Brook haven lab interview

1
Quasicrystals (QC)?
- Materials showing five-fold diffraction pattern
which is forbidden in terms of solid state and
crystallographical concepts. - First discovered
by Shechtman et. al, (Phy. Rev. Lett, 53,
1951, 1984 ). - Lack of long-range order, but
having a rotational symmetry!! - They are not
amorphous, nor crystal ! gt Quasi-crystal Mor
e than 500 QCs found in Al- or Ti-base alloys
Al-based Al-Cu-Mn, Al-Pd-Mn, Al-Cu-Fe,
Al-Ni-Co Ti-based Ti-TM-Si-O, Ti-Zr-Ni
5-fold QC
Icosahedron
3-fold QC
3-fold cubic
SAD of Ti56(TiO)4Mn32Ge8 QC
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Forbidden Symmetry
where n is an integer, allowed values of
cos(2?/n) are between 1 to -1. Therefore, the
allowed values of n are n 1, 2, 3, 4, 6.
5-fold
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Quasiperiodicity and Rotational Approximant
? Golden mean
S gt L starting sequence L L gtSL 1st
iteration LS 2nd iteration LS L 3rd
iteration LS L LS 4th iteration LS L LS LSL

B ?A, C ?B ?2A 2.618A D ?C ?3A
4.236A E ?D ?4A 6.854A and the ratios
of adjacent lengths are
Diffraction spots
4
Illustration of quasi periodic model
Cut-and-project method
Penrose tiling method
Tiling model in 3-d are explained by using a
oblate (63.43o) and prolate (116.57o)
rhombohedra, A.L. Mackay, Physica A 114, 609
(1982)
5
XRD patterns from i-phase and 1/1-phase
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Experimental Procedure (Arc-melting and Quenching)

• Two kinds of samples having different
  composition were prepared
• Ti45 Zr38 Ni17, and Ti50 Zr31 Ni17.
• Samples were arc-melted on the Cu-hearth in an
  Ar atmosphere.
• Melted samples were subsequently quenched on the
  high-speed rotating stainless wheel.
• Wheel speeds were varied from 800 to 4000 rpm.
• Metallic ribbons (about 20 µm thick) were
  obtained.
• Structures of the ribbons were analyzed by using
  XRD.

S.S wheel
Tungsten electrode (-)
Copper hearth
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XRD patterns of Ti45 Zr38 Ni17 as quenched ribbons
8
XRD patterns of Ti50 Zr31 Ni17 as quenched ribbons
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Conclusions
1) Studied the formation of quasicrystals in
Ti-based alloys by varying the composition
and cooling rates from 1000 to 3000 rpm. 2)
Discovered that the alloys having composition of
Ti50 Zr31 Ni17 showed relatively a better
i-phase, compare to the Ti45 Zr38 Ni17 one which
is known to be the most meta-stable
composition. 3) It is shown that quasicrystals
are formed in a broad range of atomic
compositions suggesting relatively easy to
produce. 4) Samples showed a tendency to forms
the quasicrystals as increasing the quenching
rates. 5) For understanding the structure of this
new composition, need to get more data in many
ways. 6) Our preliminary study on distribution of
site energies for hydrogen in Ti-Zr-Ni alloys
imply - structural similarities between the
quasicrystal and 1/1 approximant phase -
the width of the energy distribution 1/1
approximant lt quasicrystals lt glass. 7) Hydrogen
atom is further expected be used to probe the
local structure of quasicrystals. 8) Obtaining a
phase pure quasicrystals, however, is an urgent
task to realize the candidate materials for the
hydrogen storage applications.