Upload
john-k-borchardt
View
218
Download
2
Embed Size (px)
Citation preview
RESEARCH NEWS
September 200410
There is increasing interest in using
nanoscale reinforcing fillers to improve
polymer flammability properties.
Takashi Kashiwagi and coworkers at
the US National Institute of Standards
and Technology and the University of
Kentucky have measured the thermal
and flammability properties of
polypropylene/multi-walled carbon
nanotube (PP/MWNT) nanocomposites
[Kashiwagi et al., Polymer (2004) 45
(12), 4227].
The MWNT content of the
nanocomposite was varied from 0.5%
to 4% by weight and the flammability
properties measured. The peak heat
release rate was observed at an
MWNT content of 1%. In contrast, the
addition of carbon black powder to PP
did not reduce the heat release rate as
much. This suggests that the size and
shape of MWNTs is crucial to reducing
PP flammability. The radiative ignition
delay time of nanocomposites with
<2% MWNTs by mass is less than
that of PP because the radiant flux
absorptivity at infrared wavelengths
increases significantly with the addition
of MWNTs. Ignition delay time and the
peak heat release rate of the
nanocomposite increases with MWNT
content above 1% by weight. The
lowest heat release rate is observed
with the PP/MWNT (1%) sample.
Scanning electron and optical
microscopy studies indicate that the
PP/MWNT flame retardancy results
from formation of a relatively uniform
network-structured MWNT layer, which
covers the entire sample surface
without any cracks or gaps. This layer
re-emits much of the incident radiation
from its hot surface, thereby reducing
the transmitted flux to the PP layers
below. Formation of this nanotube
network layer is critical in improving
flammability properties. John K. Borchardt
Nanotubesretard fire COMPOSITES
A simple but effective synthesis route for polymernanowires is a major challenge for materialschemists. However, progress is being made andnow Erkang Wang and colleagues at ChangchunInstitute of Applied Chemistry in China report thelarge-scale room-temperature synthesis of uniform poly(o-phenylenediamine) nanobelts from an o-phenylenediamine-HAuCl4 aqueous solution withoutusing templates or surfactants to direct thesynthesis.The nanobelts are several hundred microns long,several hundred nanometers wide, and tens ofnanometers thick [Sun et al., Chem. Commun.(2004) 10, 1182]. Scanning electron microscopy(SEM) of the precipitate formed when aqueoussolutions of the two chemicals are combined revealsa large quantity of uniform one-dimensionalstructures. Higher magnification indicates that
these structures are transparent nanobeltsseparated from one another. Despite their length,the nanobelts are very straight, suggesting thatthey are quite rigid. Nanoparticles were alsoobserved in the precipitate.Elemental analysis of the nanobelts andnanoparticles using secondary electron SEManalysis reveals peaks from Au, C, and N. X-raydiffraction analysis confirms that that thenanoparticles are Au and the nanobelts are purelypoly(o-phenylenediamine).The researchers suggest that o-phenylenediamineacts as an electron donor for HAuCl4, which is apowerful oxidant with a high reduction potential.They propose that spontaneous nanobelt formationand growth is the result of Au nanoparticle catalysisof poly(o-phenylenediamine).John K Borchardt
Synthesizing polymer nanobelts POLYMERS
Growing magnetic cables on the nanoscaleNANOTECHNOLOGY
Researchers at the University of SouthernCalifornia have developed a generic synthesistechnique for the growth of composite‘nanocables’ [Han et al., Nano Lett. (2004),4 (7), 1241].The nanocables consist of dense arrays ofultrafine MgO wires coated with a uniform,precisely controlled layer of a transitionmetal oxide (TMO). First, Chongwu Zhou andcoworkers created arrays of MgO nanowiresby condensing MgO vapor onto MgO plates
using an Au catalyst, following a vapor-solid-liquid mechanism. The resulting nanowiresare 30-100 nm in diameter and 3 µm long.Next pulsed laser deposition was used todeposit a layer of a TMO onto the MgOcores, which act as a template. “The trick iswe can preserve the TMO composition duringthis technique,” says Zhou, “while othertechniques cannot.” The resulting structuresare single crystalline core-shell MgO/TMOnanowires with precisely controlled layerthickness. The researchers grew nanocablesof MgO with an outer layer of thesuperconductor YBa2Cu3O6.66,La0.67Ca0.33MnO3, which shows colossalmagnetoresistance, the ferroelectricPbZr0.58Ti0.42O3, and magnetic mineral formof magnetite, Fe3O4. The La0.67Ca0.33MnO3/MgO nanowires show that the metal-insulator transition and magnetoresistanceare preserved down to the nanoscale.The versatile synthesis method could allowthe production of many different nanowirecompositions. “We expect that these TMOnanowires may offer enormous opportunitiesto explore intriguing physics at nanoscaledimensions,” says Zhou. “Future work will bedirected toward in-depth understanding anddevice applications of these new materials.”Cordelia Sealy
Left: Schematic diagram of the pulsed laser deposition setup,
where MgO nanowires are used as templates for the coating of a
second layer, therefore rendering core-shell nanocables. Upper
right: Scanning electron micrograph of MgO/YBCO core-shell
nanocables. Lower right: Transmission electron micrograph
revealing the MgO core and the YBCO shell. YBCO is known to be
a high-transition-temperature superconductor.