1. Field of the Invention
The present invention relates to a method for producing transparent PLZT ceramics, which have a uniform composition and a high light-transmittance. More particularly, the present invention relates to a method for producing a calcined PLZT powder which can yield the transparent PLZT ceramics mentioned above. Transparent PLZT ceramics are intended for use in the field of opto-electronics as the materials for an optical shutter, an optical valve, an optical switch, an image discharge-storage device, an optical modulation component, and the like, by utilizing the electro-optic effect, electro-optic scattering effect, piezoelectric effect, and the like of these ceramics.
2. Description of the Related Arts
The transparent PLZT ceramics are produced by preparing the powder, pressing the powder to form a green compact, and hot-pressing the green compact in air or an oxygen atmosphere, or sintering the green compact under a normal pressure and in a mixed atmosphere of oxygen gas and PbO vapor.
In the case of sintering, an excessive amount of PbO is frequently included in the green compact, to generate a liquid phase therein and thus promote the densification thereof.
To obtain a PLZT sintered body having a chemically high uniformity and a high light transmittance, it is important that the PLZT powder have a uniformity in shape and chemical composition, and that the powder can be easily sintered. A sintered body having a nonuniform chemical composition is optically non-uniform, with the result that light transmittance and electro-optic coefficient vary greatly and cause serious problems when in practical use. In addition to produce the PLZT ceramics at a low cost, inexpensive starting materials must be used, and the process must be simple and have a high workability.
As previous methods for producing the starting material of transparent PLZT ceramics, there are the dry method, the alkoxide-coprecipitation method, the oxalate method in ethanol solution, and the multi-stage coprecipitation method. The dry method, for example, is described in Journal of the American Ceramic Society, Vol. 54, No. 1, page 1-11 (1971). The alkoxide-coprecipitation method, for example, is described in Ferroelectrics, Vol. 3, page 269-280 (1972). The oxalate method in ethanol solution is described in Japanese Unexamined Patent Publication No. 60-180959. The multi-stage coprecipitation method is described in Japanese Patent Application No. 59-228760, corresponding to Japanese Unexamined Patent Publication No. 61-106457 entitled, A Method for Producing PLZT Light-Transmitting Ceramics.
In the dry method, the constituent compounds are mixed and then calcined, but it is difficult to ensure a uniform mixing of the four constituent compounds in the dry method, and thus the resultant calcined powder is liable to have a nonuniform chemical composition. In addition, since the calcination is carried out at a high temperature to provide a uniform composition, the particles sinter together due to the neck formation, and this leads to the formation of coarse particles. As a result, it is also difficult to obtain a starting material which can be easily sintered.
In the alkoxide-coprecipitation method, the lead oxide powder, zirconium alkoxide solution, titanium alkoxide solution, and lanthanum acetate solution, for example, are mixed, coprecipitated, dried, calcined, and then grinded. According to this method, the obtained powder has a uniform composition and is easily-sintered, but because of the high cost of the alkoxide and acetate used in this method, an inexpensive PLZT powder cannot be obtained.
In the oxalate method in ethanol solution, ethanol is added to the aqueous mixed solution containing nitrates of lead, lanthanum, zirconium, and titanium, and an ethanol solution in which oxalic acid has been dissolved is dropped into the aqueous mixed solution, to precipitate the oxalates of the respective components. These coprecipitates are thermally decomposed, calcined, and then grinded. The PLZT powder obtained by this method is fine and is easily-sintered, but the cost is high because of a large amount of ethanol and oxalic acid must be used, which is comparatively expensive.
The development of the alkoxide-coprecipitation method and the oxalate method in ethanol solution lies in the provision of methods in which titanium tetrachloride is not used, as when titanium tetrachloride is used in a precipitation method, the chlorine therein reacts with lead to yield a white precipitate of lead chloride, with the result that the desired co-precipitation is not realized. Therefore, the above two methods, in which titanium tetrachloride is not used, were devised.
In the multi-stage coprecipitation method as disclosed in Japanese Unexamined Patent Publication No. 61-106,457 it is possible to use titanium tetrachloride as the titanium source, as follows. Four aqueous solutions each containing one of the components, i.e., lead, lanthanum, zirconium, and titanium, are prepared, and then the lanthanum aqueous solution and the zirconium aqueous solution are mixed with either the lead aqueous solution or the titanium aqueous solution, thereby obtaining two kinds of aqueous solutions. One of these aqueous solutions is mixed with a liquid precipitant in an excess amount to form the precipitates. Subsequently, the remaining aqueous solution is uniformly mixed with the aqueous solution containing the dispersed precipitates, thereby forming homogeneous precipitates of all of the components. The precipitates are then dried calcined, formed, and sintered. This method is advantageous in that titanium tetrachloride, which cannot be used in the conventional co-precipitation method, can be used as the titanium source, and in addition, since the precipitation is carried out in two stages, the kind and concentration of the precipitant-liquid can be selected while ensuring that they are appropriate to the composition of precipitates to be formed at the respective stages. Accordingly, the secondary particles do not grow, the sintering is easy.
When the PLZT is used as an optically functional material, the chemical composition of a sintered body must be microscopically uniform. To provide such a microscopically uniform chemical composition, the chemical composition of the PLZT powder must be uniform, that is, in the PLZT, which is expressed by ABO.sub.3 of perovskite, the quantity ratio of Pb and La of the A-site constituent elements, as well as the quantity ratio of Zr and Ti of the B-site constituent elements must be uniform in all particles of the powder, and must have particular values.
The conventional, powder-synthetizing methods are now evaluated from the viewpoints of a necessity for the composition to be uniform, and the powder to be inexpensively produced by a simple process. First, the process of the dry method is simple but raw materials having a high purity must be used. In addition, since the mixing method by, for example, a ball mill, is limited with regard to a uniform mixing, the composition of the obtained powder has a poor uniformity. Second, in the wet methods, such as the alkoxide-coprecipitation method and the oxalate method in ethanol solution, it is easy to obtain a powder having a uniform composition, but expensive raw materials must be used, the production process is complicated, and the workability is poor. Third, in the multi-stage coprecipitation method, the fine particles are obtained by using inexpensive raw materials, such as titanium tetrachloride, but it is difficult to obtain a powder having a uniform composition.