ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 7, pp. 799–805. © Pleiades Publishing, Inc., 2006. Original Russian Text © G.M. Kaleva, N.V. Golubko, S.V. Suvorkin, G.V. Kosarev, I.P. Sukhareva, A.K. Avetisov, E.D. Politova, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 7, pp. 881–887.
Preparation and Microstructure of ZrO2- and LaGaO3-Based High-Porosity Ceramics G. M. Kaleva, N. V. Golubko, S. V. Suvorkin, G. V. Kosarev, I. P. Sukhareva, A. K. Avetisov, and E. D. Politova Karpov Research Institute of Physical Chemistry (Russian State Scientific Center), ul. Vorontsovo pole 10, Moscow, 105064 Russia e-mail:
[email protected] Received December 27, 2005
Abstract—We have studied the effects of the morphology and concentration of pore formers on the microstructure and gas permeability of porous zirconia- and lanthanum-gallate-based oxygen-ion-conducting ceramics. The results have been used to optimize the preparation conditions and composition of the ceramics. The resultant dense, fine-grained materials have porosities of up to 56%. DOI: 10.1134/S0020168506070193
INTRODUCTION There is currently considerable research interest in porous ceramics which are potentially attractive as materials for high-temperature gas separation membranes, gas sensors, electrodes of solid oxide fuel cells, and catalytic membrane reactors. Moreover, oxygenion-conducting porous ceramics can be used as substrates for producing thin-film membranes [1, 2]. Inorganic membranes have a number of important advantages over polymer membranes, in particular, higher chemical, structural, and thermal stability and, accordingly, a longer service life [1]. Among the many ceramic oxygen-ion conductors, the best known one is yttria-stabilized zirconia (YSZ), which is used as electrolyte in high-temperature solid oxide fuel cells and oxygen sensors. Zirconia doped with more than 8 mol % Y2O3 has a cubic fluorite-type structure, which is stable in a broad temperature range, from room temperature to 3000 K, and a thermal expansion coefficient comparable to that of dense membrane materials [2]. Porous inorganic oxygen-ionconducting membrane materials, such as YSZ, can be used as substrates for thin, dense ceramic or metallic membranes and as filters for liquid-phase filtration. Lanthanum-gallate-based perovskites containing strontium and magnesium cations on some lanthanum and gallium sites, respectively, offer high oxygen-ion conductivity σi in the range 900–1100 K, comparable to the σi of well-known zirconia-, bismuth-oxide-, and ceria-based solid electrolytes. Such ceramics possess high chemical and thermodynamic stability in various gaseous environments (oxygen, hydrogen, water vapor) at high temperatures [3–9]. Because of the lower σi of such materials in comparison with bismuth oxide, thin layers of oxygen-ion-conducting oxides must be pro-
duced on porous substrates having a sufficiently small thermal expansion mismatch with the oxide. It is of current interest to prepare high-porosity materials based on stabilized zirconia and lanthanum gallate and to determine their gas permeability. In creating dense ceramics with controlled pore structure, potentially attractive as substrates for producing thinfilm membranes, an important step is detailed microstructural characterization and evaluation of the porosity of the material, pore size, and pore shape. One way of fabricating porous ceramic materials is by adding a natural or synthetic pore former to oxide powder [10, 11]. In a certain temperature range, the pore former in the synthesized material reacts with atmospheric oxygen to form a gaseous substance. In this work, we analyze the effects of the nature and morphology of the pore former and the morphology of the starting oxide powders on the microstructure and gas permeability of porous ZrO2 (I), CaO-stabilized ZrO2 (II), Y2O3-stabilized ZrO2 (III), (La0.9Sr0.1)(Ga0.8Mg0.2)O2.85 (IV), (La0.9Sr0.1)(Ga0.4Fe0.4Mg0.2)O3 – y (V), and (La0.9Sr0.1)(Ga0.4Fe0.4Mg0.2)O3 – y + Y2O3-stabilized ZrO2 (1 : 1 weight ratio) (VI) ceramics. EXPERIMENTAL Porous ceramics were prepared by sintering lanthanum-gallate-based powders using the following chemicals as pore formers: natural potato starch (PS), (ë6H10é5)n ; ammonium citrate (AC), C6H5O7(NH4)3; and poly(methyl methacrylate) (PMMA), [−CH2C(CH3)(CO2CH3)–]n . As a plasticizer, we used a 5% polyvinyl alcohol solution in order to wet the surface of pore former particles.
799
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KALEVA et al. 2 5
1
3 4 N2
6
G
mounted in a holder (5), which was hermetically sealed with rubber gaskets. The free area of the substrate was 28.3 mm2 in all experiments. Next, the holder was connected to the gas delivery line. The inlet N2 pressure was varied using a gas regulator (2) and control valve (3) and was monitored with a precision pressure gage (4). The flow rate of the gas passing through the substrate was determined by a rotameter (6). RESULTS AND DISCUSSION
Fig. 1. Schematic of the experimental setup for determining the gas permeability of porous substrates: (1) gas cylinder, (2) gas regulator, (3) control valve, (4) precision pressure gage, (5) sample holder, (6) rotameter.
The sintering temperature was 1770 (I–VI) or 1820 K (I, III, VI), and the sintering time was varied from 0.5 to 8 h. Sol-derived La0.9Sr0.1Ga0.5Fe0.5Oy samples were sintered at T1 = 1570 K (4 h) and T2 = 1770 K (4.5 h). The ceramics were characterized by x-ray diffraction (XRD) analysis (DRON-3M diffractometer, ëuKα radiation) and electron microscopy (JEOL-35CF instrument). The elemental composition of individual grains was determined by electron-probe x-ray microanalysis (EPXMA). Porosity was determined as Π = 1 – ρmeas /ρx .
(1)
The gas permeability of the ceramics was assessed using N2 . A schematic of the experimental setup we used is shown in Fig. 1. A porous substrate was
The electron micrographs shown in Fig. 2 illustrate the microstructure of the pore formers. PS and AC consist of agglomerated particles ranging in size from 10 to 20 and from 10 to 50 µm, respectively. In contrast to PS and AC, the PMMA pore former has a uniform microstructure and consists of almost identical spherical particles 5–6 µm in size. The CaO-stabilized ZrO2 powder consists of large rectangular crystallites 50 to 130 µm in size. First, we prepared porous CaO-stabilized ZrO2 ceramics using the three pore formers. Analysis of the microstructure, particle size, and morphology of the pore-former powders and synthesized porous ceramics showed that PMMA was the most effective pore former for reproducible fabrication of porous ceramics possessing, on the one hand, high density and, on the other, porosity high enough for air delivery to gas-tight membranes not to be rate-limiting. Figure 3 shows the XRD patterns of the ceramics. Using XRD data, we determined their lattice parameters and x-ray density, which was then used to evaluate the porosity of the ceramic samples.
(a)
10 µm
(b)
(c)
10 µm
(d)
100 µm
100 µm
Fig. 2. Electron micrographs illustrating the surface microstructures of (a) PS, (b) AC, and (c) PMMA used as pore formers and (d) CaO-stabilized ZrO2 powder. INORGANIC MATERIALS
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PREPARATION AND MICROSTRUCTURE 111
1200
801 (a)
311
ZrO2〈CaO〉
220
200
800
ZrO2(M)
ZrO2(M)
400
0 20
25
30
800
35
40
45
50
55
ZrO2〈Y2O3〉
M
60 (b)
0
M
222
200
M
20
220
C
C
25
35 110
800
30
M M 40
M C
M
50
55
M
45
C
60
(La0.9Sr0.1)(Ga0.4Fe0.4Mg0.2)O3 (c)
600
111
100
200
20
211
200
400
0
311
M 400
111
Intensity
600
25
30
35
40
45
50
55 60 2θ, deg
Fig. 3. XRD patterns of porous (a) CaO-stabilized ZrO2 , (b) Y2O3-stabilized ZrO2 , and (c) La0.9Sr0.1Ga0.4Fe0.4Mg0.2Oy ceramics prepared by sintering for 2 h at 970 K + 8 h at 1770 K with the use of 50 vol % PMMA as a pore former.
Electron micrographs of fracture and sample surfaces demonstrate that the porous CaO-stabilized ZrO2 ceramics prepared by firing for 2 h at 970 K + 4 h at 1770 K with the addition of AC and PMMA as pore INORGANIC MATERIALS
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formers have a coarse-grained microstructure and contain pores from several microns to 20–30 µm in size (Fig. 4). Increasing the PMMA content from 30 to 50 vol % increases the porosity, pore size, and, hence,
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(a)
100 µm
(b)
100 µm
(c)
100 µm
(d)
100 µm
Fig. 4. Electron micrographs of (a, b) fracture and (c, d) sample surfaces illustrating the microstructure of porous CaO-stabilized ZrO2 ceramics prepared by sintering for 2 h at 970 K + 4 h at 1770 K with the use of (a, c) 30 and (b, d) 50 vol % PMMA as a pore former.
(a)
10 µm (b)
10 µm
(c)
10 µm (d)
10 µm
Fig. 5. Electron micrographs of (a, b) fracture and (c, d) sample surfaces illustrating the microstructure of porous (a, c) La0.9Sr0.1Ga0.8Mg0.2Oy and (c, d) La0.9Sr0.1Ga0.4Fe0.4Mg0.2Oy ceramics prepared by sintering for 2 h at 970 K + 8 h at 1770 K with the use of 50 vol % PMMA as a pore former.
the volume of the intergranular channels. Such ceramics had a relatively low density, were brittle, and showed little or no shrinkage during firing, clearly, because of the small particle size of the ceramic powder. To produce dense, fine-grained, porous materials, ceramics I and III–VI were prepared to different firing schedules using submicron powders of controlled com-
position (checked by EPXMA) and 50 vol % PMMA, the most effective pore former. Fine-grained, porous ceramics were obtained from powders of compositions IV and V. The grain size of ceramics IV and V was 5–7 and 1–5 µm, respectively (Fig. 5). INORGANIC MATERIALS
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Table 1. Shrinkage ∆D/D, measured density ρmeas , x-ray density ρx, and porosity Π = 1 – ρmeas /ρx of ceramics prepared with the addition of 50 vol % PMMA as a pore former Sample no.
Composition
∆D/D, %
ρmeas , g/cm3
ρx, g/cm3
Π, %
1
La0.9Sr0.1Ga0.8Mg0.2O2.85
38.9
4.44
6.70
33.7
2
La0.9Sr0.1Ga0.8Mg0.2O2.85
38.9
4.21
6.70
37.2
3
La0.9Sr0.1Ga0.4Fe0.4Mg0.2O2.85
29.9
3.37
6.55
48.6
4
La0.9Sr0.1Ga0.4Fe0.4Mg0.2O2.85
29.3
3.26
6.55
50.2
5
(Zr,Ca) O *2
0
3.13
5.9
47.0
6
(Zr,Ca) O ** 2
0
3.16
5.9
46.0
7
(Zr,Ca)O2
3.0
2.88
5.9
51.0
8
La0.9Sr0.1Ga0.5Fe0.5O2.85 (sol-derived)
32.5
9
ZrO2
24.8
2.75
5.9
53.2
10
(Zr,Y) O *** 2
25.0
2.60
5.9
55.9
11
ZrO2
26.7
2.84
5.9
51.9
12
(Zr,Y)O2
26.2
2.66
5.9
54.9
13
La0.9Sr0.1Ga0.4Fe0.4Mg0.2O2.85 + (Zr,Y)O2 (1 : 1 weight ratio)
42.9
4.45
6.2
26.6
6.55
Note: The samples were sintered in two steps; samples 1–7: 2 h at 970 K + 8 h at 1770 K; samples 8–10: 1 h at 970 K + 4.5 h at 1770 K, prefiring for 2 h at 970 K + 4 h at 1570 K; samples 11–13: 1.5 h at 770 K + 2 h at 1820 K, heating from 1170 to 1820 K and cooling from 1820 to 1370 K at a rate of 5 K/min. * 30 vol % pore former, 12 at % Ca. ** 40 vol % pore former, 12 at % Ca. *** 5 mol % Y2O3 .
Prefiring at 770–970 K for 1.5–2 h ensured complete burnout of the pore former, and high-temperature (1770–1820 K) heat treatment for 2–8 h ensured densification of the ceramic samples. The shrinkage, density, and porosity data for the resultant ceramics are presented in Table 1 (samples 1–4, 8, 10, 13). In selecting sintering conditions, we took into account the temperature-dependent shrinkage data obtained by Kim and Lin [1] for porous Y2O3-stabilized ZrO2 ceramics using dilatometry. Their results showed that densification occurred mainly in the temperature range 1300–1800 K. Therefore, low heating and cooling rates are of considerable importance in heat treatment of porous ceramics. Analysis of the shrinkage data for our samples allowed us to optimize the sintering conditions: twostep sintering, with 1.5-h low-temperature (770 K) holding, ensuring complete burnout of the pore former, and high-temperature (1770 K) firing. In the peakshrinkage temperature range, the samples were heated and cooled at a rate of 5 K/min. In contrast to ZrO2-based ceramics, porous solderived (La0.9Sr0.1) · (Ga0.5Fe0.5)O3 – y ceramics prepared by two-step high-temperature firing, 4 h at 1570 K + INORGANIC MATERIALS
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4.5 h at 1770 K, have a fine-grained microstructure (grain size far below 1 µm) and contain submicronsized pores (Fig. 6). These results are consistent with the conclusion drawn by Kim and Lin [1] that the pore size and shape in porous Y2O3-stabilized ZrO2 ceramics are governed by the morphology of the pore former. Moreover, electron micrographs of fracture and sample surfaces indicate that the morphology of the ceramic powder has a significant effect on the pore size and porosity in ceramics IV and V. The porosity data obtained by Eq. (1) are presented in Table 1. The porosity of singlephase samples I and III is 50–51%, and that of samples IV and V is 34–37 and 48–50%, respectively. The present porosity data correlate with those reported by Kim and Lin [1] for porous ceramics prepared using different pore formers: PMMA, graphite, polyethylene, and a PMMA–graphite composite. Our results demonstrate that the ceramics prepared with the use of PMMA as a pore former have the highest porosity (Table 1), in line with earlier data [1]. Table 2 presents gas permeability data for porous ceramics I–V. As seen the highest gas permeability is offered by ceramics I and III, which have the highest porosity: Π 53–56%. Analysis of the N2 permeability
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Table 2. N2 permeability P of porous ceramics prepared with the addition of 50 vol % PMMA as a pore former Composition and preparation conditions (Zr,Ca)O2 970 K (2 h) + 1770 K (4 h) ZrO2 770 K (1.5 h) + 1820 K (2 h) (Zr,Y)O2 770 K (1.5 h) + 1820 K (2 h) (La0.9Sr0.1)(Ga0.8Mg0.2)O2.85 970 K (2 h) + 1770 K (4 h) (La0.9Sr0.1)(Ga0.4Fe0.4Mg0.2)O3 970 K (2 h) + 1770 K (4 h)
∆p, kPa
P, cm3/(cm2 min)
P/∆p, cm3/(cm2 min kPa)
Average P/∆p, cm3/(cm2 min kPa)
4.05 8.11 12.16 4.05 8.11 12.16 4.05 8.11 12.16 4.05 8.11 12.16 4.05 8.11 12.16
25.2 51.0 89.2 892.6 2009.8 2903.0 78.8 159.5 238.4 2.91 5.7 8.5 0.8 1.6 2.7
6.2 6.3 7.3 220.4 247.8 238.7 19.5 19.7 19.6 0.7 0.7 0.7 0.2 0.2 0.2
6.6
data for the lanthanum-gallate-based ceramics IV and V in relation to their microstructures indicates that the gas permeability is proportional to the pore size.
235.6
19.6
0.7
0.2
Note that composite VI showed no N2 permeability in the range of pressure drops studied, which is attributable to the high density of the ceramic and its low porosity (26%), due to the small (below 1 µm) pore size. This finding suggests that there is a critical porosity below which ceramics have no N2 permeability. CONCLUSIONS We prepared porous ceramics that offer the desired combination of density and porosity, an optimal finegrained microstructure, and gas permeability high enough not to limit air delivery to gas-tight membranes.
(a)
1 µm
ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 05-03-18127. REFERENCES
(b)
1 µm
Fig. 6. Electron micrographs of (a) fracture and (b) sample surfaces illustrating the microstructure of porous sol-derived La0.9Sr0.1Ga0.5Fe0.5Oy ceramics prepared by two-step hightemperature firing, 4 h at 1570 K + 4.5 h at 1770 K.
1. Kim, J. and Lin, Y.S., Synthesis and Characterization of Suspension-Derived, Porous Ion-Conducting Ceramic Membranes, J. Am. Ceram. Soc., 1999, vol. 82, no. 10, pp. 2641–2646. 2. Boaro, M., Vohs, J.M., and Gorte, R.J., Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting Methods, J. Am. Ceram. Soc., 2003, vol. 86, no. 3, pp. 395–400. 3. Ishihara, T., Matsuda, H., and Takita, Y., Doped LaGaO3 Perovskite Type Oxide as a New Oxide Ionic Conductor, J. Am. Ceram. Soc., 1994, vol. 116, no. 9, pp. 3801–3803. 4. Huang, P. and Petric, A., Superior Oxygen Ion Conductivity of Lanthanum Gallate Doped with Strontium and INORGANIC MATERIALS
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PREPARATION AND MICROSTRUCTURE Magnesium, J. Electrochem. Soc., 1996, vol. 143, no. 5, pp. 1644–1648. 5. Huang, K., Tichy, R., and Goodenough, J., Superior Perovskite Oxide-Ion Conductor; Strontium- and Magnesium-Doped LaGaO3: Phase Relationships and Electrical Properties, J. Am. Ceram. Soc., 1998, vol. 81, no. 10, pp. 2565–2575. 6. Aleksandrovskii, V.V., Venskovskii, N.U., Kaleva, G.M., et al., Phase Composition, Microstructure, and Thermal Expansion of B2O3-Modified LaGaO3-Based Ceramics, Neorg. Mater., 2001, vol. 37, no. 6, pp. 758–763 [Inorg. Mater. (Engl. Transl.), vol. 37, no. 6, pp. 641–646]. 7. Glavatskikh, T.Yu., Venskovskii, N.U., Aleksandrovskii, V.V., et al., Preparation and Properties of Oxygen-Selective LaGaO3-Based Ceramics with Mixed Ionic–Electronic Conduction, Neorg. Mater., 2003,
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