As an electrode of a capacitor made of a sintered metal, those which are composed of sintered aluminum, tantalum and alloys thereof are known. These capacitors have widely used in various fields. For example, for a capacitor used in a smoothing circuit for obtaining a direct current from an alternating current, it is desired that the capacitor possesses a low impedance and a large capacitance at a high frequency for suppressing the occurrence of spike-shaped voltage and enhancing the efficiency of conversion to a direct current.
The above-mentioned sintered metals used as a capacitor electrode have problems. Namely, sintered aluminum has poor environmental characteristics such as moisture resistance and chemical characteristics, and sintered tantalum is expensive. Sintered niobium is also known as a material used for a capacitor electrode, and not possessing the problems encountered with sintered aluminum and tantalum, but, it has another problem that oxygen adsorbed on its surface influences dielectrics as mentioned below, and thus, the leak current characteristics are not satisfactory and it is of poor practical use.
To provide a capacitor used in a smoothing circuit and having an enhanced capacitance at a high frequency, the volume of a sintered metal substrate made of, for example, tantalum or aluminum, should be increased. The increase in volume of the sintered metal substrate is inconsistent with a requirement of miniaturization of a capacitor. Among others, tantalum gives a relatively satisfactory for the requirements of an enhanced capacitance at a high frequency and a miniaturization of a capacitor, but, it is still not completely satisfactory for these requirements. Usually a tantalum oxide is used as a dielectric substance for a capacitor with an electrode composed of sintered tantalum bodies. However, if a material having a dielectric constant larger than that of tantalum oxide is used as a dielectric substance, the capacitor can be more miniaturized. As examples of the material having a large dielectric constant, there can be mentioned titanium oxide and niobium oxide. But, these materials exhibit poor leak current (hereinafter abbreviated to “LC”) characteristics.