High‐performance hexagonal boron nitride/bismaleimide composites with high thermal conductivity,low coefficient of thermal expansion,and low dielectric loss

High‐performance insulating materials have been increasingly demanded by many cutting‐edge fields. A new kind of high‐performance composites with high thermal conductivity, low coefficient of thermal expansion (CTE), and low dielectric loss was successfully developed, consisting of hexagonal boron nitride (hBN) and 2,2′‐diallylbisphenol A (DBA)‐modified 4,4′‐bismaleimidodiphenylmethane (BDM) resin. The effects of hBN and its content on the integrated properties, including curing behavior of uncured system, the CTE, thermal conductivity, dielectric properties, and thermal resistance of cured composites, are systematically investigated and discussed. Results show that there are amino groups on the surface of hBN, which supply desirable interfacial adhesion between hBN and BDM/DBA resin and a good dispersion of hBN in the resin. With the increase of the hBN content, the thermal conductivity increases linearly, whereas the CTE value decreases linearly; in addition, dielectric loss gradually decreases and becomes more stable over the whole frequency from 10 to 10⁹ Hz. In the case of the composite with 35 wt% hBN, its thermal conductivity, CTE in glassy state, and dielectric loss are about 3.3, 0.63, and 0.5 times of the corresponding value of BDM/DBA resin, respectively. These attractive integrated properties suggest that hBN/BDM/DBA composites are high‐performance insulating materials, which show great potential in applications, especially for electronics and aerospace industries. Copyright © 2011 John Wiley & Sons, Ltd.     

Polymer/boron nitride nanosheet composite with high thermal conductivity and sufficient dielectric strength

An efficient method was reported to fabricate boron nitride (BN) nanosheets using a sonication–centrifugation technique in DMF solvent. Then non‐covalent functionalization and covalent functionalization of BN nanosheets were performed by octadecylamine (ODA) and hyperbranched aromatic polyamide (HBP), respectively. Then, three different types of epoxy composites were fabricated by incorporation of BN nanosheets, BN‐ODA, and BN‐HBP. Among all three epoxy composites, the thermal conductivity and dielectric strength of epoxy composites using BN‐HBP nanosheets display the highest value, efficiently enhancing to 9.81 W/m K at 50 vol% and 34.8 kV/mm at 2.7 vol% (increase by 4057% and 9.4% compared with the neat epoxy), respectively. The significantly improved thermal conductivity and dielectric strength are attributed to the large surface area, which increases the contact area between nanosheets and nanosheets, as well as enhancement of the interfacial interaction between nanosheets and epoxy matrix. Copyright © 2015 John Wiley & Sons, Ltd.     

Simultaneous enhancement of thermal conductivity and flame retardancy for epoxy resin thermosets through self‐assemble of ammonium polyphosphate surface with graphitic carbon nitride

Conferring the flame retardant performance and thermal conductivity simultaneously for epoxy resin (EP) thermosets was significant for fire safety and thermal management applications of electrical and electronic devices. Herein, the graphitic carbon nitride (g‐C₃N₄) with desired amount was assembled on the surface of ammonium polyphosphate (APP), and the obtained APP/g‐C₃N₄ (CN‐APP) was characterized and confirmed by X‐ray diffraction, Fourier transform infrared spectroscopy tests, scanning electron microscopy, and transmission electron microscopy. CN‐APP was incorporated into EP and then cured with m‐phenylenediamine. The thermal conductive value of EP/CN‐APP thermosets achieved 1.09 W·mK^?1, and the samples achieved UL‐94 V‐0 grade during vertical burning tests with the limiting oxygen index of 30.1% when 7 wt% CN‐APP with the mass fraction of APP/g‐C₃N₄ of 9/1 was incorporated. For comparative investigation, equal amount of individual g‐C₃N₄ was introduced into EP thermosets, and the thermal conductivity was only 0.4 W·mK^?1. Compared with pure EP, the addition of CN‐APP enhanced the glass transition temperature of EP/CN‐APP thermosets and promoted the generation of more expanded, coherent, and compact char layer during combustion. Consequently, the heat release and smoke production of EP/CN‐APP thermosets were greatly suppressed and led to the improvement of fire safety of materials. It was an alternative and promising approach for preparing high‐performance polymeric materials especially used in integrated electronic devices.     

Sn(20.5) square(3.5)As(22)I(8): a largely disordered cationic clathrate with a new type of superstructure and abnormally low thermal conductivity

Sn(20.5)As(22)I(8), a new cationic clathrate, has been prepared by using an ampoule technique. According to the X-ray powder diffraction data, it crystallizes in the face-centered cubic space group F23 or Fm(-)3 with a unit-cell parameter of a=22.1837(4) A. Single-crystal X-ray data allowed solution of the crystal structure in the subcell with a unit-cell parameter of a(0)=11.092(1) A and the space group Pm(-)3n (R=5.7 %). Sn(20.5)As(22)I(8) (or Sn(20.5) square(3.5)As(22)I(8), accounting for the vacancies in the framework) possesses the clathrate-I type crystal structure, with iodine atoms occupying the cages of the cationic framework composed of tin and arsenic atoms. The crystal structure is strongly disordered. The main features are a random distribution of vacancies, and shifts of the tin and arsenic atoms away from their ideal positions. The coordination of the tin atoms has been confirmed by using (119)Sn M?ssbauer spectroscopy. Electron diffraction and high-resolution electron microscopy (HREM) analyses have confirmed the presence of the superstructure ordering, which results in a doubling of the unit-cell parameter and a change of the space group from Pm(-)3n to either F23 or Fm(-)3. Analysis of the crystal structure has led to the construction of four ordering models for the superstructure, which have been corroborated by HREM, and has also led to the identification of disordered regions originating from overlap of the different types of ordered domains. Sn(20.5)As(22)I(8) is a diamagnetic semiconductor with an estimated band gap of 0.45 eV; it displays abnormally low thermal conductivity, with the room temperature value being just 0.5 W m(-1) K(-1).