title: Hydrogen crystallization in low-density aerogels.
abstract: Crystallization of liquids confined in disordered low-density nanoporous scaffolds is poorly understood. Here, we use relaxation calorimetry to study the liquid-solid phase transition of H2 in a series of silica and carbon (nanotube- and graphene-based) aerogels with porosities ≳94%. Results show that freezing temperatures of H2 inside all the aerogels studied are depressed but do not follow predictions of the Gibbs-Thomson theory based on average pore diameters measured by conventional gas sorption techniques. Instead, we find that, for each material family investigated, the depression of average freezing temperatures scales linearly with the ratio of the internal surface area (measured by gas sorption) and the total pore volume derived from the density of aerogel monoliths. The slope of such linear dependences is, however, different for silica and carbon aerogels, which we attribute to microporosity of carbons and the presence of macropores in silica aerogels. Our results have important implications for the analysis of pore size distributions of low-density nanoporous materials and for controlling crystallization of fuel layers in targets for thermonuclear fusion energy applications.
authors.affiliation: Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
authors.forename: S O
authors.initials: SO
authors.lastname: Kucheyev
authors.affiliation: Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
authors.forename: E
authors.initials: E
authors.lastname: Van Cleve
authors.affiliation: Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
authors.forename: L T
authors.initials: LT
authors.lastname: Johnston
authors.affiliation: Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
authors.forename: S A
authors.initials: SA
authors.lastname: Gammon
authors.affiliation: Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
authors.forename: M A
authors.initials: MA
authors.lastname: Worsley