An assessment is undertaken for the formation of anorthite crystalline phase in a melilite-based glass composition (CMAS: 38.7CaO–9.7MgO–12.9Al 2 O 3 –38.7SiO 2 mol %), used as a sealing material in solid oxide fuel cells, in view of the detrimental effect of anorthite on the sealing properties. Several advanced characterization techniques are employed to assess the material after prolonged heat treatment, including neutron powder diffraction (ND), X-ray powder diffraction (XRD), 29 Si and 27 Al magic-angle spinning nuclear magnetic resonance (MAS-NMR), and in situ Raman spectroscopy. ND, 29 Si MAS-NMR, and 27 Al MAS-NMR results revealed that both Si and Al adopt tetrahedral coordination and participate in the formation of the network structure. In situ XRD measurements for the CMAS glass demonstrate the thermal stability of the glass structure up to 850 °C. Further heat treatment up to 900 °C initiates the precipitation of melilite, a solid solution of akermanite/gehlenite crystalline phase. Qualitative XRD data for glass-ceramics (GCs) produced after heat treatment at 850 °C for 500 h revealed the presence of anorthite along with the melilite crystalline phase. Rietveld refinement of XRD data indicated a high fraction of glassy phase (∼67%) after the formation of crystalline phases. The 29 Si MAS-NMR spectra for the CMAS-GC suggest the presence of structural units in the remaining glassy phase with a polymerization degree higher than dimer units, whereas the 27 Al MAS-NMR spectra revealed that most Al 3+ cations exhibit a 4-fold coordination. In situ Raman spectroscopy data indicate that the formation of anorthite crystalline phase initiated after 240 h of heat treatment at 850 °C owing to the interaction between the gehlenite crystals and the remaining glassy phase.

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO and SiO structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-AlO-SiO (CMAS) glass composition was modified by substituting MgO for CaO and SiO for AlO to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), Si andAl magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO and AlO tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO and AlO tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al-O-Al bonds; this region coexists with a predominantly SiO-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO and AlO structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO-rich region at higher temperatures, leading to the formation of additional crystalline phases.

The electrochemical performance of the Ni-containing cermet anodes, widely used for all types of SOFCs, is essentially governed by the triple phase boundary (TPB) formed by metal, solid electrolyte and gaseous phase. Although the TPB length and electrode surface area may be drastically increased using nanostructured components, information on the resultant effects in terms of the cermet properties and SOFC production technology is still scarce. The present work is centered on the appraisal of NiO morphologies and optimization of pre-treatment conditions for the powders used for anode screen-printing, with commercial nanocrystalline nickel (II) oxide as a model starting material. A series of NiO powders were prepared via annealing at various temperatures (300 -1100 o C) followed by chemical, structural and morphological characterization. The results of X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/ TEM), thermogravimetry and Raman spectroscopy reveal coreshell structure of nanosized NiO grains, formed due to surface hydration and oxidation under ambient conditions. Thermally induced desorption, and likely oxidation of organic components of the screen-printing pastes by the hyperstoichiometric oxygen, lead to poor quality of the electrode layers, thus making it necessary to introduce an additional powder pre-annealing step. The optimum pre-treatment temperature enabling to remove the absorbates, simultaneously preserving submicron grain size in the electrodes, corresponds to approximately 700 o C.

The present work is centered on the development of a new technique for in-situ Raman spectroscopy studies of local chemical and electrochemical reactions, phase transitions, strains and morphological alterations in the SOFC electrodes under working conditions, combined with electrochemical measurements. An appropriate selection of the electrodes geometry makes it possible to directly collect Raman spectra from the triple-phase boundary zone by passing the beam through transparent single-crystal solid electrolyte onto the interface, varying current density, temperature and atmosphere over the working electrode. The results of case studies, focused on reduction of Ni-containing cermet anodes, are presented.

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