A new technique to study transient conductivity under pulsed monochromatic light in Cr-doped GaAs using acoustoelectric voltage measurement

The transient conductivity of high-resistivity, Bridgman grown, Cr-doped GaAs under pulsed monochromatic light is monitored using transverse acoustoelectric voltage (TAV) at 83 K. Keeping the photon flux constant, the height and transient time constant at the TAV are used to calculate the energy dependence of the trap density and its cross section, respectively. Two prominent trap profiles with peak trap densities of approximately 10(17) cm(-3) eV(-1 ) near the valence and the conduction bands are detected. These traps have very small capture cross sections in the range of 10(-23 )-10(-21 )cm(2). A phenomenon similar to the persistent photoconductivity with transient time constants in excess of a few seconds (in some cases, a few hundred seconds) in high-resistivity GaAs at T=83 K is also detected using this technique. These long relaxation times are readily explained by the spatial separation of the photo-excited electron-hole pairs and the small capture cross section and large density of trap distribution near the conduction band. The technique is nondestructive and, because of the dependence of the polarity of the acoustoelectric voltage on the carrier type, it yields information about the charge of the transient carriers and the type of deep traps involved in the release or trapping of these carriers.     

Effects of Ti2448 half-pin with low elastic modulus on pin loosening in unilateral external fixation

The objective of this study was to compare the benefits of titanium 2448 (Ti2448) half-pin and titanium-6 aluminium-4 vanadium (TAV) half-pin on reducing pin loosening during external fracture fixation. Although having similar strength, Ti2448 half-pin had even lower elastic modules(33 GPa)when compared with TAV half-pin (110 GPa), which was similar to that of cortical bone (20 GPa). In the external fixation of tibial model fractures and canine cadaveric tibia fractures, Ti2448 half-pin had greater recoverable deformation and less stress concentration at the pin–bone interface in compression, torsion, and four-points bending test. Then, tibial fractures were created in 24 dogs and stabilized with four half-pins of either Ti2448 or TAV in each animal. At 4 and 8 weeks postoperatively, fracture healing and pin loosening was assessed by radiographic grading scale. The scores of Ti2448 group were significantly higher than those of TAV group. Micro-CT analysis also indicated larger quantity and higher quality of newly formed bone at pin–bone interface in Ti2448 group. Histology observation showed the newly formed bone integrated well into the threads of Ti2448 half-pins. In contrast, there was a layer of necrotic tissue between the bone tissue and TAV half-pin at pin–bone interface in TAV group. The extraction torque values of Ti2448 half-pins near the fracture line were significantly higher than those TAV pins. In conclusion, the Ti2448 half-pin with low elastic modulus could enhance osseointegration and reduce pin loosening when compared with TAV half-pin. It is a promising biomaterial for constructing external fixation system in clinical application.     

大功率声电转换材料的研究及其应用

本文简要叙述了大功率声电转换材料在声学及电子工程领域中的意义和作用，论述研制和开发新型、高性能大功率声电转换材料是大功率声电转换器件研制和开发工作的需要，还介绍了作者研制的系列大功率压电陶瓷Ｆ１－ＰＺＴ、Ｆ２－ＰＺＴ、ＦＤ３－ＰＺＴ及ＦＤ４－ＰＺＴ的有关情况。     

In situ observation and measurement of the SAW thin-film acoustoelectric effect

The thin-film acoustoelectric effect in SAW devices describes the interaction of electrical energy between a SAW in a piezoelectric medium and a thin film in the wave's propagation path. The real-time observation of the thin-film acoustoelectric interaction is useful in the design and characterization of SAW sensors (i.e., temperature, humidity, viscosity, voltage, current, Hall effects, etc.). An in situ test fixture was designed to be mechanically, thermally, and electrically stable. Data acquisition software and an electron beam evaporation system were configured for real-time thin-film characterization during film growth. Data have been observed for more than 20 SAW devices and over a wide range of frequencies (i.e., 62 MHz to 1 GHz). The results suggest that the use of the in situ procedure yielded good agreement between theoretical predictions and the measured data, which demonstrates a method for the characterization of a SAW H(2)-gas sensor in real-time.     

Gate-controlled nonlinear conductivity of Dirac fermion in graphene field-effect transistors measured by terahertz time-domain spectroscopy

We present terahertz spectroscopic measurements of Dirac fermion dynamics from a large-scale graphene that was grown by chemical vapor deposition and on which carrier density was modulated by electrostatic and chemical doping. The measured frequency-dependent optical sheet conductivity of graphene shows electron-density-dependence characteristics, which can be understood by a simple Drude model. In a low carrier density regime, the optical sheet conductivity of graphene is constant regardless of the applied gate voltage, but in a high carrier density regime, it has nonlinear behavior with respect to the applied gate voltage. Chemical doping using viologen was found to be efficient in controlling the equilibrium Fermi level without sacrificing the unique carrier dynamics of graphene.     

热氧化对钛合金表面氧化层的影响

对钛合金Ti6Al7Nb(以下简称TAN)和Ti6Al4V(以下简称TAV)通过热氧化处理获得表面氧化层,用金相显微镜、维氏硬度计、XRD和XPS等手段对氧化层的性能进行表征。结果表明:相同氧化温度和氧化时间,TAN合金较TAV合金表面形成的氧化层更薄、致密度更好;合金表面硬度随热氧化温度的升高和氧化时间的延长而增加,但温度过高(TAN:1000℃;TAV:900℃)会使氧化层疏松多孔,硬度反而会下降;通过XPS对不同温度短时间氧化样品进行表面分析发现,氧化温度对合金中Nb添加元素的氧化行为影响较小,对V添加元素的氧化行为影响剧烈。两种合金元素对氧化过程有着显著的影响。     

Viscoelastic Model of Cross-Linked Polyethylene Including Effects of Temperature and Crystallinity

Characterization of the mechanical behavior of cross-linked polyethylene (XLPE) commonly used in high voltage cable insulation was performed by an extensive set of isothermal uniaxial tensile relaxation tests. Tensile relaxation experiments were complemented by pressure-volume-temperature experiments as well as density and crystallinity measurements. Based on the experimental results, a viscoelastic power law model with four parameters was formulated, incorporating temperature and crystallinity dependence. It was found that a master curve can be developed by both horizontal and vertical shifting of the relaxation curves. The model was evaluated by making comparisons of the predicted stress responses with the measured responses in relaxation tests with transient temperature histories.     

The current–voltage characteristics of heterostructures formed by MEH-PPV spin-coated on n-type GaAs and n-type porous GaAs

P–N heterostructures are formed by depositing the poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) MEH-PPV on n-type GaAs(100) substrate and on n-type porous GaAs. The elaborated heterostructures are studied by current–voltage measurements. Thermionic emission is used to model the heterostructures behaviors and to extract parameters as ideality factor and zero bias barrier high. Such model also appears to be useful as a new approach for calculating hole concentration in MEH-PPV deposited on n-type GaAs (N_a = 2.2 × 10¹⁷ cm^− 3). The zero bias barrier height in both heterostructures were found to be close to the ionization energy difference of isolated MEH-PPV and n-type GaAs and the ideality factor values are found to be high. Such high values are suggested to be due to the existence of high density of trap. This is preliminary evidenced by calculating the trap density using space charge limited conductivity (SCLC) characterized by an exponential distribution of trapping levels in both heterostructures.     

Analysis of large synchronous machines with axial skew, part 1: flux density and open-circuit voltage harmonics

An approach to predict the air-gap flux density and voltage harmonics in large synchronous machines is presented. Combining a small number of solutions of magnetostatic finite-element analysis (FEA) with harmonic analysis, the approach allows designers to rapidly investigate the source of flux density harmonic predictions. The method provides the ability to trace flux density predictions to individual mmf-permeance combinations and applies to machines with both integral and fractional slot windings, with arbitrary skew. Predicted results are compared with that of measured values of open- and short-circuit air-gap flux density in one machine. Open-circuit voltage predictions for five machines are compared with measurements and predictions using commercial FEA.     

Mechanical and ellipsometry measurements of thin TiN layer prepared by PIII

Titanium nitrides have good mechanical, biomedical and optical properties, therefore they are used to harden and protect cutting and sliding surfaces and as a non-toxic exterior for bio-medical applications. Nitrogen plasma immersion implantation (PIII), in which the diffusion of nitrogen from low pressure r.f. plasma is combined with the implantation of nitrogen ions at energies up to 30 kV, is an effective tool for nitriding titanium and titanium alloys. In this work, samples of pure titanium were nitrided by PIII at different negative high voltage pulses. The properties and the characteristics of the processed samples were evaluated using X-ray diffraction (XRD), Auger electron spectroscopy (AES), ball-on-disk type tribometer, surface profilemeter, and ellipsometry measurements. The results show that, the wear resistance of the untreated sample in comparison to the PIII treated samples is extremely poor and the friction coefficient for the PIII treated samples is decreased to the half value in comparison to the untreated titanium, this attributed to the formation of the solid solution titanium α-Ti(N) and the cubic TiN phases. Ellipsometric measurements were carried out on the PIII treated samples at different negative high voltage pulses. A three layers model was used to fit the calculated data to the experimental ellipsometric spectra. The thickness, surface roughness and refractive index increase with increasing the negative high voltage pulses. The refractive index at 550 nm increases from 1.83 to 2.09 as the negative high voltage pulses increases from 10 to 30 kV.     

Modeling Temperature in Coupled Electrical and Thermal Simulations of the Electroconsolidation® Process

Electroconsolidation® is a process for densifying complex-shaped parts by using electrically conductive particulate solids as a pressure-transmitting medium. The part is immersed in a bed of the particulate medium contained in a die chamber. Sintering temperature is achieved by resistive heating of the medium while applying compaction pressure. The process is capable of ultrahigh temperatures and short cycle times and offers the potential for low processing costs.

Control of the process and selection of process conditions require knowledge of the temperatures within the die. Temperature gradients exist because of the high heating rate and because of variations of density and electrical resistivity of the medium due to the presence of the part. Direct measurement of temperature with thermocouples or other conventional means is impractical because of the high temperatures, high currents, and high pressures that are involved. Therefore, a computer model was developed to predict temperature as a function of time and applied voltage for any location in the die. The computer model is composed of three parts: a geometrical model to approximate the density and resistivity variations in the medium, a finite-element model to calculate the rate of resistive heating within each element, and a finite-difference model to calculate the temperature distribution based on solution of the heat-transfer equations. Predicted temperatures have been shown to be in excellent agreement with measurements, and numerical simulation provided encouraging consistency and reasonably accurate predictions of temperature profiles within the die. The model demonstrated the feasibility of a new process to achieve simultaneous application of pressure and heat to powder densification in Electroconsolidation.     

Modeling Temperature in Coupled Electrical and Thermal Simulations of the Electroconsolidation® Process

Electroconsolidation® is a process for densifying complex-shaped parts by using electrically conductive particulate solids as a pressure-transmitting medium. The part is immersed in a bed of the particulate medium contained in a die chamber. Sintering temperature is achieved by resistive heating of the medium while applying compaction pressure. The process is capable of ultrahigh temperatures and short cycle times and offers the potential for low processing costs. Control of the process and selection of process conditions require knowledge of the temperatures within the die. Temperature gradients exist because of the high heating rate and because of variations of density and electrical resistivity of the medium due to the presence of the part. Direct measurement of temperature with thermocouples or other conventional means is impractical because of the high temperatures, high currents, and high pressures that are involved. Therefore, a computer model was developed to predict temperature as a function of time and applied voltage for any location in the die. The computer model is composed of three parts: a geometrical model to approximate the density and resistivity variations in the medium, a finite-element model to calculate the rate of resistive heating within each element, and a finite-difference model to calculate the temperature distribution based on solution of the heat-transfer equations. Predicted temperatures have been shown to be in excellent agreement with measurements, and numerical simulation provided encouraging consistency and reasonably accurate predictions of temperature profiles within the die. The model demonstrated the feasibility of a new process to achieve simultaneous application of pressure and heat to powder densification in Electroconsolidation.     

Influence of magnetizing parameters on the magnetic Barkhausennoise

Magnetic Barkhausen noise (MBN) measurements were made on a sample of pipeline steel as functions of magnetizing frequency and AC flux density in the sample. The observed MBN responses suggest a strong dependence on these magnetizing parameters. With increasing flux density in the sample, the MBN activity initially increases but finally begins to decrease at higher magnitudes of flux density. The MBN activity also increases with increasing magnetizing frequency. Pulse-height distribution analysis of the MBN waveforms suggests that the dependence of MBN activity on these magnetization parameters is more complex than appears from the rms voltage measurements     

Investigation of charge trapping and breakdown characteristics of sputtered-Y2O3 on n-GaAs substrates

Metal-oxide-semiconductor (MOS) capacitors fabricated by depositing yttrium oxide (Y₂O₃) using radio frequency sputtering system on top of n-GaAs substrates have been investigated. To study the interface properties, charge trapping behavior and breakdown characteristics of Y₂O₃ gate dielectric, the MOS capacitors were subjected to constant current stress, high pulse voltage stress and high constant voltage stress. The average value of the cross section of generated traps during electrical stress has been determined from our experimental data. Further the trap charge density, its distribution and location have been investigated by measurements on application and subsequent withdrawal of high pulse voltage stress. Additionally, stress induced leakage current density and time dependent dielectric breakdown characteristics have been obtained and time-to-breakdown exceeding 840 s is observed for Y₂O₃ gate dielectrics directly deposited on n-GaAs. Our experimental results have been analyzed with simple analytical formulae available in the literature.     

Multilayer piezoelectric ceramic transformer with low temperature sintering

The low-fired high performance piezoelectric ceramics used for multilayer piezoelectric transformer were investigated. Based on the transient liquid phase sintering mechanism, by doping suitable eutectic additives and optimizing processing, the sintering temperature of the quaternary system piezoelectric ceramics with high piezoelectric properties could be lower to about 960–1000°C. The low-temperature sintering multilayer piezoelectric transformer (MPT) has been developed. Some characteristics of MPT were systemically studied. The measurements include the frequency response of input impedance, frequency response of phase difference between input voltage and current, frequency shifting with load, input impedance changing with load, phase difference between input voltage and current shifting with load, and phase difference between input voltage and vibration velocity. The vibration modes and resonance characters of MPT were measured by a Laser Doppler Scanning Vibrometer. Several kinds of MPT with high voltage step-up ratio, high power density, high transfer efficiency and low cost have been industrially produced and commercialized. It reveals a broad application prospect for back-light power of liquid crystal display and piezo-ionizer etc.     

微观组织对电解电容器铝箔比电容的影响

主要探讨了微观组织结构对电解电容器用铝箔比电容的影响。分析表明，高压阳极铝箔需要具有95％以上的立方织构以及一定的晶粒度；软各中低压阳极铝箔需要有75％～85％的立方织构，防止粗大的第二相产生，正品粒要求细小；硬念中低压阳极铝箔需要有高的位错密度和85％以上的(110)织构。硬态负极铝箔要求全属间化合物粒子细小弥散分布在Al基体中，均匀的位错分布和柯氏气团，从而获得均匀的海绵孔腐蚀。     

Voltage Relaxation and Its Influence on Critical Current Measurements

Based on the collective creep model, we numerically studied evolution of electric field and current density in superconductors and its influence on transport measurements of critical current. It is shown that many experimental facts, such as the dependence of V-I curves on sweeping rate of applied current and voltage relaxation are the results of this evolution. The simulation results are confirmed by electric transport measurements on Ag-sheathed Bi_2–x Pb _x Sr₂Ca₂Cu₃O _y tapes. Discussions on influences of the voltage relaxation on electric transport measurements including superconducting critical current are made.     

Current density distribution measurement of negative point-to-planecorona discharge

The current density distribution on the plate electrode of a point-to-plane corona discharge is measured with a computer-controlled measuring system. The measurements are taken by rotating the plate which consist of built-in arrays of orthogonal probes. The current-density distribution can be measured for different separation distances between the point electrode and the plane electrode while the applied high voltage can be varied from the voltage required for the onset of corona discharge to 30 kV. The current-density distribution of a single needle is symmetric around its center point within the accuracy of the measurement system. However, the current-density distribution is radially nonuniform. The computer-controlled measuring system is capable of measuring the current distribution of single point and multipoint electrodes     

Trapping lifetime and carrier mobility measurements in CuInSe(2 ) using surface-acoustic-wave technique

A novel technique based on the measurement of the frequency spectrum of the acoustoelectric current is used to determine the trapping time associated with dominant traps in p-type CuInSe(2). At room temperature, two trap levels with trapping time constant of 2x10(-4) and 6.7x10(-5) s are detected. Under white incandescent light, two more traps with trapping time constants of 1.4x10(-3) and 6x10(-4) s are detected. The minority (electron) and majority (hole) carrier mobilities in this material are also measured using the acoustoelectric technique, and they are 6+/-3 and 3.1+/-0.15 cm(2)/V-s, respectively. The hole carrier concentration was estimated to be around 5x10(15) cm(-3), and the surface of the sample was depleted.     

Si/SiGe hetero-junction solar cell with optimization design and theoretical analysis

Performances of solar cells, such as short circuit current density, open-circuit voltage, fill factor, and efficiency of solar cells on the multi-crystalline (mc)-SiGe on the Si with different Ge contents, are compared and investigated in this paper. The average Ge concentration was varied from 0% to ~ 20%. Appropriate addition of Ge in crystal Si is a very effective method to enhance the short circuit current density without degrading the open-circuit voltage owing to the modulation of the SiGe band-gap. The band-gap of the SiGe can be extracted by electron-hole plasma (EHP) model. With an optimization of Ge content and clean process condition, the overall efficiency of a Si/SiGe hetero-junction solar cell with Ge content of 8% is found to be ~ 16% and ~ 4% improvement achieved, as compared to the control multi-crystalline (mc)-Si solar cell. The theoretical simulations and analyses can help design the high efficiency Si/SiGe hetero-junction solar cell.