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Supporting Information
Introduction
Table S1. The summary of in-plane thermally conductive paper-like composites.
Sample
Fabrication method
Thermal
Conductivity
(W m-1 K-1)
Measurement method
Year [Ref]
Nanofibrillated cellulose(NFC)/BN
Vacuum filtration
145.7 (50 wt%)
Steady-state method
2014[1]
NFC/RGO
LBL assembly
12.6(--)
Laser flash technique
2017[2]
PVA/BN@PDA
Vacuum filtration
5.4 (10 vol% )
Laser flash technique
2015[3]
PET/RGO
Graphene ink coating method
90 (--)
Optothermal raman technique
2014 [4]
CNF /BN nanotube
Vacuum filtration
21.39(25 wt%)
Laser flash technique
2017[5]
Cellulose/ RGO
Vacuum filtration
9(6 %)
Laser flash technique
2017[6]
Poly (vinylidene fluoride-co-hexafluoropropylene) / RGO
Solvent casting
19.5(27.2 wt%)
Laser flash technique
2016 [7]
Polyimides (PI)/BN
Spin casting
17.5, 5.4(60 vol%)
Temperature wave analysis (TWA) method, Laser flash technique
2013 [8]
SiC nanowire/cellulose microcrystal
Vacuum filtration
34(50 vol%)
Laser flash technique
2016 [9]
PDDA/BN nanosheets
Vacuum filtration
212.8(90 wt%)
Steady-state method
2017 [10]
RGO paper
Solvent casting
61
Laser flash technique
2015 [11]
RGO paper
Solvent casting
1100
Laser flash technique
2014 [12]
RGO paper
Vacuum filtration
1043.5
Laser flash technique
2014[13]
RGO paper
Electro-spray deposition
1238.3
Self-heating method
2014 [14]
Graphitic paper
Polyimide graphitization
1750
Laser flash technique
2014 [12]
RGO paper
Vacuum filtration
1529
Self-heating method
2017 [15]
RGO paper
Vacuum filtration
1940
Laser flash technique
2017 [16]
RGO paper
Vacuum filtration
1390
Laser flash technique
2015 [17]
RGO/carbon fiber paper
Vacuum filtration
977
Laser flash technique
2014 [18]
Fig. S1. Schematic representation of the preparation process of NR/BN and NR/GNPs composites.
Experimental section
Materials. Graphene nanoplatelets (cz-030, Xiamen Knano Graphene Technology, Co., Ltd, China) with the particle size of 20-30 um and the density of 2.25 g/cm3, BN flakes (Qinghuangdao Yinuo Advanced Material Co., Ltd, China) with the particle size of 10-15 um and the density of 2.25 g/cm3 and NR lateral (Yiwu Taiyou Hu Trade Co., Ltd., China) with the solid content of 61.79 % were used. Polyvinyl Alcohol (PVA1788) as dispersant was purchased from Chengdu Kelong Chemical reagent Co., Ltd. The curing agents including sulfur, zinc diethyl dithiocarbamate (ZDC), Zink oxide (ZnO) were commercially available industrial products. Commercial natural graphite paper (NG paper) with the thickness of 100 um and of 288 W m-1 K-1 was purchased from Beijing Jinglong Tetan Co Ltd. Cu foil with the thickness of 76 um and of 380 W m-1 K-1 was purchased from Shanghai Huhong Metal Material Co Ltd. The materials were used as received without any purification and chemical treatment. The SEM images of fillers and optical images of the comparison samples were shown in the supporting information Fig. S2.
Preparation of NR/BN and NR/GNPs composites
NR/GNPs and NR/BN composites were fabricated by simple vacuum filtration method. As shown in the supporting information Fig. S1, a certain amount of BN or GNPs and PVA (BN or GNPs: PVA=100: 5, w/w) were mixed in the deionized water via ultrasonic bath treatment for 30 min, followed by adding the NR latex. Then, a certain amount of curing agents including sulfur (2 phr), ZDC (1 phr) and ZnO (0.5 phr) was mixed into the suspension. The suspension was stirred for 12 h at room temperature. To form composite films, the mixture was vacuum filtrated with filter membrane (220 nm) and then allowed for water evaporation at room temperature for 12 h. Lastly, the resulted films was heat compressed for 6 h at 60 °C and 10 MPa. Mechanical compression improved the contact of the thermally conductive fillers and chased away the air bubbles. The composites were designated according to the volume content of thermally conductive fillers.
Characterization. The as-obtained composite films were characterized by scanning electron microscope (SEM, JOEL JSM-5900LV, Japan), X-ray diffraction (XRD, Cu Kα radiation, Rigaku, Ultima IV, Japan), insulation resistance tester (U2683, Eucol Electronic Technology Co., Ltd, China) and universal material testing machine (Instron 5967, USA). The k of the composites was tested at 25 °C with the laser flash technique (Netzsch, Pyroceram 9606 as reference sample, LFA 467, USA). The k was calculated by , where λ, α, ρ and Cp represent the k, thermal diffusivity, density and specific heat capacity of the composites, respectively. The EMI shielding performance of the composites in the X-band (8.2-12.4 GHz) was characterized by a vector network analyzer (Agilent N5230, USA). The hardness and modulus of the composite films were tested by a nano-indentation tester (Keysight G200, USA) equipped with Berkovich indenter. Tensile properties of the composites were tested at 25° C by a universal material testing machine (Instron 5967, USA) with a cross-heat speed of 0.4 mm/min.
Results and Discussions
Fig. S2. SEM images of (a) pure NR, (b) pure GNPs and (c) BN, optical images of (d) commercial NG paper and (e) copper foil.
The orientation of fillers with hexagonal crystal system such as BN can be characterized by XRD, due to that the diffraction patterns obtained from the top and side planes of hexagonal crystal are different. We found that the orientation of GNPs in polymer matrix can also been characterized by XRD patterns as shown in the Fig. S3. The XRD pattern obtained from cross section and surface of 27.48GNPs composites are quite different. The XRD pattern obtained from surface show sharp (002) and (004) diffraction peaks, while the XRD pattern obtained from cross section show sharp (002), (100), (101), (110) and (112) diffraction peaks.
Fig. S3. XRD patterns of pure GNPs filler and 27.48GNPs composites.
Fig. S4. (a) Thermal conductivity of 27.48GNPs and 55.85BN composites as function of temperature, (b) thermal diffusivity and specific heat of 27.48GNPs and 55.85BN composites as function of temperature.
Fig. S4a shows the and of 27.48GNPs and 55.85BN composites as function of temperature. The thermal conductivity of 55.85BN decreased slightly with increasing temperature, consistent with the anhar-monic phonon–phonon scattering that is dominant in highly crystalline materials (the content of BN is 55.85 vol%).[16] However, the thermal conductivity of 27.48GNPs increases for a certain degree with increasing temperature. In non-crystalline materials the change of thermal conductivity is also dependent on the change of specific heat. One can see that the specific heat of 27.48GNPs rapidly increases with increasing temperature.
Fig. S5. Volume resistivity of NR/BN composites with different content of BN.
As presented in Fig. S5, the volume resistivity obtained from NR/BN composites was in the range of 3.081014-3.631015, which is far beyond the electrical insulation range (), which guarantees the application of such TIMs in some special electrical device fields.
Table. S2 Comparison of EMI shielding effectiveness of composites in the X-band frequency range.
Sample
Density(g cm-3)
Thickness(mm)
SE(dB)
SSE(dB cm2 g-1)
[Ref]
Cellulose/CNTs
1.35
0.15
35
~1372.4
[19]
PEEK/GPPS/MWCNT
1.34
0.18
10.5
~435.3
[20]
PLLA/MWCNT
>1.25
1.5
30
<160
[21]
Epoxy/CNTs
>1.2
2
33
<137.5
[22]
PS/ RGO
>1.04
2.5
45.1
<173.5
[23]
PEDOT:PSS/Graphene
0.96
0.8
70
~911.5
[24]
Graphene/Fe3O4 film
0.78
0.3
24
~1025.6
[25]
Copper film
8.9
0.013
115
~9939.9
[26]
RGO film
1.63
0.0084
20
~14607.1
[12]
RGO film
1.63
0.031
130
~25727.3
[26]
NR/GNPs
1.14
0.071
33.96
~4195.7
This work
Comparison of EMI shielding effectiveness of composites in the X-band frequency range was made in Table S2. Clearly, our NR/GNPs composites show the highest value of SSE in the polymer composites. Noted that RGO film possess the highest value of SSE in all the composites (25727.3 dB cm2 g-1).[26]
Fig. S6. The temperature difference ∆ at 240 s which was be used to calculate the contact thermal resistance.
Fig. S7. Schematic illustration of contact thermal resistance of TIMs application.
Fig. S7 presented the schematic illustration of contact thermal resistance of TIMs application. Contact resistance is composed of two parallel contact resistances: (1) conduction resistance at the contact points and (2) conduction resistance across the air gap of non-contacting area.
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