## 2 Type of Ball Mills
### Plasma Assisted Milling (P-Milling)  
Nowadays, in ball milling process, magnetic, ultrasonic, electric energy fields, and temperature are applied. Plasma assisted milling technology, known as p-milling, involves introduction of dielectric barrier discharge plasma in the milling process for improving the milling efficiency, capacity, and process controllability. This involves concurrent action of heating by plasma and the impact resulting in ball-milling for material processing. Plasma is considered to be the fourth state of matter, and is composed of positive ions, neutral particles, electrons, and free radicals. The high energy possessed by these particles and ions can effectively interact with materials. Moreover, p-milling results in the formation of fractures, chemical decomposition, and recrystallization on materials leading to increased surface area formation and different bulk defects compared to conventional ball milling. Hence, they become more attractive in materials processing. Therefore, plasma assisted milling techniques have received significant research interest and are explored for the synthesis of electrode materials, graphene materials, lubricating materials, cermet materials, and hydrogen storage materials [36].  
The p-milling route increased the synthesis efficiency of aluminum nitride (AlN) nano powders and reduced the precursor temperatures significantly [37]. The effect of precursors Al\({}_{2}\)O\({}_{3}\) and Al\({}_{2}\)O\({}_{3}\)/C on AlN nano powder synthesis were investigated. Compared with conventional milling (c-milling), p-milling reduced the calcination temperature due to synergistic effects between rapid heating by plasma and impact stress of milling thereby improved the synthesis efficiency.  
[PERSON] et al. fabricated nano silicon/graphene nanocomposites as anode materials by embedding nano silicon in graphene using p-milling method [38]. This route converted graphite into graphene nanosheets with the integration of nano silicon in the in situ formed graphene. Twenty hours of p-milling of nano silicon/graphene nanocomposite resulted in a unique structure with homogeneous distribution of nano silicon along with abundant nanosized free spaces in the composite yielded enhanced conductivity and cycle performance. In another study, lithium storage performance of MgB\({}_{2}\) is investigated [39]. A composite consists of MgB\({}_{2}\)/expanded graphite/TiO\({}_{2}\) is fabricated by p-milling for about 20 h and the resultant composite material exhibited a discharge capacity of 305.5 mA h g\({}^{-1}\), even after 100 cycles. A facile route for the synthesis of germanium nanoparticles wrapped with few-layers of graphene by plasma assisted ball milling for anode applications is presented [40]. The precursor materials used are commercial grade germanium powder and natural graphite. The electrodes made using germanium/graphene nanocomposite material showed better electrical conductivity, low initial loss, and better cycling capacity compared to conventional milling process.  
Although plasma assisted milling is reported to have several advantages, there is also room for discussion and improvement. Owing to the complexity and non-thermal equilibrium nature of plasma, it is difficult to monitor/measure the active particles, such as ions and free radicals, that are present in it and how they affect the materials properties. Secondly, the effect of plasma on the formation of any intermediate products during the course of milling and their effects on mechano-chemical properties of materials is yet to be studied.