Doping of ferroelectric solutions of (Ba, Sr Ti O3) with In2 O3 to improve the RC time constant

Abstract

The principle of conversion of solar energy into electrical energy using ferroelectric materials has been discussed by Hoh [1]. A capacitor made of ferroelectric material is subjected to heat cycling above the Curie temperature. During heating the permittivity across the ferroelectric material decreases and hence the voltage across a charged capacitor increases. The heat energy supplied to the capacitor is converted into electrical energy and could be obtained by discharging the capacitor.

It is known that the Curie temperature of Ba Ti O₃ can be decreased by doping it with Sr TiO₃. It has been observed by us that each 1 per cent by weight of Sr Ti O₃ lowers the Curie temperature of Ba Ti O₃ by 3·5 deg.

It has been pointed out by Hoh [1] that Ba Ti O₃ is not suitable for ferroelectric energy conversion devices because of semiconduction. Solid solutions of various percentages of Sr Ti O₃ in Ba Ti O₃ are also not suitable for such devices because of semiconduction.

The efficiency of the device depends upon the RC time constant of the capacitor. The larger the RC time constant, the more efficient is the device. The product RC = ϱε, where ϱ is the resistivity in Ω-m and ε is the permittivity in F/m. Since changes of ε in orders of magnitude are not conceivable, it is essential that the resistivity determines the RC time constant.

In this work a method for increasing the resistivity of (Ba, Sr) Ti O₃ solid solutions was developed by utilizing the principles of electrical compensation. It was found that (Ba_0.77, Sr_0.23) Ti O₃ solid solution with a Curie temperature of nearly 30°C showed an N-type of conduction. To neutralize the excess free electrons, an appropriate amount of indium was added in the form of In₂ O₃. The resistivity was observed to increase by two orders of magnitude with optimum doping. Excess doping impaired the compensation.