Abstract
Several boron nitride (BN)/aluminum oxide (Al2O3) composite powders were synthesized by liquid-solid phase co-mixture method. BN was modified by the freeze-thaw method and surface modification. The precursor and as-sintered BN/Al2O3 composite powders were obtained by changing the type of surface modification materials and the molar ratios. Additionally, the thermal conductivity of the epoxy resin (EP)-based BN/Al2O3 composite materials prepared by mechanical mixture was measured and analyzed. Results show that the dispersion and interfacial compatibility of BN after freeze-thaw process are obviously better than those without freeze-thaw process. The modification effect of dopamine on BN is better than that of polyethylene glycol. When the dopamine is used as the surface modification material and the molar ratio of BN to Al(NO3)3 is 1:1, BN powder at micro-level with uniform nano-Al2O3 deposition can be obtained, namely the BN/Al2O3 micro/nano-composite powder. The thermal conductivity coefficient of this EP-based BN/Al2O3 composite reaches 0.62 W·
High-integration, high-power, and miniaturized electronic components are rapidly developing in the modern electronics, electricity, and light-emitting diode fields. With the reduction in size and volume of electronic components as well as the continuous increase in density, the generated heat is greatly increased, which has an inevitable impact on the working stability and reliability of electronic components, even decrea-sing the service lif
Polymers have the advantages of electrical insulation, high flexibility, small mass, low cost, excellent mechanical proper-ties, and good processibility, and they are widely used in various industrial field
Bian et a
The h-BN with layered structure belongs to the hexagonal crystal system. The layers are bonded to each other by Van der Waals force. As a result, h-BN is prone to agglomerate in the matri
In this research, the BN/Al2O3 composite powder and its EP-based composites were prepared and studied. The BN was treated by freeze-thaw process and chemically modified by different surface modifiers. The BN/Al2O3 composite powders were prepared with different molar ratios through the liquid-solid phase mixture method. The microstructure, morphology, and phase were characterized by optical microscope (OM), X-ray diffractometer (XRD), or scanning electron microscope (SEM). The thermal conductivity and mechanism of composites were studied and analyzed.
The materials used in this research were as follows: micron-sized hexagonal boron nitride (BN, Zibo Jingyi Ceramics Technology Co., Ltd) with layered structure and anisotropy properties which are similar to those of graphite; aluminum nitrate (Al(NO3)3, Tianjin Damao Chemical Reagent Factory), which is a white substance and soluble in water; citric acid (C6H8O7, Tianjin Damao Chemical Reagent Factory); sodium hydroxide (NaOH); hydrochloric acid (Tianjin Damao Chemical Reagent Factory); dopamine hydrochloride (Hubei Maoerwo Biomedical Co., Ltd); polyethylene glycol 6000 (PEG, Fuchen Chemical Reagent Co., Ltd); trismetyl-aminomethane (Amresco Co., Ltd); bisphenol-A EP (Epikote-828 EL, 186-190g/eq); methyl-tetrahydrophthalic anhydride curing agent; graphite spray (Tianjin Damao Chemical Reagent Factory).
The synthesis of composite materials was divided into two steps: the synthesis of BN/Al2O3 composite powder and the synthesis of EP-based BN/Al2O3 composites.
(1) Synthesis of BN/Al2O3 composite powders
The BN/Al2O3 composite powder was synthesized by the solid-liquid phase mixture method.
Step 1: BN was treated by freeze-thaw method. BN was frozen at -20 ℃ for 24 h and then thawed for 12 h. This pro-cess was repeated for 10 times. BN treated by freeze-thaw meth- od and BN without freeze-thaw treatment were used for com- parison, and they were named as BN# and BN, respectively.
Step 2: BN and BN# were then modified by DA or PEG, and the resultant products were named as DA@BN, DA@BN#, PEG@BN, and PEG@BN#.
Firstly, 10vol% dilute hydrochloric acid solution was dropped into the 0.01 mol/L trismetyl-aminomethane solution, and the pH value was adjusted to about 8.5 to obtain the Tris-HCI solution. Then, 54 mL Tris-HCl solution, 6 mL ethanol, and 0.03 g dopamine hydrochloride were added to the beaker and stirred for 1 h to obtain the DA solution. Finally, 1.24 g BN or BN# was added to the DA solution for 24 h to obtain the DA@BN or DA@BN#, respectively.
Besides, 1.68 g PEG was dissolved in 100 mL deionized water and stirred for 1 h. Then, 1.24 g BN or BN# powder was added into the mixture. The obtained solution was stirred for 1 h and ultrasonically dispersed for 3 h to obtain PEG@BN and PEG@BN#.
Step 3: BN/AlO(OH) powder, namely the precursor of BN/Al2O3 composite powder, was prepared. Firstly, 20 mL citric acid (2.5 mol/L) and 50 mL NaOH solution (10 mol/L) were added into 100 mL Al(NO3)3 solution (1 mol/L). Then, the mixed solution was stirred evenly to obtain AlO(OH) solution. Afterwards, the BN, BN#, DA@BN, DA@BN#, PEG@BN, and PEG@BN# solutions were separately added to the AlO(OH) solution based on certain molar ratios of BN to Al(NO3)3. The solution was stirred for 1 h and mechanically milled for 6 h. Finally, different BN/AlO(OH) powders were obtained after several centrifugation treatments followed by cleaning and drying for 48 h.
Step 4: the obtained BN/AlO(OH) powders were sintered at 1200 °C for 2 h in argon atmosphere to prepare the BN/Al2O3 powders.
A series of BN/Al2O3 powders were prepared at the molar ratio of BN to Al(NO3)3 of 1:1 according to the abovemen-tioned processes. Additionally, the experiments with molar ratio of BN to Al(NO3)3 of 1:0.5 and 1:2 were also conducted to determine the optimal preparation parameters.
The abovementioned precursors were named as DA@xBN/AlO(OH), PEG@xBN/AlO(OH), DA@xBN#/AlO(OH), and PEG@xBN#/AlO(OH). The sintered composite powders were named as DA@xBN/Al2O3, PEG@xBN/Al2O3, DA@xBN#/Al2O3, and PEG@xBN#/Al2O3. When x=2, 1, and 0.5, the molar ratio of BN to Al(NO3)3 was 1:0.5, 1:1, and 1:2, respec-tively. Normally, without specific instruction, the discussed products are referred to the ones with molar ratio of 1:1.
(2) EP-based BN/Al2O3 composites
According to Ref.[
The DA@BN#/Al2O3 powder was filled into EP with the methyl-tetrahydrophthalic anhydride curing agent, then forming the EP-based BN/Al2O3 composite. The content of DA@BN#/Al2O3 filler was 23wt%, and the mass ratio of EP to methyl-tetrahydrophthalic anhydride curing agent was 3:1. The specimen diameter was 12.5±2 mm and the thickness was about 2.5 mm. The upper and lower surfaces of specimen were sprayed by carbon to ensure the electrical conductivity of the specimen.
The as-prepared composites with molar ratio of BN to Al(NO3)3 of 1:0.5, 1:1, and 1:2 were named as 2BNAl/EP, 1BNAl/EP, and 0.5BNAl/EP, respectively. EP-based BN composites were also prepared and were named as BN/EP.
XRD analyzer (XRD-7000, Shimadzu, Japan) was used to analyze the phase constitutes with scanning steps from 10° to 90°. The observation of microstructure and morphology of the specimens was conducted by SEM (JEOL). The thermal conductivity of the composites was measured by German NETZSCH LFA467 flash thermal conductivity meter through the laser flash technique, where the heat was transferred from the upper surface to the lower surface of the specimens.
XRD patterns of different BN/AlO(OH) powders are shown in
Fig.1 XRD patterns of different BN/AlO(OH) powders
For the DA@BN/AlO(OH) powder, it can be seen that the positions of diffraction peak hardly change, but their diffraction intensity is decreased with increasing the Al(NO3)3 content. The diffraction peak intensity of BN/AlO(OH) powder is further decreased by the DA-modification, compared with that after PEG-modification. An organic layer is formed on BN surface after DA- or PEG-modification, which reduces the intensity of diffraction peaks. It is found that the diffraction peak at 2θ>60° almost disappears with increasing the content of Al(NO3)3. Thus, it can be inferred that the modification effect on BN for the BN/AlO(OH) powder becomes worse with increasing the Al(NO3)3 content.
The morphologies of BN/AlO(OH) powders with different modifiers (PEG, DA) are shown in
Fig.2 SEM morphologies of PEG@BN/AlO(OH) powder (a) and DA@BN/AlO(OH) powder (b)
It is noticed that the BN powder is a layered material and has clustered agglomeration structure, and it is difficult to achieve uniform deposition of AlO(OH) on BN powder surface. Therefore, the freeze-thaw method was used to pre-treat BN powder before the preparation of BN/AlO(OH) powder.
To investigate the effect of freeze-thaw treatment, XRD patterns of the DA@xBN#/AlO(OH) powders are shown in
Fig.3 XRD patterns of different DA@xBN#/AlO(OH) powders
As shown in
SEM morphologies of DA@xBN#/AlO(OH) powders are shown in
Fig.4 SEM morphologies of DA@2BN#/AlO(OH) powder (a), DA@1BN#/AlO(OH) powder (b), and DA@0.5BN#/AlO(OH) powder (c)
Fig.5 XRD patterns of different DA@xBN#/Al2O3 powders after sintering at 1200 °C
With increasing the Al(NO3)3 content, the intensity of Al2O3 diffraction peaks is increased continuously, whereas that of BN is decreased. It can be concluded that when the molar ratio of BN to Al(NO3)3 is 1:1, the diffraction intensity of BN and Al2O3 is moderate. Then, with increasing the Al(NO3)3 content, the deposition of Al2O3 on BN surface becomes more obvious firstly and then decreased.
SEM morphologies of different BN/Al2O3 powders after sintering at 1200 °C are shown in
Fig.6 SEM morphologies of DA@2BN#/Al2O3 powder (a), DA@1BN#/Al2O3 powder (b), and DA@0.5BN#/Al2O3 powder (c) after sintering at 1200 °C
The thermal conductivity of EP-based DA@xBN#/Al2O3 composites with x=2, 1, and 0.5 is 0.44, 0.62, and 0.33 W·
In this research, DA and PEG were used to modify BN. Some strong polar groups, such as -OH and -NH3, are introduced onto the BN surface during modification. Thus, the surface adhesion, dispersion, and interfacial compatibility of BN are improved. Additionally, the introduction of functional groups increases the active sites on BN surface, which is helpful for the adhesion of Al2O3 particles onto the BN surface. In the preparation process of BN/Al2O3 composite powder, the freeze-thaw treatment and the mechanical ball milling process can improve the dispersion of filler and the compatibility of matrix interface to a certain exten
The thermal conductivity of the polymer composites containing thermal conductive fillers is achieved by forming continuous transmission network or path through the filler of effectively high thermal conductivity. In the EP-based DA@xBN#/Al2O3 composites, the main thermal conduction paths are formed by BN, and other thermal conduction paths can be formed when Al2O3 particles are deposited on the BN surface. Therefore, multiple thermal conduction paths are formed and the thermal conductivity is improved. The thermal conductivity of EP-based composites is improved by the combination of BN and Al2O3 filler at low filler content.
The results show that the thermal conductivity of EP-based BN#/Al2O3 composites is greater than that of pure EP-based BN composites. Compared with the single BN filler, the composite filler of Al2O3 and BN can form a 3D structure in EP matrix, which is beneficial to establish an effective heat conduction and transmission path. It can be seen that the synergistic effect of fillers at different scales improves the thermal conductivity of material
Fig.7 Schematic diagram of production and multi-thermal conduction paths of EP-based BN/Al2O3 composites
When the molar ratio of BN to Al(NO3)3 is 1:1, the BN/Al2O3 composite filler can form an effective heat conduction path in EP. However, when the Al(NO3)3 content is insufficient or excessive, it may not be able to form the effective thermal conductivity paths, thereby leading to the decrease in thermal conductivity.
1) The freeze-thaw treatment is beneficial to change the BN properties. The dispersion and interfacial compatibility of BN are obviously improved by freeze-thaw treatment.
2) The modification effect of dopamine (DA) on BN is better than that by polyethylene glycol (PEG).
3) The microstructures of BN/Al2O3 are composed of the micron-sized BN with deposited nano-Al2O3 particles. When the molar ratio of BN to Al(NO3)3 is 1:1, the deposition of Al2O3 on BN is uniform and optimal.
4) When BN is treated by freeze-thaw method, DA is used as the surface modifier, and the molar ratio of BN to Al(NO3)3 is 1:1, the thermal conductivity of this epoxy resin (EP)-based BN/Al2O3 composite is 3 times and 1.5 times higher than that of the pure EP composite and EP-based composite material prepared by pure micro-sized BN powder. The BN with deposited Al2O3 can form multiple thermal conduction paths, which improve the thermal conductivity of EP-based BN/Al2O3 composites.
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