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71.
Alvarez-Merino MA Joly JP Carrasco-Marín F Moreno-Castilla C 《Journal of colloid and interface science》2003,260(2):449-453
Intermittent temperature-programmed desorption of ammonia was used to study the strength and population of surface acid sites of tungsten oxide supported on activated carbon pretreated at 350 and 700 degrees C. Catalysts pretreated at 350 degrees C showed two types of surface acid sites and desorption occurred with free readsorption until a temperature of around 300 degrees C was reached. Pretreatment at 700 degrees C produced three different states of ammonia adsorbed on the catalysts and desorption occurred with free readsorption. 相似文献
72.
Frank J. Millero Sara Sotolongo David J. Stade Carmen A. Vega 《Journal of solution chemistry》1991,20(11):1079-1092
The oxidation of Fe(II) with H2O2 has been measured in NaCl and NaClO4 solutions as a function of pH, temperature T (K) and ionic strength (M, mol-L–1). The rate constants, k (M–1-sec–1), d[Fe(II)]/DT=-k[Fe(II)][2O2]at pH=6.5 have been fitted to equations of the formlog k = log k0+ AI
1/2+BI+CI
1/2/T
Where log k0=15.53-3425/T in water; A=–2.3, –1.35; B=0.334, 0.180; and C=391, 235, respectively, for NaCl (=0.09) and NaClO4 ( =0.08). Measurements made in NaCl solutions with added anions yield rates in the order B(OH)
4
–
>HCO
3
–
>ClO
4
–
>Cl–>NO
3
–
>SO
4
2–
and are attributed to the relative strength of the interactions of Fe2+ or FeOH+ with these anions. The FeB(OH)
4
+
species is more reactive while the FeCO
3
0
, FeCl+, FeNO
3
+
and FeSO
4
0
species are less reactive than the FeOH+ ion pair. The general trend is similar to our earlier studies of the oxidation of Fe(II) with O2 except for B(OH)
4
–
. The effect of pH on the logk was found to be a quadratic function of the concentration of H+ or OH– from pH=4 to 8. These results have been attributed to the different rate constants for Fe2+ (k0) and FeOH+ (k1) which are related to the measured k by, k=k0Fe + k1FeOH, where i is the molar fraction of species i. The rates increase due to the greater reactivity of FeOH+ compared to Fe2+. k0 is independent of composition and ionic strength but k1 is a function of ionic strength and composition due to the interactions of FeOH+ with various anions. 相似文献
73.
This research provides a fundamental understanding of the early stage hydration of Portland cement paste, tricalcium aluminate
(C3A) paste at water to cement ratio of 0.5 and C3A suspension at water to cement ratio of 5.0 modified by 2 or 4 mass% of sodium carbonate. A high conversion of unreacted
clinker minerals to gel-like hydration products in the cement-Na2CO3 pastes takes place rapidly between 1st to 24th h. Contrary the Ca(OH)2 formation within the same time interval is retarded in the excess of CO32− ions due to intensive rise and growth of CaCO3 crystals in hydrated cement. Later, the conversion of clinker minerals to the hydrate phase is reduced and higher contents
of calcite and vaterite relative to that of Ca(OH)2 in comparison with those found in the Portland cement paste are observed. As a consequence a decrease in strength and an
increase in porosity between hardened Portland cement paste without sodium carbonate and those modified by Na2CO3 are observed. C3A hydrates very quickly with sodium carbonate between 1st and 24th h forming hydration products rich in bound water and characterized also by complex salts of (x)C3A·(y)CO2·(zH2O type, whereas C3A-H2O system offers C3AH6 as the main hydration product. Higher content of the formed calcium aluminate hydrates in C3A-Na2CO3-H2O system also contributes to early strength increase of Portland cement paste. 相似文献
74.
Julie L. S. Bell David J. Wesolowski Donald A. Palmer 《Journal of solution chemistry》1993,22(2):125-136
The dissociation quotients of formic acid were measured potentiometrically from 25 to 200°C in NaCl solutions at ionic strengths of 0.1, 0.3 1.0, 3.0, and 5.0 mol-kg–1. The experiments were carried out in a concentration cell with hydrogen electrodes. The resulting molal acid dissociation quotients for formic acid, as well as a set of infinite dilution literature values and a calorimetrically-determined enthalpy of reaction, were fitted by an empirical equation involving an extended Debye Hückel term and seven adjustable parameters involving functions of temperature and ionic strength. This regressional analysis yielded the following thermodynamic quantities for 25°C: logK=–3.755±0.002, Ho=–0.09±0.15 kJ-mol–1, So=–72.2±0.5 J-K–1-mol–1, and C
p
o
=–147±4 J-K–1-mol–1. The isocoulombic form of the equilibrium constant is recommended for extrapolation to higher temperatures. 相似文献
75.
Allison A. Calhoun Patrick D. Nicholson Alexander B. Barnes 《Polymer Degradation and Stability》2006,91(9):1964-1971
Thermo-oxidative effects on the surface energy of polypropylene were measured by inverse gas chromatography as a function of exposure time and temperature. Unaltered polypropylene had a surface energy of 33 mJ/m2. Oxidized polypropylene, after exposure to air at temperatures of 100 °C and 110 °C, had a range of maximum surface energies from 38 to 41 mJ/m2. Comparisons between FTIR carbonyl peak growth and the surface energy showed that both methods detect oxidation, though the increase in surface energy is detected before the carbonyl peak growth is noticeable. The work of adhesion predicted by the surface free energies obtained in this work between a coated calcium carbonate and polypropylene changes by 10% due to the oxidation of the polymer at 110 °C. 相似文献
76.
以去离子水与乙二醇的混合液(体积分数分别为36.5%,48.7%,59.0%及71.2%,以下简称混合液)作为同轴传输线的绝缘介质,进行了μs级高电压负充电条件下的正电极击穿实验,研究了混合液的击穿电压、击穿时间、相对介电常数及电阻率与体积分数的关系。实验结果表明:在充电电压为20kV时,71.2%的混合液比36.5%的混合液的平均击穿电压提高25.1%,平均击穿时间延长10.49%,而相对介电常数减小868%。同时,随着充电时间的缩短,混合液的击穿电压提高。 相似文献
77.
探头对共聚焦内窥成像系统层析能力的影响 总被引:2,自引:1,他引:1
根据透镜成像规律和单模光纤的传输特性,获得基于单模光纤的共聚焦内窥成像系统轴向光强分布,发现系统的成像过程由于单模光纤的介入呈现非线性,照明光路系统和接收光路系统仅影响系统接收信号的总强度,系统层析能力取决于探头和单模光纤性能.通过建立的共聚焦内窥成像实验装置,测量出在三组不同物镜组合情况下系统的归一化轴向光强分布.结果显示已建立的共聚焦内窥成像系统的理论模型是正确的,扫描光点的定位精度对系统层析能力影响很大,系统设计应该综合考虑光学、机械和电子方面的性能要求. 相似文献
78.
大多数结构用铝合金通常要经过热处理或加工硬化以得到更高的力学性能.这种合金焊接后,焊接热会使焊缝附近局部区域(称为热影响区HAZ)强度降低.这一特点使得焊接铝合金梁构件极限承载力的研究与焊接钢梁构件相比变得更为复杂.由于我国目前还没有关于铝合金结构的设计规范,为此本文对两种典型的纵向焊接工字型截面铝合金梁构件进行了试验研究,并在参考国外规范的基础上,提出了适用于静力设计的纵向焊接铝合金梁设计公式的建议.通过对试验构件进行数值分析,验证了有限元分析焊接梁的可靠性.在此基础上进行了大量的数值分析,并将试验结果及有限元计算结果与公式计算结果进行了比较,可以看出建议公式计算结果富有较大安全度,从而验证了其适用性,为我国《铝合金结构设计规范》的编制提供了依据和参考. 相似文献
79.
In this study, densified wood was prepared by hot pressing after partial lignin and hemicellulose were removed through alkaline solution cooking. The tensile strength and elastic modulus of densified wood were improved up to 398.5 MPa and 22.5 GPa as compared with the original wood, and the characterization of its supramolecular structures showed that the crystal plane spacing of the densified wood decreased, the crystallite size increased, and the maximum crystallinity (CI) of cellulose increased by 15.05%; outstandingly, the content of O(6)H⋯O(3′) intermolecular H-bonds increased by approximately one-fold at most. It was found that the intermolecular H-bond content was significantly positively correlated with the tensile strength and elastic modulus, and accordingly, their Pearson correlation coefficients were 0.952 (p < 0.01) and 0.822 (p < 0.05), respectively. This work provides a supramolecular explanation for the enhancement of tensile strength of densified wood. 相似文献
80.