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Exploring a novel ceramic (Ti,W)3SiC2 for interconnect of intermediate temperature solid oxide fuel cell
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 4 3 ( 2 0 1 8 ) 7 4 8 3 e7 4 9 1
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Exploring a novel ceramic (Ti,W)3SiC2 for interconnect of intermediate temperature solid oxide fuel cell Lili Zheng a,b,*, Qingsong Hua a, Xichao Li b,c,**, Meishuan Li b, Yuhai Qian b, Jingjun Xu b, Zuoqiang Dai a, Hongxin Zhang a, Tiezhu Zhang d, Junwei Wu e a
National Engineering Research Centre for Intelligent Electrical Vehicle Power System (Qingdao), College of Mechanical & Electronic Engineering, Qingdao University, Qingdao, 266071, China b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China c Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, China d Shandong University of Technology, Zibo, 255000, China e Department of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen, 518055, China
article info
abstract
Article history:
A solid solution (Ti,W)3SiC2 possessing good oxidation resistance and low area-specific
Received 3 January 2018
resistance (ASR) after oxidation has been synthesized by an in-situ hot pressing process.
Received in revised form
The oxidation rate constant at 800 C in air is 6.29 1014 g2 cm4 s1 for (Ti,W)3SiC2. The
14 February 2018
formed single-layer oxide is composed of W doped rutile TiO2 and amorphous SiO2. SiO2 is
Accepted 22 February 2018
evenly inlaid in the communicative body frame of TiO2. W doped in TiO2 mainly exists as
Available online 17 March 2018
W6þ. W doping not only hinders the outward diffusion of Ti by decreasing the concentration of native Ti interstitials in TiO2, but also restrains the inward diffusion of oxygen by
Keywords:
decreasing the concentration of O vacancies. Furthermore, W dopant in TiO2 enhances the
Ti3SiC2
electrical conductivity of TiO2 by increasing the concentration of semi-free electron.
W doping
Therefore, the low ASR of (Ti,W)3SiC2 after oxidation owes to high electrical conductivity of
Interconnect
TiO2 as well as the reduced thickness of oxide scale. All the results render (Ti1-xWx)3SiC2
Oxidation resistance
promising as interconnects for the intermediate temperature solid oxide fuel cell.
Area-specific resistance
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are promising candidate for future energy conversion equipment due to their advantages
of low production of pollutants, fuel flexibility and high efficiency [1e4]. Interconnect is the main component to build up the SOFC-stack, which is located between each individual cell in the cell-stacks. It plays two roles in the stacks, one is acting
* Corresponding author. National Engineering Research Centre for Intelligent Electrical Vehicle Power System (Qingdao), Qingdao University, 308 Ningxia Road, Qingdao, 266701, China. ** Corresponding author. Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, China. E-mail addresses: [email protected] (L. Zheng), [email protected] (X. Li). https://doi.org/10.1016/j.ijhydene.2018.02.162 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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as bipolar plate electrically connecting adjacent cells in the series; and the other is acting as physical separator of fuels in the anode and air or oxygen in the cathode [5e8]. During operation, interconnect realizes simultaneous dual atmosphere (wet reducing and oxidizing) exposure up to about 800 C. Therefore, interconnect materials must meet harsh requirements: good electrical conductivity; good oxidation resistance; suitable coefficient of thermal expand (CTE) with other cell components; adequate stability in term of dimension, microstructure, chemistry and phase at operating temperature in oxidizing and reduction environments; gas tight; and no reaction and interdiffusion with adjacent components. Developing a novel interconnect material which can meet all of the requirements is a challenge for the commercialization of SOFC. When SOFCs operates at the temperature range of 800e1000 C, LaCrO3 or the doped LaCrO3 are adopted as interconnects material [2]. However, with the development of electrolyte, the operating temperature of SOFC decreases to 600e800 C, which results in the insufficient electrical conductivity of LaCrO3 or doped LaCrO3 as interconnect. The descending operating temperature [7e9] makes it feasible for metallic interconnects to supplant LaCrO3 materials. Metallic interconnects are Ni-based, Cr-based and Fe-based alloys, which have many advantages, such as high electrical and thermal conductivity, low cost [5,8,10]. However, the oxidation resistance and superior electrical conductivity of these metallic interconnects mainly depend on the formation of Cr2O3, which can poison the cathode and cathode/electrolyte interface due to its vaporization, and then cause the performance degradation of SOFC [8,11e13]. Furthermore, the high thermal expansion coefficient of Ni-based and Cr-based alloys restricts their application, and the oxidation resistance of Febased alloy needs to be enhanced [14]. To overcome these problems, various kinds of alloys have been investigated [15e22], such as Crofer 22 APU, Hitachi K44M and FeCro. Moreover, many kinds of coatings [23e31] are designed and deposited on interconnect to block the diffusion of volatile Cr (VI) species. Although these efforts have certain effect, it is still a crucial issue for further preventing Cr evaporation, and coating takes the extra cost and complexity for preparation interconnect [14,32]. MAX phases are a group of layered ternary compounds with the general formula of Mnþ1AXn (M: early transition metal, A: IIIA or IVA element, X: C and/or N). They have attracted significant attention due to the combination of merits of both ceramics and metals. Ti3SiC2, one of the most typical MAXs, possesses unique properties, such as high electrical and thermal conductivity, good resistance to thermal shock below 1100 C, easy machinability, high modulus and fracture toughness [33,34]. More importantly, its thermal expansion coefficient (9.2 106 K1 (20e1000 C)) [33] matches with that of yttria stabilized zirconia (YSZ, 10.5 106 K1). All of the above aspects meet the requirements of interconnects. Ti3SiC2 is a potential material as interconnect for IT-SOFC. Previous works [35] exhibit that when Ti3SiC2 is oxidized at 600e800 C in air, its oxidation kinetics roughly follows parabolic law, and the formed oxide scales has a duplex structure with an outer layer of rutile-TiO2 (r-TiO2) and an inner layer of
mixture of r-TiO2 and amorphous SiO2 (a-SiO2). The oxidation rate constant of Ti3SiC2 (5.64 1013 g2/cm4 s) is higher than that of metallic interconnect, such as crofer 22 APU (1.71 1013 g2/cm4 s). What's more, the electrical conductivity of Ti3SiC2 after oxidation needs to be improved [35]. To resolve these problems, Nb and Ta doped Ti3SiC2 is designed and studied in the previous works [35e38]. The results revealed that Nb and Ta doping into the rutile TiO2 lattice in the formed oxide scale, and improved both the oxidation resistance and post electrical conductivity of Ti3SiC2 after oxidation. It is found that the one more charger of Ta5þ, and Nb5þ than Ti4þ is the key factor to play the doping effect. Therefore, it is inferred that W doping with two more charger of W6þ than Ti4þ will exhibit better effect than Nb and Ta, leading to the better performance of (Ti,W)3SiC2 as interconnect of SOFC. In this paper, the W doped Ti3SiC2 solid solution is successfully synthesized by an in-situ hot pressing process. The solid solution exhibits better oxidation resistance than that of the typical metallic interconnect, such as Crofer 22 APU, Ebrite, and Nb and Ta doped Ti3SiC2. The electrical conductivity of the W doped Ti3SiC2 after oxidation is suitable for the application of interconnect. Furthermore, the effect of W doping on improving oxidation behaviors and electrical property of Ti3SiC2 after oxidation is studied. The results reveal that the W doped Ti3SiC2 is a promising interconnects for SOFC.
Experimental procedure 2.5 at.% and 5 at.% W doped Ti3SiC2 bulk was fabricated by insitu hot pressing process, with the elements molar ratio of 2.925:0.075:1:2 and 2.85:0.15:1:2 for Ti:W:Si:C, respectively. The above mixed powders were compacted uniaxially under 5 MPa in a graphite die with a diameter of 50 mm, and then hot pressed at 1580 C under 30 MPa for approximately 1 h in a flowing Ar atmosphere. The electrical conductivity of the W doped Ti3SiC2 bulk was measured by 4-point method, with the sample size of 4 4 36 mm3. The thermal conductivity was measured by the United States FlashlineTM-5000 Thermal Properties Analyzer, and Non-steady state method was adopted. The sample size was F12.7 2 mm3. The coefficient of thermal expansion of (Ti,W)3SiC2 was tested on the Setsys-24 thermomechanical analyzer (Setaram, Caluire, France). The sample size was F6 8 mm3. The test temperature range was from room temperature to 1273 K. Oxidation test was carried on tubular resistance furnace at 800 C in air atmosphere. The test sample of 10 10 2 mm3 was cut from the as-synthesized bulks by electrical discharge method. Prior to oxidation, the surfaces of the samples were grounded down to 2000 SiC paper, chamfered, polished using 1 mm diamond paste, and then degreased in ethanol and distilled water. The sample was suspended in a silica crucible, and then heated up with furnace temperature increasing. In the whole oxidation period, after every 100 h isothermal oxidation the samples were cooled down rapidly to room temperature in air, and weighed using a microbalance with the accuracy of 1 105 g, and then put into the hot furnace
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Fig. 1 e X-ray diffraction pattern of the as-synthesized (Ti0.95W0.05)3SiC2, (Ti0.925W0.025)3SiC2 and Ti3SiC2.
again. At the same time, the silica crucibles were also weighted after every cycle to evaluate the spallation resistance of the oxide scale on samples. The total oxidation time for studying the oxidation kinetics and the ASR of (Ti,W)3SiC2 was 700 h. The oxidation products 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 of the oxide scale 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. The cross-section sample was prepared through: fixed the crosssection by resin, grounded and then polished the crosssection. Before SEM examination, a thin layer of Au was prepared on the surface of the oxidized sample by vacuum deposition. The microstructure of the oxide scale was further observed by a 300-kV Tecnai G 2 F30 high-resolution transmission electron microscope (HRTEM, FEI, Eindhoven, The Netherlands),
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equipped with an energy-dispersive spectroscopy (EDS) detector in the STEM system and a post-column Gatan imaging filter system, which was used for high-resolution TEM, selective area electron diffraction (SAED), and electron energy loss spectroscopy (EELS) analysis. The chemical composition of the oxide scale formed on W doped Ti3SiC2 was determined by X-ray photoelectron spectroscopy (XPS). XPS measurements were performed on the sample surfaces using a surface analysis system ESCALAB250 equipped with an Mg radiation (Ka ¼ 1253.6 eV). All binding energies used in this study were referenced to the binding energy of the carbon C1s peak at 284.6 eV. The peak fitting was performed using a Gaussian/Lorentzian peak shape after a Shirley background removal with XPSPEAK 4.1 software. The electrical resistance of coupons after oxidation was determined by the 2-probe 4-point method in air atmosphere. The ASR, depending on both the electrical resistivity and thickness of the oxide scale, was adopted to evaluate the electrical resistance of the oxidized coupons. A constant current of 10 mA was supplied by a Precision Programmable Current Source (YL4012), and the corresponding voltage drop was recorded continuously by the FLUKE 8845A 6-1/2 Digit Programmable Multimeter. The ASR was calculated according to the following equation [18], ASR ¼ 1=2 RS
(1)
Where R is the electrical resistance calculated from the Ohm's law, and S is the effective area of conductivity (Pt paste area). The activation energy for electrical conductivity was calculated from the ASR data according to the following equation [39,40], ASR=T ¼ A expðEa=kTÞ
(2)
Results and discussion Characterization of the synthesized ceramic Fig. 1 presents the X-ray diffraction pattern of the assynthesized ceramics. For comparison, the result for Ti3SiC2
Fig. 2 e (a) Oxidation kinetics and (b) square of mass gain per unit area vs. oxidation time of (Ti0.95W0.05)3SiC2, (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU [20] during oxidation at 800 C in air.
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Table 1 e Parabolic oxidation rate constants of (Ti0.95W0.05)3SiC2, (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2, Crofer 22 APU and Ti3SiC2 at 800 C in air. Materials (Ti0.95W0.05)3SiC2 (Ti0.95Nb0.05)3SiC2 (Ti0.95Ta0.05)3SiC2 Crofer 22 APU Ti3SiC2
kp (g2/cm4 s) 6.29 8.86 7.33 1.71 5.64
1014 1014 1014 1013 1013
is also presented. It can be see that the main phase for (Ti0.975W0.025)3SiC2 and (Ti0.95W0.05)3SiC2 is Ti3SiC2. A little of TiC appears as second phase when the doping content of W reaches 5 at.%, and a little of (W,Ti)Si2 appears after W doping. The density of the as-synthesized (Ti0.975W0.025)3SiC2 and (Ti0.95W0.05)3SiC2 bulk are 4.68 and 4.81 g/cm3 determined by Archimedes method, and the relative density are 99.12% and 99.18%, respectively. The electrical and thermal conductivities are determined to be 2.5 106 S m1 and 27.5 W m1 K1, which well meet the requirement of interconnect. The thermal expansion coefficient of (Ti0.95W0.05)3SiC2 is determined to be 9.0 106/K, which is similar to that of YSZ.
Oxidation behaviors of (Ti,W)3SiC2 Oxidation kinetics The oxidation experiments of (Ti0.975W0.025)3SiC2 and (Ti0.95W0.05)3SiC2 are carried out at 800 C in air for 100 h. After oxidation, the mass gain per unit area of is approximately 0.25 mg/cm2 for (Ti0.975W0.025)3SiC2, while it is only 0.17 mg/ cm2 for (Ti0.95W0.05)3SiC2. Hence, it is known that the oxidation resistance of (Ti0.95W0.05)3SiC2 is better than that of (Ti0.975W0.025)3SiC2. Therefore, the (Ti0.95W0.05)3SiC2 is adopted for further investigation in the later sections. Fig. 2 shows the mass gains per unit area of (Ti0.95W0.05)3SiC2 during the oxidation at 800 C in air up to 700 h. For comparison, the results of (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU [20] are also exhibited in Fig. 2. In the whole oxidation period, the mass gain of (Ti0.95W0.05)3SiC2 increases
continuously, and meanwhile, no weight gain for the crucible after oxidation test, suggesting no spallation of oxide scale happens. Compared to (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU, (Ti0.95W0.05)3SiC2 shows the lowest weight gain, indicating (Ti0.95W0.05)3SiC2 possesses better oxidation resistance than that of Nb or Ta doped Ti3SiC2 and Crofer 22 APU. Fig. 2b presents the square of mass gain per unit area vs. oxidation time of (Ti0.95W0.05)3SiC2. It can be seen the curve is approximately straight, which reflects that the oxidation kinetics of (Ti0.95W0.05)3SiC2 well obeys the parabolic law, described by: ðDw=AÞ2 ¼ kp $t
(3)
where, Dw is the sample weight gain, and A is the samples area, kp is the oxidation rate constant, t is the oxidation time. Thereby, it can be know that the ions diffusion through the oxide scale is the oxidation control step for (Ti0.95W0.05)3SiC2 during the oxidation process. For comparison, the results of (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU are also exhibited in Fig. 2b. By fitting these plots, the oxidation rate constants of (Ti0.95W0.05)3SiC2, (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU are obtained and listed in Table 1. Meanwhile, for comparison, the oxidation rate constants of Ti3SiC2 is also listed in Table 1. From the above results, it can be known that the oxidation rate constant of (Ti0.95W0.05)3SiC2 decreases approximately one order of magnitude than that of Ti3SiC2, indicating that W doping can greatly increases the oxidation resistance of Ti3SiC2. Between these solid solutions, W doping plays a much greater role in increasing the oxidation resistance of Ti3SiC2 than that of the Nb and Ta dopant.
Phase components and morphologies of the oxide scales Fig. 3 presents the XRD patterns of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h. The determined oxide scales on (Ti0.95W0.05)3SiC2 is rutile TiO2 (r-TiO2). No W oxide is identified in the oxide products. The diffraction peaks originated from substrate are strong, suggesting that the formed oxide scale is very thin. Meanwhile, the Pt diffraction peaks appear which comes from the Pt paste on the sample surface for electrical conductivity test. For comparison, Fig. 3b
Fig. 3 e (a) XRD pattern of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h; (b) Comparison of (101) peak position of TiO2 formed on (Ti0.95W0.05)3SiC2 and Ti3SiC2.
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Fig. 4 e TEM image of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h and the EDS results on the crystal.
exhibits the (110) peak of r-TiO2 formed on (Ti0.95W0.05)3SiC2 and Ti3SiC2, respectively. It can be clearly seen that the (110) peak of r-TiO2 formed on (Ti0.95W0.05)3SiC2 shifts to lower angle compared with that on Ti3SiC2, and the deviation value is about 0.045 . Hence, it can be concluded that the inter-planar distance of (110) is enlarged, indicating that W has doped into r-TiO2 lattice. Fig. 4 shows the TEM image of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h and the EDS results on the crystal. Combined with the XRD and this EDS results, it is known that the crystal where the EDS point marked is the r-TiO2. W exists in the crystal with the atomic ratio of W to (Ti þ W) is about 7 at.%, which can prove that W has doped into r-TiO2 lattice. The value of (110) peak shift for r-TiO2 formed on (Ti0.95Nb0.05)3SiC2 and (Ti0.95Ta0.05)3SiC2 compared to Ti3SiC2 is 0.072 and 0.020 , respectively. The ionic radius for Ti4þ, W6þ, W4þ, Nb5þ and Ta5þ are 0.068 nm, 0.062 nm, 0.070 nm, 0.069 nm and 0.068 nm, respectively. Ion with bigger radius dopes into TiO2 lattice can increase the crystal lattice. Meanwhile, highly charged ion doped into TiO2 lattice can also induce the crystal lattice increase. Therefore, it needs to further study the valence state of W doped in r-TiO2 lattice to find the reason of (110) crystal lattice increase. Fig. 5 illustrates the XPS fine spectra of W 4f in the oxide scale formed on (Ti0.95W0.05)3SiC2. The XPS fine spectra of W 4f is composed of four peaks, and the binding energy are 37.25 eV, 35.25 eV, 33.67 eV and 31.49 eV, referring as W1, W2, W3, and W4, respectively. Table 2 lists the peak analysis for W 4f of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation test. The binding energy of W1 and W2 peak are assigned to W 4f5/2 and W 4f7/2, respectively, which are attributed to the presence of Wþ6 [41]. The binding energy of W3 and W4 peak are W 4f5/2 and W 4f7/2, respectively, which are attributed to the presence of Wþ4 [42]. The peak area integral corresponding to Wþ6 is stronger than those to Wþ4, which reflects that W doped in rutile TiO2 mainly exists as Wþ6. Therefore, it can be known that the increased crystal lattice constant of (110) surface is the results of combinatory factors. Wþ4 doping increases the crystal lattice constant; and Wþ6 doping decreases the lattice constant, while the addition of two chargers increases the value. Fig. 6a exhibits the surface morphologies of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h. It can be seen that
Fig. 5 e XPS fine spectra of W 4f in the oxide scale formed on oxidized (Ti0.95W0.05)3SiC2.
Table 2 e Peak analysis for W 4f in the oxide scale formed on the (Ti0.95W0.05)3SiC2 solid solution. Peak W1 W2 W3 W4
Binding energy (eV) 37.25 35.25 33.67 31.49
Proposed species W W W W
4f5/2 for 4f7/2 for 4f5/2 for 4f7/2 for
W6þ W6þ W4þ W4þ
the oxide scale surface is flat, and no peeling phenomenon appears. The contour of the grains of (Ti0.95W0.05)3SiC2 substrate is clearly demonstrated, which indicates that the formed oxide scale is very thin. In the whole surface of the sample, some abnormal areas is marked in rectangular in the Fig. 6a, which is the oxidation products of (W,Ti)Si2 impurity identified by EDS (for abbreviate, the EDS results is not shown here). Fig. 6b illustrates the cross-section morphology and corresponding EDS line-scanning profiles along the red line for the oxide scales on (Ti0.95W0.05)3SiC2. It can be seen that the formed oxide scale consists of a single layer, and the thickness
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Fig. 6 e (a) Surface morphology, (b) Cross-section morphology and EDS line scanning taken across the oxide scales, and (c) HRTEM image of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h.
is approximately 3 mm. No void or spallation is observed at the oxide/substrate interface. The oxide scale has good adherence with the substrate after seven thermal cycles (100 h/per cycle) from 800 C to room temperature in air. The EDS line scanning results exhibit that the oxide scale is rich in Ti, Si, O and trace of W. W approximately uniformly distributes in the oxide scale. As SiO2 is not detected by XRD, it can be deduced that SiO2 exists as amorphous state. The formation of amorphous SiO2 has already been found during oxidation of Ti3SiC2 [43e45], Ti5Si3 [46], MoSi2 [47], and Si3N4 [48]. Therefore, the monolithic oxide scale on (Ti0.95W0.05)3SiC2 is composed of a mixture of crystallized r-TiO2 and amorphous SiO2 (a-SiO2). The crystallized r-TiO2 is doped by W. Previous work [35] revealed that the oxide scale formed on Ti3SiC2 after oxidation at 800 C in air is comprised by duplex layer of a TiO2 outer layer and a TiO2þ SiO2 inner layer. Therefore, it can draw the conclusion that after W doping in Ti3SiC2, the oxide scale structure is changed from the duplex layer to a single layer of TiO2þ SiO2 mixture. A detailed analysis of the distribution of crystallized TiO2 and amorphous SiO2 in the oxide scale of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h is carried out by highresolution TEM. In Fig. 6c, the substance in red coil is amorphous SiO2, where the atomic arrangement is disorganized. In contrast, the role area with atomic arrangement is rutile TiO2. It can be seen that the amorphous SiO2 is approximately evenly distributed in the oxide scale, and the amorphous SiO2 region is inlaid in the crystallized TiO2 region. Crystalline TiO2 region forms a communicative body frame.
Electrical conductivity Like metallic interconnect, the electrical conductivity of MAX phase is very high even at 800 C [49]. And after W doping, the electrical conductivity of (Ti0.95W0.05)3SiC2 is 2 106 S m1; while for metallic interconnect, the electrical conductivity is in the order of magnitude of 104 S m1. Therefore, after oxidation the electrical resistance of (Ti0.95W0.05)3SiC2 mainly comes from the formed oxide scale on the surface. Usually, the ASR is adopted to reflect the electrical conductivity of oxide scale formed on interconnect after oxidation. Fig. 7 presents the ASR of (Ti0.95W0.05)3SiC2 vs. temperature after oxidized at 800 C in air for 700 h. The ASR of (Ti0.95W0.05)3SiC2 at 800 C is about 55 mU cm2, which is smaller than that of undoped Ti3SiC2 [35,36]. With the temperature increasing, the ASR decreases smoothly. For comparison, the plot for the ASR of Ebrite vs. temperature after oxidation at 800 C in air for 500 h [40] is also presented in Fig. 7. It can be seen that the slope for the curve of (Ti0.95W0.05)3SiC2 is smaller than that of Ebrite, indicating that the ASR of Ebrite is more affected by temperature. Although oxidized for longer time, the ASR of (Ti0.95W0.05)3SiC2 is smaller than that of Ebrite at the temperature range of 500e680 C. Meanwhile the slope of the curve reflects the electrical conductivity activation energy. The smaller slope indicates that the electrical conductivity activation energy of oxide scale on (Ti0.95W0.05)3SiC2 is smaller than that oxide scale on the Ebrite. That to say, the electrical conductivity activation energy of the W doped r-TiO2 is smaller than that of Cr2O3. By fitting the curves, the activation
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oxygen vacancies and Ti interstitials in the n-type r-TiO2 will take place. The following reactions are: 00 00
VTi þ Ti4þ i 4TiTi
(5)
00 00
VTi þ 2V0 4nil
(6)
Ti4þ i
means Ti interstitial with four positive charges, TiTi where means Ti occupies the Ti site, while VO is oxygen vacancy. Moreover, the defect reaction (7) may also happens after reaction of (6), 0 1 O o 4Vo þ 2e þ =2O
2
(7)
The equilibrium constant of the reaction (7) can be expressed as: 1 K ¼ Vo $n2 $po22 =
Fig. 7 e Temperature dependence of ASR of (Ti0.95W0.05)3SiC2 and Ebrite metallic interconnect [40] after oxidation at 800 C in air.
energy for (Ti0.95W0.05)3SiC2 and Ebrite are calculated as 0.22 and 0.48 ev, respectively.
Mechanism of W doping for increasing the oxidation resistance and electrical conductivity From the above results, it is known that the oxide scale formed on (Ti0.95W0.05)3SiC2 are composed of mixed r-TiO2 and a-SiO2, while a-SiO2 approximately evenly inlaid in communicative r-TiO2 body frame. W doped in r-TiO2 mainly exists as W6þ. The oxidation process of (Ti0.95W0.05)3SiC2 is controlled by ion diffusion trough the oxide scale. The oxidation resistance of SiO2 is better than that of TiO2, it is deduced that the ions transportation through TiO2 is the main path for the oxidation process. Meanwhile, the electrical resistance of SiO2 (2 105 U cm at 900 C) is higher than that of TiO2 (1 102 U cm at 900 C) [50,51]. Thereby, the ASR depends on the transport of electrons in r-TiO2 rather than in SiO2. Hence, it is reasonable to propose that W doping mainly plays its role through affecting the transmission of both the ions and electrons in r-TiO2 to increase the oxidation resistance and electrical conductivity simultaneously after oxidation. TiO2 is an n-type semiconductor, and the native defects are oxygen vacancies, tri- and quadri-valent Ti interstitials. When W dopes in r-TiO2 as W6þ, the donation energy level of W will form in TiO2 energy band. Based on the defect chemistry theory, the following defect chemistry reaction will occur in air atmosphere [52e54]: TiO2
00 00
þ 6O 2WO3 ! 2WTi o þVTi
(4)
where WTi means W occupying the Ti site with two positive 00 00 charges, Oxo means O occupying the O site and VTi means Ti vacancy. The equilibrium (4) presents that the vacancy of Ti is generated in r-TiO2 after W doping. When Ti vacancy gener00 00 ated, the recombination reaction of VTi with native defects of
(8)
where, n denotes the concentration of semi-free electrons, the brackets [] denotes the concentration of defects in site. Po2 denotes the oxygen pressure. Therefore, it can be known that the W doping can decrease the concentration of Ti interstitial and O vacancy, resulting in the increased concentration of semi-free electrons. The oxidation of Ti3SiC2 is controlled by the outward diffusion of Ti and inward diffusion of O [35]. The previous section show that no TiO2 out-layer forms during oxidation of (Ti0.95W0.05)3SiC2, suggesting that the outward diffusion of Ti is restrained totally. In another words, W doping block off the outward diffusion of Ti by decreasing the concentration of native Ti interstitials in r-TiO2 lattice. At the same time, W doping also decreases the concentration of O vacancies in r-TiO2 lattice, which can hinder the inward diffusion flux of oxygen. The constrained diffusion of Ti and O will lead to the decreased oxidation rate. Hence, after W doping, the oxide scale structure of Ti3SiC2 changes from a duplex oxide scale to a single mixture layer, and the oxidation resistance of Ti3SiC2 increased remarkably. At the same time, from the formula of (8) it is known that after W doping, the concentration of semi-free electrons increases, which is resulted from the decreased concentration O vacancy. When the concentration of semi-free electrons increases, the electrical conductivity of the W doped r-TiO2 will increase. Therefore, it can be concluded that the lower ASR of (Ti0.95W0.05)3SiC2 after oxidation benefits from two aspects effect: On one hand, W doping changes the oxides structure from a dual-layer to a TiO2þSiO2 mixed single-layer structure. As a result, the continuous electrical conductive network of TiO2 forms. Furthermore, W doping enhances the electrical conductivity of r-TiO2. Hence the conductivity of the form oxide is significantly enhanced. On the other hand, the oxidation resistance of (Ti0.95W0.05)3SiC2 is improved by the W doping, and a thinner oxide scale forms after oxidation, leading to the reduced total electrical resistance of oxide scale.
Conclusion A novel ceramic (Ti1-xWx)3SiC2 (x ¼ 0.025, 0.05) is explored for interconnect of intermediate temperature solid oxide fuel cell.
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The characteristic, oxidation resistance and electrical conductivity of (Ti1-xWx)3SiC2 has been investigated. The following conclusions are drawn: (1) The oxidation kinetics of (Ti0.95W0.05)3SiC2 follows parabolic raw, while its oxidation rate constant at 800 C is 6.29 1014 g2 cm4 s1, which is lower than that of Crofer 22 APU, Ti3SiC2, (Ti0.975W0.025)3SiC2, (Ti0.95Nb0.05)3SiC2 and (Ti0.95Ta0.05)3SiC2. The single oxide layer formed on (Ti0.95W0.05)3SiC2 is composed of W doped rutile TiO2 and amorphous SiO2. (2) W doping in r-TiO2 not only hinder the outward diffusion of Ti, but also restrain the inward diffusion of oxygen, which leads to the enhanced oxidation resistance of Ti3SiC2 and the changed oxide layer structure from a double-layer to a single mixture layer. (3) The ASR of (Ti0.95W0.05)3SiC2 at 800 C after oxidation at 800 C in air for 700 h is 55 mU cm2, which is much lower than that of Ti3SiC2. The r-TiO2 doped with W6þ forms the conductive network in the oxide single layer. W doping increases the electrical conductivity of r-TiO2, as well as increase the oxidation resistance (decrease the oxide growth), which benefits the low ASR for (Ti0.95W0.05)3SiC2 after oxidation.
Acknowledgement Funding: This work was supported by the National Science Foundation of China under Grant No. 51571205, National Key Research and Development Program of China No. 2017YFB0102004, Shandong Province Key Research and Development Program No. 2017CXGC0502, Qingdao postdoctoral research project, and the Scientific Research Start-up Fund of Qingdao University No. 41117010089.
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Exploring a novel ceramic (Ti,W)3SiC2 for interconnect of intermediate temperature solid oxide fuel cell Lili Zheng a,b,*, Qingsong Hua a, Xichao Li b,c,**, Meishuan Li b, Yuhai Qian b, Jingjun Xu b, Zuoqiang Dai a, Hongxin Zhang a, Tiezhu Zhang d, Junwei Wu e a
National Engineering Research Centre for Intelligent Electrical Vehicle Power System (Qingdao), College of Mechanical & Electronic Engineering, Qingdao University, Qingdao, 266071, China b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China c Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, China d Shandong University of Technology, Zibo, 255000, China e Department of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen, 518055, China
article info
abstract
Article history:
A solid solution (Ti,W)3SiC2 possessing good oxidation resistance and low area-specific
Received 3 January 2018
resistance (ASR) after oxidation has been synthesized by an in-situ hot pressing process.
Received in revised form
The oxidation rate constant at 800 C in air is 6.29 1014 g2 cm4 s1 for (Ti,W)3SiC2. The
14 February 2018
formed single-layer oxide is composed of W doped rutile TiO2 and amorphous SiO2. SiO2 is
Accepted 22 February 2018
evenly inlaid in the communicative body frame of TiO2. W doped in TiO2 mainly exists as
Available online 17 March 2018
W6þ. W doping not only hinders the outward diffusion of Ti by decreasing the concentration of native Ti interstitials in TiO2, but also restrains the inward diffusion of oxygen by
Keywords:
decreasing the concentration of O vacancies. Furthermore, W dopant in TiO2 enhances the
Ti3SiC2
electrical conductivity of TiO2 by increasing the concentration of semi-free electron.
W doping
Therefore, the low ASR of (Ti,W)3SiC2 after oxidation owes to high electrical conductivity of
Interconnect
TiO2 as well as the reduced thickness of oxide scale. All the results render (Ti1-xWx)3SiC2
Oxidation resistance
promising as interconnects for the intermediate temperature solid oxide fuel cell.
Area-specific resistance
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) are promising candidate for future energy conversion equipment due to their advantages
of low production of pollutants, fuel flexibility and high efficiency [1e4]. Interconnect is the main component to build up the SOFC-stack, which is located between each individual cell in the cell-stacks. It plays two roles in the stacks, one is acting
* Corresponding author. National Engineering Research Centre for Intelligent Electrical Vehicle Power System (Qingdao), Qingdao University, 308 Ningxia Road, Qingdao, 266701, China. ** Corresponding author. Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101, China. E-mail addresses: [email protected] (L. Zheng), [email protected] (X. Li). https://doi.org/10.1016/j.ijhydene.2018.02.162 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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as bipolar plate electrically connecting adjacent cells in the series; and the other is acting as physical separator of fuels in the anode and air or oxygen in the cathode [5e8]. During operation, interconnect realizes simultaneous dual atmosphere (wet reducing and oxidizing) exposure up to about 800 C. Therefore, interconnect materials must meet harsh requirements: good electrical conductivity; good oxidation resistance; suitable coefficient of thermal expand (CTE) with other cell components; adequate stability in term of dimension, microstructure, chemistry and phase at operating temperature in oxidizing and reduction environments; gas tight; and no reaction and interdiffusion with adjacent components. Developing a novel interconnect material which can meet all of the requirements is a challenge for the commercialization of SOFC. When SOFCs operates at the temperature range of 800e1000 C, LaCrO3 or the doped LaCrO3 are adopted as interconnects material [2]. However, with the development of electrolyte, the operating temperature of SOFC decreases to 600e800 C, which results in the insufficient electrical conductivity of LaCrO3 or doped LaCrO3 as interconnect. The descending operating temperature [7e9] makes it feasible for metallic interconnects to supplant LaCrO3 materials. Metallic interconnects are Ni-based, Cr-based and Fe-based alloys, which have many advantages, such as high electrical and thermal conductivity, low cost [5,8,10]. However, the oxidation resistance and superior electrical conductivity of these metallic interconnects mainly depend on the formation of Cr2O3, which can poison the cathode and cathode/electrolyte interface due to its vaporization, and then cause the performance degradation of SOFC [8,11e13]. Furthermore, the high thermal expansion coefficient of Ni-based and Cr-based alloys restricts their application, and the oxidation resistance of Febased alloy needs to be enhanced [14]. To overcome these problems, various kinds of alloys have been investigated [15e22], such as Crofer 22 APU, Hitachi K44M and FeCro. Moreover, many kinds of coatings [23e31] are designed and deposited on interconnect to block the diffusion of volatile Cr (VI) species. Although these efforts have certain effect, it is still a crucial issue for further preventing Cr evaporation, and coating takes the extra cost and complexity for preparation interconnect [14,32]. MAX phases are a group of layered ternary compounds with the general formula of Mnþ1AXn (M: early transition metal, A: IIIA or IVA element, X: C and/or N). They have attracted significant attention due to the combination of merits of both ceramics and metals. Ti3SiC2, one of the most typical MAXs, possesses unique properties, such as high electrical and thermal conductivity, good resistance to thermal shock below 1100 C, easy machinability, high modulus and fracture toughness [33,34]. More importantly, its thermal expansion coefficient (9.2 106 K1 (20e1000 C)) [33] matches with that of yttria stabilized zirconia (YSZ, 10.5 106 K1). All of the above aspects meet the requirements of interconnects. Ti3SiC2 is a potential material as interconnect for IT-SOFC. Previous works [35] exhibit that when Ti3SiC2 is oxidized at 600e800 C in air, its oxidation kinetics roughly follows parabolic law, and the formed oxide scales has a duplex structure with an outer layer of rutile-TiO2 (r-TiO2) and an inner layer of
mixture of r-TiO2 and amorphous SiO2 (a-SiO2). The oxidation rate constant of Ti3SiC2 (5.64 1013 g2/cm4 s) is higher than that of metallic interconnect, such as crofer 22 APU (1.71 1013 g2/cm4 s). What's more, the electrical conductivity of Ti3SiC2 after oxidation needs to be improved [35]. To resolve these problems, Nb and Ta doped Ti3SiC2 is designed and studied in the previous works [35e38]. The results revealed that Nb and Ta doping into the rutile TiO2 lattice in the formed oxide scale, and improved both the oxidation resistance and post electrical conductivity of Ti3SiC2 after oxidation. It is found that the one more charger of Ta5þ, and Nb5þ than Ti4þ is the key factor to play the doping effect. Therefore, it is inferred that W doping with two more charger of W6þ than Ti4þ will exhibit better effect than Nb and Ta, leading to the better performance of (Ti,W)3SiC2 as interconnect of SOFC. In this paper, the W doped Ti3SiC2 solid solution is successfully synthesized by an in-situ hot pressing process. The solid solution exhibits better oxidation resistance than that of the typical metallic interconnect, such as Crofer 22 APU, Ebrite, and Nb and Ta doped Ti3SiC2. The electrical conductivity of the W doped Ti3SiC2 after oxidation is suitable for the application of interconnect. Furthermore, the effect of W doping on improving oxidation behaviors and electrical property of Ti3SiC2 after oxidation is studied. The results reveal that the W doped Ti3SiC2 is a promising interconnects for SOFC.
Experimental procedure 2.5 at.% and 5 at.% W doped Ti3SiC2 bulk was fabricated by insitu hot pressing process, with the elements molar ratio of 2.925:0.075:1:2 and 2.85:0.15:1:2 for Ti:W:Si:C, respectively. The above mixed powders were compacted uniaxially under 5 MPa in a graphite die with a diameter of 50 mm, and then hot pressed at 1580 C under 30 MPa for approximately 1 h in a flowing Ar atmosphere. The electrical conductivity of the W doped Ti3SiC2 bulk was measured by 4-point method, with the sample size of 4 4 36 mm3. The thermal conductivity was measured by the United States FlashlineTM-5000 Thermal Properties Analyzer, and Non-steady state method was adopted. The sample size was F12.7 2 mm3. The coefficient of thermal expansion of (Ti,W)3SiC2 was tested on the Setsys-24 thermomechanical analyzer (Setaram, Caluire, France). The sample size was F6 8 mm3. The test temperature range was from room temperature to 1273 K. Oxidation test was carried on tubular resistance furnace at 800 C in air atmosphere. The test sample of 10 10 2 mm3 was cut from the as-synthesized bulks by electrical discharge method. Prior to oxidation, the surfaces of the samples were grounded down to 2000 SiC paper, chamfered, polished using 1 mm diamond paste, and then degreased in ethanol and distilled water. The sample was suspended in a silica crucible, and then heated up with furnace temperature increasing. In the whole oxidation period, after every 100 h isothermal oxidation the samples were cooled down rapidly to room temperature in air, and weighed using a microbalance with the accuracy of 1 105 g, and then put into the hot furnace
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 4 3 ( 2 0 1 8 ) 7 4 8 3 e7 4 9 1
Fig. 1 e X-ray diffraction pattern of the as-synthesized (Ti0.95W0.05)3SiC2, (Ti0.925W0.025)3SiC2 and Ti3SiC2.
again. At the same time, the silica crucibles were also weighted after every cycle to evaluate the spallation resistance of the oxide scale on samples. The total oxidation time for studying the oxidation kinetics and the ASR of (Ti,W)3SiC2 was 700 h. The oxidation products 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 of the oxide scale 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. The cross-section sample was prepared through: fixed the crosssection by resin, grounded and then polished the crosssection. Before SEM examination, a thin layer of Au was prepared on the surface of the oxidized sample by vacuum deposition. The microstructure of the oxide scale was further observed by a 300-kV Tecnai G 2 F30 high-resolution transmission electron microscope (HRTEM, FEI, Eindhoven, The Netherlands),
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equipped with an energy-dispersive spectroscopy (EDS) detector in the STEM system and a post-column Gatan imaging filter system, which was used for high-resolution TEM, selective area electron diffraction (SAED), and electron energy loss spectroscopy (EELS) analysis. The chemical composition of the oxide scale formed on W doped Ti3SiC2 was determined by X-ray photoelectron spectroscopy (XPS). XPS measurements were performed on the sample surfaces using a surface analysis system ESCALAB250 equipped with an Mg radiation (Ka ¼ 1253.6 eV). All binding energies used in this study were referenced to the binding energy of the carbon C1s peak at 284.6 eV. The peak fitting was performed using a Gaussian/Lorentzian peak shape after a Shirley background removal with XPSPEAK 4.1 software. The electrical resistance of coupons after oxidation was determined by the 2-probe 4-point method in air atmosphere. The ASR, depending on both the electrical resistivity and thickness of the oxide scale, was adopted to evaluate the electrical resistance of the oxidized coupons. A constant current of 10 mA was supplied by a Precision Programmable Current Source (YL4012), and the corresponding voltage drop was recorded continuously by the FLUKE 8845A 6-1/2 Digit Programmable Multimeter. The ASR was calculated according to the following equation [18], ASR ¼ 1=2 RS
(1)
Where R is the electrical resistance calculated from the Ohm's law, and S is the effective area of conductivity (Pt paste area). The activation energy for electrical conductivity was calculated from the ASR data according to the following equation [39,40], ASR=T ¼ A expðEa=kTÞ
(2)
Results and discussion Characterization of the synthesized ceramic Fig. 1 presents the X-ray diffraction pattern of the assynthesized ceramics. For comparison, the result for Ti3SiC2
Fig. 2 e (a) Oxidation kinetics and (b) square of mass gain per unit area vs. oxidation time of (Ti0.95W0.05)3SiC2, (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU [20] during oxidation at 800 C in air.
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Table 1 e Parabolic oxidation rate constants of (Ti0.95W0.05)3SiC2, (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2, Crofer 22 APU and Ti3SiC2 at 800 C in air. Materials (Ti0.95W0.05)3SiC2 (Ti0.95Nb0.05)3SiC2 (Ti0.95Ta0.05)3SiC2 Crofer 22 APU Ti3SiC2
kp (g2/cm4 s) 6.29 8.86 7.33 1.71 5.64
1014 1014 1014 1013 1013
is also presented. It can be see that the main phase for (Ti0.975W0.025)3SiC2 and (Ti0.95W0.05)3SiC2 is Ti3SiC2. A little of TiC appears as second phase when the doping content of W reaches 5 at.%, and a little of (W,Ti)Si2 appears after W doping. The density of the as-synthesized (Ti0.975W0.025)3SiC2 and (Ti0.95W0.05)3SiC2 bulk are 4.68 and 4.81 g/cm3 determined by Archimedes method, and the relative density are 99.12% and 99.18%, respectively. The electrical and thermal conductivities are determined to be 2.5 106 S m1 and 27.5 W m1 K1, which well meet the requirement of interconnect. The thermal expansion coefficient of (Ti0.95W0.05)3SiC2 is determined to be 9.0 106/K, which is similar to that of YSZ.
Oxidation behaviors of (Ti,W)3SiC2 Oxidation kinetics The oxidation experiments of (Ti0.975W0.025)3SiC2 and (Ti0.95W0.05)3SiC2 are carried out at 800 C in air for 100 h. After oxidation, the mass gain per unit area of is approximately 0.25 mg/cm2 for (Ti0.975W0.025)3SiC2, while it is only 0.17 mg/ cm2 for (Ti0.95W0.05)3SiC2. Hence, it is known that the oxidation resistance of (Ti0.95W0.05)3SiC2 is better than that of (Ti0.975W0.025)3SiC2. Therefore, the (Ti0.95W0.05)3SiC2 is adopted for further investigation in the later sections. Fig. 2 shows the mass gains per unit area of (Ti0.95W0.05)3SiC2 during the oxidation at 800 C in air up to 700 h. For comparison, the results of (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU [20] are also exhibited in Fig. 2. In the whole oxidation period, the mass gain of (Ti0.95W0.05)3SiC2 increases
continuously, and meanwhile, no weight gain for the crucible after oxidation test, suggesting no spallation of oxide scale happens. Compared to (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU, (Ti0.95W0.05)3SiC2 shows the lowest weight gain, indicating (Ti0.95W0.05)3SiC2 possesses better oxidation resistance than that of Nb or Ta doped Ti3SiC2 and Crofer 22 APU. Fig. 2b presents the square of mass gain per unit area vs. oxidation time of (Ti0.95W0.05)3SiC2. It can be seen the curve is approximately straight, which reflects that the oxidation kinetics of (Ti0.95W0.05)3SiC2 well obeys the parabolic law, described by: ðDw=AÞ2 ¼ kp $t
(3)
where, Dw is the sample weight gain, and A is the samples area, kp is the oxidation rate constant, t is the oxidation time. Thereby, it can be know that the ions diffusion through the oxide scale is the oxidation control step for (Ti0.95W0.05)3SiC2 during the oxidation process. For comparison, the results of (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU are also exhibited in Fig. 2b. By fitting these plots, the oxidation rate constants of (Ti0.95W0.05)3SiC2, (Ti0.95Nb0.05)3SiC2, (Ti0.95Ta0.05)3SiC2 and Crofer 22 APU are obtained and listed in Table 1. Meanwhile, for comparison, the oxidation rate constants of Ti3SiC2 is also listed in Table 1. From the above results, it can be known that the oxidation rate constant of (Ti0.95W0.05)3SiC2 decreases approximately one order of magnitude than that of Ti3SiC2, indicating that W doping can greatly increases the oxidation resistance of Ti3SiC2. Between these solid solutions, W doping plays a much greater role in increasing the oxidation resistance of Ti3SiC2 than that of the Nb and Ta dopant.
Phase components and morphologies of the oxide scales Fig. 3 presents the XRD patterns of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h. The determined oxide scales on (Ti0.95W0.05)3SiC2 is rutile TiO2 (r-TiO2). No W oxide is identified in the oxide products. The diffraction peaks originated from substrate are strong, suggesting that the formed oxide scale is very thin. Meanwhile, the Pt diffraction peaks appear which comes from the Pt paste on the sample surface for electrical conductivity test. For comparison, Fig. 3b
Fig. 3 e (a) XRD pattern of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h; (b) Comparison of (101) peak position of TiO2 formed on (Ti0.95W0.05)3SiC2 and Ti3SiC2.
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Fig. 4 e TEM image of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h and the EDS results on the crystal.
exhibits the (110) peak of r-TiO2 formed on (Ti0.95W0.05)3SiC2 and Ti3SiC2, respectively. It can be clearly seen that the (110) peak of r-TiO2 formed on (Ti0.95W0.05)3SiC2 shifts to lower angle compared with that on Ti3SiC2, and the deviation value is about 0.045 . Hence, it can be concluded that the inter-planar distance of (110) is enlarged, indicating that W has doped into r-TiO2 lattice. Fig. 4 shows the TEM image of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h and the EDS results on the crystal. Combined with the XRD and this EDS results, it is known that the crystal where the EDS point marked is the r-TiO2. W exists in the crystal with the atomic ratio of W to (Ti þ W) is about 7 at.%, which can prove that W has doped into r-TiO2 lattice. The value of (110) peak shift for r-TiO2 formed on (Ti0.95Nb0.05)3SiC2 and (Ti0.95Ta0.05)3SiC2 compared to Ti3SiC2 is 0.072 and 0.020 , respectively. The ionic radius for Ti4þ, W6þ, W4þ, Nb5þ and Ta5þ are 0.068 nm, 0.062 nm, 0.070 nm, 0.069 nm and 0.068 nm, respectively. Ion with bigger radius dopes into TiO2 lattice can increase the crystal lattice. Meanwhile, highly charged ion doped into TiO2 lattice can also induce the crystal lattice increase. Therefore, it needs to further study the valence state of W doped in r-TiO2 lattice to find the reason of (110) crystal lattice increase. Fig. 5 illustrates the XPS fine spectra of W 4f in the oxide scale formed on (Ti0.95W0.05)3SiC2. The XPS fine spectra of W 4f is composed of four peaks, and the binding energy are 37.25 eV, 35.25 eV, 33.67 eV and 31.49 eV, referring as W1, W2, W3, and W4, respectively. Table 2 lists the peak analysis for W 4f of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation test. The binding energy of W1 and W2 peak are assigned to W 4f5/2 and W 4f7/2, respectively, which are attributed to the presence of Wþ6 [41]. The binding energy of W3 and W4 peak are W 4f5/2 and W 4f7/2, respectively, which are attributed to the presence of Wþ4 [42]. The peak area integral corresponding to Wþ6 is stronger than those to Wþ4, which reflects that W doped in rutile TiO2 mainly exists as Wþ6. Therefore, it can be known that the increased crystal lattice constant of (110) surface is the results of combinatory factors. Wþ4 doping increases the crystal lattice constant; and Wþ6 doping decreases the lattice constant, while the addition of two chargers increases the value. Fig. 6a exhibits the surface morphologies of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h. It can be seen that
Fig. 5 e XPS fine spectra of W 4f in the oxide scale formed on oxidized (Ti0.95W0.05)3SiC2.
Table 2 e Peak analysis for W 4f in the oxide scale formed on the (Ti0.95W0.05)3SiC2 solid solution. Peak W1 W2 W3 W4
Binding energy (eV) 37.25 35.25 33.67 31.49
Proposed species W W W W
4f5/2 for 4f7/2 for 4f5/2 for 4f7/2 for
W6þ W6þ W4þ W4þ
the oxide scale surface is flat, and no peeling phenomenon appears. The contour of the grains of (Ti0.95W0.05)3SiC2 substrate is clearly demonstrated, which indicates that the formed oxide scale is very thin. In the whole surface of the sample, some abnormal areas is marked in rectangular in the Fig. 6a, which is the oxidation products of (W,Ti)Si2 impurity identified by EDS (for abbreviate, the EDS results is not shown here). Fig. 6b illustrates the cross-section morphology and corresponding EDS line-scanning profiles along the red line for the oxide scales on (Ti0.95W0.05)3SiC2. It can be seen that the formed oxide scale consists of a single layer, and the thickness
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Fig. 6 e (a) Surface morphology, (b) Cross-section morphology and EDS line scanning taken across the oxide scales, and (c) HRTEM image of the oxide scale formed on (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h.
is approximately 3 mm. No void or spallation is observed at the oxide/substrate interface. The oxide scale has good adherence with the substrate after seven thermal cycles (100 h/per cycle) from 800 C to room temperature in air. The EDS line scanning results exhibit that the oxide scale is rich in Ti, Si, O and trace of W. W approximately uniformly distributes in the oxide scale. As SiO2 is not detected by XRD, it can be deduced that SiO2 exists as amorphous state. The formation of amorphous SiO2 has already been found during oxidation of Ti3SiC2 [43e45], Ti5Si3 [46], MoSi2 [47], and Si3N4 [48]. Therefore, the monolithic oxide scale on (Ti0.95W0.05)3SiC2 is composed of a mixture of crystallized r-TiO2 and amorphous SiO2 (a-SiO2). The crystallized r-TiO2 is doped by W. Previous work [35] revealed that the oxide scale formed on Ti3SiC2 after oxidation at 800 C in air is comprised by duplex layer of a TiO2 outer layer and a TiO2þ SiO2 inner layer. Therefore, it can draw the conclusion that after W doping in Ti3SiC2, the oxide scale structure is changed from the duplex layer to a single layer of TiO2þ SiO2 mixture. A detailed analysis of the distribution of crystallized TiO2 and amorphous SiO2 in the oxide scale of (Ti0.95W0.05)3SiC2 after oxidation at 800 C in air for 700 h is carried out by highresolution TEM. In Fig. 6c, the substance in red coil is amorphous SiO2, where the atomic arrangement is disorganized. In contrast, the role area with atomic arrangement is rutile TiO2. It can be seen that the amorphous SiO2 is approximately evenly distributed in the oxide scale, and the amorphous SiO2 region is inlaid in the crystallized TiO2 region. Crystalline TiO2 region forms a communicative body frame.
Electrical conductivity Like metallic interconnect, the electrical conductivity of MAX phase is very high even at 800 C [49]. And after W doping, the electrical conductivity of (Ti0.95W0.05)3SiC2 is 2 106 S m1; while for metallic interconnect, the electrical conductivity is in the order of magnitude of 104 S m1. Therefore, after oxidation the electrical resistance of (Ti0.95W0.05)3SiC2 mainly comes from the formed oxide scale on the surface. Usually, the ASR is adopted to reflect the electrical conductivity of oxide scale formed on interconnect after oxidation. Fig. 7 presents the ASR of (Ti0.95W0.05)3SiC2 vs. temperature after oxidized at 800 C in air for 700 h. The ASR of (Ti0.95W0.05)3SiC2 at 800 C is about 55 mU cm2, which is smaller than that of undoped Ti3SiC2 [35,36]. With the temperature increasing, the ASR decreases smoothly. For comparison, the plot for the ASR of Ebrite vs. temperature after oxidation at 800 C in air for 500 h [40] is also presented in Fig. 7. It can be seen that the slope for the curve of (Ti0.95W0.05)3SiC2 is smaller than that of Ebrite, indicating that the ASR of Ebrite is more affected by temperature. Although oxidized for longer time, the ASR of (Ti0.95W0.05)3SiC2 is smaller than that of Ebrite at the temperature range of 500e680 C. Meanwhile the slope of the curve reflects the electrical conductivity activation energy. The smaller slope indicates that the electrical conductivity activation energy of oxide scale on (Ti0.95W0.05)3SiC2 is smaller than that oxide scale on the Ebrite. That to say, the electrical conductivity activation energy of the W doped r-TiO2 is smaller than that of Cr2O3. By fitting the curves, the activation
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oxygen vacancies and Ti interstitials in the n-type r-TiO2 will take place. The following reactions are: 00 00
VTi þ Ti4þ i 4TiTi
(5)
00 00
VTi þ 2V0 4nil
(6)
Ti4þ i
means Ti interstitial with four positive charges, TiTi where means Ti occupies the Ti site, while VO is oxygen vacancy. Moreover, the defect reaction (7) may also happens after reaction of (6), 0 1 O o 4Vo þ 2e þ =2O
2
(7)
The equilibrium constant of the reaction (7) can be expressed as: 1 K ¼ Vo $n2 $po22 =
Fig. 7 e Temperature dependence of ASR of (Ti0.95W0.05)3SiC2 and Ebrite metallic interconnect [40] after oxidation at 800 C in air.
energy for (Ti0.95W0.05)3SiC2 and Ebrite are calculated as 0.22 and 0.48 ev, respectively.
Mechanism of W doping for increasing the oxidation resistance and electrical conductivity From the above results, it is known that the oxide scale formed on (Ti0.95W0.05)3SiC2 are composed of mixed r-TiO2 and a-SiO2, while a-SiO2 approximately evenly inlaid in communicative r-TiO2 body frame. W doped in r-TiO2 mainly exists as W6þ. The oxidation process of (Ti0.95W0.05)3SiC2 is controlled by ion diffusion trough the oxide scale. The oxidation resistance of SiO2 is better than that of TiO2, it is deduced that the ions transportation through TiO2 is the main path for the oxidation process. Meanwhile, the electrical resistance of SiO2 (2 105 U cm at 900 C) is higher than that of TiO2 (1 102 U cm at 900 C) [50,51]. Thereby, the ASR depends on the transport of electrons in r-TiO2 rather than in SiO2. Hence, it is reasonable to propose that W doping mainly plays its role through affecting the transmission of both the ions and electrons in r-TiO2 to increase the oxidation resistance and electrical conductivity simultaneously after oxidation. TiO2 is an n-type semiconductor, and the native defects are oxygen vacancies, tri- and quadri-valent Ti interstitials. When W dopes in r-TiO2 as W6þ, the donation energy level of W will form in TiO2 energy band. Based on the defect chemistry theory, the following defect chemistry reaction will occur in air atmosphere [52e54]: TiO2
00 00
þ 6O 2WO3 ! 2WTi o þVTi
(4)
where WTi means W occupying the Ti site with two positive 00 00 charges, Oxo means O occupying the O site and VTi means Ti vacancy. The equilibrium (4) presents that the vacancy of Ti is generated in r-TiO2 after W doping. When Ti vacancy gener00 00 ated, the recombination reaction of VTi with native defects of
(8)
where, n denotes the concentration of semi-free electrons, the brackets [] denotes the concentration of defects in site. Po2 denotes the oxygen pressure. Therefore, it can be known that the W doping can decrease the concentration of Ti interstitial and O vacancy, resulting in the increased concentration of semi-free electrons. The oxidation of Ti3SiC2 is controlled by the outward diffusion of Ti and inward diffusion of O [35]. The previous section show that no TiO2 out-layer forms during oxidation of (Ti0.95W0.05)3SiC2, suggesting that the outward diffusion of Ti is restrained totally. In another words, W doping block off the outward diffusion of Ti by decreasing the concentration of native Ti interstitials in r-TiO2 lattice. At the same time, W doping also decreases the concentration of O vacancies in r-TiO2 lattice, which can hinder the inward diffusion flux of oxygen. The constrained diffusion of Ti and O will lead to the decreased oxidation rate. Hence, after W doping, the oxide scale structure of Ti3SiC2 changes from a duplex oxide scale to a single mixture layer, and the oxidation resistance of Ti3SiC2 increased remarkably. At the same time, from the formula of (8) it is known that after W doping, the concentration of semi-free electrons increases, which is resulted from the decreased concentration O vacancy. When the concentration of semi-free electrons increases, the electrical conductivity of the W doped r-TiO2 will increase. Therefore, it can be concluded that the lower ASR of (Ti0.95W0.05)3SiC2 after oxidation benefits from two aspects effect: On one hand, W doping changes the oxides structure from a dual-layer to a TiO2þSiO2 mixed single-layer structure. As a result, the continuous electrical conductive network of TiO2 forms. Furthermore, W doping enhances the electrical conductivity of r-TiO2. Hence the conductivity of the form oxide is significantly enhanced. On the other hand, the oxidation resistance of (Ti0.95W0.05)3SiC2 is improved by the W doping, and a thinner oxide scale forms after oxidation, leading to the reduced total electrical resistance of oxide scale.
Conclusion A novel ceramic (Ti1-xWx)3SiC2 (x ¼ 0.025, 0.05) is explored for interconnect of intermediate temperature solid oxide fuel cell.
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The characteristic, oxidation resistance and electrical conductivity of (Ti1-xWx)3SiC2 has been investigated. The following conclusions are drawn: (1) The oxidation kinetics of (Ti0.95W0.05)3SiC2 follows parabolic raw, while its oxidation rate constant at 800 C is 6.29 1014 g2 cm4 s1, which is lower than that of Crofer 22 APU, Ti3SiC2, (Ti0.975W0.025)3SiC2, (Ti0.95Nb0.05)3SiC2 and (Ti0.95Ta0.05)3SiC2. The single oxide layer formed on (Ti0.95W0.05)3SiC2 is composed of W doped rutile TiO2 and amorphous SiO2. (2) W doping in r-TiO2 not only hinder the outward diffusion of Ti, but also restrain the inward diffusion of oxygen, which leads to the enhanced oxidation resistance of Ti3SiC2 and the changed oxide layer structure from a double-layer to a single mixture layer. (3) The ASR of (Ti0.95W0.05)3SiC2 at 800 C after oxidation at 800 C in air for 700 h is 55 mU cm2, which is much lower than that of Ti3SiC2. The r-TiO2 doped with W6þ forms the conductive network in the oxide single layer. W doping increases the electrical conductivity of r-TiO2, as well as increase the oxidation resistance (decrease the oxide growth), which benefits the low ASR for (Ti0.95W0.05)3SiC2 after oxidation.
Acknowledgement Funding: This work was supported by the National Science Foundation of China under Grant No. 51571205, National Key Research and Development Program of China No. 2017YFB0102004, Shandong Province Key Research and Development Program No. 2017CXGC0502, Qingdao postdoctoral research project, and the Scientific Research Start-up Fund of Qingdao University No. 41117010089.
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