Nonmetallic Inorganic Materials

Optimizing the Matrix Strength of ZnO Varistor Ceramics

Contact: Peter Kocher

ZnO-based varistors are strongly nonlinear resistors whose conductivity increases by several orders of magnitude when a characteristic voltage is exceeded. Therefore, they are used as surge arresters in power transmission and for the protection of electronic devices (Fig. 1). Their current-voltage curve consists of three regions:
(1) pre-switch, where the behavior is ohmic and the resistance controlled by the low grain-boundary conductivity,
(2) switch or breakdown, where the behavior is nonlinear and
(3) a high-current ohmic region where the resistance is controlled by the grain conductivity.

The nonlinear region is generally described by

I = (U/C)Alpha

with I being current, U voltage, C a constant and Alpha; the nonlinearity exponent.

ZnO varistors trace back to 1971 when Matsuoka published his results on the nonlinear properties of ZnO containing different amounts of Bi2O3, CoO, MnO, Cr2O3 and Sb2O3. He found the best Alpha; value (50) for a composition containing all five additives. Today's commercial ZnO varistors contain about 10 different additives, most important of which are Bi2O3 and Sb2O3.
Bi2O3 is needed for liquid-phase sintering. Upon cooling, the melt crystallizes as a bismuth-rich phase mainly located in the triple-grain junctions. Sb2O3 is needed to control the ZnO grain growth by forming a spinel phase of nominal composition Zn7Sb2O12, but containing small amounts of Cr, Mn, Co, and Bi as well. Larger spinel grains act as pinning for the ZnO grain growth. Thus, the microstructure of a ZnO varistor ceramic consists of ZnO grains, bismuth-rich phase and spinel grains either imbedded in bismuth-rich phase or included in ZnO grains or ZnO-ZnO grain boundaries (typical example in Fig. 2).

The distribution of these latter two "secondary" phases is important for the mechanical stability of the material. When the varistor experiences a high-current pulse, an inhomogeneous microstructure allows the formation of preferred current paths, leading to local overheating. This and the resonances of resulting acustic waves in the block may lead to micro-cracks and finally to mechanical failure of the whole block. Fracture, once initiated, proceeds preferentially along the secondary phases and is thus eased by an inhomogeneous microstructure.

The connection between critical (fracture-originating) flaw, microstructure and strength is given by the Irwin relation:

sigmac = KIc / (Y a1/2)

with KIc being the fracture toughness and Y the shape factor.
For a given critical flaw size (critical flaws being for example large pores, hollow sprays granules or triple-points between granules), the strength can still vary widely especially with different grain size and phase distribution, which both determine the fracture toughness.

In the current project, together with ABB High Voltage Technology (Wettingen, Switzerland), we study:


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