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81.
Ji Yeon Ko Sin-Young Park Duk Yong Yoon Suk-Joong L. Kang 《Journal of the American Ceramic Society》2004,87(12):2262-2267
When TiC–20 wt% Ni powder mixtures are sintered at 1400°C, relatively large TiC grains possibly containing some Ni form with near-equilibrium shapes. When these specimens are heat-treated again at 1400°C in contact with sintered WC–20 wt% Ni pieces, the liquid films between the TiC grains in the contact region migrate against their increasing curvatures, forming (Ti,W)C solid solution behind them. These migrating liquid films reverse their directions on further heat-treatment. As in other alloys, this liquid film migration must be driven by the coherency strain energy produced by W diffusion at the surface of the dissolving TiC grains. Shells of (Ti,W)C solid solution also form around the cores of TiC grains near the contact region, and this process is probably driven by both coherency strain energy and free energy of mixing. At some contact regions, (Ti,W)C precipitates nucleate and grow, probably driven mainly by the free energy of mixing. In powder mixtures, the formation of core-shell grains is expected to be driven by the coherency strain energy, the free energy of mixing, and the capillary effect. 相似文献
82.
A poly(perfluoroalkylethyl methacrylate) and a series of poly(n-alkyl methacrylate)s such as poly(methyl methacrylate), poly(ethyl methacrylate), and poly(n-butyl methacrylate) were prepared and used to investigate the surface properties of polymer mixtures containing a fluorinated homopolymer and a nonfluorinated homopolymer and the effect of the side-chain length of poly(n-alkyl methacrylate) on the surface free energy for the polymer mixtures. Contact angles were measured for the surfaces of polymer mixtures by varying the concentration of poly(perfluoroalkylethyl methacrylate). From the contact angle data, it can be inferred that most of the poly(perfluoroalkylethyl methacrylate) added to poly(n-alkyl methacrylate)s is located in the outermost layer of polymer-mixture surface. Surface free energies for the outermost surfaces of polymer mixtures were calculated from the contact angle data using Owen and Wendt's equation. The decrease in the surface free energy for the polymer mixture with the poly(perfluoroalkylethyl methacrylate) addition is more pronounced as the side-chain length of poly(n-alkyl methacrylate) decreases. Due to the steric effect of the side chain of poly(n-alkyl methacrylate), the arrangement of the perfluoroalkylethyl group of poly(perfluoroalkylethyl methacrylate) to the air side is considerably hindered. The ESCA analysis of atomic compositions of the surface for the polymer mixture verified that poly(perfluoroalkylethyl methacrylate) is preferentially arranged and concentrates at the polymer mixture–air interface. The results of functional group compositions obtained by ESCA showed that the functional group composition of CF3 for the outermost layer has a more important effect on the surface free energy than that of CF2 and confirmed the hindrance of the arrangement of perfluoroalkylethyl group to the air side by the side chain of poly(n-alkyl methacrylate). © 1994 John Wiley & Sons, Inc. 相似文献
83.
Carbon/carbon composites were made through the pyrolysis of stabilized PAN felt and phenolic resin with the addition of 5 or 10 wt % carbon black to the matrix and then heat treatment at 600–2500°C. The effects of adding carbon black to the matrix precursor on the physical properties, microstructure, and mechanical properties of the resultant composites were investigated. Adding carbon black not only reduced the weight loss but also limited the shrinkage of the resultant composites. Adding carbon black also accelerated the formation of carbon basal planes in the matrix. At 2500°C, the crystalline stacking height in the composite with 10 wt % added carbon black was 200% greater than that with no additive. The flexural strength of the composite also increased from 15 to 42 MPa (almost 300%). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 333–337, 2006 相似文献
84.
Tse-Hao Ko 《应用聚合物科学杂志》1991,42(7):1949-1957
A continuous stabilization and carbonization process was used to prepare polyacrylonitrile (PAN)-based carbon fibers. The stepwise stabilization of PAN fibers was tried at various temperatures. The effect of stepwise stabilization on the physical properties and microstructure of the final carbon fibers is reported in this article. The fixed temperature in stepwise stabilization is kept below the fusion temperature of PAN precursors to avoid overstabilization of the fibers. The optimum stepwise stabilization process not only increases the amount of ladder polymer in stabilized fiber but also improves the physical and mechanical properties of the resultant carbon fibers. The formation of closed pores from open pores in carbon fiber occurs at 1100°C, but the formation of closed pores occurs at 200°C lower for carbon fiber developed from overstabilized fiber. The effect of continuous stepwise stabilization on the properties of resulting stabilized fibers and the variation in physical properties, element composition, and microstructure of carbon fibers during the carbonization process are also reported in this article. 相似文献
85.
Polyurethane macroiniferter/poly(methyl methacrylate) block copolymer dispersions with inverse core‐shell morphologies were obtained from 1,1,2,2,‐tetraphenylethane‐1,2‐diol, dimethylol propionic acid, 4,4′‐diphenylmethane diisocyanate, and poly(propylene glycol) via a living radical mechanism. Molecular weight, particle size and dispersion viscosity, and thermal, mechanical, and dynamic mechanical properties of the dispersion cast films are reported as a function of copolymerization time. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1971–1975, 2003 相似文献
86.
Tae?Joon?Chung Min?Chan?Kim Chang?Kyun?ChoiEmail author 《Korean Journal of Chemical Engineering》2004,21(1):41-47
The temporal development of thermal disturbances in the fluid layer heated isothermally from below is investigated, based
on propagation theory. This theory is examined by using scaling. To examine the behavior of thermal instability the mean-field
approximation is employed and resulting equations are solved by Galerkin method. The stability criteria to mark the onset
of convective instability are newly suggested as the intersection point of the growth rate of averaged temperature with that
of its fluctuation. The resulting critical time is close to that derived from propagation theory. By considering the nonlinear
effects, the characteristic times to represent the detection time of manifest convection and also to exhibit the minimum Nusselt
number are discussed. 相似文献
87.
The hydrogels composed of chitosan and eugenol were prepared to enhance and sustain antioxidant activities. The vinyl groups of eugenol monomer were directly grafted on the amino groups of chitosan, using ceric ammonium nitrate. The graft of eugenol onto chitosan was confirmed by using Fourier‐transform infrared and proton nuclear magnetic resonance spectroscopies. Results from the swelling behavior, thermal stability, and wide‐angle X‐ray diffraction revealed that the equilibrium water content decreased with increase of graft yields, because of the hydrophobicity of eugenol, although the introduction of eugenol as a side chain disturbed the ordered arrangement of chitosan's crystalline structure. The eugenol‐grafted chitosan hydrogels showed lower pH sensitivity in comparison with chitosan alone, because the amino groups, which were pH sensitive, of chitosan were grafted with eugenol. The scavenging activity of the tested hydrogels increased with graft yield of eugenol, because phenolic groups in the eugenol could play a major role as potent free‐radical terminators, in the results of improved antioxidant activity in eugenol‐grafted chitosan hydrogel in comparison with chitosan alone. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99:3500–3506, 2006 相似文献
88.
Hydrogen sulfide gas was removed in a 2-dimensional gas-lift reactor by the photosynthetic microorganismChlorobium thiosulfatophilum using light emitting diodes (LEDs) as a light source. LEDs saved light energy by 99% compared with the incandescent light
source. The plate-type gas-lift reactor removed hydrogen sulfide five times better per unit mg of protein, and performed two
times better in the maximum performance per unit luminous flux, compared with cylindrical fermentors. 相似文献
89.
Summary A series of polyblends of poly(1,4-phenylene vinylene), PPV, and poly(2,5-dimethoxy-1,4-phenylene vinylene), PDMPV, were prepared in film form from precursor polyblends of the respective sulfonium salt polymers, which were separately prepared from the respectivebis(sulfonium salt) monomers. The blend films were doped with I2 at room temperature to obtain a wide range of electrical conductivities (10–2 to 102Scm–1) depending on the blend composition. The higher the content of PDMPV in the blends the higher was the conductivity. 相似文献
90.
Jun-Bin Ko Sung Wook Lee Dong Eun Kim Young Un Kim Gang Li Seung Goo Lee Tae-Sun Chang Dojin Kim Yong Lak Joo 《Journal of Porous Materials》2006,13(3):325-330
(1 − x)SiO2-(x)ZrO2 (x = 0.1, 0.2) composite fiber mats were prepared by electrospinning their sol-gel precursors of zirconium acetate and tetraethyl
orthosilicate (TEOS) without using a polymer binder. The electrospun composite fibers were characterized by powder X-ray diffraction
(XRD), scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR) and mercury porosimetry. The composite
fibers having a tetragonal crystalline ZrO2 were obtained by calcining the electrospun composite fibers at high temperatures. The results show that the structure and
crystallization of ZrO2 in the composite fibers can be controlled by sintering temperature, while the porosity and morphology of the fiber mats did
not depend on the sintering temperature. 相似文献