Green preparation of boron nitride nanosheets and their application in thermal conductivity composites
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摘要: 聚偏氟乙烯(PVDF)等聚合物因具有较低的热导率限制了其使用范围，添加高导热填料可以提升聚合物材料的导热性能，所制备的聚合物基导热复合材料在热管理领域具有重要的应用价值。本文采用六方氮化硼纳米片(BNNS)和球形氧化铝(Al2O3)作为导热填料，通过热压的方法制备出Al2O3-BNNS/PVDF导热复合材料。首先，在氯化胆碱(ChCl)与植酸(PA)水溶液组成的绿色溶剂中，高效剥离制备得到厚度3~5 nm、直径1~5 μm的BNNS纳米填料。再利用BNNS、Al2O3杂化填料的协同作用，采用溶液共混-热压的方式制得具有类似豌豆荚结构的导热复合材料，构建出良好的导热网络。当添加30wt%Al2O3与20wt%BNNS时，复合材料面内热导率高达11.54 W/(m·K)，垂直热导率为5.70 W/(m·K)，复合材料的热导率大幅提升，用作热界面材料表现出优异的散热性能。Abstract: Polymers such as polyvinylidene fluoride (PVDF) are limited by their low thermal conductivity, and it is important to enhance the thermal conductivity of polymer-based composites by adding thermally conductive fillers. In this paper, Al2O3-BNNS/PVDF composites with enhanced thermal conductivity were prepared by hot-compaction process using hexagonal boron nitride nanosheets (BNNS) and spherical alumina (Al2O3) as thermally conductive fillers. Firstly, BNNS nanofillers with thickness of 3-5 nm and diameter of 1-5 μm were prepared by exfoliation in green solvents consisting of choline chloride (ChCl) and aqueous phytic acid (PA). Then, based on the synergistic effect of BNNS and Al2O3 hybrid fillers, the thermally conductive composites with a pea pod-like structure were fabricated by solution blending-hot pressing, and a good three-dimensional heat conduction network was constructed. When 30wt%Al2O3 and 20wt%BNNS were added, the in-plane thermal conductivity of the composite was as high as 11.54 W/(m·K) and the vertical thermal conductivity was 5.70 W/(m·K). The thermal conductivity of the composite was greatly improved, showing excellent thermal performance.
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Key words:
- green solvent /
- h-BN /
- exfoliation /
- polyvinylidene fluoride /
- thermal conductivity
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图 1 Al2O3-六方氮化硼纳米片(BNNS)/聚偏氟乙烯(PVDF)复合材料制备示意图
DMF—N, N-dimethylformamide
Figure 1. Schematic diagram of the preparation of Al2O3-hexagonal boron nitride nanosheets (BNNS)/polyvinylidene fluoride (PVDF) composite
图 2 不同氯化胆碱(ChCl)-植酸(PA)摩尔比时h-BN剥离效果：((a)、(b)) BNNS分散液放置3天前后的数码照片；(c) BNNS产率；(d) 不同ChCl-PA摩尔比时溶剂体系的表面张力
Figure 2. Exfoliation of h-BN with different choline chloride (ChCl)-phytic acid (PA) molar ratio: ((a), (b)) Digital photographs of BNNS dispersion before and after standing for 3 days; (c) Yields of BNNS; (d) Surface tension of the solvents with different ChCl-PA molar ratio
图 3 h-BN (a)与BNNS (b)的SEM图像；(c) BNNS的TEM图像；BNNS的AFM图像(d)及相应的厚度曲线(e)；(f) h-BN与BNNS的XRD图谱
H—Height; X—Distance
Figure 3. SEM images of h-BN (a) and BNNS (b); (c) TEM image of BNNS; AFM image (d) and corresponding height profile (e) of the BNNS; (f) XRD patterns of h-BN and BNNS
图 4 h-BN及BNNS的XPS图谱：(a) XPS全谱；(b) B1s图谱
Figure 4. XPS spectra of h-BN and BNNS: (a) Full spectra; (b) B1s
图 5 PVDF基复合材料脆断面的SEM图像：(a) PVDF；(b) Al2O3；(c) Al2O3/PVDF；(d) Al2O3-BNNS5/PVDF；(e) Al2O3-BNNS10/PVDF；(f) Al2O3-BNNS15/PVDF；(g) Al2O3-BNNS20/PVDF；(h) BNNS20/PVDF
Figure 5. SEM images for the fracture surfaces of PVDF based composites: (a) PVDF; (b) Al2O3; (c) Al2O3/PVDF; (d) Al2O3-BNNS5/PVDF; (e) Al2O3-BNNS10/PVDF; (f) Al2O3-BNNS15/PVDF; (g) Al2O3-BNNS20/PVDF; (h) BNNS20/PVDF
图 6 Al2O3-BNNS/PVDF复合材料的面内(a)和垂直方向(b)热导率
Figure 6. Thermal conductivity of Al2O3-BNNS/PVDF composites in-plane (a) and out-of-plane (b)
图 7 Al2O3-BNNS/PVDF复合材料红外热成像分析：(a) 红外热成像图像；(b) LED灯表面温度随加热时间的变化；(c)自制散热装置图
A-G are PVDF, Al2O3/PVDF, Al2O3-BNNS5/PVDF, Al2O3-BNNS10/PVDF, Al2O3-BNNS15/PVDF, Al2O3-BNNS20/PVDF and BNNS20/PVDF, respectively
Figure 7. Infrared thermal imaging of Al2O3-BNNS/PVDF composites: (a) Infrared thermal images; (b) Surface temperature variation with heating time of LED; (c) Illustration of self-made heat dissipation device
表 1 Al2O3-BNNS/PVDF复合材料物料配比
Table 1. Formulation of Al2O3-BNNS/PVDF composites
Sample Al2O3/wt% BNNS/wt% PVDF/wt% Al2O3/PVDF 30 0 70 Al2O3-BNNS5/PVDF 30 5 65 Al2O3-BNNS10/PVDF 30 10 60 Al2O3-BNNS15/PVDF 30 15 55 Al2O3-BNNS20/PVDF 30 20 50 BNNS20/PVDF 0 20 80 
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表 2 h-BN与BNNS的XPS元素含量分析
Table 2. Analysis of the XPS element content about h-BN and BNNS
C/at% N/at% B/at% O/at% BN 14.34 30.97 52.52 2.16 BNNS 45.40 19.44 29.09 6.07
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表 3 Al2O3-BNNS/PVDF复合材料比热、密度及热扩散系数
Table 3. Heat capacity, density and thermal diffusivity of Al2O3-BNNS/PVDF composites
Sample Cp/(J·g−1·K−1) ρ/(g·cm−3) In-plane α/(mm2·s−1) Out-of-plane α/(mm2·s−1) PVDF 1.342 1.800 0.091 0.091 Al2O3/PVDF 1.099 2.107 2.639 0.925 Al2O3-BNNS5/PVDF 1.060 2.128 3.054 1.289 Al2O3-BNNS10/PVDF 1.013 2.151 4.075 1.387 Al2O3-BNNS15/PVDF 0.978 2.174 4.758 2.424 Al2O3-BNNS20/PVDF 1.035 2.197 5.073 2.507 BNNS20/PVDF 0.990 1.869 3.920 0.194 Note: Cp, ρ and α are the heat capacity, density and thermal diffusivity of the composites, respectively.
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表 4 文献中BN填充PVDF基导热复合材料热导率比较
Table 4. Comparison for the thermal conductivity of BN-filled PVDF based composites in literature
Materials BN Loading Other filler loading In-plane TC/(W·m−1·K−1) Ref. BN-f-SiC/PVDF 20wt%BN 26wt%f-SiC 1.41 [25] BNNS/PVDF 4wt%BNNS — 4.69 [26] GNP-BN/PVDF 30wt%BN 2.5wt%GNP 0.72 [27] BN-CNT/PVDF 30wt%BN 2.5wt%CNT 2.18 [28] Al2O3-BNNS/PVDF 20wt%BNNS 30wt%Al2O3 11.54 This work Notes: TC—Thermal conductivity; f-SiC—Surface functionalized silicon carbide; GNP—Graphite nanoplatelets; CNT—Carbon nanotube.
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