A capacity sensor for measuring very low temperatures

The inner space of the working chamber, the outer vessel, and cryostat 8 are pumped and helium gas is let in. The liquid refrigerant is poured into the cryostat. The specimen is cooled to the temperature of the refrigerant through thermal contact between the lower wall of the jacket and the outer vessel, which is obtained by compressing bellows 9. The specimen is heated to intermediate temperatures by the heater. Heat contact between the working chamber and the outer vessel is then reduced to a minimum by pumping the exchange gas out of the outer vessel. It takes 18 to 20 hours to determine the temperature dependence of the natural frequency of longitudinal oscillations of a specimen, from 4.2 K to room temperature. The apparatus is being used to determine Young's modulus for metals and alloys between 4.2 and 300 K. Figure 3 gives examples of the temperature dependence of Young's modulus for armco iron and titanium iodide. Comparison shows good agreement with other results 9,10.

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2.18 u~ 1.285

1.21

1'11

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50

100

150 T, K

200

250

REFERENCES

1. F6RSTER,F. Z. Metallkunde 29, 109 (1937) 2. MA:rVEEV,A. S., RIVP, E. KH., and FREIMAN,L. S. Industrial Lab. 18, 632 (1952) 3. POLOTSKn,I. G., and TABOROV,V. F. Industrial Lab. 18, 632 (1952) 4. MEL'N[CI-IUK,P. I., and TIMOSHENKO,V. G. Measuring Technique No. 4, 11 (1962) 5. KOVTUN,S. F., and UL'YANOV,R. A. Industrial Lab. 30, 1414 (1964) 6. KONRADI,G. G., and ZAMILATSKI],E. P. IndustrialLab. 32, 1410 (1966)

A capacity sensor for measuring very low temperatures V. I. M A K H A N ' K O V ,

I. S. S I D O R E N K O ,

and G. P. S H E M O N A E V

T HE AIM of the work was to find a sensor with which temperatures from room temperature down to 1 K could be measured, to an accuracy of not worse than + 0.1 K over the whole range. The sensor should stand sharp temperature changes of ~ 300 K, should be simple, reliably fixed to a specimen, readily available, and capable of repeated use. These demands can be met by a sensor based on the change of dielectric constant with temperature. A number of experiments were made with ceramic condensers of various types, for example, KTK, KDS, K M - 4 A NZO. The capacities were measured with a commercial IIEV-1 instrument (range 0 to 5 150 pF). The results of the measurements are shown in Table 1. The authors are with the Physico-Technical Institute, Academy of Sciences, Ukrainian SSR, Khar'kov. Prib. i Tekh. ~ksper. No. 4, 210 (1968). Received 14 January 1969. C R Y O G E N I C S • A U G U S T 1969

300

Figure 3. Temperature dependence of Young's modulus: 1 armcoiron, and 2 titanium iodide

7. LOZINSKII,M. G. High Temperature Metallography (Mashgiz, 1956) 8. KUZ'MENKO,V. A. Industrial Lab. 28, 726 (1962) 9. KOSTER,W . Z. Metallkunde 39, 1 (1948) 10. KOVTUN,S. F., and UL'VANOV,R. A. F M M 19, 263 (1965)

K T K condensers are unsuitable for temperature measurement as the low initial capacity reduces the accuracy. These condensers withstand sharp temperature changes. The sensitivity to temperature changes is 0.5 p F / K for a condenser of 680 pF nominal capacity. KDS condensers have an appreciably higher initial capacity, their sensitivity to temperature change is sufficiently high: 1-76 pF/K for a condenser of 3 nF nominal capacity, but the ceramic of several specimens was broken on cooling to 4.2 K many times and are unreliable in this respect. These two types of condenser are thus not of interest. Table 1 shows the changes in capacity of condensers of various types when cooled by liquid nitrogen and helium. A K M - 4 A NZO condenser satisfies all the requirements for a temperature sensor. The initial capacity is high. This leads to a small error in measuring temperature (capacity). The sensor withstands rapid temperature changes well. The temperature range measured is T A B L E 1. THE TEMPERATURE DEPENDENCE OF THE C A P A C I T Y OF V A R I O U S TYPES OF CONDENSER

Type of condenser and nominal capacity

Actual capacity at T = 300 K, pF

Capacity at T = 77.36 K, pF

Capacity at T = 4.2 K, pF

KTK 680 KDS 3000 KM-4A NZO

755 2 463 34 150

947 243.7 3 520

986.5 115.5 2 996.5

285

T = 300-1 K. It can be attached to the specimen either by soldering to a lead or by glueing. The calibration stays constant after repeated warming up to room temperature. The sensitivity between 77 and 4.2 K is 60 pF/K, and from 4.2 to 1 K, it is 20 pF/K. Only KM-4A NZO condensers were used in subsequent experiments. Ten were taken from two different sources; they were all calibrated at the following points: 273.15 K--the temperature of melting ice (primary fixed point), 178 K--the melting point of acetone, 77.36 K ~

the boiling point of nitrogen (secondary fixed point), 4.2 K - - t h e boiling point of helium. Calibration below 4-2 K was achieved by pumping liquid helium in a standard cryostat. The IIEV-1 instrument, which had some drift, was set to zero before each measurement of capacity. The zero was set while a switch placed directly on the cryostat was open, so that the leads should not affect the accuracy of the readings. We should note that the condenser losses increased on lowering the temperature. For example, if tan ~ = 0.02 at 300 K, it is 0.04 at the nitrogen boiling point. Relatively high losses prevent oscillations of the IIEV-I instruments' oscillator, so that it is necessary to connect a capacity of 1.5 to 2 nF in series with the condenser being measured. The series condenser can be disconnected at temperatures below 50 to 60 K, since the capacity of the measuring condenser decreases and the

T A B L E 2. C A P A C I T Y OF A SENSOR BETWEEN 4.2 A N D 1"7K

Temperature, K

Capacity, pF

1.7 2.4 3.3 4.2

2 949.7 2 961.5 2 974.4 2 999.0

T A B L E 3. TEMPERATURE DEPENDENCE OF THE C A P A C I T Y OF SEVERAL KM-4 A N Z O SENSORS

Capacity, pF

2.9

Temperature, K

Sensor I

Sensor II

Sensor III

1.7 4.2 77.3 178

2 921.6 2 978.0 7 314.3 14 506.6

2 611.0 2 660.2 6 073.6 14 000.8

2 459.1 2 500.3 5 694.6 13 435.8

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60 2.7

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20

2.6

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2.5

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4

6 8 10

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20 "1"I K

I

40

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60 60100

200 2-4

, I 3

2

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T,K

I nominal capacity 39.2 nF II nominal capacity 32.5 nF III nominal capacity 30.3 nF Figure 1. The temperature dependence of the capacity of KM-4ANZO condensers

Figure 2. The section of the curves of Figure 1 in the range T = 1 to 4.2 K

T A B L E 4. REPRODUCIBILITY OF THE C A P A C I T Y OF SENSOR I ON COOLING DOWN TO T = 4.2 K

Sequence of measurement Temperature, K

1

4.2 1.7

2 978.0 2 921.4

286

2

3

4

5

6

7

8

9

10

2 978.2 2 920.8

2 978.2 2 921.0

2 978.3 2 921.3

2 978.2 2 921.6

Capacity, pF 2 978.0 2 921.5

2 977.6 2 921.4

2 978.0 2 921.6

-2 920.9

2 977.4 2 921.3

CRYOGENICS

- AUGUST

1969

conditions for oscillation of the generator are satisfied. The results of experiments are given in Tables 2 and 3. Curves of the change of capacity with temperature are given for three condensers of different nominal capacity (Figures 1 and 2). The study of K M - 4 A NZO condensers thus shows that they entirely fulfil the requirements of a sensor for measuring low and very low temperatures• After thirty repeated coolings to helium temperatures, including three coolings to 1 K, none of the condensers changed its

calibration within the accuracy of measurement, or suffered mechanical damage when the temperature was changed rapidly (from 300 to 77 K and from 77 to 4"2 K). The reproducibility of the capacity of one condenser is shown in Table 4. We recommend these sensors for continuous measurement of temperature in the range from 300-I K using a commercial I IEV-I instrument. If the temperature of a liquid has to be measured, it is not necessary to enclose the sensors in protective covers.

A cryostat for studying dielectric and acoustic relaxation in the temperature range + 50 to --150 C

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V. D. P A P O N O V

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and N. V. C H E K A L I N {lllllll

THE MAJORITY of cryostats operating in the range from room temperature to - 1 8 0 C have the following drawbacks: small accuracy in temperature control (0.5-1 C), involved design, and a lengthy cool down time to the lowest temperatures (6-7 h). 1 We have tried to eliminate these defects in the design proposed here. The thermostated cryostat chamber (see Figure 1) consists of two cylindrical copper waveguides, one inside the other, with the liquid to be studied between their walls and inside one of them. The waveguides, which lead from the thermostat chamber in both directions, are made of stainless steel (waveguide length 15 cm) for thermal insulation. A four start thread is cut on the surface of the outer waveguide, and a heater of 0.1 mm diameter nichrome wire and a resistance wire are wound onto it. The thread is coated with bakelite varnish to insulate the wires from the waveguide. The thermometer is made of 0.03 mm diameter 'Pobeda' wire by the standard method, z There is a copper cylinder over the thermostatic chamber with a coil of 3 mm diameter copper tubing wound on it. The whole construction is surrounded with a 20 cm thick foam plastic layer to insulate the chamber• Rough temperature control is obtained as follows. A stainless steel tube is placed in a metal dewar filled with liquid nitrogen so that its end reaches nearly to the bottom of the dewar. Its other end is connected to the copper coil surrounding the chamber. A stainless steel tube is also soldered to the second end of the coil and is connected to a needle valve, after passing through the foam plastic. The temperature inside the chamber can be varied by changing the pressure in the dewar• A bellows regulator 3 (see Figure 2) is used to control the nitrogen vapour pressure in the dewar. Before starting operations, tap 5 is shut and tap 6 is opened. As a result of the evaporation of liquid nitrogen, the pressure The authors are with the Department of Chemistry, Moscow State University, USSR. Prib. i Tekh. ~ksper. No. 6, 201 (1968). Received 10 February 1969. CRYOGENICS - A U G U S T 1969

1 insulating stainless steel waveguide 2 inner waveguide 3 copper coil 4 outer waveguide with liquid under investigation inside

5 free volume for liquid seeping out of the working volume 6 porcelain sealing bungs 7 washer for displacing the inner waveguide 8 foam plastic 9 retainer

Figure 1. Measuring cell

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5

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Figure 2. Cooling system with bellows control of the pressure

in dewar 1 increases and liquid nitrogen rises through tube 2 into coil 3. After attaining a certain pressure P, tap 6 is shut and tap 5 is opened. The pressure inside and outside bellows 7 is then equal to P. The pressure outside the bellows increases as a result of further evaporation of nitrogen and compresses it. Needle 11 is then withdrawn, opening the exit to the atmosphere. Glass taps 5 and 6 are connected to the regulator, and to the dewar, by Kovar 287