Abstract
Based on the thermal activation and the spin-polarized tunneling, which depend on temperature range and grain separation, the transport phenomenon is discussed for polycrystalline manganites. By averaging over the distribution of grain separation, we derive a general expression of resistivity which in form is similar to a two-component description. A quantitative agreement with the experimental data obtained in polycrystalline La2/3Ca1/3MnO3 gives a strong support to this model. Some main features dealing with the grain boundary effect in polycrystalline manganites are discussed on the basis of this model.