Alan J. Hurd

Bio: Alan J. Hurd is an academic researcher from Los Alamos National Laboratory. The author has contributed to research in topics: Light scattering & Scattering. The author has an hindex of 36, co-authored 98 publications receiving 5532 citations. Previous affiliations of Alan J. Hurd include Brandeis University & Sandia National Laboratories.

Review of sol-gel thin film formation

Abstract: Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.

Scattering from fractals

Abstract: The concepts of dilation symmetry and the fractal dimension are introduced, and from these basic concepts scattering functions are computed for surface and mass fractals. It is then shown how the fractal structure of various random media has been elucidated from scattering measurements, and how these observations relate to specific models of fractal geometry. Examples include colloidal aggregates, gels, soot and the fractal porosity of rocks of various sorts.

Silica aerogel films prepared at ambient pressure by using surface derivatization to induce reversible drying shrinkage

Abstract: HIGHLY porous inorganic films have potential applications as dielectric materials, reflective and anti-reflective coatings, flat-panel displays, sensors, catalyst supports and super-insulating architectural glazings1–3. Aerogels4 are the most highly porous solids known, and can now be prepared from inorganic5 and organic6,7 precursors with volume-fraction porosities of up to 99.9% (ref. 8). Aerogels are normally prepared by supercritical extraction of the pore fluid from a wet gel1, which prevents the network collapse that is otherwise induced by capillary forces. But supercritical processing is expensive, hazardous and incompatible with the processing requirements of many potential applications,thus severely restricting the commercial exploitation of aerogels. Here we describe a means of preparing aerogels by a simple dip-coating method at ambient pressure without the need for supercriti-cal extraction. We add surface groups to the inorganic gel which make drying shrinkage reversible9: as the solvent is withdrawn, the gel springs back to a porous state. We can generate aerogel films with 98.5% porosity using this approach. We anticipate that it will greatly expand the commercial applications of these materials.

Single colloidal crystals

Abstract: Colloidal suspensions of highly charged, monodisperse polymer spheres exhibit long-range (crystalline) translational ordering in appropriate conditions of charge, number density, counterion concentration and temperature1–5. These ‘colloidal crystals’, which can conveniently be made to have lattice parameters comparable to or greater than optical wavelengths, offer unique opportunities for the study of the collective static and dynamic behaviour of strongly interacting spherical particles. For example, such systems may readily be probed by relatively simple but powerful light scattering spectroscopic techniques6,7. Furthermore, the ordering itself offers intrinsic advantages, both experimental (for example, fluctuations normally observable only about the k-space origin appear about Bragg spots, with reduced stray light and multiple scattering effects), and theoretical (the simplicity of calculating on the basis of a known rather than only statistically defined structure is well appreciated from experience with atomic solids and liquids). To exploit these ordered structures fully, reliable methods of producing orientated single crystals suitable for light scattering and other optical studies must be developed. For light scattering from bulk samples the primary requirement is that the ratio of interparticle spacing to diameter, a/d, be sufficiently large. This reduces interference by multiple scattering and renders the particle scattering factor nearly constant through those scattering angles [|k|<(2→5)2π/a] where the most useful information concerning crystal structure and dynamics appears. We now describe a technique whereby single body-centred cubic (b.c.c.) colloidal crystals, well suited for light scattering and having particular orientations, may be produced.

Ordered porous materials for emerging applications

Abstract: "Space—the final frontier." This preamble to a well-known television series captures the challenge encountered not only in space travel adventures, but also in the field of porous materials, which aims to control the size, shape and uniformity of the porous space and the atoms and molecules that define it. The past decade has seen significant advances in the ability to fabricate new porous solids with ordered structures from a wide range of different materials. This has resulted in materials with unusual properties and broadened their application range beyond the traditional use as catalysts and adsorbents. In fact, porous materials now seem set to contribute to developments in areas ranging from microelectronics to medical diagnosis.

Bond-orientational order in liquids and glasses

Abstract: Bond-orientational order in molecular-dynamics simulations of supercooled liquids and in models of metallic glasses is studied. Quadratic and third-order invariants formed from bond spherical harmonics allow quantitative measures of cluster symmetries in these systems. A state with short-range translational order, but extended correlations in the orientations of particle clusters, starts to develop about 10% below the equilibrium melting temperature in a supercooled Lennard-Jones liquid. The order is predominantly icosahedral, although there is also a cubic component which we attribute to the periodic boundary conditions. Results are obtained for liquids cooled in an icosahedral pair potential as well. Only a modest amount of orientational order appears in a relaxed Finney dense-random-packing model. In contrast, we find essentially perfect icosahedral bond correlations in alternative "amorphon" cluster models of glass structure.

Mechanisms of catalyst deactivation

Abstract: The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts. © 2001 Elsevier Science B.V. All rights reserved.

Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles

Abstract: Nanostructured carbon materials are potentially of great technological interest for the development of electronic1,2, catalytic3,4 and hydrogen-storage systems5,6. Here we describe a general strategy for the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters (typically 6 nanometres inside and 9 nanometres outside). These structures are formed by using ordered mesoporous silicas as templates, the removal of which leaves a partially ordered graphitic framework. The resulting material supports a high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials (such as carbon black, charcoal and activated carbon fibres). The platinum cluster diameter can be controlled to below 3 nanometres, and the high dispersion of these metal clusters gives rise to promising electrocatalytic activity for oxygen reduction, which could prove to be practically relevant for fuel-cell technologies. These nanomaterials can also be prepared in the form of free-standing films by using ordered silica films as the templates.

Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles

Abstract: We present an approach to fabricate solid capsules with precise control of size, permeability, mechanical strength, and compatibility. The capsules are fabricated by the self-assembly of colloidal particles onto the interface of emulsion droplets. After the particles are locked together to form elastic shells, the emulsion droplets are transferred to a fresh continuous-phase fluid that is the same as that inside the droplets. The resultant structures, which we call "colloidosomes," are hollow, elastic shells whose permeability and elasticity can be precisely controlled. The generality and robustness of these structures and their potential for cellular immunoisolation are demonstrated by the use of a variety of solvents, particles, and contents.