Liquid–solid phase transition of hydrogen and deuterium in silica aerogel

Abstract

Behavior of hydrogen isotopes confined in disordered low-density nanoporous solids remains essentially unknown. Here, we use relaxation calorimetry to study freezing and melting of H2 and D2 in an ∼85%-porous base-catalyzed silica aerogel. We find that liquid–solid transition temperatures of both isotopes inside the aerogel are depressed. The phase transition takes place over a wide temperature range of ∼4 K and non-trivially depends on the liquid filling fraction, reflecting the broad pore size distribution in the aerogel. Undercooling is observed for both H2 and D2 confined inside the aerogel monolith. Results for H2 and D2 are extrapolated to tritium-containing hydrogens with the quantum law of corresponding states.

Hydrogen Crystallization in Low-Density Aerogels

“…Also shown are average depression temperatures of freezing of H 2 (⟨Δ T freeze ⟩). Data for controlled pore glass (CPG) powders and SiO 2 -340 sample are taken from refs and , respectively.…”

“…The H 2 gas used was obtained from Matheson Tri-Gas with a purity of ≳99.999% and natural isotopic content. The details of the custom-built apparatus and the measurement procedure can be found elsewhere. , In particular, the calorimeter was designed to minimize the contributions to the effective heat capacity from the addendum and from the heat of condensation of H 2 on cooling. , In all the cases, porous specimens were overfilled with H 2 prior to cooling in order to allow the crystallization front to invade the porous matrix from outside the monolith and, hence, to minimize possible kinetic undercooling events inside the porous matrix. The total amount of H 2 in the cell and the thermal link (Cu wires) between the calorimeter cell and the heat sink were selected so that the crystallization of H 2 took ≳3 min.…”

“…Moreover, all the previous studies of freezing in nanoconfinement that we are aware of have dealt with relatively high density solids, with typical porosities ≲50% . In this paper, we investigate the crystallization of H 2 in low-density nanoporous aerogels (AGs) with porosities ≳94% and compare results with our recent studies of Vycor-type glasses and a relatively high-density silica AG with ∼85% porosity . In particular, we investigate two most common classes of low-density nanoporous materials: silica and carbon AGs.…”

“…Despite the general agreement of experimental observations with predictions of the GT theory (i.e., Δ T freeze ∝ D pore –1 ) for prototypical porous materials such as porous Vycor-type silica glasses, − there have been numerous reports of deviations from eq . , Many aspects of the physics of the liquid–solid phase transition in nanoconfinement are still not well understood. In particular, there is still a limited understanding of mechanisms that are responsible for the freezing–melting hysteresis, ,,, consequences of changes in the density of the pore filling fluid on the phase transition and associated mass transport, the crystallographic structure of the confined solid, , kinetic effects related to the dependence of crystal growth rates on the crystallographic direction, kinetic nucleation phenomena inside pores, and effects of pore geometry potentially leading to pore blocking and a percolation-like propagation of the crystallization front. ,,, All these are beyond the simple phenomenological GT theory leading to eq .…”

Atomistic simulation of size‐dependent heat capacity of liquid in molecular scale confinement at different temperatures

“…For 0-300 K temperature, an enhanced heat capacity of hydrogen inside a norbornene-based aerogel has been recently reported by the experimental works of Kucheyev and Lenhardt [14]. Cleve et al revealed the size and thermal effect on the effective heat capacity of hydrogen and deuterium entrapped in silica-aerogel [15]. However, none of the earlier researchers investigated the size dependency of the heat capacity of liquid argon under nanoscale confinement at different temperatures.…”