Nanocarbon-Based Composites and Their Thermal, Electrical, and Mechanical Properties

Carbon materials have played a pivotal role in humanity’s progress since ancient times. Their significance can be traced throughout history, shaping the evolution of civilization. One of the earliest uses of carbon materials was artistic expression, in which charcoal served as a pigment for cave paintings, contributing to the social and symbolic development of early societies [1]. In the 18th century, coal became a driving force behind scientific and technological advancements, ultimately leading to the First Industrial Revolution [2]. The invention of the steam engine, powered by coal, revolutionized manufacturing and transportation, and fueled economic growth and industrialization. More recently, carbon fibers have been widely explored for commercial applications, particularly in the form of polymeric composites [3]. These advanced composite materials exhibit a synergistic effect between their components, which allows them to outperform in industries such as the transportation and prosthetics industries, among others.

The discovery of carbon nanomaterials has expanded the potential of carbon-based materials for high-tech applications. This journey began in 1985 with the identification of cage-like fullerenes [4]. Another significant breakthrough occurred in 1991 when Sumio Iijima discovered multi-walled carbon nanotubes (CNTs) consisting of multiple carbon sheets rolled into tubes with closed ends [5]. This discovery ignited a global search for new allotropic forms of carbon. Today, a vast array of carbon nanostructures, including nano-onions, carbon nanohorns, graphene and its derivatives, and carbon dots, amongst others, are applied across diverse fields, including in healthcare, environmental science, energy storage, sensing, and electronics.

The importance of these discoveries was recognized by the scientific community with the awarding of the 1996 Nobel Prize in Chemistry to Professor Robert F. Curl of Rice University, Houston, USA [6]. Later, in 2010, the Nobel Prize in Physics was awarded to Andre Geim and Konstantin Novoselov of the University of Manchester (UK) for their groundbreaking discovery of graphene, the first two-dimensional material with atomic thickness to be isolated under room conditions [7,8].

Carbon nanomaterials, with their diverse dimensions and structures, exhibit excellent mechanical strength, electrical and thermal conductivities, and show a high versatility for chemical surface functionalization [9,10]. This adaptability is crucial for modulating interfaces at the atomic level and enhancing dispersibility and compatibility with various matrices. X-ray photoelectron spectroscopy (XPS) is a powerful tool for analyzing the atomic composition of carbon-based materials. David J. Morgan recently reviewed XPS spectral analysis of graphitic materials, presenting a simple and logical approach to increase confidence in functionalization insights derived from carbon core-level spectra [11].

Graphene and its derivatives have attracted significant interest in the development of advanced nanocomposites with enhanced performances [12]. Navaratnarajah K. et al. recently reviewed major synthetic methods for graphene-based materials, providing a systematic analysis of their physicochemical properties and applications, including batteries, supercapacitors, electrochemical sensors, transparent electrodes, and environmental remediation [13].

Graphene-based materials exhibit high thermal conductivity, making them ideal for developing multifunctional nanocomposites with enhanced thermal stability and for use as solid lubricants for heat dissipation [14]. Alexander A. Balandin investigated the use of graphene as a reinforcing agent in epoxy resin to improve thermal diffusivity and conductivity. This work demonstrated that epoxy nanocomposites containing randomly oriented few-layer and single-layer graphene achieved a remarkable thermal conductivity enhancement of 15% to 25% [15]. Jacob S. Lewis studied the equilibrium temperatures of engineered device substitutes fixed to passive heat sink solutions with varying concentrations of few-layer graphene (FLG) as thermal interface materials (TIMs). His results showed that increasing FLG TIM concentration from 0 to 7.3 vol.% reduced the passively cooled heat sink’s thermal resistance from 4.23 °C/W to 2.93 °C/W, leading to a decrease in operating temperature from approximately 108 °C to 85 °C at a heat dissipation rate of 20 W [16].

Graphene-based materials have also been explored as electrode materials for electrochemical capacitors or supercapacitors (SCs) for applications in energy storage devices, electrical vehicles, and flexible electronics [17,18]. Alexander Tkach et al. developed a self-supported paper of reduced graphene oxide (rGO) with polycarbonate (PC) as a freestanding single electrode. The flexible graphene-based symmetric supercapacitors showed a higher areal capacitance value then PET/Cu/C substrate [19]. rGO attracted considerable attention for the development of high-performance sensing materials. A recent study demonstrated the development of rGO-based electrodes using laser irradiation for thin flexible electrochemical sensors. The surface impedance of the rGO electrode was sensitive to the humidity of the working environment. However, it still presented some operational limitations, including difficulty in recovering after exposure to environmental humidity [20]. It was also observed that graphene can be utilized as a processing aid in molten polystyrene. A recent study compared the performance of different graphene materials and clays with different lateral structures to reduce viscosity. Graphene in a loose agglomerated configuration showed a higher reduction in polymer viscosity. This decrease in viscosity was attributed to the superlubricity effects generated by the atomic structure of graphene nanoparticles. Another example reported by Grigoriev et al. is the synthesis of a novel rGO-doped platinum nanoparticle electrocatalyst. Their study found that prior modification of rGO with nitrogen-containing heteroatoms (pyridine and quaternary) promoted the nucleation of Pt nanoparticles while preventing their agglomeration. This enhancement resulted in a high catalytic efficiency of the Pt nanoparticles, surpassing that of the bare rGO-supported catalyst and comparable to that of Pt/C [21].

CNTs are one-dimensional nanomaterials classified based on the number of graphene layers into single-walled (SWCNTs) [22], double-walled (DWCNTs) [23], or multi-walled (MWCNTs) [24]. SWCNTs have diameters ranging from 0.4 to 3.0 nm and can reach lengths of several micrometers. Composed of a single sp²-hybridized carbon lattice, SWCNTs exist in three distinct chiral forms: armchair, chiral, and zigzag. Indeed, their structure directly influences their electrical, thermal, and mechanical properties [25]. Recently, Chensong Dong reported a new, simple method for determining the effective elastic modulus of wavy SWCNTs by finite element analysis [26]. MWCNTs typically have an inner diameter ranging from 0.4 nm to a few nanometers, with outer diameters reaching up to 30 nm. Their length can vary from 1 μm to several centimeters [27]. Rosario A. Gerhardt recently reviewed MWCNTs and thin films’ fabrication methodologies, as well as their applications as electrode materials for electrochemical double-layer supercapacitors [28].

CNTs exhibit remarkable properties, including a high mechanical tensile modulus, strength (~1 TPa), and excellent electrical (>10⁶ S/m) and thermal conductivity (>3000 W/m K) [29]. However, their performance and reactivity are highly dependent on factors such as structure, surface area, surface charge, size distribution, surface chemistry, agglomeration state, and purity. Andrew R. Barron et al. investigated the effects of both thermal and voltage-induced annealing on the electrical conductivity of CNT fibers. Their study found that fibers annealed at 500 °C or subjected to voltage annealing under similar conditions exhibited a three-order-of-magnitude reduction in electrical resistance. This improvement was primarily attributed to the selective removal of non-conductive or less-conductive amorphous carbon and the self-repair mechanisms of CNTs, which enhanced their conductivity [30]. To expand their range of applications, CNTs can undergo chemical functionalization on their surface (exohedral) or within their internal cavity (endohedral) to create various novel hybrid materials for different applications [31]. Kenji Hata et al. investigated the correlation between CNT/Cu electrical performance by preparing three different types of CNT-Cu composites using Cu electrodeposition: SWCNT/Cu wires, SWCNT/Cu pillars, and MWCNT/Cu wires [32]. Their study revealed that SWCNT/Cu wires and pillars outperformed MWCNT/Cu wires, exhibiting approximately three times higher room-temperature, measured using a four-point probe (reaching 30–40% of pure Cu conductivity). Additionally, the SWCNT/Cu composites demonstrated up to four times lower temperature coefficients of resistance, indicating more stable conductivity across temperature variations compared to MWCNT/Cu. These findings also suggest that few-walled, small-diameter nanotubes contribute to superior temperature-stable CNT/Cu conductivities.

Many other examples of carbon-based composite materials with enhanced performance have been reported over the years. Several carbon-reinforcing agents have been studied, including carbon fibers [33], carbon black [34], diamond [35,36], and nano graphite [37]. Additionally, more unconventional allotropes, such as tetragonal hybrid sp³/sp² carbon structures [38], have also been explored. These studies highlight the exceptional mechanical, thermal, and electrical properties that emerge from synergistic interactions between carbon nanostructures and various matrices. These properties have significant scientific and technological relevance across multiple fields.

This Special Issue, Nanocarbon-Based Composites and Their Thermal, Electrical, and Mechanical Properties, published in C—Journal of Carbon Research, compiles 20 original research articles and review papers. These contributions provide valuable insights into the development of carbon-based composite materials with improved performance in diverse applications. It is anticipated that articles on this Special Issue will offer readers a strong foundational understanding of this rapidly advancing field.