G. Witz

The influence of thermal precompression on the mechanical behaviour of Ag-sheathed (Bi,Pb)2223 tapes with different matrices

Abstract The behaviour of the critical current in longitudinally strained Bi,Pb(2223) tapes shows a strain-insensitive plateau up to an irreversible strain limit e irr . For higher strains, the formation of cracks induces an abrupt decrease of the critical current. We investigate the relationship between precompression and irreversible strain limit with a set of tapes made with different filling factors as well as an in situ Inconel 600-reinforced tape. I c ( e ) curves were measured in a longitudinal strain apparatus at 77 K. The precompression at the measurement temperature is numerically estimated for each sample as well as the evolution of precompression during the cool-down. These calculated values are compared to the fracture susceptibility of extracted filaments, which gives an empirical estimate of the precompression. The main hypothesis of the “Irreversible I c Reduction Model” is confirmed, i.e. the irreversible strain limit essentially depends on precompression. However, we also found that the regime where I c remains constant contains a tensile component: the plateau extends beyond the external tensile strain needed to relieve the thermal precompression and includes a regime where the ceramic is further elongated non-destructively. This non-destructive deformation can be understood as a “connected-grains” behaviour, and extends the strain-insensitive plateau ∼0.1% beyond the precompression strain. This value is confirmed with a three points bending experiment performed on single filaments which gives a similar value for the bending failure strain. A comparable regime was found to exist also under compressive strain. These non-destructive regimes are of great importance for practical applications since up to a certain level the precompression can develop without any I c degradation.

Reduced filament coupling in Bi(2223)/BaZrO3/Ag composite tapes

Multifilamentary Bi(2223)/Ag tapes often exhibit AC loss levels comparable to those measured in monofilamentary samples, which is partly due to the large coupling currents induced in the low resistive sheath material. Surrounding the individual filaments by electrically insulating barrier layers suppresses these currents and strongly reduces the coupling. We demonstrate this effect with various types of magnetic and self-field AC loss measurements on a series of Bi(2223)/BaZrO3/Ag tapes. We also discuss the influence of barrier thickness, twist pitch length and filament arrangement on the measured losses in these composite conductors.

ac Losses in Bi, Pb(2223) barrier tapes

Abstract For many applications of Bi,Pb(2223) tapes involving alternative currents and/or fields, their ac losses are still too high. Several methods have been tried for reducing these losses and at present the most successful one is the introduction of a resistive barrier between filaments combined with filament twisting. At the University of Geneva, we tested an extensive series of different materials for their suitability as barrier materials. In this paper, we start by treating the different criteria these materials should meet for their use as barrier: no reactivity with Bi,Pb(2223) precursor, good deformability and low cost. We show why we used first BaZrO 3 and later SrZrO 3 as barriers and explain the differences between these two materials. We next discuss which modifications of deformation process are needed in order to have tapes with smooth and continuous barrier layers and to avoid too high a degradation of the critical current density. Finally, we show the effect of barriers on the ac losses and the effective matrix resistivity.

Properties of the plasma channel in liquid discharges inferred from cathode local temperature measurements

The properties of the plasma channel at the cathode surface in a liquid discharge have been studied by means of temperature measurements and heat transfer numerical analysis. The studied discharge (current: 5 A; duration: 100μs; gap: 10μm) is typical of electrical discharge machining (EDM) in the semifinishing operation. The temperature information is obtained from two independent experiments: (1) microthermocouples patterned on the cathode, close to the discharge have been used to record the temperature variation caused by a single discharge with a high local resolution and large bandwidth; (2) the geometry of the resolidified layer, which gives the maximum extension of the melting point temperature isotherm, has been measured. These temperature data have then been compared to numerical simulation using inverse calculations allowing the experimental determination of two fundamental quantities of the discharge cathode interaction: (1) the power fraction transferred from the discharge to the sample, which ...

Local temperature response to pulsed discharges in electronic discharge machining (EDM) environment

We report on local temperature measurements performed during single discharges in electronic discharge machining (EDM) environment. Measurements have been performed with two different setups. One being a thermal device consisting of 14 thin film Au-Pd thermocouples. This device with 4-/spl mu/m/sup 2/ large junctions was patterned using standard photolithography. The distance between the junctions (i.e., the local resolution of the device) is 20 /spl mu/m, whereas the sensitivity is 10 /spl mu/V/K at 150/spl deg/C. The other setup being a fast infrared charge-coupled device (CCD) camera with home made control electronics. An acquisition rate of 80 000 pictures/s with a 10-/spl mu/m local resolution was obtained. Using standard electron discharge machining (EDM) equipment, the temperature response due to a single discharge was measured with both the equipment, which show excellent agreement. The maximum temperature measured on the backside of a 20-/spl mu/m-thick foil is 590/spl deg/C. Analysis of the thermovoltage noise pattern allowed precise determination of the beginning and duration of the discharge. The numerical simulation of the process is presented and discussed.

Hot isostatic pressure reaction treatment of Ag-sheathed Bi,Pb(2223) tapes

Abstract One of the parameters of power in tube (PIT) processed Bi,Pb(2223) tapes which still leaves room for improvement is the density of the filaments, presently reported to be around 80–90%. To some extent this can be addressed with optimised intermediate deformation methods, such as periodic pressing, but this solution is limited by the necessity to heal the damage after deformation. The synthesis of the Bi,Pb(2223) phase whilst applying a hot isostatic pressure (HIP) has been found to be an attractive alternative route to obtain higher ceramic density. In this work we compare the properties of tapes processed under various isostatic pressure up to 1000 bar. High pressure was found to fasten the kinetics of phase formation. SEM observation showed an increased density of the ceramic core in HIP processed tapes. 20 h annealing under 100 bar improved the critical current density by 30% compared to the standard first heat treatment in PIT process.

Oxide barriers and their effect on AC losses of Bi,Pb(2223) multifilamentary tapes

The transverse electrical resistivity in multifilamentary Ag/Bi, Pb(2223) tapes is considerably enhanced after introducing inert oxide barriers, a new concept in which each single filament is surrounded by a highly resistive BaZrO/sub 3/ layer of <2 /spl mu/m thickness. With these oxide barriers, we have so far obtained a shift of the AC loss maximum from 5 Hz to >100 Hz. This corresponds to a marked lowering of AC coupling losses. The highest critical current density of these tapes is actually 15000 A/cm/sup 2/ at 77 K, 0 T, i.e. still below that of our tapes without barriers (35000 A/cm/sup 2/). The fabrication processes leading to Bi,Pb(2223) tapes with oxide barriers is described, with an emphasis on new deformation processes developed in our laboratory for the fabrication of long multifilamentary Bi,Pb(2223) tapes, comprising four roll (or two-axes) rolling and periodic pressing. The developed tapes with oxide barriers are promising in view of their use in transformers and cables.

Fabrication of square and round Ag/Bi(2223) wires and their ac loss behaviour

Square and round wires have been fabricated by using a combination of two-axial rolling and drawing in order to reduce the effect of field anisotropy and to confirm the decrease of ac losses predicted by calculation. Critical current measurements, as a function of magnetic field angle, confirmed that the symmetrical configuration of filaments inside the wire leads to an almost angle-insensitive behaviour of Jc(B). The Bi(2223) grains in our wires have a higher misalignment angle (around 8°) with respect to flat tapes, due to the particular wire configuration and the use of two-axial rolling. Therefore, the Jc values in our square wire are somewhat lower, 11 kA cm−2 at 77 K and zero field. AC loss measurements showed that the combination of wire geometry and twisting leads to strongly reduced ac losses in square Ag/Bi(2223) wires when compared to flat tapes: the value of 0.53 mW A−1 m−1 was measured, regardless of the field direction.

Improved J/sub c/ of multifilamentary Bi,Pb(2223)/Ag tapes by periodic pressing

Critical current densities of multifilamentary Ag-sheathed Bi(2223) tapes up to about 35 000 A/cm/sup 2/ have been achieved at 77 K and self field for lengths of several meters using an improved route: periodic pressing. This corresponds to an increase by 30-40% compared to the values obtained for conventionally rolled tapes starting from the same powders. Several pressing steps have been introduced during the anneal instead of the only standard rolling step (based on previous studies performed on both mono- and multifilamentary tapes). In contrast to earlier attempts by pressing techniques, periodic pressing is a practical and scaleable process for the fabrication of long lengths of Bi(2223) conductor as the standard intermediate rolling step. Engineering critical current densities of 8 000 A/cm/sup 2/ have successfully been obtained for tapes with high filling factor (35%).

Development of Bi(2223) multifilamentary tapes with low ac losses

A significant reduction of ac losses in twisted Bi(2223) multifilamentary tapes with Ag sheaths has been achieved by using oxide (BaZrO3 and SrZrO3) barriers between filaments. These barriers have two important effects: they increase the transverse resistivity, which suppresses induced coupling currents, and they reduce filament bridging, which in pure Ag sheath tapes largely cancels the beneficial effect of filament twisting. The decoupling can be gauged by the frequency at which loss shows a maximum in a low-amplitude ac field applied perpendicular to the tape. So far, the frequency of the loss maximum, fm, in Ag-sheathed tapes has been enhanced from 5 Hz (untwisted) to 82 Hz (11 mm in twist pitch length). Different ways to introduce oxide barriers in tapes with 19–95 filaments are presented. The critical current density in the filaments varied between 10,000 and 20,000 A/cm2. Ac loss measurements as well as the electrical and mechanical characterization are discussed in detail. The variation of the critical current density with bending strain is shown to be similar to that of tapes without barriers.