THE DEVELOPMENT OF GAS TIGHT THIN FILMS OF (La,Sr)(Ga,Fe), (La,Sr)(Co,Fe) AND La 2 FOR OXYGEN SEPARATION R. Muydinov, M. Novojilov, O. Gorbenko, I. Korsakov, A. Kaul Moscow State University, Russia S. Samoilenkov, G. Wahl IOPW, TU braunscweig, Germany V. Vashook Dresden University of Technology, Germany Abstract: A number of MIEC layers, La 1-x, Sr 1-x and La 2, with a thickness 3-30 (m were obtained on the porous substrates by MOCVD. The film composition, microstructure and gas tightness were studied. Using postannealing with oxide fluxes the membranes of gas tightness high enough for application were obtained. Specific oxygen permeability of obtained membranes were measured. Key words: MIEC, film oxygen membranes INTRODUCTION Oxygen membranes based on mixed oxygen-ionic and electronic conductors (MIECs) are of great interest for oxygen separation and catalytic membrane reactors for partial oxidation of hydrocarbons. Perspective membrane materials are complex oxides La 1-x [1] and Sr 1-x [2] with perovskite structure and La 2 with K 2 NiF 4 - type structure [3]. As the conducting oxygen flow through gas tight membrane in diffusion limited regime is inversely proportional to its thickness the film membranes are most promising for application. For mechanical stability the film should be deposited on the substrate which in
one s turn should be porous for easy gas access. The most appropriate method for obtaining such films is the Metal Organic Chemical Vapor Deposition (MOCVD) which allows to realize uniform coatings at high deposition rate. The goal of the present work is to develop the MOCVD of gas tight films on porous substrates and to test their oxygen permeation at high temperatures. EXPERIMENTAL The deposition experiments were carried out in setup with ultrasonic aerosol feeder connected with the cold wall reactor (Fig. 1). O 2 Figure 1. The scheme of experimental MOCVD setup. The precursors volatile coordination compounds La(thd) 3, (Sr(thd) 2 ) 3 *Hthd, Ga(thd) 3, Fe(thd) 3, Co(thd) 2 and Ni(thd) 2 are solved in diglyme and then solution is nebulized. The aerosol evaporates in the heated tube (200 250 o C) and vapors are transported by Ar into the reactor where mixing with O 2 and decomposition on the heated substrate (500 700 o C) occurs. The total pressure of 6 10 mbar in the reactor was controlled by a Digitric pressure controller, oxygen pressure was (3 mbar. Typical deposition rates are 10-15?m/h. A number of commercially available ceramic porous substrates with different total porosity and pore size were tested. The substrates with surface densified layer of low pore size (0.1-0.4?m) were found to be unstable: at high temperatures ((600(C) the surface layer peels off the ceramic membrane. Finally, Al 2 membrane (produced by HITK, Germany) with pore size 5? m was chosen, as most suitable porous substrate. The room temperature gas tightness of film samples was controlled by measurement of N 2 leakage flow through them (Fig. 2).
Figure 2. The scheme of gas tightness measurements. The samples were investigated by SEM, EDX (CamScan 4M scanning electron microscope, EDAX 9600 and Microspec WDX 3PC analytical systems) and XRD (Siemens D5000 diffractometer). Specific oxygen permeability was measured using experimental setup shown at Fig. 3. Al 2 gold ring sample Air OXYLIT He thermocouple Figure 3. Experimental setup for permeability measurements. The film membrane was pressed from film side by sapphire tube using gold steak. The oxygen flow (mol cm -2 sec -1 ) was calculated from oxygen concentration measurements in He flow V (l/sec) with solid electrolyte sensor (OXYLIT): were E(( and E( are the e.m.f. (V) and temperature for sensors in outlet and inlet gas respectively, T V, T((, T( are the temperatures (K) of gas flow controller and sensors, t and s are the time (sec) and the sectional area (cm 2 ) respectively. RESULTS AND DISCUSSION Fig. 4 shows SEM pictures of the films obtained. The films grow according to three-dimentional mechanism forming columns on the substrate particles. The X-ray diffraction patterns verify the formation of the perovskite phase for La 1-x and Sr 1-x, and in the case of La 2 the formation of the tetragonal phase.
Figure 4. The typical morphology of as-deposited films: left cross section, right surface of film. The permeation flow of N 2 through as deposited films was in the range of 6 10 10-4 mol cm -2 s -1 bar -1 only a factor of 2 less than the substrate. The films with higher gas tightness could be obtained for La 2 system by decreasing deposition temperatures. A most drastic densification effect was achieved however by postannealing of the as-deposited films in contact with Bi 2 and PbO powders formed from corresponding hydroxogels. The recrystallization of the as-deposited fine grain film structure takes place in Bi 2 and PbO fluxes, the resulting film morphology is illustrated by Fig. 5. Figure 5. Morphology of the films annealed with flux: left cross section, right surface of film annealed with Bi 2. Evident change in microstructure after liquid recrystallization process results in rise of gas tightness (Fig. 6): the permeation flow of N 2 through densified films was 350 times less than through as deposited films. Figure 6. Pressure drop kinetics showing gas density of the films.
La 0.8 Sr 0.2 Ga 0.6 Fe 0.4 films were densified successfully by annealing with Bi 2 and films by annealing with Bi 2 or PbO. According to XRD and EDX analyses annealing with Bi 2 is accompanied by chemical interaction for both membrane oxides: Bi 2 enriched phases are formed, the penetration of aluminum from substrate into the film and some doping of membrane oxide occurs. Doping of gallate can be ascribed to gallium substitution by aluminum but parameter s change of ferrite cannot be explained with confidence. Annealing with PbO doesn t change the phase and element composition of ferrite and seems to be preferable. The results of high temperature selective oxygen permeability measurements are summarized in Table. 1. Table 1. Oxygen permeability of dense film membranes. composition / flux thickness,?m J(O 2 ), mol cm -2 sec -1 p(o 2 )-T conditions E a, kj/mol La 0.8 Sr 0.2 Ga 0.6 Fe 0.4 10 2.6-4.0 10-7 / Bi 2 21.28kPa/4Pa, 800-900ºC 46±2 10 0.5-3.8 10-8 / Bi 2 21.28kPa/1Pa, 800-1000ºC 107±3 / PbO 10 7.2-8.5 10-8 21.28kPa/4Pa, 800-880ºC 52±10 The flows observed are close to those for ceramic samples with the thickness 1-2 mm [1,3]. This fact displays that oxygen penetration through our film membranes is limited by surface reaction but not the oxygen ion diffusion. It is known however that for similar systems the boundary thickness is less than 1 mm [4]. Slow oxygen surface exchange in the films obtained may be caused by small surface area and Bi 2 presence. The influence of Bi 2 is also realized in low activation energies of permeability [5]. Permeability measurements (Fig. 7) also show that thermo cycling results in some loss of gas tightness at low temperatures but don t affect the specific penetration at high temperatures. Only the high temperature branches of the curvers are characteristics of oxygen selective permeation while low temperature branches are due to nonhermeticity of cold steaks.
Figure 7. Permeability of gallate film (10? m thick) densified with Bi 2. CONCLUSIONS The 1-30?m thick films of La 1-x, Sr 1-x and La 2 as selective oxygen membrane materials were obtained by MOCVD method on the porous substrate (Al 2, pore size 5?m). A novel approach to densify as deposited films by annealing with the flux oxides Bi 2 and PbO is proposed. Molecular nitrogen permeability through densified membranes was less than 2.5?10-6 mol?cm -2?sec -1??bar -1, that is low enough to observe selective oxygen penetration. Oxygen flows through film membranes are close to that for ceramic ones which can be explained by low surface exchange reaction. ACKNOWLEDGMENT The work was supported by Volkswagen Foundation (grant I/77821). REFERENCES T. Ishihara, T. Yamada, H. Arikawa, H. Nishiguchi, Y. Takita, Sol. St. Ionics, 135 (2000) 631 636. S. J. Xu, W. J. Thomson, Chem. En. Science, 54 (1999) 3839-3850. V.V. Vashook, Diss. Habil, Minsk-Dresden 2000 V.V. Vashook et.al., Inorg. Materials, 36,? 8 (2000) 1016-1022 (in Russia) V.V. Kharton, E.N. Naumovich, A.A. Yaremchenko, F.M.B. Marques, J. Sol. St. Electrochem, 5 (2001) 160-187