Hump peak formation and the crystallization in amorphous Al87Co10Ce3 alloy

A distinct hump peak was observed at the scattering angle 2θ ≈ 44° in the X-ray diffraction (XRD) pattern of rapidly quenched amorphous Al₈₇Co₁₀Ce₃ alloy. From the XRD patterns of amorphous Al-Co-Ce alloys with different compositions, it is found that the intensity of the hump peak increases as the Co/Ce atomic ratio increases, while the prepeak which characterizes the medium-range order (MRO) becomes weak. The hump peak has a close relationship with the glass forming ability in the Al-Co-Ce system. Furthermore, the crystallization behavior of Al₈₇Co₁₀Ce₃ alloy was investigated by the DSC and XRD methods. Compared with Al₈₇Ni₁₀Ce₃ alloy, Al₈₇Co₁₀Ce₃ alloy did not show a single process of grain growth for fcc-Al particle. The formation of a hump peak is presumed to be associated with the presence of pre-existing nuclei of fcc-Al and Co₂Al₉ in as-quenched amorphous Al₈₇Co₁₀Ce₃ alloy.     

Electrochemical behavior of partially crystallized amorphous Al86Ni9La5 alloys

The Al₈₆Ni₉La₅ amorphous ribbons were annealed at 503 K for different time to obtain partially crystallized alloys with the different volume fractions of α-Al phase, and the effect of the crystallization extent on the electrochemical behavior of the ribbon was investigated in 0.01 M NaCl solution. The results show that the corrosion resistance of the partially crystallized ribbons is higher than that of the as-spun ribbon with the fully amorphous structure, and the corrosion resistance of the partially crystallized alloy is strongly dependent on the volume fraction of α-Al phase. The partially crystallized ribbon containing about 20 vol% α-Al phase exhibits the highest corrosion resistance.     

Enhanced thermal stability of amorphous aluminum alloys through microalloying

Amorphous aluminum alloy powders with compositions of Al₈₅Y₇Fe₈, Al₈₃Y₇Fe₈Ti₂, and Al₇₉Y₇Fe₈Ni₃Ti₂Nd₁ were synthesized with crystallization temperatures of 342 °C, 446 °C, and 457 °C, respectively. In-situ high-temperature synchrotron diffraction results were correlated with differential scanning calorimetry studies to investigate the structural evolution during the crystallization. The results show that, through microalloying, the onset-of-crystallization temperature was increased by 115 °C, and the resulting crystalline products changed from a mixture of fcc-Al and an intermetallic phase in the case of Al₈₅Y₇Fe₈ to only intermetallic phases.     

Thermal stability and crystallization kinetics of sputtered amorphous Si3N4 films

The crystallization of thin silicon nitride (Si₃N₄) films deposited on polycrystalline SiC substrates was investigated by X-ray diffractometry as a function of annealing time. The amorphous Si₃N₄ films were produced by means of reactive r.f. magnetron sputtering. Annealing at temperatures between 1300 and 1700 °C led to the formation of crystalline films composed of -Si₃N₄ and β-Si₃N₄. The fraction of β-Si₃N₄ in the films reaches approximately 40% at temperatures above 1550 °C. Both polymorphic modifications were formed simultaneously during the crystallization process. A transformation of -Si₃N₄ to β-Si₃N₄ could not be observed in the time and temperature range investigated. The crystallization process of amorphous Si₃N₄ can be described according to the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism, assuming a three-dimensional, interface controlled grain growth from pre-existing nuclei. The rate constants show an Arrhenius behaviour with an activation enthalpy of approximately 5.5 eV.     

Nanoindentation behavior of bulk metastable Al65Cu20Ti15 alloy prepared by consolidation of the ball milled powder

Present study concerns assessment of nanomechanical property in the bulk Al₆₅Cu₂₀Ti₁₅ alloy with varying microstructure synthesized by consolidation of mechanically alloyed powder at different temperature. The microstructure after consolidation at room temperature and 500 °C exhibits completely amorphous and nanocrystalline states respectively. Nanoindentation experiments suggest that the maximum strength and hardness values are achieved in the sample sintered at 450 °C. The corresponding microstructure revealed dispersion of nanocrystalline intermetallic phase (<50 nm) in the amorphous matrix.     

Crystallization behavior and resistance change in eutectic Si15Te85 amorphous films

Crystallization process and the corresponding electrical resistance change were investigated in eutectic Si₁₅Te₈₅ amorphous thin films. The Si₁₅Te₈₅ amorphous film showed two-stage crystallization process upon heating. In the first stage, the Si₁₅Te₈₅ amorphous crystallized into Te crystals at 175 °C. In the second stage, the residual amorphous phase crystallized into Si₂Te₃ crystals at above 300 °C accompanying the resistance drop. Before the second crystallization, the electrical resistance once increased in the temperature range of about 250-295 °C. This phenomenon can be explained by considering the formation of amorphous phase with a high electrical resistivity.     

Highly amorphous Fe90Zr10 thin films, and the influence of crystallites on the magnetism

A method for depositing highly amorphous, iron-rich Fe_100 − xZr_x thin films on to room temperature substrates is presented. The method involves co-depositing Fe and Zr on to an amorphous AlZr layer. Experimental proof that the structures are completely amorphous is given by transmission electron microscopy and polarized neutron reflectometry. The reflectometry measurements also give an indication of the impact that Fe crystallite impurities have on the magnetic structure and properties of amorphous FeZr. The results are consistent with previous investigations on bulk samples, which showed that crystalline impurities make the magnetic structure more non-collinear.     

Crystallization of amorphous Si thin films by the reaction of MoO3/Al nanoengineered thermite

In this work, we investigated a new crystallization method for amorphous silicon (a-Si) using a mixture of nano-energetic materials: molybdenum oxide and aluminum (MoO₃/Al). The purpose of using nano-energetic materials is to improve the performance of a-Si films with a self-propagating exothermic reaction over a period of microseconds without any substrate damage. The mixture of MoO₃/Al nanopowders was used for a thermite reaction for crystallization of a-Si thin films.Characterization results showed that a-Si thin films were successfully crystallized to poly-Si as evidenced by a Raman peak near 519 cm^− 1. The crystalline volume fraction of poly-Si after the nanoengineered thermite reaction was about 94.7% and poly-Si grains was uniformly distributed with an average grain size of around 40-50 nm. These results indicate that high quality poly-Si thin films were successfully prepared on the substrate.     

Crystallization process in Ge2Sb2Te5 amorphous films

The aim of this work is to investigate the isokinetic and isothermal amorphous-to-crystalline phase transformation process in Ge₂Sb₂Te₅ ternary alloys. The experiments were carried out using electrical impedance, X-ray diffraction and reflection measurements. The results have shown that, upon annealing, the crystallization process in amorphous Ge₂Sb₂Te₅ films starts with nuclei which were identified as the Ge₁Sb₄Te₇ crystalline phase. As temperature increases (or time of isothermal annealing) these nuclei are transformed into the fcc-Ge₂Sb₂Te₅ phase. In order to establish the mechanism of crystallization for this system, a stochastic lattice model was implemented to analyze nucleation and growth of the two phases involved (i.e., the metastable Ge₁Sb₄Te₇ nuclei followed by the stable fcc-Ge₂Sb₂Te₅). The results of the simulations demonstrate close agreement with experimental results. Furthermore, the crystallization process in amorphous films with the Ge₁Sb₄Te₇ composition shows the existence of only one phase during the whole process and can be described by the classical Johnson-Mehl-Avrami-Kolmogorov model.     

Deposition of amorphous Fe-Zr alloys by magnetron co-sputtering

In the last decades, amorphous metal alloys were under intensive research because of their specific properties. Furthermore, amorphous magnetic metallic alloys, also known as metallic glasses, are important because of their application in electronic industry, information technology, recording media, etc. [Shen J, Kirschner J. Surf. Sci. 2002;500:300-322; Bass J, Pratt Jr WP. Physica 2002;B321:1-8; Dugaev VK, Vygranenko Yu, Vieira M, et. al. Physica 2003;E16:558-562]. Fe_1−xZr_x amorphous films co-condensed by magnetron sputtering were studied. X-ray diffraction methods were applied for studying a glass forming range vs. the composition of the elements. Electrical properties of the samples were measured by so-called four-probe and Van der Pauw methods. The surface morphology was investigated by SEM. The results show that amorphous Fe_1−xZr_x alloys with 1.5 nm crystallites could be synthesized by magnetron co-sputtering on the substrates at room temperature when the alloy's composition was Fe_0.91Zr_0.09 with a very smooth surface. The grain size of alloys decreased increasing the Zr concentration. The resistivity of the thin films of these alloys depends on the crystalline size and structure.     

Effects of contacts on the electrical characterization of amorphous chalcogenide alloys

The comparison of the electrical parameters in amorphous chalcogenide, obtained using three different methods (electrical impedance, d.c. and the four-point resistivity measurements), has shown that impedance measurement is the most appropriate because it is able to separate the contributions of the bulk and contacts. For that reason, impedance measurements provide the real values for the electrical parameters of the material. In addition, the impedance measurements were able to detect the initial stages of the amorphous-to-crystalline phase transition.     

Estimation of Mott parameters in amorphous Se80Te20 and Se80Te10M10 (Cd, In, Sb) alloys

In the present communication, d.c. conductivity of a-Se₈₀Te₂₀ and a-Se₈₀Te₁₀M₁₀ (M = Cd, In, Sb) alloys has been studied in the temperature range 225–311 K in order to identify the conduction mechanism and to analyze the effect of different metallic additives on d.c. conduction in a-Se₈₀Te₂₀ alloy below the room temperature. An analysis of the experimental data confirms that conduction in low temperature region is due to variable range hopping in localized states near the Fermi level. The Mott parameters have been calculated in a-Se₈₀Te₂₀ and a-Se₈₀Te₁₀M₁₀ (M = Cd, In, Sb) alloys. The experimental data is found to fit well with Mott condition of variable range hopping conduction.     

Thermal stability and crystallization of amorphous Si1−xBx, Si1−xPx and Si1−xSbx alloy thin films

The thermal stability and crystallization of the amorphous Si_1−xP_x, Si_1−xB_x and Si_1−xSb_x alloy systems have been studied by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). It was found that the amorphous Si-P and Si-B alloys have a high thermal stability and crystallize at temperatures as high as 1150°C, whereas amorphous Si-Sb alloys are unstable and crystallize already at 350°C. The thermal stability is explained by strong Si-P and Si-B bonds, whilst the instability is explained by the weakening of Si-Si bonds by the presence of Sb atoms (< 17 at.% Sb) and by weak Si-Sb and Sb-Sb bonds (> 17 at.% Sb). The thermal stability and instability of the amorphous alloys were correctly predicted from calculated Gibbs free energy diagrams (GFED). The calculated GFED were also used together with the observed crystallization temperatures to successfully predict the first crystalline phase to form in the Si-P and Si-B systems. The first crystalline phases that formed were Si (< 40 at.% B and < 30 at.% P), a newly reported phosphide, Si₁₂P₅ (30 at.% P), and SiP (> 30 at.% P).     

Crystallization kinetics and martensitic transformation of Ti49Ni46.5Ce4.5 alloy thin film

The Ti₄₉Ni_46.5Ce_4.5 alloy thin film was prepared by direct current (DC) magnetron sputtering system for the first time. Crystallization kinetics, phase composition and the behaviors of martensitic transformation were studied. The results by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) demonstrated that the primary second phase of TiNiCe alloy thin films was Ce₂Ni₇ phase, apparent activation energy was determined to be 510 kJ/mol at the continuous heating process, Avrami exponents for different isothermal temperature were in the range of 1.1-1.88 between 713 and 730 K, one-step martensitic transformation was observed in the crystallized Ti₄₉Ni_46.5Ce_4.5 alloy thin films. The influence of thermal process on martensitic transformation temperature was investigated with non-isothermal and isothermal crystallization. The reason behind the transformation temperature change was also discussed.     

Medium-term thermal stability of amorphous Ge2Sb2Te5 flash-evaporated thin films with regards to change in structure and optical properties

The stability of flash-evaporated amorphous Ge₂Sb₂Te₅ thin films has been studied under medium-term temperature treatment (30 - 80 °C, with a step of 10 °C) in ten subsequent heating and cooling cycles. The significant changes in structure and optical properties are reported. The temperature cycling of the films resulted in formation of an isolated 5 - 7 nm nano-crystalline phase in the amorphous phase. The corresponding increase in refractive index and change in optical bandgap energy and sheet resistance are also presented. The formation of Ge₂Sb₂Te₅ nano-crystals (~ 5 - 7 nm) even under temperature below 80 °C could contribute to the explanation of mechanism of resistivity fluctuation (drift) of the “amorphous phase” films. We also show that the optical and electrical properties of flash evaporated Ge₂Sb₂Te₅ thin films are very similar to those reported for sputtered films.     

Studies on crystallization behaviour of 3Y-TZP/Al2O3 composite powders

The present work deals with the characterization of 3.0 mol.% yttria stabilized zirconia (3Y-TZP) powders prepared as composite precursors containing up to 20.0 vol.% of alumina. Following the wet-chemistry route of co-precipitation, a homogeneous dispersion of component hydroxides was obtained. Effect of calcination treatment on t-phase and primary particle size evolution of zirconia was investigated by XRD technique. A marked difference with respect to inhibition of crystallization reaction of 3Y-TZP was observed in the presence of alumina. This is attributed to the increased diffusion lengths of the two developing phases. The powders could be retained in the ultrafine and quasi-amorphous state by a suitable choice of alumina content and calcination treatment.     

Crystallization and Hardness of Melt Spun Fe73Si13B9Nb4Cu1 Alloy

An alloy having composition Fe₇₃Si₁₃B₉Nb₄Cu₁ was synthesized by melt spinning to investigate the kinetics of crystallization. Techniques of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Mössbauer spectroscopy were employed to characterize the phases produced due to annealing at various temperatures. High temperature DSC revealed two stage crystallization reactions. First stage, crystallization occurs at temperature around 514°C with the production of α-Fe (bcc) and Fe₃Si phases. In the second stage, Fe₂B and α-Fe (Si,Nb) phases were produced. Mössbauer results revealed the formation of Fe₃Si, Fe₁₃Si₃ and Fe₇Si₁ in the first stage and Fe₃Si, Fe₁₃Si₃, Fe₂B and α-Fe (Si,Nb) phases in the second stage of crystallization. An abrupt change in average internal magnetic field was observed at 500°C. The maximum hardness value was found for the sample heat-treated at 500°C.     

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs.

The kinetics of formation of Al₂O₃ in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol⁻¹. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.     

Nanocrystallization in amorphous Al83Ni10La7 and Al83.5Ni9.5La5.6 Si1.4 alloys during thermal annealing and flash lamp processing

Nanocrystallization in amorphous Al₈₃Ni₁₀La₇ and Al_83.5Ni_9.5La_5.6Si_1.4 alloys during heat treatment and flash lamp processing has been studied using X-ray diffraction, transmission electron microscopy, scanning calorimetry, and internal friction, modulus of elasticity, and microhardness measurements. The results demonstrate that thermal annealing and flash lamp processing have identical effects on the formation of a nanocrystalline structure in the amorphous alloys and their phase transformations. Original Russian Text ? O.K. Belousov, V.V. Vavilova, V.M. Ievlev, Yu.E. Kalinin, S.B. Kushchev, N.A. Palii, S.A. Pokazan’eva, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 8, pp. 941–948.     

Electronic and atomic structure of Ge2Sb2Te5 phase change memory material

Electronic structure calculations are presented for various model structures of the crystalline and amorphous phases of Ge₂Sb₂Te₅. The structures are all found to possess a band gap of order 0.5 eV, indicating closed shell behaviour. It is pointed out that structural vacancies in A7-like Ge₂Sb₂Te₅ are not electronically active. In addition, A7-like structures do not support valence alternation pair defects, which are one model of the conduction processes in the amorphous phase in non-volatile memories.