As the year draws to a close, many car manufacturers are releasing their sales performance for 2023. Earlier on November 12, 2023, Cui Dongshu, the secretary-general of the China Passenger Car Association, wrote: "The full-year car sales are expected to reach 29.5 million units in 2023, setting a historical record."

According to statistics from the Traffic Management Bureau of the Ministry of Public Security, the number of motor vehicle drivers in the country has reached 520 million, with 24.85 million new licenses (including additional driving) obtained in the first three quarters of 2023. There is no doubt that cars are becoming the "standard configuration" for Chinese people.

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For cars, the shock absorption system can alleviate the impact brought by the road surface. For the car shock absorption system, the damper is one of the indispensable parts.

Traditional dampers generally adopt hydraulic design or electromagnetic valve design. In comparison, magnetorheological dampers have a faster response speed and a larger range of damping variation, thus achieving better shock absorption effects.

With the support of unmanned driving technology, magnetorheological dampers are very likely to become the standard configuration for future transportation tools.At the same time, magnetorheological technology is also currently the fastest known response speed and can achieve mass production of automotive shock absorption technology solutions. And a magnetorheological fluid with good comprehensive performance is the foundation for the magnetorheological damper to exert maximum performance.

Based on this, Professor Dong Xuefeng and Professor Huang Hao from the School of Materials Science and Engineering at Dalian University of Technology and their team have created a low-cost, high-performance magnetorheological fluid.

At the same time, they also proposed a new type of nano-particle system. If this system can be further improved, it can create a particle system based on the next generation of new magnetorheological fluids, serving the industry that requires higher yield strength and more stable sedimentation performance of magnetorheological fluids.

Through the change of the particle system, they have achieved a relatively ideal balance in the magnetorheological fluid in terms of shear yield strength, sedimentation stability, and zero-field viscosity, and have the potential for mass production.

Recently, the relevant paper was published in Advanced Functional Materials with the title "Balanced Devil Triangle: A Satisfactory Comprehensive Performance Magnetorheological Fluids with Cross-Scale Particles", Du Tianxiang is the first author, Professor Dong Xuefeng and Professor Huang Hao serve as co-corresponding authors.The Three Major Bottlenecks of Magnetorheological Fluids

In fact, as early as 2002, scholars proposed the concept of micro-nano dual-dispersed system magnetorheological fluids and used them to improve the sedimentation stability of magnetorheological fluids.

However, there have always been several bottleneck issues that have not been overcome:

Firstly, the nanoparticles in the micro-nano dual-dispersed systems that have been reported previously mostly use particles with low saturation magnetization, which leads to the prepared magnetorheological fluids having some improvement in anti-sedimentation performance, but the enhancement effect of shear yield strength is not very ideal.

Secondly, when people introduced nanoparticles before, they mostly used the method of directly doping them into the existing micro-particle system, that is, when the content of micro-particles is determined, a certain amount of nanoparticles is added to it to improve performance.This approach would increase the total mass of the particles, leading to an increase in the viscosity of the zero-field magnetorheological fluid, which not only reduces the relative adjustment range of the magnetorheological fluid but also causes difficulties in filling the dampers.

Secondly, since this type of research is generally oriented towards actual production, the choice of nano-particle preparation method is extremely important. Previously, people mainly used chemical methods to prepare magnetic nano-particles, and for actual implementation, this method is difficult to achieve large-scale production.

For the first issue, they adopted the practice of using nanoparticles with high saturation magnetization to compensate for the loss of shear yield strength brought by the introduction of nanoparticles as much as possible.

For the second issue, under the premise of keeping the total content of particles unchanged, they used nanoparticles to partially replace micro-particles, in order to minimize the introduction of nanoparticles and avoid the phenomenon of increased zero-field viscosity.

For the third issue, they used the method of direct current arc plasma to prepare nanoparticles. The reason for adopting this method is that the starting point of this research is to aim at the application of magnetorheological fluids. Therefore, it is necessary to consider the efficiency of the preparation method and the performance of the prepared products.According to the introduction, this method is based on the high-temperature plasma generated by direct current arc discharge, which causes the raw materials to evaporate and decompose into gaseous atoms. These atoms then recombine with ionized active gases and cool down to form nanoparticles.

This method has the advantages of high purity and large output, thus enabling the mass production of magnetic nanoparticles. It also gives the potential for mass production of the micro-nano dual-dispersed system magnetorheological fluid in this study.

"Tiny powders are scattered with the micro-light."

So, how did they specifically complete this research?Firstly, the determination of materials was essential. Previously, the team had attempted to prepare iron nanoparticles, but at that time, the particle size was too small, the saturation magnetization was too low, and the coercive force was too high, making them unsuitable for magnetorheological fluids.

Therefore, they carefully considered the material selection and believed that only by choosing materials with the highest saturation magnetization could the performance of the magnetorheological fluid be fundamentally improved. Ultimately, they chose the iron-cobalt alloy with a high saturation magnetization as the raw material.

After fabricating the iron-cobalt nanoparticles, they found that the coercive force of these particles was too high, which was precisely the problem brought about by the particles reaching the nanoscale. The occurrence of this issue would, in turn, have a negative impact on the reproducibility and stability of the magnetorheological fluid performance.

Thus, they began to consider how to reduce the coercive force present in the particles. After research, they found that there were many ways to eliminate the internal stress within metal bulk or thin film materials.

The most easily implemented operation was to perform low-temperature annealing under the protection of an inert gas. After comprehensively considering factors such as preparation efficiency and operational complexity, they found that this was also the most reasonable approach.Through this, they obtained a type of iron-cobalt nanoparticles, which possess high saturation magnetization, lower coercivity, and lower remanent magnetism.

Subsequently, they found that this micro-nano dual-dispersed system can significantly improve the yield strength and sedimentation stability of magnetorheological fluids. However, the zero-field viscosity also increases substantially.

Therefore, they used high-performance nanoparticles to partially replace micro-particles, thereby improving the comprehensive performance of the magnetorheological fluid.

Before preparing the nanoparticles, when they purchased raw materials from the manufacturer, the manufacturer stated that they usually sell in tons.

Dong Xuefeng said: "When it was our turn, we informed them that we only intended to purchase one kilogram, and the merchants were shocked after hearing this, thinking we were joking. After a long negotiation, we finally convinced them for the purpose of scientific research and bought this one kilogram of raw materials."When preparing nanoparticles, it is necessary to grind and polish the bulk material, and then quickly transfer it to the equipment for vacuum operation.

These steps are all to prevent the oxidation of the bulk material in order to obtain better particle performance. During the grinding process, friction between the metal block and the grinding wheel will produce a large number of sparks, which can be as far as about one meter and are relatively dense.

"This scene looks very spectacular. In addition, the scene of tiny powders floating with a faint light in the plasma equipment furnace is also very unforgettable," said Dong Xuefeng.

In the future, they will continue to try other high-performance particles. To achieve practical application, it is necessary to reduce costs, so they hope to obtain a low-cost, high-performance magnetorheological fluid.

Dong Xuefeng said: "After all, nano powder is a bit expensive in terms of price. How to make this system more universal is what we need to focus on in the future."Specifically, they will start by adjusting the composition and size of the particles. In addition, when the particle system transitions from monodisperse to bidisperse, the composition of the corresponding carrier fluid and additives also needs to be adjusted accordingly.

The main focus of this study is to determine a particle content indicator for the bidisperse system, but the interaction between the magnetorheological fluid particles and the liquid phase system is also a key issue to be resolved.

When particles are nanosized, they often exhibit significant differences compared to micrometer-sized particles, such as differences in specific surface area, density, and other parameters.

Therefore, they will conduct further research on the carrier fluid and additives to adapt to the micro-nano bidisperse system.

As mentioned earlier, Dong Xuefeng places great emphasis on the practical application of results. Currently, he is mainly responsible for the scientific research work at the School of Materials Science and Engineering at Dalian University of Technology.The statement indicates: "The scientific research work in colleges and universities needs to cooperate closely with enterprises, and should also focus on solving the practical problems faced by enterprises in production. Many engineering problems are often not solved by one or two technological breakthroughs, but require the integration of multiple technologies."

Therefore, his main job is to contact enterprises and assist in aligning the research achievements of the college teachers with the needs of enterprises. At the same time, he helps the college teachers to form project teams to carry out organized scientific research around the major needs of the country.