IEC 61788-4:2016
(Main)Superconductivity - Residual resistance ratio measurement - Residual resistance ratio of Nb-Ti and Nb3Sn composite superconductors
Superconductivity - Residual resistance ratio measurement - Residual resistance ratio of Nb-Ti and Nb<sub>3</sub>Sn composite superconductors
IEC 61788-4:2016 specifies a test method for the determination of the residual resistance ratio (RRR) of Nb-Ti and Nb3Sn composite superconductors with Cu, Cu-Ni, Cu/Cu-Ni and Al matrix. This method is intended for use with superconductor specimens that have a monolithic structure with rectangular or round cross-section, RRR value less than 350, and cross-sectional area less than 3 mm2. In the case of Nb3Sn, the specimens have received a reaction heat-treatment. This fourth edition cancels and replaces the third edition published in 2011. This edition constitutes a technical revision.
Supraconductivité - Partie 4: Mesurage du rapport de résistance résiduelle - Rapport de résistance résiduelle des composites supraconducteurs de Nb-Ti et de Nb<sub>3</sub>Sn
L'IEC 61788-4:2016 spécifie une méthode d'essai pour la détermination du rapport de résistance résiduelle (RRR) des composites supraconducteurs de Nb-Ti et de Nb3Sn avec une matrice de Cu, Cu-Ni, Cu/Cu-Ni et Al. Cette méthode est destinée à être utilisée avec des éprouvettes de supraconducteurs présentant une structure monolithique avec une section rectangulaire ou circulaire, un RRR de valeur inférieure à 350 et une surface de section inférieure à 3 mm2. Dans le cas de Nb3Sn, les éprouvettes ont subi un traitement thermique de réaction. Cette quatrième édition annule et remplace la troisième édition parue en 2011. Cette édition constitue une révision technique.
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Standards Content (Sample)
IEC 61788-4 ®
Edition 4.0 2016-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 4: Residual resistance ratio measurement – Residual resistance ratio of
Nb-Ti and Nb Sn composite superconductors
Supraconductivité –
Partie 4: Mesurage du rapport de résistance résiduelle – Rapport de résistance
résiduelle des composites supraconducteurs de Nb-Ti et de Nb Sn
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IEC 61788-4 ®
Edition 4.0 2016-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Superconductivity –
Part 4: Residual resistance ratio measurement – Residual resistance ratio of
Nb-Ti and Nb Sn composite superconductors
Supraconductivité –
Partie 4: Mesurage du rapport de résistance résiduelle – Rapport de résistance
résiduelle des composites supraconducteurs de Nb-Ti et de Nb Sn
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.220.20; 29.050 ISBN 978-2-8322-3129-6
– 2 – IEC 61788-4:2016 © IEC 2016
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 Material of measurement mandrel or of measurement base plate . 8
5.2 Diameter of the measurement mandrel and length of the measurement base
plate . 8
5.3 Cryostat for the resistance (R ) measurement . 9
6 Specimen preparation . 9
7 Data acquisition and analysis . 9
7.1 Resistance (R ) at room temperature . 9
*
7.2 Resistance (R or R ) just above the superconducting transition . 9
2 2
7.2.1 Correction of strain effect . 9
7.2.2 Data acquisition of cryogenic resistance . 10
7.2.3 Optional acquisition methods . 12
*
7.3 Correction on measured R of Nb-Ti composite superconductor for bending
strain . 12
7.4 Residual resistance ratio (RRR) . 12
8 Uncertainty and stability of the test method . 12
8.1 Temperature . 12
8.2 Voltage measurement . 12
8.3 Current . 13
8.4 Dimension . 13
9 Test report . 13
9.1 RRR value . 13
9.2 Specimen . 13
9.3 Test conditions . 14
9.3.1 Measurements of R and R . 14
1 2
9.3.2 Measurement of R . 14
9.3.3 Measurement of R . 14
Annex A (informative) Additional information relating to the measurement of RRR . 15
A.1 Recommendation on specimen mounting orientation . 15
A.2 Alternative methods for increasing temperature of specimen above
superconducting transition temperature . 15
*
A.3 Alternative measurement methods of R or R . 15
2 2
A.4 Bending strain dependency of RRR for Nb-Ti composite superconductor . 18
A.5 Procedure of correction of bending strain effect . 21
Annex B (informative) Uncertainty considerations . 23
B.1 Overview. 23
B.2 Definitions. 23
B.3 Consideration of the uncertainty concept . 23
B.4 Uncertainty evaluation example for TC 90 standards . 25
Annex C (informative) Uncertainty evaluation in test method of RRR for Nb-Ti and
Nb Sn composite superconductors . 27
C.1 Evaluation of uncertainty . 27
C.2 Summary of round robin test of RRR of a Nb-Ti composite superconductor . 30
C.3 Reason for large COV value in the intercomparison test on Nb Sn composite
superconductor . 31
Bibliography . 32
Figure 1 – Relationship between temperature and resistance. 8
Figure 2 – Voltage versus temperature curves and definitions of each voltage . 10
Figure A.1 – Definition of voltages . 17
Figure A.2 – Bending strain dependency of RRR value for pure Cu matrix of Nb-Ti
composite superconductors (comparison between measured values and calculated
values) . 19
Figure A.3 – Bending strain dependency of RRR value for round Cu wires . 19
Figure A.4 – Bending strain dependency of normalized RRR value for round Cu wires . 20
Figure A.5 – Bending strain dependency of RRR value for rectangular Cu wires . 20
Figure A.6 – Bending strain dependency of normalized RRR value for rectangular Cu
wires . 21
Figure C.1 – Distribution of observed r of Cu/Nb-Ti composite superconductor . 31
RRR
Table A.1 – Minimum diameter of the measurement mandrel for round wires . 21
Table A.2 – Minimum diameter of the measurement mandrel for rectangular wires. 21
Table B.1 – Output signals from two nominally identical extensometers . 24
Table B.2 – Mean values of two output signals . 24
Table B.3 – Experimental standard deviations of two output signals . 24
Table B.4 – Standard uncertainties of two output signals . 25
Table B.5 – COV values of two output signals . 25
Table C.1 – Uncertainty of each measurement . 30
Table C.2 – Obtained values of R , R and r for three Nb Sn samples . 31
1 2 RRR 3
– 4 – IEC 61788-4:2016 © IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SUPERCONDUCTIVITY –
Part 4: Residual resistance ratio measurement –
Residual resistance ratio of Nb-Ti and Nb Sn
composite superconductors
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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.
International Standard IEC 61788-4 has been prepared by IEC technical committee 90:
Superconductivity.
This fourth edition cancels and replaces the third edition published in 2011. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) the unification of similar test methods for residual resistance ratio (RRR) of Nb-Ti and
Nb Sn composite superconductors, the latter of which is described in IEC 61788-11.
The text of this standard is based on the following documents:
FDIS Report on voting
90/359/FDIS 90/360/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.
A list of all 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 publication will remain unchanged until
the stability date indicated on the IEC website 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.
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.
– 6 – IEC 61788-4:2016 © IEC 2016
INTRODUCTION
Copper, Cu/Cu-Ni or aluminium is used as matrix material in Nb-Ti and Nb Sn composite
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 and AC losses 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.
This part of IEC 61788 specifies the test method for residual resistance ratio of Nb-Ti and
Sn composite superconductors. The curve method is employed for the measurement of
Nb
the resistance just above the superconducting transition. Other methods are described in A.3.
SUPERCONDUCTIVITY –
Part 4: Residual resistance ratio measurement –
Residual resistance ratio of Nb-Ti and Nb Sn
composite superconductors
1 Scope
This part of IEC 61788 specifies a test method for the determination of the residual resistance
ratio (RRR) of Nb-Ti and Nb Sn composite superconductors with Cu, Cu-Ni, Cu/Cu-Ni and Al
matrix. This method is intended for use with superconductor specimens that have a monolithic
structure with rectangular or round cross-section, RRR value less than 350, and cross-
sectional area less than 3 mm . In the case of Nb Sn, the specimens have received a
reaction heat-treatment.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. 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: www.electropedia.org)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-815 and the
following apply.
3.1
residual resistance ratio
RRR
ratio of resistance at room temperature to the resistance just above the superconducting
transition
Note 1 to entry: This note applies to the French language only.
Note 2 to entry: In this part of IEC 61788 for Nb-Ti and Nb Sn composite superconductors, the room temperature
is defined as 293 K (20 °C), and the residual resistance ratio is obtained in Formula (1), where the resistance (R )
at 293 K is divided by the resistance (R ) just above the superconducting transition.
R
r = (1)
RRR
R
Here r is a value of the residual resistance ratio, R is a value of the resistance measured in a strain-free
RRR 2
condition and zero external magnetic field.
Figure 1 shows schematically a resistance versus temperature curve acquired on a specimen while measuring the
cryogenic resistance.
– 8 – IEC 61788-4:2016 © IEC 2016
(b)
A
R
(a)
T * T
c
IEC
The cryogenic resistance, R , is determined by the intersection, A, of two straight lines (a) and (b) at
*
temperature T .
c
Figure 1 – Relationship between temperature and resistance
4 Principle
The resistance measurement both at room and cryogenic temperatures shall be performed
with the four-terminal technique. All measurements are done without an applied magnetic field.
The target relative combined standard uncertainty of this method is defined as an expanded
uncertainty (k = 2) not to exceed 5 %.
The maximum bending strain induced during mounting and cooling the Nb-Ti specimen shall
not exceed 2 %. The measurement shall be conducted in a strain-free condition or in a
condition with allowable thermal strain for the Nb Sn specimen.
5 Apparatus
5.1 Material of measurement mandrel or of measurement base plate
Material of the measurement mandrel for a coiled Nb-Ti specimen or of the measurement
base plate for a straight Nb-Ti or Nb Sn specimen shall be copper, aluminium, 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 polyethylene terephthalate, polyester, polytetrafluoroethylene, etc.)
whose thickness is 0,1 mm or less.
5.2 Diameter of the measurement mandrel and length of the measurement base plate
The diameter of the measurement mandrel shall be large enough to keep the bending strain of
the specimen less than or equal to 2 % for the Nb-Ti specimen. The Nb Sn specimen on a
base plate shall be measured in a strain-free condition or a condition with allowable thermal
strain.
The measurement base plate shall be at least 30 mm long in one dimension.
Resistance
5.3 Cryostat for the resistance (R ) measurement
The cryostat shall include a specimen support structure and a liquid helium reservoir for
measurement of the resistance R . The specimen support structure shall allow the specimen,
which is mounted on a measurement mandrel or a measurement base plate, to be lowered
into and raised 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.
6 Specimen preparation
The test specimen shall have no joints or splices with a length of 30 mm or longer. The
specimen shall be instrumented with current contacts near each of its ends and a pair of
voltage contacts over its central portion. 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 should be taken during instrumentation
and installation of the specimen on the measurement mandrel or base plate so that no
excessive force, which may cause undesired bending strain or tensile strain, would be applied
to the specimen. Ideally, it is intended that the Nb Sn specimen be as straight as possible;
however, this is not always the case, thus care should be taken to measure the specimen in
its as received condition.
The specimen shall be mounted on a measurement mandrel or on a measurement base plate
for these measurements. Both resistance measurements, R and R , shall be made on the
1 2
same specimen and the same mounting.
7 Data acquisition and analysis
7.1 Resistance (R ) at room temperature
The mounted specimen shall be measured at room temperature (T (K)), where T satisfies
m m
the following condition: 273 K ≤ T ≤ 308 K. A specimen current (I (A)) shall be applied so
m 1
2 2
that the 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. Formula (2)
1 1 m
) at room temperature. The resistance (R )
below shall be used to calculate the resistance (R
m 1
at 293 K (20 °C ) shall be calculated using Formula (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 a
1 m,
pure Cu component.
U
(2)
R =
m
I
R
m
R = (3)
1+×0,00393 T – 293
( )
m
*
7.2 Resistance (R or R ) just above the superconducting transition
2 2
7.2.1 Correction of strain effect
*
Under a strained condition of the Nb-Ti specimen, the measured cryogenic resistance, R , is
not a correct value for R . The corresponding correction of the strain effect is described in 7.3.
– 10 – IEC 61788-4:2016 © IEC 2016
7.2.2 Data acquisition of cryogenic resistance
The specimen, which is still mounted as it was for the room temperature measurement, shall
be placed in the cryostat for electrical measurement specified in 5.3. Horizontal mounting of
the specimen is recommended in A.1. Alternate cryostats that employ a heating element to
sweep the specimen temperature are described in A.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.
*
During the acquisition phases of the low-temperature R measurements, a specimen current
2 2
(I ) shall be applied so that the current density is in the range 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 absolute value of 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.
U
U *
2+
(b)
A
(a)
U
0+
U
20+
U U
0rev 20–
T
U
0–
U *
2–
IEC
NOTE Voltages with subscripts + and – are those obtained in the first and second measurements under positive
and negative currents, respectively, and U and U are those obtained at zero current. For clarity, U
20+ 20– 0rev
measured at zero current is not shown coincident with U . Straight line (a) is drawn in the transition region with a
0–
sharp increase in the voltage with temperature and straight line (b) is drawn in the region with a nearly constant
voltage.
Figure 2 – Voltage versus temperature curves
and definitions of each voltage
When the specimen is in the superconducting state and the 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 measurement requires that excessive interfering voltages are not
R
present and that the specimen is initially in the superconducting state. Thus, the following
condition shall be met for a valid measurement:
UU−
0+ 0rev
< 1 % (4)
U
where U is the average voltage for the specimen in the normal state at cryogenic
temperature, which is defined by Formula (5).
The specimen shall be gradually warmed so that it changes to the normal state completely.
When the cryostat for the resistance measurement specified in 5.3 is used, this can be
achieved simply by raising the specimen to an appropriate position above the liquid helium
level. 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. The voltage versus
temperature curve shall continue to be recorded during the transition into the normal state, up
to a temperature somewhat less than 15 K for the Nb-Ti specimen and less than 25 K for the
Nb Sn specimen. Then, the specimen current shall be decreased to zero and the
corresponding voltage, U , shall be recorded at a temperature below 15 K for the Nb-Ti
20+
specimen and below 25 K for the Nb Sn specimen.
The specimen shall then be slowly lowered into the liquid helium bath and cooled to within
±1 K from the temperature at which the initial voltage signal U was recorded. A specimen
0+
, with the same magnitude but negative polarity (polarity opposite that used for the
current, I
initial curve) shall be applied and the voltage U shall be recorded at this temperature. The
0–
procedural steps shall be repeated to record the voltage versus temperature curve with this
negative current. In addition, when the measurement current, I , decreases to 0, the
recording of U shall be made at within ±1 K from the temperature at which U was
20– 20+
recorded.
Each of the two voltage versus temperature curves shall be analysed by drawing a line (a)
through the data where the absolute value of 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 for Nb-Ti or raised gradually and almost linearly for Nb Sn with
* *
temperature increase. U and U in Figure 2 shall be determined at the intersection of
2+ 2−
these two lines for the positive and negative polarity curves, respectively.
The corrected voltages, U and U , shall be calculated using the following equations:
2+ 2–
* *
U = U – U and U = U – U . The average voltage, U , shall be defined as
2+ 2+ 0+ 2– 2− 0–
||UU−
22+−
(5)
U =
*
A valid R measurement requires that the shift of thermoelectric voltage be within acceptable
limits during the measurements of U and U . Thus, the following condition shall be met for
2+ 2–
a valid measurement:
| ∆ − ∆ |
+ −
< 3 % (6)
U
*
where ∆ and ∆ are defined as ∆ = U – U and ∆ = U – U . If the measurement
R
+ – + 20+ 0+ – 20– 0– 2
does not meet the validity requirements in 7.2.2, specifically either in Formula (4) or (6), then
improvement steps either in hardware or experimental operation shall be taken to meet these
requirements before results are reported.
– 12 – IEC 61788-4:2016 © IEC 2016
*
Formula (7) shall be used to calculate the measured resistance ( R ) just above the
superconducting transition.
U
*
R = (7)
I
7.2.3 Optional acquisition methods
The method described in the body of this part of IEC 61788 is the “reference” method and
optional acquisition methods are outlined in A.3.
*
7.3 Correction on measured R of Nb-Ti composite superconductor for bending
strain
*
If there is no pure Cu component in the superconductor, then R shall be set equal to R .
For a specimen with a pure Cu component, the bending strain shall be defined by
ε = 100 × (h/r) (%), where h is a half of the specimen thickness and r is the bending radius. If
b
the bending strain is less than 0,3 %, then no correction is necessary, and R shall be set
*
equal to R .
If neither of the above two situations applies, then the resistance R just above the
superconducting transition under the strain-free condition shall be estimated by
L
*
RR −∆ρ ×
(8)
S
Cu
where ∆ρ is defined below and S and L are defined in 8.4. The increase in the resistivity of
Cu
pure copper at 4,2 K due to tensile strain, ε (%), is expressed by
−12 −14 2
∆ρ (Ωm) = 6,24 × 10 ε − 5,11 × 10 ε ; ε ≤ 2 % (9)
The calculation of Formula (9) shall be carried out assuming that the equivalent tensile strain
ε is (1/2)ε and (4/3 π)ε for rectangular and round wires, respectively. The bending strain
b b
dependency of residual resistance ratio for pure copper is described in A.4.
7.4 Residual resistance ratio (RRR)
The RRR value shall be calculated using Formula (1).
8 Uncertainty and stability of the test method
8.1 Temperature
The room temperature shall be determined with a standard uncertainty not exceeding 0,6 K,
while holding the specimen, which is mounted on the measurement mandrel or on the
measurement base plate, at room temperature.
8.2 Voltage measurement
For the resistance measurement, the voltage signal shall be measured with a relative
standard uncertainty not exceeding 0,3 %.
=
8.3 Current
When the current is directly applied to the specimen with a programmable DC current source,
the specimen test current shall be determined with a relative standard uncertainty not
exceeding 0,3 %.
When the specimen test current is determined from a voltage-current characteristic of a
standard resistor by the four-terminal technique, the standard resistor, with a relative
combined standard uncertainty not exceeding 0,3 %, shall be used.
The fluctuation of DC specimen test current, provided by a DC power supply, shall be less
than 0,5 % during every resistance measurement.
8.4 Dimension
The distance along the specimen between the two voltage taps (L) shall be determined with a
relative combined standard uncertainty not exceeding 5 %.
For correction of the bending strain effect in the case of the wire with pure Cu matrix, the
cross-sectional area of Cu matrix (S ) shall be determined using a nominal value of copper to
Cu
non-copper ratio and nominal dimensions of the specimen. The wire diameter (d) and mandrel
radius (R ) shall be determined with relative standard uncertainty not exceeding 1 % and 3 %,
d
respectively.
9 Test report
9.1 RRR value
The obtained RRR value (r ) shall be reported as
RRR
r (1 ± U ) (n = ∙∙∙), (10)
RRR re
where
U is the expanded relative uncertainty:
re
U = 2u (k = 2)
re r
where
u denotes the relative combined standard uncertainty,
r
k is a coverage factor, and
n is the sampling number.
It is desired that n be larger than 4 so that the normal distribution can be assumed for
observed results to estimate the standard deviation. If n is not sufficiently large, a rectangular
distribution shall be assumed.
9.2 Specimen
The test report for the result of the measurements shall also include the following items, if
known:
a) Manufacturer;
b) Classification and/or symbol;
c) Shape and area of the cross-section;
– 14 – IEC 61788-4:2016 © IEC 2016
d) Dimensions of the cross-sectional area;
e) Number of filaments or subelements;
f) Diameter of the filaments or subelements;
g) Cu to Nb-Ti volume ratio, Cu-Ni to Nb-Ti volume ratio, or Cu, Cu-Ni to Nb-Ti volume ratio,
or Al, Cu to Nb-Ti volume ratio or volume ratio among Cu-Ni, Cu, and Nb-Ti or among Al,
Cu, and Nb-Ti for Nb-Ti specimen;
h) Cu to non-Cu volume ratio for Nb Sn specimen;
i) Cross-sectional area of the Cu matrix (S ).
Cu
9.3 Test conditions
9.3.1 Measurements of R and R
1 2
The following test conditions shall be reported for the measurements of R and R :
1 2
a) Total length of the specimen;
b) Distance between the voltage measurement taps (L);
c) Length of each current contact;
d) Transport currents (I and I );
1 2
e) Current densities (I and I divided by the nominal total wire cross-sectional area);
1 2
* *
f) Voltages (U , U , U , U , U , U , U , U and U );
2+ 2−
1 0+ 0rev 20+ 0– 20– 2
*
g) Resistances (R , R , R and R );
m 1 2
h) Resistivities (ρ = (R × S )/L and ρ = (R × S )/L);
1 1 Cu 2 2 Cu
i) Material, shape, and dimensions of the mandrel or the base plate;
j) Installation method of the specimen in the mandrel or the base plate;
k) Insulating material of the mandrel or the base plate.
9.3.2 Measurement of R
The following test conditions shall be reported for the measurement of R :
a) Temperature setting and holding method of the specimen;
b) T : Temperature for measurement of R .
m m
9.3.3 Measurement of R
The following test conditions shall be reported for the measurement of R :
a) Rate of increasing temperature;
b) Method of cooling down and heating up.
Additional information relating to the measurement of RRR is given in Annex A. Annex B
describes definitions and an example of uncertainty in measurement. Uncertainty evaluation
in the reference test method of RRR for composite superconductors is given in Annex C.
Annex A
(informative)
Additional information relating to the measurement of RRR
A.1 Recommendation on specimen mounting orientation
When a specimen is in the form of straight wire, horizontal mounting of the wire on the base
plate is recommended since this mounting orientation can reduce possible thermal gradient
along the wire compared to the vertical mounting orientation. Here the horizontal mounting
orientation means that the wire axis is parallel to the surface of liquid helium.
A.2 Alternative methods for increasing temperature of specimen above
superconducting transition temperature
The following methods are also recommended for increasing temperature above the
superconducting transition of the specimen. The rate of increasing temperature of the whole
specimen within a range between 0,1 K/min and 10 K/min should be applied for these
methods. In order to dampen the rate of increasing temperature and to avoid a large
temperature gradient, special care should be taken in selecting heater power, heat capacity
(the specimen with the measurement mandrel or the measurement base plate) and the
distance between the heater and the specimen.
a) Heater method
The specimen can be heated above the superconducting transition by a heater installed in
the measurement mandrel or in the measurement base plate after taking the specimen out
of the liquid helium bath in the cryostat.
b) Adiabatic methods
1) Adiabatic method
In this method, the cryostat holds a chamber in which the specimen, a sample holder,
a heater and so on are contained. Before the chamber is immersed in the liquid helium
bath, air inside the chamber is pumped out and helium gas is filled. Then, the chamber
is immersed in the liquid helium bath and the specimen is cooled to a temperature of
5 K or lower. After the helium gas is pumped out, the specimen can be heated above
the superconducting transition by the heater under adiabatic condition.
2) Quasi-adiabatic method
In this method, the cryostat holds the specimen a certain distance above the liquid
helium bath for the entire cryogenic measurement. A thermal anchor from the
measurement mandrel or the measurement base plate to the liquid helium bath allows
the specimen to be cooled to a temperature of 5 K or lower. The specimen can be
heated above the superconducting transition by a heater located in the measurement
mandrel or the measurement base plate under quasi-adiabatic condition.
c) Refrigerator method
In this method, an electromechanical apparatus (a refrigerator) is used to cool the
specimen, which is mounted on a measurement mandrel or a measurement base plate, to
a temperature of 6 K or lower. The specimen can be heated above the superconducting
transition by a heater or by controlling the refrigerator power.
*
A.3 Alternative measurement methods of R or R
2 2
*
The following methods can optionally be used for acquisition of R or R .
2 2
– 16 – IEC 61788-4:2016 © IEC 2016
a) Modified reference method
This is a simplified method with acquisition of only one voltage-temperature curve and is
used only for Nb-Ti composite superconductors. The voltage of the specimen is measured
in the superconducting state under a desired direction of current (I ) and then with current
in the opposite direction. These values are U and U as shown in Figure A.1. The
0+ 0rev
current is then changed back to the initial direction. After the transition to the normal state,
'
the voltage is measured as U in a plateau region of the curve within about 4 K above
2+
the transition. Then the voltage is read under a zero current (U ). The current direction is
'
then reversed and the voltage is measured again ( U ). The cryogenic resistance is
2−
obtained from
U
* 2
R = (A.1)
I
with
''
UU−
22+−
U = (A.2)
This approximately compensates for the effect of thermoelectric voltage. The following
conditions sho
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