转子轴承系统振动响应谱的仿真研究

在转子轴承系统振动信号处理中,针对平稳信号的传统傅里叶变换精度较低、快变启动过程的非平稳信号频谱分析方法较复杂的问题,本文仿真构造了两类响应信号。通过对比给定信号参数与信号识别参数的误差研究了几种谱分析方法或过程的简便性和准确性。对转子系统振动平稳信号离散频谱分析时存在的误差进行了定量分析,利用比例插值法对误差进行校正,开发了高精度谱分析测试软件;分析了转子轴承系统快变过程非平稳振动信号的特征,探索了一种将t时空域非平稳信号转变为tn时空间域平稳信号的办法或过程,然后结合比例插值校正法对其进行频谱分析,再返回到t时空域获得某时刻的谱特征参数;构造了转子系统振动仿真信号检验了上述过程的准确性。研究结果表明:比例插值法提取的谱特征数据近乎与仿真信号设定值相等;针对本文构造的快变过程非平稳仿真信号,利用本文给出的谱分析过程产生的频率误差最大值为0.47%,幅值误差最大值为0.2%。本文的仿真研究为提出和考证新的谱分析方法提供了手段。     

Molecular distribution and seasonal variation of hydrocarbons in PM2.5 from Beijing during 2006

Normal (n)-alkanes and polycyclic aromatic hydrocarbons (PAHs) in PM_2.5 were collected from Beijing in 2006 and analyzed using a thermal desorption-GC/MS technique. Annual average concentrations of n-alkanes and PAHs were 282 ± 96 and 125 ± 150 ng/m³, respectively: both were highest in winter and lowest in summer. C₁₉–C₂₅ compounds dominated the n-alkanes while benzo[b]fluoranthene, benzo[e]pyrene, and phenanthrene were the most abundant PAHs. The n-alkanes exhibited moderate correlations with organic carbon (OC) and elemental carbon (EC) throughout the year, but the relationships between the PAHs, OC and EC differed between the heating and non-heating seasons. The health risks associated with PAHs in winter were more than 40 times those in spring and summer even though the PM_2.5 loadings were comparable. Carbon preference index values (<1.5) indicated that the n-alkanes were mostly from fossil fuel combustion. The ratios of indeno[123-cd]pyrene to benzo[ghi]pyrelene in summer and spring were 0.58 ± 0.12 and 0.63 ± 0.09, respectively, suggesting that the PAHs mainly originated from motor vehicles, but higher ratios in winter reflected an increased influence from coal, which is extensively burned for domestic heating. A comprehensive comparison showed that PAH pollution in Beijing has decreased in the past 10 years.     

碱在过渡金属催化的有机化学反应中的作用

碱, 包括无机碱和有机碱, 在过渡金属催化的有机反应中发挥着重要作用. 本文概括地介绍和讨论相关文献中碱的作用机制. 碱的作用与许多因素有关, 包括: 碱性、溶解度、电离度、溶剂、聚集度、金属离子大小、金属离子Lewis酸性、金属离子的“软硬”度、阴离子的大小、阴离子的配位作用等. 碱可用于攫取质子、中和反应体系中的酸、活化催化剂、促进催化剂再生等, 碱中金属阳离子的作用主要集中在其对碱在有机溶剂中溶解度的影响和其与底物或溶剂间相互作用力的强弱上, 碱中阴离子的作用主要表现在离子与金属的配位方式和稳定性上; 有机碱和无机碱的主要区别体现在溶解度和空间位阻的不同上. 另外, 商品碱中极少量的过渡金属杂质也有可能对反应产生影响.     

A population-level model from the microscopic dynamics in Escherichia coli chemotaxis via Langevin approximation

Recent extensive studies of Escherichia coli (E. coli) chemotaxis have achieved a deep understanding of its mi- croscopic control dynamics. As a result, various quantitatively predictive models have been developed to describe the chemotactic behavior of E. coli motion. However, a population-level partial differential equation (PDE) that rationally incorporates such microscopic dynamics is still insufficient. Apart from the traditional Keller-Segel (K-S) equation, many existing population-level models developed from the microscopic dynamics are integro-PDEs. The difficulty comes mainly from cell tumbles which yield a velocity jumping process. Here, we propose a Langevin approximation method that avoids such a difficulty without appreciable loss of precision. The resulting model not only quantitatively repro- duces the results of pathway-based single-cell simulators, but also provides new inside information on the mechanism of E. coli chemotaxis. Our study demonstrates a possible alternative in establishing a simple population-level model that allows for the complex microscopic mechanisms in bacterial chemotaxis.     

Boron/nitrogen pairs Co-doping in metallic carbon nanotubes: a first-principle study

By using the first-principles calculations, the electronic structure and quantum transport properties of metallic carbon nanotubes with B/N pairs co-doping have been investigated. It is shown that the total energies of metallic carbon nanotubes are sensitive to the doping sites of the B/N pairs. The energy gaps of the doped metallic carbon nanotubes decrease with decreasing the concentration of the B/N pair not only along the tube axis but also around the tube. Moreover, the I--V characteristics and transmissions of the doped tubes are studied. Our results reveal that the conducting ability of the doped tube decreases with increasing the concentrations of the B/N pairs due to symmetry breaking of the system. This fact opens a new way to modulate band structures of metallic carbon nanotubes by doping B/N pair with suitable concentration and the novel characteristics are potentially useful in future applications.     

Self-calibration of a phase-shifting adapter for Fizeau interferometers

Phase-shifting interferometers with three piezoelectric transducers (PZTs) on their phase-shifting adapter (PSA) are widely used in precision measurement. The guarantee of the PSA to stretch as a flat is very important to the accuracy of the measurement. In this paper, we propose a systematic self-calibration method to calibrate the different displacements of the PZTs so that the PSA can be moved evenly as a flat. The proposed method uses only the interference patterns to calibrate the three PZTs and information about the exact position of the PZTs and the instrument parameters of the interferometer are not required. Experimental results show that, after calibration by the proposed method, the deviation of the measured phase shift error could be reduced about 43.1 times from 3.492 to 0.081°.     

A method to obtain pulse contrast on a single shot

A novel method for obtaining a single shot multi-point high dynamic range pulse contrast measurement is presented. We use Dammann gratings to generate multiple beamlets by division of amplitude on ultrashort laser pulses. The analysis results show that this method can achieve high dynamic range in pulse contrast measurement on a single shot by using photomultiplier tube （PMT） detectors and the long working distances to minimize cross talk between channels. Some distortion of pulse shape is also analyzed detailedly with the Dammann grating and its compensation grating, which may degrade the pulse contrast measurement in some degree by pulse stretching and spectrum clipping.     

Ab initio studies on n-type and p-type Li4Ti5O12

This paper studies the structure and electronic properties of Li4Ti5O12, as anode material for lithium ion batteries, from first principles calculations. The results suggest that there are two kinds of unit cell of Li4Ti5O12: n-type and p-type. The two unit cells have different structures and electronic properties:the n-type with two 16d site Li ions is metallic by electron, while the p-type with three 16d Li ions is metallic by hole. However, the Li4Ti5O12 is an insulator. It is very interesting that one n-type cell and two p-type cells constitute one Li4Ti5O12 supercell which is insulating. The results show that the intercalation potential obtained with a p-type unit cell with one additional electron is quite close to the experimental value of 1.5 V.     

Luminescence spectra of stilbene-3 doped lead-tin-fluorophosphate glass excited by VUV-UV synchrotron radiation

The stilbene-3 doped lead-tin-fluorophosphate glasses were successfully prepared and the luminescence spectra obtained from VUV-UV synchrotron radiation were investigated. The emission peaked about 440 nm from the undoped glass is observed at low temperature but quenches at room temperature. The doped glass exhibits strong characteristic emission of stilbenen-3 with 467 and 436 nm Gaussian bands under ultraviolet 250-330 nm excitation (characteristic excitation of stilbene-3) but weak emission under vacuum ultraviolet 190 nm excitation (host excitation). It is suggested that the energy transfer from host to the stilbene-3 dye can occur but the efficiency is still fairly low.     

Solid-phase microextraction under controlled agitation conditions for rapid on-site sampling of organic pollutants in water

An electric drill coupled with a solid-phase microextraction (SPME) polydimethylsiloxane (PDMS) fiber or a PDMS thin film was used for rapid sampling of polycyclic aromatic hydrocarbons (PAHs) in aqueous samples. Laboratory experiments demonstrated that the sampling rates of SPME fiber and thin film can be predicted theoretically. Compared with the SPME fiber, the PDMS thin film active sampler exhibited a higher sampling rate and much better sensitivity due to its higher surface-to-volume ratio and its larger extraction phase volume. The amount of the analytes extracted by the thin film was around 100 times higher than those obtained by fiber, for both 5 min rapid sampling and equilibrium extraction. A new thin film active sampler was then developed for rapid on-site water sampling. The sampling kit included a portable electric drill, a copper mesh pocket, a piece of thin film, and a liner. Laboratory experiments indicated that the sampling remained in the linear uptake phase with this sampler to 8 min for the PAHs. Field test illustrated that this novel sampler was excellent for rapid on-site water sampling due to its short sampling period, high sampling efficiency and durability The thin film sampling kit facilitates on-site sampling, sample preparation, storage and transport. This new sampler is more user-friendly and easier to commercialize than previous samplers.