NMI3 - Levitation techniques

In order to prospect the possibilities of levitation techniques for high temperature neutron scattering, we have organized a short satellite workshop to the fifth NMI3 meeting in Bilbao. Financial support was provided by NMI3 for most of the participants. The meeting took place at the Hotel Abando, Colón de Larreategi 9, 48001 Bilbao (Spain).

There are presently two major routes to levitation: the electromagnetic and gas flow techniques. Both have been presented during this workshop. A third technique, namely acoustic levitation, has also been explained and the developments performed in the USA were briefly presented. Very briefly, the electromagnetic techniques applies successfully to metallic samples over a temperature range where the resistivity remains optimum, i.e. from about 800 to 2000°C. With the aerodynamic or gas-flow technique, temperatures up to 3000°C can be accessed with a wider range of samples. In both cases, a number of issues remain to be solved so as to optimize their application to neutron scattering. In particular, the sample size is often too small, the sample sometimes moves around a main position and temperature gradients are not negligible. As regards the acoustic technique, it might be of interest when high purity is required at lower temperatures (up to 400°C).

L. Hennet has described the aerodynamic levitation technique from a live experiment and some movies. The actual  setup which has been installed on D4, D22 and IN8 at ILL consists of three CO2 laser units, two of which reaching 240W. The nozzle is made of vanadium or B4C and the mass flow controller required to levitate the sample delivers about 0.5 l/min of gas for standard samples (maximum of 2.5 l/min). The temperature is monitored with optical pyrometers which are calibrated from the melting point of the studied sample. The shortest counting times are about  10min on D4, compared with about 100ms at the ESRF. The maximum size of the samples is roughly Ø5mm, Ø7mm for the electromagnetic case, and the use of focusing neutron lenses would be valuable. The injected gas is rejected after action and it is possible to feed particles into the sample upon cooling. It is important to note that this technique is not suited to solids. I. Pozdnyakova has presented a selected list of scientific examples. In particular, she has shown the investigation on the coordination states of Al-(Fe/Ti/Cu/Ni) alloys, the crystalline growth of silicon, the intermediate range ordering in Al-Ni and Al-Cu alloys, the quadratic behavior of the viscosity in Al2O3, MgAl4O7 and MgAl2O4. One advantage in using neutron scattering is the possibility to cover larger Q ranges. There remains a number of questions to solve, in particular to understand the nature of the SANS signal in liquids.

I. Egry has followed with an overview of the work carried out with the electromagnetic technique, in particular with the work made to understand the structure of undercooled melts: density, viscosity, short range ordering and dynamics. The electromagnetic technique uses the Lorentz force and therefore applies only to conducting samples. When the sample is a too good conductor, it levitates but it is not possible to melt it. For a 1g sample, one observes about 5Hz oscillations of the melt with 30Hz surface oscillations. A number of experiments have been performed with X-rays and neutrons. For example, the neighboring distances in CoPd are shown to be temperature independent while the coordination number decreases with the temperature, the ordering is silicon remains unclear between 1100 and 1600°C and there are interestinf dynamics in liquid nickel. Technically, the oscillating electromagnetic field is produced by a water cooled coil connected to a 5kW RF generator. The shape of the coil depends on the nature of the sample but they are essentially 2 types of coils. The temperature is also monitored with a pyrometer. It varies a little with the vertical movements of the samples (about 0.1-0.3mm amplitude). The sample is placed in a chamber which is evacuated or under 1bar of inert gas. At present, the gas flow is tuned manually.

L. Holitzner has then presented the acoustic technique which applies to almost all types of materials (chamber filled with argon). The levitation is obtained using ultrasonic standing waves produced by an ultrasonic transducer and a reflector. Briefly, a Ø6mm water drop requires about 2.5W/cm2 at 20°C and a Ø6mm tin drop 37.2W/cm2 at 700°C. For solid samples, the optimum size is about half the wavelength (less for liquids). Using a lamp, one can warm up bio samples up to about 70°C. It is also possible to circulate a gas and control the temperature between -100 and +80°C. Temperatures of up to 400°C is attainable using the mirror technique. This technique is inexpensive compared with the other ones (roughly 50k€).

H. Lee has revealed that SNS was very interested by the levitation techniques and that it was planned to inject some manpower starting next year. IPNS has some expertise with acoustic and aerodynamic levitators. At SNS, they are quite interested to collaborate with them and to develop pump probe techniques synchronized with neutron pulses.

During the discussions, the scientific results have been reviewed and discussed. It has commonly been agreed that there was a need for furnaces taking advantage of all techniques presented and that hybrid versions should be developed at a later stage. The furnaces, if possible, should also be compatible with polarized beams and feature a top-loading design. FRM-II and ILL have decided to design and build furnaces taking advantage of these two techniques in collaboration with experts from the Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (Köln, Germany) and the Centre de Recherche sur les Matériaux à Hautes Températures (Orléans, France). Different financial supports are envisaged (a quick estimation leads to about 60k€, 300k€ and 360k€ for respectively the acoustic, aerodynamic and electromagnetic techniques). In particular, this work will be part of the Sample Environment Joint Research Activity that will perhaps be financed by the European Commission under FP7. Both DLR and CRMHT are interested by the projects and would be pleased not to have to organize the transport and installation of their existing equipment at neutron facilities (resulting in losses of beam times).