Cu-Fe substituted Mn-Co spinels by High Energy Ball ... · In this work, a simple High Energy Ball...

12th European SOFC & SOE Forum www.EFCF.com/Lib ISBN 978-3-905592-21-4 5 - 8 July 2016, Lucerne/Switzerland

Electrolytes, interconnects, seals Chapter 07 - Session B06 - 171/186

B0631

Cu-Fe substituted Mn-Co spinels by High Energy Ball Milling for interconnect coatings: insight on sintering

properties

Andrea Masi (1,2,3), Jong-Eun Hong (3), Robert Steinberger-Wilckens (3), Maurizio Carlini (2), Mariangela Bellusci (1), Franco Padella (1), Priscilla Reale (1)

(1) ENEA C.R. Casaccia, Via Anguillarese 301 00123 Rome, Italy (2) DAFNE, University of Tuscia, via San Camillo de Lellis snc 01100 Viterbo, Italy (3) Centre for Fuel Cell and Hydrogen Research, School of Chemical Engineering,

University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK Tel.: +39-06-3048-3248 [email protected]

Abstract

Corrosion effects on metallic interconnects of Solid Oxide Fuel Cells (SOFC), including the growth of insulating scales and chromium evaporation, can be reduced through the deposition of protective coatings. Mn-Co oxide coatings obtained by wet chemical coating techniques are competitive on large-scale manufacture due to low costs and high producing volumes. These methods however require the formulation of powders and inks as well as high temperature sintering treatments for reducing residual porosity being one of the main drawbacks. Metal doping of the traditional Co-Mn composition has been suggested as an option to improve the sintering behaviour and the final coating properties. In this work, a simple High Energy Ball Milling (HEBM) process of metal oxides is exploited to produce Cu and Fe substituted Mn-Co spinel nanostructured precursors. Among the powder production techniques, HEBM is an environmentally friendly and low cost mechano-chemical processing methodology, which exploits hitting balls as energy transfer media. The HEBM treatment produces a very fine grinding and intimate mixing of particles, which leads to particle activation, nucleation of stable or metastable phases, amorphisation processes, or chemical reactions. The high temperature behaviour of the produced powders has been studied by means of thermogravimetric and dilatometric analyses, while investigating morphological and structural properties of the processed powders and pellets, in order to evaluate the role of the additives in the sintering behaviour.

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Introduction The use of metallic interconnects in Solid Oxide Fuel Cells brings several advantages with respect to traditional ceramic parts, such as ease of production, robustness and costs. Due to the low cost of ferritic stainless steels and the exhibited compatibility in thermal expansion coefficient with the other cell components, these alloys are considered the most promising candidates as interconnect materials in SOFC stacks [1]. At high temperature these alloys develop a protective Cr2O3 layer on the surface, ensuring an intrinsic corrosion resistance when exposed to high temperature. When exploited in SOFCs interconnects, the electrical resistance of the passivation layer seems acceptable at 800°C, in the range of 10-3‒10-2 S/cm. Lowering temperature towards 650°C, however, the conductivity decreases about one order of magnitude [2], and the scale contribution becomes predominant in the stack overall resistivity, decreasing significantly cell performances [3]. Furthermore, the formation of volatile chromium containing species is a threat for common cathode materials, giving rise to the growth of insulating phases and the decrease in the active area of the electrode [4]. Thus, the growth and exposure of chromia scales in SOFC cathode operating environment must be avoided. The application of protective coatings on the interconnects is an effective solution to reduce the oxide scale growth rate and chromium volatilization, Mn-Co spinels (1:1<Co:Mn<2:1 at. ratio) possess characteristics (i.e. thermal expansion, electrical conductivity and long-term chemical stability) that can be usefully exploited in ferritic steel protective coatings [5]. In addition, the substitution with transition metals such as Fe, Ti, Ni and Cu can further enhance the electrical conductivity or the chromium retention capability [1,6–10]. In view of large scale application, the application of cheap wet-powder coating techniques should be considered preferred. These methods consist in the formulation of inks, the application of the coating with techniques like spraying, slurry deposition, dip-coating or ink-jet printing, and a final sintering step to densify the coating layer. The effectiveness of the coating layer is strictly related to an efficient sintering step. Mn-Co spinel powders require sintering at high temperatures (e.g. >1000°C) to achieve a density sufficient enough to prevent Cr evaporation [11], which is superior to the maximum temperatures acceptable for the steel substrates. To overcome this issue, thermal treatment in a reducing atmosphere can be applied to sintering of Mn-Co spinel layers deposited on ferritic stainless steels, which allows to achieving high densities at temperature lower than 900°C by exploiting the superior sintering properties of the metals, with respect to the spinel phase. The reducing step is then followed by an oxidation treatment to produce again the spinel phase [12,13]. Alternatively, improvements in the sintering behaviour of the mixed spinels can be obtained by the introduction of other elements such as Cu or Ni [9,10]. This approach could represent an alternative to the reactive sintering process, requiring just a change in the initial powder composition rather than a multi-step sintering treatment involving the use of purpose-designed furnaces and dangerous gases. HEBM is a cost-effective and environmental friendly powder processing technique, in which powder mixtures are subjected to energy transfer phenomena repeatedly carried out by hitting balls. The kinetic energy of the balls transferred to the powders induces several physico-chemical phenomena such as fine comminution of particles and formation of new active surfaces that lead to a significant enhancement of the powder reactivity. Nucleation of stable and metastable phases, amorphization, re-crystallization, interdiffusion phenomena or atomic rearrangements can also occur. Due to the impulsive nature of the energy transfer, HEBM is mainly a “non equilibrium” process, giving rise to materials often characterized by metastable products and showing unique characteristics [14–16]. In a previous work on undoped Mn and Co oxides, we observed that 10h milling treatment can



be enough to produce highly reactive compounds that quickly evolve in single phase products at 700–800°C [17]. This suggests that the desired spinel can be obtained in-situ during the sintering step of the compound coating layer. In this work, the sintering behaviour of a Cu and Fe doped Mn-Co oxide prepared by HEBM process is evaluated. The obtained material has been characterized evaluating high temperature and sintering behaviour by means of thermogravimetric analysis and dilatometric measurements, following morphological and structural evolution with X-ray diffraction (XRD) analysis and Scanning Electron Microscopy (SEM).

1. Experimental Mn3O4 (Sigma Aldrich, 97%), Co3O4 (Sigma Aldrich, 99%), CuO (Carlo Erba, 99%) and Fe2O3 (Carlo Erba, 99%) were mixed to obtain a metal ratio equal to Mn:Co:Fe:Cu=1:1.6:0.2:0.2. About 8 grams of the powder mixture and steel balls at a ball:powder ratio of 10:1 were sealed in a 60cm3 steel vial under Ar atmosphere. The HEBM process was carried out for 10 hours. To evaluate phase composition, XRD analysis was carried on a 120° angular dispersion X-ray diffractometer (Italstructure, curved position sensitive detector from INEL), equipped with Fe Kα1 radiation source. Phase identification was performed on collected patterns using the PDF-2 database [18] as reference data. The morphology of samples was evaluated with SEM (LEO 130 HRSEM). Thermogravimetric analyses were carried out on a thermobalance (Pyris Diamond TG/DTA, Perkin Elmer) in air up to 1200°C with 5°C/min heating rate. To evaluate sintering properties, dilatometric experiments were carried out in air on consolidated pellets (6mm diameter, 250-300mg of powder, 3.5 T/cm2 pressure) up to 1200°C with 5°C/min heating rate (DIL 402 C, NETZSCH).

2. Results and Discussion The 10 hours HEBM process produced a black fine powder. The XRD pattern of the milled powder is reported in Figure 17. The diffractogram is characterized by the presence of broad peaks, due to the nanostructuration of crystallites and strain induced during the mechanochemical process. Main peaks are ascribed to the presence of a cubic spinel phase, compatible with Co3O4 compound. Reflections of Mn3O4 and CuO phases are not evident, while the small broadened peak close to 2θ of 42 degrees is attributed to the presence of highly destructured Fe2O3 phase (JCPDS 33-0664). The high asymmetry of Co3O4 peaks towards lower angle can be related to the nucleation of a cubic spinel phase characterized by higher lattice parameter, compatibly with the formation of a mixed spinel similar to MnCo2O4.



Figure 17: XRD pattern of the oxide sample after 10 hours of milling.

In Figure 18 SEM images of the ball milled powder are reported. After 10 hours of mechanochemical processing, the powder is formed by micrometric aggregates of sub-micrometric particles. The fine structure can be observed in the high magnification image, where the presence of small nanometric particles is evident.

Figure 18: SEM image of the 10 hours milled powder

To evaluate the high temperature behaviour of the compound, thermogravimetric analysis was carried out on the milled powder up to 1200°C in air. In Figure 19 Thermogravimetric (TG) and Derivative Thermogravimetric (DTG) curves as a function of temperature are reported. An initial weight loss can be observed, due to adsorbed or loosely bound volatile molecules departure. Between 200‒500°C a weight gain step is observable. After the milling in argon atmosphere, powders are likely to be characterized by highly anion defective lattices, and already at a relatively low temperature react with oxygen. Similar phenomena have been observed with highly nanostructured mixed valence spinels, and was associated with the existence of non-stoichiometric metastable states[19].



Following this stage, a weight loss step observed is ascribed to the homogenization and reaction among the oxidized compounds. The formation of the equilibrium spinel phase is confirmed by the plateau observable over 800°C. Raising the temperature above 1050°C, a second abrupt weight loss phenomenon is observed, due to oxygen loss upon metal reduction. Metal (II) phases are stable in the high temperature region of Co-Cu and Mn-Co oxides phase diagrams [20–22], and the results here reported suggest that a similar behaviour is retained for mixed compositions. Upon cooling, this high temperature weight loss step is recovered (data not shown).

Figure 19: thermogravimetric curve of the 10 hours milled sample carried out in air.

To evaluate sintering behaviour, the milled powder was formed as pellets by uniaxial cold pressing, with a resulting average geometrical density ρ=3.57±0.07cm3/g. Considering a theoretical density similar to MnCo2O4 (ρtheor=5.56cm3/g), a green density of 64±1% is obtained. Results of dilatometric analysis performed on the consolidated pellets are reported in Figure 20, where the shrinkage and the derivative curves are reported as a function of temperature. For comparison, results related to a similarly processed undoped Mn:Co=1:2 oxide mixture are reported in the same figure (MnCo2 curves). The undoped sample exhibits a slight expansion up to about 1050°C, ascribable to thermal expansion of the compressed powders. For higher temperature a shrinkage phenomenon is observed, with 5.5% contraction at 1200°C, suggesting that the sintering of the powder occurred. Regarding the Fe-Cu doped compound, the shrinkage curve exhibits an initial slight contraction between 200°C and 600°C, probably ascribable to the homogenization of the milled powder to form the equilibrium spinel oxide. A plateau region is observed upon increasing temperature up to approximately 950°C. At higher temperatures, a large and abrupt shrinkage occurs, with relative shrinkage of 11% at 1200°C. From the analysis of the shrinkage rate curve, two separate peaks appeared in the temperature range of 950-1200°C: a first well-defined peak with a maximum at 1000°C and a second broader peak with a maximum at 1060°C approximately. Comparing these results with TGA analysis, it could be therefore deduced that two separate processes are concurring to the shrinking phenomenon. The first phenomenon, occurring in a weight plateau region, corresponds to the conventional sintering process of the spinel compound. The latter process occurs instead in correspondence with the high temperature weight loss, suggesting that the reduction phenomenon concurs to the enhancement of the compound sintering.



Figure 20: dilatometric curves of the 10 hours milled MnCo1.6Fe0.2Cu0.2 (solid lines) and

a reference MnCo2 (dashed lines) samples in air with 5°C/min heating rate. The X-Ray diffraction patterns of Cu-Fe doped pellets treated for 4 hours at 1000°C and 1200°C are reported in Figure 21. The sample sintered at lower temperature exhibits a single cubic phase, compatible with the presence of a doped MnCo2O4-like spinel phase. The sample sintered at 1200°C shows a multi-phase structure, with a cubic MnCo2O4-like phase as main component, and additional reflections assigned to a second cubic spinel phase with a lattice parameter compatible with a Co-Cu oxide spinel phase.

Figure 21: XRD patterns of the samples heat treated at 1000°C and 1200°C in air.



Figure 22: SEM image of the surface of a pellet sintered at 1000°C.

In Figure 22 the SEM image of the sample heat treated at 1000°C is reported. This sample shows homogeneous micron-size particles, suggesting that a significant grain growth took place, taking into account the fine structure of the milled powder. Morphology of the sample sintered at 1200°C is reported in Figure 23, together with the corresponding EDX maps for Mn, Fe, Co and Cu. Larger grains and irregular and corrugated extrusions were observed on the surface and at grain boundaries. EDX analysis shows that Fe is retained in the Mn-Co spinel phase, while Cu is concentrated around the corrugated structures. Considering also TGA and dilatometric results, it can be suggested that copper oxides segregation from the spinel phase occurs with the formation of a melt between 1000°C and 1200°C, promoting densification and crystal growth. Upon cooling in the selected conditions (5°C/min cooling rate), the secondary phase is not solubilized in the spinel phase, with the formation of the irregular and corrugated phase observed in the SEM image.

Figure 23: SEM image of the surface of a pellet sintered at 1200°C for 4 hours and

corresponding EDX maps for Mn, Fe, Co and Cu.



3. Conclusions A highly reactive nanostructured mixture of Fe-Cu doped Mn-Co spinels is obtained through a HEBM process of oxide powders. The milling treatment produces a multi-phase compound that is easily transformed to a single phase spinel material when exposed to intermediate temperatures (i.e. 700‒800°C). The cubic spinel is stable up to 1060°C approximately, where partial demixing and formation of Cu-rich phases are observed. The sintering behaviour of the compound is enhanced with respect to the undoped sample: the simultaneous addition of Fe and Cu lowers the sintering temperature by 100°C. High shrinkage is obtained on the doped material at 1000°C, and a secondary densification phenomenon at temperature above 1060°C can be related to a sintering enhancement induced by the Cu-rich phase segregation. The doping did not succeed however in lowering the sintering temperature of Mn-Co spinel enough for a practical use of protective layer on ferritic stainless steel substrates. Sintering treatments in reducing atmosphere may therefore be needed to obtain dense coatings suitable for the SOFC applications.

Acknowledgments This work was supported by the FCH JU within the project SCORED 2:0 under contract no. 325331.

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Cu-Fe substituted Mn-Co spinels by High Energy Ball ... · In this work, a simple High Energy Ball...

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