Thermally Conductive Aluminum Nitride–Multiwalled Carbon

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Thermally Conductive Aluminum Nitride−Multiwalled Carbon Nanotube/Cyanate Ester Composites with High Flame Retardancy and Low Dielectric Loss Ya-nan Mi, Guozheng Liang,* Aijuan Gu,* Feipeng Zhao, and Li Yuan Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Materials Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China S Supporting Information *

ABSTRACT: New high-performance composites with high thermal conductivity, good flame retardancy, and low dielectric loss using cyanate ester (CE) resin as the matrix and hybrid fillers consisting of aluminum nitride (AlN) and multiwalled carbon nanotubes (MCNTs) as the functional phase were developed. In addition to the original fillers, surface-treated AlN (kAlN) and MCNTs (eMCNTs) were prepared to develop four types of hybrid fillers and the corresponding composites. The structures and properties of the ternary composites can be adjusted by controlling the interaction between the nanotubes and the AlN. Both AlN and kAlN fillers can improve the dispersion of nanotubes in the CE resin, regardless of whether the nanotubes are modified, whereas only eMNCTs can improve the dispersion of AlN or kAlN in the matrix. The kAlN−eMCNT hybrid was found to have the highest synergistic effect, endowing CE resin with outstanding thermal conductivity, low dielectric loss, significantly improved processing characteristics, and flame retardancy.

1. INTRODUCTION High-performance insulating resins with high thermal conductivity have attracted great attention worldwide owing to their great potential in many cutting-edge industries, including microelectronics, aerospace, and aviation.1−4 Many researchers have found that adding thermally conductive fillers to a polymer is an effective and easy method of developing thermally conductive insulating resins. Many conductive fillers have been used, such as oxides (Al2O3, SiO2, ZnO, and BeO),5−7 carbides (SiC),8 and nitrides (AlN and BN),9,10 but to obtain high thermal conductivity, a very large loading of fillers, usually 60 wt % or even higher, should be used to form a fully conductive network in the matrix. Obviously, this usually results in poor processing characteristics;11,12 therefore, reducing the content of conductive fillers is a key issue for conductive composites. In recent years, some scholars prepared ternary composites by adding multiwalled carbon nanotubes (MCNTs) to the traditional thermal conductive ceramic/polymer composites and found that the resulting ternary composites had much higher thermal conductivities than the corresponding binary composites.13−15 However, some key and basic topics were not studied, and consequently, it is still difficult to develop highperformance thermal conductive resins with expected performance. First, it is worth noting that, compared with traditional high-performance insulating resins, new high-performance resins should have better and more desirable properties; among them, thermal conductivity, dielectric property, and flame retardancy are three new representative features in addition to outstanding thermal stability, moisture resistance, and toughness.16,17 This means that the new thermally conductive resins should also have good dielectric properties and flame retardancy, but unfortunately, previous works © 2013 American Chemical Society

focused only on thermal conductivity and did not consider dielectric properties and flame retardancy. For example, a high content of MCNTs was used to obtain high thermal conductivity, but this also resulted in high dielectric loss, and thus would not meet the more stringent requirements for modern high-performance insulating resins for the electronics industry.18 Second, it is known that MCNTs have very high thermal conductivity at room temperature [up to 3000 W/(m K)],19 however, the thermal conductivities of MCNT/polymer composites reported in the literature are not as high as expected owing to the phonon scattering effect at the interface of the composites.20,21 Theoretically, the thermal conductivity is proportional to the transfer velocity and the mean free path of the phonons;22 the latter decreases at the interface of a composite, thus leading to a decreased thermal conductivity. Obviously, the trend will be remarkably enhanced in a nanocomposite or a multiphase composite owing to the large amount of interfaces. However, unfortunately, previous works paid attention only to the interface between the fillers and the matrix and did not consider the interaction between the MCNTs and the ceramic fillers. This phenomenon is not beneficial for developing high-performance conductive composite because the interaction between the nanotubes and the ceramic fillers plays important role in the nature of the interface and the formation of thermally conductive paths in a composite. Therefore, studying these topics is of great Received: Revised: Accepted: Published: 3342

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filtered at room temperature and subsequently dried at 80 °C for 24 h in a vacuum oven to obtain acidulated MCNTs (denoted as aMCNTs). aMCNTs (4.5 g) and 4,4′-dimethylformamide (50 mL) were blended in a reactor and then sonicated at room temperature for 0.5 h. Next, a solution consisting of E51 (100 g), 4,4′dimethylformamide (300 mL), and triphenyl phosphine (0.5 g) was added to the mixture in the reactor. After that, the reactor was heated to 80 °C and maintained at that temperature with stirring for 48 h; the mixture was then filtered with an excess of dichloromethane. The filter cake was dried in a vacuum oven to obtain epoxy-functionalized nanotubes, denoted as eMCNTs. 2.4. Preparation of Cured CE Resin. Appropriate amounts of CE and E51 (CE/E51 weight ratio of 100:1) were thoroughly blended at 150 °C for 2 h with vigorous stirring by a homogeneous agitator to form a prepolymer, and then the prepolymer was degassed to remove entrapped air at 130 °C for 1 h in a vacuum oven. After that, the mixture was cast into a mold for curing and postcuring at the temperatures of 180 °C for 2 h, 200 °C for 2 h, 220 °C for 2 h, and 240 °C for 4 h. Finally, the cured resin was demolded; it is denoted as CE resin. 2.5. Preparation of Binary and Ternary Composites. Appropriate amounts of CE and E51 (CE/E51 weight ratio of 100:1) were thoroughly blended at 150 °C for 10 min with vigorous stirring to form a homogeneous liquid, into which preweighed AlN was added with stirring for about 2 h to form a prepolymer [for differential scanning calorimetry (DSC) tests]. After that, the prepolymer was degassed to remove entrapped air at 130 °C for 1 h in a vacuum oven and then cured and postcured at the temperatures of 180 °C for 2 h, 200 °C for 2 h, 220 °C for 2 h, and 240 °C for 4 h. Finally, the cured composite was demolded; it is denoted as AlNn/CE, where n is the weight fraction of AlN in the composite. Composites based on eMCNTs were also prepared using the same procedure except that AlN was replaced by eMCNTs, and the corresponding composites are denoted as eMCNTn′/CE, where n′ is the weight fraction of eMCNTs in the composite. Appropriate amounts of eMCNTs and CE at a weight ratio of 10:100 were blended in a reactor containing acetone (50 mL) at 60 °C for 0.5 h, and then kAlN particles were added to the reactor with vigorous stirring at 60 °C and maintained under these conditions for 0.5 h. After that, E51 resin (1 wt % of CE resin) and the residual weight of CE were added to the reactor, which was then heated to 140 °C and maintained at that temperature for 1 h to obtain a prepolymer. The prepolymer was degassed to remove acetone and entrapped air at 130 °C for 1 h in a vacuum oven and then cured and postcured at the temperatures of 150 °C for 2 h, 180 °C for 2 h, 200 °C for 2 h, 220 °C for 2 h, and 240 °C for 4 h. Finally, the cured composite was demolded; it is denoted as (kAlN− eMCNT)m/CE composites, where m is the total weight fraction of fillers in the composite. Composites based individually on AlN−MCNT, kAlN− MCNT, and AlN−eMCNT were also prepared using the same procedure except that the fillers were AlN−MCNT, kAlN− MCNT, and AlN−eMCNT, respectively, and the corresponding composites are denoted as (AlN−MCNT)m/CE, (kAlN− MCNT)m/CE, and (AlN−eMCNT)m/CE, respectively, where m is the total weight fraction of fillers in the composite. Note that the amount of MCNTs or eMCNTs in each ternary composite was fixed at 2.5 wt %.

importance and interest, which is the motivation for the research reported herein. In this article, cyanate ester (CE) resin was selected as the base resin for developing high-performance resin because it has outstanding integrated properties, such as excellent dielectric properties, thermal resistance, and good processing characteristics, and thus is regarded as the candidate with the greatest competition for the fabrication of advanced functional/ structural materials for microelectronic, aerospace, transportation, and so on, in the 21st century.23,24 On the other hand, AlN was selected as the conductive ceramic owing to its high thermal conductivity [about 160 W/(m K)] and wide applications.25−27 Using these basic components, four types of ternary composites based on the hybrid fillers (including both AlN particles and MCNTs) with different surface natures were prepared, and then the effects of the interaction between the fillers on the structure and integrated properties such as thermal conductivity, flame retardancy, and dielectric properties of the composites were intensively studied for the first time. A new type of high-performance insulating composite with outstanding thermal conductivity, good flame retardancy, and low dielectric loss was developed.

2. EXPERIMENTAL SECTION 2.1. Materials. The cyanate ester (CE) used was 2,2′-bis(4cyanatophenyl)isopropylidene, which was purchased from Jiangdu Resin Company, Jiangsu, China. AlN particles with an average diameter of 4 μm were purchased from Jiankun Chemical Co. Inc., Hefei, China. MCNTs (average diameter, 10−20 nm; length, 5−15 μm) with a purity of more than 98 wt % were obtained from Shenzhen Nanoport Company, Shenzhen, China. Bisphenol-A diglycidyl ether (E51) was made by Sanmu Group (Jiangsu, China); its average molecular weight was about 380 g mol−1, and its epoxy value was 0.52 mol per 100 g. γ-Aminopropyl triethoxy silane (KH550) was supplied by Jingzhou Jianghan Fine Chemicals Ltd. Jingzhou, China. 4,4′-Dimethylformamide, triphenyl phosphine, fuming nitric acid, and dichloromethane were commercial products obtained in analytical grades. 2.2. Preparation of Surface-Modified AlN Particles. Dried AlN particles were added to a 10 wt % sodium hydroxide solution, which was then heated to 90 °C with stirring and maintained at that temperature for 20 min. After that, the AlN particles were centrifuged, washed with acetone several times, and dried at 60 °C for 24 h. The resultant product was aluminum nitride hydroxide. Aluminum nitride hydroxide (100 g), KH550 (3 g), and ethanol (100 mL) were blended with the aid of ultrasonic dispersion for 5 h to form a mixture. This mixture was then stirred at 55 ± 2 °C for 5 h, washed with absolute ethanol several times, and dried at 60 °C for 8 h. The resultant product is denoted as kAlN, for which the preparation mechanism is shown in Figure S1 of the Supporting Information 2.3. Preparation of Epoxide-Functionalized MCNTs. The mechanism of preparing epoxide-functionalized MCNTs includes acidulation and esterification, as shown in Figure S2 of the Supporting Information. Specifically, 5 g of MCNTs was mixed in 100 mL of HNO3 (60 wt %) with the help of an ultrasonic bath at room temperature, and then the mixture was heated to 75 °C and maintained at that temperature with stirring for 48 h. After that, the pH value of the solution was adjusted to 7 using deionized water. The mixture was then 3343

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2.6. Characterizations. Fourier transform infrared (FTIR) spectra were recorded between 400 and 4000 cm−1 at a resolution of 2 cm−1 on a Prostar LC240 infrared spectrometer (Agilent Technologies, Palo Alto, CA). The sample was pressed into a pellet with KBr. X-ray photoelectron spectroscopy (XPS) was performed on a Shimadzu-KRATOS Analytical-Axis ULTRA-DLD spectrometer (Manchester, U.K.) with monochromatized Al Kα X-ray source (hν = 1486.71 eV) with a pressure in the analysis chamber of 10−9 Torr. Thermogravimetric (TG) analyses were performed using a Perkin-Elmer TGA-7 instrument (Wellesley, MA) at a heating rate of 20 °C/min in an air atmosphere from 25 to 800 °C. Transmission electron microscopy (TEM) was performed on a Hitachi 800 microscope (Tokyo, Japan) at an accelerating voltage of 20 kV. UV−vis transmittance spectra from 200 to 800 nm were recorded on a Shimadzu RF540 spectrophotometer (Kyoto, Japan). The morphologies of composites were observed on a scanning electronic microscope (Hitachi S-4700, Tokyo, Japan) equipped with an energy dispersive spectrometry (EDS) attachment. The samples were sputter coated with a thin layer (about 10 nm) of gold. All samples were dried at 100 °C for 6 h before tests. Differential scanning calorimetry (DSC) measurements were performed with a DSC 2010 apparatus (TA Instruments, New Castle, DE) ranging from room temperature to 320 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. Thermal conductivity was measured using a DRP-II thermal conductivity tester (Xiangtan Apparatus & Instruments, Xiangtan, China) by means of a steady-state method. The upper and lower surfaces of each sample were coated with a layer of heat-conducting silicone grease before tests. The dimensions of each sample were Φ(70 ± 0.02) mm × (3 ± 0.02) mm. Three specimens were tested for each formulation. Each sample was tested three times at the room temperature. Typical results were reproducible to within about 10%, and the data reported here are the averages of triplicate measurements. X-ray diffraction (XRD) analyses were carried out on a MERCURY CCD X-ray diffractometer (Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 0.15406 nm). The 2θ angle ranged from 10° to 60°, and the scanning rate was 2°/min. Dielectric properties were measured using a broadband dielectric spectrometer (Novocontrol Concept 80, Hundsangen, Germany) at room temperature over a wide frequency range (from 10 to 106 Hz). The dimensions of each sample were (25 ± 0.02) × (25 ± 0.02) × (3 ± 0.02) mm3. Limited oxygen index (LOI) values were measured on a Stanton Redcraft flame meter (London, U.K.) according to ASTM D2863/77. The dimensions of each sample were (100 ± 0.02) × (6.5 ± 0.02) × (3 ± 0.02) mm3. Three samples were tested for each composite, the values were reproducible within 0.5%, and the average value was used as the final result. Raman spectra were obtained using an Almega dispersive raman spectrometer (Thermo Nicolet, Madison, WI) with an Ar+ laser (514.5 nm) at room temperature. Dynamic mechanical analysis (DMA) scans were recorded in single-cantilever blending mode using a TA Q800 dynamic mechanical analyzer (TA Instruments, New Castle, DE) from room temperature to 330 °C at a heating rate of 10 °C/min at 1 Hz. The dimensions of each sample were (35 ± 0.02) × (13 ± 0.02) × (3 ± 0.02) mm3.

3. RESULTS AND DISCUSSION 3.1. Characterization of kAlN. Good dispersion of inorganic fillers in the organic matrix and desirable interfacial adhesion are two necessary characteristics in developing highperformance composites. KH550 was chosen as the coupling agent to functionalize AlN (Figure S1 in the Supporting Information) because −OCN groups can react with −NH2,28 so kAlN is expected to have not only good dispersion in CE resin but also desirable interface adhesion with CE resin. Figure 1 shows FTIR spectra of AlN and kAlN. Each spectrum has a peak at 700 cm−1 attributed to the absorption

Figure 1. FTIR spectra of AlN and kAlN.

band of Al−N groups.29 However, compared with the spectrum of AlN, that of kAlN has additional absorptions at 1044 and 1131 cm−1, representing the asymmetric stretching vibration of Si−O−Si groups. The characteristic peak of Si−O−Al groups cannot be distinguished because it appears at ca. 950 cm−1, a region that tends to be overlaid by the wide characteristic peak of Si−O−Si groups.30 Note that the FTIR spectrum of AlN has absorptions at 3150, 3441, and 1640 cm−1, suggesting that AlN has −NH2, −OH, and −NH groups. This is because AlN has strong reactivity with water (or moisture in air) at ambient temperature to produce −OH, −NH, and −NH2 groups on the surfaces of AlN particles.31,32 To obtain more evidence about the modification of AlN, XPS was used because of its high sensitivity to changes in the chemical environment of an atom. The general XPS spectra of AlN and kAlN are shown in Figure 2, and the percentages of each atom type on the surfaces of AlN and kAlN particles are

Figure 2. XPS survey scan spectra of AlN and kAlN. 3344

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Figure S3 in the Supporting Information shows the Raman spectra of MCNTs and eMCNTs, which have similar patterns: Both exhibit two peaks at ca. 1576 and 1324 cm−1, reflecting the G and D bands, respectively. It is known that the G band corresponds to sp2 hybridization during the formation of the aryl−nanotube bond, and the D band results from the multiple phonon scattering of defects or amorphous carbon,35 so the change of the intensity ratio between the G and D bands, IG/ID, can be used to evaluate the variety in the chemistry of carbon nanotubes.36 The IG/ID values of MCNTs and eMCNTs were 0.79 and 0.69, respectively, indicating that the eMCNTs had more local defects resulting from the formation of carboxyl and epoxide groups during the surface treatment. To determine the influence of the chemical treatment on the crystallinity of the nanotubes, the XRD patterns of MCNTs, aMCNTs, and eMCNTs were recorded. The results show that the MCNTs, aMCNTs, and eMCNTs had very similar XRD patterns (Figure S4 in the Supporting Information): Each had two peaks at 26.0° and 43.0°, but the intensities of these peaks in the patterns of aMCNTs and eMCNTs were lower than that in the pattern of MCNTs, suggesting that the acidulation and esterification did not destroy the graphite structure of the MCNTs, but increased the disorder of the structure. The TEM images of MCNTs and eMCNTs (Figure S5 in the Supporting Information) also confirm this statement. Specifically, the structure of the nanotubes was almost not destroyed after the treatment; however, upon careful observation, it can be seen that the eMCNTs had larger diameters than the MCNTs, reflecting the existence of a layer of epoxy resin on the surfaces of the eMCNTs. Through TG analyses (Figure S6 in the Supporting Information), the grafting yield of bisphenol A diglycidyl ether on the surfaces of nanotubes was calculated to be 6.6 wt %. 3.3. Design of the Formulations of Ternary Composites. Figure 5 shows the thermal conductivities of MCNT/CE

summarized in Table 1. Compared with AlN particles, kAlN particles have significantly higher contents of both C and N atoms, as well as a lower content of Al atoms. Table 1. Surface Compositions (wt %) of AlN and kAlN O 1s N 1s C 1s Al 2p and 2s Si 2p and 2s

AlN

kAlN

54.7 2.0 32.9 9.5 0.9

37.1 4.9 49.9 6.9 1.2

The N 1s spectrum of kAlN can be divided into two separate peaks as shown in Figure 3. The binding energy peak at 397.5

Figure 3. N 1s core-level spectrum of kAlN.

eV is strong and is attributed to N−Al bonds;33 whereas the peak at 400.1 eV is relatively weak and belongs to the −CH2− NH2 bonds associated with the −NH2 groups on the surfaces of kAlN particles,34 suggesting that the modification of AlN was successful. 3.2. Characterization of eMCNTs. Figure 4 shows the FTIR spectra of MCNTs, eMCNTs, and epoxy resin. From a

Figure 5. Thermal conductivities of MCNT/CE and eMCNT/CE composites. Figure 4. FTIR spectra of MCNTs, eMCNTs, and E51.

and eMCNT/CE composites. Although the eMCNT0.5/CE and MCNT0.5/CE composites had similar thermal conductivities, the other eMCNT/CE composites had higher thermal conductivities than the corresponding MCNT/CE composites with the same content of fillers, and the gap increased as the content of fillers increased, suggesting that the surface nature of the fillers plays an important role on the thermal conductivity of the composites. Specifically, for MCNT/CE composites, the MCNTs tended to aggregate and form many small “islands” in

comparison, it can be seen that there are absorptions at 2896 and 1377 cm−1 due to the stretching and bending vibrations of C−H for epoxy resin; a peak at 1628 cm−1 representing the CO stretching vibration of ester groups; and three peaks at 1091, 867, and 791 cm−1 reflecting the vibration of epoxide groups in the FTIR spectrum of eMCNTs, clearly demonstrating that epoxy resin was grafted onto the surfaces of the MCNTs. 3345

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was interesting to find that the AlN−eMCNT/alcohol solution had much lower transmittance than kAlN/alcohol, eMCNT/ alcohol, and kAlN−MCNT/alcohol solutions. Comparatively, AlN−MCNT/alcohol, AlN/alcohol, and MCNT/alcohol solutions had the same transmittances; moreover, kAlN−MCNT/ alcohol solution had the same transmittance as AlN/alcohol solution. These data clearly demonstrate that eMCNTs have a synthetic effect with both original and modified AlN particles, but kAlN particles do not show such a synthetic effect with MCNTs. Reconsidering the surface chemistry of fillers, these interesting results can be attributed to the interaction between nanotubes and ceramic fillers. Specifically, because eMCNTs have good dispersion in alcohol and the −OH groups on the surfaces of AlN particles can react with the epoxy groups on the surfaces of eMCNTs, this will endow AlN particles with good dispersion, and the resultant kAlN−eMCNT/alcohol solution will have lower transmittance than kAlN/alcohol solution. On the other hand, no reaction exists between kAlN particles and MCNTs, so the kAlN−MCNT/alcohol solution will have a similarly high transmittance as the MCNT/alcohol solution. Raman spectroscopy offers an effective tool for probing the structural characteristics and the interaction of MCNT-based materials. When MCNTs suffer from extra stress, the vibration of the C−C bond changes, and the peaks in the Raman spectrum are affected.39 Therefore, the change in the wavelength at which the G-band peak appears has been used to characterize the variety in the dispersion of nanotubes in a resin matrix.40,41 The Raman spectra of CE resin, original and modified nanotubes, eMCNT2.5/CE, and ternary composites are shown in Figure 7. The G-band peaks of MCNTs and eMCNTs appear at 1576 and 1583 cm−1, respectively. An asymmetrically broad peak (1450−1750 cm−1) appears in the Raman spectrum of each ternary composite and can be divided into the characteristic peak (at 1609 cm−1) of CE resin and that (at about 1587 cm−1) representing the G-band peak of nanotubes used. The G-band peak of the nanotubes in each ternary composite shifts to about 4−9 cm−1 higher wavelength compared with that of the nanotubes used, demonstrating that original or modified AlN particles can improve the dispersion of nanotubes in the CE resin regardless of whether the nanotubes are modified. This interesting phenomenon can be attributed to the interaction between nanotubes and AlN particles (or kAlN particles). The interaction between eMCNTs and AlN particles (or kAlN particles) mainly belongs to the chemical interaction as already discussed, whereas that between MCNTs and AlN particles (or kAlN particles) mainly results from the physical aspects. Specifically, AlN particles occupy the major volume of the resin matrix; hence, some AlN particles are located between different MCNTs, consequently blocking the aggregation of the MCNTs and improving the dispersion of the MCNTs. 3.4.2. Structures of Ternary Composites. For a composite based on inorganic fillers and thermosetting resin, its chemical structure is mainly determined by the curing mechanism of the resin matrix, and its morphological structure is principally represented by the cross-linking density and the dispersion of fillers. In the case of the CE resin and related composites reported herein, the variety in the structure is chiefly reflected in the morphological aspect. Figure 8 shows the DSC curves of CE and the binary and ternary composite prepolymers. Each prepolymer has a single exothermic peak, but the peak of each composite prepolymer

the CE resin owing to the lack of active groups on the surfaces of the MCNTs. This means that these MCNT islands were separated by the CE resin, and the channels for thermal conduction were mainly made up of the matrix but not MCNTs. Therefore, the thermal conductivity of the MCNT/ CE composite increased slowly as the content of MCNTs increased.37,38 In contrast, the situation was greatly changed in the eMCNT/CE composites, as the functional groups on the surfaces of the eMCNTs endowed the eMCNTs with good dispersion in the CE resin, and thus, the channels for thermal conduction consisted of both the matrix and fillers. Accordingly, the eMCNTs had a greater influence on the thermal conductivity of the composite. In the case of the eMCNT/CE composites, functional groups on the surfaces of the eMCNTs were found to be beneficial for homogeneously dispersing the eMCNTs in the resin. On the other hand, Figure 5 also shows that the dependence of the thermal conductivity on the content of fillers for both types of composites can be divided into two parts, with 2.5 wt % as the turning point between the two parts. When the content of fillers was lower than 2.5 wt %, as the content of fillers increased, the thermal conductivity of the composite increased linearly and then almost leveled off when the content of fillers was larger than 2.5 wt %. Based on the data shown in Figure 5, 2.5 wt % was selected as the suitable content for both original and modified MCNTs for preparing ternary composites. 3.4. Interaction between Fillers and Its Influence on the Structure of Ternary Composites. 3.4.1. Interaction between Fillers. To evaluate the interaction between fillers and its relation to the surface chemistry of the fillers, original (AlN, kAlN, MCNT, eMCNT) and hybrid (AlN−MCNT, kAlN− MCNT, AlN−eMCNT, and kAlN−eMCNT with the same weight ratio of 19:1) fillers were dispersed into alcohol with stirring for 20 min and then left free-standing for 1 week. During the storage period, the UV−vis spectra of each sample after storage for different periods were recorded, and the results are shown in Figure 6. Neither AlN particles nor MCNTs

Figure 6. UV−vis transmittances of single and hybrid fillers in alcohol after being stored for different lengths of time.

exhibited good dispersion in alcohol: They quickly precipitated at the bottom of the solution once stirring was stopped, so their solutions had higher transmittances. This phenomenon changed greatly for modified fillers (especially eMCNTs), and the corresponding solutions showed greatly decreased transmittance. In the case of solutions containing hybrid fillers, it is expected that the kAlN−eMCNT/alcohol solution would have the best dispersion and lowest transmittance; however, it 3346

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Figure 7. Raman spectra of MCNTs, eMCNTs, CE resin, and ternary composites and enlarged Raman spectrum of ternary composites.

composites containing at least one type of original fillers do not show this phenomenon, suggesting that a synergistic effect exists between two types of modified fillers, which results from the coreaction between the amine groups of kAlN and epoxide groups of eMCNTs. In addition to chemical structure, the cross-linking density is another index representing the structure of a cross-linked resin. The cross-linking densities of cured CE resin and the binary and ternary composites were calculated by the classical equation based on the statistical theory of rubber elasticity46

ρ(E ′) =

G′ 3ΦRT

(1)

where ρ(E′) is the cross-linking density; G′ is the storage modulus (Figure 9) of the sample at temperature T from DMA; Φ is the front factor, assumed to be 1;46 T is the absolute temperature at which the sample is in rubbery state, selected

Figure 8. DSC thermograms of various prepolymers.

shifted toward lower temperature compared with that of CE prepolymer. This is because the curing reaction of CE is very sensitive to the material surroundings.42 The main curing mechanism of CE is the cyclotrimerization of the −OCN groups to form cross-linked triazine rings,43 which are catalyzed by hydrogen donors (e.g., phenols, acids).44 Therefore, it is reasonable to find that the composite containing eMCNTs and AlN and kAlN particles had reduced curing temperature. The MCNTs used in this investigation were synthesized by employing a complex catalyst system containing oxide inclusions of cobalt, magnesium, and molybdenum.45 These catalysts are difficult to remove completely, and the trace residual catalysts will catalyze the cyclotrimerization of the −OCN groups to form cross-linked triazine rings. An interesting phenomenon can be observed from Figure 8; specifically, the exothermic peak of kAlN−eMCNT50/CE appears at significantly lower temperature than those of kAlN50/CE and eMCNT2.5/CE, whereas other ternary

Figure 9. Overlay plots of storage modulus−temperature plots for cured CE resin and composites. 3347

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herein to be the temperature 40 °C higher than the glass transition temperature; and R is the gas constant. Table 2 summarizes the calculated ρ(E′) values of cured CE resin and composites. Each composite has a greater ρ(E′) value

physical cross-linking points whereas the latter can only supply physical cross-linking points. Second, in the case of ternary composites, that based on eMCNTs had a greater ρ(E′) than that containing MCNTs, regardless of whether the AlN particles used were original or modified, suggesting that the surface nature of the nanotubes had a greater effect on the structure of the ternary composite than that of the AlN particles. This is because the MCNTs tend to aggregate in the resin whereas the eMCNTs have good dispersion in the resin, thus providing a large amount of physical cross-linking points. On the other hand, there are some −OH groups on the surfaces of AlN particles, meaning that the difference in the surface nature between original and modified AlN particles is not as great as that between MCNTs and eMCNTs, and thus the modification of the MCNTs will lead to a greater variety in the structure of the ternary composite. Third, the cross-linking densities of the ternary composites were lower than those of the AlN50/CE and kAlN50/CE composites. MCNTs have a lower density (∼1.8 g/cm3) than AlN particles (∼3.235 g/ cm3), so for the same weight content of fillers (50 wt %), the volume of the hybrid fillers was much greater than that of AlN particles, leading to poor processing features. Generally, it is difficult for filler-containing thermosetting resin to attain the same level of curing as pure resins owing to the higher degree of network-interlocking and greater diffusion control effects

Table 2. Cross-Linking Densities of CE Resin and Composites sample

cross-linking density (mol/m3)

CE MCNT2.5/CE eMCNT2.5/CE AlN50/CE kAlN50/CE AlN−MCNT50/CE kAlN−MCNT50/CE AlN−eMCNT50/CE kAlN−eMCNT50/CE

1123 1166 2087 5040 6138 2353 3207 3835 4431

than CE resin. This is expected because the presence of inorganic fillers provides the physical and chemical cross-linking points47 and, thus, increases the ρ(E′) value. Three interesting phenomena can be found from Table 2. First, the modified fillers had a greater effect on increasing ρ(E′) than the original fillers, because the former can provide both chemical and

Figure 10. Al mappings of binary and ternary composites. 3348

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during curing processing, especially at the end stage of curing.48 A larger volume of fillers has a greater effect on the curing of the resin, so the ternary composites had lower cross-linking densities than the AlN50/CE and kAlN50/CE composites. To investigate the influence of the interaction between fillers on the morphological structure, the morphologies of each type of composite were observed. Figure 10 shows the Al mapping of the four types of composites observed using scanning electronic microscopy (SEM) and EDS techniques, where the bright yellow dots are the Al atoms distributed in the CE resin. In the case of composites based on AlN particles, many Al aggregates existed in AlN47.5/CE. This phenomenon also appeared in the AlN−MCNT50/CE composite, but only a small amount of Al aggregates were found in the AlN− eMCNT50/CE composite. These images demonstrate that the addition of eMNCTs (but not MCNTs) improved the dispersion of AlN particles in the CE resin; a similar statement can also be made about the composites based on kAlN particles. Therefore, eMCNTs and AlN particles (kAlN particles) have an interaction resulting from the active groups on the surfaces of these fillers. The dispersions of fillers in the resin matrix for the composites with different interaction of fillers are schematically depicted in Figure 11.

3.5. Properties of Ternary Composites and Their Relations with the Interaction between Fillers. 3.5.1. Thermal Conductivity. Figure 12 shows the thermal

Figure 12. Thermal conductivities of CE-resin-based composites.

conductivities of ternary and binary composites. For the same content of fillers, ternary composites had higher thermal conductivities than the kAlN/CE and AlN/CE composites; moreover, the kAlN−eMCNT/CE composites had the highest thermal conductivities among the four types of ternary composites. To evaluate whether there was a synergistic effect between the nanotubes and the AlN particles, we modified the traditional “mixture rule” to obtain the calculated thermal conductivity of the ternary composites according to the equation λc = λ(f1/CE)(1 − wt 2) + λ(f2/CE)(1 − wt1)

(2)

where λc is the theoretical thermal conductivity of the ternary composite; λ(f1/CE) and λ(f2/CE) are the measured thermal conductivities of the f1/CE and f2/CE composites, respectively; and wt1 and wt2 are the respective weight percentages of the two types of fillers (f1 and f2) in the ternary composite The calculated and measured thermal conductivities are summarized in Table S1 in the Supporting Information, where it can be seen that the measured values of AlN−MCNT50/CE, kAlN−MCNT50/CE, AlN−eMCNT50/CE, and kAlN− eMCNT50/CE composites were respectively 50%, 59%, 76%, and 78% higher, respectively, than the calculated values, indicating that there was an obvious synergistic effect in the ternary composites and that the composite based on eMCNTs had the greatest synergistic effect. These interesting data can be attributed to the interaction of hybrid fillers. MCNTs have a high aspect ratio and can easily form a thermal conducting path in the corresponding composite; hence, a composite based on MCNTs tends to have high thermal conductivity. Accordingly, factors that can improve the dispersion of MCNTs in the matrix are beneficial for increasing the thermal conductivity. According to the above discussions, these factors include the surface modification of MCNTs and the formation of hybrid fillers with AlN or kAlN particles. Obviously, the combination of the two factors should lead to the best results, and this is why the kAlN−eMCNT/CE system showed the highest thermal conductivity among the composites prepared herein. 3.5.2. Flame Retardancy. LOI is the minimum oxygen concentration in an oxygen/nitrogen mixture that will just

Figure 11. Schematics of the dispersion of fillers in the resin matrix: (a) MCNT/CE, (b) eMCNT/CE, (c) AlN−MCNT/CE, (d) kAlN− MCNT/CE, (e) AlN−eMCNT/CE, (f) kAlN−eMCNT/CE. 3349

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(kAlN−eMCNT) had higher LOI values than other composites. To further elucidate the effect of kAlN−eMCNT on the flame retardancy of CE resin, the elemental compositions of the residual chars for cured CE resin and eMCNT2.5/CE, kAlN50/CE, and kAlN−eMCNT50/CE composites were measured using the EDS technique. EDS analyses (Table 4)

support combustion according to ASTM D286; hence, this property is used to evaluate the flammability of materials.49 Table 3 reports the LOI values of CE resin and eMCNT2.5/ CE, kAlN50/CE, and kAlN−eMCNT/CE composites. All Table 3. LOI Values of CE Resin and Binary and Ternary Composites sample CE eMCNT2.5/CE kAlN−eMCNT10/CE kAlN−eMCNT20/CE kAlN−eMCNT30/CE kAlN−eMCNT40/CE kAlN−eMCNT50/CE kAlN50/CE

LOI (%) 26 27 29 32 33 34 37 29

± ± ± ± ± ± ± ±

Table 4. Elemental Compositions of the Residual Chars for Cured CE and Binary and Ternary Composites by EDS Measurements

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

elemental composition (wt %) CE eMCNT2.5/CE

composites had larger LOI values than CE resin, and the LOI increased dramatically from 29 to 37 when the content of hybrid fillers increased from 10 to 50 wt %. Note that, when the content of hybrid fillers was greater than 10 wt %, the kAlN− eMCNT/CE composites had higher LOI values than eMCNT2.5/CE and kAlN50/CE composites, indicating that the mixture of kAlN particles and eMCNTs provided a synergistic effect on flame retardancy. To further confirm the effect of kAlN−eMCNT on the flame retardancy, the thermal-oxidation stabilities of both cured CE resin and composites were evaluated because the flame retardancy of a polymer is directly related to whether the thermal-oxidation degradation proceeds easily. Figure 13 shows the TG and DTG curves in air atmosphere. The thermaloxidation degradations of both CE resin and composite displayed two-stage processes; however, each composite, especially the kAlN−eMCNT/CE composite with the largest content of fillers, had a higher initial decomposition temperature at which the weight loss of the sample reached 5 wt %, a higher maximum thermal decomposition rate, and a higher char yield at 700 °C than CE resin. These interesting data can be attributed to the double roles of fillers that have good dispersions. Briefly, these fillers with good dispersion form a full barrier that can hinder the transport of heat into the polymer, and once the polymer suffers from heat, these fillers quickly transport the heat and avoid the accumulation of heat in the resin owing to the high heat capacity and thermal conductivity of these fillers. Consequently, it is easy to understand that the composites based on hybrid fillers

kAlN50/CE kAlN−eMCNT50/CE

exterior interior exterior interior exterior interior exterior interior

C

N

O

Al

Si

75.4 78.9 82.9 86.5 53.1 62.9 54.1 63.7

8.3 7.9 8.2 7.6 9.5 8.4 8.9 7.1

16.3 13.2 8.9 5.9 21.1 15.3 16.6 13.8

0.0 0.0 0.0 0.0 14.6 13.1 18.0 15.0

0.0 0.0 0.0 0.0 1.7 0.3 2.4 0.4

show that the atom percentages of Al (Si) in the exterior and interior of the residual of kAlN−eMCNT50/CE composite were 18.0 and 15.0 wt % (2.4 and 0.4 wt %), respectively, which were higher than those of kAlN50/CE, indicating that, compared with the kAlN particles in the kAlN50/CE composite, the kAlN particles in the kAlN−eMCNT50/CE composite had a greater possibility to accumulate on the surface of the composite during burning, thus forming a continuous protective barrier on the surface of kAlN−eMCNT/CE composite. This interesting result can be attributed to the properties of carbon nanotubes and the good interaction between the kAlN particles and eMCNTs. Specifically, previous research demonstrated that carbon nanotubes migrate toward the flaming surface and thus cover and shield the underlying polymer during the burning process,50 so the kAlN particles were driven toward the surface of the composite owing to the good interaction between eMCNTs and AlN particles. The preceding analyses were supported by following visual evidence. Figure 14 presents SEM micrographs of the surfaces of residual chars of CE resin and kAlN−eMCNT50/CE composite. The surface of residual char from the CE resin had many crevasses and holes, whereas that of the kAlN− eMCNT50/CE composite was compact.

Figure 13. TG and DTG curves of cured CE resin and binary and ternary composites in an air atmosphere. 3350

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the insulating and conductive phases have a greater difference in electrical properties.55 Because the difference in electrical properties between kAlN particles and CE resin is greater than that between eMCNTs and CE resin, the decreased number of interfaces between eMCNT and CE resin induced by the addition of kAlN particles into eMCNT/CE tended to decrease the interfacial polarization effect, so that the resultant kAlN− eMCNT/CE composites had lower dielectric constants than the eMCNT2.5/CE composite. On the other hand, because kAlN particles have greater dielectric constants than CE resin, both kAlN−eMCNT/CE and kAlN/CE composites were stable and gradually increased as the content of kAlN particles increased. Figure 16 shows the dependence of dielectric loss on frequency for the kAlN/CE, eMCNT2.5/CE, and kAlN−

Figure 14. SEM micrographs of the surfaces of the residual chars of cured (a) CE resin and (b) kAlN−eMCNT50/CE composite.

3.5.3. Dielectric Properties. For signal propagation, a material should have a desirable dielectric constant and extremely low loss to obtain fast and effective propagation.51 CE resin is known for its outstanding dielectric properties that endow CE resin with great potential in microelectronics applications.52 Therefore, it is interesting to evaluate the dielectric properties of any CE resin and related materials. Figure 15 presents the frequency dependence of the dielectric constants of the kAlN/CE, eMCNT2.5/CE, and

Figure 16. Dependence of dielectric loss on frequency for binary and ternary composites at room temperature.

eMCNT/CE composites. The eMCNT2.5/CE composite exhibited a high dielectric loss that was closely dependent on the frequency; this is a typical curve of a composite based on an electrical conductor.56 In contrast, the kAlN/CE and kAlN− eMCNT/CE composites had curves similar to that of CE resin. AlN is an insulator, which has a very low and stable dielectric loss (∼0.002), so the kAlN/CE composite had a very low dielectric loss. In general, the parameters that affect the dielectric constant also play role in dielectric loss. The dielectric loss of a conductor/polymer composite mainly consists of the loss of electrical conduction and the loss of interfacial polarization,57 so the dispersion of the conductors and the magnitude of the interfacial polarity are two key factors for determining the dielectric loss. As mentioned previously, the addition of kAlN particles to the eMCNT2.5/CE composite caused two changes: improving the dispersion of eMCNTs in the CE resin and reducing the interfacial polarity. The former led to a decreased loss of electrical conduction, and the latter resulted in decreased interfacial polarization. Therefore, the resultant kAlN− eMCNT/CE composites had similarly low dielectric loss as the kAlN/CE composites. These data are very attractive for actual applications.

Figure 15. Dependence of dielectric constant on frequency for binary and ternary composites at room temperature.

kAlN−eMCNT/CE composites at room temperature. Over the whole frequency, CE resin had a stable dielectric constant with the lowest value, whereas eMCNT2.5/CE had the highest dielectric constant, which was highly dependent on the frequency because MCNTs are electrical conductors. Interestingly, the kAlN−eMCNT/CE composites had stable dielectric constants that were much lower than that of the eMCNT2.5/ CE composite. The dielectric constant of the former gradually increased as the content of kAlN particles increased; however, even when the total content of hybrid fillers was as high as 50 wt %, the dielectric constant of kAlN−eMCNT50/CE was only 4.9 (at 10 Hz), which fits the requirement of high-performance copper-clad circuits.53 These attractive data can be attributed to the structure of the kAlN−eMCNT/CE composites. It is known that the dielectric properties of an electrical conductor/polymer composite are mainly determined by the dispersion of electrical conductors and the interface polarity.54 For the eMCNT2.5/CE composite, there were two phases, so there was only one type of interface, namely, the interface between the eMCNTs and the CE resin. When kAlN particles were added to the eMCNT2.5/CE composite, an additional interface (the kAlN−eMCNT interface) appeared; however, the number of eMCNT/CE resin interfaces decreased. Generally, a greater interfacial polarization effect occurs when

4. CONCLUSIONS The chemical and morphological structures of ternary composites are closely related to the surface natures of the fillers. The addition of eMNCTs (but not MCNTs) can improve the dispersion of AlN or kAlN particles in CE resin and alcohol; both AlN and kAlN particles can improve the 3351

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dispersion of both MCNTs and eMCNTs in CE resin. Hybrid fillers consisting of kAlN particles and eMCNTs were found to have the highest synergistic effect. New thermal conductive composites with high flame retardancy and very low dielectric loss can be developed by controlling the interaction between fillers in composites.

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ASSOCIATED CONTENT

S Supporting Information *

Theoretical and measured thermal conductivities of different composites (Table S1). Mechanism of the interaction between AlN and KH550 (Figure S1). Reaction between aMCNT and epoxy resin (Figure S2). Raman spectra of MCNTs and eMCNTs (Figure S3). XRD patterns of MCNTs, aMCNTs, and eMCNTs (Figure S4). TEM images of MCNTs and eMCNTs (Figure S5). TG curves of MCNTs and eMCNTs (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 512 61875156. Fax: +86 512 65880089. E-mail: [email protected] (A. Gu), [email protected] (G. Liang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Science Foundation of China (21274104, 51173123), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Major Program of Natural Science Fundamental Research Project of Jiangsu Colleges and Universities (11KJA430001), and Suzhou Applied Basic Research Program (SYG201141) for financially supporting this project.

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