الفهرس 
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Abstract
In recent years, due to reduction of operating temperatures (e.g., 500-800 oC), IT-SOFCs have been drawn a lot of attention. At lower working temperatures, metallic materials as interconnects replaced conventional LaCrO3-based ceramics. Feritic stainless steel alloys such as Crofer 22 and AISI 430 are vastly utilized for metallic interconnects, because of their relatively high conductivity and appropriate thermal expansion behaviors. However, exposure of those alloys to an oxidizing atmosphere during operating hours leads to Cr2O3 scales formation as interface layer between interconnect and cathode (Cr poisoning phenomenon). The continuous growth of the oxide scale allows other Cr-containing species deposition that prevent normal flow of electrons diffusion between different electrodes and block the active sites on cathode side so retard the oxygen reduction process allow lower conductive species layer formation. The most reliable solution to these problems is applying a protective layer with specific criteria on the surface of metallic interconnect. The acquired criteria for protective layer assure appropriate thermal and mechanical stability, high density, excellent electrical conductivity and tune with interconnect substrate. The most common types of the coated materials are rare earth perovskites, spinels, and MAlCrYO (M stands for a metal). Divalent spinel oxides composed of Mn, Co, Ni,Cu, and Fe have been attracted significant attention. In particular, the most studies focused on spinel structure of (Mn,Co)3O4 as a promising composition where it showed a good effect on prevention Cr2O3 scale growth at long-term evaluations. Several studies pay a lot of efforts to improve the coating layer properties by modification with different transition elements such as Cu, Ni and Fe. Mn-Co oxides coatings can also be synthesized by simple and cost-effective processes, such as screen printing, slurry coating, and electrodeposition. In this context, optimizing the synthesis conditions of the spinel structure by screen printing for pure and modified Mn-Co oxides was focused. Besides, close investigation of all synthesized phases with deep study of the modified elements effects on various phase properties. Moreover, designing heat treated coating with high quality matching very well with the utilized alloy.
This investigation includes three main stages:
A: Pure and modified MCO spinel structure synthesis and characterization
• Spinel powder preparation
Five different spinel powder compositions of Mn1.5Co1.5O4, MnFe0.25Cu0.25CoNi0.5O4, MnFe0.25Cu0.5CoNi0.2Na0.05O4, MnFe0.25Cu0.5CoNi0.25O4 and MnFe0.25Cu0.5CoNi0.2Mg0.05O4 were prepared by sol-gel auto-combustion and solid-state reaction techniques. Based on the results obtained regarding this section, the following conclusions could be derived: chemical and physical analysis of the synthesized powders by the XRD, XPS, Raman and HRSEM techniques which confirmed the presence of the cubic spinel structure for all modified MCO samples and mixed cubic and tetragonal for pure MCO sample powder. The results showed stability for all modified MCO compositions without any impurities for long time of calcination at different high temperature, which probably due to formation and stabilization of cubic spinel structures at lower temperature that result in the modification process.
• Pellets of spinel powder preparation for thermal and electrochemical measurements
Five samples were prepared as pellets of MCO spinel powders after long wet grinding process to get powder with nan sizes. Pellets with specific dimensions and different weight ratios were formed by pressing the powder by uniaxial pressure of 40 MPa for 15 min in a cylindrical mold with a diameter of 0.7 mm. The formed pellets were painted with platinum paste on the upper and lower sides. The coating was dried at 70 °C for 30 min then sintered at 850 °C at heating rate of (100 °C/min). Pellets were sintered again at temperatures of 1050 °C at very slow heating rate (50 °C/h) to avoid decomposition of the formed phases or formation of impurities.
B: Coating layer optimization and preparation
Five samples of stainless steel 316 L were prepared after cutting to specific dimensions and mechanically treated surface to become completely smooth. Five samples of pure and modified spinel were prepared as ink by mixing specific weights with organic substances of (KD) as a solvent and placing specific weights of polyvinyledene diflouride (PVDF) as a binder to help the coating adhesion. The inks of different compositions were coated as dense layer on surface of clean stainless-steel chips by screen printing technique, and the thickness of the coating layer was (10 microns). The prepared layers were treated in a medium of argon gas only at 1050°C with heating rate 100°C/min.
C: Powder, pellets and coating Layer of spinel phase characterization
• Phase evolution
XRD analysis revealed that irrespective to sintering temperature, pure MCO showed the characteristic peaks of the Mn1.5Co1.5O4 mixed cubic and tetragonal oxide phase up to 900oC. For the modified MCO, the transition element beside Na and Mg stabilized the phase at lower temperature (850-900oC), while the characteristic peaks for the cubic pure spinel oxide phased. With the temperature increase from 900 to 1100 oC, the characteristic peaks of completely pure tetragonal oxide phase of Mn1.5Co1.5O4 appears, while decomposition of cubic spinel oxide phase of modified samples starts. Very little amounts of secondary phase of different metal oxides (doesn’t exceed 5%) starts in appearance. After 120 h of sintering at 850 oC, all composition showed high stability with little amounts of decomposed single metal oxides formed. In tuned with XRD, XPS and Raman analysis confirm the single-phase formation at earlier stage for modified compounds. Transition element with Na and Mg spinel addition proved to have strong stability for long time at high temperature. Modifications in spinel phase allow limited grain growth of spinel and thereby facilitate sintering. Moreover, the densification and cubic structure stabilization behavior at much less temperature were achieved.
• Thermal stability and densification investigation
The obtained results indicated that the ball milling of the prepared powder mixtures proved to be an effective way to induce densification process of MCO pure and modified compositions by reducing particle size and increasing sintering activity, enabling an apparent porosity reduction and volume shrinkage enhancement. Also, the transition metal like sodium and magnesium addition showed an improvement in sintering rate and densification properties of modified spinel structures in comparison with the pure form as follows:
Pure MCO sintered at 1050 ºC showed porosity of " ~ "31.5%, then decreased to " ~ "12.3% at 1200 oC.
Modified MCO sintered at 1050 ºC showed porosity content less than 1.5%.
Pure MCO composition sintered at 900 ºC showed CTE value (10.8x10-6K-1) less than that of stainless steel (12.33-14.5x10-6K-1).
Modified MCO composition sintered at 900 ºC showed CTE values (12.4-14.1x10-6K-1) so close to that of ferritic stainless steel (12.33-14.5x10-6K-1).
According to TGA measurements, all modified MCO samples which have been sintered at 850 oC showed stability at weight up to 750 oC in contrary to the pure MCO samples that showed continues weight gain up to 1000 oC which probably due to continues oxidation process and transformation from cubic to tetragonal spinel.
• Electrical conductivity measurements
For all pure and modified spinel structures, The Ea and electrical conductivity values were calculated for pure and modifiedMCO samples. Electrical conductivity increased by increasing temperature from 500oC to 750oC. The results can be summarized as follow:
Pure MCO sintered at 1050 ºC showed an Electrical conductivity at 750oC of " ~ "23.1 Scm-1.
MnFe0.25Cu0.25CoNi0.5O4, MnFe0.25Cu0.5Na0.05CoNi0.2O4 MnFe0.25Cu0.5Mg0.05CoNi0.2O4 andMnFe0.25Cu0.5CoNi0.25O4 samples that sintered at 1050 ºC showed the best electrical conductivity values at 750oC of 23.1, 54.1,37.8,36.1 and 36.3Scm-1 respectively.
All modified samples that sintered at 1050 ºC showed the semiconductor manner and improvement at electrical conductivity relative to pure MCO sample.
The overall results of the modification with the Cu, Ni and Fe transition elements or Na and Mg as earth elements may indicate this spinel has a wide spinel stability zone similar to that observed in Co-Mn binary phase diagram in air. In reverse to that happened at pure MCO, the addition of Cu into MCO not only leads to the formation of cations with different valence states on octahedral sites (e.g., Cu+ and Cu2+) but also induces the transition of Mn2+ and Mn3+ to Mn3+ and Mn4+ form, which is essential to maintain the charge neutrality, thus facilitating the electrical conductivity via small polaron hopping. Even when copper was substituted with nickel, improvement in conductivity was observed. The Fe ions with higher ionic radius replace the Co ions. This substitution tends to higher lattice packing factor thus reduction in activation energy(Ea) and hopping distance and therefore higher electronic conductivity. Results revealed Na and Mg havepositive effect on electronic conductivity and activation energy (Ea).
• Area specific resistance measurements
The area specific resistance (ASR, Ω.cm2) for the coated alloy samples was measured by 2 points method at temperature range between 250 up to 750 oC in air. For all specimens, by increasing temperature the ASR steadily decreases. For the modified MCO coated USS 316L alloy demonstrates an initial electrical resistance lower than that of pure MCO coated USS 316L alloy which reflect the important role of the modification with the transition and earth elements that improve the protection of the surface of the alloy. The highest ASR value was recorded for pure MCO sampleat 750 oC of 0.947 Ω.cm2. For other modified MCO samples, they showed improvement at ASR values which improve the effectiveness of the substitution process. The best ASR value was recorded for Na modified sample at 750 oC of 0.066 Ω.cm2.
• Microstructure investigation
FESEM images of sintered powder composites at 1000 ºC showed no remarkable change in morphology before and after modification. However, the crystal grain increases with increasing the concentration of dopant. It can be seen that the crystal grains have a wide particle size distribution. The average of the particle size for pure MCO is lower than 200 nm. Crystal growth in the particle size for modified MCO can be noticed. The crystal growth of the modified MCO is probably due to the substitution with elements decrease the sintering temperature so allow wide range for grain growth. Thus, controlling the microstructure needs a very keen control of heating rate and sintering temperature at narrow temperature interval.