Liquid Mixtures of Sodium and Sodium Hydride at High Pressures and Temperatures

Abstract

The hydrogen pressure in equilibrium with liquid phases of the system sodium‐sodium hydride has been measured from 650 to 900 °C and up to 900 bar. A detailed description of the apparatus is given. The autoclave was of a nickel base superalloy and contained an outer and an inner compartment with equilibrated hydrogen pressures. At 900°C and 650 bar the maximum solubility of NaH in liquid Na is 19 mole percent. Sieverts' law is closely obeyed to 5% NaH. Sodium solubility in the liquid NaH is below 2%. The melting point of NaH was found at 632 °C and 106 bar, the enthalpy and entropy of fusion as 26.2 ± 0.7 kJ mol−1 and 29.0 ± 0.8 JK−1 mol−1. The enthalpy of solution of liquid NaH in liquid Na is 27.4 kJ mol−1. Equations and data for solubility, activity coefficients etc. of NaH are given. The critical solution point for Na and NaH is estimated to be at 1500 ± 70 K, 2500 ± 600 bar and x(NaH) = 0.65 ± 0.05.

Sodium-based hydrides for thermal energy applications

“…Again, this technique is unsuitable when considering Na/NaH as part of a TES system that operates above 400°C due to the temperature limitations of hydrocarbons [32]. A further complication of Category II SBHs is the high vapour pressure of molten sodium metal that causes its distillation at the temperatures relevant for TES [41,42]. This problem was solved by sealing NaH within thin walled iron tubes that are permeable to H 2 but not to sodium vapour [7,41,42].…”

“…Fortuitously, sodium hydride has a large number of attributes that make it attractive as a hightemperature TES system for CSP. It has a respectable theoretical hydrogen storage capacity (4.2 wt% H 2 ), a high enthalpy of H 2 absorption (DH abs = -114 kJ mol -1 H 2 ) [41] and, from pressurecomposition isotherms, a wide and flat plateau with limited H 2 solubility [42]. In addition, a significant level of research into the sodium-hydrogen system has been driven by the use of liquid sodium as a coolant in liquidmetal fast-breeder reactors and, as a result, a large body of knowledge exists pertaining to the safe production, handling, containment and use of high-temperature sodium on an industrial scale [43].…”

“…A further complication of Category II SBHs is the high vapour pressure of molten sodium metal that causes its distillation at the temperatures relevant for TES [41,42]. This problem was solved by sealing NaH within thin walled iron tubes that are permeable to H 2 but not to sodium vapour [7,41,42]. The relatively slow diffusion of H 2 through the iron walls means that temperatures greater than 550°C are required for reasonable H 2 desorption kinetics, and this technique does not address the difficulties associated with hydriding molten Na metal to form solid NaH [7,41,42].…”

Complex hydrides as thermal energy storage materials: characterisation and thermal decomposition of Na2Mg2NiH6

“…Only the rst step of this process (eqn (7)) was measured as to inhibit the decomposition of NaH, which inevitably leads to the distillation of Na metal to cooler parts of the instrument. [47][48][49] In regards to technological application, the formation of Na metal is undesirable and the thermodynamics of that process are understood. 30 The pressure of the equilibrium plateaux were measured at 0.6 wt%, which allowed for a van't Hoff plot with a corresponding R 2 factor of 0.999. larger due to the additional stability achieved by the incorporation of Na + and H À into the compound.…”

The electronic structure of metal-molten salt solutions: A tight-binding approach

“…Alkali metalalkali hydride melts exhibit a similar behavior. 3 Since experimental problems caused by the high corrosivity and chemical reactivity of the melts at high temperatures have been overcome, a large amount of experimental information on the microscopic and electronic structure is available. For the prototypal K-KCl system, experimental work covers neutron scattering, 4 NMR studies on the related Cs halides, 5 conductivity measurements, 6,7 ESR measurements, 8 the determination of optical absorption properties, 9,10 ellipsometric studies, 11 small-angle X-ray scattering, 12 and the measurement of thermodynamic data, 13,14 to mention a few.…”