The Low‐Temperature Cosintering of Cobalt Ferrite and Lead Zirconate Titanate Ceramic Composites

Magneto‐electric (ME) ceramic composites of cobalt ferrite (CoF) and lead zirconate titanate (PZT) were prepared by mechanical mixing of the constituent powders followed by cosintering. The cosintering conditions for nano‐sized CoF and submicrometer‐sized PZT powders were studied in detail. It was found that the CoF powder needs to be presintered at 700°C for 2 h to minimize the differences in the sintering kinetics of the constituent powders. Despite the low cosintering temperatures (900°C–1000°C) the interdiffusion of the cations from both phases was confirmed with energy‐dispersive X‐ray analysis and X‐ray diffraction. Efforts were made to optimize the cosintering conditions to prepare dense ceramic ME composites, which showed the converse ME effect.     

Processing and properties of 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thick films

We have investigated the processing of 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ (PMN–PT) thick films on platinised alumina substrates. Nanosized PMN–PT powder with 2 mol% of excess PbO was prepared by high-energy milling and deposited on the substrate using screen-printing technology. The films were then sintered at 950 °C in a PbO-rich atmosphere. The influence of the sintering time and the amount of PbO-containing packing powder was studied and related to the structural, microstructural, dielectric and piezoelectric properties of the film. In order to obtain a homogeneous and dense thick film without any secondary phase, the PMN–PT films had to be sintered in the presence of a PbO-based liquid phase that had to be completely removed from the thick film during the final stage of the sintering. Under optimal sintering conditions we obtained a room temperature relative dielectric permittivity of 3600, dielectric losses of 0.036, a T_m of 174 °C, a permittivity at the T_m of 21,000 and a d₃₃ of 140 pC/N.     

0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thick films prepared by electrophoretic deposition from an ethanol-based suspension

We have investigated the processing of 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ (denoted PMN–PT) thick films using an electrophoretic deposition process (denoted EPD), with the PMN–PT particles suspended in an ethanol-based suspension. The zeta-potential and the viscosity were measured to identify the conditions for the preparation of a stable suspension suitable for the EPD. The applied voltage, the deposition time, the chemical composition of the suspension and the concentration of the powder were investigated in order to obtain a high-quality PMN–PT deposit with a target thickness of about 50 μm. The PMN–PT thick films prepared from stoichiometric and PbO-excess suspensions by sintering at 950 and 1100 °C were examined by X-ray powder-diffraction analysis and scanning electron microscopy. The highest functional response was obtained for a homogeneous, dense, crack-free PMN–PT thick film prepared from a PMN–PT suspension with excess PbO. The film was deposited at a constant voltage of 10 V and for a time of 120 s, followed by sintering at 1100 °C in a PbO-rich atmosphere. The film's properties were as follows: a dielectric permittivity ? of 20,250 at a T_m of 172 °C, a remanent polarization of 17 μC/cm² and a coercive field of 5 kV/cm.     

Processing and electromechanical properties of lead zirconate titanate thick films by electrophoretic deposition

Electrophoretic deposition (EPD) was used for the fabrication of piezoelectric [lead zirconate titanate (PZT)] thick films on alumina substrates. The EPD was performed in constant current mode from an ethanol based suspension consisting of PZT and PbO particles. The influence of addition of ethyl cellulose (EC) and sintering temperature on the thickness, density, homogeneity and functional response of PZT thick films is studied. Results show that the highest electromechanical performance is obtained for the PZT thick films sintered at 900 or 950°C, with a thickness coupling factor k_t of 50%. The addition of EC influenced the thickness of the PZT thick films but had only minor effect on the porosity content for sintering temperatures over 900°C. Moreover, elastic constants of the thick films based on the suspension with EC were lower, which leads to lower acoustic impedance (15?MRa) while maintaining high (k_t) value. In this last case, a better acoustic matching can be expected with propagation media such as water or biological tissues for ultrasound medical imaging applications.     

Spatially Resolved Photoemission and Electrochemical Characterization of a Single-Chamber Solid Oxide Fuel Cell

The present paper pinpoints the feasibility of in operando near-ambient pressure characterization of an operating single chamber solid oxide fuel cell (SC-SOFC) by means of spatially resolved X-ray photoelectron spectroscopy. The results are based on an innovative and unique setup expanding the capabilities of the scanning photoelectron microscopes. In particular, the impact of getting access to the low mbar range on the very observability of chemical-state changes in an LSM cathode have been highlighted. Ex situ and in situ electrochemical measurements at OCP and close circuit cell conditions complemented the photoemission mapping and spectroscopy. The latter has evidenced the evolution of the chemical state of Sr upon the exposure of the typical SC-SOFC CH₄–O₂ gas feed.     

Characterization of the Amorphous Phase and the Nanosized Crystallites in High-Energy-Milled Lead–Magnesium–Niobate Powder

We have studied the formation of lead–magnesium–niobate Pb(Mg_1/3Nb_2/3)O₃ perovskite powder produced by high-energy milling of the constituent oxides. By applying a low ball-impact energy and a low ball-impact frequency we were able to identify the reaction sequences. The crystal structure and the amount of crystalline and amorphous phases in the powder were determined using a Rietveld refinement. The morphology, structure and chemical composition of the powder were investigated by transmission electron microscopy. The surface composition, possible contamination and the chemical states of the elements were analyzed by X-ray photoelectron spectroscopy. In the early stage of milling the constituent oxides comminute, refine to nanosize dimensions and become amorphous. In the second step the Pb(Mg_1/3Nb_2/3)O₃ nucleates from the amorphous regions where the stoichiometry corresponds to the perovskite. The perovskite phase nucleates and subsequently grows over the course of the milling. Simultaneously, the pyrochlore phase nucleates from the Pb- and Nb-rich regions; however, its nucleation and recrystallization stop after a certain milling time. The powder mixture consists of nanosized Pb(Mg_1/3Nb_2/3)O₃ particles and an amorphous phase when milled for 94 h. No traces of contamination from the milling media were detected.     

Formation of 0.65 Pb(Mg1/3Nb2/3)O3–0.35 PbTiO3 Using a High-Energy Milling Process

We have investigated the synthesis of lead-magnesium-niobate–lead-titanate perovskite powder using high-energy milling of the constituent oxides. The compositions of the crystalline and amorphous phases as a function of milling time were determined with X-ray powder diffraction using a Rietveld refinement. The perovskite can be formed by two reaction routes. In the initial stage of milling, it is formed directly from the highly activated nano-sized constituent oxides, and after a certain milling time it is mainly formed from a pyrochlore phase. The obtained as-milled powder consists of crystalline perovskite and an amorphous phase that crystallizes into the perovskite after heating at 800°C.     

High-performance PMN-PT thick films

This article describes some of our work on ?.??Pb(Mg?/?Nb(?/?)O?-?.??PbTiO? (0.65PMN-0.35PT) thick films printed on alumina substrates. These thick films, with the nominal composition ?.??Pb(Mg?/?Nb(?/?)O?-?.??PbTiO?, were produced by screen-printing and firing a paste prepared from an organic vehicle and pre-reacted fine particles of avery chemically homogeneous powder. To improve the adhesion of the 0.65PMN-0.35PT to the platinized alumina substrate,a Pb(Zr?.??Ti?.??)O? layer was deposited between the electrode and the substrate. The samples were then sintered at 950 °C for 2 h with various amounts of packing powder on the alumina (Al?O?) substrates. The sintering procedure was optimized to obtain dense 0.65PMN-0.35PT films. The films were then characterized using scanning electron microscopy as well as measurements of the dielectric and piezoelectric constants.The electrostrictive behavior of the 0.65PMN-0.35PT thick films was investigated using an atomic force microscope(AFM). Finally, substrate-free, large-displacement bending type actuators were prepared and characterized, and the normalized displacement (i.e., the displacement per unit length) of the actuators was determined to be 55 μm/cm at 3.6 kV/cm.     

A- and B-compensated PLZT x/90/10: Sintering and microstructural analysis

Structure–property relationship has been performed in the lanthanum-modified lead–zirconate–titanate (PLZT) with a lanthanum content of 6 and 10 at.% and a Zr/Ti ratio of 90/10 as a function of A- and B-compensation model. The X-ray powder-diffraction analysis, electron microscopy, energy-dispersive spectroscopy, density measurements and sintering behavior have evidently demonstrated that it is possible to tailor the microstructural properties of PLZT with using selected type of the compensation. The densification of B-site compensated PLZT occurs at lower temperatures and leads to higher density when compared to A-site compensated PLZT. A presence of PbO at the grain boundaries in B-site compensated PLZT and the lack of it in A-site compensated PLZT leads to different sintering mechanism. The dielectric response characteristics of A- and B-site compensation model do not vary significantly.