Design Of New Oxide Ceramic Materials And

Mat. Res. Soc. Symp. Proc. Vol. 755 © 2003 Materials Research Society

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Design Of New Oxide Ceramic Materials And Nanocomposites With Mixed Conductivity By Using Mechanical Activation Route. Vladimir V. Zyryanov*, Nikolai F. Uvarov*, Vitalii G. Kostrovskii* Vladislav A. Sadykov**, Tatiana G. Kuznetsova**, V. A. Rogov**, V. I. Zaikovskii**, Y.V. Frolova**, G. M. Alikina***, G. S. Litvak**, E. B. Burgina**, E. M. Moroz**, S. Neophytides***. *- Inst. of Solid State Chemistry SB RAS, Novosibirsk, Russia; **- Boreskov Inst. Catalysis SB RAS, Novosibirsk, Russia; ***- Inst. Chem. Eng. & High Temperature Proc., Patras, Greece. ABSTRACT Some perovskite –like Fe (Co) -containing and fluorite-like Bi –containing solid solutions considered as promising materials for high oxygen flux membranes application have been synthesized by the mechanochemical activation route. Structural/microstructural features of those systems characterized by TEM, XRD, IR, and Raman spectroscopy are compared with the data on their mixed conductivity and oxygen mobility. Some properties of nanocomposites comprised of those materials and such oxides as Fe2O3, BaTiO3, BaBiO3 etc are elucidated as well. Densification of those powdered samples by sintering in the 7001400 K temperature range is accompanied by annealing of metastable phases generating “core-shell” nanocomposites with oxygen transport properties different from those of traditional materials. INTRODUCTION Conventional approaches for synthesis of ceramic nanopowders (including mixed conductivity complex oxides for membrane application [1]) are sol-gel method, coprecipitation, polymerized complex precursor route (Pechini method [2]) etc. Mechanochemical approach (high energy ball milling, MA) appears to be an attractive alternative to those methods since it is more reproducible, efficient, fast, and allows to easily produce nanocomposites and metastable compounds with unique properties [3-5]. Membranes made of nanosize grain ceramics/nanocomposites are expected to have better mechanical properties due to superplasticity [6] as well as a higher conductivity due to increased interface/bulk ratio [7]. This work presents results for MA synthesis of some complex oxides with perovskite-like and fluorite-like structures and characterization of their properties as related to possible application for the oxygen separating membranes. For comparison, samples of Bi1.5Y0.3Sm0.2O3 system were prepared by citrate and Pechini routes and studied as well. EXPERIMENTAL Complex oxides in Sr-Ba-Fe-Co-O system were prepared by 5 min mechanical milling of the starting reagents (SrCO3, BaCO3, Fe2O3, CoO) blends in AGO-2 planetary mill with the powder/steel ball ratio ~12/200 followed by calcination at 1000°C÷1100°C. To decrease drastically the iron contamination and increase the activated mixture homogeneity, a proprietary procedure of the MA treatment was used [8]. The milling and firing procedures were repeated for 3 times, and finally a ceramic powder was milled for 10 min. Bi containing complex oxides were obtained by MA synthesis in identical conditions with annealing at 1000 oC () or without it (Ca-Bi-Sb-O). For comparison, samples of Bi-YSm-O system were prepared by citrate and Pechini routes (final calcinations in air at 700 oC). The microstructure of samples was studied using transmission electron microscopy (TEM, Jeol 200 C, 200 kv). The samples were analyzed by powder X-ray diffraction

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(DRON-3.0, CuKα and CoKα irradiation) combined with Infra-red spectroscopy (FTIRS, a Fourier-transform BOMEM MB-102 IR spectrometer) using samples pressed as wafers with KBr, and Raman spectroscopy (a Bruker spectrometer). The thermal analysis was carried out on a DQ-1500 and a MOM devices. Samples (200 mg) were heated with a ramp of 10 grad/min up to 1100 oC under atmosphere or in He. Samples reduction by H2 (10% in Ar) or CH4 (1% in He) in a temperatureprogrammed mode up to 900 oC was carried out using earlier described procedures [9]. The specific surface area was determined by a routine BET procedure using the Ar thermal desorption data. The conductivity of samples pressed into pellets (6-16 mm diameter, 1-3 mm thickness) was measured in air at temperatures in the range of 300-800 K using a 4284 A Precision LCR and a BM-507 Impedance instruments in the frequency range of 5 Hz -1 MHz. Measurements were carried out either in the isothermal or heating-cooling mode with the temperature ramp 3 K/min. To prepare pellets, ceramic powders with a mean particle size ~100 nm as estimated from the BET specific surface value were separated by using an electro-mass-classifier (EMC) [10]. A fraction of those powders aggregates with typical sizes < 1 µm was pressed under 100 MPa uniaxial pressure into pellets which were then sintered in air at temperatures between 1050 and 1150°C for 6 - 60 min. Two silver paste electrodes were used for electrical connections. For metastable Bi-containing fluorites (vide infra), pellets (6 mm diameter, 2 mm thickness) were prepared by hot pressing under 1GPa for 30 min at 250°C. RESULTS AND DISCUSSION Perovskite-like systems A number of single-phase perovskites of a complex composition known for their high oxygen mobility and mixed conductivity has been synthesized by MA route. Fig 1. shows typical diffraction patterns for some systems containing only reflexes corresponding to a cubic perovskite.

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Fig.1. Powder X-Ray diffraction profiles of sintered samples: (1) Sr0.88Ba0.12Fe0.62Co0.38Oz, 1100°C, 6 min; (2) 4SrFe0.5Co0.5Oz + Fe2O3, 1100°C, 6 min; (3) 4SrFe0.5Co0.5Oz + Fe2O3, 1100°C, 60 min.

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Only for Ba-containing Sr1-xBaxFe1-yCoyOz system samples with x≥0.25 were found to be multiphase. However, it was possible to accommodate simultaneously up to 12 elements (Ba, Sr, Ca, Bi, La, Sm, Y, Fe, Co, Ti, Zr, Mn) in the single –phase cubic perovskite structure

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(P*). Data presented in Table 1 demonstrate that a high density of perovskite-based ceramics is achieved by sintering at 1100 oC for 60 min. Table 1. Perovskite cell parameter and density of sintered in air at 1100°C ceramics Sintering 6 min Sintering 60 min Composition before milling and sintering Shrinkage a, Å Shrinkage Density a, Å % % % Sr0.88Ba0.12Fe0.62Co0.38Oz 3.891(3) 13.6 96.1(5) 3.894(3) 14.0 4SrFe0.5Co0.5Oz + Fe2O3 a=3.881(6). 13.22 93.3(5) 3.892(3) 15.21 c=3.919(6) SrFe0.25Co0.75Oz 3.868(3) 13.64 96.3(5) 3.880(3) 14.38 SrFe0.25Co0.75Oz+P*,85:15 3.883(3) 13.83 96.9(5) 3.884(3) 14.84 SrFe0.25Co0.75Oz+P*,55:45 3.879(3) 14.1 97.8(5) 3.881(3) 14.84

Density % 97.5(5) 99.9(5) 98.8(5) 99.9(5) 99.5(5)

The values of the density obtained here for Sr-Fe-Co-O system after sintering at 1100 C exceed the density (~ 95 % of theoretical value) obtained for a ceramic sample of a similar composition annealed at 1200 oC for 5 h [11]. Preliminary investigation of sintering and conductivity of 18 samples in the Sr-BaFe-Co-O system as well as their composites with Fe2O3, BaTiO3, BaBiO3, SrBiO3 and complex perovskite P* has been carried out. BaTiO3 addition demands enhanced temperature of sintering. In contrary, composites with BaBiO3 and SrBiO3 melt at relatively low temperatures, which promotes the grain growth, so nanocrystalline ceramics could not be obtained. The best samples from this series are presented in Table 2. o

Table 2. Density and conductivity (700°C) of ceramic samples in the Sr-Ba-Fe-Co-O system Composition before milling Density, % Conductivity, log(σ[S/cm]) and sintering 1050° 1100° 1150° 1050° 1100° 1150° SrFe0.25Co0.75Oz 92 95 97 -0.8 -1.2 -0.75 SrFe0.5Co0.5Oz 89 93 96 -1.0 -0.2 -0.2 Sr0.75Ba0.25Fe0.75Co0.25Oz 83 89 80 -0.7 -1.5 0 4SrFe0.5Co0.5Oz + Fe2O3 86 95 98 -1.0 -0.65 -0.5 SrFe0.5Co0.5Oz + P*, 50:50 92 94 -1.2 -1.3 P* 88 90 -0.7 -1.7 The sintering ability of nanocomposites (SrFe0.5Co0.5Oz + P*) and (4SrFe0.5Co0.5Oz + Fe2O3) appears to be better than that for individual compounds. Conductivity certainly correlates with the density reaching high values for samples annealed at 1150 oC . The increase of the perovskite cell parameter in the course of sintering indicates the chemical interaction between the components of nanocomposite (Table 1). A higher sintering ability of complex oxide powders subjected to high energy ball milling is related to a high concentration of vacancies and chemical inhomogeneity in their particles which promote bulk diffusion mass transfer [4]. Another important factor is disordering (polyhedra distortion, high density of extended defects etc). This feature appears to be reflected in the absence of characteristic vibronic bands in Raman spectra of perovskite samples up to relatively high temperatures of annealing (not shown for brevity). Such a disordering is thought to be preserved in a part in the dense ceramics as well due to a soft sintering conditions of MA powders. This provides wide prospects for tailoring the grain structure and composition of materials for the electrochemical membrane reactors. Such a ceramics could be even comprised of a metastable phase which is not possible for materials prepared via traditional routes. Thus, in our case, for 4SrFe0.5Co0.5Oz + Fe2O3 composition,, a stable compound Sr2Fe3-xCoxOz known as Balachandran’s phase [12] , was not revealed as

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a major component of this ceramics (specific reflexes of this phase different from that of perovskite are not observed, Fig. 1). Other examples of the relative stability of metastable compounds obtained by mechanochemical synthesis include pyrochlore Cd2Sb2O7 (exists up to beginning of sublimation [13]), and sillenite PbBi4O7 (stable up to temperature T=Tm-50° [14]). Fluorite-like systems The fields of fluorite formation were established for compositions Bi0.6Sb1.4O3 ÷ Bi2O3 ÷ Bi1.5Ca0.5O2.75, Ca0.5Sb1.5O2.75 ÷ Ca0.4Sb1.6O2.8. Single –phase cubic sample of Bi1.5Y0.3Sm0.2O3+x was obtained via MA route as well. After 30 min of mechanical activation, monoclinic Bi2O3 transforms into a metastable phase with the orthorhombic structure very close to that of cubic fluorite δ-Bi2O3. According to thermal analysis data (not shown for brevity), structural and chemical transformations under air in these metastable fluorites but Bi1.5Y0.3Sm0.2O3+x begins at ~400°C. For the latter system, no phase transformations were observed up to 1100 oC. Some parameters for Bi containing samples are presented in Table 3. Table 3. Cell parameter, density and conductivity (400°C) of new metastable fluorites. Sample Color a, Å Density, Density, Conductivity, % TD g/cm3 log(σ[S/cm]) orange ~5.532(10) 7.280 82.4 -4.2 ~δ-Bi2O3 Bi1.6Ca0.4O2.8 Bi1.5Sb0.5O3

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Bi1.5Sb0.5O3.5, yellow annealed at 470° Bi1.5Y0.3Sm0.2O3+x black

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A specific feature of the fluorite-like phases prepared via MA route is a highly disordered stacking of domains within particles, which forms a high density of linear and planar defects. (Fig. 2). In XRD pattern (not shown for brevity) high-angle reflexes (531 etc) were absent

a b c Fig. 2. Typical images of Bi1.5Y0..3Sm0.2O3+x sample particles prepared by citrate (a) or MA (b) route; high resolution image of (b) revealing small-angle boundaries and twins (c). thus indicating a high density of microstrains. In contrary, particles of samples prepared via citrate or Pechini route are comprised of rather regularly shaped platelets (Fig. 2a).

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Such a disordering of MA sample appears to be reflected in the first coordination sphere of cations as well. Thus, for Bi1.5Y0.3Sm0.2O3+x sample prepared via MA route, the band corresponding to the lattice modes is rather symmetric which is expected for the cubic fluorite structure (Fig. 3). For samples with identical chemical composition prepared via citrate or Pechini route, splitting of this band occurs, and the space structure corresponds to a tetragonal one of the Bi1.4Sm0.6O3 type (JCPDS 44-0043). 0.6 302

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Fig. 3. IR spectra of powders with composition Bi1.5Y0.3Sm0.2O3 obtained by different techniques: (1) synthesis by Pechini method; (2) by citric route; (3) by mechanochemical route (cubic fluorite). Samples 1 and 2 have tetragonal structure like Bi1.4Sm0.6O3.

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The temperature dependence of conductivity for metastable fluorites is shown in Fig. 4.

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Fig. 4. Temperature dependence of conductivity of hot pressed at 250°C metastable compounds with fluorite structure

Fig. 5. Typical H2 TPR data for Bi1.5Y0.3Sm0.2O3 samples obtained by Pechini (1), citrite(2) and MA route (3)

Conductivity of complex fluorites reversibly changes within this temperature range, while for δ-Bi2O3 there is a hysteresis, probably, due to a phase transformation [3]. The values of conductivity extrapolated to 1000 K are close to those measured in [3] for similar Bicontaining metastable fluorite-like phases and appear to be promising enough for the practical application. A high value of mixed conductivity at moderate temperatures implies a high oxygen mobility in the lattice. Indeed, as follows from Fig. 5, fluorite-like systems are easily reduced by hydrogen at moderate temperatures. Such a reduction corresponds to transformation of all Bi into a metallic state. It is interesting to note that the method of preparation or the type of phase does not affect appreciably the reducing ability of those samples.

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As for the specific application of Bi-containing systems as catalytic membranes for syn-gas generation at high (up to 1000 oC) temperatures and under reducing conditions (He, methane), Bi1.5Y0.3Sm0.2O3 systems were found to be phase stable up to 1100 C in air or under the He flow losing no more than 1.5 % of weight after keeping for 2h in He at that temperature. Moreover, those systems were found to be not reducible at all by the mixture of 1% of CH4 in He up to 900 oC. In contrary, complex perovskites described above were found to be intensively reduced by methane at temperatures exceeding 800 oC. Hence, Bi-Y-Sm containing fluorites are reasonably stable in reducing conditions and can be used at least in combination with other less reducible fluorites such as Ce-Sm –O systems [3] etc. For this purpose, combining non-expensive mechanochemical technique with Pechini method to design highly conducting dense nanocomposites comprised of particles with different shapes and sizes of identical or different chemical and phase composition appears to be an attractive option. CONCLUSIONS Mechanochemical approach (applying a correct procedure of high energy ball milling and separating a fine fraction of powders from the dense aggregates) was shown to be a suitable technique for preparation of full dense metal oxide ceramics with a high oxygen and electronic conductivity for such application as membranes in electrochemical devices and catalytic membrane reactors. Due to a high sintering activity of powders prepared via the mechanochemical route, it is possible in some cases to prepare nanograin ceramics comprised of metastable compounds. For such a case, full dense ceramics at soft conditions of sintering may be obtained by combination of MA approach with wet techniques. Application of the mechanochemical technique is more preferable for preparation of nanocomposites from compounds with different structure and/or cell parameters. ACKNOWLEDGEMENTS This work was in part supported by INTAS under grant 01-2162 and Russian Fund for Basic Researches under grant 02-03-33330. REFERENCES 1. T. J. Mazanec, T. L. Cable, and J. G. Frye, Jr. Solid State Ionics, 111, 53 (1992) 2. M. P. Pechini, U.S. Patent 3 330 697 (1967). 3. R.Vitlov-Audino, F.J.Lincoln, Mat. Res. Soc. Symp. Proc., 453, 561 (1997). 4. V.V.Zyrianov, Proc. Inter. Congress “Materiaux-2002”, Tours, France, CM-01007. 5. V.V.Zyryanov, in the book “Mechanochemical synthesis in inorganic chemistry”, (Ed. E.G.Avvakumov, Novosibirsk, Nauka, pp.102-125, 1991 (Rus.). 6. L.Radonjic, Mater. Sci. Forum, 282-283, 11 (1998). 7. P.Knauth, H.L.Tuller, Mat. Res. Soc. Symp. Proc., 548, 429 (1999). 8. V.V.Zyryanov, V.F.Sysoev, V.V.Boldyrev, T.V.Korosteleva. Inventor’s Certificate No 1375328, 1988 (USSR). 9. V.A.Sadykov, T.G.Kuznetsova, V.V.Lunin, E. Kemnitz, React. Kinet. Catal. Lett. 76, 83 (2002). 10. V.V.Zyryanov, Inventor’s Certificate No 1403439, 1988 (USSR). 11. B. Ma, U. Balachandran, C.C.Chao, and J.-H. Park, Mat. Res. Soc. Symp. Proc., 453, 579 (1997). 12. B.Ma, J.-H.Park, C.U.Segre, U.Balachandran, Mater. Res. Soc. Symp. Proc., 393, 49 (1995). 13. V.V.Zyryanov, Inorg. Mater., 37, No 11, 1138 (2001). 14. V.V.Zyryanov, Inorg. Mater., in press.