Sintering of the Ferroelectric Ceramics by Hot-Pressing and Their Electrical Properties
1967 Volume 75 Issue 857 Pages 12-20
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1967 Volume 75 Issue 857 Pages 12-20
In order to clarify the sintering process of the ferroelectric or antiferroelectric ceramics when the hot-pressing was applied, and the relation between the sintering conditions and the electrical properties, systematic experiments were made on the following solid reactions containing PbO,
4PbO+Ta2O5+Fe2O3→4Pb(Ta0.5-Fe0.5)O3 (PTF)…(1)
PbO+ZrO2→PbZrO3…(2)
0.94PbO+0.06SrCO3+0.53ZrO2+0.47TiO3→Pb0.94Sr0.06(Zr0.53-Ti0.47)O3(PZT)…(3)
At first, the variations of apparent density, when the raw powders of reactions (1)-(3) were heated, were examined as functions of the maximum firing temperature (Tm), the hot-pressing pressure (Ph) and of the duration of hot-pressing at Tm (tp). In this report, the following procedures were mainly taken for hot-pressing:
SP method; Ph was applied for tp (min) after the temperature reached to Tm in the firing process.
OP method; Ph was applied from a certain temperature (Ts) (Ts<Tm) in the firing process, and after the temperature reached to Tm, Ph was successively applied for tp (min).
DP method; Ph was applied during all firing process.
The results obtained are summarized as follows.
(1) When the mixture of raw powder of equation (1) was heated by normal sintering, remarkably anomalous expansion was observed at 600°-900°C with the maximum linear expansion of 22% and the reduction of density was occurred in this temperature range. However, when Ph of 100-700kg/cm2 was applied to the specimen by SP method, the densification was considerably promoted from about 500°C. Almost sintered ceramic was obtained by hot-pressing of (SP) Ph=500kg/cm2, Tm=800°C and tp=30 min.
This hot-pressing temperature is 400°C lower than the temperature of normal sintering. The specimens prepared by hot-pressing at relatively lower temperature and higher pressure (e.g. Tm=800°C, Ph=700kg/cm2) showed the broad temperature dependency of permittivity at about ferroelectric Curie temperature.
(2) PbZrO3 has been known as antiferroelectrics. When the normal sintering was used for the reaction (2), it was very difficult to obtain the dense ceramic body because of the strong evaporation of PbO. However, when the SP method was applied to PhZrO3, the remarkable effect on the increase of the densification was observed, and very dense body with 99.2% of theoretical density was obtained by the hot-pressing of Tm=1200°C, Ph=200kg/cm2 and tp=30 min.
The measurements of the permittivity (εs) vs. temperature characteristics during heating and cooling and of D-E hysteresis characteristics were made, and also X-ray diffraction analysis and electron micrographic observation of the etched surfaces were made on the specimens prepared by various hot-pressing conditions.
In PbZrO3 prepared by hot-pressing, the second transition (Tt) occurred at the lower side of the Curie point (Tc) and the ferroelectric behaviors were observed in the temperature range between Tc→Tt. These properties were considerably differed from the result of normally sintered PbZrO3 reported by Sawaguchi. When the hot-pressing was applied to PbZrO3 containing a few mole
