The intention of the present study is to review and compare the effect of various well-studied alloying elements on the microstructure and soft magnetic properties of the Fe-based ...amorphous/nanocrystalline system. The state-of-the-art Fe-based amorphous/nanocrystalline alloys have been developed because of their unique soft magnetic properties such as low core loss, high permeability(104–105 at 1kHz) and low magnetostriction (<10ppm) as compared to conventional silicon steels which are also called electrical steels. In Fe-based amorphous/nanocrystalline system, the chemical composition and microstructural features particularly grain size play an indispensable role on the saturated magnetization (Bs) and coercivity (Hc) values. An ideal Fe-based soft magnetic material is defined as a material possessing higher Bs and lower Hc. The problem of the new material is its low Bs value (for commercial material is 1.4T) than silicon steels (≈2T). In addition to Bs content of new material, many attempts have been made to reduce the Hc value which could be achieved via a decrease of grain size (<50nm). To reach this goal (Bs↑ and Hc↓), the effect of a variety of elements on the microstructure, crystallization process and soft magnetic properties of the Fe-based amorphous/nanocrystalline alloys has been investigated so far. The aim of all these studies is to find an appropriate replacement for conventional silicon steels because of their high core loss and low permeability. Effect of alloying elements including Si, B, Cu, Nb, Zr, N-doping, P, Ni, Co, H-doping, Ge and W on the microstructure and magnetic properties is the main subject of this study in order to shed light on the dependence of magnetic properties with composition and grain size.
•N-doped alloys showedsuperior soft magnetic properties.•Cu and Nb are the most effective elements in refining grain size.•Soft magnetic properties highly depends on the primary composition and grain size.
By utilizing ultrasonic annealing at a temperature below (or near) the glass transition temperature Tg, we revealed a microstructural pattern of a partially crystallized Pd-based metallic glass with ...a high-resolution electron microscopy. On the basis of the observed microstructure, we inferred a plausible microstructural model of fragile metallic glasses composed of strongly bonded regions surrounded by weakly bonded regions (WBRs). The crystallization in WBRs at such a low temperature under the ultrasonic vibrations is caused by accumulation of atomic jumps associated with the beta relaxation being resonant with the ultrasonic strains. This microstructural model successfully illustrates a marked increase of elasticity after crystallization with a small density change and a correlation between the fragility of the liquid and the Poisson ratio of the solid.
A deracemization technique using periodic temperature fluctuations on a conglomerate forming system undergoing a swift racemization in solution is demonstrated. The method uses heating and cooling ...periods of the suspension in order to create cycles of partial dissolution of the crystal phase followed by crystal regrowth: this enables symmetry breaking in the solid phase. The technique is an effective, simple, and cheap operation, and can promote understanding of the effects of dissolution and recrystallization on chiral symmetry breaking in the solid phase. The heating period leads to the decrease of the size of crystals and the destruction of small crystals; the surviving crystals can then grow during the cooling period. A succession of such cycles allows the autocatalytic transformation from a racemic suspension into pure enantiomer, with an enantiomeric excess (ee) > 99% within a few days. The results demonstrate a possible mechanism for the emergence of homochirality of molecules of biological significance on Earth.
Phase transformation generally begins with nucleation, in which a small aggregate of atoms organizes into a different structural symmetry. The thermodynamic driving forces and kinetic rates have been ...predicted by classical nucleation theory, but observation of nanometer-scale nuclei has not been possible, except on exposed surfaces. We used a statistical technique called fluctuation transmission electron microscopy to detect nuclei embedded in a glassy solid, and we used a laser pump-probe technique to determine the role of these nuclei in crystallization. This study provides a convincing proof of the time- and temperature-dependent development of nuclei, information that will play a critical role in the development of advanced materials for phase-change memories.
•Tin oxide nanopowders were synthesized by sol–gel method.•The structural and surface properties are studied using XRD, TEM, and Raman.•A1g Raman mode used to determine the nanocrystal sizes by ...phonon confinement model.•The changes in the low frequency Raman spectra is used to study the oxidation state.•Optical sensing response of tin oxide to ammonia gas and 2-nitrophenol are testified using Raman.
Tin oxide nanopowders were prepared by a sol–gel method and annealed at different temperatures in an oxygen atmosphere. The amorphous-to-crystalline transformation, the evolution of the nanocrystals, the sizes and their distribution, the surface characteristics, and the degree of oxidation as a function of annealing temperature are all determined from the Raman spectra. The changes in the Raman spectra between 100 and 200cm−1 represent the change in the oxidation state. The A1g mode, which is a signature of a crystalline SnO2 phase, is used to determine the nanocrystal sizes. The Raman spectra clearly reveal the gradual transformation of Sn(OH)2 to SnOx and ultimately to SnO2 with annealing. The crystallinity and crystallite sizes are also confirmed with X-ray diffraction and transmission electron microscopy measurements. Furthermore, Raman spectroscopy is utilized to study the sensing response of the thus synthesized SnOx to NH3 gas and 2-nitrophenol. Raman spectroscopy is thus demonstrated to be a rapid and comprehensive technique to determine a multitude of parameters of tin oxide relevant for sensing applications.
Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC). ...In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu46Zr45Al7Y2 bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson-Mehl-Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu46Zr45Al7Y2 bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu46Zr45Al7Y2 bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions. (C) 2011 Elsevier B.V. All rights reserved.
The CaO–MgO–Al2O3–SiO2 (CMAS) glass–ceramics containing binary complex nucleation agents were prepared by body crystallization process, and the effects of complex nucleation agents on the ...crystallization and microstructure of CMAS glass–ceramics were investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The complex nucleation agents consist of constant fluorine (8.0wt.% CaF2) and different oxides (3.0wt.% TiO2, ZrO2, or P2O5). Compared with the CMAS glass with only CaF2, the respective addition of oxides promotes the crystallization of CMAS glass, especially TiO2 or P2O5. The addition of TiO2 or ZrO2 has no obvious effect on the compositions of main crystalline phases, while P2O5 results in the precipitation of pyroxene phase instead of diopside phase, with the existence of more small crystals. The CMAS glass–ceramic containing CaF2+TiO2 or CaF2+P2O5 achieves full body crystallization with high crystallization ratio, and has high hardness, good chemical resistance and water absorption.
•The CaO–MgO–Al2O3–SiO2 (CMAS) glass–ceramics were prepared by body crystallization process.•CaF2+TiO2 or CaF2+P2O5 was used as a complex nucleation agent for the CMAS glass–ceramics.•The CMAS glass–ceramics possess full body crystallization and high crystallization ratio.•The CMAS glass–ceramics exhibit high hardness, good chemical resistance and water absorption.
We report the results of a systematic study of the thermal and optical properties of a new family of tellurite glasses, TeO2–ZnO–BaO (TZBa), as a function of the barium oxide mole fraction and ...compare them with those of TeO2–ZnO–Na2O (TZN). The characteristic temperatures of this new glass family (glass transition, Tg, crystallization, Tx, and melting, Tm) increase significantly with BaO content and the glasses are more thermally stable (greater ΔT=Tx−Tg) than TZN glasses. Relative to these, Raman gain coefficient of the TZBa glasses also increases by approximately 40% as well as the Raman shift from ~680cm−1 to ~770cm−1. The latter shift is due to the modification of the glass with the creation of non-bridging oxygen ions in the glass network. Raman spectroscopy allows us to monitor the changes in the glass network resulting from the introduction of BaO.
► Prepared TeO2–ZnO–BaO glasses with different molar concentrations of Ba. ► Measured properties: DSC, optical transmission, refractive index and Raman spectra. ► Showed these to be promising glasses for optical applications in the mid-infrared.
Amorphous calcium carbonate (ACC) is a critical transient phase in the inorganic precipitation of CaCO3 and in biomineralization. The calcium carbonate crystallization pathway is thought to involve ...dehydration of more hydrated ACC to less hydrated ACC followed by the formation of anhydrous ACC. We present here computational studies of the transition of a hydrated ACC with a H2O/CaCO3 ratio of 1.0 to anhydrous ACC. During dehydration, ACC undergoes reorganization to a more ordered structure with a significant increase in density. The computed density of anhydrous ACC is similar to that of calcite, the stable crystalline phase. Compared to the crystalline CaCO3 phases, calcite, vaterite, and aragonite, the computed local structure of anhydrous ACC is most-similar to those of calcite and vaterite, but the overall structure is not well described by either. The strong hydrogen bond interaction between the carbonate ions and water molecules plays a crucial role in stabilizing the less hydrated ACC compositions compared to the more hydrated ones, leading to a progressively increasing hydration energy with decreasing water content.
Understanding the mechanisms that govern the crystallization of natural minerals such as calcium carbonate, calcium oxalate, or hydroxyapatite and its control by biological and synthetic polymers can ...help to guide the design of new biomimetic materials. In this paper, the adsorption behavior of oligomers of polystyrene sulfonate (PSS) on calcite surfaces was investigated by molecular dynamics simulations. The binding strengths of PSS oligomers to different calcite surfaces were computed via potential of mean force calculations, and the binding modes were analyzed in detail. These results could be set in relation to and serve as a molecular-level explanation of the experimentally observed PSS-stabilized exposure of (001) surfaces during calcite mineralization. The simulations show that oligomers of PSS preferentially bind to the polar calcite (001) surface, much stronger than to the nonpolar (104) surface. While sharing in common a dominant role of solvent-induced forces, the mode of binding to the two surfaces is different. The interaction of the sulfonate group with the (001) surface is dominated by both direct and solvent-mediated binding, while the binding of the styrene sulfonate to the (104) surface is mediated by one or two layers of water molecules. Moreover, local solvent density variations at the interface impact the geometry of binding which vastly differs between the two surfaces. In particular, these last effects have important further implications for the preferential binding of PSS polymers (compared to monomers or oligomers) and specific material recognition by synthetic polymers and peptides in general.
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