Formation of lactose-mannitol molecular alloys by solid state vitrification

In this paper, we report the possibility to form glassy molecular alloys (α-lactose)_1−x(mannitol)_x for x<0.5 by co-milling two crystalline powders of pure α-lactose and pure mannitol β. The results have been established by differential scanning calorimetry and by powder X-ray diffraction. The concentration dependence of the glass transition temperature is found to obey the Gordon Taylor rule expected for regular solutions. It is also shown that the milling of pure mannitol β (x=1) leads to a polymorphic transformation towards the metastable form α of mannitol.     

Magnetic behavior of nanocrystalline LaMn2Ge2

The compound, LaMn₂Ge₂, crystallizing in ThCr₂Si₂-type tetragonal crystal structure, has been known to undergo ferromagnetic order below (T_C=) 326 K. In this article, we report the magnetic behavior of nanocrystalline form of this compound, obtained by high-energy ball milling. T_C of this compound is reduced marginally for the nanoform, whereas there is a significant reduction of the magnitude of the saturation magnetic moment with increasing milling time. The coercive field however increases with decreasing particle size. Thus, this work provides a route to tune these parameters by reducing the particle size in this ternary family.     

Short-to-medium-range order in Mg65Cu25Y10 metallic glass

The atomic configurations of liquid and glassy Mg₆₅Cu₂₅Y₁₀ alloy have been simulated in the temperature range of 300 K to 2000 K via ab initio molecular dynamics. The variations of pair correlation function (PCF), structure factor (SF), coordination number (CN) and bond pairs with the temperature for this alloy are characterized. It has been shown that the atoms are near densely packed and icosahedral type of short-range order (SRO) is predominant in the glass state. Icosahedral medium range order (MRO) can be formed by vertex or intercross connection of icosahedral SROs. In this work, an icosahedral MRO which is composed of 55 atoms has been found. It has been also clarified that Mg and Cu occupy the centre or vertex, and Y atoms only occupy the vertex of the icosahedron in this glassy alloy. It is believed that these findings have implication for understanding the glass forming mechanism of magnesium based metallic glasses.     

Synthesis and characterization of the xZnO-(1−x)α-Fe2O3 nanoparticles system

The xZnO-(1−x)α-Fe₂O₃ nanoparticles system has been obtained by mechanochemical activation for x=0.1, 0.3 and 0.5 and for ball milling times ranging from 2 to 24 h. Structural and morphological characteristics of the zinc-doped hematite system were investigated by X-ray diffraction (XRD) and Mössbauer spectroscopy. The Rietveld structure of the XRD spectra yielded the dependence of the particle size and lattice constant on the amount x of Zn substitutions and as function of the ball milling time. The x=0.1 XRD spectra are consistent with line broadening as Zn substitutes Fe in the hematite structure and the appearance of the zinc ferrite phase at milling times longer than 4 h. Similar results were obtained for x=0.3, while for x=0.5 the zinc ferrite phase occurred at 2 h and entirely dominated the spectrum at 24 h milling time. The Mössbauer spectra corresponding to x=0.1 exhibit line broadening as the ball milling time increases, in agreement with the model of local atomic environment. Because of this reason, the Mössbauer spectrum for 12 h of milling had to be fitted with two sextets. For x=0.3 and 12 milling hours, the Mössbauer spectrum reveals the occurrence of a quadrupole-split doublet, with the hyperfine parameters characteristic to zinc ferrite, ZnFe₂O₄. This doublet clearly dominates the Mössbauer spectrum for x=0.5 and 24 h of milling, demonstrating that the entire system of nanoparticles consists finally of zinc ferrite. As ZnO is not soluble in hematite in the bulk form, the present study clearly demonstrates that the solubility limits of an immiscible system can be extended beyond the limits in the solid state by mechanochemical activation. Moreover, this synthesis route allowed us to reach nanometric particle dimensions, which would make the materials very important for gas sensing applications.     

Thermally Activated Photoconduction and Alternating-Current Conduction in Se75Ge20Ag5 Chalcogenide Glass: Investigation of Meyer--Neldel Rule

We report on the observation of Meyer-Neldel rule in glassy Se75Ge20Ag5 alloy where AE is varied by two different methods. In the first approach, the intensity of light varies while measuring the photoconductivity in amorphous thin films of Se75Ge20Ag5 instead of changing composition of the glassy system. In the second approach, the variation of ac conductivity with temperature is found to be exponential and the activation energy is found to vary with frequency.     

Milling and dispersion of multi-walled carbon nanotubes in texanol

Rheological results were used to determine the optimum type of dispersant and its concentration for six commercial dispersants for the dispersion of multi-walled carbon nanotube (MWCNT) agglomerates in texanol. An unsaturated polycarboxylic acid copolymer (BYK P-104) exhibited the optimum performance with the lowest MWCNT slurry viscosity in texanol. The cutting and dispersion efficiencies of MWCNTs with 20 wt.% of BYK P-104 dispersant were compared using conventional ball milling and high energy milling, whereby the latter was found to be more effective. High energy milling for 2 h produced a large portion of MWCNT agglomerates smaller than 150 nm, showing a drastic increase in slurry viscosity due to the dispersion into individual CNTs. On the other hand, 120 h ball milling was required to achieve the agglomerate size of 300 nm with less viscosity increase upon milling. Decrease in the degree of MWCNT crystallinity was observed by both milling, even though 2 h high energy milling showed slightly less damage than 120 h ball milling based on XRD and Raman spectroscopy results.     

Influence of the modulating interfacial state on Sr2FeMoO6 powder magnetoresistance properties

In this paper, we studied the effects of the modulating interfacial state, which was introduced by mechanical ball milling, on the magnetoresistance (MR) properties of Sr₂FeMoO₆ polycrystalline material. The X-ray diffraction analysis showed that the crystal structure of Sr₂FeMoO₆ polycrystalline material was not changed in the process of ball milling, but the SrMoO₄ impurity phase was introduced at grain boundaries, and its quantity increased with the milling time. The results of resistance measurements at different temperatures indicated that ball milling had a very important influence on the MR properties. At due to the enhancement of the tunneling among adjacent grains by introducing the insulating SrMoO₄ phase at grain boundaries, the MR was enhanced with increasing the milling time. However, at the MR decreased rapidly with the increase in milling time. This phenomenon was mainly caused by the inelastic hopping of electrons through the localized states introduced at grain boundaries.     

The influence of ball-milling on structural and magnetic properties of Co-based powders

The melt-spun Co- and Fe-based amorphous alloys have been investigated extensively for applications in magnetic devices, which require magnetically soft materials. Although these alloys exhibit excellent soft magnetic properties, their thin sheet shape, which is a consequence of the low glass forming ability, limits significantly their engineering applications. A powder metallurgy is thus an alternative way of producing bulk and, at the same time, soft magnetic materials, having desired shape. In our case, Co₅₆Fe₁₆Zr₈B₂₀ and Co_70.3Fe_4.7Si₁₀B₁₅ amorphous ribbons have been ball-milled for a short time and subsequently compacted (by hot pressing) into disc-shaped specimens with the aim to achieve samll values of resulting coercivity. This work is focused only on the first preparation step i.e. on structural and magnetic properties of ball-milled powders obtained by ball-milling of Co-based melt-spun ribbons at different conditions. Two different ways of milling were employed in order to obtain a powder form of the material: the ribbons were either continuously ball-milled for up to 12 hours or, after each half an hour of ball-milling, the vials were cooled in liquid nitrogen bath for half an hour. Mössbauer spectroscopy, X-ray diffraction and differential scanning calorimetry were employed to compare and to present the differences between these two different ways of milling.     

Effects of Raw Material Content on Efficiency of TiN Synthesized by Reactive Ball Milling Ti and Urea

Ti and urea mixed according to the molar ratios of 2：1, 3：1 and 4：1 axe milled under the same condition. The structures of the as-synthesized powders are analyzed by an x-ray diffractometer （XRD）. The decomposed temperature of the urea and the products decomposed are characterized by differential scanning calorimetry （DSC） and thermogravimetry analysis-Fourier transform infrared （TG-FTIR） spectrometry. The results show that the reaction progress is a diffusion reaction. The efficiency of TiN synthesized by reactive ball milling can be increased by increasing the content of Ti. The reactive ball milling time decreases from more than 90 h to 40 h correspond- ing to the content ratio between Ti and urea increasing from 2：1 to 4：1. Ammonia gas （NH3） and cyanic acid （HNCO）, the decomposed products of urea, react with the refined Ti to form TiN. The grain refinement of Ti has a significant effect on the efficiency of reactive ball milling.     

Physical aging of glassy PMMA/toluene films: Influence of drying/swelling history

Gravimetry experiments in a well-controlled environment have been performed to investigate aging for a glassy PMMA/toluene film. The temperature is constant and the control parameter is the solvent vapor pressure above the film (i.e. the activity). Several experimental protocols have been used, starting from a high activity where the film is swollen and rubbery and then aging the film at different activities below the glass transition. Desorption and resorption curves have been compared for the different protocols, in particular in terms of the softening time, i.e. the time needed by the sample to recover an equilibrium state at high activity. Non-trivial behaviors have been observed, especially at small activities (deep quench). A model is proposed, extending the Leibler-Sekimoto approach to take into account the structural relaxation in the glassy state, using the Tool formalism. This model well captures some of the observed phenomena, but fails in describing the specific kinetics observed when aging is followed by a short but deep quench.     

Effect of Yttrium Addition on Glass Forming Ability of ZrCuAlSi Alloy

The effect of yttrium addition on glass formation of a ZrCuAlSi alloy is investigated. The maximum diameter 8mm of the glassy rods for （Zr46.3Cu43.3Al8.9Si1.5）100-xYx alloy with x = 2.5 is obtained by copper mould casting. Apparent enhancement of the glass formation ability is found with addition of yttrium, mainly due to the purification of the alloy melt and the suppression of formation of the primary phases by yttrium.     

Local icosahedral order and thermodynamics of simulated amorphous Fe

Local icosahedral order and thermodynamics of amorphous Fe have been analyzed in detail for models containing 3000 atoms, which were obtained by the molecular dynamics (MD) method. Models were obtained by cooling from the melt. Local order in models has been analyzed by using the technique proposed by Honeycutt and Andersen; we found an existence of icosahedral order in the system. Moreover, structural properties of models were also studied via radial distribution function (RDF), static structure factor, mean atomic distances, coordination number and bond-angle distributions. Glass transition temperature, heat capacity and potential energy of the system were found in addition to the evolution of structure and mean-squared displacement (MSD) of atoms upon cooling from the melt toward the glassy state. We found the glass transition temperature of simulated liquid Fe via temperature dependence of potential energy and it is close to that observed previously in the literature, i.e. T_g≈1070 K. Calculations showed that structural properties of amorphous Fe models with the Pak-Doyama interatomic potential agreed well with the experimental data.     

Crystallization kinetics of amorphous SiC films: Influence of substrate

The crystallization kinetics of amorphous silicon carbide films was studied by means of X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The films were deposited by radio frequency (r.f.) magnetron sputtering on glassy carbon and single crystalline silicon substrates, respectively. TEM micrographs and XRD patterns show the formation of nano-crystalline β-SiC with crystallite sizes in the order of 50 nm during annealing at temperatures between 1200 and 1600 °C. A modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) formalism was used to describe the isothermal transformation of amorphous SiC into β-SiC as an interface controlled, three-dimensional growth processes from pre-existing small crystallites in the order of 10 nm. These pre-existing crystallites are formed in a transient process in the early stages of crystallization. For films deposited on the silicon substrate, the obtained rate constants of crystallite growth obey an Arrhenius behavior with an activation enthalpy of 4.1 ± 0.5 eV in accordance with literature data. Films deposited on glassy carbon show an increased stability of amorphous SiC films, which is reflected in smaller rate constants of crystallite growth of several orders of magnitude at low temperatures and a higher activation enthalpy of 8.9 ± 0.9 eV. A model is proposed, where the faster crystallization of films on silicon substrates can be explained with the presence of superabundant point defects, which diffuse from the substrate into the film and accelerate the incorporation of atoms from the amorphous into the crystalline phase.     

Low temperature dependence of elastic moduli and internal friction for the glassy alloy Zr55Cu30Al10Ni5

Longitudinal and transverse wave velocities, eight kinds of elastic parameters, and dilational and shear internal frictions of Zr₅₅Cu₃₀Al₁₀Ni₅ glassy alloy were simultaneously measured as a function of temperature in the range from 77 to 373 K, using an ultrasonic pulse method. The inflections at around 150 K for wave velocities, anisotropy factor and Poisson's ratio, and the 150 K peak of shear friction seem to correspond to one topological change (pseudo‐transition) associated with an interatomic readjustment or vacancy rearrangements. The behaviors from 77 to 125 K and 125 to 373 K are due to thermal relaxation of squeezed free volumes and entropy elasticity associated with vibrational motions of clusters, respectively, accompanied by an increase in atomic distance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and magnetic properties of nanocrystalline Fe75Si25 powders prepared by mechanical alloying

Nanocrystalline Fe₇₅Si₂₅ powders were prepared by mechanical alloying in a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling process were studied by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer measurements. The evolution of non-equilibrium solid solution Fe (Si) during milling was accompanied by refinement of crystallite size down to 10 nm and the introduction of high density of dislocations of the order of 10¹⁷ m⁻². During the milling process, Fe sites get substituted by Si. This structural change and the resulting disorder are reflected in the lattice parameters and average magnetic moment of the powders milled for various time periods. A progressive increase of coercivity was also observed with increasing milling time. The increase of coercivity could be attributed to the introduction of dislocations and reduction of powder particle size as a function of milling time.     

Study on the bonding state for carbon-boron nitrogen with different ball milling time

The varied bonding state and microstructure characterization were discussed for carbon-boron nitrogen (CBN) with abundant phase structure and nanostructure, which were synthesized directly by mechanical alloying technique at room temperature. According to the results of SEM and X-ray photoelectron spectroscopy (XPS) of CBN with different ball milling time, it is substantiated that the bonding state and microstructure for CBN were closely related to the ball milling time. With the increase of the ball milling time, some new chemical bonding states of CBN were observed, which implies that some new bonding state and microstructures have been formed. The results of XPS are accordance with that of X-ray diffraction of CBN.     

Phase formation in ion bombarded metallic films

The interplay of several events, ranging from production, migration and interaction of defects, to irradiation enhanced atomic diffusion and chemical mixing, is responsible for phase formation in surface layers of ion bombarded metallic alloys. The problem is so complicated that even the interpretation and the prediction of extreme cases such as the attainment of a crystalline, or a glassy product are presently beyond the possibilities of first principle approaches, and empirical criteria have been proposed to this end. In this work we limit ourselves to the very beginning of phase formation, i.e. thenucleation stage, in the frame of an atomistic model. In a binary alloy, after formation of collision cascades, the relaxation to metastable equilibrium of the locally altered compositional profile due to preferential migration to the cascade-matrix interface of one alloy component, is schematized by charge transfer events As a result, dimers of an effective alloy are formed. Conditions specific of glass and respectively crystal formation are extracted from an analysis of surface and thermochemical properties of starting and effective alloys.     

Ion transport and diffusion in nanocrystalline and glassy ceramics

In the present paper we review experimental studies on ion transport and diffusion in nanocrystalline and glassy ceramics of LiNbO₃ and LiAlSi₂O₆ and report on new ones on LiBO₂ using the measurement of dc conductivities and ⁷Li nuclear magnetic resonance spin-lattice relaxation rates. Nanocrystalline ceramics, with an average particle size of 50 nm and less, often show an enhanced diffusivity compared to their microcrystalline (μm-sized) counterparts. This increase is due to the large fraction of atoms or ions located in the interfacial regions. A key for understanding the structure-mobility relations in nanocrystalline ceramics is to clarify the microscopic structure of the grain boundaries and also the morphology of the grain boundary network. In this context it is useful to study not only the ion transport properties of the nano- and microcrystalline materials but also those of the corresponding glassy forms. Such comparative studies gave strong evidence that in some cases the interfacial regions are of amorphous structure. For example, this was recently shown for nanocrystalline lithium niobate which was prepared by high-energy ball milling.     

Onset of chaotic dynamics in a ball mill: Attractors merging and crisis induced intermittency

In mechanical treatment carried out by ball milling, powder particles are subjected to repeated high-energy mechanical loads which induce heavy plastic deformations together with fracturing and cold-welding events. Owing to the continuous defect accumulation and interface renewal, both structural and chemical transformations occur. The nature and the rate of such transformations have been shown to depend on variables, such as impact velocity and collision frequency that depend, in turn, on the whole dynamics of the system. The characterization of the ball dynamics under different impact conditions is then to be considered a necessary step in order to gain a satisfactory control of the experimental set up. In this paper we investigate the motion of a ball in a milling device. Since the ball motion is governed by impulsive forces acting during each collision, no analytical expression for the complete ball trajectory can be obtained. In addition, mechanical systems exhibiting impacts are strongly nonlinear due to sudden changes of velocities at the instant of impact. Many different types of periodic and chaotic impact motions exist indeed even for simple systems with external periodic excitation forces. We present results of the analysis on the ball trajectory, obtained from a suitable numerical model, under growing degree of impact elasticity. A route to high dimensional chaos is obtained. Crisis and attractors merging are also found. (c) 2002 American Institute of Physics.