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Highly enhanced infrared spectral emissivity of porous CeO2 coating
Materials Letters 85 (2012) 57–60
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Highly enhanced infrared spectral emissivity of porous CeO2 coating Jianping Huang, Yibin Li n, Guangping Song, Xinjiang Zhang, Yue Sun, Xiaodong He, Shanyi Du Center for Composite Materials and Structures, Harbin Institute of Technology, 150080 Harbin, China
a r t i c l e i n f o
abstract
Article history: Received 22 May 2012 Accepted 23 June 2012 Available online 2 July 2012
A porous CeO2 coating was prepared by electron beam physical vapor deposition technique. The microstructure and infrared spectral emissivity in wavelength range 2.5 25 mm were investigated. The surface morphology could be tuned by deposition power density. The as-deposited coating shows (311)-preferred orientation. The great enhancement of infrared spectral emissivity in the whole wavelength range was observed in the porous CeO2 coating, compared to a dense CeO2 coating. The porous surface structure provides multiple reflection and absorption, thus improves the spectral emissivity. Moreover, the CeO2 coating also shows excellent adhesion property to nickel-based alloy substrate. These results show a potential application at high temperature. & 2012 Elsevier B.V. All rights reserved.
Keywords: Adhesion Cerium oxide Spectral emissivity Physical vapor deposition Porous materials
1. Introduction High emissivity coatings have attracted intensive interest to be used in high temperature alloys as thermal protective coatings [1]. Especially, considerable attention has been paid to the development of high infrared emissivity coatings [2–5]. In real application, the adhesion between thermal protective coating and the high temperature alloy should be very strong. Otherwise, it could not meet the critical thermal shock requirement. However, the conventional deposition methods, like dipping, brushing or spraying are susceptible to damage in high temperature service condition for the poor adhesion between ceramic coatings and metal substrates. Electron beam physical vapor deposition (EB-PVD) has been extensively used to prepare advanced thermal barrier coatings (TBC) for components of gas turbines. The EB-PVD derived TBC coatings exhibited excellent thermal shock resistance due to their columnar structures [6]. CeO2 is a promising candidate of such application for its extraordinary high thermal expansion coefficient, about 11.8 10 6 K 1 at 800 K [7], largely reducing the difference to that of the Ni based alloys (12 18 10 6 K 1 at 800 K) [8,9]. CeO2 coating has been investigated as protective material for various Ni-, Co- and Al-based alloys [10,11]. In the past, much more attention has been focused on optical properties of CeO2 coatings. The CeO2 coatings show high absorption in ultraviolet region and high transparency in visible and near infrared (near-IR) regions [12,13]. In this letter, we report the spectral emissivity of CeO2 coatings deposited by EB-PVD method and have found the spectral emissivity could be tuned by
manipulating the surface morphology of the coating. The porous CeO2 coating shows the enhanced spectral emissivity than the dense one.
2. Experimental procedure The CeO2 coatings were deposited on Ni based alloy (Haynes214) by EB-PVD technique. Pure CeO2 powder (99.99% in purity) was pressed into billets and sintered at 1573 K for 2 h as targets. The substrates were polished and then cleaned in ultrasonic bath before deposition. The target-to-substrate distance was 160 mm. The depositions were carried out at electron gun power of 3 kW (sample 1#) and 5 kW (sample 2#) with deposition rates of 0.6 mm/min for 12 min and 1.14 mm/min for 6 min, respectively. During the whole deposition process, the substrate temperature was maintained at 1173 K, the chamber pressure 2 3 10 2 Pa and the accelerating voltage 20 kV. The crystal structures were examined by X-ray diffraction (XRD, Rigaku D/Max 2200) with Cu Ka radiation. The surface and cross-section morphologies were observed by scanning electron microscope (SEM, Hitachi S1000). The chemical states of as-deposited coatings were analyzed by X-ray photoelectron spectra (XPS, K-Alpha) using Al Ka radiation at 1486.6 eV. The normal spectral emissivity in the wavelength range from 2.5 to 25 mm was measured by Fourier transform infrared spectrometer (JASCO FTIR-6100) at 873 K, 1073 K and 1273 K, respectively.
3. Results and discussion n
Corresponding author. Tel./fax: þ 86 451 86402326. E-mail address: [email protected] (Y. Li).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.084
Fig. 1 shows the XRD patterns of the CeO2 powder and coatings. All diffraction peaks can be indexed as the cubic fluorite
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J. Huang et al. / Materials Letters 85 (2012) 57–60
˚ which are consistent structure CeO2 with lattice constant 5.410 A, with the values in the standard card (JCPDS 34-0394). By comparing with diffraction pattern of CeO2 powder, the coatings show (311)preferred orientation. Texture coefficient (TC) f(311)¼87.42% for sample1# and f(311)¼59.84% for sample 2#, as calculated based on P f ðhklÞ ¼ ððIhkl =I0hkl Þ= ðIhkl =I0hkl ÞÞ [14], where Ihkl is the peak intensity
for the (hkl) reflection of CeO2 coating and I0hkl is the standard peak intensity of CeO2. This indicates that CeO2 coatings grew quickly along o311 4 direction during deposition, but the preferred growth direction was challenged by other directions through competition at higher deposition power. The deposition power determines the kinetic energy of the evaporated gas and intensively
Fig. 1. XRD patterns of CeO2 powder and coatings by EB-PVD (a, pure CeO2 powder; b, sample 1# deposited at 3 kW; c, sample 2# deposited at 5 kW).
Fig. 2. Surface and cross-section SEM images of CeO2 coatings, (a) low-magnification surface image of sample 1#, (b) high-magnification surface image of sample 1#, (c) cross-section image of sample 1#, (d) low-magnification surface image of sample 2#, (e) high-magnification surface image of sample 2#, (f) cross-section image of sample 2#.
J. Huang et al. / Materials Letters 85 (2012) 57–60
59
affects the growth mechanism of the coating. When the deposition power is low, the kinetic energy of adatoms or clusters is not high enough to diffuse or coalesce adequately. Therefore, the crystal planes with higher interfacial free energy, such as plane (311), may grow faster and form (311) texture. So the sample 1# deposited at 3 kW have stronger (311) texture than that of sample 2# deposited at 5 kW. Fig. 2 shows the surface and cross-section SEM images of the CeO2 coatings. Fig. 2(a) reveals that a large number of pores distribute uniformly on the surface of sample 1#. The higher magnification photograph in Fig. 2(b) shows the porous structure. The CeO2 grains show the laminated pyramidal shape. The root mean square roughness measurement by laser confocal microscopy demonstrates that the porous CeO2 coating has a roughness of 0.31 mm. When the electron beam power increases to 5 kw, the surface in Fig. 2(d) is dense and also shows the pyramid-like morphology (0.4 0.5 mm in size), which can be clearly seen in the higher magnification microscope in Fig. 2(e). The sample 2# roughness is 0.23 mm, slightly lower than the porous sample 1#. Fig. 2(c) and (f) shows cross-sectional images of CeO2 coatings. Sample 1# in Fig. 2(c) reveals the feather-like columnar structure with thickness 7.2 mm. The pores exhibit many cavities among the columns. On the contrary, sample 2# in Fig. 2(f) shows the square columnar structure with thickness 6.8 mm. The surface microstructure and cross-section columnar structure benefits the thermal shock resistance [6]. The initial thermal cycling testing results show that the CeO2 coatings present excellent thermal shock resistance, which is evident that no spallation occurs when the coatings suffer from 480 thermal cycles between 1273 K and room temperature. The Ce3 þ and oxygen vacancies may improve the free carrier absorption, and thus improve the emissivity of CeO2 coatings [15]. The Ce chemical states was checked by X-ray photoelectron spectroscope as shown in Fig. 3. The Ce3 þ and Ce4 þ percentages are evaluated by equation [16]: Cei þ % ¼ SACei þ =ðSACe3 þ þ SACe4 þ Þ, where ACei þ (i¼3 or 4) denotes the total intensity of peaks by area in the Ce3d component relevant to Cei þ . The calculation reveals that 15.3% Ce3 þ and 84.7% Ce4 þ exist in sample 1#, while 17.5% Ce3 þ and 82.5% Ce4 þ in sample 2#. The oxygen vacancy concentration in the coatings could be deduced by: 2CexCe þ OxO ¼ 2Ce0Ce þ V 00o þ1=2O2 , where Ce0Ce is reduced Ce3 þ and V 00o is the oxygen vacancy. So oxygen vacancy concentration is half of trivalent Ce3 þ . That is to say, the Ce3 þ and oxygen vacancy concentration in both CeO2 coatings have little differences. The free carrier absorption in both coatings has almost the same contribution to the infrared emissivity of both coatings. Fig. 4 shows the normal spectral emissivity curves of both CeO2 coatings in mid-infrared (2.5 25 mm) at 873 K, 1073 K and 1273 K, Fig. 4. Spectral emissivity of CeO2 coatings at 873 K (a), 1073 K (b), and 1273 K (c).
Fig. 3. Ce3d XPS spectra of CeO2 coatings (Ce3 þ peak fitting spectra are marked . and Ce4 þ peak fitting spectra are marked ’).
respectively. The spectral emissivity of sample 1# is obviously much higher than that of sample 2#, especially in low wavelength band. The total spectral emissivity is calculated by equation: R R e ¼ ll12 eðlÞMb ðlÞdl= ll12 Mb ðlÞdl, (where, e is the emissivity, l is the wavelength and Mb(l) is the blackbody emission spectrum) [3]. The results shown that the total emissivity of sample 1# is 0.784 at 873 K, 0.871 at 1073 K and 0.643 at 1273 K, about 30 50% higher than those (0.595, 0.584 and 0.466) of sample 2# at the same temperature. The total emissivity of porous CeO2 coating is close to that of the carbon nanotubes doped SiO2/SiO2–PbO double layer [3]. Emissivity is influenced by many factors, such as the composition, thickness, free carrier concentration, temperature, etc. In this work, as confirmed above, the composition, thickness (7.2 vs. 6.8 mm) and free carriers (Ce3 þ and O vacancies) concentrations
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J. Huang et al. / Materials Letters 85 (2012) 57–60
are comparable for both coatings. So it is reasonable to neglect the effects of these factors on the spectral emissivity of the coatings. The great emissivity enhancement of sample 1# is mainly attributed to its porous structure, as seen in Fig. 2. It was reported that the porosity had the positive effect on the emissivity in Si, SiO2 and Al2O3 materials [3,4,17]. The pores in the CeO2 coating are similar to blackbody cavities. The electromagnetic waves, especially infrared rays from thermal emission, are multiply reflected in the pores and strongly absorbed by CeO2. According to Kirchhoff’s law, a good absorber is also a good emitter. Therefore, the multiple reflection and absorption greatly enhance the emissivity of CeO2 coatings. 4. Conclusions The CeO2 coatings were prepared on Ni based substrate by electron beam physical vapor deposition technique. The electron beam power has a great effect on surface morphology of CeO2 coating. The lower power results in porous surface of CeO2 coating. Compared to dense surface of CeO2 coating, the porous coating shows highly enhanced infrared spectral emissivity in the wavelength range 2.5 25 mm at high temperatures. These results show that the CeO2 coatings have the promising application in the high temperature alloys.
Acknowledgments This research is supported by National Natural Science Foundation of China (Grant No. 90816005).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
He XD, Li YB, Wang LD, Sun Y, Zhang S. Thin Solid Films 2009;517:5120–9. Liu HZ, Liu ZG, Ouyang JH, Wang YM. Mater Lett 2011;65:2614–7. Zhao X, He XD, Sun Y, Wang LD. Mater Lett 2011;65:2592–4. Feng GJ, Wang Y, Li YA, Zhu JT, Zhao L. Mater Lett 2011;65:1238–40. Zolotarev VM. Opt Spectrosc 2007;103:592–602. Wada K, Yamaguchi N, Matsubara H. Surf Coat Technol 2004;184:55–62. Hayashi H, Kanoh M, Quan CJ, Inaba H, Wang SR, Dokiya M, et al. Solid State Ionics 2000;132:227–33. Pavlovic AS, Babu VS, Seehra MS. J Phys—Condens Matter 1996;8:3139–49. Liu Y, Zhu J. Int J Hydrogen Energy 2010;35:7936–44. Haugsrud R. Corros Sci 2002;44:1569–82. Hamdy AS. Mater Lett 2006;60:2633–7. Elidrissi B, Addou M, Regragui M, Monty C, Bougrine A, Kachouane A. Thin Solid Films 2000;379:23–7. Tsunekawa S, Wang JT, Kawazoe Y, Kasuya A. J Appl Phys 2003;94:3654–6. Goto T, Zhao P, Ito A, Tu R. Surf Coat Technol 2010;204:3619–22. Rogne H, Timans PJ, Ahmed H. Appl Phys Lett 1996;69:2190–2. Truffault L, Ta MT, Devers T, Konstantinov K, Harel V, Simmonard C, et al. Mater Res Bull 2010;45:527–35. Rozenbaum O, Meneses DD, Echeggut P. Int J Thermophys 2009;30:580–90.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Highly enhanced infrared spectral emissivity of porous CeO2 coating Jianping Huang, Yibin Li n, Guangping Song, Xinjiang Zhang, Yue Sun, Xiaodong He, Shanyi Du Center for Composite Materials and Structures, Harbin Institute of Technology, 150080 Harbin, China
a r t i c l e i n f o
abstract
Article history: Received 22 May 2012 Accepted 23 June 2012 Available online 2 July 2012
A porous CeO2 coating was prepared by electron beam physical vapor deposition technique. The microstructure and infrared spectral emissivity in wavelength range 2.5 25 mm were investigated. The surface morphology could be tuned by deposition power density. The as-deposited coating shows (311)-preferred orientation. The great enhancement of infrared spectral emissivity in the whole wavelength range was observed in the porous CeO2 coating, compared to a dense CeO2 coating. The porous surface structure provides multiple reflection and absorption, thus improves the spectral emissivity. Moreover, the CeO2 coating also shows excellent adhesion property to nickel-based alloy substrate. These results show a potential application at high temperature. & 2012 Elsevier B.V. All rights reserved.
Keywords: Adhesion Cerium oxide Spectral emissivity Physical vapor deposition Porous materials
1. Introduction High emissivity coatings have attracted intensive interest to be used in high temperature alloys as thermal protective coatings [1]. Especially, considerable attention has been paid to the development of high infrared emissivity coatings [2–5]. In real application, the adhesion between thermal protective coating and the high temperature alloy should be very strong. Otherwise, it could not meet the critical thermal shock requirement. However, the conventional deposition methods, like dipping, brushing or spraying are susceptible to damage in high temperature service condition for the poor adhesion between ceramic coatings and metal substrates. Electron beam physical vapor deposition (EB-PVD) has been extensively used to prepare advanced thermal barrier coatings (TBC) for components of gas turbines. The EB-PVD derived TBC coatings exhibited excellent thermal shock resistance due to their columnar structures [6]. CeO2 is a promising candidate of such application for its extraordinary high thermal expansion coefficient, about 11.8 10 6 K 1 at 800 K [7], largely reducing the difference to that of the Ni based alloys (12 18 10 6 K 1 at 800 K) [8,9]. CeO2 coating has been investigated as protective material for various Ni-, Co- and Al-based alloys [10,11]. In the past, much more attention has been focused on optical properties of CeO2 coatings. The CeO2 coatings show high absorption in ultraviolet region and high transparency in visible and near infrared (near-IR) regions [12,13]. In this letter, we report the spectral emissivity of CeO2 coatings deposited by EB-PVD method and have found the spectral emissivity could be tuned by
manipulating the surface morphology of the coating. The porous CeO2 coating shows the enhanced spectral emissivity than the dense one.
2. Experimental procedure The CeO2 coatings were deposited on Ni based alloy (Haynes214) by EB-PVD technique. Pure CeO2 powder (99.99% in purity) was pressed into billets and sintered at 1573 K for 2 h as targets. The substrates were polished and then cleaned in ultrasonic bath before deposition. The target-to-substrate distance was 160 mm. The depositions were carried out at electron gun power of 3 kW (sample 1#) and 5 kW (sample 2#) with deposition rates of 0.6 mm/min for 12 min and 1.14 mm/min for 6 min, respectively. During the whole deposition process, the substrate temperature was maintained at 1173 K, the chamber pressure 2 3 10 2 Pa and the accelerating voltage 20 kV. The crystal structures were examined by X-ray diffraction (XRD, Rigaku D/Max 2200) with Cu Ka radiation. The surface and cross-section morphologies were observed by scanning electron microscope (SEM, Hitachi S1000). The chemical states of as-deposited coatings were analyzed by X-ray photoelectron spectra (XPS, K-Alpha) using Al Ka radiation at 1486.6 eV. The normal spectral emissivity in the wavelength range from 2.5 to 25 mm was measured by Fourier transform infrared spectrometer (JASCO FTIR-6100) at 873 K, 1073 K and 1273 K, respectively.
3. Results and discussion n
Corresponding author. Tel./fax: þ 86 451 86402326. E-mail address: [email protected] (Y. Li).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.06.084
Fig. 1 shows the XRD patterns of the CeO2 powder and coatings. All diffraction peaks can be indexed as the cubic fluorite
58
J. Huang et al. / Materials Letters 85 (2012) 57–60
˚ which are consistent structure CeO2 with lattice constant 5.410 A, with the values in the standard card (JCPDS 34-0394). By comparing with diffraction pattern of CeO2 powder, the coatings show (311)preferred orientation. Texture coefficient (TC) f(311)¼87.42% for sample1# and f(311)¼59.84% for sample 2#, as calculated based on P f ðhklÞ ¼ ððIhkl =I0hkl Þ= ðIhkl =I0hkl ÞÞ [14], where Ihkl is the peak intensity
for the (hkl) reflection of CeO2 coating and I0hkl is the standard peak intensity of CeO2. This indicates that CeO2 coatings grew quickly along o311 4 direction during deposition, but the preferred growth direction was challenged by other directions through competition at higher deposition power. The deposition power determines the kinetic energy of the evaporated gas and intensively
Fig. 1. XRD patterns of CeO2 powder and coatings by EB-PVD (a, pure CeO2 powder; b, sample 1# deposited at 3 kW; c, sample 2# deposited at 5 kW).
Fig. 2. Surface and cross-section SEM images of CeO2 coatings, (a) low-magnification surface image of sample 1#, (b) high-magnification surface image of sample 1#, (c) cross-section image of sample 1#, (d) low-magnification surface image of sample 2#, (e) high-magnification surface image of sample 2#, (f) cross-section image of sample 2#.
J. Huang et al. / Materials Letters 85 (2012) 57–60
59
affects the growth mechanism of the coating. When the deposition power is low, the kinetic energy of adatoms or clusters is not high enough to diffuse or coalesce adequately. Therefore, the crystal planes with higher interfacial free energy, such as plane (311), may grow faster and form (311) texture. So the sample 1# deposited at 3 kW have stronger (311) texture than that of sample 2# deposited at 5 kW. Fig. 2 shows the surface and cross-section SEM images of the CeO2 coatings. Fig. 2(a) reveals that a large number of pores distribute uniformly on the surface of sample 1#. The higher magnification photograph in Fig. 2(b) shows the porous structure. The CeO2 grains show the laminated pyramidal shape. The root mean square roughness measurement by laser confocal microscopy demonstrates that the porous CeO2 coating has a roughness of 0.31 mm. When the electron beam power increases to 5 kw, the surface in Fig. 2(d) is dense and also shows the pyramid-like morphology (0.4 0.5 mm in size), which can be clearly seen in the higher magnification microscope in Fig. 2(e). The sample 2# roughness is 0.23 mm, slightly lower than the porous sample 1#. Fig. 2(c) and (f) shows cross-sectional images of CeO2 coatings. Sample 1# in Fig. 2(c) reveals the feather-like columnar structure with thickness 7.2 mm. The pores exhibit many cavities among the columns. On the contrary, sample 2# in Fig. 2(f) shows the square columnar structure with thickness 6.8 mm. The surface microstructure and cross-section columnar structure benefits the thermal shock resistance [6]. The initial thermal cycling testing results show that the CeO2 coatings present excellent thermal shock resistance, which is evident that no spallation occurs when the coatings suffer from 480 thermal cycles between 1273 K and room temperature. The Ce3 þ and oxygen vacancies may improve the free carrier absorption, and thus improve the emissivity of CeO2 coatings [15]. The Ce chemical states was checked by X-ray photoelectron spectroscope as shown in Fig. 3. The Ce3 þ and Ce4 þ percentages are evaluated by equation [16]: Cei þ % ¼ SACei þ =ðSACe3 þ þ SACe4 þ Þ, where ACei þ (i¼3 or 4) denotes the total intensity of peaks by area in the Ce3d component relevant to Cei þ . The calculation reveals that 15.3% Ce3 þ and 84.7% Ce4 þ exist in sample 1#, while 17.5% Ce3 þ and 82.5% Ce4 þ in sample 2#. The oxygen vacancy concentration in the coatings could be deduced by: 2CexCe þ OxO ¼ 2Ce0Ce þ V 00o þ1=2O2 , where Ce0Ce is reduced Ce3 þ and V 00o is the oxygen vacancy. So oxygen vacancy concentration is half of trivalent Ce3 þ . That is to say, the Ce3 þ and oxygen vacancy concentration in both CeO2 coatings have little differences. The free carrier absorption in both coatings has almost the same contribution to the infrared emissivity of both coatings. Fig. 4 shows the normal spectral emissivity curves of both CeO2 coatings in mid-infrared (2.5 25 mm) at 873 K, 1073 K and 1273 K, Fig. 4. Spectral emissivity of CeO2 coatings at 873 K (a), 1073 K (b), and 1273 K (c).
Fig. 3. Ce3d XPS spectra of CeO2 coatings (Ce3 þ peak fitting spectra are marked . and Ce4 þ peak fitting spectra are marked ’).
respectively. The spectral emissivity of sample 1# is obviously much higher than that of sample 2#, especially in low wavelength band. The total spectral emissivity is calculated by equation: R R e ¼ ll12 eðlÞMb ðlÞdl= ll12 Mb ðlÞdl, (where, e is the emissivity, l is the wavelength and Mb(l) is the blackbody emission spectrum) [3]. The results shown that the total emissivity of sample 1# is 0.784 at 873 K, 0.871 at 1073 K and 0.643 at 1273 K, about 30 50% higher than those (0.595, 0.584 and 0.466) of sample 2# at the same temperature. The total emissivity of porous CeO2 coating is close to that of the carbon nanotubes doped SiO2/SiO2–PbO double layer [3]. Emissivity is influenced by many factors, such as the composition, thickness, free carrier concentration, temperature, etc. In this work, as confirmed above, the composition, thickness (7.2 vs. 6.8 mm) and free carriers (Ce3 þ and O vacancies) concentrations
60
J. Huang et al. / Materials Letters 85 (2012) 57–60
are comparable for both coatings. So it is reasonable to neglect the effects of these factors on the spectral emissivity of the coatings. The great emissivity enhancement of sample 1# is mainly attributed to its porous structure, as seen in Fig. 2. It was reported that the porosity had the positive effect on the emissivity in Si, SiO2 and Al2O3 materials [3,4,17]. The pores in the CeO2 coating are similar to blackbody cavities. The electromagnetic waves, especially infrared rays from thermal emission, are multiply reflected in the pores and strongly absorbed by CeO2. According to Kirchhoff’s law, a good absorber is also a good emitter. Therefore, the multiple reflection and absorption greatly enhance the emissivity of CeO2 coatings. 4. Conclusions The CeO2 coatings were prepared on Ni based substrate by electron beam physical vapor deposition technique. The electron beam power has a great effect on surface morphology of CeO2 coating. The lower power results in porous surface of CeO2 coating. Compared to dense surface of CeO2 coating, the porous coating shows highly enhanced infrared spectral emissivity in the wavelength range 2.5 25 mm at high temperatures. These results show that the CeO2 coatings have the promising application in the high temperature alloys.
Acknowledgments This research is supported by National Natural Science Foundation of China (Grant No. 90816005).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
He XD, Li YB, Wang LD, Sun Y, Zhang S. Thin Solid Films 2009;517:5120–9. Liu HZ, Liu ZG, Ouyang JH, Wang YM. Mater Lett 2011;65:2614–7. Zhao X, He XD, Sun Y, Wang LD. Mater Lett 2011;65:2592–4. Feng GJ, Wang Y, Li YA, Zhu JT, Zhao L. Mater Lett 2011;65:1238–40. Zolotarev VM. Opt Spectrosc 2007;103:592–602. Wada K, Yamaguchi N, Matsubara H. Surf Coat Technol 2004;184:55–62. Hayashi H, Kanoh M, Quan CJ, Inaba H, Wang SR, Dokiya M, et al. Solid State Ionics 2000;132:227–33. Pavlovic AS, Babu VS, Seehra MS. J Phys—Condens Matter 1996;8:3139–49. Liu Y, Zhu J. Int J Hydrogen Energy 2010;35:7936–44. Haugsrud R. Corros Sci 2002;44:1569–82. Hamdy AS. Mater Lett 2006;60:2633–7. Elidrissi B, Addou M, Regragui M, Monty C, Bougrine A, Kachouane A. Thin Solid Films 2000;379:23–7. Tsunekawa S, Wang JT, Kawazoe Y, Kasuya A. J Appl Phys 2003;94:3654–6. Goto T, Zhao P, Ito A, Tu R. Surf Coat Technol 2010;204:3619–22. Rogne H, Timans PJ, Ahmed H. Appl Phys Lett 1996;69:2190–2. Truffault L, Ta MT, Devers T, Konstantinov K, Harel V, Simmonard C, et al. Mater Res Bull 2010;45:527–35. Rozenbaum O, Meneses DD, Echeggut P. Int J Thermophys 2009;30:580–90.