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Preparation of RuO2–YSZ nano-composite films by MOCVD
Surface and Coatings Technology 167 (2003) 240–244
Preparation of RuO2 –YSZ nano-composite films by MOCVD Teiichi Kimura*, Takashi Goto Institute for Materials Research, Tohoku University, 2-1-1 Katahira, AOBA, Sendai, Miyagi 980-8577, Japan
Abstract RuO2 –yttrium stabilized zirconia (YSZ) nano-composite films were prepared by MOCVD on YSZ substrates and their electrical properties were investigated by AC impedance spectroscopy. The composite films consisted of RuO2 and YSZ particles both several tens of nanometer in diameter. The electrical conductivity at the RuO2 –YSZ interface increased with increasing RuO2 content up to 20–25 vol.% RuO2 and became constant at more than 30 vol.% RuO2 . The emf values of the oxygen concentration cell constructed from the RuO2 film electrode and YSZ electrolyte showed the Nernstian theoretical values at 650 8C. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrode materials; MOCVD; Oxygen sensors; Composite film
1. Introduction Oxygen sensors are playing an important role to control chemical processes and environmental monitoring because they can directly indicate the presence and concentration of oxygen in their environment. A Galvanic cell-type oxygen sensor is widely used, where yttrium stabilized zirconia (YSZ) is commonly employed as a solid electrolyte due to high ionic conductivity and mechanical strength. Electrodes, on the other hand, should have high electronic conductivity, high chemicalythermal stability and catalytic activity for the dissociation of oxygen molecules. Platinum group metals, particularly Pt, could satisfy these requirements, and then Pt has been generally applied to oxygen sensors. However, Pt electrodes could not be catalytic below 500 8C, and easily react with molten metals. We have been developing the electrode material which could work well at low temperatures below 500 8C and in molten metals such as Bi–Pb as a nuclear rector coolant w1x. This material could be also applied to oxygen sensors because of the high electrical conductivity (;105 S my1), superior chemicalythermal stability at high temperatures, and excellent catalytic activity w2– *Corresponding author. Tel.: q81-22-215-2106; fax: q81-22-2152107. E-mail addresses: [email protected] (T. Kimura), [email protected] (T. Goto).
6x. By making composites of RuO2 and YSZ, the composite materials would have better thermal stability and catalytic activity as the electrode for oxygen sensors. In this study, RuO2 –YSZ composites were prepared by MOCVD, and their microstructure and electrical properties were investigated. 2. Experimental Fig. 1 shows a schematic diagram of the horizontal hot-wall type MOCVD apparatus used to prepare films. RuO2 and RuO2 –YSZ composite films were deposited on silica-glass and YSZ substrates using Ru(dpm)3, Y(dpm)3 and Zr(dpm)4 as precursors. Each precursor was vaporized and carried with argon gas separately. All precursor vapors were mixed with oxygen gas in a quartz room and conducted to the substrates through a hole of the mixing room. Compositions of the composite films were controlled by varying evaporation temperatures of precursors. Deposition conditions are summarized in Table 1. The composition of the films was determined by electron probe microanalysis (EPMA). The microstructure was observed using scanning electron microscope (SEM), and their crystalline phase was investigated by X-ray diffraction (XRD). The electrical properties were measured by AC impedance spectroscopy with a twoprove method in the frequency range of 0.1 Hz to 10 MHz in air. The oxygen concentration cell was con-
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(02)00913-1
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
241
Fig. 1. Schematic diagram of the horizontal hot-wall CVD apparatus. (1) Electric durance; (2) zirconium reservoir and precursor powder; (3) quarts mixing room; (4) substrate holder; (5) substrate; (6) o-ring seal. Table 1 Deposition conditions
3. Results and discussion
Deposition temperature, Tdep (K) Total gas pressure, Ptot (kPa)
923 0.4
Precursor temperature Zr(dpm)4, TZr (K) Y(dpm)3, TY (K) Ru(dpm)3, TRu (K)
533–543 423–443 523–573
Gas flow rates (a) Argon gas Zr(dpm)4, FRZr (10y7 m3 sy1) Y(dpm)3, FRY (10y7 m3 sy1) Ru(dpm)3, FRRu (10y7 m3 sy1) (b) Oxygen gas, FRO2 (10y7 m3 sy1)
8.0 4.0 8.0 3.2
Time, t (ks)
9–12
structed using the RuO2 film electrodes and YSZ electrolyte. The RuO2 film electrodes were attached to the both side of YSZ electrolyte.
3.1. Structure Fig. 2 shows the XRD patterns of RuO2 –YSZ composite film (32 vol.% RuO2). Every peak was indexed by Rutile-type RuO2 and tetragonal YSZ. The compositions of RuO2 –YSZ composite films calculated from the elemental results of EPMA are summarized in Table 2. The surface SEM images of RuO2 film and the composite film (98 vol.% RuO2) are shown in Fig. 3. RuO2 films consisted of spherical particles of approximately 0.5 mm in diameter. In contrast, the composite films consisted of spherical particles several tens of nanometer in diameter. The decrease of particle size was observed even though the composite film contained only 2 vol.% YSZ. The co-deposition RuO2 and YSZ particles might accelerate the nucleation of deposition and inhibit the grain. The morphologies and the electrical properties of RuO2 film and RuO2 –YSZ nano-composite films did not change by a heat-treatment at 900 8C for 24 h in air.
Table 2 Compositions of the films prepared in the present study
Fig. 2. XRD pattern of RuO2 –YSZ nano-composite film (32 vol.% RuO2).
Sample name
RuO2 content (vol.%)
YSZ content (vol.%)
R5 R20 R30 R50 R75 R98 R100
4 21 32 51 77 98 100
96 79 68 49 23 2 0
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
242
Fig. 3. Surface images of (a) RuO2 –YSZ nano-composite film (32 vol.% RuO2) and (b) RuO2 film.
3.2. Electrical properties Fig. 4 depicts an AC impedance spectrum of the composite film electrode (32 vol.% RuO2) deposited on a YSZ substrate. There are three semicircles in the spectrum. The two semicircles near the original point were assigned to the bulk and the grain boundary responses of YSZ substrate, because they were independent of the kinds of electrodes and the associated capacitances (shown in Fig. 4) were close to reported values w7x. The largest semicircle at the high Z9 region was assigned to the response of RuO2 –YSZ interface (i.e. the charge transfer reaction at electrolyteyelectrodey gas triple phase boundaries).
The interfacial conductivities for several composite film electrodes are summarized in Fig. 5. The figure includes the values of conventional sputtered-Pt electrode reported in Ref. w8x for comparison. The interfacial conductivity for the RuO2 film electrode was almost 100 times higher than that of sputtered-Pt electrodes. Activation energies for the interfacial conductivity of the RuO2, RuO2 –YSZ composites and Pt electrode were 140–150 kJ moly1 in agreement with the activation energy of the dissociation of oxygen molecules. Fig. 6 shows effect of RuO2 contents in the composite films on the interfacial conductivity at 773 K. The interfacial conductivity increased linearly with increasing RuO2 contents up to 25 vol.%, and became constant
Fig. 4. AC impedance spectrum of YSZ using RuO2 –YSZ nano-composite electrode (32 vol.% RuO2) at 723 K.
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
243
Fig. 5. Temperature dependence of the interfacial conductivity for Pt, RuO2 and RuO2 –YSZ composite films.
at more than 30 vol.%. The percolation threshold connecting conducting RuO2 particles could be approximately 20 vol.% w9–13x which was in well agreement with present study. Fig. 7 shows the emf values of oxygen concentration cell using the RuO2 film electrode at 927 K. The emf values were almost in agreement with near to the Nernstian theoretical value in the range between P9O2 yP0O2s1.1 and 4.0. The emf values were quickly stabilized within a few tens of seconds.
YSZ solid electrolytes was measured. The interfacial conductivity for the composite films was approximately 100 times higher than that of commercial Pt sputter electrode. The composite films of RuO2 content more than 25 vol.% had an almost constant interfacial conductivity which agreed with that of RuO2 films. The emf values of the oxygen concentration cell constructed from the RuO2 film electrodes and YSZ electrolyte showed the Nernstian theoretical values at 650 8C.
4. Conclusion RuO2 –YSZ composite films were prepared by MOCVD, and the electrical property as an electrode for
Fig. 6. Effect of RuO2 contents in the composite films on the interfacial conductivity at 773 K.
Fig. 7. Emf values of the oxygen concentration cell using the RuO2 film electrodes.
244
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
References w1x G. Benamati, C. Latgeand, J.U. Knebel, Technologies for Lead Alloys, TECLA Project within the Fifth European Framework Program, Ref. No. FIS5-(1999)-00308 (2000). w2x M.L. Green, M.E. Gross, L.E. Papa, K.J. Schnoes, D. Brasen, J. Electrochem. Soc. 132 (1985) 2677. w3x F.C.T. So, E. Kolawa, X.-A. Zhao, E.T.-S. Pan, M.-A. Nicolet, J. Vac. Sci. Technol. B 5 (1987) 1784. w4x E. Kolawa, F.C.T. So, E.T.-S. Pan, M.-A. Nicolet, Appl. Phys. Lett. 50 (1987) 54. w5x L. Krusin-Elbaum, M. Wittmer, D.S. Yee, Appl. Phys. Lett. 50 (1879) 1987.
w6x A. Belkind, Z. Orban, J.L. Vossen, J.A. Woollam, Thin Solid Films 50 (1992) 242. w7x S.P.S. Badwalle, H.J. de Bruin, Phys. Stat. Sol., (a) 54 (1979) 261. w8x J.T.S. Irvine, D.C. Sinclair, A.R. West, Adv. Mater. 2 (1990) 132. w9x A. Bunde, W. Dieterech, E. Roman, Phys. Rev. Lett. 55 (1985) 5. w10x A. Bunde, Solid State Ionics 75 (1995) 147. w11x W. Hu, H. Guan, X. Sun, S. Li, M. Fukumoto, I. Okane, J. Am. Ceram. Soc. 81 (1998) 2209. w12x K. Kato, T. Ota, H. Miyazaki, et al., J. Ceram. Soc. Jpn. 109 (2001) 122. w13x M. Wildan, H.J. Edrees, A. Hendry, Mater. Chem. Phys. 75 (2002) 276.
Preparation of RuO2 –YSZ nano-composite films by MOCVD Teiichi Kimura*, Takashi Goto Institute for Materials Research, Tohoku University, 2-1-1 Katahira, AOBA, Sendai, Miyagi 980-8577, Japan
Abstract RuO2 –yttrium stabilized zirconia (YSZ) nano-composite films were prepared by MOCVD on YSZ substrates and their electrical properties were investigated by AC impedance spectroscopy. The composite films consisted of RuO2 and YSZ particles both several tens of nanometer in diameter. The electrical conductivity at the RuO2 –YSZ interface increased with increasing RuO2 content up to 20–25 vol.% RuO2 and became constant at more than 30 vol.% RuO2 . The emf values of the oxygen concentration cell constructed from the RuO2 film electrode and YSZ electrolyte showed the Nernstian theoretical values at 650 8C. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrode materials; MOCVD; Oxygen sensors; Composite film
1. Introduction Oxygen sensors are playing an important role to control chemical processes and environmental monitoring because they can directly indicate the presence and concentration of oxygen in their environment. A Galvanic cell-type oxygen sensor is widely used, where yttrium stabilized zirconia (YSZ) is commonly employed as a solid electrolyte due to high ionic conductivity and mechanical strength. Electrodes, on the other hand, should have high electronic conductivity, high chemicalythermal stability and catalytic activity for the dissociation of oxygen molecules. Platinum group metals, particularly Pt, could satisfy these requirements, and then Pt has been generally applied to oxygen sensors. However, Pt electrodes could not be catalytic below 500 8C, and easily react with molten metals. We have been developing the electrode material which could work well at low temperatures below 500 8C and in molten metals such as Bi–Pb as a nuclear rector coolant w1x. This material could be also applied to oxygen sensors because of the high electrical conductivity (;105 S my1), superior chemicalythermal stability at high temperatures, and excellent catalytic activity w2– *Corresponding author. Tel.: q81-22-215-2106; fax: q81-22-2152107. E-mail addresses: [email protected] (T. Kimura), [email protected] (T. Goto).
6x. By making composites of RuO2 and YSZ, the composite materials would have better thermal stability and catalytic activity as the electrode for oxygen sensors. In this study, RuO2 –YSZ composites were prepared by MOCVD, and their microstructure and electrical properties were investigated. 2. Experimental Fig. 1 shows a schematic diagram of the horizontal hot-wall type MOCVD apparatus used to prepare films. RuO2 and RuO2 –YSZ composite films were deposited on silica-glass and YSZ substrates using Ru(dpm)3, Y(dpm)3 and Zr(dpm)4 as precursors. Each precursor was vaporized and carried with argon gas separately. All precursor vapors were mixed with oxygen gas in a quartz room and conducted to the substrates through a hole of the mixing room. Compositions of the composite films were controlled by varying evaporation temperatures of precursors. Deposition conditions are summarized in Table 1. The composition of the films was determined by electron probe microanalysis (EPMA). The microstructure was observed using scanning electron microscope (SEM), and their crystalline phase was investigated by X-ray diffraction (XRD). The electrical properties were measured by AC impedance spectroscopy with a twoprove method in the frequency range of 0.1 Hz to 10 MHz in air. The oxygen concentration cell was con-
0257-8972/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(02)00913-1
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
241
Fig. 1. Schematic diagram of the horizontal hot-wall CVD apparatus. (1) Electric durance; (2) zirconium reservoir and precursor powder; (3) quarts mixing room; (4) substrate holder; (5) substrate; (6) o-ring seal. Table 1 Deposition conditions
3. Results and discussion
Deposition temperature, Tdep (K) Total gas pressure, Ptot (kPa)
923 0.4
Precursor temperature Zr(dpm)4, TZr (K) Y(dpm)3, TY (K) Ru(dpm)3, TRu (K)
533–543 423–443 523–573
Gas flow rates (a) Argon gas Zr(dpm)4, FRZr (10y7 m3 sy1) Y(dpm)3, FRY (10y7 m3 sy1) Ru(dpm)3, FRRu (10y7 m3 sy1) (b) Oxygen gas, FRO2 (10y7 m3 sy1)
8.0 4.0 8.0 3.2
Time, t (ks)
9–12
structed using the RuO2 film electrodes and YSZ electrolyte. The RuO2 film electrodes were attached to the both side of YSZ electrolyte.
3.1. Structure Fig. 2 shows the XRD patterns of RuO2 –YSZ composite film (32 vol.% RuO2). Every peak was indexed by Rutile-type RuO2 and tetragonal YSZ. The compositions of RuO2 –YSZ composite films calculated from the elemental results of EPMA are summarized in Table 2. The surface SEM images of RuO2 film and the composite film (98 vol.% RuO2) are shown in Fig. 3. RuO2 films consisted of spherical particles of approximately 0.5 mm in diameter. In contrast, the composite films consisted of spherical particles several tens of nanometer in diameter. The decrease of particle size was observed even though the composite film contained only 2 vol.% YSZ. The co-deposition RuO2 and YSZ particles might accelerate the nucleation of deposition and inhibit the grain. The morphologies and the electrical properties of RuO2 film and RuO2 –YSZ nano-composite films did not change by a heat-treatment at 900 8C for 24 h in air.
Table 2 Compositions of the films prepared in the present study
Fig. 2. XRD pattern of RuO2 –YSZ nano-composite film (32 vol.% RuO2).
Sample name
RuO2 content (vol.%)
YSZ content (vol.%)
R5 R20 R30 R50 R75 R98 R100
4 21 32 51 77 98 100
96 79 68 49 23 2 0
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
242
Fig. 3. Surface images of (a) RuO2 –YSZ nano-composite film (32 vol.% RuO2) and (b) RuO2 film.
3.2. Electrical properties Fig. 4 depicts an AC impedance spectrum of the composite film electrode (32 vol.% RuO2) deposited on a YSZ substrate. There are three semicircles in the spectrum. The two semicircles near the original point were assigned to the bulk and the grain boundary responses of YSZ substrate, because they were independent of the kinds of electrodes and the associated capacitances (shown in Fig. 4) were close to reported values w7x. The largest semicircle at the high Z9 region was assigned to the response of RuO2 –YSZ interface (i.e. the charge transfer reaction at electrolyteyelectrodey gas triple phase boundaries).
The interfacial conductivities for several composite film electrodes are summarized in Fig. 5. The figure includes the values of conventional sputtered-Pt electrode reported in Ref. w8x for comparison. The interfacial conductivity for the RuO2 film electrode was almost 100 times higher than that of sputtered-Pt electrodes. Activation energies for the interfacial conductivity of the RuO2, RuO2 –YSZ composites and Pt electrode were 140–150 kJ moly1 in agreement with the activation energy of the dissociation of oxygen molecules. Fig. 6 shows effect of RuO2 contents in the composite films on the interfacial conductivity at 773 K. The interfacial conductivity increased linearly with increasing RuO2 contents up to 25 vol.%, and became constant
Fig. 4. AC impedance spectrum of YSZ using RuO2 –YSZ nano-composite electrode (32 vol.% RuO2) at 723 K.
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
243
Fig. 5. Temperature dependence of the interfacial conductivity for Pt, RuO2 and RuO2 –YSZ composite films.
at more than 30 vol.%. The percolation threshold connecting conducting RuO2 particles could be approximately 20 vol.% w9–13x which was in well agreement with present study. Fig. 7 shows the emf values of oxygen concentration cell using the RuO2 film electrode at 927 K. The emf values were almost in agreement with near to the Nernstian theoretical value in the range between P9O2 yP0O2s1.1 and 4.0. The emf values were quickly stabilized within a few tens of seconds.
YSZ solid electrolytes was measured. The interfacial conductivity for the composite films was approximately 100 times higher than that of commercial Pt sputter electrode. The composite films of RuO2 content more than 25 vol.% had an almost constant interfacial conductivity which agreed with that of RuO2 films. The emf values of the oxygen concentration cell constructed from the RuO2 film electrodes and YSZ electrolyte showed the Nernstian theoretical values at 650 8C.
4. Conclusion RuO2 –YSZ composite films were prepared by MOCVD, and the electrical property as an electrode for
Fig. 6. Effect of RuO2 contents in the composite films on the interfacial conductivity at 773 K.
Fig. 7. Emf values of the oxygen concentration cell using the RuO2 film electrodes.
244
T. Kimura, T. Goto / Surface and Coatings Technology 167 (2003) 240–244
References w1x G. Benamati, C. Latgeand, J.U. Knebel, Technologies for Lead Alloys, TECLA Project within the Fifth European Framework Program, Ref. No. FIS5-(1999)-00308 (2000). w2x M.L. Green, M.E. Gross, L.E. Papa, K.J. Schnoes, D. Brasen, J. Electrochem. Soc. 132 (1985) 2677. w3x F.C.T. So, E. Kolawa, X.-A. Zhao, E.T.-S. Pan, M.-A. Nicolet, J. Vac. Sci. Technol. B 5 (1987) 1784. w4x E. Kolawa, F.C.T. So, E.T.-S. Pan, M.-A. Nicolet, Appl. Phys. Lett. 50 (1987) 54. w5x L. Krusin-Elbaum, M. Wittmer, D.S. Yee, Appl. Phys. Lett. 50 (1879) 1987.
w6x A. Belkind, Z. Orban, J.L. Vossen, J.A. Woollam, Thin Solid Films 50 (1992) 242. w7x S.P.S. Badwalle, H.J. de Bruin, Phys. Stat. Sol., (a) 54 (1979) 261. w8x J.T.S. Irvine, D.C. Sinclair, A.R. West, Adv. Mater. 2 (1990) 132. w9x A. Bunde, W. Dieterech, E. Roman, Phys. Rev. Lett. 55 (1985) 5. w10x A. Bunde, Solid State Ionics 75 (1995) 147. w11x W. Hu, H. Guan, X. Sun, S. Li, M. Fukumoto, I. Okane, J. Am. Ceram. Soc. 81 (1998) 2209. w12x K. Kato, T. Ota, H. Miyazaki, et al., J. Ceram. Soc. Jpn. 109 (2001) 122. w13x M. Wildan, H.J. Edrees, A. Hendry, Mater. Chem. Phys. 75 (2002) 276.