Databáze nanotech:
1)
Scientists rope in nanotubes for stronger conducting yarns
22 November 2004
Researchers from the University of Texas at Dallas, US, and CSIRO Textile and
Fibre Technology, Australia, have dry-spun multiwalled carbon nanotubes into
twisted yarns that are both strong and good electrical conductors. The fibres
could have applications in areas such as structural composites, protective
clothing, artificial muscles, electronic textiles, heat pipes
and supercapacitors.
"Our yarns are strong, tough, extremely flexible; they are knot,
creep, chemical and radiation resistant; electrically and
thermally conducting; sewable; weavable; and can be used
from near absolute-zero to ultra-high temperatures," said Ray
Baughman of the University of Texas at Dallas NanoTech
Spinning a yarn
Institute.
To make the yarns, the scientists adapted the traditional textile spinning techniques
that have been around since at least the late Stone Age. They drew 10 nm-diameter
multiwalled carbon nanotubes from a "forest" of similar length tubes deposited on a
substrate, applying a twist at the same time.
The technique builds on a previous dry-spinning method developed in China but
produces yarns that the scientists say are 1000 times stronger.
"The trick is that we use yarn twist and the resulting nanoscale friction to provide
inter-nanotube mechanical coupling leading to yarn strength, rather than weak van
der Waals interactions or a polymer binder," said Baughman. "The absence of this
polymer is important for maximizing properties for multifunctional applications, such
as for a yarn used for structural purposes that also functions as an artificial muscle,
supercapacitor, heat pipe or fuel cell."
The team drew the yarns by hand while they were twisted with a motor at about 2000
rpm. This limited the length of the yarn to about 1 m because of "the arm length of
the person doing the drawing". But the researchers say the spinning process is
amenable to automation, which would enable the production of continuous yarns.
"We see no barrier to commercially practicing our spinning process - and
modifications of it that are described in our pending patent application," said
Baughman. "We are working with CSIRO to upscale the process."
To adjust yarn diameter, the team altered the width of the forest sidewall that they
used to generate an initial wedge-shaped ribbon. Using forest sidewall widths from
less than 150 microns to around 3 mm gave yarn diameters of between 1 and 10
microns.
The team typically applied a twist of around 80,000 turns per metre. This compares to
about 1000 turns per metre for a highly twisted conventional textile yarn with a
diameter 80 times larger.
The researchers also made two-ply yarns by overtwisting a singles yarn and allowing
it to untwist until it reached a torque-balanced state. Then they made four-ply yarns
by repeating the procedure with a two-ply yarn, this time twisting in the opposite
direction.
Despite the good conductivity of individual nanotubes and pure nanotube yarns,
composite fibres containing nanotubes and insulating polymers generally have low
conductivities. But Baughman and colleagues found that the "intertube mechanical
coupling" brought about by twisting helped maintain the electrical conductivity of the
yarn after the infiltration of polyvinyl alcohol (PVA).
Introducing PVA decreased the yarn's electrical conductivity by around 30%,
resulting in nanotube/PVA composite yarns with an electrical conductivity more than
150 times that of coagulation-spun nanotube composite fibres containing PVA.
"Although not yet quite as tough as the Kevlar used for antiballistic vests, our
nanotube-based yarns are tougher than graphite fibre," added Baughman.
"Moreover, our multiwalled nanotube yarns have advantages over Kevlar in terms of
thermal stability, resistance to creep and resistance to ultraviolet-induced
degradation."
The yarns also showed extremely large Poisson's ratios: 4.2 compared to the typical
value for a solid of around 0.3. As a result, applying a strain to the yarns produces a
considerable densification. The team say this might be used for tuning the absorption
and permeability of the yarns.
The researchers reported their work in Science.
2)
Nanotube yarn toughs it out over spider silk
12 June 2003
Scientists at the University of Texas at Dallas Richardson and Trinity College,
Dublin, have spun super-tough carbon nanotube fibres. The fibres, which are
suitable for weaving into electronic cloth, are four times tougher than spider
silk and 17 times tougher than the Kevlar fibres used in
bullet-proof vests.
"To our knowledge, no other material of any type - natural or
synthetic - has a toughness comparable to that of the
nanotube composite fibres," said Ray Baughman of the
University of Texas at Dallas. "Possible applications for this
super-toughness include safety harnesses, explosion-proof
blankets for aircraft cargo areas, and antiballistic vests and
shields."
Electronic textile
Baughman and colleagues made the fibres using a
coagulation-based carbon nanotube spinning technique. They spun surfactantdispersed single-walled carbon nanotubes from a rotating bath of aqueous polyvinyl
alcohol to produce nanotube gel fibres that they then converted to solid nanotube
composite fibres at a rate of more than 70 cm per minute.
The resulting 100 m long fibres were 50 µm in diameter and contained around 60%
nanotubes by weight. They had a tensile strength of 1.8 GPa and an energy-to-break
value of 570 J/g.
Scanning electron microscopy showed that the polyvinyl
alcohol formed a largely amorphous coating on the nanotubes.
Scientists believe that the toughness of spider silk is due to
chain extension in amorphous regions between relatively rigid
crystalline protein blocks; the polyvinyl alcohol may serve a
similar function in the carbon nanotube composite fibres.
Since submitting their work to Nature, the researchers have
further improved the properties of the spun fibres. They have
doubled the strength of the fibres so that they are stronger than Kevlar and twice as
strong as the highest performance spider silk, as well as improving the fibre
toughness to four times that of spider silk at the strain-to-break of spider silk.
Nanotube composite fibre
"We are developing applications that exploit both these mechanical properties and
the novel electronic and electrochemical properties of the carbon nanotube fibres,"
said Baughman. "One example is electronic textiles, where the nanotube fibres
provide a structural function, in addition to one or more other functions."
To demonstrate one such application, the team made supercapacitors from spun
nanotube fibres by coating them with electrolytes. They wove these capacitors into
textiles, thus enabling the materials to store electrical energy. Baughman reckons
that other promising electronic textile applications include distributed sensors,
electronic interconnects, electromagnetic shielding, antennas and batteries. Last
year, the team reported the use of nanotube fibres as artificial muscles that develop
100 times the force of natural muscle with the same diameter.
"We are currently making our fibres on the laboratory scale, producing hundreds of
metres of fibre per run," added Baughman. "This basic fibre-spinning process is
amenable to upscaling, which will involve increasing the spinning rate and going from
single filament to multifilament spinning."
The scientists reported their work in the journal Nature.
About the author
Liz Kalaugher is editor of nanotechweb.org.
3)
Scientists spy on nanotube growth
30 January 2004
Researchers in Denmark have taken the first high-resolution videos of the
growth of carbon nanofibres in a transmission electron microscope (TEM). The
scientists, from Haldor Topsøe and the Technical University of Denmark, made
the nanofibres by methane decomposition over nickel nanocrystals.
“Recent advances in in situ techniques now allow gas-solid
interactions to be studied at the atomic-level in the course of a
catalytic reaction or a nanomaterial synthesis,” Stig Helveg told
nanotechweb.org. “The movies directly show elementary steps
involved in the catalytic growth reaction.”
The videos revealed that the graphitic nanofibres grew from
nickel nanoclusters about 5 to 20 nm in diameter. The nickel
catalyst particles actually changed shape during the growth
process, becoming periodically more and less elongated. The
nanocrystals elongated until they reached a length:width ratio Growing nanofibres
of up to about four, before contracting to a spherical shape
within less than half a second. The elongation appeared to correspond to the
formation of more graphene sheets at the graphene-nickel interface: the scientists
say the reshaping of the nanocrystals assists the alignment of graphene layers into a
tubular structure. If the graphene layers completely surrounded the nickel particle,
growth of the nanofibre stopped.
The TEM images also showed the presence of mono-atomic steps at the nickel
surface, with a graphene sheet terminating at each of the steps. These nickel step
edges appeared to play a key role in the nucleation and growth of graphene sheets,
with graphene layers growing between pairs of step edges as the steps moved
towards the ends of the nickel cluster and vanished.
“By combining the atomic-scale observations with density functional theory (DFT)
calculations, we derived a detailed and coherent growth mechanism describing
carbon nanofibre formation in terms of atomic-scale surface transport and
restructuring of the nickel nanocrystals,” said Helveg. “What’s more, the direct
observations and DFT calculations show that the active site/growth centre is
associated with step edges at the nickel surface, mainly because carbon binds more
strongly to such sites than to sites at close-packed facets.”
The scientists, who reported their work in Nature, believe that their discovery that the
metallic step sites exhibit spatiotemporal dynamic behaviour “may be important for
understanding catalytic reactions and nanomaterial syntheses, which usually assume
a fixed number of stationary active sites.”
About the author
Liz Kalaugher is editor of nanotechweb.org.
4)
Carbon nanotubes head for brain repair
26 May 2005
Researchers in Italy have grown nerve cells from the hippocampus region of
the brain on substrates containing networks of carbon nanotubes. The team,
from the University of Trieste, University of Ferrara, International School for
Advanced Studies (SISSA/ISAS) and the National Consortium of Materials
Science and Technology (INSTM), found that the nanotubes improved neural
signal transfer between the cells.
“The idea of putting together carbon nanotubes and neurones
came first of all because of their structural similarities,” Laura
Ballerini and Maurizio Prato of the University of Trieste told
nanotechweb.org. “Neurite elongations are reminiscent of the
cylindrical shape of carbon nanotubes. And since carbon
nanotubes can be either conducting or semiconducting, in
principle they could be used as assistive devices to functionally
and structurally re-connect neurones that do not talk to each
Nanotube researchers
other anymore.”
In order to deposit multi-walled carbon nanotubes onto a glass substrate, the
researchers functionalized the tubes with pyrrolidine groups, boosting their solubility
in the organic solvent dimethylformamide. The team then placed small drops of a
solution of the nanotubes onto glass coverslips. Once the solvent had evaporated,
the application of a heat treatment defunctionalized the nanotubes, leaving a coating
of nonfunctionalized nanotubes on the glass.
The researchers attached hippocampal neurones both to nanotube-coated glass
coverslips and to uncoated coverslips. Then they monitored the growth of the
neurones for eight to ten days. The amount of growth on both substrates appeared
similar.
The neurones developed on carbon nanotubes and directly on glass also had similar
electrophysiological characteristics - for example, resting membrane potential, input
resistance and capacitance - and similar intrinsic excitability. But neurones grown on
carbon nanotubes displayed a six-fold increase in the frequency of postsynaptic
currents.
“We demonstrate here for the first time a large improvement in neural-signal efficacy
due to the presence of the carbon nanotube substrate,” said Ballerini. “In the long
term, our results will prompt the development of new tissue engineering strategies ...
such as the development of materials suited to functionally reconnecting injured
neurones or to directly improving neural signal transfer.”
The researchers say they can foresee an immediate impact of their findings in the
design of chronic neural implants. “In the field of spinal cord injury, investigating
nanomaterial interactions with nervous tissue will also favour the design of
acceptably small electrodes to provide spinal microstimulation without causing
significant neural damage,” said Ballerini.
The researchers reported their work in Nano Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.
5)
Carbon nanotubes fill up with magnetic nanoparticles
1 April 2005
Researchers at Drexel University and TRI/Princeton, US, have filled carbon
nanotubes with magnetic nanoparticles. The resulting magnetic
nanostructures could have applications in memory devices, medicine and
wearable electronics.
“After successfully filling multiwalled carbon nanotubes with a
variety of polar and nonpolar liquids, such as water, glycerin,
alcohols, benzene, and cyclohexane, it was tempting to try
filling large-diameter nanotubes with a particulate fluid,” Yury
Gogotsi of Drexel University told nanotechweb.org. “Ferrofluid
was a natural choice because it contains small particles (about
10 nm in diameter); numerous ferrofluids are commercially
available; and the practical benefits from filling nanotubes with
magnetic particles were obvious.”
Magnetic nanotubes
Gogotsi and colleagues made the carbon nanotubes by
chemical vapour deposition into the pores of an alumina
membrane. The result was 300 nm-diameter tubes that were open at either one or
both ends. Next, the researchers filled the nanotubes with either organic- or waterbased ferrofluids containing paramagnetic magnetite (Fe3O4) nanoparticles with an
average diameter of 10 nm.
Capillary action enabled the ferrofluid to enter the nanotubes. The carrying fluid then
dried to leave magnetic particles. The scientists used sodium hydroxide to remove
the alumina template and expose the individual carbon nanotubes. They performed
this step either before or after filling the nanotubes with ferrofluid.
“Filling of nanotubes with the ferrofluids appeared to be easier than anticipated,” said
Gogotsi. “We initially used strong magnets to guide the fluid into the tubes, but the
effect of spontaneous penetration of wetting fluids into capillaries was sufficient. We
have also recently demonstrated penetration of the same ferrofluid into 40 nm
nanotube channels.”
The team did use a magnetic field to control the magnetic anisotropy of the
structures. They estimated that around 70 000 nanoparticles entered each tube.
When suspended in liquid, the resulting paramagnetic nanotubes aligned with an
applied magnetic field. The researchers were able to use gold electrodes to orient the
nanotubes in the plane of a silicon wafer or to make them stand up perpendicular to
the surface.
“The filled nanotubes can be used as nanosubmarines externally driven through
blood vessels by a magnetic field and transporting attolitres of drugs to specific
locations in the body, as well as for medical diagnostics without surgical
interference,” said Gogotsi. “They can also be incorporated into smart textiles or films
for magnetic recording.”
According to the researchers, other applications for the magnetic nanotubes include
cantilever tips in magnetic force microscopes, magnetic stirrers or magnetic valves in
nanofluidic devices. And aligned arrays of magnetic nanotubes could be used instead
of nanoposts in fluidic chips for DNA separation. “We should also be able to control
the properties of nanotube-covered material surfaces by applying a magnetic field,”
said Gogotsi.
Now the team is exploring biomedical applications for the nanotubes and carrying out
cytotoxicity studies. “We are also working on modification of tube surfaces to control
their hydrophilicity,” said Gogotsi. “We believe that we can create a large number of
magnetically-controlled and nanopositioned tools for delivery and diagnostics on
cellular and subcellular levels using our magnetic nanotubes.”
The scientists, who reported their research in Nano Letters, have filed for a patent on
their work.
About the author
Liz Kalaugher is editor of nanotechweb.org.
6)
Lotus effect shakes off dirt
8 November 2002
The lotus - a flowering wetland plant native to Asia - may not, at first glance, be
of interest to the nanotechnologist. But researchers at German chemical
company BASF are developing a spray-on coating that mimics the way lotus
leaves repel water droplets and particles of dirt.
Lotus plants have superhydrophobic surfaces: water droplets falling onto them bead
up and, if the surface slopes slightly, will roll off. As a result, the surfaces stay dry
even during a heavy shower. What's more, the droplets pick up small particles of dirt
as they roll, so that the lotus leaves are self-cleaning.
Wilhelm Barthlott, a botanist from the University of Bonn in
Germany, first explained the phenomenon and now owns a
patent and the Lotus Effect trademark. The effect arises
because lotus leaves have a very fine surface structure and
are coated with hydrophobic wax crystals of around 1 nm in
Dirty water
diameter. Surfaces that are rough on a nanoscale tend to be
more hydrophobic than smooth surfaces because of the
reduced contact area between the water and solid. In the lotus plant, the actual
contact area is only 2-3% of the droplet-covered surface.
The nanostructure is also essential to the self-cleaning effect - on a smooth
hydrophobic surface, water droplets slide rather than roll and do not pick up dirt
particles to the same extent.
BASF's lotus-effect aerosol spray combines nanoparticles with
hydrophobic polymers such as polypropylene, polyethylene
and waxes. It also includes a propellant gas. As it dries, the
coating develops a nanostructure through self-assembly.
BASF says that the spray particularly suits rough surfaces
such as paper, leather, textiles and masonry: the self-cleaning
shoe may soon be a reality.
That said, in its current form, the spray may affect the colour of
Water-repellant wood
dark surfaces as its layers are slightly opaque. The coating can
also be mechanically unstable on smooth surfaces. But BASF is working to overcome
these problems. The company even aims to develop a product that will retain its lotus
effect after abrasion with sandpaper. Dubbed lotus stone, the material has potential
for use in the construction industry, in applications such as facing tiles.
About the author
Liz Kalaugher is editor of nanotechweb.org.
7)
Researchers spin carbon nanotube yarns
24 October 2002
A team of Chinese researchers has come up with a new technique for making
long threads of carbon nanotubes. The scientists, from the department of
physics and Tsinghua-Foxconn Nanotechnology Research Center at Tsinghua
University, drew out yarns up to 30 cm long from superaligned arrays of
nanotubes.
"The unique electrical and mechanical properties of a carbon
nanotube are mainly exhibited along its axis direction,"
Shoushan Fan of Tsinghua University told nanotechweb.org.
"If long nanotubes can be obtained, they can be used to create
macroscopic nanotube structures that maintain the unique
properties of the nanotube. It's difficult to infinitely increase the
Nanotube yarn experts
length of nanotubes by growth - our work offers an alternative
method to obtain nanotube yarns of any desired length, which should help the
remarkable properties of carbon nanotubes to be realized at a macroscopic level."
The scientists discovered the technique whilst attempting to pull a bundle of carbon
nanotubes out of an array of nanotubes several hundred microns high on a silicon
substrate. Instead, they managed to draw out a continuous yarn of nanotubes, a
process they compare to drawing a thread from a silk cocoon.
The researchers found they could only draw continuous yarns from superaligned
arrays in which the nanotubes are aligned parallel to one another and held together
in bundles by van der Waals forces. The yarns appear as thin ribbons a few hundred
microns wide that contain parallel threads with diameters of several hundred
nanometres.
A light bulb filament made from the yarn emitted light for 3 hours at 70 V. After this
treatment, the yarn's conductivity and tensile strength both increased, indicating that
some welding may have occurred at the connection points between the tubes. The
scientists also made a polarizer from the yarn.
"Our pure carbon nanotube yarn, after proper heat treatment, should be able to be
woven into a variety of macroscopic objects for various applications, just like silk
thread in the textile industry," added the researchers.
Now the team is working to increase the strength of the joints between the nanotubes
in the yarn and looking at ways to extend the yarn's applications. The scientists
reported their work in Nature.
8)
UK study calls for extra safety measures for nanotechnology
29 July 2004
The UK's Royal Society and Royal Academy of Engineering today released
their long-awaited report on the potential risks and benefits of nanotechnology.
The document recommends additional safety testing for nanoparticles and
nanotubes.
"There is a gap in the current regulation of nanoparticles," said Ann Dowling, chair of
the working group that carried out the study. "They have different properties from the
same chemical in larger form, but currently their production does not trigger
additional testing. It is important that the regulations are tightened up so that
nanoparticles are assessed, both in terms of testing and labelling, as new
chemicals."
The report proposes that UK and European legislation should treat nanoparticles and
nanotubes as new chemicals. In addition, it recommends avoiding "as far as
possible" the release of such nanomaterials into the environment until more is known
about their impact, and that the UK's Health and Safety Executive considers setting
lower exposure levels for people who work with manufactured nanoparticles.
"The lack of evidence about the risk posed by manufactured nanoparticles and
nanotubes is resulting in considerable uncertainty," states the report. With this in
mind, it suggests that a new interdisciplinary centre should research the "toxicity,
epidemiology, persistence and bioaccumulation of manufactured nanoparticles and
nanotubes as well as their exposure pathways" and instruments for monitoring the
materials in the environment. It's likely that such a centre would bring together
several existing research institutions.
Nanoparticles and nanotubes should also be approved by an independent scientific
safety committee before their use in consumer products such as cosmetics,
according to the study. And industry should make details of its nanomaterial safety
tests publicly available if the toxicological data available in peer-reviewed journals is
incomplete.
In addition, the working group proposes a public dialogue about the development of
nanotechnologies "before deeply entrenched or polarized positions appear". Market
research conducted during the course of the study indicated that only 29% of the UK
population had heard of nanotechnology.
The UK government commissioned the two bodies to conduct a study on
nanotechnology back in June 2003. According to UK science minister David
Sainsbury, the government plans to respond formally to the report by the end of the
year.
• The UK Health and Safety Laboratory is holding a conference on the occupational
health implications of nanomaterials in October.
About the author
Liz Kalaugher is editor of nanotechweb.org.
9)
Nanotechnologies in textiles-for now and in the future
Ian Holme. Technical Textiles International : TTI. Holwell: Sep 2004.Vol.13, Iss. 6; pg. 11, 4 pgs
Abstract (Document Summary)
Varying the feedstock, the thiophene concentration, the flow rate of hydrogen and the synthesis
temperature determines if the spun material is composed of single-wall carbon nanotube (SWNT) or multiwall carbon nanotube (MWNT) fibres. The SWNT fibres have diameters of 1.6-3.5 nm and are organized in
bundles with a lateral dimension of 30 nm. The MWNT fibres have a nanotube diameter of 30 nm with an
aspect ratio of about 1000.