Mixed Molten Salt Research Articles

Enhancing hot corrosion resistance of N-doping NiCoCr-based superalloy coatings fabricated by directed energy deposition in mixed molten salts

Here, a NiCoCr-based superalloy coating has been developed in ultra-supercritical environments. However, optimizing the coating composition to enhance the anti-corrosion performance remains a formidable challenge. This research introduced an innovative N-doping strategy to enhance the hot corrosion resistance of the coating by utilizing directed energy deposition. The hot corrosion behavior at 700 °C of the coating with N-doping reinforcement was investigated. Detailed characterizations of the microstructural evolution and corrosion products were conducted. The results showed that the improvement of hot corrosion resistance was attributed to the continuous protective film retained in the N-doping reinforced coating, which inhibited the inward diffusion of corrosive media to retard the chlorination and sulfuration reaction. This work provides new insight into the design of corrosion-resistant superalloys and highlights the potential of N-doping superalloy coating in extreme environments.

Growth orientation control in rapid solid-state crystal growth of (K,Na)NbO3 single crystals using plate-like NaNbO3 single crystal particles

The rolling-extended orientation technique and plate-like NaNbO3 (NN) single crystal particles prepared by a single-step molten salt synthesis, both of which have been developed to fabricate (K, Na)NbO3 (KNN) textured ceramics, were utilized to control the growth orientation of KNN single crystals synthesized by a rapid solid-stated crystal growth (RSSCG) method. As the seed crystals, two kinds of NN single crystal particles were synthesized using pure NaCl and KCl-NaCl mixed molten salts. Plate-like KNN single crystals of about 1 cm squares with the upper and lower faces almost parallel to the (100) (001) planes were obtained with a probability exceeding 50% when NN single crystal particles were synthesized from mixed salts and were subsequently thermal-treated again in Na2CO3-NaCl mixed molten salts under appropriate conditions to remove the Bi element, which is known as the suppression factor of the crystal growth. The average crystal growth rate was 0.6–1.2 mm h−1. Controlling the growth orientation of KNN single crystals produced by the SSCG method using seed crystals other than KTaO3 single crystals was successfully accomplished for the first time.

Cyclic hot corrosion behavior and mechanism of GH4738 superalloy in mixed molten salt

The research on the hot corrosion resistance of nickel-based superalloys is critical to determining and enhancing their service life. Therefore, this study investigates the cyclic hot corrosion behavior of GH4738 alloy coated with a mixed salt (75 wt% Na2SO4+25 wt% NaCl) at 750 °C and 800 °C. X-ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and transmission electron microscope (TEM) are used to analyze the phase compositions and film structures of the corrosion products, as well as to explore the corrosion mechanism. The results show that a thicker oxide layer is formed at higher temperatures, indicating poorer hot corrosion resistance at elevated temperatures. The corrosion products form at both temperatures to show a distinct layered structure, the outer layer is mainly composed of Cr2O3 and a small amount of TiO2, while the inner layer consists of Al2O3, CrS, and TiS. In addition, it is found that the corrosion mechanism of O, S, and Cl elements in the corrosion process is synergistic and accompanied by alkaline melting.

Experiment and Analysis of Compatibility between New Phase Change Mixed Molten Salt and 316L Stainless Steel

Experiment and Analysis of Compatibility between New Phase Change Mixed Molten Salt and 316L Stainless Steel

Mitigating kinetic hindrance of single-crystal Ni-rich cathodes through morphology modulation, nickel reduction, and lithium vacancy generation achieved by terbium doping

Single crystallization has proven to be effective in enhancing the capacity and stability of Ni-rich LiNi1−x−yCoxMnyO2 (SNCM) cathode materials, particularly at high cut-off voltages. Nevertheless, the synthesis of high-quality single-crystal particles remains challenging because of severe particle agglomeration and irregular morphologies. Moreover, the limited kinetics of solid-phase Li+ diffusion pose a significant concern because of the extended diffusion path in large single-crystal particles. To address these challenges, we developed a Tb-doped single-crystal LiNi0.83Co0.11Mn0.06O2 (SNCM-Tb) cathode material using a straightforward mixed molten salt sintering process. The Tb-doped Ni-rich single crystals presented a quasi-spherical morphology, which is markedly different from those reported in previous studies. Tb4+ doping significantly enhanced the dynamic transport of Li+ ions in the layered oxide phase by reducing the Ni valence state and creating Li vacancies. A SNCM-Tb material with 1 at% Tb doping shows a Li+ diffusion coefficient up to more than 9 times higher than pristine SNCM in the non-diluted state. In situ X-ray diffraction analysis demonstrated a significantly facilitated H1-H2-H3 phase transition in the SNCM-Tb materials, thereby enhancing their rate capacity and structural stability. SNCM-Tb exhibited a reversible capacity of 186.9 mA h g−1 at 5 C, retaining 94.6% capacity after 100 cycles at 0.5 C under a 4.5 V cut-off. Our study elucidates the Tb4+ doping mechanisms and proposes a scalable method for enhancing the performance of single-crystal Ni-rich NCM materials.

Effect of the SiO2 instead of Al2O3 in the embedding powder on the microstructure and hot corrosion behavior of silicon-modified aluminide coatings

The effects of Si and SiO2 content in the embedding powder on the equilibrium partial pressure of halide gas were compared by material thermodynamic calculation, and higher SiO2 content facilitated the preparation of silicon-modified aluminide coatings. The microstructure and hot corrosion behavior of silicon-modified aluminide coating prepared by different SiO2 contents were studied. As the SiO2 content in the embedding powder increased, the Al, Si, and Cr content was elevated in the prepared coating surface, which promotes the Cr9.1Si0.9 phase precipitate in the coating. Besides, the corrosion weight loss of the coating was the lowest, after 50 wt% NaCl + 50 wt%Na2SO4 mixed molten corrosion at 900 °C for 100 h, showing the excellent corrosion resistance. At the initial corrosion stage, the higher Si content induces the formation of a SiO2 oxide film on the coating surface, which could resist the mixed molten salt reaction. As the corrosion process, the SiO2 oxide film and the below Al2O3 oxide film peel off due to thermal expansion mismatch. Meanwhile, the coating relies on the dispersed precipitation below the oxide film to hinder the corrosive elements and the consumption of Al elements at the final corrosion stage.

Concentration effects on dynamic fluctuations in structure and thermodynamic properties of LiCl–AlCl3 molten Salt: Insights from ab initio molecular dynamics

The short–range and mid–range structural features, as well as thermodynamic properties of a LiCl–AlCl3 mixed molten salt at 873 K, were investigated as a function of AlCl3 concentration using ab initio molecular dynamics methods. In all solutions, the short-range structure of Al3+ reveals a regular tetrahedral [AlCl4]– unit. These tetrahedral units exist in isolation within dilute AlCl3 solution and polymerize into dimers or trimers through Al–Cl–Al bridge bonds in concentrated solutions. The dynamic dissociation analysis of Al3+ and Li+ structures indicates that the Al–Cl bond considerably stronger than Li–Cl bond. The polymerization of the [AlCl4]– units leads to the dissociation of the Li+ coordination shells into freely isolated cations. Furthermore, we examined changes in density, self-diffusion coefficient, and ionic conductivity with varying AlCl3 concentration in terms of thermodynamic properties. The conductivity and self-diffusion coefficient of Li+ showed a noticeable peak at 50 % concentration. This peak may be associated with the aggregation of [AlCl4]– units and the dissociation of Li+ coordination shells.

A facile eutectic mixed molten salt method for synthesizing LiNi0.5Co0.2Mn0.3O2 cathode material for lithium-ion batteries

LiNi0.5Co0.2Mn0.3O2 as a kind of layered oxide, has garnered significant interest due to its low cost and facile preparation method. Nevertheless, fast discharge capacity attenuation, poor cycle stability restricts it further application. To optimize its properties, in this paper, a novel eutectic mixed molten salt method is proposed to synthesize LiNi0.5Co0.2Mn0.3O2. The results demonstrate that the LiNi0.5Co0.2Mn0.3O2 obtained through LiOH-LiNO3 as eutectic mixed molten salt exhibited exceptional electrochemical properties. It had a reversible capacity of 151.7 mAh·g−1 after 100 cycles at 0.2 C, with a capacity retention of 89.3%, the discharge capacities of 165.6, 135.8, 121.8 and 111.2 mAh·g−1 at 0.5, 1.0, 2.0 and 5.0 C, respectively. This study highlights a convenient method for preparing LiNi0.5Co0.2Mn0.3O2 and demonstrates the effectiveness of the eutectic mixed molten salt approach in enhancing electrochemical properties.

Heteroatoms-Doped Mesoporous Carbon Nanosheets with Dual Diffusion Pathways for Highly Efficient Potassium Ion Storage.

The high potassization/depotassization energy barriers and lack of efficient ion diffusion pathways are two serious obstacles for carbon-based materials to achieve satisfactory potassium ion storage performance. Herein, a facile and controllable one-step exfoliation-doping-etching strategy is proposed to construct heteroatoms (N, O, and S)-doped mesoporous few-layer carbon nanosheets (NOS-C). The mixed molten salts of KCl/K2SO4 are innovatively used as the exfoliators, dopants, and etching agents, which enable NOS-C with expanded interlayer spacing and uniformly distributed mesopores with the adjusted electronic structure of surrounding carbon atoms, contributing efficient dual (vertical and horizontal) K-ion diffusion pathways, low potassization/depotassization energy barriers and abundant active sites. Thus, the NOS anodes achieve a high reversible capacity of 516.8 mAh g-1 at 0.05 A g-1, superior rate capability of 202.8 mAh g-1 at 5 A g-1 and excellent long-term cyclic stability, and their practical application potential is demonstrated by the assembled potassium-ion full batteries. Moreover, a surface-interlayer synergetic K+ storage mechanism is revealed by a combined theoretical and experimental approach including in situ EIS, in situ Raman, ex situ XPS, and SEM analysis. The proposed K+ storage mechanism and unique structural engineering provide a new pathway for potassium-ion storage devices and even beyond.

Real-time control of distillation process to improve the recovery efficiency of ThF4-LiCl-KCl molten salt.

The purification and recovery of chloride electrolyte molten salts are vital to reuse this valuable species and reduce the waste. Vacuum distillation method was used to investigate the recovery efficiency of LiCl-KCl mixed molten salt containing 20 wt% ThF4. Rapid mass loss in the initial stage and the subsequent lower evaporation rate were significantly observed under 1173 K and 20 Pa due to the coordination species such as Li3ThF7 and LiTh2F9. To achieve the recovery efficiency of chloride salts, a real-time control distillation was proposed. The distillation was terminated when about 80% mixture salt was evaporated according to the transient weight loss curve. The evaporation ratio of LiCl-KCl reached 91% and the decontamination factors for Th and rare earth elements Nd and Sm were more than 103 and 102, respectively. The results provided a simple and effective scheme to separate ThF4 and recover molten salts from waste electrolyte salts.

Corrosion behavior of iron-based and Ni-based alloys melted in NaCl–MgCl2–KCl mixed molten salt under vacuum atmosphere

In this work, static immersion and gravimetric methods were used to study the corrosion behavior of three common stainless steels and four Ni-based alloys in a NaCl–MgCl2–KCl mixed molten salt under vacuum atmosphere. The corrosion test was conducted at 700 °C, with a maximum exposure time of 100 h. Corrosion behavior was determined by measuring the mass loss of the sample at different time intervals and the corrosion rates at 100 h. The microstructure of the alloy was analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscope (AFM), and X-ray diffraction (XRD). The results showed that stainless steels were generally more severely corroded than Ni-based alloys. 304 stainless steel and In800H exhibited the worst corrosion resistance, whereas Ni-based alloy In625 had the best corrosion resistance. Fe and Cr were dissolved in the molten chloride salts and reacted with the molten chloride salts. Mo played a positive role in suppressing the corrosion of the alloys in molten chloride salts by inhibiting the outward diffusion of the elements.

Preparation and thermal properties of a novel ternary molten salt/expanded graphite thermal storage material

In thermal energy storage, the use of phase change materials (PCM) is a very efficient energy storage method. In the field of medium temperature thermal storage, nitrate PCM have always been a research hotspot, but their relatively high melting point and relatively low latent heat of phase change severely limit their application in thermal energy storage. In this paper, LiNO3 and NaCl with higher latent heat value were selected to improve the thermal properties of nitrate. A novel ternary mixed molten salt of NaNO3-LiNO3-NaCl with high latent heat was prepared and studied. NaNO3-LiNO3-NaCl/EG composite phase change material (CPCM) was prepared by selecting expanded graphite (EG) with a large thermal conductivity as the carrier adsorption material for molten salt. The thermophysical properties, thermal stability, and heat transfer ability of PCM and CPCM were tested. The composition and microstructure of the materials were also characterized. The experimental results showed that compared with pure NaNO3, the phase change temperature of NaNO3-LiNO3-NaCl was reduced by 45.3 °C, and the latent heat of phase change was increased by 59.4 J/g. NaNO3, LiNO3, NaCl and EG had good compatibility, without chemical reaction between them. From the microscopic morphology of the composite material, it can be seen that the PCM was uniformly dispersed into the porous structure of EG without agglomeration. The ternary new molten salt had higher thermal decomposition temperature and better thermal stability. When the addition ratio of EG was 20 %, the thermal conductivity of the ternary molten salt increased from 1.350 to 5.017 W/m·K. EG greatly improved the heat transfer capacity of PCM. And the PCM added with EG still had a high latent heat of phase change. The results show that NaNO3-LiNO3-NaCl/EG has broad application prospects in the field of medium and high temperature heat storage.

Two‐Dimensional Amorphous Iridium Oxide for Acidic Oxygen Evolution Reaction

AbstractThe increasing popularity of proton exchange membrane electrolysis technology for hydrogen production has brought attention to the electrolytic water reaction. However, the slow kinetics of the oxygen evolution reaction (OER) at the anode have great influence on the overall efficiency of the reaction. While iridium oxide shows excellent stability under acidic conditions, its OER activity still needs to be improved. Here, we synthesized two‐dimensional amorphous iridium oxide (Am−IrO2) nanosheets with the thickness of only 6 nm by a mixed molten salt method. Such nanosheets show an ultralow overpotential of only 230 mV at 10 mA cm−2 in 0.5 M H2SO4. The overpotential increases only 40 mV after 90 hours of the stability test at this current density. Am−IrO2 can maintain the current density of ~400 mA cm−2 after 120 hours of test at 1.8 V in the PEM device, demonstrating good industrial prospects. Density functional theoretical calculations show that the oxygen vacancies, together with the upshift of the O 2p band center, are responsible for the improvement of OER in Am−IrO2.

Synergistic regulation of the formation of yttrium oxyfluoride layer on yttrium oxide substrate by fluoride-chloride molten salts

Fluoride molten salt (KAlF4) can modify Y2O3 to form YOF, but the reaction degree of this process is difficult to control and tends to form a large amount of yttrium fluoride (YF3) with low-strength. The reaction degree of the modification process of fluoride molten salt with Y2O3 can be suppressed by the synergistic effect of chlorides (KCl-CaCl2), which is mainly determined by the diffusion of fluorine-containing complex ions (AlF4-) in the fluoride-chloride mixed molten salts (FCMMS). The low diffusion rate of AlF4- in FCMMS directly leads to the decrease of the fluorination reaction rate and delays or even prevents the deeper fluorination process. After the synergistic modulation of Y2O3 by FCMMS, only YOF is formed, and the formation of YF3 is completely avoided. The strategy and mechanism may be applied to the economical preparation of YOF protective layer or YOF powder on low-cost Y2O3 template.

Preparation and thermal properties of LiNO3-NaNO3-NaCl/EG composite heat storage material

Nitrate phase change materials (PCM) have important applications in the field of thermal energy storage, but their latent heat value is relatively low. A novel LiNO3-NaNO3-NaCl ternary molten salt with high latent heat was prepared and studied. LiNO3-NaNO3-NaCl/expanded graphite (EG) composite phase change material (CPCM) was prepared by using high thermal conductivity EG as the carrier adsorption material. The performance of PCM and CPCM were tested and analyzed. Compared with LiNO3-NaNO3, the newly prepared ternary nitrate-chloride mixed molten salt has a lower phase change temperature, a higher latent heat value, and a higher thermal decomposition temperature. At the same time, the supercooling degree of LiNO3-NaNO3-NaCl is about half lower than that of LiNO3-NaNO3. There is no chemical reaction between different molten salts or between mixed molten salts and EG. The mixed molten salt is uniformly dispersed into the pore structure of EG, and no agglomeration occurs. Both LiNO3-NaNO3-NaCl and LiNO3-NaNO3-NaCl/EG have good cyclic thermal stability. The addition of EG greatly improves the thermal conductivity of LiNO3-NaNO3-NaCl. When the addition ratio of EG is 20%, the thermal conductivity of ternary molten salt increases from 1.378 to 5.453 W/m·K. The phase change materials added with EG still have high latent heat of phase change. The results show that LiNO3-NaNO3-NaCl/EG has broad application prospects in medium temperature heat storage.

Effect of potassium and silver ion-exchange on the strengthening effect and properties of aluminosilicate glass

In this work, the aluminosilicate glass was subjected to ion-exchange using the KNO3-AgCl mixed molten salt in order to strengthen the glass while imparting antimicrobial properties. The concentration distribution of K+ ions and Ag+ ions of the ion-exchanged glasses was characterized by EDS, the effects of ion-exchange temperature (460-500 °C), ion-exchange time (0.5-3 h) and AgCl concentration (0–2.5 wt%) in the mixed molten salt on the strengthening effect and properties of the glass were investigated. The results showed that Ag+-Na+ ion-exchange, K+-Na+ ion-exchange existed simultaneously, and Ag+-Na+ ion-exchange occurred preferentially. Due to the presence of metallic silver, the appearance of the Ag+ ion-exchanged glass was light yellow and its transmittance showed a decrease. The surface compressive stress trended up and then down with increasing temperature and time because of the stress relaxation effect. The Vickers hardness of ion-exchanged glass increased by 15%, and the densities and chemical stability were also increased. Ions leaching experiments showed that the Ag+ ions release concentration of silver-loaded glass in aqueous environment can reach the bactericidal level. It has been shown that ion-exchange of glass in KNO3-AgCl mixed molten salts allowed the glass to be strengthened and incorporated with antimicrobial active ions, its chemical stability was improved, too.

Effects of TiC addition on the hot corrosion behavior of IN718 fabricated by laser direct metal deposition

The hot corrosion behavior and mechanism of the IN718 nickel-chromium alloy (Sample D1) and 1 wt % TiC-reinforced IN718 (Sample D2) fabricated by laser direct metal deposition (DMD) in a 75 wt % Na2SO4 + 25 wt % NaCl saturated mixed molten salt at 700 °C were studied. After a 100-h test, the weight loss ratio of Sample D1 was 2.4 times that of Sample D2, and the corrosion rate of Sample D1 was 1.5 times that of Sample D2. Chlorination, sulphidation, and oxidation, as well as a change in the acidity at the molten salt interface of the alloy, led to serious spalling of Sample D1. However, the addition of TiC refined the grains, resulting in rapid diffusion of Cr to the alloy surface and formation of a Cr2O3 protective layer at high temperatures. In addition, the diffusion of Ni, Fe, etc., was hindered by TiC particles inside the superalloy, which reduced the thickness of the hot corrosion product layer by about 26% and improved the hot corrosion resistance of Sample D2.

THE INFLUENCE OF CERAMIC PARTICLES ON THE CORROSION PROPERTIES OF NOVEL TI750 MATRIX ALLOY

This paper explores the corrosion behavior of matrix Ti750 alloy and its composites prepared using SiCp, B4C, SiCw, and GPLs particles as reinforcement. The consolidation of the metallic powders was by spark plasma sintering method. The study chose mixed molten salt of 25% NaCl + 75% Na2SO4 as the corrosive medium and the hot corrosion behavior of the solid-solution aging at 800 °C. The study found that at the same corrosion temperature, the rate of change of the weight gain curves of all the materials was similar. The corrosion weight gain of the base material was the highest (14.10 mg.cm-2), whiles the 5vol.% SiCp reinforced composite had the lowest weight gain of 5.1 mg.cm-2, which is 36.17% of the base alloy. The corrosion products of all the materials were mainly composed of TiO2, Al2O3, Na2TiO3, and Al2TiO5. The peak of SiO2 was enhanced when SiCp was added, and there was no distinct peeling off of the corrosion layers, unlike the base material, which had partial shedding, deep cracks, and holes. The 5vol.% SiCp reinforced composite therefore possessed the best corrosion performance.

Low-temperature and efficient preparation of starfish-like Mo2C/C composites from waste biomass

In this study, starfish-like Mo2C/C biomass ceramic composites were successfully prepared using waste pine as a C template precursor through rapid heat and mass transfer in a molten salt environment. The effects of the reaction temperature and molten salt on the formation of the Mo2C/C ceramic composites were investigated. The mechanism involved in the preparation of Mo2C/C ceramic composites in a molten salt liquid environment was determined. The results showed that the mixed molten salt environment comprising KF/KCl with a low KF content could promote the formation of Mo2C. The mixed molten salt environment was beneficial for the formation of regular Mo2C flakes, which interwove together to form a “starfish-like” shape as the reaction temperature increased to significantly increase the proportion of micropores and specific surface area of the composites. In addition, the formation of Mo2C in the C template occurred in two steps, where Mo first reacted with C to form MoC and the liquid Mo then reacted with MoC to form Mo2C.

NaNO3-KNO3-KCl/K2CO3 with the elevated working temperature for CSP application: Phase diagram calculation and machine learning

Mixed nitrates with relative low melting point have widely been used as thermal storage medium in the concentrated solar power stations. However, their maximum working temperatures aren’t high enough to integrate with more efficient power generation systems such as the supercritical CO2 cycle. In the present work, mixed nitrates with elevated working temperature were obtained by adding KCl/K2CO3. With the help of FactSage software and back-propagation (BP) algorithm combined with genetic algorithm (GA), the eutectic compositions and melting points of NaNO3-KNO3-KCl and NaNO3-KNO3-K2CO3 were predicted, and the validity of BP algorithm combined with GA was verified. Based on DSC and TG curves, the melting points of the as-prepared NaNO3-KNO3-KCl and NaNO3-KNO3-K2CO3 are 206.12 °C and 207.24 °C, respectively, which are in agreement with predicted values. Moreover, their decomposition temperatures were increased to more than 640 °C. In addition, the specific heat capacity of both mixed molten salts was measured, and it also was calculated by using FactSage software. This work provides two new ternary molten salts based on nitrates with higher working temperature and develops the prediction method on phase diagram calculation and machine learning.