Proton microbeam induced damages in glasses: Ca and Na distribution modification

Nowadays, nuclear microprobes are commonly used in Earth Sciences as an analytical tool to detect a large number of trace elements in geological samples. However, radiation damages induced in natural glasses during an analysis by ionization are not completely controlled. Some difficulties remain in defining the appropriate analytical conditions without modifying the alkaline distribution in the matrix. Our approach consists of using a synthetic glass to quantify these modifications. With a 2 MeV proton microbeam, the Na and Ca signal evolution has been followed as a function of the deposited charge. Na and Ca distribution maps in and around the beam impact evidence a complex remobilization of both elements. The use of three different proton energies (1.32 MeV, 2MeV and 3MeV) permits the Na and Ca distribution profiles to be realized for increasing depth layers. A similar behavior of alkaline elements has been found within complex natural glasses.     

Influence of Stoichiometry on the Electrical and Mechanical Behavior of Yttrium Oxide Ceramics

Because of their method of preparation, ceramics contain point defects, the concentration of which may vary from the center to the boundary of the grains, which may change the bulk properties of the material. The electronic structure of the oxygen-related defects has been studied by electron spectroscopy in yttria ceramics submitted to various heating conditions under controlled atmospheres. The differences in the mechanical and electrical behaviors are observed to correlate with the defect content.     

α-Al2O3 (0001) Surfaces: Atomic and Electronic Structure

New results on the     R ± 9° reconstructed α-Al₂O₃ (0001) surface, which can be obtained after heating at high temperature (1400°C) under vacuum, are presented. The atomic structure has been studied by combining low-energy electron diffractometry and grazing incidence X-ray scattering. The surface structure is found to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-packed Al (111) planes separated by a defect of hexagonal periodicity with a parameter of 26.44 Å. This model is consistent with previous surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ray absorption spectroscopy at the O K edge, electron energy loss spectroscopy, and X-ray-induced Auger electron spectroscopy. Interpretation of these experimental data in the frame of a self-consistent, tight-binding calculation leads to the conclusion that the     R ± 9° reconstructed surface is more covalent than the (1 × 1) surface. Significant changes in the; Al-O hybridizations are observed; these are likely due to a difference in the interatomic distances along the [0001] axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstructed surface.     

Influence of Oxygen Vacancies on the Electronic Structure of Yttrium Oxide

The influence of oxygen vacancies on the electronic structure of yttrium oxide was investigated both experimentally and theoretically. By means of X-ray absorption spectroscopy, at the yttrium K and L ₁₁ edges and at the oxygen K edge, information on the local densities of states of various orbital symmetries, around each type of atom, was obained. The total density of states in the valence band was studied using X-ray photoelectron spectroscopy. Experimental results were compared with those obtained on stoichiometric yttria. The evolution of the densities of states is well reproduced by self-consistent, semiempirical tight-binding calculations applied to clusters of increasing size, in which oxygen vacancies are introduced. The charge transfer between oxygen and yttrium is modified, as experimentally observed from the study of the O KLL Auger line shape. The decrease of the local atomic charge on oxygen is theoretically confirmed by the tight-binding calculations, showing a tendency toward a more covalent bond in nonstoichiometric yttria.     

Effect of Nonstoichiometry on Fracture Toughness and Hardness of Yttrium Oxide Ceramics

This work deals with the changes in mechanical properties of yttrium oxide ceramics induced by nonstoichiometry. The maximum fracture toughness, K_lc, is observed at the stoichiometric composition. For an oxygendeficient ceramic, a decrease of K_lc is observed from 3.5 to 2.3 MPa.m^1/2. On the other hand, the Vickers hardness seems to be less dependent on stoichiometry. These results are discussed in the frame of the evolution of the Y-O bond with the stoichiometry. They set in particular the problem of the role of electrostatic energy stored in a brittle material containing charged defects in the energy balance controlling crack propagation.     

Use of the α → β quartz transition to monitor the temperature increase produced by a proton microbeam

Ion microbeams induce in certain cases local heating of a sample under investigation, especially in insulators. To obtain experimental values of this temperature increase, we used a quartz sample which presents its own internal thermometer with the α → β transition observed at 573°C. At this temperature, the transition is easily observed from the rapid variation of the bubble size contained in a melted inclusion trapped in the quartz. We measured the temperature increase produced by a 3 MeV proton microbeam. For this purpose, the nuclear microprobe chamber was equipped with a sample heating stage. The decrease of the power injected by the heater element (beam on) required to observe the transition has been interpreted as a local increase of the temperature under microbeam exposure.     

Nonstoichiometry in Pure and Zr-Doped Yttria Ceramics: An EXAFS and XPS Study

Nonstoichiometry is exhibited by many ceramics at very high temperatures. The high-temperature treatment of yttria, in particular, can induce a large concentration of point defects that changes the electrical and mechanical behavior of the solid. In the present study, the electronic and geometric structures of nonstoichiometric pure and Zr-doped yttria are examined by X-ray photoelectron spectroscopy and X-ray absorption. The results show that the doping atom influences the level of stoichiometry, as well as the induced disorder and the change in the partial ionic character of the Y-O bond.