Stabilization of transmission function: theory for an ultrathin endoscope of one multimode fiber

Propagation of a monochromatic light through a multimode fiber is considered in connection with the problem of image transmission. We discuss the variations of throughput scrambling, i.e., of the pixel-to-pixel Greens' function for transmission under the bends of the fiber axis. Two main statements are elaborated theoretically, which in principle allow the stabilization of the throughput scrambling. First, if the deformations are smooth enough, the modes are adiabatically modified without intermixing. Second, the variation deltaphi(m,n) of the phase of a mode M (m,n) can be expressed as a power series in a small parameter (rho/R)(2), where rho is the core radius and R(-1) is the curvature of bending. A scheme for the compensation of those phase shifts is suggested based on prebending of the fiber with subsequent controllable adjustment of that prebending.     

Magnetoacoustic measurements on pure and Zn doped YBCO crystals

Investigations on the temperature dependence in the range from 4 to 100 K of the ultrasonic attenuation (T, B) and the relative change Δv(T,B)/v of the sound velocity for various applied magnetic fields have been made. The ultrasonic attenuation was measured by the pulse-echo technique and the relative change of the sound velocity Δv/v by a phase-comparison method. The measurements serve for the examination of the elastic behaviour of the flux line lattice (FLL) of pure and Zn doped YBCO crystals and YBCO/Ag composites. These crystals are important for applications as permanent magnets. In the framework of the thermally assisted flux flow model (TAFF) the activation energies U(B) were deduced from the ultrasonic data.     

Picosecond grating spectroscopy for characterizing the acoustic properties of biological material

We present measurements of the ultrasound attenuation and sound velocity of a number of liquids, transparent biological materials (the vitreous and lens of the bovine eye), and biological fluids (whole blood) at frequencies between 925 and 1020 MHz by using a picosecond thermal grating. Sound velocity and attenuation measurements of liquids (e.g., methanol and ethanol) agree very well with those reported in the literature. The sound velocity in the biological materials studied also agrees with the reported values in the literature. In contrast, the attenuation coefficients measured for biological materials, 2000-5000 dB/cm, are much higher than would be extrapolated from published low-frequency data.     

Experimental investigation of computed tomography sound velocity reconstruction using incomplete data

An approach for reconstructing the sound velocity distribution in the breast was previously proposed and verified by simulations, and the present study investigated the approach experimentally. The experimental setup comprised a 5-MHz, 128-channel linear array, a programmable digital array system, a phantom containing objects with differing physical properties, and a computer. The array system was used to collect channel data for simultaneous B-mode image formation and limited-angle tomographic sound velocity reconstruction. The phantom was constructed from materials mimicking the following tissues in the breast: glandular tissue, fat, cysts, high-attenuation tumors, and irregular tumors. The sound velocities in these materials matched those in the corresponding real tissues. The imaging setup is similar to that of x-ray mammography, in which a linear array is placed at the top of the breast and a metal plate is placed at the bottom for reflecting sound waves. Thus, both B-mode images and the sound velocity distribution can be acquired using the same setup. An algorithm based on a convex programming formulation was used to reconstruct the sound velocity images. By scanning the phantom at different positions, nine cases were evaluated. In each of the nine cases, the image object comprised a background (glandular tissue) and one or three regions of interest (fat, tumor, or cyst). The sound velocity was accurately estimated in the nine cases evaluated, with sound velocity errors being less than 5 m/s in 8 of 11 regions of interest. Thus, obtaining the sound velocity distribution is feasible with a B-mode imaging setup using linear arrays. Knowledge of the sound velocity distribution in the breast can be used to complement B-mode imaging and to enhance the detection of breast cancer.     

ZK61S镁合金微弧氧化膜层的结构及耐腐蚀性能

为了提高ZK61S镁合金的耐腐蚀性能,采用微弧氧化方法以不同电压(300,380,450 V)在ZK61S镁合金表面制备氧化膜并进行封孔处理。利用金相显微镜、扫描电镜、X射线衍射仪分析膜层的形貌、结构和组成;通过腐蚀电位试验、中性盐雾腐蚀试验及抗剥落腐蚀试验进行耐腐蚀性能考核。结果表明:微弧氧化呈现疏松多孔形态且均匀覆盖于基材表面,主要由Mg、MgO和Al_2Si_2O_5(OH)4相组成;微弧氧化处理后试样的腐蚀电位显著提升,且380 V所得微弧氧化试板的腐蚀电位达到-881.53 m V,经过408 h的中性盐雾腐蚀试验后的腐蚀速率为0.012g/(m~2·h),耐蚀性能比未进行表面处理的基材提高了88倍;经封孔处理的微弧氧化试板经过456 h的中性盐雾腐蚀试验后腐蚀速率降低到0.003 g/(m~2·h);封孔处理使微弧氧化膜的抗剥落腐蚀性能由微弧氧化后的EB级提升到EA级。     

Improved field emission properties of thiolated multi-wall carbon nanotubes on a flexible carbon cloth substrate

In this paper we report the observation of enhanced field emission properties from thiolated multi-wall carbon nanotubes (MWCNTs) produced by a simple and effective two-step chemical surface modification technique. This technique implements carboxylation and thiolation on the MWCNTs synthesized by microwave plasma chemical vapor deposition (MPCVD) on the flexible carbon cloth substrate. The resulting thiolated MWCNTs were found to have a very low threshold field value of 1.25?V?μm(-1) and a rather high field enhancement factor of 1.93 × 10(4), which are crucial for applications in versatile vacuum microelectronics.     

Thermal conductivity and sound velocity measurements of plasma enhanced chemical vapor deposited a-SiC:H thin films

We report ultrafast optical measurements of the thermal conductivity and longitudinal sound velocity for a-SiC:H thin films deposited by plasma enhanced chemical vapor deposition (PECVD). Porous and non-porous films with mass densities ranging from 1.0-2.5 g/cm³ were obtained by intentionally varying the PECVD process conditions. The longitudinal sound velocities for these materials as determined by picosecond ultrasonics ranged from 2370 m/s to 10460 m/s, and the Young's modulus determined from the sound velocity measurements ranged from 5-200 GPa. Time domain thermoreflectance measurements determined the thermal conductivity to range from 0.0009 W/cmK to 0.042 W/cmK.     

Sound velocity and sound attenuation measurements by dynamic light scattering

Dynamic light scattering (photon correlation spectroscopy) has been applied to the determination of sound velocity and sound attenuation from the Brillouin component of the frequency spectrum scattered from a fluid sample transversed by a laser beam. In this paper the time-resolved determination of the Brillouin component is described. The measurement of the linewidth allows an accurate determination of the sound attenuation, while the central frequency is connected to the adiabatic sound velocity. Sound attenuation and sound velocity measurements are presented for the new refrigerant pentafluorethane (R125). The accuracy and possible systematic errors of this technique are discussed and compared to those obtained from other spectroscopic and acoustic techniques.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado. U.S.A.     

Field-induced strain behavior for potassium sodium bismuth titanate ceramics

Data are reported for the dielectric, piezoelectric, electrostrictive, and ferroelectric properties of potassium-substituted sodium bismuth titanate, [(K(x)Na(1-x))(0.5)Bi(0.5)]TiO3. For the morphotropic phase boundary composition x = 0.2, relaxor-type behavior was observed at room temperature with piezoelectric (effective d(333) = 325 x 10(-12) m/V) and ferroelectric properties (P(R) = 25 microC/cm(2), E(C) = 30 kV/cm). A transition to a relatively frequency-independent, diffuse phase transformation region occurred with increasing temperature, with no remanent strain or coercive field. Above the transition temperature, the field-induced strain was consistent with contributions from electrostriction and field induced piezoelectricity (M(3333) = 1.9 x 10(-16) m2/V2 and d333 = 81 x 10(-12) m/V at 100 degrees C). Information is given for the temperature dependence of properties, e.g., 0.14% strain induced at 50 kV/cm at 200 degrees C. Higher potassium content x = 0.6 stabilized the ferroelectric piezoelectric region to temperatures above 200 degrees C, with a relatively stable d(333) = 150-145 x 10-12 m/V between 25 degrees C and 200 degrees C. Pb-free KNBT ceramics appear competitive with PZT, especially for higher temperature electromechanical applications.     

超声水浸法高精度测量玻璃材料声速的研究

材料的声速与其杨氏模量和密度有关,通过测量材料的声速可以评价材料的特性。相关研究表明,超低膨胀玻璃的声速是评价其热膨胀系数的关键参数,通过测量声速可以实现对超低膨胀玻璃热膨胀系数的间接无损测量。针对市售的超声声速测量仪器存在系统复杂且不易集成化的问题,结合高精度的数据采集卡设计了超声信号采集显示软件,并基于超声水浸脉冲反射法搭建了高精度的声速测量系统,系统结构简单,操作方便且较易集成化。采用该测量系统对制备的超低膨胀玻璃样品的声速进行了测量,结果表明该系统具有较高的声速测量分辨率,声速分辨率可达为0.2 m.s^-1,为使用超声声速法高精度测量超低膨胀玻璃的热膨胀系数奠定了研究基础。     

A piezoelectric, flexural-disk, neutrally buoyant, underwater accelerometer

A piezoelectric, flexural-disk accelerometer for underwater use is composed of two PZT-5A lead zirconate-titanate disks that are bonded to an aluminum substrate. The substrate is edge-supported inside an aluminum housing. The housing is enclosed in syntactic foam so that the sensor is neutrally buoyant. The overall height is 1.0 in. (26 mm), the overall diameter is 1.9 in. (49 mm), and the total mass is 0.054 kg. With 25 ft (7.6 m) of (230 pF/m) cable attached, the sensitivity is -42 dB re 1 V-s(2)/m (-22 dB re 1 V/g), the capacitance is 5.0 nF, and the resonance frequency is 11 kHz. When used in conjuction with a Micro Networks MN3210 preamplifier, the spectral noise-equivalent acceleration floor is approximately -171 dB re 1 m/s(2)- radicalHz (-151 dB re 1 g/ radicalHz) at 5 kHz.     

Ultrasound attenuation in cylindrical micro-pores: Nondestructive porometry of ion-track membranes

The propagation of ultrasonic waves in the cylindrical micro-pores (pore diam. <1 μm) of ion-track membranes (ITMs) is studied. This membrane fabrication technique provides unique possibilities to obtain cylindrical micro-pores with a very high degree of accuracy in pore shape, size, and orientation. Several ITMs were specially produced having the same pore diameter, orientation, and geometry, but different thickness. Porosity, pore diameter, and shape were determined using scanning electron microscopy, and then the coefficient of ultrasound transmission was measured using air coupling and spectral analysis. These experimental conditions permit us to eliminate the influence of the boundary conditions and to achieve a strong decoupling between the fluid filling the pores and the solid constituent of the membrane. Hence, the velocity and the attenuation coefficient for ultrasound propagation in the pores can be measured. These parameters are compared with the predictions made by conventional theories for sound propagation in porous media and in cylindrical channels. The conclusions of this work provide a better understanding of wave propagation in micro-pores and establish the basis of an ultrasonic porometry technique for ITMs.     

Tb0.3Dy0.7(Fe,M)1.95合金的声速和弹性模量与磁化磁场的关舷

研究了Tb0.3Dy0.7(Fe,M)1.95合金的声速υ和弹性模量E与磁化磁场的关系,首次观测到出现Vmin和Emin值的磁场与出现Xmax和d33的磁场是一致的,均在20kA/m左右,达在工程上有重要意义,但这种关系的物理机制尚不清楚。     

High performance horizontal gate-all-around silicon nanowire field-effect transistors

Semiconducting nanowires have been pointed out as one of the most promising building blocks for submicron electrical applications. These nanometer materials open new opportunities in the area of post-planar traditional metal-oxide-semiconductor devices. Herein, we demonstrate a new technique to fabricate horizontally suspended silicon nanowires with gate-all-around field-effect transistors. We present the design, fabrication and electrical measurements of a high performance transistor with high on current density (～150?μA?μm(-1)), high on/off current ratio (10(6)), low threshold voltage (～?-?0.4?V), low subthreshold slope (～100?mV /dec) and high transconductance (g(m)?～?9.5?μS). These high performance characteristics were possible due to the tight electrostatic coupling of the surrounding gate, which significantly reduced the Schottky-barrier effective height, as was confirmed experimentally in this study.     

Experimental study of the ultrasonic velocity in liquid cesium at high temperatures and pressures

The sound velocity in liquid cesium under pressures up to 60 MPa and temperatures to 1500 K is measured using a modified-pulses phase-sensitive technique. The sound velocity (at frequency 10 MHz) is determined by means of the pulse propagation time measurement through the cesium sample. The experimental error is 0.2%. The results obtained are discussed.Paper submitted to the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

基于声速核查方法的天然气能量计量标准装置

为解决天然气能量测量中组分数据测量频率低带来的测量周期内发热量变化无法核查的问题,利用声速核查天然气组分变化导致发热量、压缩因子的变化,研究了声速核查能量模型,搭建了具备10 MPa、2 000 m³/h测量能力的天然气能量计量标准装置,该装置测量能量的时间由原有的分钟级提升到秒级。通过天然气实流测量,对天然气能量计量标准装置的不确定度进行系统评定,其相对扩展不确定度为0.33%(k=2)。     

Liquid4He. I. Binding energy and excitation energy spectrum from two-body correlations

As a first step in a theoretical study of the properties of liquid ⁴ He we have calculated the binding energy from two-body correlations in the system. Using an effective interaction or reaction matrix obtained by a modified Brueckner theory, low-temperature properties such as the binding energy, the elementary excitation energy spectrum, and the velocity of first (ordinary) sound are calculated or estimated. For simplicity we use the approximation of a reference energy spectrum with a quadratic momentum dependence for the input single-particle energy spectrum, which in principle should be fitted to self-consistent single-particle energies. The intermediate-state potential energies are, however, chosen to be equal to zero. Hence, the three-body energy contribution and also higher order energy contributions must be estimated by separate calculations. A self-consistent solution is obtained through the depletion of the zero-momentum state, which is also calculated. The calculations are done for two different two-body potentials, an Yntema-Schneider potential given by Brueckner and Gammel, and a Frost-Musulin potential given by Bruch and McGee. The theoretical results are –3.1 to –4.0°K for the binding energy, 39–44% for the depletion, and 176–217 m/sec for the sound velocity. The corresponding experimental results are –7°K, 83%, and 238.3 m/sec, respectively, i.e., the difference is generally within a factor of two. The agreement with experimental results is reasonably good (or bad), especially since three-body and higher order cluster terms are not included in this first approximation.     

基于冰内孔隙率的冰中声速经验公式的研究

冰介质中的声速特性受温度、盐度、密度等因素影响,是一个随时间和空间变化的不稳定值。研究了冰介质中温度、盐度、密度三个物理量在垂直方向上的变化特性,引入孔隙率作为冰中声速特性的表征因子,利用孔隙率与温度、盐度、密度已有的函数关系,通过大量天然冰的实验数据,得到淡水冰中声速关于温度、盐度和密度的经验公式。进行了河冰声速测量的外场试验,验证了淡水冰中声速的经验公式的有效性,并利用该声速经验公式研究了淡水冰垂直方向上的声速剖面。文中所做的相关研究可为以后研究利用冰介质中的声速特性提供借鉴,为以后利用声学方法,在极地或我国北方高纬度地区进行冰层厚度监测,了解寒地冰层的更替、演化以及气候变迁等工作奠定研究基础。