EFFECTS OF SINTERING TEMPERATURE ON THE PERFORMANCE OF SrSc_0.1Co_0.9O3-δOXYGEN SEMIPERMEABLE MEMBRANE

Abstract

Our study investigates the influence of sintering temperature on the microstructure (grain size distribution, grain boundary length), electrical conductivity, and oxygen permeation properties of permeation membranes. For this purpose, SrSc0.1Co0.9O3-δ samples with different microstructures were prepared by varying the sintering temperature from 1100 to 1250 ˚C. The average grain sizes were gradually increased, thus the grain boundary lengths decreased with increased sintering temperatures. The influence of the ceramic microstructure on total electrical conductivity was found to be negligible. The oxygen transport properties of the samples were characterized by permeation measurements as a function of temperature in an air/helium oxygen partial pressure gradient. The decrease of the sintering temperature, meaning a decrease of grain size and thus the increase of grain boundary length, leads to an enhanced oxygen permeation flux and a reduced activation energy. This implies that oxygen exchange and transport in the SrSc0.1Co0.9O3-δ membranes occur more rapidly along grain boundaries than in the grain bulk.

Keywords:
Oxygen permeation membrane;; Perovskite; Grain size; Sintering temperatur; Strontium cobalt oxide

INTRODUCTION

Due to their intrinsic mixed conductivity, mixed ionic-electronic conductors (MIECs) with a perovskite structure are of great interest for many industrial processes in which a constant supply, or removal of, oxygen to or from gas mixtures is required (Burggraaf, 1996). Main applications include the cathode for solid oxide fuel cells (SOFCs), ultra-pure oxygen production, and hollow fiber membrane reactors for the partial oxidation of hydrocarbons to value-added products (Shao and Haile, 2004; Wang et al., 2005; Cheng et al., 2003; Zhu et al., 2005; Wei et al., 2007). Almost infinite permselectivity and remarkably high oxygen fluxes are two major advantages of these perovskite membranes (Burggraaf, 1996).

The main reason for the various applications of perovskite materials in oxygen production, or SOFCs, is their oxygen transport properties (Burggraaf, 1996; Mercle et al., 2004; Hamel et al., 2006) which are significantly influenced by differences in the microstructure of the material (i.e., grain size and homogeneousness, grain boundary distribution, and boundary composition). Recently, several studies were carried out by different groups to elucidate its importance and to address the dependence of the oxygen permeation on the microstructure (Diethelm et al., 2005; Zhang et al., 1999; Etchegoyen et al., 2006; Kim et al., 2000; Kharton et al., 2000; Kharton et al., 2001; Kharton et al., 2002; Arnold et al., 2008; Wang et al., 2005; Zeng et al., 2007; Shaula et al., 2005). Different perovskite systems were studied in many of these reports; however, no clear trend was visible relative to whether the transport along grain boundaries displays a barrier, or acts as a pathway for fast oxygen transport. Diethelm et al. (2005) found for La_0.5Sr_0.5FeO_3-δ that the larger the obtained grains, and thus the fewer the grain boundaries, the smaller the observed oxygen permeation performance. Similar findings were reported for SrCo_0.8Fe_0.2O_3-δ membranes by Zhang et al. (1999). They observed that increasing the grain size by increasing the sintering temperature resulted in a considerable decrease in the oxygen permeation flux. Again, La_0.6Sr_0.4Fe_0.9Ga_0.1O_3-δ was reported by Etchegoyen et al. (2006) that exhibited lower oxygen permeability when the grain size of the membrane was larger. Contrary to the above findings, for the ceramics LaCoO_3-δ, La_0.3Sr_0.7CoO_3-δ, La0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ and CaTi_0.8Fe_0.2O_3-δ, the increase of grain size leads to an enhanced oxygen permeation (Kharton et al., 2000; Kharton et al., 2001; Kharton et al., 2002; Arnold et al., 2008; Wang et al., 2005; Zeng et al., 2007; Shaula et al., 2005). Determining how microstructure influences the permeation fluxes of these mixed conducting ceramic membranes is complicated. Both the preparation procedures of the ceramic powder and the final membrane-sintering program may have significant effects on microstructure and, consequently, they will influence the oxygen permeation behavior of the resulting membranes.

In order to develop a membrane exhibiting high oxygen flux, better mechanical strength and good phase stability in a reducing atmosphere, extensive studies have been conducted in more than 20 laboratories worldwide (Teraoka et al., 1985; Teraoka et al., 1988; Diethelm et al., 2003 Yan et al., 2010; Wang et al., 2005; Kim et al., 1997; Ishihara et al., 2000; Schiestel et al., 2005; Liu et al., 2005; Li et al., 2006; Tan et al., 2005). Among the MIECs, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) is considered to be one of the most promising materials because its membranes show high oxygen permeation fluxes and excellent cathode performance for SOFCs (Shao and Haile, 2004; Shao et al., 2000; McIntosh et al., 2006). Recently, Zeng et al. (2008, 2009) developed and reported a new perovskite system SrSc_xCo_1-xO_3-δ (SSCx), which also exhibits excellent cathode performance and remarkably high oxygen fluxes. Within these SSCx perovskites, SrSc_0.1Co_0.9O_3-δ (SSC) shows even better cathode performance and oxygen permeability than those of BSCF. Consequently, SSC is of special interest and its microstructure and oxygen permeation performance were investigated in the present study. In this paper, we present a detailed investigation of the significant effects of sintering temperature on the microstructure (i.e., grain size and grain boundary) and the oxygen permeation behavior of the SrSc_0.1Co_0.9O_3-δ membrane with the powder synthesized by the EDTA-citrate complexing sol-gel method.

MATERIAL AND METHODS

SrSc_0.1Co_0.9O_3-δ (SSC) oxide was synthesized by a combined EDTA-citrate complexing sol-gel process. Sr(NO₃)2, Co(NO₃)2.xH₂O, and SC₂O₃ (all in A.R. grade) were selected as the raw materials for the metal-ion sources. SC₂O₃ powder was first dissolved in HNO₃ solution at 90 ºC under stirring. The as-obtained Sc(NO₃)₃ solution and Sr(NO₃)₂ and Co(NO₃)₂ were mixed with deionized water to create a mixed solution under stirring. EDTA acid powder was dissolved in NH₃.H₂O solution, which was then added to the mixed solution, followed by the addition of a citric acid crystal. NH₃.H₂O was used to maintain the pH value of the system at around 6. Under stirring and gentle heating at 90 ºC, a clear pink solution was obtained. A clear sticky gel was finally obtained upon evaporating the water. The gel was pre-treated at 250 ºC for primary decomposition and to facilitate the formation of a gray solid precursor, which was then fired at various temperatures in air for 5 h to result in the final product of SSC. The 900 ºC calcined powder was pressed into disk-shaped green membranes with a diameter of 15 mm, which were then sintered, respectively, into dense membranes at 1100, 1150, 1200, and 1250 ºC for 5 h under stagnant air. The sintered membranes had a diameter of 12-13 mm and thickness of 0.3 mm.

The phase structure of the samples was investigated using a Bruker D8 Advance diffractometer with Cu Kα radiation. The experimental diffraction patterns were collected at room temperature by step scanning in the range of 10^o ≤ 2θ≤ 90^o.

The electrical conductivity was measured by the four-probe D. C. method on sintered bars under air upon cooling from 900 ºC to 300 ºC, lowering 10 ºC per step. Silver paste was painted on the square cross sectional edges of the sample, or along the rectangular edges (separated by a distance L), to form current and voltage electrodes. Four silver wires were used in total with two as current contacts and the other two as the voltage contacts. The four silver wires were attached to the electrodes using silver paste. The sample was placed in a vertical split tube furnace. A constant current was applied to the two current wires and the voltage response on the two voltage wires were recorded using a Keithley 2420 sourcemeter.

The morphological features of the prepared membranes were examined using Environmental Scanning Microscopy (ESEM, QUANTA-2000). Based on these SEM pictures, the grain size distribution was statistically calculated by the manual measurement of the sizes of individual grains.

Permeation properties of the membranes were investigated using a high-temperature oxygen permeation cell. Silver paste was used as the sealant to fix the membrane disk onto a dense quartz tube. An effective surface area of around 0.61 cm² was exposed for the permeation study. The sidewall of the membrane disk was also covered with silver paste to avoid radial contributions to the oxygen permeation flux. Helium was used as the sweep gas to carry the permeated oxygen to a SRI 8610C gas chromatograph equipped with a 5 Å molecular sieve capillary column for in-situ gas composition analysis. Helium also acted as the diluted gas to create the oxygen partial pressure gradient across the membrane. The test temperature was measured by a thermocouple inserted in the quartz tube at the fed gas side with an accuracy of ±1 ºC. All experiments were conducted in the temperature range of 775-900 ºC. The samples were first heated up to 900 ºC and kept at this temperature until a steady state was reached (i.e., oxygen concentration varied less than 1%). The temperature was then dropped to the next measurement point. The oxygen permeation flux at each temperature was monitored periodically and recorded when the steady state was reached. The reported oxygen permeation fluxes are based on the steady-state values.

RESULTS AND DISCUSSION

Figure 1 shows the room-temperature X-ray diffraction (XRD) patterns of SSC calcined at 1100-1250 ºC for 5 h under air. All patterns can be indexed based on a cubic symmetry. Also Zeng et al. (2008, 2009) have reported in previous studies that even the SSC powder calcined at a lower temperature (900 ºC) shows a structure of cubic perovskite.

Figure 1
Room-temperature X-ray diffraction patterns of SSC calcined at 1100-1250 ºC for 5 h under air.

Thus, our findings suggest that no phase transition occurred from lower to higher temperatures during the calcination or sintering process of the SSC powder precursors synthesized by the current EDTAcitrate technique. This suggests that the sintering temperature has a negligible effect on the phase structure of the current SSC membranes. The lattice parameters of all four samples were determined based on the XRD patterns and were included in Table 1. It is apparent that the lattice constant increased slightly with the increase of sintering temperature. Since both Sr²⁺ and SC³⁺ have a fixed valence and radii of 1.44 Å and 0.745 Å, respectively, the lattice constant increase could be ascribed to the ionic radius change of cobalt ions which are Co³⁺, 0.545 Å (LS), 0.61 Å (HS), and Co⁴⁺, 0.53 Å (Dronskowski et al., 2005). Further, the cobalt ion was prone to adopt a high spin state at higher sintering temperatures.

Table 1
The lattice constants of the SSC samples sintered at different temperatures.

T (ºC)	Symmetry	D (Å)	V (Å3)
1100	Pm3m	3.8856	58.6643
1150	Pm3m	3.8894	58.8366
1200	Pm3m	3.8951	59.0956
1250	Pm3m	3.8990	59.2733

The membrane morphologies sintered at different temperatures were observed by SEM. The corresponding microstructure and grain size distribution of the various SSC membrane samples prepared at different conditions are shown in Figure 2. Obviously, the sintering temperature has a significant effect on the grain growth or size distribution of the resultant membranes. All samples displayed similar polyhedral shaped grain, while the size increased steadily with the increase of sintering temperature. The grain size distribution of the sintered membranes at various temperatures is further illustrated in Figure 3. The membrane sintered at 1100 ºC had a size range of 4-40 μm with a dominant (> 90%) grain size range of 8-28 μm. When the sintering temperature was increased to 1150 ºC, the grain enlarged to a range of 6-58 μm, with most of the grain within the range of 12-36 μm. The further increase of the sintering temperature to 1200 ºC led to the enhancement of grain size to the range of 8-61 μm, with the majority of the grain within the range of 15-50 μm. The grain size of membranes sintered at 1250 ºC spans from 16-129 μm, with the majority within the range of 30-75 μm. A substantial grain growth with the increase of the sintering temperature was also observed for other ceramics like LaCoO_3-δ, SrCo_0.8Fe_0.2O_3-δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ membranes (Zhang et al., 1999; Kharton et al., 2001; Wang et al., 2005). The grain size growth decreased the Grain Boundary Length (GBL) of the sintered ceramics. For instance, the relative GBLs of the SSC membranes sintered at 1100, 1150, 1200, and 1250 ºC for 5 h are 3.0, 2.27, 1.96 and 1.0, respectively. The GBL calculation was based on the SEM results given that the grain geometry is cubic in shape. The measurement of grain size and the calculation of GBL based on the hypothesis that grain geometry is cubic in shape were previously reported in the literature (Diethelm et al., 2005; Zhang et al., 1999; Zeng et al., 2007). The GBL was estimated from half of the perimeter of the grains per surface area. In this study, 185, 157, 147, and 156 grains of membranes sintered at 1100, 1150, 1200, and 1250 ºC for 5 h, respectively, were selected to calculate the GBLs.

Figure 2
SEM microstructures of the various SSC membranes sintered at 1100-1250 ºC for 5 h under air.

Figure 3
The temperature dependence of the grain size distribution of the membranes sintered at 1100-1250 ºC.

The electrical conductivity of different membranes was measured by the 4-probe DC method under air, and the results are shown in Figure 4. All samples sintered at different temperatures displayed a similar trend in the change of electrical conductivities, with an initial increment, then a maximum to a subsequent decrease when the operating temperature increased from 300 to 900 ºC. For a doped perovskite oxide, the electrical neutrality is compensated by the valence change of B site metal ions at low temperature. However, at a certain high temperature satisfying the formation energy of oxygen vacancies, the electrical neutrality is sustained by the formation of oxygen vacancies, and this turning temperature exhibits the peak electrical conductivity. The electronic conductivity is only associated with the valence change of B site ions, so the electrical conductivity increases with test temperatures lower than the turning temperature. However, above the turning temperature, more and more oxygen vacancies are formed, meaning less and less valence change of B site ions, resulting in the gradual reduction of the total electrical conductivity. It can be seen that the corresponding temperature of peak electrical conductivity decreases mildly with the increase of sintering temperature. This may be due to the decrease in formation energy of oxygen vacancies in samples sintered at higher temperature (Ganduglia-Pirovano et al., 2007). In addition, the increase of the sintering temperature from 1100 to 1250 ºC resulted in a slight increase in the electrical conductivity. For example, the maximum values were 119, 129, 135 and 137 S.cm^-1 for the bars sintered at 1100, 1150, 1200 and 1250 ºC, respectively. Since it is well known that the ionic conductivity σion of mixed ionic-electronic conductors (MIECs) is typically smaller than 1 S.cm^-1, which is significantly smaller than the electronic conductivity σe, it could be deduced that the differences in total electrical conductivity σtotal between samples sintered at different temperatures are mainly due to the increases in electronic conductivity. Therefore, the slight increase of electrical conductivity (predominantly electronic) with the increase of sintering temperature, or decrease of grain boundary length, suggests that the grain boundary acts as a barrier for electron transport. This finding agrees with Maier et al.'s reports regarding the electrical blocking effect of the grain boundaries in doped cerium dioxide and acceptor-doped SrTiO₃ (Göbel et al., 2012; Guo et al., 2002). According to the report of Göbel et al. (2012; 2002), the following three causes can be considered to be the origin of this behavior: (i) the presence of insulating layers at the boundaries (owing to segregations of impurities); (ii) a change of the electron mobility (e.g., due to strain or related structural effects) and (iii) an increase of negatively charged point defects in the GB core; this is a reasonable hypothesis because it has been pointed out that a space charge situation with a positive space charge potential was the reason for an opposite case in which the grain boundary showed higher electrical conductivity (Gregori et al., 2011).

Figure 4
The electrical conductivity of the SSC membranes sintered at different temperatures.

According to the fact that significant decreases of GBLs only resulted in low increases of the electrical conductivity, we may deduce that the blocking effect of the grain boundary on the electron transport is very moderate and could even be neglected. Kharton et al. (2000) reported similar results for LaCoO_3-δ membranes, in which the influence of the ceramic microstructure on total electrical conductivity of LaCoO_3-δ ceramics was negligible. However, a very different phenomenon was observed for La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) by Garcia-Belmonte et al. (1998), who found that increasing the grain size significantly enhanced the electrical conductivity of LSCF. Therefore, this suggests that the effect of microstructure on the conductivity is also dependent on the composition of the individual membrane material.

Figure 5 depicts the dependence of the oxygen permeability of SSC membranes on the test temperatures and membrane sintering temperatures. All of the oxygen fluxes included in Figure 5 were obtained at the constant helium sweep rate of 100 mL.min^-1. The oxygen permeation fluxes of all the samples clearly increase with the increase of test temperatures from 775 to 900 ºC. For example, the SSC membrane sintered at 1100 ºC exhibits an oxygen permeation flux of only 1.36 mL.cm^-2.min^-1 at 775 ºC, but a much higher flux of 3.86 mL.cm^-2.min^-1 at 900 ºC. Zeng et al. (2009) reported that a 1100 ºC sintered SSC membrane with a thickness of 0.76 mm could permeate 3.2 mL oxygen per unit centimeter per minute. The membranes tested in this study are much thinner (0.3 mm) than theirs, so it is reasonable that the membrane exhibits a higher oxygen permeability of 3.86 mL.cm^-2.min^-1 under the same test condition. Thus, under identical chemical activity gradients, the effective permeation rate should increase with the reciprocal of the membrane thickness. It was also reported (Zeng et al., 2009) that the oxygen permeation process in the 0.76 mm thick membrane was rate-limited by the relatively slower membrane gas-solid surface exchange rate (compared to the faster membrane bulk diffusion rate). Since the current membrane has a much thinner thickness of 0.3 mm, the membrane bulk diffusion rate must be much faster than that in the thicker one, which means tha the slower membrane gas-solid surface exchange process should still be the rate-limiting step for oxygen permeation in the 0.3 mm membrane.

Moreover, the oxygen permeability gradually decreases with the increase of membrane sintering temperature, and thus the grain size. For example, oxygen permeation fluxes of 3.86, 3.46, 3.18, and 2.95 mL.cm^-2.min^-1 were reached for the membranes sintered at 1100, 1150, 1200 and 1250 ºC, respectively, at the same test temperature of 900 ºC. Figure 6 correlates the normalized GBLs of the different membranes with the oxygen fluxes measured under the same operating temperature of 900 ºC. Figure 6 strongly suggests that SSC membranes with larger GBL sintered at lower temperatures possess higher oxygen permeation fluxes. For example, the corresponding oxygen fluxes through SSC membranes with relative GBLs of 3.0 and 1.0 are 3.86 and 2.95 mL.cm^-2.min^-1, respectively. Thus, this suggests that the grain boundary that has higher defect concentrations and/or higher defect jump rates than in the grain bulk acts as a faster pathway for oxygen ion exchange and transportation. This can be further proved by the decrease of the activation energy with the decrease of sintering temperature. Figure 7 shows the Arrhenius plot for the oxygen permeation flux as a function of the inverse of the temperature. The activation energies for the oxygen permeation through the membranes sintered at 1100, 1150, 1200, and 1250 ºC are calculated to be 69.0, 70.0, 72.1, and 82.1 kJ.mol^-1, respectively, indicating that, the lower the sintering temperature, the lower the activation energy observed under similar operating conditions.

Figure 5
Temperature dependence of oxygen permeation fluxes of the SSC membranes sintered at various temperatures.

Figure 6
Dependence of oxygen fluxes permeated through various SSC samples at 900 °C on the normalized grain boundary lengths.

Figure 7
Arrhenius plot for the oxygen permeation flux depending on the inverse temperature.

CONCLUSION

SrSc_0.1Co_0.9O_3-δ samples with different microstructures were prepared by varying the sintering temperature from 1100 to 1250 ºC. All samples present a cubic perovskite structure, and the lattice parameters increase moderately with the increase of sintering temperature. The microstructure of the material is affected significantly by the sintering temperature, such that the average grain size of the samples is gradually increased with the increase of sintering temperature. The influence of the ceramic microstructure on total electrical conductivity (predominantly electronic) was found to be negligible. The oxygen transport properties of the samples were characterized by permeation measurements as a function of temperature in an air/helium oxygen partial pressure gradient. The decrease of the sintering temperature, which decreases the grain size and thus increases the grain boundary length, leads to an enhanced oxygen permeation flux and reduced activation energy. This implies that oxygen exchange and transport in the SrSc_0.1Co_0.9O_3-δ membranes occurs more rapidly along grain boundaries than in the grain bulk. Lastly, it was deduced that the oxygen permeation process is predominantly rate-controlled by the rate of oxygen surface exchange.

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Publication Dates

• Publication in this collection
  Sept 2015

History

• Received
  06 Feb 2014
• Reviewed
  03 Nov 2014
• Accepted
  30 Nov 2014