Flexible 3D printed silicones for gamma and neutron radiation shielding
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.
Section snippets
Materials
The general polymer matrix used throughout this work consisted of vinyl terminated (4–6% diphenylsiloxane)-dimethylsiloxane copolymer (Gelest, PDV series), trimethylsiloxy terminated methylhydrosiloxane-dimethylsiloxane copolymer (Gelest, HMS series), in addition a cure inhibitor such as 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (Gelest, SIT7900.0) or 1-ethynyl-1-cyclohexanol (Sigma Aldrich, 99%). Polysiloxane formulations and preparation are discussed in detail in our previous
Highly filled siloxane-based resins printed using direct ink write techniques
Resin viscoelasticity is one of the most important parameters to consider when utilizing DIW printing techniques. The resin must exhibit shear-thinning behavior with a yield stress that can be achieved using either a pneumatic dispenser or a mechanical screw syringe pump. Practically, this corresponds to a resin that will not flow under its own weight (or the weight of a 7-layer printed part) but will flow through a syringe nozzle when the appropriate pressure is applied. The resin must exhibit
Conclusion
This work demonstrated a new DIW ink that exhibits good resistance to radiolysis and can be easily modified to contain high wt.% loading of radiation shielding filler materials. The basic ink formulation consists of polysiloxanes with sufficient diphenyl functionality for radiation resistance as well as 40 wt% elemental Bi or 60 wt% elemental 10B. Bi-filled and 10B-filled pads were exposed to gamma and neutron radiation, respectively. Slight variations in cross-link density and mechanical
Author statement
Samantha J. Talley: investigation, data curation, writing-original draft, writing-review & editing, Tom Robison: conceptualization, methodology, Alexander M. Long: data curation, So Young Lee: investigation, Zachary Brounstein: investigation, data curation, Kwan-Soo Lee: investigation, data curation, Drew Geller: data curation, Ed Lum: software, and Andrea Labouriau: funding acquisition, methodology, project administration, writing- original draft, writing-review & editing.
All authors have read
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Henry Pearson and Bethany Wilburn at the Kansas City National Security Campus for their generous contributions toward sample preparation and printing method development. This work was performed, in part, at the Los Alamos Neutron Science Center (LANSCE), a NNSA User Facility operated for the U.S. Department of Energy (DOE) by Los Alamos National Laboratory (Contract 89233218CNA000001). Don Hanson and Maryla Wasiolek from Sandia National Laboratories are thanked for their help with
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