A simple and consecutive melt extrusion method to fabricate thermally

the extrusion plane, as a result, the anisotropic index and the thermal ... extrusion with powerful shear flow field is an outstanding method to fabri...
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Simple and Consecutive Melt Extrusion Method to Fabricate Thermally Conductive Composites with Highly Oriented Boron Nitrides Xiaomeng Zhang, Jiajia Zhang, Lichao Xia, Chunhai Li, Jianfeng Wang, Fang Xu, Xianlong Zhang, Hong Wu,* and Shaoyun Guo* The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: In the region of thermally conductive polymer composites, forcing anisotropic fillers into the highly oriented structure is the most effective method to improve thermal conductivity and mechanical properties simultaneously. However, up to now, such highly oriented structure was mainly achieved in low viscosity polymer matrix or solutions. For the purpose of expanding the range of applications, in the present work, a new strategy, the consecutive and powerful shear flow field, was applied to introduce highly oriented boron nitride (BN) into high viscosity polymer matrix. Results indicated that BN was almost totally oriented along the extrusion plane; as a result, the anisotropic index and thermal conductivity of the composite filled with 40 wt % BN reached as high as 480% and 3.57 W/(m K), respectively. Furthermore, compared with the samples with randomly oriented BN, elongations at break were improved more than 50-fold at the same filler content. Finite element analysis was also applied to systematically investigate the effect of the orientation direction of BN on heat dissipation property of the composites, and results indicated that orienting the longitudinal direction of BN parallel to the heat source is the best way to reduce the heat source temperature to a low level. Therefore, the simple, consecutive, and environmentally friendly melt extrusion with powerful shear flow field is an outstanding method to fabricate high efficiency thermally conductive composites, and the simulative results also have important significance on designing such composites for different applications. KEYWORDS: thermal conductivity, highly oriented structure, boron nitride, melt extrusion, finite element analysis

1. INTRODUCTION With the rapid development and down-scaling of electronic devices, power densities and hot-spot temperatures are increased sharply.1−6 Meanwhile, because high electrical conductivity can result in electrical leakage and shortage, therefore, for safety and stability, the unwanted heat is mainly dissipated by thermally conductive yet electrically insulating composites.7−9 Among them, polymer-based composites are widely used due to their low cost, light weight, good processing ability, and recyclability.10 The conventional methods to fabricate such composites are introducing thermally conductive and electrically insulating fillers into the polymer matrix, such as boron nitride (BN), aluminum nitride (AlN), alumina (Al2O3), silicon carbide (SiC), etc.11−19 Among these fillers, BN is the most effective one in enhancing the thermally conductive property of the polymer matrix due to its much higher thermal conductivity along the in-plane direction and peculiar 2D structure, which makes them easier to form the network of fillers.20−23 However, in reality, to obtain the excellent heat dissipation performance to meet the increasing requirements, a large amount of BN © 2017 American Chemical Society

should be added, which has serious influence on mechanical properties of the final composites.24−26 The main reason for the low efficiency thermal conductivity of the final composite is that the potential of BN, especially the in-plane thermal conductivity, cannot be released completely with the random distribution.27 Therefore, forcing BN into the alignment structure has been treated as an effective route to improve the thermal conductivity with low filler loading. Among the existing methods, the most popular one is coating sensitive layers onto BN to make them respond to the external magnetic field. There is no doubt that the magnetic alignment is a feasible and convenient method to regulate the orientation direction of BN through remote control; unfortunately, the surface modification of BN leads to environmental pollution and enormous energy consumption because a large amount of hazardous organic solvent is always needed.15,20,21,28 Mechanical stretching is Received: April 26, 2017 Accepted: June 15, 2017 Published: June 15, 2017 22977

DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of morphology evolutions of BN in the LME during the multistage stretching extrusion.

stretching extrusion. Orientation degrees of BN, anisotropic thermal conductivity, anisotropic index, and mechanical properties of the final composites were investigated systematically. Furthermore, thermally conductive simulation based on finite element analysis was applied to give a deep insight into the effect of the orientation direction of BN on heat dissipation properties of the composites.

another good method to reorient BN in polymer matrix, and the anisotropic thermal conductivity can be improved significantly, while the most urgent problem of this method is the low efficiency for mass production.29 Flow-induced alignment is the simple method to massively produce thermally conductive and electrically insulating composites with highly oriented BN. However, low viscosity polymer matrix or solutions are highly required in this method. Furthermore, such method was mainly applied to fabricate film materials due to the sharply decreased shear force with increasing thickness, which seriously restricts their applications.30,31 Therefore, a simple, consecutive, and environmentally friendly method which can fabricate high viscosity polymer-based thermally conductive composites with highly oriented BN structure with adjustable thickness is urgently demanded in this region. To achieve this goal, in the present work, a simple melt extrusion method with a consecutive and powerful shear flow field is applied to force BN to align along the extrusion direction. The mechanism is that there is a series of laminating multiplying elements (LMEs) equipped on a conventional extruder, which is named as the multistage stretching extrusion (MSE). In each LME, polymer melts are sliced into two left and right parts by a divider and then flow through two up and down thinner and wider channels, respectively. At last, they recombine vertically, as shown in Figure 1a. Notably, highly oriented structure can be obtained after the melt flows through a series of thinner and wider channels due to the effect of biaxial stretching, as shown in Figure 1b. At the same time, the dispersion state of fillers will also be improved significantly, which has already been proved by our previous work.32 Furthermore, it should be mentioned that the thickness and width of the extruded samples could be adjusted by changing the size of the die. In this study, high density polyethylene (HDPE) with rather high viscosity (1 g/10 min) was applied to fabricate the high efficiency thermally conductive and electrically insulating composites with highly oriented BN through multistage

2. EXPERIMENTAL SECTION 2.1. Materials. HDPE (5000s) with a melt index of 1.0 g/10 min (measured at 190 °C and 2.16 kg) was obtained commercially from Lanzhou Petroleum Chemical (China). Hexagonal BN with an average particle size of 10−20 μm was purchased from Dandong Rijin Technology Co., Ltd. (China). All of these materials were used as received. 2.2. Specimen Preparation. First, HDPE and BN were dried in a vacuum oven at 80 °C for 12 h and then compounded together with different weight ratios of 95/5, 90/10, 80/20, 70/30, and 60/40 (HDPE/BN) in a corotating twin screw extruder. The temperatures from hopper to die of the extruder were maintained at 155, 185, 195, and 195 °C, and compounds were cut into small particles and dried again with the same conditions after the extrusion. Next, these particles were extruded by the multistage stretching extrusion equipped with 8 LMEs to fabricate sheet samples for characterization. The temperatures from hopper to the last LME were set at 155, 180, 195, 195, 200, and 200 °C. It should be noted that these temperatures or other processing conditions could be tuned for different devices or composites, and the thickness and width of the samples were 2 and 80 mm, respectively. The contrastive samples with the thickness of 2 mm were prepared by compression molding at 15 MPa and 190 °C using the same particles obtained by the corotating twin screw extruder. 2.3. Morphology Characterization. A scanning electron microscope (SEM, JEOL JSM-5900LV) was used to observe the morphologies of samples, especially the dispersion state and the orientation of fillers. All of the specimens were quenched in liquid nitrogen for over 8 h; then, the multistage stretching extruded samples were cryogenically fractured along two directions, parallel and vertical to the melt flow direction. Compression molded samples were cryogenically fractured along one direction because they are isotropic. 22978

DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984

Research Article

ACS Applied Materials & Interfaces An accelerating voltage of 20 kV was used during the observation, and the fractured surfaces of samples were coated with a thin layer of gold by ion sputtering prior to visualization. To evaluate the orientation degrees of BN in composites accurately, wide angle x-ray diffraction (WAXD, DX2500, Liuzhou Zinc Products Co., Ltd., China) was used with a reflection mode. The instrument was run at a scanning rate of 0.06°/s ranging from 20 to 60°. 2.4. Thermal Conductivity Measurement. Thermal diffusivities (α) along two different directions and specific heat capacity (Cp) of the multistage stretching extruded composites were measured by a Netzsch LFA467 Light Flash Apparatus at 25 °C, and thermal conductivities (λ) were calculated by λ = αCpρ, where ρ is the density of the composite. At least three samples were tested for each composition, and all tested samples were cut from the center of the extruded sheets. 2.5. Thermally Conductive Simulation. Finite element analysis (ANSYS) was used to investigate the heat dissipation process of the models with different oriented structure of BN, and the content of BN was about 10 wt % (low), 30 wt % (middle), and 53 wt % (high) in different models. In addition, uncertainties in the simulation were lower than 0.05 K. 2.6. Mechanical Properties. A characterization of tensile mode was used to evaluate the mechanical properties of the composites, and measurements were performed on a testing machine (SANS CMT4104) at a strain rate of 20 mm/min in accordance with ASTM D638. All of the specimens for tests were cut from the center of the sheets along the extrusion direction into the standard dumbbell shape, and there were at least five specimens in each group. Finally, tensile strengths and elongations at break were obtained directly after the measurement.

3. RESULTS AND DISCUSSION 3.1. Morphologies of BN in Different Composites. SEM was used to observe the oriented structure and the dispersion state of BN, as shown in Figure 2. On the basis of the views along the parallel and vertical directions, it can be clearly seen that in the multistage stretching extruded systems, BN platelets were aligned along the extruded plane (Z plane in Figure 2) completely, rather than oriented along a single line, which was difficult to obtain through magnetic assisted method or other means.20,21 In contrast, in compression molded systems, BN platelets were oriented randomly. Therefore, these results strongly proved that the multistage stretching extrusion with a consecutive, powerful shear flow field and the special biaxial stretching effect was an effective method to introduce highly oriented BN into high viscosity polymer matrix. To further verify that BN was almost totally oriented along the melt extrusion plane (Z plane in Figure 2), XRD analysis was applied. As is well-known, BN is a typically anisotropic lamellate filler; the parallel and vertically oriented BN correspond to the (002) peak and (100) peak, respectively. According to previous reports, the ratio between the intensity (I) of the (002) peak and the (100) peak could represent the orientation degrees of BN.20,21,28 The bigger the ratio is, the higher orientation degree BN has. As shown in Figure 3, along the extrusion direction, the intensities of (002) peaks of the multistage stretching extruded samples were much higher than those of compression molded ones. However, due to the high aspect ratio of BN, the intensities of (100) peaks were not obvious; thus, magnified images ranging from 40 to 42° were obtained and are shown in Figures 3c and f. As expected, the I(100) values of compression molded samples were over four times larger than those of the multistage stretching extruded ones. Then, to give a detailed and clear comparison, the statistical data were calculated and are listed in Table 1. It can be clearly seen that ratios between I(002) and I(100) of the

Figure 2. SEM images of the compression molded (CM) samples (k− o) and MSE samples along the parallel (a−e) and vertical directions (f−j). (PS: red bars represent the orientation direction of the adjacent BN).

multistage stretching extruded samples along the extrusion direction were over 24 times larger than those of compression molded samples, which meant that the oriented structure of BN became much better through the multistage stretching extrusion. At last, to further verify that the BN in multistage stretching extrusion system were nearly completely oriented, XRD analysis along the thickness direction was also characterized, as shown in Figures 3g−i. It can be seen clearly that I(100) was much larger than I(002) along this direction, and I(002)/I(100) was only 0.1, which was over 10 000 times lower than that along the extrusion direction. Therefore, combining the results of SEM and XRD, it can be concluded that BN platelets were almost oriented completely in high viscosity polymer matrix with the assistance of the consecutive and powerful shear flow field. 3.2. Anisotropic Thermal Conductivities of the Composites. To evaluate the effect of the highly oriented structure of BN on thermally conductive properties of the multistage stretching extruded samples, thermal conductivities along the in-plane and through-plane directions were 22979

DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984

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Figure 3. XRD patterns of the MSE and CM samples along the in-plane direction (a−f) and the thickness direction (g−i) with various BN content.

Table 1. Ratios between I(002) and I(100) of the Multistage Stretching Extruded and Compression Molded Samples extrusion direction mass fraction (%) 5 10 20 30 40

RMSE I(002)/I(100) (A) 1628 2020 1579 1256 2250

(±3) (±3) (±2) (±2) (±5)

thickness direction

RCM I(002)/I(100)

RMSE/RCM

RMSE I(002)/I(100) (B)

RMSE/RCM

A/B

25 (±0.5) 36 (±1.1) 22 (±0.3) 39 (±1) 94 (±2)

65 56 71 32 24

0.11 (±0.05) 0.15 (±0.1) 0.1 (±0.06) 0.12 (±0.04) 0.16 (±0.1)

0.004 0.003 0.001 0.004 0.002

14 800 12 652 15 790 10 467 14 063

characterized. For comparison, thermal conductivities of the compression molded samples with the same filler content were also measured. As shown in Figure 4, thermal conductivities along the through-plane direction were increased slightly, while the in-plane thermal conductivities were enhanced sharply with the increase in BN, and it could reach as high as 3.57 W/(m K) when the content of BN was 40 wt %, which was 2 times larger than that of the isotropic sample. Furthermore, compared with isotropic samples, it also can be clearly seen that to achieve the same thermal conductivity, the filler loading could be reduced on average about 10 wt % with the assistance of the highly oriented structure. Therefore, to take full advantage of the inplane thermal conductivity of BN by orienting them along one direction is an effective method to sharply improve the final thermal conductivity of the composites. In addition, compared with those of other nonfilm composites reported in previous works, thermally conductive properties of the multistage stretching extruded samples were also much better, as shown in Table S1.

Then, thermally conductive enhancement along parallel (α) and vertical (β) directions and anisotropic index (AI) based on the following equations, where λin‑plane, λthrough‑plane, and λisotropy correspond to thermal conductivities of the multistage stretching extruded samples along in-plane direction, throughplane direction, and isotropic samples respectively, were obtained to evaluate the effect of the oriented structure of BN on final thermal conductivities.33,34 α=

β=

AI = 22980

(λ in ‐ plane − λ isotropy ) λ isotropy

(1)

(λthough ‐ plane − λ isotropy ) λ isotropy

(2)

(λ in ‐ plane − λthrough ‐ plane) λthrough ‐ plane

(3) DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984

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Figure 6. Thermally conductive models with differently oriented fillers.

Transient thermally conductive simulation based on the following equation, where T, T0, τ, CP, ρ, and λ represent the final temperature, initial temperature, time, specific heat capacity, density, and thermal conductivity of the node, respectively, was applied to calculate the heat dissipation process of different models, and results are shown in Figure 7.

Figure 4. Thermal conductivities of the compression molded samples and the multistage stretching extruded samples with different content of BN along the in-plane and through-plane directions.

From Figure 5, it can be seen clearly that the anisotropic index increased monotonously, and the maximum was over 480%. Therefore, it can also support the viewpoint that the multistage stretching extrusion is a simple and effective method to prepare high efficiency thermally conductive composites with highly oriented filler structures, which could have particular applications in various fields.34 3.3. Thermally Conductive Simulation of the Composites. As is well-known, BN or other anisotropic thermally conductive fillers could be aligned along different directions through many effective methods. However, which oriented direction of the anisotropic filler is the most effective one on reducing the heat source temperature is still unclear due to the limit of technology. In this part, three ideal models with differently oriented fillers were created to solve this problem, and the size of the model is the same with the tested sample, as shown in Figure 6. Heat sources with the same initial temperature were applied at the bottom of each model, and major parameters used in this part are shown in Table S2 (in the Supporting Information). Then, thermally conductive properties of all these models were investigated systematically through the finite element analysis method. In addition, it should be mentioned that, because the model is symmetrical, the spatial model can be simplified to a 2D model.

Figure 7. Time dependence of heat source temperature for models with differently oriented fillers.

It can be clearly seen that the data should be divided into two parts. Within 0.9 s, the model with vertically oriented BN had the highest efficiency in reducing the heat source temperature;

Figure 5. (a) Thermally conductive enhancement and (b) anisotropic index of the multistage stretching extruded samples. 22981

DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984

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Figure 8. Sum of heat flux of different models at (a) 0.1, (b) 0.9, and (c) 3 s.

unchanged with the variation of temperature, the temperature gradient in the system was the main factor. Therefore, to make this mechanism clear, temperature distribution contours of different models at 0.1 and 0.9 s were also obtained to make an in-depth analysis on the change regularity of the heat flux, as shown in Figures S2 and S3 (in the Supporting Information). Compared with the results of 0.1 s, when the time was 0.9 s, the temperature gradient of models along the parallel direction had very small differences. However, along the vertical direction, the temperature gradient of the model with vertically oriented BN decreased sharply, which would lead to the enormous reduction of the heat flux; as a result, the efficiency on reducing the heat source temperature became much lower. As time increases to 3 s, temperature gradients along the vertical direction of the models with randomly and vertically oriented BN were almost nonexistent (Figure S4 in the Supporting Information); thus, the heat dissipation mainly depended on the in-plane thermal conduction. As a result, for the model with parallel oriented BN, the heat flux was much larger, and the heat source temperature was always lower than those of the other two models due to the much higher in-plane thermal conductivity. Furthermore, to obtain the universal conclusion, effects of filler content and size of the heat source on the thermally conductive property of the composites were investigated systematically using the same method, as shown in Figures S5−S11 (in the Supporting Information). From all of these results, it can be clearly seen that regardless of filler content and heat source size, the trend was same, that in the anisotropic models with bigger length, the vertically oriented BN was always more efficient in dissipating heat in a short time.

after 0.9 s, the model with parallel oriented BN was more effective. ∂(T − T0) ⎛ λ ⎞⎛ ∂(T − T0) ⎞ ⎟⎟⎜⎜ ⎟⎟ = ⎜⎜ ∂τ ⎝ ρ × Cp ⎠⎝ ∂(x , y) ⎠

(4)

To make the mechanism of this phenomenon clear, at 0.1, 0.9, and 3 s, the sum of heat flux, which represents the power of thermal conduction, was calculated based on eqs 5−7. As shown in Figure 8a, at 0.1 s, the sum of heat flux of the model with vertically oriented BN was much larger than those of the other two systems; thus, the temperature of the heat source was reduced quickly. However, when the time increased to 0.9 s, the sum of heat flux of the model with vertically oriented BN decreased sharply and was lower than those of the other two systems, as shown in Figure 8b. As time increased to 3 s, such phenomenon became much more significant, as shown in Figure 8c. qxi = −λ

∂T ∂x

(5)

qyi = −λ

∂T ∂y

(6)

n

qtotal =

∑ (qxi + qyi) 1

(7)

According to the Fourier law (eq 5), heat flux was determined by thermal conductivity and temperature gradient. Because thermal conductivities of HDPE and BN were 22982

DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984

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Figure 9. (a) Tensile strength and (b) elongation at break of the composites with different BN content.

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However, as time increased, the parallel oriented BN was more effective in further reducing the heat source temperature to a lower level, especially under the big heat source. Therefore, such conclusion is an important guidance to design different materials for various applications. 3.4. Mechanical Properties. At last, mechanical properties including yield strength and elongation at break were measured, as shown in Figure 9. Because the compression molded samples were prepared under 15 MPa for 10 min, thus they were denser than multistage stretching extruded samples, which made the yield strengths better at low filler loading. With further increasing the filler content to 20 wt %, the effect of filler orientation became more remarkable. As a result, yield strengths of the multistage stretching extruded samples were higher than those of compression molded ones.35,36 Furthermore, elongations at break of the multistage stretching extruded samples were always much larger. This phenomenon was mainly attributed to the fact that the BN in the multistage stretching extruded samples was uniformly dispersed and highly oriented, which could significantly eliminate defects in the final composites.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05866. XRD patterns of multistage stretching extruded samples with various filler loadings along the thickness direction (Figure S1); thermal conductivities along the orientation direction and AI in the region of thermally conductive and electrically insulating composites (focusing on nonfilm materials, thickness ≥1 mm) (Table S1); major parameters used in the simulation (Table S2); temperature distribution contours of the composites at different times (Figures S2−S4); effect of filler content on the thermally conductive property of the composites (Figures S5−S7); effect of heat source size on the thermally conductive property of the composites (Figures S8−S11) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.W.). *E-mail: [email protected] (S.G.).

4. CONCLUSIONS Highly oriented structure of BN was successfully introduced into high viscosity polymer matrix through a novel strategy: multistage stretching extrusion. In the resulted composites, BN was almost totally aligned along the extrusion plane, and the inplane thermal conductivity of the composite with 40 wt % BN could reach as high as 3.57 W/(m K), which was 2 times larger than that of the isotropic sample and 3.5 times bigger than that of the through-plane direction. Meanwhile, mechanical properties of final composites were also enhanced sharply. Furthermore, from the results of finite element analysis, it can be clearly seen that in the anisotropic model (the length was bigger than the thickness), vertically oriented BN had the highest efficiency in dissipating heat in a short time; however, parallel oriented BN played a more critical role in reducing the heat source temperature to a lower level. Therefore, all these results demonstrated that the multistage stretching extrusion is a potential route to fabricate high performance thermally conductive composites with high anisotropic thermal conductivities and balanced mechanical properties, which could shed light on the design and preparation of functional composites for electronic industries or other thermal management devices.

ORCID

Hong Wu: 0000-0001-9972-0873 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grants 51573118, 51227802, and 51421061), Program for New Century Excellent Talents in University (Grant NCET-13-0392), and the Sichuan Province Youth Science Fund (Grant 2015JQ0015) is gratefully acknowledged.

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ABBREVIATIONS HDPE, high density polyethylene BN, boron nitride MSE, multistage stretching extrusion LMEs, laminating multiplying elements CM, compression molding SEM, scanning electron microscopy DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b05866 ACS Appl. Mater. Interfaces 2017, 9, 22977−22984