Mohammad Naraghi

Carbon nanofibers (CNFs) made via pyrolysis of polymeric precursors are attractive for a wide range of load bearing applications. However, by virtue of network of covalent bonds, they are prone to catastrophic failure under complex loading. Fracture toughness provides a reliable parameter for assessing structural integrity. In this study, polyacrylonitrile (PAN) precursor fibers were electrospun and carbonized at 900C and 1400C to establish a detailed structureproperty relationship in carbon nanofibers. For the first time, the fracture toughness of individual carbon nanofibers was directly measured and quantitatively linked to their inherent microstructural defects. Fracture toughness measurements showed a size-dependent toughening, increasing from 0.41 to 1.36 M P a m as fiber diameter decreased from 550nm to 280nm. In contrast, carbonization temperature had no measurable influence on fracture toughness. This is despite the fact that the carbon nanofibers made at higher temperatures were measurably more graphitic, indicating that the failure takes place within the amorphous regions of the material. Phase field continuum models provided further evidence in support of this failure mechanism. We showed that the Griffith theory of fracture adequately attributes, rather quantitatively, their fracture and failure to the surface flaws which were observed via high resolution imaging. These findings highlight the importance of surface-controling processing for enhancing the toughness of carbon nanofibers, while also providing fundamental insights into their flaw sensitivity and structureproperty relationships.

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Lightweight polymer-derived ceramics offer exceptional thermal and chemical stability but exhibit limited performance in electromagnetic wave (EMW) absorption due to low dielectric loss. This study presents a layered -SiC/SiOC nanocomposite architecture fabricated via direct ink writing (DIW), with carbon nanotubes (CNTs) spatially confined to the central layer. -SiC functioned as a rheology modifier, making the inks DIW-printable, while simultaneously enhancing dielectric loss. CNTs contributed to improved conductivity and interfacial polarization losses. The three-layer design, with porous -SiC/SiOC top and bottom layers, allows EMW penetration and reduces surface reflections, while the CNTs in the middle layer promote internal reflections and prolong EMW interaction within the absorber. Transmission electron microscopy revealed coreshell-like CNT-SiOC interfaces, highlighting the role of interfacial polarization in energy dissipation. Among the sample compositions tested (samples with 0, 1, and 2 wt % CNT content), the intermediate CNT content (CNT-1) exhibited the highest performance, achieving a minimal reflection loss of 59.47 dB at a thickness of 3.39 mm, maximum effective absorption bandwidth covering 88% of the X-band, and high absorption efficiency per unit thickness (EABmax/d 1.16 GHz/mm). The CNT-1 also demonstrated superior compressive strength (1.46 MPa), nearly double that of the CNT-free composite. However, excessive CNT loading (CNT-2) led to agglomeration and reduced performance. These findings demonstrate that controlled filler distribution and architectural design enable lightweight, thin, and broadband EMW absorbers, providing a versatile strategy for next-generation functional ceramics.

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Cellular materials are widely used for impact mitigation in various applications, such as helmets. They are studied under two broad categories of foams and (micro-)lattices with respectively stochastic and deterministic internal geometries. Here, we demonstrate for the first time that systematically modifying the internal structure of a stochastic foam by injecting thermosetting struts with deterministic geometry using a customized 3D printer results in markedly enhanced material properties, especially as related to mechanical energy absorption and dissipation. We refer to this method as In-Foam Additive Manufacturing (IFAM). The mechanics of the resulting foams with embedded struts are investigated experimentally to unravel the unique deformation mechanisms stemming from the interactions between struts and the foams. The design parameters of the resulting composite structure include strut spacing, diameter, and inclination angle. These parameters are examined via compressive mechanical testing, and they were compared with conventional state-of-the-art foams. The results show tunable performance and excellent energy absorption efficiency exceeding the neat foam by as much as nearly an order of magnitude. Our study demonstrates that this significant improvement is partly owed to the synergistic load bearing mechanisms in these cellular composite materials, leading to nearly 10x improvement in mechanics of the foams.

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