Fabrication and application of graphene-based silicone grease

With the increasing power requirements of integrated circuits, the demand for efficient cooling has followed suit. Silicone grease is commonly used due to its thermal stability and ability to fill in airgaps between the electronic components and radiators. Previous works attempted to increase the grease’s thermal conductivity by adding various additives such as boron nitride or functionalized carbon nanotubes. Functionalized graphene was chosen in this study due to its exceptional physical and chemical properties. Results show that the functionalization with several acid mixtures combined with ball milling resulted in a compound chemically equivalent to graphene and thoroughly dispersed in silicone grease. An optimal grease was produced, containing 1 wt% Gr-COOH and possessing a thermal conductivity of 6.534 W mK−1. The resulting grease’s performance in thermal dissipation and approximated lifespan improvements was compared to a commercially available silicone grease using a 200 W LED. Results indicated a 4.5 °C decrease in saturation temperature of LED chip along with a 257% increase in thermal conductivity.


Introduction 1.Research overview
With the development of microelectronic technology, electronics, and nanoelectronics have increased sharply in both component density, capacity, and speed of operation [1].However, electronic components, especially high-power ones, such as High Brightness LED (HB-LED), high-power photoluminescent diodes, or computer processors (CPUs) operating for extended periods will consume energy and release a large amount of heat, reducing their lifetime [2].Therefore, improving the efficiency of heat dissipation will help extend their service life, increase the efficiency and luminous power of LEDs, and boost the operating speed of CPUs and other electronic components [3,4].
Along with the development of nanotechnology, many new types of nanomaterials were born, including graphene (Gr), which has many superior mechanical and physical properties.In particular, it has one of the largest known thermal conductivities (k Graphene ∼5300 W mK −1 on the high end) [5], making it apt for heat dissipation in high-power electronic components and devices.
As previously mentioned, heat dissipation is one of the most important issues today regarding micro and nanoelectronics technology.Solving the heat dissipation problem will aid the continued increase in component density and applications of electronic components and optoelectronics [3].For example, high-powered laser components used in material processing or as an optical source faces a large obstacle: excess heat, which can reduce their accuracy, performance, and operational lifetime [6].More relevant to our daily lives is heat dissipation in Light Emitting Diodes.LEDs are gradually replacing conventional lighting due to their outstanding energy efficiency, high luminosity, and long lifespan [7].However, they need to be of high wattage and brightness and possess an adequate lighting field to properly illuminate.With a small component size (less than 1 mm 2 ), the amount of heat emitted per unit volume is quite large, making it difficult to utilize high-power LED lighting systems without remedying the heat dissipation problem.
Similarly, the speed of computer operations depends on the density of transistors in the processor, memory, etc [8].However, operating components will emit a large amount of heat, directly affecting the speed of the device.Using the right heat dissipation material and configuration will directly enhance the quality and speed of computer processors.Thus, to expand the applicability of high-powered electronic components, it is necessary to tackle the problem of heat dissipation.The search for new thermal interface materials, therefore, is becoming an urgent matter in science and has substantial significance [9].This study will discuss the research and production of a heat-dissipating grease containing graphene.Furthermore, the efficiency of the grease will be tested on LED streetlights with potential future assessments on computer processors.The hypothesis is that functionalized graphene with a COOH functional group combined with high-energy ball grinding will improve the dispersion and heat transfer interaction between the grease and radiator.Using a two-step process of dispersing graphene by combining high-energy ball grinding and mechanical stirring will enable the synthesis of graphene-silicone grease in large quantities and different concentrations.

Research background
Graphene is a monolayer plane of carbon atoms tightly arranged in a 2-dimensional (2D) honeycomb-shaped crystal lattice.The graphene can be rolled to form fullerene (0D), wrapped to form carbon nanotubes (1D), or stacked to create graphite (3D) [10].Studies on graphene began in the 1940s and in 1946, P.R. Wallace published the first paper about the energy zone structure of graphene and outlined the anomalous properties of this material [11].In 2004, Novoselov et al (University of Manchester, UK) successfully separated single-layer graphene in large quantities from graphite.Single layer of graphene was transferred onto a SiO 2 base by the process of 'micromechanical separation', also known as the 'Scotch Tape Technique'.SiO 2 layer interacts weakly and can be considered electrically isolated from graphene, so the graphene layer was considered neutral and has its own characteristic properties [12].Since then, graphene has attracted the attention of many scientists in various fields due to its exceptional physical and chemical properties.In particular, the thermal properties of graphene are superior to other materials at normal temperatures.In its pure form, graphene conducts heat faster than any other substance at normal temperatures, possessing a room temperature thermal conductivity of ∼5000 W mK −1 [5], higher than any known configurations of carbon such as carbon nanotubes, graphite, and diamond [13][14][15].As electronics get scaled down and integrated circuit density increases, heat dissipation requirements for components become more important.With an exceptional thermal conductivity, graphene promises to be a potential material for future thermal applications.
However, due to the 2D nature of graphene and defects from different synthesis methods, it has been a challenge to take advantage of the exceptional thermal properties of graphene.In particular, the through-plane thermal conductivity of graphene is severely restricted by interfacial thermal resistance, which impedes phonon transmission between adjacent graphene layers [16].Additionally, defects in the graphene can lead to an order of magnitude or more decrease in thermal conductivity [17].Furthermore, their application is limited due to the difficulties of large-scale production [18,19].Low yield, high impurities, and time consumption inhibit the scalability of CNT and Gr production.To circumvent these issues, many scientists have incorporated Gr into various materials, such as copper and nickel, to create different composites.Nonetheless, these composites, depending on the method of synthesis, can exhibit worse thermal performance than the original materials.This is due to the distribution of nanocarbon [20] and thermal interface resistance [21] in the composites.A major disadvantage of these composites is that they cannot easily fill airgaps like some viscous substances can.
With this in mind, we seek to incorporate graphene into heat-dissipating grease, also commonly known as thermal grease, a typical interface material, consisting of two main components: a silicone base and an additive.Silicone is a polymer compound that is often used as a substrate due to its temperature stability, wet characteristics, and low surface energy [22][23][24], therefore, it can be spread evenly on the surface of the contiguous layer between the electronic component and the heat dissipation system.The substrates and additives are blended to form compounds that are applied to interface surfaces.When applied, the thermal grease fills the air gaps that have low thermal conductivity (0.026 W mK −1 ) [25], facilitating efficient thermal contact and transfer.The main thermally conductive component in silicone grease is heat conductors: micrometer-sized particles with high thermal conductivity dispersed evenly in silicon oil substrates such as inorganic substances or metallic materials such as aluminum oxide, zinc oxide, graphite, aluminum powder, etc.The thermal conductivity of the grease increases with an increase in the number of heat conductors.However, they tend to clump together and become large clusters that increase the grease's viscosity, inhibiting heat dissipation [26,27].Particle size is also a factor in the compound because the particles can act as pads between surfaces and affect the bond thickness.Becker et al [28] showed that by mixing particles of different sizes, it is possible to achieve much lower viscosities than the same volume fraction of particles of the same size.Additionally, Lin et al [29] have shown that there are 3 groups of materials used in the production of thermal grease: metals (nickel, copper, aluminum, silver, etc), ceramics, and carbon groups.Thus, we seek to augment silicone grease with graphene while keeping in mind the importance of graphene size, dispersion, and thermal interface resistance.This augmented grease is suitable for use in cooling electronic components due to its enhanced thermal conductivity and ability to fill air gaps.

Thermal grease synthesis
The graphene used in this study is graphene nanoplatelets (GNP) from ACS Material with a carbon content greater than 99%, a thickness of 2-10 nm, a diameter of 5 μm, a density of 2.3 g cm −3 , and a specific surface area of 20-40 m −2 g −1 .
The silicone base used in the study is a commercial silicone grease manufactured by Tianmu (TM-801) with a thermal conductivity of 1.829 W mK −1 .The silicone oil used in the study was Momentive's polydimethylsiloxane with a viscosity of 350 cst and an evaporation temperature of about 300 °C.For the COOH functionalization process of graphene, we used a ratio of 3:1 H 2 SO 4 (98%) and HNO 3 (98%).Both acids were purchased from Merck.
Preliminary processing and modification of graphene for making the heat-dissipating grease is as follows: the graphene nanoplatelets is functionalized in a strong acid mixture of H 2 SO 4 and HNO 3 (3:1 ratio) to form Gr-COOH.The functionalized graphene is added to silicone grease, followed by silicone oil to ensure an adequate viscosity with 5 wt% of Gr-COOH.The resulting mixture is transferred into a high-energy ball crusher (8000D Mixer/Mill) to evenly disperse the graphene content.This is followed by mechanical stirring to obtain a lower desired Gr-COOH content.It is prudent to differentiate the two separate processes of 'grinding' and 'stirring'.The former serves to reduce the sizes of the graphene to enhance dispersion and prevent nanocarbon agglomeration in the grease.The latter enables us to create greases with different concentrations (lower than 5 wt%) while simultaneously maintaining an even Gr distribution.Figure 1 summarizes the process of creating augmented silicone grease.

Surface morphology survey
To study surface morphology, we utilized a Field Emission Scanning Electron Microscope (FESEM).The FESEM images in the study were measured on the S-4800 field emission scanning electron microscope from Hitachi.

Graphene survey
To study the fabricated thermal grease samples, we measured the Micro-Raman scattering spectrum.The samples were measured with a Micro-Raman LABRAM-1B spectrometer from Jobin-Yvon.Fourier transform infrared spectroscopy analysis was used to study the formation of COOH functional groups on the surface of graphene after acid processing.FTIR spectroscopy measurement was carried out on the IMPAC 410 Nicolet spectrometer.

Thermal conductivity measurements
To survey the thermal conductivity of the samples, we chose the Transient Hot Bridge method (using a QSS HT sensor from Linseis), which utilizes a hot wire technique optimized to measure the thermal conductivity of both solids, greases, and liquid materials with high accuracy and fast measurement times.In this method, a heat source combined with a very thin-shaped sensor is embedded between two pieces of material.The entire surface area of the sensor was covered in thermal grease and any excess was cleaned off.The current and temperature changes were measured by the sensor and thermal conductivity was calculated in software.Air conditioners and humidifiers ran for an extensive period before measurements to keep conditions similar between trials.

Thermal grease efficiency survey
To study the efficiency of the grease, we analyzed its performance in LED street lighting, which was manufactured in collaboration with HALEDCO.Our compound is directly compared to the commercial silicone grease to ascertain the former's effectiveness in heat dissipation.LED temperature measurements were completed using a KIMO TK62 thermocouple, placed between the metal core printed circuit board (MCPCB) and heatsink.Air conditioners and humidifiers ran for an extensive period before measurements to keep conditions similar between trials.

COOH functionalized graphene
Figure 2 is the Raman scattering spectrum of graphene before and after COOH functionalization.
The most prominent characteristics of graphene at 1584 cm −1 are the G-band (graphite) and the 2D band at 2682 cm −1 .The G-peaks are generated from the graphene lattice, which characterizes the orderliness of the structure where carbon atoms arranged hexagonally.The 2D peak is the characteristic band of graphene, which is formed from the oscillations of carbon atoms in the sp 2 state.Ordinary graphene nanoplatelet results showed no D-band peaks, indicating that it is of high purity.It should be noted that the GNP indicated in figure 2 is pristine, therefore, an absence of a peak in the D-band is expected.On the contrary, the Raman results presented a D-band peak at 1340 cm −1 for Gr-COOH, representing the defects of graphene.This indicates the effect of acids on the graphene lattice in facilitating the anchoring of COOH groups onto Gr.The amplitude ratio of the D-band and G-band (I D /I G ) represents graphene impurities.The increase in peak amplitude ratio I D /I G demonstrated a transformation from sp 2 bond (C=C) to sp 3 bond (C-C) on the graphene's surface after functionalizing in a mixture of HNO 3 and H 2 SO 4 .
Figure 3 is the FTIR measurement results of graphene and Gr-COOH.The results showed that there exist characteristic peaks at 3448 cm −1 , which characterize the prolonged oscillation of the O-H bond in H 2 O. Oscillation peaks in the 3348 cm −1 region tend to expand towards low frequencies after functionalization due to the influence of O-H bonds in the COOH group.Furthermore, the peak at 1583 cm −1 is due to the in-plane vibration (C=C) of graphite.Additionally, the 1085 cm −1 peak demonstrates vibrations of the C-O bond.The infrared spectral results of Gr-COOH presented an additional peak at 1700 cm −1 , corresponding to oscillations of the C=O bond in the COOH group.Characteristic peaks on the FTIR spectrum have shown the existence of carboxyl groups on graphene surfaces after processing GNP with a mixture of HNO 3 and H 2 SO 4 .Thus, the defects represented by the D-band in figure 2, are a result of the breakage of C=C bonds and COOH functionalization.
Figure 4 contains a FESEM image of the silicone grease used in the study, the results show that it contains several large and small buffers.Their existence and varying sizes help form dense alternating structures in the grease, which is favorable for thermal conduction.

Functionalized graphene dispersion in thermal grease
Figure 4 also shows FESEM images of Gr-COOH-containing grease with grinding times of 1 to 5 h.The results show that after grinding for 1-3 h, there were still large clumps of graphene in the grease.With increased time in the crusher, the graphene clumping decreases, and good dispersion was observed after at least 4 h.Further grinding showed similar dispersion; thus, we concluded that 4 h is the minimum grinding time for adequate dispersion.It should be noted that grinding the grease breaks up clumps in the graphene and increases dispersion.
Figure 5 presents FESEM images of Gr-COOH-containing grease with mechanical stirring times of 1 to 4 h.The images showed that after stirring for 1 h, there was still a clumping of graphene in the grease.With increased stirring time, graphene clumping decreased, and good dispersion was observed after at least 3 h.After 3 h of, there are similarities in dispersion; thus, we concluded that the minimum mechanical stirring time for dispersing graphene in the silicon grease is 3 h for adequate dispersion.To reiterate, stirring ensures adequate graphene dispersion after grinding and enables the synthesis of grease with low concentrations of Gr-COOH.
After grinding and stirring for at least four and three hours respectively, the grease was determined to be extremely stable, showing little to no agglomeration after 24 months.Figure 6 shows FESEM images of a Gr-COOH grease after 4 h of grinding and 3 h of stirring.After 2 years, there was no discernable clumping in the grease.This stability ensures that an adequate dispersion of grease is maintained; and thus, a fixed thermal conductivity over time.
Figure 7 shows the Raman spectrum of thermal grease containing 1 wt% Gr-COOH and commercial silicone grease.The Raman results of the former possess the characteristic peak of graphene (G-peak) at 1584 cm −1 , and the peak characteristic for the sp 2 (2D) bond at 2682 cm −1 .The Gr-COOH augmented grease also contains characteristic Raman peaks for silicone oil as well.The peak at 1084 cm −1 represents the Si-O-Si and Si-O-C stretching virabtions and at 1404 cm −1 are related to methyl groups asymmetric deformation.Furthermore, the peak at 2906 cm −1 are associated with the stretching vibration bands of CH x aliphatic groups [30,31].Additionally, the wavenumber shift compared to values determined by Mandrile et al And Osterle et al [30,31] can be attributed to the chemical bonds formed through the incorporation of Gr-COOH into silicone grease.With 1 wt% Gr-COOH loading, the augmented grease possessed characteristic Raman peaks of both graphene and silicone grease, confirming that graphene is compatible, well dispersed, and chemically unchanged when mixed into silicon grease.

Thermal conductivity
Figure 8 illustrates thermal conductivity measurements of 0.8 wt% graphene grease with a grinding time ranging from 1.5 to 4.5 h.The measurements illustrated that thermal conductivity increases with grinding time.After 4 h, the conductivity reaches a maximum value of 6.05 W mK −1 .This shows that the effect of graphene dispersion on the thermal conductivity of grease is significant.Increasing the grinding time by more than 4 h does not substantially change, suggesting a saturation value, and confirms the minimum grinding time of 4 h when considering figure 4. We suspect that after extended grinding, there won't be any major difference in dispersion, and therefore, the sample won't conduct heat any better.Additionally, further grinding can change the alignment of graphene and affect the grease's thermal performance.In particular, due to the low through- plane thermal conductivity of graphene [16], additional misalignment can reduce the grease's overall thermal conductivity.
Figure 9 details the thermal conductivity of silicone grease with different Gr-COOH content.The results suggested that an increase in graphene content corresponds to an increase in thermal conductivity.A 1.0 wt% functionalized graphene content correlates with the maximum thermal conductivity achieved of 6.534 W mK −1 .If the graphene content continues to increase, the conductivity does not significantly increase.This could be explained by the fact that raising the content of graphene will simultaneously enhance the viscosity of the grease, thereby decreasing the efficiency of the heat exchange between graphene and silicone base, leading to a decline in thermal conductivity.Thus, in terms of economic and technical efficiency, the optimal graphene content is determined to be 1.0%.

LED heat dissipation
To survey the heat dissipation efficiency, we used a thermal sensor to measure the temperature of a 200 W LED streetlight with different thermal greases.The temperature measurements of the LED chip will help evaluate the thermal dissipation efficiency of the greases.During the temperature measurements, we used air conditioners, humidifiers and sensors to mitigate variations between experiments.We chose an ambient temperature of 27 °C and a humidity of 60%.The content of Gr in the grease surveyed was 1.0 wt%.
Figure 10 is the temperature measurements of commercial silicone grease and its graphene-augmented version.The results showed that the average temperature of the LED chip increased from ambient to a saturated state after 55 min of operation, reaching 69.3 °C for the commercial grease and 64.8 °C for graphene enhanced grease.This shows that when including 1.0 wt% functionalized Gr in silicone grease, the temperature of the chip decreases by approximately 4.5 °C compared to the commercial compound.This is a satisfactory result, which can extend the operating life of electronics.
To quantify the impact of increased heat dissipation on the lifetime of the chip, we utilize a general relationship between component life and operating temperature: when the operating temperature drops by 10 °C, the life of the LED chip increases two-fold.Therefore, the life of the LED chip is approximated by the following equation: L, L 0 , and ΔT are the extended service life, the ordinary service life in hours, and the temperature drop of the LED chip in °C, respectively.Therefore, the percentage of extended life of LEDs is determined by the following: a decrease of 4.5 °C approximately equates to a 36.6% increase in maximum operating hours.With commercial 200 W LEDs having a typical lifespan of 50,000 h, using Gr-COOH augmented silicone grease could theoretically increase maximum operating times to 68,300 h, an increase of 18,300 h.
It should be noted that this estimation is purely theoretical.In practice, the lifetime of LEDs is not solely dependent on temperature but on numerous factors such as the LED design, component failure rates, etc as well.Several models like those proposed by Yang et al [32] and Wang et al [33] consider more factors, such as component failure rates, are much more accurate.Regardless, there is consensus on the impact of temperature on the operating lifetime of LEDs.In particular, high temperatures often result in significant decreases in light output and lifetime [34,35].Determining accurate lifetime improvements from the reduced operating temperature would require extensive testing spanning several years and is outside the scope of this study.While the lifetime increase is purely theoretical, our calculation highlights the importance of thermal management and the potential of augmented grease.

Conclusion
Thermal grease containing Gr-COOH has been successfully fabricated and tested.Raman and FTIR analysis suggested the existence of carboxyl groups on the graphene surface after processing with a mixture of HNO 3 and H 2 SO 4 .FESEM images demonstrated that Gr-COOH is well dispersed in the grease using a high-energy ball crusher.The results of the grinding time survey indicated an optimal grinding time of 4 h and stirring time of 3 h for adequate dispersion of graphene in the thermal grease.Additionally, 1.0 wt% of Gr-COOH leads to a thermal conductivity increase of to 257% in silicone grease.This thermal conductivity increase also corresponds to a decrease in operating temperature of approximately 4.5 °C compared to commercial grease when applied to a 200 W LED Streetlight.Theoretical calculation results showed that the lifetime of LEDs is increased by 36.6% when using graphene thermal grease.The results confirmed that graphene is the preeminent additive for silicone grease and has great potential for applications in heat dissipation for high-power electronic devices.

Figure 1 .
Figure 1.The process of creating Gr-COOH augmented silicone grease.

Figure 6 .
Figure 6.FESEM images of Gr-COOH grease after 4 h of grinding and 3 h of stirring.

Figure 8 .
Figure 8. Thermal conductivity results at different grinding times of graphene augmented silicone grease.

Figure 9 .
Figure 9. Thermal conductivity of augmented silicone grease with different Gr-COOH content.