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Submitted on 1 Jan 1978
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INVESTIGATION OF THERMAL CONDUCTIVITY
OF SUPERCONDUCTING MATERIALS WITHIN
THE TEMPERATURE RANGE FROM 4.2 TO 300 K
J. Bischof, A. Ryska
To cite this version:
INVESTIGATION OF THERMAL CONDUCTIVITY OF SUPERCONDUCTING MATERIALS WITHIN THE
TEMPERATURE RANGE FROM 4.2 TO 300 K.
J. Bischof and A. Ryska
Research Institute of Electrical Engineering, Bechoviee, 260 97 Praha 9, Czechoslovakia
Abstract.- The dependences on temperature of superconducting materials bulk samples were measured using the continuous - flow helium cryostat developed in Research Institute of Electrical Enginee-ring with the application of the steady - state method. The materials used were NbTi and ZrNb. The temperature gradient on the samples was measured by means of the direct method and method of diffe-rences. Relative error of thermal conductivity measurement did not exceed 4 %.
INTRODUCTION.- The development of new superconduc-ting materials, the technology of their prepara-tion, as well as their technical applications, re-quire the knowledge of the values of thermal con-ductivity received from the results of experiments. Basic physical values can be determined, and cha-racteristics of dynamics of a cooling process, es-pecially in the cases of locally limited transient from superconducting state into normal state, can be derived from thermal dependences, behaviours of which are obtainable from measurements of ther-mal conductivity.
RESULTS OF EXPERIMENTS.- In the Research Institute of Electrical Engineering for the use of experimen-tal study of thermal characteristics of solid ma-terials within the temperature range from 4.2 to 300 K was developed in the continuous-flow cryostat. The design of the mentioned cryostat enables its insertion into another cryostat with superconduc-ting magnet system, the inner opening of which has the diameter of 50 mm /I/.
On the lower heat exchanger of the cryostat, serving as a cold-state source for the space where the sample is inserted, the temperature is maintai-ned within the range from 4 to 300 K with the ac-curacy of 1 mK by using the helium flow and elec-trical resistance heater thermally anchored on the heat exchanger. Electrical heating is controlled by a precise Oxford Instruments regulator, equipped with a thermal sensing device CLTS.
The samples used for thermal conductivity mea-surements had the form of bars, the length of which did not reach over 70 mm. The samples were thermal-ly contacted over indium with the lower thermal ex-changer of the cryostat. The steady-state method with axial heat flux was chosen for measurement ar-ranged suitably so as to enable measuring of tem-perature gradient on the sample, either directly scanned by two thermal sensing devices, located alongside the measured sample (direct method), or derived from two data, obtained from the same sen-sing device by subsequent usen-sing two different hea-ter windings on the sample (method of differences)
121.
The given value of specific thermal conductivi-ty k can be calculated, supposing the fulfilling of several conditions from the expression
1r/Ti + T2 = 4J, UI "• 2 ; ifcF ' T2-Ti
where Ti and T2 are the temperatures and Z is the distance between thermoelectric terminals at hea-ting output H' = U.I on the lower end of the sam-ple, and d is the diameter of the sample. When the method of differences was used, thermal gradient on the sample was measured by means of only one ther-mometer R2. The heater winding H" of the same out-put Q as that of the winding H', is mounted on the terminal with thermometer Ri. When H' heating is quickly changed over on H" heating, thermal flux from the sample into the cryostat is not disturbed and therefore neither the steady-state of the com-plete system. The thermal gradient on the sample JOURNAL DE P H Y S I Q U E Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-675
Résumé.- Emploi de la méthode thermo-potentiomëtrique à flux axial de chaleur pour la mesure de la dépendance thermique de la conductibilité thermique d'échantillons massifs de matériaux supraconduc-teurs en alliage NbTi et ZrNb. Les mesures ont été effectuées dans le cryostat à flux d'hélium conti-nu développé à l'Institut de Recherches Electrotechniques de Courants Forts. La chute de température a été mesurée par la méthode directe et par la méthode différentielle. L'erreur relative dans la me-sure de conductibilité thermique est inférieure à 4 pour cent.
can be determined from the difference Rz(T1)-R2(T2).
References
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