IEC 61788-4:2001

INTERNATIONAL IEC
STANDARD
61788-4
First edition
2001-07
Superconductivity –
Part 4:
Residual resistance ratio measurement –
Residual resistance ratio of Nb-Ti
composite superconductors
Supraconductivité –
Partie 4:
Mesure de la résistivité résiduelle –
Taux de résistivité résiduelle des supraconducteurs
composites au Nb-Ti
Reference number
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INTERNATIONAL IEC
STANDARD
61788-4
First edition
2001-07
Superconductivity –
Part 4:
Residual resistance ratio measurement –
Residual resistance ratio of Nb-Ti
composite superconductors
Supraconductivité –
Partie 4:
Mesure de la résistivité résiduelle –
Taux de résistivité résiduelle des supraconducteurs
composites au Nb-Ti
 IEC 2001  Copyright - all rights reserved
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– 2 – 61788-4  IEC:2001(E)
CONTENTS
FOREWORD . 3

INTRODUCTION .4

1 Scope . 5

2 Normative references. 5

3 Terminology. 5

4 Definition . 5
5 Requirements . 6
6 Apparatus . 6
6.1 Material of measuring mandrel or of measuring base plate. 6
6.2 Diameter of the measuring mandrel and length of the measuring base plate . 6
6.3 Cryostat for the resistance, R , measurement. 6
7 Specimen preparation . 6
8 Data acquisition .7
8.1 Resistance (R ) at room temperature . 7
8.2 Resistance (R *) just above the superconducting transition . 7
8.3 Correction on measured R * for bending strain. 9
8.4 Residual resistance ratio (RRR). 9
9 Accuracy and stability in the test method . 9
9.1 Temperature . 9
9.2 Voltage measurement . 9
9.3 Current . 9
9.4 Dimension. 9
10 Test report . 10
10.1 Specimen. 10
10.2 Reported RRR values . 10
10.3 Report of test conditions . 10
Annex A (informative) Additional information relating to the measurement of RRR . 12
Figure 1 – Relationship between temperature and voltage. 11
Figure 2 – Voltage versus temperature curves and definitions of each voltage. . 11
Figure A.1 – Bending strain dependency of RRR for pure Cu matrix of Nb-Ti composite
superconductors (comparison between measured values and calculated values) . 16
Figure A.2 – Bending strain dependency of RRR for round Cu wires . 16
Figure A.3 – Bending strain dependency of normalized RRR for round Cu wires . 17
Figure A.4 – Bending strain dependency of RRR for rectangular Cu wires. 17
Figure A.5 – Bending strain dependency of normalized RRR for rectangular Cu wires. 18
Figure A.6 – Distribution of observed RRR of Cu/Nb-Ti composite superconductor. 18
Figure A.7 – Definition of voltages. 19

61788-4  IEC:2001(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION

___________
SUPERCONDUCTIVITY –
Part 4: Residual resistance ratio measurement –

Residual resistance ratio of Nb-Ti composite superconductors

FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International
Organization for Standardization (ISO) in accordance with conditions determined by agreement between the
two organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61788-4 has been prepared by IEC technical committee 90:
Superconductivity.
The text of this standard is based on the following documents:
FDIS Report on voting
90/96/FDIS 90/104/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 3.
Annex A is for information only.
The committee has decided that the contents of this publication will remain unchanged until
2005. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
•
amended.
A bilingual version of this standard may be issued at a later date.

– 4 – 61788-4  IEC:2001(E)
INTRODUCTION
Copper is used as a matrix material in multifilamentary superconductors and works as an

electrical shunt when the superconductivity is interrupted. It also contributes to recovery of

the superconductivity by conducting heat generated in the superconductor to the surrounding

coolant. The cryogenic-temperature resistivity of copper is an important quantity, which

influences the stability of the superconductor. The residual resistance ratio is defined as a

ratio of the resistance of the superconductor at room temperature to that just above the

superconducting transition.
In this International Standard the test method of residual resistance ratio of Nb-Ti composite

superconductors is described. The curve method is employed for the measurement of the
resistance just above the superconducting transition. Other methods are described in
clause A.4.
61788-4  IEC:2001(E) – 5 –
SUPERCONDUCTIVITY –
Part 4: Residual resistance ratio measurement –

Residual resistance ratio of Nb-Ti composite superconductors

1 Scope
This part of IEC 61788 covers a test method for the determination of the residual resistance

ratio (RRR) of a composite superconductor comprised of Nb-Ti filaments and Cu, Cu-Ni or
Cu/Cu-Ni matrix. This method is intended for use with superconductors that have a
rectangular or round cross-section, RRR less than 350, and cross-sectional area less than
3 mm . All measurements shall be done without an applied magnetic field.
The method described in the body of this standard is the “reference” method and optional
acquisition methods are outlined in annex A.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this International Standard. For dated references, subsequent
amendments to, or revisions of, any of these publications do not apply. However, parties to
agreements based on this International Standard are encouraged to investigate the possibility
of applying the most recent editions of the normative documents indicated below. For undated
references, the latest edition of the normative document referred to applies. Members of IEC
and ISO maintain registers of currently valid International Standards.
IEC 60050-815:2000, International Electrotechnical Vocabulary – Part 815: Superconductivity
3 Terminology
For the purpose of this part of IEC 61788, the definitions given in IEC 60050-815 and the
following definition apply.
3.1
residual resistance ratio
the ratio of resistance at room temperature to the resistance just above the superconducting
transition
4 Definition
The residual resistance ratio of the composite wire shall be obtained in equation (1) below
where the resistance (R ) at room temperature (20 °C) is divided by the resistance (R ) just
1 2
above the superconducting transition.
R
RRR = (1)
R
Figure 1 shows schematically a voltage versus temperature curve acquired on a specimen
while measuring the cryogenic resistance. Draw a line in figure 1 where the voltage sharply
increases (a), and draw also a line in figure 1 where the temperature increases but the
resistance remains almost the same (b). The value of resistance at the intersection of these
two lines, A, is defined as resistance (R ) just above the superconducting transition.
– 6 – 61788-4  IEC:2001(E)
5 Requirements
The resistance measurement both at room and cryogenic temperatures shall be performed

with the four probe technique.

The target precision of this method is that the coefficient of variation (COV) in the inter-

comparison test shall be 5 % or less.

The maximum bending strain, induced during mounting the specimen, shall not exceed 2 %.

6 Apparatus
6.1 Material of measuring mandrel or of measuring base plate
Material of the measuring mandrel for a coiled specimen or of the measuring base plate for a
straight specimen shall be copper, aluminum, silver, or the like whose thermal conductivity is
equal to or better than 100 W/(m⋅K) at liquid helium temperature (4,2 K). The surface of the
material shall be covered with an insulating layer (tape or a layer made of mylar, polyester,
teflon, etc.) whose thickness is 0,1 mm or less.
6.2 Diameter of the measuring mandrel and length of the measuring base plate
Diameter of the measuring mandrel shall be large enough to keep bending strain of the
specimen less than or equal to 2 %.
The measuring base plate shall be at least 30 mm long in one dimension.
6.3 Cryostat for the resistance, R , measurement
The cryostat shall include a specimen support structure and a liquid helium reservoir for the
resistance, R , measurement. The specimen support structure shall allow the specimen,
which is mounted on a measurement mandrel or a measurement base plate, to be lowered
and raised into, and out of, a liquid helium bath. In addition, the specimen support structure
shall be made so that a current can flow through the specimen and the resulting voltage
generated along the specimen can be measured.
7 Specimen preparation
The test specimen shall have no joints or splices, and shall be 30 mm or longer. The distance
between two voltage taps (L) shall be 25 mm or longer. A thermometer for measuring

cryogenic temperature shall be attached near the specimen.
Some mechanical method shall be used to hold the specimen against the insulated layer of
the measurement mandrel or base plate. Special care shall be taken during instrumentation
and installation of the specimen on the measurement mandrel or on the measurement base
plate so that there is no excessive force, which may cause undesired bending strain or tensile
strain, being applied to the specimen.
The specimen shall be instrumented with current contacts near each end of the specimen and
a pair of voltage contacts over a central portion of the specimen. The specimen shall be
mounted on a measurement mandrel or on a measurement base plate for these measure-
ments. Both resistance measurements, R and R , shall be made on the same specimen and
1 2
the same mounting.
61788-4  IEC:2001(E) – 7 –
8 Data acquisition
R
8.1 Resistance ( ) at room temperature
The mounted specimen shall be measured at room temperature (T (°C)), where T satisfies
m m
the following condition, 0 ≤ T ≤ 35. A specimen current (I (A)) shall be applied so that the
m 1
2 2
current density is in the range of 0,1 A/mm to 1 A/mm based on the total wire cross-

sectional area, and the resulting voltage (U (V)), I and T shall be recorded. Equation (2)
1 1 m
below shall be used to calculate the resistance (R ) at room temperature. The resistance (R )
m 1
at 293 K (20 °C) shall be calculated using equation (3) for a wire with Cu matrix. The value of

R shall be set equal to R , without any temperature correction, for wires that do not contain
1 m
a pure Cu component.
U
R = (2)
m
I
R
m
R = (3)
[]1 + 0,00393 × ()T – 20
m
8.2 Resistance (R *) just above the superconducting transition
Under a strained condition of the specimen, the measured cryogenic resistance, R *, is not a
correct value for R . The corresponding correction of the strain effect will be described in
subclause 8.3.
8.2.1 The specimen, which is still mounted as it was for the room temperature measurement,
shall be placed in the cryostat for electrical measurement specified under 6.3. Alternate
cryostats that employ a heating element to sweep the specimen temperature are described in
clause A.2.
8.2.2 The specimen shall be slowly lowered into the liquid helium bath and cooled to liquid
helium temperature over a time period of at least 5 min.
8.2.3 During the acquisition phases of the low-temperature R * measurements, a specimen
current (I ) shall be applied so that the current density is in the range of 0,1 A/mm to
10 A/mm based on the total wire cross-sectional area, and the resulting voltage (U(V)), I
(A), and specimen temperature (T (K)) shall be recorded. In order to keep the ratio of signal to
noise high enough, the measurement shall be carried out under the condition that the
resulting voltage above the superconducting transition exceeds 10 μV. An illustration of the
data to be acquired and its analysis is shown in figure 2.

8.2.4 When the specimen is in superconducting state and test current (I ) is applied, two
voltages shall be measured nearly simultaneously, U (the initial voltage recorded with a
0+
positive current polarity) and U (the voltage recorded during a brief change in applied
0rev
current polarity). A valid R * measurement requires that excessive interfering voltages are not
present and that the specimen is initially in the superconducting state. Thus, the following
condition shall be met for a valid measurement:
U − U
0+ 0 rev
< 1 % (4)
U
where U is the average voltage for specimen in normal state at cryogenic temperature,
which is defined at 8.2.10.
– 8 – 61788-4  IEC:2001(E)
8.2.5 The specimen shall be gradually warmed so that it changes to the normal state
completely. When the cryostat for the resistance measurement specified under 6.3 is used,

this can be achieved simply by raising the specimen to an appropriate position above the

liquid helium level.
8.2.6 The specimen voltage versus temperature curve shall be acquired with the rate of

temperature increase maintained between 0,1 K/min and 10 K/min.

8.2.7 The voltage versus temperature curve shall continue to be recorded during the

transition and into the normal state, up to a temperature somewhat less than 15 K. Then the

specimen current shall be decreased to zero and the corresponding voltage, U , shall be
20+
recorded at a temperature below 15 K.
8.2.8 The specimen shall then be slowly lowered into the liquid helium bath and cooled to
the same temperature, within ±1 K, where the initial voltage signal U was recorded. A
0+
specimen current, I , with the same magnitude but negative polarity (polarity opposite that
used for the initial curve) shall be applied and the voltage U shall be recorded at this
0–
temperature. The procedural steps 8.2.5 to 8.2.7 shall be repeated to record the voltage
versus temperature curve with this negative current. In addition, the recording of U shall be
20–
made at the same temperature, within ±1 K, where U was recorded.
20+
8.2.9 Each of the two voltage versus temperature curves shall be analyzed by drawing a line
(a) through the data where the voltage sharply increases with temperature (see figure 2) and
drawing a second line (b) through the data above the transition where the voltage is nearly
constant with temperature. U * and U * shall be determined at the inter
...