Showing papers by "Tsinghua University published in 1996"

Extraction of Am(III) and Eu(III) from Nitrate Solution with Purified Cyanex 301

Abstract: The extraction of tracer amounts of Am(III) and Eu(III) from nitrate medium with purified Cyanex 301™ in kerosene was studied. The slopes of logD/pH dependencies for Am and Eu are close to 3 and the logD/extractant concentration dependencies for Am and Eu are close to 2, respectively. The calculated extraction constants are 10−7.85 for Am and 10−11.62 for Eu at 25°C. Both the extraction constants and the separation factor of Am/Eu decrease with increasing extraction temperature. The extraction enthalpies of Am and Eu are 18.10 kJmol−1 and 43.65 kJmol−1, respectively. This extraction system provides a high selectivity for Am(III) over Eu(III) (at 25°C, separation factor SF=5.9×l03) which arises from greater covalent bonding tendency between sulfur donor atom and Am(III) in comparison with Eu(III).

Transformation-Induced Plasticity (TRIP)

Abstract: This review article attempts to explain TRIP generally and its appearance in different materials. Continuum mechanics formulations are mainly used to explain this phase change phenomenon. An overview is given of the published literature which is often not very easily accessible. Technological aspects are presented, such as constitutive equations of materials showing TRIP. Aspects of material selection and future material design are also treated. This article contains 315 references.

Decentralized nonlinear optimal excitation control

Abstract: A design method to lay emphasis on a differential geometric approach for decentralized nonlinear optimal excitation control of multimachine power systems is suggested in this paper. The control law achieved is implemented via purely local measurements. Moreover, it is independent of the parameters of power networks. Simulations are performed on a six-machine power system. It has been demonstrated that the nonlinear optimal excitation control could adapt to the conditions under large disturbances. Besides, this paper has verified that the optimal control in the sense of LQR principle for the linearized system is equivalent to an optimal control in the sense of a quasi-quadratic performance index for the primitive nonlinear control system.

Hypersurfaces with constant scalar curvature in space forms

Abstract: Let Rn+l(c) be an (n + 1)-dimensional Riemannian manifold with constant sectional curvature c, we also call it space form. When c -1, R"+l(c) = S "+l (i.e. (n + 1)-dimensional unit sphere space); when c = O, Rn+t(c) = E n+l (i.e. (n + 1)-dimensional Euclidean space). Let M be an n-dimensional compact hypersurface in Rn+l(c), and el . . . . . e,, a local orthonormal frame field on M, col . . . . . con its dual coframe field. Then the second fundamental form of M is

Characterization of precursors of methanol synthesis catalysts, copper/zinc/aluminum oxides, precipitated at different pHs and temperatures

Abstract: Catalyst precursors for copper/zinc aluminum mixed oxide catalysts with an atomic ratio of 6/3/1 were prepared by co-current precipitation at different pH levels and different temperatures. The precursors, catalysts and reduced catalysts were characterized by means of infrared spectroscopy, X-ray diffraction, thermal analysis, temperature-programmed oxidation and BET surface measurements. The catalysts were also tested for methanol synthesis activity in a synthesis gas containing a small amount of carbon dioxide. When the precipitation was conducted at pH= 7.0, the precipitates mainly consisted of the malachite-like phase and the catalysts therefrom were more active. Precipitation at pH≤ 6 favored the formation of hydroxynitrate, which led to less active catalysts. Although the composition of the precursors obtained at pH= 7.0 and temperatures lower than 50°C was the same as those at pH= 7.0 and T = 70°C, the activity of the catalysts was much lower. It was also found that the aging temperature was an important factor in determining the interdispersion and activity of the final catalysts. Although the effects of both the pH value and the temperature on activity of the catalysts can be ascribed to a similar reason, i.e., the different interdispersion of copper and zinc, it seems that the pH value exerts its effect through altering the phase composition of the precursors, while the temperature functions through the precipitation kinetics of the precursors with a same composition.

Improvement in the catalyst activity for direct synthesis of dimethyl ether from synthesis gas through enhancing the dispersion of CuO/ZnO/γ-Al2O3 in hybrid catalysts

Abstract: Based on the knowledge of the mechanisms of methanol synthesis and methanol dehydration, it was predicted that if the dispersion of CuO/ZnO/γ-Al2O3 in hybrid catalysts for direct synthesis of dimethyl ether from synthesis gas was enhanced, the formation rate of dimethyl ether would be increased. In order to justify the prediction, five preparation methods, including two mechanical mixing ones and three co-precipitation ones were used to prepare the catalysts. It was found that co-precipitation impregnation and co-precipitation of Cu/Zn by NaAlO2 provided the catalyst with the highest activity and co-precipitation of Cu/Zn/Al by Na2CO3 resulted in the worst catalyst. Combined with the XRD results it is deduced that the active phases for direct synthesis of dimethyl ether from synthesis gas are highly-dispersed fine crystallites of CuO/ZnO/γ-Al2O3. Calcination temperature of the precursor of γ-alumina used in the methods of mechanical mixing was also studied. It was found that a calcination temperature of 550°C brought about the most active dehydration catalyst with the largest surface area and nearly pure γ-alumina of poor crystallinity. So it might be concluded that the active phase for methanol dehydration is fine crystallites of γ-alumina.

Hydroisomerization of light normal paraffins over series of platinum-loaded mordenite and beta catalysts

Abstract: The hydroisomerization of n-pentane, n-hexane and n-heptane on platinum-hydrogen beta catalysts (Pt/HBEA) was compared with that on platinum-hydrogen mordenite catalysts (Pt/HMOR) of various silica-aluminum ratios at temperatures ranging from 190 to 310°C. For Pt/HMOR, the isomerization yields were found to depend on both the carbon number of feed and the framework Si/Al ratio of HMOR and to be enhanced by exchanging with magnesium ions, while for Pt/HBEA high selectivity of isomerization and conversion were observed. This may be caused by the open structure of tri-dimensional channels in beta zeolite.

Design and fabrication of silicon condenser microphone using corrugated diaphragm technique

Abstract: A novel silicon condenser microphone with a corrugated diaphragm has been proposed, designed, fabricated and tested. The microphone is fabricated on a single wafer by use of silicon anisotropic etching and sacrificial layer etching techniques, so that no bonding techniques are required. The introduction of corrugations has greatly increased the mechanical sensitivities of the microphone diaphragms due to the reduction of the initial stress in the thin films, For the purpose of further decreasing the thin film stress, composite diaphragms consisting of multilayer (polySi/Si/sub x/N/sub y//polySi) materials have been fabricated, reducing the initial stress to a much lower level of about 70 MPa in tension. Three types of corrugation placements and several corrugation depths in a diaphragm area of 1 mm/sup 2/ have been designed and fabricated. Microphones with flat frequency response between 100 Hz and 8/spl sim/16 kHz and open-circuit sensitivities as high as 8.1/spl sim/14.2 mV/Pa under the bias voltages of 10/spl sim/25 V have been fabricated in a reproducible way. The experimental results proved that the corrugation technique is promising for silicon condenser microphone.

Levenberg-Marquardt method for ANFIS learning

Abstract: Presents the results of applying the Levenberg-Marquardt method (K. Levenberg, 1944, and D.W. Marquardt, 1963), which is a popular nonlinear least-squares method, to the ANFIS (Adaptive Neuro-Fuzzy Inference System) architecture proposed by Jang (IEEE Trans. on Systems, Man and Cybernctics, vol. 23, no. 3, pp 665-685, May 1993). Through empirical studies, we discuss the strengths and weaknesses of using such an efficient nonlinear regression technique for neuro-fuzzy modeling, and explain the tradeoffs between mapping precision and membership function interpretability.

A new approach to convex corner compensation for anisotropic etching of (100) Si in KOH

Abstract: The method of convex corner compensation with a 〈100〉 bar for (100) silicon rectangular etching proposed by Mayer et al. ( J. Electrochem. Soc., 137 (1990) 3947–3951) has been investigated. Limitations of the method are discussed, and a modified method is put forward. Both the theoretical analysis and the experimental result are given in this paper.

The Separation of Am from Lanthanides by Purified Cyanex 301 Extraction

Abstract: Separation factors of tracer amounts of Am from micro lanthanides (La, Ce, Pr, Nd, and Sm) by purified Cyanex 301 extraction in nitrate media have been determined: SFAm/La ∼ 3500, SFAm/Ce,pr ∼ 1000, SFAm/Nd ∼ 1900, and SFAm/Sm∼ 4500, with an average value >2300. The distribution ratio decreases with increasing lanthanide concentration in the aqueous phase. In the presence of a macro amount of Pr + Nd (0.1 ∼ 0.6 M) the separation factors SFAm/Eu and SFAm/pr+Ndare about 4.7 × 103 and 2.1 × 103, respectively. The results of the countercurrent fractional process show that by using three extraction stages and two scrubbing stages, >99.99% Am can be separated from a tracer amount of Eu with 99.99% Am and <0.6% macro amount of Pr ± Nd are extracted into the organic phase.