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Thermal and dielectric properties of the aluminum particle reinforced linear low-density polyethylene composites.

INTRODUCTION

Electronic systems in general consist of both the active and passive components. The technologies concerning the development of the passive components such as resistors, inductors, and capacitors are steadily growing in the electronic industries (1). Among these passive components, the capacitors is the one which attracts special attentions due to its variety of functions including decoupling, bypassing, filtering, and timing capacitors as well as their capability to enhance the electrical performance and to reduce the size and cost of an electronic system (2-4). Embedded-capacitor technology is an important emerging technology that will enable significant improvement of the performance and functionality of future electronic devices (3). Embedded capacitors are specially printed portions of printed circuit board (PCB) laminates which perform the charge-storing function but do not require space on the surface on the PCB. One major technical challenge for implementing this technology is the development of appropriate dielectric materials with good electric and mechanical properties, because traditional ceramic dielectrics cannot be applied by current PCB manufacturing methodologies (3).

Functional polymer composites have the ability to meet these needs. In recent years much work has been done on functional polymeric composites. Owing to the continuous development towards the miniaturization of electronics, high dielectric constant polymeric composites have become promising materials for embedded capacitors applications (1). The incorporation of various functional fillers into polymer has been extensively explored and shown to improve the dielectric permittivity, such as metallic powders (i.e., aurum, silver, copper, aluminum) (2), (3), (5), (6), carbon (carbon black, carbon nano-tube, carbon fiber, and graphite) (7-10), and ceramic particles (i.e., barium titanate, cadmium oxide, tungstic oxide) (4), (11-15).

Until now, extensive attentions have been paid to the preparation of polymeric composite with a high dielectric constant but a low dielectric loss, and good processability for the applications in embedded capacitor (16-19); however, the thermal conductivity of these dielectric materials has been seldom investigated and reported before. It is essentially crucial for the heat generated from electronics to be dissipated as quickly and effectively as possible, to maintain the operating temperatures of the electronics at a desired level because the dielectric strength decreases obviously with increasing working frequency owing to a poor thermal conduction of these dielectric materials (20-24). So, the development of a polymer composite possessing a high thermal conductivity and a dielectric constant but a low dielectric loss is very important because a high thermal conductivity facilitates prolonging the lifespan of polymer dielectrics at operating temperature and frequency (25).

The object of this paper is to prepare a high thermal conductive polymer composite with a high dielectric constant but a low dielectric loss. To meet the requirements above, a surface-passivated aluminum (Al) particle was used as filler. This surface-passivated Al particle has a core-shell structure (2). The core is metallic Al, and the nanoscale shell is insulating aluminum oxide. Such core-shell structured Al particles give their composite a high dielectric constant as a percolative system owing to the metallic core but a low dielectric loss comparable to that of a neat epoxy resin due to the insulating alumina ceramic shell (2). Linear low-density polyethylene (LLDPE) was selected as matrix due to its better strength, toughness, cold resistance, environmental stress cracking resistance, and tearing resistance properties (26), and easy procession. The thermal conductivity of Al particles filled polymer such as polyethylene had been investigated (27-30). However, up until now, the investigations of the dielectric properties of the Al/LLDPE composites in a wide frequency range have seldom been reported until now. So, our purposes expect to give a deeper insight into the influence of the content, particle size and shape of Al particles on the thermal conductivity, and dielectric properties of Al/LLDPE composite.

EXPERIMENTS

Materials

The matrix component used here was LLDPE with a brand of Hyundai SR646 from Hyundai Group (Seoul, South Korea), which has a density of 0.935 g/[cm.sup.3], a melt index of 45 g/10 min. The spherical Al powder supplied by Xi'an Sunwards Aerospace Material Co. (Xi'an, Shaanxi) had a density of 2.7 g/[cm.sup.3], a thermal conductivity of 250 W/m K, and the average particle sizes of 7 and 20 [micro]m, respectively. The flaky Al powder with average length in 10 [micro]m and thickness in 1.0 [micro]m, was purchased from Shenyang Hangda Technology Co. The [gamma]-Aminopro-pyltriethoxysilane silane coupling agent with a purity of 99% was from Nanjing Xiangfei Chemical Co. (Nanjing, China). The polyethylene wax (type: H112), offered by

Guangzhou Shencai trading Co., China, was used to improve the processing property of the Al/LLDPE composites. Other additives such as stearic acid, antioxidant, and ethanol, oxalic acid were all available from market.

Surface Modification of Al Particles

Surface treatment for Al particles using the silane coupler involved the following steps: (a) making an ethanol aqueous solution at a 95 wt % concentration; (b) adding the silane coupling agent (1.2% of the Al filler mass) to the solution and stirring for 15 min in a flask with reflux setting, adjusting the ethanol aqueous solution pH to 3-5 using diluted oxalic acid and stirring for 20 min; (c) adding Al particles to the solution made in (b) and stirring while ultrasonicating for 60 min; (d) heating to 80[degrees]C and refluxing for 6 h while stirring and then cooling to room temperature, letting it set for 2 h; (e) rinsing with ethanol by filtration at least three times; and (f) drying the mixture at 120[degrees]C for 10 h.

Preparation of All LLDPE Composites

LLDPE and Al particles were dried in vacuum at 50[degrees]C for 5 h and at 120[degrees]C for 8 h prior to use, respectively. The LLDPE was first mixed with polyethylene wax, stearic acid, antioxidant at 150[degrees]C, followed by the addition of Al particles. The compounding was carried out on a two-roll mixing mill (Type: SK-106B, Shanghai, China), and the total mixing time for all the different concentrations was kept at 20 min. After that, the resulting mixture was transferred to a stainless steel die and melt pressed at 170[degrees]C in an electrically heated hot press machine (type: SL-45, Shanghai, China) with a pressure of 15 MPa for 15 min, and was allowed to cool for 30 min at room temperature. Composites with Al powder concentrations ranging from 0 to 70 wt% were prepared.

Characterization

The structures of pure LLDPE and Al/LLDPE composites were determined with a Shimadzu XRD-6000 diffractometer. The X-ray diffraction (XRD) is equipped with a graphite homochromatic instrument and a Cu anti-cathode (40 kV, 30 mA, scanning rate 2[degrees]/min, 26 = 10 ~ 80[degrees]). The experiments were conducted at ambient temperature (25 [degrees]C).

The differential scanning calorimeter (DSC), (Model, DSC 200PC, Netzsh Corp., Selb, Germany), was used to analyze the influence of Al content on the melting temperature and melting enthalpy of the LLDPE samples (5 ~ 10 mg). Measurements were conducted in a nitrogen atmosphere, from 20[degrees]C to 200[degrees]C, at a heating rate of 15[degrees]C/min.

The microstructures of composites were observed by scanning electron microscope (SEM) (model: JSM-7000F, JEOL, Japan). The fractured surfaces were prepared in liquid [N.sub.2]. The samples were sputtered with gold in vacuum prior to observation. The observation was carried out on the cross-sections of samples to study filler distribution.

Thermal diffusivity of the samples was measured on Netzsch system (Model, LFA427, Netzsh Corp., Selb, Germany), Then, the thermal conductivity was calculated from thermal diffusivity according to Eq. 1:

k = [alpha] * [rho] * [C.sub.p] (1)

where k, [alpha], [rho], [C.sub.p] are the thermal conductivity (W/mK), thermal diffusivity ([cm.sup.2]/s), density (g/[cm.sup.3]), and specific heat capacity (J/kg K) of the material under constant pressure. The thermal diffusivities of samples were measured at room temperature (in air) and elevated temperature (in argon). The specimens for thermal diffusivity measurement were made in the form of circular discs with ~1 mm in thickness and 12.7 mm in diameter.

The dielectric measurement was performed on a broadband dielectric spectrometer (Novocontrol Technology Company, Germany) with Alpha-A high performance frequency analyzer. The measurement was earned out in the frequency range from [10.sup.-1] to [10.sup.7] Hz under room temperature. The specimens for dielectric measurement were made in the form of circular discs with ~1 mm in thickness and 20 mm in diameter. All measurements were carried out in the cryostat to avoid possible surrounding effects.

RESULTS AND DISCUSSION

X-Ray Diffraction

Figure 1 shows the X-ray diffraction (XRD) patterns of the as received Al powder, hot-pressed LLDPE and its composites with various Al particles loading.

[FIGURE 1 OMITTED]

In Fig. la the peaks at 20 = 38.43[degrees], 44.78[degrees], 65.07[degrees] correspond, respectively, to the (111), (200),) and (220) diffraction planes of the Al crystals. The pure LLDPE shows several distinct diffraction peaks in its XRD curves as seen in Fig. le. The peaks at 20 = 21.67[degrees], 24.01[degrees], 30.16[degrees], 36.28[degrees], 39.85[degrees], 40.81[degrees], 44.13[degrees] correspond, respectively, to the (110), (200), (210), (020), (011), (310), and (220) diffraction planes of the [alpha]-form LLDPE crystals. The diffraction peaks positions among various samples do not exhibit any obvious shifts, implying that the addition of Al has negligible effect on the distance between the diffraction planes of crystallites in the LLDPE matrix (7). Meanwhile, the intensity and width of some of the diffraction peaks decrease or disappear with additional Al particles loading. For example, the relative intensity of the diffraction peaks corresponding to the (110), (200), and (020) crystal planes decrease, whereas, the diffraction peaks corresponding to the (210) and (220) disappear. Further, no new diffraction peak can be observed for the Al/LLDPE composites, as seen in Fig. lb ~ d. It is evident from these patterns that the use of Al particles does not has any appreciable influence the crystallization behavior of LLDPE; however, the changes in the relative intensity of diffraction peaks of the LLDPE matrix brought about by the addition of Al particles will influence the degree of crystallinity, which may affect the final physical properties of the Al/LLDPE, including electrical, thermal, and mechanical properties (7).

Melting Temperature and Degree of Crystallinity

It is well known that LLDPE is a semicrystalline polymer. The portion of the crystalline phase places an important influence on almost all physical properties of polymers (24), (30). Therefore, it is necessary to investigate the influence of Al particles on the change of degree of crystallinity ([X.sub.c]) of LLDPE.

The DSC curves of native LLDPE and its Al composites are plotted in Fig. 2, and the corresponding thermal dynamic data values are listed in Table 1. From Fig. 2 and Table 1 it can be seen that the Al concentration does not seem to have much influence on the melting temperature of LLDPE, and the peak melting temperature ([T.sub.m]) very slightly shifts toward a lower temperature with increasing A1 loading. The reason may be that the A1 filler probably slightly reduces the lamellar thickness of crystallites, which leads to a decrease in the melting temperature (31), (32), because some Al particles will fill themselves in the interlamellar space due to a higher degree of crystallity of pure LLDPE.

[FIGURE 2 OMITTED]
TABLE 1. DSC data of native LLDPE and AI/LLDPE composites.

AI/LLDPE/w/w  [DELTA][H.sub.m]/J/g  [T.sub.m]/[degrees]C  [X.sub.c]/%

   0/100              154.0                130.2              53.1
   10/90              133.7                128.3              51.0
   20/80              114.3                129.0              49.2
   40/60               87.2                130.1              50.1
   70/30               36.9                128.5              42.7


Table 1 suggests that with increasing Al content the [X.sub.c] of LLDPE decreases as compared to native LLDPE. The reason may be that the LLDPE has a relative high crystalinity, leading to a major amorphous part in which the Al particles can be accommodated. At low filler loading the filer particles locate themselves in the interlamellar space, which leave little space for additional crystallization, and the presence of filler may even inhibit crystallization. At high filler content there is probably a change in crystallization mechanism (32), apart from the filler inhibiting crystallization. It may be conjectured without detailed microscopic evidence that the crystalline domains formed by LLDPE are rendered smaller in the presence of Al particles and reduce the overall crystallinity as the filler increases. It is therefore possible for the fillers to decrease the mobility of LLDPE chains in the formation of crystallites and, as a result, the domains of crystalline phase are reduced in size (16), (31). It is also likely that imperfection of crystals in the presence of the Al inhomogeneities contributes to the decrease in crystallinity (32).

It is generally known that the amorphous polymers have similar thermal conductivity (4). In the case of semi-crystalline polymers, an increase of thermal conductivity with an increase in crystalline part content was observed as a consequence of a better transport of the heat in a crystalline phase (31-33). The simple relation between thermal conductivity of semicrystalline polymers and the weight portion of crystalline phase is expressed in terms of the Eq. 2

[k.sub.m] = [k.sub.c][w.sub.c] + [k.sub.a](1 - [w.sub.c]) (2)

where [k.sub.m], [k.sub.c], [k.sub.a] is thermal conductivity of polymer and its crystalline and amorphous part, respectively, [w.sub.c] is weight portion of crystalline phase of polymeric matrix.

For all Al contents, the change in degree of crystallinity calculated according to Eq. 2 is too small and has negligible influence on the thermal conductivity of LLDPE matrices as well as composites, especially if the average experimental error of thermal conductivity measurements being approximately 5-7% (31).

Thermal Conductivity

The thermal conductivity values of LLDPE containing spherical and flaky Al particles at various levels of filler are presented in Fig. 3. Figure 3 shows that the thermal conductivity of Al/LLDPE increases obviously with an increase in filler loading due to the high thermal conductivity of Al particles. The Al particles at low volume fraction can disperse randomly in the LLDPE matrix and has weak interaction each other to present a little increase of thermal conductivity, whereas, at high filler loading, the thermal conductivity remarkably increases. This is because the heat-conductive Al particles surrounded or encapsulated by a polymer matrix cannot touch one another at a low loading. The result is low thermal conductivity due to the high interfacial thermal contact resistance between filler particles and the polymer matrix. On the other hand, at a high filler loading, the filler particles begin to touch one another and form particle clusters or a more compact packing structure within the matrix. This leads to an improved thermal conductivity because of the decreased interfacial thermal contact resistance (21), (22). For example, at 45 vol% Al content, the thermal conductivity of the composite reach 1.10 ~ 1.63 W/m K, near 4 ~ 6 times that of native LLDPE.

[FIGURE 3 OMITTED]

The shape of Al particle has an influence on the thermal conductivity of filled LLDPE composites. Figure 3 suggests that the flaky Al particles filled LLDPE composites exhibit an obviously higher thermal conductivity than that of the spherical fillers reinforced one. It can be seen that the thermal conductivity values of the LLDPE containing 34 vol% of flaky Al particles nearly equals to those of that filled with 45 vol% of spherical Al particles with an average particle size of 7 [micro]m and 20 [micro]m, respectively.

Hamilton and Crosser (34) derived a modified Maxwell Equation from the original Maxwell's equation by only taking the filler content and shape into consideration:

[k.sub.c] = [k.sub.m][[[k.sub.f] + 2[k.sub.m] + (n - 1)[V.sub.f]([k.sub.f] - [k.sub.m])]/[[k.sub.f] + 2[k.sub.m] - [V.sub.f]([k.sub.f] - [k.sub.m])]] (3)

where [k.sub.c], [k.sub.m], and [k.sub.f] stand for the thermal conductivity of the composites, matrix, and filler particle, respectively, [V.sub.f] is the volume fraction of filler particle, n is the particle shape factor which is related to the particle sphericity [delta] with [delta] = n/3.

If the shape of dispersed particles is a sphere, i.e., n = 3, then Eq. 3 reduces to the original Maxwell Equation. For spherical Al particles, since the [delta] = 1, and then n = 3, whereas, for flaky Al particles, n is greater than 3 because of [delta] being less than 1. Therefore, using Eq. 2 it can been explained that the flaky Al particles filled LLDPE shows a higher thermal conductivity as compared to the spherical filler reinforced one.

Since the flaky Al with large aspect ratio easily form the bridges between them as compared to the spherical particles, known as conductive network, the aspect ratio of the filler is more considerable that dictates the thermal conductivity of composites. The formation of random bridges or networks from conductive particles facilitates phonon transfer, which leads to a high conductivity (34).

The Al particle size also affects the thermal conductivity of filled LLDPE composites. Figure 3 reveals that at high filler concentration the smaller size Al particles filled LLDPE shows higher thermal conductivity than that of the larger size particles reinforced one under the same filler content. For a given volume fraction of Al with various particle sizes the thermal conductivity of filled LLDPE are ranked as follows: 7.0 [micro]m > 20 [micro]m. The thermal conductivities of the composites with Al (7 [micro]m) are 0.98 and 1.23 W/m K at 34 vol% and 45 vol% filler content, respectively, corresponding to 0.75 and 1.15 W/m K of those with Al (20 [micro]m). At high Al-particle content, smaller-sized particles with high specific surface areas are desired to minimize the scattering of phonons; moreover, they tend to form fewer thermally resistant junctions in the LLDPE matrix layer than the large-sized particles (24-26). Additionally, because the smaller-sized filler particles has more numbers of particles than the larger-sized one, and can form more conductive channels or pathways under same filler loading. Therefore, the LLDPE containing smaller-sized particles exhibits slightly higher thermal conductivity than that with larger particles.

The experimentally determined thermal conductivity values and those predicted from Maxwell model are also shown in Fig. 3. The Maxwell's equation is an exact solution for the effective conductivity of randomly distributed and noninteracting spheres in a continuous medium. However, it does not take into account the mutual interaction between the particles; thus it is not a satisfactory treatment for composites in the high volume fraction range (i.e., greater than 20 vol%). Form Fig. 3 we can see that at low Al particles content Maxwell model predicts the thermal conductivity of the composites well, whereas, at high filler concentration it tends to underestimate the thermal conductivity because filler particles to touch one another. At low filler concentration the thermal conductivity of LLDPE reinforced with flaky Al is obviously higher than that predicted from Maxwell mode because of the flaky shape of Al particles.

The influence of the [gamma]-Aminopropyltriethoxysilane silane coupling agent on the thermal conductivity of Al/LLDPE is summarized in Table 2. Table 2 suggests that the use of silane coupling agent to functionalize the surface of Alobviously improves the thermal conductivity compared to the untreated filler reinforced LLDPE at various Al content. It can be supposed that silane can form an organic active monomolecular layer on the interface of Al particles. Just like a bridge, one end of silane coupler reacts with free proton on the Al surface to form firm chemical bond, and the other end tangles with LLDPE chains by Van der Waals force. Thus, the organic interface layer between Al and LLDPE is generated (34). So, the silane coupling agent enhanced the phase interfacial bonding, and improved the thermal conductivity.
TABLE 2. Effect of coupler on the thermal conductivity of Al/LLDPE.

                                         Thermal conductivity/W/mK

           Materials                    50 wt% (A1)  60 wt%  70 wt%
                                                      (A1)    (A1)

Spherical A1 (7.0 [micro]n)  Untreated      0.56      0.98    1.23
                             Treated        0.68      1.09    1.36

Spherical A1 (20 [micro]m)   Untreated      0.51      0.75    1.15
                             Treated        0.56      0.87    1.26

Flaky A1                     Untreated      0.76      1.21    1.63
                             Treated        0.85      1.34    1.69


For a two phase system like Al/LLDPE composites, interfacial physical contact between polymer and filler is very critical, since phonons are very sensitive to surface defects (16), (31). Thermal resistance is caused by various types of phonon scattering processes, and the interfacial thermal barriers in composites is mainly due to the scattering of phonons resulting from acoustic mismatch and flaw associated with the matrix-filler interface (33). The interface between the two-phase composites acts as a barrier of heat transmission. So, the surface treatment of Al with silane improved the interfacial bonding between Al and matrix, and reduced the voids at the filler-matrix interface, which facilitates enhancing the thermal conductivity of composites.

The micro-structure images of LLDPE filled with 70 wt% spherical Al particles and 60 wt% flaky Al particles are displayed in Fig. 4. It can be observed that the Al powder distributions are found to be relatively rather uniform. The homogeneous dispersion of Al decreases the thermal contact resistance, facilitating an improved thermal conductivity.

[FIGURE 4 OMITTED]

SEM micrographs of transversally cut samples containing 70 wt% Al particles (7.0 [micro]m) without or with surface treatments are observed in Fig. 4a ~ b. From Fig. 4a it can be seen clearly that the smooth rounded holes observed in the micrograph are related to the Al particles that were withdrawn from the surface, indicating that the phase interaction between the LLDPE and Al particles is weak, and the energy needed to pull out Al particles from the matrix is low. The reasons for this may be ascribed to the poor phase interface wettability between filler and matrix. Figure 3b represents the image of coupling agents treated Al particles/LLDPE composites. It can be observed that appropriate wetting characteristics appeared at the interface between polymer and fillers, since there is little void found in the composites. Moreover, the phase interface is rather indistinct, and even phase interface layer can be observed, which proves a nicer interfacial adherence between filler and matrix. So, the use of coupling agent effectively improved the uniform dispersion of Al particles, eliminated the agglomerate of filler, and decreased the air voids and defects between filler particles.

Dielectric Property

The frequency dependence of dielectric constants of the three kinds of Al particles filled LLDPE composites with different Al content is presented in Fig. 5. As expected, the effective dielectric constant increases with an increase in Al content in LLDPE at all the frequencies under study. In all the cases, the dielectric constants obtained are higher than that of pure LLDPE, but much lower than that of pure Al filler (about 1145) (5). Al is a self-passivation metal, which leads it to the formation of a core-shell structure (nanoscale Al oxide insulating shell and metallic Al core, as seen in Fig. 6) in Al particles. The low dielectric constant as compared with that of Al observed for Al/LLDPE composites may be due to the nonpolar nature of LLDPE and the constrained polymer chain hindering the contribution of electrical polarization, apart from the absence of connectivity as seen in Fig. 4; so, the dielectric constant is much lower than that of A1.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The particle size of A1 has an effect on the dielectric constant of the composites at high filler loading. As shown in Fig. 5, the dielectric constants of the LLDPE with 7 [micro]m A1 particles at 60 and 70 wt% are higher than those of the composites with 20 [micro]m Al particles. It can be seen from Fig. 4 that the LLDPE is self-connected into a continuous phase, while Al particles are randomly dispersed in the matrix, and the filler particles are surrounded by the matrix. We assume that there is a higher chance for Al particles to form a continuous, random cluster with a decrease in particle size. Moreover, the smaller size Al particles/LLDPE composite shows a stronger interfacial polarization effect, compared to the larger size particles filled one.

Furthermore, the particle shape of Al has an effect on the dielectric constant of the composite also. From Fig. 5c we can see that the flaky Al particles reinforced LLDPE exhibits obviously higher dielectric constants than those of LLDPE filled with spherical Al particles. For example, the dielectric constant values of the LLDPE containing 50 wt% flaky Al nearly equal to those of one filled with 70 wt% spherical Al. At 60 and 70 wt% Al content the dielectric constants of flaky Al/LLDPE are 40 and 50, respectively, much higher than those of LLDPE filled with spherical Al in same filler concentration. The enhanced dielectric constant can be ascribed to that the flaky Al filled LLDPE shows a stronger interfacial polarization effect owing to its huge specific surface area, compared to spherical Al, further, the high length-diameter ratio facilitates to the formation of particles cluster, or partial connectivity, leading to an improvement in dielectric constant.

Figure 5 reveals that the dielectric permittivity for a given loading of Al is nearly frequency independent, and the dielectric permittivity vs. frequency curves is completely parallel to the frequency axis in the log scale for lower weight fractions of Al. The dielectric permittivity frequency independence behavior, indicating that the major polarization mechanisms contributing to their dielectric constants do not change over the measured range, can be ascribed to the nanoscale- Al-oxide insulating shell (as depicted in Fig. 6) (2) and the uniform dispersion of Al particles in the LLDPE matrix as shown in Fig. 4, however, the slope of the line slightly increases with an increase in the flaky Al content (i.e., 60 and 70 wt% Al). The reason for increased low frequency dispersion may be due to the space change effects and high dielectric loss associated with Al.

The frequency dependence of the dielectric losses of Al/LLDPE composites with various weight percent of Al is illustrated in Fig. 7. The loss undergoes two relations; one in the low frequency region and the other at high frequencies. The relaxation peak appears beyond 1 MHz is an obvious relaxation loss process related to the LLDPE matrix. The relaxation peak that appears below 1 Hz may be associated with molecular motion in the crystalline regions of LLDPE (1). It is observed that the dielectric loss continues to decrease as frequency increases to a certain high frequency, and subsequently increases remarkably. From Fig. 7a and b it can be seen that the content of spherical Al does not has a appreciable influence on the dielectric loss in the frequency range of 1 ~ [10.sup.6] Hz; the dissipation factor is rather low (generally less than 0.020) for both smaller and larger particle size Al/LLDPE composites because of the insulating alumina ceramic shell. For the flaky Al particles filled LLDPE composites, there is no appreciable change in the loss behavior up to 50 wt% Al loading, after that, the loss factor increases obviously at low frequencies as the Al content increases up to 70 wt% in the composites. It is believed that up to 50 wt% of Al loading, the uniform distribution of Al particles in the immobile matrix hinders the formation of networks of filler and results in a decrease in the dielectric loss in the composites. However, at high content of Al particles there are a well established particles clusters or partial connectivity of filler, resulting in an increase in the dielectric loss as observed in Fig. 7c (1). Therefore, the dielectric analysis demonstrates that the Al/LLDPE composites possessed high dielectric constant and rather low dielectric loss in the measured frequency range from 10[degrees] Hz to [10.sup.7] Hz due to the core-shell-structured Al particles.

[FIGURE 7 OMITTED]

CONCLUSION

The Al particle decreases the degree of crystallinity of LLDPE, and has no appreciable influence on the melting temperature of LLDPE. The thermal conductivity of the LLDPE composites increases with increase in Al content. The thermal conductivity of the LLDPE filled with flaky Al is 1.63 W/mK, much higher than the 1.25 W/mK and 1.13 W/mK for the spherical Al (7.0 [micro]m) and spherical Al (20.0 [micro]m) reinforced one at 70 wt% Al content, respectively. Moreover, the surface treatment of Alpar-ticles with [gamma]-Aminopropyltriethoxysilane silane coupler facilitates improving the thermal conductivity.

The dielectric constant and dissipation factor increase with increase in Al content at all frequencies under investigation, especially remarkable for the flaky Al/LLDPE composites. However, dissipation loss still remain at relatively low levels at wider frequency range from 10 ~ [10.sup.6] Hz. Dielectric constant as high as 50 for the flaky Al/ LLDPE composites is achieved, much higher than those of LLDPE filled with spherical Al under same filler concentration. The dielectric permittivity frequency independence in the measured frequency range is observed due to the nanoscale-Al-oxide insulating shell of Al.

The obtained Al/LLDPE composites have possessed both high thermal conductivity, and high dielectric constant but low dielectric loss at the measured frequency range from 1 Hz to [10.sup.6] Hz.

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Correspondence to: W.Y. Zhou; e-mail: wyzhou2004@163.com

Contract grant sponsor: China Postdoctoral Science Foundation; contract grant numbers: 20070421113, 200801434; contract grant sponsor: Scientific Research Program Funded by Shaanxi Provincial Education Commission; contract grant numbers: 2010JK694; contract grant sponsor: Scientific Research Foundation of Xi'an University of Science and Technology; contract grant numbers: 2009017; contract grant sponsor: National Science Foundation of China; contract grant number: 51073180.

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2011 Society of Plastics Engineers

Wenying Zhou (1), (2)

(1) School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an, 710054, People's Republic of China

(2) School of Electrical Engineering, State Key Laboratory of Electrical Insulation and Power Equipments, Xi'an Jiaotong University, Xi'an, 710049, People's Republic of China

DOI 10.1002/pen.21913
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Author:Zhou, Wenying
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9CHIN
Date:May 1, 2011
Words:5597
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