Thermal,electrical,and mechanical properties of hexagonal boron nitride

Abstract

Flaky hexagonal boron nitride particles-reinforced epoxy composites were prepared. The investigation on the thermal, electrical, and mechanical properties of hexagonal boron nitride/epoxy composites indicated that the incorporation of silane coupling agent-treated hexagonal boron nitride into epoxy slightly improved the glass transition temperature and the thermal stability of the composites; the epoxy containing silane treated hexagonal boron nitride exhibited higher thermal conductivity compared to the untreated ones. The dielectric permittivity increased slowly with an increase in hexagonal boron nitride content, as well as with a decrease in frequency. The obtained 50 wt% hexagonal boron nitride–filled epoxy composite had a low dielectric permittivity (less than 5.4) and dielectric loss (less than 0.02) in all frequencies ranging from 10⁻¹ to 10⁷Hz, a high volume resistivity of 6.3 × 10¹⁴ Ω·cm, and a high dielectric strength of 16 kV/mm, together with moderate mechanical properties, which are of great significance for practical electrical materials applications.

Keywords

Polymer composites dielectric properties mechanical properties thermal conductivity

Introduction

The increasingly miniaturized integral circuit leads to an escalation of power dissipation as well as an increase in heat flux for the electronics working under high frequency.^1–3 As such, the heat dissipation becomes an important issue in microelectronic parts since the increase of device density drastically increases the electrical energy consumption and thus raises the need for better heat dissipation.^4–6 Hence, heat dissipation capability of the electronic packaging, substrate, and thermal interface materials becomes critical not only to the life span but also to the performance and reliability of the electronic device^7–9 because it is crucial that the heat generated should be dissipated as quickly and as effectively as possible, while maintaining the device’s a desired operating temperature.^10–12

In comparison with commonly used metals and ceramics, polymers possess rather lower thermal conductivity (about ∼0.2 W/m K) because of their relatively low atomic density. Such a deficiency could be improved by adding inorganic filler with a high thermal conductivity to the polymers.¹³ This versatile method synergistically integrates the advantages of polymers and inorganic particles; thus, the thermal, electrical, and mechanical properties of the composites can be tuned by proper selection of the filler type, shape, size, and concentration. In recent years, high thermal conductive and electrically insulating polymeric composites have gained wide applications in various occasions such as packaging, printed circuit boards, heat exchangers, thermal interface materials, and phase change materials, etc. by virtue of their clear advantages over ceramic materials, such as the ease of forming, flexibility, low density, and low cost.¹⁴ The literature concerning thermal conductive polymer composites is particularly focused on the use of different kinds of fillers, such as hexagonal boron nitride (hBN),^14,15 aluminum nitride (AlN),^16,17 silicone nitride (Si₃N₄),^18,19 alumina (Al₂O₃),^20,21 silicon carbide (SiC),^22,23 and diamond.²⁴ For example, Yu et al.³ studied the structures and properties of AlN/PS composites; He et al.⁸ investigated the preparation and properties of Si₃N₄/PS composites for electronic packaging; Zhou et al.¹⁹ explored Si₃N₄/Al₂O₃/blended polyethylene thermal conductive composite plastics.

Generally speaking, low value of dielectric constant and dissipation factor ensures high signal propagation speed and diminishes the effect of capacitive coupling. Therefore, a low dielectric constant and loss dissipation is required for polymer packaging materials since a high dielectric constant will cause a strong negative effect on the signal propagation by increasing the delay time. The dielectric constants of SiC, Al₂O₃, and AlN are relatively high (greater than eight), which are marginal satisfactory for electronic encapsulation.⁵ hBN was considered to be an ideal candidate due to its relative high thermal conductivity, low coefficient of thermal expansion (CTE), stable crystal structure similar to graphite (a = 0.25040, c = 0.66612), relatively low dielectric constant about 4–5, nontoxicity, and high electrical resistivity in a wide temperature range.²⁵ Epoxy resin was adopted as the matrix due to its good mechanical properties, including low water uptake and CTE, easy processing, and excellent chemical resistance. In this study, hBN was selected to improve the thermal conductivity of epoxy while preserving a low dielectric constant, a low dissipation factor, and the excellent electrically insulating property.

It is well known that the use of coupling agent benefits by improving the phase interfacial bonding strength between filler and matrix of a composites, which enhances the thermal conductivity, electrical insulation, and mechanical properties of the composites.^26–31 In this work, to enhance the filler-matrix interface bonding strength, a silane coupling agent, γ–glycidoxypropyl-trimethoxysilane, was employed to improve the interfacial adherence between the hBN and epoxy matrix. A conventional mixing method was adopted for preparing the samples without using any inert organic solvents. The thermal, electrical, and mechanical properties were investigated in terms of the hBN content, surface modification, and frequency.

Experimental

Materials

The polymer matrix was a diglycidyl ether of bisphenol A-type epoxy resin (D.E.R-383, Dow Chemical Corp.) with an epoxy value of 0.50–0.52. A flexible epoxy resin (Trade name: TA30, Shanghai Xinhua Resin Co.) with a very low viscosity, was used as a reactive toughening and diluting agent to overcome the brittleness of the matrix. The structure of epoxy D.E.R-383 and TA30 are shown in Scheme 1. The curing agent was methyl hexahydrophthalic anhydride (MeHHPA) from Shanghai Shengyuan Co., China, and the cure accelerator was benzyl dimethylamine (BDMA) purchased from Shanghai Haitai Co., China. The hBN powder was supplied by Liaoning Liaobin Fine Chemicals Co., China. The physical properties of D.E.R-383and hBN are summarized in Table 1. The silane coupling agent was γ–glycidoxypropyl-trimethoxysilane from Nanjing Xiangfei Chemical, China.

Scheme l.

Chemical structure of the epoxy D.E.R-383 and flexible epoxy TA30.

Table 1.

Physical properties of epoxy and hBN.

Raw materials	epoxy	hBN
Density/g/cm³	1.2	2.17
Thermal conductivity/W/m K	0.2	X-Y plane 400, Z-3-6
Electrical resistivity/Ω·cm	∼10¹⁵	10¹⁵–10¹⁶
Dielectric constant/1 MHz	4	4–5
Dissipation factor/1 MHz	<0.02	0.0003
Particle size/µm	0.5

hBN: hexagonal boron nitride.

Samples preparation

Surface modification of hBN particles

The hBN particles were surface-treated with γ–glycidoxypropyl-trimethoxysilane (1.5% of the whole hBN mass) by adding them to the silane coupling reagent in a 95 wt% aqueous ethanol solution adjusted to pH 4–5 with diluted hydrochloric acid. The mixture was stirred while ultrasonicating for 30 min, then heated to 80℃ and refluxed, with stirring, for 6 h. After cooling to room temperature for 3 h, the mixture was rinsed with ethanol at least three times. Finally, the hBN particles were dried at 120℃ for 10 h.

Preparation of the hBN/epoxy composites

The composite resin was prepared by first stirring the mixture of epoxy D.E.R-383, TA30, MeHHPA, and accelerator according to the mass-fraction ratio of 80:20:95:1 for 2 h. Then, the hBN filler was added into the organic mixture via a milling process using a tripe rollers mill and then stirred for 30 min. The obtained homogeneous mixture was degassed for about 30 min in a vacuum to get rid of bubbles. After that, the liquid mixture was poured into a clean glass plate mold kept at 60℃ and was cured in an oven at 100℃ for 1 h and 150℃ for 5 h.

Characterization

The surface chemistry of the hBN particles was determined using a Fourier transform infrared (FT-IR) spectrometer (Perkin-Elmer, Paragon1000). The hBN particles were first dried in an oven at 120℃ for 3 h. KBr pellets were then mixed with the dried hBN particles. The spectrum in the 4000 cm⁻¹ to 400 cm⁻¹ range was recorded.

Differential scanning calorimetry (DSC) was performed to analyze the influence of the hBN content on the glass transmission temperature (T_g) of the samples using a Netzsh DSC 200PC equipment. Measurements were conducted in a nitrogen atmosphere, from 20℃ to 200℃, at a heating rate of 10℃/min.

Weight loss of the samples upon heating was measured using a thermogravimetric analyzer (TGA: Model: SDTA851, Swiss). Measurements were conducted in a nitrogen atmosphere, from 25℃ to 600℃, at a heating rate of 15℃/min. The observed weight loss was analyzed.

The morphologies of the samples were inspected with a scanning electron microscope (SEM) (Model: JSM-7000F, JEOL, Japan). Samples were frozen with liquid nitrogen, cut to the desired size, and surface-coated with gold prior to viewing. A representative cross-section of samples was examined to study filler distribution and morphology.

A Hot Disk thermal analyzer (Hot Disk AB, Uppsala/Sweden) was used to measure the thermal conductivity of the samples using the transient place source (TPS) method. A disk-shaped TPS sensor with a diameter of 7 mm and a thickness of 0.07 mm is placed between two circular sample pieces with diameters of 20 mm and thicknesses of ∼2 mm.

The dielectric measurement was performed on a broadband dielectric spectrometer (Novocontrol Technology Company, Germany) with an Alpha-A high-performance frequency analyzer. The measurement was carried out in the frequency range from 10⁻¹ to 10⁷Hz at room temperature. The specimens for dielectric measurement were circular discs ∼1 mm in thickness and 20 mm in diameter.

The dielectric breakdown strength of the samples was measured using a ball–rod electrode arrangement under a continuous alternating current (AC) voltage loading supplied by a 50 kV, 50 Hz transformer. The arrangement and sample were immerged in insulated oil to prevent surface discharges and flashovers. The test for each sample was performed at room temperature; the sample was placed directly between two copper-ball electrodes opposing each other. The lower electrode was connected to the earth, and an increasing AC voltage with a rate of 2 kV/s was applied to the upper electrode until the sample failed. Volume resistivity measurements were measured with an ultrahigh electric resistor (Model: ZC-36, China) at room temperature. Samples for the measurement were 80 mm in diameter and ∼1 mm in thickness.

Tensile and flexural measurements of the samples were determined with a screw-driven universal testing machine (Type ZMGI 250, Shenzhen New SANS Co., China) at a crosshead speed of 2 mm/min adopting standards ASTM D-638 and ASTM D-790, respectively. Impact strength of the specimens was measured with a pendulum impact tester (Type ZBC1251, Shenzhen SANS Co.) according to ASTM D-256.

Results and discussion

FT-IR characterization

Figure 1 shows the FT-IR spectra of both the untreated and silane coupling agent surface-treated hBN particles. The absorption band of the unmodified hBN at about 3200 to 3600 cm⁻¹ is attributed to the vibration of the –OH group on the hBN, which could be the reactive site interacting with the coupling agent. The peak at 808 cm⁻¹ represents the out-of-plane bending vibration, and the broad peak around 1385 cm⁻¹ could indicate in-plane stretching vibration of hBN. Compared with the spectrum of the untreated hBN, the absorption band of the modified hBN at 3200 cm⁻¹–3600 cm⁻¹ decreases due to the reaction between the –OH group and the coupler. Meanwhile, the spectrum of the treated hBN shows a new adsorption band at the wave number of about 2916 and 2850 cm⁻¹, corresponding to the characteristic peaks of the –CH₂- unit from the silane coupler, and the new adsorption band at about 1142 cm⁻¹, indicative of the characteristic peak of the stretching vibration of the -Si-O-unit form the coupler.¹⁵ The important information above supports the occurrence of chemical reaction between the hBN and silane coupler.

Figure 1.

FT-IR Spectrums of raw hBN and silane coupling agent treated hBN. hBN: hexagonal boron nitride; FT-IR: Fourier transform infrared.

A schematic diagram showing the reaction mechanism of the silane coupler with the –OH groups on the surface of hBN is shown in Figure 2. The methoxy groups of the silane coupler were first hydrolyzed by water to form three silanol groups; then, these silanol groups were condensed with the hydroxyl groups on the hBN particles to form the Si-O-BN bond via acid-catalyzed reactions. The silane-treated hBN show good compatibility with epoxy matrix because the epoxy groups grafted on the surface of hBN either chemically react with the anhydride curing agent or entangle with the polymer long chains, thereby providing strong interfacial interaction between the two components.

Figure 2.

Schematic showing the reactions of silane coupling agent with hBN particle surface, and the compatibility between the surface modified hBN and epoxy matrix. hBN: hexagonal boron nitride.

Thermal properties

Figure 3(a) presents the T_gs of the silane coupler-treated hBN/epoxy composites from the DSC measurements. It can be seen that T_gs of the composites were slightly higher than that of the pure epoxy before filler loading levels reached 40 wt%, after 40 wt%, T_g began to decrease. The glass transition in polymers is a complex phenomenon that is affected by many factors including free volume, molecular mobility, molecular weight, and crosslink density.³² In this study, we expect that the T_gs of epoxy are only influenced by the free volume and interaction between the hBN and the matrix. The epoxy groups of silane coupling on the surface of hBN particles can either react with the curing agent or entangle with the epoxy chains, leading to enhanced interfacial interactions between the matrix and the fillers. The enhanced phase interface bonding strength decreases the free volume for the epoxy chains to either rotate or move in matrix, resulting in a slight increase in T_gs compared to pure epoxy. Thermal properties analysis using DSC indicated that below 40 wt% hBN loading the addition of hBN filler to epoxy resin effectively increases the conversion and T_gs of composite resin. This evidenced that the amine groups on the plane edges of hBN filler are able to promote the polymerization and therefore enhance the crosslinking of composite resins subjected to thermal curing. However, as the hBN exceeds 40 wt%, T_g of the epoxy composite began to decrease. The lower T_g of the epoxy composite might be ascribed to that the higher viscosity of the mixture brings about defects such as air voids and poor bonding at the phase interfaces because no any inert solvent was used during processing, which facilitates the large-scale mobility of epoxy chain segment.

Figure 3.

DSC(a) and TGA(b) curves of epoxy with various hBN concentrations. DSC: differential scanning calorimetry; hBN: hexagonal boron nitride; TGA: thermogravimetric analysis.

Figure 3(b) presents the TGA curves of the cured pure epoxy and hBN/epoxy composites. As the figure shows, the introduction of hBN particles to epoxy had no remarkable influence on the thermal stability behavior of the composites. The thermal stability properties such as the onset and maximum thermal degradation temperatures were improved slightly with increasing hBN content. The reason can be ascribed to the high heat capacity of hBN and the enhanced interfacial interactions between the filler and the matrix.

The thermal conductivities of the hBN/epoxy composites at various levels of filler loading are plotted in Figure 4. Thermal conductivity generally increased with increasing hBN content due to the much higher thermal conductivity of filler particles as compared to that of the pure epoxy. The thermal conductivity of the epoxy containing 50 wt% hBN is 1.2–1.34 W/m K, more than 6–7 times that of the pure epoxy. It is known that the thermal conductivity of a composite is controlled by the intrinsic thermal conductivity of filler and matrix, the shape, size, and concentration of filler.³³ The ability to form a conductive network of filler in the matrix depends on the aspect ratio (diameter/thickness ratio) and concentration of filler. At a low hBN concentration, despite a high thermal conductivity, only limited amount of hBN was shared by the composite due to the presence of epoxy (0.22 W/m K) used as the matrix, i.e. the fillers were isolated and placed so far apart that there was no interaction between them, so, the thermal conductivity increased rather slowly. The thermal conductivity obviously increased at higher filler amount because the filler particles tended to touch one another and formed particle clusters within the matrix, which facilitates the heat propagation.

Figure 4.

Thermal conductivity of hBN/epoxy composites with different filler contents. hBN: hexagonal boron nitride.

Figure 4 also suggests that the use of silane coupling agent improved the thermal conductivity of the composites. The thermal conductivities of epoxy containing 40 and 50 wt% hBN particles with surface treatment were 1.04 and 1.34 W/m K, respectively, compared to 0.90 and 1.20 W/mK of untreated hBN-filled epoxy composites. As we know, for hBN/epoxy two-phase system the interfacial status becomes very crucial to thermal conductivity since phonons are very sensitive to interface defects. Thermal resistance is caused by various types of phonon-scattering processes, and the interfacial thermal barriers in composites is mainly due to the scattering of phonons resulting from acoustic mismatch and flaws associated with the matrix–filler interface. The interface between the two-phase composites acts as a barrier to heat transmission. Figure 4 illustrates that the surface modification of hBN with silane improves and enhances the interfacial bonding strength between the filler and the matrix, resulting in an enhanced thermal conductivity due to the reduced interface defects and air void volume (k_air = 0.0024 W/m K) at the filler-matrix interface.

Electrical properties

The dielectric properties of hBN/epoxy composites as a function of filler weight percentages at room temperature are presented in Figure 5. Figure (a) shows that the dielectric permittivities (ɛ_r) of the composites slightly decreased with the increase of frequency in the measured range from 10⁻¹ to 10⁷Hz, which is normally observed in polar resins containing functional groups. Anhydride curing agent MeHHPA and TA30 toughening agent were used in the epoxy resin to reduce the viscosity. However, the hydroxyl (–OH) groups form in the anhydride-cured epoxy resins after the opening of an epoxide ring in further reactions.³⁴ Though epoxy resins have no specific molecular unit to raise the dielectric permittivity before curing, secondary alcoholic hydroxyl groups have an adverse effect on the dielectric characteristics and raise the moisture absorption in the cured resins. So, most thermoset resins tend to exhibit high ɛ_r due to their polar functionality. The reason for the dependence of ɛ_r on the frequency can be ascribed to the interfacial polarization having no time enough to orient themselves in the direction of the alternating field.

Figure 5.

Dielectric properties of hBN/epoxy composites as a function of frequency and hBN concentration: (a) dielectric permittivity and (b) dissipation factor. hBN: hexagonal boron nitride.

Thought the ɛ_r of epoxy and hBN are below 5, the composites exhibited a higher ɛ_r than that of each component. Figure 5(a) suggests that the ɛ_r of composites resin increase with the increase of hBN content. The crystal structure of hBN is similar to that of graphite and mica that they have molecularly smooth basal plane and no surface functional group is available for chemical bonding or interaction. However, at the edge of the hBN platelets, there are amine and hydroxyl groups available for chemical bonding.³⁵ So, high hBN filler concentration indicates more hydroxyl groups were incorporated and hence the increase of ɛ_r was observed.³⁶ Over-filled composites result in high porosity, moisture adsorption, and low mechanical strength, hence an optimum filler loading in the epoxy matrix is preferred for circuit application.⁴ In this work, at 50 wt% hBN the maximum ɛ_r of the composites were still less than 5.5, so, we controlled the maximum hBN loading to be 50 wt%.

The data in Figure 5(b) indicate that the dissipation factor (δ) of the composites very slightly increased with hBN under various applied sweep frequencies despite very low δ value of hBN. Such a phenomenon might result from the adsorption of moisture on the surface of hBN. Moisture adsorption significantly influences electrical and mechanical properties of the composites. Water is a polar molecule and has high ɛ_r of 80 and high δ of 35.⁵ Hence the ɛ_r and δ of composite resin increase with the amount of hBN loading. The δ of the samples all exhibited initially decrease with increasing frequency, followed by an increase to a certain frequency about 10⁶ Hz. The δ tended to decrease with the increase of testing frequency since the dipole motion can no longer keep up with the frequency increment of bias field. A peak of δ appeared at about 10⁶Hz for the composites, which is an obvious dipole relaxation loss process related to the epoxy resin.³⁷ In general, the δ of the composites remained at a rather low level, i.e. less than 0.02 for the composites with 50 wt% hBN.

For an electrically insulating application, the dielectric strength’s ability to withstand a high electric field should be considered. Figure 6(a) presents the dielectric strength’s dependence of the composites on the hBN loading. It can be seen that the dielectric strength decreased from 32 to 16 kV/mm with increasing the hBN concentration from 0 to 50 wt%. Spatial charges generated at the interface between the hBN and epoxy under the applied electric field may cause the dielectric strength to decrease. The dependence of electrical insulation properties of hBN/epoxy on the filler content is illustrated in Figure 6(b). Figure 6(b) demonstrates that the volume resistivity continuously declined to 6.3 × 10¹⁴ Ω·cm at 50 wt% hBN content. The reason may originate from the impurities and defects inside the cured networks of hBN/epoxy composites.

Figure 6.

The breakdown strength (a) and volume resistivity (b) of epoxy composites as a function of hBN content. hBN: hexagonal boron nitride.

From Figures 5 and 6, it is worth noting that the epoxy with 50 wt% hBN content still possessed a high dielectric breakdown strength, and high electric insulating properties characteristics, as well as a low dielectric constant and δ, which are of great significance in practical engineering applications, such as substrate and packaging materials.

Mechanical properties

The dependence of mechanical properties of pure epoxy and its composites on the hBN (treated with silane coupling agent) content is indicated in Table 2. Increasing hBN particles concentration increased both the tensile and flexural modulus of the composites due to the higher rigidity of inorganic fillers when compared to the epoxy matrix, and decreased the tensile and flexural strength, and impact strength compared to the pure matrix. With increasing hBN content to a certain level, the viscosity of the mixture began to increase rapidly due to the absence of inertia solvent such as acetone.³⁸ Consequently, some voids or interfacial defects, poor interfacial wettability between the hBN and the matrix might appear in the composites, resulting in the deterioration in mechanical property.

Table 2.

Mechanical properties of neat epoxy and its hBN composites.

Mechanical properties	hBN concentration/wt%
Mechanical properties	0	20	30	40	50
Tensile strength/MPa	68.2 ± 5.6	63.6 ± 4.2	58.2 ± 4.5	51.2 ± 3.9	45.1 ± 3.7
Tensile modulus/GPa	2.68 ± 0.21	2.82 ± 0.30	2.93 ± 0.24	3.12 ± 0.29	3.14 ± 0.31
Elongation at break/%	5.40 ± 0.26	5.21 ± 0.22	4.74 ± 0.18	4.1 ± 0.19	2.84 ± 0.11
Flexural strength/MPa	112 ± 9	108 ± 8	95 ± 7	86 ± 6	81 ± 6
Flexural modulus/GPa	2.71 ± 0.13	2.79 ± 0.15	2.88 ± 0.16	3.05 ± 0.14	3.07 ± 0.18
Flexural strain at break/%	4.64 ± 0.35	4.04 ± 0.25	3.51 ± 0.21	2.63 ± 0.14	1.62 ± 0.08
Impact strength/kJ/m²	16.2 ± 0.9	14.5 ± 0.8	12.3 ± 0.7	10.3 ± 0.5	8.2 ± 0.4

hBN: hexagonal boron nitride.

Figure 7 shows the micrographs of hBN/epoxy composites, in which hBN particles treated with silane coupling agent were basically homogeneously dispersed in the epoxy matrix. The uniform dispersion of hBN particles facilitated to eliminate the agglomerate of filler particles and to decrease the air voids and defects between filler particles. Therefore, the thermal conductivity and mechanical properties were improved.

Figure 7.

SEM images of the fractured surfaces of the epoxy composites with different hBN loading (a) neat epoxy, (b) 20 wt%, (c) 30 wt%, (d) 40 wt%. hBN: hexagonal boron nitride; SEM: scanning electron microscope.

Conclusion

The incorporation of hBN particles into the composites slightly improved the T_g and the thermal stability of the epoxy. The FT-IR results confirm that the occurrence of chemical reaction between the hBN and silane coupler.

The epoxy with hBN treated with silane coupler exhibited an enhanced thermal conductivity compared to the untreated hBN/polymer due to the improved interfacial bonding and decreased void volume at the phase interface.

The ɛ_r decreases with increase of frequency but increases with the increase of filler concentration. The ɛ_r and δ of composites resin were correlated with the presence of polar groups in the composites. The epoxy with 50 wt% hBN exhibited a high dielectric breakdown strength (16 kV/mm), and a high volume resistivity, as well as a low ɛ_r (less than 5.5) and a low δ (less than 0.02) in the measure frequency range from 10⁻¹ to 10⁷Hz.

The mechanical strengths and toughness of the composites decreased with increasing the hBN concentration.

Footnotes

Conflict of interest

None declared.

Funding

The authors gratefully acknowledge the financial supports from the Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology (JZK201301), the Foundation for Key Program of Ministry of Education, China (212175), the National Science Foundation of China (No.51073180), and the China Postdoctoral Science Foundation (No. 200801434).

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Thermal,electrical,and mechanical properties of hexagonal boron nitride–reinforced epoxy composites

Abstract

Keywords

Introduction

Experimental

Materials

Samples preparation

Surface modification of hBN particles

Preparation of the hBN/epoxy composites

Characterization

Results and discussion

FT-IR characterization

Thermal properties

Electrical properties

Mechanical properties

Conclusion

Footnotes

Conflict of interest

Funding

References