Superconductivity - Part 17: Electronic characteristic measurements - Local critical current density and its distribution in large-area superconducting films

IEC 61788-17:2021 is available as IEC 61788-17:2021 RLV which contains the International Standard and its Redline version, showing all changes of the technical content compared to the previous edition.IEC 61788-17:2021 specifies the measurements of the local critical current density (Jc) and its distribution in large-area high-temperature superconducting (HTS) films by an inductive method using third-harmonic voltages. The most important consideration for precise measurements is to determine Jc at liquid nitrogen temperatures by an electric-field criterion and obtain current-voltage characteristics from its frequency dependence. Although it is possible to measure Jc in applied DC magnetic fields [20] [21], the scope of this document is limited to the measurement without DC magnetic fields. This technique intrinsically measures the critical sheet current that is the product of Jc and the film thickness d. The range and measurement resolution for Jcd of HTS films are as follows.
- Jcd: from 200 A/m to 32 kA/m (based on results, not limitation).
- Measurement resolution: 100 A/m (based on results, not limitation).

General Information

Status
Published
Publication Date
27-Apr-2021
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
28-Apr-2021
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IEC 61788-17
Edition 2.0 2021-04
INTERNATIONAL
STANDARD
colour
inside
Superconductivity –
Part 17: Electronic characteristic measurements – Local critical current density
and its distribution in large-area superconducting films
IEC 61788-17:2021-04(en)
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IEC 61788-17
Edition 2.0 2021-04
INTERNATIONAL
STANDARD
colour
inside
Superconductivity –
Part 17: Electronic characteristic measurements – Local critical current density
and its distribution in large-area superconducting films
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 17.220.20; 29.050 ISBN 978-2-8322-9663-9

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

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC 61788-17:2021 © IEC 2021
CONTENTS

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

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

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms and definitions ...................................................................................................... 8

4 Requirements .................................................................................................................. 9

5 Apparatus ...................................................................................................................... 10

5.1 Measurement equipment ....................................................................................... 10

5.2 Components for inductive measurements .............................................................. 11

6 Measurement procedure ................................................................................................ 12

6.1 General ................................................................................................................. 12

6.2 Determination of the experimental coil coefficient ................................................. 12

6.3 Measurement of J in sample films........................................................................ 16

6.4 Measurement of J with only one frequency .......................................................... 16

6.5 Examples of the theoretical and experimental coil coefficients .............................. 17

7 Uncertainty in the test method ....................................................................................... 18

7.1 Major sources of systematic effects that affect the U measurement ..................... 18

7.2 Effect of deviation from the prescribed value in the coil-to-film distance ................ 19

7.3 Uncertainty in the experimental coil coefficient and the obtained J ...................... 20

7.4 Effects of the film edge ......................................................................................... 20

7.5 Specimen protection ............................................................................................. 20

8 Test report ..................................................................................................................... 21

8.1 Identification of test specimen ............................................................................... 21

8.2 Report of J values ............................................................................................... 21

8.3 Report of test conditions ....................................................................................... 21

Annex A (informative) Additional information relating to Clauses 1 to 8 ................................ 22

A.1 Comments on other methods for measuring the local J of large-area HTS

films ...................................................................................................................... 22

A.2 Requirements ....................................................................................................... 22

A.3 Theory of the third-harmonic voltage generation ................................................... 23

A.4 Calculation of the induced electric fields ............................................................... 24

A.5 Theoretical coil coefficient k and experimental coil coefficient k′ ............................ 25

A.6 Scaling of the U –I curves and the constant-inductance criterion to
3 0

determine I ........................................................................................................ 25

A.7 Effects of reversible flux motion ............................................................................ 27

Annex B (informative) Optional measurement systems ......................................................... 28

B.1 Overview............................................................................................................... 28

B.2 Harmonic noises arising from the power source and their reduction ...................... 29

Annex C (informative) Evaluation of the uncertainty ............................................................. 33

C.1 Evaluation of the uncertainty in the experimental coil coefficient ........................... 33

C.2 Uncertainty in the calculation of induced electric fields.......................................... 34

C.3 Experimental results on the effect of the deviation of the coil-to-film distance ....... 35

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IEC 61788-17:2021 © IEC 2021 – 3 –
C.4 Examples of the Type-A uncertainties of J and n-values, originating from

the experimental uncertainty in the U measurement ............................................ 35

C.5 Evaluation of the uncertainty in the obtained J .................................................... 36

C.6 Experimental results that reveal the effect of the film edge ................................... 37

Bibliography .......................................................................................................................... 39

Figure 1 – Diagram for an electric circuit used for inductive J measurement

of HTS films .......................................................................................................................... 10

Figure 2 – Illustration showing techniques to press the sample coil to HTS films ................... 11

Figure 3 – Example of a calibration wafer used to determine the coil coefficient ................... 12

Figure 4 – Illustration of the sample coil and the magnetic field during measurement ............ 13

Figure 5 – Illustration of the sample coil and its magnetic field generation ............................ 14

Figure 6 – E-J characteristics measured by a transport method and the U inductive

method ................................................................................................................................. 16

Figure 7 – Illustration of coils 1 and 3 in Table 2 ................................................................... 17

Figure 8 – The coil-factor function F(r) = 2H /I calculated for the three coils....................... 18

0 0

Figure 9 – The coil-to-film distance Z dependence of the theoretical coil coefficient k ......... 19

Figure A.1 – Illustration of the sample coil and the magnetic field during measurement ........ 24

Figure A.2 – U and U /I plotted against I in a YBCO thin film measured in applied

3 3 0 0

DC magnetic fields, and the scaling observed when normalized by I (insets) ..................... 26

Figure A.3 – Example of the normalized third-harmonic voltages (U /fI ) measured
3 0

with various frequencies ....................................................................................................... 26

Figure B.1 – Schematic diagram for the variable-RL-cancel circuit ........................................ 29

Figure B.2 – Diagram for an electrical circuit used for the two-coil method ............................ 29

Figure B.3 – Harmonic noises arising from the power source ................................................ 30

Figure B.4 – Noise reduction using a cancel coil with a superconducting film ........................ 30

Figure B.5 – Normalized harmonic noises (U /fI ) arising from the power source ................. 31

3 0

Figure B.6 – Normalized noise voltages after the reduction using a cancel coil with a

superconducting film ............................................................................................................. 31

Figure B.7 – Normalized noise voltages after the reduction using a cancel coil without

a superconducting film .......................................................................................................... 32

Figure B.8 – Normalized noise voltages with the two-coil system shown in Figure B.2 .......... 32

Figure C.1 – Effect of the coil position against a superconducting thin film on the

measured J values .............................................................................................................. 38

Table 1 – Specifications and theoretical coil coefficients k of sample coils ............................ 14

Table 2 – Specifications and coil coefficients of typical sample coils ..................................... 17

Table C.1 – Uncertainty budget table for the experimental coil coefficient k′ ......................... 34

Table C.2 – Examples of repeated measurements of J and n-values ................................... 36

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– 4 – IEC 61788-17:2021 © IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SUPERCONDUCTIVITY –
Part 17: Electronic characteristic measurements –
Local critical current density and its distribution
in large-area superconducting films
FOREWORD

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indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent

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

IEC 61788-17 has been prepared by IEC technical committee 90: Superconductivity. It is an

International Standard.

This second edition cancels and replaces the first edition published in 2013. This edition

constitutes a technical revision.

This edition includes the following a significant technical change with respect to the previous

edition:

a) A simple method to calculate theoretical coil coefficient k is described in 6.2.1.

---------------------- Page: 6 ----------------------
IEC 61788-17:2021 © IEC 2021 – 5 –
The text of this International Standard is based on the following documents:
FDIS Report on voting
90/462/FDIS 90/464/RVD

Full information on the voting for the approval of this International Standard can be found in the

report on voting indicated in the above table.
The language used for the development of this International Standard is English.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in

accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available

at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are

described in greater detail at www.iec.ch/standardsdev/publications.

A list of all the parts of 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 document will remain unchanged until the

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

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it

contains colours which are considered to be useful for the correct understanding of its

contents. Users should therefore print this document using a colour printer.
---------------------- Page: 7 ----------------------
– 6 – IEC 61788-17:2021 © IEC 2021
INTRODUCTION

Over thirty years after their discovery in 1986, high-temperature superconductors are now

finding their way into products and technologies that will revolutionize information transmission,

transportation, and energy. Among them, high-temperature superconducting (HTS) microwave

filters, which exploit the extremely low surface resistance of superconductors, have already

been commercialized. They have two major advantages over conventional non-superconducting

filters, namely: low insertion loss (low noise characteristics) and high frequency selectivity

(sharp cut) [1] . These advantages enable a reduced number of base stations, improved speech

quality, more efficient use of frequency bandwidths, and reduced unnecessary radio wave noise.

Large-area superconducting thin films have been developed for use in microwave devices [2].

They are also considered for use in emerging superconducting power devices, such as resistive-

type superconducting fault-current limiters (SFCLs) [3] [4] [5], superconducting fault detectors

used for superconductor-triggered fault current limiters [6] [7] and persistent-current switches

used for persistent-current HTS magnets [8] [9]. The critical current density J is one of the key

parameters that describe the quality of large-area HTS films. Nondestructive, AC inductive

methods are widely used to measure J and its distribution for large-area HTS films [10] [11]

cos(3ωt + θ) is the most
[12] [13], among which the method utilizing third-harmonic voltages U

popular [10] [11], where ω, t and θ denote the angular frequency, time, and initial phase,

respectively. However, these conventional methods are not accurate because they have not

considered the electric-field E criterion of the J measurement [14] [15] and sometimes use an

inappropriate criterion to determine the threshold current I from which J is calculated [16]. A

th c

conventional method can obtain J values that differ from the accurate values by 10 % to 20 %

[15]. It is thus important to establish standard test methods to precisely measure the local

critical current density and its distribution, to which all involved in the HTS filter industry can

refer for quality control of the HTS films. Background knowledge on the inductive J

measurements of HTS thin films is summarized in Annex A.

In these inductive methods, AC magnetic fields are generated with AC currents I cosωt in a

small coil mounted just above the film, and J is calculated from the threshold coil current I ,

c th

at which full penetration of the magnetic field to the film is achieved [17]. For the inductive

method using third-harmonic voltages U , U is measured as a function of I , and the I is

3 3 0 th

determined as the coil current I at which U starts to emerge. The induced electric fields E in

0 3

the superconducting film at I = I , which are proportional to the frequency f of the AC current,

0 th

can be estimated by a simple Bean model [14]. A standard method has been proposed to

precisely measure J with an electric-field criterion by detecting U and obtaining the n-value

c 3

(index of the power-law E-J characteristics) by measuring I precisely at various frequencies

[14] [15] [18] [19]. This method not only obtains precise J values, but also facilitates the

detection of degraded parts in inhomogeneous specimens, because the decline of n-value is

more noticeable than the decrease of J in such parts [15]. It is noted that this standard method

is excellent for assessing homogeneity in large-area HTS films, although the relevant parameter

for designing microwave devices is not J , but the surface resistance. For application of large-

area superconducting thin films to SFCLs, knowledge on J distribution is vital, because J

c c
distribution significantly affects quench distribution in SFCLs during faults.

The International Electrotechnical Commission (IEC) draws attention to the fact that it is claimed

that compliance with this document may involve the use of a patent. IEC takes no position

concerning the evidence, validity, and scope of this patent right.
___________
Numbers in square brackets refer to the Bibliography.
---------------------- Page: 8 ----------------------
IEC 61788-17:2021 © IEC 2021 – 7 –

The holder of this patent right has assured IEC that s/he is willing to negotiate licences under

reasonable and non-discriminatory terms and conditions with applicants throughout the world.

In this respect, the statement of the holder of this patent right is registered with IEC. Information

may be obtained from the patent database available at http://patents.iec.ch.

Attention is drawn to the possibility that some of the elements of this document may be the

subject of patent rights other than those in the patent database. IEC shall not be held

responsible for identifying any or all such patent rights.
---------------------- Page: 9 ----------------------
– 8 – IEC 61788-17:2021 © IEC 2021
SUPERCONDUCTIVITY –
Part 17: Electronic characteristic measurements –
Local critical current density and its distribution
in large-area superconducting films
1 Scope

This part of IEC 61788 specifies the measurements of the local critical current density (J ) and

its distribution in large-area high-temperature superconducting (HTS) films by an inductive

method using third-harmonic voltages. The most important consideration for precise

measurements is to determine J at liquid nitrogen temperatures by an electric-field criterion

and obtain current-voltage characteristics from its frequency dependence. Although it is

possible to measure J in applied DC magnetic fields [20] [21], the scope of this document is

limited to the measurement without DC magnetic fields.

This technique intrinsically measures the critical sheet current that is the product of J and the

film thickness d. The range and measurement resolution for J d of HTS films are as follows.

– J d: from 200 A/m to 32 kA/m (based on results, not limitation).
– Measurement resolution: 100 A/m (based on results, not limitation).
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 – Part 815: Superconductivity

(available at )
3 Terms and definitions

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

some of which are repeated here for convenience.

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

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
critical current

maximum direct current that can be regarded as flowing without resistance practically

Note 1 to entry: I is a function of magnetic field strength, temperature and strain.

[SOURCE: IEC 60050-815:2015, 815-12-01]
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IEC 61788-17:2021 © IEC 2021 – 9 –
3.2
critical current criterion
I criterion

criterion to determine the critical current, I , based on the electric field strength, E, or the

resistivity, ρ
-14

Note 1 to entry: E = 10 µV/m or E = 100 µV/m is often used as electric field criterion, and ρ = 10 Ω · m or

-13
ρ = 10 Ω · m is often used as resistivity criterion.
[SOURCE: IEC 60050-815:2015, 815-12-02]
3.3
critical current density

electric current density at the critical current using either the cross-section of the whole

conductor (overall) or of the non-stabilizer part of the conductor if there is a stabilizer

Note 1 to entry: The overall current density is called engineering current density (symbol: J ).

[SOURCE: IEC 60050-815:2015, 815-12-03]
3.4
transport critical current density
critical current density obtained by a resistivity or a voltage measurement
[SOURCE: IEC 60050-815:2015, 815-12-04]
3.5
n-value

exponent obtained in a specific range of electric field strength or resistivity

when the voltage/current U (I) curve is approximated by the equation
UI∝
[SOURCE: IEC 60050-815:2015, 815-12-10]
4 Requirements

The critical current density J is one of the most fundamental parameters that describe the

quality of large-area HTS films. In this document, J and its distribution are measured non-

destructively via an inductive method by detecting third-harmonic voltages U cos(3ωt + θ). A

small coil, which is used both to generate AC magnetic fields and detect third-harmonic voltages,

is mounted just above the HTS film and used to scan the measuring area. To measure J

precisely with an electric-field criterion, the threshold coil currents I , at which U starts to

th 3

emerge, are measured repeatedly at different frequencies and the E-J characteristics are

determined from their frequency dependencies.

The target relative combined standard uncertainty in the method used to determine the absolute

value of J is less than 10 %. However, the target uncertainty is less than 5 % for the purpose

of evaluating the homogeneity of J distribution in large-area superconducting thin films.

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– 10 – IEC 61788-17:2021 © IEC 2021
5 Apparatus
5.1 Measurement equipment

Figure 1 shows a schematic diagram of a typical electric circuit used for the third-harmonic

voltage measurements. This circuit is comprised of a signal generator, power amplifier, digital

multimeter (DMM) to measure the coil current, band-ejection filter to reduce the fundamental

wave signals and lock-in amplifier to measure the third-harmonic signals. It involves the single-

coil approach in which the coil is used to generate an AC magnetic field and detect the inductive

voltage. This method can also be applied to double-sided superconducting thin films with no

obstacles. In the methods proposed here, however, there is an additional system to reduce

harmonic noise voltages generated from the signal generator and the power amplifier [14]. In

an example of Figure 1, a cancel coil of specification being the same as the sample coil is used

for cancelling. The sample coil is mounted just above the superconducting film, and a

superconducting film with a J d sufficiently larger than that of the sample film is placed below

the cancel coil to adjust its inductance to that of the sample coil. Note that the inductance of

the sample coil decreases by 20 % to 30 % due to the superconducting shielding current when

it is mounted on a superconducting film. Both coils and superconducting films are immersed in

liquid nitrogen (a broken line in Figure 1). Other optional measurement systems are described

in Annex B.

NOTE In this circuit, coil currents of about 0,1 A (RMS) and power source voltages of > 6 V (RMS) are needed to

measure the superconducting film of J d ≈ 10 kA/m while using coil 1 or 2 of Table 2. A precision power amplifier

with sufficiently high power is used to supply such large currents and voltages.
NOTE The broken line surrounds elements immersed in liquid nitrogen.
Figure 1 – Diagram for an electric circuit used
for inductive J measurement of HTS films
---------------------- Page: 12 ----------------------
IEC 61788-17:2021 © IEC 2021 – 11 –
5.2 Components for inductive measurements
5.2.1 Coils

Currently available large-area HTS films are deposited on areas as large as about 25 cm in

diameter, while films about 5 cm in diameter are commercially used to prepare microwave

filters [22]. Larger YBa Cu
...

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