- Email: [email protected]
Fabrication and characterization of Ni–YSZ anode functional coatings by electron beam physical vapor deposition
Thin Solid Films 517 (2009) 4975–4978
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Fabrication and characterization of Ni–YSZ anode functional coatings by electron beam physical vapor deposition B. Meng a, Y. Sun b, X.D. He b,⁎, J.H. Peng a a b
Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, No. 68 Wenchang Street, Wuhua District, Kunming, 650093, PR China Center for Composite Materials, Harbin Institute of Technology, No. 2 Yikuang Street, Nangang District, Harbin, 150080, PR China
a r t i c l e
i n f o
Available online 21 March 2009 Keywords: EB–PVD Ni–YSZ Graded coatings Porosity
a b s t r a c t Two kinds of NiO–YSZ (yttria-stabilized zirconia) coatings, respectively with uniform and gradient distributions of NiO content along the coating thickness direction, were prepared by electron beam physical vapor deposition (EB–PVD) via adjusting electron beam currents. Then uniform and graded Ni–YSZ coatings were obtained from corresponding NiO–YSZ coatings after a reduction treatment. For uniform Ni–YSZ coating, the composition and porosity distributions along the coating thickness were uniform. The specific surface area and total pore volume for this coating could reach up to 4.330 m2 g − 1 and 0.0346 cm3 g − 1 respectively. The area specific resistance (ASR) of this coating kept increasing with the rise in temperature and an ASR of 2.1 × 10 − 5Ω cm2 was obtained at 600 °C. For graded Ni–YSZ coating, a gradient in Ni content and porosity was realized along the coating thickness. A high porosity of up to 33% was achieved in the part of the coating close to the substrate, while a low porosity of 10% was obtained in the part close to coating surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In order to reduce manufacture cost and improve long-term stability of solid oxide fuel cell (SOFC), the reduction of operation temperature of planar SOFCs to an intermediate temperature range is a significant objective in current research work [1,2]. A low operation temperature opens up the possibility of using porous metallic substrates as SOFC supports to replace expensive anode or electrolyte supports [3]. Metal-supported SOFCs offer many advantages over conventional electrode- and electrolyte-supported SOFCs [4]. For metal-supported SOFCs, porous metal substrates have been employed as structure supports so that the anode layers can be fabricated in thin films for catalysis only. The anode with a thin film structure contributes to the decrease of gas diffusion impedance and concentration polarization loss. Ni–YSZ (yttria-stabilized zirconia) cermet has been widely employed as anode coating material for SOFCs because of its low cost, good catalytic activity and suitable thermal expansion coefficient [5]. Therefore, many processes have been investigated to prepare Ni–YSZ coatings for SOFCs, such as screen printing, slurry coating, tape casting, plasma spraying, and so on [6,7]. The coatings fabricated by these processes commonly show a single-layered structure and a uniform pore distribution, which often results in the particle agglomeration, an insufficient adhesion between anode and electrolyte layer, an intense internal stress and consequently a high degra⁎ Corresponding author. Tel.: +86 451 86402928; fax: +86 451 86402440. E-mail address: [email protected] (X.D. He). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.098
dation rate [8]. So the anode coating with a multi-layered structure has been developed to replace the single-layered anode and exhibits many excellent properties [9]. But another problem in multi-layered anode is the occurrence of cracks between different layers [10]. A reasonable method to resolve this problem is a design with a gradient structure for the anode coating. In this design, the anode shows a gradient in chemical content, porosity, electrical conductivity and coefficient of thermal expansion along the cross section of the coating [11]. Although the properties of anode coating will be improved dramatically by this method of gradient structure design, this structure is difficult to be realized by traditional methods. As a typical process for coating preparation, electron beam physical vapor deposition (EB–PVD) technique has exhibited a unique superiority in fabrication of microporous coatings with the advantages of high deposition rate, large deposition area and good adherence to substrates [12]. At the same time, the current cost of EB–PVD equipment, especially produced by E.O. Paton Welding Institute, has been reduced to 1/10 of that of forty years ago[13]. So for preparation of porous anode coating of SOFCs, EB–PVD is prospective to be an alternative to traditional wet chemical methods. When this Ni–YSZ coating is prepared by EB–PVD through simultaneously evaporating two adjacent ingots of Ni and YSZ, a limited porosity of 5–15% is obtained. Considering the volumetric shrinkage and additional pore formation during the reduction course from NiO to Ni, the porosity of final Ni–YSZ coating can be increased through fabricating a NiO–YSZ coating combined with a reduction treatment in hydrogen. Furthermore, due to the effects of NiO reduction on coating porosity, the gradient in NiO content for NiO–YSZ coatings will lead to a gradient in
4976
B. Meng et al. / Thin Solid Films 517 (2009) 4975–4978
Fig. 1. The adjustment of electron beam currents for preparations of uniform and graded coatings.
porosity along the coating thickness direction for final Ni–YSZ coating, which is obtained from NiO–YSZ coating after a reduction treatment. The gradient in NiO content for NiO–YSZ coating can be realized by EB–PVD through adjusting electron beam currents for evaporating different ingots. This adjustment of electron beam currents is more convenient and without the complex course of ingot preparation compared with traditional EB–PVD process to prepare graded coating, in which an ingot with graded constituent distribution along its axial line was employed [14]. The gradient structure in porosity not only can help the fuel gas diffuse in the anode and decrease the concentration polarization, but also is in favor of the following preparation of the dense electrolyte films. So in this investigation, NiO–YSZ anode coatings were prepared by EB–PVD through simultaneously evaporating two adjacent ingots of NiO and 8YSZ. The uniform or gradient distribution of NiO content along the coating thickness direction was realized by adjusting the electron beam currents during evaporation course. Then uniform and graded Ni–YSZ coatings were obtained from NiO–YSZ coatings after a reduction in hydrogen. The performances of these two kinds of Ni–YSZ coatings were analyzed and characterized. 2. Experimental An EB–PVD equipment of type L5 was employed to prepare NiO– YSZ coatings with a deposition rate of 0.6 µm/min by simultaneously evaporating two adjacent ingots of NiO and 8YSZ. The final Ni–YSZ coatings were obtained from NiO–YSZ coatings after a reduction at 800 °C in hydrogen for 2 h. A plate of stainless steel (SUS430) with a size of Φ100 × 1.2 mm was used as the substrate because of its suitable thermal expansion coefficient close to NiO–YSZ coating. The substrate was firstly cleaned by a supersonic wave in ethanol and acetone bathes respectively and then was suspended above the middle position of two ingots in a non-rotation mode. The distance between the ingots and substrate was 300 mm and the length between the centers of two ingots was 150 mm. During coating preparation, the substrate temperature was kept at 650 °C by a radiation heater and the pressure level of deposition chamber was kept less than 1 × 10 − 2 Pa. Two kinds of NiO–YSZ coatings, respectively with uniform and gradient distributions of NiO content along the coating thickness direction, were prepared by EB–PVD. According to repeated experiments, when the currents of electron beams for evaporating NiO and YSZ ingots were 0.18 A and 0.48 A respectively and the area of melting pool kept constant at 10 mm2, the equality between evaporation rates of NiO and YSZ was realized. So for preparation of uniform NiO–YSZ coating, the currents of electron beams for evaporating NiO and YSZ ingots were kept constant at 0.18 A and 0.48 A respectively. At the
same time, in order to realize the gradient distribution of NiO content along the coating thickness direction, the currents of electron beams for evaporating NiO and YSZ ingots were designed and shown in Fig. 1. Through this adjustment of electron beam currents for different ingots, a continuous decrease of NiO content as well as a continuous increase of YSZ content in this coating could be realized along the direction from the interface between the coating and substrate to coating surface. Considering the volumetric shrinkage and additional pore formation during the reduction course from NiO to Ni, a gradient in NiO content for NiO–YSZ coatings would lead to a gradient in porosity along the thickness direction for the final Ni–YSZ coating, which was obtained from NiO–YSZ coating after a reduction treatment. After coating preparation, the phase composition of this coating was characterized by Philips X’Pert diffractometer (XRD) with parallel Cu Kα radiation. The morphology of fracture surface as well as element distribution of the coating was analyzed by scanning electron microscope (SEM) (Hitachi S4700) combined with an electron probe microanalysis (EPMA). The specific surface area of the open pores, average pore diameter and pore diameter distribution of this coating were measured with a Quantachrome Autosorb-1 analyzer employing N2 as working gas. The area specific resistance (ASR) of the coating was measured in the direction perpendicular to coating surface by direct current method in hydrogen atmosphere. Before this ASR measurement, the silver glue was pasted and sintered as electrodes at 850 °C. For graded Ni–YSZ coating, the coating porosity in different regions along the coating thickness direction was measured by a digital image analysis method (a software of Imagepro plus), which was based on quantitative stereological theory to characterize microstructure parameters such as porosity and its spatial distribution. 3. Results and discussion 3.1. Phase composition of uniform coating Uniform NiO–YSZ coatings were prepared by EB–PVD via simultaneously evaporating two adjacent ingots of NiO and YSZ and then the coatings were reduced in hydrogen at 800 °C for 2 h. The phase compositions of the coatings before and after reduction treatment were analyzed and exhibited in Fig. 2. This analysis result demonstrates that the as-deposited coating shows strong diffraction peaks of cubic YSZ and NiO besides a very small amount of Ni. Due to the poor thermal stability of NiO compared with cubic YSZ, the existence of a little amount of Ni originates from the decomposition of NiO during the evaporation course. By contraries, the phase compositions of the coating after reduction treatment only consist of cubic YSZ and Ni and no NiO can be found in this coating. Considering the limited penetration depth of X-ray, the phase composition information in the inner
Fig. 2. Phase compositions of NiO–YSZ coating before and after reduction treatment.
B. Meng et al. / Thin Solid Films 517 (2009) 4975–4978
4977
Fig. 5. ASR of uniform Ni–YSZ coating in temperature ranges from 30 °C to 600 °C. Fig. 3. SEM image of fracture surface of uniform Ni–YSZ coating and the element distribution.
part of the anode coating would not be detected. So XRD analysis is performed at different depths along the coating thickness direction and the analysis result shows that NiO has been entirely transferred into metallic Ni after this reduction treatment. Due to the volumetric shrinkage during the reduction of NiO grains, many pores and even some channels can be formed to transfer the hydrogen to the inner part of the coating, resulting in the reduction of NiO grains inside the coating. So all of NiO grains have been reduced after reduction treatment and the Ni–YSZ coating can be obtained by EB–PVD through simultaneously evaporating NiO and YSZ ingots combined with a reduction in hydrogen. 3.2. Uniform Ni–YSZ coating Two ingots of YSZ and NiO were evaporated to prepare NiO–YSZ coating by EB–PVD and the currents of electron beams for evaporating YSZ and NiO ingots were kept constant at 0.48 A and 0.18 A respectively during preparation course, just as described in Fig. 1 (1). Then a Ni–YSZ coating with the composition of 50 wt.% of Ni and 50 wt.% of YSZ was obtained from NiO–YSZ coating after a reduction treatment. The SEM image of fracture surface of this Ni–YSZ coating is exhibited in Fig. 3. The coating presents a uniform thickness of 11 µm and a good bonding to the substrate. In this coating, there are many pores with diameters from several nanometers to about one micrometer. The big pores originate from the aggregations of small pores during the
Fig. 4. Pore diameter distribution of uniform Ni–YSZ coating.
reduction course and all these pores are uniformly distributed in the coating along the coating thickness direction. In this Ni–YSZ coating, the pore formation mainly comes from the volumetric shrinkage of NiO grains, but not the shadowing effect. Therefore, the columnar structure is not found in this coating, although it commonly appears in ceramic coatings produced by EB–PVD [15]. At the same time, the element distribution along the coating thickness direction has been illustrated in Fig. 3. The elements of Ni, Zr and O in this coating exhibit a uniform distribution except some limited fluctuations. These fluctuations mainly attribute to the variations in melting pool area during the course of feeding the ingots. These variations have a decisive effect on evaporation rates of different ingots so that the fluctuations of element content along the coating thickness direction occur. In order to measure the specific surface area of open pores and the pore diameter distribution in this coating, N2 adsorption-desorption isotherm was measured. According to an accurate calculation on the basis of BET theory, the specific surface area of open pores in this coating is up to 4.330 m2 g − 1. At the same time, an average pore diameter of 38 nm as well as a total pore volume of 0.0346 cm3 g − 1 is obtained. Furthermore, the distribution of pore diameters in this coating is illustrated in Fig. 4. In this figure, the pore diameter distribution shows three peaks, maximized at the value of 2 nm, 4 nm and 6 nm respectively. These pores with diameters of several nanometers mainly originate from the reduction of NiO grains, leading to a high specific surface area as well as a high catalytic activity for this Ni–YSZ
Fig. 6. SEM image of fracture surface of graded Ni–YSZ coating and the element distribution.
4978
B. Meng et al. / Thin Solid Films 517 (2009) 4975–4978
nificant variation in the porosity and pore diameter for final Ni–YSZ coating. In order to analyze the porosity of different regions along the coating thickness direction quantitatively, a digital image analysis method was employed and the measurement result is shown in Fig. 7. This figure demonstrates that a high porosity of up to 33% is obtained in the part of the coating close to the substrate because there is a high content of NiO before the reduction in this part according to the gradient design. At the same time, a low porosity of 10% is obtained in the part close to coating surface. The high porosity in one side of the anode coating can help the fuel gas diffuse in the anode and decrease the concentration polarization, while a low porosity in the other side is in favor of the following preparation of the dense electrolyte films. In the middle part between the coating surface and substrate, a continuous variation in porosity appears. 4. Conclusions Fig. 7. Porosity distribution at different regions in graded coating.
coating. Besides these small pores, there are also many pores with diameters in the range from 10 to 600 nm in this coating, which originates from the aggregations of nano-sized pores. The ASR of uniform Ni–YSZ coating was measured in temperature range from 30 °C to 600 °C and shown in Fig. 5. Obviously, the ASR of this coating keeps increasing with the rise in temperature and this characteristic corresponds to the behavior of electrical conductivity of Ni. This exhibits that Ni grains in this Ni–YSZ coating have connected with each other and the channel has been formed to transport electrons. At 600 °C, an ASR of 2.1 × 10 − 5Ω cm2 is obtained, which is much lower than the ASR of YSZ electrolyte at this temperature [16]. A low ASR of this coating contributes to a small ohmic polarization loss from this anode coating and a performance improvement of SOFC. 3.3. Graded Ni–YSZ coating According to the gradient design, NiO–YSZ coating was prepared by adjusting electron beam currents for evaporating YSZ and NiO targets, just as described in Fig. 1 (2). Fig. 6 exhibits the cross-sectional morphologies of graded Ni–YSZ coating, which is obtained from this NiO–YSZ coating after a reduction treatment. In this SEM image, the coating shows loose and porous characteristics. In the direction across the coating thickness, there is an obvious variation in porosity as well as pore diameter. In the part of the coating close to the substrate, a high porosity appears and the pore diameters are with the magnitude of about one micron. In contrast, in the part close to coating surface, the porosity is low and the pore diameters are in the range of tens of nanometers. The variations in porosity and pore diameter mainly originate from the changes of Ni content along the thickness direction. Just as shown in Fig. 6, the element contents of Ni and Zr show gradient characteristics obviously. From the coating surface to the interface between the coating and substrate, a continuous reduction of Zr content as well as a continuous increase of Ni content in the coating is observed. Considering the effects of volumetric shrinkage of NiO grains on the porosity of final Ni–YSZ coating obtained from NiO–YSZ coating, the gradient distribution of Ni content will lead to a sig-
Two kinds of Ni–YSZ coatings, respectively with uniform and gradient distributions of element content and porosity along the coating thickness direction, were prepared by EB–PVD combined with a reduction treatment. For uniform Ni–YSZ coatings, the specific surface area of open pores could reach up to 4.330 m2 g − 1. At the same time, average pore diameter and total pore volume in this coating were 38 nm and 0.0346 cm3 g − 1 respectively. The pore diameter distribution showed three peaks, respectively maximized at the value of 2 nm, 4 nm and 6 nm besides large numbers of pores with the diameters from 10 to 600 nm. The ASR of this coating kept increasing with the rise in temperature and an ASR of 2.1 × 10 − 5Ω cm2 was obtained at 600 °C. For Graded Ni–YSZ coatings, a high porosity of up to 33% was achieved in the part of the coating close to the substrate, while a low porosity of 10% was obtained in the part close to coating surface. Acknowledgements The authors gratefully acknowledge the financial assistance from the New Century Excellent Talent Plan of China (NCET2004). References [1] P. Charpentier, P. Fragnaud, D.M. Schleich, E. Gehain, Solid State Ion. 135 (2000) 373. [2] C.J. Fu, K.N. Sun, N.Q. Zhang, X.B. Chen, D.R. Zhou, Thin Solid Films 516 (2008) 1857. [3] I. Villarreal, C. Jacobson, A. Leming, Y. Matus, S. Visco, L. De Jonghe, Electrochem. Solid State 6 (9) (2003) 178. [4] R. Hui, Z.W. Wang, O. Kesler, L. Rose, J. Jankovic, S. Yick, R. Maric, D. Ghosh, J. Power Sources 167 (2007) 318. [5] H. Abe, K. Murata, T. Fukui, W.-J. Moon, K. Kaneko, M. Naito, Thin Solid Films 496 (2006) 49. [6] F. Tietz, H.-P. Buchkremer, D. Stöver, Solid State Ion. 152-153 (2002) 378. [7] C.S. Hwang, C.H. Yu, Surf. Coat. Technol. 188-189 (2004) 85. [8] A. Ringuede, J.A. Labrincha, J.R. Frade, Solid State Ion. 141-142 (2001) 551. [9] A.C. Müller, D. Herbstritt, E.I. Tiffee, Solid State Ion. 152-153 (2002) 539. [10] T. Hatae, N. Kakuda, T. Taniyama, Y. Yamazaki, J. Power Sources 135 (2004) 27. [11] B. Ferrari, R. Moreno, Adv. Eng. Mater. 6 (2004) 12. [12] G. Laukaitis, J. Dudonis, D. Milcius, Thin Solid Films 515 (2006) 678. [13] B.A. Movchan, F.D. Lemkey, Surf. Coat. Technol. 165 (2003) 90. [14] B.A. Movchan, K.Y. Yakovchuk, Surf. Coat. Technol. 188-189 (2004) 86. [15] A.F. Renteria, B. Saruhan, U. Schulz, H.-J.R. Scheibe, Surf. Coat. Technol. 201 (2006) 2612. [16] L. Jia, Z. Lü, X.Q. Huang, Z.G. Liu, Z. Zhi, X.Q. Sha, G.Q. Li, W.H. Su, Ceram. Int. 33 (2007) 635.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Fabrication and characterization of Ni–YSZ anode functional coatings by electron beam physical vapor deposition B. Meng a, Y. Sun b, X.D. He b,⁎, J.H. Peng a a b
Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, No. 68 Wenchang Street, Wuhua District, Kunming, 650093, PR China Center for Composite Materials, Harbin Institute of Technology, No. 2 Yikuang Street, Nangang District, Harbin, 150080, PR China
a r t i c l e
i n f o
Available online 21 March 2009 Keywords: EB–PVD Ni–YSZ Graded coatings Porosity
a b s t r a c t Two kinds of NiO–YSZ (yttria-stabilized zirconia) coatings, respectively with uniform and gradient distributions of NiO content along the coating thickness direction, were prepared by electron beam physical vapor deposition (EB–PVD) via adjusting electron beam currents. Then uniform and graded Ni–YSZ coatings were obtained from corresponding NiO–YSZ coatings after a reduction treatment. For uniform Ni–YSZ coating, the composition and porosity distributions along the coating thickness were uniform. The specific surface area and total pore volume for this coating could reach up to 4.330 m2 g − 1 and 0.0346 cm3 g − 1 respectively. The area specific resistance (ASR) of this coating kept increasing with the rise in temperature and an ASR of 2.1 × 10 − 5Ω cm2 was obtained at 600 °C. For graded Ni–YSZ coating, a gradient in Ni content and porosity was realized along the coating thickness. A high porosity of up to 33% was achieved in the part of the coating close to the substrate, while a low porosity of 10% was obtained in the part close to coating surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In order to reduce manufacture cost and improve long-term stability of solid oxide fuel cell (SOFC), the reduction of operation temperature of planar SOFCs to an intermediate temperature range is a significant objective in current research work [1,2]. A low operation temperature opens up the possibility of using porous metallic substrates as SOFC supports to replace expensive anode or electrolyte supports [3]. Metal-supported SOFCs offer many advantages over conventional electrode- and electrolyte-supported SOFCs [4]. For metal-supported SOFCs, porous metal substrates have been employed as structure supports so that the anode layers can be fabricated in thin films for catalysis only. The anode with a thin film structure contributes to the decrease of gas diffusion impedance and concentration polarization loss. Ni–YSZ (yttria-stabilized zirconia) cermet has been widely employed as anode coating material for SOFCs because of its low cost, good catalytic activity and suitable thermal expansion coefficient [5]. Therefore, many processes have been investigated to prepare Ni–YSZ coatings for SOFCs, such as screen printing, slurry coating, tape casting, plasma spraying, and so on [6,7]. The coatings fabricated by these processes commonly show a single-layered structure and a uniform pore distribution, which often results in the particle agglomeration, an insufficient adhesion between anode and electrolyte layer, an intense internal stress and consequently a high degra⁎ Corresponding author. Tel.: +86 451 86402928; fax: +86 451 86402440. E-mail address: [email protected] (X.D. He). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.098
dation rate [8]. So the anode coating with a multi-layered structure has been developed to replace the single-layered anode and exhibits many excellent properties [9]. But another problem in multi-layered anode is the occurrence of cracks between different layers [10]. A reasonable method to resolve this problem is a design with a gradient structure for the anode coating. In this design, the anode shows a gradient in chemical content, porosity, electrical conductivity and coefficient of thermal expansion along the cross section of the coating [11]. Although the properties of anode coating will be improved dramatically by this method of gradient structure design, this structure is difficult to be realized by traditional methods. As a typical process for coating preparation, electron beam physical vapor deposition (EB–PVD) technique has exhibited a unique superiority in fabrication of microporous coatings with the advantages of high deposition rate, large deposition area and good adherence to substrates [12]. At the same time, the current cost of EB–PVD equipment, especially produced by E.O. Paton Welding Institute, has been reduced to 1/10 of that of forty years ago[13]. So for preparation of porous anode coating of SOFCs, EB–PVD is prospective to be an alternative to traditional wet chemical methods. When this Ni–YSZ coating is prepared by EB–PVD through simultaneously evaporating two adjacent ingots of Ni and YSZ, a limited porosity of 5–15% is obtained. Considering the volumetric shrinkage and additional pore formation during the reduction course from NiO to Ni, the porosity of final Ni–YSZ coating can be increased through fabricating a NiO–YSZ coating combined with a reduction treatment in hydrogen. Furthermore, due to the effects of NiO reduction on coating porosity, the gradient in NiO content for NiO–YSZ coatings will lead to a gradient in
4976
B. Meng et al. / Thin Solid Films 517 (2009) 4975–4978
Fig. 1. The adjustment of electron beam currents for preparations of uniform and graded coatings.
porosity along the coating thickness direction for final Ni–YSZ coating, which is obtained from NiO–YSZ coating after a reduction treatment. The gradient in NiO content for NiO–YSZ coating can be realized by EB–PVD through adjusting electron beam currents for evaporating different ingots. This adjustment of electron beam currents is more convenient and without the complex course of ingot preparation compared with traditional EB–PVD process to prepare graded coating, in which an ingot with graded constituent distribution along its axial line was employed [14]. The gradient structure in porosity not only can help the fuel gas diffuse in the anode and decrease the concentration polarization, but also is in favor of the following preparation of the dense electrolyte films. So in this investigation, NiO–YSZ anode coatings were prepared by EB–PVD through simultaneously evaporating two adjacent ingots of NiO and 8YSZ. The uniform or gradient distribution of NiO content along the coating thickness direction was realized by adjusting the electron beam currents during evaporation course. Then uniform and graded Ni–YSZ coatings were obtained from NiO–YSZ coatings after a reduction in hydrogen. The performances of these two kinds of Ni–YSZ coatings were analyzed and characterized. 2. Experimental An EB–PVD equipment of type L5 was employed to prepare NiO– YSZ coatings with a deposition rate of 0.6 µm/min by simultaneously evaporating two adjacent ingots of NiO and 8YSZ. The final Ni–YSZ coatings were obtained from NiO–YSZ coatings after a reduction at 800 °C in hydrogen for 2 h. A plate of stainless steel (SUS430) with a size of Φ100 × 1.2 mm was used as the substrate because of its suitable thermal expansion coefficient close to NiO–YSZ coating. The substrate was firstly cleaned by a supersonic wave in ethanol and acetone bathes respectively and then was suspended above the middle position of two ingots in a non-rotation mode. The distance between the ingots and substrate was 300 mm and the length between the centers of two ingots was 150 mm. During coating preparation, the substrate temperature was kept at 650 °C by a radiation heater and the pressure level of deposition chamber was kept less than 1 × 10 − 2 Pa. Two kinds of NiO–YSZ coatings, respectively with uniform and gradient distributions of NiO content along the coating thickness direction, were prepared by EB–PVD. According to repeated experiments, when the currents of electron beams for evaporating NiO and YSZ ingots were 0.18 A and 0.48 A respectively and the area of melting pool kept constant at 10 mm2, the equality between evaporation rates of NiO and YSZ was realized. So for preparation of uniform NiO–YSZ coating, the currents of electron beams for evaporating NiO and YSZ ingots were kept constant at 0.18 A and 0.48 A respectively. At the
same time, in order to realize the gradient distribution of NiO content along the coating thickness direction, the currents of electron beams for evaporating NiO and YSZ ingots were designed and shown in Fig. 1. Through this adjustment of electron beam currents for different ingots, a continuous decrease of NiO content as well as a continuous increase of YSZ content in this coating could be realized along the direction from the interface between the coating and substrate to coating surface. Considering the volumetric shrinkage and additional pore formation during the reduction course from NiO to Ni, a gradient in NiO content for NiO–YSZ coatings would lead to a gradient in porosity along the thickness direction for the final Ni–YSZ coating, which was obtained from NiO–YSZ coating after a reduction treatment. After coating preparation, the phase composition of this coating was characterized by Philips X’Pert diffractometer (XRD) with parallel Cu Kα radiation. The morphology of fracture surface as well as element distribution of the coating was analyzed by scanning electron microscope (SEM) (Hitachi S4700) combined with an electron probe microanalysis (EPMA). The specific surface area of the open pores, average pore diameter and pore diameter distribution of this coating were measured with a Quantachrome Autosorb-1 analyzer employing N2 as working gas. The area specific resistance (ASR) of the coating was measured in the direction perpendicular to coating surface by direct current method in hydrogen atmosphere. Before this ASR measurement, the silver glue was pasted and sintered as electrodes at 850 °C. For graded Ni–YSZ coating, the coating porosity in different regions along the coating thickness direction was measured by a digital image analysis method (a software of Imagepro plus), which was based on quantitative stereological theory to characterize microstructure parameters such as porosity and its spatial distribution. 3. Results and discussion 3.1. Phase composition of uniform coating Uniform NiO–YSZ coatings were prepared by EB–PVD via simultaneously evaporating two adjacent ingots of NiO and YSZ and then the coatings were reduced in hydrogen at 800 °C for 2 h. The phase compositions of the coatings before and after reduction treatment were analyzed and exhibited in Fig. 2. This analysis result demonstrates that the as-deposited coating shows strong diffraction peaks of cubic YSZ and NiO besides a very small amount of Ni. Due to the poor thermal stability of NiO compared with cubic YSZ, the existence of a little amount of Ni originates from the decomposition of NiO during the evaporation course. By contraries, the phase compositions of the coating after reduction treatment only consist of cubic YSZ and Ni and no NiO can be found in this coating. Considering the limited penetration depth of X-ray, the phase composition information in the inner
Fig. 2. Phase compositions of NiO–YSZ coating before and after reduction treatment.
B. Meng et al. / Thin Solid Films 517 (2009) 4975–4978
4977
Fig. 5. ASR of uniform Ni–YSZ coating in temperature ranges from 30 °C to 600 °C. Fig. 3. SEM image of fracture surface of uniform Ni–YSZ coating and the element distribution.
part of the anode coating would not be detected. So XRD analysis is performed at different depths along the coating thickness direction and the analysis result shows that NiO has been entirely transferred into metallic Ni after this reduction treatment. Due to the volumetric shrinkage during the reduction of NiO grains, many pores and even some channels can be formed to transfer the hydrogen to the inner part of the coating, resulting in the reduction of NiO grains inside the coating. So all of NiO grains have been reduced after reduction treatment and the Ni–YSZ coating can be obtained by EB–PVD through simultaneously evaporating NiO and YSZ ingots combined with a reduction in hydrogen. 3.2. Uniform Ni–YSZ coating Two ingots of YSZ and NiO were evaporated to prepare NiO–YSZ coating by EB–PVD and the currents of electron beams for evaporating YSZ and NiO ingots were kept constant at 0.48 A and 0.18 A respectively during preparation course, just as described in Fig. 1 (1). Then a Ni–YSZ coating with the composition of 50 wt.% of Ni and 50 wt.% of YSZ was obtained from NiO–YSZ coating after a reduction treatment. The SEM image of fracture surface of this Ni–YSZ coating is exhibited in Fig. 3. The coating presents a uniform thickness of 11 µm and a good bonding to the substrate. In this coating, there are many pores with diameters from several nanometers to about one micrometer. The big pores originate from the aggregations of small pores during the
Fig. 4. Pore diameter distribution of uniform Ni–YSZ coating.
reduction course and all these pores are uniformly distributed in the coating along the coating thickness direction. In this Ni–YSZ coating, the pore formation mainly comes from the volumetric shrinkage of NiO grains, but not the shadowing effect. Therefore, the columnar structure is not found in this coating, although it commonly appears in ceramic coatings produced by EB–PVD [15]. At the same time, the element distribution along the coating thickness direction has been illustrated in Fig. 3. The elements of Ni, Zr and O in this coating exhibit a uniform distribution except some limited fluctuations. These fluctuations mainly attribute to the variations in melting pool area during the course of feeding the ingots. These variations have a decisive effect on evaporation rates of different ingots so that the fluctuations of element content along the coating thickness direction occur. In order to measure the specific surface area of open pores and the pore diameter distribution in this coating, N2 adsorption-desorption isotherm was measured. According to an accurate calculation on the basis of BET theory, the specific surface area of open pores in this coating is up to 4.330 m2 g − 1. At the same time, an average pore diameter of 38 nm as well as a total pore volume of 0.0346 cm3 g − 1 is obtained. Furthermore, the distribution of pore diameters in this coating is illustrated in Fig. 4. In this figure, the pore diameter distribution shows three peaks, maximized at the value of 2 nm, 4 nm and 6 nm respectively. These pores with diameters of several nanometers mainly originate from the reduction of NiO grains, leading to a high specific surface area as well as a high catalytic activity for this Ni–YSZ
Fig. 6. SEM image of fracture surface of graded Ni–YSZ coating and the element distribution.
4978
B. Meng et al. / Thin Solid Films 517 (2009) 4975–4978
nificant variation in the porosity and pore diameter for final Ni–YSZ coating. In order to analyze the porosity of different regions along the coating thickness direction quantitatively, a digital image analysis method was employed and the measurement result is shown in Fig. 7. This figure demonstrates that a high porosity of up to 33% is obtained in the part of the coating close to the substrate because there is a high content of NiO before the reduction in this part according to the gradient design. At the same time, a low porosity of 10% is obtained in the part close to coating surface. The high porosity in one side of the anode coating can help the fuel gas diffuse in the anode and decrease the concentration polarization, while a low porosity in the other side is in favor of the following preparation of the dense electrolyte films. In the middle part between the coating surface and substrate, a continuous variation in porosity appears. 4. Conclusions Fig. 7. Porosity distribution at different regions in graded coating.
coating. Besides these small pores, there are also many pores with diameters in the range from 10 to 600 nm in this coating, which originates from the aggregations of nano-sized pores. The ASR of uniform Ni–YSZ coating was measured in temperature range from 30 °C to 600 °C and shown in Fig. 5. Obviously, the ASR of this coating keeps increasing with the rise in temperature and this characteristic corresponds to the behavior of electrical conductivity of Ni. This exhibits that Ni grains in this Ni–YSZ coating have connected with each other and the channel has been formed to transport electrons. At 600 °C, an ASR of 2.1 × 10 − 5Ω cm2 is obtained, which is much lower than the ASR of YSZ electrolyte at this temperature [16]. A low ASR of this coating contributes to a small ohmic polarization loss from this anode coating and a performance improvement of SOFC. 3.3. Graded Ni–YSZ coating According to the gradient design, NiO–YSZ coating was prepared by adjusting electron beam currents for evaporating YSZ and NiO targets, just as described in Fig. 1 (2). Fig. 6 exhibits the cross-sectional morphologies of graded Ni–YSZ coating, which is obtained from this NiO–YSZ coating after a reduction treatment. In this SEM image, the coating shows loose and porous characteristics. In the direction across the coating thickness, there is an obvious variation in porosity as well as pore diameter. In the part of the coating close to the substrate, a high porosity appears and the pore diameters are with the magnitude of about one micron. In contrast, in the part close to coating surface, the porosity is low and the pore diameters are in the range of tens of nanometers. The variations in porosity and pore diameter mainly originate from the changes of Ni content along the thickness direction. Just as shown in Fig. 6, the element contents of Ni and Zr show gradient characteristics obviously. From the coating surface to the interface between the coating and substrate, a continuous reduction of Zr content as well as a continuous increase of Ni content in the coating is observed. Considering the effects of volumetric shrinkage of NiO grains on the porosity of final Ni–YSZ coating obtained from NiO–YSZ coating, the gradient distribution of Ni content will lead to a sig-
Two kinds of Ni–YSZ coatings, respectively with uniform and gradient distributions of element content and porosity along the coating thickness direction, were prepared by EB–PVD combined with a reduction treatment. For uniform Ni–YSZ coatings, the specific surface area of open pores could reach up to 4.330 m2 g − 1. At the same time, average pore diameter and total pore volume in this coating were 38 nm and 0.0346 cm3 g − 1 respectively. The pore diameter distribution showed three peaks, respectively maximized at the value of 2 nm, 4 nm and 6 nm besides large numbers of pores with the diameters from 10 to 600 nm. The ASR of this coating kept increasing with the rise in temperature and an ASR of 2.1 × 10 − 5Ω cm2 was obtained at 600 °C. For Graded Ni–YSZ coatings, a high porosity of up to 33% was achieved in the part of the coating close to the substrate, while a low porosity of 10% was obtained in the part close to coating surface. Acknowledgements The authors gratefully acknowledge the financial assistance from the New Century Excellent Talent Plan of China (NCET2004). References [1] P. Charpentier, P. Fragnaud, D.M. Schleich, E. Gehain, Solid State Ion. 135 (2000) 373. [2] C.J. Fu, K.N. Sun, N.Q. Zhang, X.B. Chen, D.R. Zhou, Thin Solid Films 516 (2008) 1857. [3] I. Villarreal, C. Jacobson, A. Leming, Y. Matus, S. Visco, L. De Jonghe, Electrochem. Solid State 6 (9) (2003) 178. [4] R. Hui, Z.W. Wang, O. Kesler, L. Rose, J. Jankovic, S. Yick, R. Maric, D. Ghosh, J. Power Sources 167 (2007) 318. [5] H. Abe, K. Murata, T. Fukui, W.-J. Moon, K. Kaneko, M. Naito, Thin Solid Films 496 (2006) 49. [6] F. Tietz, H.-P. Buchkremer, D. Stöver, Solid State Ion. 152-153 (2002) 378. [7] C.S. Hwang, C.H. Yu, Surf. Coat. Technol. 188-189 (2004) 85. [8] A. Ringuede, J.A. Labrincha, J.R. Frade, Solid State Ion. 141-142 (2001) 551. [9] A.C. Müller, D. Herbstritt, E.I. Tiffee, Solid State Ion. 152-153 (2002) 539. [10] T. Hatae, N. Kakuda, T. Taniyama, Y. Yamazaki, J. Power Sources 135 (2004) 27. [11] B. Ferrari, R. Moreno, Adv. Eng. Mater. 6 (2004) 12. [12] G. Laukaitis, J. Dudonis, D. Milcius, Thin Solid Films 515 (2006) 678. [13] B.A. Movchan, F.D. Lemkey, Surf. Coat. Technol. 165 (2003) 90. [14] B.A. Movchan, K.Y. Yakovchuk, Surf. Coat. Technol. 188-189 (2004) 86. [15] A.F. Renteria, B. Saruhan, U. Schulz, H.-J.R. Scheibe, Surf. Coat. Technol. 201 (2006) 2612. [16] L. Jia, Z. Lü, X.Q. Huang, Z.G. Liu, Z. Zhi, X.Q. Sha, G.Q. Li, W.H. Su, Ceram. Int. 33 (2007) 635.