Impact Factors of Mechanical Ball Milling in the Preparation of Nano Hydrogen Storage Materials

In the process of mechanical ball milling, the energy required for powder fragmentation and refinement comes from the impact and shear action of the milling balls. When the ball milling speed is low, the movement of zirconium oxide beads is primarily due to friction, with a minimal proportion of impacts, making friction and shear the main mechanisms of ball milling. Selecting appropriate ball milling parameters such as speed, duration, milling media, additives, atmosphere, and more plays a crucial role in the preparation of nano materials.

1. Ball Milling Speed

Mechanical ball milling was employed to prepare Fe3O4 nano magnetic particles. The results indicate that controlling the milling speed between 180 to 220 rpm yields optimal grinding effects. Cai Xiaolan, in an inert gas environment, used milling to prepare finely lamellar zinc powder. By controlling the speed and atmosphere, they achieved good raw material particle sizes ranging from 7.0 to 15.0 μm.

2. Ball Milling Duration

In the process of mechanical ball milling, the duration of milling has a significant impact on the particle size, specific surface area, crystal structure, and hydrogen release performance of nano hydrogen storage materials. With increasing milling time, the particle size of magnesium decreases, but if milling time is too long, the crushing effect becomes less pronounced. When the milling time increased from 3 hours to 80 hours, diffraction peaks for magnesium and nickel significantly broadened, and a new phase, Mg2Ni, appeared. Huo et al. found that as milling time increased from 2 hours to 20 hours, the MgH2 crystalline structure transformed. Hu Xiuying and others studied the impact of milling time on the structure and performance of magnesium carbon composite hydrogen storage materials (40Mg60C). The results showed that a milling time of 2 hours was sufficient to achieve nanoscale particle sizes (10-20 nm), and further extending the milling time increased material agglomeration. Barkhordarian et al. studied the effect of milling time on the hydrogen release performance of magnesium hydride, and found that extending milling time from 2 hours to 100 hours reduced the time required for complete hydrogen release at 300°C from 3000 seconds to 300 seconds. Properly extending the milling time can increase the content of amorphous nano phases in the material, reduce the platform pressure and enthalpy change of hydrogen release, thereby improving the structural stability of the material and enhancing its dehydrogenation capability.

3. Ball Milling Media

The commonly used milling media is zirconium oxide beads, while materials for making milling media mostly consist of specially processed cast iron or alloys, followed by ceramics, aluminum oxide, and others. Khan's research results indicate that sodium chloride, as a milling medium, can effectively suppress the aggregation of aminated nano diamonds (DNDs). Lu Guojian et al. reacted microcrystalline carbon and magnesium powder through wet milling in an H2 atmosphere, and a milling duration of 3 hours achieved particle sizes ranging from 20 to 120 nm, demonstrating that the introduction of an appropriate amount of microcrystalline carbon is beneficial for rapid magnesium powder nanosizing.

4. Milling Additives

When hydrogen storage materials are hard and difficult to refine, the addition of an appropriate amount of grinding aid is necessary. Song et al. separately added Cr2O3, Al2O3, and CeO2 as grinding aids to magnesium-based hydrogen storage materials, resulting in significant changes in hydrogen absorption properties through milling, producing nano Mg-based multiphase alloys. When hydrogen storage materials are prone to agglomeration, an appropriate amount of dispersant needs to be added. Common dispersants include MoS2, graphite, microcrystalline carbon, and more. Kondo et al. used Mg and TiFe0.92Mn0.08 as raw materials to wet mill Mg-50% TiFe0.92Mn0.08 composite hydrogen storage materials in n-hexane. The results showed that TiFe0.92Mn0.08 was uniformly dispersed in Mg, and the material began absorbing hydrogen at 25°C. Hydrogen absorption and desorption properties improved with changes in dispersion.

When hydrogen storage materials are prone to refinement and exceed the desired particle size range, an appropriate amount of lubricant should be added. In an H2 atmosphere, adding 30% microcrystalline carbon and milling magnesium powder for 3 hours resulted in magnesium-based hydrogen storage materials ranging from 20 to 60 nm. The MgH2 grain size remained nearly unchanged with increasing milling time, indicating that microcrystalline carbon provided effective lubrication.

5. Milling Atmosphere

Due to the significant energy output during mechanical ball milling, the energy output may affect the gas environment inside the milling jar. When materials are relatively stable in the air, atmospheric conditions can be directly utilized for milling. However, when materials are prone to oxidation, it is necessary to evacuate the milling jar or replace the internal air with an inert gas. For example, in an air atmosphere, metallic magnesium is easily oxidized into MgO, resulting in the loss of effective hydrogen storage components. Therefore, when preparing nano hydrogen storage materials through mechanical ball milling, it is crucial to select the appropriate milling atmosphere based on the nature of the materials.

Conclusion

Mechanical ball milling enables the preparation of nano hydrogen storage materials through grinding, dispersion, and induced chemical reactions. By adjusting milling parameters, controlled preparation of nano hydrogen storage materials is achievable. Nevertheless, uneven particle size distribution poses practical challenges to the application of mechanical ball milling. With the continuous improvement of milling processes and advancements in nanotechnology, mechanical ball milling, with its advantages of low cost, high efficiency, and simplicity of operation, is poised to carve a niche in the field of nano hydrogen storage material preparation.