Eutectic Fluoride Research Articles

The Microstructure and Mechanical Properties of High Entropy Alloy CoCrFeNiMn Matrix with Cr3C2 Reinforcement and Ag, BaF2/CaF2 Solid Lubrication

A series of CoCrFeNiMn high-entropy alloy matrix self-lubricating composites were prepared by spark plasma sintering. The composites are composed of an FCC phase, Cr7C3, Ag, and eutectic fluoride BaF2/CaF2 phases. The microstructure of the composites is uniform. The additional phases distribute along the boundary of equiaxed grains of the FCC phase. The compressive yield strength and fracture toughness decrease with the increase of eutectic fluoride BaF2/CaF2. The composites are susceptible to brittle cleavage fracture.

Corrosion Behavior of Incoloy®800H, Hastelloy®G35® and 316L Stainless Steel in the Molten Eutectic Fluoride Mixture FLiNaK and Its Vapors

This paper discusses the findings of a corrosion experiment which investigates a transition area which is between the lower area and upper area of the sample. In this experiment, the lower half of the sample surface is exposed to molten salt and the upper half of the sample surface is exposed to the vapors generated by molten FLiNak salt. Incoloy®800H and Hastelloy®G35® alloys and 316L stainless steel were selected for the corrosion experiment on the basis of their Cr content. The experiment was conducted at 600 °C for a period of 100 h. The results of the experiment show that, in the transition area, no abrupt change in corrosion mechanism takes place; the experiments also give us information on how the degree of degradation varies across the three areas of the samples. The experiment also showed two distinct corrosion mechanisms operating in the test samples: intergranular corrosion in the SS316L stainless steel test sample and continuous corrosion in Hastelloy®G35®. The results also show a progressive reduction in the concentrations of F, K and Na in the upper areas compared to the lower areas for Hastelloy©G35®. Cr is shown to have a critical role in the corrosion process, even when the sample surface is not in direct contact with the molten salt and is only in contact with the vapors generated by the molten salt.

Thermodynamic analysis and optimization of an integrated solar thermochemical hydrogen production system

In this study, thermodynamic analysis of solar-based hydrogen production via copper-chlorine (Cu–Cl) thermochemical water splitting cycle is presented. The integrated system utilizes air as the heat transfer fluid of a cavity-pressurized solar power tower to supply heat to the Cu–Cl cycle reactors and heat exchangers. To achieve continuous operation of the system, phase change material based on eutectic fluoride salt is used as the thermal energy storage medium. A heat recovery system is also proposed to use the potential waste heat of the Cu–Cl cycle to produce electricity and steam. The system components are investigated thoroughly and system hotspots, exergy destructions and overall system performance are evaluated. The effects of varying major input parameters on the overall system performance are also investigated. For the baseline, the integrated system produces 343.01 kg/h of hydrogen, 41.68 MW of electricity and 11.39 kg/s of steam. Overall system energy and exergy efficiencies are 45.07% and 49.04%, respectively. Using Genetic Algorithm (GA), an optimization is performed to evaluate the maximum amount of produced hydrogen. The optimization results show that by selecting appropriate input parameters, hydrogen production rate of 491.26 kg/h is achieved.

Microstructure and molten salt impregnation characteristics of a micro-fine grain graphite for use in molten salt reactors

The microstructure and molten salt impregnation characteristics of a micro-fine grain isotropic graphite ZXF-5Q from Poco Inc. was investigated. The microstructural characteristics of the pores caused by gas evolution, calcination cracks, Mrozowski cracks, and the crystal structure were characterized by optical microscopy, mercury porosimetry, helium pycnometry, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Results show that the ZXF-5Q has uniformly-distributed pores caused by gas evolution with very small entrance diameters (∼0.4 μm), and numerous lenticular Mrozowski cracks. Molten salt impregnation with a molten eutectic fluoride salt at 650 °C and 1, 3 and 5 atm, indicate that ZXF-5Q could not be infiltrated even at 5 atm due to its very small pore entrance diameter. Some scattered global salt particles found inside the ZXF-5Q are possibly formed by condensation of the fluoride salt steam during cooling.

Design, Fabrication, and Testing of Ceramic Plate-Type Heat Exchangers with Integrated Flow Channel Design

Novel plate-type heat exchangers (HXs) with high-power densities are proposed based on novel integrated flow channel designs. Net-shape plates were achieved by the molding of powder mixtures to wood-based composites (WBC's). HX stacks are built by plate-to-plate joining followed by pyrolysis and silicon melt infiltration. The C/SiSiC provided with PyC–SiC coating was gas-tight to helium up to 5.5 MPa. Corrosion tests with coupons were performed using a ternary eutectic fluoride salt (FLiNaK) as the intermediate heat transfer fluid. While SiC is vulnerable to corrosion by the salt, the coating offers a high degree of protection to the ceramic substrate.

Post-test corrosion analysis of high-temperature thermal energy storage capsules

Thermal energy storage capsules have been freeze-thaw cycled in vacuum at 1000 ± 100 K. The capsules were fabricated from Inconel 617® (Inco Alloys International, Inc.) and contained eutectic fluoride mixtures of sodium, magnesium, lithium, and potassium. Samples that were thermal cycled for 20,000 and 30,000 hr were analyzed for corrosion effects. Radiography indicated neither flaws nor inhomogeneities, and there were no significant microstructural changes in the container alloy. The microstructural damage penetrating the inside surface was as deep as 120 m and that penetrating the outside surface was approximately 150 m. Microprobe results on the containers revealed a concentration gradient of alloying elements. The aluminum concentration was reduced from 1.34% in the original matrix to 0.4% at 10 m from the inside surface, and chromium was reduced from a nominal value of 23% to 10% at the outer surface. The depletion of aluminum and chromium from the outer surface was due to vacuum vaporization at elevated temperature. X-ray diffraction revealed the formation of possible protective films consisting of MgNaF3 and MgF2. The measured and theoretically predicted concentrations of aluminum and chromium were in good agreement. It was concluded that the corrosion process is a solid-state diffusion-dominated process and an expected lifetime of 5 to 7 years is a reasonable estimate.

High temperature corrosion analysis of Inconel 617® thermal energy storage capsules

The corrosion degradation of three Inconel 617 Alloy Thermal Energy Storage capsules filled with eutectic fluoride salts LiF-MgF2-KF, LiF-MgF2-NaF and LiF-MgF2 were analyzed. The ultrahigh purity fluoride salts were previously sealed inside the Inconel capsules under controlled processing conditions and thermally cycled in vacuum over a temperature differential of 200 K near 1000 K for 10,000 hr. It was determined that the microstructural degradation is minimal but some aluminum from the alloy may have been displaced into the salt. Comparisons are made with previously reported equilibrium calculations and a semiquantitative diffusion model is presented to support the possibility of diffusion control over the corrosion. The results indicate that Inconel 617 may be a suitable material for fluoride salts in thermal energy storage applications.

Thermal Energy Storage and Control

Eutectic fluoride mixtures of alkali and alkaline earth metals posses extremely high thermal storage capacities, and are therefore suitable for space heating as well as for powering heat engines. Their melting points ranging from 449 to 832° C are well adapted to any high level heat supply. The compounds are of high chemical stability and show no corrosive action with stainless steel. In order to make efficient use of the stored heat, good thermal insulation and controllable heat transfer are achieved by a multifoil insulation jacket with a controllable hydrogen filling. For powering engines energy transfer is realized by heat pipes.

Controlled solidification of the eutectic LiF-MgF2

A two-phase “model” composite material of LiF and MgF2 has been prepared by unidirectional solidification of the mixed eutectic fluoride. The microstructure has been examined by optical and electron microscopy. The morphology is that of regularly arranged rods of the MgF2 phase (rutile structure type) in a continuous LiF matrix. This microstructure is restricted to high purity materials with a planar solid-liquid interface and progressively degrades into a cellular lamellar structure associated with constitutional supercooling. The crystallographic orientation relation between the phases for the regular morphology has been determined as: Growth axis //[001]MgF2//[001]LiF, Preferred interfaces //(110)MgF2//(100)LiF and (1¯10)MgF2//(010)LiF.