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Oxygen semipermeability of mixed-conductive oxide thick-film prepared by slip casting
SOLID
ELSEVIER
STATE IONICS Solid State Ionics 79 (1995) 195-200
Oxygen semipermeability of mixed-conductive prepared by slip casting
oxide thick-film
Norio Miura ‘, Yasuo Okamoto, Jun Tamaki, Kenji Morinaga, Noboru Yamazoe Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan
Abstract A dense thick film of mixed-conductive perovskite-type oxide (La,,Sr,,Co,,Fe,,O,) was prepared by means of a slip-casting technique. Film thickness could be easily controlled by changing the casting time. Oxygen semipermeability through the film increased with decreasing film thickness. An acid treatment of the film was found greatly to improve the oxygen semipermeability, because the treatment could remove surface impurities like SrO. Keywords: Perovskite-type oxides; Oxygen permeation;
Mixed conductivity;
1. Introduction Some of the oxygen-deficient perovskite-type oxides (ABO,) containing a transition metal at B sites are excellent mixed conductors of oxide ions and electrons (or holes) especially at elevated temperatures [1,2]. Such mixed-conductive oxides have been extensively studied for application to an oxygen-permeating-membrane which can work without the need for electrodes and external electrical circuits [2-91. Oxide ions move through the membrane from the high to low oxygen partial pressure side under the driving force of a gradient in oxygen chemical potential, while electrons (or holes) migrate to compensate the electric charge. In our previous studies using a sintered disk type membrane [3-51, A and B site partially substituted
T
81-92-573-9611
(ext. 331); Fax: + 81-92-575-2318.
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00061-5
Slip casting; Thick film
lanthanum cobaltites, Lal_,Sr,Co,_,Fe,O,, were found to show large oxygen permeation coefficients at temperatures higher than 500X’, and thus appeared to be promising materials for the oxygen semipermeation membrane. From another viewpoint, the rate of oxygen permeation sharply depends on the thickness of membrane, usually being inversely proportional to the thickness, so that the availability of a thin membrane is also an important key to constructing an oxygen permeation system. However, the fabrication of a thin membrane of perovskite-type oxide presents considerable difficulties. Previously we tried to fabricate a thick-film of La,,Sr,,+,CoO, (LSCO) on a porous LSCO substrate by means of sputtering or spray-deposition, but the films obtained were not completely gas-tight and, in addition, not free from a change in chemical composition at the film surface [lO,ll]. Slip casting is a well-known traditional technique for fabricating ceramic forms of complex shape. This technique has recently been applied for the fabrica-
196
N. Mum
et al. /Solid
State Ionics 79 (1995) 195-200
tion of various thick-films, such as Y,O,-stabilized zirconia films for solid oxide fuel cells [12,13] and lamella Al,O, ceramics as structural materials [14]. Such a situation motivated us to apply this technique to the fabrication of a perovskite-type oxide thickfilm. This paper deals with the fabrication of thick-films of La,,Sr,,,Co,.,Fe,.,O, by using the slip casting technique and the oxygen-permeation characteristics of the resultant films.
2. Experimental
I
Pouring slip
2.1. Slip casting for thick-film membrane The slip casting technique begins with the preparation of a slip (or slurry) which is a water-based suspension of the finely pulverized ceramic material to be cast. The present work aimed at fabricating a thick-film membrane of the perovskite-type oxide, La,,,Sro.4Co,,Fe,,0~. However, rather than suspending this oxide directly, a precursor of it was suspended in the slip. The precursor was prepared from constituent metal salts, i.e., La(CH,COO), . 1.5H,O, Sr(CH,COO), .0.5H,O, Co(CH,COO), . 4H,O and Fe(NO,), .9H,O: An aqueous solution dissolving these metal salts at the cationic composition of the oxide was evaporated to dryness, followed by calcination at 350°C for 3 h in air. Because of the low calcination temperature applied, the precursor remained almost amorphous. This precursor was added to water together with a polycarbonatebased dispersant (A6301, Toa Gosei Chemical Co., Ltd.) to obtain the slip. Fig. 1 shows the procedures of slip casting to prepare a concave-shaped green body. The above slip was first poured into a square mold (12 X 12 X 5 mm) made of urethane bars on a gypsum block. After being cast for a desired period, excess slurry was drained out to leave the cast film on the gypsum block. Then with the film being masked at the central part by a silicone rubber rod of 9 mm 0, the slip was again poured into the same mold and kept standing about 40 min to cast the thick outer section of the green body. The green body was calcined at 1200°C for 5 h in air. The square comers of the periphery were polished off to finally obtain a thick-
0
Casting
Fig. 1. Slip-casting
procedure
for producing
thick-film
membrane.
film membrane supported on a round bulky periphery, as illustrated by the photographs of its top and cross-sectional view in Fig. 2. In this particular case, the membrane was 130 pm thick and 6 mm in diameter.
2.2. Characterization
of thick-film
The thick-film membranes thus obtained were subjected to structural investigations by means of XRD (4011, Rigaku), SEM (S-510, Hitachi), and XPS (ESCA3-Mk II, Vacuum Generator). Oxygen semipermeation experiments were carried out in an apparatus similar to that reported before [3,4]. Each thick-film membrane was welded at the round periphery to an MgO tube (13 mm 0) by using a silver ring as a welding material. With the outside and inside of the membrane being exposed to air and flowing He, respectively, the rate of oxygen permeation from air to He was monitored by means of gas chromatography.
N. Miura et al. /Solid
197
State Ionics 79 (1995) 195-200
3. Results and discussion 3.1. Preparation and control of thick-film membrane It was found that the thickness and quality of the slip-cast film depended strongly on the concentration of the precursor powder of the slip (or slurry) and the casting time. When the slurry was too dilute, the film could not be free of cracks and sometimes it was even difficult to remove it safely from the gypsum block. From an excessively dense slurry, on the other hand, it was difficult to control the film thickness due to too large thickening rate of the film. Thus an optimum slurry was found to contain the precursor powder at a concentration around 1.27 g. cme3. Fig. 3 depicts the thickness of the green films casted from this particular slurry as a function of the square root of casting time. An almost linear correlation is observed there, being consistent with the
5mm Fig. 2. Top view (upper) and cross sectional view (lower) of La,,,Sr,,,Co,,,Fe,,,O~ thick-film membrane (after polishing the comers).
Fig. 3. Thickness of slip-cast root of the casting time.
green film as a function
of square
theory, which is expressed by the following equation [15]: L2 = k. t.
(1)
Here L and t are film thickness and casting time, respectively, and k is a constant. It seems that the film-thickening process is determined by the rate of diffusion of water through the film already cast. In any case, the result of Fig. 2 ensures that, under the conditions used, the green film thickness can be controlled in the range of 190-850 pm by changing the casting time from 25 to 290 s. However, films thinner than 190 p,rn could not be obtained, even if the casting time was set at less than 25 s, due to the formation of cracks and/or the difficulty of its removal from the mold. Fig. 4 shows X-ray diffraction patterns of the thick-film membrane as cast and as calcined. The film as cast remained almost amorphous, showing only weak diffraction peaks ascribable to the constituent metal oxides of La,O,, Fe,O,, Co,O,, or SrO. On the other hand, the calcined film showed an XRD pattern ascribable to a single-phasic perovskite-type oxide. As seen from the SEM photograph of its cross-section in Fig. 5, the thick-film membrane looked free from cracks and holes; the relative density of the film as evaluated by the Archimedes method was as high as 99%. These results indicate that the slip-casting technique starting from the precursor material gives rise to a sufficiently dense thick-film of perovskite-type oxide. As
N. Miura et al. /Solid
State Ionics 79 (I 995) 195-200
p!
E n
* -
u
2; ._
0 :
I
l:
co103
,:
Fez03
34 .
0: Perovskite phase
(b)
.
E
La203 ,,:S& . : co304
0
5-
.B 3 2 . e E 2-
30
40
50
60
28 I deg.
& 2’ 3 . % 33 O .-.-.-...-
Fig. 4. XRD patterns of the thick-film at 1200°C (b).
as cast (a) and as calcined
revealed by SEM observation, the precursor powder consisted of finer particles than those of the oxide after calcination. This demonstrates an advantage of using the precursor rather than the oxide itself in the fabrication of a dense thick-film membrane. 3.2. Oxygen semipermeability To understand the oxygen-permeating properties of the thick-film membrane of LaO,,Sro,~Coo~,Fe,~,O, prepared by the present slip-casting technique, the rate of oxygen permeation through a 240 p.rn thick membrane as prepared (A) is shown as a function of
Fig. 5. SEM photograph (cross section) of La,,Sr,,,Co,,,Fe,,,O~ thick film calcined at 1200°C.
500
600
I 700
,
800
I 900
, 1000
Temperature I “C Fig. 6. Oxygen semipermeability of La,,Sr,,,Co,_,Feo,zO, thick film as a function of operation temperature: (A) 240 p,m film as calcined; (B) 1300 pm film after polishing (C) 200 pm film after acid-treatment.
operation temperature in the range of 500~875°C in Fig. 6. The rate increased with increasing temperature monotonically, reaching 1.5 cm3 min-’ cm-* at 875°C. For comparison, a 1300 km thick membrane (B) was also prepared by slip-casting, and polishing its surfaces with emery paper. As also plotted in Fig. 6, the rate of oxygen permeation through this membrane also increased with increasing temperature, reaching 0.6 cm3 min-’ cmP2 at 875°C. Obviously, membrane A was more oxygenpermeating than membrane B, with their permeation rates being different by 2.5 times at 875°C. This confirms the tendency that the rate increases with decreasing film thickness. However, the observed difference in rate between membranes A and B was far smaller than would be expected from the difference in film thickness. It has already been shown for the disc membrane that high temperature calcination induces a compositional change in the surface region and that the removal of the surface layer by mechanical polishing is necessary to realize an intrinsic rate of oxygen permeation [4,11]. It was found that such a surface compositional change also took place for the slip-cast membrane, as mentioned later. In the present case, an acid treatment consisting of dipping the mem-
N. Miura et al. /Solid
100
State tonics 79 (I 995) 195-200
loo0
Film thickness / pm Fig. 7. Influence of film thickness on the oxygen semipermeability of La,,,Sr,,,Co,,sFe~,20~ thick-films at three selected temperatures (after acid-treatment).
0. 0
1
199
2
3
4
5
Acid treatment time / h Fig. 8. Influence of acid treatment time on the oxygen semipermeability of 200 @rn thick film of La,,,Sr,,,Co,,sFe,, ?07 at various temperatures.
brane in 2 M HNO, solution (ca. 10 cm3> at 25°C for 2 h followed by washing with water, was effective in removing the surface layer. A 200 p,rn thick membrane after this treatment (C) gave much improved oxygen permeability as shown in Fig. 6. The rate at 875”C, 3.8 cm3 mine* cm-‘, was about six times higher than that for membrane B (1300 km thick) in good agreement with the ratio in the reciprocal film thickness (6.5 times). The apparent activation energy of oxygen permeation was about 30-34 kcal . mol-’ for the three membranes. Fig. 7 shows the dependence of oxygen permeation rate at 750, 800 and 850°C on the thickness for the films thus treated with acid. At each temperature, logarithm of the rate decreased linearly with an increase in logarithm of the thickness. The slope equal to - 1 in each temperature suggests that oxide ion migration through the films follows Fick’s first law. 3.3. Effects of acid treatment In order to elucidate the effects of acid treatment, 200 p,m thick membranes were treated with 2 M HNO, solution at 25°C for varying periods of time. Fig. 8 shows the rate of oxygen permeation as a function of acid treatment time. The rate increased with increasing treatment time up to 2 h, while a prolonged treatment for 5 h brought a significant loss
Fig. 9. SEM photographs of the surface of La,,,Sr, thick-film before (a) and after (b) acid treatment.
,Co,,,Fe,,,O,
N. Miura et al. /Solid
200
State Ionics 79 (1995) 195-200
4. Conclusions A dense, thick-film membrane of perovskite-type oxide (La,,Sr,,,Co,.sFe,.,0,) could be prepared by casting a slip containing a precursor of the oxide into a concaved-shaped green body, followed by calcining at 1200°C. The film membrane, formed the bottom of the calcined body, and its thickness was easily controlled by changing the casting time. The oxygen semipermeability was much improved with acid treatment, which removed surface impurities like SrO from the membrane. The rate of oxygen permeation through the acid-treated membrane followed Fick’s first law well. The oxygen semipermeability of 3.8 cm3 min-’ cm-* at 875°C was marked with a 200 pm thick, acid-treated membrane. I
140
1
I
135
I
I
I
130
Binding energy / eV Fig. 10. KF’S spectra at SDd level for Lao,sSro,aCo,.sFe,,,O, thick-film before (al and after (b) acid treatment.
in the oxygen permeation rate. It seems that a treatment for 2 h is an optimum for removing the surface impurity layer, while a further treatment may damage the perovskite oxide in the inner region of the film. Thus the acid treatment was set to 2 h unless otherwise noted. Fig. 9 shows SEM photographs of the film (top view) before and after the acid treatment. It is seen that the surface was rather rough before the treatment, while it became smooth after the treatment, indicating that the surface region dissolved out by the treatment. Finally, the surface state of the films was observed by means of XPS. A noticeable change in XPS spectrum with the acid treatment was observed for Sr3d, while La3d,/,,, C02p~,~, and Fe2p,,, remained almost unchanged. Fig. 10 shows the SDd spectra before and after the acid treatment. The film as calcined showed two well-dissolved peaks at 134 and 132 eV, and a shoulder around 135 eV. With the treatment, the peak at 132 eV almost disappeared, while the other peak and shoulder hardly changed. The peak at 132 eV is likely to be assigned to Sr3d5,, of SrO or SrCO,, the binding energies of which are reportedly 131.8 eV [16].
References [II Y. Teraoka, H.M. Zhang, K. Okamoto and N. Yamazoe, Mater. Res. Bull. 23 (19881 51. 121Y. Teraoka, T. Nobunaga, K. Okamoto, N. Miura and N. Yamazoe, Solid State Ionics 48 (19911 207. 131 Y. Teraoka, H.M. Zhang, S. Furukawa and N. Yamazoe, Chem. Lett. 1985 (19851 1743. [41 Y. Teraoka, S. Furukawa, H.M. Zhang and N. Yamazoe, J. Chem. Sot. Japan. 1988 (1988) 1084. [51 Y. Teraoka, T. Nobunaga and N. Yamazoe, Chem. Lett. 1988 (19881503. i’51H. lwahara, T. Esaka and T. Mangahara, J. Appl. Electrochem. 18 (1988) 173. [71 B.A. van Hassel, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya and H.J.M. Bouwmeester, Solid State lonics 66 (19931 295. [81 H. Kruidhof, H.J.M. Bouwmeester, R.H.E. v. Doorn and A.J. Burggraaf, Solid State Ionics 63-65 (1993) 816. [91 H.W. Brinkman, H. Kruidhof and A.J. Burggraaf, Solid State Ionics 68 (1994) 173. [lOI Y. Teraoka, T. Fukuda, N. Miura and N. Yamazoe, J. Ceram. Sot. Japan. 97 (1989) 467. [ill Y. Teraoka, T. Fukuda, N. Miura and N. Yamazoe, J. Ceram. Sot. Japan. 97 (1989) 533. 1121 H. Takebe, N. Yoshihara and K. Morinaga, J. Ceram. Sot. Japan. 98 (1990) 136. 1131 T. Setoguchi, T. Inoue, H. Takebe, K. Eguchi, K. Morinaga and H. Arai, Solid State Ionics 37 (19901 217. 1141 H. Takebe and K. Morinaga, J. Ceram. Sot. Japan. 96 (19881 1149. 1151 D.S. Adcock and I.C. McDowall, J. Am. Ceram. Sot. 40 (1957) 355. il61 V. Young and T. Otagawa, Appl. Surface Sci. 20 (19851228.
ELSEVIER
STATE IONICS Solid State Ionics 79 (1995) 195-200
Oxygen semipermeability of mixed-conductive prepared by slip casting
oxide thick-film
Norio Miura ‘, Yasuo Okamoto, Jun Tamaki, Kenji Morinaga, Noboru Yamazoe Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan
Abstract A dense thick film of mixed-conductive perovskite-type oxide (La,,Sr,,Co,,Fe,,O,) was prepared by means of a slip-casting technique. Film thickness could be easily controlled by changing the casting time. Oxygen semipermeability through the film increased with decreasing film thickness. An acid treatment of the film was found greatly to improve the oxygen semipermeability, because the treatment could remove surface impurities like SrO. Keywords: Perovskite-type oxides; Oxygen permeation;
Mixed conductivity;
1. Introduction Some of the oxygen-deficient perovskite-type oxides (ABO,) containing a transition metal at B sites are excellent mixed conductors of oxide ions and electrons (or holes) especially at elevated temperatures [1,2]. Such mixed-conductive oxides have been extensively studied for application to an oxygen-permeating-membrane which can work without the need for electrodes and external electrical circuits [2-91. Oxide ions move through the membrane from the high to low oxygen partial pressure side under the driving force of a gradient in oxygen chemical potential, while electrons (or holes) migrate to compensate the electric charge. In our previous studies using a sintered disk type membrane [3-51, A and B site partially substituted
T
81-92-573-9611
(ext. 331); Fax: + 81-92-575-2318.
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00061-5
Slip casting; Thick film
lanthanum cobaltites, Lal_,Sr,Co,_,Fe,O,, were found to show large oxygen permeation coefficients at temperatures higher than 500X’, and thus appeared to be promising materials for the oxygen semipermeation membrane. From another viewpoint, the rate of oxygen permeation sharply depends on the thickness of membrane, usually being inversely proportional to the thickness, so that the availability of a thin membrane is also an important key to constructing an oxygen permeation system. However, the fabrication of a thin membrane of perovskite-type oxide presents considerable difficulties. Previously we tried to fabricate a thick-film of La,,Sr,,+,CoO, (LSCO) on a porous LSCO substrate by means of sputtering or spray-deposition, but the films obtained were not completely gas-tight and, in addition, not free from a change in chemical composition at the film surface [lO,ll]. Slip casting is a well-known traditional technique for fabricating ceramic forms of complex shape. This technique has recently been applied for the fabrica-
196
N. Mum
et al. /Solid
State Ionics 79 (1995) 195-200
tion of various thick-films, such as Y,O,-stabilized zirconia films for solid oxide fuel cells [12,13] and lamella Al,O, ceramics as structural materials [14]. Such a situation motivated us to apply this technique to the fabrication of a perovskite-type oxide thickfilm. This paper deals with the fabrication of thick-films of La,,Sr,,,Co,.,Fe,.,O, by using the slip casting technique and the oxygen-permeation characteristics of the resultant films.
2. Experimental
I
Pouring slip
2.1. Slip casting for thick-film membrane The slip casting technique begins with the preparation of a slip (or slurry) which is a water-based suspension of the finely pulverized ceramic material to be cast. The present work aimed at fabricating a thick-film membrane of the perovskite-type oxide, La,,,Sro.4Co,,Fe,,0~. However, rather than suspending this oxide directly, a precursor of it was suspended in the slip. The precursor was prepared from constituent metal salts, i.e., La(CH,COO), . 1.5H,O, Sr(CH,COO), .0.5H,O, Co(CH,COO), . 4H,O and Fe(NO,), .9H,O: An aqueous solution dissolving these metal salts at the cationic composition of the oxide was evaporated to dryness, followed by calcination at 350°C for 3 h in air. Because of the low calcination temperature applied, the precursor remained almost amorphous. This precursor was added to water together with a polycarbonatebased dispersant (A6301, Toa Gosei Chemical Co., Ltd.) to obtain the slip. Fig. 1 shows the procedures of slip casting to prepare a concave-shaped green body. The above slip was first poured into a square mold (12 X 12 X 5 mm) made of urethane bars on a gypsum block. After being cast for a desired period, excess slurry was drained out to leave the cast film on the gypsum block. Then with the film being masked at the central part by a silicone rubber rod of 9 mm 0, the slip was again poured into the same mold and kept standing about 40 min to cast the thick outer section of the green body. The green body was calcined at 1200°C for 5 h in air. The square comers of the periphery were polished off to finally obtain a thick-
0
Casting
Fig. 1. Slip-casting
procedure
for producing
thick-film
membrane.
film membrane supported on a round bulky periphery, as illustrated by the photographs of its top and cross-sectional view in Fig. 2. In this particular case, the membrane was 130 pm thick and 6 mm in diameter.
2.2. Characterization
of thick-film
The thick-film membranes thus obtained were subjected to structural investigations by means of XRD (4011, Rigaku), SEM (S-510, Hitachi), and XPS (ESCA3-Mk II, Vacuum Generator). Oxygen semipermeation experiments were carried out in an apparatus similar to that reported before [3,4]. Each thick-film membrane was welded at the round periphery to an MgO tube (13 mm 0) by using a silver ring as a welding material. With the outside and inside of the membrane being exposed to air and flowing He, respectively, the rate of oxygen permeation from air to He was monitored by means of gas chromatography.
N. Miura et al. /Solid
197
State Ionics 79 (1995) 195-200
3. Results and discussion 3.1. Preparation and control of thick-film membrane It was found that the thickness and quality of the slip-cast film depended strongly on the concentration of the precursor powder of the slip (or slurry) and the casting time. When the slurry was too dilute, the film could not be free of cracks and sometimes it was even difficult to remove it safely from the gypsum block. From an excessively dense slurry, on the other hand, it was difficult to control the film thickness due to too large thickening rate of the film. Thus an optimum slurry was found to contain the precursor powder at a concentration around 1.27 g. cme3. Fig. 3 depicts the thickness of the green films casted from this particular slurry as a function of the square root of casting time. An almost linear correlation is observed there, being consistent with the
5mm Fig. 2. Top view (upper) and cross sectional view (lower) of La,,,Sr,,,Co,,,Fe,,,O~ thick-film membrane (after polishing the comers).
Fig. 3. Thickness of slip-cast root of the casting time.
green film as a function
of square
theory, which is expressed by the following equation [15]: L2 = k. t.
(1)
Here L and t are film thickness and casting time, respectively, and k is a constant. It seems that the film-thickening process is determined by the rate of diffusion of water through the film already cast. In any case, the result of Fig. 2 ensures that, under the conditions used, the green film thickness can be controlled in the range of 190-850 pm by changing the casting time from 25 to 290 s. However, films thinner than 190 p,rn could not be obtained, even if the casting time was set at less than 25 s, due to the formation of cracks and/or the difficulty of its removal from the mold. Fig. 4 shows X-ray diffraction patterns of the thick-film membrane as cast and as calcined. The film as cast remained almost amorphous, showing only weak diffraction peaks ascribable to the constituent metal oxides of La,O,, Fe,O,, Co,O,, or SrO. On the other hand, the calcined film showed an XRD pattern ascribable to a single-phasic perovskite-type oxide. As seen from the SEM photograph of its cross-section in Fig. 5, the thick-film membrane looked free from cracks and holes; the relative density of the film as evaluated by the Archimedes method was as high as 99%. These results indicate that the slip-casting technique starting from the precursor material gives rise to a sufficiently dense thick-film of perovskite-type oxide. As
N. Miura et al. /Solid
State Ionics 79 (I 995) 195-200
p!
E n
* -
u
2; ._
0 :
I
l:
co103
,:
Fez03
34 .
0: Perovskite phase
(b)
.
E
La203 ,,:S& . : co304
0
5-
.B 3 2 . e E 2-
30
40
50
60
28 I deg.
& 2’ 3 . % 33 O .-.-.-...-
Fig. 4. XRD patterns of the thick-film at 1200°C (b).
as cast (a) and as calcined
revealed by SEM observation, the precursor powder consisted of finer particles than those of the oxide after calcination. This demonstrates an advantage of using the precursor rather than the oxide itself in the fabrication of a dense thick-film membrane. 3.2. Oxygen semipermeability To understand the oxygen-permeating properties of the thick-film membrane of LaO,,Sro,~Coo~,Fe,~,O, prepared by the present slip-casting technique, the rate of oxygen permeation through a 240 p.rn thick membrane as prepared (A) is shown as a function of
Fig. 5. SEM photograph (cross section) of La,,Sr,,,Co,,,Fe,,,O~ thick film calcined at 1200°C.
500
600
I 700
,
800
I 900
, 1000
Temperature I “C Fig. 6. Oxygen semipermeability of La,,Sr,,,Co,_,Feo,zO, thick film as a function of operation temperature: (A) 240 p,m film as calcined; (B) 1300 pm film after polishing (C) 200 pm film after acid-treatment.
operation temperature in the range of 500~875°C in Fig. 6. The rate increased with increasing temperature monotonically, reaching 1.5 cm3 min-’ cm-* at 875°C. For comparison, a 1300 km thick membrane (B) was also prepared by slip-casting, and polishing its surfaces with emery paper. As also plotted in Fig. 6, the rate of oxygen permeation through this membrane also increased with increasing temperature, reaching 0.6 cm3 min-’ cmP2 at 875°C. Obviously, membrane A was more oxygenpermeating than membrane B, with their permeation rates being different by 2.5 times at 875°C. This confirms the tendency that the rate increases with decreasing film thickness. However, the observed difference in rate between membranes A and B was far smaller than would be expected from the difference in film thickness. It has already been shown for the disc membrane that high temperature calcination induces a compositional change in the surface region and that the removal of the surface layer by mechanical polishing is necessary to realize an intrinsic rate of oxygen permeation [4,11]. It was found that such a surface compositional change also took place for the slip-cast membrane, as mentioned later. In the present case, an acid treatment consisting of dipping the mem-
N. Miura et al. /Solid
100
State tonics 79 (I 995) 195-200
loo0
Film thickness / pm Fig. 7. Influence of film thickness on the oxygen semipermeability of La,,,Sr,,,Co,,sFe~,20~ thick-films at three selected temperatures (after acid-treatment).
0. 0
1
199
2
3
4
5
Acid treatment time / h Fig. 8. Influence of acid treatment time on the oxygen semipermeability of 200 @rn thick film of La,,,Sr,,,Co,,sFe,, ?07 at various temperatures.
brane in 2 M HNO, solution (ca. 10 cm3> at 25°C for 2 h followed by washing with water, was effective in removing the surface layer. A 200 p,rn thick membrane after this treatment (C) gave much improved oxygen permeability as shown in Fig. 6. The rate at 875”C, 3.8 cm3 mine* cm-‘, was about six times higher than that for membrane B (1300 km thick) in good agreement with the ratio in the reciprocal film thickness (6.5 times). The apparent activation energy of oxygen permeation was about 30-34 kcal . mol-’ for the three membranes. Fig. 7 shows the dependence of oxygen permeation rate at 750, 800 and 850°C on the thickness for the films thus treated with acid. At each temperature, logarithm of the rate decreased linearly with an increase in logarithm of the thickness. The slope equal to - 1 in each temperature suggests that oxide ion migration through the films follows Fick’s first law. 3.3. Effects of acid treatment In order to elucidate the effects of acid treatment, 200 p,m thick membranes were treated with 2 M HNO, solution at 25°C for varying periods of time. Fig. 8 shows the rate of oxygen permeation as a function of acid treatment time. The rate increased with increasing treatment time up to 2 h, while a prolonged treatment for 5 h brought a significant loss
Fig. 9. SEM photographs of the surface of La,,,Sr, thick-film before (a) and after (b) acid treatment.
,Co,,,Fe,,,O,
N. Miura et al. /Solid
200
State Ionics 79 (1995) 195-200
4. Conclusions A dense, thick-film membrane of perovskite-type oxide (La,,Sr,,,Co,.sFe,.,0,) could be prepared by casting a slip containing a precursor of the oxide into a concaved-shaped green body, followed by calcining at 1200°C. The film membrane, formed the bottom of the calcined body, and its thickness was easily controlled by changing the casting time. The oxygen semipermeability was much improved with acid treatment, which removed surface impurities like SrO from the membrane. The rate of oxygen permeation through the acid-treated membrane followed Fick’s first law well. The oxygen semipermeability of 3.8 cm3 min-’ cm-* at 875°C was marked with a 200 pm thick, acid-treated membrane. I
140
1
I
135
I
I
I
130
Binding energy / eV Fig. 10. KF’S spectra at SDd level for Lao,sSro,aCo,.sFe,,,O, thick-film before (al and after (b) acid treatment.
in the oxygen permeation rate. It seems that a treatment for 2 h is an optimum for removing the surface impurity layer, while a further treatment may damage the perovskite oxide in the inner region of the film. Thus the acid treatment was set to 2 h unless otherwise noted. Fig. 9 shows SEM photographs of the film (top view) before and after the acid treatment. It is seen that the surface was rather rough before the treatment, while it became smooth after the treatment, indicating that the surface region dissolved out by the treatment. Finally, the surface state of the films was observed by means of XPS. A noticeable change in XPS spectrum with the acid treatment was observed for Sr3d, while La3d,/,,, C02p~,~, and Fe2p,,, remained almost unchanged. Fig. 10 shows the SDd spectra before and after the acid treatment. The film as calcined showed two well-dissolved peaks at 134 and 132 eV, and a shoulder around 135 eV. With the treatment, the peak at 132 eV almost disappeared, while the other peak and shoulder hardly changed. The peak at 132 eV is likely to be assigned to Sr3d5,, of SrO or SrCO,, the binding energies of which are reportedly 131.8 eV [16].
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