Experimental Study on the Granulation and the Forming for Raw Powders of Ceramic Dielectrics

The granulation and the Forming are very important processes in the fabrication of the ceramic dielectrics. Generally, in the case of dry pressing, the raw powder is mixed with an organic binder and water and then the second particle (i.e. granule) is prepared by a method such as screening or spray drying in order to flow quickly from the hopper into the die cavity. The authors carried out systematic experiments on the effects of the binder influencing upon the electrical and physical properties of the final products and on some factors affecting to the properties of granules.The results obtained are summarized as follows.(1) The mechanical strength (S) of the pressed body was measured as a function of the heating temperature (T) from room temperature to the sintering temperature. As a result, it was found that the S-T curve has a minimum point at about 400°C, which corresponds just to the decomposing temperature of the binder and this characteristic is fairly influenced by the quantity of the binder.(2) By the same method as above, electrical resistivity (ρ) at 150°C versus heating temperature (T) curve was measured with the results that the ρ-T curve has a minimum point at about 1000°C.It can be interpreted as a resultant curve of two effect, that with highter heating temperature, the contact area between particles is increased and the concentration of the non-reacted ion is decreased accompanying with the soild reaction.(3) It was clarified that the void remaining inside ceramics was increased by increasing the quantity of the binder and this affected to the breakdown characteristics of the dielectric ceremics.(4) The experiments on the PZT ceramics containing PbO under a hot-pressing condition were carried out with the results that the organic binder has a considerable reducing action such as separating of metalic Pb.(5) The fluidity of the granules was correlative to the bulk density for any granules prepared by various conditions. It was concluded that the most important charactor of a well-granulated body is to have a high bulk density, with an exception of the case of dry pressing for a very thin body.