Papers by Keyword: Amorphous Materials

Abstract: In this paper there are presented results of structure analyses of the Fe78Nb2B20 amorphous alloy after annealing. Basing on the initial analyses the “hypothetical” model structure was chosen as the combination of different kind of the Fe structure deformations: change of unit cell parameters (change of unit cell shape) and change of line broadening (change of crystallite size). The crystallite size (in meaning of the size of the ordered regions) for the amorphous state is in a range of 7 – 37 Å and for the crystalline state is about of 1100 Å. The comparison of the values of the unit cell parameters shows that the value of the basic – hypothetical unit cell in an amorphous state is bigger of 0.2 Å than in a crystalline state. After crystallisation there are detected Fe and Fe3B phases.

Abstract: The aim of the work was to provide information on structure development and change of properties at elevated temperatures in Fe41Ni39P10Si5B5 amorphous alloy. The alloy was characterized by X-ray diffraction. The changes of properties were characterized with use of dynamic mechanical thermal analysis (DMTA) and the resistivity measurements at elevated temperatures. The microstructure of the melt spun ribbon was investigated with use of transmission electron microscope (TEM) at different stages of phase transformations after heating to different temperatures. The initially amorphous structure undergoes phase transformations due to glass transition and crystallization of the alloy. The appearance of glass transition region results in decrease of storage modulus and in a reversible change of temperature coefficient of resistivity (TCR). The phases are characterized with use of TEM. The crystallization was found to have the two stages. Formation of bcc crystals and Ni12P5 is followed by transformation of the products into fcc crystals and Ni3P. Temporary changes of the storage modulus and elongation of the sample suggest formation of hard phases during crystallization.

Abstract: The effect of bismuth (Bi) additions upon the physical properties, coordination number (m), constraints (Nc), density (ρ), molar volume (Vm), cohesive energy (CE), lone pair electrons (L) and glass transition temperature (Tg) of Ge20Te80-xBix (x = 0, 1.5, 2.5, 5.0) bulk glassy alloy has been investigated. The density and molar volume of the glassy alloys has been found to increase with increasing Bi content. The CE of the investigated samples has been calculated by using the chemical bond approach (CBA) and is correlated with a decrease in the optical band-gap with increasing Bi content. The glass transition temperature has been estimated by using the Tichy–Ticha approach and was found to increase with an increase in the Bi content.

Abstract: A series of shock recovery experiments up to ~50 GPa were performed on reactions to form carbon nitrides. Nitrogen-rich starting materials, included a C-N-O amorphous precursor, dicyandiamide, melamine, and a mixture of carbon tetrahalide and sodium dicyanoamide, were used and the recovered samples were investigated by X-ray diffraction technique, elemental analysis, transmission electron microscopy and so on. Experimental results showed formation of a new carbon nitride, high stability of melamine up to a shock pressure of 37 GPa, and production of amorphous C-N materials with a highest N/C ration of 1.26 from the reaction between carbon tetrahalide and sodium dicyanoamide. We extended to the system C3N4-Si3N4 based on the recent results on synthesis of spinel-type nitrides. Shock wave chemical reactions provide a route for synthesizing novel materials including not only high-pressure phases but also metastable, unique substances.

Abstract: Electrodes for electric double layer supercapacitors (EDLS) usually made of a mixture of carbon black and binder material. However the binding mixture causes loss in the capacitance because after the polymerization it obstructs the pores of the carbon black, thus decreasing the active surface of the capacitor. The capacitance of the electrode can be increased by use of additive materials, which improve the electrochemical performance of the active material. The effect of amorphous carbon nano-particles, added to the carbon black on the performance of the supercapacitor electrodes was studied earlier. It was found that maximal value was obtained for the composition of 30 wt.% of amorphous carbon nano-particles and 70 wt.% of carbon black [1]. In this work test-piece supercapacitors (TPS) fabricated with non-aqueous Li-based electrolyte were studied to analyze the influence of the size of the TPS to the specific capacitance. These systems were examined by impedance, charge-discharge measurements and cyclic voltammetry. The working potential window of the capacitor was found to be the 0-1 V region. Increasing the area of the electrode by using Al foils of larger size resulted in proportional increase of the capacitance.

Abstract: Phase transformation induced by ball-milling was studied in this work. It was found that amorphous Fe90Zr10 ribbons undergo crystallization into BCC α-Fe(Zr) under milling in an AGO-2 mill. The decomposition degree of the amorphous phase increased with increasing milling time and intensity. Analyses of samples milled at different speeds suggested that the observed crystallization is a deformation-induced process rather than a thermally induced one. In addition, the decomposition behavior of a FeSn intermetallic under ball-milling was carefully studied. Upon milling a large amount of the FeSn intermetallic decomposed into Fe5Sn3 and FeSn2, where the average grain size of the product phases stayed nearly constant with milling-time. It is suggested that the mechanically driven decomposition of FeSn results from local melting of powder particles due to high temperature pulses during ball collisions.

Abstract: Mechanically induced crystallization of an amorphous Fe90Zr10 alloy was studied by means of X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Under high-energy ball-milling in an AGO-2 mill, melt-spun Fe90Zr10 ribbons undergo crystallization into BCC α- Fe(Zr). Zr atoms are found to be solved in the Fe(Zr) grains up to a maximum supersaturation of about 3.5 at.% Zr, where it can be presumed that the remaining Zr atoms are segregated in the grainboundaries. The decomposition degree of the amorphous phase increases with increasing milling time and intensity. It is proposed that the observed crystallization is deformation-induced and rather not attribute to local temperature rises during ball-collisions.

Abstract: The Herzer model suggests that superior magnetic properties can be observed in magnetic alloys provided a suitable microstructure consisting of soft magnetic nanometre size precipitates separated by nanometre size distances is obtained. In order to explore new magnetic alloy compositions which can satisfy the Herzer model, we studied selected alloys in which we wished to obtain suitable microstructures, one such alloy has the composition Fe40Ni38B18Mo4. This alloy is amorphous as received, the heat treatment schedule required to obtain nanoprecipitates was designed based on DSC, resistivity and x-ray results. Heat treatment at temperatures between 420 0C and 500 0C and different heat times was carried out on the amorphous ribbon. Transmission Electron Microscopy (TEM) results of the early stage of crystallization behavior showed that soft magnetic precipitates of nano-size were indeed observed within the amorphous matrix. The crystal structure, composition analysis and thermal stability of the precipitates were studied by XRD, EDS and TEM. These results will be presented and the implication of these results to the production of new soft magnetic alloys will be emphasized.