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Investigation on the properties of Nb and Al doped Ti3SiC2 as a new interconnect material for IT-SOFC
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 7 ( 2 0 1 2 ) 1 0 8 4 e1 0 8 8
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journal homepage: www.elsevier.com/locate/he
Investigation on the properties of Nb and Al doped Ti3SiC2 as a new interconnect material for IT-SOFC L.L. Zheng a,b, J.J. Li a,b, M.S. Li a,*, Y.C. Zhou a a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China b Graduate School of Chinese Academy of Sciences, Beijing 100039, China
article info
abstract
Article history:
A modified layered ternary compound (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 was evaluated as a new
Received 20 November 2010
interconnect material for intermediate temperature solid oxide fuel cells (IT-SOFC). The
Received in revised form
results indicated that the compound has low density (4.50 g/cm3), closer matched thermal
27 January 2011
expansion coefficient with Y2O3 stabled ZrO2 (9.0 10 6/K), low room temperature elec-
Accepted 14 February 2011
trical resistivity (24 mU cm), and good mechanical properties and machinability. During the
Available online 15 March 2011
isothermal oxidation at 600 Ce800 C, a monolithic layer with the mixture of rutile TiO2 and amorphous SiO2 formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. Its oxidation kinetics roughly
Keywords:
follows a parabolic law, the rate constant is 0.45, 1.26, 1.57 10
7
mg2/cm4 s at 600 C,
MAX phase
700 C, 800 C, respectively. After the oxidation at 700 C and 800 C in air for 100 h, its area
Interconnect
special resistance is 5 and 12 mU cm2 at the corresponding oxidation temperature,
SOFC
respectively. In oxidizing environment, (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 is very stable, and has
Oxidation
not Cr or other element volatilization. All above determined properties of (Ti0.98Nb0.02)3
Area specific resistance
(Si0.95Al0.05)C2 are favorable for its application as interconnect in IT-SOFC. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) are electrochemical devices which can convert chemical energy of a fuel directly into electricity and heat without combustion [1]. SOFCs have gained significant attention due to their advantages of low production of pollutants, fuel flexibility and high efficiency. In order to perform their intended functions, interconnects are needed to build the multiple cell stacks. As physical separator of fuels and oxygen and bipolar plate of connecting adjacent cells, interconnects should fulfill rigorous selecting criteria. One of the challenges for the commercialization of the SOFC is the development of suitable interconnect material. Over the past several decades, most efforts for the development of
interconnect have been focused on the materials of LaCrO3 and doped LaCrO3. However, this kind of materials has many drawbacks, e.g., difficulty in sintering dense chromite parts and high cost of raw materials and fabrication. Recent progresses make it feasible for metallic interconnects to supplant LaCrO3 and doped LaCrO3 [3]. Compared to LaCrO3 and doped LaCrO3, metallic interconnects have many advantages, such as high electrical and thermal conductivity, low cost, easy manufacture and good workability [4]. However, chromium oxide vaporization can poison the cathode and then lead to the degradation of SOFC performance [5]. To overcome this problem, many kinds of alloys [6,7] and coatings [8e10] have been investigated. However, alloys together with coatings are not well satisfied for inhibiting Cr evaporation.
* Corresponding author. E-mail address: [email protected] (M.S. Li). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.02.083
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 7 ( 2 0 1 2 ) 1 0 8 4 e1 0 8 8
Ti3SiC2, belonging to the family of MAX phases, exhibits low density, high modulus and fracture toughness, high electrical and thermal conductivity, easy machinability, good resistance to oxidation and thermal shock below 1100 C [11], and matched thermal expansion coefficient with yttria stabilized zirconia (YSZ) electrolyte. These unique properties endow it a promising interconnect material for intermediate temperature SOFC (IT-SOFC). In order to further improve the high temperature conductivity of T3SiC2 in oxidizing environment, solid solution modification may be an effect way. According to Wagner’s theory for n-type and p-type oxides, doping penta-valent oxide into n-type TiO2 (main oxide formed on T3SiC2 [12]) can form donor-type addition, thus the electrical conductivity of TiO2 may be increased. Therefore in this work, a novel Nb doped Ti3(Si0.95Al0.05)C2 solid solution was synthesized, its relevant properties for applications of interconnect in IT-SOFC have been investigated.
2.
Experimental procedure
2.1. Preparation and characterization of the as-synthesized material Nb doped Ti3(Si0.95Al0.05)C2 bulk material was fabricated by insitu solideliquid reaction/hot pressing process at 1550 C under a pressure of 30 MPa for 1 h in flowing Ar atmosphere. The raw powders of titanium, niobium, silicon, aluminum and graphite were selected as the molar ratio of 2.94:0.06:0.95:0.05:2. The final solid solution can be denoted as (Ti0.98Nb0.02)3(Si0.95Al0.05) C2 according to the nominal apportions of the raw powders. The density of the as-synthesized sample was determined by the Archimedean method (ISO 18754), and the composition was identified using a step-scanning X-ray diffraction (Rigaku D/max-2400, Tokyo, Japan).
degreased in ethanol and finally cleaned in distilled water. Isothermal oxidation tests were conducted in Setsys 16/18 microbalance (SETRAM, Caluire, France). The coupons were suspended by a Pt wire in the vertical furnace, oxidized at 600 Ce800 C for 20 h in flowing air with the inlet rate of 30 ml/min. The mass gain of the sample was recorded continuously as a function of oxidation time by the personal computer. The cyclic oxidation tests were carried out in the tube furnace at 800 C in static air up to 100 h. The sample was kept at the required temperature for 20 h then cooled down to room temperature in air, which is defined as one cycle. Based on the measurement of sample weight, the mass change of the sample with cyclic times (or oxidation time) can be obtained. After oxidation tests, the samples were identified by X-ray diffraction in a D/max 2500PC diffractometer (Rigaku, Tokyo, Japan) with Cu Ka radiation. The surface and cross-section morphologies were observed by the SUPRA35 scanning electron microscope (SEM, LEO, Oberkochen, Germany), equipped with an energy-dispersive spectroscopy (EDS, INCA, Oxford Instrument, Oxford, U.K.) system.
2.4.
Results and discussion
Determination of basic properties
The bulk electrical conductivity was measured by the standard 4-probe D.C. method at room temperature. The thermal expansion coefficient conductivity (TEC) of the dense bulk material was determined by high-temperature dilatometer (Setaram Setsys 24, Caluire, France). Cylindrical samples with dimensions of F6 mm 8 mm were prepared by electricaldischarge machining method. The sample was heated from room temperature (RT) to 1373 K as a rate of 2 K/min in air atmosphere. After measurement, the thermal expansion data were corrected by subtracting the base line. The flexural strength of the samples was determined using four-point bending method. The measurement was conducted at 800 C by an electrical universal testing machine, with the dimensions of coupon samples 3 mm 4 mm 36 mm.
2.3.
High temperature electrical conductivity
Electrical conductivity of samples after isothermal oxidation at 700 C and 800 C for 100 h was measured using the 2-probe 4-point method in static air atmosphere at 700 C and 800 C, respectively. Pt-paste electrodes were applied on the oxidized sample surfaces with the Pt wire as the current and voltage probes. A constant current of 10 mA was applied by a Precision Programmable Current Source, and the corresponding voltage drop was recorded continuously by the FLUKE 8845A 6-1/2 Digit Programmable Multimeter.
3. 2.2.
1085
Oxidation tests
For oxidation tests, rectangular coupons with dimensions of 10 10 2 mm3 were cut from the as-synthesized material by electrical-discharge machining method. Prior to oxidation, the surfaces of the samples were grounded down to 2000 SiC paper, polished using 1 mm diamond paste, chamfered, then
3.1. Phase and basic properties of the as-synthesized material Fig. 1 shows the XRD pattern of the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. It can be seen, this material is only composed of Ti3SiC2, and no diffraction peak of Nb or Al containing compounds is detected. The insert in Fig. 1 shows the position of the XRD peak (101) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 and Ti3(Si0.95Al0.05)C2. It is clear that comparing with Ti3(Si0.95Al0.05) C2, the diffraction peak (101) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 shifts to low angle. Combined with the computational modeling of the crystal structure of Ti3SiC2, we can conclude that Nb atoms have been doped onto the sites of Ti. The determined density of the as-synthesized (Ti0.98Nb0.02)3 (Si0.95Al0.05)C2 is 4.50 0.01 g/cm3, and its relative density is 98.6%, hence the as-synthesized material is distinctly dense. Since the cell stacks of IT-SOFC will experience cyclic heating and cooling procedure, the thermal expansion behavior of interconnect is significant for the performance of cell stacks. The thermal expansion test was conducted, the result indicates that the expansion strain increases almost linearly as increasing the temperature from RT to 1100 C, and the thermal expansion coefficient of (Ti0.98Nb0.02)3(Si0.95Al0.05)
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Fig. 1 e The XRD pattern of the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. The insert shows the position of the diffraction peak (101) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 and Ti3(Si0.95Al0.05)C2.
3.2.
Oxidation resistance
3.2.1.
Oxidation kinetics
Fig. 2 shows the isothermal oxidation kinetics (mass gain per unit area as a function of oxidation time) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 at 600 C, 700 C and 800 C in air for 20 h. It can be seen that the mass gain per unit area increases with increasing the oxidation temperature. By fitting the mass gain curves in Fig. 2, it can be known that the oxidation kinetics roughly obey the parabolic law at 600 Ce800 C, the parabolic rate constant is determined to be 0.45, 1.26 and 1.57 10 7 mg2/cm4 s at 600 C, 700 C and 800 C, respectively, which is in the same order of magnitude with some alloys, e.g., AL 453 and SUS 430 [2]. The cyclic oxidation at 800 C in air for 100 h presents that no spallation happened during the oxidation, and the mass gain curve obeys parabolic law. By fitting the cyclic oxidation kinetics, the parabolic rate constant is determined to be 1.67 10 7 mg2/cm4 s, which is in accordance with the data obtained during the isothermal oxidation at 800 C.
3.2.2. C2 is determined to be (9.0 0.2) 10 6/K, which is very close to the value of YSZ (10.5 10 6/K). Therefore, when YSZ is used as electrolyte in IT-SOFC, the thermal stress resulting from the mismatch of the TEC of interconnect with YSZ can be minimized. The flexural strength of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 is determined to be 328 MPa at 800 C, which is higher than that of LaCrO3. The electrical resistivity of (Ti0.98Nb0.02)3(Si0.95Al0.05) C2 at RT in air is 24 mU cm, which is even lower than that of some stainless steels. Meanwhile, the thermal conductivity and elastic modulus of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 have been determined, are 37 W m 1 K 1 and 330 10 GPa (RT) respectively. All measured data are summarized in Table 1. The good mechanical properties, high electrical and thermal conductivity are favorable for the application of the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 as interconnect.
Oxidation products
The XRD pattern of the oxide scale formed on (Ti0.98Nb0.02)3 (Si0.95Al0.05)C2 at 800 C in air for 100 h is shown in Fig. 3. Only rutile TiO2 crystal was identified in the oxide scale. It should be noted that the diffraction peaks associated with the substrate of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 are strong, suggesting that the oxide scale is quite thin, and the oxidation resistance of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 at 800 C is distinctively well. Meanwhile, the peaks from the substrate also can exhibit that the material is stable without decomposition under the oxidation atmosphere. After oxidation, the oxide scales formed on Nb doped and Nb free Ti3(Si0.95Al0.05)C2 were scraped off from the substrate and rubbed to be powders, thereafter the powders were heated to 800 C for 1 h to eliminate the thermal stress. To avoid experimental error, the XRD patterns were obtained from the same experiment rig. The diffraction peaks of rutile TiO2 (110) formed on the two substrates oxidized at 800 C are inserted in Fig. 3. The (110) peak of rutile
Table 1 e The determined values of relevant properties of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. Property Density Thermal expansion coefficient Thermal conductivity Flexural strength Elastic modulus Electrical resistivity Parabolic oxidation rate constant in air
ASR after oxidation in air for 100 h at 700 C and 800 C
Measured value 4.50 0.01 g/cm3 (9.0 0.2) 10 6/K (RTe1000 C) 37 W m 1 K 1 328 MPa (800 C) 330 10 GPa (RT) 24 mU cm (RT) 0.45 10 7 (600 C), 1.26 10 7 (700 C), 1.57 10 7 (800 C) mg2/cm4 s 5 mU cm2 (700 C) 12 mU cm2 (800 C)
Fig. 2 e Weight gain per unit area vs. oxidation time for (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 during oxidation at 600 C, 700 C and 800 C in air for 20 h.
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 7 ( 2 0 1 2 ) 1 0 8 4 e1 0 8 8
1087
Fig. 3 e XRD pattern of the oxide scale formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 800 C in air for 100 h. The insert shows the shift of TiO2 (110) on Ti3(Si0.95Al0.05)C2 after Nb doping under the same oxidation condition.
TiO2 formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 shifts to low angle, which suggests that the Nb has been doped into rutile lattice.
3.2.3.
Morphology of the oxide scale
The surface morphology of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 800 C in air for 100 h is presented in Fig. 4(a). It can be seen that the substrate grains are distinctly visible, which suggests that the oxide scale is very thin. The grain size of the oxides formed along the substrate grain boundaries is larger than that formed on the substrate grains. Based on the EDS analysis (not shown here), the oxide grains are rich in Ti, Si and O. The oxides formed on the surface of substrate grains are composed of Ti and Si with weight ratio of 3:1, however, this value becomes 6:1 for the oxides formed along the substrate grain boundaries. Fig. 4(b) and (c) illustrate the cross-section morphology and EDS line scanning taken across the oxide scale formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 at 800 C in air for 100 h, respectively. Fig. 4(b) shows that the oxide scale consists of monolithic oxide scale layer, and the thickness is about 2 mm. EDS results indicate that the scale is rich in Ti, Si and O, and trace of Al and Nb uniformly distribute in the scale. Again, Si was detected in the oxide scale by EDS. However, SiO2 has not been identified by XRD, Si may exist as amorphous SiO2. XPS analysis was further conducted, and identified that SiO2 existed in the oxide scale (The XPS spectrum has not been shown here). Therefore, it can be concluded that the monolithic layer of the oxide scale is composed of mixture of rutile TiO2 and amorphous SiO2. Moreover, Si Ka curve exhibits the same shape as the element of Ti (see Fig. 4(c)), which indicates the uniform distribution of mixture of rutile TiO2 and amorphous SiO2 in the oxide scale. The uniform mixture of rutile TiO2 and amorphous SiO2 is a benefit to the electrical conductivity of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 in oxidizing environment, because the uniform distribution can avoid the formation of continuous insulating SiO2 layer.
Fig. 4 e (a) Surface morphology, (b) cross-section and (c) EDS line scanning of the oxide scale on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 800 C in air for 100 h.
3.3.
High temperature electrical resistivity
The electrical resistance of one material after oxidized under the simulated cathode atmosphere is very important to assess its application as interconnect. Generally, the conductive substrate has a much lower electrical resistance than its oxide scale, the measured ASR of the oxidized sample mainly denotes the electrical resistance of its oxide scale. The ASR of
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the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 700 C and 800 C in air for 100 h has been determined to be 5 and 12 mU cm2 at the corresponding oxidation temperature, respectively. The value obtained at 800 C is lower than the calculated value from the electrical resistivity of bulk rutile TiO2 at 800 C (120 U m), which may result from doping effect. TiO2 is a n-type semiconductor, and the native defects are oxygen vacancies, tri- and quadri-valent Ti interstitials. When penta-valent niobium oxide is used as dopant, donor-type addition will form and the concentration of charge carrier ([n]) will increase. Therefore, Nb doping can improve the high temperature electrical conductivity of Ti3(Si0.95Al0.05)C2 after oxidation in air atmosphere.
4.
Conclusion
A modified ternary layered compound of (Ti0.98Nb0.02)3(Si0.95Al0.05) C2 was evaluated as a new interconnect material for IT-SOFC. This material has low density, closer matched thermal expansion coefficient with YSZ, good mechanical properties and machinability. The compound also exhibits good oxidation resistance at 600 Ce800 C in air, its parabolic rate constant is 0.45, 1.26, 1.57 10 7 mg2/cm4 s at 600 C, 700 C, 800 C, respectively. The uniform oxide scale with the mixture of rutile TiO2 and amorphous SiO2 formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 during oxidation. After the oxidation at 700 C and 800 C in air for 100 h, its area special resistance is 5 and 12 mU cm2 at the corresponding oxidation temperature, respectively. All evaluated properties of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 are favorable for its application as interconnect, further studies on its practical performance in ITSOFC are in progress.
Acknowledgment This work was supported by the National Science Foundation of China under Grant No.50771099.
references
[1] Minh NQ. Ceramic fuel cells. J Am Ceram Soc 1993;76(3): 563e88. [2] Yang ZG, Weil KS, Paxton DM, Stevenson JW. Selection and evaluation of heat-resistant alloys for SOFC interconnect applications. J Electrochem Soc 2003;150(9):A1188e201. [3] Fergus JW. Metallic interconnects for solid oxide fuel cells. Mater Sci Eng A 2005;397:271e83. [4] Huang KQ, Hou PY, Goodenough JB. Characterization of iron-based alloy interconnects for reduced temperature solid oxide fuel cells. Solid State Ionics 2000;129: 237e50. [5] Badwal SPS, Deller R, Foger K, Ramprakash Y, Zhang JP. Interaction between chromia forming alloy interconnects and air electrode of solid oxide fuel cells. Solid State Ionics 1997;99:297e310. [6] Jablonski PD, Alman DE. Oxidation resistance of novel ferritic stainless steels alloyed with titanium for SOFC interconnect applications. J Power Sources 2008;180:433e9. [7] Geng SJ, Zhu JH, Brady MP, Anderson HU, Zhou XD, Yang ZG. A low-Cr metallic interconnect for intermediatetemperature solid oxide fuel cells. J Power Sources 2007; 172:775e81. [8] Shaigana N, Qu W, Ivey DG, Chen WX. A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects. J Power Sources 2010;195:1529e42. [9] Yoo J, Woo SK, Yu JH, Lee S, Park GW. La0.8Sr0.2MnO3 and (Mn1.5Co1.5)O4 double layer coated by electrophoretic deposition on Crofer22APU for SOFC interconnect applications. Int J Hydrogen Energy 2009;34:1542e7. [10] Chua CL, Lee J, Lee TH, Cheng YN. Oxidation behavior of metallic interconnect coated with LaeSreMn film by screen painting and plasma sputtering. Int J Hydrogen Energy 2009; 34:422e34. [11] Barsoum MW, El-Raghy T. Synthesis and characterization of a remarkable ceramic: Ti3SiC2. J Am Ceram Soc 1996;79: 1953e6. [12] Sun ZM, Zhou YC, Li MS. High temperature oxidation behavior of Ti3SiC2-based material in air. Acta Mater 2001;49: 4347e53.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Investigation on the properties of Nb and Al doped Ti3SiC2 as a new interconnect material for IT-SOFC L.L. Zheng a,b, J.J. Li a,b, M.S. Li a,*, Y.C. Zhou a a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China b Graduate School of Chinese Academy of Sciences, Beijing 100039, China
article info
abstract
Article history:
A modified layered ternary compound (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 was evaluated as a new
Received 20 November 2010
interconnect material for intermediate temperature solid oxide fuel cells (IT-SOFC). The
Received in revised form
results indicated that the compound has low density (4.50 g/cm3), closer matched thermal
27 January 2011
expansion coefficient with Y2O3 stabled ZrO2 (9.0 10 6/K), low room temperature elec-
Accepted 14 February 2011
trical resistivity (24 mU cm), and good mechanical properties and machinability. During the
Available online 15 March 2011
isothermal oxidation at 600 Ce800 C, a monolithic layer with the mixture of rutile TiO2 and amorphous SiO2 formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. Its oxidation kinetics roughly
Keywords:
follows a parabolic law, the rate constant is 0.45, 1.26, 1.57 10
7
mg2/cm4 s at 600 C,
MAX phase
700 C, 800 C, respectively. After the oxidation at 700 C and 800 C in air for 100 h, its area
Interconnect
special resistance is 5 and 12 mU cm2 at the corresponding oxidation temperature,
SOFC
respectively. In oxidizing environment, (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 is very stable, and has
Oxidation
not Cr or other element volatilization. All above determined properties of (Ti0.98Nb0.02)3
Area specific resistance
(Si0.95Al0.05)C2 are favorable for its application as interconnect in IT-SOFC. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solid oxide fuel cells (SOFCs) are electrochemical devices which can convert chemical energy of a fuel directly into electricity and heat without combustion [1]. SOFCs have gained significant attention due to their advantages of low production of pollutants, fuel flexibility and high efficiency. In order to perform their intended functions, interconnects are needed to build the multiple cell stacks. As physical separator of fuels and oxygen and bipolar plate of connecting adjacent cells, interconnects should fulfill rigorous selecting criteria. One of the challenges for the commercialization of the SOFC is the development of suitable interconnect material. Over the past several decades, most efforts for the development of
interconnect have been focused on the materials of LaCrO3 and doped LaCrO3. However, this kind of materials has many drawbacks, e.g., difficulty in sintering dense chromite parts and high cost of raw materials and fabrication. Recent progresses make it feasible for metallic interconnects to supplant LaCrO3 and doped LaCrO3 [3]. Compared to LaCrO3 and doped LaCrO3, metallic interconnects have many advantages, such as high electrical and thermal conductivity, low cost, easy manufacture and good workability [4]. However, chromium oxide vaporization can poison the cathode and then lead to the degradation of SOFC performance [5]. To overcome this problem, many kinds of alloys [6,7] and coatings [8e10] have been investigated. However, alloys together with coatings are not well satisfied for inhibiting Cr evaporation.
* Corresponding author. E-mail address: [email protected] (M.S. Li). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.02.083
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 7 ( 2 0 1 2 ) 1 0 8 4 e1 0 8 8
Ti3SiC2, belonging to the family of MAX phases, exhibits low density, high modulus and fracture toughness, high electrical and thermal conductivity, easy machinability, good resistance to oxidation and thermal shock below 1100 C [11], and matched thermal expansion coefficient with yttria stabilized zirconia (YSZ) electrolyte. These unique properties endow it a promising interconnect material for intermediate temperature SOFC (IT-SOFC). In order to further improve the high temperature conductivity of T3SiC2 in oxidizing environment, solid solution modification may be an effect way. According to Wagner’s theory for n-type and p-type oxides, doping penta-valent oxide into n-type TiO2 (main oxide formed on T3SiC2 [12]) can form donor-type addition, thus the electrical conductivity of TiO2 may be increased. Therefore in this work, a novel Nb doped Ti3(Si0.95Al0.05)C2 solid solution was synthesized, its relevant properties for applications of interconnect in IT-SOFC have been investigated.
2.
Experimental procedure
2.1. Preparation and characterization of the as-synthesized material Nb doped Ti3(Si0.95Al0.05)C2 bulk material was fabricated by insitu solideliquid reaction/hot pressing process at 1550 C under a pressure of 30 MPa for 1 h in flowing Ar atmosphere. The raw powders of titanium, niobium, silicon, aluminum and graphite were selected as the molar ratio of 2.94:0.06:0.95:0.05:2. The final solid solution can be denoted as (Ti0.98Nb0.02)3(Si0.95Al0.05) C2 according to the nominal apportions of the raw powders. The density of the as-synthesized sample was determined by the Archimedean method (ISO 18754), and the composition was identified using a step-scanning X-ray diffraction (Rigaku D/max-2400, Tokyo, Japan).
degreased in ethanol and finally cleaned in distilled water. Isothermal oxidation tests were conducted in Setsys 16/18 microbalance (SETRAM, Caluire, France). The coupons were suspended by a Pt wire in the vertical furnace, oxidized at 600 Ce800 C for 20 h in flowing air with the inlet rate of 30 ml/min. The mass gain of the sample was recorded continuously as a function of oxidation time by the personal computer. The cyclic oxidation tests were carried out in the tube furnace at 800 C in static air up to 100 h. The sample was kept at the required temperature for 20 h then cooled down to room temperature in air, which is defined as one cycle. Based on the measurement of sample weight, the mass change of the sample with cyclic times (or oxidation time) can be obtained. After oxidation tests, the samples were identified by X-ray diffraction in a D/max 2500PC diffractometer (Rigaku, Tokyo, Japan) with Cu Ka radiation. The surface and cross-section morphologies were observed by the SUPRA35 scanning electron microscope (SEM, LEO, Oberkochen, Germany), equipped with an energy-dispersive spectroscopy (EDS, INCA, Oxford Instrument, Oxford, U.K.) system.
2.4.
Results and discussion
Determination of basic properties
The bulk electrical conductivity was measured by the standard 4-probe D.C. method at room temperature. The thermal expansion coefficient conductivity (TEC) of the dense bulk material was determined by high-temperature dilatometer (Setaram Setsys 24, Caluire, France). Cylindrical samples with dimensions of F6 mm 8 mm were prepared by electricaldischarge machining method. The sample was heated from room temperature (RT) to 1373 K as a rate of 2 K/min in air atmosphere. After measurement, the thermal expansion data were corrected by subtracting the base line. The flexural strength of the samples was determined using four-point bending method. The measurement was conducted at 800 C by an electrical universal testing machine, with the dimensions of coupon samples 3 mm 4 mm 36 mm.
2.3.
High temperature electrical conductivity
Electrical conductivity of samples after isothermal oxidation at 700 C and 800 C for 100 h was measured using the 2-probe 4-point method in static air atmosphere at 700 C and 800 C, respectively. Pt-paste electrodes were applied on the oxidized sample surfaces with the Pt wire as the current and voltage probes. A constant current of 10 mA was applied by a Precision Programmable Current Source, and the corresponding voltage drop was recorded continuously by the FLUKE 8845A 6-1/2 Digit Programmable Multimeter.
3. 2.2.
1085
Oxidation tests
For oxidation tests, rectangular coupons with dimensions of 10 10 2 mm3 were cut from the as-synthesized material by electrical-discharge machining method. Prior to oxidation, the surfaces of the samples were grounded down to 2000 SiC paper, polished using 1 mm diamond paste, chamfered, then
3.1. Phase and basic properties of the as-synthesized material Fig. 1 shows the XRD pattern of the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. It can be seen, this material is only composed of Ti3SiC2, and no diffraction peak of Nb or Al containing compounds is detected. The insert in Fig. 1 shows the position of the XRD peak (101) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 and Ti3(Si0.95Al0.05)C2. It is clear that comparing with Ti3(Si0.95Al0.05) C2, the diffraction peak (101) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 shifts to low angle. Combined with the computational modeling of the crystal structure of Ti3SiC2, we can conclude that Nb atoms have been doped onto the sites of Ti. The determined density of the as-synthesized (Ti0.98Nb0.02)3 (Si0.95Al0.05)C2 is 4.50 0.01 g/cm3, and its relative density is 98.6%, hence the as-synthesized material is distinctly dense. Since the cell stacks of IT-SOFC will experience cyclic heating and cooling procedure, the thermal expansion behavior of interconnect is significant for the performance of cell stacks. The thermal expansion test was conducted, the result indicates that the expansion strain increases almost linearly as increasing the temperature from RT to 1100 C, and the thermal expansion coefficient of (Ti0.98Nb0.02)3(Si0.95Al0.05)
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Fig. 1 e The XRD pattern of the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. The insert shows the position of the diffraction peak (101) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 and Ti3(Si0.95Al0.05)C2.
3.2.
Oxidation resistance
3.2.1.
Oxidation kinetics
Fig. 2 shows the isothermal oxidation kinetics (mass gain per unit area as a function of oxidation time) of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 at 600 C, 700 C and 800 C in air for 20 h. It can be seen that the mass gain per unit area increases with increasing the oxidation temperature. By fitting the mass gain curves in Fig. 2, it can be known that the oxidation kinetics roughly obey the parabolic law at 600 Ce800 C, the parabolic rate constant is determined to be 0.45, 1.26 and 1.57 10 7 mg2/cm4 s at 600 C, 700 C and 800 C, respectively, which is in the same order of magnitude with some alloys, e.g., AL 453 and SUS 430 [2]. The cyclic oxidation at 800 C in air for 100 h presents that no spallation happened during the oxidation, and the mass gain curve obeys parabolic law. By fitting the cyclic oxidation kinetics, the parabolic rate constant is determined to be 1.67 10 7 mg2/cm4 s, which is in accordance with the data obtained during the isothermal oxidation at 800 C.
3.2.2. C2 is determined to be (9.0 0.2) 10 6/K, which is very close to the value of YSZ (10.5 10 6/K). Therefore, when YSZ is used as electrolyte in IT-SOFC, the thermal stress resulting from the mismatch of the TEC of interconnect with YSZ can be minimized. The flexural strength of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 is determined to be 328 MPa at 800 C, which is higher than that of LaCrO3. The electrical resistivity of (Ti0.98Nb0.02)3(Si0.95Al0.05) C2 at RT in air is 24 mU cm, which is even lower than that of some stainless steels. Meanwhile, the thermal conductivity and elastic modulus of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 have been determined, are 37 W m 1 K 1 and 330 10 GPa (RT) respectively. All measured data are summarized in Table 1. The good mechanical properties, high electrical and thermal conductivity are favorable for the application of the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 as interconnect.
Oxidation products
The XRD pattern of the oxide scale formed on (Ti0.98Nb0.02)3 (Si0.95Al0.05)C2 at 800 C in air for 100 h is shown in Fig. 3. Only rutile TiO2 crystal was identified in the oxide scale. It should be noted that the diffraction peaks associated with the substrate of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 are strong, suggesting that the oxide scale is quite thin, and the oxidation resistance of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 at 800 C is distinctively well. Meanwhile, the peaks from the substrate also can exhibit that the material is stable without decomposition under the oxidation atmosphere. After oxidation, the oxide scales formed on Nb doped and Nb free Ti3(Si0.95Al0.05)C2 were scraped off from the substrate and rubbed to be powders, thereafter the powders were heated to 800 C for 1 h to eliminate the thermal stress. To avoid experimental error, the XRD patterns were obtained from the same experiment rig. The diffraction peaks of rutile TiO2 (110) formed on the two substrates oxidized at 800 C are inserted in Fig. 3. The (110) peak of rutile
Table 1 e The determined values of relevant properties of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2. Property Density Thermal expansion coefficient Thermal conductivity Flexural strength Elastic modulus Electrical resistivity Parabolic oxidation rate constant in air
ASR after oxidation in air for 100 h at 700 C and 800 C
Measured value 4.50 0.01 g/cm3 (9.0 0.2) 10 6/K (RTe1000 C) 37 W m 1 K 1 328 MPa (800 C) 330 10 GPa (RT) 24 mU cm (RT) 0.45 10 7 (600 C), 1.26 10 7 (700 C), 1.57 10 7 (800 C) mg2/cm4 s 5 mU cm2 (700 C) 12 mU cm2 (800 C)
Fig. 2 e Weight gain per unit area vs. oxidation time for (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 during oxidation at 600 C, 700 C and 800 C in air for 20 h.
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Fig. 3 e XRD pattern of the oxide scale formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 800 C in air for 100 h. The insert shows the shift of TiO2 (110) on Ti3(Si0.95Al0.05)C2 after Nb doping under the same oxidation condition.
TiO2 formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 shifts to low angle, which suggests that the Nb has been doped into rutile lattice.
3.2.3.
Morphology of the oxide scale
The surface morphology of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 800 C in air for 100 h is presented in Fig. 4(a). It can be seen that the substrate grains are distinctly visible, which suggests that the oxide scale is very thin. The grain size of the oxides formed along the substrate grain boundaries is larger than that formed on the substrate grains. Based on the EDS analysis (not shown here), the oxide grains are rich in Ti, Si and O. The oxides formed on the surface of substrate grains are composed of Ti and Si with weight ratio of 3:1, however, this value becomes 6:1 for the oxides formed along the substrate grain boundaries. Fig. 4(b) and (c) illustrate the cross-section morphology and EDS line scanning taken across the oxide scale formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 at 800 C in air for 100 h, respectively. Fig. 4(b) shows that the oxide scale consists of monolithic oxide scale layer, and the thickness is about 2 mm. EDS results indicate that the scale is rich in Ti, Si and O, and trace of Al and Nb uniformly distribute in the scale. Again, Si was detected in the oxide scale by EDS. However, SiO2 has not been identified by XRD, Si may exist as amorphous SiO2. XPS analysis was further conducted, and identified that SiO2 existed in the oxide scale (The XPS spectrum has not been shown here). Therefore, it can be concluded that the monolithic layer of the oxide scale is composed of mixture of rutile TiO2 and amorphous SiO2. Moreover, Si Ka curve exhibits the same shape as the element of Ti (see Fig. 4(c)), which indicates the uniform distribution of mixture of rutile TiO2 and amorphous SiO2 in the oxide scale. The uniform mixture of rutile TiO2 and amorphous SiO2 is a benefit to the electrical conductivity of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 in oxidizing environment, because the uniform distribution can avoid the formation of continuous insulating SiO2 layer.
Fig. 4 e (a) Surface morphology, (b) cross-section and (c) EDS line scanning of the oxide scale on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 800 C in air for 100 h.
3.3.
High temperature electrical resistivity
The electrical resistance of one material after oxidized under the simulated cathode atmosphere is very important to assess its application as interconnect. Generally, the conductive substrate has a much lower electrical resistance than its oxide scale, the measured ASR of the oxidized sample mainly denotes the electrical resistance of its oxide scale. The ASR of
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the as-synthesized (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 oxidized at 700 C and 800 C in air for 100 h has been determined to be 5 and 12 mU cm2 at the corresponding oxidation temperature, respectively. The value obtained at 800 C is lower than the calculated value from the electrical resistivity of bulk rutile TiO2 at 800 C (120 U m), which may result from doping effect. TiO2 is a n-type semiconductor, and the native defects are oxygen vacancies, tri- and quadri-valent Ti interstitials. When penta-valent niobium oxide is used as dopant, donor-type addition will form and the concentration of charge carrier ([n]) will increase. Therefore, Nb doping can improve the high temperature electrical conductivity of Ti3(Si0.95Al0.05)C2 after oxidation in air atmosphere.
4.
Conclusion
A modified ternary layered compound of (Ti0.98Nb0.02)3(Si0.95Al0.05) C2 was evaluated as a new interconnect material for IT-SOFC. This material has low density, closer matched thermal expansion coefficient with YSZ, good mechanical properties and machinability. The compound also exhibits good oxidation resistance at 600 Ce800 C in air, its parabolic rate constant is 0.45, 1.26, 1.57 10 7 mg2/cm4 s at 600 C, 700 C, 800 C, respectively. The uniform oxide scale with the mixture of rutile TiO2 and amorphous SiO2 formed on (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 during oxidation. After the oxidation at 700 C and 800 C in air for 100 h, its area special resistance is 5 and 12 mU cm2 at the corresponding oxidation temperature, respectively. All evaluated properties of (Ti0.98Nb0.02)3(Si0.95Al0.05)C2 are favorable for its application as interconnect, further studies on its practical performance in ITSOFC are in progress.
Acknowledgment This work was supported by the National Science Foundation of China under Grant No.50771099.
references
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