Ultra-highly electrically insulating carbon materials and their use for thermally conductive and electrically insulating polymer composites

Highlights

• Carbon materials with high electrical insulation are prepared by scalable and nondestructive layer-by-layer self-assembly.

• Water glass-coated carbon nanotubes (WG-CNTs) have firm and uniform coatings after calcination, showing high heat resistance.

• The WG-CNT/polymer composites show ultra-high electrical resistivity at high CNT loadings and enhanced thermal conductivity (TC).

• The TC is much superior to those of previously reported CNT/polymer composites with electrical insulation.

Abstract

Anisotropic carbon materials such as carbon nanotubes (CNTs) and carbon fibers exhibit extremely high thermal conductivity (TC). However, due to their high electrical conductivity, they have not been used in applications that require both high TC and electrical insulation. Herein, ultra-highly electrically insulating CNTs were prepared by layer-by-layer self-assembly method using various oxide nanomaterials (ONMs) such as titania nanosheets, silica nanoparticles, and water glasses (WGs). ONM-coated CNTs (ONM-CNTs) showed high surface resistivity and ultra-high volume resistivity in acrylate polymer matrices (∼10¹⁷ Ω cm) even at high CNT loadings (∼10 vol%). WG-coated CNTs (WG-CNTs) after calcination exhibited the highest heat resistance, showing high electrical insulation even after heat treatment at 500 °C under air. WG-CNTs were stable in the poly (1,4-phenylenesulfide) (PPS) matrix during melt-mixing at high temperature (300 °C), giving the WG-CNT/PPS composites with extremely high volume resistivity (≥10¹⁵ Ω cm) even at high WG-CNT loadings (∼16.7 vol%) and enhanced through-plane TC (∼2.0 W m⁻¹ K⁻¹). The TC is much superior to those of previously reported CNT/polymer composites with electrical insulation (≤0.57 W m⁻¹ K⁻¹). This method is scalable, nondestructive, and applicable to various carbon materials. The presented approach is promising for preparing electrically insulating materials for various thermal management applications.

Introduction

Highly electrical insulating polymer composite materials with high thermal conductivity (TC) are strongly required for miniaturization, increasing power output and reliability of electronics and electric power systems in current and future vehicles, robots, and communication equipment. Anisotropic carbon materials such as carbon nanotubes (CNTs) [[1], [2], [3], [4], [5], [6], [7], [8]], graphenes [2,[8], [9], [10], [11], [12], [13]] and carbon fibers (CFs) [14,15], and boron nitride (BN) materials including BN nanotubes [16,17], BN nanosheets [[18], [19], [20], [21], [22]] and BN fibers [23] possess excellent TCs and high mechanical properties. Among these materials, CNTs, especially multi-walled CNTs (MWCNTs), are relatively cheap and widely produced except for their extremely high TC. In addition, many methods have been developed to exfoliate CNT bundles into individual CNTs and improve their dispersibility in solvents and polymer matrices [3,7,8,13,[24], [25], [26]]. However, owing to large electrical conductivity of CNTs, they cannot be used as additives for highly electrically insulating polymeric materials for the above-mentioned applications because the addition of even a few CNTs to these polymers led to a drastic decrease in electrical insulation [4,6,8,24]. Therefore, we previously fabricated MWCNT/polymer composites having a controlled morphology, which exhibited enhanced TC without losing electrical insulation [4,6]. These composites comprise polymer blends with sea-island morphology in which MWCNTs are selectively localized in the dispersed island polymers in the sea of a polymer matrix [4], or MWCNT-dispersed polymers in which both ends of each MWCNT are capped with small insulating polymer domains [6]. Controlling the affinities of MWCNTs and constituent polymers to each other is key to obtaining this morphology, and therefore, this method is limited to specific polymer combinations. On the other hand, using an insulating material coating on CNT surfaces is an effective way of preparing electrically insulating CNTs [27]. Introducing hydrophilic groups such as –COOH, –OH, or –NH₂ on the CNT surface and using surface-initiated polymerization have been reported [24,[28], [29], [30], [31]]. CNTs were coated with poly (cyclohexyl methacrylate) (PCHMA) [30] or poly (methyl methacrylate) (PMMA) [31], and the volume resistivity was increased. However, acrylate polymers such as PCHMA and PMMA decompose from ∼270 °C because of their zipper-like decomposition of main chains started from ∼270 °C [3], and the graphene structure of the CNT surface is partially destroyed due to formed covalent bonding. The introduced surface defects degrade several properties of CNTs, particularly their TC properties [3,32]. Therefore, as nondestructive modification approaches, the noncovalent functionalization of CNTs with polymers has been developed [3,[24], [25], [26],33]. However, the electric resistivity of the functionalized CNTs was not high (surface resistivity <10⁴ Ω) due to thin polymer layers on the CNT surface. As an inorganic polymer precursor, tetraalkoxysilane (TAOS) was also used for CNT surface modification; however, CNTs were not uniformly covered with formed polysiloxane [34], and many large polysiloxane particles were independently formed. Therefore, to form uniform silica coating on the CNT surface, carboxylic group-functionalized CNTs and 3-aminopropyltrialkoxysilane were used [34,35]. In this approach, however, forming uniform silica coating is not easy, which largely depends on experimental condition such as pH of the solution and surface condition (e.g. acid value) of CNTs. Moreover, the quantity of silica-coated CNTs was small (up to tens of milligrams) [34,35] for using for bulk applications including CNT/polymer composites, and the thickness was ultra-thin (∼3 nm), which was unsuitable to achieve higher electrical insulation. That thickness is not enough for preventing electron hopping between CNTs [36], although the upper limit separation distance for CNTs for forming conductive networks is suggested as 1.8 nm [36]. In addition, forming carboxylic groups on the CNT surface damages the intrinsic CNT structure and, in turn, leads to decreased TC. Others have coated CNTs by alumina (Al₂O₃) shell using the atomic layer deposition (ALD) technique [37] or by inorganic BN nano-layers [38]. However, functionalized CNTs with carboxylic groups were also used for these methods, and the production scale of this ALD technique was up to tens of milligrams per batch. In addition, the layer thickness of BN nano-layers was thin (∼3 nm) [38], and therefore, the volume resistivity of BN-coated CNT/polyimide (PI) (3/97 wt%) composites was ∼8 × 10⁹ Ω cm, which was much lower than that of pure PI (∼4 × 10¹⁴ Ω cm) [38]. Meanwhile, oxidized CNTs (oCNTs) immobilized on the edges of silane functionalized hexagonal BNs (fBN) via electrostatic interaction were used to improve the TC while maintaining the electrical insulation properties of the epoxy composites [39]. The TC of the epoxy composites containing 20 wt% of oCNTs@fBN10 (oCNTs:fBN, 1:10 w/w) was 1.26 W m⁻¹ K⁻¹. However, the volume resistivity of the epoxy composites was less than 10¹³ Ω cm, which decreased as increasing the oCNT content. In addition, this method also needed oxidation of CNTs for the electrostatic interaction.

To prevent surface damage and degradation of properties and to form thicker insulating layers, CNTs were modified by the layer-by-layer (LBL) self-assembly of poly (diallyldimethylammonium chloride) (PDADMAC) and poly (sodium 4-styrenesulfonate) (PSS), respectively [40]. Self-assembly is an essential process in nature and is associated with the spontaneous formation of well-defined nanostructures driven by noncovalent interactions under some certain condition, which is recently important in the field of material science [41,42]. LBL self-assembly was also used for the growth of porous indium oxide on CNT surfaces [43] and the coating of graphene-crosslinked CNTs with polyimide layers [44]. However, the electrical resistivity of these functionalized CNTs is low (surface resistivity <10⁴ Ω) for dielectric applications. Moreover, for thermal management toward obtaining further miniaturization and high-power output in future electronics and electric power systems, maintaining electrical insulation after treatments at high temperatures (e.g., ≥300 °C) is strongly demanded. Therefore, forming thick and uniform layers made of electrically insulating high-heat-resistant oxide nanomaterials (ONMs), such as titania nanosheets (TNSs) [[45], [46], [47], [48]], clay nanosheets (CNSs) [49,50], silicon oxide nanomaterials (silica nanoparticles, SNPs) [51], and sodium silicates (water glasses, WGs) [52,53] onto CNT surfaces is expected to provide electrical resistivity increase.

Here, we fabricated multigram-scale LBL self-assembled TNS-, CNS-, SNP-, and WG-coated CNTs (TNS-CNTs, CNS-CNTs, SNP-CNTs, and WG-CNTs) and SNP-coated CFs (SNP-CFs) showing high electrical resistivity. In particular, the WG-CNTs showed extremely high electrical insulation even after high-temperature heat treatment. These insulating CNTs have been used to fabricate CNT/high-heat-resistant polymer (super engineering plastic) composites, which offer enhanced TC in addition to extremely high volume resistivity (≥10¹⁵ Ω cm) even at high CNT loadings.