Significantly Enhanced Dielectric Performances and High Thermal

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Significantly Enhanced Dielectric Performances and High Thermal Conductivity in Poly(vinylidene fluoride)-based Composites Enabled by SiC@SiO2 Core-shell Whiskers Alignment Dalong He, Yao Wang, Silong Song, Song Liu, and Yuan Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14751 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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ACS Applied Materials & Interfaces

Significantly Enhanced Dielectric Performances and High Thermal Conductivity in Poly(vinylidene fluoride)-based Composites Enabled by SiC@SiO2 Core-shell Whiskers Alignment Dalong He,† Yao Wang,*,† Silong Song,† Song Liu,† and Yuan Deng*,†,‡ †

School of Materials Science and Engineering, Beihang University, Beijing, 100191, China

‡

Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology,

Beijing, 100191, China KEYWORDS: Core-shell structure; Polymer-matrix composites; Anisotropy; Ordered structure; Dielectric performances; Thermal conductivity; Numerical simulation

ABSTRACT: Design of composites with ordered fillers arrangement results in anisotropic performances with greatly enhanced properties along specific direction, which is a powerful tool to optimize physical properties of composites. Well aligned core-shell SiC@SiO2 whiskers in poly(vinylidene fluoride) (PVDF) matrix has been achieved via a modified spinning approach. Due to high aspect ratio of SiC whiskers, strong anisotropy and significant enhancement in dielectric constant were observed with permittivity 854 along parallel direction vs. 71 along

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perpendicular direction at 20 vol% SiC@SiO2 loading, while little increase in dielectric loss was accompanied owing to the highly insulating SiO2 shell. The anisotropic dielectric behavior of the composite is perfectly understood macroscopically to be originated from anisotropic intensity of interfacial polarization based on an equivalent circuit model of two parallel RC circuits connected in series. Furthermore, finite element simulations on three-dimensional distribution of local electric field, polarization and leakage current density in oriented SiC@SiO2/PVDF composites under different applied electrical field directions unambiguously revealed that aligned core-shell SiC@SiO2 whiskers with high aspect ratio significantly improved dielectric performances. Importantly, the thermal conductivity of the composite was synchronously enhanced over 7 times compared with that of PVDF matrix along parallel direction at 20 vol% SiC@SiO2 whiskers loading. This study highlights an effective strategy to achieve excellent comprehensive properties for high-k dielectrics.


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INTRODUCTION Advanced polymer-based dielectric composites are receiving great attention due to the everincreasing demand in high-performance modern electronics and power devices.1-10 To achieve high dielectric permittivity, percolation phenomenon is widely utilized so that dramatic changes in the dielectric properties of composites occur when fillers form a percolating network through the composite, particularly when the difference in permittiviy between the filler and the polymer matrix is large.10-14 The dielectric constant of the conductor-insulator composite is expressed as:

ε ∝ ε m f − fc

−s

(1)

where εm is the dielectric constant of the matrix and s is a critical exponent of approximately 1. fc is percolation threshold which is the most important parameter to describe the properties of composites, moreover, it strongly depends on the microstructure of the composites. The geometric parameters of the fillers, such as particle size, shape, orientation, and the distribution of fillers in the matrix are crucial to determine the percolation behavior. Despite the excellent dielectric properties achieved in various elaborately designed nancomposites, the intrinsic drawback of the percolative composites is obvious. The accumulated interfacial charges that contribute to the high permittivity of the composite would meanwhile cause a huge conductive loss under high electric field, which is mostly converted to waste heat. Thus the heat dissipation capability of dielectric materials is also an important parameter that needs consideration. Coating the conductive fillers with insulating shell has been proposed to overcome the drawback, considering that the shell layer not only improves the compatiblity between fillers and polymer matrix, but more importantly serves as a barrier layer blocking the migration of charge carriers. Dramatically suppressed dielectric losses have been achieved in various material systems.15-23


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Meanwhile, spatial distribution of the fillers is one of the geometry parameters that determines the macroscopic physical properties of the composites, so it shall provide another promising route to manipulate the dielectric performances of the percolative composites.24-26 When fillers with strong shape anisotropy are aligned in one direction in the matrix, the nanocomposites would present enhanced dielectric permittivity along the direction in which the fillers are oriented with respect to the external field and improved breakdown strength in the direction that aligned fillers lie perpendicular to the applied field. For examples, Liu et al. fabricated the multi-wall carbon nanotubes (MWCNTs) array/polysulfone composites showing high dielectric constant and low dielectric loss (tanδ = 0.06, ε=58 @ MWCNT 25 vol %) due to the isolation and preorientation of carbon nanotubes.27 Xie tuned the orientation of 3 vol % BaTiO3 nanowires (BT NWs) in poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDFCTFE) matrix and an improved energy storage performance (Ue=10.8 J cm-3 at 240 kV mm-1) was obtained when BT NWs aligned in the direction of the applied electric field compared with that in the perpendicular direction (Ue=10.1 J cm-3 at 340 kV mm-1).28 Tang et al. reported the composites with aligned lead zirconate titanate (PZT) nanowires exhibiting much higher dielectric permittivities than those of samples with randomly dispersed PZT NWs, resulting in energy density up to 51.6% increment compared with that of random nanocomposite at 20 vol % loading.29 According to our previous study, alignment of one-dimensional nanofillers greatly suppressed the dielectric loss of the PVDF based nanocomposite with conductive filler.30 Despite the excellent mechnical properties of β-SiC whisker, which is widely used to strengthen or toughen the polymers, it is also a one-dimensional semiconductor ceramic with large aspect ratio. The bandgap of β-SiC is around 2.3 eV with high electron mobility (~1000 cm2 V-1 s-1),31 making it a good candidate for fillers to tune internal microstructure of high-k


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composites. Here, we first took the advantage of SiC@SiO2 core-shell architecture to suppress the loss brought by condutive SiC whisker; next, SiC@SiO2 whiskers were aligned using modified spinning process to simultanously increase the dielectric constant and suppress the dielectric loss of the conductive filler loaded composites. Since the heat dissipation has become a key issue for designing the electronic devices, especially for those with high power density, the thermal transport behavior was explored meanwhile. 3D numberical simulation on the distribution of local electric field, polarization and leakage current together with equivalent circuit model was empolyed to deeply understand the anisotropic dielectric behaviors of the composites.

EXPERIMENTAL SECTION Material PVDF polymer was purchased from Shanghai 3F company, China. Raw SiC whiskers with an average diameter of 500 nm and aspect ratio larger than 40 were supplied by Xuzhou Jiechuang New Material Technology Co., Ltd. N,N-dimethylformamide (DMF) was supplied by Beijing BlueYi Chemical Products Co., LTD. Tetraethoxysilane (TEOS, 28%), ethyl alcohol and ammonia (28%) were obtained from Alfa Aesar Chemical Company. All the chemicals were used as received without further purification. Deionized water was used in all experiments. Preparation of SiC@SiO2 whisker Coating SiC whisker with uniform amorphous SiO2 shell was achieved via a modified sol-gel method.32 SiC whiskers were added into ethanol, followed by an ultra-sonication of 1 h. Ammonia was then added to adjust the pH. Subsequently, a certain amount of TEOS was added dropwisely into the solution and the solution was continuously stirred for 12 h at room temperature. Then, the solution was centrifugated and washed with ethanol twice, and finally


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dried in a vacuum oven at 60 °C for 12 h. The schematic diagram of the fabrication process was illustrated in Scheme 1.

Scheme 1. Schematic diagram for the fabrication of SiC@SiO2 core-shell structured whiskers. Preparation of oriented SiC@SiO2/PVDF composites The SiC@SiO2/PVDF composite were fabricated by a simple direct spinning and hotpressing method. SiC@SiO2 whisker and PVDF powders were proportionally dispersed in DMF under ultrasonication for 30 min followed by stirring for 12 h to form a stable suspension. The suspension was transfered into the syringe to inject on a rotating drum, where the temperature and rotate speed were 80 ºC and 300 rpm/min, respectivly. Subsequently, these spun composite nanofibers were collected in the form of membranes with thickness ranging from 100 to 200 µm, and then these membranes were cut into small pieces with square size 10 mm × 10 mm and were stacked along the membrane thickness direction with the same fiber orientation. These membranes were finally molded by hot-pressing at 240 ºC under a pressure of 3 MPa perpendicular to the membranes to form the SiC@SiO2/PVDF composite bulk sample. The geometry of the final bulk sample is a 10 mm × 10 mm × 10 mm cube. The schematic diagram of the fabrication process is shown in Scheme 2.


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Scheme 2. Schematic illustration for preparation of SiC@SiO2/PVDF composites via modified spinning process. Characterizations X-ray diffraction (XRD) measurement was carried out at room temperature on a D/max2200/PC diffractometer using Cu Kα radiation (λ=0.154056 nm) at a scanning rate of 6 º min-1 in the 2θ range from 15 º to 75 º. The microstructures of core-shell nanostructures were identified by transmission electron microscopy (TEM, JEM-2100F JEOL, Japan). The morphologies of raw SiC whiskers and SiC@SiO2/PVDF composites were observed by a field emission scanning electron microscope (FESEM, Quanta 250 FEG, FEI, Czech). To measure the electric properties, silver electrodes were pasted on both sides of the samples. The dielectric constant, dielectric loss and AC conductivity of the composites were tested by HP 4294A precision impedance analyzer (Agilent) at room temperature in the frequency ranging from 1 kHz to 10 MHz. The thermal conductivity of the composites in parallel and perpendicular direction are measured on the instrument TPS2500S (Hot Disk, Sweden).

RESULTS AND DISCUSSION Microstructures of SiC and core-shell SiC@SiO2 whisker


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The morphology of the commercial raw SiC whiskers was observed by SEM as shown in Figure 1 a & b. The whiskers present a size distribution of diameters among 0.3-1 µm and lengths ranging from a few tens to hundreds of µm. Notably, the surface of most SiC whiskers is uniform and smooth (see Figure 1b), which is prerequisite to form tight shell layer outside the SiC core. As identified from the TEM image of SiC@SiO2 core-shell whisker, the SiO2 shell is uniformly and densely coated outside SiC whisker with thickness around 25 nm. A closer look around SiC/SiO2 interface is presented in the inset of Figure 1c, revealing coherent and tightly bonded interface. Figure 1d presents the XRD patterns of raw SiC and core-shell structured SiC@SiO2 whiskers. Both samples show four peaks at 36 °, 42 °, 60 ° and 72 ° which are derived from the (111), (200), (220) and (311) planes of pure β-SiC without other detectable crystalline phase peaks, indicating that SiO2 shell is amorphous.

Figure 1. (a) SEM images of raw SiC whiskers. (b) Magnified image of a single SiC whisker, (c) TEM image of the SiC@SiO2 core-shell whisker with the inset showing the interface between SiC core and SiO2 shell and (d) XRD patterns of pristine SiC and SiC@SiO2 whiskers.


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Anisotropy in microstructures of oriented SiC@SiO2/PVDF composites The morphologies of composites with different volume fraction of SiC@SiO2 whiskers are shown in Figure 2. Seen from front view (Figure 2a-d), the high aspec ratio of SiC@SiO2 whiskers is retained in the composites and they are well dispersed and aligned along one direction. Seen from the side view (Figure 2e-h), clear whikser ends are observed, demonstrating strong structure anisotropy of the composite.

Figure 2. Cross-sectional SEM images of SiC@SiO2/PVDF composites with various SiC@SiO2 volume fraction viewed from different direction. (a), (e) f =0.05; (b), (f) f =0.1; (c), (g) f =0.15; (d), (h) f =0.2. (a-d) Front view, and (e-h) side view as indicated by the sketch. Dielectric Properties of the SiC@SiO2/PVDF Composite According to literature and our previous study, empolying highly insulating SiO2 shell outside conductive filler results in greatly suppressed Maxwell-Wagner-Sillars interfacial polarization, thus greatly reduced loss could be realized in conductor/PVDF composite.16, 33 As presented in Figure S1 (supporting materials), high dielectric constant (~ 458 @ 1 kHz) was achieved in SiC/PVDF composite with 20 vol% SiC, accompanying with the high dielectric loss and high ac conductivity as expected. For the composites containing aligned core-shell SiC@SiO2 whiskers, the dielectric performances were tested along two directions, i.e.,


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perpendicular and parallel to the axial direction of whisker, respectively. As shown in Figure 3 (a-1) and (a-2), the dielectric constants in perpendicular and parallel directions all show good stability in the frequency range from 1 kHz to 100 kHz and increase with the increasing of SiC@SiO2 content when the filler loading is below 15 vol%. However, the dielectric constant increases sharply and exhibits strong relaxion when the filler loading reaches 20 vol%. Frequency dependent dielectric losses of SiC@SiO2/PVDF composites in both the directions show similar behavior as PVDF matrix [see Figure 3(b-1) and (b-2)], and the losses increase slowly with the increasing of SiC@SiO2 whisker content, and the maximum values are no more than 0.16 (@1 kHz), which are much lower than SiC/PVDF counterpart (i.e., tanδ=12.9 @ 20 vol% SiC) and composites with similar fillers.34, 35 It is also noticed that at low frequency losses are slightly higher than PVDF matrix, which is probably contributed from the interfacial polarization relaxation happened at the weak bonding between whiskers and PVDF as seen from Figure 2(e-h). Figure 3 (c-1) and (c-2) show the ac conductivities of composites in perpendicular and parallel directions, respectively, which increase linearly with the increasing frequency, and gradually increase with the increasing of SiC@SiO2 whisker volume fraction with the maximum conductivity of 4.5×10-7 S m-1 in perpendicular direction and 5.1×10-6 S m-1 in parallel direction at 20 vol% loading, indicating the excellent insulation of the composites.


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Figure 3. Frequency-dependent changes of (a) dielectric constant, (b) dielectric loss and (c) ac conductivity of oriented SiC@SiO2/PVDF composites with different volum fraction of SiC@SiO2 whisker. (a-1), (b-1), (c-1) perpendicular and (a-2), (b-2), (c-2) parallel directions. The comparison in dielectric properties of SiC@SiO2/PVDF composites with various filler contents between perpendicular and parallel directions is presented in Figure 4 to clearly uncover the strong dielectric anisotropy. Seen from Figure 4a, the change in dielectric constant of SiC@SiO2/PVDF composite along parallel direction with filler content shows a typical behavior of percolative composite where dielectric constant increases quickly to an ultrahigh value, while, in the perpendicular direction, the dielectric constant slowly increases with increasing filler content. Finally the dielectric constant reaches 854 for parallel direction vs. 71 for perpendicular direction at 20 vol% whisker loading @1 kHz, which is about 95 and 9 times larger than that of pure PVDF, respectivly. Little disparity in dielectric losses along different directions is shown in Figure 4b, since the interfacial polarization from Maxwell−Wagner−Sillars effect between conductive filler SiC and PVDF matrix is greatly mitigated by coating highly insulating SiO2 layer. Most of the losses come from the imperfect interfaces between SiC@SiO2 whiskers and PVDF matrix, which are isotropic. Meanwhile, the ac conductivities of composites are shown in


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Figure 4c with consistent anisotropy and low values (i.e., less than 6×10-6 S m-1), indicating that the significantly increased permittivity of composite is not caused by elevated conductivity.

Figure 4. Anisotropy in (a) dielectric constant, (b) dielectric loss and (c) ac conductivity @ 1 kHz of oriented SiC@SiO2/PVDF composites in perpendicular and parallel directions with increasing filler loading. For the composite, the dielectric constant can be expressed as : ε’=ε’MWS + ε’DIPOLE, where ε’MWS and ε’DIPOLE represent the dielectric contant contributed by the interfacial polarization process (dominant in low frequency range) and dipole polarization process (dominant in high frequency range), respectively.36 Then, the enhancement of dielectric constant in the low frequency range is mainly due to the large contribution from interfacial polarization. The polarization intensity P is defined as:

∑ ∑

where µ represent the dipolar moment, V corresponds to the volume of the sample, q and l are denoted as positive or negative charge and the displacement between positive and negative charges under applied electric field, respectively.36 For a given cubic sample, V is usually considered as a constant independent of directions. In the case of conductive filler, such as Bi2S3, positive charges +q will be separated from the negative space charges -q to move along the direction of electric field to the two ends of the filler with no charges inside the filler. As shown


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in Figure 5, the displacement l in the parallel direction (along the axial direction of whisker) is tens of times longer than that of perpendicular direction, leading to much stronger intensity of interfacial polarization and higher dielectric constant accordingly. Therefore, the anisotropic dielectric constant could be ascribed to the anisotropic intensity of interfacial polarization.

Figure 5. The distribution of positive and negative charge inside SiC whisker under different electric field dierection (a) parallel and (b) perpendicular. Impedance Analysis for the Composites The dielectric properties of solid materials can be explored by its equivalent circuit model composed of three elements: resistor (R), capacitor (C), and inductor (L) elements.38-40 An equivalent circuit model containing two parallel RC circuits in series was established for our SiC@SiO2/PVDF composites as illustrated in the inset of Figure 6. The R1 and constant phase element 1 (CPE1) represent the resistance and capacitance at high frequency, respectively, meanwhile, R2 and CPE2 stand for the corresponding parts at low frequency. All the parameters were calculated by fitting experimental impedance data to equivalent circuit model according to the following equation: ∗

1 1

where τ = RC is the time constant of circuits. According to our recent research on interfacial polarzation in semiconductor-insulator interface, the insulating SiO2 layer tremendously reduces the charge density at the interface between the filler and the PVDF matrix regardless of the layer


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thickness.33 The carriers are thus localized inside the conductive fillers by the insulating shell. Therefore, CPE2 in the equivalent circuit model established here, stands for the interfacial polarization between SiC@SiO2 whisker filler and PVDF matrix, and the fitted values of CPE2 changing with SiC@SiO2 content at both directions are shown in Figure 6. It can be concluded that, the interfacial polarization in the parallel direction is obviously more intense than that in the perpendicular direction, which simultaneously verifies the explanation on the anisotropic dielectric constant.

Figure 6. Variation of simulated constant phase element 2 with different volume fraction of SiC@SiO2 in the parallel and perpendicular directions, and the inset is the equivalent circuit model with two parallel RC circuits in series. Finite element simulations of electrical properties of SiC@SiO2/PVDF composites Computer modeling is employed to further clarify the effects of filler alignment on the distribution of local electric field, polarization and leakage current density. The finite element simulation (FEA) for the anisotropic composites with 5 vol% of SiC@SiO2 whisker was performed with COMSOL Multiphysics and MATLAB. The distribution of SiC whiskers in the


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PVDF matrix was randomly generated via MATLAB with aligned arrangement. The model of simulation system was built as displayed in Figure S2. The dielectric constant of SiC whisker, SiO2 and PVDF is assigned as 1000, 3.9 and 8.5, respectively, and the electrical conductivity of SiC whisker, SiO2 and PVDF for the modeling is 1 S m-1, 10-15 S m-1 and 1013

S m-1, respectively. In the simulation process, the applied electric field decreases gradually from top to bottom

and from left to right in the range from 5000 V to 0 V, and the electric potential distribution is shown in Figure S3. The distribution of the electric field along parallel and perpendicular directions are illustrated in Figure 7a & b, respectively. Distorted regions of electric field could be observed clearly with large contrast between the adjacent regions around two ends and the middle part of the whisker when electric field is applied along the parallel direction; while, much more uniform distribution is observed when electric field is applied along perpendicular direction of the composite, which unambiguously indicates that the local electric field concentration along the parallel direction is caused by oriented SiC@SiO2 whiskers. Cross-sectional images of electric field distribution are obtained according to the modelling displayed in Figure S4 and are shown in Figure 7c & d for parallel and perpendicular directions, respectively. Clearly, the local electric field along the applied field direction is significantly enhanced in the parallel direction in contrast to the perpendicular direction (see Figure 7b&d), where the field concentration is strongly dependent on the geometry of the fillers. Seen from Figure 7a&c, when applied electric field is along parallel direction with whiskers, the local field all aggregate at two ends of whiskers resulting in ultrahigh electric field concentration, while applied electric field is in the direction perpendicular to the whiskers, the electric field is homogeneously distributed in the matrix with slightly increased intensity around surface of whiskers. Accordingly, the polarization


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distribution in the composites at different external electric field directions show similar results (see Figure S5, supporting information). As for the leakage current distribution shown in Figure 8, the whole composite is basically insulating in consistent with the dielectric loss behavior of the composite with 5 vol% whisker loading, due to the encapsulation of SiC whisker by highly insulating SiO2 layer. The core-shell structure has more remarkable effect in lowering the leakage current when external electric field is perpendicular to the whiskers as presented in Figure 8b&d.

Figure 7. Distribution of electric field strength simulated for 5 vol% SiC@SiO2/PVDF composite in (a) parallel and (b) perpendicular directions, and (c), (d) are the corresponding cross-sectional images of (a) and (b), respectively.


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Figure 8. Distribution of leakage current simulated for 5 vol% SiC@SiO2/PVDF composite in (a) parallel and (b) perpendicular directions, and (c), (d) are the corresponding cross-sectional images of (a) and (b), respectively. Thermal conductivities of SiC@SiO2/PVDF composites As dielectric loss is converted into waste heat, efficient dissipation of heat from dielectric materials is of crucial importance for capacitors, especially for polymer-based capacitors due to the intrinsically poor heat transfer capability of polymer, usually in the range of 0.1–0.5 W m-1 K-1.41, 42 The thermal conductivities of oriented SiC@SiO2/PVDF composites as a function of filler loading is given in Figure 9a. The thermal conductivity increases with SiC@SiO2 whisker content and shows strong anisotropy along parallel and perpendicular directions, where much higher thermal conductivity is obtained along the direction that SiC@SiO2 whiskers are aligned; for instance, 1.256 W m-1 K-1 in parallel direction vs. 0.585 W m-1 K-1 in perpendicular direction at 20 vol % SiC@SiO2 whiskers loading. The enhancement in thermal conductivity of the composites is ascribed to the mixing rule between β-SiC whisker with high intrinsic thermal conductivity (~100 W m-1 K-1) and pure PVDF matrix (~0.2 W m-1 K-1).12,

43

While, the


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anisotropy of thermal transport behavior of the composites could be explained in terms of large aspect ratio of the SiC whiskers35, 44 and the well aligned whiskers in PVDF matrix, which favor the formation of thermal transport pathways in the parallel direction in contrast to the perpendicular direction as demonstrated in Figure 9b.24, 45-47

Figure 9. (a) Thermal conductivities of SiC@SiO2/PVDF composites with various SiC@SiO2 wiskers loadings along parallel and perpendicular two directions. (b) Schematic illustration of thermal transport in oriented SiC@SiO2/PVDF composites along two directions. The red lines stand for heat flows where the solid lines indicate fast heat transfer while the dash lines indicate slow heat transfer. Table 1 presents the comparison in dielectric properties and thermal conductivity of polymer based composites reported in literature and this study. From the standpoint of dielectric constant, alignment of SiC whiskers significantly enhances the permittivity in the parallel direction compared to randomly distributed SiC whiskers. While, the SiC@SiO2 core-shell structure ensures largely suppressed dielectric loss compared with composites containing highly conductive, low dimensional carbon materials such as carbon nanotube or graphene.48,

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Meanwhile, when compared to the composites empolying boron nitride (BN) which is a widely


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studied material employed in composites for high thermal conductivity, this study show a fairly competitive thermal conducting performance.50 Finally, excellent comprehensive properties have been achieved in SiC@SiO2/PVDF composite enabled by rational microstructure design. Table 1. Comparison in dielectric properites and thermal conductivities between literature and this study. Thermal Dielectric Dielectric Filler loading conductivity Reference constant Loss -1 -1 (W m K ) 51 AlN@PI/epoxy 20 wt% 1.65 4.5 0.04 12 SiC whisker/PVDF 20 vol% 0.6 253 1.46 52 GNPs/epoxy 1.892 vol% 0.54 ~230 ~10 53 ZnO/PVDF 40 wt% ~0.6 ~100 / CNTs 2wt% 54 CNTs/BN/PVDF ~0.5 ~2300 ~0.8 BN 5 wt% BNNS /P(VDF-TrFE55 14 wt% 1.4 ~37 ~0.025 CFE) 1.3 (//) 854 (//) 0.16 (//) Oriented 20 vol% This study SiC@SiO2/PVDF 0.6 (⊥) 71 (⊥) 0.14 (⊥) Composites

CONCLUSION Aligning core-shell structured SiC@SiO2 whiskers in PVDF matrix was realized via modified spinning process. Preservation of the high aspect ratio of SiC whiskers in PVDF matrix with highly-ordered alignment renders strong anistopy in dielectric constant and thermal conductivity observed along directions parallel and perpendicular to the arrangement orientation of SiC whiskers. Incorporation of core-shell structured SiC@SiO2 whiskers effctively suppressed the dielectric loss (tanδ

Significantly Enhanced Dielectric Performances and High Thermal

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