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Capillary pressure in graphene oxide nanoporous membranes for enhanced heat transport in Loop Heat Pipes for aeronautics
Accepted Manuscript Capillary pressure in Graphene oxide nanoporous membranes for enhanced heat transport in Loop Heat Pipes for aeronautics Cosimo Buffone, Jerome Coulloux, Beatriz Alonso, Mark Schlechtendahl, Vincenzo Palermo, Amaia Zurutuza, Thomas Albertin, Simon Martin, Marco Molina, Sergey Chikov, Rolf Muelhaupt PII: DOI: Reference:
S0894-1777(16)30095-4 http://dx.doi.org/10.1016/j.expthermflusci.2016.04.019 ETF 8756
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
25 November 2015 21 April 2016 23 April 2016
Please cite this article as: C. Buffone, J. Coulloux, B. Alonso, M. Schlechtendahl, V. Palermo, A. Zurutuza, T. Albertin, S. Martin, M. Molina, S. Chikov, R. Muelhaupt, Capillary pressure in Graphene oxide nanoporous membranes for enhanced heat transport in Loop Heat Pipes for aeronautics, Experimental Thermal and Fluid Science (2016), doi: http://dx.doi.org/10.1016/j.expthermflusci.2016.04.019
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Capillary pressure in Graphene oxide nanoporous membranes for enhanced heat transport in Loop Heat Pipes for aeronautics
Cosimo Buffone1, i, Jerome Coulloux2, Beatriz Alonso3, Mark Schlechtendahl4,5, Vincenzo Palermo6, Amaia Zurutuza3, Thomas Albertin2, Simon Martin2, Marco Molina7, Sergey Chikov1, Rolf Muelhaupt4,5
1
Microgravity Research Centre, Universitè Libre de Bruxelles, Avenue F.D. Roosevelt 50, 1050 Bruxelles, Belgium 2
3
4
Atherm, Z.I. – 1 rue Charles Morel, 38420 Domène, France
Graphenea, Tolosa Hiribidea, 76, 20018 Donostia, Gipuzkoa, Spain
Freiburg Materials Research Center, FMF, Albert-Ludwigs-University of Freiburg, Stefan-Meier-Str. 21, 79104 Freiburg, Germany
5
Institute for Macromolecular Chemistry, Albert-Ludwigs-University of Freiburg, Stefan-Meier-Str. 31, 79104 Freiburg, Germany
6
Institute for Organic Synthesis and Photoreactivity, CNR - National Research Council of Italy, Via P. Gobetti 101, 40129 Bologna, Italy
i
Corresponding author: Tel. 0032 2650 3029, Fax. 0032 2650 3126, Email [email protected]
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Selex ES, Viale Europa, 20014 Nerviano (MI), Italy
ABSTRACT We describe a novel application of graphene-based materials to enhance heat transport in sintered metal wicks, which are the core components for Loop Heat Pipe (LHP) evaporators. Standard metal wicks limit the applicability of LHP to about 8-10 m of transport length and around few meters of gravitational head. This is due to the typical average pore size (about 1 µm) in the sintered metal wicks made of nickel or titanium, which are the most commonly used materials. The idea of the present work is to bond a layer of graphene on top of the wick facing the vapour side of the LHP evaporator. The much smaller pore sizes of graphene (around tens of nanometers) would produce a significant increase in capillary force, while at the same time minimising the pressure drop due to its microscopic thickness (few microns). The wicking height (i.e., capillary rise of a liquid inside a pore) measurements demonstrate that there is an improvement of at least more than 3.5 times when the graphene coating is used, compared to the standard nickel sintered powder wick. This means that the heat transfer of a graphene LHP could work in a spatial range in excess of 28-35 m, which would allow breakthrough applications such as anti-icing of aircraft wings and propellers, as well as wind turbines that cannot be addressed by standard LHP technology.
Keywords: graphene; wicking height; Loop Heat Pipe.
INTRODUCTION Heat Pipes (HP) are passive heat transfer devices capable of transferring heat between two remote locations kept at different temperatures 1-2. HPs are evacuated vessels inside which a working fluid undergoes evaporation and condensation; due to the very large latent heat of evaporation, a
2
small quantity of working fluid is required to transfer large heat loads. In addition, because evaporation and condensation are nearly isothermal processes, there are relatively small temperature gradients in the HP with limited thermal stresses of the materials involved. The core of the HP is the wick structure which provides capillary pressure to overcome all the pressure losses inside the HP. There are different types of wicks for HPs 1-2, the most commonly being based on: 1. Groove arrays; 2. Screen meshes; and, 3. Sintered powders. The best performing wicks are made of sintered powder because the average pore size is much smaller than that of grooves or screen meshes. However, the manufacturing process for sintered powder is more complicated and costly. HPs have been used in a variety of applications, from electronic cooling 3, to aerospace 4-6, to core cooling for nuclear power plants 7, just to mention but a few. There are some important limitations with standard HPs which are connected with the fact that vapour and liquid are in intimate contact and at high loads this can result in reaching the “entrainment limit” during which the evaporator is starved of liquid that cannot return from the condenser by capillary action. The other limitation is connected with the fact that in standard HP the wick is present all over its length, which limits the shape that the HP can have and makes it much heavier. These important limitations were overcome in the ‘80s by the invention of Loop Heat Pipes by Maidanik and co-workers 8, who developed this concept for space applications. Loop Heat Pipes (LHP) 9-10 can transport heat over distances of 8-10 m compared to a few meters for standard grooved and sintered HP, and up to few meters against gravitational head compared with less than a few cm for grooved HP and less than 1 meter for sintered HP, when working against gravity. LHP are more complex to predict and much more complicated to build, resulting in higher manufacturing costs compared with standard HP technology. However, the LHP allows in certain circumstances to control the evaporator temperature 10, something that is not possible with standard heat pipes that operate in a “constant conductance” mode. LHP have been also used in demanding high g-force environments such as aboard fighter jets. The most complex and costly part
3
of a LPH is the evaporator where the wick structure resides. The complicated network of superficial grooves renders the LHP evaporator a very challenging part to make using the current manufacturing techniques (mainly sintering and machining). The limitation on transfer distance has kept to date LHP technology out of applications such as antiicing of aeroplanes and aeroengines where there is a lot of waste heat generated inside the core of aeroengines which could be used to heat up cold spots such as nacelle lips, Outlet Guide Vanes and even wing leading edge of aircrafts. If the improvement in capillary pressure is substantial, then LHP could be also used to anti-ice propeller blades, which typically suffer dramatically from icing because propeller aircrafts fly relatively lower than turbojets, just at the same height where subcooled droplets are more common in the atmosphere. The revolutionary work of 2004 by Geim and Novoselov 11-12, demonstrated the possibility to process and manipulate thin sheets with mesoscopic lateral size and atomic thickness. This has allowed the production of a new class of 2-dimensional (2D) materials, where billions of sheets shall be stacked over each other to form highly anisotropic membranes and coatings. New electronic and chemical properties shall be achieved by creating complex multilayered structures of chemically functionalized graphene sheets 13-15 or by stacking together different kinds of inorganic, 2dimensional sheets 16-18. Such kind of materials features a highly interconnected, 2D porous structure, where both the chemical nature of the 2D walls and the average pore size can be tuned 19. Molecules of liquids diffusing through these 2D pores will feature new properties 20-21. To this aim, exciting results have been recently published on the capillary transport of gases and small molecules inside 2D materials such as graphene or graphene oxide (GO). Geim and co-workers demonstrated that GO membranes can block very effectively some gases and be selectively permeable to others, such as water 22-23 or small ions 24. These barrier properties 4
are due to the 2D multilayer nature of GO membranes, which creates highly tortuous, 2D diffusion paths of the molecules in between the sheets. Furthermore, a recent work suggests that selective transport of protons is possible through the atomic hexagonal voids of single sheets of boron nitride [24]. Applications still more futuristic regards the use of a single sheet of graphene, capable of withstanding particular high pressures, with well-defined holes to desalinate seawater 25-26 or to sequence DNA fragments extremely fast 27. This work describes the processing of different types of graphene onto a nickel sintered wick coupon and measure the wicking height using a standard method. We present Scanning Electron Microscope characterization of the standard nickel wick and of the wick coated with graphene. We demonstrate that the addition of graphene improves significantly the capillary pressure generated by the wick by a factor of at least 3.5, which opens up many more opportunities for LHP in aerospace and ground based applications.
WICK COUPONS INTEGRATED WITH GRAPHENE We have tried two different methods of integrating graphene inside a porous wick structure made of Nickel. The first approach was to suck inside the wick coupon a graphene solution and the second method was to bond only one surface of the wick coupon with graphene oxide. The different wick coupon samples with different types of graphene are summarised in Table 1.
Table 1. Graphene coated wick coupons.
sample
Material used
Coating method
number
5
1
TRGO
vacuum suction (method 1)
2
TRGO
vacuum suction (method 1)
3
TRGO
vacuum suction (method 1)
4
GO graphenea
Slurry deposition (method 2)
5
GO graphenea
Slurry deposition (method 2)
6
GO graphenea
Slurry deposition (method 2)
Figure 1 shows the nickel wick coupon (having 12 mm diameter and 45 mm length) coated with graphene slurries only on top of the upper circular surface, that is the one where the wicking height will be measured.
Method 1: Coating by vacuum suction inside the wick Graphene was dispersed by sonication (Bandelin Sonopuls K76, Berlin, Germany, 30 min sonication at 40 % amplitude) in Acetone (5 g∙L-1) and further processed with a high pressure homogenizer at 1000 bar (GEA NIRO SOAVI NS100 1L 2K, Parma, Italy). The dispersion was then diluted to different concentrations before being poured into the setup in which the wick was fixed as in Figure 2A. The setup was used with a Buchner funnel attached to vacuum pump. The graphene solution was sucked through the wick. Depending on the solutions concentration and the total amount of graphene used a graphene layer formed on top of the wick (Figure 2 B, C, D). Samples with different graphene types were produced and analysed by SEM (ElectroScan Corp. Environmental Scanning Electron Microscope Mod. 2020, Wilmington, MA, USA). We tested two different materials for this method: 6
A) thermally reduced graphite oxide [28-29] (TRGO) and, B) milled graphene with different functionalization [30-31]. TRGO was synthesised based on a modified Hummers [31-32] process, i.e. oxidation of graphite (KFL 99.5) by KMnO4 in sulfuric acid milieu as reported previously [28]. The purified GO was dried and ground and then thermally reduced by rapid heat treatment (750°C) in a rotary tube furnace (Nabertherm, Lilienthal, Germany) under nitrogen atmosphere. The milled graphene was instead produced by milling Graphite under argon and carbon dioxide pressure using a planetary ball mill PM 100 from Retsch, Haan (Germany). The milling chamber was filled with graphite (9.3 g) and 50 ZrO2 balls (Yttrium stabilized, diameter 10 mm) both were dried in vacuum at 60°C. The chamber was evacuated and then pressurized with argon or CO2 (13 bar), respectively. Milling was performed at 250 rpm for the duration of 48 h. Here we present only the most promising results obtained with TRGO.
Method 2: Deposition of GO slurry on the wick We used a graphene oxide slurry that was synthesised using a modified Hummers method [33-35] and investigated in different applications [36-38]. The as-prepared slurry was then deposited on top of the wicks, the surface was activated prior to the deposition. After the deposition and depending on the thickness of the coating the samples were dried under vacuum at 60 °C for a few hours until the graphene oxide was completely dried. The prepared samples contained approximately 20 mg of graphene oxide coating.
In SEM images of Figure 3 it can be observed that the GO coating is very homogeneous. At high magnifications the stacked monolayer GO flakes can be distinguished.
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MEASUREMENT OF WICKING HEIGHT There is a widely accepted method to measure wicking height using the “bubble point method”. The set-up is presented in Figure 4 and is described in detail in Fahgri 39. In the bubble point method is based on the principle that the gas pressure that forces a bubble through a wick on top of which there is the working liquid, is inversely proportional to the average pore size of the wick. In practice, gas pressurize the bottom surface of the wick before the liquid is put in contact with the top surface of the wick, in order to avoid that liquid gets inside the wick making it difficult to get dislodged by the gas pressure. Then, the liquid is poured on the top surface of the wick and columns of bubbles are seen raising in the liquid. The gas pressure is reduced till single bubbles are observed coming out of the top surface of the wick; this gas pressure defines the Pg in the formulae below from which the average pore size of the wick is calculated. The reading of the gas pressure when the bubbles disappear from the methanol liquid column is used in the following formula to evaluate the maximal effective pore size of the wick:
where: h Pg
experimental methanol height
[m]
experimental argon pressure
[Pa]
methanol density
[kg.m-3]
methanol surface tension
[N.m-1]
hcap,max
maximal capillary pressure
[Pa]
reff,max
maximal effective pore size
[m]
8
The methanol thermodynamic properties are taken at 20°C:
= 22.6 10-3 N m-1 = 791 kg m-3
We have performed three measurements with nickel wicks where the graphene solution was sucked inside the wick (samples 1 to 3) and then three measurements of the composite wick with graphene bonded as a coating (samples 4 to 6). Prior to the wicking height measurements, for samples 4 to 6, we deposited few drops of methanol on the composite wick in order to see if methanol wets and permeates the GO and does not dissolve the GO. Visual observation confirmed that methanol permeates the GO and does not dissolve it. The results are reported in Table 2, where we report the characteristics of the uncoated nickel wicks and then the same measurements with the addition of graphene. It is worth noting that there is no improvement of h cap for the first three samples where the graphene solution has been sucked inside the nickel wick. In addition, this type of graphene resulted to be relatively fragile. Instead, the last three samples with the graphene coating showed an improvement and if sample 4 is disregarded, the improvement in hcap of the last two samples is remarkable and repeatable. It should be pointed out that the maximum pressure measured is at the end of the scale for the wicking height measurement kit, therefore the actual pressure might well be higher than this threshold. If we assume that the maximum wicking height is that measured, we get an effective pore size (reff,max) of around 180 nm which is almost an order of magnitude less than the pore dimension with standard sintering powder technology.
Wick coupon with graphene
Comments
r
be
m
nu
Uncoated wick coupon e
pl
m
Sa
Table 2. Wicking height measurements of porous structures without and with graphene.
9
1
h
Peq
Patm
hcap
h
Peq
Patm
hcap
(mm)
(mbar)
(mbar)
(Pa)
(mm)
(mbar)
(mbar)
(Pa)
541
1133.6
990.5
10112
516
1116
991.4
8466
Graphene layer deterioration
2
541
1133.6
990.5
10112
510
1137
1004.2
9323
O-ring detached graphene around the pressure area
3
635
1140.0
990.5
10023
490
1139
1004.2
9678
Graphene layer deterioration
4
522
1133
1003.5
8899
560
1400
973.5
38304
Possible leaks from the wick cylindrical surface
5
556
1111.3
1000.6
6756
622
3500
973.6
247813
hcap limited by tool threshold 3.5 bars
6
494
1099.9
1003.5
5807
715
3500
990
245452
hcap limited by tool threshold of 3.5 bars
Sucking graphene inside the nickel matrix should lead to a bimodal pore size distribution 40-42 which improves the heat transfer capabilities. From Table 2 it is clear that the graphene solution sucked inside the wick does not lead to a noticeable increase of wicking height. When graphene slurries are used instead to coat only the top surface of the nickel wick coupon, there is a significant increase in wicking height. Table 2 shows that the wicking height jumps from around 1 bar for standard nickel wick to 3.5 bars for nickel wick coated with graphene slurries. This remarkable finding suggests that a small graphene coating on top of the network of superficial grooves in a real LHP would increase its transfer length by at least 3.5 times. This means that a graphene coated LHP wick can transport heat passively in aerospace applications such as anti-icing of aircraft leading edges, propeller blades and wind turbines all suffering from severe icing issues, replacing today’s other energy hungry solutions.
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A previous attempt to increase the capillary pressure of LHP using Carbon NanoTubes (CNT) [43] has shown no major improvement. In particular the authors used a ceramic wick (made of zircon) and CNT grown on the outer surface of the ceramic wick. The authors found that the effective pore diameter decreased from 0.54 µm for the ceramic wick to 0.31 µm for the ceramic/CNT wick, a reduction of about 42%. The consequent capillary pressure was increased of an equivalent value, from 90 to 150 kPa. In the present case, instead, the improvement in capillary pressure is much more dramatic. The improvement of capillary force obtained with method 2 could be explained looking at the nanoscopic structure of the material obtained. Method 1 yields a structure where graphene sheets are deposited inside the metal wick, but without forming a continuous layer (Figure 5); thus, methanol wetting and gas transport will be influenced only minimally by the presence of the nanosheets. Conversely, method 2 yields a very dense, uniform coating of GO, which will stop effectively the macroscopic transport of liquid, allowing only nanoscale diffusion of the methanol in between GO sheets. The uniformity of the coating was verified by OM and SEM. In principle, in the SEM analysis no holes were observed. Previous work demonstrated that Graphite oxide is selectively intercalated by methanol when exposed to liquid water/methanol mixtures with methanol fraction in the range 20−100%. Insertion of water into the GO structure occurs only when the content of water in the mixture with methanol is increased up to 90% [21]. The wick obtained with method 2 will thus feature a multiscale hierarchical structure, in which the liquid is first driven forward by capillarity of the mesoscopic metal wick; then, in the very end of the wick, an additional nano-scale capillary effect is due to the 2D pores of the GO membrane, and this combined multiscale effect shall account for the significant improvement observed.
11
A schematic representation of the 3 cases considered is shown in Figure 5.
CONCLUSIONS In this work we have bonded a graphene coating on a nickel wick coupon for a LHP with the aim of generating large capillary pressures and open up new applications of LHP that cannot be serviced by existing LHP technology relying on sintered powder. We have demonstrated a large improvement on capillary pressure generated by graphene slurries bonded on the external surface of the nickel wick coupon. The wicking height measurements (a proxy of capillary pressure) increases from around 1 bar for the nickel wick to at least 3.5 bars for the nickel wick coated with graphene slurries. This would increase the operational range of a LHP from 8-10 m to more than 28-35 m and/or withstand much higher centrifugal forces than those of standard LHP technology (between 4 and 6-g). The next phase of the project would be to produce a fully functional LHP with graphene coating on the primary wick and test it in an hypergravity facility up to 20-g alongside a standard LHP. Standard LHP technology would not withstand more than about 6-g, so it is expected that the graphene LHP would overcome without problems the 20-g limit of the centrifuge.
ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n°604391 Graphene Flagship.
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[43] E. Terrado, R. Molina, E. Natividad, M. Castro, P. Erra, D. Mishkinis, A. Torres, M.T. Martinez, Modifying the heat transfer and capillary pressure of Loop Heat Pipe wicks with Carbon Nanotubes, Journal of Physical Chemistry C 115, 9312-9319, 2011.
18
Figure_1
5 mm Figure 1. Nickel wick coupon coated with graphene slurries only on one circular surface (seen as black ink).
Figure_2
Figure 2. Insertion of graphene solution inside the wick and SEM pictures.
A) Profile of the wick coupon B) Wick coupon without graphene setup with a) lid with screw b) wick and c) rubber seal
C) Wick coupon with 0,5mg graphene
D) Wick coupon with 1,5mg graphene
Figure_3
Figure 3. SEM images of the Ni wick before and after the GO coating.
Figure_4
Figure 4. Experimental apparatus to measure wicking height of porous materials. Right picture shows details of how the wick samples are accommodated.
Figure_5
Figure 5. Schematic representation of the three wick types of present study.
HIGHLIGHTS
A graphene coating has been bonded to a sintered metal wick for loop heat pipe;
The graphene coating improves the wicking height of at least 3,5 times;
The findings opens up applications that were not accessible to loop heat pipe.
19
S0894-1777(16)30095-4 http://dx.doi.org/10.1016/j.expthermflusci.2016.04.019 ETF 8756
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
25 November 2015 21 April 2016 23 April 2016
Please cite this article as: C. Buffone, J. Coulloux, B. Alonso, M. Schlechtendahl, V. Palermo, A. Zurutuza, T. Albertin, S. Martin, M. Molina, S. Chikov, R. Muelhaupt, Capillary pressure in Graphene oxide nanoporous membranes for enhanced heat transport in Loop Heat Pipes for aeronautics, Experimental Thermal and Fluid Science (2016), doi: http://dx.doi.org/10.1016/j.expthermflusci.2016.04.019
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Capillary pressure in Graphene oxide nanoporous membranes for enhanced heat transport in Loop Heat Pipes for aeronautics
Cosimo Buffone1, i, Jerome Coulloux2, Beatriz Alonso3, Mark Schlechtendahl4,5, Vincenzo Palermo6, Amaia Zurutuza3, Thomas Albertin2, Simon Martin2, Marco Molina7, Sergey Chikov1, Rolf Muelhaupt4,5
1
Microgravity Research Centre, Universitè Libre de Bruxelles, Avenue F.D. Roosevelt 50, 1050 Bruxelles, Belgium 2
3
4
Atherm, Z.I. – 1 rue Charles Morel, 38420 Domène, France
Graphenea, Tolosa Hiribidea, 76, 20018 Donostia, Gipuzkoa, Spain
Freiburg Materials Research Center, FMF, Albert-Ludwigs-University of Freiburg, Stefan-Meier-Str. 21, 79104 Freiburg, Germany
5
Institute for Macromolecular Chemistry, Albert-Ludwigs-University of Freiburg, Stefan-Meier-Str. 31, 79104 Freiburg, Germany
6
Institute for Organic Synthesis and Photoreactivity, CNR - National Research Council of Italy, Via P. Gobetti 101, 40129 Bologna, Italy
i
Corresponding author: Tel. 0032 2650 3029, Fax. 0032 2650 3126, Email [email protected]
1
7
Selex ES, Viale Europa, 20014 Nerviano (MI), Italy
ABSTRACT We describe a novel application of graphene-based materials to enhance heat transport in sintered metal wicks, which are the core components for Loop Heat Pipe (LHP) evaporators. Standard metal wicks limit the applicability of LHP to about 8-10 m of transport length and around few meters of gravitational head. This is due to the typical average pore size (about 1 µm) in the sintered metal wicks made of nickel or titanium, which are the most commonly used materials. The idea of the present work is to bond a layer of graphene on top of the wick facing the vapour side of the LHP evaporator. The much smaller pore sizes of graphene (around tens of nanometers) would produce a significant increase in capillary force, while at the same time minimising the pressure drop due to its microscopic thickness (few microns). The wicking height (i.e., capillary rise of a liquid inside a pore) measurements demonstrate that there is an improvement of at least more than 3.5 times when the graphene coating is used, compared to the standard nickel sintered powder wick. This means that the heat transfer of a graphene LHP could work in a spatial range in excess of 28-35 m, which would allow breakthrough applications such as anti-icing of aircraft wings and propellers, as well as wind turbines that cannot be addressed by standard LHP technology.
Keywords: graphene; wicking height; Loop Heat Pipe.
INTRODUCTION Heat Pipes (HP) are passive heat transfer devices capable of transferring heat between two remote locations kept at different temperatures 1-2. HPs are evacuated vessels inside which a working fluid undergoes evaporation and condensation; due to the very large latent heat of evaporation, a
2
small quantity of working fluid is required to transfer large heat loads. In addition, because evaporation and condensation are nearly isothermal processes, there are relatively small temperature gradients in the HP with limited thermal stresses of the materials involved. The core of the HP is the wick structure which provides capillary pressure to overcome all the pressure losses inside the HP. There are different types of wicks for HPs 1-2, the most commonly being based on: 1. Groove arrays; 2. Screen meshes; and, 3. Sintered powders. The best performing wicks are made of sintered powder because the average pore size is much smaller than that of grooves or screen meshes. However, the manufacturing process for sintered powder is more complicated and costly. HPs have been used in a variety of applications, from electronic cooling 3, to aerospace 4-6, to core cooling for nuclear power plants 7, just to mention but a few. There are some important limitations with standard HPs which are connected with the fact that vapour and liquid are in intimate contact and at high loads this can result in reaching the “entrainment limit” during which the evaporator is starved of liquid that cannot return from the condenser by capillary action. The other limitation is connected with the fact that in standard HP the wick is present all over its length, which limits the shape that the HP can have and makes it much heavier. These important limitations were overcome in the ‘80s by the invention of Loop Heat Pipes by Maidanik and co-workers 8, who developed this concept for space applications. Loop Heat Pipes (LHP) 9-10 can transport heat over distances of 8-10 m compared to a few meters for standard grooved and sintered HP, and up to few meters against gravitational head compared with less than a few cm for grooved HP and less than 1 meter for sintered HP, when working against gravity. LHP are more complex to predict and much more complicated to build, resulting in higher manufacturing costs compared with standard HP technology. However, the LHP allows in certain circumstances to control the evaporator temperature 10, something that is not possible with standard heat pipes that operate in a “constant conductance” mode. LHP have been also used in demanding high g-force environments such as aboard fighter jets. The most complex and costly part
3
of a LPH is the evaporator where the wick structure resides. The complicated network of superficial grooves renders the LHP evaporator a very challenging part to make using the current manufacturing techniques (mainly sintering and machining). The limitation on transfer distance has kept to date LHP technology out of applications such as antiicing of aeroplanes and aeroengines where there is a lot of waste heat generated inside the core of aeroengines which could be used to heat up cold spots such as nacelle lips, Outlet Guide Vanes and even wing leading edge of aircrafts. If the improvement in capillary pressure is substantial, then LHP could be also used to anti-ice propeller blades, which typically suffer dramatically from icing because propeller aircrafts fly relatively lower than turbojets, just at the same height where subcooled droplets are more common in the atmosphere. The revolutionary work of 2004 by Geim and Novoselov 11-12, demonstrated the possibility to process and manipulate thin sheets with mesoscopic lateral size and atomic thickness. This has allowed the production of a new class of 2-dimensional (2D) materials, where billions of sheets shall be stacked over each other to form highly anisotropic membranes and coatings. New electronic and chemical properties shall be achieved by creating complex multilayered structures of chemically functionalized graphene sheets 13-15 or by stacking together different kinds of inorganic, 2dimensional sheets 16-18. Such kind of materials features a highly interconnected, 2D porous structure, where both the chemical nature of the 2D walls and the average pore size can be tuned 19. Molecules of liquids diffusing through these 2D pores will feature new properties 20-21. To this aim, exciting results have been recently published on the capillary transport of gases and small molecules inside 2D materials such as graphene or graphene oxide (GO). Geim and co-workers demonstrated that GO membranes can block very effectively some gases and be selectively permeable to others, such as water 22-23 or small ions 24. These barrier properties 4
are due to the 2D multilayer nature of GO membranes, which creates highly tortuous, 2D diffusion paths of the molecules in between the sheets. Furthermore, a recent work suggests that selective transport of protons is possible through the atomic hexagonal voids of single sheets of boron nitride [24]. Applications still more futuristic regards the use of a single sheet of graphene, capable of withstanding particular high pressures, with well-defined holes to desalinate seawater 25-26 or to sequence DNA fragments extremely fast 27. This work describes the processing of different types of graphene onto a nickel sintered wick coupon and measure the wicking height using a standard method. We present Scanning Electron Microscope characterization of the standard nickel wick and of the wick coated with graphene. We demonstrate that the addition of graphene improves significantly the capillary pressure generated by the wick by a factor of at least 3.5, which opens up many more opportunities for LHP in aerospace and ground based applications.
WICK COUPONS INTEGRATED WITH GRAPHENE We have tried two different methods of integrating graphene inside a porous wick structure made of Nickel. The first approach was to suck inside the wick coupon a graphene solution and the second method was to bond only one surface of the wick coupon with graphene oxide. The different wick coupon samples with different types of graphene are summarised in Table 1.
Table 1. Graphene coated wick coupons.
sample
Material used
Coating method
number
5
1
TRGO
vacuum suction (method 1)
2
TRGO
vacuum suction (method 1)
3
TRGO
vacuum suction (method 1)
4
GO graphenea
Slurry deposition (method 2)
5
GO graphenea
Slurry deposition (method 2)
6
GO graphenea
Slurry deposition (method 2)
Figure 1 shows the nickel wick coupon (having 12 mm diameter and 45 mm length) coated with graphene slurries only on top of the upper circular surface, that is the one where the wicking height will be measured.
Method 1: Coating by vacuum suction inside the wick Graphene was dispersed by sonication (Bandelin Sonopuls K76, Berlin, Germany, 30 min sonication at 40 % amplitude) in Acetone (5 g∙L-1) and further processed with a high pressure homogenizer at 1000 bar (GEA NIRO SOAVI NS100 1L 2K, Parma, Italy). The dispersion was then diluted to different concentrations before being poured into the setup in which the wick was fixed as in Figure 2A. The setup was used with a Buchner funnel attached to vacuum pump. The graphene solution was sucked through the wick. Depending on the solutions concentration and the total amount of graphene used a graphene layer formed on top of the wick (Figure 2 B, C, D). Samples with different graphene types were produced and analysed by SEM (ElectroScan Corp. Environmental Scanning Electron Microscope Mod. 2020, Wilmington, MA, USA). We tested two different materials for this method: 6
A) thermally reduced graphite oxide [28-29] (TRGO) and, B) milled graphene with different functionalization [30-31]. TRGO was synthesised based on a modified Hummers [31-32] process, i.e. oxidation of graphite (KFL 99.5) by KMnO4 in sulfuric acid milieu as reported previously [28]. The purified GO was dried and ground and then thermally reduced by rapid heat treatment (750°C) in a rotary tube furnace (Nabertherm, Lilienthal, Germany) under nitrogen atmosphere. The milled graphene was instead produced by milling Graphite under argon and carbon dioxide pressure using a planetary ball mill PM 100 from Retsch, Haan (Germany). The milling chamber was filled with graphite (9.3 g) and 50 ZrO2 balls (Yttrium stabilized, diameter 10 mm) both were dried in vacuum at 60°C. The chamber was evacuated and then pressurized with argon or CO2 (13 bar), respectively. Milling was performed at 250 rpm for the duration of 48 h. Here we present only the most promising results obtained with TRGO.
Method 2: Deposition of GO slurry on the wick We used a graphene oxide slurry that was synthesised using a modified Hummers method [33-35] and investigated in different applications [36-38]. The as-prepared slurry was then deposited on top of the wicks, the surface was activated prior to the deposition. After the deposition and depending on the thickness of the coating the samples were dried under vacuum at 60 °C for a few hours until the graphene oxide was completely dried. The prepared samples contained approximately 20 mg of graphene oxide coating.
In SEM images of Figure 3 it can be observed that the GO coating is very homogeneous. At high magnifications the stacked monolayer GO flakes can be distinguished.
7
MEASUREMENT OF WICKING HEIGHT There is a widely accepted method to measure wicking height using the “bubble point method”. The set-up is presented in Figure 4 and is described in detail in Fahgri 39. In the bubble point method is based on the principle that the gas pressure that forces a bubble through a wick on top of which there is the working liquid, is inversely proportional to the average pore size of the wick. In practice, gas pressurize the bottom surface of the wick before the liquid is put in contact with the top surface of the wick, in order to avoid that liquid gets inside the wick making it difficult to get dislodged by the gas pressure. Then, the liquid is poured on the top surface of the wick and columns of bubbles are seen raising in the liquid. The gas pressure is reduced till single bubbles are observed coming out of the top surface of the wick; this gas pressure defines the Pg in the formulae below from which the average pore size of the wick is calculated. The reading of the gas pressure when the bubbles disappear from the methanol liquid column is used in the following formula to evaluate the maximal effective pore size of the wick:
where: h Pg
experimental methanol height
[m]
experimental argon pressure
[Pa]
methanol density
[kg.m-3]
methanol surface tension
[N.m-1]
hcap,max
maximal capillary pressure
[Pa]
reff,max
maximal effective pore size
[m]
8
The methanol thermodynamic properties are taken at 20°C:
= 22.6 10-3 N m-1 = 791 kg m-3
We have performed three measurements with nickel wicks where the graphene solution was sucked inside the wick (samples 1 to 3) and then three measurements of the composite wick with graphene bonded as a coating (samples 4 to 6). Prior to the wicking height measurements, for samples 4 to 6, we deposited few drops of methanol on the composite wick in order to see if methanol wets and permeates the GO and does not dissolve the GO. Visual observation confirmed that methanol permeates the GO and does not dissolve it. The results are reported in Table 2, where we report the characteristics of the uncoated nickel wicks and then the same measurements with the addition of graphene. It is worth noting that there is no improvement of h cap for the first three samples where the graphene solution has been sucked inside the nickel wick. In addition, this type of graphene resulted to be relatively fragile. Instead, the last three samples with the graphene coating showed an improvement and if sample 4 is disregarded, the improvement in hcap of the last two samples is remarkable and repeatable. It should be pointed out that the maximum pressure measured is at the end of the scale for the wicking height measurement kit, therefore the actual pressure might well be higher than this threshold. If we assume that the maximum wicking height is that measured, we get an effective pore size (reff,max) of around 180 nm which is almost an order of magnitude less than the pore dimension with standard sintering powder technology.
Wick coupon with graphene
Comments
r
be
m
nu
Uncoated wick coupon e
pl
m
Sa
Table 2. Wicking height measurements of porous structures without and with graphene.
9
1
h
Peq
Patm
hcap
h
Peq
Patm
hcap
(mm)
(mbar)
(mbar)
(Pa)
(mm)
(mbar)
(mbar)
(Pa)
541
1133.6
990.5
10112
516
1116
991.4
8466
Graphene layer deterioration
2
541
1133.6
990.5
10112
510
1137
1004.2
9323
O-ring detached graphene around the pressure area
3
635
1140.0
990.5
10023
490
1139
1004.2
9678
Graphene layer deterioration
4
522
1133
1003.5
8899
560
1400
973.5
38304
Possible leaks from the wick cylindrical surface
5
556
1111.3
1000.6
6756
622
3500
973.6
247813
hcap limited by tool threshold 3.5 bars
6
494
1099.9
1003.5
5807
715
3500
990
245452
hcap limited by tool threshold of 3.5 bars
Sucking graphene inside the nickel matrix should lead to a bimodal pore size distribution 40-42 which improves the heat transfer capabilities. From Table 2 it is clear that the graphene solution sucked inside the wick does not lead to a noticeable increase of wicking height. When graphene slurries are used instead to coat only the top surface of the nickel wick coupon, there is a significant increase in wicking height. Table 2 shows that the wicking height jumps from around 1 bar for standard nickel wick to 3.5 bars for nickel wick coated with graphene slurries. This remarkable finding suggests that a small graphene coating on top of the network of superficial grooves in a real LHP would increase its transfer length by at least 3.5 times. This means that a graphene coated LHP wick can transport heat passively in aerospace applications such as anti-icing of aircraft leading edges, propeller blades and wind turbines all suffering from severe icing issues, replacing today’s other energy hungry solutions.
10
A previous attempt to increase the capillary pressure of LHP using Carbon NanoTubes (CNT) [43] has shown no major improvement. In particular the authors used a ceramic wick (made of zircon) and CNT grown on the outer surface of the ceramic wick. The authors found that the effective pore diameter decreased from 0.54 µm for the ceramic wick to 0.31 µm for the ceramic/CNT wick, a reduction of about 42%. The consequent capillary pressure was increased of an equivalent value, from 90 to 150 kPa. In the present case, instead, the improvement in capillary pressure is much more dramatic. The improvement of capillary force obtained with method 2 could be explained looking at the nanoscopic structure of the material obtained. Method 1 yields a structure where graphene sheets are deposited inside the metal wick, but without forming a continuous layer (Figure 5); thus, methanol wetting and gas transport will be influenced only minimally by the presence of the nanosheets. Conversely, method 2 yields a very dense, uniform coating of GO, which will stop effectively the macroscopic transport of liquid, allowing only nanoscale diffusion of the methanol in between GO sheets. The uniformity of the coating was verified by OM and SEM. In principle, in the SEM analysis no holes were observed. Previous work demonstrated that Graphite oxide is selectively intercalated by methanol when exposed to liquid water/methanol mixtures with methanol fraction in the range 20−100%. Insertion of water into the GO structure occurs only when the content of water in the mixture with methanol is increased up to 90% [21]. The wick obtained with method 2 will thus feature a multiscale hierarchical structure, in which the liquid is first driven forward by capillarity of the mesoscopic metal wick; then, in the very end of the wick, an additional nano-scale capillary effect is due to the 2D pores of the GO membrane, and this combined multiscale effect shall account for the significant improvement observed.
11
A schematic representation of the 3 cases considered is shown in Figure 5.
CONCLUSIONS In this work we have bonded a graphene coating on a nickel wick coupon for a LHP with the aim of generating large capillary pressures and open up new applications of LHP that cannot be serviced by existing LHP technology relying on sintered powder. We have demonstrated a large improvement on capillary pressure generated by graphene slurries bonded on the external surface of the nickel wick coupon. The wicking height measurements (a proxy of capillary pressure) increases from around 1 bar for the nickel wick to at least 3.5 bars for the nickel wick coated with graphene slurries. This would increase the operational range of a LHP from 8-10 m to more than 28-35 m and/or withstand much higher centrifugal forces than those of standard LHP technology (between 4 and 6-g). The next phase of the project would be to produce a fully functional LHP with graphene coating on the primary wick and test it in an hypergravity facility up to 20-g alongside a standard LHP. Standard LHP technology would not withstand more than about 6-g, so it is expected that the graphene LHP would overcome without problems the 20-g limit of the centrifuge.
ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement n°604391 Graphene Flagship.
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Figure_1
5 mm Figure 1. Nickel wick coupon coated with graphene slurries only on one circular surface (seen as black ink).
Figure_2
Figure 2. Insertion of graphene solution inside the wick and SEM pictures.
A) Profile of the wick coupon B) Wick coupon without graphene setup with a) lid with screw b) wick and c) rubber seal
C) Wick coupon with 0,5mg graphene
D) Wick coupon with 1,5mg graphene
Figure_3
Figure 3. SEM images of the Ni wick before and after the GO coating.
Figure_4
Figure 4. Experimental apparatus to measure wicking height of porous materials. Right picture shows details of how the wick samples are accommodated.
Figure_5
Figure 5. Schematic representation of the three wick types of present study.
HIGHLIGHTS
A graphene coating has been bonded to a sintered metal wick for loop heat pipe;
The graphene coating improves the wicking height of at least 3,5 times;
The findings opens up applications that were not accessible to loop heat pipe.
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