Experimental and numerical investigation of fractal-tree-like heat exchanger manufactured by 3D printing

© 2018 Elsevier Ltd The manufacturing difficulties of complex fractal-tree-like heat exchangers have limited their industrial applications, although many evidences have shown that they have significant advantages in heat transfer. Nevertheless, the emerging 3D printing technology has brought great opportunity for the development of complex structured device. In the present study, three-dimensional (3D) fractal-tree-like heat exchangers were designed and manufactured using 3D printing technology. Their performance was evaluated from both thermal and hydrodynamic perspectives, the flow characteristics were investigated in detail. The results show that a fractal-tree-like heat exchanger can improve hydrodynamic performance, reduce pressure drops and has great heat transfer ability. In general, the fractal-tree-like heat exchanger has a comprehensive advantage over the traditional spiral-tube exchangers as it has a higher value of coefficient of performance (COP). Furthermore, the 3D printing provides a visual, efficient, and precise approach in the present research

Self-Assembled Monolayer Growth of Phospholipids on Hydrophobic Surface toward Mimetic Biomembranes: Scanning Probe Microscopy Study

Atomic force microscopy (AFM) and lateral force microscopy (LFM) were used simultaneously to analyze a model membrane bilayer structure consisting of a phospholipid outer monolayer deposited onto organosilane-derivatized mica surfaces, which were constructed by using painting and self-assembly methods. The phospholipid used as outer monolayer was dimyristoylphosphatidylcholine (DMPC). The hydrocarbon-covered substrate that formed the inner half bilayer was composed of a self-assembly monolayer (SAM) of octadecyltrichloroorganosilane (OTS) on mica. SAMs of DMPC were formed by exposing hydrophobic mica to a solution of DMPC in decane/isobutanol and subsequently immersing into pure water. AFM images of samples immersed in solution for varying exposure times showed that before forming a complete monolayer the molecules aggregated into dense islands (2.2−2.6 nm high) on the surface. The islands had a compact and rounded morphology. LFM, coupled with topographic data obtained with the atomic force mode, had made possible the distinction between DMPC and OTS. The rate constant of DMPC growth was calculated. This is the first systematic study of the SAM formation of DMPC by AFM and LFM imaging. It reveals more direct information about the film morphology than previous studies with conventional surface analytical techniques such as infrared spectroscopy, X-ray, or fluorescence microscopy

Semiconductor Nanoparticles on Solid Substrates: Film Structure, Intermolecular Interactions, and Polyelectrolyte Effects

Citrate-stabilized CdSe or CdSe/CdS core−shell nanoparticles (NPs) were adsorbed on the standard silicon wafers bearing either a short-chain covalently bound adsorption promoter (3-aminopropyl)triethoxylsilane (APTES) or macromolecular adsorption promoterspolyethylenimine (PEI) or poly(diallydimethylammonium) chloride (PDDA). The aim of this study is 2-fold:  (1) to compare different methods of NP processing into thin films and (2) to elucidate the effect of the long-chain dynamics on the NP film structure. Systematic atomic force microscopy study of the films revealed that both types of NPs produced densely packed films on PDDA, while rarified films with significant clustering formed on PEI and APTES. The difference in NP layer morphologies was rationalized on the basis of intermolecular NP−polyelectrolyte interactions. Importantly, we observed that the adsorption layer of the weak polyelectrolyte PEI could alter its chain distribution by partial wrapping around the NPs, while no disturbance in APTES and PDDA monolayer by NP was observed. This was attributed to more labile binding of PEI to the solid substrate than for other adsorption promoters

Twisted Metal−Amino Acid Nanobelts: Chirality Transcription from Molecules to Frameworks

We have studied the self-assembly process of a typical biocoordination polymer, Ag(I)/cysteine (Cys), with different chiralities of the amino acid. Self-assembly of Ag(I)/l-Cys leads to production of pure right-handed helical nanobelts, whereas Ag(I)/d-Cys gives rise to the “mirror image”, i.e., pure left-handed helical nanobelts. As a comparison, racemic Ag(I)/dl-Cys forms a totally different product, two-dimensional achiral nanosheets. Density functional theory simulation revealed that the molecular chirality of Cys is originally programmed in the specific lattice twisting, which further determines the chirality and dimensionality of the assembly products. This understanding will shed light on comprehending chirality transcription in metal−organic frameworks as well as designing chirality-regulated nanosuperstructures

Perspective of Chiral Colloidal Semiconductor Nanocrystals: Opportunity and Challenge

Chiral colloidal semiconductor nanocrystals (NCs) are an emerging type of chiral materials. These chiral NCs exhibit unique quantum confinement-determined optical activity and have aroused much interest in the multidisciplinary fields of chemistry, physics and biology. Herein, the state-of-the-art progresses of their rational synthesis, fundamental understanding and potential application are summarized. In addition, a personal view about the future development of chiral semiconductor NCs is offered

Modeling Fischer–Tropsch to Olefins in Pilot Slurry Process with a Method of Multiscale Bubbles Hybrid Injection

A pilot Fischer–Tropsch to olefin (FTO) slurry bubbling process CFD model was established based on the Eulerian multifluid model and species transport equation. The gas holdup agreed with the pilot design, and the simulated reaction products met the laboratory experimental data. Further, a two-scale bubbles model was built with the single-size (500 μm) microbubble phase by considering the self-pressurizing effect and the multisize (millimeter-scale) large-bubble phase with a population balance model (PBM) to intensify the gas–liquid–solid FTO reaction. Simulation results revealed the two-scale bubbles intake ratios to greatly impact the uniformity of catalyst mixing, mass transfer rate, and FTO reaction. A ratio of 50% microbubble (MB) intake was identified as the optimal value for the pilot FTO reaction to simultaneously provide good mixing with the lower mean solid fraction standard deviation, better mass transfer rate (twice more than without microbubbles), and intensified reaction process (40% CO conversion rate, equivalent to 10% higher than without microbubbles)

Hot Zone Empowered Highly Sensitive Surface-Enhanced Raman Scattering Analysis Using Au@HOF Nanoparticles

Metal–organic frameworks (MOFs)-based core–shell hybrid substrates have drastically progressed in developing surface-enhanced Raman spectroscopy (SERS) detection. However, these substrates still face reliability issues, owing to the random adsorption of targets onto the active metal center of the outer MOFs structure. Herein, a strategy for analyzing SERS called the “hot zone” empowered analysis has been presented. This strategy involves the preparation of core–shell structures by using hydrogen-bonded organic frameworks (HOFs). By coating the designed HOF structure onto a Au nanoparticle, the probe molecules were concentrated and preferentially accumulated on the hot zone of the plasmonic Au core instead of competitively adsorbing on the metal-free HOF shell. Experimental and theoretical investigations revealed that the excellent SERS performance of the as-prepared Au@HOF is attributed to the effective delivery of the analyte to the hot zone and its subsequent enhancement. Moreover, when the proposed Au@HOF film was fabricated as an SERS chip, it demonstrated reliability in detecting pesticide residues in a real system. This study may stimulate further research on the functionalization of HOFs for broader applications. Notably, the developed “hot zone” empowered strategy provides a scientific framework for advancing the development of SERS sensing

Resonance Tunneling Diode Structures on CdTe Nanowires Made by Conductive AFM

Variation of band gap across the nanowire length would be an exceptionally attractive property for the fabrication of on-nanowire devices such as resonance tunneling diodes (RTD). Band gap variation can be achieved by selective thinning of semiconductor wires by scanning probe lithography (SPL) technique. The external bias applied to a conductive AFM tip during scanning of CdTe nanowires was chosen so as to exceed the threshold of electric field-assisted evaporation of CdTe, estimated to be 5.5 V. Relatively high external voltages of 10−11 V cause fast and complete disintegration of a nanowire portion under the tip. In this way the nanowire can be cut to a desired length. Selection of a voltage between 5.5 and 10 V allows one to control the speed of CdTe evaporation. Thus, one can modulate the thickness of the semiconductor with angstrom scale precision along the nanowire length. Smaller diameter of the nanowire results in increase of quantum confinement in selected areas. The double barrier quantum well valence band profile necessary for the manufacturing of RTD Esaki diodes was demonstrated

Facile Synthesis of Surfactant-Free Au Cluster/Graphene Hybrids for High-Performance Oxygen Reduction Reaction

Non-Pt noble metal clusters like Au clusters are believed to be promising high performance catalysts for the oxygen reduction reaction (ORR) at the cathode of fuel cells, but they still suffer big problems during the catalysis reactions, such as a large amount of the capping agents being on the surface and easy occurrence of dissolution and aggregation. To overcome these obstacles, here, we present a novel and general strategy to grow ultrafine Au clusters and other metal (Pt, Pd) clusters on the reduced graphene oxide (rGO) sheets without any additional protecting molecule or reductant. Compared with the currently generally adopted nanocatalysts, including commercial Pt/C, rGO sheets, Au nanoparticle/rGO hybrids, and thiol-capped Au clusters of the same sizes, the as-synthesized Au cluster/rGO hybrids display an impressive eletrocatalytic performance toward ORR, for instance, high onset potential, superior methanol tolerance, and excellent stability

Simple Preparation Strategy and One-Dimensional Energy Transfer in CdTe Nanoparticle Chains

One-dimensional aggregates of CdTe nanoparticles were prepared by an exceptionally simple method of self-assembly initiated by partial removal of the stabilizing shell. The driving force of the particle self-organization is likely to be the dipole−dipole attraction between the nanoparticle cores. The obtained nanoparticle chains can be easily transferred on any substrate. Steady-state and time-resolved luminescent spectroscopes revealed strong Forster resonance energy transfer (FRET) between the particles, which led to the migration of excitation along the chain similarly to the waveguiding of light observed for chains of metal nanoparticles. The efficiency of FRET quenching in this one-dimensional system is comparable to that in three-dimensional packed nanoparticle solids despite a substantially smaller number of adjacent particles. The strong coupling of the donor and acceptor excited states is likely due to short inter-nanoparticle distances and partial ground-state dipole alignment taking place during the chain formation