We study more than 10^{4} random aggregates of 10^{6} monodisperse sticky hard spheres each, generated by various static algorithms. Their packing fraction varies from 0.370 up to 0.593. These aggregates are shown to be based on two types of disordered structures: random regular polytetrahedra and random aggregates, the former giving rise to δ peaks on pair distribution functions. Distortion of structural (Delaunay) tetrahedra is studied by two parameters, which show some similarities and some differences in terms of overall tendencies. Isotropy of aggregates is characterized by the nematic order parameter. The overall structure is then studied by distinguishing spheres in function of their contact coordination number (CCN). Distributions of average CCN around spheres of a given CCN value show trends that depend on packing fraction and building algorithms. The radial dependence of the average CCN turns out to be dependent upon the CCN of the central sphere and shows discontinuities that resemble those of the pair distribution function. Moreover, it is shown that structural details appear when the CCN is used as pseudochemical parameter, such as various angular distribution of bond angles, partial pair distribution functions, Ashcroft-Langreth and Bhatia-Thornton partial structure factors. These allow distinguishing aggregates with the same values of packing fraction or average tetrahedral distortion or even similar global pair distribution function, indicative of the great interest of paying attention to contact coordination numbers to study more precisely the structure of random aggregates.

Sequentially-built random sphere-packings have been numerically studied in the packing fraction interval 0329 < gamma < 0.586. For that purpose fast running geometrical algorithms have been designed in order to build about 400 aggregates, containing 10(6) spheres each one, which allowed a careful study of the local fluctuations and an improved accuracy in the calculations of the pair distribution P(r) and structure factors S(Q) of the aggregates.Among various parameters (Voronoi tessellation, contact coordination number distribution,...), fluctuations were quantitatively evaluated by the direct evaluation of the fluctuations of the local sphere number density, which appears to follow a power law. The FWHM of the Voronoi cells volume shows a regular variation over the whole packing fraction range.Dirac peaks appear on the pair correlation function as the packing fraction of the aggregates decreases, indicating the growth of larger and larger regular polytetrahedra, which manifest in two ways on the structure factor, at low and large Q values. These low PF aggregates have a composite structure made of polytetrahedra embedded in a more disordered matrix. Incidentally, the irregularity index of the building tetrahedron appears as a better parameter than the packing fraction to describe various features of the aggregates structure. (C) 2014 Published by Elsevier B.V.

Since the first application of chalcogenide glasses in xerography, they have found numerous applications in nanotechnology, due to the different electrical conductivity of their crystalline and amorphous phases, thus giving rise to the development of optoelectronic devices. The process takes place by a local amorphisation and re-crystallisation of a chalcogenide glassy film. The crystallised area is heated up to a temperature beyond its melting point through a laser, then his hottest spot is cooled down at a rate of 1010 - 1011 K/s, which is higher than the critical rate for the material to become amorphous. The optical reflectivity is then measured on the localised amorphous area and it is possible to extract the information registered through the whole material. Matsushita began with the manufacture of DVD and Intel and Samsung were the first to introduce RAM memories capable to reach much higher speeds needing of lower energy and delivering a higher density of information.La primera aplicación de los vidrios de calcogenuro tuvo lugar en el campo de la xerografía, pero ya en los años 60 encontraron utilidad en el campo de las nano-tecnologías aprovechando la diferencia de conductividad eléctrica entre una fase vítrea y otra cristalina de los mismos, lo que dio lugar a su primera incorporación en circuitos opto-electrónicos. El proceso tiene lugar por la amorfización y re-cristalización local de una capa de vidrio de calcogenuro. Se calienta localmente la capa cristalizada hasta una temperatura superior a su punto de fusión por medio de un impulso láser. Después del impulso láser la zona caliente se enfría con una velocidad del orden de 1010 - 1011 K/s, muy superior a la velocidad crítica de amorfización del calcogenuro, dando lugar a la formación en la matriz cristalizada de una marca amorfa con una conductividad eléctrica diferente. Mediante la medida de la reflectividad óptica es posible leer la información registrada en el disco. Fue primeramente la empresa MATSUSHITA la que comenzó la fabricación de DVD y posteriormente INTEL y SAMSUNG las que introdujeron nuevas memorias RAM capaces de alcanzar velocidades de conmutación más grandes con consumos eléctricos más bajos y densidades de información altísimas.

Since the first application of chalcogenide glasses in xerography, they have found numerous applications in nanotechnology, due to the different electrical conductivity of their crystalline and amorphous phases, thus giving rise to the development of optoelectronic devices. The process takes place by a local amorphisation and re-crystallisation of a chalcogenide glassy film. The crystallised area is heated up to a temperature beyond its melting point through a laser, then his hottest spot is cooled down at a rate of 10(10) - 10(11) K/s, which is higher than the critical rate for the material to become amorphous. The Optical reflectivity is then measured oil the localised amorphous area and it is possible to extract the information registered through the whole material. Matsushita began With the manufacture of DVD and Intel and Samsung were the first to introduce RAM memories capable to reach much higher speeds needing of lower energy and delivering a higher density of information.

Droplets of quasicrystal forming alloys with average atomic composition Al(63)Cu(25)Fe(12) have been produced by a drop tube solidification process which allows large liquid undercoolings and leads to special microstructures. Approximate cooling laws for this drop tube process are first derived as a function of droplet diameter and cooling gas properties. Depending on these cooling conditions it is shown that the droplets contain either two (i + beta) or three (i + beta + lambda) major phases, with a predominant icosahedral i-phase whose (metastable) composition Al(76)Cu(15)Fe(18) differs noticeably from the reported equilibrium value. An annealing treatment at 1073K transforms these multiphased droplets into an Al(63)Cu(25)Fe(12) Single phase with a crystalline approximant i'-structure of higher density. It is concluded that the composition of (quasi)crystalline Al-Cu-Fe phases strongly depends on the solidification path and solid cooling rate involved in the elaboration process and that the phase diagram to be considered is often a metastable one.

Abstract A model for the liquid-glass transition, based on a percolation blocking of local chemical order, is proposed. The case of metallic liquids and glasses, whose structure is dominated by first neighbour chemical arrangement, is first treated. The chemical ordering "reaction" of the liquid phase is studied at thermodynamical equilibrium and the increase of the chemical order parameter with decreasing temperature is calculated. Within a given composition interval, however, a geometrical percolation process is shown to block this reaction below a "percolation temperature" (corresponding to null cooling rate) where the liquid is irreversibly frozen into a glass. The liquid-glass "phase diagram" is established and kinetic arguments, involving "frustrated" finite clusters which are formed close to the percolation threshold, provide an evaluation of the experimentally measured "glass transition temperature" as a function of cooling rate. The validity of this one order parameter model is then discussed with the help of the irreversible thermodynamics theory of Prigogine. The formation of tetracoordinated glasses is explained by the formation of tetrahedral bonds, when the liquid temperature decreases, and represented by a "hole ordering" reaction. A general description of the structure of tetracoordinated glasses is thus achieved, which applies to amorphous silicon and germanium, 111 -V compounds, silica, amorphous water etc. Furthermore, an estimation of the temperature interval for the glass transformation of silica is obtained, which agrees well with experiment. The existence of frustrated clusters gives to glasses a composite structure in the "medium distance order", which could explain the "fractal nature" of glass fracture surfaces down to the nanometer scale.

First-neighbour geometrical models are presented and shown to provide a good approximation for the structural description of binary disordered systems with metallic, covalent or ionic bonding. Sphere mixture models taking into account atomic size and chemical order effects are first studied and analytical expressions for the partial coordination numbers are derived which only involve packing fraction, diameter ratio, concentration and chemical order parameter. Relations between the concentration thresholds separating different chemical order regimes and site percolation theory are established. Close-packed sphere mixtures are finally shown to be a good representation of liquid or glassy alloys. Hole sphere models are then introduced and shown to provide a satisfactory approximation to the structure of loose-packed covalent glasses of silicon and germanium. Finally, first-neighbour distance models are presented and the variations of their partial structure factors with non-additivity parameter are studied. These models provide a satisfactory approximation to the structure of ionic molten salts.

Multiple scattering by plane slab samples is calculated for any scattering geometry with the aid of Vineyard's theory.

Variations of the liquid alloy resistivity with local chemical order are studied within the framework of Faber-Ziman nearly free electron theory. Electron-ion interactions are presented by Ashcroft's empty-core pseudopotentials and structural changes arising from chemical ordering are evaluated with the help of geometrical models. These calculations show that chemical ordering cannot produce resistivity variations larger than about 20% whatever the valency of the alloy components may be. It is therefore concluded that discrepancies between measured and calculated resistivities larger than 20% should be attributed to electronic rearrangements producing changes in the pseudopotentials, or to departures from the nearly free electron approximation.

espanolLos procedimientos de pirolisis para el deposito de capas delgadas experimentan una expansion constante en la industria del vidrio plano. Una descripcion rapida de dichos procedimientos nos permite precisar sus ventajas y las cualidades de los productos obtenidos. Se analizan las funciones nuevas que estas capas pirolizadas pueden conferior a los acristalamientos. Esta exposicion es ilustrada finalmente con el ejemplo de capas poco emisivas destinadas a la construccion o al automovil. EnglishPyrolisis procedures for depositing thin layers are experiencing a constant growt in the plate glass industry. A quick description of those procedures enables to state their advantages and quality of the products obtained. The new functions that these pyrolized layers may confer to glasses are analyzed. This review is completed with the example of low emissive layers intented for the building and car industry. francaisLes procedes de pyrolyse pour le depot de couches minces sont en constant developpement dans l'industrie du verre plat. Une description rapide de ces methodes permet de preciser leurs avantages ainsi que les qualites des produits obtenus. L'auteur analyse les nouvelles fonctions que peuvent remplir les vitrages grace aux couches pyrolysees. L'expose est enfin illustre a l'aide de l'exemple de couches peu emissives destinees au bâtiment ou a l'automobile. DeutschDie Pyrolyseverfahren zur Aufbringung dunner Schichten kommen in der Flachglasindustrie in zunehmendem Mase zur Anwendung. Eine kurzgefaste Beschreibung dieser Verfahren gestattet, ihre Vorteile und die Gute der damit erhaltenen Erzeugnisse zu prazisieren. Es werden die neuen Funktionen untersucht, die mit pyrolysierten Beschichtungen versehene Verglasungen ubernehmen konnen. Als Beispiel werden dann die Beschichtungen mit geringer Emissivitat angefuhrt, die im Baugewerbe und in der Automobilindustrie Verwendung finden.

Two similar high temperature furnaces (up to 2 000 °C) have been developed for large angle and small angle neutron scattering (SANS) experiments respectively. They are vacuum furnaces with a thin tungsten foil heating element supplied by two tantalum leads, and shielded by thin tungsten foils maintained in a tantalum box. In a neutron beam, the furnaces produce a very low scattering level (without any Bragg peaks) due to the openings in the heating element, the shields and the tantalum box for the incident, unscattered beam and the scattered beam at low angles. Thus the furnace absorption correction is also much easier. A platinum thermocouple controls temperature, very high temperatures are measured optically. A vacuum chamber has been especially designed for SANS experiments with two windows in polished sapphire. The furnaces have been successfully used for the study of liquid alloys on the D4, D11 and D17 spectrometers at the ILL.

Resume •-Les trois facteurs de structure partiels (FSP) d'un alliage Co •, _ B _ _ amorphes ont ete mesures simultanement par diffraction de neutrons polarises.' Un prepic de surstructure apparait vers Q =2 A sur le facteur de structure nucleaire.Le FSP concentration-concentration montre que les atomes de bore sont uniquement entoures d'atomes de cobalt.On deduit les distances entre premiers voisins suivantes : R(Co-Co) = 2,54 A, R(Co-B) = 2,3 A, R(B-B) ^ 3,6 A. Enfin, les FSP calcules pour le modele de GASKELL sont en accord satisfaisant avec les mesures tandis qu'apparaxt un sensible disaccord entre 1'experience et le FSP S calcule pour un empilement de spheres adhesives.

Nous montrons ici les possibilites et les limites des modeles de spheres adhesives pour decrire la structure des verres metalliques, covalents ou ioniques (I).

Resume •- Les trois facteurs de structure partiels (FSP) d'un alliage Co •, _ B _ _ amorphes ont ete mesures simultanement par diffraction de neutrons polarises. ' Un prepic de surstructure apparait vers Q =2 A sur le facteur de structure nucle- aire. Le FSP concentration-concentration montre que les atomes de bore sont unique- ment entoures d'atomes de cobalt. On deduit les distances entre premiers voisins suivantes : R(Co-Co) = 2,54 A, R(Co-B) = 2,3 A, R(B-B) ^ 3,6 A. Enfin, les FSP cal- cules pour le modele de GASKELL sont en accord satisfaisant avec les mesures tandis qu'apparaxt un sensible disaccord entre 1'experience et le FSP S calcule pour un empilement de spheres adhesives. Abstract - The three partial structure factors (PSF) of a Co i:LB " glass were simultaneously measured by polarized neutron diffraction. A " ' prepeak appears around Q = 2 A on the nuclear structure factor. The concentration-concen­ tration PSF shows that the boron atoms are only surrounded by cobalt atoms. The fol­ lowing pair distances are deduced from PSF : R(Co-Co) = 2.54 A, R(Co-B) = 2.3 A, R(B-B) ^3.6 A. Finally the PSF calculated for the model of GASKELL are in satisfac­ tory agreement with the measurements while a slight discrepancy is observed between experiment and the S FSP calculated for a packing of sticky spheres.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. STUDY OF LIQUID NICKEL-VANADIUM ALLOYS BY NEUTRON DIFFRACTION AND MODEL SIMULATION J. Lemarchand, J. Bletry, P. Desre

Abstract The structure of monoatomic liquids in the static approximation is described by a soft sphere model and the corresponding equation of state is derived. This geometrical model is then extended to binary liquid or amorphous substitution alloys where both components have the same size. The structure of these alloys is shown to be determined by the first neighbour order parameter which is positive, negative or zero for segregated, ordered or disordered alloys respectively. The variations of the partial structure factors with concentration are presented for these three cases. Size effects are then studied by varying the ratio of atomic diameters. Many experimental results are shown to fit this simple framework.