Showing papers by "Hamid Garmestani published in 2005"

Adhesion energy in carbon nanotube-polyethylene composite: Effect of chirality

Abstract: This work presents a study of the adhesion energy between carbon nanotube-polyethylene matrix based on molecular dynamics simulations. Specifically, the study focuses on the influence of carbon nanotube chirality on adhesion energy. It is observed that composites that utilize nanotubes with smaller chiral angles achieve higher adhesion energy, and tend to have smaller diameter and longer cylindrical axes compared to those with larger chiral angles. A zigzag nanotube (zero-chiral angle) undergoes considerable deformation to achieve an equilibrium configuration that has relatively maximum adhesion energy. On the other hand, the armchair nanotube (30° chiral angle) deforms moderately to reach equilibrium with minimal adhesion bonds to the polyethylene matrix.

FeCo nanoalloy formation by decomposition of their carbonyl precursors

Abstract: In this paper we describe the synthesis of iron and cobalt nanoalloys by the co-decomposition of iron and cobalt carbonyl precursors in the presence of polystyrene as a surface stabilizing agent. In order to prevent the preferential aggregation of metal atoms in the iron–cobalt nanoalloys, which results in phase segregation, the decomposition kinetics of the Fe(CO)5 and Co2(CO)8 precursors had to be firmly established and controlled. The kinetics of cobalt cluster formation have been thoroughly investigated and documented, but data for Fe(CO)5 decomposition are relatively scarce. To fully explore the kinetic characteristics of the formation of iron nanoclusters, initial Fe(CO)5 concentrations and reaction media solvents were varied and hence reaction order and rate constants were established. Our results suggest that this decomposition is a higher-order process (not first-order as previously assumed), with a complicated intermediate mechanism, which has been postulated and experimentally verified. By using this kinetic data, we were able to predict the necessary conditions for the in situ creation of new iron–cobalt nanoalloys using carbonyl precursors. Equal initial concentrations of both precursors generated nanoalloys with a crystalline core–shell dense morphology, while precursor concentrations corresponding to initial equal rates of decomposition generated polycrystalline nanoalloys with a diffuse morphology.

Magnetization reversal and nanostructure refinement in magnetically annealed Nd2Fe14B∕α-Fe-type nanocomposites

Abstract: Nanostructure refinement, magnetic anisotropy and hard magnetic property enhancement have been observed in melt-spun Nd2.4Pr5.6Dy1Fe84Mo1B6 nanocomposites annealed in an in-plane or out-of-plane field of 1.2T. The magnetic annealing results in an enhancement of an out-of-plane (110) crystal texture of α-Fe and an in-plane uniaxial magnetic anisotropy of the 2:14:1 phase. Magnetic annealing also introduces finer, less angular and more homogeneously distributed soft and hard nanograins. Field dependent torque measurement indicates a complex magnetization reversal mechanism in these nanocomposites. Compared with the sample annealed without a field, there is a noticeable improvement in the hard magnetic properties for the magnetically annealed samples. Especially, the energy product (BH)max was enhanced by 26.6% (from 94to119kJ∕m3). The improvement in the magnetic properties is a result of the enhanced crystallographic texture, nanostructure refinement, and in-plane uniaxial magnetic anisotropy enhancement.

Processing Path Model to Describe Texture Evolution during Mechanical Processing

Abstract: Using a processing path model based on the conservation principle in the orientation space explicit solutions can be formed linking any final (desired) microstructure to a given initial state for polycrystalline materials. The model uses texture coefficients in spherical harmonics expansion to as materials descriptors to represent the texture state of polycrystalline materials. In this work, the effect of increasing the maximum number of texture coefficients used in the series expansion (represented by Lmax) on the prediction of texture and its accuracy is fully studied.

Magnetic Field Effect on the Microstructure of Low Silicon Steel

Abstract: Magnetic annealing at five different magnitudes of field was conducted to evaluate the effect of the field on the recrystallized microstructure of Fe-0.75%Si samples. At higher fields the retardation during recrystallization is compensated by the magnetic filed driving force that causes an increase in the grain boundary mobility of grains that have a certain relationship with the direction of the field

High magnetic field effect on texture and grain growth of GNO silicon steel

Abstract: Magnetic annealing at 800 °C for 3, 15 and 30 minutes was conducted to evaluate the effect of a 17T magnetic field on the microstructure formation of cold rolled Fe-0.75 %Si samples. According to texture measurements magnetic field did not seem to significantly affect the development of grains with Goss orientation, however it showed to affect the nucleation and growth process of {001}, {110} and {111} grains by increasing the volume fraction of the η fiber and by decreasing the fraction of the alfa and gamma fibers. ESEM/OIM and optical microscope results indicate that although magnetic field may retard nucleation it also promotes grain boundary displacement and for longer annealing times the retardation effect is compensated by the magnetic filed driving force. Hochmagnetische Feldeffekte auf Gefuge und Kornwachstum in GNO Silizium-Stahl Magnetische Gluhungen bei 800 °C fur 3, 15 und 30 Minuten wurden durchgefuhrt, um die Wirkung eines 17T-Magnetfeldes auf die Mikrostrukturbildung von kaltgewalzten Fe-0,75 %-Si-Proben zu untersuchen. Die Gefugeuntersuchungen zeigten, dass das Magnetfeld praktische keinen Einfluss auf die Bildung von Kornern mit Goss-Orientierung hat, jedoch die Bildung und der Wachstumsprozess von {001}, {110} und {111} Kornern durch Erhohung des Volumenanteils der η-Faser und durch Erniedrigung des Anteils der alfa- und gamma-Fasern beeinflusst wird. Messungen mittels ESEM/OIM und optischem Mikroskop lassen erkennen, dass das magnetische Feld zwar die Bildung verzogern kann, jedoch auch die Verschiebung von Korngrenzen begunstigt wird und fur langere Gluhzeiten der Effekt der Verzogerung durch die Wirkung der magnetischen Feldkraft kompensiert wird.

On Improving Predictions of Texture Evolution Using Processing Path Model

Abstract: The processing path model based on the conservation principle in the orientation space allows us to optimize processing path from a given initial state to a desired final microstructure for polycrystalline materials. This model uses texture coefficients in spherical harmonics expansion as descriptors to represent the texture state of polycrystalline materials. In this work, the effect of increasing the number of texture coefficients used in the series expansion (decided by lmax) on the prediction accuracy of texture evolution is investigated. Uber die verbesserte Vorhersage der Gefugeentwicklung mittels des Processing-Path-Modells Das Processing-Path-Modell zur Gefugeentwicklung, das in dieser Studie entwickelt wurde, basiert auf dem Erhaltungsprinzip im Orientierungsraum und ermoglicht die endgultige Mikrostruktur - aus den ursprunglichen Mikrostrukturdaten eines polykristallinen Materials - vorauszusagen. Um den Gefugestatus eines polykristallinen Materials zu beschreiben, verwendet dieses Modell zur Materialbeschreibung Gefugekoeffizienten in spharisch-harmonischer Reihenentwicklung. In dieser Arbeit wurde die Auswirkung einer Erhohung der Gefugekoeffizienten in der Reihenentwicklung (dargestellt durch lmax) auf die Voraussage des End-gefuges untersucht.

Adhesion Energy of Single Wall Carbon Nanotube-Polyethylene Composite: Effect of Magnetic Field

Abstract: In this paper, we investigate the adhesion energy at the interface between single wall carbon nanotubes and polyethylene matrix with and without an external magnetic field. The carbon nanotubes are of two different chiralities—armchair (10, 10), and zigzag (10, 0), and the external high magnetic field is of 25 Tesla intensity. The study employs molecular dynamics simulations and concludes that the magnetic field decreases the interfacial adhesion energy although it increases the individual potential energies of the nanotubes, the polyethylene, and the composite.