Pierre Lhuissier

Towards stiffness prediction of cellular structures made by electron beam melting (EBM)

Abstract Additive manufacturing is a novel way of processing metallic cellular structures from a powder bed. However, differences in geometry have been observed between the CAD and the produced structures. Struts geometry has been analysed using X-ray microtomography. From the 3D images, a criterion of ‘mechanically efficient volume’ is defined for stiffness prediction. The variation of this criterion with process parameters, strut size and orientation has been studied. The effective stiffness of struts is computed by finite element analysis on the images obtained by X-ray tomography. Comparison between the predicted stiffness and the effective one tends to show that the efficient volume ratio leads to a slight underestimation of the stiffness. Finally, the effective stiffness is used at the scale of a unit cell. This can help define the build orientation and loading direction that lead to the highest stiffness.

MHz frame rate hard X-ray phase-contrast imaging using synchrotron radiation

Third generation synchrotron light sources offer high photon flux, partial spatial coherence, and ~10-10 s pulse widths. These enable hard X-ray phase-contrast imaging (XPCI) with single-bunch temporal resolutions. In this work, we exploited the MHz repetition rates of synchrotron X-ray pulses combined with indirect X-ray detection to demonstrate the potential of XPCI with millions of frames per second multiple-frame recording. This allows for the visualization of aperiodic or stochastic transient processes which are impossible to be realized using single-shot or stroboscopic XPCI. We present observations of various phenomena, such as crack tip propagation in glass, shock wave propagation in water and explosion during electric arc ignition, which evolve in the order of km/s (µm/ns).

Pore morphology of polar firn around closure revealed by X-ray tomography

Abstract. Understanding the slow densification process of polar firn into ice isnessential in order to constrain the age difference between the ice matrix andnentrapped gases. The progressive microstructure evolution of the firn columnnwith depth leads to pore closure and gas entrapment. Air transport models innthe firn usually include a closed porosity profile based on available data.nPycnometry or melting–refreezing techniques have been used to obtain thenratio of closed to total porosity and air content in closed pores,nrespectively. X-ray-computed tomography is complementary to these methods, asnit enables one to obtain the full pore network in 3-D. This study takesnadvantage of this nondestructive technique to discuss the morphologicalnevolution of pores on four different Antarctic sites. The computation ofnrefined geometrical parameters for the very cold polar sites Domexa0C andnLockxa0In (the two Antarctic plateau sites studied here) provides newninformation that could be used in further studies. The comparison of thesentwo sites shows a more tortuous pore network at Lockxa0In than at Domexa0C, whichnshould result in older gas ages in deep firn at Lockxa0In. A comprehensivenestimation of the different errors related to X-ray tomography and to thensample variability has been performed. The procedure described here may benused as a guideline for further experimental characterization of firnnsamples. We show that the closed-to-total porosity ratio, which isnclassically used for the detection of pore closure, is strongly affected bynthe sample size, the image reconstruction, and spatial heterogeneities. Innthis work, we introduce an alternative parameter, the connectivity index,nwhich is practically independent of sample size and image acquisitionnconditions, and that accurately predicts the close-off depth and density. Itsnstrength also lies in its simple computation, without any assumption of thenpore status (open or close). The close-off prediction is obtained for Domexa0Cnand Lockxa0In, without any further numerical simulations onnimages (e.g., by permeability orndiffusivity calculations).

Solvent-free preparation of porous poly(l-lactide) microcarriers for cell culture

Porous polymeric microcarriers are a versatile class of biomaterial constructs with extensive use in drug delivery, cell culture and tissue engineering. Currently, most methods for their production require potentially toxic organic solvents with complex setups which limit their suitability for biomedical applications and their large-scale production. Herein, we report an organic, solvent-free method for the fabrication of porous poly(l-lactide) (PLLA) microcarriers. The method is based on the spherulitic crystallization of PLLA in its miscible blends with poly(ethylene glycol) (PEG). It is shown that the PLLA spherulites are easily recovered as microcarriers from the blends by a water-based process. Independent control over microcarrier size and porosity is demonstrated, with a higher crystallization temperature leading to a larger size, and a higher PLLA content in the starting blend resulting in a lower microcarrier porosity. Microcarriers are shown to be biocompatible for the culture of murine myoblasts and human adipose stromal/stem cells (hASC). Moreover, they support not only the long-term proliferation of both cell types but also hASC differentiation toward osseous tissues. Furthermore, while no significant differences are observed during cell proliferation on microcarriers of two different porosities, microcarriers of lower porosity induce a stronger hASC osteogenic differentiation, as evidenced by higher ALP enzymatic activity and matrix mineralization. Consequently, the proposed organic-solvent-free method for the fabrication of biocompatible porous PLLA microcarriers represents an innovative methodology for ex vivo cell expansion and its application in stem cell therapy and tissue engineering.nnnSTATEMENT OF SIGNIFICANCEnWe report a new solvent-free method for the preparation of porous polymeric microcarriers for cell culture, based on biocompatible poly(l-lactide), with independently controllable size and porosity. This approach, based on the spherulitic crystallization in polymer blends, offers the advantages of simple implementation, biological and environmental safety, easy adaptability and up-scalablility. The suitability of these microcarriers is demonstrated for long-term culture of both murine myoblasts and human adipose stromal/stem cells (hASCs). We show that prepared microcarriers support the osteogenic differentiation of hASCs, provided microcarriers of properly-tuned porosity are used. Hence, this new method is an important addition to the arsenal of microcarrier fabrication techniques, which will contribute to the adoption, regulatory approval and eventually clinical availability of microcarrier-based treatments and therapies.

Coupling in-situ X-ray micro- and nano-tomography and discrete element method for investigating high temperature sintering of metal and ceramic powders

The behaviour of various powder systems during high temperature sintering has been investigated by coupling X-ray microtomography and discrete element method (DEM). Both methods are particularly relevant to analyse particle interactions and porosity changes occurring during sintering. Two examples are presented. The first one deals with a copper powder including artificially created pores which sintering has been observed in situ at the European synchrotron and simulated by DEM. 3D images with a resolution of 1.5 μ m have been taken at various times of the sintering cycle. The comparison of the real displacement of particle centers with the displacement derived from the mean field assumption demonstrates significant particle rearrangement in some regions of the sample. Although DEM simulation showed less rearrangement, it has been able to accurately predict the densification kinetics. The second example concerns multilayer ceramic capacitors (MLCCs) composed of hundreds of alternated metal electrode and ceramic dielectric layers. The observation of Ni-based MLCCs by synchrotron nanotomography at Argon National Laboratory with a spatial resolution between 10 and 50 nm allowed understanding the origin of heterogeneities formed in Ni layers during sintering. DEM simulations confirmed this analysis and provided clues for reducing these defects.