159 research outputs found
Role of the particle size polydispersity in the electrical conductivity of carbon nanotube-epoxy composites
Carbon nanotubes (CTNs) with large aspect-ratios are extensively used to
establish electrical connectedness in polymer melts at very low CNT loadings.
However, the CNT size polydispersity and the quality of the dispersion are
still not fully understood factors that can substantially alter the desired
characteristics of CNT nanocomposites. Here we demonstrate that the electrical
conductivity of polydisperse CNT-epoxy composites with purposely-tailored
distributions of the nanotube length L is a quasiuniversal function of the
first moment of L. This finding challenges the current understanding that the
conductivity depends upon higher moments of the CNT length. We explain the
observed quasiuniversality by a combined effect between the particle size
polydispersity and clustering. This mechanism can be exploited to achieve
controlled tuning of the electrical transport in general CNT nanocomposites.Comment: 9 pages, 5 figure
Electrical conductivity of multi-walled carbon nanotubes-SU8 epoxy composites
We have characterized the electrical conductivity of the composite which
consists of multi-walled carbon nanotubes dispersed in SU8 epoxy resin.
Depending on the processing conditions of the epoxy (ranging from
non-polymerized to cross-linked) we obtained tunneling and percolating-like
regimes of the electrical conductivity of the composites. We interpret the
observed qualitative change of the conductivity behavior in terms of reduced
separation between the nanotubes induced by polymerization of the epoxy matrix.Comment: 4 pages, 3 figure
The Role of Transport Agents in MoS2 Single Crystals
We report resistivity, thermoelectric power and thermal conductivity of MoS2
single crystals prepared by chemical vapour transport (CVT) method using I2,
Br2 and TeCl4 as transport agents. The material presents low-lying donor and
acceptor levels, which dominate the in-plane charge transport. Intercalates
into the Van der Waals gap strongly influence the inter-plane resistivity.
Thermoelectric power displays the characteristics of strong electron-phonon
interaction. Detailed theoretical model of thermal conductivity reveals the
presence of high number of defects in the MoS2 structure. We show that these
defects are inherent to CVT growth method, coming mostly from the transport
agent molecules inclusion as identified by Total Reflection X-ray Fluorescence
analysis (TXRF) and in-beam activation analysis (IBAA).Comment: 17 pages, 5 figure
Synthesis of Homogeneous Manganese-Doped Titanium Oxide Nanotubes from Titanate Precursors
We report a novel synthesis route of homogeneously manganese-doped titanium
dioxide nanotubes in a broad concentration range. The scroll-type trititanate
(H(2)Ti(3)O(7)) nanotubes prepared by hydrothermal synthesis were used as
precursors. Mn2+ ions were introduced by an ion exchange method resulting
Mn(x)H(2-x)Ti(3)O(7). In a subsequent heat-treatment they were transformed into
Mn(y)Ti(1-y)O(2) where y=x/(3+x). The state and the local environment of the
Mn2+ ions in the precursor and final products were studied by Electron Spin
Resonance (ESR) technique. It was found that the Mn2+ ions occupy two
positions: the first having an almost perfect cubic symmetry while the other is
in a strongly distorted octahedral site. The ratio of the two Mn2+ sites is
independent of the doping level and amounts to 15:85 in Mn(x)H(2-x)Ti(3)O(7)
and to 5:95 in Mn(y)Ti(1-y)O(2). SQUID magnetometry does not show long-range
magnetic order in the homogeneously Mn2+-doped nanotubes.Comment: 7 pages, 6 figure
Management of nanomaterials safety in research environment
Despite numerous discussions, workshops, reviews and reports about responsible development of nanotechnology, information describing health and environmental risk of engineered nanoparticles or nanomaterials is severely lacking and thus insufficient for completing rigorous risk assessment on their use. However, since preliminary scientific evaluations indicate that there are reasonable suspicions that activities involving nanomaterials might have damaging effects on human health; the precautionary principle must be applied. Public and private institutions as well as industries have the duty to adopt preventive and protective measures proportionate to the risk intensity and the desired level of protection. In this work, we present a practical, 'user-friendly' procedure for a university-wide safety and health management of nanomaterials, developed as a multi-stakeholder effort (government, accident insurance, researchers and experts for occupational safety and health). The process starts using a schematic decision tree that allows classifying the nano laboratory into three hazard classes similar to a control banding approach (from Nano 3 - highest hazard to Nano1 - lowest hazard). Classifying laboratories into risk classes would require considering actual or potential exposure to the nanomaterial as well as statistical data on health effects of exposure. Due to the fact that these data (as well as exposure limits for each individual material) are not available, risk classes could not be determined. For each hazard level we then provide a list of required risk mitigation measures (technical, organizational and personal). The target 'users' of this safety and health methodology are researchers and safety officers. They can rapidly access the precautionary hazard class of their activities and the corresponding adequate safety and health measures. We succeed in convincing scientist dealing with nano-activities that adequate safety measures and management are promoting innovation and discoveries by ensuring them a safe environment even in the case of very novel products. The proposed measures are not considered as constraints but as a support to their research. This methodology is being implemented at the Ecole Polytechnique de Lausanne in over 100 research labs dealing with nanomaterials. It is our opinion that it would be useful to other research and academia institutions as well
Direct observation of the exchange anisotropy in the helimagnetic insulator CuOSeO
The helical magnetic structures of cubic chiral systems are well-explained by
the competition among Heisenberg exchange, Dzyaloshinskii-Moriya interaction,
cubic anisotropy, and anisotropic exchange interaction (AEI). Recently, the
role of the latter has been argued theoretically to be crucial for the
low-temperature phase diagram of the cubic chiral magnet CuOSeO, which
features tilted conical and disordered skyrmion states for a specific
orientation of the applied magnetic field (). In this study, we exploit transmission resonant x-ray scattering
(REXS) in vector magnetic fields to directly quantify the strength of the
AEI in CuOSeO, and measure its temperature dependence. We find that the
AEI continuously increases below 50\,K, resulting in a conical spiral pitch
variation of in the (001) plane. Our results contribute to establishing
the interaction space that supports tilted cone and low-temperature skyrmion
state formation, facilitating the goals for both a quantitative description and
eventual design of the diverse spiral states existing amongst chiral magnets
Influence of the initial state of carbon nanotubes on their colloidal stability under natural conditions
The colloidal stability of CNTs varies a lot depending on the initial state of the CNTs (dry vs. pre-dispersed), the applied dispersant for pre-suspension, and the composition of the medium. a r t i c l e i n f o Keywords: Carbon nanotubes Surfactants (anionic, non-ionic, cationic) Natural organic matter (humic/fulvic acid) Long-term colloidal stability a b s t r a c t The colloidal stability of dry and suspended carbon nanotubes (CNTs) in the presence of amphiphilic compounds (i.e. natural organic matter or surfactants) at environmentally realistic concentrations was investigated over several days. The suspensions were analyzed for CNT concentration (UVevis spectroscopy), particle size (nanoparticle tracking analysis), and CNT length and dispersion quality (TEM). When added in dry form, around 1% of the added CNTs remained suspended. Pre-dispersion in organic solvent or anionic detergent stabilized up to 65% of the added CNTs after 20 days of mild shaking and 5 days of settling. The initial state of the CNTs (dry vs. suspended) and the medium composition hence are critical determinants for the partitioning of CNTs between sediment and the water column. TEM analysis revealed that single suspended CNTs were present in all suspensions and that shaking and settling resulted in a fractionation of the CNTs with shorter CNTs remaining predominantly in suspension
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