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Spray pyrolytic synthesis of samarium doped ceria (Ce0.8Sm0.2O1.9) films for solid oxide fuel cell applications
Applied Surface Science 253 (2007) 4994–5002 www.elsevier.com/locate/apsusc
Spray pyrolytic synthesis of samarium doped ceria (Ce0.8Sm0.2O1.9) films for solid oxide fuel cell applications B.B. Patil a, S.H. Pawar a,b,* a
School of Energy Studies, Department of Physics, Shivaji University, Kolhapur 416 004, India b Tatyasaheb Kore Institute of Engineering and Technology, Warananagar 416 113, India
Received 14 August 2006; received in revised form 7 November 2006; accepted 7 November 2006 Available online 8 December 2006
Abstract Uniform, adherent, single phase samarium doped ceria films have been successfully deposited by spray pyrolysis technique for their application in solid oxide fuel cell. These films have been deposited at different substrate temperatures on glass substrate and subsequently heat treated in tube furnace. Effect of substrate temperature and annealing temperature on phase formation was studied with thermo-gravimetric analysis and differential temperature analysis, X-ray diffraction, scanning electron microscope, and energy dispersive X-ray analysis techniques. These studies showed the formation of single phase Ce0.8Sm0.2O1.9 films, at substrate temperature 400 8C and annealing temperature 550 8C. Electrical resistivity of the films, at room temperature was of the order of 107 V cm while at 400 8C it is found to be of the order of 101 V cm. This reveals the use of these films for making low temperature solid oxide fuel cells. # 2006 Elsevier B.V. All rights reserved. PACS : 82.47.Ed Keywords: Solid oxide fuel cell; CeO2 based ceramics; Spray deposition technique; Samarium doped ceria films
1. Introduction Fuel cell is an electrochemical device, which converts chemical energy of fuel directly into electrical energy. It will not run down as long as fuel and oxidant are supplied to it. It consists of three important parts as anode, cathode and electrolyte. The electrolyte is sandwiched between the two electrodes. Depending on type of electrolyte the fuel cells are classified as alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), polymer electrolyte membrane fuel cell (PEMFC), phosphoric acid fuel cell and solid oxide fuel cell (SOFC). Among all these types of fuel cells SOFC offers many advantages over conventional power generating systems in terms of efficiency, reliability, modularity, fuel flexibility and environmental friendliness [1]. Since the ionic conductivity of rare-earth or alkaline earth doped ceria has been known to
* Corresponding author at: School of Energy Studies, Department of Physics, Shivaji University, Kolhapur 416 004, India. E-mail address: [email protected] (S.H. Pawar). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.11.007
possesses higher ionic conductivity than YSZ [2–4], the higher efficiencies can be achieved at low temperature below 800 8C which will help to increase the stack life time and also reduce the fabrication cost by relieving the material constraints. Rare earth-doped ceria is known to exhibit mixed ionic and electronic conductivity under reducing atmospheres. In fact the maximum ionic conductivity in fluorite type oxides is observed when the dopants ion radius is close to the radius of host cations [5]. Therefore among the aliovalent cation doped ceria, samarium doped ceria possesses high ionic conductivity [6]. Due to high ionic conductivity this material is used as an electrolyte [7–9] and due to mixed conductivity is also used as anode or in the synthesis of composite anode for SOFC such as Ru–SDC [10], Ni–SDC [11]. There are different methods for the synthesis of samarium doped ceria material, such as sol–gel [12,13], solid-state [14– 16], pulse laser deposition, etc. In the present work spray pyrolysis technique was used for the synthesis of samarium doped ceria. Spray pyrolysis is an integrated process, which consists of three consecutive steps, namely atomization of liquid into droplets, traveling of droplets with atomization gas
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and deposition of droplets on to three-dimensional reform [17]. This technique has some advantages such as: 1. It is simple and low cost. 2. It has capability to produce large area of high quality films of uniform thickness. 3. Doping of the film is simple, since it is accomplished by merely addition of dopants to the spray solution. 4. It is easy to prepare film of any composition by simply mixing the components in appropriate ratios.
from 10 to 1000 8C in air environment. For DTA/TG analysis, SDC thin films were scratched out and powder was taken for the further study. The films TA300, TA350, TA400 and T400 were characterized by X-ray diffraction (XRD) technique using Phillips diffractometer PW1710 with Cu Ka radiation having wavelength ˚ . The spectra were obtained over the 2u range 208– 1.5424 A 1008. The diffractometer was operated at 40 kV and 30 mA. The crystallite size was determined by using Scherrer’s formula d¼
Very little data is available on the spray pyrolytic synthesis of samarium doped ceria (Ce0.8Sm0.2O1.9) films. However in the present investigation thin films of single phase samarium doped ceria have been deposited successfully by spray pyrolysis technique. The films are characterized for their structural, morphological and electrical properties and the results are reported in this paper. 2. Experimental In order to prepare films of samarium doped ceria (SDC) having composition Ce0.8Sm0.2O1.9 stoichiometric amount of reagent grade samaria oxide Sm2O3 (99.9% pure, Loba chemicals) and cerium nitrate Ce(NO3)36H2O (99.9% pure, Loba chemicals) were dissolved in double distilled water. Ultrasonically cleaned plane glass plates were used as substrates. Concentration of precursor solution was varied between 0.025, 0.050, 0.075 and 0.1 M. The optimized concentration was found to be 0.1 M. In spray pyrolysis technique in order to get good quality, adherent films there is need to optimize certain preparative parameters such as: spray rate, air pressure, concentration of precursor solution, substrate temperature, quantity of solution, etc. optimized parameters for the deposition of thin film of samaria doped ceria material are spray rate 3 ml/min, concentration of precursor solution 0.1 M, quantity 25 ml. Substrate temperature was varied from 300 to 450 8C with an interval of 50 8C and these films were described as T300, T350, T400 and T450. Optimized substrate temperature for the deposition of SDC films was found to be 400 8C. Deposited films then allowed to cool slowly, as slow cooling produces films with higher resistivity, possibly because of the reaction with oxygen in air over a larger time used in cooling. Films were then annealed in tube furnace at 550 8C for 3 h and were described as TA300, TA350, TA400 and TA450, respectively. Film thickness was calculated by using weight difference method by using the formula: t¼
weight difference Ar
where A is the area of the film and r is the density of the bulk material and for Ce0.8Sm0.2O1.9 density was taken as 7.15 g/ cm3. Differential thermal analysis/thermo-gravimetric analysis (DTA/TG) were carried out by using an instrument TG-DTADSC, SDT-2960, TA Inc., USA, with a heating rate 10 8C/min
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0:9l b cos u
where, d is the crystallite size of the material, l is the wavelength of the X-ray radiation, b is the full width at half maximum and u is the angle of diffraction. In order to find out the optimized annealing temperature, the film T400 was further heat treated at various temperatures such as 400, 450, 500, 550, and 600 8C and were described as T400, TH400, TH450, TH500, TH550 and TH600 (here TA400 = TH550). The description of all samples is recapped in Table 1. Scanning electron microscope (SEM) images of the films T400, TH400, TH450, TH500, TH550 and TH600 were taken by using scanning electron microscope mode JEOL JSM 636D. Composition of the deposited film was determined by energy dispersive X-ray analysis (EDAX) technique using X-ray diffractometer attested JEOL JSM 636D SEM model. Two probe resistivity method was used to study the effect of substrate temperature on the electrical property of SDC film in the temperature range 27–425 8C. Table 1 Description of the samples T300 T350 T400 T450 TA300 TA350 TA400 TA450 TH400 TH450 TH500 TH550 TH600 Here TH550 = TA400
As deposited SDC film at 300 8C substrate temperature As deposited SDC film at 350 8C substrate temperature As deposited SDC film at 400 8C substrate temperature As deposited SDC film at 450 8C substrate temperature SDC film deposited at 300 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 350 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 450 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 400 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 450 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 500 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 600 8C
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3. Results and discussions 3.1. Optimization of preparative parameters and effect of substrate temperature on the thickness of the Ce0.8Sm0.2O1.9 film The Ce0.8Sm0.2O1.9 films deposited at different substrate temperatures ranging from 300 to 450 8C by using spray pyrolysis technique. It is found that the films deposited at substrate temperatures 300, and 350 8C were not completely formed. This might be due to the incomplete decomposition of the precursor solution at these substrate temperatures. The films deposited at the substrate temperature 450 8C were of less thickness and also dry and powdery in nature. But at 400 8C temperature uniform, smooth, adhesive and milky white films of samarium doped ceria were formed and therefore this temperature is considered as optimized temperature. The variation of the thickness of the films with substrate temperature is shown in Fig. 1. It is observed that, the thickness of the films is found to decrease with increase in the substrate temperature. This is attributed to the increase in the evaporation rate of the initial products with increase in the substrate temperature. Concentration of precursor solution was varied between 0.025, 0.050, 0.075 and 0.1 M. At concentrations 0.025, 0.050, and 0.075 M the films of lower thickness were formed and also their thickness was not uniform through out the film. This might be due to excess evaporation of precursor solution at optimized substrate temperature. However at 0.1 M concentration uniform films of SDC were formed. Therefore this concentration is considered as optimized concentration for SDC film formation. 3.2. DTA/TG Simultaneous DTA/TG was carried out to find out the minimum temperature required for Ce0.8Sm0.2O1.9 phase formation. In order to determine the crystallization process of a Ce0.8Sm0.2O1.9 material, thermal analysis of spray deposited
Fig. 1. Effect of substrate temperature on the thickness of Ce0.8Sm0.2O1.9 films.
Fig. 2. The simultaneous DTA/TG curves of spray deposited Ce0.8Sm0.2O1.9 powder.
SDC powder was carried out. Fig. 2 shows the DTA/TG curve of the Ce0.8Sm0.2O1.9 spray deposited powder scratched from the glass substrate. The TG curve reveals a weight loss of 2.5% below 350 8C, which corresponds to the formation of SDC phase by the decomposition of the precursor solution. DTA curve shows broad exothermic peak at 279 8C and after at about 350 8C no further change in DTA curve was observed. Since instead of using starting material directly the material scratched from the deposited film was taken, so very little weight loss of 4% was observed and this might be due to the loss of residue remains from the precursor solution. In accordance with the results of the DTA/TG, crystallization of the film occurred by post heat treatment to the film at 550 8C/3 h and almost all the characteristic peaks corresponding to the cubic fluorite structure of the Ce0.8Sm0.2O1.9 have appeared [18]. Films annealed at these conditions did not show peaks corresponding to Sm2O3 indicating direct formation of Ce0.8Sm0.2O1.9 phase. This has been evidenced by XRD results as reported in latter section. 3.3. XRD characterization 3.3.1. Effect of substrate temperature on the XRD patterns of the Ce0.8Sm0.2O1.9 films The Ce0.8Sm0.2O1.9 films deposited at different substrate temperatures were characterized by using X-ray diffractometer. Fig. 3 shows the XRD patterns of TA300, TA350 and TA400 and T400 films. XRD patterns of all films TA300, TA350 and TA400 and T400 shows peaks corresponding to cubic phase of Ce0.8Sm0.2O1.9, confirming its poly-crystalline nature. All peaks have been identified and indexed from the known patterns of the standard data files [pdf no. Ce (hexagonal): 03065-3368, Ce (tetragonal): 03-065-5410, SDC (cubic): ICSD data base code: 28792]. From the XRD patterns, lattice constant ‘a’ of TA300, TA350, TA400, and T400 have been calculated and are ˚ , respectively. found to be 5.443, 5.4572, 5.4258, and 5.4678 A ˚ of These values match well with the standard ‘a’ value 5.433 A the Ce0.8Sm0.2O1.9 phase having cubic fluorite structure.
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002
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Fig. 3. The XRD patterns of: (a) TA300, (b) TA350 and (c) TA400 and (d) T400 Ce0.8Sm0.2O1.9 films.
The film TA300 and TA350 shows some unidentified impurities peak. Also film TA350 shows peaks corresponding to hexagonal (pdf no. Ce (hexagonal): 03-065-3368) and tetragonal cerium (Ce (tetragonal):03-065-5410) along with cubic Ce0.8Sm0.2O1.9 phase (ICSD data base code: 28792). This might be due to the incomplete decomposition of the precursor solution on the surface of the substrate. Impurity peak (1 0 3) due to elemental cerium is indexed as ‘‘103 Ce’’ in Fig. 3. The observed and standard ‘d’ values of the spray deposited Ce0.8Sm0.2O1.9 thin films having cubic fluorite structure are listed in Table 1A. From Table 1A, it is observed that for all films (1 1 1) reflection is most intense, also number of peaks corresponding to the SDC phase found to increase with increase in substrate temperature. All major reflections corresponding to SDC cubic fluorite phase like, (1 1 1), (0 0 2), (0 2 2), (1 1 3), (2 2 2), (0 0 4), (1 3 3), (0 2 4), and (2 2 4) were obtained for the film TA400. Thus for the SDC film deposited at the substrate temperature 400 8C and post heat treated at 550 8C/3 h (TA400) no phase corresponding to Sm2O3 or cerium were observed confirming the direct formation of Ce0.8Sm0.2O1.9 phase.
Comparison of the intensities of TA300, TA350, TA400 and T400 SDC films is listed in Table 1B. The highest intensity X-ray diffraction (1 1 1) peak lying at about 2u = 288 has been further analyzed to yield the crystallite size of the SDC material by using the Scherer’s formula. Full width half maxima (b) of the (1 1 1) peak for TA300, TA350 and TA400 and T400 were determined from XRD and by using these b values crystallite size of SDC film was calculated. Crystallite size of TA300, TA350, TH400 and T400 films are tabulated in Table 2. Fig. 4 shows variation of the crystallite size of Ce0.8Sm0.2O1.9 film with substrate temperature. It is observed from Fig. 4 that, at lower substrate temperature such as 300, and 350 8C films exhibit smaller crystallite size than the film deposited at 400 8C substrate temperature. Thus the crystallite size increases with increase in substrate temperature. Also comparison of the crystallite size of the TA400 and T400 films reveals that the annealing of the films at the higher temperature increases the crystallite size of the material.
Table 1A Comparison of the d values of TA300, TA350, TA400 and T400 SDC films
Table 1B Comparison of the intensities of TA300, TA350, TA400 and T400 SDC films
hkl
111 002 022 113 222 004 133 024 224 115
˚) d (std.) (A
3.1367 2.7165 1.9208 1.6381 1.5683 1.3582 1.2464 1.2148 1.1090 1.0415
˚) d (calculated) (A
hkl
TA300
TA350
TA400
T400
3.1429 2.7203 1.9251 – – – – – – –
3.1494 2.7231 – 1.6569 – 1.3636 – – 1.1327 1.0396
3.1091 2.6999 1.9123 1.6386 1.6386 1.5708 1.3577 1.2459 1.2148 1.1161
3.1532 2.7251 – 1.6581 – 1.3651 – – – –
111 002 022 113 222 004 133 024 224 115
100 I/I0 (calculated) TA300
TA350
TA400
T400
100 49.5 35.1 – – – – – – –
100 48.9 – 13.6 – 5.4 – – 5 8.5
100 42.3 41.4 35.3 6.2 4 7.2 3.8 4 –
100 23.4 – 12.6 – 5.5 – – – –
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B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002 Table 2 Variation of crystallite size of TA300, TA350, TA400 and T400 Ce0.8Sm0.2O1.9 films with substrate temperature
Fig. 4. Variation of the crystallite size of Ce0.8Sm0.2O1.9 film with substrate temperature.
3.3.2. Effect of annealing temperature on the XRD patterns of the Ce0.8Sm0.2O1.9 films Fig. 5 shows XRD patterns of T400, TH400, TH450, TH500, and TH550 thin films. XRD shows peaks confirming the polycrystalline nature of the films. All peaks have been identified and indexed from the known patterns of the standard data files SDC (cubic): ICSD database code: 28792. From the XRD patterns, lattice constant ‘a’ of
Film
Crystallite size 0.2 (nm)
TA300 TA350 TH400 T400
7 8 11 7
T400, TH400, TH450, TH500, and TH550 have been calculated and it is found that the value of the lattice constant ‘a’ for these films ˚ match well with 5.4441, 5.4678, 5.4569, 5.4445, and 5.4258 A ˚ standard ‘a’ value 5.433 A of the Ce0.8Sm0.2O1.9 phase having cubic fluorite structure. The d values and the intensities of different planes for the films deposited at 400 8C substrate temperature and annealed at different temperatures are listed, respectively, in Tables 3A and 3B. It is seen that the intensity of (1 1 1) plane is strongest one for all the films. However the intensity of (0 0 2) plane is found to decrease with annealing temperature. This can be understood with the changes in the crystallographic unit cell of cerium oxide. The schematic picture of crystallographic unit cell of CeO2 is shown in Fig. 6. The structure is FCC with cerium atoms placed at the corners and at the center of the faces of the unit cell. Each
Fig. 5. The XRD patterns of: (a) T400, (b) TH400, (c) TH450, (d) TH500, and (e) TH550 Ce0.8Sm0.2O1.9 films.
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002 Table 3A Comparison of d values of T400, TH400, TH450, TH500 and TH550 Ce0.8Sm0.2O1.9 films ˚) d (std.) (A
hkl
111 002 022 113 222 004 133 024 224 115
3.1367 2.7165 1.9208 1.6381 1.5683 1.3582 1.2464 1.2148 1.1090 1.0415
˚) d (calculated) (A T400
TH400
TH450
TH500
TH550
3.1423 – 1.9258 1.6411 – – – – – –
3.1532 2.7251 – 1.6581 – 1.3651 – – – –
3.1516 2.7381 1.9276 1.6419 – 1.3608 1.2538 – – –
3.1521 2.7312 1.9149 1.6445 1.5700 1.36 – – 1.112 1.0452
3.1091 2.6999 1.9123 1.6386 1.5708 1.3577 1.2459 1.2148 1.1161 –
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Table 4 Variation of crystallite size of T400, TA400, TA450, TA500, and TA550 SDC films with substrate temperature Annealing temperature (8C)
Crystallite size 0.2 (nm)
Without annealing 400 450 500 550
7 8 9 11 11
cerium atom is surrounded by eight oxygen atoms. From this structure the density of the atoms in (1 1 1) plane is maximum, which is in support of the maximum X-ray diffraction intensity observed in all the samples. The intensities as well as the d values of (0 0 2) plane are found to decrease with increase in
annealing temperature. This indicates that there might be vacancies created due to the replacement of the face centered cerium atoms. This is in favor of creating vacancy defects and helps to increase the conduction mechanism, which is needed for its application as the ionic conductor electrolyte. The observed and standard ‘d’ values and intensities for T400, TH400, TH450, TH500 and TH550 SDC films are listed in Tables 3A and 3B, respectively. From the Table 3A it is observed that for the film TA400 no phase corresponding to the Sm2O3 or cerium has been observed confirming the direct formation of Ce0.8Sm0.2O1.9 phase. XRD studies also reveal that the number of peaks corresponding to SDC phases increase with increase in the annealing temperature. This might be due to the increase in the crystalline nature of the film with annealing temperature. Full width half maxima (b) of the (1 1 1) peak for T400, TH400, TH450, TH500, and TH550 Ce0.8Sm0.2O1.9 thin films were determined from XRD. By using these b values crystallite size of SDC films were calculated by using Scherer’s formula and the crystallite size for above films are tabulated in Table 4. It is observed from Fig. 7 that, crystallite size of the deposited film is found to increase with the increase in annealing temperature. This might be due to the increase in the crystalline nature of the film with annealing temperature. From Tables 1 and 3 optimized substrate and annealing temperature for the single-phase deposition of the SDC thin film were found to be 400 and 550 8C, respectively.
Fig. 6. The schematic picture of crystallographic unit cell of CeO2.
Fig. 7. Variation of the crystallite size of Ce0.8Sm0.2O1.9 film with annealing temperature.
Table 3B Comparison of intensities of T400, TH400, TH450, TH500 and TH550 Ce0.8Sm0.2O1.9 films 100 I/I0 (calculated)
hkl
111 002 022 113 222 004 133 024 224 115
T400
TH400
TH450
TH500
TH550
100 – 32.7 20.4 – – – – – –
100 23.4 – 12.6 – 5.5 – – – –
100 47.8 36.5 28.8 5.8 6.9 – – – –
100 72.2 32.3 28.9 3.5 5.3 – – 3.1 3.8
100 42.3 41.4 35.3 6.2 4 7.2 3.8 4 –
5000
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002
Fig. 8. SEM images of SDC films (a) as deposited an annealed at (b) 400 8C, (c) 450 8C, (d) 500 8C, (e) 550 8C, and (f) 600 8C.
3.4. Morphological studies Morphology of the as deposited and SDC films annealed at different annealing temperatures were studied. Fig. 8 shows the scanning electron microscope images of (8a) T400 and, (8b) TH400, (8c) TH450, (8d) TH500, (8e) TH550 and (8f) TH600 SDC films, respectively. SEM images of these films show formation of uniform and dense films. Film T400, post heat treated at 600 8C shows many cracks, these cracks are due to the melting of the glass substrate. It is observed that, substrate starts to melt above 550 8C. From the SEM graph of SDC film heat treated at lower temperature, cracking of the film on the substrate surface can be seen. This might be due to shrinkage and compaction of the film upon heat treatment. Such cracking was also observed in case of CeO2 films deposited by electrophoresis [19] and anodic electrodeposition [20].
Fig. 9 shows EDAX pattern of the spray deposited Ce0.8Sm0.2O1.9 film post heat treated at 600 8C (TA600). Fig. 9 shows peaks corresponding to the elements Ce, Sm, O, along with the elemental peaks corresponding to Na, Si, Mo, and Pt. Peaks corresponding to the elements Na, Si, Mo, and Pt are due to melting of the glass substrate. Table 5 presents the
3.5. Compositional study In order to find out the composition of the deposited SDC film energy dispersive X-ray analysis plot of TA400 film was carried out. SEM of the same film showed many cracks due to melting of glass substrate. This cracking is also confirmed by EDAX studies.
Fig. 9. EDAX pattern of the spray deposited Ce0.8Sm0.2O1.9 film.
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002 Table 5 Composition of the spray deposited Ce0.8Sm0.2O1.9 film post heat treated at 600 8C (TA600) Element
Atomic wt.(%)
Error (%)
O Ce Sm Na Si Mo Pt
44 30 8 7 5 5 1
0.2 0.2 0.2 0.4 0.5 0.8 0.6
composition of the above film in weight percent. From Table 5, composition ratio of Ce:Sm is found out to be 29:7 and it is approximately equals to 4:1. Thus EDAX studies showed the formation of stoichiometric Ce0.8Sm0.2O1.9 thin films. 3.6. Electrical resistivity study Two-probe resistivity method was used to study the electrical properties of Ce0.8Sm0.2O1.9 films deposited at different substrate temperatures and heat treated at 550 8C. Two-probe resistivity technique is the DC measurement technique. In this technique a brass block was used as a sample holder cum heater. The films of the size 3.7 cm 1.0 cm on the glass substrate were used for this study. Two press contacts were made to the film with the help of pointed brass screws. The area of the film on the glass substrate was defined as 1 cm 1 cm and silver paste was applied to ensure the ohmic contact to the film. Cromel–Alumel thermocouple was used to measure the temperature. Transistorized power supply unit (TPSU) model LVA 30/1 Aplab was used as a constant source for voltage. Current in the circuit was measured with digital nanometer model DNM-121. The resistivity of the film was studied in the temperature range 27–425 8C. Measurements were carried out under air atmosphere. It is observed that, the resistivity of all the films decreases with increase in the temperature showing semiconducting nature. The typical plot of log r versus 1/T for TA400 is shown in
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Table 6 The activation energies for TA300, TA350, TA400 and TA450 films Films
TA300 TA350 TA400 TA450
Activation energies 0.03 eV Low temperature
High temperature
0.48 0.56 0.21 0.52
0.73 0.78 0.72 0.74
Fig. 10. The plot shows two regions corresponding to high and low temperatures. The limit of both domains was found at 50 8C. Ceria is usually considered to provide mixed electronic and ionic conductivity [21]. Pure ceria oxide is a poor oxygen vacancy conductor [22]. Substitution of aliovalent cations of lower valance for Ce results in the formation of oxygen vacancies to compensate the charge balance in sub-lattice. Thus the solid solution becomes predominantly ionic conductive for oxygen vaccines over an extended temperature range. It is reported that among cations doped ceria Sm doped ceria found to have highest ionic conductivity. There is formation of large amount of oxygen vacancies, VO, in the CeO2 fluorite lattice [23–25], due to the addition of Sm2O3 in as expressed by the following reaction: 2SmO1:5 ! 2Sm0 Ce þ O þ VO where Sm0 Ce is a Sm3+ ion in a Ce4+ lattice with negative charge. Increase in the conductivity in the film with increasing temperature is due to introduction of oxide ion vacancies by charge compensation. Similar results were reported by Eguchi [26,27]. Therefore in this case electrical conductivity measured is considered to be completely due to ionic conduction. The room temperature resistivity of TA300, TA350, TA400 and TA450 films is of the order of 107 V cm and the resistivity of the film was found to be of the order of 101 V cm at 400 8C. This very large decrease in the resistivity is due to the role of ionic conductivity. Activation energies of the deposited films were calculated by using the relation: Ea r ¼ r exp ; kT where r is the resistivity, Ea the activation energy, k the Boltzmann constant and T is the absolute temperature. Table 6 shows the activation energies for TA300, TA350, TA400 and TA450 films, corresponding to high and low temperature region. Activation energy values were found to be in the range 0.7–0.8 eV which is similar to the value of 0.78 eV for the activation energy reported by others [6,27]. 4. Conclusions
Fig. 10. Plot of log r vs. 1/T for TA400 SDC film.
Optimized substrate and annealing temperatures for the synthesis of uniform, adherent, single phase samarium doped ceria films by spray deposition technique was found to be 400
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and 550 8C, respectively. Crystallite size was found to increase with increase in substrate and annealing temperatures. Resistivity studies showed that deposited films are of semiconducting nature with activation energy 0.72 eV. The room temperature resistivity of TA300, TA350, TA400 and TA450 films is of the order of 107 V cm and the resistivity of the film was found to be of the order of 101 V cm at 400 8C. This very large decrease in the resistivity is due to the role of ionic conductivity. Composition of the film was found to be Ce:Sm 8:2. Thus stiochiometric, uniform, single-phase SDC films suitable for SOFC application are successfully deposited by a low cost spray pyrolysis method. Acknowledgements One of the authors BBP is thankful to the Ministry of Non Conventional Energy Sources for providing financial assistance. They are also indebted to U.G.C., Delhi for financial support. References [1] [2] [3] [4] [5] [6]
N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. H. Yahiro, T. Ohuchi, K. Eguchi, H. Arai, J. Mater. Sci. 23 (1988) 1036. H. Yahiro, K. Eguchi, H. Arai, Solid State Ionics 21 (1986) 37. H. Yahiro, K. Eguchi, H. Arai, Solid State Ionics 36 (1989) 71. B.C.H. Steele, Solid State Ionics 129 (2000) 95. H. Yahira, Y. Egushi, K. Eguchi, H. Aria, J. Appl. Electrochem. 18 (1988) 527.
[7] G.B. Jung, T.J. Huang, M.H. Huang, C.L. Chang, J. Mater. Sci. 36 (2001) 5839–5844. [8] D.-S. Bae, E.-J. Kim, S.-W. Park, K.-S. Han, J. Ceram. Process. Res. 5 (3) (2004) 244–246. [9] J.-G. Li, T. Ikegami, T. Mori, Acta Mater. 52 (2004) 2221–2228. [10] H. Uchida, H. Suzuki, M. Watanabe, J. Electrochem. Soc. 145 (2) (1998) 615–620. [11] S. Ohara, R. Maric, X. Zhang, K. Mukai, T. Fukui, H. Yoshida, T. Inagaki, K. Miura, J. Power Sources 86 (2000) 455–458. [12] K.Q. Huang, M. Feng, J.B. Goodenough, J. Amer. Ceram. Soc. 81 (2) (1998) 357. [13] J.V. Herle, T. Horita, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, Solid State Ionics 86–88 (1996) 1255. [14] O.A. Marina, M. Mogensen, Appl. Cata. A 189 (1999) 117. [15] R.J. Gorte, H. Kim, J.M. Vohs, J. Power Sources 106 (2002) 10. [16] B.C.H. Steele, P.H. Middleton, R. Rudkin, Solid State Ionics 40 (1990) 388. [17] A.R.E. Singer (1972) British Patent No. 1262471. [18] ICSD database code 28792. [19] I. Zhitomirsky, A. Petric, Mater. Lett. 40 (1999) 263. [20] A.Q. Wang, T.D. Golden, J. Electrochem. Soc. 150 (9) (2003) C616– C620. [21] E.P. Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27. [22] H. Yahiro, Y. Baba, K. Eguchi, H. Arai, J. Electrochem. Soc. 135 (1998) 2077. [23] Y. Egushi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics 52 (1992) 165. [24] H. Yahiro, K. Eguchi, H. Arai, J. Appl. Electrochem. 18 (1988) 527. [25] G.B. Balazs, R.S. Glass, Solid State Ionics 76 (1995) 155. [26] K. Eguchi, J. Alloys Compd. 250 (1997) 486–491. [27] G.-B. Jung, T.-J. Huang, C.L. Chang, J. Solid State Electrochem. 6 (2002) 225–230.
Spray pyrolytic synthesis of samarium doped ceria (Ce0.8Sm0.2O1.9) films for solid oxide fuel cell applications B.B. Patil a, S.H. Pawar a,b,* a
School of Energy Studies, Department of Physics, Shivaji University, Kolhapur 416 004, India b Tatyasaheb Kore Institute of Engineering and Technology, Warananagar 416 113, India
Received 14 August 2006; received in revised form 7 November 2006; accepted 7 November 2006 Available online 8 December 2006
Abstract Uniform, adherent, single phase samarium doped ceria films have been successfully deposited by spray pyrolysis technique for their application in solid oxide fuel cell. These films have been deposited at different substrate temperatures on glass substrate and subsequently heat treated in tube furnace. Effect of substrate temperature and annealing temperature on phase formation was studied with thermo-gravimetric analysis and differential temperature analysis, X-ray diffraction, scanning electron microscope, and energy dispersive X-ray analysis techniques. These studies showed the formation of single phase Ce0.8Sm0.2O1.9 films, at substrate temperature 400 8C and annealing temperature 550 8C. Electrical resistivity of the films, at room temperature was of the order of 107 V cm while at 400 8C it is found to be of the order of 101 V cm. This reveals the use of these films for making low temperature solid oxide fuel cells. # 2006 Elsevier B.V. All rights reserved. PACS : 82.47.Ed Keywords: Solid oxide fuel cell; CeO2 based ceramics; Spray deposition technique; Samarium doped ceria films
1. Introduction Fuel cell is an electrochemical device, which converts chemical energy of fuel directly into electrical energy. It will not run down as long as fuel and oxidant are supplied to it. It consists of three important parts as anode, cathode and electrolyte. The electrolyte is sandwiched between the two electrodes. Depending on type of electrolyte the fuel cells are classified as alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), polymer electrolyte membrane fuel cell (PEMFC), phosphoric acid fuel cell and solid oxide fuel cell (SOFC). Among all these types of fuel cells SOFC offers many advantages over conventional power generating systems in terms of efficiency, reliability, modularity, fuel flexibility and environmental friendliness [1]. Since the ionic conductivity of rare-earth or alkaline earth doped ceria has been known to
* Corresponding author at: School of Energy Studies, Department of Physics, Shivaji University, Kolhapur 416 004, India. E-mail address: [email protected] (S.H. Pawar). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.11.007
possesses higher ionic conductivity than YSZ [2–4], the higher efficiencies can be achieved at low temperature below 800 8C which will help to increase the stack life time and also reduce the fabrication cost by relieving the material constraints. Rare earth-doped ceria is known to exhibit mixed ionic and electronic conductivity under reducing atmospheres. In fact the maximum ionic conductivity in fluorite type oxides is observed when the dopants ion radius is close to the radius of host cations [5]. Therefore among the aliovalent cation doped ceria, samarium doped ceria possesses high ionic conductivity [6]. Due to high ionic conductivity this material is used as an electrolyte [7–9] and due to mixed conductivity is also used as anode or in the synthesis of composite anode for SOFC such as Ru–SDC [10], Ni–SDC [11]. There are different methods for the synthesis of samarium doped ceria material, such as sol–gel [12,13], solid-state [14– 16], pulse laser deposition, etc. In the present work spray pyrolysis technique was used for the synthesis of samarium doped ceria. Spray pyrolysis is an integrated process, which consists of three consecutive steps, namely atomization of liquid into droplets, traveling of droplets with atomization gas
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002
and deposition of droplets on to three-dimensional reform [17]. This technique has some advantages such as: 1. It is simple and low cost. 2. It has capability to produce large area of high quality films of uniform thickness. 3. Doping of the film is simple, since it is accomplished by merely addition of dopants to the spray solution. 4. It is easy to prepare film of any composition by simply mixing the components in appropriate ratios.
from 10 to 1000 8C in air environment. For DTA/TG analysis, SDC thin films were scratched out and powder was taken for the further study. The films TA300, TA350, TA400 and T400 were characterized by X-ray diffraction (XRD) technique using Phillips diffractometer PW1710 with Cu Ka radiation having wavelength ˚ . The spectra were obtained over the 2u range 208– 1.5424 A 1008. The diffractometer was operated at 40 kV and 30 mA. The crystallite size was determined by using Scherrer’s formula d¼
Very little data is available on the spray pyrolytic synthesis of samarium doped ceria (Ce0.8Sm0.2O1.9) films. However in the present investigation thin films of single phase samarium doped ceria have been deposited successfully by spray pyrolysis technique. The films are characterized for their structural, morphological and electrical properties and the results are reported in this paper. 2. Experimental In order to prepare films of samarium doped ceria (SDC) having composition Ce0.8Sm0.2O1.9 stoichiometric amount of reagent grade samaria oxide Sm2O3 (99.9% pure, Loba chemicals) and cerium nitrate Ce(NO3)36H2O (99.9% pure, Loba chemicals) were dissolved in double distilled water. Ultrasonically cleaned plane glass plates were used as substrates. Concentration of precursor solution was varied between 0.025, 0.050, 0.075 and 0.1 M. The optimized concentration was found to be 0.1 M. In spray pyrolysis technique in order to get good quality, adherent films there is need to optimize certain preparative parameters such as: spray rate, air pressure, concentration of precursor solution, substrate temperature, quantity of solution, etc. optimized parameters for the deposition of thin film of samaria doped ceria material are spray rate 3 ml/min, concentration of precursor solution 0.1 M, quantity 25 ml. Substrate temperature was varied from 300 to 450 8C with an interval of 50 8C and these films were described as T300, T350, T400 and T450. Optimized substrate temperature for the deposition of SDC films was found to be 400 8C. Deposited films then allowed to cool slowly, as slow cooling produces films with higher resistivity, possibly because of the reaction with oxygen in air over a larger time used in cooling. Films were then annealed in tube furnace at 550 8C for 3 h and were described as TA300, TA350, TA400 and TA450, respectively. Film thickness was calculated by using weight difference method by using the formula: t¼
weight difference Ar
where A is the area of the film and r is the density of the bulk material and for Ce0.8Sm0.2O1.9 density was taken as 7.15 g/ cm3. Differential thermal analysis/thermo-gravimetric analysis (DTA/TG) were carried out by using an instrument TG-DTADSC, SDT-2960, TA Inc., USA, with a heating rate 10 8C/min
4995
0:9l b cos u
where, d is the crystallite size of the material, l is the wavelength of the X-ray radiation, b is the full width at half maximum and u is the angle of diffraction. In order to find out the optimized annealing temperature, the film T400 was further heat treated at various temperatures such as 400, 450, 500, 550, and 600 8C and were described as T400, TH400, TH450, TH500, TH550 and TH600 (here TA400 = TH550). The description of all samples is recapped in Table 1. Scanning electron microscope (SEM) images of the films T400, TH400, TH450, TH500, TH550 and TH600 were taken by using scanning electron microscope mode JEOL JSM 636D. Composition of the deposited film was determined by energy dispersive X-ray analysis (EDAX) technique using X-ray diffractometer attested JEOL JSM 636D SEM model. Two probe resistivity method was used to study the effect of substrate temperature on the electrical property of SDC film in the temperature range 27–425 8C. Table 1 Description of the samples T300 T350 T400 T450 TA300 TA350 TA400 TA450 TH400 TH450 TH500 TH550 TH600 Here TH550 = TA400
As deposited SDC film at 300 8C substrate temperature As deposited SDC film at 350 8C substrate temperature As deposited SDC film at 400 8C substrate temperature As deposited SDC film at 450 8C substrate temperature SDC film deposited at 300 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 350 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 450 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 400 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 450 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 500 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 550 8C SDC film deposited at 400 8C substrate temperature and post heat treated at 600 8C
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3. Results and discussions 3.1. Optimization of preparative parameters and effect of substrate temperature on the thickness of the Ce0.8Sm0.2O1.9 film The Ce0.8Sm0.2O1.9 films deposited at different substrate temperatures ranging from 300 to 450 8C by using spray pyrolysis technique. It is found that the films deposited at substrate temperatures 300, and 350 8C were not completely formed. This might be due to the incomplete decomposition of the precursor solution at these substrate temperatures. The films deposited at the substrate temperature 450 8C were of less thickness and also dry and powdery in nature. But at 400 8C temperature uniform, smooth, adhesive and milky white films of samarium doped ceria were formed and therefore this temperature is considered as optimized temperature. The variation of the thickness of the films with substrate temperature is shown in Fig. 1. It is observed that, the thickness of the films is found to decrease with increase in the substrate temperature. This is attributed to the increase in the evaporation rate of the initial products with increase in the substrate temperature. Concentration of precursor solution was varied between 0.025, 0.050, 0.075 and 0.1 M. At concentrations 0.025, 0.050, and 0.075 M the films of lower thickness were formed and also their thickness was not uniform through out the film. This might be due to excess evaporation of precursor solution at optimized substrate temperature. However at 0.1 M concentration uniform films of SDC were formed. Therefore this concentration is considered as optimized concentration for SDC film formation. 3.2. DTA/TG Simultaneous DTA/TG was carried out to find out the minimum temperature required for Ce0.8Sm0.2O1.9 phase formation. In order to determine the crystallization process of a Ce0.8Sm0.2O1.9 material, thermal analysis of spray deposited
Fig. 1. Effect of substrate temperature on the thickness of Ce0.8Sm0.2O1.9 films.
Fig. 2. The simultaneous DTA/TG curves of spray deposited Ce0.8Sm0.2O1.9 powder.
SDC powder was carried out. Fig. 2 shows the DTA/TG curve of the Ce0.8Sm0.2O1.9 spray deposited powder scratched from the glass substrate. The TG curve reveals a weight loss of 2.5% below 350 8C, which corresponds to the formation of SDC phase by the decomposition of the precursor solution. DTA curve shows broad exothermic peak at 279 8C and after at about 350 8C no further change in DTA curve was observed. Since instead of using starting material directly the material scratched from the deposited film was taken, so very little weight loss of 4% was observed and this might be due to the loss of residue remains from the precursor solution. In accordance with the results of the DTA/TG, crystallization of the film occurred by post heat treatment to the film at 550 8C/3 h and almost all the characteristic peaks corresponding to the cubic fluorite structure of the Ce0.8Sm0.2O1.9 have appeared [18]. Films annealed at these conditions did not show peaks corresponding to Sm2O3 indicating direct formation of Ce0.8Sm0.2O1.9 phase. This has been evidenced by XRD results as reported in latter section. 3.3. XRD characterization 3.3.1. Effect of substrate temperature on the XRD patterns of the Ce0.8Sm0.2O1.9 films The Ce0.8Sm0.2O1.9 films deposited at different substrate temperatures were characterized by using X-ray diffractometer. Fig. 3 shows the XRD patterns of TA300, TA350 and TA400 and T400 films. XRD patterns of all films TA300, TA350 and TA400 and T400 shows peaks corresponding to cubic phase of Ce0.8Sm0.2O1.9, confirming its poly-crystalline nature. All peaks have been identified and indexed from the known patterns of the standard data files [pdf no. Ce (hexagonal): 03065-3368, Ce (tetragonal): 03-065-5410, SDC (cubic): ICSD data base code: 28792]. From the XRD patterns, lattice constant ‘a’ of TA300, TA350, TA400, and T400 have been calculated and are ˚ , respectively. found to be 5.443, 5.4572, 5.4258, and 5.4678 A ˚ of These values match well with the standard ‘a’ value 5.433 A the Ce0.8Sm0.2O1.9 phase having cubic fluorite structure.
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002
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Fig. 3. The XRD patterns of: (a) TA300, (b) TA350 and (c) TA400 and (d) T400 Ce0.8Sm0.2O1.9 films.
The film TA300 and TA350 shows some unidentified impurities peak. Also film TA350 shows peaks corresponding to hexagonal (pdf no. Ce (hexagonal): 03-065-3368) and tetragonal cerium (Ce (tetragonal):03-065-5410) along with cubic Ce0.8Sm0.2O1.9 phase (ICSD data base code: 28792). This might be due to the incomplete decomposition of the precursor solution on the surface of the substrate. Impurity peak (1 0 3) due to elemental cerium is indexed as ‘‘103 Ce’’ in Fig. 3. The observed and standard ‘d’ values of the spray deposited Ce0.8Sm0.2O1.9 thin films having cubic fluorite structure are listed in Table 1A. From Table 1A, it is observed that for all films (1 1 1) reflection is most intense, also number of peaks corresponding to the SDC phase found to increase with increase in substrate temperature. All major reflections corresponding to SDC cubic fluorite phase like, (1 1 1), (0 0 2), (0 2 2), (1 1 3), (2 2 2), (0 0 4), (1 3 3), (0 2 4), and (2 2 4) were obtained for the film TA400. Thus for the SDC film deposited at the substrate temperature 400 8C and post heat treated at 550 8C/3 h (TA400) no phase corresponding to Sm2O3 or cerium were observed confirming the direct formation of Ce0.8Sm0.2O1.9 phase.
Comparison of the intensities of TA300, TA350, TA400 and T400 SDC films is listed in Table 1B. The highest intensity X-ray diffraction (1 1 1) peak lying at about 2u = 288 has been further analyzed to yield the crystallite size of the SDC material by using the Scherer’s formula. Full width half maxima (b) of the (1 1 1) peak for TA300, TA350 and TA400 and T400 were determined from XRD and by using these b values crystallite size of SDC film was calculated. Crystallite size of TA300, TA350, TH400 and T400 films are tabulated in Table 2. Fig. 4 shows variation of the crystallite size of Ce0.8Sm0.2O1.9 film with substrate temperature. It is observed from Fig. 4 that, at lower substrate temperature such as 300, and 350 8C films exhibit smaller crystallite size than the film deposited at 400 8C substrate temperature. Thus the crystallite size increases with increase in substrate temperature. Also comparison of the crystallite size of the TA400 and T400 films reveals that the annealing of the films at the higher temperature increases the crystallite size of the material.
Table 1A Comparison of the d values of TA300, TA350, TA400 and T400 SDC films
Table 1B Comparison of the intensities of TA300, TA350, TA400 and T400 SDC films
hkl
111 002 022 113 222 004 133 024 224 115
˚) d (std.) (A
3.1367 2.7165 1.9208 1.6381 1.5683 1.3582 1.2464 1.2148 1.1090 1.0415
˚) d (calculated) (A
hkl
TA300
TA350
TA400
T400
3.1429 2.7203 1.9251 – – – – – – –
3.1494 2.7231 – 1.6569 – 1.3636 – – 1.1327 1.0396
3.1091 2.6999 1.9123 1.6386 1.6386 1.5708 1.3577 1.2459 1.2148 1.1161
3.1532 2.7251 – 1.6581 – 1.3651 – – – –
111 002 022 113 222 004 133 024 224 115
100 I/I0 (calculated) TA300
TA350
TA400
T400
100 49.5 35.1 – – – – – – –
100 48.9 – 13.6 – 5.4 – – 5 8.5
100 42.3 41.4 35.3 6.2 4 7.2 3.8 4 –
100 23.4 – 12.6 – 5.5 – – – –
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B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002 Table 2 Variation of crystallite size of TA300, TA350, TA400 and T400 Ce0.8Sm0.2O1.9 films with substrate temperature
Fig. 4. Variation of the crystallite size of Ce0.8Sm0.2O1.9 film with substrate temperature.
3.3.2. Effect of annealing temperature on the XRD patterns of the Ce0.8Sm0.2O1.9 films Fig. 5 shows XRD patterns of T400, TH400, TH450, TH500, and TH550 thin films. XRD shows peaks confirming the polycrystalline nature of the films. All peaks have been identified and indexed from the known patterns of the standard data files SDC (cubic): ICSD database code: 28792. From the XRD patterns, lattice constant ‘a’ of
Film
Crystallite size 0.2 (nm)
TA300 TA350 TH400 T400
7 8 11 7
T400, TH400, TH450, TH500, and TH550 have been calculated and it is found that the value of the lattice constant ‘a’ for these films ˚ match well with 5.4441, 5.4678, 5.4569, 5.4445, and 5.4258 A ˚ standard ‘a’ value 5.433 A of the Ce0.8Sm0.2O1.9 phase having cubic fluorite structure. The d values and the intensities of different planes for the films deposited at 400 8C substrate temperature and annealed at different temperatures are listed, respectively, in Tables 3A and 3B. It is seen that the intensity of (1 1 1) plane is strongest one for all the films. However the intensity of (0 0 2) plane is found to decrease with annealing temperature. This can be understood with the changes in the crystallographic unit cell of cerium oxide. The schematic picture of crystallographic unit cell of CeO2 is shown in Fig. 6. The structure is FCC with cerium atoms placed at the corners and at the center of the faces of the unit cell. Each
Fig. 5. The XRD patterns of: (a) T400, (b) TH400, (c) TH450, (d) TH500, and (e) TH550 Ce0.8Sm0.2O1.9 films.
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002 Table 3A Comparison of d values of T400, TH400, TH450, TH500 and TH550 Ce0.8Sm0.2O1.9 films ˚) d (std.) (A
hkl
111 002 022 113 222 004 133 024 224 115
3.1367 2.7165 1.9208 1.6381 1.5683 1.3582 1.2464 1.2148 1.1090 1.0415
˚) d (calculated) (A T400
TH400
TH450
TH500
TH550
3.1423 – 1.9258 1.6411 – – – – – –
3.1532 2.7251 – 1.6581 – 1.3651 – – – –
3.1516 2.7381 1.9276 1.6419 – 1.3608 1.2538 – – –
3.1521 2.7312 1.9149 1.6445 1.5700 1.36 – – 1.112 1.0452
3.1091 2.6999 1.9123 1.6386 1.5708 1.3577 1.2459 1.2148 1.1161 –
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Table 4 Variation of crystallite size of T400, TA400, TA450, TA500, and TA550 SDC films with substrate temperature Annealing temperature (8C)
Crystallite size 0.2 (nm)
Without annealing 400 450 500 550
7 8 9 11 11
cerium atom is surrounded by eight oxygen atoms. From this structure the density of the atoms in (1 1 1) plane is maximum, which is in support of the maximum X-ray diffraction intensity observed in all the samples. The intensities as well as the d values of (0 0 2) plane are found to decrease with increase in
annealing temperature. This indicates that there might be vacancies created due to the replacement of the face centered cerium atoms. This is in favor of creating vacancy defects and helps to increase the conduction mechanism, which is needed for its application as the ionic conductor electrolyte. The observed and standard ‘d’ values and intensities for T400, TH400, TH450, TH500 and TH550 SDC films are listed in Tables 3A and 3B, respectively. From the Table 3A it is observed that for the film TA400 no phase corresponding to the Sm2O3 or cerium has been observed confirming the direct formation of Ce0.8Sm0.2O1.9 phase. XRD studies also reveal that the number of peaks corresponding to SDC phases increase with increase in the annealing temperature. This might be due to the increase in the crystalline nature of the film with annealing temperature. Full width half maxima (b) of the (1 1 1) peak for T400, TH400, TH450, TH500, and TH550 Ce0.8Sm0.2O1.9 thin films were determined from XRD. By using these b values crystallite size of SDC films were calculated by using Scherer’s formula and the crystallite size for above films are tabulated in Table 4. It is observed from Fig. 7 that, crystallite size of the deposited film is found to increase with the increase in annealing temperature. This might be due to the increase in the crystalline nature of the film with annealing temperature. From Tables 1 and 3 optimized substrate and annealing temperature for the single-phase deposition of the SDC thin film were found to be 400 and 550 8C, respectively.
Fig. 6. The schematic picture of crystallographic unit cell of CeO2.
Fig. 7. Variation of the crystallite size of Ce0.8Sm0.2O1.9 film with annealing temperature.
Table 3B Comparison of intensities of T400, TH400, TH450, TH500 and TH550 Ce0.8Sm0.2O1.9 films 100 I/I0 (calculated)
hkl
111 002 022 113 222 004 133 024 224 115
T400
TH400
TH450
TH500
TH550
100 – 32.7 20.4 – – – – – –
100 23.4 – 12.6 – 5.5 – – – –
100 47.8 36.5 28.8 5.8 6.9 – – – –
100 72.2 32.3 28.9 3.5 5.3 – – 3.1 3.8
100 42.3 41.4 35.3 6.2 4 7.2 3.8 4 –
5000
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002
Fig. 8. SEM images of SDC films (a) as deposited an annealed at (b) 400 8C, (c) 450 8C, (d) 500 8C, (e) 550 8C, and (f) 600 8C.
3.4. Morphological studies Morphology of the as deposited and SDC films annealed at different annealing temperatures were studied. Fig. 8 shows the scanning electron microscope images of (8a) T400 and, (8b) TH400, (8c) TH450, (8d) TH500, (8e) TH550 and (8f) TH600 SDC films, respectively. SEM images of these films show formation of uniform and dense films. Film T400, post heat treated at 600 8C shows many cracks, these cracks are due to the melting of the glass substrate. It is observed that, substrate starts to melt above 550 8C. From the SEM graph of SDC film heat treated at lower temperature, cracking of the film on the substrate surface can be seen. This might be due to shrinkage and compaction of the film upon heat treatment. Such cracking was also observed in case of CeO2 films deposited by electrophoresis [19] and anodic electrodeposition [20].
Fig. 9 shows EDAX pattern of the spray deposited Ce0.8Sm0.2O1.9 film post heat treated at 600 8C (TA600). Fig. 9 shows peaks corresponding to the elements Ce, Sm, O, along with the elemental peaks corresponding to Na, Si, Mo, and Pt. Peaks corresponding to the elements Na, Si, Mo, and Pt are due to melting of the glass substrate. Table 5 presents the
3.5. Compositional study In order to find out the composition of the deposited SDC film energy dispersive X-ray analysis plot of TA400 film was carried out. SEM of the same film showed many cracks due to melting of glass substrate. This cracking is also confirmed by EDAX studies.
Fig. 9. EDAX pattern of the spray deposited Ce0.8Sm0.2O1.9 film.
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002 Table 5 Composition of the spray deposited Ce0.8Sm0.2O1.9 film post heat treated at 600 8C (TA600) Element
Atomic wt.(%)
Error (%)
O Ce Sm Na Si Mo Pt
44 30 8 7 5 5 1
0.2 0.2 0.2 0.4 0.5 0.8 0.6
composition of the above film in weight percent. From Table 5, composition ratio of Ce:Sm is found out to be 29:7 and it is approximately equals to 4:1. Thus EDAX studies showed the formation of stoichiometric Ce0.8Sm0.2O1.9 thin films. 3.6. Electrical resistivity study Two-probe resistivity method was used to study the electrical properties of Ce0.8Sm0.2O1.9 films deposited at different substrate temperatures and heat treated at 550 8C. Two-probe resistivity technique is the DC measurement technique. In this technique a brass block was used as a sample holder cum heater. The films of the size 3.7 cm 1.0 cm on the glass substrate were used for this study. Two press contacts were made to the film with the help of pointed brass screws. The area of the film on the glass substrate was defined as 1 cm 1 cm and silver paste was applied to ensure the ohmic contact to the film. Cromel–Alumel thermocouple was used to measure the temperature. Transistorized power supply unit (TPSU) model LVA 30/1 Aplab was used as a constant source for voltage. Current in the circuit was measured with digital nanometer model DNM-121. The resistivity of the film was studied in the temperature range 27–425 8C. Measurements were carried out under air atmosphere. It is observed that, the resistivity of all the films decreases with increase in the temperature showing semiconducting nature. The typical plot of log r versus 1/T for TA400 is shown in
5001
Table 6 The activation energies for TA300, TA350, TA400 and TA450 films Films
TA300 TA350 TA400 TA450
Activation energies 0.03 eV Low temperature
High temperature
0.48 0.56 0.21 0.52
0.73 0.78 0.72 0.74
Fig. 10. The plot shows two regions corresponding to high and low temperatures. The limit of both domains was found at 50 8C. Ceria is usually considered to provide mixed electronic and ionic conductivity [21]. Pure ceria oxide is a poor oxygen vacancy conductor [22]. Substitution of aliovalent cations of lower valance for Ce results in the formation of oxygen vacancies to compensate the charge balance in sub-lattice. Thus the solid solution becomes predominantly ionic conductive for oxygen vaccines over an extended temperature range. It is reported that among cations doped ceria Sm doped ceria found to have highest ionic conductivity. There is formation of large amount of oxygen vacancies, VO, in the CeO2 fluorite lattice [23–25], due to the addition of Sm2O3 in as expressed by the following reaction: 2SmO1:5 ! 2Sm0 Ce þ O þ VO where Sm0 Ce is a Sm3+ ion in a Ce4+ lattice with negative charge. Increase in the conductivity in the film with increasing temperature is due to introduction of oxide ion vacancies by charge compensation. Similar results were reported by Eguchi [26,27]. Therefore in this case electrical conductivity measured is considered to be completely due to ionic conduction. The room temperature resistivity of TA300, TA350, TA400 and TA450 films is of the order of 107 V cm and the resistivity of the film was found to be of the order of 101 V cm at 400 8C. This very large decrease in the resistivity is due to the role of ionic conductivity. Activation energies of the deposited films were calculated by using the relation: Ea r ¼ r exp ; kT where r is the resistivity, Ea the activation energy, k the Boltzmann constant and T is the absolute temperature. Table 6 shows the activation energies for TA300, TA350, TA400 and TA450 films, corresponding to high and low temperature region. Activation energy values were found to be in the range 0.7–0.8 eV which is similar to the value of 0.78 eV for the activation energy reported by others [6,27]. 4. Conclusions
Fig. 10. Plot of log r vs. 1/T for TA400 SDC film.
Optimized substrate and annealing temperatures for the synthesis of uniform, adherent, single phase samarium doped ceria films by spray deposition technique was found to be 400
5002
B.B. Patil, S.H. Pawar / Applied Surface Science 253 (2007) 4994–5002
and 550 8C, respectively. Crystallite size was found to increase with increase in substrate and annealing temperatures. Resistivity studies showed that deposited films are of semiconducting nature with activation energy 0.72 eV. The room temperature resistivity of TA300, TA350, TA400 and TA450 films is of the order of 107 V cm and the resistivity of the film was found to be of the order of 101 V cm at 400 8C. This very large decrease in the resistivity is due to the role of ionic conductivity. Composition of the film was found to be Ce:Sm 8:2. Thus stiochiometric, uniform, single-phase SDC films suitable for SOFC application are successfully deposited by a low cost spray pyrolysis method. Acknowledgements One of the authors BBP is thankful to the Ministry of Non Conventional Energy Sources for providing financial assistance. They are also indebted to U.G.C., Delhi for financial support. References [1] [2] [3] [4] [5] [6]
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