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Facile auto-combustion synthesis for oxygen separation membrane application
Journal of Membrane Science 329 (2009) 219–227
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Facile auto-combustion synthesis for oxygen separation membrane application Lei Ge a , Ran Ran a , Wei Zhou a , Zongping Shao a,∗ , Shaomin Liu b , Wanqin Jin a , Nanping Xu a a b
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing, JiangSu, 210009, PR China ARC Centre for Functional Nanomaterials, School of Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia
a r t i c l e
i n f o
Article history: Received 30 July 2007 Received in revised form 4 December 2008 Accepted 21 December 2008 Available online 30 December 2008 Keywords: Combustion synthesis La0.6 Sr0.4 Co0.2 Fe0.8 O3−␦ Perovskite membranes Oxygen permeability Electrical conductivity
a b s t r a c t The potential application of combustion synthesis of La0.6 Sr0.4 Co0.2 Fe0.8 O3−␦ (LSCF) based on a modified ethylenediaminetetraacetic acid (EDTA)–citrate complexing method with NH4 NO3 as combustion aid for ceramic oxygen separation membrane was systematically investigated. Depending on the relative amount of NH4 NO3 applied during the synthesis, the combustion can proceed in three different modes: self-propagating combustion, volumetric combustion and smothering combustion. As a whole, electrical conductivity and oxygen permeation fluxes of derived LSCF membrane increased with sintering temperature (1000–1300 ◦ C). Among three different combustion modes, LSCF from self-propagating combustion showed the highest sintering ability, electrical conductivity and oxygen permeability. Under optimized conditions, the derived membrane exhibited the permeation fluxes comparable to that of LSCF membrane prepared from the normal EDTA–citrate complexing method. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Oxygen is one of the most important raw materials for chemical industry. Nowadays, there are considerable interests in one kind of dense ceramic membranes based on mixed oxygen ionic and electronic conducting oxides for oxygen separation from air. Many technical/scientific research papers and patents about such membranes have appeared in literature during the past two decades. Among them, considerable investigations have been conducted on the development of new membrane materials with high oxygen permeability and phase/chemical stability [1–5], the oxygen permeating mechanism [6–10], the application of oxygen permeating membrane reactor for coupling reaction such as methane partial oxidation to syngas [11–15], and the reactor design and fabrication [16–20]. The enthusiasm about such membranes is mainly driven by their potential reduction in energy consumption and capital cost for oxygen production significantly as compared to the traditional industrial scale of cryogenic distillation of air [21–23]. The ceramic oxygen separation membranes are the dense-type sintering body of the mixed conducting oxides. They are usually fabricated from the powder samples via shaping followed by high-temperature sintering [24–27]. In more details, first the oxide powders are prepared by any of the various powder synthesis techniques including solid-state reaction, hydrothermal synthesis, co-precipitation, amorphous complexing process, and combustion synthesis; the derived powders are then fabricated into membranes
∗ Corresponding author. Tel.: +86 25 83587722; fax: +86 25 83365813. E-mail address: [email protected] (Z. Shao). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.12.040
with desired shape such as disk or tube by pressing, extrusion, or other shaping techniques; the green membranes are finally sintered at high temperatures to form the dense ceramic with high mechanical strength by sintering the particles. Economical and mass production of high-quality oxide powders is one of the key steps in realizing the practical industrial scale application of oxygen separation membranes. Sol–gel process based on combined EDTA–citrate complexing method has proved to be an excellent lab-scale powder synthesis technique for oxygen permeating membrane application [28–32]. Combustion synthesis, an energy-efficient and easy-to-scale-up process, has proved to be a convenient and versatile way for the preparation of large varieties of functional composite oxides [33–37]. It can not only eliminate the step of high-temperature sintering, but also greatly shorten the synthesis time. Recently, we have demonstrated that the combined EDTA–citrate complexing process could be changed into a low-temperature auto-combustion process via introducing a proper amount of NH4 NO3 as the combustion aid [38]. Therefore, the synthesis process based on EDTA–citrate combined process was greatly simplified, and highly promising for large-scale application. On the other hand, the properties of functional oxides are also closely related with the synthesis technique [39–42]. A slight change of the synthesis parameter(s) might have a significant impact on the properties of the derived oxide powders and their following application. We observed that the simple treatment of the EDTA–citrate solid precursor of Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−␦ perovskite oxide by HNO3 resulted in the change of relative concentration of various metal ions inside the perovskite structure and also the decrease of the oxygen surface exchange and bulk diffusion rates of the oxide [39], therefore a sharp decrease in cathode performance
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for solid-oxide fuel cells was observed. It was also observed that the pH value during the citric acid process had a significant impact on the microstructure and the oxygen permeability of the corresponding membranes [40]. The synthesis techniques should therefore be carefully tailored for the optimized performance of the membrane. In this study we presented a systematic investigation of the possible application of the NH4 NO3 -assisted auto-combustion synthesis based on combined EDTA–citrate complexing process for oxygen separation membrane application. La0.8 Sr0.2 Co0.4 Fe0.6 O3−␦ was selected as the membrane composition for this investigation. 2. Experimental 2.1. Synthesis and fabrication Cobalt nitrate, ferric nitrate, lanthanum nitrate, and strontium nitrate, all in analytical grades, were used as the metal sources. EDTA powder and crystallized citric acid were used as the chelants and also the fuels, which both have purities higher than 99.5%. The typical combustion process, in this study, includes the preparation of the mixed solution, the gellation, and the auto-combustion under the low-temperature heating. The stoichiometric amounts of metal nitrates were dissolved into a mixed metal nitrate solution. The required amount of EDTA dissolved in NH3 solution, the solid citric acid, and NH4 NO3 were introduced into the mixed solution under stirring in sequence. The mixed solution was then heated at ∼90 ◦ C over a hot plate under magnetic stirring until a gel was obtained, which was then transferred to a pre-heated oven at various temperatures (150–250 ◦ C) for a possible auto-combustion. The mole ratio of EDTA to citric acid in this study was fixed at 1:2 unless stated else. The auto-combustion experiment was conducted within a 250 mL Pyrex beaker, which was put inside a 2 L beaker for security issue. An electrical fan was used during the heat treatment to ensure a homogeneous temperature distribution, and also to enhance the gas flow inside the oven. For fabricating dense membranes, the obtained powder from the auto-combustion process was either directly pressed into diskshaped membranes using a diameter 15 mm stainless steel die under a hydraulic pressure of 420 MPa, or pre-calcined the powder at 600 ◦ C for 5 h before pressing. The as-obtained green membrane disks were then sintered between 1000 and 1300 ◦ C for 5 h in air, with the heating and cooling rate of 5 ◦ C min−1 . 2.2. Oxygen permeation measurement In our oxygen permeation test, both surfaces of the LSCF membranes were carefully polished to a thickness of 0.70 mm. Permeation properties of the membranes were investigated by the gas chromatography (GC) method using a high-temperature oxygen permeation apparatus via helium sweep gas method as shown in [43,44]. The effective membrane geometric area was around 0.4 cm2 at the sweep side for permeation study. The oxygen permeation flux was calculated by
−2 −1
JO2 (mol cm
s
) = CO2 − CN2
0.21 × × 0.79
28 1/2 32
×
F S
where CO2 and CN2 are the measured concentrations of oxygen and nitrogen in the gas on the sweep side (mol mL−1 ), respectively, F is the flow rate of the exit gas on the sweep side (mL s−1 ), and S is the membrane geometric area of the sweep side (cm2 ). 2.3. Characterization A thermocouple, interfaced with computer via Yudian 708P temperature indicator with the help of Labview program, was used
to monitor the temperature–time profiles of the auto-combustion process at the sampling rate of 10 s−1 [38]. It was immersed into the gel to measure the temperature around the powder during the auto-combustion process. The crystal structure of the synthesized powders or sintered membranes was characterized with an X-ray diffractometer (XRD, Bruker D8 Advance) using Cu K␣ radiation. The experimental diffraction patterns were collected at room temperature by step scanning in the range of 10◦ ≤ 2 ≤ 90◦ . The specific surface area of the oxide powders was characterized by N2 adsorption using a BELSORP II instrument at the temperature of liquid nitrogen. Before the measurement, the samples were pretreated at 200 ◦ C for 2 h under vacuum to remove the surface adsorbed species. The weight loss of the prepared samples was examined by the Thermogravimetric Analysis (TGA) using a NETZSCH STA 409 thermal gravity analyzer under flowing air at the rate of 20 mL min−1 . The surface or cross-section morphologies of the powders or sintered pellets were examined using an environmental scanning electron microscopy (ESEM), QUANTA-2000. The electrical conductivity was measured by four-probe DC method on sintered bars of approximate dimensions 2 mm × 5 mm × 12 mm. The measurements were performed under air atmosphere upon cooling from 900 to 300 ◦ C, 10 ◦ C per step [43,44]. 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 source meter. The current was increased from 1 A to a maximum value of 2 A.
3. Results and discussion 3.1. Auto-combustion behavior and powder properties It was observed that the combustion behavior of the solid precursors was closely related with the relative amount of NH4 NO3 applied during the synthesis and also the heating temperature. For the precursor from the normal EDTA–citrate process (no NH4 NO3 applied), combustion was not happened even at the heating temperature of 250 ◦ C. However, the introduction of proper amount of NH4 NO3 into the precursor resulted in a self-sustained combustion. As shown in Fig. 1, three different combustion modes were observed, they are volumetric combustion (VCS), self-propagating combustion (SPS) and smothering combustion (SCS), similar to that observed based on a glycine–nitrate process (GNP) [45]. Compared with the normal combined EDTA–citrate complexing sol–gel synthesis (ECS), the total time for the powder synthesis by the NH4 NO3 -assisted auto-combustion process was greatly shortened. The self-propagating combustion was characterized by starting at the top of the precursor and propagating towards the bottom of the beaker; the reaction was really fast, it just took about 20–25 s for the synthesis of 0.005 mol LSCF. The more NH4 NO3 applied in the synthesis, the more vagarious combustion was observed. While the volumetric combustion happened simultaneously throughout the whole precursor; the reaction was so fast that finished within 4–5 s. Flames were observed for both the self-propagating synthesis and the volumetric combustion synthesis. On the other hand, only occasional sparks can be observed during the non-flame smothering combustion. The reaction was in a much gentle manner and lasted several minutes for the auto-combustion. Fig. 2 shows the temperature profiles of the various combustion modes. The increasing amount of NH4 NO3 applied during the synthesis resulted in the increasing combustion temperature. The smothering combustion had the lowest temperature, followed by the volumetric combustion, and then the self-propagating combustion. It should be mentioned that the temperatures reported here could be lower than the real flame temperature, since the heat transferred from the flame to the thermocouple has time lag.
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Fig. 1. Experimental procedure for low temperature auto-combustion preparation of LSCF powder (SPS: self-propagating combustion; VCS: volumetric combustion, SCS: smothering combustion).
The powders investigated in Figs. 1 and 2 were then selected for the further investigation. Two types of powders from the self-propagating combustion synthesis, named as LSCF-SPS-A and LSCF-SPS-B, represent the synthesis of 0.005 mol LSCF by applying 10 g and 35 g NH4 NO3 as combustion aid, respectively, and under a heating temperature of 250 ◦ C for initialization of the auto-combustion; while LSCF-VCS represents the powder from the volumetric combustion process with 10 g NH4 NO3 applied in the synthesis of 0.005 mol LSCF under the heating temperature of 175 ◦ C; and LSCF-SCS represents the powder synthesized from the smothering combustion with 7.5 g NH4 NO3 applied in the synthesis of 0.005 mol LSCF under the heating temperature of 250 ◦ C. Fig. 3 shows the corresponding XRD patterns of the respective assynthesized powders. All samples formed the perovskite structure
with no other impurity phases detected. The powders from the self-propagating combustions (LSCF-SPS-A and LSCF-SPS-B) had the highest crystallization degree, which was agreeable with the highest combustion temperature as shown in Fig. 2. Most of the as-synthesized powders were very light in weight, and easily to be crashed into very fine powder, while the LSCF-SPS-B powder was in much denser state and relatively harder to be crashed. The surface morphologies of the various powders are shown in Fig. 4. As discussed in our previous report [38], the packing density of SPSA is only about 1/1000 of the structural density of LSCF as well as sample SCS and VCS. With well agreement, highly porous morphological states of powders LSCF-SPS-A, LSCF-VCS, and LSCF-SCS were
Fig. 2. Temperature profiles of the LSCF bulk oxides during the auto-combustion process at different combustion modes.
Fig. 3. The room-temperature X-ray diffraction patterns of LSCF prepared from the auto-combustion process at different combustion modes.
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Fig. 4. SEM morphologies of the various LSCF powders synthesized by auto-combustion process with different combustion modes.
Fig. 5. SEM morphologies of the various LSCF membranes synthesized by different auto-combustion types sintered at 1200 ◦ C without pre-calcination.
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Fig. 6. TG curves of the as-prepared powders via different auto-combustion processes.
observed (specific surface area of 10–18 m2 g−1 ), while the LSCFSPS-B powder resulted somewhat sintered (specific surface area of 1.5 m2 g−1 ). 3.2. Sintering behavior of corresponding membranes The as-prepared powders from the various combustion modes were first directly applied to fabricate disk-shape membranes for sintering behavior investigation. It was found that the membranes had very large shrinkage after the high-temperature sintering. After the sintering at 1200 ◦ C for 5 h under air, the shrinkage reached 53, 61 and 56% for the membranes prepared from the powders of LSCF-SPS-A, LSCF-VCS, and LSCF-SCS, respectively. Cracks around the edges of the membranes were also observed for most of the samples, especially for membranes with the powders made from the SCS and VCS modes. Moreover, the relative density of these membranes measured by the Archimedes method was below 90% which was not dense enough for permeation test. Fig. 5 shows the surface morphologies of the sintered membranes. Obviously, large amounts of pores with the diameter of 2–10 m were appeared in the membranes composed of LSCF-SCS and LSCF-VCS. Membrane made from LSCF-SPS-A had the highest degree of density, while the membrane made from LSCF-SPS-B displayed a poor sintering behavior. Fig. 6 shows the TGA curves of the LSCF powders synthesized from the various combustion modes. The weight loss reached as large as 65% for the sample from the smothering combustion synthesis (LSCF-SCS), about 45% for LSCF-VCS, and ∼15% for LSCFSPS-A. It suggests there were still large amounts of organic residues inside the LSCF powder synthesized from the auto-combustion synthesis, especially for the powders from the smothering and volumetric combustion synthesis. The decreasing amounts of organic residues in the sequence of LSCF-SCS, LSCF-VSC, to LSCF-SPS-A coincided with the increased combustion temperature as shown in Fig. 2. A much different TGA curve was observed for the LSCF-SPS-B powder. The sample first gained weight of ∼0.5% between the temperature of 300 and 450 ◦ C, then started to loss weight with the further increase in temperature, though the weight loss was also very weak. The gain of weight for the LSCF-SPS-B can be explained by the oxidization of the cobalt and iron in LSCF to a higher valence state. During the auto-combustion, the sample temperature raised suddenly to a very high value, some of which could be higher than 1000 ◦ C, therefore a thermally induced reduction of the cobalt and iron ions in LSCF structure to a lower valence state was likely happened. On the other hand, since the solid precursor was in a fuel-rich condition, the instant high-temperature combustion
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could produce a lot of highly reductive species with small molecule weight, which could also lead to the partial reduction of LSCF. Once the combustion reaction was ceased, the as-synthesized powder cooled down to room temperature so fast that the partially reduced LSCF powder did not have sufficient time to be re-oxidized by air to its most stable oxidation state. Therefore, the re-oxidation was happened during the TGA experiment. Furthermore, the small weight change also demonstrated there was almost no carbon residue inside the LSCF-SPS-B sample. The substantial amount of organic residues inside the oxides of LSCF-SPS-A, LSCF-SCS, and LSCF-VCS were likely to perform as pore formers, which might be the reason for the porous states of the membranes then fabricated. While the porous membrane made from LSCF-SPS-B might be attributed to the poor sintering ability of the powder. Based on above results, the best powder synthesis condition for achieving a high-density membrane is the self-propagating combustion at the condition that 10 g NH4 NO3 applied in the synthesis of 0.005 mol LSCF (SPS-A). Similar porous membranes were observed by direct application of the GNP combustion-synthesized La0.8 Sr0.2 CrO3 powder for membrane fabrication [45]. It was reported that the pre-calcination of the oxide from autocombustion resulted in the improved membrane density [45]. Therefore, in this study, we also investigated the effects of the pre-calcination of the as-synthesized powders at 600 ◦ C for 5 h under air for following application. The shrinkage after the sintering at 1200 ◦ C was 36, 37, and 50% for the membranes made from pre-calcined LSCF-SPS-A, LSCF-VCS, and LSCF-SCS, respectively, which was slightly smaller than that made from the un-calcined powders. However, the integrity of the membranes was greatly improved at this time. No crack was formed for most of the sintered membranes. Furthermore, the relative density of the membrane derived by auto-combustion and pre-calcination was significantly improved (>95%) for further permeation tests. Fig. 7 shows the corresponding SEM photos of the membranes made from LSCF-SPS-A after pre-calcination then sintered between 1100 and 1300 ◦ C. The membranes were much better densified as compared with the membranes made from the un-calcined powders sintered at the same temperature as shown in Fig. 5. It suggests that the elimination of organic residue is critical for obtaining the dense membranes. The grain size of the membranes increased steadily with the increase of the sintering temperature, which was 0.1–0.2 m, 2–3 m, and ∼10 m for the membranes calcined at 1100, 1200 and 1300 ◦ C, respectively. The influence of combustion mode on the membrane morphology was also investigated. Fig. 8 shows the SEM morphologies of the membranes prepared from different combustion-synthesized LSCF powders with pre-calcination and sintered at 1200 ◦ C for 5 h. As a whole, all the samples have the similar sintering behavior after eliminating the organic residues. 3.3. Electrical conductivity and oxygen permeation properties Under the bulk diffusion controlling, the oxygen permeation through mixed conducting membranes can be expressed by the equation: ln P
JO2
RT =− 2 2 4 F L
O2
ln P
i e d[ln PO2 ] i + e
O2
where F is Faraday’s constant, L is the membrane thickness, i and e are the oxygen ionic and electrical conductivity, respectively. PO 2 and PO are the equivalent oxygen partial pressures at the feed side 2 and sweep side, respectively. It demonstrates that the oxygen permeation flux is closely related with the electrical conductivity of the membranes. In the
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Fig. 7. Sectional SEM morphologies of the various LSCF membranes synthesized by auto-combustion process (SPS-A) sintered at 1100–1300 ◦ C with powder pre-calcination.
Fig. 8. Sectional SEM morphologies of the various LSCF membranes synthesized by different auto-combustion types sintered at 1200 ◦ C with powder pre-calcination.
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Fig. 9. The temperature dependence of the electrical conductivities of the various membranes prepared from different combustion modes and sintered at different temperatures.
conductivity investigation, all the powders were pre-calcinated before pressing bar-shaped samples. The effects of sintering temperature and combustion mode during the powder synthesis on the electrical conductivities of the corresponding membranes were then measured by the four-terminal DC conductivity technique, and the results were shown in Fig. 9. In a previous paper, we observed that the electrical conductivity of LSCF membrane prepared from normal EDTA–citrate complexing method increased with the increase of the sintering temperature [43]. As a whole, an increase in the sintering temperature also resulted in the increasing electrical conductivity, similar to that observed for the normal EDTA–citrate derived LSCF [43], except for the LSCF-SPS-A sample, which experienced a decrease in electrical conductivity with the sintering temperature elevated from 1200 to 1300 ◦ C, this may be due to the melting or element volatilization at the high sintering temperature of 1300 ◦ C as badly melting was observed. At sintering temperature of 1100 or 1200 ◦ C, the membrane made from LSCFSPS-A powder had the highest conductivity. The 1100 ◦ C sintered one reached a maximum value of ∼250 S cm−1 at an operating temperature of 550 ◦ C, followed by the membrane with the powder prepared from the volumetric combustion (LSCF-VCS) (145 S cm−1 at 550 ◦ C), and then the smothering combustion synthesis (LSCFSCS) (91 S cm−1 at 550 ◦ C). The powders from the self-propagating combustion with 35 g NH4 NO3 applied in the synthesis (LSCF-SPSB) had the lowest conductivity of <40 S cm−1 , which might be due to the porous state of the membrane. However, at the sintering temperature of 1300 ◦ C, the membrane from smothering combustion synthesis showed the highest conductivity with a maximum value of ∼270 S cm−1 at 550 ◦ C. An electrical conductivity of ∼300 S cm−1 was observed for the 1300 ◦ C calcined sample with the powder prepared from the normal EDTA–citrate complexing process [43]. It suggests that the powders from the auto-combustion synthesis could have comparable conductivity to that from the normal EDTA–citrate complexing synthesis. The oxygen permeation properties of the membranes made from LSCF-SPS-A with powder pre-calcination were chiefly investigated.
As shown in Fig. 10, an increase in the oxygen permeation flux was observed with the increase of the sintering temperature. A maximum value of ∼4.46 × 10−7 mol cm−2 s−1 [STP] was obtained for the membrane sintered at 1300 ◦ C for 5 h. As we demonstrated before, the increasing oxygen permeation flux was closely related with the increase of the grain size of the membranes [43]. Fig. 10 also presents the temperature dependence of the oxygen permeation fluxes through LSCF membrane prepared from the normal EDTA–citrate complexing process sintered at 1300 ◦ C for 5 h. Similar oxygen permeation flux was observed for the membranes made from LSCF-SPS-A powder or ECS powder. It suggests that the addition of NH4 NO3 to the EDTA–citrate system to change the sol–gel synthesis into an auto-combustion process did not cast significant influence on the transporting properties of the oxide. The influence of combustion mode on the oxygen permeation flux of the corresponding membranes was also investigated. Fig. 11 shows a
Fig. 10. The temperature dependence of permeation fluxes through LSCF-SPS-A membranes sintered at different temperatures for 5 h with powder pre-calcination.
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synthesis. As a result, perovskite-type composite oxides prepared by optimized auto-combustion method exhibited comparable performance in practical applications such as oxygen permeation membrane for its facile synthesis process and easy product treatment. Acknowledgements This work was supported by the National Natural Science Foundation of China under contracts No. 20646002 and No. 20676061, and also Natural Science Foundation of JiangSu Province, under contract No. BK2006180. References Fig. 11. The temperature dependence of permeation fluxes through LSCF membranes with the powder from different combustion modes and sintered at 1200 ◦ C for 5 h with powder pre-calcination.
comparison of the oxygen permeation flux through the different membranes sintered at 1200 ◦ C for 5 h with powder pre-calcination. The membranes made from the LSCF-SPS-A and LSCF-VCS have better performance, while the membrane from the LSCF-SCS had the worst oxygen permeation fluxes. Due to the crack formation of the membranes with the powder from direct auto-combustion synthesis without pre-calcination powder, we were not able to get the oxygen permeability information of the corresponding sintered membranes. Actually, synthesis condition makes an important effect on the amount of organic residues. In order to ensure low level of organic residues, the pre-calcination process is still necessary for membrane application based on the auto-combustion synthesis. However, current auto-combustion process still has great advantage over the normal EDTA–citrate process for membrane application. For the normal EDTA–citrate complexing process, due to the presence of tremendous amount of organic inside the precursor, a huge amount of gases were produced during the calcination at elevated temperature, which could easily blow away the synthesized powder from the container and contaminate the furnace. Furthermore, simultaneously a huge amount of oxygen would be consumed during the synthesis; it then had a strict requirement on the atmosphere inside the furnace, especially for the large-scale application; once the O2 and CO2 could not be provided or removed in time, it would have a significant impact on the powder properties. While the precalcination process was much easier to handle since most of the organic in the oxide has already burned out from the combustionsynthesized powder. 4. Conclusions A facile EDTA–citrate complexation/auto-combustion method was developed for the preparation of pervoskite-type oxygen permeable materials. The combustion process could be smothering, volumetric or self-propagating, depending on the NH4 NO3 addition amount and the pre-heating temperature. Well-crystallized LSCF can be obtained through volumetric and self-propagating combustion. At sintering temperature of 1200 ◦ C, the membrane made from LSCF-SPS-A powder had the highest conductivity (>250 S cm−1 at 550 ◦ C). An increasing in the oxygen permeation flux was observed with the increase of the sintering temperature. A maximum value of ∼4.46 × 10−7 mol cm−2 s−1 [STP] was observed for the LSCF-SPSA membrane sintered at 1300 ◦ C for 5 h. Due to large shrinkage and the cracks formation of the membranes caused by organic residues in the as-obtained oxides, the pre-calcination process is preferred for membrane application based on the auto-combustion
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Facile auto-combustion synthesis for oxygen separation membrane application Lei Ge a , Ran Ran a , Wei Zhou a , Zongping Shao a,∗ , Shaomin Liu b , Wanqin Jin a , Nanping Xu a a b
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing, JiangSu, 210009, PR China ARC Centre for Functional Nanomaterials, School of Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia
a r t i c l e
i n f o
Article history: Received 30 July 2007 Received in revised form 4 December 2008 Accepted 21 December 2008 Available online 30 December 2008 Keywords: Combustion synthesis La0.6 Sr0.4 Co0.2 Fe0.8 O3−␦ Perovskite membranes Oxygen permeability Electrical conductivity
a b s t r a c t The potential application of combustion synthesis of La0.6 Sr0.4 Co0.2 Fe0.8 O3−␦ (LSCF) based on a modified ethylenediaminetetraacetic acid (EDTA)–citrate complexing method with NH4 NO3 as combustion aid for ceramic oxygen separation membrane was systematically investigated. Depending on the relative amount of NH4 NO3 applied during the synthesis, the combustion can proceed in three different modes: self-propagating combustion, volumetric combustion and smothering combustion. As a whole, electrical conductivity and oxygen permeation fluxes of derived LSCF membrane increased with sintering temperature (1000–1300 ◦ C). Among three different combustion modes, LSCF from self-propagating combustion showed the highest sintering ability, electrical conductivity and oxygen permeability. Under optimized conditions, the derived membrane exhibited the permeation fluxes comparable to that of LSCF membrane prepared from the normal EDTA–citrate complexing method. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Oxygen is one of the most important raw materials for chemical industry. Nowadays, there are considerable interests in one kind of dense ceramic membranes based on mixed oxygen ionic and electronic conducting oxides for oxygen separation from air. Many technical/scientific research papers and patents about such membranes have appeared in literature during the past two decades. Among them, considerable investigations have been conducted on the development of new membrane materials with high oxygen permeability and phase/chemical stability [1–5], the oxygen permeating mechanism [6–10], the application of oxygen permeating membrane reactor for coupling reaction such as methane partial oxidation to syngas [11–15], and the reactor design and fabrication [16–20]. The enthusiasm about such membranes is mainly driven by their potential reduction in energy consumption and capital cost for oxygen production significantly as compared to the traditional industrial scale of cryogenic distillation of air [21–23]. The ceramic oxygen separation membranes are the dense-type sintering body of the mixed conducting oxides. They are usually fabricated from the powder samples via shaping followed by high-temperature sintering [24–27]. In more details, first the oxide powders are prepared by any of the various powder synthesis techniques including solid-state reaction, hydrothermal synthesis, co-precipitation, amorphous complexing process, and combustion synthesis; the derived powders are then fabricated into membranes
∗ Corresponding author. Tel.: +86 25 83587722; fax: +86 25 83365813. E-mail address: [email protected] (Z. Shao). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.12.040
with desired shape such as disk or tube by pressing, extrusion, or other shaping techniques; the green membranes are finally sintered at high temperatures to form the dense ceramic with high mechanical strength by sintering the particles. Economical and mass production of high-quality oxide powders is one of the key steps in realizing the practical industrial scale application of oxygen separation membranes. Sol–gel process based on combined EDTA–citrate complexing method has proved to be an excellent lab-scale powder synthesis technique for oxygen permeating membrane application [28–32]. Combustion synthesis, an energy-efficient and easy-to-scale-up process, has proved to be a convenient and versatile way for the preparation of large varieties of functional composite oxides [33–37]. It can not only eliminate the step of high-temperature sintering, but also greatly shorten the synthesis time. Recently, we have demonstrated that the combined EDTA–citrate complexing process could be changed into a low-temperature auto-combustion process via introducing a proper amount of NH4 NO3 as the combustion aid [38]. Therefore, the synthesis process based on EDTA–citrate combined process was greatly simplified, and highly promising for large-scale application. On the other hand, the properties of functional oxides are also closely related with the synthesis technique [39–42]. A slight change of the synthesis parameter(s) might have a significant impact on the properties of the derived oxide powders and their following application. We observed that the simple treatment of the EDTA–citrate solid precursor of Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−␦ perovskite oxide by HNO3 resulted in the change of relative concentration of various metal ions inside the perovskite structure and also the decrease of the oxygen surface exchange and bulk diffusion rates of the oxide [39], therefore a sharp decrease in cathode performance
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for solid-oxide fuel cells was observed. It was also observed that the pH value during the citric acid process had a significant impact on the microstructure and the oxygen permeability of the corresponding membranes [40]. The synthesis techniques should therefore be carefully tailored for the optimized performance of the membrane. In this study we presented a systematic investigation of the possible application of the NH4 NO3 -assisted auto-combustion synthesis based on combined EDTA–citrate complexing process for oxygen separation membrane application. La0.8 Sr0.2 Co0.4 Fe0.6 O3−␦ was selected as the membrane composition for this investigation. 2. Experimental 2.1. Synthesis and fabrication Cobalt nitrate, ferric nitrate, lanthanum nitrate, and strontium nitrate, all in analytical grades, were used as the metal sources. EDTA powder and crystallized citric acid were used as the chelants and also the fuels, which both have purities higher than 99.5%. The typical combustion process, in this study, includes the preparation of the mixed solution, the gellation, and the auto-combustion under the low-temperature heating. The stoichiometric amounts of metal nitrates were dissolved into a mixed metal nitrate solution. The required amount of EDTA dissolved in NH3 solution, the solid citric acid, and NH4 NO3 were introduced into the mixed solution under stirring in sequence. The mixed solution was then heated at ∼90 ◦ C over a hot plate under magnetic stirring until a gel was obtained, which was then transferred to a pre-heated oven at various temperatures (150–250 ◦ C) for a possible auto-combustion. The mole ratio of EDTA to citric acid in this study was fixed at 1:2 unless stated else. The auto-combustion experiment was conducted within a 250 mL Pyrex beaker, which was put inside a 2 L beaker for security issue. An electrical fan was used during the heat treatment to ensure a homogeneous temperature distribution, and also to enhance the gas flow inside the oven. For fabricating dense membranes, the obtained powder from the auto-combustion process was either directly pressed into diskshaped membranes using a diameter 15 mm stainless steel die under a hydraulic pressure of 420 MPa, or pre-calcined the powder at 600 ◦ C for 5 h before pressing. The as-obtained green membrane disks were then sintered between 1000 and 1300 ◦ C for 5 h in air, with the heating and cooling rate of 5 ◦ C min−1 . 2.2. Oxygen permeation measurement In our oxygen permeation test, both surfaces of the LSCF membranes were carefully polished to a thickness of 0.70 mm. Permeation properties of the membranes were investigated by the gas chromatography (GC) method using a high-temperature oxygen permeation apparatus via helium sweep gas method as shown in [43,44]. The effective membrane geometric area was around 0.4 cm2 at the sweep side for permeation study. The oxygen permeation flux was calculated by
−2 −1
JO2 (mol cm
s
) = CO2 − CN2
0.21 × × 0.79
28 1/2 32
×
F S
where CO2 and CN2 are the measured concentrations of oxygen and nitrogen in the gas on the sweep side (mol mL−1 ), respectively, F is the flow rate of the exit gas on the sweep side (mL s−1 ), and S is the membrane geometric area of the sweep side (cm2 ). 2.3. Characterization A thermocouple, interfaced with computer via Yudian 708P temperature indicator with the help of Labview program, was used
to monitor the temperature–time profiles of the auto-combustion process at the sampling rate of 10 s−1 [38]. It was immersed into the gel to measure the temperature around the powder during the auto-combustion process. The crystal structure of the synthesized powders or sintered membranes was characterized with an X-ray diffractometer (XRD, Bruker D8 Advance) using Cu K␣ radiation. The experimental diffraction patterns were collected at room temperature by step scanning in the range of 10◦ ≤ 2 ≤ 90◦ . The specific surface area of the oxide powders was characterized by N2 adsorption using a BELSORP II instrument at the temperature of liquid nitrogen. Before the measurement, the samples were pretreated at 200 ◦ C for 2 h under vacuum to remove the surface adsorbed species. The weight loss of the prepared samples was examined by the Thermogravimetric Analysis (TGA) using a NETZSCH STA 409 thermal gravity analyzer under flowing air at the rate of 20 mL min−1 . The surface or cross-section morphologies of the powders or sintered pellets were examined using an environmental scanning electron microscopy (ESEM), QUANTA-2000. The electrical conductivity was measured by four-probe DC method on sintered bars of approximate dimensions 2 mm × 5 mm × 12 mm. The measurements were performed under air atmosphere upon cooling from 900 to 300 ◦ C, 10 ◦ C per step [43,44]. 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 source meter. The current was increased from 1 A to a maximum value of 2 A.
3. Results and discussion 3.1. Auto-combustion behavior and powder properties It was observed that the combustion behavior of the solid precursors was closely related with the relative amount of NH4 NO3 applied during the synthesis and also the heating temperature. For the precursor from the normal EDTA–citrate process (no NH4 NO3 applied), combustion was not happened even at the heating temperature of 250 ◦ C. However, the introduction of proper amount of NH4 NO3 into the precursor resulted in a self-sustained combustion. As shown in Fig. 1, three different combustion modes were observed, they are volumetric combustion (VCS), self-propagating combustion (SPS) and smothering combustion (SCS), similar to that observed based on a glycine–nitrate process (GNP) [45]. Compared with the normal combined EDTA–citrate complexing sol–gel synthesis (ECS), the total time for the powder synthesis by the NH4 NO3 -assisted auto-combustion process was greatly shortened. The self-propagating combustion was characterized by starting at the top of the precursor and propagating towards the bottom of the beaker; the reaction was really fast, it just took about 20–25 s for the synthesis of 0.005 mol LSCF. The more NH4 NO3 applied in the synthesis, the more vagarious combustion was observed. While the volumetric combustion happened simultaneously throughout the whole precursor; the reaction was so fast that finished within 4–5 s. Flames were observed for both the self-propagating synthesis and the volumetric combustion synthesis. On the other hand, only occasional sparks can be observed during the non-flame smothering combustion. The reaction was in a much gentle manner and lasted several minutes for the auto-combustion. Fig. 2 shows the temperature profiles of the various combustion modes. The increasing amount of NH4 NO3 applied during the synthesis resulted in the increasing combustion temperature. The smothering combustion had the lowest temperature, followed by the volumetric combustion, and then the self-propagating combustion. It should be mentioned that the temperatures reported here could be lower than the real flame temperature, since the heat transferred from the flame to the thermocouple has time lag.
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Fig. 1. Experimental procedure for low temperature auto-combustion preparation of LSCF powder (SPS: self-propagating combustion; VCS: volumetric combustion, SCS: smothering combustion).
The powders investigated in Figs. 1 and 2 were then selected for the further investigation. Two types of powders from the self-propagating combustion synthesis, named as LSCF-SPS-A and LSCF-SPS-B, represent the synthesis of 0.005 mol LSCF by applying 10 g and 35 g NH4 NO3 as combustion aid, respectively, and under a heating temperature of 250 ◦ C for initialization of the auto-combustion; while LSCF-VCS represents the powder from the volumetric combustion process with 10 g NH4 NO3 applied in the synthesis of 0.005 mol LSCF under the heating temperature of 175 ◦ C; and LSCF-SCS represents the powder synthesized from the smothering combustion with 7.5 g NH4 NO3 applied in the synthesis of 0.005 mol LSCF under the heating temperature of 250 ◦ C. Fig. 3 shows the corresponding XRD patterns of the respective assynthesized powders. All samples formed the perovskite structure
with no other impurity phases detected. The powders from the self-propagating combustions (LSCF-SPS-A and LSCF-SPS-B) had the highest crystallization degree, which was agreeable with the highest combustion temperature as shown in Fig. 2. Most of the as-synthesized powders were very light in weight, and easily to be crashed into very fine powder, while the LSCF-SPS-B powder was in much denser state and relatively harder to be crashed. The surface morphologies of the various powders are shown in Fig. 4. As discussed in our previous report [38], the packing density of SPSA is only about 1/1000 of the structural density of LSCF as well as sample SCS and VCS. With well agreement, highly porous morphological states of powders LSCF-SPS-A, LSCF-VCS, and LSCF-SCS were
Fig. 2. Temperature profiles of the LSCF bulk oxides during the auto-combustion process at different combustion modes.
Fig. 3. The room-temperature X-ray diffraction patterns of LSCF prepared from the auto-combustion process at different combustion modes.
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Fig. 4. SEM morphologies of the various LSCF powders synthesized by auto-combustion process with different combustion modes.
Fig. 5. SEM morphologies of the various LSCF membranes synthesized by different auto-combustion types sintered at 1200 ◦ C without pre-calcination.
L. Ge et al. / Journal of Membrane Science 329 (2009) 219–227
Fig. 6. TG curves of the as-prepared powders via different auto-combustion processes.
observed (specific surface area of 10–18 m2 g−1 ), while the LSCFSPS-B powder resulted somewhat sintered (specific surface area of 1.5 m2 g−1 ). 3.2. Sintering behavior of corresponding membranes The as-prepared powders from the various combustion modes were first directly applied to fabricate disk-shape membranes for sintering behavior investigation. It was found that the membranes had very large shrinkage after the high-temperature sintering. After the sintering at 1200 ◦ C for 5 h under air, the shrinkage reached 53, 61 and 56% for the membranes prepared from the powders of LSCF-SPS-A, LSCF-VCS, and LSCF-SCS, respectively. Cracks around the edges of the membranes were also observed for most of the samples, especially for membranes with the powders made from the SCS and VCS modes. Moreover, the relative density of these membranes measured by the Archimedes method was below 90% which was not dense enough for permeation test. Fig. 5 shows the surface morphologies of the sintered membranes. Obviously, large amounts of pores with the diameter of 2–10 m were appeared in the membranes composed of LSCF-SCS and LSCF-VCS. Membrane made from LSCF-SPS-A had the highest degree of density, while the membrane made from LSCF-SPS-B displayed a poor sintering behavior. Fig. 6 shows the TGA curves of the LSCF powders synthesized from the various combustion modes. The weight loss reached as large as 65% for the sample from the smothering combustion synthesis (LSCF-SCS), about 45% for LSCF-VCS, and ∼15% for LSCFSPS-A. It suggests there were still large amounts of organic residues inside the LSCF powder synthesized from the auto-combustion synthesis, especially for the powders from the smothering and volumetric combustion synthesis. The decreasing amounts of organic residues in the sequence of LSCF-SCS, LSCF-VSC, to LSCF-SPS-A coincided with the increased combustion temperature as shown in Fig. 2. A much different TGA curve was observed for the LSCF-SPS-B powder. The sample first gained weight of ∼0.5% between the temperature of 300 and 450 ◦ C, then started to loss weight with the further increase in temperature, though the weight loss was also very weak. The gain of weight for the LSCF-SPS-B can be explained by the oxidization of the cobalt and iron in LSCF to a higher valence state. During the auto-combustion, the sample temperature raised suddenly to a very high value, some of which could be higher than 1000 ◦ C, therefore a thermally induced reduction of the cobalt and iron ions in LSCF structure to a lower valence state was likely happened. On the other hand, since the solid precursor was in a fuel-rich condition, the instant high-temperature combustion
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could produce a lot of highly reductive species with small molecule weight, which could also lead to the partial reduction of LSCF. Once the combustion reaction was ceased, the as-synthesized powder cooled down to room temperature so fast that the partially reduced LSCF powder did not have sufficient time to be re-oxidized by air to its most stable oxidation state. Therefore, the re-oxidation was happened during the TGA experiment. Furthermore, the small weight change also demonstrated there was almost no carbon residue inside the LSCF-SPS-B sample. The substantial amount of organic residues inside the oxides of LSCF-SPS-A, LSCF-SCS, and LSCF-VCS were likely to perform as pore formers, which might be the reason for the porous states of the membranes then fabricated. While the porous membrane made from LSCF-SPS-B might be attributed to the poor sintering ability of the powder. Based on above results, the best powder synthesis condition for achieving a high-density membrane is the self-propagating combustion at the condition that 10 g NH4 NO3 applied in the synthesis of 0.005 mol LSCF (SPS-A). Similar porous membranes were observed by direct application of the GNP combustion-synthesized La0.8 Sr0.2 CrO3 powder for membrane fabrication [45]. It was reported that the pre-calcination of the oxide from autocombustion resulted in the improved membrane density [45]. Therefore, in this study, we also investigated the effects of the pre-calcination of the as-synthesized powders at 600 ◦ C for 5 h under air for following application. The shrinkage after the sintering at 1200 ◦ C was 36, 37, and 50% for the membranes made from pre-calcined LSCF-SPS-A, LSCF-VCS, and LSCF-SCS, respectively, which was slightly smaller than that made from the un-calcined powders. However, the integrity of the membranes was greatly improved at this time. No crack was formed for most of the sintered membranes. Furthermore, the relative density of the membrane derived by auto-combustion and pre-calcination was significantly improved (>95%) for further permeation tests. Fig. 7 shows the corresponding SEM photos of the membranes made from LSCF-SPS-A after pre-calcination then sintered between 1100 and 1300 ◦ C. The membranes were much better densified as compared with the membranes made from the un-calcined powders sintered at the same temperature as shown in Fig. 5. It suggests that the elimination of organic residue is critical for obtaining the dense membranes. The grain size of the membranes increased steadily with the increase of the sintering temperature, which was 0.1–0.2 m, 2–3 m, and ∼10 m for the membranes calcined at 1100, 1200 and 1300 ◦ C, respectively. The influence of combustion mode on the membrane morphology was also investigated. Fig. 8 shows the SEM morphologies of the membranes prepared from different combustion-synthesized LSCF powders with pre-calcination and sintered at 1200 ◦ C for 5 h. As a whole, all the samples have the similar sintering behavior after eliminating the organic residues. 3.3. Electrical conductivity and oxygen permeation properties Under the bulk diffusion controlling, the oxygen permeation through mixed conducting membranes can be expressed by the equation: ln P
JO2
RT =− 2 2 4 F L
O2
ln P
i e d[ln PO2 ] i + e
O2
where F is Faraday’s constant, L is the membrane thickness, i and e are the oxygen ionic and electrical conductivity, respectively. PO 2 and PO are the equivalent oxygen partial pressures at the feed side 2 and sweep side, respectively. It demonstrates that the oxygen permeation flux is closely related with the electrical conductivity of the membranes. In the
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Fig. 7. Sectional SEM morphologies of the various LSCF membranes synthesized by auto-combustion process (SPS-A) sintered at 1100–1300 ◦ C with powder pre-calcination.
Fig. 8. Sectional SEM morphologies of the various LSCF membranes synthesized by different auto-combustion types sintered at 1200 ◦ C with powder pre-calcination.
L. Ge et al. / Journal of Membrane Science 329 (2009) 219–227
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Fig. 9. The temperature dependence of the electrical conductivities of the various membranes prepared from different combustion modes and sintered at different temperatures.
conductivity investigation, all the powders were pre-calcinated before pressing bar-shaped samples. The effects of sintering temperature and combustion mode during the powder synthesis on the electrical conductivities of the corresponding membranes were then measured by the four-terminal DC conductivity technique, and the results were shown in Fig. 9. In a previous paper, we observed that the electrical conductivity of LSCF membrane prepared from normal EDTA–citrate complexing method increased with the increase of the sintering temperature [43]. As a whole, an increase in the sintering temperature also resulted in the increasing electrical conductivity, similar to that observed for the normal EDTA–citrate derived LSCF [43], except for the LSCF-SPS-A sample, which experienced a decrease in electrical conductivity with the sintering temperature elevated from 1200 to 1300 ◦ C, this may be due to the melting or element volatilization at the high sintering temperature of 1300 ◦ C as badly melting was observed. At sintering temperature of 1100 or 1200 ◦ C, the membrane made from LSCFSPS-A powder had the highest conductivity. The 1100 ◦ C sintered one reached a maximum value of ∼250 S cm−1 at an operating temperature of 550 ◦ C, followed by the membrane with the powder prepared from the volumetric combustion (LSCF-VCS) (145 S cm−1 at 550 ◦ C), and then the smothering combustion synthesis (LSCFSCS) (91 S cm−1 at 550 ◦ C). The powders from the self-propagating combustion with 35 g NH4 NO3 applied in the synthesis (LSCF-SPSB) had the lowest conductivity of <40 S cm−1 , which might be due to the porous state of the membrane. However, at the sintering temperature of 1300 ◦ C, the membrane from smothering combustion synthesis showed the highest conductivity with a maximum value of ∼270 S cm−1 at 550 ◦ C. An electrical conductivity of ∼300 S cm−1 was observed for the 1300 ◦ C calcined sample with the powder prepared from the normal EDTA–citrate complexing process [43]. It suggests that the powders from the auto-combustion synthesis could have comparable conductivity to that from the normal EDTA–citrate complexing synthesis. The oxygen permeation properties of the membranes made from LSCF-SPS-A with powder pre-calcination were chiefly investigated.
As shown in Fig. 10, an increase in the oxygen permeation flux was observed with the increase of the sintering temperature. A maximum value of ∼4.46 × 10−7 mol cm−2 s−1 [STP] was obtained for the membrane sintered at 1300 ◦ C for 5 h. As we demonstrated before, the increasing oxygen permeation flux was closely related with the increase of the grain size of the membranes [43]. Fig. 10 also presents the temperature dependence of the oxygen permeation fluxes through LSCF membrane prepared from the normal EDTA–citrate complexing process sintered at 1300 ◦ C for 5 h. Similar oxygen permeation flux was observed for the membranes made from LSCF-SPS-A powder or ECS powder. It suggests that the addition of NH4 NO3 to the EDTA–citrate system to change the sol–gel synthesis into an auto-combustion process did not cast significant influence on the transporting properties of the oxide. The influence of combustion mode on the oxygen permeation flux of the corresponding membranes was also investigated. Fig. 11 shows a
Fig. 10. The temperature dependence of permeation fluxes through LSCF-SPS-A membranes sintered at different temperatures for 5 h with powder pre-calcination.
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synthesis. As a result, perovskite-type composite oxides prepared by optimized auto-combustion method exhibited comparable performance in practical applications such as oxygen permeation membrane for its facile synthesis process and easy product treatment. Acknowledgements This work was supported by the National Natural Science Foundation of China under contracts No. 20646002 and No. 20676061, and also Natural Science Foundation of JiangSu Province, under contract No. BK2006180. References Fig. 11. The temperature dependence of permeation fluxes through LSCF membranes with the powder from different combustion modes and sintered at 1200 ◦ C for 5 h with powder pre-calcination.
comparison of the oxygen permeation flux through the different membranes sintered at 1200 ◦ C for 5 h with powder pre-calcination. The membranes made from the LSCF-SPS-A and LSCF-VCS have better performance, while the membrane from the LSCF-SCS had the worst oxygen permeation fluxes. Due to the crack formation of the membranes with the powder from direct auto-combustion synthesis without pre-calcination powder, we were not able to get the oxygen permeability information of the corresponding sintered membranes. Actually, synthesis condition makes an important effect on the amount of organic residues. In order to ensure low level of organic residues, the pre-calcination process is still necessary for membrane application based on the auto-combustion synthesis. However, current auto-combustion process still has great advantage over the normal EDTA–citrate process for membrane application. For the normal EDTA–citrate complexing process, due to the presence of tremendous amount of organic inside the precursor, a huge amount of gases were produced during the calcination at elevated temperature, which could easily blow away the synthesized powder from the container and contaminate the furnace. Furthermore, simultaneously a huge amount of oxygen would be consumed during the synthesis; it then had a strict requirement on the atmosphere inside the furnace, especially for the large-scale application; once the O2 and CO2 could not be provided or removed in time, it would have a significant impact on the powder properties. While the precalcination process was much easier to handle since most of the organic in the oxide has already burned out from the combustionsynthesized powder. 4. Conclusions A facile EDTA–citrate complexation/auto-combustion method was developed for the preparation of pervoskite-type oxygen permeable materials. The combustion process could be smothering, volumetric or self-propagating, depending on the NH4 NO3 addition amount and the pre-heating temperature. Well-crystallized LSCF can be obtained through volumetric and self-propagating combustion. At sintering temperature of 1200 ◦ C, the membrane made from LSCF-SPS-A powder had the highest conductivity (>250 S cm−1 at 550 ◦ C). An increasing in the oxygen permeation flux was observed with the increase of the sintering temperature. A maximum value of ∼4.46 × 10−7 mol cm−2 s−1 [STP] was observed for the LSCF-SPSA membrane sintered at 1300 ◦ C for 5 h. Due to large shrinkage and the cracks formation of the membranes caused by organic residues in the as-obtained oxides, the pre-calcination process is preferred for membrane application based on the auto-combustion
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