Halide and Oxy-Halide Eutectic Systems for High Performance High Temperature Heat Transfer Fluids

Project: Research project

Description

1. PROJECT OBJECTIVES Eutectic molten salts are proposed as the high temperature heat transfer fluids (HTF) in this proposal. The research team chooses halide salts (primarily chloride) and oxy-halide salts as two major parallel systems for study. Among the halides, the chlorides have the most favorable cost and availability. The boiling temperatures of major species of halide salts (for example, Cl with Na, K, Li, Ca, Mg, Mn) are all above 900 oC as seen in Table 1 (Janz, 1967). The intrinsically high boiling point of these major salts suggests that the eutectic mixture might also exhibit high temperature stability. With high temperature stability relatively secured, the team can thus put its main effort on lowering the melting temperature by selecting a proper eutectic system, and resolve other challenges such as less chemical corrosion, and adequate thermophysical properties as HTF. Research work (Etter& Wiedenheft, 1980) has demonstrated the promise of the current proposed core idea. For example, an eutectic system by 66 mol% of CuCl and KCl shows a melting point of 136 oC and high temperature stability above 800 oC, although issues of chemical corrosion of the eutectic system to steel devices still remain to be challenging. Borates are attractive as additives to halide molten salts based on the facts that B2O3 melts at 300oC, and halo borates are also low melting, plus borates are chemically and electrochemically stable. B2O3 is also an example of network glass former. It is most commonly found in vitrous form and it is well known for its propensity to not crystallize. The structure of vitrous B2O3 is believed to take a polymer-like form with 3-coordinated Boon and twocoordinated O. Boron oxides are well known high temperature fluxes but there is sparse information from the literature about boron oxides and boron halides as heat transfer fluids. Lack of evidence from the literature does not mean that borates and halo-borates might not be useful as heat transfer fluids. Still, in the absence of literature precedent for using borates as heat transfer fluids, the best use of the boron oxides and boron halides (oxo and halo borates) may be as components to add to other better known metal halides during attempts at combinatorial discovery of a high stability low melting eutectic of borates mixed with metal and alkali metal chlorides. Phosphate and phosphoryl halides are also attractive as a dopant because they binds strongly to metal halides, are stable at high temperature and may for polymer like structure that can stabilize halide melts.
StatusFinished
Effective start/end date10/1/129/30/16

Funding

  • US Department of Energy (DOE): $196,172.00

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Eutectics
Heat transfer
Fluids
Boron
Salts
Metal halides
Temperature
Oxides
Melting point
Molten materials
Melting
Corrosion
Borates
Boiling point
Alkali metals
Polymers
Boiling liquids
Phosphates
Thermodynamic properties
Doping (additives)