Molten Salt Infiltration Research Articles

Enhancing the high-temperature oxidation resistance of C-103 niobium alloy via hybrid treatment combining nickel plating with molten-salt siliconising

ABSTRACT A multi-phase silicide protective coating was prepared by electroplating nickel on the surface of C-103 niobium alloy (89Nb–10Hf–1Ti), followed by molten-salt non-electrolytic siliconising to enhance the high-temperature oxidation resistance of the substrate. The phase composition, microscopic morphology and elemental analysis of the coating were analysed via X-ray diffraction, scanning electron microscope, energy-dispersive spectrometer and hardness distribution. Oxidation characteristics were evaluated by comparing the oxidation inhibition effect of the coating on niobium alloy substrate and that of coating prepared via molten-salt infiltration alone at a constant temperature of 1100°C for 9 h. Results show that a silicide infiltration coating containing about 200-μm-thick compound layer with Ni2Si, NiSi, Nb5Si3, NbSi2, Ni3Si2 and TiNiSi multi-phase composition and ∼10–15-μm-thick diffusion layer were dense with few defects. In the hybrid synthesis process, the upper electroplated nickel coating can promote and modify the growth of subsequent siliconised coating. The duplex prepared silicon infiltration coating can reduce the oxidation rate of niobium alloy substrate by about 1/10, indicating good high-temperature oxidation resistance. This is because the siliconised coating is considerably thick, has a dense blocking structure and generates Ni4Si7Ti4, thereby blocking oxygen.

Excellent oxyacetylene ablation performance of C/C-Me-Zr-Hf-C (Me=Ta, Si) composites prepared by a novel two-step molten salt infiltration

In this study, to overcome the difficulties associated with the preparation of PyC-ceramic interface and controllable deposition of the structure, we initially used a two-step molten salt infiltration method to prepare a C/C-Me-Zr-Hf (Me = Ta, Si) composite with excellent oxyacetylene ablation performance. The microstructure and the behavior of ablative were addressed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as well as by X-ray diffraction（XRD）. The results reveals that C/C-ZrHfC-SiC composite prepared by two-step molten salt infiltration method have a controllable ultra-high temperature ceramic structure and better ablation resistance.

Molten salt infiltration–oxidation synergistic controlled lithium extraction from spent lithium iron phosphate batteries: an efficient, acid free, and closed-loop strategy

Based on a molten salt Li extraction from spent LiFePO4, which constitutes a one-step preparation with a LiFePO4 product, a closed-loop process was conducted to regenerate a spent LiFePO4 cathode.

The Effect of Molten Salt Infiltration on 2D SiCf/SiC Composite by Chemical Vapour Infiltration.

The SiCf/SiC composite manufactured by chemical vapour infiltration (CVI) is a kind of porous material. Liquid molten salt in a Molten Salt Reactor (MSR) may enter into the porous composites and affect their performance. Through the study of the internal pores in the material, the permeability behaviour of the material can be investigated, which is of great significance to the analysis of the properties of the material itself. However, there is less investigation on effects of molten salt infiltration on the internal pore structure of SiCf/SiC composites. In this paper, a molten salt infiltration experiment of 2D woven SiCf/SiC composites was implemented at 650 °C, 3 atm. SEM, CT and XRD were used to characterize it. The results indicated that the microstructure could be affected by partial molten salt infiltration and temperature change. The distribution of porosity of the composite showed an obvious transformation. The lattice spacing of SiC showed an increased tendancy of stress relaxation.

Ablation properties of C/C-UHTCs and their preparation by reactive infiltration of K2MeF6 (Me = Zr, Ti) molten salt

Ultra-high temperature ceramic-modified C/C composites (C/C-UHTCs) were prepared by the reactive infiltration of K2MeF6 (Me = Zr, Ti) mixed with Si and Zr-Si powders. Molten salt infiltration can be divided into two stages: salt ion melt and Me-Si alloy melt. In the temperature range below 1400 °C, Zr and Si dissolve in the molten salt, are carried by the ion melt, and precipitate at the PyC interface to form carbides. Above 1400 °C, a large amount of molten salt volatilises and thermally decomposes. The Me-Si alloy forms a melt and infiltrates the C/C matrix, and finally forms C/C-ZrC-SiC, C/C-Ti3SiC2-SiC, and C/C-ZrC-TiC-SiC composites. The C/C-ZrC-SiC composite with the highest ZrC content exhibited the lowest mass rate (2.6 ± 0.02 mg/s) and linear ablation rate (0.82 ± 0.04 μm/s), which were reduced by 43.5 and 50.8 %, respectively, compared to the unmodified C/C-ZrC-SiC composite.

Densification of A3-3 matrix graphite to inhibit the infiltration of liquid FLiBe salt in molten salt reactors

In this study, a novel graphite was prepared by adding MCMBs (mesocarbon microbeads) to densify matrix graphite A3−3, a typical fuel element and moderator material in high-temperature gas-cooled reactors. The densified graphite was evaluated as a potential fuel element material for molten salt reactors (MSRs). In particular, the effects of MCMB loading on physical properties and the corresponding effects on the infiltration of liquid fluoride salt in MSRs were studied, as molten salt infiltration can impact safe reactor operation. MCMBs with a mean particle size of approximately 3 μm were added to matrix graphite A3−3 at loading levels of 1 wt%, 5 wt%, 10 wt%, and 15 wt%, and samples were prepared by a quasi-isostatic pressing process at 250 MPa. Mercury porosimetry data showed that MCMB-densified graphite (MDGs) had lower porosity (approximately 15 vol%) and median pore diameters (ranging from 105 to 306 nm) compared to A3−3 itself (22.9 vol% porosity and 573 nm median pore diameter). Importantly, the MDGs exhibited greatly reduced infiltration by FLiBe molten salt compared to unmodified A3−3. For instance, A3−3 modified with 1 wt% MCMB exhibited a 2.1 % weight increase after treatment at 1 MPa for 20 h, while unmodified A3−3 exhibited a weight increase of 11.6 %. This study revealed that MDGs could be promising candidate materials for the graphite matrix of fuel elements as well as a neutron moderator and reflector in MSRs.

Scale-up of Metal-Supported Solid Oxide Fuel Cells

Metal-supported solid oxide fuel cells (MS-SOFC) offer the advantages of low-cost structural materials (e.g. stainless steel), mechanical ruggedness, excellent tolerance to redox cycling, and extremely fast start-up capability. Because of rapid-start capability, MS-SOFCs may be well-suited for portable, ruggedized, fast-start, intermittent-fuel, transportation, or other unique and innovative applications. The LBNL approach focuses on symmetric-architectured, co-sintered, ScSZ-based MS-SOFCs with porous metal supports on both anode and cathode sides, and catalysts deposited into both electrodes via infiltration (Fig 1). These features provide for a mechanically rugged cell that can be processed with low-cost scalable techniques, and for high surface-area catalysts that are deposited after sintering, thereby avoiding interdiffusion or coarsening during cell sintering.LBNL’s previous work on MS-SOFCs includes demonstration of A) Over 1000h operation in both hydrogen fuel cell and steam electrolysis modes [1,2], B) stable operation with direct internal reforming of ethanol-water blend fuel [3], C) startup within 10 sec for a bare cell and more than 200 rapid thermal cycles for a cell mounted on a test rig manifold [1,4], D) dynamic load-following with operating temperature fluctuating rapidly under load [5], and E) tolerance to multiple deep redox cycles [1].Previous demonstrations were achieved with button cells in the range 3 to 7 cm2 total cell area. In this work, the MS-SOFC cell area is scaled up to 50 cm2 and higher, Fig 2. Scale-up of the stainless steel and zirconia cell architecture is straightforward. The cell structure is prepared by tapecasting large sheets and laser cutting to size, which is amenable to making any size or shape cell. Due to the symmetric nature of the cell architecture, the large cells remain flat after sintering.Uniform catalyst deposition over a large area is, however, challenging. A number of improvements to the infiltration technique were developed, including: pre-oxidation of the stainless steel to enhance wetting of the aqueous catalyst precursor solution into the porous cell surface; development of highly concentrated Pr-oxide and Ni/doped-ceria precursor solutions that are shelf stable and low-viscosity; and, aerosol spray deposition of those solutions to uniformly coat the entire cell. With the improved infiltration, nearly identical performance for button and large cells is achieved. Spray deposition of shelf-stable aqueous solutions is also anticipated to be much more relevant to industrial-scale cell manufacturing, relative to LBNL’s previous molten salt infiltration technique.AcknowledgementsThe information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under work authorization numbers 13/CJ000/04/03 and 18/CJ000/04/01. Funding for this work was provided by LBNL and the U.S. Department of Energy Advanced Manufacturing Office through a Technology Commercialization Fund grant number TCF-18-15740. This work was funded in part by the U.S. Department of Energy under contract no. DE-AC02-05CH11231.References Tucker, M.C., J. Power Sources 369 (2017) 6-12Shen, F., R. Wang, and M.C. Tucker, in preparationDogdibegovic, E., Y. Fukuyama, M.C.Tucker, Journal of Power Sources, 449 (2020) 227598Tucker, M.C., and A.S. Ying, International Journal of Hydrogen Energy, 38 (2017) 24426-24434Tucker, M.C., J. Power Sources, 395 (2018) 314-317 Figure 1

Facile preparation and tribological property of alloyed Fe3Si coatings on stainless steels surface by molten-salt infiltration method

Alloyed Fe3Si infiltration coatings on the surface of 2Cr13 and 0Cr18Ni9 stainless steels were easily prepared via non-electrolysis infiltration in a molten salt. The infiltrated coatings on a 20 carbon-steel surface were used for comparative study. The microstructure of each infiltration coating was examined via XRD and SEM. The hardness and toughness of Fe3Si coatings were investigated by using a microscopic hardness tester, nano-indentation and static-indentation tests. Furthermore, the wear resistance of the coatings was evaluated using a ball-on-disk wear tester. The results indicated that alloying elements in the stainless-steel substrates participate in the mutual thermal diffusion of Fe-Si and alloyed Fe3Si coatings containing elemental Cr and Cr, Ni can be easily obtained. Cr and Ni alloying elements can increase the growth rate of the infiltration coatings, help to reduce the defect density, and improve the compactness of these coatings. In addition, the surface wear resistance capacity of alloyed Fe3Si coatings was 2.3 times and 3.7 times that of the corresponding 2Cr13 and 0Cr18Ni9 stainless-steel substrates, respectively. The reason lie in that Cr- or (Ni, Cr)- alloying has endowed Fe3Si coating higher hardness and surface toughness simultaneously.

Study on Microstructures of 2D and 3D Chemical Vapor Infiltration SiC/SiC Composites

The chemical vapor infiltration (CVI) SiC/SiC composites are porous materials, and the porosity affects the molten salt infiltration and may affect the mechanical properties. Therefore, the internal microstructures of the 2D and 3D silicon carbide composites are studied by synchrotron radiation micro‐computed tomography (micro‐CT) technology. The cross‐sectional slice and 3D volume image of the composite are reconstructed, then the pores of the composites are visualized in three dimensions, and the porosity is quantitatively analyzed. The results indicate that most of small pores (micrometer) exist between filaments and larger pores (millimeter) occur between fiber bundles, and the internal pores of 2D and 3D composites are greatly influenced by the fabric structure. In contrast, the 3D braided composites have lower porosity, smaller pore distribution fluctuations, and more stable internal structure.

Hot corrosion behavior of NdYbZr2O7 exposed to V2O5 and Na2SO4 + V2O5 molten salts

In order to evaluate the application prospects of NdYbZr2O7 as a novel TBC material, NdYbZr2O7 ceramic was synthesized via a solid-state reaction sintering method, and its hot corrosion behavior exposed to V2O5 and Na2SO4 + V2O5 molten salts at 900 °C, 1000 °C, and 1100 °C was comparatively investigated. For the V2O5 salt, the primary corrosion products were granular (Nd,Yb)VO4 as well as cube-like m-ZrO2. The corrosion layer consisted of two distinct layers, one of which was Zr-rich layer and another was V-rich layer. In the case of Na2SO4 + V2O5, NaVO3, as an intermediate product, played an important role in dissolving the NdYbZr2O7 ceramic. Herein, the (Nd,Yb)VO4 exhibited a rod/plate-like morphology, which could be attributed to the synergistic effect of low driving force and low nucleation rate. Since the molten salt infiltration rate was superior to the pore filling rate throughout the hot corrosion, the thickness of corrosion layer increased with the rise of temperature. The hot corrosion mechanisms of NdYbZr2O7 ceramic in various molten salts were discussed based on the phase diagram, Lewis acid-base rule and chemical thermodynamics. On this basis, the NdYbZr2O7 coating was prepared by atmospheric plasma spray (APS) and it exhibits a higher corrosion resistance compared to YSZ coating.

Preparation, microstructure and electrical property of GdSmZr2O7-(Li0.52Na0.48)2CO3 composite electrolyte via carbonate infiltration

Porous GdSmZr2O7 (GSZ) with different porosities has been prepared by the solid state reaction, and GSZ-carbonates composites have been prepared by infiltrating (Li0·52Na0.48)2CO3 (LNC) molten carbonates. Phase structure, microstructure and electrical property have been analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS). SEM results show that the voids in porous GSZ are uniformly distributed. The relative density of GSZ-LNC composites obtained by molten salt infiltration is above 93%. The electrical conductivity of the GSZ-LNC composites obtained by molten salt infiltration reaches the highest value of 0.50 S cm−1 at 600 °C, which is much higher than that of GSZ-LNC composites prepared by traditional mechanical mixing method. This excellent electrical property strongly indicates that the GSZ-LNC composite obtained by molten salt infiltration is a promising ionic conductor.

Impermeable CeO2 overlay for the protection of plasma sprayed YSZ thermal barrier coating from molten sulfate-vanadate salts

Yttria stabilized Zirconia thermal barrier coatings with around 200 μm overlay of Al2O3 and CeO2 were deposited on Inconel substrate using atmospheric plasma spray technique. Hot corrosion tests were carried out on the thermal barrier coating in molten salt mixtures (45 wt% Na2SO4 + 55 wt% V2O5) at 1150 °C for 30 h. Although the YSZ and Al2O3/YSZ coatings were gradually degraded by the infiltration of the molten salt, the CeO2/YSZ coatings remarkably restrained the infiltration of the molten salt into the YSZ coating. For the YSZ coating without any overlay, the YSZ reacted with V2O5 to form YVO4 during exposure to the molten salt, leading to the formation of a large amount of monoclinic phase of ZrO2 due to the leaching of Y2O3 from YSZ. For the Al2O3/YSZ coating, the Al2O3 overlay reduced the rigorous infiltration of the molten salt, which caused in lesser amount of leaching of Y2O3 from YSZ. However, for the CeO2/YSZ coatings, CeO2 not only physically prevented the molten salt infiltration, but also completely reacted with V2O5 to form CeVO4 and completely restrained the infiltration of the molten salt into the YSZ coating. Additionally, the Al2O3/YSZ and CeO2/YSZ showed 17% and 52% increase in thermal shock resistance respectively compared to YSZ coating.

Corrosion characteristics of Fe3Si intermetallic coatings prepared by molten salt infiltration in sulfuric acid solution

Alloyed Fe3Si intermetallic compound coatings were prepared through non-electrolysis high-temperature infiltration in molten salt on the surface of AISI420 and AISI304 stainless steel containing Cr and Cr, Ni elements, compared with Fe3Si coating on A3 carbon steel surface. The microstructure of infiltration coatings was examined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and glow discharge optical emission spectroscopy. The electrochemical corrosion properties and behavior of Fe3Si coatings in H2SO4 solution were evaluated via electrochemical techniques and static immersion tests. The results revealed that the permeation coatings prepared on the stainless steel surface are composed of a Cr- and Cr-, Ni-alloyed Fe3Si intermetallic-compound layer and a diffusion layer. Owing to the presence of alloying elements Cr and Ni from the substrate, the permeation coatings were relatively thick, dense, and contained only a few defects. The Cr-, Ni-alloyed Fe3Si coating prepared on AISI304 surface was the most compact, contained fewer defects, and was thicker than the coating prepared on AISI420. The Fe3Si infiltration coatings exhibited significant passivation characteristics in 10 vol% H2SO4 aqueous solution. Alloyed Fe3Si coatings of AISI420 and AISI304 exhibit better corrosion resistance in 10 vol% and 20 vol% H2SO4 than the Fe3Si coating of carbon steel. Therefore, these coatings can protect both types of stainless steel from surface corrosion. The best corrosion resistance was realized for the alloyed Fe3Si coating on AISI304 surface. The corrosion resistance and protection performance of Fe3Si intermetallic-compound coatings in H2SO4 solution are directly correlated with the thickness, compactness, defect characteristics, and passivation properties of the infiltration coatings. However, alloying elements from the alloy steel substrates had a significant effect on these four factors, during the infiltration process in the molten salt.

Stress analysis of the TMSR graphite component under irradiation conditions

TMSR uses nuclear graphite as a neutron moderator, a reflector, and the structural material, and utilizes molten salt as a coolant. When running normally, the graphite components are immersed in the molten salt. Thus, the nuclear graphite comes into direct contact with the molten salt, which infiltrates the open pores of the nuclear graphite. This infiltration may influence the stress analysis of the graphite component. In this study, a User Material subroutine was used to analyze the stress distribution of the graphite component, both with and without molten salt infiltration. Many influence factors were taken into consideration, such as the dose gradient, the shape of the permeation zone, and the permeation area. The results show that the dose gradient, shape, and area of the permeation zone all significantly influence the stress distribution. Furthermore, the results of the stress analysis indicate that for a regular graphite component with a square cross section, the peak maximum principal stress value occurs at the center of the cross section, and the symmetry of the maximum principal stress distributions was modified by quarter circle and half ellipse permeation zones.

The effect of molten FLiNaK salt infiltration on the strength of graphite

Graphite has been identified as a structural and neutron-moderator material in the molten salt reactor (MSR). In order to support the structural material development for MSR, the influence of molten salt infiltration on the mechanical strength of graphite was studied. The high-temperature mechanical strength of graphite grades IG-110 and NG-CT-10 decrease with the increasing of weight gain ratio during the molten FLiNaK salt infiltration. It shows a noticeable high-temperature mechanical strength degradation, which can be evidenced by the softening coefficients. The smaller softening coefficient of compressive strength compared with that of tensile strength at the same infiltration pressure indicates the compressive strength is more vulnerable to the molten salt infiltration than the tensile strength. The major fragments analysis of the samples after compressive strength tests shows that the molten FLiNaK salt infiltrated graphite samples experience are longitudinal splitting fracture rather than the shear fracture of virgin graphite. The increase of thermal strain after molten salt infiltration indicates an extra stress in infiltrated graphite, which may be one of the reasons for the failure mechanism change.

Microstructure and properties of fine-grained isotropic graphite based on mixed fillers for application in molten salt breeder reactor

Fine-grained isotropic graphite (FG) derived from mixed fillers with different weight ratio of natural graphite flake (NG) and calcined coke was prepared by cold isostatic pressing method. The average diameters of NG and coke were both 10 μm. The structure and properties of FG were investigated and compared with IG-110 graphite from Toyo Tanso Co., Ltd. The structure and properties of FG changed regularly when the weight percentage of NG in filler varied from 0 to 100%. The average pore diameter and porosity respectively minimized from 1.05 to 0.183 μm and 17.1 ± 0.1% to 11.8 ± 0.1%. Nevertheless, thermal conductivity at 298 K and density respectively increased from 93 ± 2 to 130 ± 2 W/m·K and 1.74 ± 0.02 to 1.85 ± 0.02 g/cm3. Result of molten salt infiltration experiment indicated that FG exhibited much better molten salt barrier property than IG-110 once the NG in filler excess 50 wt%. Especially, only 0.085 wt% fluoride salt could be detected for the graphite derived from 100% NG even up to 10 atm, which implied it could nearly totally inhibit the molten fluoride salt permeation and NG might be used as filler to fabricate moderator or reflector graphite materials for molten salt breeder reactor.

Preparation and Property of LSGM-Carbonate Composite Electrolyte for Low Temperature Solid Oxide Fuel Cell

The LSGM-carbonate composite electrolyte is a new type of medium and low temperature SOFC electrolyte material, which has great application potential. In this paper, the molten salt infiltration method was used to prepare the LSGM-carbonate composite electrolyte. The results of SEM test proved that the molten salt infiltration method was more appropriate in preparing the LSGM-carbonate composite electrolyte comparing with direct mixing method. The influence of the type and content of pore forming agent was investigated. The result showed that the polymethyl methacrylate (PMMA) had an excellent pore forming performance and could create interconnected pore structures successfully in LSGM matrix. The XRD result indicated that the LSGM-carbonate composite electrolyte showed almost a single LSGM phase and the carbonate remained glass state. Four terminal method was used to measure the conductivity. The result showed that the conductivity of the LSGM-carbonate composite electrolytes was increased by one order of magnitude compared with pure LSGM. The conductivity of LSGM-carbonate composite electrolytes increased firstly and then decreased with the increasing of PMMA. The LSGM-carbonate composite electrolyte prepared by 25 wt.% PMMA addition has the highest conductivity during the whole range of test temperature and reached 0.3 S.cm-1 at 600°C.

Mesocarbon microbead based graphite for spherical fuel element to inhibit the infiltration of liquid fluoride salt in molten salt reactor

Mesocarbon microbeads (MCMB) and quasi-isostatic pressing method were used to prepare MCMB based graphite (MG) for spherical fuel element to inhibit the infiltration of liquid fluoride salt in molten salt reactor (MSR). Characteristics of mercury infiltration and molten salt infiltration in MG were investigated and compared with A3-3 (graphite for spherical fuel element in high temperature gas cooled reactor) to identify the infiltration behaviors. The results indicated that MG had a low porosity about 14%, and an average pore diameter of 96 nm. Fluoride salt occupation of A3-3 (average pore diameter was 760 nm) was 10 wt% under 6.5 atm, whereas salt gain did not infiltrate in MG even up to 6.5 atm. It demonstrated that MG could inhibit the infiltration of liquid fluoride salt effectively. Coefficient of thermal expansion (CTE) of MG lies in 6.01 × 10−6 K−1 (α∥) and 6.15 × 10−6 K−1 (α⊥) at the temperature range of 25–700 °C. The anisotropy factor of MG calculated by CTE maintained below 1.02, which could meet the requirement of the spherical fuel element (below 1.30). The constant isotropic property of MG is beneficial for the integrity and safety of the graphite used in the spherical fuel element for a MSR.

Effects of FLiNaK infiltration on thermal expansion behavior of graphite

The changes in both the crystallographic coefficient of thermal expansion (CCTE) and the coefficient of thermal expansion (CTE) of FLiNaK-infiltrated nuclear graphite were studied using in situ-heating X-ray diffraction analysis and horizontal push-rod dilatometry. It was found that, at temperatures lower than the melting point of FLiNaK, the CCTE of the d(002) spacing of graphite decreases with an increase in the weight of the graphite sample because of FLiNaK infiltration. On the other hand, the CTE of bulk graphite increases after molten salt infiltration. The CCTEs of the as-prepared FLiNaK salt, and the salt in the graphite pores were compared. The decrease in the CCTE of the FLiNaK salt after it had infiltrated into the graphite pores confirmed that interactions occur between the graphite and the salt. These interactions are probably induced by the difference in the CTEs of graphite and the solidified salt. Further, it is likely that the crystallization pressure also plays an important role here. Thus, both the causes need to be considered when using nuclear graphite in molten salt reactors.

Molten FLiNaK salt infiltration into degassed nuclear graphite under inert gas pressure

Infiltration of molten FLiNaK salt into degassed nuclear graphite samples under inert gas pressure was studied. The weight gain of different grades (2020, 2114, IG-110, NBG-8, G1 and G2) of nuclear graphite during infiltration with different pressures was measured. Molten salt infiltration was compared with mercury intrusion porosimetry where it was found that mercury infiltration was a useful predictor of the threshold pressure and infiltration volume per gram graphite for molten salt infiltration. The distribution and morphology of salt in the graphite were observed by scanning electron microscopy, with very little difference between the molten salt content at the center and edge of samples for samples infiltrated at pressure higher than the threshold pressure. Increased molten salt infiltration with increased pressure resulted from the occupation of smaller pores and full occupation of the larger irregular pores. The similarity of weight gain between molten salt infiltration equilibrated at 20 and 100h showed 20h was adequate to obtain equilibrium.