Different Degrees Of Crystallization Research Articles

Melt‐crystallization mechanism of poly(ethylene terephthalate)/multi‐walled carbon nanotubes prepared by in situ polymerization

AbstractA series of poly(ethylene terephthalate)/multi‐walled carbon nanotubes (PET/MWCNTs) nanocomposites were prepared by in situ polymerization using different amounts of multi‐walled carbon nanotubes (MWCNTs). The polymerization of poly(ethylene terephthalate) (PET) was carried out by the two‐stage melt polycondensation method. The intrinsic viscosity (IV) of the composites is ranged between 0.31 and 0.63 dL/g depending on the concentration of the MWCNTs. A decrease of IV was found by increasing MWCNTs content. This is due to the reactions taking place between the two components leading to branched and crosslinked macromolecules. These reactions are, mainly, responsible for thermal behavior of nanocomposites. The melting point of the nanocomposites was shifted to slightly higher temperatures by the addition till 0.55 wt % of MWCNTs while for higher concentration was reduced. The degree of crystallinity in all nanocomposites was, also, reduced by increasing MWCNTs amount. However, from crystallization temperature, it was found that MWCNTs till 1 wt % can enhance the crystallization rate of PET, whereas at higher content (2 wt %), the trend is the opposite due to the formation of crosslinked macromolecules. From the extended crystallization analysis, it was proved that MWCNTs act as nucleating agents for PET crystallization. Additionally, the crystallization mechanism due to the existence of MWCNT becomes more complicated because two mechanisms with different activation energies are taking place in the different degrees of crystallization, depending on the percentage of MWCNT. The effect of molecular weight also plays an important role. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1452–1466, 2009

Noise of MgO-based magnetic tunnel junctions

Low-frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. We present here the noise for different degrees of crystallization and CoFeB/MgO interface quality depending on the annealing temperature. For optimized annealing temperature, an extremely low 1/ f noise, compared to magnetic junctions with Al 2O 3 barriers, has been observed. The origin of the low-frequency noise can be explained in terms of localized charge traps with the MgO barriers. Results for very thin CoFeB are presented in the second part as a function of temperature. Despite the absence of coercivity at room temperature for thinner free-layer structures, an important increase of H c appears under 180 K. Meanwhile, the random telegraph noise present at room temperature is suppressed due to magnetic domains freezing. These results are discussed in view of various sensors applications of MgO-MTJ, giving advantages and drawbacks in terms of signal-to-noise ratio with respect to the operating temperature.

Ferromagnetic and spin-glass behaviour of nanosized oriented pyrolytic graphite in Pb–C nanocomposites

Pb–C nanocomposites consisting of pyrolytic carbon nanospheres and Pb nanoparticles were produced by discharging a mixture of Pb and graphite powders in an ethanol and Ar atmosphere. Raman spectrum and x-ray photoelectron spectroscopy indicated that the carbons in the Pb–C nanocomposites are in both ordered and disordered forms. The Pb atoms in the Pb–C nanocomposites were removed by the treatment of a 65% HNO3 solution to form carbon nanocomposites in order to have a comparison. X-ray diffraction data and magnetic measurements revealed the magnetism of the nanosized pyrolytic carbon with different degrees of crystallization, in which disordered carbon with a (0 0 2) interlayer spacing of 0.342 nm is diamagnetic, while the oriented pyrolytic graphite with a spacing of 0.337 nm shows ferromagnetism. The saturation magnetization of highly oriented pyrolytic graphite (HOPG) nanospheres is enhanced by about two orders of magnitude, compared with that of bulk HOPG. Spin-glass behaviour of HOPG nanospheres may be caused by complex competing interactions between magnetic moments in nanosized HOPG.

Persistent activity and violent strombolian eruptions at Vesuvius between 1631 and 1944

During the period 1631–1944, Vesuvius was in persistent activity with alternating mild strombolian explosions, quiet effusive eruptions, and violent strombolian eruptions. The major difference between the predominant style of activity and the violent strombolian stages is the effusion rate. The lava effusion rate during major eruptions was in the range 20–100 m 3/s, higher than during mild activity and quiet effusion (0.1–1 m 3/s). The products erupted during the mild activity and major paroxysms have different degree of crystallization. Highly porphyritic lava flows are slowly erupted during years-long period of mild activity. This activity is fed by a magma accumulating at shallow depth within the volcanic edifice. Conversely, during the major paroxysms, a fast lava flow precedes the eruption of a volatile-rich, crystal-poor magma. We show that the more energetic eruptions are fed by episodic, multiple arrival of discrete batches of magma rising faster and not degassing during the ascent. The rapidly ascending magma pushes up the liquid residing in the shallow reservoir and eventually reaches the surface with its full complement of volatiles, producing kilometer-high lava fountains. Rapid drainage of the shallow reservoir occasionally caused small caldera collapses. The major eruptions act to unplug the upper part of the feeding system, erupting the cooling and crystallizing magma. This pattern of activity lasted for 313 y, but with a progressive decrease in the number of more energetic eruptions. As a consequence, a cooling plug blocked the volcano until it eventually prevented the eruption of new magma. The yearly probability of having at least one violent strombolian eruption has decreased from 0.12 to 0.10 from 1944 to 2007, but episodic seismic crises since 1979 may be indicative of new episodic intrusions of magma batches.

Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes: Influence of annealing temperature

Low frequency noise has been measured in magnetic tunnel junctions with MgO barriers and magnetoresistance values up to 235%. The authors investigated the noise for different degrees of crystallization and CoFeB∕MgO interface quality depending on the annealing temperature. The authors report an extremely low 1∕f noise, compared to magnetic junctions with Al2O3 barriers. The origin of the low frequency noise is discussed and it is attributed to localized charge traps with the MgO barriers.

Nanocrystallization and magnetic anisotropy in [formula omitted

A study of the magnetic anisotropy originated by the nanocrystallization of the amorphous Co 66 Si 16 B 12 Fe 4 Mo 2 is presented. Mechanically polished samples of this alloy were submitted to various thermal treatments to obtain different degrees of crystallization. The results show that an homogeneous distribution of nanocrystallites of about 10 nm originates a magnetic anisotropy of 18 J m - 3 while the larger crystals that appear, due to the coalescence of the small crystallites, with annealings over 500 ∘ C produce an increase of the magnetic anisotropy up to values of about 100 J m - 3 , that are similar to the ones of the as-quenched material. From the value of the effective magnetic anisotropy constant we deduce that in the samples annealed below the onset temperature, the majority of the crystallites are of Co 2 B and Co 3 B .

In situ study of hydriding–dehydriding properties in some Pd/Mg thin films with different degree of Mg crystallization

Abstract A new in situ system with the functions of thin film formation and analysis of hydrogen absorption–desorption properties has been developed to clarify hydrogen storage properties in nano-scaled composites. In this work, some Pd/Mg films (Pd (25 nm)-coated Mg (200 nm) films) with different degree of crystallization in the Mg layer were prepared in different sputtering conditions by changing RF coil powers and argon pressures. Hydrogenation under hydrogen gas pressure of 0.1 MPa at 373 K for 24 h indicated that MgH2 and non-crystalline Mg hydrides were formed in all the Pd/Mg films and the hydrogen content reached 2.9∼6.6 mass% independent of the degree of Mg crystallization. From the thermal desorption spectrum, it was deduced that the dehydriding temperature decreased with decreasing the degree of crystallization in the Mg layer in Pd/Mg films and the Pd/Mg film with lowest crystallization absorbed 5.6 mass% of hydrogen and all the hydrogen desorbed at a temperature lower than 463 K in vacuum.

Surface crystallization and frequency dependence of AC coercivity in Fe 80B 20 amorphous alloy

We report here, for the first time, the results of a study of the frequency dependence of coercivity ( H c(ƒ) ) at the surface of amorphous and partially crystallized Fe 80B 20 samples with different degrees of crystallization. H c(ƒ) shows a shape which does not depend on either the surface or bulk structure (i.e. crystalline or amorphous), consisting of a nearly constant value of H c up to 100 Hz followed by a knee and a rapid increase as frequency is further increased. Our data are well fitted by a functional law H c(ƒ) ∝ ƒ α with α = 0.45 for the amorphous sample and α = 0.49 for the partially crystallized samples.

Magnetic correlations in Fe 80B 20 studied by magnetic small angle neutron scattering

Fe 80B 20 alloys with different degrees of crystallization were investigated by small angle neutron scattering. From the analysis of the obtained scattering patterns with and without magnetic field, we were able to separate the magnetic and the nuclear scattering contributions and to study the evolution of both structural and magnetic parameters of the iron particles in the various steps of the crystallization process.

EXAFS and Mössbauer Study of the Crystallization of Fe91Zr9Metallic Glass

The crystallization process of the Fe 91 Zr 9 amorphous alloy has been followed by means of two experimental techniques, EXAFS and Mossbauer spectroscopy. X-ray absorption spectra on Fe K-edge and Mossbauer spectra of six samples with different degrees of crystallization have been measured at room temperature in order to obtain information about the structural changes that take place. From the beginning of the crystallization process it is clearly seen that a crystalline phase with a bcc-type structure appears. From the obtained data we can conclude that this phase is nearly pure bcc-Fe, with small amount of impurities in its structure.

Magnetization dynamics of pinned domain walls in partially crystallized ribbons

The frequency dependence of the permeability of Co- and Fe-rich amorphous ribbons (Vitrovac 6025(R) and Metglas(R) 2605 SC, respectively), submitted to a variety of thermal treatments involving different degrees of crystallization, was measured in the 5 Hz-13 MHz frequency range. The quasi-static initial permeability, /spl mu//sub i/, and the domain wall relaxation frequency, f/sub x/, exhibited an inverse relationship, /spl mu//sub i//spl middot/f/sub x/=constant. These results are interpreted in terms of variations in the average distance between pinning points of domain walls, and the effects of such variations on the vibrational properties of pinned domain walls.

Magnetic properties of partially crystallized Fe80B20 amorphous alloys

Magnetization measurements have been performed on amorphous Fe80B20 ribbons with different degrees of crystallization. Samples were obtained by isothermal treatment using a differential scanning calorimeter in order to achieve a nucleation-and-growth process only. The temperature dependence of the saturation magnetization and of the low field magnetization is interpreted in terms of the effects of the interfacial component formed by the atoms on the surface of bcc Fe nanocrystals.

Copper sorption in clay fractions of alluvial soils

The influence of copper sorption by iron hydroxides with different degrees of crystallization in clay fractions of several alluvial cultivated soils of the fertile lowlands of the Manzanares river (Madrid) has been studied. Mineralogical study of the < 2-μm clay fraction allows us to differentiate two major zones, which are also different as far as their contents of copper and iron oxide and their degree of crystallization is concerned. There is a generalized tendency of Cu association towards the amorphous and microcrystalline iron oxides. This fact is confirmed through the copper sorption studies on the untreated clay fraction and on those treated with oxalate and citrate-bicarbonate-dithionite. Sorption studies also highlight the influence of the different phyllosilicates on the sorption process of this metal.

Local and Long‐Range Diffusion of Hydrogen in Amorphous and Partially Crystallized Fe90Zr10 Alloys

AbstractThe magnetic after‐effect spectra of hydrogen (H) charged amorphous Fe90Zr10 alloys containing different degrees of crystallization (10% and 50%) are measured in the temperature range between 4.2 and 480 K. For the partially crystallized specimens one broadened asymmetric relaxation maximum is observed. With decreasing H content a shift of the maximum temperature from 140 to 196 K takes place due to the increase of the average activation enthalpy from 0.33 up to 0.49 eV. Using a phenomenological diffusion model for H in amorphous alloys, which takes into account a broad distribution of equilibrium sites as well as of saddle point energies the H‐induced relaxation maxima and their annealing behaviour are assigned to local jumps of hydrogen in the amorphous phase. In addition, the long‐range H diffusion above 300 K is measured by thermal degassing at different heating rates. The observed similar H degassing kinetics of the amorphous and partially crystallized samples show that the long‐range diffusion is exclusively determined by H atoms in the amorphous phase. The average activation parameters Qdiff = (0.53 ± 0.1) eV and Do = 7 × 10−7±2 m2/s are evaluated for a H concentration of 1.7 at%.

On the Phase Transformation between the Quasicrystalline to Crystalline Phases in Alloys of Al-Cu-Fe-Co

We report in this investigation preliminary results on the phase transformations between the quasicrystalline phases in quaternary alloys of Al-Cu-Fe-Co to crystalline structures. In these alloys obtained by normal casting methods under inert atmospheres both quasicrystalline phases; icosahedral and decagonal are found to coexist. By means of annealing, transformation of the quasicrystalline phases to crystalline structures are induced.Although not quite clear from diffraction patterns along the five-fold and threefold axis, experimental evidence on this crystallization process is clearly obtained along the two-fold axis. The diffraction patterns obtained from the annealed specimens show sign of different degrees of crystallization for different quasicrystalline grains.

クロム酸水溶液中で生成されたアルミニウムアノード酸化皮膜の格子像観察

Stripped porous anodic film formed on aluminum in chromic acid solution was thinned with an argon ion beam and examined in a high-resolution electron microscope. Diffraction patterns obtained from several different areas of the film showed different degree of crystallization. TEM images of areas where many diffraction spots were detected showed clear lattice images of almost the entire field except for the vicinity of the pore walls. Both from the lattice images, which showed distances and angles between crystal planes, and from the analysis of diffraction patterns, the film was identified as γ-alumina. Other diffraction spots and lattice distances were also found and were presumably caused by the slightly mixed other crystal forms of alumina in the film. Careful observation showed many disordered mosaic fractions in the lattice images of the crystalline film, and even taking into account the effect of crystallization by beam irradiation, this fact suggests that the original film was composed of a mixture of ordered and disordered structures.

The growth of FeOOH microcrystals and chemisorption rate of NO

AbstractThe adsorption of NO over a period of 1 ms–1 min on α‐ and γ‐FeOOH with different degrees of crystallization has been examined in order to clarify the fast chemisorption phenomenon. The FeOOH crystals were prepared under the conditions of various reaction temperatures and ageing times. The changes in X‐ray diffraction patterns, particle size, specific surface area and electrical conductivity were examined. The fast chemisorption rate constant, kf, for NO was obtained at 30°C from the conductivity change in the initial stage of NO adsorption. The α‐FeOOH aged for 2 days and 6 days, the crystallite size of which approximately 100 Å gave the highest kf and the greatest amount of chemisorption; the high activity is assumed to be caused by a special surface structure accepting NO molecules. The maximum kf for α‐FeOOH was also for a crystallite size of approximately 100 Å. As the change of kf with crystal growth for γ‐FeOOH is similar to that of electrical conductivity, an electronic factor seems to be most important in the fast chemisorption process of NO on γ‐FeOOH.

Influence of preparation condition on catalytic activity for oxygen reduction of lanthanum nickel oxide and related oxides

The catalytic activities for oxygen reduction of La 1− x Ln x NiO 3 (Ln: Nd, Sm) and of La−NiO 3 prepared by various methods were studied. La 0.9Nd 0.1NiO 3 was only a single phase perovskite type oxide in La 1− x Ln x NiO 3, and its catalytic activity was the same as that of LaNiO 3, as expected. All the other oxides of La 1− x Ln x NiO 3 investigated were mixtures consisting of a deformed perovskite structure oxide and a monoclinic lanthanoid oxide, and the catalytic activity decreased with promotion of the substitution, i.e., with increase of x. The deformation of the perovskite structure brings a decrease of the catalytic activity, and this was confirmed by comparison of the activities of three kinds of oxides of the single phase LaNiO 3 which had different degrees of crystallization. A fairly high current density of several tens of mA cm −2 was obtained on an electrode with a form for practical use, when the oxide was prepared by decomposition of nitrates.

Positron lifetimes in lithium disilicate glass with different degrees of crystallization

Positron annihilation has been studied in samples of lithium disilicate glass heat treated to produce different degrees of internal crystallization. It has been found that the intensity of the longer positron lifetime component (I2) decreases with percentage crystallinity. This decrease can be correlated linearly with the reduction in free volume on crystallization in this system. A decrease in the lifetime, tau 2, with crystallinity has also been observed.

Petrography of crystalline basement of Russian platform

Precambrian rock types consist of metasediments and metavolcanics and occur generally throughout the entire Russian platform and adjacent Ukrainian crystalline shield. Rocks of different ages and belonging to different structural stages and diastrophic periods show different degrees of crystallization. From Archean rocks to Proterozoic rocks, the degree of recrystallization decreases greatly. Reliable correlation criteria are the degree of crystallization and the similarity of rocks, of the same age, which have been subject to identical tectonic conditions. The petrography of the ortho- and pararocks corroborates the hypothesis of granite forming from partially molten, ancient, sedimentary rocks subsequently altered by various metasomatic processes. Study of the mineral composition of gabbros and pyroxene paragneisses indicates the actual manifestations of basic magma were effective as early as Archean and have been active to the present. R.M. Hutchinson