Mechanism of heat transport in nanofluids

We have calculated thermal conductivity of alumina nanofluids (with water and ethylene glycol as base fluids) using temperature as well as concentration-dependent viscosity, η. The temperature profile of η is obtained using Gaussian fit to the available experimental data. In the model, the interfacial resistance effects are incorporated through a phenomenological parameter α. The micro-convection of the alumina nanoparticle (diameter less than 100 nm) is included through Reynolds and Prandtl numbers. The model is further improved by explicitly incorporating the thermal conductivity of the nanolayer surrounding the nanoparticles. Using this improved model, thermal conductivity of copper nanofluid is calculated. These calculations capture the particle concentration-dependent thermal conductivity and predict the dependence of the thermal conductivity on the size of the nanoparticle. These studies are significant to understand the underlying processes of heat transport in nanofluids and are crucial to design superior coolants of next generation.     

Nanocoating individual cohesive boron nitride particles in a fluidized bed by ALD

Experiments are conducted with alumina (Al₂O₃) deposition on a wide size range of hexagonal boron nitride (BN) platelet-like particles. Successful deposition of alumina films on these particles, with film thickness controllable at the Angstrom level, is observed based upon TEM imaging, ICP-AES, particle size distributions, and surface area analysis. While fluidizing, fine BN particles aggregate in the bed. The aggregates are the entities fluidizing, not the primary particles. However, individual particles are coated using Atomic Layer Deposition (ALD), not aggregates. Since ALD is a surface chemistry phenomenon, the films grow uniformly on every primary particle. BN particles are small platelets with different functional groups on the basal planes and edge planes. A small exposure to reagents [2.5×10⁶ Langmuir (L) per reagent per cycle], will only coat the edge planes of uncoated BN particles. A larger dose of 1×10⁸ L will coat the entire uncoated BN particle (edge and basal planes). After 10 ALD cycles of the 1×10⁸ L dose, the exposures can be reduced to 1×10⁶ L as the film is then growing on alumina and not BN. Peel strength data indicate that adhesion between the coated particles and a cured epoxy in a filled composite is ∼25% stronger than that of uncoated particles and the epoxy. The overall thermal conductivity drops ∼17% for an identical filler loading as expected due to the additional thermal resistance added by the film. However, the viscosity of an epoxy resin loaded with coated BN is as much as five times lower than that of the resin loaded with the same amount of uncoated BN. These results indicate that the loading of Al₂O₃ nanocoated BN particles in an epoxy matrix can be substantially increased relative to that of uncoated particles. The thermal conductivity of the more highly filled composite will be increased without adversely impacting filled resin viscosity or the peel strength of the cured material. This is the first reported study indicating that cohesive primary particles that fluidize as aggregates in a fluidized bed can be individually coated with a nano-thick ceramic film using ALD.     

(Cr1‐x,Alx)N ein Review über ein vielseitig einsetzbares Schichtsystem

(Cr_1‐x,Al_x)N a review about a multi‐purpose coating system Nitride based coatings claimed a big market share for PVD‐coatings. Especially in the field for high temperature die casting and cutting operations chromium based coatings are well used. These coatings are also used in low temperature processes for the coating of machine parts. In the beginning of the nineties first examinations are done on the ternary system Chromium‐Aluminium‐Nitride. This system shows an excellent corrosion behaviour against many different liquids, but gains also a high hardness for a good wear behaviour. By changing the AlN to CrN content and the coating design CrAlN opens up a wide range for different coating applications. A major step for machine parts was the reducing of coating process temperature beneath 200 °C. This was only possible by using pulsed power supplies. CrAlN shows a very good performance on the fast growing market of coated machine parts e.g. on spindle bearings.     

Nanolayer treatment to realize suitable configuration for electrochemical allopurinol sensor based on molecular imprinting recognition sites on multiwall carbon nanotube surface

In this work, nanolayer MIP (molecularly imprinted polymer) is used to achieve suitable conformation for catalyzing AP (allopurinol) redox reaction. Also, a sensitive electrochemical sensor was fabricated for AP based on MIP immobilized on multiwall carbon nanotube (MWCNT) surface. Thin film of MIP immobilized on MWCNT surface (MIPCNT) with specific recognition sites for AP was cast on glassy carbon electrode (GCE). The morphology and features of the film were characterized by field emission scanning electron microscopy (FE-SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometric measurements (I–t) in detail. The near equilibrium time to adsorb AP on the surface of electrode is about 9 min. The modified electrode was used to detect the concentration of AP with a linear range and detection limit (S/N = 3) of 0.01–1.0 μM and 6.88 nM, respectively. The MIPCNT film displayed an excellent selectivity toward AP. Finally, the modified electrode was successfully applied to determine AP in the human serum sample and two brand tablets.     

LaF3 nanolayer surface modified spinel LiNi0.5Mn1.5O4 cathode material for advanced lithium-ion batteries

As an attractive high power-density cathode material for lithium-ion batteries, spinel-structured LiNi_0.5Mn_1.5O₄ (LNMO) so far still suffers fast capacity decay during repeated cycling due to transition metal (TM) dissolution and structure degradation. In this work, a thin nanolayer of LaF₃ is applied to modify the surface of LNMO. Electrochemical and thermal tests indicate that 4 wt% LaF₃ surface modification could greatly improve the electrode performances in terms of cycling stability and rate capability as well as thermal stability of LNMO compared with the pristine electrode, without influencing the crystallographic structure of bulk material. Further analysis for understanding the intrinsic mechanism reveals that the growth of solid electrolyte interface (SEI) film could be effectively suppressed by the surface LaF₃ nanolayer, which meanwhile stabilizes the bulk structure through retarding continuous TM dissolution from intensive chemical aging measurements at elevated temperature. This work, theoretically and technically, provides a promising alternative approach for enhancing electrochemical performances of high voltage LNMO cathode material.     

Tribological behavior of CrN/WN multilayer coatings grown by ion-beam assisted deposition

CrN monolayer coating and CrN/WN multilayer coatings were deposited on the silicon (100) substrate by ion-beam assisted deposition process. The bilayer period of these coatings was controlled at 8 nm and 30 nm. The cross-sectional morphology of nanoscaled multilayer coatings was characterized by scanning electron microscopy and transmission electron microscopy. The wear resistance of CrN/WN multilayer coatings and CrN monolayer coating was investigated using a pin-on-disc tribometer. The surface roughness (Ra) of the coatings was evaluated by atomic force microscopy, and that of CrN and WN monolayer coating was 6.7 and 5.9 nm, respectively. The employment of multilayer configuration in CrN/WN coating with bilayer period of 8 nm and 30 nm effectively reduced the surface roughness down to 1.9 and 2.2 nm, respectively. The friction coefficient of CrN monolayer coating and CrN/WN multilayer film with a bilayer period of 30 nm was 0.63 and 0.31, respectively. Owing to the high hardness/elastic modulus ratio, as well as the dense structure and the smooth surface roughness, the CrN/WN multilayer coatings exhibited better wear resistance in the consideration of friction coefficient and the worn surface morphology.     

The effect of the carbon nanotubes surface oxidation on the morphology and properties of poly(N-vinylcarbazole) coated multi-walled carbon nanotube nanocables

This article describes the effect of carbon nanotubes (CNTs) outer surface oxidation on the morphology and properties of poly(N-vinylcarbazole) (PNVC)-coated individual multi-walled CNT (MWCNT) nanocables. Surface oxidation of MWCNTs has been carried out by refluxing MWCNTs with 5 M nitric acid (HNO₃) at 80 °C for 1 h. The PNVC-coated MWCNT nanocables are synthesized by in situ solid-state polymerization of N-vinylcarbazole monomers in the presence of oxidised MWCNTs (o-MWCNTs) at an elevated temperature. The PNVC-coated MWCNT nanocables are characterized by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, field-emission scanning and transmission electron microscopes, photo-luminescence spectroscopy, and direct-current conductivity measurements. Results show that the uniform nanolayer coating of PNVC decreases the inherent bulk conductivity of MWCNTs, but significantly increases the optical properties of MWCNTs.     

Characterizations of CrN/a-CNx nanolayered coatings deposited by DC reactive magnetron sputtering of Cr and graphite targets

CrN/a-CN_x nanolayered coatings have been deposited by DC reactive magnetron sputtering of pure Cr and graphite targets. The total thickness is 1 μm and that of a-CN_x layers is kept constant at 3.5 nm. The period (bilayer thickness) is in the range 8-16 nm. CrN and a-CN_x layers are crystalline and amorphous respectively. The decrease of CrN layers’ thickness (decrease of period) in the stack leads to refinement of CrN microstructure associated with (200) preferred orientation. The hardness of nanolayered films is independent of the period’s thickness, while internal compressive stress, which remains between that of each elementary layer, follows an evolution close to that of the law of mixtures. The best tribological behaviours are reached for a periods’ thickness of 8 nm.     

Coating different thickness nickel-boron nanolayers onto boron carbide particles

This work is focused on electroless coating of nickel-boron (Ni-B) onto boron carbide (B₄C) particles. Using NiSO₄ as Ni²⁺ source, SnCl₂ as sensitizing agent, PdCl₂ as activation agent, and NaBH₄ as reducing agent, Ni-B nanolayers of different thicknesses have been successfully coated onto the B₄C particles. The B₄C particles are around 2 μm in size and the Ni-B coating thickness can be adjusted by changing the Ni²⁺:B₄C ratio. For the targeted 1 nm Ni-B thickness, the layer is discontinuous. When the targeted Ni-B layer increases to 5 nm, the coating layer becomes continuous and completely covers the B₄C particle surfaces. When the targeted Ni-B coating thickness increases to 10 nm or higher, Ni-B nodules start to form with the mesh-like structures between the Ni-B nodules. The Ni-B nodule size increases with the Ni-B layer thickness. EDS results show the presence of oxygen in the Ni-B coating; oxygen content decreases as the Ni-B coating thickness increases. XPS confirms that B₂O₃ forms in the coating and the Ni:B ratio decreases with the Ni-B layer thickness. Ni-B electroless coating processes and morphological changes on the B₄C particle surfaces with different Ni-B coating thickness are analyzed.     

Determination of nanolayer thickness for a nanofluid

Recent core–shell–medium models as a modification of the traditional effective medium theories for explaining the enhanced thermal conductivity of nanofluids are semi-empirical. Generally, the resulting thickness and conductivity of the nanolayer both have to be chosen to match the measured thermal conductivity of the nanofluid. Here, we attempt to find a more systematic procedure to determine the nanolayer thickness and the thermal conductivity profile within the nanolayer. An expression for the nanolayer thickness is derived by manipulation of the three basic heat conduction regions. Comparison of the estimated thermal conductance with known experimental data and thermal diffusion length are made.