Dynamic aspects of cerium dioxide sintering: HT-ESEM study of grain growth and pore elimination

International audienceSintering of CeO2 is studied in situ by high temperature scanning environmental microscopy (HT-ESEM) at T = 1400 °C. The morphological modifications of a single grains population are recorded for 6 h. Kinetic parameters are extracted from image series. The local grain growth determined from the single population studied in situ is compared to the general grain growth obtained by classical ex situ technique. Using HT-ESEM for sintering study is validated. The grain boundary velocities range between 0 and 5 μm h−1, with a mean value of about 1 μm h−1. The migration of the intragranular surface pores is described. Their velocities range between 0.4 and 1.2 μm h−1 and depend on pore diameters: the smaller the pore, the faster the pore velocity. The time required to fill a pore that arises at the sample surface is determined as a function of pore diameter. The time for pore elimination dependence with the pore diameters is also established

Oppurtunities for Functional Oxides in YO1.5-TiO2-ZrO2 System: Applications for Novel Thermal Barrier Coatings

[to be added

Development of Piezoelectric BCZT Ceramics as Electroactive Bone Implant Material

The functionalisation of current load-bearing bone implant metals with piezoelectric ceramics is envisioned as a novel opportunity for promoting tissue healing and improving the strength of the bone-implant interface through the in vivo application of power-source independent electrical cell stimulation. Calcium- and zirconium-doped barium titanate ((Ba1-xCax)(ZryTi1-y)O3 or BCZT) is a piezoelectric ceramic displaying exceptional piezoelectric responses. However, it has yet to be utilised as an implantable biomaterial and therefore, this thesis aims to assess several questions regarding its relevance to the biomedical field. Cell responses are triggered by specific electrical stimulation requirements. As such, it is important to assess the magnitude of the piezoelectric response of BCZT and the ability to tailor the stimulation for the local physiological environment. In the first part of the thesis, good piezoelectric responses were obtained for four compositions of BCZT, influenced by the contained proportions of tetragonal, orthorhombic, and rhombohedral phases. The implementation of BCZT as an implantable biomaterial requires an understanding of the material interactions with human cells and liquid physiological environments. The biocompatibility of BCZT was therefore assessed over a 10-day period with respect to human osteoblast and endothelial cells. Cell viability and proliferation tests showed BCZT to be not cytotoxic. In addition, an evaluation of ion release from BCZT in deionised water and sodium chloride solution over 90 days revealed an incongruent dissolution behaviour, suggesting the potential of a conditioning treatment that can be performed for safe implantation. Physiological mechanical loads on BCZT may influence the stability of the piezoelectric response. As such, the mechanical depolarisation resistance was investigated with respect to composition and grain size by static compressive mechanical loading over a 60 min period. Both composition and grain size were observed to influence the mechanical depolarisation resistance, highlighting routes for optimising the response without toxic elemental dopants. In the last part of the thesis, a process for integrating BCZT with existing load-bearing metal implant materials was investigated. BCZT ceramics were spray-coated onto stainless steel and Ti-alloy. Several parameters were modified with the aim of developing mechanically robust ceramic coatings. However, a highly hygroscopic region was formed during heat-treatment, affecting the stability of the ceramic coatings. It is expected that the metal/ceramic attachment and stability can be improved through the implementation of several optimised synthesis parameters. The work in this thesis demonstrates that BCZT is promising as a relevant electrically stimulative biomaterial. It displays a good piezoelectric response that, in addition to the mechanical depolarisation resistance, can be optimised by compositional or grain size tailoring. The material is also non-cytotoxic and a feasible pre-implantation treatment has been proposed. By careful monitoring of the synthesis procedure, it is expected that a reliable metal/ceramic attachment should be achievable for the successful integration of BCZT ceramics as functional coatings on existing implant metals

Advanced materials from gels. Proceedings of the Sixth International Workshop on Glasses and Ceramics from Gels

International workshop on glasses and ceramics from gels (6th. 1991. Sevilla

Extrinsic contributions to the piezoelectric response of lead-based ferroelectrics

The present study aims at a better understanding of the high piezoelectric properties encountered in lead-based ferroelectrics by focusing on the extrinsic contributions to the response. The main characteristics of these materials are the highly nonlinear character of the electro-mechanical response and the presence of a morphotropic phase boundary (MPB) where properties are reaching a maximum. Thus, our approach was first to develop a new description for the piezoelectric hysteresis and nonlinearities and second to investigate MPB effects on the extrinsic contributions to the piezoelectric response. For these purposes, lead titanate (PT), lead zirconate titanate (PZT), and lead nickel niobate-lead zirconate titanate solid solution (PNN-PZT) were chosen as prototype compositions. The sample preparation was classical for the first two compounds whereas two new synthesis routes of PNN-PZT were developed. The first consists in the preparation of a B-site precursor combining all the perovskite B-site cations before calcination with lead oxide, enhancing chemical homogeneity and insuring good reproducibility in the final properties. The second processing method aimed at more time efficiency: the use of a nickel hydroxy-carbonate instead of nickel oxide permitted to obtain in one single calcination step a pure perovskite phase exhibiting properties as high as those obtained for two firing steps procedures. The piezoelectric hysteresis and nonlinearity description was undertaken using the Preisach formalism, first developed in ferromagnetism. This approach considers that each hysteretic system satisfying the wiping-out and the congruency properties can be seen as composed of bistable units characterized by distributed coercive and bias fields. In our case, the use of a general expression for the unit parameters distribution function permitted to describe the most significant piezoelectric coefficient nonlinearities such as applied field dependence, saturation or threshold fields. Moreover, a minimal expression describing the piezoelectric nonlinearity in lead-based compounds was derived from experimental determination of the distribution function topography. Advanced piezoelectric hysteresis modelling was attained by coupling the classical viscous descriptions with Preisach-inspired loops expressions. This allowed to separate piezoelectric losses into viscous and field dependent parts. Moreover, the possibility of extracting distribution function parameters from the loops was established. The versatility of the Preisach hysteresis description was also demonstrated by deriving pinched ferroelectric loops from the supposed microscopical mechanisms in hard-doped ferroelectrics. Morphotropic phase boundary study started with the derivation of a composition-temperature phase diagram for PNN-PZT using dielectric and pyroelectric measurements along with X-ray diffraction. The MPB of this solid solution was shown to be strongly curved toward the rhombohedral side. Therefore, certain compositions undergo a tetragonal to rhombohedral transition upon cooling. The influence of electric field and thermal history on this morphotropic transition was first investigated in view of optimizing the poling conditions of such materials. Then, the piezoelectric extrinsic contributions at MPB for both PZT and PNN-PZT were studied as a function of composition and temperature. The peak of properties occurring in this region was assigned to intrinsic effects coupled with an extension of the irreversible contributions. Using the developed hysteresis and nonlinearity formalism, this extrinsic contributions increase could be related to an extended response of each defect rather than to an increase in the defects density