Dynamical features of methane hydrate around 12 K

https://doi.org/10.1016/j.physb.2004.03.105Get rights and content

Abstract

Methane hydrate is a possible candidate material for use as a cold neutron moderator in pulsed neutron sources. Our aim was to perform inelastic neutron scattering measurements with sufficient range in energy and momentum to create a scattering kernel to simulate its behavior as a moderator material. Measurements were made on samples of D2O–CH4 and H2O–CH4 at 12 K covering a range up to 500 meV and 28 Å−1. These data clearly shows the almost free recoil of the CH4 rotational and vibrational modes and confirms that the dynamics of the ice and methane are largely decoupled on the energy scales that we are studying. From these initial measurements we can conclude that methane hydrate will be a good moderator over a wide range of neutron energies. We are now able to produce models of sufficient quality to create scattering kernels for the Monte-Carlo simulation codes that are used to design neutron sources.

Introduction

Clathrate hydrates have many properties that are interesting not only for scientific reasons but also in the fields of geology and nuclear engineering. Gas clathrate hydrates are a non-stoichiometric inclusion of elements or molecules within a host framework or “cage” composed of water molecules. There are many elements and molecules that are able to act as guest molecules, including: Ar, O2, N2, CH4, and CO2, but in this study we are specifically interested in methane hydrate (MH).

There has recently been a surge of interest in MH due to its potential use as an energy source, and for geo-scientific or environmental reasons. However, our interest is in its potential use as a moderator on a pulsed neutron source. A good moderator material should have a high hydrogen density and a large density-of-states in and around the thermal region (1–100 meV) in order to produce a well defined Maxwellian. Pure solid methane is already used in various pulsed sources around the world and is excellent for producing a high flux of cold neutrons. In contrast, water ice lacks low-energy vibrational modes and is much better at producing thermal neutrons [1]. Because of its high content of methane, MH potentially combines the properties of ice and methane to produce a moderator with a high performance over a wide energy range.

Although several neutron scattering studies of MH have already been undertaken both looking at the dynamics [2], [3], [4] and the structure [5]; the measurements of the dynamics are of insufficient range in momentum and energy for our purposes. Our ultimate aim is to create a scattering kernel that can be used in the Monte-Carlo codes that are used to design neutron sources. To this end we wish to derive a physical model that can reproduce the scattering function with sufficient accuracy.

In this paper we describe the characteristic features of the scattering function of MH over a wide range of Q–E at a temperature of 12 K.

Section snippets

Experimental

Two kinds of MH samples were made at the Hokkaido National Industrial Research Institute (HNIRI): CH4–D2O (MH–D2O) and CH4–H2O (MH–H2O). They were prepared by grinding crushed ice at 268 K whilst it was in intimate contact with methane gas at 5 MPa for 72 h. The methane stoichiometries were 80% for MH–D2O and 87% for MH–H2O, respectively. Crushed ice Ih samples of H2O and D2O were prepared for comparison. The samples were stored under liquid nitrogen. For the neutron measurements an annular

Results and discussion

The resultant dynamical structure factor of MH–D2O, from the 500 meV incident energy run, is shown in Fig. 1. The strong recoil of the rotational and vibrational levels of the CH4 molecules can be seen clearly. The lowest energy peak (A in Fig. 1) is assigned to a series of CH4 rotational levels. These have been studied in more detail by Tse [2] and Gutt [3]. Peaks B and C in Fig. 1 are related to CH4 bending and stretching vibrations, respectively. By extrapolating these modes to Q=0 we can

Conclusion

In summary, we measured the dynamical structure of MH over a much wider range Q−E space than in previous neutron inelastic experiments. The rotational and vibrational levels of CH4 molecules show recoil like behavior in the high Q region. Although the acoustic and molecular optic modes of the ice in the clathrate look similar to those in ice Ih there are differences in the librational modes. All these facts indicate that the methane molecule only weakly interacts with its H2O host and other

References (9)

  • K. Inoue et al.

    Nucl. Instrum. Methods

    (1982)
  • J.S. Tse et al.

    J. Phys. Chem. A

    (1997)
  • G. Gutt et al.

    Europhys. Lett.

    (1999)
  • H. Conrad, W.F. Kuhs, K. Nuighoff, C. Pohl, M. Prager, Proceedings of ICANS-XVI, May...
There are more references available in the full text version of this article.

Cited by (5)

View full text
Copyright © 2004 Elsevier B.V. All rights reserved.