954 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



diminish in length as they approach the twin-plane normal. This is perhaps 

 more obvious in Fig. 6 where only one pair of twins is shown. 



Finally, the streaks from bent crystals are more uniform in intensity 

 than those with intertwin-oriented material, but if the bending were non- 

 uniform or the intertwin material more abundant this might not be so. 



X-RAY Evidence of Effect of High Field 



The Laue photograph of crystal a, taken after the poling process had 

 caused it to become a single untwinned crystal. Figs. 3A and 4A, is 

 shown in Fig. 9A. It shows a pattern of single spots. The absence of 

 streaks agrees with the paucity of birefringent regions in Fig. 4A in indi- 

 cating very little inhomogeneous strain in the crystal. 



The Laue photograph of crystal 6, taken after the poling attempt had 

 produced only a regular multiple-twin pattern in it, Figs. 3B and 4B, is 

 shown in Fig. 9B. The inhomogeneous strain has not disappeared, as 

 indicated also by the birefringent regions in the photograph of Fig. 4B. 



Summary of Results of Single-Crystal Experiments 



From the experiments described above it is concluded that barium- 

 titanate crystals with only twin-boundary strain can, under the influence 

 of a high field at elevated temperatures, be caused to have a single crystal- 

 lographic orientation whereas barium titanate crystals otherwise strained 

 cannot. 



Application to Barium Titanate Ceramics 



Single crystals of barium titanate large enough for practical applications 

 have not yet been grown. Therefore all practical applications using barium 

 titanate have so far used it in the ceramic form. 



Ceramics have been made for two dififerent applications: condensers and 

 electromechanical transducers. For the first, the maximum electrical po- 

 larizability for a given applied electric field is desired, since this results in 

 a high dielectric constant. For the second application, however, it is desira- 

 ble to have a ceramic which will deform mechanically in an electric field 

 according to its own polarity. With this end in view, ceramics intended for 

 electromechanical transducers have been poled by being subjected to a high 

 field (roughly 15000 v/cm.) as they were cooled through the Curie tempera- 

 ture to room temperature. 



In a series of unpublished experiments W. P. Mason and R. F. Wick of 

 the Bell Laboratories have found that certain barium titanate ceramics, 

 when poled in this way, retain their polarization in spite of high reverse 

 fields (J to J the poling fields), i.e. require a high coercive force to change 



