Nondestructive measurement and high-precision evaluation of the electrical conductivity of doped GaAs wafers using microwaves

A nondestructive method for measuring the electrical conductivity of doped GaAs wafers using a compact microwave instrument is presented. Based on the characteristics of the microwave detector and the fact that the microwave measurement is independent of the thickness of the wafer, the analytical and explicit expressions to evaluate the electrical conductivity of the wafer are derived. Using this method, only the voltages of the reflected signals for two wafer samples whose conductivities are known are required to calibrate the two undetermined constants in the equation. Then, the conductivity of any other wafer can be evaluated by substituting the measured voltage of the reflected signal into the explicit expressions. Seven different doped GaAs wafers with thickness larger than 350 μm and conductivities in the range of 1.3 × 10(4) S/m to 7.6 × 10(4) S/m are measured in the experiment, two of which are used for calibration. The evaluated results agree well with those obtained by the conventional Hall effect measurement method, with an evaluation error less than ±4.5%. The proposed method is potentially useful for the contactless and nondestructive evaluation of the electrical conductivity of any kind of semiconductor wafer.     

Automatic torque magnetometer for vacuum-to-high-pressure hydrogen environments

An automatic torque magnetometer has been developed for use in high-pressure hydeogen. It will contain pressures ranging from vacuum to 200 atm of hydrogen gas at sample temperatures greater than 400 degrees C. This magnetometer, which uses an optical lever postion sensor and a restoring force technique has an operating range of 2.0x10(3) dyn cm to l.6x10(-4) dyn cm. An accompanying digital data collection system extends the sensitivity to 1x10(-5) dyn cm as well as increasing the data handling capacity of the system. The magnetic properties of thin films in high-temperature and high-pressure hydrogen environments can be studied using this instruments.     

Metastable transfer emission spectroscopy-method and instrument for detection and measurement of trace materials in gas flows

A simple and relatively inexpensive new technique for qualitatively and quantitatively measuring various gas-phase species in a flow and the apparatus for implementation are described. Detection of atomic species has been demonstrated from concentrations greater than 10(10) atoms/cm(3) to approximately 10(4) atoms/cm(3). Several classes of molecules can also be detected quatitatively with the method, although with somewhat reduced sensitivity. The method, metastable transfer emission spectroscopy (MTES), is particularly useful in materials analysis. Possible applications including the analysis of gas, liquid, and solid samples and the determination of vapor-pressure curves are discussed.     

The measurement of thermal conductivity variation with temperature for solid materials

An apparatus was designed to routinely measure the thermal conductivity variation with temperature for solid materials. The apparatus was calibrated by measuring the thermal conductivity variations with temperature for aluminum, zinc, tin and indium metals. The variations of thermal conductivity with temperature for the Zn-[x] wt.% Sb alloys (x = 10, 20, 30 and 40) were then measured by using the linear heat flow apparatus designed in present work. From experimental results it can be concluded that the linear heat flow apparatus can be used to measure thermal conductivity variation with temperature for multi component metallic alloys as well as pure metallic materials and for any kind of alloys. Variations of electrical conductivity with temperature for the Zn-[x] wt.% Sb alloys were determined from the Wiedemann–Franz (W–F) equation by using the measured values of thermal conductivity. Dependencies of the thermal and electrical conductivities on composition of Sb in the Zn–Sb alloys were also investigated. According to present experimental results, the thermal conductivity and electrical conductivity for the Zn-[x] wt.% Sb alloys decrease with increasing the temperature and the composition of Sb.     

An Automatic Range-Switching Modification for Direct-Reading Specific Conductance Measurements

ABSTRACT

An analysis system was developed to automatically determine specific conductance on samples at the rate of 30 per hour over a range of 1-15,000 μmho/cm with a precision of 1 percent or less. The system consists of custom designed digital electronic circuitry which modifies a commercially available direct-reading conductivity meter. This modification permits automatic range switching of the meter so that measurements are made under optimum conditions.     

空间光谱成像仪热设计参数的灵敏度

针对空间光学遥感器在轨运行期间其热物理属性的实际参数与热设计参数之间存在一定的偏差,从而影响整机热设计的问题,本文基于系统灵敏度理论,对空间光学遥感器的热设计进行了分析,并建立了在轨条件下的热平衡方程组.通过分析热平衡方程组的设计变量,总结出影响整机温度分布的热设计参数.以某空间光谱成像仪热设计为例,分析了上述影响整机温度分布的设计参数的灵敏度.灵敏度分析结果表明:整机平均温度对太阳吸收系数的灵敏度几乎为零;对红外半球发射率的灵敏度为2.2～14.55℃;对内部热源的灵敏度为1.8～2℃/W;对导热率的灵敏度为2.25×10-3～4.39×10-2 m℃2/W:对接触导热系数的灵敏度为0～1.1×10-3 m2℃2/W.试验验证结果表明,基于灵敏度分析结果的热控设计方案有效且可行.     

A simple and sensitive apparatus for measuring the magnetization of small ferrite samples

This paper describes an apparatus which allows the routine measurement of the magnetization of small ferrite samples using the well-known extraction method. In contrast to the classical extraction method, however, this device makes use of a high-permeability magnetic circuit that serves both to apply the bias field and to collect the sample flux. This design, together with a signal sampling technique, allows an unusually high sensitivity for this type of method (magnetic moment resolution is 5 x 10(-5) microem). The apparatus is described in detail and a brief theory of its function is given. Finally, the device usefulness is illustrated by some measurement results referring to Ga YIG spheres that are used in ferrimagnetic resonators.     

Magnetic induction tomography guided electrical capacitance tomography imaging with grounded conductors

Electrical capacitance tomography (ECT) is an imaging technique commonly used for imaging dielectric permittivity of insulating objects. In applications such industrial process tomography and non-destructive testing (NDT), the objects under test may exhibit variations in both dielectric permittivity and electrical conductivity. In particular, a sample that includes high conductivity, such as metal, can cause a large change in electrical field in ECT. The metal sample in imaging area will cause a large change in the sensitivity map of ECT compared to free space, which will make the ECT image reconstruction inaccurate. This effect is more severe in grounded conductor than floating conductors, so this paper focuses on grounded conductor. In order to update the sensitivity map, one needs to gain information about the conductivity distribution in ECT problem. Magnetic induction tomography (MIT) is sensitive to electrical conductivity and not sensitive to permittivity variations; therefore, it can be used to visualize the conductivity distribution of the target under test. In this paper, a dual-modality MIT and ECT system is proposed to image a medium including conductors and dielectrics. Both simulated and experimental results are presented, which demonstrate the feasibility of the proposed method.     

Development of eddy current microscopy for high resolution electrical conductivity imaging using atomic force microscopy

We present a high resolution electrical conductivity imaging technique based on the principles of eddy current and atomic force microscopy (AFM). An electromagnetic coil is used to generate eddy currents in an electrically conducting material. The eddy currents generated in the conducting sample are detected and measured with a magnetic tip attached to a flexible cantilever of an AFM. The eddy current generation and its interaction with the magnetic tip cantilever are theoretically modeled using monopole approximation. The model is used to estimate the eddy current force between the magnetic tip and the electrically conducting sample. The theoretical model is also used to choose a magnetic tip-cantilever system with appropriate magnetic field and spring constant to facilitate the design of a high resolution electrical conductivity imaging system. The force between the tip and the sample due to eddy currents is measured as a function of the separation distance and compared to the model in a single crystal copper. Images of electrical conductivity variations in a polycrystalline dual phase titanium alloy (Ti-6Al-4V) sample are obtained by scanning the magnetic tip-cantilever held at a standoff distance from the sample surface. The contrast in the image is explained based on the electrical conductivity and eddy current force between the magnetic tip and the sample. The spatial resolution of the eddy current imaging system is determined by imaging carbon nanofibers in a polymer matrix. The advantages, limitations, and applications of the technique are discussed.     

Inverse and Double Electromagnetic-Acoustic Conversion in Ferromagnetic Plates for an Arbitrary Ratio between Wavelengths of Acoustic Vibrations and Electromagnetic Oscillations

Secondary magnetic fields generated due to the inverse and double electromagnetic-acoustic conversion (EMAC) of any mode of the Lamb waves have been calculated. The behavior of these fields in transforming zeroth modes has been analyzed as a function of the thickness, electrical conductivity, and magnetic susceptibility of plates. It has been established that the dependences of the secondary field amplitudes on the plate thickness and conductivity are interrelated. It has been shown that the possibility of increase in the effectiveness of conversion due to electromagnetic-acoustic resonance in low-conducting materials revealed in the direct EMAC is conserved in the inverse and double EMAC.     

High sensitivity magnetometer for measuring the isotropic and anisotropic magnetisation of small samples

We describe how the full, isotropic and anisotropic, magnetisation of samples as small as tens of micrometers in size can be sensitively measured using a piezoresistive microcantilever and a small, moveable ferromagnet. Depending on the position of the ferromagnet, a strong but highly local field gradient of up to ～4200 T/m can be applied at the sample or removed completely during a single measurement. In this way, the magnetic force and torque on the sample can be independently determined without moving the sample or cycling the experimental system. The technique can be used from millikelvin temperatures to ～85 K and in magnetic fields from 2 T to the highest fields available. We demonstrate its application in measurements of the semimagnetic semiconductor Hg(1 - x)Fe(x)Se, where we achieved a moment sensitivity of better than 2.5 × 10(-14) J/T for both isotropic and anisotropic components.     

Measurement of Electromagnetic Parameters of Metal-Coating Thickness Measures

Thickness measures for electrically conductive coatings applied to electrically conductive bases are described, for which not only geometrical but also electrophysical parameters such as the specific electrical conductivity and complex relative permeability of the measure base and the specific electrical conductivity of the measure coating are prescribed. The procedure and a technique for measuring the specific electrical conductivity of the base of a measure using the van der Pauw method, the complex relative magnetic permeability of metal on ring-shaped samples using a permeameter, a technique for transferring the value of this parameter to the measure base of an arbitrary shape with a flat surface, and also the procedure and a method for measuring the specific electrical conductivity of the coating-measure material with the use of an eddy current probe with a wave-like drive winding are described.     

Optical extinction monitor using cw cavity enhanced detection

We present details of an apparatus capable of measuring optical extinction (i.e., scattering and/or absorption) with high precision and sensitivity. The apparatus employs one variant of cavity enhanced detection, specifically cavity attenuated phase shift spectroscopy, using a near-confocal arrangement of two high reflectivity (R approximately 0.9999) mirrors in tandem with an enclosed cell 26 cm in length, a light emitting diode (LED), and a vacuum photodiode detector. The square wave modulated light from the LED passes through the absorption cell and is detected as a distorted wave form which is characterized by a phase shift with respect to the initial modulation. The amount of that phase shift is a function of fixed instrument properties-cell length, mirror reflectivity, and modulation frequency-and of the presence of a scatterer or absorber (air, particles, trace gases, etc.) within the cell. The specific implementation reported here employs a blue LED; the wavelength and spectral bandpass of the measurement are defined by the use of an interference filter centered at 440 nm with a 20 nm wide bandpass. The monitor is enclosed within a standard 19 in. rack-mounted instrumentation box, weighs 10 kg, and uses 70 W of electrical power including a vacuum pump. Measurements of the phase shift induced by Rayleigh scattering from several gases (which range in extinction coefficient from 0.4-32 Mm(-1)) exhibit a highly linear dependence (r(2)=0.999 97) when plotted as the co-tangent of the phase shift versus the expected extinction. Using heterodyne demodulation techniques, we demonstrate a detection limit of 0.04 Mm(-1) (4 x 10(-10) cm(-1)) (2sigma) in 10 s integration time and a base line drift of less than +/-0.1 Mm(-1) over a 24 h period. Detection limits decrease as the square root of integration time out to approximately 150 s.     

Thermodynamic measurements of submilligram bulk samples using a membrane-based "calorimeter on a chip"

Calorimetry offers a direct measurement of thermodynamic properties of materials, including information on the energetics of phase transitions. Many materials can only be prepared in thin film or small crystal (submilligram) form, negating the use of traditional bulk techniques. The use of micromachined, membrane-based calorimeters for submilligram bulk samples is detailed here. Numerical simulations of the heat flow for this use have been performed. These simulations describe the limits to which this calorimetric technique can be applied to the realm of small crystals (1-1000 microg). Experimental results confirm the feasibility of this application over a temperature range from 2 to 300 K. Limits on sample thermal conductivity as it relates to the application of the lumped and distributed tau 2 models are explored. For a typical sample size, the simulations yield 2.5% absolute accuracy for the heat capacity of a sample with thermal conductivity as low as 2 x 10(-5) W/cm K at 20 K, assuming a strong thermal link to the device. Silver paint is used to attach (both thermally and physically) the small samples; its heat capacity and reproducibility are discussed. Measurements taken of a submilligram single crystal of cobalt oxide (CoO) compare favorably to the results of a bulk calorimetric technique on a larger sample.     

Senftleben-Beenakker thermal conductivity effect apparatus

An apparatus for measuring the influence of magnetic fields on the thermal conductivity of gases, the Senftleben-Beenakker effect, is described. Conventional 'cold finger type' cooling with appropriate cryogenic fluids is utilized with a helium exchange gas chamber to establish the general temperature level of the cell holder. Automatic temperature control of the trim heater maintains the desired temperature to +/-10(-3) degrees C in the 77-300 K range. A concentric cylinder cell adaptable for use with either conventional electromagnet or superconducting solenoid is described. The use of thin Mylar polyester film end seals minimizes cell 'end effect' corrections and provides a resolution of 5x10(-6) in gas thermal conductivity.     

高灵敏度的全自动低值磁通测量系统

介绍了一种高灵敏度的全自动低值磁通测量方法，对于从测量线圈得到的感应电动势信号利用了高灵敏度的纳伏计作为前置放大器进行放大，因而测量低值磁通的分辨率达到了1nWb，比国外同类仪器高出了10倍，同时还研制了V／F变换器及数字积分器作为后续的积分系统。消除了常用的RC积分放大器法无法避免的回复误差，包括激励电源在内的测量仪器构成了一个基于计算的全自动测量系统，使用十分方便。这样的低值磁通测量系统配上一个探测线圈时可探测很微弱的磁场。当探测线圈的磁面积为0.2m^2（直径1cm，匝数约为4000）时，探测弱磁场的分辨率可达高5nT.     

Use of an electromagnetic-acoustic technique in the practice of testing tubes of main gas pipelines

The results of development, design, and industrial testing of an electromagnetic-acoustic (EMA) technique are considered. An ultrasonic EMA pig flaw detector has higher sensitivity to cracks and can work in gas pipelines as well as a magnetic pig flaw detector does. The EMA pig flaw detector reliably detects cracks with a 10-μm opening, which are almost invisible for a magnetic pig flaw detector.     

A Zeeman slower design with permanent magnets in a Halbach configuration

We describe a simple Zeeman slower design using permanent magnets. Contrary to common wire-wound setups, no electric power and water cooling are required. In addition, the whole system can be assembled and disassembled at will. The magnetic field is however transverse to the atomic motion and an extra repumper laser is necessary. A Halbach configuration of the magnets produces a high quality magnetic field and no further adjustment is needed. After optimization of the laser parameters, the apparatus produces an intense beam of slow and cold (87)Rb atoms. With typical fluxes of (1-5) × 10(10)?atoms/s at 30 m s(-1), our apparatus efficiently loads a large magneto-optical trap with more than 10(10) atoms in 1 s, which is an ideal starting point for degenerate quantum gas experiments.     

电磁轴承及其系统设计方法

电磁轴承是磁悬浮原理在机械领域中的一个应用实例,其工作机理是依靠电磁力使转子非接触地“支承”于轴承体内,有运动阻力小、无接触摩擦和磨损、功耗低以及寿命长等优点。国外在这方面已有成熟的产品应用于生产实际。介绍了电磁轴承及其系统设计的特点、系统控制器的设计方法、刚性转子系统的设计方法和柔性转子系统的动力学设计方法,并提供了几个实际设计的数据和部分试验结果。