Activation of ferroelectrics

Abstract

Abstract 776,568. Ferroelectric materials. AMERICAN LAVA CORPORATION. Dec. 9, 1954 [Dec. 11, 1953], No. 35714/54. Class 37. [Also in Group XL (b)] A method of activating a ferroelectric ceramic comprises (i) applying an activating current to the ceramic with the ceramic heated to not more than 10‹ C. above its transition temperature and (ii) continuing to supply the activating current whilst allowing the ceramic to cool. The term " activated " means treatment to produce increased dielectric constant. The term " transition temperature " means the temperature at which the first crystal transformation above room temperature is effected. The current is D.C. and is preferably that which produces a change of at least 15 per cent and more preferably the maximum change in dielectric constant. The current may be maintained. substantially constant while the ceramic cools to at least 65‹ C. below its transition temperature. The maximum change in dielectric constant for a particular ceramic is found by using sample pieces of the material and subjecting each piece to activation until that current is found which produces the largest. change in dielectric constant. Detailed embodiment.-A ceramic cylinder 10, shown in Fig. 1, has silver electrode coatings 11, 12 and leads 13,14. The ceramic material comprises 96 per cent barium titanate and 4 per cent lead titanate. A number of such cylinders are immersed in chlorinated diphenyl and slowly heated to 10‹ C. above the transition temperature (120‹ C). A D.C. is then applied to leads 13, 14 and increased gradually until it reaches 14 micro-amperes per square inch of electrode surface. After a short time the temperature is lowered slowly and the current maintained constant by increasing the applied voltage as the cylinders cool to about 65‹ C. below the transition temperature. After complete cooling the diphenyl is removed by dipping the cylinders in acetone. The activating current value of 14 micro-amperes is that which produces the greatest change in dielectric constant for the given ceramic. This value is found by treating groups of cylinders using different current values for each group and measuring the capacity values and compiling results. Fig. 1 has application as a condenser or piezo-electric device. Ceramic materials.-The invention may be performed using the following ferro-electric ceramics : titanates of alkali metals and alkaline earth metals; tungstates, bismuth oxides, strontium cerate, boron phosphate, arsenic sesquioxide, lithium ferrite, lithium sulphate and antimony iodide; also titanate compositions comprising selectively as additives or minor proportions the stannates and zirconates of barium, lead and calcium and the titanates of calcium, strontium and lead.

Inventors

• 👤 John D Wallace

Bibliographic Details