Download paper - Indico

STUDIES ON THERMAL CONDUCTIVITY OF EPOXY-ALUMINIUM
COMPOSITES IN THE RANGE OF 300 K TO 4.5 K FOR THE
DEVELOPMENT OF CRYOSORPTION PUMPS
Ravi Verma1,2, N C Shivaprakash2, Upendra Behera1,
S Kasthurirengan1, Bharath G J 1and R Gangradey3
1
2
3
Centre for Cryogenic Technology, Indian Institute of Science, India
Instrumentation and Applied Physics, Indian Institute of Science, India
Institute for Plasma Research, India
[email protected]
Abstract
Cryosorption pumps are the only solution in nuclear
fusion environment where helium and hydrogen are the
by-products of reaction between deuterium and tritium.
Cryosorption pumps are used to achieve ultra-high
vacuum in such harsh conditions. An important aspect
in their development is the proper adhesion of the
activated carbon granules onto the metallic cryopanel
and their cooling to the lowest possible temperature by
using high thermal conductivity adhesives to adhere the
activated carbons. Hence, the thermal conductivity data
of the select adhesives and activated carbons down to
4.5 K are quite essential, but they are not available in
open literature. Maintaining the activated carbon
granules at lower temperatures is an important factor
for achieving high pumping speed of the cryosorption
pump. This paper deals with the studies on thermal
conductivity of epoxy-aluminium composites to
enhance the pumping speed of cryosorption pump.
Towards this, GM cryocooler based experimental setup
has been developed to measure the thermal
conductivities of epoxy-aluminium composite samples
from 300 K to 4.5 K. The thermal conductivity data for
pure epoxy and epoxy mixed with different percentage
of aluminium powder will be presented in this paper.
The above studies will enable to make the right choice
of adhesives for the development of cryosorption
pumps.
INTRODUCTION
Thermal conductivity is one of the fundamental
thermal properties of a material which indicates the
rate at which heat energy can propagate through a
material at a particular temperature.
In low temperature technology, measurement of
thermal conductivity is utmost important because it is
required for thermal design of components and
systems. Thermal conductivity of material depends on
temperature, crystal structure, density of states, Fermi
energy, impurities etc. Now days with the advancement
in technology, new compound materials with low
temperature applications are being created whose
thermal conductivity values are still unknown,
especially at low temperature. Therefore thermal
conductivity measurement places a vital role in filling
blank of low temperature thermal property data base.
Epoxy and epoxy-aluminium composites are such
materials whose low temperature thermal properties are
unknown. They are one of the essential components in
the design of high efficiency cryosorption pumps. So
in this article, we present an experimental system to
measure the thermal conductivity of epoxy. Thermal
conductivity values of epoxy and epoxy-aluminium
composites with varied volume fractions of aluminium
powder in epoxy from 300 K TO 4.5 K.
Measurement of thermal conductivity values of
epoxy and epoxy-aluminium composites is useful in
development of cryosorption pump. Cryosorption
pumps are only solution in nuclear fusion systems to
achieve high vacuum in the environment of hydrogen
and helium. High thermal conductivity value of epoxy
is an important factor for development of cryosorption
pump such that it transfers cold from liquid helium to
activated carbon via SS panel. That’s why
determination of thermal Conductivity is very
important for cryogenics application.
In this paper, the experimental principle and the
apparatus are illustrated first and the measurement of
the thermal conductivity of Al-2024 T4 is carried out
for illustration. Experiments have been performed in
the temperature range from 300 K to as low as 4.5 K.
Experimental setup is calibrated by measuring thermal
conductivity of known sample.
Figure 1. Stainless steel panel with liquid
helium flowing through it.
Figure 2. Activated carbon pasted on
stainless steel panel with liquid helium
flowing through it.
EXPERIMENTAL PRINCIPLE
Thermal conductivity is measured by longitudinal
steady heat flow method on the basis of one
dimensional Fourier heat conduction law in this
measurement.
A heating power Q is supplied to the heater
sandwiched between two samples. One side we have
sample whose thermal conductivity value is known. On
the other side we have sample whose thermal
conductivity value is unknown.
∆𝑻
∆𝑻
Q=k1(T)A1(∆𝒙𝟏 )+k2(T)A2(∆𝒙𝟐 )
𝟏
Figure 3. Cut-section view of Cryocooler
(1)
𝟐
Where;
Q = heating power in Watt
k1 = thermal conductivity as a function of temperature
of known sample in watt per meter Kelvin
∆T1= Temperature gradient of known sample in Kelvin
∆𝑥1 = effective length of known sample across which
temperature is measured by sensor
A1 = cross sectional area of known sample in square
meter
k2 = thermal conductivity as a function of temperature
of unknown sample in watt per meter Kelvin
∆T2 = Temperature gradient of unknown sample in
Kelvin
∆𝑥2 = effective length of unknown sample across
which temperature is measured by sensor
A2 = cross sectional area of unknown sample in square
meter
Figure 4. Experimental Set-up of Cryo-cooler
EXPERIMENTAL APPARATUS AND
PROCEDURE
The experimental apparatus designed for the thermal
conductivity measurement was based on above
principle. The experimental setup is:
variable temperatures G.M. Cryocooler
G.M. cryocooler with a refrigeration power of 1 watt at
4.2 K is used for performing the experiment. Vacuum
pump is used to evacuate the space inside the sample
chamber so that when experiment is performed heat
should not dissipate through convection via air
molecules.
Special type of cryogenic sensors are used to
sensing temperature like Si 410B & DT670 especially
designed for higher accuracy at low temperature. For
indicating the reading scientific instrument 9302
temperature indicator is used. For giving current to the
sample heater KEITHLEY 6220 instrument is used.
The voltage across the heater is measured by
KEITHLEY 2000. Lakeshore 332 PID controllers are
used to maintain the temperature of sample chamber at
different temperature. LabVIEW 2011 software is used
for data acquisition.
Experimental Procedure of G.M. Cryocooler
based experimental setup for thermal
conductivity measurement:
Fix the test sample inside sample chamber, Keep
sample chamber inside cryocooler. Evacuate sample
chamber up to vacuum level 10-6 mbar at room
temperature & close the evacuation valve. Using
temperature controller, sample temperature is
controlled & maintained at desired value. When sample
is maintained at desired temperature, the heater
sandwiched between samples is energized. (DC current
is applied to the heater).
Figure 5. Comparison of Thermal conductivity of
‘Al 2024 T4’ with NIST data.
RESULTS AND DISCUSSIONS
Figure 6 shows the thermal conductivity of pure epoxy
from 4.5K TO 300K, figure 7 shows the comparison
between pure epoxy and commercially available epoxy
stycast 2850FT. Figure 8 shows the variation of
thermal conductivity with aluminium volume fraction
in the base matrix of epoxy at 300K and figure 9
explains the variation of thermal conductivity with
aluminium volume fraction in the base matrix of epoxy
from 4.5K to 300K.
Temperature difference (T) along the axial direction
of the sample, is measured with the help of temperature
sensors. The heater voltage and the current is measured
with the help of the DMM. Thermal conductivity
determined by the Fourier’s law of heat conduction.
Thermal conductivity is measured at different
temperatures by maintaining the sample chamber at
appropriate temperatures.
We have performed experiment on Al 2024 T4 to
calibrate the experimental setup, and we compare the
experiment result obtained with NIST published data.
From graph we can see that repeatability is good and
thermal conductivity data is within 10% of NIST data.
Figure 6. Thermal conductivity of epoxy.
CONCLUSION:
Figure 7. Comparision of thermal conductivity
between Epoxy and Stycast.
The thermal conductivity of pure epoxy and
epoxy-alumimium composite is measured by GM
cryocooler based experimental setup from 4.5 K
to 300 K. It was observed that thermal
conductivity of pure epoxy increases significantly
with addition of fine aluminium powder.
Variation of thermal conductivity with aluminium
volume fraction in the base matrix of epoxy at 300
K is also presented in this paper. The value of
thermal conductivity of developed epoxy is
relatively higher than the present epoxy used at
low temperature like Stycast 2850, because of this
higher value of thermal conductivity it will be
useful in the development of cryosorption pumps.
REFERENCES:
[1]
Figure 8. Variation of thermal conductivity with
aluminium volume fraction in the base matrix of
epoxy at 300 K.
Figure 9. Variation of thermal conductivity with
aluminium volume fraction in the base matrix
of epoxy from 4.5 K to 300 K
Zhang P, Chen Y J, Ren X J, Wu A B, Zhao
Y et. al. Thermal conductivity measurement of
the epoxies and composite material for low
temperature superconducting magnet design.
2011 Cryogenics 51 534–540
[2] Baudouy B. Low temperature thermal
conductivity of aluminum alloy 1200. 2011.
Cryogenics.
51: 617–620
[3]
Woodcraft A L., Predicting the thermal
conductivity of aluminium alloys in the
cryogenic
to
room temperature range. 2005. Cryogenics; 45
(6) 421-431
[4] Yeon Suk Choi , Dong Lak Kim . Thermal
property measurement of insulating material
used in HTS power device.2012 Journal of
Mechanical Science and Technology; 26 (7)
2125-2128
[5]
Nunes W, Santos D, Mummery P and
Wallwork A. Thermal diffusivity of polymers
by the laser flash technique.2005 Polymer
testing; 25 628-634
[6] White G K. Experimental techniques in low
temperature physics. 3rd ed. Oxford:
Clarendon Press; 1979
[7]
Hust J G. Low temperature thermal
conductivity measurements on longitudinal
and transvers sections of a superconducting
coil.1975. Cryogenics; 15:8-11.
[8]
Cryocomp v 3.06 Cryodata Inc , Eckels
Engineering, Florence SC, USA 29501. G10
“normal to cloth layer”