Scientific Events

High-throughput screening is a well-established method for scientific experimentation in chemistry and biology. Examples are heterogeneous catalysts, drug developments and nanoparticle toxicology. These methods involve the synthesis of small sample volumes often in form of particles that are quickly tested. These tests are designed to quickly obtain easily accessible data (called descriptors) that are related with a predictor function to the desired properties. The descriptor-predictor-relation is found through mathematical modelling and calibration. One particle based high-throughput concept for the evaluation of potential toxicological hazards will be presented in more detail. Furthermore, a new concept is presented which transfers high-throughput screening to the exploration of new structural metals. The method comprises the synthesis of many small alloy samples in form of particles. These samples obtain a defined microstructure by fast or parallel thermal and mechanical treatments and are subsequently subjected to novel fast descriptor tests while a mathematical algorithm develops the predictor function. The method presented here is a collaborative approach among many researchers and also involves sample routing and automation considerations as well as process modelling. [more]

The development of metallic alloys is arguably one of the oldest of sciences, dating back at least 3,000 years. It is therefore very surprising when a new class of metallic alloys is discovered. High Entropy Alloys (HEA) appear to be such a class furthermore, one that is receiving a great deal of attention in terms of the underlying physics responsible for their formation as well as unusual physical,mechanical and radiation resistance properties that make them candidates for technological applications. The term HEA typically refers to alloys that are comprised of 5, 6, 7… elements, each in in equal proportion, that condense onto simple underlying crystalline lattices but where the different atomic species are distributed randomly on the different sites -face centered cubic (fcc) Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2 and body-centered-cubic (bcc) V_0.2Nb_0.2Mo_0.2Ta_0.2W_0.2 being textbook examples. The naming of these alloys originates from an early conjecture that these unusual systems are stabilized as disordered solid solutions alloys by the high entropy of mixing associated with the large number of components  - a conjecture that has since proved insufficient. In the first part of the presentation I will describe a model that allows us to predict which combinations of N elements taken from the periodic table are most likely to yield a HEA that is based on the results of modern high-throughput ab initio electronic structure computations. In the second part I will broaden the discussion to a wider class of equiatomic fcc concentrated solid solution alloys that is based on the 3d- and 4d-transition metal elements Cr, Mn, Fe, Co, Ni, Pd that range from simple binary alloys, such as Ni_0.5Co_0.5 and Ni_0.5Fe_0.5, to the quinary high entropy alloys Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2 and Cr_0.2Pd_0.2Fe_0.2Co_0.2Ni_0.2 themselves. Here I will discuss the role that increasing chemical complexity and disorder has on the underlying electronic structure and the magnetic and transport properties. Finally, I will argue that the manipulation of chemical complexity may offer a new design principle for more radiation tolerant structural materials for energy applications. [more]

Quantification of the mechanical properties of crystalline materials at micro and nano length-scales is as important as it is challenging. In situ mechanical testing inside the SEM using a micro-indenter offers the great advantage of a direct observation of the progressive deformation in materials and has been applied for many years to assess the deformation mechanisms at small scale. This is now reinforced by the capability of doing in situ EBSD and HR-EBSD in order to map the evolution of the microstructure, the stress field and the GNDs distribution in the materials at several steps during progressive deformation. We apply this technique to estimate the size of the plastic zone underneath the crack tip during micro-cantilever bending in tungsten and NiAl intermetallic for fracture toughness determination. HR-EBSD is used to map the evolution of the stress field around the notch tip and to estimate the GNDs in the plastically deformed zone. In situ EBSD has been also applied to micropillar compression in alpha Titanium in order to study the formation and evolution of compressive twins during the deformation. The results show dislocation driven twin formation and twin propagation and thickening according to the local resolve shear stress. [more]

The design of Atom probe tomography (APT) at Oxford and Rouen universities for 25 years ago has been an outstanding breakthrough in the microscopy world. APT is the only analytical microscope able to provide 3D images of a material at the atomic scale [1]. Because of its ultimate spatial resolution (0.1 nm in depth, a few tenths of a nm at the sample surface), combined with its quantitativity of composition measurements, APT has played a major role in the investigation of the early stages of phase separation in solids. APT has also been the first instrument to show Cottrell atmospheres (tiny clouds of impurity atoms around dislocations in crystals) at the atomic-scale in the three dimensions of space [2]. A new breakthrough has been achieved ten years ago with the implementation of ultrafast pulsed laser (duration < 1ps) to atom probe tomography [3]. This new generation of laser-enhanced atom probe tomograph, designed in our lab and at Madison, USA, has opened the instrument to semi-conductors and oxides that are key materials in micro-electronics and nanosciences [4,5]. Correlative approaches combining TEM with APT has been shown to be crucial for more accurate APT reconstructions of microelectronics devices [6]. A key force of APT is that 3D reconstructions can be confronted at the same scale to kinetic Monte-Carlo simulations conducted on rigid lattice. This dual approach has been recently applied to phase separation in self-organised GeMn magnetic thin films [5]. In this talk, APT results will be confronted to simulations but also to analytical models dealing with precipitation kinetics (non-classical nucleation [7], coarsening in ternary systems, influence of precipitate size on their composition [8]). A recently developed analytical model dealing with nucleation, growth and coarsening in ternary systems including diffusion coupling between chemical species has revealed that the kinetic pathway does not necessarily follow the tie lines of phase diagram in agreement with APT experiments on model nickel base superalloys [9]. [1] D. Blavette, A. Bostel, J.M. Sarrau, B. Deconihout and A. Menand, 1993, Nature 363, 432 [2] D. Blavette, E. Cadel, A. Fraczkiewicz, A. Menand, 1999, Science 17, 2317 [3] B. Gault, F. Vurpillot, A. Vella, M. Gilbert, A. Menand, D. Blavette, B., 2006, Rev. Sci. Instr. 77, 043705 [4] S. Duguay, T. Philippe, F. Cristiano, D. Blavette, Applied Physics Letter (2010) 97, 242104 [5] I. Mouton, R, Larde, E. Talbot, C. Pareige, D. Blavette, JAP 115, 053515 (2014) [6] A. Grenier, R. Serra, G. Audoit, Jp Barnes, S. Duguay, D. Blavette, N. Rolland, F. Vurpillot, P. Morin, P. Gouraud, Applied Physics Letters 106, 213102 (2015) [7] T. Philippe, D. Blavette, Journal Of Chemical Physics, 135, 134508 1-3 (2011) [8] M. Bonvalet, T. Philippe, X. Sauvage, D. Blavette, Phil. Mag Vol. 94, N°26, 2956-2966 (2014) [9] M. Bonvalet, T. Philippe, X. Sauvage, D. Blavette, Acta Materialia 100 (2015) 169-177 [more]

Perovskite (ABO₃) oxides are by no exaggeration an extremely versatile class of materials, exhibiting a broad spectrum of fascinating physical properties: superconductivity, ferromagnetism, ferroelectricity, multiferroic behavior. Scaling down from bulk single crystals to thin and ultrathin (few unit cell thick) epitaxial films the first question is to what extent we are capable to preserve the properties of the bulk and understand the role played by epitaxial growth in changing the physical properties. Another degree of freedom and of complexity as well arises when we interface coherently thin films of two or more chemically and physically different oxides. On one hand, the interfacing poses challenges in terms of finding the fabrication conditions that satisfy the needs of all partners. On the other hand, new physical properties may arise at the interfaces: these are the driving forces for heterostructures and superlattices of perovskite oxides, with a boom of efforts in the last decades, supported by the progress in molecular beam epitaxy and pulsed-laser deposition of complex oxides. Simultaneously the advances of high resolution and scanning transmission electron microscopy with analytical accessoires (EELS, EDX) have been of tremendous help in investigating such heterostructures with unprecedented spatial resolution, contributing directly to the understanding of the physical properties at a unit cell level. In order to underline these statements, I shall give example of the physical properties of heterostructures and superlattices of ferromagnetic and ferroelectric perovskites from my work. [more]

Transition metal oxide superlattices have been widely investigated during recent years as they are one of the largest material groups where physical and chemical properties such as ferroelectricity, magnetism, ionic and electronic conductivity are closely coupled to structural parameters. Cation sub¬sti¬tution in complex oxides is an effective way to develop the functionalities through carrier doping, band engineering, or application of chemical pressure. For example, the coupling between charge and spin degrees of freedom across the interfaces and the local charge carrier concentration profiles have profound influences on the occurrence of superconductivity in low dimensional systems. Super-conductivity arises when a parent insulator compound is doped beyond some critical con-centration. Furthermore, the magnetic behaviour and conductivity of complex oxide superlattices can be tuned by controlling the layer thickness and by selecting appropriate intervening layer materials. Various methods for growing controlled superlattice structures exist, a favourite has been pulsed laser deposition (PLD), but molecular beam epitaxy (MBE) is now also popular because of the controlled deposition rate and the flexibility allowed by the use of individual element sources. In theory, this allows composition control to the level of individual atomic layers. The PLD process requires higher temperatures and pressures than MBE. It also involves significantly higher energies for the impinging particles, which has potential implications for the interface roughness. In this presentation, I will discuss mapping of the local structure and interfacial chemistry of various complex oxide hetero-interfaces through advanced scanning transmission electron microscopy (STEM) in combination with energy-dispersive x-ray (EDX) analysis and electron energy-loss spectroscopy (EELS).1 EELS allows for local probing of chemical composition and bonding, as well as electronic and magnetic structure, making the combination of STEM and EELS ideal for discovery of structure-property correlations at the atomic scale.2,3 References 1 F. Baiutti et al., Nature Comm. (2015), DOI: 10.1038/ncomms9586, in press. A.V. Boris et al., Science 332 (2011) 937-940. E. Detemple et al., Appl. Phys. Lett. 99 (2011) 211903. E. Detemple et al., J. Appl. Phys. 112 (2012) 013509. A. Frano et al., Adv. Mater. 26 (2014) 258-262. F. Wrobel et al., submitted (2015). K. Song et al., APL Materials 2 (2014) 032104. D. Zhou et al., APL Materials 2 (2014) 127301. D. Zhou et al., Adv. Mater. Interfaces 2 (2015) 1500377. D. Zhou et al., Ultra¬micro¬sco¬py 160 (2016) 110–117. 2 PAvA gratefully acknowledges the intense collaboration with the following people without their contributions this work wouldn’t have been possible: F. Baiutti, E. Benckiser, C. Bernhard, A.V. Boris, M. Castro-Colin, G. Cristiani, E. Detemple, K. Du, A. Frano, E. Gilardi, G. Gregori, H.-U. Habermeier, V. Hinkov, B. Keimer, M. Kelsch, F.F. Krause, G. Logvenov, Y. Lu, J. Maier, V.K. Malik, A.F. Mark, Y. Matiks, M. Morenzoni, K. Müller-Caspary, E. Okunishi, P. Popovich, T. Prokscha, Q.M. Ramasse, M. Reehuis, A. Rosenauer, Z. Salman, H. Schmid, W. Sigle, K. Song, V. Srot, A. Suter, Y. Wang, P. Wochner, F. Wrobel, M. Wu, D. Zhou. 3 The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007/2013] under grant agreement no 312483 (ESTEEM2). [more]

Materials with more non-metallic bonding are brittle, but are widely used, for instance as protective coatings. These often fail by cracking, so if their fracture resistance were increased, by making plastic flow easier, their lifetime could be extended. Some non-metallic materials deform readily, on a limited number of crystal planes, such as the ternary carbide Ti3SiC2 as well as Nb2Co7, W2B5 and Ta4C3. However, at present the understanding of how to design crystal structures for easy plastic flow is guided only by desirable ratios of elastic constants. Here, it is shown that flow is predicted to become very much easier if there are electronegativity differences within a crystal's unit cell, which cause non-uniform elastic deformation. Very substantial changes in flow behavior appear possible, suggesting this is a first step in developing a simple way of controlling plastic flow in non-metallic crystals. [more]