Dissertation / PhD Thesis

PUBDB-2018-05505

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Interfacial Premelting of Ice in Nanocomposite Materials

Li, H. (Corresponding author)MPG* ; Mezger, M. (Thesis advisor)MPG*

2017
Mainz

Abstract: In this thesis, the interfacial melting phenomenon of ice in nanocomposite materials is studied by high energy X-ray diffraction (HEXRD) and quasi elastic neutron scattering (QENS) technologies. In order to conduct experiments under hydrostatic pressure, a new high-pressure cell which can sustain pressures up to 5 kbar was designed. The main part of this setup is the inner sample cell, a tube made of grade 5 titanium alloys. Another important component is the new cooling and heating part. It is designed to control the sample temperature between -60 oC and +25 oC with a temperature stability better than ±0.005 oC.Using this new cell, HEXRD experiments are conducted to investigate the interfacial premelting in ice/clay nanocomposites. The surface property effect on ice interfacial melting is studied by choosing surface charged vermiculite and uncharged kaolin as substrates which have a large surface to volume ratio. Below the melting point of bulk water, the formation of a quasi-liquid layer (qll) is observed for both samples. The thickness of this interfacial premelting layer is gradually increasing with temperature. For both minerals, a similar thickness of about 1 nm is reached 3 K below the bulk melting point. This corresponds to one and a half times the ice Ih lattice spacing along the c-axis. In this regime, continuum models might be not applicable any more. For higher temperatures, the qll thickness is compared with theoretical predictions from continuum models. The data is best described by a logarithmic growth law, originating from short range interactions. Pronounced differences in the decay length of the short range interactions are observed between the charged vermiculite and uncharged kaolin minerals. Using well defined and characterized ice/clay nanocomposite samples, this work bridges the gap between studies on single crystalline ice/solid model interfaces and naturally occurring soils and permafrost.Complementary to the static information achieved by X-ray studies, we conduct QENS experiments to achieve information about the dynamic of water molecules within the qll. To investigate the influence of the surface properties of the substrates, a series of charged hydrophilic (vermiculite), uncharged hydrophilic (kaolin), and uncharged hydrophobic (talc) ice/clay nanocomposites are studied. The interfacial melting is again observed at a temperature lower than the melting point. The diffusion coefficient D of water in qll is calculated from the quasi elastic spectrum. At lower temperatures T_m-T > 4 oC, we find D_vermiculite<D_kaolin<D_talc<D_(supercooled-water). The reduced mobility for water in the qll is the result of cooperation of nano confinement and intermolecular interactions between the water molecules and the solid substrates. The high viscosity calculated from D indicates the much dense water in the qll. Water molecules in the qll become more mobile due to the diminished nano confinement effect and intermolecular interactions when increasing temperature. After that we move on to investigate how hydrostatic pressure on the order of several kbar affects the interfacial melting behavior. High pressure HEXRD experiments were conducted on vermiculite/ice sample under pressures up to 5 kbar. For ice Ih, the suppression phenomenon of interfacial melting is observed for pressure up to 900 bar. With increasing pressure, the decay length extracted from the slope of the qll growth law gets smaller. In contrast, for high pressure ice phase V a different premelting behavior was observed. Here, the qll thickness attenuates upon approaching the bulk melting point. One remarkable difference between ice Ih and ice V is their mass density relative to the density of liquid water at the melting curve. This might be an indication that the different premelting behavior is related to the qll density.

Note: Dissertation, University of Mainz, 2017

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Contributing Institute(s):

1. DOOR-User (DOOR)

Research Program(s):

1. 6G3 - PETRA III (POF3-622) (POF3-622)

Experiment(s):

1. PETRA Beamline P07 (PETRA III)

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