Isothermal Annealing Experiments Research Articles

Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy

A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150°C–500°C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤300°C, whereas heat treatment at temperatures of ≥400°C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400°C–500°C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate.

Curie temperatures of titanomagnetite in ignimbrites: Effects of emplacement temperatures, cooling rates, exsolution, and cation ordering

AbstractPumices, ashes, and tuffs from Mt. St. Helens and from Novarupta contain two principal forms of titanomagnetite: homogeneous grains with Curie temperatures in the range 350–500°C and oxyexsolved grains with similar bulk composition, containing ilmenite lamellae and having Curie temperatures above 500°C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement temperatures exceeded 600°C, and in laboratory experiments, heating to over 600°C in air causes the homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured during cooling, but little or no change is observed in room temperature susceptibility. We interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although we cannot conclusively rule out an alternative interpretation involving fine‐scale subsolvus unmixing. Short‐range ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the Curie temperature have important implications for the acquisition, stabilization, and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites.

Study of the optical properties of etched alpha tracks in annealed and non-annealed CR-39 polymeric detectors

The UV–visible absorption spectra of virgin and alpha particle-irradiated, annealed and non-annealed CR-39 polymeric track detectors were investigated using a UV–visible spectrometer (Shimadzu mini 1240). Isothermal annealing experiments were carried out on poly allyl diglycol carbonate (PADC) films based nuclear track detectors (NTDs) exposed to a 241Am source. A shifting and broadening of the UV–visible peaks was observed as a result of the etched alpha tracks in the non-annealed and annealed PADC films. The UV–visible spectra of the virgin and non-annealed α-irradiated PADC polymer films displayed a decreasing trend in their optical energy band gaps, both direct and indirect, whereas those measured for the annealed α-irradiated ones showed no significant change. This drop in the energy band gap with increasing fluence is discussed on the basis of the alpha particle- and thermal annealing-induced modifications in the PADC polymeric detector. The results clearly showed that the values for the indirect energy band gap were lower than the corresponding values for the direct band gap. In addition, the Urbach energy was estimated from the Urbach edge, and exhibited roughly the same trend as the optical band gap. Finally, this study presents new results showing that the annealed PADC films were highly insensitive to alpha particles.

The effect of thermal treatment of radiation-induced EPR signals of different polymorphic forms of trehalose

Electron paramagnetic resonance (EPR) signals induced by γ-radiation in different polymorphic forms of trehalose were studied with dosimetry applications in view. Dose response of trehalose in terms of the concentration of induced paramagnetic centers was studied in the dose range from 0.5 to 50kGy. The dependences of the dose responses of anhydrous β-crystalline trehalose (TREβ) and glassy trehalose (TREg) on dose are linear up to 15kGy, whereas the linearity of the dependence for trehalose dihydrate (TREh) is limited to about 10kGy. At doses above 15kGy, the dependences get saturated for all three forms. The relative radiation sensitivities pointed to the following order of decreasing concentrations of radiation-induced paramagnetic centers in the forms: TREg>TREβ>TREh. The results showed that at all three trehalose polymorphic forms are suitable for dosimetry, especially for retrospective dose measurements. Also, thermal stability and decay kinetics of the EPR signals of the different forms of trehalose were studied in isothermal annealing experiments. The kinetic parameters, which had been derived by fitting the Arrhenius function to the measured decay rate constants, indicated that the fading of the EPR signals varied from one polymorphic form of trehalose to another. This emphasizes the impact of the molecular packing in the vicinity of the radiation-induced paramagnetic centers on their stability.

Investigation of the phase transition of Ge2Sb2Te5 films using internal friction method

The phase transition of Ge2Sb2Te5 has been studied by internal friction measurement. Two Q−1 peaks were observed during the process of heating amorphous Ge2Sb2Te5 from room up to 400°C. With a heating rate of 5.5°C/min, the first peak (P1) is located at 180°C and the second peak is (P2) located at 330°C. Only the peak P1 appears when sample has been beforehand annealed at 230°C for 1hour. Both the peaks P1 and P2 don’t appear when sample has been beforehand annealed at 380°C for 1hour. The positions of P1 and P2 are both affected by the heating rate, and they follow Kissinger relation. The activation energy corresponding to the peak P1 is 1.94±0.18eV, and the activation energy corresponding to the peak P2 is 3.49±0.24eV. The isothermal annealing experiment revealed that Q−1 monotonously decreases with time t in the process of phase change, and the internal friction Q−1 versus time t can be represented as Q−1(t)=Q−1(∞)−[Q−1(∞)−Q−1(0)] exp (−t/τ). The characteristic time τ is affected by temperature, and lnτ increase linearly with T−1.

Kinetics of Tritium Release from Thermal Neutron-Irradiated Li0.17Pb0.83

Tritium release behavior for thermal neutron-irradiated Li0.17Pb0.83 eutectic alloy was studied. Main tritium release peak was observed in the temperature just a little higher than melting point in a thermal desorption spectrometry (TDS) experiment. Most of tritium release from Li0.17Pb0.83 eutectic alloy was found to be governed by diffusion process from the results of isothermal annealing experiments. Tritium diffusivity in a liquid state of Li0.17Pb0.83 eutectic alloy was evaluated to be D = 4.7 × 10-8 exp(-0.13 eV/kT) m2 s-1. Tritium diffusivity was increased by the phase transition of Li0.17Pb0.83 eutectic alloy from a solid state to a liquid state, resulting in the sharp tritium release peak that appeared in TDS spectrum. In addition, about 4% of tritium was trapped in Li0.17Pb0.83 eutectic alloy as Li-T bond.

Modifications of the registration properties of charged particles in a CR-39 polymeric track detector induced by thermal annealing

The thermal annealing behavior of alpha and fission fragment tracks, following exposure pre- and post-annealing, in a high polymer material of poly allyl diglycol carbonate (PADC), a form of CR-39, has been investigated as a function of temperature and time. Isothermal and isochronal annealing experiments were carried out on CR-39 polymeric detectors exposed to a 252Cf source. The yields of fission fragment and surviving alpha tracks were measured as a function of annealing time and temperature, as was the variation of fission track diameter with annealing time. The bulk and track etch rates were measured using alpha and fission fragment track diameters and the bulk etch rate was also measured by the weight loss method. The activation energy of annealing was determined using two different models based on the experimental results. This paper presents novel information showing that the annealed CR-39 detectors were demonstrated to be highly sensitive to the fission fragments, but not to alpha particles.

Release kinetics of tritium generated in lithium-enriched Li2+xTiO3 by thermal neutron irradiation

Tritium release kinetics for Li2+xTiO3, the lithium-enriched Li2TiO3, was investigated by isochronal and isothermal annealing experiments. Tritium release by isochronal annealing showed that the dominant release stage was found at around 600K. An additional release stage at lower temperature side was appeared with increasing excess lithium, which was attributed to the release of tritium trapped in Li4TiO4 structure. The dominant release stage was considered to a release of tritium trapped by irradiation defects. Isothermal annealing experiments indicated that tritium releases were controlled by diffusion process. The diffusion coefficient of Li2.0TiO3 was one order of magnitude as large as those of Li2.2TiO3 and Li2.4TiO3, although their activation energies were almost the same. These results showed that rate-determining step was the diffusion process of tritium in Li2TiO3 structure for Li2+xTiO3 and excess lithium would make diffusion coefficient smaller. Simulation of tritium-TDS spectra for Li2.0TiO3 has clarified that the TDS spectrum forLi2.0TiO3 can be demonstrated by using Arrhenius diffusion parameters obtained by isothermal annealing experiment in the present study.

Impact of Rapid Crystallization of Si Using Nickel-Metal-Induced Lateral Crystallization on Thin-Film Transistor Characteristics

Metal-induced lateral crystallization (MILC) of amorphous Si using a nickel disilicide catalyst at temperatures up to 770 °C is investigated to produce high-quality polycrystalline Si films in a short period, while 670 °C is the maximum temperature allowed for processing using non alkaline glass substrates. Investigation of crystallization kinetics by isothermal annealing experiments provides activation energy values of 2.3 and 3.6 eV for MILC growth and spontaneous nucleation of amorphous Si, respectively. These values indicate that a MILC region of about 30 µm, which is large enough to place transistor circuits, can be grown by annealing for 15 min at 670 °C, which almost agrees with experimental results. N-channel thin-film transistors are fabricated on MILC films. An average carrier mobility of about 200 cm2·V-1·s-1 is obtained from the MILC film crystallized at 670 °C.

A phenomenological model for lifetime design of Ni2Al3/Ni hybrid coating formed on creep resistant ferritic steels

The phase layer transformation kinetics in the Ni2Al3/Ni hybrid coating formed on creep resistant steel P92 has been studied via a series of prolonged isothermal annealing experiments at 650 °C. All the intermediate phase layers of NiAl, Ni5Al3 and Ni3Al formed in the coating by interdiffusion during isothermal annealing process. The phase layers of NiAl and Ni3Al formed at the very beginning of isothermal annealing at the interface between Ni2Al3 and Ni, but the Ni5Al3 phase layer formed at the interface between the NiAl and Ni3Al phase layer only at an annealing time at which the outer Ni2Al3 phase layer was completely consumed. The growth and consumption of the Ni2Al3 and NiAl phase layers and the growth of the Ni3Al5 phase layer were all parabolic, but the growth of the Ni3Al phase layer obeyed the power rate law d = kt1/n. The growth kinetics of an intermediate phase layer was found to be faster than the kinetics of its subsequent consumption. The rate constants in both the growth and consumption kinetics need to be determined for each of the intermediate phase layers at a particular temperature. The lifetime of the coating with an outer Ni2Al3 phase layer of any specified initial thickness and a sufficiently thick inner Ni layer can then be estimated using the lifetime design model delineated in this study.

Degradation kinetics at 650 °C and lifetime prediction of Ni 2Al 3/Ni hybrid coating for protection against high temperature oxidation of creep resistant ferritic steels

The degradation kinetics of the Ni 2Al 3/Ni hybrid coating formed on creep resistant ferritic steel is studied by prolonged isothermal annealing experiments at 650 °C. The sequence of formation and subsequent consumption of all the intermediate phase layers are determined by intermittent analysis of the specimen at various annealing intervals. It is shown that the Ni 5Al 3 phase layer starts to form only when the outer Ni 2Al 3 phase layer is completely consumed and the growth kinetics of each of the intermediate layers differs from that of its subsequent consumption. A model is subsequently formulated to predict the lifetime of the coating studied.

Polytypic transformations of the HfCr2 Laves phase – Part II: Kinetics of the polymorphic C14 → C15 transformation

The progress of the polymorphic phase transformation C14 → C15 of the HfCr2 Laves phase has been traced using two different parameters: (i) the normalized intensity of the 100 reflection of the C14 phase and, (ii) the ratio of certain lattice spacings common to all Laves phases. These tracing parameters allow determination of the degree of transformation. The effective activation energy of the phase transformation has been determined using the equivalent-time method on the basis of the isothermal annealing experiments for three alloys of different composition within the homogeneity range of the HfCr2 phase. For the stoichiometric alloy a value of 235–240 kJ/mol for the effective activation energy of the C14 → C15 transformation has been obtained. The corresponding values for the non-stoichiometric alloys vary from 180 kJ/mol to 300 kJ/mol; the activation energy increases with decreasing Hf content, adopting the kinetic data determined from the lattice spacings. The magnitude of the values obtained for the effective activation energy indicates that jumps of individual atoms at kinks of gliding synchro-Shockley partial dislocation determine the kinetics of the polytypic shear transformation C14 → C15 of the HfCr2 Laves phase.

Thermal stability of ESR signals in hydrothermal barites

Abstract Thermal stability of the ESR signals from barites in chimneys deposited from hydrothermal vents is investigated using isothermal and isochronal annealing experiments. A combination of first and second order kinetics is required to explain the results. The Arrhenius plots of the decay rate constants give the activation energies of 1.0–1.3 eV. From the estimated decay rate constants at the sea bottom (3 °C), the decay rate of the signal was calculated to be less than 2% for the period of 20 ka, suggesting the applicability of the ESR method for dating barites up to about twenty thousand years.

Kinetics of Fe-oxidation/deprotonation process in Fe-rich phlogopite under isothermal conditions

ab S tract The kinetics of the Fe-oxidation/deprotonation process in a natural Fe-rich phlogopite from Mt. Vulture (Potenza, Italy) was studied under isothermal conditions by in situ high-temperature singlecrystal X-ray diffraction. Isothermal annealing experiments were performed at five temperatures in the range 640–750 °C on five crystals with similar chemical composition and lattice parameters. The Fe-oxidation/deprotonation process at high temperature occurs with a reduction of unit-cell parameters and cell volume. The changes in unit-cell parameters measured at high temperature and during cooling show that the same degree of Fe-oxidation/deprotonation process was achieved at all temperatures. Changes in unit-cell parameters with temperature and time show that the kinetics of Fe-oxidation/ deprotonation in phlogopite follows an exponential law, and the temperature dependence follows the Arrhenius relation. A kinetic analysis was performed and good agreement was obtained with the onedimensional diffusion model. An apparent activation energy of 195(4) kJ/mol was determined.

Single-crystal EPR and ENDOR study of an AlO center in prehnite: implications for aluminum-associated oxyradicals in layer silicates

Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated prehnite (Jeffrey mine, Quebec, Canada) measured at 298 and 160 K reveal an aluminum-associated oxygen hole center (Al—O − ). Spin Hamiltonian parameters g and A ( 27 Al) fitted from the 298 K spectra suggest that this Al—O − center represents hole trapping on an apical hydroxyl oxygen atom (after removal of the proton) coordinated to an octahedral Al 3+ ion ( i.e. , an [ • OAlO 4 (OH)] center from the [(OH)AlO 4 (OH)] precursor, where • denotes the unpaired spin). Pulsed electron nuclear double resonance (ENDOR) spectra measured at 25 K allow the identification and quantitative analysis of two sets of 27 Al hyperfine structures and five proton hyperfine structures, which are all consistent with the proposed structural model. Isothermal and isochronal annealing experiments show that this center is bleached out completely at 375 °C, but can be readily restored by gamma-ray irradiation, and exhibits second-order decay kinetics. These results from the Al—O − center in prehnite provide support for and new insights into Clozel et al . (1995)’s VI Al—O − — VI Al model for B-centers in kaolinite.

Single-crystal EPR and DFT studies of a [BO4]0 center in datolite: electronic structure, formation mechanism and implications

A natural datolite CaBSiO4(OH) (Bergen Hill, NJ, USA), before and after gamma-ray irradiation (up to ~70 kGy), has been investigated by single-crystal and powder electron paramagnetic resonance (EPR) spectroscopy from 10 to 295 K. EPR spectra of gamma-ray-irradiated datolite show the presence of a boron-associated oxygen hole center (BOHC) and an atomic hydrogen center (H0), both of which grow with increasing radiation dose. The principal g and A(11B) values of the BOHC at 10 K are: g 1 = 2.04817(3), g 2 = 2.01179(2), g 3 = 2.00310(2), A 1 = −0.401(7) mT, A 2 = −0.906(2) mT, A 3 = −0.985(2) mT, with the orientations of the g 1 and A 1 axes approximately along the B–OH bond direction. These experimental results suggest that the BOHC represents hole trapping on the hydroxyl oxygen atom after the removal of the proton (i.e. a [BO4]0 center): via a reaction O3BOH → O3BO· + H0, where · denotes the unpaired electron. Density functional theory (DFT) calculations (CRYSTAL06, B3PW, all-electron basis sets, and 1 × 2 × 2 supercell) support the proposed structural model and yield the following 11B hyperfine coupling constants: A 1 = −0.429 mT, A 2 = −0.901 mT, A 3 = −0.954 mT, in excellent agreement with the experimental results. The [BO4]0 center undergoes the onset of thermal decay at ~200°C and is completely annealed out at 375°C but can be restored readily by gamma-ray irradiation. Isothermal annealing experiments show that the [BO4]0 center exhibits a second-order thermal decay with an activation energy of 0.96 eV. The confirmation of the [BO4]0 center (and its formation from the O3BOH precursor) in datolite has implications for not only understanding of BOHCs in alkali borosilicate glasses but also their applications to nuclear waste disposal.

Effect of copper addition on crystallization and properties of hafnium containing HITPERM alloys (invited)

This paper presents properties and nanocrystallization characteristics of HITPERM based Fe68.8Co17.2Hf7Cu1B6 and Fe69.6Co17.4Hf7B6 alloys. Both alloys differing only in copper content were subjected to series of characterizations in order to study the effect of copper. Evolution of magnetic properties as a function time has been investigated by isothermal annealing experiments and optimum annealing conditions of amorphous precursors have been reported. Resulting magnetic properties (ac and dc) measured at room temperature are presented. Nanocrystallization kinetics studied by using time dependant magnetization measurements was discussed in the framework of Johnson–Mehl–Avrami model. Results of thermal stability measurements conducted in air at 500 K for up to 1000 h are reported.

Stability and oxygen-storage characteristics of Al-substituted YBaCo 4O 7+δ

A series of Al-substituted YBa(Co 1− x Al x ) 4O 7+ δ samples was synthesized and characterized with respect to the capability to store large amounts of oxygen at low temperatures (at 200–400 °C) and the phase decomposition upon heating under oxidizing conditions at higher temperatures (above 550 °C). It was revealed that increasing the Al-substitution level up to x≈0.10 boosts up the phase-decomposition temperature from ∼550 to ∼700 °C, while the unique oxygen absorption/desorption characteristics remain nearly the same as those of the pristine YBaCo 4O 7+ δ phase. The maximum amount of excess oxygen absorbed by the Al-substituted YBa(Co 1− x Al x ) 4O 7+ δ samples was determined to be as large as δ≈1.45 for x=0.10 (in 100 atm O 2 at 320 °C). Isothermal annealing experiments carried out for the same x=0.10 phase at 300 °C revealed that it could be reversibly charged and discharged with 1.2 oxygen atoms per formula unit by switching the gas flow from N 2 to O 2 and vice versa.

Studies of defects in photonic materials

Abstract Indium tin oxide (ITO) films have high optical transmission and infrared reflectance, good electrical conductivity, excellent substrate adherence, hardness and chemical inertness. These properties lead to many applications in the area of photonics. Bombardment of ITO films with 1 MeV protons has been carried out resulting in an observed darkening. Insights into the darkening mechanism that consists of three growth stages as a function of fluence are provided by a study of the optical absorption and X-ray lattice parameter. A new interpretation is provided for the darkening mechanism in terms of the production of defect clusters resulting from the atomic displacements during implantation. CsI crystals are very effective scintillator materials for particle detectors in high energy physics. Although radiation hard, radiation damage produces colour centres in CsI that reduce light emission and can negatively affect the luminescent centres. Using a combination of Raman and optical absorption spectroscopy applied to CsI crystals bombarded with 1 MeV protons at 300 K, the resulting defects are shown to be F -type centres and interstitial V -centres having the I 3 − structure and being responsible for absorption bands at 2.7 and 3.4 eV. Isochronal and isothermal annealing experiments show a mutual decay of the F and V -centres. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides.

Radiation-induced defects in quartz. IV. Thermal properties and implications

The thermal stabilities and decay kinetics of three peroxy radicals (Centers #1, B and B′) and three other radiation-induced defects (#3, C′ and E1′) in natural quartz from the high-grade McArthur River uranium deposit (Athabasca basin, Canada) have been investigated by isochronal and isothermal annealing experiments and electron paramagnetic resonance (EPR) spectroscopy. Single-crystal EPR spectra of isochronally (2 h) annealed quartz show that these centers all grow in intensity to 280°C and then decay with further increase in temperature, but their disappearance temperatures differ markedly and depend on the initial concentrations (e.g., Center #1 in a dark smoky quartz is annealed out at 380°C, B and B′ at 420°C and #3 and C′ at 580°C). The isothermal decay processes of these centers are all of the second order type. The calculated activation energies for the peroxy radicals [#1 and B + B′ at 0.36 (9) and 0.83 (8) eV, respectively] are smaller than those of Centers #3, C′ and E1′ [1.09 (8), 1.24 (8) and 1.45 (7) eV, respectively]. Gamma-ray irradiations of thermally bleached quartz restore a fraction of the peroxy radicals, suggesting that their diamagnetic precursors are stable up to at least 800°C. The unusual decay characteristics of “peroxy radicals” in quartz reported in the literature are shown to most likely arise from multiple radiation-induced defects. These results have implications for not only applications of peroxy radicals in quartz for EPR dating but also better understanding of thermoluminescence and cathodoluminescence spectra of this mineral.