Crystallization Of Amorphous Matrix Research Articles

Non-isothermal nano-crystallization kinetics in amorphous Ni55Nb35Si10 alloy

Abstract The non-isothermal crystallization kinetics of Ni55Nb35Si10 amorphous alloy, prepared by mechanical alloying, was studied using differential scanning calorimetry. The amorphous alloy showed one-stage crystallization on heating, which led to the formation of nano-intermetallic crystals in amorphous matrix. The apparent activation energy for the crystallization of the alloy, determined by the Kissinger equation, was relatively high (468 kJ/mol), indicating that this amorphous alloy has high thermal stability. Changes in the activation energy during the crystallization process, were also evaluated by iso-conversional methods. The results showed that it decreases slowly from the beginning to crystallized fraction α=0.35 and it remains almost constant to the end of the process. The nano-crystallization mechanism for the non-isothermal crystallization of the amorphous alloy was explained by determining Avrami exponents. Transmission electron microscopy studies revealed the microstructural modification of amorphous alloy via nano-crystallization during annealing. The results suggest that the nucleation rate decreases with increasing time and the crystallization mechanism is governed dominantly by a three-dimensional diffusion-controlled growth. A predictive equation was obtained based on the Sestak−Berggren autocatalytic model to describe quantitatively the non-isothermal crystallization kinetics.

Transformation of sputtered calcium copper titanate thin film into nanorods by sequential annealing

Rapid synthesis of long calcium copper titanate (CCTO) nanorods was carried out by sequential annealing. CCTO thin films have been deposited on p-Si substrate by RF sputtering technique and afterwards, the samples were thermally treated using a preheated furnace by varying the annealing temperature from 850°C to 1100°C. CCTO nanorods of 12µm lengths and 400–600nm diameters were synthesized at 1100°C. Based on the FESEM observations, a plausible growth mechanism has been proposed to explain the formation of nanorods. The (220) XRD peak of the CCTO film became prominent for the annealing temperature of 950°C. The presence of nanoscale crystals in amorphous matrix has been observed by HRTEM studies. The elemental mapping of CCTO nanorod has shown a spatial variation of elements throughout the nanorod. The oxide and interface charge density was found to be increased with the rise in annealing temperature.

Microstructure evolution and hardening mechanisms of Ni–P electrodeposits

Ni–P alloy coatings with different phosphorus contents were prepared by electroplating in a nickel sulfate bath containing phosphorous acid (H3PO3). Hardening mechanism, such as dispersion hardening of Ni nano crystals in amorphous matrix of the as-deposited Ni–P coating and coarsening weakening of Ni3P for the high P coating after 1h of heat treatment at 400°C were concluded from the experimental data. Hardening mechanism of Ni–P alloys were further discussed based on the microstructure evolution with increasing deposit P content and during the heat treatment by using high resolution TEM (HR-TEM). A maximum hardness was observed for the as-deposited and heat-treated Ni–P alloys with 4wt.% and 6wt.% P, respectively. These composition ranges corresponding to the microstructure with high hardness proposed are believed to be useful for the industrial applications and further study.

Development of crystal texture in Nd-lean amorphous Nd9Fe85B6 under hot deformation

A key challenge in synthesizing anisotropic nanocomposite magnets is to form crystallographic texture for R2Fe14B crystals in R-lean alloys (R=rare earth, R<10 at. %). Here, we demonstrate a (00l) texture development for Nd2Fe14B nanocrystals in a Nd-lean amorphous Nd9Fe85B6 under a hot deformation at a large uniaxial stress of ∼310 MPa. The unusual texture formation is attributed to a preferential nucleation of Nd2Fe14B crystals in amorphous matrix at the large stress. The present study provides an opportunity to yield anisotropic nanocomposite magnets from Nd-lean alloys and thus is of wide interest.

Effect of synthesis route on the microstructure of SiO2 doped bismuth titanate ceramics

Synthesis and characterization of SiO2 doped bismuth titanate ceramics were investigated. Four investigated compositions belonging to the system Bi2O3-TiO2-SiO2 are located in the section Bi4Ti3O12-SiO2, near by it and in the binary Bi2O3-TiO2 system. Melt quenching was applied for synthesis of SiO2 doped bismuth titanate ceramics and for the sample 40Bi2O3?60TiO2. The binary sample 70Bi2O3?30TiO2 is prepared by gradient heating of bulk materials near the liquidus temperature (modified solid state reaction). The influence of the thermal treatment on the phase formation and microstructure was evaluated using XRD, EDS and SEM. The binary samples prepared by solid state reaction at low temperature (1000?C) possess poly-phased dense microstructure, while secondary crystallization combined with porosity formation is typical for the sample obtained at high temperature (1150?C). The ternary Bi2O3-TiO2-SiO2 samples, obtained by supercooling of the melts down to room temperature, are thermally treated at 700, 800?C. They consist of elongated crystals in amorphous matrix. The crystals have lower Bi2O3-content and higher TiO2-content than the nominal batch composition. The XRD data show that the main crystalline phases in the ceramics produced by melt quenching method and solid state reaction are ?-Bi2O3, Bi4Ti3O12 and one unknown new phase. It is proved that the applied methods of synthesis are suitable for generation of different microstructures in the bulk SiO2 doped bismuth titanate ceramics, which is promising basis for modification of their electrical properties. .

On determination of volume fraction of crystalline phase in partially crystallized amorphous and nanocrystalline materials

The method for determination of volume fraction of crystalline phase in amorphous-crystalline materials is proposed. The method is based on the analysis of X-ray patterns obtained under study of structure-phase changes in nanocrystalline Finemet- type alloys. Verification of the method was carried out with the use of X-ray diffraction data (including small-angle X-ray scattering) of specimens in as-quenched amorphous state as well as after annealing at various temperatures, which provided formation and growth of crystals in amorphous matrix with sizes in a range from 2 to 15 nm. The method is the most effective under nanocrystals' size exceeding 5–6 nm, when their further increase does not affect the height and width of diffraction reflexes.

Influences of ion species (Ge +, Sb +, Au 2+) on crystallization of amorphous Ge films induced by high energy ion irradiation

Amorphous Ge films deposited on CaF 2 substrates were irradiated with Ge, Sb and Au ions and their structural changes were examined. Samples during irradiations were kept at a constant temperature about 380°C since a formation of randomly oriented crystals in amorphous matrix was closely related to temperature. It was found that the crystal quality of the epitaxial Ge films became good and the formation of randomly oriented crystals was retarded with increasing ion mass. These results presumably suggested that the crystal quality was correlated with the amount of randomly oriented crystals. Furthermore, it was assumed that the formation of randomly oriented crystals were affected by the single collision cascade whose size was increased with ion mass. From these results, it was concluded that heavy ion irradiation was effective for the enhancement of the epitaxial crystallization and the suppression of the formation of randomly oriented crystals.