Dual Asymmetric Centrifuge Research Articles

Influence of Carbon-Based Fillers on the Electromagnetic Shielding Properties of a Silicone-Potting Compound.

The effect of carbon-based additives on adhesives and potting compounds with regard to electrical conductivity and electromagnetic interference (EMI) shielding properties is of great interest. The increasing power of wireless systems and the ever-higher frequency bands place new demands on shielding technology. This publication gives an overview of the effect of carbon-based fillers on electrical conductivity, electromagnetic shielding properties, and the influence of different fillers and filler amounts on rheological behavior. This work focuses on carbon black (CB), recycled carbon fibers (rCF), carbon nanotubes (CNTs), and complex nanomaterials. Therefore, silicon samples with different fillers and filler amounts were prepared using a dual asymmetric centrifuge and a three-roll mill. It has been found that even with small filler amounts, the electromagnetic shielding properties were drastically raised. The filler content as well as the dispersion technique have a significant influence on most of the fillers. It has also been found that the complex viscosity is strongly influenced by the dispersion technique as well as by the choice and amount of filler. In the experiments carried out, shielding values of over 20 dB were achieved with several fillers, whereby even 43 dB were reached with complex, pre-crosslinked fillers. This signal reduction of up to 99.99% enables almost complete shielding of the related frequency.

Preparation of Cerium-Bismuth Oxide Catalysts for Diesel Soot Oxidation Including Evaluation of an Automated Soot-Catalyst Contact Mode.

Cerium‐bismuth oxides have emerged as promising candidates for Diesel soot oxidation. The catalysts are synthesized via automated co‐precipitation methods. T50 values, where 50 % of soot is oxidized, and the dynamic oxygen storage capacity (OSCdyn) are used to compare the catalytic activity. The activity is measured by thermogravimetric methods. The synthesized catalysts are characterized through powder X‐ray diffraction (PXRD), Raman spectroscopy, and specific surface area (SBET) measurements. This work investigates the influence of the contact mode between soot and catalyst. The literature‐known manual contact modes “loose”, “tight”, and “wet” are compared with our developed automated contact mode, using a dual asymmetric centrifuge. The rotation speed rs and mixing time tM have been varied independently. Both factors influence the T50 value. A continuous transition from loose to tight contact mode with increasing rotation speed rs can be shown. Furthermore, the automated contact mode shows better reproducibility behavior compared to manual contact modes.

Carbon Nanotubes Readily Disperse in Linear Silicones and Improve the Thermal Stability of Dimethylsilicone Elastomers.

Stable silicone fluid-carbon nanotube dispersions were prepared in minutes by simple mixing processes, without the addition of solvents or surfactants and without the chemical modification of the nanotubes. With linear silicones of sufficient viscosity, a dual asymmetric centrifuge (SpeedMixer) was sufficient for dispersion; lower viscosity silicones required a brief ultrasound treatment. Optical microscopy indicates a homogeneous dispersion of multiwalled carbon nanotube (MWCNT) bundles in linear poly(dimethylsiloxane) (PDMS) oils. The facile dispersion of carbon nanotubes in PDMS has been reported in several previous publications and this appears to be general for silicones. MWCNTs also disperse readily, and to a greater extent, as assessed by optical microscopy, in poly(methylphenylsiloxane) and, in particular, poly(diethylsiloxane). Linear PDMS/MWCNT dispersions are stable against agglomeration for months. Platinum-catalyzed hydrosilylation of MWCNT-containing vinyl-/hydride-functionalized PDMS liquids yielded filled elastomers that unexpectedly exhibit significantly increased thermal stability. This enhancement occurs with only fractions of a weight percent of MWCNTs. Thermal gravimetric analysis shows a 54 °C increase in peak weight loss temperature (446-500 °C), an increased decomposition activation energy (158-233 kJ/mol), a second higher temperature decomposition process, and doubled char formation (20-40%) with only 0.5 wt %-added MWCNT. Pyrolysis combustion flow calorimetry confirmed the enhancement in thermal stability. Improvements in electrical conductivity were observed at loadings as low as 0.025 wt %. Spontaneous adsorption of dialkylsiloxane chains to MWCNT surfaces (wetting) and the resulting changes in the composite structure are implicated as the basis for dispersion and thermal behavior changes.

FEEDSTOCK FLOW CHARACTERIZATION AND PROCESSING OF POROUS NITI BY METAL INJECTION MOULDING (MIM)

Porous NiTi alloy with a nearly three-dimensionally interconnected pore structure has been successfully fabricated by a transient liquid phase sintering following the metal injection moulding (MIM) process, using a mixture of Ni and Ti elemental powders. The elemental Ni and Ti powders mixture was mixed with a binder system, comprised mainly polyethylene-glycol (PEG) in an alternative technique using a speed mixer, principally incorporating a dual asymmetric centrifuge (DAC). The powder-binder mixture was then characterized using a capillary rheometer at various temperatures and shear rates. It was found that the feedstock exhibited pseudo-plastic behaviour, which is favourable for the MIM process. A temperature range of 120oC - 130oC was considered as the optimum operating condition for the injection moulding processing. The parts were moulded into cylindrical shapes, leached in warm water (60oC for 10 hours), thermally debound in argon and subsequently sintered in a vacuum furnace at four different temperatures ranging from 950oC to 1250oC. All samples underwent expansion in both diameter and height after water leaching and sintering. The XRD results showed that increasing the sintering temperature resulted in a major fraction of the B2 NiTi phase due to phase homogenization and subsequently decreased the amount of secondary phases such as NiTi2 and Ni3Ti. Besides that, the formation of the transient liquid phase during sintering enabled major fraction of pores to be developed with porosity and average size of 39 - 45% and 100 - 120 μm, respectively. The porous parts produced have a great potential to be used as an implant in biomedical applications.

Design of multifunctional adhesives by the use of carbon nanoparticles

We investigated the influence of carbon materials on the properties of adhesives. With the aim of the development of conductive and mechanically improved adhesives, different types of multi-walled carbon nanotubes (MWNT), single-layer graphene, graphene nanoplatelets, graphite, and carbon black were dispersed into an epoxy adhesive. For inserting particles within the viscous matrices and to obtain homogenous and stable dispersions, two different methods namely a three-roll mill and a dual asymmetric centrifuge have been compared The results demonstrate that filling epoxies with carbon nanoparticles improves the conductivity differently. Measuring the electrical resistivity of MWNT-filled composites resulted in an electrical percolation starting underneath 0.1 wt.%. Further, with increasing the MWNT content, conductivity sharply increases. In contrast to the MWNT composites, other carbon nanoparticles require a higher filling content to reach similar values. With a filling ratio up to 17.0 wt.% for ACS graphene nanoplatelets 2–10 nm and the filling of 3.0 wt.% with MWNTs from Nanocyl, the lowest volume resistivities have been reached.

Effect of processing technique on the transport and mechanical properties of graphite nanoplatelet/rubbery epoxy composites for thermal interface applications

Graphite nanoplatelet (GNP)/rubbery epoxy composites were fabricated by mechanical mixer (MM) and dual asymmetric centrifuge speed mixer (SM). The properties of the GNP/rubbery epoxy were compared with GNP/glassy epoxy composites. The thermal conductivity of GNP/rubbery epoxy composite (25 wt.% GNP, particle size 15 μm) reached 2.35 W m −1 K −1 compared to 0.1795 W m −1 K −1 for rubbery epoxy. Compared with GNP/rubbery epoxy composite, at 20 wt.%, GNP/glassy epoxy composite has a slightly lower thermal conductivity but an electrical conductivity that is 3 orders of magnitude higher. The viscosity of rubbery epoxy is 4 times lower than that of glassy epoxy and thus allows higher loading. The thermal and electrical conductivities of composites produced by MM are slightly higher than those produced by SM due to greater shearing of GNPs in MM, which results in better dispersed GNPs. Compression and hardness testing showed that GNPs increase the compressive strength of rubbery epoxy ∼2 times without significantly affecting the compressive strain and hardness. The GNP/glassy epoxy composites are 40 times stiffer than the GNP/rubbery epoxy composites. GNP/rubbery epoxy composites with their high thermal conductivity, low electrical conductivity, low viscosity before curing and high conformability are promising thermal interface materials.

Characterisation of graphite nanoplatelets and the physical properties of graphite nanoplatelet/silicone composites for thermal interface applications

Thermally conducting and highly compliant composites were developed by dispersing graphite nanoplatelets (GNPs) into a silicone matrix by mechanical mixing. X-ray diffraction (XRD) indicates that the average thickness of the GNPs decreased from 60 to 35 nm during mechanical mixing. XRD-texture analysis demonstrated that GNP/silicone composites at 8 wt.% GNPs have a higher degree of basal plane alignment than at 20 wt.%. Differential scanning calorimetry showed that GNPs raised the curing temperature of silicone with no significant effect on the glass transition temperature. The thermal conductivity of the 20 wt.% composites reached 1.909 W/m.K, an 11-fold increase over silicone suggesting an improved dispersion compared to similar composites prepared by dual asymmetric centrifuge mixing. The percolation threshold for electrical conductivity of the composites was at ∼15 wt.%. The compressive modulus of the composite increased to twice that of silicone at 20 wt.%. The corresponding strength decreased by a factor of two compared to silicone and this can be attributed to the weak bonding at the GNP-silicone interface. Overall, these GNP/silicone composites, with a high thermal conductivity, low electrical conductivity and compliant nature are promising materials for use as thermal pads for thick gap filling thermal interface applications.

Structure & strength of silica-PDMS nanocomposites

Silica nanoparticles having specific surface area ( SSA) 50–300 m 2 g −1 were admixed into vinyl-terminated dimethylsiloxy monomer with a dual asymmetric centrifuge (planetary mixer) and cured to form PDMS-based nanocomposites containing up to 12 vol% SiO 2. Thin sections of cured nanocomposites obtained by a cryostate-microtome were analyzed by TEM while small and ultra small angle X-ray scattering (U/SAXS) was used to determine nanocomposite structure: filler primary particle, aggregate (chemically or sinter-bonded particles) and agglomerate (physically-bonded particles) size as a function of mixing duration and filler concentration. More aggregated silicas with higher SSA exhibited denser crosslinking than less aggregated ones regardless of crosslinker content as determined by swelling nanocomposites in toluene at equal filler content. The nanocomposite strength was determined by tensile tests (Young's modulus and elongation at break). Consistent with “bound rubber” theory, the Young's modulus of the nanocomposites increased non-linearly with increasing filler volume fraction.

Vesicular phospholipid gel-based depot formulations for pharmaceutical proteins: Development and in vitro evaluation

The object of this study was to evaluate the potential of vesicular phospholipid gels (VPGs) as an alternative delivery system for therapeutic proteins. Therefore, the model protein erythropoietin (EPO) was incorporated into various VPG formulations by dual asymmetric centrifugation. In order to characterize and to quantify the incorporated protein, different extraction protocols were investigated. Among several detergents and organic solvents an extraction method applying chloroform was found most suitable. SDS-PAGE analysis of EPO extracted from VPGs revealed excellent protein stability which was maintained in the VPGs' matrix after an incorporation process with dual asymmetric centrifuge. Irrespective of the investigated formulation all VPGs delivered the protein over prolonged periods of time at close to linear kinetics without any burst effect. Both the lipid content and the lipid charge had a strong influence on the release behavior. For instance formulations based on 300 mg/g lipid delivered 83% EPO after 280 h while gels based on 550 mg/g lipid liberated 64% within 400 h. From the parallelism of release and erosion kinetics found for all formulations it was concluded that erosion rather than diffusion is the dominant release controlling mechanism for these macromolecule-loaded VPGs. For the first time the present study presents VPGs as promising alternative depot systems for protein drugs.

Dual asymmetric centrifugation (DAC)—A new technique for liposome preparation

This is the first report on the use of a “dual asymmetric centrifuge (DAC)” for preparing liposomes. DAC differs from conventional centrifugation by an additional rotation of the sample around its own vertical axis: While the conventional centrifugation constantly pushes the sample material outwards, this additional rotation constantly forces the sample material towards the center of the centrifuge. This unique combination of two contra rotating movements results in shear forces and thus, in efficient homogenization. We demonstrated that it is possible to prepare liposomes by DAC, by homogenizing a rather concentrated blend of hydrogenated phosphatidylcholine and cholesterol (55:45 mol%) and 0.9% NaCl-solution, which results in a viscous vesicular phospholipid gel (VPG). The resulting VPG can subsequently be diluted to a conventional liposome dispersion. Since DAC is intended to make sterile preparations of liposomes, or to entrap toxic/radioactive compounds, the process was performed within a sealed vial. It could be shown that the DAC speed, the lipid concentration, the homogenization time and the addition of a mixing aid (glass beads) are all critical for the size of the liposomes. Optimized conditions resulted in liposomes of 60 ± 5 nm and a trapping efficacy of 56 ± 3.3% for the model compound calcein.

Effect of dispersion method on tribological properties of carbon nanotube reinforced epoxy resin composites

Epoxy/CNT nanocomposites were synthesized in various ways to examine the effects of dispersion methods on its tribological properties. The carbon nanotubes (CNTs) were pre-treated in three different ways: no pre-treatment, activation in HNO 3, and activation in HNO 3 plus application of a coupling agent. The dispersion and mixing methods used were: dual asymmetric centrifuge, sonication, hand mixing. The CNTs were mixed either into the hardener or the resin. The curing behaviour was studied via DSC, and the thermomecanical properties were determined using a DMA. The tribological properties were investigated in a ball-on-prism test rig under unidirectional continuous sliding against austenitic stainless steel. For the untreated CNTs, it seems that the wear resistance improves with increasing effort put into dispersion: sonication has a positive effect, sonication plus dual asymmetric centrifuge proves even better. A pre-treatment with HNO 3 or a silane-coupling agent can improve the wear resistance of the composite. However, the pre-treated CNTs should be dispersed without the use of ultrasound, which seems to damage the pre-treated CNTs. The results showed that the wear resistance in general increases with improved dispersion and integrity of the CNTs.

Wear behaviour of carbon nanotube reinforced epoxy resin composites

This paper deals with the effect of a multi-walled carbon nanotube (MWCNT) reinforcement on the wear behaviour of Epoxy (EP) composites. Firstly, various dispersion methods were compared. Secondly, the optimum CNT amount was evaluated. In a third step, dry lubricants were added to the optimised EP/CNT composite. Finally, the influence of the steel counterpart (martensitic bearing steel 100Cr6 and austenitic stainless steel X5CrNi18-10) was studied. The preparation method was found to have a decisive effect on the wear behaviour of the composite. A pre-treatment of the CNTs in concentrated nitric acid proved beneficial. Even more important was the mixing method. A dual asymmetric centrifuge delivered so good wear results that the pre-treatment could be skipped. The optimum CNT content was at about 1 wt%, regardless of the preparation method. The lowest wear rates were found after addition of 10 wt% graphite. MoS2 proved to be less effective and Polytetrafluorethylene (PTFE) even increased the wear. The wear rates against the unalloyed martensitic steel were far higher than against austenitic stainless steel.