Progress in material selection for solid oxide fuel cell technology: A review

Porcelain represents the foundation of the ceramics discipline and one of the most complex ceramic materials. Composed primarily of clay, feldspar, and quartz, porcelains are heat-treated to form a mixture of glass and crystalline phases. This review focuses on raw materials, processing, heat treatment, and mechanical behavior. Because of the complexities of the porcelain system and despite the substantial amount of research already conducted within the field, there remain significant opportunities for research and study, particularly in the areas of raw material understanding, processing science, and phase and microstructure evolution.

Porcelain—Raw Materials, Processing, Phase Evolution, and Mechanical Behavior

Sealants for solid oxide fuel cells

High temperature hermetic seal is essential for utilizing the full potentials of planar solid oxide fuel/electrolyzer cells. A seal glass needs to have excellent thermal and chemical stabilities, mechanical integrity, and sealing ability in stringent oxidizing and reducing environments and for hundreds of thermal cycles. Comprehensive analysis and understanding are needed in the design of a seal glass in order to meet the demanding requirements. In this review, seal requirements and the advantages of glass-based seals are first discussed. Different glass compositions are reviewed from thermal, chemical, mechanical, and electrical property point of view. Based on these considerations, glass composition design approaches are provided that aid in search of the best seal glass that can offer all the desired properties and stabilities. Required thermal properties such as thermal expansion coefficient, glass transition temperature, and softening temperature have been achieved in several alkaline earth borosilicate glass systems. Interfacial compatibility with other cell components has also been obtained for several alkaline earth borosilicate glass systems. However, long-term thermal and chemical stabilities are yet to be achieved. Among all the glass systems studied, a boron-free SrO–La2O3–Al2O3–SiO2 seal glass has been specifically discussed because it has met all the thermal and chemical properties along with high thermal and chemical stabilities. For future endeavors, the relationships between seal glass constituents, glass network structures, required thermal, chemical, mechanical, and electrical properties need to be established in order to improve sealing performance while maintaining design flexibility and low fabrication cost.

Glass-based seals for solid oxide fuel and electrolyzer cells - A review

Structural complexity of minerals is characterized using information contents of their crystal structures calculated according to the modified Shannon formula. The crystal structure is considered as a message consisting of atoms classified into equivalence classes according to their distribution over crystallographic orbits (Wyckoff sites). The proposed complexity measures combine both size- and symmetry-sensitive aspects of crystal structures. Information-based complexity parameters have been calculated for 3949 structure reports on minerals extracted from the Inorganic Crystal Structure Database. According to the total structural information content, I G,total , mineral structures can be classified into very simple (0–20 bits), simple (20–100 bits), intermediate (100–500 bits), complex (500–1000 bits), and very complex (> 1000 bits). The average information content for mineral structures is calculated as 228(6) bits per structure and 3.23(2) bits per atom. Twenty most complex mineral structures are ( I G,total in bits): paulingite (6766.998), fantappieite (5948.330), sacrofanite (5317.353), mendeleevite-(Ce) (3398.878), bouazzerite (3035.201), megacyclite (2950.928), vandendriesscheite (2835.307), giuseppetite (2723.097), stilpnomelane (2483.819), stavelotite-(La) (2411.498), rogermitchellite (2320.653), parsettensite (2309.820), apjohnite (2305.361), antigorite ( m = 17 polysome) (2250.397), tounkite (2187.799), tschoertnerite (2132.228), farneseite (2094.012), kircherite (2052.539), bannisterite (2031.017), and mutinaite (2025.067). The following complexity-generating mechanisms have been recognized: modularity, misfit relationships between structure elements, and presence of nanoscale units (clusters or tubules). Structural complexity should be distiguished from topological complexity. Structural complexity increases with decreasing temperature and increasing pressure, though at ultra-high pressures, the situation may be different. Quantitative complexity measures can be used to investigate evolution of information in the course of global and local geological processes involving formation and transformation of crystalline phases. The information-based complexity measures can also be used to estimate the ‘ease of crystallization’ from the viewpoint of simplexity principle proposed by J.R. Goldsmith (1953) for understanding of formation of simple and complex mineral phases under both natural and laboratory conditions. According to the proposed quantitative approach, the crystal structure can be viewed as a reservoir of information encoded in its complexity. Complex structures store more information than simple ones. As erasure of information is always associated with dissipation of energy, information stored in crystal structures of minerals must have an important influence upon natural processes. As every process can be viewed as a communication channel, the mineralogical history of our planet on any scale is a story of accumulation, storage, transmission and processing of structural information.

Structural complexity of minerals: information storage and processing in the mineral world

Barium aluminosilicate glasses are being investigated as matrix materials in high-temperature ceramic composites for structural applications. Kinetics of crystallization of two refractory glass compositions in the barium aluminosilicate system were studied by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). From variable heating rate DTA, the crystallization activation energies for glass compositions (wt percent) 10BaO-38Al2O3-51SiO2-1MoO3 (glass A) and 39BaO-25Al2O3-35SiO2-1MoO3 (glass B) were determined to be 553 and 558 kJ/mol, respectively. On thermal treatment, the crystalline phases in glasses A and B were identified as mullite (3Al2O3-2SiO2) and hexacelsian (BaO-Al2O3-2SiO2), respectively. Hexacelsian is a high-temperature polymorph which is metastable below 1590 C. It undergoes structural transformation into the orthorhombic form at approximately 300 C accompanied by a large volume change which is undesirable for structural applications. A process needs to be developed where stable monoclinic celsian, rather than hexacelsian, precipitates out as the crystal phase in glass B.

Crystallization kinetics of BaO-Al2O3-SiO2 glasses

Crystallization kinetics of BaOAl2O32Si02 (BAS) and SrOAl2O32SiO2 (SAS) glasses in bulk and powderforms have been studied by non-isothermal differential scanning calorimetry (DSC) The crystal growth activation energies were evaluated to be 473 and 451 kJ mol−1 for bulk samples and 560 and 534 kJ mol−1 for powder specimens in BAS and SAS glasses, respectively Development of crystalline phases on thermal treatments of glasses at various temperatures has been followed by powder X-ray diffraction Powder samples crystallized at lower temperatures than the bulk and the crystallization temperature was lower for SAS glass than BAS Crystallization in both glasses appeared to be surface nucleated The high temperature phase hexacelsian, MAl2Si2O8 (M = Ba or Sr), crystallized first by nucleating preferentially on the glass surface Also, monoclinic celsian does not nucleate directly in the glass, but is formed at higher temperatures from the transformation of the metastable hexagonal phase In SAS the transformation to monoclinic celsian occurred rapidly after 1 h at 1100 °C In contrast, in BAS this transformation is sluggish and difficult and did not go to completion even after 10 h heat treatment at 1400 °C The crystal growth morphologies in the glasses have been observed by optical microscopy Some of the physical properties of the two glasses are also reported

Crystal growth kinetics in BaOAl2O32SiO2 and SrOAl2O32SiO2 glasses

Powders of roller quenched (Sr,Ba)O-Al2O3-2SiO2 glasses of various compositions were uniaxially pressed into bars and hot isostatically pressed at 1350 C for 4 hours or cold isostatically pressed and sintered at different temperatures between 800 to 1500 C for 10 or 20 hours. Densities, flexural strengths, and linear thermal expansion were measured for three compositions. The glasss transition and crystallization temperatures were determined by Differential Scanning Calorimetry (DSC). The liquidus and crystallization temperature from the melt were measured using high temperature Differential Thermal Analysis (DTA). Crystalline phases formed on heat treatment of the glasses were identified by powder x ray diffraction. In Sr containing glasses, the monoclinic celsian phase always crystallized at temperatures above 1000 C. At lower temperatures, the hexagonal analog formed. The temperature for orthorhombic to hexagonal structure transformation increased monotonically with SrO content, from 327 C for BaO-Al2O3-2SiO2 to 758 C for SrO-Al2O3-2SiO2. These glass powders can be sintered to almost full densities and monoclinic celsian phase at a relatively low temperature of 1100 C.

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Crystallization and properties of Sr-Ba aluminosilicate glass-ceramic matrices

Monophasic and diphasic xerogels have been prepared as precursors for mullite (3Al2O3-2SiO2). Monophasic xerogel was synthesized from tetraethyl orthosilicate and aluminium nitrate nanohydrate and the diphasic xerogel from colloidal suspension of silica and boehmite. The chemical and structural evolutions, as a function of thermal treatment in these two types of sol-gel-derived mullite precursor powders, have been characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and infrared spectroscopy (IRS). Monophasic xerogel transforms to an aluminium-silicon spinel from an amorphous structure at ∼980 ° C. The spinel then changes into mullite on further heating. Diphasic xerogel forms mullite at ∼1360 ° C. The components of the diphasic powder react independently up to the point of mullite formation. The transformation in the monophasic powder occurs rapidly and yields strongly crystalline mullite with no other phases present. The diphasic powder, however, transforms rather slowly and contains remnants of the starting materials (α-Al2O3, cristobalite) even after heating at high temperatures for long periods (1600 ° C, 6 h). The diphasic powder could be sintered to high density but not the monophasic powder, in spite of its molecular-level homogeneity.

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Phase transformations in xerogels of mullite composition

The crystallization of a celsian glass composition was investigated as a possible high-temperature ceramic matrix material. Heat treatments invariably resulted in crystallization of the hexaclesian phase unless a flux, such as lithia, was added or a nucleating agent used (e.g., celsian seeds). TEM analysis revealed complex microstructures. Glasses with Mo additions contained hexacelsian, mullite, and an Mo-rich glass. Li2O additions stabilized celsian but mullite and Mo-rich glass were still present.

Mark J. Hyatt

Papers

Crystallization kinetics of BaO-Al2O3-SiO2 glasses

Crystal growth kinetics in BaOAl2O32SiO2 and SrOAl2O32SiO2 glasses

Crystallization and properties of Sr-Ba aluminosilicate glass-ceramic matrices

Phase transformations in xerogels of mullite composition

Crystallization of a barium-aluminosilicate glass