- Email: [email protected]
LaNbO4 toughened NiO–Y2O3 stabilized ZrO2 composite for the anode support of planar solid oxide fuel cells
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
LaNbO4 toughened NiOeY2O3 stabilized ZrO2 composite for the anode support of planar solid oxide fuel cells Ben Ma, Bo Chi, Jian Pu, Li Jian* Center for Fuel Cell Innovation, School of Materials Science and Engineering, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan, China
article info
abstract
Article history:
NiOeY2O3 stabilized ZrO2 (YSZ) composite is the state-of-the-art material for the anode
Received 9 January 2013
support of planar solid oxide fuel cells (SOFCs). To improve its fracture toughness (KC),
Received in revised form
lanthanum orthoniobate LaNbO4 is synthesized by the method of solid state reaction and
31 January 2013
added to the mixture of YSZ and NiO at the weight ratio of 47:53. The content of LaNbO4 in
Accepted 7 February 2013
the composite is in the range between 5 and 30 wt%. The microstructure of the composites
Available online 5 March 2013
is examined by scanning electron microscopy (SEM); and the chemical compatibility among the components is evaluated by X-ray diffraction (XRD) and energy dispersive X-ray
Keywords:
spectroscopy (EDS). Vickers hardness test is performed for estimating the KC of the
Lanthanum orthoniobate
composites. The results indicate that the KC increases with the addition of LaNbO4 in the
Anode support
composites; and the toughening effect is associated with the grain-refinement in the
Composite
composite and the domain switch in the monoclinic LaNbO4.
Hardness
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Fracture toughness
1.
Introduction
Solid oxide fuel cells (SOFCs) are an emerging technology that generates clean electrical power from fossil or hydrocarbon fuels for various kinds of applications, such as portable devices, transportations, distributed power plants and centralized power stations. Conventionally, there are two kinds of design for SOFCs, i.e., the tubular and the planar. Comparing with the tubular design, planar SOFCs are more attractive due to their low manufacturing cost and high power density [1], especially for the anode-supported cells where the thin electrode and electrolyte are built in sequence on the anode substrate. However, such planar cells are facing challenge of cracking or fracture during stack assembling and operation under a compressive load, leading to mixing of fuel and oxidant gases and in turn the failure of stack performance.
reserved.
The state-of-the-art anode material is NieY2O3 doped ZrO2 (YSZ) cermet which is usually reduced from NiOeYSZ ceramic composite during the process of stack start-up in reducing atmosphere. Thus the substrate of the cell is a ceramic composite of NiOeYSZ rather than the cermet of NieYSZ when the cells are assembled into a stack. At this stage, the cells are most susceptible to cracking under the applied compressive load; therefore, the mechanical property of the cell at room temperature is more critical than that at the operating temperature. Previous efforts have been made to optimize the microstructure of [2,3] or add Al2O3 [4] into the NiOeYSZ substrate, which manipulates its stress state for the enhancement of the strength. However, the cracking of the anode-supported cells cannot be solely attributed to the insufficient strength of the substrate; and more likely, it is due to its weak toughness. Improving the fracture toughness of
* Corresponding author. Tel.: þ86 27 87557496; fax: þ86 27 87558142. E-mail address: [email protected] (L. Jian). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.033
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
the cell substrate is a promising way to solve the issue of cell fracture. A conventional approach to toughen ceramic materials is to add a second phase in various shapes into the ceramic matrix, such as particles, whiskers and fibers [5e7]. The toughening effect appears to be associated with phase transformation, microcracking, residual thermal stress, fiber/ whisker reinforcement or grain bridging. In recent years, another toughening mechanism associated with the domain switch in the second phase has been reported. In this scenario, a certain amount of ceramic ferroics, such as ferroelectric, piezoelectric and ferroelastic, is introduced into ceramic materials to arrest the crack propagation by the orientation switch of domains existing in the ferroic phase, dissipating energy and stress build-up at the tip of a crack. Traditional ferroic materials, including BaTiO3 [8e10], Sr2Nb2O7 [11], Nd2Ti2O7 [12] and LiTaO3 [13], have been proved to be effective in enhancing the toughness of 3 mol% Y2O3eZrO2 (3Y-TZP), PdZrO3 (LZ) and Al2O3, respectively. In this study, lanthanum orthoniobate (LaNbO4) was used as the secondary phase to improve the toughness of NiOeYSZ composite for the anode substrate. LaNbO4 is a kind of ferroelastic material, undergoes a reversible polymorphic transformation at approximately 520 C from low temperature monoclinic phase to high temperature tetragonal phase, and presents a rubber-like behavior in the monoclinic form that contains domains in two crystallographic orientations separated by highly mobile boundaries [14e19]. This paper reports the enhanced toughness of NiOeYSZ composite achieved by the addition of LaNbO4, and discusses the possible toughening mechanisms.
2.
Experimental
2.1.
Synthesis of LaNbO4 powder
LaNbO4 powder was prepared by solid state reaction using lanthanum oxide La2O3 (99.9% purity, Sino-Pharm Chemical Reagent) and niobium oxide Nb2O5 (99.99% purity, Aladdin Chemistry) as the raw materials, according to the chemical reaction of La2 O3 þ Nb2 O5 /2LaNbO4
4777
ethanol for 24 h with zirconia beads as the media, followed by drying and pelleting. Disc specimens of 18 mm in diameter were obtained by press forming the powdery pellets under a load of 50 MPa and sintering at 1500 C in air for 6 h.
2.3.
Characterizations
The crystal structure of the synthesized LaNbO4 powder and sintered LaNbO4eNiOeYSZ composites was analyzed by X-ray diffraction (XRD, X’Pert PRO Diffractometer, PANalytical) with Cu Ka emission. The scanning angle covered 2q values from 20 to 80 . The morphology and microstructure of the sintered composites were examined by using a scanning electron microscope (SEM, Quanta 200, Micro FEI Philips) equipped with energy dispersive X-ray spectroscopy (EDS). The density of the sintered LaNbO4eNiOeYSZ composites was determined by the Archimedes method in distilled water. A Vickers hardness tester (HV-432SVD, Qingdao Dizhong Measuring Instruments) was employed for the hardness measurement of the sintered specimens under a load of 10 kgf for 15 s. And their fracture toughness was converted from the geometry of the indentation and indentation-induced crack. Before the indentation test, the surfaces of the sintered specimens were polished with a diamond suspension by using a metallographic polishing machine (P-1, Laizhou Weiyi Testing Machine).
3.
Results and discussion
Fig. 1 shows the XRD pattern of the synthesized LaNbO4 powder. The diffraction peaks was well matched to those displayed in the JCPDS file 86-0909 for LaNbO4, indicating that pure monoclinic LaNbO4 was formed at room temperature by calcining La2O3 and Nb2O5 powder mixture at 1200 C in air for 4 h. Fig. 2 presents the XRD patterns of the sintered LaNbO4eNiOeYSZ composites with various LaNbO4 contents. Comparing with the JCPDS files for LaNbO4 (86-0909), NiO (711179) and YSZ (89-9069) revealed that the peak intensity of monoclinic LaNbO4 was gradually amplified by increasing the amount of LaNbO4 and no XRD evidence of extra phases other
(1)
To obtain a uniform powder mixture for the reaction, La2O3 and Nb2O5 powders were mixed at 1 to 1 molar ratio and ballmilled in ethanol for 24 h by using a satellite mill (QM-2SP12, Nanjing Nanda Instruments) with zirconia beads as the milling media. The ball-milled mixture was dried in an oven at 80 C and then heated at a rate of 3 C min1 to 1200 C. Pure LaNbO4 phase was formed after 4 h calcination at 1200 C in air.
2.2.
Preparation of LaNbO4eNiOeYSZ composite
YSZ (TZ-8YS, Tosoh) and NiO (Type Standard, Inco) powders were mixed at a weight ratio of 47:53 to form a NiOeYSZ powder mixture, to which LaNbO4 powder was added at various contents between 5 and 30 wt%, respectively. The LaNbO4eNiOeYSZ powder mixtures were ball-milled in
Fig. 1 e XRD pattern of LaNbO4 powder synthesized by solid reaction at 1200 C in air for 4 h. The JCPD file 86-0909 for LaNbO4 is inserted for comparison.
4778
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
Fig. 2 e XRD patterns of the LaNbO4eNiOeYSZ composites sintered at 1500 C in air for 6 h. The content of added LaNbO4 is between 5 and 30 wt%.
than LaNbO4, NiO and YSZ was found in the spectra. This result is consistent with that no chemical reaction occurred between NiO and LaNbO4 at temperatures up to 1400 C as observed by Magraso et al. [20]. In a study of YNbO4-added 3 mol% Y2O3eZrO2 (3YZ), Yuh et al. [13] reported that less than 10 mol% of YNbO4 added in the 3YZ was fully dissolved in the matrix after sintering at 1600 C for 1 h, and the XRD peaks were not significantly shifted. Maschio et al. [21] also reported Ce enrichment in LaNbO4 grains in LaNbO4-added CeO2eZrO2 composites sintered at 1500 C. Therefore, the XRD results showing in Fig. 2 may not exclude the possibility of element inter-diffusion between LaNbO4 and YSZ, although no noticeable change in the peak position was detected. The morphology of the sintered LaNbO4eNiOeYSZ composites with different amounts of added LaNbO4 is shown in Fig. 3. As confirmed by the EDS analysis shown in Fig. 4, the white grains were LaNbO4, the gray grains were YSZ and the
dark grains were NiO; and Y in LaNbO4 and La in YSZ were detected (the presence of Au in Fig. 4d was due to the coating used on the SEM specimen to minimize charging). This result suggests the element inter-diffusion between LaNbO4 and YSZ. However, such insignificant element inter-diffusion did not change their crystallographic structures and is not expected to adversely affect the performance and stability of the composite substrate, since the function of the reduced substrate is to support the thin electrolyte and electrodes of the cell and deliver the electrons generated by the anode reaction to the interconnect. These three phases are uniformly distributed, and the average grain size of the composite was refined from 1.96 to 1.35 mm as the LaNbO4 content increased from 5 to 30 wt% (Table 1), implying that LaNbO4 as the secondary phase hindered the grain growth of the composite. Fig. 5 shows the dependence of the hardness Hv and hardness-converted fracture toughness KC of the composite on its LaNbO4 content at room temperature. The relative density of the samples was approximately 99%, measured by the Archimedes method in distilled water. The value of KC was determined from the geometry of the indentation and indentation-induced crack according to the following widely accepted empirical equation [22], pffiffiffi KC F=HV a ¼ 0:15kðc=aÞ3=2
(2)
where a is the half length of the indentation diagonal and c is the half length of the indentation crack, as shown in Fig. 6; F is the constraint factor (z3) and k is the correction factor (z3.2 for large c/a values as shown in the case herein) [22]. Each KC value shown in Fig. 5 is the average value obtained from five different measurements. The error bars represent the range from the lowest to highest KC value. The hardness of the sintered LaNbO4eNiOeYSZ composites decreased with the addition of LaNbO4; and conversely the fracture toughness KC increased and gradually approached to the maximum at LaNbO4 contents above 25 wt%. Comparing with KC of NiOeYSZ composite, 21% increase in KC was obtained for the
Fig. 3 e SEM back-scattered electron micrographs of the LaNbO4eNiOeYSZ composites with various LaNbO4 contents between 5 and 30 wt%. The white grains are LaNbO4, the dark grains are NiO and the gray grains are YSZ.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
4779
Fig. 4 e EDS compositional analysis of the LaNbO4eNiOeYSZ composite with 30 wt% added LaNbO4: a) SEM image; b) EDS spectrum from spot A in a); c) EDS spectrum from spot B in a); and d) EDS spectrum from spot C in a).
LaNbO4eNiOeYSZ composites containing more than 25 wt% of LaNbO4, which demonstrates the toughening effect of LaNbO4 on the LaNbO4eNiOeYSZ composite. Maschio et al. [21] studied the influence of LaNbO4 addition on mechanical properties of polycrystal CeO2-doped tetragonal ZrO2 and proposed that the observed toughening was due to the crack wake bridging by the elongated LaNbO4 grains in the composite. The elongated LaNbO4 grains were formed during sintering at 1500 C in the presence of silica and transient liquid phase. And in another study of YNbO4 toughened 3YZ, Yuh et al. [13] noticed toughness increase for specimens sintered at 1600 C for 1 h and containing less than 5 mol% of YNbO4; and the toughening mechanism was not clearly elucidated by either the domain switch or the tetragonal-tomonoclinic phase transformation in the 3YZ. However, the above-mentioned possible mechanisms seem not applicable for the explanation of the enhanced fracture toughness
demonstrated by the LaNbO4eNiOeYSZ composites. NiO and YSZ are stable cubic phases without domains inside their grains; therefore, both phase transformation and domain switch in NiOeYSZ matrix are not expected to occur. The possible reasons for the improved toughness are likely to be the grain refinement in the composite and domain switch in
Table 1 e Average grain size (mm) of LaNbO4eNiOeYSZ composites at various LaNbO4 contents (wt%). LaNbO4 content
5%
10%
15%
20%
25%
30%
Grain size
1.96
1.90
1.70
1.56
1.55
1.35
Fig. 5 e Fracture toughness and Vickers hardness of the LaNbO4eNiOeYSZ composites with various LaNbO4 contents ranging from 5 to 30 wt%.
4780
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
Fig. 7 e Schematic illustration showing domain switch in monoclinic LaNbO4 in the LaNbO4eNiOeYSZ composite under an applied stress. Domains at preferred orientation grow at expense of that unfavored.
Fig. 6 e Representative hardness indentation showing the geometry of the indentation and indentation-induced crack.
LaNbO4. As shown in Table 1, more than 31% of grain refinement was achieved by increasing the LaNbO4 content from 5 to 30 wt%, which will improve the fracture toughness by limiting the size of pre-existing microcracks and promoting crack deflection [23]. Apart from the grain size contribution, the added LaNbO4 is also expected to play an important role in toughening the LaNbO4eNiOeYSZ composite. LaNbO4 is in the monoclinic form at room temperature and its grains consist of domains in various crystallographic orientations separated by a boundary located at a lattice plane in the {20 4}I/{402}II family [17,18]; consequently LaNbO4 exhibits ferroelasticity and domain switchability in response to the applied stresses [19]. A domain packet contains parallel domains in two crystallographic orientations, and different packets can meet in various configurations determined by the requirement of energy minimization [14]. Under the influence of an external uniaxial stress sij, the Gibbs free-energy density G for a domain can be written in differential form as [18] dG ¼ SdT 3 ij dsij
(3)
where S, T and 3 ij denote the entropy, temperature and strain, respectively. And the strain can be further expressed as 3 ij
¼ esij þ sijkl skl
(4)
where esij is the component of the spontaneous strain tensor, and sijkl is the component of the elastic compliance tensor. Substituting Eq. (4) into Eq. (3), followed by integration, gives the free energy density G of a domain. And the difference of the free energy density (DG) for domains in two different orientations DG ¼ G2 G1 ¼ esij sij
the initiation of microcracks by absorbing locally accumulated energy needed for crack formation; and also provide the resistance to the extension of microcracks by releasing the stress build-up at the tip of cracks. Thus both the intrinsic and extrinsic toughness [23] of the composite are promoted by the addition of LaNbO4.
4.
Conclusion
To improve the fracture toughness of the anode support material NiOeYSZ of planar SOFC cells, LaNbO4 powder was added into the mixture of NiO and YSZ to form LaNbO4eNiOeYSZ composites with various LaNbO4 contents between 5 and 30 wt%. From the obtained results, the following conclusions can be drawn. 1) LaNbO4, NiO and YSZ are uniformly distributed in the LaNbO4eNiOeYSZ composites, and the microstructure is refined with the increase of LaNbO4 content. They are chemically stable and compatible without forming new phases after sintering at 1500 C in air for 6 h. 2) The fracture toughness of the LaNbO4eNiOeYSZ composites is considerably enhanced by the addition of LaNbO4. The toughening effect is attributed to both the grain refinement in the microstructure and the domain switch in monoclinic LaNbO4.
Acknowledgment The authors gratefully acknowledge the financial support to this study provided by the National Natural Science Foundation of China under the project 51172082. The XRD and SEM characterizations were performed with the assistance of the Analytical and Testing Center of Huazhong University of Science and Technology.
(5)
acts as the driving force for domain switching between these two orientations. Under the external stress the domain with the preferred orientation will grow at the expense of the unfavored one, as schematically presented in Fig. 7. By this mechanism, the embedded LaNbO4 will provide resistance to
references
[1] Steele BCH, Heizel A. Materials for fuel-cell technologies. Nature 2001;414:345e52.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
[2] Wang Y, Walter ME, Sabolsky K, Seabaugh MM. Effects of powder sizes and reduction parameters on the strength of NieYSZ anodes. Solid State Ionics 2006;177:1517e27. [3] Radovic M, Lara-Curzio E. Elastic properties of nickel-based anodes for solid oxide fuel cells as a function of the fraction of reduced NiO. J Am Ceram Soc 2004;87:2242e6. [4] Choi RS, Bansal NP. Flexure strength, fracture toughness and slow crack growth of YSZ/alumina composites at high temperatures. J Am Ceram Soc 2005;88:1474e80. [5] Baldacim SA, Cairo CAA, Silva CRM. Mechanical properties of ceramic composites. J Mater Process Technol 2001;119:273e6. [6] Reimanis IE. A review of issues in the fracture of interfacial ceramics and ceramic composites. Mater Sci Eng A 1997;237: 159e67. [7] Kostopoulos V, Markopoulos YP. On the fracture toughness of ceramic matrix composites. Mater Sci Eng A 1998;250: 303e12. [8] Yang B, Chen XM, Liu XQ. Effect of BaTiO3 addition on structures and mechanical properties of 3Y-TZP ceramics. J Eur Ceram Soc 2000;20:1153e8. [9] Zhan GD, Kuntz J, Wan J, Garay J, Mukherjee AK. Spark-plasma-sintered BaTiO3/Al2O3 nanocomposites. Mater Sci Eng A 2003;356:443e6. [10] Li JY, Dai H, Zhong XH, Zhang YF, Ma XF, Meng J, et al. Lanthanum zirconate ceramic toughened by BaTiO3 secondary phase. J Alloy Compd 2008;452:406e9. [11] Liu YG, Zhou Y, Jia DC, Meng QC, Chen YH. Domain switching toughening in a LiTaO3 dispersed Al2O3 ceramic composite. Scr Mater 2002;47:63e8. [12] Yang B, Chen XM. Alumina ceramics toughened by a piezoelectric secondary phase. J Eur Ceram Soc 2000;20: 1687e90.
4781
[13] Yuh SD, Lai YC, Chou CC. YNbO4-addition on the fracture toughness of ZrO2 (3Y) ceramics. J Mater Sci 2001;36:2303e11. [14] Li J, James RD. Prediction of microstructure in monoclinic LaNbO4 by energy minimization. Acta Mater 1997;45: 4271e81. [15] Tsunekawa S, Takei H. Domain switching behaviour of ferroelastic LaNbO4 and NdNbO4. J Phys Soc Jpn 1976;40: 1523e4. [16] Tsunekawa S, Hara K, Nishitani R, Kasuya A, Fukuda T. Observation of ferroelastic domains in LaNbO4 by atomic force microscope. Mater Trans JIM 1995;36:1188e91. [17] Li J, Huang CM, Xu GB, Wayman CM. The domain structure of LaNbO4 in the low temperature monoclinic phase. Mater Lett 1994;21:105e10. [18] Li J, Wayman CM. Domain boundary and domain switching in a ceramic rare-earth orthoniobate LaNbO4. J Am Ceram Soc 1996;79:1642e8. [19] Li J, Wayman CM. Compressive behavior and domain-related shape memory effect in LaNbO4 ceramics. Mater Lett 1996;26: 1e7. [20] Magraso A, Haugsrud R, Norby T. Preparation and characterization of NieLaNbO4 cermet anode supports for proton-conducting fuel cell applications. J Am Ceram Soc 2010;93:2650e5. [21] Maschio S, Pezzotti G, Sbaizero O. Effect of LaNbO4 addition on the mechanical properties of ceria-tetragonal zirconia polycrystal matrices. J Eur Ceram Soc 1998;18: 1779e85. [22] Evans AG, Charles EA. Fracture toughness determination by indentation. J Am Ceram Soc 1976;59:371e2. [23] Launey ME, Ritchie RO. On the fracture toughness of advanced materials. Adv Mater 2009;21:2103e10.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
LaNbO4 toughened NiOeY2O3 stabilized ZrO2 composite for the anode support of planar solid oxide fuel cells Ben Ma, Bo Chi, Jian Pu, Li Jian* Center for Fuel Cell Innovation, School of Materials Science and Engineering, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan, China
article info
abstract
Article history:
NiOeY2O3 stabilized ZrO2 (YSZ) composite is the state-of-the-art material for the anode
Received 9 January 2013
support of planar solid oxide fuel cells (SOFCs). To improve its fracture toughness (KC),
Received in revised form
lanthanum orthoniobate LaNbO4 is synthesized by the method of solid state reaction and
31 January 2013
added to the mixture of YSZ and NiO at the weight ratio of 47:53. The content of LaNbO4 in
Accepted 7 February 2013
the composite is in the range between 5 and 30 wt%. The microstructure of the composites
Available online 5 March 2013
is examined by scanning electron microscopy (SEM); and the chemical compatibility among the components is evaluated by X-ray diffraction (XRD) and energy dispersive X-ray
Keywords:
spectroscopy (EDS). Vickers hardness test is performed for estimating the KC of the
Lanthanum orthoniobate
composites. The results indicate that the KC increases with the addition of LaNbO4 in the
Anode support
composites; and the toughening effect is associated with the grain-refinement in the
Composite
composite and the domain switch in the monoclinic LaNbO4.
Hardness
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Fracture toughness
1.
Introduction
Solid oxide fuel cells (SOFCs) are an emerging technology that generates clean electrical power from fossil or hydrocarbon fuels for various kinds of applications, such as portable devices, transportations, distributed power plants and centralized power stations. Conventionally, there are two kinds of design for SOFCs, i.e., the tubular and the planar. Comparing with the tubular design, planar SOFCs are more attractive due to their low manufacturing cost and high power density [1], especially for the anode-supported cells where the thin electrode and electrolyte are built in sequence on the anode substrate. However, such planar cells are facing challenge of cracking or fracture during stack assembling and operation under a compressive load, leading to mixing of fuel and oxidant gases and in turn the failure of stack performance.
reserved.
The state-of-the-art anode material is NieY2O3 doped ZrO2 (YSZ) cermet which is usually reduced from NiOeYSZ ceramic composite during the process of stack start-up in reducing atmosphere. Thus the substrate of the cell is a ceramic composite of NiOeYSZ rather than the cermet of NieYSZ when the cells are assembled into a stack. At this stage, the cells are most susceptible to cracking under the applied compressive load; therefore, the mechanical property of the cell at room temperature is more critical than that at the operating temperature. Previous efforts have been made to optimize the microstructure of [2,3] or add Al2O3 [4] into the NiOeYSZ substrate, which manipulates its stress state for the enhancement of the strength. However, the cracking of the anode-supported cells cannot be solely attributed to the insufficient strength of the substrate; and more likely, it is due to its weak toughness. Improving the fracture toughness of
* Corresponding author. Tel.: þ86 27 87557496; fax: þ86 27 87558142. E-mail address: [email protected] (L. Jian). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.033
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
the cell substrate is a promising way to solve the issue of cell fracture. A conventional approach to toughen ceramic materials is to add a second phase in various shapes into the ceramic matrix, such as particles, whiskers and fibers [5e7]. The toughening effect appears to be associated with phase transformation, microcracking, residual thermal stress, fiber/ whisker reinforcement or grain bridging. In recent years, another toughening mechanism associated with the domain switch in the second phase has been reported. In this scenario, a certain amount of ceramic ferroics, such as ferroelectric, piezoelectric and ferroelastic, is introduced into ceramic materials to arrest the crack propagation by the orientation switch of domains existing in the ferroic phase, dissipating energy and stress build-up at the tip of a crack. Traditional ferroic materials, including BaTiO3 [8e10], Sr2Nb2O7 [11], Nd2Ti2O7 [12] and LiTaO3 [13], have been proved to be effective in enhancing the toughness of 3 mol% Y2O3eZrO2 (3Y-TZP), PdZrO3 (LZ) and Al2O3, respectively. In this study, lanthanum orthoniobate (LaNbO4) was used as the secondary phase to improve the toughness of NiOeYSZ composite for the anode substrate. LaNbO4 is a kind of ferroelastic material, undergoes a reversible polymorphic transformation at approximately 520 C from low temperature monoclinic phase to high temperature tetragonal phase, and presents a rubber-like behavior in the monoclinic form that contains domains in two crystallographic orientations separated by highly mobile boundaries [14e19]. This paper reports the enhanced toughness of NiOeYSZ composite achieved by the addition of LaNbO4, and discusses the possible toughening mechanisms.
2.
Experimental
2.1.
Synthesis of LaNbO4 powder
LaNbO4 powder was prepared by solid state reaction using lanthanum oxide La2O3 (99.9% purity, Sino-Pharm Chemical Reagent) and niobium oxide Nb2O5 (99.99% purity, Aladdin Chemistry) as the raw materials, according to the chemical reaction of La2 O3 þ Nb2 O5 /2LaNbO4
4777
ethanol for 24 h with zirconia beads as the media, followed by drying and pelleting. Disc specimens of 18 mm in diameter were obtained by press forming the powdery pellets under a load of 50 MPa and sintering at 1500 C in air for 6 h.
2.3.
Characterizations
The crystal structure of the synthesized LaNbO4 powder and sintered LaNbO4eNiOeYSZ composites was analyzed by X-ray diffraction (XRD, X’Pert PRO Diffractometer, PANalytical) with Cu Ka emission. The scanning angle covered 2q values from 20 to 80 . The morphology and microstructure of the sintered composites were examined by using a scanning electron microscope (SEM, Quanta 200, Micro FEI Philips) equipped with energy dispersive X-ray spectroscopy (EDS). The density of the sintered LaNbO4eNiOeYSZ composites was determined by the Archimedes method in distilled water. A Vickers hardness tester (HV-432SVD, Qingdao Dizhong Measuring Instruments) was employed for the hardness measurement of the sintered specimens under a load of 10 kgf for 15 s. And their fracture toughness was converted from the geometry of the indentation and indentation-induced crack. Before the indentation test, the surfaces of the sintered specimens were polished with a diamond suspension by using a metallographic polishing machine (P-1, Laizhou Weiyi Testing Machine).
3.
Results and discussion
Fig. 1 shows the XRD pattern of the synthesized LaNbO4 powder. The diffraction peaks was well matched to those displayed in the JCPDS file 86-0909 for LaNbO4, indicating that pure monoclinic LaNbO4 was formed at room temperature by calcining La2O3 and Nb2O5 powder mixture at 1200 C in air for 4 h. Fig. 2 presents the XRD patterns of the sintered LaNbO4eNiOeYSZ composites with various LaNbO4 contents. Comparing with the JCPDS files for LaNbO4 (86-0909), NiO (711179) and YSZ (89-9069) revealed that the peak intensity of monoclinic LaNbO4 was gradually amplified by increasing the amount of LaNbO4 and no XRD evidence of extra phases other
(1)
To obtain a uniform powder mixture for the reaction, La2O3 and Nb2O5 powders were mixed at 1 to 1 molar ratio and ballmilled in ethanol for 24 h by using a satellite mill (QM-2SP12, Nanjing Nanda Instruments) with zirconia beads as the milling media. The ball-milled mixture was dried in an oven at 80 C and then heated at a rate of 3 C min1 to 1200 C. Pure LaNbO4 phase was formed after 4 h calcination at 1200 C in air.
2.2.
Preparation of LaNbO4eNiOeYSZ composite
YSZ (TZ-8YS, Tosoh) and NiO (Type Standard, Inco) powders were mixed at a weight ratio of 47:53 to form a NiOeYSZ powder mixture, to which LaNbO4 powder was added at various contents between 5 and 30 wt%, respectively. The LaNbO4eNiOeYSZ powder mixtures were ball-milled in
Fig. 1 e XRD pattern of LaNbO4 powder synthesized by solid reaction at 1200 C in air for 4 h. The JCPD file 86-0909 for LaNbO4 is inserted for comparison.
4778
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
Fig. 2 e XRD patterns of the LaNbO4eNiOeYSZ composites sintered at 1500 C in air for 6 h. The content of added LaNbO4 is between 5 and 30 wt%.
than LaNbO4, NiO and YSZ was found in the spectra. This result is consistent with that no chemical reaction occurred between NiO and LaNbO4 at temperatures up to 1400 C as observed by Magraso et al. [20]. In a study of YNbO4-added 3 mol% Y2O3eZrO2 (3YZ), Yuh et al. [13] reported that less than 10 mol% of YNbO4 added in the 3YZ was fully dissolved in the matrix after sintering at 1600 C for 1 h, and the XRD peaks were not significantly shifted. Maschio et al. [21] also reported Ce enrichment in LaNbO4 grains in LaNbO4-added CeO2eZrO2 composites sintered at 1500 C. Therefore, the XRD results showing in Fig. 2 may not exclude the possibility of element inter-diffusion between LaNbO4 and YSZ, although no noticeable change in the peak position was detected. The morphology of the sintered LaNbO4eNiOeYSZ composites with different amounts of added LaNbO4 is shown in Fig. 3. As confirmed by the EDS analysis shown in Fig. 4, the white grains were LaNbO4, the gray grains were YSZ and the
dark grains were NiO; and Y in LaNbO4 and La in YSZ were detected (the presence of Au in Fig. 4d was due to the coating used on the SEM specimen to minimize charging). This result suggests the element inter-diffusion between LaNbO4 and YSZ. However, such insignificant element inter-diffusion did not change their crystallographic structures and is not expected to adversely affect the performance and stability of the composite substrate, since the function of the reduced substrate is to support the thin electrolyte and electrodes of the cell and deliver the electrons generated by the anode reaction to the interconnect. These three phases are uniformly distributed, and the average grain size of the composite was refined from 1.96 to 1.35 mm as the LaNbO4 content increased from 5 to 30 wt% (Table 1), implying that LaNbO4 as the secondary phase hindered the grain growth of the composite. Fig. 5 shows the dependence of the hardness Hv and hardness-converted fracture toughness KC of the composite on its LaNbO4 content at room temperature. The relative density of the samples was approximately 99%, measured by the Archimedes method in distilled water. The value of KC was determined from the geometry of the indentation and indentation-induced crack according to the following widely accepted empirical equation [22], pffiffiffi KC F=HV a ¼ 0:15kðc=aÞ3=2
(2)
where a is the half length of the indentation diagonal and c is the half length of the indentation crack, as shown in Fig. 6; F is the constraint factor (z3) and k is the correction factor (z3.2 for large c/a values as shown in the case herein) [22]. Each KC value shown in Fig. 5 is the average value obtained from five different measurements. The error bars represent the range from the lowest to highest KC value. The hardness of the sintered LaNbO4eNiOeYSZ composites decreased with the addition of LaNbO4; and conversely the fracture toughness KC increased and gradually approached to the maximum at LaNbO4 contents above 25 wt%. Comparing with KC of NiOeYSZ composite, 21% increase in KC was obtained for the
Fig. 3 e SEM back-scattered electron micrographs of the LaNbO4eNiOeYSZ composites with various LaNbO4 contents between 5 and 30 wt%. The white grains are LaNbO4, the dark grains are NiO and the gray grains are YSZ.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
4779
Fig. 4 e EDS compositional analysis of the LaNbO4eNiOeYSZ composite with 30 wt% added LaNbO4: a) SEM image; b) EDS spectrum from spot A in a); c) EDS spectrum from spot B in a); and d) EDS spectrum from spot C in a).
LaNbO4eNiOeYSZ composites containing more than 25 wt% of LaNbO4, which demonstrates the toughening effect of LaNbO4 on the LaNbO4eNiOeYSZ composite. Maschio et al. [21] studied the influence of LaNbO4 addition on mechanical properties of polycrystal CeO2-doped tetragonal ZrO2 and proposed that the observed toughening was due to the crack wake bridging by the elongated LaNbO4 grains in the composite. The elongated LaNbO4 grains were formed during sintering at 1500 C in the presence of silica and transient liquid phase. And in another study of YNbO4 toughened 3YZ, Yuh et al. [13] noticed toughness increase for specimens sintered at 1600 C for 1 h and containing less than 5 mol% of YNbO4; and the toughening mechanism was not clearly elucidated by either the domain switch or the tetragonal-tomonoclinic phase transformation in the 3YZ. However, the above-mentioned possible mechanisms seem not applicable for the explanation of the enhanced fracture toughness
demonstrated by the LaNbO4eNiOeYSZ composites. NiO and YSZ are stable cubic phases without domains inside their grains; therefore, both phase transformation and domain switch in NiOeYSZ matrix are not expected to occur. The possible reasons for the improved toughness are likely to be the grain refinement in the composite and domain switch in
Table 1 e Average grain size (mm) of LaNbO4eNiOeYSZ composites at various LaNbO4 contents (wt%). LaNbO4 content
5%
10%
15%
20%
25%
30%
Grain size
1.96
1.90
1.70
1.56
1.55
1.35
Fig. 5 e Fracture toughness and Vickers hardness of the LaNbO4eNiOeYSZ composites with various LaNbO4 contents ranging from 5 to 30 wt%.
4780
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
Fig. 7 e Schematic illustration showing domain switch in monoclinic LaNbO4 in the LaNbO4eNiOeYSZ composite under an applied stress. Domains at preferred orientation grow at expense of that unfavored.
Fig. 6 e Representative hardness indentation showing the geometry of the indentation and indentation-induced crack.
LaNbO4. As shown in Table 1, more than 31% of grain refinement was achieved by increasing the LaNbO4 content from 5 to 30 wt%, which will improve the fracture toughness by limiting the size of pre-existing microcracks and promoting crack deflection [23]. Apart from the grain size contribution, the added LaNbO4 is also expected to play an important role in toughening the LaNbO4eNiOeYSZ composite. LaNbO4 is in the monoclinic form at room temperature and its grains consist of domains in various crystallographic orientations separated by a boundary located at a lattice plane in the {20 4}I/{402}II family [17,18]; consequently LaNbO4 exhibits ferroelasticity and domain switchability in response to the applied stresses [19]. A domain packet contains parallel domains in two crystallographic orientations, and different packets can meet in various configurations determined by the requirement of energy minimization [14]. Under the influence of an external uniaxial stress sij, the Gibbs free-energy density G for a domain can be written in differential form as [18] dG ¼ SdT 3 ij dsij
(3)
where S, T and 3 ij denote the entropy, temperature and strain, respectively. And the strain can be further expressed as 3 ij
¼ esij þ sijkl skl
(4)
where esij is the component of the spontaneous strain tensor, and sijkl is the component of the elastic compliance tensor. Substituting Eq. (4) into Eq. (3), followed by integration, gives the free energy density G of a domain. And the difference of the free energy density (DG) for domains in two different orientations DG ¼ G2 G1 ¼ esij sij
the initiation of microcracks by absorbing locally accumulated energy needed for crack formation; and also provide the resistance to the extension of microcracks by releasing the stress build-up at the tip of cracks. Thus both the intrinsic and extrinsic toughness [23] of the composite are promoted by the addition of LaNbO4.
4.
Conclusion
To improve the fracture toughness of the anode support material NiOeYSZ of planar SOFC cells, LaNbO4 powder was added into the mixture of NiO and YSZ to form LaNbO4eNiOeYSZ composites with various LaNbO4 contents between 5 and 30 wt%. From the obtained results, the following conclusions can be drawn. 1) LaNbO4, NiO and YSZ are uniformly distributed in the LaNbO4eNiOeYSZ composites, and the microstructure is refined with the increase of LaNbO4 content. They are chemically stable and compatible without forming new phases after sintering at 1500 C in air for 6 h. 2) The fracture toughness of the LaNbO4eNiOeYSZ composites is considerably enhanced by the addition of LaNbO4. The toughening effect is attributed to both the grain refinement in the microstructure and the domain switch in monoclinic LaNbO4.
Acknowledgment The authors gratefully acknowledge the financial support to this study provided by the National Natural Science Foundation of China under the project 51172082. The XRD and SEM characterizations were performed with the assistance of the Analytical and Testing Center of Huazhong University of Science and Technology.
(5)
acts as the driving force for domain switching between these two orientations. Under the external stress the domain with the preferred orientation will grow at the expense of the unfavored one, as schematically presented in Fig. 7. By this mechanism, the embedded LaNbO4 will provide resistance to
references
[1] Steele BCH, Heizel A. Materials for fuel-cell technologies. Nature 2001;414:345e52.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 4 7 7 6 e4 7 8 1
[2] Wang Y, Walter ME, Sabolsky K, Seabaugh MM. Effects of powder sizes and reduction parameters on the strength of NieYSZ anodes. Solid State Ionics 2006;177:1517e27. [3] Radovic M, Lara-Curzio E. Elastic properties of nickel-based anodes for solid oxide fuel cells as a function of the fraction of reduced NiO. J Am Ceram Soc 2004;87:2242e6. [4] Choi RS, Bansal NP. Flexure strength, fracture toughness and slow crack growth of YSZ/alumina composites at high temperatures. J Am Ceram Soc 2005;88:1474e80. [5] Baldacim SA, Cairo CAA, Silva CRM. Mechanical properties of ceramic composites. J Mater Process Technol 2001;119:273e6. [6] Reimanis IE. A review of issues in the fracture of interfacial ceramics and ceramic composites. Mater Sci Eng A 1997;237: 159e67. [7] Kostopoulos V, Markopoulos YP. On the fracture toughness of ceramic matrix composites. Mater Sci Eng A 1998;250: 303e12. [8] Yang B, Chen XM, Liu XQ. Effect of BaTiO3 addition on structures and mechanical properties of 3Y-TZP ceramics. J Eur Ceram Soc 2000;20:1153e8. [9] Zhan GD, Kuntz J, Wan J, Garay J, Mukherjee AK. Spark-plasma-sintered BaTiO3/Al2O3 nanocomposites. Mater Sci Eng A 2003;356:443e6. [10] Li JY, Dai H, Zhong XH, Zhang YF, Ma XF, Meng J, et al. Lanthanum zirconate ceramic toughened by BaTiO3 secondary phase. J Alloy Compd 2008;452:406e9. [11] Liu YG, Zhou Y, Jia DC, Meng QC, Chen YH. Domain switching toughening in a LiTaO3 dispersed Al2O3 ceramic composite. Scr Mater 2002;47:63e8. [12] Yang B, Chen XM. Alumina ceramics toughened by a piezoelectric secondary phase. J Eur Ceram Soc 2000;20: 1687e90.
4781
[13] Yuh SD, Lai YC, Chou CC. YNbO4-addition on the fracture toughness of ZrO2 (3Y) ceramics. J Mater Sci 2001;36:2303e11. [14] Li J, James RD. Prediction of microstructure in monoclinic LaNbO4 by energy minimization. Acta Mater 1997;45: 4271e81. [15] Tsunekawa S, Takei H. Domain switching behaviour of ferroelastic LaNbO4 and NdNbO4. J Phys Soc Jpn 1976;40: 1523e4. [16] Tsunekawa S, Hara K, Nishitani R, Kasuya A, Fukuda T. Observation of ferroelastic domains in LaNbO4 by atomic force microscope. Mater Trans JIM 1995;36:1188e91. [17] Li J, Huang CM, Xu GB, Wayman CM. The domain structure of LaNbO4 in the low temperature monoclinic phase. Mater Lett 1994;21:105e10. [18] Li J, Wayman CM. Domain boundary and domain switching in a ceramic rare-earth orthoniobate LaNbO4. J Am Ceram Soc 1996;79:1642e8. [19] Li J, Wayman CM. Compressive behavior and domain-related shape memory effect in LaNbO4 ceramics. Mater Lett 1996;26: 1e7. [20] Magraso A, Haugsrud R, Norby T. Preparation and characterization of NieLaNbO4 cermet anode supports for proton-conducting fuel cell applications. J Am Ceram Soc 2010;93:2650e5. [21] Maschio S, Pezzotti G, Sbaizero O. Effect of LaNbO4 addition on the mechanical properties of ceria-tetragonal zirconia polycrystal matrices. J Eur Ceram Soc 1998;18: 1779e85. [22] Evans AG, Charles EA. Fracture toughness determination by indentation. J Am Ceram Soc 1976;59:371e2. [23] Launey ME, Ritchie RO. On the fracture toughness of advanced materials. Adv Mater 2009;21:2103e10.