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Perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides: Structural, reduction-tolerant, sintering, and electrical properties
Solid State Ionics 209–210 (2012) 24–29
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Perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides: Structural, reduction-tolerant, sintering, and electrical properties Hui Lu a,⁎, Linlin Zhu a, Jong Pyo Kim b, Sou Hwan Son b, Jung Hoon Park b,⁎⁎ a b
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Greenhouse Gas Research Center, Climate Change Technology Research Division, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 September 2011 Received in revised form 3 November 2011 Accepted 3 January 2012 Available online 26 January 2012 Keywords: Perovskite oxides Structural stability Reduction-tolerance Sintering Electrical conductivity
a b s t r a c t The perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) powders have been synthesized by the citrate gel method. Structural, reduction-tolerant, and sintering properties of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides were studied by x-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscope (SEM). The enhanced structural stability of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Cr, Ti) oxides is ascribed to the higher metal–oxygen bonding energy of the Ti and Cr cations in the oxides. Electrical conductivities of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) ceramics were measured, and increased as B = Co> Cr> Ti among the La0.6Sr0.4B0.2Fe0.8O3 − δ oxides. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Mixed-conducting perovskite oxides have been paid much attention with regard to functional ceramics for oxygen separation/catalysis at high temperatures, and cathode materials for solid oxide fuel cells (SOFCs) [1–18]. The functional ceramics made from the perovskite oxides may also possess potential applications for the reduction and capture of CO2 in the oxy-fuel process for power generation [14,19–24]. Accompanying with the predominantly electronic conductivities (~102 S/cm), the perovskite oxides possess high oxygen ionic conductivities (~10 − 1 S/cm) depending on the intrinsic characters and operating conditions. The perovskite SrCo0.8Fe0.2O3 − δ (SCF) oxide had been studied extensively by many researchers [2,3,11–14]. However, the poor structural stability of SCF limits its practical application in the fields of chemical engineering and clean energy technologies, e.g. in the catalytic dense membrane reactors for syngas production by partial oxidation of methane, and for CO2 capture in the oxy-fuel combustion process. In addition, the SCF oxide also exhibits other serious disadvantages, e.g. the high thermal expansion coefficients (TECs), the insufficient mechanical strength, and the poor structural and chemical stability [2,3,11,14]. The (La,Sr)(Co,Fe)O3 − δ oxides (based on the SrCoO3 − δ oxide) are known to possess high electrical conductivity and excellent oxygen transport characteristics at high temperatures (>700 °C) [1–3,14]. However, the Co-containing perovskite oxides may exhibit structural ⁎ Corresponding author. Tel.: + 86 411 8437 9180; fax: + 86 411 8469 4447. ⁎⁎ Corresponding author. Tel.: + 82 42 860 3766; fax: + 82 42 860 3134. E-mail addresses: [email protected] (H. Lu), [email protected] (J.H. Park). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.01.001
and chemical instability (e.g. the phase decomposition/transformation and the structural deterioration) during long-term operations. Thus the perovskite oxides with high structural and chemical stability are desired for the practical applications. The Ti- and Cr-substituted oxides may offer the possibility to improve the thermal, structural and chemical stability of the perovskite oxides. In this work, the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Co, Cr) oxides have been synthesized by the citrate gel method. Crystal structure, structural and chemical stability, and sintering properties of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Co, Cr) oxides have been investigated x-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscope (SEM). The electrical conductivities of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides were measured by a dc four-probe method. 2. Experimental The perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxide powders were synthesized by the citrate-gel method. Reagent grade La(NO3)2· 6H2O, Sr(NO3)2, Co(NO3)3·6H2O (or Cr(NO3)3·6H2O as Cr source and Ti[OCH(CH3)2]4 as Ti source, respectively, for La0.6Sr0.4B0.2Fe0.8O3 − δ (B=Cr, Ti) oxides), Fe(NO3)3·9H2O and the citric acid were used as starting materials. The corresponding nitrate salts were weighed according to the nominal composition of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co, Cr, Ti) oxides. Subsequently, the nitrate salts were dissolved completely in the de-ionized water for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co, Cr) (or in the ethylene glycol for B = Ti) and then a certain amount of citric acid (the molar ratio of the citric acid/metal: 1.20) was added into the nitrates solution. Finally a dark brown gel was obtained, and dried at
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
110 °C. The as-synthesized precursors were calcined at different temperatures (900–1300 °C) to investigate the phase evolution of oxides. The calcined powders were then compressed into green disks/pellets with a thickness of ~1–2 mm. The green pellets were sintered at 1300–1400 °C for 5–10 h at heating/cooling rates of 2–5 °C/min. The crystal structures of the oxide powders and sintered ceramics were characterized by XRD using Cu Kα radiation. TG experiments were carried out in a flowing 5% (or 10%) H2/helium atmosphere with a heating rate of 10 °C/min up to 1000 °C. The morphologies and elemental compositions of the sintered ceramics were observed by SEM-EDS. To measure the electrical conductivities of the sintered ceramics, the silver wires were painted on the rectangular specimens by silver paste. The electrical conductivities as a function of temperature (100–900 °C) were measured by a dc four-probe method in a synthetic air stream with a total flow rate of 100 mL/min. The equilibration duration at each measuring temperature point was 1 h, and a heating/cooling rate was 1–2 °C/min.
a
25
p
B = Ti p
p
p
p
p
p 1100 oC 1000 oC 900 oC 800 oC 700 oC o
600 C 500 oC 400 oC 300 oC o 200 C
20
40
60
b
80 B = Cr
3. Results and discussion 1100 oC
3.1. Crystal structure of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides
Relative intensity, a. u.
Fig. 1a shows the XRD patterns of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) powders calcined at 1300 °C for 5 h. The La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides possess the perovskite structure. The diffraction peaks of the Ti- and Cr-substituted oxide powders shift slightly to a relatively low 2θ angle compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide, indicating the lattice expansion of the Cr or Ti-doped oxides as B = Ti> Cr> Co among the La0.6Sr0.4BM0.2Fe0.8O3 − δ oxides (Fig. 1b). It is ascribed to the larger Ti 3+/4 + (ionic radius: 81 pm/74.5 pm) and Cr3+/4 + (ionic radius: 75.5 pm/69 pm) ions are substituted for Co3+/4 + ions (ionic radius: 68.5 pm/67 pm) in the B-site of the perovskite oxide. Fig. 2 shows the high-temperature XRD (HT-XRD) patterns of the perovskite La0.6Sr0.4M0.2Fe0.8O3 − δ (M = Ti, Cr, Co) oxides up to 1100 °C, in a stagnant air atmosphere. The HT-XRD results show that the perovskite structure of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides is kept completely in a wide range of the temperatures investigated, indicating the absence of the phase transformation and/ or decomposition of these perovskite oxides in air. Thermal expansion coefficients (TECs) of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides were calculated based on the HT-XRD experiments, and summarized in Table 1. TECs of the La0.6Sr0.4B0.2Fe0.8O3 − δ
1000 oC o 900 C
800 oC o 700 C o 600 C
500 oC o 400 C
300 oC o 200 C
20
40
60
c
80 B = Co 1100 oC 1000 oC o 900 C
800 oC 700 oC 600 oC
a
b
La0.6Sr0.4B0.2Fe0.8O3-δ citrate gel method
500 oC
Relative intensity, a. u.
400 oC
p
p
o 300 C
B = Co
p
o 200 C
p p
p
p
p
p
20
p
40
60
80
80
Fig. 2. High-temperature XRD patterns of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxide powders in air, (a) B = Ti, (b) B = Cr and (c) B = Co, respectively. (p: perovskite).
B = Ti
2 Theta, degree
60
2 Theta, degree
B = Cr
20
40
30
33
2 Theta, degree
Fig. 1. Room temperature XRD patterns of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxide powders synthesized by the citrate method. (p: perovskite).
(B = Ti, Cr, Co) oxides are 15.8, 17.8 and 22.3 × 10 − 6/°C, respectively, at a high temperature range of 700–1100 °C, which is smaller than that (26.0 × 10 − 6/°C) of the Ba0.5Sr0.5Co0.8Fe0.2O3 − δ oxide obtained by HT-XRD. The Ti- and Cr-doped La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) perovskite oxides possess relatively lower TECs compared to the Cocontaining La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) and SCF-based oxides, implying that they may be beneficial for the thermal compatibility with ceramic sealing modules in the functional modules and YSZ electrolyte in SOFCs.
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H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
Table 1 Thermal expansion coefficients (TECs) of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides. La0.6Sr0.4B0.2Fe0.8O3 − δ
Synthesizing route
Measuring method
B = Ti
Citrate
XRD
B = Cr
Citrate
XRD
B = Co
TECs
10−6 =C 200−600C 13:4 200−600C
Citrate
XRD
Citrate
Dilatometry
Commercial
XRD Ref. [24]
respectively, for the perovskite-type La0.6Sr0.4B0.2Fe0.8O3 − δ (B=Ti, Cr, Co) oxides. The weight loss process is mainly originated from two kinds of contribution, the crystal oxygen loss (reaction a) and chemical reduction (reaction b) of the perovskite oxides under H2-containing atmospheres at high temperatures:
Weight, %
600
B = Cr
450 B = Co
Temperature, oC
in a flowing 5% H2/He stream
97.5
300 94.5 150 93.0 180
240
300
Time, minutes
b in a flowing 10% H2/He stream
99.0
Weight, %
B = Ti
97.5
96.0
B = Cr
94.5 B = Co
93.0
500
600
700
800
→ La High temperature
0:6 Sr0:4 B0:2 Fe0:8 O3−δ−x
þ x=2O2
ðaÞ
→La O
La0:6 Sr0:4 B0:2 Fe0:8 O3−δ 750
400
425−1000C 24:5
900
B = Ti
300
24:2
30−1000C 17:5
99.0
100.5
18:7
Dilatometry Ref. [26]
900oC
120
700−1000C 24:5 700−1100C
Solid state
100.5
60
200−600C 19:3 200−600C
XRD Ref. [25]
La0:6 Sr0:4 B0:2 Fe0:8 O3−δ
96.0
17:8 700−1100C 22:3
Commercial
Fig. 3a shows the steady-state TG curves of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B=Ti, Cr, Co) oxide powders in a flowing 5% H2/He atmosphere at 900 °C for 3 h. The total weight loss is ~5.88%, 5.52% and 1.20%,
a
12:8 200−600C 15:4
20−425C 16:3
3.2. Reduction-tolerance of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides
700−1100C 15:8 700−1100C
900 1000
Temperature, oC Fig. 3. TG curves of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides (a) in a flowing 5% H2/He stream at 900 °C for 3 h, and (b) in a flowing 10% H2/He stream up to 1000 °C.
High temperature; H2
2
3
þ SrO þ B=BOα þ Fe=FeOβ :
ðbÞ
As for the Co-containing La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) oxide, the weight loss process is pronounced, and approached basically the steady-state at 900 °C (here note that the perovskite structure was still not destroyed fully, as demonstrated by XRD). A slight mass loss still occurs during the quasi steady-state at 900 °C for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) oxide, as shown in Fig. 3a. For the Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Cr) oxide, the weight loss process occurs gradually at 900 °C during 3 h. The large weight loss process (after an initial small weigh loss process) was not observed at 900 °C for the Ti-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti) oxide, as illustrated in Fig. 3a. The steady-state TG results (at 900 °C for 3 h in a flowing 5% H2/He stream) also indicate that the reduction-tolerance is increased as B = Ti> Cr> Co for the La0.6Sr0.4B0.2Fe0.8O3 − δ oxides. Fig. 3b shows the dynamic TG results for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides up to 1000 °C in a flowing 10% H2/helium stream, which also demonstrates the enhanced chemical stability of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co, Cr) perovskite oxides. Fig. 4a shows the room temperature XRD patterns of the oxides (after TG experiments) at 900 °C for 3 h in a flowing 5% H2/He stream. For the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co, Cr) oxides after TG, some additional diffraction peaks appear though the perovskite characteristic peaks are still kept. For the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Cr) oxide, fewer additional peaks are observed compared to those in the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) oxide. However, the perovskite structure of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti) oxide remains stable. Furthermore, Fig. 4b gives the room temperature XRD patterns of the perovskite powders after TG experiments up to 1000 °C in a flowing 10% H2/He stream, and the similar XRD patterns were also observed as those in Fig. 4a. XRD results confirm the enhanced chemical stability of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) oxides compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide. The presence of Cr 3+/4 + or Ti 3+/4 + in the B-site of the perovskite may further stabilize the neighboring
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
27
a o
Relative intensity, a. u.
In a flowing 5 % H2/He stream at 900 C for 3 h
P: perovskite *: Fe #: La2O3 s: SrO +: unknown
p
p p
p
p
a
p
p
p
p
B = Ti B = Cr s
+# #
20
+ +*
30
s +
40
50
s *+
60
+
B = Co
70
80
2 Theta, degree
b
Relative intensity, a. u.
p
o
In a flowing 10 % H2/He stream up to 1000 C
P: perovskite *: Fe #: La2O3 s: SrO +: unknown
p
b
p p
p
p p
p p B = Ti
B = Cr # +#
20
#
30
s+
+ +*
40
s ++
50
s
*+
60
++
70
B = Co
80
2 Theta, degree Fig. 4. XRD patterns (at room temperature) of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides after TG experiments (a) in a flowing 5% H2/He stream at 900 °C for 3 h, and (b) in a flowing 10% H2/He stream up to 1000 °C.
oxygen octahedral, resulting in the improved structural stability of the Cr- and Ti-substituted perovskite oxides. TG and XRD results reveal that the Ti- and Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ oxides possess improved reduction-tolerance compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide at high temperatures. Thus they are more suitable applied in functional membrane reactors under reducing atmospheres for syngas (CO + H2) production from partial oxidation of light hydrocarbons (e.g. methane), and auto-thermal reforming of gasoline and biomass at high temperatures (>750 °C).
c Fig. 5. Surface SEM micrographs of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics, (a) B = Ti, (b) B = Cr and (c) B = Co, respectively.
La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide. Table 2 summarizes the metal atomic compositions (by EDS) and the calculated atomic ratios for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) ceramics. The metal molar ratios measured by EDS are in good agreement with the aimed composition ratio (i.e. La:Sr:B:Fe= 0.6:0.4:0.2:0.8) for these oxides. 3.4. Electrical conductivities of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics
3.3. Sintering of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics Fig. 5 shows the surface SEM micrographs of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics for electrical conductivity measurements. The sintered pellets exhibit a small grain size of ~0.5–1.0 μm for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr), and ~ 0.2– 0.5 μm for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) in the surface. The sintered pellets show a typical characteristic of the dense ceramics, exhibiting a near fusion state in the cross-section although some micro-pores and/or holes (~ 1.0 μm in diameter) exist (Fig. 6). The diameters of these micro-pores/holes are significantly less than the thickness (~1–2 mm) of the sintered pellets. Thus, these micro-pores/ holes in the bulk should be closed in the sintered ceramics. The surface grain size and cross-sectional morphology show that the sintering properties of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) oxides were improved significantly compared to the Co-containing
The electrical conductivity (σ) can be regarded as the electronic conductivity of the mixed-conducting ceramics, due to the very low oxygen ionic transport number (generally, ti b 0.01) in the perovskite oxides. Fig. 7a gives the electrical conductivities of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides under a synthetic air atmosphere. The Tiand Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) oxides exhibit lower electrical conductivities compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide at given temperatures, and the electrical conductivity is increased as B = Co > Cr > Ti among the sintered perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ ceramics. The reduced electrical conductivity of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) oxides is ascribed to the presence of the relatively higher oxidation state of the Ti or Cr cations in the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides, resulting in the lowered electrical conductivities in the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) oxides compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide [3,27,28]. The decrease
28
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
a Co Cr Ti
400
σ, S/cm
300
200
100
a 0 400
600
800
1000
1200
T, K
b Co Cr Ti
5.5
Log (σ*T, S.cm-1.K)
5.0
b
4.5
4.0
3.5
3.0 0.5
1.0
1.5
2.0
2.5
3.0
1000/T, K-1 Fig. 7. Electric conductivities of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics.
c Fig. 6. Cross-sectional SEM micrographs of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics, (a) B = Ti, (b) B = Cr and (c) B = Co, respectively.
in conductivity on replacing Co in the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide is mainly due to an increase in the charge-transfer gap between the metal:3d and O2p bands, and the decrease in the covalency of the metal–oxygen bonds in the perovskite oxides.
Table 2 EDS results and the metal atomic ratio of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides. La0.6Sr0.4B0.2Fe0.8O3 − δ
a
B = Ti B = Crb B = Cob a b
Atomic ratio by EDS
Calculated atomic ratio
La
Sr
B
Fe
La
Sr
B
Fe
30.38 11.82 13.67
19.75 6.89 7.67
10.35 3.38 4.58
41.32 14.30 17.31
0.63 0.63 0.64
0.37 0.37 0.36
0.20 0.19 0.21
0.80 0.81 0.79
Based on the metals elements (%). Based on the metals and oxygen elements (%).
In addition, these perovskite oxide ceramics exhibit a similar trend in electrical conductivities accompanying with a maximum value observed, as the measuring temperature was increased up to 900 °C. The change in electrical conductivities is remarkably obvious for the Cocontaining La0.6Sr0.4Co0.2Fe0.8O3 − δ ceramics with a maximum value of 404 S/cm at ~ 550 °C, which is significantly higher than that (~290 S/cm) for the La0.6Sr0.4Co0.2Fe0.8O3 − δ synthesized by the EDTA-citrate complexing sol–gel method. The discrepancy is ascribed to the great effect of the sintering temperature, ceramic microstructure and powder synthesis method etc. on the electrical conductivity [29–33]. Correspondingly, the maximum electrical conductivity is 57 and 71 S/cm at ~ 650 °C, respectively, for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) ceramics. The results indicate that the Co-free perovskite ceramics possess enough electronic conductivity for potential applications as the cathode materials of SOFCs. Furthermore, Fig. 7b shows the Arhennius plots of the electrical conductivity of the sintered perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics. Equation σT = A exp(− Ea/kT), where A is a material constant, T is the absolute temperature, Ea is the activation energy for small polaron hopping and k is the Boltzmann constant. The activation energy for the electrical conduction (up to 700 °C) for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) ceramics is calculated to be 19.9, 13.9 and 7.4 kJ/mol, respectively.
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
4. Conclusions The reduction-tolerance capability of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides were studied by the combined TG and XRD, demonstrating that the structural and chemical stabilities of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Cr, Ti) oxides are improved greatly. Electrical conductivity of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxide ceramics is increased as B = Co > Cr > Ti among the La0.6Sr0.4B0.2Fe0.8O3 − δ oxides. The Ti- and Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) perovskite oxides possess lower TECs and higher structural stability in comparison with the Cocontaining La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co) oxide, indicating that the Co-free oxides may be potentially applied in the functional modules for membrane separation/catalysis and SOFCs cathodes. Acknowledgments This work was partially supported by Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS), China, and the Ministry of Education, Science and Technology of Korea. References [1] X.Y. Tan, K. Li, AICHE J. 55 (2009) 2675. [2] L. Qiu, T.H. Lee, L.M. Liu, Y.L. Yang, A.J. Jacobson, Solid State Ionics 76 (1995) 321. [3] H.J.M. Bouwmeester, A.J. Burggraaf, in: A.J. Burggraaf, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, 1996, p. 435. [4] M. Arnold, H.H. Wang, A. Feldhoff, J. Membr. Sci. 293 (2007) 44. [5] H.J.M. Bouwmeester, Catal. Today 82 (2003) 141. [6] H. Lu, Y. Cong, W.S. Yang, Mater. Sci. Eng. B 41 (2007) 55.
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[7] H. Lu, J.H. Tong, Y. Cong, W.S. Yang, Catal. Today 104 (2005) 154. [8] V.N. Tikhonovich, O.M. Zharkovskaya, E.N. Naumovich, I.A. Bashmakov, V.V. Kharton, A.A. Vecher, Solid State Ionics 160 (2003) 259. [9] J. Hu, H.S. Hao, C.P. Chen, D.L. Yang, X. Hu, J. Membr. Sci. 280 (2006) 809. [10] L. Yang, X.H. Gu, L. Tian, L.X. Zhang, C.Q. Wang, N.P. Xu, Sep. Purif. Technol. 32 (2003) 301. [11] S. Kim, Y.L. Yang, A.J. Jacobson, B. Abeles, Solid State Ionics 106 (1998) 189. [12] M.R. Dassonneville, S. Rosini, A.C. van Veen, D. Farrusseng, C. Mirodatos, Catal. Today 104 (2005) 131. [13] Z.P. Shao, W.S. Yang, Y. Cong, H. Dong, J.H. Tong, G.X. Xiong, J. Membr. Sci. 172 (2000) 177. [14] J. Sunarso, S. Baumann, J.M. Serra, W.A. Meulenberg, S.M. Liu, Y.S. Lin, J.C. Diniz da Costa, J. Membr. Sci. 320 (2008) 13. [15] X. Qi, Y.S. Lin, S.L. Swartz, Ind. Eng. Chem. Res. 39 (2000) 646. [16] H. Lu, J.H. Tong, Z.Q. Zeng, Y. Cong, W.S. Yang, Mater. Res. Bull. 41 (2006) 683. [17] M. Sφgaard, P.V. Hendriksen, M. Mogensen, J. Solid State Chem. 180 (2007) 1489. [18] H. Lu, Y. Cong, W.S. Yang, Solid State Ionics 177 (2006) 595. [19] R. Bredesena, K. Jordal, A. Bolland, Chem. Eng. Process. 43 (2004) 1129. [20] Z.G. Shen, P.X. Lu, G. Yan, X. Hu, Mater. Lett. 64 (2010) 980. [21] Q. Zeng, Y.B. Zuo, C.G. Fan, C.S. Chen, J. Membr. Sci. 335 (2009) 140. [22] H. Lu, J.P. Kim, S.H. Son, J.H. Park, Mater. Sci. Eng. B 166 (2010) 135. [23] J.H. Park, K.Y. Kim, S.D. Park, Desalination 245 (2009) 559. [24] H. Lu, J.P. Kim, S.H. Son, J.H. Park, Mater. Sci. Eng. B 163 (2009) 151. [25] G. Corbel, S. Mestiri, P. Lacorre, Solid State Sci. 7 (2005) 1216. [26] A. Petric, P. Huang, F. Tietz, Solid State Ionics 135 (2000) 719. [27] V.V. Kharton, A.P. Viskup, A.A. Yaremchenko, R.T. Baker, B. Gharbage, G.C. Mather, F.M. Figueiredo, E.N. Naumovich, F.M.B. Marques, Solid State Ionics 132 (2000) 119. [28] K.T. Lee, A. Manthiram, Solid State Ionics 178 (2007) 995. [29] P.Y. Zeng, R. Ran, Z.H. Chen, H.X. Gu, Z.P. Shao, J.C. Diniz da Costa, S.M. Liu, J. Membr. Sci. 302 (2007) 171. [30] S.R. Wang, M. Katsuki, M. Dokiya, T. Hashimoto, Solid State Ionics 159 (2003) 71. [31] J.A. Lane, S.J. Benson, D. Waller, J.A. Kilner, Solid State Ionics 121 (1999) 201. [32] B. Zhu, J. Power Sources 93 (2001) 822. [33] Z. Liu, M.F. Han, W.T. Miao, J. Power Sources 173 (2007) 837.
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Perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides: Structural, reduction-tolerant, sintering, and electrical properties Hui Lu a,⁎, Linlin Zhu a, Jong Pyo Kim b, Sou Hwan Son b, Jung Hoon Park b,⁎⁎ a b
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Greenhouse Gas Research Center, Climate Change Technology Research Division, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 September 2011 Received in revised form 3 November 2011 Accepted 3 January 2012 Available online 26 January 2012 Keywords: Perovskite oxides Structural stability Reduction-tolerance Sintering Electrical conductivity
a b s t r a c t The perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) powders have been synthesized by the citrate gel method. Structural, reduction-tolerant, and sintering properties of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides were studied by x-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscope (SEM). The enhanced structural stability of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Cr, Ti) oxides is ascribed to the higher metal–oxygen bonding energy of the Ti and Cr cations in the oxides. Electrical conductivities of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) ceramics were measured, and increased as B = Co> Cr> Ti among the La0.6Sr0.4B0.2Fe0.8O3 − δ oxides. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Mixed-conducting perovskite oxides have been paid much attention with regard to functional ceramics for oxygen separation/catalysis at high temperatures, and cathode materials for solid oxide fuel cells (SOFCs) [1–18]. The functional ceramics made from the perovskite oxides may also possess potential applications for the reduction and capture of CO2 in the oxy-fuel process for power generation [14,19–24]. Accompanying with the predominantly electronic conductivities (~102 S/cm), the perovskite oxides possess high oxygen ionic conductivities (~10 − 1 S/cm) depending on the intrinsic characters and operating conditions. The perovskite SrCo0.8Fe0.2O3 − δ (SCF) oxide had been studied extensively by many researchers [2,3,11–14]. However, the poor structural stability of SCF limits its practical application in the fields of chemical engineering and clean energy technologies, e.g. in the catalytic dense membrane reactors for syngas production by partial oxidation of methane, and for CO2 capture in the oxy-fuel combustion process. In addition, the SCF oxide also exhibits other serious disadvantages, e.g. the high thermal expansion coefficients (TECs), the insufficient mechanical strength, and the poor structural and chemical stability [2,3,11,14]. The (La,Sr)(Co,Fe)O3 − δ oxides (based on the SrCoO3 − δ oxide) are known to possess high electrical conductivity and excellent oxygen transport characteristics at high temperatures (>700 °C) [1–3,14]. However, the Co-containing perovskite oxides may exhibit structural ⁎ Corresponding author. Tel.: + 86 411 8437 9180; fax: + 86 411 8469 4447. ⁎⁎ Corresponding author. Tel.: + 82 42 860 3766; fax: + 82 42 860 3134. E-mail addresses: [email protected] (H. Lu), [email protected] (J.H. Park). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2012.01.001
and chemical instability (e.g. the phase decomposition/transformation and the structural deterioration) during long-term operations. Thus the perovskite oxides with high structural and chemical stability are desired for the practical applications. The Ti- and Cr-substituted oxides may offer the possibility to improve the thermal, structural and chemical stability of the perovskite oxides. In this work, the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Co, Cr) oxides have been synthesized by the citrate gel method. Crystal structure, structural and chemical stability, and sintering properties of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Co, Cr) oxides have been investigated x-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscope (SEM). The electrical conductivities of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides were measured by a dc four-probe method. 2. Experimental The perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxide powders were synthesized by the citrate-gel method. Reagent grade La(NO3)2· 6H2O, Sr(NO3)2, Co(NO3)3·6H2O (or Cr(NO3)3·6H2O as Cr source and Ti[OCH(CH3)2]4 as Ti source, respectively, for La0.6Sr0.4B0.2Fe0.8O3 − δ (B=Cr, Ti) oxides), Fe(NO3)3·9H2O and the citric acid were used as starting materials. The corresponding nitrate salts were weighed according to the nominal composition of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co, Cr, Ti) oxides. Subsequently, the nitrate salts were dissolved completely in the de-ionized water for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co, Cr) (or in the ethylene glycol for B = Ti) and then a certain amount of citric acid (the molar ratio of the citric acid/metal: 1.20) was added into the nitrates solution. Finally a dark brown gel was obtained, and dried at
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
110 °C. The as-synthesized precursors were calcined at different temperatures (900–1300 °C) to investigate the phase evolution of oxides. The calcined powders were then compressed into green disks/pellets with a thickness of ~1–2 mm. The green pellets were sintered at 1300–1400 °C for 5–10 h at heating/cooling rates of 2–5 °C/min. The crystal structures of the oxide powders and sintered ceramics were characterized by XRD using Cu Kα radiation. TG experiments were carried out in a flowing 5% (or 10%) H2/helium atmosphere with a heating rate of 10 °C/min up to 1000 °C. The morphologies and elemental compositions of the sintered ceramics were observed by SEM-EDS. To measure the electrical conductivities of the sintered ceramics, the silver wires were painted on the rectangular specimens by silver paste. The electrical conductivities as a function of temperature (100–900 °C) were measured by a dc four-probe method in a synthetic air stream with a total flow rate of 100 mL/min. The equilibration duration at each measuring temperature point was 1 h, and a heating/cooling rate was 1–2 °C/min.
a
25
p
B = Ti p
p
p
p
p
p 1100 oC 1000 oC 900 oC 800 oC 700 oC o
600 C 500 oC 400 oC 300 oC o 200 C
20
40
60
b
80 B = Cr
3. Results and discussion 1100 oC
3.1. Crystal structure of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides
Relative intensity, a. u.
Fig. 1a shows the XRD patterns of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) powders calcined at 1300 °C for 5 h. The La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides possess the perovskite structure. The diffraction peaks of the Ti- and Cr-substituted oxide powders shift slightly to a relatively low 2θ angle compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide, indicating the lattice expansion of the Cr or Ti-doped oxides as B = Ti> Cr> Co among the La0.6Sr0.4BM0.2Fe0.8O3 − δ oxides (Fig. 1b). It is ascribed to the larger Ti 3+/4 + (ionic radius: 81 pm/74.5 pm) and Cr3+/4 + (ionic radius: 75.5 pm/69 pm) ions are substituted for Co3+/4 + ions (ionic radius: 68.5 pm/67 pm) in the B-site of the perovskite oxide. Fig. 2 shows the high-temperature XRD (HT-XRD) patterns of the perovskite La0.6Sr0.4M0.2Fe0.8O3 − δ (M = Ti, Cr, Co) oxides up to 1100 °C, in a stagnant air atmosphere. The HT-XRD results show that the perovskite structure of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides is kept completely in a wide range of the temperatures investigated, indicating the absence of the phase transformation and/ or decomposition of these perovskite oxides in air. Thermal expansion coefficients (TECs) of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides were calculated based on the HT-XRD experiments, and summarized in Table 1. TECs of the La0.6Sr0.4B0.2Fe0.8O3 − δ
1000 oC o 900 C
800 oC o 700 C o 600 C
500 oC o 400 C
300 oC o 200 C
20
40
60
c
80 B = Co 1100 oC 1000 oC o 900 C
800 oC 700 oC 600 oC
a
b
La0.6Sr0.4B0.2Fe0.8O3-δ citrate gel method
500 oC
Relative intensity, a. u.
400 oC
p
p
o 300 C
B = Co
p
o 200 C
p p
p
p
p
p
20
p
40
60
80
80
Fig. 2. High-temperature XRD patterns of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxide powders in air, (a) B = Ti, (b) B = Cr and (c) B = Co, respectively. (p: perovskite).
B = Ti
2 Theta, degree
60
2 Theta, degree
B = Cr
20
40
30
33
2 Theta, degree
Fig. 1. Room temperature XRD patterns of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxide powders synthesized by the citrate method. (p: perovskite).
(B = Ti, Cr, Co) oxides are 15.8, 17.8 and 22.3 × 10 − 6/°C, respectively, at a high temperature range of 700–1100 °C, which is smaller than that (26.0 × 10 − 6/°C) of the Ba0.5Sr0.5Co0.8Fe0.2O3 − δ oxide obtained by HT-XRD. The Ti- and Cr-doped La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) perovskite oxides possess relatively lower TECs compared to the Cocontaining La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) and SCF-based oxides, implying that they may be beneficial for the thermal compatibility with ceramic sealing modules in the functional modules and YSZ electrolyte in SOFCs.
26
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
Table 1 Thermal expansion coefficients (TECs) of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides. La0.6Sr0.4B0.2Fe0.8O3 − δ
Synthesizing route
Measuring method
B = Ti
Citrate
XRD
B = Cr
Citrate
XRD
B = Co
TECs
10−6 =C 200−600C 13:4 200−600C
Citrate
XRD
Citrate
Dilatometry
Commercial
XRD Ref. [24]
respectively, for the perovskite-type La0.6Sr0.4B0.2Fe0.8O3 − δ (B=Ti, Cr, Co) oxides. The weight loss process is mainly originated from two kinds of contribution, the crystal oxygen loss (reaction a) and chemical reduction (reaction b) of the perovskite oxides under H2-containing atmospheres at high temperatures:
Weight, %
600
B = Cr
450 B = Co
Temperature, oC
in a flowing 5% H2/He stream
97.5
300 94.5 150 93.0 180
240
300
Time, minutes
b in a flowing 10% H2/He stream
99.0
Weight, %
B = Ti
97.5
96.0
B = Cr
94.5 B = Co
93.0
500
600
700
800
→ La High temperature
0:6 Sr0:4 B0:2 Fe0:8 O3−δ−x
þ x=2O2
ðaÞ
→La O
La0:6 Sr0:4 B0:2 Fe0:8 O3−δ 750
400
425−1000C 24:5
900
B = Ti
300
24:2
30−1000C 17:5
99.0
100.5
18:7
Dilatometry Ref. [26]
900oC
120
700−1000C 24:5 700−1100C
Solid state
100.5
60
200−600C 19:3 200−600C
XRD Ref. [25]
La0:6 Sr0:4 B0:2 Fe0:8 O3−δ
96.0
17:8 700−1100C 22:3
Commercial
Fig. 3a shows the steady-state TG curves of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B=Ti, Cr, Co) oxide powders in a flowing 5% H2/He atmosphere at 900 °C for 3 h. The total weight loss is ~5.88%, 5.52% and 1.20%,
a
12:8 200−600C 15:4
20−425C 16:3
3.2. Reduction-tolerance of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides
700−1100C 15:8 700−1100C
900 1000
Temperature, oC Fig. 3. TG curves of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides (a) in a flowing 5% H2/He stream at 900 °C for 3 h, and (b) in a flowing 10% H2/He stream up to 1000 °C.
High temperature; H2
2
3
þ SrO þ B=BOα þ Fe=FeOβ :
ðbÞ
As for the Co-containing La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) oxide, the weight loss process is pronounced, and approached basically the steady-state at 900 °C (here note that the perovskite structure was still not destroyed fully, as demonstrated by XRD). A slight mass loss still occurs during the quasi steady-state at 900 °C for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) oxide, as shown in Fig. 3a. For the Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Cr) oxide, the weight loss process occurs gradually at 900 °C during 3 h. The large weight loss process (after an initial small weigh loss process) was not observed at 900 °C for the Ti-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti) oxide, as illustrated in Fig. 3a. The steady-state TG results (at 900 °C for 3 h in a flowing 5% H2/He stream) also indicate that the reduction-tolerance is increased as B = Ti> Cr> Co for the La0.6Sr0.4B0.2Fe0.8O3 − δ oxides. Fig. 3b shows the dynamic TG results for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides up to 1000 °C in a flowing 10% H2/helium stream, which also demonstrates the enhanced chemical stability of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co, Cr) perovskite oxides. Fig. 4a shows the room temperature XRD patterns of the oxides (after TG experiments) at 900 °C for 3 h in a flowing 5% H2/He stream. For the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co, Cr) oxides after TG, some additional diffraction peaks appear though the perovskite characteristic peaks are still kept. For the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Cr) oxide, fewer additional peaks are observed compared to those in the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) oxide. However, the perovskite structure of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti) oxide remains stable. Furthermore, Fig. 4b gives the room temperature XRD patterns of the perovskite powders after TG experiments up to 1000 °C in a flowing 10% H2/He stream, and the similar XRD patterns were also observed as those in Fig. 4a. XRD results confirm the enhanced chemical stability of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) oxides compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide. The presence of Cr 3+/4 + or Ti 3+/4 + in the B-site of the perovskite may further stabilize the neighboring
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
27
a o
Relative intensity, a. u.
In a flowing 5 % H2/He stream at 900 C for 3 h
P: perovskite *: Fe #: La2O3 s: SrO +: unknown
p
p p
p
p
a
p
p
p
p
B = Ti B = Cr s
+# #
20
+ +*
30
s +
40
50
s *+
60
+
B = Co
70
80
2 Theta, degree
b
Relative intensity, a. u.
p
o
In a flowing 10 % H2/He stream up to 1000 C
P: perovskite *: Fe #: La2O3 s: SrO +: unknown
p
b
p p
p
p p
p p B = Ti
B = Cr # +#
20
#
30
s+
+ +*
40
s ++
50
s
*+
60
++
70
B = Co
80
2 Theta, degree Fig. 4. XRD patterns (at room temperature) of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides after TG experiments (a) in a flowing 5% H2/He stream at 900 °C for 3 h, and (b) in a flowing 10% H2/He stream up to 1000 °C.
oxygen octahedral, resulting in the improved structural stability of the Cr- and Ti-substituted perovskite oxides. TG and XRD results reveal that the Ti- and Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ oxides possess improved reduction-tolerance compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide at high temperatures. Thus they are more suitable applied in functional membrane reactors under reducing atmospheres for syngas (CO + H2) production from partial oxidation of light hydrocarbons (e.g. methane), and auto-thermal reforming of gasoline and biomass at high temperatures (>750 °C).
c Fig. 5. Surface SEM micrographs of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics, (a) B = Ti, (b) B = Cr and (c) B = Co, respectively.
La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide. Table 2 summarizes the metal atomic compositions (by EDS) and the calculated atomic ratios for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) ceramics. The metal molar ratios measured by EDS are in good agreement with the aimed composition ratio (i.e. La:Sr:B:Fe= 0.6:0.4:0.2:0.8) for these oxides. 3.4. Electrical conductivities of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics
3.3. Sintering of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics Fig. 5 shows the surface SEM micrographs of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics for electrical conductivity measurements. The sintered pellets exhibit a small grain size of ~0.5–1.0 μm for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr), and ~ 0.2– 0.5 μm for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Co) in the surface. The sintered pellets show a typical characteristic of the dense ceramics, exhibiting a near fusion state in the cross-section although some micro-pores and/or holes (~ 1.0 μm in diameter) exist (Fig. 6). The diameters of these micro-pores/holes are significantly less than the thickness (~1–2 mm) of the sintered pellets. Thus, these micro-pores/ holes in the bulk should be closed in the sintered ceramics. The surface grain size and cross-sectional morphology show that the sintering properties of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) oxides were improved significantly compared to the Co-containing
The electrical conductivity (σ) can be regarded as the electronic conductivity of the mixed-conducting ceramics, due to the very low oxygen ionic transport number (generally, ti b 0.01) in the perovskite oxides. Fig. 7a gives the electrical conductivities of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides under a synthetic air atmosphere. The Tiand Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) oxides exhibit lower electrical conductivities compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide at given temperatures, and the electrical conductivity is increased as B = Co > Cr > Ti among the sintered perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ ceramics. The reduced electrical conductivity of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) oxides is ascribed to the presence of the relatively higher oxidation state of the Ti or Cr cations in the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) oxides, resulting in the lowered electrical conductivities in the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) oxides compared to the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide [3,27,28]. The decrease
28
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
a Co Cr Ti
400
σ, S/cm
300
200
100
a 0 400
600
800
1000
1200
T, K
b Co Cr Ti
5.5
Log (σ*T, S.cm-1.K)
5.0
b
4.5
4.0
3.5
3.0 0.5
1.0
1.5
2.0
2.5
3.0
1000/T, K-1 Fig. 7. Electric conductivities of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics.
c Fig. 6. Cross-sectional SEM micrographs of the sintered La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics, (a) B = Ti, (b) B = Cr and (c) B = Co, respectively.
in conductivity on replacing Co in the La0.6Sr0.4Co0.2Fe0.8O3 − δ oxide is mainly due to an increase in the charge-transfer gap between the metal:3d and O2p bands, and the decrease in the covalency of the metal–oxygen bonds in the perovskite oxides.
Table 2 EDS results and the metal atomic ratio of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides. La0.6Sr0.4B0.2Fe0.8O3 − δ
a
B = Ti B = Crb B = Cob a b
Atomic ratio by EDS
Calculated atomic ratio
La
Sr
B
Fe
La
Sr
B
Fe
30.38 11.82 13.67
19.75 6.89 7.67
10.35 3.38 4.58
41.32 14.30 17.31
0.63 0.63 0.64
0.37 0.37 0.36
0.20 0.19 0.21
0.80 0.81 0.79
Based on the metals elements (%). Based on the metals and oxygen elements (%).
In addition, these perovskite oxide ceramics exhibit a similar trend in electrical conductivities accompanying with a maximum value observed, as the measuring temperature was increased up to 900 °C. The change in electrical conductivities is remarkably obvious for the Cocontaining La0.6Sr0.4Co0.2Fe0.8O3 − δ ceramics with a maximum value of 404 S/cm at ~ 550 °C, which is significantly higher than that (~290 S/cm) for the La0.6Sr0.4Co0.2Fe0.8O3 − δ synthesized by the EDTA-citrate complexing sol–gel method. The discrepancy is ascribed to the great effect of the sintering temperature, ceramic microstructure and powder synthesis method etc. on the electrical conductivity [29–33]. Correspondingly, the maximum electrical conductivity is 57 and 71 S/cm at ~ 650 °C, respectively, for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr) ceramics. The results indicate that the Co-free perovskite ceramics possess enough electronic conductivity for potential applications as the cathode materials of SOFCs. Furthermore, Fig. 7b shows the Arhennius plots of the electrical conductivity of the sintered perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) ceramics. Equation σT = A exp(− Ea/kT), where A is a material constant, T is the absolute temperature, Ea is the activation energy for small polaron hopping and k is the Boltzmann constant. The activation energy for the electrical conduction (up to 700 °C) for the La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr, Co) ceramics is calculated to be 19.9, 13.9 and 7.4 kJ/mol, respectively.
H. Lu et al. / Solid State Ionics 209–210 (2012) 24–29
4. Conclusions The reduction-tolerance capability of the La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxides were studied by the combined TG and XRD, demonstrating that the structural and chemical stabilities of the Co-free La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Cr, Ti) oxides are improved greatly. Electrical conductivity of the perovskite La0.6Sr0.4B0.2Fe0.8O3 − δ (B = Ti, Cr, Co) oxide ceramics is increased as B = Co > Cr > Ti among the La0.6Sr0.4B0.2Fe0.8O3 − δ oxides. The Ti- and Cr-substituted La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Ti, Cr) perovskite oxides possess lower TECs and higher structural stability in comparison with the Cocontaining La0.6Sr0.4B0.2Fe0.8O3 − δ (B= Co) oxide, indicating that the Co-free oxides may be potentially applied in the functional modules for membrane separation/catalysis and SOFCs cathodes. Acknowledgments This work was partially supported by Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS), China, and the Ministry of Education, Science and Technology of Korea. References [1] X.Y. Tan, K. Li, AICHE J. 55 (2009) 2675. [2] L. Qiu, T.H. Lee, L.M. Liu, Y.L. Yang, A.J. Jacobson, Solid State Ionics 76 (1995) 321. [3] H.J.M. Bouwmeester, A.J. Burggraaf, in: A.J. Burggraaf, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, 1996, p. 435. [4] M. Arnold, H.H. Wang, A. Feldhoff, J. Membr. Sci. 293 (2007) 44. [5] H.J.M. Bouwmeester, Catal. Today 82 (2003) 141. [6] H. Lu, Y. Cong, W.S. Yang, Mater. Sci. Eng. B 41 (2007) 55.
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