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

Cubic BC2N was synthesized from graphite-like BC2N at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of c-BC2N at ambient conditions is 3.642(2) A, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of c-BC2N is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and c-BN. The hardness of c-BC2N is higher than that of c-BN single crystals which indicates that the synthesized phase is only slightly less hard than diamond.

Synthesis of superhard cubic BC2N

BaTiO3 powders with various crystallite sizes, which were prepared through microemulsion-mediated synthesis, were thoroughly studied by Raman spectroscopy. Clear evidence for the presence of the tetragonal phase was found for ultrafine powders with an average crystallite size above 30 nm. The lifetime of phonons that are specifically assigned to the tetragonal phase decreased with decreasing crystallite size below a critical size of 100 nm. In particles as fine as 100 nm, the short mean free path of phonons, mainly due to internal pressure, causes decoupling of the coupled A1 (TO) phonons and a diffuse phase transition behaviour (TC = 115 °C). Coupled A1(TO) phonons, which give a spectral dip at around 180 cm−1 and a lesser extent of diffuseness, were revealed for powders consisting of particles as large as 0.17 µm (TC = 123 °C). Further coarsening upon annealing induced the formation of aggregates, resulting in the shift of phase transition points to higher temperatures for the rhombohedral to orthorhombic and the orthorhombic to tetragonal transitions and to lower temperatures for the tetragonal to cubic transition, respectively. Phase stability in powders is discussed by considering possible factors such as internal pressure in isolated particles and internal stress in aggregates. Copyright © 2007 John Wiley & Sons, Ltd.

Raman scattering studies on nanocrystalline BaTiO3 Part I—isolated particles and aggregates

https://onlinelibrary.wiley.com/doi/epdf/10.1002/adfm.201909260

Synthesis, Structure, Properties, and Applications of Bimetallic Nanoparticles of Noble Metals

Carbon-supported Pt–Ru–WO
 x
 /C catalysts for application in PEMFC anodes were synthesized by a modified Bonnemann method. Their electrocatalytic activity for the oxidation of H2/CO mixtures and CH3OH was measured by E/i-curves in PEM single cell arrangements under working conditions. Information about composition, microstructure and nanomorphology was obtained by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence analysis (XFA) and transmission electron microscopy (TEM). X-ray diffraction data at room temperature show only one single Pt f.c.c. phase; no evidence of Ru, W and their oxides, respectively, is found. Hence, the presence of W and Ru as amorphous oxide species seems likely. Surface-sensitive XPS measurements detect Pt0, platinum oxide and hydroxide species, metallic Ru, ruthenium oxide, hydrous ruthenium oxide and WO3. For the crystalline platinum phase particle sizes of less than 2 nm were determined by TEM images and XRD patterns via solving the Scherrer equation. Temperature-dependent XRD measurements were performed to show the influence of ageing on the catalyst structure.

Synthesis and characterization of carbon-supported Pt-Ru-WOx catalysts by spectroscopic and diffraction methods

Change of the Unit Cell Parameters in Highly Dispersed Gold, Silver, and Copper Powders

A monotonous increase of the lattice parameters with the decrease of particle size is established by an X-ray study for highly disperse nickel powders in the interval of sizes from 4.9 to 35 nm. The relative changes of lattice parameters are from 4.9 × 10−3 ± 5 × 10−4 up to 3 × 10−4 ± 1 × 10−4. The effect is explained by the decrease of the intracrystalline pressure in small particles stipulated by electrostatic interaction of the elements of crystal charge lattice. A calculated dependence of the lattice parameters which agrees with experimental curve is obtained in the frameworks of the model suggested by the charge lattice represented by an ion—electron lattice of positive ions and collectivized electrons with regard of the lattice of atomic neutral cores (the contribution of the latter is proved very small as found from the calculations).



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Change of Lattice Parameters in Highly Disperse Nickel Powders

A relative increase of the lattice parameters equal to 2.05 × 10−3 ± 1.8 × 10−4 is established by X-ray analysis in ultradisperse powder diamond with average size of crystallites equal to 4.6 ± 0.4 nm as compared with the lattice parameter of a large-crystal sample. The effect found for diamond as well as the experimental data of the increase of the lattice parameters in ultradisperse samples of silicon established previously are explained by the size change of intracrystalline pressure stipulated by the interaction of the excess negative charges arranged on the bond lines CC (or SiSi), and the positive cores of the atoms of the covalent crystal. The size dependences of the relative change of the lattice parameters are calculated in the frameworks of the point model of the charge lattice for diamond and silicon. The dependences show a monotonous increase of the lattice parameters when decreasing the sizes of the crystals. Calculated dependences are in good agreement with experimental data.



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Size Changes of Lattice Parameters in Ultradisperse Diamond and Silicon

It is shown that the volume forces of electrostatic interaction of the crystal lattice elements — ions and electrons — result in intracrystalline pressure (IP) which depends on the size of particles and stipulates the size dependence of the unit cell parameters. Analytical expressions are obtained for the IP by the crystal energy and it is shown that the IP can increase or decrease in small particles depending on the crystal structure, i.e. has tensile or compressing action. The results of the theoretical calculations for NaCl and CeO2 give evidence for the compressing effect of IP in small particles of NaCl and for the tensile effect of IP in small particles of CeO2. These results are in good agreements with experimental data of the changes of the unit cell parameters in small particles of NaCl and CeO2.



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Tensile and Compressive Effect of Intracrystalline Pressure in Small Particles

The nature of size-related changes of lattice parameters is determined by the difference in bond energies of surface and inner unit cells of a particle, which (the difference) leads to an excess or a deficit of the intracrystalline pressure in a small particle, stipulating a compession or tension of its lattice. It is shown that the size-related change of the length of each lattice edge is determined by crystal sizes in perpendicular direction and practically does not depend on the crystal size along this edge. This property, called transversality, is explained by an energetic non-equivalence of different unit cells, which is realized within cell monolayers or columns perpendicular to the edge. It is shown that the sign of the size effect – decrease or increase of the lattice parameter – is determined by the sign of the interaction energy v of a unit cell with crystal unit cells excepting the nearest ones. To the positive sign of this energy there corresponds compression, to the negative sign tension of a small particle. The structural features are analysed for substances with ionic, covalent, and metallic bond types, determining the sign of v and therefore the sign of the size effect.



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M. Ya. Gamarnik

Papers

Change of the Unit Cell Parameters in Highly Dispersed Gold, Silver, and Copper Powders

Change of Lattice Parameters in Highly Disperse Nickel Powders

Size Changes of Lattice Parameters in Ultradisperse Diamond and Silicon

Tensile and Compressive Effect of Intracrystalline Pressure in Small Particles

The Physical Nature of Changes of Lattice Parameters in Small Particles