Robust 2D layered MXene matrix-boron carbide hybrid films for neutron radiation shielding

The increasing use of powerful electronic devices in high-radiation environments (aerospace, automotive, nuclear, medical) necessitates radiation-tolerant technologies. Neutron radiation, in particular, poses a significant challenge due to its high penetrating power and ability to induce further ionizing radiation. Traditional neutron shielding materials, such as water, concrete, and polyethylene, require considerable thickness due to the reliance on neutron scattering. Boron carbide (B₄C) offers a superior alternative due to its high neutron absorption cross-section, high melting point, and low density. However, achieving homogeneous high-content B₄C in flexible, processable composites has proven difficult due to the challenges of boron solubility in metallic matrices and the limited processability of B₄C-reinforced polymers. This research addresses these limitations by utilizing the unique properties of two-dimensional (2D) MXenes as a matrix for B₄C, aiming to create lightweight, shape-controllable, and structurally stable neutron shielding composites.

Existing neutron shielding materials and approaches have limitations. Hydrogenous materials rely on scattering and require significant thickness. Boron compounds, especially B₄C, are favored for their high neutron absorption capacity. However, incorporating high B₄C concentrations into flexible and processable matrices presents challenges. Metallic matrices suffer from heterogeneous structures and low boron content, while polymer matrices require expensive surface modifications for enhanced interfacial interactions. Previous work on B₄C composites often involves complex and costly processing methods, hindering scalability and commercial viability. This study leverages the recent advancements in 2D MXene materials, highlighting their potential for creating advanced composites with tailored properties.

The research involved several key steps. First, Ti₃AlC₂ MAX phase was synthesized via ball milling and high-temperature sintering. This was then chemically exfoliated using a mixed etchant (HF+HCl) and an inorganic intercalant (LiCl) to produce large, high-quality Ti₃C₂Tx MXene flakes. Next, nano-sized B₄C (n-B₄C) particles (<300 nm) were obtained from commercially available B₄C via sonication and centrifugation to ensure homogeneity. The surface of n-B₄C was modified to enhance its electrostatic dispersibility. A stable and homogeneous Ti₃C₂Tx MXene/B₄C (MB) hybrid colloid was created by mixing the MXene flakes and n-B₄C particles, leveraging the repulsive forces between their negatively charged surfaces. Polyvinyl alcohol (PVA) was added as a binder to the MB hybrid solution to create a Ti₃C₂Tx/n-B₄C/PVA (MBP) hybrid solution. Freestanding MBP composite films were fabricated via vacuum-assisted filtration, while painted MBP films were produced by blade-coating a concentrated MBP hybrid paint. Various characterization techniques were employed including XRD, SEM, EDS, Raman spectroscopy, tensile testing, nitrogen adsorption/desorption analysis, and neutron attenuation tests. Monte Carlo N-Particle (MCNP) simulations were used to further validate the neutron shielding performance. The mechanical properties of the films were investigated using tensile and bending tests. The neutron shielding ability was assessed using an Am-Be neutron source and a He proportional counter.

The study successfully fabricated freestanding and painted MBP hybrid films with high B₄C content (up to 60 wt%). XRD and SEM analysis confirmed the homogeneous distribution of B₄C particles within the layered MXene matrix. The films exhibited excellent mechanical flexibility and stability, even at high B₄C loading. The tensile strength decreased with increasing B₄C content, but remained comparable to other B₄C composite coatings. Nitrogen adsorption/desorption analysis showed that the MBP films possessed a mesoporous structure, indicating high boron capacity. The painted MBP films demonstrated excellent adhesion to various substrates (stainless steel, glass, nylon fabric) and thickness controllability through multiple coating layers. The neutron attenuation tests showed that the films exhibit significant neutron shielding ability, with the half-value layer (HVL) consistent with MCNP simulations and significantly lower than that of comparable commercial shielding materials. The MCNP simulations confirmed the impact of B₄C isotope composition on neutron absorption, with isotope-enriched B₄C showing higher absorption compared to naturally occurring B₄C. The freestanding MBP films showed thermal stability below 180 °C. The painted MBP film on the nylon fabric maintained good flexibility even after 20,000 bending cycles, indicating high structural stability.

The results demonstrate the successful development of highly effective and flexible neutron shielding films based on a 2D MXene matrix and B₄C nanoparticles. The use of solution-processable MXene flakes facilitates the homogeneous dispersion of high B₄C content while maintaining film flexibility. The simple and scalable fabrication methods, both vacuum filtration and blade coating, offer significant advantages for large-scale production and practical applications. The superior neutron shielding properties of the MBP films, combined with their flexibility and processability, represent a significant advance over existing technologies, offering a path toward lightweight, adaptable neutron shielding solutions. The observed mechanical properties suggest that optimizing the PVA content or exploring alternative high-temperature polymers could further enhance the films' thermal stability and overall performance. The findings open new avenues for the development of multifunctional MXene-based coatings.

This research successfully developed robust and flexible neutron shielding films using a novel 2D Ti₃C₂Tx MXene-B₄C hybrid material. Both freestanding and painted films demonstrate superior neutron absorption capacity, high mechanical flexibility, and scalable fabrication methods. The approach addresses limitations of conventional shielding materials by combining the advantages of B₄C's high neutron absorption with the unique properties of MXene as a high-capacity matrix. Future studies could focus on optimizing the polymer binder for enhanced thermal stability and exploring other MXene compositions for further improved shielding performance. The versatility of MXene coatings opens doors for diverse applications beyond neutron shielding.

The current study primarily focused on thermal neutron shielding. Further investigation is needed to assess the effectiveness against other neutron energies. The long-term stability of the MBP films under continuous neutron irradiation requires additional evaluation. The mechanical properties of the painted films were largely dependent on the substrate material; therefore, future work should explore the effects of various substrates on neutron shielding performance. The thermal stability of the PVA binder might limit the applicability of the films in extremely high-temperature environments.