Microfluidic Platform for Preparation and Screening of Narrow Size

Subscriber access provided by Kaohsiung Medical University

Process Systems Engineering

A Microfluidic Platform for Preparation and Screening of Narrow SizeDistributed Nanoscale Explosives and Super-mixed Composite Explosives Shuangfei Zhao, jiawei wu, Peng Zhu, huanming xia, cong chen, and Ruiqi Shen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03434 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

A Microfluidic Platform for Preparation and Screening of Narrow Size-Distributed Nanoscale Explosives and Super-mixed Composite Explosives Shuangfei Zhao[a], Jiawei Wu[b], Peng Zhu*[a], Huanming Xia[b], Cong Chen[a] and Ruiqi Shen[a] [a]

School of Chemical Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China [b]

School of Mechanical Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China *Corresponding author: Peng Zhu, E-mail address: [email protected]

Graphical Abstract:

Microfluidic platform combined by a microfluidic oscillator and a single-swirling micro-chip

reactor can be successfully applied for preparation and screening of nanoscale single-compound explosive (HNS) and composite explosive (HNS & HMX composite). 1/42

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: A microfluidic platform suitable for the research, preparation and screening of nanoscale single-compound and composite explosives was established. The microfluidic platform consisting of a microfluidic oscillator and a micro-chip reactor has the characteristics of excellent mixing performance, fast preparation and less sample consumption. Furthermore, the recrystallization growth kinetics of explosives under microscale conditions was studied. Here we use typical explosives HNS (2,2’,4,4’,6,6’-hexanitrostibene) and HMX (cyclotetramethylenete tranitramine) as samples to study the applicability of the microfluidic platform in preparation and screening of nanoscale and composite explosives. Using the microfluidic platform, the optimal preparation conditions were screened efficiently. Additionally, narrow size-distributed nanoscale HNS and super-mixed HNS & HMX composite were prepared with production rate of about 45mg/min. This study demonstrates the feasibility of an efficient, low-cost and safe way to prepare and screen nanoscale single-compound and composite explosives. KEYWORDS: Microfluidic platform, Recrystallization growth kinetics, Screening and preparation, Nanoscale explosive, Composite explosive

1 Introduction Explosives, also called high-energy materials, are widely used in engineering blasting, space applications, explosive forming, and weapon systems, playing a key role in promoting the progress of human society.1, 2 Researches are currently directed 2/42


Page 2 of 42

Page 3 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


toward preparation of nanoscale explosives and composites explosives because of their appropriate sensitivities and excellent explosive properties.3, 4 Optimization for preparation of nanoscale explosives and composite explosives requires a large number of repeatable experiments. However, complex stepwise operation, waste of resources (time, reagent, equipment, and labor), poor reproducibility and potential danger make the existing methods not suitable for rapid preparation and screening of nanoscale and composite explosives. There are several ways for preparation of nanoscale explosives. Generally, sol-gel method5 can be used for preparation of nanoscale explosives, but the sol-gel process takes a long time and often takes days or weeks. Mechanical ball milling6, 7 and spray drying8, 9 are two rapid methods, and easy to achieve mass production. However, the large quantities consumption of explosives limits the application of mechanical ball milling in fundamental research and screening. Spray drying is easy to cause nozzle clogging and produce static electricity, which limits its use in explosives with large particle or high electrostatic sensitivity. Preparation of nanoscale explosives in batch reactors is time-consuming, and has poor size distribution with large batch-to-batch variation, which shows a negative impact on the functionalities as rapid preparation and screening method. Composite explosives can combine the advantages of two or more explosives by adjusting the proportion of different explosives. Composite explosives offer an easy way to achieve a fine balance between high detonation performance and low 3/42



Page 4 of 42

sensitivity. Composite explosives with excellent sensitivity and high energy can be obtained by compounding a high sensitivity explosive with a high energy explosive. Their current applications will mainly focus on the use of slapper detonators. Some researchers have compounded explosives for slapper detonator with high-energy explosives to produce composite explosives with moderate sensitivity and high energy output and easy to be initiated by short pulse. The uniformity of explosive component plays a significant role in properties of composite explosives. Composite explosives with high homogeneity have more stable sensitivity and explosion power, which makes the composite explosives have more stable detonation conditions in application. Conventionally, composite explosives can be obtained by mechanical mixing. The method is time-consuming, with poor mixing uniformity and poor repeatability. To obtain composite explosives with excellent properties using recrystallization method, a large number of preparation and screening experiments are required. Consequently, a new recrystallization platform with high mixing efficiency, fast preparation efficiency and less sample consumption is preferable for preparation and screening of nanoscale single-compound and composite explosives. In recent decades, explosives have been prepared and handled by batch reactors. The reaction parameters and environments of batch reactors are hard to control precisely down to nm and pL levels. Therefore, batch reactors have mediocre performance in obtaining desired products with controllable structures and properties.10,

11

Additionally, low mixing efficiency in batch reactors leads to variations in 4/42


Page 5 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature and concentration distribution, which challenges the reproducible preparation of explosives, and trends to produce products with poor size distribution. Furthermore, synthesis of explosives using batch reactors poses a potential safety hazard in operation process. In recent years, microreactors have been widely used for preparation of microscale and nanoscale particles.12-15 Continuous synthesis of explosives can be achieved with enhanced mixing efficiency and improved safety using microreactors.16-19 Microreactor needs small quantities of reagents, and the reaction parameters can be precisely controlled, which promotes the development of high throughput screening.20,

21

However, few studies focused on preparation and

screening of nanoscale single-compound explosives and composite explosives using microfluidic technology. Considering stepwise operation, resource consumption, reproducibility and safety, microfluidic technology offers a new feasibility for preparation and screening of nanoscale single-compound explosives and composite explosives. In this work, we propose a combinatorial microfluidic platform consisted of a microfluidic oscillator and a micro-chip reactor for preparation and screening of nanoscale single-compound and composite explosives. The microfluidic oscillator is based on our previous works

22-24

, which can convert a stable laminar flow to an

oscillatory flow. The micro-chip reactor with a circular chamber was designed to extend the applicable flow rate range of the microfluidic oscillator. The combination of the microfluidic oscillator and the micro-chip reactor is aim to enhance the mixing 5/42



efficiency during recrystallization process and improve the production rate of explosives. Recrystallization growth kinetics in microscale was studied in order to guide the preparation of nanoscale explosives. To verify the applicability of the proposed microfluidic platform, HNS (2,2’,4,4’,6,6’-hexanitrostibene) and HMX (cyclotetramethylenete tranitramine) as two typical explosives are used to prepare and screen nanoscale HNS particles and HNS & HMX composites.

2 Design and fabrication of the combinatorial microfluidic platform 2.1 Theoretical basis for design of the microfluidic platform The mixing performance of the combinatorial microfluidic platform was studied by numerical simulation using ANSYS FLUENT. Liquid water with density of 9.998×102kg/m3 and viscosity of 9×10-4kg/(m⋅s) was selected as the working fluid for mixing characterization because solvent and anti-solvent have the similar properties to water. The kinematic diffusivity of water was 2.0×10-9m2/s. All meshes in the simulations were composed of hexahedral cells. The diameter and depth of the chamber are 5mm and 0.5mm, respectively. The width of the inlet channels is 0.5mm and the width of the outlet channel is 1mm. The wider outlet channel is designed to avoid channel blockage. The flow rate of inlet-1 (Q1) was fixed at 1.8mL/min, which is suitable for the normal production of most explosives. The mass fraction of water at inlet-1 boundary was set as 0 and the corresponding color was blue. The flow rate of inlet-2 (Q2) was variable. The mass fraction at inlet-2 boundary was set as 1 and the corresponding color was red. The mixing performance can be represented by the 6/42


Page 6 of 42

Page 7 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mixing index A.

25

The value of A ranges from 0 for non-mixing to 1 for complete

mixing. The greater A value means the better mixing quality and any A value has a color corresponding to it. The value of σ can be calculated using Eq. (1). A = 1 − ∑Ni=1

Ci -Cmix Cunmix -Cmix

/N 2

(1)

Where N is the number of cells of the selected area, Ci is the concentration of one cell in the selected area, Cmix is the concentrateion after completely mixing and Cunmix is the concentration before mixing. The swirling micro-chip reactor with single chamber was simulated. Figure 1 shows the simulation results of the single-swirling micro-chip reactor using single-swirling flow and oscillatory-swirling flow at Q2/Q1=5/1 and t=350ms. At the time of 350ms, the single-swirling flow mixing as shown in Figure 1(a) is inhomogeneous, while the oscillatory-swirling flow mixing as shown in Figure 1(b) is almost completely mixed at the chamber and the outlet area. It is indicated that the mixing is clearly intensified through fluid perturbations and the mixing time is shortened. For the single-swirling flow, though swirling flows occur in the chamber, they are too weak to cause a substantial enhancement of the mixing. For the oscillatory-swirling flow, the stable laminar flow can be autonomously converted to an oscillatory flow, which can highly enhance the mixing efficiency. Therefore, the microfluidic platform will combine the single-swirling flow micro-chip reactor with an oscillatory flow to enhance the mixing performances.

7/42



Figure 1. Simulation results of (a) single-swirling flow and (b) oscillatory-swirling flow at Q2/Q1=5/1 and t=350ms.

The oscillatory mixing in a microchannel mainly attributes to the sequential fluid segmentation which shortens the axial mixing path. The oscillatory flow with higher frequency can generate shorter fluid segments, which contributes to improve the mixing efficiency and increase the mixing index. At a fixed frequency, a lower flow rate will generate shorter fluid segments. But at a lower flow rate, the yield will decrease. Therefore, the flow rate of oscillatory flow should be screened to make a fine balance between mixing efficiency and yield. In order to further study the mixing performance of the combinatorial microfluidic platform and screen flow rate of oscillatory flow, the theoretical calculations with the 8/42


Page 8 of 42

Page 9 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


flow rate ratio (Q2/Q1) of 1, 5 and 10 were conducted. The results are shown in Figure 2. The outlet area as the research object is used for study on mixing efficiency. A is calculated using Eq. (1). With the increase of time, the mixture in the outlet area becomes more and more uniform, and finally tends to a stable A value. The stability A value represents the mixing efficiency of the micro-chip reactor. It can be clearly seen that the oscillatory-swirling flow has a faster mixing speed and a higher mixing efficiency than the single-swirling flow at the same Q2/Q1. In addition, with the increase of Q2/Q1, the mixing speed becomes faster and the mixing efficiency becomes higher. As for single-swirling flow, A is below 0.8 even though Q2/Q1 is up to 10/1. The reactor volume calculated by structure size of microchip is about 0.11mL. Therefore, the residence times at Q2/Q1 of 1/1, 5/1 and 10/1 are 1833.3ms, 611.1ms and 333.3ms, respectively. As for oscillatory-swirling flow, just 350ms is needed for completely mixed at Q2/Q1=5/1 and A reaches 0.92; for Q2/Q1=10/1, just 200ms is needed and A reaches 0.95. It indicates that the fluids are almost fully mixed at Q2/Q1=5/1 and Q2/Q1=10/1 using oscillatory-swirling flow. These conditions with high mixing efficiency are likely to be beneficial for preparation of nanoscale explosives and composites explosives. Considering practical application, one mixing chamber can shorten the residence time of recrystallization, and it is enough for mixing of reagents used for preparation of explosives.

9/42



Page 10 of 42

Figure 2. The change with time of A of single-swirling flow and oscillatory-swirling flow at different Q2/Q1.

2.2 Construction of the combinatorial microfluidic platform

The microfluidic platform consists of a microfluidic oscillator and a single-swirling micro-chip reactor. Figure 3(a) shows the photograph of microfluidic oscillator. The functional part of the microfluidic oscillator is an elastic diaphragm held by a small chamber. The elastic diaphragm is made of a spring metal (Copper/Beryllium (Cu98/Be2) foil, Goodfellow, UK). The working principle of the oscillator is a negative-differential-resistance

mechanism.

The

elastic

diaphragm

produces

self-excited oscillations when driven by an incoming flow and above a critical pumping pressure, converting the steady flow to oscillatory flow. The microfluidic oscillator works in a passive way without resorting to actively-controlled elements although there is a moving part (i.e. the oscillating diaphragm). The averaged flow rate and oscillation frequency depends on its geometric structure, elasticity of the 10/42


Page 11 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


diaphragm, the fluid viscosity and the operating pressure. For more detailed description of the microfluidic oscillator, please refer to our previous articles 22-24. The fluids can be easily mixed in a microchannel using the microfluidic oscillator at high flow rates. However, higher flow rate means more energy consumption and more waste liquid production in the preparation process of explosive. In order to extend the applicable flow rate range, the single-swirling micro-chip reactor (Figure 3(b)) with a circular mixing chamber was designed in the microfluidic platform. Combined with the single-swirling micro-chip reactor, the microfluidic oscillator can work with enough mixing efficiency at a lower flow rate. Furthermore, the mixing efficiency can be further strengthened by generating single-swirling when the fluid inertial plays a role at moderate Reynolds numbers (Re=UL/ν, where U is the fluid velocity, L is the characteristic dimension of the channel, ν is the kinematic viscosity of the fluid). The micro-chip reactor is made of transparent PMMA. Conventional micro-milling and thermal bonding methods were employed to fabricate the micro-chip reactor. The micro-chip reactor consists of two inlet channels, one outlet channel, and one mixing chambers. The length, width and thickness of the micro-chip reactor are 65mm, 30mm and 2mm, respectively. The width of the inlet channels is 0.5mm and the width of the outlet channel is 1mm. The wider outlet channel is designed to avoid channel blockage. The diameter and depth of the chamber are 5mm and 0.5mm, respectively. Figure 3(c) displays the schematic of the microfluidic platform. The anti-solvent is 11/42



stored in a high-pressure container (Nordson Corp., USA). A compressed nitrogen source together with a pressure regulator is used to control the pumping pressure. Oscillatory flow is produced after the anti-solvent flowing out form the oscillator. For solvent, its flow rate is controlled using a syringe pump (YSP-101, YMC Co., Ltd.). During the recrystallization process, the experimental operations of the microfluidic platform are very simple, just needing to control the switches of the pressure regulator and the syringe pump. The solvent and anti-solvent come into contact and generate eddy currents rapidly in the single-swirling micro-chip reactor. Then, the mixture and the precipitated nanoparticles flow along an ETFE (ethylene-tetra-fluoro-ethylene) tube and are collected using a beaker. The whole experimental process is carried out at room temperature. To improve the purity of the products and reduce the influence of cleaning agents on the crystal morphology and particle size, the products are washed using deionized water for 3 times and then filtration. Finally nanoscale HNS particles are obtained after freeze-drying. In order to obtain the optimal experimental parameters for preparation of nanoscale and composite explosives, the influences of flow rate ratio, surfactant, solvent and concentration will be screened.

12/42


Page 12 of 42

Page 13 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Photographs of (a) the microfluidic oscillator and (b) the single-swirling micro-chip

reactor. (c) The schematic of the microfluidic platform. 2.3 Mechanism of fast recrystallization in microscale of nanoscale explosive

The recrystallization of explosive is a fast crystallization process, consisting of nucleation process (Figure 4(a)) and crystal growth process (Figure 4(b)). The diffusion of explosive molecules has great influence on the nucleation process and crystal growth process. Under the condition of microscale, the transport rate of explosive molecules can be easily improved by increasing the mixing efficiency. In order to study the recrystallization growth kinetics of explosives in microscale, the relationship between crystal growth rate and microscale mixing efficiency was established. 13/42



Figure 4. (a) Nucleation process and (b) crystal growth process of explosive

recrystallization. (c) Saturation states of explosive solution. C is the concentration of explosive in solvent; S is the steady zone; M1 is the first metastable zone; M2 is the second metastable zone; L is the unstable zone. (d) The relationship between particle size of nanoscale explosive and mixing index of the microfluidic platform. The nucleation process can be divided into two steps: one is diffusion of explosive molecules; the other is nucleation formation. There are three states (Figure 4(c)) of 14/42


Page 14 of 42

Page 15 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


explosive solution for recrystallization, namely steady zone (S), metastable zone (M, including first metastable zone (M1) and second metastable zone (M2)) and unstable zone (L). At S, the solution of explosive is not saturated yet, and there is no possibility of crystallization. At L, the solution is supersaturated and extremely unstable, and spontaneous nucleation occurs immediately. At M1, the nuclei will not form spontaneously, but when the seeds are added the crystals will grow on the seeds. At M2, the solution can spontaneously nucleate, but the nucleation does not occur

immediately, requiring a certain delay time interval. High mixing efficiency contributes to fast diffusion and collision of the explosive molecules, which is beneficial to the formation of nucleation. Therefore, the higher mixing efficiency in the microfluidic platform can promote the formation of nucleation. In the nucleation stage of the explosive crystal, nucleation rate equation can be expressed in the form of Arrhenius reaction rate equation (see Eq. (2)). J = A' exp[-

16πσ3 υ2 3K3 T3 (lns)2

]

(2)

Where J is nucleation rate; A' is the frequency factor; σ is the surface tension of solid liquid two phase; υ is the volume of the explosive molecule; K is the reaction rate constant; T is the temperature; s is the supersaturation. Surface tension can be calculated by Mersmann’ s method26 using Eq. (3). σ=0.414kT(ρc NA /M)2/3ln(

ρc ceq

)

(3)

Where k is Boltzmann constant; ρc is crystal density; NA is Avogadro's number; M is molar mass of crystals; ceq is solubility of crystal. 15/42



Page 16 of 42

Saturation can be calculated by Eq. (4). s=(C-C0 )/C0

(4)

Where C is concentration of explosive in solvent; C0 is saturation concentration. Faster nucleation rate helps reduce the size of explosives. According to Eq. (2), the factors affecting the nucleation rate are σ, υ, T and s. For recrystallization system of explosive, υ is a determined value. Therefore, the nucleation rate can be improved by adjusting the value of σ, T and s. To obtain nanoscale explosive with finer size, σ can be reduced by selection of suitable solvents and surfactants; T can be improved properly (high temperatures can lead to a decline in production); s can be increased by increasing the concentration of explosives in the solvent. The selection of optimal recrystallization conditions requires a large number of screening experiments. Besides the nucleation rate, the final size of the recrystallized nanoscale explosive is also affected by the crystal growth rate. The growth process of explosive crystals can also be divided into two steps: the first is the diffusion process of explosive molecules toward the crystal nucleus; the second is the recrystallization of explosive molecules on the surface of the crystal nucleus. The diffusion can be indicated as Eq. (5), and the recrystallization can be indicated as Eq. (6). dn dt dn dt

=-

4πrr+δ)D δ

(C-C' )

(5)

= k' 4πr2 (C' -C* )

(6)

Where n is amount of substance in time t; D is the diffusion coefficient of explosive 16/42


Page 17 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


molecular; δ is the thickness of the retention layer; r is the radius of the nucleus; C is the concentration of explosive molecules in the liquid phase; C' is the interface concentration; k' is the precipitation reaction coefficient; C* is the saturation concentration. The thickness of the retention layer can be calculated by Eq. (7). δ=k''D1/3 γ1/6 ω-1/2

(7)

Where k’’ is detention layer coefficient; γ is kinematic viscosity coefficient; ω is crystal speed. Generally, the recrystallization of the explosive is very fast. The growth rate of crystal is determined by the diffusion process. Thus the growth rate of the crystal can be indicated as Eq. (8). dn dt

=

4πrr+δ)D δ

(C-C* )

(8)

In the microfluidic platform, the diffusion includes convection diffusion and free diffusion. In microscale, the strength of free diffusion is very weak. Therefore, the diffusion in the platform can be regarded as the diffusion caused by oscillatory-swirling flow. The diffusion can be indicated by the mixing index as Eq. (9). dn dt

=

V dc

M

V

=(1-A) M C0

(9)

Where A is the mixing index showing in Eq. (1); V is the volume of solvent; M is the relative molecular mass of explosive molecules, C0 is initial concentration of explosives. 17/42



Page 18 of 42

Based on Eq. (8) and Eq. (9), the relationship between diffusion rate of the explosive molecules caused by the microfluidic platform and crystal growth rate of the recrystallized nanoscale explosives can be built (Eq. (10)). Then, we obtain the formula between final size of explosive particles and mixing performance of the microfluidic platform (Eq. (11)). It can be seen from Eq. (11) that higher mixing index can promote crystal growth. V

(1-A) C0 = M r=

4πrr+δ)D δ

(1-A)Vδ *

4πM(C-C )

−

(C-C* )

-

(10)

(11)

2

The nucleation rate and growth rate determine the final size of the explosive. Higher mixing efficiency promotes both nucleation and crystal growth, which means there is a critical mixing index (AC) corresponding to minimize particle size (dmin) of the explosive. The relationship between particle size (d) and A of the explosive is displayed in Figure 4(d). When A < AC, strengthening the mixing efficiency is beneficial to reducing the size of explosive; when A = AC, explosive with optimal size can be obtained; when A > AC, strengthening the mixing efficiency is not beneficial to reducing the size of explosive. Therefore, nanoscale explosives with the optimum size can be prepared by the microfluidic platform with proper mixing efficiency.

3 Feasibility for preparation and screening of nanoscale explosives and composite explosives 3.1 Screening of flow rate ratios

In order to determine the appropriate velocity ratio, the influence of flow rate ratios 18/42


Page 19 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of anti-solvent to solvent (Q2/Q1) on nanoscale HNS preparation using the microfluidic platform was studied. The concentration of HNS was fixed at 28g/L and the flow rate of HNS/DMF solution (HNS solved in DMF) was fixed at 1.8mL/min. Deionized water was used as anti-solvent. OP-10 with the concentration of 0.5% was used as surfactant. All waste liquid was recycled because of security concerns. Three experiments with different Q2/Q1 (1/1, 5/1 and 10/1) were conducted using the microfluidic platform through anti-solvent recrystallization method. The yields of nanoscale HNS prepared at Q2/Q1 of 1/1, 5/1 and 10/1 were 95.6%, 93.1% and 91.7%, respectively. The crystal morphology was characterized by scanning electron microscope (SEM, Zeiss Merlin at 10kV). Figure 5 shows the crystal morphology of nanoscale HNS produced using the platform with different Q2/Q1. The crystal morphology of nanoscale HNS produced at Q2/Q1=1/1 is flaky and the particle size ranges from around 100nm to 1000nm. When Q2/Q1=5/1, the crystal morphology becomes ellipsoidal because the length and width of the flaky crystals get smaller. The particle size becomes smaller and the particle size distribution (PSD) becomes narrower compared with Q2/Q1=1/1. Furthermore, the particle size ranges from around 200nm to 500nm. When Q2/Q1=10/1, the crystal morphology is also ellipsoidal and the morphology of the nanoscale HNS crystal becomes more uniform. Though the HNS particles intend to agglomerate, the size of individual HNS particle is in the level of several hundred nanometers and the particle size ranges from around 150nm to 450nm. 19/42



Compared with Q2/Q1=1/1 and Q2/Q1=5/1, the SEM images show that nanoscale HNS prepared at Q2/Q1=10/1 has the most homogeneous crystal morphology, the most minimum particle size and the narrowest PSD. The reason for the crystal morphology becoming homogeneous is the enhancement of mixing efficiency with the increase of Q2/Q1.

Figure 5. SEM images of nanoscale HNS with different flow rate ratios. (a and b) Q2/Q1 =1/1; (c and d) Q2/Q1 =5/1; (e and f) Q2/Q1 =10/1.

The corresponding PSD results of nanoscale HNS prepared at these three flow rate ratios are shown in Figure 6. The PSD of nanoscale HNS were tested by particle size analyzer (Zetasizer Nano ZS9, Malvern). The tested particle size is the equivalent diameter of the particles. For Q2/Q1 =1/1, the particles are highly polydisperse with the size varying from 122.4nm to 1281.0nm. For Q2/Q1 =5/1, the size of HNS particles varies from 220.2nm to 531.2nm. For Q2/Q1 =10/1, the size of HNS particles varies from 164.2nm to 458.7nm. The mean deviations of nanoscale HNS obtained at 20/42


Page 20 of 42

Page 21 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Q2/Q1 of 1/1, 5/1 and 10/1 are 1158.6, 331.0 and 296.3nm, respectively. The

relationship between flow rate ratio and particle size of nanoscale HNS are shown in Figure 6(d). It is can be clearly seen that the average particle size reduces and PSD becomes narrow with the increase of flow rate ratio. It indicates that the PSD results are in accordance with the SEM data. With the increasing of Q2, the residence time is shortened, which means growth period is shortened and the yield will be reduced. Therefore, the particle size of HNS will be reduced. Furthermore, the particle size of HNS has a sharp reduction when Q2/Q1 changes from 1/1 to 5/1. The particle size of HNS reduces slower when Q2/Q1 changes from 5/1 to 10/1. The higher Q2/Q1 has been proved with high mixing efficiency, and the high mixing efficiency contributes to nanoscale HNS preparation. It also indicates that particle size of nanoscale HNS can be controlled by flow rate ratio using the microfluidic platform. Considering energy consumption and waste liquid production, it is a good choice for nanoscale HNS preparation to use 5/1 as the most suitable flow rate ratio.

21/42



Figure 6. Particle size distribution of nanoscale HNS prepared at (a) Q2/Q1 =1/1, (b) Q2/Q1 =5/1 and (c) Q2/Q1 =10/1. (d) The relationship between flow rate ratio and particle size of nanoscale HNS.

22/42


Page 22 of 42

Page 23 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanoscale HNS particles with three different size prepared at Q2/Q1=1/1 (585.9nm), Q2/Q1=5/1 (382.8nm) and Q2/Q1=10/1 (274.0nm) are used to study the relationship

between particle size and heat output as well as apparent activation energy (Ea). The thermal decomposition process of nanoscale HNS particles prepared at different flow rate ratio under the condition of flowing N2 gas was carried by differential scanning calorimeter (DSC, DSC-823e, Mettler Toledo). Ea was calculated using Kissinger’s method (Eq. (12)).27 ln 2 =ln - a E RT β

TP

AR a

E

(12)

P

Where β is the heating rate; Tp is the peak temperature of thermal decomposition curve; A is the preexponential factor; R is the gas constant 8.314 J·mol-1·k-1. Figure 7 presents the DSC curves and the relationships. The nanoscale HNS with size of 382.8nm has the maximum heat releases compared with those of 274.0nm and 585.9nm whatever the heat flow rate is 5°C/min, 10°C/min, 15°C/min or 20°C/min. Furthermore, with the increase of particle size of nanoscale HNS, Ea reduces linearly. The linear relationship between particle size and Ea of nanoscale HNS is Ea =201.6-0.0334r (where r represents particle size of nanoscale HNS, 274.0nm< r < 585.9nm, R-square is 0.9919). The results show that the thermal decomposition property of HNS is seriously affected by the particle size. The nanoscale HNS obtained at Q2/Q1 =5/1 has the maximum heat release and Ea. Therefore, 5/1 was selected as the optimal Q2/Q1.

23/42



Figure 7. DSC curves of nanoscale HNS with flow rate ratio of (a) Q2/Q1 =1/1, (b) Q2/Q1 =5/1 and (c) Q2/Q1 =10/1. (d) The relationship between particle size of nanoscale HNS and heat output as 24/42


Page 24 of 42

Page 25 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


well as apparent activation energy.

3.2 Screening of surfactant, solvent and concentration

Surfactant, concentration and solvent have great influences on the particle size and morphology of recrystallized HNS. To obtain the optimal nanoscale HNS, the microfluidic platform was used for screening of experimental conditions. Figure 8 exhibits the crystal morphology and PSD of recrystallized HNS obtained by the microfluidic platform. In present study, OP-10 (alkylphenolspolyoxyethylene) and CTAC (hexadecyltrimethyl ammonium chloride) were used to tune the crystal morphology of HNS. The crystal morphology of HNS recrystallized using OP-10 (Figure 8(a)) as surfactant is flaky, while the shape of HNS recrystallized using CTAC (Figure 8(e)) as surfactant is ellipsoid under the situation of other conditions being equal. From PSD data, the particle size of HNS obtained using these two surfactants were both nanoscale, but the particle size of recrystallized HNS using CTAC (Figure 8(f)) is much smaller and the size distribution is much narrower than OP-10 (Figure 8(b)). For OP-10, the particle size ranges from 190.1nm to 531.2nm and the nanoscale HNS has a little agglomeration. For CTAC, the particle size ranges from 182nm to 346nm. CTAC has a better effect on reducing the size of HNS, because CTAC as a cationic surfactant can significantly reduce the surface tension between solvent and the recrystallized nanoscale HNS. It means that CTAC is better than OP-10 for nanoscale HNS preparation considering morphology, PSD and agglomeration of nanoscale HNS.

25/42



Figure 8. Scanning electron microscopy and particle size distribution of HNS recrystallized with 26/42


Page 26 of 42

Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surfactant, concentration and solvent of (a and b) OP-10, 28g/L and DMSO; (c and d) CTAC, 14g/L and DMSO; (e and f) CTAC, 28g/L and DMSO; (g and h) CTAC, 56g/L and DMSO; (I and j) CTAC, 28g/L and DMF.

Furthermore, the crystal morphology of nanoscale HNS obtained with different concentrations of HNS solved in DMSO, such as 14g/L, 28g/L and 56g/L, were compared. Under the same conditions, 14g/L (Figure 8(c)), 28g/L (Figure 8(e)) and 56g/L (Figure 8(g)) have the same morphology (ellipsoidal crystal), but the particle size and PSD differ. For 14g/L (Figure 8(d)), the particle size ranges from 141.8nm to 531.2nm. For 28g/L (Figure 8(f)), it is 182nm to 346nm. For 56g/L (Figure 8(h)), it is 122.4nm to 162.4nm. The average particle size of 14g/L, 28g/L and 56g/L HNS are 307.6nm, 265.2nm and 158.1nm, respectively. It is obvious that the PSD became narrower and the average particle size became smaller with the increase of concentration of HNS. Moreover, the yields of nanoscale HNS prepared under these three concentrations were 91.3%, 93.8% and 95.0%, respectively, which are all higher than 90%. Too many use of raw materials can easily lead to waste of resources and environmental pollution. Therefore, 28g/L is the most suitable for screening conditions other than concentration. In addition, the effect of DMSO and DMF on crystal morphology and particle size of the recrystallized HNS has been studied under the same conditions. The concentration of HNS in DMF is 28g/L. For DMSO, the morphology is ellipsoidal (Figure 8(e)), the average particle size of nanoscale HNS is 265.2nm and the size ranges from 182nm to 346nm (Figure 8(f)). For DMF, the morphology of nanoscale 27/42



HNS is flaky (Figure 8(i)), the average particle size is 435.7nm and the size ranges from 220.3nm to 615.1nm (Figure 8(j)). Compared with DMF, DMSO is the better solvent considering morphology, average particle size and PSD. Consequently, the optimal solvent for nanoscale HNS preparation is DMSO. HNS with higher heat release and higher decomposition may have the better energy output and higher thermal stability. The thermal analysis process can be used for screening for HNS with good thermal decomposition performance. Therefore, the thermal decomposition performance of the nanoscale HNS was studied. Figure 9 displays the DSC curves of the nanoscale HNS prepared at different surfactants, solvents and concentrations. Each DSC curve of HNS has one endothermic peak and one exothermic peak. The endothermic peak is caused by melting of HNS. The exothermic peak is caused by decomposition of HNS. Comparing the DSC results, the heat release of recrystallized HNS using CTAC as surfactant is 162.4J/g higher than that of OP-10. The exothermic peaks of OP-10 and CTAC samples are observed 347.5 and 350.3°C, respectively. The heat releases of 14g/L, 28g/L and 56g/L HNS were 1892.5, 1928.7 and 1661.2J/g, respectively. The recrystallized HNS with the concentration of 28g/L had maximum heat release than 14g/L and 56g/L. They had little difference in decomposition temperature. Additionally, the comparison between the two solvents DMSO and DMF indicated that using DMSO as the solvent had 336.9J/g more heat than that achieved using DMF. The DSC results show that the recrystallization of HNS exhibits good compatibility with CTAC as surfactant and 28/42


Page 28 of 42

Page 29 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DMSO as solvent. Therefore, CTAC, DMSO and 28g/L as well as Q2/Q1 =5/1 are screened as the optimal conditions for nanoscale HNS preparation.

Figure 9. DSC measured curves for recrystallized HNS with surfactant, concentration and solvent

of (a) OP-10, 28g/L and DMSO; (b) CTAC, 14g/L and DMSO; (c) CTAC, 28g/L and DMSO; (d) CTAC, 56g/L and DMSO; (e) CTAC, 28g/L and DMF.

Using the screened optimal conditions, the production rate is up to 45.36mg/min (calculated by flow rate of 1.8 mL/min, concentration of 28g/L and yield of 90%) which means the preparation and screening of the microfluidic platform is very fast. 10mg is enough for basic tests of the explosive sample, screening speed of the microfluidic platform is about 4 samples per minute.

29/42



Page 30 of 42

3.3 Preparation of narrow size distributed nanoscale HNS

Nanoscale HNS was prepared using the screened optimal experimental conditions to verify the superiority of the microfluidic platform for nanoscale explosive preparation. The performances of the nanoscale HNS was compared with the raw HNS. The comparison data of crystal morphology, particle size and thermal decomposition performance are shown in Table 1. Table 1.The comparison between nanoscale HNS and raw HNS Sample

nanoscale HNS

Raw HNS

Crystal morphology

ellipsoidal

needle-like

Average particle size/µm

0.2652

10.4

Size range/µm

0.182~0.346

0.9~40.1

Endothermic peak temperature/°C

318.5

318.8

Heat absorption/J⋅g-1

57.4

86.6

Exothermic peak temperature/°C

350.8

353.0

Heat release/J⋅g-1

1928.7

2015.3

The morphology and PSD of raw HNS and nanoscale HNS are shown in Figure 10. It can be clearly seen that the morphology of raw HNS is needle-like and the particle size is with the diameter about 8µm and length about 50µm. The morphology of the nanoscale HNS is ellipsoidal and the particle size is about 200nm. The average particle size of raw HNS is 10.4µm and the particle size ranges from 0.9µm to 45µm, 30/42


Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


while the average particle size of the nanoscale HNS is 265.2nm and the size ranges from 182nm to 346nm. The particle size of recrystallized HNS is much smaller than raw HNS, and the PSD is much narrower.

Figure 10 SEM images (a) and PSD (b) of the raw HNS; SEM images (c) and PSD (d) of the nanoscale HNS.

The thermal analysis data of raw HNS and nanoscale HNS are displayed in Figure 11(a) and Figure 11(b), respectively. Both the HNS particles experience the process of melting and thermal decomposition with the increase of temperature. At the heating rate, the temperatures of molten endothermic peak (Tm) and thermal decomposition exothermic peak (Tp) of nanoscale HNS particles are a little lower than raw HNS. At 5°C /min, the Tm of nanoscale HNS is 317.9°C, which is 0.7°C lower than that of raw HNS (i.e, ∆Tm=-0.7°C). For Tp, it is 340.9°C, 0.9°C lower than that of raw HNS (i.e, ∆Tp=-0.9°C). At 10°C /min, ∆Tm=-0.3°C, ∆Tp=-2.2°C. At 15°C /min, ∆Tm=-0.6°C, 31/42



∆Tp=-1.9°C. At 20°C /min, ∆Tm=-0.2°C, ∆Tp=-1.0°C. That is because nanoscale HNS particle has a smaller size and a larger specific surface area. Therefore, the heat transfer of nanoscale HNS is more efficient than that of raw HNS.

Figure 11. DSC curves with flow rate of 5°C /min, 10°C /min, 15°C /min and 20°C /min of the raw HNS(a) and the nanoscale HNS (b). (c) Kissinger’s plot of raw HNS and nanoscale HNS.

The apparent activation energy of the raw HNS and the nanoscale HNS were calculated using Eq. (9). Linear regression analysis of ln(β/Tp2) on 1/TP was carried 32/42


Page 32 of 42

Page 33 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


out showing in Figure 11(c). The slope of the line was (-Ea/R) and the intercept of the line on the ordinate axis was ln(AR/Ea). Finally, the value of Ea was calculated using the fitted data. Ea of the raw HNS and the nanoscale HNS are 218.8kJ/mol and 191.1kJ/mol, respectively, which means the Ea of the nanoscale HNS is 27.7 kJ/mol (or 12.4%) lower than that of raw HNS. That is to say, the nanoscale HNS has higher reactive activity than the raw HNS. The nanoscale HNS prepared using the microfluidic platform has better crystal morphology, narrower PSD and lower Ea, which proves that the microfluidic platform has advantages in the preparation of nanoscale explosives. 3.5 Preparation of super-mixed HNS & HMX composite explosives

In order to verify the feasibility of preparing and screening composite explosives by the microfluidic platform, HNS and HMX were used to prepare super-mixed composite explosive. 5 proportions of HNS and HMX with total mass of 200mg were solved in 10mL DMSO, respectively. CTAC with concentration of 0.5% was used as surfactant and deionized water was used as anti-solvent. The concentrations of HNS in DMSO are 18g/L, 16g/L, 14g/L, 12g/L and 10g/L with the corresponding HMX concentrations of 2g/L, 4g/L, 6g/L, 8g/L and 10g/L, respectively. The solvent is driven by the injection pump with flowing rate of 1.1mL/min. The anti-solvent is driven by the N2 pneumatic pump combined with oscillatory-swirling flow and the flowing rate is 16.5mL/min. The morphology of the HNS & HMX and the raw material of HMX are shown in Figure 12(a and b) and Figure 12(c), respectively. The 33/42



raw HMX has the appearance of columnar with size of approximately 400µm. After anti-solvent recrystallization, the sizes of HNS and HMX reduce to 200nm and 20µm, respectively. Furthermore, the nanoscale HNS particles are evenly coated on the surface of HMX. FIIR spectroscopy (NICOLET IS10, Thermo-Nocilet) was used to study the surface structure of the obtained composites. Figure 12(d) shows the FIIR spectra of HNS, HMX and HNS & HMX composite. The characteristic peaks of HNS are 3100.0cm-1 and 1601.6cm-1, while the characteristic peaks of HMX are 3050.3cm-1 and 601.2cm-1. Both the featured positions of HNS and HMX are found in the spectrum of the obtained HNS & HMX composite. To further confirm the relationship between the two individual units, XRD patterns (Bruker D8-advanced diffractometerequipped with a Cu-Kα X-ray source operating at 40 kV and40 Ma) of the above samples were obtained. From the XRD patterns shown in Figure 12(e), the feature peaks of HNS & HMX includes those of HNS and HMX, but the intensity of some peaks are different, which indicates that the morphology and size of HNS & HMX change after recrystallization. Combining the SEM, FIIR and XRD data, it is demonstrated that HNS & HMX composites are obtained using the microfluidic platform.

34/42


Page 34 of 42

Page 35 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12. (a and b) SEM images of recrystallized HNS & HMX composite, (c) SEM image of the raw HMX, (d) FIIR patterns and (e) XRD patterns of HNS, HMX and HNS & HMX composite.

Thermal decomposition properties of the HNS & HMX composites recrystallized by the microfluidic platform and prepared by mechanical mixing are compared. To illustrate the uniformity and repeatability of the composite explosive, 4 groups of repeated DCS experiments were carried out. Smaller deviation of the DSC results means more homogeneity of HNS and HMX composite. These two composites are randomly selected for four 0.45mg samples. The four samples were tested with the flow rate of 10°C/min under the condition of flowing N2. Figure 13(a) and Figure 13(b) show the thermal analysis data of the recrystallized and mechanical mixing HNS & HMX, respectively. It can be clearly seen that the DSC curves of these HNS & HMX

35/42



composites have two exothermic peaks. The first exothermic reaction belongs to the exothermic decomposition of HNS and the second belongs to HMX. Additionally, the four samples of recrystallized HNS & HMX composites have low deviation in initial exothermic temperature and heat release than those of physical mixing HNS & HMX composite. Figure 13(c) displays the comparison between initial exothermic temperature and heat release ranges of recrystallized and physical mixing HNS & HMX. For the recrystallized HNS & HMX composite, the ranges of initial exothermic temperature and heat release of first peak are 0.35°C and 95.65mW/mg, and those of second peak are 0.58°C and 53.25mW/mg. For mechanical mixing HNS & HMX, those are 2.69°C, 582.08mW/mg, 3.29°C and 556.05mW/mg, respectively. These results indicate that the degree of mixing of the recrystallized HNS & HMX composites are much better than that of mechanical mixing.

36/42


Page 36 of 42

Page 37 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13. (a) DSC curves of the recrystallized HNS & HMX composites with 4 samples, (b) DSC curves of the mechanical mixing HNS & HMX with 4 samples, (c) comparison between the recrystallized HNS & HMX and the mechanical mixing HNS & HMX with thermal decomposition performance.

Combustion heats of recrystallized and physical mixing HNS & HMX with different content of HMX were tested using the oxygen bomb calorimeter (ZDHW-2, 37/42



Hebishi Longhua Instrument Sales Co. Ltd). The mass of each sample is 150mg and tested for three times. Figure 14 displays the average combustion heats and the corresponding error range. The two samples at no HMX content are different. The physical mixing one with no HMX is the raw HNS. The recrystallization one with no HMX is HNS. The purity of HNS has been improved after solvent-antisolvent recrystallization. Therefore, the recrystallized HNS has the higher combustion heat than the raw HNS. With the increase of HMX content, the combustion heat of the composite explosive decreases gradually, because the combustion heat of HMX is lower than that of HNS. Compared with the physical mixing HNS & HMX, the recrystallized HNS & HMX has higher combustion heat and narrower error range. The higher combustion heat is due to higher purity of the recrystallized HNS & HMX. The narrower error range can further prove the super homogeneity of the recrystallized HNS & HMX composites. Therefore, the microfluidic platform can be used not only for the preparation and screening of nanoscale explosives, but also for the preparation and screening of super-mixed explosives.

38/42


Page 38 of 42

Page 39 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 14. Combustion heats of recrystallization and physical mixing HNS & HMX with 0, 10%, 20%, 30%, 40% and 50% of HMX.

4 Conclusion A microfluidic platform for preparation and screening of nanoscale and composite explosives was developed by combining a single-swirling micro-chip reactor with a microfluidic oscillator. The platform has good mixing performance and the applicable flow rate range can be extend at moderate low flow rate to prepare explosives. Furthermore, the recrystallization growth kinetics of explosives prepared under microscale was studied. Using the platform, approximately 45mg nanoscale HNS can be prepared in one minute and the optimal preparation conditions of nanoscale HNS can be quickly screened with less sample consumption, good reproducibility and safe operation. The prepared nanoscale HNS has good crystal morphology, narrow particle size distribution and low apparent activation energy. Furthermore, the super-mixed HNS & HMX composite prepared using the platform has more uniform distribution of components, higher repeatability and higher combustion heat. In conclusion, the 39/42



microfluidic platform can be used for the preparation and screening of various nanoscale single-compound explosives and composite explosives. For higher throughput screening, it can be realized by parallel connection of the platform. Through parallel connection, the production of small quantities of high quality nanoscale explosives and composite explosives can also be achieved. All the findings reported provide useful references for proper preparation and screening of other nanoscale and composite materials.

Acknowledgement This work was supported by the Fundamental Research Funds for the Central Universities of China (No. 30916012101) and the National Natural Science Foundation of China (No. 51575282).

Reference 1. Zhang, C.; Sun, C.; Hu, B.; Yu, C.; Lu, M., Synthesis and characterization of the pentazolate anion cyclo-N5- in (N5)6(H3O)3(NH4)4Cl. Science 2017, 355, (6323), 374. 2. Xu, Y.; Wang, Q.; Shen, C.; Lin, Q.; Wang, P.; Lu, M., A series of energetic metal pentazolate hydrates. Nature 2017, 549, (7670), 78-81. 3. Spitzer, D.; Comet, M.; Baras, C.; Pichot, V.; Piazzon, N., Energetic nano-materials: Opportunities for enhanced performances. J. Phys. Chem. Solids 2010, 71, (2), 100-108. 4. Rossi, C., Two Decades of Research on Nano‐Energetic Materials. Propellants, Explos., Pyrotech. 2014, 39, (3), 323-327. 5. Tillotson, T. M.; Gash, A. E.; Simpson, R. L.; Hrubesh, L. W.; Jr, J. H. S.; Poco, J. F., Nanostructured energetic materials using sol–gel methodologies. J. Non-Cryst. Solids 2001, 285, (1), 338-345. 6. Guo, X.; Ouyang, G.; Liu, J.; Li, Q.; Wang, L.; Gu, Z.; Li, F., Massive Preparation of Reduced-Sensitivity Nano CL-20 and Its Characterization. J. Energ. Mater. 2015, 33, (1), 24-33. 7. An, C.; Xu, S.; Zhang, Y.; Ye, B.; Geng, X.; Wang, J., Nano-HNS Particles: Mechanochemical Preparation and Properties Investigation. J. Nanomater. 2018, 2018, (4), 1-7. 8. Risse, B.; Spitzer, D.; Hassler, D.; Schnell, F.; Comet, M.; Pichot, V.; Muhr, H., Continuous 40/42


Page 40 of 42

Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formation of submicron energetic particles by the flash-evaporation technique. Chem. Eng. J. 2012, 203, (5), 158-165. 9. Wang, J.; Li, J.; An, C.; Hou, C.; Xu, W.; Li, X., Study on Ultrasound- and Spray-Assisted Precipitation of CL-20. Propellants, Explos., Pyrotech. 2012, 37, (6), 670–675. 10. Emory M. Chan; Richard A. Mathies, a.; A. Paul Alivisatos, ‡, Size-Controlled Growth of CdSe Nanocrystals in Microfluidic Reactors. Nano Lett. 2003, 3, (2), 199-201. 11. Visaveliya, N.; Hoffmann, C.; Gross, A.; Taeuscher, E.; Ritter, U.; Koehler, J. M., Micro-flow assisted synthesis of fluorescent polymer nanoparticles with tuned size and surface properties. Nanotechnol. Rev. 2016, 5, (2), 259-272. 12. Knauer, A.; Schneider, S.; Moeller, F.; Csaki, A.; Fritzsche, W.; Koehler, J. M., Screening of plasmonic properties of composed metal nanoparticles by combinatorial synthesis in micro-fluid segment sequences. Chem. Eng. J. 2013, 227, 80-89. 13. Zhang, T.; Zhang, X.; Yan, X.; Kong, L.; Zhang, G.; Liu, H.; Qiu, J.; Yeung, K. L., Synthesis of Fe3O4@ZIF-8 magnetic core-shell microspheres and their potential application in a capillary microreactor. Chem. Eng. J. 2013, 228, 398-404. 14. Zhang, L.; Feng, Q.; Wang, J.; Sun, J.; Shi, X.; Jiang, X., Microfluidic synthesis of rigid nanovesicles for hydrophilic reagents delivery. Angew. Chem., Int. Ed. 2015, 54, (13), 3952. 15. Liu, Y.; Jiang, X., Why microfluidics? Merits and trends in chemical synthesis. Lab Chip 2017, 17, (23), 3960. 16. Delville, M. M. E.; Nieuwland, P. J.; Janssen, P.; Koch, K.; van Hest, J. C. M.; Rutjes, F. P. J. T., Continuous flow azide formation: Optimization and scale-up. Chem. Eng. J. 2011, 167, (2-3), 556-559. 17. Vaccaro, L.; Lanari, D.; Marrocchi, A.; Strappaveccia, G., Flow approaches towards sustainability. Green Chem. 2014, 16, (8), 3680-3704. 18. Movsisyan, M.; Delbeke, E. I.; Berton, J. K.; Battilocchio, C.; Ley, S. V.; Stevens, C. V., Taming hazardous chemistry by continuous flow technology. Chem. Soc. Rev. 2016, 45, (18), 4892-4928. 19. Singh, R.; Lee, H.-J.; Singh, A. K.; Kim, D.-P., Recent advances for serial processes of hazardous chemicals in fully integrated microfluidic systems. Korean J. Chem. Eng. 2016, 33, (8), 2253-2267. 20. Agresti, J. J.; Antipov, E.; Abate, A. R.; Ahn, K.; Rowat, A. C.; Baret, J. C.; Marquez, M.; Klibanov, A. M.; Griffiths, A. D.; Weitz, D. A., Ultrahigh-throughput screening in drop-based microfluidics for directed evolution. oc. Natl. Acad. Sci. U. S. A. 2010, 107, (9), 4004. 21. Liu, Y.; Du, J.; Choi, J. S.; Chen, K. J.; Hou, S.; Yan, M.; Lin, W. Y.; Chen, K. S.; Ro, T.; Lipshutz, G. S., A High-Throughput Platform for Formulating and Screening Multifunctional Nanoparticles Capable of Simultaneous Delivery of Genes and Transcription Factors. Angew. Chem. 2016, 55, (1), 169. 22. Xia, H. M.; Wang, Z. P.; Fan, W.; Wijaya, A.; Wang, W.; Wang, Z. F., Converting steady laminar flow to oscillatory flow through a hydroelasticity approach at microscales. Lab Chip 2012, 12, (1), 60-4. 23. Xia, H. M.; Wang, Z. P.; Nguyen, V. B.; Ng, S. H.; Wang, W.; Leong, F. Y.; Le, D. V., Analyzing the transition pressure and viscosity limit of a hydroelastic microfluidic oscillator. Appl. 41/42



Phys. Lett. 2014, 104, (2), 491. 24. Xia, H. M.; Wu, J. W.; Wang, Z. P., The negative-differential-resistance (NDR) mechanism of a hydroelastic microfluidic oscillator. . Micromech. Microeng. 2017, 27, (7). 25. Lin, Y., Numerical characterization of simple three-dimensional chaotic micromixers. Chem. Eng. J. 2015, 277, 303-311. 26. Mersmann, A., Calculation of interfacial tensions. . Cryst. Growth 1990, 102, (4), 841-847. 27. Kissinger, H. E., Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 1957, 29, (11), 1702-1706.

42/42


Page 42 of 42

Microfluidic Platform for Preparation and Screening of Narrow Size

Recommend Documents