The migration behavior of atomic clusters in early nanocrystalline process of soft magnetic Finemet alloy

The Finemet alloys are commonly used as cores in transformers and generators, stress and field sensors in technological application for their excellent soft magnetic characteristics. To clarify the nanocrystallization mechanism of Finemet especially about the atomic migration in early stage is very essential for developing their distinctive characteristics. In this study, we investigate the migration behavior of atoms in order to clarify the mechanism of the early-stage nanocrystallization in amorphous Finemet alloys. The Fe(73.5)Si(13.5)B9Nb3Cu1 amorphous ribbons were prepared by single-roller melt-spinning process in argon atmosphere, and then annealed at 350 degrees C-400 degrees C for 10 minutes in vacuum. The atom force microscope (AFM) and the coincidence Doppler broadening spectra (CDB) were used to characterize the migration behavior of different atoms in Fe(73.5)Si(13.5)B9Nb3Cu1 amorphous alloy during the early-stage nanocrystallization. The X-ray diffraction (XRD) patterns show that all annealed samples are in the amorphous state. But the AFM observation shows clearly that there are many small atomic clusters (nuclei) which distribute in the amorphous matrix of the annealed samples. With increasing annealing temperature, there is a significant increase in the amount of atomic clusters and a dramatic drop in the average size of clusters with very limited Cu contention in the samples, which reflect the structural evolution into more homogeneity. The CDB spectrum indicates that the peaks of positron annihilation spectrum are gradually reduced, which means the number of grain boundary and the defects in samples are gradually increased. It can be concluded that more defects are introduced by the formation of atomic clusters through atomic migration during the early-stage nanocrystallization in Fe(73.5)Si(13.5)B9Nb3Cu1 amorphous alloys.     

Positron annihilation study on repeated deformation/precipitation aging in Fe–Cu–B–N alloys

We have investigated the influence of deformation-induced defects on the isothermal Cu precipitation at 550 °C in an as-quenched (solute supersaturated) and annealed (solute depleted) Fe–Cu–B–N alloy by positron annihilation spectroscopy. Preferred precipitation of supersaturated Cu at defects sites was recently proposed as a self-healing mechanism for deformation damage. Repeated deformation/aging cycles result in a significant increase in tensile strength for as-quenched samples. Positron Annihilation Lifetime Spectroscopy revealed a larger positron lifetime for the annealed samples compared to the as-quenched samples caused by the additional open-volume defects at incoherent interfaces between Cu precipitates and the matrix. For both as-quenched and annealed samples deformation generates new defects (mainly dislocations), while subsequent aging leads to a reduction in the density of these defects. Simultaneous Coincidence Doppler Broadening measurements monitored the evolution of local environment at the positron annihilation sites (defects, Cu precipitates and matrix) during the repeated deformation/aging cycles. For the as-quenched samples, aging after deformation results in a sharp decrease in the defect contribution combined with a strong increase in the copper signature, which can be attributed to preferential copper precipitation at the defects introduced by plastic deformation. For the annealed samples, aging after deformation leads to a much weaker evolution of the Cu signature.     

Effect of sintered grain growth on chemical ordering in binary FePt/Cu nanoparticle arrays

Recent studies have shown a strong correlation between grain growth and chemical ordering in chemically synthesized FePt nanoparticles. In order to study this effect, we have prepared a series of samples in which 3.5 nm FePt nanoparticles are dispersed in a matrix of Cu nanoparticles. The samples were annealed at 600 degrees C and at 800 degrees C. Grain size was determined by XRD Scherrer analysis and time-dependent remanent coercivity measurements were made to determine the intrinsic remanent coercivity, Hcr0. For samples annealed at 600 degrees C, Hcr0 increases strongly with grain size up to approximately 5 nm and increases weakly with additional grain growth. By contrast, after annealing at 800 degrees C, Hcr0 appears nearly independent of grain size. The results suggest that isolated 3.5 nm FePt nanoparticles can be weakly ordered when annealed at 600 degrees C and sintering is necessary for significant chemical ordering.     

FeTaC magnetic soft underlayer for L1(0) FePt based perpendicular recording media

FeTaC magnetic soft underlayer under elevated temperature process conditions for L1(0) FePt based perpendicular recording media has been investigated. After annealing FeTaC for 40 min at 350 degrees C, saturation moment increases to 750 emu/cm3 and, coercivity and remanent moment reduce to 2.3 Oe and 166 emu/cm3 respectively. The microstructure of FeTaC annealed at 350 degrees C for 40 min composes of Fe nanocrystals with random orientations immersed in an amorphous matrix. FeTaC surface roughness due to elevated temperature process is reduced by 100 W RF plasma etching and CrRu with (200) orientation is developed. It is found that changing elemental composition due to C diffusion into the CrRu layer and RF preferential etching over Fe, Ta and C has the influence on the magnetic properties of FeTaC.     

Photocatalytic activity of Ag-TiO2 nanoparticles encapsulated with titanium oxide layer

The deposition of nanosized Ag on the TiO2 matrix was first prepared by the sonochemical reduction of silver precursor and the Ag-deposited TiO2 was subsequently encapsulated with titanium oxide layer through an ultrasonic-assisted sol-gel process. The resulting Ag-TiO2 nanocomposites (Ag-TiO2@TiOx) exhibited the lower PL spectra and higher photocatalytic activity as compared to pristine TiO2, indicating the increased separation efficiency of electron-hole pairs at the optimal thickness of TiOx. The excessive encapsulation of TiOx, however, induced the decrease of photocatalytic activity of Ag-TiO2@TiOx owing to the formation of amorphous titanium oxide with less photocatalytic activity. The annealed Ag-TiO2@TiOx at 500 degrees C for 2 hrs exhibited an enhanced photocatalytic activity as compared to non-annealed samples because of the partial transformation of amorphous TiOx into crystalline TiO2.     

Thermal annealing dependence of high-frequency magnetoimpedance in amorphous and nanocrystalline FeSiBCuNb ribbons

The magnetoimpedance (MI) effect in Fe73.5Si13.5B9Nb3Cu1 melt-spun amorphous ribbons has been studied in the frequency range (1-500 MHz). Isothermal heating treatments in a furnace have been employed to nanocrystallize the ribbons (1 h at 565 degrees C in a vacuum of 10(-3) mbar), while other samples were annealed at lower temperatures (400 and 475 degrees C during 1 h), in order to evaluate the influence of the annealing temperature on the MI effect. The high-frequency impedance was measured using a technique based on the reflection coefficient measurements of a specific transmission line by using a network analyzer. Frequency dependence of the MI ratio, DeltaZ/Z, and both resistive, DeltaR/R, and reactive, DeltaX/X, components of magnetoimpedance were measured in the amorphous and annealed states, at different temperatures. A maximum value of the MI ratio of about 50% at a driving frequency of 18 MHz is obtained in the nanocrystalline (annealed at 565 degrees C) ribbon. Maxima for DeltaR/R of about 81% at 85 MHz and DeltaX/X around 140% at 5 MHz were also achieved. It is revealed that the microstructural evolution in the nanocrystalline sample leads to a magnetic softening, an optimum domain structure and a permeability which is sensitive to frequency and applied magnetic field, generating a large MI response.     

The effect of hydrogen in the mechanism of aluminum-induced crystallization of sputtered amorphous silicon using scanning Auger microanalysis

The metal-induced crystallization (MIC) of hydrogenated sputtered amorphous silicon (a-Si:H) using aluminum has been investigated using X-ray diffraction (XRD) and scanning Auger microanalysis (SAM). Hydrogenated, as well as non-hydrogenated, amorphous silicon (a-Si) films were sputtered on glass substrates, then capped with a thin layer of Al. Following the depositions, the samples were annealed in the temperature range 200 °C to 400 °C for varying periods of time. Crystallization of the samples was confirmed by XRD. Non-hydrogenated films started to crystallize at 350 °C. On the other hand, crystallization of the samples with the highest hydrogen (H₂) content initiated at 225 °C. Thus, the crystallization temperature is affected by the H₂ content of the a-Si. Material structure following annealing was confirmed by SAM. In this paper, a comprehensive model for MIC of a-Si is developed based on these experimental results.     

Stress Evolution With Temperature in Titanium‐Rich NiTi Shape Memory Alloy Films

Abstract: The effect of deposition temperature on residual stress evolution with temperature in Ti‐rich NiTi films deposited on silicon substrates was studied. Ti‐rich NiTi films were deposited on 3″ Si (100) substrates by DC magnetron sputtering at three deposition temperatures (300, 350 and 400 °C) with subsequent annealing in vacuum at their respective deposition temperatures for 4 h. The initial value of residual stress was found to be the highest for the film deposited and annealed at 400 °C and the lowest for the film deposited and annealed at 300 °C. All the three films were found to be amorphous in the as‐deposited and annealed conditions. The nature of the stress response with temperature on heating in the first cycle (room temperature to 450 °C) was similar for all three films although the spike in tensile stress, which occurs at ～330 °C, was significantly higher in the film deposited and annealed at 300 °C. All the films were also found to undergo partial crystallisation on heating up to 450 °C and this resulted in decrease in the stress values around 55–60 °C in the cooling cycle. The stress response with temperature in the second thermal cycle (room temperature to 450 °C and back), which is reflective of the intrinsic film behaviour, was found to be similar in all cases and the elastic modulus determined from the stress response was also more or less identical. The three deposition temperatures were also not found to have a significant effect on the transformation characteristics of these films such as transformation start and finish temperatures, recovery stress and hysteresis.     

Thermal stability of nanocrystalline diamond films grown by biased enhanced microwave plasma chemical vapor deposition

The thermal stability of nanocrystalline diamond (NCD) films grown on mirror-polished silicon substrates by biased enhanced microwave plasma chemical vapor deposition was investigated. Different pieces of a NCD sample were annealed for 1 h in an ambient argon atmosphere at 200, 400, 600, and 800 degrees C. The structural and mechanical properties of as-grown and annealed samples were assessed. The surface roughness and high hardness of the samples remained fairly constant with annealing temperature.     

Polycrystalline silicon films on glass grown by amorphous-liquid-crystalline transition at temperatures below 330 °C

An approach to deposit polycrystalline silicon layers on amorphous substrates is presented. It is shown that metastable amorphous silicon can be transformed into its more stable microcrystalline structure at a temperature below 330 °C via an intermediate liquid solution stage. In particular, the interaction of liquid indium nanodroplets in contact with amorphous silicon is shown to lead to the formation of circular polycrystalline domains. Crystallinity of these domains is analyzed by micro-Raman spectroscopy. The droplet size necessary for the onset of crystallization is related to the temperature of the film. Full coverage of the substrate with microcrystalline silicon has been obtained at 320 °C within less than one hour. These films might act as seeding layers for further enlargement by steady-state liquid phase epitaxy.     

Concurrent multiscale modeling of amorphous materials in 3D

A general three‐dimensional concurrent multiscale modeling approach is developed for amorphous materials. The material is first constructed as a tessellation of hexahedral amorphous cells. For regions of linear deformation, the number of degrees of freedom is reduced by computing the displacements of the vertices of the amorphous cells only instead of the atoms within. This is achieved by determining, a priori, the atom displacements within such pseudoamorphous cells associated with orthogonal deformation modes of the cell. Actual atom displacements are calculated using traditional molecular mechanics for regions of nonlinear deformation. Computational implementation of the coupling between pseudoamorphous cells and molecular mechanics regions and stiffness matrix formulation are elucidated. Multiscale simulations of nanoindentation on polymer and crystalline substrates show good agreement with pure molecular mechanics simulations. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of thermal annealing in oxygen plasma for buffer layers on properties of ZnO thin films

ZnO thin films with ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si(100) substrates. Before the growth of the ZnO thin films, the ZnO buffer layers were deposited on the Si substrates for 20 minutes and then annealed at the different substrate temperature ranging from 600 to 800 degrees C in oxygen plasma. The structural and optical properties of the ZnO thin films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and room-temperature (RT) photoluminescence (PL). A narrower full width at half maximum (FWHM) of the XRD spectra for ZnO(002) and a larger grain are observed in the samples with the thermal annealed buffer layers in oxygen plasma, compared to those of the as-grown sample. The surface morphology of the samples is changed from rugged to flat surface. In the PL spectra, near-band edge emission (NBEE) at 3.2 eV (380 nm) and deep-level emission (DLE) around 1.77 to 2.75 eV (700 to 450 nm) are observed. By increasing the annealing temperatures up to 800 degrees C, the PL intensity of the NBEE peak is higher than that of the as-grown sample. These results imply that the structural and optical properties of ZnO thin films are improved by the annealing process.     

Fabrication of periodic nickel silicide nanodot arrays using nanosphere lithography

The interfacial reactions of the 2D-ordered nickel metal nanodots that were prepared by polystyrene nanosphere lithography (NSL) on Si substrates after different heat treatments have been investigated. Epitaxial NiSi₂ nanodot arrays were found to form at a temperature as low as 350 °C. The results indicated that the growth of epitaxial NiSi₂ is more favorable for the Ni metal dot array samples. The sizes of these epitaxial NiSi₂ nanodots in samples annealed at 350–800 °C are in the range of 84–110 nm. The shape of the epitaxial NiSi₂ nanodot was found to be pyramidal. Furthermore, for the samples annealed at 900 °C, amorphous SiO_x nanowires were found to grow on individual nickel silicide nanoparticles. The diameters of these nanowires are in the range of 15–20 nm. As the size of metal nanodot can be adjusted by tuning the diameter of the polystyrene (PS) spheres, the NSL technique promises to be an effective patterning method without complex lithography.     

Influence of oxygen in the deposition and annealing atmosphere on the characteristics of ITO thin films prepared by sputtering at room temperature

Indium tin oxide (ITO) thin films have been grown onto glass substrates by sputtering at room temperature with various controlled oxygen and argon ratios used as reactive and sputter gases, respectively. After deposition, the samples have been annealed at 350 °C in the same sputtering chamber in vacuum or in air. The structure, morphology and electro-optical characteristics of the ITO coatings have been analyzed as a function of the oxygen added during deposition and of the annealing atmosphere by X-ray diffraction, atomic force microscopy, four points electrical measurements and spectrophotometry. It has been found that the as-deposited amorphous samples crystallize in the cubic structure by annealing. The optical transmittance and the electrical resistance decrease when the oxygen content in the deposition and the annealing atmosphere is reduced.     

Coating of LaCoO3 thin film with sol-gel dip coating method

LaCoO3 thin film was coated on Al2O3 single crystal by sol-gel route. Appropriate composition of precursors, chelating agents and the solvent put together into a flask and magnetically stirred on a magnetic stirrer. After having the red transparent solution, it was stirred for 12 hours before coating. Ultrasonically cleaned substrate is dipped into the solution and taken immediately into vertical furnace which is preheated at 550 degrees C. A dense amorphous film is coated on the substrate. Fired amorphous films are annealed at temperature between 900 degrees C and 1000 degrees C for 20 minutes in the air. Then coated film was characterized by means of XRD, AFM, and SEM. Conductivity of the film was measured to be -0.1819 for 881 degrees C for the log sigma value by assuming the thickness as     

Thin transparent W-doped indium-zinc oxide (WIZO) layer on glass

Annealing effect on structural and electrical properties of W-doped IZO (WIZO) films for thin film transistors (TFT) was studied under different process conditions. Thin WIZO films were deposited on glass substrates by RF magnetron co-sputtering technique using indium zinc oxide (10 wt.% ZnO-doped In2O3) and WO3 targets in room temperature. The post annealing temperature was executed from 200 degrees C to 500 degrees C under various O2/Ar ratios. We could not find any big difference from the surface observation of as grown films while it was found that the carrier density and sheet resistance of WIZO films were controlled by O2/Ar ratio and post annealing temperature. Furthermore, the crystallinity of WIZO film was changed as annealing temperature increased, resulting in amorphous structure at the annealing temperature of 200 degrees C, while clear In2O3 peak was observed for the annealed over 300 degrees C. The transmittance of as-grown films over 89% in visible range was obtained. As an active channel layer for TFT, it was found that the variation of resistivity, carrier density and mobility concentration of WIZO film decreased by annealing process.     

Structural and Mössbauer effect investigations of amorphous FeCuNbSiB alloys nanocrystallized (VITROPERM) in magnetic field

Melt-spun amorphous ribbons of nominal composition Fe73Cu1Nb3Si16B7, annealed at 560-580 degrees C for 1 hour in a magnetic field (H) applied along the width in the ribbon plane, develop uniaxial magnetic anisotropy with easy axis along H and exhibit several novel attributes. The samples labelled as S20 and S150 are nanocomposites consisting of ferromagnetic nanocrystalline grains (volume fraction approximately equal to 84% and 81%) of mean size d = 13(2) nm embedded in a ferromagnetic amorphous matrix and possess a magnetic permeability as large as 20,000 and 150,000, respectively. While nearly 55% of the nanocrystalline grains have a cubic DO3 Fe3Si-like structure with actual Si concentration of about 22 at.%, the remaining 45% nanocrystalline grains have tetragonal Fe3B and hexagonal Fe2Si structure. Since the crystalline volume fraction of Fe3B and Fe2Si nanocrystals is more in the sample S20, this sample exhibits stronger local magnetic anisotropy and hence lower permeability.     

Study of surface morphology and optical properties of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films with annealing

Nb₂O₅ films have been deposited on variety of substrates using the sol-gel dip coating technique. As-deposited films on all substrates are amorphous. Films were annealed under controlled ambience at 300, 400 and 600°C for 5 h. As-deposited films on glass substrate show uniform surface structure. The crystal structure and surface topography are found to depend strongly on the annealing temperature and nature of the substrates. The average grain size of 40 nm is observed in films annealed at 300°C. On annealing at 400°C increasing grain size and resulting fusing of them, enhanced surface roughness. Films deposited on NaCl substrates crystallized into a stable monoclinic phase and those deposited on single crystal Si substrates crystallized into hexagonal phase after annealing at 600°C. The as-deposited films show very high transmittance (>90%) in the visible region. The optical band gap is observed to increase from 4.35 eV when the films are in amorphous state to 4.87 eV on crystallization.     

Growth and vacuum post-annealing effect on the properties of the new absorber CuSbS2 thin films

Post-growth treatments in vacuum atmosphere were performed on CuSbS₂ films prepared by the single-source thermal evaporation method on glass substrates. The films were annealed in vacuum atmosphere for 2 h in temperature range 130-200 °C. The effect of this thermal treatment on the structural, optical and electrical properties of the films was studied. X-ray diffraction (XRD) patterns indicated that the films exhibited an amorphous structure for annealing temperature below 200 °C and a polycrystalline structure with CuSbS₂ principal phase. For the films annealed at temperatures below 200 °C one direct optical transition in range 1.8-2 eV was found. For the films annealed at 200 °C, two optical direct transitions emerged at 1.3 and 1.79 eV corresponding to the CuSbS₂ and Sb₂S₃ values respectively. The electrical measurements showed a conversion from low resistivities (3.10^− 2-9.10^− 2) Ω cm for the samples annealed at temperatures below 200 °C to relatively high resistivities (2 Ω cm) for the samples annealed at 200 °C. In all cases the samples exhibited p-Ztype conductivity.     

Characterization of ultrasonic spray pyrolyzed tungsten oxide thin films

The ultrasonic spray pyrolysis (USP) technique has been employed to deposit tungsten oxide (WO₃) thin films. The films were prepared by spraying 0.02 M ammonium metatungstate solution onto amorphous glass substrates kept at 250°C. These films were further annealed at 400°C for different time periods (1–5 h) in air. The films were characterized for structural, electrical and opto-electronic properties. X-ray diffraction technique was used to determine the crystallinity of the WO₃ films and identify the phases that form as a function of annealing time. The as-prepared WO₃ films were amorphous and crystallize when annealed at 400°C in air for 2 or more hours. From TEM, the grain size and lattice plane spacing are estimated. The films were further characterized by using time resolved microwave conductivity (TRMC) technique and decay time of the photogenerated charge carriers is calculated to be about 154 ns. The concentration and mobility of charge carriers are estimated from thermoelectric power (TEP) measurements.