Effect of filler size distribution on the mechanical and physical properties of alumina-filled silicone rubber.
Recent advancement in electronics technology has resulted in the miniaturization of transistors, allowing more transistors to be integrated into a single device (1-3). Nevertheless, integration and cramming of transistors leads to the escalation of power dissipation as well as an increase in heat flux at the devices. So, the heat dissipation problem is of great importance to the lifespan of the higher performance electronic device (4), (5). Therefore, it is essentially crucial for the heat generated from the devices to be dissipated as quickly and effectively as possible, to maintain the operating temperatures of the device at a desired level (6-8).
Indeed, the inability to adequately conduct heat from the chip or transistor has imposed another engineering constraint in many new product designs (9). Traditionally, thermal problem in encapsulated devices has been addressed by the use of high cost-embedded heat sink (10-13). However, without good thermal contacts, the performance of a heat sink to dissipate heat is sharply limited due to interfacial thermal resistance arising from nonsurface flatness and surface roughness of both the devices and heat sink, resulting in as much as 99% of the interfaces being separated by air gaps (1). Interstitial air gaps trapped due to improper mating of the surfaces significantly reduce the capability to dissipate heat, because of the very low thermal conductivity value of air ([k.sub.air] = 0.026 W/m K). Under this circumstance, one method widely used to reduce the thermal contact resistance between the two surfaces is to include an additional material, commonly referred as thermal interface materials (TIMs), to provide an effective heat path (as seen in Fig. 1) (1), (14-16).
[FIGURE 1 OMITTED]
An ideal TIM should possess both high thermal conductivity and low coefficient of thermal expansion (CTE). In addition, the material must be easily deformed by small contact pressure to contact all the uneven areas of the mating surfaces (1). Thus, the rubbers filled with thermally conductive fillers are considered an effective way to solve the thermal management issue (17), but for the poor thermal conductivity of the elastomeric thermal pad, a kind of TIM, the incorporation of highly thermally conductive ceramics fillers into rubbers to develop high performance thermal-conductive TIM has been mostly desired (18-21).
Among the rubbers used in industry, the vinyl end-blocked polymethylsiloxane (silicone rubber) was primarily chosen to fabricate the elastomeric thermal pad due to its excellent high temperature-resistant and environmental-friendly property. Aluminum nitride, alumina, boron nitride, silicone carbide, etc., are often used as heat-conductive fillers for the heat-conductive silicone rubber serving as heat dissipation of the high performance electronic devices [1, 3, 5, 7, 15, 16, 18, 20]. For example, Sim et al. (1) studied the thermal-conductive [A1.sub.2][O.sub.3]- and ZnO-filled silicone rubber; Wang and Xie (22) explored the heat-conductive silicone rubber filled with hybrid [A1.sub.2][O.sub.3] and AIN particles; Zhou et al. (4) investigated the alumina-reinforced heat-conductive composite silicone rubber.
It was reported that a mixture of fillers of hybrid sizes has advantages over a single particle size in obtaining high thermal conductivity due to the higher packing volume fraction of hybrid (4), (22). The higher packing volume fraction leads to more conductive pathways or networks from filler particles, because smaller particles easily enter into the space that larger particles cannot occupy and form a higher packing density of filler in the rubber matrix (4), (21). Therefore, to achieve a high packing density, hybrid fillers of different particle sizes are suggested. The use of hybrid fillers would result in a more compact packing structure in the rubber matrix and the easy formation of random bridges or networks from conductive particles, which facilitate phonon transfer and lead to higher thermal conductivity; therefore, the thermal conductivity of silicone rubber filled with hybrid fillers of different particle sizes is higher than that of silicone rubber filled with a single particle size (23-25). Moreover, the use of hybrid fillers with different particle sizes has an influence on the other physical properties of filled silicone rubber, such as CTE, dielectric constant, tensile strength, viscosity, etc. For instance, Bae et al. (26) investigated the effect of hybrid AIN fillers of binary particle sizes on the thermal conductivity, CTE, flexural strength, and water absorption properties of filled epoxy molding compound. Zhou et al. (27) explored the influence of a mixture of [S[i.sub.3][N.sub.4] particles with 2 and 25 [micro]m particle sizes on the thermal conductivity of filled silicone rubber. Zhou et al. (28) also studied the effect of hybrid alumina fillers with four kinds of particle sizes (25 [micro]m, 5 [micro]m, 0.5 [micro]m, and 50 nm) at various weight ratios on the thermal conductivity, elongation at break, shore A, and tensile strength properties of composite silicone rubber. At the same total filler content, the use of hybrid particle size fillers at preferable weight ratio can improve the thermal conductivity, tensile strength, and other properties of composites, compared with single particle size filler. Thus, the content of hybrid fillers, which provide the composites with the same thermal conductivity as the filler of single particle size used alone, could be decreased. The decrease in the filler concentration could improve the mechanical strength and reduce the processing viscosity of filled silicone rubber. Therefore, to obtain high thermal conductivity and other improved physical properties of filled conductive silicone rubber, it is very needed to investigate the effect of hybrid fillers of different particle sizes at varying weight ratios on the physical property of the composites.
Up to now, the studies on heat-conductive silicone rubber filled with hybrid alumina fillers of binary particle size distribution have seldom been reported before. Understanding the changes in silicone rubber's various physical properties according to the type, size, and content of filler is very necessary for the development of excellent elastomeric thermal pad. Therefore, the aim of this paper is to investigate the effect of hybrid alumina fillers of various binary particle size distributions on the thermal conductivity, CTE, dielectric constant, tensile strength, and elongation at break of filled silicone rubber, as potential applications as elastomeric TIMs.
The silicone rubber was vinyl endblocked polymethyl-siloxane (type 101B) manufactured by Chenguang Chemical Co. (Zigong, China), whereas the curing agent used was 2,5-bis (tert-butyl peroxy)-2,5-dimethylhexane, and the processing oil was dimethyl hydroxy silicone oil emulsion, both purchased from Northwest Research Academy of Rubber (Xianyang, China). The thermal-conductive filler used was [beta]-phase alumina (A[l.sub.2][O.sub.3]) with a purity of 99.5% and average particle sizes of 0.5, 5.0, 10, and 30 [micro]m from Sanhe Powder Technology Co. (Sanhe, China).
The chemical structures of silicone rubber and curing agent used are shown in Scheme 1. The properties of the main raw materials used are listed in Table 1.
TABLE 1. Properties of alumina and silicone rubber. Formula [Al.sub.2][O.sub.3] Silicone rubber Density (g [cm.sup.-3]) 3.97 1.05 CTE(X [10.sup.-6]) ([K.sup.1]) 6.9-7.4 285 Dielectric constant 6-7 3.5 Electrical resistivity [greater than or [greater than or ([ohm] cm) equal to] [10.sup.14] equal to] [10.sup.16] Thermal 33 0.2 conductivity (W/m K) Mean particle size ([mu]m) 0.5, 5.0, 10, and 30
The silicone rubber was mixed with alumina fillers and processing oil, and this was followed by the addition of curing agent. The compounding was carried out on a two roll mixing mill (S K-106B, Shanghai No. 1 Rubber Machinery. China), and the total mixing time for all the different concentrations was 45 min.
The gross silicone rubber mixture was placed in a stainless steel mold and was compression-molded at 170[degrees]C and at a pressure of 10 MPa for 15 min in an electrically heated hot press (SL-45, Shanghai No. 1 Machine Plant, China). Then, the stain steel mold with sample was cooled down to room temperature in the air, and the sample was removed from the mold.
Thermal conductivity (W/m K) of the composites was measured using the guarded heater meter apparatus (as seen in Fig. 2), which was fabricated according to the ASTM D-5470-06. Thermal conductivity measurements were conducted on specimens with a diameter of 12 mm and thickness of 1 mm in size, cut from the hot pressed samples. Measurements were done by heating the heater blocks up to a previously set temperature, with the samples clamped in between the two calorimeters to produce an average specimen temperature. For this, a chiller plate with circulating water at a set temperature was placed above the cooling calorimeter to create a thermal gradient from the heating calorimeter to the cooling calorimeter (1). During measurements, an onset pressure of about 1.5 bar was applied to reduce the effects of contact resistance between the specimen and the calorimeters due to minor surface irregularities. Readings from the thermocouples are recorded when thermal equilibrium is achieved, whereby two successive set of temperature readings are taken at 15-min interval, which shows a difference of [+.(or).-]0.1[degrees]C.
[FIGURE 2 OMITTED]
The thermal impedance (R) of the samples were calculated from the Eq. 1. according to the Fourier's law based on the assumption that the heat flow is one dimensional in the perpendicular direction, and no hear loss occurs in the lateral direction.
The thermal conductivity was obtained from a plot of thermal impedance for single and multiple layered specimens against the respective specimen thickness (plot values of the specimen thickness on the x axis and specimen thermal impedance on the y axis). The thermal conductivity [lambda] was calculated from the Eq. 2; the curve is a straight line whose slope is the reciprocal of the thermal conductivity. The intercept at zero thickness is the thermal interfacial resistance.
R = [[2d]/[[[lambda].sub.12]([T.sub.1] - [T.sub.2]) + [[lambda].sub.34]([T.sub.3] - [T.sub.4])]]X[([T.sub.2] - [T.sub.3]) + [[d.sub.D]/[d.sub.C]]([T.sub.3] - [T.sub.4]) - [[d.sub.B]/[d.sub.A]]([T.sub.1] - [T.sub.2])] (1)
[lambda] = [([[delta]R]/[[delta]d]).sup. - 1] (2)
where [[lambda].sub.12] is thermal conductivity of the hot meter bar material (W/m K), [[lambda].sub.34] is the thermal conductivity of the cold meter bar material (W/m K), A is the area of the reference calorimeter ([m.sup.2]). [T.sub.1] - [T.sub.2] is the temperature difference between temperature sensors of the hot meter bar (K), [T.sub.3] - [T.sub.4] is the temperature difference between temperature sensors of the cold meter bar (K), d is the distance between temperature sensors in the meter bars (m), [T.sub.1] is the warmer temperature of the hot meter bar (K), [T.sub.2] is the cooler temperature of the hot meter bar (K), [d.sub.A] is the distance between [T.sub.1] and [T.sub.2] (m), [d.sub.B] is the distance from [T.sub.2] to the surface of the hot meter bar in contact with the specimen (m), [T.sub.3] is the warmer temperature of the cold meter bar (K), [T.sub.4] is the cooler temperature of the cold meter bar (K), [d.sub.C] is the distance between [T.sub.3] and [T.sub.4] (m), and [d.sub.D] is the distance from [T.sub.3] to the surface of the cold meter bar in contact with the specimen (m).
Morphological observations on the composite silicone rubber were performed by means of the scanning electron microscope (SEM, model KYKY-2000, Instrument Co., China Academy of Science, China). Observations were carried out on the cross-sections of the samples to study alumina distribution and morphology affecting the thermal conductivity of the system.
The dielectric property measurement of composites was conducted on a electric property apparatus (Model: S914, testing frequency: 1 MHz, Shanghai Aishi Electronic Co., Shanghai, China) at room temperature, following the standard GB/T1410-1989. The samples for dielectric constant measurement are of a diameter of 80 mm and of thickness of 1 mm in size. The dielectric constant can be calculated by Eq. 3 by measuring the dielectric capacitance (c) (26):
N = [c/[(A/d)[N.sub.0]]] (3)
where N and c are dieletric constant and dielectric capacitance of the specimen, respectively, A and d are the area of cross-section surface and thickness of the specimen, respectively, and [N.sub.0] is the electric constant in a vacuum.
The CTE of the composite was measured by using a bar-shaped specimen in a linear dilatometer (Model IDM C0007, Shanghai Lidu Instrument Co., Shanghai, China) with a heating rate of 10 C/min, from room temperature up to 250 C according to the Standard GB1036-89. The samples for CTE measurements are 6 mm X 6 mm X 100 mm in size and were prepared from the hot pressed panels. Equation 4 was used to calculate CTE:
a = [[[DELTA]L]/[[L.sub.0][DELTA]T]] (4)
where a is the CTE, [DELTA]T is the difference of temperature, and [DELTA]L/[L.sub.0] is the thermal expansion ratio of a sample.
The mechanical property tests of the samples were conducted on a screw-driven universal testing machine (Model ZMGI 250. Shenzhen SANS Co., Shenzhen, China) at a cross-head speed of 50 mm min (1) to determine the tensile strength with standard GB/T528-1998. The samples were prepared according to standard GB/T9865.1; the thickness is 2 mm, and the length of tested part is 25 mm for all samples.
RESULTS AND DISCUSSION
The Effect of Filler Size Distribution on Thermal Conductivity
Cumberland and Crawford (29) calculated the maximum packing volume fraction of binary mixture of spheres as a function composition with diameter ratios as a parameter. The result is plotted in Fig. 3. It can be seen from Fig. 3 that the packing volume fraction reaches the maximum value when the volume fraction of small spheres is 26.5%, and it increases with increasing the diameter ratio of filler particles could improve the packing volume faction of hybrid fillers and enhance the thermal conductivity of composites owing to the formation of more conductive pathways or networks from filler particles.
[FIGURE 3 OMITTED]
In this paper, four kinds of alumina powders with a different particle sizes of 0.5, 5.0, 10, and 30 [micro]m, respectively, were used as candidate thermal-conductive fillers for silicone rubber. Based on those alumina particles with a single size, three kinds of binary mixtures of alumina fillers with different particle size distribution were obtained by mixing the 0.5 [micro]m alumina particles with the 30 [micro]m alumina filler or the 10 [micro]m alumina filler or the 5 [micro]m alumina particles, respectively, i.e., 30 [micro]m + 0.5 [micro]m, 10 [micro]m + 0.5 [micro]m, and 5 [micro]m + 0.5 [micro]m. The obtained three kinds of hybrid alumina fillers have different average diameter ratios, namely, [D.sub.L]/[D.sub.S] = 30 [micro]m/0.5 [micro]m = 60, [D.sub.L]/[D.sub.s] = 10 [[micro]m]/0.5 [[micro]m] 20, [D.sub.L]/[D.sub.s] = 5.0 [[um]m]]/0.5 [[micro]m]=10 (here, [D.sub.L] and [D.sub.S] are the respective average diameters of large and small alumina particles). Keeping the total hybrid filler content fixed at 55 vol%, the thermal conductivity of filled silicone rubbers was investigated as a function of relative volume fraction of the 0.5 [[micro]m] small particles in the binary mixture of alumina ([V.sub.s]). The thermal conductivity as a function of [V.sub.s] is depicted in Fig. 4.
[FIGURE 4 OMITTED]
It can be seen from Fig. 4 that the maximum thermal conductivity value appeared at different [V.sub.s], for the three cases with varying [D.sub.L]/[D.sub.S]. For the case of a [D.sub.L]/[D.sub.S] = 60 binary mixture of fillers, the highest thermal conductivity of 1.78 W/m K was obtained at [V.sub.s] = 0.2, which is 1.20 times higher than that of the 30 [[micro]m] alumina-filled silicone rubber (1.48 W/m K). For the case of a [D.sub.L]/[D.sub.S] = 20 binary mixture, the highest thermal conductivity of 1.62 W/m K was obtained at [V.sub.S] = 0.3, which is 1.27 times higher than that of the 10 [[micro]m] alumina-filled silicone rubber (1.28 W/m K.) For the case of a [D.sub.L]/[D.sub.S] = 10 binary mixture, the highest thermal conductivity of 1.50 W/m K was obtained at [V.sub.S] = 0.35. This value is 1.32 times higher than that of the 5 [[micro]m] alumina filler-reinforced silicone rubber (1.14 W/m K). Comparatively speaking, the improvement of thermal conductivity by filler size distribution was pronounced for the case of [D.sub.L]/[D.sub.S] = 60 binary mixture of fillers. The obtained highest thermal conductivity could be ascribed to the fact that small particles easily enter into space that large particles cannot occupy and form a higher packing density of the filler in the matrix; thus, the thermal conductivity increases because of the decreased thermal resistance among adjacent conductive fillers. The use of hybrid alumina fillers would result in a more compact packing density structure in the silicone rubber matrix and the easy formation of random bridges or pathways from conductive particles (see Fig. 5), which facilitate phonon transfer and lead to higher thermal conductivity (28), (30); therefore, to achieve a high thermal conductivity, hybrid fillers of different particle sizes are suggested (26).
[FIGURE 5 OMITTED]
According to Cumberland and Crawford (29), the maximum packing volume fraction of binary mixture of spheres appeared at the volume fraction of small spheres of 0.265. In our experiments, the maximum thermal conductivity (corresponding to the maximum packing volume fraction of fillers) appeared at [V.sub.S] = 0.20, 0.30, and 0.35, respectively, for the three cases of [D.sub.L]/[D.sub.S] = 60, 20, and 10 binary mixtures. The highest thermal conductivity shifts towards the higher relative content of 0.5 [[micro]m] small particles with decreasing the [D.sub.L]/[D.sub.S]. The main reason for the deviations may be due to the fact that practically the shape of alumina particle is not sphere, on which the Cumberland's assumption was based, additionally, the Cumberland's model (29), (30) did not take the interactions between the rubber matrix and filler particles into consideration, which limited the freely packing of filler particles in matrix.
[FIGURE 6 OMITTED]
It is found from Fig. 4 that at [V.sub.S] = 0 (0.5 [micro]m] particles content being zero) the thermal conductivity of the larger filler particles-filled rubber is higher than that of the smaller particles-filled system, namely, 1.48, 1.28, and 1.14 W/m K for the 30,10, and 5 [[micro]m] alumina particlesfilled systems, respectively. Clearly, the size of alumina particles has an effect on the thermal conductivity of composite rubber at the same filler content. So, the use of larger particles is an effective way of increasing the thermal conductivity with increasing particles size might be attributed to greater stability of thermal-conductive pathways for the larger particles. With increasing the size of particles, the interfacial area per unit volume of rubber between the particles and rubber matrix will decrease. Therefore, there may be less rubber layer around each particle at the same content of rubber compared with the smaller particles. So, the heat-conductive pathways can be considered more stable for the larger particles, because the thicker conductive pathways have less chance of being disrupted by contacting grains. Therefore, to maximize the formation of conductive networks while minimizing the heat resistance along the heat flow path, it is recommended to decrease the amount of thermally resistant junctions involving a polymer layer between adjacent filler units by using large filller (28).
The Effect of Filler Size Distribution on CTE
The CTE of filled silicone rubbers as a function of [V.sub.S] is described in Fig. 6. Figure 6 reveals that the value of CTE decreased with increasing the [V.sub.S] for the three cases above. The CTE of filled silicone rubbers are 80-59, 78-56 and 75-53 X [10.sup.-6]/K, respectively, for the three cases of [D.sub.L]/[D.sub.S] = 60, 20, and 10 binary mixture of fillers at the [V.sub.S] = 0-50%. It is observed that, to reduce the CTE of composites, the use of small filler particles is of advantage over large particles, because small particles possess higher specific surface area, which resulted in more physical crosslinking points and stronger mechanical interactions between fillers and rubber matrix bonding the matrix together. Therefore, by varying the [V.sub.S] in the binary mixture of alumina, the desired CTE could be obtained (26). For elastomeric thermal pad, the low CTE of composite silicone rubber facilitates effective transferring heat from heat resource to the aluminum heat spreader due to the CTE match between the two kinds of materials when subjected to a predetermined number of cycles of heating and cooling to test its reliability during temperature cycle [1,28].
From Fig. 6, a conclusion can be drawn that both the particle sizes and size distribution of alumina have an influence on the CTE of composite rubber.
The Effect of Filler Size Distribution on Dielectric Constant
The dielectric constant of filled silicone rubbers as a function of [V.sub.S] is plotted in Fig. 7. Figure 7 shows that the dielectric constant of composite rubber first decreases to a low value with increasing [V.sub.S], then increases with increasing [V.sub.S]. For both [D.sub.L]/[D.sub.S] = 20 and 10 binary mixture of fillers, the lowest values of dielectric constant. i.e., 3.8 and 3.4, respectively, were obtained at [V.sub.S] = 0.3. For the case of a [D.sub.L]/[D.sub.S] = 60 binary mixtures, dielectric constant slightly varies with increasing [V.sub.S]. According to Fig. 7. the dielectric constant of composite rubber filled with large particles was slightly higher than that of the one filled with small particles at [V.sub.S] = 0. The reason could be ascribed to the fact that there exist many microvoids at the phase interface between small particles and rubber matrix owing to the hyper specific surface area of the small particles, and such microvoids can decrease the dielectric constant of the material, because the dielectric constant of voids is approximately one (26). At [V.sub.S] greater than 0.2 or 0.3, the effect of interfacial polarization between small particles and matrix overrides the micro voids effect of filler size distribution in the dielectric constant of filled systems. So, the dielectric constant increased as the [V.sub.s] continued to increase.
[FIGURE 7 OMITTED]
The Effect of Filler Size Distribution on Tensile Strength
The tensile strength of filled silicone rubbers as a function of [V.sub.s] is shown in Fig. 8. It is found that in both [D.sub.L]/ [D.sub.s] = 20 and 10 binary mixture of fillers cases, the tensile strength of filled silicone rubber as a function of [V.sub.s] exhibited the maximum values at [V.sub.s] = 0.2; for the case of [D.sub.l/ [D.sub.s] = 60, the tensile strength showed the highest values at [V.sub.s] = 0.3, and the improvement of tensile strength of system by filler size distribution was higher than those of the [D.sub.L] / [D.sub.s] = 20 and 10 binary filler systems, i.e., 133% vs. 67% and 85% of tensile strength improvement, respectively. When [V.sub.s] = 0.3, the tensile strength of large particles-filled rubber was lower than that of one filled with small particles for the three cases, because there exhibited stronger interactions between small particles and matrix resulting from the higher surface energy of small particles, compared to large particles [4, 26].
The Effect of Filler Size Distribution on Elongation at Break
The elongation at break of filled silicone rubbers as a function of [V.sub.s] is depicted in Fig. 9. It is observed that in both [D.sub.L]/[D.sub.s] = 60, 20 binary mixture of fillers cases, elongation at break of filled silicone rubber basically decline with increasing [V.sub.s]; for the case of [D.sub.L]/[D.sub.s] = 10, [V.sub.s] has little influence on the elongation at break of filled system. When [V.sub.s] = 0, the large particles-filled silicone rubber showed higher elongation at break than that of the one filled with smaller particles for the three cases. The results demonstrate that filler with a larger particle size is preferable to increase the elongation at break and to decrease the tensile strength. The reinforcing mechanism caused by filled particles is due to the linking of aggregates by polymer matrix. It is believed that a single polymer chain is adsorbed on two separate aggregates, resulting in the formation of physical crosslinkage; moreover, filler with larger size leads to less contact sites and decreasing physical crosslink density. As a result, the increase in [V.sub.s] in binary mixture of fillers decreases the elongation at break of filled rubber (4).
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
The three kinds of binary mixtures of alumina particles with different diameter ratios at the fixed total volume fraction of 55%, i.e., 30 [micro]m/0.5 [micro]m = 60, 10 [micro]m/5 /[mu]m = 20, 5 [micro]m/0.5 [micro]m = 10, were employed to reinforce silicone rubber. The results indicate that silicone rubber filled with a binary mixture of alumina particles exhibited improved thermal conductivity, tensile strength, and decreased dielectric constant, compared to a single particle size filler-reinforced one; moreover, the maximum improvement was obtained when the [V.sub.s] is 20-35%; whereas the CTE of filled silicone rubber obviously reduced with increasing the [V.sub.s] and the elongation at break slightly decreased. When the [V.sub.s] is zero, the larger particles-filled silicone rubber showed higher thermal conductivity, CTE, dielectric constant and elongation at break, and lower tensile strength compared to those of the smaller particles-filled one. Therefore, both filler particle sizes and particle size distribution have an influence on the physical properties of a binary mixture of alumina filler-reinforced silicone rubber composites.
The authors sincerely thank Mr. Jingli Kou and Mr. Yi Jiang for their assistance during the experimental procedures.
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Correspondence to: Prof. Demei Yu; e-mail: email@example.com
Contract grant sponsor: the 4th Research Academy of China Aerospace Science and Technology Corporation (CASC).
Published online in Wiley InterScience (www.interscience.wiley.com).
[c] 2008 Society of Plastics Engineers
Wenying Zhou, (1) Demei Yu, (1,2) Caifeng Wang, (3) Qunli An, (4) Shuhua Qi(4)
(1) State Key Laboratory of Electrical Insulation and Power Equipments, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
(2) Department of Applied Chemistry, School of Science, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China
(3) Xi'an Sunward Aerospace Materials Co. Ltd., the 4th Research Academy of Aerospace, CASC, Xi'an 710025, People's Republic of China
(4) Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an 710072, People's Republic of China
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|Author:||Zhou, Wenying; Yu, Demei; Wang, Caifeng; An, Qunli; Qi, Shuhua|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Jul 1, 2008|
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