Sintering Process of Ceramic Dielectric Materials by Hot Pressing

Abstract

The purpose of this study is to clear the sintering process of raw powders of ceramic dielectric materials by hot pressing and the electrical property of hot pressed ferroelectric ceramics.
The results obtained are summarized as follows.
(1) At first, systematic experiments were carried out on the variation of the apparent density (ρ) in the sintering process of the following solid reaction,
Bi₂O₃+3ZrO₂→Bi₂(ZrO₃)₃…(1)
As a function of the maximum firing temperature (T_m), the hot pressing pressure (P_h) and the duration of the hot pressing (t_p) in the temperature range from room temperature to the temperature when the material turns into dense ceramic body.
When the mixture of raw powders of the equation (1) was heated by ordinary firing method, the value of ρ increased from about 700°C and the sintering was completed at about 900°C. When P_h was applied, the above solid reaction was promoted above 500°C and ρ-T_m curves were shifted to lower temperature side according to the value of P_h. The density of the ceramic body prepared by hotpressing under the condition T_m=800°C, P_h=1000kg/cm² and t_p=30min, was higher than that prepared by ordinary firing at 900°C for 1hr. Next, the relation between the porosity (p) of the hot-pressed ceramics and t_p at constant T_m and P_h was examined.
(2) The same experiments were made on the following solid reaction,
0.94PhO+0.06SrCO₃+0.53ZrO₂+0.47TiO₂
→Pb_0.94Sr_0.06(Zr_0.53-Ti_0.47)O₃…(2)
In the sintering process of the equation (2) by ordinary firing method, anomalous expansion was observed at 500°-600°C. We examined ρ-T_m curves under the constant pressures, P_h=100, 300 and 700kg/cm². Consequently, the degree of the anomalous expansion was decreased by hot-pressing. At P_h of 300kg/cm², slight anomalous expansion was observed. Furthermore, the anomalous expansion was not observed at P_h of 700kg/cm² and the apparent density increased at above 600°C. According to the above experiments, it can be assumed that the pressure to prevent the reduction of density (P_a) is about 400-500kg/cm². This value seems to be useful to discuss the cause of the anomalous expansion. The density of the specimens prepared by hot-pressing under the condition T_m=900°C, P_h=700kg/cm² and t_p=30min was 7.6g/cm³ and this value is the same as that prepared by ordinary firing at 1300°C for 1hr in the PbO atmosphere.
(3) The dielectric property in the temperature range 0°-400°C was examined on several specimens of (Pb-Sr) (Zr-Ti)O₃ prepared by various methods. The specimens prepared by ordinary firing showed the sharp Curie peak at 310°C. The specimens prepared by hot pressing showed the flat temperature dependency of permittivity. After annealing the hot-pressed ceramics at higher temperature, the variation of dimension was measured in the direction of thickness and radius. Consequently, it was found that the ceramic body sintered by hot-pressing has a considerable residual strain in the thickness direction and this residual strain was released by annealing.
The value of the permittivity at Curie temperature was increased by annealing. The existence of the residual strain in the hot-pressed ceramics was also supported by the X-ray diffraction pattern.