HEWMASATOOLFORPREPARATIONOFSUBMICROMETRICBIMETALLICMECHANICALMIXTURES

HEWMASATOOLFORPREPARATIONOFSUBMICROMETRICBIMETALLICMECHANICALMIXTURES

Authors: VBiondo; AOSouza, Ph.D.; BHallouche, Ph.D.; APJr., Ph.D.
Departamento de Física – Universidade Estadual de Maringá – Jardim Universitário
87.020-900  Av. Colombo, 5790  Maringá PR Brazil
Departamento de Química e Física – Universidade de Santa Cruz do Sul – RS  Brazil

ABSTRACT
Submicrometric bimetallic mechanical mixtures were prepared by high energy milling
metallic powder  mixtures of M-Fe type  (with M =  Al,  Cr,  Cu, Nb,  Ti  or  Zn).  The asmilled material presents morphology of foils or flakes of non-reacted elements, with high
aspect  ratio.  Structural  analyzes  revealed  that  the  bimetallic  systems  investigated  are
structured so that iron is encrusted in the M metal, but is not mixed at atomic scale. This
is attributed to the use of a "process control agent” which characterizes the milling as
"wet"  and  avoids  the  mechanosynthesis.  It  was  found  that  the  orientation  of  the  iron
magnetic domains lies preferentially in the foil plane. The results indicate great potential
for  technological  applications  for  these  bimetallic  particles,  made  by  a  one-step  and
inexpensive method, suitable for large scale production.
Keywords: bi-metallic particles; wet-milling; mechanical mixture
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1. INTRODUCTION
Materials  at  least  with  one  of  its  dimensions  lying  in  the  submicron  or  nano
scales  (i.e., less than 1x10-6 m or 1x10-7m, respectively)  have  attracted  much  attention
because  they  present  new  properties  with  applications  in  the  state-of-the-art
technologies [1].
Especially,  submicrometric  and nanometric particles with a bi or multi-metallic
composition are considered a particular class of materials with functionalities that make
them  useful  in  catalysis  [2,3],  electrocatalysis  [4,5]  and  magnetic  hyperthermia  in
biomedicine  [6,7].  These  functional  attributes  are  designed  varying  the  chemical
composition, dimension and morphology  of the particles,  aiming  to assembly desirable
individual  features  of  the  constituent  metals.  In  addition,  both  a  fine  particulate  pure
metals and  metallic  mixture  in the flake or foil morphology have a high "diameter -tothickness" ("aspect ratio") and, consequently, large surface área [8].
In  most  cases,  the  preparation  of  these  materials  is  non-trivial.  Synthesis
methods (chemical in most cases) of such multi-metallic particles must be very precise to
accomplish the right  compositions  of metals  (usually supported)  not-atomically  mixed,
and  often  are  quite  laborious  and  expensive  [1,2,3,4].  Therefore,  a  new  or  improved
method  to  produce  multi-metallic  materials  of  high  superficial  area  that  are  low  cost
and  easy  to  produce  in  large  quantities  may  be  exceptionally  interesting  for  many
applications, such as environmental remediation in areas polluted by contaminants (e.g.,
toxic chemicals and heavy metals)  [9,10,11,12,13].  If  these particulate materials exhibit
magnetic  properties,  the  contaminants  can  be  recovered  using  magnets  in  the
remediation process [14].
Among other processing methods,  high-energy  ball  milling  could be a valuable
tool  for preparing  fine metallic  mixtures  [15].  It is  a simple and inexpensive routine  for
comminution of materials and mechanosynthesis of metallic alloys. As a rule, dry milling
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metallic powders of different metals  together  results in intermetallic compounds, solid
solutions and, even, amorphous alloys [15,16].
The as-milled  particles can be  nano structured depending on the setting of the milling
process.  In  addition,  if  carried  out  on  a  single  powdered  metal  in  the  presence  of  a
lubricant  –    or  a  “process  controlling  agent”  –    the  process  is  called  “wet  milling”
[17,18,19] –  the material can be conformed to very fine sheets or flakes.
In order to investigate  the  preparation  of  bi-metallic submicrometric  mixtures,
in the present study  metallic  pairs of  M-Fe  type, where  M =  Al, Cr, Cu, Nb, Ti  or  Zn,
were high-energy  milled together, in the presence of  a  lubricant. Iron was  chosen  as a
common  element  aiming  to  permit  the  Mössbauer  spectroscopy  characterization,  a
technique especially suitable to identify  iron-containing  phases, even in nanoscale.  Also,
because iron is magnetic and, plausibly,  it  is a strong candidate to  constitute particles
that can be used as catalysts.
The as-milled materials were structurally characterized and the results revealed
that wet milling is  definitely  appropriate  for preparation of submicrometric bimetallic
mixtures.
2. EXPERIMENTAL DETAILS
Transition  metal  (M)  precursors  –  Al  (99.97%),  Cr  (99%),  Cu  (99.9%),  Nb
(99.8%), Ti (99.9%) or Zn (99.9%)  –  were  manually pre-mixed with iron (99.9%) at  the
0.7M-0.3Fe  molar ratio.  Further, each  binary  mixture was  wet milled in  a  high-energy
planetary  mill  (Fritsch  –  model  Pulverisette  6),  using  a  hardened  80  cm
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steel  vial,
charged  with  10  mm  diameter  hardened  steel  balls.  Ethanol  was  used  as  a  lubricant,
placed  with  the  balls and metallic powders,  in an amount  to cover all the material.  The
mechanical  milling  process  was  conducted  for  12  hours,  with  400/300  rpm  rotation
speed and ball-to-powder mass ratio of 60 : 1. Samples of elemental metals (M) were also
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milled  to  verify  possible  contamination  from  the  milling  vial.  After  the  high-energy
milling, the solid part of the as-milled material was  separated from the liquid fraction
first  by using a  small mesh  sieve  and  then  naturally dried in  a  free atmosphere,  at room
temperature. All  samples were characterized  immediately  after the drying process.  The
morphology  of  as-milled  samples  was  observed  by  scanning  electron  microscopy.  The
present  phases  were  checked  by  powder  X-ray  diffraction  (PXRD)  and  Mössbauer
spectroscopy (MS).
SCANNING ELECTRON MICROSCOPY
Scanning electron microscopy was performed in a Shimadzu SuperScan SS-550,
used  for  taking  images  of  the  as-milled  powder.    Prior  to  analysis,  the  samples  were
coated with a conductive gold film by a sputtering process.
X-RAY DIFFRACTION
The  PXRD  characterization  was done  at room temperature,  using an ordinary
diffractometer,  operating  in  the  Bragg-Brentano reflection geometry  (θ  -  2θ), with Cu
Kα  radiation (λ  = 1.5418 Å).  Data were collected between 30°  and 90°, in steps of 0.02°
and  2.4  s per step.  The diffraction peaks were identified  with the support  of the  JCPDS
files  [20].  The  diffractograms presented  ahead  show  vertical  colored bars that  indicate
the angular  position for the peaks of  each present  phase. Their  length is  proportional to
the peak relative intensity, according to the reported in the respective JCPDS files.
MÖSSBAUER
The  MS  was  conducted  at  room  temperature  (RT),  in  transmission  geometry,
through  a conventional Mössbauer spectrometer, operating  in  the  constant acceleration
mode.  The  14.4  keV    rays  were  provided  by  a
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Co(Rh)  source  with  25  mCi  initial
activity. The velocity scale was calibrated by using a standard iron foil absorber   (-Fe).
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The spectra were analyzed using a non-linear least-square routine, with Lorentzian line
shapes.  The  Mössbauer  measurement  obtained  for  the  mono-elemental  as-milled
samples  revealed  the  inexistence  of  resonant  absorption,  which  means  that  no
contamination of M with iron took place from the milling process.
3. RESULTS AND DISCUSSION
Figure 1  shows representative images obtained by electron microscopy  for some
as-milled  samples.  In  general,  the  resultant  material  presents  morphology  of  foils  or
flakes  apparently  of  non-reacted  elements  (XRD  and  Mössbauer  characterization  will
confirm  that).  The  area  of  the  foils  (or  flakes)  is  measured  in  micrometers,  but  the
thicknesses  are  submicrometric.  Evidently,  the  aspect  ratio  is  high,  which  is  a  very
attractive attribute for, e.g., catalysis applications.
The  X-ray  diffractograms  of  all  the  samples  are  presented  in  Figure  2.  It  is
observed for every M that only peaks belonging to the elemental phases –  M and -Fe –
are present. There is no evidence for intermetallic compound formation, at least in the
resolution of the diffractometry technique. However, in some diffractograms peaks are
more (M = Cr, Nb) or less (M  = Al, Zn) broadened. Therefore, solid solutions of  M(Fe)
or Fe(M) types could be mechanosynthesized. According to the binary phase diagram of
the Al-Fe [21], for example, it is possible to dissolve until ~18%at. of aluminum in iron,
without  changing  the  crystallographic  structure  of  the  -Fe.  Thus,  at  this  point,  solid
solution  formation cannot be ruled out. Furthermore,  the relation of peak  intensities  in
each diffractogram, in general, does not obey what is expected for powders randomically
oriented. This is attributed to their preferential orientation in the sample holder.
Mössbauer  spectra  for all samples prepared  are shown in Figure 3.  Invariably,
all of them present a  well defined  magnetic component  and were fitted using a  discrete
sextet. The  mean  hyperfine parameters  fitted  are:    =  -0.01 mm/s;    =  0.0 mm/s;  Bhf
=
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330 kOe;      = 0.34 mm/s.  These  values  correspond  closely  to the  -Fe  at RT.  According
to that,  the  dissolution of M in iron, forming a  Fe(M) solid solution,  is minimum,  if any.
Some  Mössbauer  spectra  also  reveal  magnetic  texture  for  the  respective  samples.
Particularly,  for  M  =  Al,  Cu  and  Ti  lines  2  and  5  of  those  spectra  are  more  intense,
which is due to the magnetization of iron lying preferentially in the foil plane.
According  to  the  respective  phase  diagram,  aluminum,  niobium  and  titanium
may  form  intermetallic  compounds  when  alloyed  with  iron.  Chromium  and  zinc  may
present solid solutions or stable phases with wide solubility, whereas copper and iron are
immiscible at any practical temperature.  However, all the results converge to  a common
description of the as-milled samples, as schematically shown in Figure 4.  Metallic iron is
adhered  to  the  surface  of  M  (superficial  welding)  or  results  mixed  to  M  (amalgam),
without any significant combination at atomic level.  Thus,  individual properties of the
metals can be  retained in as-milled samples,  or  a fast solid state reaction  involving both
metals can be accomplished  by heat  treatment  (i.e., when thermodynamically possible),
depending on the proposed specific application for the produced mixture.
4. CONCLUSIONS
The  above  results  showed  that  it  is  possible  to  prepare  a  submicrometric
bimetallic  mechanical  mixture  of  M-Fe  type,  from  a  single  high-energy  wet  milling
process.  The  process  control  agent  prevents  the  formation  of  binary  alloys  by
mechanosynthesis, even when both elements have full miscibility  between  them  or could
form intermetallic compounds.  This fact  is very important from the metallurgical point
of  view,  because  it  permits  the  large  scale  preparation  of  submicrometric  bi-metallic
mechanical mixtures, using a very simple routine.
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