Abstract: Low-frequency acoustic mode atomic force microscopy was successfully developed based on the commercial AFM. The experimental results on nanometer scale elastic response at the grain boundaries in ZnO varistors assessed by the technique were presented. Different acoustic contrast of the individual micro-grains at grain boundaries were examined which reflected the distribution of additives. The acoustic contrasts enhanced at the grain boundaries can be observed clearly, which present possibility of crystal lattice expanding of Bi-rich phase caused by phase transition in the heat treatment. The lateral resolution of acoustic image is down to nanometer scale. The results show the application perspective of low-frequency scanning probe acoustic microscopy in functional materials at submicro- or nanometer scale.

Key words: scanning probe microscopy, acoustic, local elasticity, ZnO varistor, grain boundary