Synchrotron Radioscopy and Tomography

The achievement of keeping films liquid to study particle and liquid movements, was based up on the process of the single liquid model system. The aimed visualisation of those particles in-situ was achieved by using high brilliance X-ray synchrotron radioscopy at the European Synchrotron Radiation Facility (ESRF), beamline ID 19, Grenoble Cedex, France. The provided whitebeam was used in phase contrast mode (energy 10-35 keV), what allows the separation of similar density materials, in the case at our issue, Al alloy and ceramic particles. The photonbeam was further transformed to visible light using a scintillating screen made of a YAG:Ce single crystal and directly transferred via a folded beam path onto a digital camera, to avoid damage by ionising radiation. As a camera system a pco.dimax S was used, which is working with a resolution of 2016 x 2015 pixel, 2.7 µm pixel size, framerate up to 12 fps and an exposure time of 1.5 µs- 40 ms. For our purpose the gap of the wiggler was adjusted to 14.5 mm and the exposure time to 10 ms to aquire images with 50 fps, which was sufficient to track particles in the liquid Al. The following image processing and visualisation of particles was performed using the software "Image J".

European Synchrotron Radiation Facility (ESRF), Grenoble Cedex, France (left) and the experimental hall at BESSY II, Helmholtz-Centre Berlin, Germany (right)

High resoluted tomographies of the solidified films and foams (each 2000 images, at half rotation, every 0.09°) were obtained at the ESRF (ID19) as well as at the BAMline of BESSY, Helmholtz-Centre Berlin, Germany. At latter beamline an energie of 23 keV, an exposure time of 2 s at full resolution (4008 x 2672 pixel) and a pco.4000 camera (14 bit system) were used. The received projections were reconstructed to a 3D model using the software "Octopus 8.6" and analysed using the image processing software "VGStudioMax2.2". At both beamlines the visualisation of the particle distribution inside the films and Plateau borders was the main motivation.

Due to brilliant synchrotron X-rays at ESRF and the possibility to work in phase contrast mode, particle trajectories in liquid films based on the QPF model system could be visualised and analysed. Have a look at such a radioscopy sequence on the lower right. One outstanding detected effect was the incident that single particles and clusters flow continuous, whereby others stop for unrecognizable reasons and don´t move any further. This behaviour was surprising and studied in more detail for AlSi9Mg/SiC at low oxygen concentration and 21 % O2 as well as for AlSi/TiB2 at ambient atmosphere to discuss the influence of Magnesium, particles as well as the oxygen content. To visualise this effect more concrete, several image processing steps (background correction, image subtraction and integration over several images, etc.) were performed. One exemplary result of this phenomenon can be seen in the left figure, whereby SiC particles respectively clusters in AlSiMg/SiC at 21 % O2 flow vertical downwards forced by gravity (blurred black lines, integrated over 0.2 s) and stop surprisingly (black solid dots), see the video on the left. Furthermore it could be observed, that some already fixed cluster could even catch a further particle and hinder its further movement.

Synchrotron X-ray radiographies show moving SiC particles and clusters as well as the phenomenon of particle attachment in an AlSiMg/SiC film drawn at 21 % O2.
Processed and integrated images over 0.2 s per image of the left radioscopy of SiC clusters in AlSiMg/SiC at 21 % O2. Blurred lines indicate particles in motion. Sharp, clearly recognisable spots mark particles in idle state (fixed).

The question, why Air improves foam stability so tremendously was aimed to be answered by optical investigations methods, see figure right. Therefore it could be detected, that SiC particles of AlSiMg/SiC are distributed over the entire Plateau border if Air is used as source for gas injection, in comparison to Argon, whereby almost none particles could be visualised in the centre of the Plateau border, see the lower figure.

Synchrotron tomography of an AlSiMg/SiC Plateau Border of an Argon injected foam. Red spots indicate SiC particles and their tendency to stay at the solid-gas interface.

Individual metallic films were pulled from molten aluminium alloys containing different types of stabilising particles (0-20 vol.% of SiC or TiB2). A chamber with a controlled atmosphere (1000ppm-2.1×105 ppm O2) and different molybdenum wire frame structures were used. The objective was to discuss stabilisation of metallic foams based on the same melts [Heim, 2013]. Beside individual flat films, a modification of the wire frame structure allowed us to create artificial metallic Plateau borders and keep them liquid in order to study melt dynamics. Drainage, the velocity of the propagating rupture as well as the behaviour of the ceramic particles inside the melt were analysed in-situ both visually via a high-speed camera system and by means of synchrotron X-ray radiography. The oxide surface on the films was studied ex-situ by energy-filtered TEM and the particle distribution by metallography and SEM to discuss their role in stabilisation. We found that the particles and the immobile oxide skin, whose formation depends on the level of oxygen in the surrounding atmosphere and the presence of magnesium, are strongly interrelated. Phenomena such as the healing of thin regions in a film or the fixation of moveable clusters at the immobile gas-liquid interface could be discerned. Interactions of particles and oxygen were also visible during film collapse, where at the oxidised surface fixed particles hinder further film retraction, see Fig. 1 and the lower video. We discuss and compare the results obtained to analogous ones related to both aqueous systems and to metallic foams made from the same alloys by injecting air or Ar into a melt.

K. Heim, Soft Matter, 10:4711-4716, 2014

Synchrotron X-ray radiography showing the rupture of an AlSi/TiB2 film pulled in air.