Molten metal chloride salts have gained academic and industrial interest as candidates for the next generation of high-temperature heat transfer fluids (HTF). (1) Ideal fluids for HTF applications have a low vapor pressure, moderate viscosity, high boiling point, high heat capacity and thermal conductivity, and structural resilience against radiation damage. (2) Molten salts contain most of these physical properties, making them an ideal fluid for use in nuclear reactor systems among other applications such as nuclear fuel reprocessing, electroextraction, and CO[SUB[2 capture. (3−6) Metal chloride salts have the advantages over other salt mixtures such as the lowest production costs of all salts being considered for high-temperature thermal energy transfer and storage. (7) Furthermore, metal chloride based molten salts do not have decomposition concerns associated with polyatomic anions and fewer health and environmental safety concerns than the fluorides. (8)

Methods to optimize the specific heat capacity (C[sub[p) of molten salts are currently an area of research focus, including the current work. Addition of nanoparticles to molten salt can form a stable colloid, or nanofluid, and increase the C[sub[p of the fluid. (9-12) The increase is attributed to a new semiordered phase that is more dense than the base fluid and enhances the molten salt’s C[sub[p. (13,14) This new phase is semisolid and forms due to strong interactions between the nanoparticle and solution. (15) While preparing for this work, it became apparent that there are relatively few published works which examined the impact of adding metallic nanoparticles to molten chloride salts and particularly the potential of high-temperature magnetic properties. It has been shown extensively that the addition of magnetic nanoparticles to solutions can dramatically affect a variety of bulk solution properties. (16-26) Thus, the overarching goal of this work was to create and investigate the first molten metal chloride salt based ferrofluid.

Ferrofluids, first produced by NASA in the 1960s, contain magnetic nanoparticles that are suspended in a liquid matrix that causes the entire solution to become responsive to an external magnetic field. (27) Previous works have prepared ferrofluids by suspending nanoparticles, typically 1.0–10.0% weight, in aqueous or organic base fluids and using surfactants to modify the surface of the nanoparticles, preventing agglomeration and stabilizing the suspension. (28,29) The use of aqueous and organic base fluids has limited the operational temperature. Even so, ferrofluids have found many applications, including use as heat transfer fluids related to small electronic devices. (30) Most applications have been microscale, although scaling to industrial applications is possible. (31)
The lack of research effort devoted to high-temperature ferrofluids could be due to the relatively low Curie temperature (T[SUB[C) of most magnetic materials. Upon reaching T[SUB[C, thermal effects dominate over the magnetic coupling, resulting in a loss of long-range magnetic polarization. (32) A challenge with ferrofluids is the smaller particles tend to result in a more stable colloid, but the thermodynamic stability of the magnetic ordering and saturation magnetization of the nanoparticles scales with the particle volume, thus there is an inherent dichotomy between suspension stability and magnetic ordering. (33−35) Previous calculations of ferrofluid stability while exposed to an external magnetic field showed that maximum colloidally stable particle size increases as temperature increases. (36) Building on this result, a high-temperature molten salt ferrofluid should support a stable colloidal suspension of larger nanoparticles with a correspondingly improved thermal stability. In this way, high-temperature ferrofluids remain a possibility.

Altogether, the no-contact control of the thermophysical and magnetic properties in ferrofluids presents a unique opportunity for a new class of magnetic high-temperature HTFs with applications beyond nuclear reactors and concentrated solar power systems. We propose that the candidate ferrofluid must demonstrate three qualities to be practically deployed: (1) chemical stability at operational temperatures, (2) sufficient response to applied magnetic fields, and (3) colloidal stability of the suspension. This work evaluates the first two requirements by experimentally examining two molten metal–chloride salt ferrofluids for potential application as high-temperature HTFs....

Figure 6. Magnetometry measurements of the magnetization versus temperature for (A) 25–30 nm cobalt particles, (B) 2.0 wt % 25–30 nm cobalt particles in 57% NaCl 43% MgCl2, and (C) 2.0 wt % 25–30 nm cobalt particles in 45% KCl 55% ZnCl2.

Figure 7. Magnetometry measurements of the field response for (A) pure 25–30 nm cobalt particles, (B) 2.0 wt % 25–30 nm cobalt particles in 57% NaCl–43% MgCl2, and (C) 2.0 wt % 25–30 nm cobalt particles in 45% KCl–55% ZnCl2.

Figure 10. Results of MTG force measurements for 28 and 20 nm cobalt nanoparticles in KCl–ZnCl2 and NaCl–MgCl2, respectively. Plots show 28 nm cobalt particles in (A) KCl–ZnCl2 and (B) 20 nm cobalt particles in NaCl–MgCl2. Data is normalized to the force measured at room temperature (approx 30 °C).

This work represents the first-time creation of a molten chloride salt ferrofluid that can operate at temperatures higher than those of previous ferrofluids. Experiments demonstrated that magnetic nanoparticles could be used to generate an appreciable force even at elevated temperatures up to 800 °C in MTG measurements and remain strongly magnetic, with little reduction in the moment up to our maximum measured magnetometry measurement at 726 °C. The high TC and magnetocrystalline anisotropy allow the cobalt to maintain a large magnetic moment even at very high temperatures. Solubility experiments showed that the particles maintained some degree of chemical inertness to the molten salt matrix once they reached equilibrium after initial solubilization. While colloidal stability was not quantitatively examined, the Tyndall effect was observed in the solutions, indicating qualitative particle dispersion and phase separation was not observed. Lastly, a surprising increase measured in the VSM and MTG experiments at elevated temperatures was observed, and the origin of this increase is not fully understood.