Person:
Thadhani, Naresh N.

Associated Organization(s)
ORCID
ArchiveSpace Name Record

Publication Search Results

Now showing 1 - 6 of 6
  • Item
    Explosive shock processing of Pr₂Fe₁₄B/α–Fe exchange-coupled nanocomposite bulk magnets
    (Georgia Institute of Technology, 2005-03) Jin, Z. Q. ; Thadhani, Naresh N. ; McGill, M. ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Chen, M. ; Zeng, Hao ; Chakka, V. M. ; Liu, J. Ping
    Explosive shock compaction was used to consolidate powders obtained from melt-spun Pr₂Fe₁₄B/α–Fe nanocomposite ribbons, to produce fully dense cylindrical compacts of 17–41-mm diameter and 120-mm length. Characterization of the compacts revealed refinement of the nanocomposite structure, with approximately 15 nm uniformly sized grains. The compact produced at a shock pressure of approximately 1 GPa maintained a high coercivity, and its remanent magnetization and maximum energy product were measured to be 0.98 T and 142 kJ/m³, respectively. The compact produced at 4–7 GPa showed a decrease in magnetic properties while that made at 12 GPa showed a magnetic softening behavior. However, in both of these cases, a smooth hysteresis loop implying exchange coupling and a coercivity of 533 kA/m were fully recovered after heat treatment. The results illustrate that the explosive compaction followed by post-shock heat treatment can be used to fabricate exchange-coupled nanocomposite bulk magnets with optimized magnetic properties.
  • Item
    Amorphization and ultrafine-scale recrystallization in shear bands formed in shock-consolidated Pr₂Fe₁₄B/α -Fe nanocomposite magnets
    (Georgia Institute of Technology, 2004-09-20) Wang, Z. L. (Zhong Lin) ; Jin, Z. Q. ; Liu, J. Ping ; Thadhani, Naresh N.
    Amorphization and ultrafine-scale recrystallization within shear bands formed in shock-consolidated Pr₂Fe₁₄B/α -Fe nanocomposite magnetic powder compacts have been observed using transmission electron microscopy. The shear bands span through multiple grain lengths and truncate preexisting ~25 nm hard and soft magnetic phase grains, resulting in further grain size refinement. The shear bands contain nanocrystallites (<10 nm size) interdispersed in an amorphous matrix, which suggests the occurrence of shock-induced phase transition in localized regions of the shear bands, and provides insight into the process of deformation of nanocrystalline materials under coupled high-strain-rate and high-pressure conditions.
  • Item
    Grain size dependence of magnetic properties in shock synthesized bulk Pr₂Fe₁₄B/α-Fe nanocomposites
    (Georgia Institute of Technology, 2004-09-15) Jin, Z. Q. ; Thadhani, Naresh N. ; McGill, M. ; Li, Jing ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Zeng, Hao ; Chen, M. ; Cheng, S.-F. ; Liu, J. Ping
    The structural and magnetic properties of the melt-spun Pr₂Fe₁₄B/α-Fe nanocomposite powders consolidated via shock-wave compression and subjected to postshock thermal treatment were investigated. Shock compression results in grain refinement, which leads to a reduction of an effective anisotropy and therefore an increase in the ferromagnetic exchange length, resulting in an enhanced exchange coupling in fully consolidated bulk magnets. A small amount of amorphous phase formed during the shock compression were observed to crystallize into Pr₂Fe₁₄B upon annealing above 600 °C. The heat treatment also results in the recovery of coercivity partially lost during the consolidation, which can be related directly to the dependence of the effective anisotropy on the grain size, as illustrated by the transmission electron microscopy observation of grain refinement in the shock-consolidated bulk samples. A uniform grain morphology is suggested as a means for further increasing the magnetic properties of bulk nanocomposites.
  • Item
    Bulk nanocomposite magnets produced by dynamic shock compaction
    (Georgia Institute of Technology, 2004-07-15) Chen, K. H. ; Jin, Z. Q. ; Li, Jing ; Kennedy, G. ; Wang, Z. L. (Zhong Lin) ; Thadhani, Naresh N. ; Zeng, Hao ; Cheng, S.-F. ; Liu, J. Ping
    Exchange-coupled R₂Fe₁₄B/α -Fe (R = Nd or Pr) nanocomposite bulk magnets with nearly full density have been successfully produced by shock compaction of melt-spun powders. X-ray diffraction and transmission electronic microscopy analyses of the shock-consolidated compacts showed no grain growth upon compaction, in fact, a decrease in the crystallite size of both the hard and soft phases was observed. As a consequence, magnetic properties were retained and even improved after compaction. Hysteresis loops of the shock-consolidated powder compacts showed a smooth single-phaselike behavior, indicating effective exchange coupling between hard and soft magnetic phases.
  • Item
    Shock consolidation of magnetic powers
    (Georgia Institute of Technology, 2004-06-15) Thadhani, Naresh N.
  • Item
    Controlling the crystallization and magnetic properties of melt-spun Pr₂Fe₁₄B/α Fe nanocomposites by Joule heating
    (Georgia Institute of Technology, 2004-05-12) Jin, Z. Q. ; Cui, B. Z. ; Liu, J. Ping ; Ding, Yong ; Wang, Z. L. (Zhong Lin) ; Thadhani, Naresh N.
    Pr₂Fe₁₄B/α Fe based nanocomposites have been prepared through crystallization of melt-spun amorphous Pr₇Tb₁Fe₈₅Nb ₀.₅ Zr ₀.₅ B₆ ribbons by means of ac Joule heating while simultaneously monitoring room-temperature electrical resistance R. The R value shows a strong variation with respect to applied current I, and is closely related to the amorphous-to-nanocrystalline phase transformation. The curve of R versus I allows one to control the crystallization behavior during Joule heating and to identify the heat-treatment conditions for optimum magnetic properties. A coercivity of 550 kA/m and a maximum energy product of 128 kJ/m³ have been obtained upon heating the amorphous ribbons at a current of 2.0 A. These properties are around 30% higher than the values of samples prepared by conventionally (furnace) annealed amorphous ribbons.