Internal Tin Research Articles

A thermo-mechanical model forNb3Sn filaments and wires: strain field for different strand layouts

In Nb3Sn CIC conductors, the superconducting compound is distributed into finefilaments and embedded in a resistive matrix for electrical and thermal stability.Nb3Sn formation requires a solid state diffusion reaction at high temperature, whichcauses an Sn gradient inside the filaments. It is well known that the criticalparameters vary with composition (Sn content) and strain state. In this workthe complete 3D strain field is computed for different wire layouts. First, therelation between the grade of filament reaction and strain is investigated:superconducting wires are studied, taking into consideration non-homogeneousNb3Sn filaments, i.e. considering an unreacted core of pure Nb. Furthermore, the case when thefilaments agglomerate together to give a ‘macrofilament’ is also taken into consideration(internal tin wires).A finite element discretization fine enough to take into consideration non-homogeneousfilaments would result in a very high number of unknowns, which could be beyond thecapacity of today’s computers. Therefore a thermo-mechanical model is formulated, basedon the generalized self-consistent method, suitably developed to deal with the materialnonlinearity and the coupling between the thermal and mechanical fields. In this way,equivalent homogeneous properties are obtained and the analysis of the wires becomesfeasible. An appropriate unsmearing technique finally gives the strain state in the real, nothomogenized, materials.

Evidence for highly localized damage in internal tin and powder-in-tubeNb3Sn strands rolled before reaction obtained from coupled magneto-optical imaging and confocallaser scanning microscopy

Nb3Sn strands for high-current, high-field magnets must be cabled before reaction while theconductor is still composed of ductile components. Even though still in the ductile,deformable state, significant damage can occur in this step, which expresses itself byinhomogeneous A15 formation, Sn leakage or even worse effects during later reaction. Inthis study, we simulate cabling damage by rolling recent high performance powder-in-tube(PIT) and internal tin (IT) strands in controlled increments, applying standardNb3Sn reaction heat treatments, and then examining the local changes using magneto-opticalimaging (MOI), scanning electron microscopy (SEM) and confocal laser scanningmicroscopy (CLSM). These combined characterizations allow any local damage to thefilament architecture to be made clear. MOI directly reveals the local variation ofsuperconductivity while CLSM is extremely sensitive in revealing Sn leakagebeyond the diffusion barrier into the stabilizing Cu. These techniques reveal amarkedly different response to deformation by the PIT and IT strands. The studydemonstrates that these tools can provide a local, thorough, and detailed view of howstrands degrade and thus complement more complex extracted strand studies.

Microstructure and superconducting properties comparison of bronze and internal tin process Nb 3Sn strands for ITER

The long multifilamentary Nb 3Sn strands for international thermonuclear experimental reactor (ITER) have been successfully fabricated by bronze and internal Sn process, respectively. Adopted in this work, the bronze process Nb 3Sn strand with a Cu/non-Cu area ratio of 1:1.09 and the internal tin process strand with a Cu/non-Cu area ratio of 1:1.15 were designed. The microstructure details of two kinds of strands before and after heat treatment have been investigated by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). The non-Cu J c (12 T, 4.2 K) value of 752 A/mm 2 for bronze process strand and 1185 A/mm 2 for internal tin process strand have been obtained. The influence of microstructure on transport property of strands has been discussed.

Internal Tin ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Conductors Engineered for Fusion and Particle Accelerator Applications

The critical current density (Jc) of Nb3Sn strand has been significantly improved over the last several years. For most magnet applications, high Jc internal tin has displaced bronze process strand. The highest Jc values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved Jc values of 3000 A/mm2 (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm2 at 15 T. To reduce the effective filament diameter in these high Jc strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu Jc values over 1100 A/mm2 (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm3 over non-Cu volume. The Jc-strain behavior of such composites is also presented.

The Influence of Bending Strain on the Critical Current of ${\rm Nb}_{3}{\rm Sn}$ Strands With Different Filament Twist Pitch

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The effect of bending strain on the transport properties of <formula formulatype="inline"><tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> strands has been the object of several investigations in the last years. We report on the performances of internal tin <formula formulatype="inline"><tex Notation="TeX">${\rm Nb}_{3}{\rm Sn}$</tex></formula> strands with different filament twist pitches, pre-compressed by swaging into thin stainless steel tubes before the reaction heat treatment and subject to pure bending strain. We have previously reported on the comparison between the performances of the technological OST-Dipole strand (TW-strand) with those of the same strand, but with untwisted superconducting filaments (UNTW-strand). The critical current, as well as the n-index of the samples with twisted filaments gradually degraded with the applied bending strain, while an enhancement of the performances has been observed on UNTW-strands. Here, results on further experimental measurements carried out on OKSC internal tin strands with different filament twist pitches are discussed, in order to clarify some aspects of the current transfer process within the strands. Finally, we carried out an analysis through the second derivative of the V-I curve, which evidenced a peaked critical current distribution for the UNTW-strands, while TW-strands under bending showed a higher degree of non-homogeneity, proven by broader distributions. </para>

Characterization of High Current RRP Wires as a Function of Magnetic Field, Temperature, and Strain

A new instrument for the characterization of superconducting materials as a function of Magnetic Field, Temperature and Strain, was designed, constructed and tested at Lawrence Berkeley National Laboratory (LBNL). A U-shaped bending spring was selected, since that design has proven to enable accurate characterizations of a multitude of superconducting materials for more than a decade. The new device is validated though measurements on very high current Rod Restack Processed (RRP) Internal-Tin (IT) wires, for which we will present initial results, including parameterizations of the superconducting phase boundaries and comparisons with other wire types. Accurate parametrization of modern high magnetic field conductors is important for the analysis of the performance of magnet systems.

Non-Uniform Bronze Formation in Internal Tin ${\rm Nb}_{3}{\rm Sn}$ Wire

The bronze phase transformations during the heat treatment of two multifilamentary internal tin Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn wires for high field applications made by Luvata are investigated and compared with some model single core wires. Non-uniform bronze profiles, non-uniform Sn distributions in different bundles, and Sn concentration gradient inversion were observed. Their possible formation mechanisms and consequences are discussed in relation to crystal orientation, Ti addition and microstructure. Wire manufacturing processes such as wire drawing impart preferred orientations on the components. The effect of wire texture on Cu-Sn interdiffusion and non-uniform bronze formation was investigated and believed not to be the cause of the non-uniform bronze profiles. 3-dimensional X-ray microtomography is applied to explain the non-uniformity in different bundles by analyzing the porosity formed during reactive diffusion of the low-melting phases. Ti is often added as a ternary addition, either in the Sn core or as Nb-Ti. In the Luvata wires investigated, Ti is present in Sn as the intermetallic compound Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> . Its dissolution through a ternary CuSnTi phase, possibly via reaction with CU <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , has been observed. EDS mapping of the Ti distribution suggests that it is likely to be responsible for the inversion of the Sn concentration gradient. The potential advantage of adding Ti in Sn over using Nb-Ti alloy is discussed. Critical temperature measurements were performed to relate superconducting properties to the phase transformations, composition changes and non-uniform bronze formation.

Titanium Doped ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Superconductor Fabricated by the Internal Tin Tube (ITT) Approach

Multifilament Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconductor has been produced by the internal tin tube (ITT) approach. The process employs tubular niobium filaments with cores consisting of tin or high tin alloy inside a copper sheath. The niobium annulus of each filament contained eight Nb-47Ti ldquoislands.rdquo During the subsequent reaction heat treatments the titanium was diffused out of these islands and throughout the niobium annulus resulting in a calculated titanium concentration of 1.5 wt%. The process holds the promise to minimize the amount of non-superconducting area in the conductor cross section and thereby maximize non-copper critical current density. Niobium tube fabrication, wire processing, filament quality, and microstructure are discussed, and superconducting properties are presented.

Synchrotron Radiation Techniques for the Characterization of ${\rm Nb}_{3}{\rm Sn}$ Superconductors

The high flux of high energy X-rays that can be provided through state-of-the-art high energy synchrotron beam lines has enabled a variety of new experiments with the highly absorbing Nb3Sn superconductors. We report different experiments with Nb3Sn strands that have been conducted at the ID15 High Energy Scattering beam line of the European Synchrotron Radiation Facility (ESRF). Synchrotron X-ray diffraction has been used in order to monitor phase transformations during in-situ reaction heat treatments prior to Nb3Sn formation, and to monitor Nb3Sn growth. Fast synchrotron micro-tomography was applied to study void growth during the reaction heat treatment of Internal Tin strands. The elastic strain in the different phases of fully reacted Nb3Sn composite conductors has been measured by high resolution X-ray diffraction during in-situ tensile tests.

Electromagnetic Cycling and Strain Effects on ${\rm Nb}_{3}{\rm Sn}$ Cable-in-Conduit Conductors With Variations in Cabling Design and Conduit Material Properties

A parametric study has been conducted to quantify the effect in performance of cable-in-conduit conductors (CICC's) to changes in cable and conduit design. Measurements of current sharing temperature and critical current as a function of electromagnetic cycling and longitudinal strain were systematically performed on CICC's with common Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn internal tin strand. The designs varied in void fraction (0.30 or 0.36), long or short cable twist pitch, cable core patterns (6 around 1 or triplet), and conduit material property (stainless steel 316 or Haynes 242). Measurements were performed at the NHMFL in a test facility for conductor characterization with capability to 12 T, 20 kA, and 250 kN axial tensile load now modified to deliver temperature controlled supercritical helium to the CICC samples.

Systematic Study on Filament Fracture Distribution in ITER ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Strands

In ITER CICCs, the strands experience varying mechanical strain levels due to thermal compression and electromagnetic loading, cumulating into severe periodic bending and contact strain. Depending on the choice of the cabling pattern, the strain may exceed the irreversibility limit leading to cracks in the Nb3Sn filaments and degraded performance of the conductors. We present a systematic microscopy study of filament fracture at gradually varied axial tensile strain levels in a bronze and internal tin processed type of Nb3Sn ITER strand. The axial stress-strain relation of the wires was measured at 4.2 K, up to a different peak load for each sample, successively increasing from 0.0% to 0.7%. Longitudinal cross sections were prepared from cut sections of the specimens. The crack pattern and distribution in the isolated filaments could thus be studied as a function of applied strain, revealing different fracture mechanisms. Electrical measurements were performed with applied strain with special attention for the tensile strain range. Finally, the microscopic information was correlated back to the observed macroscopic I c degradation.

The key role of twist pitch and mechanical pre-treatment in the transport properties of Nb 3Sn wires subject to bending strain

In modelling the transport properties of multi-filamentary Nb 3Sn strands, the knowledge of the geometrical parameters of the superconducting filaments and the electrical and mechanical properties of the different materials composing the wire are required. In particular, the filament twist pitch and the transverse resistivity between filaments have a crucial role in the definition of the current transfer length and, consequently, in the simulation of the transport performances of superconducting wires subject to mechanical loads, as in cable-in-conduit conductors (CICC) for fusion magnets during operation. We have measured the critical current and the n-index of internal tin Nb 3Sn wires with different values of the filament twist pitch, having inserted the strand into a stainless steel jacket and under the application of pure bending strain. Results show that the degradation of the transport properties is affected by the twist pitch value and, in the limit case of non-twisted filaments, even an improvement is observed in presence of bending. Moreover, the reversibility of critical current after relaxing the mechanical load has also been checked and an improvement of the performances has been observed after pre-bending applications, presumably due to the strain relaxation. In addition, the differential analysis through the second derivative of the V– I curve evidenced a peaked critical current distribution for the UNTW-strands, while TW-strands under bending showed a higher degree of non-homogeneity, proven by broader distributions.

Phase Transformations During the Reaction Heat Treatment of Internal Tin Nb&lt;formula formulatype="inline"&gt; &lt;tex Notation="TeX"&gt;$_{3}$&lt;/tex&gt; &lt;/formula&gt;Sn Strands With High Sn Content

The phase transformations that occur during the reaction heat treatment (HT) of Nb3Sn superconductors depend on the overall elemental composition of the strand subelements. In the case of modern high Jc strands with a relatively low Cu content, liquid phases are present during large temperature intervals and phases that can be detrimental for the microstructural and microchemical homogeneity of the fully reacted strand are formed. We report synchrotron X-ray diffraction measurements during in-situ reaction HT of a state-of-the-art high Jc Nb3 Sn internal tin strand. In this strand, Cu3Sn is formed upon Cu 6Sn 5 decomposition at 415degC, a Sn-rich ternary Cu-Nb-Sn phase is detected in the approximate temperature interval 345degC-575degC, and NbSn2 is present in the temperature interval 545degC-630degC. The formation of voids in the strand subelements has been monitored by synchrotron microtomography during in-situ reaction HT.

Manufacturing of the ITER TF Full Size Prototype Conductor

The experience gained in the past for the ITER toroidal field model coil conductor and the results obtained so far have led to the definition of an upgraded full size prototype conductor, based on advanced Nb3Sn strand, and entirely manufactured in the European Union (EU). Samples for the characterization in the Sultan facility have been prepared by Luvata (Italy) following the conductor layout defined by ITER. ENEA was responsible for conductor fabrication. Since the conductor layout was new, a full size copper dummy conductor has been preventively produced for the setting of the cabling and jacketing tools. Then, a total of four full size superconducting cables have been prepared by using Nb3Sn advanced strands produced by Oxford Instruments (OST) and European advanced superconductors (EAS), by internal tin and bronze technology, respectively. The details of manufacturing procedures will be described in this paper.

Characterization of ITER $\hbox{Nb}_{3}\hbox{Sn}$ Strands Under Strain-Applied Conditions

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Japan Atomic Energy Agency has developed four types of <formula formulatype="inline"> <tex>$\hbox{Nb}_{3}\hbox{Sn}$</tex></formula> strand which can be used in the ITER TF coils. One is a strand made by an internal tin process strand and the others are bronze process strands. The achieved critical current density is more than 790 <formula formulatype="inline"><tex>$\hbox{A/mm}^{2}$</tex> </formula> in the bronze process strands and more than 980 <formula formulatype="inline"> <tex>$\hbox{A/mm}^{2}$</tex></formula> in the internal tin process strand under 4.2 K temperature and 12 T magnetic field and there is hysteresis loss of less than 770 mJ/cc under <formula formulatype="inline"><tex>$\pm$</tex> </formula> 3 T cycle. Since these strands are utilized with an external strain, it is necessary to evaluate strain dependency to confirm the ITER conductor design. An apparatus to measure the strain dependency was newly developed. It has a horseshoe-shaped ring to produce uniform axial compressive or tensile strain along the strand length, a strand being soldered on the outer surface of the ring. The detailed strand characteristics were investigated subjecting the developed strands to a magnetic field from 10 T to 13 T, a strain from about <formula formulatype="inline"><tex>$-$</tex></formula>0.8% to 0.5%, and a temperature from 4.2 K to the critical temperature. When the critical current is normalized to that under the conditions where strain is intrinsically zero, the bronze process strands exhibit better performance than the internal tin process strand. However, the three bronze process strands do not exhibit the same <formula formulatype="inline"><tex>${\rm I}_{c}$</tex></formula>-strain characteristics. Two types of scaling relations are applied to the data, and good expressions of strand performance were obtained by the least square method within 3 A as RMS. </para>

Development of a New RF Produced Internal Tin ITER TF Conductor Sample for Testing in SULTAN Facility

The results are presented on characterization of internal-tin Nb3Sn strands designed and fabricated for use in full scale ITER TF conductor sample. The microstructure of strands has been investigated and interpreted in correlation with measured dependences of Jc (B) and Jc(B, T). The process of cabling in accordance with ITER recommended scheme have been successfully performed. The alternative new design of TF conductor with lower voids fraction was proposed and experimentally verified. The analysis of ITER related performance of both designed and fabricated conductors has been done on the base of SULTAN samples testing.

Strain influence on Jc behavior of Nb 3Sn multifilamentary strands fabricated by internal tin process for ITER

Superconducting properties of Nb 3Sn strands are much affected by the strain state of strands. More attention is concentrated on the influence of axial strain and bending test on the performance of Nb 3Sn strands and cables for practical applications. J c values of Nb 3Sn strands fabricated by internal tin process were measured in the axial strain range from −0.8% (compressive) to +0.7% (tensile) at 4.2 K, 12 T for ITER use. The different strain sensitivity of J c to the strain state was elaborated for the strands. The degradation of the superconducting critical current density and the n-value due to strain effect were analyzed.

Status of Nb 3Sn strand development in Korea

Although there were many research activities for the development of superconducting Nb 3Sn strands, the major one started under KSTAR (Korea Superconducting Tokamak Advanced Research) project in 1996. After the success of a large scale production test of Nb 3Sn strand using the internal tin route, a new mass production facility is under operation since 2004. KAT (Kiswire Advanced Technology Ltd.), an affiliate of Kiswire Ltd., manufactured various types of Nb 3Sn strands using the internal tin process optimized for fusion magnets. For the Nb 3Sn strand of the KSTAR PF coil, each module has ∼190 niobium filaments and 19 modules are restacked for the strand production. For the ITER TF strand, there are two types of basic design. One of them has 37and 19 modules with 169–219 niobium filaments in each module. The other has 19 modules with 164–190 niobium filaments in each module. Both the designs satisfy the requirements for ITER TF strand with enough margins. The characterization of the strands is performed by hysteresis loss measurement, RRR (Residual Resistivity Ratio), n-value, and critical current density measurement vs. temperature, magnetic field, and strain. The critical current density of the strands reached around 1100 A/mm 2 at 12 T and 4.2 K. A well defined quality assurance program helped to produce a high quality strand with a piece length of more than 15 km. KAT has been provided Nb 3Sn strand for KSTAR PF Coil and ready to produce the Nb 3Sn strand for ITER TF coil. In this paper, the design concept, the fabrication procedure and the result of the strand performance test are discussed.

Nb 3Sn material development in Russia

In the USSR and later in Russia, the main activities in technical superconductivity were concentrated in the institutes that belonged to the Ministry of Atomic Energy (Minatom). The development of new technologies shortly transferred to the large-scale industrial production of NbTi and Nb 3Sn superconductors in early 1970s. Two main technologies for multifilamentary Nb 3Sn strands were under investigation during that time – bronze-process and internal tin method. More than 25 ton of Nb 3Sn bronze-processed strands were produced for the fabrication of 90 ton of conductors for application in the magnet system of first in the world fusion facility (tokamak T-15) with magnet system based on the intermetallic compound. The characteristics of these strands and conductors have been briefly described. The requirements for the Nb 3Sn strands constantly increased and the main R&D on the enhancement of critical current density have been reviewed. For bronze-processed strands the increase of the tin content in large ingots was the crucial factor. The artificial doping of niobium filaments by niobium–titanium alloy was invented, which enabled to improve the workability of Nb 3Sn strands, with enhanced critical current density in high fields. For internal tin Nb 3Sn strands the main R&D were concentrated on the optimization of the layouts of the strand and on the multistage heat treatment because of the inevitable liquid phase formation which could result in severe distortion of the geometrical arrangement of the filaments and even in destruction of the whole strand. The main results of these investigations have been presented. The corresponding impact of these R&D on the design of bronze-processed and internal tin strands has been analyzed. The quantitative estimations of the grain size were made for bronze-processed and internal tin strands. It was shown that in bronze-processed and internal tin strands subjected to the standard ITER heat treatment characterized by two stages at 575 °C and 650 °C, the variation of Nb 3Sn grain size in the range of 30–300 nm could be observed. The correlations of microstructure and superconducting properties have been discussed. The ITER connected activities in Russia on the development of Nb 3Sn strands, which met the HP-II specification, have been outlined. The results of the ITER Model Coil Program have shown a degradation of the critical current of large cable-in-conduit conductors (CICC) built with Nb 3Sn strands. For this reason, the investigation on the strain dependence of critical current density in Nb 3Sn strands of different designs is of high interest and priority. The R&D on development of bronze-processed and internal tin Nb 3Sn strands with enhanced, by the nanostructured Cu–Nb material, mechanical strength have been reviewed.

Microstructure development in Nb3Sn(Ti) internal tin superconducting wire

The authors have studied the phase formation sequences in a Nb3Sn ‘internal tin’ process superconductor. Heat treatments were performed to convert the starting materials of tin, Ti–Sn, copper and niobium, to bronze and Nb3Sn. Specimens were quenched at different points of the heat treatment, followed by metallography to identify the phases present and X-ray microtomography (XMT) to investigate the void volume and distribution. An unexpected observation of the microstructure development was the uphill diffusion of tin during the Cu–Sn reactive diffusion. Some defects likely to affect the superconducting performance of the wires were observed. Microscopy revealed the presence of a Ti–Sn intermetallic compound displacing the niobium filaments, and XMT revealed the formation of long pores in the longitudinal direction. Two types of pore formation mechanism, in addition to Kirkendall pores, are proposed. The phase and microstructure development suggests that low-temperature heat treatment (below 415 °C) will have significant influence on optimising the final superconducting properties.