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Metal-organic chemical vapor deposition of Sr–Co–Fe–O films on porous substrates Changfeng Xia, Timothy L. Ward, and Paolina Atanasova Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131 Robert W. Schwartz Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Boulevard SE, Albuquerque, New Mexico 87106 (Received 30 August 1996; accepted 2 May 1997) Aerosol-assisted chemical vapor deposition using the b-diketonate precursors Sr(tmhd)2 ? 2H2O, Fe(tmhd)3, and Co(tmhd)3 was investigated for depositing thin films of the mixed-conducting ceramic SrCoyFe1–yO32d onto porous a –Al2O3 substrates. Single-phase SrCoyFe1–yO32d perovskite films were obtained at a deposition temperature of 550 ±C and pressure of 15 mm Hg, whereas deposition at atmospheric pressure produced mixed-phase films. The Co͞Fe elemental ratios in the films reflected those in the precursor solution, but the films were depleted in Sr. Reduced-pressure deposition provided a more uniform film morphology than atmospheric pressure, and led to a supported film which was leak-tight to N2 flow.

I. INTRODUCTION dense mixed-conducting ceramics has been done with Perovskite and other related phases from the relatively thick (ϳ 1 mm) membranes made by pressing 2,6 La1–xSrxCo1–yFeyO32d family have been found to have or extruding, followed by sintering. Thick membrane high O2 permeation rates at elevated temperatures walls in these types of membranes often limit the oxy- which may be useful for applications such as gas gen flux achievable due to diffusional resistance of the separations, fuel cells, and sensors. Many of these membrane. In these cases, limited oxygen flux may materials possess relatively high ionic conductivity as contribute to membrane phase stability problems in well as electronic conductivity, hence they are referred to membrane reactor operation if the flux is insufficient to as mixed conductors. Since ionic conductivity in these prevent reduction of the membrane material by chemical materials stems from oxygen ion conduction (which reactions at the membrane surface.6,7 occurs via oxygen ion vacancy diffusion), a dense layer One strategy to alleviate these problems is the fab- of these materials may serve as a perfectly selective rication of thin films of the dense membrane material membrane for oxygen separation; the ionic conductivity to reduce the diffusional transport resistance. In order allows oxygen transport through the material, while to maintain mechanical strength and stability with thin the electronic conductivity prevents polarization of the film membranes, the separation layer should be deposited membrane. At the surfaces of the membrane, charge- on a mechanically strong and compatible support which transfer reactions facilitate the conversion of ionic to contributes minimally to the overall transport resistance molecular oxygen. SrCo0.8Fe0.2O3–x is one example of of the composite structure. There have been recent a mixed conductor which has been reported to have reports which indicate that the oxygen permeation rate a high oxygen permeability, about one to two orders through some perovskite-type mixed conductors may of magnitude higher than that of the well-known ionic become limited by the oxygen surface exchange reaction 1,2 8–10 conductor, stabilized ZrO2. rate for sufficiently thin films. However, the criti- The high oxygen permeability, perfect gas- cal thickness at which surface reaction effects become separation selectivity, and high-temperature stability important depends strongly on the material composition of ceramic mixed conductors have generated consid- and its catalytic properties, and relatively little data are erable interest for membrane reactor, fuel cell and gas currently available. Thus, fabrication of supported thin separation applications.3–7 For example, thick-walled films continues to be a prominent objective in dense tubular membranes of La–Sr–Co –Fe–O composi- ceramic membrane research. tions have already been demonstrated to provide high Chemical vapor deposition is an attractive method conversion and selectivity to CO for the oxidative to deposit thin, conformal, crystalline metal oxide reforming of methane.6 Much of the previous work coatings on complex topography substrates. In this on membrane fabrication and characterization using paper, we report on the fabrication and charac-

J. Mater. Res., Vol. 13, No. 1, Jan 1998  1998 Materials Research Society 173 C. Xia et al.: Metal-organic chemical vapor deposition of Sr–Co– Fe–O films terization of Sr–Co–Fe–O films on porous and by a thermocouple inserted into the stainless steel sub- dense substrates using aerosol-assisted chemical vapor strate holder, and an Omega CN-9000A controller was deposition (AACVD) of metal-organic precursors. used to regulate the temperature. The thermocouple Though considerable literature exists on metal-organic temperature in the substrate holder was calibrated to the CVD of metals and oxides, there is little prior work actual surface temperature of the substrates in separate on the use of AACVD to deposit multicomponent experiments using another thermocouple touching the oxides of controlled composition, and even less on surface of the substrate. A TSI 3076 Aerosol Generator, film morphology for deposition on porous substrates. operated with a gas supply pressure of 30 psig and flow These are both important issues for the fabrication rate of 1500 sccm, was used to atomize the precursor of dense supported films or membranes. Several solution. b-diketonate precursors were investigated for the Several different metal b-diketonate compounds codeposition of mixed-oxide material at atmospheric were investigated as possible precursors. These included pressure and reduced pressures of 5–15 mm Hg. Sr(hfac)2, Co(hfac)3, Fe(acac)3, Sr(tmhd)2 ? xH2O, Empirical manipulation of the film composition by Co(tmhd)3, and Fe(tmhd)3 (hfac ෇ hexafluoroacetyl- control of the precursor solution composition was acetonate; acac ෇ acetylacetoneate; tmhd ෇ tetra- demonstrated. The effect of deposition pressure on methylheptanedionate); all precursors were obtained morphology of deposits produced on porous supports from Strem and used as-received. The precursors were was investigated, with the ultimate goal of achieving dissolved in toluene to give a total metal concentration leak-tight micron-thick mixed conductor top layers on of 0.05 mol͞l. The ratios of precursors in solutions porous ceramic supports. were varied in order to control the concentrations of the components in the deposited films. II. EXPERIMENTAL Sr–Co–Fe–O films were deposited under atmo- spheric pressure (ϳ 630 mm Hg) and reduced pressure Sr–Co–Fe–O perovskite films were deposited in an (5–15 mm Hg) in order to explore the effects of pressure AACVD system which consisted of an aerosol generator, on the deposition rate, film quality, and morphology. evaporation tube, and a cold wall CVD reactor, shown Under atmospheric pressure AACVD, 1 to 5% O2 in in Fig. 1. In the AACVD process, precursors were N2 was used to atomize the precursor solution. For dissolved into a common solvent, which was atomized to the experiments using the tmhd compounds, evapora- provide small particles of the mixed precursors. These tion zone temperatures of 180–220 ±C were used, and submicron particles were transported by a carrier gas deposition was conducted in the 500–650 ±C range. The through a heated evaporation zone which should fully average residence time in the evaporation zone was evaporate the precursors prior to delivery to the cold-wall approximately 1.5 s. Under reduced pressure AACVD, CVD reactor. The substrate in the reactor was directly the pressure in the CVD reactor was maintained at heated by a lamp (Research Inc. Model 4085) from 5–15 Torr, while the pressure in the evaporation zone beneath and the precursor-carrier gas mixture impinged was at 400–600 mm Hg. The solution was atomized downward onto the top of the substrate in a stagnation- using pure N2, and pure O2 was injected into the system flow geometry. The substrate temperature was monitored after the evaporation zone. In this manner, the oxygen content of the carrier gas was varied over the range of 0 to 50%. The deposits were cooled from the deposi- tion temperature to below 100 ±C under an atmospheric pressure O2 flow over a period of approximately 5 min. Deposition was conducted on porous a –Al2O3 disks and, in some cases, onto the native oxide surface of sil- icon. The porous a –Al2O3 substrates were made in our laboratory. These porous disks were either 0.5 or 1 inch in diameter and approximately 1 mm in thickness. They possessed an average pore diameter of approximately 0.5 mm and a porosity of about 50%. The mean pore size was not quantified, but is based on knowledge of the starting powder size and SEM micrographs. The disks were made by pressing high purity a –Al2O3 powder (Sumitomo AKP-15), followed by sintering at 1100 ±C for 4 h. Some of the disks were coated on the outside FIG. 1. Apparatus used for aerosol-assisted chemical vapor edges with a commercial ceramic glaze (Duncan IN deposition. 1653). Two layers of the glaze formed an impermeable

174 J. Mater. Res., Vol. 13, No. 1, Jan 1998 C. Xia et al.: Metal-organic chemical vapor deposition of Sr–Co– Fe–O films barrier that prevented gas leakage from the edge of the sensitivity factors were employed in Auger analysis of disk when mounted in the gas permeation or membrane the unknown film compositions. This approach proved reactor test modules. to be much more accurate than the use of handbook X-Ray Diffractometry (XRD) (Siemens D5000, sensitivity factors derived for pure metals.11 Cu Ka radiation) was conducted to determine the crystalline phase content. The deposition rate was calculated from the weight increase after deposition, III. RESULTS AND DISCUSSION and was confirmed by cross-sectional Scanning Electron Several Sr, Co, and Fe b-diketonate compounds Microscopy (SEM) (Hitachi S-800). “Gas-tightness” of were investigated in early experiments, generally with the membranes was checked by helium or nitrogen the objective of producing films of SrCo0.8Fe0.2Ox permeation at room temperature with pressures up composition. Deposition using Sr(hfac)2, Co(hfac)3, to 40 psig. A defect-free or “gas-tight” membrane and Fe(acac)3 dissolved in toluene with Sr : Co : Fe ෇ should have no leakage at room temperature. Auger 1 : 0.8 : 0.2 resulted in a complex mixture of oxides under Electron Spectroscopy (AES) data were collected with all conditions investigated (evaporation zone temperature a cylindrical mirror analyzer (PHI 10-155), with an of 200 ±C and deposition temperature of 500–700 ±C), electron beam energy of 3 keV, at a base pressure of without formation of the desired perovskite-phase 10–9 mm Hg. The Ar+ ion gun with ion beam current material. However, deposition at 550 ±C and atmospheric 2 intensity of 10 mA͞cm at 1 keV beam energy and pressure using Sr(tmhd)2, Co(tmhd)3, and Fe(tmhd)3 4 3 1025 Torr pressure was applied for etching the precursors in the same ratios produced films which surface impurities and for depth profiling of the films. possessed the desired perovskite phase Sr(Co, Fe)O32d Determination of the elemental composition of the by XRD (Fig. 2). The individual oxide Co3O4 was also deposits by AES was based on experimentally deter- identified in the XRD patterns of the film (Fig. 2). mined sensitivity factors. The Auger peaks of C (KLL) We are not certain whether the observed perovskite at 271 eV, O (KLL) at 510 eV, Fe (LMM) at 598 eV, phase was deposited directly or formed during cooling Co (LMM) at 776 eV, and Sr (LMM) at 1652 eV were under oxygen. Under conditions of low O2 partial used to estimate the composition and stoichiometry of pressure (below ϳ 5%O2 at atmospheric pressure), the samples. The low energy Fe (LMM) peak at 598 eV an orthorhombic brownmillerite phase is expected rather and high energy Co (LMM) peak at 776 eV are the than the cubic perovskite phase; however, the perovskite only ones from the LMM family of lines of iron and phase is expected under air or oxygen.12,13 Since the cobalt which do not overlap, and were therefore used deposits were thin (ϳ 1–2 mm), it is possible that for the quantitative estimation of the film composition. the exposure to pure O2 during the brief cooling may The experimental sensitivity factors were obtained from have been sufficient to convert much of the material Auger spectra of standard stoichiometric compounds to the perovskite phase. Published oxygen diffusion 14 containing the elements of interest—SrCo0.8Fe0.2Ox and coefficients for the perovskite SrCo0.8Fe0.2Ox phase 2 SrCo0.5FeOx powders (nominal compositions). The bulk indicate a diffusional relaxation time (t ෇ x ͞D, where composition of these powders was determined with x is thickness and D is the diffusion coefficient) on Atomic Absorption Spectroscopy (AAS) and the results the order of 1 s at ϳ 500 ±C for a 1 mm film. Thus, were used to estimate the experimental sensitivity fac- equilibration of the perovskite phase with O2 at the tors for the Auger surface analysis (Table I). Using the beginning of cooling would be rapid; however, whether known composition (by AAS) of the SrCo0.5FeOx pow- the perovskite phase is produced during or prior to der, sensitivity factors were determined which matched cooling is still not established. the AES composition to that determined by AAS. When The XRD pattern from the film deposited at reduced these experimental sensitivity factors were applied for pressure showed single perovskite-phase Sr(Co, Fe)O32d SrCo0.8Fe0.2Ox powder, the estimated composition from together with peaks from the glaze coated on the outside AES was in excellent agreement with the bulk compo- edge of the disk (Fig. 3). The O2 partial pressure in the sition determined by AAS (Table II). Therefore, these deposition region for these experiments was approxi-

TABLE I. Comparison of experimentally determined and handbook sensitivity factors for Auger analysis.

Auger peak (energy) O (KLL) (510 eV) Fe (LMM) (598 eV) Co (LMM) (776 eV) Sr (LMM) (1652 eV) Sensitivity factor Handbook 0.50 a 0.27 0.025 Experimental 0.45 0.10 0.22 0.075 aHandbook value not given for 598 eV line.

J. Mater. Res., Vol. 13, No. 1, Jan 1998 175 C. Xia et al.: Metal-organic chemical vapor deposition of Sr–Co– Fe–O films

TABLE II. Comparison of elemental composition of standard powders as determined by atomic absorption spectroscopy and Auger electron spectroscopy using experimentally determined sensitivity factors.

Nominal composition Composition by atomic Composition by Auger electron spectroscopy of standard powders absorption spectroscopy using experimental sensitivity factors

SrCo0.5FeOx Sr1.00Co0.50Fe1.18O3.70 Sr1.00Co0.48Fe1.20O3.69 SrCo0.8Fe0.2Ox Sr1.00Co0.76Fe0.20O3.48 Sr0.98Co0.76Fe0.22O3.48 mately 7 Torr, and cooling was done under atmospheric- the solid precursor aerosol particles or in the gas phase, pressure O2. Although no secondary phases were present, both of which have been reported for the group 2 metal 16,17 the Sr(Co, Fe)O32d perovskite phase exists over a rel- tmhd compounds. Solid state oligomer formation in atively broad range of Fe͞Co composition. Since the the precursor particles could prevent complete evapora- Fe͞Co composition affects the oxygen permeability and tion of the Sr precursor, leading to reduced gas-phase catalytic properties of the material,2,15 it is desirable to concentration and a reduced CVD rate relative to the know whether a relationship exists between the precursor other precursors. Gas-phase oligomers, if formed, would solution composition and the CVD film composition. have reduced diffusion coefficients which could decrease Therefore, a series of Sr–Co–Fe–O films was de- the transport rate to the substrate compared to the other posited using different metal ratios in the precursor precursors. These effects would all tend to reduce the Sr solution in order to determine whether an empirical content in the films. relationship could be found to allow control of the film Solution-film composition relationships (such as composition by appropriately manipulating the solution Fig. 4) can generally depend on many other factors and composition. These experiments were conducted using cannot necessarily be extrapolated to other conditions silicon substrates and tmhd precursors with the following or experimental arrangements. For example, modeling conditions: evaporation zone temperature ෇ 200 ±C, of impinging-flow CVD reactor designs has shown deposition temperature ෇ 500 ±C, deposition pressure that hydrodynamic streamlines (and, hence, diffusional ෇ 16 Torr, 50% O2 in N2 carrier gas. boundary layer thickness) and the thermal boundary re- AES was used to determine the elemental composi- gion adjacent to the substrate are dramatically impacted tion of the CVD films. The metal atom ratios, expressed by natural convection in the downward flow configura- as Co͞Fe and Sr͞Fe, for the film versus solution are tion compared to upward flow.18 Such system-dependent shown in Fig. 4. Typically, the Co͞Fe ratio in the films factors could dramatically impact multicomponent film was approximately equal to that in solution, whereas Sr composition and deposition rates due to their possible was generally somewhat depleted in the film relative to effects on gas-phase reactions and precursor transport solution. The agreement of Co͞Fe ratios between the film to the substrate. However, if precursor evaporation is and solution would be consistent with the deposition rate complete without significant precursor depletion prior being limited by either diffusion to the substrate or by to the substrate, then transport-limited deposition is the feed rate to the reactor. The reduced Sr content of the likely to provide a film composition which roughly films could be related to Sr(tmhd)2 oligomer formation in reflects the gas-phase and solution composition, though

FIG. 2. X-ray diffraction pattern of Sr –Co–Fe–O film deposited FIG. 3. X-ray diffraction pattern of Sr –Co–Fe–O film deposited onto a porous a –Al2O3 support using tmhd precursors at atmospheric onto a porous a –Al2O3 support using tmhd precursors at deposition pressure and deposition temperature of 550 ±C. pressure of 15 mm Hg and temperature of 550 ±C.

176 J. Mater. Res., Vol. 13, No. 1, Jan 1998 C. Xia et al.: Metal-organic chemical vapor deposition of Sr–Co– Fe–O films

Under reduced pressure deposition (ϳ 15 mm Hg), the structure of the deposited films changed from nodular to more columnar (Fig. 6). The surface of the deposit was much smoother and more uniform than seen at atmospheric pressure. The plan view SEM still revealed some apparent void channels in the film, but they are clearly reduced in size and number compared to the atmospheric pressure deposit. A gas-tightness test was conducted on one membrane which showed that the membrane was leak-tight to nitrogen up to a pressure of 40 psig. However, the fabrication of a leak-tight film was difficult to reproduce. We believe that the difficulty in reproducing leak-tight films is largely due to variations in the roughness and particularly large pores which may be present in some supports. The ability to fully block or FIG. 4. Elemental ratios (metal͞Fe) in precursor solution and CVD bridge the pores in these supports is clearly dependent deposit. Deposit analysis by Auger electron spectroscopy. Films were on these factors. It must be conceded, however, that our deposited using tmhd precursors, deposition temperature of 500 ±C, and pressure of 16 mm Hg. understanding of the evolution of deposit morphology in CVD on porous supports is poor, and there are many possible reasons for the difficulties in fabricating the deposition rate and film thickness uniformity are gas-tight films. Relatively subtle effects may determine likely to be very dependent on the reactor geometry and whether the channels seen in the plan view of Fig. 6 conditions. provide leakage paths through the film or not. Another issue that is always of concern with metal- It was evident that deposition pressure is a very organic CVD of oxides, especially under low O2 partial important variable for controlling the morphology and pressures, is the incorporation of carbon into the de- permeation defects of Sr–Co –Fe–O films deposited posits. AES indicated that the carbon contents of the onto porous supports. With reduced pressure deposition, films presented in Fig. 4 ranged from zero to approx- pore closure of relatively large pores (ϳ 0.5 mm) was imately 3 at. %. These analyses were obtained after achievable by a relatively thin CVD layer (ϳ 2 mm), sputtering down approximately 150 A˚ below the surface. whereas pore closure was not obtained using Though the carbon levels appeared constant at this atmospheric-pressure deposition. Though we have depth, they may not reflect its amount in the entire film not yet developed a thorough conceptual framework thickness because of the O2 exposure during cooling. to explain these effects, some insight is provided The effect of C contamination on oxygen permeation by consideration of the classical view of diffusion and stability of mixed conducting ceramics has also not and reaction in nonuniform or porous materials. At been determined. atmospheric pressure, the mean free path of the gas For membrane or fuel cell applications, the mor- is very short compared to the support and pore size. phology and elimination of voids in the films is just In this case, if precursor delivery and surface reaction as important as the film purity and composition. The rates are high enough, the deposit growth is controlled films deposited under atmospheric pressure possessed a by diffusion of precursors through a boundary layer coarse cauliflower-like morphology with a topography to the surface and into the pores of the substrate. characterized by prominent nodules (Fig. 5). SEM plan Under diffusion-limited deposition on nonuniform views (Fig. 5) also revealed large void channels in the substrates, concentration gradients and deposition film, although it is not possible to determine from rates are accentuated at high or exposed regions, and the SEM whether these voids extended through the decreased in recessed regions and pores. This would film thickness to the support. Room temperature N2 tend to promote nodule formation above the high permeation tests indicated that the films were porous or points and the continuation of substrate pores into did not fully cover the porous support. The N2 leakage the film, while inhibiting infiltration into the support. flux was reduced to approximately 25% of that of the However, at sufficiently low pressure, the gas-phase bare porous support, although this is still much too high mean free path is longer and concentration gradients for use in oxygen permeation measurements. Based on do not exist on the small length scales of the support SEM observation, the failure of these films to be leak- pores or particles. This tends to alleviate the localized tight appeared to be due to nonuniform CVD growth at concentration gradients which lead to the nonuniform atmospheric pressure which left void channels through growth patterns seen at higher pressures, and should the film. provide more uniform deposits, which is consistent

J. Mater. Res., Vol. 13, No. 1, Jan 1998 177 C. Xia et al.: Metal-organic chemical vapor deposition of Sr–Co– Fe–O films

FIG. 5. Scanning electron micrographs [(a) plan view and (b) cross-sectional view] of Sr –Co–Fe–O film deposited at atmospheric pressure ± onto porous a –Al2O3. Precursors were tmhd compounds and deposition temperature was 550 C.

FIG. 6. Scanning electron micrographs [(a) plan view and (b) cross-sectional view] of Sr–Co–Fe –O film deposited at reduced pressure ± (ϳ 15 Torr) onto porous a –Al2O3. Precursors were tmhd compounds and deposition temperature was 550 C. with our observations on the reduced pressure deposits. nitrogen flow, though leak-tight films were difficult Theoretical modeling and model experimental studies to reproduce. Using atmospheric-pressure deposition, have been done that illuminate some of these qualitative deposit morphology was much less uniform, and leak- relationships on more well-defined nonuniform, dense tight films were never obtained. Pore blockage and substrates.19 However, considerably more experimental deposit morphology were found to depend on several and theoretical work remains to be done to understand parameters, including total pressure deposition tempera- the relationships that determine deposit characteristics ture, precursor concentration, and support pore size and on granular porous supports. morphology. In particular, uniformity of the porous support surface and pore openings at the surface appear to be critical factors for reproducible fabrication of ceramic membranes by this approach. Reduction of the IV. CONCLUSION support pore size below the pore size used in this work Aerosol-assisted MOCVD of a toluene solution of (roughly 0.5 mm), coupled with appropriate control of Sr(tmhd)2 ? 2H2O, Co(tmhd)3, and Fe(tmhd)3 was used temperature and pressure, should enable more reliable to deposit Sr(Co, Fe)O32d perovskite films of thickness and reproducible fabrication of leak-tight membrane 1–2 mm upon porous a –Al2O3 supports. Aerosol pre- layers. cursor delivery enabled the precursor compounds to be delivered and decomposed together in controlled ratios from a single solution. For dense membrane applica- ACKNOWLEDGMENTS tions, deposit purity, composition control, and com- This research was supported by the United States plete blockage of the support pores are all important Department of Energy, Office of Industrial Technolo- issues. Under reduced pressure deposition conditions gies (OIT), through a contract with Sandia National (ϳ 15 mm Hg), and after cooling in O2, a single-phase Laboratories (DE-AC04-94AL85000). Sandia National perovskite film was obtained which was leak-tight to Laboratories is a multi-program laboratory operated by

178 J. Mater. Res., Vol. 13, No. 1, Jan 1998 C. Xia et al.: Metal-organic chemical vapor deposition of Sr–Co– Fe–O films

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