## 2 Type of Ball Mills
### Ultrasonic Milling  
Mechanical alloying by ultrasonic milling benefits the combined advantages of ultrasonic, mechanical, and cavitation processes on powder materials that leads to diffusion, mass-transfer, and solid-phase reactions [51]. In this technique, cavitation process plays a vital role in grain size of metal powders or alloys. Compared to conventional milling in planetary ball mills, it is possible to make pseudo alloys from immiscible components, initiate phase transformations and substitution and interstitial reactions in a much shorter time by ultrasonic assisted milling. Schematic illustration of a typical ultrasonic ball mill is shown in Figure 4.  
A simple, economic, and eco-friendly synthesis of manganese ferrite nanoparticles (MnFe\({}_{2}\)O\({}_{4}\)) with a mean particle size of 20 nm without calcination (low temperature \(\leq\) 100 \({}^{\circ}\)C) by ultrasound wave assisted ball-milling is reported [52]. Iron balls with diameters of 1.0-1.5 mm were used for milling. Ultrasonic ball milling technology was employed for the synthesis of large, ultra-thin, 2D materials: h-BN, graphene, MoS\({}_{2}\), WS\({}_{2}\), and BCN. The sizes ranged from 1 to 20 \(\upmu\)m, thickness of \(\sim\)1-3 nm, and a yield of over 20% [53].  
[PERSON] et al. studied the dispersion capability of nano La\({}_{2}\)O\({}_{3}\) suspensions prepared by four dispersion methods, namely, ball milling, ultrasonic, ultrasonic followed by ball milling, and ball milling followed by ultrasonic [54]. The authors declare that the dispersion capability of nano La\({}_{2}\)O\({}_{3}\) suspensions were superior in the ultrasonic followed by ball milling method. Through this route, the secondary aggregation of nanoparticles can be minimized, and the aggregates formed if any can be de-agglomerated. They assessed the dispersion stability through particle size analysis (DLS), zeta potential, sedimentation test, and transmission electron microscopy (TEM). Recently, oleic acid modified graphene was dispersed in oil by ultrasonic assisted ball milling and are thickened by a lubricating grease [55]. The graphene modified lubricating grease exhibited improved corrosion resistance and tribological properties. The lubrication mechanism and corrosion resistance mechanism also discussed.  
Figure 4: Laboratory Ultrasonic grinding mill for powder processing. (**a**) Scheme: (1) ultrasonic generator; (2) vibrator; (3) step like horn; (4) container; (5) metallic balls; (6) gas inlet; (7) thermocouple. (**b**) Photo of chamber with transparent walls. “Reprinted/adapted with permission from Ref. [42]. 2004, Elsevier Publications”. More details on “Copyright and Licensing are available via the following link: [[http://Creativecommons.org/license/by/4.0/](http://Creativecommons.org/license/by/4.0/)]([http://Creativecommons.org/license/by/4.0/](http://Creativecommons.org/license/by/4.0/)) (accessed on 29 August 2022).  
Ultrasonic assisted ball milling for Congo red (azo dye) degradation in wastewater is studied [56]. The researchers used ultrasonic cavitation and mechano chemistry for dye degradation. The effect of zero valence iron significantly increased the hydroxyl radical concentration. By reacting with nano iron, the oxidation of hydroxyl radical accelerated and 99% degradation of dye happened in 2 min. Most of the organic matter was converted into minerals and removal rate of total carbon (TOC) reached 80% within 10 min.