Johnson Mehl Avrami Exponent Research Articles

Optical and thermal properties of Sb/Bi-modified mixed Ge-Ga-Se-Te glasses

The non-isothermal crystallization kinetics of novel BixGa5Ge20Sb10-xSe45Te20 (x = 1,3,5,10) glasses are studied with differential scanning calorimetry method, and analyzed using Frazer-Suzuki fitting function. The applicability of Johnson-Mehl-Avrami (JMA) model to describe the crystallization kinetics in these materials is verified, and JMA exponent is determined for the relevant crystallization processes. It is found, that substitution of Sb with Bi in glass composition leads to a decrease in optical gap and thermal stability of the glass, and increase in the density and crystallization activation energy for glass-ceramics formation.

Crystallization behavior of phosphate glasses and its impact on the glasses’ bioactivity

The crystallization behavior of phosphate glasses of the composition 50P2O5–(40−x)CaO−xSrO–10Na2O with x = 0, 20, and 40 was investigated using non-isothermal differential thermal analysis. The study was performed on three different size fractions (fine powder ( 500 μm)). The DTA thermograms recorded for all glasses exhibited a single and symmetrical crystallization peak. The position and shape of the crystallization varies with the particles size. The activation energy for crystallization, Ec, was calculated using equations proposed by Kissinger and Friedman. The calculated Ec values were similar for both techniques. Ec decreased with increasing the particle size for all glass compositions. The Johnson–Mehl–Avrami exponent n expressing the crystal growth dimensionality was quantified using the methods proposed by Augis–Bennett (AB) and Ozawa. In general, the AB method gave higher value than the Ozawa technique. For all glass composition the n value for the coarse powder suggested bulk crystallization whereas with decreasing particle size, the value of n indicated a complex surface crystallization. Glass monoliths were heat treated in electric oven at several temperatures for various times. XRD suggested crystallization of two phases: Ca(PO3)2 and NaCa(PO3)3 for the glass with x = 0, Sr(PO3)2 and NaSr(PO3)3 for the glass with x = 40, and a solid solution of (Ca,Sr)(PO3)2 and Na(Ca,Sr)(PO3)3 for the glass with x = 20. All crystals were found to preferentially nucleate and grow from the surface. Partially to fully crystallized glass particles were immersed in simulated body fluid for 24, 48, and 72 h. SEM/EDS of the particles and changes in the pH and ion concentrations of the solution (ICP-OES) suggested that crystallization prevents the CaP layer formation for all partially crystallized glasses.

Kinetics of Hematite to Wüstite by Hydrogen for Chemical Looping Combustion

Kinetics analysis of hematite (Fe2O3) reduction by hydrogen was evaluated by the thermogravimetric analyser (TGA) in the temperature range of 700–950 °C, using continuous streams of 5, 10, and 20% H2 concentrations in N2. A number of kinetic models have been considered, including the single and multi-step models to describe the experimental reduction data. The details of nucleation and growth during the isothermal reduction are described in terms of the local Johnson–Mehl–Avrami (JMA) exponent and local activation energy. The variations of n values (JMA exponent) and activation energies for the reduction conversion indicate the presence of the multi-step reaction process. The reduction was shown to be one-dimensional (1D) growth with a decrease in the nucleation rate.

Crystallization Mechanism of the Bioactive Glasses, 45S5 and S53P4

The crystallization kinetics of the two commercial bioactive glasses, 45S5 and S53P4, was studied using differential thermal analysis (DTA), optical microscopy, and scanning electron microscopy (SEM). The thermal properties, the activation energy of crystallization, and the Johnson‐Mehl‐Avrami (JMA) exponent were determined for two glass fractions: fine powder (<45 μm) and coarse powder (300–500 μm). The crystallization behavior of 45S5 was significantly different for the two fractions, whereas the particle size did not affect the crystallization behavior of S53P4. The JMA exponent of S53P4 suggested surface crystallization for both size fractions. However, for 45S5, the JMA exponent suggested that, with increasing particle size, crystallization evolves from predominantly surface crystallization to predominantly bulk crystallization. Surprisingly, SEM imaging did not support this conclusion. A method based on the crystallization rate dα/dt showed that the JMA approach could not be employed for 45S5. The crystallization mechanism of 45S5 appears to be more complex than a simple nucleation and growth process. Nucleation‐like curves were measured for both fractions of the two glasses. The maximum nucleation rate occurred at 566 ± 4°C and 608 ± 4°C for the coarse powders of 45S5 and S53P4, respectively. The higher maximum nucleation temperature of S53P4 was attributed to the higher SiO2 content. The nucleation temperature range of these two glasses together with DTA data makes it possible to develop guidelines for tailoring thermal treatment parameters to achieve desired glass‐to‐crystal ratios.

T–T–T behaviour of bioactive glasses 1–98 and 13–93

Abstract In this work crystallization kinetics of bioactive glasses 1–98 and 13–93 are discussed. Within a certain temperature–time window these glasses can be hot worked into various products without interfering with crystallization. The crystallization was studied isothermally by heating glass plates at different temperatures for different times. Phases in the samples were studied through XRD and SEM analyses. The nucleation-like curves and crystallization characteristics were measured with DTA. The temperature of maximum nucleation was measured for glass 1–98 at 725 °C and for 13–93 at 700 °C. The activation energy of crystallization of both glasses was 280 kJ/mol. The Johnson–Mehl–Avrami exponent and the SEM micrographs of the samples suggested surface crystallization. The primary crystalline phase was wollastonite. The growth rate of the crystallized surface layer was 1 order of magnitude higher in the plates of 1–98 than in 13–93. The results can be utilized to optimize the parameters in hot-working of the glasses.

Nucleation and growth behavior of glasses in the TeO 2–Bi 2O 3–ZnO glass system

The kinetics of crystallization of glasses in the (90 − x)TeO 2–10Bi 2O 3–xZnO system with x = 15, 17.5, 20 and 25 have been studied using differential thermal analysis (DTA), hot stage XRD and optical microscopy. Thermal properties, activation energy for crystallization, Johnson–Mehl–Avrami exponent, nucleation and growth regimes and rates were determined and shown to depend on glass composition. These variations were related to the domain of glass formation in the TeO 2–Bi 2O 3–ZnO ternary glass system: the glasses with x = 15 and 25 are at the limit of the glass formation domain while the glasses with x = 17.5 and 20 are in the middle of the glass-forming region. From the Johnson–Mehl–Avrami parameter, which increases slightly from 1.7 to 2.2 when x increases from 15 to 25, and from optical micrographs, the predominant crystallization in the investigated glasses is expected to be 2 dimensional bulk crystallization governed by diffusion. Using XRD, we found that Bi 2O 3 crystals are the first crystals to form in the glasses when heat treated at their respective temperatures of maximum nucleation. When heat treated at a higher temperature the Bi 2O 3 crystals were found to transform into Bi 2Te 4O 11. We observed an increase in the crystal size when the glasses were heat treated at their respective temperatures of maximum nucleation revealing a probable overlap between the nucleation and growth regimes in all the glasses, the largest overlap being for the glass with x = 15. Thus, this glass is not an appropriate material candidate for applications which require controlled nucleation and growth as a large distribution of crystal sizes can be obtained in the glass when heat treated at its temperature of maximum nucleation rate. The glass with x = 25 is suspected to be also an inappropriate candidate for this kind of applications as it has a higher tendency to fully crystallize in a small temperature range than the other glasses as evidenced by its high N q and I n.

Thermal stability and crystallization kinetics of Cu-reinforced Cu 47Ti 33Zr 11Ni 8Si 1 metallic glass composite powders synthesized by ball milling: the effect of particulate reinforcement

The thermal stability and crystallization kinetics of Cu 47Ti 33Zr 11Ni 8Si 1 gas atomized powder (GAP) and composite powder reinforced with 25 vol.% nanosized Cu second phase particles introduced by ball milling was studied by differential scanning calorimetry in the mode of continuous heating and isothermal annealing. Ball milling leads to successful incorporation of Cu particles within the GAP matrix and hence offers itself as a useful technique to prepare composite microstructures. In the case of continuous heating, both the glass transition temperature T g and the crystallization temperature T x display a strong dependence on the heating rate. The activation energy, as determined by the Kissinger equation, for the first order transformation was found to be almost identical for the GAP (3.53±0.12 eV) and composite powders (3.59±0.06 eV). The isothermal transformation kinetics were modeled by the Johnson–Mehl–Avrami (JMA) equation. The values of the JMA exponent imply that the crystallization of the metallic glass GAP as well as the composite powder is governed by diffusion-controlled three-dimensional growth. The activation energy of crystallization derived from isothermal annealing experiments is nearly the same for the GAP (3.69±0.20 eV) and the composite powders (3.78±0.10 eV). The addition of Cu particles does not noticeably affect the crystallization kinetics of the amorphous matrix composites.

Limitations of the Augis and Bennett Method for Kinetic Analysis of the Crystallization of Glasses and Conditions for Correct Use

The equation proposed by Augis and Bennett for determining the kinetic exponent of the Johnson–Mehl–Avrami (JMA) model is thoroughly analyzed; a new expression, calculated accurately with no assumptions introduced, is proposed. This new method of calculation has been extended to the different kinetic models more commonly used in the literature for describing solid-state reactions. However, determining the JMA exponent from the Augis and Bennett method can lead to an incorrect interpretation of the reaction mechanism unless an additional, independent test is used. A testing method for verifying the applicability of the Augis and Bennett method is proposed. The kinetic analysis of the crystallization of Ge0.3Sb1.4S2.7 has been used for checking this method.

Estimation of Incubation Time for the Erasure Process in Phase Change Materials for Optical Memories

A general approach has been given to estimate incubation time (τ) which was experimentally observed for the first time by Sugiyama et al. for InTe-based optical memories. Prior to this observation it was established that τ was dependent upon the frequency factor (k 0 ), the Johnson-MehlAvrami exponent (n) and the activation energy required for crystallization (E a ). In the present work it is shown that interfacial strain energy (σ); E a and the change in Gibbs free energy (AC,.) play vital roles in deciding τ. It has also been shown that for the case of laser induced (a → c) transitions only the grain boundary nucleation process explains the incubation time observed. Calculations have been presented for a typical Sb 2 Te 3 system and τ has been found to decrease with increase in σ and to increase with increase in E a .

Heteroepitaxial growth of lanthanum aluminate films derived from mixed metal nitrates

Epitaxial lanthanum aluminate (LaAlO3) thin films were deposited on single-crystal substrates by pyrolysis of spin-on mixed nitrate precursors. The films are epitaxial without any second phase. TEM micrographs show that all of these films have pores with sizes ranging from 5 to 30 nm. Grain boundaries are not observed. Selected area diffraction shows that the films are single-crystal-like, despite the porosity. All the films are smooth and crack-free. The precursors first decompose into an amorphous mixture. Heterogeneous nucleation occurs on the lattice-matched, single-crystal substrate surface. The epitaxial films grow upward and consume the amorphous regions. The crystallization temperature of LaAlO3 is lower for thin films than for bulk samples due to nucleation on the substrate. The crystallization of LaAlO3 does not exhibit linear growth kinetics. The Johnson–Mehl–Avrami exponent of growth is between 1.4 and 1.5. This deviation from the linear growth model (n = 1) can be attributed to continuous nucleation on the substrate/film interface.

The complex characteristics of crystallization of the Fe75Si15B10 glassy ribbon

The Fe75Si15B10 glassy ribbon was examined in the course of various isothermal and dynamic heat treatments. The number, type and mechanism of formation of the crystallization products formed during two crystallization stages in this alloy were determined by differential scanning calorimetry (DSC), thermomagnetometry, transmission electron microscopy and X-ray diffraction measurements. It is concluded that the first DSC peak is due to the crystallization of α-Fe(Si) or Fe3Si and the composite microcrystals containing Fe3B cores with α-Fe(Si) envelopes. The metastable Fe3B subsequently transforms into the stable Fe2B in the second DSC peak. The remaining amorphous matrix crystallizes by the eutectic reaction also in the second DSC peak forming the Fe3Si and Fe2B eutectic structures. The apparent activation energy E1⊛ decreases during the first crystallization stage from 480 kJ (g atom-1), characteristic of the Johnson–Mehl–Avrami (JMA) nucleation-and-growth kinetics of the as-quenched sample, to 350 kJ (g atom)-1 implying a dominance of the diffusion-controlled growth of iron in the already pre-nucleated sample. Simultaneously, the JMA exponent n1 decreases from 2.5 to 1.5. The apparent activation energy E2⊛ depends on the temperature of the first crystallization stage. It decreases with increasing Ta from 384 to 282 kJ (g atom)-1. This phenomenon was attributed to modification of the chemical composition of the remaining eutectic after the temperature dependent primary crystallization of Fe3B within the composites. The value of n2 is 3.

Kinetics for changes in anisotropy, coercivity, and argon content of transition metal-rare earth films

Amorphous films of transition metal-rare earth alloys were prepared by rf bias sputtering ∼1000 Å thick, and were protected with ∼1000-Å SiO2, under conditions to provide an easy axis perpendicular to the film plane. Some of the kinetics of the changes in the parameters affecting the domain size were measured, namely the anisotropy, the coercivity Hc and the argon content. Both isothermal and isochronal exposures were used. The decrease in the perpendicular anisotropy K⊥ and Hc and the increase in the planar anisotropy all follow first-order kinetics, that is, the Johnson–Mehl–Avrami exponent n=1.0. The activation energy for the change in K⊥ was 1.0±0.2 eV. The changes in anisotropy were the rate limiting factor in the lifetime of the films. The Hc decreased with ΔE=0.6±0.2 eV while the Ar content decreased with ΔE=0.08±0.05 eV. Because of the large difference in activation energy for the change in Ar compared to the changes in K or Hc we conclude that the rate limiting step for the change in Ar is different than the rate limiting step for the changes in K and Hc. At least two mechanisms have been associated with the changes on annealing. One is the oxidation of the rare earth component even through the SiO2 overlayer; the other is the release of Ar which changes the local stress in the film or changes the degree of isolation of the columnar structure by the Ar.