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Hydrogen bonding enhanced thermally conductive carbon nano grease
Synthetic Metals 259 (2020) 116213
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Hydrogen bonding enhanced thermally conductive carbon nano grease c
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Greg Christensen , Hammad Younes , George Hong , Ding Lou , Haiping Hong *, Christian Widenerb, Craig Baileyc, Rob Hrabec a b c
Department of Advanced Material Processing Lab, South Dakota School of Mines and Technology, Rapid City, SD 57701, United States VRC Metal Systems, Box Elder SD 57719, United States Novum Nano, Rapid City SD 57701, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon nano grease Carbon nanomaterial Hydrogen bonding
Grease made from carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have shown superior thermal conductivity. Thermal conductivity enhancements up to 545.9% over the base fluid are reported by loading 12 wt% of carbon nanofiber into NYE Blank Grease. The unexpected results in this paper also lead to an improved way to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. A carbon nanofiber loading of only 5 wt% leads to a 163.3% increase in thermal conductivity in NYE 758G grease. A loading of 1.4 wt% hydroxyl functionalized multi-wall nanotube (MWNT-OH) in Krytox XHT750 oil leads to a 37.8% increase in thermal conductivity. The new discovery detailed in this paper is that hydrogen bonding between nanotube and oil is the key element for a good conductivity performance. The introduction of hydrogen bonding in any form into the grease increases thermal conductivity. The grease structure is created by the sole thickeners which are carbon nanotubes and nanofibers. This makes the grease unique and valuable.
1. Introduction
found that the addition of multi-walled carbon nanotubes (MWNTs) to calcium grease was optimal at 3% [7]. The same concentration was found for graphene nanosheets [8]. Ideal geometries for carbon nanotubes are also being investigated. Yujun et al. introduced MWNTs to silicone thermal grease, discovering a way to reduce MWNT entanglement and improve grease performance [9]. Others have found a mixture of alumina particles and graphene sheets in grease that yields a maximum thermal conductivity of 3.45 W/m K at only 1 wt.% graphene, through creating a unique packing structure [10] Chen et al. reduced the thermal impedance of silicone grease by as much as 35% by introducing functionalized carbon nanotubes (CNTs) alongside micronsized Al2O3 and submicron-sized ZnO [11]. Heat dissipation is also a serious problem for many machines as it impairs the performance, efficiency, and accuracy and limits the lifetime of the machines. Moreover, the reliability of machines is dependent exponentially on the operating temperature of the junction. Therefore, the generated heat by machines should be removed as fast as possible to keep the machine temperature within the acceptable range and avoid any effect on the performance of the machines [12]. CNTs grease with high thermal conductivity can have a dual role in machines (i) it can act as a lubricant that fills gap between parts to reduce wear and friction [13,14]. (ii) It can be used effectively between solid surface to generate a thermal connection between the heat
Applications requiring improved thermal performance have become widespread. The use of thermal interface materials (TIMs) can improve performance, stability, and longevity in high-power electronic devices from LEDs to CPUs [1]. The addition of metal particles to improve thermal performance has been well explored. Verma et al. investigated the thermal conductivity (TC) of lithium multipurpose grease, mixed with Fe, Cu, and Al particles, discovering ideal weight fractions from 0.3 to 0.4 [2]. Du et al. explored the contribution of the thermal conductive filler’s geometry, finding that T-shaped ZnO has superior thermal transport properties [3]. Another study investigated the use of cupric oxide (CuO) structures as fillers in thermal grease; CuO microdisk thermal networks, increasing thermal conductivity by 139% at 9 vol%, were more effective than nanoblocks and nanospheres [4]. Liquid metal micron-droplets in thermal greases have also been investigated by Mei et al, discovering that methyl silicone oil with 81.8 vol.% Galistan possesses a thermal conductivity of 5.27 W/m K [5]. In comparison, the addition of carbon nanotubes, even at low loadings, can yield promising results. Nguyen et al. analytically and experimentally confirmed that a 2 wt.% CNT thermal grease could have 1.4 times its normal thermal conductivity and reduce the saturation temperature of an Intel Pentium IV processor by 5 °C [6]. Kamel et al.
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Corresponding author. E-mail address: [email protected] (H. Hong).
https://doi.org/10.1016/j.synthmet.2019.116213 Received 5 August 2019; Received in revised form 4 October 2019; Accepted 16 October 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Petro-Canada N650HT, Krytox XHT750, Ethylene Glycol, Glycerol, NYE Blank grease, NYE Ester oil, Used Silicon oil from water bath heater, Valvoline Cerulean Grease, MG Chemicals Silicone Heat Transfer Compound, DURASYN® 166 is a commercial polyalphaolefin oil product purchased from Chemcentral (Chicago, IL). A three-roll mill (Ross Engineering Inc. New York) was used to prepare the stable and homogeneous greases. Base oils and carbon nano materials were mixed and milled for a couple of times until it appeared well dispersed, which can be observed by naked eyes. Thermal conductivity data was obtained by a Hot Disk™ thermal constants analyzer (Mathis Instruments Ltd.) using the following parameters: 6 mm measurement depth, room temperature, 0.012 W power, 10 s measurement time, 3.189 mm sensor radius, 0.0471/K TCR, Kapton disk type, temperature drift record. Thermal conductivity measurement range for a standard sample is 0.005–500 W/mK.
generating parts and heat sinks to increase the heat transfer efficacy [10]. Many groups research have tried to improve the thermal conductivity of grease using nanomaterials, Hong et al, fabricated nanogrease using PAO and 11 wt.% of CNTs. The fabricated grease showed a 60–70% improvement in the thermal conductivity [15,16, p.]. This previous research suggests that these nanomaterials are excellent for the development of high-performing thermal grease. This increase in thermal conductivity, after the introduction of a sufficient number of CNTs, is thought to be the result of the formed 3D structure between the grease and CNTs [17]. Hongtao and Hongmin et al. studied the tribological properties of the CNT grease made of the PAO and 11 wt.% of CNTs. The study revealed that CNT greases demonstrated better lubricating performance and wear resistance than the base oil grease [18]. In his paper Hammad et al. answered the question regarding why CNTs make stable homogenous grease but other carbon nanomaterials don’t. The stability and the homogeneity of the CNTs grease were attributed to the 3D network structure that the CNTs can make [19]. In addition, Hammad et al. studied the thermal conductivity for grease made of CNTs and other carbon nanomaterials, grease made of CNTs have the highest improvement in the thermal conductivity which gave another evidence about the 3D network structure necessary to create the stable grease [19]. Furthermore, Hong et al. studied the rheological properties of the grease. The study found that the viscoelastic response and evidence of creep recovery support the theory of the stable threedimensional network (3D) formation in the CNT-grease. The elastic response indicates that significant energy is needed to dismember the network structure and initiate viscous flow [20]. In previous work it was found that the formation of stable three-dimensional network (3D) is essential to fabricate the CNT-grease. However, that finding didn’t give a clear explanation about the high thermal conductivity of the fabricated grease. Dispersion of carbon nanotubes plays a key role in performance of the grease as well, therefore the optimization of dispersion conditions has been investigated [21]. One team subjected MWNTs, TiO2, Al2O3, and CuO in a guarded hot-plate method-based apparatus to test thermal conductivity and found an approximately 28% enhancement in thermal conductivity with the addition of 0.10 wt % MWNTs [22]. In this paper, high thermally conductive grease made of carbon nanotubes and carbon nanofibers have been successfully fabricated. Different base oils were used to fabricate the high thermally conductive grease in order to understand more about the interactions that occur between the base oil and the carbon nanomaterials that led to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. The hydrogen bonding between nanotube and oil is the key element for a good conductivity performance.
3. Result and discussion
2. Materials and methods
Many of these grease samples use hydroxyl functionalized multiwall carbon nanotubes (MWNT-OH). The role of the functional group is to introduce hydrogen bonding capabilities into the 3D network structure formed by the carbon nanotubes. Hydrogen bonding has been previously used to demonstrate its application in the dispersion and alignment of iron oxide nanoparticles [23]. It is thought that this enhances the integrity of the network, strengthens the nanotube-nanotube attraction as well as the nanotube-base fluids attraction, which in turn provides a significant thermal conductivity enhancement. An example of hydrogen bonding of a water molecule to a MWNT-OH is given in Fig. 1. Water can be replaced with any other hydrogen bonding base fluid such as ethylene glycol or a fluorinated fluid as well as another MWNT-OH. Fig. 2 (A) shows that the CNTs used to fabricate this grease is MWNTs. The CNTs are agglomerates in a way that forms the threedimensional network structure. The SEM and AFM images B, and C and D show that the CNTs are interconnected with each other through a three-dimensional network structure that we believe it occurs either (1) tube–tube within the structure or (2) between neighbor bundles through their contacts. Van der Waals forces of the nanotubes and the strong attractive forces experienced between adjacent nanotube bundles determine the 3D network formed. The AFM image (see Fig. 2) provides a visualization of the structure, including relative sizes of the nanotubes and the degree of entanglement. This sample was prepared on a silicon substrate to get a high magnification image of individual CNTs. Some impurities are visible, and may be PAO oil, catalyst particles, or amorphous carbon. Grease made from carbon nanotubes and carbon nanofibers have
Single-wall carbon nanotubes (SWNTs), multiwall carbon nanotubes (MWNTs), and hydroxyl functionalized multiwall carbon nanotubes (MWNT-OH) were purchased from a variety of companies and used as purchased. Industrial-grade MWNT-OH, 20–40 nm diameter, 10–30 μm length, 88+% (> 90%), and Industrial-grade MWNT, 20–40 nm diameter, 10–30 μm length, > 90% were purchased from Nanostructured & Amorphous Materials, Inc. Houston, Texas. Multi-Wall Carbon Nanotube 10–20 nm diameter, 0.5–40 μm length, Purity > 95 was purchased from Helix Material solutions, Inc. Richardson, TX. Single Walled Carbon nanotubes, Purity of CNTs > 90%, Content of SWNTs > 80%, Diameter < 2 nm, Length < 20 μm, were purchased from Beijing Boda Green High Tech. Co. Ltd. Beijing, China. Graphene Grade 3 nanoplatelets were purchased from Cheaptubes, Cambridge, VT. MWNT NC7000 was purchased from Nanocyl, Rue de l'Essor, 4 B5060 Sambreville, Belgium. Carbon nanofibers PR-19-XT-HHT were purchased from Pyrograf Products, Inc. (Cedarville, Ohio). Average fiber diameter of 150 nm with a length range of 50–200 μm. A variety of base oils and greases were used. ROYCO 500 and 808,
Fig. 1. Schematic of hydrogen bonding between MWNT-OH and a water molecule. 2
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Fig. 2. (A) TEM image for pristine CNTs used to fabricate the CNTs-Grease, (B) SEM image for the fabricated CNT grease, (C) Nanoscope image for grease made of CNTs, (D) CNTs 3D network structure. The above figure is reproduced with permission from H. Younes TC Study of Manufacturable Nano Grease: Evidence of 3D Network Structure, Nanomanufacturing and Metrology (2018), page 150.
attributed to the OH groups that introduce hydrogen bonding into the grease sample. Krytox is a fluorinated synthetic oil mad for high temperature applications. The fluorine atoms are also able to participate in hydrogen bonding with the functionalized MWNTs, increasing the grease network. PAO Durasyn 166 is a very common synthetic base oil used in many commercial lubricants. Even with what is considered to be a non-hydrogen bonding base oil, we see that good thermal conductivity enhancements can be obtained. MWNT-OH provides up to an 83% enhancement at 7.5 wt.%. The CNFs provide and impressive 347.6% percent enhancement at 12 wt.%. Generally, a higher wt.% of CNFs is required to from a stable grease when compared to MWNT-OH, but the thermal enhancement also benefits greatly from this as well. Greases made from the base oil N650HT are listed in Table 2. N650HT has a thermal conductivity of 0.1837 W/mK. As far as thermal conductivity enhancement improvement is concerned, CNFs outperformed with a 259% enhancement, but at 10 wt.% loading. MWNTOH were able to produce a 56.1% enhancement at 7.5 wt.% loading, while even at 8.4 wt.% the CNT-MWNTs only produced a 32.2% enhancement in thermal conductivity. There also other notable differences, for example the Nano C SWNT at 2 wt.% produced a 23.4% enhancement while at 1.9 wt.% NC7000 MWNT only produced a 0.6% enhancement. This difference is likely due to a combination of quality differences and also the performance differences of SWNT vs. MWNT.
shown superior thermal conductivity. The unexpected results in this paper also lead to an improved way to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. The new discovery here is that hydrogen bonding between nanotube and oil is the key element for a good conductivity performance. The introduction of hydrogen bonding in any form into the grease increases thermal conductivity. There are multiple hydrogen bonding scenarios that aid the conductivity of the grease. The oil molecules may hydrogen bond with themselves, with carbon nanomaterials, or both. The carbon nanomaterials may also hydrogen bond with themselves if they have the appropriate functional groups. While hydrogen bonding is not measured directly for these samples there is strong evidence that conductivity is enhanced by the presence of hydrogen bonding. The grease structure is created by the sole thickeners which are either carbon nanotubes or nanofibers. This makes the grease unique and valuable. Table 1 contains data for the Krytox XHT 750 and PAO Durasyn 166 based grease samples. Pure Krytox XHT 750 has a thermal conductivity value of 0.1131 W/mK. We see that for the Helix MWNT a high wt.% of 15 is required to obtain a 69.9% increase in thermal conductivity While a 1.4 wt.% loading of CNT-MWNT yielded a 10.3% increase. The Industrial MWNT-OH have surprisingly high thermal conductivity improvement of 37.8% with only 1.4 wt.%. This dramatic difference is 3
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Table 1 Thermal enhancement of Krytox XHT750 and PAO Durasyn 166 based grease. Base Oil
Carbon Material
Carbon wt. %
TC (W/mK)
TC Percent Increase
Krytox XHT750 Krytox XHT750 Krytox XHT750 PAO Durasyn 166 PAO Durasyn 166 PAO Durasyn 166 PAO Durasyn 166
Helix MWNT MWNT-OH (Industrial) CNT-MWNT MWNT-OH (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-24-XT-HHT CNF
15 1.4 1.4 6 7.5 12 10
0.19 0.16 0.12 0.29 0.31 0.76 0.68
69.9% 37.8% 10.3% 70.6% 83.0% 347.6% 301.4%
Table 2 Thermal enhancement of Petro-Canada N650HT based grease. Base Oil Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada
N650HT N650HT N650HT N650HT N650HT N650HT N650HT N650HT N650HT
Carbon Material
Carbon wt. %
% additive (based on carbon wt.)
TC (W/mK)
TC Percent Increase
MWNT-OH (Industrial) MWNT-OH (Industrial) SWNT CNT-MWNT MWNT-OH (Industrial) NC7000 (Nanocyl MWNT) MWNT-OH (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF
6 7.5 2 8.4 7.5 1.9 5.3 6.8 10
N/A N/A N/A N/A 5.4 % Fe2O3 N/A 2.25 wt.% Fe2O3 0.75 wt.% IMERYS Super 65 Carbon Black N/A
0.26 0.29 0.23 0.24 0.28 0.18 0.28 0.30 0.66
39.6% 56.1% 23.4% 32.2% 54.2% 0.6% 50.6% 65.9% 259.0%
Table 3 Thermal enhancement of ROYCO oils. Base Oil ROYCO ROYCO ROYCO ROYCO
808 500 500 500
Carbon Material
Carbon wt. %
TC (W/mK)
TC Percent Increase
MWNT-OH (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-19-XT-HHT CNF
7.5 7.5 6 10
0.32 0.24 0.66 1.00
87.9% 34.4% 273.9% 470.7%
Table 4 Thermal enhancement of Ethylene Glycol, Glycerol, and Water fluids. Base Oil
Carbon Material
Carbon wt. %
TC (W/mK)
TC Percent Increase
Ethylene Glycol Ethylene Glycol Glycerol Glycerol 50%Ethylene Glycol/50%Water Glycerol 50%Glycerol/50%Water 50%Glycerol/50%Water 50%Glycerol/50%Water 25%Glycerol/75%Water
MWNT-OH (Industrial) MWNT (Industrial) MWNT-OH (Industrial) MWNT (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-19-XT-HHT CNF and MWNT-OH (Industrial) Cheaptubes Grade 3 Graphene Nanoplatelets Pyrograf Pr-19-XT-HHT CNF
4.5 12.5 4.5 12.5 5.7 12 12 6 each 12 6.5
0.41 0.38 0.48 0.45 0.68 1.50 2.64 1.78 0.57 1.95
58.6% 48.4% 54.9% 43.0% 62.1% 381.4% 489.9% 296.5% 27.1% 261.5%
Fig. 3. (a) Glycerol sample with 4.5 wt.% of MWNT-OH(industrial) (54.9% TC Enhancement); (b)NYE blank grease sample with 12 wt.% of CNF-19 (545.9%TC Enhancement).
4
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Table 5 Thermal conductivity enhancement of Used Silicon Oil and some common commercial grease. Base Fluid
1st particles
2nd particles
TC (W/mK)
TC Percent Increase
Used Silicon oil from water bath heater Used Silicon oil from water bath heater Used Silicon oil from water bath heater NYE 758 G grease Valvoline Cerulean Grease Valvoline Cerulean Grease
CNF-19 (4.9%) Silica nano (5.1%) MWNT-OH (7%) CNF-19 (5%) CNF-19 (3.7%) CNF-19 (1.8%)
N/A N/A N/A N/A N/A Graphene nano platelets (1.83%)
0.51 0.20 0.34 0.48 0.35 0.21
180.5% 8% 86.6% 163.3 108.7% 28%
Carbon wt. %
TC (W/mK)
TC Percent Increase
12 6 10 6 5 3 1
1.17 0.70 1.08 0.35 1.70 1.33 0.99
545.9% 286.1% 504.9% 95.5% 143.4% 91% 41.4%
Table 6 Thermal enhancement of NYE blank grease and ester oil and MG Chemicals Silicone Heat Transfer Compound. Base Oil
Carbon Material
NYE Blank Grease NYE Blank Grease NYE ESTER NYE ESTER MG Chemicals Silicone Heat Transfer Compound MG Chemicals Silicone Heat Transfer Compound MG Chemicals Silicone Heat Transfer Compound
Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT
CNF CNF CNF CNF CNF CNF
enhancement of ethylene glycol was noted while at 12.5 wt.% only 48.8% enhancement was noted. The 50:50 water:ethylene glycol sample with 5.68 wt.% MWNT-OH added produced a thermal conductivity enhancement of 62.1% which is the best enhancement of all the glycol samples. As was noted in a previous paper [21], the 50:50 mixture of water and ethylene glycol may produce an ideal combination of hydrogen bonding for nanoparticle interaction. Greases based on glycerol and glycerol/water were also prepared with carbon nanofibers, along with the addition of MWNT-OH, and graphene nanoplatelets. At 12 wt.% CNF it was noted that the best thermal conductivity enhancement of 490% (2.64 W/mK) was for the 50:50 water:glycerol sample. Perhaps the water is able to lessen the intra-molecular bonding or the glycerol enough to increase the interaction with the nanofibers. Even pure glycerol at 12 wt.% produced an enhancement of 381.4%. When the ratio is changed to 25%glycerol:75%water we see that even 6.54 wt.% CNF gives a 261.5% thermal conductivity enhancement. The graphene nanoplatelets only produced a 27% enhancement at 12 wt.% loading. A 50:50 water:glycerol sample with 50:50 CNF:MWNT-OH at 6 wt.% each provided a 296.5% thermal conductivity enhancement over the base fluid showing that even though the CNFs do outperform the nanotubes, a combination of the two can give very good results. Fig. 3 contains pictures of two grease samples. These pictures demonstrate the appearance difference between a lower performing grease and a greatly enhanced grease sample. Silicon oil was taken from a water bath heater to see how much the thermal conductivity could be improved. Used silicon oil has a thermal conductivity of 0.1817 W/mK. A 180.5% enhancement was noted for 4.9 wt.% CNF, see Table 5. This is very high improvement as low carbon loading. Silicon nano particles at a similar loading only produced and 8% enhancement. MWNT-OH at 7 wt.% loading gave a thermal conductivity enhancement of 86.6%. Common commercial greases were also looked at and the results are shown in Table 5. NYE 758 G grease has a thermal conductivity of 0.1816, Valvoline Cerulean Grease has a thermal conductivity of 0.1667 W/mK. NYE 758G grease had an enhancement of 163% when 5 wt.% of CNFs were added. Valvoline Cerulean grease improved 108.7% with 3.7 wt.% CNFs added. When CNFs (1.8%) and Graphene (1.8%) were added, only a 28% enhancement was noted. It seems that the particular graphene used did not add much to the thermal conductivity enhancement in this case. Much work can still be done in order to determine how various grades and functionalization of graphene would influence these results. Carbon nanofibers in NYE blank grease were able to increase the
Some of the samples contained an extra additive such iron oxide or other carbon nanomaterials. When 5.4 wt.% Fe2O3 was added to the 7.5 wt.% MWNT-OH sample the thermal conductivity decreased a little. While this may be within error of the measurement, it may also be that the iron oxide somewhat disrupted the carbon nanotube network. Adding 0.75 wt.% Super 65 carbon black to a 6.75 wt.% MWNT-OH sample enhances thermal conductivity over that of the base fluid by 65.9%. While that is a total carbon wt.% of 7.5, Its thermal performance is better than that of the sample containing 7.5 wt.% MWNT-OH. ROYCO oils are ester based and used in turbine engine applications. Royco 500 has a thermal conductivity of 0.1754 W/mK, Royco 800 has a thermal conductivity of 0.1685 W/mK. Turbine engine oils must maintain performance for extended periods of time at high temperatures. From Table 3 we see that the CNFs were able to provide a thermal conductivity enhancement of 470.7% at 10 wt.%. This is a huge performance enhancement over the base fluid, even at 6 wt.% an enhancement of 273.9% was noted for the ROYCO 500 oil. Both ROYCO 500 and 808 oils were compared by adding 7.5 wt.% MWNT-OH, with the 808 oil showing a much higher enhancement of 87.9% compared to 34.4% for the 500 oil. While we do not know the exact composition of the two oils, we see that the ROYCO 808 had much more favorable interactions with the MWNT-OH, perhaps it was able to more significantly hydrogen bond with the functionalized nanotubes. Table 4 contains data for greases made from based fluids that are known for their ability to form hydrogen bonds with themselves as well as other chemicals. Ethylene glycol has a thermal conductivity of 0.258 W/mK, while 50:50 ethylene glycol:H2O has a thermal conductivity of 0.42 W/mK. Glycerol has a thermal conductivity of 0.3116 W/mK, while 50:50 Glycerol:Water has a thermal conductivity of 0.4477 W/mK, and 25:75 Glycerol:Water has a thermal conductivity of 0.5390 W/mK.To this end, we introduce functionalized MWNT-OH with the intent of forming hydrogen bonds between the base fluids and the nanotubes. For glycerol, which has the most hydrogen bonding capabilities, samples were made containing 4.5 and 12.5 wt.% MWNTOH. The 4.5 wt.% sample showed a 54.9% enhancement while the 12.5 Wt.% samples only showed a 43% enhancement. It seems that a large number of nanotubes are not needed to get a large thermal conductivity enhancement and may even be counterproductive in terms of both cost and performance. Ethylene glycol is often used in antifreeze as a 50:50 mix with water to produce a fluid that has a low freezing point and good cooling ability with a reasonable boiling point. Once again MWNT-OH was added in both 4.5 wt.% and 12.5 wt.% samples producing a similar trend to the glycerol samples. At 4.5 wt.%. a 58.6% 5
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thermal conductivity by 545.9% with 12 wt.% loading, and by 286.1% at 6 wt.% loading at stated in Table 6. NYE ester oil has thermal conductivity of 0.1787 W/mK. When added to NYE ester oil, CNFs produced a 504% enhancement at 10 wt.% loading, while MWNT-OH produced a 95.5% enhancement at 6 wt.% loading. These results are exceptional and show how much room there is for thermal performance enhancements in commercial products. Another area of interest is that of commercial greases sold specifically for the purpose of aiding in heat transfer applications. MG chemicals silicone heat transfer compound, which has a measured thermal conductivity of 0.6981 W/Mk, was selected for this part of the research work. In this case only CNFs were added in varying concentrations from 1, 3, and 5 wt.% as stated in Table 6. The results vary from a 41.4% enhancement up to a 143.4% enhancement. At these low weight percentages, the viscosity is very similar to the original grease so the application of the grease in an end product is still very feasible. This does show that there is a lot of room for improvement in commercial heat transfer products. Further testing of this new grease in commercial devices is needed in order to show that the thermal conductivity will greatly enhance its performance as is anticipated.
materials: a review, Int. Mater. Rev. 63 (1) (2018) 22–45. [2] K. Verma, M. Dabas, A. Upadhyay, R. Singh, Effective thermal conductivity of lithium multipurpose grease filled with metal particles, J. Reinf. Plast. Compos. 33 (October (19)) (2014) 1794–1801. [3] H. Du, Y. Qi, W. Yu, J. Yin, H. Xie, T-shape ZnO whisker: a more effective thermal conductive filler than spherical particles for the thermal grease, Int. J. Heat Mass Transf. 112 (September) (2017) 1052–1056. [4] W. Yu, J. Zhao, M. Wang, Y. Hu, L. Chen, H. Xie, Thermal conductivity enhancement in thermal grease containing different CuO structures, Nanoscale Res. Lett. 10 (March (1)) (2015) 113. [5] S. Mei, Y. Gao, Z. Deng, J. Liu, Thermally conductive and highly electrically resistive grease through homogeneously dispersing liquid metal droplets inside methyl silicone oil, J. Electron. Packag. 136 (January (1)) (2014) 011009. [6] M.H. Nguyen, et al., Thermo-mechanical properties of carbon nanotubes and applications in thermal management, Adv. Nat. Sci. Nanosci. Nanotechnol. 7 (June (2)) (2016) 025017. [7] B.M. Kamel, A. Mohamed, M.E. Sherbiny, K.A. Abed, Rheology and thermal conductivity of calcium grease containing multi-walled carbon nanotube, Fuller. Nanotub. Carbon Nanostruct. 24 (April (4)) (2016) 260–265. [8] B.M. Kamel, A. Mohamed, M. El-Sherbiny, K.A. Abed, M. Abd-Rabou, Rheological characteristics of modified calcium grease with graphene nanosheets, Fuller. Nanotub. Carbon Nanostruct. 25 (July (7)) (2017) 429–434. [9] G. Yujun, L. Zhongliang, Z. Guangmeng, L. Yanxia, Effects of multi-walled carbon nanotubes addition on thermal properties of thermal grease, Int. J. Heat Mass Transf. 74 (July) (2014) 358–367. [10] W. Yu, H. Xie, L. Chen, Z. Zhu, J. Zhao, Z. Zhang, Graphene based silicone thermal greases, Phys. Lett. A 378 (January (3)) (2014) 207–211. [11] H. Chen, H. Wei, M. Chen, F. Meng, H. Li, Q. Li, Enhancing the effectiveness of silicone thermal grease by the addition of functionalized carbon nanotubes, Appl. Surf. Sci. 283 (October) (2013) 525–531. [12] W. Yu, H. Xie, L. Yin, J. Zhao, L. Xia, L. Chen, Exceptionally high thermal conductivity of thermal grease: synergistic effects of graphene and alumina, Int. J. Therm. Sci. 91 (May) (2015) 76–82. [13] H. Chen, et al., Friction and wear behaviour of in-situ transformed Cf/Al2O3 composite under different lubrication conditions, Wear 332–333 (May) (2015) 918–925. [14] J. Ren, et al., Tribological performance of in-situ transformed Cf/Al2O3 self-lubricating composite, Wear 376–377 (April) (2017) 363–371. [15] H. Hong, D. Thomas, A. Waynick, W. Yu, P. Smith, W. Roy, Carbon nanotube grease with enhanced thermal and electrical conductivities, J. Nanopart. Res. 12 (February (2)) (2010) 529–535. [16] H. Hong, F.D.S. Marquis, J.A. Waynick, Carbon Nanoparticle-Containing Lubricant and Grease, US20070158609A1, 12 July (2007). [17] H. Younes, G. Christensen, M. Horton, A. Al Ghaferi, H. Hong, Y. Qiang, TC study of manufacturable nano grease: evidence of 3D network structure, Nanomanuf. Metrol. 1 (September (3)) (2018) 148–155. [18] L. Hongtao, J. Hongmin, H. Haiping, H. Younes, Tribological properties of carbon nanotube grease, Ind. Lubr. Tribol. (August) (2014). [19] H. Younes, G. Christensen, L. Groven, H. Hong, P. Smith, Three dimensional (3D) percolation network structure: key to form stable carbon nano grease, Int. J. Appl. Res. Inf. Technol. Comput. 14 (December (6)) (2016) 375–382. [20] H. Hong, H. Younes, G. Christensen, M. Horton, Y. Qiang, A rheological investigation of carbon nanotube grease, J. Nanosci. Nanotechnol. 19 (July (7)) (2019) 4046–4051. [21] M. Rahman, H. Younes, N. Subramanian, A.A. Ghaferi, Optimizing the dispersion conditions of SWCNTs in aqueous solution of surfactants and organic solvents, J Nanomater. 2014 (January) (2014) 145:145–145:145. [22] L. Peña-Parás, et al., Thermal transport and tribological properties of nanogreases for metal-mechanic applications, Wear 332–333 (May) (2015) 1322–1326. [23] G. Christensen, H. Younes, H. Hong, P. Smith, Effects of solvent hydrogen bonding, viscosity, and polarity on the dispersion and alignment of nanofluids containing Fe2O3 nanoparticles, J. Appl. Phys. 118 (December (21)) (2015) 214302.
4. Conclusion Grease made from carbon nanotubes and carbon nanofibers have shown superior thermal conductivity. Thermal conductivity enhancements up to 545.9% over the base fluid are reported. The unexpected results in this paper also lead to an improved way to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. A carbon nanofiber loading of only 5% leads to a 163.3% increase in thermal conductivity in NYE 758 G grease. Loading 1.4 wt.% MWNT-OH in Krytox XHT750 oil leads to a 37.8% increase in thermal conductivity. The new discovery here is that hydrogen bonding between nanotube and oil is the key element for a good conductivity performance. The introduction of hydrogen bonding in any form into the grease increases thermal conductivity. The grease structure is created by the sole thickeners which are carbon nanotubes and nanofibers. This makes the grease unique and valuable. Declaration of competing interests We have no conflicts of interest. Acknowledgements The financial support of Army Research Lab (Cooperative agreement W911NF 15-2-0034-S, South Dakota State funding, Advanced Manufacturing Process Technology Transition and Training Center (AMPTECH), BOR Proof of Concept Grant are acknowledged. References [1] J. Hansson, T.M.J. Nilsson, L. Ye, J. Liu, Novel nanostructured thermal interface
6
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Hydrogen bonding enhanced thermally conductive carbon nano grease c
a
a
a
T
a,
Greg Christensen , Hammad Younes , George Hong , Ding Lou , Haiping Hong *, Christian Widenerb, Craig Baileyc, Rob Hrabec a b c
Department of Advanced Material Processing Lab, South Dakota School of Mines and Technology, Rapid City, SD 57701, United States VRC Metal Systems, Box Elder SD 57719, United States Novum Nano, Rapid City SD 57701, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon nano grease Carbon nanomaterial Hydrogen bonding
Grease made from carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have shown superior thermal conductivity. Thermal conductivity enhancements up to 545.9% over the base fluid are reported by loading 12 wt% of carbon nanofiber into NYE Blank Grease. The unexpected results in this paper also lead to an improved way to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. A carbon nanofiber loading of only 5 wt% leads to a 163.3% increase in thermal conductivity in NYE 758G grease. A loading of 1.4 wt% hydroxyl functionalized multi-wall nanotube (MWNT-OH) in Krytox XHT750 oil leads to a 37.8% increase in thermal conductivity. The new discovery detailed in this paper is that hydrogen bonding between nanotube and oil is the key element for a good conductivity performance. The introduction of hydrogen bonding in any form into the grease increases thermal conductivity. The grease structure is created by the sole thickeners which are carbon nanotubes and nanofibers. This makes the grease unique and valuable.
1. Introduction
found that the addition of multi-walled carbon nanotubes (MWNTs) to calcium grease was optimal at 3% [7]. The same concentration was found for graphene nanosheets [8]. Ideal geometries for carbon nanotubes are also being investigated. Yujun et al. introduced MWNTs to silicone thermal grease, discovering a way to reduce MWNT entanglement and improve grease performance [9]. Others have found a mixture of alumina particles and graphene sheets in grease that yields a maximum thermal conductivity of 3.45 W/m K at only 1 wt.% graphene, through creating a unique packing structure [10] Chen et al. reduced the thermal impedance of silicone grease by as much as 35% by introducing functionalized carbon nanotubes (CNTs) alongside micronsized Al2O3 and submicron-sized ZnO [11]. Heat dissipation is also a serious problem for many machines as it impairs the performance, efficiency, and accuracy and limits the lifetime of the machines. Moreover, the reliability of machines is dependent exponentially on the operating temperature of the junction. Therefore, the generated heat by machines should be removed as fast as possible to keep the machine temperature within the acceptable range and avoid any effect on the performance of the machines [12]. CNTs grease with high thermal conductivity can have a dual role in machines (i) it can act as a lubricant that fills gap between parts to reduce wear and friction [13,14]. (ii) It can be used effectively between solid surface to generate a thermal connection between the heat
Applications requiring improved thermal performance have become widespread. The use of thermal interface materials (TIMs) can improve performance, stability, and longevity in high-power electronic devices from LEDs to CPUs [1]. The addition of metal particles to improve thermal performance has been well explored. Verma et al. investigated the thermal conductivity (TC) of lithium multipurpose grease, mixed with Fe, Cu, and Al particles, discovering ideal weight fractions from 0.3 to 0.4 [2]. Du et al. explored the contribution of the thermal conductive filler’s geometry, finding that T-shaped ZnO has superior thermal transport properties [3]. Another study investigated the use of cupric oxide (CuO) structures as fillers in thermal grease; CuO microdisk thermal networks, increasing thermal conductivity by 139% at 9 vol%, were more effective than nanoblocks and nanospheres [4]. Liquid metal micron-droplets in thermal greases have also been investigated by Mei et al, discovering that methyl silicone oil with 81.8 vol.% Galistan possesses a thermal conductivity of 5.27 W/m K [5]. In comparison, the addition of carbon nanotubes, even at low loadings, can yield promising results. Nguyen et al. analytically and experimentally confirmed that a 2 wt.% CNT thermal grease could have 1.4 times its normal thermal conductivity and reduce the saturation temperature of an Intel Pentium IV processor by 5 °C [6]. Kamel et al.
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Corresponding author. E-mail address: [email protected] (H. Hong).
https://doi.org/10.1016/j.synthmet.2019.116213 Received 5 August 2019; Received in revised form 4 October 2019; Accepted 16 October 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Petro-Canada N650HT, Krytox XHT750, Ethylene Glycol, Glycerol, NYE Blank grease, NYE Ester oil, Used Silicon oil from water bath heater, Valvoline Cerulean Grease, MG Chemicals Silicone Heat Transfer Compound, DURASYN® 166 is a commercial polyalphaolefin oil product purchased from Chemcentral (Chicago, IL). A three-roll mill (Ross Engineering Inc. New York) was used to prepare the stable and homogeneous greases. Base oils and carbon nano materials were mixed and milled for a couple of times until it appeared well dispersed, which can be observed by naked eyes. Thermal conductivity data was obtained by a Hot Disk™ thermal constants analyzer (Mathis Instruments Ltd.) using the following parameters: 6 mm measurement depth, room temperature, 0.012 W power, 10 s measurement time, 3.189 mm sensor radius, 0.0471/K TCR, Kapton disk type, temperature drift record. Thermal conductivity measurement range for a standard sample is 0.005–500 W/mK.
generating parts and heat sinks to increase the heat transfer efficacy [10]. Many groups research have tried to improve the thermal conductivity of grease using nanomaterials, Hong et al, fabricated nanogrease using PAO and 11 wt.% of CNTs. The fabricated grease showed a 60–70% improvement in the thermal conductivity [15,16, p.]. This previous research suggests that these nanomaterials are excellent for the development of high-performing thermal grease. This increase in thermal conductivity, after the introduction of a sufficient number of CNTs, is thought to be the result of the formed 3D structure between the grease and CNTs [17]. Hongtao and Hongmin et al. studied the tribological properties of the CNT grease made of the PAO and 11 wt.% of CNTs. The study revealed that CNT greases demonstrated better lubricating performance and wear resistance than the base oil grease [18]. In his paper Hammad et al. answered the question regarding why CNTs make stable homogenous grease but other carbon nanomaterials don’t. The stability and the homogeneity of the CNTs grease were attributed to the 3D network structure that the CNTs can make [19]. In addition, Hammad et al. studied the thermal conductivity for grease made of CNTs and other carbon nanomaterials, grease made of CNTs have the highest improvement in the thermal conductivity which gave another evidence about the 3D network structure necessary to create the stable grease [19]. Furthermore, Hong et al. studied the rheological properties of the grease. The study found that the viscoelastic response and evidence of creep recovery support the theory of the stable threedimensional network (3D) formation in the CNT-grease. The elastic response indicates that significant energy is needed to dismember the network structure and initiate viscous flow [20]. In previous work it was found that the formation of stable three-dimensional network (3D) is essential to fabricate the CNT-grease. However, that finding didn’t give a clear explanation about the high thermal conductivity of the fabricated grease. Dispersion of carbon nanotubes plays a key role in performance of the grease as well, therefore the optimization of dispersion conditions has been investigated [21]. One team subjected MWNTs, TiO2, Al2O3, and CuO in a guarded hot-plate method-based apparatus to test thermal conductivity and found an approximately 28% enhancement in thermal conductivity with the addition of 0.10 wt % MWNTs [22]. In this paper, high thermally conductive grease made of carbon nanotubes and carbon nanofibers have been successfully fabricated. Different base oils were used to fabricate the high thermally conductive grease in order to understand more about the interactions that occur between the base oil and the carbon nanomaterials that led to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. The hydrogen bonding between nanotube and oil is the key element for a good conductivity performance.
3. Result and discussion
2. Materials and methods
Many of these grease samples use hydroxyl functionalized multiwall carbon nanotubes (MWNT-OH). The role of the functional group is to introduce hydrogen bonding capabilities into the 3D network structure formed by the carbon nanotubes. Hydrogen bonding has been previously used to demonstrate its application in the dispersion and alignment of iron oxide nanoparticles [23]. It is thought that this enhances the integrity of the network, strengthens the nanotube-nanotube attraction as well as the nanotube-base fluids attraction, which in turn provides a significant thermal conductivity enhancement. An example of hydrogen bonding of a water molecule to a MWNT-OH is given in Fig. 1. Water can be replaced with any other hydrogen bonding base fluid such as ethylene glycol or a fluorinated fluid as well as another MWNT-OH. Fig. 2 (A) shows that the CNTs used to fabricate this grease is MWNTs. The CNTs are agglomerates in a way that forms the threedimensional network structure. The SEM and AFM images B, and C and D show that the CNTs are interconnected with each other through a three-dimensional network structure that we believe it occurs either (1) tube–tube within the structure or (2) between neighbor bundles through their contacts. Van der Waals forces of the nanotubes and the strong attractive forces experienced between adjacent nanotube bundles determine the 3D network formed. The AFM image (see Fig. 2) provides a visualization of the structure, including relative sizes of the nanotubes and the degree of entanglement. This sample was prepared on a silicon substrate to get a high magnification image of individual CNTs. Some impurities are visible, and may be PAO oil, catalyst particles, or amorphous carbon. Grease made from carbon nanotubes and carbon nanofibers have
Single-wall carbon nanotubes (SWNTs), multiwall carbon nanotubes (MWNTs), and hydroxyl functionalized multiwall carbon nanotubes (MWNT-OH) were purchased from a variety of companies and used as purchased. Industrial-grade MWNT-OH, 20–40 nm diameter, 10–30 μm length, 88+% (> 90%), and Industrial-grade MWNT, 20–40 nm diameter, 10–30 μm length, > 90% were purchased from Nanostructured & Amorphous Materials, Inc. Houston, Texas. Multi-Wall Carbon Nanotube 10–20 nm diameter, 0.5–40 μm length, Purity > 95 was purchased from Helix Material solutions, Inc. Richardson, TX. Single Walled Carbon nanotubes, Purity of CNTs > 90%, Content of SWNTs > 80%, Diameter < 2 nm, Length < 20 μm, were purchased from Beijing Boda Green High Tech. Co. Ltd. Beijing, China. Graphene Grade 3 nanoplatelets were purchased from Cheaptubes, Cambridge, VT. MWNT NC7000 was purchased from Nanocyl, Rue de l'Essor, 4 B5060 Sambreville, Belgium. Carbon nanofibers PR-19-XT-HHT were purchased from Pyrograf Products, Inc. (Cedarville, Ohio). Average fiber diameter of 150 nm with a length range of 50–200 μm. A variety of base oils and greases were used. ROYCO 500 and 808,
Fig. 1. Schematic of hydrogen bonding between MWNT-OH and a water molecule. 2
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Fig. 2. (A) TEM image for pristine CNTs used to fabricate the CNTs-Grease, (B) SEM image for the fabricated CNT grease, (C) Nanoscope image for grease made of CNTs, (D) CNTs 3D network structure. The above figure is reproduced with permission from H. Younes TC Study of Manufacturable Nano Grease: Evidence of 3D Network Structure, Nanomanufacturing and Metrology (2018), page 150.
attributed to the OH groups that introduce hydrogen bonding into the grease sample. Krytox is a fluorinated synthetic oil mad for high temperature applications. The fluorine atoms are also able to participate in hydrogen bonding with the functionalized MWNTs, increasing the grease network. PAO Durasyn 166 is a very common synthetic base oil used in many commercial lubricants. Even with what is considered to be a non-hydrogen bonding base oil, we see that good thermal conductivity enhancements can be obtained. MWNT-OH provides up to an 83% enhancement at 7.5 wt.%. The CNFs provide and impressive 347.6% percent enhancement at 12 wt.%. Generally, a higher wt.% of CNFs is required to from a stable grease when compared to MWNT-OH, but the thermal enhancement also benefits greatly from this as well. Greases made from the base oil N650HT are listed in Table 2. N650HT has a thermal conductivity of 0.1837 W/mK. As far as thermal conductivity enhancement improvement is concerned, CNFs outperformed with a 259% enhancement, but at 10 wt.% loading. MWNTOH were able to produce a 56.1% enhancement at 7.5 wt.% loading, while even at 8.4 wt.% the CNT-MWNTs only produced a 32.2% enhancement in thermal conductivity. There also other notable differences, for example the Nano C SWNT at 2 wt.% produced a 23.4% enhancement while at 1.9 wt.% NC7000 MWNT only produced a 0.6% enhancement. This difference is likely due to a combination of quality differences and also the performance differences of SWNT vs. MWNT.
shown superior thermal conductivity. The unexpected results in this paper also lead to an improved way to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. The new discovery here is that hydrogen bonding between nanotube and oil is the key element for a good conductivity performance. The introduction of hydrogen bonding in any form into the grease increases thermal conductivity. There are multiple hydrogen bonding scenarios that aid the conductivity of the grease. The oil molecules may hydrogen bond with themselves, with carbon nanomaterials, or both. The carbon nanomaterials may also hydrogen bond with themselves if they have the appropriate functional groups. While hydrogen bonding is not measured directly for these samples there is strong evidence that conductivity is enhanced by the presence of hydrogen bonding. The grease structure is created by the sole thickeners which are either carbon nanotubes or nanofibers. This makes the grease unique and valuable. Table 1 contains data for the Krytox XHT 750 and PAO Durasyn 166 based grease samples. Pure Krytox XHT 750 has a thermal conductivity value of 0.1131 W/mK. We see that for the Helix MWNT a high wt.% of 15 is required to obtain a 69.9% increase in thermal conductivity While a 1.4 wt.% loading of CNT-MWNT yielded a 10.3% increase. The Industrial MWNT-OH have surprisingly high thermal conductivity improvement of 37.8% with only 1.4 wt.%. This dramatic difference is 3
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Table 1 Thermal enhancement of Krytox XHT750 and PAO Durasyn 166 based grease. Base Oil
Carbon Material
Carbon wt. %
TC (W/mK)
TC Percent Increase
Krytox XHT750 Krytox XHT750 Krytox XHT750 PAO Durasyn 166 PAO Durasyn 166 PAO Durasyn 166 PAO Durasyn 166
Helix MWNT MWNT-OH (Industrial) CNT-MWNT MWNT-OH (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-24-XT-HHT CNF
15 1.4 1.4 6 7.5 12 10
0.19 0.16 0.12 0.29 0.31 0.76 0.68
69.9% 37.8% 10.3% 70.6% 83.0% 347.6% 301.4%
Table 2 Thermal enhancement of Petro-Canada N650HT based grease. Base Oil Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada Petro-Canada
N650HT N650HT N650HT N650HT N650HT N650HT N650HT N650HT N650HT
Carbon Material
Carbon wt. %
% additive (based on carbon wt.)
TC (W/mK)
TC Percent Increase
MWNT-OH (Industrial) MWNT-OH (Industrial) SWNT CNT-MWNT MWNT-OH (Industrial) NC7000 (Nanocyl MWNT) MWNT-OH (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF
6 7.5 2 8.4 7.5 1.9 5.3 6.8 10
N/A N/A N/A N/A 5.4 % Fe2O3 N/A 2.25 wt.% Fe2O3 0.75 wt.% IMERYS Super 65 Carbon Black N/A
0.26 0.29 0.23 0.24 0.28 0.18 0.28 0.30 0.66
39.6% 56.1% 23.4% 32.2% 54.2% 0.6% 50.6% 65.9% 259.0%
Table 3 Thermal enhancement of ROYCO oils. Base Oil ROYCO ROYCO ROYCO ROYCO
808 500 500 500
Carbon Material
Carbon wt. %
TC (W/mK)
TC Percent Increase
MWNT-OH (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-19-XT-HHT CNF
7.5 7.5 6 10
0.32 0.24 0.66 1.00
87.9% 34.4% 273.9% 470.7%
Table 4 Thermal enhancement of Ethylene Glycol, Glycerol, and Water fluids. Base Oil
Carbon Material
Carbon wt. %
TC (W/mK)
TC Percent Increase
Ethylene Glycol Ethylene Glycol Glycerol Glycerol 50%Ethylene Glycol/50%Water Glycerol 50%Glycerol/50%Water 50%Glycerol/50%Water 50%Glycerol/50%Water 25%Glycerol/75%Water
MWNT-OH (Industrial) MWNT (Industrial) MWNT-OH (Industrial) MWNT (Industrial) MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-19-XT-HHT CNF Pyrograf Pr-19-XT-HHT CNF and MWNT-OH (Industrial) Cheaptubes Grade 3 Graphene Nanoplatelets Pyrograf Pr-19-XT-HHT CNF
4.5 12.5 4.5 12.5 5.7 12 12 6 each 12 6.5
0.41 0.38 0.48 0.45 0.68 1.50 2.64 1.78 0.57 1.95
58.6% 48.4% 54.9% 43.0% 62.1% 381.4% 489.9% 296.5% 27.1% 261.5%
Fig. 3. (a) Glycerol sample with 4.5 wt.% of MWNT-OH(industrial) (54.9% TC Enhancement); (b)NYE blank grease sample with 12 wt.% of CNF-19 (545.9%TC Enhancement).
4
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Table 5 Thermal conductivity enhancement of Used Silicon Oil and some common commercial grease. Base Fluid
1st particles
2nd particles
TC (W/mK)
TC Percent Increase
Used Silicon oil from water bath heater Used Silicon oil from water bath heater Used Silicon oil from water bath heater NYE 758 G grease Valvoline Cerulean Grease Valvoline Cerulean Grease
CNF-19 (4.9%) Silica nano (5.1%) MWNT-OH (7%) CNF-19 (5%) CNF-19 (3.7%) CNF-19 (1.8%)
N/A N/A N/A N/A N/A Graphene nano platelets (1.83%)
0.51 0.20 0.34 0.48 0.35 0.21
180.5% 8% 86.6% 163.3 108.7% 28%
Carbon wt. %
TC (W/mK)
TC Percent Increase
12 6 10 6 5 3 1
1.17 0.70 1.08 0.35 1.70 1.33 0.99
545.9% 286.1% 504.9% 95.5% 143.4% 91% 41.4%
Table 6 Thermal enhancement of NYE blank grease and ester oil and MG Chemicals Silicone Heat Transfer Compound. Base Oil
Carbon Material
NYE Blank Grease NYE Blank Grease NYE ESTER NYE ESTER MG Chemicals Silicone Heat Transfer Compound MG Chemicals Silicone Heat Transfer Compound MG Chemicals Silicone Heat Transfer Compound
Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT MWNT-OH (Industrial) Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT Pyrograf Pr-19-XT-HHT
CNF CNF CNF CNF CNF CNF
enhancement of ethylene glycol was noted while at 12.5 wt.% only 48.8% enhancement was noted. The 50:50 water:ethylene glycol sample with 5.68 wt.% MWNT-OH added produced a thermal conductivity enhancement of 62.1% which is the best enhancement of all the glycol samples. As was noted in a previous paper [21], the 50:50 mixture of water and ethylene glycol may produce an ideal combination of hydrogen bonding for nanoparticle interaction. Greases based on glycerol and glycerol/water were also prepared with carbon nanofibers, along with the addition of MWNT-OH, and graphene nanoplatelets. At 12 wt.% CNF it was noted that the best thermal conductivity enhancement of 490% (2.64 W/mK) was for the 50:50 water:glycerol sample. Perhaps the water is able to lessen the intra-molecular bonding or the glycerol enough to increase the interaction with the nanofibers. Even pure glycerol at 12 wt.% produced an enhancement of 381.4%. When the ratio is changed to 25%glycerol:75%water we see that even 6.54 wt.% CNF gives a 261.5% thermal conductivity enhancement. The graphene nanoplatelets only produced a 27% enhancement at 12 wt.% loading. A 50:50 water:glycerol sample with 50:50 CNF:MWNT-OH at 6 wt.% each provided a 296.5% thermal conductivity enhancement over the base fluid showing that even though the CNFs do outperform the nanotubes, a combination of the two can give very good results. Fig. 3 contains pictures of two grease samples. These pictures demonstrate the appearance difference between a lower performing grease and a greatly enhanced grease sample. Silicon oil was taken from a water bath heater to see how much the thermal conductivity could be improved. Used silicon oil has a thermal conductivity of 0.1817 W/mK. A 180.5% enhancement was noted for 4.9 wt.% CNF, see Table 5. This is very high improvement as low carbon loading. Silicon nano particles at a similar loading only produced and 8% enhancement. MWNT-OH at 7 wt.% loading gave a thermal conductivity enhancement of 86.6%. Common commercial greases were also looked at and the results are shown in Table 5. NYE 758 G grease has a thermal conductivity of 0.1816, Valvoline Cerulean Grease has a thermal conductivity of 0.1667 W/mK. NYE 758G grease had an enhancement of 163% when 5 wt.% of CNFs were added. Valvoline Cerulean grease improved 108.7% with 3.7 wt.% CNFs added. When CNFs (1.8%) and Graphene (1.8%) were added, only a 28% enhancement was noted. It seems that the particular graphene used did not add much to the thermal conductivity enhancement in this case. Much work can still be done in order to determine how various grades and functionalization of graphene would influence these results. Carbon nanofibers in NYE blank grease were able to increase the
Some of the samples contained an extra additive such iron oxide or other carbon nanomaterials. When 5.4 wt.% Fe2O3 was added to the 7.5 wt.% MWNT-OH sample the thermal conductivity decreased a little. While this may be within error of the measurement, it may also be that the iron oxide somewhat disrupted the carbon nanotube network. Adding 0.75 wt.% Super 65 carbon black to a 6.75 wt.% MWNT-OH sample enhances thermal conductivity over that of the base fluid by 65.9%. While that is a total carbon wt.% of 7.5, Its thermal performance is better than that of the sample containing 7.5 wt.% MWNT-OH. ROYCO oils are ester based and used in turbine engine applications. Royco 500 has a thermal conductivity of 0.1754 W/mK, Royco 800 has a thermal conductivity of 0.1685 W/mK. Turbine engine oils must maintain performance for extended periods of time at high temperatures. From Table 3 we see that the CNFs were able to provide a thermal conductivity enhancement of 470.7% at 10 wt.%. This is a huge performance enhancement over the base fluid, even at 6 wt.% an enhancement of 273.9% was noted for the ROYCO 500 oil. Both ROYCO 500 and 808 oils were compared by adding 7.5 wt.% MWNT-OH, with the 808 oil showing a much higher enhancement of 87.9% compared to 34.4% for the 500 oil. While we do not know the exact composition of the two oils, we see that the ROYCO 808 had much more favorable interactions with the MWNT-OH, perhaps it was able to more significantly hydrogen bond with the functionalized nanotubes. Table 4 contains data for greases made from based fluids that are known for their ability to form hydrogen bonds with themselves as well as other chemicals. Ethylene glycol has a thermal conductivity of 0.258 W/mK, while 50:50 ethylene glycol:H2O has a thermal conductivity of 0.42 W/mK. Glycerol has a thermal conductivity of 0.3116 W/mK, while 50:50 Glycerol:Water has a thermal conductivity of 0.4477 W/mK, and 25:75 Glycerol:Water has a thermal conductivity of 0.5390 W/mK.To this end, we introduce functionalized MWNT-OH with the intent of forming hydrogen bonds between the base fluids and the nanotubes. For glycerol, which has the most hydrogen bonding capabilities, samples were made containing 4.5 and 12.5 wt.% MWNTOH. The 4.5 wt.% sample showed a 54.9% enhancement while the 12.5 Wt.% samples only showed a 43% enhancement. It seems that a large number of nanotubes are not needed to get a large thermal conductivity enhancement and may even be counterproductive in terms of both cost and performance. Ethylene glycol is often used in antifreeze as a 50:50 mix with water to produce a fluid that has a low freezing point and good cooling ability with a reasonable boiling point. Once again MWNT-OH was added in both 4.5 wt.% and 12.5 wt.% samples producing a similar trend to the glycerol samples. At 4.5 wt.%. a 58.6% 5
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thermal conductivity by 545.9% with 12 wt.% loading, and by 286.1% at 6 wt.% loading at stated in Table 6. NYE ester oil has thermal conductivity of 0.1787 W/mK. When added to NYE ester oil, CNFs produced a 504% enhancement at 10 wt.% loading, while MWNT-OH produced a 95.5% enhancement at 6 wt.% loading. These results are exceptional and show how much room there is for thermal performance enhancements in commercial products. Another area of interest is that of commercial greases sold specifically for the purpose of aiding in heat transfer applications. MG chemicals silicone heat transfer compound, which has a measured thermal conductivity of 0.6981 W/Mk, was selected for this part of the research work. In this case only CNFs were added in varying concentrations from 1, 3, and 5 wt.% as stated in Table 6. The results vary from a 41.4% enhancement up to a 143.4% enhancement. At these low weight percentages, the viscosity is very similar to the original grease so the application of the grease in an end product is still very feasible. This does show that there is a lot of room for improvement in commercial heat transfer products. Further testing of this new grease in commercial devices is needed in order to show that the thermal conductivity will greatly enhance its performance as is anticipated.
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4. Conclusion Grease made from carbon nanotubes and carbon nanofibers have shown superior thermal conductivity. Thermal conductivity enhancements up to 545.9% over the base fluid are reported. The unexpected results in this paper also lead to an improved way to significantly enhance the thermal conductivity of greases while reducing the nanotube loading percentage. A carbon nanofiber loading of only 5% leads to a 163.3% increase in thermal conductivity in NYE 758 G grease. Loading 1.4 wt.% MWNT-OH in Krytox XHT750 oil leads to a 37.8% increase in thermal conductivity. The new discovery here is that hydrogen bonding between nanotube and oil is the key element for a good conductivity performance. The introduction of hydrogen bonding in any form into the grease increases thermal conductivity. The grease structure is created by the sole thickeners which are carbon nanotubes and nanofibers. This makes the grease unique and valuable. Declaration of competing interests We have no conflicts of interest. Acknowledgements The financial support of Army Research Lab (Cooperative agreement W911NF 15-2-0034-S, South Dakota State funding, Advanced Manufacturing Process Technology Transition and Training Center (AMPTECH), BOR Proof of Concept Grant are acknowledged. References [1] J. Hansson, T.M.J. Nilsson, L. Ye, J. Liu, Novel nanostructured thermal interface
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