Les mécanismes de l'oxydation interne dans les alliages à base d'argent

Internal oxidation mechanism in silver alloys was studied by residual resistivity, gravimetric and calorimetric measurements, X-Ray diffraction and by transmission electron microscopy. It has been established in Ag-Mg alloys that the oxygen fixation process involves three steps : firstly, substoichiometric species are initially formed ; secondly these elementary species evolve toward irregular clusters including an important excess of oxygen ; thirdly, a part of this excess oxygen is released and clusters evolve toward a more compact structure. The mechanism of internal oxidation in Ag-Mg alloys can be generalized to the other studied alloys such as Ag-Cu, Ag-Zn and Ag-Sn

INTERFACIAL STRUCTURE MODIFICATIONS INDUCED BY SULPHUR IN Ag-Ni ALLOYS

L'influence de la ségrégation du soufre à l'interface de deux métaux de faible solubilité mutuelle (Ag/Ni) est étudiée par microscopie électronique en transmission (M.E.T.). Les interfaces (100) sont obtenues par évaporation successive d'argent et de nickel sur substrat monocristallin de NaCl clivé. Les observations effectuées en M.E.T. montrent que le nickel est en bonne épitaxie sur l'argent alors que les dislocations de raccordement ne sont pas visibles. L'introduction de soufre provoque une décohésion de cette interface.In this paper we present the first results concerning the effect of an impurity of low solubility on the interfacial structure of simple model system : the Ag/Ni system with sulphur as the impurity common to the two metals. The Ag/Ni interfaces are produced by vapour deposition and epitaxial growth on single crystal substrate of NaCl cleaved. T.E.M. observations of these interfaces show that nickel is in good epitaxy on silver while misfit dislocations network is not visible. After annealing in pure hydrogen, epitaxy is reinforced : this indicates that the (100) interface is preserved. Quite the contrary, after annealing under H2/H2S ambient, the (100) interface disappears

Relations entre dilatation et premiers stades de l'oxydation interne

Variation of the lattice parameter during interna1 oxidation of dilute Ag(Mg) alloys is closely related to the successive steps of the oxidation process at low temperature. Models of elementary reactions are proposed by taking into account a potential barrier due to the strain field near the reacting species. First, substoichiometric species ([O]/[Mg]<1) form in a non expanded zone ; then excess oxygen by comparison with the amount necessary to form stoichiometric MgO oxide and over stoichiometric clusters are responsible of the highly expanded zone observed. The decrease of deformation and of excess oxygen which occurs at last is related to an evolution toward more compact clusters

Laterally graded division parameter layered synthetic microstructures

A laterally graded γ layered synthetic microstructure (LSM), (where the division parameter γ is the ratio between the thickness of the high refractive index material and the LSM's period), has been manufactured by means of a sputtering technique. This special LSM has been characterized by specular reflectivity X-ray measurements using a diffractometer equipped with an incident beam monochromator and Cu-K α (1.5418 A) radiation. It exhibits the properties expected by the dynamical theory of X-ray diffraction. The results are shown to be in good agreement with computer simulations and transmission electron microscopy (TEM) observations. The object of this paper is to prove the feasibility of such a structure whose the interest is the selection on one sample of the division parameter γ desired for different applications. We give a calibration curve of the sample by means of an experimental law of laterally γ variation.Des microstructures synthétiques en couches minces à variation latérale de γ (oú γ est le rapport entre l'épaisseur de matériau de haut indice et le paramètre de la multicouche) ont été fabriquées par pulvérisation. Elles ont été caractérisées par réflectivité spéculaire X avec un diffractomètre fonctionnement sous rayonnement primaire monochromatique Cu Kα. Les résultats sont en bon accord avec ceux prévus par la théorie dynamique de la diffraction X, ainsi qu'avec des simulations sur ordinateur et des observations en MET. Nous montrons ainsi qu'il est possible de réaliser des structures dont l'intérêt est de disposer sur un échantillon de valeurs du paramètre γ correspondant à des applications diverses

Structural and Composition Effects on Electronic and Magnetic Properties in Thermoelectric Mn1-x-yCo1+xGe1+y Materials

Magnetocaloric effects are of great interest for magnetic refrigeration as well as thermomagnetic energy production. Materials based on MnCoGe have been shown to be promising candidates in this respect. In this study, structural, electronic, and magnetic properties of MnCoGe alloys with several compositions (stoichiometric and Mn-depleted) have been investigated using first-principles calculations based on density functional theory. Their stability in the orthorhombic and hexagonal structure has been determined for different spin configurations. In all materials, the ferromagnetic orthorhombic phase is most stable; however, variation of the composition can lead to a decrease of the energy difference between the equilibrium orthorhombic and hexagonal structure. The magnetic moment is primarily found on the Mn atoms leading to a decrease of the total magnetic moment with decreasing amount of Mn. Using the Boltzmann transport model and the electronic density of states, the spin Seebeck coefficient has been estimated. At room temperature, it is found to be highest in stoichiometric MnCoGe in the orthorhombic crystal structure

Growth and Characterization of Shape Memory Cu-Zn-Al Single Crystals

To provide a better insight of shape memory alloys (SMA) behavior, Cu- Zn-Al single crystals were grown by the Bridgman technique. Melting was performed under pure argon in a graphite crucible allowing to obtain samples (total length 40 mm) with the final shape directly suitable for mechanical tests (cylinders with taper heads : 25 mm useful length, 4 mm in diameter). The chemical composition was checked by Differential Scanning Calorimetry and resistivity. It was found that a small zinc concentration gradient exists along the samples : it induces a difference less than 5 K for transformation temperatures in the useful length. The morphology, the shape deformation, crystal structure, sub-structure and orientation have been investigated by optical, transmission and scanning electron microscopy and by X-Ray diffraction. Pseudoelastic isothermal tensile tests and training under constant applied stress and variable temperatures were carried out. The training efficiency for these elaborated alloys is very large (about 80-90%)

On the mechanism ruling the morphology of silicon nanowires obtained by one-pot metal-assisted chemical etching

One-pot Ag-assisted chemical etching (SACE) of silicon provides an effective, simple way to obtain Si nanowires (NWs) of potential interest for technological applications ranging from photovoltaics to thermoelectricity. The detailed mechanism ruling the process has not been yet fully elucidated, however. In this paper we report the results of an extended analysis of the interplay among doping level and type of silicon, nanowire nanomorphology and the parameters controlling the chemistry of the etching process. We provide evidence that the SACE mechanism entirely occurs at the interface between the etching solution and the Si substrate as a result of Si extrusion by sinking self-propelled Ag particles. Also, a rationale is advanced to explain the reported formation of (partially) porous NWs at high doping levels in both p- and n-type Si. A model not relying on the asserted formation of potential barriers enables to recover full consistency between SACE electrochemistry and the mechanism of formation of porous silicon in electrochemical cells