Formation Of Nanocrystalline Phase Research Articles

Microstructure and Electrochemical Performance of CeMg12/Ni/TiF3 Composites for Hydrogen Storage

Ball milling was used to prepare CeMg12/Ni/TiF3 composite alloys in this study. Microstructures of experimental alloys were analyzed by scanning electron microcopy (SEM), x-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) with electron diffraction (ED). The electrochemical and kinetic characteristics of the ball-milled composite alloys were further evaluated based on their galvanostatic charge–discharge measurement, high-rate dischargeability (HRD), hydrogen diffusion behavior, as well as electrochemical impedance spectrum (EIS). Results indicated that the ball-milled alloy with 3 wt.% TiF3 exhibited the best electrochemical discharge capacity, which was not only related to the formation of amorphous and/or nanocrystalline phase but to the formation of MgF2 because it can effectively reduce the thermodynamic stability of hydride. With increasing TiF3, the cycle degradation rates of the milled alloys were ameliorated remarkably. This improvement was attributed to the formation of an amorphous phase (which possesses strong anticorrosive and antioxidation abilities) as well as the formation of MgF2 and TiNi secondary phases. The ball-milled alloy with 3 wt.% TiF3 additive exhibited the strongest electrochemical kinetic properties, related to the highest hydrogen diffusion rate in the alloy (which was associated with multiple defects and grain boundary of amorphous and/or nanocrystalline phases) and the electrochemical reaction on the surface of alloy (which was related to the lowest apparent activation energy).

Enhanced upconverted luminescence and the optical thermometry behavior of Er3+-doped BaYbF5 transparent glass ceramics

Transparent SiO2 - Al2O3 - Na2O - CaO - BaF2 - YbF3 glass ceramics (GC) doped with Er3+ ions were successfully fabricated by a melt-quenching technique with subsequent heat treatment. The formation of BaYbF5 nano-crystalline phase was confirmed by X-ray diffraction and transmission electron microscopy. Compared to the precursor glass (PG), the clearer Stark splitting and greatly enhanced up-conversion (UC) emission in GC indicate that Er3+ ions mainly enter into BaYbF5 nanocrystals with low phonon energy after crystallization. The temperature dependent on purple UC emission ratio (which is due to the Er3+ 4G11/2→4I15/2 and 2H9/2→4I15/2 transitions) and common green UC emission ratio with low-power excitation in BaYbF5 GC have been studied respectively. In addition, the UC mechanisms in PG and GC are illustrated and analyzed. The outstanding properties of Er3+-doped BaYbF5 transparent GC may present potential applications in all-solid-state UC lasers and optical fiber temperature sensors.

Swift Heavy Ion irradiation induced nanocrystallisation in Te/Cd/Te trilayer thin films

In this paper, we report Swift Heavy Ion (SHI) irradiation induced nanocrystallisation in Te/Cd/Te trilayer thin films. In the present investigation, thermally evaporated Te/Cd/Te trilayer thin films were irradiated by 100MeV 107Ag ion beam at varying fluence. The structural characterization of Te/Cd/Te trilayer thin films was carried out using X-ray diffraction (XRD) technique. XRD studies revealed that SHI irradiation results in Te/Cd/Te layer mixing leading to formation of CdTe nanocrystalline phase. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were employed for surface studies. SEM studies indicate ion beam induced surface modifications. Further, AFM pictures indicate nanograin formation in the irradiated films. The elemental analysis of trilayer films before and after irradiation was carried out using Energy Dispersive X-ray Spectroscopy (EDX). EDX analysis indicates that percentage ratio of Cd and Te remains unaffected after SHI irradiation. Optical characterization was done using UV–Vis spectroscopy. Band gap values determined on the basis of UV–Vis spectroscopy of all the irradiated films were found to be more than the reported bulk value of 1.5eV. The increase in bandgap value is attributed to the combined effect of ion induced strain and quantum confinement effect. SHI irradiation induced formation of nanocrystallisation in Te/Cd/Te trilayer films is explained mainly in the framework of Ion Beam Mixing (IBM).

Effect of substrate temperature on the morphological, structural, and optical properties of RF sputtered Ge1−xSnx films on Si substrate**Project supported by the Universiti Sains Malaysia (Grant No. 1001/PFIZIK/846072).

In this study, Ge1−xSnx alloy films are co-sputtered on Si(100) substrates using RF magnetron sputtering at different substrate temperatures. Scanning electron micrographs, atomic force microscopy (AFM), Raman spectroscopy, and x-ray photoemission spectroscopy (XPS) are conducted to investigate the effect of substrate temperature on the structural and optical properties of grown GeSn alloy films. AFM results show that RMS surface roughness of the films increases from 1.02 to 2.30 nm when raising the substrate temperature. This increase could be due to Sn surface segregation that occurs when raising the substrate temperature. Raman spectra exhibits the lowest FWHM value and highest phonon intensity for a film sputtered at 140 °C. The spectra show that decreasing the deposition temperature to 140 °C improves the crystalline quality of the alloy films and increases nanocrystalline phase formation. The results of Raman spectra and XPS confirm Ge–Sn bond formation. The optoelectronic characteristics of fabricated metal-semiconductor-metal photodetectors on sputtered samples at room temperature (RT) and 140 °C are studied in the dark and under illumination. The sample sputtered at 140 °C performs better than the RT sputtered sample.

Formation of nanocrystalline and amorphization phase of Fe–Dy2O3 powder mixtures induced by ball milling

Abstract

Erosion-wear and intergranular corrosion resistance properties of AISI 304L austenitic stainless steel after low-temperature plasma nitriding

In this paper, AISI 304L austenitic stainless steel has been subjected to plasma nitriding at 673 and 681 K lasted for 6, 18, 24, and 36 h. The morphology, microhardness, phase, and microstructure evolution of the nitrided layer were studied. Not only an expanded austenitic phase (γN) but also Cr-nitrides were formed in the nitrided layer. Transmission Electron Microscopy (TEM) study showed the formation of amorphous and nanocrystalline phases in the surface layers of the samples. The erosion-wear and intergranular corrosion (IGC) resistance behaviors of the nitrided samples were studied in detail. As compared to the non-nitrided sample, the erosion-wear resistance of the nitrided layer at normal impact angle doubles after nitriding at 673 K for 6 h, as a result of the formation of γN phase. Degree of sensitization (DOS) of IGC is gradually enhanced with the increase of nitriding time as indicated by a double loop electrochemical potentiokinetic reactivation (DL-EPR) measurement. This is assigned to the nanostructuring and to the Cr-depletion in the areas adjacent to the Cr-nitrides.

Amorphization and nanocrystallization of Ni–Nb-Si Alloys

New Ni–Nb-Si amorphous alloys with enhanced thermal stability and toughness were developed, and the effect of Si on the amorphization, nanocrystallization, and mechanical properties of these alloys were examined. The mechanical properties and thermal stability of Ni60Nb40, Ni57.5Nb37.5Si5, and Ni55Nb35Si10 amorphous alloys were improved as the Si content was increased. Nanocrystallization of the Ni55Nb35Si10 amorphous alloy led to the formation of intermetallic crystals with sizes below 10nm in the amorphous matrix. Formation of Nb7Ni6, NbNi3, and Nb3Ni2Si nano-crystalline phases significantly increased the hardness and the elastic modulus of amorphous/nano-crystalline alloys. The toughness of the alloys were observed to increase with the addition of Si in the amorphous and amorphous/nano-crystalline state.

The Effect of Ag Addition on the Enhancement of the Thermal and Mechanical Properties of CuZrAl Bulk Metallic Glasses

In this study, the thermal and mechanical properties of Cu50−xZr43Al7Agx (x = 0, 3, 4, 5, 6) bulk metallic glasses (BMGs) are investigated by using an X-ray diffractometer (XRD), a differential scanning calorimeter (DSC), differential thermal analysis (DTA), a Vickers hardness tester, a material test system (MTS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cu50−xZr43Al7Agx (x = 0, 3, 4, 5, 6) BMGs were made by arc-melting and an injection casting process. The results revealed that the glass transition temperature (Tg) and the crystallization temperature (Tx) of CuZrAl alloy decreased with the Ag addition. Hence, the supercooled liquid region and γ of Cu45Zr43Al7Ag5 alloy increased to 76 K and 0.42, respectively. The thermal stability and glass forming ability of CuZrAlAg BMG alloys were enhanced by the microalloyed Ag content. The room temperature compressive fracture strength and strain measured of Cu47Zr43Al7Ag3 were about 2200 MPa and 2.1%, respectively. The distribution of vein patterns and the formation of nanocrystalline phases on the fracture surface of Cu47Zr43Al7Ag3 alloy can be observed by SEM and TEM to be significant, indicating a typical ductile fracture behavior and an improved plasticity of alloys with the addition of microalloyed Ag from 0 to 6 atom %.

Formation of nanocrystalline GeSn thin film on Si substrate by sputtering and rapid thermal annealing

Nanocrystalline Ge1-xSnx thin films have been formed after rapid thermal annealing of sputtered GeSn layers. The alloy films were deposited onto the Silicon (100) substrate via low cost radio frequency magnetron sputtering. Then, the films were annealed by rapid thermal annealing at 350 °C, 400 °C, and 450 °C for 10 s. The morphological, structural, and optical properties of the layers were investigated with field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, and high-resolution X-ray diffraction (HR-XRD). The Raman analysis showed that the only observed phonon mode is attributed to Ge-Ge vibrations. Raman phonon intensities of GeSn thin films were enhanced with increasing the annealing temperature. The results clearly revealed that by increasing the annealing temperature the crystalline quality of the films were improved. The XRD measurements revealed the nanocrystalline phase formation in the annealed films with (111) preferred orientation. The results showed the potentiality of using the sputtering technique and rapid thermal anneal to produce crystalline GeSn layer.

Effect of fluoride ions on the optical properties of Eu3+:PbF2 nanocrystals embedded into sol–gel host materials

In this research, the effects of fluoride ions concentration on luminescence properties of Eu3+:PbF2 nanocrystals in silicate sol–gel materials have been investigated. Optical and structural properties of Eu3+:PbF2 nanocrystals have been examined using X-ray diffraction analysis (XRD) and luminescence spectroscopy. Formation of the cubic β-PbF2 nanocrystalline phase was confirmed. The crystal size was estimated using Scherrer's equation and Williamson-Hall formula. The optical behavior of Eu3+ ions in studied materials is strongly dependent on amount of incorporated fluorides in the host lattice and the optimal concentration about 3.0 mass. % give the more intense emission of Eu3+. The luminescence spectra showed the characteristic orange-red emission due to the 5D0 → 7F1 and 5D0 → 7F2 electron transitions, respectively. The luminescence intensity ratio R (I(5D0 → 7F2)/I(5D0 → 7F1)) and luminescence lifetimes for the 5D0 excited state of Eu3+ ions were determined for each sample before and after heat treatment process. Obtained results indicate the partially incorporation of Eu3+ ions into β-PbF2 nanocrystalline phase during ceramization process.

Development of Mo‐Ni‐Si‐B metallic glass with high thermal stability and H versus E ratios

We report a novel Mo‐Ni‐Si‐B metallic glass which can be solidified into fully amorphous state by melt-spinning process, with high crystallization onset temperature of over 1100K, extremely high Vickers hardness of 27.5±2.2GPa and relatively low Young's modulus of 364.3±6.6GPa. The dense cluster-packing model suggests that the addition of boron up to 10at.% can occupy vacant cluster-interstices of (Mo, Ni)-Si cluster arrays, which results in a more efficiently dense-packed cluster structure, destabilizes the formation of nanocrystalline phases, and systematically increases the glass-forming ability (GFA) in Mo‐Ni‐Si‐B alloys. The GFA parameters that do not directly rely on Tg, such as ΔT* and ε parameter, show greater reliability to evaluate GFA for Mo‐Ni‐Si‐B metallic glass exhibiting no clear Tg. The H/E and H2/(2E) ratios of the newly developed Mo‐Ni‐Si‐B metallic glass, which reflect wear resistance and resilience, exhibit the highest values among various hard ceramic materials as well as metallic glass-forming alloys developed up to now. These advantages of Mo‐Ni‐Si‐B metallic glass can be used more widely to form a high temperature wear-resistant coating layer on various substrates. Furthermore, the same idea might be used to form a metallic glass-nitride nanocomposite coating layer by reactive deposition in N2 ambient, with highly lubricative property and high wear-resistance, especially at high temperature.

Comparative Thermodynamic Analysis of Nanocrystalline and Amorphous Phase Formation in Nb/Al and Nb/Sn Systems During Mechanical Alloying

Elemental powders of niobium–tin and niobium–aluminum were mechanically alloyed by Spex ball mill, respectively, in order to fabricate disordered nanocrystalline Nb3Sn and Nb3Al. The solid solution phase transitions of MA powders before and after heat treatment were characterized using X-ray diffraction (XRD) analysis. The microstructural analysis was performed using scanning electron microscopy (SEM). Results showed that mechanical alloying (MA) of Nb75Sn25 for 28 h led to the formation of the Nb3Sn intermetallic phase, while mechanical alloying of Nb75Al25 until 41 h did not show formation of the intermetallic phase. A thermodynamic analysis was performed based on the semiempirical theory of Miedema. The theory’s results showed that the intermetallic phase has a minimum Gibbs free energy compared to solid solution and amorphous states in both systems. Therefore, the most stable phase is intermetallic compound. But in case of the Nb/Al, the Nb3Al intermetallic compound was not formed during milling and needs heat treatment.

Resputtering Effect on Nanocrystalline Ni-Ti Alloy Films

We report on the effect of resputtering on the properties of nanocrystalline Ni-Ti alloy thin films deposited using co-sputtering of Ni and Ti targets. In order to facilitate the formation of nanocrystalline phases, films were deposited at room temperature and 573 K (300 °C) with substrate bias voltage of −100 V. The influence of substrate material on the composition, surface topography microstructure, and phase formations of nanocrystalline Ni-Ti thin films was also systematically investigated. The preferential resputtering of Ti adatoms was lesser for Ni-Ti films deposited on quartz substrate owing to high surface roughness of 4.87 nm compared to roughness value of 1.27 nm for Si(100) substrate.

Magnetism of coexisting rhombohedral and orthorhombic Hf2Co11 phases in rapidly quenched Hf2Co11B

Evolution of crystalline structure caused by heat treatment and its influence on magnetic properties of two melt-spun Hf2Co11B alloys in fully amorphous and partially crystalline states have been investigated. The as-cast ribbons were annealed in order to improve the hard magnetic properties by inducing the formation of nanocrystalline phases with high anisotropy. The X-ray diffraction and magnetic measurements confirmed the coexistence of two Hf2Co11 phases, crystallizing in different structures, with the rhombohedral one showing hard magnetic properties. Annealing of fully amorphous alloy resulted in the lower coercive field in comparison to that of annealed partially crystalline ribbon. From the point of view of exchange coupling between magnetic phases and magnetic hardening, the optimum annealing temperature and time were determined to be 570 °C and 60 min, for partially crystalline sample. Annealing at 650 °C for 1 h leads to excess of the HfCo3B2 phase above the critical value and results in a decrease in the coercive field. Isothermal annealing increases the magnetocrystalline anisotropy constant K1 from 7.52 for the as-quenched sample to 12.11 Merg/cm3 for the partially crystalline ribbon. The existence of quenched-in nanocrystals resulted in higher coercive fields, up to 3.23 kOe, after annealing.

In English

Mesoporous nanosized titania films and powders modified with Co 2+ , Ni 2+ , Mn 3+ and Cu 2+ ions have been produced by templated sol-gel method and characterized by optical spectroscopy, XRD, and BET surface area measurment. After calcinations at 400°C, XRD patterns showed the anatase nanocrystalline phase formation (8–20 nm). The characteristic bands of Co 2+ and Co 3+ in octahedral and tetrahedral oxygen environment registered in diffusion reflectance spectra indicated the formation of Co 3 O 4 spinel phase; crystallization of M/TiO 2 powders after heat treatment at 650°C led to an appearance of absorption bands belonging to Ni 2+ or Mn 3+ ions in an octahedral environment. Band gap energy and the position of flat band potentials were estimated by photoelectrochemical measurements. The films doped with transition metals possessed higher photocurrent quantum yield than that for undoped samples.

Formation and analysis of amorphous and nanocrystalline phases in Al–Cu–Mg alloy under friction stir processing

Homogeneous amorphous and nanocrystalline phases formed in the nugget zone of a friction stir-processed Al–Cu–Mg alloy have been studied. X-ray diffraction analysis indicated a diffuse scattering peak with characteristics of an amorphous phase existed in the range 15°–29°. Further, TEM analysis proved the existence of an amorphous structure. Friction stir processing provides special physical conditions, such as high temperature, high hydrostatic pressure and large shear stress, which could induce the amorphization of the alloy.

Study of structural and spectral properties of neodymium-doped lead-yttrium oxyfluoride nano-glass-ceramics

Processes of nanocrystalline phase formation in transparent yttrium oxyfluoride nano-glass-ceramics doped with neodymium ions are studied. An optimal heat treatment regime for a given glass composition is determined using differential thermal analysis (DTA). Glasses are heat-treated for 30, 60, and 120 min; the sizes of crystals are calculated, and the unit cell parameters are determined. The physicochemical and spectral properties of yttrium oxyfluoride glasses doped with neodymium ions, as well as of nano-glass-ceramics based on these glasses, are studied.

Temperature Dependence of the Elastic Modulus of (Ni0.6Nb0.4)1−xZrx Membranes: Effects of Thermal Treatments and Hydrogenation

Amorphous (Ni0.6Nb0.4)1−xZrx membranes were investigated by means of X-ray diffraction, thermogravimetry, differential thermal analysis and tensile modulus measurements. Crystallization occurs only above 673 K, and even after hydrogenation the membranes retain their mainly amorphous nature. However, after exposure to gaseous hydrogen, the temperature dependence of the tensile modulus, M, displays large variations. The modulus of the hydrogen reacted membrane is higher with respect to the pristine samples in the temperature range between 298 K and 423 K. Moreover, a sharp drop in M is observed upon heating to approximately 473 K, well below the glass transition temperature of these glasses. We propose that the changes in the moduli as a function of temperature on the hydrogenated samples are due to the formation of nanocrystalline phases of Zr hydrides in (Ni0.6Nb0.4)1−xZrx-H membanes.

Structural transition and softening in Al–Fe intermetallic compounds induced by high energy ball milling

In the present investigation, powders of as-cast ingots of Al–25at%Fe and Al–34.5at%Fe alloys close to Al3Fe and Al2Fe intermetallic phases are subjected to high energy ball milling to understand the possibility of formation of amorphous and/or nanocrystalline phases or any other metastable phases. The development of microstructure, evolution of various metastable phases and their stability are investigated by x-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Mechanical milling of the alloys, up to 50h, was carried out in high energy planetary ball mill. It resulted in phase transformation from monoclinic and triclinic structures of Al3Fe and Al2Fe, respectively, to orthorhombic structure pertaining to Al5Fe2 phase and structural transformation from crystalline to amorphous phase. Hardness measurements revealed a transition from hardening to softening behavior in these mechanically milled alloys undergoing prolonged milling. The softening effect in the milled powders, having a composite structure involving nanocrystalline and amorphous phases, is attributed to the competing phenomenon of grain size reduction and amorphous phase formation with increasing milling time.

Development of Nanostructure Formation of Fe<sub>73.5</sub>Cu<sub>1</sub>Nb<sub>3</sub>Si<sub>13.5</sub>B<sub>9</sub> Alloy from Amorphous State on Heat Treatment

Iron-based amorphous alloys have attracted technological and scientific interests due to their excellent soft magnetic properties. The typical nanocrystalline alloy with the composition of Fe73.5Cu1Nb3Si13.5B9 known as FINEMENT has been studied for structural properties analysis. Recently, it is found that after proper annealing the amorphous alloy like Fe73.5Cu1Nb3Si13.5B9 has a transition to the nanocrystalline state, thus exhibiting good magnetic properties. The alloy in the form of ribbon of 10 mm width and 25mm thickness with the composition of Fe73.5Cu1Nb3Si13.5B9 was prepared by rapid quenching method. The prepared ribbon sample has been annealed for 30 min in a controlled way in the temperature range 490℃ - 680℃. By analyzing X-ray diffraction (XRD) patterns, various structural parameters such as lattice parameters, grain size and silicon content of the nanocrystalline Fe(Si) grains, crystallization behavior and nanocrystalline phase formation have been investigated. In the nanocrystalline state, Cu helps the nucleation of α-Fe(Si) grains while Nb controls their growth, Si and B has been used as glass forming materials. Thus on the residual amorphous, the nanometric Fe(Si) grains develops. From broadening of fundamental peaks, the optimum grain size has been determined in the range of 7 - 23 nm.