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Electron beam physical vapor deposition of YSZ electrolyte coatings for SOFCs
Applied Surface Science 254 (2008) 7159–7164
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electron beam physical vapor deposition of YSZ electrolyte coatings for SOFCs Xiaodong He *, Bin Meng, Yue Sun, Bochao Liu, Mingwei Li Center for Composite Materials, Harbin Institute of Technology, PO Box 3010, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 March 2008 Received in revised form 12 May 2008 Accepted 12 May 2008 Available online 20 May 2008
YSZ electrolyte coatings were prepared by electron beam physical vapor deposition (EB-PVD) at a high deposition rate of up to 1 mm/min. The YSZ coating consisted of a single cubic phase and no phase transformation occurred after annealing treatment at 1000 8C. A typical columnar structure was observed in this coating by SEM and feather-like characteristics appeared in every columnar grain. In columnar grain boundaries there were many micron-sized gaps and pores. In TEM image, many white lines were found, originating from the alignment of nanopores existing within feather-like columnar grains. The element distribution along the cross-section of the coating was homogeneous except Zr with a slight gradient. The coating exhibited a characteristic anisotropic behavior in electrical conductivity. In the direction perpendicular to coating surface the electrical conductivity was remarkably higher than that in the direction parallel to coating surface. This mainly attributed to the typical columnar structure for EBPVD coating and the existence of many grain boundaries along the direction parallel to coating surface. For as-deposited coating, the gas permeability coefficient of 9.78 105 cm4 N1 s1 was obtained and this value was close to the critical value of YSZ electrolyte layer required for solid oxide fuel cell (SOFC) operation. ß 2008 Elsevier B.V. All rights reserved.
PACS: 76.61.r Keywords: EB-PVD YSZ electrolyte coatings SOFC Electrical conductivity
1. Introduction Solid oxide fuel cell (SOFC) is a clean and high efficient electrochemical energy converter which directly transforms the chemical energy of the fuel gas into electrical energy [1]. In order to reduce manufacture cost and improve long-term stability of SOFCs, the reduction of operating temperature of planar SOFCs to an intermediate temperature range of 650–800 8C is a significant objective in current research work [2]. It is one of the most effective strategies to decrease the thickness of solid electrolyte to maintain a lower operation temperature and a lower ohmic loss derived from solid electrolyte [3,4]. Therefore, many processes have been investigated to prepare the electrolyte films, such as sol–gel, slurry coating, tape casting, electrochemical vapor deposition (EVD), chemical vapor deposition (CVD), physical vapor deposition (PVD, including magnetron sputtering, pulsed laser deposition and ebeam deposition) and so on [5,6]. In the last two decades, there has been a growing interest in the synthesis of solid electrolyte films by PVD due to its strict control of film microstructure, porosity, stoichiometry and growth rate during the course of deposition [7].
* Corresponding author. Tel.: +86 451 86402928; fax: +86 451 86402440. E-mail address: [email protected] (X.D. He). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.271
As one form of PVD techniques, electron beam physical vapor deposition (EB-PVD) has been widely applied to fabricate conventional ceramic coatings, such as thermal barrier coatings (TBCs), with the advantages of high deposition rate, large deposition area and good adherence to substrates [8,9]. At the same time, owing to the improvement of electron gun technology, the current cost of EB-PVD equipment, especially produced by the International Center for Electron Beam Technology of E.O. Paton Welding Institute, has been reduced to one-tenth of that of 40 years ago through replacing the magnetic-field-bending-type electron guns by Pierce-type guns [10]. Another remarkable advantage of EB-PVD process is the accomplishment of continuous deposition of different components, such as SOFC electrodes and electrolytes. So for preparation of SOFC materials, EB-PVD process is prospective to be an attractive alternative to traditional powder technology. But on the other hand, research work about applying EB-PVD to prepare SOFC solid electrolyte coatings was seldom done by far and mainly limited in aspect of the influences of process parameters on the microstructure of deposited coatings, especially when the preparation was performed at a low deposition rate and using electron guns with low power [11,12]. Due to the intrinsic characteristics of EB-PVD, non-equilibrium condensation of vapor phase will lead to the development of microporous structure according to so-called ‘‘shadowing effect’’ [13]. This microporous
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structure will have a profound influence on the coating properties, such as the electrical conductivity and gas tightness. It is important to note that these two properties are critical to determine the cell performances when this electrolyte coating is applied in SOFCs. However, these properties of EB-PVD electrolyte coatings have seldom been investigated in past research work. Cubic zirconia stablilized by 8 mol% yttria (8YSZ) is commonly used as solid electrolyte for SOFCs due to its attractive ionic conductivity at high operating temperature and good thermal and chemical stability in both oxidizing and reducing environments [8]. So in this work, the 8YSZ coating was prepared by EB-PVD technique at a high deposition rate of 1 mm/min. Then the microstructure of this coating was characterized and the properties, such as electrical conductivity and gas tightness, were measured.
the silver glue was pasted and sintered at 850 8C as electrodes. When the electrical conductivity in the direction parallel to coating surface was measured, a quartz plate was used as the substrate because the electrical resistance of quartz was much high than that of YSZ in the measuring temperature range. At the same time, in order to measure the electrical conductivity in the direction perpendicular to coating surface, a metal sheet of 316L stainless steel was used as the substrate in hydrogen atmosphere because its electrical conductivity was six orders higher than that of the YSZ coating so that the electrical resistance from metal substrate could be ignored during this measurement. For the measurements of electrical conductivities in both directions, the YSZ electrolyte coatings had the same thickness of 11 mm. The detailed electrode configurations for measurements of electrical conductivities in both directions were shown in Fig. 1.
2. Experimental
2.4. Evaluation of gas tightness
2.1. Preparation of as-deposited coatings
The gas tightness of the coating was evaluated using the apparatus just as shown in Fig. 2. In order to confirm the gas tightness of this testing system, a gas-tight glass plate was firstly adhered to the cross-section of the quartz tube by a silica sealant and this system was evacuated to less than 103 Pa by a mechanical pump. Then the valve was shut off and the pressure difference between ambient atmosphere and inside this testing system kept constant within 24 h. So the absence of gas leakage either through sealed junction or other connections was confirmed. Then the coating sample was adhered to the quartz tube by the same sealant and the same vacuum state in this testing system was achieved before the valve was cut off. Due to the air leakage through the coating, the pressure difference between this system and atmosphere would change with time. According to Darcy’s law [14]:
A large-scale EB-PVD equipment of L5 type was employed to prepare YSZ electrolyte coatings on porous NiO-8YSZ substrates at a high deposition rate of up to 1 mm/min, via evaporating ceramic powders of 8YSZ. The porous substrates with the composition of 56 wt.% of NiO and 44 wt.% of 8YSZ were prepared by tape-casting method. Pore-formers were added in to increase the porosity of the substrates and a total porosity of 30% was obtained after a reduction in hydrogen at 800 8C for 2 h. A radiation heater was adopted to heat the substrates because traditional heating method by electron beam had a high energy density and often resulted in too fast rate of rise in temperature and cracks of ceramic substrates. Due to the limited power of the radiation heater adopted in this study, a substrate temperature, 750 8C, was achieved during coating deposition. In this EB-PVD equipment, the magnetic-field-bending-type electron guns were replaced by the Pierce-type electron guns. An accelerating voltage of up to 20 kV between the cathode and evaporation target provided the equipment with a higher power, leading to a higher deposition rate during coating preparation. The deposition was performed in a high vacuum of less than 102 Pa by a turbo-molecular pump and the distance between the substrate and evaporation target was 250 mm. During preparation, the deposition rate could be adjusted by changing the currents of electron beams.
Q ¼
kAD p
d
(1)
where Q is the gas leakage flux (cm3/s), k is the gas permeability of the as-deposited coating (cm4 N1 s1), A is the area of the coating (cm2), Dp is the pressure difference between this system and atmosphere (N cm2), d is the thickness of the coating (cm).
2.2. Microstructure analysis After coating preparation, the phase composition of this coating was characterized by Philips X’Pert diffractometer (XRD) with parallel Cu Ka radiation. The surface and cross-sectional morphologies were observed by scanning electron microscopy (SEM) with a Hitachi S4700. Before the transmission electron microscopy (TEM) analysis was conducted using a Philips EM420 TEM, two coating samples were firstly adhered face to face and then inserted into a copper tube with a diameter of 3 mm. Then a slice was cut down from this tube and was thinned in the direction perpendicular to the cross-section of the coating by mechanical grinding combined with Ar ion etching process. The element distribution of the coating was characterized by electron probe microanalysis (EPMA). 2.3. Measurement of electrical conductivity The electrical conductivity of YSZ coating was measured by direct current four-probe method in the temperature range from 500 to 800 8C. In order to determine the area of working electrodes,
Fig. 1. Electrode configurations for measurements of electrical conductivities of the YSZ coating in two directions: (a) in direction parallel to coating surface, (b) in direction perpendicular to coating surface.
X. He et al. / Applied Surface Science 254 (2008) 7159–7164
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Fig. 2. Schematic of the apparatus evaluating the gas tightness of electrolyte coating.
3. Results and discussion
panied by a significant volume expansion, which often leads to cracks and even a failure of the coating. A further observation from Fig. 3 demonstrates that there is an obvious shift for the diffraction peak positions between the asdeposited and annealed coatings. In fact, the diffraction peaks shift to low angles after annealing treatment. Due to the differences in thermal expansion coefficients (TEC) between the NiO-YSZ substrate and YSZ coating (for example, the mean TEC of NiOYSZ substrate is 13.0 106 K1, while that of YSZ coating is 10.8 106 K1), a compressive stress is formed during preparation course. After annealing, the compressive stress is relieved and this leads to an increment in crystal cell parameters of YSZ coating. So a shift of the diffraction peaks to low angles appears after annealing.
3.1. Phase compositions
3.2. Surface and cross-sectional morphologies
Considering the higher operation temperature of more than 850 8C for traditional SOFCs using YSZ coatings as electrolytes, the as-deposited coating was treated by annealing in air at 1000 8C for 2 h combined with a furnace cooling in order to confirm the stability of phase composition. Therefore, the phase compositions of the as-deposited and annealed coatings are analyzed and shown in Fig. 3 together. It can be found from this analysis that the XRD patterns of both as-deposited and annealed coatings all show the lines of cubic fluorite-type structure and no lines of tetragonal or monoclinic phase are observed. So no phase transformation occurs during the course of annealing treatment and this is beneficial for the volumetric and thermal stability of the coating because the phase transformation from tetragonal to monoclinic is accom-
The surface and cross-sectional morphologies of the asdeposited coating are shown in Fig. 4. The surface SEM image exhibits there are many micron-sized particles in the coating surface. Moreover, every particle consists of many crystal clusters, which originates from the crystal grain growth in mode of threedimensional islands during coating deposition. Between the particle boundaries, there are many gaps and pores, which will have a significant effect on the gas tightness and electrical conductivity of this coating. A further cross-sectional SEM observation shows that the coating, with a thickness of 11 mm, presents a good adhesion to the substrate and a characteristic columnar structure normal to the substrate. The diameters of columnar grains are in the range from several microns to 10 mm. In the magnified cross-sectional image of Fig. 4(c), the columnar grains exhibit feather-like characteristics and some distinct gaps as well as pores exist between the boundaries of columnar grains. According to the growth pattern of coatings prepared by EBPVD, which was presented by Movchan and Demchishin, the substrate temperature has a significant influence on the coating growth pattern and morphology. Because of the adoption of a radiation heater and its limited power, the substrate temperature, 750 8C, is the highest temperature for this equipment, which is slightly higher than one-third of the melting point of YSZ material. So a characteristic columnar structure forms in the coating.
Then the total gas volume charged from leakage during time from t0 to t can be expressed as: Z Z t kA t Q dt ¼ D pðtÞ dt (2) t0
d
t0
Consequently, when the change of the pressure difference with time is measured, the gas permeability can be calculated according to the following formula: R d tt Q dt k ¼ Rt 0 (3) A t0 D pðtÞ dt
3.3. TEM observation
Fig. 3. XRD patterns of the as-deposited and annealed YSZ coatings.
Fig. 5 shows a bright-field TEM image of the YSZ coating, in which many white lines are observed in the YSZ grain in a certain direction. According to the SEM image in Fig. 4(c), a feather-like structure forms in the columnar grain of the YSZ coating. In the edge and inner of the feather-like grains, many nanopores are
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Fig. 4. Surface and cross-sectional morphologies of the as-deposited coating: (a) surface morphology, (b) cross-sectional morphology, (c) magnified cross-sectional morphology.
created due to the secondary shadowing effect. These pores are mainly controlled by the incidence angle of vapor flow and aligned in a certain direction, so the white lines are observed in the TEM image. Nanopores in the YSZ coating are considered to have a
profound influence on coating properties, such as thermal conductivity and electrical conductivity. These pores, which have a lower thermal or electrical conductivity compared with the matrix, decrease the mean thermal or electrical conductivity and alter the conduction paths of photons or electrical charges. So it is necessary to control the quantity and distribution of the nanopores in order to obtain ideal coating properties. 3.4. Element distribution
Fig. 5. A bright-field TEM image of the YSZ coating.
The element distribution of YSZ coating was measured by the method of EPMA and shown in Fig. 6. The element distribution along the cross-section of the coating is homogeneous except Zr with a slight gradient. Due to the selective evaporation behavior for EB-PVD process, the component with higher saturation evaporation pressure at the same temperature will be evaporated preferentially when multi components are evaporated simultaneously in one crucible. Considering the yttria with a higher saturation evaporation pressure compared with zirconia, the yttria will be preferentially evaporated from the ingot at a higher evaporation rate at the beginning. This will lead to an enrichment of zirconia in the melting pool, which results in an increment of evaporation rate for zirconia unless an evaporation rate balance is achieved. As a result, the element of zirconium exhibits a slight gradient distribution and the concentration of zirconium at the interface between the coating and substrate is slightly lower than
X. He et al. / Applied Surface Science 254 (2008) 7159–7164
Fig. 6. Element distribution of YSZ coating by EPMA.
that at the coating surface. Due to a low concentration of yttrium in the coating, a slight variation in yttrium content along the crosssection of the coating is difficult to be detected by EPMA. So the element of yttrium shows a uniform distribution relative to zirconium. 3.5. Electrical conductivity Electrical conductivities of the YSZ coating in temperature range from 500 to 800 8C were measured in both directions perpendicular and parallel to coating surface and shown in Fig. 7. Obviously, the electrical conductivity in the direction perpendicular to coating surface is remarkably higher than that in the direction parallel to coating surface. For example, the electrical conductivity in the direction parallel to coating surface at 800 8C is lower than half of that in the direction perpendicular to coating surface. The anisotropy and significant differences in the electrical conductivities between these two directions originate from the typical columnar structure characteristics for EB-PVD coating. When the electrical charges are transferred in the direction parallel to coating surface, there are many gaps and pores in the columnar
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Fig. 8. Arrhenius plots and the activation energies in both directions parallel and perpendicular to coating surface.
grain boundaries along the transmission paths. On the contrary, in the direction perpendicular to coating surface, many columnar grains penetrate through the coating to form parallel transmission paths, with fewer grain boundaries and defects. Therefore, the electrical conductivity in the direction perpendicular to coating surface is much higher than that in the direction parallel to coating surface. During the YSZ electrolyte operation in SOFCs, the oxygen ions are transferred in the direction perpendicular to coating surface, so a higher electrical conductivity is prospective to be obtained by EB-PVD process. The typical columnar structure for EB-PVD coating also determines a remarkable difference in the activation energies for ion transmission between these two directions. Fig. 8 shows the Arrhenius plots and the activation energies for ion transmission in both directions. For example, the activation energy for ion conduction in the direction perpendicular to coating surface is 0.84 eV, while that in the direction parallel to coating surface is 1.14 eV. Furthermore, an electrical conductivity of 2.6 S m1 is obtained in the direction perpendicular to coating surface at 800 8C for the as-deposited coating. According to the result reported by Yamamoto and Ivers-Tiffee [15,16], 8YSZ bulk shows an electrical conductivity from 3.5 to 4.2 S m1 at 800 8C. So the electrical conductivity of as-deposited coating in direction perpendicular to coating surface is still lower than that of 8YSZ bulk. This can be explained by the lower density of YSZ coating compared with YSZ bulk and the existence of nanopores within the feather-like columnar grains. The existence of nanopores reduces the area of the transmission paths, resulting in a lower electrical conductivity. 3.6. Gas tightness
Fig. 7. Electrical conductivities of YSZ coating measured in both directions parallel and perpendicular to coating surface.
The gas tightness of YSZ electrolyte coating was evaluated employing the apparatus in Fig. 2. The change of pressure difference between this testing system and atmosphere with time is illustrated in Fig. 9. It can be found that the pressure difference decreases rapidly and the pressure balance is achieved within 280s. The gas leakage through EB-PVD coating mainly originates from the existence of gaps and pores in the columnar grain boundaries, just as shown in the Fig. 4(c). Those nanopores within feather-like columnar grains, especially those closed discontinuous nanopores, have little influence on the gas tightness of this coating. According to an accurate calculation, the coefficient of gas permeability for as-deposited coating is 9.78 105 cm4 N1 s1.
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Fig. 9. The change of pressure difference between this testing system and atmosphere with time and gas permeability coefficient of the as-deposited coating.
Although there is an obvious gas leakage for as-deposited coating, this value of gas permeability coefficient is remarkably lower than that of the coating prepared by atmospheric plasma spraying (APS). Li et al. had reported that the value of gas permeability coefficient, 2.96 104 cm4 N1 s1, was obtained by APS for tubular SOFC. There has been some argument about the critical value of gas permeability of YSZ electrolyte layer required for SOFC operation [17]. Some investigations have demonstrated that the value should be less than 1.02 104 cm4 N1 s1 [12,18]. In this study, the value of gas permeability coefficient is close to the critical value so that the normal operation and long-term stability for SOFCs can not be ensured. So a post densification treatment, such as heat treatment or a sol infiltration, is suggested to improve the gas tightness of EB-PVD coating. 4. Conclusions YSZ electrolyte coating with a thickness of 11 mm was prepared by EB-PVD process at a high deposition rate of up to 1 mm/min. The YSZ coating consisted of a single cubic phase and no phase transformation occurred after annealing treatment at 1000 8C. A typical columnar structure was formed in this coating and featherlike characteristics appeared in every columnar grain. In the columnar grain boundaries there were many micron-sized gaps
and pores. Many white lines were found in TEM observation, which originated from the alignment of nanopores existing within feather-like columnar grains. The element distribution along the cross-section of the coating was homogeneous except Zr with a slight gradient due to the selective evaporation behavior for EBPVD process. The coating exhibited a characteristic anisotropic behavior in electrical conductivity. The value of electrical conductivity in the direction perpendicular to coating surface was remarkably higher than that in the direction parallel to coating surface. This mainly attributed to the typical columnar structure characteristics for EBPVD coating and the existence of many grain boundaries along the direction parallel to coating surface. Moreover, the electrical conductivity of as-deposited coating in the direction perpendicular to coating surface was still lower than that of 8YSZ bulk due to the existence of many nanopores within the feather-like columnar grains. For the as-deposited coating, the gas permeability coefficient of 9.78 105 cm4 N1 s1 was obtained and this value was close to the critical value of YSZ electrolyte layer required for SOFC operation. Acknowledgement The authors gratefully acknowledge the financial assistance from the New Century Excellent Talent Plan of China (NCET2004). References [1] C. Brahim, A. Ringuede´, M. Cassir, M. Putkonen, L. Niinisto¨, Appl. Surf. Sci. 253 (2007) 3962–3968. [2] P. Charpentier, P. Fragnaud, D.M. Schleich, E. Gehain, Solid State Ionics 135 (2000) 373. [3] J.R. Kong, K.N. Sun, D.R. Zhou, N.Q. Zhang, J. Mu, J.S. Qiao, J. Power Sources 166 (2007) 337. [4] L.R. Pederson, P. Singh, X.D. Zhou, Vacuum 80 (2006) 1066. [5] N. Pryds, B. Toftmann, J.B. Bilde-Sørensen, J. Schou, S. Linderoth, Appl. Surf. Sci. 252 (2006) 4882–4885. [6] J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, L.J. Gauckler, Solid State Ionics 131 (2000) 79. [7] G. Laukaitis, J. Dudonis, D. Milcius, Appl. Surf. Sci. 254 (2008) 2980–2987. [8] J. Yi, X.D. He, Y. Sun, Y. Li, Appl. Surf. Sci. 253 (2007) 4361–4366. [9] B.A. Movchan, K.Y. Yakovchuk, Surf. Coat. Technol. 188–189 (2004) 85. [10] B.A. Movchan, F.D. Lemkey, Surf. Coat. Technol. 165 (2003) 90. [11] G. Laukaitis, J. Dudonis, D. Milcius, Thin Solid Films 515 (2006) 678. [12] G. Laukaitis, J. Dudonis, D. Milcius, Vacuum 81 (2007) 1288–1291. [13] B.A. Movchan, Surf. Eng. 22 (1) (2006) 35–46. [14] C.J. Li, X.J. Ning, C.X. Li, Surf. Coat. Technol. 190 (2005) 60. [15] E. Ivers-Tiffee, A. Weber, D. Herbstritt, J. Eur. Ceram. Soc. 21 (2001) 1805. [16] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [17] X.J. Ning, C.X. Li, C.J. Li, G.J. Yang, Mater. Sci. Eng. A 428 (2006) 98. [18] C.J. Li, C.X. Li, X.J. Ning, Vacuum 73 (2004) 699.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electron beam physical vapor deposition of YSZ electrolyte coatings for SOFCs Xiaodong He *, Bin Meng, Yue Sun, Bochao Liu, Mingwei Li Center for Composite Materials, Harbin Institute of Technology, PO Box 3010, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 March 2008 Received in revised form 12 May 2008 Accepted 12 May 2008 Available online 20 May 2008
YSZ electrolyte coatings were prepared by electron beam physical vapor deposition (EB-PVD) at a high deposition rate of up to 1 mm/min. The YSZ coating consisted of a single cubic phase and no phase transformation occurred after annealing treatment at 1000 8C. A typical columnar structure was observed in this coating by SEM and feather-like characteristics appeared in every columnar grain. In columnar grain boundaries there were many micron-sized gaps and pores. In TEM image, many white lines were found, originating from the alignment of nanopores existing within feather-like columnar grains. The element distribution along the cross-section of the coating was homogeneous except Zr with a slight gradient. The coating exhibited a characteristic anisotropic behavior in electrical conductivity. In the direction perpendicular to coating surface the electrical conductivity was remarkably higher than that in the direction parallel to coating surface. This mainly attributed to the typical columnar structure for EBPVD coating and the existence of many grain boundaries along the direction parallel to coating surface. For as-deposited coating, the gas permeability coefficient of 9.78 105 cm4 N1 s1 was obtained and this value was close to the critical value of YSZ electrolyte layer required for solid oxide fuel cell (SOFC) operation. ß 2008 Elsevier B.V. All rights reserved.
PACS: 76.61.r Keywords: EB-PVD YSZ electrolyte coatings SOFC Electrical conductivity
1. Introduction Solid oxide fuel cell (SOFC) is a clean and high efficient electrochemical energy converter which directly transforms the chemical energy of the fuel gas into electrical energy [1]. In order to reduce manufacture cost and improve long-term stability of SOFCs, the reduction of operating temperature of planar SOFCs to an intermediate temperature range of 650–800 8C is a significant objective in current research work [2]. It is one of the most effective strategies to decrease the thickness of solid electrolyte to maintain a lower operation temperature and a lower ohmic loss derived from solid electrolyte [3,4]. Therefore, many processes have been investigated to prepare the electrolyte films, such as sol–gel, slurry coating, tape casting, electrochemical vapor deposition (EVD), chemical vapor deposition (CVD), physical vapor deposition (PVD, including magnetron sputtering, pulsed laser deposition and ebeam deposition) and so on [5,6]. In the last two decades, there has been a growing interest in the synthesis of solid electrolyte films by PVD due to its strict control of film microstructure, porosity, stoichiometry and growth rate during the course of deposition [7].
* Corresponding author. Tel.: +86 451 86402928; fax: +86 451 86402440. E-mail address: [email protected] (X.D. He). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.271
As one form of PVD techniques, electron beam physical vapor deposition (EB-PVD) has been widely applied to fabricate conventional ceramic coatings, such as thermal barrier coatings (TBCs), with the advantages of high deposition rate, large deposition area and good adherence to substrates [8,9]. At the same time, owing to the improvement of electron gun technology, the current cost of EB-PVD equipment, especially produced by the International Center for Electron Beam Technology of E.O. Paton Welding Institute, has been reduced to one-tenth of that of 40 years ago through replacing the magnetic-field-bending-type electron guns by Pierce-type guns [10]. Another remarkable advantage of EB-PVD process is the accomplishment of continuous deposition of different components, such as SOFC electrodes and electrolytes. So for preparation of SOFC materials, EB-PVD process is prospective to be an attractive alternative to traditional powder technology. But on the other hand, research work about applying EB-PVD to prepare SOFC solid electrolyte coatings was seldom done by far and mainly limited in aspect of the influences of process parameters on the microstructure of deposited coatings, especially when the preparation was performed at a low deposition rate and using electron guns with low power [11,12]. Due to the intrinsic characteristics of EB-PVD, non-equilibrium condensation of vapor phase will lead to the development of microporous structure according to so-called ‘‘shadowing effect’’ [13]. This microporous
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structure will have a profound influence on the coating properties, such as the electrical conductivity and gas tightness. It is important to note that these two properties are critical to determine the cell performances when this electrolyte coating is applied in SOFCs. However, these properties of EB-PVD electrolyte coatings have seldom been investigated in past research work. Cubic zirconia stablilized by 8 mol% yttria (8YSZ) is commonly used as solid electrolyte for SOFCs due to its attractive ionic conductivity at high operating temperature and good thermal and chemical stability in both oxidizing and reducing environments [8]. So in this work, the 8YSZ coating was prepared by EB-PVD technique at a high deposition rate of 1 mm/min. Then the microstructure of this coating was characterized and the properties, such as electrical conductivity and gas tightness, were measured.
the silver glue was pasted and sintered at 850 8C as electrodes. When the electrical conductivity in the direction parallel to coating surface was measured, a quartz plate was used as the substrate because the electrical resistance of quartz was much high than that of YSZ in the measuring temperature range. At the same time, in order to measure the electrical conductivity in the direction perpendicular to coating surface, a metal sheet of 316L stainless steel was used as the substrate in hydrogen atmosphere because its electrical conductivity was six orders higher than that of the YSZ coating so that the electrical resistance from metal substrate could be ignored during this measurement. For the measurements of electrical conductivities in both directions, the YSZ electrolyte coatings had the same thickness of 11 mm. The detailed electrode configurations for measurements of electrical conductivities in both directions were shown in Fig. 1.
2. Experimental
2.4. Evaluation of gas tightness
2.1. Preparation of as-deposited coatings
The gas tightness of the coating was evaluated using the apparatus just as shown in Fig. 2. In order to confirm the gas tightness of this testing system, a gas-tight glass plate was firstly adhered to the cross-section of the quartz tube by a silica sealant and this system was evacuated to less than 103 Pa by a mechanical pump. Then the valve was shut off and the pressure difference between ambient atmosphere and inside this testing system kept constant within 24 h. So the absence of gas leakage either through sealed junction or other connections was confirmed. Then the coating sample was adhered to the quartz tube by the same sealant and the same vacuum state in this testing system was achieved before the valve was cut off. Due to the air leakage through the coating, the pressure difference between this system and atmosphere would change with time. According to Darcy’s law [14]:
A large-scale EB-PVD equipment of L5 type was employed to prepare YSZ electrolyte coatings on porous NiO-8YSZ substrates at a high deposition rate of up to 1 mm/min, via evaporating ceramic powders of 8YSZ. The porous substrates with the composition of 56 wt.% of NiO and 44 wt.% of 8YSZ were prepared by tape-casting method. Pore-formers were added in to increase the porosity of the substrates and a total porosity of 30% was obtained after a reduction in hydrogen at 800 8C for 2 h. A radiation heater was adopted to heat the substrates because traditional heating method by electron beam had a high energy density and often resulted in too fast rate of rise in temperature and cracks of ceramic substrates. Due to the limited power of the radiation heater adopted in this study, a substrate temperature, 750 8C, was achieved during coating deposition. In this EB-PVD equipment, the magnetic-field-bending-type electron guns were replaced by the Pierce-type electron guns. An accelerating voltage of up to 20 kV between the cathode and evaporation target provided the equipment with a higher power, leading to a higher deposition rate during coating preparation. The deposition was performed in a high vacuum of less than 102 Pa by a turbo-molecular pump and the distance between the substrate and evaporation target was 250 mm. During preparation, the deposition rate could be adjusted by changing the currents of electron beams.
Q ¼
kAD p
d
(1)
where Q is the gas leakage flux (cm3/s), k is the gas permeability of the as-deposited coating (cm4 N1 s1), A is the area of the coating (cm2), Dp is the pressure difference between this system and atmosphere (N cm2), d is the thickness of the coating (cm).
2.2. Microstructure analysis After coating preparation, the phase composition of this coating was characterized by Philips X’Pert diffractometer (XRD) with parallel Cu Ka radiation. The surface and cross-sectional morphologies were observed by scanning electron microscopy (SEM) with a Hitachi S4700. Before the transmission electron microscopy (TEM) analysis was conducted using a Philips EM420 TEM, two coating samples were firstly adhered face to face and then inserted into a copper tube with a diameter of 3 mm. Then a slice was cut down from this tube and was thinned in the direction perpendicular to the cross-section of the coating by mechanical grinding combined with Ar ion etching process. The element distribution of the coating was characterized by electron probe microanalysis (EPMA). 2.3. Measurement of electrical conductivity The electrical conductivity of YSZ coating was measured by direct current four-probe method in the temperature range from 500 to 800 8C. In order to determine the area of working electrodes,
Fig. 1. Electrode configurations for measurements of electrical conductivities of the YSZ coating in two directions: (a) in direction parallel to coating surface, (b) in direction perpendicular to coating surface.
X. He et al. / Applied Surface Science 254 (2008) 7159–7164
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Fig. 2. Schematic of the apparatus evaluating the gas tightness of electrolyte coating.
3. Results and discussion
panied by a significant volume expansion, which often leads to cracks and even a failure of the coating. A further observation from Fig. 3 demonstrates that there is an obvious shift for the diffraction peak positions between the asdeposited and annealed coatings. In fact, the diffraction peaks shift to low angles after annealing treatment. Due to the differences in thermal expansion coefficients (TEC) between the NiO-YSZ substrate and YSZ coating (for example, the mean TEC of NiOYSZ substrate is 13.0 106 K1, while that of YSZ coating is 10.8 106 K1), a compressive stress is formed during preparation course. After annealing, the compressive stress is relieved and this leads to an increment in crystal cell parameters of YSZ coating. So a shift of the diffraction peaks to low angles appears after annealing.
3.1. Phase compositions
3.2. Surface and cross-sectional morphologies
Considering the higher operation temperature of more than 850 8C for traditional SOFCs using YSZ coatings as electrolytes, the as-deposited coating was treated by annealing in air at 1000 8C for 2 h combined with a furnace cooling in order to confirm the stability of phase composition. Therefore, the phase compositions of the as-deposited and annealed coatings are analyzed and shown in Fig. 3 together. It can be found from this analysis that the XRD patterns of both as-deposited and annealed coatings all show the lines of cubic fluorite-type structure and no lines of tetragonal or monoclinic phase are observed. So no phase transformation occurs during the course of annealing treatment and this is beneficial for the volumetric and thermal stability of the coating because the phase transformation from tetragonal to monoclinic is accom-
The surface and cross-sectional morphologies of the asdeposited coating are shown in Fig. 4. The surface SEM image exhibits there are many micron-sized particles in the coating surface. Moreover, every particle consists of many crystal clusters, which originates from the crystal grain growth in mode of threedimensional islands during coating deposition. Between the particle boundaries, there are many gaps and pores, which will have a significant effect on the gas tightness and electrical conductivity of this coating. A further cross-sectional SEM observation shows that the coating, with a thickness of 11 mm, presents a good adhesion to the substrate and a characteristic columnar structure normal to the substrate. The diameters of columnar grains are in the range from several microns to 10 mm. In the magnified cross-sectional image of Fig. 4(c), the columnar grains exhibit feather-like characteristics and some distinct gaps as well as pores exist between the boundaries of columnar grains. According to the growth pattern of coatings prepared by EBPVD, which was presented by Movchan and Demchishin, the substrate temperature has a significant influence on the coating growth pattern and morphology. Because of the adoption of a radiation heater and its limited power, the substrate temperature, 750 8C, is the highest temperature for this equipment, which is slightly higher than one-third of the melting point of YSZ material. So a characteristic columnar structure forms in the coating.
Then the total gas volume charged from leakage during time from t0 to t can be expressed as: Z Z t kA t Q dt ¼ D pðtÞ dt (2) t0
d
t0
Consequently, when the change of the pressure difference with time is measured, the gas permeability can be calculated according to the following formula: R d tt Q dt k ¼ Rt 0 (3) A t0 D pðtÞ dt
3.3. TEM observation
Fig. 3. XRD patterns of the as-deposited and annealed YSZ coatings.
Fig. 5 shows a bright-field TEM image of the YSZ coating, in which many white lines are observed in the YSZ grain in a certain direction. According to the SEM image in Fig. 4(c), a feather-like structure forms in the columnar grain of the YSZ coating. In the edge and inner of the feather-like grains, many nanopores are
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Fig. 4. Surface and cross-sectional morphologies of the as-deposited coating: (a) surface morphology, (b) cross-sectional morphology, (c) magnified cross-sectional morphology.
created due to the secondary shadowing effect. These pores are mainly controlled by the incidence angle of vapor flow and aligned in a certain direction, so the white lines are observed in the TEM image. Nanopores in the YSZ coating are considered to have a
profound influence on coating properties, such as thermal conductivity and electrical conductivity. These pores, which have a lower thermal or electrical conductivity compared with the matrix, decrease the mean thermal or electrical conductivity and alter the conduction paths of photons or electrical charges. So it is necessary to control the quantity and distribution of the nanopores in order to obtain ideal coating properties. 3.4. Element distribution
Fig. 5. A bright-field TEM image of the YSZ coating.
The element distribution of YSZ coating was measured by the method of EPMA and shown in Fig. 6. The element distribution along the cross-section of the coating is homogeneous except Zr with a slight gradient. Due to the selective evaporation behavior for EB-PVD process, the component with higher saturation evaporation pressure at the same temperature will be evaporated preferentially when multi components are evaporated simultaneously in one crucible. Considering the yttria with a higher saturation evaporation pressure compared with zirconia, the yttria will be preferentially evaporated from the ingot at a higher evaporation rate at the beginning. This will lead to an enrichment of zirconia in the melting pool, which results in an increment of evaporation rate for zirconia unless an evaporation rate balance is achieved. As a result, the element of zirconium exhibits a slight gradient distribution and the concentration of zirconium at the interface between the coating and substrate is slightly lower than
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Fig. 6. Element distribution of YSZ coating by EPMA.
that at the coating surface. Due to a low concentration of yttrium in the coating, a slight variation in yttrium content along the crosssection of the coating is difficult to be detected by EPMA. So the element of yttrium shows a uniform distribution relative to zirconium. 3.5. Electrical conductivity Electrical conductivities of the YSZ coating in temperature range from 500 to 800 8C were measured in both directions perpendicular and parallel to coating surface and shown in Fig. 7. Obviously, the electrical conductivity in the direction perpendicular to coating surface is remarkably higher than that in the direction parallel to coating surface. For example, the electrical conductivity in the direction parallel to coating surface at 800 8C is lower than half of that in the direction perpendicular to coating surface. The anisotropy and significant differences in the electrical conductivities between these two directions originate from the typical columnar structure characteristics for EB-PVD coating. When the electrical charges are transferred in the direction parallel to coating surface, there are many gaps and pores in the columnar
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Fig. 8. Arrhenius plots and the activation energies in both directions parallel and perpendicular to coating surface.
grain boundaries along the transmission paths. On the contrary, in the direction perpendicular to coating surface, many columnar grains penetrate through the coating to form parallel transmission paths, with fewer grain boundaries and defects. Therefore, the electrical conductivity in the direction perpendicular to coating surface is much higher than that in the direction parallel to coating surface. During the YSZ electrolyte operation in SOFCs, the oxygen ions are transferred in the direction perpendicular to coating surface, so a higher electrical conductivity is prospective to be obtained by EB-PVD process. The typical columnar structure for EB-PVD coating also determines a remarkable difference in the activation energies for ion transmission between these two directions. Fig. 8 shows the Arrhenius plots and the activation energies for ion transmission in both directions. For example, the activation energy for ion conduction in the direction perpendicular to coating surface is 0.84 eV, while that in the direction parallel to coating surface is 1.14 eV. Furthermore, an electrical conductivity of 2.6 S m1 is obtained in the direction perpendicular to coating surface at 800 8C for the as-deposited coating. According to the result reported by Yamamoto and Ivers-Tiffee [15,16], 8YSZ bulk shows an electrical conductivity from 3.5 to 4.2 S m1 at 800 8C. So the electrical conductivity of as-deposited coating in direction perpendicular to coating surface is still lower than that of 8YSZ bulk. This can be explained by the lower density of YSZ coating compared with YSZ bulk and the existence of nanopores within the feather-like columnar grains. The existence of nanopores reduces the area of the transmission paths, resulting in a lower electrical conductivity. 3.6. Gas tightness
Fig. 7. Electrical conductivities of YSZ coating measured in both directions parallel and perpendicular to coating surface.
The gas tightness of YSZ electrolyte coating was evaluated employing the apparatus in Fig. 2. The change of pressure difference between this testing system and atmosphere with time is illustrated in Fig. 9. It can be found that the pressure difference decreases rapidly and the pressure balance is achieved within 280s. The gas leakage through EB-PVD coating mainly originates from the existence of gaps and pores in the columnar grain boundaries, just as shown in the Fig. 4(c). Those nanopores within feather-like columnar grains, especially those closed discontinuous nanopores, have little influence on the gas tightness of this coating. According to an accurate calculation, the coefficient of gas permeability for as-deposited coating is 9.78 105 cm4 N1 s1.
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Fig. 9. The change of pressure difference between this testing system and atmosphere with time and gas permeability coefficient of the as-deposited coating.
Although there is an obvious gas leakage for as-deposited coating, this value of gas permeability coefficient is remarkably lower than that of the coating prepared by atmospheric plasma spraying (APS). Li et al. had reported that the value of gas permeability coefficient, 2.96 104 cm4 N1 s1, was obtained by APS for tubular SOFC. There has been some argument about the critical value of gas permeability of YSZ electrolyte layer required for SOFC operation [17]. Some investigations have demonstrated that the value should be less than 1.02 104 cm4 N1 s1 [12,18]. In this study, the value of gas permeability coefficient is close to the critical value so that the normal operation and long-term stability for SOFCs can not be ensured. So a post densification treatment, such as heat treatment or a sol infiltration, is suggested to improve the gas tightness of EB-PVD coating. 4. Conclusions YSZ electrolyte coating with a thickness of 11 mm was prepared by EB-PVD process at a high deposition rate of up to 1 mm/min. The YSZ coating consisted of a single cubic phase and no phase transformation occurred after annealing treatment at 1000 8C. A typical columnar structure was formed in this coating and featherlike characteristics appeared in every columnar grain. In the columnar grain boundaries there were many micron-sized gaps
and pores. Many white lines were found in TEM observation, which originated from the alignment of nanopores existing within feather-like columnar grains. The element distribution along the cross-section of the coating was homogeneous except Zr with a slight gradient due to the selective evaporation behavior for EBPVD process. The coating exhibited a characteristic anisotropic behavior in electrical conductivity. The value of electrical conductivity in the direction perpendicular to coating surface was remarkably higher than that in the direction parallel to coating surface. This mainly attributed to the typical columnar structure characteristics for EBPVD coating and the existence of many grain boundaries along the direction parallel to coating surface. Moreover, the electrical conductivity of as-deposited coating in the direction perpendicular to coating surface was still lower than that of 8YSZ bulk due to the existence of many nanopores within the feather-like columnar grains. For the as-deposited coating, the gas permeability coefficient of 9.78 105 cm4 N1 s1 was obtained and this value was close to the critical value of YSZ electrolyte layer required for SOFC operation. Acknowledgement The authors gratefully acknowledge the financial assistance from the New Century Excellent Talent Plan of China (NCET2004). References [1] C. Brahim, A. Ringuede´, M. Cassir, M. Putkonen, L. Niinisto¨, Appl. Surf. Sci. 253 (2007) 3962–3968. [2] P. Charpentier, P. Fragnaud, D.M. Schleich, E. Gehain, Solid State Ionics 135 (2000) 373. [3] J.R. Kong, K.N. Sun, D.R. Zhou, N.Q. Zhang, J. Mu, J.S. Qiao, J. Power Sources 166 (2007) 337. [4] L.R. Pederson, P. Singh, X.D. Zhou, Vacuum 80 (2006) 1066. [5] N. Pryds, B. Toftmann, J.B. Bilde-Sørensen, J. Schou, S. Linderoth, Appl. Surf. Sci. 252 (2006) 4882–4885. [6] J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, L.J. Gauckler, Solid State Ionics 131 (2000) 79. [7] G. Laukaitis, J. Dudonis, D. Milcius, Appl. Surf. Sci. 254 (2008) 2980–2987. [8] J. Yi, X.D. He, Y. Sun, Y. Li, Appl. Surf. Sci. 253 (2007) 4361–4366. [9] B.A. Movchan, K.Y. Yakovchuk, Surf. Coat. Technol. 188–189 (2004) 85. [10] B.A. Movchan, F.D. Lemkey, Surf. Coat. Technol. 165 (2003) 90. [11] G. Laukaitis, J. Dudonis, D. Milcius, Thin Solid Films 515 (2006) 678. [12] G. Laukaitis, J. Dudonis, D. Milcius, Vacuum 81 (2007) 1288–1291. [13] B.A. Movchan, Surf. Eng. 22 (1) (2006) 35–46. [14] C.J. Li, X.J. Ning, C.X. Li, Surf. Coat. Technol. 190 (2005) 60. [15] E. Ivers-Tiffee, A. Weber, D. Herbstritt, J. Eur. Ceram. Soc. 21 (2001) 1805. [16] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [17] X.J. Ning, C.X. Li, C.J. Li, G.J. Yang, Mater. Sci. Eng. A 428 (2006) 98. [18] C.J. Li, C.X. Li, X.J. Ning, Vacuum 73 (2004) 699.