Experimental setup for the study of thermal conductivity of elastomeric material at cryogenic temperature

Experimental setup for the study of thermal conductivity of elastomeric material at cryogenic temperature T. B h o w m i c k and S. P a t t a n a y a k Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur-721302, India

Received 26 November 1987; revised 26 September 1988 In this work a new convenient experimental setup is made with the help of a GiffordMcMahan cryorefrigerator to study the variation of thermal conductivity with the temperature of any solid material in the range of 35-300 K. The GM refrigerator can produce a temperature down to 35 K with a refrigeration capacity of 29.20 W. The principle of this experiment is based on a guarded hotplate method. The thermal gradient along the thickness of the sample is achieved with the help of electrical heating in the range of 100-230 K. The sample has been insulated from the ambient heat inleak with the help of a vacuum of 10 -6 Torr and with a number of super insulation layers. Three samples of elastomer with different composition suitable for cryogenic appliance have been prepared in cylindrical shapes of 6.00 cm diameter and 1.2 cm thickness. The variation of thermal conductivity is studied at a mean temperature of 1 00, 120, 130, 1 50, 170, 190 and 210 K. From the experimental study it is seen that the thermal conductivity of all the samples increases with the increase of temperature and reaches a peak value in the range of 0.00165 to 0.00173 W cm -1 K -1 and then decreases, as temperature is further increased to a constant value. This is in full agreement with the published literature. The variation of peak values of thermal conductivity for different samples is found to be 1 to 6% with respect to the published data. This has ensured the reliability and accuracy of the experimental setup.

Keywords: thermal conductivity; elastomeric material; guarded hotplate

With the invention of different types of elastomeric and polymeric materials for cryogenic application there has been an urgent need for the study and determination of physical properties of such materials. The manufacturing technology of such materials has been developed by a number of organizations, though the design data required for any cryogenic system are not available. In this work, an attempt has been made to design an experimental setup to study the thermal conductivity of such materials. There are many scientists 1-9 involved in the measurement of this parameter and they have studied the thermal conductivity of elastomers. In all their experimental setups, they used gas flow or liquid flow cryostat(cryogenic) for their desired low temperatures. In this work the G M refrigerator, which can produce a temperature down to 35 K with a cooling capacity of 29.20 W, is used. The use of a cryorefrigerator has the following advantages: 1 it eliminates the requirement of LN2 which is used in most such low temperature work; 2 it has a wide experimental temperature range from 35 to 300 K. 3 it gives very effective control on the variation of test temperature up to room temperature;

4 5

simple operation and easy maintenance; compact, less complicated, does not need expert persons for conducting the experiment.

In this experimental setup the heat inleak has been decreased by evacuating (to 10 -6 Torr) and putting I0 to 15 layers of multilayer insulation at cold temperature.

Experimental setup The schematic diagram of the experimental setup is shown in Figure 1. In this experiment, the elastomeric samples of different composition are taken in the form of a disk 6.00 cm diameter and 1.20 cm thick. For the production of cryogenic temperatures in the range of 80 K, a Cryomech AL05 ® cryorefrigerator is used. The refrigerator is capable of producing a cooling capacity of 29.20 W. It is a G M cycle cryorefrigerator with helium gas used as refrigerant. The copper block (the heater) is 6.00 cm diameter and 1.50 em thick and is wound by a nichrome wire of 0.02 cm diameter and 80.00 cm length, with 6.80 ~ total resistance. The temperature of the copper block is controlled by regulating the heating current through the nichrome wire. The control circuit is also shown in Figure 1. The resistance

0011-2275/89/040463-04 $03.00 © 1989 Butterworth & Co (Publishers)Ltd Cryogenics 1989 Vol 29_April

463

Thermal conductivity of elastomeric material: T. Bhowmick and S. Pattanayak

V=cuurngauge

1

(~ (~ J Sta~lizer Rotary S,W piece

Heater

Voltmeter Figure 1

Experimental procedure

Layout of the experimental setup

Determination refrigerator

of the nichrome wire at different cryogenic temperatures was determined earlier for the calculation of heat input to the system. The resistance is measured with the help of a digital multimeter (Kithley 177 Microvolt DMM) with a precision of 0.001 f~. The current and voltage for the heating element are measured with the help of the same digital multimeter (precision: 1/aV). To maintain good thermal contact the test sample is held rigidly between the cold and hot surfaces by means of two thin Teflon screws. Teflon is used because its thermal conductivity is much smaller than that of the test sample. The hot and cold temperature of the test sample is measured by means of a Cu-Constantan thermocouple. The e.m.f.s produced in the thermocouples are measured with the help of a Kithley microvoltmeter. Ice is used as a reference point for the thermocouples. The entire assembly is kept inside a vacuum chamber of 34.00cm height and 9.50 cm diameter. The inside connections for the thermocouples and current input leads were taken through a vacuum tight lead connector. All the above leads are silver soldered with the above connector. To minimize the radiation heat the test sample, with its cold head, and the copper block, is wrapped with ten layers of super insulation. These layers are kept in contact with the cold head temperature. Assembly with the test sample is thermally insulated by creating a vacuum of 10 - 6 Torr around it in the vacuum chamber. This vacuum is achieved with the help of a HIND - - HI - - VAC (VS-4) vacuum module using liquid nitrogen in the cold trap of the diffusion pump. With this arrangement it is seen that there is practically no cold loss from the test assembly. Three samples ofelastomer are considered in this work. The composition of the elastomeric samples are shown in Table 1. The elastomeric samples are prepared in the form of disk 6.00 cm diameter and 1.20 cm thick. To prepare the samples natural rubber is first masticated (temperature: 80°C) in a two-roll mill to reduce its viscosity. Then Table 1

Composition of the elastomeric samples

Natural rubber

Stearic acid Neozone D CBS Zinc oxide Sulphur Altax

464

zinc oxide, stearic acid and Neozone-D are added to it. Then it is cooled to 30 °C in the mill itself by passing cold water over it. The accelerator material (CBS/Altax) and vulcanizing agent sulphur is then added and the mixture is dispersed uniformly, placed in a disk type mould and cured in an electrically heated press at 150°C for 20 min. Then the sample, in the form of a disk, is taken from the mould.

Rubber 1

Rubber 2

Rubber 3

100 PHR 1 PHR 1 PHR -5 PHR 2.5 PHR 1 PHR

100 PHR 2 PHR -0.8 PHR 6 PHR 2 PHR --

100 PHR 2 PHR -0.8 PHR 5 PHR 2 PHR --

Cryogenics 1989 Vol 29 April

of

refrigeration

capac~y

of

the

The refrigeration capacity of the cryorefrigerator (Cryomech AL05 ®) is determined in the following way. The hot surface of the heater block and the cold surface of the refrigerator are kept in close contact with no sample between them under a vacuum of 10 -6 Torr. All six thermocouples are placed between the two surfaces. The heater circuit is kept non-functioning. To maintain a stable voltage of 200 V the cryorefrigerator is connected to the mains through a stabilizer. The cryorefrigerator is then charged with helium gas up to capacity (to a pressure difference of 150 psig), the refrigerator switched on and the pressure difference between the high pressure and the low pressure side is controlled to approximately 180 psig by the pressure control valve. The cryorefrigerator is started and the e.m.f.s produced by the thermocouples are recorded at intervals of 5 min. This process continues (approximately 7 h) till the steady state is reached. The heater circuit is switched on and a small amount of heat input (0.40 W) is introduced. The thermocouple readings are observed for 30 min to note changes in the readings. The heat input is gradually increased at steps of one V and the thermocouple readings are recorded continuously to see any change in reading. At a heat input of 25.20 W it is observed that the thermocouple reading has just begun to change. Hence, the refrigeration capacity is calculated as a sum of the heat input and the external heat inleak to the cold head through the leads.

To find the external heat inleak to the experimental setup through the leads To find the heat inleak, a Teflon sample (6.00 cm diameter and 0.65 cm thick) of known thermal conductivities at different temperature is introduced into the sample position. Now, after evacuating the system to 10 -6 Tort, the cryorefrigerator is started. The temperatures of the two surfaces of the Teflon sample are recorded continuously through the thermocouples by means of a recorder. When the steady state is reached the temperatures are taken down for the measurement of heat inleak to the system. This experiment is repeated a number of times to ensure the reliability of the recorded data. To measure the thermal conductivity of the sample at different cryogenic temperatures

The test sample is placed between the two hot and cold isothermal surfaces. The system is evacuated with the help of a vacuum module to the extent of 10 - 6 Torr.

Thermal conductivity of elastomeric material." T. Bhowmick and S. Pattanayak Table 2

Variation of peak temperature and peak value of thermal conductivity for elastomers with respect to the published curve3

Sample Rubber 1 Rubber 2 Rubber 3 Established curve

Peak temperature (K) (in present work)

Variation of peak temperature (Eiermann's work 3)

Peak value thermal conductivity (W cm-1 K - l ) (in present work)

Variation of thermal conductivity (Eiermann's work 3)

175 170 167

12.5% 15% 16.5 %

0.00173 0.00169 0.00165

5.78% 3.68% 1.22 %

200

0.00170

The cryorefrigerator is started, keeping the heater circuit non-functioning. The temperature at the two surfaces of the sample are recorded continuously through the thermocouples by means of a recorder. When the steady state is reached, the temperature at the two surfaces is noted and thermal conductivity of the sample is calculated. Now the heater circuit is changed to a higher input (in steps of 0.1 W). The changes of temperature at the two surfaces are recorded continuously until the next steady state is achieved. At steady state the temperature at the two surfaces are noted and the thermal conductivity is calculated again in the same way. The mean temperature corresponding to this value of thermal conductivity will be the average temperature of the two isothermal surfaces. This process is repeated for different heater inputs ranging from 1 to 3 W. As the heater input

IE 1.8 0

~1.7 ×

~ 1.6 C

8

~

1.5

..C

I

70

,

90

1

,

110

I

=

130

I

=

I

,

I

150 170 190 Temperature (K)

a

I

210

,

I

230

250

Figure 2 Temperature v e r s u s thermal conductivity curve for the three elastomeric samples with the published curve3. O, Rubber 1 ; A, Rubber 2; 0 , Rubber 3; - - - , published curve

Table 3

changes, the steady state temperature also changes. Hence, the thermal conductivities at different heat inputs represent the thermal conductivities at different temperatures. The sample is changed and the same process repeated a number of times to make the data more reliable. After calculating the thermal conductivities at different steady state temperatures, curves are drawn relating mean temperature and thermal conductivity for all the test samples.

Results and discussion Temperature-thermal conductivity curves for the elastomeric samples are shown in Figure 2. It is observed that the thermal conductivity increases with temperature up to a peak value of about 170 K, after which it decreases. This phenomenon is quite in agreement with other published work 3. The peak values in those cases are observed approximately at 200 K. The actual peak values observed in this work for all the samples are given in Table 2. The variances with respect to the published data are also included in Table 2. The shifting of peak values of temperature from 170 to 200 K may be due to the differences in composition of the samples. One more point which is to be observed is that the increase of thermal conductivity with temperature before peak value is almost linear. This is in agreement with the recent study on elastomers at low temperature performed by Freeman and Greig 9. Among the three elastomeric samples studied in the experiment, it is seen that the values of thermal conductivity in the present work vary from 0 to 5.8 % with respect to the published work. This may be due to the large temperature difference between the hot and cold faces of the test sample. It is also observed that the values of thermal conductivity at different temperature are higher for Rubber 1 than Rubber 2 and Rubber 3. Rubber 3 has the lowest value of thermal conductivity in this group. This can be explained from the study of their composition. For Rubber 1 it is noted from its composition that it has a higher PHR of sulphur (2.5) than Rubber

Variation of thermal conductivity of different elastomers with respect to the published data3 T=130K K (W c m - l K

Rubber 1 Rubber 2 Rubber 3 Established curve

0.00160 0.001 57 0.00155 0.001 58

T=150K

T=160K

T=170K

T=190K

K K K K 1) Variance (W cm 1K-1) Variance (W c m - l K -1) Variance (W c m - l K -1) Variance (W c m - l K -1) Variance 1,25% 0,64 % 2,13%

0.00167 0.00165 0.00167 0.00164

1.8% 0.6 % 1.2%

0.00170 0.00168 0.00164 0.00165

5.8% 4.2 % 2.4%

0.00172 0.00169 0.00165 0.00166

3.6% 1.77 % 0.61%

0.00171 0.001 67 0.00163

2.3% 0 2.5%

0.00167

Cryogenics 1989 Vol 29"April

465

Thermal conductivity of elastomeric material: T. Bhowmick and S. Pattanayak 2 and Rubber 3. It is known from the characteristics of elastomer that a 10% increase in sulphur concentration in the sample will increase the thermal conductivity by 2 %. For this reason the thermal conductivity of Rubber 1 is increased. For Rubber 2 and 3, although their compositions are the same, the amount of zinc oxide is less (by 1 PHR) in Rubber 3. It is known from the published literature 6 that a reduction of 1 P H R of zinc oxide will reduce thermal conductivity by 0.7 %; this may be the reason for the lower K values of Rubber 3. The values of thermal conductivity at a particular temperature for all elastomeric samples studied and their variance with respect to the published curve are tabulated in Table 3.

Conclusions The following conclusions can be drawn from this work: 1.

466

the thermal conductivity of any solid material can be

Cryogenics 1989 Vol 29 April

2.

3.

tested in the form of a disk; spongy or gas contained sample can also be tested, if some special attachment is provided to protect the sample from vacuum atmosphere; the apparatus is found to be very convenient in testing thermal conductivity because it does not require cryogenic fluid.

References 1 2 3 4 5

6 7 8 9

Rehner, R. J Polymer Sci (1947) 2 163 Ueberreiter, K. and Nens, S. Kolloid-2 (1951) 92 123 Eiermann, K. and Hellwege, K.H. J Polymer Sci (1962) 57 99 Hansen,D. and Ho, C.C. J Polymer Sei (1965) A-3 659 Reese, W. J Appl Phys (1966) 35 8 Lois, C.K.K.and Hoge, J. J Rubber Chem Technol (1966) 39 166 Sandberg, O. and Backstorm, G. J Appl Phys (1979) 50 7 Hands,D. Rubber Chem Technol (1980) 53 1 Freeman, J.J. and Greig, D. Adv Cryog Eng (1983) 31 105