Optimisation of charge ratios for ball milling synthesis: agglomeration and refinement of coconut shells
Agglomeration is an attraction of fine particles for one another due to their high surface energy, leading to formation of particle colonies known as agglomerates. When a polymeric or metallic matrix is reinforced with particles, agglomerates usually create regions of discontinuity or weak particle adhesion within the matrix and degrade mechanical properties of the resulting composites. In ball-milling synthesis of nanoparticles, formation of agglomerates can be controlled through optimisation of milling parameters. In this study, coconut shell (lignocellulosic) nanoparticles were synthesised by varying the charge ratios from 2.5 to 40 at constant milling duration (70 hours), speed in terms of drum/vial rotation (194 revolution per minute) and ball sizes (5- 60 mm). Assessment of the effects of charge ratios (CRs) on the morphologies and particles sizes of uncarbonised coconut shell nanoparticles (UCSnp) was studied. The synthesised UCSnp were characterised using electron microscopy and X-ray diffractometry (XRD). The results showed various morphologies and orientations of UCSnp with changes in the CRs. Size determination using XRD and SEM revealed a reduction in particle size as the CR increased up to a value of 10. At higher CRs, further reduction in the average particle size was not observable. This could be linked to a balance between particle refinement and agglomeration at these higher CRs. Although particle agglomeration was apparent above CR values of 10, sizes of the UCSnp obtained above this CRs were much smaller than the initial size (37 μm) of the coconut shell precursor particles. This affirmed the ball milling synthesis as a particle refinement process, but not a coarsening/agglomeration process. The results obtained from statistical analyses show agreement with experimental results.