Phase change materials coated with modified graphene-oxide as fillers for silicone rubber used in thermal interface applications

NEW CARBON MATERIALS Volume 34, Issue 2, Apr 2019 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2019, 34(2): 188-195

Phase

change

materials

RESEARCH PAPER

coated

with

modified

graphene-oxide as fillers for silicone rubber used in thermal interface applications Jing Feng1,2, Zhan-jun Liu1, Dong-qing Zhang1,, Zhao He1,2, Ze-chao Tao1, Quan-gui Guo1 1

CAS Key Laboratory of Carbon Materials，Institute of Coal Chemistry，Chinese Academy of Sciences，Taiyuan 030001，China;

2

University of Chinese Academy of Sciences，Beijing 100049，China

Abstract:

Graphene oxide prepared by the Hummers method was chemically modified by 3-aminopropyltriethoxysilane and spherical

paraffin@modified graphene-oxide particles (P@m-GO) with a core-shell structure were obtained by an emulsion method. P@m-GO filler/silicone rubber (SIR) matrix composites (P@m-GO/SIR) were prepared by dispersing different amounts of P@m-GO in the SIR precursors, followed by curing and used as thermal interface materials (TIMs). Results indicate that the best TIM had a P@m-GO loading of 60 wt.% and had both a high thermal conductivity (1.248 W·m-1·K-1) and a high latent heat (88.7 J·g-1). Its compression elastic modulus (1.01 MPa) was only one-eighth of that of the pristine SIR (8.16 MPa) due to the plasticity of the paraffin. The paraffin leakage under pressure was low (below 3.98 wt.%) before and after thermal cycling 50 times. These favorable thermal and mechanical properties together with the good cycling stability make it a promising TIM for electronic devices. Key Words:

Thermal interface materials; Phase change fillers; Graphene-oxide; Thermal transfer; Flexibility

1 Introduction Thermal management becomes a critical problem in electronic devices with the rapid development of microelectronic package [1]. When transferring from the device to heat sink, the heat flow would be impeded by the solid-solid interface. Therefore, the solid connection plays a vital role in thermal management. However, the punctate contact between the solids could bring a high interface thermal resistance to the integrated circuit, lead to overheating of the electron components [2, 3]. Thermal interface materials (TIMs) are the materials that are used to reduce the thermal contact resistance between electronic devices [4-6]. Silicone rubber (SIR) is widely used as a material of TIMs for its good flexibility, convenient use and excellent electrical insulation [7,8] . Nevertheless, SIR cannot meet the requirement of the interfacial thermal transfer due to the low thermal conductivity. In the light of efficient heat transfer, current TIMs with SIR matrix are usually filled by thermally conducting fillers or phase change materials (PCMs) to get a higher thermal conductivity or melting latent heat, respectively. Thermally conducting fillers with a high thermal conductivity, such as copper nanowires, functionalized carbon nanotubes, highly

oriented hexagonal boron nitride nanosheets and graphite nanoplatelets are regularly used to improve the thermal conductivity of SIR, which is generally in the range of 1- 4 W·m-1·K-1 [9-11]. In these work, adequate filler content must be achieved to get a high thermal conductivity. However, this causes the poor flexibility and inefficient insert of TIMs. At the same time, PCMs are also used in commercial thermal interface practice by virtue of their huge latent heat during solid-liquid phase change. As a significant component of organic PCMs, paraffin is extensively applied in the form of microcapsule. In traditional research, the melting latent heat of microencapsulated paraffin phase change fillers is on the order of 64-101 J·g-1 with a polymer coating layer, such as difunctional olefin block copolymer and [12, 13] polymethylmethacrylate . Due to the heat insulation characteristic of the polymer shell, the melting latent heat of the paraffin microcapsule cannot meet the requirement of commercial applications. Hence, there is a strong demand to improve thermal conductivity, melting latent and flexibility of TIMs simultaneously in practical applications. Graphene oxide (GO) could be selected as a proper paraffin coating material to improve the thermal conductivity of phase change fillers owing to the relatively high thermal conductivity, thin laminar structure and activative surface functional groups. Many researches have been devoted to the TIMs based on GO

Received date: 01 Jan 2019; Revised date: 20 Mar 2019 *Corresponding author. E-mail: [email protected] Copyright©2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(19)60011-9

Jing Feng et al. / New Carbon Materials, 2019, 34(2): 188-195

and paraffin, but industrial use has not been realized due to their inadequate thermal conductivity or latent heat [14-17]. Yuan et al. [16] produced a paraffin@SiO2/GO composite via in-situ hydrolysis and polycondensation. They reported that the composite had a thermal conductivity of 1.162 W·m-1·K-1 and melting latent heat of 81.6 J·g-1. Mehrali and his coworkers prepared PCMs composites of paraffin within GO sheets by a vacuum impregnation method. The thermal conductivity and melting latent heat of the composites could only reach 0.985 W·m-1·K-1 and 63.8 J·g-1, respectively [17].

A GO aqueous solution was prepared according to the modified Hummers method[18]. The diameter of GO particles in the solution ranges from 1 to 5 μm. The paraffin was purchased from Shanghai Joule Wax Co., Ltd. Its melting point and melting latent heat is 46.6 oC and 225.3 J·g-1, respectively. 3-Aminopropyltriethoxysilane (3-APTS) and hydroxypropyl cellulose (HPC) were obtained from Aladdin Industrial Corporation. Sylgard 184 Silicone Elastomer was bought from Dow Corning and used as a matrix material in P@m-GO/SIR composites.

In this research, the core-shell paraffin@modified GO (P@m-GO) fillers were dispersed into SIR matrix to obtain P@m-GO/SIR composites as TIMs. With the introduction of m-GO shell, P@m-GO fillers are prone to be spherical and provide the P@m-GO/SIR composites with favorable mechanical properties. On the other hand, P@m-GO could increase the thermal conductivity and melting latent heat of the composite. The obtained TIMs have higher thermal conductivity, larger latent heat and better flexibility compared with previous TIMs based on SIR alone, which are promising TIMs for industrial applications.

2.2

2

Experimental

2.1

Materials

Preparation of P@m-GO/SIR composites

HPC and paraffin at a volume ratio of 4:6 were dispersed in deionized water, followed by heating at 60 °C for 1 h under magnetic stirring. Different volumes (10, 15, 20 and 25 mL) of the GO solution (10 mg·mL-1) were added into different volumes of 3-APTS (1, 1.5, 2.0 and 2.5 mL), respectively, diluted with absolute ethyl alcohol in argon atmosphere at 60 °C and stirred for 2 h to obtain the m-GO solutions. Then the m-GO solutions were added into the as-received paraffin mixture under a constant argon flow, kept at 60 °C for another 5 h and then put in a vacuum oven at 35 °C for 12 h to remove the solvent. The resulting P@m-GO fillers were named as P@m-GO-5, P@m-GO-10, P@m-GO-15 and P@m-GO-20, respectively. Fig. 1a shows the schematic illustration of the preparation process of the P@m-GO fillers by the emulsion method.

Fig. 1 Preparation process of (a) P@m-GO fillers and (b) P@m-GO/SIR TIMs.

Jing Feng et al. / New Carbon Materials, 2019, 34(2): 188-195

2.3

Fabrication of P@m-GO/SIR TIMs

The P@m-GO/SIR TIMs were prepared by the conventional mechanical mixing method. The two liquid parts for preparing SIR could be cured to flexible SIR by thoroughly mixing them at the volume ratio of 10:1. P@m-GO-15 was added into the above SIR mixture with weight loadings of 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.% and 60 wt.% before curing, corresponding to sample names, P@m-GO/SIR-2, P@m-GO/SIR-3, P@m-GO/SIR-4, P@m-GO/SIR-5 and P@m-GO/SIR-6, respectively. Adding P@m-GO-15 above 60 wt.% would inhibit the curing of SIR. The P@m-GO/SIR samples were shaped in a mold (50 × 50 × 2 mm3) and cured for 48 h at room temperature to obtain the P@m-GO/SIR TIMs. Fabrication process of the P@m-GO/SIR TIMs is displayed in Fig. 1b. 2.4 Characterization The P@m-GO/SIR TIMs with different P@m-GO-15 loadings were cut into the dimension of 10 mm × 10 mm × 2 mm and placed into a Laser flash thermal analyzer (NETSCH LF447) to measure the thermal diffusivity in an argon flow of 10 mL·min-1. The phase transition temperature and melting latent heat value of the P@m-GO fillers and P@m-GO/SIR TIMs were investigated by a differential scanning calorimeter (DSC, NETSCH 200 F3). Each piece of the samples was inserted into the aluminum crucible in argon atmosphere at a flow of 10 mL·min-1. The dynamic test was carried out from 30 to 65 oC at a heating rate of 5 oC·min-1. The specific heat of the P@m-GO/SIR TIMs was also tested by DSC. The paraffin leakage of the P@m-GO/SIR TIMs was directly expressed by a weight loss ratio. The samples were put into a vacuum oven at atmospheric pressure for 24 h at 60 oC, which is higher than the melting point of paraffin. Compression test of the P@m-GO/SIR samples (10mm × 10mm × 2 mm) were conducted by a universal testing machine (CMT 4304) at a

strain rate of 0.5 mm·min-1 according to the Chinese standard GB/T 1431-2009. The compressive elastic modulus of the samples was calculated with the average slope of the stress-strain curves between 0 and 30 % strains. The modification mechanism of P@m-GO fillers was measured by the infrared spectroscopy (FTIR, VERTEX 70). The samples were pressed into KBr pellets and scanned in the range of 1800 to 600 cm-1. The adhesion of the m-GO in the P@m-GO fillers was certified by Raman spectroscopy (Raman, Horiba Labram 880). Field emission scanning electron microscopy (FESEM ， JSM 7001F) was used to observe the morphology and microstructure of P@m-GO/SIR TIMs.

3

Results and discussion

3.1 Modification mechanism of P@m-GO fillers Functional groups of the materials were characterized by FT-IR, as shown in Fig. 2a. In pristine GO spectra, there are two peaks at 1 263 cm-1 and 1 735 cm-1, which are ascribed to νC–O and νCOOH. After modification, the bands of CO and COOH disappeared while the characteristic peak of NH appeared at 1 563 cm-1. These changes demonstrate that the amide reaction occurs between COOH of GO and NH2 of 3-APTS [19]. At the same time, Si(OC2H5)3 of 3-APTS also converts to -Si(OH)3 with the addition of GO solution due to the hydrolysis reaction. Due to the unstable characteristics of -Si(OH)3 at high temperatures, a part of them would condense with hydrolyzed 3-APTS and the other parts would dehydrate with -OH group of HPC. The reaction is associated with the peaks located at Si-O-Si of 883 cm-1 and SiOC of 1 022 cm-1. Briefly, 3-APTS is not only used to reduce the hydrophilic of GO, but also improve the connection among paraffin, HPC and GO.

Fig. 2 (a)FT-IR spectra of GO, HPC, paraffin and P@m-GO-15. (b) Raman spectra of P@m-GO-15 and para-HPC.

For comparison, a composite of paraffin and HPC (para-HPC) was prepared similar to the preparation of P@m-GO-15 but without the addition of m-GO. And Figure 2b exhibits the Raman spectra of para-HPC and P@m-GO. No obvious carbon peaks could be seen in the spectra of para-HPC. Comparatively, two characteristic peaks at 1 355

cm-1 (D band) and 1 593 cm-1 (G band) appear in the curve of P@m-GO-15, suggesting that m-GO is successfully introduced into P@m-GO-15. The D peak of the P@m-GO-15 curve corresponds to sp3 defective carbon peak of m-GO, while the G peak is caused by the vibration of C=C bond. 3.2

Morphology of P@m-GO fillers

Jing Feng et al. / New Carbon Materials, 2019, 34(2): 188-195

In order to explore the influence of m-GO content on the amount of superficially coated paraffin, different contents of m-GO were added to prepare the P@m-GO fillers. Fig. 3 shows the surface structure of paraffin, m-GO and as-received composites (P@m-GO-5, P@m-GO-10, P@m-GO-15 and P@m-GO-20). It can be seen that paraffin is amorphous material with a large and thick layer while m-GO presents a small and curly laminar structure. Because of the different morphologies of paraffin and m-GO, the m-GO adhesion could be seen intuitively from the SEM photos. From Fig. 3c-f, it can be easily observed that the amount of m-GO adhesion increases gradually with the volume of the m-GO suspension.

Fig. 3

Without sufficient m-GO addition, there would be incomplete coating and a large leakage of molten paraffin below a volume of the m-GO suspension. Adding too much m-GO would give rise to uneven thermal transmission. P@m-GO-15 possesses a proper loading among these samples. Fig. 3g is the picture of incomplete coating of the m-GO. From this picture, the morphological difference between the m-GO shell and paraffin core could be distinguished visually. Fig. 3h shows the low magnification pictures of P@m-GO-15 from which the diameters of spherical P@m-GO-15 fillers could be seen around 50 μm. P@m-GO-15 is chosen to fill SIR for further characterizations.

SEM images of (a) paraffin, (b) m-GO, (c) P@m-GO-5, (d) P@m-GO-10, (e)P@m-GO-15 and (h)P@m-GO-20.

Jing Feng et al. / New Carbon Materials, 2019, 34(2): 188-195

3.3

𝜆 = а 𝜌 𝐶𝑝

Thermal properties of P@m-GO/SIR TIMs

P@m-GO/SIR samples with different P@m-GO-15 loadings (0 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.%, and 60 wt.%) were prepared. Table 1 displays the physical properties of these samples. It can be seen that P@m-GO-15 presents a high thermal conductivity of 1.662 W·m-1·K-1. The thermal diffusivity, specific heat and thermal conductivity of P@m-GO/SIR TIMs increase with the P@m-GO-15 loading. The increase of thermal diffusivity is ascribed to the thermal transport networks brought by the m-GO coating with a considerable thermal conductivity. And the presence of paraffin wax gives rise to an increase of the specific heat according to the equation Table 1

where λ, ρ, а and Cp correspond to the thermal conductivity, bulk density, thermal diffusivity and specific heat of P@m-GO/SIR, respectively. The thermal conductivity of P@m-GO/SIR TIMs also increases even though the bulk density decreases with increasing P@m-GO-15 loading. Besides, P@m-GO-15 filler with different diameters increases the maximum P@m-GO-15 content for P@m-GO/SIR TIMs [20] . When 60 wt.% P@m-GO-15 was added, the thermal conductivity of the P@m-GO/SIR TIM reaches 1.248 W·m-1·K-1, more than eleven-fold increase as compared with SIR.

Thermal properties of P@m-GO/SIR TIMs.

P@m-GO-15 weight content %

ρ /g·cm-3

а /mm2·s-1

Cp /J·g-1·K-1

λ /W·m-1·K-1

0

1.030

0.082

1.277

0.108

20

0.982

0.174

1.691

0.288

30

0.971

0.204

1.835

0.364

40

0.958

0.262

1.919

0.482

50

0.946

0.470

2.070

0.920

60

0.935

0.612

2.181

1.248

100

0.875

0.733

2.592

1.662

A key property for PCM is its heat storage capability. The DSC curves of P@m-GO-15, P@m-GO/SIR-2, P@m-GO/SIR-3, P@m-GO/SIR-4, P@m-GO/SIR-5 and P@m-GO/SIR-6 are shown in Fig. 4a. It can be concluded from the image that the peak value of these samples ranges from 45 to 48 oC and the enthalpy of P@m-GO/SIR TIMs increases with the P@m-GO-15 content. This increase is attributed to the huge latent heat brought by paraffin. The experimental latent heat of P@m-GO-15 could reach 149.5 J·g-1 while the theoretical value reaches 156.2 J·g-1 which is estimated according to the paraffin mass percentage in P@m-GO-15. Similarly, the measured and theoretical melting latent heat values of P@m-GO/SIR-6 are 88.7 and 93.7 J·g-1, respectively. The addition of P@m-GO-15 improves apparently heat storage ability for P@m-GO/SIR TIMs. When paraffin is used as a phase change filler, its leakage would pollute the electronic component because of its low melting temperature. Therefore, the leakage of SIR, P@m-GO/SIR-2, P@m-GO/SIR-3, P@m-GO/SIR-4, P@m-GO/SIR-5 and P@m-GO/SIR-6 were measured by putting them into a vacuum oven at 60 oC for 24 h. The weight loss ratio of TIMs was calculated by 𝜂=

(1)

𝑀1 −𝑀2 𝑀1

× 100%

(2)

where η is the weight loss percentage of P@m-GO/SIR TIMs, and M1, M2 are the weights of P@m-GO/SIR before and after the heating process. Fig. 4b shows the weight loss percentages of P@m-GO/SIR TIMs with different

P@m-GO-15 contents. In order to explore the thermal cycling stability of P@m-GO/SIR TIMs, the weight loss percentage of the P@m-GO/SIR TIMs after 50 heat cycles is also provided. The experimental result indicates that the weight loss of these samples positively correlates with the filling amount of P@m-GO-15. Most of important, the weight loss percentages of all samples are at a low level, even the leakage of P@m-GO/SIR-6 is only 3.07%. After 50 heat cycles, the weight loss percentage of the P@m-GO/SIR is still in a low level. It verifies that the m-GO coating could effectively prevent molten paraffin from leaking. 3.4

Mechanical properties of P@m-GO/SIR TIMs

As we all known, TIMs are used to fill the gap between electronic devices and heat sink to decrease the interfacial thermal resistance. Low elastic modulus of compression could improve the flexibility of TIMs and benefit to interfacial filling efficiency. Compressive strength of P@m-GO/SIR-2, P@m-GO/SIR-3, P@m-GO/SIR-4, P@m-GO/SIR-5 and P@m-GO/SIR-6 are shown in Fig. 5. For contrast, the stress-strain curve of pristine SIR was also provided. It can be seen from the curves that the compression elastic modulus of P@m-GO/SIR TIMs is in inverse proportion to the addition amount of P@m-GO-15. The value of P@m-GO/SIR-6 could be diminished to 1.01 MPa, which is significantly lower than the value (8.16 MPa) for pristine SIR. To some extent, this abnormal phenomenon is ascribed to the fact that paraffin acts as a plasticizer, which could reduce the intermolecular forces of SIR. On the other hand, a simulated interaction brought by

Jing Feng et al. / New Carbon Materials, 2019, 34(2): 188-195

the global structure of P@m-GO-15 between phase change fillers could also make it easier for silicone molecule chain to move and bring down the compression elastic modulus of the P@m-GO/SIR TIMs. In short, the compression elastic

modulus of P@m-GO/SIR TIMs decreases significantly with the content of P@m-GO-15. The insert picture in Fig. 5a howed the macrostructure of P@m-GO/SIR-6 that could be folded easily.

Fig. 4 (a) DSC curves of P@m-GO-15 and P@m-GO/SIR TIMs and (b) weight loss percentages of P@m-GO/SIR TIMs.

Fig. 5 (a) Compressive strength of SIR and P@m-GO/SIR TIMs, (b) A schematic illustration of the mechanical test.

4

Conclusions

Dual-enhanced phase change fillers P@m-GO were successfully loaded into SIR matrix to prepare the P@m-GO/SIR TIMs. The thermal conductivity and melting latent of P@m-GO-15 could reach 1.662 W·m-1·K-1 and 149.5 J·g-1, respectively. With the addition of P@m-GO-15, the thermal properties and flexibility of P@m-GO/SIR TIMs are significantly improved. With a 60 wt.% loading of P@m-GO-15, thermal conductivity and melting latent heat of the P@m-GO/SIR-6 could be increased to 1.248 W·m-1·K-1 and 88.7 J·g-1, respectively. Moreover, the compressive elastic modulus of the P@m-GO/SIR-6 is reduced to 1.01 MPa, nearly a factor of one-eighth as compared with the pure SIR. In general, the high thermal conducting efficiency and good flexibility facilitate P@m-GO/SIR TIMs to be promising materials in thermal interface applications.

Acknowledgements We acknowledge the financial supports from the Youth Innovation Promotion Association CAS (Grant No. 2017205) and National Nature Science Foundation of China (Grant No. 51303198).

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