Dissociation of basal dislocations in hexagonal barium titanate

Abstract

Summary Dislocation substructure in hot‐pressed hexagonal BaTiO3 ceramics was analysed by transmission electron microscopy. Two dislocation networks each consisting of dissociated half‐partials were determined for the Burgers vectors (b) using the g · b = 0 effective invisibility criteria, and the true directions (u) by trace analysis. Each of the networks contains three partial nodes that are in the form: 1/3[010]+1/3[100]+1/3[100]+1/3[100] = 0, where four partials meet at a point and the Burgers vectors are conserved, as analysed by the weak‐beam dark field technique. Basal dislocation with bb = 1/3<110> is dissociated into two prism plane half‐partials with bhp = 1/3<100> by: 1/3<110> → 1/3<010> + 1/3<100>. Dissociation of basal dislocation by a glide mechanism creates a stacking fault when shear occurs along <100> in the c‐layer of (000l), where l = 1, 3, 4 and 6, of the (chc)1(chc)2 (or (CBC)(ABA)) stacking sequence. The slip system of 1/3<110>(0001) in hexagonal BaTiO3 has been activated at 1300 °C by hot‐pressing under ∼25.8 MPa. Plastic flow contributing to the densification of hexagonal BaTiO3 ceramics occurs through glide of half‐partials in the basal plane by a glide‐controlled dislocation glide mechanism. Dislocation motion governed by the Peierls mechanism, where velocity is determined by both correlated and uncorrelated double‐kink nucleation on two half‐partials, is discussed.

Migration of Half Partial Dislocations in the Planar Fault Plane of Hexagonal Barium Titanate

“…Figure 1(a) shows tetragonal and metastably retained hexagonal phases coexisted 18 in the region where dislocations 10 and one of the planar faults 6,9 have been analyzed. Several faults were clearly discerned by weak‐beam dark‐field (WBDF) imaging.…”

“…Perfect basal dislocations with Burgers vector b B = 1/3 〈1100〉 and a pair of prism plane half partials b ′ and b ″ with b Pr = 1/3 〈1100〉, all lying in the basal plane, were identified 10–13 . Basal dislocation dissociating into a pair of half partials has also occurred 8,10 in h ‐BaTiO 3 by the classical reaction …”

“…Although the rate‐determining mechanism for the hot pressing of h ‐BaTiO 3 has not yet been studied systematically, similar to creep deformation, 16 the mechanism being either diffusion or dislocation controlled was deduced 17 experimentally from the stress exponent ( n ) and grain‐size exponent ( p ) 14 combining with microstructure observation 15 . Microstructure analysis 10,14,15 is indispensable since only then are grain boundaries free of second phase(s) and corresponding dislocation substructure confirmed 15 . Dislocation‐controlled mechanisms (with n ≥3 and p =0) are further divided 16 into glide‐controlled glide (as suggested 10 for h ‐BaTiO 3 ) or climb‐controlled glide.…”

“…Microstructure analysis 10,14,15 is indispensable since only then are grain boundaries free of second phase(s) and corresponding dislocation substructure confirmed 15 . Dislocation‐controlled mechanisms (with n ≥3 and p =0) are further divided 16 into glide‐controlled glide (as suggested 10 for h ‐BaTiO 3 ) or climb‐controlled glide. The latter has an activation enthalpy (Δ H ) similar to that of the Nabarro–Herring (N–H) creep (with n =1 and p =2) with diffusion being the rate‐determining step, e.g., the lattice diffusion of oxygen via vacancies in the hot pressing of MgAl 2 O 4 14…”

“…The 6H‐polytype obtained by hot pressing 5,6 of undoped nano‐size BaTiO 3 powder was analyzed 10 before for its dislocation substructure where plastic flow by a glide‐controlled glide mechanism of prism plane half partials has contributed to the densification of the ceramic. Here, we report observation and analysis of such half partials dissociated in an EPSF fault plane where such fault planes were found 6,9 ubiquitously in hot‐pressed h ‐BaTiO 3 .…”