into siliceous intergranular phase [16,17]. This means that alumina can improve the grain boundary conduction only when the sintering temperature is sufficiently low. The effect of the soaking time during sintering on the electrical conductivity was similar to the increase of the sintering temperature, i.e., long soaking time promoted higher bulk conductivity and lower grain boundary conductivity. This behavior was also observed for samples sintered by TSS (discussed next section) and a possible reason for that is: i) the longer the soaking time, the better the dissolution of the dopant in the bulk, increasing the bulk conductivity; ii) on the other side, longer soaking time can increase impurities' concentration on the grain boundary region, and also, improve the distribution of insulating phases in the grain boundary region [7,18], lowering the ratio between diameter of the grain-to-grain contact area, i.e., the effective conducting contact area, and the mean grain size [19]. Table 3 shows the data for the effect of soaking time on the bulk, grain boundary and total conductivity of CZY and CZYA sintered at different conditions, and measured at 500 °C. In a peculiar way, the grain boundary conductivity of CZY samples sintered at 1480 °C does not follow this behavior which requires further investigation. This anomaly can be related to the very dynamic nature of the liquid phase formed during sintering. Parameters of the liquid phase such as composition, location, viscosity and wetting properties can be strongly modified with a small change of the sintering temperature or soaking time. 3.2.2. Two-step sintering (TSS) mode The purpose of subjecting CZY and CZYA samples to two-step sintering was to control the second phase distribution, thereby minimizing the electrical blocking behavior of the grain boundaries. Two-
Sintering condition denomination (see Table 1)
Conductivity (mS cm− 1)
% True density
10 9 4
1.78 1.69 1.62
95.3 94.4 95.6
step sintering can be divided into three parts: (i) reaching a higher temperature T1 to conduct first-step sintering, (ii) getting a density around 75% of the true density to produce unstable pores, and (iii) lowering the temperature to T2 to conduct second-step sintering. The choice of the condition of 1500 °C–0.1 h for T1 is to meet condition (ii), i.e., to achieve a density of more than 75% of the true density. The value obtained in this condition was slightly higher than 80% of the true density. At the T2 temperature, the choice was based on the isothermal sintering results. Since temperatures below 1480 °C produced the highest grain boundary conductivity, two T2 temperatures were used, 1420 and 1450 °C. A long soaking time (N10 h) was tried for T2 = 1420 °C in order to produce high density samples. The effect of soaking time on both compositions, CZY and CZYA, sintered at T2 = 1450 °C and T2 = 1420 °C was the same, i.e., increased bulk conductivity and decreased grain boundary conductivity (see Table 3), as discussed in the previous section. The total conductivity of samples sintered at each T2 temperature was also very similar, but the advantage of sintering at T2 = 1420 °C–10 h was the greater density attained, which exceeded 95%, as indicated in Table 4. A comparison of the total conductivity and bulk density of CZYA samples sintered in the isothermal mode at 1480 °C for 2 h, and in the two-step mode at T2 = 1420 °C for 10 h indicates that the two-step mode is a convenient approach, since it can produce samples with high electrical conductivity and density at lower temperature, as shown in Table 4. 3.3. Microstructural analysis Fig. 5 shows the effect of the sintering temperature on the average grain size of CZY and CZYA sintered isothermally. As can be seen, for temperatures below 1450 °C both samples show similar grain size, and grain growth takes place only for sintering temperature above 1450 °C. The average grain size of the CZY sample showed a linear dependence on the sintering temperature, while the CZYA samples exhibited an anomalous behavior between 1450 and 1500 °C. In this region, the average grain size of the CZYA samples was slightly smaller than that of the CZY samples. This behavior can be attributed to: i) a lower oxygen vacancy diffusion coefficient due to the presence of aluminum in the ceria lattice, or ii) due to a pinning effect which
Table 3 Effect of soaking time on the bulk, grain boundary and total conductivity of CZY and CZYA sintered at different conditions, measured at 500 °C. Sintering condition denomination (see Table 1)
2 3 4 5 8 9 10 11
Conductivity (mS cm− 1) Bulk
Grain boundary
Total
CZY
CZYA
CZY
CZYA
CZY
CZYA
0.44 0.66 0.70 0.78 0.28 0.44 0.74 0.87
0.37 0.56 0.47 0.61 0.28 0.39 0.38 0.62
0.35 0.11 0.08 0.23 0.69 0.23 0.18 0.11
1.39 0.43 0.69 0.09 1.46 1.07 1.11 0.39
0.20 0.09 0.07 0.17 0.20 0.15 0.14 0.10
0.28 0.25 0.28 0.08 0.24 0.29 0.28 0.24
Fig. 5. Effect of sintering temperature on the average grain size of (■) CZY and (□) CZYA samples sintered in the isothermal mode.
G.S. Godoi, D.P.F. De Souza / Solid State Ionics 180 (2009) 1587–1592
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reduces mobility of the grain boundary during the sintering process. The additive did not affect the average grain size of the samples sintered at 1600 °C. Concerning the microstructure of samples sintered at 1480 °C, it can be noticed that a larger amount of small particles over the grains of CZY and CZYA samples are shown in Fig. 6 (A) and (C). In some regions of CZYA samples only, the presence of second phase not as small particles but as a continuous phase was verified (Fig. 6 B). The microstructure of CZY samples sintered at 1500 °C–2 h, Fig. 7 (A), also presents a large amount of second phase over the grains. The possible source of this second phase can be the spelling of liquid phase from the grain boundaries during the thermal etching resulting in small glass drops (b500 nm) over the grains. Because of the limitations of the chemical analysis (EDS) performed, it was not possible to verify the composition of these particles, but since the ceria raw material used contains Si and Ca as main impurities, it is possible that these
Fig. 7. SEM images of polished and thermally etched surfaces of samples sintered at 1500 °C–2 h: (A) CZY, and (B) and (C) CZYA (A and B bar = 5 µm; C bar = 2 µm).
Fig. 6. SEM images of the fractured surface of samples sintered at 1480 °C–2 h: CZY (A) (bar = 5 µm), and CZYA (B) and (C) (bar = 2 µm).
particles consist of some calcium-silicate glass. This phenomenon has already been reported in the literature in different systems, for example, in RE2O3:YSZ ceramics (RE = rare earth) [20]. For CZYA samples sintered at the same temperature (1500 °C–2 h) no second phase present as small particles or continuous phase was observed over the grains. However, a secondary phase distributed preferentially along the grain boundaries was observed, as depicted in Fig. 7 (B) and (C). The composition analysis by EDS showed that this phase consisted of some oxide containing the following elements: Al, Y, Zr and Ce. Fig. 8 illustrates the evolution of the grain boundary thickness of the CZYA samples sintered under different conditions. The grain boundary thickness increased with the increasing of the sintering temperature, i.e., t1500 °C N t1450 °C N t1420 °C. This behavior may be related to the evolution of grain boundary conductivity, σgb 1500 °C b σgb1450 °C b σgb 1420 °C.
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than 1500 °C. This result is the same for both sintering condition, i.e., isothermal and two-step sintering. For sintering temperatures exceeding 1500 °C (samples sintered by IS), the grain boundary conductivity in alumina-doped samples is lower than undoped samples. This result shows that alumina can improve the grain boundary conduction only when the sintering temperature is sufficiently low enough. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (N95%) at low temperature and long soaking time. For alumina-doped samples isothermally sintered at 1480 °C–2 h, and two-step sintered at T1 = 1500 °C–0.1 h and T2 = 1420 °C–10 h, the total conductivities at 600 °C were 1.62 and 1.78 mS/cm, respectively. Acknowledgement The financial support of the Brazilian research-funding agency CNPq (grant 401227/03) is gratefully acknowledged. References
Fig. 8. TEM images of CZYA samples sintered under different conditions: (A) and (B): CZYA samples sintered in two-step mode at T2 = 1420 °C–10 h and T2 = 1450 °C– 2 h respectively; (C): CZY samples sintered in isothermal mode at 1500 °C–2 h.
4. Conclusions Alumina as an additive in Y2O3–ZrO2-doped ceria improves the grain boundary conductivity of samples sintered at temperatures lower
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Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%) G.S. Godoi a,⁎, D.P.F. De Souza b a b
Department of Chemistry, Federal University of Sergipe, São Cristovão, Brazil Department of Materials Engineering, Federal University of São Carlos, São Carlos, Brazil
a r t i c l e
i n f o
Article history: Received 21 July 2008 Received in revised form 18 June 2009 Accepted 8 October 2009 Keywords: Doped ceria Microstructure Electrical conductivity SOFC
a b s t r a c t Yttria–zirconia doped ceria, 10% ZrO2–10% Y2O3–CeO2 (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (N 95%) at relatively low sintering temperature and long soaking time. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ceria-based ceramics are an alternative to yttria-stabilized zirconia as electrolyte materials in solid oxide fuel cells (SOFCs) [1]. However, doped ceria such as yttria-doped ceria has the disadvantage of increasing the electronic conductivity in a reductive atmosphere, i.e., the atmosphere prevailing on the anode side of SOFCs. The parameter used to establish the degree of electrolyte reducibility is the electrolytic domain boundary (EDB), which is defined as the partial pressure of oxygen (PO2) below which the electronic conductivity exceeds the ionic conductivity. When the ionic conductivity is constant, if the electronic conductivity increases the SOFC performance will decrease. To devise an electrolyte with a wide EDB, attempts have been made to use ternary electrolytes such as ceria–zirconia–yttria solid solutions as substitutes for ceria doped with only one type of dopant. The EDB can extend to pO2 = 10− 20 in these ternary ceramics [2]. Zirconia-rich systems were studied initially but were later replaced by ceria-rich systems because of the latter's better properties. Lee et al. [3], who studied the nY2O3–10 mol% ZrO2–CeO2 system (n = 0, 2, 4, 6, 8 and 10%) prepared by coprecipitation, obtained an almost threefold increase in the ionic conductivity, i.e., 4 × 10− 2 S cm− 1 at 800 °C, compared with the zirconia-rich system. For a material to be used as an electrolyte it must display not only high ionic conductivity but also high sinterability. Powders prepared by chemical routes normally meet these requirements, but at a high fabrication cost. On the other hand, electrolytic material produced
⁎ Corresponding author. E-mail address: [email protected] (G.S. Godoi). 0167-2738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2009.10.006
from powders prepared by the traditional oxide mixture process is a good option due to its low fabrication cost. However, oxide raw materials usually contain impurities that may be deleterious to electrical conductivity [4–7]. To avoid this problem in some systems, for example yttria-stabilized zirconia (YSZ), an additive such as alumina has already been used, which optimizes the electrical properties by acting as a silica scavenger [8,9]. In this work, alumina was used as an additive in ceria-based ceramics and an evaluation was made of its effect on the electrical and microstructural properties of these ceramics. A two-step sintering procedure was also tested to minimize some deleterious effects resulting from high temperature sintering, which are common in powders prepared by the traditional oxide mixture process. This procedure was originally designed to obtain materials with very small grain sizes [10], but our aim here was to control the distribution of second phase at the grain boundaries by sintering at low temperatures. 2. Experimental 2.1. Preparation of specimens Two ceria-based ceramic compositions, 10 mol%Y2O3–10 mol% ZrO2–CeO2 (CZY) and 0.5 mol%Al2O3–10 mol%Y2O3–10 mol%ZrO2– CeO2 (CZYA), were prepared by the oxide mixture process. The following raw materials were used: ZrO2 TZ-0 (99.9%) and Y2O3 (99.99%) from TOSOH and Alfa-Aesar, respectively, and Al(NO3)3–9H2O (N98% purity) and CeO2 (99.9% purity with 140 ppm SiO2) from Aldrich. The appropriate amount of each raw material was mixed in a vibratory mill in isopropanol alcohol containing 2 wt.% of polyvinyl butyral (Butvar B98 Solutia) for 12 h. The slurry was dried at room temperature and
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the powder obtained was calcined at 800 °C for 2 h. After two milling and calcination steps, the slurry was ball-milled again and dried at room temperature following the same procedure described. The dried powder was deagglomerated and sieved to 80# mesh. Pellets of CZY and CZYA powders were prepared by uniaxial pressing (die diameter = 10 mm) followed by isostatic pressing at 200 MPa. The pellets were sintered using isothermal sintering (IS) and two-step sintering (TSS) as described in Table 1. The heating rate of 15 °C/min was used for all the sintering procedures. The characteristics of the sintered samples were examined to ascertain the effect of sintering temperature and soaking time. 2.2. Characterization The true densities of CZY and CZYA solid solutions were determined by the gas pycnometer technique (Pycnometer Micromeritics Accu Pyc 1330). The samples used in this measurement were prepared by crushing the sintered bodies and sieved them through a 600mesh nylon sieve. The bulk densities of the sintered samples were measured by the Archimedes method, using a sensitive analytical balance (Metler Toledo AX 204). The electrical conductivity of the sintered samples was measured by electrochemical impedance spectroscopy (EIS) (HP 4192A impedance analyzer). Prior to deposition of Pt electrodes (Demetron 308A Platinum paste) on both sides of the pellets, a 600-mesh paper-grit was used to polish pellets' surface to improve the adhesion of the paste and the quality of the contacts. After the electrode deposition the samples were fired at 1100 °C for 0.5 h. The geometric areas of the electrodes were 0.60–0.65 cm2. Measurements were made in twoelectrode configuration at a temperature range of 250 to 700 °C in air, over a frequency range of 5 Hz to 13 MHz, with an applied peak-topeak voltage of 50 mV. The temperature dependence of the grain (bulk) and grain boundary resistances, Rg and Rgb respectively, were obtained by fitting the complex impedance data using equivalent circuits (fitting performed by specific software). The resistance values were converted to conductivity values (σ) by the equation σ = L / (RA), where L is the sample thickness and A the geometric area of the electrode [5]. The microstructures of polished and thermally etched surfaces were analyzed by scanning electron microscopy (Philips XL30 SEMFEG) equipped with an X-ray energy dispersive spectrometer (EDS) in order to evaluate the modifications caused by the additive and sintering procedure. The average grain size of sintered samples was
Table 1 Sintering schedules used for CZY and CZYA samples. Sintering condition denomination Isothermal sintering
Two-step sintering
1 2 3 4 5 6 7 8 9
10 11
Sintering temperature (°C)
Soaking time (h)
1400 1450 1450 1480 1480 1500 1600 1st step sintering (1500 °C–0.1 h) + 2nd step sintering at reduced temperature of 1450 °C 1st step sintering (1500 °C–0.1 h) + 2nd step sintering at reduced temperature of 1420 °C
2 2 10 2 10 2 2 1 2
calculated based on an examination of about 500 grains using an imaging software. The measured grain diameters were multiplied by 4/π to get a better approximation of the real grain diameter (under the spherical grain approximation) [11]. Selected samples were also analyzed by transmission electron microscopy (Philips CM 120). 3. Results and discussion 3.1. Densities of sintered samples According to the procedure described in Section 2.2, the true density obtained for CZY and CZYA was practically the same, i.e., 6.87 g cm− 3. Table 2 shows the percentage of true densities of sintered samples. The highest values were obtained for isothermal sintering at 1500 °C–2 h. 3.2. Electrical properties of CZY and CZYA The electrical conductivity of CZY and CZYA samples sintered under isothermal and two-step conditions was measured by two-electrode configuration ac impedance spectroscopy. One of the main advantages of this technique is that the contributions of the grain boundary and the bulk to the sample's total electrical conductivity can be evaluated separately. To highlight the effect of the sintering conditions on the electrical properties of the samples, this section is separated into two parts: the results obtained with the isothermal sintering mode (2 h of soaking time at all the sintering temperatures), and those obtained by two-step sintering. 3.2.1. Isothermal sintering (IS) mode Fig. 1 shows the impedance spectra of CZY and CZYA samples sintered at different temperatures (soaking time = 2 h), measured at 400 °C in air. This measurement temperature was chosen to make comparison among all the samples' impedance spectra easier. The ac impedance of an ionic conductor measured by two-electrode configuration contains the contributions from bulk (high frequency), grain boundaries (intermediate frequency), and electrode–electrolyte interface (low frequency), that can be represented in a complex plane by three consecutive semicircles [12]. For impedance analysis, a serial equivalent circuit, with a series of two RC (resistance/capacitance) parallel circuits was assumed, each one representing bulk and grain boundary contributions. For a more detailed conductivity analysis, Fig. 2 shows the bulk, grain boundary and total conductivity obtained at 500 °C for CZY and CZYA samples sintered at 1400 °C ≤ T ≤ 1600 °C. For all samples the measured conductivities were corrected for the porosity by dividing with the true density fraction (% true density/100) [13]. For samples sintered in high temperature region (T ≥ 1500 °C), the bulk electrical conductivity of both compositions was insensitive to the sintering temperature. However, the grain boundary conductivity was sensitive to both sintering temperature and additive, as follows: i) the increase
Table 2 Bulk densities of CZY and CZYA sintered samples.
Isothermal sintering
10 20 Two-step sintering
Sintering condition denomination (see Table 1)
% True density CZY
CZYA
1 2 3 4 5 6 7 8 9 10 11
81.6 90.9 98.2 98.1 97.6 98.5 97.9 90 94 97.7 98.4
85.6 91.4 96.6 95.6 97.8 98.1 97.8 93.4 94.4 95.3 96.3
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Fig. 1. Impedance spectra of CZY and CZYA samples sintered at different temperatures (soaking time = 2 h), measured at 400 °C in air: (■) 1400 °C, (●) 1450 °C, (▲) 1480 °C, (□) 1500 °C, and (○) 1600 °C. The numbers (2, 3, 5, and 7) give the log10(signal frequency) for the corresponding point.
of sintering temperature decreased the grain boundary conductivity; ii) the additive decreased the grain boundary conductivity. This effect is more pronounced for samples sintered at 1500 °C; samples sintered at 1600 °C were insensitive to the additive. The total conductivity fol-
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lows the same behavior of the grain boundary because low grain boundary conductivity, even in a conductor with high bulk conduction, will result in poor overall conductivity hence the grain and grain boundary impedance contributions occur in series. For samples sintered in low temperature region (T b 1500 °C), Fig. 2 indicates the following relationship for bulk conductivity: σCZY, CZYA 1480 °C N σCZY, CZYA 1450 °C N σCZY, CZYA 1400 °C. Two possible reasons for this behavior are the high density of samples sintered at high temperatures and the better dissolution of the dopant in the ceria matrix. Concerning the lower bulk conductivity for CZYA samples sintered at 1450 and 1480 °C, a possible reason is the partial dissolution of alumina in the bulk producing large lattice distortion. This effect can decrease the oxygen vacancy mobility, and consequently the bulk conductivity. Similar result was obtained for alumina-doped YSZ as reported by Feighery and Irvine [13]. With regard to the grain boundary conductivity, an interesting fact was observed. Unlike what occurred with the samples sintered in the high temperature range, the additive had a positive effect here, i.e., increased the grain boundary conductivity. Similar results were reported by Butler and Drennan for yttria-stabilized zirconia [8], who suggested that alumina acts as a scavenger of silica impurities located at the grain boundaries. The total conductivity was strongly affected by the grain boundary conductivity, as was discussed for samples sintered in the high temperature region. An important point to keep in mind is that even when calculating the grain boundary conductivity considering the microstructural features, i.e., the conductance per square centimeter of grain boundary surface (Cgb, S cm− 2) [14], Cgb = σgb / d, where σgb is the grain boundary conductivity obtained directly from the impedance data and d is the average grain size, the results of all samples maintained the same relationship, as indicated in Fig. 3, because the average grain size was similar in both samples sintered at the same temperature. Dessemond [15] demonstrated that the blocking effect on the grain boundaries can be evaluated consistently by the blocking factor αR, which is defined from either admittance diagram parameters or impedance diagram parameters. Using the impedance approach, αR can be calculated by: αR = Rgb = ðRg + RbgÞ; where Rg and Rgb are the electrical resistance of the grain (bulk) and grain boundary, respectively. Based on this factor, different samples can be compared with regard to the fraction of electrical charge carriers being blocked at the impermeable internal surfaces, under the measuring conditions, with respect to the total number of electric carriers in the sample [15]. Fig. 4 shows the effect of the sintering temperature on the CZY and CZYA blocking factor measured at 500 °C. As can be seen, alumina promotes the increase of grain boundary conductivity only for samples sintered below 1500 °C. A similar result was obtained for calcia-stabilized zirconia (CSZ) samples [16,17], in
Fig. 2. Effect of sintering temperature on bulk, grain boundary and total conductivity of (■) CZY and (□) CZYA samples, measured at 500 °C in air.
Fig. 3. Effect of sintering temperature on conductance per square centimeter of grain boundary surface of: (■) CZY and (□) CZYA samples, measured at 500 °C in air.
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Fig. 4. Effect of sintering temperature on the blocking factor of (■) CZY and (□) CZYA samples sintered at different temperatures, measured at 500 °C in air.
which alumina was also used as an additive and promoted an increase of grain boundary conductivity only for samples sintered below 1600 °C. The deterioration effect in the grain boundary conduction was attributed to the dissolution of alumina into siliceous intergranular phase [16,17]. This means that alumina can improve the grain boundary conduction only when the sintering temperature is sufficiently low. The effect of the soaking time during sintering on the electrical conductivity was similar to the increase of the sintering temperature, i.e., long soaking time promoted higher bulk conductivity and lower grain boundary conductivity. This behavior was also observed for samples sintered by TSS (discussed next section) and a possible reason for that is: i) the longer the soaking time, the better the dissolution of the dopant in the bulk, increasing the bulk conductivity; ii) on the other side, longer soaking time can increase impurities' concentration on the grain boundary region, and also, improve the distribution of insulating phases in the grain boundary region [7,18], lowering the ratio between diameter of the grain-to-grain contact area, i.e., the effective conducting contact area, and the mean grain size [19]. Table 3 shows the data for the effect of soaking time on the bulk, grain boundary and total conductivity of CZY and CZYA sintered at different conditions, and measured at 500 °C. In a peculiar way, the grain boundary conductivity of CZY samples sintered at 1480 °C does not follow this behavior which requires further investigation. This anomaly can be related to the very dynamic nature of the liquid phase formed during sintering. Parameters of the liquid phase such as composition, location, viscosity and wetting properties can be strongly modified with a small change of the sintering temperature or soaking time. 3.2.2. Two-step sintering (TSS) mode The purpose of subjecting CZY and CZYA samples to two-step sintering was to control the second phase distribution, thereby minimizing the electrical blocking behavior of the grain boundaries. Two-
Sintering condition denomination (see Table 1)
Conductivity (mS cm− 1)
% True density
10 9 4
1.78 1.69 1.62
95.3 94.4 95.6
step sintering can be divided into three parts: (i) reaching a higher temperature T1 to conduct first-step sintering, (ii) getting a density around 75% of the true density to produce unstable pores, and (iii) lowering the temperature to T2 to conduct second-step sintering. The choice of the condition of 1500 °C–0.1 h for T1 is to meet condition (ii), i.e., to achieve a density of more than 75% of the true density. The value obtained in this condition was slightly higher than 80% of the true density. At the T2 temperature, the choice was based on the isothermal sintering results. Since temperatures below 1480 °C produced the highest grain boundary conductivity, two T2 temperatures were used, 1420 and 1450 °C. A long soaking time (N10 h) was tried for T2 = 1420 °C in order to produce high density samples. The effect of soaking time on both compositions, CZY and CZYA, sintered at T2 = 1450 °C and T2 = 1420 °C was the same, i.e., increased bulk conductivity and decreased grain boundary conductivity (see Table 3), as discussed in the previous section. The total conductivity of samples sintered at each T2 temperature was also very similar, but the advantage of sintering at T2 = 1420 °C–10 h was the greater density attained, which exceeded 95%, as indicated in Table 4. A comparison of the total conductivity and bulk density of CZYA samples sintered in the isothermal mode at 1480 °C for 2 h, and in the two-step mode at T2 = 1420 °C for 10 h indicates that the two-step mode is a convenient approach, since it can produce samples with high electrical conductivity and density at lower temperature, as shown in Table 4. 3.3. Microstructural analysis Fig. 5 shows the effect of the sintering temperature on the average grain size of CZY and CZYA sintered isothermally. As can be seen, for temperatures below 1450 °C both samples show similar grain size, and grain growth takes place only for sintering temperature above 1450 °C. The average grain size of the CZY sample showed a linear dependence on the sintering temperature, while the CZYA samples exhibited an anomalous behavior between 1450 and 1500 °C. In this region, the average grain size of the CZYA samples was slightly smaller than that of the CZY samples. This behavior can be attributed to: i) a lower oxygen vacancy diffusion coefficient due to the presence of aluminum in the ceria lattice, or ii) due to a pinning effect which
Table 3 Effect of soaking time on the bulk, grain boundary and total conductivity of CZY and CZYA sintered at different conditions, measured at 500 °C. Sintering condition denomination (see Table 1)
2 3 4 5 8 9 10 11
Conductivity (mS cm− 1) Bulk
Grain boundary
Total
CZY
CZYA
CZY
CZYA
CZY
CZYA
0.44 0.66 0.70 0.78 0.28 0.44 0.74 0.87
0.37 0.56 0.47 0.61 0.28 0.39 0.38 0.62
0.35 0.11 0.08 0.23 0.69 0.23 0.18 0.11
1.39 0.43 0.69 0.09 1.46 1.07 1.11 0.39
0.20 0.09 0.07 0.17 0.20 0.15 0.14 0.10
0.28 0.25 0.28 0.08 0.24 0.29 0.28 0.24
Fig. 5. Effect of sintering temperature on the average grain size of (■) CZY and (□) CZYA samples sintered in the isothermal mode.
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reduces mobility of the grain boundary during the sintering process. The additive did not affect the average grain size of the samples sintered at 1600 °C. Concerning the microstructure of samples sintered at 1480 °C, it can be noticed that a larger amount of small particles over the grains of CZY and CZYA samples are shown in Fig. 6 (A) and (C). In some regions of CZYA samples only, the presence of second phase not as small particles but as a continuous phase was verified (Fig. 6 B). The microstructure of CZY samples sintered at 1500 °C–2 h, Fig. 7 (A), also presents a large amount of second phase over the grains. The possible source of this second phase can be the spelling of liquid phase from the grain boundaries during the thermal etching resulting in small glass drops (b500 nm) over the grains. Because of the limitations of the chemical analysis (EDS) performed, it was not possible to verify the composition of these particles, but since the ceria raw material used contains Si and Ca as main impurities, it is possible that these
Fig. 7. SEM images of polished and thermally etched surfaces of samples sintered at 1500 °C–2 h: (A) CZY, and (B) and (C) CZYA (A and B bar = 5 µm; C bar = 2 µm).
Fig. 6. SEM images of the fractured surface of samples sintered at 1480 °C–2 h: CZY (A) (bar = 5 µm), and CZYA (B) and (C) (bar = 2 µm).
particles consist of some calcium-silicate glass. This phenomenon has already been reported in the literature in different systems, for example, in RE2O3:YSZ ceramics (RE = rare earth) [20]. For CZYA samples sintered at the same temperature (1500 °C–2 h) no second phase present as small particles or continuous phase was observed over the grains. However, a secondary phase distributed preferentially along the grain boundaries was observed, as depicted in Fig. 7 (B) and (C). The composition analysis by EDS showed that this phase consisted of some oxide containing the following elements: Al, Y, Zr and Ce. Fig. 8 illustrates the evolution of the grain boundary thickness of the CZYA samples sintered under different conditions. The grain boundary thickness increased with the increasing of the sintering temperature, i.e., t1500 °C N t1450 °C N t1420 °C. This behavior may be related to the evolution of grain boundary conductivity, σgb 1500 °C b σgb1450 °C b σgb 1420 °C.
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than 1500 °C. This result is the same for both sintering condition, i.e., isothermal and two-step sintering. For sintering temperatures exceeding 1500 °C (samples sintered by IS), the grain boundary conductivity in alumina-doped samples is lower than undoped samples. This result shows that alumina can improve the grain boundary conduction only when the sintering temperature is sufficiently low enough. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (N95%) at low temperature and long soaking time. For alumina-doped samples isothermally sintered at 1480 °C–2 h, and two-step sintered at T1 = 1500 °C–0.1 h and T2 = 1420 °C–10 h, the total conductivities at 600 °C were 1.62 and 1.78 mS/cm, respectively. Acknowledgement The financial support of the Brazilian research-funding agency CNPq (grant 401227/03) is gratefully acknowledged. References
Fig. 8. TEM images of CZYA samples sintered under different conditions: (A) and (B): CZYA samples sintered in two-step mode at T2 = 1420 °C–10 h and T2 = 1450 °C– 2 h respectively; (C): CZY samples sintered in isothermal mode at 1500 °C–2 h.
4. Conclusions Alumina as an additive in Y2O3–ZrO2-doped ceria improves the grain boundary conductivity of samples sintered at temperatures lower
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