Li Chenyang put on a gas mask and carefully weighed out 0.465 grams of amorphous boron, plus 1.395 grams of potassium nitrate, and then added it to the polyvinylidene fluoride (PVDF) binder solution. After mixing, the paste-like drug is extruded from the visualized 3D printing equipment. Under the witness of the high-speed camera, he captured the moment of the burning of these energetic threads one by one, and the dazzling light lit up the entire experimental table again and again. He repeated this scenario hundreds of times.
Chenyang Li is a PhD candidate in Ordnance Science and Technology at the School of Environmental and Safety Engineering, North University of China. For the first time, he used 3D micro-straight writing technology to realize the integrated and controllable construction of high solid boron/potassium nitrate (BPN) ignition energetic ink, studied its chemical energy release law in the linear integrated state, revealed the energy coupling mechanism between energetic components, and determined the reactivity regulation method of the “assembly-structure” dual path of BPN energetic line, which provided a reference for the study of the release control of the agent under microscale and the reactivity of the microenergy device array.
The study was published in the Chemical Engineering Journal with a paper entitled “Reactivity regulation of B/KNO3/PVDF energetic sticks prepared by direct ink writing”, with North University of China as the only unit and Professor An Chongwei as the corresponding author. This paper is the first high-level academic paper published by a doctoral student in North University of China as the first author in this field.
Li Chenyang uses 3D printing equipment to customize the molding of energetic inks. Photo courtesy of interviewee
Explore the law of interpretation
Chemical energy is a class of substances that contain explosive groups or contain oxidants and combustibles, and can independently carry out chemical reactions and output energy, called energetic materials. This material has the characteristics of high energy density and high energy release power, and has become an important material basis in the fields of aerospace, engineering blasting and military damage. To realize the fine and multi-functional application of energetic materials, it is the most important to realize the controllable adjustment of their energy release. Only controllable and adjustable release energy can meet the application needs of different fields.
From large and small satellite attitude adjustment, small to car airbags, there are boron/potassium nitrate figures everywhere in life, boron is fuel, potassium nitrate is an oxidizer, and the combination they form is the world’s most widely used and most common energy-containing material.
Ignition is an intermolecular energetic complex rich in chemical energy, often used as the starting charge in the detonation sequence. It does not need the “sensitive and restless” gunpowder used in the warhead, nor can it be too “dull”, boron/potassium nitrate is “milder” than the manic and dangerous CL-20, Hessokin, and Okto, can produce gas, high calorific value of injection, high mechanical sensitivity, moderate burning speed, just meet the requirements of ignition.
Since boron/potassium nitrate was applied to ignition, scientists have limited their research to drug formulation, performance characterization, etc., and lack of in-depth exploration of how to accurately control it. In recent years, the use of fluorine-containing polymer content and components to regulate the reactivity of energetic materials has become the mainstream research direction, Li Chenyang’s research group uses physical arrays or geometric structures to achieve chemical energy release, which has also become an effective means to regulate the reactivity of energetic materials. These studies use convection of gases and heat from energetic complex reactions, or advection of hot particles, to optimize energy output, and linear burn rates to measure the reactivity of building structures.
Li Chenyang, a doctoral student at North University of China, said that this research has not been applied in potassium boron nitrate, and the mechanism of its energy release regulation has not been revealed. In this classic formulation, boron has a higher volumetric and mass energy density than other active metals, and this “gentle temperament” pair is safer and more stable than other reactive metal/oxide combinations. Therefore, studying the energy release mechanism of boron/potassium nitrate has long-term demonstration significance.
In the future, Li Chenyang also hopes to use ignition powder as energy through more accurate energy regulation to realize the conversion of various energy sources such as sound, light, electricity, force, and heat. By adjusting the energy intensity of the ignition agent, the strength of the electrical signal or optical signal is changed, so as to realize the transmission of the signal.
Customized molding of energetic materials
The energy release of energetic materials has been widely concerned, but the energy release law of energetic materials with powder properties is not fully applicable to energetic materials in the integrated state. Whether cast or pressed into a column, the energetic material in its integrated state is very different from its initial powder form. It is necessary to realize the integration of energetic materials first, and then study their energy release laws.
Since 2021, with the goal of controlling the reactivity regulation of boron/potassium nitrate, Li Chenyang has carried out experiments on a very small scale.
For example, if you want to make a cylindrical gunpowder with a diameter of 1 mm, powdered boron and potassium nitrate drugs need a container, fill them into the container, and also need to use pressure or spiral method of charging, the process is inevitably lost, and its density is uniform in the process is difficult to operate. Li Chenyang said that in order to achieve a millisecond-level delay after ignition, accurately grasp the combustion time, and is very sensitive to errors, even if a specified form of ignition is manufactured, but when encountering specific characteristics and specific structural scenarios, when loading gunpowder in miniature devices, the traditional pressure charge mode is completely inadequate.
“The two difficulties of shape and density can actually be achieved through 3D printing”, Li Chenyang used micro-straight writing technology to create the ideal powder line, which they call energetic wire. First of all, boron and potassium nitrate are matched in the specified proportion, supplemented by polyvinylidene fluoride (PVDF) as a binder, 3D printer in the set program to produce different forms of network structure, not only no shape and structure restrictions, but also can maintain the internal structure density uniformity, network charge greatly improves the stability of printing rod combustion.
Among them, the binder is the core key to the role of bond molding, lubrication, flame retardancy or combustion support. When polyvinylidene fluoride (PVDF) is used as a binder, combustion produces hydrogen fluoride gas and other fluorocarbon fragments, and can also peel off the very stubborn inert oxide layer on the outside of the printing rod.
The corresponding author of the paper, Professor An Chongwei’s research group of North University of China, has long been committed to the research of 3D printing technology of energetic materials, especially in the research of micro-size explosion transmission sequence charges, and is a pioneer in the field of energetic materials. He said that although the application of 3D printing technology to energetic materials is not new, it is not common to construct an array structure through 3D printing to regulate the energy release characteristics of energetic materials from the perspective of structure. Moreover, we are not satisfied with the printing and molding of energetic materials, in the future, we hope to apply energetic arrays to energetic devices, and realize the integrated manufacturing of energetic devices including energetic arrays, device shells, ignition units, etc. with the help of 3D printing equipment.
The traditional bulk gunpowder and powder integrated pharmaceutical line have huge differences in all aspects, the research objects change, the research process is very different, and the printing rod opens up a new field for the regulation of energy release of energetic materials.
Fast or slow burning, I have the final say
Linear combustion rate is the most important indicator for studying ignition powder, because due to the manufacturing process, ideal chemical reaction conditions are almost non-existent in reality, and in order to find the pattern, the test has to be repeated many times. Li Chenyang carried out the combustion experiment in an explosion-proof box equipped with a shooting window, and captured the combustion process of the sample in the air through a high-speed camera equipment of 2000 frames per second.
Experiments show that under the premise of maintaining ink stability and structural integrity of the printing rod, it is found that when the particle load is 93wt% (the mass proportion of potassium boron nitrate, the rest of the material is binder), the linear combustion rate can reach 60 mm per second, and the reactivity of the printing rod is the best. Subsequently, the particle loading rate (mass percentage of potassium boronitrate) was reduced from 97 wt% to 85 wt%, and the linear burn rate of the print rod was reduced from 126.34 mm per second to 28.12 mm per second.
“This speed is not fast in ignition, we do not pursue extreme indicators, but pay more attention to the control and observation of the combustion process.” Li Chenyang said that the more binders, the slower the linear burning rate, the less binder, the burning rate will increase, but the molding effect will be worse. It can be comprehensively regulated according to demand, and the combustion speed can be determined by itself.
Li Chenyang also inferred from the data that the linear combustion rate of the printing rod showed a strong dependence on the combustion oxygen ratio. Conventionally speaking, according to the stoichiometric ratio of boron and potassium nitrate, theoretically the most efficient, the most violent chemical reaction, boron and potassium nitrate can all disappear after the end of combustion. Li Chenyang said that the practice and theory are not consistent, and we found through the data that due to the active factors of boron, the slightly more fueled formula (that is, a little more boron) burns more efficiently.
It is not easy to conclude that the linear burning rate of the ignition powder line is about 60mm/s, and Li Chenyang carried out repeated experiments in order to grasp accurate data, and could not confirm whether the amplitude of the observed data was an error in operation or a problem with its own formula and production process. “At that time, I repeated it too many times, and I never stabilized the number at 60mm/s, and my mentor was afraid that I would ‘go crazy’ and thought I was too serious,” Li recalls.
Li Chenyang said that there is a critical size of the energetic line, beyond this size can be combusted, below the size of the combustion can not be passed on, so studying the combustion speed and making the thickness of the energetic line are the key to controlling combustion. This is one of his most important discoveries in the experiment, the burning rate of the printing rod has an obvious size effect, all combustion should be studied within the thickness, when the thickness is less than 110 μm, the combustion can not be stably transmitted, when the thickness is between 750μm ~ 1400μm (micron), the burning rate-thickness curve shows a platform effect, and the burning rate is always stable.
In addition to polyvinylidene fluoride (PVDF), the research group has also introduced a variety of polymer materials such as HPMC (hydroxypropyl methylcellulose), PVA (polyvinyl alcohol), etc., and has realized the powder integration of energetic materials such as explosives, propellants, and nano-thermite agents.
In addition, the control of the ignition velocity of the ignition powder line through the energetic structure is another important discovery of Li Chenyang. It establishes the energy coupling between the drug line channels, which is of great significance for the release regulation of energy-containing arrays. The research of the research group is at the forefront of this field in China.
An Chongwei said that the application of 3D printing technology to the field of energetic materials is only a means, and exploring the law is the original intention of unchanged. (Source: Li Qingbo, China Science News)
Related paper information:https://doi.org/10.1016/j.cej.2022.138376