Molten salt eutectics from atomistic simulations

Abstract

Despite their importance for solar thermal power applications, phase-diagrams of molten salt mixture heat transfer fluids (HTFs) are not readily accessible from first principles. We present a molecular dynamics scheme general enough to identify eutectics of any HTF candidate mixture. The eutectic mixture and temperature are located using the liquid mixture free energy and the pure component solid-liquid free energy differences. The liquid mixture free energy is obtained using thermodynamic integration over particle identity transmutations sampled with molecular dynamics at a single temperature. According to Carnot's cycle, the maximum efficiency of solar thermal power facilities increases with the temperature difference between melting and thermal decomposition of employed heat transfer fluids (HTFs). Consequently, new formulations of low melting but temperature resistant molten salt mixtures are actively sought [1]. Competitive formulations currently include eutectics of alkali and alkali-metal earth nitrates and nitrite mixtures. Unfortunately, the dimensionality of the combinatorial multicomponent space of all possible cations and anions prohibits exhaustive scanning using computer simulation. Once an optimal HTF formulation is identified, however, its realization through simple mixing is readily accomplished in experiment. Identifying new HTFs therefore represents a rewarding challenge for atomistic first principles materials design efforts using theory and computation. Ab initio (AI) based design, through an efficient combination of first principles methods and optimization algorithms [2], has already been applied to inverting band structures [3], identifying stable alloys [4], designing heterogeneous catalysts [5,6], or discovering ternary metal oxides for batteries [7]. Identity transformations, also called "alchemical" transmutations [8], have been successfully applied to compute compositions in the earth's core [9], phase stability in solid solutions [10], and a wide variety of biochemical applications [11], among others.The Gibbs criteria for phase equilibrium dictates that temperature, pressure, and chemical potential of individual components must be equal in coexisting phases. Therefore, knowledge not only of average liquid configurations but also of the solid mixture's structure is essential. For phase transitions of mixtures, the problem is hence strongly linked to the challenge of predicting molecular crystals from first principles [12], which has been addressed for co-crystals only recently [7,13] in mapping out entire phase diagrams. For the identification of HTF mixtures with minimal melting points, however, it is sufficient to simply focus on eutectics, rather than having to screen the entire solid-liquid phase diagram. Here, we present a first principles approach for directly estimating eutectic compositions and temperatures for any number of components, relying on a combination of only a couple of molecular dynamics (MD) calculations per mole fraction at a single temperature and knowledge ab...

Thermostatic properties of nitrate molten salts and their solar and eutectic mixtures

“…The characterization of their thermostatic properties is important not only for a fundamental understanding of the physics of ionic liquids at high temperature, but also for energy applications in any system containing a heat storage component. In this respect, it is also important the development of viable simulation methods to determine technological relevant binary and ternary molten salt eutectics 37 . In the following we will calculate, in the liquid phase only, the thermostatic properties of two relevant nitrate molten salt mixed systems: the eutectic and the “solar” mixture.…”

Quantum Mechanical Treatment of Variable Molecular Composition: From 'Alchemical' Changes of State Functions to Rational Compound Design

“…[17][18][19][20] Smith and van Gunsteren found that non-linear alchemical coupling not necessarily leads to linear free energy changes. [21] Further applications of al-chemical coupling to the estimation of free energy difference include the free energy of hydration of ions using ab initio molecular dynamics, [22] differences in free energy of binding between various host-guest complexes, [23] free energy differences at phase boundaries to predict melting points, [24,25] the free energy of mixing to identify eutectics in ternary mixtures of molten alkali-nitrate salts, [26] kinetic isotope effects, [27] as well as constraints on the composition of the Earth's core. [28] But also from the solid state point of view the concept of alchemical coupling is used for the prediction of properties of disordered materials, such as co-crystals, solid solutions, or solid mixtures, as a function of molefraction.…”

Structure Transformation of Na-Mg Based Salts for CO2 Capture and Storage at High Temperature Probed with Variable Temperature X-ray Powder Diffraction

“…In order to demonstrate it, the mixed metal salt consisting of the mixture of 12 wt% NaNO 3 and 8 wt% KNO 3 known as "Solar salt" or eutectic mixture [25][26][27] of which the melting point was 495 K, was impregnated onto the MgO because the eutectic composition was difficult to obtain when the coprecipitation method was applied because the filtering and subsequent thorough washing deviated the desired composition. The melting point of NaNO 3 -KNO 3 impregnated on MgO was also determined to be 496 K from differential scanning calorimeter measurement.…”