An Estimating Method for Parametric Spectral Densities of Gaussian Time Series

An estimating method for spectral densities of Gaussian time series that belong to a parametric model is proposed. Spectral density estimators are evaluated by using average Kullback–Leibler divergence from the true spectral density to estimated spectral densities. In the classical approach, unknown spectral densities are estimated by replacing the unknown parameters by asymptotically efficient estimates. In the alternative method introduced in the present paper, spectral density estimates usually do not belong to the model. The alternative spectral density estimators asymptotically dominate the classical ones. The difference in average Kullback–Leibler divergence between them can be regarded as the mixture mean curvature of the model in the space of all spectral densities. The explicit expression for the proposed estimators of spectral densities of autoregressive processes is obtained. The accuracy of prediction can be improved by using predictors that correspond to the alternative spectral density estimators.     

The effect of environmental temperature upon the physical characteristics of the smoke produced by burning wood and PVC samples

Data are presented which show the effect of elevated environmental temperature upon smoke particle characteristics of wood (Douglas fir) and a rigid PVC, under both flaming and nonflaming combustion in air. The physical data obtainbed include the smoke particle average diameter, the smoke optical densities in red and blue light, and the weight loss of the sample. The average particle dimeter, the smoke optical densities in red and blue light, and the weight loss of the sample. The average particle diameter and optical densities in red and blue light, and the weight loss of the sample. The average particle dimater and optical densities were obtained uwing in in situ optical system which measures forward scattered light at two angles and transmitte light at two wavelengths. Data were taken for vertically mounted samples exposed to a radiant flux of 5 W cm^?2 in environments at a room temperatures, 100°C, 200°C and 300°C. The result indicate that in flaming test higher environmental temperatures general result in greater smoke optical densities and larger smoke particles, while in nonflaming tests higherl temperatures in lower smoke densities and smaller smoke particles, for wood. These results may explain why small-scale test data (room temperature environment) do not often correlate well with full-scale data (often at elevated environmental temperatures).     

A Computational Study of Density of Some High Energy Molecules

The detonation pressure depends quadratically on the loading density of the explosives. A precise estimate of the density is thus crucial to decide if a novel energetic material is worth pursuing. In this work we investigate theoretically the crystal densities of the energetic compounds RDX, TNT, NTO, DNAM, CL‐20, DADNE, and HMX. We calculate the crystal densities by using Materials Studio 7.0 Polymorph Predictor, employing force fields and exploring molecular packing arrangements with minima in total energy. Geometry optimized molecular structures computed by density functional theory (DFT) are used as input to the density predictions. In an additional DFT study we apply two functionals, B3LYP and M06 with the 6‐31G(d) and the 6‐31G(d,p) basis sets, and the program package GAUSSIAN09. In this part of the work crystal densities are calculated by using the molecular isosurface volume (defined by the volume within a surface with an electron density of 0.001 electrons per Bohr³) alone or combined with the variance of the electrostatic potential (ESP). The Polymorph Predictor seems to overestimate the densities, but the values are very dependent on the force field strength determined by charges assigned to atoms. In the GAUSSIAN09 DFT study the densities derived by using the M06 functional are in similar agreement with experimental data as what we experienced for the B3LYP results, although both functionals appear to give slightly lower densities than reported experimentally for the majority of the molecules. On average, the densities derived by the ESP method correlate equally well with measured values as the results obtained by the isosurface method.     

制备高纯超细氧化铝粉体新方法

以99.99% 高纯铝板为阳极原料,采用电化学方法制备氧化铝前体氢氧化铝,讨论分析了氢氧化铝的焙烧温度、保温时间对制备高纯超细氧化铝的影响,考察了不同电流密度放电对氧化铝形貌和粒度的影响。结果表明：在70 mA·cm^-2的较高电流密度下铝/空气电池放电过程可得到平均粒度为268 nm的氢氧化铝;制备的氢氧化铝经洗涤,在1400℃焙烧,保温3 h,可得到平均粒度为200 nm,形貌为近似球状的99.99% 的超细氧化铝粉体;而低电流密度所得氧化铝颗粒团聚严重。主要原因是高电流密度使放电过程中产生的氢氧化铝晶体的介稳区宽度变窄所致。     

外推法与Lorentz-Lorenz公式在溶液密度测算中的应用

提出了一种计算不同温度、压强下溶液密度的新方法:先用液体状态方程外推法计算各种温度、压强下溶液各纯组分的密度,然后再利用Lorentz-Lorenz公式对各纯组分密度进行合成,从而获得相同条件下溶液的密度。以多种二元溶液为例进行了计算,结果表明:在多种摩尔分数、温度和压强下,计算值与实验测量值吻合得非常好,平均绝对偏差仅为0.005 3 g/cm3,最大绝对偏差也不超过0.011 8 g/cm3。表明该方法的计算值准确、可靠,这为科研和生产中,特别是高温、高压下,强腐蚀性、有毒性溶液密度的获得提供了方便,并大大降低了实验费用和测试的劳动强度。     

在共存相中饱和液体及饱和蒸气密度的计算

本文证实了Hanson计算方法的可能性,并应用了跨界压缩因子解决了Hanson方法中存在的缺点.计算饱和液体的密度,其误差在1%以内,本文尚提供了计算饱和蒸气的密度和混合液体的密度的方法.导出了一个计算汽化热的普遍公式.这些结果证明了在两相区域内可以应用对比状态的规律.     

Development of a slurry injection technique for continuous fibre ultra-high temperature ceramic matrix composites

A simple and effective slurry injection method for producing dense and uniform ultra-high ceramic matrix composites from preforms of high fibre density was developed. As this method is based on slurry injection the homogeneity is not constrained to small preform sizes; dense components of high fibre volume can be produced in theoretically any size and shape. Samples produced by this method demonstrated high and consistent densities, with the injection method obtaining densities an average 27% higher and 87% lower in variability when compared to conventional vacuum impregnation. Tomography demonstrated no bias in the ceramic powder distribution for samples produced by injection, whereas samples produced by vacuum impregnation alone displayed poor powder penetration to the centre of large samples. The new approach yielded composites that were as strong and/or more consistent in strength compared to vacuum impregnation. Thermo-ablative testing demonstrated significant improvements in protective capability for materials produced by this route.     

Monte Carlo simulation on the adsorption properties of carbon tetrachloride, neopentane, and cyclohexane in MCM-41

The adsorption properties of carbon tetrachloride, neopentane, and cyclohexane in MCM-41 with heterogeneous and cylindrical pores have been studied by using grand canonical ensemble Monte Carlo simulation. The adsorption isotherm, average potential of adsorbate, isosteric heat of adsorption, and number density of molecules in MCM-41 were calculated. The simulated isotherms were compared with experimental ones. Also, different adsorption behaviors in MCM-41 with pore diameter of 2.2 and 3.2 nm were discussed. The capillary-condensation pressure increased for a given adsorbate with an increase in pore diameter. The average densities of carbon tetrachloride, neopentane, and cyclohexane in the two different pores above the capillary-condensation pressure were smaller than the corresponding liquid densities by about 12%. The adsorbate molecules did not form the multilayer in pore below the capillary-condensation pressure. The number of adsorption layers of molecules was constant in a given pore for the three adsorbates above the capillary-condensation pressure. Carbon tetrachloride molecules in pore were also ordered along the pore axis.     

Heat and mass transfer in apple cubes in a microwave-assisted fluidized bed drier

In the present study, a pilot scale microwave assisted fluidized-bed dryer was designed and used to dry apple cubes. A model was developed to describe heat and mass transfer in apple cubes during drying in a combined microwave-assisted fluidized-bed dryer. A numerical solution based on the finite difference method was used to develop the model for moisture distribution and temperature variation of samples. The model was validated using experimental data, including average moisture content, center and surface temperature at various air temperatures and microwave power densities.     

Additive group contributions to density and glass transition temperature in polyurethanes

The method of additive properties was applied to the density and glass transition temperature, T_g, of linear amorphous polyurethanes. This method assumes that certain polymer properties result from the additive effects of unique constituent group properties and that these group properties are independent of their environment. To determine the component properties, 14 model polyurethanes were synthesized. The polymers contained only the following four groups: urethane, phenylene, methylene, and ether oxygen. The densities and T_g's of these polymers were measured. Using these measured values, the known composition of each polymer, and an appropriate additive from, the molar component properties were calculated using a matrix least-squares simultaneous fit algorithm. An error analysis confirmed that the component values were determined with greater accuracy than literature results. The densities ranged from 1.0 to 1.2 g/cm³ and measured versus predicted densities agreed to within 0.3% on the average. T_g's ranged from ?70°C to 70°C and measured versus predicted T_g's agreed to within 1.3% on the average.     

电压对钛阳极氧化薄膜形貌的影响

采用两步施加电压的阳极氧化方法制备了具有不同形貌的氧化钛薄膜,使用扫描电子显微镜对在不同初始电压、不同放电电压下制备的氧化钛薄膜的形貌进行了分析。结果表明,在相同的放电电压下,通过改变初始电压可以制备出具有不同平均孔径和平均孔密度的阳极氧化钛薄膜;在一定的初始电压下,放电电压有一定的极限值,超过这个极限值电极将被烧毁;在初始电压60V,放电电压140V条件下制备的阳极氧化钛薄膜的纳米孔最规整、分布最均匀、孔径最均一。     

The steady-state behavior of multivariate exponentially weighted moving average control charts

Abstract

Multivariate exponentially weighted moving average (MEWMA) charts are popular, handy, and effective procedures to detect distributional changes in a stream of multivariate data. For doing appropriate performance analysis, dealing with the steady-state behavior of the MEWMA statistic is essential. Going beyond early papers, we derive quite accurate approximations of the respective steady-state densities of the MEWMA statistic. It turns out that these densities could be rewritten as the product of two functions depending on one argument only that allows feasible calculation. For proving the related statements, the presentation of the noncentral chi-square density deploying the confluent hypergeometric limit function is applied. Using the new methods it was found that for large dimensions, the steady-state behavior becomes different from what one might expect from the univariate monitoring field. Based on the integral equation driven methods, steady-state and worst-case average run lengths are calculated with higher accuracy than before. Eventually, optimal MEWMA smoothing constants are derived for all considered measures.     

硬球链流体在平板和硬球表面分布的密度泛函理论

采用Yethiraj和Woodward的密度泛函理论方法,结合胡英和刘洪来等发展的硬球链流体状态方程,得到了自由连接硬球链流体在平板狭缝中和球形固体颗粒表面附近的密度分布表达式,并计算了在两平行壁所组成的狭缝中和直径大小不同的球形固体颗粒周围硬球链分子的链节密度分布.理论计算结果与作者采用Dickman 和Hall 的方法进行Monte Carlo计算机模拟结果非常吻合.颗粒直径对链状分子的密度分布有一定的影响,随着固体颗粒直径的增加,靠近颗粒表面附近的链节密度降低.     

Extending the Capabilities of Single Particle Mass Spectrometry: II. Measurements of Aerosol Particle Density without DMA

Particle density is an important and useful property that is difficult to measure because it usually requires two separate instruments to measure two particle attributes. As density measurements are often performed on size-classified particles, they are hampered by low particle numbers, and hence poor temporal resolution. We present here a new method for measuring particle densities using our single particle mass spectrometer, SPLAT. This method takes advantage of the fact that the detection efficiency in our single particle mass spectrometer drops off very rapidly as the particle size decreases below 100 nm creating a distinct sharp feature on the small particle side of the vacuum aerodynamic size distribution. Thus, the two quantities needed to determine particle density, the particle diameter and vacuum aerodynamic diameter, are known. We first test this method on particles of known compositions and densities to find that the densities it yields are accurate. We then apply the method to obtain the densities of particles that were characterized during instrument field deployments. We illustrate how the method can also be used to measure the density of chemically resolved particles. In addition, we present a new method to characterize the instrument detection efficiency as a function of particle size that relies on measuring the mobility and vacuum aerodynamic size distributions of polydisperse spherical particles of known density. We show that a new aerodynamic lens used in SPLAT II improves instrument performance, making it possible to detect 83 nm particles with 50% efficiency.     

CORRELATIONS FOR ESTIMATING THE DIFFUSIVITIES OF NITROUS OXIDE IN AQUEOUS SOLUTIONS OF DIETHANOLAMINE AND N-METHYLDIETHANOLAMINE AND THE SOLUTION DENSITIES AND VISCOSITIES

Experimentally measured diffusion coefficients of nitrous oxide in water, aqueous solutions of diethanolamine, aqueous solutions of N-methyldiethanolamine, and aqueous blends of diethanolamine and N-methyldiethanolamine have been used to arrive at a modified Stokes-Einstein correlation by using nonlinear least-squares filling. The average deviation of this correlation from the experimentally measured diffusion coefficients is about 13%. Comparisons are also made to the estimated diffusivities of nitrous oxide in the amine solutions which are obtained from the Wilke-Chang and Hayduk-Minhas correlations. The average deviations of the Wilke-Chang and Hayduk-Minhas correlations from the measured values are about 13% and 19%, respectively. Correlations were also presented for estimating the solution densities and viscosities. Predicted values for the solution densities and viscosities from these correlations deviate from experimentally measured values on average by 1.2% and 4.8% respectively.     

匀泡剂对硬质聚氨酯泡沫孔径及冲击性能的影响

通过改变配方中匀泡剂的用量，制备了一系列具有相同密度的硬质聚氨酯泡沫塑料(RPUF)。使用二维图像分析方法对其孔径进行了表征，结果显示其孔径分布在210～624μm范围。研究了冲击性能与孔径之间的关系，发现材料的冲击强度随孔径增大呈线性下降的趋势，同时其脆性增大而韧性降低。     

Selective synthesis of TiO2 nanopowders

Nanosized anatase, rutile, brookite, and mixtures of these materials taken in different ratios are synthesized using the detonation method with variations in the densities and ratios of explosives composed of the TiO₂ precursor, NH₄NO₃, and C₃H₆N₃(NO₂)₃. It is shown that the phase composition, the phase content, and the average particle size of TiO₂ nanopowders depend on the composition of the explosive mixtures and their densities. When the weight ratio between the C₃H₆N₃(NO₂)₃ compound and the TiO₂ precursor lies in the range 0.695–1.270, the average size of rutile particles is larger than that of anatase particles by a factor of approximately two.     

Additivity rules for the prediction of the density of aqueous solutions containing mixed-type solutes

Densities of ternary aqueous solutions of mixed-type solutes have been measured at 25°C to high concentration. Two methods, the one isopycnotic (same density) and the other based on ionic strength, have been used to predict the densities of these solutions. The isopycnotic method proved to be the more accurate. The two methods have been applied in reverse to calculate densities of supersaturated binary aqueous solutions of NaCl and KCl. The methods give concordant results for each salt Deviation parameters for the methods are presented, with a discussion of their use.     

Additivity rules for the prediction of the density of aqueous solutions containing mixed-type solutes

Densities of ternary aqueous solutions of mixed-type solutes have been measured at 25°C to high concentration. Two methods, the one isopycnotic (same density) and the other based on ionic strength, have been used to predict the densities of these solutions. The isopycnotic method proved to be the more accurate. The two methods have been applied in reverse to calculate densities of supersaturated binary aqueous solutions of NaCl and KCl. The methods give concordant results for each salt Deviation parameters for the methods are presented, with a discussion of their use.     

Improved electrochemical CO removal via potential oscillations in serially connected PEM fuel cells with PtRu anodes

The polarization performance of two PEM fuel cells (with anode PtRu/C catalyst) connected either in parallel or serial, was compared to the performance of a single PEM fuel cell in galvanostatic operation using CO-free H₂ or 200 ppm CO-containing H₂ stream as anode feed at ambient temperature. Spontaneous potential oscillations were observed experimentally for the coupled configuration with two cells connected in serial or parallel using CO-containing H₂ feed at various current densities applied. The potential oscillations are ascribed by the dynamic CO adsorption and subsequent electrochemical CO oxidation on the anode. The measured anode outlet CO concentration was found to decrease with the order: single cell > parallel cells > serial cells at various current densities and anodic flow rates. The low anode outlet CO concentration (<10 ppm) at high current densities applied showed that CO in the anode feed was removed efficiently by the electrochemical CO oxidation occurring on the PtRu anode. The anode outlet CO concentration decreased as follows: a single cell > the parallel cells > the serial cells at broad range of current densities and anodic flow rates. The highest CO conversion and the highest average power output at equal hydrogen recovery degree were obtained with serially coupled fuel cells.