Structural,thermal and optical study of nanocrystalline silicon produced by ball milling

The ball milling process was used to obtain nanocrystalline cubic silicon. Between 5 and 10 h of milling, amorphous silicon was also formed. The differential scanning calorimetry (DSC) spectrum of a powder milled for 10 h showed that the amorphous–crystalline phase transition occurs at about 450 °C. According to Raman spectroscopy and X‐ray diffraction results, volume fractions of the crystalline, interfacial and amorphous phases were about 30, 39 and 31%, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Iron nanoparticles produced by high-energy ball milling

In this investigation, the chemical and structural characteristics of Fe nanoparticles synthesized by high-energy ball milling have been explored. After the milling process the nanoparticles were collected using a magnetic field. The structure, morphology and composition of the powders were obtained using high-resolution electron microscopy. HREM images confirmed the nanoparticles’ presence with approximately 2–4 nm in size. It was found that using this method allowed the formation of nanoparticles in a smaller size range than other synthesis methods. Also, it was confirmed by HREM images that the obtained nanoparticles were mainly of the fcc nature and some of them of the MTP type.     

Preparation of magnesium ferrite nanoparticles by ultrasonic wave-assisted aqueous solution ball milling

Magnesium ferrite, MgFe₂O₄ nanoparticles with high saturation magnetization were successfully synthesized using ultrasonic wave-assisted ball milling. In this study, the raw materials were 4MgCO₃·Mg(OH)₂·5H₂O and Fe₂O₃ powders and the grinding media was stainless steel ball. The average particle diameter of the product MgFe₂O₄ powders was 20 nm and the saturation magnetization of them reached 54.8 emu/g. The different results of aqueous solution ball milling with and without ultrasonic wave revealed that it was the coupling effect of ultrasonic wave and mechanical force that played an important role during the synthesis of MgFe₂O₄. In addition, the effect of the frequency of the ultrasonic wave on the ball milling process was investigated.     

Characterization of Fe–Nb sputtered thin films

Structure, composition and chemical behavior of co-sputtered Fe–Nb thin films are analyzed by different techniques, as conversion electron Mössbauer spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction. It is shown that oxygen is determinative in hindering the Fe–Nb alloy formation and, as a result, Fe_1−xO and Nb₂O₅ occur in significant amounts, even in vacuum. In spite of the oxygen role, a Fe–Nb alloy is formed in little amounts, which increase as the Nb content is increased. The increase of the Nb content is also related with the increase of Fe_1−xO and with a decreasing of the metallic Fe present. Mössbauer data indicate the Fe–Nb phase present is the Fe₂Nb Laves phase.     

Mechanomaking of nanostructure in nitrided Fe–Cr alloys by cyclic “dissolution–precipitation” deformation-induced transformations

Structural-phase transformations in surface layers of iron and Fe? Cr? (Ni) alloys subjected to ion-plasma nitriding and subsequent cold plastic compression shear deformation in Bridgman anvils have been investigated by the methods of Mössbauer spectroscopy, transmission electron microscopy and X-ray analysis. It has been shown that the deformation-induced cyclic phase “dissolution–precipitation” transformations of nitrides in alloys lead to the formation of nitrogen oversaturated solid solutions, precipitation of secondary nitrides and nanostructurization of the metal matrix.     

Mössbauer, X-ray diffraction and magnetization studies of Fe–Mn–Al–Nb alloys prepared by high energy ball milling

Fe₆₀Mn₁₀Al₂₀Nb₁₀, (Fe₆₀Mn₁₀Al₃₀)₉₅Nb₅ and (Fe₆₀Mn₁₀Al₃₀)₉₀Nb₁₀ ball milled powdered alloys were investigated using X-ray diffraction, Mössbauer spectrometry, thermomagnetic (TGM) and magnetization measurements. We studied the influence of Nb content and of different milling times on the structural and magnetic properties. Two main features can be concluded: (1) the FeAlMn induces a BCC phase whatever the Nb content is, and (2) as both increasing Nb content and milling time give rise to an highly disordered state in conjunction with a decrease of the ferromagnetic behavior.     

Characterization of nanocrystalline FeSiCr powders prepared by ball milling

FeSi₁₀Cr₁₀ powder was mechanically alloyed by high energy planetary ball milling, starting from elemental powders. The microstructural and magnetic properties of the milled powders were characterized by scanning electron microscopy, X-ray diffraction, ⁵⁷Fe Mössbauer spectrometry and a vibratory sample magnetometer.After 3 h of milling, the formation of two bcc solid solutions α-Fe₁ (Si, Cr) and α-Fe₂ (Si, Cr) is observed. Their grain sizes decrease with increase in milling time attaining, at 15 h of milling, 23 and 11 nm, respectively. Mössbauer spectra of the milled powder show the presence of two components. One is a ferromagnetic type with a broad sextuplet. Its distribution of hyperfine field is characterized by high and low hyperfine field’s peaks and a mean value of 26.5 T. The other is a single paramagnetic peak. Its low concentration increases to ∼4% at 15 h of milling. These results can be explained by different atomic environments affected by Si or/and Cr elements, as well as the increased disordered grain boundaries.Magnetic measurements of the milled FeSi₁₀Cr₁₀ alloy powder exhibit a soft ferromagnetic character with a decrease of both magnetization at saturation (Ms) and coercive force (Hc) with milling time attaining values of Ms=151 emu/g and Hc=2500 A/m at 30 h of milling time.     

Short-range order clustering in BCC Fe–Mn alloys induced by severe plastic deformation

The effect of severe plastic deformation, namely, high-pressure torsion (HPT) at different temperatures and ball milling (BM) at different time intervals, has been investigated by means of Mössbauer spectroscopy in Fe_100–xMn_x (x = 4.1, 6.8, 9) alloys. Deformation affects the short-range clustering (SRC) in BCC lattice. Two processes occur: destruction of SRC by moving dislocations and enhancement of the SRC by migration of non-equilibrium defects. Destruction of SRC prevails during HPT at 80–293 K; whereas enhancement of SRC dominates at 473–573 K. BM starts enhancing the SRC formation at as low as 293 K due to local heating at impacts. The efficiency of HPT in terms of enhancing SRC increases with increasing temperature. The authors suppose that at low temperatures, a significant fraction of vacancies are excluded from enhancing SRC because of formation of mobile bi- and tri-vacancies having low efficiency of enhancing SRC as compared to that of mono vacancies. Milling of BCC Fe_100–xMn_x alloys stabilises the BCC phase with respect to α → γ transition at subsequent isothermal annealing because of a high degree of work hardening and formation of composition inhomogeneity.     

Study of the glassy magnetic behaviour and charge-ordering phase transitions in La0.75Ca0.25FeO3−δ perovskite

Abstract

In this work, La_0.75Ca_0.25FeO_3?δ perovskite sample was prepared by the coprecipitation method. The nanoparticle was found to crystallize in the orthorhombic (Pbnm) phase as confirmed by X-ray diffraction (XRD) and transmission electron microscopic (TEM). The oxygen non-stoichiometry (δ) and magnetic states of iron ions (three magnetic sextets and non-magnetic doublet) were investigated by Mössbauer spectroscopy at room temperature (RT). The shape of the magnetic hysteresis loop of the sample reveals the existence of a weak ferromagnetism at RT. The magnetization vs. temperature curves, measured in the 9 to 200 K range, showed that the sample exhibits two magnetic-phase transition temperatures at 29 K (T_g) and 120 K (T_CO). The magnetization isotherms, M (H), around these magnetic-phase transition temperatures for the sample are analyzed.     

Structural phase transitions during high-energy ball milling of BaTiO3

Microstructural characterization of ball milled perovskite BaTiO₃ powders has been done by the modeling of X-ray diffraction profiles. The study reveals that on size reduction, BaTiO₃ powders undergo a continuous, displacive, and diffusionless dynamic phase transitions involving tetragonal (T), monoclinic (M), and orthorhombic (O) symmetry via the second-order type [T?→?(T?+?M)?→?(M?+?O)?→?O] when stimulated by a high-power pulse of pressure in a planetary mill. The order parameter, a phenomenological quantity to describe the general behavior of a system going through phase transitions has been estimated using spontaneous strain calculated from lattice parameters or physical distortions derived from atomic coordinates or both. At room temperature, BaTiO₃ nanoparticle achieved an orthorhombic phase when a critical size (<15?nm) has been reached at later stage of milling (≥70?h). Raman's study reveals similar structural phase transitions sequence on size reduction and TEM study reveals the corresponding particle diameter.     

High-Energy Ball Milling of NiAl(Fe) System

High-energy ball milling was used to promote the solubilization of iron into NiAl powder for an iron concentration range of 10–30 wt.%. The microstructural evolution induced by the intense mechanical deformations, under different milling conditions, was followed by X-ray diffraction and Mössbauer spectroscopy. The Mössbauer spectra are dominated by a magnetic sextet of about 33 T. Increasing the time and the speed of milling gives rise to a non-resolved doublet, having parameters typical of a NiAl compound with Fe atoms in solution. At the same time a reduction of lattice parameter occurs, which can be correlated to composition variations and partial disordering of the NiAl structure. Subsequent annealing modifies the Mössbauer spectra noticeably. In particular, the non-magnetic component becomes a broad singlet. Both diffraction analysis and Mössbauer spectroscopy indicate that a fcc Ni(Al,Fe) solid solution is forming in samples milled in agate. It is observed that the grain size of the milled products remains in the nanometric range even after thermal treatment, which adds interest to possible applications.     

High-Energy Ball Milling of Some Intermetallics

High-energy ball milling of metallic powders has been used in recent years for the synthesis of alloys through reactions mainly occurring in solid state. The diffusive phenomena accompanying and promoting the reactions of formation are related to the microstructure acquired by the powders as a consequence of the intense mechanical deformations. The process induces a remarkable comminution of powder particles, inside of which domains of nanometric size are formed and compositional variations often occur. Several analytical techniques are suitable for following the structural evolution of the powders during milling. Among them, Mössbauer spectroscopy is suitable for obtaining detailed local information on the atomic arrangement of the treated materials, if one of the constituents is a Mössbauer isotope, and for detecting little changes occurring at an atomic scale. For these reasons Mössbauer spectroscopy is more sensitive than other analytical techniques especially in the early stages of the process. Some recent results are presented regarding in particular the Fe–Cu, Fe–Al, Fe–Al–Cu, NiAl(Fe) and Fe–Mn systems.     

Mixing of iron with various metals by high-energy ball milling of elemental powder mixtures

The alloying of Fe with T=V, Cr and Mn by high-energy ball milling of elemental powder mixtures has been studied from the scale of a powder particle down to the atomic scale using X-ray and neutron diffraction, Mössbauer spectrometry and magnetic measurements for Fe_1?x T _x alloys with x=0.50, 0.65 for T=V, x=0.50, 0.70 for T=Cr and x=0.72 for T=Mn. Different alloying behaviours are observed according to T once powder particles have the final composition. The rather fast mechanical alloying of Fe with Mn reflects the statistical nature of the milling process in contrast to the slow mixing of Fe with V and of Fe with Cr. Hyperfine magnetic field distributions remain stationary in shape in the last milling stage at room temperature both for T=V and T=Cr. Magnetic measurements evidence the persistence with milling time of a large population of nanometer-sized Fe-Cr zones that are superparamagnetic at room temperature and at 400 K. By contrast, room-temperature Mössbauer spectra show only a single line for long milling times. The unmixed stationary state of milled p-Fe_0.7Cr_0.3 is discussed in the light of a recent model of systems driven by competing dynamics.     

Phase transformation of strontium hexagonal ferrite

The phase transformation of strontium hexagonal ferrite (SrFe₁₂O₁₉) to magnetite (Fe₃O₄) as main phase and strontium carbonate (SrCO₃) as secondary phase is reported here. SrFe₁₂O₁₉ powder was obtained by a heat treatment at 250 °C under controlled oxygen flow. It was observed that the phase transformation occurred when the SrFe₁₂O₁₉ ferrite was heated up to 625 °C in confinement conditions. This transformation took place by a combination of three factors: the presence of stresses in the crystal lattice of SrFe₁₂O₁₉ due to a low synthesis temperature, the reduction of Fe³⁺ to Fe²⁺ during the heating up to 625 °C, and the similarity of the coordination spheres of the iron atoms present in the S-block of SrFe₁₂O₁₉ and Fe₃O₄. X-ray diffraction analysis confirmed the existence of strain and crystal deformation in SrFe₁₂O₁₉ and the absence of them in the material after the phase transformation. Dispersive X-ray absorption spectroscopy and Fe⁵⁷ Mössbauer spectroscopy provided evidences of the reduction of Fe³⁺ to Fe²⁺ in the SrFe₁₂O₁₉ crystal.     

Characterization of nanostructured ceramics prepared by both high energy ball milling and fast firing sintering processes

High-energy ball milling technique was successfully applied to calcinated lead zirconate titanate (PZT 60/40) powders. After 20?h of ball milling, large PZT particles were completely broken down, reducing its initial size in three orders of magnitude. Experimental results show a huge sinterability enhancement of the PZT powders by using this technique, achieving its maximum sintering rate at ～800°C. Relatively low densities (～91%) were achieved in stoichiometric samples, while in 3% lead excess samples sintered at 950°C for 30, 45, 60, 90 and 120?min using a fast firing process and a post-annealing treatment at 800°C for 4 h, densities of ～97% of the theoretical were achieved. PZT nanostructured ceramics prepared under optimized processing conditions (60?h of powder milling, 950°C of sintering temperature, 60?min of sintering time and a post-annealing process at 800°C during 4?h) show high dielectric constant (ε′) values (900) and low dielectric loss (tan?δ) at room temperature and a ferroelectric-paraelectric transition temperature at 375°C.     

X-ray diffraction and Mössbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mössbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (～10¹⁵/m²) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of ?1?1?1? screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.     

Spin-glass behaviour of novel ternary uranium aluminide U3Co4+ x Al12− x (x = 0.55)

Experimental results on dc and ac susceptibility, magnetization and magnetic relaxation, specific heat, electrical resistivity and magnetoresistivity up to 8?T are reported for the novel ternary uranium aluminide U₃Co_4.55Al_11.45. The temperature dependence of the dc susceptibility shows a cusp at a characteristic temperature T _f?=?8–10?K that depends weakly on the applied magnetic field. The observed pronounced difference between the ZFC and FC magnetizations, as well as the decay in the remanent, both give evidence that a highly irreversible, frozen state is formed below T _f which is reminiscent of spin-glass behaviour. The real and imaginary parts of the ac susceptibility show that the corresponding T _f peaks are only slightly dependent on frequency. Electrical resistivity measured at zero and in fields up to 8?T indicates that the Kondo-like state becomes dominant at temperatures above T _f.     

Mössbauer effect studied in rare-earth oxides (REu)O3 (R = Pr,Nd, Sm)

Abstract

Both X-ray diffraction and the Mössbauer effect have been used to study the double rare-earth oxide solid solutions (REu)O³ (R = Pr, Nd, Sm), which were synthesized with a combination of high pressure and high temperature for the first time. It is shown that all products are of the B-type (monoclinic) structures of single phase. The hyperfine interactions depend strongly on the ligand volume of Eu ions.     

The effect of crystal atomic chain discontinuity upon channeling

It is shown that diffusion of transversal energy due to discontinuity of atomic chains decreases exponentially, when the transversal energy decreases.