Effect Of Mechanical Milling Research Articles

Study of structure and luminescence of CdSe Nanocrystals obtained by ball milling

The effect of mechanical milling on the structure and luminescent properties of CdSe powder has been investigated by means of cathodoluminescence (CL) in the scanning electron microscope, transmission electron microscopy (TEM) and x-ray diffraction. The starting powders were ball milled for times between 5 and 20h. TEM of the milled samples reveals the presence of nanocrystals with a wide range of sizes including nanocrystals of 10nm and smaller. The CL spectra of the untreated samples show the characteristic band edge emission centered at 1.8eV. In the milled samples a band centered at about 2.5eV is observed which appears to be associated to the presence of nanocrystals. The evolution of defect structure with milling time has been monitored by recording infrared cathodoluminescence spectra. A partial phase transformation from wurtzite to zinc blende structure also has been observed.

Factors influencing the kinetics of electrochemical reactions in milling

Input of mechanical energy at a high rate can drive a system and induce phase transformations and chemical reactions away from equilibrium. The evolution of such a change depends on both thermodynamic as well as kinetic factors. Besides the microstructural changes like attainment of nanostructure, which alters the overall free energy, the high rate of mechanical energy input also changes the kinetics by influencing the mass transport and related processes. In order to understand these factors, we have recently started a programme of looking at the influence of mechanical energy on driving simple chemical reactions in solid state. In this presentation we shall present and discuss the results of two kinds of situation that we have studied. The first one is simple electrochemical replacement reactions between metals and metal sulphates in solid state. We show that the mechanical milling alters the kinetics of these reactions, which can be rationalized by considering the phenomena taking place at the microscopic level. For example we will show that the crystal structure of the sulphate and the nature of the reaction product at the interface influence the mechanochemistry significantly. It is even possible in some special cases to alter the direction of the chemical reaction. In the second set of results we shall present the effect of mechanical milling on the site occupancy in ferrites, which can lead to a significant change in magnetic behaviour.

Free volume changes in mechanically milled PS and PC studied by positron annihilation lifetime spectroscopy (PALS)

AbstractThe effect of mechanical milling on free volume was studied by means of positron annihilation lifetime spectroscopy (PALS) in polystyrene (PS) as a typical brittle polymer and in polycarbonate (PC) as a tough representative. Long‐time milling increases the free volume, while a decrease is observed for short milling times. The changes are mostly irreversible in PS. The irreversible fraction is much smaller for PC. Gel permeation chromatography (GPC) measurements show a decrease of the molecular weight, which is much more pronounced in PS. The milling‐induced irreversible changes in free volume are attributed to chain‐end defects resulting from chain scission. In PC, other deformation‐induced defects that anneal upon heat treatment above the glass transition temperature dominate. Polym. Eng. Sci. 44:1351–1359, 2004. © 2004 Society of Plastics Engineers.

Understanding milling induced changes: Some results

The effect of mechanical milling on materials has been studied using simple model systems. The results show that milling leads to enhancement in both thermodynamic driving force and transport kinetics. A study of some characteristic physical properties of the milled samples in comparison to the bulk shows how milling affects the properties.

Effect of mechanical milling on the structural, magnetic and dielectric properties of coprecipitated ultrafine zinc ferrite

Nanosized ZnFe 2O 4 particles containing traces of α-Fe 2O 3 by intent were produced by low temperature chemical coprecipitation methods. These particles were subjected to high-energy ball milling. These were then characterised using X-ray diffraction, magnetisation and dielectric studies. The effect of milling on zinc ferrite particles have been studied with a view to ascertaining the anomalous behaviour of these materials in the nanoregime. X-ray diffraction and magnetisation studies carried out show that these particles are associated with strains and it is the surface effects that contribute to the magnetisation. Hematite percentage, probably due to decomposition of zinc ferrite, increases with milling. Dielectric behaviour of these particles is due to interfacial polarisation as proposed by Koops. Also the defects caused by the milling produce traps in the surface layer contributes to dielectric permittivity via spin polarised electron tunnelling between grains. The ionic mechanism is enhanced in dielectrics with the rise in temperature which results in the increase of dielectric permittivity with temperature.

Luminescence properties of mechanically milled and laser irradiated ZnO

The effect of mechanical milling on the luminescence properties of ZnOmicrocrystalline samples has been studied by means of cathodoluminescence in ascanning electron microscope. The samples consisted of pressed pellets ofcommercially available ZnO powder which were ball milled to investigate thepossibility of nanocrystalline ZnO formation. Changes observed in the relativeintensities of the characteristic ultraviolet and green band of ZnO are discussed interms of defects generated during milling. The effect of nano- and picosecondpulsed laser irradiation on the particle size and luminescence of the milled sampleshas been also investigated.

The effect of mechanical milling on the formation of nanocrystalline Mg 2Si through solid-state reaction

Mechanical milling effects on the formation of nanocrystalline Mg 2Si through solid-state reaction were investigated. The results indicate that mechanical milling lowers the formation temperature to ∼280 °C. Correspondingly the reaction process is found to change into two sub-processes whose activation energies are much smaller than that of the original one.

The effect of mechanical milling on the solid state reactions in the barium oxalate–iron(III) oxide system

Abstract The formation of barium hexaferrite, BaFe 12 O 19 , from a 1:6 molar ratio mixture of barium oxalate and iron oxide has been investigated. Thermogravimetry (TGA), high temperature X-ray powder diffraction (HT-XRPD), differential scanning calorimetry (DSC) and micro-Raman spectroscopy have been used to determine the effect of mechanical activation on the solid state reactions occurring during heating. The resulting magnetic properties were investigated measuring hysteresis loops. For the activated mixtures, the mass loss is over at ≈600 °C i.e. well below the temperature where BaCO 3 spontaneous decomposition is complete ( T >850 °C). Such a noticeable temperature lowering is a consequence of the high energy milling enhancing the formation of BaFe 2 O 4 . After heating the milled mixture to 850 °C, BaFe 12 O 19 was rapidly formed from the BaFe 2 O 4 and residual Fe 2 O 3 . Starting from an unmilled mixture, only minor amounts of BaFe 12 O 19 were formed by heating to 850 °C. The favourable formation of barium hexaferrite, when starting from milled powders, has been confirmed by micro-Raman spectroscopy. The powder from the activated sample was shown to have far better magnetic properties than the unactivated sample.

Effect of mechanical milling on solid state formation of BaTiO 3 from BaCO 3–TiO 2 (rutile) mixtures

Barium metatitanate (BaTiO 3) is widely used because of its high dielectric constant, ferroelectric properties and positive temperature coefficient of electrical resistivity. The present work reports the results obtained in the set up of a preparation method of BaTiO 3 by milling and annealing mixtures of BaCO 3 and TiO 2 (rutile). High energy milling of a BaCO 3–TiO 2 (rutile) equimolecular mixture resulted in a noticeable temperature drop of the temperature where the reaction can occur. The formation of single phase BaTiO 3 was confirmed by thermogravimetric analysis (TGA) (both alone and coupled with Fourier transform infrared spectroscopy (FT-IR) evolved gas analysis), by differential scanning calorimetry (DSC) and X-ray powder diffractometry (XRPD). The formation of phases like Ba 2TiO 4 (orthotitanate), which are known to be detrimental for BaTiO 3 end performances, has only been revealed as an intermediate minor phase that disappears by annealing at temperatures as low as 750°C. At such temperatures the crystal size of the product is 400 Å. BaTiO 3 formation occurs either by a slow heating (2 K/min) of the milled mixture or by a rapid heating followed by an isothermal annealing of 12 h.

Effect of Carbon Additives on the Electrochemical Properties of AB 5 Graphite Composite Electrodes Prepared by Mechanical Milling

The effect of mechanical milling on powder mixtures consisting of graphite and alloys, prepared either by mechanical alloying or by a high‐temperature melting process, has been investigated. The resulting hydride‐forming composite electrodes show a 10 and 40% capacity enhancement for arc‐melted and mechanically prepared alloys, respectively. Such an increase in capacity is suggested to be the result of several cumulative effects: (1) a mechanically induced reducing role of graphite which eliminates the particles of oxide coatings, enabling a better hydrogen adsorption/absorption and diffusion into the insertion sites of the alloy, (2) the appearance of an increasingly important double‐layer capacitance on each particle with increased milling time that adds to the faradic component, and (3) the improved electronic conductivity between the active material and the graphite that allows a better utilization of the alloy. © 1999 The Electrochemical Society. All rights reserved.

Transmission electron microscopy studies on carbon materials prepared by mechanical milling

The effects of mechanical milling on the multiscale organization (structure and microtexture) of various carbon materials were investigated by means of Transmission Electron Microscopy. We show that mechanical grinding generates an increasing amount of disordered carbon at a rate depending on the type of grinding mode used (shear- or shock-type grinding). When the shock-type grinding is used, the triperiodic structure and the lamellar microtexture of the graphite completely break down to give microporous and turbostratic carbons made of misoriented nanometric Basic Structural Units (BSUs). Graphite grinding permits the elaboration of disordered carbons. The involved mechanism is different from a simple reverse graphitization, since not only structure but also microtexture are strongly modified by the grinding. After heat treatment at 2800°C, the graphite organization is not recovered, and a mesoporous turbostratic carbon is mainly obtained. All the carbon precursors studied, submitted to strong grinding, leads to similar microporous carbons. Shear grinding is less effective since remnants of graphitic carbon are still present within the disordered carbon.

Lithium insertion in ball-milled graphite

The effects of mechanical milling on the microstructure, morphology and electrochemical performance of graphite powders with respect to lithium insertion are studied. After 150 h of ball-milling, the well-graphitized graphite has been pulverized into small particles with a size of about 50 nm, in which there are a lot of excess vacancies, microcavities and metastable carbon interstitial phases with sizes around 13 Å. Due to the large surface energy, the merging of single particles is favoured and results in the formation of agglomerates with average size about 1 μm. Voids are formed among the agglomerated particles. The ball-milled graphite shows reversible specific capacity for lithium of 700 mA h g −1 (L 1.88 C 6 ) with large hysteresis. The large reversible capacity is due mainly to Li doping at vacancies, microcavities (or at the edges of the metastable carbon interstitial phase) and voids. The bonding change between the interstitial carbon and the carbon in the aromatic plane that is induced by insertion of Li atoms leads to hysteresis. During charge–discharge cycles, the reversible capacity above 1 V decreases rapidly, which may be due to some vacancies and microcavities being annihilated by moveable and some bound interstitial carbon and to electrolyte penetrating gradually into voids formed by agglomerated particles during the Li insertion and desertion process.

Chemical order and structure of the mechanically milled Laves phase

The effects of mechanical milling on the chemical order and structure of the C14 Laves phase have been studied by x-ray diffraction and Mössbauer spectroscopy. The results show that ball milling leads to a segregation of two phases. One, with a lower degree of chemical disorder, maintains the structure of the original intermetallic phase and the other, with a higher degree of chemical disorder, exhibits a disordered structure. At 4.2 K, the two phases exhibit similar hyperfine magnetic fields which are smaller than that of the original intermetallic phase, but much larger than that of the amorphous alloy with the same composition, prepared by sputtering deposition. Differing from the crystalline phase, the structurally disordered phase shows a Curie temperature lower than room temperature.

Raman Study on Carbonaceous Materials Prepared by Mechanical Milling

Through Raman spectroscopy studies, we show that mechanical grinding generates an increasing amount of unorganized carbon at a rate depending on the type of grinding mode used (Shear and Shock-type grinding). The first-order Raman spectrum for pure unground graphite has a well-known G sharp band at 1579 cm−1, which corresponds to the E2g vibration while the ground samples present a broadened G band accompanied by new components at about 1610 cm−1 (D′), 1510 cm−1 (D″) and 1348 cm−1 (D), usually explained as arising from disorder and defects1. Shock-type grinding produces a faster disorder increase than shear-type grinding. The latter preserves part of the graphitic character. The general effect of mechanical milling remains however opposite to that of Thermal Treatment (Graphitization).

Martensitic transformation and structural inhomogeneity in Fe 72Pt 28 induced by mechanical milling

The effect of mechanical milling on the structure and magnetic properties of ordered Fe 72Pt 28 has been studied. The X-ray diffraction results suggest the presence of two kinds of transformation under milling; order—disorder transformation and the martensitic one. Moreover, the mechanically milled Fe 72Pt 28 was transformed into a FCC structure by heating up to 750 K. Structural inhomogeneity in heated samples, that were FCC Fe 72Pt 28, is found to depend on milling time. Magnetic behavior is suggested to depend on, primarily, the structures and the structural inhomogeneity induced by milling.

Mechanical Alloying of Fullerene with Various Metal Elements

The effect of mechanical milling on C 60 (C 70 ) (a mixture of 87 vol.% C 60 and 13 vol.% C 70 ) and mechanical alloying on metal-0.3 mol% C 60 (C 70 ) (M-C 60 (C 70 ) ), where M=Al,Cu,Fe,Ni,Sn or Zn was studied. Liquid chromatography and infrared absorption measurements done on the mechanically milled C 60 (C 70 ) showed that when milled the C 60 (C 70 ) was stable for 3600 ks. However, milling results in a reduced fraction of C 60 and for the same time, showed the formation of probably a dimer of C 60 and C 60 polymer. After milling C 60 (C 70 ) with a metal for 720ks, a small fraction of the initial C 60 (C 70 ) retained its identity in Al- and Zn-C 60 (C 70 ) systems, whereas the molecular structure of C 60 (C 70 ) is lost when the metal was Cu, Fe, Ni or Sn. The sintered compacts made from the MA powder of a M-C 60 (C 70 ) and a M-graphite showed similar hardness in the cases of M=Cu, Fe and Ni. However, the sintered compacts made from an Al-C 60 (C 70 ) MA powder showed about a 40% increase in hardness compared with that made from an Al-graphite MA powder. While in the case of Zn, the hardness of the sintered compact made from the Zn-C 60 (C 70 ) MA powder was less than half of that made from a Zn-graphite MA powder. The stability of C 60 in a metal after MA was discussed according to which element group the metal belonged to on the periodic table.

Effect of mechanical milling on the structure and magnetic properties of

The effect of mechanical milling on the structure and magnetic properties of has been studied. Decomposition during milling was found to occur via a two-stage reaction involving partial disordering and amorphization of the phase, followed by the formation of . Magnetic behaviour was found to depend on the microstructure developed during milling. Exchange spring behaviour was observed in partially amorphized samples. The influence of annealing on the structure and magnetic properties of milled powders was also investigated.

The effect of mechanical milling on the structure and magnetic properties of Fe2Sc Laves phase

Abstract The Fe2Sc intermetallic compound with C14 Laves phase structure was mechanically milled. The structural evolution and the change in atomic order were followed by X-ray diffraction and Mossbauer spectroscopy. The influence of mechanical milling on the magnetic properties was studied by Mossbauer spectroscopy and magnetization measurement. The results indicated that ball milling induced a partial chemical disorder followed by an amorphization. The partial chemical disorder and the amorphization lead to an increase in magnetic moment of Fe in the materials. The amorphous phase exhibits a ferromagnetic behavior with Curie temperature of about 250 K, which is much lower than that of 540 K for the original compound. The crystallization temperature of the amorphous phase is about 873 K, as determined by DSC measurement.

Amorphization of graphite induced by mechanical milling and subsequent crystallization of the amorphous carbon upon heat treating

The effects of mechanical milling (MM) on the phase transformation of graphite carbon were investigated using high resolution electron microscopy (HREM), x-ray diffraction, and differential thermal analysis (DTA). Amorphization of graphite as a result of prolonged high-energy ball milling was directly observed with HREM. The exothermic peak in the DTA trace of the ∼200 h ball-milled sample indicated a crystallization onset temperature of about 670 °C and crystallization activation energy of 234 kJ/mole.

Induced magnetic anisotropy in Fe80B20 metallic glass by mechanical milling

By means of transmission Mössbauer spectroscopy, the effect of mechanical milling on the magnetic anisotropy of Fe80B20 metallic glass has been studied. On the basis of variation of the areal ratio R21 of the second to the first Mössbauer line with milling time, it was found that a rotation of the average magnetic-moment directions from the in-plane to an out-of-plane orientation may take place by mechanical milling without onset of bulk crystallization, and that the distribution of the magnetic-moment directions becomes random after the bulk crystallization has occurred. The induced out-of-plane magnetic anisotropy by mechanical milling can be attributed to the occurrence of the chemical short-range order and surface crystallization in the Fe80B20 metallic glass. The latter effect may cause the formation of compressive stresses in the bulk and therefore a rotation of magnetic-moment directions in the Fe80B20 metallic glass.