Applied Mechanics and Materials Vol. 819 (2016) pp 601-605
© (2016) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.819.601
Submitted: 2014-09-08
Revised: 2015-02-01
Accepted: 2015-07-16
Fabrication of lotus-type porous copper using slip casting and sintering
techniques for heat pipe applications
Sugeng Supriadi1a*, Nandy Putra2b, Bambang Ariantara2c, Sunaryo1d,
Dadit D. Rahmanto1e
1
Manufacturing Laboratory, Department of Mechanical Engineering University of Indonesia,
Kampus UI, Depok 16424, Indonesia
2
Heat Transfer Laboratory, Department of Mechanical Engineering University of Indonesia,
Kampus UI, Depok 16424, Indonesia
a
sugeng@eng.ui.ac.id, bnandyputra@eng.ui.ac.id, cbambang.ariantara@ui.ac.id,
d
sunaryo99adhi@gmail.com, edaditdamar@gmail.com
Keywords: lotus-type porous copper, slip casting, sintering, heat pipe.
Abstract. A lotus type-porous copper will be used to substitute wick materials from tabulate coral
capillary for heat pipe applications. The lotus type-porous material are normally fabricated using
Gasar process, in which unidirectional solidification was applied to a metal-eutectic systems to
obtain long cylindrical pores in the direction of solidification. A new process is proposed to
fabricate the lotus type-porous material using slip casting and sintering. As an initial work, this
paper is aimed to obtain appropriate processing parameters of fabrication of lotus-type porous
copper using slip casting and sintering techniques. Nylon strings were used as pore formers to form
cylindrical pores. They were coated with copper slurry consisting of copper powder and binding
agent. The coated nylon strings were arranged in a mold and the slurry were poured. After drying,
these castings were sintered at various sintering temperatures and sintering times. The results
showed that starch as the binding agent is superior to PVA. The mixture of starch-water with starch
content of 97%, the copper slurry with copper content of 50% by volume, the copper powder size of
200 µm, the sintering temperatur of 900°C, and the sintering time of 60 minutes are able to produce
lotus-type porous copper similar to that produced through Gasar process.
Introduction
A heat pipe is a heat exchanger that is widely used in many fields not only for thermal
management system of spacecraft and mobile devices such as laptops but also for waste heat
utilization for HVAC applications. The main considerations in applying a heat pipe are heat flux
intensity, compact design, lightweight and not requiring an external power supply [1]. The high heat
flux is due to heat transfer which involves a phase change of working fluid. The compact design,
lightweight and does not need external power are possible because of heat pipes require just few
working fluid and not requiring pumps and compressors to circulate the working fluid. The simplest
heat pipe is the straight heat pipe comprising an evaporator, adiabatic section and condenser. The
inner surface of the heat pipe is coated with capillary wick material. Heat absorbed by the
evaporator will heats the working fluid until reaches the saturation temperature and turns it into a
vapor phase. The vapor flows through the adiabatic section to the condenser where the heat is
released to the surrounding and causes the vapor to condense. The condensate is pumped to the
evaporator by capillary action of the wick to form a heat pipe cycle.
Fig1. Straight heat pipe [2]
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Advances in Heat Transfer, Flow Engineering and Energy Installations
Capillary wick is a very important component of a heat pipe for heat pipe performance of which
is determined by capillary wick parameters, namely porosity, capillarity and permeability [2].
Capillary wick of a heat pipe can be a groove, gauze, metal foam and sintered metal. Sintered metal
are the most popular capillary wick of a heat pipe. Some studies have been carried out to improve
the performance of sintered metal capillary wick [3, 4]. Li et al. [5] and Chen et al. [6, 7] developed
biporous capillary wicks for loop heat pipes using cold press sintering technique with nickel as a
base metal and a mixture of sodium carbonate-nickel powder as pore formers. The pore-forming
material has a lower melting point than the base metal so it will melt and leaves empty spaces
forming large pores. In addition, there are recently several studies on the use of other materials as
the capillary wick material for heat pipe applications. Nandy Putra et al. [8, 9] found that the use of
tabulate coral as the capillary wick combined with nanofluids as working fluid can lower the
thermal resistance of loop heat pipes significantly. This performance improvement is closely related
to the pore structure of the tabulate coral which has long and homogeneous cylindrical pores that
produce good capillarity and permeability. However, the use of tabulate coral in large quantities can
lead to some environmental problems, so it is necessary to study to mimic the pore structure of the
tabulate coral.
Lotus-type porous copper is a porous copper which has long cylindrical pore structure which
resembles pore structure of a lotus stem. The long cylindrical pore structure is obtained through a
process where unidirectional solidification is applied to a metal-eutectic system known as Gasar
process [10, 11]. Gasar process is a fairly complicated process and requires expensive equipment. In
this study, the fabrication of lotus-type porous copper using slip casting and sintering techniques
which are much simpler than Gasar process is proposed. Process parameters of fabrication of lotustype porous copper using slip casting and sintering techniques include the copper powder size, the
viscosity of the slurry, the sintering temperature and the sintering time. The objective of this study
is to obtain an appropriate process parameters for fabrication of lotus-type porous copper using slip
casting and sintering techniques as an initial work.
Methodology
In this study, lotus-type porous copper is made using pore formers similar to the fabrication of
biporous metal. To obtain long cylindrical pores structure nylon strings are used as pore former.
Prior to casting the nylon strings are coated with copper slurry consisting of copper powder, binding
agent and water. The coated nylon strings are dried, and after drying they are arranged in parallel
and bound to form a beam resembling a broom stick. Copper slurry-coated nylon string is inserted
into a mold, and at the same time the copper slurry is poured into the mold so that the coated nylon
string submerged by the slurry to form a green product. After drying, the green product is sintered.
The flow chart of fabrication process of lotus-type porous copper is shown in Fig2. Materials used
in fabrication of lotus-type porous copper using slip casting and sintering techniques are copper
powder of diameter 200 µm as base material, poly vinyl alcohol (PVA) and starch powder as
binding agents and nylon strings of diameter 120 µm as pore former.
Preparation of copper slurry
Coating on pore former
Slip casting
Drying
Sintering
Fig2. Flow chart of fabrication of lotus-type porous copper
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Preparation of Copper Slurry. Binding agent is mixed with 20 ml of water. The mixture is
stirred until homogeneous. For the mixture with starch powder, stirring is done while heating at
about 80°C. Binding composition are arranged to produce three kinds of mixtures, e.g. dilute
mixture with a viscosity of 225 P, medium mixture with a viscosity of 1380 P and thick mixture
with a viscosity of 2420 P. Copper powder is added into each mixture with a composition of 40%,
50% and 60% by volume so that there are nine compositions of copper slurry mixture for each
binding agent. Furthermore, copper slurry mixture are stirred until homogeneous.
Coating on Pore Former. Nylon string was used as pore former for lotus type pores material.
The Nylon string that have been cut at a length of about 60 mm are bonded to form a beam
resembling a broom stick. Furthermore, these nylon strings are coated with copper slurry evenly and
dried.
Slip Casting. The copper coated nylon string is placed in a 14.7 mm diameter and 20 mm height
cylindrical mold and laid lengthwise direction of the cylinder axis. The copper slurry is poured into
the mold to fill the mold evenly. Furthermore, the upper section of the coated nylon strings are cut
to spread evenly then put in the vacuum chamber of 20 kPa for 10 minutes to remove trapped air.
The next step is drying to remove moisture at 35 ° C for 2-5 days.
Sintering. Samples were removed from the mold by pushing it from one side to release it. The
sample is heated up in a furnace at temperatures of 700, 800 and 900 ° C with a heating rate of 20 °
C / min and sintering time of 20, 40 and 60 minutes. The process of fabricating of lotus-type porous
copper is shown schematically in Fig3.
Fig3. Schematic of fabrication of lotus-type porous copper
The green poduct lotus-type porous copper produced should have a good similarity with the
shape of the mold and a good mechanical properties. Furthermore, the finished product should have
long cylindrical pore structure. Macro structure analysis are conducted to observe the pore structure
of the finished product. To see the structure of the pore formed, finished products were cut
transversely, and the cutting surface is cleaned with ultrasonic processor for 10 minutes to remove
particles covering the pores. To determine the ability of a sample to pass water, droplet test was
done by measuring the time required by a droplet of water to seep into the sample. The results are
compared to results of droplet test of sintered copper. Observations of droplet test are performed
using a high speed video camera with frame rate of 480 fps.
Results and Discussion
All mixture made using PVA as binding agent, either the dilute mixture, the medium mixture
and the thick mixture, produce slurry with poor fluidity. It can be caused by the PVA-water
mixtures that are difficult to homogenize. In addition, drying of the green product made using PVA
requires quite a long time which is up to 5 days. After sintering, all product made using PVA have
fairly solid structure, but there are still cavities and some white powder. The cavities can be caused
by the occurrence of a large shrinkage, while the white powder may be caused by the presence of
PVA which dries. In general, the green products made using PVA have poor shape similarity with
the mold.
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Advances in Heat Transfer, Flow Engineering and Energy Installations
On the other hand, the starch-water mixture is more easily to homogenize to obtain slurry with
good fluidity. This facilitates the process of casting, even for the thick mixtures. By using starch
powder the drying time of castings becomes shorter, i.e. 1-3 days. Shrinkage is smaller, especially if
thick mixtures are used. In addition, shrinkage is more regularly so that the cavities formed also
become more regularly. The cavities can be reduced by using a thicker mixture of starch. In general,
the green products made using starch powder have better shape similarity with the mold. Medium
mixtures with viscosity of 1380 P provide a fairly small shrinkage and ease of casting process. Fig4
shows the green product after pouring and after drying using a medium mixture with viscosity of
1380 P and copper powder content of 50%.
Coated nylon
strings
Coated nylon
strings
Using PVA
After pouring After Drying
Using Starch
After pouring
After Drying
Fig4. Green products using slurry with viscosity of 1380 P and copper content of 50%
The combination of low copper content (40%) and low viscosity mixture (225 P) produces large
shrinkage resulting in cavities formation. Shrinkage decreased when the content of copper is raised,
however, the drying time increased and the bond between the grains decreased. The increase in
viscosity will increase the bond between copper powder and the adhesive so as to increase the
strength of the product. The combination of copper content 50% by volume with medium viscosity
mixture (1380 P) gives a small shrinkage and a good similarity between castings and the mold.
Directional pores
Fig5. Pore distribution after sintering at
900°C for 60 minutes
Fig6. Influence of sintering temperature and
sintering time to pore diameter
Observation on the pore structure of products after sintering shows that the products have long
cylindrical pores, similar to the pore structure of the lotus-type porous copper fabricated through
Gasar process [10, 11]. The sintering temperatur of 900°C with heating rate of 20°C/min and
sintering time of 60 min provide fairly uniform cylindrical pore diameter. Fig5 shows the pore
distribution of a sample made using coper slurry with viscosity of 1380 P, copper content of 50%
after sintering at 900 °C with a sintering time of 60 minutes. Obtained also that the higher the
sintering temperature the smaller cylindrical pore sizes are formed. Similarly, the longer the
sintering time the smaller cylindrical pores are also formed as shown in Fig6. These can be caused
by shrinkage that increased with temperature and sintering time.
The best result of droplet test on finished products is 0.0625 sec/droplet, meanwhile the result of
droplet test on conventional sintered copper is 0.2375 sec/droplet. This indicates that the finished
product of lotus-type porous copper have a good ability to pass water as required by capillary wicks
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of heat pipes. This ability is associated with the increase in porosity produced by the directional
pores.
Conclussions
In this study, the lotus-type porous copper with long cylindrical pore structure has been
successfully fabricated using slip casting and sintering techniques. The products have good ability
to pass water as required by capillary wicks of heat pipes. These results are promising for heat pipe
application. Here are some conclusions about the materials and process parameters for used in
subsequent studies to optimize the process.
1. Starch as binding agent in terms of fluidity, drying time and similarity of the castings with
the mold is superior to PVA.
2. Starch-water mixture with a viscosity of 1380 P prepared using starch content of 97%
provides good fluidity, short drying time and small shrinkage.
3. Copper slurry with a copper content of 50% by volume gives small shrinkage and good
similarity of castings and the mold.
4. Sintering temperature of 900 °C with heating rate of 20 ° C / min and sintering time of 60
minutes resulted in fairly uniform cylindrical pore diameter.
Acknowledgment
The authors would like to thank LPDP Department of Finance of Republic of Indonesia under
RISPRO 2014.
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