R Formation of Sn-dispersed Si Nanoparticles by Co-grinding

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Journal of the Korean Ceramic Society Vol. 46, No. 6, pp. 545~547, 2009.

−545⁻

Review

DOI:10.4191/KCERS.2009.46.6.545

†Corresponding author : Bong-Chull Kim, Ph.D E-mail : [email protected] Tel : +82-41-560-3732 Fax : +82-41-560-3696

Formation of Sn-dispersed Si Nanoparticles by Co-grinding

Bong-Chull Kim

^†

, Hiroyuki Uono

^*

, and Makoto Ue

^*

, and Mamoru Senna

^**

Sasmung SDI Co. Ltd., Cheonan 330-300, Korea

*Mitsubishi Chemical Corporation MCC-Group Science &Technology Research Center, Ibaraki 300-0332, Japan

**Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan (Received April 9, 2009; Revised September 22, 2009; Accepted September 30, 2009)

ABSTRACT

An immiscible Si/Sn (=7/3 by volume) powder mixture was subjected to simple grinding and subsequent leaching process to give Sn nanopowder reinforced or dispersed in Si powder. Crystallite and their agglomerates of Si were ^ca. 15 nm and 100 nm, respectively. Sn remained at 4.5 vol% in Si powder after aqueous HCl leaching, dispersively occluded in Si matrix as confirmed by ICP analysis and cross sectional TEM observation.

Key words^:Mechanical milling, Nanocomposite, Powder processing, Co-grinding

1. Introduction

ecently, Si and Sn have gathered enormous interest because of their large capacity as anodes of Li-ion batteries.^1,2) However, they usually exhibit rapid capacity loss during cycling because of their volume change up to 400%

during charge-discharge cycling.²⁾ A decrease of the particle size of these powders, down to a nano-size regime, is known to be very effective in overcoming these problems by reduc- ing mechanical stresses due to macroscopic volume changes of the reactive phase.^3,4)

Nanoparticles have been fabricated by various methods such as chemical vapor deposition,⁵⁾ laser ablation,⁶⁾ high energy milling with or without a mechanochemical reaction⁷⁾ and the wet chemical method.⁸⁾ The high energy mill is attractive in view of mass production.⁹⁾However, drawbacks such as low energy efficiency and severe aggregation are still to be eliminated.¹⁰⁾ We have improved this process by combining two dry milling processes with different stress modes and have achieved downsizing of Si powders to the nano-scale, as shown in an earlier paper.¹¹⁾

In this study, we conducted co-grinding of Si with Sn for further downsizing. We chose Sn as an immiscible metal with Si as co-grinding ingredient. Starting from our pri- mary attempt to downsize Si particles with the aid of Sn as a kind of grinding media, we observed the formation of nanocomposites by co-grinding. In the case of co-grinding with other metals, various microstructures develop through the repeated cold welding and fracture of the particles. In this study, we therefore preferentially analyze the proper-

ties of Sn / Si nanocomposites as a product of co-grinding.

2. Experimental Procedure

The starting materials used in this study were polycrystalline Si powder (purity 99%, mean particle size 6^µm, Yamaishi Metals, Tokyo, Japan) and Sn powder (purity 99.9%, mean particle size 100^µm, Soekawa Chemicals, Japan). We milled a Si/Sn mixture at 7/3 volume with a multi-ring-type mill (model MICROS MIC-0, Nara Machinery Co. LTD, Tokyo, Japan; denoted MIC hereafter).

The main body of MIC is composed of three parts: the first, a revolving main shaft in a ZrO2-lined casing; the second, six ZrO2

rod-type subshafts interlocked between two plates attached to the main shaft; and the third, many ZrO2 milling rings inserted within the subshafts. Being pressed toward the inner wall of the casing, particles are held between the revolving ring and the wall surface. They are subjected to compression by the centrifugal force of the ring and the shear stress developed by the rotating ring.¹²⁾ The machine was operated batchwise at 2,000revolutions of the main shaft/min in N2 gas flow (100cc/min) to inhibit oxida- tion of Si, particularly on the new surface created by milling. The amount of each batch was kept constant at 40g.

We conducted an HCl leaching process after milling to remove Sn in Si/Sn composite powder. The sample was washed several times with deionized water and finally rinsed with eth- anol (purity 99%) and dried for two days in an oven at 50^oC.

The remaining Sn amount in the sample was analyzed by inductively coupled plasma atomic emission spectrometry (ICP- AES; Model IRIS/AP, NIPPON JARRELL-ASH, Kyoto, Japan).

For determining crystallite size of Si powder, X-ray diffracto- metry (XRD) was preformed with a diffractometer (Model RINT 2200, Rigaku, Yokohama, Japan) with Cu-K radiation and a monochromater. Particle morphology was observed with a field emission scanning electron microscope (FE-SEM; Model S-4700, Hitachi, Ibaraki, Japan). The powders were mixed with

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546 Journal of the Korean Ceramic Society - Bong-Chull Kim^{et al}. Vol. 46, No. 6

an epoxy resin, impregnated in a vacuum chamber for 10 min, solidified and cut into 50 nm-thin slices using a microtome (Model MT-XL, RMC, AZ, USA). The slices were observed under a transmission electron microscope (TEM; Model JEM2010, JEOL, Tokyo, Japan) at 200 KV with an energy dis- persive x-ray spectrometer (EDS, Noran, MA, USA).

3. Results and Discussion

Fig. 1 shows scanning electron micrographs of the as-milled Si/Sn mixture before leaching, (a); after leaching, (b); and pure Si, (c). Large particles of ca. 1^µm and small particles of ca.

100 nm are coexisting in the as-milled Si/Sn composite (Fig.

1(a). Large particles disappear and homogeneous small particles (ca. 100 nm) remain in the leached sample (Fig. 1 (b)) but

pure Si is shown to be severely agglomerated. The XRD profiles shown in Fig. 2 exhibit significant broadening of Si peaks due to grinding. The crystallite size of Si calculated by the Scherrer equation was approximately 15 nm for the co-ground and leached Si/Sn. The average particle size is smaller than that of pure Si powder milled under the same conditions (50 nm), as summarized in Table 1. After leaching, Sn peaks almost disap- peared and only very weak peaks remained near 2^Θ = 31^o and 44^o. The Sn amount in the leached sample is determined to be 4.5 vol% by ICP analysis.

Microstructure of the leached sample was analyzed by cross-sectional TEM techniques and elemental analyses. As shown in Fig. 3, elemental Sn was detected by EDX spot analyses at the interior, dark positions (a and c) of a particle but not at the bright positions such as position b. We therefore regard the dark points as Sn metal. Nano-sized Sn particles are dispersed in an Si powder, the latter being in the nanometer regime as well. From the TEM image, we determined the dark particle size to be 14.0^±3.7 nm.

By co-grinding Si with Sn, Si crystallite size becomes smaller than that of Si milled without Sn under the same milling conditions. By leaching the co-ground Si with HCl solution in an attempt to remove Sn, we obtained homogeneous Si powders of ca. 100 nm with Sn dispersed, as mentioned above.

The MIC milling machine generates the compressive stress and the shear stress concurrently and the comminution is prac- tically achieved by shear stress.¹²⁾ The shear stress is also pro- portional to the interfacial area acquired by compression.¹³⁾The comminution behavior of MIC milling with or without co-grinding additives is schematically described in Fig. 4. Without an additive, the contact interface area is small because the rough surface is easily created during milling (Fig. 4(a)). When a ductile co-grinding additive such as Sn metal exists between the brittle Si particles, the interfacial area contacted with compressive force increases due to the plastic deformation of the additive (Fig. 4(b)). At the same rate of revolutions of the grinding ring, the strong shear stress is, therefore, generated in the case of Sn additive due to the efficient transfer of the shear stress.

This explains why co-grinding of Sn is effective at obtaining smaller nano-sized Si powders.

4. Summary

By co-grinding Si with Sn and after subsequent leaching, we not only achieve further downsizing of Si but also obtain nanocomposite particles with Sn particles in the Si matrix.

Fig. 1.Scanning electron micrographs of silicon powders (a) co-ground with Sn, (b) leached after co-grinding, and (c) as-milled without additive.

Fig. 2.X-ray diffraction patterns of silicon powders (a) co- ground with Sn, (b) leached after co-grinding, and (c) as-milled without additive.

Table 1.Particle Sizes and Contamination of Nano-sized Si Powders.

Samples Crystalite size (nm)

By X-ray Particle size (nm) Impurity contamination By SEM

observation By laser P.S.A

Non additive 50 270 350 ZrO2

(0.3 wt%)

Sn additive 13 110 150 Sn(13 wt%)

ZrO2

(0.9 wt%)

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November 2009 Formation of Sn-dispersed Si Nanoparticles by Co-grinding 547

The size of the crystallites and their aggregate of Si are 15 nm and 100 nm, respectively.

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Fig. 3.Bright-field TEM image of cross-sectioned Si powder leached after co-grinding with Sn and EDX point analyses of the areas indicated by arrows in TEM image (a) and (b) EDX spectra of dark positions, (c) EDX spectra of a bright position.

Fig. 4.Schematic diagram describing milling behavior of Si powder milled by MIC generates compressive and shear forces (a) non- additive, (b) ductile additive.