Carbon nanohorns (CNHs) are a unique class of carbon-based nanomaterials characterized by their horn-like conical structures, typically aggregated into spherical clusters. These materials exhibit exceptional mechanical strength, high thermal conductivity, and excellent electrical properties, making them attractive candidates for reinforcing composite materials. Their incorporation into polymer, ceramic, or metal matrices can significantly enhance performance in structural, thermal management, and conductive applications. However, achieving uniform dispersion and strong interfacial bonding remains a critical challenge that directly influences the final properties of the composite.

The dispersion of carbon nanohorns within a matrix is a primary factor determining the effectiveness of reinforcement. Due to their strong van der Waals interactions, CNHs tend to aggregate, leading to uneven distribution and reduced mechanical or electrical performance. Several techniques have been developed to address this issue. Sonication is commonly employed to break apart agglomerates, particularly in liquid-phase processing. High-power ultrasonication can disperse CNHs in solvents or polymer solutions, though excessive energy input may damage their structure. Surfactants and functionalization are also used to improve compatibility with the matrix. Covalent modifications, such as oxidation or grafting of polymer chains, enhance dispersibility by introducing polar groups that reduce agglomeration. Non-covalent methods, including wrapping with polymers or biomolecules, preserve the intrinsic properties of CNHs while improving dispersion.

Interfacial interactions between carbon nanohorns and the matrix are crucial for load transfer and property enhancement. In polymer composites, weak interfacial adhesion can lead to slippage under stress, diminishing mechanical reinforcement. Chemical functionalization, such as carboxylation or amination, creates active sites for covalent bonding with the matrix, improving stress transfer. For example, epoxy composites with amine-functionalized CNHs exhibit higher tensile strength due to covalent crosslinking between the nanohorns and the resin. In ceramic or metal matrices, interfacial reactions during processing can form strong bonds. Spark plasma sintering of CNH-reinforced alumina ceramics results in improved fracture toughness, as the nanohorns bridge microcracks and deflect their propagation. Similarly, in metal matrix composites, CNHs can enhance hardness and wear resistance when uniformly dispersed and well-bonded to the metal phase.

The mechanical properties of CNH-reinforced composites are among the most studied performance metrics. Tensile strength, Young’s modulus, and toughness are significantly influenced by the loading fraction and dispersion quality. For instance, adding just 1 wt% of well-dispersed CNHs to a polypropylene matrix can increase tensile strength by up to 30%, while higher loadings may lead to agglomeration and diminished returns. Flexural strength and impact resistance also improve, particularly in thermoset polymers, where CNHs act as nano-reinforcements that hinder crack propagation. In ceramic composites, CNHs contribute to toughening mechanisms such as crack bridging and pull-out, often leading to a 20-40% increase in fracture toughness compared to unreinforced materials.

Thermal properties of composites benefit from the high intrinsic thermal conductivity of carbon nanohorns. When incorporated into polymers, they create conductive pathways that enhance heat dissipation, which is critical for electronic packaging and heat exchanger applications. A 5 wt% CNH loading in epoxy resin can double the thermal conductivity, reducing thermal resistance in devices. Additionally, CNH composites exhibit improved thermal stability, with higher decomposition temperatures due to the barrier effect of nanohorns that retard polymer degradation. In metal matrix composites, CNHs help reduce thermal expansion mismatch, enhancing dimensional stability under thermal cycling.

Electrical conductivity is another area where CNH composites excel. The sp² carbon structure of nanohorns facilitates electron transport, making them suitable for antistatic coatings, conductive adhesives, and electromagnetic shielding materials. A percolation threshold as low as 0.5 wt% has been reported in some polymer-CNH systems, beyond which conductivity increases sharply. This low threshold is advantageous for applications requiring lightweight conductive materials. Furthermore, the combination of electrical and thermal conductivity in CNH composites is beneficial for flexible electronics and energy storage devices, where efficient heat and charge management are required.

Performance metrics for CNH composites are often evaluated through standardized mechanical testing (tensile, compression, flexural), thermal analysis (conductivity, stability), and electrical measurements (resistivity, impedance). Dynamic mechanical analysis reveals enhanced viscoelastic properties, with higher storage modulus and glass transition temperatures in polymer composites. Microscopy techniques, such as scanning electron microscopy and transmission electron microscopy, are essential for assessing dispersion quality and interfacial bonding at the nanoscale.

Despite their advantages, challenges remain in scaling up CNH composite production. Achieving consistent dispersion in large-volume manufacturing requires optimized processing techniques, such as melt mixing or in-situ polymerization. Cost-effective functionalization methods are also needed to balance performance with industrial feasibility. Nevertheless, carbon nanohorns offer a promising pathway for developing advanced composites with tailored mechanical, thermal, and electrical properties, opening new possibilities in aerospace, automotive, electronics, and energy applications. Continued research into interfacial engineering and processing innovations will further unlock their potential in next-generation materials.