Plastic deformation in metallic glasses

Lithium can be inserted reversibly within most carbonaceous materials. The physical mechanism for this insertion depends on the carbon type. Lithium intercalates in layered carbons such as graphite, and it adsorbs on the surfaces of single carbon layers in nongraphitizable hard carbons. Lithium also appears to reversibly bind near hydrogen atoms in carbonaceous materials containing substantial hydrogen, which are made by heating organic precursors to temperatures near 700°C. Each of these three classes of materials appears suitable for use in advanced lithium batteries.

Mechanisms for Lithium Insertion in Carbonaceous Materials

In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

The pronounced glass-forming tendencies of alloys of S and Se with Ge and/or As are discussed topologically. An atomic model is introduced which for predominantly covalent forces constitutes the first microscopic realization of Kauzmann's description of the glass transition as an entropy (not enthalpy or volume) crisis. The model contains no adjustable parameters and predicts the glass-forming tendency as a function of composition in excellent agreement with experiment. Several related properties, including phase diagrams, radial distribution functions and crystal structures are discussed in the context of chemical bonding and short-range order in the non-crystalline covalent networks of these materials.

Topology of covalent non-crystalline solids I: Short-range order in chalcogenide alloys

The straightforward and scalable synthesis and patterning of graphene-based nanomaterials remains a technological challenge. Here, the authors use a CO2 infrared laser, under ambient conditions, to directly produce and pattern porous graphene films with three-dimensional networks from commercial polymer films.

/pdf/laser-induced-porous-graphene-films-from-commercial-polymers-p07oxxf95p.pdf

Laser-induced porous graphene films from commercial polymers

A new study of the structure of vitreous silica has been made under greatly improved conditions. Using Rh Kα radiation with the method of fluorescence excitation, reliable intensity values were measured to 4π sinθ/λ = 20. The interpretation was in terms of pair functions, thereby eliminating the approximations in earlier work. Each silicon is tetrahedrally surrounded by 4 oxygen atoms, with a Si–O distance which is closely 1.62 A. Each oxygen atom is bonded to 2 silicon atoms. The Si–O–Si bond angle α shows a distribution V(α) extending all the way from 120° to 180°, with a maximum at α = 144°. This wide variation in α is an important distinction between the vitreous and the crystalline forms of silica, and it furnishes an important criterion for any proposed model of vitreous silica. Good agreement with the measured pair function distribution curve was obtained by assuming a random orientation about the Si–O bond directions. The interpretation leads to the familiar random network model, but with the new results the model is more precise.

The structure of vitreous silica

Random layer lattice structures are considered which consist of layers arranged parallel and equidistant, but random in translation parallel to the layer, and rotation about the normal. We call $a$ and $b$ the axes in the layer, and $c$ the axis normal to the layer. In this notation there will be crystalline reflections of type ($00l$), two-dimensional lattice reflections of type ($\mathrm{hk}$), and no general reflections ($\mathrm{hkl}$). Equations are developed for the intensity distribution in a two-dimensional powder reflection, and for the integrated intensity. Equations are also de-developed for the particle size in terms of the peak breadth, and for the displacement of the peak. The powder pattern of a heat treated carbon black is presented as an illustration of two-dimensional lattice reflections.

X-Ray Diffraction in Random Layer Lattices

The X-ray scattering curves for vitreous SiO2 and B2O2 are obtained in a vacuum camera, using monochromatic Mo and Cu radiation. A Fourier analysis of the scattering curve gives directly a curve representing the distribution of neighboring atoms about any one atom. For SiO2, the distribution curve establishes definitely the tetrahedral silicon-oxygen network. The distribution curve for B2O3 indicates a triangular coordination, each boron surrounded by three oxygens, and each oxygen shared between two borons. The interatomic distances which are found are in good agreement with the values found in crystalline silicates and borates. The importance of the new Fourier method of analysis of glass diffraction patterns is emphasized by the fact that the distribution curves which are obtained are unique, no assumptions as to structure being involved.

FOURIER ANALYSIS OF X‐RAY PATTERNS OF VITREOUS SiO2 AND B2O3*

X-ray diffraction patterns were obtained for five compositions of soda-silica glass The patterns were made in an evacuated camera, using MoKα radiation monochromated by reflection from rock salt The patterns show a perfectly continuous change in appearance with increasing soda content From a Fourier analysis of the X-ray scattering curves, radial distribution curves for the five glasses were obtained The distribution curves uniquely establish several important points Each silicon atom is surrounded by 4 oxygens at a distance of about 162A and each sodium by about 6 oxygens at 235A The existence in the glass of simple discrete molecules, such as SiO2, NaaO, Na2Si2O6, and Na2SiO3, is uniquely ruled out by the distribution curves

The X-ray results lead to the following picture of soda-silica glass Each silicon is tetrahedrally surrounded by 4 oxygens at a distance of about 162A Part of the oxygens are bonded to 2 silicons and part only to 1 silicon The sodium ions Na+ are arranged in various holes in the silicon-oxygen network, being surrounded on the average by about 6 oxygens at a distance 235A Although this is a definite scheme of structure, it does not repeat identically at regular intervals, and the material, therefore, is noncrystalline

Fourier analysis of x‐ray patterns of soda‐silica glass

X-ray diffraction patterns have been made of three samples of soda-boric oxide glass of the following molal compositions: 0.114 Na2O, 0.225 Na2O, and 0.333 Na2O. The patterns were made in an evacuated camera, using MoK α radiation monochromatic by reflection from rock salt. Radial distribution curves for the three compositions were obtained by Fourier analysis. The first peak of the distribution curves is due to the boron-oxygen separation. The distances are 1.37, 1.42, and 1.48A. From the peak areas, the number of oxygens about a boron is found to be 3.2, 3.7, and 3.9. The continuous change, both in interatomic distance and number of surrounding oxygens, indicates that the borons are partly in triangular and partly in tetrahedral coordination, the fraction in the latter increasing with increase of soda. Six oxygens about each sodium at 2.4A and from five to six oxygens about each oxygen at 2.4A are consistent with the distribution curves. The results can be interpreted in terms of a random network picture of vitreous soda-boric oxide, in which the borons are bonded either to three or four oxygens and the sodium atoms are randomly distributed in holes in the boron-oxygen network. Maxima and minima in the physical properties of glasses containing boric oxide and soda are explained in terms of the ability of the boron atom to change to tetrahedral coordination when some oxide such as Na2O is present to supply the necessary extra oxygen.

B. E. Warren

Papers

The structure of vitreous silica

X-Ray Diffraction in Random Layer Lattices

FOURIER ANALYSIS OF X‐RAY PATTERNS OF VITREOUS SiO2 AND B2O3*

Fourier analysis of x‐ray patterns of soda‐silica glass

X-ray diffraction study of soda-boric oxide glass *