Wansoo Nah

Effects of silver additions on the mechanical properties and resistance to thermal shock of YBa2Cu3O7–δ superconductors

Abstract We evaluated the effects of silver (Ag) additions on the mechanical properties and resistance to thermal shock of bulk YBa2Cu3O7–δ (YBCO) superconductors. Various amount of Ag was added to YBCO by two different mixing techniques, i.e. in the form of metallic Ag powder and AgNO3 powder. It was observed that the bending strength (strength), fracture toughness and resistance to thermal shock of YBCO increased with increasing Ag content, probably due to strengthening mechanisms by the addition of Ag. In addition, YBCO-Ag composites made by mixing with AgNO3 showed slightly higher strength, fracture toughness and resistance to thermal shock as compared with those made by mixing with metallic Ag powder. The improvement in the resistance to thermal shock is believed to be due to improvements in the strength, fracture toughness and thermal conductivity of YBCO-Ag composites resulting from Ag additions.

3-D optimal shape design of ferromagnetic pole in MRI magnet of open permanent-magnet type

This paper presents 3-D optimal shape design method of ferromagnetic pole pieces for magnetic resonance imaging (MRI) devices of open permanent magnet type. The design method is based on a continuum sensitivity analysis in conjunction with the boundary integral equation method (BIEM). The magnetic poles of an open magnet assembly are designed by the proposed method and the conventional shimming method. Magnetic field strength, ppm order homogeneity and working space volume are compared between the proposed method and conventional one.

A study on joining method of Bi–Pb–Sr–Ca–Cu–O multifilamentary tape

We evaluated the electrical properties of 37 multifilamentary jointed tapes processed by superconducting joint. In the superconducting-joint method, a lap-joint was used; tapes were selectively etched, and exposed superconducting cores of the two tapes were brought into contact with each other and then only the joined region was uniaxially pressed. The critical current ratio (CCR) of the jointed tape was evaluated as a function of number of step in the contacting region. It was observed that the CCR was dependent on the number of step, and it steadily increased to 58% as the number of step increased to four. The microstructural evolution was evaluated and correlated to the resultant property of jointed tape.

Inverted-load network for high-power Doherty amplifier

In this article, a high-power, high-efficiency inverted Doherty power amplifier (PA), having a more compact load network than that of the conventional Doherty amplifier, was designed and implemented for wide-band code-division multiple access (WCDMA) base-station applications. Its configuration and working principle are compared with the conventional Doherty amplifier. For experimental verification, we implemented an inverted Doherty amplifier, using a 190 W peak-envelope-power (PEP) laterally diffused metal-oxide-semiconductor (LDMOS) field-effect transistors (FETs). Using a four-carrier down-link WCDMA signal, we achieved a high power-added efficiency (PAE) of 32% and an average output power level as high as 46.3 dBm at a given adjacent channel leakage ratio (ACLR) level of -30 dBc. This is a 9.5% improvement in efficiency and 1 dB improvement in output power under the same ACLR conditions from those of the balanced class-AB operation using the same devices.

AC transport current loss of horizontally attached Bi-2223/Ag tapes

In this paper we investigate AC-transport current loss and current distribution of Bi-2223/Ag tapes which are arranged in edge-to-edge configuration. A numerical code was developed to estimate the transport current loss and the current distribution across the tapes. AC-loss measurements were also performed and the coincidence between the calculated and measured values is excellent. AC losses of the horizontally-attached tapes were larger than just a simple summation of each tapes loss but not as large as the loss from Norris equation. Discussions on these results are addressed using the current distribution obtained from numerical analysis.

Comparison of shape and topology optimization methods for HTS solenoid design

This paper compares the advantages and shortcomings of shape optimization method and topology optimization method for HTS solenoid design. For both methods critical current condition of superconducting material is taken into account for accurate analysis and design. Details of each method are explained including design variables and sensitivity analysis. With these methods, it is possible to design HTS solenoid that makes efficient use of superconducting material. Design results show that it is best to use topology optimization method for basic layout design and use shape optimization method for the fine-tuning of the basic layout.

High-Efficiency Class-F Amplifier Design In the Presence of Internal Parasitic Components of Transistors

In this paper, the authors present an efficient design method for high efficiency class-F amplifiers considering active devices with internal parasitic components. Considering internal drain current/voltage waveforms observed at the internal drain current source of the transistor, harmonics control and fundamental matching circuits are optimized to have higher power-added efficiency (PAE). Two 1.2GHz amplifiers were designed using a large-signal model of an LDMOSFET: one had harmonics control circuits optimized for external current/voltage waveforms and another had them optimized for internal waveforms. Simulation of the class-F amplifiers with two different output networks was conducted. Class-F amplifiers having an output network optimized for internal waveforms had 19% higher PAE (79% versus 60%)

Electrical Parameter Extraction of High Performance Package Using PEEC Method

This paper proposes a novel electrical characterization approach for a high-performance package system using an improved Partial Element Equivalent Circuit (PEEC). As the effect of interconnects becomes a pivotal factor for the performance of high-speed electronic systems, there is a great demand for an accurate equivalent model for interconnects. In particular, an equivalent model of interconnects is established in this paper for the Fine-Pitch Ball Grid Array (FBGA) package using the improved PEEC method. Based on the equivalent model, electrical characteristics are analyzed; furthermore, these are verified through the measurement results of a Vector Network Analyzer (VNA).

Measurement of Multi-Port S-Parameters using Four-Port Network Analyzer

An efficient measurement methodology is proposed to construct the scattering parameters of a multi-port device using a four-port vector network analyzer (VNA) without the external un-terminated ports. By using the four-port VNA, the reflected waves from the un-terminated ports could be minimized. The proposed method significantly enhances the accuracy of the S-parameters with less number of measurements compared to the results of classical renormalization technique which uses twoport VNA. The proposed method is validated from the measured data with the coupled 8-port micro-strip lines.

Load line analysis of Bi-2223 tape-stacked-cable for self field effects

In general, the critical current of a Bi-2223 tape-stacked-cable is much less than the total summation of critical currents of each tape, which is mainly due to the self magnetic fields of the cable itself. Therefore, to predict the critical current of a Bi-2223 tape-stacked-cable, we need to analyze the self field effects of the stacked cable as well as the critical current density data (J/sub c/) of one tape. To make it more complex, the critical current degradation of a Bi-2223 tape is anisotropic; the critical current is lower in the normal magnetic field (to the tape surface) than in the parallel field. In this paper, a novel approach to predict the critical current of a Bi-2223 tape-stacked-cable from a J/sub c/-B curve of one tape is presented. The approach basically includes the load line analysis of the stacked tapes, and its usefulness is confirmed by the experimental data.