JMEPEG (2018) 27:3276–3289 The Author(s) https://doi.org/10.1007/s11665-018-3422-7 1059-9495/$19.00 Synthesis, Processing and Properties of Calcium- and Nickel-Doped Yttrium Chromates(III) Y Ca Cr Ni O (x = 0-0.3) and Studies on Their 0.8 0.2 1x x 3 Potential Application as Coatings for SOFC Interconnects M. Stygar, W. Tejchman, J. Da˛ browa, A. Kruk, and T. Brylewski (Submitted February 15, 2018; in revised form April 22, 2018; published online May 29, 2018) In the present study, a calcium- and nickel-doped yttrium chromates (YCCN)-based, conductive–protective layers for metallic interconnects used in the intermediate-temperature solid oxide fuel cells (IT-SOFCs) were investigated. Synthesis of Y Ca Cr Ni O (x = 0; 0.15 and 0.3) powders was performed using a 0.8 0.2 12x x 3 wet chemistry method with two different complexing agents: ethylenediaminetetraacetic acid and glycine. Based on the result of thermal analysis of obtained precursors, optimal conditions of the calcination process were determined. Powders were then milled, compacted and sintered at different temperatures using free sintering method, into series of dense, polycrystalline sinters. The use of glycine precursor allowed obtaining a single-phase material in all cases. Based on the electrical and sintering properties, the Y Ca Cr Ni O material was selected for further studies. It was deposited using cost-effective 0.8 0.2 0.85 0.15 3 screen-printing method on the Crofer 22APU ferritic stainless steel. To investigate properties and suitability of the resulting layer/steel system for IT-SOFCs applications, the high-temperature, dual-atmosphere studies were carried out for the ﬁrst time for ceramic/metallic system, in conditions as close as possible to actual working conditions of the fuel cell. The layer exhibited high stability and good protective properties. The area-speciﬁc resistance of the studied ceramic layer/metallic substrate composite was determined, with 2 2 the obtained value of 0.0366 X cm being within the arbitrary limit set for these materials (0.1 X cm ). The results show that the investigated materials are suitable for the projected application. stacks to multiply these parameters. Materials for interconnects Keywords doped yttrium chromite, interconnect, perovskite are chosen accordingly to very rigorous requirements such as sol–gel process, solid oxide fuel cell value of thermal expansion coefﬁcient compatible with those of the other cells components, gas tightness, high electrical and thermal conductivity, good mechanical properties in elevated temperatures and chemical stability in the working condition of 1. Introduction fuel cell. Due to the high-working temperatures of the cells, the ﬁrst choice for materials for interconnects was lanthanum In the era of growing demand of worldwide energy, the solid chromites materials such as LaCrO ; (La,Sr)CrO ; (La,Mg)- 3 3 oxide fuel cells (SOFCs) are considered to be highly promising CrO (Ref 1, 2). The main problems with these materials are devices that might become efﬁcient sources of electrical energy their low gas tightness, moderate mechanical properties and and heat in the future. Their main advantages are very high very high cost of production. The reduction of SOFC operating energy conversion efﬁciency and relatively low emissivity, temperatures to the 600-800 C range made it possible to which makes them very attractive for countries characterized by employ cost-effective metallic interconnects such as ferritic highly industrialized economy, such as Japan, Germany or stainless steel (FSS) (Ref 3), which are currently intensively Scandinavian countries. The efﬁciency of these devices strictly studied as potential interconnect materials for IT-SOFCs due to depends on the properties of the applied materials and their low cost, ease of production and favorable mechanical construction of the cells. Since singular fuel cell does not offer properties. FSS interconnects are exposed to the aggressive adequate voltage and current density, single cells are grouped in environment of both air (on the cathodic side) and hydrogen/ steam (on the anodic side) atmospheres in the mentioned Electronic supplementary material The online version of this temperature range, which severely deteriorates their electrical article (https://doi.org/10.1007/s11665-018-3422-7) contains supple and mechanical properties, due to the formation of a chromia mentary material, which is available to authorized users. scale with low electrical conductivity on the metallic surface. In order to limit the inﬂuence of this phenomenon, ceramic-based M. Stygar, J. Da ˛browa, and T. Brylewski, Faculty of Materials conductive–protective layers are used (Ref 4). Their main task Science and Ceramics, AGH University of Science and Technology, al. is to improve the electrical properties of the interconnects over Mickiewicza 30, 30-059 Krako´w, Poland; W. Tejchman, Faculty of Geography and Biology, Pedagogical University of Cracow, ul. the time of fuel cell action by both reducing the oxidation rate Podchora˛z _ych 2, 30-084 Krako´w, Poland; and A. Kruk, Faculty of of the metallic base and increasing the electrical conductivity of Applied IT, University of Information Technology and Management, the metal/ceramic system. Calcium yttrium chromate ul. Sucharskiego 2, 35-225 Rzeszow, Poland. Contact e-mail: (Y Ca CrO ; called YCC) and calcium–nickel-doped 0.8 0.2 3 email@example.com. 3276—Volume 27(7) July 2018 Journal of Materials Engineering and Performance yttrium chromate (Y Ca Cr Ni O , where x = 0.15 or 0.3; (referred to as the ‘‘glycine method’’), respectively. The 0.8 0.2 1x x 3 called YCCN0.15 or YCCN0.3) perovskites are considered as a procedures used to prepare the above-speciﬁed powders are one of the potential types of materials with regard to such illustrated in Fig. 1. Initial ingredients used for metal–organic application (Ref 4). They are characterized by high physico- precursors preparation were procured in the 99.99% purity chemical stability and adequate electrical conductivity in the grade from Sigma-Aldrich and Avantor PMP. operating temperatures of IT-SOFCs. The proposed composite The obtained precursors were studied with the use of DTA– of YCCNs layers and Crofer 22APU Ferritic Stainless Steel TG method (differential thermal analysis and thermogravime- base should allow obtaining suitable material for SOFC try—using NETZSCH STA 449 F3) to determine optimal interconnect, which would combine the advantages of both conditions of further thermal treatment. The samples were materials groups while minimizing their drawbacks. heated with the use of alumina crucible in a synthetic air 3 1 Based on the available literature data (Ref 5-15), it can be atmosphere (10 dm h ﬂow), with the heating rate of said that the most important factors inﬂuencing physicochem- 10 C min . The experimental temperature range was 22- ical properties of Y Ca CrO ,Y Ca Cr Ni O and 1050 C. 0.8 0.2 3 0.8 0.2 0.85 0.15 3 Y Ca Cr Ni O materials are applied methods of pow- After calcination procedure, which parameters were deter- 0.8 0.2 0.7 0.3 3 ders preparation and conditions of sintering process. In Table 1, mined based on the DTA–TG results and XRD measurements a short review of the methods used in synthesis of these (see section 3.1), powders were ﬁrstly dried in the room materials is presented. As it can be seen, the most commonly temperature and then formed into pellets, using the uniaxial applied method of synthesis of YCC and YCCN materials is the pressing method in a rigid form under the pressure of 100 MPa. one in which glycine is used as a complexing agent in the sol– The ﬁnal pressing was performed using cold isostatic pressing gel process. method (CIP) under the pressure of 250 MPa. After calcination, As indicated by the previously published studies, the upper milling and compacting the powders were sintered at a limit of doping of YCC materials with calcium is about 23- temperature of 1600 C for 1 h in air, and a number of dense, 24 at.%, as introduction of Ca in higher amounts into the polycrystalline sinters were obtained. They were in a form of structures leads to the decrease in chemical stability due to the pellets of 10 mm diameter and thickness varying from 2 to formation of additional CaCrO phase (Ref 5). At the same 3 mm. The inﬂuence of the composition of Y Ca Cr 4 0.8 0.2 1x time, the doping of Y Ca Cr Ni O with nickel can be Ni O sinters on their physicochemical properties was analyzed 0.8 0.2 1x x 3 x 3 done with the x values varying from 0 to 0.2. If the x is based on the results of XRD, SEM–EDS and electrical exceeding 0.2, additional oxide phases, namely Y O and NiO, conductivity measurements, with possible applications as 2 3 are observed (Ref 5, 6). conductive coatings considered to be the main criteria. In the current paper, a synthesis of YCC–YCCN com- pounds, with the doping amount of nickel being, respectively, 2.2 Preparation of the Coatings x = 0; 0.15; 0.3, was conducted by means of a wet chemistry The pastes for the screen-printing method were obtained method that utilized two different complexing agents: EDTA through mixing of the chosen ﬁne-grained perovskite powder and glycine. After optimization of sintering process, all with the organic carrier in a mass ratio of 5:1. As an organic materials were studied with respect to their electrical properties. carrier, a 5% solution of ethyl cellulose in terpineol was used. Inﬂuence of the excess amount of Ni was investigated as well. The prepared pastes then were homogenized and their viscosity Resulting materials were then studied in a form of coatings on was adjusted if necessary, through the content of organic the FSS substrates in the dual-atmosphere setup to simulate the support. actual conditions of the working fuel cell. After obtaining, the paste was deposited on both sides of the previously prepared Crofer 22APU sample (ﬁnal grinding with SiC paper grit 600; cleaning in the ultrasonic bath; cleaning 2. Experimental with acetone). The coating was deposited using manual screen printer, with the 42-mesh screen. Coated samples were dried for 2 h in 90 C and then To synthesize the ceramic powders of perovskite-type thermally annealed in the air atmosphere. The latter process structures, two different ‘‘soft chemical route’’ methods were was conducted in two stages. The ﬁrst one was conducted at selected. The biggest advantages of such approach are: control temperature of 600 C (heating rate 1.5 C min ), with its over the grain size and the purity of the multicomponent main goal being vaporization/burning of the organic solvent. systems, small variance of the grain sizes and high homogene- After 1 h of isothermal annealing, the samples were cooled ity of the ﬁnal product. down to room temperature at rate of 2 C min . During the The composite consisting of YCCNs layers and Crofer second stage, the samples were heated again in Ar-H -H O 2 2 22APU Ferritic Stainless Steel base was obtained using the mixture to 1000 C and kept in this temperature for 10 h. screen-printing method. [The chemical composition of the used Finally, they were cooled down to room temperature at a rate of steel is presented in Ref 16] The method was chosen based on 5 C min . its simplicity, universality and low cost in comparison with other methods. 2.3 Dual-Atmosphere Exposure: Experimental and Setup The studies of the physicochemical properties of the 2.1 Powder Synthesis and Sinter Preparation coating/substrate layered system were conducted in single The synthesis of Y Ca CrO ,Y Ca Cr Ni O 0.8 0.2 3 0.8 0.2 0.85 0.15 3 atmospheres, namely synthetic air and Ar-H -H O mixture, but 2 2 and Y Ca Cr Ni O powders was carried out using two 0.8 0.2 0.7 0.3 3 also in a dual Ar-H -H O/air atmosphere setup [shown 2 2 variants of a wet chemistry method with different chelating elsewhere (Ref 17, 18)], which consists of a reaction furnace, agents: EDTA (referred to as the ‘‘EDTA method’’) or glycine two quartz tubes (which allowed to create two different, Journal of Materials Engineering and Performance Volume 27(7) July 2018—3277 3278—Volume 27(7) July 2018 Journal of Materials Engineering and Performance Table 1 Summary on the methods of powders preparation and sintering conditions of various YCC and YCCN compounds (Ref 5-15) Compositions of perovskites Sintering Method of synthesis/ Authors Y Ca Cr Ni O temperature, C/time, h complexing agent Results/most important properties 0.8 0.2 12x x 3 Yoon et al. (Ref 5) x = 0, 0.05; 0.1; 0.15 1400-1700/12 Glycine Conductivity of dense sinters at 900 C in air increased from 10 to 34 S cm with x = 0.15 Yoon et al. (Ref 6, 7) x = 0; 0.05; 0.1; 0.15; 0.2; 0.3 1400-1700/12 Glycine Secondary phase formation of yttrium oxide and nickel oxide was observed with higher than 20% Ni-doping Li et al. (Ref 8) x = 0, 0.1; 0.2 1200-1300/4 Citric acid; EDTA Resistance of high frequency in electrochemical impedance (ethylenediaminetetraacetic spectroscopy (EIS) is found to be dependent on H content acid) Wang et al. (Ref 9)Y Ca CrO 1400/4 Citric acid; Pure orthorhombic perovskite phase; relative density of 96.6% 0.7 0.3 3 microwave-aided sol–gel process Li et al. (Ref 10) x = 0.1 1400/4 Citric acid; EDTA The obtained slurry was screen-printed onto both sides of YSZ or SSZ Paulik (Ref 11) x = 0 1600-1690/2-6 Glycine Fracture toughness measured by the indentation technique 1350-1675/2-5 decreased with increasing calcium substitution Armstrong x =0(Y Ca CrO ; y = 0-0.4) 1400-1650/5 Glycine Presence of CaCrO was observed in samples with 1y y 3 4 et al. (Ref 12) greater than 25 mol.% Ca substitution Stevenson x = 0.1 1200-1400/2 Glycine High purity with no detectable second phases et al. (Ref 13) (Y Ca Cr Ni O ; z = 0.18-0.25) 0.8 z 0.9 0.1 3 Jianyi (Ref 14) x = 0.15 1350-1450/2-6 Solid-state reaction Dense sinters 6 1 Seokgon (Ref 15) x = 0-0.2 (Y Ca Cr Ni O ) 1400/up to 10 Glycine TEC = 10.26 9 10 [C ] for x = 0.1 0.75 0.25 1x x 3 Fig. 1 Procedures used to synthesize YCCN powders via: (a) EDTA method and (b) glycine method separate atmospheres on both sides of the examined samples), term (200 h) measurements taken for coated/uncoated steels. gas ﬂowmeter system and a water vapor saturator. The most The electrical resistance of oxidized coated/uncoated steels is important experimental parameters for both types of investiga- usually measured in terms of its area-speciﬁc resistance (ASR), tions are presented in Table 2. which is deﬁned as the product of resistance and nominal contact surface area of the oxide and steel. Due to the 2.4 Experimental Methodology symmetrical design of the test, the area-speciﬁc resistance of the samples was calculated based on the obtained resistance The phase composition of the powders, bulk samples and values using the following formula: metal–ceramics layered systems was analyzed by means of the PANalytical XPert Pro PW 3710 XCelerator using CuK R A ASR ¼ ðEq 1Þ radiation, collected over a 2H range of 20-90 at scan rate of 0.008 and 80 s. count. Qualitative and quantitative analyses of where R—electrical resistance [X], and A—surface area of the samples compositions were conducted using XPert the Pt layer [cm ]. HighScore Plus 2.0, Crystal Impact—Match!2 and Crys- Samples for long-term ASR measurements were separately talDiffract 6.0.5. (with PDF-2 ICDD database) software. oxidized before the measurements for 50 h at temperature of Scanning electron microscopy (FEI Nova NanoSEM 200; 800 C [which was explained elsewhere (Ref 19)]. For this JEOL JSM-6610LV) coupled with an EDAX Genesis XM x- kind of examination, both sides of sinters and composite ray microanalysis system was used to examine the morphology systems were coated with Pt paste using screen-printing. The and chemical composition of the studied samples. The rheo- apparatus and the methods used for measuring electrical logical characterization of YCC-YCCN powders was per- resistance are described in Ref 20. The sinters were also formed based on the grain size distribution and BET speciﬁc investigated using electrochemical impedance spectroscopy surface area measurements (Brunauer–Emmett–Teller—using (EIS) method. In this case, the measurements were conducted Nova 1200e Quantachrome Instr., adsorption of nitrogen). The in the air atmosphere at temperatures varying from 25 to electrical conductivity of the sinters and electrical resistance of 800 C, using potentiostat–galvanostat Metrohm AutoLab type coated steel were measured using the DC 2-probe 4-point- HP PGSTAT302 N in a frequency range 1 mHz to 1 MHz. During 34401A multimeter with a constant current of 10 mA and measurement, the thermodynamic equilibrium was assumed, within temperature range: 500-800 C for sinters and coated/ with the values of resistance being constant in a function of uncoated steels, and, in addition, at 800 C in the case of long- time at given temperature. Journal of Materials Engineering and Performance Volume 27(7) July 2018—3279 Table 2 Experimental conditions of studies in both single and dual atmospheres Parameter Single atmosphere Dual atmosphere Substrate material Crofer 22APU [W.-No. 1.4760 X1CrTiLa22 —DIN EN standard; UNS S44535—ASTM standard)]; ThyssenKrupp VDM GmbH; Germany Substrate specimen Steel coupons in the form of Steel coupons in the form of disks; shape disks; ø 10 mm; 1 mm thickness ø 28 mm; 1 mm thickness Surface treatment Grinding with 100-600-grid SiC of substrate papers; ultrasonically degreasing and washing in acetone and ethanol Reaction temperature 800 C Exposure time 100; 250 h 250 h Atmosphere Ar-10%H -H O (ratio of partial pressures Ar-10%H -H O left side/right synthetic air 2 2 2 2 p(H /pH O) = 94/6) or (ratio of partial pressures p(H /pH O) = 94/6 2 2 2 2 Synthetic air (partial pressure of oxygen with partial pressure of oxygen = 0.21 atm) p(O ) = 0.21 atm) range of 800-1200 C, exhibiting single-phase structure. In the 3. Results and Discussion material with x = 0.3, additional Y O and NiO phases were 2 3 detected. In the case of materials obtained using EDTA method, 3.1 Optimization of Calcination Parameters the samples with x = 0.15 and 0.3 were found to be unstable. The results of the DTA–TG measurements are shown in The detailed discussion of these results is presented in section Fig. 2. Based on the shape of obtained TG curves (Fig. 2a), all 3.1. For further studies, powders calcinated in 1200 C organic precursors, obtained with the use of EDTA as chelating (materials for sintering) and 1000 C (materials for coatings) agents, decomposed into the desired compounds with overall were selected, with the latter value being dictated by reaching weight loss of approximately 90 wt.%. Small endothermic the limit of free sintering (Ref 6). effects suggest loss of water at the early stages of heating, while the presence of big, exothermic peaks could be related to the 3.2 Morphology Characterization and XRD Phase Structure oxidation of the organic precursors. The weight did not change Analysis any further in case of all examined samples, after occurrence of In order to obtain information on the shape and size of the last exothermic peak in the range of 300-340 C. grains and their degree of agglomeration in powders with a In the case of compounds obtained with the usage of glycine perovskite structure, observations of their morphology were (Fig. 2b), all samples were dried out for 24 h in 120 Cto made using scanning electron microscopy as shown in remove the water from the hydrated coordination complexes. Fig. 3. The DTA of these samples was conducted directly after the SEM micrographs of the Y Ca Cr Ni O (x = 0; 0.15; 0.8 0.2 x1 x 3 drying process, due to their highly hygroscopic properties. In 0.3) powders, obtained with the use of EDTA method and the temperature range of 153-180 C for each of the examined calcinated at 1200 C for 10 h, show the presence of two compounds, a single exothermal peak is observed, which different types of grains: spherical grains of diameter smaller position is strongly correlated with the drastic loss of mass than 1 lm, which exhibit tendency toward the formation of big, observed on the TG curve. However, it should be noted that the porous agglomerates of diameters of tens of micrometers, and loss of mass in each case continued up to a temperature of needle-like grains, most probably of b-CaCr O phase (Fig. 3a, 2 4 about 600 C. The total mass loss in the samples obtained by b, and c). On the other hand, the powders obtained with the use glycine method varies from 73 to 79%. of the glycine method and after similar thermal treatment as in Based on the DTA–TG results, optimal conditions of the previous case consisted only of spherical grains, with their thermal treatment for precursors obtained by both described diameter in the range of 0.29-1.33 lm (Fig. 3d, e, and f). What methods were found. For the precursor obtained by means of is more, in the powders obtained with the use of glycine a the glycine method, it was necessary to use temperatures in macroscopic change in morphology can be observed, which excess of 600 C. In the case of precursor obtained by EDTA indicates the start of the sintering process. method, the optimal thermal conditions were found to be higher In Table 3, the results of the BET speciﬁc surface area than 350 C. measurements, crystalline size based on the XRD results The choice of optimal calcination parameters was based on [estimated with the use of Scherrer equation (Ref 21)] and the the result of XRD phase composition studies of samples size of grains estimated from the SEM observations are calcinated for 10 h in temperatures: 800, 900, 1000, 1100 and summarized, for materials obtained by both EDTA and glycine 1200 C. Materials with the Ni content x = 0 and 0.15 obtained methods and calcinated at 1200 C for 10 h in air. via glycine method were found to be stable in the temperature 3280—Volume 27(7) July 2018 Journal of Materials Engineering and Performance Fig. 2 DTA–TG curves obtained for precursors synthesized via: (a) EDTA method and (b) glycine method Fig. 3 SEM micrographs of Y Ca Cr Ni O powders synthesized and calcinated at 1200 C using: the EDTA method—for (a) x = 0, (b) 0.8 0.2 1x x 3 x = 0.15, (c) = 0.3, and the glycine method—for (d) x = 0, (e) x = 0.15, (f) = 0.3 (mag. 10009) The calculated crystallite size with a minimum of 20 measurements of crystallite sizes, especially for the EDTA reﬂections assigned to the YCrO phase for each sample is in method, indicated the deviation from the spherical shape of the range from 34 to 50 nm, with the typical size for the the crystallites. majority of measurements being around 48 nm. The rela- The morphologies of the sinters obtained from powders tively high degree of variation in the value of single prepared by means of both EDTA and glycine methods and Journal of Materials Engineering and Performance Volume 27(7) July 2018—3281 subjected to 1 h of thermal treatment in air at 1600 C are 4CaCrO þ 2Cr O ¼ 4ðÞ a; b CaCr O þ 3O ðEq 2Þ 4 2 3 2 4 2 presented in Fig. 4. As it can be seen, in the case of sinters synthesized with EDTA method, fraction of grains of consid- Both CaCr O phases, which presence is unfavorable from 2 4 erably larger size was present, which is especially evident in the point of view of electrical conductivity (especially the b Fig. 4(b). This may be an indication that more than one phase phase), decomposed during sintering in 1600 C. In the case of occurred within the system. The sinters obtained using glycine the compound undoped by nickel, the decomposition/desolu- method exhibited completely different morphologies in com- tion of CaCr O phases was practically complete. In the case of 2 4 parison with the previously described polycrystalline materials compound with nickel content of x = 0.15, a partial decompo- (Fig. 4d, e, and f). sition was observed, while for the compound with x = 0.3 no Independent of the nominal chemical compositions, all visible changes were recorded. This situation is especially materials obtained using the glycine method had well- interesting, as in the temperature range of 1570-1600 Ca developed grains of characteristic isometric shape. What is phase transformation of CaCr O from orthorhombic (b)to 2 4 more, in contrast to the previous group of sinters, in this case tetragonal (a) phase is reported (Ref 22, 23). only a few inter-agglomerate pores were observed. On the The occurrence of additional phases in compounds synthe- surface of the Y Ca Cr Ni O sinter (Fig. 4f), additional 0.8 0.2 0.7 0.3 3 sized using the EDTA method is rather surprising, considering irregular crystals (probably NiO phase) were sporadically the number of successful synthesis of other similar perovskites observed. and spinels [e.g., Mn Co O (x = 0-1.5) (Ref 24); 1+x 2x 4 The observed higher grain reﬁnement of the sinters obtained La Ca CrO and La Sr CoO (Ref 25); La Sr CrO 0.8 0.2 3 0.6 0.4 3 0.8 0.2 3 using the glycine method was slightly surprising considering (Ref 26); Cu Mn Co O (Ref 27)]. The most probable 0.3 1.1 1.8 4 the results of speciﬁc surface area BET measurements, which reason of observed phenomenon seems to be a trouble with indicated that the powders obtained by EDTA methods are keeping the pH of the solution on a steady value of 8 despite characterized by larger surface area (in the range of constant addition of ammonia during the process. This may 2 1 2.28 ± 0.07 to 6.69 ± 0.04 m g ), while the value for the indicate that these compounds are extremely vulnerable to even powders obtained for glycine method was in the range of small deviations from the optimal conditions of pH. 2 1 1.57 ± 0.06 to 3.99 ± 0.04 m g . The main reason for such Similarly to the results obtained by Yoon et al. (Ref 5-7), discrepancy can be attributed to the tendency of ﬁne powders, both powders and sinters obtained using glycine method were obtained by glycine method, toward agglomeration, which was characterized by orthorhombic structure. Based on the results of conﬁrmed by SEM observations. XRD analysis (Fig. 5b), it can be said that the pure YCrO Figure 5 shows the diffraction patterns for powders calci- phase occurred only for the compound with nickel content nated for 10 h in air at 1200 C and for sinters obtained from x = 0.15. For the material without nickel (x = 0) additionally a these powders, sintered for 1 h in air at 1600 C. Based on the Y O was found, and for the case of x = 0.3 both Y O and 2 3 2 3 ex situ phase analysis of powders obtained by means of EDTA NiO were present in both powders and sinters [which was method, it can be said that all examined materials exhibited explained elsewhere (Ref 13)]. multiphase composition (Fig. 5a). The main phase observed in It should be noted that the substitution of the trivalent all materials synthesized by both methods was the orthorhom- chromium ion with divalent nickel ion does not inﬂuence values bic YCrO one (Fig. 5a and b). Besides, the following phases of the lattice constant of the Y Ca Cr Ni O (x = 0; 0.15 0.8 0.2 1x x 3 were also identiﬁed: NiO (in compounds with x = 0.3, a result and 0.3) compounds and as a result does not inﬂuence the volume of substitution limit for nickel), tetragonal CaCr O and 2 4 of primitive cell. This phenomenon can be explained by similar 3+ 2+ orthorhombic b-CaCr O . 2 4 ionic radius of both Cr (0.62Å) and Ni (0.69Å) ions. Yoon The explanation of the presence of CaCr O in two different 2 4 et al. suggested (Ref 5) that another important factor may be a phases may be analogical to the one presented by Armstrong partial substitution by trivalent nickel ion of low spin conﬁgu- et al. (Ref 12) for similar compounds in reducing atmosphere. 6 1 ration (t e ðs ¼ 1=2Þ). Such compensating mechanism explains 2g g The CaCrO phase, which at temperatures in excess of 1000 C why the differences in lattice constants are so small, that even is a liquid one, may decompose into two different CaCr O 2 4 XRD methods do not allow detecting them. phases, which is shown in the following equation: Table 3 Results of the BET speciﬁc surface area measurements, crystalline size based on the XRD results and the size of grains estimated from SEM micrographs of the studied powders Mass fraction, % Physicochemical parameter Ni = 0.0 Ni = 0.15 Ni = 0.3 Powders synthesized via EDTA method 2 1 BET speciﬁc surface area, m g 4.64 ± 0.06 2.28 ± 0.07 6.69 ± 0.04 Average size of crystallites—YCrO [nm] 46.0 47 49 Estimated grain size—YCrO lm 0.29-0.61 0.20-0.60 0.14-0.80 3, Powders synthesized via glycine method 2 1 BET speciﬁc surface area, m g 1.57 ± 0.06 1.86 ± 0.051 3.99 ± 0.04 Average size of crystallites, nm 49 49 50 Estimated grain size, lm 0.57-1.33 0.29-1.30 0.62-0.91 3282—Volume 27(7) July 2018 Journal of Materials Engineering and Performance Fig. 4 SEM micrographs of Y Ca Cr Ni O sinters obtained from powders synthesized using EDTA method: (a) x = 0; (b) x = 0.15; 0.8 0.2 1x x 3 (c) = 0.3; and glycine method: (d) x = 0; (e) x = 0.15; (f) = 0.3(mag. 1000x) Fig. 5 XRD spectra of powders and sinters synthesized via: (a) EDTA method and (b) glycine method (Ref 5) Using the relation between intensity of refracted beams and with the temperature increase, conﬁrming their semiconducting quantity of the given phase in the measured sample, a content character. At the temperature of 800 C, which is close to the of each phase in all examined compositions of Y Ca Cr working conditions of SOFCs, the sample characterized by the 0.8 0.2 1x Ni O powders, calcinated for 10 h in 1200 C in air atmo- highest electrical conductivity of 50.3 SÆcm was x 3 sphere, was evaluated using software for phase identiﬁcation Y Ca Cr Ni O sinter, obtained based on the powders 0.8 0.2 0.85 0.15 3 mentioned before. The results are presented in Table 4. synthesized by the glycine method. At the same time, the worst electrical properties were observed in the samples undoped by 3.3 Electrical Properties Ni, with the value of electrical conductivity measured for the sinter, obtained using the same method as in the previous case, The studies conducted using DC 2-probe 4-point technique being 11.9 SÆcm . revealed that the electrical conductivity of all sinters increased Journal of Materials Engineering and Performance Volume 27(7) July 2018—3283 In Fig. 6, the temperature dependence of the electrical After taking the logarithm of both sides of Eq 3, a following conductivity of all examined Y Ca Cr Ni O sinters equation is obtained: 0.8 0.2 1x x 3 (x = 0; 0.15; 0.3, both methods of synthesis, calcination for E 1 10 h at 1200 C, sintering for 1 h at 1600 C) is presented. ln rT ¼ þ ln r ðEq 4Þ k T Using the Arrhenius relation for the electrical conductivity in a form presented below, it was possible to calculate the Presenting the data using the Arrhenius plot, the slope of the activation energy of the process: line (equal to E /k in this case) was estimated using the linear regression. Based on this, the energies of activation for each r E 0 a case were found. In all cases, measuring error of energies of r ¼ exp ðEq 3Þ T kT activation does not exceed 0.01 eV. Their values are pre- 1 1 sented in Table 5. where r—electrical conductivity of the sinter [X cm ]; 1 1 1 As it can be seen, the values of activation energy at r —pre-exponential factor [X cm K ]; E energy of o a temperatures below 700 C are signiﬁcantly lower than for activation [eV]; k—Boltzmanns constant [eV K ]. temperatures higher than 700 C, with the exception of the two sinters that had been obtained via the EDTA method and had compositions corresponding to x = 0 and 0.3. This may be a Table 4 Mass fraction of each phase in the Y Ca 0.8 0.2 result of the change in the electrical conductivity mechanism, Cr Ni O powders with x = 0; 0.15 and 0.3 calcinated 12x x 3 for example, from the grain boundary mechanism to the for 10 h in air at 1200 C conduction through the bulk of the grains. In general, the ﬁrst type is characterized by lower energies of activation and occurs Mass fraction, % in the low temperature range, which is in agreement with the Identiﬁed phase Ni = 0.0 Ni = 0.15 Ni = 0.3 presented results. Furthermore, it can be seen that in the case of EDTA, a change in the activation energy is associated with Powders synthesized via EDTA method increasing nickel content. YCrO 30.2 25.5 43.2 To the further studies, including the test of coating/metallic a-CaCr O 20.4 17.4 17.1 2 4 substrate layered systems, only the Y Ca Cr Ni O 0.8 0.2 0.85 0.15 3 b-CaCr O 49.4 57.1 31.1 2 4 compound was selected, due to its preferable electrical NiO …… 8.6 properties. While at this stage it was clear that the sinter Identiﬁed reﬂexes, % 96.6 92.8 94.9 obtained using glycine method was characterized by better Powders synthesized via glycine method YCrO 99.1 100 98.6 properties, both sinters of this type were further investigated for Y O 0.9 … 0.9 2 3 comparison. NiO …… 0.5 The studies using electrochemical impedance spectroscopy Identiﬁed reﬂexes, % 99.6 99.5 99.1 method (EIS) were conducted on both Y Ca Cr Ni O 0.8 0.2 0.85 0.15 3 sinters, prepared by both previously described methods (EDTA and glycine), calcinated for 10 h in 1200 C and sintered for 1 h in 1600 C. The data were analyzed using the equivalent circuit method, which allowed extracting the basic electrical properties and separating contributions of different phases. Firstly, the form of equivalent circuit was proposed; then, using the optimization methods (least square errors + conjugate gradients) parameters of each electrical element were deter- mined. The applied equivalent circuit allowed very close approximation of the experimental data. In Fig. 7(a), an exemplary set of spectra, measured at low and medium temperatures for the sample obtained using EDTA method, is presented. In this case, the equivalent circuit was postulated in a form of series circuit of two parallel RC connections. Additional resistor was added to compensate for Fig. 6 Temperature dependence of electrical conductivity for the resistance of the experimental setup. (The capacity effect Y Ca Cr Ni O (x = 0; 0.15 and 0.3) sinters obtained by both 0.8 0.2 x1 x 3 EDTA and glycine methods presented using Arrhenius plot was deemed to be negligible.) Each RC circuit was used to Table 5 Activation energy of electrical conductivity determined for the examined sinters 3284—Volume 27(7) July 2018 Journal of Materials Engineering and Performance mimic the properties of a different phase: the ﬁrst one described metallic substrates, all coatings were characterized by good properties of YCrO and the second of CaCr O . Such macroscopic properties: uniformity of thickness and color, as 3 2 4 interpretation is also consistent with the result of XRD, which well as lack of cracks. showed that both these phases coexist in the measured sample. The main part of studies was conducted in a condition close In the case of the sinter, based on the glycine method to the working environment of the solid oxide fuel cells to obtained powders, analogical studies were conducted. The properly assess the potential of the investigated materials in the exemplary spectra for this sample are presented in Fig. 7(b). In role of conductive–protective coatings. The oxidation tests this case, an extremely simple equivalent circuit was postulated, were carried out in dual-atmosphere setup at temperature of in a form of series connection of resistor (describing properties 800 C for 250 h. It should be noted that to the best of authors of the experimental setup) and parallel RC circuit (samples knowledge, this was the ﬁrst time when dual-atmosphere properties), giving an excellent ﬁt quality. The EIS results studies were conducted on the layer/steel system instead of conﬁrmed the results of XRD analysis, showing that only one studies on the pure metallic substrate. main phase is present in the sample, namely the YCrO . Despite the presence of high gradients of chemical potential Based on the obtained data and using Eq 4, the electrical of both oxygen and hydrogen, no signiﬁcant structural changes conductivity was calculated for each phase in both studied were observed within the layer or on the coating/substrate samples. Using the Arrhenius relation (Eq 2), the energies of interface. The XRD results shown in Fig. 8(a) clearly indicate activation were calculated for each phase. The results are the lack of phase transitions and single-phase structure within presented in Fig. 7 and summarized in Table 6. the whole volume of coating. The obtained values of energies of activation show the Studies of morphology combined with the EDS results, importance of the base YCrO perovskite phase in terms of which are visible in Fig. 8(b), (c), (d), and (e), conﬁrmed good general electrical properties of the examined compounds, as agreement between the obtained composition and the nominal well as the inferior electrical properties of CaCr O phase. one, namely Y Ca Cr Ni O . Supplementary Fig. 10 2 4 0.8 0.2 0.85 0.15 3 (included in a separate document) shows the EDS line scan proﬁle from the sample annealed for 250 h at 800 C in a dual 3.4 General Properties of the Conductive–Protective Coating atmosphere corresponding to the air side of a SOFC. Results System are consistent with the results of EDS point measurements. To obtain dense, uniform coating, a screen-printing method, The observed structure was characterized by relatively high combined with the suitable thermal treatment, was used. The porosity and consisted mainly of irregularly shaped grain details are described in section 2.2. After deposition on the agglomerates of size 5-15 lm. Based on the SEM observations, Fig. 7 Examples of impedance spectra and temperature dependence of electrical conductivity measured for Y Ca Cr Ni O sinters 0.8 0.2 0.85 0.15 3 obtained from powders prepared via: (a), (c) EDTA method; and: (b), (d) glycine method Journal of Materials Engineering and Performance Volume 27(7) July 2018—3285 Table 6 Activation energy of electrical conductivity measured for Y Ca Cr Ni O sinters 0.8 0.2 0.85 0.15 3 Activation energy of electrical conductivity [eV] with the value of measurement error YCrO phase CaCr O phase 3 2 4 EDTA method 0.33 ± 0.02 0.24 ± 0.04 Glycine method 0.37 ± 0.03 …… Fig. 8 XRD analysis of the YCCN layers after 250 h of annealing in a dual atmosphere at 800 C (a). Morphology and cross-sectional micro- graphs with EDS analysis: (b), (d) air side and (c), (e) Ar-H -H O side 2 2 3286—Volume 27(7) July 2018 Journal of Materials Engineering and Performance the size of grains was estimated to be between 0.1 and 0.8 lm from the metallic substrate. The presence of intermediate layer (Fig. 8d and e). increased the adhesion of the coating, as well as improved The appliance of screen-printing method resulted with thick, electrical properties of the whole system (Ref 27, 28). It is porous layer characterized by good adhesion to the metallic possible that in our case analogical processes took place. substrate. Observations of the cross sections of coating/sub- To examine the suitability of the perovskite coating for the strate systems oxidized in dual-atmosphere conditions allowed surface modiﬁcation of ferritic stainless-steel Crofer 22APU in to determine the thickness of the ceramic coating after the the role of protective–conductive layer for interconnects, process, with the values varying from 60 to 100 lm (Fig. 8d measurements of the area-speciﬁc resistance (ASR) were taken and e). The obtained thickness, while relatively big, is typical for two different systems: oxide scale/Crofer 22APU and for conductive–protective layers obtained by screen-printing Y Ca Cr Ni O coating/Crofer 22APU. The measure- 0.8 0.2 0.85 0.15 3 method (Ref 27). On the coating/substrate interface, an ments were taken in the air atmosphere, using a DC 2-probe 4- additional intermediate layer was observed, of thickness point method. Based on the measured resistance of the varying from 1.7 to 7.6 lm. Beneath this layer, no signs of examined systems, their area-speciﬁc resistance was calculated internal oxidation or oxides precipitations were observed, from the relation shown in Eq 1. which are characteristic for the uncoated Crofer 22APU steel. In Fig. 9(a), a temperature dependence of the determined ASR values for both systems is presented. As it can be seen, in EDS analysis revealed that this reactive layer consisted mostly of Fe, Y, Ca and Cr, with the oxygen being present as well. the low-to-medium temperature range (up to 575 C), the ASR Similar intermediate layers were observed in, e.g., of the scale/uncoated steel system was much higher, although it Mn Co O /AL453 or Cu Mn Co O /DIN 50049 sys- was quickly decreasing with the increasing temperature. In the 1.5 1.5 4 0.3 1.1 1.8 4 tems. In these cases, the intermediate layer was formed as a case of layer/steel system, the behavior was extremely different, result of chemical reaction between the coating and chromium with the ASR being nearly constant within the whole examined temperature range, with only slight decrease in its value in the high temperature range being visible. It should be noted that the ASR of the coated steel was lower than for the uncoated one within the temperature range of interest, although the differ- ences at temperatures higher than 725 C were very small. At temperature of 800 C, values of area-speciﬁc resistance for both scale/steel and Y Ca Cr Ni O layer/steel system 0.8 0.2 0.85 0.15 3 2 2 were, respectively, 0.037 X cm and 0.0368 X cm . It is worth mentioning that inﬂuence on ASR value has also a number of factors related to physical phenomena, mainly caused by: chemical heterogeneity of the material (oxidant-associated formation of easy conductive paths, diversiﬁed microstructure of the material and polarization effects at the phase boundaries). The values of ASR were also measured at constant temperature of 800 C in air atmosphere as a function of time. After oxidation for 200 h, results (presented in Fig. 9b) indicated that for the whole duration of measurement, the value was at a constant level of 0.0366 X cm , which is far below the acceptable limit (0.1 X cm ) (Ref 4). High potential of the examined coatings for their projected application is conﬁrmed by comparison of the measured ASR values with results obtained for other ceramic/metallic systems dedicated for IT-SOFC applications after similar experimental Fig. 9 Value of ASR measured for Crofer 22APU-YCCN layered time (Table 7). The improvement of electrical properties may not system in a function of: (a) temperature; (b) time look substantial in comparison with the pure scale/Crofer Table 7 ASR values of various ceramic/metallic layered systems oxidized for different times in air at 800 C (Ref 26-33) Layer system Deposition method ASR, X cm Authors Y Ca Cr Ni O /Crofer 22APU Screen-printing 0.0366 Own research 0.8 0.2 0.85 0.15 3 Scale/Crofer 22APU … 0.0374 Own research La Sr CrO /DIN 50049 Screen-printing 0.09 (Ref 26) 0.8 0.2 3 Scale/DIN 50049 … 1.22 (Ref 26) Cu Mn Co O /DIN 50049 Screen-printing 0.08 (Ref 27) 0.3 1.1 1.8 4 Mn Co O /AL453 Screen-printing 0.0616 (Ref 28) 1.5 1.5 4 Scale/AL453 … 0.0993 (Ref 28) Y Mn Co O /E-Brite Screen-printing 0.0076 (Ref 29) 0.1 1.45 1.45 4 Scale/E-Brite … 0.0114 (Ref 29) Co/DIN 50049 Pulsed laser deposition 0.0540 (Ref 30) Y/DIN 50049 Y-implantation 0.0637 (Ref 31) La O /Crofer 22APU MOCVD 0.05 (Ref 32) 2 3 Mn Co O /Crofer 22APU PVD 0.004 (Ref 33) 1.5 1.5 4 Journal of Materials Engineering and Performance Volume 27(7) July 2018—3287 22APU. However, it should be remembered that due to the References protective qualities of chemically stable coating, the difference 1. S.C. Singhal, Solid Oxide Fuel Cells for Stationary, Mobile, and would be much more visible in favor of the Y Ca Cr 0.8 0.2 0.85 Military Applications, Solid State Ionics, 2003, 152–153, p 405–410 Ni O /Crofer 22APU system if longer times of operations 0.15 3 2. J.W. Fergus, Lanthanum Chromite-Based Materials for Solid Oxide were considered. It should also be noted that presented results Fuel Cell Interconnects, Solid State Ionics, 2004, 171, p 1–15 were obtained using a simple screen-printing method. The 3. T. Brylewski, M. Nanko, T. Maruyama, and K. Przybylski, Application of Fe-16Cr Ferritic Alloy to Interconnector for a Solid Oxide Fuel Cell, appliance of more advanced methods, such as pulsed laser Solid State Ionics, 2001, 143, p 131–150 deposition, should allow further improvement of the electrical 4. N. Shaigan, W. Qu, D.G. Ivey, and W. Chen, A Review of Recent properties. Progress in Coatings, Surface Modiﬁcations and Alloy Developments for Solid Oxide Fuel Cell Ferritic Stainless Steel Interconnects, J. Power Sources, 2010, 195, p 1529–1542 5. K.J. Yoon, J. Stevenson, and O. Marina, Effect of Nickel Substitution 4. Conclusions on Defect Chemistry, Electrical Properties, and Dimensional Stability of Calcium-Doped Yttrium Chromite, Solid State Ionics, 2011, 193,p 60–65 As a result of the studies, a steel/layer composite system 6. K.J. Yoon, C. Cramer, J. Stevenson, and O. Marina, Advanced Ceramic suitable for the application in IT-SOFCs interconnects was Interconnect Material for Solid Oxide Fuel Cells: Electrical and created. The ﬁrst-time application of YCCNs materials in the Thermal Properties of Calcium- and Nickel-Doped Yttrium Chromites, role of conductive–protective layers for ferritic stainless-steel J. Power Sources, 2010, 195, p 7587–7593 substrates was proved to be successful. The presented studies 7. K.J. Yoon, J. Stevenson, O. Marina, Calcium- and Nickel-Doped Yttrium Chromite as an Advanced Ceramic Interconnect Material for allowed selecting the optimal synthesis method, composition Solid Oxide Fuel Cells (SOFCs) (Abstract #1150) (218th Electrochem- and thermal treatment of the perovskites, showing how greatly ical Society Meeting, Las Vegas, 2010) the properties of these materials can be affected by mentioned 8. W. Li, M. Gong, and X. Liu, H Oxidation on Doped Yttrium factors. The usage of glycine as a chelating agent led to much Chromites/Yttrium Stabilized Zirconia Anode of Solid Oxide Fuel better ﬁnal materials, characterized by single-phase structure Cell, J. Power Sources, 2013, 241, p 494–501 and good electrical properties. 9. S. Wang, B. Lin, Y. Dong, D. Fang, H. Ding, X. Liu, and G. Meng, Stable, Easily Sintered Ca-Zn-Doped YCrO as Novel Interconnect The usage of cost-effective screen-printing method allowed Materials for Co-Fired Yttrium-Stabilized Zirconia-Based Solid Oxide creating a sufﬁciently solid layer on the metallic substrate, Fuel Cells, J. Power Sources, 2009, 188, p 483–488 which exhibited both protective and conductive properties. 10. W. Li, M. Gong, and X. Liu, Characterization of Doped Yttrium Studies of the microstructure and chemical composition by Chromites as Electrodes for Solid Oxide Fuel Cell by Impedance SEM and EDS methods of the cross section of Y Ca Cr Method, J. Electrochem. Soc., 2014, 161, p F551–F560 0.8 0.2 0.85 Ni O -Crofer 22APU layered system after oxidation revealed 11. S.W. Paulik, S. Baskaran, and T.R. Armstrong, Mechanical Properties 0.15 3 of Calcium-Substituted Yttrium Chromite, J. Mater. Sci. Lett., 1999, the presence of the additional intermediate layer on the 18, p 819–822 coating/substrate interface. 12. T. Armstrong, J. Stevenson, D. McCready, S. Paulik, and P. Raney, The The electrical properties determined by the ASR (area- Effect of Reducing Environments on the Stability of Acceptor speciﬁc resistance) measurements are well within the limits for Substituted Yttrium Chromite, Solid State Ionics, 1996, 92, p 213–223 the conductive–protective layers, with the ASR value equal to 13. J. Stevenson, B. Koeppel, FY13 Annual Progress Report for SECA Core Technology Program, Prepared for the U.S. Department of 0.366 X cm , even after prolonged annealing. Energy under Contract DE-AC05-76RL01830, Washington, 2013) The fact that the layers proved satisfying despite usage of 14. Y. Jianyi, Practical Approach to Characteristic Reaction Sintering with the rather simple method of deposition suggests that the Ceramics Y Ca Cr Zn O and Y Ca Cr Ni O , Master 0.7 0.3 0.9 0.1 3 0.8 0.2 0.85 0.15 3 potential of these materials may be very high, and introduction thesis (Kunshan University of Science and Technology, Taiwan, 2012). of more advanced deposition methods such as pulsed laser Accessed 10 Jan 2018. http://ir.lib.ksu.edu.tw/handle/987654321/16305 15. C. Seokgon, Doping Effect of Fe, Co or Ni on YCC(Y Ca CrO ) deposition (PLD) or physical vapor deposition (PVD) may lead 0.75 0.25 3d Perovskites as the SOFC Interconnect, PhD thesis (Pohang University to further improvement of their mechanical and electrical of Science and Technology, South Korea, 2011). Accessed 10 Jan properties (i.e., due to the obtaining of thinner, denser layers). 2018. http://oasis.postech.ac.kr/handle/2014.oak/1114 16. M. Stygar, K. Matsuda, S. Lee, and T. Brylewski, Corrosion Behavior of Crofer 22APU for Metallic Interconnects in Single and Dual Atmosphere Exposures at 1073 K, Acta Phys. Pol. A, 2017, 131,p Acknowledgments 1394–1398 17. M. Stygar, T. Brylewski, A. Kruk, and K. Przybylski, Oxidation The authors would like to express their gratitude to Mr Properties of Ferritic Stainless Steel in Dual Ar–H –H O/Air Atmo- 2 2 Krzysztof Mroczka from Pedagogical University of Cracow for sphere Exposure with Regard to SOFC Interconnect Application, Solid assistance in SEM–EDS observations. Financial support from the State Ionics, 2014, 262, p 449–453 National Science Centre for PRELUDIUM 6 (Grant No. 2013/11/ 18. M. Stygar, J. Da˛browa, P. Dziembaj, and T. Brylewski, Inﬂuence of N/ST5/01391) is gratefully acknowledged. Gaseous Media Flow in the Dual Ar-H -H O/Air Atmosphere Setup on 2 2 the Scale Growth Kinetics of Crofer 22APU Ferritic Stainless Steel, J. Mater. Eng. Perform., 2017, 26, p 540–546 19. K. Huang, P.Y. Hou, and J.B. Goodenough, Characterization of Iron- Based Alloy Interconnects for Reduced Temperature Solid Oxide Fuel Open Access Cells, Solid State Ionics, 2000, 129, p 237–250 20. S. Chevalier, G. Caboche, K. Przybylski, and T. Brylewski, Effect of This article is distributed under the terms of the Creative Commons Nano-Layered Coatings on the Electrical Conductivity of Oxide Scale Attribution 4.0 International License (http://creativecommons.org/ Grown on Ferritic Steels, J. Appl. Electrochem., 2009, 39, p 529–534 licenses/by/4.0/), which permits unrestricted use, distribution, and 21. P. 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Muan, Liquidus-Solidus Phase Relations in the 29. A. Kruk, M. Stygar, M. Krauz, M. Homa, A. Adamczyk, W. Kucza, P. System CaO-CrO-Cr O -SiO , J. Am. Ceram. Soc., 1992, 75(6), p Rutkowski, M. Bobruk, A. Gil, and T. Brylewski, Microstructure and 2 3 2 1333–1341 Electrical Properties of a Spinel Coatings on Selected Ferritic Stainless Steels, Ceram. Mater., 2014, 67, p 235–244 ((in polish)) 24. T. Brylewski, W. Kucza, A. Adamczyk, A. Kruk, M. Stygar, M. Bobruk, and J. Da˛browa, Microstructure and Electrical Properties of 30. A. Kruk, A. Adamczyk, A. Gil, S. Ka˛c, J. Da ˛bek, M. Zia ˛bka, and T. Mn Co O (0 <= x <= 1.5) Spinels Synthesized Using EDTA- Brylewski, Effect of Co Deposition on Oxidation Behavior and 1+x 2x 4 Gel Processes, Ceram. Int., 2014, 40, p 13873–13882 Electrical Properties of Ferritic Steel for Solid Oxide Fuel Cell Interconnects, Thin Solid Films, 2015, 590, p 184–192 25. K. Przybylski, T. Brylewski, and J. 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Published: May 29, 2018
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