Molten Carbonate Fuel Cells (MCFCs) are a relatively recent development in fuel cell technology, with applications ranging from small to large scale power generation systems. The interconnect is the part of the MCFC to which the anode and cathode are attached and through which the electrical current generated by the cell is conducted. Therefore, the interconnect must not only be mechanically strong and resistant to oxygen and hydrogen, but also maintain high electrical conductivity and corrosion resistance (including on the surface of the interconnect) at high temperatures (550...650°C) for long periods.

Interconnections (0.3-0.5 mm thick) made of stainless steel (which contains 16-18% Cr and has a high density γ ~ 8 g/cm3) lose surface electrical conductivity due to oxidation. MAX phases Ti₂AlC₃ and Ti₂AlC have two times lower density (γ ~ 4.1 - 4.3 g/cm³) than stainless steel, are stable in oxygen and hydrogen atmosphere at high temperature, and have high electrical conductivity. Therefore, the MAX phase based coatings are potentially promising for this application. OT4-1 titanium alloy substrates with protective coatings are being developed for use as interconnects for MCFCs to replace 316L stainless steel.

Ti-Al-C, (Ti,Mo)-Al-C and (Ti,Cr)-Al-C coatings were deposited on OT4-1 alloy substrates by hybrid magnetron sputtering and cathodic arc evaporation. For magnetron sputtering, a MAX phase (Ti₂AlC - 63 wt.% and Ti₃AlC₂ - 37 wt.%) target prepared by hot pressing of TiC, Al and TiH₂ under 20 МPа, at 1350 °С for 10 min was used. Simultaneously with magnetron sputtering of the MAX phase target, chromium or molybdenum was deposited using a cathodic arc plasma source. Three types of coatings were deposited: Ti-Al-C magnetron-only and hybrid (Ti,Mo)-Al-C and (Ti,Cr)-Al-C. The thickness of the deposited coatings was 5-11 µm.

X-ray diffraction analysis showed that all deposited coatings are close to amorphous state. The SEM-EDX study indicated that the average composition of the coatings obtained from the MAX phase based target was Ti₂Al_1.0-1.1C_1.1-1.3 (close to 211), for the coating with additions of Mo: Ti₂ Mo_2.1Al_0.9C_2.8 (close to 413) and Cr: Ti₂Cr_2.6Al_0.8C_1.5. The nanohardness of the coatings varied from 11 to 15 GPa and the Young's modulus from 188 to 240 GPa.

The (Ti,Cr)-Al-C coating showed the highest stability against electrochemical corrosion in 3.5 wt.% NaCl aqueous solution at 20 °C: corrosion potential E_corr = 0.044 V, corrosion current density i_corr = 2.48×10^-9 A/cm², anodic current density i_anodic (at 0.25 V vs. SCE) = 5.18×10^-9 A/cm². This coating also showed the highest long-term oxidation resistance and after heating in air at 600 °C, 1000 h its electrical conductivity s= 9.84×10⁶ S/m was slightly higher than before heating s= 4.35×10⁵ S/m, the nanohardness and Young's modulus are in the range of 15 GPa and 240 GPa, respectively. The increase in electrical conductivity after long-term heating at 600 °C can be explained by the observed crystallization of the amorphous phase in the structure of the coating.

Thus, the hybrid deposited (Ti,Cr)-Al-C coatings exhibit high corrosion and oxidation resistance while maintaining electrical conductivity and can be used to protect titanium alloy interconnects in lightweight MCF cells.

Acknowledgments The work was supported by the III-7-22 (0785) Project of the National Academy of Sciences of Ukraine "Development of wear-resistant electrically conductive composite materials and coatings based on MAX phases for the needs of electrical engineering, aviation, and hydrogen energy"; by the NATO project SPS G6292 “Direct liquid fuelled molten carbonate fuel cell for energy security (DIFFERENT)”, and by the MES Ukraine project №0122U001258 “Development of nanotechnological methods to prevent corrosion of structural materials in thermal and nuclear power plants”.

Ultra-high temperature (UHTC) transition metal borides can be used for a wide range of mechanical applications - as components of military and commercial equipment operating in extreme conditions, for rocket propulsion, hypersonic flights, atmospheric reentry, protective coatings on graphite, for using in abrasive, erosive, corrosive and high-temperature environments, which requires materials with significantly improved physical properties. To UHTC belongs TaB₂ which exhibits high melting point (3200 °C), hardness (24.5 GPa -25.6 GPa), fracture toughness (4.5 MPa m^0.5), bending strength (555 MPa), excellent chemical stability, electrical (308×10⁴  Ω^-1×ｍ^-1)  and thermal (0.160 -0.161 W×cm^-1×K^-1 at 300-1300 ^oC) conductivity, good corrosion resistance [1-5]. To increase the oxidation resistance and mechanical characteristics TaB₂ can be modified by silicides [1]. In the present study we investigated modification of TaB₂ by MoSi₂, ZrSi₂ and Si₃N₄ in the amount of 20-30 wt.% and its sintering process under hot pressing conditions (30 MPa, 1750-1950 ^oC) and high pressure (4.1 GPa) - high temperature (1800 ^oC) conditions. The highest Vickers hardness H_V=31.5 GPa and fracture toughness K_1C=6 MPa×m^0.5 under P= 9.8 N load was obtained for the composite sintered at 30 MPа, 1750 °C, 20 min from TaB₂+20 wt.% ZrSi₂, the increase of amount of ZrSi₂ up to 30 wt.% leads to further increase in K_1C= 6.9 MPa×m^0.5, but to the reduction of microhardness  down to H_V=23.5 GPa. The composite sintered under 30 GPa at 1950 °C for 40 min from TaB₂+20 wt.% MoSi₂ showed H_V=28.2 GPa and K_1C= 5.42 MPa×m^0.5. TaB₂+30 wt.% Si₃N₄ sintered at 4.1 GPa, 1800 ^oC for 7 min had H_V=18.8 GPa and K_1C= 4.82 MPa×m^0.5. The specific weight of the materials prepared from TaB₂+20 wt.% MoSi₂ was g=10.82 g/cm³, TaB₂+30 wt.% Si₃N₄ - g=8.77 g/cm³, TaB₂+20 wt.% ZrSi₂ - g=9.35 g/cm³, TaB₂+30 wt.% ZrSi₂ - g=10.12 g/cm³.

AcknowledgementS: This work was supported by the Project of the National Academy of Sciences of Ukraine III-5-23 (0786) “Study of regularities and optimization of sintering parameters of composite materials based on refractory borides and carbides, their physical and mechanical properties in order to obtain products of complex shape for high-temperature equipment with an operating temperature of up to 2000 ^oC”

In the electrodynamic properties study of new AlN- 5wt.%C(diamond powder)-5wt.%Mo composite materials the values ​​and   behavior of the real and imaginary parts of the microwave permittivity were determined and the optimal technological process for materials synthesis was established. The materials with density 3.16 g/cm³ and 3.30 g/cm³ were obtained by the pressureless sintering (PS) at the temperature of 1850 °C and by the hot pressing (HP) at the temperature of 1820 °C (pressure of 15 MPa), respectively.            

The microstructure and phase composition of the composite materials were studied using a scanning electron microscope and the X-ray diffractometer Ultima IV (Rigaku, Japan)  ​​with the Rietveld method for data analysis. The imaginary and real parts of the permittivity were measured by the microwave vector network analyzer Keysight PNA N5227A in the frequency range of 26 - 40 GHz.

The results of structural studies showed that sintering of composites by different methods results in  almost unchanged their phase composition, and the main phases are AlN, Al₃(O,N)₄, Mo₂C, Y₃Al₅O₁₂, and C (graphite) - as in [1].

It was determined that the real part of the permittivity (ε') for both composites obtained by the PS and HP methods is practically frequency independent between 26 to 36 GHz with the value of about 8 and 27 , respectively. At  the same time, the imaginary part of the permittivity (ε″) increases from 0.29 to 1 and from 3.46 to 5.56 for materials made by the PS and HP methods, respectively. When the test frequency is increased from 36 to 40 GHz, significant fluctuations in the permittivity values ​​ are observed, which is possibly related to the greater intensity of electromagnetic wave internal reflections due to the peculiarities of the formation of the materials structure.

As a result of the research, it was established that composite materials obtained by the hot pressing method are characterized by higher density and higher values ​​of the real part of the permittivity in the frequency range 26 - 36 GHz.

REBCO (Re=Y, Eu, Gd) coated conductors (CC) based on biaxially textured, thick and homogeneous nanoengineered multilayer structures opened up new application opportunities, such as dissipation-free energy transmission in superconducting grids, highly efficient engines for electrical aviation or compact fusion reactors beyond ITER. However, current carrying capacities of CC could be further improved because they are still far from theoretical limits. As it has been shown, one of the possible robust ways to increase current carrying capacity of CC  is overdoping with oxygen the REBCO structure of CC. 

Treatment of GdBCO_CC under 100 bar of O₂ at 600 °C for 3 h led to an increase in J_c (77K, 0 T) by 6% and a decrease in the c-parameter of Gd123 to 1.17310 nm, which may be associated not only with overdoping with oxygen, but also with silver diffusion into Gd123. No correlation was observed between J_c, T_c, c-parameter of RE123 (RE=Eu, Gd) and carrier density n_H of EuBCO_CC and GdBCO_CC treated at 300-800 ^oC, 1-160 bar O₂ for 3-12 h.

We have started investigating high oxygen pressure treatments of GdBCO and EuBCO-BHO Coated Conductors previously oxygenated with standard treatments. For our experiments we used commercially produced GdBCO and EuBCO-BHO coated conductors provided by Fujikura. The CC samples have been previously oxygenated with their standard process. For the high oxygen pressure post-treatment,  GdBCO and EuBCO-BHO coated conductors with 2 mm Ag layer on the surface of GdBCO CC and without and with Ag layer on the surface of  EuBCO-BHO CC were used. Therefore, before experiments the upper Cu layer (and in some cases the Ag layer) was chemically removed from CCs tapes. In the case of EuBCO-BHO_CCs  the Ag upper layer was removed chemically as well, but for some experiments it was preserved as indicated. The architecture (from  top) of the studied CCs was as follows: (1) FYSC : Ag (2μm)/ GdBCO (1.8μm)/ CeO₂ (700 nm)/ MgO/ / Al₂O₃/Y₂O₃ /Hastelloy (75 μm); (2) FESC : EuBCO (2.5μm)+BHO Nanorods/ CeO₂ (700 nm)/ /MgO/ Al₂O₃/Y₂O₃ / Hastelloy (50 μm); (3) Ag (2μm)/ EuBCO (2.5μm)+BHO Nanorods/ CeO₂ (700 nm)MgO/ / Al₂O₃/Y₂O₃/ Hastelloy (50 μm).

A specially designed tube furnace was used for the oxygenation process. The oxygen pressure in the furnace varied from 1 to 160 bar, the temperature from 300 to 800 °C, and the heating rate was 5 ºC/min. After, a dwell of temperature for a certain time held at the required pressure was attained, and afterwards the heater was turned off.

Results on critical current density, superconducting transition temperature, charge carrier densities, c-lattice parameter and Auger, indicate that high oxygen pressures of 100 and 160 bar can be sustained by these materials when high temperatures are used and that this can be a route to overdope these Coated Conductors. Furthermore, we evidence that these post oxygen treatements should be done without Ag etching so that the REBCO material is not exposed to air during the high pressure treatments. Futher work is on-going to obtain the best conditions to increase the charge carrier concentration and consequently the criticial current density in a robust way.

AcknowledgementS: We acknowledge funds from MUGSUP, UCRAN20088 project from CSIC programme from the scientific cooperation with Ukraine, the Spanish Ministry of Science and Innovation and the European Regional Development Fund, MCIU/AEI/FEDER for SUPERENERTECH (PID2021-127297OB-C21), FUNFUTURE “Severo Ochoa” Program for Centers of Excellence in R&D (CEX2019-000917-S), and HTS-JOINTS (PDC2022-133208-I00),  NAS of Ukraine Project III-7-24 (0788)  Authors also thanks Fujikura for supplying the samples