Heat Transport in Aluminum-Based Quasicrystals i-AlPdMn,i-AlCuFe,and d-AlCoNi

We present a semi-quantitative model used to analyze thermal conductivity data of single-grain aluminum-based icosahedral i-AlPdMn and i-AlFeCu, and decagonal d-AlCoNi quasicrystals in the quasiperiodic plane. The analysis is based on the validity of the Wiedemann–Franz law and applicability of the Debye model of quasilattice thermal conductivity at low temperatures, where the main phonon scattering centers are assumed to be structural defects in the form of the stacking faults and phenomenological quasiumklapp. At high temperature, electron and quasilattice contributions underestimate the data. This has motivated us to consider two possible additional heat-carrying channels: the activation of localized lattice vibrations and modifications of the Wiedemann–Franz law.     

Thin Film Growth on Quasicrystalline Surfaces

The surfaces of quasicrystals have proved to be a very interesting playground for thin film growth. They offer a complex potential energy surface where heterogeneous nucleation of islands at specific quasilattice sites is frequently observed. These islands tend to locally adopt the symmetry of the quasicrystalline substrate. For some specific adsorbates, a complete 2-dimensional quasiperiodic metal overlayer is even formed. Other interesting phenomena are also observed in the multilayer regime. This includes the formation of novel structures, like 1-dimensional quasiperiodic Cu films or Bi allotropes, the formation of nanoscale crystalline domains with 5-fold rotational epitaxy, or the occurrence of quantum size effects influencing the film morphology. This article presents a short review of some of the achievements in thin film growth on quasicrystalline surfaces that the discovery of quasicrystals by Dan Shechtman has enabled.     

Dodecagonal Quasicrystals Still in Progress

The dodecagonal quasiperiodic order is rather rare in the material world, but has been observed in various systems from alloys to aggregates of cylindrical polymers and in a wide range of dimensions. They exhibit uniaxial 12-fold diffraction symmetry, and have common structural properties originating from the arrangement of structural units on a square–triangle tiling. All dodecagonal quasicrystals known so far were classified into the so-called random-tiling type and not into a mathematically ideal quasiperiodic type. The observed dodecagonal quasicrystals deviated from the ideal state with respect to the limited size of their quasicrystalline region and their structural inhomogeniety. This may be caused by the structural metastability that prevents synthesis of high quality quasicrystals. The effort toward an ideal dodecagonal quasicrystal, in particular in alloys, is reviewed here. There may be two key points: one is taking advantage of the entropy for stabilization, namely the tiling configurational entropy, and the other is taking advantage of so-called second generation tiles scaled up by a factor of 2+√3 as observed in Ta_1.6Te quasicrystal and Mn-Si-(Cr, V) approximant systems.     

A Cluster–Resonance Criterion for Al-TM Quasicrystal Compositions

Compositions of binary Al-TM (TM=Cr to Ni) quasicrystals are interpreted with a unified cluster formula [icosahedron](glue)₁ using the newly developed cluster–resonance model and the e/a formalism. The icosahedra are chosen from the corresponding approximants by considering large radial atomic density, high degree of isolation, and narrow distribution of the shell atoms. Icosahedral quasicrystals are expressed by an icosahedron plus one averaged icosahedron atom as the glue atom [icosahedron](icosahedron/13)₁, characterized by e/a ∼1.83–1.85, while decagonal quasicrystals are expressed by an icosahedron plus one TM atom, with e/a ∼1.71–1.78. The total electrons accommodated in unit cluster formulas of different Al-TM quasicrystals have the same value approaching 24, which implies that the cluster formulas are both chemical and electronic structural units.     

Gas–Surface Interactions on Quasicrystals

To commemorate the awarding of the Nobel Prize for Chemistry to Daniel Shechtman for his discovery of quasicrystals, this paper reviews our recent studies of the interaction of rare gases and hydrocarbon gases with the tenfold surface of quasicrystalline decagonal Al-Co-Ni.     

Some Aspects of Stability and Nanophase Formation in Quasicrystals during Mechanical Milling

Non-equilibrium processing and other related techniques can activate quasicrystalline structures to a higher energy level and enable them to attain various stable/metastable structures. During mechanical milling of quasicrystalline alloys, the stability of the complex structures is of immense importance from a scientific and technological point of view. The evolution of nanoquasicrystalline, nanocrystalline, and amorphous phases and their composites has been observed in the course of milling. The experimental results for mechanical alloying of elemental powders (required for the synthesis of quasicrystals) and mechanical milling of quasicrystals are discussed. The salient features observed in quasicrystals during other non-equilibrium processing are highlighted. Nanospinel formation from the Al-based quasicrystalline precursor is mentioned.     

Metadislocations in Complex Metallic Alloys and their Relation to Dislocations in Icosahedral Quasicrystals

Complex metallic alloys and quasicrystals are closely related. Both possess icosahedral local order, but while in quasicrystals the long-range order and hence the macroscopic structure is also icosahedral, in complex metallic alloys a periodic structure with large lattice parameters is formed. Metadislocations are novel and highly complex structural defects in complex metallic alloys. Focusing on metadislocations in ε-type phases, we will review the basic geometric concepts of metadislocations and the relation between the number of phason planes, Burgers vector, and the elastic energy. The properties of dislocations in icosahedral Al-Pd-Mn will briefly be recalled and discussed in relation to metadislocations.     

Self‐Propagating High‐Temperature Synthesis of Quasicrystals

With an example of Al–Ni–Co systems of two different compositions, the possibility of obtaining stable decagonal quasicrystals by the method selfpropagating hightemperature synthesis is demonstrated. The burning rate and temperature are determined. Results of an electronmicroscopic and xray diffraction study of the quasicrystals are reported.     

Characterization of C–S–H and C–A–S–H phases by electron microscopy imaging,diffraction, and energy dispersive X‐ray spectroscopy

Improving concrete sustainability by increasing durability requires a detailed knowledge about microstructural properties. Due to the nanoscale nature of hydrate phases that determine concrete properties, microstructural characterization remains a challenge. Analytical electron microscopy offers promising techniques to characterize cement hydrates. In this study, electron microscopy imaging, diffraction, and energy dispersive X‐ray spectroscopic information are combined in order to compare the structural properties of calcium silicate hydrate (C–S–H) and calcium aluminum silicate hydrate (C–A–S–H) phases. Results are shown for 28 days hydrated C–(A)–S–H of portland cement and cement containing ground granulated blast‐furnace slag (GGFBS). Electron diffraction patterns of single fibrous C–S–H and foil‐like C–A–S–H phases reveal a nanocrystalline structure. Also, it is shown by electron diffraction pattern that the crystal structures of C–S–H and C–A–S–H phases are similar. It is confirmed that the crystal structure of 14 Å tobermorite serves as good base for the structure of C–S–H. The electron diffraction patterns of fibrous C–S–H show streaks which indicate stacking faults, proofing that polymerization of silicate chains in C–S–H is limited. Here, we demonstrate for the first time that the dreierketten silicate chains contained in the C–S–H structure are oriented in parallel to the long axis of C–S–H fibers. This finding should be implemented in modeling of crystal growth of C–S–H.     

A structural model of an accessibility of polycarbonate melt to thermooxidative degradation processes

It is shown that the “accessibility” of a polymer for oxidation is the only structural factor and quantitative estimate within the framework of the fractional derivation. The oxidation rate of the “accessible” part of a macromolecular coil is independent on its structure, which allows to assume its dependence only on the chemical constitution of polymer. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 3765–3768, 2006     

Thermal shock resistance of additively manufactured gas-permeable SiO2 ceramic structures for HIP-quench applications

The increasing demand for hot isostatic pressing (HIP) means that a reliable and efficient operation of modern HIP units with fast cool capability is indispensable. A key factor for efficient operation is the ceramic crucible used as the load basket. Its task is to keep as much of the HIPed parts as possible effectively within the hot zone and to prevent them touching the furnace wall. This work focuses on designing a gas-permeable ceramic structure with a high thermal shock resistance that can be scaled up to a load basket for future HIP applications. Stereolithography (SL) 3D printing of a ceramic resin is employed to build various scalable framework structures inspired by nature and by existing engineering applications. Thermal shock tests with water quenching reveal that framework structures with evenly distributed triangular bracings offer the highest flexural strength, whereas auxetic structures are best at retaining their flexural strength after thermal shock.     

The microstructure of stabilized fibers

The morphology and microstructure of a stabilized fiber, nonburning fiber, has been investigated under optical and electron microscopes, and a model for the structure of the stabilized fiber is presented. Skin-core morphology was found in the cross section of the stabilized fiber. The proposed model is consistent with the lamellar-plate-like structure, measuring about 400–700 Å in thickness, 500–4000 Å in breadth and 0.1–0.5 μ m in length along the fiber axis, which is composed of microfibrils. The structure has some circumferential distribution in the outer zone, while the inner zone has a radical distribution. Evidence for the lamellar-plate structure, including two chemical structures and having two different crystalline phases, which are the original AN units and the ladder polymer, were obtained from electron microscopy, wide-angle X-ray, and electron diffraction analyses.     

Mechanism of in vitro reaction of a new scaffold ceramic similar to porous bone

A new way of obtaining bioactive and biodegradable 3D scaffold ceramics is presented with possible bone tissue engineering requests. This implies achieving eutectoid structures from certain systems by considering the different conduct of phases. In this study, the silicocarnotite-tricalcium phosphate subsystem was selected because silicocarnotite is bioactive and tricalcium phosphate is biodegradable. A biphasic porous calcium silicophosphate scaffold with high porosity and an interconnected macro- and micropores structure is presented. The scaffold’s morphology shows a eutectoid lamellae-type microstructure formed of alternating silicocarnotite and α-tricalcium phosphate layers. The eutectoid scaffold material, when placed in simulated body fluid, responds firstly by dissolving the silicocarnotite phase and then developing a hydroxyapatite microporous structure by pseudomorphic transformation of α-tricalcium phosphate lamellae. The achieved microstructure is like that of porous bone. Afterwards, Si-hydroxyapatite precipitation formed a layer on the scaffold surface by plugging the microporous structure and maintaining the scaffold’s 3D structure intact.     

Excellent lubrication properties of 3D printed ceramic bionic structures

Three-dimensional (3D) printed ceramic structures loaded with lubricating materials could exhibit excellent lubrication effect. In this paper, various bionic petal structures and tree frog toe end structure of 3D-printed alumina (Al₂O₃) ceramic are designed to study the effects of lubricating structures and tungsten disulfide (WS₂) solid lubricant on the friction performance. Compared to the friction properties of various bionic lubricating structures loaded with WS₂, the bionic small petal structure with hexagonal arrangement exhibits a lowest friction coefficient of 0.411 and moderate wear resistance due to more storage of lubricant and debris. The maximum friction coefficient of 1.177 and optimum anti-wear ability are offered by the bionic tree frog toe end structure. In particular, the friction coefficient of the bionic petal lubricating structures loaded with WS₂ are lower than that of blank printed Al₂O₃ ceramic, indicating that the lubricating structures provide positive effect on improving friction performance. Therefore, the 3D printed ceramic lubricating structures provided a novel strategy for achieving lubrication in advanced applications.