Recently, researcher Wang Xianlong and his team from the Hefei Institute of Physical Sciences of the Chinese Academy of Sciences successfully synthesized a cubic deflection polymerized nitrogen.
The synthesized sample has a thermal decomposition temperature of 488°C, which is in good agreement with the theoretical prediction of 477°C. At a temperature of 488°C, the sample exhibits a sharp decomposition peak upon thermal decomposition and exhibits the typical thermal decomposition characteristics of a high energy density material.
Laser plasma-driven microburst testing showed a significant increase in sample detonation rate and sample storage for more than 2 months.
At the same time, the synthesis method developed this time has the advantages of safer and cheaper precursors, so it has the potential to realize macro-preparation, which can promote the basic and applied research of cubic deflection polymerized nitrogen, and can also bring some inspiration to the development of the field of high energy density materials.
Figure | Wang Xianlong (Source: Wang Xianlong)
Researchers said that as a new environmentally friendly high-energy-density material, cubic deflection polymerized nitrogen will play an application in civil blasting, aerospace and other fields.
For example:
In construction or mountain blasting projects, cubic deflection polymerized nitrogen can replace traditional explosives, providing the advantages of higher blasting efficiency and low environmental pollution.
In the aerospace field, cubic deflection polymerized nitrogen can be used as a propellant in spacecraft, thereby storing more energy in a limited space and reducing the overall weight of the spacecraft, ultimately improving the performance of the spacecraft.
（来源：Science Advances）
All-nitrogen energetic materials: an emerging high-energy-density material
High energy density materials are a class of materials that can release a large amount of energy in a short period of time, and are widely used in military, aerospace and mining fields.
Based on the method of chemical synthesis, since the invention of trinitrotoluene in 1863, the development of high-energy-density materials has gone through multiple stages of development, represented by trinitrotoluene, hesokin, and octogen.
With the development of related fields, there is an urgent need for high-energy-density materials with better performance.
All-nitrogen energetic materials, an emerging high-energy-density material, have attracted extensive attention in recent years. In the chemical structure of all-nitrogen energetic materials, nitrogen atoms are linked to nitrogen atoms by single bonds of nitrogen and nitrogen.
Due to the large energy difference between nitrogen-nitrogen single bonds and nitrogen-nitrogen triple bonds, total nitrogen energetic materials have extremely high energy densities.
In addition, when the energy of the all-nitrogen energetic material is released, the resulting product is nitrogen with nitrogen-nitrogen triple bonds, which has environmentally friendly properties.
Among them, cubic gauche nitrogen (CG-N) with diamond-like structure composed of single bonds of nitrogen and nitrogen is one of the typical representatives of new high-energy-density materials.
Previously, most of the research on the synthesis of cubic deflection polymeric nitrogen focused on the field of high-pressure synthesis. However, the pressures required for high-pressure synthesis are extremely high, and the synthesized polymeric nitrogen cannot be stored under atmospheric conditions.
In 2017, a scholar used plasma chemical vapor deposition method to synthesize trace levels of cubic deflection polymerized nitrogen using highly toxic and highly sensitive sodium azide as raw material. However, it is still necessary to improve the conversion rate through the confinement effect of carbon nanotubes.
（来源：Science Advances）
Research "Secrets": Use precursors containing potassium
In recent years, Xianlong Wang's team has focused on elucidating the instability mechanism of high-pressure synthesis of cubic deflected polymerized nitrogen during depressurization, and is committed to developing a safer, more efficient method suitable for macro-preparation of cubic deflected polymerized nitrogen.
Based on a first-principles approach, they have simulated the stability of the cube-deflected polymerized nitrogen surface at different saturation states, under different pressure conditions, and at different temperatures.
In this way, it is clarified that the instability mechanism of cubic deflection polymerized nitrogen at pressure reduction is due to surface instability.
Based on this, the research group proposed a method to stabilize the cubic deflection polymerized nitrogen to 477°C at atmospheric pressure by saturating the surface suspension bond and transferring the charge [1].
Until 2020, the team has been working on the study of nitrogen-based energetic materials that may exist at high pressure through first-principles methods, and the synthesis of nitrogen-based energetic materials at high temperatures and pressures based on a diamond-to-anvil high-pressure device and laser heating method.
At that time, the research group was able to combine cubic deflection polymerized nitrogen at high temperature and high pressure based on the diamond-to-anvil device in the high-pressure laboratory.
But as with many high-pressure synthesis results at the time, they were unable to store nitrogen-based high-energetic materials at atmospheric pressure, and the sample always began to decompose when the pressure dropped to about 40 GPa.
After 2020, they began to focus on the stability of cubic deflection polymerized nitrogen, as well as exploring new synthesis pathways, hoping to solve the following two key questions:
First, the instability mechanism of cubic deflection polymerized nitrogen during pressure reduction was developed and developed to be safer and more efficient and suitable for macro-preparation of cubic deflection polymerized nitrogen, and based on first-principles calculations, the decomposition causes of cubic deflection polymerized nitrogen synthesized under high pressure were clarified.
Second, a safer and more efficient method for the preparation of cubic deflection polymerized nitrogen at atmospheric pressure was found by plasma-enhanced chemical vapor deposition.
"Computational materials physics research based on first-principles methods is a direction that our team is very good at," the researchers said. The research department where our team is located has been studying this area since the early 80s of the 20th century, so it has a good accumulation and heritage in the field of computational materials physics. ”
However, when it comes to experiments with plasma-enhanced chemical vapor deposition, the team admits that they have little to no experience.
In order to develop a better plasma-enhanced chemical vapor deposition scheme and design a device with better performance, they also went to the University of Science and Technology of China and other units to study many times, and communicated with equipment manufacturers for many times to design the plan.
With this, they developed a device and began to increase the yield of cubic deflection polymerized nitrogen through nanotube confinement.
However, despite the fact that they have tried a variety of processing methods and different types of nanotubes, such as carbon nanotubes and titanium dioxide nanotubes, they have never been able to obtain a synthesis method with the potential for macrofabrication.
（来源：Science Advances）
Allow the cubic deflection polymer nitrogen to stabilize to 477 °C at atmospheric pressure
Fortunately, the turning point has finally come: through the method of theoretical calculation, the research group has made a certain breakthrough in the research on the stability of cubic deflection polymerized nitrogen.
Through first-principles simulation, it is found that the instability of cubic deflection polymerized nitrogen under low pressure conditions is caused by its surface decomposition.
If you suspend the bond on the saturated surface while transferring electrons to its surface, you can stabilize the cubic deflection polymer nitrogen to 477°C at atmospheric pressure.
Inspired by the results of this theoretical calculation, the team hypothesized that potassium has less electronegativity than sodium and lithium in the alkali metal group elements.
So, can the use of a precursor containing potassium transfer more electrons to the surface of the saturated cubic deflection polymerized nitrogen while it suspens the bond on its surface, thereby enhancing the stability of the cubic deflection of the polymerized nitrogen?
Through further first-principles calculations, they found that potassium saturation was more effective than sodium saturation to promote the surface stability of cubic deflection polymerized nitrogen, which also verified the above hypothesis.
Therefore, they used potassium-containing potassium azide as a raw material and used plasma-enhanced chemical vapor deposition to prepare cubic deflected polymerized nitrogen.
Soon, they obtained solid spectral evidence that they had successfully synthesized cubic deflected polynitrogen.
Then, through the thermal decomposition property test and the laser plasma-driven micro-explosion method, the research group characterized its thermal decomposition and other properties, so as to complete the results.
日前，相关论文以《环境压力下可稳定存在至 760K 的无束缚立方偏转聚合氮》（Free-standing cubic gauche nitrogen stable at 760 K under ambient pressure）为题发在 Science Advances（11.7）。 Yuxuan Xu 是第一作者，王贤龙担任通讯作者 [2]。
Figure | Related papers (Source: Science Advances)
At the same time, after the theoretical and experimental data have accumulated to a certain extent, they will try to use AI technology to guide the research on the synthesis of cubic deflection polymeric nitrogen.
In their previous research, they used machine learning methods to solve the problem of calculating Hubbard U-values in strongly associated systems, and the software developed was copyrighted at the end of 2023 and has been used for structure search under high pressure of transition metal oxides. Currently, they are trying to use this technology in the study of high-energy transition metal nitrides.
Based on this research, the team will continue to optimize the synthesis scheme of cubic deflection polymerized nitrogen based on first-principles simulation method and plasma-enhanced chemical vapor deposition method, and further improve the cubic deflection polymerized nitrogen and its environmental stability.
In addition, a series of stability tests will be carried out to fully grasp its physical and chemical properties, and then promote the process of practical application.
Resources:
1.Chin. Phys. Lett.(Express Letter)40, 086102(2023)
2.Xu, Y., Chen, G., Du, F., Li, M., Wu, L., Yao, D., ... & Wang, X. (2024). Free-standing cubic gauche nitrogen stable at 760 K under ambient pressure. Science Advances, 10(39), eadq5299.
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