Superconductivity - Part 26: Critical current measurement - DC critical current of RE-Ba-Cu-O composite superconductors

IEC 61788-26:2020 specifies a test method for determining the DC critical current of short RE (rare earth)-Ba-Cu-O (REBCO) composite superconductor specimens that have a shape of straight flat tape. This document applies to test specimens shorter than 300 mm and having a rectangular cross section with an area of 0,03 mm2 to 7,2 mm2, which corresponds to tapes with width ranging from 1,0 mm to 12,0 mm and thickness from 0,03 mm to 0,6 mm.
This method is intended for use with superconductor specimens that have critical current less than 300 A and n-values larger than 5 under standard test conditions: the test specimen is immersed in liquid nitrogen bath at ambient pressure without external magnetic field during the testing. Deviations from this test method that are allowed for routine tests and other specific restrictions are given in this document.

Supraconductivité - Partie 26: Mesurage du courant critique - Courant critique continu des composites supraconducteurs de RE-Ba-Cu-O

L’IEC 61788-26:2020 spécifie une méthode d'essai pour la détermination du courant critique continu des échantillons de composites supraconducteurs RE (terre rare)-Ba-Cu-O (REBCO) courts en forme de ruban plat et rectiligne. Le présent document s'applique aux échantillons d’essai d'une longueur inférieure à 300 mm et d'une superficie de section rectangulaire de 0,03 mm2 à 7,2 mm2, ce qui correspond aux rubans d’une largeur de 1,0 mm à 12,0 mm et d’une épaisseur de 0,03 mm à 0,6 mm.
Cette méthode est destinée à être utilisée avec des échantillons de supraconducteurs caractérisés par un courant critique inférieur à 300 A et des valeurs n supérieures à 5 dans les conditions d’essai normalisées: l’échantillon d’essai est immergé dans un bain d’azote liquide à pression ambiante sans champ magnétique externe pendant l’essai. Le présent document spécifie les écarts par rapport à cette méthode d’essai qui sont admis dans les essais individuels de série, ainsi que d'autres restrictions spécifiques.

General Information

Status
Published
Publication Date
10-Jun-2020
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
11-Jun-2020
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IEC 61788-26
Edition 1.0 2020-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Superconductivity –
Part 26: Critical current measurement – DC critical current of RE-Ba-Cu-O
composite superconductors
Supraconductivité –
Partie 26: Mesurage du courant critique – Courant critique continu des
composites supraconducteurs de RE-Ba-Cu-O
IEC 61788-26:2020-06(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 61788-26
Edition 1.0 2020-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Superconductivity –
Part 26: Critical current measurement – DC critical current of RE-Ba-Cu-O
composite superconductors
Supraconductivité –
Partie 26: Mesurage du courant critique – Courant critique continu des
composites supraconducteurs de RE-Ba-Cu-O
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.220.20; 19.080; 29.050 ISBN 978-2-8322-8436-0

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 61788-26:2020 © IEC 2020
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms and definitions ...................................................................................................... 7

4 Principle .......................................................................................................................... 8

5 Apparatus ........................................................................................................................ 8

5.1 General ................................................................................................................... 8

5.2 Critical current measuring system ........................................................................... 8

6 Specimen preparation and setup ..................................................................................... 8

6.1 Length .................................................................................................................... 8

6.2 Mounting of the specimen ....................................................................................... 9

7 Critical current measurement ........................................................................................... 9

8 Calculation of results ....................................................................................................... 9

8.1 Critical current criteria............................................................................................. 9

8.2 n-value (optional) .................................................................................................. 11

9 Uncertainty of measurement .......................................................................................... 11

10 Test report ..................................................................................................................... 11

10.1 Identification of test specimen ............................................................................... 11

10.2 Reporting of I values ........................................................................................... 11

10.3 Reporting of I test conditions ............................................................................... 11

Annex A (informative) Additional information relating to measurement, apparatus, and

calculation ............................................................................................................................ 12

A.1 General information .............................................................................................. 12

A.2 Measurement condition ......................................................................................... 12

A.3 Apparatus ............................................................................................................. 13

A.3.1 Measurement holder material ........................................................................ 13

A.3.2 Measurement holder construction .................................................................. 13

A.4 Specimen preparation ........................................................................................... 14

A.5 Measurement procedure ....................................................................................... 14

A.5.1 Voltage leads ................................................................................................. 14

A.5.2 Cooling process ............................................................................................. 14

A.5.3 Temperature of liquid nitrogen bath ............................................................... 14

A.5.4 System noise and other contributions to the measured voltage ...................... 15

A.6 Calculation of n-value ........................................................................................... 16

Annex B (informative) Evaluation of combined standard uncertainty for REBCO

I measurement [8] ............................................................................................................... 17

B.1 Practical critical current measurement .................................................................. 17

B.2 Model equation ..................................................................................................... 18

B.3 I measurement results ......................................................................................... 19

B.4 Combined standard uncertainty [11] ...................................................................... 21

B.5 Type B uncertainty evaluation ............................................................................... 22

B.5.1 General ......................................................................................................... 22

B.5.2 Uncertainty of L measurement ...................................................................... 22

B.5.3 Uncertainty of voltage measurement .............................................................. 22

---------------------- Page: 4 ----------------------
IEC 61788-26:2020 © IEC 2020 – 3 –

B.5.4 Uncertainty of current measurement .............................................................. 23

B.5.5 Uncertainty of temperature measurement ...................................................... 23

B.5.6 Uncertainty coming from intrinsic non-uniformity of I .................................... 24

B.5.7 Comparison between types A and B combined standard uncertainties ........... 25

B.6 Influence of current ramp rate on the total uncertainty .......................................... 26

Bibliography .......................................................................................................................... 27

Figure 1 – Schematic view of measurement setup ................................................................... 9

Figure 2 – Intrinsic U-I characteristic .................................................................................... 10

Figure 3 – U-I curve with a current transfer component ......................................................... 10

Figure A.1 – Illustration of a measurement configuration for a short specimen of a few

hundred amperes class REBCO conductor ........................................................................... 13

Figure A.2 – Temperature dependence of I for commercial REBCO superconductors

(data from [9]) ....................................................................................................................... 14

Figure A.3 – Pressure dependence of boiling temperature of liquid nitrogen ......................... 15

Figure B.1 – Typical circuit to measure I .............................................................................. 17

Figure B.2 – Typical voltage–current (U-I) characteristic of a superconductor ....................... 18

Figure B.3 – Ramp time dependence of total RSU of I for conductors B, C, and D ............... 26

Table A.1 – Thermal contraction data of superconductor

and sample‑holder materials [1] ............................................................................................ 13

Table B.1 – Conductors distributed in the international RRT ................................................. 19

Table B.2 – I data for conductor A ....................................................................................... 19

Table B.3 – I data for conductor B ....................................................................................... 20

Table B.4 – I data for conductor C ....................................................................................... 20

Table B.5 – I data for conductor D ....................................................................................... 20

Table B.6 – Statistics for each conductor .............................................................................. 21

Table B.7 – ANOVA results for each conductor ..................................................................... 21

Table B.8 – Atmospheric pressure from 1 January 2014 to 31 December 2014 ..................... 24

Table B.9 – Intrinsic I non-uniformity evaluated by RTR-SHPM ........................................... 24

Table B.10 – Budget table of SUs of I measurements for conductor C ................................. 25

Table B.11 – Comparison of the relative standard uncertainties for

conductors B, C, and D ......................................................................................................... 25

---------------------- Page: 5 ----------------------
– 4 – IEC 61788-26:2020 © IEC 2020
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SUPERCONDUCTIVITY –
Part 26: Critical current measurement –
DC critical current of RE-Ba-Cu-O composite superconductors
FOREWORD

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rights. IEC shall not be held responsible for identifying any or all such patent rights.

International Standard IEC 61788-26 has been prepared by IEC technical committee
90: Superconductivity.
The text of this standard is based on the following documents:
FDIS Report on voting
90/455/FDIS 90/458/RVD

Full information on the voting for the approval of this standard can be found in the report on

voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

---------------------- Page: 6 ----------------------
IEC 61788-26:2020 © IEC 2020 – 5 –

A list of all parts in the IEC 61788 series, published under the general title Superconductivity,

can be found on the IEC website.

The committee has decided that the contents of this publication will remain unchanged until the

stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to

the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
---------------------- Page: 7 ----------------------
– 6 – IEC 61788-26:2020 © IEC 2020
INTRODUCTION

In 1986, superconductivity in some perovskite type materials containing copper oxides at

temperatures far above the critical temperatures of metallic superconductors was discovered.

In 1987, it was discovered that Y-Ba-Cu-O (YBCO) has a critical temperature (T ) of 93 K. After

a quarter century, the RE-Ba-Cu-O (REBCO, RE = rare earth) superconductors became

commercially available.

In 2013, VAMAS-TWA 16 started working on the critical current measurement methods in

REBCO superconductors. In 2014, an international round robin test (RRT) on the critical current

measurement method for REBCO superconductors was conducted that was led by VAMAS-

TWA 16. 10 institutions/universities/industries from five countries participated. The pre-

standardization work of VAMAS was taken as a base for this document, on the DC critical

current test method of REBCO composite superconductors.

The test method covered in this document is intended to give an appropriate and accepted

technical base to engineers working in the field of superconductivity technology.

---------------------- Page: 8 ----------------------
IEC 61788-26:2020 © IEC 2020 – 7 –
SUPERCONDUCTIVITY –
Part 26: Critical current measurement –
DC critical current of RE-Ba-Cu-O composite superconductors
1 Scope

This part of IEC 61788 specifies a test method for determining the DC critical current of short

RE (rare earth)-Ba-Cu-O (REBCO) composite superconductor specimens that have a shape of

straight flat tape. This document applies to test specimens shorter than 300 mm and having a

2 2

rectangular cross section with an area of 0,03 mm to 7,2 mm , which corresponds to tapes

with width ranging from 1,0 mm to 12,0 mm and thickness from 0,03 mm to 0,6 mm.

This method is intended for use with superconductor specimens that have critical current less

than 300 A and n-values larger than 5 under standard test conditions: the test specimen is

immersed in liquid nitrogen bath at ambient pressure without external magnetic field during the

testing. Deviations from this test method that are allowed for routine tests and other specific

restrictions are given in this document.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies.

For undated references, the latest edition of the referenced document (including any

amendments) applies.

IEC 60050-815, International Electrotechnical Vocabulary (IEV) – Part 815: Superconductivity

3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC 60050-815 and the

following apply.

ISO and IEC maintain terminological databases for use in standardization at the following URLs:

• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp/
3.1
constant sweep rate method

U-I data acquisition method where a current is swept at a constant rate from zero to a current

above I , and where the U-I data are acquired continuously or frequently
3.2
ramp-and-hold method

U-I data acquisition method where a current is swept in stages from zero to a current above I ,

where the current is held for an appropriate amount of time at each stage, and where the U-I

data are acquired continuously or frequently
---------------------- Page: 9 ----------------------
– 8 – IEC 61788-26:2020 © IEC 2020
4 Principle

The critical current of a composite superconductor specimen shall be determined from a

voltage–current (U-I) characteristic measured in a liquid nitrogen bath at ambient pressure. To

get a U-I characteristic, a direct current is applied to the superconductor specimen and the

voltage generated along the specimen is measured. The current is increased from zero and the

U-I characteristic is recorded. The critical current shall be determined as the current at which a

specific electric field strength criterion (electric field criterion) (E ) is reached. For any selected

, there shall be a corresponding voltage criterion (U ) for a specified voltage tap separation.

c c
5 Apparatus
5.1 General

The apparatus required for the present test method includes the critical current measuring

system. Additional information relating to the apparatus is given in Annex A.
5.2 Critical current measuring system

The apparatus to measure the U-I characteristic should consist of a specimen probe, an open

bath and a U-I measurement system.

The specimen probe, which consists of a specimen and a measurement holder, is inserted in

the open bath filled with liquid nitrogen. The U-I measurement system consists of a DC current

source and necessary data acquisition system, preamplifiers, filters or voltmeters, or a

combination thereof. Suitable measurement holder materials are recommended in A.3.1.

A computer assisted data acquisition system is recommended.
6 Specimen preparation and setup
6.1 Length
An example of a schematic view of measurement setup is shown in Figure 1.
The length (L) of specimen to be measured shall be defined as follows:
L = L + 2 × L + 2 × L + L ≥ 5 × W (1)
1 2 3 4
L , L , L ≥ W (2)
1 2 3
where
L is the distance between the voltage taps;
L is the length of the current contact;

L is the shortest distance from a current contact to the neighbouring voltage tap;

L is the width of a voltage tap.
W is the width of a specimen to be measured.

The larger the current-carrying capacity of the specimen, the larger shall be L . L shall be

2 2

increased for a specimen that has a stainless steel or other high-resistivity material backing or

jacket. For a measurement that needs the higher voltage sensitivity, L shall be increased. For

some practical values for L through L , see A.3.2.
1 4
---------------------- Page: 10 ----------------------
IEC 61788-26:2020 © IEC 2020 – 9 –
Figure 1 – Schematic view of measurement setup
6.2 Mounting of the specimen

The specimen shall be mounted to the flat surface of the holder. Both ends shall be fastened

or soldered to the current contact blocks.
The voltage taps shall be placed in the central part with or without solder.

The current contacts and the voltage taps shall be on the superconducting layer side.

Voltage leads shall be twisted as close to the voltage taps as possible.
7 Critical current measurement
The critical current shall be measured while minimizing mechanical strain.

The specimen shall be inserted slowly into the liquid nitrogen bath. The volume of the liquid

nitrogen bath shall be sufficiently larger than the specimen and the measurement holder. The

depth of the bath shall be sufficiently higher than the height of the measurement holder. The

specimen shall be cooled from room temperature to liquid nitrogen temperature until the

specimen and the measurement holder are sufficiently cooled by liquid nitrogen that boils with

microbubbles, i.e. steady state. It takes several tens of seconds.

When using the constant sweep rate method, the sweep rate shall be selected not to influence

the voltage measurement.

When using the ramp-and-hold method, the current sweep rate between stages shall be lower

than the equivalent of ramping from zero current to I in 3 s. Data acquisition at each stage

shall be started as soon as the flow or creep voltage generated by the current ramp can be

disregarded. The current drift during each current set point shall be less than 1 % of I .

Record the U-I characteristic with increasing current.
After measurement, the specimen shall be warmed up to room temperature.
Additional information relating to the measurement is given in Annex A.
8 Calculation of results
8.1 Critical current criteria

The critical current I shall be determined by using an electric field criterion E .

c c
---------------------- Page: 11 ----------------------
– 10 – IEC 61788-26:2020 © IEC 2020
I shall be determined at E = 100 μV/m. I determined at E = 10 μV/m is optional.
c c c c

The I shall be determined as the current corresponding to the point on the U-I curve where the

voltage U is measured (see Figure 2 and Figure 3):
U = L E (3)
c 1 c
where
U is the voltage criterion in microvolts (μV);
L is the voltage tap separation in metres (m);
E is the electric field criterion in microvolts per metre (μV/m).

U and I are the corresponding voltage and current values at the intersecting point of the

c c
straight lines with the U-I curve as shown in Figure 2.
Figure 2 – Intrinsic U-I characteristic

If the measured U-I curve includes a resistive component, it is recommended to increase L to

minimize the current transfer component voltage.
Figure 3 – U-I curve with a current transfer component
---------------------- Page: 12 ----------------------
IEC 61788-26:2020 © IEC 2020 – 11 –
8.2 n-value (optional)

The n-value shall be calculated as the slope of the plot of log U versus log I in the region where

the I is determined. The corresponding electric field region is recommended to be from 10 µV/m

to 100 µV/m.
The electric field region used to determine the n-value shall be reported.
Additional information relating to the calculation is given in Annex A.
9 Uncertainty of measurement

Unless otherwise specified, measurements shall be carried out in a liquid nitrogen bath whose

temperature can range from 76,8 K to 77,7 K. A voltmeter having 7,5 digits resolution,

providing 1 nV sensitivity on 10 mV setting shall be used to measure specimen voltage.

According to the international round robin test (see Annex B), the relative standard uncertainty

is less than 3 %. The target measurement uncertainty shall be 6 % with a coverage factor of 2.

10 Test report
10.1 Identification of test specimen
The test specimen shall be identified by the following:
a) name of the man
...

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