1,075 research outputs found
Simulation of electric field-assisted nanowire growth from aqueous solutions
The present work is aimed at investigating the mechanisms of nanowire growth from aqueous solutions through a physical and chemical modeling. Based on this modeling, deriving an optimized process control is intended. The work considers two methods of nanowire growth. The first is the dielectrophoretic nanowire assembly from neutral molecules or metal clusters. Secondly, in the directed electrochemical nanowire assembly metal-containing ions are reduced in an AC electric field in the vicinity of the nanowire tip and afterwards deposited at the nanowire surface.
To describe the transport and growth processes, continuum models are employed. Furthermore, it has been necessary to consider electro-kinetic fluid flows to match the experimental observations. The occurring partial differential equations are solved numerically by means of finite element method (FEM).
The effect of the process parameters on the nanowire growth are analyzed by comparing experimental results to a parameter study. The evaluation has yielded that an AC electro-osmotic fluid flow has a major influence on the dielectrophoretic nanowire assembly regarding the growth velocity and morphology. In the case of directed electrochemical nanowire assembly, the nanowire morphology can be controlled by the applied AC signal shape. Based on the nanowire growth model, an optimized AC signal has been designed, whose parametrization allows to adjust to the chemical precursor and the desired nanowire diameter.Ziel der vorliegenden Arbeit ist es, mittels physikalischer und chemischer Modelle die Mechanismen des Nanodrahtwachstums aus wĂ€ssrigen Lösungen zu erforschen und daraus eine optimierte Prozesskontrolle abzuleiten. Dabei werden zwei Verfahren des Nanodrahtwachstums nĂ€her betrachtet: Dies sind die dielektrophoretische Assemblierung von neutralen MolekĂŒlen oder Metallclustern sowie die gerichtete elektrochemische Nanodrahtabscheidung (engl. directed electrochemical nanowire assembly), bei der metallhaltige Ionen im elektrischen Wechselfeld an der Nanodrahtspitze zunĂ€chst reduziert und anschlieĂend als Metallatome abgeschieden werden.
Zur Beschreibung der Transport- und Wachstumsprozesse werden Kontinuumsmodelle eingesetzt. DarĂŒber hinaus hat es sich als notwendig erwiesen, elektrokinetische Fluidströmungen zu berĂŒcksichtigen, um die experimentellen Beobachtungen zu reproduzieren. Die auftretenden partiellen Differenzialgleichungen werden mittels der Finiten Elemente Methode (FEM) numerisch gelöst.
Die Auswirkungen der Prozessparameter auf das Nanodrahtwachstum werden durch den Vergleich von experimentellen Ergebnissen mit Parameterstudien analysiert. Die Auswertung hat ergeben, dass fĂŒr das dielektrophoretische Wachstum ein durch Wechselfeldelektroosmose (engl. AC electro-osmosis) angetriebener Fluidstrom die Drahtwachstumsgeschwindigkeit und -morphologie maĂgeblich beeinflusst. Im Falle der gerichteten elektrochemischen Nanodrahtabscheidung lĂ€sst sich die Drahtmorphologie ĂŒber das angelegte elektrische Wechselsignal steuern. Unter Verwendung des Wachstumsmodells ist ein optimiertes Signal generiert worden, dessen Parametrisierung eine gezielte Anpassung auf den chemischen Ausgangsstoff und den gewĂŒnschten Drahtdurchmesser erlaubt
Deformation and orientation during shear and elongation of a polycarbonate/carbon nanotubes composite in the melt
In this study, we focused on the elongational rheology and the morphology of an electrically conductive polycarbonate/multiwalled carbon nanotubes (2 wt%) composite in the melt. In shear and melt elongation, the influence of the carbon nanotubes was large when the externally applied stress was small. Consequently, the elastic interactions resulting from the carbon nanotubes dominated in the low frequency range of the shear oscillations. The elongational viscosity of the composite was only moderately influenced by the addition of 2wt% carbon nanotubes. Transmission electron microscopy investigations of the stretched composite showed that isolated carbon nanotubes were oriented in elongation. In recovery after melt elongation, the recovered stretch of the composite was much smaller than the recovered stretch of pure polycarbonate. This effect is caused by the carbon nanotubes network, which prohibited large extensions of the macromolecules and led to a yield stress of the composit
Polyethylene Glycol as Additive to Achieve N-Conductive Melt-Mixed Polymer/Carbon Nanotube Composites for Thermoelectric Application
The development of thermoelectric (TE) materials based on thermoplastic polymers and carbon nanotubes is a focus of current TE research activities. For a TE module, both p- and n-conductive composites are required, whereby the production of n-conductive materials is a particular challenge. The present study investigates whether adding polyethylene glycol (PEG) as n-dopant during the melt-mixing of the conductive composites based on polycarbonate, poly(ether ether ketone), or poly(butylene terephthalate) with singlewalled carbon nanotubes (0.5 to 2 wt%) is a possible solution. It was shown that for all three polymer types, a change in the sign of the Seebeck coefficient from positive to negative could be achieved when at least 1.5 wt% PEG was added. The most negative Seebeck coefficients were determined to be â30.1 ”V/K (PC), â44.1 ”V/K (PEEK), and â14.5 ”V/K (PBT). The maximal power factors ranged between 0.0078 ”W/m·K2 (PC), 0.035 ”W/m·K2 (PEEK), and 0.0051 ”W/m·K2 (PBT)
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Polymer - Carbon nanotube composites for thermoelectric applications
The thermoelectric (TE) performance of electrically conductive thermoplastic composites prepared by melt mixing was investigated. A cost effective widely used in industry polymer, namely polypropylene (PP), was chosen as the matrix to fabricate the composites. Singlewalled carbon nanotubes (SWCNTs), the amount (2 wt%) of which was selected to be above the electrical percolation threshold (< 0.2 wt%), were used to form an electrical conducting network. Besides as-produced SWCNTs plasma modified tubes were employed to study the influence of the functionalization on the morphology, dispersion and TE properties of the PP composites. In addition, melt processing conditions, e.g. temperature, rotation speed, and time during mixing in a small-scale compounder were varied. Furthermore, an ionic liquid (IL, 1-methyl-3-octylimidazolium tetrafluoroborate) was used as a processing additive during melt mixing, which was confirmed to improve the electrical conductivity of the composites. Simultaneous increase in the Seebeck coefficient up to a value of 64 ΌV/K was recorded, leading to a much better power factor of 0.26 ΌW/(m·K2) compared to composites without IL. This melt mixing strategy opens new avenues for solvent-free, large scale fabrication of polymer based TE materials
Improvement of electrical resistivity of highly filled graphite/pp composite based bipolar plates for fuel cells by addition of carbon black
Novel material solutions for polymer based bipolar plates in fuel cells require adapted ways to develop suitable material compositions. The common pathway to develop materials with at the same time high electrical as well as thermal conductivity is the use of conductive graphite as filler with contents up to 80-85 wt.%. However, there is a need to develop recipes with maximized conductive behavior at lowest possible content of conductive filler to enhance the mechanical properties and allow good processability. In this study, composites based on polypropylene (PP) and different filler systems were melt-mixed using a lab scale co-rotating twin-screw extruder and compression molded to bipolar type plates. As fillers synthetic (G) or expanded (EG) graphites were incorporated. At the overall filler content of 60 wt.% or 80 wt% part of the graphite was replaced by highly conductive carbon black (CB, 2.5 wt.%, 5.0 wt.%). It was found that the addition of CB significantly reduced the electrical volume as well as the surface resistivity up to values of 0.12 Ωcm or 4 mΩ/square, respectively. For the values of thermal conductivity the kind and particle size of the selected graphite was important. If expanded graphite was partially replaced by CB, the thermal conductivity of PP/EG+CB composites decreased significantly. Otherwise, the combination of synthetic graphite and CB changed the thermal conductivity of PP composites only marginal at the same overall filler content. For both graphite types the filler with larger particle size resulted in higher thermal conductivity
Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots
A systematic variation of the exciton fine-structure splitting with quantum
dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor
deposition is observed. The splitting increases from -80 to as much as 520
eV with quantum dot size. A change of sign is reported for small quantum
dots. Model calculations within the framework of eight-band k.p theory and the
configuration interaction method were performed. Different sources for the
fine-structure splitting are discussed, and piezoelectricity is pinpointed as
the only effect reproducing the observed trend.Comment: 5 pages, 5 figure
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Blend Structure and n-Type Thermoelectric Performance of PA6/SAN and PA6/PMMA Blends Filled with Singlewalled Carbon Nanotubes
The present study investigates how the formation of melt-mixed immiscible blends based on PA6/SAN and PA6/PMMA filled with single walled nanotubes (SWCNTs) affects the thermoelectric (TE) properties. In addition to the detailed investigation of the blend morphology with compositions between 100/0 wt.% and 50/50 wt.%, the thermoelectric properties are investigated on blends with different SWCNT concentrations (0.25â3.0 wt.%). Both PA6 and the blend composites with the used type of SWCNTs showed negative Seebeck coefficients. It was shown that the PA6 matrix polymer, in which the SWCNTs are localized, mainly influenced the thermoelectric properties of blends with high SWCNT contents. By varying the blend composition, an increase in the absolute Seebeck coefficient, power factor (PF), and figure of merit (ZT) was achieved compared to the PA6 composite which is mainly related to the selective localization and enrichment of SWCNTs in the PA6 matrix at constant SWCNT loading. The maximum PFs achieved were 0.22 ”W/m·K2 for PA6/SAN/SWCNT 70/30/3 wt.% and 0.13 ”W/m·K2 for PA6/PMMA/SWCNT 60/40/3 wt.% compared to 0.09 ”W/m·K2 for PA6/3 wt.% SWCNT which represent increases to 244% and 144%, respectively. At higher PMMA or SAN concentration, the change from matrix-droplet to a co-continuous morphology started, which, despite higher SWCNT enrichment in the PA6 matrix, disturbed the electrical conductivity, resulting in reduced PFs with still increasing Seebeck coefficients. At SWCNT contents between 0.5 and 3 wt.% the increase in the absolute Seebeck coefficient was compensated by lower electrical conductivity resulting in lower PF and ZT as compared to the PA6 composites
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Melt mixed PCL/MWCNT composites prepared at different rotation speeds: Characterization of rheological, thermal, and electrical properties, molecular weight, MWCNT macrodispersion, and MWCNT length distribution
Composites of poly(caprolactone) (PCL) and 0.5 wt.% multiwalled carbon nanotubes (MWCNT) were prepared by melt-mixing in a conical twin-screw micro-compounder by varying the rotation speed between 25 and 400 rpm at constant mixing time and temperature. The state of dispersion analyzed by light microscopy was improved with increasing rotation speed but levels off starting at about 100 rpm. PCL molecular weight as well as crystallization and melting behavior did show only insignificant difference when varying the rotation speed. Concerning melt rheological properties, storage modulus GâČ and complex viscosity η* at 0.1 rad/s increased up to a rotation speed of about 75 rpm illustrating improved dispersion. When further increasing the speed GâČ and η* decreased which was attributed to more pronounced nanotube shortening as quantified by TEM measurements. Both effects - improved dispersion and nanotube shortening - are also reflected in the electrical resistivity values of compression molded samples which show a minimum of resistivity at the rotation speed of 75 rpm corresponding to a specific mechanical energy input of 0.47 kWh/kg. © 2013 Elsevier Ltd. All rights reserved
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Thermal conductivity and electrical resistivity of melt-mixed polypropylene composites containing mixtures of carbon-based fillers
Melt-mixed composites based on polypropylene (PP) with various carbon-based fillers were investigated with regard to their thermal conductivity and electrical resistivity. The composites were filled with up to three fillers by selecting combinations of graphite nanoplatelets (GNP), carbon fibers (CF), carbon nanotubes (CNT), carbon black (CB), and graphite (G) at a constant filler content of 7.5 vol%. The thermal conductivity of PP (0.26 W/(m·K)) improved most using graphite nanoplatelets, whereas electrical resistivity was the lowest when using multiwalled CNT. Synergistic effects could be observed for different filler combinations. The PP composite, which contains a mixture of GNP, CNT, and highly structured CB, simultaneously had high thermal conductivity (0.5 W/(m·K)) and the lowest electrical volume resistivity (4 Ohm·cm)
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