Non-Crystalline and Nanoscale Materials

The following sections are included:

• PREFACE

• CONTENTS

Diffusion and relaxation experiments have been performed for binary, ternary and quarternary alloys including metal-metal as well as metal-metalloid compositions. Diffusion data were obtained by the radio-tracer technique for a large number of alloys showing distinct differences in the diffusion parameters concerning the atomic radii of the diffusing atoms. The parameters of thermally activated structural relaxation processes were obtained from internal friction, magnetic relaxation and induced magnetic anisotropy measurements. A review of the experimental results will be given and the present state of the theory of short range and long range diffusion in amorphous and heterogeneous materials will be discussed in the framework of the effective medium approximation. In particular the random walk of a particle will be discussed for the random barrier model, the random trap model and the mixed model with randomness of barriers and traps. From a comparison of experimental and theoretical results it is shown that diffusion in relaxed amorphous alloys takes place by direct collective mechanisms where thermally activated displacement chains play a major role.

A combustion method has been adapted and optimized for the efficient preparation of single-phase small-particle La_1-xSr_xCoO₃(0 ≤ x ≤ 0.50) materials. It is based on the exothermic reaction of the corresponding metal nitrates and a reducing agent, oxalic acid dihydrazide (ODH) that yields fine-grained ash that readily converts into pure La_1-xSr_xCoO₃ when treated thermally at 800°C/2 h.

The effect of the different alloying modes of the constituents in FeBSi ingot elaboration on amorphous ribbons casting process and on their magnetic properties is described. The investigated alloy was prepared by induction melting a). pure elements and b). pre-alloyed components Fe-B+ Si with ferroboron obtained by carbothermic reaction method. The tendency to glass forming for a), and b). respectively type of alloy was evaluated by means of the average value of the activation energy for viscous flow in the overheating range of (T_liquidus, 1500°C). The position of the maximum magnetic permeability, saturation, coercive force and crystallization temperatures were found to be slightly affected by using different casting procedures of the ingot.

Two kinds of materials of special properties were obtained using the sol-gel method: 1- bulk glasses with gradient refraction (r-GRIN) of the SiO₂-TiO₂ system; 2-sintered bioactive materials of the CaO-P₂O₅-SiO₂ system. In the first case the special methods of the gel preparation as well as the thermal treatment of gels were applied. The wet gels were leached in hydrochloric acid in order to obtain the gradient of the TiO₂ concentration. As a result the GRIN-glasses of gradient refraction ▵n=0,06 have been obtained. In the second case the gel-derived powders of the grain fraction about 2 μm were produced and the dense sinters were obtained using the hot-pressing of powders in the temperature 800°C. As a result of the contact of sinters with simulated body fluid the hydroxyapatite layer on theirs surface was formed after 24 h. This indicates on the high bioactivity of the obtained gel-derived material of CaCO-P₂O₅-SiO₂ system.

By applying the alcoholane sol-gel method there have been obtained amorphous and glass crystalline powders of the following chemical composition (in mole%): A-2: 54CaO, 6P₂0₅, 40 SiO₂; AAl-2: 54CaO, 6P₂O5, 5Al₂O₃, 35SiO₂. The obtained gels were heated to the temperature 800°C, and, next, powdered during 3h and 10h in a rotary ball mill, applying balls made of ZrO₂. The obtained materials contained glass as the main phase and some amounts of apatite and wollastonite. Dense sinters were obtained from the powders ground during 3 h. In the case of powders ground during 10h no-completely densification was observed. Densification of sinters leads to the improvement their mechanical properties and to decreasing of chemical reactivity. As a result of examination sintered materials of high bioactivity and good mechanical properties were produced.

The crystallization kinetics of some Ge-Se-Sb glassy alloys obtained by quenching in air were investigated by means of differential scanning calorimetry, scanning electron microscopy and X ray diffraction. Two main crystallization processes were identified that correspond to the formation of the stable solid compounds. Both isothermal and continuous heating calorimetric data were obtained. The overall experimental information is discussed in terms of the mechanisms that control the crystallization activated by heat treatment. A tentative parametrization of the relevant quantities such as nucleation frequency and crystal growth is proposed and related to the phase diagram information on the GeSe₂-Sb₂Se₃ system. Crystallization is viewed as resulting from homogeneous/heterogeneous nucleation and interface/diffusion controlled crystal growth.

Glasses of different compositions in the system SiO₂-CaO-MgO-P₂O₅ have been obtained by the sol-gel method using tetraethyl orthosilicate, triethyl phosphate, calcium nitrate and magnesium nitrate as precursors. A bone-like apatite layer was formed on the glass surface when soaked in a solution with ionic concentration similar to that of human plasma at 37°C. Formation of the apatite layer was analysed by XRD, SEM, variation of the ionic concentrations and pH in the solution. The apatite layer was formed in the first week of treatment on all the glasses studied.

Models of the ions PO₄^3- and H₂PO₄^- which are involved in bone bonding of bioactive silica glasses and glass-ceramics, have been obtained using the molecular orbital semi-empirical methods AMI and PM3. Heat of formation and optimum geometry of phosphate ions were calculated. The use of a solvent model allows to study the influence of water molecules in the properties of ions. Due to the important role of interfacial charges in nucleation and growth of the calcium phosphate which bound by proteins and collagen forms the bonding layer, the net atomic charges and the dipole moment of phosphate ions were also calculated. From AMI in the presence of water, a dipole of 4.2 Debye and a net charge of +2.6 on the phosphorus atom were obtained for HPO₄^2- ion. From these results it is possible to propose that the condensation of the HPO₄^2- ions with Si-OH groups, formed on the surface of glasses and glass-ceramics, could be the initial stage for nucleation of the calcium phosphate layer.

Two liquid source CVD derived techniques have been employed to prepare thin films of perovskite type: SrTiO₃, LaA10₃, onto different substrates : Si (100), MgO (100), Y-ZrO₂ and metallic substrates (Inconel, Hastelloy, Nickel). Different deposition conditions such as nature and concentration of both precursor and solvent, pyrolysis temperature, carrier and reaction gas nature, and their influence on the thin films characteristics, have been comparatively considered on both systems : atmospheric and low pressure.

The aim of this study is to present the results of investigations of amorphous halide-chalcogenide materials, methods of their preparation and the characteristics of their properties with respect to the possibility of their application in modern optics, electronics and fibre technique. Special techniques of synthesis enabled to obtain stable glasses, a few mm thick, in the system Sb₂Se₃-BaCl₂-PbCl₂. The thermal and optical properties of the glasses have been determined. Attempts have also been made to examine the non-linear properties of these glasses. The investigations results have shown that the glasses are characterized by the light transmittance in the range 0.65-50 μm, at the level 38-60%, and the refractive index equals 4.06 depending in the wave length.

In the summarizing remarks the possibility of the application of these glasses as glasses transmitting infrared radiation is presented.

The mechanical alloying processes of Ni-43 at.% Mo powder mixtures were performed applying low energy ball mill and different milling conditions. The alloys were synthesized in argon and air atmospheres, as well as ethanol was added as a source of oxygen. In all cases the formation of Ni(Mo) solid solution was confirmed. However, the observed increase of lattice parameter varied from 0.85 % for alloying in argon to 0.2 % with ethanol addition and 0.1 % only for milling in air. Also mean crystallite size strongly depended on milling environment, e.g. for Mo after 300 h of processing it was 12 nm for synthesis in argon, 30 nm with ethanol addition and 70 nm in air. Prolonged milling in argon atmosphere led to an amorphous phase formation.

The area of the occurrence of glasses in the system Li₂O-CdO-SiO₂ nucleated with CaF₂ and the method of their preparation are determined. Detailed investigations were carried out for 4 glasses and glass-ceramics obtained from these glasses. The DTA method and the gradient furnace were used to determine the parameters of thermal treatment of the starting glasses. The specific electric resistance as a function of temperature, density, the microhardness and chemical durability on water action of the starting glasses and of glass-ceramics obtained from them were investigated. Using the method of X-ray diffraction the phase composition of the glass-ceramics was determined. The obtained glass-ceramics demonstrate very great microhardness, high electric resistance at elevated temperature and good chemical durability on water action.

Titania gels have been prepared by TiCl₄ hydrolysis and neutralization with NaOH or NH₄OH. The gels so obtained were shaped as rods or coatings of cylindrical substrates of a Ti64VAl alloy, “in vitro” studies effectuated by immersing the samples in Simulated Body Fluid (SBF) at 37°C showed the formation of an amorphous calcium phosphate layer. This process was followed by XRD and monitoring the variations of ionic concentration and pH in the fluid.

In this work, four different ligands : acetylacetone, dipivaloylmethane, benzoylacetone and dibenzoylmetane were used to prepare the corresponding barium and strontium β-diketonates. These compounds were employed as precursors for the preparation of thin films by a liquid source CVD derived technique: AAMOCVD.

We present a systematic Electron Spin Resonance (ESR) and magnetization study of the R₁-_xA_xMnO₃ (R = La,Pr; A = Ca,Sr) perovskites, up to 1100 K. Special care was taken in the data obtained in the region of T immediately above the ferromagnetic ordering temperature (Tc ≲ T ≲ 1.2T_C). In this interval of T, for most samples, the resonance line broadens, distorts, and splits into several lines. In conjunction with it, small hysteresis loops, and a susceptibility which is field and T dependent are measured. The origin of these findings are associated to non-stoichiometric samples due to inhomogeneities, probably, result of a non-random distribution of vacancies, defects, and oxygen content. It is shown that those features are easily observed by ESR. We also discuss the possible origin of the T dependence of the ESR linewidth for all the region of T measured.

The observation of effective diamagnetic susceptibility of -3.4 10-6 emu.gr^-1//Oe up to temperatures above 1000K in highly deformed ball milled Sr_0.6Ca_0.4CuO₂ is reported. As produced samples are magnetically paramagnetic with an additional ferromagnetic contribution. A positive susceptibility remains after milling but it is progressively masked by an increasing linear diamagnetic component, as the analysis of the thermal dependence of susceptibility shows. The diamagnetic susceptibility presents a maximum for a critical milling time. Finally, we propose that the wave functions of the holes in excess, introduced during milling mainly at the interfaces, are extended over the CuO₂ planes of the crystallites and are weakly modified by the magnetic field, so giving rise to this strong diamagnetism.

Magneto-impedance effect has been studied in ribbons of composition Co_66.3 Fe_3.7 Si₁₂ B₁₈ which where submitted to different annealing processes, in order to obtain a near to zero saturation magnetostriction constant and different induced anisotropies. Sensitivities to the applied bias Held of the order of 40%/Oe were obtained at 3 MHz for ribbons of 10 cm lin length. The best results were obtained for the sample that has the lowest and negative value of saturation magnetostriction coefficient and the smallest induced anisotropy.

The magnetoimpedance measurements extension to the microwave frequency region (45 MHz ÷ 3 GHz) allows one to study the behaviour of the magnetic permeability in soft magnetic ribbons and wires in the high frequency range. Measurements have been performed using a network analyser in order to obtain the magnetic field dependence of the reflection parameters connected to the sample impedance. In this work a study of magnetoimpedance behaviour as a function of frequency has been performed on CO_68.15Fe_4.35Si_12.5B₁₅ amorphous wires and ribbons produced by means of rapid solidification. The samples are characterised by similar cross section. Room temperature measurements confirms that the MI effect is in general more sensitive in wires with respect with amorphous ribbons also in the microwave frequency interval.

The frequency response of as-cast, amorphous ferromagnetic wires of nominal composition (Co_0.96Fe_0.04)_72.5B₁₅Si_12.5 was investigated in the 5 Hz-13 MHz frequency range, and 0.3 mA (RMS) ac current amplitude. By analyzing the complex impedance behavior, we show that circumferential domain walls in the wires exhibit relaxation phenomena very similar to other magnetic materials such as ferrites (with electrical resistivities larger by several orders of magnitude). The main source of impedance is therefore the inductance coupling between the circular ac field and the circumferential domains. Finally, it appears that microscopic eddy currents are more significant than macroscopic eddy currents (or the so called, skin-depth effect).

Electrical and magnetic properties of Fe_77.5B₁₅Si_7.5 wires, submitted to a modified current annealing technique in air are reported. Several heat sinkers along the sample length have been inserted in order to modify the heat conduction along the sample. The exploited method has turned out to induce more than one magnetic phase characterised by markedly different coercivity values. Selected samples have been submitted to the same electrical current flow varying the number of heat sinkers along the sample length. The measurements of dc hysteresis loops have revealed the presence of at least two different coercivity values, varying with the number of heat sinkers. The magnetotransport properties in dc regime and as a function of frequency (up to 1 MHz) have been studied in detail. The impedance variation △Z/Z as a function of the applied field turned out to increase up to 20% as a function of the number of heat sinkers. Moreover, the magnetoimpedance curve as a function of the applied field is shown to have a striking hysteretic behaviour.

The criticality of magnetic Instability in amorphous materials of different shapes : wires, ribbons and microwires is discussed. Temperature and field dependences of velocity of domain wall propagation has been measured in the range of temperatures 5-300 K in amorphous Fe₆₅B₁₅Si₁₅C₅ microwires. Domain wall velocity within the experimental range of magnetic fields (65-100 A/m) and temperatures are between 35 and 65 m/s. Domain wall mobility was estimated from these dependences. The value of critical field for domain wall propagation is lower than the switching field reflecting the existance of an energy barrier for domain wall to be depinned or critical size of reversed domain. Common characteristic features of switching in both bistable magnetic materials and domain-wall junctions are discussed.

Results on the giant magneto-impedance effect in CoFeSiB amorphous glass-covered wires and FeCuNbSiB nanocrystalline ones are reported. GMI measurements were performed on both glass-covered samples and on samples obtained after the glass removal. The sensitivity of the effect in nanocrystalline wires after glass removal was found to be higher than in amorphous CoFeSiB ones in the studied frequency range. The results are explained by taking into account the specific magnetic properties of each type of wire.

The results of our preliminary work in the construction of a general micromagnetic model for studying the coercivity in soft ferromagnetic amorphous alloys are presented. In these materials, agglomeration of the free volume during the rapid quenching process leads to short-range stresses that are the main pinning centers for domain walls. We discuss the features of our model and show that, without taking into account magnetoelastic interaction, there is a coercivity associated with the presence of a non-magnetic region.

The influence of a partial substitution of Fe by Ni on the structural and magnetic properties of melt spinning alloys with nominal composition Ni_xFe_73.5-xSi_13.5B₉Nb₃Cu₁ (0 ≤ x ≤ 25) has been investigated by different characterization methods on samples submitted to thermal treatments up to 750°C. Different steps of the crystallization process and the evolution of magnetic parameters measured at room temperature (coercive field, H_c, saturation magnetostriction constant, λ_s, etc.) have been correlated.

We have studied the influence of the short range order on the magnetic properties of (FeyCo1-y)₇₅Si₁₅B₁₀ metallic glasses. The distances between metallic atoms decrease for y≤0.5 and the corresponding to Co shows a minimun at y=0.06. These features have been correlated with the behaviour of some magnetic properties as Curie Temperature, spin wave stiffeness, and the isomer shift.

The magnetic properties of the metallic glass Fe₆₀Ru₂₀B₂₀ have been investigated in the vicinity of the paramagnetic to spin glass transition temperature (T_g). Both the variation of the linear and the non-linear terms with temperature have been studied. Like for other concentrated spin glasses, the field-dependent non-linear term has been described by means of a scaling function. As previously demonstrated for the system _a-Fe_xNi_75–x(PBAl)₂₅, analyzing several isotherms around T_g, it can be precisely located from the peaks in χ0 y χH (for a fixed B) , the latter being normally sharper. Tg has been found to be ≈ 70 K. The temperature dependence for χ0 implies a Curie behavior far from T_g, divergence at closer temperatures and saturation at T = T_g.

CoP amorphous multilayers, with thicknesses between 0.4 μm and 4 μm, have been electrolytically grown over sputtered copper thin films, exhibiting in-plane magnetic anisotropy. Depositing samples with different thicknesses, we observe a continuous variation in the easy direction for the magnetization, from the transversal direction to intermediate and to the longitudinal one. Therefore, controlling the parameters of the deposition, it is possible concomitantly modify the anisotropy direction, without any further treatment, and consequently, to control the magnetization processes of these samples.

In this work we present studies on the frequency dependence of the magneto-impedance in the range of 0.1 to 2MHz for Fe_73.5Si_13.5B₉Nb₃Cu₁ Fe_73.5Si_16.5B₆Nb₃Cu₁ nanocrystalline ribbons. As cast samples were annealed in Ar atmosphere at 560°C, with and without an axial dc magnetic field. At a fixed frequency, an improvement in the field annealed 13.5% Si samples, when compared with the zero field annealed ones, can be observed. On the 16.5 % Si field annealed samples only a reduction of magneto-impedance ratio could be observed, when compared to the non-field annealed ones. Analysis of the magnetic properties and X-ray data shows that the observed changes in magneto-impedance effect are consequence of the induced magnetic anisotropy.

The electrical resistivities of the amorphous matrix and the nanocrystaline phase in the Al-Y-Ni-Co alloys were separated. Calculations show that the decrease of total resistivity during crystallization is related to the structural changes of amorphous matrix. The observed electrical resistivity suppression in the amorphous matrix is consistent with the Ziman model. The temperature dependence of the nanocrystalline fraction was evaluated and discussed.

The variations of coercive force and the saturation magnetization with the applied stress in Fe_73.5Si_13.5B₉Cu₁X₃ (X= Nb, Ta and Mo) and Fe₈₇Zr₇B₆ amorphous alloys are investigated. The variations of magnetization and coercive force in the FeZrB alloy could be linked to the changes of Fe-Fe distance with the applied stress while in the Si-containing ones the stress dependence of the coercivity can be interpreted by considering two different mechanisms: the pinning of the domain walls on the stress centers and the domain wall energy dependence on the surface roughness.

Fully-selfconsistent spin-polarized nonlocal (GGA) Density Functional calculations of the three atom Co-Cu-Co and Cu-Co-Cu systems are performed with confined to their linear structure geometry optimizations. The core electrons of both atoms have been described by a model potential. An important 3p – 3d correlations have been taken fully into account by promoting the 3p shells into a valence basis sets. An analysis of essential d type bonding mechanisms is made by means of Mulliken populations. The bonding/antibonding nature of HOMO-LUMO interface is discussed in detail.

Nanocrystalline alloys Fe₇₀Al₃₀ prepared by mechanical alloying studied by various structurally sensitive contain the disordered bcc Fe(Al) solid solution with minimum crystallites size of 7 nm at longest milling periods. Magnetization and Mässbauer data show that the amorphized layers developing at grain boundaries suppress a dimensionality of the magnetic exchange interactions leading to the intermediate magnetization decrease. The most common nearest neighbor surrounding of the Fe is composed of 5 and more Fe atoms.

In this work we extend our previous simulations to study the magnetization processes and the effect of temperature on the domain kinetics. We have carried out extensive Monte Carlo simulations with different simple cubic lattice samples of size 20×20×20. If the simulation is allowed to come to equilibrium at high temperature and then thermally quenched by reducing T to well below T_c, domains are frequently formed. In zero field, such domains may persist or may vanish with time. In general, the results obtained show that small closed shapes are not stable, but structures involving lamellae or rectangular prisms of one sign of spins average embedded in a matrix of the opposite spin are stable indefinitely at sufficiently low temperatures.

The increasing of 13% of the reduced remanence M_r/M_s by applying an external magnetic field during annealing of the Nd₈Fe₇₇Co₅B₆CuNb₃ melt spun ribbons suggests an uniform mixture of the soft and hard magnetic component phases developed inside the ribbons. The best magnetic properties of the investigated ribbons were achieved by applying the external magnetic field during heating at 700°C for 10 min. The Curie temperatures of the hard and soft magnetic phases are not affected by the magnetic field annealing.

We report on the temperature dependencies of the static magnetic properties as well as the hyperfine magnetic characteristics observed for two kinds of nanocrystalline alloys: Fe₆₆Cr₈CuNb₃B₉Si₁₃ which exhibits a low fraction of the crystalline grains of bcc-FeSi (14at.% Si) and Fe₈₉Zr₇B₄ with high volumetric fraction of crystalline bcc-Fe grains. We discuss the role of the magnetic interactions in these nanoscale magnetic systems.

Magnetic measurements performed on Ni clusters imbedded in AlN are presented. The Ni particles were prepared by ion implantation, followed by an annealing treatment. Via X-ray absorption measurements at the Ni edge, it was possible to characterize the clusters : determine the crystallographic structure and measure the lattice parameters. In a spherical assumption, average diameters, in the range 0.9 nm to 2.5 nm, are calculated. The particle size is controlled by the incident fluence and the post-annealing conditions. Depth distributions in the matrix are measured by Rutherford Backscattering of He ions. Magnetization measurements with a SQUID magnetometer show that these particles display a superparamagnetic behavior with blocking temperatures in the range 20 K to 300 K.

As a coarse-grained model for dense polymer melts undergoing glassification we use the bond-fluctuation model on a simple cubic lattice with a Hamiltonian favoring long bonds. This model does not only reproduce the phenomenology of slowing down near the glass transition (Kohlrausch relaxation functions, time-temperature superposition, Vogel-Fulcher laws for relaxation times, etc.), but is also suitable for testing many theoretical concepts, such as the Gibbs-Di Marzio entropy theory, mode-coupling theory, and the idea that there exists a growing glass correlation length. In particular, evidence is discussed that shows that the configurational entropy of the polymer chain system significantly decreases when the glass transition is approached, but stays non-vanishing there. And although from the growth of long-range oscillations in the pair correlation function (or related density oscillations near hard walls) one can extract an increasing correlation length, the polymer self-diffusion in the respective temperature range is not affected by this length scale.

ε-Fe₂O₃ was obtained as an intermediate phase in the transformation of γ-Fe₂O₃ particles dispersed in silica into α-Fe₂O₃. The main features of the crystal structure (a=509.5, b=878.9, c=943.7pm, Pna2₁ isomorphous with GaFeO₃) and magnetic structure of ε-Fe₂O₃ are presented. The thermal properties of γ-Fe₂O₃ particles in powders and dispersed in silica matrices are compared. The role played by space confinement conditions in stabilizing metastable phases is emphasized.

Metastable multiphase materials are currently obtained by controlled crystallization of amorphous samples. This paper deals with the thermodynamic and kinetic analysis of primary or eutectic crystallization from the disordered phase as a function of the processing conditions, assuming thermodynamic and kinetic competition to form a crystalline phase by nucleation and growth. The evolution of the process on both heating the initial amorphous alloy or on isothermal annealing follows a sequence mostly consequent with the different temperature dependence of the several factors driving crystal formation.

In this work, the structural study of some Fe-Ni-P-Si amorphous ribbons prepared by the melt spinning method was performed as a function of the P/Si ratio. Metallic glass Fe₄₀Ni₄₀P₂₀-_xSi_x (x = 6, 10, 14) ribbons were prepared in the same conditions. Morphological study was performed by means of scanning electron microscopy with EDX microanalysis. X-ray diffraction and transmission Mössbauer spectroscopy were used to check the amorphous state of samples. The analysis shows that, irrespective of the composition, all the materials obtained are fully amorphous. The magnetic interaction depended on the P/Si ratio. Besides, the differential scanning calorimetry results revealed several exothermic peaks, indicating the crystallization of the amorphous state.

Changes in magnetic properties along devitrification of FeSiB-XNb alloys have been studied. Thermomagnetic plots of as-cast and annealed samples are qualitatively similar to that found for conventional FINEMET alloys, except for the presence of some additional metastable magnetic phases. Coercive field, derived from quasi-static hysteresis loops, slowly decreases with annealing at temperatures below the crystallization onset, to abruptly increase for annealing temperatures in the range of the first crystallization stage. This steep hardening is followed by a significant lowering and a new rapid increase of coercive field for higher annealing temperatures.

X-ray diffraction and Mössbauer spectra on pre-annealed samples and magnetization variation during in-situ annealing were used to derive nanocrystalline fractions. A reasonable good agreement between values obtained from different experimental techniques is obtained.

The microstructure of amorphous materials obtained by first order phase transitions influences greatly their macroscopic properties. Particle size distributions can be now obtained for different growth mechanisms giving a deep understanding of these properties. In this paper we show that an adequate dimensionless formulation allows to obtain simple but accurate estimations of the most interesting average parameters of the grain size distribution, namely the average and maximum grain size and the grain density.

Primary crystallization of the excess metal in some amorphous alloys gives rise to nanocrystalline materials of great technological interest, but the kinetics of such process is reluctant to fit any of the commonly used expressions. In this work, the kinetics of the nanocrystallization has been studied by means of new equations taking into account the changes in composition of the amorphous matrix during the process. Experimental DSC data have been obtained from FeNbSiBCu alloys of the FINEMET type. A simplified model which allows a continuous change of the apparent activation energy during the nanocrystallization, is able to fit both isothermal and non isothermal DSC peaks with single sets of parameters for each composition.

The Segregation Charge Transfer (SCT) atomistic model is used to study phase nucleation in binary covalent films irradiated under conditions such that dense colllision cascades are formed along the ion trajectory. At the cascade-matrix interface a non-equilibrium compositional profile evolves, which is associated to a local electronic density profile. Relaxation to (meta)stable equilibrium is simulated by inter-atomic charge transfer reactions, involving dissimilar atoms, which result in the formation of an effective compound. The energy balance associated to electron transfer between two atoms, the absolute value of the percent enthalpy change at the formation of a molecule of effective compound, the percent variation of bonding ionicity degree and the absolute variation of component electronegativity differences in initial and effective compounds are calculated and compared. The analysis is performed on twenty-five compounds, among which thirteen were vitrified, while twelve remained crystalline under ion bombardment; qualitative differences in the parameter trends are found between the two families of systems.

The determination of Avrami exponents is a common tool to analyse the kinetics of a first order phase transition. The values of the Avrami exponents evaluated from the slope of log(-log(1-χ)) vs. log(t) are used to determine the mechanism driving the transformation. A general analysis of the calculation of Avrami exponents for the usual case of radius dependent growth and increasing or decreasing nucleation rates is performed from two points of view, namely the effect of the evaluation procedure itself and the effect of complex nucleation and growth protocols.

Using the removal of surface layers method, the internal stress distribution in as-quenched Co₆₆Si₁₆Fe₄Mo₂ amorphous ribbons with thickness of 25 and 40 μm has been obtained. The results show that the thickness affects strongly the value of the residual stress in the wheel-side of the ribbon and the depth of the zones in compressive and tensile stress, but does not affect significantly neither the value of the residual stress in the air-side nor the tensile stress in the central part of the ribbon.

On nanocrystalline ribbons of the composition Fe₈₆Zr₇Cu₁B₆ and Fe₈₅Zr₇Cu₂B₆ the temperature dependence of the hysteresis loops was measured using a new type of hysteresigraph. The ribbons were heat treated at Ta = 673K, 723K and 833K for 1h. These nanocrystalline materials with the Curie temperature T_c(am.) of the remaining amorphous matrix near the room temperature, show superparamagnetic behavior at higher temperatures, where the exchange interactions between the nanocrystals vanish. The occurrence of this phenomenon has been analysed using a M/Ms vs. H/T plot of the magnetization curves.

The influence of the Si_x/B_18.5-x (x=9.5, 6.5, 4.5, 2.5, 0.5) content on the amorphous-to-nanocrystalline transformation was monitored by thermal, structural, and magnetic measurements. Calorimetric results obtained in continuous heating regime reveal several exothermic events appear during the whole crystallization process and the presence of a peak at the amorphous Curie temperatures ranging from 560K for x=9.5 to 513K for x=0.5. In addition, thermomagnetic measurements show the Curie temperatures of the amorphous, nanocrystalline structure as well as those corresponding to the different magnetic phases that appear during crystallization. The apparent activation energies for the nanocrystalline transformation have values ranging from 3.4 eV for x=9.5 and 2.8 eV for x=0.5. The crystallized phases were identified by X-ray diffraction. Particular attention has been paid to how the magnetic properties of a nanostructured sample changes with thermal treatment carried out on glassy precursors.

The Ga-Te system, when quenched from the melt, has a small glass forming composition range centered in the Ga₂₀Te₈₀ composition. In this work, the crystallization of amorphous GaTeSn alloys rich in Te has been studied. Amorphous samples have been obtained by means of rapid quenching from the melt with a piston and anvil device. X-Ray Diffraction and Differential Scanning Calorimetry have been used to study the crystallization kinetics of these amorphous alloys. The activation energy, crystallization temperature, crystallization enthalpy and crystallization products are reported. The addition of small quantities of Sn to the Ga₂₀Te₈₀ binary alloy generates more stable amorphous phases.

Soft-magnetic FIMEMET-type FeSiB−Nb−Cu amorphous precursors nanocrystallize into a duplex structure of primary nanocrystals within a residual amorphous matrix. The basic physics of this process can be understood through the role of elemental additions using a simple nucleation and growth model provided that the accumulation of the slower diffusing Nb atoms at the growth interface of the Fe(Si) nanocrystals is considered.

While the soft-magnetic properties of nanocrystallized FIMEMET-type Fe₇₄Si₁₄B₈Nb₃Cu₁ amorphous precursors are better than those of FeSiB amorphous alloys, nanocrystallization of the more recent generation of amorphous precusors such as Fe₉₀B₃Z_r7-type alloys with or without Cu addition leads to higher saturation induction and still very attractive soft-magnetic properties. These alloys are now commercialized as NANOPERM in Japan, with thicknesses of the order of 20 μm with optimum properties. In this paper we present a study of the nanocrystallization and magnetic properties of a laboratory-made 34 μm thick and 1 inch wide Fe₉₀B₇Nb₃ amorphous tape.

Negative magnetostrictive amorphous ribbons have been annealed by means of short current pulses. The temperature of the ribbons was determined by magnetic measurements and by using a thermocouple attached to the samples for temperatures below the Curie point. To estimate the temperature above the Curie point, we have used the local intensity of the infrared emission of the sample with a CCD nitrogen liquid cooled camera. So we have studied the thermal process kinetics in static and dynamic states, and we have performed a theoretical model that predicts the sample temperature for each annealing current intensity. The results are in good agreement with those obtained experimentally by flash annealing.

In a previous work¹, the glass forming ability and the atomic structure of the Al-rich Al-Fe-Nb amorphous alloys have been analysed. The aim of this work is to study the crystallisation behaviour of these systems, mainly using differential scanning calorimetry. These amorphous alloys present high thermal stability and their crystallisation process takes places in two stages at quite different temperatures, the first stage corresponds to α-Al crystallisation. The apparent activation energy and the transformation diagrams are determined, and the mechanisms that control the crystallisation are discussed.

An analysis of the atomic radial distribution function of the amorphous alloy Sb_0.20 Ge_0.20Se_0.60 is carried out using X-ray diffraction data. A model of this alloy has been studied by means of the random Montecarlo method, assuming tetracoordinated Ge, tricoordinated Sb and dicoordinated Se. The structure of short-range order for the alloy could be described as a three-dimensional network of covalent bonds with tricoordinated antimony atoms at one of the vertices of the triangular pyramids, and the tetrahedral structural units formed by one germanium atom in the centre of the tetrahedron, while the vertices are usually occupied by selenium atoms. These structural units are joined directly or by chalcogenide atoms. The stretching force constants are calculated using the relation put forward different authors. We assume the valence force field which allows the determination ot the vibrational frequencies of the possible structural units. By using the expressions derived by Herzberg, the vibrational frequencies are calculated with the tetrahedral and pyramidal units which appear in the structural model. These frequencies let us simulate the infrared absorption bands for the alloy above. Thus, the infrared features are assigned to Ge-Se (190 cm⁻¹ and 250 cm⁻¹) bonds in GeSe₄ tetrahedral units and Sb-Se (120 cm⁻¹ and 135 cm⁻¹)bonds in pyramidal molecules. The band at 90 cm⁻¹ is considered as being due to Se atoms chains.

A procedure has been considered for determining expressions for the volume fraction crystallized and for the kinetic parameters in non-isothermal reactions in solid system involving the formation and growth of nuclei. This method makes use of an equation for the evolution with time of the volume fraction crystallized. This equation has been integrated under non-isothermal conditions and assuming an Arrhenian temperature dependence of the nucleation frequency and of the crystal growth rate, thus obtaining a general expression for the volume fraction crystallized for each value of the related parameter with the dimensionality of the crystal. The kinetic parameter have been deduced, obtaining the maximum crystallization rate, bearing in mind the fact that, in the non-isothermal processes, the reaction rate constant is a time function through its Arrhenian temperature dependence. Finally, the theoretical expressions of the kinetic parameters have been applied to the experimental data corresponding to a set of glassy alloys, quoted in the literature, thus obtaining mean values that agree very satisfactorily with the published data. This fact shows the reliability of the theoretical method developed.

The glass formation and devitrification of alloys in the Cu-As-Se system were studied by differential scanning calorimetry. A comparison of various simple quantitative methods to assess the level of stability of the glassy materials in the above mentioned system is presented. All of these methods are based on characteristic temperatures, such as the glass transition temperature, T_g, the temperature at which crystallization begins, T_in, the temperature corresponding to the maximun crystallization rate, T_p, or the melting temperature, T_m. In this work a new parameter K_r(T) is added to the stability criteria. The stability of some ternary compounds of the Cu_x As_0.5-xSe_0.5 type has been evaluated experimentally and correlated with the activation energies of crystallization by this new kinetic criterion and compared with those evaluated by other criteria.

Crystallization of Fe_77.5Si_7.5B₁₅ amorphous ribbons has been monitored by thermomagnetic and resistivity measurements during furnace and Joule heating. X-ray diffraction and magnetic measurements have been performed at different stages of transformation to identify crystalline phases that are formed during crystallization and at the end of isothermal and constant current annealing. The results of these measurements indicate that after the crystallization is completed, further transformation takes place in the structure of crystalline phases.

Crystallization of M₇₈Si₉ (M=Fe,Co,Ni) glasses has been studied by differential scanning calonmetry (DSC), thermomagnetic gravimetry (TMG) and X-ray diffraction. Devitrification for Fe and Co alloys occurs in two main stages: a primary precipitation of metal-rich (Fe,Si or Co, Si) phase and a polymorphic reaction to give a Fe₂B- or Co₂B-type phase. For Ni alloy a single crystallization stage was detected by DSC, and Ni and Ni₃B-type phases were found in crystallized samples.

Diffraction techniques are used for the investigation of the microscopic structure of the amorphous material. In this work the local order of the amorphous semiconductor As₂Se₃ from data obtained from the radial distribution function (RDF) determined from X-ray diffraction intensities were analyzed. Interatomic distances and coordination number were determined as a result of experiment.

The paper gives a discussion on the kinetics of nucleation and crystal growth of nanocrystalline grains in amorphous AINiNd alloys. Experimental procedure includes the determination of the kinetics of devitrification under isothermal and continuous heating conditions by means of differential scanning calorimetry, X-Ray diffraction and transmission electron microscopy. The kinetic data are analyzed in terms of the evolution in temperature of the Gibbs free energies of formation of the competing phases from the undercooled liquid. The sequence of two processes, each one with a characteristic temperature range for occurring, is connected with the thermodynamic data coming from the assessment of the rich Al part of the quaternary system. Both microstructural evolution and calorimetric accidents are discussed from the experimental calorimetric and structural data and a first unified view of the crystallization behaviour is presented.

Possible physical and chemical changes induced in pure amorphous polymers were investigated. No chemical changes were observed which implies that the covalent bonds remain intact. Physical changes include a decrease in magnitude of the endothermic glass-transition peak for longer milling times, which compares well to the effect found when the polymer is quench-cooled. Recent research revealed that in an amorphous alloy the same effect occurs both when the alloy is quench-cooled and when it is stretched. An explanation which is based on the amount of free volume in an alloy can thus be applied to polymers as well, which would imply that the kinetics of the transition are typical of the amorphous structure rather than of the specific chemical constituents.

Amorphous transition metal-metalloid alloy particles can be prepared by chemical preparation techniques. We discuss the preparation of transition metal-boron and iron-carbon particles and their magnetic properties. Nanometer-sized particles of both crystalline and amorphous magnetic materials are superparamagnetic at finite temperatures. The temperature dependence of the superparamagnetic relaxation time and the influence of inter-particle interactions is discussed. Finally, some examples of studies of surface magnetization of α-Fe particles are presented.

The dynamical behavior of γ-Fe₂O₃ particles in a polymer having the same volume distribution and interparticle interactions of different strengths were studied by means of a.c. susceptibility measurements (5 < v < 10⁴ Hz) and Mössbauer spectroscopy. The blocking temperature measured by a.c. susceptibility increases with increasing interactions, and that measured by Mossbauer spectroscopy decreases. The energy barrier increases, unambiguously, in both cases, in agreement with our model based on a statistical calculation of the interaction energy for a disordered assembly of particles with volume distribution and easy axes in random directions.

The dependence of the structural properties of mechanically alloyed Fe₃₀Co₂₀Cu₅₀ with the milling time has been studied by X-ray Absorption Spectroscopy (EXAFS and XANES). In order to determine the short range order around the three components, experiments have been performed at the three Fe, Co and Cu K-edges. It can be observed that after short milling times, Co and Cu form an alloy with fcc structure, while Fe atoms enters the alloyed phase only after longer milling times.

The results of self-consistent nonlocal (GGA) density functional calculations are reported for small Cobalt clusters (n ≤ 6). An emphasis is made on a proper treatment of correlation effects. The magnetic as well as the bonding properties of these clusters are discussed in terms of a delicate balance between intra- and interatomic exchange and correlation. The enhancement of magnetic moments is analysed in terms of cluster symmetry. Finally, the present results are compared with other theoretical calculations obtained for these systems.

We present a simulation model in order to study the formation of nanoparticles in microemulsions. It has been found that the size of the particles depends on the mode of the synthesis: particle size changes with concentration of reactants, with surfactant and oil type (film flexibility), and with the size of the microemulsion droplet (water/surfactant ratio). This paper is focussed on the influence of droplet size on particle size distribution and sizes of nanoparticles. We have compared the simulation results with experimental data taken from different authors. The high degree of concordance between both kind of results supports the conclusions of this study.

Ductile Al₈₆Y₅Ni_9-xFe_x (x = 2, 4 at.%) alloys with high microhardness obtained by rapid quenching were found to be in the structural state characterized by nanoscale fcc-Al particles embedded in an amorphous matrix. Variation of cooling rate controlled by the rotational wheel speed (15 to 55 s⁻¹) causes the formation of different amount of the volume fraction of the nanoscale particles. The replacement of Ni by Fe causes a decrease in crystallite size from 20 nm (x = 2) to 10 nm (x = 4).

In this work we used the Voter and Chen version of the embedded atom model in a molecular dynamics study of the ground-state structures and ordering tendencies of Ni_26–xAl_x clusters, for all concentrations x. The results reported here supplement those of previous work in which we computed the structures of Ni-Al clusters of other sizes using the same model potential.

We have investigated the magnetoresistive properties of Cu rich-Co thin films. Samples with nominal composition Cu₉₅Co₅ have been deposited on glass substrates, at room temperature, by pulsed laser ablation of the pure elements from rotating circular targets. Post isothermal annealing optimizes the magnetoresistive effect to give a non saturated value of 10% in 1 Tesla at 77 K. X-ray diffraction analysis of the films show broad diffraction peaks, slightly shifted from pure Cu (111) reflection peak, indicating that both constituents form nanometer sized grains, and some Co is dissolved in the Cu rich grains. Scanning tunnelling microscopy investigations, in the topographic mode, have revealed pyramidal growth with basal dimensions about 100 nm. The atomic force microscopic analyses show the existence of nanoparticles with an average height value of about ≈ 9-10 nm. Furthermore, we observe a highly anisotropic magnetoresistive behaviour for samples obtained at low rotation velocities, 10-15 rpm. A reversible magnetoresistance is observed when the magnetic field is applied perpendicular to the film plane. Coercive fields as large as ≤1.7 kOe are found when the magnetic field is applied at angles to the film plane in the range 70 - 80 deg. On the other hand values of 0.4 kOe are observed for the case when the magnetic field is in the plane of the film. These properties are discussed in terms of the shape anisotropy of magnetic granules.

Microemulsion techniques have been used to prepare Co nanoparticles subsequently covered with a Ag coating. The resulting particles were characterized by neutron activation analysis to determine the actual Co/Ag ratio and the organic proportion in as-prepared samples. XRD' and TEM were used to characterize the degree of crystallization and size of the particles. TEM results show grain sizes of ~10 nm and a small size variation among the nanoparticles. As-prepared samples show a small degree of crystallization which improves with the annealing temperature in a H₂ reducing atmosphere. Magnetization versus temperature measurements showed the same blocking temperature, T_B=4.0±0.5 K, in all the as-prepared samples independently of the Co/Ag ratios. Upon annealing at low temperatures (T_A≥200°C), T_B shows a moderate increase and a saturation magnetization (M_s) enhancement of near 40 % over the Co bulk value. We propose that in this range of annealing temperatures, the reduction of the Co cores, initially partially oxidized, is the main process. When T_A≤250°C, the separation between zero field cooled (ZFC) and field cooled (FC) magnetization data begins at temperatures over the maximum in ZFC and M_s decreases with T_A. These results can be interpreted as the formation of larger particles.

Starting with ferric alcoxide, α-Fe₂O₃ (hematite) nanoparticles were synthesized. XRD, TG, TEM and magnetization measurements were employed for characterizing the samples. The results show nanoparticles of 55Å and 73Å mean diameter for the diverse series with a narrow size distribution. We present correlations between the synthesis parameters and the final particle size.

We have synthesized iron particles encapsulated in graphite by an electric arc decomposing simultaneously a graphite electrode and iron. We have characterized magnetically the sample observing the superparamagnetic and blocked behavior of the particles. We have also observed the ferromagnetic transition corresponding to the Curie temperature of the iron nanoparticles at T_c = 1137K. We have estimated the composition and size of the nanoparticles. The results show a Fe particle mean diameter < ϕ > = 9 nm with broad range of size (from 4 nm to 11 nm). These samples have a 6% Fe / 94% C composition and thus result in a 15 nm of carbon mean cover.

Room temperature Mossbauer spectroscopy and magnetisation measurements as a function of temperature have been performed on nanocrystalline iron powders produced by high-energy ball milling with different grain size in the 8-19 nm range. The results indicate that the magnetic behaviour is strongly affected by the structural configuration of the nanocrystalline samples.

Silica gels containing metallic iron particles are prepared from a mixture of tetraethyl orthosilicate, ethanol, 0.1N nitric acid and formamide in the proportion 1:4:4.5:1 in which iron is added in the form of nitrate. The range of Fe/Si molar ratio was varied from 0.01 to 0.2. After drying, the xerogels are thermally treated under hydrogen in the temperature range 600 to 1000°C. X-ray patterns and Mössbauer spectra show that the non ferromagnetic phase Fe₂SiO₄ (Fayalite) is formed, whatever iron concentration, in the annealing temperature range 700 to 850°C. In addition, some metal particles are also formed by partial reduction. For treatment at lower temperatures, the particles formed are reoxidized when the samples are exposed to air. A nearly total reduction of the metal is only obtained at high temperatures (≥ 1000°C). The particles are then large enough to prevent reoxidation. Furthermore, transmission electron microscopy shows the presence of a coating at the interface metallic particles-silica which can act as passivating layer.

A theoretical model describing the surface anisotropy as well as surface magnetoelastic properties (“surface magnetostriction”) is proposed. The model is based on the magnetic dipole - dipole interactions. The magnetic nanoparticles of perfect spherical shape in nonmagnetic environment are considered. The calculations are limited to the simple ferromagnets of a high symmetry such as body centered cubic (bcc) and face centered cubic (fcc). It is shown that the surface contribution to both, effective magnetic anisotropy (due to the internal stresses, via the surface magnetostriction) and effective magnetostriction (due to the dipolar effects) is responsible for some peculiar properties of nanoparticles.

Using a many-body potential based on the second moment approximation to the tight binding method, we have studied the structures and melting of Ti_N and Zr_N clusters (N = 2–25) by means of molecular dynamics simulations. The results are compared with those obtained previously for Ni_N, Pd_N, AU_N and Ag_N clusters on the basis of the same model potential.

Granular ribbons of composition Co₁₀Cu₉₀ show giant magnetoresistance (GMR) when heat treated. In order to understand the kinetics of the transformation of the original ribbons into the aggregate with a Co cluster distribution, we have measured the resistivity of these materials during anneals at different heating rates. These anneals show a clear two step process, associated to transformations related to the Co (or Co-rich) grains. The hysteresis curves and magnetoresistance for these samples, measured at room temperature, show a clear change in size for the magnetic grains considering superparamagnetic interactions. The dependence of the magnetoresistence on the magnetization is investigated using a a model developed by taking explicitly into consideration the magnetic moment distribution and the spin-dependent scattering at the interfaces and within the grains. Fits to a pure superparamagnetic model show that the expected dependence of the GMR with the square of the magnetization is not followed. We have also measured, at low temperatures, the thermopower, and resistivity of these alloys at different stages of the annealing process. The low temperature resistivity shows a distinct minimum around 20 K for the “as-made sample, which disappears upon annealing at even the lowest stages.

Diffusion and relaxation properties of interacting systems, which include supercooled liquids, glasses, polymers, ionically conducting glasses and melts, and concentrated colloidal particles, show very similar behavior. These remarkable similarities point to the possibility that they originate from common basic physics proposed by the coupling model. In this work, the diffusion of concentrated colloidal particles are discussed, reviewed and compared with the predictions of the coupling model.

The plastic deformation of amorphous Pd₄₀Ni₄₀P₂₀ and La₅₀Al₂₅Ni₂₅ is investigated as a function of temperature, strain rate and preannealing time. The structural state of the material after the deformation is investigated using Differential Scanning Calorimetry. It is found that amorphous Pd₄₀Ni₄₀P₂₀ and La₅₀Al₂₅Ni₂₅ when preannealed into the metastable equilibrium state, show strain softening when deformed at temperatures close to the glass transition temperature. The DSC traces show that the structural state of the material after the deformation differs from the metastable equilibrium state and depends on the strain rate, but is independent of the structural state of the material prior to the deformation. It is shown that the strain softening effect is caused by the creation of additional free volume during the deformation.

An experimental device to measure simultaneously the rotation speed and the longitudinal displacement has been developed. The operation principle is based on magnetic measurements, so there is no contact between the movable member and the sensing element. There is an excellent linearity in the speed response and it is possible to determine the position in a wide range of displacement.

Acid aqueous solutions of hydroquinone have been treated by electrochemical methods in order to obtain the oxidation of this toxic compound to Co₂ and water. The reaction rate of hydroquinone decomposition at the different Ni-Nb-Pt-Sn amorphous alloys has been obtained by means of the measurement of amount of hydroquinone disappeared during the electrochemical oxidation. The disappearance rate increases when tin and platinum are present in the amorphous alloy. The synergistic effect of tin-platinum is demonstrated by these results.

Blends of polycarbonate and poly(acrylonitrile-co-butadiene-co-styrene) with different sample compositions have been studied by dynamic-mechanical, dielectrical and infrarred spectrometries, and by differential scanning calorimetry. The samples show phase separation but partial miscibility is ascertained by shifts in the temperatures of the main relaxations of both phases. Interactions (secondary forces) between both molecules are of non-polar nature, and can be enhanced by the preparation procedure. The dynamic-mechanical spectra have been modellized with a simple equation which takes into account the temperature displacements of the main peaks. Comparison with the experimental results permits to determine which of the two polymers acts as a continuous phase in the blends.

Many ionic conducting systems, both crystalline and glasses, show an “universal dynamic response” in conductivity, in which the ac conductivity follows the law σ(ω) = σ(0) + Aω^s. The vitreous Ag-S-Ge system has been studied with alloys of 15, 20 and 33 Ag at. %. All alloys show, like many other disordered materials, a non-Debye electrical behavior, with a strong increase in conductivity with Ag content. Crystalline counterpart for the different alloys were obtained by isothermal crystallization. It is observed an increase in conductivity in these crystalline samples. On the other hand both crystalline and glassy, show an arrhenian temperature dependence in dc conductivity. The activation energies obtained increase with silver content in both types of samples. This energy is lower for crystalline alloys than for amorphous samples. The results are discussed in terms of theoretical ionic conduction models.

The electrochemical behaviour of three different Co-containing FINEMET alloys has been studied using cyclic voltammetry. In presence of big amounts of cobalt, oxidation processes are almost negligible at low anodic potentials. When the samples are submitted to termal relaxation and nanocrystallization processes, the oxidation charge decreases, probably due to the formation of a SiO₂ film on their surface. Polarization experiments show that the addition of Co shifts the corrosion potential anodically.

In this work we present experimental results of phase separation of an epoxy resin modified with elastomers and of full IPNs formed by blends of epoxy resin with unsaturated polyester. The elastomeric modifiers used change the reaction kinetics because they dissolve the plasticizer of the hardener, accelerating the curing reaction and increasing the glass transition temperature of the epoxy resin. The IPNs show only one glass transition when they are not cured. However, some compositions exhibit two or more transition regions when they are fully cured.

In the present work we study relaxation processes by differential scanning calorimetry DSC of Ge₁₀Se₉₀ glassy alloy prepared by melt-quenching technique and annealed for 0.16 to 1344 h at temperatures in the range 323 to 400 K. A distribution of activation energies gives an accurate description of the measurements. The corresponding activation energy spectrum characterizing the relaxation processes was obtained. The energy range approximately between 1.00 and 1.25 eV and has a peak energy of 1.14 eV. Given the complexity of the relaxation process, the experimental results have been analyzed using the empirical Kohlrasch-Williams-Watts relaxation model. The experiences utilized to perform such analysis are the ones obtained in the isothermal regime.

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