Flexible 3D printed silicones for gamma and neutron radiation shielding

Highlights

• Formulation of a new inks for Direct Ink Write additive manufacturing.

• Inks contain very high loadings of metal fillers such as bismuth and boron.

• Demonstration of flexible gamma and neutron radiation shielding printed materials.

• Printed materials exhibit good resistance to radiolysis and attenuate radiation.

Abstract

In this work, resin formulations for direct ink write (DIW) additive manufacturing (AM) were developed for facile customization to shield radiation signatures consisting of a range of gamma radiation and neutron energies in a variety of applications. The resin developed in this work has been formulated to accommodate very high loadings of shielding filler materials: up to 40 wt% bismuth (Bi) metal for gamma radiation shielding, and up to 60 wt% boron-10 (¹⁰B) metal for neutron shielding. We report the first instance of a printable resin containing 60 wt% ¹⁰B, which remains flexible after curing despite the high solids content. We demonstrate that these resins can not only tolerate formulation modulation for radiation profile-matched shielding, but they can also be printed in a variety of geometries such that mechanical properties of the printed shield are readily tuned to specific applications. In contrast, most existing shielding materials are rigid, bulky, and do not allow for the complex shapes and mechanical flexibility needed in high-usage applications. This work demonstrates the successful development of a tunable resin for additively manufactured, flexible radiation shielding material, containing high loadings of shielding filler, which can be utilized in harsh radiation environments associated with nuclear materials.

Introduction

Ionizing radiation, such as gamma and neutron radiation, is produced by nuclear materials and can result in detrimental chemical, electronic, and biological effects. As nuclear technologies spread across medical, energy, and space applications there is a growing need to develop shielding materials to maintain safety and limit radiation exposure. Additive manufacturing techniques can address this growing need to employ agile manufacturing strategies by providing a range of custom shielding materials for distinct radiation environments and unique protections for workers and equipment.

Shielding materials are often hard, rigid materials, which presents a critical shortcoming when it comes to personal protective equipment (PPE) or frequently-handled equipment coverings and casings (McIlwain et al., 2014; Mirzaei et al., 2019; McCaffrey et al., 2007). Additionally, commercially available radiation shields are comprised of relatively low loadings of the active shielding fillers, requiring larger volumes of shielding material. It is therefore desirable to develop customizable, flexible radiation shielding materials containing high loadings of selected shielding fillers. Highly filled composite materials are most easily fabricated using traditional casting methods with molds, which require expensive tooling and dies. This manufacturing limitation results in processes that are time and labor intensive, and cannot respond rapidly to a growing demand toward customized radiation shielding materials with smaller manufacturing footprints. Glasses doped with heavy metals fabricated through casting techniques have been proposed as attractive materials for shielding radiation due to, for instance, good optical properties and thermal stability (Rammah et al., 2020a, 2020b; El-Agawany et al., 2020a, 2020b; Ali et al., 2020; Alalawi et al., 2020; Kavaz et al., 2020). However, additive manufacturing techniques, rather than casting, are a promising approach to accelerate customized manufacturing to prepare specialty shielding materials suitable for PPE or shielding/casings for equipment with unique or complex geometries. Direct Ink Write (DIW) 3D printing is a versatile additive manufacturing technique used to prepare flexible composites, and can accommodate higher loadings of filler materials (Lewis, 2006). DIW silicones have been extensively reported with a variety of filler materials, and display favorable mechanical compliance with minimal compression-set (Maiti et al., 2016; Wu et al., 2017). Our team's previous work has demonstrated that DIW silicones specially formulated with high phenyl content increases material resistance to radiolysis when exposed to gamma radiation (Schmalzer et al., 2017), making this type of phenyl-rich silicone resin ideal as the polymer matrix for flexible composite materials used in radiation environments.

Lead has been utilized for gamma radiation shielding, but is unfortunately associated with many hazards and manufacturing challenges due to its high toxicity (Nambiar et al., 2013; Singh et al., 2014; Mann et al., 2015). Bismuth is an effective gamma-shielding element with much lower toxicity than lead (Singh et al., 2002). Composite shielding materials filled with bismuth have been reported (Singh et al., 2014; Mehnati et al., 2018; Malekzadeh et al., 2019; Brounstein et al., 2020), along with several proposed flexible bismuth filled materials (Rammah et al., 2020b; El-Fiki et al., 2015; Güngör et al., 2018; Özdemir and Yılmaz, 2018a; Tijani and Al-Hadeethi, 2019).B is an effective neutron shielding material due to its large neutron capture cross-section (Harvey et al., 1978). Boron nitride composite shield materials have been reported (Fang et al., 2017; Zhou et al., 2007; Özdemir and Yılmaz, 2018b; Harrison et al., 2008), with relatively few reported instances of elemental boron-filled composites (Labouriau et al., 2018). Neither Bi-filled or ¹⁰B-filled commercially available composite materials have been reported with high loadings of the shielding filler; furthermore, shielding composites consist of metal oxides rather than elemental Bi or ¹⁰B, which results in materials containing significantly fewer radiation-attenuating nuclei. This compositional limitation could be due to consumer demand or difficult manufacturability of brittle or rigid materials with high filler content (Wang et al., 2017).

In this work we report a versatile and printable silicone resin formulated to contain up to 40 wt% Bi metal and up to 60 wt% ¹⁰B metal for gamma and neutron radiation shielding applications, respectively. These 3D printed shielding materials were evaluated for resistance to radiolysis, neutron attenuation (¹⁰B-filled only), and mechanical performance following exposure to ionizing radiation. A broad range of material and mechanical characterization techniques were used to evaluate the chemical and physical stability of the printed materials prior to and following irradiation.