Abstracts of Scandinavian Society/or Electron Microscopy 425

energy region in EELS. In the quantita- tive analysis by the technique proposed by the present paper, there is a neces- sity to give double partial cross-section.

COMBINED CBED AND HREM STUDIES OF MATERIALS

A. Olsen

Institute of Physics, University of Oslo, P. O. Box 1048, Oslo 3, Norway

High resolution electron microscopy (HREM) and convergent-beam electron dif- fraction (CBED) are well established methods for structural investigations. However, the information from these two techniques is mainly complementary. By direct imaging HREM gives information about the atomic arrangements of period- ic and non-periodic structures. On the other hand, CBED provides information about crystal symmetry, lattice paramet- ers and structure factors. Except for a CBED and HREM study by Olsen and Goodman ! very few combined studies have been re- ported in the literature, and most were carried out in two different kinds of instruments. However, with the new gen- eration of transmission electron micro- scopes these techniques can be combined in one instrument in a much better way than previously. The present paper re- ports some combined CBED and HREM stud- ies using two different kinds of instru- ments and discusses the new possibilities for such combined work by using one of the new generation of transmission elec- tron microscopes.

i. A. Olsen and P. Goodman, Ultramicroscopy 6 (1981) i01.

ANALYTICAL ELECTRON MICROSCOPY OF ZnO VARISTOR MATERIAL

E. Olsson, L. K. L. Falk and G. L. Dunlop

Department of Physics, Chalmers Uni- versity of Technology, S-412 96 G~teborg, Sweden

Polycrystalline ZnO materials con- taining small additions of certain oxides such as Bi203 and Sb203 exhibit extremely non-linear current/voltage characteris- tics. Below a certain critical voltage only a small leakage current is transmit- ted throuqh the material, but above this

critical voltage the current increases significantly. These electrical proper ties make the material very suitable for use as surge protectors for high voltage equipment.

A thorough evaluation of the micro- structure of a ZnO varistor material has been carried out by analytical el- ectron microscopy. The bulk of the microstructure consists of ZnO grains of approximately 20 ~m in diameter in- terspersed with smaller particles of the spinel phase Zn7Sb2012. The inter- granular regions of the microstructure, considered to be of vital importance for the electrical performance of the material, are dominated by Bi-rich crystalline or amorphous phases. These have been fully characterized by elec- tron diffraction and STEM/EDX. Usually the Bi-rich phase at ZnO/ZnO grain boundaries is a thin amorphous film. Boundaries which did not have adiscrete Bi-rich phase associated with them con- tained instead a significant segrega- tion of Bi.

THE DEVELOPMENT OF OXIDE SCALE ON POLYPHASE Si3N 4

C. O'Meara and G. L. Dunlop

Department of Physics, Chalmers University of Technology, S-412 96 G~teborg, Sweden

High-temperature oxidation of sin- tered 8-Si3N 4 results in the formation of a polyphase oxide scale, whose com- position and phase content is related to the amount and composition of the intergranular glassy phase in the bulk of the material. This amorphous phase, which is rich in additive and impurity ions, is responsible for the reduction of high-temperature strength. Examina- tion by SEM together with EDX analysis has been used to study the structure and phase composition of the oxide scale which forms on a commercial B-Si3N 4 material indifferent high- temperature regimes.

The intergranular glass phase is not at equilibrium with the SiO 2 film, which forms at the beginning of oxida- tion. The process of continued oxida- tion is controlled by the outward dif- fusion of metal cations from the glass to the oxide scale, driven by the con- centration gradient between these two phases. The cations react with SiO 2 to form various silicate phases and the