Carbon nanohorns are a unique class of carbon-based nanomaterials characterized by their horn-like conical structures, typically aggregated into spherical clusters. These structures exhibit distinct mechanical properties that make them promising for applications in nanocomposites, energy storage, and biomedical devices. Understanding their tensile strength, elasticity, and fracture behavior is critical for optimizing their performance in these applications.

The tensile strength of carbon nanohorns is influenced by their structural morphology and the presence of defects. Individual nanohorns consist of single-walled carbon nanotubes with conical tips, where the angle of the cone plays a significant role in mechanical stability. Studies suggest that the tensile strength of a single carbon nanohorn can reach up to 100 GPa, comparable to that of carbon nanotubes, due to the strong sp² hybridized carbon-carbon bonds. However, the aggregated form of nanohorns, often seen in practical applications, exhibits lower effective tensile strength due to weaker inter-horn van der Waals interactions. The spherical clusters, typically 80-100 nm in diameter, show a collective tensile strength in the range of 1-10 GPa, depending on the density of the assembly and the degree of covalent cross-linking between individual horns.

Elasticity is another key mechanical property of carbon nanohorns. The conical structure introduces unique deformation mechanisms under stress. Unlike cylindrical carbon nanotubes, which deform uniformly under axial loads, nanohorns exhibit non-uniform strain distribution due to their tapered geometry. The Young’s modulus of an individual nanohorn is estimated to be around 1 TPa, similar to that of graphene and carbon nanotubes. However, the aggregated form shows reduced effective modulus, typically between 100-500 GPa, due to the compliance introduced by the porous structure of the cluster. The elasticity of nanohorn assemblies is also affected by the presence of pentagonal and heptagonal defects at the conical tips, which can act as stress concentrators and reduce the overall stiffness.

Fracture behavior in carbon nanohorns is strongly dependent on defect distribution and loading conditions. The conical tips are particularly susceptible to stress accumulation, making them the most likely sites for crack initiation. Under tensile loading, fractures often propagate along the longitudinal axis of the horn, following the hexagonal carbon lattice. However, the presence of Stone-Wales defects or vacancies can alter the fracture path, leading to more complex failure modes. In aggregated forms, fracture typically occurs at the inter-horn junctions, where weak van der Waals forces dominate. The fracture toughness of nanohorn clusters is lower than that of individual horns, often in the range of 1-5 MPa√m, due to the ease of crack propagation through the porous network.

Morphology plays a crucial role in determining the mechanical properties of carbon nanohorns. The cone angle, which typically ranges between 20° and 60°, affects both strength and flexibility. Smaller cone angles result in sharper tips and higher local stress concentrations, reducing the overall tensile strength. Larger cone angles, on the other hand, provide more uniform stress distribution but may compromise the stiffness due to increased curvature-induced strain. The length of the nanohorn also influences mechanical behavior, with longer horns exhibiting higher compliance and lower buckling resistance compared to shorter ones.

Defects are inevitable in carbon nanohorns and can significantly alter their mechanical performance. Common defects include topological imperfections such as pentagon-heptagon pairs, vacancies, and adsorbed functional groups. These defects act as stress concentrators, reducing the effective strength and promoting premature fracture. However, controlled introduction of defects can be beneficial in some cases, as they may enhance energy dissipation mechanisms or facilitate covalent bonding between horns in aggregated structures. For instance, oxidized nanohorns exhibit improved inter-horn adhesion due to the formation of carboxyl and hydroxyl groups, which can form stronger cross-links compared to pristine nanohorns.

When compared to other carbon nanomaterials, carbon nanohorns exhibit a unique combination of properties. Unlike graphene, which is a two-dimensional sheet with isotropic in-plane strength, nanohorns have anisotropic mechanical behavior due to their conical geometry. Carbon nanotubes, which share a similar cylindrical structure, outperform nanohorns in terms of tensile strength and modulus when considered as individual entities. However, nanohorn clusters offer advantages in terms of porosity and surface area, which can be beneficial in applications requiring high interfacial interactions, such as composite reinforcement or catalyst support. Fullerenes, another closely related carbon nanomaterial, lack the elongated structure of nanohorns and thus exhibit lower tensile strength and stiffness.

The mechanical properties of carbon nanohorns can be tailored through post-synthesis treatments. Thermal annealing can reduce defect density and improve crystallinity, leading to enhanced strength and stiffness. Chemical functionalization, while often reducing the intrinsic mechanical properties of individual horns, can improve the interfacial strength in composite applications. For example, hydrogenation or fluorination can modify the surface energy of nanohorns, promoting better dispersion in polymer matrices and enhancing load transfer efficiency.

In summary, carbon nanohorns exhibit a unique set of mechanical properties governed by their conical morphology and defect distribution. Their tensile strength, elasticity, and fracture behavior are intermediate between those of carbon nanotubes and fullerenes, with aggregated forms showing reduced performance due to weak inter-horn interactions. The presence of defects and variations in cone angle further influence their mechanical response. While they may not surpass carbon nanotubes in terms of absolute strength or stiffness, their clustered morphology and high surface area make them suitable for specialized applications where interfacial properties and porosity are critical. Future research aimed at controlling defect distribution and optimizing aggregation behavior could further enhance their mechanical performance for advanced technological applications.