Two step metal oxide thermochemical redox cycles have seen growing interest in the research community over the last two decades. In particular, they have often been studied as a means of converting heat from concentrated solar power to chemical energy, which can subsequently be used for a number of applications. In this work, we offer critical perspective and valuable insight on these research fields, from authors with a combined experience of 50+ years in metal oxide redox cycles. The current fundamental understanding of thermochemical redox materials, and the implications and limitations this has on the redox thermodynamics are discussed. The underlying fundamentals of the redox materials are then used to give insight into the theoretical limitations imposed on a number of applications including; solar thermochemical fuel production, solar energy storage for off sun power generation, thermochemical air separation, oxygen pumping, and thermochemical heat pumping. A number of recent novel process developments are also presented, which offer valuable motivation and direction for the respective fields. The analysis shows that oxides which undergo a stoichiometric phase change during reduction, such as ZnO or Co3O4, have much larger specific energy storage than materials undergoing partial reduction such as ceria and perovskites. The partial reduction materials generally have faster kinetics and better activity at low temperature, and the selection of materials for the various applications is often a compromise between the importance of high specific energy storage vs. fast kinetics and low temperature operation.

Applications and limitations of two step metal oxide thermochemical redox cycles; a review

Gas–solid thermochemical heat storage reactors for high-temperature applications

Mechanical performance of three oxide/oxide ceramic matrix composites (CMCs) based on Nextel 610 fibers and SiOC, alumina, and mullite/SiOC matrices respectively, is evaluated herein. Tensile strength and stiffness of all materials decreased at 1000 °C and 1200 °C, probably because of degradation of fiber properties beyond 1000 °C. Microstructural changes in the composites during exposure at 1000 °C and 1200 °C for 50 h reduce their flexural strength, fracture toughness and work of fracture. A literature review regarding mechanical properties of several oxide/oxide CMCs revealed lower influence of fiber properties on composite strength compared with elastic modulus. The tested composites exhibit comparable stiffness and strength but higher fracture toughness compared with average values determined from a literature review. Considering CMCs with different compositions, we observed an interesting linear trend between strength and fracture toughness. The validity of the linear relationship between fracture strength and flexural toughness for CMCs is discussed.

Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments

Effects of sintering temperature on mechanical properties of 3D mullite fiber (ALF FB3) reinforced mullite composites

Influence of composite powders’ microstructure on the microstructure and properties of Al2O3–TiO2 coatings fabricated by plasma spraying

High strength (2–3 GPa) of polycrystalline oxide fibers used as reinforcements of ceramic bodies is achieved as a result of submicron grain size. The fine-grained microstructure, however, is not stable at high temperatures and hence grain coarsening must be considered. Coarsening of alumina in commercial 3 M Nextel fibers occurs above 1200 °C going along with significant strength reduction. Analysis of isothermal grain growth reveals relative little time dependency which is attributed to limited grain boundary mobility. Activation energy of alumina grain growth is 660 kJ/mol. Moderate grain boundary mobility of Nextel 650 and 720 fibers is attributed to second phase blocking while in case of Nextel 610 alumina fibers sub-nanometer silica grain boundary films are assumed to affect the grain boundary mobility. Pronounced grain growth occurs in Nextel 610 fibers as soon as stabilizing silica films dissipate either by outwards diffusion or by hydroxylation and subsequent volatilization in the presence of hot water vapour. Mullite grains are less prone against coarsening than alumina as evidenced in activation energy (900 kJ/mol) and very low grain boundary mobility at temperatures below 1600 °C.

Degradation of oxide fibers by thermal overload and environmental effects

The long-term tensile creep behavior of all-oxide ceramic matrix composites (CMCs) was investigated. The accompanying fiber bundle tests revealed a strong influence of processing conditions on the creep resistance. CMCs with perpendicular fiber orientations were tested under two different load directions, moreover CMCs with nearly unidirectional fiber texture were investigated. Depending on the conditions, the absence of a steady-state creep stage was observed in most cases and due to an early onset of damage. As the majority of the samples was tested up to high strains and rupture, different failure mechanisms could be evaluated. An attempt was made to estimate fiber controlled creep rates via bundle data, but this approach also revealed problems when elastic data from unidirectional composites are transferred to CMCs with more complex fiber architecture.

Creep investigations of alumina-based all-oxide ceramic matrix composites

https://elib.dlr.de/91688/2/SolarPACES2014_Abstract_Tescari.pdf

Design of a Thermochemical Storage System for Air-operated Solar Tower Power Plants

The main aim of the investigation was to quantify the influence of production-related cross-lines on static mechanical properties (tensile, flexural and shear) of an oxide-oxide CMC as a comparison between specimens with cross-lines and specimens without cross-lines in tested regions. Investigated material was a weak-matrix oxide-oxide CMC (WHIPOX™) made of Nextel™ 610 fibers (3000 denier) and alumina matrix with a special winding pattern. Mechanical tests at room temperature revealed that cross-lines were local weak regions in a wound component. Spatial separation of the cross-line within the composite (2 mm shift from layer to layer) did not improve the negative influence of the cross-lines on mechanical properties. Fractographic investigations revealed that cross-lines acted as a trigger of material failure.

Mechanical investigation of weak regions in a wound oxide-oxide ceramic matrix composite

Highly porous and fully crystalline mullite fiber compacts were fabricated by uniaxial hot pressing of stacked short-fiber mats without binders or sintering aids. The special feature of this new fabrication method is the use of MAFTEC® organic bound mat (OBM) type fiber mats consisting of semicrystalline “MLS-2” short fibers. In contrast to conventional polycrystalline mullite fibers, semicrystalline MLS-2 fibers consist of amorphous silica and nanoscale transitional alumina. Due to this special microstructure, MLS-2 fibers are less prone to fiber breakage upon shear load and therefore can well withstand pressure-assisted consolidation. Hot pressing of stacked OBM to highly porous fiber compacts is facilitated by the thermally driven softening of amorphous silica above 900 °C; favorable deformation and consolidation rates are achieved above 1050 °C. Above 1250 °C the mullite is crystallized at the expense of amorphous silica and simultaneously both deformation and consolidation comes to an end. Obtained MAFTEC OBM-derived fiber compacts consist only of crystalline mullites, and therefore mechanical properties are favorably retained at high temperatures.

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Ferdinand Flucht

Papers

Degradation of oxide fibers by thermal overload and environmental effects

Creep investigations of alumina-based all-oxide ceramic matrix composites

Design of a Thermochemical Storage System for Air-operated Solar Tower Power Plants

Mechanical investigation of weak regions in a wound oxide-oxide ceramic matrix composite

Manufacturing of porous mullite fiber compacts by uniaxial hot pressing of semicrystalline MAFTEC® MLS-2 organic bound mats