Danjela Kuscer

Electrical and microstructural characterisation of (La0.8Sr0.2)(Fe1 − xAlx)O3 and (La0.8Sr0.2)(Mn1 − xAlx)O3 as possible SOFC cathode materials

The perovskites with nominal compositions (La0.8Sr0.2)(Fe1 − xAlx)O3 and (La0.8Sr0.2)(Mn1 − xAlx)O3 (x from 0 to 0.94) were evaluated as possible solid oxide fuel cell (SOFC) cathodes. Cell parameters of solid solutions were calculated. The electrical and microstructural characteristics and high temperature interactions with YSZ were studied. As compared with ‘pure’ perovskites, doping with strontium and aluminium decreases and increases their specific resistivity, respectively. The incorporation of alumina and strontium oxide substantially reduces the sinterability resulting in a rather porous, fine grained microstructure. The reaction rate between perovskite materials and YSZ at high temperatures is higher for lanthanum manganites than for lanthanum ferrites, and the partial exchange of cations on ‘B’ sites with aluminium decreases the reaction rate.

Interactions between a thick film LaMnO3 cathode and YSZ SOFC electrolyte during high temperature ageing

Abstract Reactions between thick film LaMnO 3 cathodes and YSZ substrates were investigated by ageing at 1450 °C. Also, subsolidus phase equilibria in the La 2 O 3 Mn 2 O 3 ZrO 2 system were confirmed by X-ray powder diffraction analysis. A La 2 Zr 2 O 7 phase formed on the YSZ/LaMnO 3 interface. The Mn 2 O 3 released in the reaction partly diffused in to YSZ and partly evaporated. Diffusion of Y and Zr into LaMnO 3 was not detected. After a prolonged period of ageing (100 h) the cathode layer is separated along most of the YSZ/LaMnO 3 interface with only a few sintered contacts. On the surface of large pores where LaMnO 3 separated from the YSZ substrate “islands” of La silicate phase was found. Silica originated from grain boundaries in YSZ. The presence of La silicate phase could be a reason for separation of the LaMnO 3 layer from YSZ substrate after prolonged high-temperature ageing.

Low-temperature sputtered mixtures of high-κ and high bandgap dielectrics for GIZO TFTs

— This paper discusses the properties of sputtered multicomponent amorphous dielectrics based on mixtures of high-κ and high-bandgap materials and their integration in oxide TFTs, with processing temperatures not exceeding 150°C. Even if Ta2O5 films are already amorphous, multicomponent materials such as Ta2O5—SiO2 and Ta2O5—Al2O3 allow an increase in the bandgap and the smoothness of the films, reducing their leakage current and improving (in the case of Ta2O5—SiO2) the dielectric/semiconductor interface properties when these dielectrics are integrated in TFTs. For HfO2- based dielectrics, the advantages of multicomponent materials are even clearer: while HfO2 films present a polycrystalline structure and a rough surface, HfO2—SiO2 films exhibit an amorphous structure and a very smooth surface. The integration of the multicomponent dielectrics in GIZO TFTs allows remarkable performance, comparable with that of GIZO TFTs using SiO2 deposited at 400°C by PECVD. For instance, with Ta2O5—SiO2 as the dielectric layer, field-effect mobility of 35 cm2/(V-sec), close to 0 V turn-on voltage, an on/off ratio higher than 106, a subthreshold slope of 0.24 V/dec, and a small/recoverable threshold voltage shifts under constant current (ID= 10 μA) stress during 24 hours are achieved. Initial results with multilayers of SiO2/HfO2—SiO2/SiO2 are also shown, allowing a lower leakage current with lower thickness and excellent device performance.

Correlation between the defect structure, conductivity and chemical stability of La1–ySryFe1–xAlxO3–δ cathodes for SOFC

Abstract The chemical compatibility of a La 1– y Sr y Fe 1– x Al x O 3- δ cathode and an yttria-stabilised ZrO 2 electrolyte (YSZ) was studied for solid-oxide fuel cell (SOFC) applications. The Al addition to the LaFe 1– x Al x O 3 reduces the reactivity with the YSZ associated with increasing tolerance factor. The Sr addition to the LaFe 1– x Al x O 3 leads to Fe 4+ and anion-vacancy formation. The Sr addition hinders the formation of La 2 Zr 2 O 7 , but at y ⩾0.2 promotes the formation of SrZrO 3 at the La 1– y Sr y Fe 1– x Al x O 3– δ /YSZ interface. At low p(O 2 ) the formation of secondary phases is pronounced what could be related to the formation of anion vacancy.

Modeling of a high frequency ultrasonic transducer using periodic structures

Solidly mounted integrated transducers with a Bragg cell inserted between the piezoelectric film and the substrate are investigated for high frequency ultrasonic applications. A numerically stable recursive one dimensional transmission/reflection model was used to analyze the behavior of the periodic structure. This theoretical analysis includes the study of the influence of the acoustic properties of the constitutive layer, the effect of the number of cells and their arrangement. A 35 MHz integrated transducer consisting in a PZT ceramic laid down on a Au/PZT Bragg cell deposited on a porous substrate was fabricated and characterized. Both theoretical and experimental results highlight the interest of using a periodic structure for high frequency ultrasonic applications.

Interactions between lead-zirconate titanate, polyacrylic acid, and polyvinyl butyral in ethanol and their influence on electrophoretic deposition behavior.

Electrophoretic deposition (EPD) is an attractive method for the fabrication of a few tens of micrometer-thick piezoelectric layers on complex-shape substrates that are used for manufacturing high-frequency transducers. Niobium-doped lead-zirconate titanate (PZT Nb) particles were stabilized in ethanol using poly(acrylic acid) (PAA). With Fourier-transform infrared spectroscopy (FT-IR), we found that the deprotonated carboxylic group from the PAA is coordinated with the metal in the perovskite PZT Nb structure, resulting in a stable ethanol-based suspension. The hydroxyl group from the polyvinyl butyral added into the suspension to prevent the formation of cracks in the as-deposited layer did not interact with the PAA-covered PZT Nb particles. PVB acts as a free polymer in ethanol-based suspensions. The electrophoretic deposition of micro- and nanometer-sized PZT Nb particles from ethanol-based suspensions onto electroded alumina substrates was attempted in order to obtain uniform, crack-free deposits. The interactions between the PZT Nb particles, the PAA, and the PVB in ethanol will be discussed and related to the properties of the suspensions, the deposition yield and the morphology of the as-deposited PZT Nb thick film.

Processing of Ferroelectric Ceramic Thick Films

The rapid development of the electronics industry has created the need for highperformance, high-reliability, miniaturised electronic components integrated into various electronic devices. Additional requirements, such as the desired size and weight, low cost, low power consumption, and portability, should be considered to make the devices user friendly and widely accessible. Attempts to miniaturise discrete elements have generally failed due to the difficulty in handling and assembly. A lot of waste material and high costs are also involved. In this approach, the ceramic parts are manufactured as a bulk ceramic, followed by a reduction in size by cutting, polishing, etc., to specified dimensions. The final step is the assembling of a thin layer of ceramic with the other components. This topdown approach imposes limits on the minimum dimensions of the manufactured parts. It constrains the geometry of the parts to simple shapes, like discs, plates, rings, cylinders, etc.

High-performance PMNߝPT thick films

This article describes some of our work on 0.65Pb(Mg_1/3Nb_2/3)O₃-0.35PbTiO₃ (0.65PMN-0.35PT) thick films printed on alumina substrates. These thick films, with the nominal composition 0.65Pb(Mg_1/3Nb_2/3)O₃-0.35PbTiO₃, were produced by screen-printing and firing a paste prepared from an organic vehicle and pre-reacted fine particles of a very chemically homogeneous powder. To improve the adhesion of the 0.65PMN-0.35PT to the platinized alumina substrate, a Pb(Zr_0.53Ti_0.47)O₃ 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 (Al₂O₃) 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.

Electrophoretic Deposition of Lead-Zirconate-Titanate Perovskite Thick Films with Low Sintering Temperature

We have studied the processing of lead-zirconate-titanate PbZr0.53Ti0.47O3 (denoted PZT) based thick films using an electrophoretic deposition (EPD) process in order to obtain the active film with desired thickness and porosity at low the sintering temperature. The colloidal suspensions were prepared by mixing the ceramic powder in ethanol with the controlled addition of a polyelectrolyte salt and an organic base. In order to optimise the deposition process, the stability of the PZT and Pb5Ge3O11 (denoted PGO) particles in ethanol-based suspensions was studied. The electrophoretic deposition of PZT-PGO was performed from a mixture of optimal PZT and PGO suspensions in an appropriate molar ratio. PZT and PGO acting as a sintering aid were deposited on an Al2O3/Au substrate at a constant applied current and sintered at 850 oC. The thickness of the deposit was controlled by the deposition time and the applied current. PZT-PGO thick films deposited at 1.2 mA for 60 seconds and sintered at 850oC for 8 hours exhibited a room-temperature dielectric permittivity of 1050, dielectric losses of 0.038, a remanent polarisation Pr of 29 C/cm2, a coercive field Ec of 21 kV/cm and a d33 of 97pC/N.

Preliminary data on subsolidus phase equilibria in the La2O3-(Al2O3/Fe2O3)-Y2O3 and La2O3-(AI2O3/Fe2O3)-ZrO2 systems

A fuel cell is a device for direct conversion of chemical energy into electrical energy. Oxidant is fed to the cathode and reducent (fuel) to the anode. The electrolyte, through which the ion current flows, also prevents the mixing of oxidant and fuel. The principle of fuel cell operation was reported in 1839 by Sir William Grove [1]. For a description of fuel cell development see, for example, [2]. High temperature solid oxide fuel cells (SOFC) work at temperatures of up to 1000 °C. Due to the high operating temperatures of SOFC, the choice of materials is mainly limited to ceramics. A comprehensive review of materials for SOFC was published by Minh [3]. The solid electrolyte in SOFC cells is usually yttria stabilized cubic zirconia (YSZ). The oxygen accepts electrons at the cathode and moves as an ion through the dense Z r Q ceramic. At the anode ions combine with fuel and release electrons. The fuel is either hydrogen, a H2/CO mixture, or hydrocarbons because the high temperature of operation makes possible the internal (in situ) reforming of hydrocarbons with water vapour [4]. The advantage of SOFC is their high efficiency, of 50-60%, while some estimations are even up to a yield of 70-80% [5-8]. Also, nitrous oxides are not produced and the amount of CO2 released per kilowatt hour is, due to the high yield, around 50% less than for power sources based on combustion. Annually, around 109 US