The amorphization of crystalline silicon by ball milling

Abstract

The transformation of crystalline silicon to amorphous silicon during ball milling was quantitatively measured by x-ray diffraction and electrochemical methods. Amorphous silicon was found to form rapidly from the very initial stages of ball milling. Simultaneously, the grain size of the crystalline silicon phase decreased. Under extended milling times it was found that a maximum of 86 % of the silicon became amorphous. Similarly, the grain size of the crystalline silicon phase could not be reduced below 6 nm. This transformation followed an Avrami kinetic model, which is consistent with a system which reaches a steady state. These observations suggest a mechanism in which ball milling generates defects, resulting in silicon amorphization and grain size reduction, where the degree of amorphization is limited in extent because there exists a limiting silicon grain size below which defects are no longer formed.

Fig. 1

XRD patterns of c-Si ball milled for different amount of time from 0 min to 32 h plotted with a (a) linear and (b) logarithmic intensity scale.

Fig. 2

(a) Calculated XRD patterns using the Debye scattering equation of a single Si unit cell (as shown in (b)), and a 2 × 2 × 2 cluster of Si unit cells (as shown in (c)).

Fig. 3

Selected fits of XRD patterns of c-Si samples after ball milling for (a) 0 min, (b) 15 min, (c) 30 min, and (d) 20 min.

Fig. 4

The relative amount of a-Si and c-Si phases in ball milled Si samples vs. the ball milling time as calculated from XRD patterns. Fits to the a-Si and c-Si phase fractions according to the Avrami kinetic equation are shown as dashed lines. Additionally shown are a-Si and c-Si phase fractions as determined by electrochemistry and the c-Si grain size as a function of milling time. These data are plotted vs. a (a) linear and (b) logarithmic time scale.

Fig. 5

SEM images of pristine c-Si and c-Si after ball milling for 7 min at two different magnifications.

Fig. 6

TEM images and a SAEDPs of c-Si after ball milling for (a,b) 7 min, (c,d) 15 min, and (e,f) 45 min. (a) and (c) are BF images. (e) is a DF image centered on a segment of the Si(111) reflection ring. In panel (c), dashed lines indicate regions containing parallel lattice fringes corresponding to the d-spacing of the (111) Si lattice planes (0.313 nm).

Fig. 7

(a) Voltage versus normalized capacity and (b) differential capacity versus voltage curves of silicon half-cells. Dashed arrows in (b) show trends in the differential capacity peak potentials as the ball milling time is increased. The peaks near 0.1 V are highlighted in blue and the peaks near 0.25 V are highlighted in yellow.