Contactless technique for the measurement of electrical resistivity in anisotropic materials

Measurement of basal plane electrical resistivities in quasi-two-dimensional materials by conventional four-point bridge techniques is extremely difficult. In making measurements of room temperature basal plane resistivities in highly conducting highly anisotropic synthetic metals we have developed an rf inductive technique which is both simple to use and provides accurate results. At the frequency we employ (100 kHz) the method is appropriate for resistivities in the range 10 (-3)-10(-7)Omega cm. Samples are thin square plates 5 mm on a side. The system is calibrated by fitting data from a series of samples of known resistivity to semi-empirical formulae. The theoretical motivation leading to these expressions is discussed and extensive calibration data are presented. The advantages of rf techniques in general have already been discussed. Two advantages commend the particular technique we describe here. First, it is simple in design and easily constructed. Second, with careful calibration, highly accurate resistivity measurements can be made in a very straightforward manner.     

A novel scaling law relating the geometrical dimensions of a photocathode radio frequency gun to its radio frequency properties

Developing a photocathode RF gun with the desired RF properties of the π-mode, such as field balance (e(b)) ~1, resonant frequency f(π) = 2856 MHz, and waveguide-to-cavity coupling coefficient β(π) ~1, requires precise tuning of the resonant frequencies of the independent full- and half-cells (f(f) and f(h)), and of the waveguide-to-full-cell coupling coefficient (β(f)). While contemporary electromagnetic codes and precision machining capability have made it possible to design and tune independent cells of a photocathode RF gun for desired RF properties, thereby eliminating the need for tuning, access to such computational resources and quality of machining is not very widespread. Therefore, many such structures require tuning after machining by employing conventional tuning techniques that are iterative in nature. Any procedure that improves understanding of the tuning process and consequently reduces the number of iterations and the associated risks in tuning a photocathode gun would, therefore, be useful. In this paper, we discuss a method devised by us to tune a photocathode RF gun for desired RF properties under operating conditions. We develop and employ a simple scaling law that accounts for inter-dependence between frequency of independent cells and waveguide-to-cavity coupling coefficient, and the effect of brazing clearance for joining of the two cells. The method has been employed to successfully develop multiple 1.6 cell BNL∕SLAC/UCLA type S-band photocathode RF guns with the desired RF properties, without the need to tune them by a tiresome cut-and-measure process. Our analysis also provides a physical insight into how the geometrical dimensions affect the RF properties of the photo-cathode RF gun.     

Imaging conductive materials with high frequency electrical capacitance tomography

Electrical capacitance tomography (ECT) is known as an imaging technique for dielectric permittivity imaging. A novel ECT sensor model at a high excitation frequency is proposed to examine the capability of the ECT system to image both conductivity and permittivity contrasts. The proposed model uses a complex impedance forward model for the ECT system. This new model indicates that in higher excitation frequency both conductive and dielectric imaging may be feasible. Normally, capacitance tomography is designed for the measurements of imaginary part and resistance tomography is used to take the measurements of real part. The drawback of a typical capacitance tomography at a low excitation frequency, such as 200 kHz is that it cannot be used to measure the conductive phase of a conductive/dielectric mixed fluid, e.g. the gas/water flow. By increasing the excitation frequency, the capacitive impedance of the conductive material decreases and dielectric phenomena of the conductive fluid dominates so that it is possible to use capacitance tomography to characterise the dielectric/conductive flows. This paper presents a development of capacitance tomography with a high excitation frequency in measuring the gas/liquid mixture i.e. gas/water and gas/oil multiphase distributions. Both theoretical and experimental results are presented to verify this feasibility study.     

射频导纳技术在蒸发器液位测量中的应用

针对海水淡化蒸发器运行工况复杂、结构设计紧凑、液位测量范围小、常规差压法安装困难等问题,提出了一种利用射频导纳技术测量蒸发器液位的方法。重点对射频导纳液位计的工作原理、主要特点、电极选型、安装注意事项以及标定方法进行了阐述和探讨,并在30t/d LT-MED中试装置中成功应用。结果表明:该仪表安装简单、测量准确、维护方便且适用范围较广,具有很好的应用前景。     

Improved circuit for impedance measurement at very high frequency and its application in testing dielectric properties of insulating materials

This paper describes improvements on an impedance measuring circuit for the measurement of very small conductance at very high frequency and one of its applications, viz., the accurate measurement of dielectric constant and loss angle of very low loss materials. In this circuit, a feedback loop for the stabilization of the output level of the high-frequency oscillator unit also provides compensation for the changes of the series resistance of resonance coil and the transfer characteristic of the detector that occur with their temperature drifts. This circuit has the sensitivity and stability to detect a 0.002 mumho change in a 16-pF specimen at 100, 150, or 200 MHz. It is capable of measuring a loss angle of several murad with the electrode system developed for the varying gap immersion method. The warming-up time is also shortened to about 10 min even for measurements requiring the highest sensitivity.     

Laser measurement of H- ions in a field-effect-transistor based radio frequency ion source

Hydrogen negative ion density measurements are required to clarify the characteristics of negative ion production and ion source performance. Both of laser photodetachment and cavity ring down (CRD) measurements have been implemented to a field-effect-transistor based radio-frequency ion source. The density ratio of negative hydrogen ions to electrons was successfully measured by laser photodetachment and effect of magnetic filter field on negative ion density was confirmed. The calculated CRD signal showed that CRD mirrors with >99.990% reflectivity are required and loss of reflectivity due to cesium contamination should be minimized.     

面向无线射频识别应用的复合订阅匹配

传统的发布/订阅系统不支持具有丰富时序关系的复合事件,不能很好地满足无线射频识别应用的需要.针对无线射频识别应用的特点,设计了面向无线射频识别应用的复合订阅语言,研究了订阅语言的组成和使用,基于时间事件、复合订阅的匹配图表示、事件检测的消费语义,给出了相应的复合订阅匹配算法.该算法已在OncePubSub系统中实现,并与Siena改造版、JESS改造版进行了性能比较.     

A Peltier cell calorimeter for the direct measurement of the isothermal entropy change in magnetic materials

We developed a calorimetric technique to measure the isothermal magnetocaloric entropy change. The method consists in the use of Peltier cells as heat flow sensor and heat pump at the same time. In this paper, we describe the setup, the constitutive equations of the Peltier cell as sensor and actuator, and the calibration procedure. The Peltier heat is used to keep the sample isothermal when magnetic field is changed. The temperature difference between the sample and the thermal reservoir is kept by a digital control within 5 mK for a magnetic field rate of 20 mT s(-1). The heat flux sensitivity around 1 microW. With this method, it is possible to measure the magnetocaloric effect in magnetic materials by tracing the curves of the exchanged entropy Delta(e)s as a function of the magnetic field H. The method proves to be, in particular, suitable to reveal the role of the entropy production Delta(i)s, which is connected with hysteresis. Measurement examples are shown for Gd, BaFe(12)O(19) ferrite, and Gd-Si-Ge.     

Efficient radio frequency filters for space constrained cryogenic setups

Noise filtering is an essential part for measurement of quantum phenomena at extremely low temperatures. Here, we present the design of a filter which can be installed in space constrained cryogenic environment containing a large number of signal carrying lines. Our filters have a -3 db point of 65 kHz and their performance at GHz frequencies is comparable to the best available RF filters.     

智能建筑的电气保护接地应用研究

随着城市化进程的日益加快,建筑技术的飞速发展,对智能建筑中大量的用电设备提出了更高的要求.电气保护接地系统也得到了广泛的应用.本文对智能建筑中几种电气保护接地的系统和应用做了分析和研究.     

Simple low-noise power supply for electrical transient measurements

A very simple means is described for rapidly applying and briefly maintaining a large, essentially noiseless, dc voltage across a capacitive load. The technique is suitable for various nonrepetitive electrical transient measurements, such as carrier mobility determination, occurring over timescales ranging from tens of microseconds to several seconds; it essentially eliminates capacitive currents caused by noise in the power supply voltage, which can interfere with measurement of the true transient current. In an example, the noise on a 10-ms transient current (driven by 2000 V dc, and observed as a voltage across a 100-kOmega load resistor) was barely more than the Johnson noise ( approximately 2x10(-11) A rms) to be expected from the resistor.     

Adaptive Chirp Transform for frequency measurement

In this paper Adaptive Chirp Transform (ACT) algorithm for power frequency measurement is presented. It is adjusted for continuous monitoring of power frequency. Signal is first preprocessed with adaptive filter to minimize the error of non-linear model parameters. The resulting parameters are used to narrow the frequency range for CZT method to be effective. Algorithm code has been analyzed and computation complexity has been significantly decreased. Using this algorithm it is possible to measure frequency transients with sufficient accuracy and low execution time. Algorithm is verified through simulations and real-time data.     

Development of a device for the measurement of thermal and fluid flow properties of heat exchanger materials

As yet, no standard equipment exists for the measurement of heat transfer through porous materials, such as metal foams (metals with a high volume fraction of porosity). Most research in this area has been carried out using bespoke test rigs. Here the creation of a test rig specifically developed for the measurement of the heat transfer of metal foams is reported. This method has been applied to laboratory made samples processed by replication and examples of commercially available aluminium foams (Duocel and Corevo), and should be suitable for the testing of all materials with comparable permeability. As this equipment is new and unique, the design will be discussed in detail, along with the various tests that were performed to ensure reliability and consistency with other methods and published data.     

Investigation of water electrical parameters as a function of measurement frequency using cylindrical capacitive sensors

In this study electrical properties of different water liquids at frequency range of 100–2000 Hz are investigated by using the short invasive and non-invasive cylindrical capacitive sensors (CCSs). Operation of the capacitance measurement module for such probes is based on the auto balancing bridge method. Comparison of the measured capacitances and measured resistances for different water liquids shows decrease by increasing the frequency. In another study the dielectric constant of distilled water, mineral water, tap water and salt water are measured. The effects of the frequency on the resistivity, permittivity and conductance of the different water liquids are also investigated.     

Resonance measurement technique for viscoelastic properties of damping materials of the porous closed cellular PE foam type

Theoretical and experimental studies of longitudinal and lateral vibrations in a 2D cavity filled with foam viscoelastic material were performed; measurement methods of Lame parameters, as well as viscosity parameters with longitudinal and lateral resonance vibrations in cellular polypropylene loaded by a final mass were proposed. Measurements of the visco-elastic properties of samples from the porous closed cellular polyethylene of a PE foam and EVA ultrathene types were made.     

采样频率对频率测量算法的影响

从算法角度探讨了正弦信号频率测量的方法.研究了含噪声情况下,各种正弦波频率算法、采样频率设置和测量精度的关系.总结了应用频率测量算法的原则,为工程实际中仅利用A/D转换和计算机实现低成本正弦波频率测量提供了一种思路.     

Low power high-performance radio frequency oscillator for driving ion traps

We report a simple, efficient, high voltage radio frequency (RF) generator powered by a single voltage source (1.5-7 V) to resonantly drive ion traps or other capacitive loads. Our circuit is able to deliver RF voltages > 500?V(p-p) at frequencies ranging from 0.1 to 10 MHz. This RF oscillator uses low-cost, commercially available components, and can be easily assembled onto a circuit board of a few cm(2). Because of its simplicity and good efficiency, this circuit is useful in applications requiring small size and low power consumption such as portable ion trap systems where the duration of operation under battery power is of concern.     

汽车生产线射频识别技术采纳的影响因素研究

针对现有射频识别采纳决策过程中缺乏科学评价方法的问题,研究了汽车生产线射频识别采纳影响因素。分析了射频识别技术在汽车生产线的应用难点,在总结现有研究成果的基础上提出了射频识别采纳影响因素集。利用模糊德尔菲法筛选出影响射频识别采纳的17个影响因素,再利用模糊层次分析法构造了各阶层的模糊互补判断矩阵并确立了各指标的相对权重。研究结果表明,汽车生产线射频识别采纳的五个最关键影响因素分别为：一次性投入成本、产品性能、信息技术能力、集成性、领导支持。最后,通过重庆某整车生产线的射频识别应用实例验证了研究成果的可行性和有效性。     

射频识别起重机械的应用技术研究

分析了现有的8种人工识别技术和3种自动识别技术及其特点,介绍了射频识别技术在气瓶等特种设备领域的应用;简述了射频识别技术的基本原理、技术特点和优势,以及射频识别起重机械系统的组成。理论论证和现场测试证明：射频识别起重机械在技术上是可行的,是技术发展的趋势。     

Compact, low power radio frequency cavity for femtosecond electron microscopy

Reported here is the design, construction, and characterization of a small, power efficient, tunable dielectric filled cavity for the creation of femtosecond electron bunches in an existing electron microscope without the mandatory use of femtosecond lasers. A 3 GHz pillbox cavity operating in the TM(110) mode was specially designed for chopping the beam of a 30 keV scanning electron microscope. The dielectric material used is ZrTiO(4), chosen for the high relative permittivity (ε(r) = 37 at 10 GHz) and low loss tangent (tan δ = 2 × 10(-4)). This allows the cavity radius to be reduced by a factor of six, while the power consumption is reduced by an order of magnitude compared to a vacuum pillbox cavity. These features make this cavity ideal as a module for existing electron microscopes, and an alternative to femtosecond laser systems integrated with electron microscopes.