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Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic
Author’s Accepted Manuscript Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic S. Ramesh, C.K. Ng, C.Y. Tan, W.H. Wong, C.Y. Ching, A. Muchtar, M.R. Somalu, S. Ramesh, Hari Chandran, P. Devaraj www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)30883-5 http://dx.doi.org/10.1016/j.ceramint.2016.06.050 CERI13066
To appear in: Ceramics International Received date: 21 May 2016 Revised date: 7 June 2016 Accepted date: 8 June 2016 Cite this article as: S. Ramesh, C.K. Ng, C.Y. Tan, W.H. Wong, C.Y. Ching, A. Muchtar, M.R. Somalu, S. Ramesh, Hari Chandran and P. Devaraj, Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic
S. Ramesh1 *, C.K. Ng1,2, C.Y. Tan1, W.H. Wong1, C.Y. Ching1, A. Muchtar3,4, M.R. Somalu4, S. Ramesh5, Hari Chandran6, P. Devaraj6
1
Centre of Advanced Manufacturing & Material Processing (AMMP), Department of
Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 2
Department of Materials Engineering, Faculty of Engineering & Built Environment,
Tunku Abdul Rahman University College, 53300 Kuala Lumpur, Malaysia 3
Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, 43600
UKM Bangi, Selangor, Malaysia 4
Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor,
Malaysia 5
Centre for Ionics University of Malaya, Department of Physics, Faculty of Science,
University of Malaya, 50603 Kuala Lumpur, Malaysia 6
Division of Neurosurgery, Faculty of Medicine, University of Malaya, 50603 Kuala
Lumpur, Malaysia Abstract The effect of conventional sintering from 1300 to 1550 °C on the properties of 1 mol% ceria-doped scandia stabilized zirconia was investigated. In addition, the influence of rapid sintering via microwave technique at low temperature regimes of 1300 C and *
Corresponding author. E-mail address: [email protected] (S. Ramesh) 1
1350 C for 15 minutes on the properties of this zirconia was evaluated. It was found that both sintering methods yielded highly dense samples with minimum relative density of 97.5%. Phase analysis by X-ray diffraction revealed the presences of only cubic phase in all sintered samples. All sintered pellets possessed high Vickers hardness (13-14.6 GPa) and fracture toughness (~ 3 MPam1/2). Microstructural examination by using the scanning electron microscope revealed that the grain size varied from 2.9 to 9.8 µm for the conventional-sintered samples. In comparison, the grain size of the microwave-sintered zirconia was maintained below 2 µm. Electrochemical Impedance Spectroscopy study showed that both the bulk and grain boundary resistivity of the zirconia decreases with increasing test temperature regardless of sintering methods. However, the grain boundary resistivity of the microwave-sintered samples was higher than the conventional-sintered ceramic at 600 C and reduced significantly at 800 C thus resulting in the enhancement of electrical conduction.
Keywords: Scandia stabilized zirconia, microwave sintering, ionic conductivity, mechanical property.
1.
Introduction
Electrochemical device such as solid oxide fuel cell (SOFC) produces electricity efficiently from the reaction between the hydrogen and air. It uses metal oxide solid ceramic electrolytes to allow the transport of oxygen vacancy (O2-). The O2- ionic conduction requirement for the ceramic electrolyte necessitates high operating temperatures (600 C -1000 C) to ensure adequate ionic conduction in the electrolyte. The high temperature operating environment promote fast reaction kinetics, therefore
2
does not require the use of noble metal that could limit the applicability of SOFC due to the scarcity of resources and high price issue in mass production [1]. It also allows reforming of hydrocarbon (CO, CH4, gasoline, etc.) within the fuel cell. However, high temperature SOFC operation on the other hand makes great demands on materials and requires very careful selection on issues concerning sealing of the cells, compatibility of thermomechanical properties between electrolyte and electrode, and stability of individual components [2, 3]. As a result, recent development emphasis has been on improving material property or modifying processing routes to reduce the working temperature of the electrolyte material. Zirconia electrolytes has high ionic conductivity, high mechanical and chemical stability, compatibility towards other components used in the SOFC and with lower electronic conductivity in oxidizing environment. Yttria stabilized zirconia (YSZ) and scandia stabilized zirconia (ScSZ) are the two most commonly used electrolytes for SOFC because of their excellent ionic conductivity [4, 5]. The ionic conductivity of ScSZ is twice that of YSZ due to similar ionic radius of Sc3+ (0.087nm) and Zr4+ (0.084nm) that leads to lower internal stress and lower activation energy, and hence results in lower operating temperatures for a comparable conductivity [6]. Co-doping of ScSZ with small amount of other oxides such as 1-2 mol% bismuth oxide [7-9], 3 mol% ytterbium oxide [10], and in particular 1 mol% ceria [11] , resulted in improved phase stability and enhanced electrical performance. More efforts in the low temperature accelerated sintering are imperative to make the electrolyte fabrication process feasible in large scale SOFC production. Conventional heating requires the heat to be transferred from the surface to the core of the material whereas microwave sintering resulted in volumetric heating and
3
thereby a promising technique for uniform and fast firing of the ceramic. It involves the absorption of microwave field to cause molecular vibrations and generates heat through direct electromagnetic to thermal energy conversion [12-14]. Previous studies on microwave sintering of various types of ceramics such as alumina [15], hydroxyapatite [16], and YSZ [17, 18] showed promising results in terms of enhancing densification at lower temperatures. The aim of this work was to study the effect of sintering temperature on the properties of ceria-doped scandia stabilized zirconia electrolyte. In addition, the influence of rapid microwave heating process at low sintering temperature on the conductivity of the zirconia was investigated for potential use as electrolyte material in intermediate-temperature SOFC.
2.
Experimental procedures
The starting material 10Sc1CeSZ (1 mol% ceria + 10 mol% scandia stabilized zirconia) was a commercially available powder obtained from Daiichi Co. Ltd. Japan. The powder has a specific surface area of 11.1 m2/g and average particle size, D50 of 0.53 µm. The as-received powders were compacted in a steel die by uniaxial pressing (5-6 MPa) followed by cold isostatic pressing at 200 MPa. The conventional sintering was performed in a box furnace at various temperatures ranging from 1300C to 1550C with 2 hours holding time at a heating rate of 10C/min. For comparison purpose, some samples were microwave sintered at 1300C and 1350C for 15 min. at a heating rate of 30C/min. in a 2.45 GHz microwave furnace. The sintered density was determined by the water immersion technique based on the Archimedes principle. The phase analysis by X-ray diffraction (XRD) of the
4
ceramics was carried out using a PANanalytical Empyran XRD with CuK as the radiation source (λ = 1.5406 Å) operated at 45 kV, 40 mA, a step size of 0.02° over a 2 of 20° to 80°. Microstructure characterization was performed by using a scanning electron microscopy (Phenom Pro SEM, Netherland) on polished and thermally etched sintered samples. The grain size of the sintered samples was determined by the line intercept technique using the SEM images. The AC impedance spectra of the sintered samples were measured using Electrochemical Impedance Spectroscopy (Metrohm Autolab EIS) at the temperature range of 600-800C. For this purpose, silver paint was applied on both sides of the disc sample and connected to two electrodes via platinum wires to a frequency response analyzer (Autolab AUT 302 FRA). The frequency was varied over the range of 0.1 Hz to 1 MHz, at an applied potential of 10 mV and the measured impedance data was fitted using the Nova software. The bulk and grain boundary resistance were subsequently obtained from equivalent circuit fitting of parallel RQ (R-Resistance, Q-Constant Phase Element) in series. Vickers Hardness was calculated from the diagonal lengths of the indentation made on the samples using a Mitutoyo Vickers hardness tester (AVK-C2, Japan) at an applied load of 10 kgf with a dwell time of 10s. For each temperature, three samples were prepared and at least 5 measurements were taken for each sample. The Young’s modulus by sonic resonance was determined for rectangular samples using a commercial testing instrument (GrindoSonic: MK5 “Industrial”, Belgium). The instrument permits determination of the resonant frequency of a sample by monitoring and evaluating the vibrational harmonics of the sample by a transducer. The modulus of elasticity or Young’s modulus was calculated using the experimentally determined resonant frequency [19].
5
3.
Results and discussion The XRD analysis of the sintered samples is shown in Fig. 1. The results
revealed the presences of only the cubic phase in the structure thus indicating that both the scandium and cerium has been incorporated in the zirconia lattice. This result is in agreement with the findings of several researchers [20-22] who reported phase-pure cubic phase exist for samples sintered above 1300C. It is believed that the incorporation of Sc3+ and Ce4+ in the zirconia stabilizes the cubic phase and effectively increased the ionic size in the cubic fluorite structure which in turn would create more oxygen vacancies within the crystal lattice [23]. SEM micrographs of the 10Sc1CeSZ samples sintered by conventional and microwave methods are shown in Fig. 2, revealing the presents of equiaxed cubic grains whereas the average grain size of the conventional-sintered samples is shown in Fig. 3. The result shows that the cubic grain size of conventional-sintered zirconia increased with increasing sintering temperature, from 2.9 ± 0.2 µm (1300 C) to 9.8 ± 0.1 µm (1550C). In terms of densification, it was found that all the conventional-sintered samples, regardless of sintering temperatures, exhibited above 97% relative densities (the relative density is calculated based on the theoretical density of 5.74 g/cm3 [24]) as shown in Fig. 4(a). The results showed that for every 100C increased in sintering temperature, the cubic grain grew at a rate of 3.5 ± 0.2 µm. The grain size obtained in the present work for the conventional-sintered samples is slightly higher than the values reported in the literature [22, 25]. On the other hand, it was found that rapid heating via microwave sintering was effective in suppressing the grain sizes to below 2 µm when sintered at
6
1300-1350 C and the samples were able to achieve compatible bulk density as with the conventional-sintered samples as shown in Table 1. The effect of sintering temperature on the Vickers hardness, fracture toughness, and Young’s modulus of the 10Sc1CeSZ samples are presented in Fig. 4 (b - d). The highest Vickers hardness of 14.6 ± 0.7 GPa was recorded for the 1300 C conventionalsintered sample. However, as the sintering temperature increases to 1550 C, this was accompanied by a reduction in the hardness to about 13 ± 0.5 GPa. This trend is in agreement with that reported by Orlovskaya et al. [26]. Higher sintering temperatures produce coarser grains with less grain boundary area per volume and therefore reduce the resistance to localized plastic deformation and results in lower hardness. On the other hand, the microwave sintered samples exhibited comparable hardness, above 13.5 GPa, when sintered at 1300 to 1350 C as shown in Table 1. The other mechanical properties, i.e. the fracture toughness and Young’s modulus of both, the conventional and microwave sintered samples were found to vary in the range of 3.2-3.7 MPa.m1/2 and 180-215 GPa, respectively with increasing sintering temperatures as shown in Fig. 4 and Table 1. The Nyquist plots from the impedance data collected over the operating temperature range of 600 C- 800 C for the samples sintered at 1300C and 1350C using both conventional and microwave methods are shown in Fig. 5. The EIS impedance measurement data are presented in the form of imaginary, Z (capacitive) against real, Z (resistive) impedances. To identify the constituent component of a material, different regions of the electrical system are usually characterized according to their electrical relaxation times by a parallel resistance and capacitance (RC) to extract the R and C values of each semicircle on the impedance plots [27]. In this study,
7
depressed arcs instead of ideal semicircles were observed on the impedance spectrum, signifying non-ideal capacitive behavior due to the distribution of currents and electroactive species [28]. Each depressed arc is therefore fitted with a parallel resistance and constant phase element (R-Q) to replace the ideal capacitive property. The associated R values obtained from the high frequency intercepts on the real axis are attributed to the bulk resistance of the sample, the intermediate frequency intercepts are attributed to grain boundary resistance, and the low frequency arcs are attributed to the electrode resistance. It is visible from the high frequency intercepts, the impedance of the bulk component of all samples tested become smaller with rising operating temperatures. This is postulated as the association energy becomes less significant due to dissociation of oxygen vacancies (O2-) from the cations at higher temperatures, and consequently activation energy for the ionic conduction is predominantly by virtue of oxygen ion migration enthalpy [23]. Migration energy needed for the free ions to hop from one crystal site to its next unoccupied lattice site would be readily available from the elevated temperature operations; hence lower activation energy and higher ionic conduction occur with raising temperatures. Both scandia and zirconia have a ratio of ionic radius of the cation (rc) and anion (ra), i.e. rc/ra of less than 0.7, thus favored in sixfold coordination in the crystal structure. Lower coordination number elicits the formation of strong anion microdomains eventuated in higher activation energy at lower temperatures, caused by additional ordering energy due to presence of the microdomains of correlated dopant-vacancy clusters [29, 30]. The contributions of bulk and grain boundary resistivities for conventionalsintered and microwave-sintered samples are presented in Fig. 6 and Fig. 7,
8
respectively. From the bulk resistivity plots in Fig. 6, the microwave-sintered samples showed no appreciable differences with the conventional-sintered samples regardless of the operating test temperature (600 – 800 °C) thus indicating that the bulk resistivity is not affected by the sintering condition. On the other hand, Fig. 7 shows that the grain boundary contribution in microwave-sintered samples are more pronounced compared to the bulk resistivity at the low temperature region (600 C) resulting in relatively high total resistivities. However, grain boundary resistivity was found to decrease significantly at higher temperature range (700 to 800C). The diminishing grain boundary resistivities show that the intrinsic grain boundary blocking effect disappeared at 800 C. This observation is in agreement with that reported by Rajeswari et al.[31]. It is believed that the fine grain size and the larger grain boundary per unit area could have contributed to the lager grain boundary resistivity in the microwave sample.
4.
Conclusions
In this work, the effect of sintering temperature on the properties of 10Sc1CeSZ was investigated and the results were also compared with samples sintered by microwave at low temperature regime. The study revealed that the sintering temperature has an effect on the bulk density as well as the grain size of the conventional-sintered zirconia. However, the mechanical properties were not significantly affected by the sintering temperatures. In contrast, the beneficial effect of microwave sintering in producing a highly dense ceria-doped scandia stabilized zirconia, characterized by having fine grain size below 2 µm coupled with equivalent mechanical properties as that obtained from the high temperature (1550 °C) conventional-sintered sample has been revealed. Grain boundary resistivities contribution was found to be dominant at lower operating
9
temperature of 600 °C for the microwave-sintered samples and diminished at high operating temperature of 800 °C. This study has demonstrated the beneficial effect of rapid sintering via microwave in promoting densification of 10Sc1CeSZ at lower temperatures without sacrificing the cubic phase and mechanical properties for SOFC application.
Acknowledgements This study was supported under the PPP grant number PG080-2013A, UMRG grant number RP024B-13AET and Esciencefund grant number SF010-2014.
References [1] A.B. Stambouli, E. Traversa, A review of an environmentally clean and efficient source of energy, Renew and Sustainable Energy Rev. 6 (2002) 433-455. [2]
E. Barsoukov, J.R. Macdonald, Impedance spectroscopy: theory, experiment, and applications, 2nd ed., John Wiley & Sons, New Jersey, 2005.
[3]
A. Orera, P. Slater, New chemical systems for solid oxide fuel cell, Chem. Mater. 22 (2009) 675-690.
[4]
S. Badwal, K. Foger, Solid oxide electrolyte fuel cell review, Ceram. Int. 22 (1996) 257-265.
[5]
S. Singhal, Advances in solid oxide fuel cell technology, Solid State Ionics 135 (2000) 305-313.
[6]
C. Haering, A. Roosen, H. Schichl, M. Schnöller, Degradation of the electrical conductivity in stabilised zirconia system: Part II: scandia-stabilized zirconia, Solid State Ionics 176 (2005) 261-268.
10
[7]
B. Bai, N. Sammes, A. Smirnova, Physical and electrochemical characterization of Bi2O3-doped scandia stabilized zirconia. J. Power Sources 176 (2008) 76-81.
[8]
M. Hirano, T. Oda, K. Ukai, Y. Mizutani, Effect of Bi2O3 additives in Sc stabilized zirconia electrolyte on a stability of crystal phase and electrolyte properties, Solid State Ionics 158 (2003) 215-223.
[9]
S. Sarat, N. Sammes, A. Smirnova, Bismuth oxide doped scandia-stabilized zirconia electrolyte for the intermediate temperature solid oxide fuel cells, J. Power Sources 160 (2006) 892-896.
[10] R. Chiba, T. Ishii, F. Yoshimura, Temperature dependence of ionic conductivity in (1-x) ZrO2−(x-y)Sc2O3−yYb2O3 electrolyte material, Solid State Ionics 91 (1996) 249-256. [11] D.- S. Lee, W. Kim, S. Choi, J. Kim, H.-W. Lee, J.- H. Lee, Characterization of ZrO2 co-doped with Sc2O3 and CeO2 electrolyte for the application of intermediate temperature SOFCs. Solid State Ionics 176 (2005) 33-39. [12] D. Agrawal, Microwave sintering of ceramics, composites and metallic materials, and melting of glasses, Trans. of the Indian Ceram. Soc. 65 (2006) 129-144. [13] W.H. Sutton, Microwave processing of ceramic materials, Am. Ceram. Soc. Bull. 68 (1989) 376-386. [14] A.R. West, Solid state chemistry and its applications, John Wiley & Sons, West Sussex, 2007. [15] S. Lefeuvre, E. Federova, O. Gomonova, J. Tao, Microwave Sintering of Microand Nano-Sized Alumina Powder, 12th Seminar Computing Modelling in Microwave Engineering & Applications, Grenoble, France, 2010, pp. 46–50. [16] S. Nath, B. Basu, A. Sinha, A comparative study of conventional sintering with
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microwave sintering of hydroxyapatite synthesized by chemical route, Trends in Biomaterials & Artificial Organs 19 (2006) 93-98. [17] M. Mazaheri, Z.R. Hesabi, F. Golestani‐Fard, S. Mollazadeh, S. Jafari, S. Sadrnezhaad, The effect of conformation method and sintering technique on the densification and grain growth of nanocrystalline 8 mol% Yttria‐Stabilized Zirconia, J. Am. Ceram. Soc. 92 (2009) 990-995. [18] K. Rajeswari, U. Hareesh, R. Subasri, D. 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. of Sinter. 42 (2010) 259-267. [19] ASTM Standard C1259-2008e1, "Standard test method for dynamic young's modulus, shear modulus, and poisson's ratio for advanced ceramics by impulse excitation of vibration," ASTM International, West Conshoshocken, PA, 2008 [20] A. Zevalkink, A. Hunter, M. Swanson, C. Johnson, J. Kapat, N. Orlovskaya, Processing and Characterization of Sc2O3-CeO2-ZrO2 Electrolyte Based Intermediate Temperature Solid Oxide Fuel Cells, In MRS Symposium Proceedings Series Volume 972 (2007) 163-168. [21] R. Grosso, E. Muccillo, Sintering, phase composition and ionic conductivity of zirconia–scandia–ceria, J. Power Sources 233 (2013) 6-13. [22] S. Yarmolenko, J. Sankar, N. Bernier, M. Klimov, J. Kapat, N. Orlovskaya, Phase stability and sintering behavior of 10 mol% Sc2O3–1mol% CeO2–ZrO2 ceramics, J. Fuel Cell Sci. Technol. 6 (2009) 021007. [23] J.T. Irvine, P. Connor, Solid Oxide Fuels Cells: Facts and Figures, Springer, London, 2013.
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[24] R. Rosten, M. Koski, E. Koppana. A Guide to the Calculation of Theoretical Densities of Crystal Structures for Solid Oxide Fuel Cells, J. Undergrad. Mater. Res. 2 (2006) 38-41. [25] O.L. Kyrpa, On the temperature dependence of oxygen ionic conductivity of 10Sc1CeSZ electrolytes, The National Academy of Sciences of Ukraine, 2013. [26] N. Orlovskaya, S. Lukich, G. Subhash, T. Graule, J. Kuebler, Mechanical properties of 10 mol% Sc2O3–1mol% CeO2–89 mol% ZrO2 ceramics, J. Power Sources 195 (2010) 2774-2781. [27] J.T. Irvine, D.C. Sinclair, A.R. West, Electroceramics: characterization by impedance spectroscopy, Adv. Mater. 2 (1990) 132-138. [28] V.F. Lvovich, Impedance spectroscopy: applications to electrochemical and dielectric phenomena, New Jersey, John Wiley & Sons, 2012. [29] T. Politova, J. Irvine, Investigation of scandia–yttria–zirconia system as an electrolyte material for intermediate temperature fuel cells—influence of yttria content in system (Y2O3)x (Sc2O3)(11− x)(ZrO2)89, Solid State Ionics 168 (2004) 153-165. [30] W. Preis, J. Waldhäusl, A. Egger, W. Sitte, E. De Carvalho, J. T. S. Irvine, Electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria, Solid State Ionics 192 (2011) 148-152. [31] K. Rajeswari, M. B. Suresh, U. S. Hareesh, Y. S. Rao, D. Das, R. Johnson, Studies on ionic conductivity of stabilized zirconia ceramics (8YSZ) densified through conventional and non-conventional sintering methodologies. Ceram. Int. 37(2011) 3557-3564.
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LIST OF TABLES Table 1. Grain Size (µm)
The properties of microwave-sintered 10Sc1CeSZ. Relative Density (%)
Vickers Hardness (GPa)
Fracture Toughness (MPam1/2)
Young’s Modulus (GPa)
13.6 ± 0.4
3.4 ± 0.2
203.1 ± 0.5
14.2 ± 0.5
3.3 ± 0.1
198.0 ± 0.3
1300 °C / 15 min. 1.2 ± 0.2
97.5
1350 °C / 15 min. 1.8 ± 0.1
98.4
LIST OF FIGURES Fig. 1.
The XRD analysis of 10Sc1CeSZ: (a) as-received powder, (b) – (g) conventional-sintered from 1300 to 1550 C for two hours and (h & i) microwave-sintered at 1300 and 1350 C for 15 minutes.
Fig. 2.
SEM images of 10Sc1CeSZ samples conventional-sintered at: (a) 1300C, (b) 1350C, (c) 1400C, (d) 1450C, (e) 1500C, (f) 1550C and microwave-sintered at (g) 1300C, (h) 1350C.
Fig. 3.
The effect of sintering temperatures on the grain size of 10Sc1CeSZ samples.
14
Fig. 4.
The effect of sintering temperatures on the (a) relative density, (b) Vickers hardness, (c) fracture toughness and (d) Young’s modulus of conventionalsintered 10Sc1CeSZ samples.
Fig. 5.
Nyquist plots taken at varying temperatures for 10Sc1CeSZ samples conventional-sintered at: (a) 1300C, (b) 1350C and microwave-sintered at: (c) 1300C, (d) 1350C.
Fig. 6.
Bulk resistivity of 10Sc1CeSZ microwave-sintered (MW) and conventionalsintered (CS) samples at 1300 C and 1350 C as a function of operating test temperatures.
Fig. 7.
Grain boundary resistivity of 10Sc1CeSZ microwave-sintered (MW) and conventional-sintered (CS) samples at 1300 C and 1350 C as a function of operating test temperatures.
15
Figure 1 (b)
(a)
16
(c)
(d)
(e)
(f)
(g)
(h)
Figure 2
17
12.0
Grain Size (µm)
10.0 8.0 6.0 4.0 2.0 0.0 1250
1300
1350
1400
1450
1500
Sintering Temperature (C)
Figure 3
18
1550
1600
Relative Density (%)
100 98 96 94 92 90 (a)
Vickers Hardness (GPa)
15 14 13 12 11 10
Young’s Modulus (GPa)
Fracture Toughness (MPam1/2)
(b)
4 3 2 1 (c)
250 200 150 100 50 (d)
Figure 4
1300
1350
1400
1450
Sintering Temperature (⁰C)
19
1500
1550
(b)
(a)
(c)
(d)
Figure 5
20
Figure 6
Figure 7
21
PII: DOI: Reference:
S0272-8842(16)30883-5 http://dx.doi.org/10.1016/j.ceramint.2016.06.050 CERI13066
To appear in: Ceramics International Received date: 21 May 2016 Revised date: 7 June 2016 Accepted date: 8 June 2016 Cite this article as: S. Ramesh, C.K. Ng, C.Y. Tan, W.H. Wong, C.Y. Ching, A. Muchtar, M.R. Somalu, S. Ramesh, Hari Chandran and P. Devaraj, Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.06.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic
S. Ramesh1 *, C.K. Ng1,2, C.Y. Tan1, W.H. Wong1, C.Y. Ching1, A. Muchtar3,4, M.R. Somalu4, S. Ramesh5, Hari Chandran6, P. Devaraj6
1
Centre of Advanced Manufacturing & Material Processing (AMMP), Department of
Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 2
Department of Materials Engineering, Faculty of Engineering & Built Environment,
Tunku Abdul Rahman University College, 53300 Kuala Lumpur, Malaysia 3
Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, 43600
UKM Bangi, Selangor, Malaysia 4
Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor,
Malaysia 5
Centre for Ionics University of Malaya, Department of Physics, Faculty of Science,
University of Malaya, 50603 Kuala Lumpur, Malaysia 6
Division of Neurosurgery, Faculty of Medicine, University of Malaya, 50603 Kuala
Lumpur, Malaysia Abstract The effect of conventional sintering from 1300 to 1550 °C on the properties of 1 mol% ceria-doped scandia stabilized zirconia was investigated. In addition, the influence of rapid sintering via microwave technique at low temperature regimes of 1300 C and *
Corresponding author. E-mail address: [email protected] (S. Ramesh) 1
1350 C for 15 minutes on the properties of this zirconia was evaluated. It was found that both sintering methods yielded highly dense samples with minimum relative density of 97.5%. Phase analysis by X-ray diffraction revealed the presences of only cubic phase in all sintered samples. All sintered pellets possessed high Vickers hardness (13-14.6 GPa) and fracture toughness (~ 3 MPam1/2). Microstructural examination by using the scanning electron microscope revealed that the grain size varied from 2.9 to 9.8 µm for the conventional-sintered samples. In comparison, the grain size of the microwave-sintered zirconia was maintained below 2 µm. Electrochemical Impedance Spectroscopy study showed that both the bulk and grain boundary resistivity of the zirconia decreases with increasing test temperature regardless of sintering methods. However, the grain boundary resistivity of the microwave-sintered samples was higher than the conventional-sintered ceramic at 600 C and reduced significantly at 800 C thus resulting in the enhancement of electrical conduction.
Keywords: Scandia stabilized zirconia, microwave sintering, ionic conductivity, mechanical property.
1.
Introduction
Electrochemical device such as solid oxide fuel cell (SOFC) produces electricity efficiently from the reaction between the hydrogen and air. It uses metal oxide solid ceramic electrolytes to allow the transport of oxygen vacancy (O2-). The O2- ionic conduction requirement for the ceramic electrolyte necessitates high operating temperatures (600 C -1000 C) to ensure adequate ionic conduction in the electrolyte. The high temperature operating environment promote fast reaction kinetics, therefore
2
does not require the use of noble metal that could limit the applicability of SOFC due to the scarcity of resources and high price issue in mass production [1]. It also allows reforming of hydrocarbon (CO, CH4, gasoline, etc.) within the fuel cell. However, high temperature SOFC operation on the other hand makes great demands on materials and requires very careful selection on issues concerning sealing of the cells, compatibility of thermomechanical properties between electrolyte and electrode, and stability of individual components [2, 3]. As a result, recent development emphasis has been on improving material property or modifying processing routes to reduce the working temperature of the electrolyte material. Zirconia electrolytes has high ionic conductivity, high mechanical and chemical stability, compatibility towards other components used in the SOFC and with lower electronic conductivity in oxidizing environment. Yttria stabilized zirconia (YSZ) and scandia stabilized zirconia (ScSZ) are the two most commonly used electrolytes for SOFC because of their excellent ionic conductivity [4, 5]. The ionic conductivity of ScSZ is twice that of YSZ due to similar ionic radius of Sc3+ (0.087nm) and Zr4+ (0.084nm) that leads to lower internal stress and lower activation energy, and hence results in lower operating temperatures for a comparable conductivity [6]. Co-doping of ScSZ with small amount of other oxides such as 1-2 mol% bismuth oxide [7-9], 3 mol% ytterbium oxide [10], and in particular 1 mol% ceria [11] , resulted in improved phase stability and enhanced electrical performance. More efforts in the low temperature accelerated sintering are imperative to make the electrolyte fabrication process feasible in large scale SOFC production. Conventional heating requires the heat to be transferred from the surface to the core of the material whereas microwave sintering resulted in volumetric heating and
3
thereby a promising technique for uniform and fast firing of the ceramic. It involves the absorption of microwave field to cause molecular vibrations and generates heat through direct electromagnetic to thermal energy conversion [12-14]. Previous studies on microwave sintering of various types of ceramics such as alumina [15], hydroxyapatite [16], and YSZ [17, 18] showed promising results in terms of enhancing densification at lower temperatures. The aim of this work was to study the effect of sintering temperature on the properties of ceria-doped scandia stabilized zirconia electrolyte. In addition, the influence of rapid microwave heating process at low sintering temperature on the conductivity of the zirconia was investigated for potential use as electrolyte material in intermediate-temperature SOFC.
2.
Experimental procedures
The starting material 10Sc1CeSZ (1 mol% ceria + 10 mol% scandia stabilized zirconia) was a commercially available powder obtained from Daiichi Co. Ltd. Japan. The powder has a specific surface area of 11.1 m2/g and average particle size, D50 of 0.53 µm. The as-received powders were compacted in a steel die by uniaxial pressing (5-6 MPa) followed by cold isostatic pressing at 200 MPa. The conventional sintering was performed in a box furnace at various temperatures ranging from 1300C to 1550C with 2 hours holding time at a heating rate of 10C/min. For comparison purpose, some samples were microwave sintered at 1300C and 1350C for 15 min. at a heating rate of 30C/min. in a 2.45 GHz microwave furnace. The sintered density was determined by the water immersion technique based on the Archimedes principle. The phase analysis by X-ray diffraction (XRD) of the
4
ceramics was carried out using a PANanalytical Empyran XRD with CuK as the radiation source (λ = 1.5406 Å) operated at 45 kV, 40 mA, a step size of 0.02° over a 2 of 20° to 80°. Microstructure characterization was performed by using a scanning electron microscopy (Phenom Pro SEM, Netherland) on polished and thermally etched sintered samples. The grain size of the sintered samples was determined by the line intercept technique using the SEM images. The AC impedance spectra of the sintered samples were measured using Electrochemical Impedance Spectroscopy (Metrohm Autolab EIS) at the temperature range of 600-800C. For this purpose, silver paint was applied on both sides of the disc sample and connected to two electrodes via platinum wires to a frequency response analyzer (Autolab AUT 302 FRA). The frequency was varied over the range of 0.1 Hz to 1 MHz, at an applied potential of 10 mV and the measured impedance data was fitted using the Nova software. The bulk and grain boundary resistance were subsequently obtained from equivalent circuit fitting of parallel RQ (R-Resistance, Q-Constant Phase Element) in series. Vickers Hardness was calculated from the diagonal lengths of the indentation made on the samples using a Mitutoyo Vickers hardness tester (AVK-C2, Japan) at an applied load of 10 kgf with a dwell time of 10s. For each temperature, three samples were prepared and at least 5 measurements were taken for each sample. The Young’s modulus by sonic resonance was determined for rectangular samples using a commercial testing instrument (GrindoSonic: MK5 “Industrial”, Belgium). The instrument permits determination of the resonant frequency of a sample by monitoring and evaluating the vibrational harmonics of the sample by a transducer. The modulus of elasticity or Young’s modulus was calculated using the experimentally determined resonant frequency [19].
5
3.
Results and discussion The XRD analysis of the sintered samples is shown in Fig. 1. The results
revealed the presences of only the cubic phase in the structure thus indicating that both the scandium and cerium has been incorporated in the zirconia lattice. This result is in agreement with the findings of several researchers [20-22] who reported phase-pure cubic phase exist for samples sintered above 1300C. It is believed that the incorporation of Sc3+ and Ce4+ in the zirconia stabilizes the cubic phase and effectively increased the ionic size in the cubic fluorite structure which in turn would create more oxygen vacancies within the crystal lattice [23]. SEM micrographs of the 10Sc1CeSZ samples sintered by conventional and microwave methods are shown in Fig. 2, revealing the presents of equiaxed cubic grains whereas the average grain size of the conventional-sintered samples is shown in Fig. 3. The result shows that the cubic grain size of conventional-sintered zirconia increased with increasing sintering temperature, from 2.9 ± 0.2 µm (1300 C) to 9.8 ± 0.1 µm (1550C). In terms of densification, it was found that all the conventional-sintered samples, regardless of sintering temperatures, exhibited above 97% relative densities (the relative density is calculated based on the theoretical density of 5.74 g/cm3 [24]) as shown in Fig. 4(a). The results showed that for every 100C increased in sintering temperature, the cubic grain grew at a rate of 3.5 ± 0.2 µm. The grain size obtained in the present work for the conventional-sintered samples is slightly higher than the values reported in the literature [22, 25]. On the other hand, it was found that rapid heating via microwave sintering was effective in suppressing the grain sizes to below 2 µm when sintered at
6
1300-1350 C and the samples were able to achieve compatible bulk density as with the conventional-sintered samples as shown in Table 1. The effect of sintering temperature on the Vickers hardness, fracture toughness, and Young’s modulus of the 10Sc1CeSZ samples are presented in Fig. 4 (b - d). The highest Vickers hardness of 14.6 ± 0.7 GPa was recorded for the 1300 C conventionalsintered sample. However, as the sintering temperature increases to 1550 C, this was accompanied by a reduction in the hardness to about 13 ± 0.5 GPa. This trend is in agreement with that reported by Orlovskaya et al. [26]. Higher sintering temperatures produce coarser grains with less grain boundary area per volume and therefore reduce the resistance to localized plastic deformation and results in lower hardness. On the other hand, the microwave sintered samples exhibited comparable hardness, above 13.5 GPa, when sintered at 1300 to 1350 C as shown in Table 1. The other mechanical properties, i.e. the fracture toughness and Young’s modulus of both, the conventional and microwave sintered samples were found to vary in the range of 3.2-3.7 MPa.m1/2 and 180-215 GPa, respectively with increasing sintering temperatures as shown in Fig. 4 and Table 1. The Nyquist plots from the impedance data collected over the operating temperature range of 600 C- 800 C for the samples sintered at 1300C and 1350C using both conventional and microwave methods are shown in Fig. 5. The EIS impedance measurement data are presented in the form of imaginary, Z (capacitive) against real, Z (resistive) impedances. To identify the constituent component of a material, different regions of the electrical system are usually characterized according to their electrical relaxation times by a parallel resistance and capacitance (RC) to extract the R and C values of each semicircle on the impedance plots [27]. In this study,
7
depressed arcs instead of ideal semicircles were observed on the impedance spectrum, signifying non-ideal capacitive behavior due to the distribution of currents and electroactive species [28]. Each depressed arc is therefore fitted with a parallel resistance and constant phase element (R-Q) to replace the ideal capacitive property. The associated R values obtained from the high frequency intercepts on the real axis are attributed to the bulk resistance of the sample, the intermediate frequency intercepts are attributed to grain boundary resistance, and the low frequency arcs are attributed to the electrode resistance. It is visible from the high frequency intercepts, the impedance of the bulk component of all samples tested become smaller with rising operating temperatures. This is postulated as the association energy becomes less significant due to dissociation of oxygen vacancies (O2-) from the cations at higher temperatures, and consequently activation energy for the ionic conduction is predominantly by virtue of oxygen ion migration enthalpy [23]. Migration energy needed for the free ions to hop from one crystal site to its next unoccupied lattice site would be readily available from the elevated temperature operations; hence lower activation energy and higher ionic conduction occur with raising temperatures. Both scandia and zirconia have a ratio of ionic radius of the cation (rc) and anion (ra), i.e. rc/ra of less than 0.7, thus favored in sixfold coordination in the crystal structure. Lower coordination number elicits the formation of strong anion microdomains eventuated in higher activation energy at lower temperatures, caused by additional ordering energy due to presence of the microdomains of correlated dopant-vacancy clusters [29, 30]. The contributions of bulk and grain boundary resistivities for conventionalsintered and microwave-sintered samples are presented in Fig. 6 and Fig. 7,
8
respectively. From the bulk resistivity plots in Fig. 6, the microwave-sintered samples showed no appreciable differences with the conventional-sintered samples regardless of the operating test temperature (600 – 800 °C) thus indicating that the bulk resistivity is not affected by the sintering condition. On the other hand, Fig. 7 shows that the grain boundary contribution in microwave-sintered samples are more pronounced compared to the bulk resistivity at the low temperature region (600 C) resulting in relatively high total resistivities. However, grain boundary resistivity was found to decrease significantly at higher temperature range (700 to 800C). The diminishing grain boundary resistivities show that the intrinsic grain boundary blocking effect disappeared at 800 C. This observation is in agreement with that reported by Rajeswari et al.[31]. It is believed that the fine grain size and the larger grain boundary per unit area could have contributed to the lager grain boundary resistivity in the microwave sample.
4.
Conclusions
In this work, the effect of sintering temperature on the properties of 10Sc1CeSZ was investigated and the results were also compared with samples sintered by microwave at low temperature regime. The study revealed that the sintering temperature has an effect on the bulk density as well as the grain size of the conventional-sintered zirconia. However, the mechanical properties were not significantly affected by the sintering temperatures. In contrast, the beneficial effect of microwave sintering in producing a highly dense ceria-doped scandia stabilized zirconia, characterized by having fine grain size below 2 µm coupled with equivalent mechanical properties as that obtained from the high temperature (1550 °C) conventional-sintered sample has been revealed. Grain boundary resistivities contribution was found to be dominant at lower operating
9
temperature of 600 °C for the microwave-sintered samples and diminished at high operating temperature of 800 °C. This study has demonstrated the beneficial effect of rapid sintering via microwave in promoting densification of 10Sc1CeSZ at lower temperatures without sacrificing the cubic phase and mechanical properties for SOFC application.
Acknowledgements This study was supported under the PPP grant number PG080-2013A, UMRG grant number RP024B-13AET and Esciencefund grant number SF010-2014.
References [1] A.B. Stambouli, E. Traversa, A review of an environmentally clean and efficient source of energy, Renew and Sustainable Energy Rev. 6 (2002) 433-455. [2]
E. Barsoukov, J.R. Macdonald, Impedance spectroscopy: theory, experiment, and applications, 2nd ed., John Wiley & Sons, New Jersey, 2005.
[3]
A. Orera, P. Slater, New chemical systems for solid oxide fuel cell, Chem. Mater. 22 (2009) 675-690.
[4]
S. Badwal, K. Foger, Solid oxide electrolyte fuel cell review, Ceram. Int. 22 (1996) 257-265.
[5]
S. Singhal, Advances in solid oxide fuel cell technology, Solid State Ionics 135 (2000) 305-313.
[6]
C. Haering, A. Roosen, H. Schichl, M. Schnöller, Degradation of the electrical conductivity in stabilised zirconia system: Part II: scandia-stabilized zirconia, Solid State Ionics 176 (2005) 261-268.
10
[7]
B. Bai, N. Sammes, A. Smirnova, Physical and electrochemical characterization of Bi2O3-doped scandia stabilized zirconia. J. Power Sources 176 (2008) 76-81.
[8]
M. Hirano, T. Oda, K. Ukai, Y. Mizutani, Effect of Bi2O3 additives in Sc stabilized zirconia electrolyte on a stability of crystal phase and electrolyte properties, Solid State Ionics 158 (2003) 215-223.
[9]
S. Sarat, N. Sammes, A. Smirnova, Bismuth oxide doped scandia-stabilized zirconia electrolyte for the intermediate temperature solid oxide fuel cells, J. Power Sources 160 (2006) 892-896.
[10] R. Chiba, T. Ishii, F. Yoshimura, Temperature dependence of ionic conductivity in (1-x) ZrO2−(x-y)Sc2O3−yYb2O3 electrolyte material, Solid State Ionics 91 (1996) 249-256. [11] D.- S. Lee, W. Kim, S. Choi, J. Kim, H.-W. Lee, J.- H. Lee, Characterization of ZrO2 co-doped with Sc2O3 and CeO2 electrolyte for the application of intermediate temperature SOFCs. Solid State Ionics 176 (2005) 33-39. [12] D. Agrawal, Microwave sintering of ceramics, composites and metallic materials, and melting of glasses, Trans. of the Indian Ceram. Soc. 65 (2006) 129-144. [13] W.H. Sutton, Microwave processing of ceramic materials, Am. Ceram. Soc. Bull. 68 (1989) 376-386. [14] A.R. West, Solid state chemistry and its applications, John Wiley & Sons, West Sussex, 2007. [15] S. Lefeuvre, E. Federova, O. Gomonova, J. Tao, Microwave Sintering of Microand Nano-Sized Alumina Powder, 12th Seminar Computing Modelling in Microwave Engineering & Applications, Grenoble, France, 2010, pp. 46–50. [16] S. Nath, B. Basu, A. Sinha, A comparative study of conventional sintering with
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microwave sintering of hydroxyapatite synthesized by chemical route, Trends in Biomaterials & Artificial Organs 19 (2006) 93-98. [17] M. Mazaheri, Z.R. Hesabi, F. Golestani‐Fard, S. Mollazadeh, S. Jafari, S. Sadrnezhaad, The effect of conformation method and sintering technique on the densification and grain growth of nanocrystalline 8 mol% Yttria‐Stabilized Zirconia, J. Am. Ceram. Soc. 92 (2009) 990-995. [18] K. Rajeswari, U. Hareesh, R. Subasri, D. 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. of Sinter. 42 (2010) 259-267. [19] ASTM Standard C1259-2008e1, "Standard test method for dynamic young's modulus, shear modulus, and poisson's ratio for advanced ceramics by impulse excitation of vibration," ASTM International, West Conshoshocken, PA, 2008 [20] A. Zevalkink, A. Hunter, M. Swanson, C. Johnson, J. Kapat, N. Orlovskaya, Processing and Characterization of Sc2O3-CeO2-ZrO2 Electrolyte Based Intermediate Temperature Solid Oxide Fuel Cells, In MRS Symposium Proceedings Series Volume 972 (2007) 163-168. [21] R. Grosso, E. Muccillo, Sintering, phase composition and ionic conductivity of zirconia–scandia–ceria, J. Power Sources 233 (2013) 6-13. [22] S. Yarmolenko, J. Sankar, N. Bernier, M. Klimov, J. Kapat, N. Orlovskaya, Phase stability and sintering behavior of 10 mol% Sc2O3–1mol% CeO2–ZrO2 ceramics, J. Fuel Cell Sci. Technol. 6 (2009) 021007. [23] J.T. Irvine, P. Connor, Solid Oxide Fuels Cells: Facts and Figures, Springer, London, 2013.
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[24] R. Rosten, M. Koski, E. Koppana. A Guide to the Calculation of Theoretical Densities of Crystal Structures for Solid Oxide Fuel Cells, J. Undergrad. Mater. Res. 2 (2006) 38-41. [25] O.L. Kyrpa, On the temperature dependence of oxygen ionic conductivity of 10Sc1CeSZ electrolytes, The National Academy of Sciences of Ukraine, 2013. [26] N. Orlovskaya, S. Lukich, G. Subhash, T. Graule, J. Kuebler, Mechanical properties of 10 mol% Sc2O3–1mol% CeO2–89 mol% ZrO2 ceramics, J. Power Sources 195 (2010) 2774-2781. [27] J.T. Irvine, D.C. Sinclair, A.R. West, Electroceramics: characterization by impedance spectroscopy, Adv. Mater. 2 (1990) 132-138. [28] V.F. Lvovich, Impedance spectroscopy: applications to electrochemical and dielectric phenomena, New Jersey, John Wiley & Sons, 2012. [29] T. Politova, J. Irvine, Investigation of scandia–yttria–zirconia system as an electrolyte material for intermediate temperature fuel cells—influence of yttria content in system (Y2O3)x (Sc2O3)(11− x)(ZrO2)89, Solid State Ionics 168 (2004) 153-165. [30] W. Preis, J. Waldhäusl, A. Egger, W. Sitte, E. De Carvalho, J. T. S. Irvine, Electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria, Solid State Ionics 192 (2011) 148-152. [31] K. Rajeswari, M. B. Suresh, U. S. Hareesh, Y. S. Rao, D. Das, R. Johnson, Studies on ionic conductivity of stabilized zirconia ceramics (8YSZ) densified through conventional and non-conventional sintering methodologies. Ceram. Int. 37(2011) 3557-3564.
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LIST OF TABLES Table 1. Grain Size (µm)
The properties of microwave-sintered 10Sc1CeSZ. Relative Density (%)
Vickers Hardness (GPa)
Fracture Toughness (MPam1/2)
Young’s Modulus (GPa)
13.6 ± 0.4
3.4 ± 0.2
203.1 ± 0.5
14.2 ± 0.5
3.3 ± 0.1
198.0 ± 0.3
1300 °C / 15 min. 1.2 ± 0.2
97.5
1350 °C / 15 min. 1.8 ± 0.1
98.4
LIST OF FIGURES Fig. 1.
The XRD analysis of 10Sc1CeSZ: (a) as-received powder, (b) – (g) conventional-sintered from 1300 to 1550 C for two hours and (h & i) microwave-sintered at 1300 and 1350 C for 15 minutes.
Fig. 2.
SEM images of 10Sc1CeSZ samples conventional-sintered at: (a) 1300C, (b) 1350C, (c) 1400C, (d) 1450C, (e) 1500C, (f) 1550C and microwave-sintered at (g) 1300C, (h) 1350C.
Fig. 3.
The effect of sintering temperatures on the grain size of 10Sc1CeSZ samples.
14
Fig. 4.
The effect of sintering temperatures on the (a) relative density, (b) Vickers hardness, (c) fracture toughness and (d) Young’s modulus of conventionalsintered 10Sc1CeSZ samples.
Fig. 5.
Nyquist plots taken at varying temperatures for 10Sc1CeSZ samples conventional-sintered at: (a) 1300C, (b) 1350C and microwave-sintered at: (c) 1300C, (d) 1350C.
Fig. 6.
Bulk resistivity of 10Sc1CeSZ microwave-sintered (MW) and conventionalsintered (CS) samples at 1300 C and 1350 C as a function of operating test temperatures.
Fig. 7.
Grain boundary resistivity of 10Sc1CeSZ microwave-sintered (MW) and conventional-sintered (CS) samples at 1300 C and 1350 C as a function of operating test temperatures.
15
Figure 1 (b)
(a)
16
(c)
(d)
(e)
(f)
(g)
(h)
Figure 2
17
12.0
Grain Size (µm)
10.0 8.0 6.0 4.0 2.0 0.0 1250
1300
1350
1400
1450
1500
Sintering Temperature (C)
Figure 3
18
1550
1600
Relative Density (%)
100 98 96 94 92 90 (a)
Vickers Hardness (GPa)
15 14 13 12 11 10
Young’s Modulus (GPa)
Fracture Toughness (MPam1/2)
(b)
4 3 2 1 (c)
250 200 150 100 50 (d)
Figure 4
1300
1350
1400
1450
Sintering Temperature (⁰C)
19
1500
1550
(b)
(a)
(c)
(d)
Figure 5
20
Figure 6
Figure 7
21