Elisa Mercadelli

Piezoelectric properties of lead-free submicron-structured (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics from nanopowders

Submicron-structured (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT6) ceramics were obtained from nanometric powder synthesized by sol?gel auto-combustion at 500??C. Hot-pressing at low temperatures and a combination of this with recrystallization, still moderate in order to reduce the loss of volatile elements, have been tested. Material properties, including all losses, were determined at the resonances of thin discs using Alemany et al software. Ceramics hot-pressed at 700?800??C for 2?h have a pseudo-cubic structure, a grain size of a few hundred nanometers and are homogeneous. Both their crystal structure and the lack of sintering prevent their poling. For ceramics hot-pressed at 950??C for 3?h, Bi or Bi0.5Na0.5 loss, together with low piezoelectric properties (d33 = 60?pC?N ? 1, kp = 8.3% and kt = 9.5%), was observed. Recrystallization at 1000??C-1?h of ceramics hot-pressed at 700 and 800??C for 2?h keeps the submicron structure, reduces porosity and prevents off-stoichiometry. Mechanical and piezoelectric losses are also reduced and coupling factors increased (kp = 24.6%, kt = 36.4%). The best piezoelectric coefficient obtained in these ceramics (d33 = 143?pC?N ? 1) is comparable with those reported for coarse-grained ceramics.

Enhanced properties for ultrasonic transduction, phase transitions and thermal depoling in 0.96(Bi0.5Na0.5)TiO3-0.04BaTiO3 submicrometre-structured ceramics

Submicrometre structured (grain size ≈500 nm), dense (Bi0.5Na0.5)0.96Ba0.04TiO3 ceramics were obtained from sol–gel auto-combustion nanopowders by hot-pressing (700–950 °C) and subsequent recrystallization (1000–1050 °C). Electromechanical coefficients were obtained by analysis of the resonance spectra of thin discs using the Alemany et al software. The real part of the room temperature set of coefficients of the best performing materials ( , d31 = (−22.68 + 0.55i)pC N−1, kt = 44.5%, kp = 21.1%) can be compared with those of coarse-grained ceramics and d33 (95 pC N−1) is higher. Shear-related coefficients were obtained from thickness poled and length excited plates ( , d15 = (108.3 − 21.4i) pC N−1 and k15 = 39.2%). At the depolarization temperature, Td = 153 °C, the dielectric loss, tan δ(T), of poled samples shows a maximum and the planar resonance virtually vanishes. Shear resonance of thickness-poled plates and weak planar electromechanical resonance are observed above Td. The relaxor behaviour extends up to the isotropization point, Ti = 238 °C. This can be understood as due to the coexistence of the room temperature ferroelectric phase in the stability range of the low-temperature non-polar phase at zero field, between Td and Ti.

Stainless steel porous substrates produced by tape casting

In this work the technological issues related to the production of tape cast large-area porous stainless steel supports for Solid Oxide Fuel Cells (SOFC) applications were carefully investigated. The slurry formulation was optimized in terms of amount and nature of the organic components needed: rice starch and polymethyl metacrylate were found to be, respectively, the most suitable pore former and binder because easily eliminated during the thermal treatment in reducing atmosphere. The compatibility of the binder system chosen with the most widely used solvents for screen printing inks was also evaluated. Finally the influence of the sintering temperature and of the refractory supports to be used during the thermal treatments onto the production of porous stainless steel supports was discussed. The whole process optimization allows to produce flat, crack-free metallic substrate 900-1000 μm thick, dimensions up to 5×5 cm and with a tailored porosity of 40% suitable for SOFCs application.

Study of reversible SOFC/SOEC based on a mixed anionic-protonic conductor

This paper deals with the fabrication and electrochemical study of a high temperature solid electrolyte supporting cell operating as SOFC (Solid Oxide Fuel Cell) and SOEC (Solid Oxide Electrolysis Cell). The cell is based on a dual membrane (DM) electrolyte design, advantageously separating the cell into three different chambers: hydrogen side, oxygen side, and dual membrane (DM), where H2O production or splitting takes place in SOFC or SOEC mode respectively. The supporting electrolyte consists of a dense/porous/dense tri-layer, exclusively made of BaCe0.85Y0.15O3−δ (monolithic design), which is a mixed anionic-protonic conductor. The assembly was fabricated by tape casting, adding pore formers to control porosity. The cell was then electrochemically studied under different operating conditions of temperature, overpotentials and gas feeding, either in SOFC and SOEC mode. From the results presented here, it can be observed that, in spite of dense and thick electrolyte layers and platinum electrodes, the electrochemical study of the cell showed: (i) promising power density, (ii) interesting SOFC/SOEC operating mode reversibility, (iii) proved H2O production in the porous dual membrane when the cell operates as a fuel cell, and proved splitting of the H2O molecules contained in the porous dual membrane when the cell operates as an electrolyser. Investigations of cell performance degradation were also conducted.

Characterization of Nanostructured Phases and Peculiar Phase Transitions in BNBT Lead-Free Piezoceramics

Submicron-structured (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT6) dense ceramics, from nanometric powder synthesized by sol gel auto-combustion at 500°C and obtained by hot-pressing (800°C-2h) and subsequent recrystallization at moderate temperature (1000-1050°C-1h), have been studied. In-situ measurements at the shear mode of electromechanical resonance of non-standard thickness-poled shear plates as a function of the temperature show higher depolarization temperature than measurements at the radial mode of thin disks. Shear mode related material coefficients are measurable up to 160°C, being k15≈30% and d15≈250 pC.N-1 at 130°C. Depolarization is a complex phenomena caused by a ferroelectric (FE) macrodomains thermal randomization and a phase transition from the field-induced FE phase to a relaxor phase. The early stage of such a transition involves a non-negligible piezoelectricity arising most probably by the percolative coexistence of ferroelectric macrodomains in the resonator under the given stress field for each resonance mode.

Anisotropy and dynamic thermal depolarization of 0.96(Bi 0.5 Na 0.5 )TiO 3 -0.04 BaTiO 3 lead-free piezoceramics

Undoped (Bi_0.5Na_0.5)_0.96Ba_0.04TiO₃ ceramics were prepared from mixed oxides and sol-gel autocombustion powder. Similar dielectric and electromechanical properties were found for both but sol-gel derived ceramics can be obtained with a lower thermal budget, which prevent losses of volatiles. Dielectric anisotropy is observed. All the features of the thermal evolution curves of ε^T₁₁ are observed at temperatures clearly displaced towards higher values with respect to those of the ε^T₃₃ curves. There is also anisotropy in the depoling process. When depoling is monitored in-situ using the shear mode of thickness poled plates, resonance is observed at T_d >; 150°C. Besides, hot in-situ measurements of time dependence of the resonaces of disks and plates at temperatures below T_d showed that the depolarization of BNBT4 is a dynamical process.

Phase Diagram of the Ferroelectric Perovskite (Na0.5Bi0.5)1−xBaxTiO3 by Anelastic and Dielectric Relaxation Measurements

Anelastic and dielectric relaxation measurements have been carried out on poled and unpoled samples of the ferroelectric perovskite (Na0.5Bi0.5)1−xBaxTiO3 (NBT-xBT), with composition in the range between pure NBT and the morphotropic phase boundary, 0 ≤ x ≤ 0.08. The complex elastic compliance spectra contain clear indications of both the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the phase diagram, which is difficult to obtain by diffraction techniques due to the very low distortions in both the tetragonal and rhombohedral phases and to the structural disorder in the Na/Bi sublattice. An extensive study is made for concentrations in the region of the morphotropic boundary (x ∼ 0.06) in order to find possible signatures of monoclinic phase, as for the case of PbZr1-xTixO3 (PZT). The main features in the anelastic curves are compared with those in the dielectric spectra and are tentatively related to different modes of octahedral rotations and polar cation shifts.

Elastic and Dielectric Measurements of the Structural Transformations in the Ferroelectric Perovskite (Na1/2Bi1/2)1-XBaXTiO3

The perovskite (Na1/2Bi1/2)TiO3 (NBT) undergoes a series of structural and polar transitions starting from the high temperature paraelectric phase: tetragonal paraelectric, tetragonal antiferroelectric, rhombohedral antiferroelectric and finally rhombohedral ferroelectric, according to neutron diffraction and dielectric spectroscopy. In solid solution with BaTiO3 (BT) the ferroelectric phase changes from rhombohedral to tetragonal, at the so-called morphotropic phase boundary, and the phases at higher temperature become ill-defined, also because of the large lattice disorder induced by the coexistence of differently charged cations in the same sublattice. Combined dielectric and anelastic spectroscopy measurements are presented, which clarify some issues related to the phase transitions in NBT-BT. The influence of Ba substitution on the tetragonal antiferroelectric phase is determined for the first time, and the possibility that a monoclinic phase, although with very short coherence length, exists near the morphotropic phase boundary is discussed in view of a large maximum of the elastic compliance.

All-ceramic asymmetric membranes with superior hydrogen permeation

BaCe0.65Zr0.20Y0.15O3−δ–Gd0.2Ce0.8O2−δ (BCZY–GDC) composite with a planar asymmetrical architecture was investigated for the first time as a hydrogen separation membrane in the 600 °C to 750 °C temperature range. Asymmetrical membranes comprising a 20 μm thick dense BCZY–GDC layer supported on a 600 μm thick porous substrate with the same composition were successfully produced through a tape casting process. The influence of the dense layer composition on hydrogen separation and the catalytic activation of the dense membrane layer and porous support were thoroughly studied. The hydrogen permeation process was further characterized by varying the environment humidification degree and temperature and by shifting the sweep and feed gas compositions. This work highlights the importance of preserving the dual proton (BCZY) and electron (GDC) conductor phases during the sintering step of the membrane to enhance H2 permeation; a value of 0.47 mL min−1 cm−2 at 750 °C was obtained, which is currently one of the highest H2 fluxes obtained for all-ceramic proton-conducting membranes. In addition, a strong improvement is achieved when the feed/sweep sides of the membrane are reversed, i.e. H2 feeding in the support side leads to a flux increase up to 0.68 mL min−1 cm−2 at 750 °C when only the sweep side is hydrated. These results demonstrate that these asymmetric dual phase BCZY–GDC membranes are the most promising candidates for H2 separation applications at high temperatures.