SEMICONDUCTIVE CERAMIC MATERIALS WITH A VOLTAGE-DEPENDENT NONLINEAR RESISTANCE, AND PROCESS FOR PREPARATION

摘要

Semiconductive ceramic materials suitable for use in varistors, consisting essentially of a major proportion of Sr(1-x)CaxTiO3, where x is from about 0.01 to about 0.50, the balance being at least one of Nb2O5, Ta2O5, WO3, La2O3, CeO2, Nd2O3, Y2O3, Sm2O3, Pr6O11 and Dy2O3, and Na2O. The ceramic materials may further comprise a minor proportion of at least one of Ag2O, CuO, MnO2 and SiO2, and/or a minor proportion of Al2O3. For the fabrication of ceramic bodies of the above compositions there are prepared mixtures of the noted ingredients in powdered form in various possible combinations, plus an organic binder. The mixtures are molded under pressure into desired shape. The moldings are first fired in a reductive or neutral atmosphere and then refired in an oxidative atmosphere. Na2O can be included in the initial mixtures in the form of Na2O itself and/or other Na compound such as NaF. Alternatively, either Na2O itself or NaF is pasted and coated on the moldings after the initial firing thereof. The subsequent refiring of the coated moldings causes thermal diffusion of Na2O therein.