Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell metallic interconnects manufactured through powder metallurgy

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Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell metallic interconnects manufactured through powder metallurgy € ¨ rk a, Alparslan Topcu b, Sultan Oztu € ¨ rk a, Omer € Bu¨lent Oztu Necati Cora c,* a

Department of Metallurgical and Materials Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey Department of Mechanical Engineering, Adana Science and Technology University, 01180, Adana, Turkey c Department of Mechanical Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey b

article info

abstract

Article history:

This study aimed to investigate oxidation, electrical and mechanical properties of solid

Received 14 August 2017

oxide fuel cell interconnects. To this goal, two different Crofer®22 interconnects samples

Received in revised form

were produced via different manufacturing routes (machining from bulk material, and

13 September 2017

powder metallurgy approach). The samples were characterized by scanning electron mi-

Accepted 15 January 2018

croscopy with energy dispersive spectroscopy (SEM-EDS), X-ray diffractions. Four-probe

Available online xxx

area specific resistance (ASR), bonding strength, leakage tests were also performed. The results indicated that interconnect sample manufactured through powder metallurgy

Keywords:

approach can be a reliable alternative to the one manufactured from commercially avail-

Metallic interconnect

able Crofer®22 alloy in bulk form.

Powder metallurgy

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Oxidation Area specific resistance Bonding strength Leakage test

Introduction Energy demand is endlessly growing along with the increase in population and industrialization. Solid oxide fuel cells (SOFCs) are energy conversion devices that can directly convert fuel into electricity without combustion [1e3]. Due to high operation temperatures (600e1000
C) of SOFCs, they do not need any precious catalyst and have several attractive features such as fuel flexibility, silent operation,

environmental friendliness and higher efficiencies that can go up to 80e90% through co-generation [4e10]. SOFCs basically consist of membrane electrode assembly (MEA) where electrochemical reaction occurs, interconnects that provide mechanical support and collects the current produced in the cells, and sealants that prevent air/fuel leakages in between MEA and interconnects [11e14]. Interconnects are used to provide distribution of the fuel and oxidant to the electrodes and collect the current as well as provide mechanical strength

* Corresponding author. € E-mail addresses: [email protected], [email protected] (O.N. Cora). https://doi.org/10.1016/j.ijhydene.2018.01.078 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. € ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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and integrity to the SOFC stack [15e20]. Ferritic stainless steels (FSS) have become the preferred choice for the interconnect material replacing more expensive ceramic counterparts in recent years [21e25]. FSS offer several advantages over the LaCrO3-based ceramic interconnects such as low material cost, good formability, higher mechanical, and electrical properties, good oxidation resistance as well as they have compatible coefficient of thermal expansion (TEC, 10.5e12.5  106 K1) with other components of SOFC stack [22,25e27]. Metallic interconnects are manufactured using the casting-rolling-(forging) route with subsequent machining (wire erosion approach) of the semi-finished products, traditionally [5,28e31]. Powder metallurgy (P/M) is another approach to manufacture metallic interconnects and has some advantages over traditional machining such as near-net shape production, fast and high production rates, elimination or reduction of machining step and scrap material [32e34]. Glatz et al. and Antepara et al. manufactured interconnect plates used in SOFCs by powder metallurgy method. Powder metallurgy approach was found much easier than traditional method [21,35e37]. Antepara et al. manufactured interconnects with porous structure and investigated the influence of porous structure on the SOFC efficiency. Same group of researchers studied also the interconnect plates manufactured using Crofer®22, ZMG232 and FeCr (70:30) metal powders and investigated the relation between electrical conductivity and porosity [38,39]. These interconnects was found compatible with electrolyte in terms of TEC. It was also indicated that near-net shape interconnects were obtained with relatively higher efficiency [40]. There are numerous studies in the literature regarding with the oxidation behavior of metallic interconnects in recent years. Different coating techniques have been employed to increase the oxidation resistance of metallic inrez et al., investigated the forterconnects [41e49]. Miguel-Pe mation of oxide scales on Crofer 22 APU, SS430 and Conicro 4023 W188 metallic interconnects at 800
C for 100 and 1000 h duration. They noted that spinel (Fe, Cr, Mn)3O4 and (Fe, Cr, Ni)3Co2O4 outer layers and a chromia (Cr2O3) inner layer were formed on the interconnects. Crofer 22 APU and Conicro 4023 W188 samples were determined to be more promising as metallic interconnects compared to SS430 due to their higher oxidation resistance [41]. Przybylski et al., coated a La0.6Sr0.4Co0.2Fe0.8O3 (LSCF48) film on the Crofer 22 APU steel using the screen-printing technique and compared to oxidation behavior of coated and pure Crofer 22 APU. The oxidation tests were performed in air, under isothermal and cyclic conditions at 800
C. The results indicated that the coated composite provides resistance against oxidation [42]. In another study, Hosseini et al., coated bare and pre-oxidized (100 h at 800
C) Crofer 22 APU samples' surfaces with CuFe2O4 spinel layers and samples were exposed to oxidation for 400 h at 800
C in air. The coating reduced the evaporation of Cr by 92% and 83% for bare þ coated and pre-oxidized þ coated alloy, respectively when compared to the uncoated substrate material [44]. Interconnects should have good electricity conductivity and low resistivity as it is responsible from collecting the current in cell stack. There are various electrical conductivity measurement techniques employed in literature including

two point-four wire probe and four point probe methods [8,44,47,50e63]. Tondo et al., coated AISI 430 stainless steel interconnects with different coatings (Y2O3, Y2O3/Co3O4 and Y2O3/Au composite films) and ASR values of coated samples were measured after oxidization in air at 800
C up to 500 h using four-point method. Results showed that ASR values of smaller than 100 mU cm2 were obtained for all samples and indicated that ASR values below the conventional acceptability limit for SOFC applications of 100 mU cm2 [50]. Safikhani and Aminfard, studied Fe-22Cr-0.5Mn FSS interconnect with addition of Ti and W at different contents. Electrical and oxidation behaviors of those samples were investigated and ASR measurements were performed using four-probe method. Results showed that the F.Ti1W2 sample with 3.98% wt of W and 0.23% wt of Ti is the best composition among the tested samples as the highest oxidation resistance and the lowest electrical resistance were obtained for this sample [52]. In another study by Molin et al., the surface of Crofer 22 APU was coated with Mn1.5Co1.5O4 coating by three methods (electrophoretic deposition (EPD), thermal co-evaporation and RF magnetron sputtering). They electrically tested the coated samples for 5000 h at 800
C and determined ASR values. After 5000 h oxidation, ASR values were determined to be in the 22e35e45 mU cm2 range for the EPD, RF sputtering and thermal co-evaporation methods, respectively. EPD method was noted to provide the best protection against Cr diffusion, contributing to the lowest ASR value and lowest increase rate [60]. Glass-ceramic sealants which bond with metallic interconnects are widely used for preventing the gas leakage. Due to their distinct thermal expansion coefficient (TEC) values, thermal stresses and cracks are generated during start-up and shutdown, and these consequently may lead to gaps at contacting surfaces, and failures. Contact loss at contacting surfaces causes gas leakages and performance loss in the cell. Therefore, an interconnect should have the sufficient strength not only to provide structural integrity but also to prevent from gas leakage. Tensile test method is the widely used to determine the bonding strength in literature [64e71]. Lin et al., developed a glass-ceramic sealant (GC-9: BaO-B2O3Al2O3-SiO2) and studied joint strength of Crofer 22 H/GC-9/ Crofer 22 H sandwich specimens using tensile test method [65,67]. Wang et al., composed a LSM/YSZ/LSM cell to reveal a correlation between the adhesion strength and the ohmic resistance at the electrolyte-cathode interface. The interfacial adhesion strength was measured by means of tensile test and the results showed that the ohmic area specific resistance decreased gradually with the increase of the adhesion strength at the interfaces [70]. The Weibull analysis is a common way to estimate the failure probability of materials [72]. This method and reliability curves are often used to define bonding strength results [69,73e79]. Sealing is one of the most important parameters influencing the fuel cell performance. The most common method is to determine the pressure drop between the gas inlet and outlet is leakage test [71,81e89]. The leakage tests are mostly performed at low temperatures and pressures. Zhang et al., carried out the leakage test, starting at 10 kPa pressure level and continued until 0,5 kPa at temperature range of 100e600
C [81]. In another study by Le et al., leakage tests

€ ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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Table 1 e Chemical composition of Crofer®22 APU powder. Element

Fe

Cr

Mn

Mo

Ni

Si

Ti

Nb

La

% (wt)

Bal.

22.8

0.44

0.1

0.03

0.5

0.2

0.1

0.1

more suitable for SOFC sealant applications [88]. On the other hand, there are some leakage studies in the literature which were performed at high pressure. For instance, Lin et al., tested a glass ceramic sealant (G6: BaO-B2O3-SiO2) at 650
C and 260 kPa under air conditions [80]. Different from existing literature, this study aimed for investigating the oxidation behaviour, area specific resistance, leakage and bonding strength of Crofer®22 metallic interconnect samples manufactured through powder metallurgy and machining from commercially available Crofer®22 alloy.

Experimental Material Fig. 1 e SEM image of Crofer®22 powder. were performed for mica based sealant at 800
C, nitrogen atmosphere and 15 kPa inlet pressure [82]. Wang et al., carried out series of studies on leakage [71,83e87]. Tiwari et al., prepared glass ceramic sealants (SZS: SrO-ZnO-SiO2)-based and various additives with B2O3, Al2O3, V2O5 and Cr2O3 by meltquench method and investigated that glasses desired TEC (9e11  106 K1) required for SOFC sealant. For leakage testing of the sealant samples, Crofer 22 APU/Sealant/Crofer 22 APU sandwiches were prepared by heating the assembly in a controlled manner at 950
C in air for 1 h and the extension tube was connected between vacuum system and sandwiches. Results showed that on the basis of thermo-physical and bonding properties, glasses having B2O3 seems to be

Crofer®22 APU powders acquired from ThyssenKrupp VDM Starck GmbH, Germany. Chemical composition and scanning electron microscope (SEM) image of Crofer®22 APU powders are given in Table 1, and Fig. 1, respectively. Powders were first compacted in a rigid die having dimensions of 30  30 mm at room temperature and 600 MPa nominal compaction pressure. The compaction test setup and the die used in current study are seen in Fig. 2. Then, sintering was performed at 1200
C for 1 h in a rectangular tube furnace in dry hydrogen atmosphere. Prior to hydrogen purge, the tube was flushed with nitrogen to remove residual oxygen in it. Sintered samples were machined to specific dimensions for using milling machine for tests. Other group of samples was prepared from commercially available bulk Crofer®22 alloy through machining.

Fig. 2 e Hydraulic press and die set used for sample manufacturing. € ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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Table 2 e Chemical composition of sealant used in the tests [80]. Element

SiO2

BaO

B2O3

%

45.06

32.4

22.54

Fabrication of the glass-ceramic sealant G6 borosilicate-based glass-ceramic sealant was used in bonding strength and leakage tests at elevated temperatures [80]. Chemical composition of glass-ceramic is given in Table 2. Glass-ceramic powders were mixed with tape cast solution 50% in weight, and slurry was ball-milled for 12 h and mixed using magnetic mixer for 3 h. Then, the slurry was tape cast with a blade gap of 100 mm via tape casting equipment. An

amount of glass-ceramic tapes were stacked together and laminated under pressure of 30 MPa for 4 min. The final thickness was 1 mm for all the sealants prepared. The laminates were cut into small rectangles (15 mm  5 mm) for bonding strength test; and the samples with inner diameter of 15 mm, and outer diameter of 25 for the fuel leakage tests via laser machining. Manufacturing procedure for sealant manufacturing is illustrated in Fig. 3.

Oxidation experiments For the oxidation behaviour, only P/M samples were tested as oxidation performance of commercial Crofer22® alloy in bulk form has been reported in several studies [38,41,42,44]. To this goal, the P/M samples were prepared in 10 mm  10 mm x 1 mm dimensions. Oxidation experiments were performed in a temperature controlled furnace at 800
C, and in air

Fig. 3 e Procedure for sealant manufacturing.

Fig. 4 e Schematic of four-point probe ASR measurement setup. € ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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Fig. 5 e (a) Schematic of bonding test specimen, (b) Heat treatment procedure employed.

Fig. 6 e (a) Fuel leakage test apparatus and ring-shaped sample, (b) Laser welded sample to the test apparatus.

Fig. 7 e Heat treatment applied in leakage test system. € ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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Fig. 8 e X-ray patterns of P/M sample after different oxidation times at 800
C in air.

atmosphere for 3, 100 and 300 h. The oxide scales formed were characterized by X-ray diffraction (XRD) at room temperature using Rigaku D diffractometer equipped with Cu Ka radiation and the patterns were recorded in 2q using a step size of 0.02
in the 10e80
range. Cross-section of the oxide scale was also characterized using a Zeiss scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) analyser.

Electrical measurements Electrical performance of machined and P/M samples was evaluated by standard four-point probe ASR test at constant temperature of 800
C for 525 h in air atmosphere. The designed ASR measurement system is shown schematically in Fig. 4. In the measurement system, A-B-C-D plates made of Ag were in 30 mm  30 mm  1 mm dimensions while 1-2-3 plates were either machined and P/M Crofer®22 samples which were in 15 mm  30 mm  1 mm dimensions. Ag wires were fixed on A-B-C-D Ag plates and plates A and D were used as current supplies while plates B and C were used

Fig. 10 e EDS analysis of the surface of P/M sample after 300 h oxidation.

as current collectors. The prepared samples were placed into ceramic die in the temperature controlled furnace. A constant current of 100 mA was supplied to plates A and D via Ag wires by Keithley 2400 Sourcemeter and the voltage was measured from plates B and C using Keithley nanovoltmeter. Acquired voltage values were evaluated using Labview software.

Bonding strength tests Test specimens (15 mm  30 mm  5 mm) were prepared from both the P/M and the machined samples. Glass-ceramic sealant were prepared in 5 mm  15 mm  1 mm dimension to conform with interconnect specimens' surfaces. Schematic of test specimens and glass-ceramic sealants are shown in Fig. 5a. Crofer®22/sealant/Crofer®22 sandwich-like structure samples were prepared with paper tape to ensure the stability of samples. Then, the samples were placed in a temperature controlled furnace and same heat treatment was applied to both. Firstly, the temperature of the samples was increased up to 500
C and kept constant for 30 min to ensure the evaporation of binders. The samples were then heated till 1000
C; and bonding of interconnect samples with glass-ceramic

Fig. 9 e SEM images of the oxide scale surface for the P/M sample after oxidation in air (a) 100 h, (b) 300 h. € ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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Fig. 11 e SEM image and EDS analysis of the cross-section of P/M sample after 300 h oxidation.

sealant was provided. After that, the temperature was decreased to 800
C and waited for 10 h at this temperature. Finally, the temperature was decreased to room temperature. Applied heat treatment process is shown in Fig. 5b. Upon heat

treatment is completed, tensile tests were carried out with samples to determine the bonding strength. Shimadzu Autograph AG-IS universal test machine was used, and tests were performed at room temperature, with a loading rate of

€ ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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was carried out along 10 cycles, and pressure drop was measured after each thermal cycle. Heat treatment process for leakage tests is shown in Fig. 7.

Results and discussion Characterization of oxide scales (XRD, SEM and EDS)

Fig. 12 e ASR (mU.cm2) values of the P/M and machined samples at 800
C in air.

0.5 mm/min. Tensile tests were repeated ten times for each sample to address repeatability and statistical analysis.

Leakage tests Fuel leakage tests were performed for both types of samples. Ring-shaped samples with inner diameter of 15 mm, and outer diameter of 25 mm were fabricated through P/M method and machining (Fig. 6). The samples were then welded on the leakage test apparatus as shown in Fig. 6b. Rigid type of glassceramic sealant was manufactured at the same dimensions with ring-shaped interconnect sample. Electrolyte, glassceramic and leakage test apparatus system was placed in the temperature controlled furnace. Test system was heated until 850
C; and temperature was kept constant for 30 min, first. After that, the system was cooled down to 200
C with natural cooling and it was heated up to 800
C. This process

The surface and cross-section of the formed oxide scale on the P/M sample after the different oxidations times (3, 100 and 300 h) were analyzed using XRD, SEM and EDS to determine the morphology and composition of the microstructures. XRD patterns of the P/M sample are provided in Fig. 8 for nonoxidized, and oxidized at 800
C, in air atmosphere for 3 h, 100 h and 300 h cases. Fig. 8 shows the XRD analysis of P/M sample after oxidation in air for 3, 100 and 300 h. The scales formed on the sample surface were chromia (Cr2O3), a spinel (M3O4 (M ¼ Mn, Fe, Cr)) and a-Fe phases. Results are in agreement with the other oxidation studies reported for Crofer®22 APU steel (in bulk form) in the literature [41,42,44]. SEM images of P/M sample are given in Fig. 9 after oxidation (a) 100 h, (b) 300 h. As it can be noticed that the surface of the samples were coated with octahedral crystal particles intensively as the oxidation exposure time increases. EDS spectra of the oxide scale surface, on the other hand, is provided in Fig. 10. It is noted that, these crystal particles consists of mainly Cr, Mn, O along with Fe. As it was also reported by Hosseini et al., these particles are cubic chromium manganese spinel with certain amount of Fe atoms infused to lattice structure [44]. SEM image of the cross-section of P/M sample after 300 h oxidation was obtained to reveal the Cr2O3 scales and spinels and EDS analysis was performed (Fig. 11). SEM images revealed the formation of chromia layer under the spinel layer. It was also observed that the bonding between chromia and spinel layer is probably not strong. This may have

Fig. 13 e Fracture surface of interconnect samples (a) machined sample, (b) P/M sample. € ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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Fig. 14 e (a) Weibull distribution of tensile results of P/M and machining samples, (b) Reliability curves of P/M and machined samples.

resulted from the fact that thermal expansion coefficients of those layers significantly different [44].

ASR measurements results Fig. 12 depicts the ASR of both P/M and machined sample at 800
C in air atmosphere for 525 h. In the case of machined sample, ASR was measured as 22 mU cm2 at initial stage (after pre-oxidation time), and decreased slowly to 10 mU cm2 till 525 h. On the other hand, P/M sample's ASR values were measured at 40 mU cm2 levels at initial stage, then rapidly decreased to 15 mU cm2 levels; and it almost stabilized at 10 mU cm2 after 525 h. The ASR values were found to be compatible with other studies in the literature [61e63]. Though P/M sample's ASR was measured higher than machined sample's ASR at the initial stage, it was observed that ASR values were in close proximity after 400 h. It was concluded that the ASR performance values obtained after 400 h are acceptable for the target life-time of SOFCs which is usually between 15 000e40 000 h [90,91].

Bonding strength results Tensile tests were carried out for both the P/M and the machined samples. P/M sample exhibited failure not only at contact interface but also glass-ceramic interface. This was interpreted as tensile strength of the P/M sample is comparable with the strength of glass-ceramic layers. Sample machined from Crofer®22 commercial alloy, on the other hand, showed failure at glass-ceramic interface, only. A pair of specimens belonging to machined and P/M samples' fractured surfaces are provided in Fig. 13. Although glass-ceramic sealant material used was the same for both samples, it appears that the fracture behaviors of these samples (leaving from glass-ceramic material) are different. Bonding strength results were analyzed with Weibull distribution method, and a reliability analysis was performed. Weibull distribution results showed that samples fabricated through P/M approach were more consistent in terms of bonding strength (Fig. 14a). Reliability curves yielded that the P/M sample is more reliable at low tensile force while the sample machined from commercial alloy is preferrable at higher tensile forces (Fig. 14b).

Leakage test results Leakage tests results showed that, the pressure steadily decreased till 10th cycle for the machined sample while the inlet pressure for P/M sample was almost constant till 6th cycle. After tenth cycle, the pressure values were recorded as 34.39 and 34.85 kPa for machined and P/M samples, respectively (Fig. 15). Even though the difference is not significant yet the prolonged tests will probably results in significant pressure drops so the performance loses for the machined sample.

Conclusions

Fig. 15 e Outlet pressure values of P/M and machined samples.

Oxidation behaviour, electrical and mechanical properties of interconnects made of commercially available bulk Crofer®22 alloy through machining as well as through powder metallurgy (P/M) approach from powders were investigated.

€ ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078

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Oxidation test results revealed the Cr2O3, spinel (M3O4 (M ¼ Mn, Fe, Cr)) and a-Fe phases were available upon 300 h of oxidation exposure at 800
C. Even though the trends were different, ASR values measured after 525 h of oxidation exposure were same for both samples (10 mU cm2). Bonding strength analyses, on the other hand, showed that the P/M sample is reliable at low tensile force while the sample machined from commercial alloy is preferrable at higher tensile forces. From gas leakage of point of view, pressure decrease in P/M sample was found to be less than that for machined sample. As an overall conclusion, the interconnect sample fabricated through powder metallurgy approach is an sound alternative to the one produced through machining from bulk Crofer®22 alloy.

Acknowledgements The authors wish to acknowledge the financial support provided by the Scientific and Technological Research Council of Turkey (TUBITAK) through the project grant #114M502.

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€ ¨ rk B, et al., Oxidation, electrical and mechanical properties of Crofer®22 solid oxide fuel cell Please cite this article in press as: Oztu metallic interconnects manufactured through powder metallurgy, International Journal of Hydrogen Energy (2018), https://doi.org/ 10.1016/j.ijhydene.2018.01.078