*3.1. X-Ray Di*ff*raction Analysis*

Figure 2 shows the XRD patterns of talc ground for up to 360 min using different ball sizes, which shows the effect of ball size on the crystallinity of ground talc.

**Figure 2.** XRD patterns of talc powder upon grinding with (**a**) 2 mm, (**b**) 1 mm, and (**c**) 0.1 mm balls. Chl: chlorite, Qtz: quartz.

In the as-received talc, (001), (002), and (003) peaks of talc with a narrow width were observed at 2θ angles of 9.4◦, 18.9◦, and 28.6◦, respectively [12]. The small peaks for chlorite and quartz were also observed, which were often found with talc as a result of carbonate alteration [33]. Upon grinding, the X-ray diffraction peak intensity gradually decreased while the peak width increased, which implied the decrease in crystallinity of ground talc. The degree and rate of reduction in peak intensity varies with the size of the balls used in grinding. When 2 mm or 1 mm balls were used for grinding, the peak intensity of the talc markedly decreased until 60 min of grinding. After 120 min of grinding, most diffraction peaks disappeared when the 2 mm balls were used. When 1 mm balls were used, small diffraction peaks were still observed, even though their intensities are very small compared with prior to grinding. A further change in the diffraction pattern was not observed for grinding by more than 120 min. These results indicated that ground talc for 120 min and more has a disordered or amorphous structure when 2 mm or 1 mm balls were used for grinding. When 0.1 mm balls were used, the decrease in peak intensity was relatively sluggish. The peak intensity consistently decreased even after 120 min of grinding, which was in contrast with the cases using 2 mm or 1 mm balls. The unique diffraction pattern of talc was still apparent even after 360 min of grinding, which indicated a crystalline structure of talc left after grinding.

To quantitatively compare the effects of the ball size on the crystallinity of ground talc, the change in the relative height of the talc (001) peak, according to grinding, was analyzed (Figure 3). When 2 mm balls were used, the peak height after 60 min of grinding decreased to approximately 9% compared with the height prior to grinding.

**Figure 3.** Changes in the relative intensities of (001) XRD peak of talc powder upon grinding with varying ball sizes for high-energy ball milling. The relative peak intensities are normalized with the (001) XRD peak of as-received talc.

Almost no peak was observed after 120 min of grinding. Similar results were obtained when 1 mm balls were used, but, after 120 min, the peak height decreased slightly to less than that obtained using 2 mm balls. When 0.1 mm balls were used, the peak height showed a slight decrease, and approximately 44% remained even after 360 min of grinding. These results indicate that the crystallinity of talc decreases more when 2 mm and 1 mm balls are used than when 0.1 mm balls are used for milling. As the ball size increases, greater size reduction of the talc particle and loss of crystallinity occur during grinding. This is due to the reduced kinetic energy of collision between rotating balls in the mill with 0.1 mm balls when compared with 2 mm and 1 mm balls [31,32].

Figure 4 shows the variation in relative peak intensities (It/I0) of talc's main diffraction peaks such as (001), (132), (003), and (020) upon grinding, which shows the effect of the ball size on the direction in the fracture of talc. For 2 mm balls, the (001) and (003) lattice planes, which are perpendicular to the c-axis of talc, decreased faster than the (132) and (020) peaks. In particular, the decrease rate of (001) and (003) peaks was very high during the first 10 min and gradually slows at 60 min of grinding. The (001) and (003) peaks mostly disappeared after 120 min of grinding. In contrast, the decrease rate of the (132) and (020) peaks is relatively low and its peak intensities are approximately 20% after 360 min of grinding compared with that prior to grinding. The use of 1 mm balls yielded similar results to those obtained using 2 mm balls. The decrease rate of (001) and (003) peaks was very high in the early stage of grinding, and most of the peaks subsequently disappear. The (132) and (020) peak intensities are approximately 50% after 360 min of grinding when compared with that prior to grinding, which indicates less amorphization when using 1 mm balls than when using 2 mm balls.

**Figure 4.** Effects of grinding time and ball size on the relative XRD intensities of lattice planes. The ball size for high-energy ball milling is (**a**) 2 mm, (**b**) 1 mm, and (**c**) 0.1 mm. The relative peak intensities are normalized with the XRD peak of as-received talc.

In contrast, when 0.1 mm balls were used, the decrease rate of (132) and (020) peaks was higher than that of the (001) and (003) peaks, especially during the first 60 min of grinding. After 360 min of grinding, all peaks except the (020) peak reached approximately 40% compared with that prior to grinding. As the ball size increases, the reduction of peak intensity prior to grinding increases for most peaks. The delamination of (00l) planes, in particular, are significantly affected by the ball size. When 2 mm or 1 mm balls were used, the peak intensity of (00l) planes reduced to less than 5% of its state prior to grinding, while it only reduced to 40% when using 0.1 mm balls. The relative intensities of (020) plane after 360 min of grinding were approximately 20%, 55%, and 60% when using 2 mm, 1 mm, and 0.1 mm balls, respectively. These results indicate that the size reduction mechanism of talc

changes with varying ball size. The contribution of delamination toward the decrease in particle size is high when 2 mm or 1 mm balls are used but low when 0.1 mm balls are used. The particle size reduction of talc occurred in all directions when using 0.1 mm balls.

#### *3.2. Analysis of Specific Surface Area*

Figure 5 shows the variation in the specific surface area of talc according to the ball size used in grinding.

**Figure 5.** Variation of BET specific surface area of talc upon grinding with varying ball sizes for high-energy ball milling.

As the grinding progresses, the increase in the specific surface area of talc is initially abrupt and then becomes gradual. The overall tendency of the increase in the specific surface area is similar for all ball sizes. However, the degree of increase varied according to the ball size. When 2 mm or 1 mm balls were used, the tendency was very similar until 60 min of grinding. The specific surface area was slightly higher when 1 mm balls were used than when 2 mm balls were used. However, since the grinding time is increased above 60 min, the specific surface area of the particles ground using 2 mm balls increased more toward the end of the grinding period. The specific surface area was approximately 6.1 m2/g before grinding and increased to approximately 419.1 m2/g for 2 mm balls and approximately 365.1 m2/g for 1 mm balls after 360 min of grinding. The grinding efficiency when using 2 mm balls was slightly lower than when using 1 mm balls for the first 60 min of grinding, but prolonged grinding yielded a higher specific surface area when 2 mm balls were used. The 0.1 mm ball showed the lowest grinding efficiency, with a specific surface area of only 171.7 m2/g even after 360 min of grinding. These results indicate that the use of 1 mm and 2 mm balls in grinding talc could yield a larger specific surface area than the use of 0.1 mm balls could. In planetary ball milling, the proper ball size could maximize the grinding efficiency with a balance of aggregation and particle size reduction [32]. Our results indicate that the specific surface area eventually increases when the 2 mm balls are used for grinding, even though it is not clearly observed during the early stage of grinding.

The estimated spherical diameters (ESDs) were calculated using the equation D = 6/ρS, where ρ is the density of talc (2.8 g/m3), *S* is the specific surface area (m2·g−1), and the constant 6 is the shape factor assuming that the talc particles are spherical. The results are listed in Table 2.


**Table 2.** Changes in mineralogical parameters as a function of grinding time.

ESD, equivalent spherical diameter. D = 6 /ρ·S, where ρ is 2.8 g/cm3.

The powder had ESD of approximately 350 nm prior to grinding, which decreased to 5.1, 5.9, and 12.5 nm after 360 min of grinding using 2 mm, 1 mm, and 0.1 mm balls, respectively. These are much lower ESDs of talc than previously reported, as shown in Table 1 [10,14]. Data from previous reports listed in Table 1 were generally acquired under dry conditions, unlike the wet milling performed in this study.

We also obtained ground talc series with a similar specific surface area, even though the ball size varied. Similar specific surface areas of 81.1, 81.5, and 87.7 m2/g (approximately 85 m2/g) were obtained after 10 min of grinding with 2 mm and 1 mm balls and after 60 min of grinding with 0.1 mm balls, respectively. Furthermore, similar specific surface areas of 165.8, 171.6, and 171.7 m2/g (approximately 170 m2/g) were obtained after 30 min of grinding with 2 mm and 1 mm balls and after 360 min of grinding with 0.1 mm balls, respectively. These two series with a similar specific surface area will be discussed below.
