A ceramic product as an electronic component has a ceramic substrate and an electrode generally formed of metal on the ceramic substrate. Examples of such a ceramic product are a laminated ceramic capacitor with a Ni electrode, Pd electrode or Pt electrode, a LTCC (low temperature co-fired ceramic) component with a Ag electrode, Cu electrode or Ag—Pd electrode, a piezo actuator with a Pd electrode, a semiconductor package with a W electrode, and a spark plug with a Ir electrode or a Pt electrode.
However, the firing of Ni, Cu, W with the ceramic substrate requires atmosphere control that makes it difficult to attain the intended performance of the ceramic substrate and results in high production cost. On the other hand, the melting point of Ag is low (962° C.) so that not only the material of the ceramic substrate applicable becomes limited, but also the properties of the ceramic substrate may be impaired by low-temperature firing. Further, noble metals such as Pd, Ir and Pt are expensive so that it is difficult to use these noble metals for electrode applications where large areas are required.
Patent Document 1 discloses, as an oxide material for an electrode, a lanthanum-cobalt oxide having negative resistance temperature characteristics in which the oxide shows a high resistance at room temperature and decreases in resistance with increasing temperature. Patent Document 2 discloses a lanthanum-cobalt oxide that shows a high resistance at around room temperature and has a B constant large in absolute value at high temperatures. Patent Document 3 discloses a non-magnetic substrate material for a magnetic head and teaches a La(Co,Ni)O3 oxide as a reference example.
However, the conductive oxides as disclosed in Patent Documents 1 and 2 are high in resistance and insufficient in conductivity at room temperature. The La(Co,Ni)O3 oxide as disclosed in Patent Document 3 is not adequate as an electrode material in light of the description that the oxide develops a crack which causes an increase in electrical resistance.
Since the ceramic component with the metal electrode faces the above various problems, the present inventors have studied the use of an oxide (ceramic) electrode as a substitute for the metal electrode. However, conventional oxides are much lower in conductivity and larger in B-value (temperature coefficient) than metals so that it is difficult to use electrodes of such oxides as substitutes for the metal oxide. Ruthenium oxides (such as RuO2 and SrRuO3) are known as high conductivity oxides, but has the problem that Ru is expensive. The present applicant has disclosed, in Patent Document 4, an oxide sintered compact having a high conductivity and small B constant (temperature coefficient) and suitable as a conductive material.