Ice crystal formation in the carbon nanotube suspension: A modelling approach

Freeze-dried porous solid foams were prepared from carbon nanotube (CNT) suspensions via either contact freezing with a heat exchanger or immersion freezing into a cryo-bath. Microstructures in the freeze-dried foam cast by ice crystals formed during the freezing step. It was found that domains of the carbon nanotubes in the freeze-dried solid foams were fairly well interconnected, and the microstructures were largely influenced by the freezing condition and freezing method. A mathematical model was proposed to simulate the observed thermal history during the freezing step; then theoretical predictions of the microstructural features were attempted. The simulated thermal history was in good agreement with experimental result. The resulting mean ice crystal sizes L* could be estimated from the calculated freezing front velocity (R) and the temperature gradient in the frozen zone (G). Interestingly, it was found that a correlation based on the power law (L*∝R^?0.2G^?0.2) was applicable to the present system for both the contact and immersion freezing methods. Though the fundamental ice crystallization phenomena were essentially the same for both freezing methods, actual temperature distribution and movement of the freezing front through the whole freezing bulk could control the morphology of ice crystal in the frozen matrix. In other words, good control of thermal flow in the freezing system would greatly contribute to rational design of microstructures of the CNT foam.     

An empirical approach in predicting biodiesel viscosity at various temperatures

A thermodynamic model is proposed for the determination of kinematic viscosities of saturated fatty acid methyl esters (FAMEs) of various chain lengths at different temperatures. The linearity of the natural logarithm of viscosity-carbon number, plot is limited to a narrow carbon number range. The predicted viscosities of FAMEs of C_12:0-C_18:0, which are commonly found in vegetable oils and used as biodiesels, agree well with the experimental values. The highest difference is 0.354 cSt (5.60%), for methyl stearate at 40 °C. When the proposed method for viscosity calculation of saturated FAMEs are used in combination with the methods for viscosities of biodiesel the mixtures, the predicted viscosities agree well with the values reported in the literatures and the measured values. The differences between the predicted viscosities and those reported in the literatures (at 40 °C) are 1.08 to 8.56% (for eight different vegetable oil methyl esters). The differences between the predicted viscosities and the measured values for coconut methyl esters, at 25, 40 and 50 °C are 9.20, 5.53 and 5.57%, respectively. The differences are slightly higher than those of palm oil methyl esters (4.48, 2.06 and 2.48%, respectively).The proposed method can also be applied to predict the viscosities of free fatty acids and it is speculated it may be applied to other homologous series as well.     

Single-step synthesis and characterization of single-walled carbon nanohorns hybridized with Pd nanoparticles using N2 gas-injected arc-in-water method

Single-walled carbon nanohorns (SWCNHs) hybridized with palladium (Pd) nanoparticles were synthesized by a single-step gas-injected arc-in-water method (GI-AIW) with a Pd wire inserted inside the anode hole. In the arc zone, carbon and Pd were vaporized simultaneously, leading to the formation of hybrid material of SWCNHs and Pd nanoparticles due to effective quenching. Based on TEM and CO chemisorption analyses, Pd nanoparticles were found to be embedded inside SWCNH aggregates. The size of Pd nanoparticles, determined by X-ray diffraction, was in the range of 3–6 nm when Pd wires with diameters of 0.1 and 0.3 mm were used. Using a Pd wire with a diameter larger than 0.5 mm results in larger Pd nanoparticles which tend to be exposed to the outer surface of the hybrid material. According to thermogravimetric analyses, the weight fraction of Pd nanoparticles is increased by increasing the Pd wire diameter although the yield of Pd nanoparticles decreased. SWCNHs hybridized with dispersed Pd nanoparticles, synthesized with 0.1 mm Pd wire, exhibited strong anti-oxidation resistance with a highly graphitic structure.     

Enhancing effect of monoolein surfactant on carbon nanoparticle synthesis by arc discharge in liquid

Effect of monoolein surfactant which promotes n-hexane to dissolve in water was investigated to enhance the carbon nanoparticle (CNP) synthesis by arc discharge in liquid. The production rate of CNPs consisting of multi-shelled fullerenes and multi-walled carbon nanotubes was increased by the addition of the n-hexane to water with support of monoolein surfactant. An optimal amount of monoolein led to the maximal production rate of CNPs which was ca. two-fold higher than that obtained from a pure water system. This effect is ascribed to the role of n-hexane and monoolein to provide carbon clusters to the arc reactive zone. The hydrodynamic particle sizes of CNPs and their crystallinity were also enhanced by the addition of these organic compounds. As the vapor pressure of the organic component is significantly suppressed, the proposed method is much safer than that using pure organic liquid.     

Selective synthesis of carbon nanotubes and nanocapsules using naphthalene pyrolysis assisted with ferrocene

Selective synthesis of well aligned multi-walled carbon nanotubes (MW-CNT) and multi-shelled carbon nanocapsule (MS-CNC) using pyrolysis of naphthalene with the presence of ferrocene was experimentally examined. With higher mole fraction of naphthalene to ferrocene, more MW-CNTs could be synthesized due to higher concentration of carbonaceous precursors emerging from the decomposed naphthalene. Based on kinetic analysis, at lower temperature, MW-CNTs could preferably be synthesized due to the controlled supply of carbonaceous clusters to get onto the surface of Fe clusters. On the other hand, at temperature higher than 900 °C the Fe clusters become more active to catalyze carbonaceous precursors to undergo self assembling process of MS-CNCs. With cheaper cost of naphthalene compared with other high-value hydrocarbons, usage of naphthalene would provide an advantage of reasonably economical synthesis of MW-CNT or MS-CNC.     

Influence of Temperature on SO2 Removal Enhanced by Water Vapor Using a Corona‐Discharge Reactor

A reactor using d.c. corona discharge of negative polarity was applied to remove sulfur dioxide from an oxygen‐nitrogen mixture in the presence or absence of water vapor for temperatures ranging from room temperature to 350 °C. It was observed that increasing the reactor temperature caused a decrease in the removal efficiency. Mixing water vapor with the process gas resulted in an increase of the removal efficiency. The effect of the presence of water vapor on improving the removal efficiency was significant under low temperature conditions, while it was relatively moderate under high temperature conditions. In addition, the solid deposit formed inside the reactor at two temperatures, room temperature and 200 °C, was analyzed with both a differential scattering calorimeter and an X ray diffractometer. The analysis indicated that SO₂ was ultimately converted to solid sulfur in both the presence and absence of water vapor in the gas flow.     

Formation of deagglomerated PLGA particles and PLGA-coated ultra fine powders by rapid expansion of supercritical solution with ethanol cosolvent

Rapid expansion of supercritical solution (RESS) was used for preparing polymer particles and polymer coating of ultra fine powders. The polymer of pharmaceutical interest was Poly(lactic-co-glycolic acid) (PLGA with PLA: PGA ratio of 85: 15 and MW of 50,000–75,000) and the simulated core particles were 1.4-μm SiO₂ and 70-nm TiO₂ particles. The supercritical solution was prepared by dissolving PLGA in supercritical carbon dioxide with ethanol as a cosolvent. Supercritical solution of CO₂-PLGA was sprayed through capillary nozzles to ambient conditions, resulting in formation of submicron PLGA particles. Similarly, rapid expansion of supercritical solution of CO₂-PLGA suspended with the core particles could provide solvent evaporation and deposition of submicron PLGA particles on the surface of the core particles, resulting in the formation of coating films on dispersed particles of SiO₂ and agglomerates of TiO₂. The influences of the core particle size, spray nozzle diameter as well as powder-to-polymer weight ratio were also investigated and discussed with respect to the coating performance.     

Improved hydrophilicity of zinc oxide-incorporated layer-by-layer polyelectrolyte film fabricated by dip coating method

ZnO nanoparticles suspended in poly(acrylic acid) (PAA) were deposited onto layer-by-layer (LBL) polyelectrolyte (PET) films fabricated from poly(allylamine hydrochloride) (PAH) and PAA by dip coating method. Effect of etching time and concentration of ZnO suspension on hydrophilicity of the LBL-PET films before and after UV irradiation was examined using water contact angle measurement. 2.0 M PAH/PAA solutions with a dipping speed of 3.0 cm/min provided stable LBL-PET films with thickness sufficient for HCl etching. Glass substrates with the etched LBL-PET film dipped into 0.2 wt.% ZnO suspension exhibited the contact angle of 10° after irradiated by UV for 60 min.     

Facile synthesis of tetrapodal ZnO nanoparticles by modified French process and its photoluminescence

Facile synthesis of tetrapodal ZnO nanoparticles was conducted using a modified French process in which oxygen and nitrogen flow rates were controlled. The synthesized ZnO nanoparticles exhibit photoluminescent characteristics depending on the synthesis conditions. Electron microscopic analysis revealed that the tetrapodal nanostructure of ZnO with high crystallinity which was confirmed by XRD analyses could be controlled by a variation of O₂/N₂ feed ratio. Typical photoluminescence with UV and blue emission of the tetrapodal ZnO nanoparticles was influenced by the particle size and crystallinity, which is manipulated by the oxidation condition.     

Generation of uniform tetrapod-shaped zincoxide nanoparticles by gas-phase reaction with using flow restrictor

Tetrapod-shaped ZnO particles are generated via gas-phase reaction of Zn vapor and oxygen in air, where they undergo homogeneous nucleation from supersaturated ZnO vapor and successive growth by surface reaction. It was found that a simple device for flow restrictor is effective in making ZnO particles of terapod-shape by leaving sufficient amounts of unreacted Zn vapor with the embryos of ZnO. In the absence of the flow restrictor, only spherical particles are formed because the oxidation reaction takes place immediately after mixing and unreacted Zn vapor does not remain for the subsequent crystal growth. The Zn vapor concentration distribution, oxygen concentration distribution, temperature, gas velocity and reaction rate in the reactor were analyzed by using a conventional computational fluid dynamic simulation package. The simulation revealed that the flow restrictor does not enhance mixing between Zn vapor and air but suppresses the mixing and reduces the residence time in the reactor so that sufficient amounts of unreacted Zn vapor remain downstream of the flow restrictor, allowing ZnO particles to grow in tetrapod-shape by abnormal crystal growth.