Synthesis and thermal transport properties of high-surface area hexagonal boron nitride foam structures

https://doi.org/10.1016/j.ijheatmasstransfer.2020.120268Get rights and content

Highlights

Abstract

Continuous and porous foam structures of thermally conducting and electrically insulating hexagonal boron nitride (h-BN) have been synthesized for heat dissipation and other applications. However, the volume fraction and effective thermal conductivity of existing h-BN foam structures are still limited by the large pore size and small specific surface area of the sacrificial reticular nickel foam templates used for chemical vapor deposition (CVD) of h-BN. Here we report experiments for increasing the volume fraction and effective thermal conductivity of CVD h-BN foams with the use of sintered nickel powder templates with reduced pore sizes. The volume fraction of the obtained high-surface area h-BN foam sample HS1 reaches 0.34 ± 0.06 %, which is an increase by a factor of 2.8 compared to a baseline h-BN foam grown on a reticulated nickel foam with the same growth time. Increasing the growth time by a factor of 3 results in a further increase of the volume fraction by an additional factor of 2 for sample HS2. With poly(methyl methacrylate) (PMMA) filled into the pore space of the h-BN foam, the room-temperature effective thermal conductivity of the composite increases from 0.31 ± 0.02 W m−1 K−1 for the baseline structure to 0.70 ± 0.05 W m−1 K−1 for HS2. Based on numerical analysis of the measured effective thermal conductivity, the solid thermal conductivity of the h-BN struts of the baseline structure is 580 ± 150 W m−1 K−1, which is comparable to a prior first principles calculation. The increases in the volume fraction and effective thermal conductivity are accompanied by a reduction of the h-BN solid thermal conductivity to 410 ± 80 W m−1 K−1 for HS1 and 250 ± 30 W m−1 K−1 for HS2 due to increased defect concentrations and surface curvature. The numerical simulation reveals that the thermal interface resistance between h-BN and PMMA plays only a small role on the effective thermal conductivity due to the dominant thermal transport contribution from the continuous high-thermal conductivity struts.

Introduction

Due to the increased transistor density in integrated circuits in the past decades, the heat dissipation density has increased considerably and reached over 100 W cm−2 [1]. This challenge opens up opportunities for innovations in thermal management materials, devices, and systems for microelectronics [2]. The superior basal-plane thermal conductivities of some two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) have motivated efforts to explore their thermal management applications. Graphite and h-BN share a similar hexagonal layered structure with only ~1.8% difference in the basal-plane lattice constant [3]. While graphite is a semimetal, the large band gap of 5.97 eV makes h-BN essentially an electrical insulator [4]. Because of its electrical insulation, high basal-plane thermal conductivity, chemical stability, mechanical robustness, and flexibility, h-BN has been explored for applications ranging from electronic packaging, electrical shielding, flexible substrates, and advanced heterostructure electronic devices [4], [5], [6], [7].

One thermal management application of high-thermal conductivity nanostructures is to serve as the filler material in low-conductivity polymeric matrix for enhancing the effective thermal conductivity of the obtained composite. However, the high thermal interface resistance of the individual nanostructures in a percolated van der Waals (vdW) network limits the thermal performance. This limitation can be overcome by replacing the vdW network with three-dimensional (3D) continuous porous foam structures. Recently, cellular h-BN foams have been grown with the use of chemical vapor deposition (CVD) for thermal management [8], [9], [10], [11], [12], [13]. Compared to liquid or mechanical exfoliation, CVD growth is unique in that it can yield continuous, large lateral size h-BN films and coatings on catalyst templates and control the film thickness by varying the growth time. The obtained continuous interconnected strut network serves as effective pathways for heat conduction from the functional materials, such as a phase change material (PCM) filled into the pore space of the foam structure for thermal energy storage [14]. In one experiment, solid borazane precursor and Ni foam templates have been used in low-pressure CVD (LPCVD) growth of free-standing h-BN foams with a very low density of 1.6 mg cm−3 [15]. Meanwhile, an atmospheric pressure CVD (APCVD) method has been used to grow h-BN foams on Ni foam templates [16], [17]. At a h-BN volume fraction of 0.076 ± 0.01%, the effective thermal conductivity of Poly(methyl methacrylate) (PMMA) composite with the h-BN foam reached 0.34 ± 0.03 W m−1 K−1 at room temperature and 0.73 ± 0.07 W m−1 K−1 at 140 K, compared to a room-temperature value of 0.21 ± 0.01 W m−1 K−1 for pure PMMA. Opposite to the trend shown by amorphous PMMA, the observed temperature dependence of the composite is attributed to the lattice thermal conductivity of crystalline h-BN, where increased umklapp phonon-phonon scattering reduces the solid thermal conductivity of the h-BN crystal with increasing temperature.

The thermal performance of the open cellular foam fillers depends on the crystalline quality, strut wall thickness, and the pore size. In existing h-BN foams grown on commercial reticulated nickel foam templates, the pore size is similar to the template pore size, which is as large as 600 µm. The thermal conductivity of a composite plate sample measured by a steady state method is a function of the volume fraction of the filler materials and does not explicitly depend on the pore size and surface area according to the foam theory [18]. However, the h-BN filler volume fraction increase with decreasing pore size and increasing specific surface area for a given strut wall thickness. Due to the limited h-BN strut wall thickness obtained by the CVD growth process, the large pore size and limited specific surface area results in a small volume fraction of the foam filler. For applications such as composite phase change materials (PCMs) for thermal energy storage and thermal management [19], moreover, the large pore size results in a large distance and resistance for 3D heat flow from the functional materials inside the pores to the high-thermal conductivity matrix grown on the template. This factor increases the time for melting or solidification of the PCMs filled in the large pores. In previous studies of ultrathin graphite foam (UGF) structures, efforts have been made to grow long carbon nanotubes on the strut wall of CVD UGF to increase the specific surface area and reduce the resistance of heat transport from the phase change material (PCM) inside the pore space to the graphite strut walls [19]. Due to a decrease of the effective pore size of the UGF, the obtained effective thermal conductivity has been increased by a factor of 1.8 compared to that of a UGF-PCM composite without nanotubes. Recently, nickel powders have been sintered to form catalytic templates for CVD growth of UGF structures [20]. The obtained nickel template can reduce the pore size and increase the specific surface area up to 68 times compared to the reticulated nickel template. Because of the large increase in the volumetric specific surface area and the volume fraction of the UGF grown on the sacrificial nickel template, the effective density and the effective thermal conductivity of the UGF have been increased as much as a factor of 8 relative to that grown on reticulated nickel foam [20].

Here, we report the synthesis and thermal transport properties of high-surface area h-BN foam structures grown on sintered nickel powder templates. Due to the small pore size of sintered nickel powder template, the volume fraction of the h-BN grown by APCVD on the template is increased by a factor of 2.8 compared to a baseline h-BN foam grown on a commercial reticular nickel foam template. A further increase of the volume fraction by an additional factor of 2 is achieved by increasing the growth time by a factor of 3. The effective room-temperature thermal conductivity of the h-BN-PMMA composite is increased from 0.31 ± 0.02 W m−1 K−1 for the baseline structure to 0.70 ± 0.05 W m−1 K−1 for the high-surface area structure with increased h-BN volume fraction and growth time. Numerical simulation is carried out to extract the solid thermal conductivity of the h-BN struts from the measured effective thermal conductivity and to understand the effect of the thermal interface resistance between the h-BN and the matrix.

Section snippets

Materials synthesis and structure characterization

As summarized in Table 1, four different types of h-BN foam composite samples are reported in this work, including baseline h-BN foams grown on a commercial reticulated Ni foam template, three high-surface area h-BN foam samples grown on sintered Ni powder templates with different growth times or procedures. Compared to the baseline samples grown on the reticular Ni foam, the sintered nickel template is used to reduce the pore size and increase the specific surface area of the other three

Thermal transport measurements and results

The thermal transport properties of h-BN-PMMA composite were measured via Laser Flash Analysis (Netzsch LFA 457 Microflash) and Differential Scanning Calorimetry (Netazch DSC 404 F1 Pegasus). Although the h-BN volume fraction was less than 1%, the interconnected h-BN open cell structure increased the effective thermal conductivity of the composite, which was obtained asκe,c=αe,cρe,cCe,cwhere the ρe,c is the measured density of the h-BN-PMMA composite, αe,c is the effective thermal diffusivity

Numerical simulation and analysis of the solid thermal conductivity

Instead of an analytical model based on the effective medium approach or the foam theory, a series of numerical simulations in the COMSOL environment is used to evaluate the h-BN solid thermal conductivity and investigate the effect of the thermal interface resistance (Ri) between h-BN and PMMA. The numerical simulation is based on the assumption of 3D isotropic heat conduction in PMMA, air voids, and thin h-BN strucs. As shown in Fig. 6(a), the h-BN foam structure is approximated as a

Conclusion

These experiments show that the effective thermal conductivity of h-BN foams can be increased with the use of high-surface area, low-cost sintered nickel powder templates for APCVD growth and by increasing the growth time. With an order of magnitude smaller pore size and an order of magnitude higher specific surface area than that of reticulated Ni foam, the high-surface area h-BN polymeric composite increases the volume fraction of h-BN from 0.12 ± 0.02% to 0.69 ± 0.03% and yields an effective

CRediT authorship contribution statement

Qianru Jia: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft, Visualization, Data curation. Li Shi: Conceptualization, Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank Evan Fleming, Arden Moore, and Taylor Ashton for helpful discussion. The baseline sample and sample HS1 were synthesized and studied with the support of National Science Foundation (NSF) award CMMI-1563382. Samples HS2 and HS3 were studied with support from NSF award EEC-1160494.

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