A cryostat enabling thermal conductivity measurements of specimens deformed plastically at liquid helium temperatures

A cryostat enabling thermal conductivity measurements of specimens deformed plastically at liquid helium temperatures

A new type o f cryostat is described which enables tensile plastic deformation of copper alloys at liquid helium temperatures and measurements of ther...

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A new type o f cryostat is described which enables tensile plastic deformation of copper alloys at liquid helium temperatures and measurements of thermal conductivity to be made without allowing the samples to age at temperatures above the liquid helium range. The cryostat has also been adapted for residual electricity resistivity measurements.

A cryostat enabling thermal conductivity measurements of specimens deformed plastically at liquid helium temperatures M.H.S. Rahim The cryostat was constructed in connection with work undertaken to investigate the effect of about 5% plastic deformation at liquid helium temperatures, and subsequent ageing at higher temperatures, on the values of lattice thermal conductivity of some copper alloys eg Cu-10 at. % A1 and Cu-10 at. % Au. The results of this work are discussed elsewhereJ Here, only the description of the cryostat used is given.

the top of the cryostat at O where the leads pass through a black wax vacuum seal. The heavy framework of the massive brass plates (P1)(Pa) are separated by three brass pillars (U) and the design is such that the helium glass dewar resting under the 'cap' (X) is not subjected to stresses when the specimen is deformed. z

A

R

General description of the cryostat

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The general form of the cryostat is shown schematically in Fig. I ; Fig. 2 shows the lower section of the cryostat in detail.

Tj_ ,

The tensile load can be applied by a screw mechanism at the top of the cryostat at (A) and the load is transmitted to the specimen (S), 1.5 mm diameter and 10 cm in length, by the stainless steel tube (B) surrounded by the evacuated 2.54 cm (1 in) stainless steel tube (C).

U

--.7--

The load is transmitted into the evacuated space through the rotating shaft vacuum seal (13) - Edwards type C 600. The specimen S is held rigidly by a massive bottom copper plate (E), and copper rod (F); the specimen being positioned in grips (G) made of copper. Plate E is fixed to the massive framework by three pairs of tubes (H) and (I); H and I are made of copper and stainless steel respectively. The massive copper plate (J) and the attached radiation shield (K) ensure that the specimen is surrounded by a vacuum space at a pressure below 1.33 x 10-TNm -2. Any heat from room temperature flowing down the tube B is diverted to the helium bath via copper wires (Y) anchored at the plate J. The apparatus is immersed in liquid helium in a helium glass dewar (not shown). The plate J has a small cavity (L) which can be filled with liquid helium via a needle-valve (M); the pressure of vapour above this helium enables the temperature of the plate to be determined and hence enables the carbon resistance thermometers (T1), (T2) fLxed on the specimen to be calibrated. The pressure of helium in the cryostat is controlled by a pump and Cartesian manostal - White 2 - , via tube (N). The design of this pumping system and that of the high vacuum specimen chamber is discussed elsewhere. 3 Thermal conductivity electrical leads pass through the high vacuum tube B, from T h e a u t h o r is at t h e D e p a r t m e n t o f Science and C o m p u t i n g , A c t o n T e c h n i c a l College, H i g h Street, A c t o n W3 6 R D , U K . Received 1 March 1978

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3 Fig. 1

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G

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Diagram of the cryostat

0011-2275/78/1806-0355 $01.00 © 1978 IPC Business Press CRYOGENICS. JUNE 1978

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The

deformation system

At the top of the cryostat the tube B is rigidly attached to a shaft (Q) and tightly fitting through the rotary screw seal D. The upper part of Q is threaded (R) to take the brass handle A with a nut (Z) - Fig. 1 - ; the threaded part never being allowed to come in contact with the rubber vacuum seal unit. As the handle is turned, longitudinal movement of the specimen mounted in the grips is caused by the action of the nut Z rotating on Q. Rotational movement of the shaft is prevented by having a strip (St.) with the thread ground away along the shaft and two pins (V1, V2) pressing against the strip.

51 N B

I 7

Specimen arrangement Here steady state methods are used to measure thermal conductivity values K of the specimen at the liquid helium bath temperature T. However, as both ends of the specimen have to be held in grips, the orthodox arrangement discussed by Charsley and Salter 4 in their standard thermal conductivity cryostat where the specimen is in thermal contact with the cryostat at one end, and with an electrical heater at the other, cannot be used. Here, instead, both ends of the specimen are thermally shortened by a short semicircular annealed copper wire (1) - Fig. 2. A short stout copper wire (2) is connected to the middle of (1) and the other ; i

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B I

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Fig. 3 3-3' is a plane view o f the copper disc E (shown in Fig. 1) t o which 3 support tubes H are soldered and arranged at the vertices of an equilateral triangle. Rod F passes through a central hole in E 4-4' is a plane view o f the copper block J shown in Fig. 1. Tube B passes through a central hole in J and the 2 . 5 4 c m (1 in) tube C soldered t o J is arranged to be concentric with tube B. Tubes I are in line with tubes H 2

5-5" is a plane view of the 'cap' X (shown in Fig. 1 ). 6 and 7 are holes provided in X t o take the liquid helium transfer tube and the vapou r pressu re release-valve

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Fig. 2

356

Detail of the lower section of the cryostat

I I

I 2

I 3

I 4 T, K

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I 6

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Fig. 4 Graph of K / T v s T 1 - Cu-10 at. % AI fully annealedspecimen B 2 -- Specimen B deformed at 3.66 K by 6.34% and measurements made w i t h o u t allowing the deformed specimen to age above 4.2 K 3 -- The deformed specimen aged at roomtemperature for 10 weeks and measurements made at liquid helium temperatures

CRYOGENICS.

JUNE

1978

end of 2 is thermally anchored to the plate J. The heater W is then placed approximately half way along the specimen and the temperature gradient along the lower half of the specimen length measured by the pair of carbon thermometers TI, T2 attached on the specimen as shown in Fig. 2.

electrically insulated from the cryostat and the grips by spraying the ends of the specimen with an insulating dupli-color satin black lacquer (supplied by Dupli-color Ltd). Standard electrical circuits similar to the design discussed by Charsley and Salter 4 are used here.

If QD is the rate of heat flow down the specimen which gives rise to the measured temperature difference, and Q is the measured heater power, then

Discussion

Otu l

O)

where l = effective length of the specimen, lu = distance of heater from the effective upper end of specimen. If T = the measured temperature difference between the two thermometers separaetd by the gauge length L then the thermal conductivity K of the specimen at temperature T is given by

K-

QDL

(2)

AT

Success was achieved in deforming specimens to about 6% strain in a pressure of about 1.33 x 10-TNm -2 at liquid helium temperatures. By pumping the gas above the liquid helium in the dewar, temperatures as low as 2.6 K have been attained. The results in this kind of work are often treated by plotting curves of KIT vs T (see for example Charsley and Salter, 4 Charsley et al s and Mitchell et a16). Fig. 4 shows three curves obtained for one of the Cu-10 at. % A1 specimens used. The dashed curve is from Salter 7 who used a similar specimen and who annealed the specimen in a similar manner to that used in the present work. Salter used a standard thermal conductivity cryostat and his results are well established. It is observed that the curve obtained for the 'annealed state' using the cryostat discussed in this paper, compares well with Salter's.

where A = cross-sectional area of the specimen.

References Adapting the cryostat for residual electrical resistivity measurements

1 2

In order to test the effect of deformation at liquid helium temperatures on the residual electrical resistivity values, the following modified arrangement is used.

3 4 5

The specimen is kept at liquid helium temperatures by doing away with the radiation shield K so that the specimen is surrounded by the liquid coolant. The specimen is

CRYOGENICS

. JUNE 1978

6 7

Charsley,P., Rahim, M.S.H. Phys Stat Sol (a) 34 (1976) 533 White,G.K. Experimental Techniques in Low Temperature Physics (1968) Clarendon, Oxford, UK Rahim,M.S.H. PhD Thesis (1975) Surrey University Charsley,P., Salter, J.A.M. Phys Stat Sol 10 (1965) 575 Charsley,P., Leaver, A.D.W., Salter, J.A.M. Phys Stat Sol 25 (1960) 531 Mitchell,M.A., Klemens, P.G., Reynolds, C.A. Phys Rev B3 (1971) 119 Salter,J.A.M. PhD Thesis (1966) London University

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