Non-métal | EPFL Graph Search

Un non-métal est un élément chimique dont les atomes du corps simple sont unis par des liaisons covalentes ou des liaisons intermoléculaires, et non par des liaisons métalliques. Ce sont de bons isolants électriques et thermiques, pour la plupart très volatils, caractérisés par une masse volumique plus faible et des températures de changement d'état généralement bien plus basses que celles des métaux, à l'exception notable du carbone. Leur énergie d'ionisation et leur électronégativité sont élevées, leurs oxydes sont acides, et ils forment des liaisons ioniques avec les métaux, acquérant ou mettant en commun des électrons lorsqu'ils réagissent avec d'autres éléments ou d'autres composés. À l'état solide, ils présentent des surfaces ternes ou faiblement brillantes (bien que celles de l'iode aient des reflets métalliques), sont plutôt fragiles et cassants (à l'exception notable du carbone diamant) et sont dépourvus de l'élasticité, de la malléabilité et de la ductilité caractéristiques

Galvanic replacement reactions of Ga and In with Cu for the synthesis of bimetallic nanoparticles

Laia Castilla Amorós

The need for efficient and selective catalysts, capable of driving important conversions to build a more sustainable society, encourages the development of synthetic approaches towards new nanomaterials. Cu-based bimetallic nanoparticles (NPs) promise to fulfill this gap by selectively producing energy-dense products via CO2 electroreduction (CO2RR). However, as small variations in its composition and configuration can enormously affect the catalyst performance, the vast diversity of bimetallic NPs challenges systematic catalyst evaluation. Colloidal bimetallic NPs serve as an ideal platform for the exploration of composition-configuration-property relationships, necessary for rational catalyst design. Although an extensive library of noble metal bimetallic NPs exists, much less has been done regarding non-noble Cu-based bimetallic NPs despite their potential as catalysts for CO2RR. The aim of this thesis is to advance in the synthesis of Cu-based NPs, with focus on Cu-Ga and Cu-In, which have been predicted or demonstrated to possess interesting catalytic properties.The thesis starts with an introduction on bimetallic NPs and the different colloidal approaches to synthesize them. Galvanic replacement reactions (GRRs) are discussed more in details, as they are the synthetic approach of choice for the Cu-Ga and Cu-In NPs. Ga and In belong to a family of liquid metals; thus, information on their general properties is also provided. The first two experimental chapters focus on the reactivity and surface chemistry of Ga NPs. Chapter 3 demonstrates that Ga NPs react with a copper-amine complex to form anisotropic Cu-Ga nanodimers (NDs). Mechanistic studies reveal that a GRR takes place. Yet, the ND morphology differs from the more typical hollow core@shell structures obtained from GRRs. This unusual morphology is attributed to the liquid nature of Ga NPs and to the presence of a native oxide shell. This discovery fostered a follow up study, which is discussed in Chapter 4. The effect of the chemical nature of the capping ligand on the oxide thickness and reactivity of the Ga NPs is investigated. The thickness of the oxide skin is found to greatly depend on the ligands. Specifically, amines and carboxylic acids promote thicker oxide shells while thiols and phosphines hinder its growth. The reaction between Ga NPs with different oxide thickness and the copper precursor leads to a surprising result, while thicker oxides cause the formation of NDs, isolated Cu NPs form with thinner oxides.Chapter 5 turns towards Cu-In NPs. Following the same synthetic scheme, In NPs with different sizes are exploited as sacrificial seeds for the GRR. Cu-In NPs with an unprecedented variety of morphologies and elemental distributions are obtained: spherical Cu11In9 intermetallic and patchy phase-segregated Cu-In NPs, as well as Cu-Cu11In9 and Cu-In NDs. Segregation of the two metals occurs as the GRR progresses, with time or with higher copper precursor concentration. Comparing the obtained results with the Cu-In phase diagram reveals that the bigger seeds stabilize the bulk-like Cu-Cu11In9 configuration before their complete segregation into Cu-In NDs. Aiding in the development of more predictive synthetic approaches.Finally, Chapter 6 concludes by summarizing the significance of the achievements, which advance the knowledge on underexplored bimetallic NPs and on the GRR when applied beyond noble metals. An outlook on future developments is also provided.

EPFL2022

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Zhen Sun

Nanocrystalline (NC) metals have attracted widespread interest in materials science due to their high strength compared to coarse-grained counterparts. It is well know that during uniaxial deformation, the stress-strain behaviour exhibits an extraordinary work-hardening followed by an early observation of constant flow stress. Information on possible deformation mechanisms have been gathered by extensive research combining in situ deformation experiments, electron microscopy observations and computational modelling. Generally, these mechanisms are categorized into two types: dislocation slip and grain boundary (GB) accommodation processes, as for instance, GB sliding based mechanisms and GB migration eventually coupled to the shear stress. However, the interplay between these mechanisms resulting in the constant deformation resistance is not fully understood. Transient testing has proven to be a suitable tool to gather information on the rate limiting deformation mechanisms that are activated during the deformation path. In particular in stress reduction experiments, after an intermediate/large stress drop thermally activated dislocation slip is suppressed so that other underlying mechanisms are brought into foreground during subsequent transient creep. Those mechanisms may play a minor role in the determination of the flow stress but might still be essential to the development of a constant deformation resistance. Within this thesis, transient testing is combined with in situ X-ray diffraction at the Swiss Light Source. Therefore, the transient responses are captured in terms of evolution of macrostrain as well as diffraction peak broadening. Since dislocation slip and GB accommodation have an opposite footprint on the peak broadening, the presence of these two types of mechanisms can be distinguished. Three electrodeposited NC materials with different grain sizes are investigated: two NC Ni batches and one NC Ni50Fe50 batch. The results reveal that the constant flow stress reached during uniaxial deformation of electrodeposited NC metals reflects a quasi-stationary balance between dislocation-based mechanisms and GB-mediated accommodation. The latter plays an important role in producing plastic strain and recovering the defects and internal stress. Depending on the magnitude of the stress drop, a non-monotonic behaviour of the diffraction peak width is observed, suggesting an alternation of mechanisms. Also, by comparing transient responses among different NC materials, the relative contributions of dislocation slip and GB accommodation mechanisms are discussed in terms of grain size and alloying. Finally, different magnitudes of stress reduction are carried out by molecular dynamics (MD) simulations with the aim to verify in situ experimental results and explore the mechanisms responsible for GB accommodation. MD simulations confirm that dislocation slip is reduced after a moderate stress drop, however can continue to operate after adaption of the GB structures by a variety of GB accommodation mechanisms explaining the non-monotonic behaviour of the peak broadening during transient creep.

EPFL2016

Ab initio study of interface states at metal contacts to III-IV semiconductors

Thomas Maxisch

We present a theoretical study of the physical characteristics of metal/semiconductor junctions. Using first principle pseudopotential calculations, we have investigated the nature of electronic states with energies within the semiconductor band gap of representative abrupt, defect-free, anion-terminated metal/III-V interfaces. Namely, we focused on Al contacts to GaAs(001), AlAs(001) and cubic GaN(001) as well as on Al, Au and Cu junctions to cubic GaN(001). Recent advances in Schottky barrier concepts emphasize the possible relationship between interface states and the formation of the Schottky barrier. We aim at understanding the atomic-scale mechanisms responsible for interface states as well as their role in the Schottky barrier formation process. At As-terminated Al/GaAs(001) and Al/AlAs(001) junctions, resonant and localized interface states occur at the J point of the interface 2D Brillouin zone near the Fermi energy in the semiconductor midgap region. They correspond to intermetallic bonds between the outermost cation atoms of the semiconductor and the interfacial Al atoms of the metal. These interface states derive from an interaction between localized states of the Al(001) surface and semiconductor conduction band states, mediated by localized states of the unreconstructed, As-terminated semiconductor (001) surface. Our results indicate that interface states of the intermetallic, bonding-like kind could play an important role in the transport properties of metal/AlxGa1-xAs junctions. We have also investigated the electronic structure of Al, Au and Cu junctions to cubic, N-terminated GaN(001). The localized interface state reported for As-terminated Al/GaAs(001) and Al/AlAs(001) junctions occurs also at metal/GaN interfaces under the condition that atoms on the outermost atomic plane of the metal are placed in front of the outermost semiconductor cation. This indicates that the formation mechanism of this state is a very general one. In contrast to Al/AlxGa1-xAs junctions, these states occur at energy much larger than EF for the contacts to GaN. Thus, they are not expected to contribute significantly to the electronic transport of the latter interfaces. However, a large number of interface states attributed to d-type orbitals occur over a wide energy range including EF at contacts of noble metals to GaN.

EPFL2003