Enhancing thermal conductivity and balancing mechanical properties of 3D-printed iPP/HDPE-based dielectric composites via the introduction of hybrid fillers and tailored crystalline structure

ABSTRACT With the development of 5G technology, the miniaturised and highly integrated electronic devices urgently require thermal management materials possessing high thermal conductivity and mechanical properties. In this work, isotactic polypropylene (iPP)/high-density polyethylene (HDPE)-based dielectric composites possessing ideal thermal conductivity and balanced mechanical properties were prepared via Fused Filament Fabrication (FFF). The advanced material properties were achieved by the introduction of hybrid fillers and tailored polymer crystalline structure. The highly oriented h-BN, oriented iPP crystalline and iPP/HDPE epitaxy crystalline were observed. Meanwhile, we studied the effect of the ratio of hybrid fillers on various properties of composites. The thermal conductivity of iPP/HDPE/h-BN/Al2O3 composites reach 1.802 W·m−1·K−1. The impact strength and tensile strength reach 13.23 KJ/m2 and 40 MPa, respectively. In addition, the composites maintain ideal dielectric properties. This work offers a feasible strategy to fabricate dielectric and thermal conductive composites with balanced mechanical properties using semicrystalline polymer through FFF process.


Introduction
In the past decades, miniaturised and highly integrated electronic devices rapidly developed with the advance of 5G (Maqbool et al. 2022).Since rapidly accumulated heat in electric devices can worsen the performance and lifespan of electronic components, the development of thermal management materials which have high thermal conduction and electric insulativity emerges as a research hotspot (Chen et al. 2016;Gao et al. 2022).Polymer-based composites have drawn great attention from researchers for their low density, high specific strength and good dielectric properties (Jiang et al. 2022;Mohan et al. 2017;Vardai et al. 2020;Zhou et al. 2016).However, their widespread utilisation in electronic industry as heat dissipation materials is limited due to inherently poor thermal conductivity (0.1-0.4 W•m −1 •K −1 ) of pure polymeric materials (Lin et al. 2022;Ma et al. 2022).The efficient approach to enhance thermal conductive property of polymeric composites is the addition of fillers (e.g.graphite (Li et al. 2016;Wieme et al. 2019), boron nitride (Li et al. 2021) and alumina (Ouyang et al. 2018, etc.).Researchers attempted to fabricate ideal thermal conductive composite materials via massive addition of fillers (Guo et al. 2019).It inevitably caused the mechanical properties degradation of materials and restricted their practical applications (Li, Ge, et al. 2017).Besides, it is hard to enhance thermal conductive property and improve the dielectric properties of composites with single filler (Agrawal and Satapathy 2014;2018).Therefore, the fabrication of polymer-based dielectric composites with desired thermally conductive and mechanical properties has emerged as a critical issue.
Studies have shown that assembling conductive fillers with different sizes and morphologies benefits the thermal conductivity and mechanical properties improvement for synergistic enhancement effect.(Han, Ruan, and Gu 2022;Hu et al. 2017;Ohnmacht et al. 2023;Zhu et al. 2019).For example, the improved thermal transport and mechanical performance were achieved by polyvinyl alcohol (PVA) composites with SiO2 nanoparticles-modified-exfoliated hexagonal boron nitride (h-BN) (Zhang et al. 2019).The use of hybrid fillers also leads to the improve of dielectric properties (Lim et al. 2022;Shi et al. 2021;Wondu, Lule, and Kim 2021).For example, increased thermal conductivity value and better dielectric properties were obtained by combining multiwalled carbon nanotubes (MWCNTs) and zinc oxide (ZnO) for PP-based composites (Russo et al. 2018).In addition, filler orientation through magnetic alignment (Lin et al. 2013), electric field alignment (Uetani et al. 2014) and shear alignment (Song et al. 2012;Wieme et al. 2022;Yang et al. 2011), etc. can be beneficial.According to this approach, the composites could gain enhanced TC for heat conduction paths construction with low filler content (Zhang, Feng, and Feng 2020).Among the filler orientation methods, shear alignment is attractive due to a wider selection of filler types and the widespread of the shear phenomenon which exists in most manufacturing process (e.g.injection moulding and extrusion moulding, etc.).However, this method has some limitations in forming functional composites with oriented fillers because it restricts the filler orientation direction and cannot be easily customised.
Additive manufacturing (AM) (Duan et al. 2019;Guo et al. 2020;Hong et al. 2019;Mitchell et al. 2018;Weng et al. 2016;Zhakeyev et al. 2017) is an innovative technology for polymer parts design and mould making owing to its rapid prototyping of complex structures.Fused Filament Fabrication (FFF) is a universally applied AM techniques (Gao et al. 2021;Jing et al. 2020) due to its distinct advantages including low cost and secure manufacturing process (Turner, Strong, and Gold 2014).In FFF, a filament is fed into the equipment and melted in a heated liquefier.The melt is extruded from the nozzle and then deposits on a platform.A complex solid part is made via the relative movement between nozzle and platform.The melt is subjected to the shear and stretching force during FFF which promotes the orientation of fillers and influences the condensed state structure development of the polymer.Meanwhile, it is convenient to control the orientation degree and direction of fillers by adjusting printing direction and velocity.Based on this, researchers fabricated some functional composites with highly oriented fillers (Ferreira et al. 2017;Geng et al. 2019) or reinforced polymer blends with tailored condensed state structure (Leng, Wu, and Zhang 2019).For example, the 3D printed thermoplastic polyurethane (TPU)-based composites with oriented BN exhibited an ideal thermal conductivity (Liu et al. 2019).Another example describes that in-situ microfibril and shish-kebab structure are observed in FFF printed PP/PS composites by selecting appropriate processing parameters (Jiang et al. 2020).
iPP and HDPE are two polymeric materials which are widely used all over the world due to their balanced properties and relatively low cost (Luo et al. 2017).Moreover, iPP/HDPE melt blending is one of the most used methods to toughen iPP which has poor impact strength.Under the ideal experimental condition, iPP/ HDPE blends can form a special crystalline structure called an epitaxy crystalline structure which is understood as the lattice matching of iPP (010) crystal plane and PE (100) crystal plane (Battegazzore, Bocchini, and Frache 2011).The epitaxy crystalline can greatly improve impact performance of products (Gu et al. 2020;Zhou et al. 2017).However, the high crystallinity and shrinkage of both polymeric materials which would cause the intense warpage deformation of printed parts restrict their wide applications in FFF.
In this work, we selected h-BN and Al 2 O 3 hybrid fillers and iPP/HDPE blend to manufacture a composite with high thermal conductivity, dielectric properties and improved mechanical performance.As two-dimensional filler with anisotropic thermal conductivity, h-BN has high in-plane thermal conductivity and electrical insulation, therefore, h-BN is commonly used to fabricate anisotropic thermally conductive composites via external field alignment (Wang et al. 2019).Alumina (Al 2 O 3 ) facilitates the construction of heat conduction paths and contributes to enhanced mechanical properties and low dielectric constant for the composite material (Wang, Wang, et al. 2021;Yetgin et al. 2020;Yuan et al. 2021).The use of iPP/HDPE blend could overcome the disadvantage of insufficient toughness of iPP.To overcome the warpage of the printed parts, we designed a platform with conical holes (as Figure S1 shows) (Li et al. 2021).Our previous work proved that the iPP/ HDPE blends could be successfully printed by a commercial FFF printer equipped with the self-developed platform.We systematically investigated the orientation of h-BN and dispersion of Al 2 O 3 and evaluated the crystalline structures of iPP and HDPE in the presence of fillers.The results showed that by combining optimised heat conduction fillers networks and reinforced polymer matrix, the thermal conductivity enhanced iPP/HDPE/ h-BN/Al 2 O 3 dielectric composites with balanced mechanical properties can be obtained by FFF.

Sample preparation
Figure 1 shows the process of iPP/HDPE-based composites manufacturing by FFF.iPP and HDPE pellets were dried at 90°C for 6 h.The materials were physically mixed by melt extrusion with h-BN and Al 2 O 3 at a hybrid fillers content of 30 wt% using a twin-screw extruder (ChengGuang Machinery Factory, China).The rotation speed of the screws was 120 rpm and processing temperatures were 150, 170, 180, 200, 200, 200, 200, 200, 190°C from hopper to die.The polymer melt was cooled and pelletised, followed by drying at 90°C for 6 h.The obtained pellets were processed by single-screw extruder (EnBeide Co., China) to fabricate filaments.The diameter was controlled at 1.75 ± 0.1 mm.The speed of single screw was 50 rpm and processing temperatures were 160, 190, 190, 190°C.For comparison, the iPP/HDPE-based composites mixed with h-BN at different filler contents of 10, 20 and 30 wt% were obtained via same process.The HDPE fraction of composites was 20 wt%.The compositions and names of prepared samples are listed in Table 1.Besides, the iPP/HDPE blend which marked as PP/PE with an iPP/HDPE mass ratio of 5/2 was also prepared by the same method.The obtained filaments were used as feedstock to fabricate printed parts by FFF-printer (HORI Z300) with a nozzle diameter of 0.8 mm.The temperatures of nozzle and platform were 220 and 80°C, respectively.The printing velocity was 4800 mm/min and the printing layer height was 0.2 mm.
For comparison of filler orientation effect, the pure iPP, iPP/HDPE blend and iPP/HDPE/h-BN/Al 2 O 3 samples were produced by a compressing machine.The materials were preheated under the temperature of 220°C for 6 min.Then the materials were compressed at 220°C and 9 MPa for 6 min.

Rheology
For the rheological measurements we used the shredded filaments of the iPP, iPP/HDPE blend and composites.As the polymer may undergo thermo-mechanical degradation during processing which results in the reduction of the molar mass and thus the viscosity (Ceretti et al. 2022), we used the samples with the same number of processing (extrusion) cycles for the correct comparison.The shear viscosities of the filaments were measured by high-pressure capillary rheometer (RG120, GOTTFERT, Germany).The aspect ratio (L/D) of the capillary is 20.The test temperature was 220°C and the shear rate was from 10 to 1000 s −1 .Storage and loss modulus were measured at 220°C from 0.1 to 100 Hz by a rotational rheometer (MCR302, Anton Paar, Austria).The disc-like test samples (diameter: 20 mm, thickness: 1.5 mm) were fabricated by compression moulding.

Scanning electron microscopy
Morphology of printed samples was obtained on scanning electron microscopy (SEM) equipment (Nova Nano SEM 450, U.S.A.).To obtain the fracture surface, all test samples were cryo-fractured and gold-sputtered before SEM characterisation.The dispersion of Al 2 O 3 and h-BN of printed samples was also detected by an energy dispersive spectroscopy (EDS) detector.

X-ray measurement
The h-BN orientation in various samples characterised by an X-ray diffraction (XRD) equipment (UItima IV, Rigaku, Japan).The range of scan angle was from 10 to 60°with a scanning speed of 10°/min.The X-ray direction was along the printing direction.

Thermal conductivity
The thermal conductivity of various samples was detected by a thermal conductivity analyzer (LFA467, Netzsch, Germany) at 25°C.The disc-like test samples (diameter: 12.5 mm, thickness: 2.5 mm) were cut from the printed parts perpendicularly to printing direction.For comparison, compression moulded samples were cut from cube-shaped compressed parts.The thermal conductivity l was calculated (Yu et al. 2008): s represents thermal diffusion coefficient, C p represents heat capacity, and r represents density of the test samples.s and C p were obtained on the thermal conductivity analyzer.r was measured by the drainage method.

Infrared thermal observation
The infrared thermal images of pure iPP and B28A2 were obtained by infrared thermal imager (Testo 870-2, Testo SE &Co., Germany).The printed test samples (size: 10 mm × 10 mm × 3 mm) were cut from the printed B28A2 samples perpendicularly to printing direction.The compression moulded samples with the same dimension were cut from cube-shaped compressed pure PP and B28A2 samples (size: 50 mm × 50 mm × 3 mm).The test samples were placed on heating stage (80°C) and heated starting from 20°C.The real-time temperatures of samples were recorded per 5 s after starting heating.
2.3.6.Two-dimensional wide-angle X-ray diffraction Two-dimensional Wide-Angle X-ray Diffraction (WAXD) experiments were conducted in the Shanghai Synchrotron Radiation Facility (SSRF) BL16B1 beamline, Shanghai, China.The X-ray wavelength was 0.124 nm.The size of the beam was 0.5 × 0.8 mm 2 .The sample was 102 mm far away detector.The test samples were cut from the intermediate zone of printed samples along the printing direction.

Mechanical properties
The tensile properties of samples were studied on testing machine (5967, INSTRON, U.S.A.) at 25°C with a tensile speed of 20 mm/s according to the ASTM D-638 test standard.The notched Izod impact strength of samples was tested on an Izod machine (XJUD-5.5,Jinjian Testing Instrument Co., China) at 25°C.The printed dumbbell tensile samples and impact samples were fabricated.The tensile direction was along the printing direction of parts.The compression-moulded tensile and impact samples were formed with the same dimension.V-shaped notch (depth of 2 mm) on the impact samples was made.The average tensile and impact strength values of five test samples were calculated as the mechanical parameters.

Dielectric properties
The dielectric properties of samples were obtained using a dielectric spectrometer (Concept 50, Novocontrol,  Germany) at 25°C.The results were recorded from 10 3 to 10 6 Hz.The breakdown strength of samples with the thickness of around 100 μm was tested according to ASTM D149-1997.The used voltage source is a DDJ-50KV (Guanmei Precision Instruments Co., China).Under the voltage increasing rate of 500 V/s, the specimen is tested until the specimen breaks down.The experiment was carried out more than 10 times for each sample.

Rheological behaviour
To access the effect of fillers addition on the rheological behavior of obtained materials, rheological measurements by high-pressure capillary rheometer and rotational rheometer were performed.Figure 2(a) shows the shear viscosities of all samples versus the shear rate (10 to 1000 s −1 ).All samples show obviously shear-thinning behaviour which ensures the smooth extrusion of polymer melt from the nozzle during the printing process (Guo et al. 2021).The composites possess higher viscosities compared to the pure iPP and iPP/HDPE blend.According to the previous study (Li et al. 2019), this phenomenon could be due to the fillers occupy the free space between the polymer chains and the chains are adsorbed on the surface of h-BN.Therefore, the composites have increasing viscosities as the movement of polymer chains is suppressed (Luo et al. 2021).The effect of h-BN content and hybrid fillers ratio on viscosities are presented clearly in Figure 2(b).The viscosities of composites increase with increased filler content.Moreover, Al 2 O 3 can further improve viscosities of composite materials.Figure 2(c,d) shows the composite materials exhibit typical viscoelastic behaviour.Furthermore, the improvement of both storage modulus and loss modulus can be observed with an increase of fillers content in the frequency range of 0.1-100 Hz.The viscosities and modulus of composites with a hybrid fillers content of 30 wt% are obviously increased compared to pure iPP.The orientation of fillers is affected by rheological properties of composite materials.During FFF process, the laminated fillers orientate along the extrusion direction which is induced by shearing force when composites melt is extruded through the printing head.The oriented structure can be well preserved in composites with hybrid fillers for the increased viscosities and modulus (Li et al. 2021).The orientation of h-BN in various samples was characterised in the next section.

Morphology of composites
To evaluate the printing quality of composites and demonstrate the predicted orientation of h-BN in printed samples, we observed the morphology (Figure 3(d)-(g)) of printed samples with single h-BN and with hybrid fillers via a SEM equipment.The viewing direction of the SEM images is indicated in Figure 3(a).From a large scale, it can be observed that the dispersion of h-BN platelets in printed samples is uniform as shown in Figure 3(d,e).The EDS results (Figure S2) of B and N element in printed B30 and B28A2 support conclusion.Meanwhile, the obtained parts possess well-dispersed and good interlayer bonding, and no gaps between adjacent filaments exist, which is the foundation of prominent mechanical properties.For the composite with hybrid fillers, the dispersion of Al 2 O 3 nanoparticles was further detected by SEM with an EDS detector as Figure 3(b,c) shows.The blue dots in the images refer to Aluminium element.In printed B28A2, Al 2 O 3 fillers are uniformly dispersed in polymer phase without obvious agglomeration, which could act as bridges to connect the oriented h-BN platelets.Moreover, the SEM images show visually that h-BN inplane direction (indicated by the red arrows in Figure 3(f,g)) in B30 and B28A2 arrange almost parallelly to in-plane direction of printed sample.It suggests that h-BN platelets show strong alignment in matrix along in-plane (X-Y plane) direction.However, local agglomeration of h-BN occurs in B30 which exerts influence on the alignment of h-BN as the yellow circle indicates in Figure 3(f).For the composite materials with hybrid fillers, the addition of Al 2 O 3 nanoparticles improves h-BN dispersion as shown in Figure 3(g).To show the generality of this dispersion difference, more SEM images of different positions in B30 and B28A2 are supplied in Figure S3.The according EDS results (Figure 3(h,i)) of N element exhibit this dispersion difference of h-BN between printed B30 and B28A2 samples more intuitively.Besides, the higher orientation degree of h-BN platelets in B28A2 compared to that of B30 sample can be observed, and the thermal conduction paths (as the red dot-lines in SEM images show) formed are denser and more regularly distributed along the inplane direction.
To further compare h-BN orientation in different composite samples, the XRD spectrums of printed samples were recorded and the results are presented in Figure 4.The 26.9°and 41.6°diffraction peaks represent h-BN ( 002) and (100) crystal planes, respectively (Wang, Liu, et al. 2021).The ( 002) and (100) crystal planes are located along thickness direction and h-BN in-plane direction, respectively (Pan et al. 2018).To indicate h-BN orientation degree along in-plane direction, the intensity ratio I (002) /I (100) of each sample was calculated.A higher ratio means higher orientation degree.The I (002) value is higher than I (100) in all printed samples, which means the h-BN oriented.For the samples with only h-BN added, the improved orientation degree of h-BN can be observed with an increase filler content.The results of XRD are consistent with that of rheological test.When hybrid fillers content is fixed at 30 wt%, with the increase added Al 2 O 3 in printed samples, the h-BN orientation degree further increases.Moreover, the value of intensity ratio I (002) /I (100) of printed B29A1 sample is already as high as 340, while that of the printed B30 sample is only 283, indicating that a small amount addition of Al 2 O 3 can greatly improve h-BN orientation of during the fused deposition process of FFF.This is mainly because increased viscosities of composites with hybrid fillers and the uniform dispersed Al 2 O 3 nanoparticles which act as lubricant can promote h-BN orientation under the action of external force (Bian et al. 2018).

Thermal conductivities
As the enhanced h-BN orientation degree has been observed, the reinforcements in the thermal conductivities (λ) of printed samples with hybrid fillers can be supposed.Therefore, the λ of various samples was measured via a thermal conductivity analyzer.As Figure 5(a) shows, for 3D-printed samples, we primarily focused λ along printing direction (l ).As the fillers are oriented along the infill path due to the shear, we expect the enhancement of the λ only along this direction.Figure 4(b) presents the l of printed iPP, iPP/ HDPE and composites.The l of pure iPP and iPP/ HDPE are 0.180 and 0.193 W•m −1 •K −1 , respectively.It confirms inherently poor thermal conduction of polyolefin.For composites with hybrid fillers, the enhanced l from 1.602 W•m −1 •K −1 for B30 to 1.730 W•m −1 •K −1 for printed B29A1 was observed.The highest l of 1.802 W•m −1 •K −1 was achieved for printed B28A2 sample with 28 wt% h- BN and 2 wt% Al 2 O 3 .It is 12.5% higher than λ of printed B30 and is 9 times higher than λ of iPP and iPP/HDPE blend.For contrast, the λ perpendicular to printing direction (l ⊥ ) of printed B30 and B28A2 have no obvious difference, which shows slight increase compared to that of pure iPP (Figure S4).As the SEM images show, the thermal conduction paths are formed along the infill direction as the orientation of the fillers is induced by shear.In our case, the infill orientation is 0°w hich means that all the layers are printed in the same direction.Consequently, the conduction path is formed only in the printing direction as well (Figure 5 (f)).However, the l reduces to 1.631 W•m −1 •K −1 for printed B26A4 when Al 2 O 3 further increased to 4 wt%.To explain this phenomenon, we investigated the internal relationship between l and h-BN volume fraction.
By recording the experimental λ of samples with various h-BN contents, the l of printed composites and  λ of compression moulded composites (l c ) versus h-BN volume fraction (V) was obtained as Figure 5(c) shows Table 1 lists the h-BN volume fraction, which was calcu- that ω 1 , ω 2 and ω 3 represent the weight fraction of iPP, HDPE and h-BN in samples shown in Table 1.ρ 1 , ρ 2 and ρ 3 are the densities of iPP, HDPE and h-BN, respectively (Liu, Huang, and Huang 2022).Figure 5(c) shows the l of printed composites improves rapidly with the increase of highly oriented h-BN.However, the l c of compression moulded composites show inapparent improvement compared with iPP/ HDPE blend.Such insignificant increase of thermal conductivity indicates that it is hard to structure heat conduction paths in composites because h-BN is randomly dispersed and individual platelets have smaller chance to contact with each other.Moreover, the λ of printed and compression moulded composites show almost linearly improvement with the increase of h-BN volume fraction.The relationship between λ and V can be obtained through a linear fitting.The results are as follows: l = 0.09V + 0.20, l c = 0.01V + 0.19.It means that the l of printed composites with highly oriented filler showed stronger dependence on the filler content.As h-BN platelets determine largely the enhancement of the l due to their orientation and overlap, the further increase of Al 2 O 3 content in printed B26A4 means the massive reduction of h-BN.Therefore, the local thermal conduction networks deteriorate which causes a decrease of thermal conductive property.In contrast, for the compression moulded samples with hybrid fillers, the ratio of hybrid fillers had less effect on the thermal conductivity as shown in Figure S5 for the randomly dispersed h-BN.
To visualise and observe the difference of heat conduction among investigated samples, the infrared thermal images (Figure 5(d)) were obtained.The compression moulded pure iPP, B28A2 and printed B28A2 were named as CPP, CB28A2 and FB28A2, respectively.Test samples were located on a heating stage (80°C) and heated from 20°C to 70°C simultaneously.The real-time temperatures of various samples were recorded per 5 s as Figure 5(e) shows.FB28A2 shows the fastest heating rate.The temperature of FB28A2 reached 70°C in 50 s, which was about 10°C higher than that of CPP and CB28A2.The printed composite with hybrid fillers exhibits excellent thermal conductivity, owing to the highly oriented h-BN platelets and massively interconnected thermal conduction networks as the schematic diagram Figure 5(f) shows.
In comparison with the previously studied composites with similar components (Cheewawuttipong et al. 2016;Gao et al. 2020;Jin et al. 2019;Li et al. 2017;Lu et al. 2021;Zha et al. 2019;Zhang et al. 2020), the present iPP/ HDPE/h-BN/Al 2 O 3 composites demonstrate satisfying thermal conductivity with relatively low filler content.The results are presented in Figure 5(g) and Table S1.

Crystalline structure
The 2D-WAXD patterns of various printed samples were obtained to characterise the crystalline structures of composites.The Miller indexes were directly annotated on the pattern of Figure 6(a).From the inside to the outside, the scattering pattern originates from the iPP (110), ( 040), ( 130), ( 111) and (−131) planes and the HDPE ( 110), ( 200) planes.In particular, the reflection patterns of iPP (111) (−131) planes and HDPE (110) plane are overlapped due to the similar diffraction angles.Figure 6(a) shows the 2D-WAXD result of compression moulded iPP/HDPE blend.The equally distributed reflection circles of iPP and HDPE can be observed.This result indicates the isotropic crystallisation of iPP and HDPE in the compression moulded iPP/HDPE sample.For compression moulded B28A2, the scattering pattern presented almost the same result.However, for the printed composites, the crystallisation of iPP and HDPE is significantly affected by the printing process.From the results, it can be found that iPP (hk0) planes show strong arc-shaped diffraction signals at equator direction, which indicates the c-axis of iPP lamellae exhibit a strong tendency to orient along the printing direction (Gu et al. 2020).It has been reported that the oriented crystalline structures contribute to the impact strength enhancement as they promote stress conduction in the polymeric materials (Leng et al. 2020).At the same time, for the HDPE crystallites, the patterns of HDPE ( 110) and ( 200) planes can be used to judge whether there exists epitaxial crystallisation in the samples (Deng, Liu, et al. 2014).Figure 6 on equator.These special reflection behaviours are HDPE lamella epitaxial growth onto iPP lamella (Deng, Whiteside, et al. 2014;Lotz and Wittmann 1986).Moreover, printed composites with hybrid fillers showed the same diffraction manners.The results indicate that the epitaxial crystalline was successfully obtained in the presence of fillers by FFF printing, which offers a new strategy to toughen composites.

Mechanical properties
As the tailored crystalline structure in printed composites had been observed, the mechanical properties of printed samples were subsequently tested.Figure 7 illustrates the mechanical properties of various samples.CPP and CPE/PE represent compression moulded iPP and iPP/HDPE samples, respectively.The other samples are FFF printed.Figure 7(a) shows stress-strain curves of various samples.The relevant results are presented in Figure 7(b).It can be noticed that the printed composites exhibit immense improvement in mechanical properties compared with compression moulded pure iPP and iPP/HDPE blend.The tensile modulus increases from 1.5 GPa of pure iPP to 2.3 GPa of printed B26A4, which is attributed to the high modulus of fillers (Balani and Agarwal 2008;Yang et al. 2021) and the oriented h-BN.The tensile strength of printed composites with single filler shows an increase from 34.7 MPa of pure iPP to 36.7 MPa of printed B30.Moreover, the tensile strength of printed composites with hybrid fillers further increases to about 40 MPa.This may be because a small amount of Al 2 O 3 particles can serve as lubricant and improve h-BN dispersion in composite samples according to SEM results.
At the same time, the impact strength results of samples are illustrated in Figure 7(c).The addition of HDPE slightly improved the impact strength of compression moulded pure iPP from 3.5 KJ/m 2 for CPP to 5.4 KJ/m 2 for CPP/PE due to the HDPE toughening effect.As for the printing composites, the impact strength shows further increase owing to the iPP shishkebab crystalline structures and iPP/HDPE epitaxy crystalline structure in printed samples (Gu et al. 2019).The impact strength of B30 is 10.9 KJ/m 2 , which is about 2 times higher than iPP.When hybrid fillers content is fixed at 30 wt%, the impact strength of composites increases with the increase of Al 2 O 3 .It reaches the maximum (13.8 KJ/m 2 ) when Al 2 O 3 content is 4 wt%.The results show satisfying tensile and impact strength of printed composite are achieved via combining iPP/HDPE blend matrix and h-BN/Al 2 O 3 hybrid fillers.Figure 7 (d) and Table S1 show the comparison of mechanical properties of composites in this work and composites with similar components in the previous relevant literatures (Gao et al. 2020;Jin et al. 2019;Lu et al. 2021;Zha et al. 2019;Zhang et al. 2018).The present iPP/HDPE/h-BN/Al 2 O 3 composites demonstrate excellent impact strength and tensile strength with ideal thermal conductivity.

Dielectric properties
The dielectric properties of various samples are illustrated in Figure 7. Ideal thermally conductive and dielectric polymer composites (TDPCs) used as thermal management materials in electronic devices require dielectric constant below 3 and dielectric loss (tangent delta) below 0.005 (Li et al. 2022).As shown in Figure 8, all the composites meet the requirement in the characterisation frequency (10 3 -10 6 Hz). Figure 8(a) shows that the dielectric constant of the blend and composites increases compared to the value of pure PP (2.50 at 1 MHz).For the PP/PE blend, it can be explained that there are more interfaces in the blend for the poor compatibility of PP and PE, which leads to the increase of the interface polarisation and finally causes increased dielectric constant (Fan et al. 2019).The value of dielectric constant for B30 (2.89 at 1 MHz) reaches the maximum among all samples.This may be because the interfacial polarisation of composite is enhanced with the addition of h-BN filler.However, when the content of the total filler is kept at 30 wt%, the composites with hybrid fillers show lower dielectric constant values than that of B30.The B28A2 sample even exhibits a lower dielectric constant (2.74 at 1 MHz) than that of PP/PE blend (2.75 at 1 MHz).The small addition of Al 2 O 3 may reduce the interfacial polarisation for the reduced h-BN content and more uniform dispersion of h-BN.Moreover, it was reported in the literature that the addition of an appropriate content of nanoparticles can suppress the charge injection and resist partial discharges (PDs) erosion of composites (Murakami et al. 2008).These phenomena benefit to the improvement of dielectric properties.Meanwhile, the dielectric loss values of composites are all below 0.005 as shown in Figure 8(b), while the values of iPP and iPP/HDPE matrix exceeded 0.005 in partial frequency.The low dielectric constant and loss of composites can improve the signal transmission quality in electronic devices by decreasing the parasitic capacitance (Li et al. 2022).Besides, the breakdown strength and frequency-dependent conductivity of various samples are presented in Figure 8(c,d).The results show that composites have comparable breakdown strength (about 400 kV/mm).And the conductivity of composites slightly increases after introduction of fillers.However, the conductivity of all samples is lower than 10 −9 S/cm at 1000 Hz, indicating their well electrically insulating properties (Xie and Yang 2023).These good features insure their extensive application in electronics industry.

Conclusion
In this study, the dielectric composites with improved thermal conductivity (λ) and satisfying mechanical properties were fabricated by FFF.h-BN/Al 2 O 3 hybrid fillers benefit to thermal conductivity enhancement for better oriented conduction networks.The iPP/HDPE blend matrix offers the ideal mechanical properties of composites due to the oriented crystalline structure of iPP and HDPE toughening effect.The epitaxial crystalline was successfully obtained in the presence of fillers due to FFF processing.Besides, it is proven that a small addition of Al 2 O 3 nanoparticles also benefits the improvement of impact strength and dielectric properties.As a result, the highest thermal conductivity of composites reaches 1.802 W•m −1 •K −1 of printed B28A2, which is 900% higher than pure iPP.The impact strength and tensile strength of printed B28A2 are 13.23 KJ/m 2 and 40 MPa, respectively, which shows immense enhancement compared with compression moulded pure iPP and iPP/ HDPE blend.The dielectric constant value and dielectric loss value of B28A2 are below 3 and 0.005, respectively, meeting the requirements of ideal thermally conductive and dielectric polymer composites (TDPCs).

Figure 2 .
Figure 2. (a) Shear viscosity versus shear rate of various samples, (b) partially enlarged detail between the shear rate of 100-450 s −1 , (c) storage modulus and (d) loss modulus of various samples.

Figure 3 .
Figure 3. (a) Schematic diagram of printed part and observation direction, (b) (c) EDS results of Al element in B28A2 sample at different magnifications, (d) (e) SEM micrographs of printed B30 and B28A2, (f) (g) the locally enlarged micrographs of (d) (e), and (h) (i) EDS results of N element in printed B30 and B28A2, respectively.

Figure 4 .
Figure 4. (a) XRD intensity of various composite samples, (b) the partially enlarged image of (a), and (c) the value of I (002) /I ( 100 ) of each sample.

Figure 5 .
Figure 5. (a) Schematic of the preparation of thermal conductivity test sample, (b) l and thermal conductivity increase ratio of pure iPP and printed composites with various fillers component, (c) l of printed composites and λ of compression moulded composites ( c ) versus h-BN volume fraction (V), (d) infrared thermal images of the compression moulded pure iPP (CPP), compression moulded B28A2 (CB28A2) and printed B28A2 (FB28A2), (e) temperature of different samples versus heating time, (f) heat transfer schematic diagram in investigated samples and (g) thermal conductivity and thermal conductivity enhancement values compared with literature reports of polymer composites.

Figure 7 .
Figure 7. (a) Stress-strain curves, (b) tensile strength and tensile modulus, (c) impact strength of compression moulded pure iPP and iPP/HDPE blend, and printed composites.Note that the CPP and CPE/PE represents compression mould pure iPP and iPP/HDPE, respectively.(d) Comparison of mechanical properties of composites in this work and composites with similar components in the previous relevant literature.

Table 1 .
The detail information of sample name and related component content.