This chapter deals with applications of neutron diffraction to the understanding of transformation processes in geomaterials with a time-resolution from seconds to several days. The choice of neutrons rather than X-rays for such time-resolved studies is due to their specific advantages like e.g., the sensitivity to light elements, the detection of volume rather than surface phenomena or the low absorption in case of bulky sample environments. General features of diffractometers suitable for time-resolved studies are discussed and the high-flux diffractometer D20 at ILL is presented in some detail. Time-resolved processes usually are studied at non-ambient pressures and temperatures. The necessary sample environment needs to be provided and matched to the diffractometer set-up. Processes in water ices and gas hydrates are taken as examples and will be used to demonstrate the capabilities and limitations of the method.

Crystals are less static than what is usually suggested by immobile drawings of their atomic or molecular arrangements representing a time-space averaged picture of atomic positions as obtained from diffraction experiments. Rather, atoms or molecules exhibit thermal displacements and some even move inside the lattice as defects. These averaged displacements are described via atomic displacement parameters (e.g., Kuhs 2003) and are accessible also using diffraction techniques. Yet, there is even more movement in a crystal. Close to structural phase transitions the collective mobility of the constituents enables the formation of new structural arrangements to lower the free energy of the system (Redfern 2006, this volume). Such solid-solid transformations are not necessarily instantaneous, in particular when the new phase needs to be reconstructed by moving molecular and atomic constituents to a new position. Following such transitions is one of the main topics of this review.

### Thermodiffractometry versus truly time-resolved studies

#### Phase transitions.

If a material is capable of existing in more than one polymorphic form, the process of transformation from one …

Time-resolved Neutron Diffraction Studies with Emphasis on Water Ices and Gas Hydrates

We present the design and operating characteristics of a vacuum furnace used for inelastic neutron scattering experiments on a time-of-flight chopper spectrometer. The device is an actively water cooled radiant heating furnace capable of performing experiments up to 1873 K. Inelastic neutron scattering studies performed with this furnace include studies of phonon dynamics and metallic liquids. We describe the design, control, characterization, and limitations of the equipment. Further, we provide comparisons of the neutron performance of our device with commercially available options. Finally we consider upgrade paths to improve performance and reliability.

/pdf/design-and-operating-characteristic-of-a-vacuum-furnace-for-1sef6hwsdp.pdf

Design and operating characteristic of a vacuum furnace for time-of-flight inelastic neutron scattering measurements.

The static $S(Q)$ liquid structure factors for binary ${\mathrm{Fe}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x},$ $x=0.90,$ 0.85, and ternary ${\mathrm{Fe}}_{0.85}{\mathrm{Ni}}_{0.05}{\mathrm{S}}_{0.15}$ molten alloys are investigated by means of the concurrent use of neutron diffraction and reverse Monte Carlo simulations. The measured $G(r)$ radial distributions reveal atomic orderings varying from that present in the ${\mathrm{Fe}}_{0.90}{\mathrm{Ni}}_{0.10}$ alloy, reminiscent of the ideal bcc structure of solid Fe, to a far more open structure found in ${\mathrm{Fe}}_{0.85}{\mathrm{Ni}}_{0.05}{\mathrm{S}}_{0.15}.$ From data at hand no clear evidence of the clustering of sulfur within the Fe-Ni matrix is found. In contrast, the addition of sulfur leads to significant changes in structural and some dynamical properties that can be inferred from knowledge of the static structure factors. Such dynamical changes seem to arise as a result of a strong decrease of the elastic moduli of the alloys resulting from interactions with a light element rather than from a density effect.

/pdf/microscopic-structure-of-fe-ni-and-fe-ni-s-molten-alloys-of-5b3xjzw9fs.pdf

Microscopic structure of Fe-Ni and Fe-Ni-S molten alloys of geophysical interest

The static S(Q) liquid structure factors for binary FexNi100−x, x = 90 and 85 and ternary Fe85Ni5S10 molten alloys are investigated by means of the concurrent use of neutron diffraction and Reverse Monte Carlo simulations. The measured g(r) radial distributions reveal atomic orderings varying from that present in the Fe90Ni10 alloy, reminiscent of the ideal b.c.c. structure of solid Fe, to a far more open structure found in Fe85Ni5S10. From data at hand no clear evidence of clustering of sulphur within the Fe–Ni matrix is found but, in contrast, its addition leads to significant changes in structural properties manifested by the loss of the intermediate range order.

/pdf/structure-of-fe-ni-and-fe-ni-s-molten-alloys-by-neutron-2fgqlyzwak.pdf

Structure of Fe-Ni and Fe-Ni-S molten alloys by neutron diffraction

Small-angle neutron scattering (SANS) is increasingly used to obtain statistically-representative data on particle or pore sizes, number and volume fractions, morphology and total surface areas in technological materials. Until recently, however, it had not been possible to perform in-situ microstructural investigations during thermal treatment. This paper reports on a new high-temperature (up to 1700°C) SANS furnace for materials research. Two interchangeable inner furnaces were built for a single outer atmosphere chamber so that either an oxidizing, a reducing, or a neutral environment can be used. Results derived during sintering of a controlled-porosity silica gel will be presented. The new furnace has made it possible for the first time to measure total porous surface areas and the evolution of pore sizes in situ during densification without interruption. Such measurements are expected to lead to improved process models offering quantitative predictability of product microstructures from the processing history of real materials.

Small-angle neutron scattering in a high-temperature furnace

Two similar high temperature furnaces (up to 2 000 °C) have been developed for large angle and small angle neutron scattering (SANS) experiments respectively. They are vacuum furnaces with a thin tungsten foil heating element supplied by two tantalum leads, and shielded by thin tungsten foils maintained in a tantalum box. In a neutron beam, the furnaces produce a very low scattering level (without any Bragg peaks) due to the openings in the heating element, the shields and the tantalum box for the incident, unscattered beam and the scattered beam at low angles. Thus the furnace absorption correction is also much easier. A platinum thermocouple controls temperature, very high temperatures are measured optically. A vacuum chamber has been especially designed for SANS experiments with two windows in polished sapphire. The furnaces have been successfully used for the study of liquid alloys on the D4, D11 and D17 spectrometers at the ILL.

https://hal.archives-ouvertes.fr/jpa-00245246/document

P. Desré

Papers

High temperature furnaces for small and large angle neutron scattering of disordered materials