Green Preparation of Boron Nitride Nanosheets and Their Application in Thermal Conductivity Composites
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摘要:
目的 聚合物基导热复合材料优异的可加工性、耐腐蚀性和电绝缘性使其在热管理领域广泛应用。然而，聚合物材料导热系数低，通常需添加大量的导热填料提高其导热性能，易导致聚合物自身的流动性和力学强度等性能劣变。通过构建有效的导热网络能够在减少填料用量的同时提升材料导热性能。六方氮化硼(h-BN)是一种常用的导热填料，将h-BN剥离制备成氮化硼纳米片(BNNS)，不仅可以大幅度提升其热导率，且大的横纵比及比表面积有利于其在聚合物基体中形成热传导通路。但传统制备BNNS的方法主要采用有机溶剂辅助液相剥离，存在剥离效率低、环境污染大等问题。因此，发展BNNS的绿色、高效制备方法，通过构建导热网络，实现BNNS填充的聚合物复合材料在较低填料填充时达到较高的导热系数，具有重要意义。 方法 本文以氯化胆碱(ChCl)与植酸(PA)组成低共熔溶剂，采用液相超声的方法，对六方氮化硼进行插层剥离，通过调节溶剂组成调控BNNS的剥离效果，研究不同溶剂组成时所得BNNS分散液的产率及稳定性。通过SEM、TEM、AFM表征剥离所得BNNS的形貌，利用XRD、XPS分析BNNS的结构和组成。将剥离所得BNNS与AlO共同用作导热填料，采用溶液共混-热压的方法与PVDF进行复合。采用SEM观察复合材料的微观结构；利用激光导热仪测试复合材料热扩散系数，进一步计算复合材料的导热系数；通过自制的散热装置测试复合材料的散热性能，系统研究了材料结构与性能之间的关系。 结果 采用氯化胆碱与植酸形成的低共熔溶剂为绿色溶剂辅助剥离六方氮化硼，当氯化胆碱与植酸摩尔比为4:1时，可制备得到横向尺寸1-5 μm，厚度3-5 nm的BNNS，剥离效率高达47.9 %。XRD测试结果表明，剥离后BNNS具有良好的结晶结构，且其002晶面衍射峰向低角度偏移，且半峰宽变宽，层间距变大，该绿色溶剂体系成功实现了h-BN的插层剥离；XPS结果显示，BNNS表面在B元素周围发生了功能化，功能化的BNNS有助于其在聚合物基体中的良好分散。以制备的高质量BNNS为导热填料，与AlO填料协同，通过溶液共混-热压制备PVDF复合材料。复合材料的断面SEM照片显示，热压作用下，基体内BNNS形成明显的取向结构，BNNS薄片之间由AlO连接，形成类似豌豆荚结构。热导率测试及散热性能结果表明，当添加30 wt% AlO与20 wt% BNNS时，复合材料的面内热导率达到11.54 W/(m·K)，垂直热导率达到5.70 W/(m·K)，作为热界面材料较纯的PVDF散热性能提升约23%。 结论 利用氯化胆碱与植酸形成的低共熔溶剂代替传统有机溶剂，可以实现BNNS的绿色、高效制备，为制备大径厚比的BNNS导热填料提供了一种新方法。利用AlO与BNNS双填料的协同作用，通过构建豌豆荚结构，形成有效的导热网络，通过“导热填料-网络结构”协同提高复合材料的导热性能，制备得到导热性能优异的PVDF基复合材料，作为热界面材料能有效降低电子器件的表面温度。本研究为制备高导热界面材料提供了新颖、简单的途径。 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.-
Key words:
- green solvent /
- h-BN /
- exfoliation /
- polyvinylidene fluoride /
- thermal conductivity
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图 1 Al2O3-BNNS/PVDF复合材料制备示意图
Figure 1. Schematic diagram of the preparation of Al2O3-BNNS/PVDF composite
图 2 不同ChCl-PA摩尔比时h-BN剥离效果: (a)和(b)分别为BNNS分散液放置3天前后的数码照片，(c) 不同氯化胆碱-植酸摩尔比时的剥离效率，(d) 不同氯化胆碱-植酸摩尔比时溶剂体系的表面张力。
Figure 2. The exfoliation of h-BN with different ChCl-PA molar ratio: Digital photographs of BNNS dispersion before (a) and after (b) standing for 3 days; (c) Yields of BNNS and (d) surface tension of the solvents with different ChCl-PA molar ratio.
图 3 h-BN(a)与BNNS(b)的SEM图片; (c) BNNS的TEM图片; (d) BNNS的AFM图片与(e)BNNS厚度曲线; (f)h-BN与BNNS的XRD谱图。
Figure 3. (a) SEM image of h-BN; (b) SEM image of BNNS; (c) TEM image of BNNS; (d) AFM image of BNNS and (e) thickness lines of BNNS; (f) XRD patterns of h-BN and BNNS
图 4 h-BN及BNNS的XPS谱图：(a) XPS全谱 (b) B1 s谱图
Figure 4. XPS patterns of h-BN and BNNS: (a) Full XPS patterns, (b) XPS spectrum of B1 s.
图 5 PVDF基复合材料脆断面的SEM照片：(a) PVDF、(b) Al2O3、(c) Al2O3/PVDF、(d) Al2O3-BNNS5/PVDF、(e) Al2O3-BNNS10/PVDF、(f) Al2O3-BNNS15/PVDF、(g) Al2O3-BNNS20/PVDF以及(h) BNNS20/PVDF。
Figure 5. SEM images for the fracture surfaces of Al2O3-BNNS/PVDF composites: (a) PVDF, (b) Al2O3,(c) Al2O3/PVDF, (d) Al2O3-BNNS5/PVDF, (e) Al2O3-BNNS10/PVDF, (f) Al2O3-BNNS15/PVDF, (g) Al2O3-BNNS20/PVDF and (h) BNNS20/PVDF.
图 6 Al2O3-BNNS/PVDF复合材料的面内(a)和垂直方向热导率(b)
Figure 6. Thermal conductivity of Al2O3-BNNS/PVDF composites: (a) In-plane and (b) out-of-plane.
图 7 Al2O3-BNNS/PVDF复合材料红外热成像分析
(a) Al2O3-BNNS/PVDF复合材料红外热成像，其中A~G分别为PVDF、Al2O3/PVDF、Al2O3-BNNS5/PVDF、Al2O3-BNNS10/PVDF、Al2O3-BNNS15/PVDF、Al2O3-BNNS20/PVDF以及BNNS20/PVDF。(b)复合材料表面温度随加热时间的变化，(c)自制散热装置图
Figure 7. Infrared thermal imaging analysis of Al2O3-BNNS/PVDFF composites, Surface temperature variation with heating time of Al2O3-BNNS/PVDF composites
表 1 Al2O3-六方氮化硼纳米片(BNNS)/PVDF复合材料物料配比
Table 1. Formulation of Al2O3- hexagonal boron nitride nanosheets (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
Element C1 s N1 s B1 s O1 s At Conc./% 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、密度以及热扩散系数
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-planeα/(mm2·s−1) Out-of-planeα/(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 Notes: Cp, ρ, In-plane α and Out-of-plane α are the heat capacity, density, In-plane thermal diffusivity and Out-of-plane thermal diffusivity of the composites.
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表 4 BN填充PVDF导热复合材料热导率比较
Table 4. Comparison for the thermal conductivity of PVDF based composites in literature
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