- Email: [email protected]
Investigation of microstructure and electrical properties of Sm doped ceria thin films
SOSI-14112; No of Pages 8 Solid State Ionics xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Investigation of microstructure and electrical properties of Sm doped ceria thin films Mantas Sriubas ⁎, Kastytis Pamakštys, Giedrius Laukaitis Physics Department, Kaunas University of Technology, Studentu str. 50, LT-51368, Kaunas, Lithuania
a r t i c l e
i n f o
Article history: Received 29 July 2016 Received in revised form 2 November 2016 Accepted 4 November 2016 Available online xxxx Keywords: Samarium doped ceria (SDC) E-beam physical vapour deposition Solid oxide fuel cells (SOFC) Thin films Ionic conductivity
a b s t r a c t Sm0.2Ce0.8O2−δ (SDC) thin films (~1.9 μm) were deposited on SiO2, Alloy 600 (Fe-Ni-Cr), and Al2O3 substrates, using e-beam evaporation technique. The deposition rate was 0.2 nm/s ÷ 1.6 nm/s, and substrate temperature during the formation of thin films was kept 323 K, 423 K, 573 K, 723 K and 873 K. SEM analysis reveals that grain size increases at 323 K, 423 K, and 573 K substrate temperatures and decreases at 723 K and 873 K temperatures. The preferential out-of-plane orientations of thin SDC films were (111) and (222). Texture coefficients of those orientations decrease at high deposition rates (1.2 nm/s and 1.6 nm/s) and high substrate temperatures (723 K and 873 K). The preferential orientation changes to (220) or (222) using SiO2 substrates (1.2 nm/s and 1.6 nm/s growth rate; 423 K, 723 K, and 873 K substrate temperature) and to (200), (220), or (311) using Alloy 600 substrates (0.2 nm/s, 0.8 nm/s, 1.2 nm/s, and 1.6 nm/s deposition rate; 723 K and 873 K substrate temperature). Crystallite size increases from 6.8 nm to 80.6 nm with increasing substrate temperatures (323 K ÷ 873 K) and influences total conductivity of SDC thin films; it increases (0.03·10−3 S/ m ÷ 1.12 S/m) with increasing crystallite size. Ce3+ concentrations change from 24.5% to 29.1% in thin SDC films and do not show clear correlation with changes of total conductivity. In addition, thin films deposited at 323 K ÷ 423 K temperatures and 0.4 nm/s ÷ 1.6 nm/s deposition rates have reduced total conductivity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFC) have been attracting a lot of attention over past few decades. It has been considered to be alternative, environmental-friendly, and efficient power generation technology from hydrogen, natural gas, etc. Military, residential, industrial, and transportation are the possible applications of this technology [1]. However, high operating temperature, long start up time, high cost, and low reliability have been slowing down the commercialization process. Obviously, the operating temperature influences this process the most. The reduction of reliability and necessity of expensive materials appear when SOFC operates at high temperature. On the other hand, the efficiency of fuel cell decreases with decreasing operating temperature. The lower conductivity of oxygen ions in the electrolyte is the reason of efficiency loss. Therefore, scientists have been making efforts to develop new materials which would allow an enhancement of oxygen conductivity at lower temperatures. At present, it is possible to group electrolyte materials into 5 groups: fluorite structured oxides, perovskites and perovskite-related oxides, LAMOX family oxides, apatites, and BIMEVOX family oxides [2]. Yttria stabilized zirconia (YSZ), samarium ⁎ Corresponding author. E-mail address: [email protected] (M. Sriubas).
doped ceria (SDC), gadolinium doped ceria (GDC), and lanthanum gallates show the highest oxygen ion conductivity [2]. However, YSZ achieves its maximum ionic conductivity (0.1 S/cm) only at ~ 1273 K and lanthanum gallates are susceptible to reduction [3]. In contrast, ceria based electrolytes exhibit about two or three times higher ionic conductivity (~ 0.01 S/cm) than YSZ at intermediate temperatures (773 K ÷ 973 K) [2,4,5]. So, this material is a good alternative to yttrium stabilized zirconia. It is already known that CeO2 achieves the highest oxygen ion conductivity by doping it with Gd3+ or Sm3+ and the optimal concentrations of dopants are ~0.15–0.20 mol% [6]. However, ionic conductivity and the quality of electrolyte also depend on grain size, surface area of grain boundaries, segregation of impurities at the grain boundaries, morphology, electrolyte thickness, density and fabrication method [7]. Thin films must be without cracks, pores and have the density higher than 94% in order to avoid short circuiting in fuel cell. Thin films (~ 2 μm) in comparison to thick (~ 50 ÷ 100 μm) have lower ohmic loses across the electrolyte. The grain boundary effect on the conductivity of oxygen ions is not well understood. It is believed that the grains and grain boundaries have ambiguous effect on the oxygen conductivity. The grain boundaries exhibit blocking effect if the sizes of the grains are large (small surface area of grain boundary) and the conductivity enhances if material consists of nano-sized grains (large surface area of grain boundary) [8,9]. One of possible explanations is that oxygen
http://dx.doi.org/10.1016/j.ssi.2016.11.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
2
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
ions migrate through the grains when they are large (the micrometer level) and the ions move along the grain boundaries and pass around the grains when the grains are nano-sized [9]. The space charge model supports this statement. According to it, the concentration of oxygen vacancy is high in the grain boundary and low (depleted zone) in the regions near the grain boundary [8,10]. This implies that the depleted zones block the diffusion of oxygen ions across them. So, oxygen ions can move freely only along grain boundary. From the statements above, it is obvious that ionic conductivity and the quality of thin film depend on the microstructure and mechanical properties. On the other hand, these properties strongly depend on the fabrication method. So, it is very important to find the best formation method and technological parameters. There are various formation techniques, such as: magnetron sputtering, chemical vapour deposition, sol-gel method, spray pyrolysis, pulsed laser deposition, ion beam assisted deposition, thermal evaporation, metal-organic vapour deposition, electrostatic spray assisted vapour deposition, and e-beam deposition [11,12]. All of them have advantages and disadvantages. For example, electron beam vapour deposition has key advantages over the other deposition methods due to high and controllable deposition rates, high density and homogeneity of formed thin film and it is relatively easy to control the stoichiometry of thin film. The goals of this work were to investigate the influence of deposition rate, substrate type and temperature on the microstructure and electrical properties of Sm doped ceria thin films, which were formed using e-beam physical vapour deposition method.
Texture coefficient T(hkl) was determined using formula [16]:
TðhklÞ ¼
" #−1 n IðhklÞ 1 X I ðhklÞ ; I0 ðhklÞ n 1 I0 ðhklÞ
where I(hkl) is intensity of the XRD peak corresponding to (hkl) planes, n is the number of the diffraction peaks taken into account, I0(hkl) denotes the intensity of the XRD peak in EVA Search–Match software database. T(hkl) = 1 corresponds to films with randomly oriented crystallites, while higher values indicate the large number of grains oriented in a given (hkl) direction. The surface topography images and cross-section images were obtained by scanning electron microscope “Hitachi S3400 N” (SEM). Elemental composition was controlled using energydispersive X-ray spectroscope “BrukerXFlash QUAD 5040” (EDS). Concentrations of elements in thin SDC films were respectively CSm ≈ 13.8 mol%, CCe = 86.2 mol%, CO = 72.3 mol%. Ce3+ and Ce4+ concentrations were determined by XPS method (PHI 5000 Versaprobe). Measurements were performed using monochromatic X-ray radiation (AlKα, 1486.6 eV). X-ray beam power was 23.2 W, diameter of the beam was 100 mm, and measurement angle was 45° during the experiments. Thin films were not sputtered before measurements in order to avoid changes of thin SDC films surface structure. Sample charging
2. Experimental Sm doped cerium oxide thin films (~1.9 μm) were deposited on SiO2, Alloy 600 (Fe-Ni-Cr), and Al2O3 substrates. Such substrates were chosen for the reason that SiO2 substrate is amorphous and it does not affect the microstructure or the morphology of thin films. Total conductivity of SDC thin films is measured in 473 K ÷ 1273 K temperature range. Al2O3 was chosen due to its high melting temperature (2345 K) and low electrical conductivity. Alloy600 (Fe-Ni-Cr) could be used as interconnect in IT-SOFC and Ni-SDC is considered to be a good choice for anode in IT-SOFC [13,14]. The goal of the investigation was to check the influence of Alloy 600 substrates to microstructure and surface morphology of SDC thin films. The substrates were ultrasonically cleaned in pure acetone for 10 min. Thin films were formed with e-beam physical vapour deposition system “Kurt J. Lesker EB-PVD 75”, using 0.2 nm/ s ÷ 1.6 nm/s deposition rate, substrate temperatures from 323 K to 873 K, and 5 ∙ 10− 7 bar work pressure. The Sm0.2Ce0.8O2–δ powder (Nexceris, LLC, Fuelcellmaterials, USA) of 6.2 m2/g surface area was used as evaporating material. It was pressed in to the pellets using mechanical press (303.5 MPa pressure). The pellets were placed into crucible and vacuum chamber was depressurised up to 2 ∙ 10− 9 bar. After that, the substrates were treated with Ar + ion plasma (10 min) and preheated up to working temperature. Thickness and deposition rate were controlled with INFICON crystal sensor. Structure of the deposited thin films was investigated using X-ray diffractometer (XRD) “Bruker D8 Discover” at 2Θ angle in a 20°–70° range using Cu Kα (λ = 0.154059 nm) radiation, 0.01° step, and Lynx eye PSD detector. EVA Search–Match software and PDF-2 database were used to identify diffraction peaks. TOPAS software was used for crystallite size calculations. Measured patterns were fitted using Pawley's method. Crystallite size was calculated using Scherrer's equation [15]:
d¼
0:9λ ; β cosθ
where d is the crystallite size of the material, λ is the wavelength of the X-ray radiation, β is the full width at half maximum, and θ is the angle of diffraction.
Fig. 1. XRD patterns of thin SDC films deposited on a) SiO2 substrates, using 1.2 nm/s growth rate and b) Alloy 600 substrates at 873 K temperature.
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
effect was compensated using radiation of low energy electrons and ions. The experimental data were curve fitted using GaussianLorentzian functions and background was eliminated using, Shirley method. MultiPak software was used for curve fitting. The concentrations of Ce3+ were calculated using these spectra and following equation: C Ce3þ ¼
AV 0 þ AU 0 þ AV 0 þ AU 0 ; AV þ AU þ AV ″ þ AU ″ þ AV ‴ þ AU ‴
where CCe3+ is Ce3 + concentration, A – the areas of particular peaks (V0 − Ce3 + 3d5/2, V − Ce4 + 3d5/2, V ′ − Ce3 + 3d5/2, V ″ − Ce4 + 3d5/2, V ‴ − Ce4 + 3d5/2, U0 − Ce3 + 3d3/2, U − Ce4 + 3d3/2, U ′ − Ce3 + 3d3/2, U ″ − Ce4+ 3d3/2, U ‴ − Ce4+ 3d3/2). Total conductivity was investigated using impedance spectrometer “NorECsAS” (EIS). Total conductivity was measured only for thin films, deposited on Al2O3 in order to avoid the influence of the substrate to the measurements. The Pt electrodes were applied on the top of thin films using mask reproducing the geometry of the electrodes. The distance between Pt electrodes was 10 mm and their dimensions were 3 mm × 10 mm. The measurements were carried out in 1 ÷ 106 Hz frequency range and in 473 K ÷ 1273 K temperature interval, using twoprobe method. 3. Results and discussion XRD measurements show that SDC thin films have fluorite structure with space group Fm3m. X-ray diffraction patterns of SDC thin films have characteristic peaks, corresponding to crystallographic orientations (111), (200), (220), (311), (222), and (400) (Fig. 1, Table 1). The preferential out-of-plane orientations are (111) and (222) for the formed thin films, because the planes of these orientations have the lowest surface energy [17]. However, preferential orientations are not the same for all investigated technological parameters. They change into (200) or (311) using particular deposition parameters (Fig. 1a, Table 1), i.e., it was noticed that the change occurs using high deposition rates (1.2 nm/s and 1.6 nm/s) and particular substrate temperatures (423 K, 723 K, 873 K) for thin films, deposited on SiO2 substrates.
3
The growth of SDC thin films is slightly different using Alloy600 substrates. The preferential orientation changes into (200), (220) or (311) at high substrate temperatures (723 K, 873 K) and certain deposition rates (0.2 nm/s, 0.8 nm/s, 1.2 nm/s, and 1.6 nm/s) (Fig. 1b, Table 1). Deeper analysis of XRD data and the calculations of texture coefficients revealed that texture coefficient of (111) and (222) orientations decreases at high deposition rates (1.2 nm/s ÷ 1.6 nm/s) and high substrate temperatures (723 K ÷ 873 K). The effect is even stronger using Alloy 600 substrates. These results correspond to theory, i.e., the adatom migration energy and length increase with increasing substrate temperature [18]. Therefore, SDC thin films grow along the orientations with higher surface energy (γ (111) b γ(200) b γ (220) ) [18–20]. However, migration length of adatoms is shorter using higher deposition rates. So, crystallite size should decrease with increasing deposition rate. In addition, other authors' experiments show that thin films grow on the orientations having higher surface energy with increasing deposition rate [20]. The crystallite size calculations prove the statements above. The crystallite size increases with increasing substrate temperature (6.8 nm ÷ 80.6 nm) (Table 1). The deposition rate influences crystallite size, also. The crystallite size decreases with increasing deposition rate if temperatures of substrates are 723 K and 873 K and the crystallite size is almost the same with increasing deposition rate if temperatures of substrates are 323 K, 423 K, and 573 K (Table 1). Moreover, the effect of substrate type on the crystallite size is negligible. SEM analysis revealed that SDC thin films deposited at low temperatures (323 K and 423 K) and 0.4 nm/s ÷ 1.6 nm/s deposition rates have cracks, leading to short circuit of the fuel cell (Fig. 2a). The deposition parameters have the influence on surface topography, also. Grain size varies according to the substrate temperature but not necessarily increases (Fig. 2a, b, and e). It increases with increasing the temperature from 323 K to 423 K. Nevertheless, the reverse effect happens at higher temperatures, i.e., the grain size decreases (Fig. 2e). It is related with increased influence of growth rate at high temperatures (723 K and 873 K) and increased contribution of (200), (220) and (311) orientations (Fig. 2c, d, e, and f) [21]. The adatom migration length at low temperatures is short and the grains grow small and the deposition rate
Table 1 Texture coefficients (T(111), T(200), T(220), T(311), and T(222)) and crystallite size (〈d〉 SiO2 and 〈d〉 Alloy) dependence on deposition rate (vg) and substrate temperature (Ts) of thin films formed on Alloy 600 and SiO2 substrates. vg, nm/s
Ts, K
SiO2 T(111)
T(200)
T(220)
T(311)
T(222)
〈d〉 SiO2, nm
T(111)
T(200)
T(220)
T(311)
T(222)
〈d〉 Alloy, nm
0.2
323 423 573 723 873 323 423 573 723 873 323 423 573 723 873 323 423 573 723 873 323 423 573 723 873
1.92 1.87 1.29 1.34 1.91 1.85 1.73 1.78 1.38 2.59 1.88 1.70 1.19 1.37 1.37 2.30 1.15 1.23 1.23 0.23 1.54 1.85 1.88 1.21 0.10
– – – – – – – – – – – – – – – 0.39 2.92 – – 0.33 0.75 – – – 3.02
0.64 0.05 – – 0.99 0.87 0.09 0.03 – 0.02 0.83 0.26 – – – 0.82 0.32 – – 0.24 1.01 0.88 0.04 – 0.27
0.09 – – – 0.23 0.19 – – – 0.01 0.23 – – – – 0.37 0.47 – – 3.20 0.71 0.17 – 1.19 0.75
1.36 1.08 0.71 0.66 0.87 1.09 1.18 1.19 0.62 1.39 1.06 1.04 0.81 0.63 0.63 1.05 0.89 0.77 0.77 – – 1.10 1.08 0.60 1.49
9.7 ± 0.1 16.0 ± 0.1 28.7 ± 0.1 80.6 ± 0.4 66.2 ± 0.6 7.9 ± 0.1 19.9 ± 0.1 30.0 ± 0.1 62.4 ± 0.3 48.7 ± 0.2 7.5 ± 0.1 12.1 ± 0.1 21.9 ± 0.1 46.1 ± 0.3 66.5 ± 0.3 10.2 ± 0.1 15.2 ± 0.1 22.7 ± 0.1 31.4 ± 0.1 50.2 ± 0.5 6.8 ± 0.1 6.9 ± 0.1 17.8 ± 0.1 25.3 ± 0.2 48.0 ± 0.3
1.64 1.86 1.25 0.59 1.00 1.69 1.19 1.34 2.40 2.38 1.56 1.18 1.10 2.08 0.11 1.88 1.85 1.18 1.27 0.06 1.55 1.58 1.26 0.07 0.03
– – – – – – – – – – – – – – 1.07 – 2.01 – – 2.43 – – – 1.48 2.82
0.17 0.06 – 2.73 0.93 0.15 – – 0.30 0.20 0.26 – – 0.27 1.17 0.63 0.20 – – 0.23 0.98 0.35 – 0.16 0.28
– – – 0.21 1.18 – – – 0.14 0.18 – – – 0.48 1.90 0.19 0.29 – – 0.76 0.46 – – 2.31 0.51
1.19 1.08 0.75 0.47 0.90 1.17 0.81 0.66 1.15 1.24 1.18 0.82 0.90 1.16 – 1.30 1.46 0.82 0.73 – – 1.06 0.74 – –
16.3 ± 0.1 25.1 ± 0.1 39.3 ± 0.1 57.0 ± 0.3 54.8 ± 0.3 13.6 ± 0.1 30.4 ± 0.1 50.7 ± 0.2 55.3 ± 0.3 67.8 ± 0.2 11.3 ± 0.1 19.0 ± 0.1 27.3 ± 0.1 58.0 ± 0.4 48.8 ± 0.3 11.0 ± 0.1 19.7 ± 0.1 31.3 ± 0.1 43.7 ± 0.2 45.7 ± 0.4 7.5 ± 0.1 9.6 ± 0.1 26.8 ± 0.1 25.8 ± 0.2 62.2 ± 0.4
0.4
0.8
1.2
1.6
Alloy 600
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
4
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
Fig. 2. Topographic SEM pictures of SDC thin films, deposited on SiO2 substrates, using a) 1.2 nm/s deposition rate, 323 K substrate temperature, b) 1.2 nm/s deposition rate, 573 K substrate temperature, c) 0.2 nm/s deposition rate, 873 K substrate temperature, d) 0.8 nm/s deposition rate, 873 K substrate temperature, e) 1.2 nm/s deposition rate, 873 K substrate temperature, and f) 1.6 nm/s deposition rate, 873 K substrate temperature.
effect is negligible. In contrast, increased growth rate has greater effect on grain size at high temperatures. The grains become smaller at higher deposition rate because adatom flux to the surface increases and migration length of adatoms becomes shorter. In addition, the grains in SEM images have the shape of triangular prism, where the height is much lower than the length of base triangle sides. It is also visible that the main differences of surface morphology are the size of the grains and their orientation in formed SDC thin films. The obtained results correspond to other authors' results [22,23]. Other factor, influencing surface morphology of thin SDC films, is substrate type. The effect is more visible at higher substrate temperatures (723 K ÷ 873 K) and deposition rates (1.2 nm/s ÷ 1.6 nm/s). The orientation of the grains changes depending on the substrate type (Fig. 3). The grains of thin films deposited on SiO2 substrates are oriented in parallel to the substrate and the grains of thin films,
deposited on Alloy 600 and Al2O3 substrates are oriented perpendicularly to the substrate. The effect of substrate is negligible at lower temperatures. Movchan-Demchishin model divides the growth of thin films into three zones: Zone I (Ts/Tm b 0.26), Zone II (0.26 b Ts/Tm b 0.45) and Zone III (Ts/Tm N 0.45). Ts and Tm represent substrate temperature and material melting temperature respectively. Grains have small grained porous structure in Zone I and dense columnar structure in Zone II and Zone III [21,24]. The cross section images of thin SDC films correspond to this model. Cross sections have small grained and hardly visible structure at low substrate temperatures (323K) (Fig. 4a). The grains' size increases with increasing substrate temperature (573 K) (Fig. 4b). It is visible that the grains of the triangular prism shape grow on the top of each other. In such manner they start to form columns growth. Finally, the grains are formed with dense columnar structure at 873 K substrate
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
5
Fig. 3. Topographic SEM pictures of thin SDC films, deposited on a) SiO2, b) Alloy 600, and c) Al2O3 substrates using 0.2 nm/s deposition rate and 723 K substrate temperature.
Fig. 4. Cross section of thin SDC films deposited on Alloy 600 substrates, using 1.2 nm/s deposition rate and different substrate temperature: a) 323 K, b) 573 K, and c) 873 K.
temperature (Fig. 4c). It means that thin SDC films grow in Zone I and Zone II according to the Movchan-Demchishin model. The impedance measurements were carried out in order to evaluate the influence of deposition parameters on total conductivity. Total conductivity increases with increasing the temperature of the substrate during deposition (Fig. 5). However, total conductivity of thin SDC films deposited at 323 K and 423 K substrate temperatures decrease
with increasing deposition rate (Fig. 5a) and b). It could be related to surface morphology. Thin films have cracks at these temperatures. On the other hand, the influence of deposition rate decreases with increasing substrate temperature (Fig. 5c and d). So, substrate temperature influencing thin film morphology has greater influence on total conductivity than deposition rate. The oxygen vacancy activation energies were calculated using Arrhenius plots (Table 2). It changes from 0.70 eV to
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
6
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
Fig. 5. Arrhenius plots of total conductivity of SDC thin films, deposited on Al2O3 substrates at different deposition rates and substrate temperatures: a) 0.2 nm/s deposition rate, b) 1.2 nm/s deposition rate, c) 423 K substrate temperature, and d) 873 K substrate temperature.
0.98 eV. In addition, it does not show clear relationship with deposition parameters. It is reasonable to relate total conductivity of thin SDC films with the properties depending similarly on deposition parameters as total conductivity. The texture, surface morphology, and oxygen vacancy activation energy do not show similar behaviour and have not clear relationship with total conductivity of formed SDC thin films. However, the crystallite size depends similarly on the temperature as total conductivity. It is seen that total conductivity increases with increasing crystallite size (Table 3). Total conductivity exhibits its lowest values (3.00·10−5 S/m ÷ 5.25·10− 2 S/m) when the crystallite size is 6.8 nm ÷ 16.0 nm and the total conductivity achieves it's the highest value (1.12 S/m) when it reaches 80.6 nm. Similar results for Sm0.2Ce0.8O2−δ pellets and thin films were obtained by other authors [5,25]. XPS measurements were carried out for SDC thin films in order to evaluate concentration of Ce3+. These SDC thin films were deposited using 0.2 nm/s deposition rate and substrate temperatures from 323 K to 873 K (Fig. 6). Typical curve fitted Ce3d spectra for thin SDC film
deposited using 0.2 nm/s deposition rate and 573 K substrate temperature are shown in Fig. 6b. Ce3d spectra exhibits three pairs of spin orbitales (V, U, V″, U″, and V‴, U‴) for Ce4 + and two pairs of spin orbitales (V0, U0 and V′, U′) for Ce3 +. Ce3 + concentration changes from 24.5% to 29.1% in thin films (Table 4). This shows that SDC thin films are mixed ionic-electronic conductors. However, correlation between changes in total conductivity and concentration of Ce3 + was not observed there. So, it is possible to say that changes in total conductivity depend on ionic conductivity component, i.e. ionic conductivity increases with increasing crystallite size. 4. Conclusions XRD analysis of SDC thin films revealed that the changes in the microstructure occur at high deposition rates (1.2 nm/s, 1.6 nm/s) and high substrate temperatures (723 K and 873 K). The texture coefficients of preferential out-of-plane orientations ((111) and (222)) decrease using these deposition parameters (SiO2 and Alloy 600 substrates). The preferential orientation changes to (220) or (222) using SiO2
Table 2 Oxygen vacancy activation energy (Ea) and total conductivity (σ) dependence on deposition rate (vg) and substrate temperature (Ts) of SDC thin films, formed on Al2O3 substrates. vg, nm/s
Ts, K 323 Ea, eV
0.2 0.4 0.8 1.2 1.6
0.90 0.83 0.85 0.88 0.82
423 σ, S/m
Ea, eV −2
5.25·10 3.00·10−5 1.50·10−4 1.70·10−4 5.00·10−5
0.98 0.86 0.91 0.98 0.90
573 σ, S/m
Ea, eV −2
2.07·10 1.46·10−1 5.36·10−2 5.80·10−4 3.50·10−2
0.91 0.82 0.81 0.80 0.85
723 σ, S/m
Ea, eV −1
1.16·10 4.22·10−1 2.74·10−1 1.30·10−1 1.80·10−1
0.70 0.89 0.89 0.84 0.84
873 σ, S/m −0
1.12·10 2.99·10−1 4.86·10−1 1.91·10−1 2.42·10−1
Ea, eV
σ, S/m
0.93 0.84 0.86 0.86 0.84
4.23·10−1 4.54·10−1 4.40·10−1 4.58·10−1 4.64·10−1
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
7
Table 3 Total conductivity (σ) measured at 873 K dependence on crystallite size (〈d〉 SiO2) of SDC thin films formed on optical quartz (SiO2) and Al2O3 substrates at different substrate temperature (Ts). vg, nm/s
Ts, K 323
0.2 0.4 0.8 1.2 1.6
423
573
723
873
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
9.7 ± 0.1 7.9 ± 0.1 7.5 ± 0.1 10.2 ± 0.1 6.8 ± 0.1
5.25·10−2 3.00·10−5 1.50·10−4 1.70·10−4 5.00·10−5
16.0 ± 0.1 19.9 ± 0.1 12.1 ± 0.1 15.2 ± 0.1 6.9 ± 0.1
2.07·10−2 1.46·10−1 5.36·10−2 5.80·10−4 3.50·10−2
28.7 ± 0.1 30.0 ± 0.1 21.9 ± 0.1 22.7 ± 0.1 17.8 ± 0.1
1.16·10−1 4.22·10−1 2.74·10−1 1.30·10−1 1.80·10−1
80.6 ± 0.4 62.4 ± 0.3 46.1 ± 0.3 31.4 ± 0.1 25.3 ± 0.2
1.12·10−0 2.99·10−1 4.86·10−1 1.91·10−1 2.42·10−1
66.2 ± 0.6 48.7 ± 0.2 66.5 ± 0.3 50.2 ± 0.5 48.0 ± 0.3
4.23·10−1 4.54·10−1 4.40·10−1 4.58·10−1 4.64·10−1
Fig. 6. XPS curves of Ce3d core levels for thin SDC films, deposited on Al2O3 substrates using a) 0.2 nm/s deposition rate and b) 573 K temperature and 0.2 nm/s deposition rate [V0 − Ce3+ 3d5/2, V−Ce4+ 3d5/2, V′ −Ce3+ 3d5/2, V″ −Ce4+ 3d5/2, V‴ −Ce4+ 3d5/2, U0 −Ce3+ 3d3/2, U−Ce4+ 3d3/2, U′ −Ce3+ 3d3/2, U″ −Ce4+ 3d3/2, U‴ −Ce4+ 3d3/2].
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
8
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
Table 4 Total conductivity (σ) measured at 873 K and Ce3+ concentrations (CCe3+) in thin SDC films, deposited on Al2O3 substrate, using 0.2 nm/s deposition rate. σ, S/m CCe3+, %
2.07·10−2 25.6
5.25·10−2 24.5
1.16·10−1 29.1
4.23·10−1 26.8
1.12·10−0 28.1
substrates (1.2 nm/s and 1.6 nm/s growth rate; 423 K, 723 K, and 873 K substrate temperature) and to (200), (220), or (311) using Alloy 600 substrates (0.2 nm/s, 0.8 nm/s, 1.2 nm/s, and 1.6 nm/s deposition rate; 723 K and 873 K substrate temperature). The crystallinity changes with deposition parameters, also. The crystallite size increases from 6.8 nm to 80.6 nm with increasing substrate temperature (323 K ÷ 873 K). However, the crystallite size decreases with increasing deposition rate (from 0.2 nm/s to 1.6 nm/s). It decreases from 80.6 nm to 25.3 nm using 723 K substrates and from 66.2 nm to 48.0 nm using 873 K substrates. These changes in microstructure can be explained by variation in adatom migration energy and length due to the influence of substrate temperature and deposition rate. SEM analysis revealed that grain size increases with increasing substrate temperature at low temperatures (323 K, 423 K, and 573 K) and decreases with increasing substrate temperature at high temperatures (723 K and 873 K). In addition, thin films deposited at 323 K ÷ 423 K temperatures and using 0.4 nm/s ÷ 1.6 nm/s deposition rates have cracks. Moreover, the orientation of grains depends on substrate type. The changes on the surface morphology are related to the different surface energies of substrates, increased influence of growth rate and increased contribution of (200), (220) and (311) orientations at high substrate temperatures (723 K and 873 K). Total conductivity of thin SDC films increases from 3.00·10−5 S/m to 1.12 S/m with increasing crystallite size from 6.8 nm to 80.6 nm, i.e. with increasing substrate temperature during the deposition of thin films. In addition, it decreases with increasing deposition rate at 323 K and 423 K substrate temperatures. However, correlation between changes in total conductivity and concentration of Ce3 + was not observed there. Therefore, it is reasonable to conclude that ionic conductivity increases with increasing crystallite size. References
[2] S.J. Skinner, J.A. Kilner, Oxygen ion conductors, Mater. Today 6 (3) (2003) 30–37. [3] J. Garcia-Barriocanal, et al., Colossal ionic conductivity at interfaces of epitaxial ZrO2: Y2O3/SrTiO3 heterostructures, Science 321 (5889) (2008) 676–680. [4] C.J. Fu, et al., Effects of transition metal oxides on the densification of thin-film GDC electrolyte and on the performance of intermediate-temperature SOFC, Int. J. Hydrog. Energy 35 (20) (2010) 11200–11207. [5] R. Sanghavi, et al., Integrated experimental and modeling study of the ionic conductivity of samaria-doped ceria thin films, Solid State Ionics 204–205 (2011) 13–19. [6] S. Zha, C. Xia, G. Meng, Effect of Gd (Sm) doping on properties of ceria electrolyte for solid oxide fuel cells, J. Power Sources 115 (1) (2003) 44–48. [7] P. Arunkumar, M. Meena, K.S. Babu, A review on cerium oxide-based electrolytes for ITSOFC, Nano Energy 1 (5) (2012) 288–305. [8] J.S. Park, et al., Oxygen diffusion across the grain boundary in bicrystal yttria stabilized zirconia, Solid State Commun. 152 (24) (2012) 2169–2171. [9] B. Zhu, Solid oxide fuel cell (SOFC) technical challenges and solutions from nanoaspects, Int. J. Energy Res. 33 (13) (2009) 1126–1137. [10] X. Guo, R. Waser, Electrical properties of the grain boundaries of oxygen ion conductors: acceptor-doped zirconia and ceria, Prog. Mater. Sci. 51 (2) (2006) 151–210. [11] G. Balakrishnan, et al., A study of microstructural and optical properties of nanocrystalline ceria thin films prepared by pulsed laser deposition, Thin Solid Films 519 (8) (2011) 2520–2526. [12] E.S. Bârcă, et al., Pyramidal growth of ceria nanostructures by pulsed laser deposition, Appl. Surf. Sci. 363 (2016) 245–251. [13] M. Chen, et al., Effect of Ni content on the microstructure and electrochemical properties of Ni–SDC anodes for IT-SOFC, Solid State Ionics 181 (23–24) (2010) 1119–1124. [14] Z. Yang, et al., Selection and evaluation of heat-resistant alloys for SOFC interconnect applications, J. Electrochem. Soc. 150 (9) (2003) A1188–A1201. [15] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Weseley Metallurgy Series, , Addison-Wesley publishing, Reading, Massachusetts, 1956 514. [16] P. Nagaraju, Y. Vijayakumar, M.V. Ramana Reddy, Optical and microstructural studies on laser ablated nanocrystalline CeO2 thin films, Glas. Phys. Chem. 41 (5) (2015) 484–488. [17] F. Bechstedt, Principles of Surface Physics, first ed.XII, Springer-Verlag, Berlin Heidelberg, 2003 342. [18] P. Arunkumar, et al., Texturing of pure and doped CeO2 thin films by EBPVD through target engineering, RSC Adv. 4 (63) (2014) 33338–33346. [19] E. Grantscharova, Texture transition in thin metal films vacuum condensed on glass: a general consideration, Thin Solid Films 224 (1) (1993) 28–32. [20] S.G. Wang, E.K. Tian, C.W. Lung, Surface energy of arbitrary crystal plane of bcc and fcc metals, J. Phys. Chem. Solids 61 (8) (2000) 1295–1300. [21] K.H. Guenther, Revisiting Structure-Zone Models for Thin-Film Growth, 1990. [22] C. Mansilla, Structure, microstructure and optical properties of cerium oxide thin films prepared by electron beam evaporation assisted with ion beams, Solid State Sci. 11 (8) (2009) 1456–1464. [23] M.S. Anwar, et al., Study of nanocrystalline ceria thin films deposited by e-beam technique, Curr. Appl. Phys. 11 (Suppl. 1) (2011) S301–S304. [24] B.A. Movchan, A.V. Demchishin, Structure and properties of thick condensates of nickel, titanium, tungsten, aluminum oxides, and zirconium dioxide in vacuum, Fiz. Met. Metalloved. 28 (1969) 653–660. [25] G. Hua, et al., Enhanced ionic doping transition metals, J. Mater. Sci. Mater. Electron. 26 (6) (2015) 3664–3669.
[1] N. Laosiripojana, et al., Reviews on Solid Oxide Fuel Cell Technology, vol. 13, 2009.
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Investigation of microstructure and electrical properties of Sm doped ceria thin films Mantas Sriubas ⁎, Kastytis Pamakštys, Giedrius Laukaitis Physics Department, Kaunas University of Technology, Studentu str. 50, LT-51368, Kaunas, Lithuania
a r t i c l e
i n f o
Article history: Received 29 July 2016 Received in revised form 2 November 2016 Accepted 4 November 2016 Available online xxxx Keywords: Samarium doped ceria (SDC) E-beam physical vapour deposition Solid oxide fuel cells (SOFC) Thin films Ionic conductivity
a b s t r a c t Sm0.2Ce0.8O2−δ (SDC) thin films (~1.9 μm) were deposited on SiO2, Alloy 600 (Fe-Ni-Cr), and Al2O3 substrates, using e-beam evaporation technique. The deposition rate was 0.2 nm/s ÷ 1.6 nm/s, and substrate temperature during the formation of thin films was kept 323 K, 423 K, 573 K, 723 K and 873 K. SEM analysis reveals that grain size increases at 323 K, 423 K, and 573 K substrate temperatures and decreases at 723 K and 873 K temperatures. The preferential out-of-plane orientations of thin SDC films were (111) and (222). Texture coefficients of those orientations decrease at high deposition rates (1.2 nm/s and 1.6 nm/s) and high substrate temperatures (723 K and 873 K). The preferential orientation changes to (220) or (222) using SiO2 substrates (1.2 nm/s and 1.6 nm/s growth rate; 423 K, 723 K, and 873 K substrate temperature) and to (200), (220), or (311) using Alloy 600 substrates (0.2 nm/s, 0.8 nm/s, 1.2 nm/s, and 1.6 nm/s deposition rate; 723 K and 873 K substrate temperature). Crystallite size increases from 6.8 nm to 80.6 nm with increasing substrate temperatures (323 K ÷ 873 K) and influences total conductivity of SDC thin films; it increases (0.03·10−3 S/ m ÷ 1.12 S/m) with increasing crystallite size. Ce3+ concentrations change from 24.5% to 29.1% in thin SDC films and do not show clear correlation with changes of total conductivity. In addition, thin films deposited at 323 K ÷ 423 K temperatures and 0.4 nm/s ÷ 1.6 nm/s deposition rates have reduced total conductivity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFC) have been attracting a lot of attention over past few decades. It has been considered to be alternative, environmental-friendly, and efficient power generation technology from hydrogen, natural gas, etc. Military, residential, industrial, and transportation are the possible applications of this technology [1]. However, high operating temperature, long start up time, high cost, and low reliability have been slowing down the commercialization process. Obviously, the operating temperature influences this process the most. The reduction of reliability and necessity of expensive materials appear when SOFC operates at high temperature. On the other hand, the efficiency of fuel cell decreases with decreasing operating temperature. The lower conductivity of oxygen ions in the electrolyte is the reason of efficiency loss. Therefore, scientists have been making efforts to develop new materials which would allow an enhancement of oxygen conductivity at lower temperatures. At present, it is possible to group electrolyte materials into 5 groups: fluorite structured oxides, perovskites and perovskite-related oxides, LAMOX family oxides, apatites, and BIMEVOX family oxides [2]. Yttria stabilized zirconia (YSZ), samarium ⁎ Corresponding author. E-mail address: [email protected] (M. Sriubas).
doped ceria (SDC), gadolinium doped ceria (GDC), and lanthanum gallates show the highest oxygen ion conductivity [2]. However, YSZ achieves its maximum ionic conductivity (0.1 S/cm) only at ~ 1273 K and lanthanum gallates are susceptible to reduction [3]. In contrast, ceria based electrolytes exhibit about two or three times higher ionic conductivity (~ 0.01 S/cm) than YSZ at intermediate temperatures (773 K ÷ 973 K) [2,4,5]. So, this material is a good alternative to yttrium stabilized zirconia. It is already known that CeO2 achieves the highest oxygen ion conductivity by doping it with Gd3+ or Sm3+ and the optimal concentrations of dopants are ~0.15–0.20 mol% [6]. However, ionic conductivity and the quality of electrolyte also depend on grain size, surface area of grain boundaries, segregation of impurities at the grain boundaries, morphology, electrolyte thickness, density and fabrication method [7]. Thin films must be without cracks, pores and have the density higher than 94% in order to avoid short circuiting in fuel cell. Thin films (~ 2 μm) in comparison to thick (~ 50 ÷ 100 μm) have lower ohmic loses across the electrolyte. The grain boundary effect on the conductivity of oxygen ions is not well understood. It is believed that the grains and grain boundaries have ambiguous effect on the oxygen conductivity. The grain boundaries exhibit blocking effect if the sizes of the grains are large (small surface area of grain boundary) and the conductivity enhances if material consists of nano-sized grains (large surface area of grain boundary) [8,9]. One of possible explanations is that oxygen
http://dx.doi.org/10.1016/j.ssi.2016.11.007 0167-2738/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
2
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
ions migrate through the grains when they are large (the micrometer level) and the ions move along the grain boundaries and pass around the grains when the grains are nano-sized [9]. The space charge model supports this statement. According to it, the concentration of oxygen vacancy is high in the grain boundary and low (depleted zone) in the regions near the grain boundary [8,10]. This implies that the depleted zones block the diffusion of oxygen ions across them. So, oxygen ions can move freely only along grain boundary. From the statements above, it is obvious that ionic conductivity and the quality of thin film depend on the microstructure and mechanical properties. On the other hand, these properties strongly depend on the fabrication method. So, it is very important to find the best formation method and technological parameters. There are various formation techniques, such as: magnetron sputtering, chemical vapour deposition, sol-gel method, spray pyrolysis, pulsed laser deposition, ion beam assisted deposition, thermal evaporation, metal-organic vapour deposition, electrostatic spray assisted vapour deposition, and e-beam deposition [11,12]. All of them have advantages and disadvantages. For example, electron beam vapour deposition has key advantages over the other deposition methods due to high and controllable deposition rates, high density and homogeneity of formed thin film and it is relatively easy to control the stoichiometry of thin film. The goals of this work were to investigate the influence of deposition rate, substrate type and temperature on the microstructure and electrical properties of Sm doped ceria thin films, which were formed using e-beam physical vapour deposition method.
Texture coefficient T(hkl) was determined using formula [16]:
TðhklÞ ¼
" #−1 n IðhklÞ 1 X I ðhklÞ ; I0 ðhklÞ n 1 I0 ðhklÞ
where I(hkl) is intensity of the XRD peak corresponding to (hkl) planes, n is the number of the diffraction peaks taken into account, I0(hkl) denotes the intensity of the XRD peak in EVA Search–Match software database. T(hkl) = 1 corresponds to films with randomly oriented crystallites, while higher values indicate the large number of grains oriented in a given (hkl) direction. The surface topography images and cross-section images were obtained by scanning electron microscope “Hitachi S3400 N” (SEM). Elemental composition was controlled using energydispersive X-ray spectroscope “BrukerXFlash QUAD 5040” (EDS). Concentrations of elements in thin SDC films were respectively CSm ≈ 13.8 mol%, CCe = 86.2 mol%, CO = 72.3 mol%. Ce3+ and Ce4+ concentrations were determined by XPS method (PHI 5000 Versaprobe). Measurements were performed using monochromatic X-ray radiation (AlKα, 1486.6 eV). X-ray beam power was 23.2 W, diameter of the beam was 100 mm, and measurement angle was 45° during the experiments. Thin films were not sputtered before measurements in order to avoid changes of thin SDC films surface structure. Sample charging
2. Experimental Sm doped cerium oxide thin films (~1.9 μm) were deposited on SiO2, Alloy 600 (Fe-Ni-Cr), and Al2O3 substrates. Such substrates were chosen for the reason that SiO2 substrate is amorphous and it does not affect the microstructure or the morphology of thin films. Total conductivity of SDC thin films is measured in 473 K ÷ 1273 K temperature range. Al2O3 was chosen due to its high melting temperature (2345 K) and low electrical conductivity. Alloy600 (Fe-Ni-Cr) could be used as interconnect in IT-SOFC and Ni-SDC is considered to be a good choice for anode in IT-SOFC [13,14]. The goal of the investigation was to check the influence of Alloy 600 substrates to microstructure and surface morphology of SDC thin films. The substrates were ultrasonically cleaned in pure acetone for 10 min. Thin films were formed with e-beam physical vapour deposition system “Kurt J. Lesker EB-PVD 75”, using 0.2 nm/ s ÷ 1.6 nm/s deposition rate, substrate temperatures from 323 K to 873 K, and 5 ∙ 10− 7 bar work pressure. The Sm0.2Ce0.8O2–δ powder (Nexceris, LLC, Fuelcellmaterials, USA) of 6.2 m2/g surface area was used as evaporating material. It was pressed in to the pellets using mechanical press (303.5 MPa pressure). The pellets were placed into crucible and vacuum chamber was depressurised up to 2 ∙ 10− 9 bar. After that, the substrates were treated with Ar + ion plasma (10 min) and preheated up to working temperature. Thickness and deposition rate were controlled with INFICON crystal sensor. Structure of the deposited thin films was investigated using X-ray diffractometer (XRD) “Bruker D8 Discover” at 2Θ angle in a 20°–70° range using Cu Kα (λ = 0.154059 nm) radiation, 0.01° step, and Lynx eye PSD detector. EVA Search–Match software and PDF-2 database were used to identify diffraction peaks. TOPAS software was used for crystallite size calculations. Measured patterns were fitted using Pawley's method. Crystallite size was calculated using Scherrer's equation [15]:
d¼
0:9λ ; β cosθ
where d is the crystallite size of the material, λ is the wavelength of the X-ray radiation, β is the full width at half maximum, and θ is the angle of diffraction.
Fig. 1. XRD patterns of thin SDC films deposited on a) SiO2 substrates, using 1.2 nm/s growth rate and b) Alloy 600 substrates at 873 K temperature.
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
effect was compensated using radiation of low energy electrons and ions. The experimental data were curve fitted using GaussianLorentzian functions and background was eliminated using, Shirley method. MultiPak software was used for curve fitting. The concentrations of Ce3+ were calculated using these spectra and following equation: C Ce3þ ¼
AV 0 þ AU 0 þ AV 0 þ AU 0 ; AV þ AU þ AV ″ þ AU ″ þ AV ‴ þ AU ‴
where CCe3+ is Ce3 + concentration, A – the areas of particular peaks (V0 − Ce3 + 3d5/2, V − Ce4 + 3d5/2, V ′ − Ce3 + 3d5/2, V ″ − Ce4 + 3d5/2, V ‴ − Ce4 + 3d5/2, U0 − Ce3 + 3d3/2, U − Ce4 + 3d3/2, U ′ − Ce3 + 3d3/2, U ″ − Ce4+ 3d3/2, U ‴ − Ce4+ 3d3/2). Total conductivity was investigated using impedance spectrometer “NorECsAS” (EIS). Total conductivity was measured only for thin films, deposited on Al2O3 in order to avoid the influence of the substrate to the measurements. The Pt electrodes were applied on the top of thin films using mask reproducing the geometry of the electrodes. The distance between Pt electrodes was 10 mm and their dimensions were 3 mm × 10 mm. The measurements were carried out in 1 ÷ 106 Hz frequency range and in 473 K ÷ 1273 K temperature interval, using twoprobe method. 3. Results and discussion XRD measurements show that SDC thin films have fluorite structure with space group Fm3m. X-ray diffraction patterns of SDC thin films have characteristic peaks, corresponding to crystallographic orientations (111), (200), (220), (311), (222), and (400) (Fig. 1, Table 1). The preferential out-of-plane orientations are (111) and (222) for the formed thin films, because the planes of these orientations have the lowest surface energy [17]. However, preferential orientations are not the same for all investigated technological parameters. They change into (200) or (311) using particular deposition parameters (Fig. 1a, Table 1), i.e., it was noticed that the change occurs using high deposition rates (1.2 nm/s and 1.6 nm/s) and particular substrate temperatures (423 K, 723 K, 873 K) for thin films, deposited on SiO2 substrates.
3
The growth of SDC thin films is slightly different using Alloy600 substrates. The preferential orientation changes into (200), (220) or (311) at high substrate temperatures (723 K, 873 K) and certain deposition rates (0.2 nm/s, 0.8 nm/s, 1.2 nm/s, and 1.6 nm/s) (Fig. 1b, Table 1). Deeper analysis of XRD data and the calculations of texture coefficients revealed that texture coefficient of (111) and (222) orientations decreases at high deposition rates (1.2 nm/s ÷ 1.6 nm/s) and high substrate temperatures (723 K ÷ 873 K). The effect is even stronger using Alloy 600 substrates. These results correspond to theory, i.e., the adatom migration energy and length increase with increasing substrate temperature [18]. Therefore, SDC thin films grow along the orientations with higher surface energy (γ (111) b γ(200) b γ (220) ) [18–20]. However, migration length of adatoms is shorter using higher deposition rates. So, crystallite size should decrease with increasing deposition rate. In addition, other authors' experiments show that thin films grow on the orientations having higher surface energy with increasing deposition rate [20]. The crystallite size calculations prove the statements above. The crystallite size increases with increasing substrate temperature (6.8 nm ÷ 80.6 nm) (Table 1). The deposition rate influences crystallite size, also. The crystallite size decreases with increasing deposition rate if temperatures of substrates are 723 K and 873 K and the crystallite size is almost the same with increasing deposition rate if temperatures of substrates are 323 K, 423 K, and 573 K (Table 1). Moreover, the effect of substrate type on the crystallite size is negligible. SEM analysis revealed that SDC thin films deposited at low temperatures (323 K and 423 K) and 0.4 nm/s ÷ 1.6 nm/s deposition rates have cracks, leading to short circuit of the fuel cell (Fig. 2a). The deposition parameters have the influence on surface topography, also. Grain size varies according to the substrate temperature but not necessarily increases (Fig. 2a, b, and e). It increases with increasing the temperature from 323 K to 423 K. Nevertheless, the reverse effect happens at higher temperatures, i.e., the grain size decreases (Fig. 2e). It is related with increased influence of growth rate at high temperatures (723 K and 873 K) and increased contribution of (200), (220) and (311) orientations (Fig. 2c, d, e, and f) [21]. The adatom migration length at low temperatures is short and the grains grow small and the deposition rate
Table 1 Texture coefficients (T(111), T(200), T(220), T(311), and T(222)) and crystallite size (〈d〉 SiO2 and 〈d〉 Alloy) dependence on deposition rate (vg) and substrate temperature (Ts) of thin films formed on Alloy 600 and SiO2 substrates. vg, nm/s
Ts, K
SiO2 T(111)
T(200)
T(220)
T(311)
T(222)
〈d〉 SiO2, nm
T(111)
T(200)
T(220)
T(311)
T(222)
〈d〉 Alloy, nm
0.2
323 423 573 723 873 323 423 573 723 873 323 423 573 723 873 323 423 573 723 873 323 423 573 723 873
1.92 1.87 1.29 1.34 1.91 1.85 1.73 1.78 1.38 2.59 1.88 1.70 1.19 1.37 1.37 2.30 1.15 1.23 1.23 0.23 1.54 1.85 1.88 1.21 0.10
– – – – – – – – – – – – – – – 0.39 2.92 – – 0.33 0.75 – – – 3.02
0.64 0.05 – – 0.99 0.87 0.09 0.03 – 0.02 0.83 0.26 – – – 0.82 0.32 – – 0.24 1.01 0.88 0.04 – 0.27
0.09 – – – 0.23 0.19 – – – 0.01 0.23 – – – – 0.37 0.47 – – 3.20 0.71 0.17 – 1.19 0.75
1.36 1.08 0.71 0.66 0.87 1.09 1.18 1.19 0.62 1.39 1.06 1.04 0.81 0.63 0.63 1.05 0.89 0.77 0.77 – – 1.10 1.08 0.60 1.49
9.7 ± 0.1 16.0 ± 0.1 28.7 ± 0.1 80.6 ± 0.4 66.2 ± 0.6 7.9 ± 0.1 19.9 ± 0.1 30.0 ± 0.1 62.4 ± 0.3 48.7 ± 0.2 7.5 ± 0.1 12.1 ± 0.1 21.9 ± 0.1 46.1 ± 0.3 66.5 ± 0.3 10.2 ± 0.1 15.2 ± 0.1 22.7 ± 0.1 31.4 ± 0.1 50.2 ± 0.5 6.8 ± 0.1 6.9 ± 0.1 17.8 ± 0.1 25.3 ± 0.2 48.0 ± 0.3
1.64 1.86 1.25 0.59 1.00 1.69 1.19 1.34 2.40 2.38 1.56 1.18 1.10 2.08 0.11 1.88 1.85 1.18 1.27 0.06 1.55 1.58 1.26 0.07 0.03
– – – – – – – – – – – – – – 1.07 – 2.01 – – 2.43 – – – 1.48 2.82
0.17 0.06 – 2.73 0.93 0.15 – – 0.30 0.20 0.26 – – 0.27 1.17 0.63 0.20 – – 0.23 0.98 0.35 – 0.16 0.28
– – – 0.21 1.18 – – – 0.14 0.18 – – – 0.48 1.90 0.19 0.29 – – 0.76 0.46 – – 2.31 0.51
1.19 1.08 0.75 0.47 0.90 1.17 0.81 0.66 1.15 1.24 1.18 0.82 0.90 1.16 – 1.30 1.46 0.82 0.73 – – 1.06 0.74 – –
16.3 ± 0.1 25.1 ± 0.1 39.3 ± 0.1 57.0 ± 0.3 54.8 ± 0.3 13.6 ± 0.1 30.4 ± 0.1 50.7 ± 0.2 55.3 ± 0.3 67.8 ± 0.2 11.3 ± 0.1 19.0 ± 0.1 27.3 ± 0.1 58.0 ± 0.4 48.8 ± 0.3 11.0 ± 0.1 19.7 ± 0.1 31.3 ± 0.1 43.7 ± 0.2 45.7 ± 0.4 7.5 ± 0.1 9.6 ± 0.1 26.8 ± 0.1 25.8 ± 0.2 62.2 ± 0.4
0.4
0.8
1.2
1.6
Alloy 600
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
4
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
Fig. 2. Topographic SEM pictures of SDC thin films, deposited on SiO2 substrates, using a) 1.2 nm/s deposition rate, 323 K substrate temperature, b) 1.2 nm/s deposition rate, 573 K substrate temperature, c) 0.2 nm/s deposition rate, 873 K substrate temperature, d) 0.8 nm/s deposition rate, 873 K substrate temperature, e) 1.2 nm/s deposition rate, 873 K substrate temperature, and f) 1.6 nm/s deposition rate, 873 K substrate temperature.
effect is negligible. In contrast, increased growth rate has greater effect on grain size at high temperatures. The grains become smaller at higher deposition rate because adatom flux to the surface increases and migration length of adatoms becomes shorter. In addition, the grains in SEM images have the shape of triangular prism, where the height is much lower than the length of base triangle sides. It is also visible that the main differences of surface morphology are the size of the grains and their orientation in formed SDC thin films. The obtained results correspond to other authors' results [22,23]. Other factor, influencing surface morphology of thin SDC films, is substrate type. The effect is more visible at higher substrate temperatures (723 K ÷ 873 K) and deposition rates (1.2 nm/s ÷ 1.6 nm/s). The orientation of the grains changes depending on the substrate type (Fig. 3). The grains of thin films deposited on SiO2 substrates are oriented in parallel to the substrate and the grains of thin films,
deposited on Alloy 600 and Al2O3 substrates are oriented perpendicularly to the substrate. The effect of substrate is negligible at lower temperatures. Movchan-Demchishin model divides the growth of thin films into three zones: Zone I (Ts/Tm b 0.26), Zone II (0.26 b Ts/Tm b 0.45) and Zone III (Ts/Tm N 0.45). Ts and Tm represent substrate temperature and material melting temperature respectively. Grains have small grained porous structure in Zone I and dense columnar structure in Zone II and Zone III [21,24]. The cross section images of thin SDC films correspond to this model. Cross sections have small grained and hardly visible structure at low substrate temperatures (323K) (Fig. 4a). The grains' size increases with increasing substrate temperature (573 K) (Fig. 4b). It is visible that the grains of the triangular prism shape grow on the top of each other. In such manner they start to form columns growth. Finally, the grains are formed with dense columnar structure at 873 K substrate
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
5
Fig. 3. Topographic SEM pictures of thin SDC films, deposited on a) SiO2, b) Alloy 600, and c) Al2O3 substrates using 0.2 nm/s deposition rate and 723 K substrate temperature.
Fig. 4. Cross section of thin SDC films deposited on Alloy 600 substrates, using 1.2 nm/s deposition rate and different substrate temperature: a) 323 K, b) 573 K, and c) 873 K.
temperature (Fig. 4c). It means that thin SDC films grow in Zone I and Zone II according to the Movchan-Demchishin model. The impedance measurements were carried out in order to evaluate the influence of deposition parameters on total conductivity. Total conductivity increases with increasing the temperature of the substrate during deposition (Fig. 5). However, total conductivity of thin SDC films deposited at 323 K and 423 K substrate temperatures decrease
with increasing deposition rate (Fig. 5a) and b). It could be related to surface morphology. Thin films have cracks at these temperatures. On the other hand, the influence of deposition rate decreases with increasing substrate temperature (Fig. 5c and d). So, substrate temperature influencing thin film morphology has greater influence on total conductivity than deposition rate. The oxygen vacancy activation energies were calculated using Arrhenius plots (Table 2). It changes from 0.70 eV to
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
6
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
Fig. 5. Arrhenius plots of total conductivity of SDC thin films, deposited on Al2O3 substrates at different deposition rates and substrate temperatures: a) 0.2 nm/s deposition rate, b) 1.2 nm/s deposition rate, c) 423 K substrate temperature, and d) 873 K substrate temperature.
0.98 eV. In addition, it does not show clear relationship with deposition parameters. It is reasonable to relate total conductivity of thin SDC films with the properties depending similarly on deposition parameters as total conductivity. The texture, surface morphology, and oxygen vacancy activation energy do not show similar behaviour and have not clear relationship with total conductivity of formed SDC thin films. However, the crystallite size depends similarly on the temperature as total conductivity. It is seen that total conductivity increases with increasing crystallite size (Table 3). Total conductivity exhibits its lowest values (3.00·10−5 S/m ÷ 5.25·10− 2 S/m) when the crystallite size is 6.8 nm ÷ 16.0 nm and the total conductivity achieves it's the highest value (1.12 S/m) when it reaches 80.6 nm. Similar results for Sm0.2Ce0.8O2−δ pellets and thin films were obtained by other authors [5,25]. XPS measurements were carried out for SDC thin films in order to evaluate concentration of Ce3+. These SDC thin films were deposited using 0.2 nm/s deposition rate and substrate temperatures from 323 K to 873 K (Fig. 6). Typical curve fitted Ce3d spectra for thin SDC film
deposited using 0.2 nm/s deposition rate and 573 K substrate temperature are shown in Fig. 6b. Ce3d spectra exhibits three pairs of spin orbitales (V, U, V″, U″, and V‴, U‴) for Ce4 + and two pairs of spin orbitales (V0, U0 and V′, U′) for Ce3 +. Ce3 + concentration changes from 24.5% to 29.1% in thin films (Table 4). This shows that SDC thin films are mixed ionic-electronic conductors. However, correlation between changes in total conductivity and concentration of Ce3 + was not observed there. So, it is possible to say that changes in total conductivity depend on ionic conductivity component, i.e. ionic conductivity increases with increasing crystallite size. 4. Conclusions XRD analysis of SDC thin films revealed that the changes in the microstructure occur at high deposition rates (1.2 nm/s, 1.6 nm/s) and high substrate temperatures (723 K and 873 K). The texture coefficients of preferential out-of-plane orientations ((111) and (222)) decrease using these deposition parameters (SiO2 and Alloy 600 substrates). The preferential orientation changes to (220) or (222) using SiO2
Table 2 Oxygen vacancy activation energy (Ea) and total conductivity (σ) dependence on deposition rate (vg) and substrate temperature (Ts) of SDC thin films, formed on Al2O3 substrates. vg, nm/s
Ts, K 323 Ea, eV
0.2 0.4 0.8 1.2 1.6
0.90 0.83 0.85 0.88 0.82
423 σ, S/m
Ea, eV −2
5.25·10 3.00·10−5 1.50·10−4 1.70·10−4 5.00·10−5
0.98 0.86 0.91 0.98 0.90
573 σ, S/m
Ea, eV −2
2.07·10 1.46·10−1 5.36·10−2 5.80·10−4 3.50·10−2
0.91 0.82 0.81 0.80 0.85
723 σ, S/m
Ea, eV −1
1.16·10 4.22·10−1 2.74·10−1 1.30·10−1 1.80·10−1
0.70 0.89 0.89 0.84 0.84
873 σ, S/m −0
1.12·10 2.99·10−1 4.86·10−1 1.91·10−1 2.42·10−1
Ea, eV
σ, S/m
0.93 0.84 0.86 0.86 0.84
4.23·10−1 4.54·10−1 4.40·10−1 4.58·10−1 4.64·10−1
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
7
Table 3 Total conductivity (σ) measured at 873 K dependence on crystallite size (〈d〉 SiO2) of SDC thin films formed on optical quartz (SiO2) and Al2O3 substrates at different substrate temperature (Ts). vg, nm/s
Ts, K 323
0.2 0.4 0.8 1.2 1.6
423
573
723
873
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
〈d〉 SiO2, nm
σ, S/m
9.7 ± 0.1 7.9 ± 0.1 7.5 ± 0.1 10.2 ± 0.1 6.8 ± 0.1
5.25·10−2 3.00·10−5 1.50·10−4 1.70·10−4 5.00·10−5
16.0 ± 0.1 19.9 ± 0.1 12.1 ± 0.1 15.2 ± 0.1 6.9 ± 0.1
2.07·10−2 1.46·10−1 5.36·10−2 5.80·10−4 3.50·10−2
28.7 ± 0.1 30.0 ± 0.1 21.9 ± 0.1 22.7 ± 0.1 17.8 ± 0.1
1.16·10−1 4.22·10−1 2.74·10−1 1.30·10−1 1.80·10−1
80.6 ± 0.4 62.4 ± 0.3 46.1 ± 0.3 31.4 ± 0.1 25.3 ± 0.2
1.12·10−0 2.99·10−1 4.86·10−1 1.91·10−1 2.42·10−1
66.2 ± 0.6 48.7 ± 0.2 66.5 ± 0.3 50.2 ± 0.5 48.0 ± 0.3
4.23·10−1 4.54·10−1 4.40·10−1 4.58·10−1 4.64·10−1
Fig. 6. XPS curves of Ce3d core levels for thin SDC films, deposited on Al2O3 substrates using a) 0.2 nm/s deposition rate and b) 573 K temperature and 0.2 nm/s deposition rate [V0 − Ce3+ 3d5/2, V−Ce4+ 3d5/2, V′ −Ce3+ 3d5/2, V″ −Ce4+ 3d5/2, V‴ −Ce4+ 3d5/2, U0 −Ce3+ 3d3/2, U−Ce4+ 3d3/2, U′ −Ce3+ 3d3/2, U″ −Ce4+ 3d3/2, U‴ −Ce4+ 3d3/2].
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007
8
M. Sriubas et al. / Solid State Ionics xxx (2016) xxx–xxx
Table 4 Total conductivity (σ) measured at 873 K and Ce3+ concentrations (CCe3+) in thin SDC films, deposited on Al2O3 substrate, using 0.2 nm/s deposition rate. σ, S/m CCe3+, %
2.07·10−2 25.6
5.25·10−2 24.5
1.16·10−1 29.1
4.23·10−1 26.8
1.12·10−0 28.1
substrates (1.2 nm/s and 1.6 nm/s growth rate; 423 K, 723 K, and 873 K substrate temperature) and to (200), (220), or (311) using Alloy 600 substrates (0.2 nm/s, 0.8 nm/s, 1.2 nm/s, and 1.6 nm/s deposition rate; 723 K and 873 K substrate temperature). The crystallinity changes with deposition parameters, also. The crystallite size increases from 6.8 nm to 80.6 nm with increasing substrate temperature (323 K ÷ 873 K). However, the crystallite size decreases with increasing deposition rate (from 0.2 nm/s to 1.6 nm/s). It decreases from 80.6 nm to 25.3 nm using 723 K substrates and from 66.2 nm to 48.0 nm using 873 K substrates. These changes in microstructure can be explained by variation in adatom migration energy and length due to the influence of substrate temperature and deposition rate. SEM analysis revealed that grain size increases with increasing substrate temperature at low temperatures (323 K, 423 K, and 573 K) and decreases with increasing substrate temperature at high temperatures (723 K and 873 K). In addition, thin films deposited at 323 K ÷ 423 K temperatures and using 0.4 nm/s ÷ 1.6 nm/s deposition rates have cracks. Moreover, the orientation of grains depends on substrate type. The changes on the surface morphology are related to the different surface energies of substrates, increased influence of growth rate and increased contribution of (200), (220) and (311) orientations at high substrate temperatures (723 K and 873 K). Total conductivity of thin SDC films increases from 3.00·10−5 S/m to 1.12 S/m with increasing crystallite size from 6.8 nm to 80.6 nm, i.e. with increasing substrate temperature during the deposition of thin films. In addition, it decreases with increasing deposition rate at 323 K and 423 K substrate temperatures. However, correlation between changes in total conductivity and concentration of Ce3 + was not observed there. Therefore, it is reasonable to conclude that ionic conductivity increases with increasing crystallite size. References
[2] S.J. Skinner, J.A. Kilner, Oxygen ion conductors, Mater. Today 6 (3) (2003) 30–37. [3] J. Garcia-Barriocanal, et al., Colossal ionic conductivity at interfaces of epitaxial ZrO2: Y2O3/SrTiO3 heterostructures, Science 321 (5889) (2008) 676–680. [4] C.J. Fu, et al., Effects of transition metal oxides on the densification of thin-film GDC electrolyte and on the performance of intermediate-temperature SOFC, Int. J. Hydrog. Energy 35 (20) (2010) 11200–11207. [5] R. Sanghavi, et al., Integrated experimental and modeling study of the ionic conductivity of samaria-doped ceria thin films, Solid State Ionics 204–205 (2011) 13–19. [6] S. Zha, C. Xia, G. Meng, Effect of Gd (Sm) doping on properties of ceria electrolyte for solid oxide fuel cells, J. Power Sources 115 (1) (2003) 44–48. [7] P. Arunkumar, M. Meena, K.S. Babu, A review on cerium oxide-based electrolytes for ITSOFC, Nano Energy 1 (5) (2012) 288–305. [8] J.S. Park, et al., Oxygen diffusion across the grain boundary in bicrystal yttria stabilized zirconia, Solid State Commun. 152 (24) (2012) 2169–2171. [9] B. Zhu, Solid oxide fuel cell (SOFC) technical challenges and solutions from nanoaspects, Int. J. Energy Res. 33 (13) (2009) 1126–1137. [10] X. Guo, R. Waser, Electrical properties of the grain boundaries of oxygen ion conductors: acceptor-doped zirconia and ceria, Prog. Mater. Sci. 51 (2) (2006) 151–210. [11] G. Balakrishnan, et al., A study of microstructural and optical properties of nanocrystalline ceria thin films prepared by pulsed laser deposition, Thin Solid Films 519 (8) (2011) 2520–2526. [12] E.S. Bârcă, et al., Pyramidal growth of ceria nanostructures by pulsed laser deposition, Appl. Surf. Sci. 363 (2016) 245–251. [13] M. Chen, et al., Effect of Ni content on the microstructure and electrochemical properties of Ni–SDC anodes for IT-SOFC, Solid State Ionics 181 (23–24) (2010) 1119–1124. [14] Z. Yang, et al., Selection and evaluation of heat-resistant alloys for SOFC interconnect applications, J. Electrochem. Soc. 150 (9) (2003) A1188–A1201. [15] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Weseley Metallurgy Series, , Addison-Wesley publishing, Reading, Massachusetts, 1956 514. [16] P. Nagaraju, Y. Vijayakumar, M.V. Ramana Reddy, Optical and microstructural studies on laser ablated nanocrystalline CeO2 thin films, Glas. Phys. Chem. 41 (5) (2015) 484–488. [17] F. Bechstedt, Principles of Surface Physics, first ed.XII, Springer-Verlag, Berlin Heidelberg, 2003 342. [18] P. Arunkumar, et al., Texturing of pure and doped CeO2 thin films by EBPVD through target engineering, RSC Adv. 4 (63) (2014) 33338–33346. [19] E. Grantscharova, Texture transition in thin metal films vacuum condensed on glass: a general consideration, Thin Solid Films 224 (1) (1993) 28–32. [20] S.G. Wang, E.K. Tian, C.W. Lung, Surface energy of arbitrary crystal plane of bcc and fcc metals, J. Phys. Chem. Solids 61 (8) (2000) 1295–1300. [21] K.H. Guenther, Revisiting Structure-Zone Models for Thin-Film Growth, 1990. [22] C. Mansilla, Structure, microstructure and optical properties of cerium oxide thin films prepared by electron beam evaporation assisted with ion beams, Solid State Sci. 11 (8) (2009) 1456–1464. [23] M.S. Anwar, et al., Study of nanocrystalline ceria thin films deposited by e-beam technique, Curr. Appl. Phys. 11 (Suppl. 1) (2011) S301–S304. [24] B.A. Movchan, A.V. Demchishin, Structure and properties of thick condensates of nickel, titanium, tungsten, aluminum oxides, and zirconium dioxide in vacuum, Fiz. Met. Metalloved. 28 (1969) 653–660. [25] G. Hua, et al., Enhanced ionic doping transition metals, J. Mater. Sci. Mater. Electron. 26 (6) (2015) 3664–3669.
[1] N. Laosiripojana, et al., Reviews on Solid Oxide Fuel Cell Technology, vol. 13, 2009.
Please cite this article as: M. Sriubas, et al., Investigation of microstructure and electrical properties of Sm doped ceria thin films, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.11.007