Polycrystalline materials with nanometer-sized grains, termed nanocrystalline materials, can be formed by crystallizing completely amorphous solids under proper heat treatment conditions. The crystallized nanocrystalline materials exhibit some unique structural characteristics and novel properties which are fundamentally different from those of the conventional coarse-grained polycrystalline materials. This article reviews the present state of the art in this field. The current status of research and developments on the nanocrystallization, microstructure and properties of the materials will be summarized. Comparisons of structural characteristics and properties are made between the crystallized nanocrystalline materials and those prepared by other methods. Further considerations of the development and applications of this new class of materials will also be presented.

Nanocrystalline metals crystallized from amorphous solids: nanocrystallization, structure, and properties

Nanoparticles are discrete nanometer (10−9 m)-scale assemblies of atoms. Thus, they have dimensions between those characteristic of ions (10−10 m) and those of macroscopic materials. They are interesting because the number of atoms in the particles is small enough, and a large enough fraction of them are at, or near surfaces, to significantly modify the particle’s atomic, electronic, and magnetic structures, physical and chemical properties, and reactivity relative to the bulk material. Nanoparticle surfaces themselves may be distinctive. Particles may be terminated by atomic planes or clusters that are not common, or not found, at surfaces of the bulk mineral. These, and other size-related effects will lead to modified phase stability and changes in reaction kinetics.

What makes a nanoparticle a nanoparticle? Definitions of the size ranges for molecules, nanoparticles, and macroscopic solids must be compound specific. However, a useful upper limit for nanoparticles is the size at which one of its properties deviates from the value for the equivalent bulk material by an amount that is significantly larger than the error of the method used to make the measurement (a few percent). In practice, some characteristic will probably be different enough to warrant description as a “nanoparticle” if it is less than a few tens of nanometers in diameter, and perhaps less than a fraction of a micron in diameter.

Because of the importance of size-dependent property changes to the materials sciences, size-property relationships have been studied in detail for some systems. For example, for semiconductors, size effects become important when the particle diameter is close to the Bohr diameter of excitons in the bulk phase. Generally, semiconductor size quantization effects (relevant for naturally occurring metal sulfides, for example) appear when particles are less than 10 nm in diameter (Vogel and Urban 1997).

Definition of the …

Nanoparticles in the Environment

Calorimetric measurements of the glass transition are presented for metallic glasses, ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$, and organic polymers with cooling and heating rates ranging over more than three orders of magnitude. Fits of the onset temperatures of the glass transition to a Vogel-Fulcher-type equation give divergence temperatures significantly above the values obtained for Vogel-Fulcher fits to the viscosity. The width of the glass transition extrapolates to a sharp step transition at a finite heating and cooling rate, which is close to the range where the data were taken. The results suggest that the glass transition becomes independent of the experimental time scale in a regime that may be accessible to measurements.

Glass transition on long time scales

Microstructure, fabrication and properties of quasicrystalline Al–Cu–Fe alloys: a review

Aluminum 5083 powder was mechanically milled under liquid nitrogen to achieve a nanocrystalline grain size in the range of 20 to 30 nm. The powder was subsequently sprayed using a nozzle designed with a validated numerical model for cold spray technology. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro- and nanoindentation. The TEM analysis shows that the nanocrystalline grain structure of the cryomilled feedstock powder was retained after the cold spray process. A significant increase in hardness from 104 to 261±8 (HV300g) was observed when comparing the nanocrystalline coating with cast, cold-worked, Al 5083. The ability to use cold spray to produce nanocrystalline large deposits was also demonstrated in this work.

https://link.springer.com/content/pdf/10.1007%2Fs11661-005-0099-y.pdf

Cold spray deposition of nanocrystalline aluminum alloys

The formalism of the Ammann bar grid with two spacings is systematically analyzed. Simple two-by-two matrices describe the self-similar deflation. The deflation and the inflation factors are the two eigenvalues of the matrix. The ratio of the two spacings is derived as another characteristic quantity of the matrices. The maximum number of spacing ratios and the ratios themselves have been derived for a given inflation factor by the matrix formalism. Applications to non-periodic patterns with eightfold, tenfold and twelvefold symmetry are demonstrated. The strong relationship to Fibonacci and Fibonacci related series is explained. The significance of the Ammann bars for matching rules, inflation/deflation procedures and phason movement is outlined.

Basic ideas of ammann bar grids

A subdivision of the stability region of decagonal Al-Co-Ni quasicrystals into eight different structural modifications is presented for the temperature against composition section along the maximum extension of the decagonal phase. The only methods identifying this subdivision are transmission electron microscopy techniques. Besides two decagonal superstructures (one of which also exists as a high-temperature modification), two variants of the basic decagonal state were observed. Also, a one-dimensionally periodic fivefold quasicrystal (together with its high-temperature superstructure modification) and a one-dimensional quasicrystal are parts of the quasicrystalline stability region. Both have a close structural relationship to the decagonal states. All samples investigated were obtained by quenching from the corresponding temperatures. The eight modifications can be differentiated by a set of attributes, especially by their characteristic diffraction patterns taken with the electron beam parallel to the unique periodic axis.

The existence regions of structural modifications in decagonal Al-Co-Ni

Heat capacities of the intermetallic compounds FeTi and Fe 2 Ti have been measured through the temperature range 120 to 700 K by differential scanning calorimetry. The Neel temperature of the magnetic transition of Fe 2 Ti is determined by means of heat capacity measurements. The data of the heat capacities are fitted with the method of least squares. The confidence limits of the heat capacities as well as of other thermodynamic functions are given for a probability of 95%. The Debye temperatures of the intermetallic compounds FeTi and Fe 2 Ti have been determined. The thermodynamic functions, the enthalpy, the entropy and the Gibbs free energy, are calculated for the temperature range from 0 to 1800 K

Heat capacities of intermetallic compounds in the iron-titanium system

The recovery of slip planes in quasicrystals is investigated on the basis of the formation of perfect tiles due to a flipping procedure. The procedure is focused on tilings with single translation classes. The minimum number of new vertices or the number of necessary flips to reform the tiles is taken as a measure of disorder of the slip process. Different ranges of the density of new vertices depending on the slip vector could be derived. Short displacements lead to the formation of a domain of ID quasicrystal.

Healing of slip planes and interfaces in quasiperiodic patterns

A single covering cluster cannot produce a rigorously quasiperiodic dodecagonal structure but may almost do so and thus be relevant to real quasicrystals. Such ‘almost covering” patches are shown for the ship, shield, square-triangle and Socolar tiling. The ‘almost covering7rdquo; of the Socolar tiling has a fractal feature.

Dodecagonal tilings almost covered by a single cluster

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