Polymer bonded explosives (PBXs) with reduced thermal stress and sensitivity by thermal conductivity enhancement with graphene nanoplatelets

Abstract

The two-dimensional (2D) graphene nanoplatelets (GNPs) were adopted to enhance the thermal conductivity of PBX. The results indicated that, the thermal conductivity of PBX was only slightly improved with very low GNP loading (0.05 wt% and 0.15 wt%). However, a remarkable enhanced effect was observed at a relatively high GNP loading (0.5 wt% and 1 wt%). The thermal conductivity of PBX had a nonlinear dependence on the GNPs loading. The nonlinear dependence of the thermal conductivity on GNPs content was fitted by an analytical model which incorporated all the effects. And the thermal conduction mechanism of GNPs based composite could change from series thermal structure to parallel thermal structure with the GNP loading increasing. Additionally, the calculated thermal shock resistance and thermal stress distribution of PBX was both enhanced with the GNP loading. Finally, the sensitivity of sensitive hexanitrohexaazaisowurtzitane (CL-20) based PBX composition could be improved by GNPs due to enhanced thermal conductivity.

Introduction

Polymer Bonded Explosives (PBX), which refer to a particle filled composite consisting of 90–95% weight of powerful explosive crystals held together by a small percentage polymer binder of 5–10% weight, are being extensively used in a variety of conventional and nuclear defense munitions [1], [2], [3]. As an important part in weapon system, PBX will be subjected to the complicated thermal physical environment during long-term storage, transportation, and usage process [4]. Moreover, after experienced rapid high-low temperature changes and broad temperature range, the heat in PBX cannot be quickly transferred due to the low thermal conductivity property of explosive crystal and polymer binder (less than 0.5 W m⁻¹ K⁻¹), resulting in a significant inhomogeneous temperature distribution and gradient. Then the generated severe thermal stress can cause cracking or damage of PBX with a low strength and toughness and further harm the security and reliability of the weapon system [5], [6]. Therefore, as a key route to reduce thermal stress and enhance the thermal environment adaptability of PBX, improving the thermal conductivity is becoming an urgent problem to be solved [7].

Generally, the thermal conductivity of polymer composites has been effectively enhanced by the addition of thermally conductive fillers. Among these fillers, Carbon-based nanofillers appear to be the best promising fillers, coupling ultrahigh thermal conductivity, lightweight and high aspect ratio [8]. For example, carbon nanotubes (CNTs) considerably improved the heat transport in polymer composites as a result of their one-dimensional (1D) structure, high thermal conductivity and high aspect ratio [9], [10]. Recently, graphene has attracted a considerable amount of interest due to its extraordinary electrical, thermal, and mechanical properties [11], [12]. As two-dimensional lattice of sp²-bonded carbon that is just one atomic layer thick, it exhibits remarkably high thermal conductivity and has been experimental established as the highest thermal conductivity material ever measured (∼3000 W/mK) [13]. Some theoretical descriptions of thermal transport in graphene revealed that thermal conductivity of graphene was actually phonon-based, since the analyzed graphene samples’ dimensions exceeded average free path of phonons (800 nm), and its electronic-based thermal conductivity represented less than 1% of the total thermal conductivity at room temperature [14], [15].

Comprised of few graphene layer G_n, where n represents the layer number, the graphene nanoplates (GNPs), show promise for application as nanofiller materials in polymer composites due to their high aspect ratio, unique two-dimensional plane nanostructure, and low manufacturing cost [16], [17], [18]. In general, the high contact area between polymer and planar GNPs can provide a 2-D path for phonon transport and maximize heat flow from polymer matrix to GNPs. And GNPs rigidity allows for better preservation of their high aspect ratio in comparison with the more flexible CNTs [19]. Thus, polymer-GNPs can be expected to exhibit better reinforcement of thermal conductivity than carbon nanotubes (CNTs) in polymer composites [20]. Many experimental studies have shown that the addition of a small amount of GNPs in polymer could result in a substantially large enhancement of the effective thermal conductivity and this enhancement was nonlinear with GNP loading [21], [22], [23]. The nonlinear dependence originated from the interaction among GNPs in matrix [24].

The application of graphene in energetic materials has been previously involved by some workers. The obtained results revealed that graphene could improve the burning rate and mechanical properties of propellant, as well as the release rate of energy [25]. On the other hand, the properties of thermal stability and sensitivity were very important for PBX formulations [26]. And the effect of carbon nanomaterials on the performances of energetic compositions has been well-summarized in the review literature of Yan [27]. It has been shown that the use of carbon nanomaterials in energetic compositions could greatly improve their combustion performances, thermal stability and sensitivity. Especially for the mechanical sensitivity, there are many carbon nanomaterials and their derivatives capable of decreasing the sensitivity of energetic materials by suppressing of hotspot formation. For example, the mechanical sensitivities of ɛ-CL-20/glue could be reduced with the incorporation of graphite and graphene oxide in PBX formulations [28].

In this study, the two-dimensional GNPs will be used in PBX to improve the low thermal conductivity. As one of the most important systems which may have potential military applications, the 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)-based PBX was adopted. The thermal conductivity was measured at the broad temperature range of 20°C–90 °C. Moreover, the nonlinear dependence of thermal conductivity of PBX on GNPs content was fitted by an analytical model. The thermal shock resistance (TSR) of PBX modified by GNPs was evaluated based on the experimental mechanical and thermal conductivity data. Meanwhile, the temperature and thermal stress distribution of TATB based PBX formulations during thermal impact process were simulated by the finite element software ANSYS. At last, to found the relationship between the sensitivity and thermal conductivity, a certain sensitive CL-20 based PBX composition was selected to further study the effect of GNPs on mechanical sensitivity.