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TECH BEAT
Dr. Neil Canter / Contributing Editor
Vulnerability of titanium to oxygen
New research reveals why oxygen causes titanium’s
physical properties to decline.
WITH THE CONTINUING TREND TO FIND APPROACHES TO INCREASE THE EFFICIENCY
OF MACHINERY, lightweight metals are
clearly being looked at more closely
as a substitute for ferrous alloys. Metals such as titanium offer a superior
strength-to-weight ratio and better
resistance to corrosion than steel. But
titanium is also more expensive.
In a previous TLT article, a new approach was made to reduce the cost of
titanium through a potentially lowercost manufacturing pathway using titanium hydride.1 The existing method
for extracting titanium is known as
the Kroll process, which is conducted
through a series of steps at elevated
KEY CO
CONCEPTS
O C
S
• Very low concentrations
concentratio
nc trat s of
oxygen in titanium can cause the
thee
metal to harden
haarden and become
more brittle.
• This effect occurs because one
dislocation in the titanium
inte
interacts
t acts
t with
th an
an oxygen
oxyygen atom
ato
tm
to form multiple
m ltip e dislocations.
• While
h l the strength
tre gth of titanium
tita um
can increase
increase
c ease by a factor
facto off three
th ee
due to multiple dislocations, this
will eventually
e ent ally cause
ca se titanium’s
titaniu ’s
tita
resi tance to cracking
resistance
acki g to decline
d cline
by six times.
18
•
M AY 2 0 1 5
temperatures in excess of 980 C. Use
of titanium hydride led to the formation of titanium in modeling studies
that involved the use of much lower
temperatures, thereby leading to the
promise of a lower-cost, more energy
efficient process.
One of the problems in finding a
better way to produce titanium is oxygen. Andrew Minor, associate professor of materials science and engineering at the University of California in
Berkeley, Calif., and faculty scientist at
Lawrence Berkeley National Laboratory, says, “Very low concentrations of
oxygen in titanium leads to the hardening of the metal, which eventually
causes a decrease in toughness in the
titanium metal that makes it less resistant to cracks.”
A better understanding of how oxygen causes titanium’s physical properties to decline is needed in order to
determine what can be done to figure
out a more cost effective method for
extracting and processing titanium.
Research has now been done showing
what oxygen does to titanium on the
atomic level.
DISLOCATION MULTIPLIER
Minor and his associates at the Berkeley Laboratory, in collaboration with
colleagues at Japan’s Nuclear Science
and Engineering Directorate and Rolls
Royce (a manufacturer of jet turbine
engines), has determined how oxygen
at very low concentrations will cause
titanium to become harder and then
eventually more brittle. The presence
T R I B O LO GY & LU B R I CAT I O N T EC H N O LO GY
The hope is that this
work will help in the
development of a lowercost process for refining
titanium without the
metal losing its optimal
characteristics.
of oxygen in titanium is similar to metals containing other components that
act as solutes to improve mechanical
properties, but the effect is far more
extreme than normal solute hardening.
Minor says, “Solutes are added to
every metal to make alloys. One of
the main reasons for doing this is to
increase the hardening of the resulting
metallic alloys. This process takes place
because the solutes are atoms with a
different size than the plane of metallic
atoms they are encountering. Solutes
end up being attracted to dislocations,
which represents extra spaces in the
metal for foreign atoms that do not fit
perfectly within the matrix lattice.”
An analogy to this process as explained by Minor is moving a rug
along a floor by applying a ripple (the
dislocation) that flows through the
fiber of the rug. There are two types
of dislocations seen in metals. One is
called an edge dislocation while the
second one is known as screw dislocation. Both are equivalently missing
W W W. ST L E .O RG
Figure 1 | A moving dislocation (shown in light blue) in titanium can interact with an oxygen atom (shown in red) to form multiple dislocations
(shown in dark blue) leading titanium to be much less resistant to cracking. (Figure courtesy of the University of California Berkeley.)
half-planes of atoms in the crystal
structure of a material.
Minor explains that the solute atoms
are typically either placed in a substitution manner in the same plane as the
metal atoms or in an interstitial fashion
in between the metal atom planes.
The researchers studied the alpha form of titanium, which is readily available commercially, doped with
oxygen at concentrations of 0.1%, 0.2%
and 0.3% by weight using a technique
known as aberration-corrected transmission electron microscopy (TEM)
to determine the effect of the oxygen
atoms on the titanium metal matrix
over time.
Minor says, “Oxygen generates a
very strong negative repulsion with
the screw dislocations in titanium at
a very small concentration. Normally,
for most alloys, a solute concentration between 0.1%-0.3% does not
make much difference in the resulting
properties. But with titanium, the difference between three and one oxygen
W W W. ST L E .O RG
atoms within 1,000 titanium atoms
will cause a very different motion of
the dislocations.”
The TEM data shows that dislocation in titanium interacts with an oxygen atom (shown in red in Figure 1) to
generate additional dislocations. Minor
says, “This oxygen atom acts as a multiplier to produce additional dislocations leading to more tangling of the
dislocations and therefore results in the
titanium becoming more brittle.”
The researchers used in situ TEM
nanocompression testing to assess the
impact of these dislocations in real
time on the mechanical properties of
titanium. Minor says, “We found that
the strength of titanium increases by
a factor of three due to the multiplier
dislocation effect of oxygen atoms—
this leads to the effect of six times less
resistance to cracking.”
Future work will concern finding
techniques for preventing oxygen atoms
from causing titanium to become brittle.
Minor says, “We will try to use compuT R I B O LO GY & LU B R I CAT I O N T EC H N O LO GY
tational means to determine how to lock
up oxygen atoms to mitigate this effect,
such as finding elements that can bind
with the oxygen preventing the dislocation multiplier effect.”
The hope of the researchers is that
this work will help in the development
of a lower-cost process for refining
titanium without the metal losing its
optimal characteristics. Additional information can be found in a recent article2 or by contacting Minor at aminor@
berkeley.edu.
REFERENCES
1. Canter, N. (2014), “Cost-effective
titanium extraction,” TLT, 70 (7),
pp. 12-13.
2. Yu, Q., Qi, L., Tsuru, T., Traylor,
R., Rugg, D., Morris, J., Asta, M.,
Chrzan, D. and Minor, A. (2015),
“Origin of dramatic oxygen solute
strengthening effect in titanium,”
Science, 347 (6222), pp. 635-639.
M AY 2 0 1 5
•
19
TECH BEAT
Removing metals from aqueous waste
streams with hydroxyfullerene
Included are a variety of main group and transition metals.
FULLERENE, WHICH IS ALSO KNOWN AS A
BUCKYBALL, exhibits a hollow sphere
cage structure similar to a soccer ball
that contains 60-carbon atoms. This
compound is prepared by vaporization
of graphite and is remarkably stable.
Since fullerene was discovered in
1985, this compound has been evaluated for use in such applications as solar cells and hydrogen gas storage. One
area where other carbon-based materi-
KEY CONCEPTS
• Hydroxyfullerenes,
Hydroxyf ll nes, a derivative
deri t e
of the 60-carbon atom
attom
buckyball structure known as
full
fullerene,
e, exhibit promise
p i e in
in
removing metals from aqueous
waste streams.
• A wide range of metals can be
extracted, but the ones
preferred are
a e +2 charge cations
ar
withh small
s all
a atomic
atom
t icc radii
d i and +3
charge cations with large
atomic radii.
• The reason for this preference
f
ce
is that hydroxyfullerenes are
mi t s of different
mixtures
d fferent molecules
mol l
with
thh hydroxyl groups placed in
multiple orientations.
20
Initial work with hydroxyfullerenes is very promising
because it shows great affinity for zinc, which is a
difficult metal to waste treat.
als such as graphene oxide have been
evaluated is in removal of heavy metals
from water through adsorption.
This application is very important
because water is a precious resource
and removal of metals from effluent
streams generated in manufacturing
plants is mandatory. In a previous TLT
article, a new compound known as
copper hydroxide ethanedisulfonate
was found to have promise in removing anions from water though ion exchange.1 Among the anions removed
were dicarboxylates and metal oxo anions such as permanganate.
Andrew R. Barron, the Charles W.
Duncan Jr.-Welch Chair of Chemistry
and professor of materials science and
nanoengineering at Rice University in
Houston, indicates that past research
has focused on looking for compounds
that extract specific precious metals.
He says, “There have been lots of studies on nanomaterials to evaluate their
ability to extract specific precious
metals. One material that has shown
promise is graphene oxide, which has
been found in the literature to remove
transition metals, lanthanides and actinides from water.”
None of these nanomaterials have
the ability to extract all of the metals
that may be present in a specific effluent stream. Barron says, “Based on the
work conducted with carbon nanomaterials, we decided to look at hydroxyfullerenes. Our initial work showed
that hydroxyfullerenes chelate ferric
ions (Fe3+) to form a water insoluble
complex.”
The other attractive reason for
working with hydroxyfullerenes is
that they function in a similar manner
to the phenolic derivative, catechol in
forming cross-linked complexes with
metals. A catechol derivative known as
3,4-dihydroxyphenylalanine is used by
marine mussel proteins to chelate metals. Barron says, “Hydroxyfullerenes are
a larger version of the phenolic ligands
seen in biological systems.”
Hydroxylation of fullerene takes
place through reaction with sodium
hydroxide in the presence of tetrabutylammonium hydroxide. Barron says,
“Depending upon the reaction conditions, over 20 hydroxyl groups can be
placed on a single fullerene molecule.
Some of the groups are alkoxides that
are neutralized with sodium cations.”
A recently completed study has now
shown that hydroxyfullerenes can extract
The Apollo 11 spacecraft consisted of the command module, Columbia, and the lunar module, Eagle.
ing of hydroxyfullerenes
with nickel and iron salts.
The choice of the anion
appears to have no effect
NANOAGGREGATES
on the ability of the hydroxyfullerene to bind to
Barron and his associthe metal. Barron says,
ates evaluated the abil“We tried a series of anity of hydroxyfullerenes
ions that included aceto cross-link a series of
tates, carbonates, citrates,
metal salts. The process
nitrates and sulfates. None
used is to mix solutions
of these anions impacted
of the metal salts and
the cross-linking process.”
the hydroxyfullerenes at
Future work will inroom temperature, obvolve
trying to boost the
serve the formation of
metal: hydroxyfullerene
a precipitate, isolate the
ration used and to work
solid and then analyze
with carbon nanotubes.
the metal ligand through
Barron says, “Our objecthe use of ultraviolettive is to determine if carvisible spectroscopy.
bon nanotubes bound to a
Experiments were
membrane filter can act as
done using individual
tentacles to bind metals in
salts and then comparing
the effluent stream.”
a specific metal salt with
Figure 2 | Nanoaggregates formed from the cross-linking of
Initial work with hyferric nitrate in a series of
hydroxyfullerenes with metals (the example shown is with iron
droxyfullerenes is very
competitive cross-linking
and nickel) explain how this fullerene derivative is effective in
extracting metals from aqueous streams. (Figure courtesy of
promising because it
reactions. The researchers
Rice
University.)
shows great affinity for
evaluated salts of alumizinc, which is a difficult
num, cadmium, calcium,
metal to waste treat. Addicobalt, copper, mangational information can be
nese, nickel, silver and
found in a recent article2
zinc.
sistent with the empirical results.”
Barron says, “We found that hyor by contacting Barron at arb@rice.
The reason for the discrepancy is
droxyfullerenes prefer to bind with +2
edu.
that hydroxyl groups attached to the
charge cations that have small atomic
fullerene cage are not only in the 1,2
radii and with +3 charge cations that
orientation but also in the 1,3 and 1,3,5
have large atomic radii. For our exREFERENCES
orientations. Barron says, “Fullerene
periments, hydroxyfullerenes have the
1. Canter, N. (2011), “Water
is functioning as a mixture of different
most affinity for zinc of the +2 charge
treatment of anions,” TLT, 67
molecules and this explains the binding
cations followed by cobalt, manganese,
(12), pp. 12-13.
behavior for +2 and +3 charged cations.”
nickel, cadmium and copper. The +3
2. Heimann, J., Morrow, L.,
Transmission electron microscope
charge cation that has most affinity to
Anderson, R. and Barron, A.
images of the cross-linked hydroxyhydroxyfullerenes is lanthanide fol(2015), “Understanding the
fullerene metal samples show nanoaglowed by iron and aluminum.”
relative binding ability of
hydroxyfullerene to divalent and
gregates that contain a series of metals
The reason for this effect was at first
trivalent metals,” Dalton Transacand fullerenes. Barron says, “In dilute
puzzling for Barron and his colleagues
tions, 44, pp. 4380-4388.
solutions, we found that these nanoagbecause they did a computational study
gregates are dozens of nanometers long
assuming that the hydroxyfullerenes
because chains initially made from one
doing the binding mainly had diols adhydroxyfullerene molecule tend to agjacent to each other in the 1,2 position.
Neil Canter heads his own
consulting company, Chemical
gregate to other chains. The aggregates
He says, “We initially did ab initio calSolutions, in Willow Grove, Pa.
can contain as many as 20-30 hydroxyculations on catechols and found that
Ideas for Tech Beat can be
fullerene molecules.”
binding is based on size as expected and
submitted to him at
Figure 2 shows an image of a nanonot on charge. For hydroxyfullerenes,
[email protected].
aggregate formed from the cross-linkthe computational studies were not cona variety of main group
and transition metals from
an aqueous environment.
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