The melting wave in a metal fast heated by a high-power current pulse

The region of a solid-liquid phase transition, which propagates at a finite velocity into a cylindrical conductor under the conditions of slightly inhomogeneous heating by a current pulse, is considered as a melting wave. The results of numerical simulation of the melting wave in a tungsten wire heated by a nanosecond high-power current pulse are presented. It is shown that the melting wave in this system cannot be treated as an infinitely thin transition region between solid and liquid metal phases.     

High-power terahertz radiation emitter with a diamond photoconductive switch array

A photoconductive switch-arrayed antenna with a chemical vapor-deposited diamond film was developed to generate high-power terahertz (THz) radiation. With this device, an electric field stress of 2 x 10(6) V/cm can be applied to photoconductive gaps because of the high breakdown threshold of diamond and the overcoated gap structure for the prevention of surface flashover. This level of field stress can alleviate the current problem of saturation in THz emission by use of a photoconductive antenna. The device consists of more than two thousand 20 mum x 2.8 mm emitters. In an experiment using an ultrashort pulse Kr*F laser, we obtained an energy density of 10 muJ/cm(2) on the emitter surface at E = 10(5) V/cm. This density was larger than that of the current large-aperture antenna. There was no severe saturation in photoconductive current up to E = 10(6) V/cm, and a focused intensity of 200 MW/cm(2) can be expected.     

高频电磁力作用下金属液内电磁超声波的生成

为解决由机械振动产生的超声波无法在高温环境中成功应用的难题，提出了利用静磁场和交流电流、静磁场和交流磁场的相互作用分别在金属液内直接生成高强度电磁超声波的方法，运用超声波压力传感器对用这两种方法在金属液内生成的电磁超声波的强度进行了测量。研究表明：电磁超声波的强度与所施加的高频电磁力的强度成正比，频率与电磁力的频率相同；实验结果和理论解析结果基本一致；利用高频电磁力的局部作用在金属液全场范围内生成的电磁超声波可以在金属精炼及凝固过程中获得广泛应用。     

Measurement of neutron dose with an organic liquid scintillator coupled with a spectrum weight function

A dose evaluation method for neutrons in the energy range of a few MeV to 100 MeV has been developed using a spectrum weight function (G-function), which is applied to an organic liquid scintillator of 12.7 cm in diameter and 12.7 cm in length. The G-function that converts the pulse height spectrum of the scintillator into the ambient dose equivalent, H*(10), was calculated by an unfolding method using successive approximation of the response function of the scintillator and the ambient dose equivalent per unit neutron fluence (H*(10) conversion coefficients) of ICRP 74. To verify the response function of the scintillator and the value of H*(10) evaluated by the G-function. pulse height spectra of the scintillator were measured in some different neutron fields, which have continuous energy, monoenergetic and quasi-monoenergetic spectra. Values of H*(10) estimated using the G-function and pulse height spectra of the scintillator were compared with those calculated using neutron energy spectra. These doses agreed with each other. From the results, it was concluded that H*(10) can be evaluated directly from the pulse height spectrum of the scintillator by applying the G-function proposed in this study.     

Operation of a Wideband Terahertz Superconducting Bolometer Responding to Quantum Cascade Laser Pulses

We make use of a niobium film to produce a micrometric vacuum-bridge superconducting bolometer responding to THz frequency. The bolometer works anywhere in the temperature range 2–7?K, which can be easily reached in helium bath cryostats or closed-cycle cryocoolers. In this work the bolometer is mounted on a pulse tube refrigerator and operated to measure the equivalent noise power (NEP) and the response to fast (μs) terahertz pulses. The NEP above 100?Hz equals that measured in a liquid helium cryostat showing that potential drawbacks related to the use of a pulse tube refrigerator (like mechanical and thermal oscillations, electromagnetic interference, noise) are irrelevant. At low frequency, instead, the pulse tube expansion-compression cycles originate lines at 1?Hz and harmonics in the noise spectrum. The bolometer was illuminated with THz single pulses coming either from a Quantum Cascade Laser operating at liquid nitrogen temperature or from a frequency-multiplied electronic oscillator. The response of the bolometer to the single pulses show that the device can track signals with a rise time as fast as about 450?ns.     

Liquid-level sensor with a high-birefringence-fiber loop mirror

A novel liquid-level sensor with a high-birefringence-fiber loop mirror (HBFLM) based on a uniform-strength cantilever beam (UCB) is proposed and demonstrated. Part of the high-birefringence fiber is pasted onto the central surface of the UCB. A hollow suspending pole is utilized to apply force at the end of the beam. The applied force varies with the change of the liquid level, leading to a change of transmission intensity. Thus the variation of liquid level can be determined via the laser wavelength within the quasi-linear transmission range of the HBFLM filter. Its sensitivity, resolution, and linear measurement range reach 0.047/cm, 10 mm, and 140 mm, respectively. The advantages of the sensor include simple structure, high sensitivity, low cost, and good repeatability, etc. The sensing signal can be directly detected by a photodetector and does not require complicated demodulation devices.     

Condensation of steam on a vertical tube in a granulated material

The heat transfer enhancement was studied during condensation of steam on a chilled vertical surface of a tube packed into a granulated material with different contact angles of wetting. The dimensionless values of heat transfer at condensation on a surface in filling, obtained for a vertical tube in the range of Reynolds numbers from 70 up to 400, exceed Nu* values for a smooth tube by the factor of 2–3. The intensification of heat transfer on a vertical tube, housed in a granulated layer, is conditioned by the several interdependent phenomena: 1 — capillary ascent of some liquid near the meniscuses, and as a consequence, reduction of the mean film thickness; 2 — burble of a film at the points of sphere contact with a surface of condensation at Re>10; 3 — removal of some film liquid by a granulated layer; accompanied by simultaneous film burble at the points of sphere contact with a cooling surface at Re>83. Results of the current research can be used for the development of heat exchanging devices under the conditions of microgravitation.     

厚壁金属管挤压变形缓冲制动响应特性研究

金属管挤压变形是高效缓冲制动的重要形式,但目前对不同结构形式在动态冲击条件下的响应特性缺少系统的认识;以厚壁金属管为对象,对其挤压变形缓冲制动响应进行数值研究。参照试验状态和结果建立厚壁金属管缓冲制动装置的有限元分析计算模型,进行准静态仿真模拟分析并校核;在此基础上利用有限元法和显示动力学算法,对厚壁金属管缓冲制动响应特性以及影响缓冲力的主要因素进行研究和分析。结果表明:①厚壁金属管缓冲制动结构具有尺寸小、冲击载荷平稳、变形能力强的特点;②利用此计算模型能够清晰地表征厚壁金属管缓冲制动装置的动态响应;③影响缓冲力大小的主要结构因素是缓冲筒下部内孔直径、上部内孔直径以及外部直径,缓冲筒上下内径比D1应控制在0.7~0.8,D2应控制在1.8~2.0,当比值越小时,冲击杆会出现反冲现象,当比值越大时,冲击杆受到较大的冲击力,缓冲筒的惯性制动段的制动也有较大波动的影响。针对工程应用中不同的缓冲制动需求,可以通过设置合理的缓冲筒内外廓直径参数来获得有效的制动载荷。可为相关研究和工程应用提供参考。     

A Flexible Loudspeaker Using the Movement of Liquid Metal Induced by Electrochemically Controlled Interfacial Tension

A flexible liquid metal loudspeaker (LML) is demonstrated consisting of a gallium‐based eutectic liquid metal (Galinstan) and basic aqueous electrolyte (NaOH_(aq)). The LML is driven by liquid metal motion induced by the electrochemically controlled interfacial tension of the Galinstan in NaOH_(aq) electrolyte under an applied alternating current (AC) voltage. The fabricated LML produces sound waves in the human audible frequency band with a sound pressure level of ≈40–50 dB at 1 cm from the device and exhibits mechanical stability under bending deformation with a bending radius of 3 mm. Various sounds can be generated with the LML from a single tone to piano notes and human voices. To understand the underlying mechanism of sound generation by the LML, motion analyses, sound measurements, and electrical characterization are conducted at various frequencies. For the first time, this work suggests a new type of liquid metal‐based electrochemically driven sound generator in the field of flexible acoustic devices that can be applied to future wearable electronics.     

An electron source with a multiarc plasma emitter for obtaining submillisecond pulsed megawatt beams

An electron source with a plasma emitter based on an arc-discharge system with six cathodes and a common cylindrical hollow anode is described. Upon synchronous initiation of vacuum-arc discharges, the space of the hollow anode is filled by dense low-temperature plasma, the emission boundary of which is stabilized by a fine-structure metal grid with a 150-cm² area. The arc-current amplitude for each cathode amounts to 100–300 A. Under the action of a constant accelerating voltage applied between the plasma emitter and grounded accelerating electrode combined with the drift tube, electrons are extracted from plasma and accelerated. At a working pressure of 0.04 Pa, an electron beam with a maximum current amplitude of 1 kA has been obtained at an initial accelerating voltage of 80 kV and pulse duration (FWHM) of 100 μs, which has been transported in a longitudinal magnetic field of 0.035 T over a distance of 80 cm.     

Radially composite piezoelectric ceramic tubular transducer in radial vibration

The radially composite piezoelectric tubular transducer is studied. It is composed of radially poled piezoelectric and a long metal tube. The electro-mechanical equivalent circuit of the radially poled piezoelectric and metal tube in radial vibration is obtained. Based on the force and velocity boundary conditions, the six-port electro-mechanical equivalent circuit for the composite tubular transducer is given and the resonance/anti-resonance frequency equations are obtained. The relationship between the resonance frequency and the dimensions is analyzed. Numerically simulated results obtained by the finite element method are compared with those from the analytical method. Composite piezoelectric tubular transducers are designed and manufactured. The resonance/anti-resonance frequencies are measured, and it is shown that the theoretical results are in good agreement with the simulated and experimental results. It is expected that radially composite piezoelectric tubular transducers can be used as high-power ultrasonic radiators in ultrasonic applications, such as ultrasonic liquid processing.     

Measurement of surface tension of tantalum by a dynamic technique in a microgravity environment

A dynamic technique has been used in a microgravity environment to measure the surface tension of tantalum at its melting point. The basic method involves resistively heating a tubular specimen from ambient temperature to temperatures above its melting point in about 1 s by passing an electrical current pulse through it, while simultaneously measuring the pertinent experimental quantities with millisecond resolution. A balance between the magnetic and the surface tension forces acting on the specimen is achieved by splitting the current after it passes through the specimen tube and returning a fraction of the current along the tube axis and the remaining fraction concentrically outside the specimen. Values for surface tension are determined from measurements of the equilibrium dimensions of the molten specimen tube and the magnitudes of the currents. Rapid melting experiments were performed during microgravity simulations with NASA's KC-135 aircraft and the results were analyzed, yielding a value of 2.07±0.06 N · m^–1 for the surface tension of tantalum at its melting point. Conditions for improving specimen stability during temperature excursions into the liquid phase are discussed.     

Simulation of liquid infiltration and semi-solid extrusion for composite tubes by quasi-coupling thermal-mechanical finite element method

As a new metal forming technology, the liquid infiltration and semi-solid extrusion process can produce various composite parts, such as tubes, bars, and shaped products, in a single process. In this paper, the liquid infiltration and semi-solid extrusion process for forming a composite tube is simulated by means of thermal rigid-plastic quasi-coupling FEM method. The key technologies such as the handling of liquid phase zone, the transition between liquid and solid phase zones, the grid re-meshing method and the establishment of the boundary condition have also been studied. Based on the FEM simulation software developed by the authors and the grid re-meshing technology, the distribution of stress field, strain field and deformation force in the liquid infiltration and semi-solid extrusion process for forming composite tubes are obtained. The deformation force simulation results accord with the experimental data, indicating the reliability of the system. Therefore, the present research is theoretically valuable in the product quality control and the process parameter choice.     

Development and experimental investigation of Stirling-type pulse tube refrigerator (PTR) below 20 K; cold compressor and colder expander

This research paper focuses on the experimental investigation of the Stirling-type pulse tube refrigerator with cold compression concept. Due to this innovative feature, the pulse tube refrigerator can reach lower temperature effectively other typical single-stage Stirling-type pulse tube refrigerators. The experiment as a proof of concept is carried out to demonstrate the capability of the pulse tube refrigerator operating between 80 K and 20 K. The cold linear compressor, which is submerged in a liquid nitrogen bath, produces cold mass flow with the efficiency of 85% for all the frequencies. At the lowest temperature part of the pulse tube refrigerator, the no-load temperature of 18.7 K is recorded and the cooling power of 0.4 W is measured at 20 K. The experimental results are analyzed in dynamic and thermal aspects by using the numerical model. The model can well explain how much losses are distributed in the system.     

Beam injection flame furnace atomic absorption spectrometry: a new flame method

A new flame method of atomic absorption spectrometry is described. The liquid sample to be analyzed is transported as a high-speed liquid jet into a heated tube which is positioned in an air/acetylene flame. The jet is generated by means of an HPLC pump which feeds a smooth jet nozzle having a diameter of 50 microns or smaller. After traveling a distance of 10 cm, the liquid jet enters a small sample introduction hole, impacts onto the opposite inner wall of the tube furnace, and immediately vaporizes (jet impact vaporization, JIV). Both the complete introduction of the entire sample and the extended residence time inside the absorption volume result in an improvement in power of detection from 6- to 202-fold for 17 elements (Ag, As, Au, Bi, Cd, Cu, Hg, In, K, Pb, Pd, Rb, Sb, Se, Te, Tl, Zn). A standard deviation of 1.7-4.0% (n = 12, 50 microL) was achieved. Sample volumes between 10 microL and 1 mL have been investigated. For 50 microL sample volumes, the sampling frequency is 4/min. The new method can also be considered a simple, effective interface between HPLC and flame AAS.     

Current induced surface diffusion on a single-crystalline silver nanowire

Scanning tunnelling microscopy was used to study the morphological changes of the surface of a single-crystalline silver nanowire caused by a lateral electron current. At current densities of about 1.5 × 10(7) A cm(-2), surface atoms are extracted from step edges, resulting in the motion of surface steps, islands and holes with a thickness or depth of one monolayer. Upon current reversal the direction of the material transport can be altered. The findings are interpreted in terms of the wind force.     

用超声脉冲反射谱评价圆管厚度的实验研究

垂直入射到固体圆管表面产生的反射超声脉冲波的频谱在圆管的厚度共振频率处产生谱陷，利用谱陷频率可以评价固体圆管的厚度。本文利用超声脉冲反射谱法在实验室内对不同形状的钢样品进行了厚度测量，测量得的厚度与样品的实际厚度的一致性很好。并且，超声波探头的频带越宽，则用此探头可以测量到的圆管的厚度范围越大。     

A liquid level sensor using the absorption of guided acoustic waves

This liquid level sensor consists of a vertically movable 10-cm-long hollow cylinder with an impedance matching conic section suspended at one end of a few-meters-long magnetorestrictive wire. The wire is automatically wound up or unwound such that the cylindrical sensor element at the bottom of the wire is immersed by just a small and measurable distance into the liquid whose level is sought. Acoustic waves are launched in the wire by a pulse of current in a coil and are transmitted to the hollow cylinder through the cylindrical horn. The carrier frequency of the wave trains and the thickness of the tube are chosen so that the elastic waves strongly leak into the liquid when the hollow cylinder is immersed. The level is then determined by measuring the time-of-flight of the echo reflected at the junction between the wire and the horn, followed by taking into account a small correction term computed from the attenuation of the end echo.     

Phase transformations of carbon heated by a high-power current pulse

The phase trajectories of carbon heated in various regimes by a high-power electric current pulse have been theoretically calculated. It is shown that diamond can be synthesized from a liquid phase in the regime of high-energy electric explosion.