Direct ink writing of 3D-Honeycombed CL-20 structures with low critical size

Bo-yun Ye , Chng-kun Song , Ho Hung , Qin-bing Li , Chong-wei An ,b,*,Jing-yu Wng ,b

a School of Environment and Safety Engineering, North University of China, 030051 Taiyuan, China

b Shanxi Engineering Technology Research Center for Ultrafine Powder, North University of China, 030051 Taiyuan, China

c China North Industries Group Corporation Limited,100821, Beijing, China

Keywords:Hexanitrohexaazaisowurtzitane (CL-20)Direct ink writing Micro-electro-mechanical system (MEMS)Pore structure Detonation performance

ABSTRACT 3D-Honeycombed CL-20 structures with low critical size of detonation have been fabricated successfully for intelligent weapon systems using a micro-flow direct ink writing(DIW)technology.The CL-20-based explosive ink for DIW technology was prepared by a two-component adhesive system with waterborne polyurethane (WPU) and ethyl cellulose (EC). Not only the preparation of the explosive ink but also the principle of DIW process have been investigated systematically.The explosive ink displayed strong shearthinning behavior that permitted layer-by-layer deposition from a fine nozzle onto a substrate to produce complex shapes. The EC content was varied to alter the pore structure distribution and rheological behavior of ink samples after curing.The deposited explosive composite materials are of a honeycombed structure with high porosity,and the pore size distribution increases with the increase of EC content.No phase change was observed during the preparation process. Both WPU and EC show good compatibility with CL-20 particles. Apparently high activation energy was realized in the CL-20-based composite ink compared with that of the refined CL-20 due to the presence of non-energetic but stable WPU. The detonation performance of the composite materials can be precisely controlled by an adjustment in the content of binders.The 3D honeycombed CL-20 structures,which are fabricated by DIW technology,have a very small critical detonation size of less than 69 μm, as demonstrated by wedge shaped charge test.The ink can be used to create 3D structures with complex geometries not possible with traditional manufacturing techniques, which presents a bright future for the development of intelligent weapon systems.

1. Introduction

A variety of intelligent weapon systems have been developed in the field of defense industry with the rapid development of technologies such as optoelectronic devices,microwave semiconductor devices, integrated circuits and information automation. The intelligent weapon systems are required to be as flexible as possible with high energy, which will lead to new challenges in fuzing mechanisms for applications and function at small geometries. At present, press-loading and cast curing have been traditionally adopted to load explosive into the fixture [1,2]. However, some defects, such as crevices, voids and cracks will emerge in the loading process using these two traditional methods,which might easily result in performance problems for fielded munitions.Furthermore,these traditional loading methods are not suitable for the mini-micro weapon systems. Therefore, a safe, reliable, effective, high-precision and mass-produced approach is urgently needed to load explosives for mini-micro weapon systems[3].

Direct ink write (DIW) technology has received considerable attention for a variety of applications due to its precision control,low cost,high efficiency,and ease of use[4,5].DIW is used to write a desired composite material into an extremely sophisticated structure through the XYZ numerical control system. The composite material can be subsequently deposited onto a heated substrate. This most commonly results in a 3D structure whose properties,including density,width and thickness,can be precisely controlled by the deposition parameters [6]. Based upon the availability of diverse binders and operating conditions,DIW can be expected to be employed for not only many kinds of materials but also different particle sizes and morphologies[7-10].In 2011,Ihnen used a drop-on-demand inkjet printer to print a RDX-based composite material in a single step with an all-liquid organic ink. Its morphology can be favorably controlled by the adjustment of parameters, among which the ink evaporation rate and microfluidic ink droplet control are most important to define crystal growth[11,12].

CL-20 (HNIW, 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) is a high-energy nitramine compound with four different forms(α,β,γ,and ε)of crystalline phases,within which the ε-CL-20 is of the largest density and best thermal stability [13-16]. Its invention has been regarded as a significant breakthrough in explosive synthesis. In view of its superior performances, ε-CL-20 is not only considered as the most powerful explosive but also applicable to be used for mini-micro weapon systems [17-19]. In 2016, Xu prepared CL-20 based composite using inkjet printing technologies[20].The composite,whose critical detonation size is ca. 0.461 mm, is of high density and low impact sensitivity.On the other hand,the crystalline phase of CL-20 is hard to control during the process of inkjet printing.

It has been already shown that superfine explosives have an exceptionally small critical detonation size,which can significantly contribute to reliable and stable detonations of energetic materials in extremely small channels[3,21].As shown in Fig.1,we describe a new DIW technology capable of printing 3D honeycombed CL-20 structure with very low critical detonation size. Briefly, the present work attempts to develop a CL-20 based energetic composite ink of waterborne polyurethane (WPU) as a binder for excellent performance in its excellent elasticity, abrasion resistance, flexibility,and broad substrate suitability[22-25],and of ethyl cellulose(EC) as a dispersant and thickener for effective control of the ink viscosity. The rheological properties of the ink can be precisely controlled by adjusting the ratio of WPU and EC. Absolute ethyl alcohol was used to dissolve the polymer to control the viscosity of the ink. The ink materials can be printed into a variety of 3D structures that are not possible by traditional methods.

2. Experimental section

2.1. Materials

CL-20 as a ε-phase (particle size 80-150 μm) was provided by Liaoning Qingyang Chemical Industry Co.Ltd of China.Waterborne polyurethane dispersions (J7091) was obtained from Newmat Environmental Materials Technology Corporation of China. The solid content and viscosity of WPUD are 30% and 70 cps, respectively. EC powders (EK-70) were bought from Guangzhou Wenjia Chemical Co. Ltd of China. 0.8 g of EC powders were dissolved in 9.2 g absolute ethanol in order to prepare 8 wt%solution in ethanol.Absolute ethanol was bought from by Tianjin Fuchen Chemical Reagents Factory of China.

2.2. Refining of raw CL-20

Submicron CL-20 (sub-CL-20) explosives were prepared via mechanical ball milling using water as a buffer solution. The grinding medium was yttria-stabilized zirconia balls with an average diameter of 0.1 mm. In detail, ca. 200 g of balls, 100 g of water and 10 g of raw CL-20 explosives were mixed together to be milled at a 300 revolutions per minute for 5 h[26,27],followed by ultrasonic separation, filtration, and freeze drying.

Fig.1. (a)The uniform dispersion of the sub-CL-20 explosives in absolute ethanol.(b)The schematic for the preparation of the CL-20/WPU/EC ink.(c)A brief representation of the apparatus for an ink patterning using DIW.

Fig. 2. Testing devices of the detonation performance of the CL-20/WPU/EC sample.

2.3. Ink preparation and direct writing

3.6 g of sub-CL-20 explosives was dispersed uniformly in absolute ethanol by an ultrasonic oscillator for 30 min to prevent from their reunion,as shown in Fig.1.Then,0.8 g of WPUD and 2 g of EC ethanol solution were simultaneously added slowly into the explosive solution with 50 rpm mechanical stirring at 40°C for 2 h,while the ultrasonic frequency and power are set at 40 kHz and 450 w, respectively. Finally, the canary yellow energetic ink with excellent stability was obtained.

The energetic ink was transferred to a dispensing syringe,which was subsequently loaded to a XYZ NC system,and then extruded at a pressure of 0.1 MPa from the syringe needle with a diameter of 0.5 mm.As is shown in Figs.5(c)and 3D structures was created via a layer-by-layer deposition by controlling the syringe needle through pre-programmed computer programs.In order to test the detonation performance, the energetic ink was written into an aluminium plate channel.The explosive ink is written in the groove by layer-by-layer deposition.All experimental procedures were in a constant temperature environment of 40°C. The excess surface explosive ink not contributing to the energetic reaction were gently removed prior to test.The test principle is shown in Fig. 2.

2.4. Characterization and testing

Fig. 3. Plot (a) and log-log plot (b) of viscosity as a function of shear rate.

Fig. 4. Pore structure distribution diagrams of the prepared inks with different EC contents of 3 wt% (a), 4 wt% (b) and 5 wt% (c).

The photographs were taken by a SLR camera (Sony, DSCRX10M3). The rheological behavior of the explosive inks was measured by rotational rheometer (Brookfield, RS-CPS) at shear rates of 0 s-1as a start and 10 s-1as an end. The pore structure distribution of ink samples after curing was characterized by mercury porosimetry (PoreMaster-60GT) with testing pressure from 0 to 228 MPa. The morphology and microstructure of the prepared CL-20 explosives and the corresponding ink samples were characterized by field emission scanning electron microscope(Tescan,MIRA3 LMH)at an acceleration voltage of 10 kV after gold sputtering coating under a vacuum of degree 10-6Pa for 50 s. The crystalline phase of the CL-20 explosives in ink samples were determined by X-ray diffractometer (XRD, Haoyuan, DX-2700) using a target material (Cu) with a tube voltage of 40 kV, a tube current of 30 mA, 5°as a start and 50°as an end diffraction angle.Differential scanning calorimetry (Setaram, DSC 131) was utilized to analyze the thermal decompositions.Samples were measured at heating rates of 5,10,15, and 20°C/min from 30 to 350°C under a nitrogen atmosphere with its flow of 30 mL/min.

A microchannel detonation experiment [20] was designed to test the detonation performances of the CL-20/WPU/EC sample,including its critical size and velocity of detonation.As is shown in Fig. 2(a), the wedge groove with length of 100 mm and width of 1 mm was etched on an aluminum substrate. The large section depth at the left end of the wedge groove is 3 mm,and the groove depth decreases linearly to 0 from left to right. The critical detonation thickness (DH) is defined the microchannel depth at the failure location of CL-20 explosive ink sample and can be calculated by measuring the distance(Lx)between the misfire position and the end of the charge. As is shown in Fig.2(b), the rectangular section groove with depth of 1 mm and width of 1 mm was etched on an aluminum substrate. Ionization probes were utilized to test the time of explosive detonation wavefront transmit from A to B,and B to C (t1and t2). The critical thickness and velocity of detonation were calculated by formula 1, and 2.

3. Results and discussions

3.1. Rheological behavior

Direct ink writing has recently been researched for a variety of applications. This research has identified some key characteristics for successful part fabrication,including shear-thinning behavior to permit extrusion at lower shear rate,a sufficiently high shear elastic modulus and shear yield strength to prevent fracture after exiting the nozzle[28],and a rapid prototyping method to fabricate porous structure.In order to develop composites with high porosity,EC as a thickener and dispersant was added to the formulation.

The effect of EC content on rheological behavior of the prepared inks was investigated. Three series of CL-20-based explosive inks were prepared containing 2 wt%,4 wt%,6 wt%of EC without impact on other compositional variables. As is shown in Fig. 3(a), all the inks exhibited shear thinning behavior to permit extrusion. The behavior can be explained by Ostwald-de Wale equation.

where τ is shear stress, K is consistency coefficient, n is flow exponent,γ is shear rate,and η is apparent viscosity.

Fig. 5. SEM images of (a) raw CL-20, (b) sub-CL-20, (c) ink writing pattern, and the corresponding ink samples with different EC contents of 3 wt% (d), 4 wt% (e), and 5 wt% (f).

Table 1 Rheological performance parameters of the prepared inks.

Fig. 6. The XRD patterns of raw CL-20, sub-CL-20, and the CL-20-based ink.

The rheological performance parameters of the prepared inks with different EC content were calculated by linear fitting as shown in Table 1.It can be seen from Fig.3 and Table 1,all the inks conform to the typical characteristics of a pseudoplastic fluid. The ink viscosity increases with the increase of EC content,and flow exponent decrease gradually with values of 0.688, 0.52, and 0.26 means a gradually stronger shear-thinning property. The feature is very conducive to ensure the extruded ink with a higher viscosity,and of excellent rheological properties to prevent a blockage of needle.

3.2. Microscopic pore characteristics

In order to obtain an explosive ink with a very low critical size of detonation, the pore structure distribution of ink samples was investigated. Three series of CL-20-based explosive inks were prepared containing 3 wt%,4 wt%,5 wt%of EC without impact on other compositional variables,the EC content was varied to alter the pore structure distribution of ink samples after curing. As can be seen from Fig. 4, all the ink samples after curing have an uneven pore size distribution range. Increasing EC content resulted in an increase in pore size distribution with values of 196-480 nm,232-708 nm,and 254-986 nm for inks with 3 wt%,4 wt%,and 5 wt% EC, and the average pore size is 304 nm, 546 nm, and 672 nm,respectively, the following SEM diagrams also illustrates this phenomenon.

Fig. 7. The DSC curves of raw CL-20, sub-CL-20, CL-20-based composite ink after curing, at a heating rate of 10°C/min.

3.3. Morphology analysis

The morphology of the sub-CL-20 explosives and the corresponding ink samples containing 3 wt% (d), 4 wt% (e), 5 wt% (f) of EC were characterized by SEM. As shown in Fig. 5(a), the shape of the raw CL-20 is spindle-shaped.After mechanical ball milling,the sub-CL-20 explosives (Fig. 5(b)) have a nearly spherical shape and smooth surface,whose median size are 140 nm.Fig.5(c)shows a 3D periodic structure obtained by writing 10 layers of the explosive ink on a substrate.After curing,the ink lines have a diameter of 780 μm and a spacing of 2.9 mm. The pattern reveals that the CL-20/WPU/EC ink can be printed in a variety of patterns with high uniformity.The corresponding ink samples have a honeycomb shape, good dispersing effect, and uniform compact cross-section. The CL-20 explosive particles were well-distributed over the binders after the ink was cured. The pore size distribution increases with the increase of EC content as mentioned earlier.The feature will have a significant impact on detonation performance of the CL-20 explosives in extremely small channel as described later.

Fig.8. The DSC curves of(a)raw CL-20,(b)sub-CL-20,(c)CL-20-based composite ink after curing;(d)Kinetic studies on the thermal decomposition of raw CL-20,sub-CL-20 and CL-20-based composite ink by Kissinger method.

Table 2 Kinetic parameters of thermal decomposition by DSC.

3.4. Polymorph stability

During the process of mechanical ball milling, and especially under the ultrasound dispersion in the ink preparation, the crystalline phase of CL-20 may result in a change [26]. In order to investigate the polymorph stability of the CL-20 explosives throughout the experiment, the samples were characterized by XRD. It can be seen from Fig. 6 that the main diffraction peaks of both the sub-CL-20 and the CL-20-based ink are basically consistent with that of the raw CL-20 [29,30]. They all have three strong characteristic peaks at 12.59°,13.82°,and 30.29°,which correspond to the crystal faces of (11-1), (200), (20-3) of ε-CL-20, respectively.All the XRD patterns are consistent with the standard PDF card(00-050-2045) of ε-CL-20. These evidences indicate that both mechanical ball milling and ultrasound adopted during the experimental process do not cause the crystalline phase transition of raw CL-20.

3.5. Thermal properties

For the CL-20-based composite ink with 3D honeycombed structures, the thermal decomposition properties are very important to directly determine its safety properties and detonation performance.Fig.7 shows DSC curves of raw CL-20,sub-CL-20,and CL-20-based composite ink at a heating rate of 10°C/min. The thermal decomposition behavior of CL-20 includes an endothermic process of solid-solid phase transition. As shown in Fig. 7, three endothermic peaks are observed at 175.1°C, 165.93°C, and 181.18°C, which are assigned to the ε →γ phase transition temperatures of raw CL-20,sub-CL-20 and CL-20-based composite ink,respectively. It should be noted here that the phase transition temperature of sub-CL-20 is lower than that of raw CL-20, since small sub-CL-20 particles inherently have a lower phase transition temperature.Interestingly,the phase transition temperature of the CL-20-based composite ink does not decline further with a decrease in its particle size. The possible reason is that sub-CL-20 particles are coated with the binders, restricting their molecular activity.

Fig.9. Photographs of detonation performance test:the critical size and velocity of detonation of explosive inks containing 3 wt%(a,and d),4 wt%(b,and e),5 wt%(c,and f)of EC.

Table 3 Detonation properties of CL-20-based composite ink.

Exothermic reactions are considered to be one of the most important properties of explosives.The DSC curve of the sub-CL-20 shows an exothermic decomposition process at the temperature range of 238-250°C (Tmax=246.41°C) at a heating rate of 10°C/min. However, the corresponding Tmaxof raw CL-20 is located at 252.83°C, and thus the sub-CL-20 exhibits a shift of 6.42°C to lower temperature.This may be due to the ratio of surface atoms to interior atoms is higher than that of raw CL-20, which leads to a higher surface energy and the decrease of decomposition peak.However, the decomposition temperature of the CL-20-based composite ink ranges from 222°C to 236°C (Tmax=230.83°C).The addition of binders causes a significant decrease in the exothermic peak from 246.41°C of sub-CL-20 to 230.83°C of the CL-20-based composite ink.The same phenomenon is observed at the rates of 5,15 and 20°C/min(Fig.8(a),(b)and(c)).The possible reason is that the addition of binder and dispersant results in a more uniform distribution of explosive particles. The loose structure causes the composite to start reacting at lower temperatures.This can be seen from the corresponding SEM images from Fig. 5.

The kinetic parameters of thermal decomposition reactions,which are listed in Table 2, have been calculated using Kissinger method (Eq. (1)) [31], as shown in Fig. 8(d).

The sub-CL-20 nearly has an equal value of activation energy with raw CL-20 after the process of mechanical ball milling.The CL-20-based composite ink after curing has to a large activation energy compared with sub-CL-20 due to the presence of non-energetic but stable WPU.

3.6. Detonation performance

The detonation performance was investigated as a function of the proportion of WPU and EC.As mentioned earlier,the explosive inks after curing with different porosity were prepared, and the corresponding critical size and velocity of detonation of explosive inks containing 3 wt%(a,and d),4 wt%(b,and e),5 wt%(c,and f)of EC are shown in Fig. 9. The theoretical density (ρTMD), calculated detonation velocity (Dmax), charge density (ρ0), actual velocity (V)and critical size (DH) of detonation are listed in Table 3. All tests were repeated three times and averaged.

Due to a similar density of WPU and EC,a change in the ratio of binders almost has no effect on the theoretical density and detonation velocity. However, there is a significant decline in charge density as the EC content increases, since EC acts as a thickening agent in the CL-20-based composite ink, which is consistent with our previous results.It is well known that the detonation velocity is closely related to the charge density. There is a drastic detonation velocity decrease of 564 m/s for Formula 2 compared with 3.Interestingly, although Formula 2 has a lower detonation velocity than Formula 1, the critical size of detonation decreases significantly from 282 μm to 69 μm. This may result from a higher porosity that facilitates the formation of an activation centers (hot spots) leading to a faster volumetric combustion rate, a narrower chemical reaction zone, and a smaller lateral energy loss, which results in a decrease of the critical detonation size, and enhancement in the detonation wave propagation capability.However,with further increase in the proportion of EC, the critical size of detonation increases again, because too low charge density leads to lower detonation energy. Therefore, it is really not suggested to increase the porosity without limit.

4. Conclusion

In summary,a controllable and high-efficiency method has been proposed here to successfully load sub-CL-20-based composite ink into extremely small grooves with a very small critical detonation size. The sub-CL-20-based explosive ink, which can be written in complex structures using DIW,has a honeycomb morphology with high porosity. The porosity of printed composites can be precisely controlled by an adjustment in the ratio of binders, which has a significant impact on the detonation performance of the printed composites. The explosive ink displayed strong shear-thinning behavior that permitted layer-by-layer deposition. Characterizations of the printed samples indicate that no phase change was observed during the preparation process.The data of SEM and DSC show that the functionalized binders have acceptable compatibility with CL-20. In conclusion, the CL-20-based composite materials fabricated by DIW technology can be competent in demands of low-cost,fine-precision and high-efficiency in energetic materials,which show a bright future for intelligent weapon systems,and not possible with traditional manufacturing techniques.

Acknowledgments

This research work was financially supported by the Advantage Disciplines Climbing Plan of Shanxi Province and Graduate Education Innovation Project in Shanxi Province (2016BY119).