Jean-Claude M’Peko

In situ characterization of the grain and grain-boundary electrical responses of zirconia ceramics under uniaxial compressive stresses

The electrical properties of ion-conducting tetragonal zirconia ceramics subjected to mechanical stresses were studied using impedance spectroscopy. The material’s overall resistance (grain and grain boundary) was found to increase when stress was applied perpendicularly to the measuring electric field (σ⊥E), while only comparatively discreet variations, involving a decreasing trend of resistance, occurred when the electric field and mechanical stress were parallel (σ∥E). The increment in electrical resistance for σ⊥E was found to be consistent with an increase of the conduction process energy barrier. The mechanical effect reported here is of elastic nature, covering a wide range of applied stresses. The electrical characteristics from cracking and fracturing at higher stresses are also presented.

Ionic transport in polycrystalline zirconia and Frenkel's space-charge layer postulation

Various physical and chemical techniques have been applied elsewhere to get insight into the peculiarities of conduction processes in zirconia-based ionic conductors. Although Frenkel’s space-charge model is popularly accepted to describe well the capability of grain boundaries for limiting ion transport throughout these materials, validating this model has generally resulted to be a difficult task provided substantial restrictions to access the subgrain nanometer-sized regions involved. The present work reports a simple but consistent method to also approach this important question, and consists of studying the bulk and grain-boundary electrical responses of these materials while they are subjected to external mechanical loads.

Dynamics of normal to diffuse and relaxor phase transition in lead metaniobate-based ferroelectric ceramics

Tetragonal tungsten bronze-structured materials based on lead metaniobate (PbNb2O6) were studied in terms of thermal dynamics of dielectric properties, showing ferroelectric-to-paraelectric phase transition of diffuse and relaxor type in some specific cases. These features are normally ascribed to defects-induced structural disorder and compositional fluctuations associated with an arbitrary lattice site occupation between dopant and host ions. Nevertheless, for these lead metaniobate-based materials, the drastic change in the phase transition from normal to diffuse and relaxor is shown to take place when dopants are able to significantly shift the transition toward low temperatures, where these compounds are known to exhibit incommensurate superstructures that naturally present diffuse and relaxor dielectric characteristics.

Electrochemically induced crystallization in lead fluoroborate glasses below the glass transition temperature

Lead fluoroborate (BO1.5–PbO–PbF2) glasses were prepared by melt quenching and studied here in terms of (micro)structural and electrical properties. The results show that electrochemically induced surface crystallization may also occur below the glass transition temperature. As previously proposed elsewhere and shown here, occurrence of this phenomenon arose from a field-induced redox reaction at the electrodes, followed by crystallite nucleation. Once nucleated, growth of β-PbF2 crystallites, with the indication of incorporating reduced lead ions (Pb+), was both (micro)structurally and electrically detectable and was analyzed. In particular, the authors introduce a procedure combining impedance spectroscopy and polarization/depolarization techniques to also monitor this phenomenon.

Linking origin of the electric field-assisted β-PbF2 crystallization in lead oxyfluoroborate glasses below Tg to simultaneous cathode/anode-compensated electrochemical reactions

Full validation of the electrochemical mechanisms so far postulated as driving force of electric field-assisted non-spontaneous crystallization development in given glasses has suffered experimental restrictions. In this work, we looked into origin of this phenomenon in lead oxyfluoroborate glasses, resulting in β-PbF2 growth even below the corresponding glass transition temperatures, through achieving a systematic study of not only Pt,Ag/Glass/Ag,Pt- but also Pt,Ag/Glass/YSZ:PbF2/Ag,Pt-type cells, where YSZ:PbF2 represents a two-phase system (formed by Y2O3-doped ZrO2 and PbF2). It is demonstrated that crystallization induction in these glasses involves Pb2+ ions reduction at the cathode, the phenomenon being, however, confirmed only when the F− ions were simultaneously also able to reach the anode for oxidation, after assuring either a direct glass–anode contact or percolation pathways for free fluoride migration across the YSZ:PbF2 mixtures. A further support of this account is that the electrochemically induced β-PbF2 phase crystallizes showing ramified-like microstructure morphology that arises, accordingly, from development of electroconvective diffusion processes under electric field action.

La3+-induced (micro)structural changes and origin of the relaxor-like phase transition in ferroelectric lead barium niobate electroceramics

Lead barium niobate (Pb1−xBaxNb2O6, PBN) ferroelectric materials have been and are the subject of numerous studies in literature due to their potential for wide-ranging applications in the electronic industry. In this work, La3+-doped Pb0.56Ba0.44Nb2O6 (PBN44) electroceramics were prepared and investigated in terms of X-ray diffraction, scanning electron microscopy, thermal spectra of dielectric permittivity, Curie–Weiss law, and hysteresis loop characteristics. It was noted that La3+ doping favors the formation of orthorhombic mm2 phase in PBN44, which originally shows only the tetragonal 4mm symmetry-related phase. In particular, the PBN44 material with 1 wt% La2O3 displays (micro)structural characteristics and dielectric properties similar to those from PBN compositions lying within their morphotropic phase boundary region. Our results also show that La3+ is able to promote a change of the ferroelectric to paraelectric phase transition appearance of PBN44 from pseudo-normal to really diffuse. However, conversion to a diffuse plus relaxor transition behavior reveals directly linked to incommensurate superstructures present and dielectrically-active in PBN materials toward low temperatures, with an intrinsically frequency-dispersive dielectric response. This statement is also supported by observation of hysteresis loops showing a transformation trend to pseudo-slim-like, even in the normal-like ferroelectrics, when moving into the temperature region of incommensuration manifestation.

Structural characterization of La3+ modified Bi4Ti3O12 ferroelectric ceramics by XRD and XAS techniques

Bi4xLaxTi3O12 (BLT) ceramics were prepared and studied in this work in terms of La3+ modified the long and short range order structure. According to the results obtained from X‐ray diffraction (XRD), the solubility limit (xL) of La3+ into the Bi4Ti3O12 (BIT) matrix was here found to locate slightly above x = 1.5 and all samples bellow this limit presents reflection peaks characteristic of the orthorhombic Bi4Ti3O12 sample. X‐ray absorption near edge spectra (XANES) shows that the local order around Ti, Bi and La atoms is not significantly affected by the increase of La amount as well as by the fact that La atoms could be located on different Bi sites.