High-temperature Energy Applications Research Articles

Tracing mechanistic pathways and reaction kinetics toward equilibrium in reactive molten salts.

In the dynamic environment of multi-component reactive molten salts, speciation unfolds as a complex process, involving multiple competing reaction pathways that are likely to face free energy barriers before reaching the reaction equilibria. Herein, we unravel intricate speciation in the AlCl3-KCl melt compositions with rate theory and ab initio molecular dynamics simulations. We find that the compositions with 100 and 50 mol% AlCl3 exclusively comprise neutral Al2Cl6 dimers and charged AlCl4- monomers, respectively. In intermediate AlCl3-KCl compositions, the chemical speciation proves to be a very complex process, requiring over 0.5 nanosecond to reach an equilibrium distribution of multiple species. It is a consequence of the competitive formation and dissociation of additional species, including charged Al dimers, trimers, and tetramers. Here, the species formation occurs through ion exchange events, which we explain by computing free energy landscapes and employing a Marcus-like rate theory. We show that both interspecies and intraspecies ion exchanges are probable and are dictated by the local structural reorganization reflected in the change of local coulombic fields. The species distributions are validated by comparing computed Raman spectra and neutron structure factors with the available experimental data. We find an excellent simulation-experiment agreement in both cases. Nevertheless, Raman spectroscopy turns out to be particularly advantageous for distinguishing between unique species distributions because of the distinct vibrational signatures of different species. The mechanistic insight into reaction dynamics gained in this study will be essential for the advancement of molten salts as reactive media in high-temperature energy applications.

High-temperature reduction thermochemistry of SrVO3−δ

Cubic SrVO3 perovskite oxide is an attractive candidate for high-temperature energy applications due to its favorable features such as multiple oxidation state cations, high structural and thermal stabilities, ability to accommodate a large number of oxygen vacancies, and cost-effectiveness. Herein, the temperature-dependent reduction properties of SrVO3 have been studied using accurate first-principles calculations to reveal the effects of oxygen vacancies and temperature on the reduction potential of SrVO3−δ , δ = 0–0.125. The reduction potential of SrVO3−δ was found to be significantly impacted by increasing oxygen vacancy concentration and temperature. Analysis of the electronic and vibrational properties of SrVO3−δ for differing δ revealed the origin of this reduction behavior. The electronic structure analysis shows that the reduction of SrVO3−δ upon oxygen vacancy formation is highly localized to the neighboring V4+ t2g states in the vicinity of the oxygen defect, irrespective of δ. A comparison of the vibrational density of states of defect-free and reduced SrVO3 demonstrated that the ionic contributions to the phonon density of states, and hence to the thermal contributions to the SrVO3−δ lattices, were significantly altered by the introduction of oxygen vacancies, which ultimately impacted the temperature-dependent reduction behavior of SrVO3−δ .

Hot corrosion behaviour of Hastelloy X and Inconel 625 in an aggressive environment for superalloys for high-temperature energy applications

Hot corrosion behaviour of Hastelloy X and Inconel 625 in an aggressive environment for superalloys for high-temperature energy applications

Preface: emergent coatings and thin films for high-temperature energy applications

Preface: emergent coatings and thin films for high-temperature energy applications

Laser processing of ceramic materials for electrochemical and high temperature energy applications

The laser is a powerful tool for materials processing, incorporated already in many industrial processes and laboratory procedures. In this work, we are concerned with laser processing applied to research and development of ceramics for electrochemical cells and other high temperature oxide ceramics for energy applications. Solidification of single crystals or composites of relevant oxides can be performed by the laser assisted floating zone method, providing samples for structural, mechanical or functional fundamental research, as well as knowledge about its manufacture by melt processes. Selective laser melting of these ceramic oxides is a very promising technology, at the research level. Successful examples of surface laser melting of oxide eutectic composites are presented. The technologies of subtractive laser processing of ceramics (cutting, drilling, structuring, cleaning, etc.) are more developed, and the research is directed towards the optimization of mechanisms, increase of resolution and efficiency and the investigation of the effects of the laser treatment on the functional performance. Different laser processes of SOC (solid oxide cell) components are shown to decrease the ohmic, concentration and activation losses. The manuscript describes the state-of-the art of the technologies as applied to oxide and composite materials present in solid oxide electrochemical devices (SOFC, SOEC, and batteries) and selective emitters for thermophotovoltaics, with emphasis on the last achievements by the authors team.

Charge and discharge behavior of elemental sulfur in isochoric high temperature thermal energy storage systems

Thermal energy storage with elemental sulfur is a low-cost alternative to molten salts for many medium to high-temperature energy applications (200–600 °C). In this effort, by examining elemental sulfur stored isochorically inside isolated pipes, we find that sulfur provides attractive charge/discharge performance since it operates in the liquid-vapor regime at the temperatures relevant to many important applications, such as combined heat and power (CHP) plants and concentrating solar power (CSP) plants with advanced power cycle systems. The isolated pipe configuration is relevant to shell-and-tube thermal battery applications where the heat transfer fluid flows over the storage pipes through the shell. We analyze the transient charge and discharge behavior of sulfur inside the pipes using detailed computational modeling of the complex conjugate heat transfer and fluid flow phenomena. The computational model is validated against experiments of a single tube with well-defined temperature boundary conditions and internal temperature measurements. The model results evaluate the influence of pipe diameter on charge and discharge times, heat transfer rate, and Nusselt number due to buoyancy driven convection currents. Depending on the Rayleigh number (pipe diameter), the average Nusselt number obtained for discharge is 3–14 times higher than proposed solid-liquid phase change technologies based on molten salt, which are limited in their performance due to conduction based solidification and low thermal conductivity. The results show competing trade-offs between increase in heat transfer coefficient, thermal energy stored in sulfur, and increase in charge and discharge time with increase in pipe diameter. A preferred pipe diameter can be determined for target applications based on their requirements and these competing trade-offs. A validated fundamental correlation for Nusselt number as a function of Rayleigh number for charge and discharge is developed that can be used to design the sulfur-based thermal storage system for transient operation.

Characterization of oxygen-dependent stability of selected mixed-conducting perovskite oxides

For high-temperature energy applications, e.g., as solid oxide fuel cell cathodes or as dense ceramic oxygen-transport membranes, mixed ionic-electronic conducting (MIEC) perovskites, namely of the composition (AxSr1−x)(CoyFe1−y)O3−δ (A=La, Pr, Ba), are promising candidates given their excellent oxygen-ionic and electronic transport properties. A high level of oxygen non-stoichiometry is an important prerequisite for a high performance; however, the structural stability of the oxide is also affected by oxygen deficiency.By coulometric titration experiments performed on selected state-of-the-art MIEC powder samples in a tubular zirconia “oxygen pump” setup, facilitating precise control of the oxygen partial pressure in the entire range between pO2=10−18…1bar at temperatures between 700…1000°C, the low-pO2 stability limits can be determined. By monitoring the electrical current necessary for pumping oxygen through the solid electrolyte into or out of the measurement chamber, the amount of oxygen transported and, thus, changes in oxygen stoichiometry can be determined. The low-pO2 stability limits for all investigated high-performance MIEC oxides were similar (below 10−10bar at 900°C).

High-surface-area ordered mesoporous oxides for continuous operation in high temperature energy applications

The collapse of nanostructures at high temperature is one of the main drawbacks for the implementation of nanomaterials in some energy applications. An exciting virtual non-degradation up to 1000 °C is presented here for ordered mesoporous gadolinia doped ceria. By using the nanocasting method based on the KIT-6 template, the long-term stability of the material is achieved when extending the self-limited grain growth regime, recently proved for thin films, to open three-dimensional structures. Contrary to widely employed high temperature stabilization treatments inside the template, this work shows the advantage of a counterintuitive and cost-effective thermal treatment at intermediate temperatures, lower than the operation temperature. The evolution of the mesostructure with time at high temperatures, ranging from 800 °C to 1100 °C, is reported in terms of the microstructure (grain size and specific surface area) and catalytic activity (redox ability and oxygen storage capacity). The possibility of extension of this methodology to almost all metal oxides and the capability of working at temperatures significantly over the state-of-the-art open a new avenue for the use of these high-surface area 3D nanostructures in up-to-now forbidden high temperature energy applications such as solid oxide fuel/electrolysis cells, gas separation membranes or high temperature catalysis.

Evaluation of Phenylnaphthalenes as Heat Transfer Fluids for High Temperature Energy Applications

The thermodynamic properties of 1- and 2-phenyl substituted polyaromatic hydrocarbons have been measured and estimated under conditions relevant to heat transport applications almost to the critical point. Densities, vapor pressures, and heat capacity measurements were used to derive critical temperature, pressure, and density for the phenylnaphthalenes. The thermal and radiolytic stability of 1-phenylnaphthalene was examined using thermogravimetric analysis, differential scanning calorimetery, and gamma irradiation. Low melting point, low vapor pressure, high critical temperature, and resistance to thermal decomposition may make phenylnaphthalenes suitable for heat transfer applications up to 800 K, including power generation and separations processes.

Thermal conductivity of plasma-sprayed W/SiC composite for high-temperature energy applications

W/SiC metal matrix composites were produced by gas tunnel type plasma spraying (GTPS) using a mixture of 12 wt.% SiC–88 wt.% W feedstock powder. This work aimed at the optimization of the plasma gun current for deposition of a W/SiC composite with fine microstructure on AISI 304 substrate. Characterization of deposits was performed in order to assess microstructure, micro-hardness, thermal diffusivity and thermal conductivity. WO 3 was detected in the composite deposits, which indicated that the tungsten partially oxidized during plasma spraying. Also, the deposit composite was dense and nearly free of pores due to the little mismatch between the coefficient of thermal expansion (CTE) for W and SiC. Microhardness values gradually decreased as a function of input current due to the formation of WO 3 and the decomposition of SiC particles in high temperature flame region. The thermal conductivity as high as ∼ 59 W/mK was obtained at gun current 80 A. It was found that both tungsten oxide and structure imperfections have a significant influence on the thermal conductivity and mechanical properties.

95/01875 Comparative study of test methodologies for thermal shock and thermal fatigue testing of ceramics in high temperature energy applications

95/01875 Comparative study of test methodologies for thermal shock and thermal fatigue testing of ceramics in high temperature energy applications