Topological interlocking of protective tiles for the space shuttle

We propose a new concept of design of heat- and impact-resistant tile covering for space shuttles based upon topological interlocking of tiles. The key features of this type of tiling are as follows: firstly, the tiles are kept in place by virtue of the geometry of their contacting surfaces alone, so that no binding agent needs to be used; secondly, failure of an individual element does not compromise the structural integrity of the tile assembly and, moreover, a failed tile will be kept in place by its neighbours, thus maintaining its thermal protection function; thirdly, topological interlocking removes the need for connectors or other stress concentrators deleterious in extreme conditions of space flight and re-entry. Topological interlocking is scale and material independent, which permits combining tiles made from different materials in any proportion.     

Arsenic adsorption onto silicon stepped surfaces: a study at semiempirical level

As adsorption onto Si surfaces vicinal to (100) has been studied using the Hartree–Fock method at a semiempirical level and a cluster model of the surface steps. The simulation conditions apply to the dilute limit of the adsorbate concentration. Accordingly, one As atom is placed on the step in a substitutional or interstitial location, either above or below the surface. The optimal configuration of the system is evaluated from a steepest-descent energy minimization. The central finding is that As is preferably adsorbed in the interstitial location and the physical explanation is that this location allows both a stronger electrostatic coupling with the surface and a lower stress than in the case of the substitutional impurity. This result represents a significant divergence with respect to the known properties of the bulk impurity. It can, however, consistently account for known features of heteroepitaxial growth of As on Si(100).     

Mechanically driven grain boundary relaxation: a mechanism for cyclic hardening in nanocrystalline Ni

Molecular dynamics simulations are used to show that cyclic mechanical loading can relax the non-equilibrium grain boundary (GB) structures of nanocrystalline metals by dissipating energy and reducing the average atomic energy of the system, leading to higher strengths. The GB processes that dominate deformation in these materials allow low-energy boundary configurations to be found through kinematically irreversible structural changes during cycling, which increases the subsequent resistance to plastic deformation.     

Grain boundary phase observed in Al–5 at.% Zn alloy by using HREM

The nature and behaviour of grain boundary (GB) phases is very important since they can control strength, plasticity, resistivity, grain growth, corrosion resistance, etc, especially in nanocrystalline materials. For nanocrystalline Al-based light alloys, extremely high plasticity has been observed in restricted temperature and concentration intervals close to the solidus line. This phenomenon is not fully understood. It can be explained by formation of GB phases not included in the bulk phase diagram. Therefore, the structure of GB phases, as well as thermodynamic conditions for their existence, has to be carefully studied. In this work the structure and composition of GBs and GB triple junctions in Al–5 at.% Zn polycrystals annealed in the temperature region above and below the bulk solidus line were studied by high-resolution electron microscopy and analytical transmission electron microscopy. Evidence has been obtained that a thin layer of a liquid-like phase exists in GBs and GB triple junctions slightly below the bulk solidus line.     

The crystal structure of Al20.6Ta22.4 and its relation to Frank–Kasper phases

The monoclinic phase in the Al–Ta system has been identified by means of X-ray single-crystal structure analysis as Al_{19+ x} Ta_{24? x} (x?=?1.6), Pearson symbol mP86. Al_20.58Ta_22.42 crystallizes in the space group P2₁/n, with a?=?987.86?pm, b?=?990.12?pm, c?=?1489.45?pm and β?=?99.958°. Twinned crystals of the title compound can be obtained by iodine-promoted reactions of the elements. In the experiments described here, the reactions took place in sealed tantalum ampoules at 1400°C. Single-phase samples were obtained by a metallurgical powder method, which minimizes aluminium loss by incongruent vapourization. The composition was found to span the range 0.51?≤?x _Ta?≤?0.53. Al_{19+ x} Ta_{24? x} decomposes in the solid state at 1450°C into Al₆₉Ta₃₉ and σ-AlTa₂. The crystal structure is analyzed in relation to tetrahedral close-packed structures. The kinship with Cr₃Si-type and H-type structures is accentuated.     

Thermal behaviour of poly(methyl methacrylate)/fullerene composite films

ABSTRACT

Poly(methyl methacrylate) films and poly(methyl methacrylate)/fullerene composite films were fabricated by casting from toluene solutions. The mass fraction of fullerene (C₆₀) was varied from 0.05–3?wt. %. The effect of the fullerene (C₆₀) content on the thermal degradation parameters of the composite films was evaluated by thermogravimetric (TG) analysis. It was found that the incorporation of C₆₀ improved the thermal stability of the polymers. The films eventually decomposed in three stages. Incorporation of fullerene caused a change in the distribution of mass loss over the stages of degradation in comparison with the pristine polymer. Using differential scanning calorimetry (DSC), the phase transitions from the glassy state to the elastic one was studied for the examined polymeric materials. New data relating to the effect of C₆₀ on the glass transition temperature of composites with low weight fractions of filler were obtained. Specifically, for films containing up to 0.1 wt. % of C₆₀, a single glass transition temperature was found, whereas for composites with a higher concentration of filler, two glass transition temperatures were recorded.     

Shear horizontal surface waves in piezoelectric materials with surface stress

ABSTRACT

The propagation of shear horizontal (SH) surface waves in piezoelectric semi-infinite bodies is investigated theoretically. The surface stress exerting on the boundary is taken into account through surface piezoelectricity theory, and the velocity ranges and existence conditions of such surface waves for both electrically open and shorted cases are derived in detail. Analysis results show that the SH surface waves occurring with surface stress are dispersive in contrast to the classical situation, and the propagation characteristics strongly depend on the relative value of surface elasticity, surface piezoelectricity and surface density.     

A general solution of elasto-hydrodynamics of two-dimensional quasicrystals

Dynamic problems of quasicrystals are analysed within the framework of continuum mechanics. Phonon excitations yield wave propagation while phason excitations yield atomic diffusion. Phonon fields obey the equations of motion and phason fields obey the diffusion equations. Governing equations of elasto-hydrodynamics of decagonal quasicrystals combine characteristics of the equations of motion and diffusion. A general solution is derived in terms of two introduced auxiliary functions. It provides an easy-to-use approach to solve initial-boundary value problems encountered in elasticity problems for quasicrystals. Explicit expressions for displacements and stresses in the phonon and phason fields can be directly obtained. A derived general solution is also applicable to some planar quasicrystals such as pentagonal quasicrystals with fivefold symmetry.     

Cascade effects on the irradiation stability of amorphous SiOC

We studied the heavy ion radiation tolerance of amorphous silicon oxycarbide (SiOC) alloys by in situ Kr ion irradiation within a transmission electron microscopy. The amorphous SiOC thin films were grown via co-sputtering from SiO₂ and SiC targets on a surface-oxidized Si (100) substrate. These films were irradiated by 1 MeV Kr ions at both room temperature and 300 °C with damage levels up to 5 displacements per atom (dpa). TEM characterization shows no sign of crystallization, void formation or segregation in all irradiated samples. Our findings suggest that SiOC alloys are a class of promising radiation tolerant materials.