## 2 Type of Ball Mills
### Temperature Assisted Ball milling In-Situ and/or Ex-Situ  
Thermal changes in a ball milling process often influence the diffusivity and defect concentration that may directly impact the phase formation and transformation [57]. Higher temperatures, therefore, result in phases with higher atomic mobility in intermetallic alloys, whereas, at lower temperatures, the amorphous or nanocrystalline phase formation are inevitable [58]. Macroscopic temperature developed during ball milling has been experimentally measured. [PERSON] et al. [59] and [PERSON] et al. [60] reported the temperature increase of 40-50 \({}^{\circ}\)C in a spex ball mill; however, [PERSON] et al. [61] and [PERSON] et al. [62] measured the temperatures range of 172-215 \({}^{\circ}\)C, respectively, in an attritor ball mill using SiC and diamond sensors. The macroscopic temperature variations of the ball mill are, in general, low, and sensitive to the ball milling type and design [63, 64]. However, the microscopic temperature of the milled powder is quite high due to the high energy impact that causes amorphization, local melting and rapid solidification [65, 66]. These microscopic temperatures have been indirectly inferred via modeling mechanism studies, as well as physicochemical characteristics, such as microhardness, crystallite sizes, structural, and microstructural information of the milled ingredients. The mechanical properties of bulk and nanocrystalline copper alloys by ball milling under room temperature and at lower temperature (cryomilling) have been demonstrated by [PERSON] et al. [67]. According to this study, about 200 MPa decrease in microhardness was found when the sample Cu-10 wt.% Zn alloy is treated and switched from cryo-temperature to room temperature (Figure 5a), and this observation is correlated with the increase in the crystallite size as shown in Figure 5b. Thus, it is clear that the temperature increase causes higher mobility of deformation that has already been created by the cryomilling.  
A very recent study of pre-treatment of the cellulose with liquid nitrogen assisted ball milling, thus enhancing conversion of cellulose structure into chemicals and fuels, namely, glucose and formic acid [67]. The effectiveness in hydrolysis reaction of cellulose into glucose and formic acid in two separate experiments demonstrated that the crystallinity and the molecular weight of the cellulose could be reduced with ease using liquid nitrogen temperature assisted ball milling compared to simple ball milling itself [68]. In another study, the nitridation of Fe powder was examined by three different temperatures, liquid nitrogen temperature, room temperature, and at 200 \({}^{\circ}\)C using two different gaseous atmospheres, namely, nitrogen and ammonia [68]. Based on the above experiment, it was concluded that the optimal iron nitride formation when Fe powder was subjected under the ammonia gas ambient and with treatment at room temperature and below (liquid nitrogen). Having seen the advantage of liquid nitrogen temperature assisted ball milling, a novel study using ultra cryo-milling method was devised by mechanically milled the poorly water-soluble drug with dry-ice beads in liquid nitrogen environment in ball mill showed improvement in the solubility with less contamination effects [69]. On the hydrogen storage to form a binary/ternary metal hydride (MgH\({}_{2}\)/Mg\({}_{2}\)FeH\({}_{6}\)), a cryo-ball milling was demonstrated with higher hydrogen absorption kinetics due to efficient interface-controlled growth processes [70].