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991.
992.
The spatial probability distribution associated with diffusion and attenuation in partially absorbing media is studied. An equivalence is established between a system with free diffusion for x>0 and partial absorption for x<0, and a semi-infinite system (x>0) with a radiation boundary condition at x=0. By exploiting this equivalence, it is shown that the effect of a partially absorbing medium in the long-time limit is equivalent to that of a perfect, virtual trap whose size is smaller than the original absorbing medium. For short times, however, there is substantial penetration of diffusing particles into the absorber. The virtual trap approach is readily generalized to higher dimensions. This allows one to obtain the density profile of diffusing particles around a partially absorbing spherical trap. An unusual crossover between short-time penetration and long-time trapping occurs in two dimensions; the size of the virtual trap is exponentially small in the case of weak absorption, corresponding to an absorption time which is exponentially large. 相似文献
993.
Direct phenol synthesis from benzene in aqueous solution where water was used as the oxidant through glow discharge plasma
(GDP) process was described for the first time. The effect of pH, ferrous and cupric ions on the phenol yield and selectivity
was examined. Phenol yield of 8.3% and selectivity of 81% has been achieved. 相似文献
994.
Olle Nilsson 《Journal of separation science》1982,5(6):311-320
It is shown theorectically that the classical formula for calculating the theoretical plate number from squared first central moment, t , and second central moment, σ2, according to n theor = t /σ2 is valid only when the capacity ratio, k approaches infinity. The general relation between the classical experimental HETP value, H = L/nm theor, and the underlying true theoretical plate height, ΔL, is found to be when (σ′)2 is the total column contribution to band broadening, L is the column length, D m is the average diffusion coefficient of the sample component in the mobile phase, D s is its value in the stationary phase, and u is the average linear velocity of the mobile phase. The mobile phase displacement, as well as the mass exchange process, is assumed to be continuous, but the application of the plate concept conditions leads to a mass balance equation that can be interpreted as belonging to a modified discontinuous plate model. The contributions 2D m/u and k 2 D s/u from longitudinal sample diffusion in the mobile and stationary phases, respectively, are consistent with the assumption that the processes are statistically independent, although the common solution technique of the differential equations does not take full account of this independence. 相似文献
995.
The performance of back-propagation artificial neural networks (NN) and partial least squares (PLS) regression for the calibration of linear and nonlinear systems has been investigated by using six types of synthetic data. Three PLS methods, conventional linear-PLS and two nonlinear-PLS methods, have been used in the study. In all but one of the synthetic data types, the band intensities varied nonlinearly with concentration. These five data types were designed to represent the effect of band shifts with increasing concentration, a nonlinear relationship between peak height and concentration, or a combination of both types of nonlinearities. The results showed that NNs perform better than PLS for all the nonlinear datasets. When a band shift is the major reason for the nonlinearity, the relative performance of NNs and PLS depends on the overlap of the absorption bands. If there is no band overlap, neither NN nor PLS can calibrate the data accurately but the results could be improved by convolving the spectral features with a Gaussian broadening function. The results indicate that a combination of peak position shift and peak height change is the most difficult nonlinearity to calibrate. NN and PLS were also used to determine the concentration of CHCl3 in pure component and mixtures of CHCl3 and CH2Cl2 using their Fourier transform infrared (FT-IR) spectra, a dataset that has been proved nonlinear in high concentrations due to the nonlinear response of the detector. The best results for the experimental data were obtained by applying one hidden layer NN to the mean-centered absorbance spectra. 相似文献
996.
Burdina K. P. Zorov N. B. Kim J. I. Kravchenko O. V. Kuzyakov Yu. Ya. 《Russian Chemical Bulletin》2002,51(9):1633-1640
A new method was developed for the preparation of bulk samples of crystalline carbon nitride on exposure of an amorphous nitrogen- and carbon-containing material to high temperature and ultrahigh pressure in the presence of crystallization seeds. Amorphous carbon nitride whose composition was close to C3N4was used as a starting material. Thin films of crystalline carbon nitrides prepared by the laser-electric discharge method were used as crystallization seeds. 相似文献
997.
一阶紫外导数光谱PLS法直接同时测定硝酸根和亚硝酸根 总被引:11,自引:1,他引:11
用偏最小二乘(PLS)法解析了NO_3~-和NO_2~-混合体系的一阶紫外导数光谱,提出了直接同时测定环境水样中NO_3~--N和NO_2~--N的计算分析法。该法对NO_3~-和NO_2~-来说,最低检测浓度分别为9×10~(-8)和2×10~(-7)mol·L~(-1),精密度(RSD)分别为2.05%和2.12%。用于测定合成水样,平均相对误差在4%以内;用于测定环境水样,结果与标准法相吻合。 相似文献
998.
Reactions of both SF4 and SF5 with F have been studied at 295 K in a gas-flow reactor sampled by a mass spectrometer. The rate coefficient for the combination reaction of F with SF4 to produce SF5 was found to increase from (0.9 to 3.0)×10–12 cm3 s–1 when the helium bath gas number density was increased from (2 to 26)×1016 cm–3. The values obtained here are three orders of magnitude higher than a recent estimate of the high-pressure value based on the modelling of photochemical studies. The experimental results have been compared with RRKM and master equation calculations in which a simplified Gorin model has been used to determine the structure of the transition state. These calculations show that reasonable agreement can be obtained between the experimental data and the calculation if a small (2 KJ/mol) activation energy is assumed. The rate coefficient for the reaction between SF5 and F to produce SF6 was found to be independent of helium bath gas number density within the range given above. The value obtained for the rate coefficient was 9×10–12 cm3 s–1 with an uncertainty of a factor of 2. This value is close to that of 1×10–11 cm3 s–1 computed from the simplified Gorin model and to the value of 1.7×10–11 cm3 s–1 deduced from modelling of photochemical experiments. 相似文献
999.
组合偏最小二乘回归方法在近红外光谱定量分析中的应用 总被引:3,自引:1,他引:3
针对近红外光谱数据局部效应显著,变量个数多,彼此间常存在严重的复共线性,并多与样品组分含量呈非线性关系,构建一种组合非线性偏最小二乘回归(E-S-QPLSR)方法。它采用无重复采样技术(subag-ging),从训练样本中生成若干子样,然后每个子样通过二次多项式偏最小二乘回归(QPLSR),建立其子模型,并实现对训练样本因变量的定量预测,再将它们交由线性PLS算法用于计算各子模型的组合权系数。将该法应用于80个玉米样品的水组分含量与其近红外光谱的定量关系建模,效果良好,显示出很强的学习能力,所建模型的预报性能也优于其它方法。 相似文献
1000.
在 QCISD ( T) / / MP2水平下 ,分别采用 6-3 1 1 ++G* * 基组和 SDD基组对重 -轻 -重反应 I+HI(ν=0 )→ IH(ν′=0 ) +I中的 H和 I的偏分势能面进行了 ab initio计算 ,指认出在 0~ 0 .5 8e V碰撞能范围内所产生的 6个散射共振态为 Feshbach共振 ,并与文献报道的量子散射理论计算与高分辨阈值光分离光谱实验结果进行了比较 . 相似文献