Activation Energy Spectrum Research Articles

Clusterization of light interstitial impurities and the 1/f noise in metals

The effect of clusterization of light interstitial impurities on the 1/f noise spectrum in metals has been studied by numerical simulation. It is shown that an increase in the intensity of the 1/f noise may serve as an indication of clusterization, and that the shape of the activation energy spectrum can provide information on the character of impurity ordering in the clusters formed.

Low-field magnetic properties of amorphous and nanocystalline FeCrCuNbSiB alloys

The AC susceptibility dependence on magnetic field, time and temperature of amorphous as well as nanocrystalline Fe 73.5− x Cr x Cu 1Nb 3Si 13.5B 9 ( x=0–4) alloys was studied. Micromagnetic model is used for calculating the activation energy spectra (AES) of the magnetic after-effect (MAE). It was observed that addition of Cr to the amorphous FeCrCuNbSiB alloys highly decreases the amplitude of the MAE so that no MAE is observed for Cr content higher than 2 at%. After annealing at 550°C, the initial susceptibility increases as a result of magnetic softening during nanocrystallization and the MAE vanishes. The nanocrystalline state was characterized by the high magnetic as well as structural stability. Moreover, addition of 1 at% Cr makes the initial susceptibility of the nanocrytalline sample higher than in the FINEMET alloy.

A new look into the pyrolysis of polystyrene

Pyrolysis of polystyrene has been investigated in the temperature range 400–650 0 K and up to 40% conversion. In this study, we used polystyrene particles (around 140 μm) suspended in an electric field and heated by radiation (very fast heating rates). The main advantage of this method is that kinetic interpretation is straightforward and unique because chemical reactions are not coupled with heat and mass transfer processes. Due to the fast heating, the initial structure of the polymer is not modified and therefore the results relate to the initial chemical structure and intrinsic rate. The following properties were determined in real time: mass, diameter, density, optical properties, and temperature. From mass loss rate versus inverse temperature, and activation energy of 103.1 kJ/mol was determined. This value is in the low spectrum of activation energies measured by other methods. From the behavior of the squared diameter with time, it was found that pyrolysis obeyed a d 2 law. Density was found to remain constant with extent of reaction, indicative of a non-charring process. The novelty of this study is the ability to determine the absorption coefficient of the particle (from the photophoretic force measured in this study). This enabled a look into the condensed phase during reaction, in real time. The absorption coefficient did not change during the first 8% of conversion, then it increased gradually with conversion. Since the absorption coefficient is directly related to the chemical structure and extent of cross-linking, constancy indicates that pyrolysis occurred without changes in the chemical structure. The gradual increase of the absorption coefficient (above 8% conversion) indicates that significant changes occurred during pyrolysis, mostly by increasing the molecular weight or cross-linking of the polystyrene chains.

Magnetic after-effect in FeNiZrCuB amorphous alloys

The magnetic after-effect (MAE) and AC susceptibility in the amorphous Fe 86 − x Ni x Zr 7Cu 1B 6 alloys with different contents of Ni ( x = 0–86 at%) were investigated. The Curie temperatures T c were estimated from the temperature dependences of the AC susceptibility measured in the temperature range 20–550°C. Additions of Ni up to x = 43 at% cause rapid increase of the Curie temperature from 71°C for Fe 86Zr 7Cu 1B 6 to 373°C for Fe 43Ni 43Zr 7Cu 1B 6 alloys. Higher concentrations of Ni cause decrease of T c down to paramagnetic state of Ni 86Zr 7Cu 1B 6 alloy at the room temperature. It was found that the additions of Ni drastically change shapes and amplitudes of MAE spectra. From the MAE spectra measured in the temperature ranges (− 197°C; T c) the activation energy spectra with the most probable value Q = 1.23 eV were obtained.

Activation energy spectra for annealing of ion irradiation induced defects in silica glasses

In situ stress measurements were performed on alkali-borosilicate glass samples during and after 2 MeV Xe ion irradiation at several temperatures between 95 and 580 K. After switching off the ion beam, stress changes are observed that are related to the annealing of ion beam generated defects. The activation energy spectra for defect annealing are obtained from the data at each irradiation temperature. Defects are observed in the energy range from 0.26 to 1.85 eV. At each temperature the spectrum increases monotonously with activation energy. At each energy the defect density per unit energy is smaller at higher temperatures. This behavior can be explained using a binary collision model. The data are contrasted against the results obtained for 4 MeV Xe ion irradiation of thermally grown SiO 2 films, which can be explained using a thermal spike model. Measurements of the radiation induced viscosity support these ideas.

Effect of metalloid substitution on the reversible structural relaxation of Fe–Ni–B(-P, Si) metallic glasses

The reversible structural relaxation of Fe 37.5Ni 37.5B 25− x P x ( x=0, 3, 6, 16, 25) and Fe 37.5Ni 37.5B 25− y Si y ( y=0, 5, 10, 15) metallic glasses was investigated using the activation energy spectra (AES) model by differential scanning calorimetry (DSC) and resistivity measurements. From the deduced activation enthalpy spectra for these amorphous alloys, it is concluded that possibilities for atomic movements with characteristic activation energies are created by the substitution of B with P or Si. It is shown that at least one Gaussian contribution to the AES appears on increasing P or Si content, superimposed on an exponential part previously found for Fe–Ni–B glasses. The amplitude of the Gaussian function increases with increasing P or Si content while its position decreases with increasing P and remains constant with increasing Si content. According to isothermal resistivity measurements, for P substitution the reversible atomic rearrangements connected to the Gaussian contribution increase the resistivity while those associated with the exponential part decrease it. For Si substitution both processes decrease the resistivity.

Time and thermal stability of magnetic properties of amorphous Fe80TM3B17 alloys

We have investigated the influence of the addition of different transition metals (Fe. Co. Cr. Mn. Mo. Ni. Os. Pt. Ru. Ta. Ti. V and W) on the time-temperature stability of amorphous Fe 80 TM 3 B 1- alloys. Time stability is represented by the Magnetic After - Effect (MAE). Each element influences near neighbourhood of other atoms by its characteristic way which results in a different chracteristics of the MAE spectra and its spectra of activation energies. It turned out from our measurements that samples can be divided into the three groups: The first one contains alloys with MAE similar to that of an FeB alloy, in the second there are alloys with different MAE, and in the third one there are alloys with almost no MAE. It was found that MAE can be successfuily descnbed on the basis of the micromagnetic model with activation parameters Q * = 1.3 - 1.7 eV and τ 0 = 10 -18 -10 -13 S. Small values of the preexponantial factor points out that MAE in Fe 80 TM 3 B 17 amorphous alloys is caused by the rearrangement of the mobile atom pairs within the domain walls.

Determination of Activation Energy Spectrum for Fe<sub>40</sub>Ni<sub>40</sub>B<sub>20</sub> Metallic Glasses

Very inherent metastable character of amorphous metal alloys, induces structure phase transformations which manifest themselves in thermally activated relaxation processes. The kinetics of these processes can be interpreted with the aid of two models: two equilibrium states and spectrum of activation energies. Structure relaxation of amorphous Fe 40 Ni 40 B 20 metallic glass has been observed by measuring changes in electrical resistivity at corresponding isothermal treatments. Electrical resistance changes have been measured at liquid nitrogen temperature (78 K) using the d.c. method and a fixed probe. By measuring the resistivity change during isothermal treatment, logarithmic kinetics has been observed. Also, Q(E) function has been derived having a symmetric distribution around the value of activation energy of 2.1 eV, determined by some other methods too.

Determination of activation energy spectra of irreversible structural relaxation of metallic glasses using non-isothermal stress relaxation data

A new method for determination of activation energy spectra of irreversible structural relaxation of metallic glasses using the kinetics of torque relaxation under linear heating has been developed. The method is simple, has acceptable accuracy, and lacks some limitations inherent to the known spectrum reconstruction methods. An experiment was carried out to determine the real structural relaxation spectrum of a Co-based metallic glass by this method. The form of this spectrum can indicate that plastic flow under intensive structural relaxation is realized via a set of atomic rearrangements in regions with `faulty' and `good' short range order.

Method to Study Crystallization Kinetics by Electrical Resistivity

A procedure of transformed material fraction evaluation from electrical resistivity isochronal measurements was characterized. The amorphous-to-crystalline transformation in Fe-B alloys was taken as the model eutectic and primary reaction. The results were applied to the experimental resistivity data obtained by Dehghan [9]. The interpretation was done with help of the latest ideas of activation energy spectrum developed for primary reaction.

Magnetic after-effect in amorphous Fe 85 − xCr xB 15 alloys

We have investigated the influence of chromium on the magnetic after-effect (MAE) of amorphous Fe 85 − x Cr x B 15 ( x = 0, 2, 3, 5 and 7) alloys. Isochronal curves were obtained for times t = 5, 10, 30, 60, 120 and 180 s in the temperature interval from 77 K up to T c. A computer analysis was performed to evaluate the spectra of activation energies from these measurements. The spectra of activation energies have a characteristic shape with a peak at the value of 1.2–1.4 eV. The pre-exponential factors are found to lie between 4 × 10 −13 and 1.1 × 10 −17 s. This indicates that the MAE can be assigned to short range atomic reorientation near free volumes. It follows from the isochronal MAE measurements that the addition of Cr atoms also causes a decrease in MAE amplitude as well as a decrease in Curie temperature. The higher the Cr-contents, the lower the MAE and the preexponential factor τ 0.

Dilatometric measurements on metallic glass ribbons with a wide glass transition range

A special dilatometric device which avoids tensile stress to the specimen is used to examine structural relaxation, glass transition and crystallization of metallic glass ribbons of Al 7.5Cu 17.5Ni 10Zr 65 and Pd 40Ni 40P 20. The relaxation kinetics can be well described either in the free volume model or by the Kohlrausch-Williams-Watts formula or by an activation energy spectrum, all indicating co-operative atomic rearrangements. The first model yields for the product of the critical free volume and overlap factor, initiating a structural relaxation event, values of 0.17 (Al 7.5Cu 17.5Ni 10Zr 65) and 0.14 (Pd 40Ni 40P 20) of an atomic volume. The coefficient of thermal expansion (and its temperature dependence) is measured and found not to change with crystallization in the case of Al 7.5Cu 17.5Ni 10Zr 65, unlike that of Pd 40Ni 40P 20.

Magnetic relaxation and soft magnetic properties of annealed amorphous FeZrCuB alloy

In the work presented, we have investigated temperature dependences of the relative permeability and the relative resistivity of amorphous FeZrCuB alloys with increasing and decreasing temperatures in the range 50–600 °C. Curie and crystallization temperatures were estimated from the analysis of measured dependences. The spectra of magnetic after-effect (MAE) were measured in the temperature range ( − 100 °C, T c). Two maxima in the relaxation spectra have occurred at the temperatures − 30 and 70 °C. The first maximum corresponds with the magnetic relaxation and the second corresponds with magnetic after-effect induced by phase transition. The spectrum of activation energy of magnetic relaxation was found by computer analysis. The most probable value of the activation energy is 1.25 eV. In this work, we have also studied microstructures of samples annealed near the crystallization temperature.

Anelastic deformation processes in metallic glasses and activation energy spectrum model

The isothermal kinetics of anelastic deformation below the glass transition temperature (so-called ‘stress induced ordering’ or ‘creep recovery’ deformation) was investigated in Ni-Si-B metallic glass. The relaxation time spectrum model and two recently developed methods for its calculation from the isothermal experimental data sets seem to be powerful tool for the study of anelastic deformation processes in disordered systems. Reliability of the used calculation methods (the first is based on the Fourier transformation technique and the second on the non-linear regression) is expressed in the fact that both provide qualitatively the same result (without prior assumptions concerning the form or nature of the spectrum): creep recovery relaxation spectrum consists of three relatively separated peaks with the well defined characteristic relaxation times. The influence of (i) loading time, (ii) the structural relaxation and (iii) unloading temperature on the spectrum is demonstrated and discussed. Using the assumption that different peaks in creep recovery spectra represent different type of elementary deformation processes (or different type of deformation defects) detected during the creep recovery, discussion concerning the eventual deformation defects will be actualized.

Analysis of the activation energy spectrum for the enthalpy relaxation of a glassy liquid crystalline polymer

The phenomenology of the physical ageing of glassy materials can be described by using the activation energy spectrum model. This model was originally developed for relaxations in metallic and oxide glasses and this paper applies it to a liquid crystalline polymer, poly(diethylene glycol p, p′-bibenzoate) (PDEB), in the glassy state.

Anelastic relaxation in amorphous Pd 40Ni 40P 20 studied with elongation and electrical resistance measurements

The anelastic strain of amorphous Pd 40Ni 40P 20 at temperatures in the vicinity of the glass transition temperature has been studied. To overcome some of the restrictions of elongation measurements, electrical resistance measurements were used for measuring the relaxation of the quenched-in anelastic strain. The validity of this method is shown. The relaxation of the anelastic strain is described with a spectrum of activation energies between 150 kJ/mol and 350 kj/mol. The rate of relaxation depends on the concentration of diffusion defects. The relation between the anelastic relaxation and the chemical short range ordering process is discussed.

Reversible structural relaxation in melt-spun Co 80− xCr xB 20 ( x = 20, 25 or 30) amorphous alloys

The structural relaxation in melt-spun Co 80− x Cr x B 20 ( x = 20, 25 or 30) amorphous alloys was investigated by measuring the residual electrical resistance (liquid-N 2 temperature). The reversibility, the specific logarithmic kinetics and the cross-over effect were confirmed for the chemical short-range ordering (CSRO) process in the Co 50Cr 30B 20 alloy. Isothermal measurements were performed mainly for this alloy, and the results were interpreted by the activation energy spectrum model. Spectra of activation energy showing three peaks were obtained by numerical calculations based on Lagrange multipliers.

Complex permeability after-effects in an annealed Co-based amorphous alloy

We have investigated the magnetic after-effects (MAEs) of the real and imaginary parts of the ac inverse permeability r [r(t) ≡ 1/μ = r1(t) + jr2(t)]. For a near-zero magnetostrictive CoFeCrSiB amorphous alloy, it was found that r1(t) after demagnetization increases, whereas r2(t) mostly decreases. The activation energy spectra which control the MAE1 and MAE2 of r1(t) and r2(t), respectively, show different distributions p1(Q) and p2(Q) in the range 0.72–1.1 eV, with maxima at 0.78 eV (τ0 = 3.6 × 10−14 s).

Study of structural relaxation in ferromagnetic amorphous Fe-Si-B ribbons by MAE investigations

The present study concerns structural relaxation in a Fe-based amorphous alloy via the influence of temperature (293-673 K) and time (3-10/sup 4/ s) on the value of the initial permeability. Magnetic after-effects at low temperatures (disaccommodation) and structural relaxation at high temperatures have been observed and analyzed. On the one hand, assuming a first order kinetics of the relaxation processes, one mode spectrum of activation energies was obtained. On the other hand, the initial permeability, correlated to anisotropy, is a good probe for investigation of structural relaxation during annealings.

Exact deduction of activation energy population distributions from evolution rate measurements during tempering

It is shown that, by using a tempering profile in which temperature increases as an exponential function of time and deduction of the moments of the evolution rate-reciprocal temperature product in the temperature domain, the moments of the initial population density amongst a continuously distributed activation energy spectrum can be deduced. The analysis is exact for first order reaction kinetics and accurate to within a few percent for second order reaction kinetics.