Application of microfluidics in the preparation of energy-containing materials

Microfluidic technology has been widely used in the fields of medical testing, fine chemicals synthesis, nano-functional materials preparation and drug synthesis, and the technology is quite mature. In view of the many advantages of microfluidic technology in material preparation, more and more studies have begun to explore its application in the synthesis of energy-containing compounds, recrystallization modification of monolithic energy-containing materials, and preparation of composite energy-containing materials.

1. Synthesis of energy-containing compounds using microfluidics

The synthesis of energy-containing materials using microfluidics has significant advantages. First, it greatly improves the efficiency. In conventional experiments, the mixing of reactants usually relies on stirring, which takes a long time to achieve the desired homogeneity.

The microfluidic system, however, realizes rapid and efficient mixing through laminar shear, distributed mixing, extended flow and molecular diffusion mechanisms. Microfluidic technology not only can precisely control the process parameters, but also requires less time for each experiment, making the screening of process conditions more rapid and effective.

The mixing efficiency varies for different structures of microfluidic chips. Due to the small diameter of the microfluidic channel, the fluid always maintains laminar flow in the linear channel, and the efficiency of two-phase mixing relying only on laminar shear and molecular diffusion is low.

In order to improve mixing efficiency, three design concepts are usually used: first, the use of special geometries such as bends and corners; second, the installation of obstacles in the channel; and third, the structural design to achieve the diversion and remixing of fluids.

Second, microfluidic systems excel in terms of reaction conversion and selectivity. The yield is usually affected by factors such as temperature and material ratio. In the microfluidic system, due to the small volume of materials in the pipeline, the reactants can be uniformly mixed in a very short time, which reduces the probability of the occurrence of side reactions and thus improves the conversion and selectivity of the reaction.

In addition, the microfluidic system requires a very small amount of sample, which can greatly improve the safety of operators, especially in the synthesis of hazardous chemicals. Thanks to these advantages, microfluidic technology has been successfully applied to the synthesis of nitroguanidine, dinitronaphthalene, isooctyl nitrate, Pb(N3)2, BaTNR, LTNR, and other explosives and energy-containing additives.

With the growing demand for new energy-containing materials in the modern battlefield, microfluidic technology has a promising application and is expected to provide more convenient and efficient experimental solutions for the synthesis of new energy-containing materials in the future.

2. Modification of monolithic energy-containing materials using microfluidic technology

Early scholars studied micro- and nanoenergy-containing materials with the main objective of enhancing their energy performance. With a significant increase in specific surface area, the mass transfer rate of the material is significantly accelerated, exhibiting higher energy release rates and combustion rates.

However, it was unexpectedly found that the sensibility of the micro- and nanoenergy-containing materials decreased during the experiment. With the in-depth study of the mechanism of sensibility reduction, the viewpoint based on the hot spot theory is gradually accepted, i.e., when the material is subjected to external stimulation, hot spots are easy to be formed at the corners of the surface of the particles with irregular shape, while the spherical crystals accumulate less hot spots due to their smooth surface and no corners, which reduces the mechanical sensibility and enhances the safety of the preparation and application of the energy-containing materials.

Therefore, in order to realize the sensibility reduction, on the one hand, it can be achieved by preparing micro- and nanoenergy-containing materials, and on the other hand, it is necessary to focus on improving the particle morphology.

In the conventional reaction system, the presence of concentration gradient and temperature gradient is difficult to avoid, leading to a large difference in the reaction environment in different regions of the reactor, which in turn leads to the emergence of different crystalline morphologies after recrystallization, with polycrystalline particles and single-crystalline particles often mixed together.

In contrast, these variables can be precisely controlled during solvent/non-solvent recrystallization at the microscale, thus contributing to the preparation of energy-containing materials with smaller particle sizes and narrower distributions. And in this process, the chip structure, two-phase flow rate ratio, and concentration are the main factors affecting the particle size regulation.

Screening of chip structures is usually accomplished by flow field simulations, which greatly reduces the amount of effort required for experiments. The two-phase flow rate ratio and concentration regulate the surface reaction rate by affecting the supersaturation of the system, thus controlling the particle size.

When a highly concentrated solution is mixed with a non-solvent, the solution is instantly and rapidly diluted, and the solventization effect is rapidly weakened so that the particles can be stabilized and precipitated, thus making it easier to obtain particles with small particle sizes and narrow distributions.

Typically, an increase in the flow rate of the non-solvent phase will lead to more effective squeezing and collision of the particles inside the fluid, but if the non-solvent flow rate is too high, the force exceeds the tolerance threshold of the particles, and instead, it will not be able to further refine the particles.

The preparation of spherical energy-containing materials using microfluidic systems involves two steps: droplet formation and solidification. For droplet formation, the commonly used chip structures differ from those used for mixing, with common structures being T-type, coaxial flow type, and flow-focused type.

The size and shape of droplets are affected by the two-phase flow rate ratio and the concentration of the dispersed phase. At a suitable two-phase flow rate ratio and dispersed-phase concentration, the droplets can be generated uniformly and stably by dropwise flow to form particles with regular morphology and uniform particle size. When the two-phase flow rate ratio decreases, the particle size increases.

Curing methods for droplets include polymerization reactions, solvent exchange, cross-linking reactions, cooling crystallization, and UV curing. Energy-containing materials are usually cured by the solvent exchange principle, supplemented by appropriate heating to accelerate the exchange rate. Some studies have reported that the addition of 3-methyl-4-nitroxide furazan (NMFO) can accelerate the curing process of droplets, or solvent evaporation curing can be carried out using rotary evaporation equipment at appropriate temperatures.

The difficulty of microfluidics in microsphere preparation is that high viscosity polymers are hard to mold and difficult to control the structure. For fluids with viscosities below 0.1 Pa-s, a higher continuous phase viscosity is usually favorable for droplet formation, while the chip structure and its hydrophilicity need to be adjusted when the viscosity is slightly higher than 0.1 Pa-s. If the viscosity is too high, it is difficult to obtain droplets with uniform size.

3. Preparation of composite energy-containing materials by microfluidic technology

Composite energy-containing materials maintain the physical, chemical and mechanical properties of monolithic energy-containing materials while improving the surface properties of the materials and reducing the sensibility, thus enhancing the safety of production and application.

In addition, composite energy-containing materials also have the ability to enhance the dispersibility and fluidity of micro and nano energy-containing materials, effectively solving the problem of particle agglomeration and poor dispersion, thus significantly improving the use of materials. Because of this, composite energy-containing materials have become a hot field of research at home and abroad.

The preparation of composite energy-containing materials using microfluidic technology mainly takes two forms: eutectic and encapsulation. Eutectic is a multi-component molecular crystal formed by combining multiple substances in the same lattice under the action of non-covalent bonding, which can fundamentally change the crystal structure and internal composition of energy-containing materials. Conventional eutectic preparation methods include solvent volatilization and cooling crystallization.

The preparation of coated composite energy-containing materials by microfluidic technology is mainly based on the solvent/non-solvent method. There are two current experimental ideas: one is to dissolve the core material in the solvent phase and the shell material in the non-solvent phase, and to realize the encapsulation through the principle of supersaturated precipitation of the core material after mixing the two phases.

In summary, although the application of microfluidics in the preparation of composite energy-containing materials is still in its infancy, it is quite mature in the field of other biochemical materials.

Especially in the preparation of core-shell composites, it is theoretically possible to precisely regulate the wall thickness of the core-shell droplets by the W/O/W or O/W/O composite droplets generated by co-flow and focusing flow, and it is also possible to obtain composite droplets with multilayers and multiple nuclei.

In the future, solving the problems of solid-containing droplets that are easy to rupture and de-encapsulate, and combining the theory with practical cases and applying them to the production and preparation of energy-containing materials are the directions that we should focus on.