Diffusion and Ionic Conduction in Nanocrystalline Ceramics
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Diffusion and Ionic Conduction in Nanocrystalline Ceramics Paul Heitjans and Sylvio Indris Institut f¨ur Physikalische Chemie und Elektrochemie, Universit¨at Hannover, 30167 Hannover, Germany
ABSTRACT Diffusion and ionic conduction in nanocrystalline ceramics, both monophase and composite, was studied by NMR relaxation and NMR lineshape as well as impedance spectroscopy. Mea surements were mainly done on ion conductors prepared by high-energy ball milling. It was possible to discriminate between mobile ions in the interfacial regions and immobile ions in the grains. In general the diffusivity and conductivity are enhanced in the nanocrystalline monophase system as compared to the microcrystalline one, e. g. by about four orders of magnitude in the case of . An exception is, e. g., where the nano- and microcrystalline forms have similar con ductivities. However, when the nanocrystalline insulator is added to nanocrystalline the conductivity of the composite increases whereas it decreases in the corresponding microcrystalline system. INTRODUCTION Nanocrystalline materials are polycrystals with a grain size of a few nanometers and a large fraction of atoms or ions located in interfacial regions. They have attracted considerable interest in recent years because of the possibility to tailor functional properties such as fast ionic conductivity, high mechanical creep rate or increased catalytic activity [1-5]. These properties are partly due to an enhanced diffusivity with respect to that in the microcrystalline counterparts. Mixing with a different phase, being an ion conductor or not, offers additional freedom to tune atomic transport by varying composition and grain sizes. We report on studies of the monophase systems [6-8],
[9,10], [11-13], [14] and the composite material !#"%$ :"& [1518]. The Li containing compounds are interesting for applications as solid electrolytes/electrodes e. g. in battery systems, and is a well-defined model compound showing anionic conduction. For comparison also microcrystalline samples (average grain size of some micrometers) were pre'( ) pared for all materials. Furthermore, amorphous samples were prepared in the cases of and . By this it was possible to study modifications with the same chemical composition but different microstructure and to examine influences of structural disorder on diffusion in these solids.
Y6.6.1
average grain diameter L0 (nm)
120 Li2O LiNbO3 B2O3 LiBO2
100 80 60 40 20
*
0 1
+milling time t
10 mill (hours)
100
Figure 1. Average grain diameter vs. milling time for different ball-milled ceramics as determined from the broadening of x-ray diffraction lines [18].
EXPERIMENTAL DETAILS Samples The nanocrystalline ceramics studied here were mainly prepared by ball milling of the coarse grained source materials in argon atmosphere. A high-energy ball mill (SPEX 8000) with an alumina vial and a single ball was used. The ball-to-powder weight ratio was ty
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