BaTi03 ceramics doped with several iron-group elements up to 2.0 mole 'Io were identically prepared and afterwards annealed at 700'C in 02 at 150 bar or in H2-H20 with an oxygen partial pressure of,10 bar. The valence states of the dopants were determined by magneticsusceptibility measurements, and the phase-transition temperatures were established by dielectric measurements.The transition temperatures and the charge of the Cr, Mn, and Co dopants were found to change reversibly upon annealing at different oxygen partial pressures whereas both remained constant for Fe, Ni, Zn, and Ga dopants. The change of the concentration of oxygen vacancies, which proved to be inherently connected with valence changes, was investigated gravimetrically.
Nm(gain)' '. Since they found reasonable agreement with this relation for the formation of a plastic zone in steel, it appears to be widely applicable, lending support to the adjustment for gain made in this paper.
V. Summary and ConclusionsThe AE that accompanies subcritical crack growth in glass is caused by the interaction between the moving crack and defects on the surface. As-received glass plates produce AE at a very low level but abrasion of the surface with S i c powder increases both the level and the reproducibility. A surface groove =4 p m deep produced one AE count in the present experiments.The relation between AE rate, N, and system sensitivity was found to be
N.((V,/V,)'(11) Lower sensitivity to specimen size and thickness were observed.Of the four AE simulators examined, spark bar, helium jet, pulser, and dropped ball, none was completely satisfactory in reproducibly simulating AE.
Acknowledgments:S. M. Wiederhorn for providing a stimulating and congenial environment.
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