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Sintering and electrical properties of Ce1−xBixO2−δ solid solution
Journal of Alloys and Compounds 617 (2014) 563–568
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Sintering and electrical properties of Ce1xBixO2d solid solution M. Prekajski a,⇑, M. Stojmenovic´ a, A. Radojkovic´ b, G. Brankovic´ b, H. Oraon c,1, R. Subasri c, B. Matovic´ a a
Vinca Institute of Nuclear Science, Material Science Laboratory, University of Belgrade, PO Box 522, Belgrade, Serbia Institute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava 1a, 11030 Belgrade, Serbia c International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad 500 005, India b
a r t i c l e
i n f o
Article history: Received 6 December 2013 Received in revised form 25 July 2014 Accepted 10 August 2014 Available online 19 August 2014 Keywords: Nanostructured materials Sintering Electrochemical impedance spectroscopy Microstructure Scanning electron microscopy
a b s t r a c t Solid solution Ce1xBixO2d nanopowders with the composition of x = 0.1–0.5 were synthesized by using Self Propagating Room Temperature procedure (SPRT). The results obtained by XRPD show that synthesized samples were single-phase solid solution at room temperature. Powders were densified by using Conventional (CS) and Microwave (MS) Sintering techniques at different temperatures, in an air atmosphere for 1 h. Scanning electron microscopy (SEM) and complex impedance method measurements were carried out on sintered samples. Maximum achieved density was for sample with Ce0.80Bi0.20O2d composition for both applied sintering techniques. The highest conductivity was obtained for the ceramic composition Ce0.80Bi0.20O2d sintered by microwave technique at 700 °C. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the context of significant interest and concern on energy and environment, nanostructured cerium oxide (CeO2)-based materials are highly desired for solving present energy and environment related issues due to the improvements in redox properties, transport properties and surface to volume ratio with respect to the bulk materials. CeO2 based materials have been widely used in: clean energy [1], environmental protection and remediation [2], as a promoter in three-way catalysts for the elimination of toxic autoexhaust gases [3], solar-driven thermochemical CO2 reduction [4], UV absorbers [5], in biotechnology [6], medicine [7] etc. Most of above applications are related to the rapid formation and elimination of oxygen vacancy in CeO2 that endows it with high oxygen storage properties. In recent years, ceria-based materials have been extensively studied as some of the most promising electrolytes for reduced temperature solid oxide fuel cell (SOFC) system due to their high ionic conductivity at moderate temperature [8–10]. CeO2 can accommodate a high oxygen deficiency by the substitution of lower valent elements on the cation sublattice. However, being a highly refractory material, ceria based materials are difficult to
⇑ Corresponding author. Postal address: PO Box 522, Belgrade, Serbia. Tel.: +381 11 340 8860; fax: +381 11 340 8224. E-mail address: [email protected] (M. Prekajski). 1 Present address: Centre for Nanotechnology, Central University of Jharkhand, Brambe, Ranchi 835205, India. http://dx.doi.org/10.1016/j.jallcom.2014.08.090 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
densify below 1500 °C [11,12]. This makes it difficult for the manufacture of ceria electrolyte and other components, such as cathode and anode, which have to be co-fired below 1500 °C. CeO2 system doped with Bi3+ can be very interesting for application in SOFC’s due to the high ion conductivity of CeO2 and Bi2O3 phases [13]. Therefore (CeO2)1x(BiO1.5)x solid solution is expected to be a novel electrolyte exhibiting new electrochemical transport properties. In addition ceria-based solid solutions doped with lower valence ions usually possess oxide ion conductivity higher than yttrium-stabilized zirconia (YSZ). Exactly that was a motivation for the study of sintering properties and ionic conductivity on this solid solution. Sintering presents a very important part of processing for almost all ceramic materials. Conventional sintering (CS) often involves mixing of corresponding oxides and heating them at elevated temperatures for extended duration [14–16]. However, very low diffusion coefficients of ions in solid reactions requires intermittent grinding and repeated heating and grinding for extended periods of time to form the desired single-phase compound. On the other side, high temperature treatment usually results in large crystal growth and hence, total grain surface area. Moreover, the conductivity in nanocrystalline grain boundary regions is greater than for larger grains [17]. In this respect, it is important to develop powders of high quality with particle size in the nanometric range, but to reduce the sintering temperatures so as to retain the nanocrystallinity. In addition sintering of bismuth-rich compounds can be quite difficult because of the properties of Bi which easily vaporizes and melts at a temperature of 835 °C. These problems may be
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overcome by using some of the contemporary methods such as microwave sintering. In the last few years, microwave sintering (MS) of ceramics, has emerged as an alternative approach to densify ceramics and has gained much attention because of the rapid heating, enhanced densification rate, and improved microstructure [18–20]. In MS electromagnetic waves interact with ceramics, leading to volumetric heating by dielectric loss. Such a volumetric heating of MS may result in ceramics with a more uniform and finer microstructure when compared to conventional sintering. Volumetric heating in MS results in a homogenous temperature distribution, both on the surface as well as in the core. The ability of many ceramics to absorb MW radiation above a critical temperature has led to the use of microwave for processing of ceramics during drying, clinkering, sintering, melting and joining [21]. In addition to shorter processing times, microwave sintering results in ceramics with improved microstructure and mechanical properties. The most intriguing effects observed during MW sintering are the higher densification [22–24] and reaction rates [25] when compared to conventional sintering. The reasons for higher densification rates have been attributed to lower activation energy for MW sintering [22,23]. During microwave heating, electrical energy from the microwaves is converted into heat. The MW power absorbed by a sample is proportional to the electric field distribution in the sample, which is determined by the electrical properties of the material. Our previous investigations [26,27] focused on a complete characterization of synthesized nanopowders of Ce1xBixO2d with composition x = 0.1–0.5 and in the present investigation, a detailed analysis of their sintering behavior when subjected to conventional and microwave sintering has been carried out and results are compared. 2. Materials and methods In the present experiments, nanocrystalline Ce1xBixO2d powders were synthesized by SPRT method [26–29]. The starting materials for the preparation of solid solutions were cerium nitrate hexahydrate, bismuth nitrate pentahydrate and sodium hydroxide. Used chemicals had a purity of 99.9% as stated by the manufacturer (Riedel-de Haën). The compositions of the starting reacting mixtures were calculated according to the nominal composition of the final reaction product, according to Eq. (1):
2½ð1 xÞCeðNO3 Þ3 6H2 O þ xBiðNO3 Þ3 H2 O þ 6NaOH þ ð2 dÞO2 ! 2Ce1x Bix O2d þ 6NaNO3 þ ð15 2xÞH2 O þ 3=2O2
ð1Þ
Synthesis of nanopowders was carried out in an alumina mortar mixing reactants for 5–7 min, allowing rapid progress of the reaction at room temperature in air. After being exposed to air for 3 h, the entire volume of the powder was dissolved in water and subjected to centrifugation at Centurion 1020D centrifuge at 3000 rpm, for 10 min. Rinsing procedure was repeated four times with distilled water and twice with ethanol, in order to eliminate NaNO3 from the synthesized powder mixture. At the end, material was dried out at 60 °C in ambient atmosphere. Synthesized powders were characterized and details can be found in our previous work [26]. X-ray powder diffraction analysis of as-synthesized samples showed that the ceria powders with up to 50% of Bi are solid solutions with fluorite type of structure [26]. All synthesized powders have particle size in nanometric range (less than 4 nm) [26]. Raman spectral studies confirmed that all synthesized powders are single-phase solid solutions [26]. Synthesized powders were uniaxially compacted at 200 MPa without any binder into disks with 8 mm in diameter and 3 mm thickness. Samples were conventionally sintered in a furnace in air at different temperatures. Heating rate was 10 °C/min and with soaking time of 1 h. Microwave sintering (MS) was carried out in a 2.45 GHz, 6 kW, multimode cavity microwave sintering furnace (model MHTD 1800-6,4/2,45) supplied by Linn Hightherm GmbH, Germany. An optical pyrometer with a measurement range of 300–2000 °C attached to the chamber, was used for temperature measurement. The furnace was interfaced with a programmable logic controller (PLC) that gets the feedback from the optical pyrometer to carry out controlled heating and cooling. In order to obtain uniform and homogeneous heating, SiC susceptors were placed around the sample. The entire assembly, i.e. the sample along with the susceptors was housed in an insulation casket made of alumina fiber board supplied by Zircar ceramics, USA. Microwave sintering was carried out in air at temperatures of
900 °C, 1050 °C and 1150 °C with a heating and cooling rate of 10 °C/min for a soaking time of 1 h. The density of all sintered samples was measured by the Archimedes method. Semi-quantitative analysis of sintered samples with the highest density was determinated by using a energy dispersive X-ray analysis (EDX) on X-Max Large Area SDD EDX spectrometer. For scanning electron microscopy (SEM) analysis of sintered samples of the most density, the electron microscope (FE-SEM, Jeol JSM 6330F, Japan) was used. The samples were pre-coated with a several nanometers thick layer of gold before observation. For coating procedure, a device Fine Coat (JFC – 1100, Ion Sputter, JEOL, Japan) was used. The images were recorded in SEI mode at a magnification 5000 with the accelerating voltage of 20 kV. EDS analysis was carried out at the invasive electron energy of 30 keV by means of QX 2000S device (Oxford Microanalysis Group, UK). The spatilal resolution was 0.4 nm. The electric conductivity was measured by complex impedance method in the temperature range 600–700 °C. The measuring cell was placed in a vertical furnace open to air atmosphere. To ensure good electrical contact, both sides of the sintered samples were coated with a silver paste. The temperature was increased step by step with the increments of 25 °C. The measuring device was a frequency response analyzer (FRA Solartron 1260 Impedance/Gain Phase Analyzer) coupled with a dielectric interface (Solartron 1296). Operating frequencies were in the range 0.1 Hz–5 MHz and the peak-to-peak AC voltage amplitude was 50 mV. The impedance plots obtained experimentally were fitted by means of the software ZView Ò for Windows (Version 3.2b).
3. Results and discussion 3.1. Densification, semi-quantitative analysis and microstructure of sintered samples The results of conventional (CS) and microwave (MS) sintering of the Ce1xBixO2d (x = 0.1–0.5) samples (calculated and measured densities) are presented in Tables 1 and 2. Theoretical densities were calculated according to Eq. (2):
qðxÞ ¼
Z MðxÞ a3 NA
ð2Þ
In Eq. (2) Z represents number of unit cell, M is molar weight of compound x, a3 is volume of unit cell, whereas a represents lattice parameter (lattice parameters of all synthesized samples were obtained by using Rietveld refinement method, and the results were published in our previous paper [27]). NA is Avogadro’s number (6.0231023 atoms/mol). The success of nanopowders consolidation is intimately related to the control of the competition between densification and coarsening. The green structure, i.e., pore size distribution plays an important role in achieving high density. Densification is retarded or inhibited for wide pore distribution. In such case, big pores became larger while small pores shrink. In case of conventional sintering, it can be observed that at a temperature of 1450 °C, the maximum density was achieved for the composition Ce0.80Bi0.20O2d (87.0%). On the other hand, it is notable that the increase (1500 °C) or decrease (1400 °C) of temperature leads to a reduction of density values. Therefore, the optimal temperature as well as composition for conventional sintering was Ce0.80Bi0.20O2d sample densified at 1450 °C (87.0%). Microwave sintered samples show much better results for densities compared with conventional sintered ones. It should be emphasized that the achieved densities were obtained at much lower temperatures when compared to conventional sintering. The highest densities were achieved for samples that were microwave sintered at 1050 °C. With further increasing of temperature (1150 °C) there is decrease in density. The reason for this behavior is the same like in previous case, i.e. at higher temperatures, there is a concern of Bi evaporation, which leads to density decrease [27]. The results presented in Table 2 show that the optimal composition in MS sintering is Ce0.80Bi0.20O2d and Ce0.50Bi0.50O2d obtained at a temperature of 1050 °C. Therefore, we can conclude that the microwave sintered samples exhibited an enhanced densification when compared to the conventionally sintered samples.
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M. Prekajski et al. / Journal of Alloys and Compounds 617 (2014) 563–568 Table 1 The theoretical and measured density of the conventionally sintered (CS) samples at different temperatures, in the air, with a soaking time of 1 h. Temperature (°C)
Composition
Theoretical density
Measured density
Relative density (%)
1400
Ce0.80Bi0.20O2d
7.63
6.49
85.1
1450
Ce0.90Bi0.10O2d Ce0.80Bi0.20O2d Ce0.70Bi0.30O2d Ce0.60Bi0.40O2d Ce0.50Bi0.50O2d
7.45 7.63 7.83 8.03 8.23
6.45 6.94 5.82 5.25 4.09
86.6 90.9 74.3 65.4 49.8
1500
Ce0.80Bi0.20O2d
7.63
6.34
83.1
Table 2 The theoretical and measured density of the microwave sintered (MS) samples at different temperatures, in the air, with a soaking time of 1 h. Temperature (°C)
Composition
Theoretical density
Measured density
Relative density (%)
900
Ce0.90Bi0.10O2d Ce0.80Bi0.20O2d Ce0.70Bi0.30O2d Ce0.60Bi0.40O2d Ce0.50Bi0.50O2d
7.45 7.63 7.83 8.03 8.23
5.44 6.97 6.26 5.07 7.78
73.1 91.4 79.9 63.2 94.6
1050
Ce0.90Bi0.10O2d Ce0.80Bi0.20O2d Ce0.70Bi0.30O2d Ce0.60Bi0.40O2d Ce0.50Bi0.50O2d
7.45 7.63 7.83 8.03 8.23
7.00 7.23 7.29 7.48 7.90
94.0 94.8 93.1 93.2 96.0
1150
Ce0.80Bi0.20O2d Ce0.50Bi0.50O2d
7.45 7.83
6.65 6.79
89.3 86.7
It is known that the sintering method might have a significant influence on final material properties. In order to find out the stability of chemical compositions of CS and MS sintered samples, EDS analyses were performed (Table 3) for samples with highest density. Sample with the composition Ce0.80Bi0.20O2d sintered by CS at 1450 °C shows that bismuth almost completely evaporated at this temperature. However, unlike previous one, in the sample with same composition obtained by MS at 1050 °C the bismuth content remains almost the same as it was in the initial as synthesized sample [27]. Since the sample with composition Ce0.80Bi0.20O2d has the highest density for both sintering methods, it was taken as representative one for microstructure analysis. This sample was thermally treated at 1450 °C in case of conventional sintering, and at 1050 °C in case of microwave sintering (Fig. 1). Microstructure of conventionally sintered sample possesses bimodal structure. The round shapes of grains indicate Ostwald ripening growth in presence of liquid phase. The porosity of sample can be explained by very high vapor pressure of bismuth at used sintering temperature [30]. On the other hand, it is clear that the mean grain size of the MS samples is smaller than that of the corresponding conventionally sintered grade. The grain size of Ce0.70Bi0.30O2d is about 10–20 lm for CS, while mean grain size of MS sintered sample is about 0.3 lm. A more narrow size distribution is found for the MS samples. The difference in grain size implies that microwave sintering has the potential to limit the grain growth and homogenize the microstructure due to a volumetric heating mode and
enhanced vacancy diffusion. Namely it is known that ceria has low thermal conductivity, and it is known that vacancy diffusion is thermally driven process, and that CS process is not uniform and results in thermal mismatch between the surface and centre of the bodies and possible vacancy clustering that lower vacancy diffusion. On the other hand, in case of MS, volumetric nature of microwave heating results in uniform kinetic transport of vacancy and thus, enhanced vacancy diffusion, and in that way affects on the slowing down of grain growth. 3.2. Electrical conductivity Impedance measurements were performed only on MS sintered samples with highest densities (>95%) due to assumption that samples with these densities possess the closed porosity. Nyquist’s diagrams (impedance graphs) of the sintered samples with compositions Ce0.80Bi0.20O2d and Ce0.50Bi0.50O2d obtained by microwave sintering at temperature 1050 °C, are presented in Fig. 2. Measurements were recorded within the temperature range of 600–700 °C, with the increments of 25 °C. Their impedance diagrams for various temperatures were selected to illustrate the impedance behavior of the investigated doped ceria samples as a whole. Generally, in the available frequency range (1 Hz–0.1 MHz), impedance semicircle is observed. Such a type of Nyquist plot is characteristic of serially connected resistance (Rb – balk resistance) Capacitive element is the only being distributed i.e. frequency dependent, with impedance that can be expressed as:
Table 3 EDS microanalysis of the microwave and conventionally sintered samples after sintered at different temperatures. Sintering method
Temperature of calcination/sintering
Nominal chemical composition
Resulting composition (EDS analysis)
CS
1450 °C
Ce0.80Bi0.20O2d
Ce0.99Bi0.01O2d
MS
900 °C 900 °C 1050 °C 1050 °C
Ce0.80Bi0.20O2d Ce0.50Bi0.50O2d Ce0.80Bi0.20O2d Ce0.50Bi0.50O2d
Ce0.83Bi0.17O2d Ce0.55Bi0.45O2d Ce0.81Bi0.19O2d Ce0.55Bi0.45O2d
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Fig. 1. SEM micrographs of the samples Ce0.8Bi0.2O2d obtained by (a) conventional sintering at 1450 °C, and (b) microwave sintering at 1050 °C.
Fig. 2. Impedance diagrams for the sintered samples presented within the different temperature range: (a) Ce0.80Bi0.20O2d (600–700 °C) and (b) Ce0.50Bi0.50O2d (650–700 °C).
ZðxÞ ¼ Re þ
1 a ðjxÞ Q
ð3Þ
where a is an exponent (0 6 a 6 1) and Q is a pre-exponential factor and of the so-called constant phase element (CPE). If value of the resistive element, R, bonded in parallel with CPE is known, the effective capacitance can be calculated by using the following equation:
C¼R
1n n
1
Qn
ð4Þ
Equivalent circuit with both constant [31–33] and distributed [34] capacitive elements has been widely applied in the literature related to the sintered ceramics. When the resistive components in the semicircle were clearly separated, i.e., RbCb RgbCgb, where bulk capacitance (Cb) is typically few orders of magnitude lower than Cgb, the values of Rb and Rgb may be read separately as intercepts of the semicircles with the real axis [35]. At impedance graph of the sample Ce0.8Bi0.2O2d (Fig. 2a) the first section of the semicircles with the real axis may be attributed to the bulk resistance crystallite grains Rb, and second section may be attributed to the intergranular capacitance Rig. Exponent a obtained from the fitting results amounted to about 0.75 and corresponds to the capacity of grain boundary, since the effective capacitances calculated from Eq. (4) are 109 F/cm, i.e. values typical for grain boundary capacitance [36]. However, as can be seen from Fig. 2b, in the sample Ce0.50Bi0.50O2d resistance decrease with the temperature increasing. Consequently, the whole region of the impedance points shifts towards the low-frequency semicircle. Namely, at higher temperatures (650–700 °C), instead of Rb and Rgb separately, only the whole
sum Rb + Rgb became readable in the available frequency range. The effective capacitances related to Cgb in this case is about two orders of magnitude smaller, while a amounted to about 0.97. Therefore, in this range of temperatures the Nyquist plot does not clearly indicate the separation between the balk and grain boundary. The appearance of a new semicircle in Fig. 2 was observed in a low – frequency region (arrows indicate the beginning of the new semicircle) and almost doubtless, it originates from the oxygen electrode reactions, O2/O2 (Eq. (2)), analysis of which is beyond the scope of this study.
1 O2 þ 2e ! O2 2
ð5Þ
The total resistance values measured for all samples were used to calculate ionic conductivity of the samples at various temperatures (Table 4).
Table 4 The ionic conductivity bulk (jb) and grain boundary (jgb) of the sample Ce0.80Bi0.20O2d, and total ionic conductivity (jb+gb) of the sample Ce0.50Bi0.50O2d, obtained by microwave sintering at temperature 1050 °C. T (°C)
Ce0.80Bi0.20O2d
jb (S cm1) 600 625 650 675 700
3
1.62 10 2.03 103 2.74 103 2.83 103 3.14 103
Ce0.50Bi0.50O2d
jgb (S cm1) 4
3.52 10 4.98 104 6.67 104 7.62 104 9.32 104
jb+gb (S cm1) 1.89 104 2.45 104 4.74 104 7.85 104 1.23 103
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Fig. 3. The dependence log j = f(1/T) for the samples: (a) Ce0.80Bi0.20O2d and (b) Ce0.50Bi0.50O2d in the form of Arrhenius plots obtained by microwave sintering.
By comparing the results obtained for the two ceramics samples, it can be seen that the highest values of ionic conductivity exhibited the sample that having 20% Bi3+ obtained by microwave sintering. The actual values at 700 °C for bulk and grain boundary amounted to 3.14 103 and 9.32 104 S cm1, respectively. On the other hand, for the sample with 50% Bi3+ at temperature of 700 °C a lower total conductivity (1.23 103) is observed. The two samples displayed a difference in density, grain size and porosity, which also may be responsible for the difference in their electrical conductivities. Based on the results listed in Table 4, the dependence log k = f(1/ T) for the microwave sintered samples having compositions Ce0.80Bi0.20O2d and Ce0.50Bi0.50O2d are compared and depicted in Fig. 3. Literature data showed [37] that the activation energies (Ea) for CeO2 obtained by MGNP (modified glycine nitrate procedure) method with two and three dopants were reported to be 0.66 and 0.81 eV, respectively. However, here the significantly lower activation energies were found, amounting to 0.209 (bulk – Ce0.80Bi0.20O2d) and 0.289 (grain boundary – Ce0.80Bi0.20O2d) and 0.600 eV (bulk + grain boundary – Ce0.50Bi0.50O2d). It is known that CeO2 doped systems with 20% concentration of dopant usually achieves the highest ionic conductivity values and a minimum values of activation energies [8,34,38,39]. With a further increase in concentration of the dopant, the conductivity decreased and increases activation energy [8,34,38,39]. 4. Conclusions In summary, samples of nanopowder solid solution Ce1xBixO2d (x = 0.1–0.5) which was previously synthesized by a simple and low cost SPRT method were sintered with two different methods. It appears that conventional sintering requires much higher temperatures for densification. However, at high temperatures there is significant loss of Bi content due to process of evaporation. This problem can be overcome by applying the microwave sintering technique. In this way high densified samples can be obtained at much lower temperatures (1050 °C) without loss of Bi concentration. A finer grain size and more uniform microstructure were obtained by microwave sintering due to the shorter sintering cycle because of the faster cooling rate and the establishment of a more uniform temperature distribution within the short sintering time, respectively. Temperature of 1050 °C seems to be an optimum sintering temperature where 93–96% of theoretical density can be achieved depending on the bismuth content. Temperatures higher than 1050 °C can lead to Bi loss, which further leads to lower densities. The lowest total resistance R was obtained for the samples Ce0.80Bi0.20O2d obtained by microwave sintering, and thus higher
ionic conductivity, because of a finer grain size and more uniform microstructure that allows for a higher density to be obtained. At 700 °C, conductivities for bulk and grain boundary amounted to 3.14 103 and 9.32 104 S cm1 for sample Ce0.80Bi0.20O2d, respectively. Ea value obtained is consistent with the results of ionic conductivity. Acknowledgment Financial support from the Serbian Education and Science Ministry in the Framework of projects No. 45012 and 176016 is gratefully acknowledged. References [1] A.B. Stambouli, E. Traversa, Solid oxide fuel cells (SOFCs). A review of an environmentally clean and efficient source of energy, Renew. Sust. Energy Rev. 6 (2002) 433–455. [2] F.B. Li, X.Z. Li, M.F. Hou, K.W. Cheah, W.C.H. Choy, Enhanced photocatalytic activity of Ce3+–TiO2 for 2-mercaptobenzothiazole degradation in aqueous suspension for odour control, Appl. Catal. A 285 (2005) 181–189. [3] A. Trovarelli, C. De Leitenburg, M. Boaro, G. Dolcetti, The utilization of ceria in industrial catalysis, Catal. Today 50 (1999) 353–367. [4] S.N. Jacobsen, L.R. Wallenberg, P.O. Larsson, A. Andersson, J.O. Bovin, S.N. Jacobsen, U. Helmersson, Carbon monoxide oxidation on copper oxide thin films supported on corrugated cerium dioxide 111 and 001 surfaces, J. Catal. 181 (1999) 6–15. [5] S. Yabe, S. Momose, Cerium dioxide-solica complex: a novel, non-reactive and transparent UV absorber for cosmetics, J. Soc. Cosmet. Chem. Jpn. 32 (1998) 372–378. [6] R.W. Tarnuzzer, J. Colon, S. Patil, S. Seal, Vacancy engineered ceria nanostructures for protection from radiation-induced cellular damage, Nano Lett. 5 (2005) 2573–2577. [7] M. Das, S. Patil, N. Bhargava, L.F. Kang, L.M. Riedel, S. Seal, J.J. Hickman, Autocatalytic ceria nanoparticles offer neuroprotection to adult rat spinal cord neurons, Biomaterials 28 (2007) 1918–1925. [8] H. Inaba, H. Tagawa, Ceria-based solid electrolytes, Solid State Ionics 83 (1996) 1–16. [9] J. Van Herle, T. Horita, N. Kawada, N. Sakai, H. Yokakava, M. Dokiya, Low temperature fabrication of (Y, Gd, Sm)-doped ceria electrolyte, Solid State Ionics 86–88 (1996) 1255–1258. [10] K. Zheng, B.C.H. Sreele, M. Sahibzada, I.S. Metcafe, Solid oxide fuel cells based on Ce(Gd)O2x electrolytes, Solid State Ionics 86–88 (1996) 1221–1244. [11] M.A. Panhans, R.N. Blumenthal, A thermodynamic and electrical conductivity study of nonstoichiometric cerium dioxide, Solid State Ionics 60 (1993) 279– 298. [12] Y.S. Zhen, S.J. Milne, R.J. Brook, Oxygen ion conductivity of La2O3 doped with alkaline earth oxides, Sci. Ceram. 14 (1988) 1025–1030. [13] Z.C. Li, H. Zhang, B. Bergman, Synthesis and characterization of nanostructured Bi2O3-doped cerium oxides fabricated by PVA polymerization process, Ceram. Int. 34 (2008) 1949–1953. [14] S.E. Dann, Reactions and Characterization of Solids, Wiley, New York, 2000. pp. 141–14. [15] S. Wang, K. Maeda, M. Awano, Direct formation of crystalline gadoliniumdoped ceria powder via polymerized precursor solution, J. Am. Ceram. Soc. 85 (2002) 1750–1753. [16] A.R. West, Solid State Chemistry and its Applications, Wiley, New York, 1991. pp. 36–64. [17] M. Oljaca, R. Maric, S. Shanmugham, A. Hunt, Nanomaterials for solid oxide fuel cells, Am. Ceram. Soc. Bull. 82 (2003) 38–47.
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[18] K. Rajeswari, U.S. Hareesh, R. Subasri, Dibyendu Chakravarty, R. Johnson, Comparative Evaluation of Spark Plasma (SPS), Microwave (MWS), Two stage sintering (TSS) and Conventional Sintering (CRH) on the densification and micro structural evolution of fully stabilized zirconia ceramics, Sci. Sinter. 42 (2010) 259–267. [19] R. Subasri, M. Asha, K. Hembram, G.V.N. Rao, T.N. Rao, Microwave sintering of doped nanocrystalline ZnO and characterization for varistor applications, Mater. Chem. Phys. 115 (2009) 677–684. [20] R. Subasri, T. Mathews, O.M. Sreedharan, V.S. Raghunathan, Microwave processing of sodium beta alumina, Solid State Ionics 158 (2003) 199–204. [21] W.H. Sutton, Microwave processing of ceramic materials, Am. Ceram. Soc. Bull. 68 (1989) 376–386. [22] M.A. Janney, H.D. Kimrey, Microwave sintering of alumina at 28 GHz, Ceram. Powder Sci. 1 (1988) 919–924. [23] J. Samuels, J.R. Brandon, Effect of composition and the enhanced microwave sintering of alumina-based ceramic composites, J. Mater. Sci. 27 (1992) 3259– 3265. [24] Z. Xie, J. Li, Y. Huang, X. Kong, Microwave sintering behavior of ZrO2–Y2O3 with agglomerate, J. Mater. Sci. Lett. 15 (1996) 1158–1160. [25] K.J. Rao, P.D. Ramesh, Use of microwaves for the synthesis and processing of materials, Bull. Mater. Sci. 18 (1995) 447–465. [26] M. Prekajski, Z. Dohcˇevic´-Mitrovic´, M. Radovic´, B. Babic´, J. Pantic´, A. Kremenovic´, B. Matovic´, Nanocrystaline solid solution CeO2–Bi2O3, J. Eur. Ceram. Soc. 32 (2012) 1983–1987. [27] M. Prekajski, V. Fruth, C. Andronescu, V.L. Trandafilovic´, J. Pantic´, A. Kremenovic´, B. Matovic´, Thermal stability of Ce1xBixO2d (x = 0.1–0.5) solid solution, J. Alloys Comp. 578 (2013) 26–31. [28] M. Stojmenovic´, S. Boškovic´, S. Zec, B. Babic´, B. Matovic´, D. Bucˇevac, Z. Dohcˇevic´-Mitrovic´, F. Aldinger, Characterization of nanometric multidoped ceria powders, J. Alloys Comp. 507 (2010) 279–285.
[29] M. Stojmenovic´, S. Boskovic´, D. Bucevac, M. Prekajski, B. Babic´, B. Matovic´, S. Mentus, Electrical characterization of multidoped ceria ceramics, Ceram. Int. 39 (2013) 1249–1255. [30] P. Rice, D.V. Ragone, Measurement of enthalpies of evaporation of Bi and Bi2 by an optical absorption technique, J. Chem. Phys. 45 (1966) 4141–4146. [31] R. Waster, R. Hagenbeck, Grain boundaries in dielectric and mixed – conducting ceramics, Acta Mater. 48 (2000) 797–825. [32] X.J. Chen, K.A. Khor, S.H. Chan, L.G. Yu, Preparation yttria – stabilized zirconia electrolyte by spark – plasma sintering, Mater. Sci. Eng. A 341 (2003) 43–48. [33] D. Pérez Coll, P. Núnez, J.R. Frade, The role of SiO2 and sintering temperature on the grain boundary properties of Ce0.8Sm0.2O2I, J. Power Sources 196 (2011) 8383–8390. [34] M.R. Kosinski, R.T. Baker, Preparation and property – performance relationships in samarium – doped ceria nanopowders for solid oxide fuel cell electrolytes, J. Power Sources 196 (2011) 2498–2512. [35] B. Matovic´, M. Stojmenovic´, J. Pantic´, A. Varela, M. Zˇunic´, N. Jiraborvornpongsa, T. Yano, Electrical and microstructural properties of Yb-doped CeO2, J. Asian Ceram. Soc. 2 (2014) 117–122. [36] D. Bucevac, A. Radojkovic, M. Miljkovic, B. Babic, B. Matovic, Effect of preparation route on the microstructure and electrical conductivity of codoped ceria, Ceram. Int. 39 (2013) 3603–3611. [37] S. Boskovic, S. Zec, G. Brankovic, Z. Brankovic, A. Devecerski, B. Matovic, F. Aldinger, Preparation, sintering and electrical properties of nano-grained multidoped ceria, Ceram. Int. 36 (2010) 121–127. [38] M. Dudek, J. Molenda, Otrzymywanie i charakterystyka elektrolitów staych na bazie CeO2, Ceramics 84 (2004) 177. [39] K.C. Anjaneya, G.P. Nayaka, J. Manjanna, G. Govindaraj, K.N. Ganesha, Preparation and characterization of Ce1xGdxO2d (x = 0.1–0.3) as solid electrolyte for intermediate temperature SOFC, J. Alloys Comp. 578 (2013) 53–59.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Sintering and electrical properties of Ce1xBixO2d solid solution M. Prekajski a,⇑, M. Stojmenovic´ a, A. Radojkovic´ b, G. Brankovic´ b, H. Oraon c,1, R. Subasri c, B. Matovic´ a a
Vinca Institute of Nuclear Science, Material Science Laboratory, University of Belgrade, PO Box 522, Belgrade, Serbia Institute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava 1a, 11030 Belgrade, Serbia c International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur, Hyderabad 500 005, India b
a r t i c l e
i n f o
Article history: Received 6 December 2013 Received in revised form 25 July 2014 Accepted 10 August 2014 Available online 19 August 2014 Keywords: Nanostructured materials Sintering Electrochemical impedance spectroscopy Microstructure Scanning electron microscopy
a b s t r a c t Solid solution Ce1xBixO2d nanopowders with the composition of x = 0.1–0.5 were synthesized by using Self Propagating Room Temperature procedure (SPRT). The results obtained by XRPD show that synthesized samples were single-phase solid solution at room temperature. Powders were densified by using Conventional (CS) and Microwave (MS) Sintering techniques at different temperatures, in an air atmosphere for 1 h. Scanning electron microscopy (SEM) and complex impedance method measurements were carried out on sintered samples. Maximum achieved density was for sample with Ce0.80Bi0.20O2d composition for both applied sintering techniques. The highest conductivity was obtained for the ceramic composition Ce0.80Bi0.20O2d sintered by microwave technique at 700 °C. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the context of significant interest and concern on energy and environment, nanostructured cerium oxide (CeO2)-based materials are highly desired for solving present energy and environment related issues due to the improvements in redox properties, transport properties and surface to volume ratio with respect to the bulk materials. CeO2 based materials have been widely used in: clean energy [1], environmental protection and remediation [2], as a promoter in three-way catalysts for the elimination of toxic autoexhaust gases [3], solar-driven thermochemical CO2 reduction [4], UV absorbers [5], in biotechnology [6], medicine [7] etc. Most of above applications are related to the rapid formation and elimination of oxygen vacancy in CeO2 that endows it with high oxygen storage properties. In recent years, ceria-based materials have been extensively studied as some of the most promising electrolytes for reduced temperature solid oxide fuel cell (SOFC) system due to their high ionic conductivity at moderate temperature [8–10]. CeO2 can accommodate a high oxygen deficiency by the substitution of lower valent elements on the cation sublattice. However, being a highly refractory material, ceria based materials are difficult to
⇑ Corresponding author. Postal address: PO Box 522, Belgrade, Serbia. Tel.: +381 11 340 8860; fax: +381 11 340 8224. E-mail address: [email protected] (M. Prekajski). 1 Present address: Centre for Nanotechnology, Central University of Jharkhand, Brambe, Ranchi 835205, India. http://dx.doi.org/10.1016/j.jallcom.2014.08.090 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
densify below 1500 °C [11,12]. This makes it difficult for the manufacture of ceria electrolyte and other components, such as cathode and anode, which have to be co-fired below 1500 °C. CeO2 system doped with Bi3+ can be very interesting for application in SOFC’s due to the high ion conductivity of CeO2 and Bi2O3 phases [13]. Therefore (CeO2)1x(BiO1.5)x solid solution is expected to be a novel electrolyte exhibiting new electrochemical transport properties. In addition ceria-based solid solutions doped with lower valence ions usually possess oxide ion conductivity higher than yttrium-stabilized zirconia (YSZ). Exactly that was a motivation for the study of sintering properties and ionic conductivity on this solid solution. Sintering presents a very important part of processing for almost all ceramic materials. Conventional sintering (CS) often involves mixing of corresponding oxides and heating them at elevated temperatures for extended duration [14–16]. However, very low diffusion coefficients of ions in solid reactions requires intermittent grinding and repeated heating and grinding for extended periods of time to form the desired single-phase compound. On the other side, high temperature treatment usually results in large crystal growth and hence, total grain surface area. Moreover, the conductivity in nanocrystalline grain boundary regions is greater than for larger grains [17]. In this respect, it is important to develop powders of high quality with particle size in the nanometric range, but to reduce the sintering temperatures so as to retain the nanocrystallinity. In addition sintering of bismuth-rich compounds can be quite difficult because of the properties of Bi which easily vaporizes and melts at a temperature of 835 °C. These problems may be
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overcome by using some of the contemporary methods such as microwave sintering. In the last few years, microwave sintering (MS) of ceramics, has emerged as an alternative approach to densify ceramics and has gained much attention because of the rapid heating, enhanced densification rate, and improved microstructure [18–20]. In MS electromagnetic waves interact with ceramics, leading to volumetric heating by dielectric loss. Such a volumetric heating of MS may result in ceramics with a more uniform and finer microstructure when compared to conventional sintering. Volumetric heating in MS results in a homogenous temperature distribution, both on the surface as well as in the core. The ability of many ceramics to absorb MW radiation above a critical temperature has led to the use of microwave for processing of ceramics during drying, clinkering, sintering, melting and joining [21]. In addition to shorter processing times, microwave sintering results in ceramics with improved microstructure and mechanical properties. The most intriguing effects observed during MW sintering are the higher densification [22–24] and reaction rates [25] when compared to conventional sintering. The reasons for higher densification rates have been attributed to lower activation energy for MW sintering [22,23]. During microwave heating, electrical energy from the microwaves is converted into heat. The MW power absorbed by a sample is proportional to the electric field distribution in the sample, which is determined by the electrical properties of the material. Our previous investigations [26,27] focused on a complete characterization of synthesized nanopowders of Ce1xBixO2d with composition x = 0.1–0.5 and in the present investigation, a detailed analysis of their sintering behavior when subjected to conventional and microwave sintering has been carried out and results are compared. 2. Materials and methods In the present experiments, nanocrystalline Ce1xBixO2d powders were synthesized by SPRT method [26–29]. The starting materials for the preparation of solid solutions were cerium nitrate hexahydrate, bismuth nitrate pentahydrate and sodium hydroxide. Used chemicals had a purity of 99.9% as stated by the manufacturer (Riedel-de Haën). The compositions of the starting reacting mixtures were calculated according to the nominal composition of the final reaction product, according to Eq. (1):
2½ð1 xÞCeðNO3 Þ3 6H2 O þ xBiðNO3 Þ3 H2 O þ 6NaOH þ ð2 dÞO2 ! 2Ce1x Bix O2d þ 6NaNO3 þ ð15 2xÞH2 O þ 3=2O2
ð1Þ
Synthesis of nanopowders was carried out in an alumina mortar mixing reactants for 5–7 min, allowing rapid progress of the reaction at room temperature in air. After being exposed to air for 3 h, the entire volume of the powder was dissolved in water and subjected to centrifugation at Centurion 1020D centrifuge at 3000 rpm, for 10 min. Rinsing procedure was repeated four times with distilled water and twice with ethanol, in order to eliminate NaNO3 from the synthesized powder mixture. At the end, material was dried out at 60 °C in ambient atmosphere. Synthesized powders were characterized and details can be found in our previous work [26]. X-ray powder diffraction analysis of as-synthesized samples showed that the ceria powders with up to 50% of Bi are solid solutions with fluorite type of structure [26]. All synthesized powders have particle size in nanometric range (less than 4 nm) [26]. Raman spectral studies confirmed that all synthesized powders are single-phase solid solutions [26]. Synthesized powders were uniaxially compacted at 200 MPa without any binder into disks with 8 mm in diameter and 3 mm thickness. Samples were conventionally sintered in a furnace in air at different temperatures. Heating rate was 10 °C/min and with soaking time of 1 h. Microwave sintering (MS) was carried out in a 2.45 GHz, 6 kW, multimode cavity microwave sintering furnace (model MHTD 1800-6,4/2,45) supplied by Linn Hightherm GmbH, Germany. An optical pyrometer with a measurement range of 300–2000 °C attached to the chamber, was used for temperature measurement. The furnace was interfaced with a programmable logic controller (PLC) that gets the feedback from the optical pyrometer to carry out controlled heating and cooling. In order to obtain uniform and homogeneous heating, SiC susceptors were placed around the sample. The entire assembly, i.e. the sample along with the susceptors was housed in an insulation casket made of alumina fiber board supplied by Zircar ceramics, USA. Microwave sintering was carried out in air at temperatures of
900 °C, 1050 °C and 1150 °C with a heating and cooling rate of 10 °C/min for a soaking time of 1 h. The density of all sintered samples was measured by the Archimedes method. Semi-quantitative analysis of sintered samples with the highest density was determinated by using a energy dispersive X-ray analysis (EDX) on X-Max Large Area SDD EDX spectrometer. For scanning electron microscopy (SEM) analysis of sintered samples of the most density, the electron microscope (FE-SEM, Jeol JSM 6330F, Japan) was used. The samples were pre-coated with a several nanometers thick layer of gold before observation. For coating procedure, a device Fine Coat (JFC – 1100, Ion Sputter, JEOL, Japan) was used. The images were recorded in SEI mode at a magnification 5000 with the accelerating voltage of 20 kV. EDS analysis was carried out at the invasive electron energy of 30 keV by means of QX 2000S device (Oxford Microanalysis Group, UK). The spatilal resolution was 0.4 nm. The electric conductivity was measured by complex impedance method in the temperature range 600–700 °C. The measuring cell was placed in a vertical furnace open to air atmosphere. To ensure good electrical contact, both sides of the sintered samples were coated with a silver paste. The temperature was increased step by step with the increments of 25 °C. The measuring device was a frequency response analyzer (FRA Solartron 1260 Impedance/Gain Phase Analyzer) coupled with a dielectric interface (Solartron 1296). Operating frequencies were in the range 0.1 Hz–5 MHz and the peak-to-peak AC voltage amplitude was 50 mV. The impedance plots obtained experimentally were fitted by means of the software ZView Ò for Windows (Version 3.2b).
3. Results and discussion 3.1. Densification, semi-quantitative analysis and microstructure of sintered samples The results of conventional (CS) and microwave (MS) sintering of the Ce1xBixO2d (x = 0.1–0.5) samples (calculated and measured densities) are presented in Tables 1 and 2. Theoretical densities were calculated according to Eq. (2):
qðxÞ ¼
Z MðxÞ a3 NA
ð2Þ
In Eq. (2) Z represents number of unit cell, M is molar weight of compound x, a3 is volume of unit cell, whereas a represents lattice parameter (lattice parameters of all synthesized samples were obtained by using Rietveld refinement method, and the results were published in our previous paper [27]). NA is Avogadro’s number (6.0231023 atoms/mol). The success of nanopowders consolidation is intimately related to the control of the competition between densification and coarsening. The green structure, i.e., pore size distribution plays an important role in achieving high density. Densification is retarded or inhibited for wide pore distribution. In such case, big pores became larger while small pores shrink. In case of conventional sintering, it can be observed that at a temperature of 1450 °C, the maximum density was achieved for the composition Ce0.80Bi0.20O2d (87.0%). On the other hand, it is notable that the increase (1500 °C) or decrease (1400 °C) of temperature leads to a reduction of density values. Therefore, the optimal temperature as well as composition for conventional sintering was Ce0.80Bi0.20O2d sample densified at 1450 °C (87.0%). Microwave sintered samples show much better results for densities compared with conventional sintered ones. It should be emphasized that the achieved densities were obtained at much lower temperatures when compared to conventional sintering. The highest densities were achieved for samples that were microwave sintered at 1050 °C. With further increasing of temperature (1150 °C) there is decrease in density. The reason for this behavior is the same like in previous case, i.e. at higher temperatures, there is a concern of Bi evaporation, which leads to density decrease [27]. The results presented in Table 2 show that the optimal composition in MS sintering is Ce0.80Bi0.20O2d and Ce0.50Bi0.50O2d obtained at a temperature of 1050 °C. Therefore, we can conclude that the microwave sintered samples exhibited an enhanced densification when compared to the conventionally sintered samples.
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M. Prekajski et al. / Journal of Alloys and Compounds 617 (2014) 563–568 Table 1 The theoretical and measured density of the conventionally sintered (CS) samples at different temperatures, in the air, with a soaking time of 1 h. Temperature (°C)
Composition
Theoretical density
Measured density
Relative density (%)
1400
Ce0.80Bi0.20O2d
7.63
6.49
85.1
1450
Ce0.90Bi0.10O2d Ce0.80Bi0.20O2d Ce0.70Bi0.30O2d Ce0.60Bi0.40O2d Ce0.50Bi0.50O2d
7.45 7.63 7.83 8.03 8.23
6.45 6.94 5.82 5.25 4.09
86.6 90.9 74.3 65.4 49.8
1500
Ce0.80Bi0.20O2d
7.63
6.34
83.1
Table 2 The theoretical and measured density of the microwave sintered (MS) samples at different temperatures, in the air, with a soaking time of 1 h. Temperature (°C)
Composition
Theoretical density
Measured density
Relative density (%)
900
Ce0.90Bi0.10O2d Ce0.80Bi0.20O2d Ce0.70Bi0.30O2d Ce0.60Bi0.40O2d Ce0.50Bi0.50O2d
7.45 7.63 7.83 8.03 8.23
5.44 6.97 6.26 5.07 7.78
73.1 91.4 79.9 63.2 94.6
1050
Ce0.90Bi0.10O2d Ce0.80Bi0.20O2d Ce0.70Bi0.30O2d Ce0.60Bi0.40O2d Ce0.50Bi0.50O2d
7.45 7.63 7.83 8.03 8.23
7.00 7.23 7.29 7.48 7.90
94.0 94.8 93.1 93.2 96.0
1150
Ce0.80Bi0.20O2d Ce0.50Bi0.50O2d
7.45 7.83
6.65 6.79
89.3 86.7
It is known that the sintering method might have a significant influence on final material properties. In order to find out the stability of chemical compositions of CS and MS sintered samples, EDS analyses were performed (Table 3) for samples with highest density. Sample with the composition Ce0.80Bi0.20O2d sintered by CS at 1450 °C shows that bismuth almost completely evaporated at this temperature. However, unlike previous one, in the sample with same composition obtained by MS at 1050 °C the bismuth content remains almost the same as it was in the initial as synthesized sample [27]. Since the sample with composition Ce0.80Bi0.20O2d has the highest density for both sintering methods, it was taken as representative one for microstructure analysis. This sample was thermally treated at 1450 °C in case of conventional sintering, and at 1050 °C in case of microwave sintering (Fig. 1). Microstructure of conventionally sintered sample possesses bimodal structure. The round shapes of grains indicate Ostwald ripening growth in presence of liquid phase. The porosity of sample can be explained by very high vapor pressure of bismuth at used sintering temperature [30]. On the other hand, it is clear that the mean grain size of the MS samples is smaller than that of the corresponding conventionally sintered grade. The grain size of Ce0.70Bi0.30O2d is about 10–20 lm for CS, while mean grain size of MS sintered sample is about 0.3 lm. A more narrow size distribution is found for the MS samples. The difference in grain size implies that microwave sintering has the potential to limit the grain growth and homogenize the microstructure due to a volumetric heating mode and
enhanced vacancy diffusion. Namely it is known that ceria has low thermal conductivity, and it is known that vacancy diffusion is thermally driven process, and that CS process is not uniform and results in thermal mismatch between the surface and centre of the bodies and possible vacancy clustering that lower vacancy diffusion. On the other hand, in case of MS, volumetric nature of microwave heating results in uniform kinetic transport of vacancy and thus, enhanced vacancy diffusion, and in that way affects on the slowing down of grain growth. 3.2. Electrical conductivity Impedance measurements were performed only on MS sintered samples with highest densities (>95%) due to assumption that samples with these densities possess the closed porosity. Nyquist’s diagrams (impedance graphs) of the sintered samples with compositions Ce0.80Bi0.20O2d and Ce0.50Bi0.50O2d obtained by microwave sintering at temperature 1050 °C, are presented in Fig. 2. Measurements were recorded within the temperature range of 600–700 °C, with the increments of 25 °C. Their impedance diagrams for various temperatures were selected to illustrate the impedance behavior of the investigated doped ceria samples as a whole. Generally, in the available frequency range (1 Hz–0.1 MHz), impedance semicircle is observed. Such a type of Nyquist plot is characteristic of serially connected resistance (Rb – balk resistance) Capacitive element is the only being distributed i.e. frequency dependent, with impedance that can be expressed as:
Table 3 EDS microanalysis of the microwave and conventionally sintered samples after sintered at different temperatures. Sintering method
Temperature of calcination/sintering
Nominal chemical composition
Resulting composition (EDS analysis)
CS
1450 °C
Ce0.80Bi0.20O2d
Ce0.99Bi0.01O2d
MS
900 °C 900 °C 1050 °C 1050 °C
Ce0.80Bi0.20O2d Ce0.50Bi0.50O2d Ce0.80Bi0.20O2d Ce0.50Bi0.50O2d
Ce0.83Bi0.17O2d Ce0.55Bi0.45O2d Ce0.81Bi0.19O2d Ce0.55Bi0.45O2d
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Fig. 1. SEM micrographs of the samples Ce0.8Bi0.2O2d obtained by (a) conventional sintering at 1450 °C, and (b) microwave sintering at 1050 °C.
Fig. 2. Impedance diagrams for the sintered samples presented within the different temperature range: (a) Ce0.80Bi0.20O2d (600–700 °C) and (b) Ce0.50Bi0.50O2d (650–700 °C).
ZðxÞ ¼ Re þ
1 a ðjxÞ Q
ð3Þ
where a is an exponent (0 6 a 6 1) and Q is a pre-exponential factor and of the so-called constant phase element (CPE). If value of the resistive element, R, bonded in parallel with CPE is known, the effective capacitance can be calculated by using the following equation:
C¼R
1n n
1
Qn
ð4Þ
Equivalent circuit with both constant [31–33] and distributed [34] capacitive elements has been widely applied in the literature related to the sintered ceramics. When the resistive components in the semicircle were clearly separated, i.e., RbCb RgbCgb, where bulk capacitance (Cb) is typically few orders of magnitude lower than Cgb, the values of Rb and Rgb may be read separately as intercepts of the semicircles with the real axis [35]. At impedance graph of the sample Ce0.8Bi0.2O2d (Fig. 2a) the first section of the semicircles with the real axis may be attributed to the bulk resistance crystallite grains Rb, and second section may be attributed to the intergranular capacitance Rig. Exponent a obtained from the fitting results amounted to about 0.75 and corresponds to the capacity of grain boundary, since the effective capacitances calculated from Eq. (4) are 109 F/cm, i.e. values typical for grain boundary capacitance [36]. However, as can be seen from Fig. 2b, in the sample Ce0.50Bi0.50O2d resistance decrease with the temperature increasing. Consequently, the whole region of the impedance points shifts towards the low-frequency semicircle. Namely, at higher temperatures (650–700 °C), instead of Rb and Rgb separately, only the whole
sum Rb + Rgb became readable in the available frequency range. The effective capacitances related to Cgb in this case is about two orders of magnitude smaller, while a amounted to about 0.97. Therefore, in this range of temperatures the Nyquist plot does not clearly indicate the separation between the balk and grain boundary. The appearance of a new semicircle in Fig. 2 was observed in a low – frequency region (arrows indicate the beginning of the new semicircle) and almost doubtless, it originates from the oxygen electrode reactions, O2/O2 (Eq. (2)), analysis of which is beyond the scope of this study.
1 O2 þ 2e ! O2 2
ð5Þ
The total resistance values measured for all samples were used to calculate ionic conductivity of the samples at various temperatures (Table 4).
Table 4 The ionic conductivity bulk (jb) and grain boundary (jgb) of the sample Ce0.80Bi0.20O2d, and total ionic conductivity (jb+gb) of the sample Ce0.50Bi0.50O2d, obtained by microwave sintering at temperature 1050 °C. T (°C)
Ce0.80Bi0.20O2d
jb (S cm1) 600 625 650 675 700
3
1.62 10 2.03 103 2.74 103 2.83 103 3.14 103
Ce0.50Bi0.50O2d
jgb (S cm1) 4
3.52 10 4.98 104 6.67 104 7.62 104 9.32 104
jb+gb (S cm1) 1.89 104 2.45 104 4.74 104 7.85 104 1.23 103
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Fig. 3. The dependence log j = f(1/T) for the samples: (a) Ce0.80Bi0.20O2d and (b) Ce0.50Bi0.50O2d in the form of Arrhenius plots obtained by microwave sintering.
By comparing the results obtained for the two ceramics samples, it can be seen that the highest values of ionic conductivity exhibited the sample that having 20% Bi3+ obtained by microwave sintering. The actual values at 700 °C for bulk and grain boundary amounted to 3.14 103 and 9.32 104 S cm1, respectively. On the other hand, for the sample with 50% Bi3+ at temperature of 700 °C a lower total conductivity (1.23 103) is observed. The two samples displayed a difference in density, grain size and porosity, which also may be responsible for the difference in their electrical conductivities. Based on the results listed in Table 4, the dependence log k = f(1/ T) for the microwave sintered samples having compositions Ce0.80Bi0.20O2d and Ce0.50Bi0.50O2d are compared and depicted in Fig. 3. Literature data showed [37] that the activation energies (Ea) for CeO2 obtained by MGNP (modified glycine nitrate procedure) method with two and three dopants were reported to be 0.66 and 0.81 eV, respectively. However, here the significantly lower activation energies were found, amounting to 0.209 (bulk – Ce0.80Bi0.20O2d) and 0.289 (grain boundary – Ce0.80Bi0.20O2d) and 0.600 eV (bulk + grain boundary – Ce0.50Bi0.50O2d). It is known that CeO2 doped systems with 20% concentration of dopant usually achieves the highest ionic conductivity values and a minimum values of activation energies [8,34,38,39]. With a further increase in concentration of the dopant, the conductivity decreased and increases activation energy [8,34,38,39]. 4. Conclusions In summary, samples of nanopowder solid solution Ce1xBixO2d (x = 0.1–0.5) which was previously synthesized by a simple and low cost SPRT method were sintered with two different methods. It appears that conventional sintering requires much higher temperatures for densification. However, at high temperatures there is significant loss of Bi content due to process of evaporation. This problem can be overcome by applying the microwave sintering technique. In this way high densified samples can be obtained at much lower temperatures (1050 °C) without loss of Bi concentration. A finer grain size and more uniform microstructure were obtained by microwave sintering due to the shorter sintering cycle because of the faster cooling rate and the establishment of a more uniform temperature distribution within the short sintering time, respectively. Temperature of 1050 °C seems to be an optimum sintering temperature where 93–96% of theoretical density can be achieved depending on the bismuth content. Temperatures higher than 1050 °C can lead to Bi loss, which further leads to lower densities. The lowest total resistance R was obtained for the samples Ce0.80Bi0.20O2d obtained by microwave sintering, and thus higher
ionic conductivity, because of a finer grain size and more uniform microstructure that allows for a higher density to be obtained. At 700 °C, conductivities for bulk and grain boundary amounted to 3.14 103 and 9.32 104 S cm1 for sample Ce0.80Bi0.20O2d, respectively. Ea value obtained is consistent with the results of ionic conductivity. Acknowledgment Financial support from the Serbian Education and Science Ministry in the Framework of projects No. 45012 and 176016 is gratefully acknowledged. References [1] A.B. Stambouli, E. Traversa, Solid oxide fuel cells (SOFCs). A review of an environmentally clean and efficient source of energy, Renew. Sust. Energy Rev. 6 (2002) 433–455. [2] F.B. Li, X.Z. Li, M.F. Hou, K.W. Cheah, W.C.H. Choy, Enhanced photocatalytic activity of Ce3+–TiO2 for 2-mercaptobenzothiazole degradation in aqueous suspension for odour control, Appl. Catal. A 285 (2005) 181–189. [3] A. Trovarelli, C. De Leitenburg, M. Boaro, G. Dolcetti, The utilization of ceria in industrial catalysis, Catal. Today 50 (1999) 353–367. [4] S.N. Jacobsen, L.R. Wallenberg, P.O. Larsson, A. Andersson, J.O. Bovin, S.N. Jacobsen, U. Helmersson, Carbon monoxide oxidation on copper oxide thin films supported on corrugated cerium dioxide 111 and 001 surfaces, J. Catal. 181 (1999) 6–15. [5] S. Yabe, S. Momose, Cerium dioxide-solica complex: a novel, non-reactive and transparent UV absorber for cosmetics, J. Soc. Cosmet. Chem. Jpn. 32 (1998) 372–378. [6] R.W. Tarnuzzer, J. Colon, S. Patil, S. Seal, Vacancy engineered ceria nanostructures for protection from radiation-induced cellular damage, Nano Lett. 5 (2005) 2573–2577. [7] M. Das, S. Patil, N. Bhargava, L.F. Kang, L.M. Riedel, S. Seal, J.J. Hickman, Autocatalytic ceria nanoparticles offer neuroprotection to adult rat spinal cord neurons, Biomaterials 28 (2007) 1918–1925. [8] H. Inaba, H. Tagawa, Ceria-based solid electrolytes, Solid State Ionics 83 (1996) 1–16. [9] J. Van Herle, T. Horita, N. Kawada, N. Sakai, H. Yokakava, M. Dokiya, Low temperature fabrication of (Y, Gd, Sm)-doped ceria electrolyte, Solid State Ionics 86–88 (1996) 1255–1258. [10] K. Zheng, B.C.H. Sreele, M. Sahibzada, I.S. Metcafe, Solid oxide fuel cells based on Ce(Gd)O2x electrolytes, Solid State Ionics 86–88 (1996) 1221–1244. [11] M.A. Panhans, R.N. Blumenthal, A thermodynamic and electrical conductivity study of nonstoichiometric cerium dioxide, Solid State Ionics 60 (1993) 279– 298. [12] Y.S. Zhen, S.J. Milne, R.J. Brook, Oxygen ion conductivity of La2O3 doped with alkaline earth oxides, Sci. Ceram. 14 (1988) 1025–1030. [13] Z.C. Li, H. Zhang, B. Bergman, Synthesis and characterization of nanostructured Bi2O3-doped cerium oxides fabricated by PVA polymerization process, Ceram. Int. 34 (2008) 1949–1953. [14] S.E. Dann, Reactions and Characterization of Solids, Wiley, New York, 2000. pp. 141–14. [15] S. Wang, K. Maeda, M. Awano, Direct formation of crystalline gadoliniumdoped ceria powder via polymerized precursor solution, J. Am. Ceram. Soc. 85 (2002) 1750–1753. [16] A.R. West, Solid State Chemistry and its Applications, Wiley, New York, 1991. pp. 36–64. [17] M. Oljaca, R. Maric, S. Shanmugham, A. Hunt, Nanomaterials for solid oxide fuel cells, Am. Ceram. Soc. Bull. 82 (2003) 38–47.
568
M. Prekajski et al. / Journal of Alloys and Compounds 617 (2014) 563–568
[18] K. Rajeswari, U.S. Hareesh, R. Subasri, Dibyendu Chakravarty, R. Johnson, Comparative Evaluation of Spark Plasma (SPS), Microwave (MWS), Two stage sintering (TSS) and Conventional Sintering (CRH) on the densification and micro structural evolution of fully stabilized zirconia ceramics, Sci. Sinter. 42 (2010) 259–267. [19] R. Subasri, M. Asha, K. Hembram, G.V.N. Rao, T.N. Rao, Microwave sintering of doped nanocrystalline ZnO and characterization for varistor applications, Mater. Chem. Phys. 115 (2009) 677–684. [20] R. Subasri, T. Mathews, O.M. Sreedharan, V.S. Raghunathan, Microwave processing of sodium beta alumina, Solid State Ionics 158 (2003) 199–204. [21] W.H. Sutton, Microwave processing of ceramic materials, Am. Ceram. Soc. Bull. 68 (1989) 376–386. [22] M.A. Janney, H.D. Kimrey, Microwave sintering of alumina at 28 GHz, Ceram. Powder Sci. 1 (1988) 919–924. [23] J. Samuels, J.R. Brandon, Effect of composition and the enhanced microwave sintering of alumina-based ceramic composites, J. Mater. Sci. 27 (1992) 3259– 3265. [24] Z. Xie, J. Li, Y. Huang, X. Kong, Microwave sintering behavior of ZrO2–Y2O3 with agglomerate, J. Mater. Sci. Lett. 15 (1996) 1158–1160. [25] K.J. Rao, P.D. Ramesh, Use of microwaves for the synthesis and processing of materials, Bull. Mater. Sci. 18 (1995) 447–465. [26] M. Prekajski, Z. Dohcˇevic´-Mitrovic´, M. Radovic´, B. Babic´, J. Pantic´, A. Kremenovic´, B. Matovic´, Nanocrystaline solid solution CeO2–Bi2O3, J. Eur. Ceram. Soc. 32 (2012) 1983–1987. [27] M. Prekajski, V. Fruth, C. Andronescu, V.L. Trandafilovic´, J. Pantic´, A. Kremenovic´, B. Matovic´, Thermal stability of Ce1xBixO2d (x = 0.1–0.5) solid solution, J. Alloys Comp. 578 (2013) 26–31. [28] M. Stojmenovic´, S. Boškovic´, S. Zec, B. Babic´, B. Matovic´, D. Bucˇevac, Z. Dohcˇevic´-Mitrovic´, F. Aldinger, Characterization of nanometric multidoped ceria powders, J. Alloys Comp. 507 (2010) 279–285.
[29] M. Stojmenovic´, S. Boskovic´, D. Bucevac, M. Prekajski, B. Babic´, B. Matovic´, S. Mentus, Electrical characterization of multidoped ceria ceramics, Ceram. Int. 39 (2013) 1249–1255. [30] P. Rice, D.V. Ragone, Measurement of enthalpies of evaporation of Bi and Bi2 by an optical absorption technique, J. Chem. Phys. 45 (1966) 4141–4146. [31] R. Waster, R. Hagenbeck, Grain boundaries in dielectric and mixed – conducting ceramics, Acta Mater. 48 (2000) 797–825. [32] X.J. Chen, K.A. Khor, S.H. Chan, L.G. Yu, Preparation yttria – stabilized zirconia electrolyte by spark – plasma sintering, Mater. Sci. Eng. A 341 (2003) 43–48. [33] D. Pérez Coll, P. Núnez, J.R. Frade, The role of SiO2 and sintering temperature on the grain boundary properties of Ce0.8Sm0.2O2I, J. Power Sources 196 (2011) 8383–8390. [34] M.R. Kosinski, R.T. Baker, Preparation and property – performance relationships in samarium – doped ceria nanopowders for solid oxide fuel cell electrolytes, J. Power Sources 196 (2011) 2498–2512. [35] B. Matovic´, M. Stojmenovic´, J. Pantic´, A. Varela, M. Zˇunic´, N. Jiraborvornpongsa, T. Yano, Electrical and microstructural properties of Yb-doped CeO2, J. Asian Ceram. Soc. 2 (2014) 117–122. [36] D. Bucevac, A. Radojkovic, M. Miljkovic, B. Babic, B. Matovic, Effect of preparation route on the microstructure and electrical conductivity of codoped ceria, Ceram. Int. 39 (2013) 3603–3611. [37] S. Boskovic, S. Zec, G. Brankovic, Z. Brankovic, A. Devecerski, B. Matovic, F. Aldinger, Preparation, sintering and electrical properties of nano-grained multidoped ceria, Ceram. Int. 36 (2010) 121–127. [38] M. Dudek, J. Molenda, Otrzymywanie i charakterystyka elektrolitów staych na bazie CeO2, Ceramics 84 (2004) 177. [39] K.C. Anjaneya, G.P. Nayaka, J. Manjanna, G. Govindaraj, K.N. Ganesha, Preparation and characterization of Ce1xGdxO2d (x = 0.1–0.3) as solid electrolyte for intermediate temperature SOFC, J. Alloys Comp. 578 (2013) 53–59.