Liquid crystal-based temperature-controlled recirculating flat jet system
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In this paper, the design and implementation of a temperature-controlled recirculating flat jet system for liquid crystals (LCs)-based experiments are presented. In these experiments, the target liquid is usually exposed to medium to high laser fluences, possibly resonant with specific excitation, thus resulting in a change of local temperature and sudden degradation. To overcome this problem, each laser pulse must interact with a new volume of liquid, preferably with flat surfaces, while avoiding the use of substrates. A well-established solution consists of impinging two identical laminar jets that force the liquid into a radial expansion perpendicular to the plane formed by the jets, resulting in a consecutive chain of flat sheets bound by thick rims. In this context, LCs pose several challenges considering their viscosity, non-Newtonian behavior, and mesophase nature. Here, a precise control of temperature, thus mesophase, and pressure is demonstrated enabling the use of LCs in an impinging jet system. In particular, the system presented here delivers stable fluid chains of different sizes and thicknesses. The viscosity and non-Newtonian behavior of the LCs have a significant impact on the thickness of the chains as a function of the nozzle inner diameter, impinging angle, and radial distance from the impinging point. The flow rate, on the other hand, primarily affects the width and length of the liquid sheet.The chemical and physical processes involved in formation of mesophase pitch are described. Carbonaceous mesophase exhibits properties of both nematic liquid crystals and glass-like pitches. Mesophase development requires polymerization of aromatic pitch components to a sufficient molecular size, and a proper concentration balance between molecules with different mesophase forming tendencies.
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Carbon fibers
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Carbon fibers
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The effect of ferrocene in pitch on nucleation and growth processes of mesophase was investigated from the behavior of iron. The quinoline soluble coal-tar pitch to which ferrocene was added, was heat-treated at 430°C for a certain period. Quinoline insoluble part in the heat-treated pitch was separated by solvent extraction with quinoline. Then, the quinoline insoluble part was treated with HCl. Iron contents in the heat-treated pitch, the quinoline insoluble part, and the quinoline insoluble part treated with HCl were examined. The obtained results are as follows.Ferrocene added in pitch was already decomposed at 430°C and changed into the fine crystals of α-Fe and FeS, which covered the surface of mesophase spherules formed in pitch. The mesophase spherules were found to have almost the same size. Amounts of mesophase formation in the ferrocene-added pitch were larger than those in the pitch having no ferrocene content. From these results, it is considered that the iron crystals controll the size of mesophase spherules and accelerate the mesophase formation. Furthermore, when the mesophase coalesces each other, the crystals not only covers the surface of the coalesced mesophase but also exists inside the mesophase.It is also made clear that the shape of mesophase spherules changes from lemon-like form to spherical form with growth of mesophase spherules.
Quinoline
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A new 13C-NMR measurement technique, named fused-state 13C-NMR, has been developed. Using this technique, many aspects of carbonaceous mesophase have been elucidated: Constituents of soft mesophase have methyl and/or naphthenic groups whereas those of hard mesophase have no aliphatic groups. The order parameter of carbonaceous mesophase in the nematic phase is about 0.7. Very small molecules such as hexamethylbenzene and pyrene orientate in a carbonaceous mesophase. In a liquid crystalline phase, all oriented molecules in a carbonaceous mesophase (including the small molecules trapped in the mesophase) fluctuate around the orientation axis in the same manner. That is, an oriented molecule behaves as if it were trapped between two walls the distance between which is about 1.0 nm. The analyzed results rule out the assertion that soft mesophase is a gel and also rule out the existence of basic structural units. A constructed model for molecular ordering in a mesophase has clarified some pending questions on carbonaceous mesophase.
Hexamethylbenzene
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A method in determination of the mesophase contents of pitches are presented. The method is based on the reflectivity difference between a mesophase part and an isotropic part. In order to get clear contrast between a mesophase part and an isotropic part, image processing is used. The method achieved accurate determination of the mesophase content from 0% to 100% in several minuets.
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A finite-difference procedure is applied to analyse the fully developed laminar flow characteristics in a pipe rotating about a perpendicular axis. We propose a new combination of two parameters of KL and number Ro as characteristic non-dimensional numbers. The four flow regimes are classified for large and srmall KL, Ro. We present a numerical formula for friction factor which is valid for wide range of KL and Ro. The formula agrees well with experimental data.
Friction Factor
Laminar sublayer
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Mesophase pitch is the pioneer body of the new carbon materials.This paper reveals recent research aspects of mesophase pitch such as transform mechanism,the composition,properties,and the influencing factors of mesophase pitch and the latest application of mesophase pitch.
Carbon fibers
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Coalescence (physics)
Reactivity
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