Esther Belin-Ferré

Complex metallic alloys : fundamentals and applications

FOREWORD I. INTRODUCTION TO THE SCIENCE OF CMAS Brief History of the Field Definition of CMAs Phase Selection and Stability Unexpected Transport Properties Plasticity and Metadislocations Outlook at Potential Applications II. PHYSICS OF CMAS: THEORY AND EXPERIMENTS Electronic Structure Chemical Bonding Effects Cluster Based Solids Physics Transport Properties: Inverse Matthiessen Rule, Wiedemann-Franz Law Phenomenological Approaches Hierarchical Scales in Reciprocal Space Anisotropic Physical Properties of CMAs: Electrical and Thermal Conductivity, Magnetic Susceptibility, Hall Coefficient, Thermoelectric Power III. SURFACE SCIENCE OF CMAS Stability of Alloy Surfaces Structural Determination of CMA Surfaces Electronic Structure Thin Film Growth on CMA Surfaces Adhesion, Friction and Wetting Properties of CMA Surfaces IV. METALLURGY, PREPARATION AND PROCESSING, THIN FILMS Introduction to Thin Film Science Deposition by MOVCD Deposition by PVD Structural Properties Mechanical Properties V. SURFACE CHEMISTRY OF CMAS Introduction Surface Chemistry of CMAs under UHV Environment Surface Chemistry of CMAs under Environmental Conditions Surface Chemistry and Reactions in Aqueous Solutions High-Temperature Corrosion VI. MECHANICAL ENGINEERING PROPERTIES OF CMAS Introduction to CMAs Designed for Mechanical Applications: Structure and Properties, Possible Applications, Single-Phase CMAs and CMA-Reinforced Composites Processing and Mechanical Properties of CMAs: Solidification, Glassy Precursors, Powder Metallurgy The Sintering Route Surface Mechanical Testing and Potential Applications VII. THERMOELECTRICITY, THE GRAAL OF CMAS Materials Properties Applications/Demonstrators VIII. CATALYSIS CATALYSIS ON INTERMETALLIC COMPOUNDS IX. CMAS: FROM BASICS TO PRODUCTS

Study of Al-Cu Hume-Rothery alloys and their relationship to the electronic properties of quasicrystals

The stability of quasicrystals is related to the Hume-Rothery mechanism known to take place in many intermetallics, especially Al-Cu alloys. In order to assess to what extent the electronic density of states in aperiodic Al-Cu-Fe compounds is sensitive to Hume-Rothery stabilization, we present experimental measurements by soft x-ray emission and absorption spectroscopies of the aluminium electronic states in a series of Al-Cu crystalline compounds. Some of them, like icosahedral Al-Cu-Fe, are characterized by nearly spherical Brillouin zones. We observe the opening of a narrow but definite pseudo-gap in such Al-Cu compounds. We point out a strong interaction between Cu d states and the Al states in the middle of the valence band which is well described by a Fano-like effect. Some other characteristic features of our spectra are interpreted on the basis of band structure calculations. A comparison to previous data published for quasicrystalline and approximant phases stresses the importance for the stability of these compounds of sp-d hybridization at the Fermi level. It also leads to the conclusion that the effect of Hume-Rothery interaction is dramatically enhanced by the hierarchical structure of quasicrystals rather than by the almost spherical shape of the Brillouin zone.

Electronic structure in quasicrystalline compounds and related crystals

This article reviews and discusses the most relevant results achieved over recent years from the study of the electronic structure of quasicrystals (QCs) and related alloys using basically densities-of-states calculations and the x-ray emission, x-ray absorption, and photoemission spectroscopic techniques, in agreement with information obtained by other experimental means. It focuses on Al-based systems, the most widely analysed so far, and also reports data for Zr–Ni–Ti as well as Mg-based compounds. The present knowledge of QCs and approximants as compared to conventional crystals of the same phase diagrams may be summarized as follows: (i) the occurrence of a pseudogap at the Fermi level that is also present at the very surface is well established for icosahedral QCs whose formation derives from both Hume-Rothery and Al–transition metal hybridization effects; (ii) the pseudogap does not exist in the total densities of states of decagonal QCs but always exists in the Al sub-bands; (iii) states in QCs and close approximants are of more localized-like character in the vicinity of the pseudogap at the Fermi level than in conventional intermetallics, and at greater distances, above as well as below the Fermi level, they are extended-like in character, signalling a clear tendency to weak electron localization especially effective on either side of the pseudogap. As a result, there is a progressive loss of the metallic character, that can be tuned intentionally, when going from usual crystals to approximants and QCs.

Al 3p occupied states in Al-Cu-Fe intermetallics and enhanced stability of the icosahedral quasicrystal

This paper concentrates on a comparison of Al 3p occupied densities of states in crystalline and quasicrystalline Al-Cu-Fe intermetallics. This comparison is made quantitative by computing the two first moments of this partial distribution, which correspond to the partial contribution of Al atoms to the cohesive energy of the material and to the orbital overlap modulated by the coordination number, respectively. From these data, we conclude that the icosahedral quasicrystal and its approximants represent a region of specifically enhanced stability in the Al-Cu-Fe phase diagram. We assign this effect to the specific role played by Al 3p states in the Hume-Rothery mechanism. We confirm the occurrence of a stronger hybridization between Al 3p and Fe 3d states in the icosahedral structure whereas we point out a weaker interaction with Cu 3d states in the icosahedral compounds as compared to stable approximants.

Electronic structure of Al-Pd-Mn crystalline and quasicrystalline alloys

An experimental and theoretical analysis of the electronic structure of Al-Pd alloys is presented. The experimental study, based on soft x-ray emission and absorption spectroscopies, is further extended to the case of quasicrystalline Al-Pd-Mn alloys and their approximants. The electronic structure is dominated by the sp-d hybridization. The Al-sp band is strongly modified by the interaction with the Pd-d states in the middle of the valence band. Close to the Fermi level, hybridization with the Mn-d states is found to enhance the structure induced pseudo-gap in the Al partial density of states, both in the icosahedral and the decagonal quasicrystalline phases.

About the Al 3p density of states in Al–Cu–Fe compounds and its relation to the compound stability and apparent surface energy of quasicrystals

Abstract We report the characteristic parameters of the Al 3p densities of states gained from a thorough study of many Al–Cu(–Fe) intermetallics including quasicrystalline and approximant phases using X-ray emission spectroscopy. These parameters are the density of states at the Fermi energy EF, the average binding energy due to Al 3p states and the second moment of their distribution. Hence, we demonstrate that the contribution to the compound stability arising from localization of Al 3p states via the Hume-Rothery mechanism is maximum for the icosahedral state. We also show that Al-transition metal sp-d hybridization is enhanced in the presence of icosahedral order. Finally, we confirm the already pointed out relationship between depth of the pseudo-gap at EF and apparent surface energy of the samples.

Electronic structure calculations of Al± Cu alloys: comparison with experimental results on Hume-Rothery phases

Abstract Electronic band-structure calculations for a set of hypothetical Al-Cu alloys are presented. The influence of the local order and of the Cu concentration on the local density of states is analysed. The Als states are repelled towards the bottom of the occupied band by the interaction with the more localized Cud states lying about 4eV below the Fermi level. Bonding and antibonding Alp states also appear, located on each side of the Cud band. The coupling between the more extended states of Al with the d band of Cu is well described by a Fano-like effect. Two calculations performed for real Al-Cu phases of the Hume-Rothery type allow direct comparison with experimental results obtained by soft-X-ray spectroscopies. A pseudogap induced by the interaction between the Fermi sphere and the Brillouin zone is observed in agreement with the experiment. The availability of the Hume-Rothery criterion in the formation of the pseudogap observed in quasicrystalline phases is also discussed.

Electronic distributions and pseudo-gap in quasicrystalline decagonal and alloys

Electronic state distributions of decagonal Al 65Cu15Co20 and Al70Co15Ni15 quasicrystals have been investigated by experimental means using the soft-x-ray spectroscopy technique complemented by photoemission spectroscopy measurements. In both quasicrystals, interaction is shown within an energy range of 2 eV below the Fermi level between the Al states and either the Co 3d states or both the Ni 3d and the Co 3d states. In Al65Cu15Co20, the Cu 3d states are observed in the middle of the occupied band. The formation of a significant pseudo-gap at the Fermi level is shown in the Al 3p and 3s-d distributions of these alloys. The comparison with available photoemission data an Al65Cu20Co15 and Al70Co15Ni15 samples shows good agreement in the energy positions of Cu, Co and Ni occupied states. The results for Al65Cu15Co20 are compared with calculations of densities of states for two models of the Al-Co-Cu decagonal quasicrystals. Very good agreement between experiment and one of these calculations is found for the Al distributions. For Al 70Co15Ni15, fair agreement between the present results and data for crystalline Al 5Co2 is shown. The Al p and Ni s-d unoccupied states are considered as well in Al70Co15Ni15. It is suggested that a slight charge transfer may occur from Al to Ni in this sample and that most of the Al p unoccupied states presumably occur from the contribution of the quasiperiodic planes in this specimen.

Surface energy of complex ? and simple ? metallic compounds as derived from friction test in vacuum

This article attempts to understand better the origin of low friction and zero solid–solid adhesion observed on quasicrystals placed in vacuum. We present an ensemble of experimental data that allows us on the one hand to estimate the true surface energy of unoxidized quasicrystals and approximants and on the other hand to point out a correlation with electronic densities of states.

Electronic structure of hexagonal

We have investigated the electronic structure of hexagonal through theoretical as well as experimental means. Two different calculations have been carried out within the LMTO and tight-binding - LMTO methods. The experimental procedure involved soft-x-ray spectroscopy complemented by photoemission measurements. Co states have been found near the Fermi level, in interaction with Al states of hybridized p, s - d character. As a result, a pseudo-gap is generated at the Fermi level. Occupied Al states beyond 5 eV form the Fermi level are found to be almost pure s in character. The comparison between the experimental results and calculations indicates a rather good agreement with results from LMTO, consistent with the fact that this is a more elaborate band structure calculation. Agreement is only acceptable with results from the TB - LMTO calculations but these can be improved with a more careful treatment of the TB structure constants.