Nanocrystalline Oxide Ceramics Prepared by High-Energy Ball Milling ...

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July 2000 , Volume 8, Issue 3, pp 245–250 ... D. & Heitjans, P. Journal of Materials Synthesis and Processing (2000) 8: 245. doi:10.1023/A:1011324429011.
Journal of Materials Synthesis and Processing, Vol. 8, Nos. 3/ 4, 2000

Nanocrystalline Oxide Ceramics Prepared by High-Energy Ball Milling S. Indris,1,2 D. Bork,1 and P. Heitjans1

Studies of grain size effects in nanocrystalline materials require a preparation technique which allows adjustment of the grain size. We prepared various nanocrystalline ceramics by high-energy ball milling. The investigated systems are the oxide ceramics Li2 O, LiNbO3 , LiBO2 , B2 O3 , TiO2 as monophase materials and the composite material Li2 O : B2 O3 . The average grain size was adjusted by variation of the milling time. It was determined via line broadening of X-ray diffraction patterns (XRD) and directly with transmission electron microscopy (TEM). Thermal stability and thermally induced grain growth of the samples can be observed with differential thermal analysis and X-ray analysis. Further information concerning the structure of these heterogeneously disordered materials was extracted from nuclear magnetic resonance (NMR) and infrared spectroscopy. Li diffusion in the lithium-containing compounds is studied with ac conductivity measurements, as well as [7 Li] NMR relaxation spectroscopy. The TiO2 is interesting for research on catalytic activity. Ball milling not only causes particle size reduction, but may also lead to phase transitions and chemical reactions. This was verified with XRD. KEY WORDS: Nanocrystalline materials; oxide ceramics; interfacial regions; ball milling; XRD.

1. INTRODUCTION

(CVD), and ball milling. The advantage of ball milling is easy handling, the possibility to produce large quantities, and the applicability to a wide range of different classes of materials. Moreover, reliable parameter control ensures reproducible results. To study grain size effects, the average grain size has to be varied over a large range, which is readily done by choosing different milling times. During milling, abrasion of the milling tools can occur. In order to avoid impurities in the samples, appropriate milling parameters have to be chosen (e.g., material of the milling vial, ball-to-powder weight ratio, optimum milling time). We studied the oxide ceramics Li2 O, LiNbO3 , LiBO2 , B2 O3 , TiO2 as monophase materials and the composite system Li2 O : B2 O3 [3–5]. Reducing the grain size and compacting the powders results in a large fraction of grain boundaries in these materials which, e.g., in the case of the Li-containing compounds, can act as fast diffusion pathways for the Li ions. Materials of that kind are discussed with respect to their future application

Nanocrystalline materials have attracted considerable interest in recent years because of the possibility of improving macroscopic properties of materials by varying the crystallite sizes [1, 2]. For example, some nanocrystalline ceramics that can be sintered at low temperatures and ductile ceramics were found that can be shaped by pressing. Nanocrystalline materials contain a large fraction of grain boundaries, which can act as fast diffusion pathways. Therefore these materials are expected to be employed as solid electrolytes for battery systems, fuel cells, or sensors. Common methods for preparation of nanocrystalline materials are, e.g., inert gas condensation (IGC), chemical vapor deposition 1 Institut

fu¨ r Physikalische Chemie und Elektrochemie, Universita¨ t Hannover, 30167 Hannover, Germany. 2 To whom all correspondence should be addressed at e-mail: indris@ mbox.pci.uni-hannover.de

245 1064-7562/ 00/ 0700-0245$18.00/ 0  2000 Plenum Publishing Corporation

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as solid electrolytes, e.g., in advanced batteries. TiO2 is considered for applications in photocatalysis. Besides the increased surface area of the nanocrystalline powder, for particles smaller than 10 nm, quantum effects should also be observable in the absorption behavior of the semiconductor particles (increased band gap) [6]. In the nanocrystalline composite system Li2 O : B2 O3 , the additional (two-phase) interfaces between the ionic conductor Li2 O and the insulator B2 O3 cause enhanced Li conductivity in comparison to pure Li2 O [7].

2. SAMPLE PREPARATION

The nanocrystalline ceramics are prepared in a highenergy ball mill Spex 8000. We use ceramic milling vials (alumina or zirconia), a single 4-g ball, and a ball-topowder weight ratio of typically 2 : 1 to produce at least 2 g of powder. The average grain size was determined by the line broadening of X-ray diffraction (XRD) patterns (cf. Fig. 1) for various milling times. A few hours of milling are sufficient to produce nanocrystalline powders of about 20-nm grain size. Longer milling does not lead to further reduction of the average grain size, as

Fig. 2. Average grain size L0 of different oxide powders for various milling times t mill determined by XRD.

shown in Fig. 2 for LiNbO3 , Li2 O, and B2 O3 . Although kinetics of grain size reduction are different, all samples show a similar saturation value for the final grain size (including rutile TiO2 where a value of 23 nm was found after 8 h of milling). Such a behavior was also found for metals [8]. The saturation value may be decreased by performing the milling in liquid nitrogen. For LiNbO3 , some experiments with steel vials were also performed, which led to results comparable with those from alumina vials. Compacted samples were produced by cold-pressing with an uniaxial pressure of approximately 1 GPa. The density was typically about 85% of that of the single crystals. For the Li-containing oxides, which are hygroscopic, all preparation steps were performed in argon atmosphere (glove box) and for the milling step, the vial was placed in a special air-tight metal box, also in argon atmosphere.

3. GRAIN SIZES

XRD serves to identify phase transitions or chemical reactions by indicating the presence of new crystalline phases. The average grain size L0 of the samples was determined by the line broadening of XRD profiles. The ball-milled samples show increasing peak widths with longer milling times (Fig. 1). As standard procedure we used the Scherrer equation L0 c Fig. 1. XRD pattern of LiNbO3 after different milling times showing the peak broadening due to the reduced crystallite sizes. Diffraction angle v refers to CuKa radiation.

K .l b . cos v

(1 )

where K is a constant of the order of 1 dependent on par-

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ticle shape (0.89 for spherical particles), l is the wavelength of the X-ray radiation (all diffraction patterns shown in this paper were performed with CuKa radia˚ ), v is the diffraction angle, and b is tion, l c 1.54056 A the width of the peak, after corrections concerning instrumental broadening. The line broadening of the nanocrystalline samples is due to (1) the small grain sizes and (2) to strains in the material. These two effects can be separated by their different dependence on the diffraction angle v after the instrumental line broadening has been eliminated by the method of Stokes [9]. The method of Warren and Averbach evaluates not only the width of the broadened peak, but also its shape [10]. Assuming spherical particles and a lognormal distribution

gLN (L) c

f

2 . exp − (ln L − ln L0 ) 2 . 2 (ln j ) 2p . L . ln j

1



冣 (2 )

of grain sizes L one can extract the average grain size L0 and the standard deviation j . This method was applied to Li2 O, where many well-separated, intense peaks (cubic symmetry) over a wide range of diffraction angles could be measured. The results for L0 are shown in Fig. 2 and the standard deviation j was about 1.4 independent of milling time [3]. The XRD results could be verified by direct observation using transmission electron microscopy (TEM). Results are displayed for microcrystalline (unmilled) and nanocrystalline Li2 O (milled for 16 h) in Fig. 3. For microcrystalline Li2 O, a picture with magnification by a factor of 6.000 is shown in Fig. 3a. Crystallites of some microns in size are visible. In Fig. 3b, the nanocrystalline material is shown with a magnification factor of 100.000. The grains have a size of about 20 nm, which is in good agreement with the XRD results. The inserts in the pictures show electron diffraction patterns that demonstrate that both samples are crystalline. The point pattern for the microcrystalline sample reveals that only one crystallographic orientaion is present, in contrast to the nanocrystalline material where many different orientations form the ring pattern.

4. GRAIN GROWTH

A major aim in ceramic engineering is to produce fully dense materials with grain sizes smaller than 100

Fig. 3. TEM pictures of (a) micro- and (b) nanocrystalline Li2 O with mangification by a factor of 6.000 and 100.000, respectively. The inserts are diffraction patterns, which prove that both samples are crystalline and that many different orientations of the crystallites are present in the nanocrystalline agglomerates. The grain sizes are in good agreement with XRD results.

nm. Nanocrystalline samples are metastable and show grain growth when they are heated. This can be observed by TEM, XRD line narrowing, or differential scanning calorimetry (DSC). Figure 4 shows DSC scans for nano- and microcrystalline LiNbO3 in the temperature range 300–960 K and a heating rate of 20 K/ min. The nanocrystalline sample exhibits an exothermic peak at about 650 K, because of grain growth and grain boundary relaxation, which is not present in the microcrystalline sample. Corresponding results are found for Li2 O and LiBO2 . More detailed studies were made for Li2 O. The ball-milled powder was heated up to different temperatures for 10 h; XRD measurements were then done. Results are shown in Fig. 5. The broadened shape of the (400) peak, which belongs to a grain size of 20 nm, is narrowed during the heat treatment and one can extract that grain growth starts at about 450 K. Chen et al.

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Indris, Bork, and Heitjans 5. PHASE TRANSITIONS

Fig. 4. DSC measurement of micro- and nanocrystalline LiNbO3 (dashed and solid line, respectively). A broad exothermic peak is observed for the nanocrystalline sample, which is not detected for the microcrystalline sample and thus can be attributed to grain boundary relaxation and grain growth.

[11] have recently shown that two-step sintering with reduced temperature in the second step can give fully dense ceramics with less grain growth.

We also studied TiO2 in its two modifications rutile (Acros, 99.95%) and anatase (Acros, 99%). The metastable phase anatase is converted to rutile at high temperatures. This material is used in many industrial applications, e.g, as color pigment, but is also discussed for catalytic applications [6]. We are interested in the influence of the grain size on the photocatalytic activity of rutile and anatase powders. Besides increased surface areas, which should enhance catalytic activity, for grain sizes