模压氮化硼/聚对苯二甲酸乙二醇酯复合材料的导热机制与散热效果

电子电器设备中元器件的高密度集成使得散热问题日益突出,对导热材料的需求不断增长。本文以聚对苯二甲酸乙二醇酯(PET)为基体,六方氮化硼(h-BN)作为导热填料,通过熔融共混法制备h-BN/PET复合材料,考察了h-BN含量和PET基体聚集态结构对复合材料导热性能的影响,分析了复合材料的导热机制,并从材料应用的角度探讨了复合材料导热系数的温度依赖性和散热效果。结果表明,PET基体的结晶度和h-BN含量对复合材料的最终导热系数均有贡献,复合材料的导热系数随结晶度和h-BN含量的增加而提升。h-BN发挥了异相成核作用,显著加快了PET的结晶速度,提高了PET的结晶度。模压成型中h-BN受剪切应力驱使在PET基体中沿流动方向取向,导致复合材料呈现明显的各向异性特征。面内方向h-BN的有序排列为声子提供了更为通畅的传导通道。当h-BN含量为50wt%时,复合材料的面内与面间导热系数分别达到最大值3.00 W·(m·K)^-1和2.19 W·(m·K)^-1。h-BN/PET复合材料具有良好的散热效果,h-BN含量越高,复合材料的冷却速率越快,散热过程中温度...     

Polyaniline nanofiber composites with amines: Novel materials for phosgene detection

Several orders of magnitude of change in resistance are observed upon chemical doping and dedoping of the conducting polymer polyaniline. This large conductivity range can be utilized to make sensitive chemical sensors. Polyaniline, in its nanofiber form, has even greater sensing capabilities due to the small fiber diameters, high surface area, and porous nanofiber network that enhances gas diffusion into the fibers. Polyaniline nanofibers have been synthesized using a rapid mixing method and dispersed in water allowing them to be easily modified with water soluble agents, making new composite materials. Polyaniline nanofiber composite materials can be used to enhance detection of analytes that unmodified polyaniline would not otherwise be able to detect. The detection mechanism involves the reaction of an additive with the analyte to generate a strong acid that is easily detected by polyaniline, resulting in orders of magnitude changes in resistance. The reaction of the additive alone with the analyte produces no electrical response, however. In this paper, an array of amine-polyaniline nanofiber composite materials is investigated for the detection of phosgene gas. The influence of environmental conditions such as humidity and temperature are examined and a detection mechanism is presented.      

Synthesis and thermal analysis of aluminium nitride filled epoxy composites and its effective application as thermal interface material for LED applications

The improvement of heat conduction in any electronic devices has become a predominant issue in which effective heat dissipation is crucial to enhance the performance of packaged devices. This paper elucidates the application of thermally conductive particles filled composites as thermal interface material for LEDs. Present work aims on reducing the junction temperature and thermal resistance of the device under test with heavily filled ceramic-epoxy composite as the interface material between the device and metal substrate. Silane treated aluminium nitride (AlN) powder was studied for its feasibility as the filler material. The thermal conductivity values obtained by hot disc method (ISO/DIS 22007-2.2) were 0.66, 0.54 and 0.44 W/mK for 60, 50 and 40 wt% AlN filled epoxy composites respectively which were described well by thermal transient measurement of LEDs. The junction temperature and total thermal resistance of the thermal set up was reduced significantly with increased filler loading. The least junction to ambient thermal resistance (R_thJ-A) was achieved for 60 wt% followed by 50 and 40 wt% AlN filled TIM with the values of 24.8, 31.98 and 34.64 K/W respectively. Characteristics of the AlN filled composites for LED applications are discussed extensively in terms of thermogravimetric and thermo-mechanical analysis.     

考虑主动散热圆柱多孔夹层材料等效热导率及散热性能

针对多孔夹层圆柱壳的主动散热性能以及等效热导率的计算问题,通过推导多孔结构的等效热导率和主动散热状态下的稳态温度控制方程,在对流换热条件下,计算了考虑主动散热后所折减的等效热导率,及其与结构相对密度和流体流速之间的关系。采用考虑主动散热的等效热导率,计算结构瞬态温度分布,并与有限元结果进行对比。同时,计算了夹层圆柱壳相对密度和结构散热性能的关系以及最大散热性能相对应的最优相对密度和最优杆件厚度。通过瞬态温度分布结果与有限元结果对比,得到采用考虑主动散热的等效热导率所得结果与有限元结果吻合,说明了计算等效热导率方法的有效性,并且可以准确地计算瞬态温度分布;同时分析得到等效热导率随着流体流速的增大而减小,随着结构相对密度的增大而增大;在相同结构质量下,正方形和正六边形构型具有较好的主动散热性能。     

Large-scale synthesis of high-quality hexagonal boron nitride nanosheets for large-area graphene electronics

Hexagonal boron nitride (h-BN) has received a great deal of attention as a substrate material for high-performance graphene electronics because it has an atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. Herein, we report the large-scale synthesis of high-quality h-BN nanosheets in a chemical vapor deposition (CVD) process by controlling the surface morphologies of the copper (Cu) catalysts. It was found that morphology control of the Cu foil is much critical for the formation of the pure h-BN nanosheets as well as the improvement of their crystallinity. For the first time, we demonstrate the performance enhancement of CVD-based graphene devices with large-scale h-BN nanosheets. The mobility of the graphene device on the h-BN nanosheets was increased 3 times compared to that without the h-BN nanosheets. The on-off ratio of the drain current is 2 times higher than that of the graphene device without h-BN. This work suggests that high-quality h-BN nanosheets based on CVD are very promising for high-performance large-area graphene electronics.     

Pressure infiltration of boron nitride preforms with molten aluminum for low density heat sink materials

Cubic boron nitride (cBN) has outstanding mechanical and thermal properties. The previous research focused on mechanical properties, to data, the thermal property of cBN has rarely been reported. In this work, a wide range of aluminum/cubic boron nitride (Al/cBN) composites were fabricated by pressure infiltration at 5.0 GPa and 960–1600 °C. The microstructure, phase composition, thermal conductivity and coefficient of thermal expansion of the Al/cBN composites were investigated. The results showed that a maximum thermal conductivity of 266 W/mK and the coefficient of thermal expansion of 4–6 × 10^?6 K^?1 which matches well to semiconductors, indicating that the Al/cBN composites are promised heat sink materials of high efficiency for the wide band gap semiconductors.     

Novel surfactant-stabilized graphene-polyaniline composite nanofiber for supercapacitor applications

In this work, we present a new synthesis method for surfactant stabilized graphene (SSG) combined with polyaniline nanofiber (PANI-Nf) and apply the composite material as supercapacitor (SC) electrodes by screen-printing technique. Surfactant stabilized graphene polyaniline nanofiber composite (PANI-SSG) was synthesized by electrolytic exfoliation of graphite and subsequent interfacial polymerization. Firstly, graphite was electrolytically exfoliated in an electrolyte containing anionic surfactant. Next, ammonium peroxydisulfate initiator and hydrochloric acid were added to the graphene dispersion to form the aqueous phase for interfacial polymerization of polyaniline nanofiber. This dispersion was then added to the water-insoluble solvent phase containing aniline monomer. The polymerization only occurred at the interface of the two immiscible phases leading to polyaniline nanofiber decorated graphene structures. Characterizations by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy suggested nanocomposite formation with intermolecular π-π bonding of graphene with polyaniline nanofibers. Pastes of the materials were screen printed on stainless steel current collectors and tested for SC performance by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements with 2 M H₂SO₄ electrolyte using a home-built two-electrode test-cell. CV results showed redox peaks of polyaniline with wide cyclic loop, indicating large pseudocapacitance of the nanocomposite. From GCD measurement, a high specific capacitance of 690 Fg⁻¹ at 1 Ag⁻¹ was achieved. Therefore, PANI-SSG nano-composite prepared by electrolytic exfoliation and interfacial polymerization is a promising candidate for SC applications.     

Preparation and properties of unidirectional boron nitride fibre reinforced boron nitride matrix composites via precursor infiltration and pyrolysis route

The unidirectional boron nitride fibre reinforced boron nitride matrix (BN_f/BN) composites were prepared via the precursor infiltration and pyrolysis (PIP) route, and the structure, composition, mechanical and dielectric properties were studied. The composites have a high content and fine crystallinity of BN. The density is 1.60 g cm⁻³ with a low open porosity of 4.66%. The composites display good mechanical properties with the average flexural strength, elastic modulus and fracture toughness being 53.8 MPa, 20.8 GPa and 6.88 MPa m^1/2, respectively. Lots of long fibres pull-out from the fracture surface, suggesting a good fibre/matrix interface. As temperature increases, both of the flexural strength and elastic modulus exhibit a decreasing trend, with the lowest values being 36.2 MPa and 8.6 GPa at 1000 °C, respectively. The desirable residual ratios of the flexural strength and elastic modulus at 1000 °C are 67.3% and 41.3%, respectively. The composites have excellent dielectric properties, with the average dielectric constant and loss tangent being 3.07 and 0.0044 at 2-18 GHz, respectively.     

Electrospun nanofiber composites with micro-/nano-particles for thermal insulation

Polyacrylonitrile (PAN) nanofiber and silica aerogel (SAG) laminated composites were prepared via electrospinning for thermal insulation. Conventional single nozzle and co-axial electrospinning were used to increase the fraction of aerogel particles in the composite sheets while maintaining the mechanical strength of the sheet. When the core-shell electrospinning technique with co-axial nozzle was applied, the proportion of aerogel particles increased two fold without a deterioration of the mechanical properties. The average thermal conductivity of the laminated composite sheet was reduced by approximately 12.5% as compared to the nanofiber composite prepared using the single-nozzle electrospinning technique. For additional reduction in thermal conductivity, hollow glass microspheres (HGM) was inserted between the interlayer spacing of the electrospun sheets to increase the interlayer spacing. When HGM particles were inserted, it was observed that the thermal conductivity decreased by approximately 20% compared to that of the specimen without particles.     

Carbon black/graphite nanoplatelet/rubbery epoxy hybrid composites for thermal interface applications

Hybrid composites were developed by dispersing carbon black (CB) nanoparticles and graphite nanoplatelets (GNPs) at 4–6 and 12–14 wt%, respectively, into rubbery epoxy resin. SEM analysis showed that CB particles improved the dispersion of GNPs in the hybrid composite. The thermal conductivity of 4 wt% CB/14 wt% GNP-15/rubbery epoxy hybrid composite, 0.81 W/m K, is ca. four times higher than that of rubbery epoxy. When silane-functionalised, the fillers reduced the viscosity of the hybrid dispersion and made the hybrid composite highly electrically insulating. Nevertheless, filler functionalisation decreased the composite’s thermal conductivity by only 16.6%. Compression testing showed that the hybrid fillers increased the compressive modulus and strength of rubbery epoxy by nearly two and three times, respectively. Overall, the hybrid composites with their thermal paste-type morphology, low viscosity, high compliance, improved thermal conductivity and, when fillers are functionalised, low electrical conductivity makes them promising materials as thermal interface adhesives.     

氮化硼填充聚酰亚胺复合材料的制备及其亚胺化的研究

采用原位聚合法制备聚酰亚胺/氮化硼(PI/BN)复合材料,并对其进行表征分析.通过一系列不同偶联剂、催化剂含量等合成参数的优化,得到最佳性能的薄膜.采用称重法对薄膜的亚胺化及固化程度进行测定,黏度法对聚合物分子量的测定,使用FTIR,DSC和TG对单体、制备过程及最终复合材料的结构和热性能进行表征分析.结果表明,聚酰亚...     

Synergetic effect of hybrid boron nitride and multi-walled carbon nanotubes on the thermal conductivity of epoxy composites

This study investigates the synergistic effect of combining multi-walled carbon nanotubes (MWCNTs) and boron nitride (BN) flakes on thermally conductive epoxy composite. The surface of the two fillers was functionalized to form covalent bonds between the epoxy and filler, thereby reducing thermal interfacial resistance. The hybrid filler provided significant enhancement of thermal conductivity, adding 30 vol% modified BN and 1 vol% functionalized MWCNTs achieving a 743% increase in thermal conductivity (1.913 W mK⁻¹, compared to 0.2267 W mK⁻¹ of neat epoxy).     

Few-atomic-layered hexagonal boron nitride: CVD growth,characterization, and applications

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Activation of boron nitride nanotubes and their polymer composites for improving mechanical performance

Boron nitride nanotubes (BNNTs) are inappropriate for further chemical derivatization because of their chemical inertness. We demonstrate covalent activation of chemically inert BNNTs by isophorone diisocyanate (IPDI) to form isocyanate group (NCO)-terminated BNNT precursors with an 'NCO anchor' ready for further functionalization. As identified by Fourier transform infrared spectroscopy, a number of molecules or polymers with -COOH, -OH or -NH? groups are readily attached to the activated IPDI-BNNTs. The IPDI-BNNT-involving polymer composites have shown mechanical properties are considerably improved due to the good dispersibility of IPDI-BNNTs in the polymer matrix and the strong interfacial interactions between BNNTs and polymers. The methodology reported here provides a promising method to promote the chemical reactivity of BNNTs and covalently modify polymer nanocomposites with improved mechanical performance.     

Biomimetic smart interface materials for biological applications

Controlling the surface chemical and physical properties of materials and modulating the interfacial behaviors of biological entities, e.g., cells and biomolecules, are central tasks in the study of biomaterials. In this context, smart polymer interface materials have recently attracted much interest in biorelated applications and have broad prospects due to the excellent controllability of their surface properties by external stimuli. Among such materials, poly(N-isopropylacrylamide) and its copolymer films are especially attractive due to their reversible hydrogen-bonding-mediated reversible phase transition, which mimics natural biological processes. This platform is promising for tuning surface properties or to introduce novel biofunctionalities via copolymerization with various functional units and/or combination with other materials. Important progress in this field in recent years is highlighted.     

Enhanced thermal conductivity and stability of boron nitride/phenyl silicone rubber composites via surface modification and grain alignment

Journal of Materials Science - With the extensive use of high-power electronic appliances, polymer-based thermal insulation composites with excellent thermal properties are utilized in the field of...     

The crystallographic texture of graphite-like and diamond-like boron nitride bulk materials

X-ray powder diffraction and quantitative texture analysis have been employed to study the crystallographic relation of parent and product phases in direct transformations of graphite-like BN polymorphs into diamond-like BN phases at a pressure of 7.7 GPa and temperatures up to 2600 K. It has been found that the transformation of both CVD-produced bulk slightly textured turbostratic BN (tBN) and highly textured graphite-like hexagonal BN (hBN) results in a preferred orientation of the crystallites of the new cubic BN (cBN) due to the (111)_cBN (001)_{tBN, hBN} epitaxial relation. Phase transformation of a CVD-produced bulk highly textured rhombohedral BN (rBN) results in the formation of highly textured wurtzitic BN (wBN) according to the (100)_wBN (001)_rBN crystallographic relation.