A thermodynamic explanation for martensitic phase stability of nanostructured Fe-Ni and Co metallic materials

In order to explain the experimental facts that start temperatures of reverse martensite transformation (A_s) for nanostructured Fe-Ni and Co are lower than those for their coarse-grained counterparts, a thermodynamic model was established in this paper. The thermodynamic analysis shows that the decrease of A_s for nanostructured Fe-Ni and Co can be attributed to the difference of surface free energy for martensite and austenite. The correlation between the decrease of A_s and start temperature of martensitic transformation (M_s) in nanostructured material is further proposed.     

Prediction of nanoparticles’ size-dependent melting temperature using mean coordination number concept

Many models have been developed to predict size-dependent melting temperature of nanoparticles. A new model based on the cluster mean coordination number (MCN) calculations is developed in this work. Results of the model for Al, Au, Pb, Ag, Cu, In, Sn, and Bi were compared with other models and experiments. The comparison indicated that the MCN model is in good agreement with available experimental values. It is also found that the melting temperature is more dependent on particle size as the atomic radius increased.     

Elastic properties of nanostructured materials with layered grain boundary structure

Atomistic calculations of the elastic constants for a bulk nanostructured material that consists of a layered structure where alternating layers meet along high angle grain boundaries and where atoms interact via a Lennard-Jones potential are presented. The calculations of the elastic constants were performed in the frame of homogeneous deformations for a wide range of layer widths ranging from 2.24 up to 74.62 nm. The results showed that the relaxation of the atomic structure affects the elastic constants for the cases where more than 5% of atoms are located in the GB region. Also it was found that the way that external stresses are applied on the system affects the values of the obtained elastic properties, with the elastic constants related to the characteristic directions of the grain boundary being the most affected ones. The findings of this work are of interest for the fabrication methods of nanostructured materials, the measurement methods of their elastic properties as well as multiscale modeling schemes of nanostructured materials.     

Size-dependent density of nanoparticles and nanostructured materials

We discuss the size-dependent density of nanoparticles and nanostructured materials keeping the recent experimental results in mind. The density is predicted to increase with decreasing size for nanoparticles but it can decrease with size for nanostructured materials that corroborates the experimental results reported in the literature.     

Gibbs free energy approach to calculate the thermodynamic properties of copper nanocrystals

Based on the thermodynamic and thermophysical properties of bulk materials, Gibbs free energy for nanocrystals is obtained and used to study the size-dependent melting point depression phenomenon. On the basis of size effects on the melting temperature of nanocrystals, thermodynamic properties of Cu nanocrystals, such as the specific heat capacity, the Debye temperature and the diffusion activation energy are investigated. Conversely, reliable predictions of these thermodynamic properties further verify the proposed approach.     

Modeling the thermodynamic properties of bimetallic nanosolids

A model has been developed to account for size, shape, surface segregation, composition and dimension dependent cohesive energy of bimetallic nanosolids, and further been extended to predict the size dependent thermodynamic properties, such as melting temperature, Curie temperatures, ordering temperature and phase diagram. The cohesive energy, melting temperature, Curie temperatures and ordering temperature of bimetallic nanosolids decrease with decreasing the particle size. The depression is dramatic in the lower range of size, while it becomes smoothly in large size. For nano phase diagram, the solidus and liquidus curves drop and the two-phase zones become small, as the size of the nanosolids decreases. The two-phase zones of the nano phase are always lower than the regions indicated in the bulk Ag-Pd alloy phase diagram, and they may deteriorate into a curve at a critical size. It is also found that the thermodynamic properties of nanosolids not only depend on the compositions, the atomic diameter and the cohesive energy of each component, but also depend on the size and the shape. The model predictions are consistent with the corresponding simulation, semi-empirical model and experimental data.     

Critical thermodynamic evaluation and optimization of the Fe-Mg-O system

A complete literature review, critical evaluation and thermodynamic modeling of the phase diagrams and thermodynamic properties at 1 bar total pressure of all oxide phases in the Fe-Mg-O system are presented. Optimized model equations for the thermodynamic properties of all phases are obtained which reproduce all available thermodynamic and phase equilibrium data within experimental error limits from 25 °C to above the liquidus temperatures at all compositions and oxygen partial pressures. The complex phase relationships in the system have been elucidated and discrepancies among the data have been resolved. The database of the model parameters can be used along with software for Gibbs energy minimization in order to calculate any type of phase diagram section. Sublattice models, based upon the compound energy formalism, were used for the spinel, pyroxene, olivine and monoxide phases. The use of physically reasonable models means that the models can be used to predict properties, phase equilibria, and cation site distributions in composition and temperature regions where data are not available.     

Synthesis and properties of new nitrogen-doped nanostructured carbon materials obtained by templating of mesoporous silicas with aminosugars

The negative templating synthesis process has been applied to prepare nanostructured carbon materials with a high nitrogen content. SBA-15 silica template was impregnated with the following carbon precursors: sucrose, glucose and amino-glucose. The structure of the materials was investigated by SAXS, WAXS and TEM. Nitrogen functions were analyzed by XPS and the textural parameters of the carbons were studied by nitrogen and CO₂ adsorption. X-ray and TEM studies confirmed that a pore nanostructure is inherited from the silica templates. XPS analysis showed that the nitrogen content of the materials can be controlled between 2 and 5 wt% and that N atoms are strongly bonded in the carbon structure in heterocycles or nitrile functions. An important result is that these nanostructured carbon materials exhibits interesting textural properties with BET surface areas ranging between 1000 and . Moreover, the study of the influence of nitrogen on the textural and structural parameters of the resulting carbon materials shows that nitrogen plays an active role during the synthesis process. This observation is also supported by the speciation of nitrogen in the nanostructured carbon materials.     

Computer-aided design of nanostructured materials containing trisaza-bridged [60]fullerene

A novel fullerene-based building block for the synthesis of nanostructured materials has been designed with the aid of electronic structure theory calculations and molecular modeling. The building block consists of four trisaza-bridged C₆₀ fullerene molecules linked to a central cubane (C₈) unit. Each C₆₀ unit is located on the vertex of a tetrahedron with edge of 2.2 nm. One possible packing mode of the building blocks to yield the nanostructured material is suggested.     

Mechanical properties of bulk and nanoscale TiO2 phases

The mechanical properties of bulk and nanoscale TiO₂ phases are examined with a view to assess the available bulk modulus and hardness data, and to understand the size-dependent behaviors. The bulk modulus values of thermodynamically stable bulk TiO₂ phases show a general correlation with Ti-O coordination number. As with the cotunnite-structured (OII) phase, it is likely that the seven-coordinated OI and eight-coordinated fluorite forms of TiO₂ are ultrahard substances. Of the nanoscale phases investigated thus far, nanocrystalline anatase displays the strongest size dependence of bulk modulus values, with possible stiffening behavior effected by incipient grain boundary amorphization under pressure. Nanocrystalline rutile and baddeleyite phases do not show appreciable size dependence in their compression behaviors.     

Development and properties of nanostructured thermal spray coatings

Nanostructured thermal spray coatings have been intensively studied because of their potential in a wide variety of industrial applications. In the present paper, current development status of nanostructured thermal spray coatings is presented, mainly based on the results of the authors. In the nanostructured WC–Co wear-resistant coatings, the influence of feedstock characteristics on the coating properties was discussed to suggest the desirable morphology of feedstock for thermal spraying. For the nanostructured Cr₂O₃ based solid-lubricant coatings, the advanced feedstock has been developed in order to solve the inhomogeneity problem of the conventional coatings. Various properties of the nanostructured coatings were evaluated and compared with those of the conventional counterparts. These results clearly demonstrate that the significant improvement in coating performance can be achieved by utilizing proper nanostructured coatings.     

Substrate-dependent structure, microstructure, composition and properties of nanostructured TiN films

Titanium nitride films of a thickness of ∼1.5 μm were deposited on amorphous and crystalline substrates by DC reactive magnetron sputtering at ambient temperature with 100% nitrogen in the sputter gas. The growth of nanostructured, i.e. crystalline nano-grain sized, films at ambient temperature is demonstrated. The microstructure of the films grown on crystalline substrates reveals a larger grain size/crystallite size than that of the films deposited on amorphous substrates. Specular reflectance measurements on films deposited on different substrates indicate that the position of the Ti-N 2s band at 2.33 eV is substrate-dependent, indicating substrate-mediated stoichiometry. This clearly demonstrates that not only structure and microstructure, but also chemical composition of the films is substrate-influenced. The films deposited on amorphous substrates display lower hardness and modulus values than the films deposited on crystalline substrates, with the highest value of hardness being 19 GPa on a lanthanum aluminate substrate.     

Empirical correlation between melting temperature and cohesive energy of binary Laves phases

A list of 143 binary Laves phases with their melting temperature and melting type is collected, and used to study a correlation between melting temperature and cohesive energy. It is found that the melting temperature of Laves phases is roughly proportional to its cohesive energy calculated by Miedema's empirical model from their intrinsic atomic properties. The average predicted error of melting temperature of compounds is as low as 8.0%. This empirical rule is consistent with the result of the universal binding energy theory of solids.     

Role of surface defects and microstructure in infrared optical properties of thermochromic VO2 materials

Thermochromic vanadium dioxide VO₂ exhibits a semi-conducting to metallic phase transition at T_c=68 °C, involving strong variations in optical transmittance, reflectance and emissivity. However, the optical contrasts observed in thin films or nanostructured compacted samples seem to depend on both surface microstructure and surface crystal texture. In the case of opaque materials, surface defects might play a drastic role in optical reflectivity. As the high temperature metallic phase of VO₂ is opaque for infrared radiations, we used aluminum samples as standards allowing us to correlate reflectivity responses with porosity and surface defects. Then, various polycrystalline and nanostructured VO₂ samples compacted at various pressures and presenting variable surface roughness were prepared. Thin films were deposited by radio frequency sputtering process. The samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Optical properties (reflectance and emissivity) were analyzed above and below the transition temperature, making use of specific FTIR equipments. In thin films, the deposited VO₂ phase was systematically oriented and surface porosity was very weak. In polycrystalline samples, as the compaction pressure increased, surface porosity decreased, and infrared optical contrast increased. In such samples, preferred orientations were favored for low applied pressures. These features clearly show that the main parameters conditioning the optical contrast should be the surface defects and porosity, not the preferred crystal orientations. As an additional interesting result, the surfaces formed from compacted nanocrystalline VO₂ powders present improved optical contrast for reflectance and emissivity properties.     

Synthesis of nanostructured polymer materials with different morphologies: Nanoporous ordered networks and hollow capsules

Nanostructured polymer materials with interesting morphological variation, which include three dimensionally interconnected uniform nanoporous network arrays (volume- and surface-templated ordered arrays) and hollow core spheres were synthesized by inducing different polymerization process of phenol and formaldehyde as a precursor over silica templates (ordered silica colloidal crystals or individual silica particles). The pore sizes of the resulting nanostructured polymer materials can be easily controlled by monitoring the sizes of silica spheres, while their morphologies were modulated by controlling the initiation sites of the acid-catalyzed condensation reaction of the same polymer precursor and by modifying silica templates.     

Distinctive thermodynamic properties of solute–solvent hydrogen bonds in self‐associated solvents

Solute–solvent hydrogen bonding affects reactivity and other properties of dissolved species. In self‐associated media, because of cooperativity and solvent reorganization, the thermodynamic functions of solute bonding with bulk solvent can be different from those of bimolecular solute–solvent complexes. Using available experimental data on the Gibbs free energies of solvation in aliphatic alcohols and water, we have determined the energies of solute–solvent hydrogen bonding for various proton accepting solutes. We show that the increase in the strength of hydrogen bonds because of the cooperative effect is strong for bonding with bulk water and significantly less so with bulk aliphatic alcohols. The hydrogen bonding Gibbs free energies for the same solute with bulk water and alcohol are correlated, but they correlate poorly with the energies of formation of the corresponding bimolecular solute–solvent complexes. Thus, the traditional hydrogen bond basicity scales, based on data for bimolecular complexes, do not correctly describe the thermodynamics of hydrogen bonding with self‐associated solvents. Our results may help to define a separate solute basicity scale for associated media. Copyright © 2012 John Wiley & Sons, Ltd.     

The relationship between chemical bond properties and Stokes shift of Eu in some silicate host lattices

The covalency of each bond in divalent europium doped hosts CaSiO₃, SrSiO₃, BaSiO₃, Sr₂LiSiO₄F, Ba₅SiO₄Cl₆ and Ba₅SiO₄Br₆ were calculated by using the complicate crystal chemical bond theory. The relationship between the Stokes shift and the bond properties of Eu²⁺ in these crystals was discussed. The result demonstrates that, in the isostructural crystals that being doped with Eu²⁺, there is a more precise connection between the magnitude of Stokes shift and the mean covalency of the dopant site.     

Investigation of thermodynamic properties of cerium dioxide by statistical moment method

The thermodynamic properties of the cerium dioxide (CeO₂) are studied using the statistical moment method, including the anharmonicity effects of thermal lattice vibrations. The free energy, linear thermal expansion coefficient, bulk modulus, specific heats at the constant volume and those at the constant pressure, C_V and C_P, are derived in closed analytic forms in terms of the power moments of the atomic displacements. The temperature dependence of the thermodynamic quantities of cerium dioxide is calculated using three different interatomic potentials. The influence of dipole polarization effects on the thermodynamic properties and thermodynamic stability of cerium dioxide have been studied in detail.     

Investigation of the effect of pressure on thermodynamic properties and thermoelastic phase transformation of CuAlNi alloys: A molecular dynamics study

Thermoelastic phase transformations and thermodynamic properties of CuAlNi alloys at 0, 1, 2 and 3 GPa pressures were investigated by using MD simulation in this study. The interactions between atoms were modelled by Sutton-Chen type of embedded atom method (SCEAM) that is based on many-body interaction. It was observed that thermoelastic phase transformation in the ternary alloy system occurred at the end of thermal process. Radial distribution function (RDF) was used in order to analysis the structures obtained from MD simulation using the simulation techniques’ thermodynamic parameters. The transformation temperatures, enthalpy and entropy of the ternary alloy system have been observed to be changing with the applied pressure. In addition, it was found that the elastic energy has been decreased about 22% by applied pressure whereas Gibbs free energy has been increased about 60% by applied pressure. The values of the thermodynamical parameters obtained in this study were observed to be in close agreement with the experimental study.     

Comparative investigation on spectroscopic properties of Er between Ce-doped and B2O3-added bismuth glasses

The comparative investigation on the spectroscopic properties of Er³⁺ in low phonon energy Bi₂O₃-GeO₂-Ga₂O₃-Na₂O glasses codoped with Ce³⁺ ion and added with B₂O₃ component, respectively, is presented. With increasing Ce₂O₃ content from 0 to 0.8 mol% or B₂O₃ content from 0 to 15 mol%, the lifetime of Er³⁺:⁴I_11/2 level decreases dramatically from 607 to 283 μs or to 197 μs, and the upconversion fluorescence is quenched in both glass samples. The nonradiative energy transfer from Er³⁺:⁴I_11/2→Ce³⁺:²F_5/2 or the enhanced multiphonon relaxation process together with the energy transfer between Er³⁺ and OH⁻ groups are, respectively, responsible for the results. Meanwhile, the lifetime of ⁴I_13/2 level remains almost unchanged in Er³⁺/Ce³⁺-codoped glasses whereas it decreases rapidly in B₂O₃-added cases. As a result, Er³⁺/Ce³⁺ codoping improves the 1.5 μm fluorescence emission intensity, however, B₂O₃ addition has a negative effect on it. The research results indicate that the Er³⁺/Ce³⁺-codoped bismuth glasses will be preferable for obtaining efficient 980 nm pumped EDFA.