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Solid oxide fuel cells are characterized by a high efficiency for converting chemical energy into electricity and fuel flexibility. This research work focuses on developing durable and efficient anodes for solid oxide fuel cells (SOFCs) based on exsolving nickel from the perovskite structure. A-site-deficient La- and Ni-doped strontium titanates (La0.2Sr0.7Ti0.95Ni0.05O3−δ, LSTN) were synthesized using four different techniques and mixed with Ce0.8Gd0.2O2−δ (GDC) to form the SOFC anode. The synthesis routes of interest for comparison included solid-state, sol-gel, hydrothermal, and co-precipitation methods. LSTN powders were characterized via XRD, SEM, TPR, BET and XPS. In situ XRD during reduction was measured and the reduced powders were analyzed using TEM. The impact of synthesis route on SOFC performance was investigated. All samples were highly durable when kept at 0.5 V for 48 h at 800 °C with H2 fuel. Interestingly, the best performance was observed for the cell with the LSTN anode prepared via co-precipitation, while the conventional solid-state synthesis method only achieved the second-best results. [ABSTRACT FROM AUTHOR]

Phase separation by the spinodal decomposition or nucleation and growth mechanisms is an established method for the generation of thermodynamically stable sub-micron features, capable of both reducing the lattice thermal conductivity, κl, and stabilizing its value, while obtaining high and stable thermoelectric (TE) figure of merit ZT values during practical applications. In the Mg2(Si,Sn,Ge) class of TE materials, a miscibility gap and a thermodynamic tendency of phase separation were reported in the Mg2Si–Mg2Sn quasi-binary section of the ternary phase diagram, capable of enhancing and stabilizing the TE performance, by κl minimization. Yet, no such tendency was ever reported for the Mg2Si–Mg2Ge quasi-binary system, prohibiting the fulfillment of its TE potential. It is currently shown that a similar (yet, less pronounced) tendency of phase separation is also apparent in the Mg2Si–Mg2Ge quasi-binary system, into Mg2Si- and Mg2Ge-rich Mg2Si0.5±δGe0.5±δ phases. This phenomenon is enhanced upon bismuth doping. Upon 1.5, 2, and 2.5% bismuth doping of Mg2Si0.5Ge0.5, following induction melting and hot-pressing, the solubility limit of Bi was found as 1.5–2%, while increasing the bismuth content resulted in significant Mg3Bi2 segregation into grain boundaries. The combined phase separation and segregation effects on κl reduction with the electronic effect of Bi doping resulted in a reasonably high maximal ZT of 0.9, which was observed upon 2.5% Bi doping. [ABSTRACT FROM AUTHOR]

The effect of a galia (Ga2O3) addition on the crystallographic phase evolution and the crystal growth behavior of zirconia (ZrO2) rich powder were investigated. Binary compositions of galia-zirconia powder (on the zirconia-rich side of the quasi-binary phase diagram) were obtained by the sol-gel method and calcined at different temperatures for 2 h. Rietveld refinement (FullProf program) was used to determine the crystallographic phase composition. Crystal size was approximated based on X-ray diffractions analysis combined with estimates of average crystal size obtained by processing high-resolution scanning electron microscopy images. The surface area of the powders obtained was measured via the Brunauer-Emmett-Teller method. Zirconia-galia sol-gel powders were found to form tetragonal or mixed tetragonal-monoclinic solid solution phases depending on both composition and calcination temperature. It was shown that galia suppressed crystal growth process in zirconia, producing finer nano-powders with higher surface area compared to pure zirconia. [ABSTRACT FROM AUTHOR]

Zirconia (ZrO2) and yttria-stabilized-zirconia (YSZ) are known as advanced oxides, widely applied in gas-turbine engines, fuel cells and other applications, requiring controlled thermal conductivity values. In the current research hot-pressed ZrO2–8YSZ composites, were synthesized. The effect of the three apparent phases, 8YSZ, ZrO2 and the pores on the thermal conductivity was analyzed using the general effective media (GEM) approach, while considering their relative amounts and morphology. [ABSTRACT FROM AUTHOR]

The effect of a galia (Ga2O3) addition on the crystallographic phase evolution and the crystal growth behavior of zirconia (ZrO2) rich powder were investigated. Binary compositions of galia-zirconia powder (on the zirconia-rich side of the quasi-binary phase diagram) were obtained by the sol-gel method and calcined at different temperatures for 2 h. Rietveld refinement (FullProf program) was used to determine the crystallographic phase composition. Crystal size was approximated based on X-ray diffractions analysis combined with estimates of average crystal size obtained by processing high-resolution scanning electron microscopy images. The surface area of the powders obtained was measured via the Brunauer-Emmett-Teller method. Zirconia-galia sol-gel powders were found to form tetragonal or mixed tetragonal-monoclinic solid solution phases depending on both composition and calcination temperature. It was shown that galia suppressed crystal growth process in zirconia, producing finer nano-powders with higher surface area compared to pure zirconia. [ABSTRACT FROM AUTHOR]

Phase separation by the spinodal decomposition or nucleation and growth mechanisms is an established method for the generation of thermodynamically stable sub-micron features, capable of both reducing the lattice thermal conductivity, κ l , and stabilizing its value, while obtaining high and stable thermoelectric (TE) figure of merit ZT values during practical applications. In the Mg 2 (Si,Sn,Ge) class of TE materials, a miscibility gap and a thermodynamic tendency of phase separation were reported in the Mg 2 Si-Mg 2 Sn quasi-binary section of the ternary phase diagram, capable of enhancing and stabilizing the TE performance, by κ l minimization. Yet, no such tendency was ever reported for the Mg 2 Si-Mg 2 Ge quasi-binary system, prohibiting the fulfillment of its TE potential. It is currently shown that a similar (yet, less pronounced) tendency of phase separation is also apparent in the Mg 2 Si-Mg 2 Ge quasi-binary system, into Mg 2 Si- and Mg 2 Ge-rich Mg 2 Si 0.5± δ Ge 0.5± δ phases. This phenomenon is enhanced upon bismuth doping. Upon 1.5, 2, and 2.5% bismuth doping of Mg 2 Si 0.5 Ge 0.5 , following induction melting and hot-pressing, the solubility limit of Bi was found as 1.5-2%, while increasing the bismuth content resulted in significant Mg 3 Bi 2 segregation into grain boundaries. The combined phase separation and segregation effects on κ l reduction with the electronic effect of Bi doping resulted in a reasonably high maximal ZT of 0.9, which was observed upon 2.5% Bi doping.

Rare-earth (RE) oxides are important in myriad fields, including metallurgy, catalysis, and ceramics. However, the phase diagram of RE oxides in the nanoscale might differ from the phase diagrams for bulk, thus attracting attention nowadays. We suggest that grain size in the nanoscale also determines the obtained crystallographic phase along with temperature and pressure. For this purpose, nanoparticles of Sm 2 O 3 and Eu 2 O 3 were mixed in an inert MgO matrix via the sol-gel method. This preparation method allowed better isolation of the oxide particles, thus hindering the grain growth process associated with increasing the temperature. The mixed oxides were compared to pure oxides, which were heat-treated using two methods: gradual heating versus direct heating to the phase transition temperature. The cubic phase in pure oxides was preserved to a higher extent in the gradual heating treatment compared to the direct heating treatment. Additionally, in MgO, even a higher extent of the cubic phase was preserved at higher temperatures compared to the pure oxide, which transformed into the monoclinic phase at the same temperature in accordance with the phase diagram for bulk. This indicates that the cubic phase is the equilibrium phase for nanosized particles and is determined also by size.

Zirconia (ZrO2) and yttria-stabilized-zirconia (YSZ) are known as advanced oxides, widely applied in gas-turbine engines, fuel cells and other applications, requiring controlled thermal conductivity values. In the current research hot-pressed ZrO2-8YSZ composites, were synthesized. The effect of the three apparent phases, 8YSZ, ZrO2 and the pores on the thermal conductivity was analyzed using the general effective media (GEM) approach, while considering their relative amounts and morphology.