Piezoelectric ceramic devices produced by forming electrodes on sintered piezoelectric ceramics (hereinafter referred to as sintered ceramics) have been used in ceramic filters, speakers, ultrasonic oscillators, etc. In general, piezoelectric ceramic devices are polarized by direct current voltage so as to orient their polarization in the same direction. In order to increase the degree of polarization of the devices, high voltage is applied to the devices, and the devices are required to be resistant to the high voltage for polarization with no dielectric breakdown.
Because piezoelectric ceramic devices are often used under severe conditions, sintered ceramics are required to have good weather resistance, especially good moisture resistance.
Sintered ceramics are obtained by calcining a piezoelectric ceramic, then mixing it with a binder, granulating the resulting mixture, molding the resulting granular pellets and firing the molding. Usually, the molded units ("moldings") are stacked in a furnace to fire as many as possible. However, there is a problem in that the stacked moldings fuse so as to be combined together at high temperatures. To solve this problem, the following methods have been proposed.
As illustrated in FIG. 1, particles 25 are provided between stacked moldings 23. The particles used include zirconia, Al.sub.2 O.sub.3 and the like. First, the powder 25 is spread over a support (not shown). Then a molding 23 is put thereon. Further, the powder 25 is spread over the upper surface of the molding 23, and the other moldings 23 are piled up thereon one after another with spacing via the powder 25.
Another method is illustrated in FIG. 2 with reference to Japanese Patent Publication (JP-B) Hei-3-2821. Coarse grains 23a having the same composition as that of the moldings 23 but having a large mean grain size are dispersed in each molding 23, by which the moldings 23 are prevented from being combined together. The is referred to as a coarse grains-mixing method. The moldings 23 can be piled up without providing powders therebetween.
In the above-methods, piles of the moldings 23 are fired and then they are separated into individual sintered moldings 23.
However, the sintered ceramics produced by the conventional methods do not sufficiently satisfy recent requests in the market for piezoelectric ceramics having much better characteristics including moisture resistance and being much low-priced.
In addition, the conventional method have the following problems:
1. An additional step is required for removing powder from the sintered moldings.
2. The sintered moldings still have traces of the powder 25.
3. It is difficult to evenly spread the powder 25 between the adjacent moldings 23 and this causes great warping of the moldings 23. Therefore, the sintered moldings must be re-heated to remove the warping.
The coarse grains-mixing method is also problematic in the following points:
1. Since the coarse grains 23a have the same composition as that of the moldings 23, some amount of the grains may be combined with the moldings 23 even under a relatively low firing temperature.
2. Under a high firing temperature, most of the grains are combined with the moldings 23.
3. The coarse grains 23a must be prepared in an additional step. In addition, different types of coarse grains must be prepared for different compositions of piezoelectric ceramics. Therefore, the method using such coarse grains is expensive.