Papers by Keyword: Amorphous Materials

Abstract: The crystallization kinetics of melt-spun Cu_64.5Zr_35.5 amorphous alloy ribbons was investigated using differential scanning calorimetry (DSC) at different heating rates. Besides, the Kissinger and isoconversional approaches were used to obtain the crystallization kinetic parameters. As shown in the results, the activation energies for glass transition and crystallization process at the onset, peak and end crystallization temperatures were obtained by means of Kissinger equation to be 577.65 ± 34, 539.86 ± 54, 518.25 ± 20 and 224.84 ± 2 kJ/mol, respectively. The nucleation activation energy E_nucleation is greater than grain growth activation energy E_growth, indicating that the nucleation process is harder than grain growth. The local activation energy E_α decreases in the whole crystallization process, which suggests that crystallization process is increasingly easy.

Abstract: The various technologically important properties of metallic glasses are intimately connected to their amorphous structure that lacks the archetypical structural defects of polycrystalline metals and alloys, i.e. dislocations and grain boundaries. However, the amorphous structure also limits the application potential of this class of materials because of a macroscopically brittle behavior and size limitations. Consequently, with some exceptions, at least one dimension for technological products is limited to a few millimeters or even less. With the presented technological approach this drawback will be addressed. Our first results on several alloys show that with a dedicated instrumentation amorphous granulates can be successfully produced. By hot pressing in the supercooled liquid region, these granulates can be compacted into bulk shapes in the cm range. Further, due to the low viscosity of the supercooled liquid state, this technology disposes of a high formability. It is demonstrated that not only compact samples but also complex shapes in near net shape geometry can be produced. Results on the mechanical properties and microstructure will be discussed and related to important processing issues. Even though this technological approach does not directly address the second drawback of bulk metallic glasses, i.e. catastrophic failure due to highly localized shear bands, it is believed that this route offers possible pathways to improve this issue as well and, most important, to offer a technological route for implementing bulk metallic glasses into products of rather arbitrary shape and larger size.

Abstract: The article states the description of the foreign experience in exploitation of power transformers with a core of amorphous materials and their constructive peculiarities. The technical and economic basis of their use in the distribution lines in the Russian Federation is represented. There are also the results of the substitution of conventional transformers by transformers with a core of amorphous steel.

Abstract: Two samples of industrial alloys on the basis of copper are experimentally studied. These alloys are used for production of amorphous solders. The main studied property is kinematic viscosity of metal melt in a liquid state. Data of viscosity dependences from temperature when heating and the subsequent cooling are obtained. Besides, viscosity dependences at constant temperatures from time are received. On viscosity dependences from temperature, for the same samples, abnormal sites and a divergence of curves of heating and cooling are found. On viscosity dependences from time, at a constant temperature, instability is found. The conclusion is drawn that the revealed features are connected with complexity of relaxation process of the studied fusions.

Abstract: The work presents a structural and corrosion resistance analysis of Mg-based bulk metallic glasses in “as-cast” state. The studies were performed on bulk glassy samples in the form of plates. The structure analysis of the samples in “as-cast” state was carried out by the XRD and DSC methods. The corrosion behavior of the bulk glassy alloys and pure magnesium samples was studied by electrochemical measurements and immersion tests in 5% NaCl solution at room temperature. For the amorphous alloys the highest corrosion potential was achieved, indicating the formation of protective surface layers with Mg and Cu oxides. The corrosion behavior of the alloys with Zn and Ni addition was found to be better than pure magnesium.

Abstract: The paper presents the possibility of using amorphous materials as cores of tensile stress sensors. Application of thermal annealing of the core to increase magnetoelastic sensitivity is presented. Important parts of measuring system are the signal processing circuit of the sensor and the sensor control system. The results of measurements of the characteristics of the sensors with signal processing circuit are presented.

Abstract: Silicon nanoparticles have extraordinary electrochemical performance for lithium-ion batteries. This paper gives an improved low temperature solution synthesis route of Si NPs. Reduced by magnesium and then passivated by four kinds of amines/amides respectively, stable yellow Si NPs ranging from 5-50 nm were prepared. When passivated by N-methyl-2-pyrrolidone, grape-like aggregation of 5-20 nm particles were generated. FTIR, XRD, SEM and Electrochemical Characterization were performed to confirm the product. The Si NPs passivated by NMP achieve good electrochemical performance with a first discharge capacity of 1154 mAhg^-1 at a current density of 200 mAg^-1 and good capacity retention of 95.3% after 5 cycles.

Abstract: The quality of metal production can be different even at identical elemental composition and similar heat treatment. The thermo-time smelting regime influences on structure of a metal melt. The structure of a melt influences on process of a hardening and quality of solid alloy. The thermo-time processing of a melt is very relevant at production of nanocristaline materials. The structure of amorphous ribbon is inherited from a melt. The long-lived relaxation processes can be exist in liquid state. They can lasts units or even tens hours. The thermo-time processing allows to receive an equilibrium melt. The properties of an equilibrium melt depend only on an elemental composition and temperature. The development of thermo-time processing is possible on the basis of analysis of different structural-sensing properties of melts. The thermo-time processing is a combination of heating temperatures and temporary ranges. But more often it is possible to determine temperature at which one a melt passes to an equilibrium state practically instantly. Such temperature is named “critical temperature”. The achievement of “critical temperature” is accompanied by anomalies on relations of properties to temperature. The quality of soft magnetic materials received from melt heated up above than “critical temperature” is higher.

Abstract: To obtain low-Si/Al-ratio geopolymers and overcome the disadvantage of impure raw materials, amorphous Al₂O₃-SiO₂ powder was first prepared using a sol-gel method in accordance with specially designed chemical compositions. The composition of the geopolymers was designed according to the NaA zeolite composition (Si/Al/Na atom ratio is 1:1:1), and the zeolite crystal structure was obtained by geopolymers with high Na/Al ratios through hydrothermal processing at 90oC for 6 h. The geopolymers with Si/Al ratios ranging from 0.9 to 1.05 exhibited amorphous structures after being cured at 60oC for 24 h; however, the geopolymer sample with a Si/Al ratio of 1.1 exhibited crystallization. After the hydrothermal treatment of these geopolymers , the XRD, SEM and NMR results show that the phase structure of the geopolymers may transform into a crystalline structure under appropriate compositions and external conditions. Hence, the alkalinity, Si/Al ratio and hydrothermal conditions determined the crystallinity of the geopolymers that were synthesized.

Abstract: Cobalt-boron nanoparticles have been synthesized by a chemical reaction between NaBH₄ and CoCl₂.₆H₂O through manipulating pH value of the reaction mixture. The morphology, structure, phase composition, and thermal behavior have been examined via FESEM, TEM, XRD, EDS, and DSC techniques, respectively. It is demonstrated that the morphology and structure of ultimate nanoparticles completely depends on the pH value of reaction mixture. While the neutral pH value favors the smallest nanoparticles with a mean particle size of 50 nm and complete amorphous structure, the acidic condition promotes the growth process and the crystal structure. Furthermore, these nanoparticles transform into cobalt nanocrystallites after heated at 600°C, and retained the discrepancies in the morphology and the structure of the parent cobalt-boron nanoparticles. A detailed characterization of the nanoparticulates, discussions on the synthesis mechanism, and subsequent formation transformation have been provided.