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fluorinated polyimide composite materials using direct current electrical fields
Thermochimica Acta 684 (2020) 178491
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Enhancing thermal conductivities of hexagonal boron nitride/fluorinated polyimide composite materials using direct current electrical fields
T
Masashi Haruki*, Jun Tada, Ren Funaki, Hajime Onishi, Yukio Tada Faculty of Mechanical Engineering, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan
ARTICLE INFO
ABSTRACT
Keywords: Thermal conductivity Electrical insulation property Composite Hexagonal boron nitride Fluorinated polyimide
The effect of the electrical field treatment using the direct current (DC) during preparation of the composite sheets on their effective thermal conductivity (TC) and electrical insulation property for the hexagonal boron nitride (hBN)/fluorinated polyimide was studied. By the electrical field treatment, the aggregation of hBNs was obviously observed in the cross-sections of the composite sheets, and the number of hBNs lying parallel to the inplane direction of the sheets was decreased based on XRD study. Moreover, the effective TC in the out-of-plain direction of the composite sheets treated with a one-way electrical field was increased approximately 1.5-fold that of the corresponding sheets prepared without electrical field treatment at 14.2 vol%. On the other hand, electrical insulation properties were deteriorated by the electrical field treatment, and the values for breakdown voltage against DC roughly approximated that following electrical field treatments to high content of hBNs regardless of their size.
1. Introduction Functional engineering plastics have various superior properties such as heat resistance, a high modulus of elasticity, mechanical strength, a low dielectric constant, and weight that is very light relative to comparable materials. Therefore, they are used in a variety of industrial fields. Moreover, they have been shown to become promising biomaterials by the inducement of functional chemical groups and skillful control of their chemical structures [1,2]. For example, cell sheets can be easily detached from the culture substrate without trypsin treatment by using a thermosensitive polymer as a ground layer [3]. In that technique, it is necessary to promptly control the temperature by enough low level of output power in order to avoid damage to cells. Therefore, a high level of thermal conductivity (TC) is required for the instruments used in such cell cultivations. Bioavailable polymer materials usually play important roles in the construction of the experimental instruments and practical equipment used in biotechnology. Since polymers generally have a low level of TC, however, a technique has been developed whereby highly conductive thermal fillers are added. Hexagonal boron nitride (hBN), aluminum nitride, silicon carbide and functional nanocarbon has received much attention as high TC fillers in reports from several investigations [4–10]. Polyimide (PI) is one of the toughest polymers and has excellent thermal stability. In particular, fluorinated PI contains fluorine groups
⁎
that exhibit excellent biocompatibility while retaining mechanical strength and heat resistance that has been demonstrated by conventional PIs [11–16]. The TC of fluorinated PI remains low, however, as is the case with both general polymers and conventional PIs. Several researchers have reported the influence of adding highly thermal conductive fillers into the PI materials to improve their TC [17–23]. In previous work, we focused on the Kapton type of PI that is obtained from the polymerization of pyromellitic dianhydride and 4,4′-diaminodiphenyl ether (ODA). hBN was used as a high TC filler in order to investigate the effect that its content and size would exert on the effective TC of a hBN/PI composite [24]. Moreover, the effect of the coaddition of hBNs and carbon nanofibers (CNFs) on the effective TC of the CNF/hBN/PI composite sheet was also investigated, and a small amount of CNFs was found to drastically increase the effective TC of the composites [25]. Recent studies on enhancing the TC of polymers have focused not only on the addition of fillers but also on controlling the orientation and alignment of the fillers in the polymer matrix via the use of additional treatments. The additional treatments involved loading shear stress [26,27] and processing in either a magnetic field [28,29] or an electrical field [30–35]. For the electrical field treatment, hBNs were an attractive high TC filler because of the high response hBNs have shown to an electrical field due to a wide band gap. And, thermally conductive paths made of hBNs were established inside hBN/polysiloxane [33,34]
Corresponding author. E-mail address: [email protected] (M. Haruki).
https://doi.org/10.1016/j.tca.2019.178491 Received 3 August 2019; Received in revised form 21 November 2019; Accepted 20 December 2019 Available online 23 December 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram of the apparatus used for electric field treatments. (a) Overall view; (b) Detail view of the electrodes. 1: Electrode equipment, 2: Stage, 3: High voltage DC source, 4: Temperature indicator, 5: Temperature controller, 6: Cartridge heaters, 7: Stainless steel block, 8: Acrylic box covered with heat insulator, 9: Fan, 10: PTFE cover, 11: Copper plate, 12: Electrodes (SUS304), 13: Sample, 14: Clearance, 15: Electric wire, T1 and T2: Thermocouples.
and hBN/polymethylsiloxane [31] composite sheets. Therefore, the application of an electrical field treatment to produce the hBN/PI composites is considered advantageous. In preparing the filler/PI composites, however, fillers cannot be added directly into the PI matrix, and therefore, the fillers are generally added into a polyamic acid (PAA) solution that is the precursor of PI. In this process, the solvent must be removed from the filler/PAA composite and the reaction from the filler/ PAA to the filler/PI requires high temperature to produce a filler/PI composite, which would cause a collapse of the orientation and alignment of the hBNs obtained by the electrical field treatment. In the present work, the fluorinated PI consisting of 2,2-bis(3,4anhydrodicarboxyphenyl)-hexafluoropropane (6FDA) and ODA was focused on as a representative fluorinated PI, and a method to apply the electrical field treatment that could control the orientation and alignment of the hBNs was studied to prepare a hBN/fluorinated PI composite with high TC. Moreover, the effect that the electrical field treatment would exert on the electrical insulation properties of the hBN/fluorinated PI composite sheet was also discussed.
composite sheet. In the drying and curing processes, the hBN/fluorinated PAA solution on the plate was let stand under an ambient air atmosphere for the first 1 h. The solution sheet was then introduced into a vacuum oven (DP33, Yamato Scientific Co.), and the temperature in the oven was first maintained at 60 °C for 24 h. The dried composite sheet on the plate was ejected from the oven and was then separated from the plate. The composite sheet was then introduced into a vacuum oven again and was dried at 100 °C for 1 h, followed by further drying at 150 °C for 1 h and at 200 °C for 1 h. Finally, the composite sheet was imidized at 250 °C for 1 h using a hot plate under an ambient air atmosphere. 2.3. Apparatus and procedures for applying a direct current electrical field A self-built apparatus was used to apply electrical field treatments to the hBN/fluorinated PAA solutions in order to orient and align the hBNs dispersed in the PAA matrix. A schematic diagram of the apparatus developed is shown in Fig. 1. The apparatus mainly consisted of a device to apply direct current (DC) to the sample and a high-voltage DC power supply unit (HER-10R6, Matsusada Precision Co.), as shown in Fig. 1(a). The electrical field treatments were carried out in a simple thermostatic air bath made of an acrylic box and a fan. A detailed diagram of the electrode parts appears in Fig. 1(b). SUS304 stainless steel plates with diameters and thicknesses of 30 and 2 mm, respectively, were used as the electrodes. The bottom side of the SUS304 plate was also used as a substrate for the application of a sample. The electrodes were surrounded by blocks made of polytetrafluoroethylene (PTFE) that served as electrical insulation. Clearance was needed between the sample surface and the upper side electrode to avoid the formation of a short-circuit due to the relatively high electrical conductivity of DMF used in the fluorinated PAA solution. The distance between the electrodes was set at 2.2 mm except when the effect of distance between the electrodes was investigated. Two types of DC electrical field treatments were used in the present work. For one, the poles of the electrodes were fixed throughout the treatment (one-way DC). For the other, the poles of the electrodes were switched at certain intervals (switching DC). The treatment times for one-way DC were from 1 to 4.5 h. On the other hand, the times for switching DC were only 2.5 and 4.5 h. The poles of the electrodes were switched every 30 min up to 1.5 h or every hour up to 3 h, and the poles were not changed from 1.5 to 2.5 h or from 3 to 4.5 h, respectively, because of the drying step for the last 30 min. In experiment, a sufficiently mixed hBN/fluorinated PAA solution was applied to the SUS304 stainless-steal plate that also served as the bottom-side electrode. The plate was set on a copper plate, and a DC electrical field was applied to the sample. The interior of the acrylic box was heated to 60 °C only for the last 30 min during the electrical field treatment. After the electrical field treatment, the sample was further dried in the vacuum oven, and heat imidization was carried out on a hot plate in the same manner as for preparation of the composite sheets without the electrical treatment, as described in subsection 2.2.
2. Experiment 2.1. Materials 6FDA with a purity > 99 mol% used as an aromatic tetracarboxylic dianhydride was supplied from Daikin Industries Co. ODA with a purity > 97 mol% was used as an aromatic diamine and was purchased from Sigma-Aldrich Co. These materials served as monomers to produce the fluorinated PI in the present work. Both monomers were dehydrated at 120 °C for 60 min before polymerization. N,N-Dimethylformamide (DMF) with a purity > 99.5 vol% was used as the reaction media to produce PAA and was purchased from Nacalai Tesque Co. hBN fillers with median diameters of 11 μm (hBN_11) and 0.2-0.8 μm (hBN_0.5) were supplied from Showa Denko Co. We adopted a theoretical density of 2.27 g/cm3 for hBN [36] to estimate the volume fraction for both hBN_11 and hBN_0.5. On the other hand, the density of commercial Kapton (1.42 g/cm3) was used as the density of the fluorinated PI. 2.2. Preparation of the hBN/fluorinated PI composite materials without an electrical field In the present work, a fluorinated PAA solution was first produced via the polymerization of 6FDA and ODA, which is the precursor of fluorinated PI. In polymerization, ODA was first dissolved in DMF, and a powdery form of 6FDA that had the same molar number as ODA was then added to the ODA + DMF solution. The solution was stirred with a magnetic stirring bar in an ice bath for 1 h and then continuously stirred at room temperature for 47 h to obtain a fluorinated PAA solution. The concentration of the fluorinated PAA solution was adjusted to 20 wt% of the fluorinated PAA + DMF mixture. Next, hBN was added to the fluorinated PAA solution, which was stirred again for 12 h, and was subsequently sonicated for 10 min. The well mixed solution was then applied to a stainless steel (SUS304) plate using a spatula to obtain a 2
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2.4. Evaluating the characteristics of composite materials
prepared with similar sheet thicknesses in order to reduce the differences in the effect of the interfacial resistance based on the methods and conditions of the sample preparations for comparison of the effective TCs. The sheet thicknesses were 80 ± 19 μm for all samples used in the effective TC measurements. Moreover, a thermal conductive paste (Sanhayato Co., SCH-20: 0.84 W/(m·K)) was also applied between the copper rods and sheet surfaces to reduce the interfacial resistance, and the effective TCs of the composite sheets were obtained from multiple measurements. The sample temperatures that were estimated as the average temperatures of both surfaces were 32 ± 4 °C for all measurements. The electrical breakdown voltage (BDV) against the DC voltage was measured using the high-voltage DC power supply that was also used for the electrical field treatment. The hand-built measuring device is shown in Fig. S1 of the supplementary material. The upper and bottom electrode plates (SUS304) had diameters of 10 and 30 mm, respectively, and were connected to the negative and positive poles of the power source, respectively. For the measurement, a sample with 20 mm in diameter was placed onto the center of the bottom electrode plate. The upper electrode plate was then placed onto the sample sheet, and DC voltage was applied to the sample sheet via the plates. The boosting speed of the DC voltage was approximately 200 V/s during the measurement.
The dispersion behaviors of the hBNs in the composite sheets were visually investigated using a digital microscope (VH-5500, Keyence Co.). Moreover, the orientational states of hBNs in the composite sheets were analyzed using an X-ray diffractometer (XRD, MiniFlex II, Rigaku Co.). Copper Kα radiation was used and the voltage and current were adjusted to 30 kV and 15 mA, respectively. The orientations of hBNs in the composite sheet were evaluated based mainly on a method reported by Cho et al. [34]. The peak area fraction expressed by Eq. (1) was used for the evaluations.
Peak area fraction=
Aa × 100 [%] Aa + A c
(1)
In Eq. (1), Aa and Ac represent the peak areas found at around 2θ = 42° and 27°, respectively. The peaks at around 2θ = 42° and 27° correspond to diffractions from the (100) and (002) planes of hBN, respectively, and they are also identified as the signals of the a-axis and caxis of hBN, respectively. The effective TC was measured using a self-built apparatus based on a temperature-gradient, steady-state method that was established in our previous work. The apparatus and measurement method are explained in detail in our previous work [24], and only a brief explanation is given here. Each sample was cut into a circular shape with a diameter of 20 mm and placed between the two copper rods. Heat from a band heater (Sakaguchi E.H VOC Co., MB1A1JN2) was supplied from the top edge of the upper-side of the copper rod and was then eluted into a water bath that maintained a constant temperature. Heat flux that passed through the composite sheet in the out-of-plane direction was estimated from the TC of the copper rods and the temperature distribution on the copper rods using Fourier’s law, as shown in Eq. (2).
q=
kCu
dTCu dx Cu
3. Results and discussion 3.1. Orientation and dispersion states of hBNs in composite sheets The orientation and dispersion behaviors of hBNs in the hBN/ fluorinated composite sheet were investigated by both visualization and XRD studies. As for the conditions of the electrical field treatments, the distance between the electrodes was set to 2.2 mm, and 1500 V of DC voltage was applied to the sample for 4.5 h. The cross-sectional microscopic images of the pristine fluorinated PI and hBN_11/fluorinated PI composite sheets are shown in Fig. 3. The composite sheets shown in the figures were made up of 14.2 vol% of hBN. The observed samples were embedded in epoxy resin, and were cut out using a microtome. A cross-section of the pristine fluorinated PI is shown in Fig. 3(a) as a reference. Small black lines that seem like defects appear in Fig. 3(a). However, similar black parts and gray parts were also found in the epoxy parts. Moreover, visible cracks were not found in the SEM images shown in Fig. S2 of the supplementary materials. Therefore, the black lines in Fig. 3(a) should not be cracks. In the composites without electrical field treatment, the hBNs were relatively uniformly dispersed inside the sheet (Fig. 3(b)). On the other hand, the cross-section of the composites prepared under a switching of the DC electrical field obviously differed from that of the composite sheets without electrical field loading, whereby aggregations of hBN grains were observed inside the sheet (Fig. 3(c)). Those aggregations were likely due to the dielectric polarization of the hBNs, as reported in the literature [31,33,34]. Moreover, in the composite sheet prepared under a oneway DC electrical field, the hBNs looked much more aggregated, and portions where hBNs were sparse were found in the vicinity of the negative electrode (Fig. 3(d)). This non-uniform distribution of hBNs could have been caused by electrophoresis due to the negative charge at the surface of the hBNs, which also was observed in the electrical field treatment of a hBN/polysiloxane composite reported by Fujihara et al. [33]. On the other hand, since the direction of the electrophoresis was switched at certain intervals, large non-uniformities in the distribution of hBNs could not be found for the switching DC treatment. Moreover, XRD analyses were carried out to obtain information about the orientation of the hBN grains, and the peak patterns obtained are shown in Fig. 4. The samples included the hBN_11/fluorinated PI composite sheet prepared without electrical field treatment, the composite sheet prepared under electrical field treatments with switching DC, and the sheet prepared with one-way DC operation. Pristine
(2)
In Eq. (2), q is the heat flux. kCu shows the TC of the tough pitch copper, and a value of 381 W/(m·K) was used in the present work. dTCu/ dxCu is the measured temperature gradient of the copper rods. The effective TC of the composite sheet was estimated using Eq. (3) [24,37].
kseff =
q/
dTs dx s
q
ts Ts
(3)
eff
In Eq. (3), ks represents the effective TC of the sample sheet, ΔTs and ts show the temperature difference between both surfaces of the sample sheet and the sheet thickness, respectively. The effective TC, kseff, obtained using Eq. (3) includes the effect of the interfacial resistance between the surfaces of the copper rods and the sheet surfaces. The relationship between kseff and the actual TC is represented by Eq. (4).
Rt = Rs + Ri =
ts t = s + Ri ks k seff
(4)
In Eq. (4), Rt indicates the total thermal resistance, and RS and ks represent the sample’s own thermal resistance and TC, respectively. Ri shows the interfacial thermal resistance. The relationships between the effective TC obtained by Eq. (3) and the sheet thickness for pristine fluorinated PI and the 17 vol%-hBN_11/ fluorinated PI are shown in Fig. 2(a). The effective TCs obtained by Eq. (3) showed similar values regardless of the sheet thicknesses that ranged between approximately 50 and 100 μm for both sheets. Moreover, the relationships between total thermal resistance and the sheet thickness are described in Fig. 2(b). The relationships obtained were approximately linear for both sheets, and the TCs obtained by Eq. (4) using slopes of the lines in Fig. 2(b) approximated the values obtained by Eq. (3). The deviations were within 10 % for all effective TCs shown in Fig. 2(a). Therefore, the discussion below was conducted using the effective TCs obtained by Eq. (3). In the experiment, the samples were 3
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Fig. 2. Influence of the sheet thickness on the effective thermal conductivity and the thermal resistance for pristine fluorinated PI and17 vol%-hBN_11/fluorinated PI. (a) Effective thermal conductivity; (b) Thermal resistance.
fluorinated PI and powdery hBN_11 were used as references. The hBN contents in the composite sheets were adjusted to 14.2 vol%. The degree of the orientation for hBNs in fluorinated PI was evaluated using the peaks at around 2θ = 27° (c-axis) and 42° (a-axis), as described above. The small differences in the angles between the samples were likely caused by the different shapes of the composite sheets rather than by a change in the crystalline structure of hBN. As for the hBN/fluorinated PI composite sheets, the peak assigned to the c-axis was dominant regardless of whether the electrical field treatments were used. By applying the electrical field treatment to the hBN/fluorinated PI composites, the peak intensity of the c-axis was notably decreased whereas the peak of the a-axis was either unchanged or was only slightly increased. Similar behavior was even more pronounced when the one-way DC electrical field treatment was used. Moreover, peak area fractions estimated by Eq. (1) were obtained at several levels of hBN content, as shown in Fig. 5. As for hBN_11, the order of the values for the peak area fractions was one-way DC
treatment > switching DC treatment > without DC treatment, and the value obtained by the one-way DC treatment was much higher than that without DC treatment. The size of hBN_11 was relatively large compared with the sheet thickness of the hBN/fluorinated PI sheet finally produced, and the hBNs had a sheet shape. Therefore, without electrical field treatment, the hBN should lie down in a fluorinated PAA matrix as the thickness of the PAA matrix was reduced during drying. On the other hand, the percentage of the supine hBNs parallel to the inplane direction of the sheet would be decreased by electrical field treatment. Some hBNs would collectively stand by the formation of aggregation structures or would rise up alone with a tilted angle. hBNs have a very large degree of thermal anisotropy such that the TC in the in-plane direction (a-axis) can be more than 20-fold higher than that in the out-of-plane direction (c-axis) [29], and XRD studies have suggested that the effective TC of the hBN/fluorinated PI composites would be improved by electrical treatment. Fig. 3. Images of the cross-sections of the pristine fluorinated PI and 14.2 vol%-hBN_11/ fluorinated PI composite sheets embedded with epoxy resins obtained by the digital microscope (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V). (a) Pristine fluorinated PI; (b) 14.2 vol%-hBN_11/fluorinated PI composite sheet without electric field treatment; (c) 14.2 vol%-hBN_11/fluorinated PI composite sheet with switching DC treatment for 4.5 h (1 h × 3 and 1.5 h × 1); (d) 14.2 vol %-hBN_11/fluorinated PI composite sheet with one-way DC treatment for 4.5 h.
4
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Fig. 4. XRD patterns of the 14.2 vol%-hBN_11/ fluorinated PI composite sheets with and without electric field treatments along with the patterns of pristine fluorinated PI and powdery hBN (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 4.5 h). (b) shows the XRD pattern enlarged at the range of the peaks of a-axis.
3.2. Effective thermal conductivity
composite sheets were lower than those of the hBN_11/fluorinated PI composite sheets at the same hBN content both with and without electrical field treatment. As the results of the XRD study shown in Fig. 5 show, however, the peak area fractions of the hBN_0.5/fluorinated PI were much higher than those of the hBN_11/fluorinated PI. The reasons of this relationship should be that total contact resistance between hBN and fluorinated PI and thermal tortuosity should increase with decreasing the hBN size [24]. The effective TC of the 14.3 vol %-hBN_0.5/fluorinated PI composite sheets obtained by the one-way DC treatment was also higher (≈1.5-fold) than that of the composite sheets prepared without the electrical field treatment, which was similar to the case of the 14.2 vol%-hBN_11/fluorinated PI composite sheet. Moreover, the effective TC of the hBN_0.5/fluorinated PI composite prepared using the electrical field treatment was higher than that of the hBN_11/fluorinated PI composite without the electrical field treatment, although the effective TC of the sheets was generally increased with increases in the size of the hBNs added to the polymer [24]. These results suggest that the relationship between hBN size and the effective TC could be changed intentionally by electrical field treatment. Moreover, the relationships between the duration of the electrical field treatment and the effective TC of the composite sheets were investigated. The results of the experiments for 14.2 vol%-hBN_11/ fluorinated PI composite sheets with an applied DC voltage of 1500 V using an electrode distance of 2.2 mm are shown in Fig. 7. As for both one-way and switching operations, the remarkable dependency of the effective TC on treatment time could not be found within a range of from 1 to 4.5 h, which indicated the morphology changes of the hBNs
The relationships between the effective TC, hBN content, hBN size and methods of electrical field application are shown in Fig. 6. The 1500 V of the one-way DC or switching DC electrical field was applied to hBN/PAA solution sheet for 4.5 h. The distance between the electrodes was set to 2.2 mm. The effective TC of the hBN/fluorinated PI composite sheets was increased as the hBN content increased regardless of the hBN size. Moreover, this result was obtained both with and without the electrical field treatment. By applying the electrical field treatment, the effective TC of the hBN/fluorinated PI composite sheets was clearly increased for both switching DC and one-way DC treatments. The one-way DC treatment increased the effective TC of the composite sheets up to a much higher level than that obtained by the switching treatment. At 14.2 vol% of hBN_11, the effective TC of the composite sheets prepared by the one-way DC treatment showed 0.59 W/(m·K), which was approximately 1.5 times higher than that of the corresponding composite sheets prepared without the electrical field treatment (0.40 W/(m·K)). These results qualitatively agreed with the results from the investigations of the orientations of hBNs based on the XRD studies although the hBN-poor layer that was formed in the direction perpendicular to the heat transfer direction shown in Fig. 3d led to low thermal conductivity [38]. Fig. 3d also indicates how the hBN grains seemed to make connections with each other. Therefore, the positive effects of the aggregations and alignment of the hBNs obtained by the one-way DC treatment on the effective TC were expected to be dominant in the present study. The values of the effective TC for the hBN_0.5/fluorinated PI
Fig. 5. Peak area fractions obtained by the XRD patterns for the hBN_11/fluorinated PI and hBN_0.5/fluorinated PI composite sheets with and without electric field treatments (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 4.5 h). (a) hBN_11/fluorinated PI; (b) hBN_0.5/fluorinated PI. 5
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Fig. 6. Relationships between hBN content and effective TC for the hBN_11/fluorinated PI and hBN_0.5/fluorinated PI composite sheets with and without the electric field treatment (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 4.5 h). (a) hBN_11/fluorinated PI; (b) hBN_0.5/fluorinated PI.
Fig. 7. Relationship between the electrical treatment duration and the effective thermal conductivity for the 14.2 vol%-hBN_11/fluorinated PI composite sheet (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V).
Fig. 8. Effects of the applied voltage and the electrode distance of the one-way DC treatment for 1 h on the effective TC of the 14.2 vol%-hBN_11/fluorinated PI composite sheet.
inside the sheets might be nearly complete within 1 h. Moreover, the effects that the application of voltage and the distance between electrodes exert on the effective TC were also investigated. The effective TCs for the 14.2 vol%-hBN_11/fluorinated PI composite sheets prepared via one-way DC treatment for 1 h are shown in Fig. 8. As shown in the results for the electrode distances of 2.2 and 5.9 mm, the effective TC depended on the application voltage, and the increment of the values appeared with increases in the load voltage of less than 1500 V. Plateau behavior appeared at voltages ranging from 1500 to 3000 V for changes in the effective TC obtained when using an electrode distance of 5.9 mm. That plateau behavior was also found in the literature for the hBN/silicone rubber composite sheet of 10 vol% of hBN with 50 Hz of the AC electrical field treatment [31], which was considered to be caused by a saturation of the chain formation of hBNs. On the other hand, the distance of the electrodes (1.2–5.9 mm) had very little impact on the effective TC under load voltages that ranged from 500 to 1500 V.
of electrical field treatment on the electrical insulation properties of the composite sheets were also investigated. The relationships between the sample thickness and the breakdown voltage of the composite sheets are shown in Fig. 9. Experiments were carried out for both the hBN_11/ fluorinated PI and hBN_0.5/fluorinated PI composite sheets. The electrical field treatments were conducted using one-way DC method under an electrode distance of 2.2 mm and a DC voltage of 1500 V for 1 h. Note that the apparatus used in the present study could measure the breakdown voltage to a value as high as 7 kV. Therefore, the values of the breakdown voltages that did not appear below 7 kV, such as the pristine fluorinated PI sheet, are posted at 7 kV in Fig. 9. As for the breakdown voltages of the hBN_11/fluorinated PI sheets, the composite sheets of 3.4 vol%-hBN_11/fluorinated PI and 7.0 vol%-hBN_11/fluorinated PI showed relatively high values in the cases without electrical treatment. However, the breakdown voltages of the composite sheets with the electrical field treatments were much lower than those without the electrical field treatments for the hBN_11/fluorinated PI of both 3.4 and 7.0 vol%. According to the analyses accomplished via digital microscope and the XRD data, those deteriorations would have been caused by an aggregation of the hBNs due to the electrical field treatment. On the other hand, the breakdown voltages of 14 vol%-hBN_11/
3.3. Dielectrical breakdown voltage The influences of the hBN content, size, and the presence or absence 6
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Fig. 9. DC breakdown voltages of the hBN_11/fluorinated PI and hBN_0.5/ fluorinated PI composite sheets prepared by the one-way DC treatment and without electric treatment (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 1 h). (a) hBN_11/fluorinated PI, (b) hBN_0.5/fluorinated PI.
CRediT authorship contribution statement
fluorinated PI were close regardless of the presence or absence of electrical field treatment, which would indicate that the electrically conductive paths were constructed even at 14 vol%, and, therefore, any further decreases in the breakdown voltage by electrical field treatment were not observed. In the case of hBN_0.5/fluorinated PI composites prepared without electrical field treatment, the breakdown voltages showed more than 7 kV for hBN_0.5 contents of 3.3 and 6.8 vol%. As in the case of the hBN_11/fluorinated PI composite, the breakdown voltages were decreased with electrical field treatment. Moreover, the breakdown voltages of the hBN_11/fluorinated PI and hBN_0.5/fluorinated PI sheets were quite close for an hBN content of around 7 vol% with electrical field treatment and 14 vol% in the presence or absence of electrical field treatment. These breakdown voltages were roughly approximated by linear relationships with a slope of 54 kV/mm, as shown in Fig. S3 of the supplementary material. As a result, the electrically conductive paths that were constructed by the fillers would be in the PI matrix after the electrical treatment for a hBN content of around 7 vol%. This should also indirectly indicates the formation of thermally conductive paths via electrical field treatment.
Masashi Haruki: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Funding acquisition. Jun Tada: Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Ren Funaki: Validation, Investigation. Hajime Onishi: Investigation, Writing - review & editing. Yukio Tada: Investigation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. Acknowledgement The authors are grateful for financial support from JSPS KAKENHI, Japan, Grant Number 17K06887.
4. Conclusions
Appendix A. Supplementary data
In the present work, the effects of hBN content, hBN size, and the presence or absence of electrical field treatment on the effective TC and insulation properties of hBN/fluorinated PI composite sheets were investigated. Cross-sections of the composite sheets prepared under electrical field treatment showed an aggregation of hBNs in the PI matrix. In addition, XRD analyses of the composite sheets indicated that the number of hBN grains that lay supine in the in-plane direction of the composite sheet was clearly decreased when electrical field treatment was applied, and the effective TC of the hBN/fluorinated PI composite sheets was evidently increased. In particular, the effective TCs of the composite sheet obtained by the application of one-way DC treatment were approximately 1.5 times higher than those of composite sheets prepared without electrical field treatment at around 14 vol% of hBN for both hBN_11/fluorinated PI and hBN_0.5/fluorinated PI. The increment of the effective TC depended on the applied voltage, but was almost independent of treatment times that ranged from 1 to 4.5 h. On the other hand, the electrical breakdown voltage was degraded by the application of electrical field treatments, which was likely caused by the formation of thermally and electrically conductive paths in the composite sheets.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.tca.2019.178491. References [1] A.J.T. Teo, A. Mishra, I. Park, Y.-J. Kim, W.-T. Park, Y.-J. Yoon, Polymeric biomaterials for medical implants and devices, ACS Biomater. Sci. Eng. 2 (2016) 454–472. [2] B.Liu Kenry, Recent advances in biodegradable conducting polymers and their biomedical applications, Biomacromolecules 19 (2018) 1783–1803. [3] K. Rennert, M. Nitschke, M. Wallert, N. Keune, M. Raasch, S. Lorkowski, A.S. Mosig, Thermo-responsive cell culture carrier: effects on macrophage functionality and detachment efficiency, J. Tissue Eng. 8 (2017) 1–17. [4] T. Ji, Y. Feng, M. Qin, W. Feng, Thermal conducting properties of aligned carbon nanotubes and their polymer composites, Composites A 91 (2016) 351–369. [5] I.-L. Ngo, S. Jeon, C. Byon, Thermal conductivity of transparent and flexible polymers containing fillers: a literature review, Int. J. Heat Mass Transf. 98 (2016) 219–226. [6] S.N. Leung, Thermally conductive polymer composites and nanocomposites: processing-structure-property relationships, Composites B 150 (2018) 78–92. [7] S. Zhai, P. Zhang, Y. Xian, J. Zeng, B. Shi, Effective thermal conductivity of polymer composites: theoretical models and simulation models, Int. J. Heat Mass Transf. 117 (2018) 358–374. [8] C. Yu, J. Zhang, W. Tian, X. Fan, Y. Yao, Polymer composites based on hexagonal boron nitride and their application in thermally conductive composites, RSC Adv. 8
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polyimide composites, Polym. Compos. 40 (2019) 3032–3039. [26] H. Jung, S. Yu, N.-S. Bae, S.M. Cho, R.H. Kim, S.H. Cho, I. Hwang, B. Jeong, J.S. Ryu, J. Hwang, S.M. Hong, C.M. Koo, C. Park, High through-plane thermal conduction of graphene nanoflake filled polymer composites melt-processed in an L-shape kinked tube, ACS Appl. Mater. Interfaces 7 (2015) 15256–15262. [27] Y. Xue, X. Li, H. Wang, F. Zhao, D. Zhang, Y. Chen, Improvement in thermal conductivity of through-plane aligned boron nitride/silicone rubber composites, Mater. Design 165 (2019) 107580. [28] F. Yuan, W. Jiao, F. Yang, W. Liu, Z. Xu, R. Wang, Surface modification and magnetic alignment of hexagonal boron nitride nanosheets for highly thermally conductive composites, RSC Adv. 7 (2017) 43380–43389. [29] J. Yuan, X. Qian, Z. Meng, B. Yang, Z.-Q. Liu, Highly thermally conducting polymerbased films with magnetic field-assisted vertically aligned hexagonal boron nitride for flexible electronic encapsulation, ACS Appl. Mater. Interfaces 11 (2019) 17915–17924. [30] H.-B. Cho, A. Konno, T. Fujihara, T. Suzuki, S. Tanaka, W. Jiang, H. Suematsu, K. Niihara, T. Nakayama, Self-assemblies of linearly aligned diamond fillers in polysiloxane/diamond composite films with enhanced thermal conductivity, Compos. Sci. Technol. 72 (2011) 112–118. [31] Y. Han, S. Lv, C. Hao, F. Ding, Y. Zhang, Thermal conductivity enhancement of BN/ silicone composites cured under electric field: stacking of shape, thermal conductivity, and particle packing structure anisotropies, Thermochim. Acta 529 (2012) 68–73. [32] H.-B. Cho, S.T. Nguyen, T. Nakayama, M.T.T. Huynh, H. Suematsu, T. Suzuki, W. Jiang, S.B. Rozali, Y. Tokoi, Y.-H. Park, K. Niihara, Oxidation of nanodiamonds and modulation of their assembly in polymer-based nanohybrids by field-inducement, J. Mater. Sci. 48 (2013) 4151–4162. [33] T. Fujihara, H.-B. Cho, M. Kanno, T. Nakayama, T. Suzuki, W. Jiang, H. Suematsu, K. Niihara, Three-dimensional structural control and analysis of hexagonal boron nitride nanosheets assembly in nanocomposite films induced by electric field concentration, Jpn. J. Appl. Phys. 53 (2014) 02BD12. [34] H.-B. Cho, T. Nakayama, H. Suematsu, T. Suzuki, W. Jiang, K. Niihara, E. Song, N.S.A. Eom, S. Kim, Y.-H. Choa, Insulating polymer nanocomposites with highthermal-conduction routes via linear densely packed boron nitride nanosheets, Compos. Sci. Technol. 129 (2016) 205–213. [35] Y. Guo, Y. Chen, E. Wang, M. Cakmak, Roll-to-roll continuous manufacturing multifunctional nanocomposites by electric-field-assisted “Z” direction alignment of graphite flakes in poly(dimethylsiloxane), ACS Appl. Mater. Interfaces 9 (2017) 919–929. [36] J. Li, S. Bernard, V. Salles, C. Gervais, P. Miele, Preparation of polyborazylenederived bulk boron nitride with tunable properties by warm-pressing and pressureless pyrolysis, Chem. Mater. 22 (2010) 2010–2019. [37] T. Ogushi, S. Yanaura, S. Watanabe, T. Hirata, Thermal conductivity measurement method for isotropic conductive adhesives, Netsu Bussei 28 (2014) 22–28. [38] O. Maruzhenko, Y. Mamunya, G. Boiteux, S. Pusz, U. Szeluga, S. Pruvost, Improving the thermal and electrical properties of polymer composites by ordered distribution of carbon micro- and nanofillers, Int. J. Heat Mass Transf. 138 (2019) 75–84.
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Enhancing thermal conductivities of hexagonal boron nitride/fluorinated polyimide composite materials using direct current electrical fields
T
Masashi Haruki*, Jun Tada, Ren Funaki, Hajime Onishi, Yukio Tada Faculty of Mechanical Engineering, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan
ARTICLE INFO
ABSTRACT
Keywords: Thermal conductivity Electrical insulation property Composite Hexagonal boron nitride Fluorinated polyimide
The effect of the electrical field treatment using the direct current (DC) during preparation of the composite sheets on their effective thermal conductivity (TC) and electrical insulation property for the hexagonal boron nitride (hBN)/fluorinated polyimide was studied. By the electrical field treatment, the aggregation of hBNs was obviously observed in the cross-sections of the composite sheets, and the number of hBNs lying parallel to the inplane direction of the sheets was decreased based on XRD study. Moreover, the effective TC in the out-of-plain direction of the composite sheets treated with a one-way electrical field was increased approximately 1.5-fold that of the corresponding sheets prepared without electrical field treatment at 14.2 vol%. On the other hand, electrical insulation properties were deteriorated by the electrical field treatment, and the values for breakdown voltage against DC roughly approximated that following electrical field treatments to high content of hBNs regardless of their size.
1. Introduction Functional engineering plastics have various superior properties such as heat resistance, a high modulus of elasticity, mechanical strength, a low dielectric constant, and weight that is very light relative to comparable materials. Therefore, they are used in a variety of industrial fields. Moreover, they have been shown to become promising biomaterials by the inducement of functional chemical groups and skillful control of their chemical structures [1,2]. For example, cell sheets can be easily detached from the culture substrate without trypsin treatment by using a thermosensitive polymer as a ground layer [3]. In that technique, it is necessary to promptly control the temperature by enough low level of output power in order to avoid damage to cells. Therefore, a high level of thermal conductivity (TC) is required for the instruments used in such cell cultivations. Bioavailable polymer materials usually play important roles in the construction of the experimental instruments and practical equipment used in biotechnology. Since polymers generally have a low level of TC, however, a technique has been developed whereby highly conductive thermal fillers are added. Hexagonal boron nitride (hBN), aluminum nitride, silicon carbide and functional nanocarbon has received much attention as high TC fillers in reports from several investigations [4–10]. Polyimide (PI) is one of the toughest polymers and has excellent thermal stability. In particular, fluorinated PI contains fluorine groups
⁎
that exhibit excellent biocompatibility while retaining mechanical strength and heat resistance that has been demonstrated by conventional PIs [11–16]. The TC of fluorinated PI remains low, however, as is the case with both general polymers and conventional PIs. Several researchers have reported the influence of adding highly thermal conductive fillers into the PI materials to improve their TC [17–23]. In previous work, we focused on the Kapton type of PI that is obtained from the polymerization of pyromellitic dianhydride and 4,4′-diaminodiphenyl ether (ODA). hBN was used as a high TC filler in order to investigate the effect that its content and size would exert on the effective TC of a hBN/PI composite [24]. Moreover, the effect of the coaddition of hBNs and carbon nanofibers (CNFs) on the effective TC of the CNF/hBN/PI composite sheet was also investigated, and a small amount of CNFs was found to drastically increase the effective TC of the composites [25]. Recent studies on enhancing the TC of polymers have focused not only on the addition of fillers but also on controlling the orientation and alignment of the fillers in the polymer matrix via the use of additional treatments. The additional treatments involved loading shear stress [26,27] and processing in either a magnetic field [28,29] or an electrical field [30–35]. For the electrical field treatment, hBNs were an attractive high TC filler because of the high response hBNs have shown to an electrical field due to a wide band gap. And, thermally conductive paths made of hBNs were established inside hBN/polysiloxane [33,34]
Corresponding author. E-mail address: [email protected] (M. Haruki).
https://doi.org/10.1016/j.tca.2019.178491 Received 3 August 2019; Received in revised form 21 November 2019; Accepted 20 December 2019 Available online 23 December 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram of the apparatus used for electric field treatments. (a) Overall view; (b) Detail view of the electrodes. 1: Electrode equipment, 2: Stage, 3: High voltage DC source, 4: Temperature indicator, 5: Temperature controller, 6: Cartridge heaters, 7: Stainless steel block, 8: Acrylic box covered with heat insulator, 9: Fan, 10: PTFE cover, 11: Copper plate, 12: Electrodes (SUS304), 13: Sample, 14: Clearance, 15: Electric wire, T1 and T2: Thermocouples.
and hBN/polymethylsiloxane [31] composite sheets. Therefore, the application of an electrical field treatment to produce the hBN/PI composites is considered advantageous. In preparing the filler/PI composites, however, fillers cannot be added directly into the PI matrix, and therefore, the fillers are generally added into a polyamic acid (PAA) solution that is the precursor of PI. In this process, the solvent must be removed from the filler/PAA composite and the reaction from the filler/ PAA to the filler/PI requires high temperature to produce a filler/PI composite, which would cause a collapse of the orientation and alignment of the hBNs obtained by the electrical field treatment. In the present work, the fluorinated PI consisting of 2,2-bis(3,4anhydrodicarboxyphenyl)-hexafluoropropane (6FDA) and ODA was focused on as a representative fluorinated PI, and a method to apply the electrical field treatment that could control the orientation and alignment of the hBNs was studied to prepare a hBN/fluorinated PI composite with high TC. Moreover, the effect that the electrical field treatment would exert on the electrical insulation properties of the hBN/fluorinated PI composite sheet was also discussed.
composite sheet. In the drying and curing processes, the hBN/fluorinated PAA solution on the plate was let stand under an ambient air atmosphere for the first 1 h. The solution sheet was then introduced into a vacuum oven (DP33, Yamato Scientific Co.), and the temperature in the oven was first maintained at 60 °C for 24 h. The dried composite sheet on the plate was ejected from the oven and was then separated from the plate. The composite sheet was then introduced into a vacuum oven again and was dried at 100 °C for 1 h, followed by further drying at 150 °C for 1 h and at 200 °C for 1 h. Finally, the composite sheet was imidized at 250 °C for 1 h using a hot plate under an ambient air atmosphere. 2.3. Apparatus and procedures for applying a direct current electrical field A self-built apparatus was used to apply electrical field treatments to the hBN/fluorinated PAA solutions in order to orient and align the hBNs dispersed in the PAA matrix. A schematic diagram of the apparatus developed is shown in Fig. 1. The apparatus mainly consisted of a device to apply direct current (DC) to the sample and a high-voltage DC power supply unit (HER-10R6, Matsusada Precision Co.), as shown in Fig. 1(a). The electrical field treatments were carried out in a simple thermostatic air bath made of an acrylic box and a fan. A detailed diagram of the electrode parts appears in Fig. 1(b). SUS304 stainless steel plates with diameters and thicknesses of 30 and 2 mm, respectively, were used as the electrodes. The bottom side of the SUS304 plate was also used as a substrate for the application of a sample. The electrodes were surrounded by blocks made of polytetrafluoroethylene (PTFE) that served as electrical insulation. Clearance was needed between the sample surface and the upper side electrode to avoid the formation of a short-circuit due to the relatively high electrical conductivity of DMF used in the fluorinated PAA solution. The distance between the electrodes was set at 2.2 mm except when the effect of distance between the electrodes was investigated. Two types of DC electrical field treatments were used in the present work. For one, the poles of the electrodes were fixed throughout the treatment (one-way DC). For the other, the poles of the electrodes were switched at certain intervals (switching DC). The treatment times for one-way DC were from 1 to 4.5 h. On the other hand, the times for switching DC were only 2.5 and 4.5 h. The poles of the electrodes were switched every 30 min up to 1.5 h or every hour up to 3 h, and the poles were not changed from 1.5 to 2.5 h or from 3 to 4.5 h, respectively, because of the drying step for the last 30 min. In experiment, a sufficiently mixed hBN/fluorinated PAA solution was applied to the SUS304 stainless-steal plate that also served as the bottom-side electrode. The plate was set on a copper plate, and a DC electrical field was applied to the sample. The interior of the acrylic box was heated to 60 °C only for the last 30 min during the electrical field treatment. After the electrical field treatment, the sample was further dried in the vacuum oven, and heat imidization was carried out on a hot plate in the same manner as for preparation of the composite sheets without the electrical treatment, as described in subsection 2.2.
2. Experiment 2.1. Materials 6FDA with a purity > 99 mol% used as an aromatic tetracarboxylic dianhydride was supplied from Daikin Industries Co. ODA with a purity > 97 mol% was used as an aromatic diamine and was purchased from Sigma-Aldrich Co. These materials served as monomers to produce the fluorinated PI in the present work. Both monomers were dehydrated at 120 °C for 60 min before polymerization. N,N-Dimethylformamide (DMF) with a purity > 99.5 vol% was used as the reaction media to produce PAA and was purchased from Nacalai Tesque Co. hBN fillers with median diameters of 11 μm (hBN_11) and 0.2-0.8 μm (hBN_0.5) were supplied from Showa Denko Co. We adopted a theoretical density of 2.27 g/cm3 for hBN [36] to estimate the volume fraction for both hBN_11 and hBN_0.5. On the other hand, the density of commercial Kapton (1.42 g/cm3) was used as the density of the fluorinated PI. 2.2. Preparation of the hBN/fluorinated PI composite materials without an electrical field In the present work, a fluorinated PAA solution was first produced via the polymerization of 6FDA and ODA, which is the precursor of fluorinated PI. In polymerization, ODA was first dissolved in DMF, and a powdery form of 6FDA that had the same molar number as ODA was then added to the ODA + DMF solution. The solution was stirred with a magnetic stirring bar in an ice bath for 1 h and then continuously stirred at room temperature for 47 h to obtain a fluorinated PAA solution. The concentration of the fluorinated PAA solution was adjusted to 20 wt% of the fluorinated PAA + DMF mixture. Next, hBN was added to the fluorinated PAA solution, which was stirred again for 12 h, and was subsequently sonicated for 10 min. The well mixed solution was then applied to a stainless steel (SUS304) plate using a spatula to obtain a 2
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2.4. Evaluating the characteristics of composite materials
prepared with similar sheet thicknesses in order to reduce the differences in the effect of the interfacial resistance based on the methods and conditions of the sample preparations for comparison of the effective TCs. The sheet thicknesses were 80 ± 19 μm for all samples used in the effective TC measurements. Moreover, a thermal conductive paste (Sanhayato Co., SCH-20: 0.84 W/(m·K)) was also applied between the copper rods and sheet surfaces to reduce the interfacial resistance, and the effective TCs of the composite sheets were obtained from multiple measurements. The sample temperatures that were estimated as the average temperatures of both surfaces were 32 ± 4 °C for all measurements. The electrical breakdown voltage (BDV) against the DC voltage was measured using the high-voltage DC power supply that was also used for the electrical field treatment. The hand-built measuring device is shown in Fig. S1 of the supplementary material. The upper and bottom electrode plates (SUS304) had diameters of 10 and 30 mm, respectively, and were connected to the negative and positive poles of the power source, respectively. For the measurement, a sample with 20 mm in diameter was placed onto the center of the bottom electrode plate. The upper electrode plate was then placed onto the sample sheet, and DC voltage was applied to the sample sheet via the plates. The boosting speed of the DC voltage was approximately 200 V/s during the measurement.
The dispersion behaviors of the hBNs in the composite sheets were visually investigated using a digital microscope (VH-5500, Keyence Co.). Moreover, the orientational states of hBNs in the composite sheets were analyzed using an X-ray diffractometer (XRD, MiniFlex II, Rigaku Co.). Copper Kα radiation was used and the voltage and current were adjusted to 30 kV and 15 mA, respectively. The orientations of hBNs in the composite sheet were evaluated based mainly on a method reported by Cho et al. [34]. The peak area fraction expressed by Eq. (1) was used for the evaluations.
Peak area fraction=
Aa × 100 [%] Aa + A c
(1)
In Eq. (1), Aa and Ac represent the peak areas found at around 2θ = 42° and 27°, respectively. The peaks at around 2θ = 42° and 27° correspond to diffractions from the (100) and (002) planes of hBN, respectively, and they are also identified as the signals of the a-axis and caxis of hBN, respectively. The effective TC was measured using a self-built apparatus based on a temperature-gradient, steady-state method that was established in our previous work. The apparatus and measurement method are explained in detail in our previous work [24], and only a brief explanation is given here. Each sample was cut into a circular shape with a diameter of 20 mm and placed between the two copper rods. Heat from a band heater (Sakaguchi E.H VOC Co., MB1A1JN2) was supplied from the top edge of the upper-side of the copper rod and was then eluted into a water bath that maintained a constant temperature. Heat flux that passed through the composite sheet in the out-of-plane direction was estimated from the TC of the copper rods and the temperature distribution on the copper rods using Fourier’s law, as shown in Eq. (2).
q=
kCu
dTCu dx Cu
3. Results and discussion 3.1. Orientation and dispersion states of hBNs in composite sheets The orientation and dispersion behaviors of hBNs in the hBN/ fluorinated composite sheet were investigated by both visualization and XRD studies. As for the conditions of the electrical field treatments, the distance between the electrodes was set to 2.2 mm, and 1500 V of DC voltage was applied to the sample for 4.5 h. The cross-sectional microscopic images of the pristine fluorinated PI and hBN_11/fluorinated PI composite sheets are shown in Fig. 3. The composite sheets shown in the figures were made up of 14.2 vol% of hBN. The observed samples were embedded in epoxy resin, and were cut out using a microtome. A cross-section of the pristine fluorinated PI is shown in Fig. 3(a) as a reference. Small black lines that seem like defects appear in Fig. 3(a). However, similar black parts and gray parts were also found in the epoxy parts. Moreover, visible cracks were not found in the SEM images shown in Fig. S2 of the supplementary materials. Therefore, the black lines in Fig. 3(a) should not be cracks. In the composites without electrical field treatment, the hBNs were relatively uniformly dispersed inside the sheet (Fig. 3(b)). On the other hand, the cross-section of the composites prepared under a switching of the DC electrical field obviously differed from that of the composite sheets without electrical field loading, whereby aggregations of hBN grains were observed inside the sheet (Fig. 3(c)). Those aggregations were likely due to the dielectric polarization of the hBNs, as reported in the literature [31,33,34]. Moreover, in the composite sheet prepared under a oneway DC electrical field, the hBNs looked much more aggregated, and portions where hBNs were sparse were found in the vicinity of the negative electrode (Fig. 3(d)). This non-uniform distribution of hBNs could have been caused by electrophoresis due to the negative charge at the surface of the hBNs, which also was observed in the electrical field treatment of a hBN/polysiloxane composite reported by Fujihara et al. [33]. On the other hand, since the direction of the electrophoresis was switched at certain intervals, large non-uniformities in the distribution of hBNs could not be found for the switching DC treatment. Moreover, XRD analyses were carried out to obtain information about the orientation of the hBN grains, and the peak patterns obtained are shown in Fig. 4. The samples included the hBN_11/fluorinated PI composite sheet prepared without electrical field treatment, the composite sheet prepared under electrical field treatments with switching DC, and the sheet prepared with one-way DC operation. Pristine
(2)
In Eq. (2), q is the heat flux. kCu shows the TC of the tough pitch copper, and a value of 381 W/(m·K) was used in the present work. dTCu/ dxCu is the measured temperature gradient of the copper rods. The effective TC of the composite sheet was estimated using Eq. (3) [24,37].
kseff =
q/
dTs dx s
q
ts Ts
(3)
eff
In Eq. (3), ks represents the effective TC of the sample sheet, ΔTs and ts show the temperature difference between both surfaces of the sample sheet and the sheet thickness, respectively. The effective TC, kseff, obtained using Eq. (3) includes the effect of the interfacial resistance between the surfaces of the copper rods and the sheet surfaces. The relationship between kseff and the actual TC is represented by Eq. (4).
Rt = Rs + Ri =
ts t = s + Ri ks k seff
(4)
In Eq. (4), Rt indicates the total thermal resistance, and RS and ks represent the sample’s own thermal resistance and TC, respectively. Ri shows the interfacial thermal resistance. The relationships between the effective TC obtained by Eq. (3) and the sheet thickness for pristine fluorinated PI and the 17 vol%-hBN_11/ fluorinated PI are shown in Fig. 2(a). The effective TCs obtained by Eq. (3) showed similar values regardless of the sheet thicknesses that ranged between approximately 50 and 100 μm for both sheets. Moreover, the relationships between total thermal resistance and the sheet thickness are described in Fig. 2(b). The relationships obtained were approximately linear for both sheets, and the TCs obtained by Eq. (4) using slopes of the lines in Fig. 2(b) approximated the values obtained by Eq. (3). The deviations were within 10 % for all effective TCs shown in Fig. 2(a). Therefore, the discussion below was conducted using the effective TCs obtained by Eq. (3). In the experiment, the samples were 3
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Fig. 2. Influence of the sheet thickness on the effective thermal conductivity and the thermal resistance for pristine fluorinated PI and17 vol%-hBN_11/fluorinated PI. (a) Effective thermal conductivity; (b) Thermal resistance.
fluorinated PI and powdery hBN_11 were used as references. The hBN contents in the composite sheets were adjusted to 14.2 vol%. The degree of the orientation for hBNs in fluorinated PI was evaluated using the peaks at around 2θ = 27° (c-axis) and 42° (a-axis), as described above. The small differences in the angles between the samples were likely caused by the different shapes of the composite sheets rather than by a change in the crystalline structure of hBN. As for the hBN/fluorinated PI composite sheets, the peak assigned to the c-axis was dominant regardless of whether the electrical field treatments were used. By applying the electrical field treatment to the hBN/fluorinated PI composites, the peak intensity of the c-axis was notably decreased whereas the peak of the a-axis was either unchanged or was only slightly increased. Similar behavior was even more pronounced when the one-way DC electrical field treatment was used. Moreover, peak area fractions estimated by Eq. (1) were obtained at several levels of hBN content, as shown in Fig. 5. As for hBN_11, the order of the values for the peak area fractions was one-way DC
treatment > switching DC treatment > without DC treatment, and the value obtained by the one-way DC treatment was much higher than that without DC treatment. The size of hBN_11 was relatively large compared with the sheet thickness of the hBN/fluorinated PI sheet finally produced, and the hBNs had a sheet shape. Therefore, without electrical field treatment, the hBN should lie down in a fluorinated PAA matrix as the thickness of the PAA matrix was reduced during drying. On the other hand, the percentage of the supine hBNs parallel to the inplane direction of the sheet would be decreased by electrical field treatment. Some hBNs would collectively stand by the formation of aggregation structures or would rise up alone with a tilted angle. hBNs have a very large degree of thermal anisotropy such that the TC in the in-plane direction (a-axis) can be more than 20-fold higher than that in the out-of-plane direction (c-axis) [29], and XRD studies have suggested that the effective TC of the hBN/fluorinated PI composites would be improved by electrical treatment. Fig. 3. Images of the cross-sections of the pristine fluorinated PI and 14.2 vol%-hBN_11/ fluorinated PI composite sheets embedded with epoxy resins obtained by the digital microscope (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V). (a) Pristine fluorinated PI; (b) 14.2 vol%-hBN_11/fluorinated PI composite sheet without electric field treatment; (c) 14.2 vol%-hBN_11/fluorinated PI composite sheet with switching DC treatment for 4.5 h (1 h × 3 and 1.5 h × 1); (d) 14.2 vol %-hBN_11/fluorinated PI composite sheet with one-way DC treatment for 4.5 h.
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Fig. 4. XRD patterns of the 14.2 vol%-hBN_11/ fluorinated PI composite sheets with and without electric field treatments along with the patterns of pristine fluorinated PI and powdery hBN (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 4.5 h). (b) shows the XRD pattern enlarged at the range of the peaks of a-axis.
3.2. Effective thermal conductivity
composite sheets were lower than those of the hBN_11/fluorinated PI composite sheets at the same hBN content both with and without electrical field treatment. As the results of the XRD study shown in Fig. 5 show, however, the peak area fractions of the hBN_0.5/fluorinated PI were much higher than those of the hBN_11/fluorinated PI. The reasons of this relationship should be that total contact resistance between hBN and fluorinated PI and thermal tortuosity should increase with decreasing the hBN size [24]. The effective TC of the 14.3 vol %-hBN_0.5/fluorinated PI composite sheets obtained by the one-way DC treatment was also higher (≈1.5-fold) than that of the composite sheets prepared without the electrical field treatment, which was similar to the case of the 14.2 vol%-hBN_11/fluorinated PI composite sheet. Moreover, the effective TC of the hBN_0.5/fluorinated PI composite prepared using the electrical field treatment was higher than that of the hBN_11/fluorinated PI composite without the electrical field treatment, although the effective TC of the sheets was generally increased with increases in the size of the hBNs added to the polymer [24]. These results suggest that the relationship between hBN size and the effective TC could be changed intentionally by electrical field treatment. Moreover, the relationships between the duration of the electrical field treatment and the effective TC of the composite sheets were investigated. The results of the experiments for 14.2 vol%-hBN_11/ fluorinated PI composite sheets with an applied DC voltage of 1500 V using an electrode distance of 2.2 mm are shown in Fig. 7. As for both one-way and switching operations, the remarkable dependency of the effective TC on treatment time could not be found within a range of from 1 to 4.5 h, which indicated the morphology changes of the hBNs
The relationships between the effective TC, hBN content, hBN size and methods of electrical field application are shown in Fig. 6. The 1500 V of the one-way DC or switching DC electrical field was applied to hBN/PAA solution sheet for 4.5 h. The distance between the electrodes was set to 2.2 mm. The effective TC of the hBN/fluorinated PI composite sheets was increased as the hBN content increased regardless of the hBN size. Moreover, this result was obtained both with and without the electrical field treatment. By applying the electrical field treatment, the effective TC of the hBN/fluorinated PI composite sheets was clearly increased for both switching DC and one-way DC treatments. The one-way DC treatment increased the effective TC of the composite sheets up to a much higher level than that obtained by the switching treatment. At 14.2 vol% of hBN_11, the effective TC of the composite sheets prepared by the one-way DC treatment showed 0.59 W/(m·K), which was approximately 1.5 times higher than that of the corresponding composite sheets prepared without the electrical field treatment (0.40 W/(m·K)). These results qualitatively agreed with the results from the investigations of the orientations of hBNs based on the XRD studies although the hBN-poor layer that was formed in the direction perpendicular to the heat transfer direction shown in Fig. 3d led to low thermal conductivity [38]. Fig. 3d also indicates how the hBN grains seemed to make connections with each other. Therefore, the positive effects of the aggregations and alignment of the hBNs obtained by the one-way DC treatment on the effective TC were expected to be dominant in the present study. The values of the effective TC for the hBN_0.5/fluorinated PI
Fig. 5. Peak area fractions obtained by the XRD patterns for the hBN_11/fluorinated PI and hBN_0.5/fluorinated PI composite sheets with and without electric field treatments (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 4.5 h). (a) hBN_11/fluorinated PI; (b) hBN_0.5/fluorinated PI. 5
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Fig. 6. Relationships between hBN content and effective TC for the hBN_11/fluorinated PI and hBN_0.5/fluorinated PI composite sheets with and without the electric field treatment (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 4.5 h). (a) hBN_11/fluorinated PI; (b) hBN_0.5/fluorinated PI.
Fig. 7. Relationship between the electrical treatment duration and the effective thermal conductivity for the 14.2 vol%-hBN_11/fluorinated PI composite sheet (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V).
Fig. 8. Effects of the applied voltage and the electrode distance of the one-way DC treatment for 1 h on the effective TC of the 14.2 vol%-hBN_11/fluorinated PI composite sheet.
inside the sheets might be nearly complete within 1 h. Moreover, the effects that the application of voltage and the distance between electrodes exert on the effective TC were also investigated. The effective TCs for the 14.2 vol%-hBN_11/fluorinated PI composite sheets prepared via one-way DC treatment for 1 h are shown in Fig. 8. As shown in the results for the electrode distances of 2.2 and 5.9 mm, the effective TC depended on the application voltage, and the increment of the values appeared with increases in the load voltage of less than 1500 V. Plateau behavior appeared at voltages ranging from 1500 to 3000 V for changes in the effective TC obtained when using an electrode distance of 5.9 mm. That plateau behavior was also found in the literature for the hBN/silicone rubber composite sheet of 10 vol% of hBN with 50 Hz of the AC electrical field treatment [31], which was considered to be caused by a saturation of the chain formation of hBNs. On the other hand, the distance of the electrodes (1.2–5.9 mm) had very little impact on the effective TC under load voltages that ranged from 500 to 1500 V.
of electrical field treatment on the electrical insulation properties of the composite sheets were also investigated. The relationships between the sample thickness and the breakdown voltage of the composite sheets are shown in Fig. 9. Experiments were carried out for both the hBN_11/ fluorinated PI and hBN_0.5/fluorinated PI composite sheets. The electrical field treatments were conducted using one-way DC method under an electrode distance of 2.2 mm and a DC voltage of 1500 V for 1 h. Note that the apparatus used in the present study could measure the breakdown voltage to a value as high as 7 kV. Therefore, the values of the breakdown voltages that did not appear below 7 kV, such as the pristine fluorinated PI sheet, are posted at 7 kV in Fig. 9. As for the breakdown voltages of the hBN_11/fluorinated PI sheets, the composite sheets of 3.4 vol%-hBN_11/fluorinated PI and 7.0 vol%-hBN_11/fluorinated PI showed relatively high values in the cases without electrical treatment. However, the breakdown voltages of the composite sheets with the electrical field treatments were much lower than those without the electrical field treatments for the hBN_11/fluorinated PI of both 3.4 and 7.0 vol%. According to the analyses accomplished via digital microscope and the XRD data, those deteriorations would have been caused by an aggregation of the hBNs due to the electrical field treatment. On the other hand, the breakdown voltages of 14 vol%-hBN_11/
3.3. Dielectrical breakdown voltage The influences of the hBN content, size, and the presence or absence 6
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Fig. 9. DC breakdown voltages of the hBN_11/fluorinated PI and hBN_0.5/ fluorinated PI composite sheets prepared by the one-way DC treatment and without electric treatment (Distance of electrodes: 2.2 mm, Applied DC voltage: 1500 V for 1 h). (a) hBN_11/fluorinated PI, (b) hBN_0.5/fluorinated PI.
CRediT authorship contribution statement
fluorinated PI were close regardless of the presence or absence of electrical field treatment, which would indicate that the electrically conductive paths were constructed even at 14 vol%, and, therefore, any further decreases in the breakdown voltage by electrical field treatment were not observed. In the case of hBN_0.5/fluorinated PI composites prepared without electrical field treatment, the breakdown voltages showed more than 7 kV for hBN_0.5 contents of 3.3 and 6.8 vol%. As in the case of the hBN_11/fluorinated PI composite, the breakdown voltages were decreased with electrical field treatment. Moreover, the breakdown voltages of the hBN_11/fluorinated PI and hBN_0.5/fluorinated PI sheets were quite close for an hBN content of around 7 vol% with electrical field treatment and 14 vol% in the presence or absence of electrical field treatment. These breakdown voltages were roughly approximated by linear relationships with a slope of 54 kV/mm, as shown in Fig. S3 of the supplementary material. As a result, the electrically conductive paths that were constructed by the fillers would be in the PI matrix after the electrical treatment for a hBN content of around 7 vol%. This should also indirectly indicates the formation of thermally conductive paths via electrical field treatment.
Masashi Haruki: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Funding acquisition. Jun Tada: Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Ren Funaki: Validation, Investigation. Hajime Onishi: Investigation, Writing - review & editing. Yukio Tada: Investigation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. Acknowledgement The authors are grateful for financial support from JSPS KAKENHI, Japan, Grant Number 17K06887.
4. Conclusions
Appendix A. Supplementary data
In the present work, the effects of hBN content, hBN size, and the presence or absence of electrical field treatment on the effective TC and insulation properties of hBN/fluorinated PI composite sheets were investigated. Cross-sections of the composite sheets prepared under electrical field treatment showed an aggregation of hBNs in the PI matrix. In addition, XRD analyses of the composite sheets indicated that the number of hBN grains that lay supine in the in-plane direction of the composite sheet was clearly decreased when electrical field treatment was applied, and the effective TC of the hBN/fluorinated PI composite sheets was evidently increased. In particular, the effective TCs of the composite sheet obtained by the application of one-way DC treatment were approximately 1.5 times higher than those of composite sheets prepared without electrical field treatment at around 14 vol% of hBN for both hBN_11/fluorinated PI and hBN_0.5/fluorinated PI. The increment of the effective TC depended on the applied voltage, but was almost independent of treatment times that ranged from 1 to 4.5 h. On the other hand, the electrical breakdown voltage was degraded by the application of electrical field treatments, which was likely caused by the formation of thermally and electrically conductive paths in the composite sheets.
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