Electron attenuation lengths in fullerene and fullerides

Using X-ray photoemission measurements, we have determined the attenuation length of C 1s photoelectrons in C₆₀ film to be 21.5 Å with the incident photon energy of Mg Kα radiation. The inelastic mean free path calculated with the TPP-2M algorithm coincides fairly well with the experimentally determined attenuation length, indicating the validity of the algorithm to fullerene and fullerides. The inelastic mean free paths for some fullerides, i.e. K₃C₆₀, K₆C₆₀, Ba₄C₆₀, Sm_2.75C₆₀ and Sm₆C₆₀ are calculated to help the quantitative analyses of the photoemission spectra for these compounds.     

滤波器的选择与安装

滤波与屏蔽、接地共同构成完善的电磁干扰防护,介绍如何正确选择与安装滤波器,利用其衰减特性,使其对电磁干扰的抑制作用达到预期的效果。     

Temporal gradients in microfluidic systems to probe cellular dynamics: A review

Microfluidic devices have found a unique place in cellular studies due to the ease of fabrication, their ability to provide long-term culture, or the seamless integration of downstream measurements into the devices. The accurate and precise control of fluid flows also allows unique stimulant profiles to be applied to cells that have been difficult to perform with conventional devices. In this review, we describe and provide examples of microfluidic systems that have been used to generate temporal gradients of stimulants, such as waveforms or pulses, and how these profiles have been used to produce biological insights into mammalian cells that are not typically revealed under static concentration gradients. We also discuss the inherent analytical challenges associated with producing and maintaining temporal gradients in these devices.     

非线性兰姆波在厚度缓慢变化和衰减下的特性分析

该文运用解析的方式推导了考虑声波衰减时兰姆波二次谐波的累积和传播规律,并用半解析方式将该理论推广到缓慢变厚度板的情况。由于色散特性,兰姆波二次谐波和基频波相速度不匹配,传播通常会产生拍频效应,使得二次谐波的振幅沿着传播距离周期性的归零。当考虑声波衰减或板的厚度缓慢变化的情况时,拍频效应将不再严格地被满足。二次谐波的振幅依然会沿着传播距离而振荡,但不会归零。该研究可以用于分析如何高效地激发和接收兰姆波的二次谐波,表征和评估不同厚度变化的结构中的微观结构损伤。     

Use of early acoustic emission to evaluate the structural condition and self-healing performance of textile reinforced cements

Textile reinforce cements (TRC) are innovative materials that are used for repair of existing structures or recently as stand-alone lightweight structural members. Fracture and thermal behavior of these materials are very complicated due to multiple failure modes. The undergoing processes cannot be described by the simple constitutive equation. In this perspective the contribution of monitoring techniques is crucial. Acoustic emission (AE) is used to check the behavior of TRC beams and plates in different states of structural health: intact, thermally pre-cracked and self-healed by polymer powder. This is the first time that the AE behaviors of intact, cracked and self-healed TRC laminates are compared in literature.     

墙体对微波脉冲的衰减特性

 研究墙体对微波脉冲的衰减特性，测量了微波脉冲垂直入射墙体后的脉冲信号，经过计算得到普通砖混墙、普通钢筋混凝土墙、钢筋网混凝土墙对窄带微波和超宽带微波的衰减值；分析了微波脉冲穿越不同墙体的频谱变化。研究结果表明，普通钢筋混凝土墙和普通砖混墙对窄带微波脉冲的衰减为0.342～0.699 dB/cm，对超宽带信号的衰减为0.134～0.183 dB/cm。钢筋网混凝土墙（厚65 cm）对超宽带信号的衰减较大（29.07～45.79 dB），同时使穿透墙体的超宽带信号频率分布向高频位移。     

Rain Rate Statistics and Fade Distribution of Millimeter Waves in Indian Continents

For any communication service operating in the Microwave/ Millimeter wave region, statistical information characterising the attenuation due to rain along satellite slant path would be required for the design of satellite communication links and for the broadcasting network above 20 Ghz. It is necessary to have a prior knowledge of the probability of exceeding different levels of rain attenuation in order to design appropriate fade margins into systems and establishing estimates of the year to year variability of rain fade margin for particular geographic regions of India so that the communication system reflects the extremes of these variabilities. Direct measurement of beacon signals from geostationary satellites have been a mean to determine the above information and experiments can be pursued with satellite such as INSAT. [1]Attenuatiuon of Millimeter Waves by rainfall restricts the path length of a communication system. A knowledge of the rain attenuation at such frequencies is therefore desirable in designing a reliable communication system. Signal level fading over line-of-sight links strongly depends on the hop length, frequency and climate. For short hops, the probability of occurance of deep fades becomes diminishingly small. However, since an extended hop length is possible for regions with little rain activity, clear weather fading can affect the link reliability in a similar way ti a rain.[2]     

Experimental study of wave dispersion and attenuation in concrete

Results from an experimental study concerning wave propagation in cementitious materials are presented in this paper. Narrow band pulses at several frequencies were introduced into specimens of cement paste, mortar and concrete allowing direct measurement of longitudinal wave velocities and amplitude for each frequency. It is shown that aggregate content play an important role in wave propagation increasing considerably the wave velocity, while the aggregate size seems to control the attenuation observed. Slight velocity variations observed with frequency are discussed in relation to the degree of inhomogeneity of the materials.     

Attenuation and depolarization of millimeter waves due to incoherent scattering in tropical rainfall

The radiative transfer equation has been applied to investigate the effects of multiple scattering on communication at the millimeter wavelength in tropical rain. Horizontal linearly polarized incident waves are assumed. The scattering characteristics are calculated using the Mie theory at frequencies of 16, 34.8, 82, 140 and and at rainfall rates of 12.5, 50 and . Some results of the directional patterns of the incoherent intensities at various points in the rain medium are presented. The frequency dependence of the propagation distance, attenuation coefficient and the variation of incoherent specific intensities in tropical rainfall are investigated. The incoherent power at the receiver relative to the coherent power, which is important in assessing the quality of signal in the receiver, is also investigated. It is observed that the maximum incoherent copolarized received power is about in tropical rainfall at rainfall rate. Also the difference between the copolarized incoherent power in tropical and temperate rainfall is about at frequencies higher than , and about for the cross-polarized incoherent power components. It may therefore be safely said that multiple scattering may not result in serious degradation of the coherent wave component even in the most intense tropical precipitation. It also appears that it is independent of regional rainfall climatology.     

Study on the lifetime and attenuation properties of microbubbles coated with carboxylic acid salts

Four kinds of surfactants, sodium laurate, sodium myristate, sodium palmitate and sodium oleate were used to study the effects of surfactant coatings on the lifetime and attenuation of microbubbles. The changes in the size distribution of microbubbles prepared with these surfactants in saline were measured with a Coulter Multisizer (Coulter Electronics Ltd., Luton, UK). Frequency characteristics of ultrasonic attenuation of the microbubble suspensions were measured between 400 kHz and 6 MHz. From the changes in attenuation in the microbubble suspensions over time, it was found that the lifetime of microbubbles in a suspension also depends on the frequency of the irradiating ultrasound. The effect of surfactants on the frequency characteristics of attenuation was also studied, and characteristics of the surfactant coating, including shell elasticity and shell friction parameters were calculated from the measurement results. Microbubbles produced with sodium palmitate had the longest lifetime and the smallest average size. The shell had very little effect on the ultrasonic properties of microbubbles produced with sodium palmitate, suggesting that the sodium palmitate microbubbles behaved ultrasonically as free microbubbles.