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51.
Masaaki Suzuki Yoshihisa Hasegawa Masayuki Aizawa Yoshinori Nakata Takeshi Okutani Kohei Uosaki 《Journal of the American Ceramic Society》1995,78(1):83-89
The structure of silicon carbide–silicon nitride (SiC–Si3 N4 ) composite particles synthesized using a CO2 laser was studied by magic angle spinning nuclear magnetic resonance (MAS-NMR) and electron spin resonance (ESR). The structure around Si atoms changed by introducing N. C atoms around Si were substituted by N atoms, and N-rich configurations around Si atoms increased stepwise as the N content increased. The low N content composite particles consisted of mainly SiC phase containing dissolved N. N atoms were partly present in β-SiC microcrystal and partly in the grain boundary layer in the particle. N atoms were tetrahedrally surrounded by four Si atoms in β-SiC microcrystal and were trivalent state bonded to three Si atoms in the grain boundary layer. The high N content particles consisted of SiC, Si3 N4 , and amorphous phases, whose amount depended on N content. 相似文献
52.
Takeshi Yagi Kazuo Shinozaki Nobuo Ishizawa Nobuyasu Mizutani Masanori Kato Akihiko Tsuge 《Journal of the American Ceramic Society》1988,71(7):334-C
The thermal diffusivity of AlN ceramics was significantly decreased by the addition of SiO2 . The AlN ceramics with 4 wt% SiO2 could not be densified by pressureless sintering in the temperature range 1400° to 1800°C. The thermal diffusivity of these samples was very low because of their porous structure. The AlN ceramics containing 2, 4, and 8 wt% SiO2 were densified by hot-pressing and also had low thermal diffusivity. In these samples, the grains of the 27R polytype that resulted from the reaction between AlN and SiO2 were dispersed, obstructing the conduction of heat. The relation between the amount of 27R polytype and the thermal diffusivity of the AlN ceramics was determined. 相似文献
53.
Active 4-dodecanoyl-2-nitrophenyl esters of β-alanine, β-alanyl-β-alanine, and β-alanyl-β-alanyl-β-alanine were prepared, and tried to polymerize in various solvents. Nonpolar solvents were found to be convenient for the polycondensation reaction. The yield of the polycondensation was high for the monopeptide ester, and less for the dipeptide and tripeptide esters. The effect of temperature on the polycondensation reaction was also studied. 相似文献
54.
A physicochemical parameter, represented by the symbol Σs*, based on molar solubility in water and molar attraction constants of Small, has been developed to express quantitatively the relative hydrophobicity, or nonpolar character, of the hydrocarbon molecule. The value of Σs* can be calculated for a hydrocarbon from its chemical structure. The scale of Σs* is consistent within each group of aromatic, cyclic, and noncyclic hydrocarbons. Reverse osmosis data have been obtained at 250 psig for single-solute aqueous feed solution systems involving low concentrations of 39 different hydrocarbons (including 13 aromatics, 10 cyclic, and 16 noncyclic compounds) and several samples of cellulose acetate membranes of different surface porosities. The effect of operating pressure on membrane performance has also been studied for two aromatic hydrocarbon solutes. The values of Σs* for the solutes used were in the range of 425 to 924 for aromatic hydrocarbons, 521 to 931 for cyclic hydrocarbons, and 369 to 960 for noncyclic hydrocarbons. The reverse osmosis data have been correlated with Σs* for each group of hydrocarbons studied. In all cases, positive solute separations were obtained, and the ratio [PR]/[PWP] was less than 1. With respect to each film, solute separation increased with increase in Σs*, and decreased with increase in operating pressure. Also, solute separation decreased in the order aromatic hydrocarbon > cyclic hydrocarbon > noncyclic hydrocarbon at any given value of Σs*. At a given operating pressure, for low values of Σs* (~500 or less) solute separation increased with progressive decrease in average pore size on the membrane surface. For high values of Σs* (~800 or more), solute separation initially increased with decrease in average pore size, then passed through a maximum and minimum with further decrease in average pore size, and again increased with still further decrease in average pore size. The results are discussed on the basis of preferential sorption of solute at the membrane–solution interface under the experimental conditions studied. 相似文献
55.
Reverse osmosis separations of eight polyethylene glycol (PEG) solutes in the average molecular weight range of 200 to 6750 in single-solute dilute aqueous solutions have been studied using porous cellulose acetate membranes at the operating pressures of 50, 75, and 100 psig. Diffusivity data for the above PEG solutes have also been obtained from experimental data on intrinsic viscosities. From an analysis of all experimental data, numerical values for the parameters representing the polar (?ΔΔG/RT), steric (δ*ΣEs), and nonpolar (ω*Σs*) forces governing reverse osmosis separations of PEG solutes have been generated. These numerical values are useful for precise characterization of cellulose acetate membranes for whose specifications sodium chloride is not the appropriate reference solute because of its low or practically negligible separation under reverse osmosis operating conditions. This work also illustrates that solute separation in reverse osmosis can predictably increase or decrease with increase in operating pressure depending on experimental conditions. 相似文献
56.
Cationic chitosan (CT) and anionic dextran sulfate (Dex) were layer-by-layer (LbL) assembled from aqueous solutions containing
1 M NaCl on a quartz crystal microbalance (QCM) substrate, and the original films ((CT-Dex)3-CT)) were end-capped with LbL assembly from CT solutions containing 1 M NaCl and Dex solutions without NaCl. The enzymatic
degradation of films by chitosanase was quantitatively analyzed by QCM in terms of numbers of end-capping steps. The degradation
of films end-capped with (Dex-CT)3 was considerably prolonged when compared to those end-capped with other end-capping steps. A mechanism for the prolonged
degradation was proposed by quantitative QCM data and zeta potential results. 相似文献
57.
In order to improve the fracture properties of p, p′-diaminodiphenylmethane-cured epoxy resin, various kinds of aromatic and aliphatic glycidyl compounds were investigated as a modifier at an amount of 30 wt %. Several compounds promoted the fracture toughness. In any glycidyl compounds, however, heat resistance was decreased by the modification. The dynamic mechanical properties of the modified epoxy resins were measured. The crosslinking density ρ was calculated from the theory of rubber elasticity, and the mechanical properties of the resins were discussed in regard to the crosslinking density. Tensile strength was scarcely affected by the crosslinking density. Elongation at break and Izod impact strength increased remarkably with decrease in crosslinking density. The fracture toughness KIc- increased with decrease in crosslinking density except at small ρ. 相似文献
58.
Summary Multi-step precipitation separation system was developed by using aqueous mixtures of some thermosensitive polymers. The following
three polymers were used here; poly(N-n-propylacrylamide), poly(N-isopropylacrylamide), and poly(N-isopropylmethacrylamide). A mixture of the three polymers showed three endothermic peaks, and the peak top temperatures were
almost consistent with that of the each polymer solution. The polymers were purified by thermal precipitation to obtain fractions
which can respond in narrow temperature ranges prior to use. In the case of the precipitation separation of two polymers mixtures,
purities of the obtained precipitate and supernatant fractions became high comparing with the case in which the unpurified
polymers were used. Parts of the polymers which were not the precipitation targets were also precipitated by the separation
procedures. This was caused not only by insolubilization of the non-targeted polymers due to their phase transitions but also
by their non-specific entanglement with the targeted polymers. The purities of the fractions also improved when the difference
of the phase transition temperature between two polymers was large enough to avoid the coprecipitation. In the case of the
precipitation separation of mixtures of the three polymers, purities of each fraction also improved when the purified polymers
were used. 相似文献
59.
Several onium cations having vinyl group formed ionic liquids after coupling with bis(trifluoromethanesulfonyl)imide. These monomers were polymerized, and the relation between onium cation structure and properties of thus polymerized ionic liquids was investigated. The polymerized ionic liquid having ethylimiadzolium cation unit showed the highest ionic conductivity of around 10−4 S cm−1 at 30 °C among the obtained polymers reflecting the lowest glass transition temperature of −59 °C. These polymers were thermally stable and their decomposition temperatures were about 350 °C. The ionic conductivity of the polymerized ionic liquids decreased by both the addition of lithium bis(trifluoromethanesulfonyl)imide and the polymerization in the presence of cross-linker. However, the polymerized ionic liquid having 1-methylpiperidinium cation structure showed good lithium ion transference number of 0.43 at room temperature. 相似文献
60.
Hideki Ishida Kaori Sasaki Yoshihiko Okada Takeshi Mitsuda 《Journal of the American Ceramic Society》1992,75(9):2541-2546
The effect of curing temperature (40°, 60°, 80°C) on the hydration behavior of β-dicalcium silicate (β-C2 S) was investigated. The β-C2 S was obtained by decomposition of hillebrandite, Ca2 (SiO3 )(OH)2 , at 600°C, has a specific surface area of about 7 m2 /g, and is in the form of fibrous crystals. The dependence of the hydration reaction on temperature continues until the reaction is completed. The hydration is completed in 1 day at 80°C and in 14 days at 14°C. The hydration mechanism is different above and below 60°C, but at a given temperature, the reaction mechanism and the silicate anion structures of C-S-H do not change significantly from the initial to the late stages of the reaction. High curing temperature and long curing times after completion of reaction promote silicate polymerization. The Ca/Si ratio of C-S-H shows high values, being almost 2.0 above 60°C and 1.95 below 40°C. 相似文献