Seiji Ikegami

Mechanism of Aging in Polycrystalline BaTiO3

Microscopic observation of the domain structure in polycrystalline BaTiO 3 is carried out. After the specimen is cooled down to room temperature through the Curie point the domain structure changes gradually from an initial irregular one to a final one consisting of fine stripes of 90°-domains. Such a change is named “90°-splitting.” The aging of dielectric constant may be due to an increase in clamping effect by the 90°-splitting. This “90°-splitting model” may explain also the agings in dielectric loss and hysteresis loop. The dielectric constant decreases linearly with logarithm of aging time. It was shown theoretically that the displacement of a 90°-wall towards the mechanically most stable position is proportional to the logarithm of time in the case of whisker crystal. The “logarithmic time dependence” of dielectric constant in polycrystalline specimen may be explained as due to the 90°-splitting which progresses at a rate proportional to logarithm of time.

Semiconductive Single Crystal of BaTiO3Reduced in Hydrogen Atmosphere

The electrical resistivity, optical transmission, effects of chemical etching, Hall effect, thermoelectric force and electron spin resonance have been studied on single crystals of BaTiO 8 reduced for five hours at temperatures from 400 to 1000°C in a hydrogen atmosphere. The reduction treatment causes a lowering of the resistivity, and enhancement of the infrared absorption centered at 2.3µ and a visible absorption near the optical absorption edge. The single crystals are not always reduced homogeneously. The resistivity of a homogeneously reduced crystal changes abruptly at the three transition temperatures. The sign of the Hall coefficient and the thermoelectric power indicates that the charge carriers in the reduced crystals are electrons. On the basis of the results obtained, a tentative model is proposed for defects in the reduced single crystal.

Oxidation Phenomena in Semiconducting BaTiO3

The effects of sintering ambient and cooling procedure on the electrical resistivity of Gd-doped polycrystalline BaTiO 3 have been investigated. Effects of thermal annealing in air on the resistivity-temperature characteristic have been also studied, of Gd-doped specimens as well as reduced BaTiO 3 both in polycrystalline and single crystal forms. The specimens sintered in nitrogen or those sintered and followed by quenching in air have been found to be semiconducting in a wide range of Gd content (0.2∼1.0 atomic %). The high resistivity of highly doped specimens, obtained by the usual sintering process, results from the oxidation during cooling in air. The resistivity anomaly at the Curie point of some reduced single crystals is shown to be surface effect. The PTC anomaly of resistivity in polycrystalline materials may be due to oxidized grain boundary. The oxygen partial pressure during cooling is more effective for the PTC anomaly than that during sintering.

Dielectric Properties of Mixed Crystals of Barium-Strontium Titanate

Mixed crystals of (Ba-Sr)TiO 3 containing up to about 10 mole per cent of Sr were grown in the same triangular plate shape as BaTiO 3 single crystals, and dielectric measurements were performed thereon. Up to 2.25 mole per cent Sr, the dielectric constant vs temperature ( e -T) curve near the cubic-tetragonal transition temperature maintains the same form as that of pure BaTiO 3 , the peak value rising with increasing Sr content. Above 5.64 mole per cent Sr, however, the e -T curve has a rounded peak and the peak value drops remarkably. The temperature of maximum dielectric constant decreases with increasing Sr content at a constant rate of 2.9°C per mole per cent in spite of this change. The Curie-Weiss temperature also drops with increasing Sr content at nearly the same rate, the difference between these two temperatures being held at about 20°C. The Curie constant also seems to decrease slightly with increasing Sr content. A comparison between the effect of replacing Ba with Sr and the effect of hydros...

Raman Spectrum of BaTiO3

Seven Raman lines of the tetragonal BaTiO 3 single crystals, excited by the Hg-c line, have been observed at 359, 416, 502, 513, 736, 823 and 1181 cm -1 from the c-line. The three lines of higher frequencies are interpreted to be corresponding to the lattice vibrations of higher mode. A possible explanation of the four lines of lower frequencies is presented as corresponding to the fundamental modes of vibration in tetragonal BaTiO 3 . A possibility is pointed out that the “ferroelectric mode” splits into two species, of which frequencies are near 340 cm -1 and near 34 cm -1 , respectively. The Raman line located at 359 cm -1 is interpreted to be corresponding to the former species.

Dielectric Breakdown of Polycrystalline BaTiO3

The temperature dependence of breakdown field, distribution of time lag. and dielectric constant in high electric field were studied on polycrystalline BaTiO 3 . The d. c. breakdown field is nearly constant in the temperature range 20 to 150°C, on the other band the pulse breakdown field changes markedly near the Curie temperature (120°C). The time lag is statistical and depends on the applied field. The “total” dielectric constant in high fields behaves differently from that in low fields, that is higher in ferroelectric state than in paraelectric state, decreasing remarkably near the Curie temperature. From these facts, it is concluded that the breakdown is of the avalanche type. It is suggested that the pulse breakdown field ralates to the high field dielectric constant and the d. c. breakdown field is affected by the transport of free charge.

Mechanism of Microwave Dielectric Dispersion in Polycrystalline BaTiO3

The frequency dependence and temperature dependence of the dielectric constant, far infrared reflectivities, grain sizes and densities of polycrystalline BaTiO 3 , SrTiO 3 and TiO 2 were measured. From the results obtained, it is shown that, in BaTiO 3 , the microwave dispersion occurs only in the ferroelectric phase and the dielectric constant at low frequencies corresponds to that under the condition of constants stress. Thus, it is concluded that the microwave dispersion in polycrystalline BaTiO 3 is due to the transition of the dielectric constant from the value at “free” state to that at “clamped” state with increasing frequency, in the neighborhood of the resonance frequencies of piezoelectric vibration of single domains in grains of this material.