Efficient Thermal Transport Highway Construction Within Epoxy Matrix via Hybrid Carbon Fibers and Alumina Particles.

Polymer composites with excellent thermal conductivity and superior mechanical strength are in high demand in the electrical engineering systems. However, achieving superior thermal conductivity and mechanical properties simultaneously at high loading of fillers will still be a challenging issue. In this work, a facile method was proposed to prepare the epoxy composite with carbon fibers (CFs) and alumina (Al2O3). This CF and Al2O3 hybrid structure can effectively reduce the interfacial thermal resistance between the matrix and the CFs. The thermal conductivity of epoxy composite with 6.4 wt % CFs and 74 wt % Al2O3 hybrid filler reaches 3.84 W/(m K), which is increasing by 2096% compared with that of pure epoxy. Meanwhile, the epoxy composite still retains outstanding thermal stability and mechanical performance at high filler loading. A cost-effective avenue to prepare highly thermally conductive and superior mechanical properties of polymer-based composites may enable some prospective application in advanced thermal management.

Introduction

Many significant electronic applications require effective heat dissipation, especially for electronic devices such as integrated circuits and light-emitting devices (LEDs), as overheating always causes rapid aging or even failure of the core chip.[1−4] Polymer-based thermal management material is widely accepted as a high-performance material in many fields for its easy processability, excellent chemical properties, and low cost, but its low thermal conductivity is still a major drawback and limits its applications.[5,6] Many methods have been developed to improve its performance. Traditionally, it is reported that a variety of thermally conductive fillers including metal nanoparticles (e.g., Ag),[7] metal oxides (e.g., Al2O3, MgO),[8,9] nitrides (e.g., AlN, BN),[10,11] and most recently graphene[12] and carbon nanotubes (CNTs)[13] can be usually included in the polymer matrix.

Among them, alumina (Al2O3) has received considerable attention due to its remarkable physical properties, such as high thermal conductivity (30 W/(m K)), abrasion resistance, corrosion resistance, thermal stability, and electrical insulation, as well as its low cost.[14] Thus, epoxy resin modified by Al2O3 filler-containing composites is considered as a promising method for improving thermal conductivity.[15] However, researchers are focused on adding above 80 wt % loading of Al2O3 filler to obtain high thermally conductive composites by the traditional blend method,[16,17] yet leading to significant degradation of mechanical strength and processability.[18] Therefore, the researchers are urged to find facile and novel approaches to prepare composites with both high thermal conductivity and excellent mechanical properties.

In recent years, two kinds of methods have been adopted to obtain superior thermal conductivity as well as excellent mechanical strength. In the first method, the functionalization of filler surface by surfactants and coupling agents[19−23] for reducing the interfacial thermal resistance between the thermally conductive filler and the polymer matrix.[24,25] Nevertheless, the effect of thermal conductivity enhancement (TCE) reported above is limited compared without functionalization.[26,27] The second is based on constructing a continuous filler architecture that can provide thermal transport channels. This network yield high thermal conductivity at a relatively low filler loading. For example, a compact packing structure of filler particles can be formed by combining different particle sizes of spherical Al2O3 at different mass ratios,[28] which slightly increases the thermal conductivity. The porous Al2O3 ceramic skeletons are constructed by a new processing technique including the gel casting, sintering, and vacuum infiltration methods.[29] This structure can dramatically improve the heat transport of the composites. But it should be noted that the entire fabrication process is very complicated and not suitable for industrial mass production.

Herein, an epoxy resin-based composite with thermal transport expressways is prepared by incorporating carbon fiber (CF) foams and Al2O3 particles. The CF foams are used as the premade template to connect the neighboring spherical Al2O3 particles, which provides the coherent thermal transport channels for CFs. A hybrid network structure composed of Al2O3 and CFs provides prolonged pathways similar to heat dissipation highways in epoxy composites and shows a much higher thermal conductivity (3.84 W/(m K)) at the loading of 74 wt % Al2O3 than that of pure epoxy (0.18 W/(m K)), which may find a potential application for thermal management of electronic devices. Furthermore, our composites present more outstanding thermal stability and excellent mechanical strength compared to pure epoxy.

Results and Discussion

The preparation process of the epoxy/CF/Al2O3 (ECA) composites is shown in Figure . First, the CF foams were mixed with Al2O3 aqueous solution and dried to obtain CF foam and Al2O3 particles (CA) hybrid fillers. The amount of Al2O3 in the CA mixture block could be controlled by adjusting the concentration of Al2O3 in the solution (as shown in Table S1). Then, the epoxy precursor was infiltrated into the CA mixture block and degassed under vacuum. Finally, the resulted mixture was precured and postcured to obtain the ECA composites with different Al2O3 ratios.

Figure 1

Preparation process of epoxy/CF/Al2O3 composites and the tailored works of the logotype of Ningbo Institute of Industrial Technology of Chinese Academy of Sciences (CNITECH) with samples.

As can be seen in Figure , the morphologies and chemical compositions of Al2O3, carbon fiber, epoxy resin, and the cross-sectional of our epoxy/CF composites with different ratios of Al2O3 were characterized by using scanning electron microscopy (SEM), ZETA, and Raman spectroscopy. The average diameter of Al2O3 particles is about 5 μm as shown in the inset of Figure a. Raman spectrum that exhibits two Raman shifts at 1354 cm–1 (D-band) and 1583 cm–1 (G-band) in the inset of Figure b demonstrates that the main component of the foam is carbon fiber[30] (the spectral range is scanning from 1000 to 3200 cm–1). The D-band intensity (ID) and the G-band intensity (IG) of carbon fiber correspond to the defect-inducing vibration mode and in-plane vibrations of graphite lattice, respectively.[31−33]Figure c shows the microstructure of pure epoxy after curing. Note that the fractured paths of river line patterns at the fracture surface indicate a brittle fracture. Figure d–i present the cross-sectional SEM image morphologies of ECA composites with different ratios of Al2O3. From Figure d, we can see the carbon fiber at the epoxy resin matrix interface. The SEM images shown in Figure e–i reveal that the Al2O3 particles and CFs have good contact with epoxy resin and this microstructure can increase the strength of ECA composites greatly. As the Al2O3 content increasing, more Al2O3 balls appeared on the fracture surface. Figure j shows the SEM image of the composite with 74 wt % Al2O3 along with the corresponding energy-dispersive spectrometry (EDS) elemental mappings of carbon (C), oxygen (O) and aluminum (Al). It further illustrates that the Al2O3 balls are gradually and closely distributed in the ECA composites.

Figure 2

SEM images of (a) Al2O3, (b) carbon fiber, (c) epoxy resin. The cross-sectional SEM images of epoxy/CF composites with different ratio with (d) 0 wt %, (e) 12 wt %, (f) 29 wt %, (g) 44 wt %, (h) 58 wt %, (i) 74 wt % Al2O3 and its corresponding, (j) EDS elemental mappings of C, O, and Al.

The thermal conductivity of the polymer composites is often determined by factors such as the interaction between thermal filler and matrix, the loading of filler, the structure and properties of filler and matrix, and so on.[16] The thermal conductivity of the ECA composites was evaluated by using a laser flash method. Figure a shows the difference in thermal diffusivity (TD) and thermal conductivity (TC) of epoxy/CF/Al2O3 composites. The TC of pure epoxy resin is only 0.18 W/(m K) at 25 °C. After adding CFs, the TC was significantly improved up to 1.48 W/(m K). In particular, when Al2O3 was added in epoxy/CF composites, the TC of the ECA composites was improved even higher and increased monotonically with the incorporation of the Al2O3 fillers. As the increase in the loading levels of Al2O3 from 12 to 74 wt %, the TC increased almost linearly from 1.97 to 3.84 W/(m K). Similarly, the curve of the TD of composites also shows the same change trend. Herein, the thermal conductivity enhancement (TCE) is defined as (λ – λm)/λ, where λ and λm are the thermal conductivity of composites with addition to fillers and the thermal conductivity of pure epoxy, respectively.[34]Figure b compares the TC and TCE of the epoxy/CF composite, the ECA composite with 74 wt % Al2O3 filler loading (ECA-74) and the pure epoxy at 25 °C. These values were more than 7 and 20 times of the pure resin, respectively. Furthermore, it is remarkable that the TC of composites prepared with two kinds of fillers, carbon fiber, and Al2O3, presented more improvement than that prepared with the individual filler, carbon fiber. The results may be attributed to three reasons. First, even a very small loading of carbon-based thermal fillers can dramatically increase the TC of composites because of their high intrinsic TC.[16,35] Thermal transport through epoxy/CF composites is dominated by thermal resistance at the interface.[36−38] The resistance results principally from interfacial defects and from phonon scattering caused by phonon spectra mismatch in two phases (fiber and matrix).[39] Second, at lower Al2O3 filler loading levels (<20 wt %), a slight increase of the TC was observed. Only at high Al2O3 loading levels, a more significant improvement can be observed. It is reported that the high filler content with above 60–70 vol % can lead to continuous thermal conduction paths of fillers in the polymer composite.[16,40] This implies that effectively thermal conductive pathways begin to form at high Al2O3 loading levels due to filler-to-filler connections, as observed in Figure e–i. Simultaneously, the CFs dispersed in epoxy, effectively connect with Al2O3 particles and form a dense thermal conductive network throughout the material. These connections reduce the chance of contact between CFs and the surrounding matrix, resulting in a decrease in their interfacial thermal resistance. As a result, an enhanced hybrid network composed of Al2O3 and CFs provides prolonged pathways similar to heat dissipation highways and shows a much higher thermal conductivity at the loading of 74 wt % Al2O3 than that of pure epoxy and epoxy/CF composites. Figure c presents the variation in TC of pure epoxy, epoxy composites at room temperature, and other possible operating ambient temperatures. The TC curves of pure epoxy, epoxy composites revealed a trend of monotonically increasing with temperature over the temperature range investigated. It is aligned with the general trend for some highly disordered dielectric materials. The increase in TC with the temperature, indicate that the temperature improves the interfacial bonding between the fillers and the polymer matrix. Figure d shows the variation in TC of pure epoxy. The EC composite and ECA-74 composite upon multiple heating and cooling cycles alternating in the range of 25 and 100 °C. The trend of TC of pure epoxy, epoxy composites with temperature is consistent with Figure c. Both EC composite and ECA composite maintained their stable and original thermal conductivities during the first 10 cycles, demonstrating stable heat dissipation capacity after multiple treatments.

Figure 3

(a) Thermal diffusivity and thermal conductivity of epoxy/CF/Al2O3 composites; (b) TCE curves of the pure epoxy, epoxy/CF, and epoxy/CF/Al2O3 composites; (c) thermal conductivity of composites as a function of test temperature; (d) thermal conductivity of heating/cooling cycles alternating between 25 and 100 °C.

To intuitively display the heat dissipation performance of epoxy composites, an infrared camera was used to record the surface temperature distribution and variation, which can characterize their heat dissipation performances. The samples were investigated by placing on a ceramic heating plate to study their thermal behavior. We used the electrified heating plate as the heat source and fixed the samples on the plate with conductive tape. From the infrared images in Figure a, the surface temperature of each sample was measured and the surface color became brighter as the temperature increasing, which means the surface temperature of the sample increases. In particular, the surface heating rate of the ECA-74 composite was faster than that of pure epoxy and EC composite. Quantitatively, the surface temperature of pure epoxy, EC composite, and ECA-74 composite were measured as 90, 94, and 102 °C, respectively, when the temperature rose to 115 °C within 240 s. These results demonstrate that the ECA-74 composite exhibits a better heating dissipation performance. Similarly, according to the data measured by the infrared camera, the curves of surface temperature with time during the heating and cooling process were made, as shown in Figure b. It is obvious that the pure epoxy, EC composite, and ECA-74 composite reached their highest temperature at 93, 97, and 104 °C, respectively. Besides, ECA-74 presented much faster heating and cooling rates compared with other samples. In summary, the ECA-74 composite shows better heating and cooling behaviors than the pure epoxy and the EC composite. As is evident in Figure c, an experiment was conducted

Figure 4

(a) Infrared images of pure epoxy resin, epoxy/CF, and epoxy/CF/Al2O3 composites; (b) surface temperature variation with time upon heating and cooling event; (c) CuSO4 solution variation with time on pure epoxy and epoxy/CF/Al2O3 composites.

to demonstrate the high thermal conductivity of the ECA-74 composite. Both the pure epoxy sample and ECA-74 sample in the same size (10 × 10 × 1.5 mm3) were put on the heater and the same size of CuSO4 saturated solution was dropped in the sample center by the same pipette gun. As the temperature of heater increasing, the solution placed on the ECA-74 sample quickly changed from blue droplet to white powder, while that placed on the epoxy sample only became slightly smaller. It shows that the surface temperature of ECA-74 increases sharply compared with pure epoxy resin and ECA-74 exhibits high thermal conductivity along with the heat dissipation performance.

In addition to thermal conductivity, thermal stability is one of the most important properties of investigation for the epoxy composites. Figure a,b shows the thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) profiles of pure epoxy resin and its composites as a function of temperature under air atmosphere at a heating rate of 10 °C/min, respectively. For pure epoxy, there was a double-step process for thermally degrading in the air as shown in the black curve in Figure a. It is believed that the main loss with a maximum rate in the first stage is due to the decomposition of the epoxy network structure at 370 °C while leaving tiny amounts of char residue that is further degraded by thermal oxidation at around 420–540 °C in the second stage.[41,42] The thermal decomposition curves of epoxy-based composites were one step more than that of pure epoxy. It is attributed to the oxidative decomposition reaction of carbon fibers at around 540–900 °C.[43] Note that all the samples exhibited similar thermal behavior except for the additional step of carbon fiber decomposition reaction, indicating that the presence of fillers do not significantly change the degradation mechanism of the epoxy matrix. With the increase in Al2O3 filler content, the second step decomposition became less obvious, which is attributed to the reduction of epoxy resin content. The temperature for 10% weight loss (T10%) and the maximum decomposition (Tmax) are selected as the characteristic thermal parameters and the results are listed in Table S2. Quantitatively, the T10% of pure epoxy resin was measured as 331.4 °C, while the T10% of EC composites with 0, 12, 29, 44, 58, and 74 wt % Al2O3 filler were 318.7, 335.4, 331.4, 334.0, 341.7, and 358.9 °C, respectively. It is obvious that all the ECA composites exhibited a slight increase in T10% compared to pure epoxy. Considering the third stage is the carbon fibers decomposition stage in the air, the carbon residue in the third step degradation reaction stage (540–1000 °C) is defined as the char yield. As can be seen that the char yields of all the ECA composites were improved when compared with that of pure epoxy. Furthermore, it is observed from Figure b that the maximum decomposition temperature (Tmax) of the composites was also mildly increased after adding thermal filler, compared with that of epoxy resin in DTG curves. It is considered that the compact chars of the polymer matrix and hybrid thermal fillers will be formed during the process of thermal decomposition, which is beneficial for the enhancement of thermal stability for the composites.[44] Meanwhile, the structure of CFs and Al2O3 increases the crosslinking between the binding sites of polymer matrix and fillers, thus limiting the mobility of the polymer segments at the interface of the epoxy and the thermal motion of the polymer chains.[45] Moreover, the stress–strain curves of pure epoxy and its composites were measured to show their mechanical properties, as shown in Figure S1. The thermomechanical properties of ECA composites are shown in Figure c,d. The storage modulus, i.e., the amount of energy stored in the polymer, is associated with the stiffness along with the dampening capacity of the material.[46] All the composite showed a superior storage modulus (E′) than pure epoxy in the glass region, as shown in Figure c. Particularly, the storage modulus of the ECA-44 composite exhibited a significant increase (8580 MPa at 40 °C) in the glassy region, which was 246.5% larger than that of pure epoxy (2476 MPa), suggesting that the thermal fillers are uniformly dispersed and have powerful interfacial adhesion with polymer matrix.[47] The storage modulus fell with the increasing temperature, which indicates that the energy dissipation occurs in the transition from glass-like to rubber-like behavior. It is seen that the storage modulus of the composite materials appeared to be much larger than that of the pure epoxy within the rubber state. The storage modulus of the composite, for instance, was about 215% higher than pure epoxy at 100 °C when Al2O3 filler content was 74 wt %. In addition, the loss factor tan δ is defined as the ratio of loss modulus to storage modulus, and it is very susceptible to structural transition for the solid material.[48] The peak temperature of tan δ is selected as the glass transition temperature (Tg). It is observed from Figure d that the Tg values of the epoxy composites reached to higher temperatures compared with pure epoxy. These results can be attributed to the existing hybrid thermal fillers in the composite material, which will increase the obstruction of the segmental motion for the epoxy chains owing to the effects of interfacial entanglement and interaction.

Figure 5

(a) TGA curves and (b) DTG curves of the pure epoxy and epoxy/CF/Al2O3 composites. (c) Storage modulus and (d) loss factors of epoxy/CF/Al2O3 composites as a function of test temperature.

Conclusions

In summary, an effective heat transport highway structure was constructed by the CF-Al2O3 hybrid three-dimensional foams in epoxy matrix, which can provide coherent bridges composed of spherical Al2O3 particles and connect dispersed CFs in an epoxy resin matrix. The epoxy composite has high conductivity (3.84 W/(m K)) with 6.4 wt % CFs and 74 wt % Al2O3 hybrid filler, which is increasing by 2096% compared to that of pure epoxy, showing great potential for thermal management in electronic applications. In addition, the composite still retains outstanding thermal stability and mechanical performance at high filler loading. Our work reports a new and cost-effective method for preparing highly thermally conductive and robust heat-conducting polymer-based composites by designing effective thermal transport channels.

Experimental Section

Materials

Al2O3 powders with an average diameter of 5 μm were produced from Yaan Bestry Performance Materials Co., Ltd. (Sichuan, China). The Al2O3 particles were irregular in shape and close to a spherical shape. Carbon fiber foams were supplied by Vulcan New Material Technology Co., Ltd. (Hangzhou, China). A cycloaliphatic epoxy resin monomer (UVR 6105, 95%) was purchased from DOW Chemicals. Neodymium(III) acetylacetonate trihydrate (Nd(III) acac) (Shanghai D&B Biological Science and Technology Co., Ltd.) and methylhexahydrophthalic anhydride (MHHPA, Zhejiang Alpharm Chemtech Co., Ltd.) were used as the catalyst and curing agent. Deionized water was utilized in all processes of aqueous solution preparations and washing.

Preparation of Alumina Aqueous Solution

The Al2O3 powder was dispersed into the deionized water in this experiment. The mass fraction of alumina powder in deionized water was 10, 20, 40, 60, and 80%, respectively. The resulted solution was mixed successively at 2500 rpm for 3 min by vacuum deaeration mixer (TM-310T, Shenzhen Smida Intelligent Equipment Co., Ltd., China). High-speed stirring can mitigate the agglomeration of Al2O3 clusters and reduce the sedimentation of Al2O3 particles, which is essential to make alumina particles more easily dispersed into CF foams.

Preparation of Epoxy/CF/Al2O3 (ECA) Composites

The CF foams were further mixed with Al2O3 aqueous solution at 2500 rpm for 3 min, followed by drying at 100 °C for 12 h in the oven to obtain CF foam and Al2O3 particles (CA) hybrid fillers. Then, the desired amount of Nd(III) acac was added into epoxy monomer (UVR 6105) by a certain weight ratio (0.1/100), stirred and degassed adequately at 80 °C for 2 h in a three-necked flask. The uniform solution was cooling down to room temperature before use. The epoxy precursor was then obtained by homogeneously mixing the curing agent (MHHPA) and pretreated epoxy resin (95/100.1). Afterward, the precursor was infiltrated into the CA mixture block and degassed under vacuum for 24 h. Finally, the resulted mixture was precured and postcured at 135 °C for 2 h and 165 °C for 14 h, respectively to obtain the epoxy/CF/Al2O3 (ECA) composites.

Characterization

The microscopic morphologies of the raw materials and the fracture surface of the composites were obtained with a field-emission scanning electron microscopy instrument (FE-SEM, Quanta 250) at an accelerating voltage of 20 kV. Zetasizer Nano ZS (ZETA, Malvern Instruments Ltd., Malvern, U.K.) was adapted to estimate the micrometric sizes of Al2O3 particles. The heat capacity and the thermal diffusivity of the ECA composites were respectively measured through the differential scanning calorimetry (DSC 214) at 10 °C/min heating rate and the LFA 467 Hyper flash instrument (NETZSCH, Germany) at room temperature. The thermal conductivity (λ, W/(m K)) of composites was calculated by density (ρ, g/cm3), heat capacity (Cp, J/(g K)), and multiplying thermal diffusivity (α, mm2/s)The density (ρ) of samples was determined by using the Archimedes water (alcohol) displacement method. An infrared camera (Fluke, Ti400) was used to collect IR images and record the thermal conductive behavior. The thermal stabilities of pure epoxy resin and the composites were evaluated through the thermogravimetric analyzer (TG 209 F3 instrument, NETZSCH, Germany). Thermogravimetric analysis (TGA) was conducted under air in the range of 50–1000 °C at a heating rate of 10 °C/min. A DMA Q800 dynamic mechanical analyzer (TA Instruments) was used to carry out dynamic mechanical analysis (DMA) to study the thermomechanical properties of composites. During the measurement, the samples were tested by carrying out in the single cantilever mode in a single-frequency pattern at 1.00 Hz from 25 to 270 °C with a 5 °C/min heating rate.