Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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Low-Melting-Point Alloy Continuous Network Construction in a Polymer Matrix for Thermal Conductivity and Electromagnetic Shielding Enhancement Ping Zhang,†,‡,§ Xin Ding,†,§ Yanyan Wang,†,‡,§ Mengting Shu,†,‡,§ Yi Gong,†,§ Kang Zheng,†,§ Xingyou Tian,†,§ and Xian Zhang*,†,§
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†
Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230088, People’s Republic of China ‡ University of Science and Technology of China, Hefei 230026, People’s Republic of China § Key Laboratory of Photovolatic and Energy Conservation Materials, Chinese Academy of Sciences, Hefei 230088, People’s Republic of China S Supporting Information *
ABSTRACT: Electronic devices require effective thermal dissipation capacity to maintain the working temperature in a safe range. Meanwhile, electromagnetic interference shielding (EMI SE) performance of electronic packaging material becomes critically important due to the broad frequency band of electronic components in the era of 5G communication. To address the above-mentioned issue, a low-melting-point alloy (LMPA) was introduced to poly(vinylidene fluoride) (PVDF) matrix as a novel electronic packaging material. The obtained composite exhibits a continuous LMPA network structure partly wrapped by PVDF microspheres. When loading of LMPA reached 50%, the composite showed a thermal conductivity of 6.38 Wm−1K−1, which was nearly a 2774% increase compared to that of pure PVDF resin. Meanwhile, excellent EMI SE performance was also achieved synchronously, and the total EMI SE was −68.79 dB at 10 GHz. This study is promising to broaden the application of LMPA in the fields of thermal management and EMI SE. KEYWORDS: low-melting-point alloy (LMPA), continuous LMPA network structure, thermal conductivity, electromagnetic interference shielding (EMI SE), thermal management limitations of carbon nanomaterials to be the filler for thermal conductivity and EMI SE enhancement.18 First, phonons dominate heat conduction in carbon nanomaterial composites, and the phonon vibrational spectra mismatch between the carbon nanomaterials and polymer matrix could create a big thermal energy barrier.19,20 The interfacial thermal resistance was considered to be the main bottleneck of heat dissipation of polymer composites.21,22 Second, the dominating EMI SE mechanism of carbon nanomaterial composites is absorption. Carbon nanomaterial composites have excellent electrical conductivity, which is beneficial to the absorption of electromagnetic waves.16,23,24 The polymer is generally an electrical insulator, and many nanofillers are needed to improve the electrical conductivity of polymers.25 However, high loading of nanomaterials generally leads to dispersion difficulties and dense agglomerates in the polymer matrix.26
1. INTRODUCTION Nowadays, the fast development of modern technology inevitably brings serious challenges of keeping the working temperature of the electronic device in a safe zone.1−3 Meanwhile, internal and external electromagnetic interference shielding (EMI SE) of the electronic packaging materials draws intensive attention due to broadening of the working frequency of 5G communication.4−6 Construction of a continuous functional filler network in polymer composite materials is efficient to improve their heat dissipation and EMI SE performances.7−11 In the past decade, the emerging composites with continuous filler network structure have been widely used as either EMI SE materials or thermally conductive materials.12−15 However, only a few materials have been reported to achieve thermal conductivity and EMI shielding performance enhancement simultaneously. Usually, composites with a continuous filler network consist of mainly carbon materials such as carbon nanotubes.9,16,17 Although carbon nanomaterials have excellent theoretical thermal conductivity and good electrical conductivity, there are still several © XXXX American Chemical Society
Received: March 19, 2019 Accepted: June 26, 2019 Published: June 26, 2019 A
DOI: 10.1021/acsapm.9b00258 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Polymer Materials
Scheme 1. (a) Schematic Drawing of Preparation of the PVDF Microspheres and (b) Schematic Drawing of the Preparation of the PVDF/SnBi58 Composites
Agglomeration of fillers could lead to a discontinuous electrical conductive path. This would affect the improvement of the electrical conductivity of the polymer resin. Therefore, metal,27 alloy,28 and magnetic fillers29−31 have been introduced into the polymer matrix for further EMI SE effectiveness promotion. Recently, use of low-melting-point alloy (LMPA) has caused interest because of its unique low melting point, high thermal conductivity coefficient, excellent thermal stability, and high electrical conductivity.32−34 Schubert reported a new kind of variable stiffness composite using a combination of the lowmelting-point alloy embedded in poly(dimethylsiloxane).35 Hu et al. developed a metallic wood through metal continuously filling the wood vessels, and it had excellent electrical, thermal, and mechanical anisotropic performances.36 Zhang et al. used elemental sulfur to achieve nanodispersed liquid metals in bulk polymers for excellent process ability and recyclability.37 Although there are some reports of LMPA in flexible devices, microfluidic chips, and stretchable electronics, there are few investigations on its application in EMI SE materials and thermally conductive materials.38,39 It is worth noting that LMPA was chosen as the filler in this work for the following three reasons: (1) LMPA could be transformed from solid into liquid during the temperature range of the polymers processing;40 (2) similar to other metal-based composites, LMPA-based composites have electron dominated heat conduction;41 and (3) LMPA exhibits excellent electrical conductivity.42 In our work, we introduced LMPA into poly(vinylidene fluoride) (PVDF) resin to build a continuous LMPA network for the improvement of both thermal conduction and EMI SE performances. Herein, PVDF resin microspheres were prepared using water vapor induced phase separation (Scheme 1a).43 LMPA was mixed with PVDF microspheres in a certain ratio, and the mixture was hot pressed in the mold (Scheme 1b). The processing temperature was set higher than the melting point of the LMPA to retain mobility, enabling the structure of the LMPA continuous network to be partly wrapped by PVDF microspheres. The composites followed an electron dominating thermally conductive mechanism and showed great reflection on incident electromagnetic waves. Therefore, the simply fabricated composites exhibit excellent thermal conductivity and outstanding EMI SE performance.
2. EXPERIMENTAL SECTION 2.1. Materials. SnBi58 (T 25, melting point:142 °C) was purchased from Beijing Kangpu Xiwei Technology Co., Ltd. with size varying from 25 to 63 μm. N,N′-Dimethylformamidel (DMF) was provided by Sinopharm Chemical Reagent Co., Ltd. PVDF (FR901 type) pellets were supplied provided by Shanghai 3F New Materials Company. All pure materials were used without further processing. 2.2. Experiment. PVDF pellets (20 g) were dissolved in DMF (112 mL) by stirring in a 70 °C water bath for 10 h until a brown solution was obtained. The solution was then used to thinly cover a clean glass plate. The glass plate was quickly put into a standard curing box and maintained for 24 h with preset humidity and temperature. The PVDF microspheres were washed with deionized water and stored for future use after drying and grinding. PVDF microspheres were mixed with designated content of SnBi58 by mechanical stirring and hot pressing at 170 °C for 10 min under 15 MPa to acquire the PVDF/SnBi58 composite. The volume fraction of SnBi58 in PVDF was set as 10, 20, 30, 40, or 50%. 2.3. Characterization. The micromorphology of the PVDF/ SnBi58 composite was observed with the Sirion 200 field emission scanning electron microscope (FESEM). Cross sections of composites were coated with Au in a sputter coater, and the FESEM was operated at an accelerating voltage of 10 kV. The energy dispersive spectrometer (EDS) mapping images of the PVDF/SnBi58 composites were obtained with a Gemini SEM 500 instrument. X-ray diffraction (XRD) of the PVDF/SnBi58 composite was tested using the Rigaku Smartlab 9KW instrument with Cu Kα radiation. The radiation X-ray diffractometer was in the range of 10−80° at room temperature. The thermal conductivity of the samples was measured using the Hot Disk Thermal Conductivity Tester (TPS2200). The measurement for each sample was repeated three times. A thermal imager was used to study the thermal dissipation performance of the composite. The sample was heated using a hot plate and then cooled in air. The thermographic images of the sample were captured using an infrared camera. Temperature changes on the sample surface during the operation time were analyzed based on the thermographic image.44 A differential scanning calorimeter (DSC) DSC-Q2000 (TA, United States) was used to investigate the thermal performance. The tests were carried out in a nitrogen atmosphere, and the heating program was set as follows: the sample was first heated from 40 to 280 °C with a heating rate of 10 °C min−1, maintained at 280 °C for 3 min, and then cooled to 40 °C with a cooling rate of 5 °C min−1. The EMI SE performance was investigated through a vector network analyzer (VNA, Keysight, E5071C) equipped with two waveguide-to-coaxial adaptors connected face to face. Samples with 2 mm thickness were tested in the frequency from 8.2 to 12.5 GHz. EMI performance, including SE total, SE reflection, and SE absorption B
DOI: 10.1021/acsapm.9b00258 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Polymer Materials
Figure 1. (a) Picture of samples taken with an Apple 8 mobile phone and (b) XRD diagram of raw material and the PVDF/30SnBi58 composites.
Figure 2. (a and b) SEM images of PVDF and SnBi58, respectively, (c and d) cross-sectional SEM images of the PVDF/30SnBi58 composite showing the core−shell structure, (e) detailed image of the inner surface of the shell comprised of many PVDF microspheres, and (f) the outer surface of the shell which consisted of many concave holes.
were γ phase, indicating that LMPA has no effect on the crystalline structure of the PVDF matrix. The characteristic peaks of the composite at 27.2°, 37.9°, 39.7°, and 48.8° of Bi and 30.6°, 32.1°, and 44.8° of Sn corresponded to (012), (104), (110), and (202) and (200), (101), and (211), respectively, in the LMPA.46 In summary, the PVDF/SnBi58 composite was successfully fabricated, and SnBi58 had no effect on the crystal structure of the polymer matrix.47 The microstructures of raw materials are shown in Figure 2a and 2b. PVDF microspheres were approximately ∼3 μm with a rough surface; while LMPA spheres were ∼30 μm with a smooth surface. In the cross-sectional SEM image of the composite, LMPA was partly wrapped by PVDF microspheres. Interestingly, the smooth surface of PVDF microspheres and rough surface of LMPA were observed in the continuous LMPA network structure of the composite (Figure 2e and 2f). The LMPA sphere surfaces had evenly distributed small concave holes, and PVDF microspheres were embedded in these holes. At the temperatures of the hot pressing, PVDF softened and merged with each other, while PVDF microspheres were distributed around the periphery of the LMPA. The adsorbed PVDF made the LMPA surface rough. The microstructure was similar to the “core−shell structure”, and the matrix and the filler constructed a continuous LMPA network structure based on such repeating units (Figure 2c and 2d). This “shell” was formed by the interconnection of
(SETotal, SEA, and SER, respectively) was obtained from the scattering parameters (S11 and S21).5,9 The volume of electrical conductivity of the samples was tested using a digital conductivity meter (Sigma 2008B1) when the electrical conductivity was above 4000 S cm−1. A four-point probe electrical resistivity measurement system (model RST-8) was used to measure the electrical conductivity of samples between 1 × 10−6 and 4000 S cm−1. All of the tests were carried out five times at room temperature. Thermomechanical analysis (TMA 402 F1) was conducted at a heating rate of 10 °C min−1 and frequency of 1 Hz in a nitrogen atmosphere. The samples were cut into 10 × 10 × 1 mm size and each tested twice to eliminate thermal history, and the coefficient of thermal expansion (a) was evaluated with temperature from 30 to 130 °C.
3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of the PVDF/SnBi58 Composites. Compared to the resin matrix, the macroscopic image of the composite exhibited a metallic luster, as shown in Figure 1a. XRD analyses were used to study the crystalline structures of raw material and the composite, as shown in Figure 1b. The XRD curve analysis of LMPA showed that LMPA was a Sn and Bi eutectic alloy. The PVDF microspheres were γ phase semicrystalline polymers.26,45 The XRD pattern of the PVDF/SnBi58 composite showed refection peak at 20.2°, which coincides with the Bragg angles of (101) in γ phase PVDF. Both the composite and the pure PVDF microspheres C
DOI: 10.1021/acsapm.9b00258 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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Figure 3. (a) SEM image of the LMPA surface and the selected area where the EDS mapping was performed, (b and c) crystal structure images of Sn and Bi of LMPA in the composite, and (d−f) EDS mapping images of the LMPA surface: the Bi element is purple, and the Sn element is green.
Figure 4. (a) EMI SE (SETotal) of PVDF/SnBi58; (b) SETotal, SEA, and SER of samples as a function of LMPA loading at the frequency of 10 GHz; (c) electrical conductivity of PVDF/SnBi58 composites; (d) model of EMI SE mechanism of PVDF/SnBi58 composites.
softened PVDF microspheres, while the “core” was formed by recrystallization of LMPA fillers. We selected the surface of LMPA in a cross-sectional SEM image (Figure 3a) to study the recrystallization of the filler during the hot pressing process. Figures 3b and 3c show the regular crystal form arrangement of Sn and Bi in composites. Phase separation and recrystallization of Sn and Bi took place during the slow cooling of composites. Although Sn and Bi were separated, they were closely in contact with each other, as shown in the EDS mapping image (Figure 3d−3f).
3.2. EMI SE Performance of PVDF/SnBi58 Composites. The EMI SE performance of the composite was tested at different LMPA contents. The EMI SETotal performance of composites is shown in Figure 4a. SETotal of the composites increased quickly compared to pure PVDF while the LMPA was loaded. The composite had a SETotal of −38.57 dB at 10 GHz when LMPA loading reached 10%. When the content of LMPA was 50%, the SETotal of PVDF/SnBi58 was up to −68.79 dB at 10 GHz. Although the SETotal of PVDF/SnBi58 increased with LMPA loading, the SETotal of PVDF/SnBi58 reached D
DOI: 10.1021/acsapm.9b00258 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Polymer Materials
Figure 5. (a) Thermal conductivity values of PVDF/SnBi58 and the matching crowding factor model of PVDF/SnBi58 composites; (b) thermal transmission model of PVDF/SnBi58 composites.
composites obviously increases at 20% filler loading, as shown in Figure 5a. When SnBi58 loading reached 50%, PVDF/50% SnBi58 had a maximum thermal conductivity of 6.38 Wm−1K−1, which was 27.74 times higher than that of pure PVDF. The thermal conductivity of the composite showed a linear increase when the volume fraction was below 15% (Figure 5a). Above 15% volume fraction, the trend becomes superlinear (Figure 5a), showing the start of the thermal percolation transport.59 Interestingly, thermal and electrical conductivities show very different trends near 15% volume fraction (electrical percolation). The electrical conductivity showed variation of several orders of magnitude near the electrical percolation. However, the thermal conductivity changed very little compared to electrical conductivity at the thermal conductivity limit.48,59 This could be explained by the competition between the continuous thermally conductive path and the increased interface thermal resistance.19 Further, the polymer matrix was electrically insulating, and only the LMPA contributes to the electrical conductivity.60 Regarding the thermal property, there was no absolute thermal insulator, so the contribution from the polymer’s thermal transport could not be ignored. It is known that the energy was transported by both the energy coupling across the material interface in the composite and the electron−phonon coupling in the metal particle.61 When the particle size is on the order of the electron−phonon coupling length, electron−phonon coupling becomes a vital factor that determines the thermal conductivity of the composite. There have been some reported models that could be applied to simulate the thermal conductivity of metal and nonmetal composite systems such as the two-temperature model,62 Nielsen empirical model,63 crowding factor model,64 and so on. The two-temperature model is applied to calculate heat conduction only at low filler content. Nielsen proposed the empirical model by definition of the maximum volumetric packing fraction of fillers. The model could explain that composites had high thermal conductivity at high volume fraction of filler loading. However, the particle interactions were stronger at high filler loading, which was not taken into account in the Nielsen empirical model. Therefore, the crowding factor model was chosen to fit our experimental
saturation at 40% LMPA loading. Further, SEA and SER were obtained from the scattering parameters (Supporting Information). Figure 4b plots the SETotal, SEA, and SER of samples as a function of LMPA loading at the frequency of 10 GHz. SETotal and SEA both increased with the loading of LMPA; however, the contribution from SER was almost unchanged. As SEA and SETotal were far greater than SER, absorption had a dominant contribution to EMI SE in the X-band frequency region.48 As one of the important parameters to evaluate electromagnetic radiation absorption of the sample, excellent electrical conductivity is vital for outstanding EMI SE performance.49 Therefore, the electrical conductivity of composites was investigated, as shown in Figure 4c. All of the samples had greatly improved electrical conductivity compared to pure PVDF. The electrical conductivity of the composite was 108 S cm−1 when the LMPA loading reached 10%. When the volume fraction of LMPA was 50%, the electrical conductivity of the composite was 10 500 S cm−1. When the LMPA loading was further increased, the LMPA conductive network became stronger, which was favorable for increasing the electrical conductivity of PVDF/SnBi 58 composites. The good electrical conductivity would discourage the impedance matching.50−52 Impedance mismatch could enhance the reflection of electromagnetic waves, leading to EMI SE improvement. The continuous LMPA network structure attenuated the incident electromagnetic wave power effectively by multiple reflections due to the large interface area in the composite.53,54 As indicated in Figure 4d, the incident power could be scattered or reflected repeatedly at the interface of LMPA and PVDF. The incident microwaves had difficulty escaping from the composite system before being transferred to heat or absorbed.55 Therefore, the incident power can be attenuated effectively. These multiple reflections enhanced the absorption loss of electromagnetic waves.56−58 As shown in Table S3, Supporting Information, the composite has the potential to be used as an EMI SE material with excellent electrical conductivity. 3.3. Thermal Conductivity of PVDF/SnBi58 Composites. Samples’ thermal conductivity was measured and presented a nonlinear increase. The thermal conductivity of E
DOI: 10.1021/acsapm.9b00258 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Polymer Materials
Figure 6. (a) Curve of detected temperature as a function of time using the thermal imager; (b) infrared image of the detected temperature of the PVDF/SnBi58 composites.
between the composite’s thermal conductivity and specific heat capacity. It is worth noting that specific heat capacity of the composite was sharply increased in PVDF/30%SnBi58 (Table S1, Supporting Information). When the filler content was below 30%, the samples’ thermal conductivity dominated the temperature change. Composites could quickly transmit heat flux from the heat source to the surface of the sample. The surfaces of the composites emitted infrared radiation which was captured by the thermal imager. Thus, the composites had fast heating or cooling rates, as shown in the infrared image. When the filler content was up to 40%, the PVDF/SnBi58 composites simultaneously had high thermal conductivity and specific heat capacity. Composites could not only rapidly transmit heat flux but also timely store the heat energy. The captured infrared image showed the surface color of PVDF/ 50SnBi58 exhibiting the slowest change. Considering that the temperature was higher than the melting point of SnBi58, we further calculated the latent heat of the composites (Table S1, Supporting Information). There was also a mutation in PVDF/ 30%SnBi58 with latent heat. The high latent heat composites could undergo a phase change at the melting point to absorb extra heat without changing the temperature of the composite.67 Therefore, the composite can maintain a relatively stable heat dissipation performance and steady working temperature. 3.4. Thermal Expansion and Tensile Properties of PVDF/SnBi58 Composites. In the study, the thermal expansion coefficient (a) of the PVDF/SnBi58 composite decreased with the increasing amounts of fillers (Figure S3, Supporting Information). The thermal expansion coefficient of the pure PVDF was 254.96 × 10−6 K−1, while that of the PVDF/50% SnBi58 composite drastically decreased to 23.44 × 10−6 K−1 (Table S2, Supporting Information). For the composites, the coefficient of thermal expansion was dependent on fillers, the polymer matrix, and the interactions between them.68 Therefore, the quick decrease in the coefficient of thermal expansion of PVDF/SnBi58 composites should originate from the core−shell structure, providing a good binding of PVDF and LMPA in the composites. As the temperature increases, the LMPA can hinder the mobility of the loose molecular bonds in the polymer chains. As shown in Table S4, Supporting Information, the tensile modulus of elasticity of the composites was greatly increased compared to that of the PVDF matrix. Moreover, the ductility and intensity
data, and it took the particle interactions into account (Supporting Information). The experimental data matched well with the exponential function (Figure 5a). The electron− phonon coupling length (d, Supporting Information) was about 50 nm at room temperature, which was much lower than the sizes of LMPA. The influence of electron−phonon coupling on the thermal conductivity of PVDF/SnBi58 composites was enhanced and could counteract the high interfacial thermal resistance due to particle interactions at high-volume fractions of LMPA.65 Here, the transport diagram of the heat flow was given in Figure 5b. Above the thermal percolation limit, heat flow transmitted by the phonons in the matrix could be ignored. Heat flow was transferred quickly along the continuous LMPA network structure by a hot carrier in composites. Although the LMPA’s intrinsic thermal conductivity was smaller than those of carbon nanomaterials or ceramic material, the PVDF/SnBi58 composite had better thermal conductivity compared to BN or graphene fillers in Table S3, Supporting Information. The high thermal conductivity of the PVDF/SnBi58 composite was attributed to not only higher loading of LMPA but also the continuous LMPA network structure. More importantly, the lower melting point of the fillers showed the excellent fluidity during hot pressing, helping the formation of the continuous LMPA network structure in the composite. Thermal imaging was executed to study the thermal behavior of the composite in a real working state.66 Composites with high thermal conductivity were warmed and cooled quickly and had a corresponding image color change (Supporting Information). Interestingly, the surface color of PVDF/50SnBi58 exhibited the slowest change compared to the other composites, while the composites of PVDF/30SnBi58 had a faster heating or cooling rate and higher maintained temperature during the same warm up time (Figure 6). The composites had a linear growth of heating or cooling rate at same operated time when the filler loading was below 30%, while the composites had a linear decline of heating or cooling rate at same operated time when the filler loading was above 30%. The DSC curve in Figure S2b, Supporting Information clearly indicates that the melting points of composites were far above 60 °C. The phase transition of the composites was not directly the reason for the surface color of PVDF/50SnBi58 exhibiting the slowest change. This phenomenon maybe attributed to the competition F
DOI: 10.1021/acsapm.9b00258 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
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of the composites were increased after loading LMPA (Figure S4, Supporting Information). The low thermal expansion coefficient and good tensile property ensured a good thermal stability of the composite.
4. CONCLUSIONS In conclusion, we introduced LMPA to a PVDF microsphere matrix by blending and hot pressing. The obtained composites had the LMPA continuous network struture partly wrapped by PVDF microspheres. When the volume fraction of LMPA was 50%, the composite had a thermal conductivity of 6.38 Wm−1K−1 and the total EMI SE of −68.79 dB at 10 GHz. The high thermal conductivity was attributed to the continuous LMPA network. The high EMI SE performance of the composite was due to the high electrical conductivity and multiple reflections of incident electromagnetic waves that increased electromagnetic absorption. In addition, the composite had good thermal stability in a real working environment which was closely related to the thermophysical properties of LMPA. This study introduced a novel application of LMPA for both thermal management and future use in EMI SE materials.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00258. Calculation formula of EMI shielding performance of PVDF/SnBi58 composites, details of the crowding factor model, schematic diagram of the thermal imager, comparison of the performance with the reported literature, DSC curve of PVDF/SnBi58 composites, TMA test curve of PVDF/SnBi58 composites, and force−strain curve of PVDF and PVDF/SnBi 58 composites (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; Tel.: +86-551-65592752; Fax: +86-551-65393564. ORCID
Yi Gong: 0000-0002-3721-2678 Xian Zhang: 0000-0002-7910-1562 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS All authors acknowledge financial support from the National Key Research and Development Program of China (Grant 2017YFB0406200). Anhui Province Key Laboratory of Environment-friendly Polymer Materials and the Key Lab of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences are also gratefully acknowledged by the authors.
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REFERENCES
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DOI: 10.1021/acsapm.9b00258 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX
