Comparative Effects of Hydrazine and Thermal Reduction Methods on Electromagnetic Interference Shielding Characteristics in Foamed Titanium Carbonitride MXene Films.

The urgent need for the development of micro-thin shields against electromagnetic interference (EMI) has sparked interest in MXene materials owing to their metallic electrical conductivity and ease of film processing. Meanwhile, postprocessing treatments can potentially exert profound impacts on their shielding effectiveness (SE). This work comprehensively compares two reduction methods, hydrazine versus thermal, to fabricate foamed titanium carbonitride (Ti3CNTx) MXene films for efficient EMI shielding. Upon treatment of ≈ 100 µm-thick MXene films, gaseous transformations of oxygen-containing surface groups induce highly porous structures (up to ≈ 74.0% porosity). The controlled application of hydrazine and heat allows precise regulation of the reduction processes, enabling tailored control over the morphology, thickness, chemistry, and electrical properties of the MXene films. Accordingly, the EMI SE values are theoretically and experimentally determined. The treated MXene films exhibit significantly enhanced SE values compared to the pristine MXene film (≈ 52.2 dB), with ≈ 38% and ≈ 83% maximum improvements for the hydrazine and heat-treated samples, respectively. Particularly, heat treatment is more effective in terms of this enhancement such that an SE of 118.4 dB is achieved at 14.3 GHz, unprecedented for synthetic materials. Overall, the findings of this work hold significant practical implications for advancing high-performance, non-metallic EMI shielding materials.

Incorporating Loss Factor Modular Design for Full Ku-Band Microwave Attenuation in Double-Layered Graphene Aerogels.

The fabrication of absorption-dominant electromagnetic interference (EMI) shielding materials is a pressing priority to prevent secondary electromagnetic pollution in miniaturized electronic devices and communication systems. Meeting this goal has remained a tough challenge to keep pace with the rapid evolution of electronics due to the complex compositional and structural design and narrow operating bands. This work articulates a sound and simple strategy to precisely modulate the electrical conductivity of reduced graphene oxide (rGO), as the building block in lightweight double-layered rGO-film/rGO-aerogel/polyvinyl-alcohol (PVA) composites, for efficient microwave absorption over the entire Ku-band frequency range. These constructs reasonably comprised a porous absorption structure built from parallel rGO sheets aligned and prepared via freeze casting followed by freeze drying. The electrical conductivity and impedance of this layer were tuned by varying the annealing temperature from 400 to 800 °C, thereby adjusting the degree of reduction and the absorption characteristic. This layer was backed by a highly conductive rGO film reduced at a high temperature of 1000 °C, with a reflectivity of 97.5%. The incorporation of this film ensured high EMI shielding effectiveness of the double-layered structure through the absorption-reflection-reabsorption mechanism, consistent with the predicted values based on calculated loss factors and the input impedance of the structure. Accordingly, at an average EMI shielding effectiveness of 57.59 dB, the reflection shielding effectiveness (SER) and reflectivity (R) of the assembled composites were optimized to be as low as 0.22 dB and 0.049, respectively. This equates to approximately 99.999% shielding (SET) and ∼95% absorptivity (A) of the incident wave. This study opens new avenues for the development of lightweight (with a density as low as 15 mg/cm3) absorption-dominant EMI shielding composite materials with promising EMI shielding efficiency and potential applications in modern electronics.

Enhanced electrical properties of microcellular polymer nanocomposites via nanocarbon geometrical alteration: a comparison of graphene nanoribbons and their parent multiwalled carbon nanotubes.

Geometric factors of nanofillers considerably govern the properties of conductive polymer composites (CPCs). This study provides insights into how geometrical alteration through nanotube-to-nanoribbon conversion affects the electrical properties of solid and microcellular CPCs. In this regard, polyvinylidene fluoride (PVDF)-based nanocomposites are synthesized using both the parent multi-walled carbon nanotube (MWCNT) and its chemically unzipped product, i.e., graphene nanoribbons (GNRs). Theoretical and experimental results show that GNR-based composites exhibit 1-4 orders greater conductivities than MWCNT-based composites at the same filler loading because of the larger number of filler-filler junctions as well as the significantly greater contact areas. On the other hand, the conductivities of MWCNT-based and GNR-based composites are significantly increased by 230 times and 121 times, respectively, through microcellular foaming. The effective rearrangements of rigid MWCNTs and flexible GNRs (having 4 and 5 orders less bending stiffness) for network formation during cellular growth are compared. The GNR-based composites also exhibit a superior dielectric permittivity (e.g., 2.6 times larger real permittivity at a representative frequency of 103 Hz and a nanofiller loading of 4.2 vol%) compared to their MWCNT-based counterparts. This study demonstrates how the modification of the carbon fillers and the polymer matrix can dramatically enhance EMI shielding.

Flexible, Thermally Stable, and Ultralightweight Polyimide-CNT Aerogel Composite Films for Energy Storage Applications.

Polyimide (PI) aerogels are promising in various fields of application, ranging from thermal insulators to aerospace. However, they are typically in the form of a bulk monolith, which suffers from a lack of conformability and drapability. Moreover, their electrical conductivity is limited, and they mainly display an insulative behavior. These shortcomings can limit the applications of PI aerogels in energy storage systems, which require ultralightweight flexible conductive films, which at the same time offer high thermal stability, ultralow density, and high surface area. To overcome these obstacles, the present study reports the fabrication of PI-carbon nanotube (PI-CNT) aerogel composite films with varying CNT content prepared through a sol-gel preparation method, followed by a supercritical drying procedure. Compared to pristine PI aerogels, which displayed a large shrinkage and density of 18.3% and 0.12 g cm-3, respectively, the incorporation of only 5 wt % CNTs resulted in a significant reduction of both shrinkage and density to only 11.5% and 0.10 g cm-3, respectively. This suggests the importance of CNTs in improving the dimensional stability of aerogels and creating a robust network. Further characterizations showed that incorporation of 5 wt % CNTs also resulted in the highest pore volume (1.25 cm3 g-1), highest surface area (324 m2 g-1), highest real permittivity (80), highest electrical conductivity (3 × 10-1 S m-1), and ultrahigh service temperature (575 °C). It was also shown that the aerogel films can withstand a large degree of bending, can be twisted, and can be fully rolled with no obvious cracks propagated in the structure. The combined outstanding properties of the developed aerogel composite films make them promising potential candidates for supercapacitor electrodes. Therefore, the electrochemical performance of the devices based on aerogel electrodes was further studied. The device demonstrated a high energy density of 2.6 Wh kg-1 at a power density of 303.8 W kg-1. The total capacitance after 5000 cycles was 91.8% of the initial capacitance, which indicated excellent stability and durability of the device. Overall, this work provides a facile yet effective methodology for the development of high-performance aerogel materials for energy storage applications.

Tailoring Multifunctional and Lightweight Hierarchical Hybrid Graphene Nanoplatelet and Glass Fiber Composites.

In this work, hybrid polypropylene (PP)-based composites reinforced with graphene nanoplatelets (GnPs) and glass fiber (GF) were fabricated by injection molding to elucidate how the hybrid approach can produce synergistic effects capable of achieving properties and functionalities not possible in biphasic composites. Synergism between the reinforcements translated to improved mechanical performance, which was attributed to the chemically and/or electrostatically assembled hierarchical structure that facilitates load transfer at the interface while simultaneously tailoring the crystalline microstructure of the matrix by inducing transcrystallization and ß-crystal formation. It was demonstrated that there exists an optimal concentration of 0.5 wt % GnP, producing the greatest mechanical properties and synergistic effect, corresponding to the highest degree of crystallinity (∼6% greater than Neat PP) and peak formation of ß-crystals within the PP matrix. The greatest synergistic effect was found to be ∼52 and ∼39% for the specific tensile strength and flexural strength, respectively. The same optimal concentration of GnPs was found to produce the highest synergistic effect for thermal conductivity of ∼68% due to the volume exclusion effect induced by the GFs combined with the higher crystallinity of the microstructure, promoting the formation of thermally conductive pathways. Ultimately, the mechanisms contributing to the synergistic effect presented in this work can be used to maximize the performance of hybrid composite systems, giving them the potential to be tailored for a variety of high-performance industrial applications to meet the rising demands for ultra-strong, thermally conductive, and lightweight materials.