Fabrication, Thermal Conductivity, and Mechanical Properties of Hexagonal-Boron-Nitride-Pattern-Embedded Aluminum Oxide Composites

As electronics become more portable and compact, the demand for high-performance thermally conductive composites is increasing. Given that the thermal conductivity correlates with the content of thermally conductive fillers, it is important to fabricate composites with high filler loading. However, the increased viscosity of the composites upon the addition of these fillers impedes the fabrication of filler-reinforced composites through conventional methods. In this study, hexagonal-boron-nitride (h-BN)-pattern-embedded aluminum oxide (Al2O3) composites (Al/h-BN/Al composites) were fabricated by coating a solution of h-BN onto a silicone-based Al2O3 composite through a metal mask with square open areas. Because this method does not require the dispersion of h-BN into the Al2O3 composite, composites with high filler loading could be fabricated without the expected problems arising from increased viscosity. Based on the coatability and thixotropic rheological behaviors, a composite with 85 wt.% Al2O3 was chosen to fabricate Al/h-BN/Al composites. The content of the Al2O3 and the h-BN of the Al/h-BN/Al-1 composite was 74.1 wt.% and 12.8 wt.%, respectively. In addition to the increased filler content, the h-BN of the composite was aligned in a parallel direction by hot pressing. The in-plane (kx) and through-plane (kz) thermal conductivity of the composite was measured as 4.99 ± 0.15 Wm−1 K−1 and 1.68 ± 0.2 Wm−1 K−1, respectively. These results indicated that the method used in this study is practical not only for increasing the filler loading but also for achieving a high kx through the parallel alignment of h-BN fillers.


Introduction
Light-weight and electrically insulating polymers are widely used as packing materials for electronics [1]. However, the very low thermal conductivity (0.1-0.5 Wm −1 K −1 ) [2] of the polymers hampers their application as thermal interface materials (TIMs), which require both high insulation resistance and high thermal conductivity. To enhance the thermal conductivity of polymer composites, many researchers have endeavored to incorporate ceramic fillers into polymers [2][3][4]. Because of its low price, spherical shape, excellent insulating resistance [3], and good thermal conductivity (~30 Wm −1 K −1 ) [2], aluminum oxide (Al 2 O 3 ) has been extensively used as a filler for the manufacturing of TIMs. To improve thermal conductivity further, hybrid filler systems have been prepared by incorporating fillers with higher thermal conductivity into Al 2 O 3 composites, wherein increased filler packing density and the formation of heat pathways were expected [5]. Hexagonal boron nitride (h-BN) has an in-plane thermal conductivity that reaches 550 Wm −1 K −1 [6] due to the strong in-plane σ-bonds stemming from the sp 2 -hybridized boron and nitrogen atoms. Some studies [5,[7][8][9][10][11] showed that the thermally conductive pathways formed by random bridging of Al 2 O 3 and BN platelets increased the thermal conductivity of their composites. Despite this synergetic effect, the increased viscosity resulting from the addition of BN platelets deteriorates the processability, limiting the filler loading of the composites [7]. Given that with a greater filler content, the thermal conductivity of the analysis was carried out using a scanning electron microscope (JSM 6390, JEOL Korea Ltd, Seoul, Korea) at an acceleration voltage of 20 KV.

Compositions of Thermally Conductive Composites
A silicone resin was produced by mixing vinyl terminated polysiloxane (1.87 g), hydride terminated polysiloxane (0.13 g), monohydroxy terminated poly(dimethylsiloxane) (0.2 g), a platinum catalyst (0.01 g), and an inhibitor (0.01 g). The h-BN mixture consisted of 11 µm h-BN (1.5 g) and 31 µm h-BN (3.5 g). The Al 2 O 3 mixture was composed of 45 µm Al 2 O 3 (15.4 g) and 5 µm Al 2 O 3 (6.6 g). All compositions listed in Table 1 were admixed using a Thinky mixer (ARE-250, Thinky Corporation, Tokyo, Japan) for 5 min to produce the composites. The Al50 and Al/BN composites were bar-coated onto a release film and cured at 150 • C for 30 min to produce 1 mm-thick samples for the thermal diffusivity measurement.

An Amplitude Sweep Test and a 3ITT
To determine the limit of the linear viscoelastic (LVE) behavior of the samples, the storage modulus (G') and loss modulus (G") were measured by applying shear strain from 0.001% to 10% to each sample; samples were placed between two plates with a 1 mm gap between them at a constant frequency of 5 Hz. The thixotropic behavior of the composites was evaluated by a 3ITT. Shear strain of 0.001% was applied in the first interval (for 60 s) and third intervals (for 40 s), and shear strain of 1% was applied in the second interval (for 20 s). During the 3ITT experiment, the complex viscosity of the composites was measured.

Fabrication of Al/h-BN/Al Composites
The fabrication process of the Al/h-BN/Al composites is illustrated in Figure 1. First, the h-BN mixture (6.0 g) was admixed with MEK (6.0 g) to prepare an h-BN solution with a solid content of 50 wt.%. The Al85 composite outlined in Table 1 was coated onto a release film using a doctor blade with a 150 µm gap to yield a u-Al film, which was subsequently heated at 150 • C for 10 min to produce a 220 µm-thick Al85 layer on the release film (c-Al film). The as-prepared h-BN solution was then knife-coated onto the obtained c-Al film through a metal mask with square open areas and dried at 150 • C for 15 min to produce an h-BN pattern/c-Al film. The product was then laminated with the u-Al film, and was subsequently pressed under a pressure of 35 MPa at 130 • C for 30 min to yield an h-BN-pattern-embedded c-Al film. After the top and bottom release films were removed, a 500 µm-thick Al/h-BN/Al composite was obtained. The thickness of the h-BN patterns of the product was measured as 130 µm using a scanning electron microscope. The Al/h-BN/Al composites fabricated using metal masks with square open areas of 3 mm × 3 mm, 5 mm × 5 mm, and 7 mm × 7 mm are denoted as Al/h-BN/Al-1, Al/h-BN/Al-2, and Al/h-BN/Al-3, respectively. Nanomaterials 2022, 12, 2815 4 of 10 × 3 mm, 5 mm × 5 mm, and 7 mm × 7 mm are denoted as Al/h-BN/Al-1, Al/h-BN/Al-2, and Al/h-BN/Al-3, respectively.

Thermal Diffusivity Measurement
The as-prepared composites were punched into 25 mm circles and 10 mm × 10 mm squares for an in-plane and through-plane thermal diffusivity measurements. Both the top and bottom sides of the cut samples were coated with graphite via spray coating. After the samples were placed on a stage, their in-plane and through-plane thermal diffusivity was measured by irradiating them with a xenon lamp with a maximum energy pulse of 10 J/s. The pulse width was set as 300 μs and 180 μs for in-plane and through-plane thermal diffusivity measurements, respectively.

Results and Discussion
Silicone-based Al50 and Al/BN composites containing various h-BN fillers were prepared according to the compositions outlined in Table 1. Given that the viscosity of the composites increases with increasing filler content, the effect of the amount of h-BN fillers on the complex viscosity was investigated through rheometry. The complex viscosity of the Al/BN composites increased with increasing h-BN loading, as shown in Figure 2. For instance, the complex viscosity measured at 0.1 Hz increased from ~2 Pa·s (for Al50) to ~57 Pa·s (for Al/BN-54) because of the higher h-BN content of the latter composite. The complex viscosity of the composites was sensitive to the amount of h-BN due to its planar structure. The complex viscosity of all Al/BN composites decreased with increasing frequency because the random orientation of fillers at low shear rates aligned in the direction of the flow at higher shear rates, leading to a decrease in flow resistance (shear thinning) [4].

Thermal Diffusivity Measurement
The as-prepared composites were punched into 25 mm circles and 10 mm × 10 mm squares for an in-plane and through-plane thermal diffusivity measurements. Both the top and bottom sides of the cut samples were coated with graphite via spray coating. After the samples were placed on a stage, their in-plane and through-plane thermal diffusivity was measured by irradiating them with a xenon lamp with a maximum energy pulse of 10 J/s. The pulse width was set as 300 µs and 180 µs for in-plane and through-plane thermal diffusivity measurements, respectively.

Results and Discussion
Silicone-based Al50 and Al/BN composites containing various h-BN fillers were prepared according to the compositions outlined in Table 1. Given that the viscosity of the composites increases with increasing filler content, the effect of the amount of h-BN fillers on the complex viscosity was investigated through rheometry. The complex viscosity of the Al/BN composites increased with increasing h-BN loading, as shown in Figure 2. For instance, the complex viscosity measured at 0.1 Hz increased from~2 Pa·s (for Al50) to~57 Pa·s (for Al/BN-54) because of the higher h-BN content of the latter composite. The complex viscosity of the composites was sensitive to the amount of h-BN due to its planar structure. The complex viscosity of all Al/BN composites decreased with increasing frequency because the random orientation of fillers at low shear rates aligned in the direction of the flow at higher shear rates, leading to a decrease in flow resistance (shear thinning) [4]. Nanomaterials 2022, 12, 2815 5 of 10 The effect of h-BN content on the thermal conductivity of the composites was investigated. Because of the anisotropic thermal conductivity of h-BN [6], both kx and kz of the 1 mm-thick composites were calculated using the thermal diffusivity (α) obtained through a laser flash method based on Equation (1) [27].
where ρ is density and Cp is specific heat. The kx and kz values of the composites increased with increasing h-BN content, and kx was more sensitive to the amount of h-BN than kz, as shown in Figure 3. Given that the test specimens were prepared by bar-coating, the h-BN fillers partially aligned in a parallel direction and thus, kx was higher than kz [4,[28][29][30][31]. The kx and kz values of the Al/BN-62 composite reached 1.62 ± 0.04 Wm -1 K -1 and 0.86 ± 0.05 Wm -1 K -1 , respectively. Because the viscosity of the composites increased drastically with increasing h-BN content, it is challenging to produce Al/BN composites with >62 wt.% fillers through conventional methods. To address this issue, Al/h-BN/Al composites were fabricated through a sequential coating process in this study. Because a solution of h-BN was coated onto a silicone-based Al2O3 composite layer through a metal mask and laminated with another silicone-based Al2O3 composite by hot pressing (Figure 1), this fabrication method could be used to circumvent the use of highly viscous Al2O3/h-BN composites to attain high thermal conductivity [24]. To fabricate a Al2O3 composite layer that would be used as a The effect of h-BN content on the thermal conductivity of the composites was investigated. Because of the anisotropic thermal conductivity of h-BN [6], both k x and k z of the 1 mm-thick composites were calculated using the thermal diffusivity (α) obtained through a laser flash method based on Equation (1) [27].
where ρ is density and Cp is specific heat.
The k x and k z values of the composites increased with increasing h-BN content, and k x was more sensitive to the amount of h-BN than k z , as shown in Figure 3. Given that the test specimens were prepared by bar-coating, the h-BN fillers partially aligned in a parallel direction and thus, k x was higher than k z [4,[28][29][30][31]. The k x and k z values of the Al/BN-62 composite reached 1.62 ± 0.04 Wm −1 K −1 and 0.86 ± 0.05 Wm −1 K −1 , respectively. The effect of h-BN content on the thermal conductivity of the composites was investigated. Because of the anisotropic thermal conductivity of h-BN [6], both kx and kz of the 1 mm-thick composites were calculated using the thermal diffusivity (α) obtained through a laser flash method based on Equation (1) [27].
where ρ is density and Cp is specific heat.
The kx and kz values of the composites increased with increasing h-BN content, and kx was more sensitive to the amount of h-BN than kz, as shown in Figure 3. Given that the test specimens were prepared by bar-coating, the h-BN fillers partially aligned in a parallel direction and thus, kx was higher than kz [4,[28][29][30][31]. The kx and kz values of the Al/BN-62 composite reached 1.62 ± 0.04 Wm -1 K -1 and 0.86 ± 0.05 Wm -1 K -1 , respectively. Because the viscosity of the composites increased drastically with increasing h-BN content, it is challenging to produce Al/BN composites with >62 wt.% fillers through conventional methods. To address this issue, Al/h-BN/Al composites were fabricated through a sequential coating process in this study. Because a solution of h-BN was coated onto a silicone-based Al2O3 composite layer through a metal mask and laminated with another silicone-based Al2O3 composite by hot pressing (Figure 1), this fabrication method could be used to circumvent the use of highly viscous Al2O3/h-BN composites to attain high thermal conductivity [24]. To fabricate a Al2O3 composite layer that would be used as a Because the viscosity of the composites increased drastically with increasing h-BN content, it is challenging to produce Al/BN composites with >62 wt.% fillers through conventional methods. To address this issue, Al/h-BN/Al composites were fabricated through a sequential coating process in this study. Because a solution of h-BN was coated onto a silicone-based Al 2 O 3 composite layer through a metal mask and laminated with another silicone-based Al 2 O 3 composite by hot pressing (Figure 1), this fabrication method could be used to circumvent the use of highly viscous Al 2 O 3 /h-BN composites to attain high thermal conductivity [24]. To fabricate a Al 2 O 3 composite layer that would be used as a substrate for h-BN patterns, three types of composites with 80 wt.% Al 2 O 3 (Al80), 85 wt.% Al 2 O 3 (Al85), and 90 wt.% Al 2 O 3 (Al90) were prepared. Their thixotropic rheological behaviors were investigated through a 3ITT [32]. An amplitude sweep test was first performed to determine the limit of the LVE of the composites [33]. The G' and G" of each sample were measured under shear strain over a range from 0.001% to 10%, as shown in Figure 4a. The G' value of the composites increased with increasing Al 2 O 3 content, indicating that the three-dimensional networks of the fillers were extended. Given that the crossover point at which G' = G" increased from 0.005% (for Al80) to 0.02% (for Al90) with increasing Al 2 O 3 fillers, the precipitation of Al 2 O 3 fillers could be expected in the Al composites with fewer fillers over long-term storage. The limits of the LVE amplitude of the Al80, Al85, and Al90 composites were determined as 0.02%, 0.01%, and 0.005%, respectively. With the limits of the LVE in hand, the thixotropic rheological behaviors of the composites were investigated by a 3ITT (Figure 4b), where the first and the third intervals reflected "stabilization" and "recovery", respectively, and the second interval mimicked the "coating process" [34,35]. The shear strain for the first and third intervals was set as 0.001%, and that for the second interval was set as 1% in the 3ITT experiments. The complex viscosity of the composites was stable in the first interval, which indicated that their structures remained undamaged. In the second interval, the high shear strain led to the deformation of the structures, and the complex viscosity drastically decreased. This shear-thinning behavior observed during the second interval was more noticeable with the Al90 composite because more fillers aligned in the direction of the flow. In the third interval, its complex viscosity increased with the partial recovery of the deformed structures, despite the degree of recovery becoming lower with increasing filler content.
Nanomaterials 2022, 12, 2815 6 of 10 substrate for h-BN patterns, three types of composites with 80 wt.% Al2O3 (Al80), 85 wt.% Al2O3 (Al85), and 90 wt.% Al2O3 (Al90) were prepared. Their thixotropic rheological behaviors were investigated through a 3ITT [32]. An amplitude sweep test was first performed to determine the limit of the LVE of the composites [33]. The G′ and G″ of each sample were measured under shear strain over a range from 0.001% to 10%, as shown in Figure 4a. The G′ value of the composites increased with increasing Al2O3 content, indicating that the three-dimensional networks of the fillers were extended. Given that the crossover point at which G′ = G″ increased from 0.005% (for Al80) to 0.02% (for Al90) with increasing Al2O3 fillers, the precipitation of Al2O3 fillers could be expected in the Al composites with fewer fillers over long-term storage. The limits of the LVE amplitude of the Al80, Al85, and Al90 composites were determined as 0.02%, 0.01%, and 0.005%, respectively. With the limits of the LVE in hand, the thixotropic rheological behaviors of the composites were investigated by a 3ITT (Figure 4b), where the first and the third intervals reflected "stabilization" and "recovery", respectively, and the second interval mimicked the "coating process" [34,35]. The shear strain for the first and third intervals was set as 0.001%, and that for the second interval was set as 1% in the 3ITT experiments. The complex viscosity of the composites was stable in the first interval, which indicated that their structures remained undamaged. In the second interval, the high shear strain led to the deformation of the structures, and the complex viscosity drastically decreased. This shearthinning behavior observed during the second interval was more noticeable with the Al90 composite because more fillers aligned in the direction of the flow. In the third interval, its complex viscosity increased with the partial recovery of the deformed structures, despite the degree of recovery becoming lower with increasing filler content. Next, the coatability of the Al80, Al85, and Al90 composites was evaluated by coating them onto a release film. While the Al80 and Al85 composites produced a coating layer, the Al90 composite failed to wet the release film because its viscosity was presumably too high ( Figure 5). As disclosed by 3ITT experiments, the complex viscosity of the Al90 composite was higher than that of the other samples under shear strain ranging from 0.001% to 1%. Given that the thermal conductivity is correlated with the Al2O3 content, the Al85 composite, rather than the Al80 composite, was chosen to fabricate a substrate for the h-BN patterns. The u-Al85 composite layer of the u-Al film was cured at 150 °C to produce a 220 μm-thick c-Al85 composite layer of a c-Al film (Figure 1). The u-Al85 and c-Al85 samples are denoted as uncured Al85 and cured Al85, respectively, in this study.  Next, the coatability of the Al80, Al85, and Al90 composites was evaluated by coating them onto a release film. While the Al80 and Al85 composites produced a coating layer, the Al90 composite failed to wet the release film because its viscosity was presumably too high ( Figure 5). As disclosed by 3ITT experiments, the complex viscosity of the Al90 composite was higher than that of the other samples under shear strain ranging from 0.001% to 1%. Given that the thermal conductivity is correlated with the Al 2 O 3 content, the Al85 composite, rather than the Al80 composite, was chosen to fabricate a substrate for the h-BN patterns. The u-Al85 composite layer of the u-Al film was cured at 150 • C to produce a 220 µm-thick c-Al85 composite layer of a c-Al film (Figure 1). The u-Al85 and c-Al85 samples are denoted as uncured Al85 and cured Al85, respectively, in this study. 130 °C under a pressure of 35 MPa. During this process, the u-Al85 composite filled the gaps between h-BN patterns, made contact with the bottom of the c-Al85 composite, and was cured to produce an h-BN-pattern-embedded Al film. After removing the top and bottom release films from the product, a 500 μm-thick Al/h-BN/Al composite was obtained. The thickness of the h-BN patterns was measured as 130 μm using a scanning electron microscope. The Al/h-BN/Al composites fabricated using metal masks with open areas of 3 mm × 3 mm, 5 mm × 5 mm, and 7 mm × 7 mm were denoted as Al/h-BN/Al-1, Al/h-BN/Al-2, and Al/h-BN/Al-3, respectively. The Al2O3 and h-BN content of the Al/h-BN/Al composites are summarized in Table 2. The h-BN content of the composites increased with increasing open areas of the metal masks. Given that it was challenging to produce Al/BN composites with >62 wt.% fillers because of the increased viscosity, the fabrication of Al/h-BN/Al composites with ≥12.8 wt.% h-BN and ≥70.2 wt.% Al2O3 is impressive.   Next, each Al/h-BN/Al composite was punched into a 10 mm × 10 mm square for the thermal conductivity measurement. While the Al/h-BN/Al-1 composite was isolated as an intact square (Figure 7a), complete delamination was observed for the other Al/h-BN/Al composites after punching (Figure 7b,c). Given that the top and bottom Al2O3 composites had created joints to hold the h-BN patterns tightly during hot pressing, the decreased joint areas with enlarged h-BN pattern areas caused the separation of h-BN interlayers in the Al/h-BN/Al-2 and Al/h-BN/Al-3 composites. Therefore, only the performance of the Al/h-BN/Al-1 composite was measured in this study. The kx and kz values of the Al/h-BN/Al-1 composite were determined as 4.99 ± 0.15 Wm -1 K -1 and 1.68 ± 0.2 Wm -1 K -1 , respectively ( Figure 3). The high kx value of the Al/h-BN/Al-1 composite was attributed to the h-BN alignment in a parallel direction during hot pressing [25]. This verified that the 130 °C under a pressure of 35 MPa. During this process, the u-Al85 composite filled the gaps between h-BN patterns, made contact with the bottom of the c-Al85 composite, and was cured to produce an h-BN-pattern-embedded Al film. After removing the top and bottom release films from the product, a 500 μm-thick Al/h-BN/Al composite was obtained. The thickness of the h-BN patterns was measured as 130 μm using a scanning electron microscope. The Al/h-BN/Al composites fabricated using metal masks with open areas of 3 mm × 3 mm, 5 mm × 5 mm, and 7 mm × 7 mm were denoted as Al/h-BN/Al-1, Al/h-BN/Al-2, and Al/h-BN/Al-3, respectively. The Al2O3 and h-BN content of the Al/h-BN/Al composites are summarized in Table 2. The h-BN content of the composites increased with increasing open areas of the metal masks. Given that it was challenging to produce Al/BN composites with >62 wt.% fillers because of the increased viscosity, the fabrication of Al/h-BN/Al composites with ≥12.8 wt.% h-BN and ≥70.2 wt.% Al2O3 is impressive.   Next, each Al/h-BN/Al composite was punched into a 10 mm × 10 mm square for the thermal conductivity measurement. While the Al/h-BN/Al-1 composite was isolated as an intact square (Figure 7a), complete delamination was observed for the other Al/h-BN/Al composites after punching (Figure 7b,c). Given that the top and bottom Al2O3 composites had created joints to hold the h-BN patterns tightly during hot pressing, the decreased joint areas with enlarged h-BN pattern areas caused the separation of h-BN interlayers in the Al/h-BN/Al-2 and Al/h-BN/Al-3 composites. Therefore, only the performance of the Al/h-BN/Al-1 composite was measured in this study. The kx and kz values of the Al/h-BN/Al-1 composite were determined as 4.99 ± 0.15 Wm -1 K -1 and 1.68 ± 0.2 Wm -1 K -1 , respectively ( Figure 3). The high kx value of the Al/h-BN/Al-1 composite was attributed to the h-BN alignment in a parallel direction during hot pressing [25]. This verified that the  Next, each Al/h-BN/Al composite was punched into a 10 mm × 10 mm square for the thermal conductivity measurement. While the Al/h-BN/Al-1 composite was isolated as an intact square (Figure 7a), complete delamination was observed for the other Al/h-BN/Al composites after punching (Figure 7b,c). Given that the top and bottom Al 2 O 3 composites had created joints to hold the h-BN patterns tightly during hot pressing, the decreased joint areas with enlarged h-BN pattern areas caused the separation of h-BN interlayers in the Al/h-BN/Al-2 and Al/h-BN/Al-3 composites. Therefore, only the performance of the Al/h-BN/Al-1 composite was measured in this study. The k x and k z values of the Al/h-BN/Al-1 composite were determined as 4.99 ± 0.15 Wm −1 K −1 and 1.68 ± 0.2 Wm −1 K −1 , respectively ( Figure 3). The high k x value of the Al/h-BN/Al-1 composite was attributed to the h-BN alignment in a parallel direction during hot pressing [25]. This verified that the suggested technique not only increased the filler content but also aligned the h-BN in a parallel direction to attain high thermal conductivity of the composites. The volume resistivity of the Al/h-BN/Al-1 composite was 6.62 × 10 14 Ω·cm, indicating that this product could be applicable as a TIM, which requires high thermal conductivity and excellent insulating properties.
suggested technique not only increased the filler content but also aligned the h-BN in a parallel direction to attain high thermal conductivity of the composites. The volume resistivity of the Al/h-BN/Al-1 composite was 6.62 × 10 14 Ω·cm, indicating that this product could be applicable as a TIM, which requires high thermal conductivity and excellent insulating properties. The mechanical properties of the Al/h-BN/Al-1 composite were measured by pulling a 20 mm × 50 mm rectangle in the tensile direction until it was torn. Figure 8a shows the stress-strain curve of the Al/h-BN/Al-1 composite. The tensile strength, Young's modulus, and elongation at break of the Al/h-BN/Al-1 composite were 1.04 ± 0.09 MPa, 95.0 ± 7.4 MPa, and 1.52 ± 0.26%, respectively. The morphology of the ruptured surface of the Al/h-BN/Al-1 composite obtained after the tensile test was analyzed using SEM, as shown in Figure 8b. The presence of the stack of the aligned h-BN fillers at the torn surface indicates that the initial crack was generated at the interface of the h-BN pattern and Al2O3 composite and then propagated into the Al2O3 composite. In addition, the craters observed in the Al2O3 composite show that the applied energy was transferred to the Al2O3 fillers and then dissipated through the pullout of the fillers [36].

Conclusions
In this study, composites with high filler loading were fabricated by coating an h-BN solution onto a silicone-based composite with 85 wt.% Al2O3 through metal masks with square open areas. This method could avoid the problems that arise from increased viscosity because the planar h-BN platelets were not added into the compositions of the Al2O3 composites. In addition, hot pressing induced alignment of h-BN in a parallel direction to The mechanical properties of the Al/h-BN/Al-1 composite were measured by pulling a 20 mm × 50 mm rectangle in the tensile direction until it was torn. Figure 8a shows the stress-strain curve of the Al/h-BN/Al-1 composite. The tensile strength, Young's modulus, and elongation at break of the Al/h-BN/Al-1 composite were 1.04 ± 0.09 MPa, 95.0 ± 7.4 MPa, and 1.52 ± 0.26%, respectively. The morphology of the ruptured surface of the Al/h-BN/Al-1 composite obtained after the tensile test was analyzed using SEM, as shown in Figure 8b. The presence of the stack of the aligned h-BN fillers at the torn surface indicates that the initial crack was generated at the interface of the h-BN pattern and Al 2 O 3 composite and then propagated into the Al 2 O 3 composite. In addition, the craters observed in the Al 2 O 3 composite show that the applied energy was transferred to the Al 2 O 3 fillers and then dissipated through the pullout of the fillers [36]. suggested technique not only increased the filler content but also aligned the h-BN in a parallel direction to attain high thermal conductivity of the composites. The volume resistivity of the Al/h-BN/Al-1 composite was 6.62 × 10 14 Ω·cm, indicating that this product could be applicable as a TIM, which requires high thermal conductivity and excellent insulating properties. The mechanical properties of the Al/h-BN/Al-1 composite were measured by pulling a 20 mm × 50 mm rectangle in the tensile direction until it was torn. Figure 8a shows the stress-strain curve of the Al/h-BN/Al-1 composite. The tensile strength, Young's modulus, and elongation at break of the Al/h-BN/Al-1 composite were 1.04 ± 0.09 MPa, 95.0 ± 7.4 MPa, and 1.52 ± 0.26%, respectively. The morphology of the ruptured surface of the Al/h-BN/Al-1 composite obtained after the tensile test was analyzed using SEM, as shown in Figure 8b. The presence of the stack of the aligned h-BN fillers at the torn surface indicates that the initial crack was generated at the interface of the h-BN pattern and Al2O3 composite and then propagated into the Al2O3 composite. In addition, the craters observed in the Al2O3 composite show that the applied energy was transferred to the Al2O3 fillers and then dissipated through the pullout of the fillers [36].

Conclusions
In this study, composites with high filler loading were fabricated by coating an h-BN solution onto a silicone-based composite with 85 wt.% Al2O3 through metal masks with square open areas. This method could avoid the problems that arise from increased viscosity because the planar h-BN platelets were not added into the compositions of the Al2O3 composites. In addition, hot pressing induced alignment of h-BN in a parallel direction to

Conclusions
In this study, composites with high filler loading were fabricated by coating an h-BN solution onto a silicone-based composite with 85 wt.% Al 2 O 3 through metal masks with square open areas. This method could avoid the problems that arise from increased viscosity because the planar h-BN platelets were not added into the compositions of the Al 2 O 3 composites. In addition, hot pressing induced alignment of h-BN in a parallel direction to increase k x of the composite further. Although the Al/h-BN/Al composites containing >15.8 wt.% h-BN were completely delaminated because of the decrease in areas jointed by Al 2 O 3 composites, the Al/h-BN/Al-1 composite containing 74.1 wt.% Al 2 O 3 and 12.8 wt.% h-BN remained undamaged after punching. The k x and k z values of the Al/h-BN/Al-1 composite were determined as 4.99 ± 0.15 Wm −1 K −1 and 1.68 ± 0.2 Wm −1 K −1 , respectively. Furthermore, the volume resistivity of the composite was measured to be 6.62 × 10 14 Ω·cm. In conclusion, the method suggested in this study has remarkable potential to realize composites with high thermal conductivity through the combination of high filler loading and parallel alignment of h-BN fillers.